The role of Methyl-CpG binding domain 3 (Mbd3) in epileptogenesis

Methyl CpG binding domain 3 (Mbd3) protein belongs to the MBD family of proteins, responsible for reading the DNA methylation pattern. MBD family proteins bind the methyl-CpG domain and are also involved in heterochromatin formation. Mbd3 protein does not have the ability to selectively recognize methyl-CpG islands, however, its characteristic feature is the ability to bind to 5-hydroxymethylcytosine and unmethylated DNA. Little is known about the role of Mbd3 in epilepsy and epileptogenesis. Our previous study (Bednarczyk et al., 2016) showed an increase in levels of NuRD complex proteins, including Mbd3 protein, in the brain of epileptic animals in a rat model of temporal epilepsy induced by electrical stimulation of the amygdala. Amygdala stimulation induced the binding of NuRD complex containing MBD3 to larger number of regions of DNA. In the present study, we investigated whether the Mbd3 protein is involved in the determination of the seizure threshold. An increase in Mbd3 protein levels was demonstrated in the entorhinal cortex/amygdala in the rat's brain 4 hours after pentylenetetrazole (PTZ)-induced seizures. No alterations in MBD3 protein levels were detected in hippocampus and somatosensory cortex. No alterations in MBD3 mRNA expression were observed at any time point in any studied brain area. Reduction of Mbd3 level using AAV vector coding shRNA against Mbd3 injected to the amygdala prolonged the latency time to the onset of an acute seizure in PTZ challenge test indicating increase in seizure threshold. This was accompanied by increased anxiety in the open field test. In the contrast, an overexpression of Mbd3 using AAV decreased anxiety and increased their excitability in the open field test. Moreover Mbd3 overexpression in the amygdala accelerated epileptogenesis in the PTZ-kindling model. In order to identify the role of Mbd3 protein in the regulation of gene expression, mRNA profiling with RNA-seq was performed in a model of magnesium deficiency-induced epileptiform discharges in vitro. Mbd3 overexpression in vitro induced changes in gene expression in a time- and state-specific manner. Our data indicate the pro-epileptic properties of the Mbd3 protein in vivo and in vitro. Overall design: RNAseq samples from neuronal cell culture: control without transfection, AAV transfected cells : AAV-SYN-GFP as a control AAV; AAV-SYN-rMBD3-GFP for Mbd3 overexpression, incubated in standard (medium with Magnesium) and epileptiform (medium without Magnesium) condition, tested in selected time points: 2h, 6h and 24h after induction of epileptiform activity