GFPT1 deficiency in muscle leads to myasthenia and myopathy in mice.

Yasmin Issop,Joachim Weis,Denisa Hathazi,Hanns Lochmüller,Muzamil Majid Khan,Andreas Roos,Sally Spendiff,Clarke R Slater,Rüdiger Rudolf

doi:10.1093/hmg/ddy225

Abstract

Glutamine-fructose-6-phosphate transaminase 1 (GFPT1) is the rate-limiting enzyme in the hexosamine biosynthetic pathway which yields precursors required for protein and lipid glycosylation. Mutations in GFPT1 and other genes downstream of this pathway cause congenital myasthenic syndrome (CMS) characterized by fatigable muscle weakness owing to impaired neurotransmission. The precise pathomechanisms at the neuromuscular junction (NMJ) owing to a deficiency in GFPT1 is yet to be discovered. One of the challenges we face is the viability of Gfpt1−/− knockout mice. In this study, we use Cre/LoxP technology to generate a muscle-specific GFPT1 knockout mouse model, Gfpt1tm1d/tm1d, characteristic of the human CMS phenotype. Our data suggest a critical role for muscle derived GFPT1 in the development of the NMJ, neurotransmission, skeletal muscle integrity and highlight that a deficiency in skeletal muscle alone is sufficient to cause morphological postsynaptic NMJ changes that are accompanied by presynaptic alterations despite the conservation of neuronal GFPT1 expression. In addition to the conventional morphological NMJ changes and fatigable muscle weakness, Gfpt1tm1d/tm1d mice display a progressive myopathic phenotype with the presence of tubular aggregates in muscle, characteristic of the GFPT1-CMS phenotype. We further identify an upregulation of skeletal muscle proteins glypican-1, farnesyltransferase/geranylgeranyltransferase type-1 subunit α and muscle-specific kinase, which are known to be involved in the differentiation and maintenance of the NMJ. The Gfpt1tm1d/tm1d model allows for further investigation of pathophysiological consequences on genes and pathways downstream of GFPT1 likely to involve misglycosylation or hypoglycosylation of NMJs and muscle targets.

Highlights

Congenital myasthenic syndromes (CMS) are a heterogeneous group of inherited disorders of neurotransmission that are commonly categorized according to the pathophysiological involvement in the presynaptic compartment, synaptic cleft and the postsynaptic basal lamina of the neuromuscular junction (NMJ)
We further identify an upregulation of skeletal muscle proteins glypican-1, farnesyltransferase/geranylgeranyltransferase type-1 subunit a and muscle-specific kinase, which are known to be involved in the differentiation and maintenance of the Received: April 22, 2018
Owing to the ubiquitous nature of Glutamine-fructose-6-phosphate transaminase 1 (GFPT1) and its expression in the early stages of embryonic development, we confirmed that complete knockout of this protein results in embryonic lethality as homozygous GFPT1 knockout mice were never obtained through breeding heterozygous knockout mice

Summary

Introduction

Congenital myasthenic syndromes (CMS) are a heterogeneous group of inherited disorders of neurotransmission that are commonly categorized according to the pathophysiological involvement in the presynaptic compartment, synaptic cleft and the postsynaptic basal lamina of the neuromuscular junction (NMJ). Mutations in genes encoding ubiquitously expressed enzymes involved in glycosylation have been implicated in CMS (5 out of 29 CMS causing genes) [1,2,3,4,5,6,7,8,9]. GFPT1 is a ubiquitous protein that catalyses the conversion of fructose-6-phosphate to glucosamine-6-phosphate and glutamate via a transamidase reaction. This is the ratelimiting step in the hexosamine biosynthetic pathway which yields precursors required for N- and O-linked glycosylation of proteins [10]. Missense mutations in GFPT1 have been found outside of the muscle-specific exon, yet impaired function seems to be restricted to the muscle and NMJ

Methods

Results

Conclusion