Automated region of interest analysis of dynamic Ca²+ signals in image sequences

Improved Workflow for Analysis of Vascular Myocyte Time-Series and Line-Scan Ca2+ Imaging Datasets

Intracellular Ca2+ signals are key for the regulation of cellular processes ranging from myocyte contraction, hormonal secretion, neural transmission, cellular metabolism, transcriptional regulation, and cell proliferation. Measurement of cellular Ca2+ is routinely performed using fluorescence microscopy with biological indicators. Analysis of deterministic signals is reasonably straightforward as relevant data can be discriminated based on the timing of cellular responses. However, analysis of stochastic, slower oscillatory events, as well as rapid subcellular Ca2+ responses, takes considerable time and effort which often includes visual analysis by trained investigators, especially when studying signals arising from cells embedded in complex tissues. The purpose of the current study was to determine if full-frame time-series and line-scan image analysis workflow of Fluo-4 generated Ca2+ fluorescence data from vascular myocytes could be automated without introducing errors. This evaluation was addressed by re-analyzing a published "gold standard" full-frame time-series dataset through visual analysis of Ca2+ signals from recordings made in pulmonary arterial myocytes of en face arterial preparations. We applied a combination of data driven and statistical approaches with comparisons to our published data to assess the fidelity of the various approaches. Regions of interest with Ca2+ oscillations were detected automatically post hoc using the LCPro plug-in for ImageJ. Oscillatory signals were separated based on event durations between 4 and 40 s. These data were filtered based on cutoffs obtained from multiple methods and compared to the published manually curated "gold standard" dataset. Subcellular focal and rapid Ca2+ "spark" events from line-scan recordings were examined using SparkLab 5.8, which is a custom automated detection and analysis program. After filtering, the number of true positives, false positives, and false negatives were calculated through comparisons to visually derived "gold standard" datasets. Positive predictive value, sensitivity, and false discovery rates were calculated. There were very few significant differences between the automated and manually curated results with respect to quality of the oscillatory and Ca2+ spark events, and there were no systematic biases in the data curation or filtering techniques. The lack of statistical difference in event quality between manual data curation and statistically derived critical cutoff techniques leads us to believe that automated analysis techniques can be reliably used to analyze spatial and temporal aspects to Ca2+ imaging data, which will improve experiment workflow.

An automated method to discover true events and classification of intracellular Ca2+ profiles for endothelium in situ injury assay

Introduction: Endothelial cells (ECs), being located at the interface between flowing blood and vessel wall, maintain cardiovascular homeostasis by virtue of their ability to integrate chemical and physical cues through a spatio-temporally coordinated increase in their intracellular Ca²⁺ concentration ([Ca²⁺]i). Endothelial heterogeneity suggests the existence of spatially distributed functional clusters of ECs that display different patterns of intracellular Ca²⁺ response to extracellular inputs. Characterizing the overall Ca²⁺ activity of the endothelial monolayer in situ requires the meticulous analysis of hundreds of ECs. This complex analysis consists in detecting and quantifying the true Ca²⁺ events associated to extracellular stimulation and classifying their intracellular Ca²⁺ profiles (ICPs). The injury assay technique allows exploring the Ca²⁺-dependent molecular mechanisms involved in angiogenesis and endothelial regeneration. However, there are true Ca²⁺ events of nearly undetectable magnitude that are almost comparable with inherent instrumental noise. Moreover, undesirable artifacts added to the signal by mechanical injury stimulation complicate the analysis of intracellular Ca²⁺ activity. In general, the study of ICPs lacks uniform criteria and reliable approaches for assessing these highly heterogeneous spatial and temporal events. Methods: Herein, we present an approach to classify ICPs that consists in three stages: 1) identification of Ca²⁺ candidate events through thresholding of a feature termed left-prominence; 2) identification of non-true events, known as artifacts; and 3) ICP classification based upon event temporal location. Results: The performance assessment of true-events identification showed competitive sensitivity = [0.9995, 0.9831], specificity = [0.9946, 0.7818] and accuracy = [0.9978, 0.9579] improvements of 2x and 14x, respectively, compared with other methods. The ICP classifier enhanced by artifact detection showed 0.9252 average accuracy with the ground-truth sets provided for validation. Discussion: Results indicate that our approach ensures sturdiness to experimental protocol maneuvers, besides it is effective, simple, and configurable for different studies that use unidimensional time dependent signals as data. Furthermore, our approach would also be effective to analyze the ICPs generated by other cell types, other dyes, chemical stimulation or even signals recorded at higher frequency.

Algorithm for biological second messenger analysis with dynamic regions of interest

Physiological function is regulated through cellular communication that is facilitated by multiple signaling molecules such as second messengers. Analysis of signal dynamics obtained from cell and tissue imaging is difficult because of intricate spatially and temporally distinct signals. Signal analysis tools based on static region of interest analysis may under- or overestimate signals in relation to region of interest size and location. Therefore, we developed an algorithm for biological signal detection and analysis based on dynamic regions of interest, where time-dependent polygonal regions of interest are automatically assigned to the changing perimeter of detected and segmented signals. This approach allows signal profiles to be rigorously and precisely tracked over time, eliminating the signal distortion observed with static methods. Integration of our approach with state-of-the-art image processing and particle tracking pipelines enabled the isolation of dynamic cellular signaling events and characterization of biological signaling patterns with distinct combinations of parameters including amplitude, duration, and spatial spread. Our algorithm was validated using synthetically generated datasets and compared with other available methods. Application of the algorithm to volumetric time-lapse hyperspectral images of cyclic adenosine monophosphate measurements in rat microvascular endothelial cells revealed distinct signal heterogeneity with respect to cell depth, confirming the utility of our approach for analysis of 5-dimensional data. In human tibial arteries, our approach allowed the identification of distinct calcium signal patterns associated with atherosclerosis. Our algorithm for automated detection and analysis of second messenger signals enables the decoding of signaling patterns in diverse tissues and identification of pathologic cellular responses.

Control of Ca2+ signals by astrocyte nanoscale morphology at tripartite synapses

Much of the Ca²⁺ activity in astrocytes is spatially restricted to microdomains and occurs in fine processes that form a complex anatomical meshwork, the so-called spongiform domain. A growing body of literature indicates that those astrocytic Ca²⁺ signals can influence the activity of neuronal synapses and thus tune the flow of information through neuronal circuits. Because of technical difficulties in accessing the small spatial scale involved, the role of astrocyte morphology on Ca²⁺ microdomain activity remains poorly understood. Here, we use computational tools and idealized 3D geometries of fine processes based on recent super-resolution microscopy data to investigate the mechanistic link between astrocytic nanoscale morphology and local Ca²⁺ activity. Simulations demonstrate that the nano-morphology of astrocytic processes powerfully shapes the spatio-temporal properties of Ca²⁺ signals and promotes local Ca²⁺ activity. The model predicts that this effect is attenuated upon astrocytic swelling, hallmark of brain diseases, which we confirm experimentally in hypo-osmotic conditions. Upon repeated neurotransmitter release events, the model predicts that swelling hinders astrocytic signal propagation. Overall, this study highlights the influence of the complex morphology of astrocytes at the nanoscale and its remodeling in pathological conditions on neuron-astrocyte communication at so-called tripartite synapses, where astrocytic processes come into close contact with pre- and postsynaptic structures.

Reinforcing Interdisciplinary Collaborations to Unravel the Astrocyte "Calcium Code"

In this review article, we present the major insights from and challenges faced in the acquisition, analysis and modeling of astrocyte calcium activity, aiming at bridging the gap between those fields to crack the complex astrocyte “Calcium Code”. We then propose strategies to reinforce interdisciplinary collaborative projects to unravel astrocyte function in health and disease.

Restricted Intimal Ca2+ Signaling Associated With Cardiovascular Disease

Endothelial dysfunction is a key feature of cardiovascular disease (CVD) including atherosclerosis. Impaired endothelial signaling leads to plaque formation, vascular wall remodeling and widespread cardiovascular dysregulation. The specific changes along the vascular intima associated with atherosclerosis, including the vulnerable circulation downstream of the flow obstruction, remain poorly understood. Previous findings from animal models suggest that preservation of a distinct Ca2+ signaling profile along the arterial endothelial network is crucial for maintaining vasculature homeostasis and preventing arterial disease. Ca2+ signaling in the intact human artery intima has not been well characterized. Here, we employed confocal imaging and a custom analysis algorithm to assess the spatially and temporally dynamic Ca2+ signaling profiles of human peripheral arteries isolated from the amputated legs of patients with advanced CVD (peripheral artery disease and/or diabetes) or patients who had lost limbs due to non-cardiovascular trauma. In all tibial artery branches (0.5-5 mm diameter) assessed, the intima consistently elicited a broad range of basal Ca2+ signals ranging from isolated focal transients to broad waves. Arteries from patients with existing CVD displayed a restricted intimal Ca2+ signaling pattern characterized by diminished event amplitude and area. Stimulation of type-4 vanilloid transient receptor potential channels (TRPV4) amplified endothelial Ca2+ signals; however, these signals remained smaller and spatially confined in arteries from patients with CVD verses those without CVD. Our findings reveal a characteristic underlying basal Ca2+ signaling pattern within the intima of human peripheral arteries and suggest a distinct truncation of the inherent Ca2+ profile with CVD.

Long-Term Hypoxia Negatively Influences Ca2+ Signaling in Basilar Arterial Myocytes of Fetal and Adult Sheep

Cerebral arterial vasoreactivity is vital to the regulation of cerebral blood flow. Depolarization of arterial myocytes elicits whole-cell Ca²⁺ oscillations as well as subcellular Ca²⁺ sparks due to activation of ryanodine receptors on the sarcoplasmic reticulum. Previous evidence illustrates that contraction of cerebral arteries from sheep and underlying Ca²⁺ signaling pathways are modified by age and that long-term hypoxia (LTH) causes aberrations in Ca²⁺ signaling pathways and downstream effectors impacting vasoregulation. We hypothesize that age and LTH affect the influence of membrane depolarization on whole-cell intracellular Ca²⁺ oscillations and sub-cellular Ca²⁺ spark activity in cerebral arteries. To test this hypothesis, we examined Ca²⁺ oscillatory and spark activities using confocal fluorescence imaging techniques of Fluo-4 loaded basilar arterial myocytes of low- and high-altitude term fetal (∼145 days of gestation) and adult sheep, where high-altitude pregnant and non-pregnant sheep were placed at 3,801 m for >100 days. Ca²⁺ oscillations and sparks were recorded using an in situ preparation evaluated in the absence or presence of 30 mM K⁺ (30K) to depolarize myocytes. Myocytes from adult animals tended to have a lower basal rate of whole-cell Ca²⁺ oscillatory activity and 30K increased the activity within cells. LTH decreased the ability of myocytes to respond to depolarization independent of age. These observations illustrate that both altitude and age play a role in affecting whole-cell and localized Ca²⁺ signaling, which are important to arterial vasoreactivity and cerebral blood flow.

Measurement of agonist-induced Ca2+ signals in human airway smooth muscle cells using excitation scan-based hyperspectral imaging and image analysis approaches

Ca²⁺ and cAMP are ubiquitous second messengers known to differentially regulate a variety of cellular functions over a wide range of timescales. Studies from a variety of groups support the hypothesis that these signals can be localized to discrete locations within cells, and that this subcellular localization is a critical component of signaling specificity. However, to date, it has been difficult to track second messenger signals at multiple locations. To overcome this limitation, we utilized excitation scan-based hyperspectral imaging approaches to track second messenger signals as well as labeled cellular structures and/or proteins in the same cell. We have previously reported that hyperspectral imaging techniques improve the signal-to-noise ratios of both fluorescence measurements, and are thus well suited for the measurement of localized Ca²⁺ signals. We investigated the spatial spread and intensities of agonist-induced Ca²⁺ signals in primary human airway smooth muscle cells (HASMCs) using the Ca²⁺ indicator Cal520. We measured responses triggered by three agonists, carbachol, histamine, and chloroquine. We utilized custom software coded in MATLAB and Python to assess agonist induced changes in Ca²⁺ levels. Software algorithms removed the background and applied correction coefficients to spectral data prior to linear unmixing, spatial and temporal filtering, adaptive thresholding, and automated region of interest (ROI) detection. All three agonists triggered transient Ca²⁺ responses that were spatially and temporally complex. We are currently analyzing differences in both ROI area and intensity distributions triggered by these agonists. This work was supported by NIH awards P01HL066299, K25HL136869, and R01HL137030 and NSF award MRI1725937.

Ion Channel-Based Reporters for cAMP Detection

In the last 20 years tremendous progress has been made in the development of single cell cAMP sensors. Sensors are based upon cAMP binding proteins that have been modified to transduce cAMP concentrations into electrical or fluorescent readouts that can be readily detected using patch clamp amplifiers, photomultiplier tubes, or cameras. Here, we describe two complementary approaches for the detection and measurement of cAMP signals near the plasma membrane of cells using cyclic nucleotide (CNG) channel-based probes. These probes take advantage of the ability of CNG channels to transduce small changes in cAMP concentration into ionic flux through channel pores that can be readily detected by measuring Ca²⁺ and/or Mn²⁺ influx or by measuring ionic currents.

Analysing Intercellular Communication in Astrocytic Networks Using "Astral"

Astrocytic networks are critically involved in regulating the activity of neuronal networks. However, a comprehensive and ready-to-use data analysis tool for investigating functional interactions between the astrocytes is missing. We developed the novel software package named "Astral" to analyse intercellular communication in astrocytic networks based on live-cell calcium imaging. Our method for analysing calcium imaging data does not require the assignment of regions of interest. The package contains two applications: the core processing pipeline for detecting and quantifying Ca⁺⁺ events, and the auxiliary visualization tool for controlling data quality. Our method allows for the network-wide quantification of Ca⁺⁺ events and the analysis of their intercellular propagation. In a set of proof-of-concept experiments, we examined Ca⁺⁺ events in flat monolayers of primary astrocytes and confirmed that inter-astrocytic interactions depend on the permeability of gap junctions and connexin hemichannels. The Astral tool is particularly useful for studying astrocyte-neuronal interactions on the network level. We demonstrate that compared with purely astrocytic cultures, spontaneous generation of Ca⁺⁺ events in astrocytes that were co-cultivated with neurons was significantly increased. Interestingly, the increased astrocytic Ca⁺⁺ activity after long-term co-cultivation with neurons was driven by the enhanced formation of gap junctions and connexin hemichannels but was not affected by silencing neuronal activity. Our data indicate the necessity for systematic investigation of astrocyte-neuronal interactions at the network level. For this purpose, the Astral software offers a powerful tool for processing and quantifying calcium imaging data.

Gestational long-term hypoxia induces metabolomic reprogramming and phenotypic transformations in fetal sheep pulmonary arteries

Gestational long-term hypoxia increases the risk of myriad diseases in infants including persistent pulmonary hypertension. Similar to humans, fetal lamb lung development is susceptible to long-term intrauterine hypoxia, with structural and functional changes associated with the development of pulmonary hypertension including pulmonary arterial medial wall thickening and dysregulation of arterial reactivity, which culminates in decreased right ventricular output. To further explore the mechanisms associated with hypoxia-induced aberrations in the fetal sheep lung, we examined the premise that metabolomic changes and functional phenotypic transformations occur due to intrauterine, long-term hypoxia. To address this, we performed electron microscopy, Western immunoblotting, calcium imaging, and metabolomic analyses on pulmonary arteries isolated from near-term fetal lambs that had been exposed to low- or high-altitude (3,801 m) hypoxia for the latter 110+ days of gestation. Our results demonstrate that the sarcoplasmic reticulum was swollen with high luminal width and distances to the plasma membrane in the hypoxic group. Hypoxic animals were presented with higher endoplasmic reticulum stress and suppressed calcium storage. Metabolically, hypoxia was associated with lower levels of multiple omega-3 polyunsaturated fatty acids and derived lipid mediators (e.g., eicosapentaenoic acid, docosahexaenoic acid, α-linolenic acid, 5-hydroxyeicosapentaenoic acid (5-HEPE), 12-HEPE, 15-HEPE, prostaglandin E3, and 19(20)-epoxy docosapentaenoic acid) and higher levels of some omega-6 metabolites (P < 0.02) including 15-keto prostaglandin E2 and linoleoylglycerol. Collectively, the results reveal broad evidence for long-term hypoxia-induced metabolic reprogramming and phenotypic transformations in the pulmonary arteries of fetal sheep, conditions that likely contribute to the development of persistent pulmonary hypertension.

A high throughput machine-learning driven analysis of Ca2+ spatio-temporal maps

High-resolution Ca²⁺ imaging to study cellular Ca²⁺ behaviors has led to the creation of large datasets with a profound need for standardized and accurate analysis. To analyze these datasets, spatio-temporal maps (STMaps) that allow for 2D visualization of Ca²⁺ signals as a function of time and space are often used. Methods of STMap analysis rely on a highly arduous process of user defined segmentation and event-based data retrieval. These methods are often time consuming, lack accuracy, and are extremely variable between users. We designed a novel automated machine-learning based plugin for the analysis of Ca²⁺ STMaps (STMapAuto). The plugin includes optimized tools for Ca²⁺ signal preprocessing, automated segmentation, and automated extraction of key Ca²⁺ event information such as duration, spatial spread, frequency, propagation angle, and intensity in a variety of cell types including the Interstitial cells of Cajal (ICC). The plugin is fully implemented in Fiji and able to accurately detect and expeditiously quantify Ca²⁺ transient parameters from ICC. The plugin's speed of analysis of large-datasets was 197-fold faster than the commonly used single pixel-line method of analysis. The automated machine-learning based plugin described dramatically reduces opportunities for user error and provides a consistent method to allow high-throughput analysis of STMap datasets.

PunctaSpecks: A tool for automated detection, tracking, and analysis of multiple types of fluorescently labeled biomolecules

Recent advances in imaging technology and fluorescent probes have made it possible to gain information about the dynamics of subcellular processes at unprecedented spatiotemporal scales. Unfortunately, a lack of automated tools to efficiently process the resulting imaging data encoding fine details of the biological processes remains a major bottleneck in utilizing the full potential of these powerful experimental techniques. Here we present a computational tool, called PunctaSpecks, that can characterize fluorescence signals arising from a wide range of biological molecules under normal and pathological conditions. Among other things, the program can calculate the number, areas, life-times, and amplitudes of fluorescence signals arising from multiple sources, track diffusing fluorescence sources like moving mitochondria, and determine the overlap probability of two processes or organelles imaged using indicator dyes of different colors. We have tested PunctaSpecks on synthetic time-lapse movies containing mobile fluorescence objects of various sizes, mimicking the activity of biomolecules. The robustness of the software is tested by varying the level of noise along with random but known pattern of appearing, disappearing, and movement of these objects. Next, we use PunctaSpecks to characterize protein-protein interaction involved in store-operated Ca2+ entry through the formation and activation of plasma membrane-bound ORAI1 channel and endoplasmic reticulum membrane-bound stromal interaction molecule (STIM), the evolution of inositol 1,4,5-trisphosphate (IP3)-induced Ca2+ signals from sub-micrometer size local events into global waves in human cortical neurons, and the activity of Alzheimer's disease-associated β amyloid pores in the plasma membrane. The tool can also be used to study other dynamical processes imaged through fluorescence molecules. The open source algorithm allows for extending the program to analyze more than two types of biomolecules visualized using markers of different colors.

Structural basis of astrocytic Ca2+ signals at tripartite synapses

Astrocytic Ca²⁺ signals can be fast and local, supporting the idea that astrocytes have the ability to regulate single synapses. However, the anatomical basis of such specific signaling remains unclear, owing to difficulties in resolving the spongiform domain of astrocytes where most tripartite synapses are located. Using 3D-STED microscopy in living organotypic brain slices, we imaged the spongiform domain of astrocytes and observed a reticular meshwork of nodes and shafts that often formed loop-like structures. These anatomical features were also observed in acute hippocampal slices and in barrel cortex in vivo. The majority of dendritic spines were contacted by nodes and their sizes were correlated. FRAP experiments and Ca²⁺ imaging showed that nodes were biochemical compartments and Ca²⁺ microdomains. Mapping astrocytic Ca²⁺ signals onto STED images of nodes and dendritic spines showed they were associated with individual synapses. Here, we report on the nanoscale organization of astrocytes, identifying nodes as a functional astrocytic component of tripartite synapses that may enable synapse-specific communication between neurons and astrocytes.

Astrocytic Ca²⁺ signals can be fast and local, supporting the idea that astrocytes have the ability to regulate single synapses. Here, the authors report the organization of astrocytes at nanoscale level and identify nodes as a functional astrocytic component of tripartite synapses.

Ablation of Endothelial TRPV4 Channels Alters the Dynamic Ca2+ Signaling Profile in Mouse Carotid Arteries

Transient receptor potential vanilloid 4 channels (TRPV4) are pivotal regulators of vascular homeostasis. Altered TRPV4 signaling has recently been implicated in various cardiovascular diseases, including hypertension and atherosclerosis. These versatile nonselective cation channels increase endothelial Ca²⁺ influx in response to various stimuli including shear stress and G protein-coupled receptor (GPCR) activation. Recent findings suggest TRPV4 channels produce localized Ca²⁺ transients at the endothelial cell plasma membrane that may allow targeted effector recruitment and promote large-scale Ca²⁺ events via release from internal stores (endoplasmic reticulum). However, the specific impact of TRPV4 channels on Ca²⁺ signaling in the intact arterial intima remains unknown. In the current study, we employ an endothelium-specific TRPV4 knockout mouse model (ecTRPV4^-/-) to identify and characterize TRPV4-dependent endothelial Ca²⁺ dynamics. We find that carotid arteries from both ecTRPV4^-/- and WT mice exhibit a range of basal and acetylcholine (ACh)-induced Ca²⁺ dynamics, similar in net frequency. Analysis of discrete Ca²⁺ event parameters (amplitude, duration, and spread) and event composite values reveals that while ecTRPV4^-/- artery endothelium predominantly produces large Ca²⁺ events comparable to and in excess of those produced by WT endothelium, they are deficient in a particular population of small events, under both basal and ACh-stimulated conditions. These findings support the concept that TRPV4 channels are responsible for generating a distinct population of focal Ca²⁺ transients in the intact arterial endothelium, likely underlying their essential role in vascular homeostasis.

Calcium Mobilization in Endothelial Cell Functions

Endothelial cells (ECs) constitute the innermost layer that lines all blood vessels from the larger arteries and veins to the smallest capillaries, including the lymphatic vessels. Despite the histological classification of endothelium of a simple epithelium and its homogeneous morphological appearance throughout the vascular system, ECs, instead, are extremely heterogeneous both structurally and functionally. The different arrangement of cell junctions between ECs and the local organization of the basal membrane generate different type of endothelium with different permeability features and functions. Continuous, fenestrated and discontinuous endothelia are distributed based on the specific function carried out by the organs. It is thought that a large number ECs functions and their responses to extracellular cues depend on changes in intracellular concentrations of calcium ion ([Ca²⁺]_i). The extremely complex calcium machinery includes plasma membrane bound channels as well as intracellular receptors distributed in distinct cytosolic compartments that act jointly to maintain a physiological [Ca²⁺]_i, which is crucial for triggering many cellular mechanisms. Here, we first survey the overall notions related to intracellular Ca²⁺ mobilization and later highlight the involvement of this second messenger in crucial ECs functions with the aim at stimulating further investigation that link Ca²⁺ mobilization to ECs in health and disease.

Calcium signals that determine vascular resistance

Small arteries in the body control vascular resistance, and therefore, blood pressure and blood flow. Endothelial and smooth muscle cells in the arterial walls respond to various stimuli by altering the vascular resistance on a moment to moment basis. Smooth muscle cells can directly influence arterial diameter by contracting or relaxing, whereas endothelial cells that line the inner walls of the arteries modulate the contractile state of surrounding smooth muscle cells. Cytosolic calcium is a key driver of endothelial and smooth muscle cell functions. Cytosolic calcium can be increased either by calcium release from intracellular stores through IP3 or ryanodine receptors, or the influx of extracellular calcium through ion channels at the cell membrane. Depending on the cell type, spatial localization, source of a calcium signal, and the calcium-sensitive target activated, a particular calcium signal can dilate or constrict the arteries. Calcium signals in the vasculature can be classified into several types based on their source, kinetics, and spatial and temporal properties. The calcium signaling mechanisms in smooth muscle and endothelial cells have been extensively studied in the native or freshly isolated cells, therefore, this review is limited to the discussions of studies in native or freshly isolated cells. This article is categorized under: Biological Mechanisms > Cell Signaling Laboratory Methods and Technologies > Imaging Models of Systems Properties and Processes > Mechanistic Models.

Long-Term High-Altitude Hypoxia and Alpha Adrenoceptor-Dependent Pulmonary Arterial Contractions in Fetal and Adult Sheep

Autonomic innervation of the pulmonary vasculature triggers vasomotor contractility predominately through activation of alpha-adrenergic receptors (α-ARs) in the fetal circulation. Long-term hypoxia (LTH) modulates pulmonary vasoconstriction potentially through upregulation of α₁-AR in the vasculature. Our study aimed to elucidate the role of α-AR in phenylephrine (PE)-induced pulmonary vascular contractility, comparing the effects of LTH in the fetal and adult periods on α-AR subtypes and PE-mediated Ca²⁺ responses and contractions. To address this, we performed wire myography, Ca²⁺ imaging, and mRNA analysis of pulmonary arteries from ewes and fetuses exposed to LTH or normoxia. Postnatal maturation depressed PE-mediated contractile responses. α₂-AR activation contracted fetal vessels; however, this was suppressed by LTH. α_1A- and α_1B-AR subtypes contributed to arterial contractions in all groups. The α_1D-AR was also important to contractility in fetal normoxic vessels and LTH mitigated its function. Postnatal maturity increased the number of myocytes with PE-triggered Ca²⁺ responses while LTH decreased the percentage of fetal myocytes reacting to PE. The difference between myocyte Ca²⁺ responsiveness and vessel contractility suggests that fetal arteries are sensitized to changes in Ca²⁺. The results illustrate that α-adrenergic signaling and vascular function change during development and that LTH modifies adrenergic signaling. These changes may represent components in the etiology of pulmonary vascular disease and foretell the therapeutic potential of adrenergic receptor antagonists in the treatment of pulmonary hypertension.

Quantifying Ca2+ signaling and contraction in vascular pericytes and smooth muscle cells

Vascular pericytes and smooth muscle cells surround many blood vessels of the body. Their primary roles include vessel stabilization and regulation of the blood flow. The high degree of heterogeneity among these cells is dictated by (1) differences in their developmental origin and (2) their location in the vascular bed. Phenotype switching contributes to this heterogeneity especially following in vitro culture. In the absence of distinguishing molecular markers, functional assays that capture their heterogeneity in vitro are needed. Spatiotemporal changes in intracellular Ca²⁺ levels and contraction in response to vasoconstrictors reflect the differences between vascular pericyte and smooth muscle cell. In order to capture this heterogeneity in vitro, large ensembles of cells need to be analyzed. Here we developed an automated image processing method to measure intracellular Ca²⁺ and contraction in large cell groups which in combination with a computational approach for integrative analysis allowed vascular pericytes and smooth muscle cells to be distinguished without knowledge of their anatomical origin.

Functional Interaction among KCa and TRP Channels for Cardiovascular Physiology: Modern Perspectives on Aging and Chronic Disease

Effective delivery of oxygen and essential nutrients to vital organs and tissues throughout the body requires adequate blood flow supplied through resistance vessels. The intimate relationship between intracellular calcium ([Ca²⁺]_i) and regulation of membrane potential (V_m) is indispensable for maintaining blood flow regulation. In particular, Ca²⁺-activated K⁺ (K_Ca) channels were ascertained as transducers of elevated [Ca²⁺]_i signals into hyperpolarization of V_m as a pathway for decreasing vascular resistance, thereby enhancing blood flow. Recent evidence also supports the reverse role for K_Ca channels, in which they facilitate Ca²⁺ influx into the cell interior through open non-selective cation (e.g., transient receptor potential; TRP) channels in accord with robust electrical (hyperpolarization) and concentration (~20,000-fold) transmembrane gradients for Ca²⁺. Such an arrangement supports a feed-forward activation of V_m hyperpolarization while potentially boosting production of nitric oxide. Furthermore, in vascular types expressing TRP channels but deficient in functional K_Ca channels (e.g., collecting lymphatic endothelium), there are profound alterations such as downstream depolarizing ionic fluxes and the absence of dynamic hyperpolarizing events. Altogether, this review is a refined set of evidence-based perspectives focused on the role of the endothelial K_Ca and TRP channels throughout multiple experimental animal models and vascular types. We discuss the diverse interactions among K_Ca and TRP channels to integrate Ca²⁺, oxidative, and electrical signaling in the context of cardiovascular physiology and pathology. Building from a foundation of cellular biophysical data throughout a wide and diverse compilation of significant discoveries, a translational narrative is provided for readers toward the treatment and prevention of chronic, age-related cardiovascular disease.

Quantitative Analysis of Intracellular Ca2+ Release and Contraction in hiPSC-Derived Vascular Smooth Muscle Cells

Vascular smooth muscle cells (vSMCs) are highly heterogeneous across different vascular beds. This is partly dictated by their developmental origin but also their position in the vascular tree, reflected in their differential responses to vasoactive agonists depending on which arteriolar or venular segment they are located. Functional assays are necessary to capture this heterogeneity in vitro since there are no markers that distinguish subtypes. Here we describe methods for determining real-time intracellular Ca²⁺ release and contraction in vSMCs of neural crest origin differentiated from human induced pluripotent stem cells using multiple protocols, and compare these with primary human brain vascular pericytes and smooth muscle cells. Open-source software was adapted for automated high-density analysis of Ca²⁺-release kinetics and contraction by tracking individual cells. Simultaneous measurements on hundreds of cells revealed heterogeneity in responses to vasoconstrictors that would likely be overlooked using manual low-throughput assays or marker expression.

VasoTracker, a Low-Cost and Open Source Pressure Myograph System for Vascular Physiology

Pressure myography, one of the most commonly used techniques in vascular research, measures the diameter of isolated, pressurized arteries to assess the functional activity of smooth muscle and endothelial cells. Despite the widespread adoption of this technique for assessing vascular function, there are only a small number of commercial systems and these are expensive. Here, we introduce a complete, open source pressure myograph system and analysis software, VasoTracker, that can be set-up for approximately 10% of the cost of commercial alternatives. We report on the development of VasoTracker and demonstrate its ability to assess various components of vascular reactivity. A unique feature of the VasoTracker platform is the publicly accessible website (http://www.vasotracker.com/) that documents how to assemble and use this affordable, adaptable, and expandable pressure myograph.

CellSpecks: A Software for Automated Detection and Analysis of Calcium Channels in Live Cells

To couple the fidelity of patch-clamp recording with a more high-throughput screening capability, we pioneered a, to our knowledge, novel approach to single-channel recording that we named "optical patch clamp." By using highly sensitive fluorescent Ca²⁺ indicator dyes in conjunction with total internal fluorescence microscopy techniques, we monitor Ca²⁺ flux through individual Ca²⁺-permeable channels. This approach provides information about channel gating analogous to patch-clamp recording at a time resolution of ∼2 ms with the additional advantage of being massively parallel, providing simultaneous and independent recording from thousands of channels in the native environment. However, manual analysis of the data generated by this technique presents severe challenges because a video recording can include many thousands of frames. To overcome this bottleneck, we developed an image processing and analysis framework called CellSpecks capable of detecting and fully analyzing the kinetics of ion channels within a video sequence. By using randomly generated synthetic data, we tested the ability of CellSpecks to rapidly and efficiently detect and analyze the activity of thousands of ion channels, including openings for a few milliseconds. Here, we report the use of CellSpecks for the analysis of experimental data acquired by imaging muscle nicotinic acetylcholine receptors and the Alzheimer's disease-associated amyloid β pores with multiconductance levels in the plasma membrane of Xenopus laevis oocytes. We show that CellSpecks can accurately and efficiently generate location maps and create raw and processed fluorescence time traces; histograms of mean open times, mean close times, open probabilities, durations, and maximal amplitudes; and a "channel chip" showing the activity of all channels as a function of time. Although we specifically illustrate the application of CellSpecks for analyzing data from Ca²⁺ channels, it can be easily customized to analyze other spatially and temporally localized signals.

Long-term hypoxia uncouples Ca2+ and eNOS in bradykinin-mediated pulmonary arterial relaxation

Bradykinin-induced activation of the pulmonary endothelium triggers a rise in intracellular Ca²⁺ that activates nitric oxide (NO)-dependent vasorelaxation. Chronic hypoxia is commonly associated with increased pulmonary vascular tone, which can cause pulmonary hypertension in responsive individuals. In the present study, we tested the hypothesis that long-term high-altitude hypoxia (LTH) diminishes bradykinin-induced Ca²⁺ signals and inhibits endothelial nitric oxide synthase (eNOS), prostacyclin (PGI₂), and large-conductance K⁺ (BK_Ca) channels in sheep, which are moderately responsive to LTH, resulting in decreased pulmonary arterial vasorelaxation. Pulmonary arteries were isolated from ewes kept near sea level (720 m) or at high altitude (3,801 m) for >100 days. Vessel force was measured with wire myography and endothelial intracellular Ca²⁺ with confocal microscopy. eNOS was inhibited with 100 μM N^G-nitro-l-arginine methyl ester (l-NAME), PGI₂ production was inhibited with 10 µM indomethacin that inhibits cyclooxygenase, and BK_Ca channels were blocked with 1 mM tetraethylammonium. Bradykinin-induced endothelial Ca²⁺ signals increased following LTH, but bradykinin relaxation decreased. Furthermore, some vessels contracted in response to bradykinin after LTH. l-NAME sensitivity decreased, suggesting that eNOS dysfunction played a role in uncoupling Ca²⁺ signals and bradykinin relaxation. The Ca²⁺ ionophore A-23187 (10 µM) elicited an enhanced Ca²⁺ response following LTH while relaxation was unchanged although l-NAME sensitivity increased. Additionally, BK_Ca function decreased during bradykinin relaxation following LTH. Western analysis showed that BK_Ca α-subunit expression was increased by LTH while that for the β₁ subunit was unchanged. Overall, these results suggest that those even moderately responsive to LTH can have impaired endothelial function.

Long-term high-altitude hypoxia influences pulmonary arterial L-type calcium channel-mediated Ca2+ signals and contraction in fetal and adult sheep

Long-term hypoxia (LTH) has a profound effect on pulmonary arterial vasoconstriction in the fetus and adult. Dysregulation in Ca²⁺ signaling is important during the development of LTH-induced pulmonary hypertension. In the present study, we tested the hypothesis that L-type Ca²⁺ channels (Ca_L), which are voltage dependent and found in smooth, skeletal, and cardiac muscle, are important in the adaptation of pulmonary arterial contractions in postnatal maturation and in response to LTH. Pulmonary arteries were isolated from fetal or adult sheep maintained at low or high altitude (3,801 m) for >100 days. The effects were measured using an L-type Ca²⁺ channel opener FPL 64176 (FPL) in the presence or absence of an inhibitor, Nifedipine (NIF) on arterial contractions, intracellular Ca²⁺ oscillations, and ryanodine receptor-driven Ca²⁺ sparks. FPL induced pulmonary arterial contractions in all groups were sensitive to NIF. However, when compared with 125 mM K⁺, FPL contractions were greater in fetuses than in adults. FPL reduced Ca²⁺ oscillations in myocytes of adult but not fetal arteries, independently of altitude. The FPL effects on Ca²⁺ oscillations were reversed by NIF in myocytes of hypoxic but not normoxic adults. FPL failed to enhance Ca²⁺ spark frequency and had little impact on spatiotemporal firing characteristics. These data suggest that Ca_L-dependent contractions are largely uncoupled from intracellular Ca²⁺ oscillations and the development of Ca²⁺ sparks. This raises questions regarding the coupling of pulmonary arterial contractility to membrane depolarization, attendant Ca_L facilitation, and the related associations with the activation of Ca²⁺ oscillations and Ca²⁺ sparks.

Permeability and calcium signaling in lung endothelium: unpack the box…

This brief review assesses the role of Ca²⁺ signaling in lung endothelium in regulation of endothelial permeability. The disconnect between experimental and clinical outcomes to date may be due, in part, to the use of tools which yield information about aggregate permeability or Ca²⁺ responses in lung or in endothelial monolayers. The teaching point of this review is to “unpack the box,” i.e. consider the many potential issues which could impact interpretation of outcomes. These include phenotypic heterogeneity and resultant segment-specific permeability responses, methodologic issues related to permeability measures, contributions from Ca²⁺ channels in cells other than endothelium—such as alveolar macrophages or blood leukocytes), Ca²⁺ dynamic patterns, rather than averaged Ca²⁺ responses to channel activation, and the background context, such as changes in endothelial bioenergetics with sepsis. Any or all of these issues might color interpretation of permeability and Ca²⁺ signaling in lung.

Changes in vascular reactivity and endothelial Ca2+ dynamics with chronic low flow

Disruption of blood flow promotes endothelial dysfunction and predisposes vessels to remodeling and atherosclerosis. Recent findings suggest that spatial and temporal tuning of local Ca2+ signals along the endothelium is vital to vascular function. In this study, we examined whether chronic flow disruption causes alteration of dynamic endothelial Ca2+ signal patterning associated with changes in vascular structure and function. For these studies, we performed surgical PL of the left carotid arteries of mice to establish chronic low flow for 2 weeks; right carotid arteries remained open and served as controls (C). Histological sections showed substantial remodeling of PL compared to C arteries, including formation of neointima. Isometric force measurements revealed increased PE-induced contractions and decreased KCl-induced contractions in PL vs C arteries. Endothelium-dependent vasorelaxation in response to ACh; 10-8 to 10-5  mol/L) was significantly impaired in PL vs C vessels. Evaluation of endothelial Ca2+ using confocal imaging and custom analysis exposed distinct impairment of Ca2+ dynamics in PL arteries, characterized by reduction in active sites and truncation of events, corresponding to attenuated vasorelaxation. Our findings suggest that endothelial dysfunction in developing vascular disease may be characterized by distinct shifts in the spatial and temporal patterns of localized Ca2+ signals.

Astrocytic IP3 Rs: Contribution to Ca2+ signalling and hippocampal LTP

Astrocytes regulate hippocampal synaptic plasticity by the Ca²⁺ dependent release of the N-methyl d-aspartate receptor (NMDAR) co-agonist d-serine. Previous evidence indicated that d-serine release would be regulated by the intracellular Ca²⁺ release channel IP₃ receptor (IP₃ R), however, genetic deletion of IP₃ R2, the putative astrocytic IP₃ R subtype, had no impact on synaptic plasticity or transmission. Although IP₃ R2 is widely believed to be the only functional IP₃ R in astrocytes, three IP₃ R subtypes (1, 2, and 3) have been identified in vertebrates. Therefore, to better understand gliotransmission, we investigated the functionality of IP₃ R and the contribution of the three IP₃ R subtypes to Ca²⁺ signalling. As a proxy for gliotransmission, we found that long-term potentiation (LTP) was impaired by dialyzing astrocytes with the broad IP₃ R blocker heparin, and rescued by exogenous d-serine, indicating that astrocytic IP₃ Rs regulate d-serine release. To explore which IP₃ R subtypes are functional in astrocytes, we used pharmacology and two-photon Ca²⁺ imaging of hippocampal slices from transgenic mice (IP₃ R2^-/- and IP₃ R2^-/- ;3^-/- ). This approach revealed that underneath IP₃ R2-mediated global Ca²⁺ events are an overlooked class of IP₃ R-mediated local events, occurring in astroglial processes. Notably, multiple IP₃ Rs were recruited by high frequency stimulation of the Schaffer collaterals, a classical LTP induction protocol. Together, these findings show the dependence of LTP and gliotransmission on Ca²⁺ release by astrocytic IP₃ Rs. GLIA 2017;65:502-513.

Acetylcholine released by endothelial cells facilitates flow-mediated dilatation

KEY POINTS

The endothelium plays a pivotal role in the vascular response to chemical and mechanical stimuli. The endothelium is exquisitely sensitive to ACh, although the physiological significance of ACh-induced activation of the endothelium is unknown. In the present study, we investigated the mechanisms of flow-mediated endothelial calcium signalling. Our data establish that flow-mediated endothelial calcium responses arise from the autocrine action of non-neuronal ACh released by the endothelium.

ABSTRACT

Circulating blood generates frictional forces (shear stress) on the walls of blood vessels. These frictional forces critically regulate vascular function. The endothelium senses these frictional forces and, in response, releases various vasodilators that relax smooth muscle cells in a process termed flow-mediated dilatation. Although some elements of the signalling mechanisms have been identified, precisely how flow is sensed and transduced to cause the release of relaxing factors is poorly understood. By imaging signalling in large areas of the endothelium of intact arteries, we show that the endothelium responds to flow by releasing ACh. Once liberated, ACh acts to trigger calcium release from the internal store in endothelial cells, nitric oxide production and artery relaxation. Flow-activated release of ACh from the endothelium is non-vesicular and occurs via organic cation transporters. ACh is generated following mitochondrial production of acetylCoA. Thus, we show ACh is an autocrine signalling molecule released from endothelial cells, and identify a new role for the classical neurotransmitter in endothelial mechanotransduction.

Dissection of local Ca(2+) signals inside cytosol by ER-targeted Ca(2+) indicator

Calcium (Ca(2+)) is a versatile intracellular second messenger that operates in various signaling pathways leading to multiple biological outputs. The diversity of spatiotemporal patterns of Ca(2+) signals, generated by the coordination of Ca(2+) influx from the extracellular space and Ca(2+) release from the intracellular Ca(2+) store the endoplasmic reticulum (ER), is considered to underlie the diversity of biological outputs caused by a single signaling molecule. However, such Ca(2+) signaling diversity has not been well described because of technical limitations. Here, we describe a new method to report Ca(2+) signals at subcellular resolution. We report that OER-GCaMP6f, a genetically encoded Ca(2+) indicator (GECI) targeted to the outer ER membrane, can monitor Ca(2+) release from the ER at higher spatiotemporal resolution than conventional GCaMP6f. OER-GCaMP6f was used for in vivo Ca(2+) imaging of C. elegans. We also found that the spontaneous Ca(2+) elevation in cultured astrocytes reported by OER-GCaMP6f showed a distinct spatiotemporal pattern from that monitored by plasma membrane-targeted GCaMP6f (Lck-GCaMP6f); less frequent Ca(2+) signal was detected by OER-GCaMP6f, in spite of the fact that Ca(2+) release from the ER plays important roles in astrocytes. These findings suggest that targeting of GECIs to the ER outer membrane enables sensitive detection of Ca(2+) release from the ER at subcellular resolution, avoiding the diffusion of GECI and Ca(2+). Our results indicate that Ca(2+) imaging with OER-GCaMP6f in combination with Lck-GCaMP6f can contribute to describing the diversity of Ca(2+) signals, by enabling dissection of Ca(2+) signals at subcellular resolution.

Transient Receptor Potential Channel 4 Encodes a Vascular Permeability Defect and High-Frequency Ca(2+) Transients in Severe Pulmonary Arterial Hypertension

The canonical transient receptor potential channel 4 (TRPC4) comprises an endothelial store-operated Ca(2+) entry channel, and TRPC4 inactivation confers a survival benefit in pulmonary arterial hypertension (PAH). Endothelial Ca(2+) signals mediated by TRPC4 enhance vascular permeability in vitro, but the contribution of TRPC4-dependent Ca(2+) signals to the regulation of endothelial permeability in PAH is poorly understood. We tested the hypothesis that TRPC4 increases vascular permeability and alters the frequency of endothelial Ca(2+) transients in PAH. We measured permeability in isolated lungs, and found that TRPC4 exaggerated permeability responses to thapsigargin in Sugen/hypoxia-treated PAH rats. We compared endothelial Ca(2+) activity of wild-type with TRPC4-knockout rats using confocal microscopy, and evaluated how Ca(2+) signals were influenced in response to thapsigargin and sequential treatment with acetylcholine. We found that thapsigargin-stimulated Ca(2+) signals were increased in PAH, and recovered by TRPC4 inactivation. Store depletion revealed bimodal Ca(2+) responses to acetylcholine, with both short- and long-duration populations. Our results show that TRPC4 underlies an exaggerated endothelial permeability response in PAH. Furthermore, TRPC4 increased the frequency of endothelial Ca(2+) transients in severe PAH, suggesting that TRPC4 provides a Ca(2+) source associated with endothelial dysfunction in the pathophysiology of PAH. This phenomenon represents a new facet of the etiology of PAH, and may contribute to PAH vasculopathy by enabling inflammatory mediator flux across the endothelial barrier.

Functional Tuning of Intrinsic Endothelial Ca2+ Dynamics in Swine Coronary Arteries

RATIONALE

Recent data from mesenteric and cerebral beds have revealed spatially restricted Ca(2+) transients occurring along the vascular intima that control effector recruitment and vasodilation. Although Ca(2+) is pivotal for coronary artery endothelial function, spatial and temporal regulation of functional Ca(2+) signals in the coronary endothelium is poorly understood.

OBJECTIVE

We aimed to determine whether a discrete spatial and temporal profile of Ca(2+) dynamics underlies endothelium-dependent relaxation of swine coronary arteries.

METHODS AND RESULTS

Using confocal imaging, custom automated image analysis, and myography, we show that the swine coronary artery endothelium generates discrete basal Ca(2+) dynamics, including isolated transients and whole-cell propagating waves. These events are suppressed by depletion of internal stores or inhibition of inositol 1,4,5-trisphosphate receptors but not by inhibition of ryanodine receptors or removal of extracellular Ca(2+). In vessel rings, inhibition of specific Ca(2+)-dependent endothelial effectors, namely, small and intermediate conductance K(+) channels (K(Ca)3.1 and K(Ca)2.3) and endothelial nitric oxide synthase, produces additive tone, which is blunted by internal store depletion or inositol 1,4,5-trisphosphate receptor blockade. Stimulation of endothelial inositol 1,4,5-trisphosphate-dependent signaling with substance P causes idiosyncratic changes in dynamic Ca(2+) signal parameters (active sites, event frequency, amplitude, duration, and spatial spread). Overall, substance P-induced vasorelaxation corresponded poorly with whole-field endothelial Ca(2+) measurements but corresponded precisely with the concentration-dependent change in Ca(2+) dynamics (linearly translated composite of dynamic parameters).

CONCLUSIONS

Our findings show that endothelium-dependent control of swine coronary artery tone is determined by spatial and temporal titration of inherent endothelial Ca(2+) dynamics that are not represented by tissue-level averaged Ca(2+) changes.

Endothelial SK3 channel-associated Ca2+ microdomains modulate blood pressure

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.

Pressure-dependent regulation of Ca2+ signalling in the vascular endothelium

Key points

Increased pressure suppresses endothelial control of vascular tone but it remains uncertain (1) how pressure is sensed by the endothelium and (2) how the vascular response is inhibited.

This study used a novel imaging method to study large numbers of endothelial cells in arteries that were in a physiological configuration and held at normal blood pressures.

Increased pressure suppressed endothelial IP₃‐mediated Ca²⁺ signals.

Pressure modulated endothelial cell shape.

The changes in cell shape may alter endothelial Ca²⁺ signals by modulating the diffusive environment for Ca²⁺ near IP₃ receptors.

Endothelial pressure‐dependent mechanosensing may occur without a requirement for a conventional molecular mechanoreceptor.

Abstract

The endothelium is an interconnected network upon which haemodynamic mechanical forces act to control vascular tone and remodelling in disease. Ca²⁺ signalling is central to the endothelium's mechanotransduction and networked activity. However, challenges in imaging Ca²⁺ in large numbers of endothelial cells under conditions that preserve the intact physical configuration of pressurized arteries have limited progress in understanding how pressure‐dependent mechanical forces alter networked Ca²⁺ signalling. We developed a miniature wide‐field, gradient‐index (GRIN) optical probe designed to fit inside an intact pressurized artery that permitted Ca²⁺ signals to be imaged with subcellular resolution in a large number (∼200) of naturally connected endothelial cells at various pressures. Chemical (acetylcholine) activation triggered spatiotemporally complex, propagating inositol trisphosphate (IP₃)‐mediated Ca²⁺ waves that originated in clusters of cells and progressed from there across the endothelium. Mechanical stimulation of the artery, by increased intraluminal pressure, flattened the endothelial cells and suppressed IP₃‐mediated Ca²⁺ signals in all activated cells. By computationally modelling Ca²⁺ release, endothelial shape changes were shown to alter the geometry of the Ca²⁺ diffusive environment near IP₃ receptor microdomains to limit IP₃‐mediated Ca²⁺ signals as pressure increased. Changes in cell shape produce a geometric microdomain regulation of IP₃‐mediated Ca²⁺ signalling to explain macroscopic pressure‐dependent, endothelial mechanosensing without the need for a conventional mechanoreceptor. The suppression of IP₃‐mediated Ca²⁺ signalling may explain the decrease in endothelial activity as pressure increases. GRIN imaging provides a convenient method that gives access to hundreds of endothelial cells in intact arteries in physiological configuration.

Increased pressure suppresses endothelial control of vascular tone but it remains uncertain (1) how pressure is sensed by the endothelium and (2) how the vascular response is inhibited.

This study used a novel imaging method to study large numbers of endothelial cells in arteries that were in a physiological configuration and held at normal blood pressures.

Increased pressure suppressed endothelial IP₃‐mediated Ca²⁺ signals.

Pressure modulated endothelial cell shape.

The changes in cell shape may alter endothelial Ca²⁺ signals by modulating the diffusive environment for Ca²⁺ near IP₃ receptors.

Endothelial pressure‐dependent mechanosensing may occur without a requirement for a conventional molecular mechanoreceptor.

Developmental acceleration of bradykinin-dependent relaxation by prenatal chronic hypoxia impedes normal development after birth

Bradykinin-induced activation of the pulmonary endothelium triggers nitric oxide production and other signals that cause vasorelaxation, including stimulation of large-conductance Ca(2+)-activated K(+) (BKCa) channels in myocytes that hyperpolarize the plasma membrane and decrease intracellular Ca(2+). Intrauterine chronic hypoxia (CH) may reduce vasorelaxation in the fetal-to-newborn transition and contribute to pulmonary hypertension of the newborn. Thus we examined the effects of maturation and CH on the role of BKCa channels during bradykinin-induced vasorelaxation by examining endothelial Ca(2+) signals, wire myography, and Western immunoblots on pulmonary arteries isolated from near-term fetal (∼ 140 days gestation) and newborn, 10- to 20-day-old, sheep that lived in normoxia at 700 m or in CH at high altitude (3,801 m) for >100 days. CH enhanced bradykinin-induced relaxation of fetal vessels but decreased relaxation in newborns. Endothelial Ca(2+) responses decreased with maturation but increased with CH. Bradykinin-dependent relaxation was sensitive to 100 μM nitro-L-arginine methyl ester or 10 μM 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, supporting roles for endothelial nitric oxide synthase and soluble guanylate cyclase activation. Indomethacin blocked relaxation in CH vessels, suggesting upregulation of PLA2 pathways. BKCa channel inhibition with 1 mM tetraethylammonium reduced bradykinin-induced vasorelaxation in the normoxic newborn and fetal CH vessels. Maturation reduced whole cell BKCa channel α1-subunit expression but increased β1-subunit expression. These results suggest that CH amplifies the contribution of BKCa channels to bradykinin-induced vasorelaxation in fetal sheep but stunts further development of this vasodilatory pathway in newborns. This involves complex changes in multiple components of the bradykinin-signaling axes.

CalQuo: automated, simultaneous single-cell and population-level quantification of global intracellular Ca2+ responses

Detecting intracellular calcium signaling with fluorescent calcium indicator dyes is often coupled with microscopy techniques to follow the activation state of non-excitable cells, including lymphocytes. However, the analysis of global intracellular calcium responses both at the single-cell level and in large ensembles simultaneously has yet to be automated. Here, we present a new software package, CalQuo (Calcium Quantification), which allows the automated analysis and simultaneous monitoring of global fluorescent calcium reporter-based signaling responses in up to 1000 single cells per experiment, at temporal resolutions of sub-seconds to seconds. CalQuo quantifies the number and fraction of responding cells, the temporal dependence of calcium signaling and provides global and individual calcium-reporter fluorescence intensity profiles. We demonstrate the utility of the new method by comparing the calcium-based signaling responses of genetically manipulated human lymphocytic cell lines.

Genetically-encoded tools for cAMP probing and modulation in living systems

Intracellular 3′-5′-cyclic adenosine monophosphate (cAMP) is one of the principal second messengers downstream of a manifold of signal transduction pathways, including the ones triggered by G protein-coupled receptors. Not surprisingly, biochemical assays for cAMP have been instrumental for basic research and drug discovery for decades, providing insights into cellular physiology and guiding pharmaceutical industry. However, despite impressive track record, the majority of conventional biochemical tools for cAMP probing share the same fundamental shortcoming—all the measurements require sample disruption for cAMP liberation. This common bottleneck, together with inherently low spatial resolution of measurements (as cAMP is typically analyzed in lysates of thousands of cells), underpin the ensuing limitations of the conventional cAMP assays: (1) genuine kinetic measurements of cAMP levels over time in a single given sample are unfeasible; (2) inability to obtain precise information on cAMP spatial distribution and transfer at subcellular levels, let alone the attempts to pinpoint dynamic interactions of cAMP and its effectors. At the same time, tremendous progress in synthetic biology over the recent years culminated in drastic refinement of our toolbox, allowing us not only to bypass the limitations of conventional assays, but to put intracellular cAMP life-span under tight control—something, that seemed scarcely attainable before. In this review article we discuss the main classes of modern genetically-encoded tools tailored for cAMP probing and modulation in living systems. We examine the capabilities and weaknesses of these different tools in the context of their operational characteristics and applicability to various experimental set-ups involving living cells, providing the guidance for rational selection of the best tools for particular needs.

Diversity of mitochondria-dependent dilator mechanisms in vascular smooth muscle of cerebral arteries from normal and insulin-resistant rats

Mitochondrial depolarization following ATP-sensitive potassium (mitoKATP) channel activation has been shown to induce cerebral vasodilation by generation of mitochondrial reactive oxygen species (ROS), which sequentially promotes frequency of calcium sparks and activation of large conductance calcium-activated potassium channels (BKCa) in vascular smooth muscle (VSM). We previously demonstrated that cerebrovascular insulin resistance accompanies aging and obesity. It is unclear whether mitochondrial depolarization without the ROS generation enhances calcium sparks and vasodilation in phenotypically normal [Sprague Dawley (SD); Zucker lean (ZL)] and insulin-resistant [Zucker obese (ZO)] rats. We compared the mechanisms underlying the vasodilation to ROS-dependent (diazoxide) and ROS-independent [BMS-191095 (BMS)] mitoKATP channel activators in normal and ZO rats. Arterial diameter studies from SD, ZL, and ZO rats showed that BMS as well as diazoxide induced vasodilation in endothelium-denuded cerebral arteries. In normal rats, BMS-induced vasodilation was mediated by mitochondrial depolarization and calcium sparks generation in VSM and was reduced by inhibition of BKCa channels. However, unlike diazoxide-induced vasodilation, scavenging of ROS had no effect on BMS-induced vasodilation. Electron spin resonance spectroscopy confirmed that diazoxide but not BMS promoted vascular ROS generation. BMS- as well as diazoxide-induced vasodilation, mitochondrial depolarization, and calcium spark generation were diminished in cerebral arteries from ZO rats. Thus pharmacological depolarization of VSM mitochondria by BMS promotes ROS-independent vasodilation via generation of calcium sparks and activation of BKCa channels. Diminished generation of calcium sparks and reduced vasodilation in ZO arteries in response to BMS and diazoxide provide new insights into mechanisms of cerebrovascular dysfunction in insulin resistance.

Decoding dynamic Ca(2+) signaling in the vascular endothelium

Although acute and chronic vasoregulation is inherently driven by endothelial Ca(2+), control and targeting of Ca(2+)-dependent signals are poorly understood. Recent studies have revealed localized and dynamic endothelial Ca(2+) events comprising an intricate signaling network along the vascular intima. Discrete Ca(2+) transients emerging from both internal stores and plasmalemmal cation channels couple to specific membrane K(+) channels, promoting endothelial hyperpolarization and vasodilation. The spatiotemporal tuning of these signals, rather than global Ca(2+) elevation, appear to direct endothelial functions under physiologic conditions. In fact, altered patterns of dynamic Ca(2+) signaling may underlie essential endothelial dysfunction in a variety of cardiovascular diseases. Advances in imaging approaches and analyses in recent years have allowed for detailed detection, quantification, and evaluation of Ca(2+) dynamics in intact endothelium. Here, we discuss recent insights into these signals, including their sources of origination and their functional encoding. We also address key aspects of data acquisition and interpretation, including broad applications of automated high-content analysis.

An algorithm for automated detection, localization and measurement of local calcium signals from camera-based imaging

Local Ca(2+) transients such as puffs and sparks form the building blocks of cellular Ca(2+) signaling in numerous cell types. They have traditionally been studied by linescan confocal microscopy, but advances in TIRF microscopy together with improved electron-multiplied CCD (EMCCD) cameras now enable rapid (>500 frames s(-1)) imaging of subcellular Ca(2+) signals with high spatial resolution in two dimensions. This approach yields vastly more information (ca. 1 Gb min(-1)) than linescan imaging, rendering visual identification and analysis of local events imaged both laborious and subject to user bias. Here we describe a routine to rapidly automate identification and analysis of local Ca(2+) events. This features an intuitive graphical user-interfaces and runs under Matlab and the open-source Python software. The underlying algorithm features spatial and temporal noise filtering to reliably detect even small events in the presence of noisy and fluctuating baselines; localizes sites of Ca(2+) release with sub-pixel resolution; facilitates user review and editing of data; and outputs time-sequences of fluorescence ratio signals for identified event sites along with Excel-compatible tables listing amplitudes and kinetics of events.

Automated analysis of dynamic Ca2+ signals in image sequences

Intracellular Ca(2+) signals are commonly studied with fluorescent Ca(2+) indicator dyes and microscopy techniques. However, quantitative analysis of Ca(2+) imaging data is time consuming and subject to bias. Automated signal analysis algorithms based on region of interest (ROI) detection have been implemented for one-dimensional line scan measurements, but there is no current algorithm which integrates optimized identification and analysis of ROIs in two-dimensional image sequences. Here an algorithm for rapid acquisition and analysis of ROIs in image sequences is described. It utilizes ellipses fit to noise filtered signals in order to determine optimal ROI placement, and computes Ca(2+) signal parameters of amplitude, duration and spatial spread. This algorithm was implemented as a freely available plugin for ImageJ (NIH) software. Together with analysis scripts written for the open source statistical processing software R, this approach provides a high-capacity pipeline for performing quick statistical analysis of experimental output. The authors suggest that use of this analysis protocol will lead to a more complete and unbiased characterization of physiologic Ca(2+) signaling.

Calcineurin/nuclear factor of activated T cells-coupled vanilliod transient receptor potential channel 4 ca2+ sparklets stimulate airway smooth muscle cell proliferation

Proliferation of airway smooth muscle cells (ASMCs) contributes to the remodeling and irreversible obstruction of airways during severe asthma, but the mechanisms underlying this disease process are poorly understood. Here we tested the hypothesis that Ca(2+) influx through the vanilliod transient receptor potential channel (TRPV) 4 stimulates ASMC proliferation. We found that synthetic and endogenous TRPV4 agonists increase proliferation of primary ASMCs. Furthermore, we demonstrate that Ca(2+) influx through individual TRPV4 channels produces Ca(2+) microdomains in ASMCs, called "TRPV4 Ca(2+) sparklets." We also show that TRPV4 channels colocalize with the Ca(2+)/calmodulin-dependent protein phosphatase calcineurin in ASMCs. Activated calcineurin dephosphorylates nuclear factor of activated T cells (NFAT) transcription factors cytosolic (c) to allow nuclear translocation and activation of synthetic transcriptional pathways. We show that ASMC proliferation in response to TRPV4 activity is associated with calcineurin-dependent nuclear translocation of the NFATc3 isoform tagged with green florescent protein. Our findings suggest that Ca(2+) microdomains created by TRPV4 Ca(2+) sparklets activate calcineurin to stimulate nuclear translocation of NFAT and ASMC proliferation. These findings further suggest that inhibition of TRPV4 could diminish asthma-induced airway remodeling.

L-type Ca2+ channel sparklets revealed by TIRF microscopy in mouse urinary bladder smooth muscle

Calcium is a ubiquitous second messenger in urinary bladder smooth muscle (UBSM). In this study, small discrete elevations of intracellular Ca(2+), referred to as Ca(2+) sparklets have been detected in an intact detrusor smooth muscle electrical syncytium using a TIRF microscopy Ca(2+) imaging approach. Sparklets were virtually abolished by the removal of extracellular Ca(2+) (0.035 ± 0.01 vs. 0.23 ± 0.07 Hz/mm(2); P<0.05). Co-loading of smooth muscle strips with the slow Ca(2+) chelator EGTA-AM (10 mM) confirmed that Ca(2+) sparklets are restricted to the cell membrane. Ca(2+) sparklets were inhibited by the calcium channel inhibitors R-(+)-Bay K 8644 (1 μM) (0.034 ± 0.02 vs. 0.21 ± 0.08 Hz/mm(2); P<0.05), and diltiazem (10 μM) (0.097 ± 0.04 vs. 0.16 ± 0.06 Hz/mm(2); P<0.05). Ca(2+) sparklets were unaffected by inhibition of P2X1 receptors α,β-meATP (10 μM) whilst sparklet frequencies were significantly reduced by atropine (1 μM). Ca(2+) sparklet frequency was significantly reduced by PKC inhibition with Gö6976 (100 nM) (0.030 ± 0.01 vs. 0.30 ± 0.1 Hz/mm(2); P<0.05), demonstrating that Ca(2+) sparklets are PKC dependant. In the presence of CPA (10 μM), there was no apparent change in the overall frequency of Ca(2+) sparklets, although the sparklet frequencies of each UBSM became statistically independent of each other (Spearman's rank correlation 0.2, P>0.05), implying that Ca(2+) store mediated signals regulate Ca(2+) sparklets. Under control conditions, inhibition of store operated Ca(2+) entry using ML-9 (100 μM) had no significant effect. Amplitudes of Ca(2+) sparklets were unaffected by any agonists or antagonists, suggesting that these signals are quantal events arising from activation of a single channel, or complex of channels. The effects of CPA and ML-9 suggest that Ca(2+) sparklets regulate events in the cell membrane, and contribute to cytosolic and sarcoplasmic Ca(2+) concentrations.

Positive feedback regulation of agonist-stimulated endothelial Ca2+ dynamics by KCa3.1 channels in mouse mesenteric arteries

OBJECTIVE

Intermediate and small conductance KCa channels IK1 (KCa3.1) and SK3 (KCa2.3) are primary targets of endothelial Ca(2+) signals in the arterial vasculature, and their ablation results in increased arterial tone and hypertension. Activation of IK1 channels by local Ca(2+) transients from internal stores or plasma membrane channels promotes arterial hyperpolarization and vasodilation. Here, we assess arteries from genetically altered IK1 knockout mice (IK1(-/-)) to determine whether IK1 channels exert a positive feedback influence on endothelial Ca(2+) dynamics.

APPROACH AND RESULTS

Using confocal imaging and custom data analysis software, we found that although the occurrence of basal endothelial Ca(2+) dynamics was not different between IK1(-/-) and wild-type mice (P>0.05), the frequency of acetylcholine-stimulated (2 μmol/L) Ca(2+) dynamics was greatly decreased in IK1(-/-) endothelium (515±153 versus 1860±319 events; P<0.01). In IK1(-/-)/SK3(T/T) mice, ancillary suppression (+Dox) or overexpression (-Dox) of SK3 channels had little additional effect on the occurrence of events under basal or acetylcholine-stimulated conditions. However, SK3 overexpression did restore the decreased event amplitudes. Removal of extracellular Ca(2+) reduced acetylcholine-induced Ca(2+) dynamics to the same level in wild-type and IK1(-/-) arteries. Blockade of IK1 and SK3 with the combination of charybdotoxin (0.1 μmol/L) and apamin (0.5 μmol/L) or transient receptor potential vanilloid 4 channels with HC-067047 (1 μmol/L) reduced acetylcholine Ca(2+) dynamics in wild-type arteries to the level of IK1(-/-)/SK3(T/T)+Dox arteries. These drug effects were not additive.

CONCLUSIONS

IK1, and to some extent SK3, channels exert a substantial positive feedback influence on endothelial Ca(2+) dynamics.

TRPV4 calcium entry and surface expression attenuated by inhibition of myosin light chain kinase in rat pulmonary microvascular endothelial cells

In previous studies, blockade or gene deletion of either myosin light chain kinase (MLCK) or the mechanogated transient receptor potential vanilloid 4 (TRPV4) channel attenuated mechanical lung injury. To determine their effects on calcium entry, rat pulmonary microvascular endothelial cells (RPMVEC) were labeled with fluo-4 and calcium entry initiated with the TRPV4 agonist, 4α-phorbol 12, 13-didecanoate (4αPDD). Mean calcium transients peaked at ∼25 sec and persisted ∼500 sec. The 4αPDD response was essentially abolished in calcium-free media, or after pretreatment with the MLCK inhibitor, ML-7. ML-7 also attenuated the 4αPDD-induced inward calcium current measured directly using whole-cell patch clamp. Pretreatment with dynasore, an inhibitor of dynamin produced an initial calcium transient followed by a 4αPDD transient of unchanged peak intensity. Automated averaging of areas under the curve (AUC) of calcium transients in individual cells indicated total calcium activity with a relationship between treatment groups of ML-7 + 4αPDD < 4αPDD only < dynasore + 4αPDD. Measurement of biotinylated surface TRPV4 protein indicated a significant reduction after ML-7 pretreatment, but no significant change with dynasore treatment. RPMVEC monolayer electrical resistances were decreased by only 3% with 10 μmol/L 4αPDD and the response was dose-related. Dynasore alone produced a 29% decrease in resistance, but neither ML-7 nor dynasore affected the subsequent 4αPDD resistance response. These studies suggest that MLCK may inhibit mechanogated calcium responses through reduced surface expression of stretch activated TRPV4 channels in the plasma membrane.

TRPC4 inactivation confers a survival benefit in severe pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is characterized by elevated pulmonary arterial pressure with lumen-occluding neointimal and plexiform lesions. Activation of store-operated calcium entry channels promotes contraction and proliferation of lung vascular cells. TRPC4 is a ubiquitously expressed store-operated calcium entry channel, but its role in PAH is unknown. We tested the hypothesis that TRPC4 promotes pulmonary arterial constriction and occlusive remodeling, leading to right ventricular failure in severe PAH. Severe PAH was induced in Sprague-Dawley rats and in wild-type and TRPC4-knockout Fischer 344 rats by a single subcutaneous injection of SU5416 [SU (semaxanib)], followed by hypoxia exposure (Hx; 10% O2) for 3 weeks and then a return to normoxia (Nx; 21% O2) for 3 to 10 additional weeks (SU/Hx/Nx). Although rats of both backgrounds exhibited indistinguishable pulmonary hypertensive responses to SU/Hx/Nx, Fischer 344 rats died within 6 to 8 weeks. Normoxic and hypertensive TRPC4-knockout rats recorded hemodynamic parameters similar to those of their wild-type littermates. However, TRPC4 inactivation conferred a striking survival benefit, due in part to preservation of cardiac output. Histological grading of vascular lesions revealed a reduction in the density of severely occluded small pulmonary arteries and in the number of plexiform lesions in TRPC4-knockout rats. TRPC4 inactivation therefore provides a survival benefit in severe PAH, associated with a decrease in the magnitude of occlusive remodeling.

TRP channel Ca(2+) sparklets: fundamental signals underlying endothelium-dependent hyperpolarization

Important functions of the vascular endothelium, including permeability, production of antithrombotic factors, and control of vascular tone, are regulated by changes in intracellular Ca(2+). The molecular identities and regulation of Ca(2+) influx channels in the endothelium are incompletely understood, in part because of experimental difficulties associated with application of patch-clamp electrophysiology to native endothelial cells. However, advances in confocal and total internal reflection fluorescence microscopy and the development of fast, high-affinity Ca(2+)-binding fluorophores have recently allowed for direct visualization and characterization of single-channel transient receptor potential (TRP) channel Ca(2+) influx events in endothelial cells. These events, called "TRP channel Ca(2+) sparklets," have been optically recorded from primary endothelial cells and the intact endothelium, and the biophysical properties and fundamental significance of these Ca(2+) signals in vasomotor regulation have been characterized. This review will first briefly discuss the role of endothelial cell TRP channel Ca(2+) influx in endothelium-dependent vasodilation, describe improved methods for recording unitary TRP channel activity using optical methods, and highlight discoveries regarding the regulation and physiological significance of TRPV4 Ca(2+) sparklets in the vascular endothelium enabled by this new technology. Perspectives on the potential use of these techniques to evaluate changes in TRP channel Ca(2+) influx activity associated with endothelial dysfunction are offered.

Recruitment of dynamic endothelial Ca2+ signals by the TRPA1 channel activator AITC in rat cerebral arteries

OBJECTIVE

Stimulation of endothelial TRP channels, specifically TRPA1, promotes vasodilation of cerebral arteries through activation of Ca2+ -dependent effectors along the myoendothelial interface. However, presumed TRPA1-triggered endothelial Ca2+ signals have not been described. We investigated whether TRPA1 activation induces specific spatial and temporal changes in Ca2+ signals along the intima that correlates with incremental vasodilation.

METHODS

Confocal imaging, immunofluorescence staining, and custom image analysis were employed.

RESULTS

We found that endothelial cells of rat cerebral arteries exhibit widespread basal Ca2+ dynamics (44 ± 6 events/minute from 26 ± 3 distinct sites in a 3.6 × 10(4) μm2 field). The TRPA1 activator AITC increased Ca2+ signals in a concentration-dependent manner, soliciting new events at distinct sites. Origination of these new events corresponded spatially with TRPA1 densities in IEL holes, and the events were prevented by the TRPA1 inhibitor HC-030031. Concentration-dependent expansion of Ca2+ events in response to AITC correlated precisely with dilation of pressurized cerebral arteries (p = 0.93 by F-test). Correspondingly, AITC caused rapid endothelium-dependent suppression of asynchronous Ca2+ waves in subintimal smooth muscle.

CONCLUSIONS

Our findings indicate that factors that stimulate TRPA1 channels expand Ca2+ signal-effector coupling at discrete sites along the endothelium to evoke graded cerebral artery vasodilation.