Mechanisms of Gain Control by Voltage-Gated Channels in Intrinsically-Firing Neurons

Gain modulation is a key feature of neural information processing, but underlying mechanisms remain unclear. In single neurons, gain can be measured as the slope of the current-frequency (input-output) relationship over any given range of inputs. While much work has focused on the control of basal firing rates and spike rate adaptation, gain control has been relatively unstudied. Of the limited studies on gain control, some have examined the roles of synaptic noise and passive somatic currents, but the roles of voltage-gated channels present ubiquitously in neurons have been less explored. Here, we systematically examined the relationship between gain and voltage-gated ion channels in a conductance-based, tonically-active, model neuron. Changes in expression (conductance density) of voltage-gated channels increased (Ca²⁺ channel), reduced (K⁺ channels), or produced little effect (h-type channel) on gain. We found that the gain-controlling ability of channels increased exponentially with the steepness of their activation within the dynamic voltage window (voltage range associated with firing). For depolarization-activated channels, this produced a greater channel current per action potential at higher firing rates. This allowed these channels to modulate gain by contributing to firing preferentially at states of higher excitation. A finer analysis of the current-voltage relationship during tonic firing identified narrow voltage windows at which the gain-modulating channels exerted their effects. As a proof of concept, we show that h-type channels can be tuned to modulate gain by changing the steepness of their activation within the dynamic voltage window. These results show how the impact of an ion channel on gain can be predicted from the relationship between channel kinetics and the membrane potential during firing. This is potentially relevant to understanding input-output scaling in a wide class of neurons found throughout the brain and other nervous systems.     

Voltage-dependent potassium channels: minK and Shaker families

In the last 4 years, the molecular identity of several types of voltage-dependent potassium channels has been discovered. These include channels that terminate action potentials and control repetitive neuronal firing, as well as channels whose biological role is not yet understood. The majority of these are encoded by genes related to the Drosophila Shaker gene. The large number of genes comprising the Shaker gene family, coupled with the existence of different channels that result from alternatively spliced messages from the same gene, provide both vertebrates and invertebrates with a wide selection of channels whose voltage-dependence and kinetics can be tailored to the needs of a specific cell. Mutagenesis experiments on such channels are providing new information on those regions of the protein that govern essential aspects of channel activity, such as gating by voltage and ion permeation. Another gene, unrelated to the Shaker family, encodes a voltage-dependent potassium channel that activates much more slowly than the Shaker channels. This has been termed the MinK channel.     

A K+-selective cGMP-gated ion channel controls chemosensation of sperm

Eggs attract sperm by chemical factors, a process called chemotaxis. Sperm from marine invertebrates use cGMP signalling to transduce incident chemoattractants into changes in the Ca2+ concentration in the flagellum, which control the swimming behaviour during chemotaxis. The signalling pathway downstream of the synthesis of cGMP by a guanylyl cyclase is ill-defined. In particular, the ion channels that are involved in Ca2+ influx and their mechanisms of gating are not known. Using rapid voltage-sensitive dyes and kinetic techniques, we record the voltage response that is evoked by the chemoattractant in sperm from the sea urchin Arbacia punctulata. We show that the chemoattractant evokes a brief hyperpolarization followed by a sustained depolarization. The hyperpolarization is caused by the opening of K+-selective cyclic-nucleotide-gated (CNG) channels in the flagellum. Ca2+ influx commences at the onset of recovery from hyperpolarization. The voltage threshold of Ca2+ entry indicates the involvement of low-voltage-activated Ca(v) channels. These results establish a model of chemosensory transduction in sperm whereby a cGMP-induced hyperpolarization opens Ca(v) channels by a 'recovery-from-inactivation' mechanism and unveil an evolutionary kinship between transduction mechanisms in sperm and photoreceptors.     

Local adaptation in host use among marine invertebrates

The study of interactions between small invertebrates and their larger plant and animal hosts has a long tradition. One persistent theme within this literature is that spatially‐segregated populations of terrestrial and freshwater invertebrates commonly adapt to local hosts across their geographic ranges. Marine examples are rare, which leaves the impression that marine populations are less likely to adapt to locally abundant hosts and more likely to evolve generalized or phenotypically‐plastic strategies. Here, I review a short but growing list of marine invertebrates that appear to display local adaptation in host use. As expected, most of the marine examples are brooded animals with weak dispersal potential. However, some species with pelagically dispersed larvae have apparently adapted to local hosts. This surprising result is consistent with recent evidence that pelagically‐dispersed larvae are not always broadly dispersed, that strong selective pressures maintain local differences in host use, or both. The presence of host‐mediated adaptation in the sea alters predictions on how marine communities respond to disturbance, supports the notion that marine consumer‐prey interactions can coevolve, and indicates that hosts play fundamental roles in the differentiation and perhaps speciation of small marine invertebrates.     

Cyclic AMP Levels,Adenylyl Cyclase Activity,and Their Stimulation by Serotonin Quantif?ied in Intact Neurons

In molluscan central neurons that express cAMP-gated Na⁺ current (I_Na,cAMP), estimates of the cAMP binding affinity of the channels have suggested that effective native intracellular cAMP concentrations should be much higher than characteristic of most cells. Using neurons of the marine opisthobranch snail Pleurobranchaea californica, we applied theory and conventional voltage clamp techniques to use I_Na,cAMP to report basal levels of endogenous cAMP and adenylyl cyclase, and their stimulation by serotonin. Measurements were calibrated to iontophoretic cAMP injection currents to enable expression of the data in molar terms. In 30 neurons, serotonin stimulated on average a 23-fold increase in submembrane [cAMP], effected largely by an 18-fold increase in adenylyl cyclase activity. Serotonin stimulation of adenylyl cyclase and [cAMP] was inversely proportional to cells'' resting adenylyl cyclase activity. Average cAMP concentration at the membrane rose from 3.6 to 27.6 μM, levels consistent with the expected cAMP dissociation constants of the I_Na,cAMP channels. These measures confirm the functional character of I_Na,cAMP in the context of high levels of native cAMP. Methods similar to those employed here might be used to establish critical characters of cyclic nucleotide metabolism in the many cells of invertebrates and vertebrates that are being found to express ion currents gated by direct binding of cyclic nucleotides.     

Ion channels in postnatal neurogenesis

Neural stem and progenitor cells (NSC/NPCs) are unspecialized cells found in the adult peri-ventricular and sub-granular zones that are capable of self-renewal, migration, and differentiation into new neurons through the remarkable process of postnatal neurogenesis. We are now beginning to understand that the concerted action of ion channels, multi-pass transmembrane proteins that allow passage of ions across otherwise impermeable cellular membranes tightly regulate this process. Specific ion channels control proliferation, differentiation and survival. Furthermore, they have the potential to be highly selective drug targets due to their complex structures. As such, these proteins represent intriguing prospects for control and optimization of postnatal neurogenesis for neural regeneration following brain injury or disease. Here, we concentrate on ion channels identified in adult ventricular zone NSC/NPCs that have been found to influence the stages of neurogenesis. Finally, we outline the potential of these channels to elicit repair, and highlight the outstanding challenges.     

Application of oscillating potentials to the Shaker potassium channel

Nonequilibrium response spectroscopy (NRS) has been proposed recently to complement standard electrophysiological techniques used to investigate ion channels. It involves application of rapidly oscillating potentials that drive the ion channel ensemble far from equilibrium. It is argued that new, so far undiscovered features of ion channel gating kinetics may become apparent under such nonequilibrium conditions. In this paper we explore the possibility of using regular, sinusoidal voltages with the NRS protocols to facilitate Markov model selection for ion channels. As a test case we consider the Shaker potassium channel for which various Markov models have been proposed recently. We concentrate on certain classes of such models and show that while some models might be virtually indistinguishable using standard methods, they show marked differences when driven with an oscillating voltage. Model currents are compared to experimental data obtained for the Shaker K+ channel expressed in mammalian cells (tsA 201).     

PKC regulation of ion channels: The involvement of PIP2

Ion channels are integral membrane proteins whose gating has been increasingly shown to depend on the presence of the low-abundance membrane phospholipid, phosphatidylinositol (4,5) bisphosphate. The expression and function of ion channels is tightly regulated via protein phosphorylation by specific kinases, including various PKC isoforms. Several channels have further been shown to be regulated by PKC through altered surface expression, probability of channel opening, shifts in voltage dependence of their activation, or changes in inactivation or desensitization. In this review, we survey the impact of phosphorylation of various ion channels by PKC isoforms and examine the dependence of phosphorylated ion channels on phosphatidylinositol (4,5) bisphosphate as a mechanistic endpoint to control channel gating.     

Ion channels in postnatal neurogenesis: potential targets for brain repair

Neural stem and progenitor cells (NSC/NPCs) are unspecialized cells found in the adult peri-ventricular and sub-granular zones that are capable of self-renewal, migration, and differentiation into new neurons through the remarkable process of postnatal neurogenesis. We are now beginning to understand that the concerted action of ion channels, multi-pass transmembrane proteins that allow passage of ions across otherwise impermeable cellular membranes tightly regulate this process. Specific ion channels control proliferation, differentiation and survival. Furthermore, they have the potential to be highly selective drug targets due to their complex structures. As such, these proteins represent intriguing prospects for control and optimization of postnatal neurogenesis for neural regeneration following brain injury or disease. Here, we concentrate on ion channels identified in adult ventricular zone NSC/NPCs that have been found to influence the stages of neurogenesis. Finally, we outline the potential of these channels to elicit repair, and highlight the outstanding challenges.     

The transient receptor potential family of ion channels

The transient receptor potential (TRP) multigene superfamily encodes integral membrane proteins that function as ion channels. Members of this family are conserved in yeast, invertebrates and vertebrates. The TRP family is subdivided into seven subfamilies: TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPP (polycystin), TRPML (mucolipin), TRPA (ankyrin) and TRPN (NOMPC-like); the latter is found only in invertebrates and fish. TRP ion channels are widely expressed in many different tissues and cell types, where they are involved in diverse physiological processes, such as sensation of different stimuli or ion homeostasis. Most TRPs are non-selective cation channels, only few are highly Ca²⁺ selective, some are even permeable for highly hydrated Mg²⁺ ions. This channel family shows a variety of gating mechanisms, with modes of activation ranging from ligand binding, voltage and changes in temperature to covalent modifications of nucleophilic residues. Activated TRP channels cause depolarization of the cellular membrane, which in turn activates voltage-dependent ion channels, resulting in a change of intracellular Ca²⁺ concentration; they serve as gatekeeper for transcellular transport of several cations (such as Ca²⁺ and Mg²⁺), and are required for the function of intracellular organelles (such as endosomes and lysosomes). Because of their function as intracellular Ca²⁺ release channels, they have an important regulatory role in cellular organelles. Mutations in several TRP genes have been implicated in diverse pathological states, including neurodegenerative disorders, skeletal dysplasia, kidney disorders and pain, and ongoing research may help find new therapies for treatments of related diseases.     

Ion currents in embryo development

Ion channels are proteins expressed in the plasma membrane of electrogenic cells. In the zygote and blastomeres of the developing embryo, electrical modifications result from ion currents that flow through these channels. This phenomenon implies that ion current activity exerts a specific developmental function, and plays a crucial role in signal transduction and the control of embryogenesis, from the early cleavage stages and during growth and development of the embryo. This review describes the involvement of ion currents in early embryo development, from marine invertebrates to human, focusing on the occurrence, modulation, and dynamic role of ion fluxes taking place on the zygote and blastomere plasma membrane, and at the intercellular communication between embryo cell stages. Birth Defects Research (Part C) 108:6–18, 2016. © 2016 Wiley Periodicals, Inc.     

Estimation of lipids in marine animals and tissues: Detailed investigation of the sulphophosphovanilun method for ‘total’ lipids

A reliable and simple method for the estimation of ‘total’ lipids in the tissue of marine animals is much needed for biochemical-ecological work. As a result of the present investigations the phosphosulphovanillin method used, hitherto, only for blood serum samples, is recommended. The method depends on using a cholesterol standard which has previously been related to gravimetric values obtained on blood sera. The colorimetric values have been compared with gravimetric results on a variety of marine tissues — largely of invertebrates.The data given in the tables may be used to convert colorimetric values, using a cholesterol standard to total lipids. For extended investigations, however, it is recommended that the cholesterol values are first calibrated against gravimetric values for the particular tissue under investigation.A summary of the preparation of reagents and procedure is given.     

Annelid epithelia as models for electrogenic Na+ transport

The electrogenic Na(+) absorption across tight epithelia from invertebrates follows the principles analog to the mechanisms found in vertebrates. Extracellular Na(+)-ions pass the apical cell membranes through highly selective Na(+) channels and follow an electrochemical gradient which is sustained by the basolateral Na(+)/K(+)-ATPases. These apical Na(+) channels are selectively blocked by amiloride and represent the rate-limiting target for the control of transcellular Na(+) uptake. Although annelids express ADH-like peptide hormones, they lack the osmoregulatory mineralocorticoid system with the vertebrate-specific key hormone aldosterone. Thus, their epithelia may represent interesting models for investigation of ion transport regulation. While the formation of urine in the nephridia of, for example, leeches had been subject to intensive studies, the investigation of ion transport across their body wall was largely neglected. We use dissected segments of integuments from the limnic leech Hirudo medicinalis and, recently, from the earthworm Lumbricus terrestris for Ussing chamber experiments. We investigate transintegumental ion transport with focus on control of electrogenic Na(+) uptake and the amiloride-sensitive part of it and identified several extracellular factors as peptide hormones, tri- and divalent cations or purinergic molecules with regulatory effects on it. Meanwhile, there exists a macroscopic view on Na(+) absorption; however, other ion transport mechanisms across annelid integuments still await scientific effort. Here we present a concise synopsis about the electrophysiology of annelid integuments to illustrate the state of science and to evaluate whether further studies in this particular field may be of interest.     

Overview of the chemical ecology of benthic marine invertebrates along the western Antarctic peninsula

Thirteen years ago in a review that appeared in the American Zoologist, we presented the first survey of the chemical and ecological bioactivity of Antarctic shallow-water marine invertebrates. In essence, we reported that despite theoretical predictions to the contrary the incidence of chemical defenses among sessile and sluggish Antarctic marine invertebrates was widespread. Since that time we and others have significantly expanded upon the base of knowledge of Antarctic marine invertebrates' chemical ecology, both from the perspective of examining marine invertebrates in new, distinct geographic provinces, as well as broadening the evaluation of the ecological significance of secondary metabolites. Importantly, many of these studies have been framed within established theoretical constructs, particularly the Optimal Defense Theory. In the present article, we review the current knowledge of chemical ecology of benthic marine invertebrates comprising communities along the Western Antarctic Peninsula (WAP), a region of Antarctica that is both physically and biologically distinct from the rest of the continent. Our overview indicates that, similar to other regions of Antarctica, anti-predator chemical defenses are widespread among species occurring along the WAP. In some groups, such as the sponges, the incidence of chemical defenses against predation is comparable to, or even slightly higher than, that found in tropical marine systems. While there is substantial knowledge of the chemical defenses of benthic marine invertebrates against predators, much less is known about chemical anti-foulants. The sole survey conducted to date suggests that secondary metabolites in benthic sponges are likely to be important in the prevention of fouling by benthic diatoms, yet generally lack activity against marine bacteria. Our understanding of the sensory ecology of Antarctic benthic marine invertebrates, despite its great potential, remains in its infancy. For example, along the WAP, community-level non-consumptive effects occur when amphipods chemically sense fish predators and respond by seeking refuge in chemically-defended macroalgae. Such interactions may be important in releasing amphipods from predation pressure and facilitating their unusually high abundances along the WAP. Moreover, recent studies on the sensory biology of the Antarctic keystone sea star Odontaster validus indicate that chemotactile-mediated interactions between conspecifics and other sympatric predatory sea stars may have significant ramifications in structuring community dynamics. Finally, from a global environmental perspective, understanding how chemical ecology structures marine benthic communities along the WAP must increasingly be viewed in the context of the dramatic impacts of rapid climatic change now occurring in this biogeographic region.     

Effect of ciguatoxin 3C on voltage-gated Na+ and K+ currents in mouse taste cells

The marine dinoflagellate Gambierdiscus toxicus produces highly lipophilic, polycyclic ether toxins that cause a seafood poisoning called ciguatera. Ciguatoxins (CTXs) and gambierol represent the two major causative agents of ciguatera intoxication, which include taste alterations (dysgeusiae). However, information on the mode of action of ciguatera toxins in taste cells is scarce. Here, we have studied the effect of synthetic CTX3C (a CTX congener) on mouse taste cells. By using the patch-clamp technique to monitor membrane ion currents, we found that CTX3C markedly affected the operation of voltage-gated Na(+) channels but was ineffective on voltage-gated K(+) channels. This result was the exact opposite of what we obtained earlier with gambierol, which inhibits K(+) channels but not Na(+) channels. Thus, CTXs and gambierol affect with high potency the operation of separate classes of voltage-gated ion channels in taste cells. Our data suggest that taste disturbances reported in ciguatera poisoning might be due to the ability of ciguatera toxins to interfere with ion channels in taste buds.     

Hysteresis in the voltage dependence of HCN channels: conversion between two modes affects pacemaker properties

Hyperpolarization-activated, cyclic nucleotide-gated (HCN) ion channels are important for rhythmic activity in the brain and in the heart. In this study, using ionic and gating current measurements, we show that cloned spHCN channels undergo a hysteresis in their voltage dependence during normal gating. For example, both the gating charge versus voltage curve, Q(V), and the conductance versus voltage curve, G(V), are shifted by about +60 mV when measured from a hyperpolarized holding potential compared with a depolarized holding potential. In addition, the kinetics of the tail current and the activation current change in parallel to the voltage shifts of the Q(V) and G(V) curves. Mammalian HCN1 channels display similar effects in their ionic currents, suggesting that the mammalian HCN channels also undergo voltage hysteresis. We propose a model in which HCN channels transit between two modes. The voltage dependence in the two modes is shifted relative to each other, and the occupancy of the two modes depends on the previous activation of the channel. The shifts in the voltage dependence are fast (tau approximately 100 ms) and are not accompanied by any apparent inactivation. In HCN1 channels, the shift in voltage dependence is slower in a 100 mM K extracellular solution compared with a 1 mM K solution. Based on these findings, we suggest that molecular conformations similar to slow (C-type) inactivation of K channels underlie voltage hysteresis in HCN channels. The voltage hysteresis results in HCN channels displaying different voltage dependences during different phases in the pacemaker cycle. Computer simulations suggest that voltage hysteresis in HCN channels decreases the risk of arrhythmia in pacemaker cells.     

A common pathway for charge transport through voltage-sensing domains

Voltage-gated ion channels derive their voltage sensitivity from the movement of specific charged residues in response to a change in transmembrane potential. Several studies on mechanisms of voltage sensing in ion channels support the idea that these gating charges move through a well-defined permeation pathway. This gating pathway in a voltage-gated ion channel can also be mutated to transport free cations, including protons. The recent discovery of proton channels with sequence homology to the voltage-sensing domains suggests that evolution has perhaps exploited the same gating pathway to generate a bona fide voltage-dependent proton transporter. Here we will discuss implications of these findings on the mechanisms underlying charge (and ion) transport by voltage-sensing domains.     

An electrostatic potassium channel opener targeting the final voltage sensor transition

Free polyunsaturated fatty acids (PUFAs) modulate the voltage dependence of voltage-gated ion channels. As an important consequence thereof, PUFAs can suppress epileptic seizures and cardiac arrhythmia. However, molecular details for the interaction between PUFA and ion channels are not well understood. In this study, we have localized the site of action for PUFAs on the voltage-gated Shaker K channel by introducing positive charges on the channel surface, which potentiated the PUFA effect. Furthermore, we found that PUFA mainly affects the final voltage sensor movement, which is closely linked to channel opening, and that specific charges at the extracellular end of the voltage sensor are critical for the PUFA effect. Because different voltage-gated K channels have different charge profiles, this implies channel-specific PUFA effects. The identified site and the pharmacological mechanism will potentially be very useful in future drug design of small-molecule compounds specifically targeting neuronal and cardiac excitability.     

Temperature and embryonic development in polar marine invertebrates

The life-history tactics of many Antarctic marine invertebrates suggest that the commonly observed slow rates of growth are adaptations to the pattern of food availability, and not due to low temperature per se. This implies that marine invertebrates have been able, over the course of evolutionary time, to compensate their rates of embryonic development for the effect of temperature. Data from north Atlantic copepods indicate that this is so. It is therefore suggested that the slow rates of embryonic development in many Antarctic marine invertebrates are the result of large egg size, and not the low temperature. Large, slowly developing eggs are part of a suite of tactics, often called K-strategies, which characterise many marine invertebrates in Antarctica.