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2. Erwin, D.H., Laflamme, M., Tweedt, S.M., Sperling, E.A., Pisani, D., and Peterson, K.J. (2011). The Cambrian conundrum: early divergence and later ecological success in the early history of animals. Science 334, 1091–1097. 3. Human Microbiome Project Consortium (2012). Structure, function and diversity of the healthy human microbiome. Nature 486, 207–214. 4. Clemente, J.C., Ursell, L.K., Parfrey, L.W., and Knight, R. (2012). The impact of the gut microbiota on human health: an integrative view. Cell 148, 1258–1270. 5. Underhill, D.M., and Iliev, I.D. (2014). The mycobiota: interactions between commensal fungi and the host immune system. Nat. Rev. Immunol. 14, 405–416. 6. Morales, D.K., and Hogan, D.A. (2010). Candida albicans interactions with bacteria in the context of human health and disease. PLoS Pathog. 6, e1000886. 7. Peleg, A.Y., Hogan, D.A., and Mylonakis, E. (2010). Medically important bacterial-fungal interactions. Nat. Rev. Microbiol. 8, 340–349. 8. Fox, E.P., Cowley, E.S., Nobile, C.J., Hartooni, N., Newman, D.K., and Johnson, A.D. (2014). Anaerobic bacteria grow within Candida albicans biofilms and induce biofilm formation
9.
10.
11.
12.
13.
14.
15.
16.
in suspension cultures. Curr. Biol. 24, 2411–2416. Mitchell, A.P. (1998). Dimorphism and virulence in Candida albicans. Curr. Opin. Microbiol. 1, 687–692. Berman, J., and Sudbery, P.E. (2002). Candida Albicans: a molecular revolution built on lessons from budding yeast. Nat. Rev. Genet. 3, 918–930. Naglik, J.R., Fidel, P.L., Jr., and Odds, F.C. (2008). Animal models of mucosal Candida infection. FEMS Microbiol. Lett. 283, 129–139. Finkel, J.S., and Mitchell, A.P. (2011). Genetic control of Candida albicans biofilm development. Nat. Rev. Microbiol. 9, 109–118. Davey, M.E., and O’Toole, G.A. (2000). Microbial biofilms: from ecology to molecular genetics. Microbiol. Mol. Biol. Rev. 64, 847–867. Costerton, J.W., Stewart, P.S., and Greenberg, E.P. (1999). Bacterial biofilms: a common cause of persistent infections. Science 284, 1318–1322. Harriott, M.M., and Noverr, M.C. (2011). Importance of Candida-bacterial polymicrobial biofilms in disease. Trends Microbiol. 19, 557–563. Bradshaw, D.J., Marsh, P.D., Allison, C., and Schilling, K.M. (1996). Effect of oxygen,
Cellular Energetics: Actin and Myosin Abstain from ATP during Starvation Energy is required for all cellular functions and diverse mechanisms allow the cell to overcome acute energy starvation. A new study reveals that starved cells inhibit type V myosins and actin depolymerization, measures that may conserve energy while temporarily retaining cell polarity. Destiney Buelto1 and Mara C. Duncan2,* Energy starvation at the cellular level is surprisingly common. In multicellular organisms, energy starvation occurs during heart attack, stroke, hypoglycemia, and in highly proliferative cancers. In microbes, energy starvation occurs whenever the microbe encounters a nutrient-poor environment. Even after decades of research, new aspects of starvation biology are still emerging. In this issue of Current Biology, Xu and Bretscher [1] expand the cellular repertoire of starvation responses by identifying two previously unknown responses to glucose starvation — inhibition of myosin V function and inhibition of actin turnover. Starvation elicits diverse responses, which are frequently ancient, highly conserved and, importantly, dictate whether a cell survives or dies in response to acute starvation [2]. These responses allow the cell to access alternative sources of energy, enter
quiescence or engage in programmed cell death, which may provide an advantage for the organism or population [3,4]. Each of these responses requires consumption of energy and other raw material upfront before any benefits are gained. For example, amino acids and ATP are needed for the synthesis and function, respectively, of transporters that allow the cell to access alternative exogenous energy sources. Similarly, quiescence and programmed cell death also require energy [2,5]. Accordingly, energy starvation immediately inhibits processes with high-energy demands, such as translation, which consumes four ATP equivalents per amino acid added, and plasma membrane ATPase activity, which can consume enormous amounts of ATP [6,7]. The prompt shutdown of these processes conserves raw material that the cell needs for other uses. In the absence of exogenous sources, the cell can also produce limited raw material for quiescence by consuming stored
inoculum composition and flow rate on development of mixed-culture oral biofilms. Microbiology 142, 623–629. 17. Rossignol, T., Ding, C., Guida, A., d’Enfert, C., Higgins, D.G., and Butler, G. (2009). Correlation between biofilm formation and the hypoxic response in Candida parapsilosis. Eukaryot. Cell 8, 550–559. 18. Stichternoth, C., and Ernst, J.F. (2009). Hypoxic adaptation by Efg1 regulates biofilm formation by Candida albicans. Appl. Environ. Microbiol. 75, 3663–3672. 19. Nobile, C.J., Fox, E.P., Nett, J.E., Sorrells, T.R., Mitrovich, Q.M., Hernday, A.D., Tuch, B.B., Andes, D.R., and Johnson, A.D. (2012). A recently evolved transcriptional network controls biofilm development in Candida albicans. Cell 148, 126–138.
Department of Microbiology and Immunology, Geisel School of Medicine at Dartmouth, 74 College Street Remsen 213, Hanover, NH 03755, USA. E-mail: [email protected] http://dx.doi.org/10.1016/j.cub.2014.09.030
carbohydrate reserves, as well as cell-surface and cytosolic components [4,8,9]. Starvation responses thus either provide necessary nutrients or perform activities needed for long-term survival of the organism or population. The new work from Xu and Bretscher [1] adds to our understanding of the responses to acute energy starvation in eukaryotes. It follows the previous observation that the cortical actin cytoskeleton becomes depolarized upon glucose starvation in the yeast Saccharomyces cerevisiae [10]. In yeast, cell polarity depends on actin cables, which are functionally distinct from the endocytic cortical actin cytoskeleton [11]. Actin cables are long bundles of many short actin filaments aligned with the fast-growing end oriented towards the site of growth (Figure 1A) [11]. Actin cables act as tracks for type V myosins, Myo2 and Myo4, which direct polarized delivery of secretory vesicles, organelles, and RNA. This polarized delivery of secretory vesicles is required to maintain cell polarity [12]. Thus, the transient depolarization of the cortical cytoskeleton was thought to reflect the depolarization of the actin cables and the subsequent loss of polarized delivery of secretory cargo. The current work challenges this model. The authors find that, upon glucose starvation, Myo2 and Myo4, but not type I or II myosins, become stably associated with polarized actin
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cables (Figure 1B) [1]. Furthermore, only type V myosins that have first interacted with cargo engage in a rigor-like state. These findings are not unexpected since all myosins require ATP to disassociate from actin filaments, and type V myosins only associate with actin cables when activated by cargo [13]. However, what is unexpected is that Xu and Bretscher [1] find that Myo2 releases cargo during starvation; this cargo-free rigor-like state has not been observed previously. Curiously, the rigor-like state of Myo2 seems to be activated only at critically low levels of ATP, as intermediate starvation conditions cause Myo2 to disengage from actin cables and become diffusely localized. ATP seems to be the key regulator of this behavior because addition of exogenous ATP to permeablized starved cells allows Myo2 motility on the cables. However, it is not clear why extremely low ATP levels cause a rigor-like state yet intermediate levels cause Myo2 disengagement. Whether the rigor-like state is responsible for the cargo release of Myo2 in starved cells is not yet known. In support of a link between rigor and cargo release, multiple cargos of Myo2 are released during glucose starvation, suggesting Myo2 could be incompetent for cargo binding in starved cells [1]. On the other hand, the displacement of cargo could easily be due to a mechanism that acts on cargo directly. Many different molecular mechanisms regulate starvation responses. Adaptation to energy starvation requires activation of AMP-activated kinase (AMPK) and inhibition of growth-promoting signaling pathways involving mTOR complex 1 and protein kinase A (PKA), which phosphorylate many proteins to remodel cell physiology [4]. In addition, a number of changes in cell chemistry also regulate starvation responses. For example, pH, which drops w1 unit upon glucose starvation in yeast, is an important regulator of several responses [14,15]. In addition, decreased ATP, GTP, and increased NAD+ all directly regulate starvation responses [16,17]. Thus, cargo release from Myo2 could be modulated by changes in phosphorylation or by any number of changes in cell chemistry acting on cargo directly. The functional significance of the cargo-free rigor-like state of type V myosin is not clear. Although myosin
A
B
Glucose present ATP levels high
Acute starvation ATP levels low
? ATP ATP ATP
? Inactive motor
Actin
Cargo-primed motor
Cargo Current Biology
Figure 1. Model of actin and myosin V responses to glucose starvation. (A) In the presence of glucose, ATP is plentiful. Actin cables, which consist of assemblies of many small filaments, polarize towards the site of growth. The filaments treadmill rapidly, releasing free actin monomers. However, rapid ATP exchange allows the actin cables to maintain steady-state polarity to act as tracks for type V myosins. Type V myosins deliver secretory vesicles, organelles, and other cargo to the site of growth. Cargo binding is a pre-requisite for actin binding. Motor activity requires ATP to release the motor domains for continued motility. (B) During glucose starvation, an unknown mechanism inhibits actin disassembly, stabilizing actin filaments, retaining their original polarity. At the same time, the depletion of ATP induced by glucose starvation forces cargo-associated motors into a rigor-like state, whereas cargo-free motors remain cytosolic. Concurrently, an unknown mechanism strips type V myosins of cargo.
is an ATPase, its ATP consumption represents only a small fraction of total cellular ATP; however, even the modest saving achieved by adopting the rigor-like state could be important. Alternatively, the cargo release may have functional importance. Release of the secretory vesicles may allow isotropic delivery of cell wall synthesis enzymes to prepare the cell for quiescence in the most energy-economical manner. Conversely, inhibiting secretion may be beneficial: such a role is supported by the observation that many cell-surface proteins are internalized during glucose starvation [8]. It will be interesting for future studies to monitor the fate of secretory cargo during starvation. The authors also report the equally surprising discovery that actin cables are stabilized during glucose starvation. Under growth-promoting conditions, the yeast actin cables turn over so rapidly that the entire structure recycles within a minute [11]. In contrast, in cells subjected to acute glucose starvation, Xu and Bretscher [1] found that actin cables are highly stable, as monitored by several criteria. This stabilization is also observed in energy-starved HeLa cells, suggesting
that this may be a conserved response to energy stress [1]. So, how is actin stabilized in starved cells? Like Myo2 cargo release, any number of signaling or cell-chemistry changes could regulate actin turnover. Because ATP is required for actin polymerization, changes in ATP levels are unlikely to act directly on actin. However, ATP or other changes could modulate the activity of an actin-binding protein, resulting in reduced turnover. One candidate is the filament-severing protein cofilin, which is required for rapid filament turnover in the presence of glucose [11]. Furthermore, cofilin is pH sensitive and can be phosphorylated, suggesting its activity could be directly modulated in starved cells [18]. Xu and Bretscher [1] found that overexpression of cofilin did not destabilize actin filaments in glucose-starved cells, suggesting that cofilin is likely inactive during starvation. This inactivity is not due to increased activity of the actin-binding protein tropomyosin because inhibition of tropomyosin failed to destabilize actin in starved cells [1]. It remains to be explored whether cofilin is the direct target of glucose starvation.
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Regardless of the molecular mechanism, the functional significance of stabilizing actin appears clear. Actin is a major consumer of ATP [19] and the observed stabilization of actin filaments therefore prevents the consumption of ATP, whilst retaining actin filaments and their polarity. This polarity retention may be important to allow growth using the existing cytoskeleton with minimal ATP consumption, if the post-starvation environment provides enough nutrients for growth. This new work provides tantalizing insights into previously unrecognized responses to starvation. However, these findings raise nearly as many questions as they answer. A key question is why only extreme energy stress induces the rigor-like state and whether this relates to cargo release. It will also be important to understand if the type V myosin responses are important for conserving energy or if the cargo-free rigor-like state provides some other advantage for adaptation to starvation. It will also be worth exploring whether the type V myosin responses are conserved and what advantage this might provide to mammalian cells that are more dependent on microtubules for polarized transport. In contrast, the advantage of the stabilization of actin is apparent, although the molecular mechanism inducing stabilization in yeast and mammalian cells needs to be
elucidated. Further investigation into the role and regulation of these two new responses will provide a clearer picture of how the cell overcomes the challenge of energy starvation. References 1. Xu, L., and Bretscher, A. (2014). Rapid glucose depletion immobilizes active myosin-V on stabilized actin cables. Curr. Biol. 24, 2471–2479. 2. Klosinska, M.M., Crutchfield, C.A., Bradley, P.H., Rabinowitz, J.D., and Broach, J.R. (2011). Yeast cells can access distinct quiescent states. Genes Dev. 25, 336–349. 3. Fuchs, Y., and Steller, H. (2011). Programmed cell death in animal development and disease. Cell 147, 742–758. 4. Zaman, S., Lippman, S.I., Zhao, X., and Broach, J.R. (2008). How Saccharomyces responds to nutrients. Annu. Rev. Genet. 42, 27–81. 5. Zamaraeva, M.V., Sabirov, R.Z., Maeno, E., Ando-Akatsuka, Y., Bessonova, S.V., and Okada, Y. (2005). Cells die with increased cytosolic ATP during apoptosis: a bioluminescence study with intracellular luciferase. Cell Death Diff. 12, 1390–1397. 6. Serrano, R. (1983). In vivo glucose activation of the yeast plasma membrane ATPase. FEBS Lett. 156, 11–14. 7. Ashe, M.P., De Long, S.K., and Sachs, A.B. (2000). Glucose depletion rapidly inhibits translation initiation in yeast. Mol. Biol. Cell 11, 833–848. 8. Lang, M.J., Martinez-Marquez, J.Y., Prosser, D.C., Ganser, L.R., Buelto, D., Wendland, B., and Duncan, M.C. (2014). Glucose starvation inhibits autophagy via vacuolar hydrolysis and induces plasma membrane internalization by down-regulating recycling. J. Biol. Chem. 289, 16736–16747. 9. Xu, Y.F., Letisse, F., Absalan, F., Lu, W., Kuznetsova, E., Brown, G., Caudy, A.A., Yakunin, A.F., Broach, J.R., and Rabinowitz, J.D. (2013). Nucleotide degradation and ribose salvage in yeast. Mol. Syst. Biol. 9, 665. 10. Uesono, Y., Ashe, M.P., and Toh, E.A. (2004). Simultaneous yet independent regulation of
Neuronal Mitophagy: Long-Distance Delivery or Eating Locally? Parkinson’s disease-associated PINK1 and Parkin maintain mitochondrial health through promoting mitophagy, the autophagic removal of damaged mitochondria, in non-neuronal cells. Whether they do so in neurons has been controversial. A new study aims to resolve the controversy. Bingwei Lu Mitochondria play essential roles in many aspects of cellular physiology, from energy production and intermediary metabolism to apoptosis [1]. Proper mitochondrial function is particularly important for the health of neurons, as suggested by the prominent manifestation of
neurological impairments in mitochondrial diseases [2]. Mitochondrial dysfunction has been observed in many of the age-related neurodegenerative diseases, including Alzheimer’s disease and Parkinson’s disease (PD) [2]. However, the origin of the mitochondrial dysfunction in most of these diseases remains elusive. Pten-induced kinase 1
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actin cytoskeletal organization and translation initiation by glucose in Saccharomyces cerevisiae. Mol. Biol. Cell 15, 1544–1556. Moseley, J.B., and Goode, B.L. (2006). The yeast actin cytoskeleton: from cellular function to biochemical mechanism. Microbiol. Mol. Biol. Rev. 70, 605–645. Savage, N.S., Layton, A.T., and Lew, D.J. (2012). Mechanistic mathematical model of polarity in yeast. Mol. Biol. Cell 23, 1998–2013. Donovan, K.W., and Bretscher, A. (2012). Myosin-V is activated by binding secretory cargo and released in coordination with Rab/exocyst function. Dev. Cell 23, 769–781. Dechant, R., Binda, M., Lee, S.S., Pelet, S., Winderickx, J., and Peter, M. (2010). Cytosolic pH is a second messenger for glucose and regulates the PKA pathway through V-ATPase. EMBO J. 29, 2515–2526. Dechant, R., Saad, S., Ibanez, A.J., and Peter, M. (2014). Cytosolic pH regulates cell growth through distinct GTPases, Arf1 and Gtr1, to promote Ras/PKA and TORC1 activity. Mol. Cell 55, 409–421. Aoh, Q.L., Hung, C.W., and Duncan, M.C. (2013). Energy metabolism regulates clathrin adaptors at the trans-Golgi network and endosomes. Mol. Biol. Cell 24, 832–847. Wellen, K.E., and Thompson, C.B. (2012). A two-way street: reciprocal regulation of metabolism and signalling. Nat. Rev. Mol. Cell Biol. 13, 270–276. Bernstein, B.W., Painter, W.B., Chen, H., Minamide, L.S., Abe, H., and Bamburg, J.R. (2000). Intracellular pH modulation of ADF/ cofilin proteins. Cell Motil. Cytoskel. 47, 319–336. Bernstein, B.W., and Bamburg, J.R. (2003). Actin-ATP hydrolysis is a major energy drain for neurons. J. Neurosci 23, 1–6.
1Curriculum
in Genetics and Molecular Biology, University of North Carolina, Chapel Hill, NC 27599, USA. 2Department of Cell and Molecular Biology, University of Michigan, Ann Arbor, MI 48109, USA. *E-mail: [email protected]
http://dx.doi.org/10.1016/j.cub.2014.09.004
(PINK1) and Parkin, two genes linked to familial PD, have been implicated in mitochondrial quality control, specifically the promotion of mitochondrial degradation by autophagy — mitophagy. Many of the studies indicating a function for PINK1 and Parkin in mitophagy were carried out in cultured non-neuronal cells, but a new study published in the Journal of Cell Biology [3] now suggests that PINK1 and Parkin can indeed act in neuronal axons to remove damaged mitochondria. PD is the most common movement disorder and the second most common neurodegenerative disease after Alzheimer’s disease. It is characterized by rather selective degeneration of
2. Erwin, D.H., Laflamme, M., Tweedt, S.M., Sperling, E.A., Pisani, D., and Peterson, K.J. (2011). The Cambrian conundrum: early divergence and later ecological success in the early history of animals. Science 334, 1091–1097. 3. Human Microbiome Project Consortium (2012). Structure, function and diversity of the healthy human microbiome. Nature 486, 207–214. 4. Clemente, J.C., Ursell, L.K., Parfrey, L.W., and Knight, R. (2012). The impact of the gut microbiota on human health: an integrative view. Cell 148, 1258–1270. 5. Underhill, D.M., and Iliev, I.D. (2014). The mycobiota: interactions between commensal fungi and the host immune system. Nat. Rev. Immunol. 14, 405–416. 6. Morales, D.K., and Hogan, D.A. (2010). Candida albicans interactions with bacteria in the context of human health and disease. PLoS Pathog. 6, e1000886. 7. Peleg, A.Y., Hogan, D.A., and Mylonakis, E. (2010). Medically important bacterial-fungal interactions. Nat. Rev. Microbiol. 8, 340–349. 8. Fox, E.P., Cowley, E.S., Nobile, C.J., Hartooni, N., Newman, D.K., and Johnson, A.D. (2014). Anaerobic bacteria grow within Candida albicans biofilms and induce biofilm formation
9.
10.
11.
12.
13.
14.
15.
16.
in suspension cultures. Curr. Biol. 24, 2411–2416. Mitchell, A.P. (1998). Dimorphism and virulence in Candida albicans. Curr. Opin. Microbiol. 1, 687–692. Berman, J., and Sudbery, P.E. (2002). Candida Albicans: a molecular revolution built on lessons from budding yeast. Nat. Rev. Genet. 3, 918–930. Naglik, J.R., Fidel, P.L., Jr., and Odds, F.C. (2008). Animal models of mucosal Candida infection. FEMS Microbiol. Lett. 283, 129–139. Finkel, J.S., and Mitchell, A.P. (2011). Genetic control of Candida albicans biofilm development. Nat. Rev. Microbiol. 9, 109–118. Davey, M.E., and O’Toole, G.A. (2000). Microbial biofilms: from ecology to molecular genetics. Microbiol. Mol. Biol. Rev. 64, 847–867. Costerton, J.W., Stewart, P.S., and Greenberg, E.P. (1999). Bacterial biofilms: a common cause of persistent infections. Science 284, 1318–1322. Harriott, M.M., and Noverr, M.C. (2011). Importance of Candida-bacterial polymicrobial biofilms in disease. Trends Microbiol. 19, 557–563. Bradshaw, D.J., Marsh, P.D., Allison, C., and Schilling, K.M. (1996). Effect of oxygen,
Cellular Energetics: Actin and Myosin Abstain from ATP during Starvation Energy is required for all cellular functions and diverse mechanisms allow the cell to overcome acute energy starvation. A new study reveals that starved cells inhibit type V myosins and actin depolymerization, measures that may conserve energy while temporarily retaining cell polarity. Destiney Buelto1 and Mara C. Duncan2,* Energy starvation at the cellular level is surprisingly common. In multicellular organisms, energy starvation occurs during heart attack, stroke, hypoglycemia, and in highly proliferative cancers. In microbes, energy starvation occurs whenever the microbe encounters a nutrient-poor environment. Even after decades of research, new aspects of starvation biology are still emerging. In this issue of Current Biology, Xu and Bretscher [1] expand the cellular repertoire of starvation responses by identifying two previously unknown responses to glucose starvation — inhibition of myosin V function and inhibition of actin turnover. Starvation elicits diverse responses, which are frequently ancient, highly conserved and, importantly, dictate whether a cell survives or dies in response to acute starvation [2]. These responses allow the cell to access alternative sources of energy, enter
quiescence or engage in programmed cell death, which may provide an advantage for the organism or population [3,4]. Each of these responses requires consumption of energy and other raw material upfront before any benefits are gained. For example, amino acids and ATP are needed for the synthesis and function, respectively, of transporters that allow the cell to access alternative exogenous energy sources. Similarly, quiescence and programmed cell death also require energy [2,5]. Accordingly, energy starvation immediately inhibits processes with high-energy demands, such as translation, which consumes four ATP equivalents per amino acid added, and plasma membrane ATPase activity, which can consume enormous amounts of ATP [6,7]. The prompt shutdown of these processes conserves raw material that the cell needs for other uses. In the absence of exogenous sources, the cell can also produce limited raw material for quiescence by consuming stored
inoculum composition and flow rate on development of mixed-culture oral biofilms. Microbiology 142, 623–629. 17. Rossignol, T., Ding, C., Guida, A., d’Enfert, C., Higgins, D.G., and Butler, G. (2009). Correlation between biofilm formation and the hypoxic response in Candida parapsilosis. Eukaryot. Cell 8, 550–559. 18. Stichternoth, C., and Ernst, J.F. (2009). Hypoxic adaptation by Efg1 regulates biofilm formation by Candida albicans. Appl. Environ. Microbiol. 75, 3663–3672. 19. Nobile, C.J., Fox, E.P., Nett, J.E., Sorrells, T.R., Mitrovich, Q.M., Hernday, A.D., Tuch, B.B., Andes, D.R., and Johnson, A.D. (2012). A recently evolved transcriptional network controls biofilm development in Candida albicans. Cell 148, 126–138.
Department of Microbiology and Immunology, Geisel School of Medicine at Dartmouth, 74 College Street Remsen 213, Hanover, NH 03755, USA. E-mail: [email protected] http://dx.doi.org/10.1016/j.cub.2014.09.030
carbohydrate reserves, as well as cell-surface and cytosolic components [4,8,9]. Starvation responses thus either provide necessary nutrients or perform activities needed for long-term survival of the organism or population. The new work from Xu and Bretscher [1] adds to our understanding of the responses to acute energy starvation in eukaryotes. It follows the previous observation that the cortical actin cytoskeleton becomes depolarized upon glucose starvation in the yeast Saccharomyces cerevisiae [10]. In yeast, cell polarity depends on actin cables, which are functionally distinct from the endocytic cortical actin cytoskeleton [11]. Actin cables are long bundles of many short actin filaments aligned with the fast-growing end oriented towards the site of growth (Figure 1A) [11]. Actin cables act as tracks for type V myosins, Myo2 and Myo4, which direct polarized delivery of secretory vesicles, organelles, and RNA. This polarized delivery of secretory vesicles is required to maintain cell polarity [12]. Thus, the transient depolarization of the cortical cytoskeleton was thought to reflect the depolarization of the actin cables and the subsequent loss of polarized delivery of secretory cargo. The current work challenges this model. The authors find that, upon glucose starvation, Myo2 and Myo4, but not type I or II myosins, become stably associated with polarized actin
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cables (Figure 1B) [1]. Furthermore, only type V myosins that have first interacted with cargo engage in a rigor-like state. These findings are not unexpected since all myosins require ATP to disassociate from actin filaments, and type V myosins only associate with actin cables when activated by cargo [13]. However, what is unexpected is that Xu and Bretscher [1] find that Myo2 releases cargo during starvation; this cargo-free rigor-like state has not been observed previously. Curiously, the rigor-like state of Myo2 seems to be activated only at critically low levels of ATP, as intermediate starvation conditions cause Myo2 to disengage from actin cables and become diffusely localized. ATP seems to be the key regulator of this behavior because addition of exogenous ATP to permeablized starved cells allows Myo2 motility on the cables. However, it is not clear why extremely low ATP levels cause a rigor-like state yet intermediate levels cause Myo2 disengagement. Whether the rigor-like state is responsible for the cargo release of Myo2 in starved cells is not yet known. In support of a link between rigor and cargo release, multiple cargos of Myo2 are released during glucose starvation, suggesting Myo2 could be incompetent for cargo binding in starved cells [1]. On the other hand, the displacement of cargo could easily be due to a mechanism that acts on cargo directly. Many different molecular mechanisms regulate starvation responses. Adaptation to energy starvation requires activation of AMP-activated kinase (AMPK) and inhibition of growth-promoting signaling pathways involving mTOR complex 1 and protein kinase A (PKA), which phosphorylate many proteins to remodel cell physiology [4]. In addition, a number of changes in cell chemistry also regulate starvation responses. For example, pH, which drops w1 unit upon glucose starvation in yeast, is an important regulator of several responses [14,15]. In addition, decreased ATP, GTP, and increased NAD+ all directly regulate starvation responses [16,17]. Thus, cargo release from Myo2 could be modulated by changes in phosphorylation or by any number of changes in cell chemistry acting on cargo directly. The functional significance of the cargo-free rigor-like state of type V myosin is not clear. Although myosin
A
B
Glucose present ATP levels high
Acute starvation ATP levels low
? ATP ATP ATP
? Inactive motor
Actin
Cargo-primed motor
Cargo Current Biology
Figure 1. Model of actin and myosin V responses to glucose starvation. (A) In the presence of glucose, ATP is plentiful. Actin cables, which consist of assemblies of many small filaments, polarize towards the site of growth. The filaments treadmill rapidly, releasing free actin monomers. However, rapid ATP exchange allows the actin cables to maintain steady-state polarity to act as tracks for type V myosins. Type V myosins deliver secretory vesicles, organelles, and other cargo to the site of growth. Cargo binding is a pre-requisite for actin binding. Motor activity requires ATP to release the motor domains for continued motility. (B) During glucose starvation, an unknown mechanism inhibits actin disassembly, stabilizing actin filaments, retaining their original polarity. At the same time, the depletion of ATP induced by glucose starvation forces cargo-associated motors into a rigor-like state, whereas cargo-free motors remain cytosolic. Concurrently, an unknown mechanism strips type V myosins of cargo.
is an ATPase, its ATP consumption represents only a small fraction of total cellular ATP; however, even the modest saving achieved by adopting the rigor-like state could be important. Alternatively, the cargo release may have functional importance. Release of the secretory vesicles may allow isotropic delivery of cell wall synthesis enzymes to prepare the cell for quiescence in the most energy-economical manner. Conversely, inhibiting secretion may be beneficial: such a role is supported by the observation that many cell-surface proteins are internalized during glucose starvation [8]. It will be interesting for future studies to monitor the fate of secretory cargo during starvation. The authors also report the equally surprising discovery that actin cables are stabilized during glucose starvation. Under growth-promoting conditions, the yeast actin cables turn over so rapidly that the entire structure recycles within a minute [11]. In contrast, in cells subjected to acute glucose starvation, Xu and Bretscher [1] found that actin cables are highly stable, as monitored by several criteria. This stabilization is also observed in energy-starved HeLa cells, suggesting
that this may be a conserved response to energy stress [1]. So, how is actin stabilized in starved cells? Like Myo2 cargo release, any number of signaling or cell-chemistry changes could regulate actin turnover. Because ATP is required for actin polymerization, changes in ATP levels are unlikely to act directly on actin. However, ATP or other changes could modulate the activity of an actin-binding protein, resulting in reduced turnover. One candidate is the filament-severing protein cofilin, which is required for rapid filament turnover in the presence of glucose [11]. Furthermore, cofilin is pH sensitive and can be phosphorylated, suggesting its activity could be directly modulated in starved cells [18]. Xu and Bretscher [1] found that overexpression of cofilin did not destabilize actin filaments in glucose-starved cells, suggesting that cofilin is likely inactive during starvation. This inactivity is not due to increased activity of the actin-binding protein tropomyosin because inhibition of tropomyosin failed to destabilize actin in starved cells [1]. It remains to be explored whether cofilin is the direct target of glucose starvation.
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Regardless of the molecular mechanism, the functional significance of stabilizing actin appears clear. Actin is a major consumer of ATP [19] and the observed stabilization of actin filaments therefore prevents the consumption of ATP, whilst retaining actin filaments and their polarity. This polarity retention may be important to allow growth using the existing cytoskeleton with minimal ATP consumption, if the post-starvation environment provides enough nutrients for growth. This new work provides tantalizing insights into previously unrecognized responses to starvation. However, these findings raise nearly as many questions as they answer. A key question is why only extreme energy stress induces the rigor-like state and whether this relates to cargo release. It will also be important to understand if the type V myosin responses are important for conserving energy or if the cargo-free rigor-like state provides some other advantage for adaptation to starvation. It will also be worth exploring whether the type V myosin responses are conserved and what advantage this might provide to mammalian cells that are more dependent on microtubules for polarized transport. In contrast, the advantage of the stabilization of actin is apparent, although the molecular mechanism inducing stabilization in yeast and mammalian cells needs to be
elucidated. Further investigation into the role and regulation of these two new responses will provide a clearer picture of how the cell overcomes the challenge of energy starvation. References 1. Xu, L., and Bretscher, A. (2014). Rapid glucose depletion immobilizes active myosin-V on stabilized actin cables. Curr. Biol. 24, 2471–2479. 2. Klosinska, M.M., Crutchfield, C.A., Bradley, P.H., Rabinowitz, J.D., and Broach, J.R. (2011). Yeast cells can access distinct quiescent states. Genes Dev. 25, 336–349. 3. Fuchs, Y., and Steller, H. (2011). Programmed cell death in animal development and disease. Cell 147, 742–758. 4. Zaman, S., Lippman, S.I., Zhao, X., and Broach, J.R. (2008). How Saccharomyces responds to nutrients. Annu. Rev. Genet. 42, 27–81. 5. Zamaraeva, M.V., Sabirov, R.Z., Maeno, E., Ando-Akatsuka, Y., Bessonova, S.V., and Okada, Y. (2005). Cells die with increased cytosolic ATP during apoptosis: a bioluminescence study with intracellular luciferase. Cell Death Diff. 12, 1390–1397. 6. Serrano, R. (1983). In vivo glucose activation of the yeast plasma membrane ATPase. FEBS Lett. 156, 11–14. 7. Ashe, M.P., De Long, S.K., and Sachs, A.B. (2000). Glucose depletion rapidly inhibits translation initiation in yeast. Mol. Biol. Cell 11, 833–848. 8. Lang, M.J., Martinez-Marquez, J.Y., Prosser, D.C., Ganser, L.R., Buelto, D., Wendland, B., and Duncan, M.C. (2014). Glucose starvation inhibits autophagy via vacuolar hydrolysis and induces plasma membrane internalization by down-regulating recycling. J. Biol. Chem. 289, 16736–16747. 9. Xu, Y.F., Letisse, F., Absalan, F., Lu, W., Kuznetsova, E., Brown, G., Caudy, A.A., Yakunin, A.F., Broach, J.R., and Rabinowitz, J.D. (2013). Nucleotide degradation and ribose salvage in yeast. Mol. Syst. Biol. 9, 665. 10. Uesono, Y., Ashe, M.P., and Toh, E.A. (2004). Simultaneous yet independent regulation of
Neuronal Mitophagy: Long-Distance Delivery or Eating Locally? Parkinson’s disease-associated PINK1 and Parkin maintain mitochondrial health through promoting mitophagy, the autophagic removal of damaged mitochondria, in non-neuronal cells. Whether they do so in neurons has been controversial. A new study aims to resolve the controversy. Bingwei Lu Mitochondria play essential roles in many aspects of cellular physiology, from energy production and intermediary metabolism to apoptosis [1]. Proper mitochondrial function is particularly important for the health of neurons, as suggested by the prominent manifestation of
neurological impairments in mitochondrial diseases [2]. Mitochondrial dysfunction has been observed in many of the age-related neurodegenerative diseases, including Alzheimer’s disease and Parkinson’s disease (PD) [2]. However, the origin of the mitochondrial dysfunction in most of these diseases remains elusive. Pten-induced kinase 1
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actin cytoskeletal organization and translation initiation by glucose in Saccharomyces cerevisiae. Mol. Biol. Cell 15, 1544–1556. Moseley, J.B., and Goode, B.L. (2006). The yeast actin cytoskeleton: from cellular function to biochemical mechanism. Microbiol. Mol. Biol. Rev. 70, 605–645. Savage, N.S., Layton, A.T., and Lew, D.J. (2012). Mechanistic mathematical model of polarity in yeast. Mol. Biol. Cell 23, 1998–2013. Donovan, K.W., and Bretscher, A. (2012). Myosin-V is activated by binding secretory cargo and released in coordination with Rab/exocyst function. Dev. Cell 23, 769–781. Dechant, R., Binda, M., Lee, S.S., Pelet, S., Winderickx, J., and Peter, M. (2010). Cytosolic pH is a second messenger for glucose and regulates the PKA pathway through V-ATPase. EMBO J. 29, 2515–2526. Dechant, R., Saad, S., Ibanez, A.J., and Peter, M. (2014). Cytosolic pH regulates cell growth through distinct GTPases, Arf1 and Gtr1, to promote Ras/PKA and TORC1 activity. Mol. Cell 55, 409–421. Aoh, Q.L., Hung, C.W., and Duncan, M.C. (2013). Energy metabolism regulates clathrin adaptors at the trans-Golgi network and endosomes. Mol. Biol. Cell 24, 832–847. Wellen, K.E., and Thompson, C.B. (2012). A two-way street: reciprocal regulation of metabolism and signalling. Nat. Rev. Mol. Cell Biol. 13, 270–276. Bernstein, B.W., Painter, W.B., Chen, H., Minamide, L.S., Abe, H., and Bamburg, J.R. (2000). Intracellular pH modulation of ADF/ cofilin proteins. Cell Motil. Cytoskel. 47, 319–336. Bernstein, B.W., and Bamburg, J.R. (2003). Actin-ATP hydrolysis is a major energy drain for neurons. J. Neurosci 23, 1–6.
1Curriculum
in Genetics and Molecular Biology, University of North Carolina, Chapel Hill, NC 27599, USA. 2Department of Cell and Molecular Biology, University of Michigan, Ann Arbor, MI 48109, USA. *E-mail: [email protected]
http://dx.doi.org/10.1016/j.cub.2014.09.004
(PINK1) and Parkin, two genes linked to familial PD, have been implicated in mitochondrial quality control, specifically the promotion of mitochondrial degradation by autophagy — mitophagy. Many of the studies indicating a function for PINK1 and Parkin in mitophagy were carried out in cultured non-neuronal cells, but a new study published in the Journal of Cell Biology [3] now suggests that PINK1 and Parkin can indeed act in neuronal axons to remove damaged mitochondria. PD is the most common movement disorder and the second most common neurodegenerative disease after Alzheimer’s disease. It is characterized by rather selective degeneration of
