ABSTRACT
Skeletal muscle is a highly ordered, yet complex tissue which contains several cell types that interact with each
other to maintain structure and homeostasis. Muscle satellite cells (SCs) are a stem cell population responsible
for muscle growth, repair and regeneration. Maintenance of the balance between SC differentiation and self-
renewal is required for muscle homeostasis. A defect in self-renewal ability leads to a decrease in SC number,
resulting in depletion of the SC pool and reduced muscle regeneration capacity, which occurs in aged and
diseased muscle, including Duchenne muscular dystrophy (DMD). DMD is a progressive disorder in which the
absence of the dystrophin protein results in loss of the dystrophin bridge at the muscle membrane. Recent work
demonstrates the angiogenic impairment of the ECs in DMD model mdx mice. We demonstrates the increased
vascular density can ameliorate phenotypes associated with DMD. However, the importance of angiogenesis in
DMD treatment has not yet been well addressed. A recent study demonstrated that SCs are preferentially located
next to capillary endothelial cells (ECs). However, the exact relationship between SCs and ECs has yet to be
examined. Recently, we established an optimized tissue clearing protocol for skeletal muscle. We utilized
fluorescent reporters for SCs and ECs along with tissue clearing to demonstrate the close proximity of SCs to
capillaries in 3-dimentional imaging, suggesting the juxtavascular niche of SCs for stem cell maintenance. SCs
express vascular endothelial cell growth factor (VEGF) and Notch receptors which is dynamically altered during
the regeneration process. Therefore, we hypothesize that SCs recruit capillary ECs via VEGF to establish a
juxtavascular niche for SC maintenance during homeostasis and regeneration. In this proposal, we will determine
the extent to which an increased vascular niche in postnatal conditional knockout of Flt1 and by blocking Flt1 via
anti-Flt1 nanobodies, created by a combination of llama-immunization and phage display technologies, in mdx
mice can increase SC number, reduced pathological endothelial-to-mesenchymal transition (EndMT), and
ameliorate phenotypes associated with DMD. Consequently, we will elucidate key molecules that regulate angio-
myogenesis, including those involved in SC homeostasis, which is fundamental to developing therapeutic
approaches to treat DMD. SCs and their juxtavascular niche are potential therapeutic targets to induce and
maintain SC populations that slow the loss of skeletal muscle function with DMD.