The exocyst complex and Rab5 are required for abscission by localizing ESCRT III subunits to the cytokinetic bridge

Deletion of Exoc7, but not Exoc3, in male germ cells causes severe spermatogenesis failure with spermatocyte aggregation in mice

Vesicular trafficking is essential for the transport of intracellularly produced functional molecules to the plasma membrane and extracellular space. The exocyst complex, composed of eight different proteins, is an important functional machinery for "tethering" in vesicular trafficking. Functional studies have been conducted in laboratory mice to identify the mechanisms by which the deletion of each exocyst factor affect various biological phenomena. Interestingly, each exocyst factor-deficient mutant exhibits a different phenotype. This discrepancy may be due to the function of the exocyst factor beyond its role as a component of the exocyst complex. Male germline-specific conditional knockout (cKO) mice of the Exoc1 gene, which encodes one of the exocyst factors EXOC1 (SEC3), exhibit severe spermatogenesis defects; however, whether this abnormality also occurs in mutants lacking other exocyst factors remains unknown. In this study, we found that exocyst factor EXOC3 (SEC6) was not required for spermatogenesis, but depletion of EXOC7 (EXO70) led to severe spermatogenesis defects. In addition to being a component of the exocyst complex, EXOC1 has other functions. Notably, male germ cell-specific Exoc7 cKO and Exoc1 cKO mice exhibited phenotypic similarities, suggesting the importance of the exocyst complex for spermatogenesis. The results of this study will contribute to further understanding of spermatogenesis from the aspect of vesicular trafficking.

Vesicle-mediated transport of ALIX and ESCRT-III to the intercellular bridge during cytokinesis

Cellular abscission is the final step of cytokinesis that leads to the physical separation of the two daughter cells. The scaffold protein ALIX and the ESCRT-I protein TSG101 contribute to recruiting ESCRT-III to the midbody, which orchestrates the final membrane scission of the intercellular bridge. Here, we addressed the transport mechanisms of ALIX and ESCRT-III subunit CHMP4B to the midbody. Structured illumination microscopy revealed gradual accumulation of ALIX at the midbody, resulting in the formation of spiral-like structures extending from the midbody to the abscission site, which strongly co-localized with CHMP4B. Live-cell microscopy uncovered that ALIX appeared together with CHMP4B in vesicular structures, whose motility was microtubule-dependent. Depletion of ALIX led to structural alterations of the midbody and delayed recruitment of CHMP4B, resulting in delayed abscission. Likewise, depletion of the kinesin-1 motor KIF5B reduced the motility of ALIX-positive vesicles and delayed midbody recruitment of ALIX, TSG101 and CHMP4B, accompanied by impeded abscission. We propose that ALIX, TSG101 and CHMP4B are associated with endosomal vesicles transported on microtubules by kinesin-1 to the cytokinetic bridge and midbody, thereby contributing to their function in abscission.

The exocyst complex regulates C. elegans germline stem cell proliferation by controlling membrane Notch levels

The conserved exocyst complex regulates plasma membrane-directed vesicle fusion in eukaryotes. However, its role in stem cell proliferation has not been reported. Germline stem cell (GSC) proliferation in the nematode Caenorhabditis elegans is regulated by conserved Notch signaling. Here, we reveal that the exocyst complex regulates C. elegans GSC proliferation by modulating Notch signaling cell autonomously. Notch membrane density is asymmetrically maintained on GSCs. Knockdown of exocyst complex subunits or of the exocyst-interacting GTPases Rab5 and Rab11 leads to Notch redistribution from the GSC-niche interface to the cytoplasm, suggesting defects in plasma membrane Notch deposition. The anterior polarity (aPar) protein Par6 is required for GSC proliferation, and for maintaining niche-facing membrane levels of Notch and the exocyst complex. The exocyst complex biochemically interacts with the aPar regulator Par5 (14-3-3ζ) and Notch in C. elegans and human cells. Exocyst components are required for Notch plasma membrane localization and signaling in mammalian cells. Our study uncovers a possibly conserved requirement of the exocyst complex in regulating GSC proliferation and in maintaining optimal membrane Notch levels.

EXOC1 plays an integral role in spermatogonia pseudopod elongation and spermatocyte stable syncytium formation in mice

The male germ cells must adopt the correct morphology at each differentiation stage for proper spermatogenesis. The spermatogonia regulates its differentiation state by its own migration. The male germ cells differentiate and mature with the formation of syncytia, failure of forming the appropriate syncytia results in the arrest at the spermatocyte stage. However, the detailed molecular mechanisms of male germ cell morphological regulation are unknown. Here, we found that EXOC1, a member of the Exocyst complex, is important for the pseudopod formation of spermatogonia and spermatocyte syncytia in mice. EXOC1 contributes to the pseudopod formation of spermatogonia by inactivating the Rho family small GTPase Rac1 and also functions in the spermatocyte syncytia with the SNARE proteins STX2 and SNAP23. Since EXOC1 is known to bind to several cell morphogenesis factors, this study is expected to be the starting point for the discovery of many morphological regulators of male germ cells.

Nucleolin regulates 14-3-3ζ mRNA and promotes cofilin phosphorylation to induce tunneling nanotube formation

Tunneling nanotubes (TNTs) mediate intercellular communication between animal cells in health and disease, but the mechanisms of their biogenesis and function are poorly understood. Here we report that the RNA-binding protein (RBP) nucleolin, which interacts with the known TNT-inducing protein MSec, is essential for TNT formation in mammalian cells. Nucleolin, through its RNA-binding domains (RBDs), binds to and maintains the cytosolic levels of 14-3-3ζ mRNA, and is, therefore, required for TNT formation. A specific region of the 3'-untranslated region (UTR) of the 14-3-3ζ mRNA is likely to be involved in its regulation by nucleolin. Functional complementation experiments suggest that nucleolin and 14-3-3ζ form a linear signaling axis that promotes the phosphorylation and inactivation of the F-actin depolymerization factor cofilin to induce TNT formation. MSec also similarly inactivates cofilin, but potentiates TNT formation independent of the nucleolin-14-3-3ζ axis, despite biochemically interacting with both proteins. We show that 14-3-3ζ and nucleolin are required for the formation of TNTs between primary mouse neurons and astrocytes and in multiple other mammalian cell types. We also report that the Caenorhabditis elegans orthologs of 14-3-3ζ and MSec regulate the size and architecture of the TNT-like cellular protrusions of the distal tip cell (DTC), the germline stem cell niche in the gonad. Our study demonstrates a novel and potentially conserved mRNA-guided mechanism of TNT formation through the maintenance of cellular 14-3-3ζ mRNA levels by the RBP nucleolin.

Meeting report - the Microtubules, Motors, Transport and Trafficking (M2T2) 2019 meeting

The Molecular Motors, Transport and Trafficking (M2T2) meeting serves as a platform for both Indian and global scientists working on the cytoskeleton, cytoskeletal motors and membrane trafficking to gather and discuss the latest developments in the field. The 2019 edition of the meeting, held from 18-20 October at the National Brain Research Centre (NBRC), Manesar, India and organised by Mahak Sharma (Indian Institute of Science Education and Research, Mohali) and Anindya Ghosh Roy (NBRC), was witness to stimulating research on a range of topics related to the cytoskeleton, including cytoskeletal organization, motor protein function and regulation, mechanical forces and vesicular transport, and trafficking in health and disease.

Regulation of the Total Cell Surface Area in Dividing Dictyostelium Cells

When a cell divides into two daughter cells, the total cell surface area should increase. There are two models for membrane supply to support cell division: (1) unfolding of small surface membrane reservoirs such as microvilli or wrinkles and (2) exocytosis of intracellular vesicles. Here, we precisely measured the total cell surface area in dividing Dictyostelium cells, flattened by the agar overlay that eliminated the complexity of unfolding surface membrane reservoirs. Because the cells divided normally under the agar overlay, unfolding of surface membrane reservoirs was not required for cell division. Under the agar overlay, the total cell surface area slightly decreased from the interphase to the metaphase and then increased about 20% during cytokinesis. Both endocytosis and exocytosis were suppressed in the early mitotic phase but recovered during cytokinesis. The imbalance of endocytosis and exocytosis could contribute to the changes observed in the cell surface area. Clathrin-dependent endocytosis was also substantially suppressed during cytokinesis, but contrary to previous reports in cultured animal cells, it did not significantly contribute to the regulation of the cell surface area. Furrowing during cytokinesis was indispensable for the cell membrane increase, and vice versa.

Rabs in Signaling and Embryonic Development

Rab GTPases play key roles in various cellular processes. They are essential, among other roles, to membrane trafficking and intracellular signaling events. Both trafficking and signaling events are crucial for proper embryonic development. Indeed, embryogenesis is a complex process in which cells respond to various signals and undergo dramatic changes in their shape, position, and function. Over the last few decades, cellular studies have highlighted the novel signaling roles played by Rab GTPases, while numerous studies have shed light on the important requirements of Rab proteins at various steps of embryonic development. In this review, we aimed to generate an overview of Rab contributions during animal embryogenesis. We first briefly summarize the involvement of Rabs in signaling events. We then extensively highlight the contribution of Rabs in shaping metazoan development and conclude with new approaches that will allow investigation of Rab functions in vivo.