- Email: [email protected]
Mitochondrial Dynamics: ER Actin Tightens the Drp1 Noose
Current Biology
Dispatches Mitochondrial Dynamics: ER Actin Tightens the Drp1 Noose Julien Prudent and Heidi M. McBride* Montreal Neurological Institute, McGill University, 3801 University Ave, Montreal, PQ H3A 2B4, Canada *Correspondence: [email protected] http://dx.doi.org/10.1016/j.cub.2016.01.009
Drp1 is an oligomeric GTPase essential for mitochondrial division. A recent study has proposed a new model whereby machinery localized to the endoplasmic reticulum drives polymerization of actin, which then acts as a platform for Drp1 oligomerization and hydrolysis at mitochondrial constriction sites. The process of organelle division, whether to generate vesicles or to drive organellar biogenesis, is a complex process requiring the co-ordinated recruitment of both structural and enzymatic proteins to complete the job. Given the large diameter of the double-membrane-bound mitochondria, the difficulty of this task for these organelles is amplified, as a great deal of restructuring is required to constrict both bilayers. Mitochondrial constriction is the primary function of dynamin-related protein 1 (Drp1, known as Dnm1 in yeast), an oligomeric mechano-enzyme whose GTP hydrolysis is critical for mitochondrial division [1]. However, it has been a challenge to unravel the mechanisms of Drp1 recruitment and assembly and of fission site selection. A breakthrough in this field occurred recently with the realization that the fission sites are determined by interactions with the endoplasmic reticulum (ER) [2], an organelle whose intimate contacts with mitochondria had already been well established as sites of lipid and calcium flux [3]. That discovery launched a series of pioneering studies into the role of the ER as a functional platform for the co-ordinated polymerization of actin filaments essential to drive the assembly of Drp1 oligomers around mitochondria [4]. The last few years have led us to a new understanding of the dynamic interplay between these two independent organelles, and how they work together to drive mitochondrial constriction. From yeast model systems we have learned that the fission GTPase Dnm1 is recruited from the cytosol to receptors on the outer mitochondrial membrane, where it assembles, oligomerizes into a ring-like structure, and drives mitochondrial
division upon GTP hydrolysis [1]. The interchangeable Dnm1 receptors Mdv1 and Caf4 [5,6] have no obvious orthologues in mammalian systems; instead, the Drp1 adaptors are mitochondrial fission factor (Mff) [7,8] and mitochondrial dynamics proteins of 49 and 51 kDa (MiD49 and MiD51, respectively) [9,10]. Recent studies have highlighted that Mff and MiD49/MiD51 may act independently on Drp1 recruitment and may represent two different pathways, further increasing the complexity of this program [11,12]. Additional factors, including the lipid cardiolipin, and changes in membrane curvature can increase the GTPase activity of Drp1 at the surface of mitochondria [13]. While the Drp1 receptors have a clear role in recruitment, understanding how the scission site is chosen was less clear. This has been partly resolved with the observation that ER membranes wrap around mitochondria prior to Drp1 recruitment, thereby defining a scission site [2]. Most of the pro-fission factors in both yeast and mammalian cells, including Drp1, Mff, MiD49 and MiD51, as well as mitochondrial DNA, colocalized within these ER–mitochondrial contact sites during constriction and division [14,15]. One mechanistic contribution of the ER is the generation of actin filaments that drive the initial bending of the mitochondrial membranes. Previous work from the Higgs lab had shown that the ER-resident formin INF2 mediates actin polymerization, leading to the recruitment of the myosin IIa motor, which pulls on the actin filaments, driving the mitochondria to constrict [16,17]. Recent studies have further expanded the role of actin in mitochondrial division, reporting that the
mitochondrial-anchored actin-nucleating protein Spire1C binds to INF2 and promotes actin assembly and division [18]. Two other actin-binding proteins, cofilin and cortactin, were also shown to play a role in actin-mediated mitochondrial division [19]. While the number of players involved in mitochondrial constriction increased, the precise mechanisms of assembly and constriction had not been established. Now, a new story from the Higgs lab has again advanced the field with evidence that links the two oligomeric systems — actin and Drp1 — and provides a unified mechanism to drive mitochondrial division [20]. In contrast to a ‘de novo’ pathway, where cytosolic Drp1 is recruited directly to a constriction site through its membrane-anchored adaptors, the authors argue for a ‘targeted equilibrium’ mechanism (see model in Figure 1) [20]. They proposed that dimeric and oligomeric forms of Drp1, observed as differently sized ‘puncta’ in high-resolution microscopy approaches, are in constant balance between the cytosol and the mitochondria. After an unknown fission signal, mitochondria-bound puncta merge into a mature-sized Drp1 complex capable of moving laterally along the mitochondrial tubule. ER contact sites where actin polymerization has occurred enhanced Drp1 maturation and conversion into the oligomeric forms. Most importantly, the authors showed that the binding of Drp1 oligomers to actin filaments stimulated Drp1 GTPase activity, driving mitochondrial division. This activity was further enhanced in the presence of Mff. Drp1 also showed some ability to bundle actin in cell-free systems, hinting at reciprocal regulation. These results reveal an active role for polymerized actin
Current Biology 26, R192–R217, March 7, 2016 ª2016 Elsevier Ltd All rights reserved R207
Current Biology
Dispatches A
Drp1 recruitment and assembly
B
Drp1 oligomer maturation
Mitochondrion
ER
C Drp1 Active Drp1 Drp1 adaptors (Mff/MiDs) INF2 Spire1C Actin Myosin IIA
D
Mitochondrial division Mff-dependent?
No mitochondrial division MiDs-dependent?
Current Biology
Figure 1. ER-derived actin filaments facilitate Drp1 maturation and hydrolysis, driving mitochondrial division. (A) Drp1 dimers are constantly in balance between cytosolic and mitochondrial-bound forms. Oligomeric Drp1 forms move along the outer mitochondrial membrane and accumulate at sites where ER-driven constriction is initiated. (B) The process of Drp1 assembly into a ring structure further constricts the mitochondria, and finally, the mature Drp1 oligomers hydrolyze GTP, resulting in fission. (C) The zoomed area highlights the factors regulating mitochondrial division. The accumulation of polymerized actin at the ER–mitochondrial contact sites drives assembly and hydrolysis of Drp1 oligomers. ERlocalized INF2 and mitochondrial Spire1C bind actin and the myosin IIa motor may act as the force generator required for the mitochondrial constriction. The Drp1 receptors, Mff and MiD49/MiD51 (MiDs), are also shown. (D) Drp1 oligomers do not always lead to productive mitochondrial scission. This difference may be attributed to the differential use of Drp1 adaptors. Mff promotes Drp1 recruitment and oligomerization and, in the presence of polymerized actin, synergistically promotes GTP hydrolysis within the Drp1 oligomer, leading to fission. The mechanisms of segregation and disassembly of Drp1 oligomers following fission remain unclear. In contrast, the preferential accumulation of MiDs within constriction sites may lead to the inactivation of Drp1, stabilizing a constriction that does not result in division.
R208 Current Biology 26, R192–R217, March 7, 2016 ª2016 Elsevier Ltd All rights reserved
filaments in the enzymatic activity of Drp1 oligomers that drive fission. In considering this work within the context of other recent advances in the field, a number of new questions arise. As with other models of GTPase recruitment, the stochastic association with the membrane becomes selective and functional only when mechanistically ‘‘locked on’’, generally through GTP loading. For Drp1, the interaction with actin filaments at sites of ER contact would drive Drp1 towards this locked-on state. The difference with other GTPases is the rapid lateral movement of smaller puncta (with a velocity of 48.5 nm/s), which were seen to both merge and dissociate (Figure 1) [20]. The authors proposed that the purpose of this motility is to ‘patrol’ the mitochondrion in search of intrinsic fission signals. However, the mechanism of Drp1 oligomer migration along the outer mitochondrial membrane is unclear. It is easy to envision how Drp1 may become trapped within a microenvironment created by the ER, which would include actin, Drp1 adaptors, and cardiolipin (Figure 1), but the forces involved in this movement are unclear. It does not appear to be Brownian in nature, leading us to question whether or not Drp1 adaptors like Mff and the MiDs may move in synergy to bring Drp1 oligomers to the constriction site. Another intriguing point raised by the authors is that only 6% of the mature Drp1 punctae localized at the constriction will induce fission within a 10-minute time span. It has long been observed that most Drp1 foci do not lead to mitochondrial division; however, this study shows that, even when Drp1 is at an ER contact site that contains actin, there may not be a fission event. Interestingly, even within the sites that do not result in division, there was a significant decrease in the diameter of the mitochondria by about 100 nm (from 320 to 215 nm), suggesting that the machinery was primed, but scission was not executed. The reason for this is unknown. We consider that non-scission contacts may have a significant functional role in stabilizing the flux of metabolites between the ER and mitochondria — a long-established concept in interorganellar contact. The authors also highlighted the significant kinetic variation in the rates of actin polymerization and the time that the
Current Biology
Dispatches merged, larger Drp1 puncta reside at these sites before fission occurs. This variation suggests additional regulatory steps that we do not yet fully understand. The most obvious source of variation may be in the differential use of Mff and the MiDs as Drp1 receptors and assembly factors (Figure 1). Mff was shown to selectively recruit Drp1 oligomers [12] and was seen to synergize with the effect of polymerized actin in the conversion of Drp1 to larger puncta and to enhance Drp1 GTPase activity [20]. In contrast, recent studies have shown how the MiDs may act independently of Mff [11,12] and sequester Drp1, but may not lead to efficient, functional oligomerization, and could even inhibit fission [9]. Perhaps the non-productive forms of Drp1 may be sequestered by a high concentration of the MiDs in these constriction sites, decreasing Drp1 activity, but allowing some level of constriction and stable contact formation. It will be interesting to test whether mitochondrial constriction sites that lead to fission are more enriched in Mff, which is known to facilitate the maturation of oligomerized Drp1. Lastly, it is unclear how oligomerized Drp1 disassembles after the scission event. The kinetics of disassembly were seen to be highly variable, further hinting at regulatory mechanisms beyond the GTP hydrolysis that drives constriction [20]. Indeed, Drp1 is heavily posttranslationally modified, from phosphorylation to SUMOylation [1]. The role of additional regulatory elements within the actin-mediated ER– mitochondrial contact sites will no doubt be investigated in future studies. REFERENCES 1. Elgass, K., Pakay, J., Ryan, M.T., and Palmer, C.S. (2013). Recent advances into the understanding of mitochondrial fission. Biochim. Biophys. Acta 1833, 150–161. 2. Friedman, J.R., Lackner, L.L., West, M., DiBenedetto, J.R., Nunnari, J., and Voeltz, G.K. (2011). ER tubules mark sites of mitochondrial division. Science 334, 358–362. 3. Naon, D., and Scorrano, L. (2014). At the right distance: ER-mitochondria juxtaposition in cell life and death. Biochim. Biophys. Acta 1843, 2184–2194. 4. Hatch, A.L., Gurel, P.S., and Higgs, H.N. (2014). Novel roles for actin in mitochondrial fission. J. Cell Sci. 127, 4549–4560. 5. Griffin, E.E., Graumann, J., and Chan, D.C. (2005). The WD40 protein Caf4p is a
component of the mitochondrial fission machinery and recruits Dnm1p to mitochondria. J. Cell Biol. 170, 237–248. 6. Koirala, S., Guo, Q., Kalia, R., Bui, H.T., Eckert, D.M., Frost, A., and Shaw, J.M. (2013). Interchangeable adaptors regulate mitochondrial dynamin assembly for membrane scission. Proc. Natl. Acad. Sci. USA 110, E1342–E1351. 7. Gandre-Babbe, S., and van der Bliek, A.M. (2008). The novel tail-anchored membrane protein Mff controls mitochondrial and peroxisomal fission in mammalian cells. Mol. Biol. Cell 19, 2402–2412. 8. Otera, H., Wang, C., Cleland, M.M., Setoguchi, K., Yokota, S., Youle, R.J., and Mihara, K. (2010). Mff is an essential factor for mitochondrial recruitment of Drp1 during mitochondrial fission in mammalian cells. J. Cell Biol. 191, 1141–1158. 9. Palmer, C.S., Osellame, L.D., Laine, D., Koutsopoulos, O.S., Frazier, A.E., and Ryan, M.T. (2011). MiD49 and MiD51, new components of the mitochondrial fission machinery. EMBO Rep. 12, 565–573. 10. Loson, O.C., Song, Z., Chen, H., and Chan, D.C. (2013). Fis1, Mff, MiD49, and MiD51 mediate Drp1 recruitment in mitochondrial fission. Mol. Biol. Cell 24, 659–667. 11. Palmer, C.S., Elgass, K.D., Parton, R.G., Osellame, L.D., Stojanovski, D., and Ryan, M.T. (2013). Adaptor proteins MiD49 and MiD51 can act independently of Mff and Fis1 in Drp1 recruitment and are specific for mitochondrial fission. J. Biol. Chem. 288, 27584–27593. 12. Liu, R., and Chan, D.C. (2015). The mitochondrial fission receptor Mff selectively recruits oligomerized Drp1. Mol. Biol. Cell 26, 4466–4477.
13. Stepanyants, N., Macdonald, P.J., Francy, C.A., Mears, J.A., Qi, X., and Ramachandran, R. (2015). Cardiolipin’s propensity for phase transition and its reorganization by dynaminrelated protein 1 form a basis for mitochondrial membrane fission. Mol. Biol. Cell 26, 3104–3116. 14. Murley, A., Lackner, L.L., Osman, C., West, M., Voeltz, G.K., Walter, P., and Nunnari, J. (2013). ER-associated mitochondrial division links the distribution of mitochondria and mitochondrial DNA in yeast. Elife 2, e00422. 15. Elgass, K.D., Smith, E.A., LeGros, M.A., Larabell, C.A., and Ryan, M.T. (2015). Analysis of ER-mitochondria contacts using correlative fluorescence microscopy and soft X-ray tomography of mammalian cells. J. Cell Sci. 128, 2795–2804. 16. Korobova, F., Gauvin, T.J., and Higgs, H.N. (2014). A role for myosin II in mammalian mitochondrial fission. Curr. Biol. 24, 409–414. 17. Korobova, F., Ramabhadran, V., and Higgs, H.N. (2013). An actin-dependent step in mitochondrial fission mediated by the ERassociated formin INF2. Science 339, 464–467. 18. Manor, U., Bartholomew, S., Golani, G., Christenson, E., Kozlov, M., Higgs, H., Spudich, J., and Lippincott-Schwartz, J. (2015). A mitochondria-anchored isoform of the actin-nucleating spire protein regulates mitochondrial division. Elife 4, e08828. 19. Li, S., Xu, S., Roelofs, B.A., Boyman, L., Lederer, W.J., Sesaki, H., and Karbowski, M. (2015). Transient assembly of F-actin on the outer mitochondrial membrane contributes to mitochondrial fission. J. Cell Biol. 208, 109–123. 20. Ji, W.K., Hatch, A.L., Merrill, R.A., Strack, S., and Higgs, H.N. (2015). Actin filaments target the oligomeric maturation of the dynamin GTPase Drp1 to mitochondrial fission sites. Elife 4, e11553.
Developmental Biology: Decapentaplegic Controls Growth at a Distance Jean-Paul Vincent*, Ruta Ziukaite, and Cyrille Alexandre The Francis Crick Institute, Mill Hill Laboratory, The Ridgeway, London NW7 1AA, UK *Correspondence: [email protected] http://dx.doi.org/10.1016/j.cub.2016.01.061
Decapentaplegic has long been thought to be a morphogen that controls patterning and growth in Drosophila wings, but hard evidence for the requisite long-range action has only now come from two new studies. Alan Turing is credited for coining the term morphogen to describe a diffusible molecule that organises biological
patterns [1]. A handful of theoretical biologists further built upon this concept to explain self-organisation, positional
Current Biology 26, R192–R217, March 7, 2016 ª2016 Elsevier Ltd All rights reserved R209
Dispatches Mitochondrial Dynamics: ER Actin Tightens the Drp1 Noose Julien Prudent and Heidi M. McBride* Montreal Neurological Institute, McGill University, 3801 University Ave, Montreal, PQ H3A 2B4, Canada *Correspondence: [email protected] http://dx.doi.org/10.1016/j.cub.2016.01.009
Drp1 is an oligomeric GTPase essential for mitochondrial division. A recent study has proposed a new model whereby machinery localized to the endoplasmic reticulum drives polymerization of actin, which then acts as a platform for Drp1 oligomerization and hydrolysis at mitochondrial constriction sites. The process of organelle division, whether to generate vesicles or to drive organellar biogenesis, is a complex process requiring the co-ordinated recruitment of both structural and enzymatic proteins to complete the job. Given the large diameter of the double-membrane-bound mitochondria, the difficulty of this task for these organelles is amplified, as a great deal of restructuring is required to constrict both bilayers. Mitochondrial constriction is the primary function of dynamin-related protein 1 (Drp1, known as Dnm1 in yeast), an oligomeric mechano-enzyme whose GTP hydrolysis is critical for mitochondrial division [1]. However, it has been a challenge to unravel the mechanisms of Drp1 recruitment and assembly and of fission site selection. A breakthrough in this field occurred recently with the realization that the fission sites are determined by interactions with the endoplasmic reticulum (ER) [2], an organelle whose intimate contacts with mitochondria had already been well established as sites of lipid and calcium flux [3]. That discovery launched a series of pioneering studies into the role of the ER as a functional platform for the co-ordinated polymerization of actin filaments essential to drive the assembly of Drp1 oligomers around mitochondria [4]. The last few years have led us to a new understanding of the dynamic interplay between these two independent organelles, and how they work together to drive mitochondrial constriction. From yeast model systems we have learned that the fission GTPase Dnm1 is recruited from the cytosol to receptors on the outer mitochondrial membrane, where it assembles, oligomerizes into a ring-like structure, and drives mitochondrial
division upon GTP hydrolysis [1]. The interchangeable Dnm1 receptors Mdv1 and Caf4 [5,6] have no obvious orthologues in mammalian systems; instead, the Drp1 adaptors are mitochondrial fission factor (Mff) [7,8] and mitochondrial dynamics proteins of 49 and 51 kDa (MiD49 and MiD51, respectively) [9,10]. Recent studies have highlighted that Mff and MiD49/MiD51 may act independently on Drp1 recruitment and may represent two different pathways, further increasing the complexity of this program [11,12]. Additional factors, including the lipid cardiolipin, and changes in membrane curvature can increase the GTPase activity of Drp1 at the surface of mitochondria [13]. While the Drp1 receptors have a clear role in recruitment, understanding how the scission site is chosen was less clear. This has been partly resolved with the observation that ER membranes wrap around mitochondria prior to Drp1 recruitment, thereby defining a scission site [2]. Most of the pro-fission factors in both yeast and mammalian cells, including Drp1, Mff, MiD49 and MiD51, as well as mitochondrial DNA, colocalized within these ER–mitochondrial contact sites during constriction and division [14,15]. One mechanistic contribution of the ER is the generation of actin filaments that drive the initial bending of the mitochondrial membranes. Previous work from the Higgs lab had shown that the ER-resident formin INF2 mediates actin polymerization, leading to the recruitment of the myosin IIa motor, which pulls on the actin filaments, driving the mitochondria to constrict [16,17]. Recent studies have further expanded the role of actin in mitochondrial division, reporting that the
mitochondrial-anchored actin-nucleating protein Spire1C binds to INF2 and promotes actin assembly and division [18]. Two other actin-binding proteins, cofilin and cortactin, were also shown to play a role in actin-mediated mitochondrial division [19]. While the number of players involved in mitochondrial constriction increased, the precise mechanisms of assembly and constriction had not been established. Now, a new story from the Higgs lab has again advanced the field with evidence that links the two oligomeric systems — actin and Drp1 — and provides a unified mechanism to drive mitochondrial division [20]. In contrast to a ‘de novo’ pathway, where cytosolic Drp1 is recruited directly to a constriction site through its membrane-anchored adaptors, the authors argue for a ‘targeted equilibrium’ mechanism (see model in Figure 1) [20]. They proposed that dimeric and oligomeric forms of Drp1, observed as differently sized ‘puncta’ in high-resolution microscopy approaches, are in constant balance between the cytosol and the mitochondria. After an unknown fission signal, mitochondria-bound puncta merge into a mature-sized Drp1 complex capable of moving laterally along the mitochondrial tubule. ER contact sites where actin polymerization has occurred enhanced Drp1 maturation and conversion into the oligomeric forms. Most importantly, the authors showed that the binding of Drp1 oligomers to actin filaments stimulated Drp1 GTPase activity, driving mitochondrial division. This activity was further enhanced in the presence of Mff. Drp1 also showed some ability to bundle actin in cell-free systems, hinting at reciprocal regulation. These results reveal an active role for polymerized actin
Current Biology 26, R192–R217, March 7, 2016 ª2016 Elsevier Ltd All rights reserved R207
Current Biology
Dispatches A
Drp1 recruitment and assembly
B
Drp1 oligomer maturation
Mitochondrion
ER
C Drp1 Active Drp1 Drp1 adaptors (Mff/MiDs) INF2 Spire1C Actin Myosin IIA
D
Mitochondrial division Mff-dependent?
No mitochondrial division MiDs-dependent?
Current Biology
Figure 1. ER-derived actin filaments facilitate Drp1 maturation and hydrolysis, driving mitochondrial division. (A) Drp1 dimers are constantly in balance between cytosolic and mitochondrial-bound forms. Oligomeric Drp1 forms move along the outer mitochondrial membrane and accumulate at sites where ER-driven constriction is initiated. (B) The process of Drp1 assembly into a ring structure further constricts the mitochondria, and finally, the mature Drp1 oligomers hydrolyze GTP, resulting in fission. (C) The zoomed area highlights the factors regulating mitochondrial division. The accumulation of polymerized actin at the ER–mitochondrial contact sites drives assembly and hydrolysis of Drp1 oligomers. ERlocalized INF2 and mitochondrial Spire1C bind actin and the myosin IIa motor may act as the force generator required for the mitochondrial constriction. The Drp1 receptors, Mff and MiD49/MiD51 (MiDs), are also shown. (D) Drp1 oligomers do not always lead to productive mitochondrial scission. This difference may be attributed to the differential use of Drp1 adaptors. Mff promotes Drp1 recruitment and oligomerization and, in the presence of polymerized actin, synergistically promotes GTP hydrolysis within the Drp1 oligomer, leading to fission. The mechanisms of segregation and disassembly of Drp1 oligomers following fission remain unclear. In contrast, the preferential accumulation of MiDs within constriction sites may lead to the inactivation of Drp1, stabilizing a constriction that does not result in division.
R208 Current Biology 26, R192–R217, March 7, 2016 ª2016 Elsevier Ltd All rights reserved
filaments in the enzymatic activity of Drp1 oligomers that drive fission. In considering this work within the context of other recent advances in the field, a number of new questions arise. As with other models of GTPase recruitment, the stochastic association with the membrane becomes selective and functional only when mechanistically ‘‘locked on’’, generally through GTP loading. For Drp1, the interaction with actin filaments at sites of ER contact would drive Drp1 towards this locked-on state. The difference with other GTPases is the rapid lateral movement of smaller puncta (with a velocity of 48.5 nm/s), which were seen to both merge and dissociate (Figure 1) [20]. The authors proposed that the purpose of this motility is to ‘patrol’ the mitochondrion in search of intrinsic fission signals. However, the mechanism of Drp1 oligomer migration along the outer mitochondrial membrane is unclear. It is easy to envision how Drp1 may become trapped within a microenvironment created by the ER, which would include actin, Drp1 adaptors, and cardiolipin (Figure 1), but the forces involved in this movement are unclear. It does not appear to be Brownian in nature, leading us to question whether or not Drp1 adaptors like Mff and the MiDs may move in synergy to bring Drp1 oligomers to the constriction site. Another intriguing point raised by the authors is that only 6% of the mature Drp1 punctae localized at the constriction will induce fission within a 10-minute time span. It has long been observed that most Drp1 foci do not lead to mitochondrial division; however, this study shows that, even when Drp1 is at an ER contact site that contains actin, there may not be a fission event. Interestingly, even within the sites that do not result in division, there was a significant decrease in the diameter of the mitochondria by about 100 nm (from 320 to 215 nm), suggesting that the machinery was primed, but scission was not executed. The reason for this is unknown. We consider that non-scission contacts may have a significant functional role in stabilizing the flux of metabolites between the ER and mitochondria — a long-established concept in interorganellar contact. The authors also highlighted the significant kinetic variation in the rates of actin polymerization and the time that the
Current Biology
Dispatches merged, larger Drp1 puncta reside at these sites before fission occurs. This variation suggests additional regulatory steps that we do not yet fully understand. The most obvious source of variation may be in the differential use of Mff and the MiDs as Drp1 receptors and assembly factors (Figure 1). Mff was shown to selectively recruit Drp1 oligomers [12] and was seen to synergize with the effect of polymerized actin in the conversion of Drp1 to larger puncta and to enhance Drp1 GTPase activity [20]. In contrast, recent studies have shown how the MiDs may act independently of Mff [11,12] and sequester Drp1, but may not lead to efficient, functional oligomerization, and could even inhibit fission [9]. Perhaps the non-productive forms of Drp1 may be sequestered by a high concentration of the MiDs in these constriction sites, decreasing Drp1 activity, but allowing some level of constriction and stable contact formation. It will be interesting to test whether mitochondrial constriction sites that lead to fission are more enriched in Mff, which is known to facilitate the maturation of oligomerized Drp1. Lastly, it is unclear how oligomerized Drp1 disassembles after the scission event. The kinetics of disassembly were seen to be highly variable, further hinting at regulatory mechanisms beyond the GTP hydrolysis that drives constriction [20]. Indeed, Drp1 is heavily posttranslationally modified, from phosphorylation to SUMOylation [1]. The role of additional regulatory elements within the actin-mediated ER– mitochondrial contact sites will no doubt be investigated in future studies. REFERENCES 1. Elgass, K., Pakay, J., Ryan, M.T., and Palmer, C.S. (2013). Recent advances into the understanding of mitochondrial fission. Biochim. Biophys. Acta 1833, 150–161. 2. Friedman, J.R., Lackner, L.L., West, M., DiBenedetto, J.R., Nunnari, J., and Voeltz, G.K. (2011). ER tubules mark sites of mitochondrial division. Science 334, 358–362. 3. Naon, D., and Scorrano, L. (2014). At the right distance: ER-mitochondria juxtaposition in cell life and death. Biochim. Biophys. Acta 1843, 2184–2194. 4. Hatch, A.L., Gurel, P.S., and Higgs, H.N. (2014). Novel roles for actin in mitochondrial fission. J. Cell Sci. 127, 4549–4560. 5. Griffin, E.E., Graumann, J., and Chan, D.C. (2005). The WD40 protein Caf4p is a
component of the mitochondrial fission machinery and recruits Dnm1p to mitochondria. J. Cell Biol. 170, 237–248. 6. Koirala, S., Guo, Q., Kalia, R., Bui, H.T., Eckert, D.M., Frost, A., and Shaw, J.M. (2013). Interchangeable adaptors regulate mitochondrial dynamin assembly for membrane scission. Proc. Natl. Acad. Sci. USA 110, E1342–E1351. 7. Gandre-Babbe, S., and van der Bliek, A.M. (2008). The novel tail-anchored membrane protein Mff controls mitochondrial and peroxisomal fission in mammalian cells. Mol. Biol. Cell 19, 2402–2412. 8. Otera, H., Wang, C., Cleland, M.M., Setoguchi, K., Yokota, S., Youle, R.J., and Mihara, K. (2010). Mff is an essential factor for mitochondrial recruitment of Drp1 during mitochondrial fission in mammalian cells. J. Cell Biol. 191, 1141–1158. 9. Palmer, C.S., Osellame, L.D., Laine, D., Koutsopoulos, O.S., Frazier, A.E., and Ryan, M.T. (2011). MiD49 and MiD51, new components of the mitochondrial fission machinery. EMBO Rep. 12, 565–573. 10. Loson, O.C., Song, Z., Chen, H., and Chan, D.C. (2013). Fis1, Mff, MiD49, and MiD51 mediate Drp1 recruitment in mitochondrial fission. Mol. Biol. Cell 24, 659–667. 11. Palmer, C.S., Elgass, K.D., Parton, R.G., Osellame, L.D., Stojanovski, D., and Ryan, M.T. (2013). Adaptor proteins MiD49 and MiD51 can act independently of Mff and Fis1 in Drp1 recruitment and are specific for mitochondrial fission. J. Biol. Chem. 288, 27584–27593. 12. Liu, R., and Chan, D.C. (2015). The mitochondrial fission receptor Mff selectively recruits oligomerized Drp1. Mol. Biol. Cell 26, 4466–4477.
13. Stepanyants, N., Macdonald, P.J., Francy, C.A., Mears, J.A., Qi, X., and Ramachandran, R. (2015). Cardiolipin’s propensity for phase transition and its reorganization by dynaminrelated protein 1 form a basis for mitochondrial membrane fission. Mol. Biol. Cell 26, 3104–3116. 14. Murley, A., Lackner, L.L., Osman, C., West, M., Voeltz, G.K., Walter, P., and Nunnari, J. (2013). ER-associated mitochondrial division links the distribution of mitochondria and mitochondrial DNA in yeast. Elife 2, e00422. 15. Elgass, K.D., Smith, E.A., LeGros, M.A., Larabell, C.A., and Ryan, M.T. (2015). Analysis of ER-mitochondria contacts using correlative fluorescence microscopy and soft X-ray tomography of mammalian cells. J. Cell Sci. 128, 2795–2804. 16. Korobova, F., Gauvin, T.J., and Higgs, H.N. (2014). A role for myosin II in mammalian mitochondrial fission. Curr. Biol. 24, 409–414. 17. Korobova, F., Ramabhadran, V., and Higgs, H.N. (2013). An actin-dependent step in mitochondrial fission mediated by the ERassociated formin INF2. Science 339, 464–467. 18. Manor, U., Bartholomew, S., Golani, G., Christenson, E., Kozlov, M., Higgs, H., Spudich, J., and Lippincott-Schwartz, J. (2015). A mitochondria-anchored isoform of the actin-nucleating spire protein regulates mitochondrial division. Elife 4, e08828. 19. Li, S., Xu, S., Roelofs, B.A., Boyman, L., Lederer, W.J., Sesaki, H., and Karbowski, M. (2015). Transient assembly of F-actin on the outer mitochondrial membrane contributes to mitochondrial fission. J. Cell Biol. 208, 109–123. 20. Ji, W.K., Hatch, A.L., Merrill, R.A., Strack, S., and Higgs, H.N. (2015). Actin filaments target the oligomeric maturation of the dynamin GTPase Drp1 to mitochondrial fission sites. Elife 4, e11553.
Developmental Biology: Decapentaplegic Controls Growth at a Distance Jean-Paul Vincent*, Ruta Ziukaite, and Cyrille Alexandre The Francis Crick Institute, Mill Hill Laboratory, The Ridgeway, London NW7 1AA, UK *Correspondence: [email protected] http://dx.doi.org/10.1016/j.cub.2016.01.061
Decapentaplegic has long been thought to be a morphogen that controls patterning and growth in Drosophila wings, but hard evidence for the requisite long-range action has only now come from two new studies. Alan Turing is credited for coining the term morphogen to describe a diffusible molecule that organises biological
patterns [1]. A handful of theoretical biologists further built upon this concept to explain self-organisation, positional
Current Biology 26, R192–R217, March 7, 2016 ª2016 Elsevier Ltd All rights reserved R209