Exploring the role of tau modulation in excitotoxicity and epileptogenesis

<p dir="ltr">Dravet syndrome (DS) is a severe, drug resistant epileptic encephalopathy of early infancy. DS emerges postnatally due to heterozygous, loss of function variants in <i>SCN1A</i>, encoding a sodium channel crucial to inhibitory interneuron function. In turn, hyperstimulation of excitatory neuronal networks (hyperexcitation) drives pro-epileptic mechanisms. Despite its extensive study, a full mechanistic picture of DS pathogenesis and progression is yet to be established, and as such, there are currently no effective treatments available. This investigation sought to better understand DS, and the role of microtubule-associated protein tau (a known mediator of hyperexcitation-induced neurotoxic mechanisms), in epilepsy progression. <i>In vivo </i>modelling of epilepsy was achieved by generation and characterisation of a mouse model of DS (by genetic depletion of <i>Scn1a</i>), and an acute seizure mouse model (by administration of a chemo-convulsant). In both cases, epilepsy-associated phenotypes were reliably recapitulated. Subsequently, a battery of phenotyping tests revealed that administration of an activity-enhancing form of the tau kinase, p38γ, was therapeutically beneficial in epilepsy. In DS mice, reduced behavioural deficits and electroencephalographic (EEG) abnormalities were observed, and prevention of seizure-induced mortality in both models when administered prior to epilepsy onset. This therapeutic benefit was lost upon inhibition of site-specific tau-p38γ engagement. Moreover, kinase treatment after chemically-induced seizure onset lessened mortality and normalised EEG profiles. The results demonstrate that modulation of tau both prior to, and following, seizure onset mitigates engagement of pathological neuronal processes triggering epilepsy and associated phenotypes, thus offering a therapeutic candidate for DS and beyond. Next, to transition rodent DS modelling into a clinically relevant system, a human <i>in vitro </i>forebrain organoid model was generated to study the prenatal dorso-ventral axis of the human telencephalon. Located in the developing ventral forebrain, the medial ganglionic eminence (MGE) supplies the largest source of cortical inhibitory interneurons, and as such gives rise to an abundance of SCN1A-expressing neuronal subtypes. Generation of ventral forebrain organoids modelling the MGE revealed early prenatal gene expression changes, indicative of fate commitment impairments, in organoids harbouring SCN1A loss of function. In turn, assembly of ventral and dorsal forebrain organoids (assembloids) revealed reduced capacity for subpallial-pallial migration of GABAergic progenitors, and impaired expression of MGE-derived interneurons. These striking early alterations posit novel disease-causing prenatal mechanisms driving DS. Future studies can harness this DS organoid platform for efficacy testing of p38γ during pre- and post- natal brain development. Furthermore, therapeutic targeting aimed at boosting early ventral forebrain fate commitment is a promising new avenue for preclinical research in DS. Overall, adoption of both <i>in vivo </i>and <i>in vitro </i>models described here will enhance our understanding of DS mechanisms, and its effective treatment.</p>