Overexpression of a Shaker-type potassium channel in mammalian central nervous system dysregulates native potassium channel gene expression

Optogenetic control of neural activity: The biophysics of microbial rhodopsins in neuroscience

Optogenetics, the use of microbial rhodopsins to make the electrical activity of targeted neurons controllable by light, has swept through neuroscience, enabling thousands of scientists to study how specific neuron types contribute to behaviors and pathologies, and how they might serve as novel therapeutic targets. By activating a set of neurons, one can probe what functions they can initiate or sustain, and by silencing a set of neurons, one can probe the functions they are necessary for. We here review the biophysics of these molecules, asking why they became so useful in neuroscience for the study of brain circuitry. We review the history of the field, including early thinking, early experiments, applications of optogenetics, pre-optogenetics targeted neural control tools, and the history of discovering and characterizing microbial rhodopsins. We then review the biophysical attributes of rhodopsins that make them so useful to neuroscience - their classes and structure, their photocycles, their photocurrent magnitudes and kinetics, their action spectra, and their ion selectivity. Our hope is to convey to the reader how specific biophysical properties of these molecules made them especially useful to neuroscientists for a difficult problem - the control of high-speed electrical activity, with great precision and ease, in the brain.

The sigma-1 receptor behaves as an atypical auxiliary subunit to modulate the functional characteristics of Kv1.2 channels expressed in HEK293 cells

Expression of Kv1.2 within Kv1.x potassium channel complexes is critical in maintaining appropriate neuronal excitability and determining the threshold for action potential firing. This is attributed to the interaction of Kv1.2 with a hitherto unidentified protein that confers bimodal channel activation gating, allowing neurons to adapt to repetitive trains of stimulation and protecting against hyperexcitability. One potential protein candidate is the sigma-1 receptor (Sig-1R), which regulates other members of the Kv1.x channel family; however, the biophysical nature of the interaction between Sig-1R and Kv1.2 has not been elucidated. We hypothesized that Sig-1R may regulate Kv1.2 and may further act as the unidentified modulator of Kv1.2 activation. In transiently transfected HEK293 cells, we found that ligand activation of the Sig-1R modulates Kv1.2 current amplitude. More importantly, Sig-1R interacts with Kv1.2 in baseline conditions to influence bimodal activation gating. These effects are abolished in the presence of the auxiliary subunit Kvβ2 and when the Sig-1R mutation underlying ALS16 (Sig-1R-E102Q), is expressed. These data suggest that Kvβ2 occludes the interaction of Sig-1R with Kv1.2, and that E102 may be a residue critical for Sig-1R modulation of Kv1.2. The results of this investigation describe an important new role for Sig-1R in the regulation of neuronal excitability and introduce a novel mechanism of pathophysiology in Sig-1R dysfunction.

Biology and Bias in Cell Type-Specific RNAseq of Nucleus Accumbens Medium Spiny Neurons

Subcellular RNAseq promises to dissect transcriptional dynamics but is not well characterized. Furthermore, FACS may introduce bias but has not been benchmarked genome-wide. Finally, D1 and D2 dopamine receptor-expressing medium spiny neurons (MSNs) of the nucleus accumbens (NAc) are fundamental to neuropsychiatric traits but have only a short list of canonical surface markers. We address these gaps by systematically comparing nuclear-FACS, whole cell-FACS, and RiboTag affinity purification from D1- and D2-MSNs. Using differential expression, variance partitioning, and co-expression, we identify the following trade-offs for each method. RiboTag-seq best distinguishes D1- and D2-MSNs but has the lowest transcriptome coverage. Nuclear-FACS-seq generates the most differentially expressed genes and overlaps significantly with neuropsychiatric genetic risk loci, but un-annotated genes hamper interpretation. Whole cell-FACS is more similar to nuclear-FACS than RiboTag, but captures aspects of both. Using pan-method approaches, we discover that transcriptional regulation is predominant in D1-MSNs, while D2-MSNs tend towards cytosolic regulation. We are also the first to find evidence for moderate sexual dimorphism in these cell types at baseline. As these results are from 49 mice (n_male = 39, n_female = 10), they represent generalizable ground-truths. Together, these results guide RNAseq methods selection, define MSN transcriptomes, highlight neuronal sex differences, and provide a baseline for D1- and D2-MSNs.

Potassium Channels in Epilepsy

This review attempts to give a concise and up-to-date overview on the role of potassium channels in epilepsies. Their role can be defined from a genetic perspective, focusing on variants and de novo mutations identified in genetic studies or animal models with targeted, specific mutations in genes coding for a member of the large potassium channel family. In these genetic studies, a demonstrated functional link to hyperexcitability often remains elusive. However, their role can also be defined from a functional perspective, based on dynamic, aggravating, or adaptive transcriptional and posttranslational alterations. In these cases, it often remains elusive whether the alteration is causal or merely incidental. With ∼80 potassium channel types, of which ∼10% are known to be associated with epilepsies (in humans) or a seizure phenotype (in animals), if genetically mutated, a comprehensive review is a challenging endeavor. This goal may seem all the more ambitious once the data on posttranslational alterations, found both in human tissue from epilepsy patients and in chronic or acute animal models, are included. We therefore summarize the literature, and expand only on key findings, particularly regarding functional alterations found in patient brain tissue and chronic animal models.

Pathway-driven discovery of epilepsy genes

Epilepsy genes deliver critical insights into the molecular control of brain synchronization and are revolutionizing our understanding and treatment of the disease. The epilepsy-associated genome is rapidly expanding, and two powerful complementary approaches, isolation of de novo exome variants in patients and targeted mutagenesis in model systems, account for the steep increase. In sheer number, the tally of genes linked to seizures will likely match that of cancer and exceed it in biological diversity. The proteins act within most intracellular compartments and span the molecular determinants of firing and wiring in the developing brain. Every facet of neurotransmission, from dendritic spine to exocytotic machinery, is in play, and defects of synaptic inhibition are over-represented. The contributions of somatic mutations and noncoding microRNAs are also being explored. The functional spectrum of established epilepsy genes and the arrival of rapid, precise technologies for genome editing now provide a robust scaffold to prioritize hypothesis-driven discovery and further populate this genetic proto-map. Although each gene identified offers translational potential to stratify patient care, the complexity of individual variation and covert actions of genetic modifiers may confound single-gene solutions for the clinical disorder. In vivo genetic deconstruction of epileptic networks, ex vivo validation of variant profiles in patient-derived induced pluripotent stem cells, in silico variant modeling and modifier gene discovery, now in their earliest stages, will help clarify individual patterns. Because seizures stand at the crossroads of all neuronal synchronization disorders in the developing and aging brain, the neurobiological analysis of epilepsy-associated genes provides an extraordinary gateway to new insights into higher cortical function.

Organization and maintenance of molecular domains in myelinated axons

Over a century ago, Ramon y Cajal first proposed the idea of a directionality involved in nerve conduction and neuronal communication. Decades later, it was discovered that myelin, produced by glial cells, insulated axons with periodic breaks where nodes of Ranvier (nodes) form to allow for saltatory conduction. In the peripheral nervous system (PNS), Schwann cells are the glia that can either individually myelinate the axon from one neuron or ensheath axons of many neurons. In the central nervous system (CNS), oligodendrocytes are the glia that myelinate axons from different neurons. Review of more recent studies revealed that this myelination created polarized domains adjacent to the nodes. However, the molecular mechanisms responsible for the organization of axonal domains are only now beginning to be elucidated. The molecular domains in myelinated axons include the axon initial segment (AIS), where various ion channels are clustered and action potentials are initiated; the node, where sodium channels are clustered and action potentials are propagated; the paranode, where myelin loops contact with the axolemma; the juxtaparanode (JXP), where delayed-rectifier potassium channels are clustered; and the internode, where myelin is compactly wrapped. Each domain contains a unique subset of proteins critical for the domain's function. However, the roles of these proteins in axonal domain organization are not fully understood. In this review, we highlight recent advances on the molecular nature and functions of some of the components of each axonal domain and their roles in axonal domain organization and maintenance for proper neuronal communication.

Heterologous expression of a glial Kir channel (KCNJ10) in a neuroblastoma spinal cord (NSC-34) cell line

Heterologous expression of Kir channels offers a tool to modulate excitability of neurons which provide insight into Kir channel functions in general. Inwardly-rectifying K+ channels (Kir channels) are potential candidate proteins to hyperpolarize neuronal cell membranes. However, heterologous expression of inwardly-rectifying K+ channels has previously proven to be difficult. This was mainly due to a high toxicity of the respective Kir channel expression. We investigated the putative role of a predominantly glial-expressed, weakly rectifying Kir channel (Kir4.1 channel subunit; KCNJ10) in modulating electrophysiological properties of a motoneuron-like cell culture (NSC-34). Transfection procedures using an EGFP-tagged Kir4.1 protein in this study proved to have no toxic effects on NSC-34 cells. Using whole cell-voltage clamp, a substantial increase of inward rectifying K+ currents as well as hyperpolarization of the cell membrane was observed in Kir4.1-transfected cells. Na+ inward currents, observed in NSC-34 controls, were absent in Kir4.1/EGFP motoneuronal cells. The Kir4.1-transfection did not influence the NaV1.6 sodium channel expression. This study demonstrates the general feasibility of a heterologous expression of a weakly inward-rectifying K+ channel (Kir4.1 subunit) and shows that in vitro overexpression of Kir4.1 shifts electrophysiological properties of neuronal cells to a more glial-like phenotype and may therefore be a candidate tool to dampen excitability of neurons in experimental paradigms.

Selective and quickly reversible inactivation of mammalian neurons in vivo using the Drosophila allatostatin receptor

Genetic strategies for perturbing activity of selected neurons hold great promise for understanding circuitry and behavior. Several such strategies exist, but there has been no direct demonstration of reversible inactivation of mammalian neurons in vivo. We previously reported quickly reversible inactivation of neurons in vitro using expression of the Drosophila allatostatin receptor (AlstR). Here, adeno-associated viral vectors are used to express AlstR in vivo in cortical and thalamic neurons of rats, ferrets, and monkeys. Application of the receptor's ligand, allatostatin (AL), leads to a dramatic reduction in neural activity, including responses of visual neurons to optimized visual stimuli. Additionally, AL eliminates activity in spinal cords of transgenic mice conditionally expressing AlstR. This reduction occurs selectively in AlstR-expressing neurons. Inactivation can be reversed within minutes upon washout of the ligand and is repeatable, demonstrating that the AlstR/AL system is effective for selective, quick, and reversible silencing of mammalian neurons in vivo.

Rapid and reversible chemical inactivation of synaptic transmission in genetically targeted neurons

Inducible and reversible silencing of selected neurons in vivo is critical to understanding the structure and dynamics of brain circuits. We have developed Molecules for Inactivation of Synaptic Transmission (MISTs) that can be genetically targeted to allow the reversible inactivation of neurotransmitter release. MISTs consist of modified presynaptic proteins that interfere with the synaptic vesicle cycle when crosslinked by small molecule "dimerizers." MISTs based on the vesicle proteins VAMP2/Synaptobrevin and Synaptophysin induced rapid ( approximately 10 min) and reversible block of synaptic transmission in cultured neurons and brain slices. In transgenic mice expressing MISTs selectively in Purkinje neurons, administration of dimerizer reduced learning and performance of the rotarod behavior. MISTs allow for specific, inducible, and reversible lesions in neuronal circuits and may provide treatment of disorders associated with neuronal hyperactivity.

Neuropathology in Drosophila membrane excitability mutants

Mutations affecting ion channels and neuronal membrane excitability have been identified in Drosophila as well as in other organisms and characterized for their acute effects on behavior and neuronal function. However, the long-term effect of these perturbations on the maintenance of neuronal viability has not been studied in detail. Here we perform an initial survey of mutations affecting Na+ channels and K+ channels in Drosophila to investigate their effects on life span and neuronal viability as a function of age. We find that mutations that decrease membrane excitability as well as those that increase excitability can trigger neurodegeneration to varying degrees. Results of double-mutant interactions with dominant Na+/K+ ATPase mutations, which themselves cause severe neurodegeneration, suggest that excitotoxicity owing to hyperexcitability is insufficient to explain the resultant phenotype. Although the exact mechanisms remain unclear, our results suggest that there is an important link between maintenance of proper neuronal signaling and maintenance of long-term neuronal viability. Disruption of these signaling mechanisms in any of a variety of ways increases the incidence of neurodegeneration.

Dissecting neural circuitry by combining genetics and pharmacology

In systems neuroscience, advances often come from lesioning and reversible inhibition of brain regions. Dissecting the circuitry of regions involves conceptually the same approach - stop a class of cell from firing action potentials, or make the cells fire more, then deduce how these components influence the performance of the circuit and animal behaviour. To perform such cell-type-specific and reversible fine-scale analysis of circuitry, and to do so on the fast signalling timescale of the brain (milliseconds to seconds), is challenging in mammals. Ingenious and diverse methods are being developed towards this goal. These new tools will encourage further synergy between molecular biologists, systems neuroscientists and electrophysiologists.

Neuronal LRP1 functionally associates with postsynaptic proteins and is required for normal motor function in mice

The LDL receptor-related protein 1 (LRP1) is a multifunctional cell surface receptor that is highly expressed on neurons. Neuronal LRP1 in vitro can mediate ligand endocytosis, as well as modulate signal transduction processes. However, little is known about its role in the intact nervous system. Here, we report that mice that lack LRP1 selectively in differentiated neurons develop severe behavioral and motor abnormalities, including hyperactivity, tremor, and dystonia. Since their central nervous systems appear histoanatomically normal, we suggest that this phenotype is likely attributable to abnormal neurotransmission. This conclusion is supported by studies of primary cultured neurons that show that LRP1 is present in close proximity to the N-methyl-D-aspartate (NMDA) receptor in dendritic synapses and can be coprecipitated with NMDA receptor subunits and the postsynaptic density protein PSD-95 from neuronal cell lysates. Moreover, treatment with NMDA, but not dopamine, reduces the interaction of LRP1 with PSD-95, indicating that LRP1 participates in transmitter-dependent postsynaptic responses. Together, these findings suggest that LRP1, like other ApoE receptors, can modulate synaptic transmission in the brain.

Light-activated ion channels for remote control of neuronal firing

Neurons have ion channels that are directly gated by voltage, ligands and temperature but not by light. Using structure-based design, we have developed a new chemical gate that confers light sensitivity to an ion channel. The gate includes a functional group for selective conjugation to an engineered K(+) channel, a pore blocker and a photoisomerizable azobenzene. Long-wavelength light drives the azobenzene moiety into its extended trans configuration, allowing the blocker to reach the pore. Short-wavelength light generates the shorter cis configuration, retracting the blocker and allowing conduction. Exogenous expression of these channels in rat hippocampal neurons, followed by chemical modification with the photoswitchable gate, enables different wavelengths of light to switch action potential firing on and off. These synthetic photoisomerizable azobenzene-regulated K(+) (SPARK) channels allow rapid, precise and reversible control over neuronal firing, with potential applications for dissecting neural circuits and controlling activity downstream from sites of neural damage or degeneration.

Cell-type-specific splicing of KChIP4 mRNA correlates with slower kinetics of A-type current

In neurons, rapidly inactivating A-type potassium currents regulate repetitive firing and sensitivity to synaptic inputs both in the soma and in the dendrites. It has been established that Kv4 family subunits with several modifying proteins such as KChIPs are responsible for A-type current in most neurons. However, it is not clear which of these modifying proteins are responsible for the observed difference in the properties of A-type currents in the neurons. For example, in globus pallidus (GP) and basal forebrain (BF) neurons in rats, A-type current possesses a slowly inactivating (tau > 80 ms) component of inactivation that is absent in the currents obtained from striatal cholinergic interneurons (StrI) and hippocampal area CA1 pyramidal neurons (HIP). It has been shown that KChIP4 splice variant A but not splice variant B can increase inactivation rates of Kv4 current to > 100 ms in Xenopus oocytes. We tested the hypothesis that cell-specific expression of KChIP4A is responsible for the slow inactivation of A-type current in these neurons. Employing single-cell RT-PCR in acutely dissociated rat neurons, KChIP4A mRNA was detected in 12/14 GP cells and in 12/14 BF neurons whereas it was not detected in any StrI or HIP cells. By contrast, the KChIP4 splice variant B was detected in all four types of cells. Moreover, deactivation rates at -100 mV were slower in BF and GP cells than in StrI and HIP neurons as expected, owing to the presence KChIP4A in BF and GP neurons. These data are consistent with our initial hypothesis.

Chromosome 21 KIR channels in brain development

Two KIR (K+ Inwardly Rectifying) channel genes have been identified on chromosome 21, in a region associated with important phenotypic features of trisomy 21, including mental retardation: KIR3.2 (GIRK2) and KIR4.2. We analysed the expression of these channel genes in developing human and mouse brains to determine the possible role of the corresponding channels in brain development and function. KIR3.2, which has been extensively studied in the mouse, was found to be expressed in the human cerebellum during development. The KIR4.2 channel is expressed later in development in both mice and humans. We compared the expression of these channels in terms of RNA and protein levels and discussed the potential synergy and consequences of the overexpression of these channels in Down's syndrome brain development.

Some aspects of the physiological role of ion channels in the nervous system

Recent analyses of the genomes of several animal species, including man, have revealed that a large number of ion channels are present in the nervous system. Our understanding of the physiological role of these channels in the nervous system has followed the evolution of biophysical techniques during the last century. The observation and the quantification of the electrical events associated with the operation of the ionic channels has been, and still is, one of the best tools to analyse the various aspects of their contribution to nerve function. For this reason, we have chosen to use electrophysiological recordings to illustrate some of the main functions of these channels. The properties and the roles of Na+ and K+ channels in neuronal resting and action potentials are illustrated in the case of the giant axons of the squid and the cockroach. The nature and role of the calcium currents in the bursting behaviour of the neurons are illustrated for Aplysia giant neurons. The relationship between presynaptic calcium currents and synaptic transmission is shown for the squid giant synapse. The involvement of calcium channels in survival and neurite outgrowth of cultured neurons is exemplified using embryonic cockroach brain neurons. This same neuronal preparation is used to illustrate ion channel noise and single-channel events associated with the binding of agonists to nicotinic receptors. Some features of the synaptic activity in the central nervous system are shown, with examples from the cercal nerve giant-axon preparation of the cockroach. The interplay of different ion conductances involved in the oscillatory behaviour of the Xenopus spinal motoneurons is illustrated and discussed. The last part of this review deals with ionic homeostasis in the brain and the function of glial cells, with examples from Necturus and squids.

The biology of epilepsy genes

Mutations in over 70 genes now define biological pathways leading to epilepsy, an episodic dysrhythmia of the cerebral cortex marked by abnormal network synchronization. Some of the inherited errors destabilize neuronal signaling by inflicting primary disorders of membrane excitability and synaptic transmission, whereas others do so indirectly by perturbing critical control points that balance the developmental assembly of inhibitory and excitatory circuits. The genetic diversity is now sufficient to discern short- and long-range functional convergence of epileptogenic molecular pathways, reducing the broad spectrum of primary molecular defects to a few common processes regulating cortical synchronization. Synaptic inhibition appears to be the most frequent target; however, each gene mutation retains unique phenotypic features. This review selects exemplary members of several gene families to illustrate principal categories of the disease and trace the biological pathways to epileptogenesis in the developing brain.

Physiological, anatomical and genetic identification of CPG neurons in the developing mammalian spinal cord

The basic motor patterns underlying rhythmic limb movements during locomotion are generated by neuronal networks located within the spinal cord. These networks are called Central Pattern Generators (CPGs). Isolated spinal cord preparations from newborn rats and mice have become increasingly important for understanding the organization of the CPG in the mammalian spinal cord. Early studies using these preparations have focused on the overall network structure and the localization of the CPG. In this review we concentrate on recent experiments aimed at identifying and characterizing CPG-interneurons in the rodent. These experiments include the organization and function of descending commissural interneurons (dCINs) in the hindlimb CPG of the neonatal rat, as well as the role of Ephrin receptor A4 (EphA4) and its Ephrin ligand B3 (EphrinB3), in the construction of the mammalian locomotor network. These latter experiments have defined EphA4 as a molecular marker for mammalian excitatory hindlimb CPG neurons. We also review genetic approaches that can be applied to the mouse spinal cord. These include methods for identifying sub-populations of neurons by genetically encoded reporters, techniques to trace network connectivity with cell-specific genetically encoded tracers, and ways to selectively ablate or eliminate neuron populations from the CPG. We propose that by applying a multidisciplinary approach it will be possible to understand the network structure of the mammalian locomotor CPG. Such an understanding will be instrumental in devising new therapeutic strategies for patients with spinal cord injury.

Selective elimination of glutamate activation and introduction of fluorescent proteins into a Caenorhabditis elegans chloride channel

Glutamate-gated chloride (GluCl) channels from invertebrates can be activated by ivermectin (IVM) to produce electrical silencing in mammalian neurons. To improve this GluCl/IVM strategy, we sought to mutate the Caenorhabditis elegans GluCl channels so that they become insensitive to glutamate but retain their sensitivity to IVM. Based on structure-function studies of nicotinic acetylcholine receptor superfamily members, we tested in oocytes 19 point mutants at 16 residues in the beta-subunit likely to be involved in the response to glutamate. Y182F reduces the glutamate response by greater than six-fold, with little change to IVM responses, when coexpressed with wild-type (WT) GluCl alpha. For GluCl alphabeta(Y182F), the EC(50) and Hill coefficient for glutamate are similar to those of WT, indicating that the mutant decreases the efficacy of glutamate, but not the potency. Also, fluorescent proteins (enhanced green fluorescent protein, enhanced yellow fluorescent protein, enhanced cyan fluorescent protein; XFP) were inserted into the M3-M4 loop of the GluCl alpha, beta and beta(Y182F). We found no significant functional difference between these XFP-tagged receptors and WT receptors. The modified GluCl channel, without glutamate sensitivity but with a fluorescent tag, may be more useful in GluCl silencing strategies.

NRSF causes cAMP-sensitive suppression of sodium current in cultured hippocampal neurons

The neuron restrictive silencer factor (NRSF/REST) has been shown to bind to the promoters of many neuron-specific genes and is able to suppress transcription of Na(+) channels in PC12 cells, although its functional effect in terminally differentiated neurons is unknown. We constructed lentiviral vectors to express NRSF as a bicistronic message with green fluorescent protein (GFP) and followed infected hippocampal neurons in culture over a period of 1-2 wk. NRSF-expressing neurons showed a time-dependent suppression of Na(+) channel function as measured by whole cell electrophysiology. Suppression was reversed or prevented by the addition of membrane-permeable cAMP analogues and enhanced by cAMP antagonists but not affected by increasing protein expression with a viral enhancer. Secondary effects, including altered sensitivity to glutamate and GABA and reduced outward K(+) currents, were duplicated by culturing GFP-infected control neurons in TTX. The striking similarity of the phenotypes makes NRSF potentially useful as a genetic "silencer" and also suggests avenues of further exploration that may elucidate the transcription factor's in vivo role in neuronal plasticity.

Overexpression of human stanniocalcin affects growth and reproduction in transgenic mice

In mammals stanniocalcin (STC) is widely expressed, and in the kidney and gut it regulates serum calcium levels by promoting phosphate reabsorption. To shed further light on its functional significance in mammals we have created several lines of mice that express a human STC (hSTC) transgene. Three lines expressed the hSTC transgene, but only two lines exhibited high expression and contained circulating hSTC, and in these animals there was a reduction in postnatal growth (30-50%) that persisted after weaning. Moreover, even wild-type pups exhibited a growth retardation phenotype when nursed by a transgenic foster mother, and this implies that hSTC overexpression deleteriously affects maternal behavior and/or lactation. The reproductive potential of female transgenic mice was also compromised, as evidenced by significantly smaller litter sizes, but transgenic male fertility was unchanged even though the transgene was most highly expressed in testes. Interestingly, transgene-derived serum hSTC increased significantly after puberty and was severalfold higher in females than in males, suggesting a gender-specific mechanism for maintaining elevated circulating levels of STC. Blood analysis revealed that both transgenic lines had elevated phosphate and decreased alkaline phosphatase levels, indicative of altered kidney and bone metabolism. These studies provide the first evidence that STC is involved in growth and reproduction and reaffirm its role in mineral homeostasis.

Molecular genetic approaches to the targeted suppression of neuronal activity

Understanding how the diverse cells of the nervous system generate sensations, memories and behaviors is a profound challenge. This is because the activity of most neurons cannot easily be monitored or individually manipulated in vivo. As a result, it has been difficult to determine how different neurons contribute to nervous system function, even in simple organisms like Drosophila. Recent advances promise to change this situation by supplying molecular genetic tools for modulating neuronal activity that can be deployed in a spatially and temporally restricted fashion. In some cases, targeted groups of neurons can be 'switched off' and back 'on' at will in living, behaving animals.

Targeted attenuation of electrical activity in Drosophila using a genetically modified K(+) channel

We describe here a general technique for the graded inhibition of cellular excitability in vivo. Inhibition is accomplished by expressing a genetically modified Shaker K(+) channel (termed the EKO channel) in targeted cells. Unlike native K(+) channels, the EKO channel strongly shunts depolarizing current: activating at potentials near E(K) and not inactivating. Selective targeting of the channel to neurons, muscles, and photoreceptors in Drosophila using the Gal4-UAS system results in physiological and behavioral effects consistent with attenuated excitability in the targeted cells, often with loss of neuronal function at higher transgene dosages. By permitting the incremental reduction of electrical activity, the EKO technique can be used to address a wide range of questions regarding neuronal function.

Functional genomics of Down syndrome: a multidisciplinary approach

The availability of the DNA sequence of human chromosome 21 (HSA21) is a landmark contribution that will have an immediate impact on the study of the role of specific genes to Down syndrome (DS). Trisomy 21, full or partial, is a major cause of mental retardation and other phenotypic abnormalities, collectively known as Down syndrome (DS), a disorder affecting 1 in 700 births. The identification of genes on HSA21 and the elucidation of the function of the proteins encoded by these genes have been a major challenge for the human genome project and for research in DS. Over 100 of the estimated 300-500 genes of HSA21 have been identified, but the function of most remains largely unknown. It is believed that the overexpression of an unknown number of HSA21 genes is directly or indirectly responsible for the mental retardation and the other clinical features of DS. For this reason, HSA21 genes that are expressed in tissues affected in DS patients are of special interest.

Gain of function mutants: ion channels and G protein-coupled receptors

Many ion channels and receptors display striking phenotypes for gain-of-function mutations but milder phenotypes for null mutations. Gain of molecular function can have several mechanistic bases: selectivity changes, gating changes including constitutive activation and slowed inactivation, elimination of a subunit that enhances inactivation, decreased drug sensitivity, changes in regulation or trafficking of the channel, or induction of apoptosis. Decreased firing frequency can occur via increased function of K+ or Cl- channels. Channel mutants also cause gain-of-function syndromes at the cellular and circuit level; of these syndromes, the cardiac long-QT syndromes are explained in a more straightforward way than are the epilepsies. G protein-coupled receptors are also affected by activating mutations.

The sequence of human chromosome 21 and implications for research into Down syndrome

The recent completion of the DNA sequence of human chromosome 21 has provided the first look at the 225 genes that are candidates for involvement in Down syndrome (trisomy 21). A broad functional classification of these genes, their expression data and evolutionary conservation, and comparison with the gene content of the major mouse models of Down syndrome, suggest how the chromosome sequence may help in understanding the complex Down syndrome phenotype.

ROMK1 (Kir1.1) causes apoptosis and chronic silencing of hippocampal neurons

Lentiviral vectors were constructed to express the weakly rectifying kidney K(+) channel ROMK1 (Kir1.1), either fused to enhanced green fluorescent protein (EGFP) or as a bicistronic message (ROMK1-CITE-EGFP). The channel was stably expressed in cultured rat hippocampal neurons. Infected cells were maintained for 2-4 wk without decrease in expression level or evidence of viral toxicity, although 15.4 mM external KCl was required to prevent apoptosis of neurons expressing functional ROMK1. No other trophic agents tested could prevent cell death, which was probably caused by K(+) loss. This cell death did not occur in glia, which were able to support ROMK1 expression indefinitely. Functional ROMK1, quantified as the nonnative inward current at -144 mV in 5.4 mM external K(+) blockable by 500 microM Ba(2+), ranged from 1 to 40 pA/pF. Infected neurons exhibited a Ba(2+)-induced depolarization of 7 +/- 2 mV relative to matched EGFP-infected controls, as well as a 30% decrease in input resistance and a shift in action potential threshold of 2.6 +/- 0.5 mV. This led to a shift in the relation between injected current and firing frequency, without changes in spike shape, size, or timing. This shift, which quantifies silencing as a function of ROMK1 expression, was predicted from Hodgkin-Huxley models. No cellular compensatory mechanisms in response to expression of ROMK1 were identified, making ROMK1 potentially useful for transgenic studies of silencing and neurodegeneration, although its lethality in normal K(+) has implications for the use of K(+) channels in gene therapy.

Overexpressed A(1) adenosine receptors reduce activation of acetylcholine-sensitive K(+) current by native muscarinic M(2) receptors in rat atrial myocytes

In adult rat atrial myocytes, muscarinic acetylcholine (ACh)-sensitive K(+) current activated by a saturating concentration of adenosine (I(K(ACh),(Ado))) via A(1) receptors (A(1)Rs) amounts to only 30% of the current activated by a saturating concentration of ACh (I(K(ACh),(ACh))) via muscarinic M(2) receptors. The half-time of activation of I(K(ACh),(Ado)) on a rapid exposure to agonist was approximately 4-fold longer than that of I(K(ACh),(ACh)). Furthermore, I(K(ACh),(Ado)) never showed fast desensitization. To study the importance of receptor density for A(1)R-I(K(ACh),(Ado)) signaling, adult atrial myocytes in vitro were transfected with cDNA encoding for rat brain A(1)R and enhanced green fluorescent protein (EGFP) as a reporter. Whole-cell current was measured on days 3 and 4 after transfection. Time-matched cells transfected with only the EGFP vector served as controls. In approximately 30% of EGFP-positive cells (group I), the density of I(K(ACh),(Ado)) was increased by 72%, and its half-time of activation was reduced. Density and kinetic properties of I(K(ACh),(ACh)) were not affected in this fraction. In approximately 70% of transfection-positive myocytes (group II), the density of I(K(ACh),(ACh)) was significantly reduced, its activation was slowed, and the fast desensitizing component was lost. Adenosine-induced currents were larger in group II than in group I, their activation rate was further increased, and a fast desensitizing component developed. These data indicate that in native myocytes the amplitude and activation kinetics of I(K(ACh),(Ado)) are limited by the expression of A(1)R. Overexpression of A(1)R negatively interferes with signal transduction via the muscarinic M(2) receptor-linked pathway, which might reflect a competition of receptors with a common pool of G proteins. Negative interference of an overexpressed receptor with physiological regulation of a target protein by a different receptor should be considered in attempts to use receptor overexpression for gene therapy.