INDUCTION OF DIFFERENTIATION IN CULTURED RAT AND HUMAN PODOCYTES

Mature podocytes are highly differentiated cells that are unable to di vide in vivo. During glomerulogenesis, podocytes develop from simple c uboidal cells into their adult phenotype, which is characterized by a complex pattern of processes. Cultivation of podocytes under standard conditions leads to dedifferentiation, including the loss of processes and of pp44, a marker of differentiated podocytes. In this study, the cell culture conditions for rat and human podocytes were modified by avoiding repeated subcultivation. This led to profound phenotypic chan ges in podocytes in vitro. The conversion of cobblestones into arboriz ed cells was directly observed, and a series of intermediate phenotype s was documented. The cells converted within 3 wk from typical cobbles tone appearance into individual arborized cells more closely resemblin g in vivo podocytes. Arborized cells were frequently binucleated and r eached a size of up to 500 mu m Both cobblestone and arborized cells o riginated from podocytes, as evidenced by the expression of a podocyte -specific O-acetylated ganglioside and of the WT-1 protein. In contras t to primary cultures and early passages of cobblestones, a cloned rat podocyte cell line did not express WT-1 and could not be induced to d ifferentiate into arborized cells. This finding indicates a role for W T-1 in maintaining differentiation of adult podocytes. The differentia tion of arborized cells led to growth arrest and was reflected by the formation of processes and the expression of pp44 and desmin, which we re never detected in cobblestones. It was concluded that partial diffe rentiation of cultured podocytes can be achieved simply by avoiding re peated subcultivation, resulting in an arborized phenotype more closel y reflecting in vivo podocytes.