Calponin is expressed by Sertoli cells within rat testes and is associated with actin-enriched cytoskeleton

Within seminiferous tubules, Sertoli cells form three types of actin-filament-containing cell junctions, viz., tight junctions, ectoplasmic specializations, and tubulobulbar complexes. These Sertoli cell junctions are involved in the formation of the blood-testis barrier, germ cell translocation, and the release of spermatozoa. Actin and actin-binding proteins are important for these functions. In the present study, a monoclonal antibody against human smooth-muscle-cell calponin detected a 38-kDa protein in a total protein extract of rat testis. The protein has a molecular weight identical to that of aorta calponin and binds calmodulin. Calponin mRNA was detected in the testis and cultured rat Sertoli cells by the reverse transcription/polymerase chain reaction method. Thus, the 38-kDa protein present in the testes is a basic isoform of calponin. In adult rats (70 days of age or older), calponin was detected within seminiferous tubules at sites of Sertoli cell junctions. Cultured Sertoli cells express calponin, whereas cultured spermatogonia do not. When rat testicular tissue was fractionated into cytosol, membrane, and cytoskeleton fractions, calponin was detected in all three fractions. Calcium, potassium, and okadaic acid reduced the amount of calponin associated with the cytoskeleton. These data suggest that Sertoli cells express calponin. The function of calponin within the testis is as yet unknown.