Inhibition of TGF-β signaling attenuates disuse-induced trabecular bone loss after spinal cord injury in male mice

Bone loss is one of the most common complications of immobilization after spinal cord injury (SCI). Whether TGF-β signaling plays a role in SCI-induced disuse bone loss has not been determined. Thus, 16-week-old male mice underwent sham or spinal cord contusion injury to cause complete hindlimb paralysis. Five days later, 10 mg/kg/day control (IgG) or anti-TGF-β1,2,3 neutralizing antibody (1D11) was administered twice weekly for 4 weeks. Femurs were examined by micro-computed tomography scanning (micro-CT) and histology. Bone marrow (BM) supernatants were analyzed by ELISA for levels of procollagen type 1 intact N-terminal propeptide (P1NP), tartrate-resistant acid phosphatase (TRAcP-5b), receptor activator of nuclear factor-kappa B ligand (RANKL), osteoprotegerin (OPG), and prostaglandin E2 (PGE2). Distal femoral micro-CT analysis showed that SCI-1D11 mice had significantly (P<0.05) attenuated loss of trabecular fractional bone volume (123% SCI-1D11 vs 69% SCI-IgG), thickness (98% vs 81%), and connectivity (112% vs 69%) and improved the structure model index (2.1 vs 2.7). Histomorphometry analysis revealed that osteoclast numbers were lower in the SCI-IgG mice than sham-IgG sham control. Biochemically, SCI-IgG mice had higher levels of P1NP and PGE2 but similar TRAcP-5b and RANKL/OPG ratio to the sham-IgG group. SCI-1D11 group exhibited higher levels of P1NP and RANKL/OPG ratio but similar TRAcP-5b and PGE2 to the sham-1D11 group. Furthermore, 1D11 treatment prevented SCI-induced hyperphosphorylation of tau protein in osteocytes, an event that destabilizes the cytoskeleton. Together, inhibition of TGF-β signaling after SCI protects trabecular bone integrity, likely by balancing bone remodeling, inhibiting PGE2 elevation, and preserving the osteocyte cytoskeleton.