Roles of Calcium Stores and Store-Operated Channels in Plasticity of Dendritic Spines

Effects of high-intensity interval training combined with dual-site transcranial direct current stimulation on inhibitory control and working memory in healthy adults

Transcranial direct current stimulation (tDCS) and high-intensity interval training (HIIT) have been demonstrated to enhance inhibitory control and working memory (WM) performance in healthy adults. However, the potential benefits of combining these two interventions have been rarely explored and remain largely speculative. This study aimed to explore the effects of acute HIIT combined with dual-site tDCS over the dorsolateral prefrontal cortex (DLPFC, F3 and F4) on inhibitory control and WM in healthy young adults. Twenty-five healthy college students (20.5 ± 1.3 years; 11 females) were recruited to complete HIIT + tDCS, HIIT + sham-tDCS, rest + tDCS, and rest + sham-tDCS (CON) sessions in a randomized crossover design. tDCS or sham-tDCS was conducted after completing HIIT or a rest condition of the same duration. The Stroop and 2-back tasks were used to evaluate the influence of this combined intervention on cognitive tasks involving inhibitory control and WM performance in post-trials, respectively. Response times (RTs) of the Stroop task significantly improved in the HIIT + tDCS session compared to the CON session across all conditions (all p values <0.05), in the HIIT + tDCS session compared to the rest + tDCS session in the congruent and neutral conditions (all p values <0.05), in the HIIT + sham-tDCS session compared to the CON session in the congruent and neutral conditions (all p values <0.05), in the HIIT + sham-tDCS session compared to the rest + tDCS session in the congruent condition (p = 0.015). No differences were found between sessions in composite score of RT and accuracy in the Stroop task (all p values >0.05) and in the 2-back task reaction time and accuracy (all p values >0.05). We conclude that acute HIIT combined with tDCS effectively improved inhibitory control but it failed to yield cumulative benefits on inhibitory control and WM in healthy adults. These preliminary findings help to identify beneficial effects of combined interventions on cognitive performance and might guide future research with clinical populations.

Intracellular microbial rhodopsin-based optogenetics to control metabolism and cell signaling

Microbial rhodopsin (MRs) ion channels and pumps have become invaluable optogenetic tools for neuroscience as well as biomedical applications. Recently, MR-optogenetics expanded towards subcellular organelles opening principally new opportunities in optogenetic control of intracellular metabolism and signaling via precise manipulations of organelle ion gradients using light. This new optogenetic field expands the opportunities for basic and medical studies of cancer, cardiovascular, and metabolic disorders, providing more detailed and accurate control of cell physiology. This review summarizes recent advances in studies of the cellular metabolic processes and signaling mediated by optogenetic tools targeting mitochondria, endoplasmic reticulum (ER), lysosomes, and synaptic vesicles. Finally, we discuss perspectives of such an optogenetic approach in both fundamental and applied research.

Cultured Rat Hippocampal Neurons Exposed to the Mitochondrial Uncoupler Carbonyl Cyanide Chlorophenylhydrazone Undergo a Rapid, Presenilin-Dependent Change in Neuronal Properties

Presenilin 1 (PS1) is a transmembrane proteolytic subunit of γ-secretase that cleaves amyloid precursor proteins. Mutations in PS1 (mPS1) are associated with early-onset familial Alzheimer's disease (AD). The link between mutated PS1, mitochondrial calcium regulation, and AD has been studied extensively in different test systems. Despite the wide-ranging role of mPS1 in AD, there is a paucity of information on the link between PS1 and neuronal cell death, a hallmark of AD. In the present study, we employed the selective mitochondrial uncoupler carbonyl cyanide chlorophenylhydrazone (CCCP) and compared the reactivity of mPS1-transfected cultured rat hippocampal neurons with PS1 and control neurons in a situation of impaired mitochondrial functions. CCCP causes a slow rise in cytosolic and mitochondrial calcium in all three groups of neurons, with the mPS1 neurons demonstrating a faster rise. Consequently, mPS1 neurons were depolarized by CCCP and measured with TMRM, a mitochondrial voltage indicator, more than the other two groups. Morphologically, CCCP produced more filopodia in mPS1 neurons than in the other two groups, which were similarly affected by the drug. Finally, mPS1 transfected neurons tended to die from prolonged exposure to CCCP sooner than the other groups, indicating an increase in vulnerability associated with a lower ability to regulate excess cytosolic calcium.

Exposure of Cultured Hippocampal Neurons to the Mitochondrial Uncoupler Carbonyl Cyanide Chlorophenylhydrazone Induces a Rapid Growth of Dendritic Processes

A major route for the influx of calcium ions into neurons uses the STIM-Orai1 voltage-independent channel. Once cytosolic calcium ([Ca2+]i) elevates, it activates mitochondrial and endoplasmic calcium stores to affect downstream molecular pathways. In the present study, we employed a novel drug, carbonyl cyanide chlorophenylhydrazone (CCCP), a mitochondrial uncoupler, to explore the role of mitochondria in cultured neuronal morphology. CCCP caused a sustained elevation of [Ca2+]i and, quite surprisingly, a massive increase in the density of dendritic filopodia and spines in the affected neurons. This morphological change can be prevented in cultures exposed to a calcium-free medium, Orai1 antagonist 2APB, or cells transfected with a mutant Orai1 plasmid. It is suggested that CCCP activates mitochondria through the influx of calcium to cause rapid growth of dendritic processes.

Dynamic Regulation of Mitochondrial [Ca2+] in Hippocampal Neurons

While neuronal mitochondria have been studied extensively in their role in health and disease, the rules that govern calcium regulation in mitochondria remain somewhat vague. In the present study using cultured rat hippocampal neurons transfected with the mtRCaMP mitochondrial calcium sensor, we investigated the effects of cytosolic calcium surges on the dynamics of mitochondrial calcium ([Ca²⁺]m). Cytosolic calcium ([Ca²⁺]c) was measured using the high affinity sensor Fluo-2. We recorded two types of calcium events: local and global ones. Local events were limited to a small, 2–5 µm section of the dendrite, presumably caused by local synaptic activity, while global events were associated with network bursts and extended throughout the imaged dendrite. In both cases, cytosolic surges were followed by a delayed rise in [Ca²⁺]m. In global events, the rise lasted longer and was observed in all mitochondrial clusters. At the end of the descending part of the global event, [Ca²⁺]m was still high. Global events were accompanied by short and rather high [Ca²⁺]m surges which we called spikelets, and were present until the complete decay of the cytosolic event. In the case of local events, selective short-term responses were limited to the part of the mitochondrial cluster that was located directly in the center of [Ca²⁺]c activity, and faded quickly, while responses in the neighboring regions were rarely observed. Caffeine (which recruits ryanodine receptors to supply calcium to the mitochondria), and carbonyl cyanide m-chlorophenyl hydrazine (CCCP, a mitochondrial uncoupler) could affect [Ca²⁺]m in both global and local events. We constructed a computational model to simulate the fundamental role of mitochondria in restricting calcium signals within a narrow range under synapses, preventing diffusion into adjacent regions of the dendrite. Our results indicate that local cytoplasmic and mitochondrial calcium concentrations are highly correlated. This reflects a key role of signaling pathways that connect the postsynaptic membrane to local mitochondrial clusters.

Store-operated calcium entry is reduced in spastin-linked hereditary spastic paraplegia

Pathogenic variants in SPAST, the gene coding for spastin, are the single most common cause of hereditary spastic paraplegia, a progressive motor neuron disease. Spastin regulates key cellular functions, including microtubule-severing and endoplasmic reticulum-morphogenesis. However, it remains unclear how alterations in these cellular functions due to SPAST pathogenic variants result in motor neuron dysfunction. Since spastin influences both microtubule network and endoplasmic reticulum structure, we hypothesized that spastin is necessary for the regulation of Ca2+ homeostasis via store-operated calcium entry. Here, we show that the lack of spastin enlarges the endoplasmic reticulum and reduces store-operated calcium entry. In addition, elevated levels of different spastin variants induced clustering of STIM1 within the endoplasmic reticulum, altered the transport of STIM1 to the plasma membrane and reduced store-operated calcium entry, which could be rescued by exogenous expression of STIM1. Importantly, store-operated calcium entry was strongly reduced in induced pluripotent stem cell-derived neurons from hereditary spastic paraplegia patients with pathogenic variants in SPAST resulting in spastin haploinsufficiency. These neurons developed axonal swellings in response to lack of spastin. We were able to rescue both store-operated calcium entry and axonal swellings in SPAST patient neurons by restoring spastin levels, using CRISPR/Cas9 to correct the pathogenic variants in SPAST. These findings demonstrate that proper amounts of spastin are a key regulatory component for store-operated calcium entry mediated Ca2+ homeostasis and suggest store-operated calcium entry as a disease relevant mechanism of spastin-linked motor neuron disease.

Differential hippocampal protein expression between normal mice and mice with the perioperative neurocognitive disorder: a proteomic analysis

OBJECTIVES

To compare differential expression protein in hippocampal tissues from mice of perioperative neurocognitive disorder (PND) and normal control mice and to explore the possible mechanism of PND.

METHODS

Mice were randomly divided into a PND group (n = 9) and a control group (n = 9).The mice in the PND group were treated with open tibial fracture with intramedullary fixation under isoflurane anesthesia, while the mice in the control group received pure oxygen without surgery. The cognitive functions of the two groups were examined using Morris water maze experiment, Open field test and Fear conditioning test. The protein expression of the hippocampus of mice was analyzed by high-performance liquid chromatography-mass spectrometry (HPLC-MS/MS). Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses were performed to explore the principal functions of dysregulated proteins.

RESULTS

A total of 21 proteins were differentially expressed between PND and control mice on days 1, 3, and 7 after the operation. These proteins were involved in many pathological processes, such as neuroinflammatory responses, mitochondrial oxidative stress, impaired synaptic plasticity, and neuronal cell apoptosis. Also, the dysregulated proteins were involved in MAPK, AMPK, and ErbB signaling pathways.

CONCLUSION

The occurrence of PND could be attributed to multiple mechanisms.

Nanoscale molecular architecture controls calcium diffusion and ER replenishment in dendritic spines

Dendritic spines are critical components of neuronal synapses as they receive and transform synaptic inputs into a succession of calcium-regulated biochemical events. The spine apparatus (SA), an extension of smooth endoplasmic reticulum, regulates slow and fast calcium dynamics in spines. Calcium release events deplete SA calcium ion reservoir rapidly, yet the next cycle of signaling requires its replenishment. How spines achieve this replenishment without triggering calcium release remains unclear. Using computational modeling, calcium and STED superresolution imaging, we show that the SA replenishment involves the store-operated calcium entry pathway during spontaneous calcium transients. We identified two main conditions for SA replenishment without depletion: a small amplitude and a slow timescale for calcium influx, and a close proximity between SA and plasma membranes. Thereby, spine’s nanoscale organization separates SA replenishment from depletion. We further conclude that spine’s receptor organization also determines the calcium dynamics during the induction of long-term synaptic changes.

Calcium Sensors STIM1 and STIM2 Regulate Different Calcium Functions in Cultured Hippocampal Neurons

There are growing indications for the involvement of calcium stores in the plastic properties of neurons and particularly in dendritic spines of central neurons. The store-operated calcium entry (SOCE) channels are assumed to be activated by the calcium sensor stromal interaction molecule (STIM)which leads to activation of its associated Orai channel. There are two STIM species, and the differential role of the two in SOCE is not entirely clear. In the present study, we were able to distinguish between transfected STIM1, which is more mobile primarily in young neurons, and STIM2 which is less mobile and more prominent in older neurons in culture. STIM1 mobility is associated with spontaneous calcium sparks, local transient rise in cytosolic [Ca²⁺]_i, and in the formation and elongation of dendritic filopodia/spines. In contrast, STIM2 is associated with older neurons, where it is mobile and moves into dendritic spines primarily when cytosolic [Ca²⁺]_i levels are reduced, apparently to activate resident Orai channels. These results highlight a role for STIM1 in the regulation of [Ca²⁺]_i fluctuations associated with the formation of dendritic spines or filopodia in the developing neuron, whereas STIM2 is associated with the maintenance of calcium entry into stores in the adult neuron.

Beyond the Mind-Serum Trace Element Levels in Schizophrenic Patients: A Systematic Review

The alterations in serum trace element levels are common phenomena observed in patients with different psychiatric conditions such as schizophrenia, autism spectrum disorder, or major depressive disorder. The fluctuations in the trace element concentrations might act as potential diagnostic and prognostic biomarkers of many psychiatric and neurological disorders. This paper aimed to assess the alterations in serum trace element concentrations in patients with a diagnosed schizophrenia. The authors made a systematic review, extracting papers from the PubMed, Web of Science, and Scopus databases according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Among 5009 articles identified through database searching, 59 of them were assessed for eligibility. Ultimately, 33 articles were included in the qualitative synthesis. This review includes the analysis of serum levels of the following trace elements: iron, nickel, molybdenum, phosphorus, lead, chromium, antimony, uranium, magnesium, aluminum, zinc, copper, selenium, calcium, and manganese. Currently, there is no consistency regarding serum trace element levels in schizophrenic patients. Thus, it cannot be considered as a reliable prognostic or diagnostic marker of schizophrenia. However, it can be assumed that altered concentrations of those elements are crucial regarding the onset and exaggeration of either psychotic or negative symptoms or cognitive dysfunctions.

Store-Operated Calcium Channels in Physiological and Pathological States of the Nervous System

Store-operated calcium channels (SOCs) are widely expressed in excitatory and non-excitatory cells where they mediate significant store-operated calcium entry (SOCE), an important pathway for calcium signaling throughout the body. While the activity of SOCs has been well studied in non-excitable cells, attention has turned to their role in neurons and glia in recent years. In particular, the role of SOCs in the nervous system has been extensively investigated, with links to their dysregulation found in a wide variety of neurological diseases from Alzheimer’s disease (AD) to pain. In this review, we provide an overview of their molecular components, expression, and physiological role in the nervous system and describe how the dysregulation of those roles could potentially lead to various neurological disorders. Although further studies are still needed to understand how SOCs are activated under physiological conditions and how they are linked to pathological states, growing evidence indicates that SOCs are important players in neurological disorders and could be potential new targets for therapies. While the role of SOCE in the nervous system continues to be multifaceted and controversial, the study of SOCs provides a potentially fruitful avenue into better understanding the nervous system and its pathologies.

Regulation of chemoconvulsant-induced seizures by store-operated Orai1 channels

KEY POINTS

Temporal lobe epilepsy is a complex neurological disease caused by imbalance of excitation and inhibition in the brain. Growing literature implicates altered Ca²⁺ signalling in many aspects of epilepsy but the diversity of Ca²⁺ channels that regulate this syndrome are not well-understood. Here, we report that mice lacking the store-operated Ca²⁺ channel, Orai1, in the brain show markedly stronger seizures in response to the chemoconvulsants, kainic acid and pilocarpine. Electrophysiological analysis reveals that selective deletion of Orai1 channels in inhibitory neurons disables chemoconvulsant-induced excitation of GABAergic neurons in the CA1 hippocampus. Likewise, deletion of Orai1 in GABAergic neurons abrogates the chemoconvulsant-induced burst of spontaneous inhibitory postsynaptic currents (sIPSCs) on CA1 pyramidal neurons in the hippocampus. This loss of chemoconvulsant inhibition likely aggravates status epilepticus in Orai1 KO mice. These results identify Orai1 channels as regulators of hippocampal interneuron excitability and seizures.

ABSTRACT

Store-operated Orai1 channels are a major mechanism for Ca²⁺ entry in many cells and mediate numerous functions including gene expression, cytokine production and gliotransmitter release. Orai1 is expressed in many regions of the mammalian brain; however, its role in regulating neuronal excitability, synaptic function and brain disorders has only now begun to be investigated. To investigate a potential role of Orai1 channels in status epilepticus induced by chemoconvulsants, we examined acute seizures evoked by intraperitoneal injections of kainic acid (KA) and pilocarpine in mice with a conditional deletion of Orai1 (or its activator STIM1) in the brain. Brain-specific Orai1 and STIM1 knockout (KO) mice exhibited significantly stronger seizures (P = 0.00003 and P < 0.00001), and higher chemoconvulsant-induced mortality (P = 0.02) compared with wildtype (WT) littermates. Electrophysiological recordings in hippocampal brain slices revealed that KA stimulated the activity of inhibitory interneurons in the CA1 hippocampus (P = 0.04) which failed to occur in Orai1 KO mice. Further, KA and pilocarpine increased the frequency of spontaneous IPSCs in CA1 pyramidal neurons >twofold (KA: P = 0.04; pilocarpine: P = 0.0002) which was abolished in Orai1 KO mice. Mice with selective deletion of Orai1 in GABAergic neurons alone also showed stronger seizures to KA (P = 0.001) and pilocarpine (P < 0.00001) and loss of chemoconvulsant-induced increases in sIPSC responses compared with WT controls. We conclude that Orai1 channels regulate chemoconvulsant-induced excitation in GABAergic neurons and that destabilization of the excitatory/inhibitory balance in Orai1 KO mice aggravates chemoconvulsant-mediated seizures. These results identify Orai1 channels as novel molecular regulators of hippocampal neuronal excitability and seizures.

Transgenic Mice Overexpressing Human STIM2 and ORAI1 in Neurons Exhibit Changes in Behavior and Calcium Homeostasis but Show No Signs of Neurodegeneration

The maintenance of proper cytosolic Ca²⁺ level is crucial for neuronal survival, and dysregulation of Ca²⁺ homeostasis is found in a variety of neurological disorders, including Alzheimer’s disease. According to the “Ca²⁺ hypothesis of aging”, Ca²⁺ disturbances precede the onset of AD symptoms and lead to neurodegeneration. STIM and ORAI proteins are involved in neuronal physiological and pathological processes as essential components of the store-operated Ca²⁺ entry. Our previous data suggested that overexpression of STIM2 and ORAI1 might increase basal neuronal cytosolic Ca²⁺ level. We generated double transgenic mice overexpressing these two genes in neurons, expecting that the increased basal Ca²⁺ concentration will lead to premature neurodegeneration. We observed changes in Ca²⁺ homeostasis and electrophysiological properties in acute brain slices of STIM2/ORAI1 neurons. However, we did not observe any augmentation of neurodegenerative processes, as tested by Fluoro-Jade^® C staining and assessment of amyloidogenesis. The battery of behavioral tests did not show any signs of accelerated aging. We conclude that changes of calcium homeostasis induced by overexpression of STIM2 and ORAI1 had no substantial adverse effects on neurons and did not lead to early neurodegeneration.

Fast calcium transients in dendritic spines driven by extreme statistics

Fast calcium transients (<10 ms) remain difficult to analyse in cellular microdomains, yet they can modulate key cellular events such as trafficking, local ATP production by endoplasmic reticulum-mitochondria complex (ER-mitochondria complex), or spontaneous activity in astrocytes. In dendritic spines receiving synaptic inputs, we show here that in the presence of a spine apparatus (SA), which is an extension of the smooth ER, a calcium-induced calcium release (CICR) is triggered at the base of the spine by the fastest calcium ions arriving at a Ryanodyne receptor (RyR). The mechanism relies on the asymmetric distributions of RyRs and sarco/ER calcium-ATPase (SERCA) pumps that we predict using a computational model and further confirm experimentally in culture and slice hippocampal neurons. The present mechanism for which the statistics of the fastest particles arriving at a small target, followed by an amplification, is likely to be generic in molecular transduction across cellular microcompartments, such as thin neuronal processes, astrocytes, endfeets, or protrusions.

Intracellular Ca2+ Release and Synaptic Plasticity: A Tale of Many Stores

Ca2+ is an essential trigger for most forms of synaptic plasticity. Ca2+ signaling occurs not only by Ca2+ entry via plasma membrane channels but also via Ca2+ signals generated by intracellular organelles. These organelles, by dynamically regulating the spatial and temporal extent of Ca2+ elevations within neurons, play a pivotal role in determining the downstream consequences of neural signaling on synaptic function. Here, we review the role of three major intracellular stores: the endoplasmic reticulum, mitochondria, and acidic Ca2+ stores, such as lysosomes, in neuronal Ca2+ signaling and plasticity. We provide a comprehensive account of how Ca2+ release from these stores regulates short- and long-term plasticity at the pre- and postsynaptic terminals of central synapses.

Lysophosphatidic Acid and Glutamatergic Transmission

Signaling through bioactive lipids regulates nervous system development and functions. Lysophosphatidic acid (LPA), a membrane-derived lipid mediator particularly enriched in brain, is able to induce many responses in neurons and glial cells by affecting key processes like synaptic plasticity, neurogenesis, differentiation and proliferation. Early studies noted sustained elevations of neuronal intracellular calcium, a primary response to LPA exposure, suggesting functional modifications of NMDA and AMPA glutamate receptors. However, the crosstalk between LPA signaling and glutamatergic transmission has only recently been shown. For example, stimulation of presynaptic LPA receptors in hippocampal neurons regulates glutamate release from the presynaptic terminal, and excess of LPA induce seizures. Further evidence indicating a role of LPA in the modulation of neuronal transmission has been inferred from animal models with deficits on LPA receptors, mainly LPA₁ which is the most prevalent receptor in human and mouse brain tissue. LPA₁ null-mice exhibit cognitive and attention deficits characteristic of schizophrenia which are related with altered glutamatergic transmission and reduced neuropathic pain. Furthermore, silencing of LPA₁ receptor in mice induced a severe down-regulation of the main glutaminase isoform (GLS) in cerebral cortex and hippocampus, along with a parallel sharp decrease on active matrix-metalloproteinase 9. The downregulation of both enzymes correlated with an altered morphology of glutamatergic pyramidal cells dendritic spines towards a less mature phenotype, indicating important implications of LPA in synaptic excitatory plasticity which may contribute to the cognitive and memory deficits shown by LPA₁-deficient mice. In this review, we present an updated account of current evidence pointing to important implications of LPA in the modulation of synaptic excitatory transmission.

Calcium Deregulation and Mitochondrial Bioenergetics in GDAP1-Related CMT Disease

The pathology of Charcot-Marie-Tooth (CMT), a disease arising from mutations in different genes, has been associated with an impairment of mitochondrial dynamics and axonal biology of mitochondria. Mutations in ganglioside-induced differentiation-associated protein 1 (GDAP1) cause several forms of CMT neuropathy, but the pathogenic mechanisms involved remain unclear. GDAP1 is an outer mitochondrial membrane protein highly expressed in neurons. It has been proposed to play a role in different aspects of mitochondrial physiology, including mitochondrial dynamics, oxidative stress processes, and mitochondrial transport along the axons. Disruption of the mitochondrial network in a neuroblastoma model of GDAP1-related CMT has been shown to decrease Ca²⁺ entry through the store-operated calcium entry (SOCE), which caused a failure in stimulation of mitochondrial respiration. In this review, we summarize the different functions proposed for GDAP1 and focus on the consequences for Ca²⁺ homeostasis and mitochondrial energy production linked to CMT disease caused by different GDAP1 mutations.

Behavioral and electrophysiological changes in female mice overexpressing ORAI1 in neurons

Orai proteins form highly selective Ca²⁺ release-activated channels (CRACs). They play a critical role in store-operated Ca²⁺ entry (SOCE; i.e., the influx of external Ca²⁺ that is induced by the depletion of endoplasmic reticulum Ca²⁺ stores). Of the three Orai homologs that are present in mammals (Orai1-3), the physiological function of Orai1 is the best described. CRACs are formed by both homomeric assemblies and heteromultimers of Orais. Orai1 and Orai2 can form heteromeric channels that differ in conductivity during SOCE, depending on their Orai1-to-Orai2 ratio. The present study explored the potential consequences of ORAI1 overexpression in neurons where the dominant isoform is Orai2. We established the Tg(ORAI1)Ibd transgenic mouse line that overexpresses ORAI1 in brain neurons. We observed seizure-like symptoms in aged (≥15-month-old) female mice but not in males of the same age. The application of kainic acid and bicuculline to slices that were isolated from 8-month-old (±1 month) female Tg(ORAI1)Ibd mice revealed a significantly lower frequency of interictal bursts compared with samples that were isolated from wildtype mice. No differences were observed in male mice of a similar age. A battery of behavioral tests showed that context recognition decreased only in female transgenic mice. The phenotype that was observed in female mice suggests that ORAI1 overexpression may affect neuronal activity in a sex-dependent manner. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

Redundancy principle and the role of extreme statistics in molecular and cellular biology

The paradigm of chemical activation rates in cellular biology has been shifted from the mean arrival time of a single particle to the mean of the first among many particles to arrive at a small activation site. The activation rate is set by extremely rare events, which have drastically different time scales from the mean times between activations, and depends on different structural parameters. This shift calls for reconsideration of physical processes used in deterministic and stochastic modeling of chemical reactions that are based on the traditional forward rate, especially for fast activation processes in living cells. Consequently, the biological activation time is not necessarily exponentially distributed. We review here the physical models, the mathematical analysis and the new paradigm of setting the scale to be the shortest time for activation that clarifies the role of population redundancy in selecting and accelerating transient cellular search processes. We provide examples in cellular transduction, gene activation, cell senescence activation or spermatozoa selection during fertilization, where the rate depends on numbers. We conclude that the statistics of the minimal time to activation set kinetic laws in biology, which can be very different from the ones associated to average times.

Spine-to-Dendrite Calcium Modeling Discloses Relevance for Precise Positioning of Ryanodine Receptor-Containing Spine Endoplasmic Reticulum

The endoplasmic reticulum (ER) forms a complex endomembrane network that reaches into the cellular compartments of a neuron, including dendritic spines. Recent work discloses that the spine ER is a dynamic structure that enters and leaves spines. While evidence exists that ER Ca²⁺ release is involved in synaptic plasticity, the role of spine ER morphology remains unknown. Combining a new 3D spine generator with 3D Ca²⁺ modeling, we addressed the relevance of ER positioning on spine-to-dendrite Ca²⁺ signaling. Our simulations, which account for Ca²⁺ exchange on the plasma membrane and ER, show that spine ER needs to be present in distinct morphological conformations in order to overcome a barrier between the spine and dendritic shaft. We demonstrate that RyR-carrying spine ER promotes spine-to-dendrite Ca²⁺ signals in a position-dependent manner. Our simulations indicate that RyR-carrying ER can initiate time-delayed Ca²⁺ reverberation, depending on the precise position of the spine ER. Upon spine growth, structural reorganization of the ER restores spine-to-dendrite Ca²⁺ communication, while maintaining aspects of Ca²⁺ homeostasis in the spine head. Our work emphasizes the relevance of precise positioning of RyR-containing spine ER in regulating the strength and timing of spine Ca²⁺ signaling, which could play an important role in tuning spine-to-dendrite Ca²⁺ communication and homeostasis.

Organelle Optogenetics: Direct Manipulation of Intracellular Ca2+ Dynamics by Light

As one of the ubiquitous second messengers, the intracellular Ca²⁺, has been revealed to be a pivotal regulator of various cellular functions. Two major sources are involved in the initiation of Ca²⁺-dependent signals: influx from the extracellular space and release from the intracellular Ca²⁺ stores such as the endoplasmic/sarcoplasmic reticulum (ER/SR). To manipulate the Ca²⁺ release from the stores under high spatiotemporal precision, we established a new method termed “organelle optogenetics.” That is, one of the light-sensitive cation channels (channelrhodopsin-green receiver, ChRGR), which is Ca²⁺-permeable, was specifically targeted to the ER/SR. The expression specificity as well as the functional operation of the ER/SR-targeted ChRGR (ChRGR_ER) was evaluated using mouse skeletal myoblasts (C2C12): (1) the ChRGR_ER co-localized with the ER-marker KDEL; (2) no membrane current was generated by light under whole-cell clamp of cells expressing ChRGR_ER; (3) an increase of fluorometric Ca²⁺ was evoked by the optical stimulation (OS) in the cells expressing ChRGR_ER in a manner independent on the extracellular Ca²⁺ concentration ([Ca²⁺]_o); (4) the ΔF/F₀ was sensitive to the inhibitor of sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) and (5) the store-operated Ca²⁺ entry (SOCE) was induced by the OS in the ChRGR_ER-expressing cells. Our organelle optogenetics effectively manipulated the ER/SR to release Ca²⁺ from intracellular stores. The use of organelle optogenetics would reveal the neuroscientific significance of intracellular Ca²⁺ dynamics under spatiotemporal precision.

Tumor necrosis factor (TNF) modulates synaptic plasticity in a concentration-dependent manner through intracellular calcium stores

The role of inflammatory signaling pathways in synaptic plasticity has long been identified. Yet, it remains unclear how inflammatory cytokines assert their pleiotropic effects on neural plasticity. Moreover, the neuronal targets through which inflammatory cytokines assert their effects on plasticity remain not well-understood. In an attempt to learn more about the plasticity-modulating effects of the pro-inflammatory cytokine tumor necrosis factor (TNF), we used two-pathway long-term potentiation (LTP) experiments at Schaffer collateral-CA1 synapses to test for concentration-dependent effects of TNF on synaptic plasticity. We report that high concentrations of TNF (1 μg/mL) impair the ability of mouse CA1 pyramidal neurons to express synaptic plasticity without affecting baseline synaptic transmission and/or previously established LTP. Interestingly, 100 ng/mL of TNF has no apparent effect on LTP, while low concentrations (1 ng/mL) promote the ability of neurons to express LTP. These dose-dependent metaplastic effects of TNF are modulated by intracellular calcium stores: Pharmacological activation of intracellular calcium stores with ryanodine (10 μM) reverses the negative effects of TNF_[high], and the plasticity-promoting effects of TNF_[low] are blocked when intracellular calcium stores are depleted with thapsigargin (1 μM). Consistent with this result, TNF does not promote plasticity in synaptopodin-deficient preparations, which show deficits in neuronal calcium store-mediated synaptic plasticity. Thus, we propose that TNF mediates its pleiotropic effects on synaptic plasticity in a concentration-dependent manner through signaling pathways that are modulated by intracellular calcium stores and require the presence of synaptopodin. These results demonstrate that TNF can act as mediator of metaplasticity, which is of considerable relevance in the context of brain diseases associated with increased TNF levels and alterations in synaptic plasticity.

KEY MESSAGES

• TNF modulates the ability of neurons to express synaptic plasticity. • High concentrations of TNF impair synaptic plasticity. • Low concentrations of TNF improve synaptic plasticity. • TNF does not affect previously established long-term potentiation. • Plasticity effects of TNF are modulated by intracellular calcium stores.

Characterization of postsynaptic calcium signals in the pyramidal neurons of anterior cingulate cortex

Calcium signaling is critical for synaptic transmission and plasticity. N-methyl-D-aspartic acid (NMDA) receptors play a key role in synaptic potentiation in the anterior cingulate cortex. Most previous studies of calcium signaling focus on hippocampal neurons, little is known about the activity-induced calcium signals in the anterior cingulate cortex. In the present study, we show that NMDA receptor-mediated postsynaptic calcium signals induced by different synaptic stimulation in anterior cingulate cortex pyramidal neurons. Single and multi-action potentials evoked significant suprathreshold Ca²⁺ increases in somas and spines. Both NMDA receptors and voltage-gated calcium channels contributed to this increase. Postsynaptic Ca²⁺signals were induced by puff-application of glutamate, and a NMDA receptor antagonist AP5 blocked these signals in both somas and spines. Finally, long-term potentiation inducing protocols triggered postsynaptic Ca²⁺ influx, and these influx were NMDA receptor dependent. Our results provide the first study of calcium signals in the anterior cingulate cortex and demonstrate that NMDA receptors play important roles in postsynaptic calcium signals in anterior cingulate cortex pyramidal neurons.

Inhibition of TRPC1-Dependent Store-Operated Calcium Entry Improves Synaptic Stability and Motor Performance in a Mouse Model of Huntington's Disease

BACKGROUND

Huntington disease (HD) is a dominantly inherited neurodegenerative disorder caused by a CAG repeat expansion in the huntingtin gene. We previously discovered that mutant Huntingtin sensitizes type 1 inositol 1,4,5-trisphosphate receptor (InsP3R1) to InsP3. This causes calcium leakage from the endoplasmic reticulum (ER) and a compensatory increase in neuronal store-operated calcium (nSOC) entry. We previously demonstrated that supranormal nSOC leads to synaptic loss in striatal medium spiny neurons (MSNs) in YAC128 HD mice.

OBJECTIVE

We sought to identify calcium channels supporting supranormal nSOC in HD MSNs and to validate these channels as potential therapeutic targets for HD.

METHODS

Cortico-striatal cultures were established from wild type and YAC128 HD mice and the density of MSN spines was quantified. The expression of candidate nSOC components was suppressed by RNAi knockdown and by CRISPR/Cas9 knockout. TRPC1 knockout mice were crossed with YAC128 HD mice for evaluation of motor performance in a beamwalk assay.

RESULTS

RNAi-mediated knockdown of TRPC1, TRPC6, Orai1, or Orai2, but not other TRPC isoforms or Orai3, rescued the density of YAC128 MSN spines. Knockdown of stromal interaction molecule 1 (STIM1), an ER calcium sensor and nSOC activator, also rescued YAC128 MSN spines. Knockdown of the same targets suppressed supranormal nSOC in YAC128 MSN spines. These channel subunits co-immunoprecipitated with STIM1 and STIM2 in synaptosomal lysates from mouse striata. Crossing YAC128 mice with TRPC1 knockout mice improved motor performance and rescued MSN spines in vitro and in vivo, indicating that inhibition of TRPC1 may serve as a neuroprotective strategy for HD treatment.

CONCLUSIONS

TRPC1 channels constitute a potential therapeutic target for treatment of HD.

Store-Operated Calcium Entry Is Required for mGluR-Dependent Long Term Depression in Cortical Neurons

Store-operated calcium entry (SOCE) is a Calcium (Ca²⁺) influx pathway activated by depletion of intracellular stores that occurs in eukaryotic cells. In neurons, the presence and functions of SOCE are still in question. Here, we show evidences for the existence of SOCE in primary mouse cortical neurons. Endoplasmic reticulum (ER)-Ca²⁺ depletion using thapsigargin (Tg) triggered a maintained cytosolic Ca²⁺ increase, which rapidly returned to basal level in the presence of the SOCE blockers 2-Aminoethoxydiphenyl borate (2-APB) and YM-58483. Neural SOCE is also engaged by activation of metabotropic glutamate receptors (mGluRs) with (S)-3,5-dihydroxyphenylglycine (DHPG) (agonist of group I mGluRs), being an essential mechanism to maintain the mGluR-driven Ca²⁺ signal. Activation of group I of mGluRs triggers long-term depression (LTD) in many brain regions, but the underlying mechanism and, specifically, the necessity of Ca²⁺ increase in the postsynaptic neuron is controversial. In primary cortical neurons, we now show that the inhibition of Ca²⁺ influx through SOCE impaired DHPG-LTD, pointing out a key function of calcium and SOCE in synaptic plasticity.

Sexual Dimorphism in a Reciprocal Interaction of Ryanodine and IP3 Receptors in the Induction of Hyperalgesic Priming

Hyperalgesic priming, a model of pain chronification in the rat, is mediated by ryanodine receptor-dependent calcium release. Although ryanodine induces priming in both sexes, females are 5 orders of magnitude more sensitive, by an estrogen receptor α (EsRα)-dependent mechanism. An inositol 1,4,5-triphosphate (IP₃) receptor inhibitor prevented the induction of priming by ryanodine. For IP₃ induced priming, females were also more sensitive. IP₃-induced priming was prevented by pretreatment with inhibitors of the sarcoendoplasmic reticulum calcium ATPase and ryanodine receptor. Antisense to EsRα prevented the induction of priming by low-dose IP₃ in females. The induction of priming by an EsRα agonist was ryanodine receptor-dependent and prevented by the IP₃ antagonist. Thus, an EsRα-dependent bidirectional interaction between endoplasmic reticulum IP₃ and ryanodine receptor-mediated calcium signaling is present in the induction of hyperalgesic priming, in females. In cultured male DRG neurons, IP₃ (100 μm) potentiated depolarization-induced transients produced by extracellular application of high-potassium solution (20 mm, K20), in nociceptors incubated with β-estradiol. This potentiation of depolarization-induced calcium transients was blocked by the IP₃ antagonist, and not observed in the absence of IP₃ IP₃ potentiation was also blocked by ryanodine receptor antagonist. The application of ryanodine (2 nm), instead of IP₃, also potentiated K20-induced calcium transients in the presence of β-estradiol, in an IP₃ receptor-dependent manner. Our results point to an EsRα-dependent, reciprocal interaction between IP₃ and ryanodine receptors that contributes to sex differences in hyperalgesic priming.SIGNIFICANCE STATEMENT The present study demonstrates a mechanism that plays a role in the marked sexual dimorphism observed in a model of the transition to chronic pain, hyperalgesic priming. This mechanism involves a reciprocal interaction between the endoplasmic reticulum receptors, IP₃ and ryanodine, in the induction of priming, regulated by estrogen receptor α in the nociceptor of female rats. The presence of this signaling pathway modulating the susceptibility of nociceptors to develop plasticity may contribute to our understanding of sex differences observed clinically in chronic pain syndromes.

Organelle-Specific Sensors for Monitoring Ca2+ Dynamics in Neurons

Calcium (Ca²⁺) plays innumerable critical functions in neurons ranging from regulation of neurotransmitter release and synaptic plasticity to activity-dependent transcription. Therefore, more than any other cell types, neurons are critically dependent on spatially and temporally controlled Ca²⁺ dynamics. This is achieved through an exquisite level of compartmentalization of Ca²⁺ storage and release from various organelles. The function of these organelles in the regulation of Ca²⁺ dynamics has been studied for decades using electrophysiological and optical methods combined with pharmacological and genetic alterations. Mitochondria and the endoplasmic reticulum (ER) are among the organelles playing the most critical roles in Ca²⁺ dynamics in neurons. At presynaptic boutons, Ca²⁺ triggers neurotransmitter release and synaptic plasticity, and postsynaptically, Ca²⁺ mobilization mediates long-term synaptic plasticity. To explore Ca²⁺ dynamics in live cells and intact animals, various synthetic and genetically encoded fluorescent Ca²⁺ sensors were developed, and recently, many groups actively increased the sensitivity and diversity of genetically encoded Ca²⁺ indicators (GECIs). Following conjugation with various signal peptides, these improved GECIs can be targeted to specific subcellular compartments, allowing monitoring of organelle-specific Ca²⁺ dynamics. Here, we review recent findings unraveling novel roles for mitochondria- and ER-dependent Ca²⁺ dynamics in neurons and at synapses.