Regulation of behavioral plasticity by systemic temperature signaling in Caenorhabditis elegans

A neuronal GPCR is critical for the induction of the heat shock response in the nematode C. elegans

In the nematode Caenorhabditis elegans, the heat shock response (HSR) is regulated at the organismal level by a network of thermosensory neurons that senses elevated temperatures and activates the HSR in remote tissues. Which neuronal receptors are required for this signaling mechanism and in which neurons they function are largely unanswered questions. Here we used worms that were engineered to exhibit RNA interference hypersensitivity in neurons to screen for neuronal receptors that are required for the activation of the HSR and identified a putative G-protein coupled receptor (GPCR) as a novel key component of this mechanism. This gene, which we termed GPCR thermal receptor 1 (gtr-1), is expressed in chemosensory neurons and has no role in heat sensing but is critically required for the induction of genes that encode heat shock proteins in non-neural tissues upon exposure to heat. Surprisingly, the knock-down of gtr-1 by RNA interference protected worms expressing the Alzheimer's-disease-linked aggregative peptide Aβ3-42 from proteotoxicity but had no effect on lifespan. This study provides several novel insights: (1) it shows that chemosensory neurons play important roles in the nematode's HSR-regulating mechanism, (2) it shows that lifespan and heat stress resistance are separable, and (3) it strengthens the emerging notion that the ability to respond to heat comes at the expense of protein homeostasis (proteostasis).

Temperature integration at the AC thermosensory neurons in Drosophila

Temperature sensation has a strong impact on animal behavior and is necessary for animals to avoid exposure to harmful temperatures. It is now well known that thermoTRP (transient receptor potential) channels in thermosensory neurons detect a variable range of temperature stimuli. However, little is known about how a range of temperature information is relayed and integrated in the neural circuits. Here, we show novel temperature integration between two warm inputs via Drosophila TRPA channels, TRPA1 and Pyrexia (Pyx). The internal AC (anterior cell) thermosensory neurons, which express TRPA1, detect warm temperatures and mediate temperature preference behavior. We found that the AC neurons were activated twice when subjected to increasing temperatures. The first response was at ∼25°C via TRPA1 channel, which is expressed in the AC neurons. The second response was at ∼27°C via the second antennal segments, indicating that the second antennal segments are involved in the detection of warm temperatures. Further analysis reveals that pyx-Gal4-expressing neurons have synapses on the AC neurons and that mutation of pyx eliminates the second response of the AC neurons. These data suggest that AC neurons integrate both their own TRPA1-dependent temperature responses and a Pyx-dependent temperature response from the second antennal segments. Our data reveal the first identification of temperature integration, which combines warm temperature information from peripheral to central neurons and provides the possibility that temperature integration is involved in the plasticity of behavioral outputs.

Molecular mechanism for trimetric G protein-coupled thermosensation and synaptic regulation in the temperature response circuit of Caenorhabditis elegans

How the nervous system controls the sensation and memory of information from the environment is an essential question. The nematode Caenorhabditis elegans is a useful model for elucidating neural information processing that mediates sensation and memory. The entire nervous system of C. elegans consists of only 302 neurons, and their wiring diagram has been revealed by electron microscopy analysis. Here, we review the molecular and physiological mechanisms responsible for the neural circuit-mediated temperature-seeking behavior (thermotaxis) in C. elegans. Recent molecular biology studies and optogenetic analyses, such as the optical manipulation of neural activity, and neural imaging have revealed the novel concept of neural calculation. Most significantly, trimetric G proteincoupled thermosensation, single sensory neuron-based memory, and the orchestrated synaptic transmission system have been elucidated.

Synaptic vesicle clustering requires a distinct MIG-10/Lamellipodin isoform and ABI-1 downstream from Netrin

The chemotrophic factor Netrin can simultaneously instruct different neurodevelopmental programs in individual neurons in vivo. How neurons correctly interpret the Netrin signal and undergo the appropriate neurodevelopmental response is not understood. Here we identify MIG-10 isoforms as critical determinants of individual cellular responses to Netrin. We determined that distinct MIG-10 isoforms, varying only in their N-terminal motifs, can localize to specific subcellular domains and are differentially required for discrete neurodevelopmental processes in vivo. We identified MIG-10B as an isoform uniquely capable of localizing to presynaptic regions and instructing synaptic vesicle clustering in response to Netrin. MIG-10B interacts with Abl-interacting protein-1 (ABI-1)/Abi1, a component of the WAVE complex, to organize the actin cytoskeleton at presynaptic sites and instruct vesicle clustering through SNN-1/Synapsin. We identified a motif in the MIG-10B N-terminal domain that is required for its function and localization to presynaptic sites. With this motif, we engineered a dominant-negative MIG-10B construct that disrupts vesicle clustering and animal thermotaxis behavior when expressed in a single neuron in vivo. Our findings indicate that the unique N-terminal domains confer distinct MIG-10 isoforms with unique capabilities to localize to distinct subcellular compartments, organize the actin cytoskeleton at these sites, and instruct distinct Netrin-dependent neurodevelopmental programs.

Regulation of lifespan by chemosensory and thermosensory systems: findings in invertebrates and their implications in mammalian aging

Many environmental factors that dynamically change in nature influence various aspects of animal physiology. Animals are equipped with sensory neuronal systems that help them properly sense and respond to environmental factors. Several studies have shown that chemosensory and thermosensory neurons affect the lifespan of invertebrate model animals, including Caenorhabditis elegans and Drosophila melanogaster. Although the mechanisms by which these sensory systems modulate lifespan are incompletely understood, hormonal signaling pathways have been implicated in sensory system-mediated lifespan regulation. In this review, we describe findings regarding how sensory nervous system components elicit physiological changes to regulate lifespan in invertebrate models, and discuss their implications in mammalian aging.

Endoplasmic reticulum stress pathway required for immune homeostasis is neurally controlled by arrestin-1

In response to pathogen infection, the host innate immune system activates microbial killing pathways and cellular stress pathways that need to be balanced because insufficient or excessive immune responses have deleterious consequences. Recent studies demonstrate that two G protein-coupled receptors (GPCRs) in the nervous system of Caenorhabditis elegans control immune homeostasis. To investigate further how GPCR signaling controls immune homeostasis at the organismal level, we studied arrestin-1 (ARR-1), which is the only GPCR adaptor protein in C. elegans. The results indicate that ARR-1 is required for GPCR signaling in ASH, ASI, AQR, PQR, and URX neurons, which control the unfolded protein response and a p38 mitogen-activated protein kinase signaling pathway required for innate immunity. ARR-1 activity also controlled immunity through ADF chemosensory and AFD thermosensory neurons that regulate longevity. Furthermore, we found that although ARR-1 played a key role in the control of immunity by AFD thermosensory neurons, it did not control longevity through these cells. However, ARR-1 partially controlled longevity through ADF neurons.

Regulation of cellular protein quality control networks in a multicellular organism

The long-term health of all metazoan cells is linked to protein quality control, which is achieved by proteostasis, a complex network of molecular interactions that determines the health of the proteome under physiological or stress conditions. Studying the regulation of cellular proteostasis in the context of the whole organism has unraveled multiple layers of cell-nonautonomous regulation, including neuronal regulation, cell-to-cell stress signals and endocrine signaling that affect growth, development and aging. Here, we discuss emerging concepts in cell-nonautonomous regulation of protein quality control networks. The identification of organismal modulators of cellular proteostasis may present novel, yet general targets for misfolding disease intervention.

Neuronal circuitry regulates the response of Caenorhabditis elegans to misfolded proteins

The consequence of chronic protein misfolding is the basis of many human diseases. To combat the deleterious effects of accumulated protein damage, all cells possess robust quality-control systems, specifically molecular chaperones and clearance machineries, that sense and respond to protein misfolding. However, for many protein conformational diseases, it is unclear why this quality-control system does not efficiently counter protein aggregation. Previous findings that the heat shock response in Caenorhabditis elegans is regulated by thermosensory neurons led us to consider whether neuronal activity could also be responsible for the inadequate response of an organism to chronic protein misfolding. Here we show, in animals expressing polyglutamine expansion proteins and mutant SOD-1(G93A) in intestinal or muscle cells, that the nervous system does indeed control the cellular response to misfolded proteins. Whereas polyglutamine expansion-expressing animals with WT thermosensory neurons readily express protein aggregates, leading to cellular dysfunction without concomitant up-regulation of molecular chaperones, modulation of the nervous system results in chaperone up-regulation that suppresses aggregation and toxicity. The neuronal signal is transmitted through calcium-activated dense core vesicle neurosecretion. Cell-nonautonomous control of chaperone expression by the thermosensory neurons allows C. elegans to respond differently to acute stress such as heat shock, and chronic stress caused by the expression of misfolded proteins, suggesting that neuronal signaling determines the course of cellular proteotoxicity.