SRGAP2C

SRGAP2C is a human-specific, partially duplicated paralog of SRGAP2A that arose ~2.4 mya and encodes a truncated F-BAR domain protein which acts as a dominant antagonist of its ancestral parent, contributing to human-specific features of neocortical development (PMID:22559944, PMID:22559943). The duplication retained only the promoter and first nine exons of SRGAP2A, producing a protein with a defective F-BAR extension (Fx) domain that is poorly soluble and unable to scaffold membranes; SRGAP2C nonetheless heterodimerizes with full-length SRGAP2A through the F-BAR/Fx and RhoGAP-SH3 interface, and the resulting heterodimers are insoluble and functionally inhibitory (PMID:28333212). In neurons SRGAP2C is intrinsically unstable, and upon heterodimerization it reduces SRGAP2A levels in a proteasome-dependent manner; human-specific arginine substitutions enhance this inhibitory capacity and drive long-lasting increases in both excitatory and inhibitory synapse density with protracted (neotenic) maturation (PMID:31822692). Functionally, expression of SRGAP2C in mouse cortex phenocopies SRGAP2 deficiency — sustained radial migration, neotenic spine maturation, and increased density of longer spines (PMID:22559944) — and specifically increases local and long-range cortico-cortical connectivity onto layer 2/3 pyramidal neurons, reshaping sensory coding and enhancing sensory-discrimination learning (PMID:34707291). Downstream, SRGAP2C enhances synaptic accumulation of its binding partner CTNND2 (delta-catenin), which slows synaptic maturation and supports synaptic SYNGAP1 retention, linking SRGAP2C activity to synaptic neoteny (PMID:39352808). Transgenic primate and zebrafish models reproduce delayed neurodevelopment and neotenic transcriptomic and circuit phenotypes (PMID:38090550, PMID:39314374).

1. 2012 High

   Established that SRGAP2C is not a pseudogene but a functional human-specific paralog that antagonizes ancestral SRGAP2A, answering whether the duplication had a biological consequence for cortical development.

   Evidence Transgenic mouse neocortical expression, reciprocal Co-IP for dimerization, and neuronal morphology assays compared against SRGAP2 loss-of-function

   PMID:22559943 PMID:22559944

   Open questions at the time

   • The structural basis for how a truncated protein inhibits the full-length parent was not resolved
   • Whether inhibition occurs by sequestration, degradation, or steric block was undefined
   • Synaptic and circuit-level consequences in adult cortex were not addressed
2. 2017 High

   Defined the molecular mechanism of antagonism by showing SRGAP2A homodimerizes through an F-BAR/Fx/RhoGAP-SH3 interface and that SRGAP2C's defective Fx domain produces insoluble, non-functional heterodimers.

   Evidence X-ray crystallography of SRGAP2A plus in vitro heterodimerization, solubility, membrane-scaffolding, and mutagenesis assays

   PMID:28333212

   Open questions at the time

   • Did not establish how insoluble heterodimers are handled in neurons (e.g. degradation)
   • No structure of the SRGAP2A:SRGAP2C heterodimer itself
   • Link between solubility defect and synaptic phenotype remained indirect
3. 2019 High

   Connected heterodimerization to a degradative outcome and identified the specific arginine substitutions that make SRGAP2C a more potent and durable inhibitor than the paralog SRGAP2B.

   Evidence Neuronal expression of SRGAP2B/2C, proteasome inhibitor assays, site-directed mutagenesis of arginine residues, and longitudinal synaptic density quantification

   PMID:31822692

   Open questions at the time

   • The E3 ligase and recognition pathway targeting SRGAP2A for degradation were not identified
   • How proteasomal turnover translates to sustained adult synaptic changes was not mechanistically traced
4. 2021 High

   Demonstrated that SRGAP2C reshapes cortical circuit architecture and behavior, moving beyond single-cell morphology to network-level function.

   Evidence Transgenic mouse expression in cortical pyramidal neurons with viral tracing, in vivo calcium imaging, sensory-discrimination behavior, and computational modelling

   PMID:34707291

   Open questions at the time

   • The molecular intermediary linking SRGAP2A inhibition to specific connectivity changes was not identified
   • Results obtained in mouse cortex; primate-specific effects untested here
5. 2024 Medium

   Identified CTNND2 as a downstream synaptic effector, defining a molecular pathway from SRGAP2C to synaptic neoteny.

   Evidence Co-IP mapping the SRGAP2-CTNND2 interaction, CTNND2 loss-of-function in neurons, and quantification of synaptic CTNND2 accumulation in human neurons expressing SRGAP2C

   PMID:39352808

   Open questions at the time

   • Whether CTNND2 accumulation is necessary for all SRGAP2C circuit phenotypes was not established
   • The mechanism by which reduced SRGAP2A increases synaptic CTNND2 was not fully resolved
   • Single lab, reciprocal Co-IP without orthogonal interaction validation
6. 2024 Medium

   Extended SRGAP2C's developmental effects to a primate model, testing whether the gene drives neotenic phenotypes in a brain closer to human.

   Evidence Transgenic cynomolgus macaque generation with longitudinal MRI, histological neuron quantification, transcriptomics, and cognitive testing

   PMID:38090550

   Open questions at the time

   • Causal link between transcriptomic neoteny and behavioral outcomes not established
   • Small cohort inherent to transgenic primate studies
   • Cell-type-specific mechanism in macaque cortex not dissected
7. 2024 Medium

   Showed via zebrafish that loss of srgap2 and gain of SRGAP2C converge on shared phenotypes, including microglial and excitatory/inhibitory balance changes, broadening the cellular scope beyond pyramidal neurons.

   Evidence Zebrafish srgap2 knockout and SRGAP2C transgenics with scRNA-seq, live microglial imaging, behavioral seizure and visual assays, and cross-species transcriptome comparison (preprint)

   PMID:39314374

   Open questions at the time

   • Preprint, not yet peer-reviewed
   • Microglial effects of SRGAP2C in mammalian systems not confirmed
   • Whether zebrafish phenotypes reflect the same molecular antagonism mechanism is inferred
8. 2025 Low

   Began addressing how SRGAP2C achieves paralog-specific expression by identifying differentially active cis-regulatory elements within the locus.

   Evidence Massively parallel reporter assay in lymphoblastoid and neuroblastoma lines with long-read expression/epigenomic data and CRE validation (preprint)

   PMID:41292788

   Open questions at the time

   • Reporter activity establishes CRE function but consequence for endogenous SRGAP2C protein is inferred; preprint, single lab
   • No demonstration that the CREs control native SRGAP2C levels in neurons
   • Regulatory contribution quantified only as moderate

• The E3 ligase and degradation machinery that clear SRGAP2A:SRGAP2C heterodimers, and whether CTNND2 accumulation is required for SRGAP2C's circuit-level and behavioral effects, remain unresolved.
• No identified ubiquitin ligase for proteasome-dependent SRGAP2A reduction
• Necessity of CTNND2 for connectivity and behavioral phenotypes untested
• Structure of the inhibitory heterodimer not determined