Visualization of astrocytic intracellular Ca2+ mobilization

Luminal transport through intact endoplasmic reticulum limits the magnitude of localized Ca2+ signals

The endoplasmic reticulum (ER) forms an interconnected network of tubules stretching throughout the cell. Understanding how ER functionality relies on its structural organization is crucial for elucidating cellular vulnerability to ER perturbations, which have been implicated in several neuronal pathologies. One of the key functions of the ER is enabling Ca[Formula: see text] signaling by storing large quantities of this ion and releasing it into the cytoplasm in a spatiotemporally controlled manner. Through a combination of physical modeling and live-cell imaging, we demonstrate that alterations in ER shape significantly impact its ability to support efficient local Ca[Formula: see text] releases, due to hindered transport of luminal content within the ER. Our model reveals that rapid Ca[Formula: see text] release necessitates mobile luminal buffer proteins with moderate binding strength, moving through a well-connected network of ER tubules. These findings provide insight into the functional advantages of normal ER architecture, emphasizing its importance as a kinetically efficient intracellular Ca[Formula: see text] delivery system.

IP3RPEP6, a novel peptide inhibitor of IP3 receptor channels that does not affect connexin-43 hemichannels

AIM

Inositol 1,4,5-trisphosphate receptors (IP3 Rs) are intracellular Ca2+ -release channels with crucial roles in cell function. Current IP3 R inhibitors suffer from off-target effects and poor selectivity towards the three distinct IP3 R subtypes. We developed a novel peptide inhibitor of IP3 Rs and determined its effect on connexin-43 (Cx43) hemichannels, which are co-activated by IP3 R stimulation.

METHODS

IP3RPEP6 was developed by in silico molecular docking studies and characterized by on-nucleus patch-clamp experiments of IP3 R2 channels and carbachol-induced IP3 -mediated Ca2+ responses in IP3 R1, 2 or 3 expressing cells, triple IP3 R KO cells and astrocytes. Cx43 hemichannels were studied by patch-clamp and ATP-release approaches, and by inhibition with Gap19 peptide. IP3RPEP6 interactions with IP3 Rs were verified by co-immunoprecipitation and affinity pull-down assays.

RESULTS

IP3RPEP6 concentration-dependently reduced the open probability of IP3 R2 channels and competitively inhibited IP3 Rs in an IC50 order of IP3 R2 (~3.9 μM) < IP3 R3 (~4.3 μM) < IP3 R1 (~9.0 μM), without affecting Cx43 hemichannels or ryanodine receptors. IP3RPEP6 co-immunoprecipitated with IP3 R2 but not with IP3 R1; interaction with IP3 R3 varied between cell types. The IC50 of IP3RPEP6 inhibition of carbachol-induced Ca2+ responses decreased with increasing cellular Cx43 expression. Moreover, Gap19-inhibition of Cx43 hemichannels significantly reduced the amplitude of the IP3 -Ca2+ responses and strongly increased the EC50 of these responses. Finally, we identified palmitoyl-8G-IP3RPEP6 as a membrane-permeable IP3RPEP6 version allowing extracellular application of the IP3 R-inhibiting peptide.

CONCLUSION

IP3RPEP6 inhibits IP3 R2/R3 at concentrations that have limited effects on IP3 R1. IP3 R activation triggers hemichannel opening, which strongly affects the amplitude and concentration-dependence of IP3 -triggered Ca2+ responses.

Neuronal and astrocyte NCX isoform/splice variants: How do they participate in Na+ and Ca2+ signalling?

NCX1, NCX2, and NCX3 gene isoforms and their splice variants are characteristically expressed in different regions of the brain. The tissue-specific splice variants of NCX1-3 isoforms show specific expression profiles in neurons and astrocytes, whereas the relevant NCX isoform/splice variants exhibit diverse allosteric modes of Na+- and Ca2+-dependent regulation. In general, overexpression of NCX1-3 genes leads to neuroprotective effects, whereas their ablation gains the opposite results. At this end, the partial contributions of NCX isoform/splice variants to neuroprotective effects remain unresolved. The glutamate-dependent Na+ entry generates Na+ transients (in response to neuronal cell activities), whereas the Na+-driven Ca2+ entry (through the reverse NCX mode) raises Ca2+ transients. This special mode of signal coupling translates Na+ transients into the Ca2+ signals while being a part of synaptic neurotransmission. This mechanism is of general interest since disease-related conditions (ischemia, metabolic stress, and stroke among many others) trigger Na+ and Ca2+ overload with deadly outcomes of downstream apoptosis and excitotoxicity. The recently discovered mechanisms of NCX allosteric regulation indicate that some NCX variants might play a critical role in the dynamic coupling of Na+-driven Ca2+ entry. In contrast, the others are less important or even could be dangerous under altered conditions (e.g., metabolic stress). This working hypothesis can be tested by applying advanced experimental approaches and highly focused computational simulations. This may allow the development of structure-based blockers/activators that can selectively modulate predefined NCX variants to lessen the life-threatening outcomes of excitotoxicity, ischemia, apoptosis, metabolic deprivation, brain injury, and stroke.

Modulating mitochondrial calcium channels (TRPM2/MCU/NCX) as a therapeutic strategy for neurodegenerative disorders

Efficient cellular communication is essential for the brain to regulate diverse functions like muscle contractions, memory formation and recall, decision-making, and task execution. This communication is facilitated by rapid signaling through electrical and chemical messengers, including voltage-gated ion channels and neurotransmitters. These messengers elicit broad responses by propagating action potentials and mediating synaptic transmission. Calcium influx and efflux are essential for releasing neurotransmitters and regulating synaptic transmission. Mitochondria, which are involved in oxidative phosphorylation, and the energy generation process, also interact with the endoplasmic reticulum to store and regulate cytoplasmic calcium levels. The number, morphology, and distribution of mitochondria in different cell types vary based on energy demands. Mitochondrial damage can cause excess reactive oxygen species (ROS) generation. Mitophagy is a selective process that targets and degrades damaged mitochondria via autophagosome-lysosome fusion. Defects in mitophagy can lead to a buildup of ROS and cell death. Numerous studies have attempted to characterize the relationship between mitochondrial dysfunction and calcium dysregulation in neurodegenerative diseases such as Alzheimer's Disease, Parkinson's Disease, Huntington's Disease, Amyotrophic lateral sclerosis, spinocerebellar ataxia, and aging. Interventional strategies to reduce mitochondrial damage and accumulation could serve as a therapeutic target, but further research is needed to unravel this potential. This review offers an overview of calcium signaling related to mitochondria in various neuronal cells. It critically examines recent findings, exploring the potential roles that mitochondrial dysfunction might play in multiple neurodegenerative diseases and aging. Furthermore, the review identifies existing gaps in knowledge to guide the direction of future research.

Simultaneous Measurement of Changes in Mitochondrial and Endoplasmic Reticulum Free Calcium in Pancreatic Beta Cells

The free calcium (Ca2+) levels in pancreatic beta cell organelles have been the subject of many recent investigations. Under pathophysiological conditions, disturbances in these pools have been linked to altered intracellular communication and cellular dysfunction. To facilitate studies of subcellular Ca2+ signaling in beta cells and, particularly, signaling between the endoplasmic reticulum (ER) and mitochondria, we designed a novel dual Ca2+ sensor which we termed DS-1. DS-1 encodes two stoichiometrically fluorescent proteins within a single plasmid, G-CEPIA-er, targeted to the ER and R-CEPIA3-mt, targeted to mitochondria. Our goal was to simultaneously measure the ER and mitochondrial Ca2+ in cells in real time. The Kds of G-CEPIA-er and R-CEPIA3-mt for Ca2+ are 672 and 3.7 μM, respectively. Confocal imaging of insulin-secreting INS-1 832/13 expressing DS-1 confirmed that the green and red fluorophores correctly colocalized with organelle-specific fluorescent markers as predicted. Further, we tested whether DS-1 exhibited the functional properties expected by challenging an INS-1 cell to glucose concentrations or drugs having well-documented effects on the ER and mitochondrial Ca2+ handling. The data obtained were consistent with those seen using other single organelle targeted probes. These results taken together suggest that DS-1 is a promising new approach for investigating Ca2+ signaling within multiple organelles of the cell.

Diffusive exit rates through pores in membrane-enclosed structures

The function of many membrane-enclosed intracellular structures relies on release of diffusing particles that exit through narrow pores or channels in the membrane. The rate of release varies with pore size, density, and length of the channel. We propose a simple approximate model, validated with stochastic simulations, for estimating the effective release rate from cylinders, and other simple-shaped domains, as a function of channel parameters. The results demonstrate that, for very small pores, a low density of channels scattered over the boundary is sufficient to achieve substantial rates of particle release. Furthermore, we show that increasing the length of passive channels will both reduce release rates and lead to a less steep dependence on channel density. Our results are compared to previously-measured local calcium release rates from tubules of the endoplasmic reticulum, providing an estimate of the relevant channel density responsible for the observed calcium efflux.

Intercellular Communication in the Central Nervous System as Deduced by Chemical Neuroanatomy and Quantitative Analysis of Images: Impact on Neuropharmacology

In the last decades, new evidence on brain structure and function has been acquired by morphological investigations based on synergic interactions between biochemical anatomy approaches, new techniques in microscopy and brain imaging, and quantitative analysis of the obtained images. This effort produced an expanded view on brain architecture, illustrating the central nervous system as a huge network of cells and regions in which intercellular communication processes, involving not only neurons but also other cell populations, virtually determine all aspects of the integrative function performed by the system. The main features of these processes are described. They include the two basic modes of intercellular communication identified (i.e., wiring and volume transmission) and mechanisms modulating the intercellular signaling, such as cotransmission and allosteric receptor–receptor interactions. These features may also open new possibilities for the development of novel pharmacological approaches to address central nervous system diseases. This aspect, with a potential major impact on molecular medicine, will be also briefly discussed.

ER-GCaMP6f: An Endoplasmic Reticulum-Targeted Genetic Probe to Measure Calcium Activity in Astrocytic Processes

Ca²⁺ is a major second messenger involved in cellular and subcellular signaling in a wide range of cells, including astrocytes, which use calcium ions to communicate with other cells in the brain. Even though a variety of genetically encoded Ca²⁺ indicators have been developed to study astrocyte calcium signaling, understanding the dynamics of endoplasmic reticulum calcium signaling is greatly limited by the currently available tools. To address this, we developed an endoplasmic reticulum-targeted calcium indicator, ER-GCaMP6f, which is anchored to the cytosolic side of the organelle and measures signaling that occurs in close proximity to the endoplasmic reticulum of astrocytes. Using a combination of confocal and super-resolution microscopy techniques, we demonstrate the localization of the indicator in the endoplasmic reticulum in both cell soma and processes of astrocytes. Combining ER-GCaMP6f with total internal reflection fluorescence microscopy, we show that Ca²⁺ fluctuations in small astrocytic processes can be detected, which are otherwise not observable with existing indicators and standard wide-field and confocal techniques. We also compared the ER-GCaMP6f indicator against currently used plasma membrane-tethered and cytosolic GCaMP6f indicators. ER-GCaMP6f identifies dynamics in calcium signaling of endoplasmic reticulum resident receptors that are missed by plasma membrane-anchored indicators. We also generated an adeno-associated virus (AAV5) and demonstrate that ER-GCaMP6f can be expressed in vivo and by measured calcium activity in brain slices. ER-GCaMP6f provides a powerful tool to study calcium signaling in close proximity to the endoplasmic reticulum in astrocyte cell soma and processes both in culture and in brain slices.

Astrocytic processes: from tripartite synapses to the active milieu

We define a new concept of 'active milieu' that unifies all components of nervous tissue (neuronal and glial compartments, extracellular space, extracellular matrix, and vasculature) into a dynamic information processing system. Within this framework, we focus on the role of astrocytic processes, classified into organelle-containing branches and organelle-free leaflets. We argue that astrocytic branches with emanating leaflets are homologous to dendritic shafts with spines. Within the active milieu, astrocytic processes are engaged in reciprocal interactions with neuronal compartments and communication with other cellular and non-cellular elements of the nervous tissue.

Novel Approaches Used to Examine and Control Neurogenesis in Parkinson's Disease

Neurogenesis is a key mechanism of brain development and plasticity, which is impaired in chronic neurodegeneration, including Parkinson’s disease. The accumulation of aberrant α-synuclein is one of the features of PD. Being secreted, this protein produces a prominent neurotoxic effect, alters synaptic plasticity, deregulates intercellular communication, and supports the development of neuroinflammation, thereby providing propagation of pathological events leading to the establishment of a PD-specific phenotype. Multidirectional and ambiguous effects of α-synuclein on adult neurogenesis suggest that impaired neurogenesis should be considered as a target for the prevention of cell loss and restoration of neurological functions. Thus, stimulation of endogenous neurogenesis or cell-replacement therapy with stem cell-derived differentiated neurons raises new hopes for the development of effective and safe technologies for treating PD neurodegeneration. Given the rapid development of optogenetics, it is not surprising that this method has already been repeatedly tested in manipulating neurogenesis in vivo and in vitro via targeting stem or progenitor cells. However, niche astrocytes could also serve as promising candidates for controlling neuronal differentiation and improving the functional integration of newly formed neurons within the brain tissue. In this review, we mainly focus on current approaches to assess neurogenesis and prospects in the application of optogenetic protocols to restore the neurogenesis in Parkinson’s disease.

Using Genetically Encoded Calcium Indicators to Study Astrocyte Physiology: A Field Guide

Ca²⁺ imaging is the most frequently used technique to study glial cell physiology. While chemical Ca²⁺ indicators served to visualize and measure changes in glial cell cytosolic Ca²⁺ concentration for several decades, genetically encoded Ca²⁺ indicators (GECIs) have become state of the art in recent years. Great improvements have been made since the development of the first GECI and a large number of GECIs with different physical properties exist, rendering it difficult to select the optimal Ca²⁺ indicator. This review discusses some of the most frequently used GECIs and their suitability for glial cell research.

Astrocytic mitochondria in adult mouse brain slices show spontaneous calcium influx events with unique properties

Astrocytes govern critical aspects of brain function via spontaneous calcium signals in their soma and processes. A significant proportion of these spontaneous astrocytic calcium events are associated with mitochondria, however, the extent, sources, or kinetics of astrocytic mitochondrial calcium influx have not been studied in the adult mouse brain. To measure calcium influx into astrocytic mitochondria in situ, we generated an adeno-associated virus (AAV) with the astrocyte-specific GfaABC₁D promoter driving expression of the genetically encoded calcium indicator, GCaMP6f tagged to mito7, a mitochondrial matrix targeted signal sequence. Using this construct, we observed AAV-mediated expression of GCaMP6f in adult mouse astrocytic mitochondria that co-localized with MitoTracker deep red (MTDR) in the dorsolateral striatum (DLS) and in the hippocampal stratum radiatum (HPC). Astrocytic mitochondria co-labeled with MTDR and GCaMP6f displayed robust, spontaneous calcium influx events in situ, with subcellular differences in calcium influx kinetics between somatic, branch, and branchlet mitochondria, and inter-regional differences between mitochondria in DLS and HPC astrocytes. Calcium influx into astrocytic mitochondria was strongly dependent on endoplasmic reticulum calcium stores, but did not require the mitochondrial calcium uniporter, MCU. Exposure to either glutamate, D1 or D2 dopamine receptor agonists increased calcium influx in some mitochondria, while simultaneously decreasing calcium influx in other mitochondria from the same astrocyte. These findings show that astrocytic mitochondria possess unique properties with regard to their subcellular morphology, mechanisms of calcium influx, and responses to neurotransmitter receptor agonists. Our results have important implications for understanding the role of astrocytic mitochondria during pathological processes.

Store-operated Ca2+ entry-dependent Ca2+ refilling in the endoplasmic reticulum in astrocytes

Astrocytes regulate various brain functions, for which Ca²⁺ release from the endoplasmic reticulum (ER) often play crucial roles. Because astrocytic ER Ca²⁺ release is robust and frequent, the ER Ca²⁺ refilling mechanism should be critical for ongoing Ca²⁺ signaling in astrocytes. In this study, we focused on the putative functional significance of store-operated Ca²⁺ entry (SOCE) in ER Ca²⁺ refilling. We expressed the ER luminal Ca²⁺ indicator G-CEPIA1er in astrocytes in acute cortical slices to directly monitor the decrease and recovery of ER Ca²⁺ concentration upon spontaneous or norepinephrine-induced Ca²⁺ release. Inhibition of SOCE significantly slowed the recovery of ER Ca²⁺ concentration after Ca²⁺ release in astrocytes. This delayed recovery resulted in a prolonged decrease in the ER Ca²⁺ content in astrocytes with periodic spontaneous Ca²⁺ release, followed by the attenuation of cytosolic Ca²⁺ responses upon Ca²⁺ release. Therefore, our results provide direct evidence for the physiological significance of SOCE in ER Ca²⁺ refilling after ER Ca²⁺ release.