Sarco/Endoplasmic Reticulum Ca2+‐ATPase 2 (SERCA2) is Present at Tubulobulbar Complexes

Abstract

Endocytic structures known as tubulobulbar complexes (TBCs) internalize intact intercellular junction proteins at sites of intercellular attachment between spermatids and Sertoli cells prior to sperm release. TBCs are highly specialized clathrin‐based endocytosis machines. They also resemble podosomes which have an elongated membrane tube and an associated dendritic actin network. However, unlike podosomes that have a membrane core of a single plasma membrane, the core of TBCs consists of two plasma membranes: the membrane of the Sertoli cell which the structure is invaginated into and the membrane of a spermatid. TBCs have a bulb region in the distal third part of the complex which does not have the cuff of actin but is closely associated with a cisterna of endoplasmic reticulum (ER). We hypothesize that the plasma membrane/ER junction is involved in Ca2+ signaling. If this is true then we would expect to find molecular machinery involved in calcium signaling mechanisms in other systems. In this study, we used immunofluorescence to probe for the calcium pump, sarco/endoplasmic reticulum Ca2+ ‐ATPase 2 (SERCA2), which pumps Ca2+ into the ER. Cryosections and epithelial fragments from perfusion‐fixed rat testes were probed with antibodies against SERCA2 and an appropriate fluorescent secondary antibody. The SERCA2 antibody has a labeling pattern that extends from the base of the epithelium to the apex which is consistent with a Sertoli cell pattern. As seen in both epithelial fragments and cryosections, the SERCA2 antibody labels regions where we would expect TBCs to be. Our results show that one possible component of a calcium signaling mechanism is present in regions containing TBCs.Support or Funding InformationSupported by the Natural Sciences and Engineering Research Council of Canada Discovery Grants program.