INFLUENCE OF HEAD-DOWN TILT BEDREST ON DNA REPAIR CAPACITY

Abstract

INTRODUCTION Major limiting factors in human spaceflight are the deleterious effects of reduced gravity and space radiation exposure on health and performance. Radiation-induced cellular DNA damage, if not repaired or not correctly repaired, increases the risk of cancer and degenerative diseases. Likewise, without appropriate countermeasures, reduced gravity will lead to musculoskeletal and cardiovascular deconditioning. We hypothesized that deconditioning per se would hinder the recovery of cells from radiation damage. We developed a terrestrial ex vivo model to investigate cellular DNA repair while simulating microgravity effects using head-down-tilt (HDT) bedrest during the Spaceflight-Associated Neuro-Ocular Syndrome Countermeasures (SANS-CM) study. METHODS Each of the four SANS CM campaigns comprised three experimental phases: 1) a 14-day baseline data collection (BDC) phase (BDC-14 through BDC-1); 2) a 30-day 6° HDT bedrest phase (HDT1 through HDT30); and 3) a 14-day recovery (R+) phase (R+0 through R+13). Twelve participants were enrolled in each campaign. Blood samples were obtained from the subjects 14 days before the bedrest (BDC-14), 10 and 28 days into head-down-tilt bedrest (HDT10 and HDT28), and after 10 days of recovery (R+10). Peripheral blood mononuclear cells (PBMC) were isolated by density gradient centrifugation using Greiner LeucosepTM tubes. We studied the ex vivo induction and repair of DNA double strand breaks, for which the cells were exposed to 1 and 4 Gy of X-rays. The cells were then harvested 0.5, 1, 2, 4, and 24 h after irradiation. DNA double strand breaks were detected via immunofluorescence staining of γH2AX that was quantified using flow cytometry. RESULTS The results for γH2AX fluorescence intensity and the percentage of cells with unrepaired DNA reached a peak value after 2 h of X-rays irradiation and was greatly reduced after 24 h. For all blood collection time-points during the study, the γH2AX fluorescence intensity did not differ significantly. Furthermore, there was no significant interpersonal variance of DNA double strand break repair capacity. CONCLUSION DNA double strand break repair activity in PBMC remained unaffected by 30 days of HDT bedrest, suggesting that physiological deconditioning likely does not modulate DNA repair capacity. ACKNOWLEDGEMENT The study was funded by the National Aeronautics and Space Administration (NASA) and the German Aerospace Center (DLR), and performed at the :envihab research facility of the DLR Institute of Aerospace Medicine. The authors wish to gratefully acknowledge the bedrest participants who volunteered their time, without whom this project would not have been possible. We also thank the SANS CM study staff for their dedicated work and tireless effort.