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Simulated Hypogravity Triggers Differentiation and Apoptosis in Preosteoclastic Cells.  M. Monici1, P.A. Bernabei2, F.Fusi3, N. Marziliano4 and A. Cogoli5. 1Center of Excellence in Optronics, 2Careggi Hospital & 3Dept. of Clinical Physiopath., Florence Univ., I; 4Applera Italia, Monza, I; 5Space Biology Group, ETZH, Zurich, CH.

   The weightlessness affects the skeletal system by inducing decrease of bone density, with clinical picture similar to that observed in the osteoporotic diseases of the elderly persons. Bone loss is due to the uncoupling between bone formation and bone resorption by osteoblasts and osteoclasts respectively. Therefore the regulation of bone cell behaviour is a key factor in osteoporosis. The aim of this study was to investigate the effect of hypogravity, simulated by a Random Positioning Machine (RPM), on the behaviour of an osteoclastic precursor model, the FLG 29.1 cell line, and to address the hypothesis that simulated hypogravity alters major cell programs, such as differentiation and apoptosis. Cell morphology and phenotypic expression of cytoskeletal components, osteoclastic markers, factors regulating apoptosis were studied by cytochemistry and immuno-cytochemistry. Gene expression was analysed by RT-PCR. Cell metabolism was monitored by autofluorescence microspectroscopy and imaging. Osteoclastic bone resorption was assayed and quantified by surface profilometry on bone slices. The results showed changes involving both phenotypic and genotypic expression of FLG29.1 cells cultured for 72h in the RPM, in comparison with controls at 1xg. We found alterations of cell morphology and cytoskeletal components, up-regulation of osteoclastic markers and differentiation factors, presence of bone resorption activity, overexpression of pro-apoptotic factors, differences in cell metabolism. The body of the results demonstrates that differentiation and apoptosis, two fundamental processes in the cell machinery, can be triggered in FLG 29.1 cells by culturing in the RPM. In conclusion, this study provides new information on the possible mechanisms leading to osteoporosis in astronauts, and prompt us to hypothesize a direct effect of weightless on osteoclastic precursors and their differentiation.  (Supported by Italian Space Agency).