Protocadherin (alpha)3 Acts at Sites Distinct from Classic Cadherins in Rat Testis and Sperm

The testis expresses a variety of cadherin superfamily members including classic cadherins and protocadherins. This report describes the first localization of a protocadherin protein in testis and sperm. After cloning rat cDNAs for protocadherin (alpha)3 and (alpha)4, isoform-specific polyclonal antibodies were generated against protocadherin (alpha)3. Western blotting of rat testis showed that protocadherin (alpha)3 was solubilized completely by Triton X-100, in contrast to the adhesion junction components N-cadherin, ?-catenin, and p120 catenin. Corroborating this data, protocadherin (alpha)3 was immunolocalized to the spermatid acrosomal area, intercellular bridge, and flagellum, but not classic cadherin-based adhesion junctions. Acrosome-associated protocadherin (alpha)3 was first detected at step 8 of spermiogenesis, and this association remained on cauda epididymal sperm. Acrosome immunostaining was reduced, but present, in acrosome-reacted sperm. Spermatid intercellular bridges became positive for protocadherin (alpha)3 coincident with the appearance of plectin, occurring at spermiogenic steps 8 to 9, and elongate spermatid bridges remained positive throughout spermatogenesis. The developing flagellum was uniformly immunostained for protocadherin (alpha)3 up to approximately spermiogenic step 17. Subsequently, flagellar immunostaining was confined to the principal piece, and this pattern continued in cauda epididymal sperm. These data show that protocadherin (alpha)3 performs functions unique from classic cadherins in spermatogenesis and suggest a role for protocadherin (alpha)3 in organizing germ cell-specific structures including the intercellular bridge, flagellum, and acrosome.