SynGAP regulates synaptic plasticity and cognition independently of its catalytic activity

SynGAP is an abundant synaptic GTPase-activating protein (GAP) critical for synaptic plasticity, learning, memory, and cognition. Mutations in SYNGAP1 in humans result in intellectual disability, autistic-like behaviors, and epilepsy. Heterozygous Syngap1-knockout mice display deficits in synaptic plasticity, learning, and memory and exhibit seizures. It is unclear whether SynGAP imparts structural properties at synapses independently of its GAP activity. Here, we report that inactivating mutations within the GAP domain do not inhibit synaptic plasticity or cause behavioral deficits. Instead, SynGAP modulates synaptic strength by physically competing with the AMPA-receptor-TARP excitatory receptor complex in the formation of molecular condensates with synaptic scaffolding proteins. These results have major implications for developing therapeutic treatments for SYNGAP1-related neurodevelopmental disorders. Editor’s summary Synaptic plasticity is the critical mechanism supporting learning, memory, and many other neurophysiological processes during brain development and in adulthood. The GTPase-activating protein SynGAP has been shown to be necessary for synaptic plasticity, and mutations have been associated with autism and other intellectual disabilities. Araki et al. found that the GAP activity of SynGAP is not required for synaptic plasticity (see the Perspective by Choquet). Instead, the protein modulates synaptic plasticity by competing with the AMPA receptor–TARP complex at excitatory synapses, influencing the formation of molecular condensates and ultimately regulating the recruitment of AMPA receptors during plasticity. These results will help in the development of treatments for SynGAP-mediated neurological disorders. —Mattia Maroso An abundant synaptic GTPase-activating protein regulates synaptic strength via phase separation. INTRODUCTION Experience-dependent changes in the strength of synaptic connections in the brain are essential for neuronal development and for brain processes such as learning and memory. Long-term potentiation (LTP) of synapses is a key form of synaptic plasticity that is widely recognized as a cellular model for the study of memory. Many forms of synaptic plasticity, including LTP, are mediated by long-lasting changes in the level of AMPA receptors (AMPARs), the major neurotransmitter receptors at excitatory synapses. Excitatory synapses contain a complex structure called the postsynaptic density (PSD), which includes hundreds of proteins that orchestrate synaptic structure and function and dynamic changes during synaptic plasticity. One of these is SynGAP, a RasGAP that binds to the major synaptic scaffolding protein PSD95 and is highly abundant in the PSD in excitatory synapses. SynGAP is essential for normal brain development and for LTP. During LTP induction, SynGAP is phosphorylated, decreasing its affinity for PSD95, resulting in its dispersion from the synapse. This disinhibits Ras activity and activates its downstream signaling processes, which were thought to be critical for synaptic potentiation. Heterozygote Syngap1-knockout mice have deficits in synaptic plasticity, learning, and memory and exhibit seizures. De novo damaging SYNGAP1 mutations in humans result in haploinsufficiency and cause SYNGAP1-related intellectual disability, characterized by intellectual disability, autistic-like features, and epilepsy. RATIONALE SynGAP is one of the most abundant proteins at excitatory synapses, suggesting that it may play a structural role in the PSD in addition to its role in regulating Ras activity. SynGAP was recently found to have unique structural properties and to undergo liquid-liquid phase separation (LLPS) with PSD95. Dispersion of SynGAP from the synapse during LTP induction would be predicted to free up PSD95-binding sites, allowing other PSD95-binding proteins to dynamically change the composition of the synapse. To differentiate the role of GAP activity from its structural properties, we examined the function of SynGAP with mutations that inactivate GAP activity in vitro in neuronal cultures and in vivo using knock-in mice containing inactivating GAP mutations. RESULTS We knocked down endogenous SynGAP in neuronal cultures and replaced it with wild-type and GAP mutant SynGAP and found that mutation of the GAP domain did not affect its ability to rescue LTP in neuronal cultures in vitro. We confirmed this in vivo using mice containing inactivating GAP mutations. These mice show normal viability, LTP, and behaviors that are deficient in the heterozygote Syngap1-knockout mice. We investigated how the structural properties of SynGAP could regulate AMPAR recruitment to synapses and mediate synaptic potentiation. Recent studies have shown that Transmembrane AMPAR Regulatory Proteins (TARPs), essential components of the AMPAR protein complex, also undergo LLPS with PSD95. A simple hypothesis was that SynGAP directly competes with the TARP-AMPAR complex, and when SynGAP is dispersed from the synapse, tthis complex could replace it and be recruited to the synapse. We tested whether SynGAP competed with TARPs in forming LLPS with PSD95 in vitro using purified proteins, heterologous cells, and neurons. We found that SynGAP directly competed with TARPs in forming LLPS with PSD95. This competition with TARPs was not dependent on GAP activity but required regions in the C-terminal domain of SynGAP responsible for LLPS with PSD95. CONCLUSION These results indicate that SynGAP’s GAP activity is not required for synaptic plasticity and several cognitive behaviors. These data do not suggest that GAP activity is unimportant, and further work with these mice is needed to understand the role of SynGAP GAP activity in brain function. Finally, these results are relevant for developing treatments for SYNGAP1-related intellectual disability. Our findings suggest that treatments that regulate Ras activity or its downstream signaling will not be sufficient as a therapy and that rescuing SYNGAP1 haploinsufficiency by increasing the expression of the normal allele will be a more effective therapeutic approach. Model of SynGAP regulation of synaptic plasticity. SynGAP regulates synapses by competing with AMPAR-TARP complexes to form LLPS condensates with PSD95. During LTP induction, phosphorylation of SynGAP promotes the dispersal of SynGAP from the synapse and is replaced with AMPAR-TARP complexes, resulting in the potentiation of synaptic transmission. ILLUSTRATION: N. CARY/SCIENCE BASED ON BILL BLAKESLEY