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The proteoglycan-containing pericellular matrix (PCM) controls both the biophysical and biochemical microenvironment of osteocytes, which are the most abundant cells embedded and dispersed in bones. As a molecular sieve, osteocytic PCMs not only regulate mass transport to and from osteocytes but also act as sensors of external mechanical environments. The turnover of osteocytic PCM remains largely unknown due to technical challenges. Here, we report a novel imaging technique based on metabolic labeling and "click-chemistry," which labels de novo PCM as "halos" surrounding osteocytes in vitro and in vivo. We then tested the method and showed different labeling patterns in young vs. old bones. Further "pulse-chase" experiments revealed dramatic difference in the "half-life" of PCM of cultured osteocytes (~70 h) and that of osteocytes in vivo (~75 d). When mice were subjected to either 3-week hindlimb unloading or 7-week tibial loading (5.1 N, 4 Hz, 3 d/week), PCM half-life was shortened (~20 d) and degradation accelerated. Matrix metallopeptidase MMP-14 was elevated in mechanically loaded osteocytes, which may contribute to PCM degradation. This study provides a detailed procedure that enables semi-quantitative study of the osteocytic PCM remodeling in vivo and in vitro. [ABSTRACT FROM AUTHOR]

Perlecan/Hspg2, a large monomeric heparan sulfate proteoglycan, is found in the basement membrane and extracellular matrix, where it acts as a matrix scaffold, growth factor depot, and tissue barrier. Perlecan deficiency leads to skeletal dysplasia in Schwartz-Jampel Syndrome (SJS) and is a risk factor for osteoporosis. In the SJS-mimicking murine model (Hypo), inferior cortical bone quality and impaired mechanotransduction in osteocytes were reported. This study focused on trabecular bone, where perlecan deficiency was hypothesized to result in structural deficit and altered response to disuse and re-loading. We compared the Hypo versus WT trabecular bone in both axial and appendicular skeletons of 8-38-week-old male mice, and observed severe trabecular deficit in Hypo mice, approximately 50% reduction of Tb.BV/TV regardless of skeletal site and animal age. Defects in endochondral ossification (e.g., accelerated mineralization), increases in osteoclast activity, and altered differentiation of bone progenitor cells in marrow contributed to the Hypo phenotype. The Hypo trabecular bone deteriorated further under three-week hindlimb suspension as did the WT. Re-ambulation partially recovered the lost trabecular bone in Hypo, but not in WT mice. The novel finding that low-impact loading could counter detrimental disuse effects in the perlecan-deficient skeleton suggests a strategy to maintain skeletal health in SJS patients. [ABSTRACT FROM AUTHOR]