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

Sphingosine-1-phosphate lyase (SPL) insufficiency syndrome (SPLIS), also known as nephrotic syndrome type 14, 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-KO HEK293T cells, and Sgpl1-/- and Sgpl1rosa+fl/fl mice. Metabolic stable isotope labeling revealed that SPL deficiency does not invariably result in S1P accumulation. Instead, SPL-deficient cells maintain near-normal S1P levels through (a) feedback regulation of de novo sphingolipid synthesis via the ORMDL-ceramide axis and (b) 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 had 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.