Abnormal Skeletal Muscle Regeneration plus Mild Alterations in Mature Fiber Type Specification in Fktn-Deficient Dystroglycanopathy Muscular Dystrophy Mice

Abstract

Glycosylated α-dystroglycan provides an essential link between extracellular matrix proteins, like laminin, and the cellular cytoskeleton via the dystrophin-glycoprotein complex. In secondary dystroglycanopathy muscular dystrophy, glycosylation abnormalities disrupt a complex O-mannose glycan necessary for muscle structural integrity and signaling. Fktn-deficient dystroglycanopathy mice develop moderate to severe muscular dystrophy with skeletal muscle developmental and/or regeneration defects. To gain insight into the role of glycosylated α-dystroglycan in these processes, we performed muscle fiber typing in young (2, 4 and 8 week old) and regenerated muscle. In mice with Fktn disruption during skeletal muscle specification (Myf5/Fktn KO), newly regenerated fibers (embryonic myosin heavy chain positive) peaked at 4 weeks old, while total regenerated fibers (centrally nucleated) were highest at 8 weeks old in tibialis anterior (TA) and iliopsoas, indicating peak degeneration/regeneration activity around 4 weeks of age. In contrast, mature fiber type specification at 2, 4 and 8 weeks old was relatively unchanged. Fourteen days after necrotic toxin-induced injury, there was a divergence in muscle fiber types between Myf5/Fktn KO (skeletal-muscle specific) and whole animal knockout induced with tamoxifen post-development (Tam/Fktn KO) despite equivalent time after gene deletion. Notably, Tam/Fktn KO retained higher levels of embryonic myosin heavy chain expression after injury, suggesting a delay or abnormality in differentiation programs. In mature fiber type specification post-injury, there were significant interactions between genotype and toxin parameters for type 1, 2a, and 2x fibers, and a difference between Myf5/Fktn and Tam/Fktn study groups in type 2b fibers. These data suggest that functionally glycosylated α-dystroglycan has a unique role in muscle regeneration and may influence fiber type specification post-injury.

Mitochondrial dysfunction in skeletal muscle of fukutin deficient mice is resistant to exercise- and AICAR-induced rescue

“…Our group has previously published a report with histological evidence suggesting that KO mice have abnormal muscle regeneration following severe muscle injury 8 . Thus, to determine whether the histological alterations in KO mice translated to functional deficits in recovery and to see if AICAR could help correct any abnormal recovery, we measured muscle function in LM and KO mice with and without AICAR treatment following an injury.…”

“…Seven-micron tissue cryosections were mounted on microscope slides and processed for immunofluorescence analysis of centrally located nuclei (CNF) and embryonic myosin heavy chain (eMHC) as similar to previous work 8…”

“…Mutations in the FKTN gene have been directly linked to αDG dysfunction and subsequent muscle dystroglycanopathy 7 . A fukutin knockout mouse (KO) model accurately recapitulates many of the characteristics observed in human dystroglycanopathies as the muscles are characterized by chronic weakness, a perpetual cycle of injury and repair, and abnormal muscle regeneration 8,9 . That said, several pathological features of dystroglycanopathies remain undefined, and herein we sought to interrogate mitochondrial function based on our previous work which eluded to possible mitochondrial alterations in fukutin KO muscle 10 .…”

Distribution of myosin heavy chain isoforms in muscular dystrophy: Insights into disease pathology

“…MyHC fiber type analyses were recently reported in a non-dystrophin DGC muscular dystrophy, namely the Fktn-deficient model of dystroglycan glycosylation-deficient muscular dystrophy. In postnatal through juvenile development, there were relatively minor shifts in the timing of type 1 fiber post-natal downregulation and in the upregulation of type 2x fibers that were resolved by 8 weeks of age [16] . Similarly, analysis of MyHC isoforms during myogenesis in a model of FKRP-deficient dystroglycan-related muscular dystrophy, found that slow MyHC was not different between knockout and wild-type animals during primary myogenesis or at birth in either the EDL or the TA muscles [17] .…”

“…In contrast, a toxin-induced injury in the TA muscle caused various MyHC remodeling, as quantified two weeks after injury. In all Fktn-knockout and littermate mice studied, type 2x fibers were significantly reduced, indicating a common failure in all muscle to remodel as MyHC 2x or that such specification takes more than two weeks to occur [16] . A unique feature of the study was its use of two different Cre models for conditional knockout of the Fktn gene: Myf5-cre/Fktn, in which the Fktn gene is disrupted during skeletal muscle development; versus Tam-cre/Fktn, in which the Fktn gene is disrupted in all cells of the mouse following tamoxifen administration to 6 week old mice (post-development).…”