Isolation and preliminary characterization of proteoglycans synthesized by skeletal muscle

The proteoglycans synthesized by cartilage both in vivo and in vitro have been well characterized and serve as a standard for comparison with proteoglycans from other tissues. Both cartilage and muscle are mesenchymally derived tissues; yet proteoglycans synthesized by skeletal muscle have not been studied. This report describes the isolation and preliminary characterization of skeletal muscle proteoglycans synthesized by embryonic chick leg muscle in vitro and in vivo. Proteoglycans extracted from skeletal muscle cultures have a lower relative buoyant density than cartilage proteoglycans, as evidenced by dissociative CsCl equilibrium density gradient centrifugation; only 25% of the muscle proteoglycans are recovered from the densest portion of such gradients. The high buoyant density skeletal muscle proteoglycans have a significantly larger monomer hydrodynamic size and substantially longer chondroitin sulfate chains than do the proteoglycans synthesized by limb bud mesenchyme-derived chondrocytes in culture. The skeletal muscle proteoglycan monomers elute from Sepharose CL-2B with a Kav = 0.17 and have chondroitin sulfate chains estimated to be greater than 50,000 daltons. The proportion of chondroitin-6-sulfate in the in vitro skeletal muscle proteoglycans is higher than that in limb bud culture chondrocyte proteoglycans: 76% versus 58%. Importantly, the chemical and structural features of skeletal muscle proteoglycans are distinct from those of the sulfated molecules produced by muscle-fibroblasts which are also present in the muscle cultures. Thus, the high buoyant density proteoglycans synthesized in vitro by embryonic chick leg myotubes appear to be unique species distinct from the proteoglycans produced in culture by chick limb bud chondrocytes and by chick leg muscle-fibroblasts. Preliminary analysis of chick leg skeletal muscle proteoglycans radiolabeled in ovo indicates that, like the proteoglycans synthesized by skeletal muscle in vitro, they are significantly larger than chondrocyte proteoglycans.