Matriglycan maintains t-tubule structural integrity in cardiac muscle.

Maintaining the structure of cardiac membranes and membrane organelles is essential for heart function. A critical cardiac membrane organelle is the transverse tubule system (called the t-tubule system) which is an invagination of the surface membrane. A unique structural characteristic of the cardiac muscle t-tubule system is the extension of the extracellular matrix (ECM) from the surface membrane into the t-tubule lumen. However, the importance of the ECM extending into the cardiac t-tubule lumen is not well understood. Dystroglycan (DG) is an ECM receptor in the surface membrane of many cells, and it is also expressed in t-tubules in cardiac muscle. Extensive posttranslational processing and O-glycosylation are required for DG to bind ECM proteins and the binding is mediated by a glycan structure known as matriglycan. Genetic disruption resulting in defective O-glycosylation of DG results in muscular dystrophy with cardiorespiratory pathophysiology. Here, we show that DG is essential for maintaining cardiac t-tubule structural integrity. Mice with defects in O-glycosylation of DG developed normal t-tubules but were susceptible to stress-induced t-tubule loss or severing that contributed to cardiac dysfunction and disease progression. Finally, we observed similar stress-induced cardiac t-tubule disruption in a cohort of mice that solely lacked matriglycan. Collectively, our data indicate that DG in t-tubules anchors the luminal ECM to the t-tubule membrane via the polysaccharide matriglycan, which is critical to transmitting structural strength of the ECM to the t-tubules and provides resistance to mechanical stress, ultimately preventing disruptions in cardiac t-tubule integrity.

Levofloxacin prophylaxis in pediatric oncology and hematopoietic stem cell transplantation: a literature review.

Bloodstream infections (BSI) are one of the leading causes of morbidity and mortality in children and young adults receiving chemotherapy for malignancy or undergoing hematopoietic stem cell transplantation (HSCT). Antibiotic prophylaxis is commonly used to decrease the risk of BSI; however, antibiotics carry an inherent risk of complications. The aim of this manuscript is to review levofloxacin prophylaxis in pediatric oncology patients and HSCT recipients. We reviewed published literature on levofloxacin prophylaxis to prevent BSI in pediatric oncology patients and HSCT recipients. Nine manuscripts were identified. The use of levofloxacin is indicated in neutropenic children and young adults receiving intensive chemotherapy for leukemia or undergoing HSCT. These results support the efficacy of levofloxacin in pediatric patients with leukemia receiving intensive chemotherapy and should be considered in pediatric patients undergoing HSCT prior to engraftment.

Nox2 Inhibition Regulates Stress Response and Mitigates Skeletal Muscle Fiber Atrophy during Simulated Microgravity.

Insufficient stress response and elevated oxidative stress can contribute to skeletal muscle atrophy during mechanical unloading (e.g., spaceflight and bedrest). Perturbations in heat shock proteins (e.g., HSP70), antioxidant enzymes, and sarcolemmal neuronal nitric oxidase synthase (nNOS) have been linked to unloading-induced atrophy. We recently discovered that the sarcolemmal NADPH oxidase-2 complex (Nox2) is elevated during unloading, downstream of angiotensin II receptor 1, and concomitant with atrophy. Here, we hypothesized that peptidyl inhibition of Nox2 would attenuate disruption of HSP70, MnSOD, and sarcolemmal nNOS during unloading, and thus muscle fiber atrophy. F344 rats were divided into control (CON), hindlimb unloaded (HU), and hindlimb unloaded +7.5 mg/kg/day gp91ds-tat (HUG) groups. Unloading-induced elevation of the Nox2 subunit p67phox-positive staining was mitigated by gp91ds-tat. HSP70 protein abundance was significantly lower in HU muscles, but not HUG. MnSOD decreased with unloading; however, MnSOD was not rescued by gp91ds-tat. In contrast, Nox2 inhibition protected against unloading suppression of the antioxidant transcription factor Nrf2. nNOS bioactivity was reduced by HU, an effect abrogated by Nox2 inhibition. Unloading-induced soleus fiber atrophy was significantly attenuated by gp91ds-tat. These data establish a causal role for Nox2 in unloading-induced muscle atrophy, linked to preservation of HSP70, Nrf2, and sarcolemmal nNOS.

Mucosal barrier injury-associated bloodstream infections in pediatric oncology patients.

BACKGROUND: Single-center reports of central line-associated bloodstream infection (CLABSI) and the subcategory of mucosal barrier injury laboratory-confirmed bloodstream infection (MBI-LCBI) in pediatric hematology oncology transplant (PHO) patients have focused on the inpatient setting. Characterization of MBI-LCBI across PHO centers and management settings (inpatient and ambulatory) is urgently needed to inform surveillance and prevention strategies. METHODS: Prospectively collected data from August 1, 2013, to December 31, 2015, on CLABSI (including MBI-LCBI) from a US PHO multicenter quality improvement network database was analyzed. CDC National Healthcare Safety Network definitions were applied for inpatient events and adapted for ambulatory events. RESULTS: Thirty-five PHO centers reported 401 ambulatory and 416 inpatient MBI-LCBI events. Ambulatory and inpatient MBI-LCBI rates were 0.085 and 1.01 per 1000 line days, respectively. Fifty-three percent of inpatient CLABSIs were MBI-LCBIs versus 32% in the ambulatory setting (P  <  0.01). Neutropenia was the most common criterion defining MBI-LCBI in both settings, being present in ≥90% of events. The most common organisms isolated in MBI-LCBI events were Escherichia coli (in 28% of events), Klebsiella spp. (23%), and viridans streptococci (12%) in the ambulatory setting and viridans streptococci (in 29% of events), E. coli (14%), and Klebsiella spp. (14%) in the inpatient setting. CONCLUSION: In this largest study of PHO MBI-LCBI inpatient events and the first such study in the ambulatory setting, the burden of MBI-LCBI across the continuum of care of PHO patients was substantial. These data should raise awareness of MBI-LCBI among healthcare providers for PHO patients, help benchmarking across centers, and help inform prevention and treatment strategies.

Outcomes after bloodstream infection in hospitalized pediatric hematology/oncology and stem cell transplant patients.

BACKGROUND: Pediatric hematology/oncology (PHO) patients receiving therapy or undergoing hematopoietic stem cell transplantation (HSCT) often require a central line and are at risk for bloodstream infections (BSI). There are limited data describing outcomes of BSI in PHO and HSCT patients. METHODS: This is a multicenter (n = 17) retrospective analysis of outcomes of patients who developed a BSI. Centers involved participated in a quality improvement collaborative referred to as the Childhood Cancer and Blood Disorder Network within the Children's Hospital Association. The main outcome measures were all-cause mortality at 3, 10, and 30 days after positive culture date; transfer to the intensive care unit (ICU) within 48 hours of positive culture; and central line removal within seven days of the positive blood culture. RESULTS: Nine hundred fifty-seven BSI were included in the analysis. Three hundred fifty-four BSI (37%) were associated with at least one adverse outcome. All-cause mortality was 1% (n = 9), 3% (n = 26), and 6% (n = 57) at 3, 10, and 30 days after BSI, respectively. In the 165 BSI (17%) associated with admission to the ICU, the median ICU stay was four days (IQR 2-10). Twenty-one percent of all infections (n = 203) were associated with central line removal within seven days of positive blood culture. CONCLUSIONS: BSI in PHO and HSCT patients are associated with adverse outcomes. These data will assist in defining the impact of BSI in this population and demonstrate the need for quality improvement and research efforts to decrease them.

The American Society of Pediatric Hematology/Oncology workforce assessment: Part 1-Current state of the workforce.

The American Society of Pediatric Hematology/Oncology (ASPHO) recognized recent changes in medical practice and the potential impact on pediatric hematology-oncology (PHO) workforce. ASPHO surveyed society members and PHO Division Directors between 2010 and 2016 and studied PHO workforce data collected by the American Board of Pediatrics and the American Medical Association to characterize the current state of the PHO workforce. The analysis of this information has led to a comprehensive description of PHO physicians, professional activities, and workplace. It is important to continue to collect data to identify changes in composition and needs of the PHO workforce.

Mitochondria in the middle: exercise preconditioning protection of striated muscle.

Cellular and physiological adaptations to an atmosphere which became enriched in molecular oxygen spurred the development of a layered system of stress protection, including antioxidant and stress response proteins. At physiological levels reactive oxygen and nitrogen species regulate cell signalling as well as intracellular and intercellular communication. Exercise and physical activity confer a variety of stressors on skeletal muscle and the cardiovascular system: mechanical, metabolic, oxidative. Transient increases of stressors during acute bouts of exercise or exercise training stimulate enhancement of cellular stress protection against future insults of oxidative, metabolic and mechanical stressors that could induce injury or disease. This phenomenon has been termed both hormesis and exercise preconditioning (EPC). EPC stimulates transcription factors such as Nrf-1 and heat shock factor-1 and up-regulates gene expression of a cadre of cytosolic (e.g. glutathione peroxidase and heat shock proteins) and mitochondrial adaptive or stress proteins (e.g. manganese superoxide dismutase, mitochondrial KATP channels and peroxisome proliferator activated receptor γ coactivator-1 (PGC-1)). Stress response and antioxidant enzyme inducibility with exercise lead to protection against striated muscle damage, oxidative stress and injury. EPC may indeed provide significant clinical protection against ischaemia-reperfusion injury, Type II diabetes and ageing. New molecular mechanisms of protection, such as δ-opioid receptor regulation and mitophagy, reinforce the notion that mitochondrial adaptations (e.g. heat shock proteins, antioxidant enzymes and sirtuin-1/PGC-1 signalling) are central to the protective effects of exercise preconditioning.

Central Line Associated Blood Stream Infections in Pediatric Hematology/Oncology Patients With Different Types of Central Lines.

BACKGROUND: Central line associated bloodstream infections (CLABSIs) are a significant cause of morbidity and mortality in pediatric hematology/oncology (PHO) patients. Understanding the differences in CLABSI rates by central line (CL) type is important to inform clinical decisions. PROCEDURE: CLABSI, using similar definitions, noted with three commonly used CL types (totally implanted catheter [port], tunneled externalized catheter [TEC], peripherally inserted central catheter [PICC]) and CL-specific line days were prospectively tracked across 15 US PHO centers from May 2012 until April 2015 and CLABSI rates (CLABSI per 1,000 CL-specific line days) were calculated. Host and organism characterstics associated with the CLABSI events were analyzed. RESULTS: Over the course of 2.8 million line days, 1,113 CLABSI events (397 in inpatients and 716 in ambulatory patients) were noted. The inpatient CLABSI rate was higher than the ambulatory CLABSI rate for each of the CL types: 1.48 versus 0.16 for ports, 3.51 versus 1.38 for TECs, and 3.07 versus 1.16 for PICCs, respectively. TECs and PICCs were associated with higher CLABSI rates than ports, inpatient and ambulatory. CONCLUSIONS: We found that CLABSI rates were significantly higher for inpatients compared to ambulatory PHO patients for all CL types. Among ambulatory patients, TECs had the highest CLABSI rate and ports the lowest. Among inpatients, TECs and PICCs had higher CLABSI rates than ports but were not statistically different from one another. Cognizant that host and underlying disease attributes may contribute to these differences, these results can still inform CL choice in clinical practice.

Child Health and the Pediatric Hematology-Oncology Workforce: 2020-2040.

Pediatric hematology-oncology (PHO) is 1 of the oldest recognized pediatric subspecialities. PHO physicians care for infants, children, adolescents, and young adults with all types of cancer and nonmalignant blood conditions, in many cases temporarily assuming the role of a primary care physician because of the complexity and intensity of treatment. However, the number of clinically active PHO subspecialists needed to care for children in the United States remains unknown. Recent papers suggest a potential oversaturation of PHO physicians in some geographic areas. This article is part of a Pediatrics supplement focused on projecting the future supply of the pediatric subspecialty workforce. It draws on information available in the literature, data from the American Board of Pediatrics, and findings from a new microsimulation model estimating the future supply of pediatric subspecialists through 2040. The model predicts a workforce growth in PHO subspecialists of 66% by 2040. Alternative scenarios, including changes in clinical time and fellowship size, resulted in a difference in growth of ±18% from baseline. The model also forecasts significant geographic maldistribution. For example, the current workforce is concentrated in the Northeast Census region and the model predicts the New England Census division will have a 2.9-fold higher clinical workforce equivalent per 100 000 children aged 0 to 18 years than the Mountain Census division by 2040. These findings suggest potential opportunities to improve the PHO subspecialty workforce and the outcomes and experiences of its patient population through educational changes, practice initiatives, policy interventions, and dedicated research.

Compliance With Central Line Maintenance Bundle and Infection Rates.

BACKGROUND: Reliable bundle performance is the mainstay of central line-associated bloodstream infections (CLABSI) prevention despite an unclear relationship between bundle reliability and outcomes. Our primary objective was to evaluate the correlation between reported bundle compliance and CLABSI rate in the Solutions for Patient Safety network. The secondary objective was to identify which hospital and process factors impact this correlation. METHODS: We examined data on bundle compliance and monthly CLABSI rates from January 11 to December 21 in 159 hospitals. The correlation (adjusting for temporal trend) between CLABSI rates and bundle compliance was done at the network level. Negative binomial regression was done to detect the impact of hospital type, central line audit rate, and adoption of a comprehensive safety culture program on the association between bundle compliance and CLABSI rates. RESULTS: During the study, hospitals reported 27 196 CLABSI on 20 274 565 line days (1.34 CLABSI/1000 line days). Out of 2 460 133 observed bundle opportunities, 2 085 700 (84%) were compliant. There was a negative correlation between the monthly bundle reliability and monthly CLABSI rate (-0.35, P <.001). After adjusting for the temporal trend, the partial correlation was -0.25 (P = .004). On negative binomial regression, significant positive interaction was only noted for the hospital type, with Hospital Within Hospital (but not freestanding children's hospitals) revealing a significant association between compliance ≥95% and lower CLABSI rates. CONCLUSIONS: Adherence to best practice guidelines is associated with a reduction in CLABSI rate. Hospital-level factors (hospitals within hospitals vs freestanding), but not process-related (central line audit rate and safety culture training), impact this association.

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

Dystroglycan (DG) requires extensive post-translational processing and O-glycosylation to function as a receptor for extracellular matrix (ECM) proteins containing laminin-G (LG) domains. Matriglycan is an elongated polysaccharide of alternating xylose (Xyl) and glucuronic acid (GlcA) that binds with high affinity to ECM proteins with LG domains and is uniquely synthesized on α-dystroglycan (α-DG) by like-acetylglucosaminyltransferase-1 (LARGE1). Defects in the post-translational processing or O-glycosylation of α-DG that result in a shorter form of matriglycan reduce the size of α-DG and decrease laminin binding, leading to various forms of muscular dystrophy. Previously, we demonstrated that protein O-mannose kinase (POMK) is required for LARGE1 to generate full-length matriglycan on α-DG (~150-250 kDa) (Walimbe et al., 2020). Here, we show that LARGE1 can only synthesize a short, non-elongated form of matriglycan in mouse skeletal muscle that lacks the DG N-terminus (α-DGN), resulting in an ~100-125 kDa α-DG. This smaller form of α-DG binds laminin and maintains specific force but does not prevent muscle pathophysiology, including reduced force production after eccentric contractions (ECs) or abnormalities in the neuromuscular junctions. Collectively, our study demonstrates that α-DGN, like POMK, is required for LARGE1 to extend matriglycan to its full mature length on α-DG and thus prevent muscle pathophysiology.

Combined effects of heavy ion exposure and simulated Lunar gravity on skeletal muscle.

BACKGROUND: The limitations to prolonged spaceflight include unloading-induced atrophy of the musculoskeletal system which may be enhanced by exposure to the space radiation environment. Previous results have concluded that partial gravity, comparable to the Lunar surface, may have detrimental effects on skeletal muscle. However, little is known if these outcomes are exacerbated by exposure to low-dose rate, high-energy radiation common to the space environment. Therefore, the present study sought to determine the impact of highly charge, high-energy (HZE) radiation on skeletal muscle when combined with partial weightbearing to simulate Lunar gravity. We hypothesized that partial unloading would compromise skeletal muscle and these effects would be exacerbated by radiation exposure. METHODS: For month old female BALB/cByJ mice were -assigned to one of 2 groups; either full weight bearing (Cage Controls, CC) or partial weight bearing equal to 1/6th bodyweight (G/6). Both groups were then divided to receive either a single whole body absorbed dose of 0.5 Gy of 300 MeV 28Si ions (RAD) or a sham treatment (SHAM). Radiation exposure experiments were performed at the NASA Space Radiation Laboratory (NSRL) located at Brookhaven National Laboratory on Day 0, followed by 21 d of CC or G/6 loading. Muscles of the hind limb were used to measure protein synthesis and other histological measures. RESULTS: Twenty-one days of Lunar gravity (G/6) resulted in lower soleus, plantaris, and gastrocnemius muscle mass. Radiation exposure did not further impact muscle mass. 28Si exposure in normal ambulatory animals (RAD+CC) did not impact gastrocnemius muscle mass when compared to SHAM+CC (p>0.05), but did affect the soleus, where mass was higher following radiation compared to SHAM (p<0.05). Mixed gastrocnemius muscle protein synthesis was lower in both unloading groups. Fiber type composition transitioned towards a faster isoform with partial unloading and was not further impacted by radiation. The combined effects of partial loading and radiation partially mitigated fiber cross-sectional area when compared to partial loading alone. Radiation and G/6 reduced the total number of myonuclei per fiber while leading to elevated BrdU content of skeletal muscle. Similarly, unloading and radiation resulted in higher collagen content of muscle when compared to controls, but the effects of combined exposure were not additive. CONCLUSIONS: The results of this study confirm that partial weightbearing causes muscle atrophy, in part due to reductions of muscle protein synthesis in the soleus and gastrocnemius as well as reduced peripheral nuclei per fiber. Additionally, we present novel data illustrating 28Si exposure reduced nuclei in muscle fibers despite higher satellite cell fusion, but did not exacerbate muscle atrophy, CSA changes, or collagen content. In conclusion, both partial loading and HZE radiation can negatively impact muscle morphology.

Eradication of factor VIII inhibitors in patients with mild and moderate hemophilia A.

In hemophilia A, up to 25% of new antifactor VIII (FVIII) inhibitory antibodies (inhibitors) occur in patients with mild or moderate disease. Once the inhibitor develops, options for management include observation, immune modulation, and immune tolerance induction (ITI). Currently, there is little data to guide a clinician's management decisions. In a case series, eight of the 26 subjects with mild or moderate hemophilia complicated by an inhibitor underwent ITI; two were successful, two were unsuccessful, and four were partially successful. In a systematic review of the literature, 12 of the 16 patients with mild or moderate hemophilia responded to rituximab for treatment to eradicate the inhibitor. To increase our understanding of treatment options for inhibitor eradication in patients with mild or moderate hemophilia A complicated by an inhibitor, a secondary analysis of clinical and treatment characteristics in a cohort of 36 patients with mild or moderate hemophilia A and inhibitor was undertaken. In multivariate analyses, rituximab alone (n = 6) and other immune-modulating treatments alone (n = 2) were significantly associated with an increased likelihood of inhibitor clearance [hazard ratio (HR) = 4.4 (95% CI = 1.06­20.03) and 10.21 (95% CI = 1.17­78.28), respectively], whereas ITI alone (n = 9) was not [HR = 1.35 (95% CI = 0.44­4.07)].

Evaluation for Bleeding Disorders in Suspected Child Abuse.

Bruising or bleeding in a child can raise the concern for child abuse. Assessing whether the findings are the result of trauma and/or whether the child has a bleeding disorder is critical. Many bleeding disorders are rare, and not every child with bruising/bleeding that may raise a concern for abuse requires an evaluation for bleeding disorders. However, in some instances, bleeding disorders can present in a manner similar to child abuse. Bleeding disorders cannot be ruled out solely on the basis of patient and family history, no matter how extensive. The history and clinical evaluation can be used to determine the necessity of an evaluation for a possible bleeding disorder, and prevalence and known clinical presentations of individual bleeding disorders can be used to guide the extent of laboratory testing. This clinical report provides guidance to pediatricians and other clinicians regarding the evaluation for bleeding disorders when child abuse is suspected.

Nox2 signaling and muscle fiber remodeling are attenuated by losartan administration during skeletal muscle unloading.

Reduced mechanical loading results in atrophy of skeletal muscle fibers. Increased reactive oxygen species (ROS) are causal in sarcolemmal dislocation of nNOS and FoxO3a activation. The Nox2 isoform of NADPH oxidase and mitochondria release ROS during disuse in skeletal muscle. Activation of the angiotensin II type 1 receptor (AT1R) can elicit Nox2 complex formation. The AT1R blocker losartan was used to test the hypothesis that AT1R activation drives Nox2 assembly, nNOS dislocation, FoxO3a activation, and thus alterations in morphology in the unloaded rat soleus. Male Fischer 344 rats were divided into four groups: ambulatory control (CON), ambulatory + losartan (40 mg kg-1  day-1 ) (CONL), 7 days of tail-traction hindlimb unloading (HU), and HU + losartan (HUL). Losartan attenuated unloading-induced loss of muscle fiber cross-sectional area (CSA) and fiber-type shift. Losartan mitigated unloading-induced elevation of ROS levels and upregulation of Nox2. Furthermore, AT1R blockade abrogated nNOS dislocation away from the sarcolemma and elevation of nuclear FoxO3a. We conclude that AT1R blockade attenuates disuse remodeling by inhibiting Nox2, thereby lessening nNOS dislocation and activation of FoxO3a.

Muscular dystrophy-dystroglycanopathy in a family of Labrador retrievers with a LARGE1 mutation.

Alpha-dystroglycan (αDG) is a highly glycosylated cell surface protein with a significant role in cell-to-extracellular matrix interactions in muscle. αDG interaction with extracellular ligands relies on the activity of the LARGE1 glycosyltransferase that synthesizes and extends the heteropolysaccharide matriglycan. Abnormalities in αDG glycosylation and formation of matriglycan are the pathogenic mechanisms for the dystroglycanopathies, a group of congenital muscular dystrophies. Muscle biopsies were evaluated from related 6-week-old Labrador retriever puppies with poor suckling, small stature compared to normal litter mates, bow-legged stance and markedly elevated creatine kinase activities. A dystrophic phenotype with marked degeneration and regeneration, multifocal mononuclear cell infiltration and endomysial fibrosis was identified on muscle cryosections. Single nucleotide polymorphism (SNP) array genotyping data on the family members identified three regions of homozygosity in 4 cases relative to 8 controls. Analysis of whole genome sequence data from one of the cases identified a stop codon mutation in the LARGE1 gene that truncates 40% of the protein. Immunofluorescent staining and western blotting demonstrated the absence of matriglycan in skeletal muscle and heart from affected dogs. Compared to control, LARGE enzyme activity was not detected. This is the first report of a dystroglycanopathy in dogs.

High-dose cyclophosphamide treatment for refractory severe aplastic anemia in children.

OBJECTIVE: To determine if high-dose cyclophosphamide is an effective therapy for children with refractory severe aplastic anemia (SAA). BACKGROUND: SAA is an illness characterized by the depletion of hematopoietic precursors associated with life-threatening complications. Hematopoietic stem cell transplant (HSCT) is the treatment of choice if a human leukocyte antigen (HLA)-related donor is available. Immunosuppression with anti-thymocyte globulin (ATG) and cyclosporine A (CSA) is an option for patients who are not HSCT candidates. Unrelated donor HSCT has been used with limited success. High-dose cyclophosphamide has been used successfully in the treatment of adults with SAA, but experience in children is limited. PROCEDURE: Five pediatric patients who had failed previous immunosuppressive therapy for SAA were treated with high-dose cyclophosphamide (45 mg/kg/day x 4 days). RESULTS: After 12 months of treatment, two of five patients experienced a complete response with high-dose cyclophosphamide therapy. The two complete responders achieved red cell recovery with a hematocrit of >36% at days 212 and 112 and platelet recovery with a platelet count of >100 x 10(9)/L at days 126 and 324. Of the remaining patients, one patient failed to respond, and two patients expired from infectious complications. CONCLUSIONS: High-dose cyclophosphamide can lead to complete responses in children with SAA who have failed to respond to traditional immunosuppressive therapy.

POMK regulates dystroglycan function via LARGE1-mediated elongation of matriglycan.

Matriglycan [-GlcA-ß1,3-Xyl-α1,3-]n serves as a scaffold in many tissues for extracellular matrix proteins containing laminin-G domains including laminin, agrin, and perlecan. Like-acetyl-glucosaminyltransferase 1 (LARGE1) synthesizes and extends matriglycan on α-dystroglycan (α-DG) during skeletal muscle differentiation and regeneration; however, the mechanisms which regulate matriglycan elongation are unknown. Here, we show that Protein O-Mannose Kinase (POMK), which phosphorylates mannose of core M3 (GalNAc-ß1,3-GlcNAc-ß1,4-Man) preceding matriglycan synthesis, is required for LARGE1-mediated generation of full-length matriglycan on α-DG (~150 kDa). In the absence of Pomk gene expression in mouse skeletal muscle, LARGE1 synthesizes a very short matriglycan resulting in a ~ 90 kDa α-DG which binds laminin but cannot prevent eccentric contraction-induced force loss or muscle pathology. Solution NMR spectroscopy studies demonstrate that LARGE1 directly interacts with core M3 and binds preferentially to the phosphorylated form. Collectively, our study demonstrates that phosphorylation of core M3 by POMK enables LARGE1 to elongate matriglycan on α-DG, thereby preventing muscular dystrophy.