Skeletal muscle makes up almost half of the human lean body mass and approximately 40% of all traumatic injuries involve skeletal muscle damage. This results in a global economic burden of roughly $6 billion. While skeletal muscle possesses an intrinsic self-regeneration capacity, in clinical scenarios of volumetric muscle loss (VML) where the muscle's natural repair mechanisms are overwhelmed, regeneration fails. Tissue engineering strategies using human skeletal muscle stem or progenitor cells combined with novel biomaterials have unprecedented potential to provide effective therapies. In this study, we propose to harness the myogenic potential and regenerative capacity of sorted skeletal muscle stem/progenitor reporter cells (PAX7::GFP+) derived from human pluripotent stem cells (hPSCs). Specifically, we hypothesize that PAX7::GFP+ myogenic progenitors grown on electrospun fibrin microfiber bundles will proliferate, upregulate their expression of myogenic genes and form aligned, multi-nucleated myotubes assembled into 3D muscle grafts. These engineered grafts will be used to regenerate skeletal muscle tissue and restore normal function following VML. We further hypothesize that the use of agrin in combination with insulin-like growth factor-1 (IGF-1) will promote the formation of more densely packed PAX7::GFP+ derived myotubes in the engineered muscle grafts and enable the formation of mature neuromuscular junctions (NMJs) in the regenerating skeletal muscle. We will test these hypotheses in three Specific Aims. In Sp. Aim 1, we will engineer uniform, densely seeded skeletal muscle grafts by (i) electrospinning PAX7::GFP+ cell aggregates into the fibrin microfiber bundles and (ii) coating the microfiber bundles with PAX7::GFP+ cell-seeded bulk fibrin. We will stimulate their maturation into contractile 3D skeletal muscle tissues using biophysical stimulation. We will quantitatively evaluate cell morphology, proliferation, multi-nucleation, and myogenic differentiation and utilize single-cell RNA-sequencing to compare the cellular heterogeneity and myogenic gene expression profiles with that of native muscle cells. In Sp. Aim 2, we will evaluate the potential of soluble and tethered agrin/IGF-1 individually and in combination to enhance the proliferation and myogenesis of PAX7::GFP+ cells. We will also characterize the effects of tethering these molecules on the physicochemical and pro-myogenic properties of the modified scaffolds. In Sp. Aim 3, we will implant PAX7::GFP+ derived muscle grafts engineered with and without soluble or tethered agrin/IGF-1 into small incisions into the tibialis anterior (TA) muscle of immunodeficient mice to assess cell survival, integration, and regenerative potential. We will use these data to optimize the engineered skeletal muscle grafts that we will implant into VML defects to quantitatively assess muscle regeneration, vascular and neural infiltration, the formation of mature neuromuscular junctions, and funct...