N-terminal domain on dystroglycan enables LARGE1 to extend matriglycan on α-dystroglycan and prevents muscular dystrophy

Sarcolemma resilience and skeletal muscle health require O-mannosylation of dystroglycan

BACKGROUND

Maintaining the connection between skeletal muscle fibers and the surrounding basement membrane is essential for muscle function. Dystroglycan (DG) serves as a basement membrane extracellular matrix (ECM) receptor in many cells, and is also expressed in the outward-facing membrane, or sarcolemma, of skeletal muscle fibers. DG is a transmembrane protein comprised of two subunits: alpha-DG (α-DG), which resides in the peripheral membrane, and beta-DG (β-DG), which spans the membrane to intracellular regions. Extensive post-translational processing and O-mannosylation are required for α-DG to bind ECM proteins, which is mediated by a glycan structure known as matriglycan. O-mannose glycan biosynthesis is initiated by the protein O-mannosyltransferase 1 (POMT1) and POMT2 enzyme complex and leads to three subtypes of glycans called core M1, M2, and M3. The lengthy core M3 is capped with matriglycan. Genetic defects in post-translational O-mannosylation of DG interfere with its receptor function and result in muscular dystrophy with central nervous system and skeletal muscle pathophysiology.

METHODS

To evaluate how the loss of O-mannosylated DG in skeletal muscle affects the development and progression of myopathology, we generated and characterized mice in which the Pomt1 gene was specifically deleted in skeletal muscle (Pomt1skm) to interfere with POMT1/2 enzyme activity. To investigate whether matriglycan is the primary core M glycan structure that provides the stabilizing link between the sarcolemma and ECM, we generated mice that retained cores M1, M2, and M3, but lacked matriglycan (conditional deletion of like-acetylglucosaminyltransferase 1; Large1skm). Next, we restored Pomt1 using gene transfer via AAV2/9-MCK-mPOMT1 and determined the effect on Pomt1skm pathophysiology.

RESULTS

Our data showed that in Pomt1skm mice O-mannosylated DG is required for sarcolemma resilience, remodeling of muscle fibers and muscle tissue, and neuromuscular function. Notably, we observed similar body size limitations, sarcolemma weakness, and neuromuscular weakness in Large1skm mice that only lacked matriglycan. Furthermore, our data indicate that genetic rescue of Pomt1 in Pomt1skm mice limits contraction-induced sarcolemma damage and skeletal muscle pathology.

CONCLUSIONS

Collectively, our data indicate that DG modification by Pomt1/2 results in core M3 capped with matriglycan, and that this is required to reinforce the sarcolemma and enable skeletal muscle health and neuromuscular strength.

Dystroglycan-HSPG interactions provide synaptic plasticity and specificity

AIM

This study examined the roles of the laminin and proteoglycan receptor dystroglycan (DG) in extracellular matrix stabilization and cellular mechanosensory processes conveyed through communication between the extracellular matrix (ECM) and cytoskeleton facilitated by DG. Specific functional attributes of HS-proteoglycans (HSPGs) are conveyed through interactions with DG and provide synaptic specificity through diverse interactions with an extensive range of cell attachment and adaptor proteins which convey synaptic plasticity. HSPG-DG interactions are important in phototransduction and neurotransduction and facilitate retinal bipolar-photoreceptor neuronal signaling in vision. Besides synaptic stabilization, HSPG-DG interactions also stabilize basement membranes and the ECM and have specific roles in the assembly and function of the neuromuscular junction. This provides neuromuscular control of muscle systems that control conscious body movement as well as essential autonomic control of diaphragm, intercostal and abdominal muscles and muscle systems in the face, mouth and pharynx which assist in breathing processes. DG is thus a multifunctional cell regulatory glycoprotein receptor and regulates a diverse range of biological and physiological processes throughout the human body. The unique glycosylation of the αDG domain is responsible for its diverse interactions with ECM components in cell-ECM signaling. Cytoskeletal cell regulatory switches assembled by the βDG domain in its role as a nuclear scaffolding protein respond to such ECM cues to regulate cellular behavior and tissue homeostasis thus DG has fascinating and diverse roles in health and disease.

The α-dystroglycan N-terminus is a broad-spectrum antiviral agent against SARS-CoV-2 and enveloped viruses

The COVID-19 pandemic has shown the need to develop effective therapeutics in preparedness for further epidemics of virus infections that pose a significant threat to human health. As a natural compound antiviral candidate, we focused on α-dystroglycan, a highly glycosylated basement membrane protein that links the extracellular matrix to the intracellular cytoskeleton. Here we show that the N-terminal fragment of α-dystroglycan (α-DGN), as produced in E. coli in the absence of post-translational modifications, blocks infection of SARS-CoV-2 in cell culture, human primary gut organoids and the lungs of transgenic mice expressing the human receptor angiotensin I-converting enzyme 2 (hACE2). Prophylactic and therapeutic administration of α-DGN reduced SARS-CoV-2 lung titres and protected the mice from respiratory symptoms and death. Recombinant α-DGN also blocked infection of a wide range of enveloped viruses including the four Dengue virus serotypes, influenza A virus, respiratory syncytial virus, tick-borne encephalitis virus, but not human adenovirus, a non-enveloped virus in vitro. This study establishes soluble recombinant α-DGN as a broad-band, natural compound candidate therapeutic against enveloped viruses.

The underlying mechanisms of arenaviral entry through matriglycan

Matriglycan, a recently characterized linear polysaccharide, is composed of alternating xylose and glucuronic acid subunits bound to the ubiquitously expressed protein α-dystroglycan (α-DG). Pathogenic arenaviruses, like the Lassa virus (LASV), hijack this long linear polysaccharide to gain cellular entry. Until recently, it was unclear through what mechanisms LASV engages its matriglycan receptor to initiate infection. Additionally, how matriglycan is synthesized onto α-DG by the Golgi-resident glycosyltransferase LARGE1 remained enigmatic. Recent structural data for LARGE1 and for the LASV spike complex informs us about the synthesis of matriglycan as well as its usage as an entry receptor by arenaviruses. In this review, we discuss structural insights into the system of matriglycan generation and eventual recognition by pathogenic viruses. We also highlight the unique usage of matriglycan as a high-affinity host receptor compared with other polysaccharides that decorate cells.

Protein O-mannosylation: one sugar, several pathways, many functions

Recent research has unveiled numerous important functions of protein glycosylation in development, homeostasis, and diseases. A type of glycosylation taking the center stage is protein O-mannosylation, a posttranslational modification conserved in a wide range of organisms, from yeast to humans. In animals, protein O-mannosylation plays a crucial role in the nervous system, whereas protein O-mannosylation defects cause severe neurological abnormalities and congenital muscular dystrophies. However, the molecular and cellular mechanisms underlying protein O-mannosylation functions and biosynthesis remain not well understood. This review outlines recent studies on protein O-mannosylation while focusing on the functions in the nervous system, summarizes the current knowledge about protein O-mannosylation biosynthesis, and discusses the pathologies associated with protein O-mannosylation defects. The evolutionary perspective revealed by studies in the Drosophila model system are also highlighted. Finally, the review touches upon important knowledge gaps in the field and discusses critical questions for future research on the molecular and cellular mechanisms associated with protein O-mannosylation functions.

Identification of a short, single site matriglycan that maintains neuromuscular function in the mouse

Matriglycan (-1,3-β-glucuronic acid-1,3-α-xylose-) is a polysaccharide that is synthesized on α-dystroglycan, where it functions as a high-affinity glycan receptor for extracellular proteins, such as laminin, perlecan and agrin, thus anchoring the plasma membrane to the extracellular matrix. This biological activity is closely associated with the size of matriglycan. Using high-resolution mass spectrometry and site-specific mutant mice, we show for the first time that matriglycan on the T317/T319 and T379 sites of α-dystroglycan are not identical. T379-linked matriglycan is shorter than the previously characterized T317/T319-linked matriglycan, although it maintains its laminin binding capacity. Transgenic mice with only the shorter T379-linked matriglycan exhibited mild embryonic lethality, but those that survived were healthy. The shorter T379-linked matriglycan exists in multiple tissues and maintains neuromuscular function in adult mice. In addition, the genetic transfer of α-dystroglycan carrying just the short matriglycan restored grip strength and protected skeletal muscle from eccentric contraction-induced damage in muscle-specific dystroglycan knock-out mice. Due to the effects that matriglycan imparts on the extracellular proteome and its ability to modulate cell-matrix interactions, our work suggests that differential regulation of matriglycan length in various tissues optimizes the extracellular environment for unique cell types.

Identification of Matriglycan by Dual Exoglycosidase Digestion of α-Dystroglycan

Matriglycan is a linear polysaccharide of alternating xylose and glucuronic acid units [-Xyl-α1,3-GlcA-β1,3]n that is uniquely synthesized on α-dystroglycan (α-DG) and is essential for neuromuscular function and brain development. It binds several extracellular matrix proteins that contain laminin-globular domains and is a receptor for Old World arenaviruses such as Lassa Fever virus. Monoclonal antibodies such as IIH6 are commonly used to detect matriglycan on α-DG. However, endogenous expression levels are not sufficient to detect and analyze matriglycan by mass spectrometry approaches. Thus, there is a growing need to independently confirm the presence of matriglycan on α-DG and possibly other proteins. We used an enzymatic approach to detect matriglycan, which involved digesting it with two thermophilic exoglycosidases: β-Glucuronidase from Thermotoga maritima and α-xylosidase from Sulfolobus solfataricus. This allowed us to identify and categorize matriglycan on α-DG by studying post-digestion changes in the molecular weight of α-DG using SDS-PAGE followed by western blotting with anti-matriglycan antibodies, anti-core α-DG antibodies, and/or laminin binding assay. In some tissues, matriglycan is capped by a sulfate group, which renders it resistant to digestion by these dual exoglycosidases. Thus, this method can be used to determine the capping status of matriglycan. To date, matriglycan has only been identified on vertebrate α-DG. We anticipate that this method will facilitate the discovery of matriglycan on α-DG in other species and possibly on other proteins. Key features • Analysis of endogenous matriglycan on dystroglycan from any animal tissue. • Matriglycan is digested using thermophilic enzymes, which require optimum thermophilic conditions. • Western blotting is used to assay the success and extent of digestion. • Freshly purified enzymes work best to digest matriglycan.