Distribution of thimet oligopeptidase (EC 3.4.24.15) in human and rat testes

Thimet oligopeptidase (TOP:E.C. 3.4.24.15) is a thiol sensitive metalloendo peptidase which is widely distributed and active in most tissues including testis, brain and pituitary, In the median eminence it is postulated to pla y a role in the degradation of GnRH released from the hypothalamus and thus to modulate LH levels. In the rat and human, the testis is the richest sou rce of TOP activity with levels 3- to 5-fold higher than that of the brain. In order to define the exact localisation of this enzyme within the rat an d human testis, the distribution of TOP in the developing and adult gonad w as examined in situ and in isolated cells by immunohistochemistry, western blotting and northern blotting analysis. Ontogeny studies have demonstrated that TOP is detectable by western blotti ng from 9 days with levels of expression increasing with the age of the ani mal. Immunolocalisation of the protein in the interstitium was positive fro m 9 days onwards but was negative within the seminiferous tubules before 35 days of age, whereas TOP mRNA was not detected within the testis until 35 days of age with subsequent stable expression levels up to 90 days. In the adult rat testis, a strong TOP immunoreactivity was observed within seminif erous tubules, in elongating and elongated spermatids and residual bodies. In the interstitial compartment, immunoreactivity was also observed in Leyd ig cells and throughout the interstitial space. Western blot analyses confi rmed the distribution of expression observed using immunochemistry, however Leydig cells display a lower signal than expected from the immunohistochem ical data, Northern hybridization showed that the transcript is present in pachytene spermatocytes, early spermatids, and residual bodies, whereas its presence was not observed in Leydig cells probably due to very low levels of expression of the message. Analyses of various human tissue extracts sho wed that the testis displays the highest levels of TOP mRNA, with immunohis tochemical experiments revealing that, as in the rat, the protein is princi pally expressed in elongated spermatids/residual bodies, and in Leydig cell s. It is concluded that in the human and rat testes, TOP is highly expressed, in particular in post-meiotic germ cells and Leydig cells. The possible inv olvement of TOP in proteolytic events associated with the process of spermi ogenesis and Leydig cell function is currently under investigation.