Manon Lebel

Chronic stress promotes blood-brain barrier (BBB) integrity loss leading to passage of inflammatory mediators in mood-regulating brain areas and establishment of depressive behaviors. Conversely, neurovascular adaptations favoring stress resilience and preventive strategies to promote them are undetermined. We report that environmental enrichment dampens stress-induced loss of endothelial tight junction Claudin-5 (Cldn5) along with anxiety- and depression-like behaviors in male mice via an increase in fibroblast growth factor 2 (Fgf2). Coping with voluntary physical exercise also protects the BBB from stress deleterious effects by increasing Fgf2. Fgf2 is mostly expressed by glial cells, and viral-mediated astrocyte-specific Fgf2 upregulation prevents stress-induced social avoidance while downregulation increases stress susceptibility and blunts physical exercise benefits. Treatment of mouse and human endothelial cells with Fgf2 prior an immune challenge reduces BBB dysfunction, Cldn5 loss, and altered signaling supporting its protective role. Circulating FGF2 level is linked with depression severity and symptomatology in men and women reinforcing involvement of this growth factor in mood disorders.

Psychiatric disorders affect millions of people worldwide. Despite widespread use of conventional treatments targeting monoaminergic systems, remission rates remain low, and many individuals experience treatment resistance or relapse. Consequently, there has been growing interest in the involvement of other systems, with exacerbated immune responses and barrier alterations reported in clinical settings and preclinical models. Indeed, emerging evidence supports disruption of the blood–brain barrier (BBB) and intestinal barrier in the etiology and progression of psychiatric conditions, notably major depression, bipolar disorder, and generalized anxiety. The BBB is a highly selective structure whose integrity is maintained by endothelial cells, astrocytes, pericytes, and cellular adhesion molecules. Loss of BBB integrity has been increasingly recognized not only as a marker of psychiatric disorders but also as a contributing factor in their development. The BBB and intestinal barrier share anatomical features and functions, especially with the gut–vascular barrier, which remains understudied. Intestinal barrier dysfunction is a hallmark of inflammatory bowel disease (IBD), a condition with a high rate of comorbidity with psychiatric disorders. Both barriers are characterized by similar cellular components and signaling pathways regulating permeability. Psychological stress, a major risk factor for psychiatric conditions and IBD, renders the BBB and intestinal barrier hyperpermeable, feeding a vicious cycle of exacerbated inflammation and ultimately, mood changes as discussed here. We highlight key signaling pathways linked to barrier development and function, including Wnt/β‐catenin, VEGF, and FGF‐2, and argue that they may contribute to the pathophysiology of mental disorders and IBD, and could be targeted to develop innovative diagnostic tools and treatments. Key limitations and knowledge gaps are reviewed. To sum up, barrier‐related alterations have long been reported in clinical studies in psychiatry and are now receiving increasing attention at the mechanistic level, as they may be relevant to uncovering new therapeutic targets beyond traditional monoamine‐focused treatments. image

BACKGROUND: Poststroke depression (PSD) affects ≈33% of individuals 1 year after a stroke. Blood-brain barrier (BBB) dysfunction in the nucleus accumbens (NAc), a hub for emotional processing, reward, and mood regulation, has been linked to stress-induced depressive-like behaviors in male mice. Neurovascular alterations were also observed in postmortem tissue samples from men with a diagnosis of major depression. Thus, we aimed to investigate if BBB changes in the NAc could contribute to PSD pathophysiology. METHODS: Stereotaxic injection of ET-1 (endothelin-1), a potent vasoconstrictor, was performed in the NAc of male mice to create a focal brain stroke, and then, infarct size and localization were assessed and quantified. We subsequently evaluated transcriptomic and morphological effects of the infarct on BBB-related genes and cells in the NAc, particularly those known to be altered after stress exposure in mice or human depression. BBB integrity was assessed with a dextran dye, and magnetic resonance imaging scans were conducted before versus after the injection of Gadovist, a contrast agent. Last, a battery of behavioral tests related to depressive- and anxiety-like behaviors was performed to determine if an infarct in the NAc is sufficient to induce a PSD-like phenotype. RESULTS: Following ET-1 injection, ≈50% of the total lesion was observed in the NAc leading to BBB hyperpermeability in this brain area. BBB gene expression was impacted by ET-1, and also surgery alone and profiles were differentially regulated throughout time up to 14 days. Gliosis in the NAc was observed with increased reactivity of astrocytes and microglia. The effect of ET-1 on PSD-like symptoms was limited. However, body weight, sociability, and activity were affected by surgery with a more pronounced impact of ET-1 on social interactions compared with naive animals. CONCLUSIONS: While no clear PSD phenotype was observed following an ET-1–induced stroke in the NAc of male mice, our study shed light on the technical complexity of focal lesions in deep brain structures, an understudied phenomenon occurring in humans. We provide technical insights for the development of a mouse model of deep brain lesions, characterize its impact at molecular, cellular, and behavioral levels, and highlight the need to control for vascular alterations when performing stroke surgeries.

Blood–brain barrier (BBB) alterations contribute to stress vulnerability and the development of depressive behaviors. In contrast, neurovascular adaptations underlying stress resilience remain unclear. Here we report that high expression of astrocytic cannabinoid receptor 1 (CB1) in the nucleus accumbens (NAc) shell, particularly in the end-feet ensheathing blood vessels, is associated with resilience during chronic social stress in adult male mice. Viral-mediated overexpression of Cnr1 in astrocytes of the NAc shell results in baseline anxiolytic effects and dampens stress-induced anxiety- and depression-like behaviors in male mice. It promotes the expression of vascular-related genes and reduces astrocyte inflammatory response and morphological changes following an immune challenge with the cytokine interleukin-6, linked to stress susceptibility and mood disorders. Physical exercise and antidepressant treatment increase the expression of astrocytic Cnr1 in the perivascular region in male mice. In human tissue from male donors with major depressive disorder, we observe loss of CNR1 in the NAc astrocytes. Our findings suggest a role for the astrocytic endocannabinoid system in stress responses via modulation of the BBB.

Chronic stress can promote loss of blood-brain barrier (BBB) integrity leading to passage of circulating inflammatory mediators in mood-regulating brain areas and establishment of depressive behaviors. Conversely, neurovascular adaptations favoring resilience to stress exposure and preventive strategies to promote them remain poorly understood. Here, we report that environmental enrichment dampens stress-induced loss of endothelial tight junction protein Claudin-5 (Cldn5) along with anxiety- and depression-like behaviors in mice via an increase in fibroblast growth factor 2 (Fgf2). Coping with voluntary physical exercise also protects the BBB from stress deleterious effects by increasing Fgf2. This growth factor acts on endothelial cells by preventing Cldn5 loss, exacerbated inflammation, and social avoidance. Fgf2 is mostly expressed by glial cells and astrocyte-specific increase of Fgf2 expression dampens stress-induced social avoidance. Treatment of mouse and human endothelial cells with Fgf2 preceding an immune challenge with the proinflammatory cytokine tumor necrosis alpha (TNFα), known to be elevated after chronic stress and in depression, reduces BBB dysfunction, and altered cell signaling supporting its role in BBB protection. Circulating FGF2 level is linked with depression severity and symptomatology in humans supporting involvement of this growth factor in mood disorders and stress-induced BBB changes.

Summary: Chronic stress can promote loss of blood-brain barrier (BBB) integrity leading to passage of circulating inflammatory mediators in mood-regulating brain areas and establishment of depressive behaviors. Conversely, neurovascular adaptations favoring resilience to stress exposure remain undetermined. Here, we report that environmental enrichment dampens stress-induced loss of endothelial tight junction protein Claudin-5 (Cldn5) along with anxiety- and depression-like behaviors in mice via an increase in fibroblast growth factor 2 (Fgf2). Treatment of mouse and human endothelial cells with Fgf2 preceding an immune challenge with the proinflammatory cytokine TNFα, elevated after chronic stress and in depression, reduces BBB dysfunction, and altered cell signaling. Coping with voluntary physical exercise also protects the BBB from stress deleterious effects by increasing Fgf2 preventing Cldn5 loss, exacerbated inflammation, and social avoidance. Circulating FGF2 level is linked with depression severity and symptomatology in humans supporting involvement of this growth factor in mood disorders and stress-induced BBB changes.

Blood-brain barrier (BBB) alterations contribute to stress vulnerability and development of depressive behaviors. In contrast, neurovascular adaptations underlying stress resilience remain unexplored. Here, we report that high expression of astrocytic cannabinoid receptor 1 (CB1) in the nucleus accumbens (NAc) shell, particularly in the endfeet ensheathing blood vessels, is associated with resilience despite chronic social stress exposure. Viral-mediated overexpression of Cnr1 in astrocytes of the NAc shell has baseline anxiolytic effects and dampened stress-induced anxiety- and depression-like behaviors. It also reduced astrocyte inflammatory response and morphological changes following an immune challenge with the cytokine interleukin-6, linked to stress susceptibility and mood disorders. At the preventive and therapeutic level, physical exercise and antidepressant treatment increased perivascular astrocytic Cnr1 in mice. Loss of CNR1 was confirmed in the NAc astrocytes of depressed individuals. These findings suggest a role for the astrocytic endocannabinoid system in stress responses and possibly, human depression, via BBB modulation.

Environment is known to substantially alter mental state and behaviour across the lifespan. Biological barriers such as the blood-brain barrier (BBB) and gut barrier (GB) are major hubs for communication of environmental information. Alterations in the structural, social and motor environment at different stages of life can influence function of the BBB and GB and their integrity to exert behavioural consequences. Importantly, each of these environmental components is associated with a distinct immune profile, glucocorticoid response and gut microbiome composition, creating unique effects on the BBB and GB. These barrier-environment interactions are sensitive to change throughout life, and positive or negative alterations at critical stages of development can exert long-lasting cognitive and behavioural consequences. Furthermore, because loss of barrier integrity is implicated in pathogenesis of mental disorders, the pathways of environmental influence represent important areas for understanding these diseases. Positive environments can be protective against stress- and age-related damage, raising the possibility of novel pharmacological targets. This review summarizes known mechanisms of environmental influence - such as social interactions, structural complexity and physical exercise - on barrier composition, morphology and development, and considers the outcomes and implications of these interactions in the context of psychiatric disorders.

Experiences are linked to emotions impacting memory consolidation and associated brain neuronal circuits. Posttraumatic stress disorder is an example of strong negative emotions affecting memory processes by flashbacks of past traumas. Stress-related memory deficits are also observed in major depressive disorder (MDD). We recently highlighted that sex-specific blood-brain barrier (BBB) alterations underlie stress responses in mice and human depression. However, little is known about the relationship between emotional valence, memory encoding and BBB function. Here, we investigated the effects of novel object recognition (NOR) test, an experience considered of neutral emotional valence, on BBB properties in dorsal vs ventral hippocampus in the context of various environmental conditions (arena size, handling, age). The hippocampus is a brain area central for learning and memory processes with the dorsal and ventral subregions being associated with working memory vs reference memory retrieval, respectively. Expression of genes related to BBB integrity are altered in line with learning and memory processes in a region- and sex-specific manner. We observed correlations between poor learning, anxiety, stress-induced corticosterone release and changes in BBB-associated gene expression. Comparison of BBB transcriptomes between sexes also revealed profound differences at baseline in both ventral and dorsal hippocampus. Finally, we identified circulating vascular biomarkers, such as sE-selectin and Mmp-9, altered following NOR exposure supporting that recognition memory formation has an impact on the neurovasculature. Although deemed as a neutral valence test, NOR experimental conditions impact performance, highlighting the need to minimize anxiety when performing this commonly used test in mice. Significance Statement With this study, we aim to investigate the blood-brain barrier’s (BBB) role in memory acquisition and consolidation to unravel new mechanisms and decipher the involvement of non-neuronal cell types in these processes. For this purpose, male and female mice were subjected to a recognition memory test associated with a neutral emotional experience and impact on the transcriptomic profile of the BBB along with blood vascular biomarkers were evaluated under various experimental conditions. Crossing the BBB remains an important challenge to develop therapeutic drugs including in the context of memory deficits driven by psychiatric disorders or neurodegenerative diseases and thus, the possibility to directly target this barrier by better understanding its biology is attractive and innovating.

Major depressive disorder (MDD) is the leading cause of disability worldwide. However, 30-50% of patients are unresponsive to commonly prescribed antidepressants, highlighting untapped causal biological mechanisms. Dysfunction in the microbiota-gut-brain axis, the bidirectional communications between the central nervous system and gastrointestinal tract that are modulated by gut microorganisms, has been implicated in MDD pathogenesis. Exposure to chronic stress disrupts blood-brain barrier integrity, still, little is known about intestinal barrier function in these conditions particularly for the small intestine where most food and drug absorption takes place. Thus, here we investigate how chronic social or variable stress, two mouse models of depression, impact the jejunum (JEJ) intestinal barrier in males and females. Mice were subjected to stress paradigms followed by analysis of gene expression profiles of intestinal barrier-related targets, fecal microbial composition, and blood-based markers. Altered microbial populations as well as changes in gene expression of JEJ tight junctions were observed depending on the type and duration of stress, with sex-specific effects. We took advantage of machine learning to characterize in detail morphological tight junction properties identifying a cluster of ruffled junctions in stressed animals. Junctional ruffling is associated with inflammation, so we evaluated if LPS injection recapitulates stress-induced changes in the JEJ and observed profound sex differences. Finally, LPS-binding protein (LBP), a marker of gut barrier leakiness, was associated with stress vulnerability in mice and translational value was confirmed on blood samples from women with MDD. Our results provide evidence that chronic stress disrupts intestinal barrier homeostasis in conjunction with the manifestation of depressive-like behaviors in a sex-specific manner in mice and possibly, human depression.