Regulation of spermatogenesis : the roles of planar cell polarity proteins, basement membrane and the downstream signalling molecules

Abstract

Spermatogenesis takes place in the seminiferous tubules in the testis. It is composed of a series of cellular and molecular processes that give rise to millions of spermatozoa per day in an adult male. During spermatogenesis, developing germ cells residing in the basal compartment must be transported across the seminiferous epithelium, from near the basement membrane (BM) to the edge of the adluminal compartment. Thus, fully developed spermatids (i.e., spermatozoa) are released into the tubule lumen at spermiation which is facilitated by the breakdown of apical ectoplasmic specialization (ES), a testis-specific atypical adherens junction between elongated spermatids and Sertoli cells in the seminiferous epithelium. In rodents and humans, preleptotene spermatocytes derived from type B spermatogonia are the only germ cells that are being transported across the blood-testis barrier (BTB) – the tissue barrier created by adjacent Sertoli cells, which also divides the seminiferous epithelium into the basal and the adluminal compartment. Thus, the BTB undergoes cyclic restructuring to support germ cell transport across the barrier. Although these junction restructuring events that take place at the opposite end of the seminiferous epithelium are known for decades, the underlying mechanism(s) remains elusive. The study reported herein aims to provide crucial information regarding these events.

The first study examines the regulation of BTB and apical ES by investigating the role of planar cell polarity (PCP) protein Vangl2. Vangl2 expressed predominantly by Sertoli cells localized at both apical ES and BTB. Vangl2 knockdown by RNAi both in vitro and in vivo was found to cause BTB tightening and apical ES disruption, which was mediated through an alteration on the organization of actin microfilaments involving actin regulatory proteins Eps8, Arp3 and Scribble, which in turn affected the function of adhesion protein complexes during spermatogenesis.

The second study focuses on the non-collagenous domain 1 (NC1) peptide derived from collagen α3 (IV) chains – which are a major constituent component of the BM in the testis. NC1 domain peptide was shown to be transported across the seminiferous epithelium via a microtubule-dependent mechanism, capable of inducing apical ES degeneration, leading to germ cell exfoliation, as well as BTB disruption.

Finally, to assess if findings obtained using rodent models can be applied to humans, cultured human Sertoli cells exposed to PFOS (perfluorooctanesulfonate), an environmental toxicant known to cause rat Sertoli cell injury, was used as a study model. PFOS-induced Sertoli cell injury in humans was similar to that in rats. Furthermore, the PFOS-induced BTB disruption in human Sertoli cells was rescued by overexpressing p-FAK-Y407E, a signaling molecule that has been shown to play a crucial role in regulating BTB and apical ES in rodent testes.

In summary, the synchronized regulation of BTB and apical ES relies on the involvement of PCP protein Vangl2 and the BM. The information on the regulation of spermatogenesis in the rodent model can be a useful guide to human study.