Increased Microglial Activity, Impaired Adult Hippocampal Neurogenesis, and Depressive-like Behavior in Microglial VPS35-Depleted Mice

Potential role of hippocampal neurogenesis in spinal cord injury induced post-trauma depression

It has been reported both in clinic and rodent models that beyond spinal cord injury directly induced symptoms, such as paralysis, neuropathic pain, bladder/bowel dysfunction, and loss of sexual function, there are a variety of secondary complications, including memory loss, cognitive decline, depression, and Alzheimer's disease. The large-scale longitudinal population-based studies indicate that post-trauma depression is highly prevalent in spinal cord injury patients. Yet, few basic studies have been conducted to address the potential molecular mechanisms. One of possible factors underlying the depression is the reduction of adult hippocampal neurogenesis which may come from less physical activity, social isolation, chronic pain, and elevated neuroinflammation after spinal cord injury. However, there is no clear consensus yet. In this review, we will first summarize the alteration of hippocampal neurogenesis post-spinal cord injury. Then, we will discuss possible mechanisms underlie this important spinal cord injury consequence. Finally, we will outline the potential therapeutic options aimed at enhancing hippocampal neurogenesis to ameliorate depression.

Intermittent hypoxia training enhances Aβ endocytosis by plaque associated microglia via VPS35-dependent TREM2 recycling in murine Alzheimer's disease

BACKGROUND

Beta-amyloid (Aβ) deposition in the brain parenchyma is a crucial initiating step in the amyloid cascade hypothesis of Alzheimer's disease (AD) pathology. Furthermore, dysfunction of plaque-associated microglia, also known as disease-associated microglia (DAM) has been reported to accelerate Aβ deposition and cognitive impairment. Our previous research demonstrated that intermittent hypoxia training (IHT) improved AD pathology by upregulating autophagy in DAM, thereby enhancing oligomeric Aβ (oAβ) clearance. Considering that oAβ internalization is the initial stage of oAβ clearance, this study focused on the IHT mechanism involved in upregulating Aβ uptake by DAM.

METHODS

IHT was administered to 8-month-old APP/PS1 mice or 6-month-old microglial vacuolar protein sorting 35 (VPS35) knockout mice in APP/PS1 background (MG VPS35 KO: APP/PS1) for 28 days. After the IHT, the spatial learning-memory capacity of the mice was assessed. Additionally, AD pathology was determined by estimating the nerve fiber and synapse density, Aβ plaque deposition, and Aβ load in the brain. A model of Aβ-exposed microglia was constructed and treated with IHT to explore the related mechanism. Finally, triggering receptor expressed on myeloid cells 2 (TREM2) intracellular recycling and Aβ internalization were measured using a fluorescence tracing technique.

RESULTS

Our results showed that IHT ameliorated cognitive function and Aβ pathology. In particular, IHT enhanced Aβ endocytosis by augmenting the intracellular transport function of microglial TREM2, thereby contributing to Aβ clearance. Furthermore, IHT specifically upregulated VPS35 in DAM, the primary cause for the enhanced intracellular recycling of TREM2. IHT lost ameliorative effect on Aβ pathology in MG VPS35 KO: APP/PS1 mice brain. Lastly, the IHT mechanism of VPS35 upregulation in DAM was mediated by the transcriptional regulation of VPS35 by transcription factor EB (TFEB).

CONCLUSION

IHT enhances Aβ endocytosis in DAM by upregulating VPS35-dependent TREM2 recycling, thereby facilitating oAβ clearance and mitigation of Aβ pathology. Moreover, the transcriptional regulation of VPS35 by TFEB demonstrates a close link between endocytosis and autophagy in microglia. Our study further elucidates the IHT mechanism in improving AD pathology and provides evidence supporting the potential application of IHT as a complementary therapy for AD.

Psilocybin for dementia prevention? The potential role of psilocybin to alter mechanisms associated with major depression and neurodegenerative diseases

Major depression is an established risk factor for subsequent dementia, and depression in late life may also represent a prodromal state of dementia. Considering current challenges in the clinical development of disease modifying therapies for dementia, the focus of research is shifting towards prevention and modification of risk factors to alter the neurodegenerative disease trajectory. Understanding mechanistic commonalities underlying affective symptoms and cognitive decline may reveal biomarkers to aid early identification of those at risk of progressing to dementia during the preclinical phase of disease, thus allowing for timely intervention. Adult hippocampal neurogenesis (AHN) is a phenomenon that describes the birth of new neurons in the dentate gyrus throughout life and it is associated with spatial learning, memory and mood regulation. Microglia are innate immune system macrophages in the central nervous system that carefully regulate AHN via multiple mechanisms. Disruption in AHN is associated with both dementia and major depression and microgliosis is a hallmark of several neurodegenerative diseases. Emerging evidence suggests that psychedelics promote neuroplasticity, including neurogenesis, and may also be immunomodulatory. In this context, psilocybin, a serotonergic agonist with rapid-acting antidepressant properties has the potential to ameliorate intersecting pathophysiological processes relevant for both major depression and neurodegenerative diseases. In this narrative review, we focus on the evidence base for the effects of psilocybin on adult hippocampal neurogenesis and microglial form and function; which may suggest that psilocybin has the potential to modulate multiple mechanisms of action, and may have implications in altering the progression from major depression to dementia in those at risk.

Prenatal SARS-CoV-2 infection results in neurodevelopmental and behavioral outcomes in mice

Prenatal exposure to viral pathogens has been known to cause the development of neuropsychiatric disorders in adulthood. Furthermore, COVID-19 has been associated with a variety of neurological manifestations, raising the question of whether in utero SARS-CoV-2 exposure can affect neurodevelopment, resulting in long-lasting behavioral and cognitive deficits. Using a human ACE2-knock-in mouse model, we have previously shown that prenatal exposure to SARS-CoV-2 at later stages of development leads to fetal brain infection and gliosis in the hippocampus and cortex. In this study, we aimed to determine whether infection of the fetal brain results in long-term neuroanatomical alterations of the cortex and hippocampus or in any cognitive deficits in adulthood. Here, we show that infected mice developed slower and weighed less in adulthood. We also found altered hippocampal and amygdala volume and aberrant newborn neuron morphology in the hippocampus of adult mice infected in utero. Furthermore, we observed sex-dependent alterations in anxiety-like behavior and locomotion, as well as hippocampal-dependent spatial memory. Taken together, our study reveals long-lasting neurological and cognitive changes as a result of prenatal SARS-CoV-2 infection, identifying a window for early intervention and highlighting the importance of immunization and antiviral intervention in pregnant women.

The major histocompatibility complex participates in Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative disease characterized by progressive loss of dopaminergic neurons in the substantia nigra and the aggregation of alpha-synuclein (α-syn). The central nervous system (CNS) has previously been considered as an immune-privileged area. However, studies have shown that the immune responses are involved in PD. The major histocompatibility complex (MHC) presents antigens from antigen-presenting cells (APCs) to T lymphocytes, immune responses will be induced. MHCs are expressed in microglia, astrocytes, and dopaminergic neurons. Single nucleotide polymorphisms in MHC are related to the risk of PD. The aggregated α-syn triggers the expression of MHCs by activating glia cells. CD4+ and CD8+ T lymphocytes responses and microglia activation are detected in brains of PD patients. In addiction immune responses further increase blood-brain barrier (BBB) permeability and T cell infiltration in PD. Thus, MHCs are involved in PD through participating in immune and inflammatory responses.

VPS35 and retromer dysfunction in Parkinson's disease

The vacuolar protein sorting 35 ortholog (VPS35) gene encodes a core component of the retromer complex essential for the endosomal sorting and recycling of transmembrane cargo. Endo-lysosomal pathway deficits are suggested to play a role in the pathogenesis of neurodegenerative diseases, including Parkinson's disease (PD). Mutations in VPS35 cause a late-onset, autosomal dominant form of PD, with a single missense mutation (D620N) shown to segregate with disease in PD families. Understanding how the PD-linked D620N mutation causes retromer dysfunction will provide valuable insight into the pathophysiology of PD and may advance the identification of therapeutics. D620N VPS35 can induce LRRK2 hyperactivation and impair endosomal recruitment of the WASH complex but is also linked to mitochondrial and autophagy-lysosomal pathway dysfunction and altered neurotransmitter receptor transport. The clinical similarities between VPS35-linked PD and sporadic PD suggest that defects observed in cellular and animal models with the D620N VPS35 mutation may provide valuable insights into sporadic disease. In this review, we highlight the current knowledge surrounding VPS35 and its role in retromer dysfunction in PD. We provide a critical discussion of the mechanisms implicated in VPS35-mediated neurodegeneration in PD, as well as the interplay between VPS35 and other PD-linked gene products. This article is part of a discussion meeting issue 'Understanding the endo-lysosomal network in neurodegeneration'.

ATG5 (autophagy related 5) in microglia controls hippocampal neurogenesis in Alzheimer disease

Macroautophagy/autophagy is the intracellular degradation process of cytoplasmic content and damaged organelles. Autophagy is strongly associated with the progression of Alzheimer disease (AD). Microglia are brain-resident macrophages, and recent studies indicate that autophagy in microglia protects neurons from neurodegeneration. Postnatal neurogenesis, the generation of new neurons from adult neural stem cells (NSCs), is impaired in AD patients as well as in AD animal models. However, the extent to which microglial autophagy influences adult NSCs and neurogenesis in AD animal models has not been studied. Here, we showed that conditional knock out (cKO) of Atg5 (autophagy related 5) in microglia inhibited postnatal neurogenesis in the dentate gyrus (DG) of the hippocampus, but not in the subventricular zone (SVZ) of a 5×FAD mouse model. Interestingly, the protection of neurogenesis by Atg5 in microglia was only observed in female AD mice. To confirm the roles of autophagy in microglia for postnatal hippocampal neurogenesis, we generated additional cKO mice to delete autophagy essential genes Rb1cc1 or Atg14 in microglia. However, these rb1cc1 cKO and atg14 cKO mice did not exhibit neurogenesis defects in the context of a female AD mouse model. Last, we used the CSF1R antagonist to deplete ATG5-deficient microglia and this intervention restored neurogenesis in the hippocampus of 5×FAD mice. These results indicate that microglial ATG5 is essential to maintain postnatal hippocampal neurogenesis in a mouse model of AD. Our findings further support the notion that ATG5 in microglia supports NSC health and may prevent neurodegeneration.Abbreviations: 5×FAD: familial Alzheimer disease; Aβ: β-amyloid; AD: Alzheimer disease; AIF1: allograft inflammatory factor 1; ATG: autophagy related; BrdU: 5-bromo-2'-deoxyuridine; CA: Cornu Ammonis; cKO: conditional knock out; CSF1R: colony stimulating factor 1 receptor; Ctrl: control; DCX: doublecortin; DG: dentate gyrus; GFAP: glial fibrillary acidic protein; GZ: granular zone; H&E: hematoxylin and eosin; IF: immunofluorescence; LD: lipid droplet; LDAM: lipid droplets accumulated microglia; LPS: lipopolysaccharides; MAP1LC3B/LC3: microtubule-associated protein 1 light chain 3 beta; NSCs: neural stem cells; RB1CC1: RB1-inducible coiled-coil 1; SOX2: SRY (sex determining region Y)-box 2; SGZ: subgranular zone; SVZ: subventricular zone; WT: wild type.

Influenza A Virus PB1-F2 Induces Affective Disorder by Interfering Synaptic Plasticity in Hippocampal Dentate Gyrus

Influenza A virus (IAV) infection, which leads to millions of new cases annually, affects many tissues and organs of the human body, including the central nervous system (CNS). The incidence of affective disorders has increased after the flu pandemic; however, the potential mechanism has not been elucidated. PB1-F2, a key virulence molecule of various influenza virus strains, has been shown to inhibit cell proliferation and induce host inflammation; however, its role in the CNS has not been studied. In this study, we constructed and injected PB1-F2 into the hippocampal dentate gyrus (DG), a region closely associated with newborn neurons and neural development, to evaluate its influence on negative affective behaviors and learning performance in mice. We observed anxiety- and depression-like behaviors, but not learning impairment, in mice injected with PB1-F2. Furthermore, pull-down and mass spectrometry analyses identified several potential PB1-F2 binding proteins, and enrichment analysis suggested that the most affected function was neural development. Morphological and western blot studies revealed that PB1-F2 inhibited cell proliferation and oligodendrocyte development, impaired myelin formation, and interfered with synaptic plasticity in DG. Taken together, our results demonstrated that PB1-F2 induces affective disorders by inhibiting oligodendrocyte development and regulating synaptic plasticity in the DG after IAV infection, which lays the foundation for developing future cures of affective disorders after IAV infection.

Chronological transitions of hepatocyte growth factor treatment effects in spinal cord injury tissue

Inflammatory responses are known to suppress neural regeneration in patients receiving stem cell-based regenerative therapy for spinal cord injury (SCI). Consequently, pathways involved in neurogenesis and immunomodulation, such as the hepatocyte growth factor (HGF)/MET signaling cascade, have garnered significant attention. Notably, various studies, including our own, have highlighted the enhanced recovery of locomotor functions achieved in SCI animal models by combining HGF pretreatment and human induced stem cell-derived neural stem/progenitor cell (hiPSC-NS/PC) transplantation. However, these studies implicitly hypothesized that the functionality of HGF in SCI would be time consistent and did not elucidate its dynamics. In the present article, we investigated the time-course of the effect of HGF on SCI, aiming to uncover a more precise mechanism for HGF administration, which is indispensable for developing crystallizing protocols for combination therapy. To this end, we performed a detailed investigation of the temporal variation of HGF using the RNA-seq data we obtained in our most recent study. Leveraging the time-series design of the data, which we did not fully exploit previously, we identified three components in the effects of HGF that operate at different times: early effects, continuous effects, and delayed effects. Our findings suggested a concept where the three components together contribute to the acceleration of neurogenesis and immunomodulation, which reinforce the legitimacy of empirically fine-tuned protocols for HGF administration and advocate the novel possibility that the time-inconsistent effects of HGF progressively augment the efficacy of combined therapy.

Association of psychological distress and DNA methylation: A 5-year longitudinal population-based twin study

AIM

To identify the psychological distress (PD)-associated 5'-cytosine-phosphate-guanine-3' sites (CpGs), and investigate the temporal relationship between dynamic changes in DNA methylation (DNAm) and PD.

METHODS

This study included 1084 twins from the Chinese National Twin Register (CNTR). The CNTR conducted epidemiological investigations and blood withdrawal twice in 2013 and 2018. These included twins were used to perform epigenome-wide association studies (EWASs) and to validate the previously reported PD-associated CpGs selected from previous EWASs in PubMed, Embase, and the EWAS catalog. Next, a cross-lagged study was performed to examine the temporality between changes in DNAm and PD in 308 twins who completed both 2013 and 2018 surveys.

RESULTS

The EWAS analysis of our study identified 25 CpGs. In the validation analysis, 741 CpGs from 29 previous EWASs on PD were selected for validation, and 101 CpGs were validated to be significant at a false discovery rate <0.05. The cross-lagged analysis found a unidirectional path from PD to DNAm at 14 CpGs, while no sites showed significance from DNAm to PD.

CONCLUSIONS

This study identified and validated PD-related CpGs in a Chinese twin population, and suggested that PD may be the cause of changes in DNAm over time. The findings provide new insights into the molecular mechanisms underlying PD pathophysiology.

N-palmitoylethanolamine modulates hippocampal neuroplasticity in rats with stress-induced depressive behavior phenotype

Bioactive lipid mediator N-palmitoylethanolamide (PEA) is an endocannabinoid-like molecule. Based on our previous data, this study aimed to further investigate the antidepressant property of PEA via the peroxisome proliferator-activated receptor alpha (PPARα) pathway, focusing on the intervention of PEA on hippocampal neuroplasticity. Behavioral tests were performed in rats induced by unpredictable chronic mild stress (uCMS) in the last week of the experiment, and then the brain tissue samples were retained for subsequent immunohistochemical detection and Western blot analysis. In vitro, the apoptosis of HT22 cells induced by CORT and apoptosis-related proteins were detected by Hoechst staining and Western blot, respectively. The results showed that PEA ameliorated the depression-like phenotype in rats induced by uCMS, prevented the uCMS-induced reduction in the number of BrdU-positive cells, and increased BrdU/NeuN co-localization in the hippocampus, and upregulated the levels of synapse associated protein NCAM, MAP2, SYN and PSD95 in the hippocampus. Hoechst staining results showed that PEA significantly increased the CORT-induced reduction in the number of hippocampal neurons. Western blot analysis showed that PEA decreased the expression of caspase-3 and c-caspase-3, and increased the ratio of Bcl-2/Bax in CORT-induced HT22 cells. MK886, a PPARα antagonist, partially or completely reversed these effects. In conclusion, the therapeutic potential of PEA for depressive mood disorders may be through targeting the hippocampal neuroplasticity, including increasing adult neurogenesis and synaptic plasticity, as well as down-regulated neuronal apoptosis, to remodel hippocampal circuitries upon functional integration and PPARα pathway may be involved in this process.

Brain Infection by Group B Streptococcus Induces Inflammation and Affects Neurogenesis in the Adult Mouse Hippocampus

Central nervous system infections caused by pathogens crossing the blood-brain barrier are extremely damaging and trigger cellular alterations and neuroinflammation. Bacterial brain infection, in particular, is a major cause of hippocampal neuronal degeneration. Hippocampal neurogenesis, a continuous multistep process occurring throughout life in the adult brain, could compensate for such neuronal loss. However, the high rates of cognitive and other sequelae from bacterial meningitis/encephalitis suggest that endogenous repair mechanisms might be severely affected. In the current study, we used Group B Streptococcus (GBS) strain NEM316, to establish an adult mouse model of brain infection and determine its impact on adult neurogenesis. Experimental encephalitis elicited neurological deficits and death, induced inflammation, and affected neurogenesis in the dentate gyrus of the adult hippocampus by suppressing the proliferation of progenitor cells and the generation of newborn neurons. These effects were specifically associated with hippocampal neurogenesis while subventricular zone neurogenesis was not affected. Overall, our data provide new insights regarding the effect of GBS infection on adult brain neurogenesis.

Adult hippocampal neurogenesis in Alzheimer's disease: A roadmap to clinical relevance

Adult hippocampal neurogenesis (AHN) drops sharply during early stages of Alzheimer's disease (AD), via unknown mechanisms, and correlates with cognitive status in AD patients. Understanding AHN regulation in AD could provide a framework for innovative pharmacological interventions. We here combine molecular, behavioral, and clinical data and critically discuss the multicellular complexity of the AHN niche in relation to AD pathophysiology. We further present a roadmap toward a better understanding of the role of AHN in AD by probing the promises and caveats of the latest technological advancements in the field and addressing the conceptual and methodological challenges ahead.

Immunogenetic Determinants of Parkinson’s Disease Etiology

Parkinson’s disease (PD) is increasingly recognised as a systemic disorder in which inflammation might play a causative role rather than being a consequence or an epiphenomenon of the neurodegenerative process. Although growing genetic evidence links the central and peripheral immune system with both monogenic and sporadic PD, our understanding on how the immune system contributes to PD pathogenesis remains a daunting challenge. In this review, we discuss recent literature aimed at exploring the role of known genes and susceptibility loci to PD pathogenesis through immune system related mechanisms. Furthermore, we outline shared genetic etiologies and interrelations between PD and autoimmune diseases and underlining challenges and limitations faced in the translation of relevant allelic and regulatory risk loci to immune-pathological mechanisms. Lastly, with the field of immunogenetics expanding rapidly, we place these insights into a future context highlighting the prospect of immune modulation as a promising disease-modifying strategy.

Critical Role of Neuronal Vps35 in Blood Vessel Branching and Maturation in Developing Mouse Brain

Vps35 (vacuolar protein sorting 35), a key component of retromer, plays a crucial role in selective retrieval of transmembrane proteins from endosomes to trans-Golgi networks. Dysfunctional Vps35/retromer is a risk factor for the development of neurodegenerative diseases. Vps35 is highly expressed in developing pyramidal neurons, both in the mouse neocortex and hippocampus, Although embryonic neuronal Vps35’s function in promoting neuronal terminal differentiation and survival is evident, it remains unclear whether and how neuronal Vps35 communicates with other types of brain cells, such as blood vessels (BVs), which are essential for supplying nutrients to neurons. Dysfunctional BVs contribute to the pathogenesis of various neurodegenerative disorders. Here, we provide evidence for embryonic neuronal Vps35 as critical for BV branching and maturation in the developing mouse brain. Selectively knocking out (KO) Vps35 in mouse embryonic, not postnatal, neurons results in reductions in BV branching and density, arteriole diameter, and BV-associated pericytes and microglia but an increase in BV-associated reactive astrocytes. Deletion of microglia by PLX3397 enhances these BV deficits in mutant mice. These results reveal the function of neuronal Vps35 in neurovascular coupling in the developing mouse brain and implicate BV-associated microglia as underlying this event.

Understanding the contributions of VPS35 and the retromer in neurodegenerative disease

Perturbations of the endolysosomal pathway have been suggested to play an important role in the pathogenesis of several neurodegenerative diseases, including Parkinson's disease (PD) and Alzheimer's disease (AD). Specifically, VPS35 and the retromer complex play an important role in the endolysosomal system and are implicated in the pathophysiology of these diseases. A single missense mutation in VPS35, Asp620Asn (D620N), is known to cause late-onset, autosomal dominant familial PD. In this review, we focus on the emerging role of the PD-linked D620N mutation in causing retromer dysfunction and dissect its implications in neurodegeneration. Additionally, we will discuss how VPS35 and the retromer are linked to AD, amyotrophic lateral sclerosis, and primary tauopathies. Interestingly, reduced levels of VPS35 and other retromer components have been observed in post-mortem brain tissue, suggesting a role for the retromer in the pathophysiology of these diseases. This review will provide a comprehensive dive into the mechanisms of VPS35 dysfunction in neurodegenerative diseases. Furthermore, we will highlight outstanding questions in the field and the retromer as a therapeutic target for neurodegenerative disease at large.

Microglial VPS35 deficiency impairs Aβ phagocytosis and Aβ-induced disease-associated microglia, and enhances Aβ associated pathology

Background

Vacuolar sorting protein 35 (VPS35), a key component of the retromer, plays an essential role in selectively retrieval of transmembrane proteins from endosomes to trans-Golgi networks. Dysfunctional retromer is a risk factor for neurodegenerative disorders, including Alzheimer’s disease (AD). Microglial VPS35 deficiency is found in AD patients’ brain; however, it remains unclear if and how microglial VPS35-loss contributes to AD development.

Methods

We used mice with VPS35 cKO (conditional knockout) in microglial cells in 5XFAD, an AD mouse model. The AD related brain pathology (Aβ and glial activation), behavior, and phagocytosis of Aβ were accessed by a combination of immunofluorescence staining analyses and neurological behavior tests.

Results

A decrease in learning and memory function, but increases in insoluble, fibrillar, and plaques of β-amyloids (Aβ), dystrophic neurites, and reactive astrocytes are observed in microglial VPS35 deficient 5XFAD mice. Further examining microglial phenotype demonstrates necessity of microglial VPS35 in disease-associated microglia (DAM) development and microglial uptake of Aβ, revealing a tight association of microglial Aβ uptake with DAM development.

Conclusions

Together, these results uncovered a mechanism by which microglial VPS35-deficiency precipitates AD pathology in 5XFAD mice likely by impairing DAM development and DAM mediated Aβ uptake and clearance, and thus accelerating the cognition decline.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02422-0.

Long-term high-fat diet consumption by mice throughout adulthood induces neurobehavioral alterations and hippocampal neuronal remodeling accompanied by augmented microglial lipid accumulation

High-fat diet (HFD) consumption is generally associated with an increased risk of cognitive and emotional dysfunctions that constitute a sizeable worldwide health burden with profound social and economic consequences. Middle age is a critical time period that affects one's health later in life; pertinently, the prevalence of HFD consumption is increasing among mature adults. Given the growing health-related economic burden imposed globally by increasing rates of noncommunicable diseases in rapidly aging populations, along with the pervasive but insidious health impairments associated with HFD consumption, it is critically important to understand the effects of long-term HFD consumption on brain function and to gain insights into their potential underlying mechanisms. In the present study, adult male C57BL/6J mice were randomly assigned a control diet (CD, 10 kJ% from fat) or an HFD (60 kJ% from fat) for 6 months (6 M) or 9 months (9 M) followed by behavioral tests, serum biochemical analysis, and histological examinations of both the dorsal and ventral regions of the hippocampus. In both the 6 M and 9 M cohorts, mice that consumed an HFD exhibited poorer memory performance in the Morris water maze test (MWM) and greater depression- and anxiety-like behavior during the open field test (OFT), sucrose preference test (SPT) and forced swim test (FST) than control mice. Compared with age-matched mice in the CD group, mice in the HFD group showed abnormal hippocampal neuronal morphology, which was particularly evident in the ventral hippocampus. Hippocampal microglia in mice in the HFD group generally had a more activated phenotype evidenced by a smaller microglial territory area and increased cluster of differentiation 68 (CD68, a marker of phagocytic activity) immunoreactivity, while the microglial density in the dentate gyrus (DG) was decreased, indicating microglial decline. The engulfment of postsynaptic density 95 (PSD95, a general postsynaptic marker) puncta by microglia was increased in the HFD groups. Histological analysis of neutral lipids using a fluorescent probe (BODIPY) revealed that the total neutral lipid content in regions of interests (ROIs) and the lipid load in microglia were increased in the HFD group relative to the age-matched CD group. In summary, our results demonstrated that chronic HFD consumption from young adulthood to middle age induced anxiety- and depression-like behavior as well as memory impairment. The negative influence of chronic HFD consumption on behavioral and hippocampal neuroplasticity appears to be linked to a change in microglial phenotype that is accompanied by a remarkable increase in cellular lipid accumulation. These observations highlighting the potential to target lipid metabolism deficits to reduce the risk of HFD-associated emotional dysfunctions.

Defects of parvalbumin-positive interneurons in the ventral dentate gyrus region are implicated depression-like behavior in mice

Depression is an increasingly common but extremely serve mood disorder that remains poorly understood and inadequately treated. Fast-spiking parvalbumin-positive interneurons (PVIs), a subpopulation of GABAergic interneurons (GABA, g-aminobutyric acid), exhibit a widespread distribution throughout the hippocampus, and has been reported to play an important role in a variety of mental disorders. However, the relationship between depression and hippocampal PVIs remains unclear. Here in this present study, a series of experiments were conducted to clarify the potential relationship. Here, chronic unpredicted mild stress (CUMS) and Lipopolysaccharide (LPS) injection were introduced to induce depression-like behavior in mice, and led to a clear decline in PVIs numbers in the ventral hippocampal (vHPC), particularly in the ventral dentate gyrus (vDG) subfield. After a selectively removal of the PVIs in PV-ires-Cre::Ai14 mice, we confirmed that ablation of PVIs from the vDG induced depression-like behavior. Furthermore, we found that the removal of vDG-PVIs induced depression likely to be accounted for upregulation of neuroinflammation. These findings facilitate us better understand the role of hippocampal PVIs in depression.

Andrographolide promotes hippocampal neurogenesis and spatial memory in the APPswe/PS1ΔE9 mouse model of Alzheimer's disease

In Alzheimer´s disease (AD) there is a reduction in hippocampal neurogenesis that has been associated to cognitive deficits. Previously we showed that Andrographolide (ANDRO), the main bioactive component of Andrographis paniculate, induces proliferation in the hippocampus of the APPswe/PSEN1ΔE9 (APP/PS1) mouse model of AD as assessed by staining with the mitotic marker Ki67. Here, we further characterized the effect of ANDRO on hippocampal neurogenesis in APP/PS1 mice and evaluated the contribution of this process to the cognitive effect of ANDRO. Treatment of 8-month-old APP/PS1 mice with ANDRO for 4 weeks increased proliferation in the dentate gyrus as evaluated by BrdU incorporation. Although ANDRO had no effect on neuronal differentiation of newborn cells, it strongly increased neural progenitors, neuroblasts and newborn immature neurons, cell populations that were decreased in APP/PS1 mice compared to age-matched wild-type mice. ANDRO had no effect on migration or in total dendritic length, arborization and orientation of immature neurons, suggesting no effects on early morphological development of newborn neurons. Finally, ANDRO treatment improved the performance of APP/PS1 mice in the object location memory task. This effect was not completely prevented by co-treatment with the anti-mitotic drug TMZ, suggesting that other effects of ANDRO in addition to the increase in neurogenesis might underlie the observed cognitive improvement. Altogether, our data indicate that in APP/PS1 mice ANDRO stimulates neurogenesis in the hippocampus by inducing proliferation of neural precursor cells and improves spatial memory performance.

Parkinson's in the bone

Patients with Parkinson’s disease (PD) exhibit systemic deficits, including arthritis and osteoporosis-like symptoms. However, the questions, how the deficits in periphery organs or tissues occur in PD patients, and what are the relationship (s) of the periphery tissue deficits with the brain pathology (e.g., dopamine neuron loss), are at the beginning stage to be investigated. Notice that both PD and osteoporosis are the products of a complex interaction of genetic and environmental risk factors. Genetic mutations in numerous genes have been identified in patients either with recessive or autosomal dominant PD. Most of these PD risk genes are ubiquitously expressed; and many of them are involved in regulation of bone metabolism. Here, we review the functions of the PD risk genes in regulating bone remodeling and homeostasis. The knowledge gaps in our understanding of the bone-to-brain axis in PD development are also outlined.

Microglia Regulate Neuronal Circuits in Homeostatic and High-Fat Diet-Induced Inflammatory Conditions

Microglia are brain resident macrophages, which actively survey the surrounding microenvironment and promote tissue homeostasis under physiological conditions. During this process, microglia participate in synaptic remodeling, neurogenesis, elimination of unwanted neurons and cellular debris. The complex interplay between microglia and neurons drives the formation of functional neuronal connections and maintains an optimal neural network. However, activation of microglia induced by chronic inflammation increases synaptic phagocytosis and leads to neuronal impairment or death. Microglial dysfunction is implicated in almost all brain diseases and leads to long-lasting functional deficiency, such as hippocampus-related cognitive decline and hypothalamus-associated energy imbalance (i.e., obesity). High-fat diet (HFD) consumption triggers mediobasal hypothalamic microglial activation and inflammation. Moreover, HFD-induced inflammation results in cognitive deficits by triggering hippocampal microglial activation. Here, we have summarized the current knowledge of microglial characteristics and biological functions and also reviewed the molecular mechanism of microglia in shaping neural circuitries mainly related to cognition and energy balance in homeostatic and diet-induced inflammatory conditions.

Neuronal VPS35 deletion induces spinal cord motor neuron degeneration and early post-natal lethality

Neurodegenerative diseases are characterized by the selective degeneration of neuronal populations in different brain regions and frequently the formation of distinct protein aggregates that often overlap between diseases. While the causes of many sporadic neurodegenerative diseases are unclear, genes associated with familial or sporadic forms of disease and the underlying cellular pathways involved tend to support common disease mechanisms. Underscoring this concept, mutations in the Vacuolar Protein Sorting 35 Orthologue (VPS35) gene have been identified to cause late-onset, autosomal dominant familial Parkinson's disease, whereas reduced VPS35 protein levels are reported in vulnerable brain regions of subjects with Alzheimer's disease, neurodegenerative tauopathies such as progressive supranuclear palsy and Pick's disease, and amyotrophic lateral sclerosis. Therefore, VPS35 is commonly implicated in many neurodegenerative diseases. VPS35 plays a critical role in the retromer complex that mediates the retrieval and recycling of transmembrane protein cargo from endosomes to the trans-Golgi network or plasma membrane. VPS35 and retromer function are highly conserved in eukaryotic cells, with the homozygous deletion of VPS35 inducing early embryonic lethality in mice that has hindered an understanding of its role in the brain. Here, we develop conditional knockout mice with the selective deletion of VPS35 in neurons to better elucidate its role in neuronal viability and its connection to neurodegenerative diseases. Surprisingly, the pan-neuronal deletion of VPS35 induces a progressive and rapid disease with motor deficits and early post-natal lethality. Underlying this neurological phenotype is the relatively selective and robust degeneration of motor neurons in the spinal cord. Neuronal loss is accompanied and preceded by the formation of p62-positive protein inclusions and robust reactive astrogliosis. Our study reveals a critical yet unappreciated role for VPS35 function in the normal maintenance and survival of motor neurons during post-natal development that has important implications for neurodegenerative diseases, particularly amyotrophic lateral sclerosis.

Expression of Low Level of VPS35-mCherry Fusion Protein Diminishes Vps35 Depletion Induced Neuron Terminal Differentiation Deficits and Neurodegenerative Pathology, and Prevents Neonatal Death

Vps35 (vacuolar protein sorting 35) is a key component of retromer that consists of Vps35, Vps26, and Vps29 trimers, and sortin nexin dimers. Dysfunctional Vps35/retromer is believed to be a risk factor for development of various neurodegenerative diseases. Vps35Neurod6 mice, which selectively knock out Vps35 in Neurod6-Cre+ pyramidal neurons, exhibit age-dependent impairments in terminal differentiation of dendrites and axons of cortical and hippocampal neurons, neuro-degenerative pathology (i.e., increases in P62 and Tdp43 (TAR DNA-binding protein 43) proteins, cell death, and reactive gliosis), and neonatal death. The relationships among these phenotypes and the underlying mechanisms remain largely unclear. Here, we provide evidence that expression of low level of VPS35-mCherry fusion protein in Vps35Neurod6 mice could diminish the phenotypes in an age-dependent manner. Specifically, we have generated a conditional transgenic mouse line, LSL-Vps35-mCherry, which expresses VPS35-mCherry fusion protein in a Cre-dependent manner. Crossing LSL-Vps35-mCherry with Vps35Neurod6 to obtain TgVPS35-mCherry, Vps35Neurod6 mice prevent the neonatal death and diminish the dendritic morphogenesis deficit and gliosis at the neonatal, but not the adult age. Further studies revealed that the Vps35-mCherry transgene expression was low, and the level of Vps35 mRNA comprised only ~5-7% of the Vps35 mRNA of control mice. Such low level of VPS35-mCherry could restore the amount of other retromer components (Vps26a and Vps29) at the neonatal age (P14). Importantly, the neurodegenerative pathology presented in the survived adult TgVps35-mCherry; Vps35Neurod6 mice. These results demonstrate the sufficiency of low level of VPS35-mCherry fusion protein to diminish the phenotypes in Vps35Neurod6 mice at the neonatal age, verifying a key role of neuronal Vps35 in stabilizing retromer complex proteins, and supporting the view for Vps35 as a potential therapeutic target for neurodegenerative diseases.

Microglia modulate stable wakefulness via the thalamic reticular nucleus in mice

Microglia are important for brain homeostasis and immunity, but their role in regulating vigilance remains unclear. We employed genetic, physiological, and metabolomic methods to examine microglial involvement in the regulation of wakefulness and sleep. Microglial depletion decreased stable nighttime wakefulness in mice by increasing transitions between wakefulness and non-rapid eye movement (NREM) sleep. Metabolomic analysis revealed that the sleep-wake behavior closely correlated with diurnal variation of the brain ceramide, which disappeared in microglia-depleted mice. Ceramide preferentially influenced microglia in the thalamic reticular nucleus (TRN), and local depletion of TRN microglia produced similar impaired wakefulness. Chemogenetic manipulations of anterior TRN neurons showed that they regulated transitions between wakefulness and NREM sleep. Their firing capacity was suppressed by both microglial depletion and added ceramide. In microglia-depleted mice, activating anterior TRN neurons or inhibiting ceramide production both restored stable wakefulness. These findings demonstrate that microglia can modulate stable wakefulness through anterior TRN neurons via ceramide signaling.

Metformin restores hippocampal neurogenesis and learning and memory via regulating gut microbiota in the obese mouse model

Numerous studies have shown that over-nutritional obesity may lead to pre-diabetes, type 2 diabetes and cognitive decline. As the degree of metabolic disorders increases, the cognitive decline is getting worse. However, the cellular events that cause this cognitive dysfunction is yet to be clarified. We used a high-fat diet (HFD) consumption-induced obesity mouse model to test the effects of metformin on the hippocampal neurogenesis and learning and memory abilities of obese mice. 5-Bromo-2'-deoxyuridine (BrdU) labelling and retrovirus labeling were applied to detect hippocampal newborn neurons. Behavioral experiments were used to detect learning and memory abilities of mice. 16S rRNA gene sequencing was performed to detect the composition of gut microbiota. The positron emission tomography (PET) was conducted to detect the energy metabolism activity of different mouse brain regions. Our results reveal that metformin restores the impairment of neurogenesis in the dentate gyrus and finally prevents the cognitive decline of the obese mice. Moreover, the therapeutic effects of metformin are achieved by regulating the composition of gut microbiota of mice, which may inhibit microglia activation and neuroinflammation in the brain of obese mice. This study suggests that metformin may be taken as a promising candidate for the intervention of cognitive decline related to imbalance of gut microbiota caused by obesity.

Endosomal recycling reconciles the Alzheimer's disease paradox

A hub-and-spoke model with endosomal recycling as the hub can reconcile the pathogenic contribution of amyloid precursor protein to Alzheimer's disease.

The Potential Role of Inflammation in Modulating Endogenous Hippocampal Neurogenesis After Spinal Cord Injury

Currently there are approximately 291,000 people suffering from a spinal cord injury (SCI) in the United States. SCI is associated with traumatic changes in mobility and neuralgia, as well as many other long-term chronic health complications, including metabolic disorders, diabetes mellitus, non-alcoholic steatohepatitis, osteoporosis, and elevated inflammatory markers. Due to medical advances, patients with SCI survive much longer than previously. This increase in life expectancy exposes them to novel neurological complications such as memory loss, cognitive decline, depression, and Alzheimer's disease. In fact, these usually age-associated disorders are more prevalent in people living with SCI. A common factor of these disorders is the reduction in hippocampal neurogenesis. Inflammation, which is elevated after SCI, plays a major role in modulating hippocampal neurogenesis. While there is no clear consensus on the mechanism of the decline in hippocampal neurogenesis and cognition after SCI, we will examine in this review how SCI-induced inflammation could modulate hippocampal neurogenesis and provoke age-associated neurological disorders. Thereafter, we will discuss possible therapeutic options which may mitigate the influence of SCI associated complications on hippocampal neurogenesis.

The gut microbiota regulates hypothalamic inflammation and leptin sensitivity in Western diet-fed mice via a GLP-1R-dependent mechanism

Mice lacking a microbiota are protected from diet-induced obesity. Previous studies have shown that feeding a Western diet causes hypothalamic inflammation, which in turn can lead to leptin resistance and weight gain. Here, we show that wild-type (WT) mice with depleted gut microbiota, i.e., germ-free (GF) and antibiotic-treated mice, have elevated levels of glucagon-like peptide-1 (GLP-1), are protected against diet-induced hypothalamic inflammation, and have enhanced leptin sensitivity when fed a Western diet. Using GLP-1 receptor (GLP-1R)-deficient mice and pharmacological inhibition of the GLP-1R in WT mice, we demonstrate that intact GLP-1R signaling is required for preventing hypothalamic inflammation and enhancing leptin sensitivity. Furthermore, we show that astrocytes express the GLP-1R, and deletion of the receptor in glial fibrillary acidic protein (GFAP)-expressing cells diminished the antibiotic-induced protection against diet-induced hypothalamic inflammation. Collectively, our results suggest that depletion of the gut microbiota attenuates diet-induced hypothalamic inflammation and enhances leptin sensitivity via GLP-1R-dependent mechanisms.

Adult Hippocampal Neurogenesis in Aging and Alzheimer's Disease

Cognitive deficits associated with Alzheimer's disease (AD) severely impact daily life for the millions of affected individuals. Progressive memory impairment in AD patients is associated with degeneration of the hippocampus. The dentate gyrus of the hippocampus, a region critical for learning and memory functions, is a site of adult neurogenesis in mammals. Recent evidence in humans indicates that hippocampal neurogenesis likely persists throughout life, but declines with age and is strikingly impaired in AD. Our understanding of how neurogenesis supports learning and memory in healthy adults is only beginning to emerge. The extent to which decreased neurogenesis contributes to cognitive decline in aging and AD remains poorly understood. However, studies in rodent models of AD and other neurodegenerative diseases raise the possibility that targeting neurogenesis may ameliorate cognitive dysfunction in AD. Here, we review recent progress in understanding how adult neurogenesis is impacted in the context of aging and AD.

Cypermethrin Impairs Hippocampal Neurogenesis and Cognitive Functions by Altering Neural Fate Decisions in the Rat Brain

Neurogenesis is a developmental process that involves fine-tuned coordination between self-renewal, proliferation, and differentiation of neural stem cells (NSCs) into neurons. However, early-life assault with environmental toxicants interferes with the regular function of genes, proteins, and other molecules that build brain architecture resulting in attenuated neurogenesis. Cypermethrin is a class II synthetic pyrethroid pesticide extensively used in agriculture, veterinary, and residential applications due to its low mammalian toxicity, high bio-efficacy, and enhanced stability. Despite reports on cypermethrin-mediated behavioral and biochemical alterations, till now, no study implicates whether cypermethrin exposure has any effect on neurogenesis. Therefore, the present study was undertaken to comprehend the effects of cypermethrin treatment on embryonic and adult neurogenesis. We found that cypermethrin exposure led to a considerable decrease in the BrdU/Sox-2+, BrdU/Dcx+, and BrdU/NeuN+ co-labeled cells indicating that cypermethrin treatment decreases NSC proliferation and generation of mature and functional neurons. On the contrary, the generation of BrdU/S100β+ glial cells was increased resulting in neurogliogenesis imbalance in the hippocampus. Further, cypermethrin treatment also led to an increased number of BrdU/cleaved caspase-3+ and Fluoro-Jade B+ cells suggesting an induction of apoptosis in NSCs and increased degeneration of neurons in the hippocampus. Overall, these results explicate that cypermethrin exposure not only reduces the NSC pool but also disturbs the neuron-astrocyte ratio and potentiates neurodegeneration in the hippocampus, leading to cognitive dysfunctions in rats.

Mechanisms of VPS35-Mediated Neurodegeneration in Parkinson's Disease

Parkinson's disease is a sporadic and common neurodegenerative movement disorder resulting from the complex interplay between genetic risk, aging and environmental exposure. Familial forms of PD account for ~10% of cases and are known to result from the inheritance of mutations in at least 15 genes. Mutations in the vacuolar protein sorting 35 ortholog (VPS35) gene cause late-onset, autosomal dominant familial PD. VPS35 is a key suunit of the pentameric retromer complex that plays a role in the retrograde sorting and recycling of transmembrane cargo proteins from endosomes to the plasma membrane and trans-Golgi network. A single heterozygous Asp620Asn (D620N) mutation in VPS35 has been identified in multiple families that segregates with PD, and a number of experimental cellular and animal models have been developed to understand its pathogenic effects. At the molecular level, the D620N mutation has been shown to impair the interaction of VPS35 with the WASH complex, that plays an accessory function in retromer-dependent sorting. In addition, the D620N mutation has been linked to the abnormal sorting of retromer cargo, including CI-M6PR, AMPA receptor subunits, MUL1, LAMP2a and ATG9A, as well as to LRRK2 hyperactivation. At the cellular level, data support an impact of D620N VPS35 on mitochondrial function, the autophagy-lysosomal pathway, Wnt signaling and neurotransmission via altered endosomal sorting. The relevance of abnormal retromer sorting and cellular pathways to PD-related neurodegenerative phenotypes induced by D620N VPS35 in rodent models is not yet clear. There is also uncertainty regarding the mechanism-of-action of the D620N mutation and whether it manifests pathogenic effects in animal models and PD through a gain-of-function and/or a partial dominant-negative mechanism. Here, we discuss the emerging molecular and cellular mechanisms underlying PD induced by familial VPS35 mutations, going from structure to cellular function to neuropathology. We further discuss studies linking reduced retromer function to other neurodegenerative diseases and potential therapeutic strategies to normalize retromer function to mitigate disease.

Diurnal Variation Induces Neurobehavioral and Neuropathological Differences in a Rat Model of Traumatic Brain Injury

Traumatic brain injury (TBI) induces two types of brain damage: primary and secondary. Damage initiates a series of pathophysiological processes, such as metabolic crisis, excitotoxicity with oxidative stress-induced damage, and neuroinflammation. The long-term perpetuation of these processes has deleterious consequences for neuronal function. However, it remains to be elucidated further whether physiological variation in the brain microenvironment, depending on diurnal variations, influences the damage, and consequently, exerts a neuroprotective effect. Here, we established an experimental rat model of TBI and evaluated the effects of TBI induced at two different time points of the light–dark cycle. Behavioral responses were assessed using a 21-point neurobehavioral scale and the cylinder test. Morphological damage was assessed in different regions of the central nervous system. We found that rats that experienced a TBI during the dark hours had better behavioral performance than those injured during the light hours. Differences in behavioral performance correlated with less morphological damage in the perilesional zone. Moreover, certain brain areas (CA1 and dentate gyrus subregions of the hippocampus) were less prone to damage in rats that experienced a TBI during the dark hours. Our results suggest that diurnal variation is a crucial determinant of TBI outcome, and the hour of the day at which an injury occurs should be considered for future research.

Chronic Pain Impairs Memory Formation via Disruption of Neurogenesis Mediated by Mesohippocampal Brain-Derived Neurotrophic Factor Signaling

BACKGROUND

Chronic pain patients often complain of their poor memory. The mechanisms underlying chronic pain-related memory impairment remain elusive, and there are few clinical therapeutic strategies available for this condition.

METHODS

In a neuropathic pain model induced by chronic constrictive injury of the sciatic nerve in male mice, we used circuit-specific electrophysiological recording, combined with chemogenetic, molecular, and pharmacologic methods, to examine the circuit and molecular mechanisms underlying chronic pain-related memory impairment.

RESULTS

Our current results show that chronic neuropathic pain impaired the acquisition of spatial memory and, meanwhile, reduced adult neurogenesis in the dentate gyrus. Experimentally reducing dentate gyrus neurogenesis mimicked this pain-induced effect on spatial memory formation in naïve mice. Furthermore, pain-associated impairments of both hippocampal neurogenesis and memory formation were rescued or mimicked by chemogenetic activation or deactivation, respectively, of the ventral tegmental area dopaminergic projection, through which ventral tegmental area-released brain-derived neurotrophic factor was required. Importantly, we found that chronic, but not acute, systematic administration of subanesthetic doses of ketamine, while without relieving pain, ameliorated chronic pain-related impairment of spatial memory formation, potentially by rescuing brain-derived neurotrophic factor-mediated dentate gyrus neurogenesis.

CONCLUSIONS

These findings provide a novel, circuit-based mechanistic link between chronic pain and memory formation deficit, and potential new therapeutic options for chronic pain-related learning deficit and memory impairment.

The effects of microglia‐ and astrocyte‐derived factors on neurogenesis in health and disease

Hippocampal neurogenesis continues throughout life and has been suggested to play an essential role in maintaining spatial cognitive function under physiological conditions. An increasing amount of evidence has indicated that adult neurogenesis is tightly controlled by environmental conditions in the neurogenic niche, which consists of multiple types of cells including microglia and astrocytes. Microglia maintain the environment of neurogenic niche through their phagocytic capacity and interaction with neurons via fractalkine‐CX3CR1 signaling. In addition, microglia release growth factors such as brain‐derived neurotrophic factor (BDNF) and cytokines such as tumor necrosis factor (TNF)‐α to support the development of adult born neurons. Astrocytes also manipulate neurogenesis by releasing various soluble factors including adenosine triphosphate and lactate. Whereas, under pathological conditions such as Alzheimer's disease, depression, and epilepsy, microglia and astrocytes play a leading role in inflammation and are involved in attenuating the normal process of neurogenesis. The modulation of glial functions on neurogenesis in these brain diseases are attracting attention as a new therapeutic target. This review describes how these glial cells play a role in adult hippocampal neurogenesis in both health and disease, especially focusing glia‐derived factors.

Role of Microglia in Modulating Adult Neurogenesis in Health and Neurodegeneration

Microglia are the resident immune cells of the brain, constituting the powerhouse of brain innate immunity. They originate from hematopoietic precursors that infiltrate the developing brain during different stages of embryogenesis, acquiring a phenotype characterized by the presence of dense ramifications. Microglial cells play key roles in maintaining brain homeostasis and regulating brain immune responses. They continuously scan and sense the brain environment to detect any occurring changes. Upon detection of a signal related to physiological or pathological processes, the cells are activated and transform to an amoeboid-like phenotype, mounting adequate responses that range from phagocytosis to secretion of inflammatory and trophic factors. The overwhelming evidence suggests that microglia are crucially implicated in influencing neuronal proliferation and differentiation, as well as synaptic connections, and thereby cognitive and behavioral functions. Here, we review the role of microglia in adult neurogenesis under physiological conditions, and how this role is affected in neurodegenerative diseases.

Shifting equilibriums in Alzheimer's disease: the complex roles of microglia in neuroinflammation, neuronal survival and neurogenesis

Alzheimer's disease is the leading cause of dementia. Its increased prevalence in developed countries, due to the sharp rise in ageing populations, presents one of the costliest challenges to modern medicine. In order to find disease-modifying therapies to confront this challenge, a more complete understanding of the pathogenesis of Alzheimer's disease is necessary. Recent studies have revealed increasing evidence for the roles played by microglia, the resident innate immune system cells of the brain. Reflecting the well-established roles of microglia in reacting to pathogens and inflammatory stimuli, there is now a growing literature describing both protective and detrimental effects for individual cytokines and chemokines produced by microglia in Alzheimer's disease. A smaller but increasing number of studies have also addressed the divergent roles played by microglial neurotrophic and neurogenic factors, and how their perturbation may play a key role in the pathogenesis of Alzheimer's disease. Here we review recent findings on the roles played by microglia in neuroinflammation, neuronal survival and neurogenesis in Alzheimer's disease. In each case, landmark studies have provided evidence for the divergent ways in which microglia can either promote neuronal function and survival, or perturb neuronal function, leading to cell death. In many cases, the secreted molecules of microglia can lead to divergent effects depending on the magnitude and context of microglial activation. This suggests that microglial functions must be maintained in a fine equilibrium, in order to support healthy neuronal function, and that the cellular microenvironment in the Alzheimer's disease brain disrupts this fine balance, leading to neurodegeneration. Thus, an understanding of microglial homeostasis, both in health and across the trajectory of the disease state, will improve our understanding of the pathogenic mechanisms underlying Alzheimer's disease, and will hopefully lead to the development of microglial-based therapeutic strategies to restore equilibrium in the Alzheimer's disease brain.

A Role of Low-Density Lipoprotein Receptor-Related Protein 4 (LRP4) in Astrocytic Aβ Clearance

Amyloid-β (Aβ) deposition occurs years before cognitive symptoms appear and is considered a cause of Alzheimer's disease (AD). The imbalance of Aβ production and clearance leads to Aβ accumulation and Aβ deposition. Increasing evidence indicates an important role of astrocytes, the most abundant cell type among glial cells in the brain, in Aβ clearance. We explored the role of low-density lipoprotein receptor-related protein 4 (LRP4), a member of the LDLR family, in AD pathology. We show that Lrp4 is specifically expressed in astrocytes and its levels in astrocytes were higher than those of Ldlr and Lrp1, both of which have been implicated in Aβ uptake. LRP4 was reduced in postmortem brain tissues of AD patients. Genetic deletion of the Lrp4 gene augmented Aβ plaques in 5xFAD male mice, an AD mouse model, and exacerbated the deficits in neurotransmission, synchrony between the hippocampus and PFC, and cognition. Mechanistically, LRP4 promotes Aβ uptake by astrocytes likely by interacting with ApoE. Together, our study demonstrates that astrocytic LRP4 plays an important role in Aβ pathology and cognitive function.SIGNIFICANCE STATEMENT This study investigates how astrocytes, a type of non-nerve cells in the brain, may contribute to Alzheimer's disease (AD) development. We demonstrate that the low-density lipoprotein receptor-related protein 4 (LRP4) is reduced in the brain of AD patients. Mimicking the reduced levels in an AD mouse model exacerbates cognitive impairment and increases amyloid aggregates that are known to damage the brain. We show that LRP4 could promote the clearance of amyloid protein by astrocytes. Our results reveal a previously unappreciated role of LRP4 in AD development.

Cyclin D2-knock-out mice with attenuated dentate gyrus neurogenesis have robust deficits in long-term memory formation

Neurobehavioral studies have produced contradictory findings concerning the function of neurogenesis in the adult dentate gyrus. Previous studies have proved inconsistent across several behavioral endpoints thought to be dependent on dentate neurogenesis, including memory acquisition, short-term and long-term retention of memory, pattern separation, and reversal learning. We hypothesized that the main function of dentate neurogenesis is long-term memory formation because we assumed that a newly formed and integrated neuron would have a long-term impact on the local neural network. We used a cyclin D2-knock-out (cyclin D2^−/−) mouse model of endogenously deficient dentate neurogenesis to test this hypothesis. We found that cyclin D2^−/− mice had robust and sustained loss of long-term memory in two separate behavioral tasks, Morris water maze (MWM) and touchscreen intermediate pattern separation. Moreover, after adjusting for differences in brain volumes determined by magnetic resonance (MR) imaging, reduced dentate neurogenesis moderately correlated with deficits in memory retention after 24 hours. Importantly, cyclin D2^−/− mice did not show deficits in learning acquisition in a touchscreen paradigm of intermediate pattern separation or MWM platform location, indicating intact short-term memory. Further evaluation of cyclin D2^−/− mice is necessary to confirm that deficits are specifically linked to dentate gyrus neurogenesis since cyclin D2^−/− mice also have a reduced size of the olfactory bulb, hippocampus, cerebellum and cortex besides reduced dentate gyrus neurogenesis.

Ependymal Vps35 Promotes Ependymal Cell Differentiation and Survival, Suppresses Microglial Activation, and Prevents Neonatal Hydrocephalus

Hydrocephalus is a pathologic condition associated with various brain diseases, including Alzheimer's disease (AD). Dysfunctional ependymal cells (EpCs) are believed to contribute to the development of hydrocephalus. It is thus of interest to investigate EpCs' development and function. Here, we report that vacuolar protein sorting-associated protein 35 (VPS35) is critical for EpC differentiation, ciliogenesis, and survival, and thus preventing neonatal hydrocephalus. VPS35 is abundantly expressed in EpCs. Mice with conditional knock-out (cKO) of Vps35 in embryonic (Vps35^GFAP-Cre and Vps35^Emx1-Cre) or postnatal (Vps35^Foxj1-CreER) EpC progenitors exhibit enlarged lateral ventricles (LVs) and hydrocephalus-like pathology. Further studies reveal marked reductions in EpCs and their cilia in both Vps35^GFAP-Cre and Vps35^Foxj1-CreER mutant mice. The reduced EpCs appear to be due to impairments in EpC differentiation and survival. Additionally, both Vps35^GFAP-Cre and Vps35^Foxj1-CreER neonatal pups exhibit increased cell proliferation and death largely in a region close to LV-EpCs. Many microglia close to the mutant LV-EpC region become activated. Depletion of the microglia by PLX3397, an antagonist of colony-stimulating factor 1 receptor (CSF1R), restores LV-EpCs and diminishes the pathology of neonatal hydrocephalus in Vps35^Foxj1-CreER mice. Taken together, these observations suggest unrecognized functions of Vps35 in EpC differentiation, ciliogenesis, and survival in neonatal LV, and reveal pathologic roles of locally activated microglia in EpC homeostasis and hydrocephalus development.SIGNIFICANCE STATEMENT This study reports critical functions of vacuolar protein sorting-associated protein 35 (VPS35) not only in promoting ependymal cell (EpC) differentiation, ciliogenesis, and survival, but also in preventing local microglial activation. The dysfunctional EpCs and activated microglia are likely to induce hydrocephalus.

Retromer subunit, VPS29, regulates synaptic transmission and is required for endolysosomal function in the aging brain

Retromer, including Vps35, Vps26, and Vps29, is a protein complex responsible for recycling proteins within the endolysosomal pathway. Although implicated in both Parkinson's and Alzheimer's disease, our understanding of retromer function in the adult brain remains limited, in part because Vps35 and Vps26 are essential for development. In Drosophila, we find that Vps29 is dispensable for embryogenesis but required for retromer function in aging adults, including for synaptic transmission, survival, and locomotion. Unexpectedly, in Vps29 mutants, Vps35 and Vps26 proteins are normally expressed and associated, but retromer is mislocalized from neuropil to soma with the Rab7 GTPase. Further, Vps29 phenotypes are suppressed by reducing Rab7 or overexpressing the GTPase activating protein, TBC1D5. With aging, retromer insufficiency triggers progressive endolysosomal dysfunction, with ultrastructural evidence of impaired substrate clearance and lysosomal stress. Our results reveal the role of Vps29 in retromer localization and function, highlighting requirements for brain homeostasis in aging.

Communication, Cross Talk, and Signal Integration in the Adult Hippocampal Neurogenic Niche

Radial glia-like neural stem cells (RGLs) in the dentate gyrus subregion of the hippocampus give rise to dentate granule cells (DGCs) and astrocytes throughout life, a process referred to as adult hippocampal neurogenesis. Adult hippocampal neurogenesis is sensitive to experiences, suggesting that it may represent an adaptive mechanism by which hippocampal circuitry is modified in response to environmental demands. Experiential information is conveyed to RGLs, progenitors, and adult-born DGCs via the neurogenic niche that is composed of diverse cell types, extracellular matrix, and afferents. Understanding how the niche performs its functions may guide strategies to maintain its health span and provide a permissive milieu for neurogenesis. Here, we first discuss representative contributions of niche cell types to regulation of neural stem cell (NSC) homeostasis and maturation of adult-born DGCs. We then consider mechanisms by which the activity of multiple niche cell types may be coordinated to communicate signals to NSCs. Finally, we speculate how NSCs integrate niche-derived signals to govern their regulation.

Endosomal sorting pathways in the pathogenesis of Parkinson's disease

The identification of Parkinson's disease (PD)-associated genes has created a powerful platform to begin to understand and nominate pathophysiological disease mechanisms. Herein, we discuss the genetic and experimental evidence supporting endolysosomal dysfunction as a major pathway implicated in PD. Well-studied familial PD-linked gene products, including LRRK2, VPS35, and α-synuclein, demonstrate how disruption of different aspects of endolysosomal sorting pathways by disease-causing mutations may manifest into PD-like phenotypes in many disease models. Newly-identified PD-linked genes, including auxilin, synaptojanin-1 and Rab39b, as well as putative risk genes for idiopathic PD (endophilinA1, Rab29, GAK), further support endosomal sorting deficits as being central to PD. LRRK2 may represent a nexus by regulating many distinct features of endosomal sorting, potentially via phosphorylation of key endocytosis machinery (i.e., auxilin, synaptojanin-1, endoA1) and Rab GTPases (i.e., Rab29, Rab8A, Rab10) that function within these pathways. In turn, LRRK2 kinase activity is critically regulated by Rab29 at the Golgi complex and retromer-associated VPS35 at endosomes. Taken together, the known functions of PD-associated gene products, the impact of disease-linked mutations, and the emerging functional interactions between these proteins points to endosomal sorting pathways as a key point of convergence in the pathogenesis of PD.

Early administration of galantamine from preplaque phase suppresses oxidative stress and improves cognitive behavior in APPswe/PS1dE9 mouse model of Alzheimer's disease

Alzheimer's disease (AD) is a common neurodegenerative disease that progressively impairs memory and cognition. Deposition of amyloid-β (Aβ) peptides is the most important pathophysiological hallmark of AD. Oxidative stress induced by generation of reactive oxygen species (ROS) is a prominent phenomenon in AD and known to occur early in the course of AD. Several reports suggest a relationship between change in redox status and AD pathology including progressive Aβ deposition, glial cell activation, and inflammation. Galantamine is an acetylcholinesterase inhibitor and has been reported to have an oxidative stress inhibitory function. In the present study, galantamine was administered orally to AD model mice from before the appearance of Aβ plaques (preplaque phase), and in vivo change in redox status of the brain was measured using electron paramagnetic resonance (EPR) imaging. Administration of galantamine from the preplaque phase ameliorated memory decline in Morris water maze test and novel object recognition test. Monitoring of the redox status of the brain using EPR imaging showed that galantamine treatment improved the unbalanced redox state. Additionally, galantamine administration enhanced microglial function to promote Aβ clearance, reducing the Aβ-positive area in the cortex and amount of insoluble Aβ in the brain. In contrast, galantamine treatment from the preplaque phase suppressed the production of proinflammatory cytokines through neurotoxic microglial activity. Therefore, galantamine administration from the preplaque phase may have the potential of clinical application for the prevention of AD. In addition, our results demonstrate the usefulness of EPR imaging for speedy and quantitative evaluation of the efficacy of disease-modifying drugs for AD.

Microglial VPS35 deficiency regulates microglial polarization and decreases ischemic stroke-induced damage in the cortex

BACKGROUND

Vacuolar sorting protein 35 (VPS35), a critical component of retromer, is essential for selective endosome-to-Golgi retrieval of membrane proteins. It is highly expressed in microglial cells, in addition to neurons. We have previously demonstrated microglial VPS35's functions in preventing hippocampal, but not cortical, microglial activation, and in promoting adult hippocampal neurogenesis. However, microglial VPS35's role in the cortex in response to ischemic stroke remains largely unclear.

METHODS

We used mice with VPS35 cKO (conditional knockout) in microglial cells and examined and compared their responses to ischemic stroke with control mice. The brain damage, cell death, changes in glial cells and gene expression, and sensorimotor deficits were assessed by a combination of immunohistochemical and immunofluorescence staining, RT-PCR, Western blot, and neurological functional behavior tests.

RESULTS

We found that microglial VPS35 loss results in an increase of anti-inflammatory microglia in mouse cortex after ischemic stroke. The ischemic stroke-induced brain injury phenotypes, including brain damage, neuronal death, and sensorimotor deficits, were all attenuated by microglial VPS35-deficiency. Further analysis of protein expression changes revealed a reduction in CX3CR1 (CX3C chemokine receptor 1) in microglial VPS35-deficient cortex after ischemic stroke, implicating CX3CR1 as a potential cargo of VPS35 in this event.

CONCLUSION

Together, these results reveal an unrecognized function of microglial VPS35 in enhancing ischemic brain injury-induced inflammatory microglia, but suppressing the injury-induced anti-inflammatory microglia. Consequently, microglial VPS35 cKO mice exhibit attenuation of ischemic brain injury response.

Reduced Hippocampal Neurogenesis in Mice Deficient in Apoptosis Repressor with Caspase Recruitment Domain (ARC)

In the adult hippocampal dentate gyrus (DG), the majority of newly generated cells are eliminated by apoptotic mechanisms. The apoptosis repressor with caspase recruitment domain (ARC), encoded by the Nol3 gene, is a potent and multifunctional death repressor that inhibits both death receptor and mitochondrial apoptotic signaling. The aim of the present study was to parse the role of ARC in the development of new granule cell neurons. Nol3 gene expression as revealed by in situ hybridization is present in the entire dentate granule cell layer. Moreover, a comparison of Nol3 expression between FACS-sorted Sox2-positive neural stem cells and Doublecortin (DCX)-positive immature neurons demonstrates upregulation of Nol3 during neurogenesis. Using ARC-deficient mice, we show that proliferation and survival of BrdU birth-dated cells are strongly reduced in the absence of ARC while neuronal-glial fate choice is not affected. Both the number of DCX-positive cells and the number of calretinin (CR)-positive immature postmitotic neurons are reduced in the hippocampus of ARC^-/- mice. ARC knockout is not associated with increased numbers of microglia or with microglia activation. However, hippocampal brain-derived neurotrophic factor (BDNF) protein content is significantly increased in ARC^-/- mice, possibly representing a compensatory response. Collectively, our results suggest that ARC plays a critical cell-autonomous role in preventing cell death during adult granule cell neogenesis.

Agrin-Lrp4-Ror2 signaling regulates adult hippocampal neurogenesis in mice

Adult neurogenesis in the hippocampus may represent a form of plasticity in brain functions including mood, learning and memory. However, mechanisms underlying neural stem/progenitor cells (NSPCs) proliferation are not well understood. We found that Agrin, a factor critical for neuromuscular junction formation, is elevated in the hippocampus of mice that are stimulated by enriched environment (EE). Genetic deletion of the Agrn gene in excitatory neurons decreases NSPCs proliferation and increases depressive-like behavior. Low-density lipoprotein receptor-related protein 4 (Lrp4), a receptor for Agrin, is expressed in hippocampal NSPCs and its mutation blocked basal as well as EE-induced NSPCs proliferation and maturation of newborn neurons. Finally, we show that Lrp4 interacts with and activates receptor tyrosine kinase-like orphan receptor 2 (Ror2); and Ror2 mutation impairs NSPCs proliferation. Together, these observations identify a role of Agrin-Lrp4-Ror2 signaling for adult neurogenesis, uncovering previously unexpected functions of Agrin and Lrp4 in the brain.

Rewiring of Memory Circuits: Connecting Adult Newborn Neurons With the Help of Microglia

New neurons are continuously generated from stem cells and integrated into the adult hippocampal circuitry, contributing to memory function. Several environmental, cellular, and molecular factors regulate the formation of new neurons, but the mechanisms that govern their incorporation into memory circuits are less explored. Herein we will focus on microglia, the resident immune cells of the CNS, which modulate the production of new neurons in the adult hippocampus and are also well suited to participate in their circuit integration. Microglia may contribute to the refinement of brain circuits during development and exert a role in physiological and pathological conditions by regulating axonal and dendritic growth; promoting the formation, elimination, and relocation of synapses; modulating excitatory synaptic maturation; and participating in functional synaptic plasticity. Importantly, microglia are able to sense subtle changes in their environment and may use this information to differently modulate hippocampal wiring, ultimately impacting on memory function. Deciphering the role of microglia in hippocampal circuitry constant rewiring will help to better understand the influence of microglia on memory function.