Laminin-111 Restores Regenerative Capacity in a Mouse Model for α7 Integrin Congenital Myopathy

Mutations in the α7 integrin gene cause congenital myopathy characterized by delayed developmental milestones and impaired mobility. Previous studies in dystrophic mice suggest the α7β1 integrin may be critical for muscle repair. To investigate the role that α7β1 integrin plays in muscle regeneration, cardiotoxin was used to induce damage in the tibialis anterior muscle of α7 integrin-null mice. Unlike wild-type muscle, which responded rapidly to repair damaged myofibers, α7 integrin-deficient muscle exhibited defective regeneration. Analysis of Pax7 and MyoD expression revealed a profound delay in satellite cell activation after cardiotoxin treatment in α7 integrin-null animals when compared with wild type. We have recently demonstrated that the muscle of α7 integrin-null mice exhibits reduced laminin-α2 expression. To test the hypothesis that loss of laminin contributes to the defective muscle regeneration phenotype observed in α7 integrin-null mice, mouse laminin-111 (α1, β1, γ1) protein was injected into the tibialis anterior muscle 3 days before cardiotoxin-induced injury. The injected laminin-111 protein infiltrated the entire muscle and restored myogenic repair and muscle regeneration in α7 integrin-null muscle to wild-type levels. Our data demonstrate a critical role for a laminin-rich microenvironment in muscle repair and suggest laminin- 111 protein may serve as an unexpected and novel therapeutic agent for patients with congenital myopathies

Myotendinous Junction Defects and Reduced Force Transmission in Mice that Lack α7 Integrin and Utrophin

The α7β1 integrin, dystrophin, and utrophin glycoprotein complexes are the major laminin receptors in skeletal muscle. Loss of dystrophin causes Duchenne muscular dystrophy, a lethal muscle wasting disease. Duchenne muscular dystrophy-affected muscle exhibits increased expression of α7β1 integrin and utrophin, which suggests that these laminin binding complexes may act as surrogates in the absence of dystrophin. Indeed, mice that lack dystrophin and α7 integrin (mdx/α7−/−), or dystrophin and utrophin (mdx/utr−/−), exhibit severe muscle pathology and die prematurely. To explore the contribution of the α7β1 integrin and utrophin to muscle integrity and function, we generated mice lacking both α7 integrin and utrophin. Surprisingly, mice that lack both α7 integrin and utrophin (α7/utr−/−) were viable and fertile. However, these mice had partial embryonic lethality and mild muscle pathology, similar to α7 integrin-deficient mice. Dystrophin levels were increased 1.4-fold in α7/utr−/− skeletal muscle and were enriched at neuromuscular junctions. Ultrastructural analysis revealed abnormal myotendinous junctions, and functional tests showed a ninefold reduction in endurance and 1.6-fold decrease in muscle strength in these mice. The α7/utr−/− mouse, therefore, demonstrates the critical roles of α7 integrin and utrophin in maintaining myotendinous junction structure and enabling force transmission during muscle contraction. Together, these results indicate that the α7β1 integrin, dystrophin, and utrophin complexes act in a concerted manner to maintain the structural and functional integrity of skeletal muscle

GGPS1 Mutations Cause Muscular Dystrophy/Hearing Loss/Ovarian Insufficiency Syndrome

Objective: A hitherto undescribed phenotype of early onset muscular dystrophy associated with sensorineural hearing loss and primary ovarian insufficiency was initially identified in 2 siblings and in subsequent patients with a similar constellation of findings. The goal of this study was to understand the genetic and molecular etiology of this condition.Methods: We applied whole exome sequencing (WES) superimposed on shared haplotype regions to identify the initial biallelic variants in GGPS1 followed by GGPS1 Sanger sequencing or WES in 5 additional families with the same phenotype. Molecular modeling, biochemical analysis, laser membrane injury assay, and the generation of a Y259C knock-in mouse were done.Results: A total of 11 patients in 6 families carrying 5 different biallelic pathogenic variants in specific domains of GGPS1 were identified. GGPS1 encodes geranylgeranyl diphosphate synthase in the mevalonate/isoprenoid pathway, which catalyzes the synthesis of geranylgeranyl pyrophosphate, the lipid precursor of geranylgeranylated proteins including small guanosine triphosphatases. In addition to proximal weakness, all but one patient presented with congenital sensorineural hearing loss, and all postpubertal females had primary ovarian insufficiency. Muscle histology was dystrophic, with ultrastructural evidence of autophagic material and large mitochondria in the most severe cases. There was delayed membrane healing after laser injury in patient-derived myogenic cells, and a knock-in mouse of one of the mutations (Y259C) resulted in prenatal lethality.Interpretation: The identification of specific GGPS1 mutations defines the cause of a unique form of muscular dystrophy with hearing loss and ovarian insufficiency and points to a novel pathway for this clinical constellation. ANN NEUROL 2020;88:332–347