Suva, Larry J.

Bone loss occurs as a consequence of exposure to microgravity. Using the hindlimb-unloaded rat to model spaceflight, this study had as its purpose to determine whether skeletal unloading and cephalic fluid shifts alter bone blood flow. We hypothesized that perfusion would be diminished in the hindlimb bones and increased in skeletal structures of the forelimbs and head. Using radiolabeled microspheres, we measured skeletal perfusion during control standing and after 10 min, 7 days, and 28 days of hindlimb unloading (HU). Femoral and tibial perfusion were reduced with 10 min of HU, and blood flow to the femoral shaft and marrow were further diminished with 28 days of HU. Correspondingly, the mass of femora (−11%, P < 0.05) and tibiae (−6%, P < 0.1) was lowered with 28 days of HU. In contrast, blood flow to the skull, mandible, clavicle, and humerus was increased with 10 min HU but returned to control levels with 7 days HU. Mandibular (+10%, P < 0.05), clavicular (+18%, P < 0.05), and humeral (+8%, P < 0.1) mass was increased with chronic HU. The data demonstrate that simulated microgravity alters bone perfusion and that such alterations correspond to unloading-induced changes in bone mass. These results support the hypothesis that alterations in bone blood flow provide a stimulus for bone remodeling during periods of microgravity.

Abstract Early diagnosis of the onset of osteoporosis is key to the delivery of effective therapy. Biochemical markers of bone turnover provide a means of evaluating skeletal dynamics that complements static measurements of BMD by DXA. Conventional clinical measurements of bone turnover, primarily the estimation of collagen and its breakdown products in the blood or urine, lack both sensitivity and specificity as a reliable diagnostic tool. As a result, improved tests are needed to augment the use of BMD measurements as the principle diagnostic modality. In this study, the serum proteome of 58 postmenopausal women with high or low/normal bone turnover (training set) was analyzed by surface enhanced laser-desorption/ionization time-of-flight mass spectrometry, and a diagnostic fingerprint was identified using a variety of statistical and machine learning tools. The diagnostic fingerprint was validated in a separate distinct test set, consisting of serum samples from an additional 59 postmenopausal women obtained from the same Mayo cohort, with a gap of 2 yr. Specific protein peaks that discriminate between postmenopausal patients with high or low/normal bone turnover were identified and validated. Multiple supervised learning approaches were able to classify the level of bone turnover in the training set with 80% sensitivity and 100% specificity. In addition, the individual protein peaks were also significantly correlated with BMD measurements in these patients. Four of the major discriminatory peaks in the diagnostic profile were identified as fragments of interalpha-trypsin-inhibitor heavy chain H4 precursor (ITIH4), a plasma kallikrein-sensitive glycoprotein that is a component of the host response system. These data suggest that these serum protein fragments are the serum-borne reflection of the increased osteoclast activity, leading to the increased bone turnover that is associated with decreasing BMD and presumably an increased risk of fracture. In conjunction with the identification of the individual proteins, this protein fingerprint may provide a novel approach to evaluate high bone turnover states.