Dysfunction of the median nerve at the elbow or proximal forearm can characterize two distinct clinical entities: pronator syndrome (PS) or anterior interosseous nerve (AIN) syndrome. PS is characterized by vague volar forearm pain, with median nerve paresthesias and minimal motor findings. AIN syndrome is a pure motor palsy of any or all of the muscles innervated by that nerve: the flexor pollicis longus, the flexor digitorum profundus of the index and middle fingers, and the pronator quadratus. The sites of anatomic compression are essentially the same for both disorders. Typically, the findings of electrodiagnostic studies are normal in patients with PS and abnormal in those with AIN syndrome. PS is a controversial diagnosis and is typically treated nonsurgically. AIN syndrome is increasingly thought to be neuritis and it often resolves spontaneously following prolonged observation. Surgical indications for nerve decompression include persistent symptoms for >6 months in patients with PS or for a minimum of 12 months with no signs of motor improvement in those with AIN syndrome.
Recombinant human BMP-2 (rhBMP-2) is a potent osteoinductive agent, but has been associated not only with bone formation, but also osteoclastogenesis and bone resorption. Osteoprotegerin (OPG) is a RANKL inhibitor that blocks differentiation and function of osteoclasts. We hypothesized that the combination of local BMP-2 (recombinant protein or a product of gene therapy) plus systemic OPG-Fc is more effective than BMP-2 alone in promoting bone repair. To test this hypothesis we used a mouse critical-sized femoral defect model. Col2.3eGFP (osteoblastic marker) male mice were treated with rhBMP-2 (group I), rhBMP-2 and systemic OPG (group II), rhBMP-2 and delayed administration of OPG (group III), mouse BM cells transduced with a lentiviral vector containing the BMP-2 gene (LV-BMP-2; group IV), LV-BMP-2 and systemic OPG (group V), a carrier alone (group VI) and administration of OPG alone (group VII). All bone defects treated with BMP-2 (alone or combined with OPG) healed, whereas minimal bone formation was noted in animals treated with the carrier alone or OPG alone. MicroCT analysis showed that bone volume (BV) in rhBMP-2 + OPG and LV-BMP-2 + OPG groups was significantly higher compared to rhBMP-2 alone (p < 0.01) and LV-BMP-2 alone (p < 0.001). Similar results were observed in histomorphometry, with rhBMP-2 alone defects exhibiting significantly lower bone area (B.Ar) compared to rhBMP-2 + OPG defects (p < 0.005) and LV-BMP-2 defects having a significantly lower B.Ar compared to all BMP-2 + OPG treated groups (p ≤ 0.01). TRAP staining demonstrated a major osteoclast response in the groups that did not receive OPG (rhBMP-2, LV-BMP-2 and sponge alone) beginning as early as 7 days post-operatively. In conclusion, we demonstrated that locally delivered BMP-2 (recombinant protein or gene therapy) in combination with systemically administered OPG improved bone healing compared to BMP-2 alone in a mouse critical-sized bone defect. These data indicate that osteoclasts can diminish healing responses to BMP-2 and that RANKL inhibition may thus accentuate BMP-2 efficacy.
The role that transduced mouse bone marrow stromal cells (mBMSCs) engineered to overexpress human bone morphogenetic protein 2 (BMP-2) play in healing critical-sized skeletal defects is largely unknown. We evaluated the interaction between host osteoprogenitor cells and donor mBMSCs transduced with either a lentiviral (LV) vector-expressing red fluorescent protein (RFP) with or without BMP-2 that were implanted into a critical-sized femoral defect. Radiographs taken at the time of killing were evaluated using a five-point scaled scoring system. Frozen histologic sections were analyzed to assess both the transduced cells' role in bone repair and the local osteoprogenitor response. There was complete radiographic bridging in 94% of group I (LV-RFPch-BMP-2-cmyc) and 100% of group III (recombinant human BMP-2) specimens. Radiographs demonstrated a lack of healing in group II (LV-RFPch). Mouse BMSCs transduced with an LV-RFPch-BMP-2 vector were able to induce host cells to differentiate down an osteoblastic lineage and heal a critical-sized defect. However, the donor cells appeared to be functioning as a delivery vehicle of BMP-2 rather than actually differentiating into osteoblasts capable of participating in bone repair as evidenced by a lack of colocalization of the transduced cells to the sites of skeletal repair where the host progenitor cells were found.
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