Neurons convert synaptic or sensory inputs into cellular outputs. It is not well understood how a single neuron senses, processes multiple stimuli, and generates distinct neuronal outcomes. Here, we describe the mechanism by which the C. elegans PVD neurons sense two mechanical stimuli: external touch and proprioceptive body movement. These two stimuli are detected by distinct mechanosensitive DEG/ENaC/ASIC channels, which trigger distinct cellular outputs linked to mechanonociception and proprioception. Mechanonociception depends on DEGT-1 and activates PVD's downstream command interneurons through its axon, while proprioception depends on DEL-1, UNC-8, and MEC-10 to induce local dendritic Ca 2+ increase and dendritic release of a neuropeptide NLP-12. NLP-12 directly modulates neuromuscular junction activity through the cholecystokinin receptor homolog on motor axons, setting muscle tone and movement vigor. Thus, the same neuron simultaneously uses both its axon and dendrites as output apparatus to drive distinct sensorimotor outcomes.

Calcium in the flagellum controls sperm navigation. In sperm of marine invertebrates and mammals, Ca2+ signalling has been intensely studied, whereas for fish little is known. In sea urchin sperm, a cyclic nucleotide-gated K+ channel (CNGK) mediates a cGMP-induced hyperpolarization that evokes Ca2+ influx. Here, we identify in sperm of the freshwater fish Danio rerio a novel CNGK family member featuring non-canonical properties. It is located in the sperm head rather than the flagellum and is controlled by intracellular pH, but not cyclic nucleotides. Alkalization hyperpolarizes sperm and produces Ca2+ entry. Ca2+ induces spinning-like swimming, different from swimming of sperm from other species. The “spinning” mode probably guides sperm into the micropyle, a narrow entrance on the surface of fish eggs. A picture is emerging of sperm channel orthologues that employ different activation mechanisms and serve different functions. The channel inventories probably reflect adaptations to species-specific challenges during fertilization.DOI: http://dx.doi.org/10.7554/eLife.07624.001

Proton (H+) channels are special: They select protons against other ions that are up to a millionfold more abundant. Only a few proton channels have been identified so far. Here, we identify a family of voltage-gated “pacemaker” channels, HCNL1, that are exquisitely selective for protons. HCNL1 activates during hyperpolarization and conducts protons into the cytosol. Surprisingly, protons permeate through the channel’s voltage-sensing domain, whereas the pore domain is nonfunctional. Key to proton permeation is a methionine residue that interrupts the series of regularly spaced arginine residues in the S4 voltage sensor. HCNL1 forms a tetramer and thus contains four proton pores. Unlike classic HCN channels, HCNL1 is not gated by cyclic nucleotides. The channel is present in zebrafish sperm and carries a proton inward current that acidifies the cytosol. Our results suggest that protons rather than cyclic nucleotides serve as cellular messengers in zebrafish sperm. Through small modifications in two key functional domains, HCNL1 evolutionarily adapted to a low-Na+freshwater environment to conserve sperm’s ability to depolarize.

The degenerin channels, epithelial sodium channels, and acid-sensing ion channels (DEG/ENaC/ASICs) play important roles in sensing mechanical stimuli, regulating salt homeostasis, or responding to acidification in the nervous system. They share a common topology with two transmembrane domains separated by a large extracellular domain and are believed to assemble as homomeric or heteromeric trimers into non-voltage gated, sodiumselective, and amiloride-sensitive ion channels. Amiloride is not the only drug that targets DEG/ENaC/ASICs, however, they are also emerging as a target of nonsteroidal antiinflammatory drugs (NSAIDs) as well as other classes of small molecules. C. elegans has about 30 genes encoding DEG/ENaC/ASIC subunits and thus offers an excellent opportunity to examine variations in sensitivity to small molecules and biophysical properties. Here, we analyzed a subset of the C. elegans DEG/ENaC/ASIC proteins in order to test the hypothesis that individual family members have distinct properties. Toward this goal, we expressed five C. elegans isoforms in Xenopus laevis oocytes (DEGT-1d, DEL-1d, UNC-8d, MEC-10d and MEC-4d) and measured current amplitude, selectivity among monovalent cations, sensitivity to amiloride and its analogs, and sensitivity to NSAIDs. Of these five proteins, only DEGT-1d, UNC-8d, and MEC-4d form homomeric channels. Unlike MEC-4d and UNC-8d, DEGT-1d channels were insensitive to amiloride and its analogs and more permeable to K + than to Na + . As reported for rat ASIC1a, NSAIDs inhibit DEGT-1d and UNC-8d. Unexpectedly, MEC-4d was strongly potentiated by NSAIDs, an effect that was decreased by mutations in the extracellular domain that affect inhibition of rat ASIC1a. Collectively, these findings reveal that not all DEG/ENaC/ASIC channels are amiloride-sensitive and sodium-selective and that NSAIDs can both inhibit and potentiate these channels.The degenerin channels, epithelial sodium channels, and acid-sensing ion channels (DEG/ENaC/ASICs) are present in most, if not all metazoans and expressed in diverse tissues, including the epithelia of several organs and in the central and peripheral nervous systems (Eastwood and Goodman, 2012; Kellenberger and Schild, 2002). These channels vary in how they are activated in vivo, although the activation mechanisms are not yet known for all family members. At least two DEG proteins are known to be mechanosensitive, ENaCs are constitutively active and can be regulated by shear stress, and ASICs are activated by proton binding (Eastwood and Goodman, 2012). The DEG and ENaC proteins were the initial members of this superfamily. The DEGs were identified in C. elegans by virtue of their role in mechanosensation and neuronal degeneration (Chalfie and Wolinsky, 1990; Driscoll and Chalfie, 1991; Huang and Chalfie, 1994). The ENaCs were identified via expression of rodent cRNAs in Xenopus oocytes followed by functional screening (Canessa et al., 1995). The proteins that form acid-sensing ion channels (ASICs) were also identified in the 1990s (Waldman...

The degenerin channels, epithelial sodium channels, and acid-sensing ion channels (DEG/ENaC/ASICs) play important roles in sensing mechanical stimuli, regulating salt homeostasis, and responding to acidification in the nervous system. They have two transmembrane domains separated by a large extracellular domain and are believed to assemble as homomeric or heteromeric trimers. Based on studies of selected family members, these channels are assumed to form nonvoltage-gated and sodium-selective channels sensitive to the anti-hypertensive drug amiloride. They are also emerging as a target of nonsteroidal anti-inflammatory drugs (NSAIDs). Caenorhabditis elegans has more than two dozen genes encoding DEG/ENaC/ASIC subunits, providing an excellent opportunity to examine variations in drug sensitivity. Here, we analyze a subset of the C. elegans DEG/ENaC/ASIC proteins to test the hypothesis that individual family members vary not only in their ability to form homomeric channels but also in their drug sensitivity. We selected a panel of C. elegans DEG/ENaC/ASICs that are coexpressed in mechanosensory neurons and expressed gain-of-function or d mutants in Xenopus laevis oocytes. We found that only DEGT‑1d, UNC‑8d, and MEC‑4d formed homomeric channels and that, unlike MEC‑4d and UNC‑8d, DEGT‑1d channels were insensitive to amiloride and its analogues. As reported for rat ASIC1a, NSAIDs inhibit DEGT‑1d and UNC‑8d channels. Unexpectedly, MEC‑4d was strongly potentiated by NSAIDs, an effect that was decreased by mutations in the putative NSAID-binding site in the extracellular domain. Collectively, these findings reveal that not all DEG/ENaC/ASIC channels are amiloride-sensitive and that NSAIDs can both inhibit and potentiate these channels.