Transient muscle paralysis degrades bone via rapid osteoclastogenesis

A unilateral injection of botulinum toxin A (BTxA) in the calf induces paralysis and profound loss of ipsalateral trabecular bone within days. However, the cellular mechanism underlying acute muscle paralysis‐induced bone loss (MPIBL) is poorly understood. We hypothesized that MPIBL arises via rapid and extensive osteoclastogenesis. We performed a series of in vivo experiments to explore this thesis. First, we observed elevated levels of the proosteoclastogenic cytokine receptor activator for nuclear factor‐κB ligand (RANKL) within the proximal tibia metaphysis at 7 d after muscle paralysis (+113%, P <0.02). Accordingly, osteoclast numbers were increased 122% compared with the contralateral limb at 5 d after paralysis ( P =0.04) and MPIBL was completely blocked by treatment with human recombinant osteoprotegerin (hrOPG). Further, conditional deletion of nuclear factor of activated T‐cells c1 (NFATc1), the master regulator of osteoclastogenesis, completely inhibited trabecular bone loss (‐2.2±11.9%, P <0.01). All experiments included negative control assessments of contralateral limbs and/or within‐animal pre‐ and postintervention imaging. In summary, transient muscle paralysis induced acute RANKL‐mediated osteoclastogenesis resulting in profound local bone resorption. Elucidation of the pathways that initiate osteoclastogenesis after paralysis may identify novel targets to inhibit bone loss and prevent fractures.—Aliprantis, A. O., Stolina, M., Kostenuik, P. J., Poliachik, S. L., Warner, S. E., Bain, S. D., Gross, T. S. Transient muscle paralysis degrades bone via rapid osteoclastogenesis. FASEB J. 26, 1110‐1118 (2012). www.fasebj.org