Prefrontal changes in the glutamate-glutamine cycle and neuronal/glial glutamate transporters in depression with and without suicide
Introduction
Suicide is an important public health problem that has a strong association with psychopathology, in particular with mood disorders (Hawton and van Heeringen, 2009). A large body of evidence suggests that neurobiological factors play an important role in the predisposition to suicide (Ernst et al., 2009b, Mann and Currier, 2012). Although there is some overlap, this neurobiological predisposition appears to differ considerably from the predisposition to mood disorders per se, especially in the case of major depressive disorder (MDD) (Egeland and Sussex, 1985, Mann and Currier, 2010, Turecki, 2014).
The prefrontal cortex (PFC) is one of the major brain structures to modulate the stress response. It can be stimulatory or inhibitory, depending on specific sub-regions or stressor types. The PFC has also been implicated in mood disorders and there is a negative correlation between the severity of post-stroke depression and the distance between the brain injury site and the frontal pole (Narushima et al., 2003). Furthermore, dysfunctioning anterior cingulate cortex (ACC) and dorsolateral prefrontal cortex (DLPFC) were shown by functional and structural imaging in MDD (Drevets, 2000, Drevets et al., 2008) as well as altered glucose metabolism and blood flow (Drevets, 1999, Mayberg, 2003, Seminowicz et al., 2004). Also postmortem data support PFC involvement in MDD: e.g. reduced glial cell density and neuronal size were observed in the ACC and DLPFC (Cotter et al., 2001, Cotter et al., 2002, Rajkowska and Miguel-Hidalgo, 2007, Rajkowska et al., 1999).
Recent large-scale gene array studies in post-mortem subjects have provided strong support for alterations in GABAergic and glutamatergic neurotransmission in the PFC of depression (Bernard et al., 2011, Choudary et al., 2005, Duric et al., 2013). However, these postmortem studies consisted largely of psychiatric patients who had committed suicide, whereas none of the subjects in the control group had a psychiatric disease during their lifetime or committed suicide. Therefore it remains unclear whether these changes in the PFC were related to depression or to suicide per se.
In a recent study we found that only few GABA and glutamate-related genes were different in the ACC of elderly depressed patients who had not committed suicide (Zhao et al., 2012), while there were obvious changes in depressed patients who had committed suicide (Zhao et al., 2015). The present study therefore studied whether changes in the ACC and DLPFC occur in terms of the glutamate-glutamine cycle and glutamate transporters, in relation to depression with and without suicide.
Glutamate is synthesized from glutamine by glutaminase in neurons. After glutamate is released from the synaptic terminal, it is taken up by astrocytes through high affinity sodium-dependent neuronal glutamate transporters, such as the excitatory amino acid transporter (EAAT) 1 and EAAT2 (Bar-Peled et al., 1997, Chaudhry et al., 1995, Kugler and Schmitt, 2003, Milton et al., 1997), where it is converted back into glutamine by glutamine synthetase (Martinez-Hernandez et al., 1977). Glutamine is then transported back to the neurons by the neutral amino acid transporter (SNAT) 1 and SNAT2, ready for reuse (Melone et al., 2004). Interestingly, brain-derived neurotrophic factor (BDNF)-induced increases in SNAT1 expression are required for the regulation of dendritic length and neuronal complexity during development (Burkhalter et al., 2007), but BDNF is also clearly linked to depression and suicide (Dwivedi et al., 2003, Ernst et al., 2009b, Grah et al., 2014, Qi et al., 2013). This suggests that the glutamate-glutamine cycle is not only an important constituent of the glutamatergic neurotransmission system in physiology, but may also be a vulnerable pathway in psychiatric disorders due to its linkage with both BDNF and glutamate and GABA neurotransmission. Intrasynaptic glutamate can also be taken up by EAAT3 and EAAT4, which are located on neurons (Danbolt, 2001, Rothstein et al., 1994, Yamada et al., 1997). The EAAT family of glutamate transporters shows homology to the neutral amino acid transporters (ASCT) 1 and ASCT2 (Arriza et al., 1993, Utsunomiya-Tate et al., 1996). Studies showed that ASCT1 in glutamate transmission mediates the efflux of glutamate from the neuron into the synaptic junction via calcium-dependent release, and that ASCT2 activity plays an important role in the glutamine-glutamate cycle between neurons and glia by facilitating the efflux of glutamine from glial cells (Broer et al., 1999).
In the present study, we hypothesized that the glutamate-glutamine cycle is impaired in the brains of depressed individuals who committed suicide and that the neuronal and glial glutamate transporters associated with this cycle are dysregulated. Thus, we used real-time quantitative PCR (qPCR) to compare the gene expression patterns in the PFC of the astrocytic glutamate-glutamine cycle and neuronal/glial glutamate transporters in depressed patients in relation to suicide.
Section snippets
Subjects from the Stanley Medical Research Institute (SMRI)
Brain material was obtained from the SMRI, after obtaining permission for a brain autopsy and for the use of the brain material and clinical data for research purposes. Diagnostic and Statistical Manual of Mental Disorders (DSM)-IV diagnoses of MDD were made independently by two senior psychiatrists based on the medical records and, when necessary, telephone interviews with family members. This systematized procedure was carried out as described before (Torrey et al., 2000). The demographic
Changes in glutamate-glutamine cycle gene expression are related to suicide, not to depression
Brain material obtained from the SMRI was analyzed for changes in the genes involved in the glutamate-glutamine cycle and neuronal/glial glutamate transporters. We determined the RNA integrity value (RIN) to assess whether the quality of the human post-mortem tissue RNA isolated from the ACC or DLPFC did not differ between patient groups (Stan et al., 2006). The RIN did not show any significant differences between the diagnostic groups in the SMRI material (RIN value of the ACC from MDD-S
Discussion
In the present study we found in the SMRI cohort that ASCT2 and neuronal located molecule transcript levels were significantly higher in the ACC of MDD-S patients than in the MDD-NS patients or the control subjects. In contrast, decreased ASCT1 and astroglia located molecule transcript level were found in the DLPFC of MDD-S patients compared to MDD-NS patients. Furthermore, most of the target genes were increased when comparing MDD-NS with control subjects in the same brain area. We did not
Funding
This investigation was supported by the Royal Netherlands Academy of Arts and Sciences (10CDP037).
Contributors
J.Zhao, P.J.Lucassen and D.F.Swaab designed the experiments and prepared the manuscript. J.Zhao, D.J. van Wamelena, X-R Qi and S-F Gao performed the experiments. J.Zhao collected data and together with R. W.H.Verwer did Statistical analysis. D.F.Swaab obtained funding.
Acknowledgement
Postmortem brain tissue was donated by The Stanley Medical Research Institute Brain Collection and by the Netherlands Brain Bank. We are indebted to them for providing us with the brain material and patient information. We thank Unga Unmehopa for her technical help, Wilma Verweij and Dr. Helmut Kessels for critical reading of the manuscript.
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