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 seminiferou s epithelium showed a cell type-specific expression of hsp60. Immunost aining 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 othe r germ cell types were completely negative for hsp60. Stage-specific e xpression of hsp60 was determined from pooled segments of stage-specif ic microdissected tubules by a combination of Western blotting and pol ymerase chain reaction (PCR). High concentrations of hsp60 were found in stages I-V and IX-XIV, and low levels were detected in the other st ages, i.e., VI-VIII. In stages with high hsp60 expression, spermatogon ia divide mitotically, whereas in stages lacking mitosis, the hsp60 le vel was much weaker. In seminiferous epithelium, two different types o f mitochondria are present. Therefore, immunoelectron microscopy was u sed to differentiate these two morphologically distinct types of mitoc hondria. The crista type of mitochondria (e.g., in Sertoli cells and s permatogonia) reacted with the antibody against hsp60, whereas hsp60 w as negative in so-called ''condensed''-type mitochondria found in midp achytene spermatocytes and more advanced germ cells. It could be shown for the first time that expression of the hsp60 gene is regulated dur ing the cycle of the seminiferous epithelium. The results indicate tha t the gene product is primarily needed during the initial steps of spe rmatogenesis in which most of the cell divisions occur, while its expr ession 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 assemb ly machinery that generates further mitochondria for the dividing cell s.