SYNGAP1 links the maturation rate of excitatory synapses to the duration of critical-period synaptic plasticity

Abstract

Critical periods of developmental plasticity contribute to the refinement of neural connections that broadly shape brain development. These windows of plasticity are thought to be important for the maturation of perception, language, and cognition. Synaptic properties in cortical regions that underlie critical periods influence the onset and duration of windows, although it remains unclear how mechanisms that shape synapse development alter critical-period properties. In this study, we demonstrate that inactivation of a single copy of syngap1, which causes a surprisingly common form of sporadic, non-syndromic intellectual disability with autism in humans, induced widespread early functional maturation of excitatory connections in the mouse neocortex. This accelerated functional maturation was observed across distinct areas and layers of neocortex and directly influenced the duration of a critical-period synaptic plasticity associated with experience-dependent refinement of cortical maps. These studies support the idea that genetic control over synapse maturation influences the duration of critical-period plasticity windows. These data also suggest that critical-period duration links synapse maturation rates to the development of intellectual ability.

Figure 1.

syngap1 inactivation induces early maturation of TC synapses through a precocious accumulation of synaptic AMAPRs that directly restricts the duration of a critical-period plasticity window. A, Low-magnification image of TC fixed WT P9 slice showing the placement of stimulating electrode and recording electrode. B, Representative traces and summary data of A/N ratios evoked in VPM and patch clamp recorded in layer IV WT stellate cells at P4 (n = 6), P6 (n = 8), and P9 (n = 6). C, Same as B but in Het P4 [P4 (n = 6), P6 (n = 7), and P9 (n = 8)]. **p < 0.01 between genotypes; ^#p < 0.01 within genotypes (2-way ANOVA; Bonferroni's pairwise comparison). Arrows and dark gray bars indicate the time point at which measurement was obtained from all the experiments. D, Sequential measurement of isolated AMPAR-mediated (gray) and NMDAR-mediated (black) currents from neurons recorded in P5 TC slices (WT, n = 10; Het, n = 6). **p < 0.01, Student's t test. The A/N ratio was calculated for each cell by measuring the peak of each component at −70 mV. E, Representative recording and summary population data of LTPs evoked in VPM and perforated patch clamp recorded in layer IV stellate cells in P5 WT (n = 4) and Het (n = 4) animals. **p < 0.01, repeated-measures ANOVA. F, Same as E but in P8 (WT, n = 4; Het, n = 5). G, Top, An example recording of a minimal stimulation protocol designed to uncover silent synapses in TC slices derived from WT or Het mice at P5. Bottom, Summary of population data demonstrating a reduction in the number of silent synapses in P5 Het animals (WT, n = 4; Het = n = 4). ***p < 0.001, Student's t test. PND, Postnatal day.

Figure 2.

syngap1 mutations accelerate GluN2A incorporation into developing TC synapses. A, Representative traces and summary data of GluN2B sensitivity to 3 μm ifenprodil recorded from layer IV stellate cells in P4–P5 WT and Het mice. B, Same as A but in P8–P9. **p < 0.01 within genotypes (wg) and ^#p < 0.01 between genotypes (2-way ANOVA, Bonferroni's pairwise comparison). Gray bar indicates the point at which measurement was obtained from all the cells. PND, Postnatal day.

Figure 3.

Early excitatory synaptic maturation in mPFC layer II/III pyramidal neurons. A, A coronal section through the mouse brain illustrating the mPFC. Low-magnification image shows the prelimbic and cingulate cortices, in which recordings were obtained. High-magnification image (rotated 90° counterclockwise for clarity) shows an example of a layer II/III neuron. Cg1, Cingulate cortex area 1; PrL, prelimbic cortex; IL, infralimbic cortex. B, Representative traces and summary data of A/N ratios evoked locally and recorded in layer II/III pyramidal cells in P4–P15 WT. Calibration: 100 pA, 300 ms. C, Same as B but in Het mice. *p < 0.05 between genotypes; ^#,&p < 0.05 within genotypes [2-way ANOVA (data from B and C were included in analysis), Bonferroni's pairwise comparison]. Arrows and dark gray bars indicate the time point at which measurement was obtained from all the experiments. D, Golgi-stained mPFC layer II/III neurons from P8 WT or HET mice. Scale bar, 50 μm. E, Higher magnification of Golgi-stained neurons demonstrating a lack of spines present in both genotypes at this age. Scale bar, 10 μm. F, Adult WT mouse allows a frame of reference for Golgi-stained neurons and dendritic spine labeling. PND, Postnatal day.