SMPDL3b in Podocytes: Decoupling Proteinuria from CKD Progression in Experimental Alport Syndrome

Alport Syndrome (AS) is a rare genetic disease with impaired production of collagen type IV alpha 3, 4 and 5 chains in the glomerular basement membranes (GBM), which results amongst others in progressive loss of kidney function. In AS, abnormalities in the GBM and associated podocyte detachment may potentially result from the dysregulation of sphingolipid metabolism. Here we investigated whether renal sphingomyelin phosphodiesterase acid-like 3b (SMPDL3b) overexpression modulates the generation of sphingosine-1-phosphate (S1P) and contributes to renal failure in Col4a3 knockout mice, a mouse model of AS. We found a 3-fold increase in SMPDL3b expression in glomeruli and murine podocytes isolated from Col4a3 knockout mice. Increased SMPDL3b expression occurred in association with increased glomerular S1P levels, while podocyte specific Smpdl3b deletion in Col4a3 knockout mice was sufficient to restore S1P levels and to reduce proteinuria and podocyte foot process but not sufficient to protect from renal failure. Thus, podocyte S1P may be a key modulator of proteinuria and podocyte integrity in AS. Our study in experimental AS suggests that SMPDL3b-derived S1P may dissociate proteinuria from renal failure, and suggests that improvement of glomerular structure and function may not always translate in protection from CKD progression in AS.