Intracellular Ca(2+) pools in neuronal signalling

Sleep restoration by optogenetic targeting of GABAergic neurons reprograms microglia and ameliorates pathological phenotypes in an Alzheimer's disease model

BACKGROUND

Alzheimer's disease (AD) patients exhibit memory disruptions and profound sleep disturbances, including disruption of deep non-rapid eye movement (NREM) sleep. Slow-wave activity (SWA) is a major restorative feature of NREM sleep and is important for memory consolidation.

METHODS

We generated a mouse model where GABAergic interneurons could be targeted in the presence of APPswe/PS1dE9 (APP) amyloidosis, APP-GAD-Cre mice. An electroencephalography (EEG) / electromyography (EMG) telemetry system was used to monitor sleep disruptions in these animals. Optogenetic stimulation of GABAergic interneurons in the anterior cortex targeted with channelrhodopsin-2 (ChR2) allowed us to examine the role GABAergic interneurons play in sleep deficits. We also examined the effect of optogenetic stimulation on amyloid plaques, neuronal calcium as well as sleep-dependent memory consolidation. In addition, microglial morphological features and functions were assessed using confocal microscopy and flow cytometry. Finally, we performed sleep deprivation during optogenetic stimulation to investigate whether sleep restoration was necessary to slow AD progression.

RESULTS

APP-GAD-Cre mice exhibited impairments in sleep architecture including decreased time spent in NREM sleep, decreased delta power, and increased sleep fragmentation compared to nontransgenic (NTG) NTG-GAD-Cre mice. Optogenetic stimulation of cortical GABAergic interneurons increased SWA and rescued sleep impairments in APP-GAD-Cre animals. Furthermore, it slowed AD progression by reducing amyloid deposition, normalizing neuronal calcium homeostasis, and improving memory function. These changes were accompanied by increased numbers and a morphological transformation of microglia, elevated phagocytic marker expression, and enhanced amyloid β (Aβ) phagocytic activity of microglia. Sleep was necessary for amelioration of pathophysiological phenotypes in APP-GAD-Cre mice.

CONCLUSIONS

In summary, our study shows that optogenetic targeting of GABAergic interneurons rescues sleep, which then ameliorates neuropathological as well as behavioral deficits by increasing clearance of Aβ by microglia in an AD mouse model.

Mitochondria as central hubs in synaptic modulation

Mitochondria are present in the pre- and post-synaptic regions, providing the energy required for the activity of these very specialized neuronal compartments. Biogenesis of synaptic mitochondria takes place in the cell body, and these organelles are then transported to the synapse by motor proteins that carry their cargo along microtubule tracks. The transport of mitochondria along neurites is a highly regulated process, being modulated by the pattern of neuronal activity and by extracellular cues that interact with surface receptors. These signals act by controlling the distribution of mitochondria and by regulating their activity. Therefore, mitochondria activity at the synapse allows the integration of different signals and the organelles are important players in the response to synaptic stimulation. Herein we review the available evidence regarding the regulation of mitochondrial dynamics by neuronal activity and by neuromodulators, and how these changes in the activity of mitochondria affect synaptic communication.

Screening and Structure-Activity Relationship of D2AAK1 Derivatives for Potential Application in the Treatment of Neurodegenerative Diseases

Neurodegenerative and mental diseases are serious medical, economic and social problems. Neurodegeneration is referred to as a pathological condition associated with damage to nerve cells leading to their death. Treatment of neurodegenerative diseases is at present symptomatic only, and novel drugs are urgently needed which would be able to stop disease progression. We performed screening of reactive oxygen species, reactive nitrogen species, glutathione and level intracellular Ca²⁺. The studies were assessed using one-way ANOVA of variance with Dunnett’s post hoc test. Previously, we reported D2AAK1 as a promising compound for the treatment of neurodegenerative and mental disorders. Here, we show a screening of D2AAK1 derivatives aimed at the selection of the compound with the most favorable pharmacological profile. Selected compounds cause an increase in the proliferation of a hippocampal neuron-like cell line, changes in the levels of reactive oxygen and nitrogen forms, reduced glutathione and a reduced intracellular calcium pool. Upon analyzing the structure–activity relationship, we selected the compound with the most favorable profile for a neuroprotective activity for potential application in the treatment of neurodegenerative diseases.

A calcium fluorescence assay for quantification of cholinergic receptor activity of clinical drugs in serum - comparison with radioactive methods

A new approach is described for quantifying cholinergic receptor activation status human blood samples, based on M1 receptor-driven mobilization of intracellular calcium stores. The assay identifies anticholinergic as well as agonist cholinergic receptor activity. As a cell-based procedure, the assay shares the high efficiency of recently developed M1 receptor binding protocols, but differs from the latter in relying on fluorescence rather than radioactivity measurements. The assay targets a true functional effect insofar as it reflects a time-dependent process of net changes in activation of cholinergic receptors. Results from experiments with M1-expressing CHO cells exposed to a fluorogenic dye and the standard cholinergic agonist carbachol revealed the assay's ability to isolate pure agonist effects of clinical compounds as well as the net effects of serum containing agonist and antagonist factors. The new protocol thus provides two additional quantitative indices of cholinergic receptor activity in human serum, namely pure agonistic effects and net agonist/antagonist effects. As such, it could constitute a very useful addition to efforts to quantify global cholinergic status in human serum in various clinical conditions. By relying on fluorescence measures it should also prove much more accessible than radioactivity-based protocols.

The therapeutic potential of mitochondrial transplantation for the treatment of neurodegenerative disorders

Mitochondrial activity is essential to support neural functions, and changes in the integrity and activity of the mitochondria can contribute to synaptic damage and neuronal death, especially in degenerative diseases associated with age, such as Alzheimer's and Parkinson's disease. Currently, different approaches are used to treat these conditions, and one strategy under research is mitochondrial transplantation. For years, mitochondria have been shown to be transferred between cells of different tissues. This process has allowed several attempts to develop transplantation schemes by isolating functional mitochondria and introducing them into damaged tissue in particular to counteract the harmful effects of myocardial ischemia. Recently, mitochondrial transfer between brain cells has also been reported, and thus, mitochondrial transplantation for disorders of the nervous system has begun to be investigated. In this review, we focus on the relevance of mitochondria in the nervous system, as well as some mitochondrial alterations that occur in neurodegenerative diseases associated with age. In addition, we describe studies that have performed mitochondrial transplantation in various tissues, and we emphasize the advances in mitochondrial transplantation aimed at treating diseases of the nervous system.

Calcium Signaling During Brain Aging and Its Influence on the Hippocampal Synaptic Plasticity

Calcium (Ca²⁺) ions are highly versatile intracellular signaling molecules and are universal second messenger for regulating a variety of cellular and physiological functions including synaptic plasticity. Ca²⁺ homeostasis in the central nervous system endures subtle dysregulation with advancing age. Research has provided abundant evidence that brain aging is associated with altered neuronal Ca²⁺ regulation and synaptic plasticity mechanisms. Much of the work has focused on the hippocampus, a brain region critically involved in learning and memory, which is particularly susceptible to dysfunction during aging. The current chapter takes a specific perspective, assessing various Ca²⁺ sources and the influence of aging on Ca²⁺ sources and synaptic plasticity in the hippocampus. Integrating the knowledge of the complexity of age-related alterations in neuronal Ca²⁺ signaling and synaptic plasticity mechanisms will positively shape the development of highly effective therapeutics to treat brain disorders including cognitive impairment associated with aging and neurodegenerative disease.

Frequency-dependent exacerbation of Alzheimer's disease neuropathophysiology

Neuronal activity patterns are disrupted in neurodegenerative disorders, including Alzheimer’s disease (AD). One example is disruption of corticothalamic slow oscillations responsible for sleep-dependent memory consolidation. Slow waves are periodic oscillations in neuronal activity occurring at frequencies of <1 Hz. The power, but not the frequency of slow oscillations is altered in a mouse model of AD. Optogenetic rescue of slow oscillations by increasing activity in cortical pyramidal neurons at the frequency of slow waves restores slow wave power, halts deposition of amyloid plaques and prevents neuronal calcium dysregulation. Here we determined whether driving this circuit at an increased rate would exacerbate the amyloid-dependent calcium dyshomeostasis in transgenic mice. Doubling the frequency of slow waves for one month with optogenetics resulted in increased amyloid beta - dependent disruptions in neuronal calcium homeostasis and loss of synaptic spines. Therefore, while restoration of physiological circuit dynamics is sufficient to abrogate the progression of Alzheimer’s disease pathology and should be considered an avenue for clinical treatment of AD patients with sleep disorders, pathophysiological stimulation of neuronal circuits leads to activity - dependent acceleration of amyloid production, aggregation and downstream neuronal dysfunction.

Alteration in NMDA Receptor Mediated Glutamatergic Neurotransmission in the Hippocampus During Senescence

Glutamate is the primary excitatory neurotransmitter in neurons and glia. N-methyl-D-aspartate (NMDA), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), and kainate receptors are major ionotropic glutamate receptors. Glutamatergic neurotransmission is strongly linked with Ca²⁺ homeostasis. Research has provided ample evidence that brain aging is associated with altered glutamatergic neurotransmission and Ca²⁺ dysregulation. Much of the work has focused on the hippocampus, a brain region critically involved in learning and memory, which is particularly susceptible to dysfunction during senescence. The current review examines Ca²⁺ regulation with a focus on the NMDA receptors in the hippocampus. Integrating the knowledge of the complexity of age-related alterations in Ca²⁺ homeostasis and NMDA receptor-mediated glutamatergic neurotransmission will positively shape the development of highly effective therapeutics to treat brain disorders including cognitive impairment.

Human carbonic anhydrase-8 AAV8 gene therapy inhibits nerve growth factor signaling producing prolonged analgesia and anti-hyperalgesia in mice

Carbonic anhydrase-8 (Car8; murine gene symbol) is an allosteric inhibitor of inositol trisphosphate receptor-1 (ITPR1), which regulates neuronal intracellular calcium release. We previously reported that wildtype Car8 overexpression corrects the baseline allodynia and hyperalgesia associated with calcium dysregulation in the waddle (wdl) mouse due to a 19 bp deletion in exon 8 of the Car8 gene. In this report, we provide preliminary evidence that overexpression of the human wildtype ortholog of Car8 (CA8^WT), but not the reported CA8 S100P loss-of-function mutation (CA8^MT); inhibits nerve growth factor (NGF)-induced phosphorylation of ITPR1, TrkA (NGF high affinity receptor); and ITPR1-mediated cytoplasmic free calcium release in vitro. Additionally, we show that gene-transfer using AAV8-V5-CA8^WT viral particles via sciatic nerve injection demonstrates retrograde transport to dorsal root ganglia (DRG) producing prolonged V5-CA8^WT expression, pITPR1 and pTrkA inhibition, and profound analgesia and anti-hyperalgesia in male C57BL/6J mice. AAV8-V5-CA8^WT mediated overexpression prevented and treated allodynia and hyperalgesia associated with chronic neuropathic pain produced by the spinal nerve ligation (SNL) model. These AAV8-V5-CA8 data provide a proof-of-concept for precision medicine through targeted gene therapy of NGF-responsive somatosensory neurons as a long-acting local analgesic able to prevent and treat chronic neuropathic pain through regulating TrkA signaling, ITPR1 activation, and intracellular free calcium release by ITPR1.

Calcium alterations signal either to senescence or to autophagy induction in stem cells upon oxidative stress

Intracellular calcium ([Ca2+]i) has been reported to play an important role in autophagy, apoptosis and necrosis, however, a little is known about its impact in senescence. Here we investigated [Ca2+]i contribution to oxidative stress-induced senescence of human endometrium-derived stem cells (hMESCs). In hMESCs sublethal H2O2-treatment resulted in a rapid calcium release from intracellular stores mediated by the activation of PLC/IP3/IP3R pathway. Notably, further senescence development was accompanied by persistently elevated [Ca2+]i levels. In H2O2-treated hMESCs, [Ca2+]i chelation by BAPTA-AM (BAPTA) was sufficient to prevent the expansion of the senescence phenotype, to decrease endogenous reactive oxygen species levels, to avoid G0/G1 cell cycle arrest, and finally to retain proliferation. Particularly, loading with BAPTA attenuated phosphorylation of the main DNA damage response members, including ATM, 53BP1 and H2A.X and reduced activation of the p53/p21/Rb pathway in H2O2-stimulated cells. Next, we revealed that BAPTA induced an early onset of AMPK-dependent autophagy in H2O2-treated cells as confirmed by both the phosphorylation status of AMPK/mTORC1 pathway and the dynamics of the LC3 lipidization. Summarizing the obtained data we can assume that calcium chelation is able to trigger short-term autophagy and to prevent the premature senescence of hMESCs under oxidative stress.

Immunotherapy with Aducanumab Restores Calcium Homeostasis in Tg2576 Mice

Calcium homeostasis plays a major role in maintaining neuronal function under physiological conditions. Amyloid-β (Aβ) initiates pathological processes that include disruption in intracellular calcium levels, so amelioration of the calcium alteration could serve as an indirect functional indicator of treatment efficacy. Therefore, calcium dynamics were used as a measure of functional outcome. We evaluated the effects of the anti-Aβ antibody aducanumab on calcium homeostasis and plaque clearance in aged Tg2576 mice with in vivo multiphoton imaging. Acute topical application of aducanumab to the brain resulted in clearance of amyloid plaques. Although chronic systemic administration of aducanumab in 22-month-old mice did not clear existing plaques, calcium overload was ameliorated over time. Therefore, this antibody likely restores neuronal network function that possibly underlies cognitive deficits, indicating promise as a clinical treatment. In addition, functional readouts such as calcium overload may be a more useful outcome measure to monitor treatment efficacy in models of Alzheimer's disease compared with amyloid burden alone.

SIGNIFICANCE STATEMENT

Alzheimer's disease (AD) is a progressive neurodegenerative disorder that is currently without a cure. Aducanumab is an anti-amyloid-β antibody being developed for the treatment of AD. Interim analyses of a phase 1b clinical trial have suggested potential beneficial effects on amyloid pathology and cognitive status in patients treated with aducanumab (Sevigny et al., 2016). Here, we show that a murine analog of aducanumab clears amyloid plaques in an acute setting and restores calcium homeostasis disrupted in a mouse model of AD upon chronic treatment. Therefore, we demonstrate that aducanumab reverses a functional outcome measure reflective of neural network activity.

Hydrogen Sulfide Regulates the [Ca2+]i Level in the Primary Medullary Neurons

In the present study, we attempted to elucidate mechanisms for the regulation of intracellular calcium levels by H₂S in primary rat medullary neurons. Our results showed that NaHS significantly increased the level of [Ca²⁺]_i in rat medullary neurons in a concentration-dependent manner. L-Cysteine and SAM significantly raised the level of [Ca²⁺]_i in the medullary neurons while HA and/or AOAA produced a reversal effect. In addition, L-cysteine and SAM significantly increased but HA and/or AOAA decreased the production of H₂S in the cultured neurons. The [Ca²⁺]_i elevation induced by H₂S was significantly diminished by EGTA-Ca²⁺-free solutions, and this elevation was also reduced by nifedipine or nimodipine and mibefradil, suggesting the role of L-type and/or T-type Ca²⁺ channels. Moreover, the effect of H₂S on [Ca²⁺]_i level in neurons was significantly attenuated by BAPTA-AM and thapsigargin, suggesting the source of Ca²⁺. Therefore, we concluded that both exogenous and endogenous H₂S elevates [Ca²⁺]_i level in primarily cultured rat medullary neurons via both increasing calcium influx and mobilizing intracellular Ca²⁺ stores from ER.

In Vitro Neuroprotective and Anti-Inflammatory Activities of Natural and Semi-Synthetic Spirosteroid Analogues

Two spirosteroid analogues were synthesized and evaluated for their in vitro neuroprotective activities in PC12 cells, against glutamate-induced excitotoxicity and mitochondrial damage in glucose deprivation conditions, as well as their anti-inflammatory potential in LPS/IFNγ-stimulated microglia primary cultures. We also evaluated the in vitro anti-excitotoxic and anti-inflammatory activities of natural and endogenous steroids. Our results show that the plant-derived steroid solasodine decreased PC12 glutamate-induced excitotoxicity, but not the cell death induced by mitochondrial damage and glucose deprivation. Among the two synthetic spirosteroid analogues, only the (25R)-5α-spirostan-3,6-one (S15) protected PC12 against ischemia-related in vitro models and inhibited NO production, as well as the release of IL-1β by stimulated primary microglia. These findings provide further insights into the role of specific modifications of the A and B rings of sapogenins for their neuroprotective potential.

Carbonic anhydrase-8 regulates inflammatory pain by inhibiting the ITPR1-cytosolic free calcium pathway

Calcium dysregulation is causally linked with various forms of neuropathology including seizure disorders, multiple sclerosis, Huntington’s disease, Alzheimer’s, spinal cerebellar ataxia (SCA) and chronic pain. Carbonic anhydrase-8 (Car8) is an allosteric inhibitor of inositol trisphosphate receptor-1 (ITPR1), which regulates intracellular calcium release fundamental to critical cellular functions including neuronal excitability, neurite outgrowth, neurotransmitter release, mitochondrial energy production and cell fate. In this report we test the hypothesis that Car8 regulation of ITPR1 and cytoplasmic free calcium release is critical to nociception and pain behaviors. We show Car8 null mutant mice (MT) exhibit mechanical allodynia and thermal hyperalgesia. Dorsal root ganglia (DRG) from MT also demonstrate increased steady-state ITPR1 phosphorylation (pITPR1) and cytoplasmic free calcium release. Overexpression of Car8 wildtype protein in MT nociceptors complements Car8 deficiency, down regulates pITPR1 and abolishes thermal and mechanical hypersensitivity. We also show that Car8 nociceptor overexpression alleviates chronic inflammatory pain. Finally, inflammation results in downregulation of DRG Car8 that is associated with increased pITPR1 expression relative to ITPR1, suggesting a possible mechanism of acute hypersensitivity. Our findings indicate Car8 regulates the ITPR1-cytosolic free calcium pathway that is critical to nociception, inflammatory pain and possibly other neuropathological states. Car8 and ITPR1 represent new therapeutic targets for chronic pain.

Upstream deregulation of calcium signaling in Parkinson's disease

Parkinson’s disease (PD) is a major health problem affecting millions of people worldwide. Recent studies provide compelling evidence that altered Ca²⁺ homeostasis may underlie disease pathomechanism and be an inherent feature of all vulnerable neurons. The downstream effects of altered Ca²⁺ handling in the distinct subcellular organelles for proper cellular function are beginning to be elucidated. Here, we summarize the evidence that vulnerable neurons may be exposed to homeostatic Ca²⁺ stress which may determine their selective vulnerability, and suggest how abnormal Ca²⁺ handling in the distinct intracellular compartments may compromise neuronal health in the context of aging, environmental, and genetic stress. Gaining a better understanding of the varied effects of Ca²⁺ dyshomeostasis may allow novel combinatorial therapeutic strategies to slow PD progression.

Plasticity of calcium signaling cascades in human embryonic stem cell-derived neural precursors

Human embryonic stem cell-derived neural precursors (hESC NPs) are considered to be a promising tool for cell-based therapy in central nervous system injuries and neurodegenerative diseases. The Ca(2+) ion is an important intracellular messenger essential for the regulation of various cellular functions. We investigated the role and physiology of Ca(2+) signaling to characterize the functional properties of CCTL14 hESC NPs during long-term maintenance in culture (in vitro). We analyzed changes in cytoplasmic Ca(2+) concentration ([Ca(2+)]i) evoked by high K(+), adenosine-5'-triphosphate (ATP), glutamate, γ-aminobutyric acid (GABA), and caffeine in correlation with the expression of various neuronal markers in different passages (P6 through P10) during the course of hESC differentiation. We found that only differentiated NPs from P7 exhibited significant and specific [Ca(2+)]i responses to various stimuli. About 31% of neuronal-like P7 NPs exhibited spontaneous [Ca(2+)]i oscillations. Pharmacological and immunocytochemical assays revealed that P7 NPs express L- and P/Q-type Ca(2+) channels, P2X2, P2X3, P2X7, and P2Y purinoreceptors, glutamate receptors, and ryanodine (RyR1 and RyR3) receptors. The ATP- and glutamate-induced [Ca(2+)]i responses were concentration-dependent. Higher glutamate concentrations (over 100 μM) caused cell death. Responses to ATP were observed in the presence or in the absence of extracellular Ca(2+). These results emphasize the notion that with time in culture, these cells attain a transient period of operative Ca(2+) signaling that is predictive of their ability to act as stem elements.

Urocortin 3 elevates cytosolic calcium in nucleus ambiguus neurons

Urocortin 3 (also known as stresscopin) is an endogenous ligand for the corticotropin-releasing factor receptor 2 (CRF(2)). Despite predominant G(s) coupling of CRF(2), promiscuous coupling with other G proteins has been also associated with the activation of this receptor. As urocortin 3 has been involved in central cardiovascular regulation at hypothalamic and medullary sites, we examined its cellular effects on cardiac vagal neurons of nucleus ambiguus, a key area for the autonomic control of heart rate. Urocortin 3 (1 nM-1000 nM) induced a concentration-dependent increase in cytosolic Ca(2+) concentration that was blocked by the CRF(2) antagonist K41498. In the case of two consecutive treatments with urocortin 3, the second urocortin 3-induced Ca(2+) response was reduced, indicating receptor desensitization. The effect of urocortin 3 was abolished by pre-treatment with pertussis toxin and by inhibition of phospolipase C with U-73122. Urocortin 3 activated Ca(2+) influx via voltage-gated P/Q-type channels as well as Ca(2+) release from endoplasmic reticulum. Urocortin 3 promoted Ca(2+) release via inositol 1,4,5 trisphosphate receptors, but not ryanodine receptors. Our results indicate a novel Ca(2+) -mobilizing effect of urocortin 3 in vagal pre-ganglionic neurons of nucleus ambiguus, providing a cellular mechanism for a previously reported role for this peptide in parasympathetic cardiac regulation.

Involvement of ryanodine receptors in tetanic sciatic stimulation-induced long-term potentiation of spinal dorsal horn and persistent pain in rats

Tetanic stimulation of the sciatic nerve induces long-term potentiation (LTP) of C-fiber-evoked field potentials in the spinal dorsal horn and persistent pain, suggesting that spinal LTP may be a substrate for central sensitization of the pain pathway. However, its cellular mechanism remains unclear. The present study provides electrophysiological and behavioral evidence for the involvement of ryanodine receptor (RyR) in the induction of spinal LTP and persistent pain in rats. The specific inhibitor of ryanodine receptor, ryanodine and dantrolene, dose dependently blocked the induction, but not maintenance, of spinal LTP and reduced persistent pain behaviors induced by tetanic sciatic stimulation. Both cyclic ADP ribose (cADPR), an endogenous agonist of RyR, and (±)-1,4-dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluromethyl)-phenyl]-3-pyridine carboxylic acid methyl ester (Bay K 8644), an agonist of L-type calcium channel, attenuated ryanodine-induced inhibition. Immunohistochemistry and electron microscopic observation showed that RyR subtypes RyR1 and RyR3 were located in the spinal dorsal horn. The results suggest that RyRs are involved in synaptic plasticity of the spinal pain pathway and may be a novel target for treating pain. © 2012 Wiley Periodicals, Inc.

Cell signaling and mitochondrial dynamics: Implications for neuronal function and neurodegenerative disease

Nascent evidence indicates that mitochondrial fission, fusion, and transport are subject to intricate regulatory mechanisms that intersect with both well-characterized and emerging signaling pathways. While it is well established that mutations in components of the mitochondrial fission/fusion machinery can cause neurological disorders, relatively little is known about upstream regulators of mitochondrial dynamics and their role in neurodegeneration. Here, we review posttranslational regulation of mitochondrial fission/fusion enzymes, with particular emphasis on dynamin-related protein 1 (Drp1), as well as outer mitochondrial signaling complexes involving protein kinases and phosphatases. We also review recent evidence that mitochondrial dynamics has profound consequences for neuronal development and synaptic transmission and discuss implications for clinical translation.

The origins of oxidant stress in Parkinson's disease and therapeutic strategies

Parkinson's disease (PD) is a major world-wide health problem afflicting millions of the aged population. Factors that act on most or all cell types (pan-cellular factors), particularly genetic mutations and environmental toxins, have dominated public discussions of disease etiology. Although there is compelling evidence supporting an association between disease risk and these factors, the pattern of neuronal pathology and cell loss is difficult to explain without cell-specific factors. This article focuses on recent studies showing that the neurons at greatest risk in PD-substantia nigra pars compacta dopamine neurons-have a distinctive physiological phenotype that could contribute to their vulnerability. The opening of L-type calcium channels during autonomous pacemaking results in sustained calcium entry into the cytoplasm of substantia nigra pars compacta dopamine neurons, resulting in elevated mitochondrial oxidant stress and susceptibility to toxins used to create animal models of PD. This cell-specific stress could increase the negative consequences of pan-cellular factors that broadly challenge either mitochondrial or proteostatic competence. The availability of well-tolerated, orally deliverable antagonists for L-type calcium channels points to a novel neuroprotective strategy that could complement current attempts to boost mitochondrial function in the early stages of the disease.

Calcium, cellular aging, and selective neuronal vulnerability in Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disease in developed countries. The core motor symptoms are attributable to the degeneration of dopamine (DA) neurons in the substantia nigra pars compacta (SNc). Why these neurons, and other restricted sets of non-dopamine neuron, succumb in PD is not clear. One potential clue has come from the observation that the engagement of L-type Ca2+ channels during autonomous pacemaking elevates the sensitivity of SNc DA neurons to mitochondrial toxins used to create animal models of PD, suggesting that Ca2+ entry is a factor in their selective vulnerability. Epidemiological data also supports a linkage between L-type Ca2+ channels and the risk of developing PD. This review examines the hypothesis that the primary factor driving neurodegenerative changes in PD is the metabolic stress created by sustained Ca2+ entry, particularly in the face of genetic or environmental factors that compromise oxidative defenses or proteostatic competence.

Hydrogen sulfide: a novel signaling molecule in the central nervous system

Hydrogen sulfide is an endogenously produced gas in the central nervous system and has been touted as the body's third gaseous signaling molecule after nitric oxide and carbon monoxide. A review of the current understanding of hydrogen sulfide is necessary in view of the current plethora of research in this area. The aim of this review is to present the current understanding of H(2)S as a signaling molecule in the central nervous system (CNS). This objective is achieved by discussing the involvement of H(2)S in the regulation of (1) intracellular signaling molecules such as protein kinase A, receptor tyrosine kinases, mitogen kinases and oxidative stress signaling, (2) ion channels such as calcium (L-type, T-type and intracellular stores), potassium (K(ATP) and small conductance channels) and cystic fibrosis transmembrane conductance regulator chloride channels, (3) the release and function of neurotransmitters such as gamma-aminobutyric acid, N-methyl-D-aspartate, glutamate and catecholamines. The role of H(2)S as an important mediator in a myriad of neural functions inclusive of neuroprotection is also discussed throughout the review.

What causes the death of dopaminergic neurons in Parkinson's disease?

The factors governing neuronal loss in Parkinson's disease (PD) are the subject of continuing speculation and experimental study. In recent years, factors that act on most or all cell types (pan-cellular factors), particularly genetic mutations and environmental toxins, have dominated public discussions of disease aetiology. Although there is compelling evidence supporting an association between disease risk and these factors, the pattern of neuronal pathology and cell loss is difficult to explain without cell-specific factors. This chapter focuses on recent studies showing that the neurons at greatest risk in PD--substantia nigra pars compacta (SNc) dopamine (DA) neurons--have a distinctive physiological phenotype that could contribute to their vulnerability. The opening of L-type calcium channels during autonomous pacemaking results in sustained calcium entry into the cytoplasm of SNc DA neurons, resulting in elevated mitochondrial oxidant stress and susceptibility to toxins used to create animal models of PD. This cell-specific stress could increase the negative consequences of pan-cellular factors that broadly challenge either mitochondrial or proteostatic competence. The availability of well-tolerated, orally deliverable antagonists for L-type calcium channels points to a novel neuroprotective strategy that could complement current attempts to boost mitochondrial function in the early stages of the disease.

Characterization of Ca2+ signaling pathways in mouse adrenal medullary chromaffin cells

In the present study, we characterized the Ca2+ responses and secretions induced by various secretagogues in mouse chromaffin cells. Activation of the acetylcholine receptor (AChR) by carbachol induced a transient intracellular Ca2+ concentration ([Ca2+](i)) increase followed by two phases of [Ca2+](i) decay and a burst of exocytic events. The contribution of the subtypes of AChRs to carbachol-induced responses was examined. Based on the results obtained by stimulating the cells with the nicotinic receptor (nAChR) agonist, 1,1-dimethyl-4-phenylpiperazinium iodide, high K(+) and the effects of thapsigargin, it appears that activation of nAChRs induces an extracellular Ca2+ influx, which in turn activate Ca(2+)-induced Ca2+ release via the ryanodine receptors. Muscarine, a muscarinic receptor (mAChRs) agonist, was found to induce [Ca2+](i) oscillation and sustained catecholamine release, possibly by activation of both the receptor- and store-operated Ca2+ entry pathways. The RT-PCR results showed that mouse chromaffin cells are equipped with messages for multiple subtypes of AChRs, ryanodine receptors and all known components of the receptor- and store-operated Ca2+ entry. Furthermore, results obtained by directly monitoring endoplasmic reticulum (ER) and mitochondrial Ca2+ concentration and by disabling mitochondrial Ca2+ uptake suggest that the ER acts as a Ca2+ source, while the mitochondria acts as a Ca2+ sink. Our results show that both nAChRs and mAChRs contribute to the initial carbachol-induced [Ca2+](i) increase which is further enhanced by the Ca2+ released from the ER mediated by Ca(2+)-induced Ca2+ release and mAChR activation. This information on the Ca2+ signaling pathways should lay a good foundation for future studies using mouse chromaffin cells as a model system.

Susceptibility to Calcium Dysregulation during Brain Aging

Calcium (Ca(2+)) is a highly versatile intracellular signaling molecule that is essential for regulating a variety of cellular and physiological processes ranging from fertilization to programmed cell death. Research has provided ample evidence that brain aging is associated with altered Ca(2+) homeostasis. Much of the work has focused on the hippocampus, a brain region critically involved in learning and memory, which is particularly susceptible to dysfunction during senescence. The current review takes a broader perspective, assessing age-related changes in Ca(2+) sources, Ca(2+) sequestration, and Ca(2+) binding proteins throughout the nervous system. The nature of altered Ca(2+) homeostasis is cell specific and may represent a deficit or a compensatory mechanism, producing complex patterns of impaired cellular function. Incorporating the knowledge of the complexity of age-related alterations in Ca(2+) homeostasis will positively shape the development of highly effective therapeutics to treat brain disorders.

An association analysis between mitochondrial DNA A10398G polymorphism and temperament in Japanese young adults

The mitochondrial (mt) DNA C5178A and A10398G polymorphisms have been reported to be associated with mental disorders such as bipolar disorder. However, the effects of these polymorphisms on temperament in healthy people are poorly understood. Evaluating healthy subjects can have the advantage of providing new strategies for maintaining psychological health and preventing mental illness. We examined the association between mtDNA polymorphisms and temperament in Japanese students. There was no significant difference in examined temperament when analysed by genotypes, 5178–10398 haplotypes, or sex. The subgroup analysis based on sex indicated that there was an interactive effect of the mtDNA A10398G polymorphism and sex on anxiety and obsession. This finding is preliminary and cannot exclude the possibility of false-positive due to small sample size (144 subjects) and multiple statistical testing. Further studies involving a larger sample size or other ethnic groups are necessary to confirm that mtDNA A10398G polymorphism can be a genetic factor for temperament.

Homer and the ryanodine receptor

Homer proteins have recently been identified as novel high-affinity ligands that modulate ryanodine receptor (RyR) Ca(2+) release channels in heart and skeletal muscle, through an EVH1 domain which binds to proline-rich regions in target proteins. Many Homer proteins can also self-associate through a coiled-coil domain that allows their multimerisation. In other tissues, especially neurons, Homer anchors proteins embedded in the surface membrane to the Ca(2+) release channel in the endoplasmic reticulum and can anchor membrane or cytosolic proteins to the cytoskeleton. Although this anchoring aspect of Homer function has not been extensively investigated in muscle, there are consensus sequences for Homer binding in the RyR and on many of the proteins that it interacts with in the massive RyR ion channel complex. In this review we explore the potential of Homer to contribute to a variety of cell processes in muscle and neurons that also involve RyR channels.

Ryanodine-Sensitive Intracellular Ca2+ Channels in Rat Suprachiasmatic Nuclei Are Required for Circadian Clock Control of Behavior

Electrophysiological and calcium mobilization experiments have suggested that the intracellular calcium release channel ryanodine receptors (RyRs) are involved in the circadian rhythmicity of the suprachiasmatic nucleus (SCN). In the present report the authors provide behavioral evidence that RyRs play a specific and major role in the output of the molecular circadian clock in SCN neurons. They measured the circadian rhythm of drinking and locomotor behaviors in dim red light before, during, and after administration of an activator (ryanodine 0.1 µM) or an inhibitor (ryanodine 100 µM) of the RyRs. Drugs were delivered directly into the SCN by cannulas connected to osmotic minipumps. Control treatments included administration of artificial cerebrospinal fluid, KCl (20 mM), tetrodotoxin (1 µM), and anysomicin (5 µg/µl). Activation of RyRs induced a significant shortening of the endogenous period, whereas inhibition of these Ca2+ release channels disrupted the circadian rhythmicity. After the pharmacological treatments the period of rhythmicity returned to basal values and the phase of activity onset was predicted from a line projected from the activity onset of basal recordings. These results indicate that changes in overt rhythms induced by both doses of ryanodine did not involve an alteration in the clock mechanism. The authors conclude that circadian modulation of RyRs is a key element of the output pathway from the molecular circadian clock in SCN neurons in rats.

Calcium homeostasis, selective vulnerability and Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative disorder of which the core motor symptoms are attributable to the degeneration of dopamine (DA) neurons in the substantia nigra pars compacta (SNc). Recent work has revealed that the engagement of L-type Ca(2+) channels during autonomous pacemaking renders SNc DA neurons susceptible to mitochondrial toxins used to create animal models of PD, indicating that homeostatic Ca(2+) stress could be a determinant of their selective vulnerability. This view is buttressed by the central role of mitochondria and the endoplasmic reticulum (linchpins of current theories about the origins of PD) in Ca(2+) homeostasis. Here, we summarize this evidence and suggest the dual roles had by these organelles could compromise their function, leading to accelerated aging of SNc DA neurons, particularly in the face of genetic or environmental stress. We conclude with a discussion of potential therapeutic strategies for slowing the progression of PD.

Mitochondrial calcium transport and mitochondrial dysfunction after global brain ischemia in rat hippocampus

Here we report effect of ischemia-reperfusion on mitochondrial Ca2+ uptake and activity of complexes I and IV in rat hippocampus. By performing 4-vessel occlusion model of global brain ischemia, we observed that 15 min ischemia led to significant decrease of mitochondrial capacity to accumulate Ca2+ to 80.8% of control whereas rate of Ca2+ uptake was not significantly changed. Reperfusion did not significantly change mitochondrial Ca2+ transport. Ischemia induced progressive inhibition of complex I, affecting final electron transfer to decylubiquinone. Minimal activity of complex I was observed 24 h after ischemia (63% of control). Inhibition of complex IV activity to 80.6% of control was observed 1 h after ischemia. To explain the discrepancy between impact of ischemia on rate of Ca2+ uptake and activities of both complexes, we performed titration experiments to study relationship between inhibition of particular complex and generation of mitochondrial transmembrane potential (DeltaPsi(m)). Generation of a threshold curves showed that complex I and IV activities must be decreased by approximately 40, and 60%, respectively, before significant decline in DeltaPsi(m) was documented. Thus, mitochondrial Ca2+ uptake was not significantly affected by ischemia-reperfusion, apparently due to excess capacity of the complexes I and IV. Inhibition of complex I is favourable of reactive oxygen species (ROS) generation. Maximal oxidative modification of membrane proteins was documented 1 h after ischemia. Although enhanced formation of ROS might contribute to neuronal injury, depressed activities of complex I and IV together with unaltered rate of Ca2+ uptake are conditions favourable of initiation of other cell degenerative pathways like opening of mitochondrial permeability transition pore or apoptosis initiation, and might represent important mechanism of ischemic damage to neurones.

"Fast" plasma membrane calcium pump PMCA2a concentrates in GABAergic terminals in the adult rat brain

The plasma membrane Ca(2+)-ATPases (PMCA) represent the major high-affinity Ca(2+) extrusion system in the brain. PMCAs comprise four isoforms and over 20 splice variants. Their different functional properties may permit different PMCA splice variants to accommodate different kinds of local [Ca(2+)] transients, but for a specific PMCA to play a unique role in local Ca(2+) handling it must be targeted to the appropriate subcellular compartment. We used immunohistochemistry to study the spatial distribution of PMCA2a-one of the two major carboxyl-terminal splice variants of PMCA2-in the adult rat brain, testing whether this isoform, with especially high basal activity, is targeted to specific subcellular compartments. In striking contrast to the widespread distribution of PMCA2 as a whole, we found that PMCA2a is largely restricted to parvalbumin-positive inhibitory presynaptic terminals throughout the brain. The only major exception to this targeting pattern was in the cerebellar cortex, where PMCA2a also concentrates postsynaptically, in the spines of Purkinje cells. We propose that the fast Ca(2+) activation kinetics and high V(max) of PMCA2a make this pump especially suited for rapid clearance of presynaptic Ca(2+) in fast-spiking inhibitory nerve terminals, which face severe transient calcium loads.

IgIII (270-280)-fragment-like H2N-DDSDEEN-COOH peptide modulates N-CAM expression via Ca2+-dependent ERK signaling during "in vitro neurogenesis"

The two major isoforms (180 kDa and 140 kDa) of the neural cell adhesion molecule (N-CAM) are crucially involved in neurogenesis and brain repair via activation of the mitogen-activated protein kinase (MAPK) cascade. Modification by glycosylation, and homophilic and heterophilic interactions regulate the function of N-CAM, but little is known about the interplay of these processes. In the neuron-like PC12 cell line, extracellular small acidic peptides have been shown to modulate the expression of N-CAM mRNA and protein and regulate its translocation to the plasma membrane. Among these peptides, a synthetic Ig-III-like short sequence (H2N-DDSDEEN-COOH), designated sSP, was particularly potent. In this study, we analyzed the cross-talk between nerve growth factor (NGF) and extracellular sSP in native and N-CAM-transfected PC12 cells to determine if these systems interact to modulate transduction pathways and regulate early steps of neurogenesis in vitro. Our results indicate that sSP accelerated the phosphorylation of extracellular regulated kinase-1 (ERK1) and -2 (ERK2) and promoted plasma membrane translocation of 180 kDa N-CAM. By stabilizing cell-cell contacts and promoting cell cluster formation, these events, which were mediated via a significant increase in intracellular Ca2+, regulated some of the early stages of the NGF-induced differentiation process.

Protective effects of interrupting the binding of calmodulin to mutant huntingtin

There is evidence suggesting that transglutaminase (TG) 2 plays a role in stabilizing monomeric and aggregated huntingtin, thereby contributing to the pathophysiology of Huntington disease. Calmodulin (CaM) regulates TG2 cross-linking of N-terminal mutant huntingtin in cells and colocalizes with TG and huntingtin in inclusions in Huntington disease cortex. The current study examined the effects of small fragments of CaM in human embryonic kidney 293T cells expressing N-terminal mutant huntingtin and transglutaminase 2. Four CaM fragments were developed: first 76 amino acids, last 72 amino acids, 77 amino acids in the center (CaM-center), and the overlapping region of last 72 amino acids and CaM-center (CaM-overlap). The last 72 amino acids, CaM-center, and CaM-overlap significantly decreased amounts of TG-modified huntingtin by 40% to 60%, and cytotoxicity decreased up to 40% compared with cells not expressing any CaM construct. Carbachol-stimulated release of intracellular calcium is significantly higher in cells expressing N-terminal mutant huntingtin and TG2 compared with vector-transfected cells; expression of either CaM-center or CaM-overlap in these cells returned the levels of carbachol-stimulated intracellular calcium release to control values. Furthermore, CaM-overlap expression significantly decreased huntingtin binding to CaM. These data further suggest that CaM regulates TG2 activity, plays a role in the disease-related modifications to mutant huntingtin, and that disruption of CaM-huntingtin interaction is potentially a new target for therapeutic intervention in Huntington disease.

Association between mitochondrial DNA 10398A>G polymorphism and the volume of amygdala

Mitochondrial calcium regulation plays a number of important roles in neurons. Mitochondrial DNA (mtDNA) is highly polymorphic, and its interindividual variation is associated with various neuropsychiatric diseases and mental functions. An mtDNA polymorphism, 10398A>G, was reported to affect mitochondrial calcium regulation. Volume of hippocampus and amygdala is reportedly associated with various mental disorders and mental functions and is regarded as an endophenotype of mental disorders. The present study investigated the relationship between the mtDNA 10398A>G polymorphism and the volume of hippocampus and amygdala in 118 right-handed healthy subjects. The brain morphometry using magnetic resonance images employed both manual tracing volumetry in the native space and voxel-based morphometry (VBM) in the spatially normalized space. Amygdala volume was found to be significantly larger in healthy subjects with 10398A than in those with 10398G by manual tracing, which was confirmed by the VBM. Brain volumes in the other gray matter regions and all white matter regions showed no significant differences associated with the polymorphism. These provocative findings might provide a clue to the complex relationship between mtDNA, brain structure and mental disorders.

Presynaptic functional trkB receptors mediate the release of excitatory neurotransmitters from primary afferent terminals in lamina II (substantia gelatinosa) of postnatal rat spinal cord

A subset of primary sensory neurons produces BDNF, which is implicated in control of nociceptive neurotransmission. We previously localized full-length trkB receptors on their terminals within lamina II. To functionally study these receptors, we here employed patch-clamp recordings, calcium imaging and immunocytochemistry on slices from 8-12 days post-natal rats. In this preparation, BDNF (100-500 ng/mL) enhances the release of sensory neurotransmitters (glutamate, substance P, CGRP) in lamina II by acting on trkB receptors expressed by primary afferent fibers of the peptidergic nociceptive type (PN-PAFs). Effect was blocked by trk antagonist K252a or anti-trkB antibody clone 47. A pre-synaptic mechanism was demonstrated after (i) patch-clamp recordings where the neurotrophin induced a significant increase in frequency, but not amplitude, of AMPA-mediated mEPSCs, (ii) real time calcium imaging, where sustained application of BDNF evoked an intense response in up to 57% lamina II neurons with a significant frequency rise. Antagonists of ionotropic glutamate receptors and NK(1) receptors completely inhibited the calcium response to BDNF. Reduction of CGRP (a specific marker of PN-PAFs) and substance P content in dorsal horn following BDNF preincubation, and analysis of the calcium response after depletion with capsaicin, confirmed that the neurotrophin presynaptically enhanced neurotransmitter release from PN-PAFs. This is the first demonstration that trkB receptors expressed by PN-PAF terminals in lamina II are functional during postnatal development. Implications of this finding are discussed considering that BDNF can be released by these same terminals and microglia, a fraction of which (as shown here) contains BDNF also in unactivated state.

Protein 600 is a microtubule/endoplasmic reticulum-associated protein in CNS neurons

There is an increasing body of literature pointing to cytoskeletal proteins as spatial organizers and interactors of organelles. In this study, we identified protein 600 (p600) as a novel microtubule-associated protein (MAP) developmentally regulated in neurons. p600 exhibits the unique feature to interact with the endoplasmic reticulum (ER). Silencing of p600 by RNA interference (RNAi) destabilizes neuronal processes in young primary neurons undergoing neurite extension and containing scarce staining of the ER marker Bip. Furthermore, in utero electroporation of p600 RNAi alters neuronal migration, a process that depends on synergistic actions of microtubule dynamics and ER functions. p600-depleted migrating neurons display thin, crooked, and "zigzag" leading process with very few ER membranes. Thus, p600 constitutes the only known MAP to associate with the ER in neurons, and this interaction may impact on multiple cellular processes ranging from neuronal development to neuronal maturation and plasticity.

Axogenic effect of estrogen in male rat hypothalamic neurons involves Ca(2+), protein kinase C, and extracellular signal-regulated kinase signaling

17-beta-Estradiol (E2) stimulates the growth of axons in male-derived hypothalamic neurons in vitro. This effect is not exerted through the classical intracellular estrogen receptor (ER) but depends on a membrane mechanism involving TrkB. In the present study, we investigate the intracellular signaling cascade that mediates the axogenic effect of E2. Treatment with an intracellular Ca(2+) chelator, a Ca(2+)-dependent protein kinase C (PKC) inhibitor, or two specific inhibitors of extracellular signal-regulated kinases (ERK) mitogen-activated protein kinases (MAPK) completely inhibited the E2-induced axogenesis. E2 and the membrane-impermeant construct E2BSA rapidly induced phosphorylation of ERK, which was blocked by the specific inhibitor of the ERK pathway UO126 but not by the ER antagonist ICI 182,780. Decrease of intracellular free Ca(2+) or disruption of PKC activation by Ro 32-0432 attenuated ERK activation, indicating the confluence of signals in the MAPK pathway. Subcellular analysis of ERK demonstrated that the phospho-ERK signal is augmented in the nucleus after 15 min of E2 stimulation. We have also shown that E2 increased phosphorylation of CREB via ERK signaling. In summary, this study demonstrates that E2, probably via a membrane-associated receptor, induces axonal growth by activating CREB phosphorylation through ERK signaling by a mechanism involving Ca(2+) and PKC activation.

Integrin regulation of cytoplasmic calcium in excitatory neurons depends upon glutamate receptors and release from intracellular stores

Integrins regulate cytoplasmic calcium levels ([Ca(2+)]i) in various cell types but information on activities in neurons is limited. The issue is of current interest because of the evidence that both integrins and changes in [Ca(2+)]i are required for Long-Term Potentiation. Accordingly, the present studies evaluated integrin ligand effects in cortical neurons. Integrin ligands or alpha5beta1 integrin activating antisera rapidly increased [Ca(2+)]i with effects greater in glutamatergic than GABAergic neurons, absent in astroglia, and blocked by beta1 integrin neutralizing antisera and the tyrosine kinase antagonist genistein. Increases depended upon extracellular calcium and intracellular store release. Ligand-induced effects were reduced by voltage-sensitive calcium channel and NMDA receptor antagonists, but blocked by tetrodotoxin or AMPA receptor antagonists. These results indicate that integrin ligation triggers AMPA receptor/depolarization-dependent calcium influx followed by intracellular store release and suggest the possibility that integrin modulation of activity-induced changes in [Ca(2+)]i contributes importantly to lasting synaptic plasticity in forebrain neurons.

Mitochondrial Dynamics and Peripheral Neuropathy

Peripheral neuropathy is perhaps the archetypal disease of axonal degeneration, characteristically involving degeneration of the longest axons in the body. Evidence from both inherited and acquired forms of peripheral neuropathy strongly supports that the primary pathology is in the axons themselves and points to disruption of axonal transport as an important disease mechanism. Recent studies in human genetics have further identified abnormalities in mitochondrial dynamics—the fusion, fission, and movement of mitochondria— as a player in the pathogenesis of inherited peripheral neuropathy. This review provides an update on the mechanisms of mitochondrial trafficking in axons and the emerging relationship between the disruption of mitochondrial dynamics and axonal degeneration. Evidence suggests mitochondria are a “critical cargo” whose transport is necessary for proper axonal and synaptic function. Importantly, understanding the regulation of mitochondrial movement and the consequences of decreased axonal mitochondrial function may define new paths for therapeutic agents in peripheral neuropathy and other neurodegenerative diseases. NEUROSCIENTIST 14(1):12—18, 2008. DOI: 10.1177/1073858407307354

Inositol 1,4,5 trisphosphate receptor and chromogranin B are concentrated in different regions of the hippocampus

Calcium (Ca(2+)) release from intracellular stores plays a crucial role in many cellular functions in the brain. These intracellular signals have been shown to be transmitted within and between cells. We report a non-uniform distribution of proteins essential for Ca(2+) signaling in acutely prepared brain slice preparations and organotypic slice cultures, both made from rat hippocampus. The Type I inositol-1,4,5 trisphosphate receptor (InsP(3)R1) is the main InsP(3)R subtype in neurons. Immunohistochemistry experiments showed a prominent expression of InsP(3)R1 in the CA1 region of the hippocampus whereas the CA3 region and dentate gyrus (DG) showed only moderate immunoreactivity. In contrast, chromogranin B (CGB), a protein binding to the InsP(3)R1 on the luminal side of the endoplasmic reticular membrane was enriched in the CA3 region whereas DG and the CA1 region showed only faint CGB signals. The neuronal kinases leading to the formation of inositol-1,4,5 trisphosphate (InsP(3)), phosphatidylinositol-4-kinase (PI4K), and phosphatidylinositol-4-phosphate-5-kinase (PIPK), showed strong immunoreactivity throughout all hippocampal cell fields with differences in the subcellular distribution. Moreover, a distinct band of strong CGB and PIPK immunoreactivity was observed in the CA3 region that coincides with the mossy fiber tract (stratum lucidum). These data show differential expression of the components of the signaling toolkit leading to InsP(3)-mediated Ca(2+) release in cells of the hippocampus. The regulation of these differences may play an important role in various neuropathologic conditions such as Alzheimer's disease, epilepsy, or schizophrenia.

Conserved lysin and arginin residues in the extracellular loop of P2X(3) receptors are involved in agonist binding

Wild-type human (h) P2X(3) receptors expressed in HEK293 cells responded to the prototypic agonist alpha,beta-methylene ATP (alpha,beta-meATP) with rapidly desensitizing inward currents and an increase in the intracellular Ca(2+) concentration. In contrast to electrophysiological recordings, Ca(2+) microfluorimetry showed a lower maximum of the concentration-response curve of alpha,beta-meATP in the transiently than in the permanently transfected HEK293 cells. However, the concentrations causing 50% of the maximum possible effect (EC(50) values) were identical, when measured with either method. In order to determine the role of certain conserved, positively charged amino acids in the nucleotide binding domains (NBD-1-4) of hP2X(3) receptors for agonist binding, the lysine-63, -65, -176 and -299 as well as the arginine-281 and -295 residues were substituted by the neutral amino acid alanine. We observed no effect of alpha,beta-meATP at the K63A, K176A, R295A, and K299A mutants, and a marked decrease of agonist potency at the K65A and R281A mutants. The P2X(3) receptor antagonist 2',3'-O-trinitrophenyl-ATP (TNP-ATP) blocked the effect of alpha,beta-meATP at the wild-type hP2X(3) receptor with lower affinity than at the mutant K65A, indicating an interference of this mutation with the docking of the antagonist with its binding sites. The use of confocal fluorescence microscopy in conjunction with an antibody raised against the extracellular loop of the hP2X(3) receptor documented the expression of all mutants in the plasma membrane of HEK293 cells. Eventually, we modelled the possible agonist and antagonist binding sites NBD-1-4 of the hP2X(3) subunit by using structural bioinformatics. This model is in complete agreement with the available data and integrates results from mutagenesis studies with geometry optimization of the tertiary structure predictions of the receptor.

Ryanodine receptor Ca2+-release channels are an output pathway for the circadian clock in the rat suprachiasmatic nuclei

Ryanodine-sensitive intracellular Ca2+ channels (RyRs) are present in suprachiasmatic nuclei (SCN) neurons, but the functions served by these channels are not known. Here we addressed whether mobilization of intracellular Ca2+ stores through the RyRs may be a link between the molecular clock and the firing rate in SCN neurons. Activation of the RyRs by administration of either 1 mM caffeine or 100 nM ryanodine increased the firing frequency, whereas inhibition of RyRs by 10 microM dantrolene or 80 microm ryanodine decreased firing rate. Similar results were obtained in experiments conducted at either midday or midnight. Furthermore, these effects were not mediated by synaptic transmission as blockade of GABA A, AMPA and NMDA receptors did not prevent the excitatory or inhibitory effects induced by either dose of ryanodine on SCN firing. We conclude that gating of RyRs is a key element of the intricate output pathway from the circadian clock within SCN neurons in rats.