Dystroglycan protein distribution coincides with basement membranes and muscle differentiation during mouse embryogenesis

The many roles of dystroglycan in nervous system development and function: Dystroglycan and neural circuit development: Dystroglycan and neural circuit development

The glycoprotein dystroglycan was first identified in muscle, where it functions as part of the dystrophin glycoprotein complex to connect the extracellular matrix to the actin cytoskeleton. Mutations in genes involved in the glycosylation of dystroglycan cause a form of congenital muscular dystrophy termed dystroglycanopathy. In addition to its well-defined role in regulating muscle integrity, dystroglycan is essential for proper central and peripheral nervous system development. Patients with dystroglycanopathy can present with a wide range of neurological perturbations, but unraveling the complex role of Dag1 in the nervous system has proven to be a challenge. Over the past two decades, animal models of dystroglycanopathy have been an invaluable resource that has allowed researchers to elucidate dystroglycan's many roles in neural circuit development. In this review, we summarize the pathways involved in dystroglycan's glycosylation and its known interacting proteins, and discuss how it regulates neuronal migration, axon guidance, synapse formation, and its role in non-neuronal cells.

Laminin α2 controls mouse and human stem cell behaviour during midbrain dopaminergic neuron development

Development of the central nervous system requires coordination of the proliferation and differentiation of neural stem cells. Here, we show that laminin alpha 2 (lm-α2) is a component of the midbrain dopaminergic neuron (mDA) progenitor niche in the ventral midbrain (VM) and identify a concentration-dependent role for laminin α2β1γ1 (lm211) in regulating mDA progenitor proliferation and survival via a distinct set of receptors. At high concentrations, lm211-rich environments maintain mDA progenitors in a proliferative state via integrins α6β1 and α7β1, whereas low concentrations of lm211 support mDA lineage survival via dystroglycan receptors. We confirmed our findings in vivo, demonstrating that the VM was smaller in the absence of lm-α2, with increased apoptosis; furthermore, the progenitor pool was depleted through premature differentiation, resulting in fewer mDA neurons. Examination of mDA neuron subtype composition showed a reduction in later-born mDA neurons of the ventral tegmental area, which control a range of cognitive behaviours. Our results identify a novel role for laminin in neural development and provide a possible mechanism for autism-like behaviours and the brainstem hypoplasia seen in some individuals with mutations of LAMA2.

β-dystroglycan is regulated by a balance between WWP1-mediated degradation and protection from WWP1 by dystrophin and utrophin

Dystroglycan is a ubiquitous membrane protein that functions as a mechanical connection between the extracellular matrix and cytoskeleton. In skeletal muscle, dystroglycan plays an indispensable role in regulating muscle regeneration; a malfunction in dystroglycan is associated with muscular dystrophy. The regulation of dystroglycan stability is poorly understood. Here, we report that WWP1, a member of NEDD4 E3 ubiquitin ligase family, promotes ubiquitination and subsequent degradation of β-dystroglycan. Our results indicate that dystrophin and utrophin protect β-dystroglycan from WWP1-mediated degradation by competing with WWP1 for the shared binding site at the cytosolic tail of β-dystroglycan. In addition, we show that a missense mutation (arginine 440 to glutamine) in WWP1-which is known to cause muscular dystrophy in chickens-increases the ubiquitin ligase-mediated ubiquitination of both β-dystroglycan and WWP1. The R440Q missense mutation in WWP1 decreases HECT domain-mediated intramolecular interactions to relieve autoinhibition of the enzyme. Our results provide new insight into the regulation of β-dystroglycan degradation by WWP1 and other Nedd4 family members and improves our understanding of dystroglycan-related disorders.

Basal lamina remodeling at the skeletal muscle stem cell niche mediates stem cell self-renewal

A central question in stem cell biology is the relationship between stem cells and their niche. Although previous reports have uncovered how signaling molecules released by niche cells support stem cell function, the role of the extra-cellular matrix (ECM) within the niche is unclear. Here, we show that upon activation, skeletal muscle stem cells (satellite cells) induce local remodeling of the ECM and the deposition of laminin-α1 and laminin-α5 into the basal lamina of the satellite cell niche. Genetic ablation of laminin-α1, disruption of integrin-α6 signaling or blocking matrix metalloproteinase activity impairs satellite cell expansion and self-renewal. Collectively, our findings establish that remodeling of the ECM is an integral process of stem cell activity to support propagation and self-renewal, and may explain the effect laminin-α1-containing supports have on embryonic and adult stem cells, as well as the regenerative activity of exogenous laminin-111 therapy.

Extracellular matrix (ECM) remodelling is thought to have effects on muscle stem cells that support muscle homeostasis. Here the authors show ECM remodeling controls satellite cell self-renewal through deposition of laminin-α1 into the satellite cell niche.

Hedgehog signalling acts upstream of Laminin alpha1 transcription in the zebrafish paraxial mesoderm

Laminin-111 (α1β1γ1) is a member of the Laminin family of extra-cellular matrix proteins that comprises 16 members, components of basement membranes. Laminin-111, one of the first Laminin proteins synthesised during embryogenesis, is required for basement membrane deposition and has essential roles in tissue morphogenesis and patterning. Yet, the mechanisms controlling Laminin-111 expression are poorly understood. We generated a zebrafish transgenic reporter line that reproduces faithfully the expression pattern of lama1, the gene encoding Laminin α1, and we used this reporter line to investigate lama1 transcriptional regulation. Our findings established that lama1 expression is controlled by intronic enhancers, including an enhancer directing expression in the paraxial mesoderm, anterior spinal cord and hindbrain, located in intron 1. We show that Hedgehog signalling is necessary and sufficient for lama1 transcription in the paraxial mesoderm and identify putative Gli/Zic binding sites that may mediate this control. These findings uncover a conserved role for Hedgehog signalling in the control of basement membrane assembly via its transcriptional regulation of lama1, and provide a mechanism to coordinate muscle cell fate specification in the zebrafish embryo.

Hanging on for the ride: adhesion to the extracellular matrix mediates cellular responses in skeletal muscle morphogenesis and disease

Skeletal muscle specification and morphogenesis during early development are critical for normal physiology. In addition to mediating locomotion, skeletal muscle is a secretory organ that contributes to metabolic homeostasis. Muscle is a highly adaptable tissue, as evidenced by the ability to increase muscle cell size and/or number in response to weight bearing exercise. Conversely, muscle wasting can occur during aging (sarcopenia), cancer (cancer cachexia), extended hospital stays (disuse atrophy), and in many genetic diseases collectively known as the muscular dystrophies and myopathies. It is therefore of great interest to understand the cellular and molecular mechanisms that mediate skeletal muscle development and adaptation. Muscle morphogenesis transforms short muscle precursor cells into long, multinucleate myotubes that anchor to tendons via the myotendinous junction. This process requires carefully orchestrated interactions between cells and their extracellular matrix microenvironment. These interactions are dynamic, allowing muscle cells to sense biophysical, structural, organizational, and/or signaling changes within their microenvironment and respond appropriately. In many musculoskeletal diseases, these cell adhesion interactions are disrupted to such a degree that normal cellular adaptive responses are not sufficient to compensate for accumulating damage. Thus, one major focus of current research is to identify the cell adhesion mechanisms that drive muscle morphogenesis, with the hope that understanding how muscle cell adhesion promotes the intrinsic adaptability of muscle tissue during development may provide insight into potential therapeutic approaches for muscle diseases. Our objectives in this review are to highlight recent studies suggesting conserved roles for cell-extracellular matrix adhesion in vertebrate muscle morphogenesis and cellular adaptive responses in animal models of muscle diseases.

Reciprocal expressions between α-dystroglycan and integrin β1, perlecan receptors, in the murine enamel organ development

Signals of perlecan, an extracellular matrix molecule, which accumulates within the intercellular spaces of the stellate reticulum of the enamel organ, are mediated by at least two receptors, dystroglycan (DG) and integrin β1, in a case-dependent manner in various events in embryogenesis and pathogenesis. This study aims to understand the expression profiles of these two perlecan receptors at both protein and gene levels in murine enamel organ development. Before birth, α-DG was immunolocalized in stellate reticulum cells, in which perlecan was colocalized, while integrin β1 was mainly distributed in the peripheral enamel organ cells as well as the dental mesenchymal cells. On and after postnatal Day 1, the expression of α-DG was dramatically decreased in the stellate reticulum, while integrin β1 was enhanced around blood vessels within the enamel organ. Furthermore, biosyntheses of α-DG and integrin β1 by dental epithelial and pulp mesenchymal cells were confirmed in vitro by using immunofluorescence and reverse-transcriptase polymerase chain reaction. The results suggest that DG is a perlecan receptor that specifically functions in the stellate reticulum of the embryonic stage, but that dental epithelial and mesenchymal cells are maturated by capturing perlecan signals differentially through integrin β1.

The myotomal basement membrane: insight into laminin-111 function and its control by Sonic hedgehog signaling

The importance of laminin-containing basement membranes (BM) for adult muscle function is well established, in particular due to the severe phenotype of congenital muscular dystrophies in patients with mutations disrupting the BM-muscle cell interaction. Developing muscles in the embryo are also dependent on an intact BM. However, the processes controlled by BM-muscle cell interactions in the embryo are only beginning to be elucidated. In this review, we focus on the myotomal BM to illustrate the critical role of laminin-111 in BM assembly and function at the surface of embryonic muscle cells. The myotomal BM provides also an interesting paradigm to study the complex interplay between laminins-containing BM and growth factor-mediated signaling and activity.

Sonic hedgehog acts cell-autonomously on muscle precursor cells to generate limb muscle diversity

How muscle diversity is generated in the vertebrate body is poorly understood. In the limb, dorsal and ventral muscle masses constitute the first myogenic diversification, as each gives rise to distinct muscles. Myogenesis initiates after muscle precursor cells (MPCs) have migrated from the somites to the limb bud and populated the prospective muscle masses. Here, we show that Sonic hedgehog (Shh) from the zone of polarizing activity (ZPA) drives myogenesis specifically within the ventral muscle mass. Shh directly induces ventral MPCs to initiate Myf5 transcription and myogenesis through essential Gli-binding sites located in the Myf5 limb enhancer. In the absence of Shh signaling, myogenesis is delayed, MPCs fail to migrate distally, and ventral paw muscles fail to form. Thus, Shh production in the limb ZPA is essential for the spatiotemporal control of myogenesis and coordinates muscle and skeletal development by acting directly to regulate the formation of specific ventral muscles.

Dystroglycan expression in human placenta: basement membrane localization and subunit distribution change between the first and third trimester

OBJECTIVES

This study describes the distribution of dystroglycan (DG) in human placenta, membranes, and uterine decidua.

STUDY DESIGN

Dystroglycan expression was characterized by Western blotting, immunohistochemistry, and immunofluorescence microscopy using human tissues and cultured cells.

RESULTS

Both α-DG and β-DG are expressed in the term syncytiotrophoblast, and α-DG is localized to the basement membrane. In first-trimester chorionic villi, α-DG and β-DG are localized to the periphery of the cytotrophoblast. Expression varies in term fetal membranes. α-Dystroglycan is not detectable in choriocarcinoma cells or HTR cells, but β-DG is present in both normal and cleaved forms.

CONCLUSION

Dystroglycan is expressed at high levels in human trophoblasts, and localization of the α- and β-subunits varies with gestational age and trophoblast differentiation. Because DG expression inversely correlates with invasiveness in many cancers, its pattern of expression in trophoblasts suggests a possible function in inhibition of placental invasion.

The extracellular matrix dimension of skeletal muscle development

Cells anchor to substrates by binding to extracellular matrix (ECM). In addition to this anchoring function however, cell-ECM binding is a mechanism for cells to sense their surroundings and to communicate and coordinate behaviour amongst themselves. Several ECM molecules and their receptors play essential roles in muscle development and maintenance. Defects in these proteins are responsible for some of the most severe muscle dystrophies at every stage of life from neonates to adults. However, recent studies have also revealed a role of cell-ECM interactions at much earlier stages of development as skeletal muscle forms. Here we review which ECM molecules are present during the early phases of myogenesis, how myogenic cells interact with the ECM that surrounds them and the potential consequences of those interactions. We conclude that cell-ECM interactions play significant roles during all stages of skeletal muscle development in the embryo and suggest that this "extracellular matrix dimension" should be added to our conceptual network of factors contributing to skeletal myogenesis.

A novel Gli3 enhancer controls the Gli3 spatiotemporal expression pattern through a TALE homeodomain protein binding site

The zinc finger transcription factor Gli3 is an essential mediator of hedgehog signaling. Gli3 has a dynamic expression pattern during embryonic development. In the neural tube, Gli3 transcripts are patterned along the anteroposterior and dorsoventral axes such that the initial broad expression in the posterior neural tube becomes dorsally restricted as neurogenesis takes place. Little is known about the molecular mechanisms that regulate this dynamic expression. Here, we report on a phylogenetic analysis of the Gli3 locus that uncovered a novel regulatory element, HCNE1. HCNE1 contains a compound Pbx/Meis binding site that binds Pbx and Meis/Prep proteins in vitro and in vivo. We show that HCNE1 recapitulates Gli3 expression in the developing neural tube and that mutations in the Pbx/Meis binding site affect the spatiotemporal control of HCNE1 transcriptional activity. Ectopic expression or loss of function of Pbx and Meis/Prep proteins in the chick and mouse embryo results in aberrant expression of endogenous Gli3 transcripts. We propose a novel role for TALE proteins in establishing the correct spatiotemporal expression pattern of Gli3 in the vertebrate spinal cord, thus implicating TALE transcription factors in early embryonic patterning events controlled by Sonic hedgehog signaling.

Differential expression profiles between α-dystroglycan and integrin β1 in ameloblastoma: two possible perlecan signalling pathways for cellular growth and differentiation

AIMS

Intercellular deposition of perlecan, an extracellular matrix molecule, results in characteristic stellate reticulum-like structures in ameloblastomas. The aims of this study were to elucidate which types of perlecan receptors function within any particular type of tissue architecture of ameloblastoma.

METHODS AND RESULTS

Protein and gene expression profiles for α-dystroglycan and integrin β1 were examined comparatively with those of their ligands in ameloblastoma using surgical specimens and cells in primary culture. In the follicular-type tumour cell foci, α-dystroglycan was localized uniformly over the stellate reticulum-like cells, while integrin β1 was restricted mainly to peripheral cells facing the stroma with the interface of the basement membrane, which was also rich in perlecan. In the plexiform-type, mRNA and protein signals for α-dystroglycan were enhanced in the periphery of tumour cell foci, especially in their invading fronts. Integrin β1 was also immunolocalized in the basal cell zone, which was considered to be the proliferation centre of ameloblastoma cells. Furthermore, biosynthesis of α-dystroglycan and integrin β1 by ameloblastoma cells was confirmed in vitro using immunofluorescence and reverse transcriptase-polymerase chain reaction.

CONCLUSIONS

Ameloblastoma cells proliferate and are differentiated by capturing perlecan differentially with α-dystroglycan and integrin β1, respectively.

Dystroglycan versatility in cell adhesion: a tale of multiple motifs

Dystroglycan is a ubiquitously expressed heterodimeric adhesion receptor. The extracellular α-subunit makes connections with a number of laminin G domain ligands including laminins, agrin and perlecan in the extracellular matrix and the transmembrane β-subunit makes connections to the actin filament network via cytoskeletal linkers including dystrophin, utrophin, ezrin and plectin, depending on context. Originally discovered as part of the dystrophin glycoprotein complex of skeletal muscle, dystroglycan is an important adhesion molecule and signalling scaffold in a multitude of cell types and tissues and is involved in several diseases. Dystroglycan has emerged as a multifunctional adhesion platform with many interacting partners associating with its short unstructured cytoplasmic domain. Two particular hotspots are the cytoplasmic juxtamembrane region and at the very carboxy terminus of dystroglycan. Regions which between them have several overlapping functions: in the juxtamembrane region; a nuclear localisation signal, ezrin/radixin/moesin protein, rapsyn and ERK MAP Kinase binding function, and at the C terminus a regulatory tyrosine governing WW, SH2 and SH3 domain interactions. We will discuss the binding partners for these motifs and how their interactions and regulation can modulate the involvement of dystroglycan in a range of different adhesion structures and functions depending on context. Thus dystroglycan presents as a multifunctional scaffold involved in adhesion and adhesion-mediated signalling with its functions under exquisite spatio-temporal regulation.

Sonic hedgehog-dependent synthesis of laminin alpha1 controls basement membrane assembly in the myotome

Basement membranes have essential structural and signalling roles in tissue morphogenesis during embryonic development, but the mechanisms that control their formation are still poorly understood. Laminins are key components of basement membranes and are thought to be essential for initiation of basement membrane assembly. Here, we report that muscle progenitor cells populating the myotome migrate aberrantly in the ventral somite in the absence of sonic hedgehog (Shh) signalling, and we show that this defect is due to the failure to form a myotomal basement membrane. We reveal that expression of Lama1, which encodes laminin alpha1, a subunit of laminin-111, is not activated in Shh(-/-) embryos. Recovery of Lama1 expression or addition of exogenous laminin-111 to Shh(-/-);Gli3(-/-) embryos restores the myotomal basement membrane, demonstrating that laminin-111 is necessary and sufficient to initiate assembly of the myotomal basement membrane. This study uncovers an essential role for Shh signalling in the control of laminin-111 synthesis and in the initiation of basement membrane assembly in the myotome. Furthermore, our data indicate that laminin-111 function cannot be compensated by laminin-511.

Distinct and dynamic myogenic populations in the vertebrate embryo

Myogenic cells in the body of vertebrates derive from the dorsal somite, the dermomyotome, where multipotent cells are present. Regulation of cell fate choice is discussed, as is that of progenitor cell self-renewal once cells have entered the myogenic programme. Ongoing research on the formation of the first skeletal muscle, the myotome, is presented with emphasis on mechanisms controlling the early segregation of slow and fast muscle lineages that characterizes this process in the zebrafish embryo. Further insights into myogenic populations that contribute to trunk and limb development at different stages are summarized and the distinct regulatory networks that underlie the formation of head muscles are discussed.

Muscular dystrophy begins early in embryonic development deriving from stem cell loss and disrupted skeletal muscle formation

Examination of embryonic myogenesis of two distinct, but functionally related, skeletal muscle dystrophy mutants (mdx and cav-3(-/-)) establishes for the first time that key elements of the pathology of Duchenne muscular dystrophy (DMD) and limb-girdle muscular dystrophy type 1C (LGMD-1c) originate in the disruption of the embryonic cardiac and skeletal muscle patterning processes. Disruption of myogenesis occurs earlier in mdx mutants, which lack a functional form of dystrophin, than in cav-3(-/-) mutants, which lack the Cav3 gene that encodes the protein caveolin-3; this finding is consistent with the milder phenotype of LGMD-1c, a condition caused by mutations in Cav3, and the earlier [embryonic day (E)9.5] expression of dystrophin. Myogenesis is severely disrupted in mdx embryos, which display developmental delays; myotube morphology and displacement defects; and aberrant stem cell behaviour. In addition, the caveolin-3 protein is elevated in mdx embryos. Both cav-3(-/-) and mdx mutants (from E15.5 and E11.5, respectively) exhibit hyperproliferation and apoptosis of Myf5-positive embryonic myoblasts; attrition of Pax7-positive myoblasts in situ; and depletion of total Pax7 protein in late gestation. Furthermore, both cav-3(-/-) and mdx mutants have cardiac defects. In cav-3(-/-) mutants, there is a more restricted phenotype comprising hypaxial muscle defects, an excess of malformed hypertrophic myotubes, a twofold increase in myonuclei, and reduced fast myosin heavy chain (FMyHC) content. Several mdx mutant embryo pathologies, including myotube hypotrophy, reduced myotube numbers and increased FMyHC, have reciprocity with cav-3(-/-) mutants. In double mutant (mdxcav-3(+/-)) embryos that are deficient in dystrophin (mdx) and heterozygous for caveolin-3 (cav-3(+/-)), whereby caveolin-3 is reduced to 50% of wild-type (WT) levels, these phenotypes are severely exacerbated: intercostal muscle fibre density is reduced by 71%, and Pax7-positive cells are depleted entirely from the lower limbs and severely attenuated elsewhere; these data suggest a compensatory rather than a contributory role for the elevated caveolin-3 levels that are found in mdx embryos. These data establish a key role for dystrophin in early muscle formation and demonstrate that caveolin-3 and dystrophin are essential for correct fibre-type specification and emergent stem cell function. These data plug a significant gap in the natural history of muscular dystrophy and will be invaluable in establishing an earlier diagnosis for DMD/LGMD and in designing earlier treatment protocols, leading to better clinical outcome for these patients.

Dystroglycan, Tks5 and Src mediated assembly of podosomes in myoblasts

Background

Dystroglycan is a ubiquitously expressed cell adhesion receptor best understood in its role as part of the dystrophin glycoprotein complex of mature skeletal muscle. Less is known of the role of dystroglycan in more fundamental aspects of cell adhesion in other cell types, nor of its role in myoblast cell adhesion.

Principal Findings

We have examined the role of dystroglycan in the early stages of myoblast adhesion and spreading and found that dystroglycan initially associates with other adhesion proteins in large puncta morphologically similar to podosomes. Using a human SH3 domain phage display library we identified Tks5, a key regulator of podosomes, as interacting with β-dystroglycan. We verified the interaction by immunoprecipitation, GST-pulldown and immunfluorescence localisation. Both proteins localise to puncta during early phases of spreading, but importantly following stimulation with phorbol ester, also localise to structures indistinguishable from podosomes. Dystroglycan overexpression inhibited podosome formation by sequestering Tks5 and Src. Mutation of dystroglycan tyrosine 890, previously identified as a Src substrate, restored podosome formation.

Conclusions

We propose therefore, that Src-dependent phosphorylation of β-dystroglycan results in the formation of a Src/dystroglycan complex that drives the SH3-mediated association between dystroglycan and Tks5 which together regulate podosome formation in myoblasts.