Date of Award

2023

Document Type

Dissertation

Degree Name

Ph.D.

Organizational Unit

College of Natural Science and Mathematics, Physics and Astronomy

First Advisor

Dinah Loerke

Second Advisor

J. Todd Blankenship

Third Advisor

Michelle Knowles

Fourth Advisor

Cedric Asensio

Fifth Advisor

Mei Yin

Keywords

Drosophila, Epithelium, Germband, Lamin, Microtubule, Nucleus

Abstract

The morphogenesis of developing tissues is contingent on an extensive array of rearrangements in cellular shape, position and identity at large and small scales. One commonly used process to reshape tissues is the cell intercalation-driven elongation of a tissue in a common axis, in which rows of epithelial cells undergo oriented intercalation in a directional fashion. In most models of intercalation, cells are treated as homogeneous objects directed in their shape changes by cortical forces localized along cell-cell interfaces or tricellular junctions. However, less attention has been paid to how inhomogeneities in mechanical resistance of their own internal structures affects cell shaping processes.

In the main study, we investigate how pulsed contractile and extension dynamics respond to the presence of the largest intracellular organelle, the nucleus. Using highly time-resolved data sets on both nuclear and interfacial behaviors, we show that the tight packing of nuclei in common apical layers presents a significant physical impediment to tissue remodeling. We further demonstrate the existence of two primary mechanisms by which cells resolve internuclear tensions – nuclear deformation and nuclear dispersion. We test the contribution of these pathways to cell-cell remodeling using a non-deformable nuclear background in which nuclei adopt a highly spherical conformation. Cells in embryos with non-deformable nuclei up-regulate the nuclear dispersion pathway to enable only mildly reduced levels of cell intercalation. Conversely, we found that we could generate non-dispersible nuclei through microtubule inhibition, suggesting that nuclei in the early epithelium actively disperse through mechanosensitive microtubule-based transport. Nuclei in non-dispersible embryos are locked in a common apical-basal plane and contractile force propagation and intercalary behaviors are deeply disrupted.

Finally, we show that compromising both the nuclear deformation and positioning pathways causes nuclei to engage in a tensile tug-of-war to occupy similar apical regions, regardless of whether sufficient space exists to accommodate them. These embryos demonstrate a near complete disruption of cell intercalation and extension and internuclear tensions eventually lead to extrusion events that force cells from the apical layers of the epithelium with a concomitant reduction in cell number and density. These results reveal the critical function that nuclear shape and positioning pathways play in determining the mechanical environment that guides the remodeling of cell topologies in a columnar epithelium.

In two separate, shorter studies, we further investigate more nuanced representations of intercalation-driven tissue remodeling. In the first, we use Monte Carlo simulations to demonstrate the viability of a model of intercalation driven by radial, intracellular tensions rather than the interfacial forces used in most other models. In the second study, we propose a framework for characterizing epithelia that centers cell topological relationships, and explore its usefulness for measuring phenomena that span the whole tissue length.

Publication Statement

Copyright is held by the author. User is responsible for all copyright compliance.

Rights Holder

Noah de Leeuw

Provenance

Received from ProQuest

File Format

application/pdf

Language

en

File Size

91 pgs

Discipline

Biophysics, Cellular biology, Developmental biology

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