A Forward Genetic Screen for Molecules Involved in Pheromone-Induced Dauer Formation in Caenorhabditis elegans

Animals must constantly assess their surroundings and integrate sensory cues to make appropriate behavioral and developmental decisions. Pheromones produced by conspecific individuals provide critical information regarding environmental conditions. Ascaroside pheromone concentration and composition are instructive in the decision of Caenorhabditis elegans to either develop into a reproductive adult or enter into the stress-resistant alternate dauer developmental stage. Pheromones are sensed by a small set of sensory neurons, and integrated with additional environmental cues, to regulate neuroendocrine signaling and dauer formation. To identify molecules required for pheromone-induced dauer formation, we performed an unbiased forward genetic screen and identified phd (pheromone response-defective dauer) mutants. Here, we describe new roles in dauer formation for previously identified neuronal molecules such as the WD40 domain protein QUI-1 and MACO-1 Macoilin, report new roles for nociceptive neurons in modulating pheromone-induced dauer formation, and identify tau tubulin kinases as new genes involved in dauer formation. Thus, phd mutants define loci required for the detection, transmission, or integration of pheromone signals in the regulation of dauer formation. © 2016 Neal et al.1

The Evolutionarily Conserved LIM Homeodomain Protein LIM-4/LHX6 Specifies the Terminal Identity of a Cholinergic and Peptidergic C. elegans Sensory/Inter/Motor Neuron-Type

The expression of specific transcription factors determines the differentiated features of postmitotic neurons. However, the mechanism by which specific molecules determine neuronal cell fate and the extent to which the functions of transcription factors are conserved in evolution are not fully understood. In C. elegans, the cholinergic and peptidergic SMB sensory/inter/motor neurons innervate muscle quadrants in the head and control the amplitude of sinusoidal movement. Here we show that the LIM homeobox protein LIM-4 determines neuronal characteristics of the SMB neurons. In lim-4 mutant animals, expression of terminal differentiation genes, such as the cholinergic gene battery and the flp-12 neuropeptide gene, is completely abolished and thus the function of the SMB neurons is compromised. LIM-4 activity promotes SMB identity by directly regulating the expression of the SMB marker genes via a distinct cis-regulatory motif. Two human LIM-4 orthologs, LHX6 and LHX8, functionally substitute for LIM-4 in C. elegans. Furthermore, C. elegans LIM-4 or human LHX6 can induce cholinergic and peptidergic characteristics in the human neuronal cell lines. Our results indicate that the evolutionarily conserved LIM-4/LHX6 homeodomain proteins function in generation of precise neuronal subtypes

Diacetyl odor shortens longevity conferred by food deprivation in C. elegans via downregulation of DAF-16/FOXO

Dietary restriction extends lifespan in various organisms by reducing the levels of both nutrients and non-nutritional food-derived cues. However, the identity of specific food-derived chemical cues that alter lifespan remains unclear. Here, we identified several volatile attractants that decreased the longevity on food deprivation, a dietary restriction regimen in Caenorhabditis elegans. In particular, we found that the odor of diacetyl decreased the activity of DAF-16/FOXO, a life-extending transcription factor acting downstream of insulin/IGF-1 signaling. We then demonstrated that the odor of lactic acid bacteria, which produce diacetyl, reduced the nuclear accumulation of DAF-16/FOXO. Unexpectedly, we showed that the odor of diacetyl decreased longevity independently of two established diacetyl receptors, ODR-10 and SRI-14, in sensory neurons. Thus, diacetyl, a food-derived odorant, may shorten food deprivation-induced longevity via decreasing the activity of DAF-16/FOXO through binding to unidentified receptors. © 2020 The Authors. Aging Cell published by the Anatomical Society and John Wiley & Sons Ltd.1

Chemosensory GPCRSRI-14 are required for concentraion dependent odor preference in C. elegans

Animals must recognize and discriminate among thousands of chemicals in order to generate the correct behavioral response. Understanding basic design of a sensory system in simple animals gives the opportunity to elucidate detailed molecular and neural mechanisms underlying sensory responses in higher animals. C. elegans detects a large number of odorants via three neurons pairs including the AWC, and elicit a multitude of olfactory behaviors (Bargmann, 1993, Cell). Previous genetic and behavioral experiments have identified set of signaling genes including olfactory receptors, but the knowledge is still limited. Specifically, the mechanisms of how the same odorants can elicit either attractive or aversive responses depending on the chemical concentration are not known yet. First, we are trying to construct a comprehensive map of odorants and their receptors in C. elegans. We screened more than 29 volatile chemicals that are not tested previously, and found that animals respond to 13volatiles. We further identified that the AWC neurons are required for chemotactic responses to these chemicals. We then performed candidate gene searches and found that the chemosensory GPCR mutants sra-13, str-2, or a un-linked mutant lsk46 exhibit specific defects in chemotactic responses to 2-Furyl methyl ketone, Ethyl pyruvate, or 1- propanol, respectively. Interestingly, we also found that chemosensory GPCR sri-14 are required for both attraction to low concentration DMTS and aversion to high concentration DMTS. The defect of DMTS chemotaxis in sri-14 mutants were restored when we expressed the wild-type sri-14 gene to the AWC neurons. We next found that Ca2+ response of AWC to low concentration DMTS was decreased in sri-14 mutants. We are currently measuring the Ca2+ response of ASH to high concentration DMTS compared to the sri-14, and trying to investigate the neural circuitry mechanism how the SRI-14 regulates both aversive and chemotactic behaviors in C. elegans

Cooperative transcriptional activation of ATP-binding cassette sterol transporters ABCG5 and ABCG8 genes by nuclear receptors including Liver-X-Receptor

The ATP-binding cassette transporters ABCG5 and ABCG8 formheterodimers that limit absorption of dietary sterols in theintestine and promote cholesterol elimination from the bodythrough hepatobiliary secretion. To identify cis-regulatoryelements of the two genes, we have cloned and analyzedtwenty-three evolutionary conserved region (ECR) fragmentsusing the CMV-luciferase reporter system in HepG2 cells. TwoECRs were found to be responsive to the Liver-X-Receptor (LXR).Through elaborate deletion studies, regions containing putativeLXREs were identified and the binding of LXRα wasdemonstrated by EMSA and ChIP assay. When the LXREs wereinserted upstream of the intergenic promoter, synergisticactivation by LXRα/RXRα in combination with GATA4, HNF4α,and LRH-1, which had been shown to bind to the intergenicregion, was observed. In conclusion, we have identified twoLXREs in ABCG5/ABCG8 genes for the first time and proposethat these LXREs, especially in the ECR20, play major roles inregulating these genes. [BMB Reports 2013; 46(6): 322-327

Neuropeptide Signaling Regulates Pheromone-Mediated Gene Expression of a Chemoreceptor Gene in C. elegans

Animals need to be able to alter their developmental and behavioral programs in response to changing environmental conditions. This developmental and behavioral plasticity is mainly mediated by changes in gene expression. The knowledge of the mechanisms by which environmental signals are transduced and integrated to modulate changes in sensory gene expression is limited. Exposure to ascaroside pheromone has been reported to alter the expression of a subset of putative G protein-coupled chemosensory receptor genes in the ASI chemosensory neurons of C. elegans (Kim et al., 2009; Nolan et al., 2002; Peckol et al., 1999). Here we show that ascaroside pheromone reversibly represses expression of the str-3 chemoreceptor gene in the ASI neurons. Repression of str-3 expression can be initiated only at the L1 stage, but expression is restored upon removal of ascarosides at any developmental stage. Pheromone receptors including SRBC-64/66 and SRG-36/37 are required for str-3 repression. Moreover, pheromone-mediated str-3 repression is mediated by FLP-18 neuropeptide signaling via the NPR-1 neuropeptide receptor. These results suggest that environmental signals regulate chemosensory gene expression together with internal neuropeptide signals which, in turn, modulate behavior.1

Electrical Properties of Metal/4H-SiC Junction Modulated by Graphene Insertion Layer

We study the electrical characteristics of metal/graphene/SiC junctions by investigating the Schottky barriers for two different metal (Ti, Au) electrodes. The Fermi-level pinning for Schottky junctions on SiC (Silicon Carbide) are known to be relatively weak. However, by inserting a graphene layer at the interface, the Schottky barrier shows the inverse dependence on the metal work function, which leads to the so-called negative Fermi-level pinning. This unusual effect is confirmed from current-voltage (I-V), internal photoemission (IPE), and capacitance-voltage (C-V) measurements. Interestingly, the Schottky barriers of metal/graphene/SiC junctions extracted from C-V measurements are somewhat larger than the barriers obtained from other measurements, considered to be related to the charge carrier transfer into the graphene layer. Additionally, we observe two energy barriers originating from the crystal field splitting of SiC in the IPE measurements and they exist commonly for both metal/SiC and metal/graphene/SiC junctions