Research in our laboratories and others has revealed disturbances at anatomic, cellular and molecular levels in the central nervous system (CNS) in major depressive disorder (MDD). The alterations include structural and molecular changes not only in neurons but also in astrocytes and oligodendrocytes, glial cells that critically support neuronal function in gray and white matter (GM and WM). Similar glial cell pathology is observed in animal models of depression-like behaviors caused by stress, a risk factor for depression. This proposal aims to bridge a gap in our knowledge about the relevance of pathological disturbances of WM astrocytes in MDD by studying the interaction of astrocytes processes at the nodes of Ranvier (NR) of myelinated axons and the behavioral effects of disturbing such an interaction. Animal studies will be used to examine whether pathology of the interaction between astrocytes and NRs is a potential mechanism underpinning WM-based connectivity disturbances in depression. We and others have detected alterations in oligodendrocyte morphology and expression of myelin components in the orbitofrontal cortex (OFC) and adjacent WM in depression and animal models of stress. Importantly, interactions of astrocyte processes with axons around NRs are seminal to myelin stability and signal propagation, and thus critical to the normal connectivity between the OFC and brain regions involved in the pathophysiology of depression. However, despite the vital role of NRs in signal propagation, it is unknown whether astrocyte processes abutting around and abutting NRs contribute to the physiopathology of WM in depression, or to depression-like behaviors in animal models such as chronic unpredictable stress (CUS). This project will test whether reduced astrocyte processes abutting NRs in the OFC WM and anterior corpus callosum (CC), as well as diminished EAAT1, proteoglycans, and their axonal adhesion molecules at NRs will increase the probability of depression-like behaviors. Likewise, we hypothesize that astrocyte and axon NR components in the OFC WM from postmortem human brain will be significantly reduced in subjects with MDD. The hypotheses will be tested with these aims:
Specific aim 1 : We will use morphometry and immunohistochemistry to test the hypothesis that MDD in humans is associated with reduced extent of astrocyte processes contacting NRs and lower levels of NR astrocyte-related ECMMs and partner axonal proteins in the OFC WM, but not in the WM of the occipital cortex.
Specific aim 2 : We will test the hypothesis that CUS decreases the extent of astrocyte processes, and the levels of ECMMs and their axonal partners at NRs in OFC WM in rats.
Specific aim 3 : We will test the hypothesis that local suppression of astrocyte-related ECMMs results in depression-like behaviors in rats. If astrocytes and axonal NRs components are involved in the cellular and behavioral pathology of depression, future research in rodents will target the intracellular pathways that regulate these components, other cells that also contribute to NRs, namely NG2 cells, and the ability of antidepressants or other novel molecules to reverse the cellular and behavioral effects of CUS.
The brain mechanisms responsible for major depression involve not only neurons, but also astrocytes and oligodendrocytes, the latter forming myelin around the axons of neurons. We propose that a mechanism based on the interaction between astrocytes and the nodes of Ranvier of myelinated axons contributes to depression and could be leveraged for new antidepressant treatments.