Noelle L'Etoile, PhD

Associate Professor
Department of Cell & Tissue Biology
+1 415 476-6850
Research Overview: 

Our focus has been on the cell biological processes that promote and maintain sensory signaling and neuronal plasticity throughout development.  Neurons are the front-line of an organism’s response to its environment.  Thus, understanding how their signaling components organize to perceive and transmit information both in response to novel and persistent cues is key to understanding behavior.  We are testing the hypothesis that signaling pathways act via small RNAs to modify chromatin thereby allowing for experience-dependent changes in the output response.  We utilize C. elegans for our investigations because this nematode, with only 1,000 cells and 302 neurons exhibits robust behavioral plasticity and we can use cell biological, genetic, behavioral, physiological and molecular techniques to understand the molecular details that underlie experience driven changes in behavior.  The molecular and cellular logic that underlies behavioral plasticity in C. elegans is likely to be utilized in the nervous system of higher organisms and humans and insight we gain from examining this nematode may inform both normal processes such as learning and memory as well as help us understand what goes awry in disease states such as addiction and perhaps attention deficit disorders. Our particular focus is on the olfactory sensory circuit of C. elegans

How does persistent signaling stably alter the physiology of a cell? We examine this process in the olfactory sensory neurons of C. elegans as they adapt to prolonged odor-exposure. A. The signaling is initiated by odor binding to a seven transmembrane G-protein coupled receptor, this leads to decreases in cGMP and subsequently calcium and after 60-80 minutes nuclear accumulation of the cGMP-dependent protein kinase EGL-4. B.  Images of the olfactory sensory neuron. The arrow head denotes the sensory cilia which is connected to the boxed cell body by a long dendrite.  The axon extends opposite the dendrite.  C-D.Image of the AWC cell body.  The cell expresses soluble mCherry and GFP-tagged EGL-4. C. In the naïve animal, GFP-EGL-4 is dispersed throughout the cytoplasm. D. In the adapted AWC, GFP-EGL-4 is concentrated in the nucleus.

Figures described in the paragraph above.

Adapted from: O’Halloran, Lee and L’Etoile, PLoS Genetics, 2009.

Organismal perspective
Our lab is interested in understanding behavioral plasticity at the molecular, cellular and circuit levels.  Animals thrive despite the unpredictability of nature by adapting innate behaviors to suit their current circumstances, for example, most animals will inhibit their innate attraction to a piquant smell if they experience this odor while starving or if they are sated.  Using the free-living soil nematode, C. elegans, we seek to understand how such innate stimulus seeking behaviors are modulated by starvation on the one hand and food on the other. Paradigms we uncover by studying the plasticity of innate behaviors in C. elegans may provide molecular and cellular insight into more complex problems encountered in the human brain.

An animal's behavior is regulated by its neural circuits, which consist of individual neurons and ultimately the plasticity that emerges is a consequence of changes within the neurons.  Our goal is to understand the molecular, cellular and circuit changes that allow animals and their progeny to adapt their behavior to changes in food abundance and intake.  C. elegans is a particularly attractive organism for this study as it has a small, well defined, completely mapped nervous system which can be imaged easily due to its size and transparency.  Incredibly powerful genetics allows molecular dissection of complex processes.  The ease of genetic manipulations, coupled with a 3-day generation time means that we can conceive of a question, create a transgenic animal, test its behavior and image its nervous system in less than a month.

Currently, we are focused on understanding the molecular, cellular and circuit changes that produce olfactory "memory" formation and how environmental stimuli regulate these changes.  The innate behavior we focus on is odor-taxis: when an innately attractive odor is experienced in the context of starvation, the worm "learns" to ignore that odor.  This behavioral plasticity is termed olfactory adaptation. As diagrammed above, odor perception in the AWC olfactory sensory neuron is likely to be initialed by GPCR signaling (Troemel 1999) and adaptation to prolonged odor exposure in the starved animal requires entry of a cGMP dependent protein kinase in to the AWC nucleus (L'Etoile et al., 2002; Lee et al., 2010). The nuclear entry of EGL-4 requires both a drop in cGMP levels (O'Halloran et al., 2012) and G alpha signaling (O'Halloran 2009).  Interestingly, odor-stimulation induces localized translation of this kinase by a Pumillio type translational regulator, which is possibly aided by a microRNA (Kaye et al., 2009).  We have evidence that once the kinase enters the nucleus, it stimulates a chromatin dependent down regulation of specific odor-signaling genes (submitted).

Areas of investigation

  1. Messenger RNA localization: how is the localization and timing of translation is regulated by activity? Currently we are interested in understanding how localized translation occurs in neurons and whether the messages that are locally translated are subject to special processing into small RNAs.
  2. How the subcellular localization of a signaling molecule (a cGMP-dependent protein kinase) affects the kinase's nuclear entry and activity. We would like to understand whether proteins that are translated in an odor-activated neuron are "marked" and thus differ from the same protein that is translated in an naive cell.
  3. How food-derived signals affect signal transduction within the neuron and alter the subcellular localization of the cGMP-dependent kinase. Food signals acting through a cGMP pathway may substantially alter the physiology of the AWC sensory neuron.  cGMP is a fascinating second messenger that has been shown to direct age dependent worker honey bee behavior as well as the over all locomotory state of flies.  Most recently, cGMP levels were shown to be important in maintaining attention in mice.  We have developed a GFP-based cyclic nucleotide reporter that we are optimizing for study of food mediated cGMP regulation of odor-seeking behavioral states.
  4. The role of small RNAs in regulating chromatin dynamics as a function of prolonged stimulation.
  5. Whether RNA-dependent chromatin changes are required for cancer formation in C. elegans and human breast cell lines.
  6. How triplet repeat RNA sequences prove toxic to neurons.
Primary Thematic Area: 
Secondary Thematic Area: 
Cancer Biology & Cell Signaling
Research Summary: 
Mechanisms of Cellular and Behavioral Plasticity