M.Sc Thesis

M.Sc StudentMoghrabi Rihan
SubjectThe Role of the Stress-Induced Protein, CHOP, in Muscle
DepartmentDepartment of Medicine
Supervisors ASSOCIATE PROF. Peleg Hasson
Full Thesis textFull thesis text - English Version


Skeletal muscles have robust capacity for regeneration following injury. The quiescent muscle precursor cells in adult muscle are called satellite cells. In response to muscle injury, satellite cells are rapidly activated, enter the cell cycle with subsequent proliferation and generating myoblasts which fuse to form new skeletal muscle fibers. During their activation and differentiation, satellite cells have increased expression of the myogenic regulatory factors (MRFs) including Myf-5, MyoD, Mrf4 and Myogenin. ER stress and inflammation are two of the important processes occurring following muscle injury. When a muscle is injured, the inflammatory cells invade the injury site. The most abundant inflammatory cells to infiltrate are the neutrophils and monocytes. Subsequently monocytes get transformed into macrophages. Macrophages have two functions: removing the necrotic myofibers by phagocytosis and producing chemotactic signals that activate the satellite cells.
Recent studies in our laboratory indicated that stress-response pathway repressing MyoD transcription is transiently activated in mouse-derived myoblasts growing under differentiation-promoting conditions and the expression of C/EBP homology protein (CHOP) was induced in myoblasts which failed to differentiate. CHOP is a transcription factor, which is associated with unfolded protein response. Sh-RNA-driven knockdown of CHOP expression caused earlier and more robust differentiation, whereas its constitutive expression delayed differentiation compared to wild type myoblasts.

The aim of the present study is to analyze the possible role of the CHOP protein in the inflammatory process occurring in muscle regeneration in vivo. Mouse model of leg muscle injury was employed to compare the regeneration process in wild type and CHOP knockout mice.

The results of this study indicate that CHOP affects the intensity and the profile of the inflammatory response in the injured muscle. Following injury, less M1 macrophages are recruited to the injured muscle of CHOP KO compared to WT mice. At the same time, there are more M2 macrophages in the injured muscle of CHOP KO compared to WT mice. In vitro, more proliferation and differentiation were observed in satellite cells isolated from CHOP KO mice compared to control satellite cells under stress condition. This finding indicates that satellite cells lacking the CHOP protein miss a checkpoint that halt their proliferation or differentiation when are exposed to conditions of stress. It can be concluded from this study that CHOP affects the inflammatory response of macrophages in the injured muscle and that in satellite cells CHOP is necessary for the response of these cells to environmental stress.