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Examining the Early Response of Osteoclasts to Ionizing Radiation.  E.R. Anderson, J.S. Willey, J. Nagatomi, T.A. Bateman. Dept. of Bioengineering, Clemson University, Clemson, SC.

   During long-duration spaceflight, astronauts will experience both microgravity and solar and cosmic radiation. It is well documented that skeletal unloading during microgravity causes bone loss. It is unknown how bone, already compromised by microgravity, will respond to ionizing radiation from solar particle events or galactic cosmic rays. We have previously demonstrated that a single 2 Gy dose of clinical and spaceflight relevant radiation types causes significant trabecular bone loss four months post-exposure. However, the cause of the observed bone loss (i.e. reduced formation, increased resorption, etc.) has yet to be determined. Preliminary data have indicated that TRAP5b levels (a marker of bone resorption) are elevated in the serum by three days after irradiation. Bone resorption and formation are coupled via cell signaling mediators, of which RANK (Receptor Activator of Nuclear Factor κ-B), osteoprotegerin (OPG), and RANK ligand (RANKL) are fundamental. During bone remodeling, RANKL on the osteoblast surface binds RANK on the pre-osteoclast to induce osteoclast differentiation from circulating monocytes, activation of mature osteoclasts, and to promote osteoclast survival by inhibiting apoptosis. Likewise, proinflammatory cytokines and inflammatory conditions can induce osteoclast activity.

   To determine if osteoclast activation can increase after irradiation, we will irradiate mice with a low (2 Gy) dose of x-rays. Marrow will be cultured on glass coverslips and bone wafers 24 hours post-exposure. We will then count osteoclasts present 7 days post-exposure and examine the resorption surfaces with light and scanning electron microscopy 14 days post-exposure. Cytometric bead arrays (CBA) and enzyme-linked immunosorbent assays (ELISA) will be used to quantify cytokines (TNFα, IL-1, RANKL, TRAP5b, etc.) and proteins in the cell culture supernatant, blood serum, and marrow that could contribute to osteoclast activation and increased resorption following irradiation.

   (Supported by NSBRI (NASA NCC9-58), Procter and Gamble Pharmaceuticals, and SC Space Grant/EPSCoR.)