Dystroglycan organizes axon guidance cue localization and axonal pathfinding

Precise patterning of axon guidance cue distribution is critical for nervous system development. Using a murine forward genetic screen for novel determinants of axon guidance, we identified B3gnt1 and ISPD as required for the glycosylation of dystroglycan in vivo. Analysis of B3gnt1, ISPD, and dystroglycan mutant mice revealed a critical role for glycosylated dystroglycan in the development of several longitudinal axon tracts. Remarkably, the axonal guidance defects observed in B3gnt1, ISPD, and dystroglycan mutants resemble several of the axon guidance defects found in mice lacking the axon guidance cue Slit and its receptor Robo. This similarity is explained by our observations that dystroglycan binds directly to Slit and is required for proper Slit localization within the basement membrane and floor plate in vivo. These findings establish a novel role for glycosylated dystroglycan as a key determinant of axon guidance cue distribution and function in the mammalian nervous system.

Role of dystrophin and utrophin for assembly and function of the dystrophin glycoprotein complex in non-muscle tissue

The dystrophin glycoprotein complex (DGC) is a multimeric protein assembly associated with either the X-linked cytoskeletal protein dystrophin or its autosomal homologue utrophin. In striated muscle cells, the DGC links the extracellular matrix to the actin cytoskeleton and mediates three major functions: structural stability of the plasma membrane, ion homeostasis, and transmembrane signaling. Mutations affecting the DGC underlie major forms of congenital muscle dystrophies. The DGC is prominent also in the central and peripheral nervous system and in tissues with a secretory function or which form barriers between functional compartments, such as the blood-brain barrier, choroid plexus, or kidney. A considerable molecular heterogeneity arises from cell-specific expression of its constituent proteins, notably short C-terminal isoforms of dystrophin. Experimentally, the generation of mice carrying targeted gene deletions affecting the DGC has clarified the interdependence of DGC proteins for assembly of the complex and revealed its importance for brain development and regulation of the 'milieu intérieur. Here, we focus on recent studies of the DGC in brain, blood-brain barrier and choroid plexus, retina, and kidney and discuss the role of dystrophin isoforms and utrophin for assembly of the complex in these tissues.

Dissecting muscle and neuronal disorders in a Drosophila model of muscular dystrophy

Perturbation in the Dystroglycan (Dg)-Dystrophin (Dys) complex results in muscular dystrophies and brain abnormalities in human. Here we report that Drosophila is an excellent genetically tractable model to study muscular dystrophies and neuronal abnormalities caused by defects in this complex. Using a fluorescence polarization assay, we show a high conservation in Dg-Dys interaction between human and Drosophila. Genetic and RNAi-induced perturbations of Dg and Dys in Drosophila cause cell polarity and muscular dystrophy phenotypes: decreased mobility, age-dependent muscle degeneration and defective photoreceptor path-finding. Dg and Dys are required in targeting glial cells and neurons for correct neuronal migration. Importantly, we now report that Dg interacts with insulin receptor and Nck/Dock SH2/SH3-adaptor molecule in photoreceptor path-finding. This is the first demonstration of a genetic interaction between Dg and InR.

Distinct functions of glial and neuronal dystroglycan in the developing and adult mouse brain

Cobblestone (type II) lissencephaly and mental retardation are characteristic features of a subset of congenital muscular dystrophies that include Walker-Warburg syndrome, muscle-eye-brain disease, and Fukuyama-type congenital muscular dystrophy. Although the majority of clinical cases are genetically undefined, several causative genes have been identified that encode known or putative glycosyltransferases in the biosynthetic pathway of dystroglycan. Here we test the effects of brain-specific deletion of dystroglycan, and show distinct functions for neuronal and glial dystroglycan. Deletion of dystroglycan in the whole brain produced glial/neuronal heterotopia resembling the cerebral cortex malformation in cobblestone lissencephaly. In wild-type mice, dystroglycan stabilizes the basement membrane of the glia limitans, thereby supporting the cortical infrastructure necessary for neuronal migration. This function depends on extracellular dystroglycan interactions, since the cerebral cortex developed normally in transgenic mice that lack the dystroglycan intracellular domain. Also, forebrain histogenesis was preserved in mice with neuron-specific deletion of dystroglycan, but hippocampal long-term potentiation was blunted, as is also the case in the Largemyd mouse, in which dystroglycan glycosylation is disrupted. Our findings provide genetic evidence that neuronal dystroglycan plays a role in synaptic plasticity and that glial dystroglycan is involved in forebrain development. Differences in dystroglycan glycosylation in distinct cell types of the CNS may contribute to the diversity of dystroglycan function in the CNS, as well as to the broad clinical spectrum of type II lissencephalies.

Both laminin and Schwann cell dystroglycan are necessary for proper clustering of sodium channels at nodes of Ranvier

Nodes of Ranvier are specialized axonal domains, at which voltage-gated sodium channels cluster. How axons cluster molecules in discrete domains is mostly unknown. Both axons and glia probably provide constraining mechanisms that contribute to domain formation. Proper sodium channel clustering in peripheral nerves depends on contact from Schwann cell microvilli, where at least one molecule, gliomedin, binds the sodium channel complex and induces its clustering. Furthermore, mice lacking Schwann cell dystroglycan have aberrant microvilli and poorly clustered sodium channels. Dystroglycan could interact at the basal lamina or at the axonglial surface. Because dystroglycan is a laminin receptor, and laminin 2 mutations [merosin-deficient congenital muscular dystrophy (MDC1A)] cause reduced nerve conduction velocity, we asked whether laminins are involved. Here, we show that the composition of both laminins and the dystroglycan complex at nodes differs from that of internodes. Mice defective in laminin 2 have poorly formed microvilli and abnormal sodium clusters. These abnormalities are similar, albeit less severe, than those of mice lacking dystroglycan. However, mice lacking all Schwann cell laminins show severe nodal abnormalities, suggesting that other laminins compensate for the lack of laminin 2. Thus, although laminins are located at a distance from the axoglial junction, they are required for proper clustering of sodium channels. Laminins, through their specific nodal receptors and cytoskeletal linkages, may participate in the formation of mechanisms that constrain clusters at nodes. Finally, abnormal sodium channel clusters are present in a patient with MDC1A, providing a molecular basis for the reduced nerve conduction velocity in this disorder.

Evidence for a role of dystroglycan regulating the membrane architecture of astroglial endfeet

The dystrophin-dystroglycan complex (DDC) is a molecular array of proteins in muscle and brain cells. The central component of the DDC is dystroglycan, which comprises α- and β-subunits. α-Dystroglycan (α-DG) binds to extracellular matrix components such as agrin, whereas β-dystroglycan (β-DG) is a membrane-spanning protein linking α-DG to the cytoskeleton and other intracellular components such as α-syntrophin. In astrocytes, α-syntrophin binds to the water channel protein aquaporin-4 (AQP4). Recently, it has been shown that AQP4 expression is unaltered in agrin-knockout mice, but that formation of orthogonal arrays of particles (OAPs), consisting of AQP4, is abnormal. As the brain-selective deletion of the DG gene causes a disorganization of the astroglial endfeet, we investigated whether DG deletion has an impact on AQP4. Western blotting revealed reduced AQP4 in the parenchymal but not in the superficial compartment of the astrocyte-conditioned DG-knockout mouse brain. Accordingly, immunohistochemical stainings of AQP4 revealed a selective loss of AQP4 in perivascular but not in superficial astroglial endfeet. In both superficial and perivascular endfeet of the DG-knockout brain, we observed a loss of OAPs. We conclude that in the absence of DG the majority of superficial AQP4 molecules did not form OAPs, and that expression of AQP4 in perivascular endfeet is compromised. However, the decreased number of perivascular AQP4 molecules obviously did form a few OAPs, even in the absence of DG.

Muscular dystrophies due to glycosylation defects

In the last few years, muscular dystrophies due to reduced glycosylation of alpha-dystroglycan (ADG) have emerged as a common group of conditions, now referred to as dystroglycanopathies. Mutations in six genes (POMT1, POMT2, POMGnT1, Fukutin, FKRP and LARGE) have so far been identified in patients with a dystroglycanopathy. Allelic mutations in each of these genes can result in a wide spectrum of clinical conditions, ranging from severe congenital onset with associated structural brain malformations (Walker Warburg syndrome; muscle-eye-brain disease; Fukuyama muscular dystrophy; congenital muscular dystrophy type 1D) to a relatively milder congenital variant with no brain involvement (congenital muscular dystrophy type 1C), and to limb-girdle muscular dystrophy (LGMD) type 2 variants with onset in childhood or adult life (LGMD2I, LGMD2L, and LGMD2N). ADG is a peripheral membrane protein that undergoes multiple and complex glycosylation steps to regulate its ability to effectively interact with extracellular matrix proteins, such as laminin, agrin, and perlecan. Although the precise composition of the glycans present on ADG are not known, it has been demonstrated that the forced overexpression of LARGE, or its paralog LARGE2, is capable of increasing the glycosylation of ADG in normal cells. In addition, its overexpression is capable of restoring dystroglycan glycosylation and laminin binding properties in primary cell cultures of patients affected by different genetically defined dystroglycanopathy variants. These observations suggest that there could be a role for therapeutic strategies to overcome the glycosylation defect in these conditions via the overexpression of LARGE.

Cell adhesion molecules, the extracellular matrix and oral squamous carcinoma

Carcinomas are characterized by invasion of malignant cells into the underlying connective tissue and migration of malignant cells to form metastases at distant sites. These processes require alterations in cell-cell and cell-extracellular matrix interactions. As cell adhesion molecules play a role in cell-cell and cell-extracellular matrix adhesion and interactions they are involved in the process of tumour invasion and metastases. In epithelial tissues, receptors of the integrin family mediate adhesion to the adjacent matrix whereas cadherins largely mediate intercellular adhesion. These and other cell adhesion molecules such as intercellular adhesion molecule-1, CD44, dystroglycans and selectins, are involved and undergo changes in carcinomas, which provide possible targets for anti-cancer drug treatments. In the extracellular matrix that is associated with tumours, laminin 5, oncofetal fibronectin and tenascin C appear. The degree of expression of some of these moieties indicates prognosis in oral cancer and offer targets for antibody-directed radiotherapy. Metalloproteases which degrade the extracellular matrix are increased in carcinomas, and their activity is necessary for tumour angiogenesis and consequent invasion and metastases. Metalloprotease inhibitors have begun to produce decreases in mortality in clinical trials. This report provides a brief overview of our current understanding of cell adhesion molecules, the extracellular matrix, tumour invasion and metastasis.

Dystroglycan loss disrupts polarity and beta-casein induction in mammary epithelial cells by perturbing laminin anchoring

Precise contact between epithelial cells and their underlying basement membrane is crucial to the maintenance of tissue architecture and function. To understand the role that the laminin receptor dystroglycan (DG) plays in these processes, we assayed cell responses to laminin-111 following conditional ablation of DG gene (Dag1) expression in cultured mammary epithelial cells. Strikingly, DG loss disrupted laminin-111-induced polarity and beta-casein production, and abolished laminin assembly at the step of laminin binding to the cell surface. Dystroglycan re-expression restored these deficiencies. Investigations of the mechanism revealed that DG cytoplasmic sequences were not necessary for laminin assembly and signaling, and only when the entire mucin domain of extracellular DG was deleted did laminin assembly not occur. These results demonstrate that DG is essential as a laminin-111 co-receptor in mammary epithelial cells that functions by mediating laminin anchoring to the cell surface, a process that allows laminin polymerization, tissue polarity and beta-casein induction. The observed loss of laminin-111 assembly and signaling in Dag1(-/-) mammary epithelial cells provides insights into the signaling changes occurring in breast carcinomas and other cancers, where the binding function of DG to laminin is frequently defective.

Vascular matrix adhesion and the blood-brain barrier

The integrity of the cerebral microvasculature depends on the interaction between its component cells and the extracellular matrix, as well as reorganized cell-cell interactions. In the central nervous system, matrix adhesion receptors are expressed in the microvasculature and by neurons and their supporting glial cells. Cells within cerebral microvessels express both the integrin and dystroglycan families of matrix adhesion receptors. However, the functional significance of these receptors is only now being explored. Endothelial cells and astrocytes within cerebral capillaries co-operate to generate and maintain the basal lamina and the unique barrier functions of the endothelium. Integrins and the dystroglycan complex are found on the matrix-proximate faces of both endothelial cells and astrocyte end-feet. Pericytes rest against the basal lamina. In the extravascular compartment, select integrins are expressed on neurons, microglial cells and oligodendroglia. Significant alterations in both cellular adhesion receptors and their matrix ligands occur during focal cerebral ischaemia, which support their functional significance in the normal state. We propose that matrix adhesion receptors are essential for the maintenance of the integrity of the blood-brain permeability barrier and that modulation of these receptors contributes to alterations in the barrier during brain injury.

Dystroglycan: important player in skeletal muscle and beyond

Dystroglycan is a transmembrane protein that connects the extracellular matrix to the cytoskeleton. Given the ubiquitous tissue expression of dystroglycan, different functional roles in various organ systems have been characterized during the past decade. More recently, aberrant glycosylation of dystroglycan has been identified as a novel pathogenetic mechanism in several forms of congenital and late onset muscular dystrophy syndromes. The current review summarizes the recent scientific achievements as they relate to the function of dystroglycan under normal and pathophysiological conditions.

Functional diversity of dystroglycan

During the last 15 years, following its identification and first detailed molecular characterization, the dystroglycan (DG) complex has taken centre stage in biology and biomedicine. Functions in different cells and tissues have been identified for this complex, ranging from its typical role in skeletal muscle as a sarcolemmal stabilizer, highlighted by the recently identified "secondary dystroglycanopathies", to a variety of very diverse functions including embryogenesis, cancer progression, virus particle entry and cell signalling. Such functional promiscuity can be in part explained when considering the multiple domain organization of the two DG subunits, the extracellular alpha-DG and the transmembrane beta-DG, that has been largely scrutinized, but only in part unraveled, exploiting a variety of recombinant and transgenic approaches. Herein, while rapidly recapitulating some of the functions that nowadays can be assigned safely to each DG domain, we also try to envisage a sort of worry list featuring and dwelling on some of the most compelling "mysteries" that should be solved to finally understand DG's functional diversity.

A laminin-2, dystroglycan, utrophin axis is required for compartmentalization and elongation of myelin segments

Animal and plant cells compartmentalize to perform morphogenetic functions. Compartmentalization of myelin-forming Schwann cells may favor elongation of myelin segments to the size required for efficient conduction of nerve impulses. Compartments in myelinated fibers were described by Ramón y Cajal and depend on periaxin, mutated in the hereditary neuropathy Charcot-Marie-Tooth disease type 4F (Charcot-Marie-Tooth 4F). Lack of periaxin in mice causes loss of compartments, formation of short myelin segments (internodes) and reduced nerve conduction velocity. How compartments are formed and maintained, and their relevance to human neuropathies is largely unknown. Here we show that formation of compartments around myelin is driven by the actin cytoskeleton, and maintained by actin and tubulin fences through linkage to the dystroglycan complex. Compartmentalization and establishment of correct internodal length requires the presence of glycosylated dystroglycan, utrophin and extracellular laminin-2/211. A neuropathic patient with reduced internodal length and nerve conduction velocity because of absence of laminin-2/211 (congenital muscular dystrophy 1A) also shows abnormal compartmentalization. These data link formation of compartments through a laminin2, dystroglycan, utrophin, actin axis to internodal length, and provide a common pathogenetic mechanism for two inherited human neuropathies. Other cell types may exploit dystroglycan complexes in similar fashions to create barriers and compartments.

The detached locus encodes Drosophila Dystrophin, which acts with other components of the Dystrophin Associated Protein Complex to influence intercellular signalling in developing wing veins

Dystrophin and Dystroglycan are the two central components of the multimeric Dystrophin Associated Protein Complex, or DAPC, that is thought to provide a mechanical link between the extracellular matrix and the actin cytoskeleton, disruption of which leads to muscular dystrophy in humans. We present the characterization of the Drosophila 'crossveinless' mutation detached (det), and show that the gene encodes the fly ortholog of Dystrophin. Our genetic analysis shows that, in flies, Dystrophin is a non-essential gene, and the sole overt morphological defect associated with null mutations in the locus is the variable loss of the posterior crossvein that has been described for alleles of det. Null mutations in Drosophila Dystroglycan (Dg) are similarly viable and exhibit this crossvein defect, indicating that both of the central DAPC components have been co-opted for this atypical function of the complex. In the developing wing, the Drosophila DAPC affects the intercellular signalling pathways involved in vein specification. In det and Dg mutant wings, the early BMP signalling that initiates crossvein specification is not maintained, particularly in the pro-vein territories adjacent to the longitudinal veins, and this results in the production of a crossvein fragment in the intervein between the two longitudinal veins. Genetic interaction studies suggest that the DAPC may exert this effect indirectly by down-regulating Notch signalling in pro-vein territories, leading to enhanced BMP signalling in the intervein by diffusion of BMP ligands from the longitudinal veins.

Agrin mediates chondrocyte homeostasis and requires both LRP4 and α-dystroglycan to enhance cartilage formation in vitro and in vivo

OBJECTIVES

Osteoarthritis (OA) is a leading cause of disability for which there is no cure. The identification of molecules supporting cartilage homeostasis and regeneration is therefore a major pursuit in musculoskeletal medicine. Agrin is a heparan sulfate proteoglycan which, through binding to low-density lipoprotein receptor-related protein 4 (LRP4), is required for neuromuscular synapse formation. In other tissues, it connects the cytoskeleton to the basement membrane through binding to α-dystroglycan. Prompted by an unexpected expression pattern, we investigated the role and receptor usage of agrin in cartilage.

METHODS

Agrin expression pattern was investigated in human osteoarthritic cartilage and following destabilisation of the medial meniscus in mice. Extracellular matrix (ECM) formation and chondrocyte differentiation was studied in gain and loss of function experiments in vitro in three-dimensional cultures and gain of function in vivo, using an ectopic cartilage formation assay in nude mice. Receptor usage was investigated by disrupting LRP4 and α-dystroglycan by siRNA and blocking antibodies respectively.

RESULTS

Agrin was detected in normal cartilage but was progressively lost in OA. In vitro, agrin knockdown resulted in reduced glycosaminoglycan content, downregulation of the cartilage transcription factor SOX9 and other cartilage-specific ECM molecules. Conversely, exogenous agrin supported cartilage differentiation in vitro and ectopic cartilage formation in vivo. In the context of cartilage differentiation, agrin used an unusual receptor repertoire requiring both LRP4 and α-dystroglycan.

CONCLUSIONS

We have discovered that agrin strongly promotes chondrocyte differentiation and cartilage formation in vivo. Our results identify agrin as a novel potent anabolic growth factor with strong therapeutic potential in cartilage regeneration.

Laminin alpha1 chain mediated reduction of laminin alpha2 chain deficient muscular dystrophy involves integrin alpha7beta1 and dystroglycan

Transgenically introduced laminin (LN) alpha1 chain prevents muscular dystrophy in LNalpha2 chain deficient mice. We now report increased integrin alpha7Bbeta1D synthesis in dystrophic LNalpha2 chain deficient muscle. Yet, immunofluorescence demonstrated a reduced expression of integrin alpha7B subunit at the sarcolemma. Transgenic expression of LNalpha1 chain reconstituted integrin alpha7B at the sarcolemma. Expression of alpha- and beta-dystroglycan is enhanced in LNalpha2 chain deficient muscle and normalized by transgenic expression of LNalpha1 chain. We suggest that LNalpha1 chain in part ameliorates the development of LNalpha2 chain deficient muscular dystrophy by retaining the binding sites for integrin alpha7Bbeta1D and alpha-dystroglycan, respectively.

Loss of LARGE2 disrupts functional glycosylation of α-dystroglycan in prostate cancer

Dystroglycan (DG) is a cell surface receptor for extracellular matrix proteins and is involved in cell polarity, matrix organization, and mechanical stability of tissues. Previous studies documented loss of DG protein expression and glycosylation in a variety of cancer types, but the underlying mechanisms and the functional consequences with respect to cancer progression remain unclear. Here, we show that the level of expression of the βDG subunit as well as the glycosylation status of the αDG subunit inversely correlate with the Gleason scores of prostate cancers; furthermore, we show that the functional glycosylation of αDG is substantially reduced in prostate cancer metastases. Additionally, we demonstrate that LARGE2 (GYLTL1B), a gene not previously implicated in cancer, regulates functional αDG glycosylation in prostate cancer cell lines; knockdown of LARGE2 resulted in hypoglycosylation of αDG and loss of its ability to bind laminin-111 while overexpression restored ligand binding and diminished growth and migration of an aggressive prostate cancer cell line. Finally, our analysis of LARGE2 expression in human cancer specimens reveals that LARGE2 is significantly down-regulated in the context of prostate cancer, and that its reduction correlates with disease progression. Our results describe a novel molecular mechanism to account for the commonly observed hypoglycosylation of αDG in prostate cancer.

Role of the extracellular matrix and its receptors in smooth muscle cell function: implications in vascular development and disease

PURPOSE OF REVIEW

Cardiovascular disease affects millions of people worldwide, while the sarcoglycan deficient cardiomyopathies are rare disorders. One important common feature, however, is the vascular smooth muscle. Here we focus on the roles of extracellular matrix components and their receptors in the functions of vascular smooth muscle cells.

RECENT FINDINGS

Recent observations highlight the importance of integrins and the dystrophin-glycoprotein complex in development and cardiomyopathy. For example, integrin alpha4 and alpha7 subunits are important for distributing vascular smooth muscle cells during blood vessel development. Studies on delta-sarcoglycan deficient animals have revealed abnormal vascular smooth muscle proliferation and apoptosis. Furthermore, data suggest that perlecan, by affecting smooth muscle cell proliferation, participates in the atherosclerotic process. Overexpression of decorin leads to reduced progression of atherosclerosis and thrombospondin-1 has been implicated in regulation of smooth muscle cell contractility via inhibition of nitric oxide. Novel findings on versican suggest that the binding of versican to fibulin is of great importance for regulating smooth muscle cell function.

SUMMARY

By regulating migration, proliferation and apoptosis as well as extracellular matrix synthesis and assembly, proteoglycans, integrins and the dystrophin-glycoprotein complex may be of great importance both during development and in vascular disease.

Cell stiffness and receptors: evidence for cytoskeletal subnetworks

Viscoelastic models of cells often treat cells as homogeneous objects. However, studies have demonstrated that cellular properties are local and can change dramatically on the basis of the location probed. Because membrane receptors are linked in various ways to the intracellular space, with some receptors linking to the cytoskeleton and others diffusing freely without apparent linkages, the cellular physical response to mechanical stresses is expected to depend on the receptor engaged. In this study, we tested the hypothesis that cellular mechanical stiffness as measured via cytoskeletally linked receptors is greater than stiffness measured via receptors that are not cytoskeletally linked. We used a magnetic micromanipulator to apply linear stresses to magnetic beads attached to living cells via selected receptors. One of the receptor classes probed, the dystroglycan receptors, is linked to the cytoskeleton, while the other, the transferrin receptors, is not. Fibronectin-coated beads were used to test cellular mechanical properties of the cytoskeleton without membrane dependence by allowing the beads to endocytose. For epithelial cells, transferrin-dependent stiffness and endocytosed bead-dependent stiffness were similar, while dystroglycan-dependent stiffness was significantly lower. For smooth muscle cells, dystroglycan-dependent stiffness was similar to the endocytosed bead-dependent stiffness, while the transferrin-dependent stiffness was lower. The conclusion of this study is that the measured cellular stiffness is critically influenced by specific receptor linkage and by cell type and raises the intriguing possibility of the existence of separate cytoskeletal networks with distinct mechanical properties that link different classes of receptors.

Differential requirement for MuSK and dystroglycan in generating patterns of neuromuscular innervation

Vertebrates display diverse patterns of neuromuscular innervation, but little is known about how such diversity is generated. In mammals, neuromuscular junctions form predominantly at equatorial locations, giving rise to a focal innervation pattern along a central endplate band. In addition, vertebrate striated muscles exhibit two nonfocal neuromuscular patterns, myoseptal and distributed innervation. Although agrin-MuSK-rapsyn signaling is essential for the focal innervation pattern, it is unknown whether the same genetic program also controls synaptogenesis at nonfocal innervation sites. Here we show that one of three transcripts generated by the zebrafish unplugged locus, unplugged FL, encodes the zebrafish MuSK ortholog. We demonstrate that UnpFL/MuSK is critical for the assembly of focal synapses in zebrafish and that it cooperates with dystroglycan in the formation of nonfocal myoseptal and distributed synapses. Our results provide the first genetic evidence that neuromuscular synapse formation can occur in the absence of MuSK and that the combinatorial function of UnpFL/MuSK and dystroglycan generates diverse patterns of vertebrate neuromuscular innervation.

Analysis of dystroglycan regulation and functions in mouse mammary epithelial cells and implications for mammary tumorigenesis

Abnormalities in the interactions of cells with the extracellular matrix (ECM) play an important role in the development and progression of many types of cancer and are a hallmark of malignant transformation. The dystroglycan (DG) complex is a transmembrane glycoprotein that forms a continuous link from the ECM to the actin cytoskeleton, providing structural integrity and perhaps transducing signal, in a manner similar to integrins. Deregulated expression of DG has been reported in a variety of human malignancies and related to tumor differentiation and aggressiveness. In breast cancer, reduced DG expression has been associated with patient survival and with loss of differentiation of tumor cells. Limited data are available on DG physiology in epithelial cells. In this study, we used the HC11 spontaneously immortalized murine mammary epithelial cells to study DG function(s) and regulation in normal cells. We found that expression of DG protein and mRNA is cell-cycle and cell-density regulated in these cells. Moreover, expression of both DG subunits increased upon lactogenic differentiation of the HC11 cells. The turnover of cell-surface-expressed DG was evaluated in the same cells and half-life of DG subunits was evaluated to be about 12 h. DG-specific small inhibitory RNAs were used to analyze the effects of a reduced expression of DG in these cells. Cells in which DG expression was suppressed were growth inhibited, accumulated in the S-phase of the cell cycle, failed to undergo lactogenic differentiation, and displayed an increase in the percentage of apoptotic cells. Moreover, changes were observed in the expression and/or activity of several molecules involved in cell growth control. These results demonstrate that DG expression is tightly regulated in normal mammary epithelial cells and support the hypothesis that DG is involved in several functions other than structural integrity in these cells. This finding provides new insight into the roles played by DG in epithelial cell physiology and will contribute to our understanding of its involvement in the process of epithelial cell transformation.

Altered expression of alpha-dystroglycan subunit in human gliomas

Dystroglycan (DG) is an integral membrane receptor of extracellular matrix proteins, composed of two subunits alpha and beta derived from a common precursor. In brain DG is expressed in neurons, glia limitans, astrocytic endfeet around vessels and endothelial cells. We investigate whether DG may play a role in brain tumors. Western blot and immunofluorescence analysis showed that, while beta-DG subunit was present, the highly glycosylated alpha-DG subunit was strongly reduced in surgically derived human glioblastoma biopsies, in low passage patient-derived cultures and in glioma cell lines, U87MG and A172MG, but not in all glioma cell lines tested. Immunohistochemistry of tumor frozen sections revealed that the loss of alpha-DG was confined in the tumor area but not around blood vessels. Overexpression of DG decreased the growth rate of the glioma cell lines lacking the highly glycosylated alpha-DG subunit and the colony-forming efficiency. Clonogenic assay in presence of temozolomide showed an additive effect between DG overexpression and drug treatment. Our data suggest that DG may be involved in the progression of primary brain tumors.

Dystroglycan regulates structure, proliferation and differentiation of neuroepithelial cells in the developing vertebrate CNS

In the developing CNS alpha- and beta-dystroglycan are highly concentrated in the endfeet of radial neuroepithelial cells at the contact site to the basal lamina. We show that injection of anti-dystroglycan Fab fragments, knockdown of dystroglycan using RNAi, and overexpression of a dominant-negative dystroglycan protein by microelectroporation in neuroepithelial cells of the chick retina and optic tectum in vivo leads to the loss of their radial morphology, to hyperproliferation, to an increased number of postmitotic neurons, and to an altered distribution of several basally concentrated proteins. Moreover, these treatments also altered the oriented growth of axons from retinal ganglion cells and from tectal projection neurons. In contrast, expression of non-cleavable dystroglycan protein in neuroepithelial cells reduced their proliferation and their differentiation to postmitotic neurons. These results demonstrate that dystroglycan plays a key role in maintaining neuroepithelial cell morphology, and that interfering with dystroglycan function influences proliferation and differentiation of neuroepithelial cells. These data also suggest that an impaired dystroglycan function in neuroepithelial cells might be responsible for some of the severe brain abnormalities observed in certain forms of congenital muscular dystrophy.

Dystroglycan function is a novel determinant of tumor growth and behavior in prostate cancer

BACKGROUND

Dystroglycan is a ubiquitously expressed cell adhesion molecule frequently found to be altered or reduced in adenocarcinomas, however the mechanisms or consequences of dystroglycan loss have not been studied extensively.

METHODS

We examined the consequence of overexpression or RNAi depletion of dystroglycan on properties of in vitro growth migration and invasion of LNCaP, PC3, and DU145 prostate cancer cell lines.

RESULTS

Using LNCaP cells we observed cell density-dependent changes in β-dystroglycan with the appearance of several lower molecular weight species ranging in size from 43 to 26 kDa. The bands of 31 and 26 kDa were attributed to proteolysis, whereas bands between 43 and 38 kDa were a consequence of mis-glycosylation. The localization of β-dystroglycan in LNCaP colonies in culture also varied, cells with a mesenchymal appearance at the periphery of the colony had more pronounced membrane localization of dystroglycan. Whereas some cells demonstrated nuclear dystroglycan. Increased dystroglycan levels were inhibitory to growth in soft agar but promoted Matrigel invasion, whereas reduced dystroglycan levels promoted growth in soft agar but inhibited invasion. Similar results were also obtained for PC3 and DU145 cells.

CONCLUSIONS

This study suggests that changes in β-dystroglycan distribution within the cell and/or the loss of dystroglycan during tumorigenesis, through a combination of proteolysis and altered glycosylation, leads to an increased ability to grow in an anchorage independent manner, however dystroglycan may need to be re-expressed for cell invasion and metastasis to occur.

Dystroglycan is required for proper retinal layering

Dystroglycan (DG) is a transmembrane receptor linking the extracellular matrix to the internal cytoskeleton. Its structural function has been mainly characterized in muscle fibers, but DG plays signaling and developmental roles also in different tissues and cell types. We have investigated the effects of dystroglycan depletion during eye development of Xenopus laevis. We have injected a specific morpholino (Mo) antisense oligonucleotide in the animal pole of one dorsal blastomere of embryos at four cells stage. Mo-mediated loss of DG function caused disruption of the basal lamina layers, increased apoptosis and reduction of the expression domains of specific retinal markers, at early stages. Later in development, morphants displayed unilateral ocular malformations, such as microphtalmia and retinal delayering with photoreceptors and ganglion cells scattered throughout the retina or aggregated in rosette-like structures. These results recall the phenotypes observed in specific human diseases and suggest that DG presence is crucial at early stages for the organization of retinal architecture.