Observations on the muscle plasma membrane-associated cytoskeletons of mdx mice by quick-freeze, deep-etch, rotary-shadow replica method.

The Duchenne muscular dystrophy gene product "dystrophin" is a sarcolemma-associated cytoskeleton which is present just inside the sarcolemma. We investigated the ultrastructure and the relationship of this cytoskeleton to the muscle plasma membrane by immunoelectron microscopy and freeze-etch electron microscopy of liquid helium-frozen fresh muscles. The immunoelectron microscopy of the extensor digitorum longus muscles of six mdx mice and six control mice showed the location of anti-dystrophin antibody along the muscle plasma membrane undercoat of all the muscle samples from the control mice without any antibody reaction in the mdx mice muscles. Fresh extensor digitorum longus muscles of seven mdx and six control mice were quick-frozen in liquid helium in the rapid-freeze device. High-magnification electron microscopy of the deep-etch, rotary-shadow replicas of the frozen muscles showed the network formation and attachment of individual rod-shaped cytoskeletons of variable size to the cytoplasmic surface of the muscle plasma membrane in both mdx mice and control mice. The length of cytoskeletons attached to the muscle plasma membranes was measured and mean length +/- SE in mdx mice and control mice were 98 +/- 4 nm and 101 +/- 3 nm, respectively. Although these values were not statistically different (P greater than 0.1), the distribution frequency of 130-150 nm muscle plasma membrane-associated cytoskeletons was 7.9% in mdx mice versus 14.5% in control mice. Since the predicted length of dystrophin is 125-150 nm, the 130- to 150-nm plasma membrane-associated cytoskeletons of mdx control mice may include dystrophin.