Expression of mitochondrial heat shock protein 60 in distinct cell types and defined stages of rat seminiferous epithelium.

Changes in the level of the gene transcript of heat shock protein (hsp)60, a mitochondrial chaperonin, during the cycle of rat seminiferous epithelium and its cellular localization were studied. The seminiferous epithelium showed a cell type-specific expression of hsp60. Immunostaining of adult rat testis revealed localization in Sertoli and Leydig cells. In germ cells, mitochondria of spermatogonia and early primary spermatocytes were immunoreactive for hsp60. Mitochondria of all other germ cell types were completely negative for hsp60. Stage-specific expression of hsp60 was determined from pooled segments of stage-specific microdissected tubules by a combination of Western blotting and polymerase chain reaction (PCR). High concentrations of hsp60 were found in stages I-V and IX-XIV, and low levels were detected in the other stages, i.e., VI-VIII. In stages with high hsp60 expression, spermatogonia divide mitotically, whereas in stages lacking mitosis, the hsp60 level was much weaker. In seminiferous epithelium, two different types of mitochondria are present. Therefore, immunoelectron microscopy was used to differentiate these two morphologically distinct types of mitochondria. The crista type of mitochondria (e.g., in Sertoli cells and spermatogonia) reacted with the antibody against hsp60, whereas hsp60 was negative in so-called "condensed"-type mitochondria found in midpachytene spermatocytes and more advanced germ cells. It could be shown for the first time that expression of the hsp60 gene is regulated during the cycle of the seminiferous epithelium. The results indicate that the gene product is primarily needed during the initial steps of spermatogenesis in which most of the cell divisions occur, while its expression during the differentiation of spermatids and sperm is obviously not necessary. The presence of hsp60 in stages with mitotic activity suggests a very active mitochondrial protein import and protein assembly machinery that generates further mitochondria for the dividing cells.