Molecular mechanisms of homeostatic synaptic plasticity

Project Summary Neurons are able to restore their activity when challenged by external or internal perturbations. This form of homeostatic response is crucial for the maintenance of neuronal or network stability during development and normal brain function. During homeostatic synaptic plasticity (HSP), chronic suppression of neuronal activity leads to a compensatory increase in synaptically localized AMPA receptors (AMPARs) and the intensity of synaptic currents. Under basal conditions, AMPARs exist as GluA2-containing heterotetramers, mostly in the form of GluA1/GluA2 or GluA2/GluA3. The GluA2-containing AMPARs are permeable to sodium but impermeable to calcium. The resistance to calcium derives from a unique posttranscriptional modification of GluA2 mRNA. Editing of adenosine to inosine by the adenosine deaminase enzyme ADAR2 switches a glutamine (Q) to arginine (R) in GluA2, which confers calcium impermeability in AMPARs. Thus, inefficient editing of GluA2 mRNA, or a lack of incorporation of GluA2 in an AMPAR channel complex, will lead to calcium-permeable AMPARs (Cp-AMPARs). While the role for the Cp-AMPAR has been well recognized as a key signaling molecule in HSP, the molecular nature and mechanisms regarding the biogenesis of the Cp- AMPAR in HSP remains largely unknown. We hypothesize that during HSP, neuronal inactivity triggers cellular responses in the location and activity of the ADAR2 enzyme, leading to unedited GluA2 and the formation of GluA2(Q)-containing Cp-AMPARs. Also, the molecular substrates of the Cp-AMPAR calcium cascade that are involved in homeostatic responses remain unknown. We hypothesize that Cp-AMPAR activity leads to GluA1 acetylation which enables AMPAR synaptic accumulation, leading to the expression of homeostatic plasticity. In this proposal, we aim to examine the molecular details underlying the regulation of GluA2 editing for the biogenesis of Cp-AMPARs, as well as the subsequent modification of AMPARs by acetylation and their contribution to the expression of homeostatic synaptic plasticity in vitro in primary neurons and in vivo in the mouse visual cortex.