Induction of Osmolyte Pathways in Skeletal Muscle Inflammation: Novel Biomarkers for Myositis

Abstract

We recently identified osmolyte accumulators as novel biomarkers for chronic skeletal muscle inflammation and weakness, but their precise involvement in inflammatory myopathies remains elusive. In the current study, we demonstrate in vitro that, in myoblasts and myotubes exposed to pro-inflammatory cytokines or increased salt concentration, mRNA levels of the osmolyte carriers SLC5A3, SLC6A6, SLC6A12, and AKR1B1 enzyme can be upregulated. Induction of SLC6A12 and AKR1B1 was confirmed at the protein level using immunofluorescence and Western blotting. Gene silencing by specific siRNAs revealed that these factors were vital for muscle cells under hyperosmotic conditions. Pro-inflammatory cytokines activated mitogen-activated protein kinases, nuclear factor κB as well as nuclear factor of activated T-cells 5 mRNA expression. In muscle biopsies from patients with polymyositis or sporadic inclusion body myositis, osmolyte pathway activation was observed in regenerating muscle fibers. In addition, the osmolyte carriers SLC5A3 and SLC6A12 localized to subsets of immune cells, most notably to the endomysial macrophages and T-cells. Collectively, this study unveiled that muscle cells respond to osmotic and inflammatory stress by osmolyte pathway activation, likely orchestrating general protection of the tissue. Moreover, pro-inflammatory properties are attributed to SLC5A3 and SLC6A12 in auto-aggressive macrophages and T-cells in inflamed skeletal muscle.

Description of Osmolyte Pathways in Maturing Mdx Mice Reveals Altered Levels of Taurine and Sodium/Myo-Inositol Co-Transporters

“…In DMD patients, upregulation of SMIT was observed at the protein level and colocalization studies showed that expression coincided with CD56+ fibers, a subset of CD206+ macrophages and T-cells, pointing towards involvement in the inflammatory and the successive regenerative response [ 6 ]. In vitro experiments have shown a significant upregulation of SLC5A3 mRNA in muscle cells exposed to pro-inflammatory cytokines [ 32 ]. Taken together, our results show increased protein levels of SMIT in mdx mice, especially in damaged muscle fibers.…”

Myo-Inositol Transporter SLC5A3 Associates with Degenerative Changes and Inflammation in Sporadic Inclusion Body Myositis

“…We reported earlier that the osmolyte system becomes activated in myositis tissues [10] and in vitro in muscle cells exposed to pro-inflammatory cytokines [11]. In the present study we elaborate on the involvement of the MI transporter SLC5A3 in the degenerative and inflammatory changes associated with IBM.…”

“…Our study shows that SLC5A3 is upregulated in IBM muscle and that the osmolyte carrier localizes to the muscle fibers, blood vessels, and inflammatory cells. Overexpressed SLC5A3 has also been observed in other inflammatory myopathies and in muscular dystrophies [11]. Yet, the co-localization with the muscle fiber inclusions is peculiar for IBM, a muscle disease in which degenerative and inflammatory characteristics are equally prominent.…”

Exploring the Therapeutic Potential of Ectoine in Duchenne Muscular Dystrophy: Comparison with Taurine, a Supplement with Known Beneficial Effects in the mdx Mouse

“…This study has several limitations that should be considered. Firstly, the effect of treatment on muscle inflammation was evaluated in CCL-136 cells exposed to cytokines [ 46 ]. In this study, the use of dystrophin-deficient muscle cells exposed to cytokines would have been a more suitable model.…”