Regulation of synaptic transmission at the Caenorhabditis elegans M4 neuromuscular junction by an antagonistic relationship between two calcium channels

Stearoyl-CoA desaturases sustain cholinergic excitation and copulatory robustness in metabolically aging C. elegansmales

Regulated metabolism is required for behaviors as adults age. To understand how lipid usage affects motor coordination, we studied male Caenorhabditis elegans copulation as a model of energy-intensive behavior. Copulation performance drops after 48 h of adulthood. We found that 12–24 h before behavioral decline, males prioritize exploring and copulation behavior over feeding, suggesting that catabolizing stored metabolites, such as lipids, occurs during this period. Because fat-6/7-encoded stearoyl-CoA desaturases are essential for converting the ingested fatty acids to lipid storage, we examined the copulation behavior and neural calcium transients of fat-6(lf); fat-7(lf) mutants. In wild-type males, intestinal and epithelial fat-6/7 expression increases during the first 48 h of adulthood. The fat-6(lf); fat-7(lf) behavioral and metabolic defects indicate that in aging wild-type males, the increased expression of stearoyl-CoA desaturases in the epidermis may indirectly modulate the levels of EAG-family K⁺ channels in the reproductive cholinergic neurons and muscles.

•

Tissue distribution of fat-6-encoded stearoyl-CoA desaturase changes in adulthood

•

Markov modeling shows reduced feeding linked with more exploring in day 2 males

•

fat-6(lf); fat-7(lf) disrupted behavior can be rescued by epidermal FAT-6

•

fat-6(lf); fat-7(lf) alters neural and muscular ERG and EAG K+ channel expression

Gap junctions: historical discoveries and new findings in the Caenorhabditiselegans nervous system

Gap junctions are evolutionarily conserved structures at close membrane contacts between two cells. In the nervous system, they mediate rapid, often bi-directional, transmission of signals through channels called innexins in invertebrates and connexins in vertebrates. Connectomic studies from Caenorhabditis elegans have uncovered a vast number of gap junctions present in the nervous system and non-neuronal tissues. The genome also has 25 innexin genes that are expressed in spatial and temporal dynamic pattern. Recent findings have begun to reveal novel roles of innexins in the regulation of multiple processes during formation and function of neural circuits both in normal conditions and under stress. Here, we highlight the diverse roles of gap junctions and innexins in the C. elegans nervous system. These findings contribute to fundamental understanding of gap junctions in all animals.

Innexins: Expression, Regulation, and Functions

The innexin (Inx) proteins form gap junction channels and non-junctional channels (named hemichannels) in invertebrates. These channels participate in cellular communication playing a relevant role in several physiological processes. Pioneer studies conducted mainly in worms and flies have shown that innexins participate in embryo development and behavior. However, recent studies have elucidated new functions of innexins in Arthropoda, Nematoda, Annelida, and Cnidaria, such as immune response, and apoptosis. This review describes emerging data of possible new roles of innexins and summarizes the data available to date.

Serotonin Disinhibits a Caenorhabditis elegans Sensory Neuron by Suppressing Ca2+-Dependent Negative Feedback

Neuromodulators, such as serotonin (5-HT), alter neuronal excitability and synaptic strengths, and define different behavioral states. Neuromodulator-dependent changes in neuronal activity patterns are frequently measured using calcium reporters because calcium imaging can easily be performed on intact functioning nervous systems. With only 302 neurons, the nematode Caenorhabditis elegans provides a relatively simple, yet powerful, system to understand neuromodulation at the level of individual neurons. C. elegans hermaphrodites are repelled by 1-octanol, and the initiation of these aversive responses is potentiated by 5-HT. 5-HT acts on the ASH polymodal nociceptors that sense the 1-octanol stimulus. Surprisingly, 5-HT suppresses ASH Ca²⁺ transients while simultaneously potentiating 1-octanol-dependent ASH depolarization. Here we further explore this seemingly inverse relationship. Our results show the following (1) 5-HT acts downstream of depolarization, through Gα_q-mediated signaling and calcineurin, to inhibit L-type voltage-gated Ca²⁺ channels; (2) the 1-octanol-evoked Ca²⁺ transients in ASHs inhibit depolarization; and (3) the Ca²⁺-activated K⁺ channel, SLO-1, acts downstream of 5-HT and is a critical regulator of ASH response dynamics. These findings define a Ca²⁺-dependent inhibitory feedback loop that can be modulated by 5-HT to increase neuronal excitability and regulate behavior, and highlight the possibility that neuromodulator-induced changes in the amplitudes of Ca²⁺ transients do not necessarily predict corresponding changes in depolarization.SIGNIFICANCE STATEMENT Neuromodulators, such as 5-HT, modify behavior by regulating excitability and synaptic efficiency in neurons. Neuromodulation is often studied using Ca²⁺ imaging, whereby neuromodulator-dependent changes in neuronal activity levels can be detected in intact, functioning circuits. Here we show that 5-HT reduces the amplitude of depolarization-dependent Ca²⁺ transients in a C. elegans nociceptive neuron, through Gα_q signaling and calcineurin but that Ca²⁺ itself inhibits depolarization, likely through Ca²⁺-activated K⁺ channels. The net effect of 5-HT, therefore, is to increase neuronal excitability through disinhibition. These results establish a novel 5-HT signal transduction pathway, and demonstrate that neuromodulators can change Ca²⁺ signals and depolarization amplitudes in opposite directions, simultaneously, within a single neuron.

Next-Generation Sequencing-Based Approaches for Mutation Mapping and Identification in Caenorhabditis elegans

The use of next-generation sequencing (NGS) has revolutionized the way phenotypic traits are assigned to genes. In this review, we describe NGS-based methods for mapping a mutation and identifying its molecular identity, with an emphasis on applications in Caenorhabditis elegans In addition to an overview of the general principles and concepts, we discuss the main methods, provide practical and conceptual pointers, and guide the reader in the types of bioinformatics analyses that are required. Owing to the speed and the plummeting costs of NGS-based methods, mapping and cloning a mutation of interest has become straightforward, quick, and relatively easy. Removing this bottleneck previously associated with forward genetic screens has significantly advanced the use of genetics to probe fundamental biological processes in an unbiased manner.

Neural Circuits: From Structure to Function and Back

The new field of connectomics aims to obtain fine-grained anatomical connectivity data for vertebrate brains. A recent study highlights the types of experiments that will be necessary in order to draw conclusions about function from anatomical connectivity.

SLO BK Potassium Channels Couple Gap Junctions to Inhibition of Calcium Signaling in Olfactory Neuron Diversification

The C. elegans AWC olfactory neuron pair communicates to specify asymmetric subtypes AWCOFF and AWCON in a stochastic manner. Intercellular communication between AWC and other neurons in a transient NSY-5 gap junction network antagonizes voltage-activated calcium channels, UNC-2 (CaV2) and EGL-19 (CaV1), in the AWCON cell, but how calcium signaling is downregulated by NSY-5 is only partly understood. Here, we show that voltage- and calcium-activated SLO BK potassium channels mediate gap junction signaling to inhibit calcium pathways for asymmetric AWC differentiation. Activation of vertebrate SLO-1 channels causes transient membrane hyperpolarization, which makes it an important negative feedback system for calcium entry through voltage-activated calcium channels. Consistent with the physiological roles of SLO-1, our genetic results suggest that slo-1 BK channels act downstream of NSY-5 gap junctions to inhibit calcium channel-mediated signaling in the specification of AWCON. We also show for the first time that slo-2 BK channels are important for AWC asymmetry and act redundantly with slo-1 to inhibit calcium signaling. In addition, nsy-5-dependent asymmetric expression of slo-1 and slo-2 in the AWCON neuron is necessary and sufficient for AWC asymmetry. SLO-1 and SLO-2 localize close to UNC-2 and EGL-19 in AWC, suggesting a role of possible functional coupling between SLO BK channels and voltage-activated calcium channels in AWC asymmetry. Furthermore, slo-1 and slo-2 regulate the localization of synaptic markers, UNC-2 and RAB-3, in AWC neurons to control AWC asymmetry. We also identify the requirement of bkip-1, which encodes a previously identified auxiliary subunit of SLO-1, for slo-1 and slo-2 function in AWC asymmetry. Together, these results provide an unprecedented molecular link between gap junctions and calcium pathways for terminal differentiation of olfactory neurons.