DNA methylation regulates neuronal glutamatergic synaptic scaling

Epigenetically mediated changes in postsynaptic glutamate receptor expression enable neurons to adjust responsiveness to their environment. DNA demethylation ramps up synaptic reception To prevent overstimulation, neurons decrease the abundance of postsynaptic receptors to reduce the strength of incoming excitatory currents. Conversely, in periods of low activity, neurons increase the abundance of receptors to increase receptiveness. This adjustment is called synaptic scaling and is critical for maintaining neuronal function. Using cultured primary rat cortical neurons, Meadows et al. found that synaptic upscaling—adjustments that increase excitatory responses—involved a decrease in DNA methylation that promoted the expression of genes encoding glutamate receptors and the proteins that deliver the receptors to the synaptic membrane. The findings have implications not only for understanding homeostatic regulation of neuronal activity but also for understanding such cognitive processes as learning and memory. Enhanced receptiveness at all synapses on a neuron that receive glutamatergic input is called cell-wide synaptic upscaling. We hypothesize that this type of synaptic plasticity may be critical for long-term memory storage within cortical circuits, a process that may also depend on epigenetic mechanisms, such as covalent chemical modification of DNA. We found that DNA cytosine demethylation mediates multiplicative synaptic upscaling of glutamatergic synaptic strength in cultured cortical neurons. Inhibiting neuronal activity with tetrodotoxin (TTX) decreased the cytosine methylation of and increased the expression of genes encoding glutamate receptors and trafficking proteins, in turn increasing the amplitude but not frequency of miniature excitatory postsynaptic currents (mEPSCs), indicating synaptic upscaling rather than increased spontaneous activity. Inhibiting DNA methyltransferase (DNMT) activity, either by using the small-molecule inhibitor RG108 or by knocking down Dnmt1 and Dnmt3a, induced synaptic upscaling to a similar magnitude as exposure to TTX. Moreover, upscaling induced by DNMT inhibition required transcription; the RNA polymerase inhibitor actinomycin D blocked upscaling induced by DNMT inhibition. Knocking down the cytosine demethylase TET1 also blocked the upscaling effects of RG108. DNMT inhibition induced a multiplicative increase in mEPSC amplitude, indicating that the alterations in glutamate receptor abundance occurred in a coordinated manner throughout a neuron and were not limited to individual active synapses. Our data suggest that DNA methylation status controls transcription-dependent regulation of glutamatergic synaptic homeostasis. Furthermore, covalent DNA modifications may contribute to synaptic plasticity events that underlie the formation and stabilization of memories.