Abstract
The human lipodystrophy gene product Berardinelli-Seip congenital lipodystrophy 2/seipin has been implicated in adipocyte differentiation, lipid droplet (LD) formation, and motor neuron development. However, the molecular function of seipin and its disease-causing mutants remains to be elucidated. Here, we characterize seipin and its mis-sense mutants: N88S/S90L (both linked to motoneuron disorders) and A212P (linked to lipodystrophy) in cultured mammalian cells. Knocking down seipin significantly increases oleate incorporation into triacylglycerol (TAG) and the steady state level of TAG, and induces the proliferation and clustering of small LDs. By contrast, overexpression of seipin reduces TAG synthesis, leading to decreased formation of LDs. Expression of the A212P mutant, however, had little effect on LD biogenesis. Surprisingly, expression of N88S or S90L causes the formation of many small LDs reminiscent of seipin deficient cells. This dominant-negative effect may be due to the N88S/S90L-induced formation of inclusions where wild-type seipin can be trapped. Importantly, coexpression of wild-type seipin and the N88S or S90L mutant can significantly reduce the formation of inclusions. Finally, we demonstrate that seipin can interact with itself and its mutant forms. Our results provide important insights into the biochemical characteristics of seipin and its mis-sense mutants, and suggest that seipin may function to inhibit lipogenesis.
Highlights
The human lipodystrophy gene product Berardinelli-Seip congenital lipodystrophy 2/seipin has been implicated in adipocyte differentiation, lipid droplet (LD) formation, and motor neuron development
The role of seipin in TAG metabolism is somewhat confusing in mammalian cells; a reduction in TAG synthesis was observed in a study using seipin-deficient lymphoblastoid cells [16], whereas proliferation of LDs was observed in seipin deficient fibroblasts [18]
Quantitative PCR analysis revealed that siBSCL2-1 reduced seipin mRNA level by ف80% (Fig. 1A-a), which resulted in a two-fold increase in oleate-induced TAG formation (Fig. 1A-b)
Summary
The human lipodystrophy gene product Berardinelli-Seip congenital lipodystrophy 2/seipin has been implicated in adipocyte differentiation, lipid droplet (LD) formation, and motor neuron development. The molecular function of seipin and its disease-causing mutants remains to be elucidated. Knocking down seipin significantly increases oleate incorporation into triacylglycerol (TAG) and the steady state level of TAG, and induces the proliferation and clustering of small LDs. By contrast, overexpression of seipin reduces TAG synthesis, leading to decreased formation of LDs. Expression of the A212P mutant, had little effect on LD biogenesis. Expression of N88S or S90L causes the formation of many small LDs reminiscent of seipin deficient cells. This dominantnegative effect may be due to the N88S/S90L-induced formation of inclusions where wild-type seipin can be trapped. Coexpression of wild-type seipin and the N88S or S90L mutant can significantly reduce the formation of inclusions.
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