Seema Shirolikar

Nanoscale bending of plasma membrane increases cell adhesion, induces cell-signalling, triggers F-actin assembly and endocytosis in tissue-cultured cells. The underlying mechanisms are not very well understood. Here, we show that stretching the plasma membrane of somatic cyst cell around rigid spermatid heads generates a stable, tubular endomembrane scaffold supported by contractile actomyosin. The structure resembles an actin-basket covering the bundle of spermatid heads. Genetic analysis suggests that the actomyosin organisation is nucleated exclusively by the Formins, Diaphanous and DAAM, downstream of Rho1, recruited by the Bin-Amphiphysin-Rvs (BAR)-domain protein, Amphiphysin, around the spermatid heads. Actomyosin activity at the actin-basket gathers the spermatid heads into a compact bundle and resists the invasion of the somatic cell by the intruding spermatids. These observations revealed a new response mechanism of nanoscale bending of the plasma membrane, which generates a novel cell adhesion strategy through active clamping. Highlights: Stretching the plasma membrane around a spermatid head recruits Amphiphysin and Rho1. Rho1 activation triggers F-actin assembly in situ through Diaphanous and DAAM. Rho1-Rok activation assembles actomyosin scaffold around the folded plasma membrane. Contractile actomyosin enables plasma membrane to clamp onto the spermatid head. Author summary: Sperm released from the somatic enclosure is essential for male fertility. During differentiation, the somatic cell membrane, associated with dense F-actin scaffold, tightly hold each spermatid head before release. Kapoor et al., showed that the bending and stretching of the plasma membrane trigger the assembly of an actomyosin scaffold around the bent membrane, which clamps the spermatids together preventing the premature release and somatic cell penetration. This finding provides new insight into the molecular networks activated by mechanical bending of the plasma membrane.

Tight junctions prevent the paracellular flow and maintain cell polarity in an epithelium. These are also essential for maintaining the blood-testis-barrier involved in regulating sperm differentiation. Septate junctions are orthologous to the tight junctions in insects. In Drosophila testis, major septate junction components co-localize at the interface of germline and somatic cells initially and then condense between the two somatic cells in a cyst after germline meiosis. Their localization is extensively remodeled in subsequent stages. We find that characteristic septate junctions are formed between the somatic cyst cells at the elongated spermatid stage. Consistent with the previous reports, knockdown of essential junctional components, Discs-large-1 and Neurexin-IV, in the somatic cyst cells, during the early stages, disrupted sperm differentiation beyond the spermatocyte stage. Somatic knockdown of these proteins during the final stages of spermatid maturation caused premature release of spermatids inside the testes, resulting in partial loss of male fertility. These results indicate the importance of maintaining mechanical integrity of the somatic enclosure during spermatid coiling and release in Drosophila testis. It also highlights the functional similarity with the tight junction proteins during spermatogenesis in mammalian testes. Summary statement: Dubey et al., showed that septate junctions stitch the somatic enclosure around maturing spermatids in Drosophila testis. Maintaining the integrity of this junction is essential for proper release of spermatids.

Background: In Drosophila, all the 64 clonally derived spermatocytes differentiate in syncytium inside two somatic-origin cyst cells. They elongate to form slender spermatids, which are individualized and then released into the seminal vesicle. During individualization, differentiating spermatids are organized in a tight bundle inside the cyst, which is expected to play an important role in sperm selection. However, actual significance of this process and its underlying mechanism are unclear. Results: We show that dynamic F-actin-based processes extend from the head cyst cell at the start of individualization, filling the interstitial space at the rostral ends of the maturing spermatid bundle. In addition to actin, these structures contained lamin, beta-catenin, dynamin, myosin VI and several other filopodial components. Further, pharmacological and genetic analyses showed that cytoskeletal stability and dynamin function are essential for their maintenance. Disruption of these F-actin based processes was associated with spermatid bundle disassembly and premature sperm release inside the testis. Conclusion: Altogether, our data suggests that the head cyst cell adheres to the maturing spermatid heads through F-actin-based extensions, thus maintaining them in a tight bundle. This is likely to regulate mature sperm release into the seminal vesicle. Overall, this process bears resemblance to mammalian spermiation.