Developmental expression of spermatid-specific thioredoxin-1 protein: transient association to the longitudinal columns of the fibrous sheath during sperm tail formation

The mammalian sperm tail presents a complex organization in which a number of additional structures, namely outer dense fibers and fibrous sheath, surround the central axoneme and are thought to regulate flagellar motility. We have previously described a novel member of the thioredoxin family of proteins with a spermatid specific expression pattern, spermatid-specific thioredoxin-1 (Sptrx-1). We report here the developmental analysis of Sptrx-1 expression during murine spermiogenesis. Immunocytochemical analysis of Sptrx-1 through the different steps of spermiogenesis in rat seminiferous tubule sections showed that its expression begins at step 9, gets progressively stronger until steps 14-16 (where a peak is reached), and then diminishes in steps 17 and 18 until practically no immunolabeling is detected in step 19 spermatid. During its transient expression in spermiogenesis, Sptrx-1 is most concentrated in the periaxonemal compartment of the tail of the elongating spermatid, except in the very last steps (steps 17-19), when periaxonemal labeling disappears and a residual buildup of Sptrx-1 occurs in the shrinking cytoplasmic lobe. Electron microscopic analysis by immunogold labeling pinpointed the localization of Sptrx-1 to the assembling longitudinal columns of the fibrous sheath, whereas the forming ribs of the fibrous sheath were unlabeled. Immunoblotting of isolated fibrous sheath and tails obtained from epididymal or ejaculated sperm of rat and human confirmed our immunocytochemical observation: Sptrx-1 is no longer a component of the mature fibrous sheath. To our knowledge, this is the first report of a protein that specifically associates to the fibrous sheath during development but does not become a permanent structural component. The expression pattern of Sptrx-1 during rat spermiogenesis suggests that it could be part of a nucleation center for the formation of the longitudinal columns and transverse ribs that bridge the latter.