Microbiome Influences Prenatal and Adult Microglia in a Sex-Specific Manner.

Microglia are embryonically seeded macrophages that contribute to brain development, homeostasis, and pathologies. It is thus essential to decipher how microglial properties are temporally regulated by intrinsic and extrinsic factors, such as sexual identity and the microbiome. Here, we found that microglia undergo differentiation phases, discernable by transcriptomic signatures and chromatin accessibility landscapes, which can diverge in adult males and females. Remarkably, the absence of microbiome in germ-free mice had a time and sexually dimorphic impact both prenatally and postnatally: microglia were more profoundly perturbed in male embryos and female adults. Antibiotic treatment of adult mice triggered sexually biased microglial responses revealing both acute and long-term effects of microbiota depletion. Finally, human fetal microglia exhibited significant overlap with the murine transcriptomic signature. Our study shows that microglia respond to environmental challenges in a sex- and time-dependent manner from prenatal stages, with major implications for our understanding of microglial contributions to health and disease.

Gut microbiota changes require vagus nerve integrity to promote depressive-like behaviors in mice.

Chronic stress constitutes a major risk factor for depression that can disrupt various aspects of homeostasis, including the gut microbiome (GM). We have recently shown that GM imbalance affects adult hippocampal (HPC) neurogenesis and induces depression-like behaviors, with the exact mechanisms being under active investigation. Here we hypothesized that the vagus nerve (VN), a key bidirectional route of communication between the gut and the brain, could relay the effects of stress-induced GM changes on HPC plasticity and behavior. We used fecal samples derived from mice that sustained unpredictable chronic mild stress (UCMS) to inoculate healthy mice and assess standard behavioral readouts for anxiety- and depressive-like behavior, conduct histological and molecular analyses for adult HPC neurogenesis and evaluate neurotransmission pathways and neuroinflammation. To study the potential role of the VN in mediating the effects of GM changes on brain functions and behavior, we used mice that sustained subdiaphragmatic vagotomy (Vx) prior the GM transfer. We found that inoculation of healthy mice with GM from UCMS mice activates the VN and induces early and sustained changes in both serotonin and dopamine neurotransmission pathways in the brainstem and HPC. These changes are associated with prompt and persistent deficits in adult HPC neurogenesis and induce early and sustained neuroinflammatory responses in the HPC. Remarkably, Vx abrogates adult HPC neurogenesis deficits, neuroinflammation and depressive-like behavior, suggesting that vagal afferent pathways are necessary to drive GM-mediated effects on the brain.

Anxiety- and Depression-Like States Lead to Pronounced Olfactory Deficits and Impaired Adult Neurogenesis in Mice.

Numerous clinical reports underscore the frequency of olfactory impairments in patients suffering from major depressive disorders (MDDs), yet the underlying physiopathological mechanisms remain poorly understood. We hypothesized that one key link between olfactory deficits and MDD lies in hypercortisolemia, a cardinal symptom of MDD. Corticosterone (CORT) is known to negatively correlate with hippocampal neurogenesis, yet its effects on olfactory neurogenesis and olfaction remain unknown. Here we used a rodent model of anxiety/depression-like states, which is based on chronic CORT administration and studied the effects of the antidepressant fluoxetine (FLX) on behavior, olfaction, and adult neurogenesis in the dentate gyrus (DG), olfactory bulb (OB), and the olfactory epithelium (OE). Chronic CORT had no effect on cell proliferation in the OE or on olfactory sensory neurons projecting to the OB, but induced pronounced deficits in olfactory acuity, fine discrimination of odorants and olfactory memory. These alterations were accompanied by a significant decrease in the number of adult-born neurons in both the DG and OB. Remarkably, FLX not only reversed depression-like states as expected, but also improved olfactory acuity, memory, and restored impaired adult neurogenesis. However, fine olfactory discrimination was not restored. Morphological analysis of adult-born neurons in both the DG and the OB showed that dendritic complexity was not significantly affected by CORT, but was increased by FLX. These findings demonstrate an essential role for glucocorticoids in triggering olfactory impairments in MDD and highlight a novel therapeutic effect of FLX. SIGNIFICANCE STATEMENT: Increasing clinical reports show that major depression is characterized by pronounced olfactory deficits, yet the underlying mechanisms remain unknown. In this work, we used an endocrine model of depression to study whether hypothalamic-pituitary-adrenal axis perturbation could be sufficient to provoke olfactory impairments. We found that chronic corticosterone not only induces marked deficits in olfactory acuity, fine discrimination and olfactory memory, but also significantly decreases bulbar and hippocampal neurogenesis. Importantly, the antidepressant fluoxetine restores both adult neurogenesis and depressive states, and improves most olfactory functions. Our data reveal that impairment of hypothalamic-pituitary-adrenal axis during depression can lead to olfactory deficits and that the neurogenic effects of selective serotonin reuptake inhibitor antidepressants can successfully restore certain olfactory functions.

CB1 and CB2 cannabinoid receptor antagonists prevent minocycline-induced neuroprotection following traumatic brain injury in mice.

Traumatic brain injury (TBI) and its consequences represent one of the leading causes of death in young adults. This lesion mediates glial activation and the release of harmful molecules and causes brain edema, axonal injury, and functional impairment. Since glial activation plays a key role in the development of this damage, it seems that controlling it could be beneficial and could lead to neuroprotective effects. Recent studies show that minocycline suppresses microglial activation, reduces the lesion volume, and decreases TBI-induced locomotor hyperactivity up to 3 months. The endocannabinoid system (ECS) plays an important role in reparative mechanisms and inflammation under pathological situations by controlling some mechanisms that are shared with minocycline pathways. We hypothesized that the ECS could be involved in the neuroprotective effects of minocycline. To address this hypothesis, we used a murine TBI model in combination with selective CB1 and CB2 receptor antagonists (AM251 and AM630, respectively). The results provided the first evidence for the involvement of ECS in the neuroprotective action of minocycline on brain edema, neurological impairment, diffuse axonal injury, and microglial activation, since all these effects were prevented by the CB1 and CB2 receptor antagonists.

Slc20a1 and Slc20a2 regulate neuronal plasticity and cognition independently of their phosphate transport ability.

In recent years, primary familial brain calcification (PFBC), a rare neurological disease characterized by a wide spectrum of cognitive disorders, has been associated to mutations in the sodium (Na)-Phosphate (Pi) co-transporter SLC20A2. However, the functional roles of the Na-Pi co-transporters in the brain remain still largely elusive. Here we show that Slc20a1 (PiT-1) and Slc20a2 (PiT-2) are the most abundant Na-Pi co-transporters expressed in the brain and are involved in the control of hippocampal-dependent learning and memory. We reveal that Slc20a1 and Slc20a2 are differentially distributed in the hippocampus and associated with independent gene clusters, suggesting that they influence cognition by different mechanisms. Accordingly, using a combination of molecular, electrophysiological and behavioral analyses, we show that while PiT-2 favors hippocampal neuronal branching and survival, PiT-1 promotes synaptic plasticity. The latter relies on a likely Otoferlin-dependent regulation of synaptic vesicle trafficking, which impacts the GABAergic system. These results provide the first demonstration that Na-Pi co-transporters play key albeit distinct roles in the hippocampus pertaining to the control of neuronal plasticity and cognition. These findings could provide the foundation for the development of novel effective therapies for PFBC and cognitive disorders.

Changes in Gut Microbiota by Chronic Stress Impair the Efficacy of Fluoxetine.

Major depressive disorders (MDDs) constitute a leading cause of disability worldwide and current pharmacological treatments are partially effective. The gut microbiota (GM) has recently emerged as a target of therapeutic interest for MDDs. In this study, we transfer GM from mice that sustained unpredictable chronic mild stress (UCMS) to healthy recipient mice. The fecal transfer induces despair-like behavior, decreases neurogenesis in the hippocampus (HpC), and impairs the antidepressant and neurogenic effects of a standard selective serotonin (5-HT) reuptake inhibitor, fluoxetine (FLX). These effects are paralleled by deficits in 5-HT bioavailability, biosynthesis, and reuptake in the HpC. Treatment with 5-hydroxytryptophan restores the levels of 5-HT and its precursors in the HpC, improves HpC neurogenesis, and alleviates despair-like symptoms. Our results reveal that stress-induced changes in GM are involved in the pathogenesis of depressive disorders and minimize FLX efficacy via alterations in the serotonergic pathway of Trp metabolism.

Effect of gut microbiota on depressive-like behaviors in mice is mediated by the endocannabinoid system.

Depression is the leading cause of disability worldwide. Recent observations have revealed an association between mood disorders and alterations of the intestinal microbiota. Here, using unpredictable chronic mild stress (UCMS) as a mouse model of depression, we show that UCMS mice display phenotypic alterations, which could be transferred from UCMS donors to naïve recipient mice by fecal microbiota transplantation. The cellular and behavioral alterations observed in recipient mice were accompanied by a decrease in the endocannabinoid (eCB) signaling due to lower peripheral levels of fatty acid precursors of eCB ligands. The adverse effects of UCMS-transferred microbiota were alleviated by selectively enhancing the central eCB or by complementation with a strain of the Lactobacilli genus. Our findings provide a mechanistic scenario for how chronic stress, diet and gut microbiota generate a pathological feed-forward loop that contributes to despair behavior via the central eCB system.

Etazolate, an α-secretase activator, reduces neuroinflammation and offers persistent neuroprotection following traumatic brain injury in mice.

Traumatic brain injury (TBI) evokes an intense neuroinflammatory reaction that is essentially mediated by activated microglia and that has been reported to act as a secondary injury mechanism that further promotes neuronal death. It involves the excessive production of inflammatory cytokines and the diminution of neuroprotective and neurotrophic factors, such as the soluble form alpha of the amyloid precursor protein (sAPPα), generated by the activity of α-secretases. Hence, the aim of this study was to examine the effects of etazolate, an α-secretase activator, on acute and belated post-TBI consequences. The mouse model of TBI by mechanical percussion was used and injured mice received either the vehicle or etazolate at the dose of 1, 3 or 10 mg/kg at 2 h post-TBI. Neurological score, cerebral œdema, IL-1ß and sAPPα levels, microglial activation and lesion size were evaluated from 6 to 24 h post-TBI. Spontaneous locomotor activity was evaluated from 48 h to 12 weeks post-TBI, memory function at 5 weeks and olfactory bulb lesions at 13 weeks post-TBI. A single administration of etazolate exerted a dose-dependent anti-inflammatory and anti-œdematous effect accompanied by lasting memory improvement, reduction of locomotor hyperactivity and olfactory bulb tissue protection, with a therapeutic window of at least 2 h. These effects were associated with the restoration of the levels of the sAPPα protein post-TBI. Taken together, these results highlight for the first time the therapeutic interest of an α-secretase activator in TBI.

Minocycline restores olfactory bulb volume and olfactory behavior after traumatic brain injury in mice.

Permanent olfactory dysfunction can often arise after traumatic brain injury (TBI) and while one of the main causes is the immediate loss of neurons in the olfactory bulb (OB), the emergent neuroinflammatory environment following TBI may further promote OB deterioration. Therefore, we examined the effects of acute anti-inflammatory treatment with minocycline on post-TBI olfactory behavior and on OB surface. The mouse model of closed-head injury by mechanical percussion was applied to anesthetized Swiss mice. The treatment protocol included three injections of minocycline (i.p.) at 5 min (90 mg/kg), 3 h, and 9 h (45 mg/kg) post-TBI. An olfactory avoidance test was run up to 12 weeks post-TBI. The mice were then sacrificed and their OB surface was measured. Our results demonstrated a post-TBI olfactory behavior deficit that was significant up to at least 12 weeks post-TBI. Additionally, substantial post-TBI OB atrophy was observed that was strongly correlated with the behavioral impairment. Minocycline was able to attenuate both the olfactory lesions and corresponding functional deficit in the short and long term. These results emphasize the potential role of minocycline as a promising neuroprotective agent for the treatment of TBI-related olfactory bulb lesions and deficits.

Evaluation of late cognitive impairment and anxiety states following traumatic brain injury in mice: the effect of minocycline.

Comorbidity of cognitive and stress disorders is a common clinical sequel of traumatic brain injury (TBI) that is essentially determined by the site and severity of the insult, but also by the extent of the ensuing neuroinflammatory response. The present study sought to examine the late effects of closed-head TBI on memory function and anxiety in mice, in order to further examine the potential efficacy of an acute anti-inflammatory treatment with minocycline. The mouse model of closed-head injury by mechanical percussion was applied on anesthetized Swiss mice. The treatment protocol included three injections of minocycline (i.p.) at 5 min (90 mg/kg), 3 h and 9 h (45 mg/kg) post-TBI. The Novel Object Recognition Test as well as the Elevated Plus Maze (EPM) and Elevated Zero Maze (EZM) tasks were employed to assess post-TBI memory and anxiety respectively. Our results revealed a recognition memory deficit that was significant up to at least 13 weeks post-TBI. However, neither EPM nor EZM revealed any alteration in post-TBI anxiety levels albeit some mild disinhibition. Most importantly, minocycline was able to attenuate the memory impairment in an effective and lasting manner, highlighting its therapeutic potential in TBI.

Minocycline restores sAPPα levels and reduces the late histopathological consequences of traumatic brain injury in mice.

Traumatic brain injury (TBI) induces both focal and diffuse lesions that are concurrently responsible for the ensuing morbidity and mortality and for which no established treatment is available. It has been recently reported that an endogenous neuroprotector, the soluble form α of the amyloid precursor protein (sAPPα), exerts neuroprotective effects following TBI. However, the emergent post-traumatic neuroinflammatory environment compromises sAPPα production and may promote neuronal degeneration and consequent brain atrophy. Hence, the aim of this study was to examine the effects of the anti-inflammatory drug minocycline on sAPPα levels, as well as on long-term histological consequences post-TBI. The weight-drop model was used to induce TBI in mice. Minocycline or its vehicle were administered three times: at 5 min (90 mg/kg, i.p.) and at 3 and 9 h (45 mg/kg, i.p.) post-TBI. The levels of sAPPα, the extent of brain atrophy, and reactive gliosis were evaluated by ELISA, cresyl violet, and immunolabeling of GFAP and CD11b, respectively. Our results revealed a post-TBI sAPPα decrease that was significantly attenuated by minocycline. Additionally, corpus callosum and striatal atrophy, ventriculomegaly, astrogliosis, and microglial activation were observed at 3 months post-TBI. All the above consequences were significantly reduced by minocycline. In conclusion, inhibition of the acute phase of post-TBI neuroinflammation was associated with the sparing of sAPPα and the protection of brain tissue in the long-term, emphasizing the potential role of minocycline as an effective treatment for TBI.