Orai1 Channels Are Essential for Amplification of Glutamate-Evoked Ca2+ Signals in Dendritic Spines to Regulate Working and Associative Memory

Periodic ER-plasma membrane junctions support long-range Ca2+ signal integration in dendrites

Neuronal dendrites must relay synaptic inputs over long distances, but the mechanisms by which activity-evoked intracellular signals propagate over macroscopic distances remain unclear. Here, we discovered a system of periodically arranged endoplasmic reticulum-plasma membrane (ER-PM) junctions tiling the plasma membrane of dendrites at ∼1 μm intervals, interlinked by a meshwork of ER tubules patterned in a ladder-like array. Populated with Junctophilin-linked plasma membrane voltage-gated Ca2+ channels and ER Ca2+-release channels (ryanodine receptors), ER-PM junctions are hubs for ER-PM crosstalk, fine-tuning of Ca2+ homeostasis, and local activation of the Ca2+/calmodulin-dependent protein kinase II. Local spine stimulation activates the Ca2+ modulatory machinery, facilitating signal transmission and ryanodine-receptor-dependent Ca2+ release at ER-PM junctions over 20 μm away. Thus, interconnected ER-PM junctions support signal propagation and Ca2+ release from the spine-adjacent ER. The capacity of this subcellular architecture to modify both local and distant membrane-proximal biochemistry potentially contributes to dendritic computations.

Targeting calciumopathy for neuroprotection: focus on calcium channels Cav1, Orai1 and P2X7

As the uncontrolled entry of calcium ions (Ca2+) through plasmalemmal calcium channels is a cell death trigger, the conjecture is here raised that mitigating such an excess of Ca2+ entry should rescue from death the vulnerable neurons in neurodegenerative diseases (NDDs). However, this supposition has failed in some clinical trials (CTs). Thus, a recent CT tested whether isradipine, a blocker of the Cav1 subtype of voltage-operated calcium channels (VOCCs), exerted a benefit in patients with Parkinson's disease (PD); however, outcomes were negative. This is one more of the hundreds of CTs done under the principle of one-drug-one-target, that have failed in Alzheimer's disease (AD) and other NDDs during the last three decades. As there are myriad calcium channels to let Ca2+ ions gain the cell cytosol, it seems reasonable to predict that blockade of Ca2+ entry through a single channel may not be capable of preventing the Ca2+ flood of cells by the uncontrolled Ca2+ entry. Furthermore, as Ca2+ signaling is involved in the regulation of myriad functions in different cell types, it seems also reasonable to guess that a therapy should be more efficient by targeting different cells with various drugs. Here, we propose to mitigate Ca2+ entry by the simultaneous partial blockade of three quite different subtypes of plasmalemmal calcium channels that is, the Cav1 subtype of VOCCs, the Orai1 store-operated calcium channel (SOCC), and the purinergic P2X7 calcium channel. All three channels are expressed in both microglia and neurons. Thus, by targeting the three channels with a combination of three drug blockers we expect favorable changes in some of the pathogenic features of NDDs, namely (i) to mitigate Ca2+ entry into microglia; (ii) to decrease the Ca2+-dependent microglia activation; (iii) to decrease the sustained neuroinflammation; (iv) to decrease the uncontrolled Ca2+ entry into neurons; (v) to rescue vulnerable neurons from death; and (vi) to delay disease progression. In this review we discuss the arguments underlying our triad hypothesis in the sense that the combination of three repositioned medicines targeting Cav1, Orai1, and P2X7 calcium channels could boost neuroprotection and delay the progression of AD and other NDDs.

Calcium channel signalling at neuronal endoplasmic reticulum-plasma membrane junctions

Neurons are highly specialised cells that need to relay information over long distances and integrate signals from thousands of synaptic inputs. The complexity of neuronal function is evident in the morphology of their plasma membrane (PM), by far the most intricate of all cell types. Yet, within the neuron lies an organelle whose architecture adds another level to this morphological sophistication - the endoplasmic reticulum (ER). Neuronal ER is abundant in the cell body and extends to distant axonal terminals and postsynaptic dendritic spines. It also adopts specialised structures like the spine apparatus in the postsynapse and the cisternal organelle in the axon initial segment. At membrane contact sites (MCSs) between the ER and the PM, the two membranes come in close proximity to create hubs of lipid exchange and Ca2+ signalling called ER-PM junctions. The development of electron and light microscopy techniques extended our knowledge on the physiological relevance of ER-PM MCSs. Equally important was the identification of ER and PM partners that interact in these junctions, most notably the STIM-ORAI and VAP-Kv2.1 pairs. The physiological functions of ER-PM junctions in neurons are being increasingly explored, but their molecular composition and the role in the dynamics of Ca2+ signalling are less clear. This review aims to outline the current state of research on the topic of neuronal ER-PM contacts. Specifically, we will summarise the involvement of different classes of Ca2+ channels in these junctions, discuss their role in neuronal development and neuropathology and propose directions for further research.

L-type Ca2+ channel activation of STIM1-Orai1 signaling remodels the dendritic spine ER to maintain long-term structural plasticity

Learning and memory require coordinated structural and functional plasticity at neuronal glutamatergic synapses located on dendritic spines. Here, we investigated how the endoplasmic reticulum (ER) controls postsynaptic Ca2+ signaling and long-term potentiation of dendritic spine size, i.e., sLTP that accompanies functional strengthening of glutamatergic synaptic transmission. In most ER-containing (ER+) spines, high-frequency optical glutamate uncaging (HFGU) induced long-lasting sLTP that was accompanied by a persistent increase in spine ER content downstream of a signaling cascade engaged by N-methyl-D-aspartate receptors (NMDARs), L-type Ca2+ channels (LTCCs), and Orai1 channels, the latter being activated by stromal interaction molecule 1 (STIM1) in response to ER Ca2+ release. In contrast, HFGU stimulation of ER-lacking (ER-) spines expressed only transient sLTP and exhibited weaker Ca2+ signals noticeably lacking Orai1 and ER contributions. Consistent with spine ER regulating structural metaplasticity, delivery of a second stimulus to ER- spines induced ER recruitment along with persistent sLTP, whereas ER+ spines showed no additional increases in size or ER content in response to sequential stimulation. Surprisingly, the physical interaction between STIM1 and Orai1 induced by ER Ca2+ release, but not the resulting Ca2+ entry through Orai1 channels, proved necessary for the persistent increases in both spine size and ER content required for expression of long-lasting late sLTP.

SOCE as a regulator of neuronal activity

Store operated Ca2+ entry (SOCE) is a ubiquitous signalling module with established roles in the immune system, secretion and muscle development. Recent evidence supports a complex role for SOCE in the nervous system. In this review we present an update of the current knowledge on SOCE function in the brain with a focus on its role as a regulator of brain activity and excitability.

ER-Ca2+ stores and the regulation of store-operated Ca2+ entry in neurons

Key components of endoplasmic reticulum (ER) Ca2+ release and store-operated Ca2+ entry (SOCE) are likely expressed in all metazoan cells. Due to the complexity of canonical Ca2+ entry mechanisms in neurons, the functional significance of ER-Ca2+ release and SOCE has been difficult to identify and establish. In this review we present evidence of how these two related mechanisms of Ca2+ signalling impact multiple aspects of neuronal physiology and discuss their interaction with the better understood classes of ion channels that are gated by either voltage changes or extracellular ligands in neurons. Given how a small imbalance in Ca2+ homeostasis can have strongly detrimental effects on neurons, leading to cell death, it is essential that neuronal SOCE is carefully regulated. We go on to discuss some mechanisms of SOCE regulation that have been identified in Drosophila and mammalian neurons. These include specific splice variants of stromal interaction molecules, different classes of membrane-interacting proteins and an ER-Ca2+ channel. So far these appear distinct from the mechanisms of SOCE regulation identified in non-excitable cells. Finally, we touch upon the significance of these studies in the context of certain human neurodegenerative diseases.

The bone transcription factor Osterix controls extracellular matrix- and node of Ranvier-related gene expression in oligodendrocytes

Oligodendrocytes are the primary producers of many extracellular matrix (ECM)-related proteins found in the CNS. Therefore, oligodendrocytes play a critical role in the determination of brain stiffness, node of Ranvier formation, perinodal ECM deposition, and perineuronal net formation, all of which depend on the ECM. Nevertheless, the transcription factors that control ECM-related gene expression in oligodendrocytes remain unknown. Here, we found that the transcription factor Osterix (also known as Sp7) binds in proximity to genes important for CNS ECM and node of Ranvier formation and mediates their expression. Oligodendrocyte-specific ablation of Sp7 changes ECM composition and brain stiffness and results in aberrant node of Ranvier formation. Sp7 is known to control osteoblast maturation and bone formation. Our comparative analyses suggest that Sp7 plays a conserved biological role in oligodendrocytes and in bone-forming cells, where it mediates brain and bone tissue stiffness by controlling expression of ECM components.

Astrocyte reactivity and inflammation-induced depression-like behaviors are regulated by Orai1 calcium channels

Astrocytes contribute to brain inflammation in neurological disorders but the molecular mechanisms controlling astrocyte reactivity and their relationship to neuroinflammatory endpoints are complex and poorly understood. In this study, we assessed the role of the calcium channel, Orai1, for astrocyte reactivity and inflammation-evoked depression behaviors in mice. Transcriptomics and metabolomics analysis indicated that deletion of Orai1 in astrocytes downregulates genes in inflammation and immunity, metabolism, and cell cycle pathways, and reduces cellular metabolites and ATP production. Systemic inflammation by peripheral lipopolysaccharide (LPS) increases hippocampal inflammatory markers in WT but not in astrocyte Orai1 knockout mice. Loss of Orai1 also blunts inflammation-induced astrocyte Ca2+ signaling and inhibitory neurotransmission in the hippocampus. In line with these cellular changes, Orai1 knockout mice showed amelioration of LPS-evoked depression-like behaviors including anhedonia and helplessness. These findings identify Orai1 as an important signaling hub controlling astrocyte reactivity and astrocyte-mediated brain inflammation that is commonly observed in many neurological disorders.

Neuronal Store-Operated Calcium Channels

The endoplasmic reticulum (ER) is the major intracellular calcium (Ca²⁺) storage compartment in eukaryotic cells. In most instances, the mobilization of Ca²⁺ from this store is followed by a delayed and sustained uptake of Ca²⁺ through Ca²⁺-permeable channels of the cell surface named store-operated Ca²⁺ channels (SOCCs). This gives rise to a store-operated Ca²⁺ entry (SOCE) that has been thoroughly investigated in electrically non-excitable cells where it is the principal regulated Ca²⁺ entry pathway. The existence of this Ca²⁺ route in neurons has long been a matter of debate. However, a growing body of experimental evidence indicates that the recruitment of Ca²⁺ from neuronal ER Ca²⁺ stores generates a SOCE. The present review summarizes the main studies supporting the presence of a depletion-dependent Ca²⁺ entry in neurons. It also addresses the question of the molecular composition of neuronal SOCCs, their expression, pharmacological properties, as well as their physiological relevance.

Swing-out opening of stromal interaction molecule 1

Stromal interaction molecule 1 (STIM1) resides in the endoplasmic reticulum (ER) membrane and senses luminal calcium (Ca2+ ) concentration. STIM1 activation involves a large-scale conformational transition that exposes a STIM1 domain termed "CAD/SOAR", - which is required for activation of the calcium channel Orai. Under resting cell conditions, STIM1 assumes a quiescent state where CAD/SOAR is suspended in an intramolecular clamp formed by the coiled-coil 1 domain (CC1) and CAD/SOAR. Here, we present a structural model of the cytosolic part of the STIM1 resting state using molecular docking simulations that take into account previously reported interaction sites between the CC1α1 and CAD/SOAR domains. We corroborate and refine previously reported interdomain coiled-coil contacts. Based on our model, we provide a detailed analysis of the CC1-CAD/SOAR binding interface using molecular dynamics simulations. We find a very similar binding interface for a proposed domain-swapped configuration of STIM1, where the CAD/SOAR domain of one monomer interacts with the CC1α1 domain of another monomer of STIM1. The rich structural and dynamical information obtained from our simulations reveals novel interaction sites such as M244, I409, or E370, which are crucial for STIM1 quiescent state stability. We tested our predictions by electrophysiological and Förster resonance energy transfer experiments on corresponding single-point mutants. These experiments provide compelling support for the structural model of the STIM1 quiescent state reported here. Based on transitions observed in enhanced-sampling simulations paired with an analysis of the quiescent STIM1 conformational dynamics, our work offers a first atomistic model for CC1α1-CAD/SOAR detachment.

Regulation of neuropathic pain by microglial Orai1 channels

Microglia are important mediators of neuroinflammation, which underlies neuropathic pain. However, the molecular checkpoints controlling microglial reactivity are not well-understood. Here, we investigated the role of Orai1 channels for microglia-mediated neuroinflammation following nerve injury and find that deletion of Orai1 in microglia attenuates Ca2+ signaling and the production of inflammatory cytokines by proalgesic agonists. Conditional deletion of Orai1 attenuated microglial proliferation in the dorsal horn, spinal cytokine levels, and potentiation of excitatory neurotransmission following peripheral nerve injury. These cellular effects were accompanied by mitigation of pain hyperalgesia in microglial Orai1 knockout mice. A small-molecule Orai1 inhibitor, CM4620, similarly mitigated allodynia in male mice. Unexpectedly, these protective effects were not seen in female mice, revealing sexual dimorphism in Orai1 regulation of microglial reactivity and hyperalgesia. Together, these findings indicate that Orai1 channels are key regulators of the sexually dimorphic role of microglia for the neuroinflammation that underlies neuropathic pain.

Mitochondrial calcium cycling in neuronal function and neurodegeneration

Mitochondria are essential for proper cellular function through their critical roles in ATP synthesis, reactive oxygen species production, calcium (Ca2+) buffering, and apoptotic signaling. In neurons, Ca2+ buffering is particularly important as it helps to shape Ca2+ signals and to regulate numerous Ca2+-dependent functions including neuronal excitability, synaptic transmission, gene expression, and neuronal toxicity. Over the past decade, identification of the mitochondrial Ca2+ uniporter (MCU) and other molecular components of mitochondrial Ca2+ transport has provided insight into the roles that mitochondrial Ca2+ regulation plays in neuronal function in health and disease. In this review, we discuss the many roles of mitochondrial Ca2+ uptake and release mechanisms in normal neuronal function and highlight new insights into the Ca2+-dependent mechanisms that drive mitochondrial dysfunction in neurologic diseases including epilepsy, Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. We also consider how targeting Ca2+ uptake and release mechanisms could facilitate the development of novel therapeutic strategies for neurological diseases.

Mapping interactions between the CRAC activation domain and CC1 regulating the activity of the ER Ca2+ sensor STIM1

Stromal interaction molecule 1 (STIM1) is a widely expressed protein that functions as the endoplasmic reticulum (ER) Ca2+ sensor and activator of Orai1 channels. In resting cells with replete Ca2+ stores, an inhibitory clamp formed by the coiled-coil 1 (CC1) domain interacting with the CRAC-activation domain (CAD) of STIM1 helps keep STIM1 in a quiescent state. Following depletion of ER Ca2+ stores, the brake is released, allowing CAD to extend away from the ER membrane and enabling it to activate Orai1 channels. However, the molecular determinants of CC1-CAD interactions that enforce the inhibitory clamp are incompletely understood. Here, we performed Ala mutagenesis in conjunction with live-cell FRET analysis to examine residues in CC1 and CAD that regulate the inhibitory clamp. Our results indicate that in addition to previously identified hotspots in CC1⍺1 and CC3, several hydrophobic residues in CC2 and the apex region of CAD are critical for CC1-CAD interactions. Mutations in these residues loosen the CC1-CAD inhibitory clamp to release CAD from CC1 in cells with replete Ca2+ stores. By contrast, altering the hydrophobic residues L265 and L273 strengthens the clamp to prevent STIM1 activation. Inclusion of the inactivation domain of STIM1 helps stabilize CC1-CAD interaction in several mutants to prevent spontaneous STIM1 activation. In addition, R426C, a human disease-linked mutation in CC3, affects the clamp but also impairs Orai1 binding to inhibit CRAC channel activation. These results identify the CC2, apex, and inactivation domain regions of STIM1 as important determinants of STIM1 activation.

Role of the endoplasmic reticulum in synaptic transmission

Neurons possess a complex morphology spanning long distances and a large number of subcellular specializations such as presynaptic terminals and dendritic spines. This structural complexity is essential for maintenance of synaptic junctions and associated electrical as well as biochemical signaling events. Given the structural and functional complexity of neurons, neuronal endoplasmic reticulum is emerging as a key regulator of neuronal function, in particular synaptic signaling. Neuronal endoplasmic reticulum mediates calcium signaling, calcium and lipid homeostasis, vesicular trafficking, and proteostasis events that underlie autonomous functions of numerous subcellular compartments. However, based on its geometric complexity spanning the whole neuron, endoplasmic reticulum also integrates the activity of these autonomous compartments across the neuron and coordinates their interactions with the soma. In this article, we review recent work regarding neuronal endoplasmic reticulum function and its relationship to neurotransmission and plasticity.

Physiological Functions of CRAC Channels

Store-operated Ca2+ entry (SOCE) is a ubiquitous Ca2+ signaling pathway that is evolutionarily conserved across eukaryotes. SOCE is triggered physiologically when the endoplasmic reticulum (ER) Ca2+ stores are emptied through activation of inositol 1,4,5-trisphosphate receptors. SOCE is mediated by the Ca2+ release-activated Ca2+ (CRAC) channels, which are highly Ca2+ selective. Upon store depletion, the ER Ca2+-sensing STIM proteins aggregate and gain extended conformations spanning the ER-plasma membrane junctional space to bind and activate Orai, the pore-forming proteins of hexameric CRAC channels. In recent years, studies on STIM and Orai tissue-specific knockout mice and gain- and loss-of-function mutations in humans have shed light on the physiological functions of SOCE in various tissues. Here, we describe recent findings on the composition of native CRAC channels and their physiological functions in immune, muscle, secretory, and neuronal systems to draw lessons from transgenic mice and human diseases caused by altered CRAC channel activity.

The calcium-iron connection in ferroptosis-mediated neuronal death

Iron, through its participation in oxidation/reduction processes, is essential for the physiological function of biological systems. In the brain, iron is involved in the development of normal cognitive functions, and its lack during development causes irreversible cognitive damage. Yet, deregulation of iron homeostasis provokes neuronal damage and death. Ferroptosis, a newly described iron-dependent cell death pathway, differs at the morphological, biochemical, and genetic levels from other cell death types. Ferroptosis is characterized by iron-mediated lipid peroxidation, depletion of the endogenous antioxidant glutathione and altered mitochondrial morphology. Although iron promotes the emergence of Ca²⁺ signals via activation of redox-sensitive Ca²⁺ channels, the role of Ca²⁺ signaling in ferroptosis has not been established. The early dysregulation of the cellular redox state observed in ferroptosis is likely to disturb Ca²⁺ homeostasis and signaling, facilitating ferroptotic neuronal death. This review presents an overview of the role of iron and ferroptosis in neuronal function, emphasizing the possible involvement of Ca²⁺ signaling in these processes. We propose, accordingly, that the iron-ferroptosis-Ca²⁺ association orchestrates the progression of cognitive dysfunctions and memory loss that occurs in neurodegenerative diseases. Therefore, to prevent iron dyshomeostasis and ferroptosis, we suggest the use of drugs that target the abnormal Ca²⁺ signaling caused by excessive iron levels as therapy for neurological disorders.

RyR-mediated Ca2+ release elicited by neuronal activity induces nuclear Ca2+ signals, CREB phosphorylation, and Npas4/RyR2 expression

The expression of several hippocampal genes implicated in learning and memory processes requires that Ca²⁺ signals generated in dendritic spines, dendrites, or the soma in response to neuronal stimulation reach the nucleus. The diffusion of Ca²⁺ in the cytoplasm is highly restricted, so neurons must use other mechanisms to propagate Ca²⁺ signals to the nucleus. Here, we present evidence showing that Ca²⁺ release mediated by the ryanodine receptor (RyR) channel type-2 isoform (RyR2) contributes to the generation of nuclear Ca²⁺ signals induced by gabazine (GBZ) addition, glutamate uncaging in the dendrites, or high-frequency field stimulation of primary hippocampal neurons. Additionally, GBZ treatment significantly increased cyclic adenosine monophosphate response element binding protein (CREB) phosphorylation-a key event in synaptic plasticity and hippocampal memory-and enhanced the expression of Neuronal Per Arnt Sim domain protein 4 (Npas4) and RyR2, two central regulators of these processes. Suppression of RyR-mediated Ca²⁺ release with ryanodine significantly reduced the increase in CREB phosphorylation and the enhanced Npas4 and RyR2 expression induced by GBZ. We propose that RyR-mediated Ca²⁺ release induced by neuronal activity, through its contribution to the sequential generation of nuclear Ca²⁺ signals, CREB phosphorylation, Npas4, and RyR2 up-regulation, plays a central role in hippocampal synaptic plasticity and memory processes.

The multifaceted role of SOCE in central synapses

Store-operated calcium entry (SOCE) is a mechanism for calcium influx through the plasma membrane in response to release of free calcium from the endoplasmic reticulum. Two recent studies revealed how SOCE regulates the exocytosis of neurotransmitter vesicles at central synapses.

Presynaptic store-operated Ca2+ entry drives excitatory spontaneous neurotransmission and augments endoplasmic reticulum stress

Store-operated calcium entry (SOCE) is activated by depletion of Ca²⁺ from the endoplasmic reticulum (ER) and mediated by stromal interaction molecule (STIM) proteins. Here, we show that in rat and mouse hippocampal neurons, acute ER Ca²⁺ depletion increases presynaptic Ca²⁺ levels and glutamate release through a pathway dependent on STIM2 and the synaptic Ca²⁺ sensor synaptotagmin-7 (syt7). In contrast, synaptotagmin-1 (syt1) can suppress SOCE-mediated spontaneous release, and STIM2 is required for the increase in spontaneous release seen during syt1 loss of function. We also demonstrate that chronic ER stress activates the same pathway leading to syt7-dependent potentiation of spontaneous glutamate release. During ER stress, inhibition of SOCE or syt7-driven fusion partially restored basal neurotransmission and decreased expression of pro-apoptotic markers, indicating that these processes participate in the amplification of ER-stress-related damage. Taken together, we propose that presynaptic SOCE links ER stress and augmented spontaneous neurotransmission, which may, in turn, facilitate neurodegeneration.

A short isoform of STIM1 confers frequency-dependent synaptic enhancement

Store-operated Ca2+-entry (SOCE) regulates basal and receptor-triggered Ca2+ signaling with STIM proteins sensing the endoplasmic reticulum (ER) Ca2+ content and triggering Ca2+ entry by gating Orai channels. Although crucial for immune cells, STIM1's role in neuronal Ca2+ homeostasis is controversial. Here, we characterize a splice variant, STIM1B, which shows exclusive neuronal expression and protein content surpassing conventional STIM1 in cerebellum and of significant abundance in other brain regions. STIM1B expression results in a truncated protein with slower kinetics of ER-plasma membrane (PM) cluster formation and ICRAC, as well as reduced inactivation. In primary wild-type neurons, STIM1B is targeted by its spliced-in domain B to presynaptic sites where it converts classic synaptic depression into Ca2+- and Orai-dependent short-term synaptic enhancement (STE) at high-frequency stimulation (HFS). In conjunction with altered STIM1 splicing in human Alzheimer disease, our findings highlight STIM1 splicing as an important regulator of neuronal calcium homeostasis and of synaptic plasticity.

Where have all the Orais gone? Commentary on "Orai1 channels are essential for amplification of glutamate-evoked Ca2+ signals in dendritic spines to regulate working and associative memory"

Orai1 channels were reported as critical contributors to the Ca²⁺ signal in hippocampal neurons underlying synaptic plasticity associated with learning and memory. We discuss the results in view of conflicting other reports that stressed the roles of Orai2 channels but failed to detect functions of Orai1 channels in these neurons.