A novel mechanism and treatment target for presynaptic abnormalities in specific striatal regions in schizophrenia

Abstract

Abnormalities of amount and function of presynaptic terminals may have an important role in the mechanism of illness in schizophrenia. The SNARE proteins (SNAP-25, syntaxin, and VAMP) are enriched in presynaptic terminals, where they interact to form a functional complex to facilitate vesicle fusion. SNARE protein amounts are altered in the cortical regions in schizophrenia, but studies of protein-protein interactions are limited. We extended these investigations to the striatal regions (such as the nucleus accumbens, ventromedial caudate (VMC), and dorsal caudate) relevant to disease symptoms. In addition to measuring SNARE protein levels, we studied SNARE protein-protein interactions using a novel ELISA method. The possible effect of antipsychotic treatment was investigated in parallel in the striatum of rodents that were administered haloperidol and clozapine. In schizophrenia samples, compared with controls, SNAP-25 was 32% lower (P=0.015) and syntaxin was 26% lower (P=0.006) in the VMC. In contrast, in the same region, SNARE protein-protein interactions were higher in schizophrenia (P=0.008). Confocal microscopy of schizophrenia and control VMC showed qualitatively similar SNARE protein immunostaining. Haloperidol treatment of rats increased levels of SNAP-25 (mean 24%, P=0.003), syntaxin (mean 18%, P=0.010), and VAMP (mean 16%, P=0.001), whereas clozapine increased only the VAMP level (mean 13%, P=0.004). Neither drug altered SNARE protein-protein interactions. These results indicate abnormalities of amount and interactions of proteins directly related to presynaptic function in the VMC in schizophrenia. SNARE proteins and their interactions may be a novel target for the development of therapeutics.

Figure 1

SNARE protein levels and protein–protein interactions in three human striatal regions. Significant post hoc F -tests are indicated ^*P<0.05, Bonferonni-adjusted. Differences that do not survive Bonferonni correction are indicated (a). All three SNARE protein levels were lower in the VMC of subjects with schizophrenia (panels a–c). Syntaxin–SNAP-25 protein–protein interactions were increased in the VMC (d). Horizontal lines indicate group means. Raw (untransformed) but re-scaled values are plotted for the three SNARE measures. NAc, nucleus accumbens; VMC, ventromedial caudate; DCd, dorsal caudate; CTL, controls; SCZ, schizophrenia.

Figure 2

Distribution of SNARE proteins in human VMC. Single-channel images appear in the first two columns, with merged images in the third column. Arrows indicate the region of interest highlighted in the inset. Differences in staining intensity may be the result of slight differences in antibody concentration, antibody penetration, image capture/processing conditions. Calretinin staining appears not to colocalize with presynaptic markers (a and b) and was used as a cell body stain for orientation within the tissue slice. Syntaxin, VAMP, and SNAP-25 show some colocalization with each other (c and d, inset). S25, SNAP-25; CR, calretinin; STX, syntaxin.

Figure 3

SNARE protein levels (a–c) and interactions (d) in three striatal regions of drug-treated animals. Significant effects of drug treatment (P<0.05) were investigated using contrasts. Statistically significant differences between treatment groups and saline controls are indicated (all P<0.02, Bonferonni-adjusted). Syntaxin–SNAP-25 protein interactions were not different between groups or striatal regions (P >0.05). Horizontal lines indicate group means (all regions combined). Raw (untransformed) but re-scaled values are plotted for the three SNARE measures.