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4. Coombs, J., van der List, D., Wang, G.Y., and Chalupa, L.M. (2006). Morphological properties of mouse retinal ganglion cells. Neuroscience 140, 123–136. 5. Helmstaedter, M., Briggman, K.L., Turaga, S.C., Jain, V., Seung, H.S., and Denk, W. (2013). Connectomic reconstruction of the inner plexiform layer in the mouse retina. Nature 500, 168–174. 6. Badea, T.C., and Nathans, J. (2004). Quantitative analysis of neuronal morphologies in the mouse retina visualized by using a genetically directed reporter. J. Comp. Neurol. 480, 331–351. 7. Kim, I.J., Zhang, Y., Yamagata, M., Meister, M., and Sanes, J.R. (2008). Molecular identification of a retinal cell type that responds to upward motion. Nature 452, 478–482. 8. Huberman, A.D., Wei, W., Elstrott, J., Stafford, B.K., Feller, M.B., and Barres, B.A. (2009). Genetic identification of an On-Off direction-selective retinal ganglion cell subtype reveals a layer-specific subcortical map of posterior motion. Neuron 62, 327–334.
9. Yonehara, K., Ishikane, H., Sakuta, H., Shintani, T., Nakamura-Yonehara, K., Kamiji, N.L., Usui, S., and Noda, M. (2009). Identification of retinal ganglion cells and their projections involved in central transmission of information about upward and downward image motion. PLoS One 4, e4320. 10. Chen, S.K., Badea, T.C., and Hattar, S. (2011). Photoentrainment and pupillary light reflex are mediated by distinct populations of ipRGCs. Nature 476, 92–95. 11. Trenholm, S., Johnson, K., Li, X., Smith, R.G., and Awatramani, G.B. (2011). Parallel mechanisms encode direction in the retina. Neuron 71, 683–694. 12. Su¨mbu¨l, U., Song, S., McCulloch, K., Becker, M., Lin, B., Sanes, J.R., Masland, R.H., and Seung, H.S. (2014). A genetic and computational approach to structurally classify neuronal types. Nat. Commun. 5, 3512. 13. Hong, Y.K., Kim, I.J., and Sanes, J.R. (2011). Stereotyped axonal arbors of retinal ganglion cell subsets in the mouse superior colliculus. J. Comp. Neurol. 519, 1691–1711.
Planar Cell Polarity: The Importance of Getting It Backwards The core and Fat–Dachsous signaling systems locally align planar cell polarities in Drosophila epithelia. Three recent papers address how coupling between these systems can be altered and reversed by the products of the gene prickle. Seth S. Blair The accurate polarization of cells along the plane of an epithelium can orient molecules and structures within single cells, regulate the direction of cell and tissue rearrangements, and bias differentiation choices. Look at the hairs on your arm. Think of your inner ear. While mechanisms for this planar cell polarity (PCP) can differ, two molecular systems involved in PCP are apparently shared from flies to vertebrates: the ‘core’ polarity system, and the Fat (Ft)–Dachsous (Ds) system. Three recent papers, including one published in this issue of Current Biology, now present interesting new details about how to strengthen, weaken, and in particular reverse the coupling between these systems in Drosophila, due to two different products of a single gene [1–3]. As in many fields of biology, PCP has moved from elegant, singular theories to the reality of multiple parallel mechanisms that intersect on several levels [4,5]. This can make things a bit hard on the casual — or even professional — fan of PCP. Complexity has a way of rendering Occam’s razor
a bit duller and less reliable. One distrusts the simplest explanation (once bitten, twice shy) but, having admitted that there are several reasonable ways to get the same result, the search for a powerful experimental test becomes more difficult. Many find themselves, like the cells, repeating and reinterpreting the work of their neighbors, albeit with twists, some subtle, some profound. What has improved, however, is our ability to look in detail at the cell-by-cell planar polarization of the proteins most intimately involved in the process, rather than the final outcome. This nicely narrows interpretations, and has confirmed and extended some old ideas in lovely detail. Protein polarizations are important because they are not just an outcome but — in tissues like the Drosophila wing, abdomen and eye — a cause of PCP. Some of the polarized proteins are also signals that can direct polarization in adjacent cells, which in turn propagate that local alignment to their neighbors. Add amplification and feedback, and any slight tendency towards polarization turns into a robust, self-reinforcing property
14. Zhang, Y., Kim, I.J., Sanes, J.R., and Meister, M. (2012). The most numerous ganglion cell type of the mouse retina is a selective feature detector. Proc. Natl. Acad. Sci. USA 109, 2391–2398. 15. Bleckert, A., Schwartz, G.W., Turner, M.H., Rieke, F., and Wong, R.O. (2014). Visual space is represented by nonmatching topographies of distinct mouse retinal ganglion cell types. Curr. Biol. 24, 310–315. 16. Portugues, R., Feierstein, C.E., Engert, F., and Orger, M.B. (2014). Whole-brain activity maps reveal stereotyped, distributed networks for visuomotor behavior. Neuron 81, 1328–1343.
Neural Circuit Laboratories, Friedrich Miescher Institute for Biomedical Research, 4058 Basel, Switzerland. *E-mail: [email protected] http://dx.doi.org/10.1016/j.cub.2014.08.009
of repeated, interlocked polarities across a field of cells. Having multiple local alignment systems likely adds another level of robustness [4,5]. In the core system, signaling between cells is carried by the Wnt receptor Frizzled (Fz), the multipass transmembrane protein Van Gogh/Strabismus (Vang/Stbm) and the homophilic cadherin Flamingo/Starry night (Fmi/Stan), which is modulated and localized by the cytoplasmic proteins Disheveled (Dsh), Diego and Prickle (Pk). Fz, Dsh and Diego concentrate on one face of a cell and Vang/Stbm and Pk concentrate on the opposite; Fmi/Stan co-concentrates with both (Figure 1A). In the Ft–Ds system, signaling is carried by heterophilic binding between the Ft and Ds protocadherins, with Ds and the myosin Dachs concentrated more reliably on one face, and Ft weakly concentrated on the opposite (Figure 1A). How — and how well — are these two systems integrated in Drosophila? It depends a bit on the type of polarity, which includes biased cell divisions, hair polarities, and polarized fate choices. Ft and Ds protein polarization appears largely unaffected by the core polarity system (although I will discuss an intriguing new exception below), and there are polarities where the Ft–Ds system seems to work largely alone. Each system can independently influence the polarity of abdominal hairs when the other system has been disrupted [4,6]. Nonetheless, in the wing and eye, core proteins polarize
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Figure 1. Sple-dependent coupling between the Ft–Ds and core protein signaling systems. (A) Heterophilic binding between Ft and Ds polarize Ft to one cell face and Ds and the myosin Dachs to opposite cell faces. At high Pk:Sple ratios, cells may accumulate core proteins Fz, Diego and Dsh on the Ds–Dachs face, while at low Pk:Sple ratios, cells accumulate core proteins Vang/Stbm, Pk and Sple on the Ds–Dachs face. The arrows show the direction of Ft–Ds (purple) or core protein (green) polarization on each cell, defined arbitrarily as pointing towards the cell face with high Ds–Dachs or high Fz–Dsh–Diego. (B) Model of Sple action based on the effects of Sple on microtubule polarity and movement of Fz- and Dsh-containing vesicles [2]. When Pk dominates (left cell), microtubule plus ends and vesicle migration are biased towards the Ds–Dachs face. When Sple dominates (right cell), microtubule plus ends and vesicle migration are biased away from the Ds–Dachs face. (C) Model of Sple action based on stabilizing interactions between Sple, Dachs, Ds and the core polarity protein Vang/Stbm. Because the protein domains common to Pk and Sple can also bind Dsh and Diego, this simple model assumes either that the binding of Sple to Dsh and Diego destabilizes Dsh and Diego at the Ds–Dachs face, or that the binding to Dsh and Diego is blocked by high levels of Sple.
less accurately without Ft and Ds and ectopic Ft–Ds signaling can reorient core polarization. Thus, the Ft–Ds system is not just parallel to, but also upstream of, the core system.
This coupling is important, because the Ft–Ds system can supply something that the core system does not: a cue that orients local signaling within the tissue [4,5]. The core proteins
are ubiquitously expressed, and, while local alignment will assure similar polarization in neighboring cells, there is nothing to say what the polarity will be. The Ft–Ds system, however, comes with an built-in orientation; Ft is uniformly expressed in tissues, but its binding partner Ds is expressed in a localized or gradient pattern, and the Golgi-resident kinase Four-jointed (Fj) is expressed in a complementary pattern to Ds. Fj phosphorylates cadherin domains in Ft and Ds, reducing Ds binding to Ft, while increasing Ft binding to Ds. A cell will have neighbors on one face that have more (and more adhesive) Ds but less adhesive Ft, and cells on the other face with less (and less adhesive) Ds but more adhesive Ft. This slight bias is likely amplified, polarizing the levels of Ds and Ft on the cell surface. The coupling between the Ft–Ds and core systems is, however, poorly understood. And there is the particularly confounding problem that the alignment between the two systems in one tissue can be completely reversed in another. Based on either the direct observation of protein polarization or the polarization inferred from the Ds and Fj expression patterns, the high Ds–Dachs face in the wing and posterior abdomen tissues has high Fz, Dsh and Diego, while the high Ds–Dachs face in anterior abdomen, eye and leg has high Vang and Pk (Figure 1A). The new papers show that this 180 rotation is caused by changes in the relative levels of two protein isoforms encoded by the pk gene — Pk and Spiny legs (Sple) [1–3]. Not that this is a new idea. While Pk is a core polarity member — it binds other core proteins, and its loss reduces polarization of the others — it has always had some intriguing peculiarities. The single locus produces alternative transcripts from different promoters, creating cytoplasmic proteins that differ in their amino termini. Work going back to 1999 using isoform-specific mutants and overexpression constructs showed that the Pk and Sple proteins compete for different polarity outcomes [7]. And the tissue-specific nature of those outcomes nicely matches the tissue-specific alignments of the core and Ft–Ds systems. The hypothesis that Pk and Sple might differentially specify how core proteins and hairs interpret the Ft–Ds system
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was advanced a decade ago [8], and recently received strong support from experiments showing that mutants that lack Pk and retain Sple change the direction with which hairs reorient along artificial Ds gradients [9]. However, all this work was based on hair polarities, and this left open the possibility that the Pk:Sple ratio might affect Ft–Ds polarization, or something independent of both PCP systems. Now, the recent studies demonstrate that Pk and Sple do not alter the critical Ft–Ds protein polarities; instead, altering the Pk:Sple ratio by removal or addition of either protein reverses the alignment of the core system along the Ft–Ds axis [1–3]. The new work also shows, for the first time, tissue-specific differences in isoform expression. In the wing the Pk:Sple ratio is high, while in eye and leg the Pk:Sple ratio is low, paralleling the forward and reverse alignments in these tissues. One paper goes further [3], arguing that Sple not only reverses the alignment between the systems, but also increases the strength of the coupling from Ft–Ds to the core system, and even from the core system back to Ft–Ds. First, during wing elongation the core polarity rotates proximo-distally, apparently diverging from the center-to-margin Ft–Ds polarity; when hairs are formed, they largely follow the core pattern. Removing Pk or increasing Sple expression during this period blocks the core polarity rotation, re-aligning the systems, although reversing the direction of alignment [3]; the hairs follow [3,10]. Second, after hair formation Pk levels drop, and Ft–Ds and core PCP polarities rotate and align with each other in a direction that is the reverse of the earlier alignment. In this case, removing Sple or Vang/Stbm reduces the rotation of the Ft–Ds polarity, more strongly after Vang/Stbm loss. To my knowledge, this is the first example of the core system affecting Ft–Ds polarization. The two other papers present intriguing, if divergent, clues about how Pk and Sple work. One [2] recalls another form of wing polarity that is sensitive to the Ft–Ds system but not the core system. Apical microtubules tend to align along the proximo–distal axis and, in the parts of the wing most sensitive to the Ft–Ds system, show a bias in their plus-end polarity [2,11,12] (Figure 1B). This provides a potential connection to the core system because
it is associated with a polarized bias in the migration of Fz-containing and Dsh-containing vesicles. Biased vesicle migration is not necessary for core protein polarization, but could in theory give it enough of a push to help orient it. Microtubule alignment is disrupted by loss of Ft and Ds, and, while altering the Ds expression pattern does not greatly alter the alignment, it does alter the microtubule polarity bias [2,12]. New data show that the direction of microtubule polarization and vesicle migration is reversed by Sple expression, just as would be expected if these processes linked the core and Ft–Ds systems [2] (Figure 1B). The biochemical clues, on the other hand, suggest a more direct connection [1]. The region common to Pk and Sple proteins binds the core proteins Dsh, Diego and Vang/Stbm and is required for PCP activity [4,13]. A fragment containing the specific Sple amino terminus also binds Ds, and both this fragment and full-length Sple can bind another polarized Ft–Ds component, the myosin Dachs [1]. Dachs binds an intracellular fragment of Ds [14], and Ds, Dachs and overexpressed Sple all co-polarize to the marginal (distal) face of wing cells [1]. Thus, Sple might physically link Vang/Stbm to the side of the cell with high Dachs and Ds (Figure 1C). Data supporting a role for Dachs in this process are contradictory, however. In dachs mutant wings, overexpressed Sple no longer affects core protein and hair polarity, and now concentrates proximally instead of distally [1]. Previous work has shown, however, that loss of Dachs causes only very weak polarity defects, even in the anterior abdomen and eye where Sple is required [15]. Still, if Dachs helps couple the systems, this may help explain an old puzzle — the quite different polarity defects that result from different disruptions to the Ft–Ds system. Loss of ft or ds strongly disrupts hair polarity in the wing and abdomen, but removing Ft–Ds polarity using uniform Ds and Fj expression causes much milder defects [4,5]. Obviously there must be other cues and systems that orient core and hair polarities in the absence of Ft–Ds polarity, such as Wnts or even mechanical stresses [16,17]. But why the different outcomes? One suggestion is that the more severe PCP defects resulting from Ft and Ds loss are caused by
misregulation of other Ft–Ds-sensitive pathways, such as the growth-suppressing Hippo pathway [18,19], or a mitochondrial function regulated by an intracellular cleavage product of Fat [20]. But another effect of losing Ft or Ds is increased Dachs levels, and fat and ds mutant polarity defects are greatly improved by removing Dachs [15,18]. While Dachs removal also improves Hippo activity, the new coupling role for Dachs raises another possibility: perhaps the extra Dachs increases the reverse coupling between the core proteins and an unpolarized Ft–Ds system? In this view, ft and ds mutants are not merely lacking polarity; they are more perfectly backwards. References 1. Ayukawa, T., Akiyama, M., MummeryWidmer, J.L., Stoeger, T., Sasaki, J., Knoblich, J.A., Senoo, H., Sasaki, T., and Yamazaki, M. (2014). Dachsous-dependent asymmetric localization of spiny-legs determines planar cell polarity orientation in Drosophila. Cell Rep. 8, 610–621. 2. Olofsson, J., Sharp, K.A., Matis, M., Cho, B., and Axelrod, J.D. (2014). Prickle/spiny-legs isoforms control the polarity of the apical microtubule network in planar cell polarity. Development 141, 2866–2874. 3. Merkel, M., Sanger, A., Gruber, F.S., Etournay, R., Blasse, C., Myers, E., Eaton, S., and Ju¨licher, F. (2014). The balance of Prickle/Spiny-legs isoforms controls the amount of coupling between Core and Fat PCP systems. Curr. Biol. 24, 2111–2123. 4. Matis, M., and Axelrod, J.D. (2013). Regulation of PCP by the Fat signaling pathway. Genes Dev. 27, 2207–2220. 5. Blair, S.S. (2012). Cell polarity: overdosing on PCPs. Curr. Biol. 22, R567–R569. 6. Casal, J., Lawrence, P.A., and Struhl, G. (2006). Two separate molecular systems, Dachsous/ Fat and Starry night/Frizzled, act independently to confer planar cell polarity. Development 133, 4561–4572. 7. Gubb, D., Green, C., Huen, D., Coulson, D., Johnson, G., Tree, D., Collier, S., and Roote, J. (1999). The balance between isoforms of the prickle LIM domain protein is critical for planar polarity in Drosophila imaginal discs. Genes Dev. 13, 2315–2327. 8. Lawrence, P.A., Casal, J., and Struhl, G. (2004). Cell interactions and planar polarity in the abdominal epidermis of Drosophila. Development 131, 4651–4664. 9. Hogan, J., Valentine, M., Cox, C., Doyle, K., and Collier, S. (2011). Two frizzled planar cell polarity signals in the Drosophila wing are differentially organized by the Fat/Dachsous pathway. PLoS Genet. 7, e1001305. 10. Doyle, K., Hogan, J., Lester, M., and Collier, S. (2008). The Frizzled planar cell polarity signaling pathway controls Drosophila wing topography. Dev. Biol. 317, 354–367. 11. Shimada, Y., Yonemura, S., Ohkura, H., Strutt, D., and Uemura, T. (2006). Polarized transport of Frizzled along the planar microtubule arrays in Drosophila wing epithelium. Dev. Cell 10, 209–222. 12. Harumoto, T., Ito, M., Shimada, Y., Kobayashi, T.J., Ueda, H.R., Lu, B., and Uemura, T. (2010). Atypical cadherins Dachsous and Fat control dynamics of noncentrosomal microtubules in planar cell polarity. Dev. Cell 19, 389–401.
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13. Lin, Y.Y., and Gubb, D. (2009). Molecular dissection of Drosophila Prickle isoforms distinguishes their essential and overlapping roles in planar cell polarity. Dev. Biol. 325, 386–399. 14. Bosveld, F., Bonnet, I., Guirao, B., Tlili, S., Wang, Z., Petitalot, A., Marchand, R., Bardet, P.L., Marcq, P., Graner, F., et al. (2012). Mechanical control of morphogenesis by Fat/Dachsous/Four-jointed planar cell polarity pathway. Science 336, 724–727. 15. Mao, Y., Rauskolb, C., Cho, E., Hu, W.L., Hayter, H., Minihan, G., Katz, F.N., and Irvine, K.D. (2006). Dachs: an unconventional myosin that functions downstream of Fat to regulate growth, affinity and gene expression in Drosophila. Development 133, 2539–2551.
16. Sagner, A., Merkel, M., Aigouy, B., Gaebel, J., Brankatschk, M., Julicher, F., and Eaton, S. (2012). Establishment of global patterns of planar polarity during growth of the Drosophila wing epithelium. Curr. Biol. 22, 1296–1301. 17. Wu, J., Roman, A.C., Carvajal-Gonzalez, J.M., and Mlodzik, M. (2013). Wg and Wnt4 provide long-range directional input to planar cell polarity orientation in Drosophila. Nat. Cell Biol. 15, 1045–1055. 18. Brittle, A., Thomas, C., and Strutt, D. (2012). Planar polarity specification through asymmetric subcellular localization of Fat and Dachsous. Curr. Biol. 22, 907–914. 19. Feng, Y., and Irvine, K.D. (2007). Fat and Expanded act in parallel to regulate growth
Attentional Selection: Mexican Hats Everywhere A recent study elegantly shows that allocating attention to a particular color not only enhances perception of the attended color but also suppresses that of similar colors, presumably giving any potentially relevant object in the visual environment a perceptual advantage by increasing its perceptual strength at the expense of similar but different stimuli. Stefan Treue Visual perception resembles the task of an Alaskan bear standing in a rushing stream, trying to catch the salmon on its way up the river: just as the bear needs to detect the fish in the swirling mass of water to ensure his meal, we need to be able to concentrate our visual processing resources on the small fraction of relevant information in the torrent of data delivered by our eyes. This attentional selection can be based on a spatial location (as when the bear concentrates on a place between the rocks that the salmon prefer for their ascent) or on a feature (such as the unique color of the salmon’s scales that the bear has learned to look out for, the salmon’s body shape or the salmon’s orientation). While the neural mechanisms of spatial selection have been in the focus of attention research for decades, feature-based selection has only been in the center of interest much more recently. A study by Sto¨rmer and Alvarez [1] published recently in Current Biology provides a big step towards bringing our understanding of feature-based attention up to par with that of spatial attention. Much of our understanding of spatial attention is well-captured by the
metaphor of the spotlight — attending covertly (without making an eye movement) to one or two particular location(s) in our visual field makes our perception faster, more accurate, of higher spatial resolution and of enhanced sensitivity for fine changes. The physiological correlate of these enhancements is a gain increase of neurons with receptive fields that overlap the attended location, similar to the sensory effect of increasing the salience of a given stimulus. While the perceptual enhancement is often assumed to fall off monotonically with distance from the attended location, there is behavioral and electrophysiological evidence [2–5] for a suppressive zone in the direct vicinity of the spotlight of attention, in line with a computational model where such a ‘Mexican hat’ profile of cortical responsiveness with an excitatory center and an inhibitory surround is a core component [6]. Sto¨rmer and Alvarez’ [1] addressed the question of whether such an inhibitory surround also exists for feature-based attention. Feature-based attention refers to an enhancement of cortical information processing and perception for attended features across the whole visual field, a process particularly useful in visual search, where features
through Warts. Proc. Natl. Acad. Sci. USA 104, 20362–20367. 20. Sing, A., Tsatskis, Y., Fabian, L., Hester, I., Rosenfeld, R., Serricchio, M., Yau, N., Bietenhader, M., Shanbhag, R., Jurisicova, A., et al. (2014). The atypical cadherin Fat directly regulates mitochondrial function and metabolic state. Cell, in press.
Department of Zoology, University of Wisconsin, Madison, WI 53706 USA. E-mail: [email protected]
http://dx.doi.org/10.1016/j.cub.2014.08.008
of the searched item, but not its location, are known, and correspondingly perception of the attended feature is enhanced across the whole visual field [7–9], or where one of multiple items needs to be attended at a given location [10]. Just as in spatial attention, feature-based attentional effects are known to exist in other sensory domains too, such as somatosensory and auditory perception [11–13]. In Sto¨rmer and Alvarez’s study [1] human subjects had to detect brief periods of coherent motion in dot patterns of one target color embedded amongst randomly moving dots of another (distractor) color. This had to be done simultaneously for such two-color motion patterns in the left and in the right visual hemifield. The target color on the left and right stimulus could differ. Not surprisingly the best performance was observed when the target color was the same in the left and right field, presumably because subjects could use a single feature (the one target color) as a selection criterion across the whole visual field. The novel finding of the study was made when the difference in hue between the two target colors was systematically varied: the subjects’ performance was worst when the two target colors were different but similar, indicating that attending to one color suppressed similar but different colors across the whole visual field. In a second experiment Sto¨rmer and Alvarez [1] looked directly at the effect of feature-based attention on neural activity by measuring steady-state visual evoked potentials (SSVEPs). The SSVEP is the oscillatory response of the visual cortex to flickering stimuli: it has the same frequency as the driving
4. Coombs, J., van der List, D., Wang, G.Y., and Chalupa, L.M. (2006). Morphological properties of mouse retinal ganglion cells. Neuroscience 140, 123–136. 5. Helmstaedter, M., Briggman, K.L., Turaga, S.C., Jain, V., Seung, H.S., and Denk, W. (2013). Connectomic reconstruction of the inner plexiform layer in the mouse retina. Nature 500, 168–174. 6. Badea, T.C., and Nathans, J. (2004). Quantitative analysis of neuronal morphologies in the mouse retina visualized by using a genetically directed reporter. J. Comp. Neurol. 480, 331–351. 7. Kim, I.J., Zhang, Y., Yamagata, M., Meister, M., and Sanes, J.R. (2008). Molecular identification of a retinal cell type that responds to upward motion. Nature 452, 478–482. 8. Huberman, A.D., Wei, W., Elstrott, J., Stafford, B.K., Feller, M.B., and Barres, B.A. (2009). Genetic identification of an On-Off direction-selective retinal ganglion cell subtype reveals a layer-specific subcortical map of posterior motion. Neuron 62, 327–334.
9. Yonehara, K., Ishikane, H., Sakuta, H., Shintani, T., Nakamura-Yonehara, K., Kamiji, N.L., Usui, S., and Noda, M. (2009). Identification of retinal ganglion cells and their projections involved in central transmission of information about upward and downward image motion. PLoS One 4, e4320. 10. Chen, S.K., Badea, T.C., and Hattar, S. (2011). Photoentrainment and pupillary light reflex are mediated by distinct populations of ipRGCs. Nature 476, 92–95. 11. Trenholm, S., Johnson, K., Li, X., Smith, R.G., and Awatramani, G.B. (2011). Parallel mechanisms encode direction in the retina. Neuron 71, 683–694. 12. Su¨mbu¨l, U., Song, S., McCulloch, K., Becker, M., Lin, B., Sanes, J.R., Masland, R.H., and Seung, H.S. (2014). A genetic and computational approach to structurally classify neuronal types. Nat. Commun. 5, 3512. 13. Hong, Y.K., Kim, I.J., and Sanes, J.R. (2011). Stereotyped axonal arbors of retinal ganglion cell subsets in the mouse superior colliculus. J. Comp. Neurol. 519, 1691–1711.
Planar Cell Polarity: The Importance of Getting It Backwards The core and Fat–Dachsous signaling systems locally align planar cell polarities in Drosophila epithelia. Three recent papers address how coupling between these systems can be altered and reversed by the products of the gene prickle. Seth S. Blair The accurate polarization of cells along the plane of an epithelium can orient molecules and structures within single cells, regulate the direction of cell and tissue rearrangements, and bias differentiation choices. Look at the hairs on your arm. Think of your inner ear. While mechanisms for this planar cell polarity (PCP) can differ, two molecular systems involved in PCP are apparently shared from flies to vertebrates: the ‘core’ polarity system, and the Fat (Ft)–Dachsous (Ds) system. Three recent papers, including one published in this issue of Current Biology, now present interesting new details about how to strengthen, weaken, and in particular reverse the coupling between these systems in Drosophila, due to two different products of a single gene [1–3]. As in many fields of biology, PCP has moved from elegant, singular theories to the reality of multiple parallel mechanisms that intersect on several levels [4,5]. This can make things a bit hard on the casual — or even professional — fan of PCP. Complexity has a way of rendering Occam’s razor
a bit duller and less reliable. One distrusts the simplest explanation (once bitten, twice shy) but, having admitted that there are several reasonable ways to get the same result, the search for a powerful experimental test becomes more difficult. Many find themselves, like the cells, repeating and reinterpreting the work of their neighbors, albeit with twists, some subtle, some profound. What has improved, however, is our ability to look in detail at the cell-by-cell planar polarization of the proteins most intimately involved in the process, rather than the final outcome. This nicely narrows interpretations, and has confirmed and extended some old ideas in lovely detail. Protein polarizations are important because they are not just an outcome but — in tissues like the Drosophila wing, abdomen and eye — a cause of PCP. Some of the polarized proteins are also signals that can direct polarization in adjacent cells, which in turn propagate that local alignment to their neighbors. Add amplification and feedback, and any slight tendency towards polarization turns into a robust, self-reinforcing property
14. Zhang, Y., Kim, I.J., Sanes, J.R., and Meister, M. (2012). The most numerous ganglion cell type of the mouse retina is a selective feature detector. Proc. Natl. Acad. Sci. USA 109, 2391–2398. 15. Bleckert, A., Schwartz, G.W., Turner, M.H., Rieke, F., and Wong, R.O. (2014). Visual space is represented by nonmatching topographies of distinct mouse retinal ganglion cell types. Curr. Biol. 24, 310–315. 16. Portugues, R., Feierstein, C.E., Engert, F., and Orger, M.B. (2014). Whole-brain activity maps reveal stereotyped, distributed networks for visuomotor behavior. Neuron 81, 1328–1343.
Neural Circuit Laboratories, Friedrich Miescher Institute for Biomedical Research, 4058 Basel, Switzerland. *E-mail: [email protected] http://dx.doi.org/10.1016/j.cub.2014.08.009
of repeated, interlocked polarities across a field of cells. Having multiple local alignment systems likely adds another level of robustness [4,5]. In the core system, signaling between cells is carried by the Wnt receptor Frizzled (Fz), the multipass transmembrane protein Van Gogh/Strabismus (Vang/Stbm) and the homophilic cadherin Flamingo/Starry night (Fmi/Stan), which is modulated and localized by the cytoplasmic proteins Disheveled (Dsh), Diego and Prickle (Pk). Fz, Dsh and Diego concentrate on one face of a cell and Vang/Stbm and Pk concentrate on the opposite; Fmi/Stan co-concentrates with both (Figure 1A). In the Ft–Ds system, signaling is carried by heterophilic binding between the Ft and Ds protocadherins, with Ds and the myosin Dachs concentrated more reliably on one face, and Ft weakly concentrated on the opposite (Figure 1A). How — and how well — are these two systems integrated in Drosophila? It depends a bit on the type of polarity, which includes biased cell divisions, hair polarities, and polarized fate choices. Ft and Ds protein polarization appears largely unaffected by the core polarity system (although I will discuss an intriguing new exception below), and there are polarities where the Ft–Ds system seems to work largely alone. Each system can independently influence the polarity of abdominal hairs when the other system has been disrupted [4,6]. Nonetheless, in the wing and eye, core proteins polarize
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Current Biology
Figure 1. Sple-dependent coupling between the Ft–Ds and core protein signaling systems. (A) Heterophilic binding between Ft and Ds polarize Ft to one cell face and Ds and the myosin Dachs to opposite cell faces. At high Pk:Sple ratios, cells may accumulate core proteins Fz, Diego and Dsh on the Ds–Dachs face, while at low Pk:Sple ratios, cells accumulate core proteins Vang/Stbm, Pk and Sple on the Ds–Dachs face. The arrows show the direction of Ft–Ds (purple) or core protein (green) polarization on each cell, defined arbitrarily as pointing towards the cell face with high Ds–Dachs or high Fz–Dsh–Diego. (B) Model of Sple action based on the effects of Sple on microtubule polarity and movement of Fz- and Dsh-containing vesicles [2]. When Pk dominates (left cell), microtubule plus ends and vesicle migration are biased towards the Ds–Dachs face. When Sple dominates (right cell), microtubule plus ends and vesicle migration are biased away from the Ds–Dachs face. (C) Model of Sple action based on stabilizing interactions between Sple, Dachs, Ds and the core polarity protein Vang/Stbm. Because the protein domains common to Pk and Sple can also bind Dsh and Diego, this simple model assumes either that the binding of Sple to Dsh and Diego destabilizes Dsh and Diego at the Ds–Dachs face, or that the binding to Dsh and Diego is blocked by high levels of Sple.
less accurately without Ft and Ds and ectopic Ft–Ds signaling can reorient core polarization. Thus, the Ft–Ds system is not just parallel to, but also upstream of, the core system.
This coupling is important, because the Ft–Ds system can supply something that the core system does not: a cue that orients local signaling within the tissue [4,5]. The core proteins
are ubiquitously expressed, and, while local alignment will assure similar polarization in neighboring cells, there is nothing to say what the polarity will be. The Ft–Ds system, however, comes with an built-in orientation; Ft is uniformly expressed in tissues, but its binding partner Ds is expressed in a localized or gradient pattern, and the Golgi-resident kinase Four-jointed (Fj) is expressed in a complementary pattern to Ds. Fj phosphorylates cadherin domains in Ft and Ds, reducing Ds binding to Ft, while increasing Ft binding to Ds. A cell will have neighbors on one face that have more (and more adhesive) Ds but less adhesive Ft, and cells on the other face with less (and less adhesive) Ds but more adhesive Ft. This slight bias is likely amplified, polarizing the levels of Ds and Ft on the cell surface. The coupling between the Ft–Ds and core systems is, however, poorly understood. And there is the particularly confounding problem that the alignment between the two systems in one tissue can be completely reversed in another. Based on either the direct observation of protein polarization or the polarization inferred from the Ds and Fj expression patterns, the high Ds–Dachs face in the wing and posterior abdomen tissues has high Fz, Dsh and Diego, while the high Ds–Dachs face in anterior abdomen, eye and leg has high Vang and Pk (Figure 1A). The new papers show that this 180 rotation is caused by changes in the relative levels of two protein isoforms encoded by the pk gene — Pk and Spiny legs (Sple) [1–3]. Not that this is a new idea. While Pk is a core polarity member — it binds other core proteins, and its loss reduces polarization of the others — it has always had some intriguing peculiarities. The single locus produces alternative transcripts from different promoters, creating cytoplasmic proteins that differ in their amino termini. Work going back to 1999 using isoform-specific mutants and overexpression constructs showed that the Pk and Sple proteins compete for different polarity outcomes [7]. And the tissue-specific nature of those outcomes nicely matches the tissue-specific alignments of the core and Ft–Ds systems. The hypothesis that Pk and Sple might differentially specify how core proteins and hairs interpret the Ft–Ds system
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was advanced a decade ago [8], and recently received strong support from experiments showing that mutants that lack Pk and retain Sple change the direction with which hairs reorient along artificial Ds gradients [9]. However, all this work was based on hair polarities, and this left open the possibility that the Pk:Sple ratio might affect Ft–Ds polarization, or something independent of both PCP systems. Now, the recent studies demonstrate that Pk and Sple do not alter the critical Ft–Ds protein polarities; instead, altering the Pk:Sple ratio by removal or addition of either protein reverses the alignment of the core system along the Ft–Ds axis [1–3]. The new work also shows, for the first time, tissue-specific differences in isoform expression. In the wing the Pk:Sple ratio is high, while in eye and leg the Pk:Sple ratio is low, paralleling the forward and reverse alignments in these tissues. One paper goes further [3], arguing that Sple not only reverses the alignment between the systems, but also increases the strength of the coupling from Ft–Ds to the core system, and even from the core system back to Ft–Ds. First, during wing elongation the core polarity rotates proximo-distally, apparently diverging from the center-to-margin Ft–Ds polarity; when hairs are formed, they largely follow the core pattern. Removing Pk or increasing Sple expression during this period blocks the core polarity rotation, re-aligning the systems, although reversing the direction of alignment [3]; the hairs follow [3,10]. Second, after hair formation Pk levels drop, and Ft–Ds and core PCP polarities rotate and align with each other in a direction that is the reverse of the earlier alignment. In this case, removing Sple or Vang/Stbm reduces the rotation of the Ft–Ds polarity, more strongly after Vang/Stbm loss. To my knowledge, this is the first example of the core system affecting Ft–Ds polarization. The two other papers present intriguing, if divergent, clues about how Pk and Sple work. One [2] recalls another form of wing polarity that is sensitive to the Ft–Ds system but not the core system. Apical microtubules tend to align along the proximo–distal axis and, in the parts of the wing most sensitive to the Ft–Ds system, show a bias in their plus-end polarity [2,11,12] (Figure 1B). This provides a potential connection to the core system because
it is associated with a polarized bias in the migration of Fz-containing and Dsh-containing vesicles. Biased vesicle migration is not necessary for core protein polarization, but could in theory give it enough of a push to help orient it. Microtubule alignment is disrupted by loss of Ft and Ds, and, while altering the Ds expression pattern does not greatly alter the alignment, it does alter the microtubule polarity bias [2,12]. New data show that the direction of microtubule polarization and vesicle migration is reversed by Sple expression, just as would be expected if these processes linked the core and Ft–Ds systems [2] (Figure 1B). The biochemical clues, on the other hand, suggest a more direct connection [1]. The region common to Pk and Sple proteins binds the core proteins Dsh, Diego and Vang/Stbm and is required for PCP activity [4,13]. A fragment containing the specific Sple amino terminus also binds Ds, and both this fragment and full-length Sple can bind another polarized Ft–Ds component, the myosin Dachs [1]. Dachs binds an intracellular fragment of Ds [14], and Ds, Dachs and overexpressed Sple all co-polarize to the marginal (distal) face of wing cells [1]. Thus, Sple might physically link Vang/Stbm to the side of the cell with high Dachs and Ds (Figure 1C). Data supporting a role for Dachs in this process are contradictory, however. In dachs mutant wings, overexpressed Sple no longer affects core protein and hair polarity, and now concentrates proximally instead of distally [1]. Previous work has shown, however, that loss of Dachs causes only very weak polarity defects, even in the anterior abdomen and eye where Sple is required [15]. Still, if Dachs helps couple the systems, this may help explain an old puzzle — the quite different polarity defects that result from different disruptions to the Ft–Ds system. Loss of ft or ds strongly disrupts hair polarity in the wing and abdomen, but removing Ft–Ds polarity using uniform Ds and Fj expression causes much milder defects [4,5]. Obviously there must be other cues and systems that orient core and hair polarities in the absence of Ft–Ds polarity, such as Wnts or even mechanical stresses [16,17]. But why the different outcomes? One suggestion is that the more severe PCP defects resulting from Ft and Ds loss are caused by
misregulation of other Ft–Ds-sensitive pathways, such as the growth-suppressing Hippo pathway [18,19], or a mitochondrial function regulated by an intracellular cleavage product of Fat [20]. But another effect of losing Ft or Ds is increased Dachs levels, and fat and ds mutant polarity defects are greatly improved by removing Dachs [15,18]. While Dachs removal also improves Hippo activity, the new coupling role for Dachs raises another possibility: perhaps the extra Dachs increases the reverse coupling between the core proteins and an unpolarized Ft–Ds system? In this view, ft and ds mutants are not merely lacking polarity; they are more perfectly backwards. References 1. Ayukawa, T., Akiyama, M., MummeryWidmer, J.L., Stoeger, T., Sasaki, J., Knoblich, J.A., Senoo, H., Sasaki, T., and Yamazaki, M. (2014). Dachsous-dependent asymmetric localization of spiny-legs determines planar cell polarity orientation in Drosophila. Cell Rep. 8, 610–621. 2. Olofsson, J., Sharp, K.A., Matis, M., Cho, B., and Axelrod, J.D. (2014). Prickle/spiny-legs isoforms control the polarity of the apical microtubule network in planar cell polarity. Development 141, 2866–2874. 3. Merkel, M., Sanger, A., Gruber, F.S., Etournay, R., Blasse, C., Myers, E., Eaton, S., and Ju¨licher, F. (2014). The balance of Prickle/Spiny-legs isoforms controls the amount of coupling between Core and Fat PCP systems. Curr. Biol. 24, 2111–2123. 4. Matis, M., and Axelrod, J.D. (2013). Regulation of PCP by the Fat signaling pathway. Genes Dev. 27, 2207–2220. 5. Blair, S.S. (2012). Cell polarity: overdosing on PCPs. Curr. Biol. 22, R567–R569. 6. Casal, J., Lawrence, P.A., and Struhl, G. (2006). Two separate molecular systems, Dachsous/ Fat and Starry night/Frizzled, act independently to confer planar cell polarity. Development 133, 4561–4572. 7. Gubb, D., Green, C., Huen, D., Coulson, D., Johnson, G., Tree, D., Collier, S., and Roote, J. (1999). The balance between isoforms of the prickle LIM domain protein is critical for planar polarity in Drosophila imaginal discs. Genes Dev. 13, 2315–2327. 8. Lawrence, P.A., Casal, J., and Struhl, G. (2004). Cell interactions and planar polarity in the abdominal epidermis of Drosophila. Development 131, 4651–4664. 9. Hogan, J., Valentine, M., Cox, C., Doyle, K., and Collier, S. (2011). Two frizzled planar cell polarity signals in the Drosophila wing are differentially organized by the Fat/Dachsous pathway. PLoS Genet. 7, e1001305. 10. Doyle, K., Hogan, J., Lester, M., and Collier, S. (2008). The Frizzled planar cell polarity signaling pathway controls Drosophila wing topography. Dev. Biol. 317, 354–367. 11. Shimada, Y., Yonemura, S., Ohkura, H., Strutt, D., and Uemura, T. (2006). Polarized transport of Frizzled along the planar microtubule arrays in Drosophila wing epithelium. Dev. Cell 10, 209–222. 12. Harumoto, T., Ito, M., Shimada, Y., Kobayashi, T.J., Ueda, H.R., Lu, B., and Uemura, T. (2010). Atypical cadherins Dachsous and Fat control dynamics of noncentrosomal microtubules in planar cell polarity. Dev. Cell 19, 389–401.
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13. Lin, Y.Y., and Gubb, D. (2009). Molecular dissection of Drosophila Prickle isoforms distinguishes their essential and overlapping roles in planar cell polarity. Dev. Biol. 325, 386–399. 14. Bosveld, F., Bonnet, I., Guirao, B., Tlili, S., Wang, Z., Petitalot, A., Marchand, R., Bardet, P.L., Marcq, P., Graner, F., et al. (2012). Mechanical control of morphogenesis by Fat/Dachsous/Four-jointed planar cell polarity pathway. Science 336, 724–727. 15. Mao, Y., Rauskolb, C., Cho, E., Hu, W.L., Hayter, H., Minihan, G., Katz, F.N., and Irvine, K.D. (2006). Dachs: an unconventional myosin that functions downstream of Fat to regulate growth, affinity and gene expression in Drosophila. Development 133, 2539–2551.
16. Sagner, A., Merkel, M., Aigouy, B., Gaebel, J., Brankatschk, M., Julicher, F., and Eaton, S. (2012). Establishment of global patterns of planar polarity during growth of the Drosophila wing epithelium. Curr. Biol. 22, 1296–1301. 17. Wu, J., Roman, A.C., Carvajal-Gonzalez, J.M., and Mlodzik, M. (2013). Wg and Wnt4 provide long-range directional input to planar cell polarity orientation in Drosophila. Nat. Cell Biol. 15, 1045–1055. 18. Brittle, A., Thomas, C., and Strutt, D. (2012). Planar polarity specification through asymmetric subcellular localization of Fat and Dachsous. Curr. Biol. 22, 907–914. 19. Feng, Y., and Irvine, K.D. (2007). Fat and Expanded act in parallel to regulate growth
Attentional Selection: Mexican Hats Everywhere A recent study elegantly shows that allocating attention to a particular color not only enhances perception of the attended color but also suppresses that of similar colors, presumably giving any potentially relevant object in the visual environment a perceptual advantage by increasing its perceptual strength at the expense of similar but different stimuli. Stefan Treue Visual perception resembles the task of an Alaskan bear standing in a rushing stream, trying to catch the salmon on its way up the river: just as the bear needs to detect the fish in the swirling mass of water to ensure his meal, we need to be able to concentrate our visual processing resources on the small fraction of relevant information in the torrent of data delivered by our eyes. This attentional selection can be based on a spatial location (as when the bear concentrates on a place between the rocks that the salmon prefer for their ascent) or on a feature (such as the unique color of the salmon’s scales that the bear has learned to look out for, the salmon’s body shape or the salmon’s orientation). While the neural mechanisms of spatial selection have been in the focus of attention research for decades, feature-based selection has only been in the center of interest much more recently. A study by Sto¨rmer and Alvarez [1] published recently in Current Biology provides a big step towards bringing our understanding of feature-based attention up to par with that of spatial attention. Much of our understanding of spatial attention is well-captured by the
metaphor of the spotlight — attending covertly (without making an eye movement) to one or two particular location(s) in our visual field makes our perception faster, more accurate, of higher spatial resolution and of enhanced sensitivity for fine changes. The physiological correlate of these enhancements is a gain increase of neurons with receptive fields that overlap the attended location, similar to the sensory effect of increasing the salience of a given stimulus. While the perceptual enhancement is often assumed to fall off monotonically with distance from the attended location, there is behavioral and electrophysiological evidence [2–5] for a suppressive zone in the direct vicinity of the spotlight of attention, in line with a computational model where such a ‘Mexican hat’ profile of cortical responsiveness with an excitatory center and an inhibitory surround is a core component [6]. Sto¨rmer and Alvarez’ [1] addressed the question of whether such an inhibitory surround also exists for feature-based attention. Feature-based attention refers to an enhancement of cortical information processing and perception for attended features across the whole visual field, a process particularly useful in visual search, where features
through Warts. Proc. Natl. Acad. Sci. USA 104, 20362–20367. 20. Sing, A., Tsatskis, Y., Fabian, L., Hester, I., Rosenfeld, R., Serricchio, M., Yau, N., Bietenhader, M., Shanbhag, R., Jurisicova, A., et al. (2014). The atypical cadherin Fat directly regulates mitochondrial function and metabolic state. Cell, in press.
Department of Zoology, University of Wisconsin, Madison, WI 53706 USA. E-mail: [email protected]
http://dx.doi.org/10.1016/j.cub.2014.08.008
of the searched item, but not its location, are known, and correspondingly perception of the attended feature is enhanced across the whole visual field [7–9], or where one of multiple items needs to be attended at a given location [10]. Just as in spatial attention, feature-based attentional effects are known to exist in other sensory domains too, such as somatosensory and auditory perception [11–13]. In Sto¨rmer and Alvarez’s study [1] human subjects had to detect brief periods of coherent motion in dot patterns of one target color embedded amongst randomly moving dots of another (distractor) color. This had to be done simultaneously for such two-color motion patterns in the left and in the right visual hemifield. The target color on the left and right stimulus could differ. Not surprisingly the best performance was observed when the target color was the same in the left and right field, presumably because subjects could use a single feature (the one target color) as a selection criterion across the whole visual field. The novel finding of the study was made when the difference in hue between the two target colors was systematically varied: the subjects’ performance was worst when the two target colors were different but similar, indicating that attending to one color suppressed similar but different colors across the whole visual field. In a second experiment Sto¨rmer and Alvarez [1] looked directly at the effect of feature-based attention on neural activity by measuring steady-state visual evoked potentials (SSVEPs). The SSVEP is the oscillatory response of the visual cortex to flickering stimuli: it has the same frequency as the driving
