Regulation of sertoli cell myotubularin (rMTM) expression by germ cells invitro

Recent studies have shown that rat myotubularin (rMTM), the homolog of huma n myotubularin, which is a putative protein tyrosine phosphatase (PTP), is expressed by Sertoli cells in the rat testis. In addition, a significant in crease in its steady-state mRNA level was detected in Sertoli cells at the time of inter-Sertoli tight junction (TJ) assembly in vitro. Since the inte rplay of protein kinases and phosphatases that determines the intracellular phosphoprotein content can, in turn, regulate the assembly and maintenance of TJ and anchoring junctions (AJ) in vitro, as demonstrated in different cell types, such as Madin-Darby canine kidney (MDCK) cells, endothelial cel ls, and Sertoli cells, rMTM may be an important molecule in regulating the assembly and maintenance of inter-Sertoli TJs during spermatogenesis. We th us sought to characterize its regulation. During testicular maturation, it was shown that the rMTM steady-state mRNA level increased drastically with aging. The expression of rMTM increased by as much as 2-4-fold in the rat t estis at 45-60 days of age versus 20 days of age, coinciding with the onset of spermiation. This result seemingly suggests that rMTM may participate i n the release of spermatids by disassembling the Sertoli-spermatid AJs, sin ce PTP inhibitor was shown to perturb the inter-Sertoli TJ permeability bar rier in vitro. Unexpectedly, when Sertoli cells were isolated from 20-, 45- , and 90-day-old rats and the steady-state rMTM level was quantified. it wa s shown that there is a drastic reduction in rMTM expression in adult Serto li cells. Studies that used Sertoli-germ cell cocultures and Sertoli cells incubated with increasing germ cell-derived proteins have shown that the hi gh level of testicular rMTM expression in the testis might be maintained by germ cells. Although work remains to be done to delineate the role of rMTM in the testis, these results illustrate that germ cells play a very active rate in regulation testicular function by altering the phosphoprotein cont ent.