Occult Drosophila calcium channels and twinning of calcium and voltage-activated potassium channels

Examining the effect of iron (ferric) on physiological processes: Invertebrate models

Iron is a common and essential element for maintaining life in bacteria, plants and animals and is found in soil, fresh waters and marine waters; however, over exposure is toxic to organisms. Iron is used in electron transport complexes within mitochondria as well as a co-factor in many essential proteins. It is also established that iron accumulation in the central nervous system in mammals is associated with various neurological disorders. Ample studies have investigated the long-term effects of iron overload in the nervous system. However, its acute effects in nervous tissue and additional organ systems warrant further studies. This study investigates the effects of iron overload on development, behavior, survival, cardiac function, and glutamatergic synaptic transmission in the Drosophila melanogaster. Additionally, physiological responses in crayfish were examined following Fe3+ exposure. Fe3+ reduced neuronal excitability in proprioceptive neurons in a crayfish model. Thus, Fe3+ may block stretch activated channels (SACs) as well as voltage-gated Na+ channels. Exposure also rapidly reduces synaptic transmission but does not block ionotropic glutamatergic receptors, suggesting a blockage of pre-synaptic voltage-gated Ca2+ channels in both crustacean and Drosophila models. The effects are partly reversible with acute exposure, indicating the cells are not rapidly damaged. This study is relevant in demonstrating the effects of Fe3+ on various physiological functions in different organisms in order to further understand the acute and long-term consequences of overload.

Calcium-activated potassium channel of the tobacco hornworm, Manduca sexta: molecular characterization and expression analysis

Large-conductance calcium- and voltage-gated potassium channels (BK or Slowpoke) serve as dynamic integrators linking electrical signaling and intracellular activity. These channels can mediate many different Ca2+-dependent physiological processes including the regulation of neuronal and neuroendocrine cell excitability and muscle contraction. To gain insights into the function of BK channels in vivo, we isolated a full-length cDNA encoding the alpha subunit of a Slowpoke channel from the tobacco hornworm, Manduca sexta (msslo). Amino acid sequence comparison of the deduced Manduca protein revealed at least 80% identity to the insect Slo channels. The five C-terminal alternative splice regions are conserved, but the cloned cDNA fragments contained some unique combinations of exons E, G and I. Our spatial profile revealed that transcript levels were highest in skeletal muscle when compared with the central nervous system (CNS) and visceral muscle. The temporal profile suggested that msslo expression is regulated developmentally in a tissue- and regional-specific pattern. The levels of msslo transcripts remain relatively constant throughout metamorphosis in the CNS, transiently decline in the heart and are barely detectable in the gut except in adults. A dramatic upregulation of msslo transcript levels occurs in thoracic but not abdominal dorsal longitudinal body wall muscles (DLM), suggesting that the msSlo current plays an important role in the excitation or contractile properties of the phasic flight muscle. Our developmental profile of msslo expression suggests that msSlo currents may contribute to the changes in neural circuits and muscle properties that produce stage-specific functions and behaviors.

Non-synaptic ion channels in insects--basic properties of currents and their modulation in neurons and skeletal muscles

Insects are favoured objects for studying information processing in restricted neuronal networks, e.g. motor pattern generation or sensory perception. The analysis of the underlying processes requires knowledge of the electrical properties of the cells involved. These properties are determined by the expression pattern of ionic channels and by the regulation of their function, e.g. by neuromodulators. We here review the presently available knowledge on insect non-synaptic ion channels and ionic currents in neurons and skeletal muscles. The first part of this article covers genetic and structural informations, the localization of channels, their electrophysiological and pharmacological properties, and known effects of second messengers and modulators such as neuropeptides or biogenic amines. In a second part we describe in detail modulation of ionic currents in three particularly well investigated preparations, i.e. Drosophila photoreceptor, cockroach DUM (dorsal unpaired median) neuron and locust jumping muscle. Ion channel structures are almost exclusively known for the fruitfly Drosophila, and most of the information on their function has also been obtained in this animal, mainly based on mutational analysis and investigation of heterologously expressed channels. Now the entire genome of Drosophila has been sequenced, it seems almost completely known which types of channel genes--and how many of them--exist in this animal. There is much knowledge of the various types of channels formed by 6-transmembrane--spanning segments (6TM channels) including those where four 6TM domains are joined within one large protein (e.g. classical Na+ channel). In comparison, two TM channels and 4TM (or tandem) channels so far have hardly been explored. There are, however, various well characterized ionic conductances, e.g. for Ca2+, Cl- or K+, in other insect preparations for which the channels are not yet known. In some of the larger insects, i.e. bee, cockroach, locust and moth, rather detailed information has been established on the role of ionic currents in certain physiological or behavioural contexts. On the whole, however, knowledge of non-synaptic ion channels in such insects is still fragmentary. Modulation of ion currents usually involves activation of more or less elaborate signal transduction cascades. The three detailed examples for modulation presented in the second part indicate, amongst other things, that one type of modulator usually leads to concerted changes of several ion currents and that the effects of different modulators in one type of cell may overlap. Modulators participate in the adaptive changes of the various cells responsible for different physiological or behavioural states. Further study of their effects on the single cell level should help to understand how small sets of cells cooperate in order to produce the appropriate output.

Genetic dissection of functional contributions of specific potassium channel subunits in habituation of an escape circuit in Drosophila

Potassium channels have been implicated in central roles in activity-dependent neural plasticity. The giant fiber escape pathway of Drosophila has been established as a model for analyzing habituation and its modification by memory mutations in an identified circuit. Several genes in Drosophila encoding K+ channel subunits have been characterized, permitting examination of the contributions of specific channel subunits to simple conditioning in an identified circuit that is amenable to genetic analysis. Our results show that mutations altering each of four K+ channel subunits (Sh, slo, eag, and Hk) have distinct effects on habituation at least as strong as those of dunce and rutabaga, memory mutants with defective cAMP metabolism (). Habituation, spontaneous recovery, and dishabituation of the electrically stimulated long-latency giant fiber pathway response were shown in each mutant type. Mutations of Sh (voltage-gated) and slo (Ca2+-gated) subunits enhanced and slowed habituation, respectively. However, mutations of eag and Hk subunits, which confer K+-current modulation, had even more extreme phenotypes, again enhancing and slowing habituation, respectively. In double mutants, Sh mutations moderated the strong phenotypes of eag and Hk, suggesting that their modulatory functions are best expressed in the presence of intact Sh subunits. Nonactivity-dependent responses (refractory period and latency) at two stages of the circuit were altered only in some mutants and do not account for modifications of habituation. Furthermore, failures of the long-latency response during habituation, which normally occur in labile connections in the brain, could be induced in the thoracic circuit stage in Hk mutants. Our work indicates that different K+ channel subunits play distinct roles in activity-dependent neural plasticity and thus can be incorporated along with second messenger "memory" loci to enrich the genetic analysis of learning and memory.

Allosteric gating of a large conductance Ca-activated K+ channel

Large-conductance Ca-activated potassium channels (BK channels) are uniquely sensitive to both membrane potential and intracellular Ca2+. Recent work has demonstrated that in the gating of these channels there are voltage-sensitive steps that are separate from Ca2+ binding steps. Based on this result and the macroscopic steady state and kinetic properties of the cloned BK channel mslo, we have recently proposed a general kinetic scheme to describe the interaction between voltage and Ca2+ in the gating of the mslo channel (Cui, J., D.H. Cox, and R.W. Aldrich. 1997. J. Gen. Physiol. In press.). This scheme supposes that the channel exists in two main conformations, closed and open. The conformational change between closed and open is voltage dependent. Ca2+ binds to both the closed and open conformations, but on average binds more tightly to the open conformation and thereby promotes channel opening. Here we describe the basic properties of models of this form and test their ability to mimic mslo macroscopic steady state and kinetic behavior. The simplest form of this scheme corresponds to a voltage-dependent version of the Monod-Wyman-Changeux (MWC) model of allosteric proteins. The success of voltage-dependent MWC models in describing many aspects of mslo gating suggests that these channels may share a common molecular mechanism with other allosteric proteins whose behaviors have been modeled using the MWC formalism. We also demonstrate how this scheme can arise as a simplification of a more complex scheme that is based on the premise that the channel is a homotetramer with a single Ca2+ binding site and a single voltage sensor in each subunit. Aspects of the mslo data not well fitted by the simplified scheme will likely be better accounted for by this more general scheme. The kinetic schemes discussed in this paper may be useful in interpreting the effects of BK channel modifications or mutations.

Intrinsic voltage dependence and Ca2+ regulation of mslo large conductance Ca-activated K+ channels

The kinetic and steady-state properties of macroscopic mslo Ca-activated K+ currents were studied in excised patches from Xenopus oocytes. In response to voltage steps, the timecourse of both activation and deactivation, but for a brief delay in activation, could be approximated by a single exponential function over a wide range of voltages and internal Ca2+ concentrations ([Ca]i). Activation rates increased with voltage and with [Ca]i, and approached saturation at high [Ca]i. Deactivation rates generally decreased with [Ca]i and voltage, and approached saturation at high [Ca]i. Plots of the macroscopic conductance as a function of voltage (G-V) and the time constant of activation and deactivation shifted leftward along the voltage axis with increasing [Ca]i. G-V relations could be approximated by a Boltzmann function with an equivalent gating charge which ranged between 1.1 and 1.8 e as [Ca]i varied between 0.84 and 1,000 microM. Hill analysis indicates that at least three Ca2+ binding sites can contribute to channel activation. Three lines of evidence indicate that there is at least one voltage-dependent unimolecular conformational change associated with mslo gating that is separate from Ca2+ binding. (a) The position of the mslo G-V relation does not vary logarithmically with [Ca]i. (b) The macroscopic rate constant of activation approaches saturation at high [Ca]i but remains voltage dependent. (c) With strong depolarizations mslo currents can be nearly maximally activated without binding Ca2+. These results can be understood in terms of a channel which must undergo a central voltage-dependent rate limiting conformational change in order to move from closed to open, with rapid Ca2+ binding to both open and closed states modulating this central step.

Pharmacological analysis of heartbeat in Drosophila

Analysis of the mechanisms underlying cardiac excitability can be facilitated greatly by mutations that disrupt ion channels and receptors involved in this excitability. With an extensive repertoire of such mutations, Drosophila provides the best available genetic model for these studies. However, the use of Drosophila for this purpose has been severely handicapped by lack of a suitable preparation of heart and a complete lack of knowledge about the ionic currents that underlie its excitability. We describe a simple preparation to measure heartbeat in Drosophila. This preparation was used to ask if heartbeat in Drosophila is myogenic in origin, and to determine the types of ion channels involved in influencing the heart rate. Tetrodotoxin, even at a high concentration of 40 microM, did not affect heart rate, indicating that heartbeat may be myogenic in origin and that it may not be determined by Na+ channels. Heart rate was affected by PN200-110, verapamil, and diltiazem, which block vertebrate L-type Ca2+ channels. Thus, L-type channels, which contribute to the prolonged plateau of action potentials in vertebrate heart, may play a role in Drosophila cardiac excitability. It also suggests that Drosophila heart is subject to a similar intervention by organic Ca2+ channel blockers as the vertebrate heart. A role for K+ currents in the function of Drosophila heart was suggested by an effect of tetraethylammonium, which blocks all the four identified K+ currents in the larval body wall muscles, and quinidine, which blocks the delayed rectifier K+ current in these muscles. The preparation described here also provides an extremely simple method for identifying mutations that affect heart rate. Such mutations and pharmacological agents will be very useful for analyzing molecular components of cardiac excitability in Drosophila.

Tissue-specific alternative splicing of hybrid Shaker/lacZ genes correlates with kinetic differences in Shaker K+ currents in vivo

Alternative splicing of the Shaker (Sh) locus of Drosophila generates several transcripts with divergent 5' and 3' domains that produce kinetically distinct K+ currents in Xenopus oocytes. Although suggestive that alternative splicing may be involved in generating K+ channel diversity, clear tissue-specific differences in the distribution of particular Sh gene products have not been demonstrated. Using lacZ as a reporter gene for accurate splicing of variable Sh3' domains, we observe differences in beta-galactosidase expression patterns in transgenic animals that indicate both temporal and spatial regulation of 3' splice choice. The differences in 3'splice choice can account for variation in recovery kinetics of Sh-encoded K+ currents recorded in adult flies. The results indicate that tissue-specific expression of functionally distinct Sh K+ channels is regulated, in part, at the level of pre-mRNA splicing and implicate sequences in or around the 3' splice sites in regulating the choice of 3' domain.

Alteration of four identified K+ currents in Drosophila muscle by mutations in eag

Voltage-clamp analysis of Drosophila larval muscle revealed that ether à go-go (eag) mutations affected all identified potassium currents, including those specifically eliminated by mutations in the Shaker or slowpoke gene. Together with DNA sequence analysis, the results suggest that the eag locus encodes a subunit common to different potassium channels. Thus, combinatorial assembly of polypeptides from different genes may contribute to potassium channel diversity.

Single-channel K+ currents in Drosophila muscle and their pharmacological block

Four types of nonvoltage-activated potassium channels in the body-wall muscles of Drosophila third instar larvae have been identified by the patch-clamp technique. Using the inside-out configuration, tetraethylammonium (TEA), Ba2+, and quinidine were applied to the cytoplasmic face of muscle membranes during steady-state channel activation. The four channels could be readily distinguished on the basis of their pharmacological sensitivities and physiological properties. The KST channel was the only type that was activated by stretch. It had a high unitary conductance (100 pS in symmetrical 130/130 mM KCl solution), was blocked by TEA (Kd approximately 35 mM), and was the most sensitive to Ba2+ (complete block at 10(-4) M). A Ca(2+)-activated potassium channel, KCF.72 pS (130/130) mM KCl), was gated open at greater than 10(-8) m Ca2+, was the least sensitive to Ba2+ Kd of approximately 3 mM) and TEA (Kd of approximately 100 mM), and was not affected by quinidine. K2 was a small conductance channel of 11 pS (130/2 KCl, pipette/bath), and was very sensitive to quinidine, being substantially blocked at 0.1 mM. It also exhibited a half block at approximately 0.3 mM Ba2+ and approximately 25 mM TEA. A fourth channel type, K3, was the most sensitive to TEA (half block less than 1 mM). It displayed a partial block to Ba2+ at 10 mM, but no block by 0.1 mM quinidine. The blocking effects of TEA, Ba2+ and quinidine were reversible in all channels studied. The actions of TEA and Ba2+ appeared qualitatively different: in all four channels, TEA reduced the apparent unitary conductance, whereas Ba2+ decreased channel open probability.

Expression of ion channel genes in Drosophila

Ion channel transcripts from three genes were localized by the method of tissue in situ hybridization. The genes examined were the Drosophila Na+ channel genes, paralytic (para) and Drosophila Sodium Channel (DSC), and the K+ channel gene, Shaker( Sh). All three of the genes were expressed in cell bodies of the fly central nervous system, including optic lobes, central brain, suboesophageal ganglion, and thoracico-abdominal ganglion. Sh was additionally expressed in the photoreceptor cells of the retina and pupal flight muscle, while para and DSC were not. The temporal expression pattern of Sh in muscle was different from that in the central nervous system: muscle expression was transient and limited to mid-pupal stage while nervous system expression was observed throughout pupation, apparently peaking at the late-pupal stage. Only one class of 5' end was found in pupal muscle, possibly indicating regulation of splicing pathways.

Mutational and gene dosage analysis of calcium-activated potassium channels in Drosophila: correlation of micro- and macroscopic currents

In Drosophila, two Ca2(+)-activated K+ currents, ICF and ICS, have previously been distinguished in conventional voltage clamp experiments. The slowpoke (slo) mutation eliminates ICF specifically. We report that in patch clamp recordings a single-channel Ca2(+)-activated K+ current is readily distinguished from other channel activities in normal larval muscle membrane, whereas no such current is observed in slo muscles. This single-channel current thus correlates with the macroscopic ICF. No obvious differences in amplitude or properties were detected between normal (+/+) and heterozygous (slo/+) ICF channels in whole-cell voltage clamp recordings or single-channel patch clamp recordings. These results are consistent with the hypothesis that slo is a structural gene for the ICF channels only under certain conditions. The selective effect of the slo mutation may reflect a defect in a regulatory mechanism that is specific for the functioning of the ICF channel protein.

Proton-induced Na+ current develops prior to voltage-dependent Na+ and Ca2+ currents in neuronal precursor cells from chick dorsal root ganglion

Proton-induced Na+ currents (INa(H] were measured in precursor cells from chick dorsal root ganglia with whole-cell patch-clamp recording. Precursor cells were isolated using the method of Rohrer et al. (EMBO. J., 4 (1985) 1709-1714) and fast pH changes were applied with a technique developed by Davies et al. (J. Physiol. (Lond.), 400 (1988) 159-187). Proton-induced transient Na+ currents showing the same properties as in more mature neurons could be elicited already early in differentiation, before high-voltage activated Ca2+ currents and voltage-dependent Na+ currents develop.

Complete separation of four potassium currents in Drosophila

A number of voltage-activated and Ca2+ activated K+ currents are known to coexist and play a major role in a wide variety of cellular processes including neuromuscular phenomena. Separation of these currents is important for analyzing their individual functional roles and for understanding whether or not they are mediated by entirely different channels. In Drosophila, we have now been able to manipulate four different K+ currents, individually and in combination with one another, by a combined use of mutations and pharmacological agents. This allows analysis of the physiological and molecular properties of different K+ channels and of the role of individual currents in membrane excitability.

Diversity of calcium ion channels in cellular membranes

Successful introduction of techniques for separation of different ionic currents and recording of single channel activity has demonstrated the diversity of membrane structures responsible for generation of calcium signal during various forms of cellular activity. In excitable cells the electrically-operated calcium channels have been separated into two types functioning in different membrane potential ranges (low- and high-threshold ones). The low-threshold channels are ontogenetically primary and may play a role in regulation of cell development and differentiation. A similar function may also be characteristic of chemically-operated channels in some highly specialized cells (lymphocytes). The high-threshold channels in excitable cells generate an intracellular signal coupling membrane excitation and intracellular metabolic processes responsible for specific cellular reactions (among them retention of traces of previous activity in neurons--"learning"--being especially important). Chemically-operated N-methyl-D-aspartate-channels also participate in this function. The calcium signal can be potentiated by activation of calcium-operated channels in the membranes of intracellular structures, resulting in the liberation of calcium ions from the intracellular stores. Although different types of calcium channels have some common features in their structure which may indicate their genetic similarity, their specific properties make them well suited for participation in a wide range of cellular mechanisms.

Ion channel expression by white matter glia: I. Type 2 astrocytes and oligodendrocytes

White matter is a compact structure consisting primarily of neuronal axons and glial cells. As in other parts of the nervous system, the function of glial cells in white matter is poorly understood. We have explored the electrophysiological properties of two types of glial cells found predominantly in white matter: type 2 astrocytes and oligodendrocytes. Whole-cells and single-channel patch-clamp techniques were used to study these cell types in postnatal rat optic nerve cultures prepared according to the procedures of Raff et al. (Nature, 303:390-396, 1983b). Type 2 astrocytes in culture exhibit a "neuronal" channel phenotype, expressing at least six distinct ion channel types. With whole-cell recording we observed three inward currents: a voltage-sensitive sodium current qualitatively similar to that found in neurons and both transient and sustained calcium currents. In addition, type 2 astrocytes had two components of outward current: a delayed potassium current which activated at 0 mV and an inactivating calcium-dependent potassium current which activated at -30 mV. Type 2 astrocytes in culture could be induced to fire single regenerative potentials in response to injections of depolarizing current. Single-channel recording demonstrated the presence of an outwardly rectifying chloride channel in both type 2 astrocytes and oligodendrocytes, but this channel could only be observed in excised patches. Oligodendrocytes expressed only one other current: an inwardly rectifying potassium current that is mediated by 30- and 120-pS channels. Because these channels preferentially conduct potassium from outside to inside the cell, and because they are open at the resting potential of the cell, they would be appropriate for removing potassium from the extracellular space; thus it is proposed that oligodendrocytes, besides myelinating axons, play an important role in potassium regulation in white matter. The conductances present in oligodendrocytes suggest a "modulated Boyle and Conway mechanism" of potassium accumulation.

Calcium channels and calcium channel antagonists

Changes in free intracellular Ca2+ levels provide signals that allow nerve and muscle cells to respond to a host of external stimuli. A major mechanism for elevating the level of intracellular Ca2+ is the influx of extracellular Ca2+ through voltage-dependent channels in the cell membrane. Recent research has yielded new insights into the physiological properties, molecular structure, biochemical regulation, and functional heterogeneity of voltage-dependent Ca2+ channels. In addition, Ca2+ channel antagonist drugs have been developed that are valuable both as probes of channel structure and function and as therapeutic agents. Preliminary evidence suggests that these drugs may be useful in the treatment of diverse neurological disorders, including headache, subarachnoid hemorrhage, stroke, and epilepsy.