The enzyme SMPDL3b in podocytes decouples proteinuria from chronic kidney disease progression in experimental Alport Syndrome.

Abstract

Background: Chronic kidney disease, including Alport Syndrome, is linked to collagen type IV mutations, lipid dysmetabolism, and altered sphingolipid pathways, with no targeted therapies currently available. Sphingomyelin phosphodiesterase acid-like 3b (SMPDL3b), a key regulator of sphingolipid metabolism and membrane receptor organization in podocytes, may drive disease via ceramide and sphingosine-1-phosphate pathways. This study tested whether altered SMPDL3b expression contributes to glomerular injury and renal decline in Alport Syndrome.

Methods: Archived Alport Syndrome human biopsies were used for immunohistochemistry and NanoString re-analysis of SMPDL3b. Murine podocytes isolated from mouse models of Alport Syndrome were profiled using Illumina. Mouse models of Alport Syndrome and models with either podocyte-specific deletion or inducible overexpression of Smpdl3b were generated to assess renal function using liquid chromatography-mass spectrometry, matrix-assisted laser desorption ionization-mass spectrometry imaging and atomic force microscopy.

Results: We found a three-fold increase in SMPDL3b expression in glomeruli, tubules and murine podocytes isolated from Col4a3 knockout mice. Increased SMPDL3b expression occurred in association with alterations affecting kidney sphingolipid metabolism, increased glomerular but not tubular sphingosine-1-phosphate levels and reduced glomerular basement membrane and podocyte stiffness. Podocyte-specific Smpdl3b deletion in Col4a3 knockout mice was sufficient to restore sphingosine-1-phosphate levels, to reduce proteinuria, podocyte foot process effacement, and improve glomerular basement membrane and podocyte stiffness, but not sufficient to protect from kidney failure.

Conclusions: Our study suggests that SMPDL3b may be a key modulator of proteinuria and podocyte integrity in Alport Syndrome, decoupling proteinuria from kidney failure, and suggests that improvement of glomerular structure and function may not always translate into protection from chronic kidney disease progression.