Prefrontal alterations in GABAergic and glutamatergic gene expression in relation to depression and suicide
Introduction
Major depressive disorder (MDD) is a serious and disabling psychiatric illness that affects 121 million individuals worldwide, and is predicted to be the second most common cause of disability, after heart disease, by 2020 (Baune et al., 2007). MDD is also the most common psychiatric diagnosis associated with suicide. Up to 15% of those who are clinically depressed eventually commit suicide, and more than half of all people who commit suicide meet the criteria for depressive disorder (Alsalman and Alansari, 2016; Cavanagh et al., 2003). Several studies on a relationship between monoaminergic neurotransmission, depression and suicide have emerged in the past decades. However, both the limited efficacy and the delayed onset of the therapeutic effects of selective serotonin reuptake inhibitors (SSRIs), the most commonly prescribed antidepressant drugs, raise doubts as to whether the monoaminergic system is indeed in general a primary affected system in depression (Lawrence et al., 2017; Thase et al., 2005; Trivedi et al., 2006).
In recent years, increasing evidence suggests that, in addition to hypothalamic neuroendocrine changes (Bao et al., 2008; Qi et al., 2013a; Swaab et al., 2005), an impaired balance between inhibition and excitation within the prefrontal cortex (PFC) and related limbic brain circuitry is involved in MDD, probably mediated by altered gamma-aminobutyric acid (GABA) and glutamate neurotransmission (Gao and Bao, 2011; Ghosal et al., 2017). Large-scale gene array studies in post-mortem tissue have provided strong support for alterations in GABAergic and glutamatergic neurotransmission in depression. For instance, Choudary and colleagues demonstrated significant down-regulation of SLC1A2 and SLC1A3, and up-regulation of several glutamate and GABA-A receptor subunits by using microarray analysis from MDD patients (Choudary et al., 2005).
Later, altered glutamate related genes, SLC1A2, SLC1A3 and GLUL were also found in the MDD patients by microarray and confirmed by quantitative real-time PCR (Q-PCR) and in situ hybridization (Bernard et al., 2011). Another study also demonstrates significant dysregulation of glutamate receptor subunit genes GRIA1 and GRIA3 in the subjects diagnosed with MDD by microarrays and a subgroup of identified genes was confirmed by Q-PCR (Duric et al., 2013). Moreover, morphometric studies have reported a reduced density and size of cortical neurons (Bernstein et al., 2016; Chana et al., 2003; Rajkowska et al., 1999), an effect that has been attributed to alterations in interneurons in some (Maciag et al., 2010; Rajkowska et al., 2007) but not all studies (Cotter et al., 2002; Lucassen et al., 2014). In particular the PFC has received considerable attention in recent years given its role in inhibitory control, emotions and mood, and prominent changes in depression and suicide (Fuster, 2008; Kekesi et al., 2012; Qi et al., 2013a, 2013b; Zhang et al., 2013).
As post-mortem studies have often been inconsistent, the exact molecular mechanisms underlying the aetiology and pathophysiology of depression and suicide, and factors that distinguish these, have so far remained obscure. One critical issue, however, for interpreting discrepancies between existing studies is the different case-control matching strategies. For instance, many studies that claimed to determine molecular alterations in relation to depression had selected depressed patient groups in which most patients had committed suicide, and compared these to non-psychiatric controls (Bernard et al., 2011; Deschwanden et al., 2011; Kimoto et al., 2015; Kunii et al., 2015; Martins-de-Souza et al., 2012; Matosin et al., 2014; Shelton et al., 2011). Without an adequate comparison to a disease control group, consisting of depressed patients who did not commit suicide, it is impossible to determine whether the reported alterations are due to depression or to suicide. On the other hand, studies that claimed to show changes in relation to suicide per se, often compared suicide cases to matched controls without any psychiatric disorder (Du et al., 2014; Kekesi et al., 2012; Lopez et al., 2014; Pandey et al., 2013; Poulter et al., 2008; Schiavone et al., 2016; Sequeira et al., 2012; Valdizan et al., 2010), thus disregarding the fact that the majority of suicide cases suffer from psychiatric disorders, including depression, anxiety, substance abuse, schizophrenia or personality disorders (Hawton and van Heeringen, 2009).
To investigate whether the aspect of suicide is relevant for the gene expression changes in depression reported before by others, we previously studied a group of well-documented depressed patients who did not commit suicide, and found that surprisingly few GABA and glutamate-related genes had changed in expression in the prefrontal cortex (PFC) (Zhao et al., 2012). This suggested that the alterations reported in this brain area before, may be related to suicide rather than to depression per se. Furthermore, we recently found indications that depressed patients who committed suicide show different expression patterns of genes related to the glutamate-glutamine cycle (Zhao et al., 2016) and of stress related molecules, such as corticotropin-releasing hormone (CRH) and nitric oxide synthase (NOS), than depressed patients who died of causes other than suicide (Zhao et al., 2015).
Here, we follow up on these initial studies in a larger, well-characterized cohort from the Stanley Medical Research Institute (SMRI). We determined the expression patterns of genes involved in different aspects of both the GABA and glutamate pathways, comparing transcript levels from MDD patients who died of suicide, to MDD patients who died of other causes, and to matched controls without a neuropsychiatric disorder. Our results show that changes in the genes involved in glutamatergic and GABAergic neurotransmission may be related either to suicide or to depression per se, depending on the brain area studied.
Section snippets
Material from the Stanley Medical Research Institute (SMRI)
Brain samples were collected from the SMRI (Bethesda, MD, USA). Permission to use the brain material was provided by the next of kin. Diagnoses were made according to the Diagnostic and Statistical Manual of Mental Disorders (DSM) IV (American Psychiatric Association, 1994). All brains were examined microscopically to exclude cases with pathological signs of neurodegeneration or other lesions. Exclusion criteria included anyone over age 70, anyone with a history of seizures or other
Transcription of genes encoding glutamatergic synapse markers
As an indication for changes in excitatory synaptic transmission in the PFC of MDD patients, we analysed Q-PCR values of 16 gene transcripts that encode proteins characteristic for glutamatergic synapses. The glutamatergic markers include genes encoding: AMPA-receptor subunits GRIA1, GRIA2, GRIA3, GRIA4; kainate-receptor subunit GRIK1; NMDA-receptor subunits GRIN1, GRIN2A, GRIN2B; metabotropic glutamate receptors GRM1 (mGluR1), GRM2 (mGluR2), GRM3 (mGluR3); glutaminase (GLS); vesicular
Discussion
Several studies have suggested an increased expression of genes involved in synaptic transmission in the PFC of MDD patients (Choudary et al., 2005; Sequeira et al., 2007, 2009). However, since a large proportion of the MDD patients in these studies were suicide victims, it was unclear whether these gene changes were due to MDD or related to suicide. In brain material of the well-characterized SMRI cohort, we found the expression of both glutamate- and GABA-related genes to be increased in the
Conflict of interest
None to declare.
Acknowledgments
Postmortem brain tissue was donated by The Stanley Medical Research Institute Brain Collection. We are indebted to Dr. Maree Webster for providing us with the brain material and patient information. We thank Unga Unmehopa for her technical help, Wilma Verweij for critical reading of the manuscript. This investigation was supported by project (10CDP037) of the Royal Netherlands Academy of Arts and Sciences. PJL is supported by the Dutch Brain Foundation.
References (90)
- et al.
Relationship of suicide ideation with depression and hopelessness
Psychiatry
(2016) - et al.
Ketamine for depression: where do we go from here?
Biol. Psychiatr.
(2012) - et al.
Effect of clozapine, haloperidol, or M100907 on phencyclidine-activated glutamate efflux in the prefrontal cortex
Biol. Psychiatr.
(2001) - et al.
Instantaneous modulation of gamma oscillation frequency by balancing excitation with inhibition
Neuron
(2009) - et al.
The stress system in depression and neurodegeneration: focus on the human hypothalamus
Brain Res. Rev.
(2008) - et al.
Medical disorders affect health outcome and general functioning depending on comorbid major depression in the general population
J. Psychosom. Res.
(2007) - et al.
Two-dimensional assessment of cytoarchitecture in the anterior cingulate cortex in major depressive disorder, bipolar disorder, and schizophrenia: evidence for decreased neuronal somal size and increased neuronal density
Biol. Psychiatr.
(2003) - et al.
The density and spatial distribution of GABAergic neurons, labelled using calcium binding proteins, in the anterior cingulate cortex in major depressive disorder, bipolar disorder, and schizophrenia
Biol. Psychiatr.
(2002) - et al.
Catechol-O-methyltransferase Val158Met polymorphism and altered COMT gene expression in the prefrontal cortex of suicide brains
Prog. Neuro Psychopharmacol. Biol. Psychiatr.
(2014) - et al.
What happens to patients with treatment-resistant depression? A systematic review of medium to long term outcome studies
J. Affect. Disord.
(2009)