Mammalian muscle-specific isoform of GFPT1: Acquisition of splicing regulation in evolution and its roles in glucose metabolisms and neuromuscular junction
Glutamine:fructose-6-phosphate transaminase 1 (GFPT1) is the rate-limiting enzyme of the hexosamine biosynthetic pathway (HBP). A 54-bp exon 9 of GFPT1 is specifically included in skeletal and cardiac muscles to generate a long isoform of GFPT1 (GFPT1-L). The splicing regulation of GFPT1 exon 9 and the role of GFPT1-L remain to be determined. Disruption of splicing cis-elements, artificial tethering of RNA-binding proteins, and isolation of an early spliceosomal complex revealed that SRSF1 and Rbfox1/2 cooperatively enhance, and hnRNP H/F suppresses, the inclusion of human GFPT1 exon 9 by modulating recruitment of U1 snRNP. The roles of GFPT1-L in skeletal muscle were analyzed in Gfpt1 exon 9 knockout (KO) mice. First, lack of GFPT1-L in skeletal muscle increased the amounts of GFPT1 and UDP-HexNAc ~4.5-fold and ~2.8-fold, respectively. Second, metabolomic analysis of skeletal muscle showed that the increased flux to the HBP in KO mouse was likely to suppress the glycolytic pathway and enhance the mitochondrial electron transport chain. Third, aged KO mice showed impaired glucose uptake in response to insulin. Fourth, aged KO mice showed muscle weakness and fatigue, small and fragmented acetylcholine receptor clusters, and simplified postsynaptic junctional folds, which were similar to, but less severe than, those observed in congenital myasthenic syndrome due to GFPT1 mutations. Taken together, GFPT1-L is likely to be acquired in evolution in mammalian striated muscles to attenuate the HBP for efficient glycolytic energy production, insulin-mediated glucose uptake, and the formation and maintenance of the neuromuscular junction.