Large-Mediated Glycosylation of Dystroglycan in Skeletal Muscle Function.

Dystroglycan is a glycosylation-dependent extracellular matrix receptor implicated in a subset of muscle diseases associated with mutations in known or putative glycosyltransferases. The glycosylation-deficient muscular dystrophies share a common biochemical defect in the glycosylation of dystroglycan and result in a multi-system muscle disease that includes impairments in nerve function and brain architecture. Mutations in the putative glycosyltransferase LARGE cause muscular dystrophy in humans and in the LARGEmyd mouse. Although dystroglycan has essential functions in skeletal muscle, whether impaired glycosylation of dystroglycan is sufficient to explain all complex pathological features associated with LARGE-deficient muscular dystrophy is less clear. A novel transgenic mouse with muscle-specific overexpression of LARGE was generated and used in combination with the LARGEmyd mouse model in order to determine the selective consequence of enhanced or impaired dystroglycan function in skeletal muscle and neuronal tissues. Muscles from LARGEmyd animals were weaker than wild-type littermates but only fast-twitch muscles were susceptible to mechanical injury. Complementary to this result, overexpression of LARGE in normal skeletal muscle resulted in hyperglycosylation and enhanced function of dystroglycan, but only fast-twitch muscle demonstrated enhanced protection from mechanical injury. Furthermore, the identification of differential expression of dystroglycan and α7β1 integrin in fast-twitch versus slow-twitch muscle suggests that the two receptors may have fiber-type specific functions. Overexpression of LARGE in LARGEmyd animals resulted in complete amelioration of muscle disease as evidenced by an absence of muscle pathology and a restoration of contractile function. While deficits in neuromuscular transmission were observed in LARGEmyd animals, these deficits were fully rescued by expression of LARGE in muscle fibers in parallel with restoration of neuromuscular junction structure. This suggests that neurotransmission contributes to muscle weakness in LARGEmyd mice and that impaired neurotransmission can be restored via expression of LARGE exclusively in skeletal muscle. These results provide evidence for critical functions of LARGE-mediated glycosylation at both the lateral membrane and at the neuromuscular junction of skeletal muscle fibers and demonstrate that neuronal deficits associated with impaired dystroglycan function may be exacerbated by impaired muscle function and/or communication at the neuromuscular junction.Ph.D.Molecular and Integrative PhysiologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/91614/1/jgumerso_1.pd

The Dystrophin-Glycoprotein Complex in the Prevention of Muscle Damage

Muscular dystrophies are genetically diverse but share common phenotypic features of muscle weakness, degeneration, and progressive decline in muscle function. Previous work has focused on understanding how disruptions in the dystrophin-glycoprotein complex result in muscular dystrophy, supporting a hypothesis that the muscle sarcolemma is fragile and susceptible to contraction-induced injury in multiple forms of dystrophy. Although benign in healthy muscle, contractions in dystrophic muscle may contribute to a higher degree of muscle damage which eventually overwhelms muscle regeneration capacity. While increased susceptibility of muscle to mechanical injury is thought to be an important contributor to disease pathology, it is becoming clear that not all DGC-associated diseases share this supposed hallmark feature. This paper outlines experimental support for a function of the DGC in preventing muscle damage and examines the evidence that supports novel functions for this complex in muscle that when impaired, may contribute to the pathogenesis of muscular dystrophy

Dystrophin-glycoprotein complex: post-translational processing and dystroglycan function.

Muscular dystrophies are genetically diverse but share common phenotypic features of muscle weakness, degeneration, and progressive decline in muscle function. Previous work has focused on understanding how disruptions in the dystrophinglycoprotein complex result in muscular dystrophy, supporting a hypothesis that the muscle sarcolemma is fragile and susceptible to contraction-induced injury in multiple forms of dystrophy. Although benign in healthy muscle, contractions in dystrophic muscle may contribute to a higher degree of muscle damage which eventually overwhelms muscle regeneration capacity. While increased susceptibility of muscle to mechanical injury is thought to be an important contributor to disease pathology, it is becoming clear that not all DGC-associated diseases share this supposed hallmark feature. This paper outlines experimental support for a function of the DGC in preventing muscle damage and examines the evidence that supports novel functions for this complex in muscle that when impaired, may contribute to the pathogenesis of muscular dystrophy

Pias3 is necessary for dorso-ventral patterning and visual response of retinal cones but is not required for rod photoreceptor differentiation

Protein inhibitor of activated Stat 3 (Pias3) is implicated in guiding specification of rod and cone photoreceptors through post-translational modification of key retinal transcription factors. To investigate its role during retinal development, we deleted exon 2-5 of the mouse Pias3 gene, which resulted in complete loss of the Pias3 protein. Pias3−/− mice did not show any overt phenotype, and retinal lamination appeared normal even at 18 months. We detected reduced photopic b-wave amplitude by electroretinography following green light stimulation of postnatal day (P)21 Pias3−/− retina, suggesting a compromised visual response of medium wavelength (M) cones. No change was evident in response of short wavelength (S) cones or rod photoreceptors until 7 months. Increased S-opsin expression in the M-cone dominant dorsal retina suggested altered distribution of cone photoreceptors. Transcriptome profiling of P21 and 18-month-old Pias3−/− retina revealed aberrant expression of a subset of photoreceptor genes. Our studies demonstrate functional redundancy in SUMOylation-associated transcriptional control mechanisms and identify a specific, though limited, role of Pias3 in modulating spatial patterning and optimal function of cone photoreceptor subtypes in the mouse retina

Soleus muscle in glycosylation-deficient muscular dystrophy is protected from contraction-induced injury

The glycosylation of dystroglycan is required for its function as a high-affinity laminin receptor, and loss of dystroglycan glycosylation results in congenital muscular dystrophy. The purpose of this study was to investigate the functional defects in slow- and fast-twitch muscles of glycosylation-deficient Largemyd mice. While a partial alteration in glycosylation of dystroglycan in heterozygous Largemyd/+ mice was not sufficient to alter muscle function, homozygous Largemyd/myd mice demonstrated a marked reduction in specific force in both soleus and extensor digitorum longus (EDL) muscles. Although EDL muscles from Largemyd/myd mice were highly susceptible to lengthening contraction-induced injury, Largemyd/myd soleus muscles surprisingly showed no greater force deficit compared with wild-type soleus muscles even after five lengthening contractions. Despite no increased susceptibility to injury, Largemyd/myd soleus muscles showed loss of dystroglycan glycosylation and laminin binding activity and dystrophic pathology. Interestingly, we show that soleus muscles have a markedly higher sarcolemma expression of β1-containing integrins compared with EDL and gastrocnemius muscles. Therefore, we conclude that β1-containing integrins play an important role as matrix receptors in protecting muscles containing slow-twitch fibers from contraction-induced injury in the absence of dystroglycan function, and that contraction-induced injury appears to be a separable phenotype from the dystrophic pathology of muscular dystrophy