Abstract
Mechanisms by which muscle regulates bone are poorly understood. Electrically stimulated muscle contraction reversed elevations in bone resorption and increased Wnt signaling in bone-derived cells after spinal cord transection. Muscle contraction reduced resorption of unloaded bone independently of the CNS, through mechanical effects and, potentially, nonmechanical signals (e.g. myokines). The study provides new insights regarding muscle-bone interactions. Muscle and bone work as a functional unit. Cellular and molecular mechanisms underlying effects of muscle activity on bone mass are largely unknown. Spinal cord injury (SCI) causes muscle paralysis and extensive sublesional bone loss and disrupts neural connections between the central nervous system (CNS) and bone. Muscle contraction elicited by electrical stimulation (ES) of nerves partially protects against SCI-related bone loss. Thus, application of ES after SCI provides an opportunity to study the effects of muscle activity on bone and roles of the CNS in this interaction, as well as the underlying mechanisms. Using a rat model of SCI, the effects on bone of ES-induced muscle contraction were characterized. The SCI-mediated increase in serum C-terminal telopeptide of type I collagen (CTX) was completely reversed by ES. In ex vivo bone marrow cell cultures, SCI increased the number of osteoclasts and their expression of mRNA for several osteoclast differentiation markers, whereas ES significantly reduced these changes; SCI decreased osteoblast numbers, but increased expression in these cells of receptor activator of NF-κB ligand (RANKL) mRNA, whereas ES increased expression of osteoprotegerin (OPG) and the OPG/RANKL ratio. A microarray analysis revealed that ES partially reversed SCI-induced alterations in expression of genes involved in signaling through Wnt, FSH, parathyroid hormone (PTH), oxytocin, and calcineurin/nuclear factor of activated T-cells (NFAT) pathways. ES mitigated SCI-mediated increases in mRNA levels for the Wnt inhibitors DKK1, sFRP2, and sclerostin in ex vivo cultured osteoblasts. Our results demonstrate an anti-bone-resorptive activity of muscle contraction by ES that develops rapidly and is independent of the CNS. The pathways involved, particularly Wnt signaling, suggest future strategies to minimize bone loss after immobilization.
Highlights
Mechanisms by which muscle regulates bone are poorly understood
A microarray analysis revealed that electrical stimulation (ES) partially reversed Spinal cord injury (SCI)-induced alterations in expression of genes involved in signaling through Wnt, FSH, parathyroid hormone (PTH), oxytocin, and calcineurin/nuclear factor of activated T-cells (NFAT) pathways
We recently reported that when administered to rats with SCI, nandrolone, a synthetic androgen, increased bone mass and expression in osteoblasts derived from bone marrow stromal cells of the osteoblast differentiation markers Runx2 and osteocalcin and reversed the SCI-induced inhibition of Enc1 expression (Sun et al [62])
Summary
Results: Electrically stimulated muscle contraction reversed elevations in bone resorption and increased Wnt signaling in bone-derived cells after spinal cord transection. Conclusion: Muscle contraction reduced resorption of unloaded bone independently of the CNS, through mechanical effects and, potentially, nonmechanical signals (e.g. myokines). Cellular and molecular mechanisms underlying effects of muscle activity on bone mass are largely unknown. Muscle contraction elicited by electrical stimulation (ES) of nerves partially protects against SCI-related bone loss. Application of ES after SCI provides an opportunity to study the effects of muscle activity on bone and roles of the CNS in this interaction, as well as the underlying mechanisms. Using a rat model of SCI, the effects on bone of ES-induced muscle contraction were characterized. Our results demonstrate an anti-bone-resorptive activity of muscle contraction by ES that develops rapidly and is independent of the CNS. The pathways involved, Wnt signaling, suggest future strategies to minimize bone loss after immobilization
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