Role of MicroRNAs in Human Skeletal Muscle Tissue Engineering In Vitro
The development of a functional tissue-engineered human skeletal muscle model in vitro would provide an excellent platform on which to study the process of myogenesis, various musculoskeletal disease states, and drugs and therapies for muscle toxicity. We developed a protocol to culture human skeletal muscle bundles in a fibrin hydrogel under static conditions capable of exerting active contractions. Additionally, we demonstrated the use of joint miR-133a and miR-696 inhibition for acceleration of muscle differentiation, elevation of active contractile force amplitudes, and increasing Type II myofiber formation in vitro.
The global hypothesis that motivated this research was that joint inhibition of miR-133a and miR-696 in isolated primary human skeletal myoblasts would lead to accelerated differentiation of tissue-engineered muscle constructs with higher proportion of Type I myofibers and that are capable of significantly increased active contractile forces when subjected to electrical stimulus. The proposed research tested the following specific hypotheses: (1) that HSkM would require different culture conditions than those optimal for C2C12 culture (8% equine serum in differentiation medium on uncoated substrates), as measured by miR expression, (2) that joint inhibition of miR-133a and miR-696 would result in 2D human skeletal muscle cultures with accelerated differentiation and increased Type I muscle fibers compared to control and individual inhibition of each miR, as measured by protein and gene expression, (3) that joint inhibition of miR-133a and miR-696 in this functional 3D human skeletal muscle model would result in active contraction significantly higher than control and individual inhibition by each miR, as measured by isometric force testing, and finally (4) that specific co-culture conditions could support a lamellar co-culture model in 3D of human cord blood-derived endothelial cells (hCB-ECs) and HSkM capable of active contraction, as measured by isometric force testing and immunofluorescence.
Major results of the dissertation are as follows. Culture conditions of 100 μg/mL growth factor reduced-Matrigel-coated substrates and 2% equine serum in differentiation medium were identified to improve human skeletal myoblast culture, compared to conditions optimal for C2C12 cell culture (uncoated substrates and 8% equine serum media). Liposomal transfection of human skeletal myoblasts with anti-miR-133a and anti-miR-696 led to increased protein presence of sarcomeric alpha-actinin and PGC-1alpha when cells were cultured in 2D for 2 weeks. Presence of mitochondria and distribution of fiber type did not change with miR transfection in a 2D culture. Joint inhibition also resulted in increased PPARGC1A gene expression after 2 weeks of 2D culture. For muscle bundles in 3D, results suggest there exists a myoblast seeding density threshold for the production of functional muscle. 5 x 106 myoblasts/mL did not produce active contraction, while 10 x 106 myoblasts/mL and above were successful. Of the seeding densities studied, 15 x 106 myoblasts/mL resulted in constructs that exerted the highest twitch and tetanus forces. Engineering of human skeletal muscle from transfected cells led to significant increases in force amplitude in joint inhibition compared to negative control (transfection with scrambled miR sequence). Joint inhibition in myoblasts seeded into 3D constructs led to decreased presence of slow myosin heavy chain and increased fast myosin heavy chain. Finally, co-culture of functional human skeletal muscle with human cord blood-derived endothelial cells is possible in 3.3% FBS in DMEM culture conditions, with significant increases in force amplitudes at 48 and 96 hours of co-culture.
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