Centralspindlin Recruits ALIX to the Midbody during Cytokinetic Abscission in Drosophila via a Mechanism Analogous to Virus Budding.

Abscission, the final step of cytokinesis, cleaves the thin intercellular bridge connecting the two daughter cells [1-6]. The scaffold protein ALIX is a core component of the abscission machinery with an evolutionarily conserved role in midbody recruitment of ESCRT-III [7-11], which mediates the final cut [1-5, 8-10, 12-14]. In mammalian cells, the centralspindlin complex recruits the major midbody organizer CEP55 that directly binds and recruits ALIX and ESCRT-I [7-9, 15-17], which in turn cooperatively recruit ESCRT-III [8, 9, 18]. However, CEP55 is missing in Drosophila melanogaster and other invertebrates [6, 9, 19], and it is unknown how the abscission machinery is recruited to the midbody in the absence of CEP55. Here, we address how Drosophila ALIX is recruited to the midbody. Surprisingly, ALIX localizes to the midbody via its V-domain, independently of the GPPX3Y motif in the proline-rich region that recruits human ALIX [8, 9]. We elucidate that the centralspindlin component Pavarotti (H.s.MKLP1) interacts with the V-domain of ALIX to recruit it to the midbody. Specifically, our results indicate that an LxxLF motif in Pavarotti directly interacts with a conserved hydrophobic pocket in the ALIX V-domain, which in human ALIX binds (L)YPXnL/LxxLF motifs of virus proteins [20-28]. Thus, our study identifies that ALIX is recruited by an analogous mechanism during abscission in Drosophila as during virus budding in mammalian cells and an ancestral role for centralspindlin in recruiting the abscission machinery to the midbody.

Centrosomal ALIX regulates mitotic spindle orientation by modulating astral microtubule dynamics.

The orientation of the mitotic spindle (MS) is tightly regulated, but the molecular mechanisms are incompletely understood. Here we report a novel role for the multifunctional adaptor protein ALG-2-interacting protein X (ALIX) in regulating MS orientation in addition to its well-established role in cytokinesis. We show that ALIX is recruited to the pericentriolar material (PCM) of the centrosomes and promotes correct orientation of the MS in asymmetrically dividing Drosophila stem cells and epithelial cells, and symmetrically dividing Drosophila and human epithelial cells. ALIX-deprived cells display defective formation of astral microtubules (MTs), which results in abnormal MS orientation. Specifically, ALIX is recruited to the PCM via Drosophila Spindle defective 2 (DSpd-2)/Cep192, where ALIX promotes accumulation of γ-tubulin and thus facilitates efficient nucleation of astral MTs. In addition, ALIX promotes MT stability by recruiting microtubule-associated protein 1S (MAP1S), which stabilizes newly formed MTs. Altogether, our results demonstrate a novel evolutionarily conserved role of ALIX in providing robustness to the orientation of the MS by promoting astral MT formation during asymmetric and symmetric cell division.

Antibody Staining in Drosophila Germaria.

Drosophila oogenesis is a powerful model for studying a wide spectrum of cellular and developmental processes in vivo. Oogenesis starts in a specialized structure called the germarium, which harbors the stem cells for both germ and somatic cells. The germarium produces egg chambers, each of which will develop into an egg. Active areas of research in Drosophila germaria include stem cell self-renewal, division, and maintenance, cell cycle control and differentiation, oocyte specification, intercellular communication, and signaling, among others. The solid knowledge base, the genetic tractability of the Drosophila model, as well as the availability and fast development of tools and imaging techniques for oogenesis research ensure that studies in this model will keep being instrumental for novel discoveries within cell and developmental biology also in the future. This chapter focuses on antibody staining in Drosophila germaria and provides a protocol for immunostaining as well as an overview of commonly used antibodies for visualization of different cell types and cellular structures. The protocol is well-suited for subsequent confocal microscopy analyses, and in addition we present key adaptations of the protocol that are useful when performing structured illumination microscopy (SIM) super-resolution imaging.

Src64 controls a novel actin network required for proper ring canal formation in the Drosophila male germline.

In many organisms, germ cells develop as cysts in which cells are interconnected via ring canals (RCs) as a result of incomplete cytokinesis. However, the molecular mechanisms of incomplete cytokinesis remain poorly understood. Here, we address the role of tyrosine phosphorylation of RCs in the Drosophila male germline. We uncover a hierarchy of tyrosine phosphorylation within germline cysts that positively correlates with RC age. The kinase Src64 is responsible for mediating RC tyrosine phosphorylation, and loss of Src64 causes a reduction in RC diameter within germline cysts. Mechanistically, we show that Src64 controls an actin network around the RCs that depends on Abl and the Rac/SCAR/Arp2/3 pathway. The actin network around RCs is required for correct RC diameter in cysts of developing germ cells. We also identify that Src64 is required for proper germ cell differentiation in the Drosophila male germline independent of its role in RC regulation. In summary, we report that Src64 controls actin dynamics to mediate proper RC formation during incomplete cytokinesis during germline cyst development in vivo.