FUNCTIONAL CHARACTERIZATION OF FAM210A PROTEIN IN SKELETAL MUSCLE AND MUSCLE STEM CELLS

Skeletal muscle accounts for 40% of total body weight and the homeostasis of muscle tissue is critical in maintaining proper body function. Skeletal muscle develops during the embryonic stages from the muscle progenitor cells derived from the dermomyotome structure. The myogenic progenitor cells contribute to the primary myogenesis by forming the primary myotubes which are the founding structures that the secondary myogenesis continues to build on. A portion of the myogenic progenitor cells makes up the adult muscle stem cells residing in homeostatic muscle tissue. The adult muscle stem cells contribute substantially for the adult muscle regeneration. Due to the significance of the muscle tissue and the importance of muscle stem cells, dysregulation of the muscle homeostasis or the muscle stem cell homeostasis will result in severe pathological conditions such as myopathy.

Mitochondria are cellular organelles that are responsible for generating energy needed for cellular processes, especially for muscle tissue where muscle contraction requires the presence of ATP. On the other hand, mitochondria also serve as signaling molecules and provide macromolecules for the biosynthesis. FAM210A (Family With Sequence Similarity 210 Member A) protein was shown to impact the lean mass of human subjects yet a detailed study on the effect of FAM210A in skeletal muscle was not performed, nor has the molecular mechanisms through which FAM210A function been elucidated. Therefore, I take on the task to unveil the function of FAM210A in muscle development, muscle homeostasis and muscle stem cell behavior by using a combination of mouse models with different myogenic promoters to target Fam210a at different developmental stages.

In the first part of the thesis, I investigated the role of FAM210A in post differentiation myofibers. Using the Myl1^Cre driven deletion of Fam210a, I found that Fam210a^MKO had normal development before 3 weeks of age, but the growth was stagnant from 4 weeks on, and the mice did not survive past 8 weeks of age. I found that the assembly of the ribosomes in the Fam210a^MKO was defective, leading to impaired translation which attenuated the muscle atrophy phenotype. I identified through proteomics that the mitochondrial autophagy and proteostatic control pathways were significantly induced yet mitochondrial organization and energetic proteins were downregulated. Metabolomics analysis showed that the signaling metabolite acetyl-CoA was increased in the Fam210a^MKO which led to increased protein acetylation, specifically, we showed that the ribosomal proteins were hyperacetylated, and that the acetylation increase was elicited by the Fam210a-null mitochondria.

In the second part of the thesis, I investigated the function of FAM210A in muscle progenitor cells. In the FamMKO mice, I found that deletion of Fam210a from embryonic myogenic progenitor cells led to developmental arrest and postnatal death at day 6. In the FamPKO mice, I found that Fam210a is needed for adult muscle stem cell to contribute to regeneration. Loss of Fam210a leads to the regenerative defects when the muscle was exposed to injury cues. We further showed that Fam210a deletion in muscle stem cells resulted in disruption of the proteostatic control over muscle stem cell activation, thereby forbidding the translational increase necessary to facilitate activation and proliferation. Furthermore, I showed that Fam210a deletion leads to excessive OPA1 cleavage, which contributes to the regenerative failure of muscle stem cells as fusion is required for the mitochondrial network remodeling during regeneration. Therefore, Fam210a safeguards the mitochondrial network and proteostasis during regeneration.

In summary, my studies characterized the functional contribution of FAM210A during embryonic muscle development, muscle mass maintenance and adult muscle stem cell homeostasis. The regulation of FAM210A in these three processes impinge on the translational regulation. My studies further demonstrated the importance of mitochondrial regulated protein translation in skeletal muscle and muscle stem cells.