A rapid PCR method for genotyping the Large(myd) mouse, a model of glycosylation-deficient congenital muscular dystrophy

Dmdmdx/Largemyd: a new mouse model of neuromuscular diseases useful for studying physiopathological mechanisms and testing therapies

Although muscular dystrophies are among the most common human genetic disorders, there are few treatment options available. Animal models have become increasingly important for testing new therapies prior to entering human clinical trials. The Dmd^mdx mouse is the most widely used animal model for Duchenne muscular dystrophy (DMD), presenting the same molecular and protein defect as seen in humans with the disease. However, this mouse is not useful for clinical trials because of its very mild phenotype. The mouse model for congenital myodystrophy type 1D, Large^myd, harbors a mutation in the glycosyltransferase Large gene and displays a severe phenotype. To help elucidate the role of the proteins dystrophin and LARGE in the organization of the dystrophin-glycoprotein complex in muscle sarcolemma, we generated double-mutant mice for the dystrophin and LARGE proteins. The new Dmd^mdx/Large^myd mouse model is viable and shows a severe phenotype that is associated with the lack of dystrophin in muscle. We tested the usefulness of our new mouse model for cell therapy by systemically injecting them with normal murine mesenchymal adipose stem cells (mASCs). We verified that the mASCs were hosted in the dystrophic muscle. The new mouse model has proven to be very useful for the study of several other therapies, because injected cells can be screened both through DNA and protein analysis. Study of its substantial muscle weakness will also be very informative in the evaluation of functional benefits of these therapies.

Central nervous system involvement in the animal model of myodystrophy

Congenital muscular dystrophies present mutated gene in the LARGE mice model and it is characterized by an abnormal glycosylation of α-dystroglycan (α-DG), strongly implicated as having a causative role in the development of central nervous system abnormalities such as cognitive impairment seen in patients. However, the pathophysiology of the brain involvement remains unclear. Therefore, the objective of this study is to evaluate the oxidative damage and energetic metabolism in the brain tissue as well as cognitive involvement in the LARGE((myd)) mice model of muscular dystrophy. With this aim, we used adult homozygous, heterozygous, and wild-type mice that were divided into two groups: behavior and biochemical analyses. In summary, it was observed that homozygous mice presented impairment to the habituation and avoidance memory tasks; low levels of brain-derived neurotrophic factor (BDNF) in the prefrontal cortex, hippocampus, cortex and cerebellum; increased lipid peroxidation in the prefrontal cortex, hippocampus, striatum, and cerebellum; an increase of protein peroxidation in the prefrontal cortex, hippocampus, striatum, cerebellum, and cortex; a decrease of complex I activity in the prefrontal cortex and cerebellum; a decrease of complex II activity in the prefrontal cortex and cerebellum; a decrease of complex IV activity in the prefrontal cortex and cerebellum; an increase in the cortex; and an increase of creatine kinase activity in the striatum and cerebellum. This study shows the first evidence that abnormal glycosylation of α-DG may be affecting BDNF levels, oxidative particles, and energetic metabolism thus contributing to the memory storage and restoring process.

RPTPζ/phosphacan is abnormally glycosylated in a model of muscle-eye-brain disease lacking functional POMGnT1

Congenital muscular dystrophies (CMDs) with associated brain abnormalities are a group of disorders characterized by muscular dystrophy and brain and eye abnormalities that are frequently caused by mutations in known or putative glycotransferases involved in protein O-mannosyl glycosylation. Previous work identified α-dystroglycan as the major substrate for O-mannosylation and its altered glycosylation the major cause of these disorders. However, work from several labs indicated that other proteins in the brain are also O-mannosylated and therefore could contribute to CMD pathology in patients with mutations in the protein O-mannosylation pathway, however few of these proteins have been identified and fully characterized in CMDs. In this study we identify receptor protein tyrosine phosphatase ζ (RPTPζ) and its secreted variant, phosphacan, as another potentially important substrate for protein O-mannosylation in the brain. Using a mouse model of muscle-eye-brain disease lacking functional protein O-mannose β-1,2-N-acetylglucosaminyltransferase (POMGnT1), we show that RPTPζ/phosphacan is shifted to a lower molecular weight and distinct carbohydrate epitopes normally detected on the protein are either absent or substantially reduced, including Human Natural Killer-1 (HNK-1) reactivity. The spatial and temporal expression patterns of these O-mannosylated forms of RPTPζ/phosphacan and its hypoglycosylation and loss of HNK-1 glycan epitopes in POMGnT1 knockouts are suggestive of a role in the neural phenotypes observed in patients and animal models of CMDs.

Differential expression of genes involved in the degeneration and regeneration pathways in mouse models for muscular dystrophies

The genetically determined muscular dystrophies are caused by mutations in genes coding for muscle proteins. Differences in the phenotypes are mainly the age of onset and velocity of progression. Muscle weakness is the consequence of myofiber degeneration due to an imbalance between successive cycles of degeneration/regeneration. While muscle fibers are lost, a replacement of the degraded muscle fibers by adipose and connective tissues occurs. Major investigation points are to elicit the involved pathophysiological mechanisms to elucidate how each mutation can lead to a specific degenerative process and how the regeneration is stimulated in each case. To answer these questions, we used four mouse models with different mutations causing muscular dystrophies, Dmd (mdx), SJL/J, Large (myd) and Lama2 (dy2J) /J, and compared the histological changes of regeneration and fibrosis to the expression of genes involved in those processes. For regeneration, the MyoD, Myf5 and myogenin genes related to the proliferation and differentiation of satellite cells were studied, while for degeneration, the TGF-β1 and Pro-collagen 1α2 genes, involved in the fibrotic cascade, were analyzed. The result suggests that TGF-β1 gene is activated in the dystrophic process in all the stages of degeneration, while the activation of the expression of the pro-collagen gene possibly occurs in mildest stages of this process. We also observed that each pathophysiological mechanism acted differently in the activation of regeneration, with distinctions in the induction of proliferation of satellite cells, but with no alterations in stimulation to differentiation. Dysfunction of satellite cells can, therefore, be an important additional mechanism of pathogenesis in the dystrophic muscle.

Animal models for genetic neuromuscular diseases

The neuromuscular disorders are a heterogeneous group of genetic diseases, caused by mutations in genes coding sarcolemmal, sarcomeric, and citosolic muscle proteins. Deficiencies or loss of function of these proteins leads to variable degree of progressive loss of motor ability. Several animal models, manifesting phenotypes observed in neuromuscular diseases, have been identified in nature or generated in laboratory. These models generally present physiological alterations observed in human patients and can be used as important tools for genetic, clinic, and histopathological studies. The mdx mouse is the most widely used animal model for Duchenne muscular dystrophy (DMD). Although it is a good genetic and biochemical model, presenting total deficiency of the protein dystrophin in the muscle, this mouse is not useful for clinical trials because of its very mild phenotype. The canine golden retriever MD model represents a more clinically similar model of DMD due to its larger size and significant muscle weakness. Autosomal recessive limb-girdle MD forms models include the SJL/J mice, which develop a spontaneous myopathy resulting from a mutation in the Dysferlin gene, being a model for LGMD2B. For the human sarcoglycanopahties (SG), the BIO14.6 hamster is the spontaneous animal model for delta-SG deficiency, whereas some canine models with deficiency of SG proteins have also been identified. More recently, using the homologous recombination technique in embryonic stem cell, several mouse models have been developed with null mutations in each one of the four SG genes. All sarcoglycan-null animals display a progressive muscular dystrophy of variable severity and share the property of a significant secondary reduction in the expression of the other members of the sarcoglycan subcomplex and other components of the Dystrophin-glycoprotein complex. Mouse models for congenital MD include the dy/dy (dystrophia-muscularis) mouse and the allelic mutant dy(2J)/dy(2J) mouse, both presenting significant reduction of alpha2-laminin in the muscle and a severe phenotype. The myodystrophy mouse (Large(myd)) harbors a mutation in the glycosyltransferase Large, which leads to altered glycosylation of alpha-DG, and also a severe phenotype. Other informative models for muscle proteins include the knockout mouse for myostatin, which demonstrated that this protein is a negative regulator of muscle growth. Additionally, the stress syndrome in pigs, caused by mutations in the porcine RYR1 gene, helped to localize the gene causing malignant hypertermia and Central Core myopathy in humans. The study of animal models for genetic diseases, in spite of the existence of differences in some phenotypes, can provide important clues to the understanding of the pathogenesis of these disorders and are also very valuable for testing strategies for therapeutic approaches.

Distribution of potassium ion and water permeable channels at perivascular glia in brain and retina of the Large(myd) mouse

The dystroglycan protein complex provides a link between the cytoskeleton and the extracellular matrix (ECM). Defective O-glycosylation of alpha-dystroglycan (alpha-DG) severs this link leading to muscular dystrophies named dystroglycanopathies. These are characterized not only by muscle degeneration, but also by brain and ocular defects. In brain and retina, alpha-DG and ECM molecules are enriched around blood vessels where they may be involved in localizing the inwardly rectifying potassium channel, Kir4.1, and aquaporin channel, AQP4, to astrocytic endfeet. To investigate in vivo the role of ECM ligand-binding to glycosylated sites on alpha-DG in the polarized distribution of these channels, we used the Large(myd) mouse, an animal model for dystroglycanopathies. We found that Kir4.1 and AQP4 are lost from astrocytic endfeet in brain whereas significant labeling for these channels is detected at similar cell domains in retina. Furthermore, while both alpha- and beta1-syntrophins are lost from perivascular astrocytes in brain, labeling for beta1-syntrophin is found in retina of the Large(myd) mouse. These findings show that while ligand-binding to the highly glycosylated isoform of alpha-DG in concert with alpha- and beta1-syntrophins is crucial for the polarized distribution of Kir4.1 and AQP4 to functional domains in brain, distinct mechanisms may contribute to their localization in retina.

Intragenic deletion in the LARGE gene causes Walker-Warburg syndrome

Intragenic homozygous deletions in the Large gene are associated with a severe neuromuscular phenotype in the myodystrophy (myd) mouse. These mutations result in a virtual lack of glycosylation of alpha-dystroglycan. Compound heterozygous LARGE mutations have been reported in a single human patient, manifesting with mild congenital muscular dystrophy (CMD) and severe mental retardation. These mutations are likely to retain some residual LARGE glycosyltransferase activity as indicated by residual alpha-dystroglycan glycosylation in patient cells. We hypothesized that more severe LARGE mutations are associated with a more severe CMD phenotype in humans. Here we report a 63-kb intragenic LARGE deletion in a family with Walker-Warburg syndrome (WWS), which is characterized by CMD, and severe structural brain and eye malformations. This finding demonstrates that LARGE gene mutations can give rise to a wide clinical spectrum, similar as for other genes that have a role in the post-translational modification of the alpha-dystroglycan protein.

Stable overexpression of human macrophage cholesteryl ester hydrolase results in enhanced free cholesterol efflux from human THP1 macrophages

Reduction of the lipid burden of atherosclerotic lesion-associated macrophage foam cells is a logical strategy to reduce the plaque volume. Since extracellular cholesterol acceptor-mediated cholesterol efflux is the only recognized mechanism of cholesterol removal from foam cells and this process is rate limited at the level of intracellular cholesterol ester hydrolysis, a reaction catalyzed by neutral cholesteryl ester hydrolase (CEH), we examined the hypothesis that CEH overexpression in the human macrophage monocyte/macrophage cell line THP1 results in increased cholesterol efflux, as well as decreased cellular cholesterol ester accumulation. We generated THP1-CEH cells with stable integration of human macrophage CEH cDNA driven by the cytomegalovirus promoter. Compared with wild-type THP1 cells (THP1-WT), THP1-CEH cells showed increased CEH mRNA expression and increased CEH activity. Efflux of free or unesterified cholesterol by acetylated LDL-loaded THP1-CEH cells to ApoA-I by an ABCA1-dependent pathway or to HDL by an ABCG1-dependent pathway was significantly higher than that in THP1-WT cells. In addition, THP1-CEH cells accumulated significantly lower amount of esterified cholesterol. CEH overexpression, therefore, not only enhances cholesterol efflux but also reduces cellular accumulation of cholesteryl esters. Taken together, these data provide evidence for evaluating CEH expression in human macrophages as a potential target for attenuation of foam cell formation and regression of atherosclerotic plaques.

Rewiring enervated: thinking LARGEr than myodystrophy

LARGE is a glycosyltransferase known to glycosylate alpha-dystroglycan, a component of the dystrophin-associated glycoprotein complex. Spontaneous deletions in the Large gene (Large(myd) and Large(vls)) result in muscular dystrophy accompanied by heart, brain, and eye defects. Another Large mouse mutant, enervated (Large(enr)), is the result of a transgene integration event that disrupts Large gene expression. In addition to myodystrophy, enr mice have been shown to display peripheral nerve abnormalities, including altered axonal sorting resulting from Schwann cell defects, poor regeneration after nerve injury, and abnormal neuromuscular junctions. These data have provided new insight into our understanding of the function of LARGE and have suggested the possibility of involvement of substrates in addition to alpha-dystroglycan in the generation of the LARGE phenotype. The Large mutants are excellent models for addressing the importance of glycosylation in neuromuscular disease.