Differential expression of myosin heavy chain isoforms in the masticatory muscles of dystrophin-deficient mice.

The dystrophin-deficient mouse (mdx) is a homologue animal model of Duchenne muscular dystrophy (DMD) and is characterized by slowly progressive muscle weakness accompanied by changes in myosin heavy chain (MyHC) composition. It is likely that the masticatory muscles undergo similar changes. The aim of this study was to examine the masticatory muscles (masseter, temporal, tongue, and soleus) of 100-day-old mdx and control mice (n = 8-10), and the fibre type distribution (by immunohistochemistry) as well as the expression of the corresponding MyHC messenger RNA (mRNA) (protein and mRNA expression, using Western blot or quantitative real-time polymerase chain reaction (RT-PCR)). Immunohistochemistry and western blot analysis revealed that the masticatory muscles in the control and mdx mice consisted mainly of type 2 fibres, whereas soleus muscle consisted of both type 1 and 2 fibres. In the masseter muscle, the mRNA in mdx mice was not different from that found in the controls. However, the mRNA content of the MyHC-2b isoform in mdx mice was lower in comparison with the controls in the temporal muscle [11.9 versus 36.9 per cent; P < 0.01; mean ± standard error of the mean (SEM), Student's unpaired t-test], as well as in the tongue muscle (65.7 versus 73.8 per cent; P < 0.05). Similarly, the content of MyHC-2x isoforms in mdx tongue muscle was lower than in the controls (25.9 versus 30.8 per cent; P < 0.05). The observed down-regulation of the MyHC-2x and MyHC-2b mRNA in the masticatory muscles of mdx mice may lead to changed fibre type composition. The different MyHC gene expression in mdx mice masticatory muscles may be seen as an adaptive mechanism to muscular dystrophy.