title: Bone Morphogenetic Protein Signaling Requirement in Centripetal Migration in Drosophila melanogaster Oogenesis
Creators: name: Mosallaei, Sheila
Publisher: name: University of Nevada, Las Vegas
Year: 2021

description: Cells interact with each other via signals to coordinate migration for proper tissue development and maintenance, immunity and tissue repair. Signals regulate migration via changes to cytoskeletal components or transcriptional regulation. Here, I used Drosophila melanogaster to investigate whether and how a transcriptional signal, Bone Morphogenetic Protein signaling, drives a cell migration, centripetal migration. The migrating cells, centripetal follicle cells, consist of two subsets of cells, “leading” and “following” centripetal follicle cells, that although adjacent to each other, migrate differently. The differences in modes of migration correlate to differential gene expression in the centripetal follicle cells. The leading centripetal follicle cells show active responses to BMP ligands. My research investigated whether the BMP signal regulates centripetal migration of the leading centripetal follicle cells. I removed the ability of the centripetal cells to respond to BMP signals by deletion of each of two BMP signaling components, either: the signal-activated transcription factor, Mad, or the BMP receptor type I receptor, Thickveins (Tkv). To eliminate Mad protein, I generated an allele that deletes the protein coding region of mad using CRISPR-Cas9 gene editing. I used the mad- knockout allele to determine whether BMP signaling is required for normal centripetal migration. Live tissue imaging of developing egg follicles showed two discernable defects in the absence of Mad or Tkv: 1) shifting to the apical region, and 2) failure of the cells to thin their basal region. These phenotypes led me to investigate whether BMP signaling could be regulating changes at the basal surface of the centripetal cells during migration. To evaluate whether thinning of the basal region indicated a shift in nuclear location, I quantified the basal surface area. To determine whether adhesion to the basement membrane might change, I quantified ßPS-integrin intensity. I quantified these in three types of follicle cells: leading centripetal follicle cells, following centripetal follicle cells, and mainbody follicle cells before and during apical elongation of the leading cells. These data showed that before apical elongation, the basal surface area of the leading and following centripetal follicle cells, and mainbody follicle cells are similar. During apical elongation, the basal surface area of the leading centripetal follicle cells is smaller than that of either the following centripetal and mainbody follicle cells. Similarly, before apical elongation, ßPS-integrin levels in the leading and centripetal follicle cells did not differ from that of the mainbody follicle cells. During apical elongation, the ßPS-integrin levels in the leading centripetal follicle cells were decreased compared to that of the following centripetal and mainbody follicle cells. Together, these data suggest a model in which ßPS-integrin levels decrease gradually with the decrease in basal surface area. With this model for normal follicle cells, I determined whether BMP signaling is required for reduction of the basal surface area and of ßPS-integrin levels in the leading centripetal follicle cells as apical elongation proceeds. I measured basal surface area and ßPS intensity in leading centripetal follicle cells with reduced levels of Mad or Tkv during apical elongation. These data showed that knockdown of BMP signaling in leading centripetal follicle cells resulted in failure to reduce basal surface area. However, the ßPS integrin levels were unaffected. Altogether, my work examined how the ingressing cells change their basal surface area and attachments to the basement membrane in early migration, and whether BMP signaling regulates these changes. I found that BMP signaling regulates ingression of the leading centripetal follicle cells, and that it is involved in the apical shift of the cell’s nucleus, and reduction of basal surface area. Supplemental Videos 1-7 pertaining to Chapter 3 are included as separate files.