In a symposium titled ‘Biological Markers in Depression: From Animals to Humans,’ original work was utilized by Dr. Gustavo Turecki (McGill University, Canada) to show how molecular changes associated with antidepressant response may be investigated by using peripherally gathered neuronal-derived exosomes. Dr. Argel Aguilar-Valles (Carleton University, Canada) discussed how regulation of mRNA translation initiation is associated with neuroinflammation and, using work from his laboratory, showed how this may have implications for depression and treatment of such. Finally, Dr. Stefano Comai (University of Padua, Italy), highlighted his group’s work investigating the relationship in people with depression between the tryptophan to serotonin or kynurenine pathways and the endocannabinoid system.
Neuronal-derived exosomes can be utilized to show antidepressant response
As exosomes are a type of extracellular vesicle (EV) that carry molecular cargo, explained Dr Turecki, they can be isolated from the blood and both their contents and molecular signature of their originating cell can be analyzed1. EVs can cross the blood-brain barrier2 and neural-derived small EVs (NDEVs) can be isolated in peripheral blood, providing a non-invasive look at the brain3.
Brain derived NDEVs from post-mortem tissue and plasma NDEVs have an almost 90% overlap in terms of their cargo, showing they are comparable. Genes found within NDEVS are mostly of neuronal and brain factors3.
Peripheral neural-derived extracellular vesicles might be used to investigate antidepressant response
In a study in Dr Turecki’s laboratory, while no difference was found in the total EV count between patients with depression and healthy controls, both total EVs and NDEVs were significantly smaller in the former. With antidepressant treatment, there was a significant increase in NDEV size in those that responded at 8 weeks, along with a significant increase in the number of EVs released3.
In the NDEV cargo, three of the significant micro RNAs (miRNA) were predictive of response to AD. These findings were replicated for two of the miRNAs in an independent antidepressant treated sample. These findings, explained Dr Turecki, point to promising miRNA targets to investigate in future studies3.
Regulation of mRNA translation involved in neuroinflammation has implications in depression
In people with major depressive disorder, there is evidence of an alteration in signaling pathways that control mRNA translation initiation, including decreased mammalian target of rapamycin (mTOR) in the prefrontal cortex (PFC) and decreased extracellular-signal-regulated kinase (ERK) in the hippocampus. In animal studies, these can be increased by antidepressants4.
Regulation of mRNA translation in neurons is important in synaptic plasticity and spine morphogenesis5. In his talk, Dr Aguilar-Valles focused on the eukaryotic initiation factor 4F complex (eIF4F), which is regulated, in part, by mTORC1 at its binding protein6. Using a mouse mutant for a single nucleotide in the 4E protein, Dr Aguilar-Valles and colleagues found this pathway was involved in activation of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kB) and subsequent overexpression of brain located inflammatory mediations such as tumor necrosis factor-a (TNFa)7.
Inflammation following aberrant mRNA translation initiation is associated with depressive symptoms
Decreased levels of the NF-kb inhibitor IkBa can lead to NF-kb overactivation, as identified in mice mutant for eIf4e phosphorylation8. In work presented here, Dr Aguilar-Valles showed that brain expression of IkBa is controlled by the MAP kinase-interacting kinase 1 (MNK1)/2-eIF4E pathway. In the eIF4E knock-in mice, they found significant increases in TNFa along with microgliosis in the hippocampus and PFC, all indicative of inflammation. These mice also presented behavior indicative of depression, such as increased latency to feed in a novelty suppressed feeding condition and decreased exploration of the center of an open field.
This group also looked at neurotransmission in the PFC and found that serotonin response was severely blunted in the eIF4E knock-in mice. Using a dominant negative protein to block TNFa, such behavioral deficits were reversed, as was the induction of excitation in the PFC.
The endocannabinoid system in depression
Dr Comai posited that endocannabinoids are involved in symptoms of depression. The two main endocannabinoids, anandamide and 2-arachidonoylglycerol (2AG), are produced post-synaptically and then function pre-synaptically as ‘retrograde neuromodulators’9.
Here he presented work carried out in his laboratory that had the aim of investigating the potential relationship in people with depression between circulating endocannabinoid biomarkers and the tryptophan to 5-HT and kynurenine pathways10. While 5-HT is not able to cross the blood brain barrier, peripheral tryptophan concentrations have been shown to accurately reflect 5-HT levels in the brain11.
There is a potential role for endocannabinoids in the tryptophan degradation pathways
In Dr Comai’s study, higher degradation of circulating tryptophan to kyneurenine and higher levels of serum 2AG were found in patients with depression. Also found was a negative correlation between tryptophan and depression severity and a positive assocation between tryptophan degradation into kyneurenine and endocannabinoids. Additionally, good discrimination between the patient and controls group was found for biomarkers of degradation of tryptophan into kyneurenine. These data, Dr Comai concluded, “confirm the crosstalk between the endocannabinoid and 5-HT systems and their involvement in depression.”
Our correspondent’s highlights from the symposium are meant as a fair representation of the scientific content presented. The views and opinions expressed on this page do not necessarily reflect those of Lundbeck.
