Abstracts

Rationale: STEP is expressed throughout the brain. However, membrane-bound STEP₆₁ is the only isoform expressed in the hippocampus (HP) and cortex. High levels of STEP₆₁ have been detected in patients and animal models of Alzheimer’s disease and schizophrenia, which display HP hyperactivity. STEP₆₁ dephosphorylates and internalizes glutamate receptors, including NMDA and AMPA receptors. Genetically knocking out STEP enhances excitatory synaptic currents and long-term potentiation in the HP. However, whether STEP₆₁ affects seizure susceptibility is unclear. Since the role of STEP₆₁ in down-regulating excitatory synaptic transmission is well-established, we hypothesize that inhibiting STEP will increase seizure susceptibility. In this study, we investigate the effects of STEP inhibitor TC-2153 on seizure propensity in a murine model displaying kainic acid (KA) induced status epilepticus (SE), as well as its effect on HP network excitability and cultured neurons.

Methods: Male and female C57BL6/J mice (8-12 weeks old) were pre-treated for 3 h with TC-2153 and subjected to systemic KA injections, inducing SE that arise from the HP. Mice were injected with either vehicle control (2.8% DMSO dissolved in saline) or TC-2153 (10 mg/kg) and were given either saline or KA (Abcam, 30 mg/kg) before being monitored for behavioral seizures every 10 min. Ovaries were removed from female C57BL6/J mice by ovariectomy (OVX) at 5 wk old. After 7-10 d, OVX’ed females were treated with TC-2153 and 3 h later with KA. Acute HP slices from GCaMP6s mice were incubated with either DMSO or TC-2153 (10 μM) for 1 h as they recovered at room temp. Slices were incubated in ACSF and potassium chloride (KCl) (15 mM) for 2 min prior to imaging. Pyramidal neurons in dissociated rat HP culture (DIV 8-10) were pre-treated with DMSO or TC 2153 (10 uM) in coverslips for 60 min before recording.

Results: Contrary to our hypothesis, STEP inhibition with TC-2153 treatment significantly reduces KA-induced seizures, with greater effects seen in females. This TC-2153-induced decrease in seizure susceptibility was abolished by OVX, which results in systemic removal of sex-derived hormones. Consistently, TC-2153 application significantly decreases the overall network activity of acute HP slices in both sexes, as measured by genetic calcium indicator GCaMP6s. Surprisingly, TC-2153 treatment hyperpolarized resting membrane potential and markedly decreased action potential firing rate and sag voltage of HP pyramidal neurons.

Conclusions: Our findings demonstrate that TC-2153 decreases seizure susceptibility in a sex-dependent manner by reducing HP network activity. This study is the first to reveal STEP₆₁ as a potent regulator of HP intrinsic excitability. We propose that anti-seizure effect of TC-2153 is mediated by this unexpected function of STEP₆₁.

Funding: Please list any funding that was received in support of this abstract.: NIH RO1 Grant (NS083402) , NIH RO1 Grant (NS097610), Carle-UI Seed Grant, ICR Start-Up Fund from the University of Illinois at Urbana-Champaign, National Science Foundation Grant #1735252 (NRT-UtB).