Muscle-specific lack of GFPT1 in knock-in mice triggers ER stress to alleviate misfolded proteins
Pathogenic variants in GFPT1, encoding a key enzyme to synthesize UDP-N-acetylglucosamine (UDP-GlcNAc), cause congenital myasthenic syndrome (CMS). We made a knock-in (KI) mouse model carrying a frameshift variant in Gfpt1 exon 9 simulating a CMS patient. As Gfpt1 exon 9 is exclusively included in striated muscles, Gfpt1-KI mice were deficient for Gfpt1 only in skeletal muscles. In Gfpt1-KI mice, (i) UDP-HexNAc, CMP-NeuAc, and protein O-GlcNAcylations were reduced in skeletal muscles; (ii) aged Gfpt1-KI mice showed poor exercise performance and abnormal neuromuscular junction structures; and (iii) markers for unfolded protein response (UPR) were elevated in skeletal muscles. Denervation-mediated enhancement of ER stress in Gfpt1-KI mice facilitated protein folding, ubiquitin-proteasome degradation, and apoptosis, whereas autophagy was not induced and protein aggregates were markedly increased. Lack of autophagy was accounted for by enhanced degradation of FoxO1 by increased Xbp1-s/u proteins. Similarly, in Gfpt1-silenced C2C12 myotubes, ER stress exacerbated protein aggregates and activated apoptosis, but autophagy was attenuated. In both skeletal muscles in Gfpt1-KI mice and Gfpt1-silenced C2C12 myotubes, maladaptive UPR failed to eliminate protein aggregates and provoked apoptosis.