Effects of HCN2 Mutations on Dendritic Excitability and Synaptic Plasticity: A Computational Study

Several reports of augmented hyperpolarisation-activated cyclic nucleotide-gated (HCN) currents in seizures have suggested a pro-convulsive identity for HCN channels. The mutations identified in one or more of the four HCN channel subunits are found to be contributing to different epileptic phenotypes. S126L, S632W, V246M and E515K are four different mutations affecting the HCN2 subunit and have been reported in febrile seizures and partial/generalised idiopathic epilepsies. From the visible outcomes in subjects with these mutations, it is evident that they must play important roles in altering dendritic excitability. Through this simulation study using NEURON, we created a three-compartmental, hippocampal CA1 pyramidal neuron synapse model expressing seven different ion channels (fast sodium (NaF), T-type calcium (CaT), R-type calcium (CaR), delayed rectifier potassium (KDR), A-type potassium (KA), small conductance potassium (SK), and HCN channels) and two glutamate receptors (AMPAR and NMDAR). We modelled an HCN2 channel and incorporated changes in it to obtain mutation kinetics. Their effects on excitability were studied by observing resting membrane potentials, input resistances and plasticity profiles for measuring the sliding modification threshold (SMT) of Bienenstock-Cooper-Munro (BCM) theory. Virtual knockouts of ion channels other than HCN were also performed to assess their role in altering excitability when they act alongside HCN2 mutations. Our results show that HCN2 mutations can potentially be a primary causative factor for excessive action potential firing through their effect on resting membrane potentials and input resistance.