Sapap3 deletion causes dynamic synaptic density abnormalities: a longitudinal [11C]UCB-J PET study in a model of obsessive-compulsive disorder-like behaviour

Abstract

Background: Currently, the evidence on synaptic abnormalities in neuropsychiatric disorders-including obsessive-compulsive disorder (OCD)-is emerging. The newly established positron emission tomography (PET) ligand ((R)-1-((3-((11)C-methyl-(11)C)pyridin-4-yl)methyl)-4-(3,4,5-trifluorophenyl)pyrrolidin-2-one) ([¹¹C]UCB-J) provides the opportunity to visualize synaptic density changes in vivo, by targeting the synaptic vesicle protein 2A (SV2A). Here, we aim to evaluate such alterations in the brain of the SAP90/PSD-95-associated protein 3 (Sapap3) knockout (ko) mouse model, showing an abnormal corticostriatal neurotransmission resulting in OCD-like behaviour.

Methods: Longitudinal [¹¹C]UCB-J µPET/CT scans were acquired in Sapap3 ko and wildtype (wt) control mice (n = 9/group) to study SV2A availability. Based on the Logan reference method, we calculated the volume of distribution (V_T(IDIF)) for [¹¹C]UCB-J. Both cross-sectional (wt vs. ko) and longitudinal (3 vs. 9 months) volume-of-interest-based statistical analysis and voxel-based statistical parametric mapping were performed. Both [¹¹C]UCB-J ex vivo autoradiography and [³H]UCB-J in vitro autoradiography were used for the validation of the µPET data.

Results: At the age of 3 months, Sapap3 ko mice are already characterized by a significantly lower SV2A availability compared to wt littermates (i.a. cortex - 12.69%, p < 0.01; striatum - 14.12%, p < 0.001, thalamus - 13.11%, p < 0.001, and hippocampus - 12.99%, p < 0.001). Healthy ageing in control mice was associated with a diffuse and significant (p < 0.001) decline throughout the brain, whereas in Sapap3 ko mice this decline was more confined to the corticostriatal level. A strong linear relationship (p < 0.0001) was established between the outcome parameters of [¹¹C]UCB-J µPET and [¹¹C]UCB-J ex vivo autoradiography, while such relationship was absent for [³H]UCB-J in vitro autoradiography.

Conclusions: [¹¹C]UCB-J PET is a potential marker for synaptic density deficits in the Sapap3 ko mouse model for OCD, parallel to disease progression. Our data suggest that [¹¹C]UCB-J ex vivo autoradiography is a suitable proxy for [¹¹C]UCB-J PET data in mice.

Keywords: Autoradiography; Grooming; Obsessive–compulsive disorder (OCD); Positron emission tomography (PET); SAP90/PSD-95-associated protein 3 (Sapap3); Synaptic vesicle protein 2A (SV2A); [11C]UCB-J; [3H]UCB-J.

Fig. 1

Overview of all the experimental procedures. (wt = wildtypes; ko = knockouts)

Fig. 2

a The averaged [¹¹C]UCB-J PET V_T(IDIF) maps superimposed on a mouse MR template at 3 and 9 mo for both genotypes, b combined with the corresponding averaged [¹¹C]UCB-J PET V_T(IDIF) values ± SD. Young adult Sapap3 ko mice are characterized by a lower synaptic density availability in the cortex, the striatum, the thalamus, and the hippocampus when compared to their wt counterparts. Also, both genotypes show a progressive decline in synaptic density availability with ageing, which was more pronounced in the wt controls. The corresponding averaged values ± SD can be consulted in Additional file 1: Table 1. (ko, knockout; mo, months; MR, magnetic resonance; wt, wildtype; V_T(IDIF), volume-of-distribution; *p < 0.05, **p < 0.01, ***p < 0.001)

Fig. 3

The results of the voxel-based statistical parametric mapping analysis of the [¹¹C]UCB-J V_T(IDIF) maps. a A representation of all analysed brain regions on a corresponding MR template. b Hypo T-map showing all voxel clusters (threshold: 100 voxels) with a significantly (p < 0.01) lower V_T(IDIF) in ko compared to wt mice at the age of 3 mo. c Hypo T-maps indicating all voxel clusters (threshold: 100 voxels) that significantly (p < 0.01) declined over time in wt mice. d Similar hypo T-maps as in panel c for the ko mice (ctx = cortex; hc = hippocampus; ko = knockout; mo = months; MR = magnetic resonance; str = striatum; th = thalamus; wt = wildtype)

Fig. 4

Representative sagittal autoradiograms of a [¹¹C]UCB-J ex vivo and b [³H]UCB-J in vitro autoradiography with the corresponding regional c standard uptake values (SUV) of [¹¹C]UCB-J ex vivo autoradiography and d specific binding (SB) values of [³H]UCB-J in vitro autoradiography at the age of 3 mo and 9 mo. The corresponding averaged values ± SD can be consulted in Additional file 1: Table 2. (ko = knockout; mo = months; NB = nonspecific binding; TB = total binding; wt = wildtype; *p < 0.05; **p < 0.01; ***p < 0.001)

Fig. 5

The averaged [¹¹C]UCB-J µPET volume of distribution (V_T(IDIF)) ± SD a versus the [¹¹C]UCB-J ex vivo autoradiography averaged standard uptake value (SUV) ± SD and b versus the [³H]UCB-J in vitro autoradiography averaged specific binding (SB) for both genotypes. Three-mo data are depicted as filled symbols, while 9-mo data are depicted as open symbols