Interplay between Na+/Ca2+ exchangers and mitochondria in Ca2+ clearance at the calyx of Held

Regulation of K+-Dependent Na+/Ca2+-Exchangers (NCKX)

Potassium-dependent sodium-calcium exchangers (NCKX) have emerged as key determinants of calcium (Ca²⁺) signaling and homeostasis, especially in environments where ion concentrations undergo large changes, such as excitatory cells and transport epithelia. The regulation of NCKX transporters enables them to respond to the changing cellular environment thereby helping to shape the extent and kinetics of Ca²⁺ signals. This review examines the current knowledge of the different ways in which NCKX activity can be modulated. These include (i) cellular and dynamic subcellular location (ii); changes in protein expression mediated at the gene, transcript, or protein level (iii); genetic changes resulting in altered protein structure or expression (iv); regulation via changes in substrate concentration (v); and post-translational modification, partner protein interactions, and allosteric regulation. Detailed mechanistic understanding of NCKX regulation is an emerging area of research with the potential to provide important new insights into transporter function, the control of Ca²⁺ signals, and possible interventions for dysregulated Ca²⁺ homeostasis.

Calretinin-Expressing Synapses Show Improved Synaptic Efficacy with Reduced Asynchronous Release during High-Rate Activity

Calretinin (CR) is a major calcium binding protein widely expressed in the CNS. However, its synaptic function remains largely elusive. At the auditory synapse of the endbulb of Held, CR is selectively expressed in different subtypes. Combining electrophysiology with immunohistochemistry, we investigated the synaptic transmission at the endbulb of Held synapses with and without endogenous CR expression in mature CBA/CAJ mice of either sex. Two synapse subtypes showed similar basal synaptic transmission, except a larger quantal size in CR-expressing synapses. During high-rate stimulus trains, CR-expressing synapses showed improved synaptic efficacy with significantly less depression and lower asynchronous release, suggesting more efficient exocytosis than non-CR-expressing synapses. Conversely, CR-expressing synapses had a smaller readily releasable pool size, which was countered by higher release probability and faster synaptic recovery to support sustained release during high-rate activity. EGTA-AM treatment did not change the synaptic transmission of CR-expressing synapses, but reduced synaptic depression and decreased asynchronous release at non-CR-expressing synapses, suggesting that CR helps to minimize calcium accumulation during high-rate activity. Both synapses express parvalbumin, another calcium-binding protein with slower kinetics and higher affinity than CR, but not calbindin. Furthermore, CR-expressing synapses only express the fast isoform of vesicular glutamate transporter 1 (VGluT1), while most non-CR-expressing synapses express both VGluT1 and the slower VGluT2, which may underlie their lagged synaptic recovery. The findings suggest that, paired with associated synaptic machinery, differential CR expression regulates synaptic efficacy among different subtypes of auditory nerve synapses to accomplish distinctive physiological functions in transmitting auditory information at high rates.SIGNIFICANCE STATEMENT CR is a major calcium-binding protein in the brain. It remains unclear how endogenous CR impacts synaptic transmission. We investigated the question at the large endbulb of Held synapses with selective CR expression and found that CR-expressing and non-CR-expressing synapses had similar release properties under basal synaptic transmission. During high-rate activity, however, CR-expressing synapses showed improved synaptic efficacy with less depression, lower asynchronous release, and faster recovery. Furthermore, CR-expressing synapses use exclusive VGluT1 to refill synaptic vesicles, while non-CR-expressing synapses use both VGluT1 and the slower isoform of VGluT2. Our findings suggest that CR may play significant roles in promoting synaptic efficacy during high-rate activity, and selective CR expression can differentially impact signal processing among different synapses.

A novel K+ -dependent Na+ uptake mechanism during low pH exposure in adult zebrafish (Danio rerio): New tricks for old dogma

AIM

To determine whether Na⁺ uptake in adult zebrafish (Danio rerio) exposed to acidic water adheres to traditional models reliant on Na⁺ /H⁺ Exchangers (NHEs), Na⁺ channels and Na⁺ /Cl^- Cotransporters (NCCs) or if it occurs through a novel mechanism.

METHODS

Zebrafish were exposed to control (pH 8.0) or acidic (pH 4.0) water for 0-12 hours during which ²² Na⁺ uptake ( J Na in ), ammonia excretion, net acidic equivalent flux and net K⁺ flux ( J H net ) were measured. The involvement of NHEs, Na⁺ channels, NCCs, K⁺ -channels and K⁺ -dependent Na⁺ /Ca²⁺ exchangers (NCKXs) was evaluated by exposure to Cl^- -free or elevated [K⁺ ] water, or to pharmacological inhibitors. The presence of NCKXs in gill was examined using RT-PCR.

RESULTS

J Na in was strongly attenuated by acid exposure, but gradually recovered to control rates. The systematic elimination of each of the traditional models led us to consider K⁺ as a counter substrate for Na⁺ uptake during acid exposure. Indeed, elevated environmental [K⁺ ] inhibited J Na in during acid exposure in a concentration-dependent manner, with near-complete inhibition at 10 mM. Moreover, J H net loss increased approximately fourfold at 8-10 hours of acid exposure which correlated with recovered J Na in in 1:1 fashion, and both J Na in and J H net were sensitive to tetraethylammonium (TEA) during acid exposure. Zebrafish gills expressed mRNA coding for six NCKX isoforms.

CONCLUSIONS

During acid exposure, zebrafish engage a novel Na⁺ uptake mechanism that utilizes the outwardly directed K⁺ gradient as a counter-substrate for Na⁺ and is sensitive to TEA. NKCXs are promising candidates to mediate this K⁺ -dependent Na⁺ uptake, opening new research avenues about Na⁺ uptake in zebrafish and other acid-tolerant aquatic species.

Developmental shift to mitochondrial respiration for energetic support of sustained transmission during maturation at the calyx of Held

A considerable amount of energy is expended following presynaptic activity to regenerate electrical polarization and maintain efficient release and recycling of neurotransmitter. Mitochondria are the major suppliers of neuronal energy, generating ATP via oxidative phosphorylation. However, the specific utilization of energy from cytosolic glycolysis rather than mitochondrial respiration at the presynaptic terminal during synaptic activity remains unclear and controversial. We use a synapse specialized for high-frequency transmission in mice, the calyx of Held, to test the sources of energy used to maintain energy during short activity bursts (<1 s) and sustained neurotransmission (30-150 s). We dissect the role of presynaptic glycolysis versus mitochondrial respiration by acutely and selectively blocking these ATP-generating pathways in a synaptic preparation where mitochondria and synaptic vesicles are prolific, under near-physiological conditions. Surprisingly, if either glycolysis or mitochondrial ATP production is intact, transmission during repetitive short bursts of activity is not affected. In slices from young animals before the onset of hearing, where the synapse is not yet fully specialized, both glycolytic and mitochondrial ATP production are required to support sustained, high-frequency neurotransmission. In mature synapses, sustained transmission relies exclusively on mitochondrial ATP production supported by bath lactate, but not glycolysis. At both ages, we observe that action potential propagation begins to fail before defects in synaptic vesicle recycling. Our data describe a specific metabolic profile to support high-frequency information transmission at the mature calyx of Held, shifting during postnatal synaptic maturation from glycolysis to rely on monocarboxylates as a fuel source.NEW & NOTEWORTHY We dissect the role of presynaptic glycolysis versus mitochondrial respiration in supporting high-frequency neurotransmission, by acutely blocking these ATP-generating pathways at a synapse tuned for high-frequency transmission. We find that massive energy expenditure is required to generate failure when only one pathway is inhibited. Action potential propagation is lost before impaired synaptic vesicle recycling. Synaptic transmission is exclusively dependent on oxidative phosphorylation in mature synapses, indicating presynaptic glycolysis may be dispensable for ATP maintenance.

Phase-Locking Requires Efficient Ca2+ Extrusion at the Auditory Hair Cell Ribbon Synapse

Proper perception of sounds in the environment requires auditory signals to be encoded with extraordinary temporal precision up to tens of microseconds, but how it originates from the hearing organs in the periphery is poorly understood. In particular, sound-evoked spikes in auditory afferent fibers in vivo are phase-locked to sound frequencies up to 5 kHz, but it is not clear how hair cells can handle intracellular Ca²⁺ changes with such high speed and efficiency. In this study, we combined patch-clamp recording and two-photon Ca²⁺ imaging to examine Ca²⁺ dynamics in hair cell ribbon synapses in the bullfrog amphibian papilla of both sexes. We found that Ca²⁺ clearance from single synaptic ribbons followed a double exponential function, and the weight of the fast component, but not the two time constants, was significantly reduced for prolonged stimulation, and during inhibition of the plasma membrane Ca²⁺ ATPase (PMCA), the mitochondrial Ca²⁺ uptake (MCU), or the sarcolemma/endoplasmic reticulum Ca²⁺ ATPase (SERCA), but not the Na⁺/Ca²⁺ exchanger (NCX). Furthermore, we found that both the basal Ca²⁺ level and the Ca²⁺ rise during sinusoidal stimulation were significantly increased by inhibition of PMCA, MCU, or SERCA. Consistently, phase-locking of synaptic vesicle releases from hair cells was also significantly reduced by blocking PMCA, MCU, or SERCA, but not NCX. We conclude that, in addition to fast diffusion mediated by mobile Ca²⁺ buffer, multiple Ca²⁺ extrusion pumps are required for phase-locking at the auditory hair cell ribbon synapse.SIGNIFICANCE STATEMENT Hair cell synapses can transmit sound-driven signals precisely in the kHz range. However, previous studies of Ca²⁺ handling in auditory hair cells have often been conducted in immature hair cells, with elevated extracellular Ca²⁺ concentration, or through steady-state stimulation that may not be physiologically relevant. Here we examine Ca²⁺ clearance from hair cell synaptic ribbons in a fully mature preparation at physiological concentration of external Ca²⁺ and at physiological temperature. By stimulating hair cells with sinusoidal voltage commands that mimic pure sound tones, we recapitulated the phase-locking of hair cell exocytosis with an in vitro approach. This allowed us to reveal the Ca²⁺ extrusion mechanisms that are required for phase-locking at auditory hair cell ribbon synapses.

Calmodulin binds and modulates K+-dependent Na+/Ca2+-exchanger isoform 4, NCKX4

The family of K+-dependent Na+/Ca2+-exchangers, NCKX, are important mediators of cellular Ca2+ efflux, particularly in neurons associated with sensory transduction. The NCKX family comprises five proteins, NCKX1-5, each being the product of a different SLC24 gene. NCKX4 (SLC24A4) has been found to have a critical role in termination and adaptation of visual and olfactory signals, melanocortin-dependent satiety signaling, and the maturation of dental enamel. To explore mechanisms that might influence the temporal control of NCKX4 activity, a yeast two-hybrid system was used to search for protein interaction partners. We identified calmodulin as a partner for NCKX4 and confirmed the interaction using glutathione-S-transferase fusion pull-down. Calmodulin binding to NCKX4 was demonstrated in extracts from mouse brain and in transfected HEK293 cells. Calmodulin bound in a Ca2+-dependent manner to a motif present in the central cytosolic loop of NCKX4 and was abolished by the double-mutant I328D/F334D. When cotransfected in HEK293 cells, calmodulin bound to NCKX4 under basal conditions and induced a ∼2.5-fold increase in NCKX4 abundance, but did not influence either cellular location or basal activity. When purinergic stimulation of NCKX4 was examined in these cells, coexpression of wild-type calmodulin, but not a Ca2+ binding-deficient calmodulin mutant, suppressed NCKX4 activation in a time-dependent manner. We propose that Ca2+ binding to calmodulin prepositioned on NCKX4 induces a slow conformational rearrangement that interferes with purinergic stimulation of the exchanger, possibly by obscuring T331, a previously identified potential protein kinase C site.

The Parvalbumin Hypothesis of Autism Spectrum Disorder

The prevalence of autism spectrum disorder (ASD)-a type of neurodevelopmental disorder-is increasing and is around 2% in North America, Asia, and Europe. Besides the known genetic link, environmental, epigenetic, and metabolic factors have been implicated in ASD etiology. Although highly heterogeneous at the behavioral level, ASD comprises a set of core symptoms including impaired communication and social interaction skills as well as stereotyped and repetitive behaviors. This has led to the suggestion that a large part of the ASD phenotype is caused by changes in a few and common set of signaling pathways, the identification of which is a fundamental aim of autism research. Using advanced bioinformatics tools and the abundantly available genetic data, it is possible to classify the large number of ASD-associated genes according to cellular function and pathways. Cellular processes known to be impaired in ASD include gene regulation, synaptic transmission affecting the excitation/inhibition balance, neuronal Ca2+ signaling, development of short-/long-range connectivity (circuits and networks), and mitochondrial function. Such alterations often occur during early postnatal neurodevelopment. Among the neurons most affected in ASD as well as in schizophrenia are those expressing the Ca2+-binding protein parvalbumin (PV). These mainly inhibitory interneurons present in many different brain regions in humans and rodents are characterized by rapid, non-adaptive firing and have a high energy requirement. PV expression is often reduced at both messenger RNA (mRNA) and protein levels in human ASD brain samples and mouse ASD (and schizophrenia) models. Although the human PVALB gene is not a high-ranking susceptibility/risk gene for either disorder and is currently only listed in the SFARI Gene Archive, we propose and present supporting evidence for the Parvalbumin Hypothesis, which posits that decreased PV level is causally related to the etiology of ASD (and possibly schizophrenia).

Inter-spike mitochondrial Ca2+ release enhances high frequency synaptic transmission

KEY POINTS

Presynaptic mitochondria not only absorb but also release Ca²⁺ during high frequency stimulation (HFS) when presynaptic [Ca²⁺ ] is kept low (<500 nm) by high cytosolic Ca²⁺ buffer or strong plasma membrane calcium clearance mechanisms under physiological external [Ca²⁺ ]. Mitochondrial Ca²⁺ release (MCR) does not alter the global presynaptic Ca²⁺ transients. MCR during HFS enhances short-term facilitation and steady state excitatory postsynaptic currents by increasing vesicular release probability. The intra-train MCR may provide residual calcium at interspike intervals, and thus support high frequency neurotransmission at central glutamatergic synapses.

ABSTRACT

Emerging evidence indicates that mitochondrial Ca²⁺ buffering contributes to local regulation of synaptic transmission. It is unknown, however, whether mitochondrial Ca²⁺ release (MCR) occurs during high frequency synaptic transmission. Confirming the previous notion that 2 μm tetraphenylphosphonium (TPP⁺ ) is a specific inhibitor of the mitochondrial Na⁺ /Ca²⁺ exchanger (mNCX), we studied the role of MCR via mNCX in short-term plasticity during high frequency stimulation (HFS) at the calyx of Held synapse of the rat. TPP⁺ reduced short-term facilitation (STF) and steady state excitatory postsynaptic currents during HFS at mature calyx synapses under physiological extracellular [Ca²⁺ ] ([Ca²⁺ ]_o  = 1.2 mm), but not at immature calyx or at 2 mm [Ca²⁺ ]_o . The inhibitory effects of TPP⁺ were stronger at synapses with morphologically complex calyces harbouring many swellings and at 32°C than at simple calyx synapses and at room temperature. These effects of TPP⁺ on STF were well correlated with those on the presynaptic mitochondrial [Ca²⁺ ] build-up during HFS. Mitochondrial [Ca²⁺ ] during HFS was increased by TPP⁺ at mature calyces under 1.2 mm [Ca²⁺ ]_o , and further enhanced at 32°C, but not under 2 mm [Ca²⁺ ]_o or at immature calyces. The close correlation of the effects of TPP⁺ on mitochondrial [Ca²⁺ ] with those on STF suggests that mNCX contributes to STF at the calyx of Held synapses. The intra-train MCR enhanced vesicular release probability without altering global presynaptic [Ca²⁺ ]. Our results suggest that MCR during HFS elevates local [Ca²⁺ ] near synaptic sites at interspike intervals to enhance STF and to support stable synaptic transmission under physiological [Ca²⁺ ]_o .

Different effects of metformin and phenformin on hypoxia-induced Ca2+ fluxes in cultured primary neurons

Recent evidence suggests that metformin and phenformin may exert beneficial effects against neuronal injury in the ischemic brain, however, the difference of action between these two drugs and the molecular mechanism of such protection is not clear. In this study, we investigated whether mild hypoxia-affected neurons exhibit changes in cytosolic calcium handling and whether metformin and phenformin exert any effect on calcium homeostasis in hypoxia-affected neurons. Cultured primary rat cortical cells were stained with calcium sensitive dye Oregon Green 488 BAPTA-1 AM and spontaneous calcium dependent changes of fluorescence were recorded. Using obtained fluorescence traces we estimated changes in relative amplitude of recorded spontaneous signals, changes in frequency of spontaneous activity, and changes in decay of fluorescence traces. We found that hypoxia caused reduction of the relative signal amplitude, increased the spontaneous activity, and slowed the decay of calcium concentration. After pre-treatment of cells with 0.1-0.5 mM metformin, the relative signal amplitude increased and the frequency of spontaneous signals decreased in hypoxia-affected neurons. However, pre-treatment with 1-25 µM phenformin neither increased the relative signal amplitude nor reduced the frequency of spontaneous signals. The decay of fluorescence traces became faster after application of metformin or phenformin comparing to neurons under hypoxic conditions. These results suggest different action of metformin and phenformin in improvement of Ca²⁺ homeostasis in hypoxia-affected neurons, which may have different effects on neuronal survival and functions after hypoxia/ischemia.

Activity and Cytosolic Na+ Regulate Synaptic Vesicle Endocytosis

Retrieval of synaptic vesicles via endocytosis is essential for maintaining sustained synaptic transmission, especially for neurons that fire action potentials at high frequencies. However, how neuronal activity regulates synaptic vesicle recycling is largely unknown. Here we report that Na⁺ substantially accumulated in the mouse calyx of Held terminals of either sex during repetitive high-frequency spiking. Elevated presynaptic Na⁺ accelerated both slow and rapid forms of endocytosis and facilitated endocytosis overshoot, but did not affect the readily releasable pool size, Ca²⁺ influx, or exocytosis. To examine whether this facilitation of endocytosis is related to the Na⁺-dependent vesicular content change, we dialyzed glutamate into the presynaptic cytosol or blocked the vesicular glutamate uptake with bafilomycin and found that the rate of endocytosis was not affected by regulating the vesicular glutamate content. Endocytosis is critically dependent on intracellular Ca²⁺, and the activity of Na⁺/Ca²⁺ exchanger (NCX) may be altered when the Na⁺ gradient is changed. However, neither NCX inhibitor nor change of extracellular Na⁺ concentration affected the endocytosis rate. Moreover, two-photon Ca²⁺ imaging showed that presynaptic Na⁺ did not affect the action potential-evoked intracellular Ca²⁺ transient and decay. Therefore, we revealed a novel mechanism of cytosolic Na⁺ in accelerating vesicle endocytosis. During high-frequency synaptic transmission, when large numbers of synaptic vesicles were fused, the rapid buildup of presynaptic cytosolic Na⁺ promoted vesicle recycling and sustained synaptic transmission.SIGNIFICANCE STATEMENT High-frequency firing neurons are widely distributed in the CNS. A large number of synaptic vesicles are released during high-frequency synaptic transmission; accordingly, synaptic vesicles need to be recycled rapidly to replenish the vesicle pool. Synaptic vesicle exocytosis and endocytosis are tightly coupled, and their coupling is essential for synaptic function and structural stability. We showed here that intracellular Na⁺ concentration at the calyx of Held terminal increased rapidly during spike activity and the increased Na⁺ accelerated endocytosis. Thus, when large numbers of synaptic vesicles are released during high-frequency synaptic transmission, Na⁺ accumulated in terminals and facilitated vesicle recycling. These findings represent a novel cellular mechanism that supports reliable synaptic transmission at high frequency in the CNS.

Amyloid pathology-produced unexpected modifications of calcium homeostasis in hippocampal subicular dendrites

INTRODUCTION

Alzheimer's disease (AD) is linked to neuronal calcium dyshomeostasis, which is associated with network hyperexcitability. Decreased expression of the calcium-binding protein cal- bindin-D28K (CB) might be a susceptibility factor for AD. The subiculum is affected early in AD, for unknown reasons.

METHODS

In AD, CB knock-out and control mice fluorescence Ca2+ imaging combined with patch clamp were used to characterize Ca2+ dynamics, resting Ca2+ , and Ca2+ -buffering capacity in subicular neurons. CB expression levels in wild-type and AD mice were also analyzed.

RESULTS

The subiculum and dentate gyrus of wild-type mice showed age-related decline in CB expression not observed in AD mice. Resting Ca2+ and Ca2+ -buffering capacity was increased in aged AD mice subicular dendrites. Modeling suggests that AD calcium changes can be explained by alterations of Ca2+ extrusion pumps rather than by buffers.

DISCUSSION

Overall, abnormal Ca2+ homeostasis in AD has an age dependency that comprises multiple mechanisms, including compensatory processes.

Neuronal Glutamatergic Synaptic Clefts Alkalinize Rather Than Acidify during Neurotransmission

The dogma that the synaptic cleft acidifies during neurotransmission is based on the corelease of neurotransmitters and protons from synaptic vesicles, and is supported by direct data from sensory ribbon-type synapses. However, it is unclear whether acidification occurs at non-ribbon-type synapses. Here we used genetically encoded fluorescent pH indicators to examine cleft pH at conventional neuronal synapses. At the neuromuscular junction of female Drosophila larvae, we observed alkaline spikes of over 1 log unit during fictive locomotion in vivo. Ex vivo, single action potentials evoked alkalinizing pH transients of only ∼0.01 log unit, but these transients summated rapidly during burst firing. A chemical pH indicator targeted to the cleft corroborated these findings. Cleft pH transients were dependent on Ca²⁺ movement across the postsynaptic membrane, rather than neurotransmitter release per se, a result consistent with cleft alkalinization being driven by the Ca²⁺/H⁺ antiporting activity of the plasma membrane Ca²⁺-ATPase at the postsynaptic membrane. Targeting the pH indicators to the microenvironment of the presynaptic voltage gated Ca²⁺ channels revealed that alkalinization also occurred within the cleft proper at the active zone and not just within extrasynaptic regions. Application of the pH indicators at the mouse calyx of Held, a mammalian central synapse, similarly revealed cleft alkalinization during burst firing in both males and females. These findings, made at two quite different non-ribbon type synapses, suggest that cleft alkalinization during neurotransmission, rather than acidification, is a generalizable phenomenon across conventional neuronal synapses.SIGNIFICANCE STATEMENT Neurotransmission is highly sensitive to the pH of the extracellular milieu. This is readily evident in the neurological symptoms that accompany systemic acid/base imbalances. Imaging data from sensory ribbon-type synapses show that neurotransmission itself can acidify the synaptic cleft, likely due to the corelease of protons and glutamate. It is not clear whether the same phenomenon occurs at conventional neuronal synapses due to the difficulties in collecting such data. If it does occur, it would provide for an additional layer of activity-dependent modulation of neurotransmission. Our findings of alkalinization, rather than acidification, within the cleft of two different neuronal synapses encourages a reassessment of the scope of activity-dependent pH influences on neurotransmission and short-term synaptic plasticity.

Cytosolic Ca2+ Buffers Are Inherently Ca2+ Signal Modulators

For precisely regulating intracellular Ca²⁺ signals in a time- and space-dependent manner, cells make use of various components of the "Ca²⁺ signaling toolkit," including Ca²⁺ entry and Ca²⁺ extrusion systems. A class of cytosolic Ca²⁺-binding proteins termed Ca²⁺ buffers serves as modulators of such, mostly short-lived Ca²⁺ signals. Prototypical Ca²⁺ buffers include parvalbumins (α and β isoforms), calbindin-D9k, calbindin-D28k, and calretinin. Although initially considered to function as pure Ca²⁺ buffers, that is, as intracellular Ca²⁺ signal modulators controlling the shape (amplitude, decay, spread) of Ca²⁺ signals, evidence has accumulated that calbindin-D28k and calretinin have additional Ca²⁺ sensor functions. These other functions are brought about by direct interactions with target proteins, thereby modulating their targets' function/activity. Dysregulation of Ca²⁺ buffer expression is associated with several neurologic/neurodevelopmental disorders including autism spectrum disorder (ASD) and schizophrenia. In some cases, the presence of these proteins is presumed to confer a neuroprotective effect, as evidenced in animal models of Parkinson's or Alzheimer's disease.

Presynaptic Mitochondria Volume and Abundance Increase during Development of a High-Fidelity Synapse

The calyx of Held, a large glutamatergic presynaptic terminal in the auditory brainstem undergoes developmental changes to support the high action-potential firing rates required for auditory information encoding. In addition, calyx terminals are morphologically diverse, which impacts vesicle release properties and synaptic plasticity. Mitochondria influence synaptic plasticity through calcium buffering and are crucial for providing the energy required for synaptic transmission. Therefore, it has been postulated that mitochondrial levels increase during development and contribute to the morphological-functional diversity in the mature calyx. However, the developmental profile of mitochondrial volumes and subsynaptic distribution at the calyx of Held remains unclear. To provide insight on this, we developed a helper-dependent adenoviral vector that expresses the genetically encoded peroxidase marker for mitochondria, mito-APEX2, at the mouse calyx of Held. We developed protocols to detect labeled mitochondria for use with serial block face scanning electron microscopy to carry out semiautomated segmentation of mitochondria, high-throughput whole-terminal reconstruction, and presynaptic ultrastructure in mice of either sex. Subsequently, we measured mitochondrial volumes and subsynaptic distributions at the immature postnatal day (P)7 and the mature (P21) calyx. We found an increase of mitochondria volumes in terminals and axons from P7 to P21 but did not observe differences between stalk and swelling subcompartments in the mature calyx. Based on these findings, we propose that mitochondrial volumes and synaptic localization developmentally increase to support high firing rates required in the initial stages of auditory information processing.SIGNIFICANCE STATEMENT Elucidating the developmental processes of auditory brainstem presynaptic terminals is critical to understanding auditory information encoding. Additionally, morphological-functional diversity at these terminals is proposed to enhance coding capacity. Mitochondria provide energy for synaptic transmission and can buffer calcium, impacting synaptic plasticity; however, their developmental profile to ultimately support the energetic demands of synapses following the onset of hearing remains unknown. Therefore, we created a helper-dependent adenoviral vector with the mitochondria-targeting peroxidase mito-APEX2 and expressed it at the mouse calyx of Held. Volumetric reconstructions of serial block face electron microscopy data of immature and mature labeled calyces reveal that mitochondrial volumes are increased to support high firing rates upon maturity.

Mitochondria at the neuronal presynapse in health and disease

Synapses enable neurons to communicate with each other and are therefore a prerequisite for normal brain function. Presynaptically, this communication requires energy and generates large fluctuations in calcium concentrations. Mitochondria are optimized for supplying energy and buffering calcium, and they are actively recruited to presynapses. However, not all presynapses contain mitochondria; thus, how might synapses with and without mitochondria differ? Mitochondria are also increasingly recognized to serve additional functions at the presynapse. Here, we discuss the importance of presynaptic mitochondria in maintaining neuronal homeostasis and how dysfunctional presynaptic mitochondria might contribute to the development of disease.

Synaptotagmin Ca2+ Sensors and Their Spatial Coupling to Presynaptic Cav Channels in Central Cortical Synapses

Ca²⁺ concentrations drop rapidly over a distance of a few tens of nanometers from an open voltage-gated Ca²⁺ channel (Ca_v), thereby, generating a spatially steep and temporally short-lived Ca²⁺ gradient that triggers exocytosis of a neurotransmitter filled synaptic vesicle. These non-steady state conditions make the Ca²⁺-binding kinetics of the Ca²⁺ sensors for release and their spatial coupling to the Ca_vs important parameters of synaptic efficacy. In the mammalian central nervous system, the main release sensors linking action potential mediated Ca²⁺ influx to synchronous release are Synaptotagmin (Syt) 1 and 2. We review here quantitative work focusing on the Ca²⁺ kinetics of Syt2-mediated release. At present similar quantitative detail is lacking for Syt1-mediated release. In addition to triggering release, Ca²⁺ remaining bound to Syt after the first of two successive high-frequency activations was found to be capable of facilitating release during the second activation. More recently, the Ca²⁺ sensor Syt7 was identified as additional facilitation sensor. We further review how several recent functional studies provided quantitative insights into the spatial topographical relationships between Syts and Ca_vs and identified mechanisms regulating the sensor-to-channel coupling distances at presynaptic active zones. Most synapses analyzed in matured cortical structures were found to operate at tight, nanodomain coupling. For fast signaling synapses a developmental switch from loose, microdomain to tight, nanodomain coupling was found. The protein Septin5 has been known for some time as a developmentally down-regulated “inhibitor” of tight coupling, while Munc13-3 was found only recently to function as a developmentally up-regulated mediator of tight coupling. On the other hand, a highly plastic synapse was found to operate at loose coupling in the matured hippocampus. Together these findings suggest that the coupling topography and its regulation is a specificity of the type of synapse. However, to definitely draw such conclusion our knowledge of functional active zone topographies of different types of synapses in different areas of the mammalian brain is too incomplete.

Presynaptic loss of dynamin-related protein 1 impairs synaptic vesicle release and recycling at the mouse calyx of Held

KEY POINTS

This study characterizes the mechanisms underlying defects in synaptic transmission when dynamin-related protein 1 (DRP1) is genetically eliminated. Viral-mediated knockout of DRP1 from the presynaptic terminal at the mouse calyx of Held increased initial release probability, reduced the size of the synaptic vesicle recycling pool and impaired synaptic vesicle recycling. Transmission defects could be partially restored by increasing the intracellular calcium buffering capacity with EGTA-AM, implying close coupling of Ca²⁺ channels to synaptic vesicles was compromised. Acute restoration of ATP to physiological levels in the presynaptic terminal did not reverse the synaptic defects. Loss of DRP1 impairs mitochondrial morphology in the presynaptic terminal, which in turn seems to arrest synaptic maturation.

ABSTRACT

Impaired mitochondrial biogenesis and function is implicated in many neurodegenerative diseases, and likely affects synaptic neurotransmission prior to cellular loss. Dynamin-related protein 1 (DRP1) is essential for mitochondrial fission and is disrupted in neurodegenerative disease. In this study, we used the mouse calyx of Held synapse as a model to investigate the impact of presynaptic DRP1 loss on synaptic vesicle (SV) recycling and sustained neurotransmission. In vivo viral expression of Cre recombinase in ventral cochlear neurons of floxed-DRP1 mice generated a presynaptic-specific DRP1 knockout (DRP1-preKO), where the innervated postsynaptic cell was unperturbed. Confocal reconstruction of the calyx terminal suggested SV clusters and mitochondrial content were disrupted, and presynaptic terminal volume was decreased. Using postsynaptic voltage-clamp recordings, we found that DRP1-preKO synapses had larger evoked responses at low frequency stimulation. DRP1-preKO synapses also had profoundly altered short-term plasticity, due to defects in SV recycling. Readily releasable pool size, estimated with high-frequency trains, was dramatically reduced in DRP1-preKO synapses, suggesting an important role for DRP1 in maintenance of release-competent SVs at the presynaptic terminal. Presynaptic Ca²⁺ accumulation in the terminal was also enhanced in DRP1-preKO synapses. Synaptic transmission defects could be partially rescued with EGTA-AM, indicating close coupling of Ca²⁺ channels to SV distance normally found in mature terminals may be compromised by DRP1-preKO. Using paired recordings of the presynaptic and postsynaptic compartments, recycling defects could not be reversed by acute dialysis of ATP into the calyx terminals. Taken together, our results implicate a requirement for mitochondrial fission to coordinate postnatal synapse maturation.

Axonal Mitochondria Modulate Neuropeptide Secretion Through the Hypoxic Stress Response in Caenorhabditis elegans

Neurons are highly dependent on mitochondrial function, and mitochondrial damage has been implicated in many neurological and neurodegenerative diseases. Here we show that axonal mitochondria are necessary for neuropeptide secretion in Caenorhabditis elegans and that oxidative phosphorylation, but not mitochondrial calcium uptake, is required for secretion. Oxidative phosphorylation produces cellular ATP, reactive oxygen species, and consumes oxygen. Disrupting any of these functions could inhibit neuropeptide secretion. We show that blocking mitochondria transport into axons or decreasing mitochondrial function inhibits neuropeptide secretion through activation of the hypoxia inducible factor HIF-1 Our results suggest that axonal mitochondria modulate neuropeptide secretion by regulating transcriptional responses induced by metabolic stress.

Diffusion of Ca2+ from Small Boutons en Passant into the Axon Shapes AP-Evoked Ca2+ Transients

Not only the amplitude but also the time course of a presynaptic Ca²⁺ transient determine multiple aspects of synaptic transmission. In small bouton-type synapses, the mechanisms underlying the Ca²⁺ decay kinetics have not been fully investigated. Here, factors that shape an action-potential-evoked Ca²⁺ transient were quantitatively studied in synaptic boutons of neocortical layer 5 pyramidal neurons. Ca²⁺ transients were measured with different concentrations of fluorescent Ca²⁺ indicators and analyzed based on a single-compartment model. We found a small endogenous Ca²⁺-binding ratio (7 ± 2) and a high activity of Ca²⁺ transporters (0.64 ± 0.03 ms^-1), both of which enable rapid clearance of Ca²⁺ from the boutons. However, contrary to predictions of the single-compartment model, the decay time course of the measured Ca²⁺ transients was biexponential and became prolonged during repetitive stimulation. Measurements of [Ca²⁺]_i along the adjoining axon, together with an experimentally constrained model, showed that the initial fast decay of the Ca²⁺ transients predominantly arose from the diffusion of Ca²⁺ from the boutons into the axon. Therefore, for small boutons en passant, factors like terminal volume, axon diameter, and the concentration of mobile Ca²⁺-binding molecules are critical determinants of Ca²⁺ dynamics and thus Ca²⁺-dependent processes, including short-term synaptic plasticity.

Presynaptic GCaMP expression decreases vesicle release probability at the calyx of Held

Synaptic vesicle (SV) exocytosis is intimately dependent on free local Ca²⁺ near active zones. Genetically encoded calcium indicators (GECIs) have become an indispensable tool to monitor calcium dynamics during physiological responses, and they are widely used as a proxy to monitor activity in neuronal ensembles and at synaptic terminals. However, GECIs’ ability to bind Ca²⁺ at physiologically relevant concentration makes them strong candidates to affect calcium homeostasis and alter synaptic transmission by exogenously increasing Ca²⁺ buffering. In the present study, we show that genetically expressed GCaMP6m modulates SV release probability at the mouse calyx of Held synapse. GCaMP6m expression for approximately three weeks decreased initial SV release for both low‐frequency stimulation and high‐frequency stimulation trains, and slowed presynaptic short‐term depression. However, GCaMP6m does not affect quantal events during spontaneous activity at this synapse. This study emphasizes the careful use of GECIs as monitors of neuronal activity and inspects the role of these transgenic indicators which may alter calcium‐dependent physiological responses.

Neurotransmitter Release Can Be Stabilized by a Mechanism That Prevents Voltage Changes Near the End of Action Potentials from Affecting Calcium Currents

At chemical synapses, presynaptic action potentials (APs) activate voltage-gated calcium channels, allowing calcium to enter and trigger neurotransmitter release. The duration, peak amplitude, and shape of the AP falling phase alter calcium entry, which can affect neurotransmitter release significantly. In many neurons, APs do not immediately return to the resting potential, but instead exhibit a period of depolarization or hyperpolarization referred to as an afterpotential. We hypothesized that presynaptic afterpotentials should alter neurotransmitter release by affecting the electrical driving force for calcium entry and calcium channel gating. In support of this, presynaptic calcium entry is affected by afterpotentials after standard instant voltage jumps. Here, we used the mouse calyx of Held synapse, which allows simultaneous presynaptic and postsynaptic patch-clamp recording, to show that the postsynaptic response is affected significantly by presynaptic afterpotentials after voltage jumps. We therefore tested the effects of presynaptic afterpotentials using simultaneous presynaptic and postsynaptic recordings and AP waveforms or real APs. Surprisingly, presynaptic afterpotentials after AP stimuli did not alter calcium channel responses or neurotransmitter release appreciably. We show that the AP repolarization time course causes afterpotential-induced changes in calcium driving force and changes in calcium channel gating to effectively cancel each other out. This mechanism, in which electrical driving force is balanced by channel gating, prevents changes in calcium influx from occurring at the end of the AP and therefore acts to stabilize synaptic transmission. In addition, this mechanism can act to stabilize neurotransmitter release when the presynaptic resting potential changes.

SIGNIFICANCE STATEMENT

The shape of presynaptic action potentials (APs), particularly the falling phase, affects calcium entry and small changes in calcium influx can produce large changes in postsynaptic responses. We hypothesized that afterpotentials, which often follow APs, affect calcium entry and neurotransmitter release. We tested this in calyx of Held nerve terminals, which allow simultaneous recording of presynaptic calcium currents and postsynaptic responses. Surprisingly, presynaptic afterpotentials did not alter calcium current or neurotransmitter release. We show that the AP falling phase causes afterpotential-induced changes in electrical driving force and calcium channel gating to cancel each other out. This mechanism regulates calcium entry at the end of APs and therefore stabilizes synaptic transmission. This also stabilizes responses when the presynaptic resting potential changes.

Dysbindin Deficiency Modifies the Expression of GABA Neuron and Ion Permeation Transcripts in the Developing Hippocampus

The neurodevelopmental factor dysbindin is required for synapse function and GABA interneuron development. Dysbindin protein levels are reduced in the hippocampus of schizophrenia patients. Mouse dysbindin genetic defects and other mouse models of neurodevelopmental disorders share defective GABAergic neurotransmission and, in several instances, a loss of parvalbumin-positive interneuron phenotypes. This suggests that mechanisms downstream of dysbindin deficiency, such as those affecting GABA interneurons, could inform pathways contributing to or ameliorating diverse neurodevelopmental disorders. Here we define the transcriptome of developing wild type and dysbindin null Bloc1s8^sdy/sdy mouse hippocampus in order to identify mechanisms downstream dysbindin defects. The dysbindin mutant transcriptome revealed previously reported GABA parvalbumin interneuron defects. However, the Bloc1s8^sdy/sdy transcriptome additionally uncovered changes in the expression of molecules controlling cellular excitability such as the cation-chloride cotransporters NKCC1, KCC2, and NCKX2 as well as the potassium channel subunits Kcne2 and Kcnj13. Our results suggest that dysbindin deficiency phenotypes, such as GABAergic defects, are modulated by the expression of molecules controlling the magnitude and cadence of neuronal excitability.

Dynamics of volume-averaged intracellular Ca2+ in a rat CNS nerve terminal during single and repetitive voltage-clamp depolarizations

KEY POINTS

The intracellular concentration of free calcium ions ([Ca²⁺ ]_i ) in a nerve terminal controls both transmitter release and synaptic plasticity. The rapid triggering of transmitter release depends on the local micro- or nanodomain of highly elevated [Ca²⁺ ]_i in the vicinity of open voltage-gated Ca²⁺ channels, whereas short-term synaptic plasticity is often controlled by global changes in residual [Ca²⁺ ]_i , averaged over the whole nerve terminal volume. Here we describe dynamic changes of such global [Ca²⁺ ]_i in the calyx of Held - a giant mammalian glutamatergic nerve terminal, which is particularly suited for biophysical studies. We provide quantitative data on Ca²⁺ inflow, Ca²⁺ buffering and Ca²⁺ clearance. These data allow us to predict changes in [Ca²⁺ ]_i in the nerve terminal in response to a wide range of stimulus protocols at high temporal resolution and provide a basis for the modelling of short-term plasticity of glutamatergic synapses.

ABSTRACT

Many aspects of short-term synaptic plasticity (STP) are controlled by relatively slow changes in the presynaptic intracellular concentration of free calcium ions ([Ca²⁺ ]_i ) that occur in the time range of a few milliseconds to several seconds. In nerve terminals, [Ca²⁺ ]_i equilibrates diffusionally during such slow changes, such that the globally measured, residual [Ca²⁺ ]_i that persists after the collapse of local domains is often the appropriate parameter governing STP. Here, we study activity-dependent dynamic changes in global [Ca²⁺ ]_i at the rat calyx of Held nerve terminal in acute brainstem slices using patch-clamp and microfluorimetry. We use low concentrations of a low-affinity Ca²⁺ indicator dye (100 μm Fura-6F) in order not to overwhelm endogenous Ca²⁺ buffers. We first study voltage-clamped terminals, dialysed with pipette solutions containing minimal amounts of Ca²⁺ buffers, to determine Ca²⁺ binding properties of endogenous fixed buffers as well as the mechanisms of Ca²⁺ clearance. Subsequently, we use pipette solutions including 500 μm EGTA to determine the Ca²⁺ binding kinetics of this chelator. We provide a formalism and parameters that allow us to predict [Ca²⁺ ]_i changes in calyx nerve terminals in response to a wide range of stimulus protocols. Unexpectedly, the Ca²⁺ affinity of EGTA under the conditions of our measurements was substantially lower (K_D  = 543 ± 51 nm) than measured in vitro, mainly as a consequence of a higher than previously assumed dissociation rate constant (2.38 ± 0.20 s^-1 ), which we need to postulate in order to model the measured presynaptic [Ca²⁺ ]_i transients.

Miro1-dependent mitochondrial positioning drives the rescaling of presynaptic Ca2+ signals during homeostatic plasticity

Mitochondrial trafficking is influenced by neuronal activity, but it remains unclear how mitochondrial positioning influences neuronal transmission and plasticity. Here, we use live cell imaging with the genetically encoded presynaptically targeted Ca²⁺ indicator, SyGCaMP5, to address whether presynaptic Ca²⁺ responses are altered by mitochondria in synaptic terminals. We find that presynaptic Ca²⁺ signals, as well as neurotransmitter release, are significantly decreased in terminals containing mitochondria. Moreover, the localisation of mitochondria at presynaptic sites can be altered during long‐term activity changes, dependent on the Ca²⁺‐sensing function of the mitochondrial trafficking protein, Miro1. In addition, we find that Miro1‐mediated activity‐dependent synaptic repositioning of mitochondria allows neurons to homeostatically alter the strength of presynaptic Ca²⁺ signals in response to prolonged changes in neuronal activity. Our results support a model in which mitochondria are recruited to presynaptic terminals during periods of raised neuronal activity and are involved in rescaling synaptic signals during homeostatic plasticity.

Functional and structural properties of ion channels at the nerve terminal depends on compact myelin

KEY POINTS

In the present study, we document the role of compact myelin in regulating the structural and functional properties of ion channels at the nerve terminals, using electrophysiology, dynamic Na(+) imaging and immunohistochemistry. The subcellular segregation of Na(+) channel expression and intracellular Na(+) dynamics at the heminode and terminal was lost in the dysmyelinated axon from Long-Evans shaker rats, which lack compact myelin. In Long-Evans shaker rats, loss of the Nav β4 subunit specifically at the heminode reduced resurgent and persistent Na(+) currents, whereas K(+) channel expression and currents were increased. The results of the present study suggest that there is a specific role for compact myelin in dictating protein expression and function at the axon heminode and in regulating excitability of the nerve terminal.

ABSTRACT

Axon myelination increases the conduction velocity and precision of action potential propagation. Although the negative effects of demyelination are generally attributed to conduction failure, accumulating evidence suggests that myelination also regulates the structural properties and molecular composition of the axonal membrane. In the present study, we investigated how myelination affects ion channel expression and function, particularly at the last axon heminode before the nerve terminal, which regulates the presynaptic excitability of the nerve terminal. We compared the structure and physiology of normal axons and those of the Long-Evans shaker (LES) rat, which lacks compact myelin. The normal segregation of Na(+) channel expression and dynamics at the heminode and terminal was lost in the LES rat. Specifically, NaV -α subunits were dispersed and NaV β4 subunit was absent, whereas the density of K(+) channels was increased at the heminode. Correspondingly, resurgent and persistent Na(+) currents were reduced and K(+) current was increased. Taken together, these data suggest a specific role for compact myelin in the orchestration of ion channel expression and function at the axon heminode and in regulating excitability of the nerve terminal.

LKB1 Regulates Mitochondria-Dependent Presynaptic Calcium Clearance and Neurotransmitter Release Properties at Excitatory Synapses along Cortical Axons

Individual synapses vary significantly in their neurotransmitter release properties, which underlie complex information processing in neural circuits. Presynaptic Ca2+ homeostasis plays a critical role in specifying neurotransmitter release properties, but the mechanisms regulating synapse-specific Ca2+ homeostasis in the mammalian brain are still poorly understood. Using electrophysiology and genetically encoded Ca2+ sensors targeted to the mitochondrial matrix or to presynaptic boutons of cortical pyramidal neurons, we demonstrate that the presence or absence of mitochondria at presynaptic boutons dictates neurotransmitter release properties through Mitochondrial Calcium Uniporter (MCU)-dependent Ca2+ clearance. We demonstrate that the serine/threonine kinase LKB1 regulates MCU expression, mitochondria-dependent Ca2+ clearance, and thereby, presynaptic release properties. Re-establishment of MCU-dependent mitochondrial Ca2+ uptake at glutamatergic synapses rescues the altered neurotransmitter release properties characterizing LKB1-null cortical axons. Our results provide novel insights into the cellular and molecular mechanisms whereby mitochondria control neurotransmitter release properties in a bouton-specific way through presynaptic Ca2+ clearance.

Ca2+ removal by the plasma membrane Ca2+-ATPase influences the contribution of mitochondria to activity-dependent Ca2+ dynamics in Aplysia neuroendocrine cells

After Ca(2+) influx, mitochondria can sequester Ca(2+) and subsequently release it back into the cytosol. This form of Ca(2+)-induced Ca(2+) release (CICR) prolongs Ca(2+) signaling and can potentially mediate activity-dependent plasticity. As Ca(2+) is required for its subsequent release, Ca(2+) removal systems, like the plasma membrane Ca(2+)-ATPase (PMCA), could impact CICR. Here we examine such a role for the PMCA in the bag cell neurons of Aplysia californica CICR is triggered in these neurons during an afterdischarge and is implicated in sustaining membrane excitability and peptide secretion. Somatic Ca(2+) was measured from fura-PE3-loaded cultured bag cell neurons recorded under whole cell voltage clamp. Voltage-gated Ca(2+) influx was elicited with a 5-Hz, 1-min train, which mimics the fast phase of the afterdischarge. PMCA inhibition with carboxyeosin or extracellular alkalization augmented the effectiveness of Ca(2+) influx in eliciting mitochondrial CICR. A Ca(2+) compartment model recapitulated these findings and indicated that disrupting PMCA-dependent Ca(2+) removal increases CICR by enhancing mitochondrial Ca(2+) loading. Indeed, carboxyeosin augmented train-evoked mitochondrial Ca(2+) uptake. Consistent with their role on Ca(2+) dynamics, cell labeling revealed that the PMCA and mitochondria overlap with Ca(2+) entry sites. Finally, PMCA-dependent Ca(2+) extrusion did not impact endoplasmic reticulum-dependent Ca(2+) removal or release, despite the organelle residing near Ca(2+) entry sites. Our results demonstrate that Ca(2+) removal by the PMCA influences the propensity for stimulus-evoked CICR by adjusting the amount of Ca(2+) available for mitochondrial Ca(2+) uptake. This study highlights a mechanism by which the PMCA could impact activity-dependent plasticity in the bag cell neurons.

Reduced endogenous Ca2+ buffering speeds active zone Ca2+ signaling

Calcium influx during action potentials triggers neurotransmitter release at presynaptic active zones. Calcium buffers limit the spread of calcium and restrict neurotransmitter release to the vicinity of calcium channels. To sustain synchronous release during repetitive activity, rapid removal of calcium from the active zone is essential, but the underlying mechanisms are unclear. Therefore, we focused on cerebellar mossy fiber synapses, which are among the fastest synapses in the mammalian brain and found very weak presynaptic calcium buffering. One might assume that strong calcium buffering has the potential to efficiently remove calcium from active zones. In contrast, our results show that weak calcium buffering speeds active zone calcium clearance. Thus, the strength of presynaptic buffering limits the rate of synaptic transmission.

Fast synchronous neurotransmitter release at the presynaptic active zone is triggered by local Ca²⁺ signals, which are confined in their spatiotemporal extent by endogenous Ca²⁺ buffers. However, it remains elusive how rapid and reliable Ca²⁺ signaling can be sustained during repetitive release. Here, we established quantitative two-photon Ca²⁺ imaging in cerebellar mossy fiber boutons, which fire at exceptionally high rates. We show that endogenous fixed buffers have a surprisingly low Ca²⁺-binding ratio (∼15) and low affinity, whereas mobile buffers have high affinity. Experimentally constrained modeling revealed that the low endogenous buffering promotes fast clearance of Ca²⁺ from the active zone during repetitive firing. Measuring Ca²⁺ signals at different distances from active zones with ultra-high-resolution confirmed our model predictions. Our results lead to the concept that reduced Ca²⁺ buffering enables fast active zone Ca²⁺ signaling, suggesting that the strength of endogenous Ca²⁺ buffering limits the rate of synchronous synaptic transmission.

Mechanisms underlying presynaptic Ca2+ transient and vesicular glutamate release at a CNS nerve terminal during in vitro ischaemia

KEY POINTS

Here we demonstrate presynaptic responses and mechanisms of increased vesicular glutamate release during in vitro ischaemia in the calyx of Held terminal, an experimentally accessible presynaptic terminal in the CNS. The ischaemia-induced increase in presynaptic Ca(2+) was mediated by both Ca(2+) influx and Ca(2+) -induced Ca(2+) release from intracellular stores. The reverse operation of the plasma membrane Na(+) /Ca(2+) exchanger (NCX) plays a key role in Ca(2+) influx for triggering Ca(2+) release from intracellular stores at presynaptic terminals during in vitro ischaemia. Ca(2+) uptake via NCX underlies the ischaemia-induced Ca(2+) rise and the consequent increase in vesicular glutamate release from presynaptic terminals in the early phase of brain ischaemia.

ABSTRACT

An early consequence of brain ischaemia is an increase in vesicular glutamate release from presynaptic terminals. However, the mechanisms of this increased glutamate release are not fully understood. Here we studied presynaptic responses and mechanisms of increased glutamate release during in vitro ischaemia, using pre- and postsynaptic whole-cell recordings and presynaptic Ca(2+) imaging at the calyx of Held synapse in rat brainstem slices. Consistent with results from other brain regions, in vitro ischaemia significantly increased the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) without affecting their amplitude, suggesting that ischaemia enhances vesicular glutamate release from presynaptic terminals. We found that ischaemia-induced vesicular glutamate release was dependent on a rise in basal Ca(2+) at presynaptic terminals, which resulted from extracellular Ca(2+) influx and Ca(2+) release from intracellular stores. During early ischaemia, increased Ca(2+) influx into presynaptic terminals was due to reverse operation of the plasma membrane Na(+) /Ca(2+) exchanger (NCX) rather than presynaptic depolarization or voltage-activated Ca(2+) currents. KB-R7943, an inhibitor of NCX, prevented the ischaemia-induced increases in presynaptic Ca(2+) and vesicular glutamate release. In addition, the removal of extracellular Na(+) completely inhibited the ischaemia-induced Ca(2+) rise. It therefore appears that a link between Na(+) accumulation and Ca(2+) uptake via NCX underlies the ischaemia-induced Ca(2+) rise and the consequent increase in vesicular glutamate release from presynaptic terminals in the early phase of brain ischaemia.

A single-compartment model of calcium dynamics in nerve terminals and dendrites

This introduction describes a single-compartment model of calcium dynamics that has been applied to fluorescence measurements of intracellular free calcium concentration ([Ca(2+)]i) changes in neurons. The model describes intracellular calcium handling under simplified conditions, for which analytical expressions for the amplitude and the time constants of [Ca(2+)]i changes can be explicitly derived. In particular, it reveals the dependence of the measured [Ca(2+)]i changes on the calcium indicator concentration. Applied to experimental data from small cells or subcellular compartments, the model equations have been extremely useful for obtaining quantitative information about essential parameters of Ca(2+) influx, buffering, and clearance. We illustrate also several changes that occur when the basic assumptions do not hold (e.g., when calcium diffusion, dye saturation, or kinetic effects become significant). Finally, we discuss how the changes in calcium dynamics, which are explained by the model, have been exploited for measuring properties of calcium-driven reactions, such as those regulating short-term synaptic enhancement, vesicle recycling, and adaptation.

Early stress prevents the potentiation of muscarinic excitation by calcium release in adult prefrontal cortex

BACKGROUND

The experience of early stress contributes to the etiology of several psychiatric disorders and can lead to lasting deficits in working memory and attention. These executive functions require activation of the prefrontal cortex (PFC) by muscarinic M1 acetylcholine (ACh) receptors. Such Gαq-protein coupled receptors trigger the release of calcium (Ca(2+)) from internal stores and elicit prolonged neuronal excitation.

METHODS

In brain slices of rat PFC, we employed multiphoton imaging simultaneously with whole-cell electrophysiological recordings to examine potential interactions between ACh-induced Ca(2+) release and excitatory currents in adulthood, across postnatal development, and following the early stress of repeated maternal separation, a rodent model for depression. We also investigated developmental changes in related genes in these groups.

RESULTS

Acetylcholine-induced Ca(2+) release potentiates ACh-elicited excitatory currents. In the healthy PFC, this potentiation of muscarinic excitation emerges in young adulthood, when executive function typically reaches maturity. However, the developmental consolidation of muscarinic ACh signaling is abolished in adults with a history of early stress, where ACh responses retain an adolescent phenotype. In prefrontal cortex, these rats show a disruption in the expression of multiple developmentally regulated genes associated with Gαq and Ca(2+) signaling. Pharmacologic and ionic manipulations reveal that the enhancement of muscarinic excitation in the healthy adult PFC arises via the electrogenic process of sodium/Ca(2+) exchange.

CONCLUSIONS

This work illustrates a long-lasting disruption in ACh-mediated cortical excitation following early stress and raises the possibility that such cellular mechanisms may disrupt the maturation of executive function.

An essential role for the K+-dependent Na+/Ca2+-exchanger, NCKX4, in melanocortin-4-receptor-dependent satiety

K(+)-dependent Na(+)/Ca(2+)-exchangers are broadly expressed in various tissues, and particularly enriched in neurons of the brain. The distinct physiological roles for the different members of this Ca(2+) transporter family are, however, not well described. Here we show that gene-targeted mice lacking the K(+)-dependent Na(+)/Ca(2+)-exchanger, NCKX4 (gene slc24a4 or Nckx4), display a remarkable anorexia with severe hypophagia and weight loss. Feeding and satiety are coordinated centrally by melanocortin-4 receptors (MC4R) in neurons of the hypothalamic paraventricular nucleus (PVN). The hypophagic response of Nckx4 knock-out mice is accompanied by hyperactivation of neurons in the PVN, evidenced by high levels of c-Fos expression. The activation of PVN neurons in both fasted Nckx4 knock-out and glucose-injected wild-type animals is blocked by Ca(2+) removal and MC4R antagonists. In cultured hypothalamic neurons, melanocyte stimulating hormone induces an MC4R-dependent and sustained Ca(2+) signal, which requires phospholipase C activity and plasma membrane Ca(2+) entry. The Ca(2+) signal is enhanced in hypothalamic neurons from Nckx4 knock-out animals, and is depressed in cells in which NCKX4 is overexpressed. Finally, MC4R-dependent oxytocin expression in the PVN, a key essential step in satiety, is prevented by blocking phospholipase C activation or Ca(2+) entry. These findings highlight an essential, and to our knowledge previously unknown, role for Ca(2+) signaling in the MC4R pathway that leads to satiety, and a novel non-redundant role for NCKX4-mediated Ca(2+) extrusion in controlling MC4R signaling and feeding behavior. Together, these findings highlight a novel pathway that potentially could be exploited to develop much needed new therapeutics to tackle eating disorders and obesity.

Purinergic stimulation of K+-dependent Na+/Ca2+ exchanger isoform 4 requires dual activation by PKC and CaMKII

K⁺-dependent Na⁺/Ca²⁺-exchanger isoform 4 (NCXK4) is one of the most broadly expressed members of the NCKX (K⁺-dependent Na⁺/Ca²⁺-exchanger) family. Recent data indicate that NCKX4 plays a critical role in controlling normal Ca²⁺ signal dynamics in olfactory and other neurons. Synaptic Ca²⁺ dynamics are modulated by purinergic regulation, mediated by ATP released from synaptic vesicles or from neighbouring glial cells. Previous studies have focused on modulation of Ca²⁺ entry pathways that initiate signalling. Here we have investigated purinergic regulation of NCKX4, a powerful extrusion pathway that assists in terminating Ca²⁺ signals. NCKX4 activity was stimulated by ATP through activation of the P2Y receptor signalling pathway. Stimulation required dual activation of PKC (protein kinase C) and CaMKII (Ca²⁺/calmodulin-dependent protein kinase II). Mutating T312, a putative PKC phosphorylation site on NCKX4, partially prevented purinergic stimulation. These data illustrate how purinergic regulation can shape the dynamics of Ca²⁺ signalling by activating a signal damping and termination pathway.

Activity of the K⁺-dependent Na⁺/Ca²⁺-exchanger, NCKX4, is stimulated by purinergic signals that depend on dual activation of two protein kinase pathways. This regulation provides a novel mechanism to shape Ca²⁺ signaling and thus to have important impact on neuronal processes.

The SLC24 gene family of Na⁺/Ca²⁺-K⁺ exchangers: from sight and smell to memory consolidation and skin pigmentation

Members of the SLC24 gene family encode K(+)-dependent Na(+)/Ca(2+) exchangers (NCKX) that utilize both the inward Na(+) and outward K(+) gradients to extrude Ca(2+) from cells. There are five human SLC24 genes that play a role in biological process as diverse as vision in retinal rod and cone photoreceptors, olfaction, skin pigmentation and at least three of the five genes are also widely expressed in the brain. Here I review the functional, physiological and structural features of NCKX proteins that have emerged in the past few years.

Separate Ca2+ sources are buffered by distinct Ca2+ handling systems in aplysia neuroendocrine cells

Although the contribution of Ca(2+) buffering systems can vary between neuronal types and cellular compartments, it is unknown whether distinct Ca(2+) sources within a neuron have different buffers. As individual Ca(2+) sources can have separate functions, we propose that each is handled by unique systems. Using Aplysia californica bag cell neurons, which initiate reproduction through an afterdischarge involving multiple Ca(2+)-dependent processes, we investigated the role of endoplasmic reticulum (ER) and mitochondrial sequestration, as well as extrusion via the plasma membrane Ca(2+)-ATPase (PMCA) and Na(+)/Ca(2+) exchanger, to the clearance of voltage-gated Ca(2+) influx, Ca(2+)-induced Ca(2+)-release (CICR), and store-operated Ca(2+) influx. Cultured bag cell neurons were filled with the Ca(2+) indicator, fura-PE3, to image Ca(2+) under whole-cell voltage clamp. A 5 Hz, 1 min train of depolarizing voltage steps elicited voltage-gated Ca(2+) influx followed by EGTA-sensitive CICR from the mitochondria. A compartment model of Ca(2+) indicated the effect of EGTA on CICR was due to buffering of released mitochondrial Ca(2+) rather than uptake competition. Removal of voltage-gated Ca(2+) influx was dominated by the mitochondria and PMCA, with no contribution from the Na(+)/Ca(2+) exchanger or sarcoplasmic/endoplasmic Ca(2+)-ATPase (SERCA). In contrast, CICR recovery was slowed by eliminating the Na(+)/Ca(2+) exchanger and PMCA. Last, store-operated influx, evoked by ER depletion, was removed by the SERCA and depended on the mitochondrial membrane potential. Our results demonstrate that distinct buffering systems are dedicated to particular Ca(2+) sources. In general, this may represent a means to differentially regulate Ca(2+)-dependent processes, and for Aplysia, influence how reproductive behavior is triggered.

Transients in global Ca2+ concentration induced by electrical activity in a giant nerve terminal

Giant nerve terminals offer a unique opportunity to learn about dynamic changes in intracellular global Ca(2+) concentration ([Ca(2+)]i) because this quantity can be measured precisely with indicator dyes and the composition of the intra-terminal ionic milieu can be controlled. We review here recent literature on [Ca(2+)]i signalling in the calyx of Held and discuss what these measurements can tell us about endogenous Ca(2+) buffers and Ca(2+) extrusion mechanisms. We conclude that in spite of the favourable experimental conditions, some unresolved questions still remain regarding absolute values for the Ca(2+)-binding ratio, the affinity of the basic fixed buffer and the Ca(2+) affinities of the major endogenous Ca(2+) binding proteins. Uncertainties about some of these presynaptic properties, including the roles of Mg(2+) and ATP (as a Mg(2+) buffer), however, extend to the point that mechanisms controlling the decay of [Ca(2+)]i signals in unperturbed terminals may have to be reconsidered.

Endocytosis of somatodendritic NCKX2 is regulated by Src family kinase-dependent tyrosine phosphorylation

We have previously reported that the surface expression of K⁺-dependent Na⁺/Ca²⁺ exchanger 2 (NCKX2) in the somatodendritic compartment is kept low by constitutive endocytosis, which results in the polarization of surface NCKX2 to the axon. Clathrin-mediated endocytosis is initiated by interaction of the μ subunit of adaptor protein complex 2 (AP-2) with the canonical tyrosine motif (YxxΦ) of a target molecule. We examined whether endocytosis of NCKX2 involves two putative tyrosine motifs (³⁶⁵YGKL and ³⁷¹YDTM) in the cytoplasmic loop of NCKX2. Coimmunoprecipitation assay revealed that the ³⁶⁵YGKL motif is essential for the interaction with the μ subunit of AP-2 (AP2M1). Consistently, either overexpression of NCKX2-Y365A mutant or knockdown of AP2M1 in cultured hippocampal neurons significantly reduced the internalization of NCKX2 from the somatodendritic surface and thus abolished the axonal polarization of surface NCKX2. Next, we tested whether the interaction between the tyrosine motif and AP2M1 is regulated by phosphorylation of the 365th tyrosine residue (Tyr-365). Tyrosine phosphorylation of heterologously expressed NCKX2-WT, but not NCKX2-Y365A, was increased by carbachol (CCh) in PC-12 cells. The effect of CCh was inhibited by PP2, a Src family kinase (SFK) inhibitor. Moreover, PP2 facilitated the endocytosis of NCKX2 in both the somatodendritic and axonal compartments, suggesting that tyrosine phosphorylation of NCKX2 by SFK negatively regulates its endocytosis. Supporting this idea, activation of SFK enhanced the NCKX activity in the proximal dendrites of dentate granule cells (GCs). These results suggest that endocytosis of somatodendritic NCKX2 is regulated by SFK-dependent phosphorylation of Tyr-365.

Mitochondria and plasma membrane Ca2+-ATPase control presynaptic Ca2+ clearance in capsaicin-sensitive rat sensory neurons

The central processes of primary nociceptors form synaptic connections with the second-order nociceptive neurons located in the dorsal horn of the spinal cord. These synapses gate the flow of nociceptive information from the periphery to the CNS, and plasticity at these synapses contributes to centrally mediated hyperalgesia and allodynia. Although exocytosis and synaptic plasticity are controlled by Ca(2+) at the release sites, the mechanisms underlying presynaptic Ca(2+) signalling at the nociceptive synapses are not well characterized. We examined the presynaptic mechanisms regulating Ca(2+) clearance following electrical stimulation in capsaicin-sensitive nociceptors using a dorsal root ganglion (DRG)/spinal cord neuron co-culture system. Cytosolic Ca(2+) concentration ([Ca(2+)]i) recovery following electrical stimulation was well approximated by a monoexponential function with a ∼2 s. Inhibition of sarco-endoplasmic reticulum Ca(2+)-ATPase did not affect presynaptic [Ca(2+)]i recovery, and blocking plasmalemmal Na(+)/Ca(2+) exchange produced only a small reduction in the rate of [Ca(2+)]i recovery (∼12%) that was independent of intracellular K(+). However, [Ca(2+)]i recovery in presynaptic boutons strongly depended on the plasma membrane Ca(2+)-ATPase (PMCA) and mitochondria that accounted for ∼47 and 40%, respectively, of presynaptic Ca(2+) clearance. Measurements using a mitochondria-targeted Ca(2+) indicator, mtPericam, demonstrated that presynaptic mitochondria accumulated Ca(2+) in response to electrical stimulation. Quantitative analysis revealed that the mitochondrial Ca(2+) uptake is highly sensitive to presynaptic [Ca(2+)]i elevations, and occurs at [Ca(2+)]i levels as low as ∼200-300 nm. Using RT-PCR, we detected expression of several putative mitochondrial Ca(2+) transporters in DRG, such as MCU, Letm1 and NCLX. Collectively, this work identifies PMCA and mitochondria as the major regulators of presynaptic Ca(2+) signalling at the first sensory synapse, and underlines the high sensitivity of the mitochondrial Ca(2+) uniporter in neurons to cytosolic Ca(2+).

Calcium-dependent isoforms of protein kinase C mediate glycine-induced synaptic enhancement at the calyx of Held

Depolarization of presynaptic terminals that arises from activation of presynaptic ionotropic receptors, or somatic depolarization, can enhance neurotransmitter release; however, the molecular mechanisms mediating this plasticity are not known. Here we investigate the mechanism of this enhancement at the calyx of Held synapse, in which presynaptic glycine receptors depolarize presynaptic terminals, elevate resting calcium levels, and potentiate release. Using knock-out mice of the calcium-sensitive PKC isoforms (PKC(Ca)), we find that enhancement of evoked but not spontaneous synaptic transmission by glycine is mediated primarily by PKC(Ca). Measurements of calcium at the calyx of Held indicate that deficits in synaptic modulation in PKC(Ca) knock-out mice occur downstream of presynaptic calcium increases. Glycine enhances synaptic transmission primarily by increasing the effective size of the pool of readily releasable vesicles. Our results reveal that PKC(Ca) can enhance evoked neurotransmitter release in response to calcium increases caused by small presynaptic depolarizations.

Miniature IPSCs in hippocampal granule cells are triggered by voltage-gated Ca2+ channels via microdomain coupling

The coupling between presynaptic Ca(2+) channels and Ca(2+) sensors of exocytosis is a key determinant of synaptic transmission. Evoked release from parvalbumin (PV)-expressing interneurons is triggered by nanodomain coupling of P/Q-type Ca(2+) channels, whereas release from cholecystokinin (CCK)-containing interneurons is generated by microdomain coupling of N-type channels. Nanodomain coupling has several functional advantages, including speed and efficacy of transmission. One potential disadvantage is that stochastic opening of presynaptic Ca(2+) channels may trigger spontaneous transmitter release. We addressed this possibility in rat hippocampal granule cells, which receive converging inputs from different inhibitory sources. Both reduction of extracellular Ca(2+) concentration and the unselective Ca(2+) channel blocker Cd(2+) reduced the frequency of miniature IPSCs (mIPSCs) in granule cells by ∼50%, suggesting that the opening of presynaptic Ca(2+) channels contributes to spontaneous release. Application of the selective P/Q-type Ca(2+) channel blocker ω-agatoxin IVa had no detectable effects, whereas both the N-type blocker ω-conotoxin GVIa and the L-type blocker nimodipine reduced mIPSC frequency. Furthermore, both the fast Ca(2+) chelator BAPTA-AM and the slow chelator EGTA-AM reduced the mIPSC frequency, suggesting that Ca(2+)-dependent spontaneous release is triggered by microdomain rather than nanodomain coupling. The CB(1) receptor agonist WIN 55212-2 also decreased spontaneous release; this effect was occluded by prior application of ω-conotoxin GVIa, suggesting that a major fraction of Ca(2+)-dependent spontaneous release was generated at the terminals of CCK-expressing interneurons. Tonic inhibition generated by spontaneous opening of presynaptic N- and L-type Ca(2+) channels may be important for hippocampal information processing.

Developmental upregulation of presynaptic NCKX underlies the decrease of mitochondria-dependent posttetanic potentiation at the rat calyx of Held synapse

The sensitivity of posttetanic potentiation (PTP) to high-frequency stimulation (HFS) steeply decays during the first 2 postnatal weeks. We investigated the underlying mechanisms for the developmental change of PTP induced by HFS (100 Hz, 2 s) at postnatal days 4-6 and 9-11 at the rat calyx of Held synapse. Low-concentration tetraphenylphosphonium (2 μM), an inhibitor of mitochondrial Na(+)/Ca(2+) exchanger, suppressed the amount of posttetanic residual Ca(2+) and PTP to a larger extent at the immature calyx synapse, indicating a developmental reduction of mitochondrial contribution to PTP. The higher amount of mitochondrial Ca(2+) uptake during HFS and consequent posttetanic residual Ca(2+) at the immature calyx of Held was associated with higher peak of HFS-induced Ca(2+) transients, most likely because the mitochondrial Ca(2+) uptake during HFS was supralinearly dependent on the presynaptic resting Ca(2+) level. Probing into the contribution of Na(+)/Ca(2+) exchangers to Ca(2+) clearance, we found a specific upregulation of the K(+)-dependent Na(+)/Ca(2+) exchanger (NCKX) activity in the mature calyx of Held. We conclude that the upregulation of NCKX limits the Ca(2+) buildup and inhibits mitochondrial Ca(2+) uptake during HFS, which in turn results in the reduction of posttetanic residual Ca(2+) and PTP at the mature calyx of Held synapse.

Expression and regulation of sodium/calcium exchangers, NCX and NCKX, in reproductive tissues: do they play a critical role in calcium transport for reproduction and development?

Plasma membrane sodium/calcium (Na(+)/Ca(2+)) exchangers are an important component of intracellular calcium [Ca(2+)](i) homeostasis and electrical conduction. Na(+)/Ca(2+) exchangers, NCX and NCKX, play a critical role in the transport of one [Ca(2+)](i) and potassium ion across the cell membrane in exchange for four extracellular sodium ions [Na(+)](e). Mammalian plasma membrane Na(+)/Ca(2+) exchange proteins are divided into two families: one in which Ca(2+) flux is dependent only on sodium (NCX1-3) and another in which Ca(2+) flux is also dependent on potassium (NCKX1-4). Both molecules are capable of forward- and reverse-mode exchange. In cells and tissues, Na(+)/Ca(2+) (and K(+)) gradients localize to the cell membrane; thus, the exchangers transport ions across a membrane potential. Uterine NCKX3 has been shown to be involved in the regulation of endometrial receptivity by [Ca(2+)](i). In the uterus and placenta, NCKX3 expression is regulated by the sex steroid hormone estrogen (E2) and hypoxia stress, respectively. In this chapter, we described the expression and regulation of these proteins for reproductive functions in various tissues including uterus, placenta, and kidney of humans and rodents. Evidence to date suggests that NCKX3 and NCX1 may be regulated in a tissue-specific manner. In addition, we focused on the molecular mechanism involved in the regulation of NCKX3 and NCX1 in mammals, based upon our recent results and those of others.

Functional and structural properties of the NCKX2 Na(+)-Ca (2+)/K (+) exchanger: a comparison with the NCX1 Na (+)/Ca (2+) exchanger

Na(+)/Ca(2+)-K(+) exchangers (NCKX), alongside the more widely known Na(+)/Ca(2+) exchangers (NCX), are important players in the cellular Ca(2+) toolkit. But, unlike NCX, much less is known about the physiological roles of NCKX, while emergent evidence indicates that NCKX has highly specialized functions in cells and tissues where it is expressed. As their name implies, there are functional similarities in the properties of the two Ca(2+) exchanger families, but there are specific differences as well. Here, we compare and contrast their key functional properties of ionic dependence and affinities, as well as report on the effects of KB-R7943 - a compound that is widely used to differentiate the two exchangers. We also review structural similarities and differences between the two exchangers. The aim is to draw attention to key differences that will aid in differentiating the two exchangers in physiological contexts where both exist but perhaps play distinct roles.

Transient reversal of the sodium/calcium exchanger boosts presynaptic calcium and synaptic transmission at a cerebellar synapse

The sodium/calcium exchanger (NCX) is a widespread transporter that exchanges sodium and calcium ions across excitable membranes. Normally, NCX mainly operates in its "forward" mode, harnessing the electrochemical gradient of sodium ions to expel calcium. During membrane depolarization or elevated internal sodium levels, NCX can instead switch the direction of net flux to expel sodium and allow calcium entry. Such "reverse"-mode NCX operation is frequently implicated during pathological or artificially extended periods of depolarization, not during normal activity. We have used fast calcium imaging, mathematical simulation, and whole cell electrophysiology to study the role of NCX at the parallel fiber-to-Purkinje neuron synapse in the mouse cerebellum. We show that nontraditional, reverse-mode NCX activity boosts the amplitude and duration of parallel fiber calcium transients during short bursts of high-frequency action potentials typical of their behavior in vivo. Simulations, supported by experimental manipulations, showed that accumulation of intracellular sodium drove NCX into reverse mode. This mechanism fueled additional calcium influx into the parallel fibers that promoted synaptic transmission to Purkinje neurons for up to 400 ms after the burst. Thus we provide the first functional demonstration of transient and fast NCX-mediated calcium entry at a major central synapse. This unexpected contribution from reverse-mode NCX appears critical for shaping presynaptic calcium dynamics and transiently boosting synaptic transmission, and is likely to optimize the accuracy of cerebellar information transfer.

Regulation of presynaptic calcium in a mammalian synaptic terminal

Ca(2+) signaling in synaptic terminals plays a critical role in neurotransmitter release and short-term synaptic plasticity. In the present study, we examined the role of synaptic Ca(2+) handling mechanisms in the synaptic terminals of mammalian rod bipolar cells, neurons that play a pivotal role in the high-sensitivity vision pathway. We found that mitochondria sequester Ca(2+) under conditions of high Ca(2+) load, maintaining intraterminal Ca(2+) near resting levels. Indeed, the effect of the mitochondria was so powerful that the ability to clamp intraterminal Ca(2+) with a somatically positioned whole cell patch pipette was compromised. The plasma membrane Ca(2+)-ATPase (PMCA), but not the Na(+)/Ca(2+) exchanger (NCX) or the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA), was an important regulator of resting Ca(2+). Furthermore, PMCA activity, but not NCX or SERCA activity, was essential for the recovery of Ca(2+) levels following depolarization-evoked Ca(2+) entry. Loss of PMCA function was also associated with impaired restoration of membrane surface area following depolarization-evoked exocytosis. Given its roles in the regulation of intraterminal Ca(2+) at rest and after a stimulus-evoked Ca(2+) rise, the PMCA is poised to modulate luminance coding and adaptation to background illumination in the mammalian rod bipolar cell.

Impaired short-term plasticity in mossy fiber synapses caused by mitochondrial dysfunction of dentate granule cells is the earliest synaptic deficit in a mouse model of Alzheimer's disease

Alzheimer's disease (AD) in the early stages is characterized by memory impairment, which may be attributable to synaptic dysfunction. Oxidative stress, mitochondrial dysfunction, and Ca²⁺ dysregulation are key factors in the pathogenesis of AD, but the causal relationship between these factors and synaptic dysfunction is not clearly understood. We found that in the hippocampus of an AD mouse model (Tg2576), mitochondrial Ca²⁺ handling in dentate granule cells was impaired as early as the second postnatal month, and this Ca²⁺ dysregulation caused an impairment of post-tetanic potentiation in mossy fiber-CA3 synapses. The alteration of cellular Ca²⁺ clearance in Tg2576 mice is region-specific within hippocampus because in another region, CA1 pyramidal neuron, no significant difference in Ca²⁺ clearance was detected between wild-type and Tg2576 mice at this early stage. Impairment of mitochondrial Ca²⁺ uptake was associated with increased mitochondrial reactive oxygen species and depolarization of mitochondrial membrane potential. Mitochondrial dysfunctions in dentate granule cells and impairment of post-tetanic potentiation in mossy fiber-CA3 synapses were fully restored when brain slices obtained from Tg2576 were pretreated with antioxidant, suggesting that mitochondrial oxidative stress initiates other dysfunctions. Reversibility of early dysfunctions by antioxidants at the preclinical stage of AD highlights the importance of early diagnosis and antioxidant therapy to delay or prevent the disease processes.