Proteoglycan synthesis in fetal tendon is differentially regulated by cyclic compression in vitro

The predominant proteoglycan in tensional regions of tendon is the small proteoglycan decorin. However, a fibrocartilaginous tissue containing large amounts of aggrecan and biglycan develops at points where tendon wraps under bone and is subjected to compressive loading in addition to tension. The hypothesis that local compression regulates the development of fibrocartilage in tendon was tested by assessing the effect of in vitro compressive loading on proteoglycan synthesis. Fetal bovine deep flexor tendon explants from the region which would have become fibrocartilage were subjected to 3 days of continuous cyclic uniaxial compression (unconfined) to 30% strain, at a frequency of 1 cycle/6 s (0.17 Hz). Compression was perpendicular to the long axis of the tendon. Large proteoglycan, biglycan, and decorin were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), and [35S]sulfate incorporated into each proteoglycan was quantitated by liquid scintillation counting of gel slices. The primary effect of compression was to stimulate selectively synthesis of large proteoglycan and biglycan. Incorporation of [35S]sulfate into large proteoglycan was increased 100-300% and incorporation into biglycan was increased 50-150% in compressed tissue compared to matched uncompressed tissue segments. Incorporation into decorin was unchanged. A similar effect on radio-sulfate incorporation was seen following loading of tissue from the tensional region of tendon, which does not normally develop into fibrocartilage. Proteoglycans from compressed tissue were larger, due to slightly longer glycosaminoglycan chains. Disaccharide analysis showed that the C6S/C4S ratio was higher in both the large and the small proteoglycan populations from compressed tissue. Aggrecan mRNA levels were increased approximately fivefold in loaded tissue, and SDS-PAGE analysis of [3H]leucine-labeled core proteins indicated that large proteoglycan core protein synthesis was increased by compression. The selective changes in large proteoglycan and biglycan synthesis, and in the sulfate composition and size of the glycosminoglycan chains, are consistent with what might be expected during development of fibrocartilage in vivo. These observations support the hypothesis that compressive force can regulate the development of fibrocartilaginous tissue in tendon.