Intracellular Ca2+ signaling and store-operated Ca2+ entry are required in Drosophila neurons for flight. Proc Natl Acad Sci U S A. 2009;106:10326-10331. [PMID: 19515818 DOI: 10.1073/pnas.0902982106]

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.

IP3Rs and nSOCE-Tied Together at Two Ends

All living organisms need to respond appropriately to changes in the extracellular milieu. Cellular mechanisms that enable such responses evolved in parallel with organismal complexity and intracellular Ca2+ signaling is one such mechanism where extracellular signals received at the cell membrane communicate with endoplasmic reticular stores of Ca2+, to stimulate appropriate Ca2+-mediated changes in cellular physiology. The amplitude and dynamics of endoplasmic reticulum (ER)-Ca2+ release in response to extracellular signals determines the nature of the cellular response. An understanding of how ER-Ca2+ channels might regulate cellular Ca2+ signaling in different cell types is lacking. In a recent paper, this question has been addressed in the context of neurons ( Chakraborty et al., 2023) and the implications of these new findings are discussed here.

STIM2 regulates NMDA receptor endocytosis that is induced by short-term NMDA receptor overactivation in cortical neurons

Recent findings suggest an important role for the dysregulation of stromal interaction molecule (STIM) proteins, activators of store-operated Ca2+ channels, and the prolonged activation of N-methyl-D-aspartate receptors (NMDARs) in the development of neurodegenerative diseases. We previously demonstrated that STIM silencing increases Ca2+ influx through NMDAR and STIM-NMDAR2 complexes are present in neurons. However, the interplay between NMDAR subunits (GluN1, GluN2A, and GluN2B) and STIM1/STIM2 with regard to intracellular trafficking remains unknown. Here, we found that the activation of NMDAR endocytosis led to an increase in STIM2-GluN2A and STIM2-GluN2B interactions in primary cortical neurons. STIM1 appeared to migrate from synaptic to extrasynaptic sites. STIM2 silencing inhibited post-activation NMDAR translocation from the plasma membrane and synaptic spines and increased NMDAR currents. Our findings reveal a novel molecular mechanism by which STIM2 regulates NMDAR synaptic trafficking by promoting NMDAR endocytosis after receptor overactivation, which may suggest protection against excessive uncontrolled Ca2+ influx through NMDARs.

Hippocampal transcriptome analysis reveals mechanisms of cognitive impairment in beagle dogs with type 1 diabetes

Diabetic encephalopathy is a common complication of type 1 diabetes. However, there have been few studies on cognitive impairment and hippocampal damage in type 1 diabetes mellitus (T1DM) using dogs as experimental animals. To investigate the effects of diabetes on the CNS, 40 adult beagles were divided into streptozotocin/alloxan type 1 diabetes model and control groups. The duration of diabetes in the model group was 120 days. A cognitive dysfunction scale was used to assess cognitive function. Hematoxylin and eosin and Golgi-Cox staining methods were used to observe morphological damage to the hippocampus. Transcriptomics was used to investigate differential gene expression in the hippocampus. The results showed that the cognitive dysfunction score of the model group was significantly higher than that of the control group. In addition, the number of normal neurons, the complexity of dendritic morphology, and the density of dendritic spines were decreased in the hippocampus of diabetic dogs. A total of 672 differentially expressed genes (DEGs) were identified, 289 of which were upregulated, and 383 were downregulated. Modified genes included DBH, IGFBP2, AVPR1A, and DRAXIN. In conclusion, type 1 diabetic dogs exhibit cognitive dysfunction. The DEGs were mainly enriched in metabolic, PI3K-Akt signaling, and neuroactive ligand-receptor interaction pathways.

Regulation of store-operated Ca2+ entry by IP3 receptors independent of their ability to release Ca2

Loss of endoplasmic reticular (ER) Ca2+ activates store-operated Ca2+ entry (SOCE) by causing the ER localized Ca2+ sensor STIM to unfurl domains that activate Orai channels in the plasma membrane at membrane contact sites (MCS). Here, we demonstrate a novel mechanism by which the inositol 1,4,5 trisphosphate receptor (IP3R), an ER-localized IP3-gated Ca2+ channel, regulates neuronal SOCE. In human neurons, SOCE evoked by pharmacological depletion of ER-Ca2+ is attenuated by loss of IP3Rs, and restored by expression of IP3Rs even when they cannot release Ca2+, but only if the IP3Rs can bind IP3. Imaging studies demonstrate that IP3Rs enhance association of STIM1 with Orai1 in neuronal cells with empty stores; this requires an IP3-binding site, but not a pore. Convergent regulation by IP3Rs, may tune neuronal SOCE to respond selectively to receptors that generate IP3.

A STIM dependent dopamine-neuropeptide axis maintains the larval drive to feed and grow in Drosophila

Appropriate nutritional intake is essential for organismal survival. In holometabolous insects such as Drosophila melanogaster, the quality and quantity of food ingested as larvae determines adult size and fecundity. Here we have identified a subset of dopaminergic neurons (THD') that maintain the larval motivation to feed. Dopamine release from these neurons requires the ER Ca2+ sensor STIM. Larvae with loss of STIM stop feeding and growing, whereas expression of STIM in THD' neurons rescues feeding, growth and viability of STIM null mutants to a significant extent. Moreover STIM is essential for maintaining excitability and release of dopamine from THD' neurons. Optogenetic stimulation of THD' neurons activated neuropeptidergic cells, including median neuro secretory cells that secrete insulin-like peptides. Loss of STIM in THD' cells alters the developmental profile of specific insulin-like peptides including ilp3. Loss of ilp3 partially rescues STIM null mutants and inappropriate expression of ilp3 in larvae affects development and growth. In summary we have identified a novel STIM-dependent function of dopamine neurons that modulates developmental changes in larval feeding behaviour and growth.

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.

Neuronal role of taxi is imperative for flight in Drosophila melanogaster

Extensive studies in Drosophila have led to the elucidation of the roles of many molecular players involved in the sensorimotor coordination of flight. However, the identification and characterisation of new players can add novel perspectives to the process. In this paper, we show that the extant mutant, jumper, is a hypermorphic allele of the taxi/delilah gene, which encodes a transcription factor. The defective flight of jumper flies results from the insertion of an I-element in the 5'-UTR of taxi gene, leading to an over-expression of the taxi. We also show that the molecular lesion responsible for the taxi¹ allele results from a 25 bp deletion leading to a shift in the reading frame at the C-terminus of the taxi coding sequence. Thus, the last 20 residues are replaced by 32 disparate residues in taxi¹. Both taxi¹, a hypomorphic allele, and the CRISPR-Cas9 knock-out (taxi^KO) null allele, show a defective flight phenotype. Electrophysiological studies show taxi hypermorphs, hypomorphs, and knock out flies show abnormal neuronal firing. We further show that neuronal-specific knock-down or over-expression of taxi cause a defect in the brain's inputs to the flight muscles, leading to reduced flight ability. Through transcriptomic analysis of the taxi^KO fly head, we have identified several putative targets of Taxi that may play important roles in flight. In conclusion, from molecularly characterising jumper to establishing Taxi's role during Drosophila flight, our work shows that the forward genetics approach still can lead to the identification of novel molecular players required for neuronal transmission.

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.

STIM and Orai Mediated Regulation of Calcium Signaling in Age-Related Diseases

Tight spatiotemporal regulation of intracellular Ca²⁺ plays a critical role in regulating diverse cellular functions including cell survival, metabolism, and transcription. As a result, eukaryotic cells have developed a wide variety of mechanisms for controlling Ca²⁺ influx and efflux across the plasma membrane as well as Ca²⁺ release and uptake from intracellular stores. The STIM and Orai protein families comprising of STIM1, STIM2, Orai1, Orai2, and Orai3, are evolutionarily highly conserved proteins that are core components of all mammalian Ca²⁺ signaling systems. STIM1 and Orai1 are considered key players in the regulation of Store Operated Calcium Entry (SOCE), where release of Ca²⁺ from intracellular stores such as the Endoplasmic/Sarcoplasmic reticulum (ER/SR) triggers Ca²⁺ influx across the plasma membrane. SOCE, which has been widely characterized in non-excitable cells, plays a central role in Ca²⁺-dependent transcriptional regulation. In addition to their role in Ca²⁺ signaling, STIM1 and Orai1 have been shown to contribute to the regulation of metabolism and mitochondrial function. STIM and Orai proteins are also subject to redox modifications, which influence their activities. Considering their ubiquitous expression, there has been increasing interest in the roles of STIM and Orai proteins in excitable cells such as neurons and myocytes. While controversy remains as to the importance of SOCE in excitable cells, STIM1 and Orai1 are essential for cellular homeostasis and their disruption is linked to various diseases associated with aging such as cardiovascular disease and neurodegeneration. The recent identification of splice variants for most STIM and Orai isoforms while complicating our understanding of their function, may also provide insight into some of the current contradictions on their roles. Therefore, the goal of this review is to describe our current understanding of the molecular regulation of STIM and Orai proteins and their roles in normal physiology and diseases of aging, with a particular focus on heart disease and neurodegeneration.

Analysis of Drosophila cardiac hypertrophy by microcomputerized tomography for genetic dissection of heart growth mechanisms

Heart failure is often preceded by pathological cardiac hypertrophy, a thickening of the heart musculature driven by complex gene regulatory and signaling processes. The Drosophila heart has great potential as a genetic model for deciphering the underlying mechanisms of cardiac hypertrophy. However, current methods for evaluating hypertrophy of the Drosophila heart are laborious and difficult to carry out reproducibly. Here, we demonstrate that microcomputerized tomography (microCT) is an accessible, highly reproducible method for nondestructive, quantitative analysis of Drosophila heart morphology and size. To validate our microCT approach for analyzing Drosophila cardiac hypertrophy, we show that expression of constitutively active Ras (Ras85D^V12), previously shown to cause hypertrophy of the fly heart, results in significant thickening of both adult and larval heart walls when measured from microCT images. We then show using microCT analysis that genetic upregulation of store-operated Ca²⁺ entry (SOCE) driven by expression of constitutively active Stim (Stim^CA) or Orai (Orai^CA) proteins also results in significant hypertrophy of the Drosophila heart, through a process that specifically depends on Orai Ca²⁺ influx channels. Intravital imaging of heart contractility revealed significantly reduced end-diastolic and end-systolic dimensions in Stim^CA- and Orai^CA-expressing hearts, consistent with the hypertrophic phenotype. These results demonstrate that increased SOCE activity is an important driver of hypertrophic cardiomyocyte growth, and demonstrate how microCT analysis combined with tractable genetic tools in Drosophila can be used to delineate molecular signaling processes that underlie cardiac hypertrophy and heart failure.NEW & NOTEWORTHY Genetic analysis of Drosophila cardiac hypertrophy holds immense potential for the discovery of new therapeutic targets to prevent and treat heart failure. This potential has been hindered by a lack of rapid and effective methods for analyzing heart size in flies. Here, we demonstrate that analysis of the Drosophila heart with microcomputerized tomography yields accurate and highly reproducible heart size measurements that can be used to analyze heart growth and cardiac hypertrophy in Drosophila.

Lighting Up Ca2+ Dynamics in Animal Models

Calcium (Ca2+) signaling coordinates are crucial processes in brain physiology. Particularly, fundamental aspects of neuronal function such as synaptic transmission and neuronal plasticity are regulated by Ca2+, and neuronal survival itself relies on Ca2+-dependent cascades. Indeed, impaired Ca2+ homeostasis has been reported in aging as well as in the onset and progression of neurodegeneration. Understanding the physiology of brain function and the key processes leading to its derangement is a core challenge for neuroscience. In this context, Ca2+ imaging represents a powerful tool, effectively fostered by the continuous amelioration of Ca2+ sensors in parallel with the improvement of imaging instrumentation. In this review, we explore the potentiality of the most used animal models employed for Ca2+ imaging, highlighting their application in brain research to explore the pathogenesis of neurodegenerative diseases.

A Comparative Perspective on Functionally-Related, Intracellular Calcium Channels: The Insect Ryanodine and Inositol 1,4,5-Trisphosphate Receptors

Calcium (Ca²⁺) homeostasis is vital for insect development and metabolism, and the endoplasmic reticulum (ER) is a major intracellular reservoir for Ca²⁺. The inositol 1,4,5- triphosphate receptor (IP₃R) and ryanodine receptor (RyR) are large homotetrameric channels associated with the ER and serve as two major actors in ER-derived Ca²⁺ supply. Most of the knowledge on these receptors derives from mammalian systems that possess three genes for each receptor. These studies have inspired work on synonymous receptors in insects, which encode a single IP₃R and RyR. In the current review, we focus on a fundamental, common question: “why do insect cells possess two Ca²⁺ channel receptors in the ER?”. Through a comparative approach, this review covers the discovery of RyRs and IP₃Rs, examines their structures/functions, the pathways that they interact with, and their potential as target sites in pest control. Although insects RyRs and IP₃Rs share structural similarities, they are phylogenetically distinct, have their own structural organization, regulatory mechanisms, and expression patterns, which explains their functional distinction. Nevertheless, both have great potential as target sites in pest control, with RyRs currently being targeted by commercial insecticide, the diamides.

Purkinje Neurons with Loss of STIM1 Exhibit Age-Dependent Changes in Gene Expression and Synaptic Components

The stromal interaction molecule 1 (STIM1) is an ER-Ca²⁺ sensor and an essential component of ER-Ca²⁺ store operated Ca²⁺ entry. Loss of STIM1 affects metabotropic glutamate receptor 1 (mGluR1)-mediated synaptic transmission, neuronal Ca²⁺ homeostasis, and intrinsic plasticity in Purkinje neurons (PNs). Long-term changes of intracellular Ca²⁺ signaling in PNs led to neurodegenerative conditions, as evident in individuals with mutations of the ER-Ca²⁺ channel, the inositol 1,4,5-triphosphate receptor. Here, we asked whether changes in such intrinsic neuronal properties, because of loss of STIM1, have an age-dependent impact on PNs. Consequently, we analyzed mRNA expression profiles and cerebellar morphology in PN-specific STIM1 KO mice (STIM1^PKO ) of both sexes across ages. Our study identified a requirement for STIM1-mediated Ca²⁺ signaling in maintaining the expression of genes belonging to key biological networks of synaptic function and neurite development among others. Gene expression changes correlated with altered patterns of dendritic morphology and greater innervation of PN dendrites by climbing fibers, in aging STIM1^PKO mice. Together, our data identify STIM1 as an important regulator of Ca²⁺ homeostasis and neuronal excitability in turn required for maintaining the optimal transcriptional profile of PNs with age. Our findings are significant in the context of understanding how dysregulated calcium signals impact cellular mechanisms in multiple neurodegenerative disorders.SIGNIFICANCE STATEMENT In Purkinje neurons (PNs), the stromal interaction molecule 1 (STIM1) is required for mGluR1-dependent synaptic transmission, refilling of ER Ca²⁺ stores, regulation of spike frequency, and cerebellar memory consolidation. Here, we provide evidence for a novel role of STIM1 in maintaining the gene expression profile and optimal synaptic connectivity of PNs. Expression of genes related to neurite development and synaptic organization networks is altered in PNs with persistent loss of STIM1. In agreement with these findings the dendritic morphology of PNs and climbing fiber innervations on PNs also undergo significant changes with age. These findings identify a new role for dysregulated intracellular calcium signaling in neurodegenerative disorders and provide novel therapeutic insights.

STIM1/ORAI1 Loss-of-Function and Gain-of-Function Mutations Inversely Impact on SOCE and Calcium Homeostasis and Cause Multi-Systemic Mirror Diseases

Store-operated Ca²⁺ entry (SOCE) is a ubiquitous and essential mechanism regulating Ca²⁺ homeostasis in all tissues, and controls a wide range of cellular functions including keratinocyte differentiation, osteoblastogenesis and osteoclastogenesis, T cell proliferation, platelet activation, and muscle contraction. The main SOCE actors are STIM1 and ORAI1. Depletion of the reticular Ca²⁺ stores induces oligomerization of the luminal Ca²⁺ sensor STIM1, and the oligomers activate the plasma membrane Ca²⁺ channel ORAI1 to trigger extracellular Ca²⁺ entry. Mutations in STIM1 and ORAI1 result in abnormal SOCE and lead to multi-systemic disorders. Recessive loss-of-function mutations are associated with CRAC (Ca²⁺ release-activated Ca²⁺) channelopathy, involving immunodeficiency and autoimmunity, muscular hypotonia, ectodermal dysplasia, and mydriasis. In contrast, dominant STIM1 and ORAI1 gain-of-function mutations give rise to tubular aggregate myopathy and Stormorken syndrome (TAM/STRMK), forming a clinical spectrum encompassing muscle weakness, thrombocytopenia, ichthyosis, hyposplenism, short stature, and miosis. Functional studies on patient-derived cells revealed that CRAC channelopathy mutations impair SOCE and extracellular Ca²⁺ influx, while TAM/STRMK mutations induce excessive Ca²⁺ entry through SOCE over-activation. In accordance with the opposite pathomechanisms underlying both disorders, CRAC channelopathy and TAM/STRMK patients show mirror phenotypes at the clinical and molecular levels, and the respective animal models recapitulate the skin, bones, immune system, platelet, and muscle anomalies. Here we review and compare the clinical presentations of CRAC channelopathy and TAM/STRMK patients and the histological and molecular findings obtained on human samples and murine models to highlight the mirror phenotypes in different tissues, and to point out potentially undiagnosed anomalies in patients, which may be relevant for disease management and prospective therapeutic approaches.

A journey into the world of insect lipid metabolism

Lipid metabolism is fundamental to life. In insects, it is critical, during reproduction, flight, starvation, and diapause. The coordination center for insect lipid metabolism is the fat body, which is analogous to the vertebrate adipose tissue and liver. Fat body contains various different cell types; however, adipocytes and oenocytes are the primary cells related to lipid metabolism. Lipid metabolism starts with the hydrolysis of dietary lipids, absorption of lipid monomers, followed by lipid transport from midgut to the fat body, lipogenesis or lipolysis in the fat body, and lipid transport from fat body to other sites demanding energy. Lipid metabolism is under the control of hormones, transcription factors, secondary messengers and posttranscriptional modifications. Primarily, lipogenesis is under the control of insulin-like peptides that activate lipogenic transcription factors, such as sterol regulatory element-binding proteins, whereas lipolysis is coordinated by the adipokinetic hormone that activates lipolytic transcription factors, such as forkhead box class O and cAMP-response element-binding protein. Calcium is the primary-secondary messenger affecting lipid metabolism and has different outcomes depending on the site of lipogenesis or lipolysis. Phosphorylation is central to lipid metabolism and multiple phosphorylases are involved in lipid accumulation or hydrolysis. Although most of the knowledge of insect lipid metabolism comes from the studies on the model Drosophila; other insects, in particular those with obligatory or facultative diapause, also have great potential to study lipid metabolism. The use of these models would significantly improve our knowledge of insect lipid metabolism.

Reduced SERCA Function Preferentially Affects Wnt Signaling by Retaining E-Cadherin in the Endoplasmic Reticulum

Calcium homeostasis in the lumen of the endoplasmic reticulum is required for correct processing and trafficking of transmembrane proteins, and defects in protein trafficking can impinge on cell signaling pathways. We show here that mutations in the endoplasmic reticulum calcium pump SERCA disrupt Wingless signaling by sequestering Armadillo/β-catenin away from the signaling pool. Armadillo remains bound to E-cadherin, which is retained in the endoplasmic reticulum when calcium levels there are reduced. Using hypomorphic and null SERCA alleles in combination with the loss of the plasma membrane calcium channel Orai allowed us to define three distinct thresholds of endoplasmic reticulum calcium. Wingless signaling is sensitive to even a small reduction, while Notch and Hippo signaling are disrupted at intermediate levels, and elimination of SERCA function results in apoptosis. These differential and opposing effects on three oncogenic signaling pathways may complicate the use of SERCA inhibitors as cancer therapeutics.

Suisse and Treisman describe genetic conditions that reduce calcium in the endoplasmic reticulum to three distinct extents. They find that Wnt signaling is more sensitive to changes in calcium levels than the Notch and Hippo pathways, potentially complicating the use of calcium pump inhibitors as cancer therapeutics.

Liraglutide suppresses the metastasis of PANC-1 co-cultured with pancreatic stellate cells through modulating intracellular calcium content

In this study, we aim to explore the anti-tumor effect of liraglutide (Lira), an anti-diabetic medicine, on pancreatic cancer cell PANC-1 co-cultured with or without pancreatic stellate cells (PSCs). The chemical count kit-8 and Annexin V-FITC apoptosis detection were conducted to investigate the effect of Lira on cell viability and proliferation of PANC-1 with or without PSCs co-culture. Then, the wound healing and transwell experiments were performed to explore the influence of Lira on PANC-1 cells' migration and invasion capabilities. To identify the potential action mechanism of Lira on PANC-1, the expression of E-cadherin and N-cadherin and the intracellular calcium content in PANC-1, after Lira administration, were detected. The results indicated that Lira in 100 and 1,000 nmol/L, effectively decreased the cell viability and dose-dependently promoted cell apoptosis of PANC-1 co-cultured with or without PSCs. Lira significantly reduced the migration and invasion of PANC-1 and also reduced the inducing effect of PSCs to PANC-1. Lira effectively induced the expression of E-cadherin and suppressed the expression of N-cadherin with a dose-dependent manner. Otherwise, Lira significantly reduced the abnormal high content of calcium in PANC-1 and also weakened the elevation of calcium in PANC-1 induced by cell-cell interaction. The current study firstly indicated that Lira suppressed the cell proliferation, migration and invasion of PANC-1 with or without PSCs co-culture. This effect was partially due to the calcium modulation of Lira and its influence on Ca²⁺-binding proteins, such as E-cadherin and N-cadherin.

Microtubules Stabilization by Mutant Spastin Affects ER Morphology and Ca2+ Handling

The endoplasmic reticulum (ER) extends as a network of interconnected tubules and sheet-like structures in eukaryotic cells. ER tubules dynamically change their morphology and position within the cells in response to physiological stimuli and these network rearrangements depend on the microtubule (MT) cytoskeleton. Store-operated calcium entry (SOCE) relies on the repositioning of ER tubules to form specific ER-plasma membrane junctions. Indeed, the tips of polymerizing MTs are supposed to provide the anchor for ER tubules to move toward the plasma membrane, however the precise role of the cytoskeleton during SOCE has not been conclusively clarified. Here we exploit an in vivo approach involving the manipulation of MT dynamics in Drosophila melanogaster by neuronal expression of a dominant-negative variant of the MT-severing protein spastin to induce MT hyper-stabilization. We show that MT stabilization alters ER morphology, favoring an enrichment in ER sheets at the expense of tubules. Stabilizing MTs has a negative impact on the process of SOCE and results in a reduced ER Ca²⁺ content, affecting the flight ability of the flies. Restoring proper MT organization by administering the MT-destabilizing drug vinblastine, chronically or acutely, rescues ER morphology, SOCE and flight ability, indicating that MT dynamics impairment is responsible for all the phenotypes observed.

Genetic analyses uncover pleiotropic compensatory roles for Drosophila Nucleobindin-1 in inositol trisphosphate-mediated intracellular calcium homeostasis

Nucleobindin-1 is an EF-hand calcium-binding protein with a distinctive profile, predominantly localized to the Golgi in insect and wide-ranging vertebrate cell types, alike. Its putative involvements in intracellular calcium (Ca²⁺) homeostasis have never been phenotypically characterized in any model organism. We have analyzed an adult-viable mutant that completely disrupts the G protein α-subunit binding and activating (GBA) motif of Drosophila Nucleobindin-1 (dmNUCB1). Such disruption does not manifest any obvious fitness-related, morphological/developmental, or behavioral abnormalities. A single copy of this mutation or the knockdown of dmnucb1 in restricted sets of cells variously rescues pleiotropic mutant phenotypes arising from impaired inositol 1,4,5-trisphosphate receptor (IP3R) activity (in turn depleting cytoplasmic Ca²⁺ levels across diverse tissue types). Additionally, altered dmNUCB1 expression or function considerably reverses lifespan and mobility improvements effected by IP3R mutants, in a Drosophila model of amyotrophic lateral sclerosis. Homology modeling-based analyses further predict a high degree of conformational conservation in Drosophila, of biochemically validated structural determinants in the GBA motif that specify in vertebrates, the unconventional Ca²⁺-regulated interaction of NUCB1 with Gαi subunits. The broad implications of our findings are hypothetically discussed, regarding potential roles for NUCB1 in GBA-mediated, Golgi-associated Ca²⁺ signaling, in health and disease.

ER-Ca2+ sensor STIM regulates neuropeptides required for development under nutrient restriction in Drosophila

Neuroendocrine cells communicate via neuropeptides to regulate behaviour and physiology. This study examines how STIM (Stromal Interacting Molecule), an ER-Ca²⁺ sensor required for Store-operated Ca²⁺ entry, regulates neuropeptides required for Drosophila development under nutrient restriction (NR). We find two STIM-regulated peptides, Corazonin and short Neuropeptide F, to be required for NR larvae to complete development. Further, a set of secretory DLP (Dorso lateral peptidergic) neurons which co-express both peptides was identified. Partial loss of dSTIM caused peptide accumulation in the DLPs, and reduced systemic Corazonin signalling. Upon NR, larval development correlated with increased peptide levels in the DLPs, which failed to occur when dSTIM was reduced. Comparison of systemic and cellular phenotypes associated with reduced dSTIM, with other cellular perturbations, along with genetic rescue experiments, suggested that dSTIM primarily compromises neuroendocrine function by interfering with neuropeptide release. Under chronic stimulation, dSTIM also appears to regulate neuropeptide synthesis.

STIM Proteins and Glutamate Receptors in Neurons: Role in Neuronal Physiology and Neurodegenerative Diseases

Neuronal calcium (Ca²⁺) influx has long been ascribed mainly to voltage-gated Ca²⁺ channels and glutamate receptor channels. Recent research has shown that it is also complemented by stromal interaction molecule (STIM) protein-mediated store-operated Ca²⁺ entry (SOCE). SOCE is described as Ca²⁺ flow into cells in response to the depletion of endoplasmic reticulum Ca²⁺ stores. The present review summarizes recent studies that indicate a relationship between neuronal SOCE that is mediated by STIM1 and STIM2 proteins and glutamate receptors under both physiological and pathological conditions, such as neurodegenerative disorders. We present evidence that the dysregulation of neuronal SOCE and glutamate receptor activity are hallmarks of acute neurodegenerative diseases (e.g., traumatic brain injury and cerebral ischemia) and chronic neurodegenerative diseases (e.g., Alzheimer's disease and Huntington's disease). Emerging evidence indicates a role for STIM proteins and glutamate receptors in neuronal physiology and pathology, making them potential therapeutic targets.

The Genetics of Mating Song Evolution Underlying Rapid Speciation: Linking Quantitative Variation to Candidate Genes for Behavioral Isolation

Differences in mating behaviors evolve early during speciation, eventually contributing to reproductive barriers between species. Knowledge of the genetic and genomic basis of these behaviors is therefore integral to a causal understanding of speciation. Acoustic behaviors are often part of the mating ritual in animal species. The temporal rhythms of mating songs are notably species-specific in many vertebrates and arthropods and often underlie assortative mating. Despite discoveries of mutations that disrupt the temporal rhythm of these songs, we know surprisingly little about genes affecting naturally occurring variation in the temporal pattern of singing behavior. In the rapidly speciating Hawaiian cricket genus Laupala, the striking species variation in song rhythms constitutes a behavioral barrier to reproduction between species. Here, we mapped the largest-effect locus underlying interspecific variation in song rhythm between two Laupala species to a narrow genomic region, wherein we find no known candidate genes affecting song temporal rhythm in Drosophila Whole-genome sequencing, gene prediction, and functional annotation of this region reveal an exciting and promising candidate gene, the putative cyclic nucleotide-gated ion channel-like gene, for natural variation in mating behavior. Identification and molecular characterization of the candidate gene reveals a nonsynonymous mutation in a conserved binding domain, suggesting that ion channels are important targets of selection on rhythmic signaling during establishment of behavioral isolation and rapid speciation.

Measurement of Store-Operated Calcium Entry in Human Neural Cells: From Precursors to Differentiated Neurons

Calcium imaging in an ex-vivo setup is used to understand the calcium status of isolated cells or tissue. In this chapter we explain the use of the ratiometric chemical indicator Fura-2 which can be loaded into isolated cells in the form of lipophilic acetomethyl (AM) esters. Fura-2 is a combination of calcium chelator and fluorophore, and can be used with dual wavelength excitation (340/380 nm) for quantitative calcium concentrations. The cells can then be viewed using a fluorescence microscope and captured by a CCD camera. We specifically discuss the technique involved in understanding the endoplasmic reticulum (ER)-driven store-operated calcium entry (SOCE) in human neural precursors (NPCs) and spontaneously differentiated neurons derived from a pluripotent human embryonic stem cell (hESC) line. The derivation of neural precursors from stem cells and their subsequent spontaneous neural differentiation is also explained. The method can be used for various non-excitable and excitable cell types including neurons, be it freshly isolated, from frozen vials, or derived from different stem cell lines.

A Multicomponent Neuronal Response Encodes the Larval Decision to Pupariate upon Amino Acid Starvation

Organisms need to coordinate growth with development, particularly in the context of nutrient availability. Thus, multiple ways have evolved to survive extrinsic nutrient deprivation during development. In Drosophila, growth occurs during larval development. Larvae are thus critically dependent on nutritional inputs; but after critical weight, they pupariate even when starved. How nutrient availability is coupled to the internal metabolic state for the decision to pupariate needs better understanding. We had earlier identified glutamatergic interneurons in the ventral ganglion that regulate pupariation on a protein-deficient diet. Here we report that Drosophila third instar larvae (either sex) sense arginine to evaluate their nutrient environment using an amino acid transporter Slimfast. The glutamatergic interneurons integrate external protein availability with internal metabolic state through neuropeptide signals. IP3-mediated calcium release and store-operated calcium entry are essential in these glutamatergic neurons for such integration and alter neuronal function by reducing the expression of multiple ion channels.SIGNIFICANCE STATEMENT Coordinating growth with development, in the context of nutrient availability is a challenge for all organisms in nature. After attainment of "critical weight," insect larvae can pupariate, even in the absence of nutrition. Mechanism(s) that stimulate appropriate cellular responses and allow normal development on a nutritionally deficient diet remain to be understood. Here, we demonstrate that nutritional deprivation, in postcritical weight larvae, is sensed by special sensory neurons through an amino acid transporter that detects loss of environmental arginine. This information is integrated by glutamatergic interneurons with the internal metabolic state through neuropeptide signals. These glutamatergic interneurons require calcium-signaling-regulated expression of a host of neuronal channels to generate complex calcium signals essential for pupariation on a protein-deficient diet.

Role of STIM1 in neurodegeneration

STIM1 is an endoplasmic reticulum (ER) protein with a key role in Ca2+ mobilization. Due to its ability to act as an ER-intraluminal Ca2+ sensor, it regulates store-operated Ca2+ entry (SOCE), which is a Ca2+ influx pathway involved in a wide variety of signalling pathways in eukaryotic cells. Despite its important role in Ca2+ transport, current knowledge about the role of STIM1 in neurons is much more limited. Growing evidence supports a role for STIM1 and SOCE in the preservation of dendritic spines required for long-term potentiation and the formation of memory. In this regard, recent studies have demonstrated that the loss of STIM1, which impairs Ca2+ mobilization in neurons, risks cell viability and could be the cause of neurodegenerative diseases. The role of STIM1 in neurodegeneration and the molecular basis of cell death triggered by low levels of STIM1 are discussed in this review.

IP3 receptors and Ca2+ entry

Inositol 1,4,5-trisphosphate receptors (IP₃R) are the most widely expressed intracellular Ca²⁺ release channels. Their activation by IP₃ and Ca²⁺ allows Ca²⁺ to pass rapidly from the ER lumen to the cytosol. The resulting increase in cytosolic [Ca²⁺] may directly regulate cytosolic effectors or fuel Ca²⁺ uptake by other organelles, while the decrease in ER luminal [Ca²⁺] stimulates store-operated Ca²⁺ entry (SOCE). We are close to understanding the structural basis of both IP₃R activation, and the interactions between the ER Ca²⁺-sensor, STIM, and the plasma membrane Ca²⁺ channel, Orai, that lead to SOCE. IP₃Rs are the usual means through which extracellular stimuli, through ER Ca²⁺ release, stimulate SOCE. Here, we review evidence that the IP₃Rs most likely to respond to IP₃ are optimally placed to allow regulation of SOCE. We also consider evidence that IP₃Rs may regulate SOCE downstream of their ability to deplete ER Ca²⁺ stores. Finally, we review evidence that IP₃Rs in the plasma membrane can also directly mediate Ca²⁺ entry in some cells.

Huntingtin-Associated Protein 1A Regulates Store-Operated Calcium Entry in Medium Spiny Neurons From Transgenic YAC128 Mice, a Model of Huntington's Disease

Huntington's disease (HD) is a hereditary neurodegenerative disease that is caused by polyglutamine expansion within the huntingtin (HTT) gene. One of the cellular activities that is dysregulated in HD is store-operated calcium entry (SOCE), a process by which Ca²⁺ release from the endoplasmic reticulum (ER) induces Ca²⁺ influx from the extracellular space. HTT-associated protein-1 (HAP1) is a binding partner of HTT. The aim of the present study was to examine the role of HAP1A protein in regulating SOCE in YAC128 mice, a transgenic model of HD. After Ca²⁺ depletion from the ER by the activation of inositol-(1,4,5)triphosphate receptor type 1 (IP₃R1), we detected an increase in the activity of SOC channels when HAP1 protein isoform HAP1A was overexpressed in medium spiny neurons (MSNs) from YAC128 mice. A decrease in the activity of SOC channels in YAC128 MSNs was observed when HAP1 protein was silenced. In YAC128 MSNs that overexpressed HAP1A, an increase in activity of IP₃R1 was detected while the ionomycin-sensitive ER Ca²⁺ pool decreased. 6-Bromo-N-(2-phenylethyl)-2,3,4,9-tetrahydro-1H-carbazol-1-amine hydrochloride (C₂₀H₂₂BrClN₂), identified in our previous studies as a SOCE inhibitor, restored the elevation of SOCE in YAC128 MSN cultures that overexpressed HAP1A. The IP₃ sponge also restored the elevation of SOCE and increased the release of Ca²⁺ from the ER in YAC128 MSN cultures that overexpressed HAP1A. The overexpression of HAP1A in the human neuroblastoma cell line SK-N-SH (i.e., a cellular model of HD (SK-N-SH HTT138Q)) led to the appearance of a pool of constitutively active SOC channels and an increase in the expression of STIM2 protein. Our results showed that HAP1A causes the activation of SOC channels in HD models by affecting IP₃R1 activity.

SEPT7-mediated regulation of Ca2+ entry through Orai channels requires other septin subunits

Orai channels are plasma membrane resident Ca²⁺ channels that allow extracellular Ca²⁺ uptake after depletion of ER-Ca²⁺ stores by a process called store-operated Ca²⁺ entry (SOCE). Septins of the SEPT2 subgroup act as positive regulators of SOCE in human nonexcitable cells. SEPT2 subgroup septins form the central core of hetero-hexameric or hetero-octameric complexes with SEPT6, SEPT7 and SEPT9 subgroup septins. The presence of fewer septin encoding genes coupled with ease of genetic manipulation allows for better understanding of septin subgroup function in Drosophila. Our earlier findings show that although dSEPT7 reduction does not alter Orai-mediated Ca²⁺ entry during SOCE, it results in constitutive activation of Orai channels in resting neurons. Here, we have investigated the role of other septin subgroup members in regulating Orai channel activation in Drosophila neurons by both cellular and functional assays. We show that dSEPT1, a SEPT2 subgroup septin can exist in a complex with dSEPT2 and dSEPT7 in the central nervous system (CNS) of Drosophila. Our findings suggest that the nature of septin filaments and heteromers obtained after reducing septins of different subgroups alters their ability to regulate Orai channel opening. The molecular mechanisms underlying this complex regulation of Orai function by septins require further cellular investigations.

STIM1 deficiency is linked to Alzheimer's disease and triggers cell death in SH-SY5Y cells by upregulation of L-type voltage-operated Ca2+ entry

Abstract

STIM1 is an endoplasmic reticulum protein with a role in Ca²⁺ mobilization and signaling. As a sensor of intraluminal Ca²⁺ levels, STIM1 modulates plasma membrane Ca²⁺ channels to regulate Ca²⁺ entry. In neuroblastoma SH-SY5Y cells and in familial Alzheimer’s disease patient skin fibroblasts, STIM1 is cleaved at the transmembrane domain by the presenilin-1-associated γ-secretase, leading to dysregulation of Ca²⁺ homeostasis. In this report, we investigated expression levels of STIM1 in brain tissues (medium frontal gyrus) of pathologically confirmed Alzheimer’s disease patients, and observed that STIM1 protein expression level decreased with the progression of neurodegeneration. To study the role of STIM1 in neurodegeneration, a strategy was designed to knock-out the expression of STIM1 gene in the SH-SY5Y neuroblastoma cell line by CRISPR/Cas9-mediated genome editing, as an in vitro model to examine the phenotype of STIM1-deficient neuronal cells. It was proved that, while STIM1 is not required for the differentiation of SH-SY5Y cells, it is absolutely essential for cell survival in differentiating cells. Differentiated STIM1-KO cells showed a significant decrease of mitochondrial respiratory chain complex I activity, mitochondrial inner membrane depolarization, reduced mitochondrial free Ca²⁺ concentration, and higher levels of senescence as compared with wild-type cells. In parallel, STIM1-KO cells showed a potentiated Ca²⁺ entry in response to depolarization, which was sensitive to nifedipine, pointing to L-type voltage-operated Ca²⁺ channels as mediators of the upregulated Ca²⁺ entry. The stable knocking-down of CACNA1C transcripts restored mitochondrial function, increased mitochondrial Ca²⁺ levels, and dropped senescence to basal levels, demonstrating the essential role of the upregulation of voltage-operated Ca²⁺ entry through Ca_v1.2 channels in STIM1-deficient SH-SY5Y cell death.

Key messages

STIM1 protein expression decreases with the progression of neurodegeneration in Alzheimer’s disease.

STIM1 is essential for cell viability in differentiated SH-SY5Y cells.

STIM1 deficiency triggers voltage-regulated Ca²⁺ entry-dependent cell death.

Mitochondrial dysfunction and senescence are features of STIM1-deficient differentiated cells.

Electronic supplementary material

The online version of this article (10.1007/s00109-018-1677-y) contains supplementary material, which is available to authorized users.

FMRFa receptor stimulated Ca2+ signals alter the activity of flight modulating central dopaminergic neurons in Drosophila melanogaster

Neuropeptide signaling influences animal behavior by modulating neuronal activity and thus altering circuit dynamics. Insect flight is a key innate behavior that very likely requires robust neuromodulation. Cellular and molecular components that help modulate flight behavior are therefore of interest and require investigation. In a genetic RNAi screen for G-protein coupled receptors that regulate flight bout durations, we earlier identified several receptors, including the receptor for the neuropeptide FMRFa (FMRFaR). To further investigate modulation of insect flight by FMRFa we generated CRISPR-Cas9 mutants in the gene encoding the Drosophila FMRFaR. The mutants exhibit significant flight deficits with a focus in dopaminergic cells. Expression of a receptor specific RNAi in adult central dopaminergic neurons resulted in progressive loss of sustained flight. Further, genetic and cellular assays demonstrated that FMRFaR stimulates intracellular calcium signaling through the IP3R and helps maintain neuronal excitability in a subset of dopaminergic neurons for positive modulation of flight bout durations.

dSTIM- and Ral/Exocyst-Mediated Synaptic Release from Pupal Dopaminergic Neurons Sustains Drosophila Flight

Manifestation of appropriate behavior in adult animals requires developmental mechanisms that help in the formation of correctly wired neural circuits. Flight circuit development in Drosophila requires store-operated calcium entry (SOCE) through the STIM/Orai pathway. SOCE-associated flight deficits in adult Drosophila derive extensively from regulation of gene expression in pupal neurons, and one such SOCE-regulated gene encodes the small GTPase Ral. The cellular mechanism by which Ral helps in maturation of the flight circuit was not understood. Here, we show that knockdown of components of a Ral effector, the exocyst complex, in pupal neurons also leads to reduced flight bout durations, and this phenotype derives primarily from dopaminergic neurons. Importantly, synaptic release from pupal dopaminergic neurons is abrogated upon knockdown of dSTIM, Ral, or exocyst components. Ral overexpression restores the diminished synaptic release of dStim knockdown neurons as well as flight deficits associated with dSTIM knockdown in dopaminergic neurons. These results identify Ral-mediated vesicular release as an effector mechanism of neuronal SOCE in pupal dopaminergic neurons with functional consequences on flight behavior.

Stable STIM1 Knockdown in Self-Renewing Human Neural Precursors Promotes Premature Neural Differentiation

Ca²⁺ signaling plays a significant role in the development of the vertebrate nervous system where it regulates neurite growth as well as synapse and neurotransmitter specification. Elucidating the role of Ca²⁺ signaling in mammalian neuronal development has been largely restricted to either small animal models or primary cultures. Here we derived human neural precursor cells (NPCs) from human embryonic stem cells to understand the functional significance of a less understood arm of calcium signaling, Store-operated Ca²⁺ entry or SOCE, in neuronal development. Human NPCs exhibited robust SOCE, which was significantly attenuated by expression of a stable shRNA-miR targeted toward the SOCE molecule, STIM1. Along with the plasma membrane channel Orai, STIM is an essential component of SOCE in many cell types, where it regulates gene expression. Therefore, we measured global gene expression in human NPCs with and without STIM1 knockdown. Interestingly, pathways down-regulated through STIM1 knockdown were related to cell proliferation and DNA replication processes, whereas post-synaptic signaling was identified as an up-regulated process. To understand the functional significance of these gene expression changes we measured the self-renewal capacity of NPCs with STIM1 knockdown. The STIM1 knockdown NPCs demonstrated significantly reduced neurosphere size and number as well as precocious spontaneous differentiation toward the neuronal lineage, as compared to control cells. These findings demonstrate that STIM1 mediated SOCE in human NPCs regulates gene expression changes, that in vivo are likely to physiologically modulate the self-renewal and differentiation of NPCs.

Neuronal Calcium Signaling in Metabolic Regulation and Adaptation to Nutrient Stress

All organisms can respond physiologically and behaviorally to environmental fluxes in nutrient levels. Different nutrient sensing pathways exist for specific metabolites, and their inputs ultimately define appropriate nutrient uptake and metabolic homeostasis. Nutrient sensing mechanisms at the cellular level require pathways such as insulin and target of rapamycin (TOR) signaling that integrates information from different organ systems like the fat body and the gut. Such integration is essential for coordinating growth with development. Here we review the role of a newly identified set of integrative interneurons and the role of intracellular calcium signaling within these neurons, in regulating nutrient sensing under conditions of nutrient stress. A comparison of the identified Drosophila circuit and cellular mechanisms employed in this circuit, with vertebrate systems, suggests that the identified cell signaling mechanisms may be conserved for neural circuit function related to nutrient sensing by central neurons. The ideas proposed are potentially relevant for understanding the molecular basis of metabolic disorders, because these are frequently linked to nutritional stress.

Store-Operated Ca2+ Entry in Drosophila Primary Neuronal Cultures

Intracellular calcium signals in neurons frequently derive from the stimulation of G protein-coupled receptors (GPCR) by neurotransmitters, neuropeptides, and neurohormones. GPCR stimulation in neurons leads to generation of inositol 1,4,5-trisphosphate (IP₃), which in turn activates endoplasmic reticulum (ER)-localized IP₃ receptors. The IP₃ receptor (IP₃R) is a ligand-gated Ca²⁺ channel, which releases Ca²⁺ from ER stores. In Drosophila neurons it has been shown that depletion of ER Ca²⁺ store is followed by store-operated Ca²⁺ entry (SOCE) through STIM and Orai, the ER Ca²⁺ sensor and the plasma membrane Ca²⁺ channel respectively. The elucidation of this Ca²⁺ signaling pathway in neurons has in part been possible due to the ease of genetic manipulation in Drosophila, which has allowed neuron-specific knockdown of various proteins of interest. This has been followed by standardization of conditions for culturing neurons from dissected brains of the relevant genotypes, such that they could be used for robust Ca²⁺ measurements by imaging with standard Ca²⁺ indicator dyes. Protocols for measurement of IP₃-mediated Ca²⁺ release, passive depletion of ER Ca²⁺ store, and SOCE in primary cultures of Drosophila neurons are described here.

Ral function in muscle is required for flight maintenance in Drosophila

Ral is a small GTPase of the Ras superfamily that is important for a number of cellular functions. Recently, we found that expression of Ral is regulated by store-operated calcium entry (SOCE) in Drosophila neurons. In this study, through genetic and behavioural experiments, we show that Ral function is required in differentiated muscles for flight. Reducing Ral function in muscles, specifically reduced duration of flight bouts but not other motor functions, like climbing. Interestingly, unlike in the nervous system, Ral expression in the muscle is not regulated by SOCE. Moreover, either knockdown or genetic inhibition of SOCE in muscles does not affect flight. These findings demonstrate that a multiplicity of signalling mechanisms very likely regulate Ral expression in different tissues.

Control of protein translation by IP3R-mediated Ca2+ release in Drosophila neuroendocrine cells

The inositol 1,4,5-trisphosphate receptor (IP3R) is one of two Ca2+ channels that gates Ca2+ release from ER-stores. The ligand IP3, generated upon specific G-protein coupled receptor activation, binds to IP3R to release Ca2+ into the cytosol. IP3R also mediates ER-store Ca2+ release into the mitochondria, under basal as well as stimulatory conditions; an activity that influences cellular bioenergetics and thus, cellular growth and proliferation. In Drosophila neuroendocrine cells expressing a hypomorphic mutant of IP3R, we observed reduced protein translation levels. Here, we discuss the possible molecular mechanism for this observation. We hypothesise that the cellular energy sensor, AMPK connects IP3R mediated Ca2+ release into the mitochondria, to protein translation, via the TOR pathway.

How does calcium interact with the cytoskeleton to regulate growth cone motility during axon pathfinding?

The precision with which neurons form connections is crucial for the normal development and function of the nervous system. The development of neuronal circuitry in the nervous system is accomplished by axon pathfinding: a process where growth cones guide axons through the embryonic environment to connect with their appropriate synaptic partners to form functional circuits. Despite intense efforts over many years to understand how this process is regulated, the complete repertoire of molecular mechanisms that govern the growth cone cytoskeleton and hence motility, remain unresolved. A central tenet in the axon guidance field is that calcium signals regulate growth cone behaviours such as extension, turning and pausing by regulating rearrangements of the growth cone cytoskeleton. Here, we provide evidence that not only the amplitude of a calcium signal is critical for growth cone motility but also the source of calcium mobilisation. We provide an example of this idea by demonstrating that manipulation of calcium signalling via L-type voltage gated calcium channels can perturb sensory neuron motility towards a source of netrin-1. Understanding how calcium signals can be transduced to initiate cytoskeletal changes represents a significant gap in our current knowledge of the mechanisms that govern axon guidance, and consequently the formation of functional neural circuits in the developing nervous system.

Spontaneous Ca2+ Influx in Drosophila Pupal Neurons Is Modulated by IP3-Receptor Function and Influences Maturation of the Flight Circuit

Inositol 1,4,5-trisphosphate receptors (IP₃R) are Ca²⁺ channels on the neuronal endoplasmic reticulum (ER) membrane. They are gated by IP₃, produced upon external stimulation and activation of G protein-coupled receptors on the plasma membrane (PM). IP₃-mediated Ca²⁺ release, and the resulting depletion of the ER store, triggers entry of extracellular Ca²⁺ by store-operated Ca²⁺ entry (SOCE). Mutations in IP₃R attenuate SOCE. Compromised IP₃R function and SOCE during pupal development of Drosophila leads to flight deficits and mimics suppression of neuronal activity during pupal or adult development. To understand the effect of compromised IP₃R function on pupal neuronal calcium signaling, we examined the effects of mutations in the IP₃R gene (itpr) on Ca²⁺ signals in cultured neurons derived from Drosophila pupae. We observed increased spontaneous Ca²⁺ influx across the PM of isolated pupal neurons with mutant IP₃R and also a loss of SOCE. Both spontaneous Ca²⁺ influx and reduced SOCE were reversed by over-expression of dOrai and dSTIM, which encode the SOCE Ca²⁺ channel and the ER Ca²⁺-sensor that regulates it, respectively. Expression of voltage-gated Ca²⁺ channels (cac, Ca-α1D and Ca-αT) was significantly reduced in itpr mutant neurons. However, expression of trp mRNAs and transient receptor potential (TRP) protein were increased, suggesting that TRP channels might contribute to the increased spontaneous Ca²⁺ influx in neurons with mutant IP₃R. Thus, IP₃R/SOCE modulates spontaneous Ca²⁺ influx and expression of PM Ca²⁺ channels in Drosophila pupal neurons. Spontaneous Ca²⁺ influx compensates for the loss of SOCE in Drosophilaitpr mutant neurons.

IP3R-mediated Ca2+ release regulates protein metabolism in Drosophila neuroendocrine cells: implications for development under nutrient stress

Successful completion of animal development is fundamentally reliant on nutritional cues. Surviving periods of nutritional insufficiency requires adaptations that are coordinated, in part, by neural circuits. As neuropeptides secreted by neuroendocrine (NE) cells modulate neural circuits, we investigated NE cell function during development under nutrient stress. Starved Drosophila larvae exhibited reduced pupariation if either insulin signaling or IP₃/Ca²⁺ signaling were downregulated in NE cells. Moreover, an IP₃R (inositol 1,4,5-trisphosphate receptor) loss-of-function mutant displayed reduced protein synthesis, which was rescued by overexpression of either InR (insulin receptor) or IP₃R in NE cells of the mutant, suggesting that the two signaling pathways might be functionally compensatory. Furthermore, cultured IP₃R mutant NE cells, but not neurons, exhibited reduced protein translation. Thus cell-specific regulation of protein synthesis by IP₃R in NE cells influences protein metabolism. We propose that this regulation helps developing animals survive in poor nutritional conditions.

Summary: Intracellular Ca²⁺ signaling regulates protein translation and can compensate for insulin signaling in specialized neuro-hormonal cells, thus enabling Drosophila larval to pupal development under acute starvation.

CRISPR-Cas-Induced Mutants Identify a Requirement for dSTIM in Larval Dopaminergic Cells of Drosophila melanogaster

Molecular components of store-operated calcium entry have been identified in the recent past and consist of the endoplasmic reticulum (ER) membrane-resident calcium sensor STIM and the plasma membrane-localized calcium channel Orai. The physiological function of STIM and Orai is best defined in vertebrate immune cells. However, genetic studies with RNAi strains in Drosophila suggest a role in neuronal development and function. We generated a CRISPR-Cas-mediated deletion for the gene encoding STIM in Drosophila (dSTIM), which we demonstrate is larval lethal. To study STIM function in neurons, we merged the CRISPR-Cas9 method with the UAS-GAL4 system to generate either tissue- or cell type-specific inducible STIM knockouts (KOs). Our data identify an essential role for STIM in larval dopaminergic cells. The molecular basis for this cell-specific requirement needs further investigation.

A pupal transcriptomic screen identifies Ral as a target of store-operated calcium entry in Drosophila neurons

Transcriptional regulation by Store-operated Calcium Entry (SOCE) is well studied in non-excitable cells. However, the role of SOCE has been poorly documented in neuronal cells with more complicated calcium dynamics. Previous reports demonstrated a requirement for SOCE in neurons that regulate Drosophila flight bouts. We refine this requirement temporally to the early pupal stage and use RNA-sequencing to identify SOCE mediated gene expression changes in the developing Drosophila pupal nervous system. Down regulation of dStim, the endoplasmic reticular calcium sensor and a principal component of SOCE in the nervous system, altered the expression of 131 genes including Ral, a small GTPase. Disruption of Ral function in neurons impaired flight, whereas ectopic expression of Ral in SOCE-compromised neurons restored flight. Through live imaging of calcium transients from cultured pupal neurons, we confirmed that Ral does not participate in SOCE, but acts downstream of it. These results identify neuronal SOCE as a mechanism that regulates expression of specific genes during development of the pupal nervous system and emphasizes the relevance of SOCE-regulated gene expression to flight circuit maturation.

Cardiovascular and Hemostatic Disorders: Role of STIM and Orai Proteins in Vascular Disorders

Store-operated Ca²⁺ entry (SOCE) mediated by STIM and Orai proteins is a highly regulated and ubiquitous signaling pathway that plays an important role in various cellular and physiological functions. Endoplasmic reticulum (ER) serves as the major site for intracellular Ca²⁺ storage. Stromal Interaction Molecule 1/2 (STIM1/2) sense decrease in ER Ca²⁺ levels and transmits the message to plasma membrane Ca²⁺ channels constituted by Orai family members (Orai1/2/3) resulting in Ca²⁺ influx into the cells. This increase in cytosolic Ca²⁺ in turn activates a variety of signaling cascades to regulate a plethora of cellular functions. Evidence from the literature suggests that SOCE dysregulation is associated with several pathophysiologies, including vascular disorders. Interestingly, recent studies have suggested that STIM proteins may also regulate vascular functions independent of their contribution to SOCE. In this updated book chapter, we will focus on the physiological role of STIM and Orai proteins in the vasculature (endothelial cells and vascular smooth muscle cells). We will further retrospect the literature implicating a critical role for these proteins in vascular disease.

Neurological and Motor Disorders: Neuronal Store-Operated Ca2+ Signaling: An Overview and Its Function

Calcium (Ca2+) is a ubiquitous second messenger that performs significant physiological task such as neurosecretion, exocytosis, neuronal growth/differentiation, and the development and/or maintenance of neural circuits. An important regulatory aspect of neuronal Ca2+ homeostasis is store-operated Ca2+ entry (SOCE) which, in recent years, has gained much attention for influencing a variety of nerve cell responses. Essentially, activation of SOCE ensues following the activation of the plasma membrane (PM) store-operated Ca2+ channels (SOCC) triggered by the depletion of endoplasmic reticulum (ER) Ca2+ stores. In addition to the TRPC (transient receptor potential canonical) and the Orai family of ion channels, STIM (stromal interacting molecule) proteins have been baptized as key molecular regulators of SOCE. Functional significance of the TRPC channels in neurons has been elaborately studied; however, information on Orai and STIM components of SOCE, although seems imminent, is currently limited. Importantly, perturbations in SOCE have been implicated in a spectrum of neuropathological conditions. Hence, understanding the precise involvement of SOCC in neurodegeneration would presumably unveil avenues for plausible therapeutic interventions. We thus review the role of SOCE-regulated neuronal Ca2+ signaling in selecting neurodegenerative conditions.

Regulation of Store-Operated Ca2+ Entry by Septins

The mechanism of store-operated Ca²⁺ entry (SOCE) brings extracellular Ca²⁺ into cells after depletion of intracellular Ca²⁺ stores. Regulation of Ca²⁺ homeostasis by SOCE helps control various intracellular signaling functions in both non-excitable and excitable cells. Whereas essential components of the SOCE pathway are well characterized, molecular mechanisms underlying regulation of this pathway need investigation. A class of proteins recently demonstrated as regulating SOCE is septins. These are filament-forming GTPases that assemble into higher order structures. One of their most studied cellular functions is as a molecular scaffold that creates diffusion barriers in membranes for a variety of cellular processes. Septins regulate SOCE in mammalian non-excitable cells and in Drosophila neurons. However, the molecular mechanism of SOCE-regulation by septins and the contribution of different subgroups of septins to SOCE-regulation remain to be understood. The regulation of SOCE is relevant in multiple cellular contexts as well as in diseases, such as the Severe Combined Immunodeficiency (SCID) syndrome and neurodegenerative syndromes like Alzheimer's, Spino-Cerebellar Ataxias and Parkinson's. Moreover, Drosophila neurons, where loss of SOCE leads to flight deficits, are a possible cellular template for understanding the molecular basis of neuronal deficits associated with loss of either the Inositol-1,4,5-trisphosphate receptor (IP₃R1), a key activator of neuronal SOCE or the Endoplasmic reticulum resident Ca²⁺ sensor STIM1 (Stromal Interaction Molecule) in mouse. This perspective summarizes our current understanding of septins as regulators of SOCE and discusses the implications for mammalian neuronal function.

AMPA Receptors Are Involved in Store-Operated Calcium Entry and Interact with STIM Proteins in Rat Primary Cortical Neurons

The process of store-operated calcium entry (SOCE) leads to refilling the endoplasmic reticulum (ER) with calcium ions (Ca²⁺) after their release into the cytoplasm. Interactions between (ER)-located Ca²⁺ sensors (stromal interaction molecule 1 [STIM1] and STIM2) and plasma membrane-located Ca²⁺ channel-forming protein (Orai1) underlie SOCE and are well described in non-excitable cells. In neurons, however, SOCE appears to be more complex because of the importance of Ca²⁺ influx via voltage-gated or ionotropic receptor-operated Ca²⁺ channels. We found that the SOCE inhibitors ML-9 and SKF96365 reduced α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-induced [Ca²⁺]_i amplitude by 80% and 53%, respectively. To assess the possible involvement of AMPA receptors (AMPARs) in SOCE, we used their specific inhibitors. As estimated by Fura-2 acetoxymethyl (AM) single-cell Ca²⁺ measurements in the presence of CNQX or NBQX, thapsigargin (TG)-induced Ca²⁺ influx decreased 2.2 or 3.7 times, respectively. These results suggest that under experimental conditions of SOCE when Ca²⁺ stores are depleted, Ca²⁺ can enter neurons also through AMPARs. Using specific antibodies against STIM proteins or GluA1/GluA2 AMPAR subunits, co-immunoprecipitation assays indicated that when Ca²⁺ levels are low in the neuronal ER, a physical association occurs between endogenous STIM proteins and endogenous AMPAR receptors. Altogether, our data suggest that STIM proteins in neurons can control AMPA-induced Ca²⁺ entry as a part of the mechanism of SOCE.

Mutant IP3 receptors attenuate store-operated Ca2+ entry by destabilizing STIM-Orai interactions in Drosophila neurons

Store-operated Ca²⁺ entry (SOCE) occurs when loss of Ca²⁺ from the endoplasmic reticulum (ER) stimulates the Ca²⁺ sensor, STIM, to cluster and activate the plasma membrane Ca²⁺ channel Orai (encoded by Olf186-F in flies). Inositol 1,4,5-trisphosphate receptors (IP₃Rs, which are encoded by a single gene in flies) are assumed to regulate SOCE solely by mediating ER Ca²⁺ release. We show that in Drosophila neurons, mutant IP₃R attenuates SOCE evoked by depleting Ca²⁺ stores with thapsigargin. In normal neurons, store depletion caused STIM and the IP₃R to accumulate near the plasma membrane, association of STIM with Orai, clustering of STIM and Orai at ER–plasma-membrane junctions and activation of SOCE. These responses were attenuated in neurons with mutant IP₃Rs and were rescued by overexpression of STIM with Orai. We conclude that, after depletion of Ca²⁺ stores in Drosophila, translocation of the IP₃R to ER–plasma-membrane junctions facilitates the coupling of STIM to Orai that leads to activation of SOCE.

Summary: In Drosophila neurons, mutant IP₃ receptors disrupt store-operated Ca²⁺ entry by destabilizing interaction of STIM with the Ca²⁺ channel, Orai. The interactions could coordinate store emptying with Ca²⁺ entry.

Apoptotic Activity of MeCP2 Is Enhanced by C-Terminal Truncating Mutations

Methyl-CpG binding protein 2 (MeCP2) is a widely abundant, multifunctional protein most highly expressed in post-mitotic neurons. Mutations causing Rett syndrome and related neurodevelopmental disorders have been identified along the entire MECP2 locus, but symptoms vary depending on mutation type and location. C-terminal mutations are prevalent, but little is known about the function of the MeCP2 C-terminus. We employ the genetic efficiency of Drosophila to provide evidence that expression of p.Arg294* (more commonly identified as R294X), a human MECP2 E2 mutant allele causing truncation of the C-terminal domains, promotes apoptosis of identified neurons in vivo. We confirm this novel finding in HEK293T cells and then use Drosophila to map the region critical for neuronal apoptosis to a small sequence at the end of the C-terminal domain. In vitro studies in mammalian systems previously indicated a role of the MeCP2 E2 isoform in apoptosis, which is facilitated by phosphorylation at serine 80 (S80) and decreased by interactions with the forkhead protein FoxG1. We confirm the roles of S80 phosphorylation and forkhead domain transcription factors in affecting MeCP2-induced apoptosis in Drosophila in vivo, thus indicating mechanistic conservation between flies and mammalian cells. Our findings are consistent with a model in which C- and N-terminal interactions are required for healthy function of MeCP2.