Caroline Menard

Early life stress (ELS) events during sensitive postnatal time periods can recalibrate future stress responsiveness and precipitate mental disorders. Neurovascular adaptations can influence cognition, mood, and stress responses. Disruption of blood-brain barrier (BBB) integrity, which is formed by endothelial cells, astrocytes, and pericytes, has been implicated in affective disorders such as depression, which often arise from chronic stress experiences. Despite the BBB undergoing critical maturation stages during development, it remains poorly known how ELS influences brain vascular function, as previously shown for adult stress, and whether it augments BBB vulnerability to subsequent challenges. First, we took advantage of a public two-hit stress RNA-sequencing dataset and filtered for vascular enriched genes in the prefrontal cortex and nucleus accumbens, the two brain regions where BBB integrity is frequently compromised. This analysis revealed BBB-related gene ontology categories modulated by either ELS alone or its combination with adult stress. Then, using a mouse model combining ELS with chronic social defeat stress (CSDS) in adulthood, we found that ELS did not exacerbate CSDS susceptibility; instead, it increased social interactions and the likelihood of a resilient profile in both males and females. Transcriptomic profiling in our cohort further identified distinct sex- and region-specific BBB gene expression patterns associated with ELS and its interaction with CSDS. Additionally, we observed a reduction of corticosterone levels, the primary stress hormone, following CSDS. Altogether, these results indicate that ELS modulates stress responses when facing emotional challenges in adulthood, possibly through long-lasting changes of BBB function via the glucocorticoid system. Highlights: RNA-seq vascular filtering reveals BBB distinct ontology categories for ELS and AS ELS increases the likelihood of a high social and resilient profile. Pericytes gene expression associated to resilience is sex- and region-specific. CORT response desensitizes after adult CSDS in both sexes.

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.

Major depressive disorder (MDD) is a common mood condition affecting multiple brain regions and cell types. Changes in astrocyte function contribute to depressive-like behaviors. However, while neuronal mechanisms driving MDD have been studied in some detail, molecular mechanisms by which astrocytes promote depression have not been extensively explored. To uncover astrocyte contributions to MDD, we subjected male mice to chronic social defeat stress precipitated by encounters with a dominant male. Animals exposed to this treatment exhibit symptoms indicative of MDD, including reduced social interactions, anxiety, despair, and anhedonia. We then measured astrocyte translating mRNA expression changes in mice that underwent chronic social defeat and control animals using ribosome affinity purification. Bioinformatic analyses reveal significant alterations in the prefrontal cortex (PFC), consistent with previous studies implicating this brain region in MDD. Expression of genes encoding extracellular matrix (ECM) proteins, cell-cell interaction proteins, and proteins controlling glutamatergic synaptic function are significantly altered. These changes correlate with perturbation of glutamatergic transmission, measured by electrophysiology, and increased synaptic cleft size. Among ECM genes, increased expression of mRNA encoding the synaptic remodeling protein s ecreted p rotein a cidic and r ich in c ysteine ( Sparc ) correlates the most with the depressive phenotype. Furthermore, presence of SPARC and other ECM proteins in synaptosomes is also increased and overexpressing Sparc in PFC partially alleviates stress symptoms. Our results raise the possibility that increased expression of Sparc may be a natural protective mechanism against stress-induced synaptic dysfunction in depression.

Major depressive disorder (MDD) is a leading cause of disease burden worldwide with low therapeutic efficacy. The blood-brain barrier (BBB) is essential to maintain brain homeostasis and regulate drug delivery. Chronic social stress, MDD’s main environmental risk factor, is associated with neurovascular pathology in mice including loss of BBB integrity in mood-regulating brain regions in a sex-specific manner, which is also observed in individuals with MDD. Pericytes ensheathe endothelial cells and modulate BBB functions under physiological and pathological conditions, but contribution of these mural cells in stress responses or mood disorders pathogenesis is unknown. Here, we report profound transcriptomic, morphological and functional alterations in pericytes in the prefrontal cortex (PFC) following stress exposure in female mice and women with MDD. Endothelial cell-to-pericyte crosstalk via PDGFB:PDGFRB signaling is critical for BBB dynamics and proper brain function. Targeted ablation of pericytes in the PFC induced anxiety- and depression-like behaviours in mice, while viral-mediated increase in pericyte-endothelial communication promoted stress coping behaviors. RNA-seq of human brain endothelial cells and pericytes exposed to MDD serum revealed changes in extracellular matrix-related genes and cell-cell communication. PDGF-BB serum level correlated with depressive symptoms and diagnosis. Altogether, our findings shed light on a novel mechanism underlying PFC dysfunction in MDD along with a blood biomarker. Our work involves pericytes in the pathophysiology of mood disorders, highlights new therapeutic targets and provides tools to manipulate these cells applicable for vascular, neurodegenerative, or inflammatory diseases all characterized by high rates of depression.

Mood disorders are an enigmatic class of debilitating illnesses that affect millions of individuals worldwide. While chronic stress clearly increases incidence levels of mood disorders, including major depressive disorder (MDD), stress-mediated disruptions in brain function that precipitate these illnesses remain largely elusive. Serotonin-associated antidepressants (ADs) remain the first line of therapy for many with depressive symptoms, yet low remission rates and delays between treatment and symptomatic alleviation have prompted skepticism regarding direct roles for serotonin in the precipitation and treatment of affective disorders. Our group recently demonstrated that serotonin epigenetically modifies histone proteins (H3K4me3Q5ser) to regulate transcriptional permissiveness in brain. However, this non-canonical phenomenon has not yet been explored following stress and/or AD exposures. Here, we employed a combination of genome-wide and biochemical analyses in dorsal raphe nucleus (DRN) of male and female mice exposed to chronic social defeat stress, as well as in DRN of human MDD patients, to examine the impact of stress exposures/MDD diagnosis on H3K4me3Q5ser dynamics, as well as associations between the mark and depression-related gene expression. We additionally assessed stress-induced/MDD-associated regulation of H3K4me3Q5ser following AD exposures, and employed viral-mediated gene therapy in mice to reduce H3K4me3Q5ser levels in DRN and examine its impact on stress-associated gene expression and behavior. We found that H3K4me3Q5ser plays important roles in stress-mediated transcriptional plasticity. Chronically stressed mice displayed dysregulated H3K4me3Q5ser dynamics in DRN, with both AD- and viral-mediated disruption of these dynamics proving sufficient to attenuate stress-mediated gene expression and behavior. Corresponding patterns of H3K4me3Q5ser regulation were observed in MDD subjects on vs. off ADs at their time of death. These findings thus establish a neurotransmission-independent role for serotonin in stress-/AD-associated transcriptional and behavioral plasticity, observations of which may be of clinical relevance to human MDD and its treatment.