Mechanical loading is necessary for proper development and maintenance of the musculoskeletal system. Yet, despite the profound effects of reduced mechanical loading on muscle atrophy and skeletal fragility, there has been little investigation into the physiological effects of clinically relevant partial weight-bearing environments, such as bed rest, immobilization, stroke, cerebral palsy, muscular dystrophy, spinal cord injury or age-related reductions in physical activity.
The major obstacle to such research has been the lack of a suitable animal model. We have developed a novel model of titrated weight-bearing that offers a unique capability for exploring the chronic effects of reduced quadrupedal loading in mice. The system allows studies with controlled exposure to 10-80% weight-bearing compared to normally loaded controls in an identical environment [link].
Our long-term goal is to take advantage of this unique model to gain insight into the mechanisms underlying the musculoskeletal response to reduced mechanical loading, thereby identifying new targets for preventing musculoskeletal deterioration in due to age-, disease- or injury-induced reductions in mechanical loading. Thus, we propose to extensively characterize the mechanical stimuli associated with our partial weight-bearing model, and to determine the timing and magnitude of the musculoskeletal response to partial weight-bearing, as compared to both normal weight-bearing and full hindlimb unloading via tail suspension. Establishment of a model where quadrupedal gait is maintained, yet loads can be reduced by prescribed amounts will provide the opportunity to test long-held views about the minimal loading stimulus necessary to maintain bone and muscle tissue under conditions of disuse. A major advantage to developing a partial weight-bearing murine model is that it will be ideally suited for future studies designed to delineate the genetic, cellular and molecular mechanisms associated with musculoskeletal adaptation to altered loading environments.
We will use our partial weight-bearing model to quantify the musculoskeletal effects of 10, 21 and 35 days exposure to 20, 40, 60 or 100% body weight loading in adult female mice, and compare the response to that of tail suspension (0% body weight). Outcome assessments will include in vivo bone mineral density, body composition, as well as ex vivo muscle weight, bone architecture by µCT, and femoral biomechanics. Serum markers of bone turnover, marrow fat assessment, histology and dynamic histomorphometry will be used to delineate mechanisms underlying the response to partial weight-bearing.
We are also testing whether new therapeutic interventions can inhibit bone loss during disuse. In particular, we are testing whether a new anti-resorptive therapy, antibody to RANKL (denosumab) and whether a new anabolic therapy, sclerostin antibody, inhibits skeletal deterioration across the spectrum of partial weight bearing environments. Denosumab, a fully human monoclonal antibody to the receptor activator of nuclear factor-B ligand (RANKL), inhibits development and activity of osteoclasts, and thereby markedly decreases bone resorption. Semi-annual subcutaneous injections of denosumab significantly reduce vertebral and non-vertebral fracture risk in postmenopausal women with osteoporosis. The secreted protein sclerostin is a key negative regulator of bone formation. Humans with genetic mutations leading to sclerostin deficiency have increased bone mass, and in rodents, inhibition of sclerostin via pharmacologic antibody treatment or genetic manipulation leads to anabolic skeletal effects. Moreover, mice deficient in sclerostin are resistant to disuse-induced bone loss. Our preliminary studies demonstrate that treatment with sclerostin antibody leads to bone formation even is a disuse model. Images at the right show 3D rendering of representative microCT images of the mouse distal femur, with the unloaded, vehicle treated animal on the top and the unloaded, sclerostin-treated animal on the bottom.
Altogether, this work will provide novel information about musculoskeletal adaptation across a continuum of reduced mechanical loading, and insights into the fundamental relationship between mechanical loading and musculoskeletal adaptation. Moreover, the studies will provide experimental data that can be used to test existing quantitative theories about skeletal adaptation to altered mechanical loading. Finally, development of this model will establish a basis for future studies designed to delineate the cellular and molecular mechanisms underlying skeletal response to reduced loading, and will enhance the development of interventions to prevent muscle and bone atrophy during a variety of clinical conditions of reduced musculoskeletal loading due to disease, injury or inactivity.
This work is funded by NIH-NIAMS R21 AR057522, NASA NNX10AE39G, and a research grant from Amgen.
Ellman R, Spatz J, Cloutier A, Palme R, Christiansen BA, Bouxsein ML. Partial reductions in mechanical loading yield proportional changes in bone density, bone architecture, and muscle mass. J Bone Miner Res 2013; 28(4): 875-85.
Ellman R, Grasso DJ, van Vliet M, Brooks DJ, Spatz JM, Conlon C, Bouxsein ML. Combined Effects of Botulinum Toxin Injection and Hind Limb Unloading on Bone and Muscle. Calcif Tissue Int 2013; 94(3): 327-337.
Wagner EB, Granzella NP, Saito H, Newman DJ, Young LR, Bouxsein ML. Partial weight suspension: a novel murine model for investigating adaptation to reduced musculoskeletal loading. J Appl Physiol 2010; 109(2):350-7. PDF