Date of Award
1-1-2024
Document Type
Dissertation
Degree Name
Ph.D. in Biological Science
First Advisor
Joshua Bloomekatz
Second Advisor
Gregg W. Roman
Third Advisor
Bradley W. Jones
School
University of Mississippi
Relational Format
dissertation/thesis
Abstract
The formation of the heart tube in vertebrates involves the coordinated movement of bilateral myocardial precursors to the midline, where they merge in a process known as cardiac fusion to form the primitive heart tube. While external influences from the anterior endoderm and the endocardium are known to be essential for cardiac fusion, intrinsic myocardial mechanisms are less understood. Here, we identify an intrinsic PI3K-dependent mechanism directing myocardial cells toward the midline during cardiac fusion. We found that inhibiting the PI3K pathway throughout the embryo using chemical inhibitors and injection of dnPI3K mRNA results in cardiac fusion defects. PI3K-inhibited embryos display normal endoderm morphology and endocardial migration; however, they show a defective endocardial sheet. Previous studies show that embryos defective in the endocardium still form a cardiac ring, suggesting that the defective endocardium is unlikely to be the primary cause of myocardial movement defects in PI3K-inhibited embryos. Therefore, we explored if PI3K is required in the myocardium for proper cardiac fusion. Inhibiting the PI3K pathway specifically in the myocardium leads to cardiac fusion defects. Time-lapse studies show that PI3K-inhibited myocardial cells are misdirected and slower in their movement. Mosaic labeling reveals that wild-type myocardial cells extend membrane protrusions toward the midline, but these protrusions are unpolarized in PI3K-inhibited embryos. Additionally, PI3K activity depends on and genetically interacts with platelet-derived growth factor receptor alpha (Pdgfra). Our findings support a model where Pdgfra-PI3K signaling facilitates directed myocardial movement toward the midline. Cardiac fusion defects observed at 20 hours post fertilization (hpf) in Pdgfra mutants and PI3K-inhibited embryos can recover, forming a single heart tube by 48hpf. To understand this recovery mechanism, we investigated cardiac differentiation and morphogenetic events in PI3K-inhibited embryos post 20 hpf. Cardiomyocyte differentiation proceeded normally in PI3K-inhibited embryos. However, cardiac development deviates from the normal sequential process seen in wild-type embryos. Anterior fusion often occurs before posterior fusion and the heart extends even before forming a complete lumen. The myocardium continues with cardiac development despite initial fusion defects, indicating the resilience of the developing heart.
Recommended Citation
Shrestha, Rabina, "PI3K signaling and its role in cardiac morphogenesis" (2024). Electronic Theses and Dissertations. 2979.
https://egrove.olemiss.edu/etd/2979
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