Dystroglycan versatility

The transcription regulator ATF4 is a mediator of skeletal muscle aging

Aging slowly erodes skeletal muscle strength and mass, eventually leading to profound functional deficits and muscle atrophy. The molecular mechanisms of skeletal muscle aging are not well understood. To better understand mechanisms of muscle aging, we investigated the potential role of ATF4, a transcription regulatory protein that can rapidly promote skeletal muscle atrophy in young animals deprived of adequate nutrition or activity. To test the hypothesis that ATF4 may be involved in skeletal muscle aging, we studied fed and active muscle-specific ATF4 knockout mice (ATF4 mKO mice) at 6 months of age, when wild-type mice have achieved peak muscle mass and function, and at 22 months of age, when wild-type mice have begun to manifest age-related muscle atrophy and weakness. We found that 6-month-old ATF4 mKO mice develop normally and are phenotypically indistinguishable from 6-month-old littermate control mice. However, as ATF4 mKO mice become older, they exhibit significant protection from age-related declines in strength, muscle quality, exercise capacity, and muscle mass. Furthermore, ATF4 mKO muscles are protected from some of the transcriptional changes characteristic of normal muscle aging (repression of certain anabolic mRNAs and induction of certain senescence-associated mRNAs), and ATF4 mKO muscles exhibit altered turnover of several proteins with important roles in skeletal muscle structure and metabolism. Collectively, these data suggest ATF4 as an essential mediator of skeletal muscle aging and provide new insight into a degenerative process that impairs the health and quality of life of many older adults.

Plasma membrane-associated superstructure: Have we overlooked a new type of organelle in eukaryotic cells?

A variety of intriguing plasma membrane-associated regions, including focal adhesions, adherens junctions, tight junctions, immunological synapses, neuromuscular junctions and the primary cilia, among many others, have been described in eukaryotic cells. Emphasizing their importance, alteration in their molecular structures induces or correlates with different pathologies. These regions display surface proteins connected to intracellular molecules, including cytoskeletal component, which maintain their cytoarchitecture, and signalling proteins, which regulate their organization and functions. Based on the molecular similarities and other common features observed, we suggest that, despite differences in external appearances, all these regions are just the same superstructure that appears in different locations and cells. We hypothesize that this superstructure represents an overlooked new type of organelle that we call plasma membrane-associated superstructure (PMAS). Therefore, we suggest that eukaryotic cells include classical organelles (e.g. mitochondria, Golgi and others) and also PMAS. We speculate that this new type of organelle might be an innovation associated to the emergence of eukaryotes. Finally we discuss the implications of the hypothesis proposed.

Ancient interaction between the teneurin C-terminal associated peptides (TCAP) and latrophilin ligand-receptor coupling: a role in behavior

Teneurins are multifunctional transmembrane proteins that are found in all multicellular animals and exist as four paralogous forms in vertebrates. They are highly expressed in the central nervous system, where they exert their effects, in part, by high-affinity binding to latrophilin (LPHN), a G-protein coupled receptor (GPCR) related to the adhesion and secretin GPCR families. The teneurin C-terminal associated peptides (TCAPs) are encoded by the terminal exon of all four teneurins, where TCAPs 1 and 3 are independently transcribed as soluble peptides, and TCAPs 2 and 4 remain tethered to their teneurin proprotein. Synthetic TCAP-1 interacts with LPHN, with an association with β-dystroglycan, to induce a tissue-dependent signal cascade to modulate cytoskeletal dynamics. TCAP-1 reduces stress-induced behaviors associated with anxiety, addiction and depression in a variety of models, in part, by regulating synaptic plasticity. Therefore, the TCAP-1-teneurin-LPHN interaction represents a novel receptor-ligand model and may represent a key mechanism underlying the association of behavior and neurological conditions.

Alpha 6 integrin is important for myogenic stem cell differentiation

A muscle progenitor cell population, other than muscle satellite cells, can be isolated and purified from porcine muscle tissue. We show the presence of at least two types of stem cells in porcine muscle: those that express α6 integrin and those that lack expression of this integrin type. By flow cytometry, we could select for myogenic stem cell populations expressing the neural cell adhesion molecule in the presence and absence of α6 integrin. The expression of α6 integrin showed an advantage in the formation of myotubes, possibly by an improved cell fusion capacity. This notion was strengthened by qRT-PCR analysis showing sustained PAX7, MYF5 and DESMIN expression and a strong myogenic differentiation capacity of this stem cell population. Selective inhibition of α6 integrin function, both by blocking antibodies and RNA interference, showed the importance of α6 integrin in myogenic differentiation of muscle stem cells. It is concluded that α6 integrin expression can be used as biomarker to select for highly myogenic cell populations in muscle tissue.

Defective glycosylation of α-dystroglycan contributes to podocyte flattening

In addition to skeletal muscle and the nervous system, α-dystroglycan is found in the podocyte basal membrane, stabilizing these cells on the glomerular basement membrane. Fukutin, named after the gene responsible for Fukuyama-type congenital muscular dystrophy, is a putative glycosyltransferase required for the post-translational modification of α-dystroglycan. Chimeric mice targeted for both alleles of fukutin develop severe muscular dystrophy; however, these mice do not have proteinuria. Despite the lack of a functional renal defect, we evaluated glomerular structure and found minor abnormalities in the chimeric mice by light microscopy. Electron microscopy revealed flattening of podocyte foot processes, the number of which was significantly lower in the chimeric compared to wild-type mice. A monoclonal antibody against the laminin-binding carbohydrate residues of α-dystroglycan did not detect α-dystroglycan glycosylation in the glomeruli by immunoblotting or immunohistochemistry. In contrast, expression of the core α-dystroglycan protein was preserved. There was no statistical difference in dystroglycan mRNA expression or in the amount of nephrin and α3-integrin protein in the chimeric compared to the wild-type mice as judged by immunohistochemistry and real-time RT-PCR. Thus, our results indicate that appropriate glycosylation of α-dystroglycan has an important role in the maintenance of podocyte architecture.

Biglycan binds to alpha- and gamma-sarcoglycan and regulates their expression during development

The dystrophin-associated protein complex (DAPC), which links the cytoskeleton to the extracellular matrix, is essential for muscle cell survival, and is defective in a wide range of muscular dystrophies. The DAPC contains two transmembrane subcomplexes-the dystroglycans and the sarcoglycans. Although several extracellular binding partners have been identified for the dystroglycans, none have been described for the sarcoglycan subcomplex. Here we show that the small leucine-rich repeat (LRR) proteoglycan biglycan binds to alpha- and gamma-sarcoglycan as judged by ligand blot overlay and co-immunoprecipitation assays. Our studies with biglycan-decorin chimeras show that alpha- and gamma-sarcoglycan bind to distinct sites on the polypeptide core of biglycan. Both biglycan proteoglycan as well as biglycan polypeptide lacking glycosaminoglycan (GAG) side chains are components of the dystrophin glycoprotein complex isolated from adult skeletal muscle membranes. Finally, our immunohistochemical and biochemical studies with biglycan null mice show that the expression of alpha- and gamma-sarcoglycan is selectively reduced in muscle from young (P14-P21) animals, while levels in adult muscle (> or = P35) are unchanged. We conclude that biglycan is a ligand for two members of the sarcoglycan complex and regulates their expression at discrete developmental ages.

The canine sarcoglycan delta gene: BAC clone contig assembly, chromosome assignment and interrogation as a candidate gene for dilated cardiomyopathy in Dobermann dogs

Dilated cardiomyopathy (DCM) is a common disease of the myocardium recognized in human, dog and experimental animals. Genetic factors are responsible for a large proportion of cases in humans, and 17 genes with DCM causing mutations have been identified. The genetic origin of DCM in the Dobermann dogs has been suggested, but no disease genes have been identified to date. In this paper, we describe the characterization and evaluation of the canine sarcoglycan delta (SGCD), a gene implicated in DCM in human and hamster. Bacterial artificial chromosomes (BACs) containing the canine SGCD gene were isolated with probes for exon 3 and exons 4-8 and were characterized by Southern blot analysis. BAC end sequences were obtained for four BACs. Three of the BACs overlapped and could be ordered relative to each other and the end sequences of all four BACs could be anchored on the preliminary assembly of the dog genome sequence (www. ensembl.org). One of the BACs of the partial contig was localized by fluorescent in situ hybridization to canine chromosome 4q22, in agreement with the dog genome sequence. Two highly informative polymorphic microsatellite markers in intron 7 of the SGCD gene were identified. In 25 DCM-affected and 13 non DCM-affected dogs seven different haplotypes could be distinguished. However, no association between any of the SGCD variants and the disease locus was apparent.

Sticky worms: adhesion complexes in C. elegans

Caenorhabditis elegans is a powerful model system for investigating the establishment, regulation and function of adhesive structures in vivo. C. elegans has several adhesion complexes related to those in vertebrates. These include: (1) epithelial apical junctions, which have features of both adherens and tight junctions; (2) dense bodies, which are muscle-attachment structures similar to focal adhesions; (3) fibrous organelles, which resemble hemidesmosomes and mediate mechanical coupling between tissues; and (4) a putative dystrophin-glycoprotein complex that has potential roles in muscle function and embryogenesis. Recent work has increased our understanding of these structures and has given new insights into the functions of their vertebrate counterparts.

Regulation of vascular permeability factor/vascular endothelial growth factor (VPF/VEGF-A) expression in podocytes

BACKGROUND

Vascular permeability factor/vascular endothelial growth factor (VPF/VEGF-A) is expressed constitutively in the adult glomerular podocytes at high levels; however, the regulation of its production is unclear. Recent data from podocyte-specific knockout mice suggest that VPF/VEGF-A is critical for the proper maintenance of glomerular filtration barrier and the glomerular endothelial fenestrae. We hypothesized that the glomerular basement membrane (GBM) matrix-podocyte interaction may play a role in the constitutive expression of VPF/VEGF-A in the adult glomerulus.

METHODS

VPF/VEGF-A mRNA levels in a human podocyte cell line grown in the presence of various extracellular matrices were quantitated by real-time polymerase chain reaction (PCR) experiments. VPF/VEGF-A protein levels in the culture supernatant from the same conditions were measured by enzyme-linked immunosorbent assay (ELISA). Promoter activity of VPF/VEGF-A gene in these cells was performed by transfecting full length (2.6 kb) VPF/VEGF-A promoter, which is fused with luciferase reporter gene. Immunoprecipitation and Western blot experiments were carried out in order to detect the association of hypoxia-inducible factor-alpha (HIF-alpha) and p300 in podocyte cells.

RESULTS

In this study, we provide preliminary evidence that signaling through the extracellular matrix proteins and, in particular, laminin and its receptor alpha(3)beta(1) integrin may regulate VPF/VEGF-A production in cultured podocytes in vitro. We also present data that increased activity of the transcription factor HIF-alphas in podocyte is not related to hypoxia and may lead to up-regulation of VPF/VEGF-A transcription. The classical type protein kinase C (PKC) may be a potential intermediate signaling molecule in this event.

CONCLUSION

These data suggest a novel nonhypoxic regulation of VPF/VEGF-A production in the glomerulus of the kidney during physiologic states. These observations may form the basis of more elaborate studies that will finally provide the detailed signaling pathway for VPF/VEGF-A synthesis in podocytes and will help our understanding of the pathogenesis of various VPF/VEGF-A-related diseases in the glomerulus of the kidney.

Dystroglycan in skin and cutaneous cells: beta-subunit is shed from the cell surface

In skin, hemidesmosomal protein complexes attach the epidermis to the dermis and are critical for stable connection of the basal epithelial cell cytoskeleton with the basement membrane (BM). In muscle, a similar supramolecular aggregate, the dystrophin glycoprotein complex links the inside of muscle cells with the BM. A component of the muscle complex, dystroglycan (DG), also occurs in epithelia. In this study, we characterized the expression and biochemical properties of authentic and recombinant DG in human skin and cutaneous cells in vitro. We show that DG is present at the epidermal BM zone, and it is produced by both keratinocytes and fibroblasts in vitro. The biosynthetic precursor is efficiently processed to the alpha- and beta-DG subunits; and, in addition, a distinct extracellular segment of the transmembranous beta-subunit is shed from the cell surface by metalloproteinases. Shedding of the beta-subunit releases the alpha-subunit from the DG complex on the cell surface into the extracellular space. The shedding is enhanced by IL-1beta and phorbol esters, and inhibited by metalloproteinase inhibitors. Deficiency of perlecan, a major ligand of alpha-DG, enhanced shedding suggesting that lack of a binding partner destabilizes the epithelial DG complex and makes it accessible to proteolytic processing.

N-terminal α-dystroglycan binds to different extracellular matrix molecules expressed in regenerating peripheral nerves in a protein-mediated manner and promotes neurite extension of PC12 cells

alpha-dystroglycan is a cell surface receptor that is expressed in many tissues including the nervous system. The study shows that a recombinant, non-glycosylated N-terminal fragment of alpha-dystroglycan comprising residues 30 to 315 [alphaDG (30-315)] bound to laminin-2/-4 and laminin-1, fibronectin and fibrinogen, all molecules highly upregulated in the regenerating peripheral nerve. The interaction was concentration dependent and saturable and could not be inhibited by heparin suggesting only minor involvement of sulfated carbohydrate moieties. In contrast to published data, addition of bivalent cations increased the binding affinity by only ten fold.alphaDG (30-315) promotes neurite extension of PC12 cells in a similar amount as described for laminin isoforms and could be inhibited in a concentration dependent manner by alphaDG (30-315) itself, soluble laminin-1, partially by heparin, EDTA, and an RGD-peptide. Furthermore, co-immunoprecipitations between alpha-dystroglycan and beta1-integrin from PC12 cell surfaces suggested complex interactions between neuronal dystroglycan, integrins, and the ECM that induce neurite extension in vitro.

Vipera lebetina venom contains two disintegrins inhibiting laminin-binding beta1 integrins

To explain the myotoxic effects of snake venoms, we searched for inhibitors of alpha7beta1 integrin, the major laminin-binding integrin in skeletal muscle. We discovered two inhibitors in the venom of Vipera lebetina. One of them, lebein-1 (known as lebein), has already been proposed to be a disintegrin because of its RGD-containing primary sequence. The other, lebein-2, is a novel protein that also interacts firmly with alpha3beta1, alpha6beta1, and alpha7beta1 integrins, but not with the collagen-binding alpha1beta1 and alpha2beta1 integrins. Ligand binding of laminin-recognizing beta1 integrins was efficiently blocked by both lebein-1 and lebein-2. In cell attachment assays, lebein-1 and lebein-2 inhibited myoblast attachment not only to laminin, but also to fibronectin. However, neither lebein-1 nor lebein-2 interacted with alpha7beta1 integrin in an RGD-dependent manner, similar to the interaction of the laminin with alpha7beta1 integrin. Identical divalent cation dependence of integrin binding to laminin and to either of the two inhibitors and their mutually exclusive binding suggest that both lebein-1 and lebein-2 interact with the ligand-binding site of laminin-binding beta1 integrins by mimicking the yet unknown integrin-binding structure of laminins. Like lebein-1, lebein-2 is a soluble heterodimeric disintegrin of low molecular mass. Together with membrane-bound ADAM-2 and ADAM-9, the two inhibitors seem to form a small group of disintegrins that can bind to laminin-binding beta1 integrins. Because of their inhibitory capability both in vitro and in vivo, lebein-1 and lebein-2 may be valuable tools in influencing laminin-induced, integrin-mediated cell functions such as cell anchorage, migration, and mechanical force transduction on laminin-rich basement membranes.

The Discovery of the Matrix Cell, the Identification of the Multipotent Neural Stem Cell and the Development of the Central Nervous System

In the early 1960s I applied 3H-thymidine autoradiography to the study of the cells constituting the neural tube, and found that its wall was composed solely of one kind of single-layered epithelial cell, which perform an elevator movement between the mitotic and DNA-synthetic zones in the wall in accord with the cell cycle. They were identified as multipotent stem cells of the central nervous sytem (CNS) to which I gave the name of matrix cells. (3)H-thymidine autoradiography also revealed the chronology of development of these matrix cells: At first they proliferate only to expand the population (stage I), then switch to differentiate specific neuroblasts in given sequences (stage II), and finally change themselves into ependymoglioblasts, common progenitors of ependymal cells and neuroglia (stage III). Based on these findings, I proposed a monophyletic view of cytogenesis of the central nervous sytem. This matrix cell theory claiming the existence of multipotent stem cells has long been the target of severe criticism and not been accepted among neuro-embryologists for a long time. Recent findings by experimental and clinical neuroscientists on the importance of stem cells have renewed interest in the nature and biology of the multipotent neural stem cells. The present paper describes how the concept of the matrix cell (multipotent neural stem cells in vivo) emerged and what has come out from this view over the last 45 years, and how the basic concept of the matrix cell theory has recently been reconfirmed after a long period of controversy and neglect.

Cell biology of the glomerular podocyte

Glomerular podocytes are highly specialized cells with a complex cytoarchitecture. Their most prominent features are interdigitated foot processes with filtration slits in between. These are bridged by the slit diaphragm, which plays a major role in establishing the selective permeability of the glomerular filtration barrier. Injury to podocytes leads to proteinuria, a hallmark of most glomerular diseases. New technical approaches have led to a considerable increase in our understanding of podocyte biology including protein inventory, composition and arrangement of the cytoskeleton, receptor equipment, and signaling pathways involved in the control of ultrafiltration. Moreover, disturbances of podocyte architecture resulting in the retraction of foot processes and proteinuria appear to be a common theme in the progression of acquired glomerular disease. In hereditary nephrotic syndromes identified over the last 2 years, all mutated gene products were localized in podocytes. This review integrates our recent physiological and molecular understanding of the role of podocytes during the maintenance and failure of the glomerular filtration barrier.

Mechanistic distinctions between agrin and laminin-1 induced aggregation of acetylcholine receptors

BACKGROUND

One of the earliest steps in synaptogenesis at the neuromuscular junction is the aggregation of nicotinic acetylcholine receptors at the postsynaptic membrane. This study presents quantitative analyses of receptor and alpha-Dystroglycan aggregation in response to agrin and laminin-1, alone or in combination.

RESULTS

Both laminin and agrin increased overall expression of receptors on the plasma membrane. Following a 24 hour exposure, agrin increased the number of receptor aggregates but did not affect the number of alpha-Dystroglycan aggregates, while the reverse was true of laminin-1. Laminin also increased receptor concentration within aggregates, while agrin had no such effect. Finally, the spatial distribution of aggregates was indistinguishable from random in the case of laminin, while agrin induced aggregates were closer together than predicted by a random model.

CONCLUSIONS

Agrin and laminin-1 both increase acetylcholine receptor aggregate size after 24 hours, but several lines of evidence indicate that this is achieved via different mechanisms. Agrin and laminin had different effects on the number and density of receptor and alpha-Dystroglycan aggregates. Moreover the random distribution of laminin induced (as opposed to agrin induced) receptor aggregates suggests that the former may influence aggregate size by simple mass action effects due to increased receptor expression.

Dystroglycan expression in hepatic stellate cells: role in liver fibrosis

Dystroglycan is a membrane component of the dystrophin-glycoprotein transmembrane complex. Its expression is required for the spatial organization of laminin on the cell surface and for basement membrane assembly. In view of the constitution of a perisinusoidal basement membrane during liver fibrosis, we studied dystroglycan expression in liver fibrosis. Dystroglycan mRNA and protein expression were investigated by immunofluorescence, Western blot, and quantitative RT-PCR (TaqMan) in normal human and rat liver and in isolated rat hepatic stellate cells. On Western blot, a 43-kd band corresponding to beta-dystroglycan was observed in protein extracted from normal and fibrotic human and rat livers. The specific 43-kd protein was also detected in lysates from rat hepatic stellate cells but not from hepatocytes. By immunofluorescence, patchy deposits of beta-dystroglycan were detected on membrane of hepatic stellate cells in culture. On Western blot and quantitative RT-PCR, an up-regulation of beta-dystroglycan was shown during spontaneous activation of hepatic stellate cells in culture. Direct evidence for the role of dystroglycan in laminin-hepatic stellate cell interaction was shown because specific antibody directed against alpha-dystroglycan inhibited partially hepatic stellate cell adhesion on laminin-coated plates. This mechanism was calcium dependent because EDTA inhibited cell/laminin adhesion, an effect reversed by addition of Ca(2+). This study shows that dystroglycan is expressed on the membrane of hepatic stellate cells and is up-regulated during liver fibrosis. Dystroglycan interaction with laminin should be implicated in the concentration of pericellular laminin and in the constitution of a perisinusoidal basement membrane during liver fibrosis.

Regulation of adhesive interaction between podocytes and glomerular basement membrane

Glomerular filtration depends on well-orchestrated cell-cell and cell-matrix contacts of glomerular podocytes. Over the last years critical constituents of these contacts have been identified via molecular approaches. Podocyte cell-matrix interactions have been shown to be mediated in part by alpha(3)beta(1)-integrin heterodimers. Disturbances of integrin matrix interaction lead to detachment of podocytes in vitro, corresponding to the critical event of foot process retraction and glomerular basement membrane (GBM) denudation in vivo. Further, dystroglycan-mediated matrix attachment appears to play a critical role for podocyte foot process architecture. Downstream signaling events are currently elucidated concentrating mainly on integrin-dependent cascades and their consequences for podocyte adhesion and proliferation. An activation of the integrin-linked kinase in podocyte damage in vivo and in vitro makes this molecule a particularly interesting candidate for integrin-mediated inside-out and outside-in signaling in podocytes. Podocyte cell-cell interaction has been characterized in a few studies in vitro, indicating the slit diaphragm to be a modified adherens junction. The structural link between the cell-matrix and cell-cell contacts is maintained by the actin cytoskeleton, which may also enable cross-talk between these two cell contact sites. Examining podocyte function in tissue culture, animal models and human expression studies should allow further detailed dissection of the molecular pathways responsible for maintenance and failure of the glomerular filtration barrier.

Differential display RT-PCR analysis of enterovirus-71-infected rhabdomyosarcoma cells reveals mRNA expression responses of multiple human genes with known and novel functions

In order to better understand cellular responses to viral infection at the transcriptional level, we employed differential display RT-PCR to analyze mRNAs from RD rhabdomyosarcoma cells following infection with a neurovirulent enterovirus 71 (EV71) strain, compared with mRNAs from uninfected cells. Of 250 expressed sequence tags (ESTs) isolated, sequenced, and identified, all were of cellular origin except 1 that was of viral origin. Of these, 156 were individual distinctive clones, comprising 45 mRNAs showing unaltered expression and 111 mRNAs exhibiting upregulation or downregulation. Of the 45 uniformly expressed mRNAs, 14 represented unknown genes. Of the 111 differentially expressed mRNAs, 63 did not match any known genes. Forty-eight of the 111 mRNAs modified by EV71 infection matched known genes, including those encoding components of cell cycle, cytoskeleton, and cell death mediators; protein degradation mediators; mitochondrial-related proteins; components of protein translation and modification; and cellular transport proteins. The altered expression profiles of representative genes were authenticated by semiquantitative RT-PCR and real-time RT-PCR. We also identified a novel alternatively spliced transcript of TRIP7 thyroid receptor interactor protein; the putative human homolog of murine mc7 mRNA predominantly expressed in the brain; and a novel mRNA similar to that encoding vacuolar protein 8 involved in protein targeting. These results underscore the applicability of the mRNA differential display technique for elucidating the expression profiles of known and even novel genes in response to cellular infection with pathogenic viruses.

Function of the tetraspanin CD151-alpha6beta1 integrin complex during cellular morphogenesis

Upon plating on basement membrane Matrigel, NIH3T3 cells formed an anastomosing network of cord-like structures, inhibitable by anti-alpha6beta1 integrin antibodies. For NIH3T3 cells transfected with human CD151 protein, the formation of a cord-like network was also inhibitable by anti-CD151 antibodies. Furthermore, CD151 and alpha6beta1 were physically associated within NIH3T3 cells. On removal of the short 8-amino acid C-terminal CD151 tail (by deletion or exchange), exogenous CD151 exerted a dominant negative effect, as it almost completely suppressed alpha6beta1-dependent cell network formation and NIH3T3 cell spreading on laminin-1 (an alpha6beta1 ligand). Importantly, mutant CD151 retained alpha6beta1 association and did not alter alpha6beta1-mediated cell adhesion to Matrigel. In conclusion, the CD151-alpha6beta1 integrin complex acts as a functional unit that markedly influences cellular morphogenesis, with the CD151 tail being of particular importance in determining the "outside-in" functions of alpha6beta1-integrin that follow ligand engagement. Also, antibodies to alpha6beta1 and CD151 inhibited formation of endothelial cell cord-like networks, thus pointing to possible relevance of CD151-alpha6beta1 complexes during angiogenesis.

A stoichiometric complex of neurexins and dystroglycan in brain

In nonneuronal cells, the cell surface protein dystroglycan links the intracellular cytoskeleton (via dystrophin or utrophin) to the extracellular matrix (via laminin, agrin, or perlecan). Impairment of this linkage is instrumental in the pathogenesis of muscular dystrophies. In brain, dystroglycan and dystrophin are expressed on neurons and astrocytes, and some muscular dystrophies cause cognitive dysfunction; however, no extracellular binding partner for neuronal dystroglycan is known. Regular components of the extracellular matrix, such as laminin, agrin, and perlecan, are not abundant in brain except in the perivascular space that is contacted by astrocytes but not by neurons, suggesting that other ligands for neuronal dystroglycan must exist. We have now identified alpha- and beta-neurexins, polymorphic neuron-specific cell surface proteins, as neuronal dystroglycan receptors. The extracellular sequences of alpha- and beta-neurexins are largely composed of laminin-neurexin-sex hormone-binding globulin (LNS)/laminin G domains, which are also found in laminin, agrin, and perlecan, that are dystroglycan ligands. Dystroglycan binds specifically to a subset of the LNS domains of neurexins in a tight interaction that requires glycosylation of dystroglycan and is regulated by alternative splicing of neurexins. Neurexins are receptors for the excitatory neurotoxin alpha-latrotoxin; this toxin competes with dystroglycan for binding, suggesting overlapping binding sites on neurexins for dystroglycan and alpha-latrotoxin. Our data indicate that dystroglycan is a physiological ligand for neurexins and that neurexins' tightly regulated interaction could mediate cell adhesion between brain cells.

Genomics and the pathophysiology of heart failure

Heart failure is not a single disease entity, but a syndrome with various causes, including hypertension, ischemic and congenital heart disease, cardiomyopathy, and myocarditis. Because of the multiple etiologies and secondary adaptations contributing to heart failure, the study of the cellular and molecular mechanisms underlying the development and progression of this syndrome has been rather challenging. Much has been learned about the remodeling processes in heart failure, which involve complex interactions among numerous mediators in signaling and regulatory pathways. The Human Genome Project and related projects have provided a preliminary database for a genome-wide analysis of complex polygenic disorders such as heart failure. With the aid of expressed sequence tag technology and microarray applications, both known and previously uncharacterized genes involved in the induction and regression of cardiac hypertrophy and its progression to heart failure can be analyzed simultaneously. Deciphering the complexity of sequence-structure-function relationships in heart failure is a goal for the future, and will require advances in structural biology, proteomics, and computational technology. In this review, we summarize the cellular and molecular aspects of heart failure, and how recent applications of genomic technologies have been successful in achieving a more complete portrait of gene expression in this pathologic state.

Polycystin: new aspects of structure, function, and regulation

Polycystin-1 is a modular membrane protein with a long extracellular N-terminal portion that bears several ligand-binding domains, 11 transmembrane domains, and a > or =200 amino acid intracellular C-terminal portion with several phosphorylation signaling sites. Polycystin-1 is highly expressed in the basal membranes of ureteric bud epithelia during early development of the metanephric kidney, and disruption of the PKD1 gene in mice leads to cystic kidneys and embryonic or perinatal death. It is proposed that polycystin-1 functions as a matrix receptor to link the extracellular matrix to the actin cytoskeleton via focal adhesion proteins. Co-localization, co-sedimentation, and co-immunoprecipitation studies show that polycystin-1 forms multiprotein complexes with alpha2beta1-integrin, talin, vinculin, paxillin, p130cas, focal adhesion kinase, and c-src in normal human fetal collecting tubules and sub-confluent epithelial cultures. In normal adult kidneys and confluent epithelial cultures, polycystin-1 is downregulated and forms complexes with the cell-cell adherens junction proteins E-cadherin and beta-, gamma-, and alpha-catenin. Polycystin-1 activation at the cell membrane leads to intracellular signaling via phosphorylation through the c-Jun terminal kinase and wnt pathways leading to activation of AP-1 and TCF/LEF-dependent genes, respectively. The C-terminal of polcystin-1 has been shown to be phosphorylated by c-src at Y4237, by protein kinase A at S4252, and by focal adhesion kinase and protein kinase X at yet-to-be identified residues. Inhibition of tyrosine phosphorylation or increased cellular calcium increases polycystin-1 focal adhesion complexes versus polycystin-1 adherens junction complexes, whereas disruption of the actin cytoskeleton dissociates all polycystin-1 complexes. Genetic evidence suggests that PKD1, PKD2, NPHP1, and tensin are in the same pathway.

The NC1/endostatin domain of Caenorhabditis elegans type XVIII collagen affects cell migration and axon guidance

Type XVIII collagen is a homotrimeric basement membrane molecule of unknown function, whose COOH-terminal NC1 domain contains endostatin (ES), a potent antiangiogenic agent. The Caenorhabditis elegans collagen XVIII homologue, cle-1, encodes three developmentally regulated protein isoforms expressed predominantly in neurons. The CLE-1 protein is found in low amounts in all basement membranes but accumulates at high levels in the nervous system. Deletion of the cle-1 NC1 domain results in viable fertile animals that display multiple cell migration and axon guidance defects. Particular defects can be rescued by ectopic expression of the NC1 domain, which is shown to be capable of forming trimers. In contrast, expression of monomeric ES does not rescue but dominantly causes cell and axon migration defects that phenocopy the NC1 deletion, suggesting that ES inhibits the promigratory activity of the NC1 domain. These results indicate that the cle-1 NC1/ES domain regulates cell and axon migrations in C. elegans.

Isolation of a mouse brain cDNA expressed in developing neuroblasts and mature neurons

A previously uncharacterized 4.5-kb mouse cDNA clone, designated mc7, was isolated and found to be predominantly expressed in brain. This cDNA predicts a 1035-bp open reading frame that encodes for a 345-amino acid polypeptide especially rich in glutamic acid residues located in the region from residues 80 to 174. Computational analysis revealed among other features, putative zinc-finger motifs and coiled-coil regions. The corresponding mc7 gene is detected in mouse, rat, pig and human genomes. In mouse the mc7 mRNA is expressed predominantly in brain and to a much lesser extent in kidney, lung and spleen. In brain it is detectable as early as embryonic day 14 while it is retained in the adult. In situ hybridization studies revealed that mc7 mRNA is widely, albeit unevenly, expressed in neurons throughout the adult brain. Developmental in situ hybridization studies in the cerebellar cortex demonstrated that at postnatal day 5 mc7 mRNA is mainly expressed in neuroblasts of the external granular layer and in developing neurons of the internal granular layer. Some staining is also present in purkinje cells becoming particularly pronounced at postnatal day 10, the time of arborarization of their dendritic tree. In the adult cerebellar cortex expression is mainly confined in purkinje cells and to a lesser extent in granule neurons. The early expression of mc7 in neuroblasts and developing neurons as well as its retention in a wide variety of mature neurons suggest that it may play a role in the process of differentiation and maturation of these cells in the brain.

Differences in affinity of binding of lymphocytic choriomeningitis virus strains to the cellular receptor alpha-dystroglycan correlate with viral tropism and disease kinetics

alpha-Dystroglycan (alpha-DG) was recently identified as a receptor for lymphocytic choriomeningitis virus (LCMV) and several other arenaviruses, including Lassa fever virus (W. Cao, M. D. Henry, P. Borrow, H. Yamada, J. H. Elder, E. V. Ravkov, S. T. Nichol, R. W. Compans, K. P. Campbell, and M. B. A. Oldstone, Science 282:2079-2081, 1998). Data presented in this paper indicate that the affinity of binding of LCMV to alpha-DG determines viral tropism and the outcome of infection in mice. To characterize this relationship, we evaluated the interaction between alpha-DG and several LCMV strains, variants, and reassortants. These viruses could be divided into two groups with respect to affinity of binding to alpha-DG, dependence on this protein for cell entry, viral tropism, and disease course. Viruses that exhibited high-affinity binding to alpha-DG displayed a marked dependence on alpha-DG for cell entry and were blocked from infecting mouse 3T6 fibroblasts by 1 to 4 nM soluble alpha-DG. In addition, high-affinity binding to alpha-DG correlated with an ability to infiltrate the white pulp (T-dependent) area of the spleen, cause ablation of the cytotoxic T-lymphocyte (CTL) response by day 7 postinfection, and establish a persistent infection. In contrast, viruses with a lower affinity of binding to alpha-DG were only partially inhibited from infecting alpha-DG(-/-) embryonic stem cells and required a concentration of soluble alpha-DG higher than 100 nM to prevent infection of mouse 3T6 fibroblasts. These viruses that bound at low affinity were mainly restricted to the splenic red pulp, and the host generated an effective CTL response that rapidly cleared the infection. Reassortants of viruses that bound to alpha-DG at high and low affinities were used to map genes responsible for the differences described to the S RNA, containing the virus attachment protein glycoprotein 1.

A new beta-1,2-N-acetylglucosaminyltransferase that may play a role in the biosynthesis of mammalian O-mannosyl glycans

Recent studies have shown that O-mannosyl glycans are present in several mammalian glycoproteins. Although knowledge on the functional roles of these glycans is accumulating, their biosynthetic pathways are poorly understood. Here we report the identification and initial characterization of a novel enzyme capable of forming GlcNAc beta 1-2Man linkage, namely UDP-N-acetylglucosamine: O-linked mannose beta-1,2-N-acetylglucosaminyltransferase in the microsome fraction of newborn rat brains. The enzyme transfers GlcNAc to beta-linked mannose residues, and the formed linkage was confirmed to be beta 1-2 on the basis of diplococcal beta-N-acetylhexosaminidase susceptibility and by high-pH anion-exchange chromatography. Its activity is linearly dependent on time, protein concentration, and substrate concentration and is enhanced in the presence of manganese ion. Its activity is not due to UDP-N-acetylglucosamine: alpha-3-D-mannoside beta-1,2-N-acetylglucosaminyltransferase I (GnT-I) or UDP-N-acetylglucosamine: alpha-6-D-mannoside beta-1,2-D-acetylglucosaminyltransferase II (GnT-II), which acts on the early steps of N-glycan biosynthesis, because GnT-I or GnT-II expressed in yeast cells did not show any GlcNAc transfer activity against a synthetic mannosyl peptide. Taken together, the results suggest that the GlcNAc transferase activity described here is relevant to the O-mannosyl glycan pathway in mammals.

Modification of the composition of polycystin-1 multiprotein complexes by calcium and tyrosine phosphorylation

Mutations in the PKD1 gene are responsible for >85% of autosomal dominant polycystic kidney disease (ADPKD). The protein product of PKD1, polycystin-1, is a large, modular membrane protein, with putative ligand-binding motifs in the extracelluar N-terminal portion, 9-11 transmembrane domains and an intracellular C-terminal portion with phosphorylation sites. A role for polycystin-1 as a cell surface receptor involved in cell-matrix and cell-cell interactions has been proposed. In this study, we have analyzed polycystin-1 and associated protein distribution in normal human epithelial cells and examined the role of cell-matrix versus cell-cell interactions in regulation of the assembly of polycystin-1 multiprotein complexes. Immunocytochemistry, sucrose density gradient sedimentation, co-immunoprecipitation analyses and in vitro binding assays have shown that polycystin-1 associates with the focal adhesion proteins talin, vinculin, p130Cas, FAK, alpha-actinin, paxillin and pp60c-src in subconfluent normal human fetal collecting tubule (HFCT) epithelia when cell-matrix interactions predominate. Polycystin-1 also forms higher S value complexes with the cell-cell adherens junction proteins E-cadherin, beta- and gamma-catenins in confluent cultures when cell-cell interactions are predominant. Polycystin-1 multiprotein complexes can be disrupted by cytochalasin D but not by colchicine, suggesting involvement of the actin cytoskeleton. Although inhibition of tyrosine phosphorylation by tyrphostin inhibits polycystin-1-FAK interactions, E-cadherin interactions are enhanced. High calcium treatment also increases polycystin-1-E-cadherin interactions.

Dystroglycan overexpression in vivo alters acetylcholine receptor aggregation at the neuromuscular junction

Dystroglycan is a member of the transmembrane dystrophin glycoprotein complex in muscle that binds to the synapse-organizing molecule agrin. Dystroglycan binding and AChR aggregation are mediated by two separate domains of agrin. To test whether dystroglycan plays a role in receptor aggregation at the neuromuscular junction, we overexpressed it by injecting rabbit dystroglycan RNA into one- or two-celled Xenopus embryos. We measured AChR aggregation in myotomes by labeling them with rhodamine-alpha-bungarotoxin followed by confocal microscopy and image analysis. Dystroglycan overexpression decreased AChR aggregation at the neuromuscular junction. This result is consistent with dystroglycan competition for agrin without signaling AChR aggregation. It also supports the hypothesis that dystroglycan is not the myotube-associated specificity component, (MASC) a putative coreceptor needed for agrin to activate muscle-specific kinase (MuSK) and signal AChR aggregation. Dystroglycan was distributed along the surface of muscle membranes, but was concentrated at the ends of myotomes, where AChRs normally aggregate at synapses. Overexpressed dystroglycan altered AChR aggregation in a rostral-caudal gradient, consistent with the sequential development of neuromuscular synapses along the embryo. Increasing concentrations of dystroglycan RNA did not further decrease AChR aggregation, but decreased embryo survival. Development often stopped during gastrulation, suggesting an essential, nonsynaptic role of dystroglycan during this early period of development.

Immunosuppression and resultant viral persistence by specific viral targeting of dendritic cells

Among cells of the immune system, CD11c(+) and DEC-205(+) splenic dendritic cells primarily express the cellular receptor (alpha-dystroglycan [alpha-DG]) for lymphocytic choriomeningitis virus (LCMV). By selection, strains and variants of LCMV that bind alpha-DG with high affinity are associated with virus replication in the white pulp, show preferential replication in a majority of CD11c(+) and DEC-205(+) cells, cause immunosuppression, and establish a persistent infection. In contrast, viral strains and variants that bind with low affinity to alpha-DG are associated with viral replication in the red pulp, display minimal replication in CD11c(+) and DEC-205(+) cells, and generate a robust anti-LCMV cytotoxic T lymphocyte response that clears the virus infection. Differences in binding affinities can be mapped to a single amino acid change in the viral glycoprotein 1 ligand that binds to alpha-DG. These findings indicate that receptor-virus interaction on dendritic cells in vivo can be an essential step in the initiation of virus-induced immunosuppression and viral persistence.

Integrins as receptors for laminins

Laminins are a family of trimeric glycoproteins present in the extracellular matrix and the major constituents of basement membranes. Integrins are alpha beta transmembrane receptors that play critical roles in both cell-matrix and cell-cell adhesion. Several members of the integrin family, including alpha 1 beta 1, alpha 2 beta 1, alpha 3 beta 1, alpha 6 beta 1, alpha 7 beta 1 and alpha 6 beta 4 heterodimers serve as laminin receptors on a variety of cell types. This review summarizes recent advances in understanding the involvement of individual integrins in cell interactions with laminins and the roles of laminin-binding integrins in adhesion-mediated events in vertebrates, including embryonic development, cell migration and tumor cell invasiveness, cell proliferation and differentiation, as well as basement membrane assembly. We discuss the regulation of integrin function via alternative splicing of cytoplasmic domains of alpha and beta subunits of the integrin receptors for laminins and present examples of functional collaboration between laminin-binding integrins and non-integrin laminin receptors. Advances in our understanding of the laminin-binding integrins continue to demonstrate the essential roles these receptors play in maintaining cell polarity and tissue architecture.

Differential requirement for individual sarcoglycans and dystrophin in the assembly and function of the dystrophin-glycoprotein complex

Sarcoglycan is a multimeric, integral membrane glycoprotein complex that associates with dystrophin. Mutations in individual sarcoglycan subunits have been identified in inherited forms of muscular dystrophy. To evaluate the contributions of sarcoglycan and dystrophin to muscle membrane stability and muscular dystrophy, we compared muscle lacking specific sarcoglycans or dystrophin. Here we report that mice lacking (delta)-sarcoglycan developed muscular dystrophy and cardiomyopathy similar to mice lacking (gamma)-sarcoglycan. However, unlike muscle lacking (gamma)-sarcoglycan, (delta)-sarcoglycan-deficient muscle was sensitive to eccentric contraction-induced disruption of the plasma membrane. In the absence of (delta)-sarcoglycan, (alpha)-, (beta)- and (gamma)-sarcoglycan were undetectable, while dystrophin was expressed at normal levels. In contrast, without (gamma)-sarcoglycan, reduced levels of (alpha)-, (beta)- and (delta)-sarcoglycan were expressed, glycosylated and formed a complex with each other. Thus, the elimination of (gamma)- and (delta)-sarcoglycan had different molecular consequences for the assembly and function of the dystrophin-glycoprotein complex. Furthermore, these molecular differences were associated with different mechanical consequences for the muscle plasma membrane. Through this in vivo analysis, a model for sarcoglycan assembly is proposed.

Deflazacort increases laminin expression and myogenic repair, and induces early persistent functional gain in mdx mouse muscular dystrophy

Deflazacort slows the progress of Duchenne muscular dystrophy (DMD) with fewer side effects than prednisone. In mdx mice, deflazacort treatment augments repair and growth of new muscle fibers. We tested the hypothesis that deflazacort improves muscle function and promotes repair by increasing myogenic cell proliferation and fiber differentiation. mdx mice (3.5 weeks old) were treated with deflazacort (1.2 mg/kg) or vehicle for 4 weeks. Forelimb grip strength was measured. After 4 weeks, the right tibialis anterior muscle (TA) was crush injured to induce synchronous regeneration. DNA was labeled using different markers 24 and 2 h before collecting tissues 4 days after injury. The expression of creatine kinase (CK) isoforms, laminin-2 (merosin) mRNA and protein, and proliferation by myogenic cells were measured and satellite cells were identified by immunolocalization of c-met receptor. Peak grip strength increased 15% within 10 days of treatment, and was maintained up to 6 weeks after the end of treatment in a second experiment. Expression of CK MM in the regenerating TA rose from 46% to 55% of total CK activity after deflazacort treatment. Satellite cells were more numerous and appeared earlier on new fibers, in concert with a threefold increase in proliferation by myogenin+ (but not MyoD+) myoblasts. alpha2-Laminin mRNA expression and protein increased 1.3-5.5-fold relative to MM CK in regenerating and dystrophic TA, respectively. These studies support the hypothesis that deflazacort promotes functional gains, myogenic differentiation, myoblast fusion, and laminin expression in regenerating dystrophic muscle. The potential to augment precursor specification, strength, and possible membrane stability may be useful in directing long-term benefits for DMD patients and short-term amplification of precursors prior to myoblast transfer.

Salmonella typhimurium infection and lipopolysaccharide stimulation induce similar changes in macrophage gene expression

Changes in macrophage phenotype induced during infection result from the recognition of bacterial products as well as the action of bacterial virulence factors. We used the unprecedented opportunity provided by gene arrays to simultaneously study the expression of hundreds of genes during Salmonella typhimurium infection of macrophages and to assess the contribution of the bacterial virulence factor, LPS, in initiating the host responses to Salmonella. We found that S. typhimurium infection caused significant changes in the expression of numerous genes encoding chemokines, cell surface receptors, signaling molecules, and transcriptional activators at 4 h postinfection of the RAW 264.7 murine macrophage cell line. Our results revealed changes in the expression of several genes that had not been previously implicated in the host responses to S. typhimurium infection, as well as changes in the expression of several genes previously shown to be regulated by S. typhimurium infection. An overlapping spectrum of genes was expressed in response to virulent S. typhimurium and purified S. typhimurium LPS, reinforcing the major role of this surface molecule in stimulating the early response of macrophages to bacterial infection. The macrophage gene expression profile was further altered by activation with IFN-gamma, indicating that host cell responses depend on the activation state of the cell.

A role for nitric oxide in muscle repair: nitric oxide-mediated activation of muscle satellite cells

Muscle satellite cells are quiescent precursors interposed between myofibers and a sheath of external lamina. Although their activation and recruitment to cycle enable muscle repair and adaptation, the activation signal is not known. Evidence is presented that nitric oxide (NO) mediates satellite cell activation, including morphological hypertrophy and decreased adhesion in the fiber-lamina complex. Activation in vivo occurred within 1 min after injury. Cell isolation and histology showed that pharmacological inhibition of nitric oxide synthase (NOS) activity prevented the immediate injury-induced myogenic cell release and delayed the hypertrophy of satellite cells in that muscle. Transient activation of satellite cells in contralateral muscles 10 min later suggested that a circulating factor may interact with NO-mediated signaling. Interestingly, satellite cell activation in muscles of mdx dystrophic mice and NOS-I knockout mice quantitatively resembled NOS-inhibited release of normal cells, in agreement with reports of displaced and reduced NOS expression in dystrophin-deficient mdx muscle and the complete loss of NOS-I expression in knockout mice. Brief NOS inhibition in normal and mdx mice during injury produced subtle alterations in subsequent repair, including apoptosis in myotube nuclei and myotube formation inside laminar sheaths. Longer NOS inhibition delayed and restricted the extent of repair and resulted in fiber branching. A model proposes the hypothesis that NO release mediates satellite cell activation, possibly via shear-induced rapid increases in NOS activity that produce "NO transients."

Genomic organization of the dog dystroglycan gene DAG1 locus on chromosome 20q15.1-q15.2

Dystroglycan is a laminin binding protein, which provides a structural link between the subsarcolemmal cytoskeleton and the extracellular matrix. It is also involved in the organization of basement membranes. So far the genomic organization of the dystroglycan gene DAG1 has not been completely investigated. Here we report the cloning and sequencing of 162 kb of dog genomic DNA containing the complete approximately 71-kb canine DAG1 gene, which consists of three exons, with the translation start codon located in exon 2. Its 2679-nucleotide ORF encodes a polypeptide of 892 amino acids, which is highly similar to human, rabbit, and bovine orthologs. To further characterize the dog DAG1 gene we determined the transcription start site and several naturally occurring polymorphisms, which partially result in amino acid substitutions of the dystroglycan protein. The dog DAG1 gene was assigned to chromosome 20q15.1-q15.2 by FISH analysis. The analysis of the entire reported sequence revealed that the genes for aminomethyltransferase (AMT), bassoon (BSN), TCTA (T-cell leukemia translocation-associated) gene, and an as yet uncharacterized protein are located very close to the DAG1 gene. Therefore, this study defines a novel syntenic region among dog chromosome 20q15, human chromosome 3p21, and murine chromosome 9F.

Glomerular expression of dystroglycans is reduced in minimal change nephrosis but not in focal segmental glomerulosclerosis

Extensive flattening of podocyte foot processes and increased permeability of the glomerular capillary filter are the major pathologic features of minimal change nephrosis (MCN) and focal segmental glomerulosclerosis (FSGS). Adhesion proteins anchor and stabilize podocytes on the glomerular basement membrane (GBM), and presumably are involved in the pathogenesis of foot process flattening. Thus far, ao3 P,-integrin was localized to basal cell membrane domains. In this report, ao- and 3-dystroglycan (DG) were detected at precisely the sa-ne location by immunoelectron microscopy. and the presence of ac- and /-DG chains was confirmed by immunoblotting on isolated human glomeruli. Because the major DG binding partners in the GBM (laminin, agrin, perlecan), and the intracellular dystrophin analogue utrophin are also present in glomeruli, it appears that podocytes adhere to the GBM via DG complexes, similar to muscle fibers in which actin is linked via dystrophin and DG to the extracellular matrix. As with muscle cells, it is therefore plausible that podocytes use precisely actin-guided DG complexes at their "soles" to actively govern the topography of GBM matrix proteins. Expression of the a//3-DG complex was reported to be reduced in muscular dystrophies. and therefore a search for similar pathologic alterations in archival kidney biopsies from patients with MCN (it = 16) and FSGS (ni = 8) was conducted by quantitative immunoelectron microscopy. The density of a-DG on the podocyte's soles was significantly reduced to 25% in MCN, whereas it was not different in normal kidneys and FSGS. The expression of 3-DG was reduced to >50% in MCN, and was slightly increased in FSGS. Levels of DG expression returned to normal in MCN after steroid treatment (7 = 4). Expression of /3-integrin remained at normal levels in all conditions. These findings point to different potentially pathogenic mechanisms of foot process flattening in MCN and FSGS.

The neurobiology of duchenne muscular dystrophy: learning lessons from muscle?

Several forms of inherited muscular dystrophy are associated with brain abnormalities and cognitive impairment. One of the most common and severe of these diseases is Duchenne muscular dystrophy (DMD). Dystrophin, the product of the DMD gene, is found in neurones, where it is associated with the postsynaptic membrane. Cognitive impairment in individuals with DMD is thought to be due to an abnormality in the neuronal membrane that is caused by lack of dystrophin. Recent experimental evidence has provided valuable clues in our understanding of the complex molecular neurobiology of muscular dystrophy.

The small leucine-rich repeat proteoglycan biglycan binds to alpha-dystroglycan and is upregulated in dystrophic muscle

The dystrophin-associated protein complex (DAPC) is necessary for maintaining the integrity of the muscle cell plasma membrane and may also play a role in coordinating signaling events at the cell surface. The alpha-/beta-dystroglycan subcomplex of the DAPC forms a critical link between the cytoskeleton and the extracellular matrix. A ligand blot overlay assay was used to search for novel dystroglycan binding partners in postsynaptic membranes from Torpedo electric organ. An approximately 125-kD dystroglycan-binding polypeptide was purified and shown by peptide microsequencing to be the Torpedo ortholog of the small leucine-rich repeat chondroitin sulfate proteoglycan biglycan. Biglycan binding to alpha-dystroglycan was confirmed by coimmunoprecipitation with both native and recombinant alpha-dystroglycan. The biglycan binding site was mapped to the COOH-terminal third of alpha-dystroglycan. Glycosylation of alpha-dystroglycan is not necessary for this interaction, but binding is dependent upon the chondroitin sulfate side chains of biglycan. In muscle, biglycan is detected at both synaptic and nonsynaptic regions. Finally, biglycan expression is elevated in muscle from the dystrophic mdx mouse. These findings reveal a novel binding partner for alpha-dystroglycan and demonstrate a novel avenue for interaction of the DAPC and the extracellular matrix. These results also raise the possibility of a role for biglycan in the pathogenesis, and perhaps the treatment, of muscular dystrophy.

Crystal structure of human sex hormone-binding globulin: steroid transport by a laminin G-like domain

Human sex hormone-binding globulin (SHBG) transports sex steroids in blood and regulates their access to target tissues. In biological fluids, SHBG exists as a homodimer and each monomer comprises two laminin G-like domains (G domains). The crystal structure of the N-terminal G domain of SHBG in complex with 5alpha-dihydrotestosterone at 1.55 A resolution reveals both the architecture of the steroid-binding site and the quaternary structure of the dimer. We also show that G domains have jellyroll topology and are structurally related to pentraxin. In each SHBG monomer, the steroid intercalates into a hydrophobic pocket within the beta-sheet sandwich. The steroid and a 20 A distant calcium ion are not located at the dimer interface. Instead, two separate steroid-binding pockets and calcium-binding sites exist per dimer. The structure displays intriguing disorder for loop segment Pro130-Arg135. In all other jellyroll proteins, this loop is well ordered. If modelled accordingly, it covers the steroid-binding site and could thereby regulate access of ligands to the binding pocket.

Sarcoglycans in muscular dystrophy

Muscular dystrophy is a heterogeneous genetic disease that affects skeletal and cardiac muscle. The genetic defects associated with muscular dystrophy include mutations in dystrophin and its associated glycoproteins, the sarcoglycans. Furthermore, defects in dystrophin have been shown to cause a disruption of the normal expression and localization of the sarcoglycan complex. Thus, abnormalities of sarcoglycan are a common molecular feature in a number of dystrophies. By combining biochemistry, molecular cell biology, and human and mouse genetics, a growing understanding of the sarcoglycan complex is emerging. Sarcoglycan appears to be an important, independent mediator of dystrophic pathology in both skeletal muscle and heart. The absence of sarcoglycan leads to alterations of membrane permeability and apoptosis, two shared features of a number of dystrophies. beta-sarcoglycan and delta-sarcoglycan may form the core of the sarcoglycan subcomplex with alpha- and gamma-sarcoglycan less tightly associated to this core. The relationship of epsilon-sarcoglycan to the dystrophin-glycoprotein complex remains unclear. Animals lacking alpha-, gamma- and delta-sarcoglycan have been described and provide excellent opportunities for further investigation of the function of sarcoglycan. Dystrophin with dystroglycan and laminin may be a mechanical link between the actin cytoskeleton and the extracellular matrix. By positioning itself in close proximity to dystrophin and dystroglycan, sarcoglycan may function to couple mechanical and chemical signals in striated muscle. Sarcoglycan may be an independent signaling or regulatory module whose position in the membrane is determined by dystrophin but whose function is carried out independent of the dystrophin-dystroglycan-laminin axis.

Structural and functional analysis of the N-terminal extracellular region of beta-dystroglycan

A protein fragment corresponding to the mouse beta-dystroglycan N-terminal extracellular region from position 654 to 750, beta-DG(654-750) was recombinantly expressed in BL21(DE3) Escherichia coli cells. Secondary structure prediction of the protein fragment reveals about 70% of random coil, as confirmed by circular dichroism analysis. Moreover, fluorescence analysis shows that the tryptophan residue in position 659 lays in a solvent-exposed fashion. These data suggest that the beta-DG(654-750) is likely to have a quite flexible structure and to be only partially folded. Interestingly, the protein still retains its biological function since using solid-phase assays we have detected binding of biotinylated beta-DG(654-750) both to native alpha-dystroglycan and to a recombinant fragment which spans the C-terminal region of alpha-dystroglycan.

The crystal structure of a laminin G-like module reveals the molecular basis of alpha-dystroglycan binding to laminins, perlecan, and agrin

Laminin G-like (LG) modules in the extracellular matrix glycoproteins laminin, perlecan, and agrin mediate the binding to heparin and the cell surface receptor alpha-dystroglycan (alpha-DG). These interactions are crucial to basement membrane assembly, as well as muscle and nerve cell function. The crystal structure of the laminin alpha 2 chain LG5 module reveals a 14-stranded beta sandwich. A calcium ion is bound to one edge of the sandwich by conserved acidic residues and is surrounded by residues implicated in heparin and alpha-DG binding. A calcium-coordinated sulfate ion is suggested to mimic the binding of anionic oligosaccharides. The structure demonstrates a conserved function of the LG module in calcium-dependent lectin-like alpha-DG binding.

Formation of the neuromuscular junction. Agrin and its unusual receptors

Synapses are essential relay stations for the transmission of information between neurones and other cells. An ordered and tightly regulated formation of these structures is crucial for the functioning of the nervous system. The induction of the intensively studied synapse between nerve and muscle is initiated by the binding of neurone-specific isoforms of the basal membrane protein agrin to receptors on the surface of myotubes. Agrin activates a receptor complex that includes the muscle-specific kinase and most likely additional, yet to be identified, components. Receptor activation leads to the aggregation of acetylcholine receptors (AChR) and other proteins of the postsynaptic apparatus. This activation process has unique features which distinguish it from other receptor tyrosine kinases. In particular, the autophosphorylation of the kinase domain, which usually induces the recruitment of adaptor and signalling molecules, is not sufficient for AChR aggregation. Apparently, interactions of the extracellular domain with unknown components are also required for this process. Agrin binds to a second protein complex on the muscle surface known as the dystrophin-associated glycoprotein complex. This binding forms one end of a molecular link between the extracellular matrix and the cytoskeleton. While many components of the machinery triggering postsynaptic differentiation have now been identified, our picture of the molecular pathway causing the redistribution of synaptic proteins is still incomplete.

Mutation of a basic sequence in the laminin alpha2LG3 module leads to a lack of proteolytic processing and has different effects on beta1 integrin-mediated cell adhesion and alpha-dystroglycan binding

A RRKRRQ sequence unique to the LG3 module of the laminin alpha2 chain was previously shown to be sensitive to endogenous proteolysis during the recombinant production of the tandem array alpha2LG1-3. Mutation of RQ surrounding the cleaved peptide bond did not prevent this processing and intracellular degradation. Alanine mutagenesis of three alternate basic residues, however, was shown to prevent the cleavage in alpha2LG1-3, allowing for the alpha2LG3 module to be obtained as a folded, globular fragment. The mutation did not change heparin and sulfatide binding or cell adhesion of alpha2LG1-3 which can be mediated by alpha3beta1 and alpha6beta1 integrins. It did, however, cause a 10-fold reduction in alpha-dystroglycan binding. The data favor the interpretation that binding epitopes for heparin/sulfatides, beta1 integrins and alpha-dystroglycan occupy different parts of the alpha2LG1-3 structure.