Tushna Kapoor

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 paracellular flow and maintain cell polarity in an epithelium. These junctions are also required for maintaining the blood-testis barrier, which is essential for sperm differentiation. Septate junctions in insects are orthologous to the tight junctions. 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 previous reports, knockdown of essential junctional components – Discs-large-1 and Neurexin-IV – during the early stages disrupted sperm differentiation beyond the spermatocyte stage. 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 the 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 mammalian spermatogenesis. This article has an associated First Person interview with the first author of the paper.