Disrupted Sphingosine-1-Phosphate Homeostasis Drives Nephrotoxicity in Sphingosine-1-Phosphate Lyase Insufficiency Syndrome (SPLIS)

Sphingosine-1-phosphate lyase insufficiency syndrome (SPLIS), also known as nephrotic syndrome type 14 (NPHS14), is an autosomal recessive disorder characterized by renal, neurological, dermatological, endocrine, and immunological symptoms. This condition is caused by loss-of-function mutations in the SGPL1 gene, which encodes sphingosine-1-phosphate lyase (SGPL1p/SPL), the enzyme responsible for the terminal degradation of sphingosine-1-phosphate (S1P) in sphingolipid catabolism.

We investigated a novel case of SPLIS associated with a recently reported SGPL1 mutation ( c . 1084T>A; p . Ser362Thr ). Using stable isotope flux analyses, we demonstrated in patient-derived fibroblasts and HEK293T SGPL1 knockout models that SGPL1p deficiency does not consistently result in pathological S1P accumulation.

Instead, SPL-deficient cells are able to maintain steady-state S1P levels through two compensatory mechanisms:

Regulation of de novo sphingolipid synthesis via the ORMDL-ceramide axis.

Increased conversion of excess ceramides into glycosphingolipids.

However, when steady-state conditions are disrupted—either by external sphingolipid supplementation or by impairing homeostatic control—a pathological increase in intracellular S1P occurs in SPL-deficient cells.

In vivo, Sgpl1 -/-mice exhibited significant urinary excretion of S1P and marked S1P enrichment in the kidneys. This pathological accumulation of S1P dysregulates cytoskeletal homeostasis, impairing renal epithelial formation. Based on these findings, we hypothesize that the reabsorption of urinary S1P contributes to toxic renal accumulation, providing an explanation for the nephrotoxicity observed in SPLIS and its association with nephrotic syndrome.

Importantly, we found that the cytoskeletal disruptions could be mitigated by inhibiting the Rho-ROCK signaling pathway using the clinically approved inhibitor Fasudil. These findings illuminate the pathophysiological basis of SPLIS nephrotoxicity and propose a promising pharmacological intervention strategy.