Altered Contraction Phenotype and Smooth Muscle Function in the Intestine of Dystrophin‐Deficient ( mdx ) Mice

Duchenne Muscular Dystrophy (DMD), characterized by progressive skeletal muscle degeneration and weakness, is caused by a deficiency in the dystrophin gene. Dystrophin is a structural protein that connects the extracellular matrix to the cytoskeletal proteins and plays a protective role in mechanotransduction. Muscle cells are more susceptible to contraction‐induced damage due to lack of functional dystrophin. Although the prominent symptoms of DMD are related to skeletal muscle, loss of dystrophin also affects cardiac, vascular and visceral smooth muscle function. The animal model that is extensively used to study the pathophysiology of DMD is mdx mice which contains a mutation in the dystrophin gene on exon 23 causing a loss of dystrophin expression. In patients with DMD and in mdx mice, loss of dystrophin is associated with gastrointestinal motility disorders such as delayed gastric emptying, decrease in intestinal transit and chronic constipation. In the gastrointestinal tract, enteric neurons, interstitial cells of Cajal and smooth muscle cells are shown to express dystrophin. The AIM of the present study was to investigate whether the lack of dystrophin affects contractile phenotype and smooth muscle function using isolated intestinal muscle cells from wild type (WT) and mdx mice. Inflammation has been identified as one of the mechanisms for the disease progression in DMD; therefore, we also investigated the effect of MG132 and IKKIV, inhibitors of pro‐inflammatory NF‐κB signaling pathway, in mdx mice. METHODS Intestinal smooth muscle cells were isolated from WT and mdx mice. Expression of the p65 subunit of NF‐κB and smooth muscle markers such as smoothelin, tropomyosin, calponin, caldesmon and SM22α were measured by qRT‐PCR and western blot. Smooth muscle function was measured as contraction (decrease in cell length) in response to a cholinergic agonist, acetylcholine, by scanning micrometry. Muscle strips from mdx mice were incubated with MG132 (10 μM) or IKKIV (10 μM) in vitro for 24 h and expression of smooth muscle markers was measured. RESULTS The mRNA and protein levels of tropomyosin, calponin, caldesmon, smoothelin, and SM22α were significantly decreased in intestinal smooth muscle of mdx mice compared to WT mice. The decrease in expression of smooth muscle markers was associated with a significant decrease in both Ca 2+ ‐dependent and ‐independent contraction in response to acetylcholine. The expression of the p65 subunit of NF‐κB was increased in smooth muscle from mdx mice, while treatment of muscle strips of mdx mice with NF‐κB inhibitors (MG132 or IKKVI) reversed the decrease in the mRNA and protein levels of smooth muscle markers to levels similar to that in smooth muscle from WT mice. CONCLUSION Lack of dystrophin causes an increase in NF‐κB expression and a decrease in contractile phenotype and smooth muscle function in the intestine. Inhibition of NF‐κB pathway in the intestine of mdx mice restores contractile phenotype. Targeting NF‐κB signaling pathway may offer a potential therapeutic strategy to treat gastrointestinal motility disorders in DMD patients. Support or Funding Information DK28300 and DK15564 This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .