Mouse embryonic stem cells as a discovery tool in skeletal muscle biology

Skeletal muscle and its progenitor cells are formed during the development. In adulthood the progenitor cells remain inactive until a differentiation signal is sensed, such as an exercise or tissue damages. Under certain conditions like aging, chemotherapy and genetic disorder, the capacity of skeletal muscle regeneration declines and progenitors can¡¯t differentiate properly, resulting in immobilizing problems. Embryonic stem (ES) cells have been highlighted as a great source which can provide amplifiable skeletal muscle progenitors. ES cells have the capacity of self-renewal and the potential of differentiation into every cell type in the human body, including skeletal muscle. Although amplifiable skeletal muscle can be differentiated from ES cells by ectopic over-expression of key transcription factors, genetic integration of cDNA into host is a prerequisite. Therefore, efforts have increasingly focused on the identification of small molecules which can induce skeletal muscle differentiation from ES cells in the field. Using small molecules for cellular differentiation provides invaluable advantages compared to other methods such as genetic modifications. Modulation of signaling by small molecules is rather
straight forward and the effect can be controlled with a fine-tuning manner by applying various concentration and time points. In spite of the prominent advantages, there has been no single molecule known to drive ES cells at high efficiency to skeletal muscle thus far.
My thesis work was focused on the identification of small molecules which can drive differentiation of mouse embryonic stem cells toward skeletal muscle. I also investigated the
biological process during embryonic skeletal muscle differentiation that manipulates the development process taking place in vivo. Embryonic stem cells have been a popular tool for studying development processes as well as a great source for cell therapy via manipulations of physiological events. In this study a small molecule was identified from a mouse embryoid body (EB) screening and used as a tool compound for skeletal muscle differentiation. The small molecule was named as SMI (Skeletal Muscle Inducer). Its chemistry is N-[4-(trifluoromethyl)-6-methoxymethyl-2-pyrimidinyl]-N-(2-methyl-6-nitrophenyl)-urea. In the screening Pax3 mRNA induction was one of the major readouts since Pax3 has been known as a key transcription factor for early skeletal muscle development. SMI1 showed a high efficiency for skeletal muscle differentiation without any extra effort such as fluorescent cell sorting step. Even though the effect of the compound driving mouse ES cells from 129 mice to skeletal muscle was very clear, it did not have the same effect in ES cells of other mouse strains, BalbC and Bl6. By discovering the mechanism of action of this small molecule, I expect that it could be applied and transferred to other cell lines as well as human pluripotent stem cells. These insights can pave way to determining the complexity of embryonic skeletal muscle differentiation.
Comprehensive gene expression level analysis with SMI incubation resulted in the discovery of three pathways which are involved and play critical roles in skeletal muscle differentiation by the molecule. Wnt pathway and Nodal pathway were identified from EB day 4, and Shh signaling was found at EB day 4+4. All the three pathways are closely related to the development process of the mouse embryo. The action of SMI1 was reproduced independently using other small molecules which are known to modulate the Wnt, Nodal and Shh pathways in all mouse ES cells from different strains tested. Taken together, these results demonstrate that the differentiation of mouse embryonic stem cells into skeletal muscle by SMI1 occurs through Wnt, Nodal and Shh pathways¡¯ modulation. Therefore, SMI1 can be used as a tool to study the skeletal muscle biology and to establish a cellular skeletal muscle disease model for therapeutic research.