Success in the pharmacological treatment of major depressive disorder (MDD) would be facilitated greatly by improved knowledge of the mechanisms underlying emotional regulation by the prefrontal cortex (PFC) and its connections. Pharmacotherapies for MDD are based on the putative effects of monoamine- related drugs on neuronal function; however, these drugs are only partially efficacious. Our research has shown that PFC glial cells, including astrocytes and oligodendrocytes, may contribute to the pathophysiology of depression. Thus, to design more effective treatments for depression that enhance prefrontal function, it is critical to determine the mechanisms underpinning glial dysfunction in the PFC and how they can be leveraged to design new therapeutic approaches. Among the morphological and neurochemical alterations in PFC astrocytes and oligodendrocytes in MDD, we have found a dramatic decrease in levels and tissue distribution of connexin 43 (Cx43), the major gap junction protein of astrocytes. In the cerebral cortex, Cx43 forms gap junctions between astrocytes and of these with the oligodendrocytes. Thus, interacting astrocyte and oligodendrocyte pathology may result in altered PFC connectivity. Importantly, rats subjected to chronic unpredictable stress (CUS), a model for depression-like behaviors, undergo significant Cx43 reduction in the PFC, while gap junction blockers applied to the PFC result in depression-like behaviors. Furthermore, suppression of astrocyte Cx43 and Cx30 disrupts myelin in animal models. Recent animal studies suggest that specific myelin or gap junction disruption in the PFC plays a significant role in depression-like behaviors. This evidence fits with findings that brain connectivity patterns can be significantly altered even after non- catastrophic, restricted changes in myelin plasticity. Thus, PFC connectivity to other brain areas could be deeply affected by connexin-related changes in myelin plasticity or maintenance, thereby contributing to depression symptoms such as motor retardation, impaired decision-making or abnormal emotional regulation. We hypothesize that reduction in Cx43 and Cx30 in the prelimbic cortex (PLC, a division of the rat PFC) in response to CUS, or by molecular means, will disrupt myelin integrity and result in depression-like behaviors. By contrast, PFC-targeted enhancement of Cx43 or myelin should mitigate depression-like behaviors induced by CUS. These hypotheses will be tested with three specific aims:
Specific aim 1 : CUS will result in the induction of depression-like behaviors and reduction in the levels of astrocyte and oligodendrocyte proteins involved in myelin maintenance.
Specific aim 2 : Disruption of Cx43 expression and myelin in the rat PLC will result in depression-like behaviors along with reduction in the levels of astrocyte and oligodendrocyte proteins involved in myelin maintenance.
Specific aim 3 : Overexpression of Cx43 or the myelin protein MBP in the PLC will result in reduction of depression-like behaviors and cognitive impairment caused by CUS along with an increase proteins involved in myelin maintenance.
The brain mechanisms responsible for major depression involve not only neurons, but also astrocytes and oligodendrocytes. We propose that a mechanism based in the interaction between astrocytes and oligodendrocytes contributes to depression and could be leveraged for new antidepressant treatments.
|Rajkowska, Grazyna; Legutko, Beata; Moulana, Mohadetheh et al. (2018) Astrocyte pathology in the ventral prefrontal white matter in depression. J Psychiatr Res 102:150-158|
|Miguel-Hidalgo, José Javier; Hall, Katherine O; Bonner, Hannah et al. (2017) MicroRNA-21: Expression in oligodendrocytes and correlation with low myelin mRNAs in depression and alcoholism. Prog Neuropsychopharmacol Biol Psychiatry 79:503-514|