The skeletal fragility that accompanies bone loss due to microgravity is a significant challenge to the success of the space mission. Mechanical signals generated during functional loading promote bone formation at load bearing sites. Mimicking exercise, high magnitude strain, preserves the bone differentiation potential of mesenchymal stem cell (MSC) by regulating the active βcatenin pool. To effectively control MSC lineage allocation, βcatenin must be transported into the nucleus, a process that is poorly understood. Here, with the ultimate goal of preventing microgravity related loss of bone differentiation potential of MSC, the focus of this fellowship is to examine the role of nuclear structure and connectivity in strain-induced (e.g. exercise) βcatenin transport and identify the effects of simulated microgravity on the nucleoskeletal structure and connectivity as well as examining the microgravity-induced reduction in βcatenin signaling.
Role of LINC complex in Maintenance of MSC ßcatenin Signaling Under Microgravity (First Award Fellowship)
Gunes Uzer, Ph.D.
University of North Carolina at Chapel Hill
Loss of load leads to dysfunctional mesenchymal stem cell (MSC) differentiation that favors adipogenesis rather than osteogenesis, a process largely due to deficient βcatenin signaling. Our preliminary data suggests that βcatenin nuclear translocation, the final step that allows active βcatenin access to gene targets, is regulated by LINC complexes (Linker of Nucleoskeleton and Cytoskeleton) that mechanically couple the nuclear and cytoplasmic cytoskeleton. As microgravity also is associated with MSC cell dysfunction, it is possible that microgravity, through decreased cell loading, also arises through disruption of force transfer from the cytoskeleton through LINC to ultimately affect structure and function of the nucleoskeleton.
Here, with the ultimate goal of identifying potential nucleoskeleton and LINC regulatory mechanisms that could be targeted to combat microgravity, we investigate that the microgravity-induced shift of MSC into adipogenic lineage may be accelerated by unloading induced alterations in the nucleoskeleton. As disruption of nucleoskeleton and LINC complexes are associated with decreased Wnt-βcatenin signaling, mimicking progeria, microgravity-induced loss of connectivity between the cell nucleus and cytoplasmic actin, may ultimately lead to inhibition of βcatenin translocation into the nucleus. As such, this fellowship hypothesizes LINC connections that maintain the nucleoskeletal integrity necessary for βcatenin signaling are compromised by microgravity. The following specific aims are proposed:
1) Identify the LINC elements and/or nucleoskeletal binding partners that are required for mechanically induced trafficking of βcatenin to the nucleus; and
2) Test whether microgravity induces alterations in the nucleoskeleton and components of LINC, resulting in diminished Wnt-βcatenin signaling in MSCs.
The proposed studies provide a first look into how the cell nucleus and its connectivity to cytoskeleton as a mechanosensitive organelle altered under microgravity and offer a potential to discover novel targets that can be targeted by both biochemical and mechanical means to mitigate microgravity or disuse induced MSC failure (↓osteogenesis, (↑adipogenesis). The studies proposed here will address fundamental gaps in understanding how microgravity/unloading leads to dysfunction of tissues that arise from mesenchymal origin (bone, muscle, heart). Perhaps indicating the importance of nuclear structure and connectivity, in some progeroid early aging syndromes as well as in healthy aging, dysfunction of the nucleoskeleton and the LINC complex lead to failure of MSC-originated tissues, causing osteoporosis, increased marrow adiposity and musculodystrophy. Data obtained will help to better define and discover new countermeasure targets for mitigating early onset osteoporosis and other catabolic musculoskeletal changes seen in microgravity/unloading as well as aging.