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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Medical Biophysics

Supervisor

Pickering, J. Geoffrey

Abstract

Understanding the cellular processes involved in skeletal muscle regeneration after damage is critical to advancing translational efforts toward the development of targeted therapeutics. The purpose of this thesis was to ascertain the dynamic interplay between regenerating muscle and regenerating blood vessels after ischemic injury.

First, using a novel systematic investigation of a widely used preclinical mouse model of limb ischemia, I generated a detailed atlas of muscle injury zones and angiogenesis zones in C57BL/6 mice subjected to femoral artery excision. This uncovered previously unrecognized regional variability and an exclusive relationship between angiogenesis and actively regenerating muscle zones. Then, an unbiased, systematic review of 509 manuscripts, criteria-selected from 5147 non-duplicate publications revealed that only 15% of all published studies in the field evaluated data using methods concordant with my findings. These data provide a quality assurance approach that could strengthen the value of this preclinical animal model.

Next, I tested if microvascular mural cells were a source of myogenic progenitor cells upon skeletal muscle ischemia. Using fluorescent lineage tracing techniques, I discovered that a small portion of cells marked by Myosin heavy chain-11 (Myh11) expression possessed the ability to generate new myofibers. The Myh11 lineage cell-derived myofibers were structurally indistinguishable from satellite cell-derived myofibers. Endogenous regulation of this reprogramming process was impaired by Sirtuin 6 (Sirt6) knockout, which provides considerations for future regenerative therapies.

Finally, I further examined if Sirt6 in mural cells influenced the overall regenerative response to hindlimb ischemia. Loss of Sirt6 in mural cells resulted in a necro-fibrotic phenotype that was associated with interstitial and perivascular scarring upon ischemic injury as well as medial fibrosis in larger arteries. I also discovered that the blunted muscle recovery was associated with abnormal regeneration of the microvascular network. Arteriole wrapping by smooth muscle cells was significantly reduced. The capillary density was also reduced along with loss of junctional pericytes and capillary bridges. As well, there was upregulation of p16INK4a, a marker of cell senescence. Together, these findings reveal a role for Sirt6 in mural cell homeostasis during muscle regeneration.

In summary, this thesis provides new insights into the relationships between the microvasculature and functional skeletal muscle regeneration after a severe ischemic insult.

Summary for Lay Audience

Peripheral artery disease (PAD) is a vascular condition that can lead to complications like gangrene and limb amputation. Therefore, understanding this disease process is critical to the development of new treatments. The purpose of this thesis was to examine the relationship between regenerating muscles and regenerating blood vessels after vascular injury.

First, I generated a detailed atlas of muscle injury zones and zones of newly formed blood vessels in mice subjected to blood vessel injuries. This revealed a previously unrecognized geographical variability and an exclusive relationship between newly formed blood vessels and actively regenerating muscle zones. Then, I reviewed all published studies using this mouse model of disease. I discovered that only 15% of all studies in the field evaluated data using methods consistent with my findings. Therefore, my data provide a new quality assurance approach that could strengthen the value of future studies.

Next, I studied a special population of cells that attach to blood vessels, called mural cells. Here, I discovered that these blood vessel mural cells possess the ability to transform into new muscle cells, similar to a stem cell. This provides exciting new data in support of treatments that take advantage of the cells that are already in our bodies. Interestingly, a protein called Sirtuin 6 (SIRT6) that is naturally found in our bodies and decreases with age, was found to be critical to this stem-cell like potential.

Finally, I further studied the role of Sirt6 in blood vessel mural cells during muscle injury. Interestingly, the loss of Sirt6 in mural cells caused a severe injury to the muscles of affected mice. The mice without Sirt6 also suffered from more scarring after injury and the blood vessels that regenerated were abnormal. Ultimately, the blood vessel mural cells were discovered to have aged significantly beyond what was expected. This means we should seek out strategies to promote the activity of these molecules.

In summary, this thesis provides new insights into PAD and uncovers critical answers to advance regenerative medicine for this debilitating problem.

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

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