Internalization of adhesion junction proteins and their association with recycling endosome marker proteins in rat seminiferous epithelium

Abstract

Tubulobulbar complexes (TBCs) are elaborate cytoskeleton-related structures that are formed in association with intercellular junctions in the seminiferous epithelium. They consist of a cylindrical double-membrane core composed of the plasma membranes of the two attached cells, cuffed by a dendritic network of actin filaments. TBCs are proposed to be subcellular machines that internalize intercellular junctions during the extensive junction remodeling that occurs during spermatogenesis. At the apical sites of attachment between Sertoli cells and spermatids, junction disassembly is part of the sperm release mechanism. In this study, we used immunological probes to explore junction internalization and recycling at apical TBCs in the rat seminiferous epithelium. We demonstrate that b1-integrin and nectin 2 were concentrated at the ends of TBCs and for the first time show that the early endosome marker RAB5A was also distinctly localized at the ends of TBCs that appear to be the 'bulbar' regions of the complexes. Significantly, we also demonstrate that the 'long-loop' recycling endosome marker RAB11A was co-distributed with nectin 2 at junctions with early spermatids deeper in the epithelium. Our results are consistent with the hypothesis that TBCs associated with late spermatids internalize adhesion junctions and also indicate that some of the internalized junction proteins may be recycled to form junctions with the next generation of spermatids.

Modulation of CD112 by the alphaherpesvirus gD protein suppresses DNAM-1–dependent NK cell-mediated lysis of infected cells

“…It will be interesting to further characterize these virus-induced internalization/degradation routes of CD112 and other nectins in future research, as this may also shed new light on the physiological role and regulation of nectins. Indeed, CD112 internalization has, to our knowledge, up to now only been reported and studied during spermatogenesis (44).…”

Actin binding proteins, spermatid transport and spermiation

“…Cortactin and N-WASP are also detected at the concave side of elongated spermatid heads at stage VII during TBCs formation. Collectively, these findings suggest that activated Arp2/3 complex leads to actin nucleation and branching, assembling actin mesh network around protrusions of apical TBCs to facilitate protein endocytosis, transcytosis and recycling [62,64–66], with the likely involvement of N-WASP since it is known to participate in endocytic vesicle-mediated protein trafficking in other mammalian cells [60]. …”

Sertoli cell anatomy and cytoskeleton

“…Some representative proteins include the claudins, occludins, zona occludens-1, junctional adhesion molecule-1, and scribbled. Other proteins, such as RHOA, are associated with only the apical disengagement complex found in the small cytoplasmic extensions of Sertoli cells that interact with small finger-like tubulobulbar complexes in the heads of elongated spermatids [35, 170,206]. Several proteins, such as F-Actin, ESPIN, connexin-43 and vinculin, have been localized at both the apical and basal ectoplasmic specializations, usually giving this appearance in a stage-specific manner.…”

“…Third, junction proteins and endocytosis markers occur together at tubulobulbar complexes and their related vesicles. At apical sites of attachment to spermatids, these structures label with probes for adhesion molecules (nectin-2 [170,206,219], nectin-3 [219], α6β1-integrin [170,206]) and for endosomal/lysosomal markers (EEA1 [206], Rab5 [170], LAMP [219], SGP1 [219], acid phosphatase [360]). In addition, the structures also label with probes for polyubiquitin [370].…”