Ca2+ signaling machinery is present at intercellular junctions and structures associated with junction turnover in rat Sertoli cells

The endoplasmic reticulum (ER) in Sertoli cells is a component of unique adhesion junctions (ectoplasmic specializations-ESs) and is closely associated with structures termed tubulobulbar complexes (TBCs) that internalize intercellular junctions during sperm release and during the translocation of spermatocytes through the blood-testis barrier. A role for the ER in Ca2+ regulation at ESs and TBCs has been suspected, but evidence for this function has proved elusive. Using electron microscopy, we define two new ER-plasma membrane (PM) contact sites in apical Sertoli cell processes. One of these sites occurs at TBCs where flattened lamellar cisternae of ER envelope the swollen bulb regions of the complexes, and where the gap between adjacent membranes is 12 nm. The other is at the periphery of apical processes where the gap between membranes is 13-14 nm. Using immunolocalization at the light and electron microscopic levels, we demonstrate that Ca2+ regulatory machinery is present at the ESs attached to spermatid heads, and at ER-PM contacts. Sarco/endoplasmic reticulum Ca2+-ATPase 2 (ATP2A2, SERCA2) is present at ESs; transient receptor potential channel subfamily M member 6 (TRPM6), Homer1 (HOMER1), and inositol 1,4,5-trisphosphate receptor (ITPR, IP3R) are present at ER-PM contacts associated with TBC bulbs; and stromal interacting molecule 1 (STIM1), Orai1 (ORAI1), and ATP2A2 are present at the ER-PM contacts around the margins of Sertoli cell apical processes. In Sertoli cells, the molecular machinery associated with ER generated Ca2+ fluxes is present in regions and structures directly related to junction remodeling-a process necessary for sperm release.