Botulinum neurotoxin E (BoNT/E) reduces CA1 neuron loss and granule cell dispersion, with no effects on chronic seizures, in a mouse model of temporal lobe epilepsy

Action of Botulinum Neurotoxin E Type in Experimental Epilepsies

Botulinum neurotoxins (BoNTs) are zinc endopeptidases produced by the Clostridium genus of anerobic bacteria, largely known for their ability to cleave synaptic proteins, leading to neuromuscular paralysis. In the central nervous system, BoNTs are known to block the release of glutamate neurotransmitter, and for this reason, researchers explored the possible therapeutic action in disorders characterized by neuronal hyperactivity, such as epilepsy. Thus, using multidisciplinary approaches and models of experimental epilepsy, we investigated the pharmacological potential of BoNT/E serotype. In this review, written in memory of Prof. Matteo Caleo, a pioneer in these studies, we go back over the hypotheses and experimental approaches that led us to the conclusion that intrahippocampal administration of BoNT/E (i) displays anticonvulsant effects if prophylactically delivered in a model of acute generalized seizures; (ii) does not have any antiepileptogenic action after the induction of status epilepticus; (iii) reduces frequency of spontaneous seizures in a model of recurrent seizures if delivered during the chronic phase but in a transient manner. Indeed, the control on spontaneous seizures stops when BoNT/E effects are off (few days), thus limiting its pharmacological potential in humans.

Microglial extracellular vesicles induce Alzheimer's disease-related cortico-hippocampal network dysfunction

β-Amyloid is one of the main pathological hallmarks of Alzheimer's disease and plays a major role in synaptic dysfunction. It has been demonstrated that β-amyloid can elicit aberrant excitatory activity in cortical-hippocampal networks, which is associated with behavioural abnormalities. However, the mechanism of the spreading of β-amyloid action within a specific circuitry has not been elucidated yet. We have previously demonstrated that the motion of microglia-derived large extracellular vesicles carrying β-amyloid, at the neuronal surface, is crucial for the initiation and propagation of synaptic dysfunction along the entorhinal-hippocampal circuit. Here, using chronic EEG recordings, we show that a single injection of extracellular vesicles carrying β-amyloid into the mouse entorhinal cortex could trigger alterations in the cortical and hippocampal activity that are reminiscent of those found in Alzheimer's disease mouse models and human patients. The development of EEG abnormalities was associated with progressive memory impairment as assessed by an associative (object-place context recognition) and non-associative (object recognition) task. Importantly, when the motility of extracellular vesicles, carrying β-amyloid, was inhibited, the effect on network stability and memory function was significantly reduced. Our model proposes a new biological mechanism based on the extracellular vesicles-mediated progression of β-amyloid pathology and offers the opportunity to test pharmacological treatments targeting the early stages of Alzheimer's disease.

Considering the Role of Extracellular Matrix Molecules, in Particular Reelin, in Granule Cell Dispersion Related to Temporal Lobe Epilepsy

The extracellular matrix (ECM) of the nervous system can be considered as a dynamically adaptable compartment between neuronal cells, in particular neurons and glial cells, that participates in physiological functions of the nervous system. It is mainly composed of carbohydrates and proteins that are secreted by the different kinds of cell types found in the nervous system, in particular neurons and glial cells, but also other cell types, such as pericytes of capillaries, ependymocytes and meningeal cells. ECM molecules participate in developmental processes, synaptic plasticity, neurodegeneration and regenerative processes. As an example, the ECM of the hippocampal formation is involved in degenerative and adaptive processes related to epilepsy. The role of various components of the ECM has been explored extensively. In particular, the ECM protein reelin, well known for orchestrating the formation of neuronal layer formation in the cerebral cortex, is also considered as a player involved in the occurrence of postnatal granule cell dispersion (GCD), a morphologically peculiar feature frequently observed in hippocampal tissue from epileptic patients. Possible causes and consequences of GCD have been studied in various in vivo and in vitro models. The present review discusses different interpretations of GCD and different views on the role of ECM protein reelin in the formation of this morphological peculiarity.

Botulinum Neurotoxins in Central Nervous System: An Overview from Animal Models to Human Therapy

Botulinum neurotoxins (BoNTs) are potent inhibitors of synaptic vesicle fusion and transmitter release. The natural target of BoNTs is the peripheral neuromuscular junction (NMJ) where, by blocking the release of acetylcholine (ACh), they functionally denervate muscles and alter muscle tone. This leads them to be an excellent drug for the therapy of muscle hyperactivity disorders, such as dystonia, spasticity, and many other movement disorders. BoNTs are also effective in inhibiting both the release of ACh at sites other than NMJ and the release of neurotransmitters other than ACh. Furthermore, much evidence shows that BoNTs can act not only on the peripheral nervous system (PNS), but also on the central nervous system (CNS). Under this view, central changes may result either from sensory input from the PNS, from retrograde transport of BoNTs, or from direct injection of BoNTs into the CNS. The aim of this review is to give an update on available data, both from animal models or human studies, which suggest or confirm central alterations induced by peripheral or central BoNTs treatment. The data will be discussed with particular attention to the possible therapeutic applications to pathological conditions and degenerative diseases of the CNS.

Reelin Is Required for Maintenance of Granule Cell Lamination in the Healthy and Epileptic Hippocampus

One characteristic feature of mesial temporal lobe epilepsy is granule cell dispersion (GCD), a pathological widening of the granule cell layer in the dentate gyrus. The loss of the extracellular matrix protein Reelin, an important positional cue for neurons, correlates with GCD formation in MTLE patients and in rodent epilepsy models. Here, we used organotypic hippocampal slice cultures (OHSC) from transgenic mice expressing enhanced green fluorescent protein (eGFP) in differentiated granule cells (GCs) to monitor GCD formation dynamically by live cell video microscopy and to investigate the role of Reelin in this process. We present evidence that following treatment with the glutamate receptor agonist kainate (KA), eGFP-positive GCs migrated mainly toward the hilar region. In the hilus, Reelin-producing neurons were rapidly lost following KA treatment as shown in a detailed time series. Addition of recombinant Reelin fragments to the medium effectively prevented the KA-triggered movement of eGFP-positive GCs. Placement of Reelin-coated beads into the hilus of KA-treated cultures stopped the migration of GCs in a distance-dependent manner. In addition, quantitative Western blot analysis revealed that KA treatment affects the Reelin signal transduction pathway by increasing intracellular adaptor protein Disabled-1 synthesis and reducing the phosphorylation of cofilin, a downstream target of the Reelin pathway. Both events were normalized by addition of recombinant Reelin fragments. Finally, following neutralization of Reelin in healthy OHSC by incubation with the function-blocking CR-50 Reelin antibody, GCs started to migrate without any direction preference. Together, our findings demonstrate that normotopic position of Reelin is essential for the maintenance of GC lamination in the dentate gyrus and that GCD is the result of a local Reelin deficiency.

The anticonvulsant and neuroprotective effects of kir2.3 activation in PTZ-induced seizures and the kainic acid model of TLE

PURPOSE

To elucidate the role of activating the inwardly rectifying K⁺ channel 2.3 (Kir2.3) in acute seizure and chronic epilepsy, we investigated the effect of a Kir2.3 agonist (tenidap) on epileptic and electrophysiological activities in mice. Neuronal excitability and damage were also evaluated.

METHODS

A Pentylenetetrazole (PTZ)-induced acute seizure model and a kainic acid (KA)-induced temporal epilepsy model were used in adult mice. The mice were given tenidap 30 min before PTZ injection or were given tenidap for 7 days after entering the chronic stage of the KA model. Video monitoring and EEG recordings were performed for comparisons. Immunofluorescence of c-fos was detected in the PTZ model, and Nissl staining was performed in the KA model.

RESULTS

Tenidap intervention significantly reduced the duration and severity of PTZ-induced acute seizures, which conformed with the power-spectrum analyses of the EEG and the quantification of spikes on EEG. C-fos expression representing neuronal excitability was also reduced with tenidap pretreatment. However, the latency time to seizure onset was unaltered. Seven days of tenidap treatment in the chronic KA model significantly attenuated seizure and spike frequencies compared to the same animal before administration. Nissl staining showed reduced hilar neuron loss in the tenidap-intervention group but showed no difference in the width of the granule cell layer.

CONCLUSION

To our knowledge, few studies have reported the relevance of Kir2.3 to epilepsy. The present data suggested that activation of Kir2.3 exerts an anticonvulsant effect in acute seizures and the chronic stage of TLE, which makes this channel a potent therapeutic target.

Glial responses during epileptogenesis in Mus musculus point to potential therapeutic targets

The Mesio-Temporal Lobe Epilepsy syndrome is the most common form of intractable epilepsy. It is characterized by recurrence of focal seizures and is often associated with hippocampal sclerosis and drug resistance. We aimed to characterize the molecular changes occurring during the initial stages of epileptogenesis in search of new therapeutic targets for Mesio-Temporal Lobe Epilepsy. We used a mouse model obtained by intra-hippocampal microinjection of kainate and performed hippocampal whole genome expression analysis at 6h, 12h and 24h post-injection, followed by multilevel bioinformatics analysis. We report significant changes in immune and inflammatory responses, neuronal network reorganization processes and glial functions, predominantly initiated during status epilepticus at 12h and persistent after the end of status epilepticus at 24h post-kainate. Upstream regulator analysis highlighted Cyba, Cybb and Vim as central regulators of multiple overexpressed genes implicated in glial responses at 24h. In silico microRNA analysis indicated that miR-9, miR-19b, miR-129, and miR-223 may regulate the expression of glial-associated genes at 24h. Our data support the hypothesis that glial-mediated inflammatory response holds a key role during epileptogenesis, and that microglial cells may participate in the initial process of epileptogenesis through increased ROS production via the NOX complex.

Exploiting Botulinum Neurotoxins for the Study of Brain Physiology and Pathology

Botulinum neurotoxins are metalloproteases that specifically cleave N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins in synaptic terminals, resulting in a potent inhibition of vesicle fusion and transmitter release. The family comprises different serotypes (BoNT/A to BoNT/G). The natural target of these toxins is represented by the neuromuscular junction, where BoNTs block acetylcholine release. In this review, we describe the actions of botulinum toxins after direct delivery to the central nervous system (CNS), where BoNTs block exocytosis of several transmitters, with near-complete silencing of neural networks. The use of clostridial neurotoxins in the CNS has allowed us to investigate specifically the role of synaptic activity in different physiological and pathological processes. The silencing properties of BoNTs can be exploited for therapeutic purposes, for example to counteract pathological hyperactivity and seizures in epileptogenic brain foci, or to investigate the role of activity in degenerative diseases like prion disease. Altogether, clostridial neurotoxins and their derivatives hold promise as powerful tools for both the basic understanding of brain function and the dissection and treatment of activity-dependent pathogenic pathways.

Combining robotic training and inactivation of the healthy hemisphere restores pre-stroke motor patterns in mice

Focal cortical stroke often leads to persistent motor deficits, prompting the need for more effective interventions. The efficacy of rehabilitation can be increased by ‘plasticity-stimulating’ treatments that enhance experience-dependent modifications in spared areas. Transcallosal pathways represent a promising therapeutic target, but their role in post-stroke recovery remains controversial. Here, we demonstrate that the contralesional cortex exerts an enhanced interhemispheric inhibition over the perilesional tissue after focal cortical stroke in mouse forelimb motor cortex. Accordingly, we designed a rehabilitation protocol combining intensive, repeatable exercises on a robotic platform with reversible inactivation of the contralesional cortex. This treatment promoted recovery in general motor tests and in manual dexterity with remarkable restoration of pre-lesion movement patterns, evaluated by kinematic analysis. Recovery was accompanied by a reduction of transcallosal inhibition and ‘plasticity brakes’ over the perilesional tissue. Our data support the use of combinatorial clinical therapies exploiting robotic devices and modulation of interhemispheric connectivity.

Dynamical properties of LFPs from mice with unilateral injection of TeNT

Local field potential (LFP) recordings were performed from the visual cortex (V1) of a focal epilepsy mouse model. Epilepsy was induced by a unilateral injection of the synaptic blocker tetanus neurotoxin (TeNT). LFP signals were simultaneously recorded from V1 of both hemispheres of each animal under acute and chronic conditions (i.e. during and after the period of TeNT action). All data were analysed by using nonlinear time series methods. Suitable values of the lag time and embedding dimension for phase space reconstruction were estimated by employing well-known methods. The results showed that lag times are sensitive to the presence of TeNT. Interestingly, TeNT promoted an increase in the level of linear and nonlinear correlation of LFP signals. The values of the embedding dimension failed to show any dependence on the presence of the neurotoxin. However, a local nonlinear prediction method showed that the presence of TeNT increases the predictability, quantified by the normalized prediction error, of the neural recordings. From a neurophysiological point of view, the above results suggest that TeNT injected in one hemisphere strongly impacts the local electrical activity of the neural populations in the opposite hemisphere. We hypothesize that this could arise from a qualitative and quantitative alteration of the transmission properties of the callosal fibers.

Time evolution of interhemispheric coupling in a model of focal neocortical epilepsy in mice

Epilepsy is characterized by substantial network rearrangements leading to spontaneous seizures and little is known on how an epileptogenic focus impacts on neural activity in the contralateral hemisphere. Here, we used a model of unilateral epilepsy induced by injection of the synaptic blocker tetanus neurotoxin (TeNT) in the mouse primary visual cortex (V1). Local field potential (LFP) signals were simultaneously recorded from both hemispheres of each mouse in acute phase (peak of toxin action) and chronic condition (completion of TeNT effects). To characterize the neural electrical activities the corresponding LFP signals were analyzed with several methods of time series analysis. For the epileptic mice, the spectral analysis showed that TeNT determines a power redistribution among the different neurophysiological bands in both acute and chronic phases. Using linear and nonlinear interdependence measures in both time and frequency domains, it was found in the acute phase that TeNT injection promotes a reduction of the interhemispheric coupling for high frequencies (12-30 Hz) and small time lag (<20 ms), whereas an increase of the coupling is present for low frequencies (0.5-4 Hz) and long time lag (>40 ms). On the other hand, the chronic period is characterized by a partial or complete recovery of the interhemispheric interdependence level. Granger causality test and symbolic transfer entropy indicate a greater driving influence of the TeNT-injected side on activity in the contralateral hemisphere in the chronic phase. Lastly, based on experimental observations, we built a computational model of LFPs to investigate the role of the ipsilateral inhibition and exicitatory interhemispheric connections in the dampening of the interhemispheric coupling. The time evolution of the interhemispheric coupling in such a relevant model of epilepsy has been addressed here.

The Chemokine CCL2 Mediates the Seizure-enhancing Effects of Systemic Inflammation

Epilepsy is a chronic disorder characterized by spontaneous recurrent seizures. Brain inflammation is increasingly recognized as a critical factor for seizure precipitation, but the molecular mediators of such proconvulsant effects are only partly understood. The chemokine CCL2 is one of the most elevated inflammatory mediators in patients with pharmacoresistent epilepsy, but its contribution to seizure generation remains unexplored. Here, we show, for the first time, a crucial role for CCL2 and its receptor CCR2 in seizure control. We imposed a systemic inflammatory challenge via lipopolysaccharide (LPS) administration in mice with mesial temporal lobe epilepsy. We found that LPS dramatically increased seizure frequency and upregulated the expression of many inflammatory proteins, including CCL2. To test the proconvulsant role of CCL2, we administered systemically either a CCL2 transcription inhibitor (bindarit) or a selective antagonist of the CCR2 receptor (RS102895). We found that interference with CCL2 signaling potently suppressed LPS-induced seizures. Intracerebral administration of anti-CCL2 antibodies also abrogated LPS-mediated seizure enhancement in chronically epileptic animals. Our results reveal that CCL2 is a key mediator in the molecular pathways that link peripheral inflammation with neuronal hyperexcitability.

SIGNIFICANCE STATEMENT

Substantial evidence points to a role for inflammation in epilepsy, but currently there is little insight as to how inflammatory pathways impact on seizure generation. Here, we examine the molecular mediators linking peripheral inflammation with seizure susceptibility in mice with mesial temporal lobe epilepsy. We show that a systemic inflammatory challenge via lipopolysaccharide administration potently enhances seizure frequency and upregulates the expression of the chemokine CCL2. Remarkably, selective pharmacological interference with CCL2 or its receptor CCR2 suppresses lipopolysaccharide-induced seizure enhancement. Thus, CCL2/CCR2 signaling plays a key role in linking systemic inflammation with seizure susceptibility.

Seizure-Induced Motility of Differentiated Dentate Granule Cells Is Prevented by the Central Reelin Fragment

Granule cell dispersion (GCD) represents a pathological widening of the granule cell layer in the dentate gyrus and it is frequently observed in patients with mesial temporal lobe epilepsy (MTLE). Recent studies in human MTLE specimens and in animal epilepsy models have shown that a decreased expression and functional inactivation of the extracellular matrix protein Reelin correlates with GCD formation, but causal evidence is still lacking. Here, we used unilateral kainate (KA) injection into the mouse hippocampus, an established MTLE animal model, to precisely map the loss of reelin mRNA-synthesizing neurons in relation to GCD along the septotemporal axis of the epileptic hippocampus. We show that reelin mRNA-producing neurons are mainly lost in the hilus and that this loss precisely correlates with the occurrence of GCD. To monitor GCD formation in real time, we used organotypic hippocampal slice cultures (OHSCs) prepared from mice which express enhanced green fluorescent protein (eGFP) primarily in differentiated dentate granule cells. Using life cell microscopy we observed that increasing doses of KA resulted in an enhanced motility of eGFP-positive granule cells. Moreover, KA treatment of OHSC resulted in a rapid loss of Reelin-producing interneurons mainly in the hilus, as observed in vivo. A detailed analysis of the migration behavior of individual eGFP-positive granule cells revealed that the majority of these neurons actively migrate toward the hilar region, where Reelin-producing neurons are lost. Treatment with KA and subsequent addition of the recombinant R3–6 Reelin fragment significantly prevented the movement of eGFP-positive granule cells. Together, these findings suggest that GCD formation is indeed triggered by a loss of Reelin in hilar interneurons.

Epilepsy, Seizures, and Inflammation: Role of the C-C Motif Ligand 2 Chemokine

Epilepsy is a chronic disorder characterized by spontaneous recurrent seizures. Several lines of evidence demonstrate that inflammatory processes within the brain parenchyma contribute to recurrence and precipitation of seizures. In both epileptic patients and animal models, seizures upregulate inflammatory mediators, which in turn may enhance brain excitability. We recently showed that the C-C motif ligand 2 (CCL2) chemokine (also known as monocyte chemoattractant protein-1 [MCP-1]) mediates the seizure-promoting effects of inflammation. Systemic inflammatory challenge in chronically epileptic mice markedly enhanced seizure frequency and upregulated CCL2 expression in the brain. Selective pharmacological blockade of CCL2 synthesis or C-C chemokine receptor type 2 (CCR2) significantly suppressed inflammation-induced seizures. These results have important implications for the development of novel anticonvulsant therapies: drugs interfering with CCL2 signaling are used clinically for several human disorders and might be redirected for use in pharmacoresistant epilepsy. Here we review the role of CCL2/CCR2 signaling in linking systemic inflammation with seizure susceptibility and discuss some open questions that arise from our recent studies.

The role of dopamine signaling in epileptogenesis

Clinical and experimental studies implicate most neuromodulatory systems in epileptogenesis. The dopaminergic system has a seizure-modulating effect that crucially depends on the different subtypes of dopamine (DA) receptors involved and the brain regions in which they are activated. Specifically, DA plays a major role in the control of seizures arising in the limbic system. Studies performed in a wide variety of animal models contributed to illustrate the opposite actions of D1-like and D2-like receptor signaling in limbic epileptogenesis. Indeed, signaling from D1-like receptors is generally pro-epileptogenic, whereas D2-like receptor signaling exerts an anti-epileptogenic effect. However, this view might appear quite simplistic as the complex neuromodulatory action of DA in the control of epileptogenesis likely requires a physiological balance in the activation of circuits modulated by these two major DA receptor subtypes, which determines the response to seizure-promoting stimuli. Here we will review recent evidences on the identification of molecules activated by DA transduction pathways in the generation and spread of seizures in the limbic system. We will discuss the intracellular signaling pathways triggered by activation of different DA receptors in relation to their role in limbic epileptogenesis, which lead to the activation of neuronal death/survival cascades. A deep understanding of the signaling pathways involved in epileptogenesis is crucial for the identification of novel targets for the treatment of epilepsy.

A macaque model of mesial temporal lobe epilepsy induced by unilateral intrahippocampal injection of kainic Acid

Objective

In order to better investigate the cause/effect relationships of human mesial temporal lobe epilepsy (mTLE), we hereby describe a new non-human primate model of mTLE.

Methods

Ten macaques were studied and divided into 2 groups: saline control group (n = 4) and kainic acid (KA) injection group (n = 6). All macaques were implanted bilaterally with subdural electrodes over temporal cortex and depth electrodes in CA3 hippocampal region. KA was stereotaxically injected into the right hippocampus of macaques. All animals were monitored by video and electrocorticography (ECoG) to assess status epilepticus (SE) and subsequent spontaneous recurrent seizures (SRS). Additionally, in order to evaluate brain injury produced by SE or SRS, we used both neuroimaging, including magnetic resonance image (MRI) & magnetic resonance spectroscopy (MRS), and histological pathology, including Nissl stainning and glial fibrillary acid protein (GFAP) immunostaining.

Results

The typical seizures were observed in the KA-injected animal model. Hippocampal sclerosis could be found by MRI & MRS. Hematoxylin and eosin (H&E) staining and GFAP immunostaining showed neuronal loss, proliferation of glial cells, formation of glial scars, and hippocampal atrophy. Electron microscopic analysis of hippocampal tissues revealed neuronal pyknosis, partial ribosome depolymerization, an abnormal reduction in rough endoplasmic reticulum size, expansion of Golgi vesicles and swollen star-shaped cells. Furthermore, we reported that KA was able to induce SE followed by SRS after a variable period of time. Similar to human mTLE, brain damage is confined to the hippocampus. Accordingly, hippocampal volume is in positive correlations with the neuronal cells count in the CA3, especially the ratio of neuron/glial cell.

Conclusions

The results suggest that a model of mTLE can be developed in macaques by intra-hippocampal injection of KA. Brain damage is confined to the hippocampus which is similar to the human mTLE. The hippocampal volume correlates with the extension of the hippocampal damage.

Botulinum neurotoxin A2 reduces incidence of seizures in mouse models of temporal lobe epilepsy

Temporal lobe epilepsy often shows pharmacoresistance, and well-known anti-convulsants sometimes are not effective for blocking chronic seizures. Botulinum neurotoxins are metalloproteases that act on presynaptic proteins and inhibit neurotransmitter release in both the peripheral and central nerve systems. That is why neurotoxins may elicit an effect for the restraint of the seizures. Meanwhile, it has been suggested that a property and the stability of neurotoxin activities differ among the types A-G, in which type A neurotoxin (ANTX) is, especially, the most stable and can continue having activity for a long term. The present study therefore investigated the effects of hippocampal injections of A2NTX on seizures derived in TLE model mice, received repeated kindling stimulations in the amygdala. The injections induced complete disappearance of grand mal seizures in half of the population of amygdala kindled mice for 4 days. The injections also induced reduction of the evoked seizure level significantly for at least 18 days after injections. Taken together, these results suggest that A2NTX prevents from epileptic seizures, proposing that A2NTX is available as a new antiepileptic reagent.

Receptor for Advanced Glycation Endproducts is upregulated in temporal lobe epilepsy and contributes to experimental seizures

Toll-like receptor 4 (TLR4) activation in neuron and astrocytes by High Mobility Group Box 1 (HMGB1) protein is a key mechanism of seizure generation. HMGB1 also activates the Receptor for Advanced Glycation Endproducts (RAGE), but it was unknown whether RAGE activation contributes to seizures or to HMGB1 proictogenic effects. We found that acute EEG seizures induced by 7ng intrahippocampal kainic acid (KA) were significantly reduced in Rage-/- mice relative to wild type (Wt) mice. The proictogenic effect of HMGB1 was decreased in Rage-/- mice, but less so, than in Tlr4-/- mice. In a mouse mesial temporal lobe epilepsy (mTLE) model, status epilepticus induced by 200ng intrahippocampal KA and the onset of the spontaneous epileptic activity were similar in Rage-/-, Tlr4-/- and Wt mice. However, the number of hippocampal paroxysmal episodes and their duration were both decreased in epileptic Rage-/- and Tlr4-/- mice vs Wt mice. All strains of epileptic mice displayed similar cognitive deficits in the novel object recognition test vs the corresponding control mice. CA1 neuronal cell loss was increased in epileptic Rage-/- vs epileptic Wt mice, while granule cell dispersion and doublecortin (DCX)-positive neurons were similarly affected. Notably, DCX neurons were preserved in epileptic Tlr4-/- mice. We did not find compensatory changes in HMGB1-related inflammatory signaling nor in glutamate receptor subunits in Rage-/- and Tlr4-/- naïve mice, except for ~20% NR2B subunit reduction in Rage-/- mice. RAGE was induced in neurons, astrocytes and microvessels in human and experimental mTLE hippocampi. We conclude that RAGE contributes to hyperexcitability underlying acute and chronic seizures, as well as to the proictogenic effects of HMGB1. RAGE and TLR4 play different roles in the neuropathologic sequelae developing after status epilepticus. These findings reveal new molecular mechanisms underlying seizures, cell loss and neurogenesis which involve inflammatory pathways upregulated in human epilepsy.

Botulinum neurotoxins A and E undergo retrograde axonal transport in primary motor neurons

The striking differences between the clinical symptoms of tetanus and botulism have been ascribed to the different fate of the parental neurotoxins once internalised in motor neurons. Tetanus toxin (TeNT) is known to undergo transcytosis into inhibitory interneurons and block the release of inhibitory neurotransmitters in the spinal cord, causing a spastic paralysis. In contrast, botulinum neurotoxins (BoNTs) block acetylcholine release at the neuromuscular junction, therefore inducing a flaccid paralysis. Whilst overt experimental evidence supports the sorting of TeNT to the axonal retrograde transport pathway, recent findings challenge the established view that BoNT trafficking is restricted to the neuromuscular junction by highlighting central effects caused by these neurotoxins. These results suggest a more complex scenario whereby BoNTs also engage long-range trafficking mechanisms. However, the intracellular pathways underlying this process remain unclear. We sought to fill this gap by using primary motor neurons either in mass culture or differentiated in microfluidic devices to directly monitor the endocytosis and axonal transport of full length BoNT/A and BoNT/E and their recombinant binding fragments. We show that BoNT/A and BoNT/E are internalised by spinal cord motor neurons and undergo fast axonal retrograde transport. BoNT/A and BoNT/E are internalised in non-acidic axonal carriers that partially overlap with those containing TeNT, following a process that is largely independent of stimulated synaptic vesicle endo-exocytosis. Following intramuscular injection in vivo, BoNT/A and TeNT displayed central effects with a similar time course. Central actions paralleled the peripheral spastic paralysis for TeNT, but lagged behind the onset of flaccid paralysis for BoNT/A. These results suggest that the fast axonal retrograde transport compartment is composed of multifunctional trafficking organelles orchestrating the simultaneous transfer of diverse cargoes from nerve terminals to the soma, and represents a general gateway for the delivery of virulence factors and pathogens to the central nervous system.

Botulinum neurotoxins are the most toxic molecules known to mankind, and as a result, are currently listed among the top bio-threats. However, their ability to bind specifically to neurons and their inhibitory effects on regulated secretion prompted their clinical use in pathologies characterised by increased muscular tone, such as dystonia and various forms of spasticity, or abnormal secretion, such as drooling and excessive sweating, to cite a few. As a consequence, botulinum neurotoxin A, which is the serotype most commonly used in human therapy, has become the treatment of choice for an ever-expanding number of pathological and non-pathological (e.g. cosmetic) conditions. All current indications show that the systemic effects and toxicity of botulinum neurotoxin A are minimised by the specific route of administration (local injection) and the low diffusion of this molecule in tissues. However, recent reports suggest that in contrast to this common belief, botulinum neurotoxin A is able to reach distal sites in the body and may have previously unanticipated effects in the central nervous system. In this study, we demonstrate that botulinum neurotoxin A and E enter alternative endocytic pathway(s) in addition to synaptic vesicle recycling, and undergo long-range transport in a non degradative compartment in spinal cord motor neurons. Our results show that axonal retrograde transport is a common pathway for the dissemination in the central nervous system of pathogens and virulence factors important for human and animal health.

Emerging opportunities for serotypes of botulinum neurotoxins

Background: Two decades ago, botulinum neurotoxin (BoNT) type A was introduced to the commercial market. Subsequently, the toxin was approved by the FDA to address several neurological syndromes, involving muscle, nerve, and gland hyperactivity. These syndromes have typically been associated with abnormalities in cholinergic transmission. Despite the multiplicity of botulinal serotypes (designated as types A through G), therapeutic preparations are currently only available for BoNT types A and B. However, other BoNT serotypes are under study for possible clinical use and new clinical indications; Objective: To review the current research on botulinum neurotoxin serotypes A-G, and to analyze potential applications within basic science and clinical settings; Conclusions: The increasing understanding of botulinal neurotoxin pathophysiology, including the neurotoxin’s effects on specific neuronal populations, will help us in tailoring treatments for specific diagnoses, symptoms and patients. Scientists and clinicians should be aware of the full range of available data involving neurotoxin subtypes A-G.

Loss of survivin in neural precursor cells results in impaired long-term potentiation in the dentate gyrus and CA1-region

In adult mammals, newborn neural precursor cells (NPCs) derived from either the subventricular zone (SVZ) or the subgranular zone (SGZ) migrate into the olfactory bulb and the dentate gyrus (DG), respectively, where some of them mature into excitatory and inhibitory neurons. There is increasing evidence that this neurogenesis process is important for some types of learning and synaptic plasticity and vice versa. Survivin, a member of the inhibitor-of-apoptosis protein (IAP) family, has been suggested to have a central role in the regulation of neurogenesis. The protein is abundantly expressed in nervous tissue during embryonic development while being restricted postnatally to proliferating and migrating NPCs in SVZ and SGZ. Here we examined adult Survivin(Camcre) mice with a conditional deletion of the survivin gene in embryonic neurogenic regions. Although the deletion of survivin had no effect on basic excitability in DG and CA1-region, there was a marked impairment of long-term potentiation (LTP) in these areas. Our data support a function of survivin in hippocampal synaptic plasticity and learning and underline the importance of adult brain neurogenesis for proper operation of the hippocampal tri-synaptic circuit and the physiological functions that depend on it.

The role of activity in synaptic degeneration in a protein misfolding disease, prion disease

In chronic neurodegenerative diseases associated with aggregates of misfolded proteins (such as Alzheimer's, Parkinson's and prion disease), there is an early degeneration of presynaptic terminals prior to the loss of the neuronal somata. Identifying the mechanisms that govern synapse degeneration is of paramount importance, as cognitive decline is strongly correlated with loss of presynaptic terminals in these disorders. However, very little is known about the processes that link the presence of a misfolded protein to the degeneration of synapses. It has been suggested that the process follows a simple linear sequence in which terminals that become dysfunctional are targeted for death, but there is also evidence that high levels of activity can speed up degeneration. To dissect the role of activity in synapse degeneration, we infused the synaptic blocker botulinum neurotoxin A (BoNT/A) into the hippocampus of mice with prion disease and assessed synapse loss at the electron microscopy level. We found that injection of BoNT/A in naïve mice caused a significant enlargement of excitatory presynaptic terminals in the hippocampus, indicating transmission impairment. Long-lasting blockade of activity by BoNT/A caused only minimal synaptic pathology and no significant activation of microglia. In mice with prion disease infused with BoNT/A, rates of synaptic degeneration were indistinguishable from those observed in control diseased mice. We conclude that silencing synaptic activity neither prevents nor enhances the degree of synapse degeneration in prion disease. These results challenge the idea that dysfunction of synaptic terminals dictates their elimination during prion-induced neurodegeneration.

Tetanus neurotoxin-induced epilepsy in mouse visual cortex

Tetanus neurotoxin (TeNT) is a metalloprotease that cleaves the synaptic protein VAMP/synaptobrevin, leading to focal epilepsy. Although this model is widely used in rats, the time course and spatial specificity of TeNT proteolytic action have not been precisely defined. Here we have studied the biochemical, electrographic, and anatomic characteristics of TeNT-induced epilepsy in mouse visual cortex (V1). We found that VAMP cleavage peaked at 10 days, was reduced at 21 days, and completely extinguished 45 days following TeNT delivery. VAMP proteolysis was restricted to the injected V1 and ipsilateral thalamus, whereas it was undetectable in other cortical areas. Electrographic epileptiform activity was evident both during and after the time window of TeNT effects, indicating development of chronic epilepsy. Anatomic analyses found no evidence for long-term tissue damage, such as neuronal loss or microglia activation. These data show that TeNT reliably induces nonlesional epilepsy in mouse cortex. Due to the excellent physiologic knowledge of the visual cortex and the availability of mouse transgenic strains, this model will be useful for examining the network and cellular alterations underlying hyperexcitability within an epileptic focus.

Cracking down on inhibition: selective removal of GABAergic interneurons from hippocampal networks

Inhibitory (GABAergic) interneurons entrain assemblies of excitatory principal neurons to orchestrate information processing in the hippocampus. Disrupting the dynamic recruitment as well as the temporally precise activity of interneurons in hippocampal circuitries can manifest in epileptiform seizures, and impact specific behavioral traits. Despite the importance of GABAergic interneurons during information encoding in the brain, experimental tools to selectively manipulate GABAergic neurotransmission are limited. Here, we report the selective elimination of GABAergic interneurons by a ribosome inactivation approach through delivery of saporin-conjugated anti-vesicular GABA transporter antibodies (SAVAs) in vitro as well as in the mouse and rat hippocampus in vivo. We demonstrate the selective loss of GABAergic--but not glutamatergic--synapses, reduced GABA release, and a shift in excitation/inhibition balance in mixed cultures of hippocampal neurons exposed to SAVAs. We also show the focal and indiscriminate loss of calbindin(+), calretinin(+), parvalbumin/system A transporter 1(+), somatostatin(+), vesicular glutamate transporter 3 (VGLUT3)/cholecystokinin/CB(1) cannabinoid receptor(+) and neuropeptide Y(+) local-circuit interneurons upon SAVA microlesions to the CA1 subfield of the rodent hippocampus, with interneuron debris phagocytosed by infiltrating microglia. SAVA microlesions did not affect VGLUT1(+) excitatory afferents. Yet SAVA-induced rearrangement of the hippocampal circuitry triggered network hyperexcitability associated with the progressive loss of CA1 pyramidal cells and the dispersion of dentate granule cells. Overall, our data identify SAVAs as an effective tool to eliminate GABAergic neurons from neuronal circuits underpinning high-order behaviors and cognition, and whose manipulation can recapitulate pathogenic cascades of epilepsy and other neuropsychiatric illnesses.

Epilepsy as a neurodevelopmental disorder

Epilepsy is characterized by spontaneous recurrent seizures and comprises a diverse group of syndromes with different etiologies. Epileptogenesis refers to the process whereby the brain becomes epileptic and can be related to several factors, such as acquired structural brain lesions, inborn brain malformations, alterations in neuronal signaling, and defects in maturation and plasticity of neuronal networks. In this review, we will focus on alterations of brain development that lead to an hyperexcitability phenotype in adulthood, providing examples from both animal and human studies. Malformations of cortical development (including focal cortical dysplasia, lissencephaly, heterotopia, and polymicrogyria) are frequently epileptogenic and result from defects in cell proliferation in the germinal zone and/or impaired neuronal migration and differentiation. Delayed or reduced arrival of inhibitory interneurons into the cortical plate is another possible cause of epileptogenesis. GABAergic neurons are generated during early development in the ganglionic eminences, and failure to pursue migration toward the cortex alters the excitatory/inhibitory balance resulting in aberrant network hyperexcitability. More subtle defects in the developmental assembly of excitatory and inhibitory synapses are also involved in epilepsy. For example, mutations in the presynaptic proteins synapsins and SNAP-25 cause derangements of synaptic transmission and plasticity which underlie appearance of an epileptic phenotype. Finally, there is evidence that defects in synapse elimination and remodeling during early "critical periods" can trigger hyperexcitability later in life. Further clarification of the developmental pathways to epilepsy has important implications for disease prevention and therapy.

Evidence for anterograde transport and transcytosis of botulinum neurotoxin A (BoNT/A)

Botulinum neurotoxin type A (BoNT/A) is a metalloprotease that blocks synaptic transmission via the cleavage of SNAP-25 (synaptosomal-associated protein of 25 kDa). BoNT/A is successfully used in clinical neurology for the treatment of several neuromuscular pathologies and pain syndromes. Despite its widespread use, relatively little is known on BoNT/A intracellular trafficking in neurons. Using the visual pathway as a model system, here we show that catalytically active BoNT/A is capable of undergoing anterograde axonal transport and transcytosis. Following BoNT/A injection into the rat eye, significant levels of BoNT/A-cleaved SNAP-25 appeared in the retinorecipient layers of the superior colliculus (SC). Anterograde propagation of BoNT/A effects required axonal transport, ruling out a systemic spread of the toxin. Cleaved SNAP-25 was present in presynaptic structures of the tectum, but retinal terminals were devoid of the immunoreactivity, indicative of transcytosis. Experiments based on sequential administration of BoNT/A and BoNT/E showed a persistent catalytic activity of BoNT/A in tectal cells following its injection into the retina. Our findings demonstrate that catalytically active BoNT/A is anterogradely transported from the eye to the SC and transcytosed to tectal synapses. These data are important for a more complete understanding of the mechanisms of action of BoNT/A.

Environmental enrichment modulates cortico-cortical interactions in the mouse

Environmental enrichment (EE) is an experimental protocol based on a complex sensorimotor stimulation that dramatically affects brain development. While it is widely believed that the effects of EE result from the unique combination of different sensory and motor stimuli, it is not known whether and how cortico-cortical interactions are shaped by EE. Since the primary visual cortex (V1) is one of the best characterized targets of EE, we looked for direct cortico-cortical projections impinging on V1, and we identified a direct monosynaptic connection between motor cortex and V1 in the mouse brain. To measure the interactions between these areas under standard and EE rearing conditions, we used simultaneous recordings of local field potentials (LFPs) in awake, freely moving animals. LFP signals were analyzed by using different methods of linear and nonlinear analysis of time series (cross-correlation, mutual information, phase synchronization). We found that EE decreases the level of coupling between the electrical activities of the two cortical regions with respect to the control group. From a functional point of view, our results indicate, for the first time, that an enhanced sensorimotor experience impacts on the brain by affecting the functional crosstalk between different cortical areas.

Adoptive transfer of T lymphocytes in immunodeficient mice influences epileptogenesis and neurodegeneration in a model of temporal lobe epilepsy

Intra-hippocampal injection of kainic acid (KA) in adult mice causes a focal lesion in the CA1 area and hilus of the dentate gyrus, as well as pronounced granule cell hypertrophy and dispersion. The lesion results in chronic focal seizures, with a two-week delay following KA-induced status epilepticus. Furthermore, seizures are preceded by infiltration of T lymphocytes into the lesioned tissue and of macrophage-like cells, strongly immunopositive for the monocyte marker F4/80, into the dentate gyrus, where they regulate granule cell dispersion and survival. Unexpectedly, depletion of CD4(+) and/or CD8(+) T lymphocytes by targeted gene deletion results in a marked shortening of the delay prior to seizure onset, suggesting a role of adaptive immunity in epileptogenesis (Zattoni et al. 2011, J. Neurosci. 31, 4037). Here, we investigated the specific role of adaptive immunity in this TLE model by adoptive i.v. transfer of immunopurified T cells in mutant mice lacking either CD4(+) T cells (MHCII-knockout), CD8(+) T cells (β2-microglobulin-knockout), or both populations (RAG1-knockout mice). EEG analysis in mutants mice injected with KA two days after the T cell transfer revealed that grafting of the missing T cell population had no influence on seizure onset, but strongly influenced F4/80(+) macrophage-like cell infiltration in the dentate gyrus. Specifically, CD8(+) T cells in β2-microgloblin-knockout mice enhanced macrophage recruitment, whereas CD4(+) T cells transferred in MHCII-knockout and in RAG1-knockout mice blocked macrophage infiltration, leading to reduced granule cell dispersion and survival, thereby worsening the KA-induced lesion. These results suggest that intact adaptive immunity is required for delayed seizure onset in this mouse model of TLE and unravel complex interactions between T cells and mononuclear phagocytes for the control of neuronal integrity and survival in the lesioned brain.

Brain infiltration of leukocytes contributes to the pathophysiology of temporal lobe epilepsy

Clinical and experimental evidence indicates that inflammatory processes contribute to the pathophysiology of epilepsy, but underlying mechanisms remain mostly unknown. Using immunohistochemistry for CD45 (common leukocyte antigen) and CD3 (T-lymphocytes), we show here microglial activation and infiltration of leukocytes in sclerotic tissue from patients with mesial temporal lobe epilepsy (TLE), as well as in a model of TLE (intrahippocampal kainic acid injection), characterized by spontaneous, nonconvulsive focal seizures. Using specific markers of lymphocytes, microglia, macrophages, and neutrophils in kainate-treated mice, we investigated with pharmacological and genetic approaches the contribution of innate and adaptive immunity to kainate-induced inflammation and neurodegeneration. Furthermore, we used EEG analysis in mutant mice lacking specific subsets of lymphocytes to explore the significance of inflammatory processes for epileptogenesis. Blood-brain barrier disruption and neurodegeneration in the kainate-lesioned hippocampus were accompanied by sustained ICAM-1 upregulation, microglial cell activation, and infiltration of CD3(+) T-cells. Moreover, macrophage infiltration was observed, selectively in the dentate gyrus where prominent granule cell dispersion was evident. Unexpectedly, depletion of peripheral macrophages by systemic clodronate liposome administration affected granule cell survival. Neurodegeneration was aggravated in kainate-lesioned mice lacking T- and B-cells (RAG1-knock-out), because of delayed invasion by Gr-1(+) neutrophils. Most strikingly, these mutant mice exhibited early onset of spontaneous recurrent seizures, suggesting a strong impact of immune-mediated responses on network excitability. Together, the concerted action of adaptive and innate immunity triggered locally by intrahippocampal kainate injection contributes seizure-suppressant and neuroprotective effects, shedding new light on neuroimmune interactions in temporal lobe epilepsy.

Interleukin-1β biosynthesis inhibition reduces acute seizures and drug resistant chronic epileptic activity in mice

Experimental evidence and clinical observations indicate that brain inflammation is an important factor in epilepsy. In particular, induction of interleukin-converting enzyme (ICE)/caspase-1 and activation of interleukin (IL)-1β/IL-1 receptor type 1 axis both occur in human epilepsy, and contribute to experimentally induced acute seizures. In this study, the anticonvulsant activity of VX-765 (a selective ICE/caspase-1 inhibitor) was examined in a mouse model of chronic epilepsy with spontaneous recurrent epileptic activity refractory to some common anticonvulsant drugs. Moreover, the effects of this drug were studied in one acute model of seizures in mice, previously shown to involve activation of ICE/caspase-1. Quantitative analysis of electroencephalogram activity was done in mice exposed to acute seizures or those developing chronic epileptic activity after status epilepticus to assess the anticonvulsant effects of systemic administration of VX-765. Histological and immunohistochemical analysis of brain tissue was carried out at the end of pharmacological experiments in epileptic mice to evaluate neuropathology, glia activation and IL-1β expression, and the effect of treatment. Repeated systemic administration of VX-765 significantly reduced chronic epileptic activity in mice in a dose-dependent fashion (12.5-200 mg/kg). This effect was observed at doses ≥ 50 mg/kg, and was reversible with discontinuation of the drug. Maximal drug effect was associated with inhibition of IL-1β synthesis in activated astrocytes. The same dose regimen of VX-765 also reduced acute seizures in mice and delayed their onset time. These results support a new target system for anticonvulsant pharmacological intervention to control epileptic activity that does not respond to some common anticonvulsant drugs.

The effect of amygdala kindling on hippocampal neurogenesis coincides with decreased reelin and DISC1 expression in the adult dentate gyrus

Temporal lobe seizures can induce the proliferation and abnormal migration of newly generated dentate granule cells, but little is known about the molecular mechanisms that govern these pathological events. Reelin and DISC1 (disrupted-in-schizophrenia 1) are proteins that play a regulatory role in the maturation and integration of new neurons in the developing and adult brain. In this study, we examined whether amygdala kindling results in aberrant neurogenesis and altered expression of reelin and DISC1 in the adult dentate gyrus. Using doublecortin immunohistochemistry, we found that short-term kindling (i.e., 30 electrical stimulations) significantly increased the number of immature neurons in the dentate subgranular zone (SGZ), whereas long-term kindling (i.e., 99 electrical stimulations) did not. However, doublecortin-labeled neurons in long-term kindled rats showed greater dendritic complexity than they did in short-term kindled or control rats. We also found that long-term kindling decreased the number of reelin-positive cells and decreased DISC1 expression in the dentate granule cell layer and subgranular zone. Interestingly, kindling-induced changes in reelin and DISC1 expression coincided with the appearance of ectopically located Prox1-labeled granule cells in the hilus. These effects occurred independently of alterations in granule cell layer length, dentate volume, or the number of hilar neurons. Taken together, these findings suggest a novel role for DISC1 in the pathophysiology of temporal lobe epilepsy and further suggest that changes in reelin and DISC1 expression may contribute to aberrant neurogenesis in the kindling model.

PSA-NCAM-dependent GDNF signaling limits neurodegeneration and epileptogenesis in temporal lobe epilepsy

Polysialylated neuronal cell adhesion molecule (PSA-NCAM), a polysialylated protein constitutively expressed in the hippocampus, is involved in neuronal growth, synaptic plasticity and neurotrophin signaling. In particular, PSA-NCAM mediates Ret-independent glial-derived neurotrophic factor (GDNF) signaling, leading to downstream FAK activation. GDNF has potent seizure-suppressant action, whereas PSA-NCAM is upregulated by seizure activity. However, the involvement of Ret-independent GDNF signaling in temporal lobe epilepsy (TLE) is not established. We tested the effects of PSA-NCAM inactivation on neurodegeneration and epileptogenesis in a mouse model of TLE. In this model, unilateral intrahippocampal kainic acid (KA) injection induced degeneration of CA1, CA3c and hilar neurons, followed by spontaneous recurrent focal seizures. In the contralateral, morphologically preserved hippocampus, a long-lasting increase of PSA-NCAM immunoreactivity was observed. Inactivation of PSA-NCAM by endoneuraminidase (EndoN) administration into the contralateral ventricle of KA-treated mice caused severe degeneration of CA3a,b neurons and dentate gyrus granule cells in the epileptic focus, and led to early onset of focal seizures. This striking trans-hemispheric alteration suggested that PSA-NCAM mediates GDNF signaling, leading to transport of neuroprotective signals into the lesioned hippocampus. This hypothesis was confirmed by injecting GDNF antibodies into the contralateral hippocampus of KA-treated mice, thereby reproducing the enhanced neurodegeneration seen after PSA-NCAM inactivation. Furthermore, contralateral EndoN and anti-GDNF treatment decreased GDNF family receptor alpha1 immunoreactivity and FAK phosphorylation in the epileptic focus. Thus, Ret-independent GDNF signaling across the commissural projection might protect CA3a,b neurons and delay seizure onset. These findings implicate GDNF in the control of epileptogenesis and offer a possible mechanism explaining lesion asymmetry in mesial TLE.

Impaired reelin processing and secretion by Cajal-Retzius cells contributes to granule cell dispersion in a mouse model of temporal lobe epilepsy

Cajal-Retzius cells play a crucial role during ontogeny in regulating cortical lamination via release of reelin. In adult brain, they comprise small calretinin-positive interneurons located in the marginal zone of the cerebral cortex and in the hippocampal fissure. Alterations of reelin signaling or expression have been involved in major neurological disorders, and they underlie granule cell dispersion (GCD) in mesial temporal lobe epilepsy (TLE). Here, we investigated in a mouse model of TLE the contribution of Cajal-Retzius cells to reelin production in epileptic hippocampus and the molecular mechanisms underlying GCD. Following unilateral intrahippocampal Kainic acid injection in adult mice to induce an epileptic focus, we observed that Cajal-Retzius cells gradually became strongly immunopositive for reelin, due to intracellular accumulation. This phenotype resembled the morphology of Cajal-Retzius cells in reeler Orleans (reln (orl/orl) ) mice, which express a secretion-deficient 310-kDa reelin fragment. The possibility that GCD might result from abnormal reelin processing in Cajal-Retzius cells, leading to a lack of reelin secretion, was confirmed by KA injection in reln (orl/+) mice, which induced severe GCD. Furthermore, Western blot analysis in KA-treated wildtype mice revealed increased production of ∼300-kDa reelin fragments, confirming abnormal proteolytic processing. This effect was not seen upon treatment with Botulinum neurotoxin E (BoNT/E), which prevents GCD in KA-lesioned hippocampus by chronic blockade of synaptic transmission. Furthermore, BoNT/E blocked upregulation of TrkB in Cajal-Retzius cells, suggesting that production of truncated reelin in KA-treated hippocampus is activity-dependent and regulated by BDNF. Altogether, these data reveal that GCD results from abnormal reelin processing in Cajal-Retzius cells under the control of BDNF. Our findings highlight the critical role played by Cajal-Retzius cells for hippocampal neuronal reorganization in TLE.

Acute neuroprotection by the synaptic blocker botulinum neurotoxin E in a rat model of focal cerebral ischaemia

Evidence indicates that accumulation of excitotoxic mediators, such as glutamate, contributes to neuronal damage after an ischaemic insult. It is not clear, however, whether this accumulation is due to excess synaptic release or to impaired uptake. To test a role for synaptic release, here we investigated the neuroprotective potential of the synaptic blocker botulinum neurotoxin E (BoNT/E), that prevents vesicle fusion via the cleavage of the SNARE (soluble NSF-attachment receptor) protein SNAP-25 (synaptosomal-associated protein of 25 kDa). Focal ischaemia was induced in vivo by infusing the potent vasoconstricting peptide endothelin-1 (ET-1) into the CA1 area of the hippocampus in adult rats; BoNT/E or vehicle were administered into the same site 20 min later. Injection of ET-1 was found to produce a transient and massive increase in glutamate release that was potently antagonized by BoNT/E. To assess whether blocking transmitter release translates into neuroprotection, the extent of the ischaemic damage was determined 24 h and 6 weeks after the insult. We found that BoNT/E administration consistently reduced the loss of CA1 pyramidal neurons at 24 h. The neuroprotective effect of BoNT/E, however, was no longer significant at 6 weeks. These data provide evidence that blockade of synaptic transmitter release delays neuronal cell death following focal brain ischaemia, and underline the importance of assessing long-term neuroprotection in experimental stroke studies.

Intrahippocampal infusion of botulinum neurotoxin E (BoNT/E) reduces spontaneous recurrent seizures in a mouse model of mesial temporal lobe epilepsy

Mesial temporal lobe epilepsy (MTLE) is one of the most common forms of human epilepsy, and it is often resistant to conventional antiepileptic drug (AED) therapy. Here we tested whether a single intrahippocampal administration of the synaptic blocker botulinum neurotoxin E (BoNT/E) is effective in reducing spontaneous recurrent seizures (SRS) in a mouse model of MTLE. Unilateral intrahippocampal injection of kainic acid (KA) in mice was used as a model of MTLE. Electroencephalography (EEG) recordings of SRS were performed during the chronic phase of epilepsy, before and after administration of either BoNT/E or vehicle. Frequency of SRS was significantly decreased for at least 5 days following BoNT/E, but not vehicle, infusion. Our findings demonstrate that BoNT/E can effectively reduce seizure incidence in a mouse model of MTLE.

Increased susceptibility to kainic acid-induced seizures in Engrailed-2 knockout mice

The En2 gene, coding for the homeobox-containing transcription factor Engrailed-2 (EN2), has been associated to autism spectrum disorder (ASD). Due to neuroanatomical and behavioral abnormalities, which partly resemble those observed in ASD patients, En2 knockout (En2(-/-)) mice have been proposed as a model for ASD. In the mouse embryo, En2 is involved in the specification of midbrain/hindbrain regions, being predominantly expressed in the developing cerebellum and ventral midbrain, and its expression is maintained in these structures until adulthood. Here we show that in the adult mouse brain, En2 mRNA is expressed also in the hippocampus and cerebral cortex. Hippocampal En2 mRNA content decreased after seizures induced by kainic acid (KA). This suggests that En2 might also influence the functioning of forebrain areas during adulthood and in response to seizures. Indeed, a reduced expression of parvalbumin and somatostatin was detected in the hippocampus of En2(-/-) mice as compared to wild-type (WT) mice, indicating an altered GABAergic innervation of limbic circuits in En2(-/-) mice. In keeping with these results, En2(-/-) mice displayed an increased susceptibility to KA-induced seizures. KA (20 mg/kg) determined more severe and prolonged generalized seizures in En2(-/-) mice, when compared to WT animals. Seizures were accompanied by a widespread c-fos and c-jun mRNA induction in the brain of En2(-/-) but not WT mice. Long-term histopathological changes (CA1 cell loss, upregulation of neuropeptide Y) also occurred in the hippocampus of KA-treated En2(-/-) but not WT mice. These findings suggest that En2(-/-) mice might be used as a novel tool to study the link between epilepsy and ASD.

Long-distance retrograde effects of botulinum neurotoxin A

Botulinum neurotoxins (designated BoNT/A-BoNT/G) are bacterial enzymes that block neurotransmitter release by cleaving essential components of the vesicle fusion machinery. BoNT/A, which cleaves SNAP-25 (synaptosomal-associated protein of 25 kDa), is extensively exploited in clinical medicine to treat neuromuscular pathologies, facial wrinkles, and various types of pain. It is widely assumed that BoNT/A remains at the synaptic terminal and its effects are confined to the injection site. Here we demonstrate that catalytically active BoNT/A is retrogradely transported by central neurons and motoneurons and is then transcytosed to afferent synapses, in which it cleaves SNAP-25. SNAP-25 cleavage by BoNT/A was observed in the contralateral hemisphere after unilateral BoNT/A delivery to the hippocampus. Appearance of cleaved SNAP-25 resulted in blockade of hippocampal activity in the untreated hemisphere. Injections of BoNT/A into the optic tectum led to the appearance of BoNT/A-truncated SNAP-25 in synaptic terminals within the retina. Cleaved SNAP-25 also appeared in the facial nucleus after injection of the toxin into rat whisker muscles. Experiments excluded passive spread of the toxin and demonstrated axonal migration and neuronal transcytosis of BoNT/A. These findings reveal a novel pathway of BoNT/A trafficking in neurons and have important implications for the clinical uses of this neurotoxin.