Presynaptic pH and vesicle fusion in Drosophila larvae neurones

Effects of carbonic anhydrase activity on the excitability of hippocampal axons during high-frequency firing

Epileptic activity is known to cause a lowering of intraneuronal pH, which has been suggested to serve as a feedback signal to terminate seizures. The mechanism of such signaling is unclear, but likely involves an altered function of several types of ligand- and voltage-gated channels in postsynaptic membranes caused by increasing cytosolic and extracellular [H+]. In addition, axonal conduction properties may be altered by endogenous pH signals, but this has not been investigated. In the present study, we have recorded the axonal compound action potential (fiber volley) in hippocampal slices in the presence of glutamatergic and GABAergic antagonists. During high-frequency stimulation (HFS) of the Schaffer collaterals, the fiber volley was depressed and its latency from stimulus to peak increased. In the CA1 stratum radiatum these changes were enhanced when the carbonic anhydrase inhibitor acetazolamide (1 mM) was co-perfused. The enhancing effect of acetazolamide was absent after lowering of [Ca2+] in the perfusion medium. Acetazolamide had no detectable effect on HFS-evoked fiber volleys recorded from a more proximal site along the Schaffer collaterals (at the CA2-CA3 border) or from axons in the alveus of CA1. Intracellular acidification imposed by washout of NH4Cl (5 mM) had qualitatively similar effects on fiber volleys evoked at low frequency as those observed with acetazolamide during HFS in CA1 stratum radiatum. The results suggest that carbonic anhydrase-dependent pH regulation counteracts activity-induced reduction of the excitability of Schaffer collateral axons in CA1. A possible influence from local synaptic terminals on this effect is discussed.

Extracellular Protons Mediate Presynaptic Homeostatic Potentiation at the Mouse Neuromuscular Junction

At the vertebrate neuromuscular junction (NMJ), presynaptic homeostatic potentiation (PHP) refers to the upregulation of neurotransmitter release via an increase in quantal content (QC) when the postsynaptic nicotinic acetylcholine receptors (nAChRs) are partially blocked. The mechanism of PHP has not been completely worked out. In particular, the identity of the presumed retrograde signal is still a mystery. We investigated the role of acid-sensing ion channels (ASICs) and extracellular protons in mediating PHP at the mouse NMJ. We found that blocking AISCs using benzamil, psalmotoxin-1 (PcTx1), or mambalgin-3 (Mamb3) prevented PHP. Likewise, extracellular acidification from pH 7.4 to 7.2 triggered a significant, reversable increase in QC and this increase could be prevented by PcTx1. Interestingly, an acidic saline (pH 7.2) also precluded the subsequent induction of PHP. Using immunofluorescence we observed ASIC2a and ASIC1 subunits at the NMJ. Our results indicate that protons and ASIC channels are involved in activating PHP at the mouse NMJ. We speculate that the partial blockade of nAChRs leads to a modest decrease in the pH of the synaptic cleft (∼0.2 pH units) and this activates ASIC channels on the presynaptic nerve terminal.

The Na+/H+ Exchanger Nhe1 Modulates Network Excitability via GABA Release

Brain functions are extremely sensitive to pH changes because of the pH-dependence of proteins involved in neuronal excitability and synaptic transmission. Here, we show that the Na+/H+ exchanger Nhe1, which uses the Na+ gradient to extrude H+, is expressed at both inhibitory and excitatory presynapses. We disrupted Nhe1 specifically in mice either in Emx1-positive glutamatergic neurons or in parvalbumin-positive cells, mainly GABAergic interneurons. While Nhe1 disruption in excitatory neurons had no effect on overall network excitability, mice with disruption of Nhe1 in parvalbumin-positive neurons displayed epileptic activity. From our electrophysiological analyses in the CA1 of the hippocampus, we conclude that the disruption in parvalbumin-positive neurons impairs the release of GABA-loaded vesicles, but increases the size of GABA quanta. The latter is most likely an indirect pH-dependent effect, as Nhe1 was not expressed in purified synaptic vesicles itself. Conclusively, our data provide first evidence that Nhe1 affects network excitability via modulation of inhibitory interneurons.

Drosophila ammonium transporter Rh50 is required for integrity of larval muscles and neuromuscular system

Rhesus glycoproteins (Rh50) have been shown to be ammonia transporters in many species from bacteria to human. They are involved in various physiological processes including acid excretion and pH regulation. Rh50 proteins can also provide a structural link between the cytoskeleton and the plasma membranes that maintain cellular integrity. Although ammonia plays essential roles in the nervous system, in particular at glutamatergic synapses, a potential role for Rh50 proteins at synapses has not yet been investigated. To better understand the function of these proteins in vivo, we studied the unique Rh50 gene of Drosophila melanogaster, which encodes two isoforms, Rh50A and Rh50BC. We found that Drosophila Rh50A is expressed in larval muscles and enriched in the postsynaptic regions of the glutamatergic neuromuscular junctions. Rh50 inactivation by RNA interference selectively in muscle cells caused muscular atrophy in larval stages and pupal lethality. Interestingly, Rh50-deficiency in muscles specifically increased glutamate receptor subunit IIA (GluRIIA) level and the frequency of spontaneous excitatory postsynaptic potentials. Our work therefore highlights a new role for Rh50 proteins in the maintenance of Drosophila muscle architecture and synaptic physiology, which could be conserved in other species.

Unraveling Synaptic GCaMP Signals: Differential Excitability and Clearance Mechanisms Underlying Distinct Ca2+ Dynamics in Tonic and Phasic Excitatory, and Aminergic Modulatory Motor Terminals in Drosophila

GCaMP is an optogenetic Ca²⁺ sensor widely used for monitoring neuronal activities but the precise physiological implications of GCaMP signals remain to be further delineated among functionally distinct synapses. The Drosophila neuromuscular junction (NMJ), a powerful genetic system for studying synaptic function and plasticity, consists of tonic and phasic glutamatergic and modulatory aminergic motor terminals of distinct properties. We report a first simultaneous imaging and electric recording study to directly contrast the frequency characteristics of GCaMP signals of the three synapses for physiological implications. Different GCaMP variants were applied in genetic and pharmacological perturbation experiments to examine the Ca²⁺ influx and clearance processes underlying the GCaMP signal. Distinct mutational and drug effects on GCaMP signals indicate differential roles of Na⁺ and K⁺ channels, encoded by genes including paralytic (para), Shaker (Sh), Shab, and ether-a-go-go (eag), in excitability control of different motor terminals. Moreover, the Ca²⁺ handling properties reflected by the characteristic frequency dependence of the synaptic GCaMP signals were determined to a large extent by differential capacity of mitochondria-powered Ca²⁺ clearance mechanisms. Simultaneous focal recordings of synaptic activities further revealed that GCaMPs were ineffective in tracking the rapid dynamics of Ca²⁺ influx that triggers transmitter release, especially during low-frequency activities, but more adequately reflected cytosolic residual Ca²⁺ accumulation, a major factor governing activity-dependent synaptic plasticity. These results highlight the vast range of GCaMP response patterns in functionally distinct synaptic types and provide relevant information for establishing basic guidelines for the physiological interpretations of presynaptic GCaMP signals from in situ imaging studies.

Serotonergic Modulation Enables Pathway-Specific Plasticity in a Developing Sensory Circuit in Drosophila

How experiences during development cause long-lasting changes in sensory circuits and affect behavior in mature animals are poorly understood. Here we establish a novel system for mechanistic analysis of the plasticity of developing neural circuits by showing that sensory experience during development alters nociceptive behavior and circuit physiology in Drosophila larvae. Despite the convergence of nociceptive and mechanosensory inputs on common second-order neurons (SONs), developmental noxious input modifies transmission from nociceptors to their SONs, but not from mechanosensors to the same SONs, which suggests striking sensory pathway specificity. These SONs activate serotonergic neurons to inhibit nociceptor-to-SON transmission; stimulation of nociceptors during development sensitizes nociceptor presynapses to this feedback inhibition. Our results demonstrate that, unlike associative learning, which involves inputs from two sensory pathways, sensory pathway-specific plasticity in the Drosophila nociceptive circuit is in part established through feedback modulation. This study elucidates a novel mechanism that enables pathway-specific plasticity in sensory systems. VIDEO ABSTRACT.

Neuronal hyperactivity causes Na+/H+ exchanger-induced extracellular acidification at active synapses

Extracellular pH impacts on neuronal activity, which is in turn an important determinant of extracellular H⁺ concentration. The aim of this study was to describe the spatio-temporal dynamics of extracellular pH at synaptic sites during neuronal hyperexcitability. To address this issue we created ex.E²GFP, a membrane-targeted extracellular ratiometric pH indicator that is exquisitely sensitive to acidic shifts. By monitoring ex.E²GFP fluorescence in real time in primary cortical neurons, we were able to quantify pH fluctuations during network hyperexcitability induced by convulsant drugs or high-frequency electrical stimulation. Sustained hyperactivity caused a pH decrease that was reversible upon silencing of neuronal activity and located at active synapses. This acidic shift was not attributable to the outflow of synaptic vesicle H⁺ into the cleft nor to the activity of membrane-exposed H⁺ V-ATPase, but rather to the activity of the Na⁺/H⁺-exchanger. Our data demonstrate that extracellular synaptic pH shifts take place during epileptic-like activity of neural cultures, emphasizing the strict links existing between synaptic activity and synaptic pH. This evidence may contribute to the understanding of the physio-pathological mechanisms associated with hyperexcitability in the epileptic brain.

Acid-sensing ion channel 1a is required for mGlu receptor dependent long-term depression in the hippocampus

Acid-sensing ion channels (ASICs), members of the degenerin/epithelial Na⁺ channel superfamily, are widely distributed in the mammalian nervous system. ASIC1a is highly permeable to Ca²⁺ and are thought to be important in a variety of physiological processes, including synaptic plasticity, learning and memory. To further understand the role of ASIC1a in synaptic transmission and plasticity, we investigated metabotropic glutamate (mGlu) receptor-dependent long-term depression (LTD) in the hippocampus. We found that ASIC1a channels mediate a component of LTD in P30-40 animals, since the ASIC1a selective blocker psalmotoxin-1 (PcTx1) reduced the magnitude of LTD induced by application of the group I mGlu receptor agonist (S)-3,5-Dihydroxyphenylglycine (DHPG) or induced by paired-pulse low frequency stimulation (PP-LFS). Conversely, PcTx1 did not affect LTD in P13-18 animals. We also provide evidence that ASIC1a is involved in group I mGlu receptor-induced increase in action potential firing. However, blockade of ASIC1a did not affect DHPG-induced polyphosphoinositide hydrolysis, suggesting the involvement of some other molecular partners in the functional crosstalk between ASIC1a and group I mGlu receptors. Notably, PcTx1 was able to prevent the increase in GluA1 S845 phosphorylation at the post-synaptic membrane induced by group I mGlu receptor activation. These findings suggest a novel function of ASIC1a channels in the regulation of group I mGlu receptor synaptic plasticity and intrinsic excitability.

Na+ /H+ exchange via the Drosophila vesicular glutamate transporter mediates activity-induced acid efflux from presynaptic terminals

KEY POINTS

Intracellular pH regulation is vital to neurons as nerve activity produces large and rapid acid loads in presynaptic terminals. Rapid clearance of acid loads is necessary to maintain control of neurotransmission, but neuronal acid clearance mechanisms remain poorly understood. Glutamate is loaded into synaptic vesicles via the vesicular glutamate transporter (VGLUT), a mechanism conserved across phyla, and this study reports a previously unknown role for VGLUT as an acid-extruding protein when deposited in the plasmamembrane during exocytosis. The finding was made in Drosophila (fruit fly) larval motor neurons through a combined pharamacological and genetic dissection of presynaptic pH homeostatic mechanisms. A dual role for VGLUT serves to integrate neuronal activity and pH regulation in presynaptic nerve terminals.

ABSTRACT

Neuronal activity can result in transient acidification of presynaptic terminals, and such shifts in cytosolic pH (pH_cyto ) probably influence mechanisms underlying forms of synaptic plasticity with a presynaptic locus. As neuronal activity drives acid loading in presynaptic terminals, we hypothesized that the same activity might drive acid efflux mechanisms to maintain pH_cyto homeostasis. To better understand the integration of neuronal activity and pH_cyto regulation we investigated the acid extrusion mechanisms at Drosophila glutamatergic motorneuron terminals. Expression of a fluorescent genetically encoded pH indicator, named 'pHerry', in the presynaptic cytosol revealed acid efflux following nerve activity to be greater than that predicted from measurements of the intrinsic rate of acid efflux. Analysis of activity-induced acid transients in terminals deficient in either endocytosis or exocytosis revealed an acid efflux mechanism reliant upon synaptic vesicle exocytosis. Pharmacological and genetic dissection in situ and in a heterologous expression system indicate that this acid efflux is mediated by conventional plasmamembrane acid transporters, and also by previously unrecognized intrinsic H⁺ /Na⁺ exchange via the Drosophila vesicular glutamate transporter (DVGLUT). DVGLUT functions not only as a vesicular glutamate transporter but also serves as an acid-extruding protein when deposited on the plasmamembrane.

Archaerhodopsin Selectively and Reversibly Silences Synaptic Transmission through Altered pH

Tools that allow acute and selective silencing of synaptic transmission in vivo would be invaluable for understanding the synaptic basis of specific behaviors. Here, we show that presynaptic expression of the proton pump archaerhodopsin enables robust, selective, and reversible optogenetic synaptic silencing with rapid onset and offset. Two-photon fluorescence imaging revealed that this effect is accompanied by a transient increase in pH restricted to archaerhodopsin-expressing boutons. Crucially, clamping intracellular pH abolished synaptic silencing without affecting the archaerhodopsin-mediated hyperpolarizing current, indicating that changes in pH mediate the synaptic silencing effect. To verify the utility of this technique, we used trial-limited, archaerhodopsin-mediated silencing to uncover a requirement for CA3-CA1 synapses whose afferents originate from the left CA3, but not those from the right CA3, for performance on a long-term memory task. These results highlight optogenetic, pH-mediated silencing of synaptic transmission as a spatiotemporally selective approach to dissecting synaptic function in behaving animals.

Mitochondrial Calcium Uptake Modulates Synaptic Vesicle Endocytosis in Central Nerve Terminals

Presynaptic calcium influx triggers synaptic vesicle (SV) exocytosis and modulates subsequent SV endocytosis. A number of calcium clearance mechanisms are present in central nerve terminals that regulate intracellular free calcium levels both during and after stimulation. During action potential stimulation, mitochondria rapidly accumulate presynaptic calcium via the mitochondrial calcium uniporter (MCU). The role of mitochondrial calcium uptake in modulating SV recycling has been debated extensively, but a definitive conclusion has not been achieved. To directly address this question, we manipulated the expression of the MCU channel subunit in primary cultures of neurons expressing a genetically encoded reporter of SV turnover. Knockdown of MCU resulted in ablation of activity-dependent mitochondrial calcium uptake but had no effect on the rate or extent of SV exocytosis. In contrast, the rate of SV endocytosis was increased in the absence of mitochondrial calcium uptake and slowed when MCU was overexpressed. MCU knockdown did not perturb activity-dependent increases in presynaptic free calcium, suggesting that SV endocytosis may be controlled by calcium accumulation and efflux from mitochondria in their immediate vicinity.

Proton-mediated block of Ca2+ channels during multivesicular release regulates short-term plasticity at an auditory hair cell synapse

Synaptic vesicles release both neurotransmitter and protons during exocytosis, which may result in a transient acidification of the synaptic cleft that can block Ca(2+) channels located close to the sites of exocytosis. Evidence for this effect has been reported for retinal ribbon-type synapses, but not for hair cell ribbon synapses. Here, we report evidence for proton release from bullfrog auditory hair cells when they are held at more physiological, in vivo-like holding potentials (Vh = -60 mV) that facilitate multivesicular release. During paired recordings of hair cells and afferent fibers, L-type voltage-gated Ca(2+) currents showed a transient block, which was highly correlated with the EPSC amplitude (or the amount of glutamate release). This effect was masked at Vh = -90 mV due to the presence of a T-type Ca(2+) current and blocked by strong pH buffering with HEPES or TABS. Increasing vesicular pH with internal methylamine in hair cells also abolished the transient block. High concentrations of intracellular Ca(2+) buffer (10 mm BAPTA) greatly reduced exocytosis and abolished the transient block of the Ca(2+) current. We estimate that this transient block is due to the rapid multivesicular release of ∼600-1300 H(+) ions per synaptic ribbon. Finally, during a train of depolarizing pulses, paired pulse plasticity was significantly changed by using 40 mm HEPES in addition to bicarbonate buffer. We propose that this transient block of Ca(2+) current leads to more efficient exocytosis per Ca(2+) ion influx and it may contribute to spike adaptation at the auditory nerve.

Evidence that protons act as neurotransmitters at vestibular hair cell-calyx afferent synapses

Present data support the conclusion that protons serve as an important neurotransmitter to convey excitatory stimuli from inner ear type I vestibular hair cells to postsynaptic calyx nerve terminals. Time-resolved pH imaging revealed stimulus-evoked extrusion of protons from hair cells and a subsequent buildup of [H(+)] within the confined chalice-shaped synaptic cleft (ΔpH ∼ -0.2). Whole-cell voltage-clamp recordings revealed a concomitant nonquantal excitatory postsynaptic current in the calyx terminal that was causally modulated by cleft acidification. The time course of [H(+)] buildup limits the speed of this intercellular signaling mechanism, but for tonic signals such as gravity, protonergic transmission offers a significant metabolic advantage over quantal excitatory postsynaptic currents--an advantage that may have driven the proliferation of postsynaptic calyx terminals in the inner ear vestibular organs of contemporary amniotes.

Carbonic anhydrases and brain pH in the control of neuronal excitability

H(+) ions are remarkably efficient modulators of neuronal excitability. This renders brain functions highly sensitive to small changes in pH which are generated "extrinsically" via mechanisms that regulate the acid-base status of the whole organism; and "intrinsically", by activity-induced transmembrane fluxes and de novo generation of acid-base equivalents. The effects of pH changes on neuronal excitability are mediated by diverse, largely synergistically-acting mechanisms operating at the level of voltage- and ligand-gated ion channels and gap junctions. In general, alkaline shifts induce an increase in excitability which is often intense enough to trigger epileptiform activity, while acidosis has the opposite effect. Brain pH changes show a wide variability in their spatiotemporal properties, ranging from long-lasting global shifts to fast and highly localized transients that take place in subcellular microdomains. Thirteen catalytically-active mammalian carbonic anhydrase isoforms have been identified, whereof 11 are expressed in the brain. Distinct CA isoforms which have their catalytic sites within brain cells and the interstitial fluid exert a remarkably strong influence on the dynamics of pH shifts and, consequently, on neuronal functions. In this review, we will discuss the various roles of H(+) as an intra- and extracellular signaling factor in the brain, focusing on the effects mediated by CAs. Special attention is paid on the developmental expression patterns and actions of the neuronal isoform, CA VII. Studies on the various functions of CAs will shed light on fundamental mechanisms underlying neuronal development, signaling and plasticity; on pathophysiological mechanisms associated with epilepsy and related diseases; and on the modes of action of CA inhibitors used as CNS-targeting drugs.