Podocytopathy and Nephrotic Syndrome in Mice with Podocyte-Specific Deletion of the Asah1 Gene: Role of Ceramide Accumulation in Glomeruli.

Lysosomal acid ceramidase (Ac) has been shown to be critical for ceramide hydrolysis and regulation of lysosome function and cellular homeostasis. In the present study, we generated a knockout mouse line (Asah1/Podo) with a podocyte-specific deletion of the alpha subunit (main catalytic subunit) of Ac. Although no significant morphologic changes in glomeruli were observed in these mice under light microscope, severe proteinuria and albuminuria were found in these podocyte-specific KO mice compared to control genotype littermates. Transmission electron microscopic analysis showed that podocytes of the KO mice had distinctive foot process effacement and microvillus formation. These functional and morphologic changes indicate the development of nephrotic syndrome in mice bearing the Asah1 podocyte-specific gene deletion. Ceramide accumulation determined by LC-MS/MS was demonstrated in isolated glomeruli of Asah1/Podo mice compared to their littermates. By cross breeding Asah1/Podo mice with Smpd1 mice, we also produced a double KO strain, Smpd1/Asah1/Podo, that also lacks Smpd1, the acid sphingomyelinase that hydrolyzes sphingomyelin to ceramide. These mice exhibited significantly lower levels of glomerular ceramide with decreased podocyte injury compared with Asah1/Podo mice. These results strongly suggest that lysosomal Ac in podocytes is essential for the maintenance of the structural and functional integrity of podocytes.