Protein integrators of gene regulatory networks controlling immunity and aging in C. elegans
- Author(s): Block, Dena Hana Sachiko
- Advisor(s): Shapira, Michael
- et al.
Immunity and aging are complex processes under the control of different genetic pathways. These pathways must be coordinated in order to respond to various inputs, but how this is achieved is not fully understood. In this dissertation, I demonstrate two examples of proteins that integrate different genetic pathways into appropriate gene expression responses in Caenorhabditis elegans.
In Chapter 1, I examined the relationship between a conserved innate immune signaling pathway and a GATA transcription factor. GATA transcription factors play roles in development and tissue differentiation, but less is known about their roles in mature tissues. In C. elegans larvae, the GATA transcription factor ELT-2 regulates terminal differentiation of the intestine, but it is also expressed in the adult intestine, where it was suggested to maintain intestinal structure and function, and where it was also shown to contribute to infection resistance. To study the function of elt-2 in adults, we characterized elt-2-dependent gene expression following its knock-down specifically in adults. Microarray analysis identified two ELT-2-regulated gene subsets: one, enriched for hydrolytic enzymes, pointed at regulation of constitutive digestive functions as a dominant role of adult elt-2; the second was enriched for immune genes that are induced in response to Pseudomonas aeruginosa infection. Focusing on the latter, we used genetic analyses coupled to survival assays and quantitative RT-PCR to interrogate the mechanism(s) through which elt-2 contributes to immunity. We show that elt-2 controls p38-dependent gene induction, cooperating with two p38-activated transcription factors, ATF-7 and SKN-1. This demonstrates a mechanism through which the constitutively nuclear elt-2 can impact induced responses, and play a dominant role in C. elegans immunity.
In Chapters 2 and 3, I describe work characterizing the role of a transthyretin-like gene in influencing lifespan. The transthyretin-like (ttr) gene family of C. elegans consists of 59 genes that are related to vertebrate transthyretin, a protein that carries lipophilic molecules such as retinol and thyroid hormone through the blood and cerebrospinal fluid. Although levels of various ttrs increase during stress stimuli and aging, the functions of ttrs remain largely uncharacterized. We found that RNAi against one ttr, ttr-1, increased levels of the FOXO transcription factor DAF-16 and extended lifespan. In agreement, ttr-1 overexpression reduced lifespan. A ttr-1::gfp fusion protein was localized to the periphery of the spermatheca, the vulva, and the intestine. Analysis of genetic interactions between ttr-1 mutants and known regulators of lifespan revealed that ttr-1 disruption compromised lifespan extension following daf-2 knock-down (although DAF-16 nuclear localization was similar to that in wildtype animals), and abolished lifespan extension in eat-2 mutants, which exhibit slow pharyngeal pumping and represent a genetic model for dietary restriction. These results, in conjunction with its expression in the intestine and somatic gonad suggest that ttr-1 may play a role in orchestrating organismal responses to nutrient availability.