Fast glutamatergic synaptic transmission is based on a precise and complex molecular organization which
requires the control of the number of AMPA-type glutamate receptors (AMPARs) at the postsynaptic sites
of glutamatergic synapses on dendritic spines. The number of AMPARs varies as a function of pre- and
postsynaptic activation history of the synapse. It is now well described that synapses can change their
number of AMPARs and therefore, their response properties through biochemical mechanisms of synaptic
plasticity. In this way, information is stored in the brain. The overall goal of this project is to use quantitative
models and experiments to answer two fundamental questions about the role of an abundant postsynaptic
protein, synGAP, in regulation of the numbers of AMPARs. Numerous experiments in intact neurons have
revealed that the level of synGAP expressed at synapses is inversely correlated with the amount of
AMPARs available at the synapses, and that synGAP helps to regulate changes in AMPAR numbers
during synaptic plasticity. The enzymatic GAP domain of synGAP acts as a ratchet to adjust the rates of
addition and removal of AMPARs from the surface of the dendrite. SynGAP also contains a sight that binds
tightly to the major scaffold protein PSD-95 via its three protein-binding PDZ domains. Important to the
mental health mission of the NIMH, SynGAP plays a critical role in learning and memory in the Brain and
mutation of SynGAP is implicated in cognitive disabilities. The project is divided into two broad Aims. In Aim
1, we will answer the question: What are the mechanisms by which synGAP controls the amount of AMPA
receptor in the postsynaptic density (PSD) - by control of surface amount and/or by control of availability of
PDZ domain binding sites in the synapse? We will improve our existing computational model of the
competition between synGAP and AMPARs for binding to PSD-95 by incorporating it into our model of
AMPAR trafficking. We will use genetics and sophisticated molecular engineering to experimentally
disentangle the two mechanisms. Effects on the nano-organization of AMPARs will be measured by super-
resolution fluorescence microscopy and electrophysiology. Results of these experiments will be used to
constrain our model of AMPAR trafficking. Aim 2, Through the synergy of experimental and computational
approaches, we will address the questions: How does the formation of the condensate between synGAP
and PSD-95, and the presence of additional PDZ domain-binding proteins (GluN2 receptor subunits,
neuroligin, nNOS, CRIPT, etc.) influence the nano-organization of AMPAR-TARPs in the PSD in the basal
state and during synaptic plasticity?
RELEVANCE (See instructions):
We propose a combination of computational and experimental work that will help clarify the role of synGAP
in regulation of AMPARs in CNS synapses, including its role in mental illness. The work will impact a
specific medical condition termed “SynGAP haploinsufficiency” or “MRD5”, in which SynGAP is mutated in
~1% of children with sporadic non-syndromic cognitive disability accompanied by autism and/or epilepsy.
The medical impacts of this work are potentially quite significant as it could help to point toward specific
molecular interventions with therapeutics that could improve the lives of patients with these afflictions.
