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The lipid sphingosine 1-phosphate (S1P) is a potent neuroprotective signalling molecule that signals through its own family of five G-protein coupled receptors. S1P signalling enhances presynaptic glutamate release and is essential for neural development. S1P is synthesized by the enzymes sphingosine kinases 1 and 2 (SPHK1 and SPHK2), of which SPHK2 mRNA and activity is more abundant in the brain. In this study we investigated the consequences of global SphK2 knockout (SphK2-/-) on basic motor capabilities, anxiety, learning, and memory in mice, using a range of tests including the elevated plus maze, the cheeseboard, contextual and cued fear conditioning, and fear extinction. Loss of SphK2 resulted in an 85-90% reduction in brain S1P levels, and was associated with a notably higher freezing response in a novel context. SphK2 knockout mice also exhibited increased contextual fear conditioning but the extinction of contextual fear memory was similar to control mice. SphK2-/- mice, contrary to their control littermates, did not respond to cue presentation with increased freezing. Anxiety measures in the elevated plus maze were not different between SphK2-/- mice and control littermates. Also, knockout mice showed no deficits in neurological reflexes or motor functions, and performed as well as their control littermates in the spatial memory test. Our findings demonstrate that SphK2 is responsible for the vast majority of S1P synthesis in the mouse brain, and plays a role in freezing responses as evaluated in the fear conditioning paradigm.

SUMMARY Berardinelli-Seip congenital lipodystrophy 2 (BSCL2) is caused by loss-of-function mutations in SEIPIN, a protein implicated in both adipogenesis and lipid droplet expansion but whose molecular function remains obscure. Here, we identify physical and functional interactions between SEIPIN and microsomal isoforms of glycerol-3-phosphate acyltransferase (GPAT) in multiple organisms. Compared to controls, GPAT activity was elevated in SEIPIN-deficient cells and tissues and GPAT kinetic values were altered. Increased GPAT activity appears to underpin the block in adipogenesis and abnormal lipid droplet morphology associated with SEIPIN loss. Over-expression of Gpat3 blocked adipogenesis, and Gpat3 knockdown in SEIPIN-deficient preadipocytes partially restored differentiation. GPAT overexpression in yeast, preadipocytes, and fly salivary glands also formed supersized lipid droplets. Finally, pharmacological inhibition of GPAT in Seipin−/− mouse preadipocytes partially restored adipogenesis. These data identify SEIPIN as an evolutionarily conserved regulator of microsomal GPAT and suggest that GPAT inhibitors might be useful for the treatment of human BSCL2 patients., In Brief Pagac et al. find that SEIPIN, which has been linked to Berardinelli-Seip congenital lipodystrophy 2, interacts with microsomal glycerol-3-phosphate acyltransferase (GPAT) and influences its activity. Increased GPAT activity appears to underlie the block in adipogenesis and abnormal lipid droplet morphology associated with SEIPIN loss.