Cellular distribution of napsin (kidney-derived aspartic protease-like protein, KAP) mRNA in the kidney, lung and lymphatic organs of adult and developing mice

Kidney-derived aspartic protease-like protein (KAP), initially identified i n the mouse kidney, is a novel aspartic protease exclusively expressed in t he lung and spleen as well as the kidney. Its orthologues have been identif ied in the human and rat, and termed napsin. We performed in situ hybridiza tion analysis to determine the cellular expression of napsin mRNA in the ki dney, lung, and lymphatic organs of adult mice and to demonstrate, for the first time, its expression patterns in ontogeny. In the adult mouse kidney, extremely intense signals for napsin mRNA were observed in the proximal st raight and convoluted tubules, in agreement with a previous study. The firs t signals for napsin mRNA during nephrogenesis occurred selectively in meso nephric tubules at embryonic day 13, and in metanephric tubules from embryo nic day 14. In the lung, a distribution restricted to type II alveolar cell s or their precursors was found from embryonic day 15, at the onset of type II cell differentiation, to the adult stage. In the spleen, the mRNA was e xpressed in lymph nodules of the white pulp and the marginal zone namely, B -lymphocyte-rich regions - from postnatal day 0 to adult. The lymph node an d Peyer's patch displayed similar expression patterns, but T cell-dependent areas in these organs and the thymus lacked such signals. These findings s uggest that mouse napsin possesses crucial functional roles not only in the kidney but also in the lung and lymphatic tissues, even during fetal stage s.