Characterizing the role of mechanotransduction on aberrant wound healing
Heterotopic ossification (HO) is the formation of endochondral bone in connective tissue of skeletal muscles, tendons and ligaments. This ultimately results in fixed joints, limited movement, and seriously affects patient quality of life in 20% of trauma, hip replacement and amputation patients. Similarly, HO afflicts patients with fibrodysplasia ossificans progressiva (FOP), an autosomal dominant disorder where mutations in a type I receptor for BMPs named Activin receptor type I/ Activin-like kinase 2 (ACVR1/ALK2) result in aberrant activation of BMP signaling by binding of Activin A. Since HO can be regarded as aberrant stem cell differentiation following inflammation, knowledge of how the stem cells interact with the surrounding matrix would provide important information about the pathogenesis of HO. In addition, it has been reported that stem cells interact with surrounding inflammatory cells such as macrophages to alter functions of those cells (immunomodulatory function). The interaction of mesenchymal stromal cells (MSCs) and immune cells with the extracellular matrix (ECM) occurs at focal adhesions, resulting in signaling through focal adhesion kinase (FAK) in which cells?. Adhesion to ECM triggers spreading and shape changes that are required for β1-integrin activation, a critical step in stem cell differentiation.
Cell shape and stretch along ECM is impacted by elastic modulus (stiffness) and collagen fiber alignment. Both rigidity and fiber alignment of the ECM have been shown to alter differentiation and immune modulatory properties of stem cells. In adipose derived stem cells (ASCs), aligned fibers result in secretion of factors that, when cultured with M0 macrophages (Mφ), polarized these cells towards an anti- inflammatory M2 phenotype. This was correlated with the activation of FAK and YAP/TAZ signaling in ASCs. However in FOP, the dynamics of MSCs, immune cells, and ECM interactions during disease pathogenesis are poorly understood. Currently, no treatments exist to prevent or slow the spread of these destructive osseous lesions, and understanding the importance of MSC/immune cell/ECM interactions would provide insights into novel therapeutic targets for FOP by suppressing pathogenic immunomodulatory factors produced by MSCs.
Our long-term goals are to develop a therapy that prevents new HO lesions, limits the spread of existing lesions and allows surgeons to safely excise contracture-inducing lesions without HO recurrence. The overall objective for this application is to demonstrate and target the impact of ECM stiffness/composition on MSC immunomodulatory function, macrophage phenotype, and HO pathogenesis.
Our central hypothesis is that ECM rigidity, through the FAK and YAP/TAZ pathways, changes the immunomodulatory functions of MSCs responsible for HO. We have developed this hypothesis based on our 10X single cell sequencing in a mouse model of traumatic HO, where we find a number of genes in the FAK and YAP/TAZ pathways upregulated in MSC subsets. These signaling pathways have been shown important in mechanotransduction, the sensing and translating of mechanical force and elastic modulus into biochemical signals. The rationale to the proposed research is that once we understand the complex interactions of the ECM, MSCs, and immune cells in HO lesions, we will be able to develop effective therapies by modulating either the response to the ECM environment or the immunomodulatory factors produced by MSCs.
This proposal brings together scientists with diverse expertise from across campus. Dr. Stephen Weiss is an expert in stem cell biology, extracellular matrix and FAK/YAP/TAZ signaling. Dr. Mishina is an expert in bone biology and heterotopic ossification in FOP animal models. Dr. Levi is a surgeon scientist with expertise in heterotopic ossification and inflammation. Together, these investigators will be able to make important gains to help patients suffering from these debilitating processes.