Differential effects of corticosterone on the colocalization of reelin and neuronal nitric oxide synthase in the adult hippocampus in wild type and heterozygous reeler mice

The Inflammation-Induced Dysregulation of Reelin Homeostasis Hypothesis of Alzheimer's Disease

Alzheimer's disease (AD) accounts for most dementia cases, but we lack a complete understanding of the mechanisms responsible for the core pathology associated with the disease (e.g., amyloid plaque and neurofibrillary tangles). Inflammation has been identified as a key contributor of AD pathology, with recent evidence pointing towards Reelin dysregulation as being associated with inflammation. Here we describe Reelin signaling and outline existing research involving Reelin signaling in AD and inflammation. Research is described pertaining to the inflammatory and immunological functions of Reelin before we propose a mechanism through which inflammation renders Reelin susceptible to dysregulation resulting in the induction and exacerbation of AD pathology. Based on this hypothesis, it is predicted that disorders of both inflammation (including peripheral inflammation and neuroinflammation) and Reelin dysregulation (including disorders associated with upregulated Reelin expression and disorders of Reelin downregulation) have elevated risk of developing AD. We conclude with a description of AD risk in various disorders involving Reelin dysregulation and inflammation.

Emotional behaviors as well as the hippocampal reelin expression in C57BL/6N male mice chronically treated with corticosterone

Depression is a common psychiatric disorder affecting around 300 million people worldwide. Serum cortisol and glucocorticoid levels in humans are reportedly higher in patients with depression compared to controls. Furthermore, rodents repeatedly treated with exogenous corticosterone (CORT), a glucocorticoid in rodents, exhibit deficits in emotional behaviors. To confirm the availability of mice with chronic CORT treatment as an animal model of depression, we investigated the effect of chronic CORT treatment on depression-like behavioral and neuropathological phenotypes in C57BL/6N male mice. Behavioral studies showed depression- and anxiety-like behaviors in mice treated with CORT compared with control mice in the forced-swim and elevated-plus maze tests. Additionally, treated mice represented anhedonia and social behavior impairments in the sucrose preference and social interaction tests, respectively. Brains of depression patients have altered expression of reelin, an extracellular matrix protein involved in neuronal development and function. Likewise, in the present study, mice with chronic CORT treatment also exhibited reelin downregulation in cells of the hippocampus. Hence, we investigated therapeutic effects of reelin supplementation on CORT-induced behavioral abnormalities in mice. Microinjections of recombinant reelin protein into the hippocampus did not rescue behavioral deficits in mice with chronic CORT treatment. These results suggest that C57BL/6N male mice chronically treated with CORT are a suitable animal depression model, in which depressive behaviors may occur independently of the alternation of hippocampal Reelin expression.

Severe life stress, mitochondrial dysfunction, and depressive behavior: A pathophysiological and therapeutic perspective

Mitochondria are responsible for providing our cells with energy, as well as regulating oxidative stress and apoptosis, and considerable evidence demonstrates that mitochondria-related alterations are prevalent during chronic stress and depression. Here, we discuss how chronic stress may induce depressive behavior by potentiating mitochondrial allostatic load, which ultimately decreases energy production, elevates the generation of harmful reactive oxygen species, damages mitochondrial DNA and increases membrane permeability and pro-apoptotic factor release. We also discuss how mitochondrial insults can exacerbate the immune response, contributing to depressive symptomology. Furthermore, we illustrate how depression symptoms are associated with specific mitochondrial defects, and how targeting of these defects with pharmacological agents may be a promising avenue for the development of novel, more efficacious antidepressants. In summary, this review supports the notion that severe psychosocial stress induces mitochondrial dysfunction, thereby increasing the vulnerability to developing depressive symptoms.

Fast-acting antidepressant-like effects of Reelin evaluated in the repeated-corticosterone chronic stress paradigm

The present report examines the effects of repeated or single intrahippocampal Reelin infusions on measures of depressive-like behavior, cognition, and hippocampal neurogenesis in the repeated-corticosterone (CORT) paradigm. Rats received subcutaneous injections of CORT for 3 weeks and Reelin was infused through an inserted canula in the left hippocampus on days 7, 14, and 21, or only on day 21 of CORT injections. CORT increased immobility in the forced-swim test and impaired object-location memory. Notably, these effects were reversed by both repeated and single-Reelin infusions. CORT decreased both the number and complexity of doublecortin-labeled maturing newborn neurons in the dentate gyrus subgranular zone, and a single-Reelin infusion increased the number but not complexity of newborn neurons, while repeated Reelin infusions restored both. Injection of the AMPA antagonist CNQX blocked the rescue of the behavioral phenotype by Reelin but did completely block the effects of Reelin on hippocampal neurogenesis. Reelin is able to rescue the deficits in AMPA, NMDA, GABAA receptors, mTOR and p-mTOR induced by CORT. These novel results demonstrate that a single intrahippocampal Reelin infusion into the dorsal hippocampus has fast-acting antidepressant-like effects, and that some of these effects may be at least partially independent of Reelin actions on hippocampal neurogenesis.

Exploring the Potential Antidepressant Mechanisms of TNFα Antagonists

Human and animal studies suggest an intriguing relationship between the immune system and the development of depression. Some peripherally produced cytokines, such as TNF-α, can cross the blood brain barrier and result in activation of brain microglia which produces additional TNF-α and fosters a cascade of events including decreases in markers of synaptic plasticity and increases in neurodegenerative events. This is exemplified by preclinical studies, which show that peripheral administration of pro-inflammatory cytokines can elicit depression-like behavior. Importantly, this depression-like behavior can be ameliorated by anti-cytokine therapies. Work in our laboratory suggests that TNF-α is particularly important for the development of a depressive phenotype and that TNF-α antagonists might have promise as novel antidepressant drugs. Future research should examine rates of inflammation at baseline in depressed patients and whether anti-inflammatory agents could be included as part of the treatment regimen for depressive disorders.

Mitochondria and Mood: Mitochondrial Dysfunction as a Key Player in the Manifestation of Depression

Human and animal studies suggest an intriguing link between mitochondrial diseases and depression. Although depression has historically been linked to alterations in monoaminergic pharmacology and adult hippocampal neurogenesis, new data increasingly implicate broader forms of dampened plasticity, including plasticity within the cell. Mitochondria are the cellular powerhouse of eukaryotic cells, and they also regulate brain function through oxidative stress and apoptosis. In this paper, we make the case that mitochondrial dysfunction could play an important role in the pathophysiology of depression. Alterations in mitochondrial functions such as oxidative phosphorylation (OXPHOS) and membrane polarity, which increase oxidative stress and apoptosis, may precede the development of depressive symptoms. However, the data in relation to antidepressant drug effects are contradictory: some studies reveal they have no effect on mitochondrial function or even potentiate dysfunction, whereas other studies show more beneficial effects. Overall, the data suggest an intriguing link between mitochondrial function and depression that warrants further investigation. Mitochondria could be targeted in the development of novel antidepressant drugs, and specific forms of mitochondrial dysfunction could be identified as biomarkers to personalize treatment and aid in early diagnosis by differentiating between disorders with overlapping symptoms.

Peripheral Etanercept Administration Normalizes Behavior, Hippocampal Neurogenesis, and Hippocampal Reelin and GABAA Receptor Expression in a Preclinical Model of Depression

Depression is a serious psychiatric disorder frequently comorbid with autoimmune disorders. Previous work in our lab has demonstrated that repeated corticosterone (CORT) injections in rats reliably increase depressive-like behavior, impair hippocampal-dependent memory, reduce the number and complexity of adult-generated neurons in the dentate gyrus, decrease hippocampal reelin expression, and alter markers of GABAergic function. We hypothesized that peripheral injections of the TNF-α inhibitor etanercept could exert antidepressant effects through a restoration of many of these neurobiological changes. To test this hypothesis, we examined the effect of repeated CORT injections and concurrent injections of etanercept on measures of object-location and object-in-place memory, forced-swim test behavior, hippocampal neurogenesis, and reelin and GABA β2/3 immunohistochemistry. CORT increased immobility behavior in the forced swim test and impaired both object-location and object-in-place memory, and these effects were reversed by etanercept. CORT also decreased both the number and complexity of adult-generated neurons, but etanercept restored these measures back to control levels. Finally, CORT decreased the number of reelin and GABA β2/3-ir cells within the subgranular zone of the dentate gyrus, and etanercept restored these to control levels. These novel results demonstrate that peripheral etanercept has antidepressant effects that are accompanied by a restoration of cognitive function, hippocampal neurogenesis, and GABAergic plasticity, and suggest that a normalization of reelin expression in the dentate gyrus could be a key component underlying these novel antidepressant effects.

Reelin-Related Disturbances in Depression: Implications for Translational Studies

The finding that reelin expression is significantly decreased in mood and psychotic disorders, together with evidence that reelin can regulate key aspects of hippocampal plasticity in the adult brain, brought our research group and others to study the possible role of reelin in the pathogenesis of depression. This review describes recent progress on this topic using an animal model of depression that makes use of repeated corticosterone (CORT) injections. This methodology produces depression-like symptoms in both rats and mice that are reversed by antidepressant treatment. We have reported that CORT causes a decrease in the number of reelin-immunopositive cells in the dentate gyrus subgranular zone (SGZ), where adult hippocampal neurogenesis takes place; that down-regulation of the number of reelin-positive cells closely parallels the development of a depression-like phenotype during repeated CORT treatment; that reelin downregulation alters the co-expression of reelin with neuronal nitric oxide synthase (nNOS); that deficits in reelin might also create imbalances in glutamatergic and GABAergic circuits within the hippocampus and other limbic structures; and that co-treatment with antidepressant drugs prevents both reelin deficits and the development of a depression-like phenotype. We also observed alterations in the pattern of membrane protein clustering in peripheral lymphocytes in animals with low levels of reelin. Importantly, we found parallel changes in membrane protein clustering in depression patients, which differentiated two subpopulations of naïve depression patients that showed a different therapeutic response to antidepressant treatment. Here, we review these findings and develop the hypothesis that restoring reelin-related function could represent a novel approach for antidepressant therapies.