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Immunolocalization of dystrophin and neurofilament protein in muscle spindles of normal, mdx-dystrophic, and denervated mice Nahirney, Patrick Charles


Dystrophin is a high molecular weight protein localized under the sarcolemma of normal extrafusal muscle fibers but absent in skeletal muscles of Duchenne muscular dystrophy patients and in the mdx mouse. Muscle spindles in the soleus of 32-week-old normal and age-matched mdx mice were examined by immunocytochemical methods to determine the localization of dystrophin in polar and equatorial regions of the intrafusal fibers. Spindles were serially-sectioned in transverse and longitudinal planes, and they were double-labelled with an antibody to dystrophin and with a 200 kD neurofilament protein antibody which revealed their sensory innervation. By fluorescence microscopy, intrafusal fibers in the soleus of mdx mice were deficient in dystrophin throughout their lengths, whereas their sensory nerve-terminals stained intensely with the nerve-specific antibody and appeared unaltered in dystrophy. In the normal soleus, polar regions of bag and chain fibers exhibited a peripheral rim of sarcolemmal staining equivalent to that seen in the neighboring extrafusal fibers. Dystrophin labelling in equatorial regions of normal intrafusal fibers, however, showed dystrophin-deficient segments alternating in a spiral fashion with positive-staining domains along the sarcolemma. Double-labelling for dystrophin and neurofilament protein showed that these dystrophin-deficient sites were subjacent to the annulospiral sensory-nerve wrappings terminating on the intrafusal fibers. Additionally, it was found that chronic denervation of muscle spindles in normal mice did not affect the expression of dystrophin either at these sites or at the non-sensory regions of the sarcolemma. The results of this study suggest that dystrophin is not an integral part of the subsynaptic sensory-membrane in equatorial regions of normal intrafusal fibers, and, that the neurotrophic effect of sensory innervation is not the principle cause of this unique arrangement of dystrophin in equatorial regions. In dystrophy, intrafusal fibers display the same primary defect in muscular dystrophy as seen in the extrafusal fibers. However, because of their small-diameters, capsular investment, and relatively low tension outputs, dystrophic intrafusal fibers may be less prone to the sarcolemmal membrane disruption that is characteristic of extrafusal fibers in this disorder.

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