Title: A genetically-encoded biosensor and a conditional gene expression system for investigating Notch activity in vivo
Description: Intercellular communication is crucial during animal development and tissue maintenance to ensure that correct patterns of cell types are generated to meet the needs of the organism. During lateral specification, intercellular communication resolves cell fate decisions between equipotent cells, creating fate patterns that are biased by external factors in some contexts, but appear stochastic in others. The Notch signaling pathway mediates lateral specification; small differences in Notch activity are amplified by regulatory feedback loops to robustly differentiate cell fates based on relative levels of Notch activity. It is often unclear how noise in the environment is processed by cells to generate differences in Notch activity that can be translated into stochastic, but robust, cell fate outcomes. The nematode Caenorhabditis elegans contains a simple, Notch-mediated, stochastic lateral specification event; a small, random difference in Notch activity between two cells, the α cells, is amplified so that one α cell assumes Anchor Cell (AC) fate and the other assumes Ventral Uterine precursor cell (VU) fate.

Two upstream factors bias the outcome of the AC/VU decision depending on the length of the time interval between the births of the α cells: the relative birth order of the α cells and the onset of expression of the transcription factor HLH-2. It is unknown how these factors create a difference in the relative Notch activity level between the two α cells, and limitations of existing Notch reporters have prevented the direct observation of Notch activity levels required for determining the relationships. In this thesis, I describe a genetically-encoded Sensor Able to detect Lateral Signaling Activity, or SALSA, which uses changes in nuclear Red:Green fluorescence to indicate Notch activity. I demonstrated that SALSA captures expected Notch activity patterns in four paradigms in C. elegans, encompassing both Notch homologs, and reports low levels of Notch activity that were predicted but undetectable with other Notch activity reporters. Using SALSA, I showed that the first-born α cell is able to develop an advantage in Notch activity prior to the birth of the other α cell when the time interval between α cell births is long, but the α cell that gains the Notch activity advantage is random with respect to birth order when the time interval between α cell births is short.

These results agree with the current model of the AC/VU decision. I also describe Flexon, a method for the conditional activation of strong gene expression in specific cell lineages using a lox-stop-lox cassette encoded into an artificial exon flanked by two artificial introns. A flexon can be placed into the coding region of a gene to prevent translation of a functional gene product; gene expression is restored to specific lineages through expression of a tissue-specific Cre driver that excises the flexon. I show that flexon can be used to make bright, long-lasting, tissue-specific fluorescent lineage markers. I also showed that the flexon could be used for conditional activation of an endogenous gene by inserting a flexon into rde-1 to severely reduce RNAi activity and restore gene function in specific tissues using Cre drivers.