Transient use of a systemic adenosine kinase inhibitor attenuates epilepsy development in mice.

OBJECTIVE: Over one-third of all patients with epilepsy are refractory to treatment and there is an urgent need to develop new drugs that can prevent the development and progression of epilepsy. Epileptogenesis is characterized by distinct histopathologic and biochemical changes, which include astrogliosis and increased expression of the adenosine-metabolizing enzyme adenosine kinase (ADK; EC 2.7.1.20). Increased expression of ADK contributes to epileptogenesis and is therefore a target for therapeutic intervention. We tested the prediction that the transient use of an ADK inhibitor administered during the latent phase of epileptogenesis can mitigate the development of epilepsy. METHODS: We used the intrahippocampal kainic acid (KA) mouse model of temporal lobe epilepsy, which is characterized by ipsilateral hippocampal sclerosis with granule cell dispersion and the development of recurrent hippocampal paroxysmal discharges (HPDs). KA-injected mice were treated with the ADK inhibitor 5-iodotubercidin (5-ITU, 1.6 mg/kg, b.i.d., i.p.) during the latent phase of epileptogenesis from day 3-8 after injury; the period when gradual increases in hippocampal ADK expression begin to manifest. HPDs were assessed at 6 and 9 weeks after KA administration followed by epilepsy histopathology including assessment of granule cell dispersion, astrogliosis, and ADK expression. RESULTS: 5-ITU significantly reduced the percent time in seizures by at least 80% in 56% of mice at 6 weeks post-KA. This reduction in seizure activity was maintained in 40% of 5-ITU-treated mice at 9 weeks. 5-ITU also suppressed granule cell dispersion and prevented maladaptive ADK increases in these protected mice. SIGNIFICANCE: Our results show that the transient use of a small-molecule ADK inhibitor, given during the early stages of epileptogenesis, has antiepileptogenic disease-modifying properties, which provides the rationale for further investigation into the development of a novel class of antiepileptogenic ADK inhibitors with increased efficacy for epilepsy prevention.

Adenosine Kinase Deficiency in the Brain Results in Maladaptive Synaptic Plasticity.

Adenosine kinase (ADK) deficiency in human patients (OMIM:614300) disrupts the methionine cycle and triggers hypermethioninemia, hepatic encephalopathy, cognitive impairment, and seizures. To identify whether this neurological phenotype is intrinsically based on ADK deficiency in the brain or if it is secondary to liver dysfunction, we generated a mouse model with a brain-wide deletion of ADK by introducing a Nestin-Cre transgene into a line of conditional ADK deficient Adkfl/fl mice. These AdkΔbrain mice developed a progressive stress-induced seizure phenotype associated with spontaneous convulsive seizures and profound deficits in hippocampus-dependent learning and memory. Pharmacological, biochemical, and electrophysiological studies suggest enhanced adenosine levels around synapses resulting in an enhanced adenosine A1 receptor (A1R)-dependent protective tone despite lower expression levels of the receptor. Theta-burst-induced LTP was enhanced in the mutants and this was dependent on adenosine A2A receptor (A2AR) and tropomyosin-related kinase B signaling, suggesting increased activation of these receptors in synaptic plasticity phenomena. Accordingly, reducing adenosine A2A receptor activity in AdkΔbrain mice restored normal associative learning and contextual memory and attenuated seizure risk. We conclude that ADK deficiency in the brain triggers neuronal adaptation processes that lead to dysregulated synaptic plasticity, cognitive deficits, and increased seizure risk. Therefore, ADK mutations have an intrinsic effect on brain physiology and may present a genetic risk factor for the development of seizures and learning impairments. Furthermore, our data show that blocking A2AR activity therapeutically can attenuate neurological symptoms in ADK deficiency. SIGNIFICANCE STATEMENT: A novel human genetic condition (OMIM #614300) that is based on mutations in the adenosine kinase (Adk) gene has been discovered recently. Affected patients develop hepatic encephalopathy, seizures, and severe cognitive impairment. To model and understand the neurological phenotype of the human mutation, we generated a new conditional knock-out mouse with a brain-specific deletion of Adk (AdkΔbrain). Similar to ADK-deficient patients, AdkΔbrain mice develop seizures and cognitive deficits. We identified increased basal synaptic transmission and enhanced adenosine A2A receptor (A2AR)-dependent synaptic plasticity as the underlying mechanisms that govern these phenotypes. Our data show that neurological phenotypes in ADK-deficient patients are intrinsic to ADK deficiency in the brain and that blocking A2AR activity therapeutically can attenuate neurological symptoms in ADK deficiency.