Regulatory mechanisms in bone following mechanical loading

Perlecan/Hspg2 deficiency impairs bone's calcium signaling and associated transcriptome in response to mechanical loading

Perlecan, a heparan sulfate proteoglycan, acts as a mechanical sensor for bone to detect external loading. Deficiency of perlecan increases the risk of osteoporosis in patients with Schwartz-Jampel Syndrome (SJS) and attenuates loading-induced bone formation in perlecan deficient mice (Hypo). Considering that intracellular calcium [Ca²⁺]_i is an ubiquitous messenger controlling numerous cellular processes including mechanotransduction, we hypothesized that perlecan deficiency impairs bone's calcium signaling in response to loading. To test this, we performed real-time [Ca²⁺]_i imaging on in situ osteocytes of adult murine tibiae under cyclic loading (8N). Relative to wild type (WT), Hypo osteocytes showed decreases in the overall [Ca²⁺]_i response rate (-58%), calcium peaks (-33%), cells with multiple peaks (-53%), peak magnitude (-6.8%), and recovery speed to baseline (-23%). RNA sequencing and pathway analysis of tibiae from mice subjected to one or seven days of unilateral loading demonstrated that perlecan deficiency significantly suppressed the calcium signaling, ECM-receptor interaction, and focal adhesion pathways following repetitive loading. Defects in the endoplasmic reticulum (ER) calcium cycling regulators such as Ryr1/ryanodine receptors and Atp2a1/Serca1 calcium pumps were identified in Hypo bones. Taken together, impaired calcium signaling may contribute to bone's reduced anabolic response to loading, underlying the osteoporosis risk for the SJS patients.

Dynamic fluid flow stimulation on cortical bone and alterations of the gene expressions of osteogenic growth factors and transcription factors in a rat functional disuse model

Recently we have developed a dynamic hydraulic stimulation (DHS) as a loading modality to induce anabolic responses in bone. To further study the functional process of DHS regulated bone metabolism, the objective of this study was to evaluate the effects of DHS on cortical bone and its alterations on gene expressions of osteogenic growth factors and transcription factors as a function of time. Using a model system of 5-month-old hindlimb suspended (HLS) female Sprague-Dawley rats, DHS was applied to the right tibiae of the stimulated rats with a loading frequency of 2Hz with 30mmHg (p-p) dynamic pressure, 5days/week, for a total of 28days. Midshafts of the tibiae were analyzed using μCT and histology. Total RNA was analyzed using RT-PCR on selected osteogenic genes (RUNX2, β-catenin, osteopontin, VEGF, BMP2, IGF-1, and TGF-β) on 3-, 7-, 14- , and 21-day. Results showed increased Cort.Th and Ct.BV/TV as well as a time-dependent fashion of gradual changes in mRNA levels upon DHS. While DHS-driven fold changes of the mRNA levels remained low before Day-7, its fold changes started to elevate by Day-14 and then dropped by Day-21. This study further delineates the underlying molecular mechanism of DHS-derived mechanical signals, and its time-dependent optimization.

The interaction of force and repetition on musculoskeletal and neural tissue responses and sensorimotor behavior in a rat model of work-related musculoskeletal disorders

Background

We examined the relationship of musculoskeletal risk factors underlying force and repetition on tissue responses in an operant rat model of repetitive reaching and pulling, and if force x repetition interactions were present, indicative of a fatigue failure process. We examined exposure-dependent changes in biochemical, morphological and sensorimotor responses occurring with repeated performance of a handle-pulling task for 12 weeks at one of four repetition and force levels: 1) low repetition with low force, 2) high repetition with low force, 3) low repetition with high force, and 4) high repetition with high force (HRHF).

Methods

Rats underwent initial training for 4–6 weeks, and then performed one of the tasks for 12 weeks, 2 hours/day, 3 days/week. Reflexive grip strength and sensitivity to touch were assayed as functional outcomes. Flexor digitorum muscles and tendons, forelimb bones, and serum were assayed using ELISA for indicators of inflammation, tissue stress and repair, and bone turnover. Histomorphometry was used to assay macrophage infiltration of tissues, spinal cord substance P changes, and tissue adaptative or degradative changes. MicroCT was used to assay bones for changes in bone quality.

Results

Several force x repetition interactions were observed for: muscle IL-1alpha and bone IL-1beta; serum TNFalpha, IL-1alpha, and IL-1beta; muscle HSP72, a tissue stress and repair protein; histomorphological evidence of tendon and cartilage degradation; serum biomarkers of bone degradation (CTXI) and bone formation (osteocalcin); and morphological evidence of bone adaptation versus resorption. In most cases, performance of the HRHF task induced the greatest tissue degenerative changes, while performance of moderate level tasks induced bone adaptation and a suggestion of muscle adaptation. Both high force tasks induced median nerve macrophage infiltration, spinal cord sensitization (increased substance P), grip strength declines and forepaw mechanical allodynia by task week 12.

Conclusions

Although not consistent in all tissues, we found several significant interactions between the critical musculoskeletal risk factors of force and repetition, consistent with a fatigue failure process in musculoskeletal tissues. Prolonged performance of HRHF tasks exhibited significantly increased risk for musculoskeletal disorders, while performance of moderate level tasks exhibited adaptation to task demands.