Functional variants in the sucrase-isomaltase gene associate with increased risk of irritable bowel syndrome.

OBJECTIVE: IBS is a common gut disorder of uncertain pathogenesis. Among other factors, genetics and certain foods are proposed to contribute. Congenital sucrase-isomaltase deficiency (CSID) is a rare genetic form of disaccharide malabsorption characterised by diarrhoea, abdominal pain and bloating, which are features common to IBS. We tested sucrase-isomaltase (SI) gene variants for their potential relevance in IBS. DESIGN: We sequenced SI exons in seven familial cases, and screened four CSID mutations (p.Val557Gly, p.Gly1073Asp, p.Arg1124Ter and p.Phe1745Cys) and a common SI coding polymorphism (p.Val15Phe) in a multicentre cohort of 1887 cases and controls. We studied the effect of the 15Val to 15Phe substitution on SI function in vitro. We analysed p.Val15Phe genotype in relation to IBS status, stool frequency and faecal microbiota composition in 250 individuals from the general population. RESULTS: CSID mutations were more common in patients than asymptomatic controls (p=0.074; OR=1.84) and Exome Aggregation Consortium reference sequenced individuals (p=0.020; OR=1.57). 15Phe was detected in 6/7 sequenced familial cases, and increased IBS risk in case-control and population-based cohorts, with best evidence for diarrhoea phenotypes (combined p=0.00012; OR=1.36). In the population-based sample, 15Phe allele dosage correlated with stool frequency (p=0.026) and Parabacteroides faecal microbiota abundance (p=0.0024). The SI protein with 15Phe exhibited 35% reduced enzymatic activity in vitro compared with 15Val (p<0.05). CONCLUSIONS: SI gene variants coding for disaccharidases with defective or reduced enzymatic activity predispose to IBS. This may help the identification of individuals at risk, and contribute to personalising treatment options in a subset of patients.

cGAL, a temperature-robust GAL4-UAS system for Caenorhabditis elegans.

The GAL4-UAS system is a powerful tool for manipulating gene expression, but its application in Caenorhabditis elegans has not been described. Here we systematically optimize the system's three main components to develop a temperature-optimized GAL4-UAS system (cGAL) that robustly controls gene expression in C. elegans from 15 to 25 °C. We demonstrate this system's utility in transcriptional reporter analysis, site-of-action experiments and exogenous transgene expression; and we provide a basic driver and effector toolkit.

Caenorhabditis elegans Male Copulation Circuitry Incorporates Sex-Shared Defecation Components To Promote Intromission and Sperm Transfer.

Sexual dimorphism can be achieved using a variety of mechanisms, including sex-specific circuits and sex-specific function of shared circuits, though how these work together to produce sexually dimorphic behaviors requires further investigation. Here, we explore how components of the sex-shared defecation circuitry are incorporated into the sex-specific male mating circuitry in Caenorhabditis elegans to produce successful copulation. Using behavioral studies, calcium imaging, and genetic manipulation, we show that aspects of the defecation system are coopted by the male copulatory circuitry to facilitate intromission and ejaculation. Similar to hermaphrodites, male defecation is initiated by an intestinal calcium wave, but circuit activity is coordinated differently during mating. In hermaphrodites, the tail neuron DVB promotes expulsion of gut contents through the release of the neurotransmitter GABA onto the anal depressor muscle. However, in the male, both neuron and muscle take on modified functions to promote successful copulation. Males require calcium-dependent activator protein for secretion (CAPS)/unc-31, a dense core vesicle exocytosis activator protein, in the DVB to regulate copulatory spicule insertion, while the anal depressor is remodeled to promote release of sperm into the hermaphrodite. This work shows how sex-shared circuitry is modified in multiple ways to contribute to sex-specific mating.

Genetic polymorphism in pathogenesis of irritable bowel syndrome.

Irritable bowel syndrome (IBS) is a complex symptom-based disorder without established biomarkers or putative pathophysiology. IBS is a common functional gastrointestinal disorder which is defined as recurrent abdominal pain or discomfort that has at least two of the following symptoms for 3 d per month in the past 3 mo according to ROME III: relief by defecation, onset associated with a change in stool frequency or onset with change in appearance or form of stool. Recent discoveries revealed genetic polymorphisms in specific cytokines and neuropeptides may possibly influence the frequencies and severity of symptoms, as well as the therapeutic responses in treating IBS patients. This review gives new insights on how genetic determinations influence in clinical manifestations, treatment responses and potential biomarkers of IBS.

Gene-environment interplay in Drosophila melanogaster: chronic nutritional deprivation in larval life affects adult fecal output.

Life history consequences of stress in early life are varied and known to have lasting impacts on the fitness of an organism. Gene-environment interactions play a large role in how phenotypic differences are mediated by stressful conditions during development. Here we use natural allelic 'rover/sitter' variants of the foraging (for) gene and chronic early life nutrient deprivation to investigate gene-environment interactions on excretion phenotypes. Excretion assay analysis and a fully factorial nutritional regimen encompassing the larval and adult life cycle of Drosophila melanogaster were used to assess the effects of larval and adult nutritional stress on adult excretion phenotypes. Natural allelic variants of for exhibited differences in the number of fecal spots when they were nutritionally deprived as larvae and well fed as adults. for mediates the excretion response to chronic early-life nutritional stress in mated female, virgin female, and male rovers and sitters. Transgenic manipulations of for in a sitter genetic background under larval but not adult food deprivation increases the number of fecal spots. Our study shows that food deprivation early in life affects adult excretion phenotypes and these excretion differences are mediated by for.

PKA controls calcium influx into motor neurons during a rhythmic behavior.

Cyclic adenosine monophosphate (cAMP) has been implicated in the execution of diverse rhythmic behaviors, but how cAMP functions in neurons to generate behavioral outputs remains unclear. During the defecation motor program in C. elegans, a peptide released from the pacemaker (the intestine) rhythmically excites the GABAergic neurons that control enteric muscle contractions by activating a G protein-coupled receptor (GPCR) signaling pathway that is dependent on cAMP. Here, we show that the C. elegans PKA catalytic subunit, KIN-1, is the sole cAMP target in this pathway and that PKA is essential for enteric muscle contractions. Genetic analysis using cell-specific expression of dominant negative or constitutively active PKA transgenes reveals that knockdown of PKA activity in the GABAergic neurons blocks enteric muscle contractions, whereas constitutive PKA activation restores enteric muscle contractions to mutants defective in the peptidergic signaling pathway. Using real-time, in vivo calcium imaging, we find that PKA activity in the GABAergic neurons is essential for the generation of synaptic calcium transients that drive GABA release. In addition, constitutively active PKA increases the duration of calcium transients and causes ectopic calcium transients that can trigger out-of-phase enteric muscle contractions. Finally, we show that the voltage-gated calcium channels UNC-2 and EGL-19, but not CCA-1 function downstream of PKA to promote enteric muscle contractions and rhythmic calcium influx in the GABAergic neurons. Thus, our results suggest that PKA activates neurons during a rhythmic behavior by promoting presynaptic calcium influx through specific voltage-gated calcium channels.

Identification of a functional TPH1 polymorphism associated with irritable bowel syndrome bowel habit subtypes.

OBJECTIVES: Alterations in 5-hydroxytryptamine (5-HT) signaling have been implicated as a factor contributing to the altered bowel habit of irritable bowel syndrome (IBS) patients. Tryptophan hydroxylase 1 (TPH1) is the rate-limiting enzyme in enterochromaffin cell 5-HT biosynthesis. We hypothesized that genetic variants affecting TPH1 gene expression might alter intestinal 5-HT bioavailability and subsequently the propensity for distinct bowel habit subtypes in IBS. In this study, we assessed the only common TPH1 proximal promoter variant (-347C/A; rs7130929) and its association with bowel habit predominance in IBS. METHODS: Electrophoretic mobility shift assays were performed to assess whether the -347C/A-allele variant affects the DNA binding of nuclear factors. Genotype distribution was determined for 422 IBS patients subtyped using the Rome III criteria and for 495 healthy controls recruited from two university medical centers. Association with bowel habit was tested using a multinomial logistic regression model controlling for race, anxiety, depression, and study site. RESULTS: Early growth response factor 1 (EGR-1) bound with higher affinity to a site comprising the minor A-allele of single-nucleotide polymorphism (SNP) -347C/A. TPH1 genotype frequencies did not differ between IBS patients and controls overall. The CC genotype was more prevalent in the IBS-D subtype (47%) than in the IBS-C (25%) and IBS-M (37%) subtypes (P=0.039) after adjusting for race and other covariates. Colonic biopsies from a small cohort of IBS patients from one center were tested for higher TPH1 mRNA expression in samples with CC compared with the CA genotype, but the results did not reach statistical significance. CONCLUSIONS: The TPH1 promoter SNP -347C/A differentially binds EGR-1 and correlates with IBS bowel habit subtypes and possibly colonic TPH1 expression consistent with its role in modulating intestinal 5-HT signaling.

The receptor TGR5 mediates the prokinetic actions of intestinal bile acids and is required for normal defecation in mice.

BACKGROUND & AIMS: Abnormal delivery of bile acids (BAs) to the colon as a result of disease or therapy causes constipation or diarrhea by unknown mechanisms. The G protein-coupled BA receptor TGR5 (or GPBAR1) is expressed by enteric neurons and endocrine cells, which regulate motility and secretion. METHODS: We analyzed gastrointestinal and colon transit, as well as defecation frequency and water content, in wild-type, knockout, and transgenic mice (trg5-wt, tgr5-ko, and tgr5-tg, respectively). We analyzed colon tissues for contractility, peristalsis, and transmitter release. RESULTS: Deoxycholic acid inhibited contractility of colonic longitudinal muscle from tgr5-wt but not tgr5-ko mice. Application of deoxycholic acid, lithocholic acid, or oleanolic acid (a selective agonist of TGR5) to the mucosa of tgr5-wt mice caused oral contraction and caudal relaxation, indicating peristalsis. BAs stimulated release of the peristaltic transmitters 5-hydroxytryptamine and calcitonin gene-related peptide; antagonists of these transmitters suppressed BA-induced peristalsis, consistent with localization of TGR5 to enterochromaffin cells and intrinsic primary afferent neurons. tgr5-ko mice did not undergo peristalsis or transmitter release in response to BAs. Mechanically induced peristalsis and transmitter release were not affected by deletion of tgr5. Whole-gut transit was 1.4-fold slower in tgr5-ko than tgr5-wt or tgr5-tg mice, whereas colonic transit was 2.2-fold faster in tgr5-tg mice. Defecation frequency was reduced 2.6-fold in tgr5-ko and increased 1.4-fold in tgr5-tg mice compared with tgr5-wt mice. Water content in stool was lower (37%) in tgr5-ko than tgr5-tg (58%) or tgr5-wt mice (62%). CONCLUSIONS: The receptor TGR5 mediates the effects of BAs on colonic motility, and deficiency of TGR5 causes constipation in mice. These findings might mediate the long-known laxative properties of BAs, and TGR5 might be a therapeutic target for digestive diseases.

Improving the efficiency of feed utilization in poultry by selection. 2. Genetic parameters of excretion traits and correlations with anatomy of the gastro-intestinal tract and digestive efficiency.

BACKGROUND: Poultry production has been widely criticized for its negative environmental impact related to the quantity of manure produced and to its nitrogen and phosphorus content. In this study, we investigated which traits related to excretion could be used to select chickens for lower environmental pollution.The genetic parameters of several excretion traits were estimated on 630 chickens originating from 2 chicken lines divergently selected on apparent metabolisable energy corrected for zero nitrogen (AMEn) at constant body weight. The quantity of excreta relative to feed consumption (CDUDM), the nitrogen and phosphorus excreted, the nitrogen to phosphorus ratio and the water content of excreta were measured, and the consequences of such selection on performance and gastro-intestinal tract (GIT) characteristics estimated. The genetic correlations between excretion, GIT and performance traits were established. RESULTS: Heritability estimates were high for CDUDM and the nitrogen excretion rate (0.30 and 0.29, respectively). The other excretion measurements showed low to moderate heritability estimates, ranging from 0.10 for excreta water content to 0.22 for the phosphorus excretion rate. Except for the excreta water content, the CDUDM was highly correlated with the excretion traits, ranging from -0.64 to -1.00. The genetic correlations between AMEn or CDUDM and the GIT characteristics were very similar and showed that a decrease in chicken excretion involves an increase in weight of the upper part of the GIT, and a decrease in the weight of the small intestine. CONCLUSION: In order to limit the environmental impact of chicken production, AMEn and CDUDM seem to be more suitable criteria to include in selection schemes than feed efficiency traits.

Stress-induced depressive behaviors are correlated with Par-4 and DRD2 expression in rat striatum.

Depression is a common mental disorder; however, its molecular mechanism has not been fully elucidated. In this study, we investigated the role of maternal deprivation (MD) and chronic mild stress (CMS) in the pathogenesis of depression in rat models. The mRNA levels of prostate apoptosis response-4 (Par-4) and dopamine receptor D2 (DRD2) genes in the striatum were measured by real-time PCR. Methylation level in the promoter of Par-4 gene was detected by bisulfite sequencing. Correlation between gene expression and depression-like behaviors were analyzed. Our results demonstrated that MD and CMS alone or their combination (dual stresses: DS) caused depression-like behaviors in rats. The mRNA levels of Par-4 and DRD2 genes in the striatum were significantly lower in MD-, CMS-, and DS-treated rats than in control rats. Importantly, Par-4 and DRD2 mRNA levels significantly correlated with depression-like behaviors. However, no significant differences in total methylation levels in the promoter of Par-4 gene were found between four groups. Our study suggested that either maternal deprivation or chronic mild stress plays a crucial role in the development of depression-like behaviors in rats. This process is associated with down-regulated Par-4 and DRD2 gene expression in the striatum through a non-methylation mechanism.

Childhood constipation; an overview of genetic studies and associated syndromes.

Constipation is a common problem in children but little is known about its exact pathophysiology. Environmental, behavioural but also genetic factors are thought to play a role in the aetiology of childhood constipation. We provide an overview of genetic studies performed in constipation. Until now, linkage studies, association studies and direct gene sequencing have failed to identify mutations in specific genes associated with constipation. We show that along with functional constipation, there are numerous clinical syndromes associated with childhood constipation. These syndromic forms of constipation appear to be the result of mutations in genes affecting all aspects of the normal physiology of human defecation. We stress that syndromic causes of childhood constipation should be considered in the evaluation of a constipated child.

ANK repeat-domain of SHN-1 Is indispensable for in vivo SHN-1 function in C. elegans.

Shank protein is one of the postsynaptic density (PSD) proteins which play a major role in proper localization of proteins at membranes. The shn-1, a homolog of Shank in Caenorhabditis elegans, is expressed in neurons, pharynx, intestine, vulva and sperm. We have previously reported a possible genetic interaction between Shank and IP3 receptor by examining shn-1 RNAi in IP3 receptor (itr-1) mutant background. In order to show the direct interaction of Shank and IP3 receptor as well as to show the direct in vivo function of Shank, we have characterized two different mutant alleles of shn-1, which have different deletions in the different domains. shn-1 mutants were observed for Ca²+-related behavioral defects with itr-1 mutants. We found that only shn-1 mutant defective in ANK repeat-domain showed significant defects in defecation, pharyngeal pumping and fertility. In addition, we found that shn-1 regulates defecation, pharyngeal pumping and probably male fertility with itr-1. Thus, we suggest that Shank ANK repeat-domain along with PDZ may play a crucial role in regulating Ca²+-signaling with IP3 receptor.

Paternal influence on female behavior: the role of Peg3 in exploration, olfaction, and neuroendocrine regulation of maternal behavior of female mice.

Genomic imprinting represents a mechanism through which parent-of-origin effects on offspring development may be mediated. However, investigation of the influence of imprinted genes on behavior has been limited. Here the authors investigate the role of the maternally imprinted/paternally expressed gene, Peg3, in several aspects of behavior using both 129Sv- and B6-Peg3 mutant female mice. Virgin Peg3 females on both genetic backgrounds were less exploratory and had higher rates of defecation with strain-dependent effects on activity levels and olfactory discrimination. Reproductive success, pup retrieval, and postnatal maternal care of pups were reduced in these females whereas indices of maternal aggression were higher among B6 Peg3-KO females. Differences in maternal care were apparent in females caring for biological or cross-fostered offspring and deficits in pup retrieval apparent beyond the immediate postpartum period. Oxytocin receptor binding in the MPOA and LS was reduced in Peg3-KO females. Thus, the authors demonstrate that disruptions to Peg3 influences aspects of female behavior that are critical for mediating maternal effects on offspring development, such as postpartum licking/grooming, and that effects of Peg3 are dependent on the maternal genetic background.

The non-neuronal syntaxin SYN-1 regulates defecation behavior and neural activity in C. elegans through interaction with the Munc13-like protein AEX-1.

We have previously shown that the AEX-1 protein, which is expressed in postsynaptic muscles, retrogradely regulates presynaptic neural activity at the Caenorhabditis elegans neuromuscular junctions. AEX-1 is similar to vertebrate Munc13-4 protein, suggesting a function for vesicle exocytosis from a kind of cells. Compared to emerging evidences of the role of Munc13 proteins in synaptic vesicle release, however, the precise mechanism for vesicle exocytosis by AEX-1 and Munc13-4 is little understood. Here we have identified SYN-1 as a candidate molecule of AEX-1-dependent vesicle exocytosis from non-neuronal cells. The syn-1 gene encodes a C. elegans syntaxin, which is distantly related to the neuronal syntaxin UNC-64. The syn-1 gene is predominantly expressed in non-neuronal tissues and genetically interacts with aex-1 for presynaptic activity. However, the two proteins did not interact physically in our yeast two-hybrid system and mutational SYN-1 did not bypass the requirement of AEX-1 for the behavioral defects in aex-1 mutants, whereas mutant UNC-64 does in unc-13 mutants. These results suggest that a novel molecular interaction between the AEX-1 and syntaxin may regulate vesicle exocytosis for retrograde signal release.

Three murine anxiety models: results from multiple inbred strain comparisons.

The literature surrounding rodent models of human anxiety disorders is discrepant concerning which models reflect anxiety-like behavior distinct from general activity and whether different models are measuring the same underlying constructs. This experiment compared the responses of 15 inbred mouse strains (129S1/SvlmJ, A/J, AKR/J, BALB/cByJ, C3H/HeJ, C57BL/6J, C57L/J, CBA/J, CE/J, DBA/2J, FVB/NJ, NZB/B1NJ, PL/J, SJL/J and SWR/J) in three anxiety-like behavioral tasks (light/dark test, elevated zero-maze and open field) to examine whether responses were phenotypically and/or genetically correlated across tasks. Significant strain differences were found for all variables examined. Principal components analyses showed that variables associated with both activity and anxiety-like behaviors loaded onto one factor, while urination and defecation loaded onto another factor. Our findings differ from previous research by suggesting that general activity and anxiety-related behaviors are linked, negatively correlated and cannot easily be dissociated in these assays. However, these findings may not necessarily generalize to other unconditioned anxiety-like behavioral tests.

Intestinal Ca2+ wave dynamics in freely moving C. elegans coordinate execution of a rhythmic motor program.

Defecation in the nematode worm Caenorhabditis elegans is a highly rhythmic behavior that is regulated by a Ca(2+) wave generated in the 20 epithelial cells of the intestine, in part through activation of the inositol 1,4,5-trisphosphate receptor. Execution of the defecation motor program (DMP) can be modified by external cues such as nutrient availability or mechanical stimulation. To address the likelihood that environmental regulation of the DMP requires integrating distinct cellular and organismal processes, we have developed a method for studying coordinate Ca(2+) oscillations and defecation behavior in intact, freely behaving animals. We tested this technique by examining how mutations in genes known to alter Ca(2+) handling [including egl-8/phospholipase C (PLC)-beta, kqt-3/KCNQ1, sca-1/sarco(endo)plasmic reticulum Ca(2+) ATPase, and unc-43/Ca(2+)-CaMKII] contribute to shaping the Ca(2+) wave and asked how Ca(2+) wave dynamics in the mutant backgrounds altered execution of the DMP. Notably, we find that Ca(2+) waves in the absence of PLCbeta initiate ectopically, often traveling in reverse, and fail to trigger a complete DMP. These results suggest that the normal supremacy of the posterior intestinal cells is not obligatory for Ca(2+) wave occurrence but instead helps to coordinate the DMP. Furthermore, we present evidence suggesting that an underlying pacemaker appears to oscillate at a faster frequency than the defecation cycle and that arrhythmia may result from uncoupling the pacemaker from the DMP rather than from disrupting the pacemaker itself. We also show that chronic elevations in Ca(2+) have limited influence on the defecation period but instead alter the interval between successive steps of the DMP. Finally, our results demonstrate that it is possible to assess Ca(2+) dynamics and muscular contractions in a completely unrestrained model organism.

Function of a STIM1 homologue in C. elegans: evidence that store-operated Ca2+ entry is not essential for oscillatory Ca2+ signaling and ER Ca2+ homeostasis.

1,4,5-trisphosphate (IP(3))-dependent Ca(2+) signaling regulates gonad function, fertility, and rhythmic posterior body wall muscle contraction (pBoc) required for defecation in Caenorhabditis elegans. Store-operated Ca(2+) entry (SOCE) is activated during endoplasmic reticulum (ER) Ca(2+) store depletion and is believed to be an essential and ubiquitous component of Ca(2+) signaling pathways. SOCE is thought to function to refill Ca(2+) stores and modulate Ca(2+) signals. Recently, stromal interaction molecule 1 (STIM1) was identified as a putative ER Ca(2+) sensor that regulates SOCE. We cloned a full-length C. elegans stim-1 cDNA that encodes a 530-amino acid protein with approximately 21% sequence identity to human STIM1. Green fluorescent protein (GFP)-tagged STIM-1 is expressed in the intestine, gonad sheath cells, and spermatheca. Knockdown of stim-1 expression by RNA interference (RNAi) causes sterility due to loss of sheath cell and spermatheca contractile activity required for ovulation. Transgenic worms expressing a STIM-1 EF-hand mutant that constitutively activates SOCE in Drosophila and mammalian cells are sterile and exhibit severe pBoc arrhythmia. stim-1 RNAi dramatically reduces STIM-1GFP expression, suppresses the EF-hand mutation-induced pBoc arrhythmia, and inhibits intestinal store-operated Ca(2+) (SOC) channels. However, stim-1 RNAi surprisingly has no effect on pBoc rhythm, which is controlled by intestinal oscillatory Ca(2+) signaling, in wild type and IP(3) signaling mutant worms, and has no effect on intestinal Ca(2+) oscillations and waves. Depletion of intestinal Ca(2+) stores by RNAi knockdown of the ER Ca(2+) pump triggers the ER unfolded protein response (UPR). In contrast, stim-1 RNAi fails to induce the UPR. Our studies provide the first detailed characterization of STIM-1 function in an intact animal and suggest that SOCE is not essential for certain oscillatory Ca(2+) signaling processes and for maintenance of store Ca(2+) levels in C. elegans. These findings raise interesting and important questions regarding the function of SOCE and SOC channels under normal and pathophysiological conditions.

What keeps C. elegans regular: the genetics of defecation.

Caenorhabditis elegans exhibits a repertoire of behaviors that can be studied by genetic, anatomical and pharmacological approaches. Defecation is one of the simpler behaviors, involving a small number of muscles, a couple of neurons and only one neurotransmitter. This simplicity enables the precise characterization of the cells and genes required for executing the behavior and has made the defecation behavior a powerful model for investigating the genetic basis of nervous system function, muscle differentiation, rhythmic behaviors and oscillatory calcium signaling, and the metabolic and environmental regulation of behavior. Our review highlights how the function of a system even this simple results from the integration of many aspects of an organism's biology and involves the action of diverse genes.

The Rho/Rac-family guanine nucleotide exchange factor VAV-1 regulates rhythmic behaviors in C. elegans.

Rhythmic behaviors are a fundamental feature of all organisms. Pharyngeal pumping, the defecation cycle, and gonadal-sheath-cell contractions are three well-characterized rhythmic behaviors in the nematode C. elegans. The periodicities of the rhythms range from subsecond (pharynx) to seconds (gonadal sheath) to minutes (defecation). However, the molecular mechanisms underlying these rhythmic behaviors are not well understood. Here, we show that the C. elegans Rho/Rac-family guanine nucleotide exchange factor, VAV-1, which is homologous to the mammalian Vav proto-oncogene, has a crucial role in all three behaviors. vav-1 mutants die as larvae because VAV-1 function is required in the pharynx for synchronous contraction of the musculature. In addition, ovulation and the defecation cycle are abnormal and arrhythmic. We show that Rho/Rac-family GTPases and the signaling molecule inositol triphosphate (IP(3)) act downstream of VAV-1 signaling and that the VAV-1 pathway modulates rhythmic behaviors by dynamically regulating the concentration of intracellular Ca(2+).

Specification of muscle neurotransmitter sensitivity by a Paired-like homeodomain protein in Caenorhabditis elegans.

The effects of neurotransmitters depend on the receptors expressed on the target cells. In Caenorhabditis elegans, there are two types of GABA receptors that elicit opposite effects: excitatory receptors that open cation-selective channels, and inhibitory receptors that open anion-selective channels. The four non-striated enteric muscle cells required for the expulsion step of the defecation behavior are all sensitive to GABA: the sphincter muscle expresses a classical GABA-sensitive chloride channel (UNC-49) and probably relaxes in response to GABA, while the other three cells express a cation-selective channel (EXP-1) and contract. Here we show that the expression of the exp-1 gene is under the control of dsc-1, which encodes a Paired-like homeodomain protein, a class of transcription factors previously associated with the terminal differentiation of neurons in C. elegans. dsc-1 mutants have anatomically normal enteric muscles but are expulsion defective. We show that this defect is due to the lack of expression of exp-1 in the three cells that contract in response to GABA. In addition, dsc-1, but not exp-1, affects the periodicity of the behavior, revealing an unanticipated role for the enteric muscles in regulating this ultradian rhythm.