Functional compensation by other voltage-gated Ca2+ channels in mouse basal forebrain neurons with Ca(V)2.1 mutations

Ion channels research in hPSC-RPE cells: bridging benchwork to clinical applications

Ion channels in retinal pigment epithelial (RPE) cells are crucial for retinal health and vision functions. Defects in such channels are intricately associated with the development of various retinopathies that cause blindness. Human pluripotent stem cells (hPSC)-derived RPE cells, including those from human-induced pluripotent stem cells (hiPSC) and human embryonic stem cells (hESC), have been used as in vitro models for investigating pathogenic mechanisms and screening potential therapeutic strategies for retinopathies. Therefore, the cellular status of hPSC-RPE cells, including maturity and physiologic functions, have been widely explored. Particularly, research on ion channels in hPSC-RPE cells can lead to the development of more stable models upon which robust investigations and clinical safety assessments can be performed. Moreover, the use of patient-specific hiPSC-RPE cells has significantly accelerated the clinical translation of gene therapy for retinal channelopathies, such as bestrophinopathies. This review consolidates current research on ion channels in hPSC-RPE cells, specifically Kir7.1, Bestrophin-1, CLC-2, and CaV1.3, providing a foundation for future research.

Reduced intrinsic excitability of CA1 pyramidal neurons in human immunodeficiency virus (HIV) transgenic rats

The hippocampus is involved in key neuronal circuits that underlie cognition, memory, and anxiety, and it is increasingly recognized as a vulnerable structure that contributes to the pathogenesis of HIV-associated neurocognitive disorder (HAND). However, the mechanisms responsible for hippocampal dysfunction in neuroHIV remain unknown. The present study used HIV transgenic (Tg) rats and patch-clamp electrophysiological techniques to study the effects of the chronic low-level expression of HIV proteins on hippocampal CA1 pyramidal neurons. The dorsal and ventral areas of the hippocampus are involved in different neurocircuits and thus were evaluated separately. We found a significant decrease in the intrinsic excitability of CA1 neurons in the dorsal hippocampus in HIV Tg rats by comparing neuronal spiking induced by current step injections and by dynamic clamp to simulate neuronal spiking activity. The decrease in excitability in the dorsal hippocampus was accompanied by a higher rate of excitatory postsynaptic currents (EPSCs), whereas CA1 pyramidal neurons in the ventral hippocampus in HIV Tg rats had higher EPSC amplitudes. We also observed a reduction of hyperpolarization-activated nonspecific cationic current (Ih) in both the dorsal and ventral hippocampus. Neurotoxic HIV proteins have been shown to increase neuronal excitation. The lower excitability of CA1 pyramidal neurons that was observed herein may represent maladaptive homeostatic plasticity that seeks to stabilize baseline neuronal firing activity but may disrupt neural network function and contribute to HIV-associated neuropsychological disorders, such as HAND and depression.

The Dynamic Basis of Respiratory Rhythm Generation: One Breath at a Time

Rhythmicity is a universal timing mechanism in the brain, and the rhythmogenic mechanisms are generally dynamic. This is illustrated for the neuronal control of breathing, a behavior that occurs as a one-, two-, or three-phase rhythm. Each breath is assembled stochastically, and increasing evidence suggests that each phase can be generated independently by a dedicated excitatory microcircuit. Within each microcircuit, rhythmicity emerges through three entangled mechanisms: ( a) glutamatergic transmission, which is amplified by ( b) intrinsic bursting and opposed by ( c) concurrent inhibition. This rhythmogenic triangle is dynamically tuned by neuromodulators and other network interactions. The ability of coupled oscillators to reconfigure and recombine may allow breathing to remain robust yet plastic enough to conform to nonventilatory behaviors such as vocalization, swallowing, and coughing. Lessons learned from the respiratory network may translate to other highly dynamic and integrated rhythmic systems, if approached one breath at a time.

Precision physiology and rescue of brain ion channel disorders

Ion channel genes, originally implicated in inherited excitability disorders of muscle and heart, have captured a major role in the molecular diagnosis of central nervous system disease. Their arrival is heralded by neurologists confounded by a broad phenotypic spectrum of early-onset epilepsy, autism, and cognitive impairment with few effective treatments. As detection of rare structural variants in channel subunit proteins becomes routine, it is apparent that primary sequence alone cannot reliably predict clinical severity or pinpoint a therapeutic solution. Future gains in the clinical utility of variants as biomarkers integral to clinical decision making and drug discovery depend on our ability to unravel complex developmental relationships bridging single ion channel structure and human physiology.

Upregulation of L-type calcium channels in colonic inhibitory motoneurons of P/Q-type calcium channel-deficient mice

Enteric inhibitory motoneurons use nitric oxide and a purine neurotransmitter to relax gastrointestinal smooth muscle. Enteric P/Q-type Ca2+ channels contribute to excitatory neuromuscular transmission; their contribution to inhibitory transmission is less clear. We used the colon from tottering mice (tg/tg, loss of function mutation in the α1A pore-forming subunit of P/Q-type Ca2+ channels) to test the hypothesis that P/Q-type Ca2+ channels contribute to inhibitory neuromuscular transmission and colonic propulsive motility. Fecal pellet output in vivo and the colonic migrating motor complex (ex vivo) were measured. Neurogenic circular muscle relaxations and inhibitory junction potentials (IJPs) were also measured ex vivo. Colonic propulsive motility in vivo and ex vivo was impaired in tg/tg mice. IJPs were either unchanged or somewhat larger in tissues from tg/tg compared with wild-type (WT) mice. Nifedipine (L-type Ca2+ channel antagonist) inhibited IJPs by 35 and 14% in tissues from tg/tg and WT mice, respectively. The contribution of N- and R-type channels to neuromuscular transmission was larger in tissues from tg/tg compared with WT mice. The resting membrane potential of circular muscle cells was similar in tissues from tg/tg and WT mice. Neurogenic relaxations of circular muscle from tg/tg and WT mice were similar. These results demonstrate that a functional deficit in P/Q-type channels does not alter propulsive colonic motility. Myenteric neuron L-type Ca2+ channel function increases to compensate for loss of functional P/Q-type Ca2+ channels. This compensation maintains inhibitory neuromuscular transmission and normal colonic motility.

Dynamic compensation mechanism gives rise to period and duty-cycle level sets in oscillatory neuronal models

Rhythmic oscillation in neurons can be characterized by various attributes, such as the oscillation period and duty cycle. The values of these features depend on the amplitudes of the participating ionic currents, which can be characterized by their maximum conductance values. Recent experimental and theoretical work has shown that the values of these attributes can be maintained constant for different combinations of two or more ionic currents of varying conductances, defining what is known as level sets in conductance space. In two-dimensional conductance spaces, a level set is a curve, often a line, along which a particular oscillation attribute value is conserved. In this work, we use modeling, dynamical systems tools (phase-space analysis), and numerical simulations to investigate the possible dynamic mechanisms responsible for the generation of period and duty-cycle levels sets in simplified (linearized and FitzHugh-Nagumo) and conductance-based (Morris-Lecar) models of neuronal oscillations. A simplistic hypothesis would be that the tonic balance between ionic currents with the same or opposite effective signs is sufficient to create level sets. According to this hypothesis, the dynamics of each ionic current during a given cycle are well captured by some constant quantity (e.g., maximal conductances), and the phase-plane diagrams are identical or are almost identical (e.g., cubic-like nullclines with the same maxima and minima) for different combinations of these maximal conductances. In contrast, we show that these mechanisms are dynamic and involve the complex interaction between the nonlinear voltage dependencies and the effective time scales at which the ionic current's dynamical variables operate.

Calcium's role as nuanced modulator of cellular physiology in the brain

Neuroscientists studying normal brain aging, spinal cord injury, Alzheimer's disease (AD) and other neurodegenerative diseases have focused considerable effort on carefully characterizing intracellular perturbations in calcium dynamics or levels. At the cellular level, calcium is known for controlling life and death and orchestrating most events in between. For many years, intracellular calcium has been recognized as an essential ion associated with nearly all cellular functions from cell growth to degeneration. Often the emphasis is on the negative impact of calcium dysregulation and the typical worse-case-scenario leading inevitably to cell death. However, even high amplitude calcium transients, when executed acutely, can alter neuronal communication and synaptic strength in positive ways, without necessarily killing neurons. Here, we focus on the evidence that calcium has a subtle and distinctive role in shaping and controlling synaptic events that underpin neuronal communication and that these subtle changes in aging or AD may contribute to cognitive decline. We emphasize that calcium imaging in dendritic components is ultimately necessary to directly test for the presence of age- or disease-associated alterations during periods of synaptic activation.

Ionic Current Variability and Functional Stability in the Nervous System

Identified neurons in different animals express ionic currents at highly variable levels (population variability). If neuronal identity is associated with stereotypical function, as is the case in genetically identical neurons or in unambiguously identified individual neurons, this variability poses a conundrum: How is activity the same if the components that generate it-ionic current levels-are different? In some cases, ionic current variability across similar neurons generates an output gradient. However, many neurons produce very similar output activity, despite substantial variability in ionic conductances. It appears that, in many such cells, conductance levels of one ionic current vary in proportion to the conductance levels of another current. As a result, in a population of neurons, these conductances appear to be correlated. Here, I review theoretical and experimental work that suggests that neuronal ionic current correlation can reduce the global ionic current variability and can contribute to functional stability.

Neurotrophin receptor p75 mediates the uptake of the amyloid beta (Aβ) peptide, guiding it to lysosomes for degradation in basal forebrain cholinergic neurons

A fascinating yet perhaps overlooked trait of the p75 neurotrophin receptor (p75(NTR)) is its ability to bind ligands with no obvious neurotrophic function. Using cultured basal forebrain (BF) neurons, this study demonstrates selective internalization of amyloid β (Aβ) 1-42 in conjunction with p75(NTR) (labelled with IgG192-Cy3) by cholinergic cells. Active under resting conditions, this process was enhanced by high K(+) stimulation and was insensitive to inhibitors of regulated synaptic activity-tetrodotoxin or botulinum neurotoxins (BoNT type/A and/B). Blockade of sarco-endoplasmic reticulum (SERCA) Ca(2+) ATPase with thapsigargin and CPA or chelation of Ca(2+) with EGTA-AM strongly suppressed the endocytosis of p75(NTR), implicating the role of ER released Ca(2+). The uptake of IgG192-Cy3 was also reduced by T-type Ca(2+) channel blocker mibefradil but not Cd(2+), an indiscriminate blocker of high voltage-activated Ca(2+) currents. A strong co-localization of IgG192-Cy3 with late endosome (Rab7) or lysosome (Lamp1) qualifier proteins suggest these compartments as the primary destination for internalized IgG192 and Aβ. Selective uptake and labeling of BF cholinergic cells with IgG192-Cy3 injected into the prefrontal cortex was verified also in vivo. The significance of these findings in relation to Aβ clearance in the cerebral cortex and pathophysiology of Alzheimer's disease is discussed.

Functional crosstalk in culture between macrophages and trigeminal sensory neurons of a mouse genetic model of migraine

Background

Enhanced activity of trigeminal ganglion neurons is thought to underlie neuronal sensitization facilitating the onset of chronic pain attacks, including migraine. Recurrent headache attacks might establish a chronic neuroinflammatory ganglion profile contributing to the hypersensitive phenotype. Since it is difficult to study this process in vivo, we investigated functional crosstalk between macrophages and sensory neurons in primary cultures from trigeminal sensory ganglia of wild-type (WT) or knock-in (KI) mice expressing the Cacna1a gene mutation (R192Q) found in familial hemiplegic migraine-type 1. After studying the number and morphology of resident macrophages in culture, the consequences of adding host macrophages on macrophage phagocytosis and membrane currents mediated by pain-transducing P2X3 receptors on sensory neurons were examined.

Results

KI ganglion cultures constitutively contained a larger number of active macrophages, although no difference in P2X3 receptor expression was found. Co-culturing WT or KI ganglia with host macrophages (active as much as resident cells) strongly stimulated single cell phagocytosis. The same protocol had no effect on P2X3 receptor expression in WT or KI co-cultures, but it largely enhanced WT neuron currents that grew to the high amplitude constitutively seen for KI neurons. No further potentiation of KI neuronal currents was observed.

Conclusions

Trigeminal ganglion cultures from a genetic mouse model of migraine showed basal macrophage activation together with enhanced neuronal currents mediated by P2X3 receptors. This phenotype could be replicated in WT cultures by adding host macrophages, indicating an important functional crosstalk between macrophages and sensory neurons.

Striatal expression of Homer1a is affected by genotype but not dystonic phenotype of tottering mice: a model of spontaneously occurring motor disturbances

Tottering (tg) mice carry a missense mutation in the gene coding for P/Q-type voltage-dependent Ca(2+) channels (VDCCs). Aberrant functioning of P/Q-type VDCCs results in molecular alterations in Ca(2+) currents and in glutamate and dopamine systems. As a consequence, tottering mice exhibit mild ataxia, spontaneous epilepsy, and paroxysmal dyskinesia. In this study, we evaluated whether the tottering mice genotype (homozygous vs. heterozygous) and abnormal movement phenotype (mice exhibiting paroxysmal dyskinesia vs. mice not exhibiting dyskinesia) may affect the expression of Homer1a. Homer1a is a gene whose expression is modulated by glutamate, dopamine and Ca(2+) concentrations. Over-expression of Homer1a has been described in epilepsy and motor dysfunctions. Thereby, changes in Homer1a expression could take place in tottering mice. Studying the expression profile of this gene may shed light on the molecular events occurring in tottering mice. Moreover, tottering mice may represent a valuable animal model for investigating Homer1a involvement in motor disorders. Homer1a expression was decreased in all striatal subregions, with the exclusion of the dorsolateral caudate-putamen, in heterozygous mice compared to wild-type and homozygous mice. Gene expression was decreased in the core of the accumbens in mice exhibiting paroxysmal dyskinesia compared to wild-type mice and to mice not exhibiting dyskinesia. These results demonstrate that the tottering mouse genotype may affect striatal expression of Homer1a, possibly as a result of imbalance between Ca(2+) channels subtypes or Ca(2+)-related molecules in heterozygous vs. homozygous mice.

Alterations in intracellular calcium ion concentrations in cerebellar granule cells of the CACNA1A mutant mouse, leaner, during postnatal development

Maintaining calcium ion (Ca²+) homeostasis is crucial for normal neuronal function. Altered Ca²+ homeostasis interferes with Ca²+ signaling processes and affects neuronal survival. In this study, we used homozygous leaner and tottering mutant mice, which carry autosomal recessive mutations in the gene coding for the α(1A) pore forming subunit of Ca(V)2.1 (P/Q-type) voltage-gated calcium channels (VGCC). Leaner mice show severe ataxia and epilepsy, while tottering mice are less severely affected. Leaner cerebellar granule cells (CGC) show extensive apoptotic cell death that peaks at postnatal (P) day 20 and continues into adulthood. Intracellular Ca²+ ([Ca²+](i)) concentrations in leaner and tottering mouse Purkinje cells have been described, but [Ca²+](i) concentrations have not been reported for granule cells, the largest neuronal population of the cerebellum. Using the ratiometric dye, Fura-2 AM, we investigated the role of Ca²+ homeostasis in CGC death during postnatal development by demonstrating basal [Ca²+](i), depolarization induced Ca²+ transients, and Ca²+ transients after completely blocking Ca(V)2.1 VGCC. From P20 onward, basal [Ca²+](i) levels in leaner CGC were significantly lower compared to age-matched wild-type CGC. We also compared basal [Ca²+](i) levels in leaner and wild-type CGC to basal [Ca²+](i) in tottering CGC. Potassium chloride induced depolarization revealed no significant difference in Ca²+ transients between leaner and wild-type CGC, indicating that even though leaner CGC have dysfunctional P/Q-type VGCC, Ca²+ transients after depolarization are the same. This suggests that other VGCC are compensating for the dysfunctional P/Q channels. This finding was further confirmed by completely blocking Ca(V)2.1 VGCC using ω-Agatoxin IV-A.

Knockdown of Cav2.1 calcium channels is sufficient to induce neurological disorders observed in natural occurring Cacna1a mutants in mice

The CACNA1A gene encodes the poreforming, voltage-sensitive subunit of the voltage-dependent Ca(v)2.1 calcium channel. Mutations in this gene have been linked to several human disorders, including familial hemiplegic migraine type 1, episodic ataxia type 2, and spinocerebellar ataxia type 6. In mice, mutations of the homolog Cacna1a cause recessively inherited phenotypes in tottering, rolling Nagoya, rocker, and leaner mice. Here we describe two knockdown mice with 28.4+/-3.4% and 13.8+/-3.3% of the wild-type Ca(v)2.1 quantity. 28.4+/-3.4% level mutants displayed ataxia, absence-like seizures and progressive cerebellar atrophy, although they had a normal life span. Mutants with 13.8+/-3.3% level exhibited ataxia severer than the 28.4+/-3.4% level mutants, absence-like seizures and additionally paroxysmal dyskinesia, and died premature around 3 weeks of age. These results indicate that knock down of Ca(v)2.1 quantity to 13.8+/-3.3% of the wild-type level are sufficient to induce the all neurological disorders observed in natural occurring Cacna1a mutants. These knockdown animals with Ca(v)2.1 calcium channels intact can contribute to functional studies of the molecule in the disease.

Hypoxic remodelling of Ca(2+) signalling in proliferating human arterial smooth muscle

Ca(2+) homeostasis in proliferating smooth muscle (SM) cells strongly influences neointima formation, which can cause failure of coronary artery bypass surgery. During surgical procedures and subsequent revascularization, SM cells are also exposed to a period of hypoxia. Problems with bypass surgery in general involve neointima formation which is in turn dependent on SM proliferation and migration. Here, we have directly monitored [Ca(2+)](i) fluorimetrically in proliferating internal mammary artery (IMA) SM cells, and investigated how this is modulated by chronic hypoxia (CH; 24 h, 2.5% O(2)). IMA is the most successful replacement conduit vessel in bypass grafts. Basal [Ca(2+)](i) was unaffected by CH, but removal of extracellular Ca(2+) evoked far smaller reductions in [Ca(2+)](i) than were seen in normoxic cells. Voltage-gated Ca(2+) entry was suppressed in CH cells, and this was attributable to activation of the transcriptional regulator, hypoxia inducible factor. Furthermore, the relative contributions to voltage-gated Ca(2+) entry of L- and T-type Ca(2+) channels was markedly altered, with T-type channels becoming functionally more important in CH cells. Agonist-evoked mobilization of Ca(2+) from intracellular stores was not affected by CH, whilst subsequent capacitative Ca(2+) entry was modestly suppressed. Our data provide novel observations of the remodelling of Ca(2+) homeostasis by CH in IMASM cells which may contribute to their superior patency as coronary bypass grafts.

Pre- and post-synaptic mechanisms of synaptic strength homeostasis revealed by slowpoke and shaker K+ channel mutations in Drosophila

We report naturally occurring, systematic variations in synaptic strength at neuromuscular junctions along the dorsal-ventral (D-V) axis of the Drosophila larval body wall. These gradual changes were correlated with differences in presynaptic neurotransmitter release regulated by nerve terminal excitability and in postsynaptic receptor composition influencing miniature excitatory junctional potential (mEJP) amplitude. Surprisingly, synaptic strength and D-V differentials at physiological Ca(2+) levels were not significantly altered in slowpoke (slo) and Shaker (Sh) mutants, despite their defects in two major repolarizing forces, Ca(2+)-activated Slo (BK) and voltage-activated Sh currents, respectively. However, lowering [Ca(2+)](o) levels revealed greatly altered synaptic mechanisms in these mutants, indicated by drastically enhanced excitatory junctional potentials (EJPs) in Sh but paradoxically reduced EJPs in slo. Removal of Sh current in slo mutants by 4-aminopyridine blockade or by combining slo with Sh mutations led to strikingly increased synaptic transmission, suggesting upregulation of presynaptic Sh current to limit excessive neurotransmitter release in the absence of Slo current. In addition, slo mutants displayed altered immunoreactivity intensity ratio between DGluRIIA and DGluRIIB receptor subunits. This modified receptor composition caused smaller mEJP amplitudes, further preventing excessive transmission in the absence of Slo current. Such compensatory regulations were prevented by rutabaga (rut) adenylyl cyclase mutations in rut slo double mutants, demonstrating a novel role of rut in homeostatic plasticity, in addition to its well-established function in learning behavior.

Plasticity and stability in neuronal output via changes in intrinsic excitability: it's what's inside that counts

The nervous system faces an extremely difficult task. It must be flexible, both during development and in adult life, so that it can respond to a variety of environmental demands and produce adaptive behavior. At the same time the nervous system must be stable, so that the neural circuits that produce behavior function throughout the lifetime of the animal and that changes produced by learning endure. We are only beginning to understand how neural networks strike a balance between altering individual neurons in the name of plasticity, while maintaining long-term stability in neural system function. The balance of this plasticity and stability in neural networks undoubtedly plays a critical role in the normal functioning of the nervous system. While mechanisms of synaptic plasticity have garnered extensive study over the past three decades, it is only recently that more attention has been turned to plasticity of intrinsic excitability as a key player in neural network function. This review will focus on this emerging area of research that undoubtedly will contribute a great deal to our understanding of the functionality of the nervous system.