Abstract
Abstract: :
Purpose: The elemental contractile unit of striated muscles, the sarcomere, has a highly specialized cytoskeletal scaffold necessary for sarcomeric stability. The phenotypically novel extraocular muscles (EOMs) may differ in their sarcomeric cytoskeletal architecture from typical skeletal muscle, since EOM fiber types do not have an M–line. Transcriptional profiling studies also have shown differences in cytoskeletal gene expression between EOM and limb muscle and between the global and orbital layers of EOM. In this context, we investigated cytoskeletal organization of adult mouse EOM and limb muscle, with special emphasis upon resolution of differences in the two EOM layers. Methods: Longitudinal sections of adult mouse EOM and limb muscle (tibialis anterior and soleus) were examined using confocal immunofluorescence microsopy for the distribution of key components of sarcomeric cytoskeleton: vinculin, spectrin, nebulin, actinin, desmin, titin and talin. Transcript differences for the major cytoskeletal proteins were also studied by qPCR and in situ hybridization. Results: While showing similar distribution of the key components of the sarcomere cytoskeleton as the limb muscle, EOM had layer specific differences in the distribution of actinin 3 and titin. By qPCR, EOM showed lower levels of actinin–3 than leg and in situ hybridization did not detect this gene in the orbital layer. Confocal microscopy demonstrated a wider I band region of titin in most of the orbital layer fibers than those of the global layer. Conclusions: These data establish that the EOM exhibits unique, layer–specific differences in the cytoskeletal protein titin and its Z–line binding protein, actinin–3. These differences suggest that the sarcomeres of the two layers have different biomechanical properties and these differences may relate to the divergent functional roles proposed for the two layers by the active pulley hypothesis, the orbital layer as the pulley mover and the global layer as the globe mover.
Keywords: extraocular muscles: structure • cytoskeleton