Cellular Imaging May Reveal Mechanisms of Seizures in Children that Could Lead to Novel Interventions

Lay Summary

Cellular imaging may reveal mechanisms of seizures in children that could lead to novel interventions

This work uses an experimental animal model of a rare form of epilepsy that may lead to a broader understanding of the mechanisms of febrile seizures, the most common seizure type. Dravet syndrome is a form of epilepsy due to genetic mutation that is characterized by seizures triggered by elevated temperature (such as a fever, or warm environment), that begins during the first year of life, is resistant to current treatments, and has a high mortality rate. Infants go on to develop intractable epilepsy, with developmental delay and features of autism spectrum disorder. Finding an effective prevention or treatment has proved elusive, even though scientists have discovered that Dravet syndrome involves a genetic mutation that affects neurons that use the inhibitory neurotransmitter called GABA.

GABA normally inhibits excessive excitation of brain cells. The genetic malfunction, the investigators hypothesize, leads to the failure of brain networks (circuits) during temperature elevation due to temperature-dependent failure of these GABA neurons. Understanding the dynamics of brain circuits requires simultaneously monitoring hundreds of neurons. The investigators will do so by using 2-photon calcium microscopy in awake-behaving mice in an animal model of Dravet syndrome that develop heat-sensitive seizures.

Using this imaging, they will see hundreds of neurons in the circuit simultaneously including these GABA cells during temperature-induced seizures. They expect to be able to define the cellular architecture of seizures and identify cellular and circuit mechanisms that underlie the onset of temperature-dependent seizures. Then they will manipulate the circuitry to try to recover normal GABA network function to prevent seizures from being generated.

Abstract

Cellular imaging may reveal mechanisms of seizures in children that could lead to novel interventions

Epilepsy is a severe neurological disease defined by recurrent seizures and is hypothesized to involve imbalance between synaptic excitation and inhibition. The most severe form of epilepsy is the early infantile epileptic encephalopathies (EIEE), defined by treatment-resistant (“intractable”) epilepsy with onset in the first year of life, accompanied by developmental disability. The EIEEs are frequently genetic in etiology, secondary to mutation in genes critical for brain development and function. The most common epileptic encephalopathy is Dravet syndrome (DS), which in most cases is due to heterozygous loss of function mutation in the gene SCN1A encoding the type 1 voltage-gated sodium channel (Nav1.1). DS is characterized by normal development in early infancy, followed by the appearance of febrile seizures which are typically prolonged, progressing to afebrile seizures, and then to intractable epilepsy with multiple seizure types, developmental delay and features of autism spectrum disorder, and epilepsy-associated death from status epilepticus or sudden unexplained death in epilepsy (SUDEP). The mechanistic basis of epilepsy and intellectual disability in DS remains poorly understood, although it is hypothesized to involve dysfunction of cerebral cortical GABAergic inhibitory interneurons due to preferential reliance of these cells on Nav1.1. Despite identification of the genetic basis of DS, current treatments remain essentially palliative, and there is no cure.

This clinically-oriented basic neuroscience study uses in vivo two-photon (2P) microscopy of genetically encoded calcium indicators (2P calcium imaging) to investigate the basis of temperature-sensitive seizures in a validated preclinical experimental animal model of Dravet syndrome (Scn1a+/- mice) that recapitulates the core features of the human disease. Our hypothesis is that the prominent temperature sensitivity that characterizes epilepsy in Dravet syndrome is due to the specific temperature-dependent failure of a defined subset of inhibitory interneuron in the brain; increased temperature (such as due to fever, or increased ambient temperature) leads to preferential failure of inhibitory interneurons, decreased inhibition, and hyperexcitability, leading to seizure. We will then attempt to manipulate the activity of this cell type to reconstitute inhibition and prevent temperature-induced seizures. The ultimate goal of this work is to inform the development of novel treatments for Dravet syndrome targeted to the prevention of temperature-sensitive seizures that could be translated to the Clinic.

To test the above hypothesis, we have developed a novel imaging approach that facilitates in vivo 2P calcium imaging of temperature-induced seizures. 2P imaging allows for optical sectioning at depth including in the light scattering environment of the intact brain; calcium imaging provides an optical readout of neuronal activity. Combined with the use of cell type-specific Cre driver lines and Cre-dependent genetically-encoded calcium indicators, the technique of 2P calcium imaging allows for the in vivo monitoring of the activity of hundreds of neurons at relatively high temporal resolution while retaining single-cell spatial resolution and neuronal identity. We will image excitatory cells and prominent defined subsets of inhibitory interneurons during temperature-induced seizures in Scn1a+/- mice to define the cellular architecture of seizures and identify cellular and circuit mechanisms that underlie seizure onset; we will then manipulate dysfunctional circuit elements to recover normal network function to prevent seizure generation.

We further hypothesize that results will have more far-reaching implications, including for febrile seizures not associated with Dravet syndrome. Febrile seizures occur in otherwise neurologically normal children and are the most common type of seizure across the lifespan, affecting 2-4% of all children. This study will advance the use of in vivo 2P calcium imaging for the study of seizures and epilepsy. The application of innovative imaging methods to address an important clinical question using a highly relevant experimental model of Dravet syndrome will yield critical insights into the pathophysiological basis of this devastating and untreatable pediatric condition.