PI(3)P regulates mitochondrial dynamics through EXC-5–dependent actin remodeling

Mitochondria play a central role in metabolic regulation. They are highly dynamic organelles that constantly remodel their structure in response to metabolic cues. These morphological changes are controlled by mitochondrial dynamics, which include mitochondrial fission (division of a single mitochondrion into two) and fusion (combining two mitochondria into a single mitochondrion). Mitochondrial dynamics are crucial to modulate energy production, calcium homeostasis, mitochondrial quality control, and stress responses that are required for cellular homeostasis (1).

At the molecular level, mitochondrial dynamics require large GTPases of the dynamin family: DRP1 drives fission, whereas MFN1/2 and OPA1 control fusion (1). In addition, both fission and fusion occur at contact sites between mitochondria and other organelles. The ER is present at nearly all fission and fusion events and plays a key role in mitochondrial dynamics by acting as a platform to recruit the fission and fusion machineries (2). Another important aspect of ER-associated mitochondrial fission and fusion is the presence of actin at ER-mitochondria contact sites (3). Fission requires the presence of actin filaments that partially constrict the mitochondrial tubule with the help of myosin, prior to DRP1 recruitment (4). Actin is also required for tip-to-side fusion events, presumably to stabilize the interaction between the two fusing mitochondria (3). While both fission and fusion require formin-dependent polymerization of actin filaments, fusion also requires Arp2/3 and branched actin (3). Importantly, the upstream signals converging on mitochondria and the ER to modulate these events remain unknown.

In addition to the ER, other organelles converge at the fission site. These include endosomes, lysosomes, and Golgi-derived vesicles. While lysosomes are thought to be primarily required for mitophagy-associated mitochondrial fission, other types of contacts likely play a broader role (5). For example, Golgi-derived vesicles containing specific phosphoinositides (PIs) are required for the late steps of mitochondrial division (6). While the exact role of the organelles recruited to sites of mitochondrial fission remains unclear, the transfer of lipids, especially PIs, seems to play an important role.

PIs are generated by the phosphorylation of phosphatidylinositol. Each specific PI is generated by dedicated lipid kinases at a specific cellular membrane to control membrane trafficking and signaling pathways. For example, PI(4)P is present on lysosomes and Golgi where it controls membrane trafficking, protein sorting, lipid transfer, and autophagy. Similarly, PI(3)P regulates endosome sorting and trafficking (7). PIs regulate these functions by recruiting specific effectors that selectively bind to these signaling lipids through conserved lipid-binding domains such as PH and FYVE domains. Recent evidence also suggests that these phospholipids are introduced on mitochondria via other organelles at mitochondrial contact sites where they contribute to mitochondrial fission (6). However, how PIs contribute to mitochondrial dynamics is unclear. A new study by Zhao et al. fills this gap by demonstrating that PI(3)P promotes CDC42-dependent actin polymerization on mitochondria (8).

Using Caenorhabditis elegans as a model system and an elegant combination of in vivo time-lapse microscopy, genetics, molecular biology, and behavioral techniques, Zhao et al. provide the first direct in vivo link between PI recruitment on mitochondria and the actin remodeling around mitochondria that is required to execute mitochondrial dynamics. They identified EXC-5, a member of faciogenital dysplasia family of protein, as a regulator of mitochondrial dynamics in C. elegans hypodermis. Crucially, EXC-5 contains PI-binding domains and a RhoGEF domain that they show links PI signaling with downstream Rho family–dependent actin polymerization.

Specifically, the authors show that EXC-5 binds to PI(3)P on mitochondria-associated endosomes. The RhoGEF domain of EXC-5 then activates CDC42, stimulating N-WASP and the Arp2/3 complex to drive actin polymerization on mitochondria, regulating mitochondrial dynamics. Consistent with an important role of this pathway, deletion of EXC-5 or its FYVE PI(3)P-binding domain, as well as removal of PI(3)P, led to mitochondrial fragmentation. They further show that that PI(3)P-driven actin remodeling around mitochondria controls the spatial distribution of DRP1 around mitochondria, thereby regulating mitochondrial network and function.

Overall, by linking PI signaling and actin polymerization, the study by Zhao et al. provides an important piece of the puzzle of the coordination of the different pathways converging on mitochondria during fission. Nevertheless, some important questions remain. First, while the different players involved in regulating fission are relatively well known, mechanistic insights in the process of mitochondrial fusion and the role of different contact sites are still missing. As the inhibition of the EXC-5 pathway leads to fragmented mitochondria and actin is required for both fusion and fission, it is likely that this pathway also regulates fusion, but this remains to be investigated.

The nature of the PI(3)P-containing membrane to which EXC-5 binds also remains unclear. Mitochondria-associated EXC-5 foci partially localizes with a PI(3)P fluorescent reporter (2xFYVE::mKate) that associates with early endosomes, indicating that EXC-5 only binds to a subset of these PI(3)P-positive membranes. It also remains possible that EXC-5 binds to PI(3)P that has been transferred to the mitochondrial outer membrane, as it was suggested for other PIs.

Additionally, endosomes have been shown to affect ER structure, thereby indirectly influencing ER-mitochondria contact sites (ERMCS) (9). As ERMCS play a key role in the regulation of mitochondrial dynamics, it will be important to determine whether this contributes to PI(3)P driven–mitochondrial dynamics. Similarly, PI(4)P and PI(4,5)P have also been implicated in mitochondrial fission, but their exact role remains unclear (6). Interestingly, EXC-5 binds to PI(4)P and PI(5)P in addition to PI(3)P, suggesting that its regulation could involve other PIs associated with mitochondrial dynamics. This complexity in mitochondrial PI signaling could also potentially explain the dual regulation of fission and fusion at the same ER–mitochondria contact sites. Indeed, both the fusion and fission machineries converge on the same site prior either event (2, 3), but the mechanisms regulating the fate of these mitochondrial dynamics hotspots remains unclear.

Together, this study reveals a key role for PI lipids as a scaffold that link inter-organelle communication to actin dynamics, providing new insight into how mitochondrial morphology is controlled.