Twenty Years of SynGAP Research: From Synapses to Cognition

Abstract

SynGAP is a potent regulator of biochemical signaling in neurons and plays critical roles in neuronal function. It was first identified in 1998, and has since been extensively characterized as a mediator of synaptic plasticity. Because of its involvement in synaptic plasticity, SynGAP has emerged as a critical protein for normal cognitive function. In recent years, mutations in the SYNGAP1 gene have been shown to cause intellectual disability in humans and have been linked to other neurodevelopmental disorders, such as autism spectrum disorders and schizophrenia. While the structure and biochemical function of SynGAP have been well characterized, a unified understanding of the various roles of SynGAP at the synapse and its contributions to neuronal function remains to be achieved. In this review, we summarize and discuss the current understanding of the multifactorial role of SynGAP in regulating neuronal function gathered over the last two decades.

Figure 1.

SynGAP localization in cultured neurons. A, Overexpression of GFP-SynGAP α1 (green) and mCherry (magenta) in a DIV 18 cultured rat hippocampal neuron. Inset, Robust enrichment of GFP-SynGAP α1 in dendritic spines. B, Distribution of SynGAP in various subcellular fractions from DIV 19 cultured rat cortical neurons. Subcellular fractionation was performed as described by Diering et al. (2014). SynGAP α1 is enriched in the PSD fraction and is found in much lower abundance in the cytosolic fraction (S2). Immunoblots are shown for SynGAP α1 and PSD-95, a PSD marker and binding partner of SynGAP. S1, Postnuclear supernatant fraction; P2, crude membrane fraction.

Figure 2.

SynGAP structural isoforms. A, Schematic diagram of SynGAP protein structure. The core region comprises the C2, GAP, and proline-rich domains (gray). The extreme N- and C-termini contain variable domains whose structure depends on transcriptional and post-transcriptional processing. The remaining pleckstrin homology (PH) domain and coiled-coil (CC) domain (both magenta) are altered in several isoforms. B, Schematic diagram of the N-terminal isoforms of SynGAP arising from the use of alternative transcriptional start sites. The PH domain is partially truncated in the C isoform. C, Schematic diagram of the C-terminal isoforms of SynGAP arising from alternative splicing. The CC domain is partially truncated in the β isoform. D, Intrinsic disorder probability plotted as a function of amino acid position along the full length of the SynGAP protein sequence starting with the beginning of the A isoform. Disorder probabilities for all four C-terminal isoforms, α1 (red), α2 (orange), β (blue), and γ (purple), are shown. Disorder probabilities were calculated using IUPred2A (Mészáros et al., 2018).

Figure 3.

SynGAP dynamics. A, Representative images of a segment of a dendrite of a cultured rat hippocampal neuron expressing GFP-SynGAP and mCherry at basal state (left column) and following chemLTP stimulation (right column). The GFP-SynGAP signal is rapidly reduced in a dendritic spine following chemLTP (yellow arrow). The volume of the same dendritic spine is increased following chemLTP, as shown by the mCherry signal. B, Quantification of the GFP-SynGAP and mCherry dynamics in A. These data are reprinted from Araki et al. (2015) with permission. C, Expression of Azurite-tagged SynGAP C terminus (Az-SynGAP CC-PBM WT) (blue) and full-length PSD-95-mCherry (red) in a HEK 293T cell. Coexpression results in the formation of spherical cytoplasmic condensates containing both proteins. Image brightness and contrast were adjusted to show the presence of the diffuse soluble phase of both proteins outside of granules. D, Enlarged channel-split images of the boxed cytoplasmic condensate in C before and after photobleaching with a 561 nm laser. The PSD-95-mCherry signal rapidly recovers fluorescence following photobleaching, indicating rapid exchange of molecular constituents with the surrounding cytoplasm, a property common to liquid-like biomolecular condensates. These data are a replication of data from Zeng et al. (2016). E, Quantification of the normalized mean fluorescence intensity following FRAP experiments plotted as the mean ± SEM (N = 2 granules in 2 cells).

Figure 4.

ID-associated SYNGAP1 mutations. SYNGAP1 mutations associated with ID as collated in Vlaskamp et al. (2019). Mutations are binned and listed according to their respective exon or intron. Exonic mutations are named for the affected amino acid residue (first letter), position (number), and the resultant change to the residue due to the mutation (following letter). Intronic mutations are named for the resultant nucleotide change relative to the SYNGAP1 reference sequence. *Change resulting in a stop codon. fs*, Frameshift that results in a stop codon downstream of the frameshift site. SynGAP protein domains are as they appear in Figure 2. Truncating mutations are listed above the SYNGAP1 gene and protein structure, and nontruncating missense mutations are listed below. Importantly, the mutations are not evenly distributed across the SYNGAP1 gene, with most identified pathogenic mutations occurring after the first two and before the final two exons.

Figure 5.

Working model of SynGAP function at the synapse. Top left, SynGAP is enriched in the PSD through binding with MAGUK family scaffold proteins, such as PSD-95. NMDARs and AMPARs also interact with MAGUK proteins. The PSD is a phase-separated macromolecular complex that remains highly packed despite the lack of enclosure by lipid membranes. SynGAP inhibits the activity of Ras and other small GTPases, which are involved in numerous biochemical signaling pathways that promote enhancement of spine growth and synaptic strength through actin polymerization and AMPAR insertion. SynGAP also occupies a significant number of MAGUK PDZ domains, potentially placing a limit on the number of synaptic AMPAR/TARP complexes at the PSD. Top right, Following an LTP-inducing stimulus, SynGAP is rapidly dispersed from the PSD. This lifts the brake on small GTPase signaling, promoting plasticity-related biochemical and structural changes to the synapse. Some freely diffusing AMPAR/TARP complexes associate with MAGUK PDZ domains previously occupied by SynGAP. Bottom left, In the case of SynGAP haploinsufficiency, small GTPase signaling is basally elevated and MAGUK PDZ domains are basally more available for the binding of AMPAR/TARP complexes, resulting in larger spines with greater AMPAR content. SynGAP-haploinsufficiency-induced enhancement of basal synaptic AMPAR number represents basal enhancement of synaptic strength. Bottom right, Following a stimulus that would normally lead to robust activity-induced SynGAP dispersion, no further enhancement of synaptic strength is observed due to occlusion of LTP.