- Pathogenic mechanisms underlying epilepsy mutations in Kv7 channels
Neuronal Kv7/KCNQ potassium channels are heterotetramers composed of Kv7.2/KCNQ2 and Kv7.3/KCNQ3 subunits (Brown and Passmore, 2009), which are found throughout the brain including the hippocampus and neocortex (Cooper et al., 2001; Pan et al., 2006). Highly concentrated at the axonal initial segments (Pan et al., 2006) and preferentially enriched at the axonal plasma membrane compared to somatodenritic surface (Chung et al., 2006), these channels give rise to slowly activating and non-inactivating voltage-dependent outward potassium currents which potently inhibit repetitive and burst firing of action potentials (Brown and Passmore, 2009).
Physiological significance of these channels are underscored by the fact that dominant mutations in their subunits cause early-onset epilepsy including benign familial neonatal epilepsy (BFNE) and epileptic encephalopathy (EE) (Maljevic et al., 2010; Weckhuysen et al., 2012; Millichap et al., 2016). In contrast to transient appearance of seizures in neonates with BFNE (Psenka and Holden, 1996), EE is characterized by drug resistant seizures, psychomotor retardation, developmental delay, and neuroradiological abnormalities including white matter reduction and enlarged ventricles (Saitsu et al., 2012; Weckhuysen et al., 2012; Millichap et al., 2016). Since the mapping of the first BFNE variants to Kcnq2 gene in 1998 (Biervert et al., 1998; Charlier et al., 1998; Singh et al., 1998), > 200 mutations in Kcnq2 and Kcnq3 genes have been associated with epilepsy. However, how these epilepsy mutations of Kcnq2 and Kcnq3 ultimately lead to epilepsy with variable clinical severity remain largely elusive.
One major research goal in Chung lab is therefore to determine the pathologic mechanisms underlying epilepsy mutations of Kcnq2 and Kcnq3. Specifically, we ask the following questions:
- Do epilepsy mutations affect voltage-dependent activation and axonal targeting of Kv7 channels and disrupt their ability to inhibit neuronal excitability?
- How does the disruption of Kv7 channel function and expression ultimately lead to epilepsy?
- Why certain mutations result in BFNE but other mutations lead to EE?
Chung lab employs interdisciplinary approaches to answer these questions. These methods include electrophysiology, imaging, biochemistry, neuronal culture, and conventional and conditional knock-out and knock-in mice.
- Identification of molecular mechanisms underlying homeostatic intrinsic plasticity
Electrical properties of neuronal membranes can undergo persistent modification in response to neuronal activity or sensory experience. Homeostatic plasticity is one such mechanism by which neurons adapt their electrical activity within a physiologic range based on their previous activity. For example, a 2-day blockade of neuronal activity in the hippocampus leads to compensatory increase in neuronal communication at synapses (termed synaptic transmission) as well as the ability of hippocampal neurons to fire action potentials at a given input signal (termed intrinsic excitability). Interestingly, activity blockade in the hippocampus for 2-4 weeks leads to temporal lobe epilepsy in rodents.
These findings raise a compelling question: “when and how do neurons exploit homeostatic plasticity to stabilize their network as a normal adaptive response, or to cause persistent neuronal hyperexcitability as a pathological manifestation in epilepsy?” To answer this question, we must first understand how homeostatic plasticity is induced in the normal brain. Whereas much has been learned about homeostatic regulation of synaptic transmission (homeostatic synaptic plasticity), our knowledge of homeostatic regulation of intrinsic excitability (homeostatic intrinsic plasticity) remains largely unknown.
Chung lab has identified the signaling pathways underlying homeostatic control of intrinsic excitability in cultured hippocampal neurons, which are distinct from homeostatic synaptic scaling (Lee and Chung, 2014; Lee et al., 2015). Using unbiased gene expression profiling, we have identified 873 genes whose protein products could potentially mediate homeostatic plasticity which includes multiple potassium channel genes (Lee et al., 2015).
Chung lab is currently exploring the upstream signaling pathways that regulate multiple potassium channel genes as well as downstream effects of their transcript levels on neuronal excitability. In addition, Chung lab is actively developing novel transgenic mouse lines in which activity blockade can be induced in hippocampal neurons without involving invasive surgical methodologies. These mice will be useful in investigating the potentially link between homeostatic plasticity and epileptogenesis.
- Identification of novel players in neuronal hyperexcitability diseases including epilepsy and Alzheimer’s disease.
Chung lab is interested in delineating the mechanisms underlying the genesis of hyperexcitability in temporal lobe epilepsy (TLE) and in early stage of Alzheimer’s disease (AD). Recently, we identified that kainite-induced status epilepticus is associated with caspase-dependent cleavage and down-regulation of G-protein coupled inwardly rectifying potassium channels (Baculis et al., 2017). Currently, we are focusing on the role of STriatal-Enriched protein tyrosine Phosphatase (STEP61) in the development of TLE.
STEP61 weakens excitatory synapses by dephosphorylating and inactivating its substrates including synaptic glutamate receptors and kinases implicated in regulating synaptic transmission. However, its role in homeostatic plasticity and epilepsy remains unknown. We discovered that STEP61 mediates homeostatic plasticity of excitatory synaptic strength by modulating tyrosine phosphorylation of AMPA and NMDA receptors (Jang et al., 2015; Jang et al., 2016). We reported that a single acute episode of electroconvulsive seizures (ECS) increases expression of STEP61, amyloid-β (Aβ), and its precursor protein and decreases tyrosine phosphorylation of NMDA receptors (Jang et al., 2016), suggesting that elevated expression of STEP61 and Aβ may contribute to compensatory weakening of synaptic strength in response to seizures.
Given that the development of drug-resistant seizures in TLE is associated with sclerosis at or close to the seizure foci, we are currently investigating if progressive hippocampal sclerosis and encephalopathy of TLE is due to pathologic accumulation of Aβ and STEP61. These studies will provide insights into pathogenesis of TLE and AD, and provide STEP61 as a therapeutic target for TLE and AD.