Targeted disruption of the Walker-Warburg syndrome gene Pomt1 in mouse results in embryonic lethality

O Mannosylation of alpha-dystroglycan is essential for lymphocytic choriomeningitis virus receptor function

Alpha-dystroglycan (alpha-DG) was identified as a common receptor for lymphocytic choriomeningitis virus (LCMV) and several other arenaviruses including the human pathogenic Lassa fever virus. Initial work postulated that interactions between arenavirus glycoproteins and alpha-DG are based on protein-protein interactions. We found, however, that susceptibility toward LCMV infection differed in various cell lines despite them expressing comparable levels of DG, suggesting that posttranslational modifications of alpha-DG would be involved in viral receptor function. Here, we demonstrate that glycosylation of alpha-DG, and in particular, O mannosylation, which is a rare type of O-linked glycosylation in mammals, is essential for LCMV receptor function. Cells that are defective in components of the O-mannosylation pathway showed strikingly reduced LCMV infectibility. As defective O mannosylation is associated with severe clinical symptoms in mammals such as congenital muscular dystrophies, it is likely that LCMV and potentially other arenaviruses may have selected this conserved and crucial posttranslational modification as the primary target structure for cell entry and infection.

Dystroglycan: from biosynthesis to pathogenesis of human disease

α- and β-dystroglycan constitute a membrane-spanning complex that connects the extracellular matrix to the cytoskeleton. Although a structural role for dystroglycan had been identified, biochemical and genetic discoveries have recently highlighted the significance of posttranslational processing for dystroglycan function. Glycosylation is the crucial modification that modulates the function of dystroglycan as a receptor for extracellular binding partners. It has become clear that perturbation of dystroglycan glycosylation is the central event in the pathogenesis of several complex disorders, and recent advances suggest that glycosylation could be modulated to ameliorate the pathological features. Our increased understanding of the mechanisms of interaction of dystroglycan with its ligands has become an essential tool in deciphering the biological processes related to the human diseases in which the proteins are implicated.

Protein O-mannosyltransferase A of Aspergillus awamori is involved in O-mannosylation of glucoamylase I

Industrially important extracellular enzymes from filamentous fungi are often O-mannosylated. The structure and function of the pmtA (AapmtA) gene encoding the protein O-D-mannosyltransferase of Aspergillus awamori were characterized. The AapmtA disruptant, designated AaPMTA, was constructed by homologous recombination. The strain AaPMTA exhibited fragile cell morphology with respect to hyphal extension, as well as swollen hyphae formation and conidia formation in potato dextrose medium. Moreover, the AapmtA disruptant showed increased sensitivity to high temperature and Congo red. Thus, the AaPmtA protein is involved in the formation of the normal cell wall. The strain AaPMTA could grow well in liquid synthetic medium and secrete glucoamylase I (GAI-AaPMTA) to a similar extent to the wild-type strain (GAI-WT). Matrix-assisted laser desorption ionization time-of-flight mass spectrometry analysis of the GAIs revealed that approximately 33 mannose moieties of GAI were absent in strain AaPMTA. This result indicates that the AaPmtA protein is responsible for the transfer of mannose to GAI. Structural analysis of the O-linked oligosaccharides of GAI also demonstrated that the AapmtA disruption resulted in a reduction of the amounts of O-linked oligosaccharides, such as D-mannose and alpha-1,2-mannotriose, in GAI-AaPMTA. However, the amount of alpha-1,2-mannobiose was comparable between GAI-WT and GAI-AaPMTA. The result suggests the presence of a compensatory mechanism in the synthetic pathway of O-mannosylation in A. awamori.

The twisted gene encodes Drosophila protein O-mannosyltransferase 2 and genetically interacts with the rotated abdomen gene encoding Drosophila protein O-mannosyltransferase 1

The family of mammalian O-mannosyltransferases includes two enzymes, POMT1 and POMT2, which are thought to be essential for muscle and neural development. Similar to mammalian organisms, Drosophila has two O-mannosyltransferase genes, rotated abdomen (rt) and DmPOMT2, encoding proteins with high homology to their mammalian counterparts. The previously reported mutant phenotype of the rt gene includes a clockwise rotation of the abdomen and defects in embryonic muscle development. No mutants have been described so far for the DmPOMT2 locus. In this study, we determined that the mutation in the twisted (tw) locus, tw(1), corresponds to a DmPOMT2 mutant. The twisted alleles represent a complementation group of recessive mutations that, similar to the rt mutants, exhibit a clockwise abdomen rotation phenotype. Several tw alleles were isolated in the past; however, none of them was molecularly characterized. We used an expression rescue approach to confirm that tw locus represents DmPOMT2 gene. We found that the tw1 allele represents an amino acid substitution within the conserved PMT domain of DmPOMT2 (TW) protein. Immunostaining experiments revealed that the protein products of both rt and tw genes colocalize within Drosophila cells where they reside in the ER subcellular compartment. In situ hybridization analysis showed that both genes have essentially overlapping patterns of expression throughout most of embryogenesis (stages 8-17), while only the rt transcript is present at early embryonic stages (5 and 6), suggesting its maternal origin. Finally, we analyzed the genetic interactions between rt and tw using several mutant alleles, RNAi, and ectopic expression approaches. Our data suggest that the two Drosophila O-mannosyltransferase genes, rt and tw, have nonredundant functions within the same developmental cascade and that their activities are required simultaneously for possibly the same biochemical process. Our results establish the possibility of using Drosophila as a model system for studying molecular and genetic mechanisms of protein O-mannosylation during development.

The dystroglycanopathies: the new disorders of O-linked glycosylation

It has become clear in the past half decade that a number of forms of congenital muscular dystrophy are in fact congenital disorders of glycosylation. Genes for Walker Warburg syndrome, muscle-eye-brain disease, Fukuyama congenital muscular dystrophy, congenital muscular dystrophy 1C and 1D, and limb girdle muscular dystrophy 21 have been identified, and gene mutations resulting in these diseases all cause the underglycosylation of alpha dystroglycan with O-linked carbohydrates. Unlike congenital disorders of glycosylation involving the N-linked pathway, these O-linked disorders possess distinctive muscle, eye, and brain phenotypes. Studies using mice and patient tissues strongly suggest that underglycosylation of dystroglycan inhibits the binding extracellular matrix proteins, effectively divorcing this important cell adhesion molecule from its extracellular environment. Moreover, defects in dystroglycan alone can account for most, if not all, cellular pathology. Thus, these disorders are now collectively referred to as dystroglycanopathies.

Protein O-mannosylation is crucial for cell wall integrity, septation and viability in fission yeast

Protein O-mannosyltransferases (PMTs) initiate the assembly of O-mannosyl glycans, which are of fundamental importance in eukaryotes. The PMT family, which is classified into PMT1, PMT2 and PMT4 subfamilies, is evolutionarily conserved. Despite the fact that PMTs are crucial for viability of baker's yeast as well as of mouse, recent studies suggested that there are significant differences in the organization and properties of the O-mannosylation machinery between yeasts and mammals. In this study we identified and characterized the PMT family of the archaeascomycete Schizosaccharomyces pombe. Unlike Saccharomyces cerevisiae where the PMT family is highly redundant, in S. pombe only one member of each PMT subfamily is present, namely, oma1+ (protein O-mannosyltransferase), oma2+ and oma4+. They all act as protein O-mannosyltransferases in vivo. oma1+ and oma2+ form heteromeric protein complexes and recognize different protein substrates compared to oma4+, suggesting that similar principles underlie mannosyltransfer reaction in S. pombe and budding yeast. Deletion of oma2+, as well as simultaneous deletion of oma1+ and oma4+ is lethal. Characterization of the viable S. pombe oma1Delta and oma4Delta single mutants showed that a lack of O-mannosylation results in abnormal cell wall and septum formation, thereby severely affecting cell morphology and cell-cell separation.

Basement membrane fragility underlies embryonic lethality in fukutin-null mice

Fukuyama-type congenital muscular dystrophy (FCMD), associated with brain malformation due to defects in neuronal migration, is caused by mutations in fukutin. Several lines of evidence suggest that the fukutin protein plays a pivotal role in synthesis of O-mannosyl sugar moieties of alpha-dystroglycan, a cell surface laminin receptor. Here, through targeted disruption of the orthologous mouse fukutin gene, we show that the fukutin protein is essential, as homozygous-null embryos die by E9.5 of gestation. Fukutin-null embryos show phenotypic diversity, features of which include growth retardation, folding of the egg cylinder, leakage of maternal red blood cells into the yolk sac cavity, and an increased number of apoptotic cells in the ectoderm. Loss of immunoreactivity against sugar moieties in alpha-dystroglycan suggests a reduced laminin-binding capacity. Ultrastructural analysis shows thin and breached basement membranes (BMs). BM fragility may underlie all of these abnormal phenotypes, and maintenance of BM function may require fukutin-mediated glycosylation of alpha-dystroglycan early in embryonic development.

Characterization of the LARGE family of putative glycosyltransferases associated with dystroglycanopathies

The Large(myd) mouse has a loss-of-function mutation in the putative glycosyltransferase gene Large. Mutations in the human homolog (LARGE) have been described in a form of congenital muscular dystrophy (MDC1D). Other genes (POMT1, POMGnT1, fukutin, and FKRP) that encode known or putative glycosylation enzymes are also causally associated with human congenital muscular dystrophies. All these diseases are associated with hypoglycosylation of the membrane protein alpha-dystroglycan (alpha-DG) and consequent loss of extracellular ligand binding. Hence, they are termed dystroglycanopathies. A paralogous gene for LARGE (LARGE2 or GYLTL1B) may also have a role in DG glycosylation. Using database interrogation and reverse-transcriptase polymerase chain reaction (RT-PCR), we identified vertebrate orthologs of each of these LARGE genes in many vertebrates, including human, mouse, dog, chicken, zebrafish, and pufferfish. However, within invertebrate genomes, we were able to identify only single homologs. We suggest that vertebrate LARGE orthologs be referred to as LARGE1. RT-PCR, dot-blot, and northern analysis indicated that LARGE2 has a more restricted tissue-expression profile than LARGE1. Using epitope-tagged proteins, we show that both LARGE1 and LARGE2 localize to the Golgi apparatus. The high similarity between the LARGE paralogs suggests that LARGE2 may also act on DG. Overexpression of LARGE2 in mouse C2C12 myoblasts results in increased glycosylation of alpha-DG accompanied by an increase in laminin binding. Thus, there may be functional redundancy between LARGE1 and LARGE2. Consistent with this idea, we show that alpha-DG is still fully glycosylated in kidney (a tissue that expresses a high level of LARGE2 mRNA) of Large(myd) mutant mice.

An autosomal recessive limb girdle muscular dystrophy (LGMD2) with mild mental retardation is allelic to Walker-Warburg syndrome (WWS) caused by a mutation in the POMT1 gene

Mutations of the protein O-mannosyltransferase (POMT1) gene affect glycosylation of alpha-dystroglycan, leading to Walker-Warburg syndrome, a lethal disorder in early life with severe congenital muscular dystrophy, and brain and eye malformations. Recently, we described a novel form of recessive limb girdle muscular dystrophy with mild mental retardation, associated with an abnormal alpha-dystroglycan pattern in the muscle, suggesting a glycosylation defect. Here, we present evidence that this distinct phenotype results from a common mutation (A200P) in the POMT1 gene. Our findings further expand the phenotype of glycosylation disorders linked to POMT1 mutations. Furthermore, the A200P mutation is part of a conserved core haplotype, indicating an ancestral founder mutation.

Expression of glutamate transporter subtypes during normal human corticogenesis and type II lissencephaly

Glutamate transporters are thought to have an important role in central nervous system (CNS) development. We investigated the expression of the sodium-dependent high-affinity glutamate transporters EAAT1, EAAT2, and EAAT3 in 11 human autopsied cases without neurological disorders and in four cases with type II lissencephaly including Walker Warburg's syndrome (WWS) and Fukuyama-type congenital muscular dystrophy (FCMD), both of which are classified as migration disorders of the human brain. Expression of glutamate transporter subtypes was differentially regulated during normal human corticogenesis. Although EAAT1 and EAAT2 were mainly localized to the cortical astrocytes in the postnatal brain, EAAT1 was enriched in the proliferative zones and radial glia from 13 gestational weeks (GW) to 20 GW. EAAT2 was abundant in the intermediate zone until 23 GW, and transiently expressed in the radial fibers of the transitional form of radial glia into mature astrocytes as well as partly in the corticofugal axonal bundles. EAAT3 immunoreactivity was robust in the apical dendrites of the pyramidal neurons in the marginal zone and cortical plate during corticogenesis, and decreased postnatally. In the individuals with type II lissencephaly, glutamate transporters were expressed in the extrusion of neuroglial tissue. Bundles of EAAT2-immunoreactive radial fibers were prominent in the specimens at 20 GW. Thus, glutamate transporters are differentially regulated during normal and impaired corticogenesis. Altered glutamate transporter expression in type II lissencephaly suggests that glutamate metabolism is involved in the formation of the normal cortex and contributes to the disorganized cortex seen in migration disorders.

Localization and functional analysis of the LARGE family of glycosyltransferases: significance for muscular dystrophy

The dystroglycanopathies are a novel group of human muscular dystrophies due to mutations in known or putative glycosyltransferase enzymes. They share the common pathological feature of a hypoglycosylated form of alpha-dystroglycan, diminishing its ability to bind extracellular matrix ligands. The LARGE glycosyltransferase is mutated in both the myodystrophy mouse and congenital muscular dystrophy type 1D (MDC1D). We have transfected various cell lines with a variety of LARGE expression constructs in order to characterize their subcellular localization and effect on alpha-dystroglycan glycosylation. Wild-type LARGE co-localized with the Golgi marker GM130 and stimulated the production of highly glycosylated alpha-dystroglycan (hyperglycosylation). MDC1D mutants had no effect on alpha-dystroglycan glycosylation and failed to localize correctly, confirming their pathogenicity. The two predicted catalytic domains of LARGE contain three conserved DxD motifs. Systematically mutating each of these motifs to NNN resulted in the mislocalization of one construct, while all failed to have any effect on alpha-dystroglycan glycosylation. A construct lacking the transmembrane domain also failed to localize at the Golgi apparatus. These results indicate that LARGE needs to both physically interact with alpha-dystroglycan and function as a glycosyltransferase in order to stimulate alpha-dystroglycan hyperglycosylation. We have also cloned and overexpressed a homologue of LARGE, glycosyltransferase-like 1B (GYLTL1B). Like LARGE it localized to the Golgi apparatus and stimulated alpha-dystroglycan hyperglycosylation. These results suggest that GYLTL1B may be a candidate gene for muscular dystrophy and that its overexpression could compensate for the deficiency of both LARGE and other glycosyltransferases.

The dystroglycan complex: From biology to cancer

Dystroglycan (DG), a non-integrin adhesion molecule, is a pivotal component of the dystrophin-glycoprotein complex, that is expressed in skeletal muscle and in a wide variety of tissues at the interface between the basement membrane (BM) and the cell membrane. DG has been mainly studied for its role in skeletal muscle cell stability and its alterations in muscular diseases, such as dystrophies. However, accumulating evidence have implicated DG in a variety of other biological functions, such as maturation of post-synaptic elements in the central and peripheral nervous system, early morphogenesis, and infective pathogens targeting. Moreover, DG has been reported to play a role in regulating cytoskeletal organization, cell polarization, and cell growth in epithelial cells. Recent studies also indicate that abnormalities in the expression of DG frequently occur in human cancers and may play a role in both the process of tumor progression and in the maintenance of the malignant phenotype. This paper reviews the available information on the biology of DG, the abnormalities found in human cancers, and the implications of these findings with respect to our understanding of cancer pathogenesis and to the development of novel strategies for a better management of cancer patients.