Density of small dendritic spines and microtubule-associated-protein-2 immunoreactivity in the primary auditory cortex of subjects with schizophrenia

Abstract

Previously, we demonstrated that dendritic spine density (DSD) in deep layer 3 of the primary auditory cortex (A1) is lower, due to having fewer small spines, in subjects with schizophrenia (SZ) than non-psychiatric control (NPC) subjects. We also previously demonstrated that microtubule-associated-protein-2 immunoreactivity (MAP2-IR) in A1 deep layer 3 is lower, and positively correlated with DSD, in SZ subjects. Here, we first sought to confirm these findings in an independent cohort of 25 SZ-NPC subject pairs (cohort 1). We used immunohistochemistry and confocal microscopy to measure DSD and MAP2-IR in A1 deep layer 3. Consistent with previous studies, both DSD and MAP2-IR were lower in SZ subjects. We then tested the hypothesis that MAP2-IR mediates the effect of SZ on DSD in a cohort of 45 SZ-NPC subject pairs (combined cohort) that included all subjects from cohort 1 and two previously studied cohorts. Based on the distribution of MAP2-IR values in NPC subjects, we categorized each SZ subject as having either low MAP2-IR (SZ MAP2-IR(low)) or normal MAP2-IR (SZ MAP2-IR(normal)). Among SZ MAP-IR(low) subjects, mean DSD was significantly lower than in NPC subjects. However, mean DSD did not differ between SZ MAP2-IR(normal) and NPC subjects. Moreover, MAP2-IR statistically mediated small spine differences, with lower MAP2-IR values associated with fewer small spines. Our findings confirm that low density of small spines and low MAP2-IR are robust SZ phenotypes and suggest that MAP2-IR mediates the effect of SZ on DSD.

Fig. 1

Density of small dendritic spines is lower in primary auditory cortex deep layer 3 of schizophrenia subjects. a A spine was defined as the colocalization of spinophilin-immunoreactive puncta (green) and phalloidin-labeled puncta (red). Representative micrographs from a non-psychiatric control (NPC) subject (left) and a schizophrenia (SZ) subject (right). Scale bar = 2 µm. b Mean dendritic spine density (DSD) in deep layer 3 of the primary auditory cortex was lower in SZ subjects. c Representative micrographs of spines in each spine size category to which spines were distributed based on volume (0.15 µm³ increments with the final size category including all spines with volume >1.35 µm³). Scale bar = 2 µm. d DSD in the small size categories (volume <0.45 µm³) was lower in SZ subjects (e), thus replicating our previous findings in cohorts 2 and 3 (adapted from ref. [12]). Error bars represent ± standard error of the mean, and * indicates Bonferroni-corrected p < 0.05

Fig. 2

Microtubule-associated-protein-2 immunoreactivity (MAP2-IR) is lower in deep layer 3 of the primary auditory cortex of schizophrenia (SZ) subjects. (Main) Relative MAP2-IR in 25 SZ-non-pscyhiatric control (NPC) subject pairs. Points on, above, or below the diagonal line indicate that SZ = NPC, SZ > NPC, and SZ < NPC values, respectively. (Inset) Mean MAP2-IR was reduced in deep layer 3 of the primary auditory cortex from SZ subjects. Error bars represent ± standard error of the mean, and * indicates p < 0.05

Fig. 3

Lower dendritic spine density in schizophrenia (SZ) subjects is limited to those with low microtubule-associated-protein-2 immunoreactivity, and due to fewer small spines. a, b SZ subjects were divided into two groups based on MAP2-IR values: the SZ MAP-IR(low) group comprised SZ subjects with MAP2-IR values below the 25th percentile for MAP2-IR values in non-psychiatric control (NPC) subjects (ln(MAP2-IR) = 6.52), and the SZ MAP-IR(normal) group comprised all the remaining SZ subjects. c Dendritic spine density (DSD) was significantly lower in SZ MAP2-IR(low), but not SZ MAP2-IR(normal), subjects. d Lower DSD in SZ MAP-IR(low) subjects is accounted for by fewer small spines (<0.45 µm³). Error bars represent ± standard error of the mean, and * indicates p < 0.05