Enrico Ferrea

Large-scale, high-resolution electrophysiological imaging of field potentials in brain slices with microelectronic multielectrode arrays

Multielectrode arrays (MEAs) are extensively used for electrophysiological studies on brain slices, but the spatial resolution and field of recording of conventional arrays are limited by the low number of electrodes available. Here, we present a large-scale array recording simultaneously from 4096 electrodes used to study propagating spontaneous and evoked network activity in acute murine cortico-hippocampal brain slices at unprecedented spatial and temporal resolution. We demonstrate that multiple chemically induced epileptiform episodes in the mouse cortex and hippocampus can be classified according to their spatio-temporal dynamics. Additionally, the large-scale and high-density features of our recording system enable the topological localization and quantification of the effects of antiepileptic drugs in local neuronal microcircuits, based on the distinct field potential propagation patterns. This novel high-resolution approach paves the way to detailed electrophysiological studies in brain circuits spanning spatial scales from single neurons up to the entire slice network.

REST/NRSF-mediated intrinsic homeostasis protects neuronal networks from hyperexcitability

Intrinsic homeostasis enables neuronal circuits to maintain activity levels within an appropriate range by modulating neuronal voltage‐gated conductances, but the signalling pathways involved in this process are largely unknown. We characterized the process of intrinsic homeostasis induced by sustained electrical activity in cultured hippocampal neurons based on the activation of the Repressor Element‐1 Silencing Transcription Factor/Neuron‐Restrictive Silencer Factor (REST/NRSF). We showed that 4‐aminopyridine‐induced hyperactivity enhances the expression of REST/NRSF, which in turn, reduces the expression of voltage‐gated Na+ channels, thereby decreasing the neuronal Na+ current density. This mechanism plays an important role in the downregulation of the firing activity at the single‐cell level, re‐establishing a physiological spiking activity in the entire neuronal network. Conversely, interfering with REST/NRSF expression impaired this homeostatic response. Our results identify REST/NRSF as a critical factor linking neuronal activity to the activation of intrinsic homeostasis and restoring a physiological level of activity in the entire neuronal network.

Epileptogenic Q555X SYN1 mutant triggers imbalances in release dynamics and short-term plasticity

Synapsin I (SynI) is a synaptic vesicle (SV) phosphoprotein playing multiple roles in synaptic transmission and plasticity by differentially affecting crucial steps of SV trafficking in excitatory and inhibitory synapses. SynI knockout (KO) mice are epileptic, and nonsense and missense mutations in the human SYN1 gene have a causal role in idiopathic epilepsy and autism. To get insights into the mechanisms of epileptogenesis linked to SYN1 mutations, we analyzed the effects of the recently identified Q555X mutation on neurotransmitter release dynamics and short-term plasticity (STP) in excitatory and inhibitory synapses. We used patch-clamp electrophysiology coupled to electron microscopy and multi-electrode arrays to dissect synaptic transmission of primary SynI KO hippocampal neurons in which the human wild-type and mutant SynI were expressed by lentiviral transduction. A parallel decrease in the SV readily releasable pool in inhibitory synapses and in the release probability in excitatory synapses caused a marked reduction in the evoked synchronous release. This effect was accompanied by an increase in asynchronous release that was much more intense in excitatory synapses and associated with an increased total charge transfer. Q555X-hSynI induced larger facilitation and post-tetanic potentiation in excitatory synapses and stronger depression after long trains in inhibitory synapses. These changes were associated with higher network excitability and firing/bursting activity. Our data indicate that imbalances in STP and release dynamics of inhibitory and excitatory synapses trigger network hyperexcitability potentially leading to epilepsy/autism manifestations.

Long-term optical stimulation of channelrhodopsin-expressing neurons to study network plasticity

Neuronal plasticity produces changes in excitability, synaptic transmission, and network architecture in response to external stimuli. Network adaptation to environmental conditions takes place in time scales ranging from few seconds to days, and modulates the entire network dynamics. To study the network response to defined long-term experimental protocols, we setup a system that combines optical and electrophysiological tools embedded in a cell incubator. Primary hippocampal neurons transduced with lentiviruses expressing channelrhodopsin-2/H134R were subjected to various photostimulation protocols in a time window in the order of days. To monitor the effects of light-induced gating of network activity, stimulated transduced neurons were simultaneously recorded using multi-electrode arrays (MEAs). The developed experimental model allows discerning short-term, long-lasting, and adaptive plasticity responses of the same neuronal network to distinct stimulation frequencies applied over different temporal windows.

Asynchronous GABA Release Is a Key Determinant of Tonic Inhibition and Controls Neuronal Excitability: A Study in the Synapsin II−/− Mouse

Idiopathic epilepsies have frequently been linked to mutations in voltage-gated channels (channelopathies); recently, mutations in several genes encoding presynaptic proteins have been shown to cause epilepsy in humans and mice, indicating that epilepsy can also be considered a synaptopathy. However, the functional mechanisms by which presynaptic dysfunctions lead to hyperexcitability and seizures are not well understood. We show that deletion of synapsin II (Syn II), a presynaptic protein contributing to epilepsy predisposition in humans, leads to a loss of tonic inhibition in mouse hippocampal slices due to a dramatic decrease in presynaptic asynchronous GABA release. We also show that the asynchronous GABA release reduces postsynaptic cell firing, and the parallel impairment of asynchronous GABA release and tonic inhibition results in an increased excitability at both single-neuron and network levels. Restoring tonic inhibition with THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol; gaboxadol), a selective agonist of δ subunit-containing GABAA receptors, fully rescues the SynII−/− epileptic phenotype both ex vivo and in vivo. The results demonstrate a causal relationship between the dynamics of GABA release and the generation of tonic inhibition, and identify a novel mechanism of epileptogenesis generated by dysfunctions in the dynamics of release that can be effectively targeted by novel antiepileptic strategies.

Functional Interaction between the Scaffold Protein Kidins220/ ARMS and Neuronal Voltage-Gated Na+ Channels

Background: Tight regulation of ion channel activity is essential for neuronal function. Results: The scaffold protein Kidins220 associates with brain voltage-gated sodium channels and modulates their activity. Conclusion: Lack of Kidins220 in mice causes deregulated sodium channel function in inhibitory neurons, ultimately leading to impaired network excitability. Significance: Kidins220 may help maintain the balance between excitation and inhibition in neural networks. Kidins220 (kinase D-interacting substrate of 220 kDa)/ankyrin repeat-rich membrane spanning (ARMS) acts as a signaling platform at the plasma membrane and is implicated in a multitude of neuronal functions, including the control of neuronal activity. Here, we used the Kidins220−/− mouse model to study the effects of Kidins220 ablation on neuronal excitability. Multielectrode array recordings showed reduced evoked spiking activity in Kidins220−/− hippocampal networks, which was compatible with the increased excitability of GABAergic neurons determined by current-clamp recordings. Spike waveform analysis further indicated an increased sodium conductance in this neuronal subpopulation. Kidins220 association with brain voltage-gated sodium channels was shown by co-immunoprecipitation experiments and Na+ current recordings in transfected HEK293 cells, which revealed dramatic alterations of kinetics and voltage dependence. Finally, an in silico interneuronal model incorporating the Kidins220-induced Na+ current alterations reproduced the firing phenotype observed in Kidins220−/− neurons. These results identify Kidins220 as a novel modulator of Nav channel activity, broadening our understanding of the molecular mechanisms regulating network excitability.

Dentate gyrus network dysfunctions precede the symptomatic phase in a genetic mouse model of seizures

Neuronal circuit disturbances that lead to hyperexcitability in the cortico-hippocampal network are one of the landmarks of temporal lobe epilepsy. The dentate gyrus (DG) network plays an important role in regulating the excitability of the entire hippocampus by filtering and integrating information received via the perforant path. Here, we investigated possible epileptogenic abnormalities in the function of the DG neuronal network in the Synapsin II (Syn II) knockout mouse (Syn II−/−), a genetic mouse model of epilepsy. Syn II is a presynaptic protein whose deletion in mice reproducibly leads to generalized seizures starting at the age of 2 months. We made use of a high-resolution microelectrode array (4096 electrodes) and patch-clamp recordings, and found that in acute hippocampal slices of young pre-symptomatic (3–6 week-old) Syn II−/− mice excitatory synaptic output of the mossy fibers is reduced. Moreover, we showed that the main excitatory neurons present in the polymorphic layer of the DG, hilar mossy cells, display a reduced excitability. We also provide evidence of a predominantly inhibitory regulatory output from mossy cells to granule cells, through feed-forward inhibition, and show that the excitatory-inhibitory ratio is increased in both pre-symptomatic and symptomatic Syn II−/− mice. These results support the key role of the hilar mossy neurons in maintaining the normal excitability of the hippocampal network and show that the late epileptic phenotype of the Syn II−/− mice is preceded by neuronal circuitry dysfunctions. Our data provide new insights into the mechanisms of epileptogenesis in the Syn II−/− mice and open the possibility for early diagnosis and therapeutic interventions.

Cell adhesion molecule L1 contributes to neuronal excitability regulating the function of voltage-gated Na+ channels

ABSTRACT L1 (also known as L1CAM) is a trans-membrane glycoprotein mediating neuron–neuron adhesion through homophilic and heterophilic interactions. Although experimental evidence has implicated L1 in axonal outgrowth, fasciculation and pathfinding, its contribution to voltage-gated Na+ channel function and membrane excitability has remained unknown. Here, we show that firing rate, single cell spiking frequency and Na+ current density are all reduced in hippocampal excitatory neurons from L1-deficient mice both in culture and in slices owing to an overall reduced membrane expression of Na+ channels. Remarkably, normal firing activity was restored when L1 was reintroduced into L1-deficient excitatory neurons, indicating that abnormal firing patterns are not related to developmental abnormalities, but are a direct consequence of L1 deletion. Moreover, L1 deficiency leads to impairment of action potential initiation, most likely due to the loss of the interaction of L1 with ankyrin G that produces the delocalization of Na+ channels at the axonal initial segment. We conclude that L1 contributes to functional expression and localization of Na+ channels to the neuronal plasma membrane, ensuring correct initiation of action potential and normal firing activity. Highlighted Article: We uncovered a new role of L1CAM in the functional expression and localization of Na+ channels to the plasma membrane, ensuring correct initiation of action potential and normal firing activity.