PI(3,4)P2-mediated cytokinetic abscission prevents early senescence and cataract formation

Description ESCRTing lenses away from senescence ESCRT proteins control membrane fusion in various key cellular processes, but the mechanisms involved are still incompletely understood. Gulluni et al. report that ESCRT recruitment at the cytokinetic bridge is mediated by the binding of an ESCRT-II subunit to the signaling lipid phosphatidylinositol 3,4-bisphosphate (see the Perspective by Brill and Wilde). This pathway acts in parallel to a known cascade driven by a protein called ALIX, but its failure is sufficient to lead to premature senescence in the lens of fish, mouse, and human eyes, where ALIX is expressed at lower levels. These results point to an evolutionarily conserved pathway for the cell-specific control of cytokinesis that serves to protect from senescence and the early onset of cataracts. —SMH An evolutionarily conserved pathway underlies the cell-specific control of cytokinesis and protects from the early onset of cataracts. INTRODUCTION In response to aging, senescence plays a major role in the onset of various degenerative diseases, including the slow development of cataracts, a clouding of the eye lens that troubles the vision of millions of people worldwide. Transparent cells that form the eye lens divide throughout lifetime, but in the elderly, this process is less efficient, and senescent cells trigger the decline in transparency. Although pathogenesis is largely unclear, patients with genetic predisposition to early-onset cataract can help to define molecular mechanisms. RATIONALE Lack of the lipid kinase PI3K-C2α (phosphatidylinositol-4-phosphate 3-kinase catalytic subunit type 2α) causes premature cataracts in fish, mice, and humans. This indicates that PI3K-C2α is involved in the maintenance of an evolutionarily conserved mechanism that protects the lens from the loss of transparency. On the basis of this, we aimed to define the role of PI3K-C2α in the lens, starting from the notion that this kinase is involved in mitosis and that correct cell division protects from senescence and eventually cataract development. RESULTS We found that PI3K-C2α played a specific role in the last step of cell division, called cytokinetic abscission. At the end of the mitotic process, the two nascent daughter cells form a contractile ring that constricts the membrane and produces an intercellular bridge that is eventually cleaved through biochemical reactions orchestrated by a specific organelle, the midbody. PI3K-C2α was found to be enriched at the midbody, where it produces a specific lipid moiety known as phosphoinositide (3,4) bisphosphate [PI(3,4)P2]. PI(3,4)P2 appeared to be required for efficient organization of the membrane-severing machinery, a multiprotein aggregate in which a cascade of three serially acting, ESCRT proteins–containing complexes eventually cleave the intercellular bridge. At the midbody, the endosomal sorting complexes required for transport I (ESCRT I) complex triggers the enucleation of the ESCRT II protein aggregate and then that of the membrane-severing ESCRT III machinery. A key component of the ESCRT II complex, named VPS36, anchored on locally preformed PI(3,4)P2 to properly stabilize the ESCRT II protein aggregate at the midbody. This was found in fish, mice, and humans, indicating that this process has been conserved throughout vertebrate evolution. Without PI3K-C2α and its lipid product PI(3,4)P2, the ESCRT machinery loses efficiency, and cytokinesis is delayed. Prolonged cytokinesis triggers the expression of p16INK4A and the activation of the transcriptional program, leading to senescence, likely protecting against the genomic instability that ensues cell division after refusion and tetraploidization. However, cytokinesis evolved as a resilient cellular process, and redundant pathways are common in the organization of the various ESCRT complexes. For example, an alternative pathway to ESCRT III organization that bypasses the need for an ESCRT II aggregate relies on the function of the linker protein ALIX. In the lens, ALIX was poorly expressed, and the PI3K-C2α, PI(3,4)P2, and VPS36 pathway was specifically required to protect lens cells from cytokinetic delay, senescence, and ultimately, cataract formation. CONCLUSION This study provides evidence that a pathway based on PI3K-C2α, PI(3,4)P2, and VPS36 positioning at the midbody improves robustness of cytokinetic abscission and is nonredundant in the eye lens. Failure of this process defines a pathogenetic mechanism of cataract development, ultimately delineating a path linking ESCRT dysfunction to senescence. During cytokinesis, PI3K-C2α produced PI(3,4)P2 (red) at the midbody, triggering the recruitment of the ESCRT-II subunit VPS36 (light blue). When PI3K-C2α, PI(3,4)P2, or VPS36 is lost, the secondary ingression, where abscission occurs, does not form properly, resulting in prolonged cytokinesis. This leads to aneuploidy and early onset of senescence, which is particularly evident in the lens epithelium, resulting in cataract. Cytokinetic membrane abscission is a spatially and temporally regulated process that requires ESCRT (endosomal sorting complexes required for transport)–dependent control of membrane remodeling at the midbody, a subcellular organelle that defines the cleavage site. Alteration of ESCRT function can lead to cataract, but the underlying mechanism and its relation to cytokinesis are unclear. We found a lens-specific cytokinetic process that required PI3K-C2α (phosphatidylinositol-4-phosphate 3-kinase catalytic subunit type 2α), its lipid product PI(3,4)P2 (phosphatidylinositol 3,4-bisphosphate), and the PI(3,4)P2–binding ESCRT-II subunit VPS36 (vacuolar protein-sorting-associated protein 36). Loss of each of these components led to impaired cytokinesis, triggering premature senescence in the lens of fish, mice, and humans. Thus, an evolutionarily conserved pathway underlies the cell type–specific control of cytokinesis that helps to prevent early onset cataract by protecting from senescence.