Premorbid characteristics of the SAPAP3 mouse model of obsessive-compulsive disorder: behavior, neuroplasticity, and psilocybin treatment

Abstract

Background: SAPAP3-knockout (SAPAP3-KO) mice develop excessive self-grooming behavior at 4-6 months of age, serving as a model for obsessive-compulsive disorder (OCD). Given that anxiety often precedes OCD diagnosis in humans, this study investigated whether juvenile SAPAP3-KO mice exhibit anxiety-like behaviors before developing the self-grooming phenotype, and whether such behaviors respond to psilocybin (PSIL) treatment. The study also examined 4 key neuroplasticity-related synaptic proteins-GAP43, PSD95, synaptophysin, and SV2A-as SAPAP3 is a postsynaptic scaffold protein that interacts with PSD95 and may affect synaptic function.

Methods: Two studies were conducted using male and female juvenile (10-13 weeks) SAPAP3-KO mice. Study 1 compared behavioral phenotypes between homozygous (HOM), heterozygous, and wild-type (WT) mice. Study 2 evaluated a different sample of HOM and WT mice and assessed the effect of PSIL (4.4 mg/kg) on identified behavioral differences. Both studies included comprehensive behavioral testing focused on anxiety-like behavior, social interaction, and cognitive function. Additionally, levels of 4 synaptic proteins were measured by western blots in the frontal cortex, hippocampus, amygdala, and striatum of juvenile and adult SAPAP3-KO mice.

Results: In both studies, juvenile HOM SAPAP3-KO mice showed significant anxiety-like behaviors compared to WT mice, spending less time in open field center, and elevated plus maze open arms. They also buried fewer marbles and found fewer buried Oreos than WT mice. Psilocybin treatment did not improve these behavioral manifestations. Analysis of synaptic proteins revealed significant increases in GAP43, synaptophysin, and SV2A across multiple brain regions in adult male HOM mice and of SV2A in the frontal cortex of HOM females compared to WT, but not in juvenile mice of either sex.

Conclusions: Juvenile SAPAP3-KO mice exhibit anxiety-like behaviors before developing the characteristic excessive self-grooming phenotype, paralleling the prodromal anxiety often seen in human OCD. Unlike in adult SAPAP3-KO mice, these manifestations were not responsive to PSIL treatment. The age-dependent increases in synaptic proteins observed in adult (but not juvenile) male SAPAP3-KO mice HOM for the deletion and to a lesser extent in female homozygotes, may represent compensatory plasticity changes in response to the phenotype. These results provide insights into the developmental trajectory of OCD-like behaviors and associated neuroplastic adaptations.

Keywords: SAPAP3; Tourette’s syndrome; head-body twitches; obsessive-compulsive disorder; self-grooming.

Figure 1.

(A) Timeline of the behavioral test in Study 1. (B) Timeline of treatment and behavioral tests in Study 2.

Figure 2.

I. Open field test comparing wild type (WT), heterozygous (HET), and homozygous (HOM) SAPAP3 knockout (SAPAP3-KO) mice: (A) distance traveled (F_Genotype = 20.11, df = 2119, P < .0001). (B)Center duration (F_Genotype = 9.538, df = 2134, P = .0001). Tukey’s post hoc test: * P < .05, **P < .01, ****P < .0001. II. Elevated plus maze comparing WT, HET, and HOM SAPAP3-KO mice: (A) time spent in open arms (F_Genotype = 8.250, df = 2122 P = .0004). (B) Number of times entered open arms (F_Genotype = 6.943, df = 2127, P = .0014). Tukey’s post hoc test: **P < .01. III. –(A) Marble burying test, comparing WT, HET, and HOM SAPAP3-KO mice (F_Genotype = 60.93, df = 2134, P < .0001). Tukey’s post hoc test: ****P < .0001. –(B) Tube Dominance test, comparing WT, HET, and HOM SAPAP3-KO (F_Genotype = 5.489, df = 2136, P = .0051). Tukey’s post hoc test: **P < .01.

Figure 3.

I. Open field test comparing WT and HOM SAPAP3-KO mice with and without treatment with psilocybin: (a) Distance traveled (F_Genotype = 39.31, df = 1, 67, P < .0001). (b) Center duration (F_Genotype = 11.37, df = 1, 66, P = .0013).** P < .01, **** P < .0001 (Tukey’s post hoc test). II. Elevated plus maze test comparing WT and HOM SAPAP3-KO mice with and without treatment with psilocybin: (a) open arms duration (F_Genotype = 50.97, df = 1, 56, P < .0001). (b) Open arm frequency (F_Genotype = 24.42, df = 1,67; P < .0001). ** P < .01, **** P < .0001 (Tukey’s post hoc test). III. (a) Marbles buried in 30 minutes comparing WT and HOM SAPAP3-KO mice with and without treatment with psilocybin (F_Genotype = 116.2, df = 1, 56, P < .0001). (b) Tube Dominance Test in WT and SAPAP3-KO mice, with our without treatment with psilocybin (F_Genotype = 18.76, df = 1,62, P < .0001). * P < .05, *** P < .001, **** P < .0001 (Tukey’s post hoc test). HET, heterozygous; HOM, homozygous; SAPAP3-KO, SAPAP3 knockout; WT, wild type.

Figure 4.

Study 1: 2-way analysis of variance (ANOVA) of SV2A levels in the: (a) frontal cortex: F_Genotype = 17.22, df = 1,55, P = .001; (b) hippocampus: F_Genotype = 14.67, df = 1,56, P = .0003; F_{Genotype × Sex} = 20,94, df = 1,56, P = .00; (c) amygdala: F_Genotype = 14.48, df = 1,53, P = .0004; F_Sex = 12.32, df = 1,53, P = .0009; F_{Genotype × Sex} = 26.75, df = 1,53, P < .0001; (d) striatum: F_Genotype = 4.59, df = 1,53, P = .03; F_Sex = 7.48, df = 1,53, P = .008; F = 15.08, df = 1,53, P = .0003. * P < .05, *** P < .001, **** P < .0001 (Tukey’s post hoc test).

Figure 5.

Study1: nested analysis of synaptic proteins (GAP43, PSD95, synaptophysin, and SV2A) by genotype (SAPAP3-KO and WT) across 4 brain areas (frontal cortex, hippocampus, amygdala, and striatum) in male (left) and female (right) mice. The X axis represents normalized synaptic protein expression levels across genotypes (Y axis). The dotted line shows the mean of the highest and lowest expression values between HOM and WT groups. Statistical significance was assessed using a t-test (2-tailed). Results are shown as mean ± SEM. ** P < .01, **** P < .0001. HOM, homozygous; SAPAP3-KO, SAPAP3 knockout; WT, wild type.

Figure 6.

Study 1: nested analysis of 4 synaptic proteins (GAP43, PSD95, synaptophysin, and SV2A) within each of 4 brain areas (frontal cortex, hippocampus, amygdala, striatum) in male (left) and female (right) HOM vs WT SAPAP3-KO mice. The X axis represents normalized synaptic protein expression levels across genotypes (Y axis). The dotted line shows the mean of the highest and lowest expression values between the HOM and WT groups. Statistical significance was assessed using a t-test (2-tailed). Results are shown as mean ± SEM. ** P < .01, *** P < .001. HOM, homozygous; SAPAP3-KO, SAPAP3 knockout; WT, wild type.