Redirection of sphingolipid metabolism drives cytoskeletal defects in SPLIS and reveals ROCK inhibition as therapy.

Abstract

Sphingosine-1-phosphate lyase (SPL) insufficiency syndrome (SPLIS) or nephrotic syndrome type 14 (NPHS14), is an autosomal recessive multisystem disorder caused by loss-of-function mutations in SGPL1, encoding the enzyme responsible for the terminal degradation of sphingosine-1-phosphate (S1P). We investigated a patient carrying a previously undescribed c.1084T>A (p.Ser362Thr) SGPL1 variant and analyzed the metabolic and cellular consequences of SPL deficiency using patient fibroblasts, SGPL1-knockout HEK293T cells, and Sgpl1-/- and Sgpl1rosa+fl/fl mice. Metabolic stable isotope labelling revealed that SPL deficiency does not invariably result in S1P accumulation. Instead, SPL-deficient cells maintain near-normal S1P levels through (i) feedback regulation of de novo sphingolipid synthesis via the ORMDL-ceramide axis and (ii) increased diversion of excess ceramides into glycosphingolipids. However, perturbation of sphingolipid homeostasis - either by exogenous sphingolipid load or disruption of compensatory regulation - induces pathological intracellular S1P accumulation. In vivo, Sgpl1-/- mice exhibited pronounced urinary S1P excretion and renal S1P enrichment, accompanied by cytoskeletal disorganization and impaired epithelial morphogenesis. Mechanistically, we identify aberrant Rho-ROCK signaling as a key mediator of S1P-driven cytoskeletal dysregulation. Pharmacological ROCK inhibition with Fasudil mitigated renal cytoskeletal defects in Sgpl1-/- and Sgpl1rosa+fl/fl mice and partially restored epithelial architecture. These findings redefine the metabolic consequences of SPL deficiency and identify S1P-driven Rho-ROCK hyperactivation as a tractable therapeutic target in SPLIS.