ESCRT-dependent control of membrane remodelling during cell division

Abstract

The Endosomal Sorting Complex Required for Transport (ESCRT) proteins form an evolutionarily conserved membrane remodelling machinery. Identified originally for their role in cargo sorting and remodelling of endosomal membranes during yeast vacuolar sorting, an extensive body of work now implicates a sub-complex of this machinery (ESCRT-III), as a transplantable membrane fission machinery that is dispatched to various cellular locations to achieve a topologically unique membrane separation. Surprisingly, several ESCRT-III-regulated processes occur during cell division, when cells undergo a dramatic and co-ordinated remodelling of their membranes to allow the physical processes of division to occur. The ESCRT machinery functions in regeneration of the nuclear envelope during open mitosis and in the abscission phase of cytokinesis, where daughter cells are separated from each other in the last act of division. Roles for the ESCRT machinery in cell division are conserved as far back as Archaea, suggesting that the ancestral role of these proteins was as a membrane remodelling machinery that facilitated division and that was co-opted throughout evolution to perform a variety of other cell biological functions. Here, we will explore the function and regulation of the ESCRT machinery in cell division.

Keywords: Abscission; Cytokinesis; ESCRT; Endosomal sorting complex required for transport; Nuclear envelope.

Fig. 1

Topological equivalence of membrane ESCRT-dependent membrane remodelling events. ESCRT-III is a membrane fission machinery that regulates a series of topologically equivalent membrane remodelling events, namely biogenesis of intraluminal vesicles on a class of late endosomes called multivesicular bodies (1), release of enveloped retroviruses such as HIV-1 (2), repair of damaged plasma membrane (3), abscission during cytokinesis (4), sealing of holes in the nuclear envelope (5), depolymerisation of spindle microtubules (6) and neuronal pruning (7). In addition, ESCRT-III has also been described to regulate NPC quality in S. cerevisiae (not depicted; see Fig. 5). Sites of ESCRT-III activity indicated by magenta asterisks. Sealing of nuclear envelope allows nuclear compartmentalisation, indicated in orange.

Fig. 2

Roles for ESCRT-III in membrane remodelling during division. A. Membrane remodelling events during mitosis. As cells complete division, daughter nuclei are enclosed by sheets of ER to generate the nuclear envelope. Where these sheets meet in three dimensions, and where these sheets negotiate spindle microtubules, they leave small holes that are sealed by ESCRT-III dependent annular fusion. Completion of annular fusion permits nuclear compartmentalisation (indicated by nucleoplasm depicted in orange). Nuclear envelope regeneration happens concurrently with ingression of a cleavage furrow between daughter nuclei, leaving cells connected by a thin tube of membrane and compressed spindle microtubules, called the midbody. The midbody is severed through ESCRT-III-dependent abscission. B. Toplogical equivalence and consequences of ESCRT-III function during ESCRT-III-dependent membrane remodelling during mitotic events. C. Molecular interactions that govern ESCRT-III assembly at the midbody and at the reforming nuclear envelope. CEP55 recruits ESCRT-III to the midbody via either ALIX or ESCRT-I/ESCRT-II. ESCRT-III coordination of the microtubule severing enzyme spastin permits disassembly of midbody microtubules. CHMP7 initiates assembly of ESCRT-III at sites of annular fusion through engagement of ER-membranes and the chromatin-binding INM protein LEM2. ESCRT-III coordination of the microtubule severing enzyme spastin permits disassembly of spindle microtubules.

Fig. 3

Morphological changes during midbody maturation. The midbody tethers daughter cells at the end of division, and upstream ESCRT-I and ALIX components assemble at the Flemming body. Prior to abscission, ESCRT-III components assemble in two rings either side of the Flemming body. Rab11-FIP3 positive vesicles and delivery of the actin remodelling proteins p50RhoGAP, SCAMP2/3, OCRL, and MICAL1 are necessary to remodel the actin cytoskeleton and form a secondary ingression that narrows the midbody to permit subsequent ESCRT-III assembly at the site of abscission. Again, ESCRT-III coordination of Spastin assists in disassembling the midbody microtubules, thus coordinating membrane and cytoskeletal remodelling.

Fig. 4

Role of ESCRT-III in the Aurora B mediated abscission checkpoint. Cartoon depicting triggers that initiate Aurora B-dependent abscission arrest. Elevated Aurora B downstream of chromosome segregation errors or NPC assembly defects (NPC in red, with indicated compartmentalisation breakdown, as per [179]) act to retard ESCRT-III-dependent abscission. Whilst a direct target of Aurora B is CHMP4C, ANCHR and ULK3 contribute to maintenance of this checkpoint. This checkpoint appears effected by spatiotemporal control over ESCRT-III assembly within the midbody; Aurora B phosphorylation directs CHMP4C to the Flemming body and ULK3 also drives ESCRT-III components onto the Flemming body. ANCHR acts to restrict VPS4 to the Flemming body and prevents it relocalising to the secondary ingression. Tension in the midbody can act to suppress ESCRT-III-assembly at the midbody and ESCRT-III-dependent abscission and this signalling acts though ULK3.

Fig. 5

Potential role for Chm7 and Heh2 in surveillance of damaged NPCs. Mis-assembled NPCs are cleared in an ESCRT-III and proteasome dependent manner. In one model of misassembly, NPCs lacking Nup116 are encapsulated by new membrane to seal them off and prevent them participating in nucleocytoplasmic transport. The N-terminal domain of the INM LEM-domain protein Heh2 interacts with both Snf7 and the CHMP-like C-terminus of Chm7 and may play a role in this, from an ESCRT-III perspective, topologically satisfying closure. Hypothesis and figure suggested by Lusk and colleagues and drawn and adapted from , .