Dynamic Expression of Proteoglycans during Skeletal Muscle Development

Proteoglycans are glycoconjugates composed of a core protein and covalcntly attached glycosaminoglycans. Skeletal muscle produces a number of different types of proteogiycans, and, importantly, the types of proteogiycans made by skeletal muscle vary during muscle development. At early stages of muscle development, large chondroiim sulfate proteogiycans of the PG-M/versican type are produced both in culture and in vivo. Localization both by autoradiography of radiolabeled material and by imrnunohistochemistry indicates that the chondroitin sulfate proteogiycans are deposited in a peri cellular region around the muscle cells. Although biosynthesis of these molecules is not detected in mature skeielal muscle, their synthesis is re-initiated during regeneration, which suggests a requirement for these molecules in some early aspect of muscle development. Al later stages of muscle development, small dermatan sulfate proteogiycans are synthesized. One of the ::iajor types of dermatan sulfate proteogiycans in skeletal muscle is decorin, which can bind to collagen and affect fibril formation. Decorin is initially localized in the fibrous connective tissue areas of skeletal muscle, but eventually, at later stages of muscle development, is also found in proximity to the myotubes. Hcparan sulfate proteogiycans are also present in skeletal muscle, and among these are molecules of the syndecan, glypican, and perlecan types. Heparan sulfate proteogiycans are significant because of their involvement in signal transduction of growth factors such as fibroblast growth factor. Evidence indicates that hcparan sulfate proteogiycans play a role in the stimulation of myoblast proliferation by fibroblast growth factor. The exact roles played by other proteogiycans in muscle development are unclear at present. However, proteogiycans have the ability to affect cell adhesion and migration, processes which are important in muscle development. Because of this, it is likely that the changing patterns of proteoglycan biosynthesis during muscle development arc part of and, indeed, influence the orchestrated cellular interactions which are essential for proper muscle formation.