In dystrophicmdxhindlimb muscles where fibrosis is limited versican haploinsufficiency transiently improves contractile function without decreasing inflammation

Abstract

The provisional matrix protein versican is upregulated in Duchenne muscular dystrophy. Versican heightens inflammation in fibrotic diseases and is involved in myogenesis. In fibrotic diaphragm muscles from dystrophic mdx mice, versican reduction attenuated macrophage infiltration and improved contractile function. We investigated the association between versican and mdx hindlimb muscle pathology, where inflammation and regeneration are increased but fibrosis is minimal. Immunohistochemistry and qRT-PCR were used to assess how fiber type and glucocorticoids (α-methylprednisolone) modulate versican expression. Female mdx and male versican haploinsufficient (hdf) mice were bred resulting in male mdx-hdf and mdx (control) pups. Versican expression, contractile function, and pathology were evaluated in fast extensor digitorum longus (EDL) and slow soleus muscles, excised under medetomidine-midazolam-fentanyl anesthesia. Versican immunoreactivity was highest in soleus muscles. Versican mRNA transcripts were reduced by α-methylprednisolone in soleus, but not EDL, muscles. Versican expression was decreased in soleus muscles from 6-week-old mdx-hdf mice leading to increased force output and a modest reduction in fatiguability. These functional benefits were not accompanied by decreased inflammation; muscle architecture, regeneration markers, and fiber type also did not differ between genotypes. Improvements in soleus function were lost in adult (20-week-old) mdx-hdf mice with no significant effect of versican haploinsufficiency on macrophage infiltration and regeneration markers. Soleus muscles from juvenile mdx mice were most responsive to pharmacological or genetic approaches targeting versican; however, the benefits of versican reduction were limited due to low fibrosis. Pre-clinical matrix research in dystrophy should account for muscle phenotype and the interdependence between the fibrosis and inflammation.