Exercise builds skeletal strength by inducing new bone formation, which is accompanied by decreased marrow adipose tissue (MAT). A lack of exercise enhances MSC adipogenesis, and increased MAT has been shown to be associated with bone fragility. Understanding mechanisms by which exercise regulates mdMSC lineage allocation to increase osteogenesis and decrease adipogenesis is thus significant for managing skeletal health in an increasingly sedentary population. We have shown that mechanical activation and nuclear trafficking of catenin prevents adipogenesis and promotes MSC entry into osteoprogenitor lineage. catenin signaling is initiated by recruitment of Fyn kinase to foca adhesions during strain, followed by Fyn activation of the mTORC2->Akt->GSK3 inhibition cascade leading to increased catenin. catenin effects depend on cellular context: in mdMSC, increased cytoskeletal structure improves osteogenesis and cytoskeletal deficiency promotes adipogenesis. The Fyn/mTORC2 proximal signal also activates RhoA to build complexity and connectivity of the cytoskeleton. We propose that mechanically induced cytoskeletal reorganization promotes nuclear entry of catenin, underwriting its anti-adipogenic, pro-osteogenic effect. Importantly, the role of focal adhesion-based Fyn in initiating catenin and RhoA effectors may be unique to the mdMSC, potentially providing a specific pharmacologic target to enhance MSC response to exercise. To test this, we will delineate the synergistic contributions of Fyn/mTORC2's two downstream effectors, RhoA and catenin, to MSC lineage fate allocation in vitro and in vivo. We will find if RhoA generated cytoskeletal reorganization is necessary or sufficient to alter mdMSC cell fate and define how mechanical load activates RhoA (SA1). In SA2, we ask if activation of catenin can alter cell fate in the face of cytoskeletal deficiency, inquiring whether the cytoskeleton drives nuclear/cytoplasmic partitioning of catenin and considering if the actin-nesprin-LINC-nuclear tether might guide catenin into the nucleus. In SA3, using an innovative method to quantify and localize MAT, combined with measures of bone formation, we will investigate mechanical control of mdMSC lineage in vivo, asking if the Fyn/mTORC2 signaling unique to mdMSC is critical during exercise stimulation of bone formation; for this we will generate a Tamoxifen driven Prx1-Cre knock out of Fyn.
Mesenchymal stem cells supply progenitors for cells that form bone as well as the fat cells in the bone marrow that are associated with a negative impact on bone health. Stem cell lineage decisions are strongly regulated by exercise and lack of exercise. Knowing how to use physiologic exercise to improve bone strength should be a goal of physician scientists, especially as the US population becomes more sedentary and obese. This project is built on our data showing that a critical mechanical signaling pathway prevents stem cells from becoming adipocytes, thus preserving stem cells for bone lineage. This signal pathway results in two effects: first catenin is generated, and secondly, the cell's cytoskeleton is reorganized providing increased structure. We propose that catenin, known to regulate MSC cell fate, requires a cytoskeletal context to effectively prevent adipogenesis and promote bone formation. The work proposed in this grant will elucidate synergism between catenin and cytoskeleton, and will use a running mouse model to show that these pathways are important for exercise to promote skeletal health.
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