Roscovitine: a novel regulator of P/Q-type calcium channels and transmitter release in central neurons

Direct Current Stimulation Modulates Synaptic Facilitation via Distinct Presynaptic Calcium Channels

Transcranial direct current stimulation (tDCS) is a subthreshold neurostimulation technique known for ameliorating neuropsychiatric conditions. The principal mechanism of tDCS is the differential polarization of subcellular neuronal compartments, particularly the axon terminals that are sensitive to external electrical fields. Yet, the underlying mechanism of tDCS is not fully clear. Here, we hypothesized that direct current stimulation (DCS)-induced modulation of presynaptic calcium channel conductance alters axon terminal dynamics with regard to synaptic vesicle release. To examine the involvement of calcium-channel subtypes in tDCS, we recorded spontaneous excitatory postsynaptic currents (sEPSCs) from cortical layer-V pyramidal neurons under DCS while selectively inhibiting distinct subtypes of voltage-dependent calcium channels. Blocking P/Q or N-type calcium channels occluded the effects of DCS on sEPSCs, demonstrating their critical role in the process of DCS-induced modulation of spontaneous vesicle release. However, inhibiting T-type calcium channels did not occlude DCS-induced modulation of sEPSCs, suggesting that despite being active in the subthreshold range, T-type calcium channels are not involved in the axonal effects of DCS. DCS modulates synaptic facilitation by regulating calcium channels in axon terminals, primarily via controlling P/Q and N-type calcium channels, while T-type calcium channels are not involved in this mechanism.

Potentiation of neuromuscular transmission by a small molecule calcium channel gating modifier improves motor function in a severe spinal muscular atrophy mouse model

Spinal muscular atrophy (SMA) is a monogenic disease that clinically manifests as severe muscle weakness owing to neurotransmission defects and motoneuron degeneration. Individuals affected by SMA experience neuromuscular weakness that impacts functional activities of daily living. We have used a mouse model of severe SMA (SMNΔ7) to test whether a calcium channel gating modifier (GV-58), alone or in combination with a potassium channel antagonist (3,4-diaminopyridine; 3,4-DAP), can improve neuromuscular function in this mouse model. Bath application of GV-58 alone or in combination with 3,4-DAP significantly restored neuromuscular transmission to control levels in both a mildly vulnerable forearm muscle and a strongly vulnerable trunk muscle in SMNΔ7 mice at postnatal days 10-12. Similarly, acute subcutaneous administration of GV-58 to postnatal day 10 SMNΔ7 mice, alone or in combination with 3,4-DAP, significantly increased a behavioral measure of muscle strength. These data suggest that GV-58 may be a promising treatment candidate that could address deficits in neuromuscular function and strength and that the addition of 3,4-DAP to GV-58 treatment could aid in restoring function in SMA.

Nifedipine Ameliorates Cellular Differentiation Defects of Smn-Deficient Motor Neurons and Enhances Neuromuscular Transmission in SMA Mice

In spinal muscular atrophy (SMA), mutations in or loss of the Survival Motor Neuron 1 (SMN1) gene reduce full-length SMN protein levels, which leads to the degeneration of a percentage of motor neurons. In mouse models of SMA, the development and maintenance of spinal motor neurons and the neuromuscular junction (NMJ) function are altered. Since nifedipine is known to be neuroprotective and increases neurotransmission in nerve terminals, we investigated its effects on cultured spinal cord motor neurons and motor nerve terminals of control and SMA mice. We found that application of nifedipine increased the frequency of spontaneous Ca²⁺ transients, growth cone size, cluster-like formations of Cav2.2 channels, and it normalized axon extension in SMA neurons in culture. At the NMJ, nifedipine significantly increased evoked and spontaneous release at low-frequency stimulation in both genotypes. High-strength stimulation revealed that nifedipine increased the size of the readily releasable pool (RRP) of vesicles in control but not SMA mice. These findings provide experimental evidence about the ability of nifedipine to prevent the appearance of developmental defects in SMA embryonic motor neurons in culture and reveal to which extent nifedipine could still increase neurotransmission at the NMJ in SMA mice under different functional demands.

Regulation of Presynaptic Calcium Channels

Voltage-gated calcium channels (VGCCs), especially Cav2.1 and Cav2.2, are the major mediators of Ca2+ influx at the presynaptic membrane in response to neuron excitation, thereby exerting a predominant control on synaptic transmission. To guarantee the timely and precise release of neurotransmitters at synapses, the activity of presynaptic VGCCs is tightly regulated by a variety of factors, including auxiliary subunits, membrane potential, G protein-coupled receptors (GPCRs), calmodulin (CaM), Ca2+-binding proteins (CaBP), protein kinases, various interacting proteins, alternative splicing events, and genetic variations.

An active vesicle priming machinery suppresses axon regeneration upon adult CNS injury

Axons in the adult mammalian central nervous system fail to regenerate after spinal cord injury. Neurons lose their capacity to regenerate during development, but the intracellular processes underlying this loss are unclear. We found that critical components of the presynaptic active zone prevent axon regeneration in adult mice. Transcriptomic analysis combined with live-cell imaging revealed that adult primary sensory neurons downregulate molecular constituents of the synapse as they acquire the ability to rapidly grow their axons. Pharmacogenetic reduction of neuronal excitability stimulated axon regeneration after adult spinal cord injury. Genetic gain- and loss-of-function experiments uncovered that essential synaptic vesicle priming proteins of the presynaptic active zone, but not clostridial-toxin-sensitive VAMP-family SNARE proteins, inhibit axon regeneration. Systemic administration of Baclofen reduced voltage-dependent Ca2+ influx in primary sensory neurons and promoted their regeneration after spinal cord injury. These findings indicate that functional presynaptic active zones constitute a major barrier to axon regeneration.

Therapy development for spinal muscular atrophy: perspectives for muscular dystrophies and neurodegenerative disorders

BACKGROUND

Major efforts have been made in the last decade to develop and improve therapies for proximal spinal muscular atrophy (SMA). The introduction of Nusinersen/Spinraza™ as an antisense oligonucleotide therapy, Onasemnogene abeparvovec/Zolgensma™ as an AAV9-based gene therapy and Risdiplam/Evrysdi™ as a small molecule modifier of pre-mRNA splicing have set new standards for interference with neurodegeneration.

MAIN BODY

Therapies for SMA are designed to interfere with the cellular basis of the disease by modifying pre-mRNA splicing and enhancing expression of the Survival Motor Neuron (SMN) protein, which is only expressed at low levels in this disorder. The corresponding strategies also can be applied to other disease mechanisms caused by loss of function or toxic gain of function mutations. The development of therapies for SMA was based on the use of cell culture systems and mouse models, as well as innovative clinical trials that included readouts that had originally been introduced and optimized in preclinical studies. This is summarized in the first part of this review. The second part discusses current developments and perspectives for amyotrophic lateral sclerosis, muscular dystrophies, Parkinson's and Alzheimer's disease, as well as the obstacles that need to be overcome to introduce RNA-based therapies and gene therapies for these disorders.

CONCLUSION

RNA-based therapies offer chances for therapy development of complex neurodegenerative disorders such as amyotrophic lateral sclerosis, muscular dystrophies, Parkinson's and Alzheimer's disease. The experiences made with these new drugs for SMA, and also the experiences in AAV gene therapies could help to broaden the spectrum of current approaches to interfere with pathophysiological mechanisms in neurodegeneration.

Topographical distance between presynaptic Ca2+ channels and exocytotic Ca2+ sensors contributes to differential facilitatory actions of roscovitine on neurotransmitter release at cerebellar glutamatergic and GABAergic synapses

Calcium influx into presynaptic terminals through voltage-gated Ca²⁺ channels triggers univesicular or multivesicular release of neurotransmitters depending on the characteristics of the release machinery. However, the mechanisms underlying multivesicular release (MVR) and its regulation remain unclear. Previous studies showed that in rat cerebellum, the cyclin-dependent kinase inhibitor roscovitine profoundly increases excitatory postsynaptic current (EPSC) amplitudes at granule cell (GC)-Purkinje cell (PC) synapses by enhancing the MVR of glutamate. This compound can also moderately augment the amplitude and prolong the decay time of inhibitory postsynaptic currents (IPSCs) at molecular layer interneuron (MLI)-PC synapses via MVR enhancement and GABA spillover, thus allowing for persistent activation of perisynaptic GABA receptors. The enhanced MVR may depend on the driving force for Ca_v 2.1 channel-mediated Ca²⁺ influx. To determine whether the distinct spatiotemporal dynamics of presynaptic Ca²⁺ influence MVR, we compared the effects of slow and fast Ca²⁺ chelators, that is, EGTA and BAPTA, respectively, on roscovitine-induced actions at GC-PC and MLI-PC synapses. Membrane-permeable EGTA-AM decreased GC-PC EPSC and MLI-PC IPSC amplitudes to a similar extent but suppressed the roscovitine-induced enhancement of EPSCs. In contrast, BAPTA-AM attenuated the effects of roscovitine on IPSCs. These results suggest that roscovitine augmented glutamate release by activating the release machinery located distally from the Ca_v 2.1 channel clusters, while it enhanced GABA release in a manner less dependent on those at distal sites. Therefore, the spatial relationships among Ca²⁺ channels, buffers, and sensors are critical determinants of the differential facilitatory actions of roscovitine on glutamatergic and GABAergic synapses in the cerebellar cortex.

Roscovitine differentially facilitates cerebellar glutamatergic and GABAergic neurotransmission by enhancing Cav 2.1 channel-mediated multivesicular release

Synaptic vesicle exocytosis is triggered by Ca²⁺ influx through several subtypes of voltage-gated calcium channels in the presynaptic terminal. We previously reported that paired-pulse stimulation at brief intervals increases Ca_v 2.1 (P/Q-type) channel-mediated multivesicular release (MVR) at glutamatergic synapses between granule cells (GCs) and molecular layer interneurons (MLIs) in rat cerebellar slices. However, it has yet to be determined how Ca_v 2 channel subtypes take part in MVR in single axon terminal. This study therefore aimed at examining the effects of roscovitine on different types of cerebellar synapses that make contacts with Purkinje cells (PCs), because this compound has been shown to enhance Ca_v 2.1 channel-mediated MVR at GC-MLI synapses. Bath application of roscovitine profoundly increased the amplitude of excitatory postsynaptic currents (EPSCs) at GC-PC synapses by a presynaptic mechanism as previously observed at GC-MLI synapses, whereas it caused a marginal effect on climbing fiber-mediated EPSCs in PCs. At MLI-PC synapses, roscovitine increased both the amplitude and decay time of inhibitory postsynaptic currents (IPSCs) by enhancing multivesicular GABA release. When extracellular Ca²⁺ concentration ([Ca²⁺ ]_e ) decreased, roscovitine became less effective in increasing GC-PC EPSCs. By contrast, roscovitine was able to augment MLI-PC IPSCs in the low [Ca²⁺ ]_e . The Ca_v 2.1 channel blocker ω-agatoxin IVA suppressed the roscovitine-induced facilitatory actions on both GC-PC EPSCs and MLI-PC IPSCs. These results demonstrate that roscovitine enhances MVR at the GC-PC excitatory synapses in a manner dependent on the driving force of Ca_v 2.1 channel-mediated Ca²⁺ influx into the nerve terminal, while it also facilitates MLI-PC inhibitory transmission via Ca²⁺ -insensitive mechanisms.

Presynaptic calcium channels: specialized control of synaptic neurotransmitter release

Chemical synapses are heterogeneous junctions formed between neurons that are specialized for the conversion of electrical impulses into the exocytotic release of neurotransmitters. Voltage-gated Ca2+ channels play a pivotal role in this process as they are the major conduits for the Ca2+ ions that trigger the fusion of neurotransmitter-containing vesicles with the presynaptic membrane. Alterations in the intrinsic function of these channels and their positioning within the active zone can profoundly alter the timing and strength of synaptic output. Advances in optical and electron microscopic imaging, structural biology and molecular techniques have facilitated recent breakthroughs in our understanding of the properties of voltage-gated Ca2+ channels that support their presynaptic functions. Here we examine the nature of these channels, how they are trafficked to and anchored within presynaptic boutons, and the mechanisms that allow them to function optimally in shaping the flow of information through neural circuits.

Zebrafish as a Model System for the Study of Severe CaV2.1 (α1A) Channelopathies

The P/Q-type Ca_V2.1 channel regulates neurotransmitter release at neuromuscular junctions (NMJ) and many central synapses. CACNA1A encodes the pore-containing α_1A subunit of Ca_V2.1 channels. In humans, de novo CACNA1A mutations result in a wide spectrum of neurological, neuromuscular, and movement disorders, such as familial hemiplegic migraine type 1 (FHM1), episodic ataxia type 2 (EA2), as well as a more recently discovered class of more severe disorders, which are characterized by ataxia, hypotonia, cerebellar atrophy, and cognitive/developmental delay. Heterologous expression of Ca_V2.1 channels has allowed for an understanding of the consequences of CACNA1A missense mutations on channel function. In contrast, a mechanistic understanding of how specific CACNA1A mutations lead in vivo to the resultant phenotypes is lacking. In this review, we present the zebrafish as a model to both study in vivo mechanisms of CACNA1A mutations that result in synaptic and behavioral defects and to screen for effective drug therapies to combat these and other Ca_V2.1 channelopathies.

R-Roscovitine Improves Motoneuron Function in Mouse Models for Spinal Muscular Atrophy

Neurotransmission defects and motoneuron degeneration are hallmarks of spinal muscular atrophy, a monogenetic disease caused by the deficiency of the SMN protein. In the present study, we show that systemic application of R-Roscovitine, a Ca_v2.1/Ca_v2.2 channel modifier and a cyclin-dependent kinase 5 (Cdk-5) inhibitor, significantly improved survival of SMA mice. In addition, R-Roscovitine increased Ca_v2.1 channel density and sizes of the motor endplates. In vitro, R-Roscovitine restored axon lengths and growth cone sizes of Smn-deficient motoneurons corresponding to enhanced spontaneous Ca²⁺ influx and elevated Ca_v2.2 channel cluster formations independent of its capability to inhibit Cdk-5. Acute application of R-Roscovitine at the neuromuscular junction significantly increased evoked neurotransmitter release, increased the frequency of spontaneous miniature potentials, and lowered the activation threshold of silent terminals. These data indicate that R-Roscovitine improves Ca²⁺ signaling and Ca²⁺ homeostasis in Smn-deficient motoneurons, which is generally crucial for motoneuron differentiation, maturation, and function.

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R-Roscovitine prolongs survival of SMA mice

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R-Roscovitine increases Ca²⁺ influx and growth cone size of SMA motoneurons

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R-Roscovitine beneficially affects neurotransmission in SMA motor nerve terminals

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R-Roscovitine wakes up dormant synapses of SMA motoneurons

A mutation in CaV2.1 linked to a severe neurodevelopmental disorder impairs channel gating

Ca²⁺ flux into axon terminals via P-/Q-type Ca_V2.1 channels is the trigger for neurotransmitter vesicle release at neuromuscular junctions (NMJs) and many central synapses. Recently, an arginine to proline substitution (R1673P) in the S4 voltage-sensing helix of the fourth membrane-bound repeat of Ca_V2.1 was linked to a severe neurological disorder characterized by generalized hypotonia, ataxia, cerebellar atrophy, and global developmental delay. The R1673P mutation was proposed to cause a gain of function in Ca_V2.1 leading to neuronal Ca²⁺ toxicity based on the ability of the mutant channel to rescue the photoreceptor response in Ca_V2.1-deficient Drosophila cacophony larvae. Here, we show that the corresponding mutation in rat Ca_V2.1 (R1624P) causes a profound loss of channel function; voltage-clamp analysis of tsA-201 cells expressing this mutant channel revealed an ∼25-mV depolarizing shift in the voltage dependence of activation. This alteration in activation implies that a significant fraction of Ca_V2.1 channels resident in presynaptic terminals are unlikely to open in response to an action potential, thereby increasing the probability of synaptic failure at both NMJs and central synapses. Indeed, the mutant channel supported only minimal Ca²⁺ flux in response to an action potential-like waveform. Application of GV-58, a compound previously shown to stabilize the open state of wild-type Ca_V2.1 channels, partially restored Ca²⁺ current by shifting mutant activation to more hyperpolarizing potentials and slowing deactivation. Consequently, GV-58 also rescued a portion of Ca²⁺ flux during action potential-like stimuli. Thus, our data raise the possibility that therapeutic agents that increase channel open probability or prolong action potential duration may be effective in combatting this and other severe neurodevelopmental disorders caused by loss-of-function mutations in Ca_V2.1.

Opposite Roles in Short-Term Plasticity for N-Type and P/Q-Type Voltage-Dependent Calcium Channels in GABAergic Neuronal Connections in the Rat Cerebral Cortex

Neurotransmitter release is triggered by Ca²⁺ influx through voltage-dependent Ca²⁺ channels (VDCCs). Distinct expression patterns of VDCC subtypes localized on the synaptic terminal affect intracellular Ca²⁺ dynamics induced by action potential-triggered Ca²⁺ influx. However, it has been unknown whether the expression pattern of VDCC subtypes depends on each axon terminal or neuronal subtype. Furthermore, little information is available on how these VDCC subtypes regulate the release probability of neurotransmitters. To address these questions, we performed multiple whole-cell patch-clamp recordings from GABAergic neurons in the insular cortex of either the male or the female rat. The paired-pulse ratio (PPR; 50 ms interstimulus interval) varied widely among inhibitory connections between GABAergic neurons. The PPR of unitary IPSCs was enhanced by ω-conotoxin GVIA (CgTx; 3 μm), an N-type VDCC blocker, whereas blockade of P/Q-type VDCCs by ω-agatoxin IVA (AgTx, 200 nm) decreased the PPR. In the presence of CgTx, application of 4 mm [Ca²⁺]_o or of roscovitine, a P/Q-type activator, increased the PPR. These results suggest that the recruitment of P/Q-type VDCCs increases the PPR, whereas N-type VDCCs suppress the PPR. Furthermore, we found that charybdotoxin or apamin, blockers of Ca²⁺-dependent K⁺ channels, with AgTx increased the PPR, suggesting that Ca²⁺-dependent K⁺ channels are coupled to N-type VDCCs and suppress the PPR in GABAergic neuronal terminals. Variance-mean analysis with changing [Ca²⁺]_o showed a negative correlation between the PPR and release probability in GABAergic synapses. These results suggest that GABAergic neurons differentially express N-type and/or P/Q-type VDCCs and that these VDCCs regulate the GABA release probability in distinct manners.SIGNIFICANCE STATEMENT GABAergic neuronal axons target multiple neurons and release GABA triggered by Ca²⁺ influx via voltage-dependent Ca²⁺ channels (VDCCs), including N-type and P/Q-type channels. Little is known about VDCC expression patterns in GABAergic synaptic terminals and their role in short-term plasticity. We focused on inhibitory synaptic connections between GABAergic neurons in the cerebral cortex using multiple whole-cell patch-clamp recordings and found different expression patterns of VDCCs in the synaptic terminals branched from a single presynaptic neuron. Furthermore, we observed facilitative and depressive short-term plasticity of IPSCs mediated by P/Q-type and N-type VDCCs, respectively. These results suggest that VDCC expression patterns regulate distinctive types of synaptic transmission in each GABAergic axon terminal even though they are branched from a common presynaptic neuron.

Synaptic roles of cyclin-dependent kinase 5 & its implications in epilepsy

There is an urgent need to understand the molecular mechanisms underlying epilepsy to find novel prognostic/diagnostic biomarkers to prevent epilepsy patients at risk. Cyclin-dependent kinase 5 (CDK5) is involved in multiple neuronal functions and plays a crucial role in maintaining homeostatic synaptic plasticity by regulating intracellular signalling cascades at synapses. CDK5 deregulation is shown to be associated with various neurodegenerative diseases such as Alzheimer's disease. The association between chronic loss of CDK5 and seizures has been reported in animal models of epilepsy. Genetic expression of CDK5 at transcriptome level has been shown to be abnormal in intractable epilepsy. In this review various possible mechanisms by which deregulated CDK5 may alter synaptic transmission and possibly lead to epileptogenesis have been discussed. Further, CDK5 has been proposed as a potential biomarker as well as a pharmacological target for developing treatments for epilepsy.

New Cav2 calcium channel gating modifiers with agonist activity and therapeutic potential to treat neuromuscular disease

Voltage-gated calcium channels (VGCCs) are critical regulators of many cellular functions, including the activity-dependent release of chemical neurotransmitter from nerve terminals. At nerve terminals, the Cav2 family of VGCCs are closely positioned with neurotransmitter-containing synaptic vesicles. The relationship between calcium ions and transmitter release is such that even subtle changes in calcium flux through VGCCs have a strong influence on the magnitude of transmitter released. Therefore, modulators of the calcium influx at nerve terminals have the potential to strongly affect transmitter release at synapses. We have previously developed novel Cav2-selective VGCC gating modifiers (notably GV-58) that slow the deactivation of VGCC current, increasing total calcium ion flux. Here, we describe ten new gating modifiers based on the GV-58 structure that extend our understanding of the structure-activity relationship for this class of molecules and extend the range of modulation of channel activities. In particular, we show that one of these new compounds (MF-06) was more efficacious than GV-58, another (KK-75) acts more quickly on VGCCs than GV-58, and a third (KK-20) has a mix of increased speed and efficacy. A subset of these new VGCC agonist gating modifiers can increase transmitter release during action potentials at neuromuscular synapses, and as such, show potential as therapeutics for diseases with a presynaptic deficit that results in neuromuscular weakness. Further, several of these new compounds can be useful tool compounds for the study of VGCC gating and function.

Lambert-Eaton myasthenic syndrome: mouse passive-transfer model illuminates disease pathology and facilitates testing therapeutic leads

Lambert-Eaton myasthenic syndrome (LEMS) is an autoimmune disorder caused by antibodies directed against the voltage-gated calcium channels that provide the calcium ion flux that triggers acetylcholine release at the neuromuscular junction. To study the pathophysiology of LEMS and test candidate therapeutic strategies, a passive-transfer animal model has been developed in mice, which can be created by daily intraperitoneal injections of LEMS patient serum or IgG into mice for 2-4 weeks. Results from studies of the mouse neuromuscular junction have revealed that each synapse has hundreds of transmitter release sites but that the probability for release at each one is likely to be low. LEMS further reduces this low probability such that transmission is no longer effective at triggering a muscle contraction. The LEMS-mediated attack reduces the number of presynaptic calcium channels, disorganizes transmitter release sites, and results in the homeostatic upregulation of other calcium channel types. Symptomatic treatment is focused on increasing the probability of release from dysfunctional release sites. Current treatment uses the potassium channel blocker 3,4-diaminopyridine (DAP) to broaden the presynaptic action potential, providing more time for calcium channels to open. Current research is focused on testing new calcium channel gating modifiers that work synergistically with DAP.

In vitro and ex vivo screening of candidate therapeutics to restore neurotransmission in nerve terminals intoxicated by botulinum neurotoxin serotype A1

Botulinum neurotoxins (BoNTs) are exceedingly potent neurological poisons that block cholinergic release in the peripheral nervous system and cause death by asphyxiation. While post-exposure prophylaxis can effectively eliminate toxin in the bloodstream, there are no clinically effective treatments to prevent or reverse disease once BoNT has entered the neuron. To address the need for post-symptomatic countermeasures, we designed and developed an in vitro assay based on whole-cell, patch-clamp electrophysiological monitoring of miniature excitatory post-synaptic currents in synaptically active murine embryonic stem cell-derived neurons. This synaptic function-based assay was used to assess the efficacy of rationally selected drugs to restore neurotransmission in neurons comprehensively intoxicated by BoNT/A. Based on clinical reports suggesting that elevated Ca²⁺ signaling promotes symptomatic relief from botulism, we identified seven candidate drugs that modulate presynaptic Ca²⁺ signaling and assessed their ability to reverse BoNT/A-induced synaptic blockade. The most effective drugs from the screen were found to phasically agonize voltage-gated calcium channel (VGCC) activity. Lead candidates were then applied to ex vivo studies in BoNT/A-paralyzing mouse phrenic nerve-hemidiaphragm (PND) preparations. Treatment of PNDs with VGCC agonists after paralytic onset transiently potentiated nerve-elicited muscle contraction and delayed progression to neuromuscular failure. Collectively, this study suggests that Ca²⁺-modulating drugs represent a novel symptomatic treatment for neuromuscular paralysis following BoNT/A poisoning.

Analysis of the Inhibitory Elements in the p5 Peptide Fragment of the CDK5 Activator, p35, CDKR1 Protein

Besides the hallmark pathology of amyloid plaques and neurofibrillary tangles, it is well documented that cyclin-dependent kinase 5 (CDK5), a critical neuronal protein kinase in nervous system development, function, and survival, when deregulated and hyperactivated induces Alzheimer's disease (AD) and amyotrophic lateral sclerosis and Parkinson's disease-like phenotypes in mice. In a recent study, we demonstrated that p5, a small, truncated fragment of 24 amino acid residues derived from the CDK5 activator protein 35 (NCK5A, p35), selectively inhibited deregulated CDK5 hyperactivity and ameliorated AD phenotypes in model mice. In this study, we identified the most inhibitory elements in the p5 peptide fragment. Each amino acid residue in p5 was systematically replaced with its homologous residues that may still be able to functionally substitute. The effects of these p5 peptide analogs were studied on the phosphotransferase activities of CDK5/p35, CDK5/p25, ERK1, and GSK3β. The mimetic p5 peptide (A/V substitution at the C-terminus of the peptide) in the sequence, KNAFYERALSIINLMTSKMVQINV (p5-MT) was the most effective inhibitor of CDK5 kinase activity of 79 tested mimetic peptides including the original p5 peptide, KEAFWDRCLSVINLMSSKMLQINA (p5-WT). Replacement of the residues in C-terminus end of the peptide affected CDK5 phosphotransferase activity most significantly. These peptides were strong inhibitors of CDK5, but not the related proline-directed kinases, ERK1 and GSK3β.

Cdk5 is a New Rapid Synaptic Homeostasis Regulator Capable of Initiating the Early Alzheimer-Like Pathology

Cyclin-dependent kinase 5 (Cdk5) is a serine/threonine kinase implicated in synaptic plasticity, behavior, and cognition, yet its synaptic function remains poorly understood. Here, we report that physiological Cdk5 signaling in rat hippocampal CA1 neurons regulates homeostatic synaptic transmission using an unexpectedly rapid mechanism that is different from all known slow homeostatic regulators, such as beta amyloid (Aβ) and activity-regulated cytoskeleton-associated protein (Arc, aka Arg3.1). Interestingly, overproduction of the potent Cdk5 activator p25 reduces synapse density, and dynamically regulates synaptic size by suppressing or enhancing Aβ/Arc production. Moreover, chronic overproduction of p25, seen in Alzheimer's patients, induces initially concurrent reduction in synapse density and increase in synaptic size characteristic of the early Alzheimer-like pathology, and later persistent synapse elimination in intact brains. These results identify Cdk5 as the regulator of a novel rapid form of homeostasis at central synapses and p25 as the first molecule capable of initiating the early Alzheimer's synaptic pathology.

Regulation of density of functional presynaptic terminals by local energy supply

Background

The density of functional synapses is an important parameter in determining the efficacy of synaptic transmission. However, how functional presynaptic terminal density is regulated under natural physiological conditions is still poorly understood.

Results

We studied the factors controlling the density of presynaptic functional terminals at single dendritic branches of hippocampal neurons and found that elevation of intracellular Mg²⁺ concentration was effective in increasing the density of functional terminals. Interestingly, the upregulation was not due to synaptogenesis, but to the conversion of a considerable proportion of presynaptic terminals from nonfunctional to functional. Mechanistic studies revealed that the nonfunctional terminals had inadequate Ca²⁺-sensitivity-related proteins, resulting in very low Ca²⁺ sensitivity within their vesicle release machinery. We identified energy-dependent axonal transport as a primary factor controlling the amount of Ca²⁺-sensitivity-related proteins in terminals. The elevation of intracellular Mg²⁺ enhanced local energy supply and promoted the increase of Ca²⁺-sensitivity-related proteins in terminals, leading to increased functional terminal density.

Conclusions

Our study suggests that local energy supply plays a critical role in controlling the density of functional presynaptic terminals, demonstrating the link between energy supply and efficacy of synaptic transmission.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-015-0132-z) contains supplementary material, which is available to authorized users.

Synaptic Pathophysiology and Treatment of Lambert-Eaton Myasthenic Syndrome

Lambert-Eaton myasthenic syndrome (LEMS) is an autoimmune disease that disrupts the normally reliable neurotransmission at the neuromuscular junction (NMJ). This disruption is thought to result from an autoantibody-mediated removal of a subset of the P/Q-type Ca(2+) channels involved with neurotransmitter release. With less neurotransmitter release at the NMJ, LEMS patients experience debilitating muscle weakness. The underlying cause of LEMS in slightly more than half of all patients is small cell lung cancer, and cancer therapy is the priority for these patients. In the remaining cases, the cause of LEMS is unknown, and these patients often rely on symptomatic treatment options, as there is no cure. However, current symptomatic treatment options, such as 3,4-diaminopyridine (3,4-DAP), can have significant dose-limiting side effects; thus, additional treatment approaches would benefit LEMS patients. Recent studies introduced a novel Ca(2+) channel agonist (GV-58) as a potential therapeutic alternative for LEMS. Additionally, this work has shown that GV-58 and 3,4-DAP interact in a supra-additive manner to completely restore the magnitude of neurotransmitter release at the NMJs of a LEMS mouse model. In this review, we discuss synaptic mechanisms for reliability at the NMJ and how these mechanisms are disrupted in LEMS. We then discuss the current treatment options for LEMS patients, while also considering recent work demonstrating the therapeutic potential of GV-58 alone and in combination with 3,4-DAP.

Complete reversal of Lambert-Eaton myasthenic syndrome synaptic impairment by the combined use of a K+ channel blocker and a Ca2+ channel agonist

Lambert-Eaton myasthenic syndrome (LEMS) is an autoimmune disorder in which a significant fraction of the presynaptic P/Q-type Ca(2+) channels critical to the triggering of neurotransmitter release at the neuromuscular junction (NMJ) are thought to be removed. There is no cure for LEMS, and the current most commonly used symptomatic treatment option is a potassium channel blocker [3,4-diaminopyridine (3,4-DAP)] that does not completely reverse symptoms and can have dose-limiting side-effects. We previously reported the development of a novel Ca(2+) channel agonist, GV-58, as a possible alternative treatment strategy for LEMS. In this study, we tested the hypothesis that the combination of GV-58 and 3,4-DAP will elicit a supra-additive increase in neurotransmitter release at LEMS model NMJs. First, we tested GV-58 in a cell survival assay to assess potential effects on cyclin-dependent kinases (Cdks) and showed that GV-58 did not affect cell survival at the relevant concentrations for Ca(2+) channel effects. Then, we examined the voltage dependence of GV-58 effects on Ca(2+) channels using patch clamp techniques; this showed the effects of GV-58 to be dependent upon Ca(2+) channel opening. Based on this mechanism, we predicted an interaction between 3,4-DAP and GV-58. We tested this hypothesis using a mouse passive transfer model of LEMS. Using intracellular electrophysiological ex vivo recordings, we demonstrated that a combined application of 3,4-DAP plus GV-58 had a supra-additive effect that completely reversed the deficit in neurotransmitter release magnitude at LEMS model NMJs. This reversal contrasts with the less significant improvement observed with either compound alone. Our data indicate that a combination of 3,4-DAP and GV-58 represents a promising treatment option for LEMS and potentially for other disorders of the NMJ.

Cyclin-dependent kinase 5 in the ventral tegmental area regulates depression-related behaviors

Dopamine neurons in the ventral tegmental area (VTA) govern reward and motivation and dysregulated dopaminergic transmission may account for anhedonia and other symptoms of depression. Cyclin-dependent kinase 5 (Cdk5) is a proline-directed serine/threonine kinase that regulates a broad range of brain functions through phosphorylation of a myriad of substrates, including tyrosine hydroxylase (TH), the rate-limiting enzyme for dopamine synthesis. We investigated whether and how Cdk5 activity in VTA dopamine neurons regulated depression-related behaviors in mice. Using the Cre/LoxP system to selectively delete Cdk5 in the VTA or in midbrain dopamine neurons in Cdk5(loxP/loxP) mice, we showed that Cdk5 loss of function in the VTA induced anxiety- and depressive-like behaviors that were associated with decreases in TH phosphorylation at Ser31 and Ser40 in the VTA and dopamine release in its target region, the nucleus accumbens. The decreased phosphorylation of TH at Ser31 was a direct effect of Cdk5 deletion, whereas decreased phosphorylation of TH at Ser40 was likely caused by impaired cAMP/protein kinase A (PKA) signaling, because Cdk5 deletion decreased cAMP and phosphorylated cAMP response element-binding protein (p-CREB) levels in the VTA. Using Designer Receptors Exclusively Activated by Designer Drugs (DREADD) technology, we showed that selectively increasing cAMP levels in VTA dopamine neurons increased phosphorylation of TH at Ser40 and CREB at Ser133 and reversed behavioral deficits induced by Cdk5 deletion. The results suggest that Cdk5 in the VTA regulates cAMP/PKA signaling, dopaminergic neurotransmission, and depression-related behaviors.

The effect of Cyclin-dependent kinase 5 on voltage-dependent calcium channels in PC12 cells varies according to channel type and cell differentiation state

Cyclin-dependent kinase 5 (Cdk5) is a Ser/Thr kinase that plays an important role in the release of neurotransmitter from pre-synaptic terminals triggered by Ca(2+) influx into the pre-synaptic cytoplasm through voltage-dependent Ca(2+) channels (VDCCs). It is reported that Cdk5 regulates L-, P/Q-, or N-type VDCC, but there is conflicting data as to the effect of Cdk5 on VDCC activity. To clarify the mechanisms involved, we examined the role of Cdk5 in regulating the Ca(2+) -channel property of VDCCs, using PC12 cells expressing endogenous, functional L-, P/Q-, and N-type VDCCs. The Ca(2+) influx, induced by membrane depolarization with high K(+) , was monitored with a fluorescent Ca(2+) indicator protein in both undifferentiated and nerve growth factor (NGF)-differentiated PC12 cells. Overall, Ca(2+) influx was increased by expression of Cdk5-p35 in undifferentiated PC12 cells but suppressed in differentiated PC12 cells. Moreover, we found that different VDCCs are distinctly regulated by Cdk5-p35 depending on the differentiation states of PC12 cells. These results indicate that Cdk5-p35 regulates L-, P/Q-, or N-type VDCCs in a cellular context-dependent manner. Calcium (Ca(2+) ) influx through voltage-dependent Ca(2+) channels (VDCCs) triggers neurotransmitter release from pre-synaptic terminal of neurons. The channel activity of VDCCs is regulated by Cdk5-p35, a neuronal Ser/Thr kinase. However, there have been debates about the regulation of VDCCs by Cdk5. Using PC12 cells, we show that Cdk5-p35 regulates VDCCs in a type (L, P/Q, and N) and differentiation-dependent manner. NGF = nerve growth factor.

Cav2.1 channels control multivesicular release by relying on their distance from exocytotic Ca2+ sensors at rat cerebellar granule cells

The concomitant release of multiple numbers of synaptic vesicles [multivesicular release (MVR)] in response to a single presynaptic action potential enhances the flexibility of synaptic transmission. However, the molecular mechanisms underlying MVR at a single CNS synapse remain unclear. Here, we show that the Cav2.1 subtype (P/Q-type) of the voltage-gated calcium channel is specifically responsible for the induction of MVR. In the rat cerebellar cortex, paired-pulse activation of granule cell (GC) ascending fibers leads not only to a facilitation of the peak amplitude (PPFamp) but also to a prolongation of the decay time (PPPdecay) of the EPSCs recorded from molecular layer interneurons. PPFamp is elicited by a transient increase in the number of released vesicles. PPPdecay is highly dependent on MVR and is caused by dual mechanisms: (1) a delayed release and (2) an extrasynaptic spillover of the GC transmitter glutamate and subsequent pooling of the glutamate among active synapses. PPPdecay was specifically suppressed by the Cav2.1 channel blocker ω-agatoxin IVA, while PPFamp responded to Cav2.2/Cav2.3 (N-type/R-type) channel blockers. The membrane-permeable slow Ca(2+) chelator EGTA-AM profoundly reduced the decay time constant (τdecay) of the second EPSC; however, it only had a negligible impact on that of the first, thereby eliminating PPPdecay. These results suggest that the distance between presynaptic Cav2.1 channels and exocytotic Ca(2+) sensors is a key determinant of MVR. By transducing presynaptic action potential firings into unique Ca(2+) signals and vesicle release profiles, Cav2.1 channels contribute to the encoding and processing of neural information.

Pharmacological correction of gating defects in the voltage-gated Ca(v)2.1 Ca²⁺ channel due to a familial hemiplegic migraine mutation

Voltage-gated ion channels exhibit complex properties, which can be targeted in pharmacological therapies for disease. Here, we report that the pro-oxidant, tert-butyl dihydroquinone (BHQ), modulates Ca(v)2.1 Ca²⁺ channels in ways that oppose defects in channel gating and synaptic transmission resulting from a familial hemiplegic migraine mutation (S218L). BHQ slows deactivation, inhibits voltage-dependent activation, and potentiates Ca²⁺-dependent facilitation of Ca(v)2.1 channels in transfected HEK293T cells. These actions of BHQ help offset the gain of function and reduced Ca²⁺-dependent facilitation of Ca(v)2.1 channels with the S218L mutation. Transgenic expression of the mutant channels at the Drosophila neuromuscular junction causes abnormally elevated evoked postsynaptic potentials and impaired synaptic plasticity, which are largely restored to the wild-type phenotypes by BHQ. Our results reveal a mechanism by which a Ca(v)2.1 gating modifier can ameliorate defects associated with a disease-causing mutation in Ca(v)2.1.

Effect of roscovitine on intracellular calcium dynamics: differential enantioselective responses

Cyclin-dependent kinases (CDKs) inhibitors have emerged as interesting therapeutic candidates. Of these, (S)-roscovitine has been proposed as potential neuroprotective molecule for stroke while (R)-roscovitine is currently entering phase II clinical trials against cancers and phase I clinical tests against glomerulonephritis. In addition, (R)-roscovitine has been suggested as potential antihypertensive and anti-inflammatory drug. Dysfunction of intracellular calcium balance is a common denominator of these diseases, and the two roscovitine enantiomers (S and R) are known to modulate calcium voltage channel activity differentially. Here, we provide a detailed description of short- and long-term responses of roscovitine on intracellular calcium handling in renal epithelial cells. Short-term exposure to (S)-roscovitine induced a cytosolic calcium peak, which was abolished after stores depletion with cyclopiazonic acid (CPA). Instead, (R)-roscovitine caused a calcium peak followed by a small calcium plateau. Cytosolic calcium response was prevented after stores depletion. Bafilomycin, a selective vacuolar H(+)-ATPase inhibitor, abolished the small calcium plateau. Long-term exposure to (R)-roscovitine significantly reduced the basal calcium level compared to control and (S)-roscovitine treated cells. However, both enantiomers increased calcium accumulation in the endoplasmic reticulum (ER). Consistently, cells treated with (R)-roscovitine showed a significant increase in SERCA activity, whereas (S)-roscovitine incubation resulted in a reduced PMCA expression. We also found a tonic decreased ability to release calcium from the ER, likely via IP3 signaling, under treatment with (S)- or (R)-roscovitine. Together our data revealed that (S)-roscovitine and (R)-roscovitine exert distinct enantiospecific effects on intracellular calcium signaling in renal epithelial cells. This distinct pharmacological profile can be relevant for roscovitine clinical use.

Evaluation of a novel calcium channel agonist for therapeutic potential in Lambert-Eaton myasthenic syndrome

We developed a novel calcium (Ca(2+)) channel agonist that is selective for N- and P/Q-type Ca(2+) channels, which are the Ca(2+) channels that regulate transmitter release at most synapses. We have shown that this new molecule (GV-58) slows the deactivation of channels, resulting in a large increase in presynaptic Ca(2+) entry during activity. GV-58 was developed as a modification of (R)-roscovitine, which was previously shown to be a Ca(2+) channel agonist, in addition to its known cyclin-dependent kinase activity. In comparison with the parent molecule, (R)-roscovitine, GV-58 has a ∼20-fold less potent cyclin-dependent kinase antagonist effect, a ∼3- to 4-fold more potent Ca(2+) channel agonist effect, and ∼4-fold higher efficacy as a Ca(2+) channel agonist. We have further evaluated GV-58 in a passive transfer mouse model of Lambert-Eaton myasthenic syndrome and have shown that weakened Lambert-Eaton myasthenic syndrome-model neuromuscular synapses are significantly strengthened following exposure to GV-58. This new Ca(2+) channel agonist has potential as a lead compound in the development of new therapeutic approaches to a variety of disorders that result in neuromuscular weakness.

Zebrafish calls for reinterpretation for the roles of P/Q calcium channels in neuromuscular transmission

A long-held tenet of neuromuscular transmission is that calcium-dependent neurotransmitter release is mediated by N-type calcium channels in frog but P/Q-type channels in mammals. The N-type assignment in frog is based principally on pharmacological sensitivity to ω-conotoxin GVIA. Our studies show that zebrafish neuromuscular transmission is also sensitive to ω-conotoxin GVIA. However, positional cloning of a mutant line with compromised neuromuscular function identified a mutation in a P/Q- rather than N-type channel. Cloning and heterologous expression of this P/Q-type channel confirmed a block by ω-conotoxin GVIA raising the likelihood that all vertebrates, including frog, use the P/Q-type calcium channel for neuromuscular transmission. In addition, our P/Q defective mutant line offered a means of testing the ability of roscovitine, known to potentiate frog neuromuscular transmission, to mediate behavioral and functional rescue. Acute treatment led to rapid improvement of both, pointing to potential therapeutic benefit for myasthenic disorders involving calcium channel dysfunction.

Balance of calcineurin Aα and CDK5 activities sets release probability at nerve terminals

The control of neurotransmitter release at nerve terminals is of profound importance for neurological function and provides a powerful control system in neural networks. We show that the balance of enzymatic activities of the α isoform of the phosphatase calcineurin (CNAα) and the kinase cyclin-dependent kinase 5 (CDK5) has a dramatic influence over single action potential (AP)-driven exocytosis at nerve terminals. Acute or chronic loss of these enzymatic activities results in a sevenfold impact on single AP-driven exocytosis. We demonstrate that this control is mediated almost entirely through Cav2.2 (N-type) voltage-gated calcium channels as blocking these channels with a peptide toxin eliminates modulation by these enzymes. We found that a fraction of nerve terminals are kept in a presynaptically silent state with no measurable Ca(2+) influx driven by single AP stimuli attributable to the balance of CNAα and CDK5 activities because blockade of either CNAα or CDK5 activity changes the proportion of presynaptically silent nerve terminals. Thus, CNAα and CDK5 enzymatic activities are key determinants of release probability.

Calcium channel agonists protect against neuromuscular dysfunction in a genetic model of TDP-43 mutation in ALS

TAR DNA binding protein (TDP-43, encoded by the TARDBP gene) has recently been shown to be associated with amyotrophic lateral sclerosis (ALS), but the early pathophysiological deficits causing impairment in motor function are unknown. Here we expressed the wild-type human gene (wtTARDBP) or the ALS mutation G348C (mutTARDBP) in zebrafish larvae and characterized their motor (swimming) activity and the structure and function of their neuromuscular junctions (NMJs). Of these groups only mutTARDBP larvae showed impaired swimming and increased motoneuron vulnerability with reduced synaptic fidelity, reduced quantal transmission, and more orphaned presynaptic and postsynaptic structures at the NMJ. Remarkably, all behavioral and cellular features were stabilized by chronic treatment with either of the L-type calcium channel agonists FPL 64176 or Bay K 8644. These results indicate that expression of mutTARDBP results in defective NMJs and that calcium channel agonists could be novel therapeutics for ALS.

P/Q-type calcium channel modulators

P/Q-type calcium channels are high-voltage-gated calcium channels contributing to vesicle release at synaptic terminals. A number of neurological diseases have been attributed to malfunctioning of P/Q channels, including ataxia, migraine and Alzheimer's disease. To date, only two specific P/Q-type blockers are known: both are peptides deriving from the spider venom of Agelenopsis aperta, ω-agatoxins. Other peptidic calcium channel blockers with activity at P/Q channels are available, albeit with less selectivity. A number of low molecular weight compounds modulate P/Q-type currents with different characteristics, and some exhibit a peculiar bidirectional pattern of modulation. Interestingly, there are a number of therapeutics in clinical use, which also show P/Q channel activity. Because selectivity as well as the exact mode of action is different between all P/Q-type channel modulators, the interpretation of clinical and experimental data is complicated and needs a comprehensive understanding of their target profile. The situation is further complicated by the fact that information on potency varies vastly in the literature, which may be the result of different experimental systems, conditions or the splice variants of the P/Q channel. This review attempts to provide a comprehensive overview of the compounds available that affect the P/Q-type channel and should help with the interpretation of results of in vitro experiments and animal models. It also aims to explain some clinical observations by implementing current knowledge about P/Q channel modulation of therapeutically used non-selective drugs. Chances and challenges of the development of P/Q channel-selective molecules are discussed.

Synthesis and biological evaluation of a selective N- and p/q-type calcium channel agonist

The acute effect of the potent cyclin-dependent kinase (cdk) inhibitor (R)-roscovitine on Ca(2+) channels inspired the development of structural analogues as a potential treatment for motor nerve terminal dysfunction. On the basis of a versatile chlorinated purine scaffold, we have synthesized ca. 20 derivatives and characterized their N-type Ca(2+) channel agonist action. Agents that showed strong agonist effects were also characterized in a kinase panel for their off-target effects. Among several novel compounds with diminished cdk activity, we identified a new lead structure with a 4-fold improved N-type Ca(2+) channel agonist effect and a 22-fold decreased cdk2 activity as compared to (R)-roscovitine. This compound was selective for agonist activity on N- and P/Q-type over L-type calcium channels.

Stabilizers of neuronal and mitochondrial calcium cycling as a strategy for developing a medicine for Alzheimer's disease

For the last two decades, most efforts on new drug development to treat Alzheimer's disease have been focused to inhibit the synthesis of amyloid beta (Aβ), to prevent Aβ deposition, or to clear up Aβ plaques from the brain of Alzheimer's disease (AD) patients. Other pathogenic mechanisms such as the hyperphosphorylation of the microtubular tau protein (that forms neurofibrillary tangles) have also been addressed as, for instance, with inhibitors of the enzyme glycogen synthase-3 kinase beta (GSK3β). However, in spite of their proven efficacy in animal models of AD, all these compounds have so far failed in clinical trials done in AD patients. It seems therefore desirable to explore new concepts and strategies in the field of drug development for AD. We analyze here our hypothesis that a trifunctional chemical entity acting on the L subtype of voltage-dependent Ca(2+) channels (VDCCs) and on the mitochondrial Na(+)/Ca(2+) exchanger (MNCX), and having additional antioxidant properties, may efficiently delay or stop the death of vulnerable neurons in the brain of AD patients. In recent years, evidence has accumulated indicating that enhanced neuronal Ca(2+) cycling (NCC) and futile mitochondrial Ca(2+) cycling (MCC) are central stage in activating calpain and calcineurin, as well as the intrinsic mitochondrial pathway for apoptosis, leading to death of vulnerable neurons. An additional contributing factor to neuronal death is the excess free radical production linked to distortion of Ca(2+) homeostasis. We propose that an hybrid compound containing a dihydropyridine moiety (to block L channels and mitigate Ca(2+) entry) and a benzothiazepine moiety (to block the MNCX and slow down the rate of Ca(2+) efflux from the mitochondrial matrix into the cytosol), as well as a polyphenol moiety (to sequester excess free radicals) could break down the pathological enhanced NCC and MCC, thus delaying the initiation of apoptosis and the death of vulnerable neurons. In so doing, such a trifunctional compound could eventually become a neuroprotective medicine capable of delaying disease progression in AD patients.

Regulation of N-type voltage-gated calcium channels and presynaptic function by cyclin-dependent kinase 5

N-type voltage-gated calcium channels localize to presynaptic nerve terminals and mediate key events including synaptogenesis and neurotransmission. While several kinases have been implicated in the modulation of calcium channels, their impact on presynaptic functions remains unclear. Here we report that the N-type calcium channel is a substrate for cyclin-dependent kinase 5 (Cdk5). The pore-forming α(1) subunit of the N-type calcium channel is phosphorylated in the C-terminal domain, and phosphorylation results in enhanced calcium influx due to increased channel open probability. Phosphorylation of the N-type calcium channel by Cdk5 facilitates neurotransmitter release and alters presynaptic plasticity by increasing the number of docked vesicles at the synaptic cleft. These effects are mediated by an altered interaction between N-type calcium channels and RIM1, which tethers presynaptic calcium channels to the active zone. Collectively, our results highlight a molecular mechanism by which N-type calcium channels are regulated by Cdk5 to affect presynaptic function.

Reversal of BoNT/A-mediated inhibition of muscle paralysis by 3,4-diaminopyridine and roscovitine in mouse phrenic nerve-hemidiaphragm preparations

Botulinum neurotoxins (BoNTs) comprise a family of neurotoxic proteins synthesized by anaerobic bacteria of the genus Clostridium. Each neurotoxin consists of two polypeptide chains: a 100kDa heavy chain, responsible for binding and internalization into the nerve terminal of cholinergic motoneurons and a 50kDa light chain that mediates cleavage of specific synaptic proteins in the host nerve terminal. Exposure to BoNT leads to cessation of voltage- and Ca(2+)-dependent acetylcholine (ACh) release, resulting in flaccid paralysis which may be protracted and potentially fatal. There are no approved therapies for BoNT intoxication once symptoms appear, and specific inhibitors of the light chain developed to date have not been able to reverse the consequences of BoNT intoxication. An alternative approach for treatment of botulism is to focus on compounds that act by enhancing ACh release. To this end, we examined the action of the K(+) channel blocker 3,4-diaminopyridine (3,4-DAP) in isolated mouse hemidiaphragm muscles intoxicated with 5pM BoNT/A. 3,4-DAP restored tension within 1-3min of application, and was effective even in totally paralyzed muscle. The Ca(2+) channel activator (R)-roscovitine (Ros) potentiated the action of 3,4-DAP, allowing for use of lower concentrations of the K(+) channel blocker. In the absence of 3,4-DAP, Ros was unable to augment tension in BoNT/A-intoxicated muscle. This is the first report demonstrating the efficacy of the combination of 3,4-DAP and Ros for the potential treatment of BoNT/A-mediated muscle paralysis.

Cyclin-dependent kinases in brain development and disease

Cyclin-dependent kinase 5 (Cdk5) is a multifaceted serine/threonine kinase protein with important roles in the nervous system. Two related proteins, p35 and p39, activate Cdk5 upon direct binding. Over the past decade, Cdk5 activity has been demonstrated to regulate many events during brain development, including neuronal migration as well as axon and dendrite development. Recent evidence also suggests a pivotal role for Cdk5 in synaptic plasticity, behavior, and cognition. Dysfunction of Cdk5 has been implicated in a number of neurological disorders and neurodegenerative diseases including Alzheimer's disease, amyotrophic lateral sclerosis, Niemann-Pick type C disease, and ischemia. Hyperactivation of Cdk5 due to the conversion of p35 to p25 by the calcium-dependent protease calpain during neurotoxicity also contributes to the pathological state. This review surveys recent literature surrounding Cdk5 in synaptic plasticity and homeostasis, with particular emphasis on Cdk5 kinase activity under neurodegenerative conditions.

Domain III regulates N-type (CaV2.2) calcium channel closing kinetics

Ca(V)2.2 (N-type) and Ca(V)1.2 (L-type) calcium channels gate differently in response to membrane depolarization, which is critical to the unique physiological functions mediated by these channels. We wondered if the source for these differences could be identified. As a first step, we examined the effect of domain exchange between N-type and L-type channels on activation-deactivation kinetics, which were significantly different between these channels. Kinetic analysis of chimeric channels revealed N-channel-like deactivation for all chimeric channels containing N-channel domain III, while activation appeared to be a more distributed function across domains. This led us to hypothesize that domain III was an important regulator of N-channel closing. This idea was further examined with R-roscovitine, which is a trisubstituted purine that slows N-channel deactivation by exclusively binding to activated N-channels. L-channels lack this response to roscovitine, which allowed us to use N-L chimeras to test the role of domain III in roscovitine modulation of N-channel deactivation. In support of our hypothesis, all chimeric channels containing the N-channel domain III responded to roscovitine with slowed deactivation, while those chimeric channels with L-channel domain III did not. Thus a combination of kinetic and pharmacological evidence supports the hypothesis that domain III is an important regulator of N-channel closing. Our results support specialization of gating functions among calcium channel domains.

Heterogeneous reallocation of presynaptic efficacy in recurrent excitatory circuits adapting to inactivity

Recurrent excitatory circuits face extreme challenges in balancing efficacy and stability. We recorded from CA3 pyramidal neuron pairs in rat hippocampal slice cultures to characterize synaptic and circuit-level changes in recurrent synapses resulting from long-term inactivity. Chronic tetrodotoxin treatment greatly reduced the percentage of connected CA3-CA3 neurons, but enhanced the strength of the remaining connections; presynaptic release probability sharply increased, whereas quantal size was unaltered. Connectivity was decreased in activity-deprived circuits by functional silencing of synapses, whereas three-dimensional anatomical analysis revealed no change in spine or bouton density or aggregate dendrite length. The silencing arose from enhanced Cdk5 activity and could be reverted by acute Cdk5 inhibition with roscovitine. Our results suggest that recurrent circuits adapt to chronic inactivity by reallocating presynaptic weights heterogeneously, strengthening certain connections while silencing others. This restricts synaptic output and input, preserving signaling efficacy among a subset of neuronal ensembles while protecting network stability.

Elevation of p-NR2A(S1232) by Cdk5/p35 contributes to retinal ganglion cell apoptosis in a rat experimental glaucoma model

Glaucoma, mainly caused by high intraocular pressure (IOP), is characterized by apoptotic death of retinal ganglion cells (RGCs). We investigated the possible involvement of cyclin-dependent kinase 5 (Cdk5) and its activator p35, which have been implicated in a variety of neurological disorders, in RGC apoptosis in a rat experimental glaucoma model reproduced by blocking episcleral veins. Cholera toxin B subunit (CTB) retrogradely labeled RGCs displayed a dramatic reduction in number both in the central and peripheral retina on day 14 (D14) (P<0.05 vs. control), D21 (P<0.01 vs. control) and D28 (P<0.001 vs. control) after operation. Terminal dUTP nick end labeling (TUNEL)-positive cells were detected on D14 both in the central and peripheral regions, and numerous TUNEL-positive cells were found on D21 and D28 in both the regions (P all<0.001 vs. control). As compared with the control eyes, the expression level of Cdk5 was significantly increased on D21 (P<0.001), whereas that of p35 displayed a marked increase on D14 (P<0.01) and D21 (P<0.001). Meanwhile, both NR2A and p-NR2A(S1232) increased from D14 onwards (P<0.01 to 0.001). Co-immunoprecipitation indicated a direct interaction between Cdk5 and p-NR2A(S1232). Intraperitoneal injection of the Cdk5 inhibitor roscovitine remarkably inhibited RGC apoptosis (P<0.001 vs. vehicle group) and increased the number of CTB-labeled RGCs (P<0.05 to 0.01 vs. vehicle group) in whole flat-mounted retinas, which was accompanied by a significant decrease in expression levels of p35 and p-NR2A(S1232) (P all<0.01 vs. vehicle group). Our results suggest that elevation of p-NR2A(S1232) by Cdk5/p35 contributes to RGC apoptotic death in experimental glaucoma rats, which could be effectively ameliorated by inhibiting Cdk5/p35.

Calcium entry through slow-inactivating L-type calcium channels preferentially triggers endocytosis rather than exocytosis in bovine chromaffin cells

Calcium (Ca(2+))-dependent endocytosis has been linked to preferential Ca(2+) entry through the L-type (α(1D), Ca(V)1.3) of voltage-dependent Ca(2+) channels (VDCCs). Considering that the Ca(2+)-dependent exocytotic release of neurotransmitters is mostly triggered by Ca(2+) entry through N-(α(1B), Ca(V)2.2) or PQ-VDCCs (α(1A), Ca(V)2.1) and that exocytosis and endocytosis are coupled, the supposition that the different channel subtypes are specialized to control different cell functions is attractive. Here we have explored this hypothesis in primary cultures of bovine adrenal chromaffin cells where PQ channels account for 50% of Ca(2+) current (I(Ca)), 30% for N channels, and 20% for L channels. We used patch-clamp and fluorescence techniques to measure the exo-endocytotic responses triggered by long depolarizing stimuli, in 1, 2, or 10 mM concentrations of extracellular Ca(2+) ([Ca(2+)](e)). Exo-endocytotic responses were little affected by ω-conotoxin GVIA (N channel blocker), whereas ω-agatoxin IVA (PQ channel blocker) caused 80% blockade of exocytosis as well as endocytosis. In contrast, nifedipine (L channel blocker) only caused 20% inhibition of exocytosis but as much as 90% inhibition of endocytosis. Conversely, FPL67146 (an activator of L VDCCs) notably augmented endocytosis. Photoreleased caged Ca(2+) caused substantially smaller endocytotic responses compared with those produced by K(+) depolarization. Using fluorescence antibodies, no colocalization between L, N, or PQ channels with clathrin was found; a 20-30% colocalization was found between dynamin and all three channel antibodies. This is incompatible with the view that L channels are coupled to the endocytotic machine. Data rather support a mechanism implying the different inactivation rates of L (slow-inactivating) and N/PQ channels (fast-inactivating). Thus a slow but more sustained Ca(2+) entry through L channels could be a requirement to trigger endocytosis efficiently, at least in bovine chromaffin cells.

CDK5 serves as a major control point in neurotransmitter release

CDK5 is an important kinase in nervous system function, controlling neural development and postsynaptic signal integration. Here we show that CDK5 plays a major role in controlling neurotransmitter release. Inhibition of CDK5 activity, by either acute or genetic means, leads to profound potentiation of presynaptic function, including unmasking of previously "silent" synapses. Removal of CDK5 activity additionally unlocks access to the resting synaptic vesicle pool, which normally remains recalcitrant to exocytosis and recycling even following prolonged action potential stimuli. Presynaptic CDK5 levels are additionally severely depleted by chronic neuronal silencing, a treatment that is functionally similar to CDK5 knockdown with regard to presynaptic potentiation. Thus CDK5 appears to be an integral element in presynaptic homeostatic scaling, and the resting vesicle pool appears to provide a potent functional presynaptic homeostatic control parameter. These studies thus pinpoint CDK5 as a major control point for modulation of neurotransmitter release in mammalian neurons.

(R)-roscovitine prolongs the mean open time of unitary N-type calcium channel currents

(R)-roscovitine (Ros) is a cyclin-dependent kinase inhibitor that also has been shown to have direct agonist and antagonist actions on Ca(v)2.1 (P/Q-type) and Ca(v) 2.2 (N-type) families of voltage-gated calcium channels. These kinase-independent effects represent a novel opportunity to advance our understanding of calcium channel function and calcium-triggered neurotransmitter release. Furthermore, such actions on calcium channels may direct the development of Ros derivatives as new therapeutic agents. We used patch clamp recordings to characterize mechanisms that underlie the agonist effects of Ros on unitary N-type calcium channel gating. We found that N-type channels normally gate with either a short or long mean open time, that Ros significantly prolonged the mean open time of the long gating component and increased the probability of observing channels that gated with a long open time, but had no effect on single channel conductance. Using Monte Carlo simulations of a single channel kinetic model and Ros interactions, we were able to reproduce our experimental results and investigate the model's microscopic dynamics. In particular, our simulations predicted that the longer open times generated by Ros were due to the appearance of a long open state combined with an increased amount of time spent in transitions between open states. Our results suggest a mechanism for agonist effects of Ros at the level of single channels, and provide a mechanistic explanation for previously reported agonist effects on whole cell calcium currents.

Open-state occupancy prevents gating charge relaxation of N-type (CaV2.2) calcium channels

N-type and L-type channels have significant gating differences, and we wondered whether some of these differences are linked to the relationship between charge movement and channel opening. The time constants for N-channel closing (tau(Deact)) and Off-gating charge movement (tauQ(Off)) were compared over a range of voltages. tauQ(Off) was significantly larger than tau(Deact) at voltages < -10 mV, and the voltage dependence of the tauQ(Off) was less steep than that for tau(Deact), which suggests that gating charge relaxation does not limit channel closing. Roscovitine, a drug that slows N-channel closing by holding the channel in a high open-probability state, was found to slow both tauQ(Off) and tau(Deact), and thus the time courses of channel closing and gating charge relaxation were similar. Our gating current results were reproduced with the addition of a voltage-independent, closed-closed transition to our previously published two-open-state N-channel model. This work suggests that, like L-type channels, there is a voltage-independent transition along the N-channel activation/deactivation pathway, but this transition occurs between closed states instead of the closed-open states of the L-channel. Also unlike L-type channels, the gating charge appears to be locked into the activated position by the N-channel open state.