**1. Introduction**

Seizures are a common manifestation of a host of neurological disorders, including epilepsy. A ffecting an estimated 1% of the US population [1], these events are characterized by the anomalous synchronization of electrical activity in the brain due to the hyperexcitability of individual neurons and neural networks. Although relatively indiscriminate in their anatomical localization, seizures often occur in the hippocampus and surrounding cortical areas of the temporal lobe. Recurrent seizures in this brain region underlie temporal lobe epilepsy (TLE), which is thought to be the most common epilepsy syndrome in adults [2]. In addition to being a prevalent form of epilepsy, TLE is frequently the most di fficult to treat [3]. These seizures are often resistant to antiepileptic drugs, which makes the a ffected region susceptible to ongoing, intractable epilepsy [4–6]. For up to one-third of patients, surgical intervention is encouraged for mitigation [1,7,8]. As a result, an emphasis on the identification of additional mechanisms underlying TLE, which can serve as potential targets for therapy, is requisite.

While the etiology of seizures underlying TLE is multifaceted, a common feature is the dysfunction of voltage-gated ion channels in hippocampal pyramidal neurons. A host of channelopathies—both inherited and acquired—are associated with TLE in this principal cell type,

including in hyperpolarization-activated cyclic nucleotide-gated (HCN) channels [9–13], voltage-gated Na<sup>+</sup> channels (Nav) [14,15], and voltage-gated Ca2+ channels (Cav) [16,17]. Featured most prominently in TLE pathologies are abnormalities in the function of voltage-gate K<sup>+</sup> channels (Kv), which are critical regulators of the intrinsic excitability of neurons throughout the brain (reviewed in [18,19]). Although members of all classes of K<sup>+</sup> channels have been shown to be altered in various epilepsy syndromes [19], there has been a steady increase in findings linking A-type K<sup>+</sup> channels (Shal subfamily) to TLE. Chief among the members of this family is Kv4.2, which has been heavily implicated in TLE in both animal models [19] and humans [20–22]. Kv4.2 is the primary pore-forming Kv channel subunit underlying the rapidly activating and inactivating somatodendritic A-current (IA) in CA1 pyramidal neurons of the hippocampus [23–25]. Operating at subthreshold voltages, Kv4.2 regulates action potential (AP) repolarization and repetitive firing, dampens AP backpropagation into dendrites, and shapes synaptic potentials, thus acting as a powerful modulator of the input–output efficacy of pyramidal neurons [23,26,27]. Although mutations in the Kv4.2 gene that impart defects intrinsic to channel function are associated with TLE [20–23], it is also evident that disruptions/modifications in Kv4.2 channel properties occur *in response* to seizures, suggesting that regulation of these channels likely contributes to the intractability of TLE. Indeed, TLE has been shown to decrease Kv4.2 availability [28,29], however, the molecular mechanisms underlying this activity-induced downregulation remain unclear.

Substantial evidence supports the notion that Kv4.2 channels function in macromolecular complexes with auxiliary subunits, including the K<sup>+</sup> channel-interacting proteins (KChIP1-4) and dipeptidyl peptidases 6 and 10 (DPP6 and DPP10) [30]. Both KChIPs and DPPs work together to exert the strong modification of Kv4.2 expression, membrane surface localization, and channel kinetics [31–34]. Evidence of increased seizure susceptibility is present in mice harboring mutations in these auxiliary subunits, including KChIP 2 [35], suggesting the maintenance of channel complexes is a key factor in moderating seizures. Likely modulators of Kv4.2 complex dynamics are protein kinases. The phosphorylation of Kv4.2 by protein kinase A (PKA), protein kinase C (PKC), and extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) downregulates IA [36–38]. In pyramidal neurons, this downregulation facilitates an increase in somatodendritic excitability, enhancing susceptibility to network hyperexcitability in the hippocampus [39–41]. We have recently identified a specific MAPK, p38<sup>α</sup>, as a potent regulator of the Kv4.2 complex [42]. The p38 phosphorylation of Kv4.2 C-terminal motifs triggers a molecular cascade that facilitates the dissociation of Kv4.2 from its auxiliary subunit DPP6 [42]. This cascade is particularly intriguing in the context of TLE, as it occurs in an activity-dependent fashion, representing a novel mechanism that may be integral in regulating seizure susceptibility [42].

In the present study, we expand on our previous findings and address how p38 kinase modulates seizure susceptibility and neuronal excitability. We use biochemical, electrophysiological, and in vivo seizure techniques in WT and a novel mouse model harboring a point mutation preventing p38 phosphorylation of Kv4.2 at C-terminal Thr607 (Kv4.2TA) to illuminate the role of p38 phosphorylation of Kv4.2 in regulating the intrinsic excitability of hippocampal pyramidal neurons and seizure intensity. We show that p38 phosphorylation of Kv4.2 at C-terminal Thr607 is integral in modulating seizure strength and may contribute to the progression of seizure intensity over time. Furthermore, we confirm previous findings that a molecular cascade triggered by p38 phosphorylation alters Kv4.2-mediated excitability of hippocampal pyramidal neurons, illuminating a novel molecular mechanism involved in network hyperexcitability in mice. The combined pharmacological and genetic manipulation of the cellular cascade described here offers insight into various avenues through which therapeutic intervention to curtail seizure progression can be pursued.
