How to split up: lessons from mitochondria.

Mitochondria are remarkably dynamic organelles undergoing frequent fusion and fission events. Impairment thereof is linked to numerous neurodegenerative disorders and dysregulation of apoptosis. The principal players mediating mitochondrial fission are considered to be well known and largely conserved between yeast and mammals. However, how the essential fission factor Drp1 is recruited to mitochondria and how its activity is regulated are far more complex than previously assumed. According to a recent study (Otera et al, 2010), recruitment of Drp1 and mitochondrial fission can be exerted by Mff. Surprisingly, these processes do not appear to require Fis1, apparently contradicting several earlier reports on the role of Fis1. Two studies reported in EMBO Reports (Palmer et al, 2011) and in this issue of The EMBO Journal (Zhao et al, 2011) help to shed light on these unexpected findings. They identified two homologous vertebrate-specific negative regulators of Drp1-dependent fission termed: MIEF1/MiD51 and MiD49. They are able to recruit Drp1 to mitochondria but, importantly, rather than promoting fission, bind and inhibit Drp1. In a mutually exclusive manner, MIEF1/MiD51 can form a complex either with Drp1 or with Fis1. Thus, Fis1 may indirectly promote mitochondrial fission by its ability to sequester MIEF1/ MiD51, preventing this novel factor from inhibiting mitochondrial fission. Future studies will have to decipher the complex interplay between these novel factors and how they regulate mitochondrial dynamics. In yeast, three factors are known to be required for mitochondrial fission: Dnm1, Fis1 and Mdv1 (Chan, 2006; Westermann, 2010). Of those only Fis1 and Dnm1 have orthologues in vertebrates, namely Fis1 and Drp1, respectively. Drp1, a dynamin-like large GTPase shows a cytosolic localization that after recruitment to mitochondria and oligomerization mediates GTP-dependent fission. Downregulation of Fis1 leads to mitochondrial elongation and overexpression promoted fission consistent with data from yeast (Chan, 2006; Westermann, 2010). Fis1 was proposed to act as the mitochondrial receptor for Drp1-dependent fission (Figure 1). So, it appeared for quite some time in the field that the molecular functions of Drp1 and Fis1 are well conserved from yeast to humans. However, the general role of Fis1 in mitochondrial fission in mammals was questioned recently, for example, with the observation that a conditional knockout of this gene in a carcinoma cell culture model did not lead to a defect in mitochondrial fission, suggesting that Fis1 is dispensable for fission (Otera et al, 2010). The same study analysed in an impressive manner the molecular function of the mitochondrial fission factor, Mff, identified earlier (Gandre-Babbe and van der Bliek, 2008). Otera et al (2010) showed that this factor is able to recruit Drp1 to mitochondria, forms a complex with Drp1 and promotes mitochondrial fission (Figure 1). This was convincingly demonstrated as for example artificially linking Mff to the plasma membrane resulted in Drp1 recruitment to exactly that membrane; and downregulation of Mff led to a reduced number of Drp1-positive foci at the mitochondrial outer membrane accompanied by impairment of mitochondrial fission. In contrast, overexpression of Mff had the opposite effects. These findings strengthened the view that Fis1 is not an essential component of the fission machinery and that Mff may act in addition or alternatively as the mitochondrial receptor for Drp1. The apparent discrepancies to earlier studies in which downregulation of Fis1 impaired fission and upregulation of Fis1 promoted mitochondrial fragmentation were attributed to different types of cell lines and RNAi sequences used (Otera et al, 2010). However, an additional, attractive hypothesis explaining these findings is now provided by two parallel studies from Zhao et al (2011) and Palmer et al (2011). The latter group identified and characterized the role of MiD49 and MiD51 (mitochondrial dynamics proteins 49/51), which share about 45% amino-acid identity. In parallel, Zhao et al (2011) have identified and characterized the molecular function of MIEF1 (mitochondrial elongation factor 1), which notably is identical to MiD51. These studies demonstrate that MIEF1/MiD51 and MiD49 are able to recruit Drp1 to mitochondria (Figure 1). Importantly, instead of promoting mitochondrial fission, they rather block it by sequestering Drp1. MIEF1/ MiD51 was further shown to have a pro-fusion activity independent of Mfn2, a known fusion factor located in the outer membrane. Interestingly, MIEF1/MiD51 is able to form two different protein complexes as it, in an apparently mutually exclusive manner, binds either to Drp1 or to Fis1. Consistently, overexpression of Fis1 partially reduces the inhibitory effect of MIEF1/MiD51 on Drp1. With these findings, these studies provide a rationale how Fis1 may, more indirectly, affect mitochondrial fission and explain part of the apparently contradicting results reported during the last years: Fis1 overexpression would sequester MIEF1/MiD51. That prevents the latter to act as a fission inhibitor as Drp1 cannot be sequestered efficiently anymore. Consequently, mitochondrial fission is promoted. Downregulation of Fis1 The EMBO Journal (2011) 30, 2751–2753 | & 2011 European Molecular Biology Organization | All Rights Reserved 0261-4189/11 www.embojournal.org