DESCRIPTION (provided by applicant): Skeletal muscle as the largest organ in human body is prone to many disorders. Muscular dystrophies, muscles wasting due to cachexia or sarcopenia and muscle mass loss due to injuries are among most common types of muscle disorders. Despite many advances in understanding the pathologic basis of these disorders, therapeutic options in these cases are unfortunately very limited or ineffective. Meanwhile, using stem cell based therapies for skeletal muscle repair has been considered as one of the potential candidates in these cases. For this reason, pluripotent stem cells are the best candidate due to their unparalleled differentiation and self-renewal potentials. With the successful isolation of human embryonic stem (ES) cells and later on, generation of induced pluripotent stem cells (iPS cells) from the somatic cells, an unprecedented opportunity has been discovered for disease modeling and designing patient specific cell therapies. Therefore, differentiation of human pluripotent stem cells (i.e. hES/ iPS cells) toward skeletal muscle lineage has been the focus of attention for developmental studies as well as stem cell based regenerative therapies for muscle disorders. Indeed, in the recent years, few methods have been developed for derivation of skeletal myogenic precursors from hES/iPS cells such as myogenic gene over-expression or mesenchymal differentiation through long-term cultures. However, these approaches cannot be utilized for clinical purposes due to unsafe cell preps using viruses or due to other shortcomings such as low efficiency or impurity of the myogenic cells. These problems mostly arise from the lack of a prospective approach to study chronological differentiation of hES/iPS cells toward skeletal muscle lineage, as most of these studies evaluate myogenic differentiation of the pluripotent stem cells retrospectively. Therefore, in this research application, experiments have been designed to overcome these shortcomings. In the 1st aim of this application knock-in reporter cell lines in human iPS cells for important genes involved in early skeletal muscle development (PAX7, Myf5) is being generated. For this purpose RNA guided Cas9 mediated homologous recombination approach is utilized to incorporate a 2A- GFP or tdTomato reporter. This will provide a unique opportunity to study the temporal pattern of skeletal myogenesis during in vitro differentiation of the human ES/iPS cells. The 2nd aim of this application is to define directed differentiation of hES/iPS cells using chemical library screen. Moreover, purified myogenic precursors will be fully characterized for surface markers and gene expression in order to determine the signature profile of the myogenic precursors derived from hES/iPS cells. This will allow applying this methodology to any other human iPS cell line without need to incorporate a reporter in the genome. The 3rd aim of this application will focus on evaluation of in v...