Biomechanics in the heart and bone [electronic resource] : the role of electromechanical forces in deriving cardiac myocytes from human embryonic stem cells and the role of focal adhesion kinase in osteoblast mechanotransduction
Keywords: Jacobs, C. R. (Christopher R.)
Mechanical Engineering

This dissertation investigated the role of biomechanics in two physiological systems, the heart and bone. Biomechanics motivates the study and characterization of how cells sense external forces and convert these signals into an intracellular response in a process called mechanotransduction. Three independent studies were designed with the goal of applying mechanical forces that mimic the in vivo microenvironment of either the heart or bone. The aim of these studies was to better under the mechanisms driving cellular processes, including cardiac myocyte differentiation and osteoblast mechanotransduction. The first study presents the design and implementation of tissue engineering approach to stem cell-based myocardial therapy. Three dimensional engineered heart tissue was formed by suspending human embryonic stem cell-derived cardiac myocytes isolated from beating embryoid bodies in a soluble extracellular matrix, and an in vitro mechanical conditioning regimen was applied at physiological levels of myocardial strain. The viability of the engineered stem cell tissue was monitored in vitro and in vivo for up to 8 weeks using molecular imaging of reporter gene activity. The application of cyclic mechanical strain in vitro resulted in cellular alignment along the axis of strain and an elongated cellular morphology with a high nuclear to cytoplasmic ratio, typical of neonatal cardiac myocytes, as well as increased expression of cardiac troponin I, in comparison to static controls. Analysis of the in vitro and in vivo bioluminescence imaging data demonstrated the viability of engineered heart tissue constructs; however, histology results showed immature cells within the implanted constructs, suggesting an inability of the stem cell-derived cardiac precursors to maintain a cardiac phenotype in vivo, as well the inherent inefficiency of the beating embryoid body method to identify and isolate cardiac myocyte precursors. The functional shortcomings exhibited by the embryoid body-based differentiation of embryonic stem cell-derived cardiac myocytes in the first study motivated further refinement of cardiac myocyte differentiation techniques. Therefore, the second study executed the design and fabrication of a microelectromechanical platform to study the role of electrical and mechanical stimulation in cardiac myocyte differentiation. The fabrication process used a combination of soft lithography and traditional microfabrication techniques to pattern thin film metal electrodes on an elastomeric polymer membrane. The completed device enabled coupled characterization and imaging of cardiac myocytes precursors, and the ability to assess the range of mechanical forces, up to 10% equibiaxial strain, that may induce or maintain a cardiac fate. Electrical continuity was demonstrated under static conditions but not under strain, and improvements in metal deposition and adhesion could address this performance defect. Beating clusters containing human embryonic stem cell-derived cardiac myocytes were plated on fabricated membranes, uncoated and coated with Matrigel, and cell viability was monitored using contrast microscopy. The third study transitioned to a different mechanical model of physiological forces, which was the application of oscillatory fluid flow-mediated fluid shear stress generated by the loading and unloading of bone. Specifically, the role of focal adhesion kinase, a protein tyrosine kinase recruited at focal adhesions and a major mediator of integrin signaling pathways, was studied in osteoblast mechanotransduction. The biochemical and transcriptional response of focal adhesion kinase mutant osteoblasts to physiological levels of shear stress induced by oscillatory fluid flow was impaired as measured by prostaglandin E2 release and cyclooxygenase-2 gene expression. Restoration of focal adhesion kinase expression with site-specific mutations at two tyrosine phosphorylation sites demonstrated that phosphorylation events play a role in prostaglandin release following oscillatory fluid flow. In conclusion, the role of mechanical forces, including the effect of cyclic mechanical strain in human embryonic stem cell-derived cardiac myocyte tissue engineering and the fluid shear stress-induced response of focal adhesion kinase mutant osteoblasts, was successfully demonstrated and quantified in this dissertation.