Background/Purpose: Increased levels of the alarmin high mobility group box one (HMGB1) have been implicated as a possible pathogenic mediator and biomarker for myositis. We have recently published that in response to stress/ inflammation, HMGB1 forms covalent protein complexes (HMGB1c) via transglutaminase-2 (TG2) dependent crosslinking.  The purpose of this study is to test the hypothesis that HMGB1c is a driver of skeletal muscle myogenesis and may play a role in the pathogenesis of myositis.

Methods: A combination of in vivo and in vitro studies were used to examine the role of HMGB1c in muscle regeneration. Muscle regeneration and myogenesis were examined in TG2 -/- vs. wild-type mice as well as in mouse C2C12 myoblasts. An in vivo muscle regeneration model using BaCl2 induced injury was used to probe muscle covalent HMGB1 complex expression and myogenic markers.  Samples were analyzed by real time RT-PCR, immunohistochemistry, and western blotting.

Results: HMGB1 formed high molecular weight, heat/denaturing resistant protein complexes (HMGB1c) in differentiating C2C12 cells. Loss of TG2 function via RNAi or genetic ablation reduced HMGB1c formation, impaired C2C12 differentiation, and delayed muscle regeneration in vivo. Impaired muscle regeneration in TG2 -/- mice also correlated with enhanced pro-inflammatory immune cell infiltration and reduced presence of myogenic M2 macrophages. 

Conclusion: Our results reveal that TG2 is a novel mediator of myogenesis in vitro and in vivo, which may occur in-part via HMGB1 transamidation and altered pro- to anti-inflammatory transitions during myogenic differentiation.  Increased presence of HMGB1c in plasma of myositis patients relative to healthy controls suggests that the crosslinked alarmin could play a role in disease pathogenesis.  

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/covalent-high-molecular-weight-hmgb1-complexes-in-muscle-regeneration-implications-in-inflammatory-myopathy/