Modeled microgravity inhibits osteogenic differentiation of human mesenchymal stem cells and increases adipogenesis.

Space flight-induced bone loss has been attributed to a decrease in osteoblast function, without a significant change in bone resorption. To determine the effect of microgravity (MG) on bone, we used the Rotary Cell Culture System [developed by the National Aeronautics and Space Administration (NASA)] to model MG. Cultured mouse calvariae demonstrated a 3-fold decrease in alkaline phosphatase (ALP) activity and failed to mineralize after 7 d of MG. ALP and osteocalcin gene expression were also decreased. To determine the effects of MG on osteoblastogenesis, we cultured human mesenchymal stem cells (hMSC) on plastic microcarriers, and osteogenic differentiation was induced immediately before the initiation of modeled MG. A marked suppression of hMSC differentiation into osteoblasts was observed because the cells failed to express ALP, collagen 1, and osteonectin. The expression of runt-related transcription factor 2 was also inhibited. Interestingly, we found that peroxisome proliferator-activated receptor gamma (PPARgamma2), which is known to be important for adipocyte differentiation, adipsin, leptin, and glucose transporter-4 are highly expressed in response to MG. These changes were not corrected after 35 d of readaptation to normal gravity. In addition, MG decreased ERK- and increased p38-phosphorylation. These pathways are known to regulate the activity of runt-related transcription factor 2 and PPARgamma2, respectively. Taken together, our findings indicate that modeled MG inhibits the osteoblastic differentiation of hMSC and induces the development of an adipocytic lineage phenotype. This work will increase understanding and aid in the prevention of bone loss, not only in MG but also potentially in age-and disuse-related osteoporosis.

Vapor transport growth of organic solids in microgravity and unit gravity: some comparisons and results to date.

Thin films of an organic nonlinear optical (NLO) material, N, N-dimethyl-p-(2,2-dicyanovinyl) aniline (DCVA), have been grown in space and on the ground by physical vapor transport in an effusive ampoule arrangement. The thin film growth technique developed on the ground is a direct result of information gleaned from experiments in microgravity. This paper covers the results of our experimental investigations for establishing "ideal" terrestrial conditions for deposition of a DCVA film. The active control during the deposition process was exercised by three deposition variables: the material source temperature, the background pressure external to the growth ampoule and the substrate temperature. Successful growth occurred when the difference in temperature between the source material and the copper substrate was 14 degrees C and the background nitrogen pressure was such that the transport was either diffusive or convective. A qualitative diffusion limited boundary was estimated to occur at a pressure of approximately 20 torr. We have probed the DCVA thin films with visible-near infrared reflection absorption spectroscopy, polarized Fourier transform infrared spectrometry, differential interference contrast optical microscopy, and stylus profilometry.

Modeled microgravity disrupts collagen I/integrin signaling during osteoblastic differentiation of human mesenchymal stem cells.

Spaceflight leads to reduced bone mineral density in weight bearing bones that is primarily attributed to a reduction in bone formation. We have previously demonstrated severely reduced osteoblastogenesis of human mesenchymal stem cells (hMSC) following 7 days culture in modeled microgravity (MMG). One potential mechanism for reduced osteoblastic differentiation is disruption of type I collagen (Col I)-integrin interactions and reduced integrin signaling. Integrins are heterodimeric transmembrane receptors that bind extracellular matrix (ECM) proteins and produce signals essential for proper cellular function, survival, and differentiation. Therefore, we investigated the effects of MMG on integrin expression and function in hMSC. We demonstrate that 7 days of culture in MMG leads to reduced expression of the ECM protein, Col I. Conversely, MMG consistently increases Col I-specific alpha2 and beta1 integrin protein expression. Despite this increase in integrin subunit expression, autophosphorylation of adhesion-dependent kinases, focal adhesion kinase (FAK) and proline-rich tyrosine kinase 2 (PYK2), is significantly reduced. Activation of Akt protein kinase (Akt) is unaffected by the reduction in FAK activation. However, reduced downstream signaling via the Ras-mitogen activated protein kinase (MAPK) pathway is evidenced by a reduction in Ras and extracellular signal-related protein kinase (ERK) activation. Taken together, our findings indicate that MMG decreases integrin/MAPK signaling, which likely contributes to the observed reduction in osteoblastogenesis.