Date of Award
Physics and Astronomy
Dinah Loerke, Ph.D.
Convergent extension, Cytokinesis, Germband extension
Oriented cell intercalation is an essential developmental process that shapes tissue morphologies through the directional insertion of cells between their neighbors. Intercalary behaviors in the early Drosophila embryo occur through a remodeling of cell topologies, with cells contracting shared AP interfaces to a single point, followed by newly juxtaposed DV cells constructing horizontally-oriented interfaces between them. Previous research has focused on properties of cell-cell interfaces, and led to a model in which actomyosin networks mediate higher line tensions at AP interfaces to direct contraction. However, the contribution of tricellular vertices to tissue elongation remains unclear. This study shows that cell intercalation uses a novel sliding vertex mechanism that physically couples vertices to radially-oriented forces. Through live imaging and quantitative analysis it was observed that the motion of vertices at contracting interfaces is not coupled, but instead vertices demonstrate strong radial coupling across the area of cells. The vertices of AP junctions show independent sliding behaviors along the cell periphery to produce the topological deformations responsible for intercalation. AP junctions undergo ratcheted length changes that are coordinated with cell area oscillations. These results suggest a model in which oscillations in cell area direct the progressive, ratcheted motion of individual vertices to drive oriented cell intercalation and tissue extension in the Drosophila epithelium.
In a second study, analysis of germband extension in 4D revealed that interface contraction and T2 formation can initiate from any point along on the apical-basal axis, including basolateral regions microns away from the apical caps that host major Myosin II populations. Intriguingly, interface contractions transition smoothly into elongations without systematic T2 waiting times and at similar contraction and elongation speeds, suggesting that a common mechanism may underlie both phases of intercalation. This study also showed that the major component of tissue elongation arises from the growth of new interfaces.
In a third study, the focus was on the role of membrane trafficking during germband extension. The results of this study showed that Rab35 compartments are enriched at contractile interfaces of intercalating cells. When Rab35 function is disrupted, apical area oscillations still occur and contractile steps are observed. However, contractions are followed by reversals and interfaces fail to shorten, demonstrating that Rab35 functions as a ratchet ensuring unidirectional movement. Finally, Rab35 represents a common contractile cell-shaping mechanism, as mesoderm invagination fails in Rab35 compromised embryos and Rab35 localizes to constricting surfaces.
In a fourth and final study, the functional requirements for exocyst complex function during cell division in vivo was investigated, and a common mechanism that directs anaphase cell elongation and cleavage furrow progression during cell division was demonstrated. The results of this study show that onion rings (onr) and funnel cakes (fun) encode the Drosophila homologs of the Exo84 and Sec8 exocyst subunits, respectively. In onr and fun mutant cells, cytokinesis is disrupted early in furrow ingression, leading to cytokinesis failure. Computational analysis was used to quantitatively compare wild-type versus onr and fun mutant cells. The results demonstrate that anaphase cell elongation is grossly disrupted in cells that are compromised in exocyst complex function. Additionally, compared to wild-type, onr and fun mutant cells have a greatly reduced rate of surface area growth specifically during cell division.
Vanderleest, Timothy E., "Quantitative Aspects of Interface Remodeling During Germband Extension" (2019). Electronic Theses and Dissertations. 1551.
Received from ProQuest
Timothy E. Vanderleest