Altered expression of synapse and glutamate related genes in post-mortem hippocampus of depressed subjects

Abstract

Major depressive disorder (MDD) has been linked to changes in function and activity of the hippocampus, one of the central limbic regions involved in regulation of emotions and mood. The exact cellular and molecular mechanisms underlying hippocampal plasticity in response to stress are yet to be fully characterized. In this study, we examined the genetic profile of micro-dissected subfields of post-mortem hippocampus from subjects diagnosed with MDD and comparison subjects matched for sex, race and age. Gene expression profiles of the dentate gyrus and CA1 were assessed by 48K human HEEBO whole genome microarrays and a subgroup of identified genes was confirmed by real-time polymerase chain reaction (qPCR). Pathway analysis revealed altered expression of several gene families, including cytoskeletal proteins involved in rearrangement of neuronal processes. Based on this and evidence of hippocampal neuronal atrophy in MDD, we focused on the expression of cytoskeletal, synaptic and glutamate receptor genes. Our findings demonstrate significant dysregulation of synaptic function/structure related genes SNAP25, DLG2 (SAP93), and MAP1A, and 2-amino-3-(5-methyl-3-oxo-1,2-oxazol-4-yl)propanoic acid receptor subunit genes GLUR1 and GLUR3. Several of these human target genes were similarly dysregulated in a rat model of chronic unpredictable stress and the effects reversed by antidepressant treatment. Together, these studies provide new evidence that disruption of synaptic and glutamatergic signalling pathways contribute to the pathophysiology underlying MDD and provide interesting targets for novel therapeutic interventions.

Fig. 1

Expression levels of synapse-related genes are altered in major depressive disorder (MDD). Microarray findings were validated by real-time polymerase chain reaction on subset of samples from the same cohort. mRNA levels of selected genes involved in synaptic functioning (a) SNAP25, (b) DLG2 (SAP93), (c) MAP1A, and (d) MAP1B) are shown. Data are expressed as mean fold change ±S.E.M.(n=6) ; * p≤0.05 compared to the healthy controls (paired Student’s t test). DG, dentate gyrus.

Fig. 2

Expression levels of glutamate α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors are altered in major depressive disorder (MDD). Microarray findings were validated by real-time polymerase chain reaction on a subset of samples from the same cohort. mRNA levels of (a) GLUR1 and (b) GLUR3 are shown. Data are expressed as mean fold change±S.E.M. (n=6) ; * p≤0.05 compared to the healthy controls (paired Student’s t test). DG, dentate gyrus.

Fig. 3

Expression levels of serotonin receptors are altered in major depressive disorder (MDD). Microarray findings were validated by real-time polymerase chain reaction on a subset of samples from the same cohort. mRNA levels of (a) HTR_2C, (b) HTR₄ and (c) HTR₇ are shown. Data are expressed as mean fold change ±S.E.M.(n=5–6) ; * p≤0.05 compared to the healthy controls (paired Student’s t test). DG, dentate gyrus.

Fig. 4

Effects of chronic unpredictable stress (CUS) and antidepressant treatment on expression of selected human target genes in the rat hippocampus. (a) Rats were exposed to CUS or control housing conditions ; saline (vehicle) or fluoxetine (Flx) were administered for the last 21 d of stress. CUS had no effect on the locomotor activity (LA), but evoked increased helpless behaviour in active avoidance test (AAT) and reduced sucrose intake in sucrose preference test (SPT) ; both of these behavioural effects of CUS were reversed by chronic Flx treatment (data not shown). Representative autoradiographs and quantitative analysis of (b) Snap25, (c) GluR1, (d) GluR3 and (e) Htr4 mRNA levels by in situ hybridization on coronal sections of dorsal rat hippocampus (scale bar, 1.0 mm). Results are expressed as mean±s.e.m. percent change over non-stressed control group (n=4or 5) ; * p<0.05 compared to the non-stressed control group, ^# p<0.05 compared to CUS group, ^$ p<0.07 compared to CUS group (two-way analysis of variance and Fisher’s PLSD post-hoc analysis).