HSN Neurons Research Articles

A specific folate activates serotonergic neurons to control C. elegans behavior

Folates are B-group vitamins that function in one-carbon metabolism. Here we show that a specific folate can activate serotonergic neurons in C. elegans to modulate behavior through a pathway that requires the folate receptor FOLR-1 and the GON-2 calcium channel. FOLR-1 and GON-2 physically interact in a heterologous system, and both are expressed in the HSN and NSM serotonergic neurons. Both the folate 10-formyl-THF and a non-metabolic pteroate induce increases in the number of Ca2+ transients in the HSN neurons and egg laying in an FOLR-1- and GON-2-dependent manner. FOLR-1 and GON-2 are required for the activation of the NSM neurons in response to 10-formyl-THF, and for full NSM-mediated stoppage of movement when starved animals encounter bacteria. Our results demonstrate that FOLR-1 acts independently of one-carbon metabolism and suggest that 10-formyl-THF acts as a dietary signal that activates serotonergic neurons to impact behavior through a pathway that involves calcium entry.

Silica Nanoparticles Disclose a Detailed Neurodegeneration Profile throughout the Life Span of a Model Organism.

The incidence of age-related neurodegenerative diseases is rising globally. However, the temporal sequence of neurodegeneration throughout adult life is poorly understood. To identify the starting points and schedule of neurodegenerative events, serotonergic and dopaminergic neurons were monitored in the model organism C. elegans, which has a life span of 2-3 weeks. Neural morphology was examined from young to old nematodes that were exposed to silica nanoparticles. Young nematodes showed phenotypes such as dendritic beading of serotonergic and dopaminergic neurons that are normally not seen until late life. During aging, neurodegeneration spreads from specifically susceptible ADF and PDE neurons in young C. elegans to other more resilient neurons, such as dopaminergic CEP in middle-aged worms. Investigation of neurodegenerative hallmarks and animal behavior revealed a temporal correlation with the acceleration of neuromuscular defects, such as internal hatch in 2-day-old C. elegans. Transcriptomics and proteomics of young worms exposed to nano silica showed a change in gene expression concerning the gene ontology groups serotonergic and dopaminergic signaling as well as neuropeptide signaling. Consistent with this, reporter strains for nlp-3, nlp-14 and nlp-21 confirmed premature degeneration of the serotonergic neuron HSN and other neurons in young C. elegans. The results identify young nematodes as a vulnerable age group for nano silica-induced neural defects with a significantly reduced health span. Neurodegeneration of specific neurons impairs signaling by classical neurotransmitters as well as neuropeptides and compromises related neuromuscular behaviors in critical phases of life, such as the reproductive phase.

A single neuron in C. elegans orchestrates multiple motor outputs through parallel modes of transmission

Animals generate a wide range of highly coordinated motor outputs, which allows them to execute purposeful behaviors. Individual neurons in the circuits that generate behaviors have a remarkable capacity for flexibility as they exhibit multiple axonal projections, transmitter systems, and modes of neural activity. How these multi-functional properties of neurons enable the generation of adaptive behaviors remains unknown. Here, we show that the HSN neuron in C.elegans evokes multiple motor programs over different timescales to enable a suite of behavioral changes during egg laying. Using HSN activity perturbations and invivo calcium imaging, we show that HSN acutely increases egg laying and locomotion while also biasing the animals toward low-speed dwelling behavior over minutes. The acute effects of HSN on egg laying and high-speed locomotion are mediated by separate sets of HSN transmitters and different HSN axonal compartments. The long-lasting effects on dwelling are mediated in part by HSN release of serotonin, which is taken up and re-released by NSM, another serotonergic neuron class that directly evokes dwelling. Our results show how the multi-functional properties of a single neuron allow it to induce a coordinated suite of behaviors and also reveal that neurons can borrow serotonin from one another to control behavior.

Serotonin signals through postsynaptic Gαq, Trio RhoGEF, and diacylglycerol to promote Caenorhabditis elegans egg-laying circuit activity and behavior.

Activated Gαq signals through phospholipase-Cβ and Trio, a Rho GTPase exchange factor (RhoGEF), but how these distinct effector pathways promote cellular responses to neurotransmitters like serotonin remains poorly understood. We used the egg-laying behavior circuit of Caenorhabditis elegans to determine whether phospholipase-Cβ and Trio mediate serotonin and Gαq signaling through independent or related biochemical pathways. Our genetic rescue experiments suggest that phospholipase-Cβ functions in neurons while Trio Rho GTPase exchange factor functions in both neurons and the postsynaptic vulval muscles. While Gαq, phospholipase-Cβ, and Trio Rho GTPase exchange factor mutants fail to lay eggs in response to serotonin, optogenetic stimulation of the serotonin-releasing HSN neurons restores egg laying only in phospholipase-Cβ mutants. Phospholipase-Cβ mutants showed vulval muscle Ca2+ transients while strong Gαq and Trio Rho GTPase exchange factor mutants had little or no vulval muscle Ca2+ activity. Treatment with phorbol 12-myristate 13-acetate that mimics 1,2-diacylglycerol, a product of PIP2 hydrolysis, rescued egg-laying circuit activity and behavior defects of Gαq signaling mutants, suggesting both phospholipase-C and Rho signaling promote synaptic transmission and egg laying via modulation of 1,2-diacylglycerol levels. 1,2-Diacylglycerol activates effectors including UNC-13; however, we find that phorbol esters, but not serotonin, stimulate egg laying in unc-13 and phospholipase-Cβ mutants. These results support a model where serotonin signaling through Gαq, phospholipase-Cβ, and UNC-13 promotes neurotransmitter release, and that serotonin also signals through Gαq, Trio Rho GTPase exchange factor, and an unidentified, phorbol 12-myristate 13-acetate-responsive effector to promote postsynaptic muscle excitability. Thus, the same neuromodulator serotonin can signal in distinct cells and effector pathways to coordinate activation of a motor behavior circuit.

Coordinated Behavioral and Physiological Responses to a Social Signal Are Regulated by a Shared Neuronal Circuit.

Successful reproduction in animals requires orchestration of behavior and physiological processes. Pheromones can induce both "releaser" (behavioral) and "priming" (physiological) effects [1] in vertebrates [2, 3] and invertebrates [4, 5]. Therefore, understanding the mechanisms underlying pheromone responses could reveal how reproduction-related behaviors and physiology are coordinated. Here, we describe a neuronal circuit that couples the reproductive system and behavior in adult Caenorhabditis elegans hermaphrodites. We found that the response of the oogenic germline to the male pheromone requires serotonin signal from NSM and HSN neurons that acts via the mod-1 receptor in AIY and RIF interneurons and is antagonized by pigment-dispersing factor (PDF). Surprisingly, the same neurons and pathways have been previously implicated in regulation of exploratory behavior in the absence of male-produced signals [6]. We demonstrate that male pheromone acts via this circuit in hermaphrodites to reduce exploration and decrease mating latency, thereby tuning multiple fitness-proximal processes. Our results demonstrate how a single circuit could coordinate behavioral and physiological responses to the environment, even those that unfold on different timescales. Our findings suggest the existence of a centralized regulatory mechanism that balances organismal resources between reproductive investment and somatic maintenance.

A transcription factor collective defines the HSN serotonergic neuron regulatory landscape

Cell differentiation is controlled by individual transcription factors (TFs) that together activate a selection of enhancers in specific cell types. How these combinations of TFs identify and activate their target sequences remains poorly understood. Here, we identify the cis-regulatory transcriptional code that controls the differentiation of serotonergic HSN neurons in Caenorhabditis elegans. Activation of the HSN transcriptome is directly orchestrated by a collective of six TFs. Binding site clusters for this TF collective form a regulatory signature that is sufficient for de novo identification of HSN neuron functional enhancers. Among C. elegans neurons, the HSN transcriptome most closely resembles that of mouse serotonergic neurons. Mouse orthologs of the HSN TF collective also regulate serotonergic differentiation and can functionally substitute for their worm counterparts which suggests deep homology. Our results identify rules governing the regulatory landscape of a critically important neuronal type in two species separated by over 700 million years.

Neuroligin tuning of pharyngeal pumping reveals extrapharyngeal modulation of feeding in Caenorhabditis elegans.

The integration of distinct sensory modalities is essential for behavioural decision making. In Caenorhabditiselegans, this process is coordinated by neural circuits that integrate sensory cues from the environment to generate an appropriate behaviour at the appropriate output muscles. Food is a multimodal cue that impacts the microcircuits to modulate feeding and foraging drivers at the level of the pharyngeal and body wall muscle, respectively. When food triggers an upregulation in pharyngeal pumping, it allows the effective ingestion of food. Here, we show that a Celegans mutant in the single gene orthologous to human neuroligins, nlg-1, is defective in food-induced pumping. This was not due to an inability to sense food, as nlg-1 mutants were not defective in chemotaxis towards bacteria. In addition, we found that neuroligin is widely expressed in the nervous system, including AIY, ADE, ALA, URX and HSN neurons. Interestingly, despite the deficit in pharyngeal pumping, neuroligin was not expressed within the pharyngeal neuromuscular network, which suggests an extrapharyngeal regulation of this circuit. We resolved electrophysiologically the neuroligin contribution to the pharyngeal circuit by mimicking food-dependent pumping and found that the nlg-1 phenotype is similar to mutants impaired in GABAergic and/or glutamatergic signalling. We suggest that neuroligin organizes extrapharyngeal circuits that regulate the pharynx. These observations based on the molecular and cellular determinants of feeding are consistent with the emerging role of neuroligin in discretely impacting functional circuits underpinning complex behaviours.

Anti-amyloid compounds protect from silica nanoparticle-induced neurotoxicity in the nematode C. elegans

Identifying nanomaterial-bio-interactions are imperative due to the broad introduction of nanoparticle (NP) applications and their distribution. Here, we demonstrate that silica NPs effect widespread protein aggregation in the soil nematode Caenorhabditis elegans ranging from induction of amyloid in nucleoli of intestinal cells to facilitation of protein aggregation in body wall muscles and axons of neural cells. Proteomic screening revealed that exposure of adult C. elegans with silica NPs promotes segregation of proteins belonging to the gene ontology (GO) group of “protein folding, proteolysis and stress response” to an SDS-resistant aggregome network. Candidate proteins in this group include chaperones, heat shock proteins and subunits of the 26S proteasome which are all decisively involved in protein homeostasis. The pathway of protein homeostasis was validated as a major target of silica NPs by behavioral phenotyping, as inhibitors of amyloid formation rescued NP-induced defects of locomotory patterns and egg laying. The analysis of a reporter worm for serotonergic neural cells revealed that silica NP-induced protein aggregation likewise occurs in axons of HSN neurons, where presynaptic accumulation of serotonin, e.g. disturbed axonal transport reduces the capacity for neurotransmission and egg laying. The results suggest that in C. elegans silica NPs promote a cascade of events including disturbance of protein homeostasis, widespread protein aggregation and inhibition of serotonergic neurotransmission which can be interrupted by compounds preventing amyloid fibrillation.

SAX-3 (Robo) and UNC-40 (DCC) regulate a directional bias for axon guidance in response to multiple extracellular cues.

Axons in Caenorhabditis elegans are guided by multiple extracellular cues, including UNC-6 (netrin), EGL-20 (wnt), UNC-52 (perlecan), and SLT-1 (slit). How multiple extracellular cues determine the direction of axon guidance is not well understood. We have proposed that an axon's response to guidance cues can be modeled as a random walk, i.e., a succession of randomly directed movement. Guidance cues dictate the probability of axon outgrowth activity occurring in each direction, which over time creates a directional bias. Here we provide further evidence for this model. We describe the effects that the UNC-40 (DCC) and SAX-3 (Robo) receptors and the UNC-6, EGL-20, UNC-52, and SLT-1 extracellular cues have on the directional bias of the axon outgrowth activity for the HSN and AVM neurons. We find that the directional bias created by the cues depend on UNC-40 or SAX-3. UNC-6 and EGL-20 affect the directional bias for both neurons, whereas UNC-52 and SLT-1 only affect the directional bias for HSN and AVM, respectively. The direction of the bias created by the loss of a cue can vary and the direction depends on the other cues. The random walk model predicts this combinatorial regulation. In a random walk a probability is assigned for each direction of outgrowth, thus creating a probability distribution. The probability distribution for each neuron is determined by the collective effect of all the cues. Since the sum of the probabilities must equal one, each cue affects the probability of outgrowth in multiple directions.

Experimental evidence for UNC-6 (netrin) axon guidance by stochastic fluctuations of intracellular UNC-40 (DCC) outgrowth activity

SummaryHow the direction of axon guidance is determined is not understood. In Caenorhabditis elegans the UNC-40 (DCC) receptor mediates a response to the UNC-6 (netrin) guidance cue that directs HSN axon development. UNC-40 becomes asymmetrically localized within the HSN neuron to the site of axon outgrowth. Here we provide experimental evidence that the direction of guidance can be explained by the stochastic fluctuations of UNC-40 asymmetric outgrowth activity. We find that the UNC-5 (UNC5) receptor and the cytoskeletal binding protein UNC-53 (NAV2) regulate the induction of UNC-40 localization by UNC-6. If UNC-40 localization is induced without UNC-6 by using an unc-53 mutation, the direction of UNC-40 localization undergoes random fluctuations. Random walk models describe the path made by a succession of randomly directed movement. This model was experimentally tested using mutations that affect Wnt/PCP signaling. These mutations inhibit UNC-40 localization in the anterior and posterior directions. As the axon forms in Wnt/PCP mutants, the direction of UNC-40 localization randomly fluctuates; it can localize in either the anterior, posterior, or ventral direction. Consistent with a biased random walk, over time the axon will develop ventrally in response to UNC-6, even though at a discrete time UNC-40 localization and outgrowth can be observed anterior or posterior. Also, axon formation is slower in the mutants than in wild-type animals. This is also consistent with a random walk since this model predicts that the mean square displacement (msd) will increase only linearly with time, whereas the msd increases quadratically with time for straight-line motion.

The novel Rac effector RIN-1 regulates neuronal cell migration and axon pathfinding inC. elegans

Cell migration and axon guidance require proper regulation of the actin cytoskeleton in response to extracellular guidance cues. Rho/Rac small GTPases are essential regulators of actin remodeling. Caenorhabditis elegans CED-10 is a Rac1 homolog that is required for various cellular morphological changes and migration events and is under the control of several guidance signaling pathways. There is still considerable uncertainty regarding events following the activation of guidance receptors by extracellular signals and the regulation of actin dynamics based on spatiotemporally restricted Rac activity. Here we show that the VPS9 domain protein RIN-1 acts as a novel effector for CED-10 in C. elegans. The orthologous mammalian Rin1 protein has previously been identified as an effector for Ras GTPase and is now known to function as a guanine nucleotide exchange factor for Rab5 GTPase. We found that RIN-1 specifically binds to the GTP-bound form of CED-10 and that mutations in rin-1 cause significant defects in migration and axon guidance of restricted neuronal cell types including AVM and HSN neurons, in contrast to the various defects observed in ced-10 mutants. Our analyses place RIN-1 in the Slit-Robo genetic pathway that regulates repulsive signaling for dorsoventral axon guidance. In rin-1 mutants, actin accumulated on both the ventral and dorsal sides of the developing HSN neuron, in contrast to its ventral accumulation in wild type. These results strongly suggest that RIN-1 acts as an effector for CED-10/Rac1 and regulates actin remodeling in response to restricted guidance cues.

The novel Rac effector RIN-1 regulates neuronal cell migration and axon pathfinding in C. elegans

Cell migration and axon guidance require proper regulation of the actin cytoskeleton in response to extracellular guidance cues. Rho/Rac small GTPases are essential regulators of actin remodeling. Caenorhabditis elegans CED-10 is a Rac1 homolog that is required for various cellular morphological changes and migration events and is under the control of several guidance signaling pathways. There is still considerable uncertainty regarding events following the activation of guidance receptors by extracellular signals and the regulation of actin dynamics based on spatiotemporally restricted Rac activity. Here we show that the VPS9 domain protein RIN-1 acts as a novel effector for CED-10 in C. elegans. The orthologous mammalian Rin1 protein has previously been identified as an effector for Ras GTPase and is now known to function as a guanine nucleotide exchange factor for Rab5 GTPase. We found that RIN-1 specifically binds to the GTP-bound form of CED-10 and that mutations in rin-1 cause significant defects in migration and axon guidance of restricted neuronal cell types including AVM and HSN neurons, in contrast to the various defects observed in ced-10 mutants. Our analyses place RIN-1 in the Slit-Robo genetic pathway that regulates repulsive signaling for dorsoventral axon guidance. In rin-1 mutants, actin accumulated on both the ventral and dorsal sides of the developing HSN neuron, in contrast to its ventral accumulation in wild type. These results strongly suggest that RIN-1 acts as an effector for CED-10/Rac1 and regulates actin remodeling in response to restricted guidance cues.

A mutation in a CLC anion channel alters serotonergic neuronal activity in C. elegans

Egg‐laying in C. elegans is controlled by the HSN neurons, which release serotonin to activate egg‐laying muscles. The dominant egl‐42(n995) mutation prevents serotonin release and inhibits egg‐laying. Intracellular Ca2+ imaging demonstrated that HSN activity is reduced in egl‐42 mutants. Whole genome sequencing revealed that egl‐42(n995) is a point mutation in the clh‐3 gene, which encodes the CLC anion channel splice variants CLH‐3a and CLH‐3b. Overexpression of both wild‐type and mutant clh‐3 phenocopies the egl‐42(n995) egg‐laying defect, while clh‐3 deletion leads to hyperactive egg‐laying. The n995 allele is a valine to alanine mutation that is located in membrane helix P, which contributes to the monomer‐monomer interface in CLC homodimers. This residue is highly conserved in all human CLC proteins except CLC‐Ka and CLC‐Kb. CLH‐3a and CLH‐3b mutant channels exhibit increased current amplitude, a strongly depolarized activation voltage, and more rapid voltage dependent activation kinetics. CLH‐3a and CLH‐3b mutants also exhibit loss of pre‐potentiation behavior and greatly reduced sensitivity to inhibition by GCK‐3 kinase, respectively. Our results demonstrate 1) that CLC helix P plays an important role in controlling channel voltage sensitivity and 2) that clh‐3 encoded channels likely regulate HSN membrane potential and hence Ca2+ entry and neuronal excitability.

The Liprin Homology Domain Is Essential for the Homomeric Interaction of SYD-2/Liprin-α Protein in Presynaptic Assembly

Synapses are asymmetric structures that are specialized for neuronal signal transduction. A unique set of proteins is present at the presynaptic active zone, which is a core structure essential for neurotransmitter release. In Caenorhabditis elegans HSN neurons, SYD-2, a Liprin-α family protein, acts together with a GAP protein SYD-1 to promote presynaptic assembly. Previous studies have shown that elevating the activity of syd-2 can bypass the requirement of syd-1. Liprin-α proteins are composed of coiled-coil-rich regions in the N-terminal half, which mediate interactions with adapter proteins at the presynaptic active zone, and three SAM domains in the C terminus, which bind proteins such as LAR receptor tyrosine phosphatase. To address the molecular mechanism by which SYD-2 activity is regulated, we performed structure-function studies. By monitoring the ability of SYD-2 transgenes to rescue syd-2(lf) and to suppress syd-1(lf) phenotypes in HSN neuron synapses, we identified the N-terminal half of SYD-2 as minimally required for rescuing syd-2(lf) phenotypes. A highly conserved short coiled-coil segment named Liprin Homology 1 (LH1) domain is both necessary and sufficient to suppress syd-1(lf) defects. We show that the LH1 domain forms a dimer and promotes further oligomerization and/or complex formation of SYD-2/Liprin-α proteins. The role of the LH1 domain in presynaptic assembly can be partially complemented by artificial dimerization. These findings suggest a model by which the self-assembly of SYD-2/Liprin-α proteins mediated by the coiled-coil LH1 domain is one of the key steps to the accumulation of presynaptic components at nascent synaptic junctions.

A circuit model of the temporal pattern generator of Caenorhabditis egg-laying behavior

BackgroundEgg-laying behavior in the nematode C. elegans displays a distinct clustered temporal pattern: egg-laying events occur primarily in bursts or active phases, separated by inactive phases during which eggs are retained. The onset of the active phase can be modeled as a Poisson process with a time constant of approximately 20 minutes, while egg-laying events within an active phase occur with a faster time constant of approximately 20 seconds. Here we propose a cellular model for how the temporal pattern of egg-laying might be generated, based on genetic and cell-biological experiments and statistical analyses.ResultsWe suggest that the HSN neuron is the executive neuron driving egg-laying events. We propose that the VC neurons act as "single egg counters" that inhibit HSN activity for short periods in response to individual egg-laying events. We further propose that the uv1 neuroendocrine cells are "cluster counters", which inhibit HSN activity for longer periods and are responsible for the time constant of the inactive phase. Together they form an integrated circuit that drives the clustered egg-laying pattern.ConclusionsThe detailed predictions derived from this model can now be tested by straightforward validation experiments.

The Roles of Multiple UNC-40 (DCC) Receptor-Mediated Signals in Determining Neuronal Asymmetry Induced by the UNC-6 (Netrin) Ligand

The polarization of post-mitotic neurons is poorly understood. Preexisting spatially asymmetric cues, distributed within the neuron or as extracellular gradients, could be required for neurons to polarize. Alternatively, neurons might have the intrinsic ability to polarize without any preestablished asymmetric cues. In Caenorhabditis elegans, the UNC-40 (DCC) receptor mediates responses to the extracellular UNC-6 (netrin) guidance cue. For the HSN neuron, an UNC-6 ventral-dorsal gradient asymmetrically localizes UNC-40 to the ventral HSN surface. There an axon forms, which is ventrally directed by UNC-6. In the absence of UNC-6, UNC-40 is equally distributed and the HSN axon travels anteriorly in response to other cues. However, we find that a single amino acid change in the UNC-40 ectodomain causes randomly oriented asymmetric UNC-40 localization and a wandering axon phenotype. With UNC-6, there is normal UNC-40 localization and axon migration. A single UNC-6 amino acid substitution enhances the mutant phenotypes, whereas UNC-6 second-site amino acid substitutions suppress the phenotypes. We propose that UNC-40 mediates multiple signals to polarize and orient asymmetry. One signal triggers the intrinsic ability of HSN to polarize and causes randomly oriented asymmetry. Concurrently, another signal biases the orientation of the asymmetry relative to the UNC-6 gradient. The UNC-40 ectodomain mutation activates the polarization signal, whereas different forms of the UNC-6 ligand produce UNC-40 conformational changes that allow or prohibit the orientation signal.

Tissue-Specific Activities of an Immune Signaling Module Regulate Physiological Responses to Pathogenic and Nutritional Bacteria in C. elegans

Microbes represent both an essential source of nutrition and a potential source of lethal infection to the nematode Caenorhabditis elegans. Immunity in C. elegans requires a signaling module comprised of orthologs of the mammalian Toll-interleukin-1 receptor (TIR) domain protein SARM, the mitogen-activated protein kinase kinase kinase (MAPKKK) ASK1, and MAPKK MKK3, which activates p38 MAPK. We determined that the SARM-ASK1-MKK3 module has dual tissue-specific roles in the C. elegans response to pathogens--in the cell-autonomous regulation of innate immunity and the neuroendocrine regulation of serotonin-dependent aversive behavior. SARM-ASK1-MKK3 signaling in the sensory nervous system also regulates egg-laying behavior that is dependent on bacteria provided as a nutrient source. Our data demonstrate that these physiological responses to bacteria share a common mechanism of signaling through the SARM-ASK1-MKK3 module and suggest the co-option of ancestral immune signaling pathways in the evolution of physiological responses to microbial pathogens and nutrients.

A Self-Regulating Feed-Forward Circuit Controlling C. elegans Egg-Laying Behavior

Egg laying in Caenorhabditis elegans has been well studied at the genetic and behavioral levels. However, the neural basis of egg-laying behavior is still not well understood; in particular, the roles of specific neurons and the functional nature of the synaptic connections in the egg-laying circuit remain uncharacterized. We have used in vivo neuroimaging and laser surgery to address these questions in intact, behaving animals. We have found that the HSN neurons play a central role in driving egg-laying behavior through direct excitation of the vulval muscles and VC motor neurons. The VC neurons play a dual role in the egg-laying circuit, exciting the vulval muscles while feedback-inhibiting the HSNs. Interestingly, the HSNs are active in the absence of synaptic input, suggesting that egg laying may be controlled through modulation of autonomous HSN activity. Indeed, body touch appears to inhibit egg laying, in part by interfering with HSN calcium oscillations. The egg-laying motor circuit comprises a simple three-component system combining feed-forward excitation and feedback inhibition. This microcircuit motif is common in the C. elegans nervous system, as well as in the mammalian cortex; thus, understanding its functional properties in C. elegans may provide insight into its computational role in more complex brains.

Sexual Dimorphism in the Hoverfly Motion Vision Pathway

Many insects perform high-speed aerial maneuvers in which they navigate through visually complex surrounds. Among insects, hoverflies stand out, with males switching from stationary hovering to high-speed pursuit at extreme angular velocities [1]. In dipterans, 50-60 large interneurons -- the lobula-plate tangential cells (LPTCs) -- detect changes in optic flow experienced during flight [2-5]. It has been predicted that large LPTC receptive fields are a requirement of accurate "matched filters" of optic flow [6]. Whereas many fly taxa have three horizontal system (HS) LPTC neurons in each hemisphere, hoverflies have four [7], possibly reflecting the more sophisticated flight behavior. We here show that the most dorsal hoverfly neuron (HS north [HSN]) is sexually dimorphic, with the male receptive field substantially smaller than in females or in either sex of blowflies. The (hoverfly-specific) HSN equatorial (HSNE) is, however, sexually isomorphic. Using complex optic flow, we show that HSN, despite its smaller receptive field, codes yaw velocity as well as HSNE. Responses to a target moving against a plain or textured background suggest that the male HSN could potentially play a role in target pursuit under some conditions.

UNC-6 expression by the vulval precursor cells of Caenorhabditis elegans is required for the complex axon guidance of the HSN neurons

Netrin is an evolutionarily conserved axon guidance molecule that has both axonal attraction and repulsion activities. In Caenorhabditis elegans, Netrin/UNC-6 is secreted by ventral cells, attracting some axons ventrally and repelling some axons, which extend dorsally. One axon guided by UNC-6 is that of the HSN neuron. The axon guidance process for HSN neurons is complex, consisting of ventral growth, dorsal growth, branching, second ventral growth, fasciculation with ventral nerve cords, and then anterior growth. The vulval precursor cells (VPC) and the PVP and PVQ neurons are required for the HSN axon guidance; however, the molecular mechanisms involved are completely unknown. In this study, we found that the VPC strongly expressed UNC-6 during HSN axon growth. Silencing of UNC-6 expression in only the VPC, using a novel tissue-specific RNAi technique, resulted in abnormal HSN axon guidance. The expression of Netrin/UNC-6 by only the VPC in unc-6 null mutants partially rescued the HSN ventral axon guidance. Furthermore, the expression of Netrin/UNC-6 by the VPC and the ventral nerve cord (VNC) in unc-6 null mutants restored the complex HSN axon guidance. These results suggest that UNC-6 expressed by the VPC and the VNC cooperatively regulates the complex HSN axon guidance.