Hindered cytoplasmic diffusion of inositol trisphosphate restricts its cellular range of action

Signalling switches maintain intercellular communication in the vascular endothelium

BACKGROUND AND PURPOSE

The single layer of cells lining all blood vessels, the endothelium, is a sophisticated signal co-ordination centre that controls a wide range of vascular functions including the regulation of blood pressure and blood flow. To co-ordinate activities, communication among cells is required for tissue level responses to emerge. While a significant form of communication occurs by the propagation of signals between cells, the mechanism of propagation in the intact endothelium is unresolved.

EXPERIMENTAL APPROACH

Precision signal generation and targeted cellular manipulation was used in conjunction with high spatiotemporal mesoscale Ca2+ imaging in the endothelium of intact blood vessels.

KEY RESULTS

Multiple mechanisms maintain communication so that Ca2+ wave propagation occurs irrespective of the status of connectivity among cells. Between adjoining cells, regenerative IP3-induced IP3 production transmits Ca2+ signals and explains the propagated vasodilation that underlies the increased blood flow accompanying tissue activity. The inositide is itself sufficient to evoke regenerative phospholipase C-dependent Ca2+ waves across coupled cells. None of gap junctions, Ca2+ diffusion or the release of extracellular messengers is required to support this type of intercellular Ca2+ signalling. In contrast, when discontinuities exist between cells, ATP released as a diffusible extracellular messenger transmits Ca2+ signals across the discontinuity and drives propagated vasodilation.

CONCLUSION AND IMPLICATIONS

These results show that signalling switches underlie endothelial cell-to-cell signal transmission and reveal how communication is maintained in the face of endothelial damage. The findings provide a new framework for understanding wave propagation and cell signalling in the endothelium.

Pacemaking in the lymphatic system

Lymphatic collecting vessels exhibit spontaneous phasic contractions that are critical for lymph propulsion and tissue fluid homeostasis. This rhythmic activity is driven by action potentials conducted across the lymphatic muscle cell (LMC) layer to produce entrained contractions. The contraction frequency of a lymphatic collecting vessel displays exquisite mechanosensitivity, with a dynamic range from <1 to >20 contractions per minute. A myogenic pacemaker mechanism intrinsic to the LMCs was initially postulated to account for pressure-dependent chronotropy. Further interrogation into the cellular constituents of the lymphatic vessel wall identified non-muscle cell populations that shared some characteristics with interstitial cells of Cajal, which have pacemaker functions in the gastrointestinal and lower urinary tracts, thus raising the possibility of a non-muscle cell pacemaker. However, recent genetic knockout studies in mice support LMCs and a myogenic origin of the pacemaker activity. LMCs exhibit stochastic, but pressure-sensitive, sarcoplasmic reticulum calcium release (puffs and waves) from IP3R1 receptors, which couple to the calcium-activated chloride channel Anoctamin 1, causing depolarisation. The resulting electrical activity integrates across the highly coupled lymphatic muscle electrical syncytia through connexin 45 to modulate diastolic depolarisation. However, multiple other cation channels may also contribute to the ionic pacemaking cycle. Upon reaching threshold, a voltage-gated calcium channel-dependent action potential fires, resulting in a nearly synchronous calcium global calcium flash within the LMC layer to drive an entrained contraction. This review summarizes the key ion channels potentially responsible for the pressure-dependent chronotropy of lymphatic collecting vessels and various mechanisms of IP3R1 regulation that could contribute to frequency tuning.

Non-inositol 1,4,5-trisphosphate (IP3) receptor IP3-binding proteins

Conventionally, myo-D-inositol 1, 4,5-trisphosphate (IP₃) is thought to exert its second messenger effects through the gating of IP₃R Ca²⁺ release channels, located in Ca²⁺-storage organelles like the endoplasmic reticulum. However, there is considerable indirect evidence to support the concept that IP₃ might interact with other, non-IP₃R proteins within cells. To explore this possibility further, the Protein Data Bank was searched using the term "IP3". This resulted in the retrieval of 203 protein structures, the majority of which were members of the IP₃R/ryanodine receptor superfamily of channels. Only 49 of these structures were complexed with IP₃. These were inspected for their ability to interact with the carbon-1 phosphate of IP₃, since this is the least accessible phosphate group of its precursor, phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂). This reduced the number of structures retrieved to 35, of which 9 were IP₃Rs. The remaining 26 structures represent a diverse range of proteins, including inositol-lipid metabolizing enzymes, signal transducers, PH domain containing proteins, cytoskeletal anchor proteins, the TRPV4 ion channel, a retroviral Gag protein and fibroblast growth factor 2. Such proteins may impact on IP₃ signalling and its effects on cell-biology. This represents an area open for exploration in the field of IP₃ signalling.

Computational investigation of IP3 diffusion

Inositol 1,4,5-trisphosphate (IP₃) plays a key role in calcium signaling. After stimulation, it diffuses from the plasma membrane where it is produced to the endoplasmic reticulum where its receptors are localized. Based on in vitro measurements, IP₃ was long thought to be a global messenger characterized by a diffusion coefficient of ~ 280 μm²s^-1. However, in vivo observations revealed that this value does not match with the timing of localized Ca²⁺ increases induced by the confined release of a non-metabolizable IP₃ analog. A theoretical analysis of these data concluded that in intact cells diffusion of IP₃ is strongly hindered, leading to a 30-fold reduction of the diffusion coefficient. Here, we performed a new computational analysis of the same observations using a stochastic model of Ca²⁺ puffs. Our simulations concluded that the value of the effective IP₃ diffusion coefficient is close to 100 μm²s^-1. Such moderate reduction with respect to in vitro estimations quantitatively agrees with a buffering effect by non-fully bound inactive IP₃ receptors. The model also reveals that IP₃ spreading is not much affected by the endoplasmic reticulum, which represents an obstacle to the free displacement of molecules, but can be significantly increased in cells displaying elongated, 1-dimensional like geometries.

High rates of calcium-free diffusion in the cytosol of living cells

Calcium (Ca²⁺) is a universal second messenger that participates in the regulation of innumerous physiological processes. The way in which local elevations of the cytosolic Ca²⁺ concentration spread in space and time is key for the versatility of the signals. Ca²⁺ diffusion in the cytosol is hindered by its interaction with proteins that act as buffers. Depending on the concentrations and the kinetics of the interactions, there is a large range of values at which Ca²⁺ diffusion can proceed. Having reliable estimates of this range, particularly of its highest end, which corresponds to the ions free diffusion, is key to understand how the signals propagate. In this work, we present the first experimental results with which the Ca²⁺-free diffusion coefficient is directly quantified in the cytosol of living cells. By means of fluorescence correlation spectroscopy experiments performed in Xenopus laevis oocytes and in cells of Saccharomyces cerevisiae, we show that the ions can freely diffuse in the cytosol at a higher rate than previously thought.

Stochastic reaction-diffusion modeling of calcium dynamics in 3D dendritic spines of Purkinje cells

Calcium (Ca²⁺) is a second messenger assumed to control changes in synaptic strength in the form of both long-term depression and long-term potentiation at Purkinje cell dendritic spine synapses via inositol trisphosphate (IP₃)-induced Ca²⁺ release. These Ca²⁺ transients happen in response to stimuli from parallel fibers (PFs) from granule cells and climbing fibers (CFs) from the inferior olivary nucleus. These events occur at low numbers of free Ca²⁺, requiring stochastic single-particle methods when modeling them. We use the stochastic particle simulation program MCell to simulate Ca²⁺ transients within a three-dimensional Purkinje cell dendritic spine. The model spine includes the endoplasmic reticulum, several Ca²⁺ transporters, and endogenous buffer molecules. Our simulations successfully reproduce properties of Ca²⁺ transients in different dynamical situations. We test two different models of the IP₃ receptor (IP₃R). The model with nonlinear concentration response of binding of activating Ca²⁺ reproduces experimental results better than the model with linear response because of the filtering of noise. Our results also suggest that Ca²⁺-dependent inhibition of the IP₃R needs to be slow to reproduce experimental results. Simulations suggest the experimentally observed optimal timing window of CF stimuli arises from the relative timing of CF influx of Ca²⁺ and IP₃ production sensitizing IP₃R for Ca²⁺-induced Ca²⁺ release. We also model ataxia, a loss of fine motor control assumed to be the result of malfunctioning information transmission at the granule to Purkinje cell synapse, resulting in a decrease or loss of Ca²⁺ transients. Finally, we propose possible ways of recovering Ca²⁺ transients under ataxia.

Modeling of Astrocyte Networks: Toward Realistic Topology and Dynamics

Neuronal firing and neuron-to-neuron synaptic wiring are currently widely described as orchestrated by astrocytes—elaborately ramified glial cells tiling the cortical and hippocampal space into non-overlapping domains, each covering hundreds of individual dendrites and hundreds thousands synapses. A key component to astrocytic signaling is the dynamics of cytosolic Ca²⁺ which displays multiscale spatiotemporal patterns from short confined elemental Ca²⁺ events (puffs) to Ca²⁺ waves expanding through many cells. Here, we synthesize the current understanding of astrocyte morphology, coupling local synaptic activity to astrocytic Ca²⁺ in perisynaptic astrocytic processes and morphology-defined mechanisms of Ca²⁺ regulation in a distributed model. To this end, we build simplified realistic data-driven spatial network templates and compile model equations as defined by local cell morphology. The input to the model is spatially uncorrelated stochastic synaptic activity. The proposed modeling approach is validated by statistics of simulated Ca²⁺ transients at a single cell level. In multicellular templates we observe regular sequences of cell entrainment in Ca²⁺ waves, as a result of interplay between stochastic input and morphology variability between individual astrocytes. Our approach adds spatial dimension to the existing astrocyte models by employment of realistic morphology while retaining enough flexibility and scalability to be embedded in multiscale heterocellular models of neural tissue. We conclude that the proposed approach provides a useful description of neuron-driven Ca²⁺-activity in the astrocyte syncytium.

The journey of Ca2+ through the cell - pulsing through the network of ER membrane contact sites

Calcium is the third most abundant metal on earth, and the fundaments of its homeostasis date back to pre-eukaryotic life forms. In higher organisms, Ca²⁺ serves as a cofactor for a wide array of (enzymatic) interactions in diverse cellular contexts and constitutes the most important signaling entity in excitable cells. To enable responsive behavior, cytosolic Ca²⁺ concentrations are kept low through sequestration into organellar stores, particularly the endoplasmic reticulum (ER), but also mitochondria and lysosomes. Specific triggers are then used to instigate a local release of Ca²⁺ on demand. Here, communication between organelles comes into play, which is accomplished through intimate yet dynamic contacts, termed membrane contact sites (MCSs). The field of MCS biology in relation to cellular Ca²⁺ homeostasis has exploded in recent years. Taking advantage of this new wealth of knowledge, in this Review, we invite the reader on a journey of Ca²⁺ flux through the ER and its associated MCSs. New mechanistic insights and technological advances inform the narrative on Ca²⁺ acquisition and mobilization at these sites of communication between organelles, and guide the discussion of their consequences for cellular physiology.

Cell shape regulates subcellular organelle location to control early Ca2+ signal dynamics in vascular smooth muscle cells

The shape of the cell is connected to its function; however, we do not fully understand underlying mechanisms by which global shape regulates a cell's functional capabilities. Using theory, experiments and simulation, we investigated how physiologically relevant cell shape changes affect subcellular organization, and consequently intracellular signaling, to control information flow needed for phenotypic function. Vascular smooth muscle cells going from a proliferative and motile circular shape to a contractile fusiform shape show changes in the location of the sarcoplasmic reticulum, inter-organelle distances, and differential distribution of receptors in the plasma membrane. These factors together lead to the modulation of signals transduced by the M3 muscarinic receptor/Gq/PLCβ pathway at the plasma membrane, amplifying Ca2+ dynamics in the cytoplasm, and the nucleus resulting in phenotypic changes, as determined by increased activity of myosin light chain kinase in the cytoplasm and enhanced nuclear localization of the transcription factor NFAT. Taken together, our observations show a systems level phenomenon whereby global cell shape affects subcellular organization to modulate signaling that enables phenotypic changes.

Intercellular communication controls agonist-induced calcium oscillations independently of gap junctions in smooth muscle cells

In this study, we report the existence of a communication system among human smooth muscle cells that uses mechanical forces to frequency modulate long-range calcium waves. An important consequence of this mechanical signaling is that changes in stiffness of the underlying extracellular matrix can interfere with the frequency modulation of Ca²⁺ waves, causing smooth muscle cells from healthy human donors to falsely perceive a much higher agonist dose than they actually received. This aberrant sensing of contractile agonist dose on stiffer matrices is completely absent in isolated smooth muscle cells, although the isolated cells can sense matrix rigidity. We show that the intercellular communication that enables this collective Ca²⁺ response in smooth muscle cells does not involve transport across gap junctions or extracellular diffusion of signaling molecules. Instead, our data support a collective model in which mechanical signaling among smooth muscle cells regulates their response to contractile agonists.

State-dependent modulation of sympathetic firing by α1-adrenoceptors requires constitutive PKC activity in the neonatal rat spinal cord

The central adrenergic and noradrenergic neurotransmitter systems diffusively affect the operation of the spinal neural network and dynamically gauge central sympathetic outflow. Using in vitro splanchnic nerve-thoracic spinal cord preparations as an experimental model, this study examined the intraspinal α₁-adrenoceptor-meidated modulation of sympathetic firing behaviors. Several sympathetic single-fiber activities were simultaneously recorded. Application of phenylephrine (Phe, an α₁-adrenoceptor agonist) increased, decreased or did not affect spontaneous firing. A log-log plot of the change ratios of the average firing rates (AFR) versus their basal AFR displays a linear data distribution. Thus, the heterogeneity in α₁-adrenoceptor-mediated responses is well described by a power law function. Phe-induced power-law firing modulation (plFM) was sensitive to prazosin (Prz, an α₁-adrenoceptor antagonist). Heparin (Hep, a competitive IP₃ receptor blocker) and chelerythrine (Che, a protein kinase C inhibitor) also caused plFM. Phe-induced plFM persisted in the presence of Hep; however, it was occluded by Che pretreatment. Pair-wise analysis of single-fiber activities revealed synchronous sympathetic discharges. Application of Phe, Hep or Che suppressed synchronous discharges in fiber pairs with apparent correlated firing (ACF) and induced or potentiated synchronous discharges in those without or with minimal ACF. Thus, the basal activities of the sympathetic preganglionic neurons participate in determining the responses mediated by the activation of α₁-adrenoceptors. This deterministic factor, which is intrinsic to spinal neural networks, helps the supraspinal adrenergic and noradrenergic systems differentially control their widely distributed neural targets.

Glial Connexins and Pannexins in the Healthy and Diseased Brain

Over the past several decades a large amount of data have established that glial cells, the main cell population in the brain, dynamically interact with neurons and thus impact their activity and survival. One typical feature of glia is their marked expression of several connexins, the membrane proteins forming intercellular gap junction channels and hemichannels. Pannexins, which have a tetraspan membrane topology as connexins, are also detected in glial cells. Here, we review the evidence that connexin and pannexin channels are actively involved in dynamic and metabolic neuroglial interactions in physiological as well as in pathological situations. These features of neuroglial interactions open the way to identify novel non-neuronal aspects that allow for a better understanding of behavior and information processing performed by neurons. This will also complement the "neurocentric" view by facilitating the development of glia-targeted therapeutic strategies in brain disease.

Intrinsic regulation of microvascular tone by myoendothelial feedback circuits

The endothelium is an important regulator of arterial vascular tone, acting to release nitric oxide (NO) and open Ca2+-activated K+ (KCa) channels to relax vascular smooth muscle cells (VSMCs). While agonists acting at endothelial cell (EC) receptors are widely used to assess the ability of the endothelium to reduce vascular tone, the intrinsic EC-dependent mechanisms are less well characterized. In small resistance arteries and arterioles, the presence of heterocellular gap junctions termed myoendothelial gap junctions (MEGJs) allows the passage of not only current, but small molecules including Ca2+ and inositol trisphosphate (IP3). When stimulated to contract, the increase in VSM Ca2+ and IP3 can therefore potentially pass through MEGJs to activate adjacent ECs. This activation releases NO and opens KCa channels, which act to limit contraction. This myoendothelial feedback (MEF) is amplified by EC Ca2+ influx and release pathways, and is dynamically modulated by processes regulating gap junction conductance. There is a remarkable localization of key signaling and regulatory proteins within the EC projection toward VSM, and the intrinsic EC-dependent signaling pathways occurring with this highly specialized microdomain are reviewed.

Regulation of bifunctional proteins in cells: Lessons from the phospholipase Cβ/G protein pathway

Some proteins can serve multiple functions depending on different cellular conditions. An example of a bifunctional protein is inositide-specific mammalian phospholipase Cβ (PLCβ). PLCβ is activated by G proteins in response to hormones and neurotransmitters to increase intracellular calcium. Recently, alternate cellular function(s) of PLCβ have become uncovered. However, the conditions that allow these different functions to be operative are unclear. Like many mammalian proteins, PLCβ has a conserved catalytic core along with several regulatory domains. These domains modulate the intensity and duration of calcium signals in response to external sensory information, and allow this enzyme to inhibit protein translation in a noncatalytic manner. In this review, we first describe PLCβ's cellular functions and regulation of the switching between these functions, and then discuss the thermodynamic considerations that offer insight into how cells manage multiple and competitive associations allowing them to rapidly shift between functional states.

Ca2+ Signaling and Homeostasis in Mammalian Oocytes and Eggs

Changes in the intracellular concentration of calcium ([Ca2+]i) represent a vital signaling mechanism enabling communication between and among cells as well as with the environment. Cells have developed a sophisticated set of molecules, "the Ca2+ toolkit," to adapt [Ca2+]i changes to specific cellular functions. Mammalian oocytes and eggs, the subject of this review, are not an exception, and in fact the initiation of embryo devolvement in all species is entirely dependent on distinct [Ca2+]i responses. Here, we review the components of the Ca2+ toolkit present in mammalian oocytes and eggs, the regulatory mechanisms that allow these cells to accumulate Ca2+ in the endoplasmic reticulum, release it, and maintain basal and stable cytoplasmic concentrations. We also discuss electrophysiological and genetic studies that have uncovered Ca2+ influx channels in oocytes and eggs, and we analyze evidence supporting the role of a sperm-specific phospholipase C isoform as the trigger of Ca2+ oscillations during mammalian fertilization including its implication in fertility.

ORP4L couples IP3 to ITPR1 in control of endoplasmic reticulum calcium release

Oxysterol-binding protein-related protein (ORP) 4L acts as a scaffold protein assembling CD3-ε, G-α_q/11, and PLC-β3 into a complex at the plasma membrane that mediates inositol (1,4,5)-trisphosphate (IP₃)-induced endoplasmic reticulum (ER) Ca²⁺ release and oxidative phosphorylation in T-cell acute lymphoblastic leukemia cells. Here, we offer new evidence that ORP4L interacts with the carboxyl terminus of the IP₃ receptor type 1 (ITPR1) in Jurkat T cells. ORP4L enables IP₃ binding to ITPR1; a truncated construct that lacks the ITPR1-binding region retains the ability to increase IP₃ production but fails to mediate IP₃ and ITPR1 binding. In association with this ability of ORP4L, it enhances Ca²⁺ release from the ER and subsequent cytosolic and mitochondrial parallel Ca²⁺ spike oscillations that stimulate mitochondrial energetics and thus maintains cell survival. These data support a novel model in which ORP4L is a cofactor of ITPR1, which increases ITPR1 sensitivity to IP₃ and enables ER Ca²⁺ release.-Cao, X., Chen, J., Li, D., Xie, P., Xu, M., Lin, W., Li, S., Pan, G., Tang, Y., Xu, J., Olkkonen, V. M., Yan, D., Zhong, W. ORP4L couples IP₃ to ITPR1 in control of endoplasmic reticulum calcium release.

Geometric principles of second messenger dynamics in dendritic spines

Dendritic spines are small, bulbous protrusions along dendrites in neurons and play a critical role in synaptic transmission. Dendritic spines come in a variety of shapes that depend on their developmental state. Additionally, roughly 14-19% of mature spines have a specialized endoplasmic reticulum called the spine apparatus. How does the shape of a postsynaptic spine and its internal organization affect the spatio-temporal dynamics of short timescale signaling? Answers to this question are central to our understanding the initiation of synaptic transmission, learning, and memory formation. In this work, we investigated the effect of spine and spine apparatus size and shape on the spatio-temporal dynamics of second messengers using mathematical modeling using reaction-diffusion equations in idealized geometries (ellipsoids, spheres, and mushroom-shaped). Our analyses and simulations showed that in the short timescale, spine size and shape coupled with the spine apparatus geometries govern the spatiotemporal dynamics of second messengers. We show that the curvature of the geometries gives rise to pseudo-harmonic functions, which predict the locations of maximum and minimum concentrations along the spine head. Furthermore, we showed that the lifetime of the concentration gradient can be fine-tuned by localization of fluxes on the spine head and varying the relative curvatures and distances between the spine apparatus and the spine head. Thus, we have identified several key geometric determinants of how the spine head and spine apparatus may regulate the short timescale chemical dynamics of small molecules that control synaptic plasticity.

Changes in Ca2+ Removal Can Mask the Effects of Geometry During IP3R Mediated Ca2+ Signals

Calcium (Ca²⁺) signals are ubiquitous. Most intracellular Ca²⁺ signals involve the release of Ca²⁺ from the endoplasmic reticulum (ER) through Inositol 1,4,5-Trisphosphate Receptors (IP₃Rs). The non-uniform spatial organization of IP₃Rs and the fact that their individual openings are coupled via cytosolic Ca²⁺ are key factors for the variety of spatio-temporal distributions of the cytosolic [Ca²⁺] and the versatility of the signals. In this paper we combine experiments performed in untreated and in progesterone-treated Xenopus laevis oocytes and mathematical models to investigate how the interplay between geometry (the IP₃R spatial distribution) and dynamics (the processes that characterize the release, transport, and removal of cytosolic Ca²⁺) affects the resulting signals. Signal propagation looks more continuous and spatially uniform in treated (mature) than in untreated (immature) oocytes. This could be due to the different underlying IP₃R spatial distribution that has been observed in both cell types. The models, however, show that the rate of cytosolic Ca²⁺ removal, which is also different in both cell types, plays a key role affecting the coupling between Ca²⁺ release sites in such a way that the effect of the underlying IP₃R spatial distribution can be modified.

A Model of [Formula: see text] Dynamics in an Accurate Reconstruction of Parotid Acinar Cells

We have constructed a spatiotemporal model of [Formula: see text] dynamics in parotid acinar cells, based on new data about the distribution of inositol trisphophate receptors (IPR). The model is solved numerically on a mesh reconstructed from images of a cluster of parotid acinar cells. In contrast to our earlier model (Sneyd et al. in J Theor Biol 419:383-393. https://doi.org/10.1016/j.jtbi.2016.04.030 , 2017b), which cannot generate realistic [Formula: see text] oscillations with the new data on IPR distribution, our new model reproduces the [Formula: see text] dynamics observed in parotid acinar cells. This model is then coupled with a fluid secretion model described in detail in a companion paper: A mathematical model of fluid transport in an accurate reconstruction of a parotid acinar cell (Vera-Sigüenza et al. in Bull Math Biol. https://doi.org/10.1007/s11538-018-0534-z , 2018b). Based on the new measurements of IPR distribution, we show that Class I models (where [Formula: see text] oscillations can occur at constant [[Formula: see text]]) can produce [Formula: see text] oscillations in parotid acinar cells, whereas Class II models (where [[Formula: see text]] needs to oscillate in order to produce [Formula: see text] oscillations) are unlikely to do so. In addition, we demonstrate that coupling fluid flow secretion with the [Formula: see text] signalling model changes the dynamics of the [Formula: see text] oscillations significantly, which indicates that [Formula: see text] dynamics and fluid flow cannot be accurately modelled independently. Further, we determine that an active propagation mechanism based on calcium-induced calcium release channels is needed to propagate the [Formula: see text] wave from the apical region to the basal region of the acinar cell.

Structure and Function of IP3 Receptors

Inositol 1,4,5-trisphosphate receptors (IP₃Rs), by releasing Ca²⁺ from the endoplasmic reticulum (ER) of animal cells, allow Ca²⁺ to be redistributed from the ER to the cytosol or other organelles, and they initiate store-operated Ca²⁺ entry (SOCE). For all three IP₃R subtypes, binding of IP₃ primes them to bind Ca²⁺, which then triggers channel opening. We are now close to understanding the structural basis of IP₃R activation. Ca²⁺-induced Ca²⁺ release regulated by IP₃ allows IP₃Rs to regeneratively propagate Ca²⁺ signals. The smallest of these regenerative events is a Ca²⁺ puff, which arises from the nearly simultaneous opening of a small cluster of IP₃Rs. Ca²⁺ puffs are the basic building blocks for all IP₃-evoked Ca²⁺ signals, but only some IP₃ clusters, namely those parked alongside the ER-plasma membrane junctions where SOCE occurs, are licensed to respond. The location of these licensed IP₃Rs may allow them to selectively regulate SOCE.

Spatial-temporal patterning of Ca2+ signals by the subcellular distribution of IP3 and IP3 receptors

The patterning of cytosolic Ca²⁺ signals in space and time underlies their ubiquitous ability to specifically regulate numerous cellular processes. Signals mediated by liberation of Ca²⁺ sequestered in the endoplasmic reticulum (ER) through inositol trisphosphate receptor (IP₃R) channels constitute a hierarchy of events; ranging from openings of individual IP₃ channels, through the concerted openings of several clustered IP₃Rs to generate local Ca²⁺ puffs, to global Ca²⁺ waves and oscillations that engulf the entire cell. Here, we review recent progress in elucidating how this hierarchy is shaped by an interplay between the functional gating properties of IP₃Rs and their spatial distribution within the cell. We focus in particular on the subset of IP₃Rs that are organized in stationary clusters and are endowed with the ability to preferentially liberate Ca²⁺.

Astrocyte calcium signaling: Interplay between structural and dynamical patterns

Inspired by calcium activity in astrocytes, which is different in the cell body and thick branches on the one hand and thin branchlets and leaflets on the other hand, we formulate a concept of spatially partitioned oscillators. These are inhomogeneous media with regions having different excitability properties, with a global dynamics governed by spatial configuration of such regions. Due to a high surface-to-volume ratio, calcium dynamics in astrocytic leaflets is dominated by transmembrane currents, while somatic calcium dynamics relies on exchange with intracellular stores, mediated by IP 3 , which is in turn synthesized in the space nearby the plasma membrane. Reciprocal coupling via diffusion of calcium and IP 3 between the two regions makes the spatial configuration an essential contributor to overall dynamics. Due to these features, the mechanisms governing the pattern formation of calcium dynamics differ from classical excitable systems with noise or from networks of clustered oscillators. We show how geometrical inhomogeneity can play an ordering role allowing for stable scenarios for calcium wave initiation and propagation.

Using two dyes to observe the competition of Ca2+ trapping mechanisms and their effect on intracellular Ca2+ signals

The specificity and universality of intracellular [Formula: see text] signals rely on the variety of spatio-temporal patterns that the [Formula: see text] concentration can display. [Formula: see text] liberation through inositol 1,4,5-trisphosphate receptors ([Formula: see text]) is key for this variety. In this paper, we study how the competition between buffers of different kinetics affects [Formula: see text] signals that involve [Formula: see text] release through [Formula: see text]. The study also provides insight into the underlying spatial distribution of the channels that participate in the signals. Previous works on the effects of [Formula: see text] buffers have drawn conclusions 'indirectly' by observing the [Formula: see text]-bound dye distributions in the presence of varying concentrations of exogenous buffers and using simulations to interpret the results. In this paper, we make visible the invisible by observing the signals simultaneously with two dyes, [Formula: see text] and [Formula: see text], each of which plays the role of a slow or fast [Formula: see text] buffer, respectively. Our observations obtained for different concentrations of [Formula: see text] highlight the dual role that fast buffers exert on the dynamics, either reducing the intracluster channel coupling or preventing channel inhibition and allowing the occurrence of relatively long cycles of [Formula: see text] release. Our experiments also show that signals with relatively high [Formula: see text] release rates remain localized in the presence of large [Formula: see text] concentrations, while the mean speed of the elicited waves increases. We interpret this as a consequence of the more effective uncoupling between [Formula: see text] clusters as the slow dye concentration increases. Combining the analysis of the experiments with numerical simulations, we also conclude that [Formula: see text] release not only occurs within the close vicinity of the centers of the clearly identifiable release sites ([Formula: see text] clusters) but there are also functional [Formula: see text] in between them.

Beyond Intracellular Signaling: The Ins and Outs of Second Messengers Microdomains

A typical characteristic of eukaryotic cells compared to prokaryotes is represented by the spatial heterogeneity of the different structural and functional components: for example, most of the genetic material is surrounded by a highly specific membrane structure (the nuclear membrane), continuous with, yet largely different from, the endoplasmic reticulum (ER); oxidative phosphorylation is carried out by organelles enclosed by a double membrane, the mitochondria; in addition, distinct domains, enriched in specific proteins, are present in the plasma membrane (PM) of most cells. Less obvious, but now generally accepted, is the notion that even the concentration of small molecules such as second messengers (Ca²⁺ and cAMP in particular) can be highly heterogeneous within cells. In the case of most organelles, the differences in the luminal levels of second messengers depend either on the existence on their membrane of proteins that allow the accumulation/release of the second messenger (e.g., in the case of Ca²⁺, pumps, exchangers or channels), or on the synthesis and degradation of the specific molecule within the lumen (the autonomous intramitochondrial cAMP system). It needs stressing that the existence of a surrounding membrane does not necessarily imply the existence of a gradient between the cytosol and the organelle lumen. For example, the nuclear membrane is highly permeable to both Ca²⁺ and cAMP (nuclear pores are permeable to solutes up to 50 kDa) and differences in [Ca²⁺] or [cAMP] between cytoplasm and nucleoplasm are not seen in steady state and only very transiently during cell activation. A similar situation has been observed, as far as Ca²⁺ is concerned, in peroxisomes.

Mitochondrial Calcium Increase Induced by RyR1 and IP3R Channel Activation After Membrane Depolarization Regulates Skeletal Muscle Metabolism

Aim: We hypothesize that both type-1 ryanodine receptor (RyR1) and IP₃-receptor (IP₃R) calcium channels are necessary for the mitochondrial Ca²⁺ increase caused by membrane depolarization induced by potassium (or by electrical stimulation) of single skeletal muscle fibers; this calcium increase would couple muscle fiber excitation to an increase in metabolic output from mitochondria (excitation-metabolism coupling).

Methods: Mitochondria matrix and cytoplasmic Ca²⁺ levels were evaluated in fibers isolated from flexor digitorium brevis muscle using plasmids for the expression of a mitochondrial Ca²⁺ sensor (CEPIA3mt) or a cytoplasmic Ca²⁺ sensor (RCaMP). The role of intracellular Ca²⁺ channels was evaluated using both specific pharmacological inhibitors (xestospongin B for IP₃R and Dantrolene for RyR1) and a genetic approach (shIP₃R1-RFP). O₂ consumption was detected using Seahorse Extracellular Flux Analyzer.

Results: In isolated muscle fibers cell membrane depolarization increased both cytoplasmic and mitochondrial Ca²⁺ levels. Mitochondrial Ca²⁺ uptake required functional inositol IP₃R and RyR1 channels. Inhibition of either channel decreased basal O₂ consumption rate but only RyR1 inhibition decreased ATP-linked O₂ consumption. Cell membrane depolarization-induced Ca²⁺ signals in sub-sarcolemmal mitochondria were accompanied by a reduction in mitochondrial membrane potential; Ca²⁺ signals propagated toward intermyofibrillar mitochondria, which displayed increased membrane potential. These results are compatible with slow, Ca²⁺-dependent propagation of mitochondrial membrane potential from the surface toward the center of the fiber.

Conclusion: Ca²⁺-dependent changes in mitochondrial membrane potential have different kinetics in the surface vs. the center of the fiber; these differences are likely to play a critical role in the control of mitochondrial metabolism, both at rest and after membrane depolarization as part of an “excitation-metabolism” coupling process in skeletal muscle fibers.

PLCζ Induced Ca2+ Oscillations in Mouse Eggs Involve a Positive Feedback Cycle of Ca2+ Induced InsP3 Formation From Cytoplasmic PIP2

Egg activation at fertilization in mammalian eggs is caused by a series of transient increases in the cytosolic free Ca²⁺ concentration, referred to as Ca²⁺ oscillations. It is widely accepted that these Ca²⁺ oscillations are initiated by a sperm derived phospholipase C isoform, PLCζ that hydrolyses its substrate PIP₂ to produce the Ca²⁺ releasing messenger InsP₃. However, it is not clear whether PLCζ induced InsP₃ formation is periodic or monotonic, and whether the PIP₂ source for generating InsP₃ from PLCζ is in the plasma membrane or the cytoplasm. In this study we have uncaged InsP₃ at different points of the Ca²⁺ oscillation cycle to show that PLCζ causes Ca²⁺ oscillations by a mechanism which requires Ca²⁺ induced InsP₃ formation. In contrast, incubation in Sr²⁺ media, which also induces Ca²⁺ oscillations in mouse eggs, sensitizes InsP₃-induced Ca²⁺ release. We also show that the cytosolic level Ca²⁺ is a key factor in setting the frequency of Ca²⁺ oscillations since low concentrations of the Ca²⁺ pump inhibitor, thapsigargin, accelerates the frequency of PLCζ induced Ca²⁺ oscillations in eggs, even in Ca²⁺ free media. Given that Ca²⁺ induced InsP₃ formation causes a rapid wave during each Ca²⁺ rise, we use a mathematical model to show that InsP₃ generation, and hence PLCζ's substate PIP₂, has to be finely distributed throughout the egg cytoplasm. Evidence for PIP₂ distribution in vesicles throughout the egg cytoplasm is provided with a rhodamine-peptide probe, PBP10. The apparent level of PIP₂ in such vesicles could be reduced by incubating eggs in the drug propranolol which also reversibly inhibited PLCζ induced, but not Sr²⁺ induced, Ca²⁺ oscillations. These data suggest that the cytosolic Ca²⁺ level, rather than Ca²⁺ store content, is a key variable in setting the pace of PLCζ induced Ca²⁺ oscillations in eggs, and they imply that InsP₃ oscillates in synchrony with Ca²⁺ oscillations. Furthermore, they support the hypothesis that PLCζ and sperm induced Ca²⁺ oscillations in eggs requires the hydrolysis of PIP₂ from finely spaced cytoplasmic vesicles.

Synaptic neuron-astrocyte communication is supported by an order of magnitude analysis of inositol tris-phosphate diffusion at the nanoscale in a model of peri-synaptic astrocyte projection

Background

Astrocytes were conceived for decades only as supporting cells of the brain. However, the observation of Ca2+ waves in astrocyte synctitia, their neurotransmitter receptor expression and gliotransmitter secretion suggested a role in information handling, conception that has some controversies. Synaptic Neuron-Astrocyte metabotropic communication mediated by Inositol tris-phosphate (SN-AmcIP3) is supported by different reports. However, some models contradict this idea and Ca2+ stores are 1000 ± 325 nm apart from the Postsynaptic Density in the Perisynaptic Astrocyte Projections (PAP’s), suggesting that SN-AmcIP3 is extrasynaptic. However, this assumption does not consider IP3 Diffusion Coefficient (Dab), that activates IP3 Receptor (IP3R) releasing Ca2+ from intracellular stores.

Results

In this work we idealized a model of a PAP (PAPm) to perform an order of magnitude analysis of IP3 diffusion using a transient mass diffusion model. This model shows that IP3 forms a concentration gradient along the PAPm that reaches the steady state in milliseconds, three orders of magnitude before IP3 degradation. The model predicts that IP3 concentration near the Ca2+ stores may activate IP3R, depending upon Phospholipase C (PLC) number and activity. Moreover, the PAPm supports that IP3 and extracellular Ca2+ entry synergize to promote global Ca2+ transients.

Conclusion

The model presented here indicates that Ca2+ stores position in PAP’s does not limit SN-AmcIP3.

Electronic supplementary material

The online version of this article (10.1186/s13628-018-0043-3) contains supplementary material, which is available to authorized users.

TRPV4 (Transient Receptor Potential Vanilloid 4) Channel-Dependent Negative Feedback Mechanism Regulates Gq Protein-Coupled Receptor-Induced Vasoconstriction

OBJECTIVE

Several physiological stimuli activate smooth muscle cell (SMC) G_qPCRs (G_q protein-coupled receptors) to cause vasoconstriction. As a protective mechanism against excessive vasoconstriction, SMC G_qPCR stimulation invokes endothelial cell vasodilatory signaling. Whether Ca²⁺ influx in endothelial cells contributes to the regulation of G_qPCR-induced vasoconstriction remains unknown. Ca²⁺ influx through TRPV4 (transient receptor potential vanilloid 4) channels is a key regulator of endothelium-dependent vasodilation. We hypothesized that SMC G_qPCR stimulation engages endothelial TRPV4 channels to limit vasoconstriction.

APPROACH AND RESULTS

Using high-speed confocal microscopy to record unitary Ca²⁺ influx events through TRPV4 channels (TRPV4 sparklets), we report that activation of SMC α₁ARs (alpha₁-adrenergic receptors) with phenylephrine or thromboxane A₂ receptors with U46619 stimulated TRPV4 sparklets in the native endothelium from mesenteric arteries. Activation of endothelial TRPV4 channels did not require an increase in Ca²⁺ as indicated by the lack of effect of L-type Ca²⁺ channel activator or chelator of intracellular Ca²⁺ EGTA-AM. However, gap junction communication between SMCs and endothelial cells was required for phenylephrine activation or U46619 activation of endothelial TRPV4 channels. Lowering inositol 1,4,5-trisphosphate levels with phospholipase C inhibitor or lithium chloride suppressed phenylephrine activation of endothelial TRPV4 sparklets. Moreover, uncaging inositol 1,4,5-trisphosphate profoundly increased TRPV4 sparklet activity. In pressurized arteries, phenylephrine-induced vasoconstriction was followed by a slow, TRPV4-dependent vasodilation, reflecting activation of negative regulatory mechanism. Consistent with these data, phenylephrine induced a significantly higher increase in blood pressure in TRPV4^-/- mice.

CONCLUSIONS

These results demonstrate that SMC G_qPCR stimulation triggers inositol 1,4,5-trisphosphate-dependent activation of endothelial TRPV4 channels to limit vasoconstriction.

Dynamic Ca2+ imaging with a simplified lattice light-sheet microscope: A sideways view of subcellular Ca2+ puffs

We describe the construction of a simplified, inexpensive lattice light-sheet microscope, and illustrate its use for imaging subcellular Ca²⁺ puffs evoked by photoreleased i-IP₃ in cultured SH-SY5Y neuroblastoma cells loaded with the Ca²⁺ probe Cal520. The microscope provides sub-micron spatial resolution and enables recording of local Ca²⁺ transients in single-slice mode with a signal-to-noise ratio and temporal resolution (2ms) at least as good as confocal or total internal reflection microscopy. Signals arising from openings of individual IP₃R channels are clearly resolved, as are stepwise changes in fluorescence reflecting openings and closings of individual channels during puffs. Moreover, by stepping the specimen through the light-sheet, the entire volume of a cell can be scanned within a few hundred ms. The ability to directly visualize a sideways (axial) section through cells directly reveals that IP₃-evoked Ca²⁺ puffs originate at sites in very close (≤a few hundred nm) to the plasma membrane, suggesting they play a specific role in signaling to the membrane.

Endogenous signalling pathways and caged IP3 evoke Ca2+ puffs at the same abundant immobile intracellular sites

The building blocks of intracellular Ca²⁺ signals evoked by inositol 1,4,5-trisphosphate receptors (IP₃Rs) are Ca²⁺ puffs, transient focal increases in Ca²⁺ concentration that reflect the opening of small clusters of IP₃Rs. We use total internal reflection fluorescence microscopy and automated analyses to detect Ca²⁺ puffs evoked by photolysis of caged IP₃ or activation of endogenous muscarinic receptors with carbachol in human embryonic kidney 293 cells. Ca²⁺ puffs evoked by carbachol initiated at an estimated 65±7 sites/cell, and the sites remained immobile for many minutes. Photolysis of caged IP₃ evoked Ca²⁺ puffs at a similar number of sites (100±35). Increasing the carbachol concentration increased the frequency of Ca²⁺ puffs without unmasking additional Ca²⁺ release sites. By measuring responses to sequential stimulation with carbachol or photolysed caged IP₃, we established that the two stimuli evoked Ca²⁺ puffs at the same sites. We conclude that IP₃-evoked Ca²⁺ puffs initiate at numerous immobile sites and the sites become more likely to fire as the IP₃ concentration increases; there is no evidence that endogenous signalling pathways selectively deliver IP₃ to specific sites.

Summary: Ca²⁺ puffs are the building blocks for IP₃-evoked Ca²⁺ signals. Ca²⁺ puffs evoked by caged IP₃ or via endogenous signalling pathways initiate at the same fixed intracellular sites.

Coupling of Airway Smooth Muscle Bitter Taste Receptors to Intracellular Signaling and Relaxation Is via Gαi1,2,3

Bitter taste receptors (TAS2Rs) are expressed on human airway smooth muscle (HASM) and evoke marked relaxation. Agonist interaction with TAS2Rs activates phospholipase C and increases compartmentalized intracellular Ca²⁺ ([Ca²⁺]_i) via inositol 1,4,5 triphosphate. In taste cells, the G protein gustducin couples TAS2R to phospholipase C; however, we find very low levels of G_αgust mRNA or protein in HASM. We hypothesized that another G protein in HASM transmits TAS2R function. TAS2R signaling to [Ca²⁺]_i, extracellular signal-regulated kinase (ERK) 1/2, and physiologic relaxation was sensitive to pertussis toxin, confirming a role for a member of the G_i family. α subunit expression in HASM was G_αi2 > G_αi1 = G_αi3 > G_αtrans1 ≈ G_αtrans2, with G_αgust and G_αo at the limits of detection (>100-fold lower than G_αi2). Small interfering RNA knockdowns in HASM showed losses of [Ca²⁺]_i and ERK1/2 signaling when G_αi1, G_αi2, or G_αi3 were reduced. G_αtrans1 and G_αtrans2 knockdowns had no effect on [Ca²⁺]_i and a minimal, transient effect on ERK1/2 phosphorylation. Furthermore, G_αgust and G_αo knockdowns did not affect any TAS2R signaling. In overexpression experiments in human embryonic kidney-293T cells, we confirmed an agonist-dependent physical interaction between TAS2R14 and G_αi2. ASM cells from transgenic mice expressing a peptide inhibitor of G_αi2 had attenuated relaxation to TAS2R agonist. These data indicate that, unlike in taste cells, TAS2Rs couple to the prevalent G proteins, G_αi1, G_αi2, and G_αi3, with no evidence for functional coupling to G_αgust. This absence of function for the "canonical" TAS2R G protein in HASM may be due to the very low expression of G_αgust, indicating that TAS2Rs can optionally couple to several G proteins in a cell type-dependent manner contingent upon G protein expression.

Voltage-dependent Ca2+ entry into smooth muscle during contraction promotes endothelium-mediated feedback vasodilation in arterioles

Vascular smooth muscle contraction is suppressed by feedback dilation mediated by the endothelium. In skeletal muscle arterioles, this feedback can be activated by Ca²⁺ signals passing from smooth muscle through gap junctions to endothelial cells, which protrude through holes in the internal elastic lamina to make contact with vascular smooth muscle cells. Although hypothetically either Ca²⁺ or inositol trisphosphate (IP₃) may provide the intercellular signal, it is generally thought that IP₃ diffusion is responsible. We provide evidence that Ca²⁺ entry through L-type voltage-dependent Ca²⁺ channels (VDCCs) in vascular smooth muscle can pass to the endothelium through positions aligned with holes in the internal elastic lamina in amounts sufficient to activate endothelial cell Ca²⁺ signaling. In endothelial cells in which IP₃ receptors (IP₃Rs) were blocked, VDCC-driven Ca²⁺ events were transient and localized to the endothelium that protrudes through the internal elastic lamina to contact vascular smooth muscle cells. In endothelial cells in which IP₃Rs were not blocked, VDCC-driven Ca²⁺ events in endothelial cells were amplified to form propagating waves. These waves activated voltage-insensitive, intermediate-conductance, Ca²⁺-activated K⁺ (IK_Ca) channels, thereby providing feedback that effectively suppressed vasoconstriction and enabled cycles of constriction and dilation called vasomotion. Thus, agonists that stimulate vascular smooth muscle depolarization provide Ca²⁺ to endothelial cells to activate a feedback circuit that protects tissue blood flow.

Intimate connections: Inositol pyrophosphates at the interface of metabolic regulation and cell signaling

Inositol pyrophosphates are small, diffusible signaling molecules that possess the most concentrated three-dimensional array of phosphate groups in Nature; up to eight phosphates are crammed around a six-carbon inositol ring. This review discusses the physico-chemical properties of these unique molecules, and their mechanisms of action. Also provided is information on the enzymes that regulate the levels and hence the signaling properties of these molecules. This review pursues the idea that many of the biological effects of inositol pyrophosphates can be rationalized by their actions at the interface of cell signaling and metabolism that is essential to cellular and organismal homeostasis.

Ca2+ Microdomains in T-Lymphocytes

Early Ca²⁺ signaling is characterized by occurrence of Ca²⁺ microdomains formed by opening of single or clusters of Ca²⁺ channels, thereby initiating first signaling and subsequently activating global Ca²⁺ signaling mechanisms. However, only few data are available focusing on the first seconds and minutes of Ca²⁺ microdomain formation and related signaling pathways in activated T-lymphocytes. In this review, we condense current knowledge on Ca²⁺ microdomain formation in T-lymphocytes and early Ca²⁺ signaling, function of Ca²⁺ microdomains, and microdomain organization. Interestingly, considering the first seconds of T cell activation, a triphasic Ca²⁺ signal is becoming apparent: (i) initial Ca²⁺ microdomains occurring in the first second of T cell activation, (ii) amplification of Ca²⁺ microdomains by recruitment of further channels in the next 5-10 s, and (iii) a transition to global Ca²⁺ increase. Apparently, the second messenger nicotinic acid adenine dinucleotide phosphate is the first second messenger involved in initiation of Ca²⁺ microdomains. Ryanodine receptors type 1 act as initial Ca²⁺ release channels in CD4⁺ T-lymphocytes. Regarding the temporal correlation of Ca²⁺ microdomains with other molecular events of T cell activation, T cell receptor-dependent microdomain organization of signaling molecules Grb2 and Src homology [SH2] domain-containing leukocyte protein of 65 kDa was observed within the first 20 s. In addition, fast cytoskeletal changes are initiated. Furthermore, the involvement of additional Ca²⁺ channels and organelles, such as the Ca²⁺ buffering mitochondria, is discussed. Future research developments will comprise analysis of the causal relation between these temporally coordinated signaling events. Taken together, high-resolution Ca²⁺ imaging techniques applied to T cell activation in the past years paved the way to detailed molecular understanding of initial Ca²⁺ signaling mechanisms in non-excitable cells.

Comparison of Ca2+ puffs evoked by extracellular agonists and photoreleased IP3

The inositol trisphosphate (IP3) signaling pathway evokes local Ca2+ signals (Ca2+ puffs) that arise from the concerted openings of clustered IP3 receptor/channels in the ER membrane. Physiological activation is triggered by binding of agonists to G-protein-coupled receptors (GPCRs) on the cell surface, leading to cleavage of phosphatidyl inositol bisphosphate and release of IP3 into the cytosol. Photorelease of IP3 from a caged precursor provides a convenient and widely employed means to study the final stage of IP3-mediated Ca2+ liberation, bypassing upstream signaling events to enable more precise control of the timing and relative concentration of cytosolic IP3. Here, we address whether Ca2+ puffs evoked by photoreleased IP3 fully replicate those arising from physiological agonist stimulation. We imaged puffs in individual SH-SY5Y neuroblastoma cells that were sequentially stimulated by picospritzing extracellular agonist (carbachol, CCH or bradykinin, BK) followed by photorelease of a poorly-metabolized IP3 analog, i-IP3. The centroid localizations of fluorescence signals during puffs evoked in the same cells by agonists and photorelease substantially overlapped (within ∼1μm), suggesting that IP3 from both sources accesses the same, or closely co-localized clusters of IP3Rs. Moreover, the time course and spatial spread of puffs evoked by agonists and photorelease matched closely. Because photolysis generates IP3 uniformly throughout the cytoplasm, our results imply that IP3 generated in SH-SY5Y cells by activation of receptors to CCH and BK also exerts broadly distributed actions, rather than specifically activating a subpopulation of IP3Rs that are scaffolded in close proximity to cell surface receptors to form a signaling nanodomain.

FCS experiments to quantify Ca2+ diffusion and its interaction with buffers

Ca²⁺ signals are ubiquitous. One of the key factors for their versatility is the variety of spatio-temporal distributions that the cytosolic Ca²⁺ can display. In most cell types Ca²⁺ signals not only depend on Ca²⁺ entry from the extracellular medium but also on Ca²⁺ release from internal stores, a process which is in turn regulated by cytosolic Ca²⁺ itself. The rate at which Ca²⁺ is transported, the fraction that is trapped by intracellular buffers, and with what kinetics are thus key features that affect the time and spatial range of action of Ca²⁺ signals. The quantification of Ca²⁺ diffusion in intact cells is quite challenging because the transport rates that can be inferred using optical techniques are intricately related to the interaction of Ca²⁺ with the dye that is used for its observation and with the cellular buffers. In this paper, we introduce an approach that uses Fluorescence Correlation Spectroscopy (FCS) experiments performed at different conditions that in principle allows the quantification of Ca²⁺ diffusion and of its reaction rates with unobservable (non-fluorescent) Ca²⁺ buffers. To this end, we develop the necessary theory to interpret the experimental results and then apply it to FCS experiments performed in a set of solutions containing Ca²⁺, a single wavelength Ca²⁺ dye, and a non-fluorescent Ca²⁺ buffer. We show that a judicious choice of the experimental conditions and an adequate interpretation of the fitting parameters can be combined to extract information on the free diffusion coefficient of Ca²⁺ and of some of the properties of the unobservable buffer. We think that this approach can be applied to other situations, particularly to experiments performed in intact cells.

Taste and smell GPCRs in the lung: Evidence for a previously unrecognized widespread chemosensory system

Taste and smell receptor expression has been traditionally limited to the tongue and nose. We have identified bitter taste receptors (TAS2Rs) and olfactory receptors (ORs) on human airway smooth muscle (HASM) cells. TAS2Rs signal to PLCβ evoking an increase in [Ca²⁺]_i causing membrane hyperpolarization and marked HASM relaxation ascertained by single cell, ex vivo, and in vivo methods. The presence of TAS2Rs in the lung was unexpected, as was the bronchodilatory function which has been shown to be due to signaling within specific microdomains of the cell. Unlike β₂-adrenergic receptor-mediated bronchodilation, TAS2R function is not impaired in asthma and shows little tachyphylaxis. HASM ORs do not bronchodilate, but rather modulate cytoskeletal remodeling and hyperplasia, two cardinal features of asthma. We have shown that short chain fatty acids, byproducts of fermentation of polysaccharides by the gut microbiome, activate HASM ORs. This establishes a non-immune gut-lung mechanism that ties observations on gut microbial communities to asthma phenotypes. Subsequent studies by multiple investigators have revealed expression and specialized functions of TAS2Rs and ORs in multiple cell-types and organs throughout the body. Collectively, the data point towards a previously unrecognized chemosensory system which recognizes endogenous and exogenous agonists. These receptors and their ligands play roles in normal homeostatic functions, predisposition or adaptation to disease, and represent drug targets for novel therapeutics.

IP3, still on the move but now in the slow lane

In this issue of Science Signaling, Dickinson et al show that the intracellular messenger inositol 1,4,5-trisphosphate (IP₃), which triggers the release of calcium (Ca²⁺) from the endoplasmic reticulum, is a slowly diffusing local signal, rather than a rapidly diffusing global one. These findings have implications for the understanding of the mechanisms of Ca²⁺ wave propagation, especially long-range, cell-to-cell propagating Ca²⁺ waves.