Cell-type specific reductions in interneuron gene expression within the cingulate gyrus of schizophrenia and bipolar disorder subjects

Abstract

Schizophrenia (SZ) and bipolar disorder (BP) patients share overlapping neurocognitive deficits of varied magnitude. Neuroimaging in patients and postmortem gene expression analyses suggest that compromised cingulate gyrus GABA-ergic interneurons may contribute to cognitive impairments in SZ and BP. To address this, we used radioactive in situ hybridization to investigate potential gene expression signatures for SZ and BP using interneuron cell-type specific markers including glutamic acid decarboxylase (GAD67), parvalbumin (PV), somatostatin (SST), and vasoactive intestinal peptide (VIP) within specific Brodmann's areas (BA) of the cingulate gyrus. We report reduced GAD67 mRNA in anterior midcingulate cortex (aMCC) of BP subjects within BA24c', the most dysregulated subregion across disorders that also demonstrated reduced PV and VIP mRNA in the SZ group. In the retrosplenial (RSC) and ectosplenial (ESC) cortices, decreases in PV expression were shared by both SZ and BP subjects. Our results show unique and shared transcription signatures of two disorders in specific cingulate gyrus regions and cell types. SZ and BP displayed divergent aMCC gene expression reductions suggesting transcriptional changes are associated with disease-specific gene/subregion signatures, potentially underlying differential subregional dysregulation within areas associated with error detection/action monitoring and the salience network. In RSC/ESC, transcriptional changes are associated with more common expression patterns, possibly related to overlapping effects on visuospatial memory processing and allocation of attentional resources involving the default mode network.

Fig. 1. Interneuron markers exhibit different distribution patterns in aMCC subregions as well as differences in gene expression between healthy control vs. psychiatric groups as revealed by radioactive in situ hybridization.

a Representative GAD67 mRNA expression in the coronal plane is shown with quantified subregions 32’, 24c’ and 24ab’ of the aMCC. b, c, e, f Monocolor images of adjacent in situ hybridization sections were pseudocolored, aligned and d overlayed to examine the relative spatial distributions in gene expression as shown. Note that the distribution of PV within aMCC tissue sections was most varied across subregions (c, d). g BP subjects had reduced levels of GAD67 vs. CTR, specifically in region 24c’ (p = 0.041). h A nonsignificant trend for decreased PV expression was observed in area 32’ for SZ subjects vs. CTR (p = 0.096). i Reduced PV expression was also noted in 24c’, where SZ subjects had reduced expression vs. CTR (p = 0.028). VIP expression was lower in the SZ group vs. CTR in all examined aMCC subregions (j p = 0.036; k p = 0.042; l p = 0.044). Each dot represents one subject; data is presented as mean ± standard deviation. *p < 0.05. aMCC anterior midcingulate cortex, cc corpus callosum, D dorsal, GAD67 glutamic acid decarboxylase 67, M medial, PV parvalbumin, SST somatostatin, VIP vasoactive intestinal peptide.

Fig. 2. Distribution and differential expression of interneuron marker genes within the dPCC/RSC/ESC of psychiatric and control subjects.

a Representative coronal plane for subregional quantification of GAD67 mRNA expression is shown in the dPCC BAs (regions d23c, d23b, and d23a), the RSC (regions 30 and 29) and ESC (region 26). b–d Monochrome images of adjacent tissues sections processed for in situ hybridization were pseudocolored, aligned and e overlayed using Adobe Photoshop to visually examine the relative distribution of signal (VIP not shown). PV mRNA differed between diagnoses in RSC and ESC, where SZ subjects had decreased PV in both the RSC (f SZ vs. CTR: p = 0.037; SZ vs. BP: p = 0.020; and g SZ vs. CTR: p < 0.0001) and ESC (h SZ vs. CTR: p < 0.0001). PV expression was also decreased in BP subjects compared to CTRs in RSC region 29 (g p = 0.021) and ESC (h p = 0.008). A non-significant trend for decreased SST expression was observed in BAd23c, 30, and 29 (i–k respectively). Each dot represents one subject; data is presented as mean ± standard deviation. ***p < 0.001; **p < 0.01; *p < 0.05. BP Bipolar Disorder subject group, cc corpus callosum, CTR non-psychiatric control subject group, D dorsal, dPCC dorsal posterior cingulate cortex, ESC ectosplenial cortex, GAD67 glutamic acid decarboxylase 67, M medial, PV parvalbumin, RSC retrosplenial cortex, SST somatostatin, SZ schizophrenia subject group.

Fig. 3. Summary of interneuron-related gene expression changes.

Decreases in the expression of interneuron-related genes (left, GAD67: purple, top to bottom, VIP: yellow, SST: red; PV: green) are summarized for both aMCC (left) and dPCC/RSC/ESC (right) regions. In aMCC, GAD67 expression was reduced in BA24c’ of BP subject vs. CTR. In contrast, aMCC VIP expression was decreased in SZ (32’, 24c’, and 24ab’) compared to CTR, presumably reflecting reduced inhibition of cortical interneurons (e.g., SST and/or PV) as well as decreased inhibition at the apical dendrite of pyramidal neurons. Compared to CTRs, there was a statistical trend for decreased SST expression in BP subjects within the dPCC (regions d23c) and RSC (regions 30 and 29). These potential decreases in SST may occur in Martinotti cells, known to synapse onto the apical tufts of excitatory pyramidal neurons. Decreased PV expression (vs. CTRs) was found in SZ subjects in the aMCC (24c’), RSC (regions 30 and 29) and ESC (region 26), while decreased PV expression in BP subjects (vs. CTRs) was found in the RSC region 29 and ESC (region 26). PV expression is localized to both basket cells and chandelier cells of the aMCC and RSC/ESC, where these neurons contact pyramidal neurons at the soma and initial axon segment, respectively.