Endogenous and exogenous Ca2+ buffers differentially modulate Ca2+-dependent inactivation of Ca(v)2.1 Ca2+ channels

Retigabine and gabapentin restore channel function and neuronal firing in a cellular model of an epilepsy-associated dominant-negative KCNQ5 variant

KCNQ5 encodes the voltage-gated potassium channel KV7.5, a member of the KV7 channel family, which conducts the M-current. This current is a potent regulator of neuronal excitability by regulating membrane potential in the subthreshold range of action potentials and mediating the medium and slow afterhyperpolarization. Recently, we have identified five loss-of-function variants in KCNQ5 in patients with genetic generalized epilepsy. Using the most severe dominant-negative variant (R359C), we set out to investigate pharmacological therapeutic intervention by KV7 channel openers on channel function and neuronal firing. Retigabine and gabapentin increased R359C-derived M-current amplitudes in HEK cells expressing homomeric or heteromeric mutant KV7.5 channels. Retigabine was most effective in restoring K+ currents. Ten μM retigabine was sufficient to reach the level of WT currents without retigabine, whereas 100 μM of gabapentin showed less than half of this effect and application of 50 μM ZnCl2 only significantly increased M-current amplitude in heteromeric channels. Overexpression of KV7.5-WT potently inhibited neuronal firing by increasing the M-current, whereas R359C overexpression had the opposite effect and additionally decreased the medium afterhyperpolarization current. Both aforementioned drugs and Zn2+ reversed the effect of R359C expression by reducing firing to nearly normal levels at high current injections. Our study shows that a dominant-negative variant with a complete loss-of-function in KV7.5 leads to largely increased neuronal firing which may explain a neuronal hyperexcitability in patients. KV7 channel openers, such as retigabine or gabapentin, could be treatment options for patients currently displaying pharmacoresistant epilepsy and carrying loss-of-function variants in KCNQ5.

Mitochondrial calcium homeostasis: Implications for neurovascular and neurometabolic coupling

Mitochondrial function is critical to maintain high rates of oxidative metabolism supporting energy demands of both spontaneous and evoked neuronal activity in the brain. Mitochondria not only regulate energy metabolism, but also influence neuronal signaling. Regulation of "energy metabolism" and "neuronal signaling" (i.e. neurometabolic coupling), which are coupled rather than independent can be understood through mitochondria's integrative functions of calcium ion (Ca²⁺) uptake and cycling. While mitochondrial Ca²⁺ do not affect hemodynamics directly, neuronal activity changes are mechanistically linked to functional hyperemic responses (i.e. neurovascular coupling). Early in vitro studies lay the foundation of mitochondrial Ca²⁺ homeostasis and its functional roles within cells. However, recent in vivo approaches indicate mitochondrial Ca²⁺ homeostasis as maintained by the role of mitochondrial Ca²⁺ uniporter (mCU) influences system-level brain activity as measured by a variety of techniques. Based on earlier evidence of subcellular cytoplasmic Ca²⁺ microdomains and cellular bioenergetic states, a mechanistic model of Ca²⁺ mobilization is presented to understand systems-level neurovascular and neurometabolic coupling. This integrated view from molecular and cellular to the systems level, where mCU plays a major role in mitochondrial and cellular Ca²⁺ homeostasis, may explain the wide range of activation-induced coupling across neuronal activity, hemodynamic, and metabolic responses.

LRRK2 Regulates Voltage-Gated Calcium Channel Function

Voltage-gated Ca²⁺ (Ca_V) channels enable Ca²⁺ influx in response to membrane depolarization. Ca_V2.1 channels are localized to the presynaptic membrane of many types of neurons where they are involved in triggering neurotransmitter release. Several signaling proteins have been identified as important Ca_V2.1 regulators including protein kinases, G-proteins and Ca²⁺ binding proteins. Recently, we discovered that leucine rich repeat kinase 2 (LRRK2), a protein associated with inherited Parkinson’s disease, interacts with specific synaptic proteins and influences synaptic transmission. Since synaptic proteins functionally interact with Ca_V2.1 channels and synaptic transmission is triggered by Ca²⁺ entry via Ca_V2.1, we investigated whether LRRK2 could impact Ca_V2.1 channel function. Ca_V2.1 channel properties were measured using whole cell patch clamp electrophysiology in HEK293 cells transfected with Ca_V2.1 subunits and various LRRK2 constructs. Our results demonstrate that both wild type (wt) LRRK2 and the G2019S LRRK2 mutant caused a significant increase in whole cell Ca²⁺ current density compared to cells expressing only the Ca_V2.1 channel complex. In addition, LRRK2 expression caused a significant hyperpolarizing shift in voltage-dependent activation while having no significant effect on inactivation properties. These functional changes in Ca_V2.1 activity are likely due to a direct action of LRRK2 as we detected a physical interaction between LRRK2 and the β3 Ca_V channel subunit via coimmunoprecipitation. Furthermore, effects on Ca_V2.1 channel function are dependent on LRRK2 kinase activity as these could be reversed via treatment with a LRRK2 inhibitor. Interestingly, LRRK2 also augmented endogenous voltage-gated Ca²⁺ channel function in PC12 cells suggesting other Ca_V channels could also be regulated by LRRK2. Overall, our findings support a novel physiological role for LRRK2 in regulating Ca_V2.1 function that could have implications for how mutations in LRRK2 contribute to Parkinson’s disease pathophysiology.

Exact stochastic simulation of a calcium microdomain reveals the impact of Ca²⁺ fluctuations on IP₃R gating

In this study, we numerically analyzed the nonlinear Ca(2+)-dependent gating dynamics of a single, nonconducting inositol 1,4,5-trisphosphate receptor (IP₃R) channel, using an exact and fully stochastic simulation algorithm that includes channel gating, Ca(2+) buffering, and Ca(2+) diffusion. The IP₃R is a ubiquitous intracellular Ca(2+) release channel that plays an important role in the formation of complex spatiotemporal Ca(2+) signals such as waves and oscillations. Dynamic subfemtoliter Ca(2+) microdomains reveal low copy numbers of Ca(2+) ions, buffer molecules, and IP₃Rs, and stochastic fluctuations arising from molecular interactions and diffusion do not average out. In contrast to models treating calcium dynamics deterministically, the stochastic approach accounts for this molecular noise. We varied Ca(2+) diffusion coefficients and buffer reaction rates to tune the autocorrelation properties of Ca(2+) noise and found a distinct relation between the autocorrelation time τac, the mean channel open and close times, and the resulting IP₃R open probability PO. We observed an increased PO for shorter noise autocorrelation times, caused by increasing channel open times and decreasing close times. In a pure diffusion model the effects become apparent at elevated calcium concentrations, e.g., at [Ca(2+)] = 25 μM, τac = 0.082 ms, the IP₃R open probability increased by ≈20% and mean open times increased by ≈4 ms, compared to a zero noise model. We identified the inactivating Ca(2+) binding site of IP₃R subunits as the primarily noise-susceptible element of the De Young and Keizer model. Short Ca(2+) noise autocorrelation times decrease the probability of Ca(2+) association and consequently increase IPvR activity. These results suggest a functional role of local calcium noise properties on calcium-regulated target molecules such as the ubiquitous IP₃R. This finding may stimulate novel experimental approaches analyzing the role of calcium noise properties on microdomain behavior.

Estimation of presynaptic calcium currents and endogenous calcium buffers at the frog neuromuscular junction with two different calcium fluorescent dyes

At the frog neuromuscular junction, under physiological conditions, the direct measurement of calcium currents and of the concentration of intracellular calcium buffers—which determine the kinetics of calcium concentration and neurotransmitter release from the nerve terminal—has hitherto been technically impossible. With the aim of quantifying both Ca²⁺ currents and the intracellular calcium buffers, we measured fluorescence signals from nerve terminals loaded with the low-affinity calcium dye Magnesium Green or the high-affinity dye Oregon Green BAPTA-1, simultaneously with microelectrode recordings of nerve-action potentials and end-plate currents. The action-potential-induced fluorescence signals in the nerve terminals developed much more slowly than the postsynaptic response. To clarify the reasons for this observation and to define a spatiotemporal profile of intracellular calcium and of the concentration of mobile and fixed calcium buffers, mathematical modeling was employed. The best approximations of the experimental calcium transients for both calcium dyes were obtained when the calcium current had an amplitude of 1.6 ± 0.08 pA and a half-decay time of 1.2 ± 0.06 ms, and when the concentrations of mobile and fixed calcium buffers were 250 ± 13 μM and 8 ± 0.4 mM, respectively. High concentrations of endogenous buffers define the time course of calcium transients after an action potential in the axoplasm, and may modify synaptic plasticity.

Mitochondrial calcium uptake capacity modulates neocortical excitability

Local calcium (Ca(2+)) changes regulate central nervous system metabolism and communication integrated by subcellular processes including mitochondrial Ca(2+) uptake. Mitochondria take up Ca(2+) through the calcium uniporter (mCU) aided by cytoplasmic microdomains of high Ca(2+). Known only in vitro, the in vivo impact of mCU activity may reveal Ca(2+)-mediated roles of mitochondria in brain signaling and metabolism. From in vitro studies of mitochondrial Ca(2+) sequestration and cycling in various cell types of the central nervous system, we evaluated ranges of spontaneous and activity-induced Ca(2+) distributions in multiple subcellular compartments in vivo. We hypothesized that inhibiting (or enhancing) mCU activity would attenuate (or augment) cortical neuronal activity as well as activity-induced hemodynamic responses in an overall cytoplasmic and mitochondrial Ca(2+)-dependent manner. Spontaneous and sensory-evoked cortical activities were measured by extracellular electrophysiology complemented with dynamic mapping of blood oxygen level dependence and cerebral blood flow. Calcium uniporter activity was inhibited and enhanced pharmacologically, and its impact on the multimodal measures were analyzed in an integrated manner. Ru360, an mCU inhibitor, reduced all stimulus-evoked responses, whereas Kaempferol, an mCU enhancer, augmented all evoked responses. Collectively, the results confirm aforementioned hypotheses and support the Ca(2+) uptake-mediated integrative role of in vivo mitochondria on neocortical activity.

Mitochondrial functional state impacts spontaneous neocortical activity and resting state FMRI

Mitochondrial Ca²⁺ uptake, central to neural metabolism and function, is diminished in aging whereas enhanced after acute/sub-acute traumatic brain injury. To develop relevant translational models for these neuropathologies, we determined the impact of perturbed mitochondrial Ca²⁺ uptake capacities on intrinsic brain activity using clinically relevant markers. From a multi-compartment estimate of probable baseline Ca²⁺ ranges in the brain, we hypothesized that reduced or enhanced mitochondrial Ca²⁺ uptake capacity would decrease or increase spontaneous neuronal activity respectively. As resting state fMRI-BOLD fluctuations and stimulus-evoked BOLD responses have similar physiological origins [1] and stimulus-evoked neuronal and hemodynamic responses are modulated by mitochondrial Ca²⁺ uptake capacity [2], [3] respectively, we tested our hypothesis by measuring hemodynamic fluctuations and spontaneous neuronal activities during normal and altered mitochondrial functional states. Mitochondrial Ca²⁺ uptake capacity was perturbed by pharmacologically inhibiting or enhancing the mitochondrial Ca²⁺ uniporter (mCU) activity. Neuronal electrical activity and cerebral blood flow (CBF) fluctuations were measured simultaneously and integrated with fMRI-BOLD fluctuations at 11.7T. mCU inhibition reduced spontaneous neuronal activity and the resting state functional connectivity (RSFC), whereas mCU enhancement increased spontaneous neuronal activity but reduced RSFC. We conclude that increased or decreased mitochondrial Ca²⁺ uptake capacities lead to diminished resting state modes of brain functional connectivity.

Developmental alterations in the biophysical properties of Ca(v) 1.3 Ca(2+) channels in mouse inner hair cells

Prior to hearing onset, spontaneous action potentials activate voltage-gated Ca_v1.3 Ca²⁺ channels in mouse inner hair cells (IHCs), which triggers exocytosis of glutamate and excitation of afferent neurons. In mature IHCs, Ca_v1.3 channels open in response to evoked receptor potentials, causing graded changes in exocytosis required for accurate sound transmission. Developmental alterations in Ca_v1.3 properties may support distinct roles of Ca_v1.3 in IHCs in immature and mature IHCs, and have been reported in various species. It is not known whether such changes in Ca_v1.3 properties occur in mouse IHCs, but this knowledge is necessary for understanding the roles of Ca_v1.3 in developing and mature IHCs. Here, we describe age-dependent differences in the biophysical properties of Ca_v1.3 channels in mouse IHCs. In mature IHCs, Ca_v1.3 channels activate more rapidly and exhibit greater Ca²⁺-dependent inactivation (CDI) than in immature IHCs. Consistent with the properties of Ca_v1.3 channels in heterologous expression systems, CDI in mature IHCs is not affected by increasing intracellular Ca²⁺ buffering strength. However, CDI in immature IHCs is significantly reduced by strong intracellular Ca²⁺ buffering, which both slows the onset of, and accelerates recovery from, inactivation. These results signify a developmental decline in the sensitivity of CDI to global elevations in Ca²⁺, which restricts negative feedback regulation of Ca_v1.3 channels to incoming Ca²⁺ ions in mature IHCs. Together with faster Ca_v1.3 activation kinetics, increased reliance of Ca_v1.3 CDI on local Ca²⁺ may sharpen presynaptic Ca²⁺ signals and improve temporal aspects of sound coding in mature IHCs.

Calretinin regulates Ca2+-dependent inactivation and facilitation of Ca(v)2.1 Ca2+ channels through a direct interaction with the α12.1 subunit

Voltage-gated Ca(v)2.1 Ca(2+) channels undergo dual modulation by Ca(2+), Ca(2+)-dependent inactivation (CDI), and Ca(2+)-dependent facilitation (CDF), which can influence synaptic plasticity in the nervous system. Although the molecular determinants controlling CDI and CDF have been the focus of intense research, little is known about the factors regulating these processes in neurons. Here, we show that calretinin (CR), a Ca(2+)-binding protein highly expressed in subpopulations of neurons in the brain, inhibits CDI and enhances CDF by binding directly to α(1)2.1. Screening of a phage display library with CR as bait revealed a highly basic CR-binding domain (CRB) present in multiple copies in the cytoplasmic linker between domains II and III of α(1)2.1. In pulldown assays, CR binding to fusion proteins containing these CRBs was largely Ca(2+)-dependent. α(1)2.1 coimmunoprecipitated with CR antibodies from transfected cells and mouse cerebellum, which confirmed the existence of CR-Ca(v)2.1 complexes in vitro and in vivo. In HEK293T cells, CR significantly decreased Ca(v)2.1 CDI and increased CDF. CR binding to α(1)2.1 was required for these effects, because they were not observed upon substitution of the II-III linker of α(1)2.1 with that from the Ca(v)1.2 α(1) subunit (α(1)1.2), which lacks the CRBs. In addition, coexpression of a protein containing the CRBs blocked the modulatory action of CR, most likely by competing with CR for interactions with α(1)2.1. Our findings highlight an unexpected role for CR in directly modulating effectors such as Ca(v)2.1, which may have major consequences for Ca(2+) signaling and neuronal excitability.

Analysis of factors affecting Ca(2+)-dependent inactivation dynamics of L-type Ca(2+) current of cardiac myocytes in pulmonary vein of rabbit

L-type Ca(2+) channels (ICaLs) are inactivated by an increase in intracellular [Ca(2+)], known as Ca(2+)-dependent inactivation (CDI). CDI is also induced by Ca(2+) released from the sarcoplasmic reticulum (SR), known as release-dependent inhibition (RDI). As both CDI and RDI occur in the junctional subsarcolemmal nanospace (JSS), we investigated which factors are involved within the JSS using isolated cardiac myocytes from the main pulmonary vein of the rabbit. Using the whole-cell patch clamp technique, RDI was readily observed with the application of a pre-pulse followed by a test pulse, during which the ICaLs exhibited a decrease in peak current amplitude and a slower inactivation. A fast acting Ca(2+) chelator, 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), abolished this effect. As the time interval between the pre-pulse and test pulse increased, the ICaLs exhibited greater recovery and the RDI was relieved. Inhibition of the ryanodine receptor (RyR) or the SR Ca(2+)-ATPase (SERCA) greatly attenuated RDI and facilitated ICaL recovery. Removal of extracellular Na(+),which inhibits the Na(+)-Ca(2+) exchange (Incx), greatly enhanced RDI and slowed ICaL recovery, suggesting that Incx critically controls the [Ca(2+)] in the JSS. We incorporated the Ca(2+)-binding kinetics of the ICaL into a previously published computational model. By assuming two Ca(2+)-binding sites in the ICaL, of which one is of low-affinity with fast kinetics and the other is of high-affinity with slower kinetics, the new model was able to successfully reproduce RDI and its regulation by Incx. The model suggests that Incx accelerates Ca(2+) removal from the JSS to downregulate CDI and attenuates SR Ca(2+) refilling. The model may be useful to elucidate complex mechanisms involved in excitation–contraction coupling in myocytes.

Three functional facets of calbindin D-28k

Many neurons of the vertebrate central nervous system (CNS) express the Ca²⁺ binding protein calbindin D-28k (CB), including important projection neurons like cerebellar Purkinje cells but also neocortical interneurons. CB has moderate cytoplasmic mobility and comprises at least four EF-hands that function in Ca²⁺ binding with rapid to intermediate kinetics and affinity. Classically it was viewed as a pure Ca²⁺ buffer important for neuronal survival. This view was extended by showing that CB is a critical determinant in the control of synaptic Ca²⁺ dynamics, presumably with strong impact on plasticity and information processing. Already 30 years ago, in vitro studies suggested that CB could have an additional Ca²⁺ sensor function, like its prominent acquaintance calmodulin (CaM). More recent work substantiated this hypothesis, revealing direct CB interactions with several target proteins. Different from a classical sensor, however, CB appears to interact with its targets both, in its Ca²⁺-loaded and Ca²⁺-free forms. Finally, CB has been shown to be involved in buffered transport of Ca²⁺, in neurons but also in kidney. Thus, CB serves a threefold function as buffer, transporter and likely as a non-canonical sensor.

Similar intracellular Ca2+ requirements for inactivation and facilitation of voltage-gated Ca2+ channels in a glutamatergic mammalian nerve terminal

Voltage-gated Ca2+ channels (VGCCs) of the P/Q-type, which are expressed at a majority of mammalian nerve terminals, show two types of Ca2+-dependent feedback regulation-inactivation (CDI) and facilitation (CDF). Because of the nonlinear relationship between Ca2+ influx and transmitter release, CDI and CDF are powerful regulators of synaptic strength. To what extent VGCCs inactivate or facilitate during spike trains depends on the dynamics of free Ca2+ ([Ca2+]i) and the Ca2+ sensitivity of CDI and CDF, which has not been determined in nerve terminals. In this report, we took advantage of the large size of a rat auditory glutamatergic synapse--the calyx of Held--and combined voltage-clamp recordings of presynaptic Ca2+ currents (ICa(V)) with UV-light flash-induced Ca2+ uncaging and presynaptic Ca2+ imaging to study the Ca2+ requirements for CDI and CDF. We find that nearly half of the presynaptic VGCCs inactivate during 100 ms voltage steps and require several seconds to recover. This inactivation is caused neither by depletion of Ca2+ ions from the synaptic cleft nor by metabotropic feedback inhibition, because it is resistant to blockade of metabotropic and ionotropic glutamate receptors. Facilitation of ICa(V) induced by repetitive depolarizations or preconditioning voltage steps decays within tens of milliseconds. Since Ca2+ buffers only weakly affect CDI and CDF, we conclude that the Ca2+ sensors are closely associated with the channel. CDI and CDF can be induced by intracellular photo release of Ca2+ resulting in [Ca2+]i elevations in the low micromolar range, implying a surprisingly high affinity of the Ca2+ sensors.

Ca2+-dependent modulation of voltage-gated Ca2+ channels

BACKGROUND

Voltage-gated (Cav) Ca2+ channels are multi-subunit complexes that play diverse roles in a wide variety of tissues. A fundamental mechanism controlling Cav channel function involves the Ca2+ ions that permeate the channel pore. Ca2+ influx through Cav channels mediates feedback regulation to the channel that is both negative (Ca2+-dependent inactivation, CDI) and positive (Ca2+-dependent facilitation, CDF).

SCOPE OF REVIEW

This review highlights general mechanisms of CDI and CDF with an emphasis on how these processes have been studied electrophysiologically in native and heterologous expression systems.

MAJOR CONCLUSIONS

Electrophysiological analyses have led to detailed insights into the mechanisms and prevalence of CDI and CDF as Cav channel regulatory mechanisms. All Cav channel family members undergo some form of Ca2+-dependent feedback that relies on CaM or a related Ca2+ binding protein. Tremendous progress has been made in characterizing the role of CaM in CDI and CDF. Yet, what contributes to the heterogeneity of CDI/CDF in various cell-types and how Ca2+-dependent regulation of Cav channels controls Ca2+ signaling remain largely unexplored.

GENERAL SIGNIFICANCE

Ca2+ influx through Cav channels regulates diverse physiological events including excitation-contraction coupling in muscle, neurotransmitter and hormone release, and Ca2+-dependent gene transcription. Therefore, the mechanisms that regulate channels, such as CDI and CDF, can have a large impact on the signaling potential of excitable cells in various physiological contexts. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signaling.

Synaptic vesicles control the time course of neurotransmitter secretion via a Ca²+/H+ antiport

We investigated the physiological role of the vesicular Ca2+/H+ antiport in rapid synaptic transmission using the Torpedo electric organ (a modified neuromuscular system). By inhibiting V-type H+-transporting ATPase (V-ATPase), bafilomycin A1 dissipates the H+ gradient of synaptic vesicles, thereby abolishing the Ca2+/H+ antiport driving force. In electrophysiology experiments, bafilomycin A1 significantly prolonged the duration of the evoked electroplaque potential. A biochemical assay for acetylcholine (ACh) release showed that the effect of bafilomycin A1 was presynaptic. Indeed, bafilomycin A1 increased the amount of radio-labelled ACh released in response to paired-pulse stimulation. Bafilomycin A1 also enhanced Ca2+-dependent ACh release from isolated nerve terminals (synaptosomes). The bafilomycin-induced electroplaque potential lengthening did not arise from cholinesterase inhibition, since eserine (which also prolonged the electroplaque potential) strongly decreased evoked ACh release. Bafilomycin A1 augmented the amount of calcium accumulating in nerve terminals following a short tetanic stimulation and delayed subsequent calcium extrusion. By reducing stimulation-dependent calcium accumulation in synaptic vesicles, bafilomycin A1 diminished the corresponding depletion of vesicular ACh, as tested using both intact tissue and isolated synaptic vesicles. Strontium ions inhibit the vesicular Ca2+/H+ antiport, while activating transmitter release at concentrations one order of magnitude higher than Ca2+ does. In the presence of Sr2+ the time course of the electroplaque potential was also prolonged but, unlike bafilomycin A1, Sr2+ enhanced facilitation in paired-pulse experiments. It is therefore proposed that the vesicular Ca2+/H+ antiport function is to shorten 'phasic' transmitter release, allowing the synapse to transmit briefer impulses and so to work at higher frequencies.

Minor contribution of cytosolic Ca2+ transients to the pacemaker rhythm in guinea pig sinoatrial node cells

The question of the extent to which cytosolic Ca(2+) affects sinoatrial node pacemaker activity has been discussed for decades. We examined this issue by analyzing two mathematical pacemaker models, based on the "Ca(2+) clock" (C) and "membrane clock" (M) hypotheses, together with patch-clamp experiments in isolated guinea pig sinoatrial node cells. By applying lead potential analysis to the models, the C mechanism, which is dependent on potentiation of Na(+)/Ca(2+) exchange current via spontaneous Ca(2+) release from the sarcoplasmic reticulum (SR) during diastole, was found to overlap M mechanisms in the C model. Rapid suppression of pacemaker rhythm was observed in the C model by chelating intracellular Ca(2+), whereas the M model was unaffected. Experimental rupturing of the perforated-patch membrane to allow rapid equilibration of the cytosol with 10 mM BAPTA pipette solution, however, failed to decrease the rate of spontaneous action potential within ∼30 s, whereas contraction ceased within ∼3 s. The spontaneous rhythm also remained intact within a few minutes when SR Ca(2+) dynamics were acutely disrupted using high doses of SR blockers. These experimental results suggested that rapid disruption of normal Ca(2+) dynamics would not markedly affect spontaneous activity. Experimental prolongation of the action potentials, as well as slowing of the Ca(2+)-mediated inactivation of the L-type Ca(2+) currents induced by BAPTA, were well explained by assuming Ca(2+) chelation, even in the proximity of the channel pore in addition to the bulk cytosol in the M model. Taken together, the experimental and model findings strongly suggest that the C mechanism explicitly described by the C model can hardly be applied to guinea pig sinoatrial node cells. The possible involvement of L-type Ca(2+) current rundown induced secondarily through inhibition of Ca(2+)/calmodulin kinase II and/or Ca(2+)-stimulated adenylyl cyclase was discussed as underlying the disruption of spontaneous activity after prolonged intracellular Ca(2+) concentration reduction for >5 min.

Cytosolic Ca2+ buffers

"Ca(2+) buffers," a class of cytosolic Ca(2+)-binding proteins, act as modulators of short-lived intracellular Ca(2+) signals; they affect both the temporal and spatial aspects of these transient increases in [Ca(2+)](i). Examples of Ca(2+) buffers include parvalbumins (α and β isoforms), calbindin-D9k, calbindin-D28k, and calretinin. Besides their proven Ca(2+) buffer function, some might additionally have Ca(2+) sensor functions. Ca(2+) buffers have to be viewed as one of the components implicated in the precise regulation of Ca(2+) signaling and Ca(2+) homeostasis. Each cell is equipped with proteins, including Ca(2+) channels, transporters, and pumps that, together with the Ca(2+) buffers, shape the intracellular Ca(2+) signals. All of these molecules are not only functionally coupled, but their expression is likely to be regulated in a Ca(2+)-dependent manner to maintain normal Ca(2+) signaling, even in the absence or malfunctioning of one of the components.

Compensatory regulation of Cav2.1 Ca2+ channels in cerebellar Purkinje neurons lacking parvalbumin and calbindin D-28k

Ca(v)2.1 channels regulate Ca(2+) signaling and excitability of cerebellar Purkinje neurons. These channels undergo a dual feedback regulation by incoming Ca(2+) ions, Ca(2+)-dependent facilitation and inactivation. Endogenous Ca(2+)-buffering proteins, such as parvalbumin (PV) and calbindin D-28k (CB), are highly expressed in Purkinje neurons and therefore may influence Ca(v)2.1 regulation by Ca(2+). To test this, we compared Ca(v)2.1 properties in dissociated Purkinje neurons from wild-type (WT) mice and those lacking both PV and CB (PV/CB(-/-)). Unexpectedly, P-type currents in WT and PV/CB(-/-) neurons differed in a way that was inconsistent with a role of PV and CB in acute modulation of Ca(2+) feedback to Ca(v)2.1. Ca(v)2.1 currents in PV/CB(-/-) neurons exhibited increased voltage-dependent inactivation, which could be traced to decreased expression of the auxiliary Ca(v)beta(2a) subunit compared with WT neurons. Although Ca(v)2.1 channels are required for normal pacemaking of Purkinje neurons, spontaneous action potentials were not different in WT and PV/CB(-/-) neurons. Increased inactivation due to molecular switching of Ca(v)2.1 beta-subunits may preserve normal activity-dependent Ca(2+) signals in the absence of Ca(2+)-buffering proteins in PV/CB(-/-) Purkinje neurons.

Stabilization of Ca current in Purkinje neurons during high-frequency firing by a balance of Ca-dependent facilitation and inactivation

Purkinje neurons fire spontaneous action potentials at ∼50 spikes/sec and generate more than 100 spikes/sec during cerebellum-mediated behaviors. Many voltage-gated channels, including Ca channels, can inactivate and/or facilitate with repeated stimulation, raising the question of how these channels respond to regular, rapid trains of depolarizations. To test whether Ca currents are modulated during firing, we recorded voltage-clamped Ca currents, predominantly carried by P-type Ca channels, from acutely dissociated mouse Purkinje neurons at 30-33°C (1 mM Ca). With 0.5 mM intracellular EGTA, 1-second trains of either spontaneous action potential waveforms or brief depolarizing steps at 50 Hz evoked Ca tail currents that were stable, remaining within 5% of the first tail current throughout the train. Higher frequency trains (100 and 200 Hz) elicited a maximal inactivation of <10%. To test whether this stability of Ca currents resulted from a lack of modulation or from an equilibrium between facilitation and inactivation, we manipulated the permeant ion (Ca vs. Ba) and Ca buffering (0.5 vs. 10 mM EGTA). With low buffering, Ba accelerated the initial inactivation evoked by 1-second trains, but reduced its extent at 200 Hz, consistent with an early calcium-dependent facilitation (CDF) and late calcium-dependent inactivation (CDI) at high frequencies. Increasing the Ca buffer favored CDF. These data suggest that stable Ca current amplitudes result from a balance of CDF, CDI, and voltage-dependent inactivation. This modest net Ca-dependent modulation may contribute to the ability of Purkinje neurons to sustain long periods of regular firing and synaptic transmission.

Facilitation of P2X7 receptor currents and membrane blebbing via constitutive and dynamic calmodulin binding

The ATP-gated P2X(7) receptor (P2X(7)R) is a highly unusual calcium-permeable cationic channel in that within seconds of its activation, dramatic and reversible cytoskeletal rearrangements with prominent membrane blebbing occurs. Agonist-induced membrane currents at hyperpolarized potentials show pronounced facilitation during the initial 30-100 s of receptor activation but mechanisms responsible have not been elucidated. We measured facilitation of ATP-gated currents in HEK cells expressing rat P2X(7)R and delineated distinct calcium-dependent and independent processes. The calcium-dependent facilitation was composed of an instantaneous (millisecond time domain) and slowly developing (time constant, 20 s with maximum agonist stimulation) component. Both components were prevented when recording with a highly specific calmodulin (CaM) inhibitory peptide but only the instantaneous component was reduced by expression of the dominant-negative EF-handless CaM mutant. Coimmunoprecipitation assays detected low levels of CaM binding to unstimulated P2X(7)R, and this increased by 50% during 45 s stimulation of the receptor. We identified a novel 1-5-16 Ca(2+)-dependent CaM binding motif in the intracellular C terminus of P2X(7)R; mutations in this domain resulted in the absence of calcium-dependent facilitation and binding of CaM to unstimulated or stimulated receptor. Blockade of CaM binding also delayed membrane blebbing by threefold. Our results demonstrate that CaM binds constitutively to closed P2X(7)R channels and dynamically during channel activation to significantly enhance and prolong calcium entry. This is the first example of CaM deregulating, rather than tightly controlling, calcium entry through an ion channel.

Modeling study of the effects of membrane surface charge on calcium microdomains and neurotransmitter release

Synchronous neurotransmitter release is mediated by the opening of voltage-gated Ca(2+) channels and the build-up of submembrane Ca(2+) microdomains. Previous models of Ca(2+) microdomains have neglected possible electrostatic interactions between Ca(2+) ions and negative surface charges on the inner leaflet of the plasma membrane. To address the effects of these interactions, we built a computational model of ion electrodiffusion described by the Nernst-Planck and Poisson equations. We found that inclusion of a negative surface charge significantly alters the spatial characteristics of Ca(2+) microdomains. Specifically, close to the membrane, Ca(2+) ions accumulate, as expected from the strong electrostatic attraction exerted on positively charged Ca(2+) ions. Farther away from the membrane, increasing the surface charge density results in a reduction of the Ca(2+) concentration because of the preferential spread of Ca(2+) ions along lateral directions. The model also predicts that the negative surface charge will decrease the spatial gradient of the Ca(2+) microdomain in the lateral direction, resulting in increased overlap of microdomains originating from different Ca(2+) channels. Finally, we found that surface charge increases the probability of vesicle release if the Ca(2+) sensor is located within the electrical double layer, whereas this probability is decreased if the Ca(2+) sensor lies at greater distances from the membrane. Our data suggest that membrane surface charges exert a significant influence on the profile of Ca(2+) microdomains, and should be taken into account in models of neurotransmitter release.

Interactions between multiple sources of short-term plasticity during evoked and spontaneous activity at the rat calyx of Held

Sustained activity at most central synapses is accompanied by a number of short-term changes in synaptic strength which act over a range of time scales. Here we examine experimental data and develop a model of synaptic depression at the calyx of Held synaptic terminal that combines many of these mechanisms (acting at differing sites and across a range of time scales). This new model incorporates vesicle recycling, facilitation, activity-dependent vesicle retrieval and multiple mechanisms affecting calcium channel activity and release probability. It can accurately reproduce the time course of experimentally measured short-term depression across different stimulus frequencies and exhibits a slow decay in EPSC amplitude during sustained stimulation. We show that the slow decay is a consequence of vesicle release inhibition by multiple mechanisms and is accompanied by a partial recovery of the releasable vesicle pool. This prediction is supported by patch-clamp data, using long duration repetitive EPSC stimulation at up to 400 Hz. The model also explains the recovery from depression in terms of interaction between these multiple processes, which together generate a stimulus-history-dependent recovery after repetitive stimulation. Given the high rates of spontaneous activity in the auditory pathway, the model also demonstrates how these multiple interactions cause chronic synaptic depression under in vivo conditions. While the magnitude of the depression converges to the same steady state for a given frequency, the time courses of onset and recovery are faster in the presence of spontaneous activity. We conclude that interactions between multiple sources of short-term plasticity can account for the complex kinetics during high frequency stimulation and cause stimulus-history-dependent recovery at this relay synapse.

Ca2+-binding proteins tune Ca2+-feedback to Cav1.3 channels in mouse auditory hair cells

Sound coding at the auditory inner hair cell synapse requires graded changes in neurotransmitter release, triggered by sustained activation of presynaptic Ca(v)1.3 voltage-gated Ca(2+) channels. Central to their role in this regard, Ca(v)1.3 channels in inner hair cells show little Ca(2+)-dependent inactivation, a fast negative feedback regulation by incoming Ca(2+) ions, which depends on calmodulin association with the Ca(2+) channel alpha(1) subunit. Ca(2+)-dependent inactivation characterizes nearly all voltage-gated Ca(2+) channels including Ca(v)1.3 in other excitable cells. The mechanism underlying the limited autoregulation of Ca(v)1.3 in inner hair cells remains a mystery. Previously, we established calmodulin-like Ca(2+)-binding proteins in the brain and retina (CaBPs) as essential modulators of voltage-gated Ca(2+) channels. Here, we demonstrate that CaBPs differentially modify Ca(2+) feedback to Ca(v)1.3 channels in transfected cells and explore their significance for Ca(v)1.3 regulation in inner hair cells. Of multiple CaBPs detected in inner hair cells (CaBP1, CaBP2, CaBP4 and CaBP5), CaBP1 most efficiently blunts Ca(2+)-dependent inactivation of Ca(v)1.3. CaBP1 and CaBP4 both interact with calmodulin-binding sequences in Ca(v)1.3, but CaBP4 more weakly inhibits Ca(2+)-dependent inactivation than CaBP1. Ca(2+)-dependent inactivation is marginally greater in inner hair cells from CaBP4(-/-) than from wild-type mice, yet CaBP4(-/-) mice are not hearing-impaired. In contrast to CaBP4, CaBP1 is strongly localized at the presynaptic ribbon synapse of adult inner hair cells both in wild-type and CaBP4(-/-) mice and therefore is positioned to modulate native Ca(v)1.3 channels. Our results reveal unexpected diversity in the strengths of CaBPs as Ca(2+) channel modulators, and implicate CaBP1 rather than CaBP4 in conferring the anomalous slow inactivation of Ca(v)1.3 Ca(2+) currents required for auditory transmission.

Differential regulation of endogenous N- and P/Q-type Ca2+ channel inactivation by Ca2+/calmodulin impacts on their ability to support exocytosis in chromaffin cells

P/Q-type (Ca(V)2.1) and N-type (Ca(V)2.2) Ca2+ channels are critical to stimulus-secretion coupling in the nervous system; feedback regulation of these channels by Ca2+ is therefore predicted to profoundly influence neurotransmission. Here we report divergent regulation of Ca2+-dependent inactivation (CDI) of native N- and P/Q-type Ca2+ channels by calmodulin (CaM) in adult chromaffin cells. Robust CDI of N-type channels was observed in response to prolonged step depolarizations, as well as repetitive stimulation with either brief step depolarizations or action potential-like voltage stimuli. Adenoviral expression of Ca2+-insensitive calmodulin mutants eliminated CDI of N-type channels. This is the first demonstration of CaM-dependent CDI of a native N-type channel. CDI of P/Q-type channels was by comparison modest and insensitive to expression of CaM mutants. Cloning of the C terminus of the Ca(V)2.1 alpha1 subunit from chromaffin cells revealed multiple splice variants lacking structural motifs required for CaM-dependent CDI. The physiological relevance of CDI on stimulus-coupled exocytosis was revealed by combining perforated-patch voltage-clamp recordings of pharmacologically isolated Ca2+ currents with membrane capacitance measurements of exocytosis. Increasing stimulus intensity to invoke CDI resulted in a significant decrease in the exocytotic efficiency of N-type channels compared with P/Q-type channels. Our results reveal unexpected diversity in CaM regulation of native Ca(V)2 channels and suggest that the ability of individual Ca2+ channel subtypes to undergo CDI may be tailored by alternative splicing to meet the specific requirements of a particular cellular function.

Calcium microdomains in regulated exocytosis

Katz and co-workers showed that Ca(2+) triggers exocytosis. The existence of sub-micrometer domains of greater than 100 microM [Ca(2+)](i) was postulated on theoretical grounds. Using a modified, low-affinity aequorin, Llinas et al. were the first to demonstrate the existence of Ca(2+) 'microdomains' in squid presynaptic terminals. Over the past several years, it has become clear that individual Ca(2+) nano- and microdomains forming around the mouth of voltage-gated Ca(2+) channels ascertain the tight coupling of fast synaptic vesicle release to membrane depolarization by action potentials. Recent work has established different geometric arrangements of vesicles and Ca(2+) channels at different central synapses and pointed out the role of Ca(2+) syntillas - localized, store operated Ca(2+) signals - in facilitation and spontaneous release. The coupling between Ca(2+) increase and evoked exocytosis is more sluggish in peripheral terminals and neuroendocrine cells, where channels are less clustered and Ca(2+) comes from different sources, including Ca(2+) influx via the plasma membrane and the mobilization of Ca(2+) from intracellular stores. Finally, also non- (electrically) excitable cells display highly localized Ca(2+) signaling domains. We discuss in particular the organization of structural microdomains of Bergmann glia, specialized astrocytes of the cerebellum that have only recently been considered as secretory cells. Glial microdomains are the spatial substrate for functionally segregated Ca(2+) signals upon metabotropic activation. Our review emphasizes the large diversity of different geometric arrangements of vesicles and Ca(2+) sources, leading to a wide spectrum of Ca(2+) signals triggering release.

Dual role of calbindin-D28Kin vesicular catecholamine release from mouse chromaffin cells

Calbindin-D(28K) is suggested to play a postsynaptic role in neurotransmission and in the regulation of the intracellular Ca(2+) concentration. However, it is still unclear whether calbindin-D(28K) has a role in the regulation of exocytosis, either as Ca(2+) buffer or as Ca(2+) sensor. Amperometric recordings of catecholamine exocytosis from wild-type and calbindin-D(28K) knockout mouse chromaffin cells reveal a strong reduction in the number of released vesicles, as well as in the amount of neurotransmitter released per fusion event in knockout cells. However, Ca(2+) current recordings and Ca(2+) imaging experiments, including video-rate confocal laser scanning microscopy, revealed that the intracellular Ca(2+) dynamics are remarkably similar in wild-type and knockout cells. The combined results demonstrate that calbindin-D(28K) plays an important and dual role in exocytosis, affecting both release frequency and quantal size, apparently without strong effects on intracellular Ca(2+) dynamics. Consequently, the possibility that calbindin-D(28K) functions not only as a Ca(2+) buffer but also as a modulator of vesicular catecholamine release is discussed.

Calmodulin kinase II accelerates L-type Ca2+ current recovery from inactivation and compensates for the direct inhibitory effect of [Ca2+]i in rat ventricular myocytes

Some studies report that the positive relationship between L-type Ca(2+) current (I(Ca-L)) and frequency in cardiac myocytes is mainly due to a direct negative feedback of sarcoplasmic reticulum Ca(2+) release on I(Ca-L) inactivation while others provide evidence for activation of calmodulin kinase II (CaMKII). To further elucidate the role of endogenous CaMKII activity, the CaMKII inhibitory peptides, autocamtide-2 relating inhibitory peptide (AIP) and myristoylated AIP were applied using conventional and perforated patch-clamp methods. AIP inhibited the normal adaptive increase in I(Ca-L) in response to abrupt increase in pacing frequency from 0.05 to 2 Hz. The positive I(Ca-L)-frequency relationship was reversed by AIP and the inhibitory effect of AIP was significantly exaggerated at fast pacing rates. The onset of inactivation of I(Ca-L) was not altered by AIP. After thapsigargin treatment, AIP slowed recovery from inactivation of I(Ca-L) and this effect was exaggerated during fast pacing. Buffering of [Ca(2+)](i) by BAPTA and EGTA accelerated recovery of I(Ca-L) from inactivation, and BAPTA partly eliminated the effect of AIP on the recovery. We conclude that: (1) [Ca(2+)](i) directly slows I(Ca-L) recovery from inactivation; and (2) Ca(2+)-dependent endogenous CaMKII activity accelerates the I(Ca-L) recovery. Thus, at fast heart rates, elevated [Ca(2+)](i) activates endogenous CaMKII and compensates for its direct inhibitory effect on I(Ca-L) recovery from inactivation. Dynamic activity of endogenous CaMKII enhances the positive I(Ca-L)-frequency relationship.