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Revolutionary View of How to Split a Mitochondrion

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Revolutionary View of How to Split a Mitochondrion News & views division pathway or to different pathways. Cell biology Kleele et al. conducted careful analysis of mitochondrial division using super-resolution microscopy, and defined two spatially distinct types of division. Midzone division is centrally Revolutionary view of how located on the organelle, whereas peripheral division takes place at the ends of mitochon- to split a mitochondrion dria (Fig. 1). The two division types occur at similar frequency in Cos-7 cells from monkeys, Rajarshi Chakrabarti & Henry N. Higgs whereas midzone division is more frequent in mouse neonatal cardiomyocyte cells. Organelles called mitochondria divide in at least two contexts: The authors demonstrate that peripheral during cell growth and in response to mitochondrial damage. and midzone divisions have substantially dif- The finding that division is different in these two contexts ferent properties. Midzone division occurs in organelles with hallmarks of healthy sheds light on the regulatory pathways involved. See p.435 mitochondria — they do not display signs of abnormalities, such as a reduction of mem- brane polarization or a change in the level of Shortly before his life was cut short by the have been hints of specific types of division, reactive oxygen species (ROS). By contrast, guillotine during the French Revolution, clear evidence has been lacking until now. The peripheral division occurs when the tip of the the chemist Antoine Lavoisier made key protein DRP1 is required for the vast majority organelle has developed a decrease in mem- discoveries about the biological energy- of cases of mitochondrial division6. DRP1 can brane potential and an increase in ROS, with generating process termed respiration1. One be activated in different ways to drive such a noticeable lack of these alterations in the of his insights was to realize that respiration division in mammals. These include: inter- other portion of the organelle. In addition, is, as he described it1, “simply a slow burning action with mitochondrial DRP1 receptors this smaller product of a peripheral division of carbon and hydrogen, which is similar to (MFF, MID49, MID51 and FIS1); DRP1 modifi- often lacks replicating DNA — which is a sign how a lamp or a lighted candle works, and, cation (post-translational alterations); inter- of an unhealthy mitochondrion. from that point of view, animals who breathe action with the actin cytoskeleton (filaments These findings suggest that peripheral are veritable flammable bodies who burn of actin protein) or the mitochondrial lipid division occurs when mitochondria are dam- and consume themselves”. But how is this cardiolipin; and contact with various orga- aged, and is a precursor to mitophagy. Indeed, ‘burning’ kept under control in cells? On nelles, including the endoplasmic reticulum the authors report that peripheral divisions page 435, Kleele et al.2 report some unexpected (ER), lysosomes and the Golgi (in the form of increased on exposure to various cellular findings about an organelle at the heart of Golgi-derived vesicles)6. It has been unclear stresses, and were associated with the accumu- respiration in animal cells. whether these factors contribute to a single lation of markers of mitophagy. By contrast, About 150 years after Lavoisier’s time, orga- nelles termed mitochondria were revealed to be where this burning takes place3,4, and a Midzone division b Peripheral division the mitochondrion is often referred to as Replicating Non-replicating the powerhouse of the cell. As with burning, mtDNA ER mtDNA respiration also causes quite a bit of damage, Lysosome and active mitochondria commonly become Actin filament defective. Some of the most serious damage DRP1 that can occur is mutation of the mitochon- drial genome, located inside the organelle. A process called mitophagy serves to remove Low membrane and degrade damaged mitochondria, and is potential a crucial mechanism for cellular homeo stasis. Mitochondrion Higher ROS Defects in mitophagy, particularly those Higher Ca2+ Normal membrane affecting long-lived cells such as neurons, are potential associated with Parkinson’s disease and other Normal ROS 2+ neuro degenerative conditions5. Normal Ca Mitochondrial Degradation of During mitophagy, damaged portions of division during smaller portion of mitochondria separate from healthy portions cell growth the mitochondrion through mitochondrial division6. However, 2 damage is not the only reason for mitochon- Figure 1 | Two pathways for mitochondrial division. Kleele et al. report microscopy studies of organelle drial division. It also occurs during cell growth division in mammalian cells, which reveal that mitochondria can divide in two ways. a, Midzone division and cell division. In this scenario, the new is associated with mitochondrial division during cell growth. The organelle divides in the middle, and this process is associated with the protein DRP1, filaments of actin protein, and contact with another organelle cellular property generated by cell division — the endoplasmic reticulum (ER). The dividing mitochondrion is healthy and has replicating mitochondrial is furnished using mitochondria generated DNA (mtDNA). b, Peripheral division is associated with damaged mitochondria. This division also requires by division. In contrast to damage-associated DRP1, but the dividing mitochondrion makes contact with a different organelle, the lysosome. This division, mitochondrial division during cell asymmetric division occurs at the tip of the mitochondrion. The dividing organelle has different properties growth is a sign that times are good. on either side of the division site in terms of the membrane potential and the level of reactive oxygen species It stands to reason that different mech- (ROS) and calcium ions (Ca2+). The authors observed that the smaller mitochondrial portion often lacked anisms control mitochondrial division for replicating mtDNA (and in 32% of the divisions it lacked any mtDNA), and that this portion of the organelle mitophagy and for cell growth. Although there was degraded. 346 | Nature | Vol 593 | 20 May 2021 ©2021 Spri nger Nature Li mited. All rights reserved. ©2021 Spri nger Nature Li mited. All rights reserved. midzone division increased after stimulation has been controversial. Kleele and colleagues’ Rajarshi Chakrabarti and Henry N. Higgs are of cell proliferation. work suggests that FIS1 is the DRP1 receptor at the Geisel School of Medicine at Dartmouth, Both types of division are associated with for peripheral divison, and another study Hanover, New Hampshire 03755, USA. DRP1 accumulation. However, there are dif- also suggests that FIS1 is a DRP1 receptor11. e-mails: [email protected]; ferences in other molecular players involved. However, other studies6 indicate that FIS1 [email protected] Midzone division is associated with contact depletion has a minimal effect on division, with the ER and with the polymerization and alternative functions for FIS1 have been of actin filaments through the ER-bound described12,13. Two explanations for this appar- actin-polymerization protein INF2. In addi- ent contradiction are that the other studies tion, the data suggest that MFF has a role on FIS1 were in contexts that did not favour in midzone, but not in peripheral, divi- peripheral division, or that the role of FIS1 sion. Peripheral division is associated with in peripheral division might be indirect. 1. Lavoisier, A. L. in Œuvres de Lavoisier Vol. II, 691–692 lysosomal contact and with FIS1. Something else to consider is the absence (Imprimerie impériale, 1862). 2. Kleele, T. et al. Nature 593, 435–439 (2021). Kleele and colleagues’ careful work is of an increase in mitochondrial calcium levels 3. Kennedy, E. P. & Lehninger, A. L. J. Biol. Chem. 179, valuable, because it clearly demonstrates that during midzone division. Previous studies8,14 957–972 (1949). 4. Ernster, L. & Schatz, G. J. Cell Biol. 91, 227s–255s (1981). there is more than one type of mitochondrial have shown that an increase in mitochondrial 5. Martinez-Vicente, M. Front. Mol. Neurosci. 10, 64 (2017). division, thus enabling a more nuanced anal- calcium precedes division events resem- 6. Kraus, F., Roy, K., Pucadyil, T. J. & Ryan, M. T. Nature 590, ysis of division factors based on the reason for bling the midzone division described by 47–66 (2021). 7. Abrisch, R. G., Gumbin, S. C., Wisniewski, B. T., division. Moreover, this work is a reminder that Kleele and colleagues.It would be interest- Lackner, L. L. & Voeltz, G. K. J. Cell Biol. 219, e201911122 we need to walk before we can run when trying ing to examine the effect of suppressing the (2020). to map complicated biological processes such mitochondrial calcium uniporter (a protein 8. Chakrabarti, R. et al. J. Cell Biol. 217, 251–268 (2018). 9. Wolf, D. M. et al. EMBO J. 38, e101056 (2019). as mitophagy. Otherwise, our understanding that pumps calcium across the membrane) 10. Peng, W., Wong, Y. C. & Krainc, D. Proc. Natl Acad. Sci. of them might be hampered by an incomplete on midzone and peripheral division. A final USA 117, 19266–19275 (2020). grasp of the earlier processes that lead up question is whether there are only two types 11. Simpson, C. L. et al. Cell Rep. 34, 108689 (2021). 12. Yamano, K., Fogel, A. I., Wang, C., van der Bliek, A. M. & to them. of mitochondrial division in mammalian cells. Youle, R. J. eLife 3, e01612 (2014). This work also raises exciting questions. Do Given the large number of regulatory mecha- 13. Yu, R., Jin, S. B., Lendahl,
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