Non-muscle cofilin is a component of tubulobulbar complexes in the testis

Tubulobulbar complexes are finger-like structures that form at the interface between maturing spermatids and Sertoli cells prior to sperm release and at the interface between two Sertoli cells near the base of the seminiferous epithelium. They originate in areas previously occupied by actin filament-associated intercellular adhesion plaques known as ectoplasmic specializations. Actin filaments also are associated with tubulobulbar complexes where they appear to form a network, rather than the tightly packed bundles found in ectoplasmic specializations. Cofilin, a calcium-independent actin-depolymerizing protein, previously has been identified in the testis, but has not been localized to specific structures in the seminiferous epithelium. To determine if cofilin is found in Sertoli cells and is concentrated at actin-rich structures, we reacted fixed frozen sections of rat testis, fixed fragmented tissue, and blots of seminiferous epithelium with pan-specific and non-muscle cofilin antibodies. in addition, GeneChip microarrays (Affymetrix, Santa Clara, CA) were utilized to determine the abundance of mRNA for all cofilin isoforms in Sertoli cells. Using the monoclonal pan-specific cofilin antibody, we found specific labeling exclusively at tubulobulbar complexes and not at ectoplasmic specializations. On one-dimensional (1D) Western blots this antibody reacted monospecifically with one band, and on 2D blots reacted with two dots, which we interpret as phosphorylated and nonphosphorylated forms of a single cofilin isotype. Messenger RNA for nonmuscle cofilin in Sertoli cells is about 8.5-fold higher than for muscle-type cofilin. To confirm that the non-muscle isoform of cofilin is present at tubulobulbar complexes, we used antibodies specific to non-muscle cofilin for immunofluorescent localization. As with the pan-specific antibody, we found that the nonmuscle cofilin antibody exclusively labeled tubulobulbar complexes. Results presented here indicate that non-muscle cofilin is concentrated at tubulobulbar complexes. Our results also indicate that cofilin is not concentrated at ectoplasmic specializations.