IP3R, TRPM6 and Homer1 are Localized to ER‐PM Contact Sites at Tubulobulbar Complexes in Rat Testis

Abstract

The seminiferous epithelium is the site of spermatogenesis; the cellular basis for male fertility. Large junction complexes between Sertoli cells act to control the movement of differentiating spermatogenic cells while those between Sertoli cells and late spermatids connect these late spermatids to the apex of the epithelium. During spermatogenesis, turnover of these junction complexes must occur so that the next generation of spermatogenic cells can move into the epithelium, and mature spermatozoa can be released into the seminiferous tubules. Structures called Tubulobulbar Complexes (TBCs) have been implicated in junction turnover due to their appearance during turnover and their necessity in allowing sperm release. TBCs likely drive the internalization of junction complexes but how Sertoli cells control the development and degradation of TBCs is unknown. TBC regulation may be a product of a well‐known yet functionally unestablished contact site between TBC bulbs and cisternae of ER. Because calcium exchange occurs at other ER‐plasma membrane contact sites such as those between ER and endosomes, we chose to probe for the IP3R calcium channel. Using bioinformatics, we found potential protein binding partners for IP3R in the form of the TRPM6 plasma membrane calcium channel and the homer1 scaffolding protein. Using a high‐resolution pre‐embedding immunoelectron microscopy technique we localized all three proteins to the TBC‐ER contact site. This data suggests the potential for scaffolding of IP3R to TRPM6 via homer1 at TBC‐ER contacts and opens up the possibility for calcium exchange as an important function of the ER at these sites.Support or Funding InformationSupported by a NSERC Discovery Grant (#155397‐2013) to AWV