Access Type

Open Access Dissertation

Date of Award

1-1-2021

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Biological Sciences

First Advisor

Joy Alcedo

Abstract

ABSTRACT

MODULATION OF PHARYNGEAL HEALTH IN BACTERIAL DIET-DEPENDENT SURVIVAL

by

DENIZ SIFOGLU

August 2021

Advisor: Dr. Joy Alcedo Major: Biological Sciences Degree: Doctor of Philosophy Both diet and bacterial microbiome modulate insulin signaling, which regulates key physiological processes that are important for survival. However, the mechanisms through which diet and the microbiome modulate insulin signaling remain unclear. To understand these mechanisms, I turned to the nematode worm C. elegans, whose diet consists of different types of bacteria. Like humans and other animals, C. elegans has to modulate its responses to its diet and to bacteria to optimize its survival. Because C. elegans is highly tractable to genetics and exhibits a large degree of conservation with other animals, including humans, C. elegans is an excellent model organism in which to explore both dietary influences on physiology and bacteria-host relationships. To study how bacterial diet affects C. elegans physiology, I measured the survival of C. elegans fed two E. coli strains—the B type OP50 and the K-12 type CS180. Wild-type C. elegans fed OP50 has a higher rate of early deaths compared to C. elegans fed CS180. These early deaths on OP50 depend on swollen pharynges (P-deaths), and worms fed CS180 are largely resistant to P-deaths. Since P-deaths are characterized by bacterial accumulation in the pharynx, this suggests that bacterial cue(s) present in OP50, but not in CS180, promote P-deaths. I have identified that at least one of these cues involves the lipopolysaccharide (LPS) structure of the bacteria. Since I found that a reduction in the animal’s insulin receptor daf-2 activity leads to fewer P-deaths on both bacterial diets, this indicates that wild-type insulin receptor signaling promotes P-deaths in a bacterial type-independent manner. Thus, to define the molecular mechanisms that mediate the bacterial type-dependent effects on C. elegans P-deaths, I focused on the C. elegans strain QZ58, which has been shown to increase early survival on OP50, but not on CS180. QZ58 carries mutations in the neuropeptide neuromedin U receptor nmur-1 and the scaffold protein filamin-2 (fln-2), which suggests nmur-1 and/or fln-2 as candidate modulators of P-deaths in response to bacterial food types. I showed that loss of nmur-1 alone leads to increased P-deaths on OP50, but not on CS180, which implies that wild-type nmur-1 inhibits P-deaths in a bacterial diet-dependent manner. In contrast, loss of fln-2 alone leads to fewer P-deaths on OP50 and, to a lesser extent, on CS180, which indicates that wild-type fln-2 promotes P-deaths in response to certain bacterial food sources. Interestingly, however, nmur-1 facilitates an opposite response when daf-2 has reduced activity, where loss of nmur-1 in a daf-2 reduction-of-function mutant background now leads to a further reduction in P-deaths. The opposing effects of nmur-1, which are both bacterial type-dependent, are also dependent on the insulin pathway effector daf-16, which encodes a FOXO transcription factor. Together these suggest that NMUR-1 ensures that the insulin pathway signals at the appropriate level to promote pharyngeal health and optimal survival in response to specific bacteria. On the other hand, the P-death-related effects of fln-2 do not depend on insulin signaling or any of the major C. elegans immunity pathways, which suggests that multiple pathways regulate the animal’s pharyngeal health. The control of host responses to bacteria and/or its diet is crucial for a healthy lifespan. The fine-tuning of these responses become more crucial over the course of aging, where the host develops increased susceptibility to infection, like in the case of P-deaths. Insulin signaling would be an important pathway to fine-tune, since it plays important roles in metabolism, immunity, and other diverse processes, like development, reproduction, and aging. Insulin pathway over- or underactivity will significantly impact an organism’s survival. Thus, elucidating the modulation of insulin signaling is going to be crucial in understanding how animals react and adapt to their changing environments.

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