With the reignited push for manned spaceflight and the development of companies focused on commercializing spaceflight, increased human ventures into space are inevitable. However, this venture would not be without risk. The lower gravitational force, known as microgravity, that would be experienced during spaceflight significantly disrupts many physiological systems. One of the most notably affected systems is the musculoskeletal system, where exposure to microgravity causes both bone and skeletal muscle loss, both of which have significant clinical implications. In this review, we focus on recent advancements in our understanding of how exposure to microgravity affects the musculoskeletal system. We will focus on the catabolic effects microgravity exposure has on both bone and skeletal muscle cells, as well as their respective progenitor stem cells. Additionally, we report on the mechanisms that underlie bone and muscle tissue loss resulting from exposure to microgravity and then discuss current countermeasures being evaluated. We reveal the gaps in the current knowledge and expound upon how current research is filling these gaps while also identifying new avenues of study as we continue to pursue manned spaceflight.

Substrate surface characteristics such as roughness, wettability and particle density are well-known contributors of a substrate's overall osteogenic potential. These characteristics are known to regulate cell mechanics as well as induce changes in cell stiffness, cell adhesions, and cytoskeletal structure. Pro-osteogenic particles, such as hydroxyapatite, are often incorporated into a substrate to enhance the substrates osteogenic potential. However, it is unknown which substrate characteristic is the key regulator of osteogenesis. This is partly due to the lack of understanding of how these substrate surface characteristics are transduced by cells. In this study substrates composed of polycaprolactone (PCL) and carbonated hydroxyapatite particles (HAp) were synthesized. HAp concentration was varied, and a range of surface characteristics created. The effect of each substrate characteristic on osteoblastic differentiation was then examined. We found that, of the characteristics examined, only HAp density, and indeed a specific density (85 particles/cm2), significantly increased osteoblastic differentiation. Further, an increase in focal adhesion maturation and turnover was observed in cells cultured on this substrate. Moreover, β-catenin translocation from the membrane bound cell fraction to the nucleus was more rapid in cells on the 85 particle/cm2 substrate compared to cells on tissue culture polystyrene. Together, these data suggest that particle density is one pivotal factor in determining a substrates overall osteogenic potential. Additionally, the observed increase in osteoblastic differentiation is a at least partly the result of β-catenin translocation and transcriptional activity suggesting a β-catenin mediated mechanism by which substrate surface characteristics are transduced.

An aging population, decreased activity levels and increased combat injuries, have led to an increase in critical sized bone defects. As more defects are treated using allografts, which is the current standard of care, the deficiencies of allografts are becoming more evident. Allografts lack the angiogenic potential to induce sufficient vasculogenesis to counteract the hypoxic environment associated with critical sized bone defects. In this study, aptamer-functionalized fibrin hydrogels (AFH), engineered to release vascular endothelial growth factor (VEGF), were evaluated for their material properties, growth factor release kinetics, and angiogenic and osteogenic potential in vivo. Aptamer functionalization to native fibrin did not result in significant changes in biocompatibility or hydrogel gelation. However, aptamer functionalization of fibrin did improve the release kinetics of VEGF from AFH and, when compared to FH, reduced the diffusivity and extended the release of VEGF several days longer. VEGF released from AFH, in vivo, increased vascularization to a greater degree, relative to VEGF released from FH, in a murine critical-sized cranial defect. Defects treated with AFH loaded with VEGF, relative to nonhydrogel loaded controls, showed a nominal increase in osteogenesis. Together, these data suggest that AFH more efficiently incorporates and retains VEGF in vitro and in vivo, which then enhances angiogenesis and osteogenesis to a greater extent in vivo than FH.

Synthetic biomimetic carbonated hydroxyapatite (CHA) has shown significant promise in bone tissue engineering for its mechanical and chemical biocompatibility and osteogenic potential. Variations in the size of hydroxyapatite particles have also been shown to contribute to the hydroxyapatite's osteogenic success. However, synthesizing biomimetic CHA with optimal osteogenic properties using a simple synthesis methodology to make highly reproducible, biomimetic, and osteogenic CHA has not been evaluated fully. The objective of this study was to synthesize submicron CHA particles using a nanoemulsion method. We hypothesized that by varying the synthesis technique we could control particle size while still creating highly biomimetic CHA typically produced during nanoemulsion synthesis. Furthermore, we hypothesized that 500 nm CHA particles would induce greater osteoblastic differentiation compared to larger or smaller CHA particles. X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and dynamic light scattering were used to characterize the chemical composition, shape, and size of CHA synthesized through variations in pH, temperature and stirring speed during synthesis. Manipulation of pH showed the ability to selectively tailor CHA particle size from 200-900 nm in a reproducible manner while maintaining the chemical composition.In addition, 500 nm particles elicited the most rapid increase in osteoblastic differentiation and did not decrease cell viability compared to 200 and 900 nm particles.

The mechanism by which substrate surface characteristics are transduced by osteoblastic cells and their progenitors is not fully known. Data from previous studies by our group suggest the involvement of β‐catenin in the mechanism by which substrate surface characteristics are transduced. This focal adhesion and β‐catenin mediated mechanism functions through the liberation of β‐catenin from focal adhesion complexes in response to pro‐osteogenic substrate (POS) characteristics. After liberation, β‐catenin translocates and facilitates upregulation of genes associated with osteogenesis. It is not known whether the observed correlation between focal adhesion turnover and β‐catenin translocation directly results from focal adhesion turnover. In this study we inhibited focal adhesion turnover using a focal adhesion kinase inhibitor PF‐573228. We found that inhibition of focal adhesion turnover resulted in an abrogation of the more rapid translocation and increased transcriptional activity of β‐catenin induced by POS. In addition, inhibition of focal adhesion turnover mitigated the increase in osteoblastic differentiation induced by a POS as measured by alkaline phosphatase enzymatic activity and osteogenic gene and protein expression. Together, these data, coupled with previous findings, suggest that the observed β‐catenin translocation is a result of focal adhesion turnover, providing evidence for a focal adhesion initiated, β‐catenin mediated mechanism of substrate surface signal transduction.