The Non‐Receptor Tyrosine Kinase, Src, and its Regulator PTP1B are Present at Tubulobulbar Complexes

Abstract

During spermiation, the intercellular junctions between Sertoli cells and the spermatids must be internalized before the spermatid can be released into the lumen of the seminiferous tubule. Prior to release from the epithelium, the spermatids are engulfed in protrusions of Sertoli cell cytoplasm which are referred to as apical processes. Unique endocytic structures termed, tubulobulbar complexes (TBCs), internalize these intercellular junctions during sperm release. TBCs have three regions: a proximal tubule, a bulbar region and a distal tubule ending in a clathrin‐coated pit. At sites of intercellular junctions between Sertoli and sperm cells, TBCs form by invaginating into the Sertoli cell and dragging the spermatid's membrane along with it through its intercellular attachments. This results in the entire structure consisting of a double membrane. The tubular regions are surrounded by a dendritic actin network, which is absent at the bulbar region. The bulbar region is a swelling in the distal third part of the TBC and is closely associated with a cisterna of endoplasmic reticulum. Eventually the bulb region of the TBC vesiculates and enters the endosomal pathway of the Sertoli cell. It has been shown by immunoprecipitation that Src, a non‐receptor tyrosine kinase, associates with components of the intercellular junctions between Sertoli cells and spermatids. One of the substrates that Src is able to phosphorylate is cortactin, a component of the dendritic actin networks at TBCs. We suspect that Src is involved in regulating the actin network at TBCs. In this study, we used immunofluorescence to evaluate whether or not Src and its phosphatase regulator, protein tyrosine phosphatase 1b (PTP1B), are associated with TBCs. Rat testes were perfusion‐fixed and cryosectioned or fragmented to get epithelial fragments. Sections and fragments were probed with antibodies against Src or PTP1B and an appropriate fluorescent secondary antibody. The Src antibody labels linear tracts in apical processes that could represent the tubular regions of TBCs. In cryosections, the PTP1B antibody has a Sertoli cell‐labeling pattern that extends from the base of the seminiferous epithelium to the apex. In fragments and cryosections, the PTP1B antibody labels regions of the Sertoli cell where TBCs would be expected to form. This evidence suggests that Src and its regulator PTP1B are present at TBCs.Support or Funding InformationSupported by the Natural Sciences and Engineering Research Council of Canada Discovery Grants program.