Nuclear envelope remodelling during rat spermiogenesis: distribution and expression pattern of LAP2/thymopoietins

Lamina-associated polypeptide 2 (LAP2) and the thymopoietins (TPs) are a family of proteins described in somatic cells of mammals, which are derived by alternative splicing from a single gene. For one of the members of the family (LAP2 = TPbeta) it has been shown that this integral membrane protein locates to the inner membrane of the nuclear envelope, and that it binds to chromatin and B-type lamins. In the present study, we observed that during the third phase of spermatogenesis (i.e. spermiogenesis), TP-labelling shifted progressively to one half of the nuclear periphery in round spermatids. In the elongating spermatid the signal then becomes restricted to one spot located at the posterior (centriolar) pole of the nucleus. Changes in localization are accompanied by the disappearance, first of TPgamma, and later on of LAP2/TPbeta. TPalpha is the only member of the family detectable in the mature sperm. Concomitantly, lamin B1, the only nuclear lamina protein known to be expressed in mammalian spermatids, showed a similar behaviour, i.e. shifted progressively to the centriolar pole of spermatid nuclei before it became undetectable in fully differentiated mature sperms. These results are the first demonstration that expression and localization patterns of TPs are coordinately and differentially regulated with lamins during a differentiation process.