Caenorhabditus elegans arrestin regulates neural G protein signaling and olfactory adaptation and recovery

Evolution of neuropeptide Y/RFamide-like receptors in nematodes

The Neuropeptide Y/RFamide-like receptors belong to the Rhodopsin-like G protein-coupled receptors G protein-coupled receptors (GPCRs) and are involved in functions such as locomotion, feeding and reproduction. With 41 described receptors they form the best-studied group of neuropeptide GPCRs in Caenorhabditis elegans. In order to understand the expansion of the Neuropeptide Y/RFamide-like receptor family in nematodes, we started from the sequences of selected receptor paralogs in C. elegans as query and surveyed the corresponding orthologous sequences in another 159 representative nematode target genomes. To this end we employed a automated pipeline based on ExonMatchSolver, a tool that solves the paralog-to-contig assignment problem. Utilizing subclass-specific HMMs we were able to detect a total of 1557 Neuropeptide Y/RFamide-like receptor sequences (1100 NPRs, 375 FRPRs and 82 C09F12.3) in the 159 target nematode genomes investigated here. These sequences demonstrate a good conservation of the Neuropeptide Y/RFamide-like receptors across the Nematoda and highlight the diversification of the family in nematode evolution. No other genus shares all Neuropeptide Y/RFamide-like receptors with the genus Caenorhabditis. At the same time, we observe large numbers of clade specific duplications and losses of family members across the phylum Nematoda.

The α-arrestin SUP-13/ARRD-15 promotes isoform turnover of actin-interacting protein 1 in Caenorhabditis elegans striated muscle

Precise arrangement of actin, myosin, and other regulatory components in a sarcomeric pattern is critical for producing contractile forces in striated muscles. Actin-interacting protein 1 (AIP1), also known as WD-repeat protein 1 (WDR1), is one of essential factors that regulate sarcomeric assembly of actin filaments. In the nematode Caenorhabditis elegans, mutation in unc-78, encoding one of the two AIP1 isoforms, causes severe disorganization of sarcomeric actin filaments and near paralysis, but mutation in sup-13 suppresses the unc-78-mutant phenotypes to restore nearly normal sarcomeric actin organization and worm motility. Here, we identified that sup-13 is a nonsense allele of arrd-15 encoding an α-arrestin. The sup-13/arrd-15 mutation suppressed the phenotypes of unc-78 null mutant but required aipl-1 that encodes a second AIP1 isoform. aipl-1 was normally expressed highly in embryos and downregulated in mature muscle. However, in the sup-13/arrd-15 mutant, the AIPL-1 protein was maintained at high levels in adult muscle to compensate for the absence of the UNC-78 protein. The sup-13/arrd-15 mutation caused accumulation of ubiquitinated AIPL-1 protein, suggesting that a normal function of sup-13/arrd-15 is to enhance degradation of ubiquitinated AIPL-1, thereby promoting transition of AIP1 isoforms from AIPL-1 to UNC-78 in developing muscle. These results suggest that α-arrestin is a novel factor to promote isoform turnover by enhancing protein degradation.

Multiple p38/JNK mitogen-activated protein kinase (MAPK) signaling pathways mediate salt chemotaxis learning in C. elegans

Animals are able to adapt their behaviors to the environment. In order to achieve this, the nervous system plays integrative roles, such as perception of external signals, sensory processing, and behavioral regulations via various signal transduction pathways. Here genetic analyses of Caenorhabditis elegans (C. elegans) found that mutants of components of JNK and p38 mitogen-activated protein kinase (MAPK) signaling pathways, also known as stress-activated protein kinase (SAPK) signaling pathways, exhibit various types of defects in the learning of salt chemotaxis. C. elegans homologs of JNK MAPKKK and MAPKK, MLK-1 and MEK-1, respectively, are required for avoidance of salt concentrations experienced during starvation. In contrast, homologs of p38 MAPKKK and MAPKK, NSY-1 and SEK-1, respectively, are required for high-salt chemotaxis after conditioning. Genetic interaction analyses suggest that a JNK family MAPK, KGB-1, functions downstream of both signaling pathways to regulate salt chemotaxis learning. Furthermore, we found that the NSY-1/SEK-1 pathway functions in sensory neurons, ASH, ADF, and ASER, to regulate the learned high-salt chemotaxis. A neuropeptide, NLP-3, expressed in ASH, ADF, and ASER neurons, and a neuropeptide receptor, NPR-15, expressed in AIA interneurons that receive synaptic input from these sensory neurons, function in the same genetic pathway as NSY-1/SEK-1 signaling. These findings suggest that this MAPK pathway may affect neuropeptide signaling between sensory neurons and interneurons, thus promoting high-salt chemotaxis after conditioning.

Arrestin-mediated desensitization enables intraneuronal olfactory discrimination in Caenorhabditis elegans

In the mammalian olfactory system, cross-talk between olfactory signals is minimized through physical isolation: individual neurons express one or few olfactory receptors among those encoded in the genome. Physical isolation allows for segregation of stimuli during signal transduction; however, in the nematode worm Caenorhabditis elegans, ∼1,300 olfactory receptors are primarily expressed in only 32 neurons, precluding this strategy. Here, we report genetic and behavioral evidence that β-arrestin-mediated desensitization of olfactory receptors, working downstream of the kinase GRK-1, enables discrimination between intraneuronal olfactory stimuli. Our findings suggest that C. elegans exploits β-arrestin desensitization to maximize responsiveness to novel odors, allowing for behaviorally appropriate responses to olfactory stimuli despite the large number of olfactory receptors signaling in single cells. This represents a fundamentally different solution to the problem of olfactory discrimination than that which evolved in mammals, allowing for economical use of a limited number of sensory neurons.

Identification, tissue specific expression analysis and functional characterization of arrestin gene (ARRDC) in the earthworm Eudrilus eugeniae: a molecular hypothesis behind worm photoreception

BACKGROUND

The arrestin domain containing proteins (ARRDCs) are crucial adaptor proteins assist in signal transduction and regulation of sensory physiology. The molecular localization of the ARRDC gene has been confined mainly to the mammalian system while in invertebrates the expression pattern was not addressed significantly. The present study reports the identification, tissue specific expression and functional characterization of an ARRDC transcript in earthworm, Eudrilus eugeniae.

METHODS AND RESULTS

The coding region of earthworm ARRDC transcript was 1146 bp in length and encoded a protein of 381 amino acid residues. The worm ARRDC protein consists of conserved N-terminal and C-terminal regions and showed significant homology with the ARRDC3 sequence of other species. The tissue specific expression analysis through whole mount in-situ hybridization denoted the expression of ARRDC transcript in the central nervous system of the worm which includes cerebral ganglion and ventral nerve cord. Besides, the expression of ARRDC gene was observed in the epidermal region of earthworm skin. The functional characterization of ARRDC gene was assessed through siRNA silencing and the gene was found to play key role in the light sensing ability and photophobic movement of the worm.

CONCLUSIONS

The neuronal and dermal expression patterns of ARRDC gene and its functional characterization hypothesized the role of the gene in assisting the photosensory cells to regulate the process of photoreception and phototransduction in the worm.

Pond Assay for the Sensory Systems of Caenorhabditis elegans: A Novel Anesthesia-Free Method Enabling Detection of Responses to Extremely Low Chemical Concentrations

Simple Summary

We propose a pond assay for the sensory systems (PASS) of Caenorhabditis elegans as a novel method of behavioral analysis. In PASS, the test solution is injected into a recess(es) formed on agar and the response of C. elegans to its odor and/or taste is examined. Once C. elegans individuals fall into recesses (ponds) filled with liquid, they cannot return to the solid medium. In this way, the animals are trapped with certainty without the use of anesthesia. The anesthesia used to keep animals in the attractant area in conventional chemotaxis assays is no longer required, allowing pure evaluation of the response to specific substances. Furthermore, the test itself can be greatly streamlined because the preparation can be completed simply by providing a recess(es) and filling the liquid. The present paper reports the detailed method and effectiveness of the novel PASS through a series of chemotaxis assays. By using the PASS method, we found that the olfactory system of C. elegans accurately senses odors even at extremely low concentrations lower than the previously known detection threshold. This method can be applied to biosensor technology that uses C. elegans to detect chemical substances present at extremely low concentrations in environmental samples and biological samples with high sensitivity.

Abstract

Chemotaxis in the nematode Caenorhabditis elegans has basically been examined using conventional assay methods. Although these can be problematic, for example, in their use of anesthesia, the method has never been improved. We propose a pond assay for the sensory systems (PASS) of C. elegans as a novel population-based method of behavioral analysis. The test solution is injected into a recess(es) formed on agar and the response of C. elegans to its odor and/or taste is examined. Once C. elegans individuals fall into recesses (ponds) filled with liquid, they cannot return to a solid medium. In this way, the animals are trapped with certainty without the use of anesthesia. The anesthesia used to keep animals in the attractant area in conventional chemotaxis assays is no longer required, allowing pure evaluation of the attractant or repellent response to specific substances. Furthermore, the assay itself can be greatly streamlined because the preparation can be completed simply by providing a recess(es) and filling the liquid. The present paper reports the detailed method and effectiveness of the novel PASS.

Investigating the molecular mechanisms of learning and memory using Caenorhabditis elegans

Learning is an essential biological process for survival since it facilitates behavioural plasticity in response to environmental changes. This process is mediated by a wide variety of genes, mostly expressed in the nervous system. Many studies have extensively explored the molecular and cellular mechanisms underlying learning and memory. This review will focus on the advances gained through the study of the nematode Caenorhabditis elegans. C. elegans provides an excellent system to study learning because of its genetic tractability, in addition to its invariant, compact nervous system (~300 neurons) that is well-characterised at the structural level. Importantly, despite its compact nature, the nematode nervous system possesses a high level of conservation with mammalian systems. These features allow the study of genes within specific sensory-, inter- and motor neurons, facilitating the interrogation of signalling pathways that mediate learning via defined neural circuits. This review will detail how learning and memory can be studied in C. elegans through behavioural paradigms that target distinct sensory modalities. We will also summarise recent studies describing mechanisms through which key molecular and cellular pathways are proposed to affect associative and non-associative forms of learning.

Chemosensory signal transduction in Caenorhabditis elegans

Chemosensory neurons translate perception of external chemical cues, including odorants, tastants, and pheromones, into information that drives attraction or avoidance motor programs. In the laboratory, robust behavioral assays, coupled with powerful genetic, molecular and optical tools, have made Caenorhabditis elegans an ideal experimental system in which to dissect the contributions of individual genes and neurons to ethologically relevant chemosensory behaviors. Here, we review current knowledge of the neurons, signal transduction molecules and regulatory mechanisms that underlie the response of C. elegans to chemicals, including pheromones. The majority of identified molecules and pathways share remarkable homology with sensory mechanisms in other organisms. With the development of new tools and technologies, we anticipate that continued study of chemosensory signal transduction and processing in C. elegans will yield additional new insights into the mechanisms by which this animal is able to detect and discriminate among thousands of chemical cues with a limited sensory neuron repertoire.

SRD-1 in AWA neurons is the receptor for female volatile sex pheromones in C. elegans males

Pheromones are critical cues for attracting mating partners for successful reproduction. Sexually mature Caenorhabditis remanei virgin females and self-sperm-depleted Caenorhabditis elegans hermaphrodites produce volatile sex pheromones to attract adult males of both species from afar. The chemoresponsive receptor in males has remained unknown. Here, we show that the male chemotactic behavior requires amphid sensory neurons (AWA neurons) and the G-protein-coupled receptor SRD-1. SRD-1 expression in AWA neurons is sexually dimorphic, with the levels being high in males but undetectable in hermaphrodites. Notably, srd-1 mutant males lack the chemotactic response and pheromone-induced excitation of AWA neurons, both of which can be restored in males and hermaphrodites by AWA-specific srd-1 expression, and ectopic expression of srd-1 in AWB neurons in srd-1 mutants results in a repulsive behavioral response in both sexes. Furthermore, we show that the C-terminal region of SRD-1 confers species-specific differences in the ability to perceive sex pheromones between C. elegans and C. remanei These findings offer an excellent model for dissecting how a single G-protein-coupled receptor expressed in a dimorphic neural system contributes to sex-specific behaviors in animals.

Neurotransmitter signaling through heterotrimeric G proteins: insights from studies in C. elegans

Neurotransmitters signal via G protein coupled receptors (GPCRs) to modulate activity of neurons and muscles. C. elegans has ∼150 G protein coupled neuropeptide receptor homologs and 28 additional GPCRs for small-molecule neurotransmitters. Genetic studies in C. elegans demonstrate that neurotransmitters diffuse far from their release sites to activate GPCRs on distant cells. Individual receptor types are expressed on limited numbers of cells and thus can provide very specific regulation of an individual neural circuit and behavior. G protein coupled neurotransmitter receptors signal principally via the three types of heterotrimeric G proteins defined by the G alpha subunits Gαo, Gαq, and Gαs. Each of these G alpha proteins is found in all neurons plus some muscles. Gαo and Gαq signaling inhibit and activate neurotransmitter release, respectively. Gαs signaling, like Gαq signaling, promotes neurotransmitter release. Many details of the signaling mechanisms downstream of Gαq and Gαs have been delineated and are consistent with those of their mammalian orthologs. The details of the signaling mechanism downstream of Gαo remain a mystery. Forward genetic screens in C. elegans have identified new molecular components of neural G protein signaling mechanisms, including Regulators of G protein Signaling (RGS proteins) that inhibit signaling, a new Gαq effector (the Trio RhoGEF domain), and the RIC-8 protein that is required for neuronal Gα signaling. A model is presented in which G proteins sum up the variety of neuromodulator signals that impinge on a neuron to calculate its appropriate output level.

Multiple Signaling Pathways Coordinately Regulate Forgetting of Olfactory Adaptation through Control of Sensory Responses in Caenorhabditis elegans

Forgetting memories is important for animals to properly respond to continuously changing environments. To elucidate the mechanisms of forgetting, we used one of the behavioral plasticities of Caenorhabditis elegans hermaphrodite, olfactory adaptation to an attractive odorant, diacetyl, as a simple model of learning. In C. elegans, the TIR-1/JNK-1 pathway accelerates forgetting of olfactory adaptation by facilitating neural secretion from AWC sensory neurons. In this study, to identify the downstream effectors of the TIR-1/JNK-1 pathway, we conducted a genetic screen for suppressors of the gain-of-function mutant of tir-1 (ok1052), which shows excessive forgetting. Our screening showed that three proteins-a membrane protein, MACO-1; a receptor tyrosine kinase, SCD-2; and its putative ligand, HEN-1-regulated forgetting downstream of the TIR-1/JNK-1 pathway. We further demonstrated that MACO-1 and SCD-2/HEN-1 functioned in parallel genetic pathways, and only MACO-1 regulated forgetting of olfactory adaptation to isoamyl alcohol, which is an attractive odorant sensed by different types of sensory neurons. In olfactory adaptation, odor-evoked Ca²⁺ responses in olfactory neurons are attenuated by conditioning and recovered thereafter. A Ca²⁺ imaging study revealed that this attenuation is sustained longer in maco-1 and scd-2 mutant animals than in wild-type animals like the TIR-1/JNK-1 pathway mutants. Furthermore, temporal silencing by histamine-gated chloride channels revealed that the neuronal activity of AWC neurons after conditioning is important for proper forgetting. We propose that distinct signaling pathways, each of which has a specific function, may coordinately and temporally regulate forgetting by controlling sensory responses.SIGNIFICANCE STATEMENT Active forgetting is an important process to understand the whole mechanisms of memories. Recent papers have reported that the noncell autonomous regulations are required for proper forgetting in invertebrates. We found that in Caenorhabditis elegans hermaphrodite, the noncell autonomous regulations of forgetting of olfactory adaptation is regulated by three conserved proteins: a membrane protein, MACO-1; a receptor tyrosine kinase, SCD-2: and its ligand, HEN-1. MACO-1 and SCD-2/HEN-1, working in coordination, accelerate forgetting by controlling sensory responses in parallel. Furthermore, temporal regulation of neuronal activity is important for proper forgetting. We suggest that multiple pathways may coordinately and temporally regulate forgetting through control of sensory responses. This study should lead to a better understanding of forgetting in higher organisms.

The cellular senescence of leukemia-initiating cells from acute lymphoblastic leukemia is postponed by β-Arrestin1 binding with P300-Sp1 to regulate hTERT transcription

Although we previously reported that the self-renewal of leukemia-initiating cells of B-lineage acute lymphoblastic leukemia (B-ALL LICs) was regulated by β-Arrestin1, a multiple-function protein, the cellular senescence is critical for LICs fate and leukemia progress, and worthy for further investigation. Here we found that depletion of β-Arrestin1 extended the population doubling time and the percentage of senile cells, the signatures of cellular senescence, of B-ALL LICs. Moreover, lack of β-Arrestin1 enhanced the expression of proteins (CBX, HIRA) and genes (P53, P16) related to senescence in leukemic Reh cells and B-ALL-LICs-derived leukemic mice. Further results showed that loss of β-Arrestin1 induced senescence of Reh cells through mediating hTERT-telomerase-telomere axis, which was reversed by BIBR1532, the telomerase activity inhibitor. Importantly, depletion of β-Arrestin1 decreased the binding of Sp1 to hTERT promoter at the region of −28 to −36 bp. The anti-sense oligonucleotide of this key region downregulated the transcription of hTERT and aggravated the senescence of Reh cells. Further data demonstrated that the depleted β-Arrestin1 reduced the interaction of P300 with Sp1, thus to reduce Sp1 binding to hTERT promoter, downregulate hTERT transcription, decrease telomerase activity, shorten telomere length, and promote Reh cell senescence. Interestingly, the percentage of senile cells in B-ALL LICs was decreased, which was negatively correlated to good prognosis and β-Arrestin1 mRNA expression in childhood B-ALL patients. Our study shed a light on the senescence of B-ALL LICs and is regulated by β-Arrestin1, providing the potential therapeutic target of leukemia by promoting cellular senescence with a key region of hTERT promoter.

FLP-4 neuropeptide and its receptor in a neuronal circuit regulate preference choice through functions of ASH-2 trithorax complex in Caenorhabditis elegans

Preference choice on food is an important response strategy for animals living in the environment. Using assay system of preference choice on bacterial foods, OP50 and PA14, we identified the involvement of ADL sensory neurons in the control of preference choice in Caenorhabditis elegans. Both genetically silencing and ChR2-mediated activation of ADL sensory neurons significantly affected preference choice. ADL regulated preference choice by inhibiting function of G protein-coupled receptor (GPCR)/SRH-220. ADL sensory neurons might regulate preference choice through peptidergic signals of FLP-4 and NLP-10, and function of FLP-4 or NLP-10 in regulating preference choice was regulated by SRH-220. FLP-4 released from ADL sensory neurons further regulated preference choice through its receptor of NPR-4 in AIB interneurons. In AIB interneurons, NPR-4 was involved in the control of preference choice by activating the functions of ASH-2 trithorax complex consisting of SET-2, ASH-2, and WDR-5, implying the crucial role of molecular machinery of trimethylation of histone H3K4 in the preference choice control. The identified novel neuronal circuit and the underlying molecular mechanisms will strengthen our understanding neuronal basis of preference choice in animals.

A synergistic approach towards understanding the functional significance of dopamine receptor interactions

The importance of the neurotransmitter dopamine (DA) in the nervous system is underscored by its role in a wide variety of physiological and neural functions in both vertebrates and invertebrates. Binding of dopamine to its membrane receptors initiates precise signaling cascades that result in specific cellular responses. Dopamine receptors belong to a super-family of G-protein coupled receptors (GPCRs) that are characterized by seven trans-membrane domains. In mammals, five dopamine receptors have been identified which are grouped into two different categories D1- and D2-like receptors. The interactions of DA receptors with other proteins including specific Gα subunits are critical in deciding the fate of downstream molecular events carried out by effector proteins. In this mini-review we provide a synopsis of known protein-protein interactions of DA receptors and a perspective on the potential synergistic utility of Caenorhabditis elegans as a model eukaryote with a comparatively simpler nervous system to gain insight on the neuronal and behavioral consequences of the receptor interactions.

Investigating the Relationship between Topology and Evolution in a Dynamic Nematode Odor Genetic Network

The relationship between biological network architectures and evolution is unclear. Within the phylum nematoda olfaction represents a critical survival tool. For nematodes, olfaction contributes to multiple processes including the finding of food, hosts, and reproductive partners, making developmental decisions, and evading predators. Here we examine a dynamic nematode odor genetic network to investigate how divergence, diversity, and contribution are shaped by network topology. Our findings describe connectivity frameworks and characteristics that correlate with molecular evolution and contribution across the olfactory network. Our data helps guide the development of a robust evolutionary description of the nematode odor network that may eventually aid in the prediction of interactive and functional qualities of novel nodes.

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.

The functional cycle of visual arrestins in photoreceptor cells

Visual arrestin-1 plays a key role in the rapid and reproducible shutoff of rhodopsin signaling. Its highly selective binding to light-activated phosphorylated rhodopsin is an integral part of the functional perfection of rod photoreceptors. Structure-function studies revealed key elements of the sophisticated molecular mechanism ensuring arrestin-1 selectivity and paved the way to the targeted manipulation of the arrestin-1 molecule to design mutants that can compensate for congenital defects in rhodopsin phosphorylation. Arrestin-1 self-association and light-dependent translocation in photoreceptor cells work together to keep a constant supply of active rhodopsin-binding arrestin-1 monomer in the outer segment. Recent discoveries of arrestin-1 interaction with other signaling proteins suggest that it is a much more versatile signaling regulator than previously thought, affecting the function of the synaptic terminals and rod survival. Elucidation of the fine molecular mechanisms of arrestin-1 interactions with rhodopsin and other binding partners is necessary for the comprehensive understanding of rod function and for devising novel molecular tools and therapeutic approaches to the treatment of visual disorders.

The arrestin fold: variations on a theme

Endocytosis of ligand-activated plasma membrane receptors has been shown to contribute to the regulation of their downstream signaling. β-arrestins interact with the phosphorylated tail of activated receptors and act as scaffolds for the recruitment of adaptor proteins and clathrin, that constitute the machinery used for receptor endocytosis. Visual- and β-arrestins have a two-lobe, immunoglobulin-like, β-strand sandwich structure. The recent resolution of the crystal structure of VPS26, one of the retromer subunits, unexpectedly evidences an arrestin fold in this protein, which is otherwise unrelated to arrestins. From a functional point of view, VPS26 is involved in the retrograde transport of the mannose 6-P receptor from the endosomes to the trans-Golgi network. In addition to the group of genuine arrestins and Vps26, mammalian cells harbor a vast repertoire of proteins that are related to arrestins on the basis of their PFAM Nter and Cter arrestin- domains, which are named Arrestin Domain- Containing proteins (ADCs). The biological role of ADC proteins is still poorly understood. The three subfamilies have been merged into an arrestin-related protein clan.

This paper provides an overall analysis of arrestin clan proteins. The structures and functions of members of the subfamilies are reviewed in mammals and model organisms such as Drosophila, Caenorhabditis, Saccharomyces and Dictyostelium.

FYVE-dependent endosomal targeting of an arrestin-related protein in amoeba

Background

Visual and β-arrestins are scaffolding proteins involved in the regulation of receptor-dependent intracellular signaling and their trafficking. The arrestin superfamilly includes several arrestin domain-containing proteins and the structurally related protein Vps26. In Dictyostelium discoideum, the arrestin-domain containing proteins form a family of six members, namely AdcA to -F. In contrast to canonical arrestins, Dictyostelium Adc proteins show a more complex architecture, as they possess, in addition to the arrestin core, other domains, such as C2, FYVE, LIM, MIT and SAM, which potentially mediate selective interactions with either lipids or proteins.

Methodology and Principal Findings

A detailed analysis of AdcA has been performed. AdcA extends on both sides of the arrestin core, in particular by a FYVE domain which mediates selective interactions with PI(3)P, as disclosed by intrinsic fluorescence measurements and lipid overlay assays. Localization studies showed an enrichment of tagged- and endogenous AdcA on the rim of early macropinosomes and phagosomes. This vesicular distribution relies on a functional FYVE domain. Our data also show that the arrestin core binds the ADP-ribosylation factor ArfA, the unique amoebal Arf member, in its GDP-bound conformation.

Significance

This work describes one of the 6 arrestin domain-containing proteins of Dictyostelium, a novel and atypical member of the arrestin clan. It provides the basis for a better understanding of arrestin-related protein involvement in trafficking processes and for further studies on the expanding roles of arrestins in eukaryotes.

β-arrestin Kurtz inhibits MAPK and Toll signalling in Drosophila development

β-Arrestins have been implicated in the regulation of multiple signalling pathways. However, their role in organism development is not well understood. In this study, we report a new in vivo function of the Drosophila β-arrestin Kurtz (Krz) in the regulation of two distinct developmental signalling modules: MAPK ERK and NF-κB, which transmit signals from the activated receptor tyrosine kinases (RTKs) and the Toll receptor, respectively. Analysis of the expression of effectors and target genes of Toll and the RTK Torso in krz maternal mutants reveals that Krz limits the activity of both pathways in the early embryo. Protein interaction studies suggest a previously uncharacterized mechanism for ERK inhibition: Krz can directly bind and sequester an inactive form of ERK, thus preventing its activation by the upstream kinase, MEK. A simultaneous dysregulation of different signalling systems in krz mutants results in an abnormal patterning of the embryo and severe developmental defects. Our findings uncover a new in vivo function of β-arrestins and present a new mechanism of ERK inhibition by the Drosophila β-arrestin Krz.

Beyond desensitization: physiological relevance of arrestin-dependent signaling

Heptahelical G protein-coupled receptors are the most diverse and therapeutically important family of receptors in the human genome. Ligand binding activates heterotrimeric G proteins that transmit intracellular signals by regulating effector enzymes or ion channels. G protein signaling is terminated, in large part, by arrestin binding, which uncouples the receptor and G protein and targets the receptor for internalization. It is clear, however, that heptahelical receptor signaling does not end with desensitization. Arrestins bind a host of catalytically active proteins and serve as ligand-regulated scaffolds that recruit protein and lipid kinase, phosphatase, phosphodiesterase, and ubiquitin ligase activity into the receptor-arrestin complex. Although many of these arrestin-bound effectors serve to modulate G protein signaling, degrading second messengers and regulating endocytosis and trafficking, other signals seem to extend beyond the receptor-arrestin complex to regulate such processes as protein translation and gene transcription. Although these findings have led to a re-envisioning of heptahelical receptor signaling, little is known about the physiological roles of arrestin-dependent signaling. In vivo, the duality of arrestin function makes it difficult to dissociate the consequences of arrestin-dependent desensitization from those that might be ascribed to arrestin-mediated signaling. Nonetheless, recent evidence generated using arrestin knockouts, G protein-uncoupled receptor mutants, and arrestin pathway-selective "biased agonists" is beginning to reveal that arrestin signaling plays important roles in the retina, central nervous system, cardiovascular system, bone remodeling, immune system, and cancer. Understanding the signaling roles of arrestins may foster the development of pathway-selective drugs that exploit these pathways for therapeutic benefit.

Neuropeptide feedback modifies odor-evoked dynamics in Caenorhabditis elegans olfactory neurons

Many neurons release classical transmitters together with neuropeptide cotransmitters whose functions are incompletely understood. Here we define the relationship between two transmitters in the olfactory system of Caenorhabditis elegans, showing that a neuropeptide-to-neuropeptide feedback loop alters sensory dynamics in primary olfactory neurons. The AWC olfactory neuron is glutamatergic and also expresses the peptide NLP-1. nlp-1 mutants have increased AWC-dependent behaviors, suggesting that NLP-1 limits the normal response. The receptor for NLP-1 is the G protein-coupled receptor NPR-11, which acts in postsynaptic AIA interneurons. Feedback from AIA interneurons modulates odor-evoked calcium dynamics in AWC olfactory neurons and requires INS-1, a neuropeptide released from AIA. The neuropeptide feedback loop dampens behavioral responses to odors on short and long timescales. Our results point to neuronal dynamics as a site of behavioral regulation and reveal the ability of neuropeptide feedback to remodel sensory networks on multiple timescales.

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

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 long-lasting 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.

Arrestin and the multi-PDZ domain-containing protein MPZ-1 interact with phosphatase and tensin homolog (PTEN) and regulate Caenorhabditis elegans longevity

Arrestins are multifunctional adaptor proteins best known for their role in regulating G protein-coupled receptor signaling. Arrestins also regulate other types of receptors, including the insulin-like growth factor receptor (IGF-1R), although the mechanism by which this occurs is not well understood. In Caenorhabditis elegans, the IGF-1R ortholog DAF-2 regulates dauer formation, stress resistance, metabolism, and lifespan through a conserved signaling cascade. To further elucidate the role of arrestin in IGF-1R signaling, we employed an in vivo approach to investigate the role of ARR-1, the sole arrestin ortholog in C. elegans, on longevity. Here, we report that ARR-1 functions to positively regulate DAF-2 signaling in C. elegans. arr-1 mutant animals exhibit increased longevity and enhanced nuclear localization of DAF-16, an indication of decreased DAF-2 signaling, whereas animals overexpressing ARR-1 have decreased longevity. Genetic and biochemical analysis reveal that ARR-1 functions to regulate DAF-2 signaling via direct interaction with MPZ-1, a multi-PDZ domain-containing protein, via a C-terminal PDZ binding domain in ARR-1. Interestingly, ARR-1 and MPZ-1 are found in a complex with the phosphatase and tensin homolog (PTEN) ortholog DAF-18, which normally serves as a suppressor of DAF-2 signaling, suggesting that these three proteins work together to regulate DAF-2 signaling. Our results suggest that the ARR-1-MPZ-1-DAF-18 complex functions to regulate DAF-2 signaling in vivo and provide insight into a novel mechanism by which arrestin is able to regulate IGF-1R signaling and longevity.

Regulators of AWC-mediated olfactory plasticity in Caenorhabditis elegans

While most sensory neurons will adapt to prolonged stimulation by down-regulating their responsiveness to the signal, it is not clear which events initiate long-lasting sensory adaptation. Likewise, we are just beginning to understand how the physiology of the adapted cell is altered. Caenorhabditis elegans is inherently attracted to specific odors that are sensed by the paired AWC olfactory sensory neurons. The attraction diminishes if the animal experiences these odors for a prolonged period of time in the absence of food. The AWC neuron responds acutely to odor-exposure by closing calcium channels. While odortaxis requires a Gα subunit protein, cGMP-gated channels, and guanylyl cyclases, adaptation to prolonged odor exposure requires nuclear entry of the cGMP-dependent protein kinase, EGL-4. We asked which candidate members of the olfactory signal transduction pathway promote nuclear entry of EGL-4 and which molecules might induce long-term adaptation downstream of EGL-4 nuclear entry. We found that initiation of long-term adaptation, as assessed by nuclear entry of EGL-4, is dependent on G-protein mediated signaling but is independent of fluxes in calcium levels. We show that long-term adaptation requires polyunsaturated fatty acids (PUFAs) that may act on the transient receptor potential (TRP) channel type V OSM-9 downstream of EGL-4 nuclear entry. We also present evidence that high diacylglycerol (DAG) levels block long-term adaptation without affecting EGL-4 nuclear entry. Our analysis provides a model for the process of long-term adaptation that occurs within the AWC neuron of C. elegans: G-protein signaling initiates long-lasting olfactory adaptation by promoting the nuclear entry of EGL-4, and once EGL-4 has entered the nucleus, processes such as PUFA activation of the TRP channel OSM-9 may dampen the output of the AWC neuron.

Caenorhabditis elegans is capable of sensing a variety of attractive volatile compounds. These odors are the worm's “best guesses” as to how to track down food. Employing calculated approximations underlies a foraging strategy that is open to failure. When C. elegans track an odor which proves unrewarding, they must modify their behavior based on this experience. They also need to prevent over-stimulating their neurons. To accomplish this, C. elegans olfactory sensory neurons adapt to odors after a sustained exposure to odor in the absence of food. Within the pair of primary odor-sensory neurons, termed the AWCs, adaptation requires the cGMP-dependent protein kinase G (PKG), EGL-4. Exposing animals to AWC–sensed odors for approximately 60 minutes results in a long-lasting (∼3 hour) adaptation that requires the nuclear translocation of EGL-4. To understand how sensory transduction and desensitization machinery converge to achieve olfactory adaptation, we asked whether odor-induced EGL-4 nuclear accumulation was affected by gene mutations that abrogate either odor sensation of or adaptation to AWC–sensed odors. We find that G-protein signaling represents the integration point where primary odor sensation and odor adaptation pathways diverge. PUFA signaling, calcium, and decreased diacylglycerol all dampen the response of the AWC neuron to odor downstream of this divergence.

Retention time of attenuated response to diacetyl after pre-exposure to diacetyl in Caenorhabditis elegans

The retention time of attenuated chemotactic response to continuous presentation of odorant diacetyl was investigated in the nematode Caenorhabditis elegans. The level of chemotactic response of nematodes pre-exposed to diacetyl for 90 min was significantly smaller than that of nonexposed control nematodes. In this study, wild-type (N2) nematodes were maintained at 15, 20 and 25 degrees C after pre-exposure to diacetyl. At 20 degrees C, there was a decrease in response to diacetyl continuing for up to 6 hr after pre-exposure to the chemical, but not up to 12 hr. Interestingly, the decrease in response to diacetyl did not continue up to 2 hr in nematodes bred at 15 degrees C, although it continued beyond 12 hr in nematodes bred at 25 degrees C. These results indicate that the retention time of attenuated chemotactic response to diacetyl is dependent on the environmental breeding temperature of nematodes. The breeding temperature correlated with aging speed of nematodes, suggesting that a short life span (higher aging speed) prolongs the retention time of attenuated chemotactic response to diacetyl after pre-exposure to diacetyl. In the long-lived daf-2, age-1, clk-1 and isp-1 mutants, the effect of diacetyl did not continue up to 2 hr. The short-lived daf-16, daf-18, mev-1 and gas-1 mutants showed a longer duration of decrease in response to diacetyl, that is, the retention time of attenuated chemotactic response to diacetyl continued beyond 12 hr. There is a possibility that the duration of decrease in response to diacetyl after pre-exposure to diacetyl was inversely related to the length of nematodes' life span.

Structure of an arrestin2-clathrin complex reveals a novel clathrin binding domain that modulates receptor trafficking

Non-visual arrestins play a pivotal role as adaptor proteins in regulating the signaling and trafficking of multiple classes of receptors. Although arrestin interaction with clathrin, AP-2, and phosphoinositides contributes to receptor trafficking, little is known about the configuration and dynamics of these interactions. Here, we identify a novel interface between arrestin2 and clathrin through x-ray diffraction analysis. The intrinsically disordered clathrin binding box of arrestin2 interacts with a groove between blades 1 and 2 in the clathrin beta-propeller domain, whereas an 8-amino acid splice loop found solely in the long isoform of arrestin2 (arrestin2L) interacts with a binding pocket formed by blades 4 and 5 in clathrin. The apposition of the two binding sites in arrestin2L suggests that they are exclusive and may function in higher order macromolecular structures. Biochemical analysis demonstrates direct binding of clathrin to the splice loop in arrestin2L, whereas functional analysis reveals that both binding domains contribute to the receptor-dependent redistribution of arrestin2L to clathrin-coated pits. Mutagenesis studies reveal that the clathrin binding motif in the splice loop is (L/I)(2)GXL. Taken together, these data provide a framework for understanding the dynamic interactions between arrestin2 and clathrin and reveal an essential role for this interaction in arrestin-mediated endocytosis.

GPC-1, a G protein gamma-subunit, regulates olfactory adaptation in Caenorhabditis elegans

Caenorhabditis elegans genome carries two Ggamma genes, gpc-1 and gpc-2, and two Gbeta genes, gpb-1 and gpb-2. Of these, gpc-2 and gpb-1 are expressed ubiquitously and are essential for viability. Through a genetic screen, we identified gpc-1 as essential for olfactory adaptation. While wild-type animals show decreased chemotaxis to the odorant benzaldehyde after a short preexposure to the odorant, gpc-1 mutants are still attracted to the odorant after the same preexposure. Cell-specific rescue experiments show that gpc-1 acts in the AWC olfactory neurons. Coexpression of GPC-1 and GPB-1, but not GPB-2, caused enhanced adaptation, indicating that GPC-1 may act with GPB-1. On the other hand, knock down of gpc-2 by cell-targeted RNAi caused reduced chemotaxis to the odorant in unadapted animals, indicating that GPC-2 mainly act for olfactory sensation and the two Ggamma's have differential functions. Nonetheless, overexpression of gpc-2 in AWC neurons rescued the adaptation defects of gpc-1 mutants, suggesting partially overlapping functions of the two Ggamma's. We further tested genetic interaction of gpc-1 with several other genes involved in olfactory adaptation. Our analyses place goa-1 Goalpha and let-60 Ras in parallel to gpc-1. In contrast, a gain-of-function mutation in egl-30 Gqalpha was epistatic to gpc-1, suggesting the possibility that gpc-1 Ggamma may act upstream of egl-30 Gqalpha.

A network of G-protein signaling pathways control neuronal activity in C. elegans

The Caenorhabditis elegans neuromuscular junction (NMJ) is one of the best studied synapses in any organism. A variety of genetic screens have identified genes required both for the essential steps of neurotransmitter release from motorneurons as well as the signaling pathways that regulate rates of neurotransmitter release. A number of these regulatory genes encode proteins that converge to regulate neurotransmitter release. In other cases genes are known to regulate signaling at the NMJ but how they act remains unknown. Many of the proteins that regulate activity at the NMJ participate in a network of heterotrimeric G-protein signaling pathways controlling the release of synaptic vesicles and/or dense-core vesicles (DCVs). At least four heterotrimeric G-proteins (Galphaq, Galpha12, Galphao, and Galphas) act within the motorneurons to control the activity of the NMJ. The Galphaq, Galpha12, and Galphao pathways converge to control production and destruction of the lipid-bound second messenger diacylglycerol (DAG) at sites of neurotransmitter release. DAG acts via at least two effectors, MUNC13 and PKC, to control the release of both neurotransmitters and neuropeptides from motorneurons. The Galphas pathway converges with the other three heterotrimeric G-protein pathways downstream of DAG to regulate neuropeptide release. Released neurotransmitters and neuropeptides then act to control contraction of the body-wall muscles to control locomotion. The lipids and proteins involved in these networks are conserved between C. elegans and mammals. Thus, the C. elegans NMJ acts as a model synapse to understand how neuronal activity in the human brain is regulated.

Targeting of beta-arrestin2 to the centrosome and primary cilium: role in cell proliferation control

Background

The primary cilium is a sensory organelle generated from the centrosome in quiescent cells and found at the surface of most cell types, from where it controls important physiological processes. Specific sets of membrane proteins involved in sensing the extracellular milieu are concentrated within cilia, including G protein coupled receptors (GPCRs). Most GPCRs are regulated by β-arrestins, βarr1 and βarr2, which control both their signalling and endocytosis, suggesting that βarrs may also function at primary cilium.

Methodology/Principal Findings

In cycling cells, βarr2 was observed at the centrosome, at the proximal region of the centrioles, in a microtubule independent manner. However, βarr2 did not appear to be involved in classical centrosome-associated functions. In quiescent cells, both in vitro and in vivo, βarr2 was found at the basal body and axoneme of primary cilia. Interestingly, βarr2 was found to interact and colocalize with 14-3-3 proteins and Kif3A, two proteins known to be involved in ciliogenesis and intraciliary transport. In addition, as suggested for other centrosome or cilia-associated proteins, βarrs appear to control cell cycle progression. Indeed, cells lacking βarr2 were unable to properly respond to serum starvation and formed less primary cilia in these conditions.

Conclusions/Significance

Our results show that βarr2 is localized to the centrosome in cycling cells and to the primary cilium in quiescent cells, a feature shared with other proteins known to be involved in ciliogenesis or primary cilium function. Within cilia, βarr2 may participate in the signaling of cilia-associated GPCRs and, therefore, in the sensory functions of this cell “antenna”.

Sensory roles of neuronal cilia: cilia development, morphogenesis, and function in C. elegans

In the free-living nematode Caenorhabditis elegans, cilia are found on the dendritic endings of sensory neurons. C. elegans cilia are classified as 'primary' or 'sensory' according to the '9+0' axonemal ultrastructure (nine doublet outer microtubules with no central microtubule pair) and lack of motility, characteristics of '9+2' cilia. The C. elegans ciliated nervous system allows the animal to perceive environmental stimuli and make appropriate developmental, physiological, and behavioral decisions. In vertebrates, the biological significance of primary cilia had been largely neglected. Recent findings have placed primary/sensory cilia in the center of cellular signaling and developmental processes. Studies using genetic model organisms such as C. elegans identified the link between ciliary dysfunction and human ciliopathies. Future studies in the worm will address important basic questions regarding ciliary development, morphogenesis, specialization, and signaling functions.

Regulation of receptor trafficking by GRKs and arrestins

To ensure that extracellular stimuli are translated into intracellular signals of appropriate magnitude and specificity, most signaling cascades are tightly regulated. One of the major mechanisms involved in the regulation of G protein-coupled receptors (GPCRs) involves their endocytic trafficking. GPCR endocytic trafficking entails the targeting of receptors to discrete endocytic sites at the plasma membrane, followed by receptor internalization and intracellular sorting. This regulates the level of cell surface receptors, the sorting of receptors to degradative or recycling pathways, and in some cases the specific signaling pathways. In this chapter we discuss the mechanisms that regulate receptor endocytic trafficking, emphasizing the role of GPCR kinases (GRKs) and arrestins in this process.

Arrestins: ubiquitous regulators of cellular signaling pathways

The arrestins are a small family of proteins that regulate the signaling and trafficking of G-protein-coupled receptors and also serve as ubiquitous signaling regulators in the cytoplasm and nucleus.

In vertebrates, the arrestins are a family of four proteins that regulate the signaling and trafficking of hundreds of different G-protein-coupled receptors (GPCRs). Arrestin homologs are also found in insects, protochordates and nematodes. Fungi and protists have related proteins but do not have true arrestins. Structural information is available only for free (unbound) vertebrate arrestins, and shows that the conserved overall fold is elongated and composed of two domains, with the core of each domain consisting of a seven-stranded β-sandwich. Two main intramolecular interactions keep the two domains in the correct relative orientation, but both of these interactions are destabilized in the process of receptor binding, suggesting that the conformation of bound arrestin is quite different. As well as binding to hundreds of GPCR subtypes, arrestins interact with other classes of membrane receptors and more than 20 surprisingly diverse types of soluble signaling protein. Arrestins thus serve as ubiquitous signaling regulators in the cytoplasm and nucleus.

Generation and modulation of chemosensory behaviors in C. elegans

C. elegans recognizes and discriminates among hundreds of chemical cues using a relatively compact chemosensory nervous system. Chemosensory behaviors are also modulated by prior experience and contextual cues. Because of the facile genetics and genomics possible in this organism, C. elegans provides an excellent system in which to explore the generation of chemosensory behaviors from the level of a single gene to the motor output. This review summarizes the current knowledge on the molecular and neuronal substrates of chemosensory behaviors and chemosensory behavioral plasticity in C. elegans.

Expression of mammalian GPCRs in C. elegans generates novel behavioural responses to human ligands

Background

G-protein-coupled receptors (GPCRs) play a crucial role in many biological processes and represent a major class of drug targets. However, purification of GPCRs for biochemical study is difficult and current methods of studying receptor-ligand interactions involve in vitro systems. Caenorhabditis elegans is a soil-dwelling, bacteria-feeding nematode that uses GPCRs expressed in chemosensory neurons to detect bacteria and environmental compounds, making this an ideal system for studying in vivo GPCR-ligand interactions. We sought to test this by functionally expressing two medically important mammalian GPCRs, somatostatin receptor 2 (Sstr2) and chemokine receptor 5 (CCR5) in the gustatory neurons of C. elegans.

Results

Expression of Sstr2 and CCR5 in gustatory neurons allow C. elegans to specifically detect and respond to somatostatin and MIP-1α respectively in a robust avoidance assay. We demonstrate that mammalian heterologous GPCRs can signal via different endogenous G_α subunits in C. elegans, depending on which cells it is expressed in. Furthermore, pre-exposure of GPCR transgenic animals to its ligand leads to receptor desensitisation and behavioural adaptation to subsequent ligand exposure, providing further evidence of integration of the mammalian GPCRs into the C. elegans sensory signalling machinery. In structure-function studies using a panel of somatostatin-14 analogues, we identified key residues involved in the interaction of somatostatin-14 with Sstr2.

Conclusion

Our results illustrate a remarkable evolutionary plasticity in interactions between mammalian GPCRs and C. elegans signalling machinery, spanning 800 million years of evolution. This in vivo system, which imparts novel avoidance behaviour on C. elegans, thus provides a simple means of studying and screening interaction of GPCRs with extracellular agonists, antagonists and intracellular binding partners.

Goalpha regulates olfactory adaptation by antagonizing Gqalpha-DAG signaling in Caenorhabditis elegans

The heterotrimeric G protein G(o) is abundantly expressed in the mammalian nervous system and modulates neural activities in response to various ligands. However, G(o)'s functions in living animals are less well understood. Here, we demonstrate that GOA-1 G(o)alpha has a fundamental role in olfactory adaptation in Caenorhabditis elegans. Impairment of GOA-1 G(o)alpha function and excessive activation of EGL-30 G(q)alpha cause a defect in adaptation to AWC-sensed odorants. These pathways antagonistically modulate olfactory adaptation in AWC chemosensory neurons. Wild-type animals treated with phorbol esters and double-mutant animals of diacylglycerol (DAG) kinases, dgk-3; dgk-1, also have a defect in adaptation, suggesting that elevated DAG signals disrupt normal adaptation. Constitutively active GOA-1 can suppress the adaptation defect of dgk-3; dgk-1 double mutants, whereas it fails to suppress the adaptation defect of animals with constitutively active EGL-30, implying that GOA-1 acts upstream of EGL-30 in olfactory adaptation. Our results suggest that down-regulation of EGL-30-DAG signaling by GOA-1 underlies olfactory adaptation and plasticity of chemotaxis.

Antagonistic sensory cues generate gustatory plasticity in Caenorhabditis elegans

Caenorhabditis elegans shows chemoattraction to 0.1-200 mM NaCl, avoidance of higher NaCl concentrations, and avoidance of otherwise attractive NaCl concentrations after prolonged exposure to NaCl (gustatory plasticity). Previous studies have shown that the ASE and ASH sensory neurons primarily mediate attraction and avoidance of NaCl, respectively. Here we show that balances between at least four sensory cell types, ASE, ASI, ASH, ADF and perhaps ADL, modulate the response to NaCl. Our results suggest that two NaCl-attraction signalling pathways exist, one of which uses Ca(2+)/cGMP signalling. In addition, we provide evidence that attraction to NaCl is antagonised by G-protein signalling in the ASH neurons, which is desensitised by the G-protein-coupled receptor kinase GRK-2. Finally, the response to NaCl is modulated by G-protein signalling in the ASI and ADF neurons, a second G-protein pathway in ASH and cGMP signalling in neurons exposed to the body fluid.

A Drosophila nonvisual arrestin is required for the maintenance of olfactory sensitivity

Nonvisual arrestins are a family of multifunctional adaptor molecules that regulate the activities of diverse families of receptors including G protein-coupled receptors, frizzled, and transforming growth factor-beta receptors. These activities indicate broad roles in both physiology and development for nonvisual arrestins. Drosophila melanogaster has a single nonvisual arrestin, kurtz, which is found at high levels within the adult olfactory receptor neurons (ORNs), suggesting a role for this gene in modulating olfactory sensitivity. Using heat-induced expression of a krz cDNA through development, we rescued krz(1) lethality. The resulting adults lacked detectable levels of krz in the olfactory system. The rescued krz(1) homozygotes have an incompletely penetrant antennal structural defect that was completely rescued by the neural expression of a krz cDNA. The krz(1) loss-of-function adults without visible antennal defects displayed diminished behavioral responsiveness to both aversive and attractive odors and also demonstrated reduced olfactory receptor potentials. Both the behavioral and electrophysiological phenotypes were rescued by the targeted expression of the krz cDNA within postdevelopmental ORNs. Thus, krz is required within the nervous system for antennal development and is required later in the ORNs for the maintenance of olfactory sensitivity in Drosophila. The reduced receptor potentials in krz(1) antenna indicate that nonvisual arrestins are required for the early odor-induced signaling events within the ORNs.