Activation of the epsilon isoform of protein kinase C in the mammalian nerve terminal

The role of calcium in neuronal membrane tension and synaptic plasticity

Calcium is a primary second messenger that plays a role in cellular functions including growth, movement and responses to drugs. The role that calcium plays in mediating communication between neurons by synaptic vesicle release is well established. This review focuses on the dependence of the physical properties of neuronal plasma membranes on calcium levels. After describing the key features of synaptic plasticity, we summarize the general role of calcium in cell function and the signaling pathways responsible for intracellular increase in calcium levels. We then present findings showing that increases in intracellular calcium levels cause neurites to contract and break synaptic connections by changes in membrane tension.

Fundamental Neurochemistry Review: GABAA receptor neurotransmission and epilepsy: Principles, disease mechanisms and pharmacotherapy

Epilepsy is a common neurological disorder associated with alterations of excitation-inhibition balance within brain neuronal networks. GABAA receptor neurotransmission is the most prevalent form of inhibitory neurotransmission and is strongly implicated in both the pathophysiology and treatment of epilepsy, serving as a primary target for antiseizure medications for over a century. It is now established that GABA exerts a multifaceted influence through an array of GABAA receptor subtypes that extends far beyond simply negating excitatory activity. As the role of GABAA neurotransmission within inhibitory circuits is elaborated, this will enable the development of precision therapies that correct the network dysfunction underlying epileptic pathology.

Chemical Genetic Identification of PKC Epsilon Substrates in Mouse Brain

PKC epsilon (PKCε) plays important roles in behavioral responses to alcohol and in anxiety-like behavior in rodents, making it a potential drug target for reducing alcohol consumption and anxiety. Identifying signals downstream of PKCε could reveal additional targets and strategies for interfering with PKCε signaling. We used a chemical genetic screen combined with mass spectrometry to identify direct substrates of PKCε in mouse brain and validated findings for 39 of them using peptide arrays and in vitro kinase assays. Prioritizing substrates with several public databases such as LINCS-L1000, STRING, GeneFriends, and GeneMAINA predicted interactions between these putative substrates and PKCε and identified substrates associated with alcohol-related behaviors, actions of benzodiazepines, and chronic stress. The 39 substrates could be broadly classified in three functional categories: cytoskeletal regulation, morphogenesis, and synaptic function. These results provide a list of brain PKCε substrates, many of which are novel, for future investigation to determine the role of PKCε signaling in alcohol responses, anxiety, responses to stress, and other related behaviors.

Possible Association Between DHEA and PKCε in Hepatic Encephalopathy Amelioration: A Pilot Study

Objective: Hepatic encephalopathy (HE) is a neuropsychiatric syndrome caused by liver failure and by an impaired neurotransmission and neurological function caused by hyperammonemia (HA). HE, in turn, decreases the phosphorylation of protein kinase C epsilon (PKCε), contributing to the impairment of neuronal functions. Dehydroepiandrosterone (DHEA) exerts a neuroprotective effect by increasing the GABAergic tone through GABA_A receptor stimulation. Therefore, we investigated the protective effect of DHEA in an animal model of HE, and the possible modulation of PKCε expression in different brain area.

Methods: Fulminant hepatic failure was induced in 18 male, Sprague–Dawley rats by i.p. administration of 3 g/kg D-galactosamine, and after 30 min, a group of animals received a subcutaneous injection of 25 mg/kg (DHEA) repeated twice a day (3 days). Exploratory behavior and general activity were evaluated 24 h and 48 h after the treatments by the open field test. Then, brain cortex and cerebellum were used for immunoblotting analysis of PKCε level.

Results: DHEA administration showed a significant improvement of locomotor activity both 24 and 48 h after D-galactosamine treatment (^****p < 0.0001) but did not ameliorate liver parenchymal degeneration. Western blot analysis revealed a reduced immunoreactivity of PKCε (^*p < 0.05) following D-galactosamine treatment in rat cortex and cerebellum. After the addition of DHEA, PKCε increased in the cortex in comparison with the D-galactosamine-treated (^***p < 0.001) and control group (^*p < 0.05), but decreased in the cerebellum (^*p < 0.05) with respect to the control group. PKCε decreased after treatment with NH₄Cl alone and in combination with DHEA in both cerebellum and cortex (^****p < 0.0001). MTS assay demonstrated the synergistic neurotoxic action of NH₄Cl and glutamate pretreatment in cerebellum and cortex along with an increased cell survival after DHEA pretreatment, which was significant only in the cerebellum (^*p < 0.05).

Conclusion: An association between the DHEA-mediated increase of PKCε expression and the improvement of comatose symptoms was observed. PKCε activation and expression in the brain could inhibit GABA-ergic tone counteracting HE symptoms. In addition, DHEA seemed to ameliorate the symptoms of HE and to increase the expression of PKCε in cortex and cerebellum.

Presynaptic Short-Term Plasticity Persists in the Absence of PKC Phosphorylation of Munc18-1

Post-tetanic potentiation (PTP) is a form of short-term plasticity that lasts for tens of seconds following a burst of presynaptic activity. It has been proposed that PTP arises from protein kinase C (PKC) phosphorylation of Munc18-1, an SM (Sec1/Munc-18 like) family protein that is essential for release. To test this model, we made a knock-in mouse in which all Munc18-1 PKC phosphorylation sites were eliminated through serine-to-alanine point mutations (Munc18-1SA mice), and we studied mice of either sex. The expression of Munc18-1 was not altered in Munc18-1SA mice, and there were no obvious behavioral phenotypes. At the hippocampal CA3-to-CA1 synapse and the granule cell parallel fiber (PF)-to-Purkinje cell (PC) synapse, basal transmission was largely normal except for small decreases in paired-pulse facilitation that are consistent with a slight elevation in release probability. Phorbol esters that mimic the activation of PKC by diacylglycerol still increased synaptic transmission in Munc18-1SA mice. In Munc18-1SA mice, 70% of PTP remained at CA3-to-CA1 synapses, and the amplitude of PTP was not reduced at PF-to-PC synapses. These findings indicate that at both CA3-to-CA1 and PF-to-PC synapses, phorbol esters and PTP enhance synaptic transmission primarily by mechanisms that are independent of PKC phosphorylation of Munc18-1.SIGNIFICANCE STATEMENT A leading mechanism for a prevalent form of short-term plasticity, post-tetanic potentiation (PTP), involves protein kinase C (PKC) phosphorylation of Munc18-1. This study tests this mechanism by creating a knock-in mouse in which Munc18-1 is replaced by a mutated form of Munc18-1 that cannot be phosphorylated. The main finding is that most PTP at hippocampal CA3-to-CA1 synapses or at cerebellar granule cell-to-Purkinje cell synapses does not rely on PKC phosphorylation of Munc18-1. Thus, mechanisms independent of PKC phosphorylation of Munc18-1 are important mediators of PTP.

Mitochondrial hydrogen peroxide positively regulates neuropeptide secretion during diet-induced activation of the oxidative stress response

Mitochondria play a pivotal role in the generation of signals coupling metabolism with neurotransmitter release, but a role for mitochondrial-produced ROS in regulating neurosecretion has not been described. Here we show that endogenously produced hydrogen peroxide originating from axonal mitochondria (mtH2O2) functions as a signaling cue to selectively regulate the secretion of a FMRFamide-related neuropeptide (FLP-1) from a pair of interneurons (AIY) in C. elegans. We show that pharmacological or genetic manipulations that increase mtH2O2 levels lead to increased FLP-1 secretion that is dependent upon ROS dismutation, mitochondrial calcium influx, and cysteine sulfenylation of the calcium-independent PKC family member PKC-1. mtH2O2-induced FLP-1 secretion activates the oxidative stress response transcription factor SKN-1/Nrf2 in distal tissues and protects animals from ROS-mediated toxicity. mtH2O2 levels in AIY neurons, FLP-1 secretion and SKN-1 activity are rapidly and reversibly regulated by exposing animals to different bacterial food sources. These results reveal a previously unreported role for mtH2O2 in linking diet-induced changes in mitochondrial homeostasis with neuropeptide secretion.

Presynaptic Kv3 channels are required for fast and slow endocytosis of synaptic vesicles

Since their discovery decades ago, the primary physiological and pathological effects of potassium channels have been attributed to their ion conductance, which sets membrane potential and repolarizes action potentials. For example, Kv3 family channels regulate neurotransmitter release by repolarizing action potentials. Here we report a surprising but crucial function independent of potassium conductance: by organizing the F-actin cytoskeleton in mouse nerve terminals, the Kv3.3 protein facilitates slow endocytosis, rapid endocytosis, vesicle mobilization to the readily releasable pool, and recovery of synaptic depression during repetitive firing. A channel mutation that causes spinocerebellar ataxia inhibits endocytosis, vesicle mobilization, and synaptic transmission during repetitive firing by disrupting the ability of the channel to nucleate F-actin. These results unmask novel functions of potassium channels in endocytosis and vesicle mobilization crucial for sustaining synaptic transmission during repetitive firing. Potassium channel mutations that impair these "non-conducting" functions may thus contribute to generation of diverse neurological disorders.

Microtubule and Actin Differentially Regulate Synaptic Vesicle Cycling to Maintain High-Frequency Neurotransmission

Cytoskeletal filaments such as microtubules (MTs) and filamentous actin (F-actin) dynamically support cell structure and functions. In central presynaptic terminals, F-actin is expressed along the release edge and reportedly plays diverse functional roles, but whether axonal MTs extend deep into terminals and play any physiological role remains controversial.

Cytoskeletal filaments such as microtubules (MTs) and filamentous actin (F-actin) dynamically support cell structure and functions. In central presynaptic terminals, F-actin is expressed along the release edge and reportedly plays diverse functional roles, but whether axonal MTs extend deep into terminals and play any physiological role remains controversial. At the calyx of Held in rats of either sex, confocal and high-resolution microscopy revealed that MTs enter deep into presynaptic terminal swellings and partially colocalize with a subset of synaptic vesicles (SVs). Electrophysiological analysis demonstrated that depolymerization of MTs specifically prolonged the slow-recovery time component of EPSCs from short-term depression induced by a train of high-frequency stimulation, whereas depolymerization of F-actin specifically prolonged the fast-recovery component. In simultaneous presynaptic and postsynaptic action potential recordings, depolymerization of MTs or F-actin significantly impaired the fidelity of high-frequency neurotransmission. We conclude that MTs and F-actin differentially contribute to slow and fast SV replenishment, thereby maintaining high-frequency neurotransmission.

SIGNIFICANCE STATEMENT The presence and functional role of MTs in the presynaptic terminal are controversial. Here, we demonstrate that MTs are present near SVs in calyceal presynaptic terminals and that MT depolymerization specifically prolongs the slow-recovery component of EPSCs from short-term depression. In contrast, F-actin depolymerization specifically prolongs fast-recovery component. Depolymerization of MT or F-actin has no direct effect on SV exocytosis/endocytosis or basal transmission, but significantly impairs the fidelity of high-frequency transmission, suggesting that presynaptic cytoskeletal filaments play essential roles in SV replenishment for the maintenance of high-frequency neurotransmission.

An organotypic slice culture to study the formation of calyx of Held synapses in-vitro

The calyx of Held, a large axo-somatic relay synapse containing hundreds of presynaptic active zones, is possibly the largest nerve terminal in the mammalian CNS. Studying its initial growth in-vitro might provide insights into the specification of synaptic connection size in the developing brain. However, attempts to maintain calyces of Held in organotypic cultures have not been fruitful in past studies. Here, we describe an organotypic slice culture method in which calyces of Held form in-vitro. We made coronal brainstem slices with an optimized slice angle using newborn mice in which calyces have not yet formed; the presynaptic bushy cells were genetically labeled using the Math5 promoter. After six to nine days of culturing, we readily observed large Math5—positive nerve terminals in the medial nucleus of the trapezoid body (MNTB), but not in the neighboring lateral superior olive nucleus (LSO). These calyx—like synapses expressed the Ca²⁺- sensor Synaptotagmin-2 (Syt-2) and the Ca²⁺ binding protein Parvalbumin (PV), two markers of developing calyces of Held in vivo. Application of the BMP inhibitor LDN-193189 significantly inhibited the growth of calyx synapses, demonstrating the feasibility of long-term pharmacological manipulation using this organotypic culture method. These experiments provide a method for organotypic culturing of calyces of Held, and show that the formation of calyx—like synapses onto MNTB neurons can be preserved in-vitro. Furthermore, our study adds pharmacological evidence for a role of BMP-signaling in the formation of large calyx of Held synapses.

Protein kinase C is a calcium sensor for presynaptic short-term plasticity

In presynaptic boutons, calcium (Ca(2+)) triggers both neurotransmitter release and short-term synaptic plasticity. Whereas synaptotagmins are known to mediate vesicle fusion through binding of high local Ca(2+) to their C2 domains, the proteins that sense smaller global Ca(2+) increases to produce short-term plasticity have remained elusive. Here, we identify a Ca(2+) sensor for post-tetanic potentiation (PTP), a form of plasticity thought to underlie short-term memory. We find that at the functionally mature calyx of Held synapse the Ca(2+)-dependent protein kinase C isoforms α and β are necessary for PTP, and the expression of PKCβ in PKCαβ double knockout mice rescues PTP. Disruption of Ca(2+) binding to the PKCβ C2 domain specifically prevents PTP without impairing other PKCβ-dependent forms of synaptic enhancement. We conclude that different C2-domain-containing presynaptic proteins are engaged by different Ca(2+) signals, and that Ca(2+) increases evoked by tetanic stimulation are sensed by PKCβ to produce PTP.DOI: http://dx.doi.org/10.7554/eLife.03011.001.

Calcium-dependent PKC isoforms have specialized roles in short-term synaptic plasticity

Posttetanic potentiation (PTP) is a widely observed form of short-term plasticity lasting for tens of seconds after high-frequency stimulation. Here we show that although protein kinase C (PKC) mediates PTP at the calyx of Held synapse in the auditory brainstem before and after hearing onset, PTP is produced primarily by an increased probability of release (p) before hearing onset, and by an increased readily releasable pool of vesicles (RRP) thereafter. We find that these mechanistic differences, which have distinct functional consequences, reflect unexpected differential actions of closely related calcium-dependent PKC isoforms. Prior to hearing onset, when PKCγ and PKCβ are both present, PKCγ mediates PTP by increasing p and partially suppressing PKCβ actions. After hearing onset, PKCγ is absent and PKCβ produces PTP by increasing RRP. In hearing animals, virally expressed PKCγ overrides PKCβ to produce PTP by increasing p. Thus, two similar PKC isoforms mediate PTP in distinctly different ways.

Munc18-1 is a dynamically regulated PKC target during short-term enhancement of transmitter release

Transmitter release at synapses is regulated by preceding neuronal activity, which can give rise to short-term enhancement of release like post-tetanic potentiation (PTP). Diacylglycerol (DAG) and Protein-kinase C (PKC) signaling in the nerve terminal have been widely implicated in the short-term modulation of transmitter release, but the target protein of PKC phosphorylation during short-term enhancement has remained unknown. Here, we use a gene-replacement strategy at the calyx of Held, a large CNS model synapse that expresses robust PTP, to study the molecular mechanisms of PTP. We find that two PKC phosphorylation sites of Munc18-1 are critically important for PTP, which identifies the presynaptic target protein for the action of PKC during PTP. Pharmacological experiments show that a phosphatase normally limits the duration of PTP, and that PTP is initiated by the action of a ‘conventional’ PKC isoform. Thus, a dynamic PKC phosphorylation/de-phosphorylation cycle of Munc18-1 drives short-term enhancement of transmitter release during PTP.

DOI:http://dx.doi.org/10.7554/eLife.01715.001

Brain function depends on the rapid transfer of information from one brain cell to the next at junctions known as synapses. Small packages called vesicles play an important role in this process. The arrival of an electrical action potential at the nerve terminal of the first cell causes some vesicles in the cell to fuse with the cell membrane, and this leads to the neurotransmitters inside the vesicles being released into the synapse. The neurotransmitters then bind to receptors on the second cell, which leads to an electrical signal in the second cell. A protein called Munc18-1 has a central role in the fusion of the vesicle at the cell membrane.

The strength of a synapse—that is, how easily the first brain cell can impact the electrical behaviour of the second—can change, and this ‘synaptic plasticity’ is thought to underlie learning and memory. Long-term changes in synaptic strength require additional receptors to be inserted into the membrane of the second cell. However, synapses can also be temporarily strengthened: the arrival of a burst of action potentials—a tetanus—causes some synapses to increase the amount of neurotransmitter they release in response to any subsequent, single, action potential.

This temporary increase in synaptic strength, which is known as post-tetanic potentiation, requires an enzyme called protein kinase C; the role of this enzyme is to phosphorylate specific target proteins (i.e., to add phosphate groups to them). Now, Genç et al. have genetically modified a mouse synapse in vivo and shown that protein kinase C brings about post-tetanic potentiation by phosphorylating Munc18-1. Furthermore, pharmacological experiments show that proteins called phosphatases, which de-phosphorylate proteins, normally terminate the post-tetanic potentiation after about one minute. Taken together, the study identifies a target protein which is phosphorylated by protein kinase C during post-tetanic potentiation. The study also suggests that in addition to its fundamental role in vesicle fusion, the phosphorylation state of Munc18-1 can change the probability of vesicle fusion in a more subtle way, thereby contributing to synaptic plasticity.

DOI:http://dx.doi.org/10.7554/eLife.01715.002

PKC-epsilon activation is required for recognition memory in the rat

Activation of PKCɛ, an abundant and developmentally regulated PKC isoform in the brain, has been implicated in memory throughout life and across species. Yet, direct evidence for a mechanistic role for PKCɛ in memory is still lacking. Hence, we sought to evaluate this in rats, using short-term treatments with two PKCɛ-selective peptides, the inhibitory ɛV1-2 and the activating ψɛRACK, and the novel object recognition task (NORT). Our results show that the PKCɛ-selective activator ψɛRACK, did not have a significant effect on recognition memory. In the short time frames used, however, inhibition of PKCɛ activation with the peptide inhibitor ɛV1-2 significantly impaired recognition memory. Moreover, when we addressed at the molecular level the immediate proximal signalling events of PKCɛ activation in acutely dissected rat hippocampi, we found that ψɛRACK increased in a time-dependent manner phosphorylation of MARCKS and activation of Src, Raf, and finally ERK1/2, whereas ɛV1-2 inhibited all basal activity of this pathway. Taken together, these findings present the first direct evidence that PKCɛ activation is an essential molecular component of recognition memory and point toward the use of systemically administered PKCɛ-regulating peptides as memory study tools and putative therapeutic agents.

Presynaptic NMDA receptor mechanisms for enhancing spontaneous neurotransmitter release

NMDA receptors (NMDARs) are required for experience-driven plasticity during formative periods of brain development and are critical for neurotransmission throughout postnatal life. Most NMDAR functions have been ascribed to postsynaptic sites of action, but there is now an appreciation that presynaptic NMDARs (preNMDARs) can modulate neurotransmitter release in many brain regions, including the neocortex. Despite these advances, the cellular mechanisms by which preNMDARs can affect neurotransmitter release are largely unknown. Here we interrogated preNMDAR functions pharmacologically to determine how these receptors promote spontaneous neurotransmitter release in mouse primary visual cortex. Our results provide three new insights into the mechanisms by which preNMDARs can function. First, preNMDARs can enhance spontaneous neurotransmitter release tonically with minimal extracellular Ca(2+) or with major sources of intracellular Ca(2+) blocked. Second, lowering extracellular Na(+) levels reduces the contribution of preNMDARs to spontaneous transmitter release significantly. Third, preNMDAR enhance transmitter release in part through protein kinase C signaling. These data demonstrate that preNMDARs can act through novel pathways to promote neurotransmitter release in the absence of action potentials.

Actin-dependent rapid recruitment of reluctant synaptic vesicles into a fast-releasing vesicle pool

Glutamatergic synaptic terminals harbor reluctant synaptic vesicles (SVs) that contribute little to synchronous release during action potentials but are release competent when stimulated by sucrose or by direct intracellular application of calcium. It has been noted that the proximity of a release-competent SV to the calcium source is one of the primary factors that differentiate reluctant SVs from fast-releasing ones at the calyx of Held synapse. It has not been known whether reluctant SVs can be converted into fast-releasing ones. Here we show that reluctant SVs are recruited rapidly in an actin-dependent manner to become fast-releasing SVs once the pool of fast-releasing SVs is depleted by a short depolarization. Recovery of the pool of fast-releasing SVs was accompanied by a parallel reduction in the number of reluctant SVs. Quantitative analysis of the time course of depletion of fast-releasing SVs during high-frequency stimulation revealed that in the early phase of stimulation reluctant SVs are converted rapidly into fast-releasing ones, thereby counteracting short-term depression. During the late phase, however, after reluctant vesicles have been used up, another process of calmodulin-dependent recruitment of fast-releasing SVs is activated. These results document that reluctant SVs have a role in short-term plasticity and support the hypothesis of positional priming, which posits that reluctant vesicles are converted into fast-releasing ones via relocation closer to Ca(2+)-channels.

KCNQ5 channels control resting properties and release probability of a synapse

Little is known about which ion channels determine the resting electrical properties of presynaptic membranes. In recordings made from the rat calyx of Held, a giant mammalian terminal, we found resting potential to be controlled by KCNQ (Kv7) K(+) channels, most probably KCNQ5 (Kv7.5) homomers. Unlike most KCNQ channels, which are activated only by depolarizing stimuli, the presynaptic channels began to activate just below the resting potential. As a result, blockers and activators of KCNQ5 depolarized or hyperpolarized nerve terminals, respectively, markedly altering resting conductance. Moreover, the background conductance set by KCNQ5 channels, together with Na(+) and hyperpolarization-activated and cyclic nucleotide-gated (HCN) channels, determined the size and time course of the response to subthreshold stimuli. Signaling pathways known to directly affect exocytic machinery also regulated KCNQ5 channels, and increase or decrease of KCNQ5 channel activity controlled release probability through alterations in resting potential. Thus, ion channel determinants of presynaptic resting potential also control synaptic strength.

Calcium-dependent isoforms of protein kinase C mediate posttetanic potentiation at the calyx of Held

High-frequency stimulation leads to a transient increase in the amplitude of evoked synaptic transmission that is known as posttetanic potentiation (PTP). Here we examine the roles of the calcium-dependent protein kinase C isoforms PKCα and PKCβ in PTP at the calyx of Held synapse. In PKCα/β double knockouts, 80% of PTP is eliminated, whereas basal synaptic properties are unaffected. PKCα and PKCβ produce PTP by increasing the size of the readily releasable pool of vesicles evoked by high-frequency stimulation and by increasing the fraction of this pool released by the first stimulus. PKCα and PKCβ do not facilitate presynaptic calcium currents. The small PTP remaining in double knockouts is mediated partly by an increase in miniature excitatory postsynaptic current amplitude and partly by a mechanism involving myosin light chain kinase. These experiments establish that PKCα and PKCβ are crucial for PTP and suggest that long-lasting presynaptic calcium increases produced by tetanic stimulation may activate these isoforms to produce PTP.

Frequency-dependent modes of synaptic vesicle endocytosis and exocytosis at adult mouse neuromuscular junctions

During locomotion, adult rodent lumbar motoneurons fire in high-frequency (80-100 Hz) 1-2 s bursts every several seconds, releasing between 10,000 and 20,000 vesicles per burst. The estimated total vesicle pool size indicates that all vesicles would be used within 30 s; thus, a mechanism for rapid endocytosis and vesicle recycling is necessary to maintain effective transmission and motor behavior. However, whether such rapid recycling exists at mouse neuromuscular junctions (NMJs) or how it is regulated has been unclear. Here, we show that much less FM1-43 dye is lost per stimulus with 100 Hz stimulation than with 10 Hz stimulation even when the same number of vesicles undergo exocytosis. Electrophysiological data using folimycin show this lesser amount of dye loss is caused in part by the rapid reuse of vesicles. We showed previously that a myosin light chain kinase (MLCK)-myosin II pathway was required for effective transmission at 100 Hz. Here, we confirm the activation of MLCK, based on increased nerve terminal phospho-MLC immunostaining, with 100 Hz but not with 10 Hz stimulation. We further demonstrate that activation of MLCK, by increased extracellular Ca(2+), by PKC (protein kinase C) activation, or by a MLCK agonist peptide, reduces the amount of dye lost even with 10 Hz stimulation. MLCK activation at 10 Hz also resulted in more vesicles being rapidly reused. Thus, MLCK activation by 100 Hz stimulation switches the mechanism of vesicle cycling to a rapid-reuse mode and is required to sustain effective transmission in adult mouse NMJs.

The micro-architecture of mitochondria at active zones: electron tomography reveals novel anchoring scaffolds and cristae structured for high-rate metabolism

Mitochondria are integral elements of many nerve terminals. They must be appropriately positioned to regulate microdomains of Ca(2+) concentration and metabolic demand, but structures that anchor them in place have not been described. By applying the high resolution of electron tomography (ET) to the study of a central terminal, the calyx of Held, we revealed an elaborate cytoskeletal superstructure that connected a subset of mitochondria to the presynaptic membrane near active zones. This cytoskeletal network extended laterally and was well integrated into the nerve terminal cytoskeleton, which included filamentous linkages among synaptic vesicles. ET revealed novel features of inner membrane for these mitochondria. Crista structure was polarized in that crista junctions, circular openings of the inner membrane under the outer membrane, were aligned with the cytoskeletal superstructure and occurred at higher density in mitochondrial membrane facing the presynaptic membrane. These characteristics represent the first instance where a subcomponent of an organelle is shown to have a specific orientation relative to the polarized structure of a cell. The ratio of cristae to outer membrane surface area is large in these mitochondria relative to other tissues, indicating a high metabolic capacity. These observations suggest general principles for cytoskeletal anchoring of mitochondria in all tissues, reveal potential routes for nonsynaptic communication between presynaptic and postsynaptic partners using this novel cytoskeletal framework, and indicate that crista structure can be specialized for particular functions within cellular microdomains.

PKC epsilon facilitates recovery of exocytosis after an exhausting stimulation

It has been well documented that protein kinase Cs (PKCs) play multifaceted roles in regulating exocytosis of neurotransmitters and hormones. But the isoform-specific PKC effects are still poorly elucidated mainly because of the large variety of PKC isoforms and the dubious specificity of the commonly used pharmacological agents. In the present study, based on overexpression of wild-type or dominant negative PKC epsilon, we demonstrate in neuroendocrine PC12 cells that PKC epsilon, but not PKC alpha, facilitates recovery of exocytosis after an exhausting stimulation. Specifically, PKC epsilon mediates fast recovery of the extent of exocytosis in a phosphatidylinositol biphosphate-dependent manner, likely through enhancing the rate of vesicle delivery and reorganization of cortical actin network. In addition, PKC epsilon promotes fast recovery of vesicle release kinetics that is slowed after a strong stimulation. These experimental results may suggest a PKC-dependent mechanism relevant to the short-term plasticity of exocytosis in both neurons and neuroendocrine cells.

GABA synapses mediate neuroprotection after ischemic and epsilonPKC preconditioning in rat hippocampal slice cultures

Delayed neuroprotection against ischemic challenges is conferred by both ischemic preconditioning (IPC) and preconditioning by activation of the epsilon-isoform of protein kinase C (epsilonPKC-PC). In vivo, ischemic preconditioning enhances GABA release and ameliorates glutamate release during lethal cerebral ischemia. We tested the hypothesis that IPC and epsilonPKC-PC confer neuroprotection by GABA synapses in rat organotypic hippocampal slices. Ischemic preconditioning or epsilonPKC-PC was induced with 15 mins oxygen-glucose deprivation (OGD) or psiepsilonRACK, a selective epsilonPKC activator; and test ischemia consisted of 40 mins OGD. At the time of peak neuroprotection (48 h after preconditioning), we recorded GABA(A) receptor-mediated miniature postsynaptic currents (GABA mPSCs) in vulnerable CA1 pyramidal neurons using whole-cell voltage clamp techniques. The frequency and amplitude of GABA mPSCs significantly increased 48 h after IPC. In contrast, epsilonPKC-PC enhanced only the amplitude of GABA mPSCs with no effect on frequency. We next asked if neuroprotection depended on these changes in GABA synapses. Weak antagonism of the GABA(A) receptor with bicuculline (100 nmol/L) decreased the amplitude of GABA mPSCs by 20.9+/-6.1%. When applied during test ischemia, 100 nmol/L bicuculline abolished neuroprotection conferred by either IPC or epsilonPKC-PC. We conclude that neuroprotection conferred by preconditioning depends on functional modifications of GABA synapses.

PKC activation sets an upper limit to the functional plasticity of GABAergic transmission induced by endogenous neurosteroids

The activity of GABAergic inhibitory interneurones located in lamina II of the spinal cord is of fundamental importance for the processing of peripheral nociceptive messages. We have recently shown that 3alpha-hydroxy ring A-reduced pregnane neurosteroids [3alpha5alpha-neurosteroids (3alpha5alphaNS)], potent allosteric modulators of GABA(A) receptors (GABA(A)Rs), are synthesized in the spinal cord and limit thermal hyperalgesia during inflammatory pain. Because changes in the expression of calcium-dependent protein kinases [protein kinase C (PKC)] are observed during pathological pain in the spinal cord, we examined the possible interactions between PKC and 3alpha5alphaNS at synaptic GABA(A)Rs. Using patch-clamp recordings of lamina II interneurones in the spinal cord of 15-20-day-old rats, we showed that synaptic inhibition mediated by GABA(A)Rs and its modulation by 3alpha5alphaNS in lamina II of the spinal cord largely depend on activation of PKC. Our experimental results suggested that activation of PKC locks synaptic GABA(A)Rs in a functional state precluding further positive allosteric modulation by endogenous and exogenous 3alpha5alphaNS. This effect was fully prevented by coadministration of chelerythrin, an inhibitor of PKC. Furthermore, application of chelerythrin alone rendered synaptic GABA(A)Rs hypersensitive to endogenously produced or exogenously applied 3alpha5alphaNS. These findings confirmed that there was a significant production of endogenous 3alpha5alphaNS in lamina II of the spinal cord but also indicated that PKC-dependent phosphorylation processes were tonically activated to control GABA(A)R-mediated inhibition under resting conditions. We therefore can conclude that PKC activation sets an upper limit to the functional plasticity of GABAergic transmission induced by endogenous neurosteroids.

Isoform specificity of protein kinase Cs in synaptic plasticity

Protein kinase Cs (PKCs) are implicated in many forms of synaptic plasticity. However, the specific isoform(s) of PKC that underlie(s) these events are often not known. We have used Aplysia as a model system in order to investigate the isoform specificity of PKC actions due to the presence of fewer isoforms and a large number of documented physiological roles for PKC in synaptic plasticity in this system. In particular, we have shown that distinct isoforms mediate distinct types of synaptic plasticity induced by the same neurotransmitter: The novel calcium-independent PKC Apl II is required for actions mediated by serotonin (5-HT) alone, while the classical calcium-dependent PKC Apl I is required for actions mediated when 5-HT is coupled to activity. We will discuss the reasons for PKC isoform specificity, assess the tools used to uncover isoform specificity, and discuss the implications of isoform specificity for understanding the roles of PKC in regulating synaptic plasticity.

Persistent activation of protein kinase Calpha is not necessary for expression of cerebellar long-term depression

Protein kinase Calpha (PKCalpha) plays a major role in the induction of long-term depression (LTD) in a cerebellar Purkinje cell (PC). The sequential activation model for classical PKC states that PKCalpha translocates to the plasma membrane by binding Ca(++) and then becomes fully activated by binding diacylglycerol (DAG), which enables estimation of the activity by monitoring its localization. Here, we performed simultaneous electrophysiological recording and fluorescence imaging in a cultured PC expressing GFP-tagged PKCalpha. When a PC was depolarized, PKCalpha transiently translocated to the plasma membrane in a Ca(++)-dependent manner. Application of membrane permeable DAG or the blocker of DAG lipase prolonged the translocation. These results suggest that the sequential activation model is applicable to PCs. Conjunctive applications of glutamate and depolarization pulse induced LTD, but did not prolong the translocation. Thus, our results imply that persistent activation of PKCalpha is not necessary for the expression of LTD.

Acute functional tolerance to ethanol mediated by protein kinase Cepsilon

A low level of response to ethanol is associated with increased risk of alcoholism. A major determinant of the level of response is the capacity to develop acute functional tolerance (AFT) to ethanol during a single drinking session. Mice lacking protein kinase C epsilon (PKCepsilon) show increased signs of ethanol intoxication and reduced ethanol self-administration. Here, we report that AFT to the motor-impairing effects of ethanol is reduced in PKCepsilon (-/-) mice when compared with wild-type littermates. In wild-type mice, in vivo ethanol exposure produced AFT that was accompanied by increased phosphorylation of PKCepsilon and resistance of GABA(A) receptors to ethanol. In contrast, in PKCepsilon (-/-) mice, GABA(A) receptor sensitivity to ethanol was unaltered by acute in vivo ethanol exposure. Both PKCepsilon (-/-) and PKCepsilon (+/+) mice developed robust chronic tolerance to ethanol, but the presence of chronic tolerance did not change ethanol preference drinking. These findings suggest that ethanol activates a PKCepsilon signaling pathway that contributes to GABA(A) receptor resistance to ethanol and to AFT. AFT can be genetically dissociated from chronic tolerance, which is not regulated by PKCepsilon and does not alter PKCepsilon modulation of ethanol preference.

The calyx of Held

The calyx of Held is a large glutamatergic synapse in the mammalian auditory brainstem. By using brain slice preparations, direct patch-clamp recordings can be made from the nerve terminal and its postsynaptic target (principal neurons of the medial nucleus of the trapezoid body). Over the last decade, this preparation has been increasingly employed to investigate basic presynaptic mechanisms of transmission in the central nervous system. We review here the background to this preparation and summarise key findings concerning voltage-gated ion channels of the nerve terminal and the ionic mechanisms involved in exocytosis and modulation of transmitter release. The accessibility of this giant terminal has also permitted Ca(2+)-imaging and -uncaging studies combined with electrophysiological recording and capacitance measurements of exocytosis. Together, these studies convey the panopoly of presynaptic regulatory processes underlying the regulation of transmitter release, its modulatory control and short-term plasticity within one identified synaptic terminal.

Protein kinase C-dependent enhancement of activity of rat brain NCKX2 heterologously expressed in HEK293 cells

Different members of the Na+/Ca2++K+ exchanger (NCKX) family are present in distinct brain regions, suggesting that they may have cell-specific functions. Many neuronal channels and transporters are regulated via phosphorylation. Regulation of the rat brain NCKXs by protein kinases, however, has not been described. Here, we report an increase in NCKX2 activity in response to protein kinase C (PKC) activation. Outward current of NCKX2 heterologously expressed in HEK293 cells was enhanced by beta-phorbol dibutyrate (PDBu), whereas PDBu had little effect on activity of NCKX3 or NCKX4. The PDBu-induced enhancement (PIE) of NCKX2 activity was abolished by PKC inhibitors and significantly reduced when the dominant negative mutant of PKCepsilon (K437R) was overexpressed. Moreover, PDBu accelerated the decay rate of the Ca2+ transient at the calyx of Held, where NCKX is the major Ca2+-clearance mechanism. Intracellular perfusion with alkaline phosphatase completely inhibited PIE. Consistently, beta-phorbol myristate acetate (PMA), but not 4alpha-PMA, induced a 3-fold stimulation of 32P incorporation into NCKX2 expressed in HEK293 cells. To investigate the sites involved, PIE of wild-type NCKX2 was compared with mutant NCKX2 in which the three putative PKC consensus sites were replaced with alanine, either individually or in combination. Double-site mutation involving Thr-476 (T166A/T476A and T476A/S504A) disrupted PIE, whereas single mutation of Thr-166, Thr-476, or Ser-504 or the double mutant T166A/S504A failed to completely prevent PIE. These findings suggest that PKC-mediated activation of NCKX2 is sensitive to mutation of multiple PKC consensus sites via a mechanism that may involve several phosphorylation events.

Involvement of protein kinase C-epsilon in activity-dependent potentiation of large dense-core vesicle exocytosis in chromaffin cells

Neurotransmitter release is modulated in an activity-dependent manner. We showed previously that repetitive stimulation of nicotinic acetylcholine receptor (nAChR) induced activity-dependent potentiation (ADP) of large dense-core vesicle (LDCV) exocytosis in chromaffin cells. Here we report that protein kinase C (PKC)-epsilon is critically involved in ADP. Stimulation of nAChR induced activation of PKC-epsilon, and inhibition of PKC-epsilon by expression of the dominant-negative mutant of PKC-epsilon (DN-PKC-epsilon) or short interfering (siRNA) against PKC-epsilon abolished ADP via decreasing the frequency and quantal size of fused vesicles without affecting basal exocytosis, suggesting that PKC-epsilon is specifically involved in ADP. Electron microscopy revealed that inhibition of PKC-epsilon disrupts activity-induced vesicle translocation required for ADP. We also suggest the involvement of myristoylated alanine-rich C kinase substrate (MARCKS), which is known as a downstream target of PKC-epsilon, in ADP of LDCV exocytosis. The level of phospho-MARCKS correlated with the time course of ADP and was reduced by transfection with DN-PKC-epsilon. Actin filament disassembly induced by MARCKS phosphorylation was also significantly blocked by transfection of DN-PKC-epsilon. Furthermore, knockdown of MARCKS by siRNA resulted in inhibition of ADP and reduction of the number of fused vesicles. Together, we provide evidence that ADP of LDCV exocytosis is regulated by PKC-epsilon and its downstream target MARCKS via modulating vesicle translocation.

Modulation of Kv3.1b potassium channel phosphorylation in auditory neurons by conventional and novel protein kinase C isozymes

In fast-spiking neurons such as those in the medial nucleus of the trapezoid body (MNTB) in the auditory brainstem, Kv3.1 potassium channels are required for high frequency firing. The Kv3.1b splice variant of this channel predominates in the mature nervous system and is a substrate for phosphorylation by protein kinase C (PKC) at Ser-503. In resting neurons, basal phosphorylation at this site decreases Kv3.1 current, reducing neuronal ability to follow high frequency stimulation. We used a phospho-specific antibody to determine which PKC isozymes control serine 503 phosphorylation in Kv3.1b-tranfected cells and in auditory neurons in brainstem slices. By using isozyme-specific inhibitors, we found that the novel PKC-delta isozyme, together with the novel PKC-epsilon and conventional PKCs, contributed to the basal phosphorylation of Kv3.1b in MNTB neurons. In contrast, only PKC-epsilon and conventional PKCs mediate increases in phosphorylation produced by pharmacological activation of PKC in MNTB neurons or by metabotropic glutamate receptor activation in Kv3.1/mGluR1-cotransfected cells. We also measured the time course of dephosphorylation and recovery of basal phosphorylation of Kv3.1b following brief high frequency electrical stimulation of the trapezoid body, and we determined that the recovery process is mediated by both novel PKC-delta and PKC-epsilon isozymes and by conventional PKCs. The association between Kv3.1b and PKC isozymes was confirmed by reciprocal coimmunoprecipitation of Kv3.1b with multiple PKC isozymes. Our results suggest that the Kv3.1b channel is regulated by both conventional and novel PKC isozymes and that novel PKC-delta contributes specifically to the maintenance of basal phosphorylation in auditory neurons.

Protein kinase C and synaptic dysfunction after cardiac arrest

It is now understood that the mechanisms leading to neuronal cell death after cardiac arrest (CA) are highly complex. A well established fact in this field is that neurons continue to die over days and months after ischemia. It has been suggested that decreases in electrophysiological activities precede the morphologic deterioration in postischemic CA1 neurons and that this deterioration may be one cause for delayed cell death. The link between synaptic dysfunction and cardiac arrest is evident by the fact that about 50% of long-term survivors of cardiac arrest exhibit impaired mental abilities, manifested as learning impairment, memory disturbance. Since PKC is known to be a key player in synaptic function and has been implicated in promoting cell death after cerebral ischemia, it is a logical candidate as a modulator of synaptic derangements after CA. In this review, we provide an overview of synaptic dysfunction following CA and the putative role of PKC on this dysfunction.

The mGluR5 antagonist 6-methyl-2-(phenylethynyl)pyridine decreases ethanol consumption via a protein kinase C epsilon-dependent mechanism

Glutamatergic neurotransmission plays a critical role in addictive behaviors, and recent evidence indicates that genetic or pharmacological inactivation of the type 5 metabotropic glutamate receptor (mGluR5) reduces the self-administration of cocaine, nicotine, and alcohol. Because mGluR5 is coupled to activation of protein kinase C (PKC), and targeted deletion of the epsilon isoform (PKCepsilon) in mice reduces ethanol self-administration, we investigated whether there is a functional link between mGluR5 and PKCepsilon. Here, we show that acute administration of the mGluR5 agonist (R,S)-2-chloro-5-hydroxyphenylglycine to mice increases phosphorylation of PKCepsilon in its activation loop (T566) as well as in its C-terminal region (S729). Increases in phospho-PKCepsilon are dependent not only on mGluR5 stimulation but also on phosphatidylinositol-3 kinase (PI3K). In addition, the selective mGluR5 antagonist 6-methyl-2-(phenylethynyl)pyridine (MPEP) reduced basal levels of phosphorylation of PKCepsilon at S729. We also show that MPEP dose dependently reduced ethanol consumption in wild-type but not in PKCepsilon-null mice, suggesting that PKCepsilon is an important signaling target for modulation of ethanol consumption by mGluR5 antagonists. Radioligand binding experiments using [(3)H]MPEP revealed that these genotypic differences in response to MPEP were not a result of altered mGluR5 levels or binding in PKCepsilon-null mice. Our data indicate that mGluR5 is coupled to PKCepsilon via a PI3K-dependent pathway and that PKCepsilon is required for the ability of the mGluR5 antagonist MPEP to reduce ethanol consumption.

Distinct roles of different neural cell adhesion molecule (NCAM) isoforms in synaptic maturation revealed by analysis of NCAM 180 kDa isoform-deficient mice

Mice that lack all three major isoforms of neural cell adhesion molecule (NCAM) (180 and 140 kDa transmembrane, and 120 kDa glycosylphosphatidylinositol linked) were previously shown to exhibit major alterations in the maturation of their neuromuscular junctions (NMJs). Specifically, even by postnatal day 30, they failed to downregulate from along their axons and terminals an immature, brefeldin A-sensitive, synaptic vesicle-cycling mechanism that used L-type Ca2+ channels. In addition, these NCAM null NMJs were unable to maintain effective transmitter output with high-frequency repetitive stimulation, exhibiting both severe initial depression and subsequent cyclical periods of total transmission failures that were of presynaptic origin. As reported here, mice that lack only the 180 kDa isoform of NCAM downregulated the immature vesicle-cycling mechanism on schedule, implicating either the 140 or 120 kDa NCAM isoforms in this important maturational event. However, 180 NCAM-deficient mice still exhibited many functional transmission defects. Although 180 NCAM null NMJs did not show the severe initial depression of NCAM null NMJs, they still had cyclical periods of complete transmission failure. In addition, several presynaptic molecules were expressed at lower levels or were more diffusely localized. Thus, the 180 kDa isoform of NCAM appears to play an important role in the molecular organization of the presynaptic terminal and in ensuring effective transmitter output with repetitive stimulation. Our results also suggest that PKC and MLCK (myosin light chain kinase) may be downstream effectors of NCAM in these processes. Together, these results indicate that different isoforms of NCAM mediate distinct and important events in presynaptic maturation.

Glutamate excitotoxicity attenuates insulin-like growth factor-I prosurvival signaling

Recent evidence suggests that impaired insulin/insulin-like growth factor I (IGF-I) input may be associated to neurodegeneration. Several major neurodegenerative diseases involve excitotoxic cell injury whereby excess glutamate signaling leads to neuronal death. Recently it was shown that glutamate inactivates Akt, a serine-kinase crucially involved in the prosurvival actions of IGF-I. We now report that excitotoxic doses of glutamate antagonize Akt activation by IGF-I and inhibit the neuroprotective effects of this growth factor on cultured neurons. Glutamate induces loss of sensitivity to IGF-I by phosphorylating the IGF-I receptor docking protein insulin-receptor-substrate (IRS)-1 in Ser(307) through a pathway involving activation of PKA and PKC in a hierarchical fashion. Administration of Ro320432, a selective PKC inhibitor, abrogates the inhibitory effects of glutamate on IGF-I-induced Akt activation in vitro and in vivo and is sufficient to block the neurotoxic action of glutamate on cultured neurons. Notably, administration of Ro320432 after ischemic insult, a major form of excitotoxic injury in vivo, results in a marked decrease ( approximately 50%) in infarct size. Therefore, uncoupling of IGF-I signaling by glutamate may constitute an additional route contributing to excitotoxic neuronal injury. Further work should determine the potential use of PKC inhibitors as a novel therapeutic strategy in ischemia and other excitotoxic insults.

Distinct roles of Kv1 and Kv3 potassium channels at the calyx of Held presynaptic terminal

Despite identification of >100 potassium channel subunits, relatively little is known about their roles in synaptic transmission. To address this issue we recorded presynaptic potassium currents (IPK) directly from the calyx of Held terminal in brainstem slices of rats. IPK was composed of a 4-aminopyridine (4-AP)-sensitive component and a smaller 4-AP-insensitive component composed of an iberiotoxin-sensitive current and an unidentified slowly activating potassium current. IPK could also be separated into a tetraethylammonium (TEA; 1 mm)-sensitive high-voltage-activated component and a margatoxin (10 nm)-sensitive low-voltage-activated component, which was also blocked by dendrotoxin-I (200 nm) and tityustoxin-Kalpha (100 nm). In outside-out patches excised from calyceal terminals, TEA (1 mm) consistently and to a large extent attenuated IPK, whereas margatoxin attenuated IPK only in a subset of patches (three of seven). Immunocytochemical examination using Kv subtype-specific antibodies indicated that multiple Kv1 and Kv3 subtypes were present at the calyceal terminal. In paired presynaptic and postsynaptic whole-cell recordings, TEA (1 mm) increased both the duration and peak amplitude of presynaptic action potentials and simultaneously potentiated EPSCs. Margatoxin alone had no such effect but reduced the amount of depolarization required for action potential generation, thereby inducing a burst of spikes when the nerve terminal was depolarized for a prolonged period. Thus, at the calyx of Held terminal, Kv3 channels directly regulate evoked transmitter release, whereas Kv1 channels reduce nerve terminal excitability, thereby preventing aberrant transmitter release. We conclude that both Kv3 and Kv1 channels contribute differentially to maintaining the fidelity of synaptic transmission at the calyx of Held.

Adenosine A(1) receptor-mediated presynaptic inhibition at the calyx of Held of immature rats

At the calyx of Held synapse in brainstem slices of 5- to 7-day-old (P5-7) rats, adenosine, or the type 1 adenosine (A1) receptor agonist N6-cyclopentyladenosine (CPA), inhibited excitatory postsynaptic currents (EPSCs) without affecting the amplitude of miniature EPSCs. The A1 receptor antagonist 8-cyclopentyltheophylline (CPT) had no effect on the amplitude of EPSCs evoked at a low frequency, but significantly reduced the magnitude of synaptic depression caused by repetitive stimulation at 10 Hz, suggesting that endogenous adenosine is involved in the regulation of transmitter release. Adenosine inhibited presynaptic Ca(2+) currents (IpCa) recorded directly from calyceal terminals, but had no effect on presynaptic K+ currents. When EPSCs were evoked by IpCa during simultaneous pre- and postsynaptic recordings, the magnitude of the adenosine-induced inhibition of IpCa fully explained that of EPSCs, suggesting that the presynaptic Ca(2+) channel is the main target of A1 receptors. Whereas the N-type Ca(2+) channel blocker omega-conotoxin attenuated EPSCs, it had no effect on the magnitude of adenosine-induced inhibition of EPSCs. During postnatal development, in parallel with a decrease in the A1 receptor immunoreactivity at the calyceal terminal, the inhibitory effect of adenosine became weaker. We conclude that presynaptic A1 receptors at the immature calyx of Held synapse play a regulatory role in transmitter release during high frequency transmission, by inhibiting multiple types of presynaptic Ca(2+) channels.

Distinct roles of Kv1 and Kv3 potassium channels at the calyx of Held presynaptic terminal

Despite identification of >100 potassium channel subunits, relatively little is known about their roles in synaptic transmission. To address this issue we recorded presynaptic potassium currents (IPK) directly from the calyx of Held terminal in brainstem slices of rats. IPK was composed of a 4-aminopyridine (4-AP)-sensitive component and a smaller 4-AP-insensitive component composed of an iberiotoxin-sensitive current and an unidentified slowly activating potassium current. IPK could also be separated into a tetraethylammonium (TEA; 1 mm)-sensitive high-voltage-activated component and a margatoxin (10 nm)-sensitive low-voltage-activated component, which was also blocked by dendrotoxin-I (200 nm) and tityustoxin-Kalpha (100 nm). In outside-out patches excised from calyceal terminals, TEA (1 mm) consistently and to a large extent attenuated IPK, whereas margatoxin attenuated IPK only in a subset of patches (three of seven). Immunocytochemical examination using Kv subtype-specific antibodies indicated that multiple Kv1 and Kv3 subtypes were present at the calyceal terminal. In paired presynaptic and postsynaptic whole-cell recordings, TEA (1 mm) increased both the duration and peak amplitude of presynaptic action potentials and simultaneously potentiated EPSCs. Margatoxin alone had no such effect but reduced the amount of depolarization required for action potential generation, thereby inducing a burst of spikes when the nerve terminal was depolarized for a prolonged period. Thus, at the calyx of Held terminal, Kv3 channels directly regulate evoked transmitter release, whereas Kv1 channels reduce nerve terminal excitability, thereby preventing aberrant transmitter release. We conclude that both Kv3 and Kv1 channels contribute differentially to maintaining the fidelity of synaptic transmission at the calyx of Held.

Dual action of hydrogen peroxide on synaptic transmission at the frog neuromuscular junction

There is evidence that reactive oxygen species (ROS) are produced and released during neuromuscular activity, but their role in synaptic transmission is not known. Using a two-electrode voltage-clamp technique, at frog neuromuscular junctions, the action H2O2 on end-plate currents (EPC) was studied to determine the targets for this membrane-permeable ROS. In curarized or cut muscles, micromolar concentrations of H2O2 increased the amplitude of EPCs. Higher (> 30 microM) doses inhibited EPCs and prolonged current decay. These effects were presynaptic since H2O2 did not change the amplitude or duration of miniature EPCs (although it reduced the rate of spontaneous release at high concentrations). Quantal analysis and deconvolution methods showed that facilitation of EPCs was due to increased quantal release, while depression was accompanied by temporal dispersion of evoked release. Extracellular recordings revealed prolonged presynaptic Ca2+ entry in the presence of high H2O2. Both low and high H2O2 increased presynaptic potentiation during high-frequency stimulation. Pro-oxidant Fe2+ did not affect facilitation by low doses of H2O2 but augmented the inhibition of EPCs by high H2O2, indicating involvement of hydroxyl radicals. High Mg2+ and the ROS scavenger N-acetylcysteine eliminated both the facilitatory and depressant effects of H2O2. The facilitatory effect of H2O2 was prevented by protein kinase C (PKC) inhibitors and 4beta-phorbol 12-myristate, 13-acetate (PMA), an activator of PKC. PKC inhibitors but not PMA also abolished the depressant effect of H2O2. Our data suggest complex presynaptic actions of H2O2, which could serve as a fast feedback modulator of intense neuromuscular transmission.

Involvement of actin polymerization in vesicle recruitment at the calyx of Held synapse

Depletion and replenishment of pools of synaptic vesicles are important determinants of short-term synaptic plasticity, but the underlying molecular mechanisms are not yet clear. As a first step toward understanding the process of vesicle recruitment, we have applied various specific agents directly to the presynaptic terminal of the calyx of Held synapse. Here we show that the nonhydrolyzable ATP analog ATP-gammaS retards the recovery from vesicle pool depletion, as does latrunculin A. Phalloidin has no effects on recovery, suggesting that dynamic actin reorganization is not necessary. Unexpectedly, neither N-ethylmaleimide nor staurosporine affected the recovery, calling into question the role of N-ethylmaleimide-sensitive factor and protein kinases. The results suggest that intact actin polymerization is involved in vesicle recruitment.

Involvement of actin polymerization in vesicle recruitment at the calyx of Held synapse

Depletion and replenishment of pools of synaptic vesicles are important determinants of short-term synaptic plasticity, but the underlying molecular mechanisms are not yet clear. As a first step toward understanding the process of vesicle recruitment, we have applied various specific agents directly to the presynaptic terminal of the calyx of Held synapse. Here we show that the nonhydrolyzable ATP analog ATP-gammaS retards the recovery from vesicle pool depletion, as does latrunculin A. Phalloidin has no effects on recovery, suggesting that dynamic actin reorganization is not necessary. Unexpectedly, neither N-ethylmaleimide nor staurosporine affected the recovery, calling into question the role of N-ethylmaleimide-sensitive factor and protein kinases. The results suggest that intact actin polymerization is involved in vesicle recruitment.

Protein kinase C activity, translocation, and selective isoform subcellular redistribution in the rat cerebral cortex after in vitro ischemia

Protein kinase C (PKC) involvement in ischemia-induced neuronal damage has been investigated in superfused rat cerebral cortex slices submitted to 15 min of oxygen-glucose deprivation (OGD) and in primary cultures of rat cortical neurons exposed to 100 microM glutamate (GLU) for 10 min. OGD significantly increased the total PKC activity in the slices, mostly translocated in the particulate fraction. After 1 hr of reperfusion, the total PKC activity was reduced and the translocated fraction dropped by 84% with respect to the control. Western blot analysis of OGD samples showed an increase in total beta(2) and epsilon PKC isoform levels. After reperfusion, the total levels of alpha, beta(1), beta(2) and gamma isoforms were significantly reduced, whereas the epsilon isoform remained at an increased level. Endogenous GLU release from OGD slices increased to about 15 times the basal values after 15 min of oxygen-glucose deprivation, and to 25 and 35 times the basal level in the presence of the PKC inhibitors staurosporine (0.1 microM) and bisindolylmaleimide (1 microM), respectively. Western blot analysis of GLU-treated cortical neurons showed a significant decrease only in the total level of beta(2) isoforms. Cell survival was reduced to 31% in GLU-treated neuronal cultures; PKC inhibitors were not able to modify this effect. These findings demonstrate that the cell response to OGD and GLU involves PKC in a complex way. The net role played by PKC during OGD may be to reduce GLU release and, consequently, neurotoxicity. The isoforms beta(2) and epsilon are affected the most and may play a significant role in the mechanisms underlying neurotoxicity/neuroprotection.

PGD(2) modulates fibroblast-mediated native collagen gel contraction

Repair of tissues is a necessary step in restoring tissue function following injury consequent to inflammation. Many inflammatory mediators are capable of modulating not only the activity of "inflammatory cells" but also of modulating functions of parenchymal cells that may contribute to repair. Disordered repair is believed to contribute to tissue dysfunction in many inflammatory diseases, including bronchial asthma. The current study evaluated the ability of prostaglandin D(2) (PGD(2)) to modulate fibroblast repair using the in vitro contraction of three-dimensional native collagen gels as a model system. PGD(2) stimulated gel contraction in a concentration- and time-dependent manner. In contrast, the PGD(2) analog BW245C inhibited contraction. Both effects were blocked by a DP-receptor blocker (AH6809). Neither TP receptor blocker SQ29548 nor protein kinase (PK) A antagonist KT5720 hand an effect on PGD(2)-stimulated contraction, suggesting action through a novel prostaglandin D receptor. PKC inhibitor calphostin-C (10(-6) M) blocked the PGD(2) stimulation of gel contraction. A calcium-independent PKC-epsilon inhibitor (Ro31-8220), but not calcium-dependent PKC-alpha and -beta inhibitors, also blocked the PGD(2) effect on contraction, implying a role for a calcium-independent pathway. This study, therefore, supports a role for PGD(2) in tissue repair and remodeling. These effects of PGD(2) appear to be mediated through receptor-signal transduction pathways different from the cAMP-PKA pathways mediating the proinflammatory activity of PGD(2), creating the possibility for selective therapeutic manipulation.

Increase in the pool size of releasable synaptic vesicles by the activation of protein kinase C in goldfish retinal bipolar cells

Secretion from neurons and neuroendocrine cells is enhanced by the activation of protein kinase C (PKC) in various preparations. We have already reported that transmitter (glutamate) release from Mb1 bipolar cells in the goldfish retina is potentiated by the activation of PKC. However, it is not yet settled whether the potentiation is ascribed to the increase in the pool size of releasable synaptic vesicles or in release probability. In the present study, Ca2+ influx and exocytosis were simultaneously monitored by measuring the presynaptic Ca2+ current and membrane capacitance changes, respectively, in a terminal detached from the bipolar cell. The double pulse protocol was used to estimate separately the changes in the pool size and release probability. The activation of PKC by phorbol 12-myristate 13-acetate (PMA) specifically increased the pool size but not the release probability. PKC was activated by PMA even after the Ca2+ influx was blocked by Co2+. In bipolar cells the releasable pool can be divided into two components: one is small and rapidly exhausted, and the other is large and slowly exocytosed. To identify which component is responsible for the increase in the pool size, the effects of PMA and a PKC-specific inhibitor, bisindolylmaleimide I (BIS), on each component were examined. The slow component was selectively increased by PMA and reduced by BIS. Thus, we conclude that the activation of PKC in Mb1 bipolar cells potentiates glutamate release by increasing the pool size of the slow component.

Different actions of protein kinase C isoforms alpha and epsilon on gastric acid secretion

1. The phorbol ester TPA, an activator of protein kinase C (PKC), inhibits cholinergic stimulation of gastric acid secretion but increases basal H(+) secretion. 2. Since these contradictory findings suggest the action of different PKC isozymes we analysed the role of calcium-dependent PKC-alpha, and calcium-independent PKC-epsilon in gastric acid secretion. 3. Inhibition of PKC-alpha by the indolocarbazole Gö 6976 revealed that about 28% of carbachol-induced acid secretion was inhibited by PKC-alpha. In the presence of Gö 6976 approximately 64% of the carbachol-induced signal transduction is mediated by Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), and 14% is conveyed by PKC-epsilon as deduced from the inhibition with the bisindolylmaleimide Ro 31-8220. 4. Inhibition of carbachol-induced acid secretion by TPA was accompanied by a decrease in CaMKII activity. 5. The stimulation of basal acid secretion by TPA was biphasic with a peak at a very low concentration (10 pM), resulting in an activation of the calcium-sensor CaMKII. The activation was determined with a phosphospecific polyclonal antibody against active CaMKII. The TPA-induced increase of H(+) secretion was sensitive to the cell-permeable Ca(2+)-chelator BAPTA/AM, Ro 31-8220, and the CaMKII-inhibitor KN-62, but not to Gö 6976. 6. Since TPA induced the translocation of PKC-epsilon but not of PKC-alpha in resting parietal cells, PKC-epsilon seems to be at least responsible for an initial elevation of free intracellular calcium to initiate TPA-induced acid secretion. 7. Our data indicate the different roles of two PKC isoforms: PKC-epsilon activation appears to facilitate cholinergic stimulation of H(+)-secretion likely by increasing intracellular calcium. In contrast, PKC-alpha activation attenuates acid secretion accompanied by a down-regulation of CaMKII activity.

Increase in the pool size of releasable synaptic vesicles by the activation of protein kinase C in goldfish retinal bipolar cells

Secretion from neurons and neuroendocrine cells is enhanced by the activation of protein kinase C (PKC) in various preparations. We have already reported that transmitter (glutamate) release from Mb1 bipolar cells in the goldfish retina is potentiated by the activation of PKC. However, it is not yet settled whether the potentiation is ascribed to the increase in the pool size of releasable synaptic vesicles or in release probability. In the present study, Ca2+ influx and exocytosis were simultaneously monitored by measuring the presynaptic Ca2+ current and membrane capacitance changes, respectively, in a terminal detached from the bipolar cell. The double pulse protocol was used to estimate separately the changes in the pool size and release probability. The activation of PKC by phorbol 12-myristate 13-acetate (PMA) specifically increased the pool size but not the release probability. PKC was activated by PMA even after the Ca2+ influx was blocked by Co2+. In bipolar cells the releasable pool can be divided into two components: one is small and rapidly exhausted, and the other is large and slowly exocytosed. To identify which component is responsible for the increase in the pool size, the effects of PMA and a PKC-specific inhibitor, bisindolylmaleimide I (BIS), on each component were examined. The slow component was selectively increased by PMA and reduced by BIS. Thus, we conclude that the activation of PKC in Mb1 bipolar cells potentiates glutamate release by increasing the pool size of the slow component.