Browsing by Subject "Loading"

Now showing 1 - 5 of 5
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    Loss of the auxiliary α2δ1 voltage-sensitive calcium channel subunit impairs bone formation and anabolic responses to mechanical loading
    (Oxford University Press, 2024-01-10) Kelly, Madison M.; Sharma, Karan; Wright, Christian S.; Yi, Xin; Reyes Fernandez, Perla C.; Gegg, Aaron T.; Gorrell, Taylor A.; Noonan, Megan L.; Baghdady, Ahmed; Sieger, Jacob A.; Dolphin, Annette C.; Warden, Stuart J.; Deosthale, Padmini; Plotkin, Lilian I.; Sankar, Uma; Hum, Julia M.; Robling, Alexander G.; Farach-Carson, Mary C.; Thompson, William R.; Physical Therapy, School of Health and Human Sciences
    Voltage-sensitive calcium channels (VSCCs) influence bone structure and function, including anabolic responses to mechanical loading. While the pore-forming (α1) subunit of VSCCs allows Ca2+ influx, auxiliary subunits regulate the biophysical properties of the pore. The α2δ1 subunit influences gating kinetics of the α1 pore and enables mechanically induced signaling in osteocytes; however, the skeletal function of α2δ1 in vivo remains unknown. In this work, we examined the skeletal consequences of deleting Cacna2d1, the gene encoding α2δ1. Dual-energy X-ray absorptiometry and microcomputed tomography imaging demonstrated that deletion of α2δ1 diminished bone mineral content and density in both male and female C57BL/6 mice. Structural differences manifested in both trabecular and cortical bone for males, while the absence of α2δ1 affected only cortical bone in female mice. Deletion of α2δ1 impaired skeletal mechanical properties in both sexes, as measured by three-point bending to failure. While no changes in osteoblast number or activity were found for either sex, male mice displayed a significant increase in osteoclast number, accompanied by increased eroded bone surface and upregulation of genes that regulate osteoclast differentiation. Deletion of α2δ1 also rendered the skeleton insensitive to exogenous mechanical loading in males. While previous work demonstrates that VSCCs are essential for anabolic responses to mechanical loading, the mechanism by which these channels sense and respond to force remained unclear. Our data demonstrate that the α2δ1 auxiliary VSCC subunit functions to maintain baseline bone mass and strength through regulation of osteoclast activity and also provides skeletal mechanotransduction in male mice. These data reveal a molecular player in our understanding of the mechanisms by which VSCCs influence skeletal adaptation.
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    Postnatal β-catenin deletion from Dmp1-expressing osteocytes/osteoblasts reduces structural adaptation to loading, but not periosteal load-induced bone formation
    (Elsevier, 2016-07) Kang, Kyung Shin; Hong, Jung Min; Robling, Alexander G.; Anatomy and Cell Biology, School of Medicine
    Mechanical signal transduction in bone tissue begins with load-induced activation of several cellular pathways in the osteocyte population. A key pathway that participates in mechanotransduction is Wnt/Lrp5 signaling. A putative downstream mediator of activated Lrp5 is the nucleocytoplasmic shuttling protein β-catenin (βcat), which migrates to the nucleus where it functions as a transcriptional co-activator. We investigated whether osteocytic βcat participates in Wnt/Lrp5-mediated mechanotransduction by conducting ulnar loading experiments in mice with or without chemically induced βcat deletion in osteocytes. Mice harboring βcat floxed loss-of-function alleles (βcat(f/f)) were bred to the inducible osteocyte Cre transgenic (10)(kb)Dmp1-CreERt2. Adult male mice were induced to recombine the βcat alleles using tamoxifen, and intermittent ulnar loading sessions were applied over the following week. Although adult-onset deletion of βcat from Dmp1-expressing cells reduced skeletal mass, the bone tissue was responsive to mechanical stimulation as indicated by increased relative periosteal bone formation rates in recombined mice. However, load-induced improvements in cross sectional geometric properties were compromised in recombined mice. The collective results indicate that the osteoanabolic response to loading can occur on the periosteal surface when β-cat levels are significantly reduced in Dmp1-expressing cells, suggesting that either (i) only low levels of β-cat are required for mechanically induced bone formation on the periosteal surface, or (ii) other additional downstream mediators of Lrp5 might participate in transducing load-induced Wnt signaling.
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    Resonance in the mouse tibia as a predictor of frequencies and locations of loading-induced bone formation
    (Springer, 2014-01) Zhao, Liming; Dodge, Todd; Nemani, Arun; Yokota, Hiroki; Biomedical Engineering, School of Engineering and Technology
    To enhance new bone formation for the treating of patients with osteopenia and osteoporosis, various mechanical loading regimens have been developed. Although a wide spectrum of loading frequencies is proposed in those regimens, a potential linkage between loading frequencies and locations of loading-induced bone formation is not well understood. In this study, we addressed a question: Does mechanical resonance play a role in frequency-dependent bone formation? If so, can the locations of enhanced bone formation be predicted through the modes of vibration? Our hypothesis is that mechanical loads applied at a frequency near the resonant frequencies enhance bone formation, specifically in areas that experience high principal strains. To test the hypothesis, we conducted axial tibia loading using low, medium, or high frequency to the mouse tibia, as well as finite element analysis. The experimental data demonstrated dependence of the maximum bone formation on location and frequency of loading. Samples loaded with the low-frequency waveform exhibited peak enhancement of bone formation in the proximal tibia, while the high-frequency waveform offered the greatest enhancement in the midshaft and distal sections. Furthermore, the observed dependence on loading frequencies was correlated to the principal strains in the first five resonance modes at 8.0-42.9 Hz. Collectively, the results suggest that resonance is a contributor to the frequencies and locations of maximum bone formation. Further investigation of the observed effects of resonance may lead to the prescribing of personalized mechanical loading treatments.
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    The significance of loading profile on the occlusion mechanics of a viscoelastic periodontal ligament analogue-supported tooth
    (2020-06) Thompson, James P.; Katona, Thomas R.; Eckert, George J.
    Background: Benchtop studies of occlusal contact forces have used teeth that were rigidly attached or supported by readily available viscoelastic (VE) materials that served as periodontal ligament (PDL) substitutes. More recent specimens have incorporated a precisely dimensioned VE PDL-behavior matched analogue. Objectives: The objectives of this study were to evaluate a modified loading protocol (step function) that is more appropriate to VE support than the previously used ramp function. The occlusion manifestations of the time-dependent behaviors (creep and recovery) of the PDL analogue were examined using the revised protocol. Methods: A mandibular 1st molar denture tooth was set into a precision-machined root/socket assembly. The PDL substitute was then cured to tight dimensional tolerances. The matching rigidly fixed maxillary denture tooth was aligned into a Class I centric molar relationship in a testing apparatus. The weighted maxillary assembly was then cycled onto and off of the load cell-supported mandibular assembly. For statistical purposes, three loading schedules were tested in 21 0.05 mm shifted occlusal relationships. Rigid attachment served as control. Results: Statistical analyses were performed on the peak values of Flateral, the net in-occlusal plane force component of the occlusal contact forces. It was found that there were statistically significant differences between: chomp-to-chomp (p < .038), PDL vs. rigid (p < .001) and loading schedules (p < .044 for PDL only). Conclusion: The loading protocol affects outcomes, and the step functions maintained consistent timing with prescribable creep and recovery periods.
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    The Wnt pathway: An important control mechanism in bone's response to mechanical loading
    (Elsevier, 2021) Choi, Roy B.; Robling, Alexander G.; Anatomy, Cell Biology and Physiology, School of Medicine
    The conversion of mechanical energy into biochemical changes within living cells is process known as mechanotransduction. Bone is a quintessential tissue for studying the molecular mechanisms of mechanotransduction, as the skeleton's mechanical competence is crucial for vertebrate movement. Bone cell mechanotransduction is facilitated by a number of cell biological pathways, one of the most prominent of which is the Wnt signaling cascade. The Wnt co-receptor Lrp5 has been identified as a crucial protein for mechanical signaling in bone, and modifiers of Lrp5 activity play important roles in mediating signaling efficiency through Lrp5, including sclerostin, Dkk1, and the co-receptor Lrp4. Mechanical regulation of sclerostin is mediated by certain members of the Hdac family. Other mechanisms that influence Wnt signaling-some of which are mechanoresponsive-are coming to light, including R-spondins and their role in organizing the Rnf43/Znrf3 and Lgr4/5/6 complex that liberates Lrp5. While the identity of the key Wnt proteins involved in bone cell mechanical signaling are elusive, the likely pool of key players is narrowing. Identification of Wnt-based molecular targets that can be modulated pharmacologically to make mechanical stimulation (e.g., exercise) more beneficial is an emerging approach to improving skeletal integrity and reducing fracture risk.