Nuclear entry of a cGMP-dependent kinase converts transient into long-lasting olfactory adaptation

Abstract

To navigate a complex and changing environment, an animal's sensory neurons must continually adapt to persistent cues while remaining responsive to novel stimuli. Long-term exposure to an inherently attractive odor causes Caenorhabditis elegans to ignore that odor, a process termed odor adaptation. Odor adaptation is likely to begin within the sensory neuron, because it requires factors that act within these cells at the time of odor exposure. The process by which an olfactory sensory neuron makes a decisive shift over time from a receptive state to a lasting unresponsive one remains obscure. In C. elegans, adaptation to odors sensed by the AWC pair of olfactory neurons requires the cGMP-dependent protein kinase EGL-4. Using a fully functional, GFP-tagged EGL-4, we show here that prolonged odor exposure sends EGL-4 into the nucleus of the stimulated AWC neuron. This odor-induced nuclear translocation correlates temporally with the stable dampening of chemotaxis that is indicative of long-term adaptation. Long-term adaptation requires cGMP binding residues as well as an active EGL-4 kinase. We show here that EGL-4 nuclear accumulation is both necessary and sufficient to induce longlasting odor adaptation. After it is in the AWC nucleus, EGL-4 decreases the animal's responsiveness to AWC-sensed odors by acting downstream of the primary sensory transduction. Thus, the EGL-4 protein kinase acts as a sensor that integrates odor signaling over time, and its nuclear translocation is an instructive switch that allows the animal to ignore persistent odors.neuron | plasticity | signaling | memory | integration S ensory neurons, the entry point for information about an animal's external environment, must both respond to relevant stimuli and dampen their response as a consequence of prolonged stimulation. This dampening or adaptation is thought to be a protective consequence of prolonged stimulation, because indeed, adaptation protects mammalian photoreceptors from stimulationinduced degeneration (1, 2). The cellular, molecular, and temporal controls that allow sensory neurons to switch from being responsive to refractory to a given stimulus are examined herein. The Caenorhabditis elegans AWC sensory neuronal pair can sense (3) and adapt (4) to inherently attractive odors, and although both sensory and interneurons within the olfactory circuit adapt to persistent stimulation (5, 6), this focus will be on the events that instruct long-lasting adaptation within the sensory neuron.The AWC neurons (3, 7) are postulated to express at least 20 different odor-responsive G protein-coupled receptors (8, 9), use cGMP as a second messenger (10-13), and hyperpolarize in response to odor (14). Odor adaptation begins within 10 min of odor exposure, and there is a mild decrease in the animal's attraction to the odor accompanied by an increase in phosphorylated MAP kinase in AWC (5, 15) that requires the G protein gamma, GPC-1 (16). After 30 min of exposure, attraction decreases further as the animal enters a phase of short-term adaptation...

Two Forms of Learning following Training to a Single Odorant in Caenorhabditis elegans AWC Neurons

“…Experiments using a transgenic constitutively nuclear allele of egl-4 (NLS::GFP:: EGL-4) demonstrated that the nuclear accumulation of the protein was itself sufficient to phenocopy the conditioned response to Bnz (O'Halloran et al, 2009;Lee et al, 2010). The results thus far suggested that both the insulin signaling pathway and egl-4 function in the associative learning pathway.…”

“…Our previous work (Lin et al, 2010) suggested the main function of insulin occurs during memory retrieval, and the work of Lee et al (2010) suggested that egl-4 functions in memory acquisition. Briefly, Lee et al (2010) demonstrated that the adaptation memory significantly outlasts the nuclear localization of EGL-4, and thus while its translocation may trigger formation of the engram, the physiological manifestation of memory, it is not the engram itself. Our result demonstrated that ins-1 is epistatic to egl-4 and is therefore consistent with the role of egl-4 in memory acquisition and insulin in memory retrieval.…”

“…of EGL-4, which is believed to mediate the latter's role in adaptation (O'Halloran et al, 2009;Lee et al, 2010). Yet it remains unclear from this work whether egl-4-mediated adaptation does indeed represent a form of habituation, upstream of osm-9, or is actually a form of associative learning.…”

A Molecular Readout of Long-term Olfactory Adaptation in <em>C. elegans</em>

“…By developing a molecular readout of olfactory plasticity that represents a nuclear translocation of a PKG in stimulated neurons, we provide a platform for rapid and detailed dissection of neuronal plasticity within an in vivo model. Using our molecular readout of olfactory adaptation we have begun to describe some of the events that shape olfactory adaptation in C. elegans 5,6,7 . However, this is just a beginning, and by studying this process as a function of age, sex, infection, or diet we may uncover important themes of neuronal plasticity at the circuit and systems level.…”

Characterization of the cGMP-dependent protein kinase SmcGK1 of Schistosoma mansoni

“…Beyond that cGKs are modulators of cell growth (Hofmann et al 2009). In Caenorhabditis elegans cGKs regulate growth processes, but also patterns of particular behavioral states (Raizen et al 2006, 2008, Fujiwara et al 2002, odor perception, and signal integration (Lee et al 2010). Finally, genome sequencing approaches have led to the identification of cGK orthologs also from protozoa.…”