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The relation of form and function, namely the impact of the synaptic anatomy on calcium dynamics in the presynaptic bouton, is a major challenge of present (computational) neuroscience at a cellular level. The Drosophila larval neuromuscular junction (NMJ) is a simple model system, which allows studying basic effects in a rather simple way. This synapse harbors several special structures. In particular, in opposite to standard vertebrate synapses, the presynaptic boutons are rather large, and they have several presynaptic zones. In these zones, different types of anatomical structures are present. Some of the zones bear a so-called T-bar, a particular anatomical structure. The geometric form of the T-bar resembles the shape of the letter "T" or a table with one leg. When an action potential arises, calcium influx is triggered. The probability of vesicle docking and neurotransmitter release is superlinearly proportional to the concentration of calcium close to the vesicular release site. It is tempting to assume that the T-bar causes some sort of calcium accumulation and hence triggers a higher release probability and thus enhances neurotransmitter exocytosis. In order to study this influence in a quantitative manner, we constructed a typical T-bar geometry and compared the calcium concentration close to the active zones (AZs). We compared the case of synapses with and without T-bars. Indeed, we found a substantial influence of the T-bar structure on the presynaptic calcium concentrations close to the AZs, indicating that this anatomical structure increases vesicle release probability. Therefore, our study reveals how the T-bar zone implies a strong relation between form and function. Our study answers the question of experimental studies (namely "Wichmann and Sigrist, Journal of neurogenetics 2010") concerning the sense of the anatomical structure of the T-bar. [ABSTRACT FROM AUTHOR]

The relation of form and function, namely the impact of the synaptic anatomy on calcium dynamics in the presynaptic bouton, is a major challenge of present (computational) neuroscience at a cellular level. The Drosophila larval neuromuscular junction (NMJ) is a simple model system, which allows studying basic effects in a rather simple way. This synapse harbors several special structures. In particular, in opposite to standard vertebrate synapses, the presynaptic boutons are rather large, and they have several presynaptic zones. In these zones, different types of anatomical structures are present. Some of the zones bear a so-called T-bar, a particular anatomical structure. The geometric form of the T-bar resembles the shape of the letter “T” or a table with one leg. When an action potential arises, calcium influx is triggered. The probability of vesicle docking and neurotransmitter release is superlinearly proportional to the concentration of calcium close to the vesicular release site. It is tempting to assume that the T-bar causes some sort of calcium accumulation and hence triggers a higher release probability and thus enhances neurotransmitter exocytosis. In order to study this influence in a quantitative manner, we constructed a typical T-bar geometry and compared the calcium concentration close to the active zones (AZs). We compared the case of synapses with and without T-bars. Indeed, we found a substantial influence of the T-bar structure on the presynaptic calcium concentrations close to the AZs, indicating that this anatomical structure increases vesicle release probability. Therefore, our study reveals how the T-bar zone implies a strong relation between form and function. Our study answers the question of experimental studies (namely “Wichmann and Sigrist, Journal of neurogenetics 2010”) concerning the sense of the anatomical structure of the T-bar.

Activation of vascular endothelial small-(KCa2.3, SK3) or intermediate-(KCa3.1, IK1) conductance Ca2+-activated potassium channels induces vasorelaxation via an endothelium-derived hyperpolarization (EDH) pathway. Although the activation of SK3 and IK1 channels converges on EDH, their subcellular effects on signal transduction are different and not completely clear. In this study, a novel endothelium-specific SK3 knockout (SK3-/-) mouse model was utilized to specifically examine the contribution of SK3 channels to mesenteric artery vasorelaxation, endothelial Ca2+ dynamics, and blood pressure. The absence of SK3 expression was confirmed using real-time quantitative PCR and Western blot analysis. Functional studies showed impaired EDH-mediated vasorelaxation in SK3-/- small mesenteric arteries. Immunostaining results from SK3-/- vessels confirmed the absence of SK3 and further showed altered distribution of transient receptor potential channels, type 4 (TRPV4). Electrophysiological recordings showed a lack of SK3 channel activity, while TRPV4-IK1 channel coupling remained intact in SK3-/- endothelial cells. Moreover, Ca2+ imaging studies in SK3-/- endothelium showed increased Ca2+ transients with reduced amplitude and duration under basal conditions. Importantly, SK3-/- endothelium lacked a distinct type of Ca2+ dynamic that is sensitive to TRPV4 activation. Blood pressure measurements showed that the SK3-/- mice were hypertensive, and the blood pressure increase was further enhanced during the 12-h dark cycle when animals are most active. Taken together, our results reveal a previously unappreciated SK3 signaling microdomain that modulates endothelial Ca2+ dynamics, vascular tone, and blood pressure. [ABSTRACT FROM AUTHOR]

The plasma membrane calcium ATPases (PMCAs) are ATP-driven primary ion pumps found in all eukaryotic cells. They are the major high-affinity calcium extrusion system for expulsion of Ca 2+ ions from the cytosol and help restore the low resting levels of intracellular [Ca 2+ ] following the temporary elevation of Ca 2+ generated during Ca 2+ signaling. Due to their essential role in the maintenance of cellular Ca 2+ homeostasis they were initially thought to be “sump pumps” for Ca 2+ removal needed by all cells to avoid eventual calcium overload. The discovery of multiple PMCA isoforms and alternatively spliced variants cast doubt on this simplistic assumption, and revealed instead that PMCAs are integral components of highly regulated multi-protein complexes fulfilling specific roles in calcium-dependent signaling originating at the plasma membrane. Biochemical, genetic, and physiological studies in gene-manipulated and mutant animals demonstrate the important role played by specific PMCAs in distinct diseases including those affecting the peripheral and central nervous system, cardiovascular disease, and osteoporosis. Human PMCA gene mutations and allelic variants associated with specific disorders continue to be discovered and underline the crucial role of different PMCAs in particular cells, tissues and organs. [ABSTRACT FROM AUTHOR]

Abstract: The processes of excitation–contraction coupling in cardiac myocytes require enormous amounts of energy in the form of ATP, which is produced by oxidative phosphorylation in mitochondria. Due to the constantly varying workloads of the heart, efficient matching of energy supply to demand is a requisite for proper heart function. Ca2+ is taken up by mitochondria via the mitochondrial Ca2+ uniporter (MCU) where it stimulates key dehydrogenases of the Krebs cycle to match regeneration of NADH to its oxidation by the respiratory chain. The kinetics of mitochondrial Ca2+ uptake, however, remain controversial due to the low Ca2+ sensitivity of the MCU. Here, we review the evidence for the existence of a “mitochondrial Ca2+ microdomain”, in which the close association of the sarcoplasmic reticulum (SR) to mitochondria provides “hot spots” of very high Ca2+ concentrations in the vicinity of mitochondria, sufficient to overcome the low Ca2+ affinity of the MCU. Mitofusins 1 and 2 play redundant roles in regulating mitochondrial dynamics by controlling fusion of mitochondria with each other. Recent work revealed a unique role for mitofusin 2 in tethering mitochondria to the sarco-/endoplasmic reticulum in various cell types, including cardiac myocytes. Disruption of SR–mitochondrial Ca2+ cross talk in heart failure through spatial and ionic alterations may give rise to energetic deficit and oxidative stress, two factors believed to play causal roles in the progression of the disease. On the other hand, excessive mitochondrial Ca2+ uptake can trigger programmed necrosis, substantiating the ambiguity of the close interplay between these cousin organelles in health and disease. This article is part of a Special Issue entitled "Focus on Cardiac Metabolism". [Copyright &y& Elsevier]

Abstract: Energy supply is the most prominent function of mitochondria, but in addition, mitochondria are indispensable for a multitude of other important cellular functions including calcium (Ca2+) signaling and buffering, the supply of metabolites and the sequestration of apoptotic factors. The efficiency of those functions highly depends on the proper positioning of mitochondria within the cytosol. In lymphocytes, mitochondria preferentially localize into the vicinity (∼200nm) of the immune synapse (IS). This localization is regulated by motor-based cytoskeleton-mediated transport, the fusion/fission dynamics of mitochondria, and probably also through tethering with the ER. IS formation also induces the accumulation of CRAC/ORAI1 Ca2+ channels, the CRAC/ORAI channel activator STIM1, K+ channels and plasma membrane Ca2+ ATPase (PMCA) within the IS. Such a large agglomeration of Ca2+ binding organelles and proteins highlights the IS as a critical cellular compartment for Ca2+ dependent lymphocyte activation. At the IS, Ca2+ microdomains generated beneath open CRAC/ORAI channels provide a rapid, robust and reliable mechanism for driving cellular responses in mast cells and T cells. Here, we discuss the relevance of motor-based mitochondrial transport, fusion, fission and tethering for mitochondrial localization in T cells and the importance of subplasmalemmal mitochondria to control local CRAC/ORAI1-dependent Ca2+ microdomains at the IS for efficient T lymphocyte activation. [Copyright &y& Elsevier]

Using the mitochondrial potential (ΔΨ) marker JC-1 (5,5′,6,6′-tetrachloro-1,1′,3,3′-tetraethylbenzimidazolylcarbocyanine iodide) and high-resolution imaging, we functionally analyzed mitochondria in cultured rat hippocampal astrocytes. Ratiometric detection of JC-1 fluorescence identified mitochondria with high and low ΔΨ. Mitochondrial density was highest in the perinuclear region, whereas ΔΨ tended to be higher in peripheral mitochondria. Spontaneous ΔΨ fluctuations, representing episodes of increased energization, appeared in individual mitochondria or synchronized in mitochondrial clusters. They continued upon withdrawal of extracellular Ca, but were antagonized by dantrolene or 2-aminoethoxydiphenylborate (2-APB). Fluo-3 imaging revealed local cytosolic Ca transients with similar kinetics that also were depressed by dantrolene and 2-APB. Massive cellular Ca load or metabolic impairment abolished ΔΨ fluctuations, occasionally evoking heterogeneous mitochondrial depolarizations. The detected diversity and ΔΨ heterogeneity of mitochondria confirms that even in less structurally polarized cells, such as astrocytes, specialized mitochondrial subpopulations coexist. We conclude that ΔΨ fluctuations are an indication of mitochondrial viability and are triggered by local Ca release from the endoplasmic reticulum. This spatially confined organelle crosstalk contributes to the functional heterogeneity of mitochondria and may serve to adapt the metabolism of glial cells to the activity and metabolic demand of complex neuronal networks. The established ratiometric JC-1 imaging-especially combined with two-photon microscopy-enables quantitative functional analyses of individual mitochondria as well as the comparison of mitochondrial heterogeneity in different preparations and/or treatment conditions. [ABSTRACT FROM AUTHOR]

Abstract: The vesicular hypothesis originally introduced to explain the quantal nature of presynaptic neurotransmitter (NT) release has been initially confirmed by the presence of NT within presynaptic vesicles and by an exo–endocytotic traffic associated with intense synaptic activity. Since then, an increasing number of synaptic transmission properties cannot be readily incorporated into the now popular model in which each quantal NT packet is prepared in a vesicle and is released by diffusion across the synaptic cleft when this vesicle fuses transiently or definitively with the plasma membrane. Interestingly, presynaptic exocytosis exhibits the characteristics of the ubiquitous secretory pathway by which all eukaryotic cells interact with their immediate environment, not just externalizing soluble products, but principally delivering at particular location of the cell surface specific glycoconjugates constituting the extracellular matrix (ECM) that mediates intercellular adhesion, recognition and signaling. Recent studies point to the involvement of vesicular glycoproteins in fast transmission after their incorporation into the transsynaptic ECM, or synaptomatrix. The notion of synaptomatrix is presented as a multimolecular tight arrangement that is dynamically remodeled in a use-dependent fashion via PKC to support synaptic morpho-functional plasticity. The data reviewed suggests that the synaptomatrix controls in a Ca2+ entry-dependent manner the solubility of the NT in the cleft to support fast transmission. [Copyright &y& Elsevier]

Sound coding at hair cell ribbon synapses is tightly regulated by Ca[sup2+]. Here, we used patch-clamp, fast confocal Ca[sup2+] imaging and modeling to characterize synaptic Ca[sup2+] signaling in cochlear inner hair cells (IHCs) of hearing mice. Submicrometer fluorescence hotspots built up and collapsed at the base of IHC5 within a few milliseconds of stimulus onset and cessation. They most likely represented Ca[sup2+] microdomains arising from synaptic Ca[sup2+] influx through Cav1.3 channels. Synaptic Ca[sup2+] microdomains varied substantially in amplitude and voltage dependence even within single IHCs. Testing putative mechanisms for the heterogeneity of Ca[sup2+] signaling, we found the amplitude variability unchanged when blocking mitochondrial Ca[sup2+] uptake or Ca[sup2+]-induced Ca[sup2+] release, buffering cytosolic Ca[sup2+] by millimolar concentrations of EGTA, or elevating the Ca[sup2+] channel open probability by the dihydropyridine agonist BayK8644. However, we observed substantial variability also for the fluorescence of immunolabeled Cav1.3 Ca[sup2+] channel clusters. Moreover, the Ca[sup2+] microdomain amplitude correlated positively with the size of the corresponding synaptic ribbon. Ribbon size, previously suggested to scale with the number of synaptic Ca[sup2+] channels, was approximated by using fluorescent peptide labeling. We propose that IHCs adjust the number and the gating of Cav1.3 channels at their active zones to diversify their transmitter release rates. [ABSTRACT FROM AUTHOR]

Abstract: The limited choice and poor performance of red-emitting calcium (Ca2+) indicators have hampered microfluorometric measurements of the intracellular free Ca2+ concentration in cells expressing yellow- or green-fluorescent protein constructs. A long-wavelength Ca2+ indicator would also permit a better discrimination against cellular autofluorescence than the commonly used fluorescein-based probes. Here, we report an improved synthesis and characterization of Calcium Ruby, a red-emitting probe consisting of an extended rhodamine chromophore (578/602nm peak excitation/emission) conjugated to BAPTA and having an additional NH2 linker arm. The low-affinity variant (K D,Ca ∼30μM) with a chloride in meta position that was specifically designed for the detection of large and rapid Ca2+ transients. While Calcium Ruby is a mitochondrial Ca2+probe, its conjugation, via the NH2 tail, to a 10,000MW dextran abolishes the sub-cellular compartmentalization and generates a cytosolic Ca2+ probe with an affinity matched to microdomain Ca2+ signals. As an example, we show depolarization-evoked Ca2+ signals triggering the exocytosis of individual chromaffin granules. Calcium Ruby should be of use in a wide range of applications involving dual- or triple labeling schemes or targeted sub-cellular Ca2+ measurements. [Copyright &y& Elsevier]

Astrocytes are a neural cell type critically involved in maintaining brain energy homeostasis as well as signaling. Like neurons, astrocytes are a heterogeneous cell population. Cortical astrocytes show a complex morphology with a highly branched aborization and numerous fine processes ensheathing the synapses of neighboring neurons, and typically extend one process connecting to blood vessels. Recent studies employing genetically encoded fluorescent calcium (Ca 2+ ) indicators have described 'spontaneous' localized Ca 2+ -transients in the astrocyte periphery that occur asynchronously, independently of signals in other parts of the cells, and that do not involve somatic Ca 2+ transients; however, neither it is known whether these Ca 2+ -microdomains occur at or near neuronal synapses nor have their molecular basis nor downstream effector(s) been identified. In addition to Ca 2+ microdomains, sodium (Na + ) transients occur in astrocyte subdomains, too, most likely as a consequence of Na + co-transport with the neurotransmitter glutamate, which also regulates mitochondrial movements locally - as do cytoplasmic Ca 2+ levels. In this review, we cover various aspects of these local signaling events and discuss how structural and biophysical properties of astrocytes might foster such compartmentation. Astrocytes metabolically interact with neurons by providing energy substrates to active neurons. As a single astrocyte branch covers hundreds to thousands of synapses, it is tempting to speculate that these metabolic interactions could occur localized to specific subdomains of astrocytes, perhaps even at the level of small groups of synapses. We discuss how astrocytic metabolism might be regulated at this scale and which signals might contribute to its regulation. We speculate that the astrocytic structures that light up transiently as Ca 2+ -microdomains might be the functional units of astrocytes linking signaling and metabolic processes to adapt astrocytic function to local energy demands. The understanding of these local regulatory and metabolic interactions will be fundamental to fully appreciate the complexity of brain energy homeostasis as well as its failure in disease and may shed new light on the controversy about neuron-glia bi-directional signaling at the tripartite synapse., (Copyright © 2017 Elsevier Inc. All rights reserved.)

Activation of vascular endothelial small- (KCa2.3, SK3) or intermediate- (KCa3.1, IK1) conductance Ca(2+)-activated potassium channels induces vasorelaxation via an endothelium-derived hyperpolarization (EDH) pathway. Although the activation of SK3 and IK1 channels converges on EDH, their subcellular effects on signal transduction are different and not completely clear. In this study, a novel endothelium-specific SK3 knockout (SK3(-/-)) mouse model was utilized to specifically examine the contribution of SK3 channels to mesenteric artery vasorelaxation, endothelial Ca(2+) dynamics, and blood pressure. The absence of SK3 expression was confirmed using real-time quantitative PCR and Western blot analysis. Functional studies showed impaired EDH-mediated vasorelaxation in SK3(-/-) small mesenteric arteries. Immunostaining results from SK3(-/-) vessels confirmed the absence of SK3 and further showed altered distribution of transient receptor potential channels, type 4 (TRPV4). Electrophysiological recordings showed a lack of SK3 channel activity, while TRPV4-IK1 channel coupling remained intact in SK3(-/-) endothelial cells. Moreover, Ca(2+) imaging studies in SK3(-/-) endothelium showed increased Ca(2+) transients with reduced amplitude and duration under basal conditions. Importantly, SK3(-/-) endothelium lacked a distinct type of Ca(2+) dynamic that is sensitive to TRPV4 activation. Blood pressure measurements showed that the SK3(-/-) mice were hypertensive, and the blood pressure increase was further enhanced during the 12-h dark cycle when animals are most active. Taken together, our results reveal a previously unappreciated SK3 signaling microdomain that modulates endothelial Ca(2+) dynamics, vascular tone, and blood pressure., (Copyright © 2016 the American Physiological Society.)