Actin dynamics and the evolution of the memory trace.

BRES : 43974

pp: 2212ðcol:fig: : NILÞ

Model7 brain research ] (]]]]) ]]]–]]]

121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 Q1 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180

Available online at www.sciencedirect.com

www.elsevier.com/locate/brainres

Research Report

Actin dynamics and the evolution of the memory trace Jerry W. Rudyn Department of Psychology and Neuroscience, University of Colorado, 345 UCB, Boulder, CO 80309, USA

art i cle i nfo

ab st rac t

Article history:

The goal of this essay is to link the regulation of actin dynamics to the idea that the

Accepted 3 December 2014

synaptic changes that support long-term potentiation and memory evolve in temporally overlapping stages—generation, stabilization, and consolidation. Different cellular/mole-

Keywords:

cular processes operate at each stage to change the spine cytoarchitecture and, in doing so,

Consolidation

alter its function. Calcium-dependent processes that degrade the actin cytoskeleton

Cytoskeleton

network promote a rapid insertion of AMPA receptors into the post synaptic density,

Spines

which increases a spine's capacity to express a potentiated response to glutamate. Other

Integrins

post-translation events then begin to stabilize and expand the actin cytoskeleton by

Maintenance

increasing the filament actin content of the spine and reorganizing it to be resistant to

Tagging

depolymerizing events. Disrupting actin polymerization during this stabilization period is a terminal event—the actin cytoskeleton shrinks and potentiated synapses de-potentiate and memories are lost. Late-arriving, new proteins may consolidate changes in the actin cytoskeleton. However, to do so requires a stabilized actin cytoskeleton. The now enlarged spine has properties that enable it to capture other newly transcribed mRNAs or their protein products and thus enable the synaptic changes that support LTP and memory to be consolidated and maintained. This article is part of a Special Issue entitled SI: Brain and Memory. & 2014 Published by Elsevier B.V.

1.

Introduction

Shortly after Bliss and Lomo (1973) discovered long-term potentiation (LTP), Eva Fifková and her colleagues discovered that the induction stimulus that produced enhanced synaptic potentials also increased the size of dendritic spines (Fifková and van Harreveld, 1977; van Harreveld and Fifková, 1975). The change occurred rapidly and endured for at least an hour. A strong implication of this finding was that synaptic activity

that produces LTP alters the structure and perhaps the function of dendritic spines. Fifková's subsequent research revealed the presence of actin filaments that formed a lattice structure in the dendritic spine head but that were organized in long strands in the spine neck and dendritic shaft. She speculated that the dynamic properties of actin could be essential to the modification of spine size and could play a major role in synaptic plasticity (Fifková, 1985). As her research foreshadowed, actin dynamics have proven to be

n

Fax: þ1 303 492 2967. E-mail address: [email protected]

http://dx.doi.org/10.1016/j.brainres.2014.12.007 0006-8993/& 2014 Published by Elsevier B.V.

Please cite this article as: Rudy, J.W., Actin dynamics and the evolution of the memory trace. Brain Research (2014), http://dx. doi.org/10.1016/j.brainres.2014.12.007

181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196

BRES : 43974

2

197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256

brain research ] (]]]]) ]]]–]]]

Fig. 1 – The synaptic changes that support LTP and memory evolve in temporally ordered overlapping stages that can be distinguished by the unique set of molecular processes that regulate actin at each stage.

here as (a) generation, (b) stabilization, and (c) consolidation (Fig. 1). The purpose of this essay is to illuminate the contribution of actin dynamics to the synaptic changes that support LTP and memory by situating its regulation into this framework. Its goal is to show that different processes regulate actin dynamics at each stage and that the function of the actin cytoskeleton differs as these synaptic changes evolve toward stability. To accomplish these goals, the organization of actin cytoskeleton in dendritic spines is briefly described and the regulation of AMPA receptor trafficking and actin dynamics will be identified as core targets of the biochemical processes that establish LTP. The two major sections that follow will discuss (a) the regulation of actin during different stages in the evolution of LTP and (b) actin dynamics and memory. A final section will speculate on how the function of the actin cytoskeleton changes at each stage and how a consolidated actin cytoskeleton in dendritic spines contributes to the maintenance of the synaptic changes in the face of molecular turnover.

2.

Fig. 2 – Actin filament bundles are prominent in the spine neck and actin networks are prominent in the spine head and linked to the plasma membrane by spectrins. The actinspectrin network forms a barrier that limits access to the postsynaptic density and the plasma membrane.

central to synaptic plasticity and memory (e.g., Bosch and Hayashi, 2012; Fortin et al., 2012; Lamprecht, 2014). Over a century ago William James (1890), the father of American psychology, proposed that the memory trace evolves in a set of overlapping stages that he called after image, primary memory, and secondary memory. A briefly lasting sensation (after image) is followed by a persisting representation of experience (primary memory) that forms part of a stream of consciousness that fades into the final stage (secondary memory)—the vast record of experiences that recede from consciousness but can later be retrieved. Although the language has changed, much of modern research is guided by the idea that the memory trace and its Q3 synaptic basis also evolve in stages (McGaugh, 2003). Thus, memory researchers distinguish between short-term memory and long-term memory, and students of synaptic plasticity distinguish between various forms of LTP such as short-lasting LTP and long-lasting LTP or early-phase versus late-phase LTP. Implicit in such terminologies is the idea that (a) at different time points synapses that support memory or LTP may have a different molecular composition, and (b) this composition may be arrived at through different signaling pathways. Lynch et al. (2007) provided a modern taxonomy that follows in the James tradition. Their framework assumes that synaptic changes that support LTP and memory evolve over three temporally ordered but overlapping stages referred to

Actin cytoskeleton in dendritic spines

Actin exists as a monomer (globular, G actin) that can interact at its head and tail (polymerize) with two other actin molecules to form filamentous actin (F actin). Cofilin is a proximal regulator of actin polymerization. In its unphosphorylated state cofilin depolymerizes actin, but this property is inhibited when cofilin is phosphorylated. Fifková's early description of the organization of actin in dendritic spines is still generally accurate. Actin filaments can organize into actin bundles and actin networks. For example, in the spine neck and dendrite, actin is organized in bundles—long strands that are cross-linked in parallel (Fig. 2). Parallel actin bundles support projections of the plasma membrane such as dendrites and spines. In contrast, in networks actin filaments are cross-linked in orthogonal arrays and are found primarily in the head region of the spine (Korobova and Svitkina, 2010). Actin networks beneath the plasma membrane provide the structural basis of the cytoskeleton through association with the actin-binding protein spectrin (also called fodrin). This protein provides an interface between actin filament and the plasma membrane by its interaction with proteins in the plasma membrane. These networks are semisolid gels that can be viewed as creating a barrier to proteins, such as AMPA receptors and scaffolding proteins, which need to access the postsynaptic density (PSD) and plasma membrane. A functional synapse requires an established relationship between the pre and postsynaptic components. Actin also contributes to this stability by providing attachment sites for cell adhesion molecules such as cadherins and integrins that are present in the pre and postsynaptic components of the synapse.

3. AMPA receptor trafficking and actin regulation In 1984 Gary Lynch and Michel Baudry proposed that enhanced synaptic excitatory potentials, recorded as LTP,


257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316

BRES : 43974 brain research ] (]]]]) ]]]–]]]

317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376

were the result of postsynaptic calcium-dependent processes that expanded the pool of glutamate receptors in the PSD. This idea became a core target for researchers and is now widely accepted with the important addition that it is the complement of AMPA receptors in the PSD that is increased (Malenka and Bear, 2004). In the past 25 years a tremendous effort has uncovered many of the signaling pathways that regulate AMPA receptor trafficking and how it is reconfigured to support LTP (Huganir and Nicoll, 2013). Pertinent to this essay, Lynch and Baudry (1984) also linked the up-regulation of glutamate receptors to a specific mechanism—the breakdown of sub-membrane cytoskeleton (actin). Their idea will be elaborated on subsequently. The general point is that they introduced actin modification as a cotarget in expanding the pool of glutamate receptors in the PSD. As the field has moved forward it has become clear that actin regulation that supports the synaptic basis of LTP and memory is far more complex than one might have initially envisioned (Baudry et al., 2011). Situating actin regulation in a temporally ordered framework that links actin dynamics with specific stages in the evolution of the synaptic changes that support LTP and memory can reduce some of this complexity and provide some coherence to this literature.

4.

Actin dynamics and LTP

As noted, the synaptic changes that support LTP evolve in temporally distinct but overlapping stages. The regulation of actin dynamics by different molecular processes is critical to each stage. This section will describe some of the key events that regulate actin as the synaptic basis of LTP is generated, stabilized, and consolidated.

4.1.

Generation: degrading the actin cytoskeleton

The generation of early-phase LTP is the result of rapidly increasing the complement of AMPA receptors in the PSD (Derkach et al., 2007; Huganir and Nicoll, 2013; Opazo et al., Q4 2010). Lynch and Baudry's (1984) founding idea was that this outcome depends on calcium-dependent changes in actin cytoskeleton. Actin networks linked to the plasma membrane by spectrins (Fig. 2) normally limit access of glutamate receptors to the PSD and plasma membrane. Lynch and Baudry proposed that glutamate released onto receptors in the postsynaptic spine raises the level of calcium in the spine head, and this activates a protease, calpain, that degrades spectrins, thereby creating openings in the actin network surrounding the PSD. They argued that this process produces several outcomes. ▪ It unmasks glutamate receptors giving them access to the plasma membrane where they can now provide a potentiated synaptic response to glutamate subsequently released by the presynaptic component. ▪ The degradation of the actin-spectrin network might create permanent new “hot spots” or openings for the insertion of glutamate receptors, which could remain for the lifespan of the synapse.

3

▪ The disruption of the actin-spectrin network might also enable the change in spine morphology produced by the induction of LTP.

Lynch and Baudry's hypothesis makes a clear prediction: inhibition of the calpain-spectrin interaction should impair the generation of LTP. This hypothesis has been supported by the application of calpain inhibitors and calpain antisense knockdown (del Cerro et al., 1990; Denny et al., 1990; Vanderlish et al., 1996). More recently, Amini et al. (2013) Q5 reported that a conditional deletion of two calpain isoforms (calpain-1 and calpain-2) impaired early phase LTP. Ouyang et al. (2005) offered a complementary idea. They proposed that actin filaments in the spine limit the access of key synaptic proteins (such as CamKII) needed to potentiate dendritic spines. Thus, a key step in the generation of LTP is to weaken the actin barrier by enhancing cofilin-mediated depolymerization. In support of their idea they observed that an LTP-inducing stimulus initially reduced both the presence of phosphorylated cofilin (pCofilin) and F-actin in hippocampal slices. Antagonizing NMDA receptors prevented the transient depolymerization of actin—thus linking the result to an increase in calcium in the spine. Gu et al. (2010) also provided support for the general idea that the early stage of actin regulation involves depolymerization. They linked the initial trafficking of AMPA receptors into the synapse to an increased presence of unphosphorylated cofilin. In summary, actin networks and actin bundles provide barriers that limit the access of plasticity proteins such as transmembrane glutamate receptors (AMPARs), enzymes (e.g., CaMKII), and anchoring proteins (e.g., PSD 95) to the postsynaptic density. These barriers would maintain a spine's potential to depolarize at a basal level. To potentiate the synapse the induction stimulus initiates processes that weaken these barriers. Two classes of calcium-dependent mechanisms contribute to this outcome: (1) the activation of calpain isoforms degrades the spectrin-actin motif that links the cytoskeleton to the plasma membrane, and (2) the level of un-phosphorylated cofilin is temporally increased to enhance the depolymerization of actin filaments. In effect the outcome of these processes is that relevant potentiating proteins rapidly gain access to the PSD. Post-translation processes are sufficient to accomplish these goals.

4.2.

Stabilization: rebuilding the actin cytoskeleton

Treatments that interfere with actin polymerization do not prevent the generation of LTP but they do reverse potentiated synapses when applied either before or shortly after the induction stimulus (Kim and Lisman, 1999; Kramár, 2006; Q6 Krucker et al., 2000; Staubli and Chun, 1996). Yet, when applied 15 min or so after LTP has been generated these same treatments have no effect on spine volume or LTP (Fig. 3). Thus, shortly after synapses have been potentiated an actin cytoskeleton foundation must be rapidly re-established and stabilized. Unless this happens LTP will not endure. Many cellular processes are needed to stabilize actin cytoskeleton, but what has to be accomplished is easily


377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436

BRES : 43974

4

437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496


understood. The stabilized foundation is produced in two overlapping phases (Rex et al., 2009). First, following LTP generation there must be a rapid increase in actin filament in the spine. Second, this newly available actin filament must be (a) organized into lattice-like networks by engaging actin nucleation and crosslinking processes, (b) capped to reduce the rate of depolymerization, and (c) relinked to the plasma membrane.

4.2.1.

GTPases

Signaling cascades dependent on calcium-activated GTPases are critical to these outcomes. One cascade goes through the RhoA-ROCK pathway. Its primary task is to phosphorylate cofilin and thereby remove cofilin's depolymerizing influence on actin. This produces a rapid increase in actin filament in the spine. The second cascade goes through GTPases that activate PAK pathways—Rac-PAK and Cdc42-PAK. Its job is to organize actin filament to make it resistant to depolymerization (Fig. 4). Until the reorganization of actin filament produced by these cascades is complete, LTP can be reversed by events that depolymerize actin (Rex et al., 2009). Murakoshi et al. (2011) were able to measure the activity of these two GTPase pathways in individual spines in response to the release of caged glutamate. Consistent with Rex et al.'s

Fig. 3 – (A) Potentiated synapses that support LTP are initially vulnerable to de-potentiation by factors that inhibit actin depolymerization, such as low-frequency stimulation, actin polymerization inhibitors, and integrin inhibitors. (B) Within about 15 min the actin cytoskeleton stabilizes and synapses become resistant to de-potentiation by these treatments.

results, obtained in hippocampal slices, they reported that the RhoA-ROCK pathway is primarily involved in the initial expansion of the spine but the Cdc42-PAK pathway contributes to sustaining the expansion. Both pathways depended on activation of CaMKII and were active for about 30 min. The stabilization phase occurs too rapidly to depend on the translation of new proteins. A series of experiments reported by Chen et al. (2007) reinforces this point. Using the theta-burst stimulus (TBS) protocol they reported that changes in spine morphology occurred within 2 min after TBS. These changes (large, ovoid synapses) were associated with the activation of signaling pathways that would be expected to increase actin polymerization and reorganize actin. Specifically, the number of spines/puncta containing both pCofilin (product of the RhoA-ROCK pathway) and pPAK (product of the Cdc42-PAK pathway) was increased. Moreover, spines containing high levels of pPAK and pCofilin had larger synapses than neighboring synapse. Unexpectedly, Chen et al. (2007) also found a few pCofilin positive spines in their non-stimulated control slices, and these spines were associated with unusually large synapses. Based on this observation and given that phosphorylation is a transient event, they speculated the threshold for potentiation may be quite low and that intrinsic neural activity on occasion was sufficient to cross this threshold. They also suggested that this intrinsically induced large synaptic state produced by pCofilin would normally revert quickly to its previous smaller synaptic state. Consistent with this hypothesis, Chen et al. reported that antagonizing AMPA receptors reduced the presence of pCofilin-containing spines/puncta by

Fig. 4 – Stabilization of the actin cytoskeleton depends on calcium-dependent recruitment of GPTase signal cascades that increase actin filament and reorganize actin networks.


497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556


557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616

50% in unstimulated synapses. The surprising implication of these findings is that a synapse may spontaneously alternate between a large potentiated state and a small un-potentiated state. To remain in a potentiated state, however, requires a signal, such as a strong TBS protocol or behavioral event that does more than cross the threshold—it must activate processes that continue the reorganization of the spine. It is tempting, then, to relate the low threshold needed to spontaneously induce spine changes to a rapidly generated shortlasting LTP produced by a weak TBS protocol. Research from Harou Kasai's laboratory (Honkura et al., 2008) is consistent with Chen et al.'s (2007) findings and supports these ideas. They captured real time images of a single spine's response to the release of caged glutamate. Within a minute or so the release of glutamate produced a large increase of actin and spine volume. Moreover, antagonizing NMDA receptors prevented this rapid change, implying that it was produced by calcium dependent on signaling events. It is significant that this increase in spine volume returned to its basal state within about 20 min, which suggests that the increased calcium in the spine was sufficient to cross the threshold to engage the RhoA-ROCK path but not to activate the Cdc42-PAK path to complete the stabilization of the actin cytoskeleton. Or, perhaps insufficient to activate cell-adhesion molecules, describe below.

4.2.2.

Cell adhesion molecules

To further stabilize the foundation for enduring LTP, additional processes must be recruited that continue to grow and reorganize the actin cytoskeleton. Actin cytoskeleton is intimately associated with cell adhesion molecules—integrins and cadherins—that connect the cell to the extracellular matrix (Brakebush and Fassler, 2003). Disrupting the function of cell adhesion molecules prior to the induction stimulus does not influence the generation of LTP. However, without a contribution of cell adhesion molecules, enlarged potentiated spines return to their pre-potentiated state (Bozdagi et al.,

5

2010). In particular, the contribution of integrins is highlighted below. Integrins contribute to a variety of functions (Brakebusch and Fassler, 2003; DeMali et al., 2003). They participate in attachment sites called focal adhesions where they attach large bundles of actin filaments (called stress fibers) to the plasma membrane and link actin to the extracellular matrix (ECM) and thereby ensure a stable interaction between receptors and molecules in the ECM. This relationship allows surface integrins to respond to ECM ligands and to interact with tyrosine kinases to continue to activate GTPase-dependent signaling that regulates actin filament reorganization (Dityatev et al., 2010). Disrupting integrin function prevents LTP from enduring but does not prevent its generation (Chun et al., 2001; le Baron et al., 2003; Staubli et al., 1998), and integrins drive actin polymerization (Kramár, 2006). Thus, it is significant that TBS, which generates a persistent LTP, also enhances integrin receptor function. One way to enhance the function of integrins is to increase their surface expression. Lin et al. (2005) linked their surface expression to AMPA-receptormediated depolarization that recruits an increase in spine compartment calcium (Fig. 5). Inhibiting protein trafficking from the endoplasmic reticulum (ER) network blocked this trafficking, which suggests that the calcium influx promotes the translocation of integrins from the ER into the plasma membrane. Surface integrins respond to ligands (integrin-activating epitopes) found in the ECM. Epitopes exist in a cryptic state and cannot interact with integrins. So they must be cleaved to expose the integrin-activating component. Matrix metaloproteinases (MMPs) in the ECM are important to this process (Dityatev et al., 2010). They can become proteolytic active in response to an LTP-inducing stimulus and cleave the cryptic form to expose the integrin-activating epitope (Fig. 5). Huntley's group (Nagy et al., 2006,2007) and others Q7 (Meighan et al., 2006) revealed a role for MMPs in LTP. For

Fig. 5 – An LTP-inducing stimulus (lightning bolt) enhances the function of integrins in two ways. (A) Prior to induction integrins are below the plasma membrane. Integrin interacting epitopes are encrypted and the activity of metaloproteinases (MMP9s) in the extracellular matrix (ECM) is low. (B) The induction stimulus increases the expression of integrins in the spine head and the activity of MMPs in the ECM. The proteolytic activity of MMPs directed at encrypted epitopes increases availability of integrin interacting epitopes. Please cite this article as: Rudy, J.W., Actin dynamics and the evolution of the memory trace. Brain Research (2014), http://dx. doi.org/10.1016/j.brainres.2014.12.007

617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676

BRES : 43974

6

677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736


example, application of an MMP-9 inhibitor prior to inducing LTP has no effect on LTP generation but prevents LTP from enduring. However, applying the inhibitor 60 min following the induction stimulus has no effect. Thus, inhibiting the effect of MMPs is similar to inhibiting integrin function. Additional studies indicate that active MMPs can be detected within 30 min of LTP induction and can return to baseline within about 2 h (Nagy et al., 2006). It is significant that Huntley's group also linked MMP-9 function to sustained spine enlargement (X.B. Wang et al., 2008). The application of an MMP-9 inhibitor had no effect on either the initial generation of LTP or the accompanying increase in spine volume. However, in the presence of the MMP-9 inhibitor, both LTP and spine volume returned to baseline. In addition, the application of MMP-9 alone also induced spine expansion and synaptic potentiation.

4.2.3.

Summary

The distinguishing feature of the stabilization phase is the recreation of a degraded spine cytoskeleton in an expanded form that is reorganized to resist depolymerization. This outcome is accomplished by modifying and rearranging existing proteins and requires less than an hour. Calciumdependent GTPase signal cascades are engaged that rapidly expand actin filament and its reorganization. Enhanced integrin function also contributes to stabilization. This is accomplished by increasing their cell surface expression and the presence of integrin-activating epitomes in the extracellular matrix that activate them. Disrupting these activities before stabilization is accomplished will return spines to their pre-potentiated state. None of these processes requires that new proteins be synthesized in response to the inducing stimulus.

5. Consolidation: new Proteins sustain actin cytoskeleton Stabilization of the actin cytoskeleton sets the stage for processes that ensure that synaptic changes persist. A potentially important feature of this consolidation stage is the synthesis of new proteins that continue the reconstruction of the actin cytoskeleton. Real time imaging of single spines by Kasai's group (Tanaka et al., 2008) illustrates this general point. As noted, release of caged glutamate rapidly produces an enlarged spine by increasing its actin content, but this increase does not persist. In contrast, spine volume continues to increase when the release of glutamate is accompanied by the delivery of postsynaptic spikes and this change persists for the duration of the experiment (about an hour). However, inhibiting protein synthesis or TrkB receptors that respond to BDNF prevents this result. Moreover, pairing the release of glutamate with an application of BDNF also mimics this result.

5.1.

The BNDF-TrkB pathway

These results indicate that the continued growth and preservation of the actin cytoskeleton depends on protein synthesis, and the generation of these proteins is likely the

product of the BDNF-TrkB signaling cascade. Thus, it is significant that inhibiting TrkB receptors or a downstream target, mTOR (mammalian target of rapamycin), prevents the enduring phase of LTP (Korte et al., 1998; Tang et al., 2002). Moreover, the incubation of hippocampal slices with BDNF (a) reduces the threshold TBS (from 8 to 2 TBS) needed to promote F-actin in stimulated spines, and (b) increases levels of both pCofilin and pPAK, targets of GTPases that regulate actin dynamics (Rex et al., 2007).

5.2.

Arc and other proteins

New proteins appear to be needed to consolidate the actin cytoskeleton and ensure that LTP will endure. What are the relevant new proteins? Surprisingly few data are available to answer this question. However, some potential candidates have been identified. Protein synthesis machinery and mRNA are present locally in the dendritic spine region (Steward, 2007; Sutton and Schuman, 2005). Clive Bramham and his collaborators argue that new protein synthesized to support the emerging actin cytoskeleton is translated locally in response to BDNFTrkB signaling (Bramham, 2008; Bramham et al., 2010; Bramham and Messaoudi, 2005; Messaoudi et al. 2007). Specifically, based on results from in vivo LTP generated in the dentate gyrus, they concluded that the local synthesis of the immediate early gene Arc (activity-regulated cytoskeleton-associated protein, also known as Arg3-1) is critical to this outcome. Arc mRNA is rapidly transcribed in response to synaptic activity and is transported to the dendritic spine region within 15–20 min, where it can be translated (Link et al., 1995; Lyford et al., 1995). Guzowski (2000) discovered that Arc contributes to the Q8 consolidation of LTP. They reported that antisense knockdown of Arc disrupted the late enduring phase of LTP. Messaoudi et al. (2007) provided several observations that support their proposal that newly synthesized Arc protein preserves the emerging actin cytoskeleton:

both Arc mRNA and protein levels are rapidly elevated by an LTP-inducing stimulus,

antisense knockdown of Arc 2h but not 4h following the LTP-inducing stimulus reverses LTP,

antisense knockdown of Arc is accompanied by a large reduction in pCofilin, and

Arc antisense did not reverse LTP when preceded by an infusion of an actin polymerizing agent, jasplakinolide.

Knockdown of Arc reversed LTP even when it occurred 2 h following the induction stimulus. This is well beyond the period during which direct inhibitors of actin polymerization reverse LTP and makes Arc a possible consolidation protein. Additional observations by Messaoudi et al. linked Arc synthesis to the effects of BDNF: (a) BDNF alone can induce potentiation but (b) this effect is reversed by antisense knockdown of Arc 2h but not 4h following induction. Thus, the potentiation produced by BDNF is dependent on Arc, which suggests that the synthesis of Arc protein is an


737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796


797 798 799 800 801 802 803 804 805 806 807 808 809 810 811 Q9 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835Q11Q10 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856

important outcome of the BDNF-TrkB cascade. It should be noted, however, that there is no evidence of a direct interaction between Arc and cofilin (Bramham et al., 2010). So just how it influences this late stage of actin regulation is unknown. Other observations also suggest a role for local protein synthesis in sustaining the actin cytoskeleton. When activated, LIMK1 phosphorylates cofilin and thereby inhibits its actin depolymerization function. LIMK1 mRNA is locally present in dendritic spines but its translation is inhibited. Application of BDNF, however, significantly increases the synthesis of Limk1, but inhibiting mTOR prevents this outcome (Schratt et al., 2006). In addition MMP-9, which cleaves integrin-active epitopes, also has been identified as a target of local synthesis (Dziembowska et al., 2012).

5.2.1.

Summary

A distinguishing feature of the consolidation phase is the synthesis of proteins that are needed to grow and preserve the newly reconstructed actin cytoskeleton. Translation of these proteins likely occurs locally in the dendritic spine region and is initiated by activating BDNF-TrkB-mTOR signaling. Inhibiting protein synthesis or the BDNF-TrkB pathway reverses the continued expansion of the actin cytoskeleton. New Arc protein synthesized locally may be critical to the survival of the emerging actin cytoskeleton. Other proteins involved in regulating actin, LIMK1 and MMP-9, are also translated locally and may participate in preserving the reconstructed spine cytoskeleton. In concluding this section it is noted that the above discussion rest on what is sometime referred to as the de nova protein synthesis hypothesis–the idea that the consolidation of memory and LTP depends on the translation of new proteins triggered events that produce memory or LTP. This idea emerged over 60 years ago (Davis and Squire, 1984) yet remains controversial to this day (Abraham & Williams, 2013, Gold, 2008). Especially disconcerting are recent reports that enduring LTP can be established in the face of virtually complete inhibition of protein synthesis (e.g., Abbas, 2013; 2009; Fonseca et al., 2006; Villers, 2012; Lynch et al., this volume). No one disputes the idea that new proteins ultimately have to be produced to sustain the synaptic basis of a memory or LTP. The controversy centers the requirement that the new protein be translated in response to the event that produces the memory or LTP.

6. Local protein synthesis requires a stable actin cytoskeleton New locally synthesized proteins may be required to sustain a stabilized actin cytoskeleton. However, it is also noted that local synthesis requires a stable actin cytoskeleton. Motor proteins transport mRNA-containing granules via microtubules to dendritic regions (Hirokawa, 2006) where they may be docked on to actin filaments (Bassell, and Singer, 1997). In this state they are translational silent. The translation of synaptic proteins critical to sustaining LTP requires the initiation of processes that spatially associate mRNAcontaining granules with essential translations factors, eIF4E

7

and ribosomes (Negrutskii et al.,1994; Stapulionis, et al., 1997). As noted, activation of BDNF-TrkB signaling is critical for local protein synthesis. Thus, as one might expect the association of mRNA with translation factors is also controlled by BDNF signaling—BDNF enhances eIF4E activity induces local translation (Aakalu et al., 2001). The important point of this discussion, however, is that Vanderklish and colleagues (Smart et al., 2003) established a relationship between BDNF initiated translation and actin cytoskeleton. They reported that (a) in the presence of BDNF eIF4E was redistributed from the dendritic region to the spine compartment where granules were bound to synaptic actin filaments, and (b) this effect was prevented by latruncilin, an actin polymerization inhibitor, and by disrupting integrin function (previously noted as critical to the stabilizing actin cytoskeleton). These findings suggest that local protein synthesis depends on a stabilized actin cytoskeleton. Kelly et al. (2007) reported a more specific result that is consistent with what one might expect from the Smart et al. (2003) data. Specifically, the maintenance of LTP and the local synthesis of a specific protein, protein kinase M zeta (PKMζ), which is thought to be important for the maintenance of LTP, are prevented by inhibiting actin polymerization. They speculated that actin might act as a platform for the function of translation factors such as eIF4E. From the above discussion one would surmise that there is a complimentary relationship between a stable actin cytoskeleton and the local translation of key synaptic proteins (Smart et al., 2003). Some locally translated proteins such as Arc might be critical to maintaining the enlarged actin cytoskeleton and maintaining LTP. However, a rapidly stabilized cytoskeleton is critical to the ongoing local translation of key synaptic proteins. The significance of this conclusion will be further discussed below.

7.

Actin dynamics and memory

Processes that regulate actin dynamics are critical to the evolution of the synaptic basis of LTP. They are also critical to memories resulting from behavioral experiences. This section addresses the relationship between actin regulation and the evolution of memory traces. Simulation that produces LTP activates actin-regulating processes to enlarge dendritic spines. It is critical that behavioral experiences that produce memories also engage processes that regulate actin dynamics. Fedulov et al. (2007) allowed rats to explore a complex environment. When reexposed the next day they displayed a reduced level of exploration, indicating that a memory was established. Other rats were sacrificed immediately after exploration. Hippocampal tissue (CA1 field) from these rats contained 30% more spines/puncta containing pCofilin than control rats. Moreover, both the behavioral and spine effects were eliminated if rats were treated with an NMDA receptor antagonist prior to exploration, thereby linking spine changes to calcium. Confirming that behavioral experiences can influence actin regulation, Nelson et al. (2013) reported that training on an object discrimination task increased levels of pCofilin in the hippocampus.


857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916

BRES : 43974

8

917 918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976


The generation of LTP features processes designed to degrade actin-spectrin networks and actin filaments that limit the access of AMPA receptors and other proteins to the PSD. Thus, one might expect that experimentally inhibiting calpains or enhancing actin depolymerization would impair the generation of a memory trace. Very few data exist to evaluate this hypothesis. However, Amini et al. (2013) have reported that mice with a conditional deletion of calpain-1/ calpain-2 activity are unable to learn the Morris placelearning task. Unfortunately, there were no analytic control experiments reported to evaluate the source of the failure. More behavioral data are needed to assess calpain's role in the generation of memory. However, like LTP, memories can be generated in the face of inhibiting actin polymerization. For example, Mantzur et al. (2009) injected the actin polymerization inhibitor cytochalasin D into the lateral amygdala prior to fear conditioning. This treatment had no influence on test performance 1hour later but impaired performance with animals tested 24 h later. Motanis and Moroun (2012) reported similar results. They injected cytochalasin D into either the dorsal hippocampus or basal lateral amygdala immediately following a contextual fear-conditioning experience. Both of these brain regions provide memory support for contextual fear. So it is significant that interfering with actin polymerization in both areas has no effect when the retention interval was only 3 h but dramatically reduces fear when the interval was 24 h. The above results suggest that a relatively short-lasting memory trace can be generated in the face of disrupting actin polymerization. They are also consistent with the hypothesis that to establish a more enduring memory trace requires that processes that stabilize the actin cytoskeleton must be engaged. New protein is required to consolidate synaptic changes supporting LTP. Moreover, as noted the work from Bramham's group (Bramham, 2008; Bramham et al., 2010; Messaoudi et al., 2007) linked the local translation of Arc to consolidation of the actin cytoskeleton and LTP. Newly synthesized Arc is also critical for memory consolidation. Guzowski (2000) reported that Arc antisense infused into the hippocampus interfered with rats' 2-day retention of the memory supporting performance in the spatial version of the Morris water task (Morris, 1982) but had no effect when the retention test was only 30 min following training. Ploski et al. (2008) reported a similar pattern for fear conditioning— Arc antisense injected into the lateral amygdala had no effect when the retention interval was only 3 h but impaired retention when the interval was 24 h. Arc protein is required for consolidation of these memories, but there are no data in these behavioral studies linking Arc protein to actin regulation. No other newly synthesized proteins have been directly linked to the consolidation of the actin cytoskeleton.

8.

Functions of the changing cytoskeleton

The production and persistence of LTP and memory depend on processes that degrade, rebuild, stabilize and consolidate an enlarged actin cytoskeleton. The process of rebuilding the actin cytoskeleton begins within minutes of stimulation and continues for at least a couple of hours. Its final form may

depend on the synthesis of new proteins. Some of the possible functions supported by the vicissitudes of actin dynamics are discussed below. Spines initially are structured to tightly regulate or limit the access of proteins that favor potentiation to the PSD and thus maintain their capacity to depolarize at a relatively low basal level. To create the synaptic changes that generate LTP and memory, however, proteins that favor potentiation (e.g., kinases, AMPA receptors, scaffolding proteins and their associated trafficking proteins) have to rapidly gain access to the PSD region. This is accomplished by degrading the existing actin cytoskeleton, thereby opening the spine compartment and PSD to these proteins. This rapid degradation of actin cytoskeleton is followed shortly by an increase in actin filaments and the rebuilding of the actin networks that produces a larger spine. A potential function of spine head expansion is an increased capacity for AMPA receptors, and it is the case that spines with larger spine heads have more AMPA receptors (Matsuzaki et al., 2001). However, this may not be the only reason why the spine head is rapidly expanded and rebuilt, because spines become strongly potentiated before expansion occurs, indicating that there is already space for the new AMPA receptors. The rapid rebuilding of the cytoskeleton might support another important function. Specifically, even though the spine is larger, a tight, newly formed cytoskeleton would end the generation stage, during which potentiating proteins have “open access” to the spine and PSD and thus limit further potentiation. This outcome also would allow endocytic processes to down-regulate over-potentiated spines to an acceptable level and other processes to again carefully control the access of potentiating proteins into the spine.

8.1.

Actin's role in consolidation: capturing new protein

The consolidation of LTP and the actin cytoskeleton depends on translation and transcription processes that may continue for hours after the initiating inducing stimulus or behavioral event has passed. It is clear that an enlarged and dynamic cytoskeleton is critical to consolidating LTP and memory (Huang et al., 2013). Much of the translation is local but new mRNA or protein has to be delivered to the relevant synaptic sites—the ones that need to be consolidated. This is the problem addressed by the synaptic tagging hypothesis (Frey and Morris, 1997,1998): how can newly generated mRNA or protein find the relevant synapses to consolidate? This problem suggests a second role for an enlarged actin cytoskeleton. Newly produced mRNA must be translocated from the nucleus into dendritic compartments and selectively delivered to synapses that should be consolidated. Newly potentiated enlarged spines exist among unpotentiated small spines. So size provides a crude basis for discriminating synapses that need consolidating. Large spines are better traps for capturing new mRNAs or their protein product, including polyribosomes that would be needed to translate relevant mRNAs (Ostroff et al., 2002). From this it follows that stabilization of an enlarged actin cytoskeleton increases the capacity of stimulated spines to capture late arriving new mRNA/proteins needed to ensure the long-term survival of LTP.


977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995 996 997 998 999 1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030 1031 1032 1033 1034 1035 1036


1037 1038 1039 1040 1041 1042 1043 1044 1045 1046 1047 1048 1049 1050 1051 1052 1053 1054 1055 1056 1057 1058 1059 1060 1061 1062 1063 1064 1065 1066 1067 1068 1069 1070 1071 1072 1073 1074 1075 1076 1077 1078 1079 1080 1081 1082 1083 1084 1085 1086 1087 1088 1089 1090 1091 1092 1093 1094 1095 1096

Several results are consistent with this prediction. For example, an enlarged cytoskeleton produced by a weak stimulus normally will return to its pre-potentiated size— the synapse will de-potentiate. However, if new mRNA/ protein is generated by strongly stimulating other synapses belonging to the same neuron, a weakly stimulated synapse can capture it and potentiation will endure. However, there is a caveat—the new product must become available before the synapses de-potentiate and the enlarged cytoskeleton has returned to its baseline state (Frey and Morris, 1997;1998). Ramachanran and Frey (2009) directly tested this hypothesis. Using the standard paradigm for studying the capture of plasticity products by weakly stimulated synapses, they found that inhibiting actin polymerization prevented them from capturing the new product produced by strongly stimulating other synapses. So without a large spine, newly produced products that preserve/consolidate synaptic changes are not effective—the product has nowhere to go. What are the properties of large spines that give them an advantage in the competition for trapping late emerging proteins? An intriguing potential answer centers on the interaction between actin and another component of the cytoskeleton, microtubules. Actin and microtubules both are part of an intracellular transport system that permits motor proteins to cargo products such as endocytic vesicles and non-membrane-bound particles, such as mRNA, and proteins involved in signaling processes (Dent et al., 2011). The dendritic spine compartment is actin rich compared to the dendritic compartment. Microtubules are prominent in dendrite compartments and provide a potential track linking the dendritic regions to the cell body. The intriguing idea is that synaptic activity that promotes a persistent LTP results in new microtubule track linking the cell body and stimulated synapses (Mitsuyama et al., 2008a,

Fig. 6 – Potentiated spines contain more AMPA receptors and an enlarged actin cytoskeleton. Enlarged spines are better traps or tags for capturing new mRNAs/proteins needed for consolidation. One possibility is that actin-associated proteins interact with microtubules to cause them to invade the spine and permit motor proteins to cargo the relevant mRNAs/proteins to the spines that have been potentiated. In addition, once consolidated, spines with an enlarged actin cytoskeleton might successfully compete for the molecules that are needed to self-maintain and thus provide part of the solution to the molecular turnover problem.

9

1097 1098 1099 1100 1101 1102 1103 1104 1105 1106 8.2. Actin and the molecular turnover problem 1107 1108 It is one thing to generate and consolidate the synaptic 1109 changes that support LTP but it is another to maintain the 1110 changes. This is the molecular turnover problem Francis 1111 Crick (1984) identified. The synaptic molecules that support 1112 memory traces are short-lived in comparison to the duration 1113 of our memories. Moreover, it has been suggested that the 1114 entire complement of synaptic proteins in mature neural 1115 circuits are replaced multiple times a day (Ehlers, 2003). As 1116 Crick (1984, p. 101) put it: “How then is memory stored in the 1117 brain so that its trace is relatively immune to molecular 1118 turnover?” The general answer Crick outlined was that “… 1119 the molecules in synapses interact in such a way that they 1120 can be replaced with new material, one at a time, without 1121 altering the overall state of the structure.” So if it is to persist 1122 for days, an autonomous self-renewing process or processes 1123 must continue to operate long after the trace is established. 1124 One approach to the maintenance problem has focused on 1125 the possibility that some special enzyme that self1126 perpetuates can be identified that will maintain the trace. 1127 Much attention has focused on PKMζ as a unique contributor 1128 to maintenance (Sacktor, 2011). PKMζ is thought to regulate 1129 AMPA receptor trafficking so as to maintain the increased 1130 complement of these receptors in the PSD, and there is 1131 evidence supporting a role for PKMζ (Sacktor, 2011) or other 1132 yet to be identified enzymes (Volk et al., 2013) in maintaining 1133 the trace. So, as previously noted, it is significant that 1134 interfering with actin polymerization prevents the synthesis 1135 of PKMζ (Kelly et al., 2007). The general implication of this 1136 finding is that processes that regulate local synthesis—asso1137 ciating mRNA in the dendritic compartment with polyribo1138 somes and translation factors require a stable actin 1139 cytoskeleton, as discussed previously (Smart et al. 2003). 1140 Thus, regardless of just what particular proteins are needed 1141 to maintain the increased complement of AMPA receptors in 1142 the PSD, without a large and stable spine potentiated spines 1143 will likely revert to their basal state. 1144 Spines come in a variety of shapes and sizes but many 1145 studies have revealed that large spines endure much longer much than smaller spines (Grutzendler, 2002; Holtmaat and Q12 1146 Svoboda, 2009; Trachtenberg et al., 2002). So a second Q13 1147 1148 approach to the maintenance problem is to consider what 1149 properties emerge in a well-stabilized large spine that might 1150 empower it with self-maintenance properties. 1151 Whether a spine is long and thin or short and stubby, its 1152 existence requires a continuous supply of synaptic proteins 1153 (e.g., receptors, scaffolding proteins, and actin monomers). 1154 Synaptic proteins turn over at a remarkable rate and when 1155 they leave sometimes they return to the same spine. For 1156 example, fluorescent PSD-95 circulates among neighboring b). If true, this would provide a path for molecules activated in stimulated spines to signal the nucleus for new mRNA and a path for these new transcripts and proteins to translocate back to the spines that need consolidating. This idea gains traction with Merriam et al.'s (2013) report that the calcium activation of spine actin dynamics can promote microtubules at the spine base to invade active spines, which was prevented when actin polymerization was inhibited (Fig. 6).


BRES : 43974

10

1157 1158 1159 1160 1161 1162 1163 1164 1165 1166 1167 1168 1169 1170 1171 1172 1173 1174 1175 1176 1177 1178 1179 1180 1181 1182 1183 1184 1185 1186 1187 1188 1189 1190 1191 1192 1193 1194 1195 1196 1197 1198 1199 1200 1201 1202 1203 1204 1205 1206 1207 1208 1209 1210 1211 1212 1213 1214 1215 1216


spines. The principle determiner of where PSD-95 goes and how long it stays is spine size, which is determined by the actin cytoskeleton (Gray et al., 2006). Large spines are more successful than smaller spines in attracting and maintaining PSD-95 proteins. It may not be a stretch to imagine that a similar principle applies to other synaptic proteins. Large spines win in the competition for the molecules needed to sustain them. The ability of large spines to compete for the proteins they need to self-maintain also may be related to the role of actin as part of the intracellular transport system that provides tracks for cargo transporting motor proteins (Fig. 6). For example, AMPA receptors are continuously recycled (Park et al., 2006). As they leave the postsynaptic density they are repackaged into endosomes. The redelivery of these AMPA receptors to the extrasynaptic region depends on myosin motor proteins that can attach to the endosome. These motor proteins then use the actin filaments as a substrate to deliver the AMPA receptors to the extracellular region (Z. Wang et al., 2008). Thus, one might imagine that a well-organized actin cytoskeleton would benefit the trafficking of AMPA receptors during the maintenance phase. More generally, actin networks and their associated myosin motor proteins may be critical for targeting many vesicle-bound membrane proteins and mRNA containing granules to their proper locations. These vesicles will be more effectively captured and maintained in spines that have wellformed actin networks (Semenova et al., 2008). Such a process could allow a continuous replacement of the key synaptic proteins without disturbing the overall state of the structure, as Crick (1984) proposed. It is noteworthy that Smart et al., (2003) reached a similar conclusion. “Particular spine morphologies and efficacy states may then persist by mechanisms of normal protein replacement, or via lasting influences of structure on the profile of mRNAs locally translated (p. 1408)”. It also has been proposed that the spine size determines its plasticity; its capacity to be modified by calcium is changed. Specifically, broad heads and short necks allow the spine to more rapidly diffuse or eliminate the increased calcium resulting from NMDA receptor activation (Hayashi and Majewska, 2005; Noguchi et al., 2005; O'donnell et al., 2011). Thus, even though these spines would contribute to exciting the neuron (because they allow more sodium to enter it), they would be resistant to modification by calciuminduced plasticity processes. In this context, it has been proposed that spines can be categorized as learning spines and memory spines (Bourne and Harris, 2007). Learning spines are thin spines that can concentrate biochemical signals, such as calcium and its targets, which can modify synapses. In contrast, memory spines are large and mushroom-shaped—designed to capture products needed for their maintenance and to be stable (protected from modification). The biochemical interactions engaged by an LTP-inducing stimulus thus might convert learning spines into memory spines that have self-sustaining properties.

9.

Summary

Almost 30 years ago Eva Fifková described the actin cytoskeleton of dendritic spines and speculated that actin dynamics

could be an essential contributor to plasticity. During the intervening years a massive literature has emerged that contains a detailed understanding of how actin spine dynamics are regulated and has confirmed Fifková's general hypothesis. This essay linked actin dynamics to the idea that the synaptic changes that support LTP and memory evolve in temporally overlapping stages—generation, stabilization, and consolidation, as proposed by Lynch et al. (2007). Different cellular/molecular processes operate at each stage to influence actin dynamics to produce changes in the spine cytoarchitecture and, in doing so, alter its function. Calcium-dependent processes that degrade the actin cytoskeleton network enable the rapid insertion of AMPAs that increases a spine's capacity to express a potentiated response to glutamate. Other post-translation events then begin to expand the actin cytoskeleton by increasing the filament actin content of the spine and reorganizing it to be resistant to depolymerizing events. Disrupting actin polymerization during this stabilization period is a terminal event—the actin cytoskeleton shrinks and potentiated synapses de-potentiate. Late-arriving, new proteins may consolidate changes in the actin cytoskeleton. The now enlarged spine has properties that enable it to capture other newly transcribed mRNAs or their protein products and thus enable the synaptic changes that support LTP and memory to be and maintained.

1217 1218 1219 1220 1221 1222 1223 1224 1225 1226 1227 1228 1229 1230 1231 1232 1233 1234 1235 1236 1237 1238 1239 1240 1241 1242 1243 1244 Uncited references Q2 1245 1246 Babayan et al. (2012), Bozdagi et al. (2007), Dityateve and 1247 Fellin (2008), Fifková and Anderson (1981), Holtmaat et al. 1248 (2005), Huntley et al. (2002), Larson et al. (1993), Matsuzaki 1249 et al. (2004), Michaluk et al. (2011), Steward and Worley (2001) 1250 and Steward and Worley (2001). 1251 1252 r e f e r e n c e s 1253 1254 1255 1256 Aakalu, G., et al., 2001. Dynamic visualization of local protein synthesis in hippocampal neurons. Neuron 30, 489–502. 1257 Abbas, A.-K., 2013. Evidence for constitutive protein synthesis in 1258 hippocampal LTP stabilization. Neuroscience 246, 301–311. 1259 Amini, M., et al., 2013. Conditional disruption of Calpain in the 1260 CNS alters dendrite morphology, impairs, LTP, and promotes 1261 neuronal survival following injury. J. Neurosci. 33, 5773–5784. 1262 Bassell, G., Singer, R.H., 1997. mRNA and cytoskeletal filaments. Curr. Opin. Cell Biol. 9, 109–115. 1263 Babayan, A.H., et al., 2012. Integrin dynamics produce a delayed 1264 stage of long-term potentiation and memory consolidation. 1265 J. Neurosci. 32, 12854–12861. 1266 Baudry, M., et al., 2011. The biochemistry of memory: the 26 year 1267 journey of a ‘new and specific hypothesis’. Neurobiol. Learn. 1268 Mem. 95, 125–133. 1269 Bliss, T.V., Lomo, T., 1973. Long lasting potentiation of synaptic transmission in the dentate area of the anaesthetized rabbit 1270 following stimulation of the perforant path. J. Physiol. 232, 1271 331–356. 1272 Bourne, J., Harris, K.M., 2007. Do thin spines learn to be 1273 mushroom spines that remember?. Curr. Opin. Neurobiol. 17, 1274 381–386. 1275 Bosch, M., Hayashi, Y., 2012. Structural plasticity of dendritic spines. Curr. Opin. Neurobiol. 22, 383–388. 1276



Bozdagi, O., et al., 2007. In vivo roles for matrix metal1277 loproteinase-9 in mature hippocampal synaptic physiology 1278 and plasticity. J. Neurophysiol. 98, 334–344. 1279 Bozdagi, O., et al., 2010. Persistence of coordinated long-term 1280 potentiation and dendritic spine enlargement at mature 1281 hippocampal CA1 synapses requires N-cadherin. J. Neurosci. 1282 30, 9984–9989. 1283 Brakebusch, C., Fa¨ssler, R., 2003. The integrin–actin connection, 1284 an eternal love affair. EMBO J. 22, 2309–2535. Bramham, C.R., et al., 2008. Local protein synthesis, actin 1285 dynamics, and LTP consolidation. Curr. Opin. Neurobiol. 18, 1286 524–531. 1287 Bramham, C.R., et al., 2010. The Arc of synaptic memory. Exp. 1288 Brain Res. 2, 125–140. 1289 Bramham, C., Messaoudi, E., 2005. BDNF function in adult 1290 synaptic plasticity: the synaptic consolidation hypothesis. 1291 Prog. Neurobiol. 76, 99–125. Chen, L.Y., et al., 2007. Changes in synaptic morphology 1292 accompanying actin signaling during LTP. J. Neurosci. 27, 1293 5363–5372. 1294 Chun, D., et al., 2001. Evidence that integrins contribute to 1295 multiple stages in the consolidation of long term potentiation. 1296 Neuroscience 105, 815–829. 1297 Davis, H.P., Squire, L.R., 1984. Protein synthesis and memory: a 1298 review. Psychol. Bull. 96, 518–559. 1299 Crick, F., 1984. Memory and molecular turnover. Nature 312, 101. 1300 Q14 del Cerro, S., et al., 1990. Development of hippocampal long-term potentiation is reduced by recently introduced calpain 1301 inhibitors. Brain Res. 530 (95-91). 1302 de Mali, K.A., et al., 2003. Integrin signaling to the actin 1303 cytoskeleton. Curr. Opin. Cell Biol. 15, 572–582. 1304 Denny, J.B., et al., 1990. Calpain inhibitors block long-term 1305 potentiation. Brain Res. 534, 317–320. 1306 Dent, E.W., et al., 2011. The dynamic cytoskeleton: backbone of dentritic plasticity. Curr. Opin Neurobiol. 21, 175–181. 1307 Derkach, V., et al., 2007. Regulatory mechanisms of AMPA 1308 receptors in synaptic plasticity. Nat. Rev. Neurosci. 8, 1309 8101–8113. 1310 Dityateve, A., Fellin, T., 2008. Extracellular matrix in plasticity and 1311 epileptogenesis. Neuron Glia Biol. 4, 235–247. 1312 Dziembowska, M., et al., 2012. Activity-dependent local 1313 translation of matrix metalloproteinase-9. J. Neurosci. 32, 14538–14547. 1314 Ehlers, M., 2003. Ubiquitin and the deconstruction of synapses. 1315 Science 302, 800–801. 1316 Fedulov, V., et al., 2007. Evidence that long-term potentiation 1317 occurs within individual hippocampal synapses during 1318 learning. J. Neurosci. 27, 8031–8039. 1319 Fifkova´, E., 1985. Actin in the nervous system. Brain Res. 356, 1320 187–215. 1321 Fifkova´, E., van Harreveld, A., 1977. Long-lasting morphological changes in dendritic spines of dentate granular cells following 1322 stimulation of the entorhinal area. J. Neurocytol. 6, 211–230. 1323 Fifkova´, E., Anderson, C.L., 1981. Stimulation-induced changes in 1324 dimensions of stalks of dendritic spines in the dentate 1325 molecular layer. Exp. Neurol. 74, 621–627. 1326 Fonseca, R., et al., 2006. A balance of protein synthesis and 1327 proteasome-dependent degradation determines the 1328 maintenance of LTP. Neuron 52, 239–245. Fortin, D.A., et al., 2012. Structural modulation of dendritic spines 1329 during synaptic plastic. Neuroscience 18, 326–341. 1330 Frey, U., Morris, R.G., 1997. Synaptic tagging and long-term 1331 potentiation. Nature 385, 533–536. 1332 Frey, U., Morris, R.G., 1998. Synaptic tagging: implications for late 1333 maintenance of hippocampal long-term potentiation. Trends 1334 Neurosci. 21, 181–188. 1335 Gold, P.E., 2008. Protein synthesis inhibition and memory; 1336 formation vs. amnesia. Neurobiol. Learn Mem. 89, 201–211.

11

Gray, N.W., et al., 2006. Rapid redistribution of synaptic PSD-95 in the neocortex in vivo. PLoS Biol. 4, 265–275. Grutzendler, J., 2002. Long-term dendritic spine stability in the adult cortex. Nature 420, 812–816. Gu, J., et al., 2010. ADF/cofilin-mediated actin dynamics regulate AMPA receptor trafficking during synaptic plasticity. Nat. Neurosci. 13, 1208–1216. Guzowski, J.F., 2000. Inhibition of activity-dependent arc protein expression in the rat hippocampus impairs the maintenance of long-term potentiation and the consolidation of long-term memory. J. Neurosci. 20, 3993–4001. Hayashi, Y., Majewska, A.K., 2005. Dendritic spine geometry: functional implication and regulation. Neuron 46, 529–532. Hirokawa, N., 2006. mRNA transport in dendrites: RNA granules, motors, and tracks. J. Neurosci. 26, 7139–7142. Holtmaat, A.J., et al., 2005. Transient and persistent dendritic spines in the neocortex in vivo. Neuron 45, 279–291. Holtmaat, A.J., Svoboda, K., 2009. Experience-dependent structural synaptic plasticity in the mammalian brain. Nat. Rev. Neurosci. 10, 647–658. Honkura, N., et al., 2008. The subspine organization of actin fibers regulates the structure and plasticity of dendritic spines. Neuron 57, 719–729. Huang, W., et al., 2013. mTORC2 controls actin polymerization required for consolidation of long-term memory. Nat. Neurosci. 16, 441–448. Huganir, R.L., Nicoll, R., 2013. AMPARs and synaptic plasticity: the last 25 years. Neuron 80 (704-217). Huntley, G.W., et al., 2002. The cadherin family of cell adhesion molecules: multiple roles in synaptic plasticity. Neuroscientist 8, 221–233. James, W., 1890. Principles of Psychology. Holt, New York. Kelly, M.T., et al., 2007. Actin polymerization regulates the synthesis of PKMzeta in LTP. Neuropharmacology 52, 41–45. Kim, C.H., Lisman, J.E., 1999. A role of actin filament in synaptic transmission and long-term potentiation. J. Neurosci. 19, 4314–4324. Korte, M., et al., 1998. A role for BDNF in the late phase of hippocampal long-term potentiation. Neuropharmacology 37, 553–559. Korobova, F., Svitkina, T., 2010. Molecular architecture of synaptic actin cytoskeleton in hippocampal neurons reveals a mechanism of dendritic spine morphogenesis. Mol. Biol. Cell 21, 165–176. Krama´r, E.A., et al., 2006. Integrin-driven actin polymerization consolidates long-term potentiation. Proc. Natl. Acad. Sci. USA 103, 5579–5584. Krucker, T., et al., 2000. Dynamic actin filaments are required for stable long-term potentiation (LTP) in area CA1 of the hippocampus. Proc. Natl. Acad. Sci. USA 6 (6856–6681). Lamprecht, R., 2014. The actin cytoskeleton in memory formation. Prog. Neurobiol. 117C, 1–19. Larson, J., et al., 1993. Reversal of LTP by theta frequency stimulation. Brain Res. 600, 97–102. LeBaron, R.G., et al., 2003. An integrin binding peptide reduces rat CA1 hippocampal long-term potentiation during the first few minutes following theta burst stimulation. Neurosci. Lett. 339, 199–202. Lin, B., et al., 2005. Theta stimulation polymerizes actin in dendritic spines of hippocampus. J. Neurosci. 25, 2062–2069. Link, W., et al., 1995. Somatodendritic expression of an immediate early gene is regulated by synaptic activity. Proc. Natl. Acad. Sci. USA 92, 5734–5738. Lyford, G.L., et al., 1995. Arc, a growth factor and activityregulated gene, encodes a novel cytoskeleton-associated protein that is enriched in neuronal dendrites. Neuron 14, 433–445.


1337 1338 1339 1340 1341 1342 1343 1344 1345 1346 1347 1348 1349 1350 1351 1352 1353 1354 1355 1356 1357 1358 1359 1360 1361 1362 1363 1364 1365 1366 1367 1368 1369 1370 1371 1372 1373 1374 1375 1376 1377 1378 1379 1380 1381 1382 1383 1384 1385 1386 1387 1388 1389 1390 1391 1392 1393 1394 1395 1396

BRES : 43974

12


Lynch, G., Baudry, M., 1984. The biochemistry of memory: a new 1397 and specific hypothesis. Science 224, 1057–1063. 1398 Lynch, G., et al., 2007. LTP consolidation: substrates, explanatory 1399 power, and functional significance. Neuropharmacology 52, 12–23. 1400 Murakoshi, H., et al., 2011. Local, persistent activation of Rho 1401 GTPases during plasticity of single dendritic spines. Nature 1402 472, 100–103. 1403 Malenka, R.C., Bear, M.F., 2004. LTP and LTD: an embarrassment of riches. Neuron 44, 5–21. 1404 Mantzur, L.G., et al., 2009. Actin polymerization in lateral 1405 amygdala is essential for fear memory formation. Neurobiol. 1406 Learn. Mem. 91, 85–88. 1407 Matsuzaki, M., et al., 2001. Dendritic spine geometry is critical for 1408 AMPA receptor expression in hippocampal CA1 pyramidal 1409 neurons. Nat. Neurosci. 4, 1086–1092. 1410 Matsuzaki, M., et al., 2004. Structural basis of long-term potentiation in single dendritic spines. Nature 429, 761–766. 1411 1412 Q15 Meighan, S.E., et al., 2006. Effects of extracellular matrixdegrading proteases matrix metalloproteinases 3 and 9 on 1413 spatial learning and synaptic plasticity. J. Neurochem. 96, 1414 1227–1241. 1415 Messaoudi, E., et al., 2007. Sustained Arc/Arg3.1 synthesis 1416 controls long-term potentiation consolidation through regulation of local actin polymerization in the dentate gyrus 1417 in vivo. J. Neurosci. 27, 10445–10455. 1418 Michaluk, P., et al., 2011. Influence of matrix metalloproteinase 1419 MMP-9 on dendritic spine morphology. J. Cell Sci. 124, 1420 3369–3380. 1421 Mitsuyama, F., et al., 2008a. Redistribution of microtubules in 1422 dendrites of hippocampal CA1 neuronsafter tetanic 1423 stimulation during long-term potentiation. Ital. J. Anat. Embryol. 113, 17–27. 1424 Mitsuyama, F., et al., 2008b. Microtubules to form memor. Ital. J. 1425 Anat. Embryol. 113, 227–235. 1426 Motanis, H., Moroun, M., 2012. Differential involvement of protein 1427 synthesis and actin rearrangement in the reacquisition of 1428 contextual fear conditioning. Hip 22, 494–500. 1429 Nagy, V., et al., 2006. Matrix metalloproteinase-9 is required for 1430 hippocampal late-phase long-term potentiation and memory. J. Neurosci. 26, 1923–1934. 1431 Nagy, V., et al., 2007. The extracellular protease matrix 1432 metalloproteinase-9 is activated by inhibitory avoidance 1433 learning and required for long-term memory. Learn Mem. 1434 25 (14), 655–664. 1435 Negrutskii, B.S., Stapulionis, R., Deutscher, M.P., 1994. 1436 Supramolecular organization of the mammalian translation system. Proc. Natl. Acad. Sci. USA 91, 964–968. 1437 O’donnell, C., et al., 2011. Dendritic spine dynamics regulate the 1438 long-term stability of synaptic plasticity. J. Neurosci. 31, 1439 16142–16156. 1440 Opazo, P., et al., 2010. CaMKII triggers the diffusional trapping of 1441 surface AMPARs through phosphorylation of stargazin. 1442 Neuron 67, 239–252. 1443 Ostroff, L.E., et al., 2002. Polyribosomes redistribute from dendritic shafts into spine with enlarged synapse during LTP 1444 in developing rat hippocampal slices. Neuron 35, 535–545. 1445 Ouyang, Y., et al., 2005. Transient decrease in F-actin may be 1446 necessary for translocation of proteins into dendritic spines. 1447 Eur. J. Neurosci. 22, 2995–3005. 1448 Park, M., et al., 2006. Recyling endosomes supply AMPA receptors 1449 for LTP. Neuron 52, 817–830. 1450 Ploski, J.E., et al., 2008. The activity-regulated cytoskeletalassociated protein (Arc/Arg3.1) is required for memory 1451 consolidation of pavlovian fear conditioning in the lateral 1452 amygdala. J. Neurosci. 28, 12383–12395. 1453 Ramachandran, B., Frey, J.U., 2009. Interfering with the actin 1454 network and its effect on long-term potentiation and synaptic 1455 1456

tagging in hippocampal CA1 neurons in slices in vitro. J. 1457 Neurosci. 29, 12167–12173. 1458 Rex, C.S., et al., 2007. Brain-derived neurotrophic factor promotes Q16 1459 long-term potentiation-related cytoskeletal changes in adult 1460 hippocampus. J. Neurosci. 27, 3017–3029. 1461 Rex, C.S., et al., 2009. Different Rho GTPase-dependent signaling 1462 pathways initiate sequential steps in the consolidation of 1463 long-term potentiation. J. Cell Biol. 186, 85–97. 1464 Sacktor, T.C., 2011. How does PKMζ maintain long-term memory?. 1465 Nat. Rev.: Neurosci. 12, 9–15. Schratt, G.M., et al., 2006. A brain-specific microRNA regulates 1466 dendritic spine development. Nature 439, 283–289. 1467 Semenova, I., et al., 2008. Actin dynamics is essential for myosin1468 based transport of membrane organelles. Curr. Biol. 18, 1469 1581–1586. 1470 Smart, F.M., et al., 2003. BDNF induces translocation of initiation 1471 factor 4E to mRNA granules: evidence for a role of synaptic 1472 microfilaments and integrins. Proc. Natl. Acad. Sci. USA 100, 1473 14403–14408. 1474 Staubli, U., Chun, D., 1996. Factors regulating the reversibility of long-term potentiation. J. Neurosci. 16, 853–860. 1475 Staubli, U., et al., 1998. Time-dependent reversal of long-term 1476 potentiation by an integrin antagonist. J. Neurosci. 18, 1477 3460–3469. 1478 Stapulionis, R., Kolli, S., Deutscher, M.P., 1997. Efficient 1479 mammalian protein synthesis requires an intact F-actin 1480 system. J. Biol. Chem. 272, 24980–24986. 1481 Steward, O., 2007. Protein synthesis at synaptic sites on dendrites. 1482 In: Lajtha, Able (Ed.), Handbook of Neurochemistry and Molecular Neurobiology. Springer, Netherlands, pp. 169–195. 1483 Steward, O., Worley, P.F., 2001. A cellular mechanism for targeting 1484 newly synthesized mRNAs to synaptic sites on dendrites. 1485 Proc. Natl. Acad. Sci. USA 98, 7062–7068. 1486 Steward, O., Worley, P.F., 2001. Selective targeting of newly 1487 synthesized Arc mRNA to active synapses requires NMDA 1488 receptor activation. Neuron 30, 227–240. 1489 Sutton, M.A., Schuman, E.R., 2005. Local translational control in 1490 dendrites and its role in long-term synaptic plasticity. 1491 J. Neurobiol. 64, 116–131. Tanaka, J., et al., 2008. Protein synthesis and neurotrophin1492 dependent structural plasticity of single dendritic spines. 1493 Science 319, 1683–1687. 1494 Tang, S.J., et al., 2002. A rapamycin-sensitive signaling pathway 1495 contributes to long-term synaptic plasticity in the 1496 hippocampus. Proc. Natl. Acad. Sci. USA 99, 467–472. 1497 Trachtenberg, J.T., et al., 2002. Long-term in vivo imaging of 1498 experience-dependent synaptic plasticity in adult cortex. 1499 Nature 420, 788–794. 1500 Vanderlish, P., et al., 1996. Translational suppression of calpain blocks long-term potentiation. Learn. Mem. 3, 209–217. 1501 Villers, A., et al., 2012. Long-lasting LTP requires neither repeated 1502 trains for its induction nor protein synthesis for its 1503 development. PLoS One e40823, 7. 1504 van Harreveld, A., Fifkova´, E., 1975. Swelling of dendritic spines 1505 in the fascia dentata after stimulation of the perforant fibers 1506 as a mechanism of post-tetanic potentiation. Exp. Neurol. 1507 49, 736–749. 1508 Volk, L.J., et al., 2013. PKMζ is not required for hippocampal 1509 synaptic plasticity, learning and memory. Nature 493, 420–423. Wang, X.B., et al., 2008. Extracellular proteolysis by matrix 1510 metalloproteinase-9 drives dendritic spine enlargement and 1511 long-term potentiation coordinately. Proc. Natl. Acad. Sci. USA 1512 105, 19520–19525. 1513 Wang, Z., et al., 2008. Myosin Vb mobilizes recycling endosomes 1514 for postsynaptic plasticity. Cell 135, 535–548.


1515 1516

Single-trace fragility theory of memory dynamics.

Memory trace replay: the shaping of memory consolidation by neuromodulation.

Biologists trace the evolution of molecules.

The actin cytoskeleton in memory formation.

Nucleotide regulation of the structure and dynamics of G-actin.

Structure and evolution of the actin crosslinking proteins.

Nucleotide hydrolysis regulates the dynamics of actin filaments and microtubules.

The evolution of the actin binding NET superfamily.

An essential memory trace found.

The pros and cons of common actin labeling tools for visualizing actin dynamics during Drosophila oogenesis.

Arc expression identifies the lateral amygdala fear memory trace.

The Role of Actin Cytoskeleton in Memory Formation in Amygdala.

Improved specificity of hippocampal memory trace labeling.

Actin dynamics in the cortical array of plant cells.

Quantitative analysis of ezrin turnover dynamics in the actin cortex.

Actin dynamics regulate subcellular localization of the F-actin-binding protein PALLD in mouse Sertoli cells.

The evolution of immune memory and germinal centers.

The evolution of phenotypic correlations and "developmental memory".

Actin filament dynamics using microfluidics.

Actin dynamics in growth cones.

The retrosplenial cortex is involved in the formation of memory for context and trace fear conditioning.

Experimental evolution and the dynamics of genomic mutation rate modifiers.

The dynamics of actin and myosin association and the crossbridge model of muscle contraction.

Epistasis and the Dynamics of Reversion in Molecular Evolution.