The Role of MreB in E. coli Shape Determination and Whole-Brain Calcium Dynamics in Freely Behaving C. elegans

Abstract

Bacteria have remarkably robust cell shape control mechanisms, but the mechanisms of self-organization for robust morphological maintenance remain unclear in most systems. Precise regulation of rod shape in Escherichia coli cells requires the MreB actin-like cytoskeleton, but the mechanism by which MreB maintains rod shape and sets the cell diameter is unknown. Here, I study both of these mechanisms using a novel method for extracting the 3D shape of cells using fluorescence image stacks and forward convolution. I then use time-lapse and 3D imaging coupled with computational analysis to map the growth, geometry, and cytoskeletal organization of single bacterial. Our results demonstrate that feedback between cell geometry and MreB localization maintains rod-like cell shape by targeting cell wall growth to regions of negative cell wall curvature.

I also study how MreB sets the diameter of a cell. Here, I perturb MreB by treating cells with the drug A22 or by creating mreb point mutants. These perturbations modify the steady state diameter of cells to between 790±30 nm to 1700±20 nm. I correlated structural characteristics of fluorescently-tagged MreB polymers to cell diameter and show that the helical pitch angle of MreB inversely correlates with the cell diameter of E. coli. These results demonstrate that the physical properties of MreB filaments are important for shape control and support a model in which MreB organizes the cell wall growth machinery to produce a chiral cell wall structure and dictates cell diameter.

After investigating the control of bacterial cell shape, I study an interesting problem in neuroscience. The ability to acquire large-scale recordings of neuronal activity in freely behaving animals is needed to provide new insights into how populations of neurons generate behavior. I present a new instrument capable of recording intracellular calcium transients from the majority of neurons in the head of a freely behaving Caenorhabditis elegans with cellular resolution while simultaneously recording the animal's behavior. We observe calcium transients from 89 neurons for nearly four minutes and correlate this activity with the animal's behavior. We show that, across worms, multiple neurons show significant correlations with, backward, and turning locomotion.