CNS-wide Sexually Dimorphic Induction of the Major Histocompatibility Complex 1 Pathway With Aging

Comprehensive alpha, beta, and delta cell transcriptomics reveal an association of cellular aging with MHC class I upregulation

OBJECTIVES

This study aimed to evaluate the efficacy of a purification method developed for isolating alpha, beta, and delta cells from pancreatic islets of adult mice, extending its application to islets from newborn and aged mice. Furthermore, it sought to examine transcriptome dynamics in mouse pancreatic endocrine islet cells throughout postnatal development and to validate age-related alterations within these cell populations.

METHODS

We leveraged the high surface expression of CD71 on beta cells and CD24 on delta cells to FACS-purify alpha, beta, and delta cells from newborn (1-week-old), adult (12-week-old), and old (18-month-old) mice. Bulk RNA sequencing was conducted on these purified cell populations, and subsequent bioinformatic analyses included differential gene expression, overrepresentation, and intersection analysis.

RESULTS

Alpha, beta, and delta cells from newborn and aged mice were successfully FACS-purified using the same method employed for adult mice. Our analysis of the age-related transcriptional changes in alpha, beta, and delta cell populations revealed a decrease in cell cycling and an increase in neuron-like features processes during the transition from newborn to adult mice. Progressing from adult to old mice, we identified an inflammatory gene signature related to aging (inflammaging) encompassing an increase in β-2 microglobulin and major histocompatibility complex (MHC) Class I expression.

CONCLUSIONS

Our study demonstrates the effectiveness of our cell sorting technique in purifying endocrine subsets from mouse islets at different ages. We provide a valuable resource for better understanding endocrine pancreas aging and identified an inflammaging gene signature with increased β-2 microglobulin and MHC Class I expression as a common hallmark of old alpha, beta, and delta cells, with potential implications for immune response regulation and age-related diabetes.

Ultrasound and neuroinflammation: immune modulation via the heat shock response

Current pharmacological therapeutic approaches targeting chronic inflammation exhibit transient efficacy, often with adverse effects, limiting their widespread use - especially in the context of neuroinflammation. Effective interventions require the consideration of homeostatic function, pathway dysregulation, and pleiotropic effects when evaluating therapeutic targets. Signalling molecules have multiple functions dependent on the immune context, and this complexity results in therapeutics targeting a single signalling molecule often failing in clinical translation. Additionally, the administration of non-physiologic levels of neurotrophic or anti-inflammatory factors can alter endogenous signalling, resulting in unanticipated effects. Exacerbating these challenges, the central nervous system (CNS) is isolated by the blood brain barrier (BBB), restricting the infiltration of many pharmaceutical compounds into the brain tissue. Consequently, there has been marked interest in therapeutic techniques capable of modulating the immune response in a pleiotropic manner; ultrasound remains on this frontier. While ultrasound has been used therapeutically in peripheral tissues - accelerating healing in wounds, bone fractures, and reducing inflammation - it is only recently that it has been applied to the CNS. The transcranial application of low intensity pulsed ultrasound (LIPUS) has successfully mitigated neuroinflammation in vivo, in models of neurodegenerative disease across a broad spectrum of ultrasound parameters. To date, the underlying biological effects and signalling pathways modulated by ultrasound are poorly understood, with a diverse array of reported molecules implicated. The distributed nature of the beneficial response to LIPUS implies the involvement of an, as yet, undetermined upstream signalling pathway, homologous to the protective effect of febrile range hyperthermia in chronic inflammation. As such, we review the heat shock response (HSR), a protective signalling pathway activated by thermal and mechanical stress, as the possible upstream regulator of the anti-inflammatory effects of ultrasound.

Asparagine endopeptidase deficiency mitigates radiation-induced brain injury by suppressing microglia-mediated neuronal senescence

Mounting evidence supports the role of neuroinflammation in radiation-induced brain injury (RIBI), a chronic disease characterized by delayed and progressive neurological impairment. Asparagine endopeptidase (AEP), also known as legumain (LGMN), participates in multiple malignancies and neurodegenerative diseases and may potentially be involved in RIBI. Here, we found AEP expression was substantially elevated in the cortex and hippocampus of wild-type (Lgmn+/+) mice following whole-brain irradiation. Lgmn knockout (Lgmn-/-) alleviated neurological impairment caused by whole-brain irradiation by suppressing neuronal senescence. Bulk RNA and metabolomic sequencing revealed AEP's involvement in the antigen processing and presentation pathway and neuroinflammation. This was further confirmed by co-culturing Lgmn+/+ primary neurons with the conditioned media derived from irradiated Lgmn+/+ or Lgmn-/- primary microglia. Furthermore, esomeprazole inhibited the enzymatic activity of AEP and RIBI. These findings identified AEP as a critical factor of neuroinflammation in RIBI, highlighting the prospect of targeting AEP as a therapeutic approach.

Bacteria-derived nanovesicles enhance tumour vaccination by trained immunity

Trained immunity enhances the responsiveness of immune cells to subsequent infections or vaccinations. Here we demonstrate that pre-vaccination with bacteria-derived outer-membrane vesicles, which contain large amounts of pathogen-associated molecular patterns, can be used to potentiate, and enhance, tumour vaccination by trained immunity. Intraperitoneal administration of these outer-membrane vesicles to mice activates inflammasome signalling pathways and induces interleukin-1β secretion. The elevated interleukin-1β increases the generation of antigen-presenting cell progenitors. This results in increased immune response when tumour antigens are delivered, and increases tumour-antigen-specific T-cell activation. This trained immunity increased protection from tumour challenge in two distinct cancer models.

Immune receptors and aging brain

Aging brings about a myriad of degenerative processes throughout the body. A decrease in cognitive abilities is one of the hallmark phenotypes of aging, underpinned by neuroinflammation and neurodegeneration occurring in the brain. This review focuses on the role of different immune receptors expressed in cells of the central and peripheral nervous systems. We will discuss how immune receptors in the brain act as sentinels and effectors of the age-dependent shift in ligand composition. Within this 'old-age-ligand soup,' some immune receptors contribute directly to excessive synaptic weakening from within the neuronal compartment, while others amplify the damaging inflammatory environment in the brain. Ultimately, chronic inflammation sets up a positive feedback loop that increases the impact of immune ligand-receptor interactions in the brain, leading to permanent synaptic and neuronal loss.

NPD1 Plus RvD1 Mediated Ischemic Stroke Penumbra Protection Increases Expression of Pro-homeostatic Microglial and Astrocyte Genes

Neuroprotection to attenuate or block the ischemic cascade and salvage neuronal damage has been extensively explored for treating ischemic stroke. However, despite increasing knowledge of the physiologic, mechanistic, and imaging characterizations of the ischemic penumbra, no effective neuroprotective therapy has been found. This study focuses on the neuroprotective bioactivity of docosanoid mediators: Neuroprotectin D1 (NPD1), Resolvin D1 (RvD1), and their combination in experimental stroke. Molecular targets of NPD1 and RvD1 are defined by following dose-response and therapeutic window. We demonstrated that treatment with NPD1, RvD1, and combination therapy provides high-grade neurobehavioral recovery and decreases ischemic core and penumbra volumes even when administered up to 6 h after stroke. The expression of the following genes was salient: (a) Cd163, an anti-inflammatory stroke-associated gene, was the most differentially expressed gene by NPD1+RvD1, displaying more than a 123-fold upregulation in the ipsilesional penumbra (Lisi et al., Neurosci Lett 645:106-112, 2017); (b) 100-fold upregulation takes place in astrocyte gene PTX3, a key regulator of neurogenesis and angiogenesis after cerebral ischemia (. Rodriguez-Grande et al., J Neuroinflammation 12:15, 2015); and (c) Tmem119 and P2y12, two markers of homeostatic microglia, were found to be enhanced by ten- and fivefold, respectively (Walker et al. Int J Mol Sci 21:678, 2020). Overall, we uncovered that protection after middle cerebral artery occlusion (MCAo) by the lipid mediators elicits expression of microglia and astrocyte-specific genes (Tmem119, Fcrls, Osmr, Msr1, Cd68, Cd163, Amigo2, Thbs1, and Tm4sf1) likely participating in enhancing homeostatic microglia, modulating neuroinflammation, promoting DAMP clearance, activating NPC differentiation and maturation, synapse integrity and contributing to cell survival.

Microglial MHC-I induction with aging and Alzheimer's is conserved in mouse models and humans

Major histocompatibility complex I (MHC-I) CNS cellular localization and function is still being determined after previously being thought to be absent from the brain. MHC-I expression has been reported to increase with brain aging in mouse, rat, and human whole tissue analyses, but the cellular localization was undetermined. Neuronal MHC-I is proposed to regulate developmental synapse elimination and tau pathology in Alzheimer's disease (AD). Here, we report that across newly generated and publicly available ribosomal profiling, cell sorting, and single-cell data, microglia are the primary source of classical and non-classical MHC-I in mice and humans. Translating ribosome affinity purification-qPCR analysis of 3-6- and 18-22-month-old (m.o.) mice revealed significant age-related microglial induction of MHC-I pathway genes B2m, H2-D1, H2-K1, H2-M3, H2-Q6, and Tap1 but not in astrocytes and neurons. Across a timecourse (12-23 m.o.), microglial MHC-I gradually increased until 21 m.o. and then accelerated. MHC-I protein was enriched in microglia and increased with aging. Microglial expression, and absence in astrocytes and neurons, of MHC-I-binding leukocyte immunoglobulin-like (Lilrs) and paired immunoglobin-like type 2 (Pilrs) receptor families could enable cell -autonomous MHC-I signaling and increased with aging in mice and humans. Increased microglial MHC-I, Lilrs, and Pilrs were observed in multiple AD mouse models and human AD data across methods and studies. MHC-I expression correlated with p16INK4A, suggesting an association with cellular senescence. Conserved induction of MHC-I, Lilrs, and Pilrs with aging and AD opens the possibility of cell-autonomous MHC-I signaling to regulate microglial reactivation with aging and neurodegeneration.

[Identification of Peripheral Blood GZMK + CD8 + T Cells As Biomarkers of Alzheimer's Disease Based on Single-Cell Transcriptome]

Objective

Based on single-cell RNA sequencing (scRNA-seq) to explore immune characteristics in the peripheral blood of patients with Alzheimer's disease (AD) as biomarkers.

Methods

GSE168522, the scRNA-seq dataset of AD peripheral blood immune cells, was downloaded from the Gene Expression Omnibus (GEO) database and was analyzed in the RAD-Blood web server (http://www.bioinform.cn/RAD-Blood/). The changes in blood cell composition in AD patients were analyzed. The abnormal communications between different types of cells in AD patients were investigated by the CellChat R package.

Results

There were two kinds of CD8 + T cells in the blood of AD patients and healthy individuals, one of which highly expressed granzyme K ( GZMK) (false discovery rate [FDR]<0.05), and the other highly expressed GZMA, GZMB, and GZMH (FDR<0.05). In the blood of AD patients, the content of GZMK + CD8 + T cells was increased by 32.9% ( P=5.15E-21), their interactions with other cell types were increased, and they might be associated with AD through the abnormal signal transduction of major histocompatibility complex class Ⅰ (MHC-Ⅰ). Erythrocyte provided the main ligands, that are, human leukocyte antigen (HLA) class Ⅰ molecules, including HLA- A, HLA- B, HLA- C, and HLA- E, for the abnormal MHC-Ⅰ signaling pathway of GZMK + CD8 + T cells. The RESISTIN signaling pathway was specifically enriched in the blood of AD patients.

Conclusion

The increased content of peripheral blood GZMK + CD8 + T cells, the increased interaction between GZMK + CD8 + T cells and erythrocytes, and the enhanced RESISTIN pathway are potential blood biomarkers of AD.

Microglial senescence contributes to female-biased neuroinflammation in the aging mouse hippocampus: implications for Alzheimer's disease

BACKGROUND

Microglia, the brain's principal immune cells, have been implicated in the pathogenesis of Alzheimer's disease (AD), a condition shown to affect more females than males. Although sex differences in microglial function and transcriptomic programming have been described across development and in disease models of AD, no studies have comprehensively identified the sex divergences that emerge in the aging mouse hippocampus. Further, existing models of AD generally develop pathology (amyloid plaques and tau tangles) early in life and fail to recapitulate the aged brain environment associated with late-onset AD. Here, we examined and compared transcriptomic and translatomic sex effects in young and old murine hippocampal microglia.

METHODS

Hippocampal tissue from C57BL6/N and microglial NuTRAP mice of both sexes were collected at young (5-6 month-old [mo]) and old (22-25 mo) ages. Cell sorting and affinity purification techniques were used to isolate the microglial transcriptome and translatome for RNA-sequencing and differential expression analyses. Flow cytometry, qPCR, and imaging approaches were used to confirm the transcriptomic and translatomic findings.

RESULTS

There were marginal sex differences identified in the young hippocampal microglia, with most differentially expressed genes (DEGs) restricted to the sex chromosomes. Both sex chromosomally and autosomally encoded sex differences emerged with aging. These sex DEGs identified at old age were primarily female-biased and enriched in senescent and disease-associated microglial signatures. Normalized gene expression values can be accessed through a searchable web interface ( https://neuroepigenomics.omrf.org/ ). Pathway analyses identified upstream regulators induced to a greater extent in females than in males, including inflammatory mediators IFNG, TNF, and IL1B, as well as AD-risk genes TREM2 and APP.

CONCLUSIONS

These data suggest that female microglia adopt disease-associated and senescent phenotypes in the aging mouse hippocampus, even in the absence of disease pathology, to a greater extent than males. This sexually divergent microglial phenotype may explain the difference in susceptibility and disease progression in the case of AD pathology. Future studies will need to explore sex differences in microglial heterogeneity in response to AD pathology and determine how sex-specific regulators (i.e., sex chromosomal or hormonal) elicit these sex effects.

Sex, sepsis and the brain: defining the role of sexual dimorphism on neurocognitive outcomes after infection

Sexual dimorphisms exist in multiple domains, from learning and memory to neurocognitive disease, and even in the immune system. Male sex has been associated with increased susceptibility to infection, as well as increased risk of adverse outcomes. Sepsis remains a major source of morbidity and mortality globally, and over half of septic patients admitted to intensive care are believed to suffer some degree of sepsis-associated encephalopathy (SAE). In the short term, SAE is associated with an increased risk of in-hospital mortality, and in the long term, has the potential for significant impairment of cognition, memory, and acceleration of neurocognitive disease. Despite increasing information regarding sexual dimorphism in neurologic and immunologic systems, research into these dimorphisms in sepsis-associated encephalopathy remains critically understudied. In this narrative review, we discuss how sex has been associated with brain morphology, chemistry, and disease, sexual dimorphism in immunity, and existing research into the effects of sex on SAE.

Does the Environmental Air Impact the Condition of the Vomeronasal Organ? A Mouse Model for Intensive Farming

Chemical communication in mammals is ensured by exchanging chemical signals through the vomeronasal organ (VNO) and its ability to detect pheromones. The alteration of this organ has been proven to impact animal life, participating in the onset of aggressive behaviors in social groups. To date, few studies have highlighted the possible causes leading to these alterations, and the farming environment has not been investigated, even though irritant substances such as ammonia are known to induce serious damage in the respiratory tract. The goal of this study was to investigate the environmental impact on the VNO structure. Thirty mice were split into three groups, one housed in normal laboratory conditions and the other two in confined environments, with or without the release of litter ammonia. VNOs were analyzed using histology and immunohistochemistry to evaluate the effect of different environments on their condition. Both restricted conditions induced VNO alterations (p = 0.0311), soft-tissue alteration (p = 0.0480), and nonsensory epithelium inflammation (p = 0.0024). There was glycogen accumulation (p < 0.0001), the olfactory marker protein was underexpressed (p < 0.0001), and Gαi2 positivity remained unchanged while Gαo expression was upregulated in confined conditions. VNO conditions seemed to worsen with ammonia, even if not always significantly. These murine model results suggest that the housing environment can strongly impact VNO conditions, providing novel insights for improving indoor farming systems.

Microglial MHC-I induction with aging and Alzheimer's is conserved in mouse models and humans

Major Histocompatibility Complex I (MHC-I) CNS cellular localization and function is still being determined after previously being thought to be absent from the brain. MHC-I expression has been reported to increase with brain aging in mouse, rat, and human whole tissue analyses but the cellular localization was undetermined. Neuronal MHC-I is proposed to regulate developmental synapse elimination and tau pathology in Alzheimer's disease (AD). Here we report that across newly generated and publicly available ribosomal profiling, cell sorting, and single-cell data, microglia are the primary source of classical and non-classical MHC-I in mice and humans. Translating Ribosome Affinity Purification-qPCR analysis of 3-6 and 18-22 month old (m.o.) mice revealed significant age-related microglial induction of MHC-I pathway genes B2m , H2-D1 , H2-K1 , H2-M3 , H2-Q6 , and Tap1 but not in astrocytes and neurons. Across a timecourse (12-23 m.o.), microglial MHC-I gradually increased until 21 m.o. and then accelerated. MHC-I protein was enriched in microglia and increased with aging. Microglial expression, and absence in astrocytes and neurons, of MHC-I binding Leukocyte Immunoglobulin-like (Lilrs) and Paired immunoglobin-like type 2 (Pilrs) receptor families could enable cell-autonomous MHC-I signaling and increased with aging in mice and humans. Increased microglial MHC-I, Lilrs, and Pilrs were observed in multiple AD mouse models and human AD data across methods and studies. MHC-I expression correlated with p16INK4A , suggesting an association with cellular senescence. Conserved induction of MHC-I, Lilrs, and Pilrs with aging and AD opens the possibility of cell-autonomous MHC-I signaling to regulate microglial reactivation with aging and neurodegeneration.

Microglial senescence contributes to female-biased neuroinflammation in the aging mouse hippocampus: implications for Alzheimer's disease

Background

Microglia, the brain's principal immune cells, have been implicated in the pathogenesis of Alzheimer's disease (AD), a condition shown to affect more females than males. Although sex differences in microglial function and transcriptomic programming have been described across development and in disease models of AD, no studies have comprehensively identified the sex divergences that emerge in the aging mouse hippocampus. Further, existing models of AD generally develop pathology (amyloid plaques and tau tangles) early in life and fail to recapitulate the aged brain environment associated with late-onset AD. Here, we examined and compared transcriptomic and translatomic sex effects in young and old murine hippocampal microglia.

Methods

Hippocampal tissue from C57BL6/N and microglial NuTRAP mice of both sexes were collected at young (5-6 month-old [mo]) and old (22-25 mo) ages. Cell sorting and affinity purification techniques were used to isolate the microglial transcriptome and translatome for RNA-sequencing and differential expression analyses. Flow cytometry, qPCR, and imaging approaches were used to confirm the transcriptomic and translatomic findings.

Results

There were marginal sex differences identified in the young hippocampal microglia, with most differentially expressed genes (DEGs) restricted to the sex chromosomes. Both sex chromosomally-and autosomally-encoded sex differences emerged with aging. These sex DEGs identified at old age were primarily female-biased and enriched in senescent and disease-associated microglial signatures. Normalized gene expression values can be accessed through a searchable web interface ( https://neuroepigenomics.omrf.org/ ). Pathway analyses identified upstream regulators induced to a greater extent in females than in males, including inflammatory mediators IFNG, TNF, and IL1B, as well as AD-risk genes TREM2 and APP.

Conclusions

These data suggest that female microglia adopt disease-associated and senescent phenotypes in the aging mouse hippocampus, even in the absence of disease pathology, to a greater extent than males. This sexually divergent microglial phenotype may explain the difference in susceptibility and disease progression in the case of AD pathology. Future studies will need to explore sex differences in microglial heterogeneity in response to AD pathology and determine how sex-specific regulators (i.e., sex chromosomal or hormonal) elicit these sex effects.

The Diversity of Astrocyte Activation during Multiple Sclerosis: Potential Cellular Targets for Novel Disease Modifying Therapeutics

Neuroglial cells, and especially astrocytes, constitute the most varied group of central nervous system (CNS) cells, displaying substantial diversity and plasticity during development and in disease states. The morphological changes exhibited by astrocytes during the acute and chronic stages following CNS injury can be characterized more precisely as a dynamic continuum of astrocytic reactivity. Different subpopulations of reactive astrocytes may be ascribed to stages of degenerative progression through their direct pathogenic influence upon neurons, neuroglia, the blood-brain barrier, and infiltrating immune cells. Multiple sclerosis (MS) constitutes an autoimmune demyelinating disease of the CNS. Despite the previously held notion that reactive astrocytes purely form the structured glial scar in MS plaques, their continued multifaceted participation in neuroinflammatory outcomes and oligodendrocyte and neuronal function during chronicity, suggest that they may be an integral cell type that can govern the pathophysiology of MS. From a therapeutic-oriented perspective, astrocytes could serve as key players to limit MS progression, once the integral astrocyte-MS relationship is accurately identified. This review aims toward delineating the current knowledge, which is mainly focused on immunomodulatory therapies of the relapsing-remitting form, while shedding light on uncharted approaches of astrocyte-specific therapies that could constitute novel, innovative applications once the role of specific subgroups in disease pathogenesis is clarified.

Chromosomal and gonadal factors regulate microglial sex effects in the aging brain

Biological sex contributes to phenotypic sex effects through genetic (sex chromosomal) and hormonal (gonadal) mechanisms. There are profound sex differences in the prevalence and progression of age-related brain diseases, including neurodegenerative diseases. Inflammation of neural tissue is one of the most consistent age-related phenotypes seen with healthy aging and disease. The pro-inflammatory environment of the aging brain has primarily been attributed to microglial reactivity and adoption of heterogeneous reactive states dependent upon intrinsic (i.e., sex) and extrinsic (i.e., age, disease state) factors. Here, we review sex effects in microglia across the lifespan, explore potential genetic and hormonal molecular mechanisms of microglial sex effects, and discuss currently available models and methods to study sex effects in the aging brain. Despite recent attention to this area, significant further research is needed to mechanistically understand the regulation of microglial sex effects across the lifespan, which may open new avenues for sex informed prevention and treatment strategies.

Plasma β2-microglobulin and cerebrospinal fluid biomarkers of Alzheimer's disease pathology in cognitively intact older adults: the CABLE study

BACKGROUND

Previous studies have suggested a correlation between elevated levels of β₂-microglobulin (B2M) and cognitive impairment. However, the existing evidence is insufficient to establish a conclusive relationship. This study aims to analyze the link of plasma B2M to cerebrospinal fluid (CSF) Alzheimer's disease (AD) biomarkers and cognition.

METHODS

To track the dynamics of plasma B2M in preclinical AD, 846 cognitively healthy individuals in the Chinese Alzheimer's Biomarker and LifestylE (CABLE) cohort were divided into four groups (suspected non-AD pathology [SNAP], 2, 1, 0) according to the NIA-AA criteria. Multiple linear regression models were employed to examine the plasma B2M's relationship with cognitive and CSF AD biomarkers. Causal mediation analysis was conducted through 10,000 bootstrapped iterations to explore the mediating effect of AD pathology on cognition.

RESULTS

We found that the levels of plasma B2M were increased in stages 1 (P = 0.0007) and 2 (P < 0.0001), in contrast to stage 0. In total participants, higher levels of B2M were associated with worse cognitive performance (P = 0.006 for MMSE; P = 0.012 for MoCA). Moreover, a higher level of B2M was associated with decreases in Aβ_1-42 (P < 0.001) and Aβ_1-42/Aβ_1-40 (P = 0.015) as well as increases in T-tau/Aβ_1-42 (P < 0.001) and P-tau/Aβ_1-42 (P < 0.001). The subgroup analysis found B2M correlated with Aβ_1-42 in non-APOE ε4 individuals (P < 0.001) but not in APOE ε4 carriers. Additionally, the link between B2M and cognition was partially mediated by Aβ pathology (percentage: 8.6 to 19.3%), whereas tau pathology did not mediate this effect.

CONCLUSIONS

This study demonstrated the association of plasma B2M with CSF AD biomarkers as well as a possible important role of Aβ pathology in the association between B2M and cognitive impairment, particularly in cognitively normal individuals. The results indicated that B2M could be a potential biomarker for preclinical AD and might have varied functions throughout various stages of preclinical AD progression.

Translatomic response of retinal Müller glia to acute and chronic stress

Analysis of retina cell type-specific epigenetic and transcriptomic signatures is crucial to understanding the pathophysiology of retinal degenerations such as age-related macular degeneration (AMD) and delineating cell autonomous and cell-non-autonomous mechanisms. We have discovered that Aldh1l1 is specifically expressed in the major macroglia of the retina, Müller glia, and, unlike the brain, is not expressed in retinal astrocytes. This allows use of Aldh1l1 cre drivers and Nuclear Tagging and Translating Ribosome Affinity Purification (NuTRAP) constructs for temporally controlled labeling and paired analysis of Müller glia epigenomes and translatomes. As validated through a variety of approaches, the Aldh1l1cre/ERT2-NuTRAP model provides Müller glia specific translatomic and epigenomic profiles without the need to isolate whole cells. Application of this approach to models of acute injury (optic nerve crush) and chronic stress (aging) uncovered few common Müller glia-specific transcriptome changes in inflammatory pathways, and mostly differential signatures for each stimulus. The expression of members of the IL-6 and integrin-linked kinase signaling pathways was enhanced in Müller glia in response to optic nerve crush but not aging. Unique changes in neuroinflammation and fibrosis signaling pathways were observed in response to aging but not with optic nerve crush. The Aldh1l1cre/ERT2-NuTRAP model allows focused molecular analyses of a single, minority cell type within the retina, providing more substantial effect sizes than whole tissue analyses. The NuTRAP model, nucleic acid isolation, and validation approaches presented here can be applied to any retina cell type for which a cell type-specific cre is available.

Modeling CSF-1 receptor deficiency diseases - how close are we?

The role of colony-stimulating factor-1 receptor (CSF-1R) in macrophage and organismal development has been extensively studied in mouse. Within the last decade, mutations in the CSF1R have been shown to cause rare diseases of both pediatric (Brain Abnormalities, Neurodegeneration, and Dysosteosclerosis, OMIM #618476) and adult (CSF1R-related leukoencephalopathy, OMIM #221820) onset. Here we review the genetics, penetrance, and histopathological features of these diseases and discuss to what extent the animal models of Csf1r deficiency currently available provide systems in which to study the underlying mechanisms involved.

Differential Regulation of Mouse Hippocampal Gene Expression Sex Differences by Chromosomal Content and Gonadal Sex

Common neurological disorders, like Alzheimer's disease (AD), multiple sclerosis (MS), and autism, display profound sex differences in prevalence and clinical presentation. However, sex differences in the brain with health and disease are often overlooked in experimental models. Sex effects originate, directly or indirectly, from hormonal or sex chromosomal mechanisms. To delineate the contributions of genetic sex (XX v. XY) versus gonadal sex (ovaries v. testes) to the epigenomic regulation of hippocampal sex differences, we used the Four Core Genotypes (FCG) mouse model which uncouples chromosomal and gonadal sex. Transcriptomic and epigenomic analyses of ~ 12-month-old FCG mouse hippocampus, revealed genomic context-specific regulatory effects of genotypic and gonadal sex on X- and autosome-encoded gene expression and DNA modification patterns. X-chromosomal epigenomic patterns, classically associated with X-inactivation, were established almost entirely by genotypic sex, independent of gonadal sex. Differences in X-chromosome methylation were primarily localized to gene regulatory regions including promoters, CpG islands, CTCF binding sites, and active/poised chromatin, with an inverse relationship between methylation and gene expression. Autosomal gene expression demonstrated regulation by both genotypic and gonadal sex, particularly in immune processes. These data demonstrate an important regulatory role of sex chromosomes, independent of gonadal sex, on sex-biased hippocampal transcriptomic and epigenomic profiles. Future studies will need to further interrogate specific CNS cell types, identify the mechanisms by which sex chromosomes regulate autosomes, and differentiate organizational from activational hormonal effects.

Spatial transcriptomic analysis reveals inflammatory foci defined by senescent cells in the white matter, hippocampi and cortical grey matter in the aged mouse brain

There is strong evidence that aging is associated with an increased presence of senescent cells in the brain. The finding that treatment with senolytic drugs improves cognitive performance of aged laboratory mice suggests that increased cellular senescence is causally linked to age-related cognitive decline. The relationship between senescent cells and their relative locations within the brain is critical to understanding the pathology of age-related cognitive decline and dementia. To assess spatial distribution of cellular senescence in the aged mouse brain, spatially resolved whole transcriptome mRNA expression was analyzed in sections of brains derived from young (3 months old) and aged (28 months old) C57BL/6 mice while capturing histological information in the same tissue section. Using this spatial transcriptomics (ST)-based method, microdomains containing senescent cells were identified on the basis of their senescence-related gene expression profiles (i.e., expression of the senescence marker cyclin-dependent kinase inhibitor p16INK4A encoded by the Cdkn2a gene) and were mapped to different anatomical brain regions. We confirmed that brain aging is associated with increased cellular senescence in the white matter, the hippocampi and the cortical grey matter. Transcriptional analysis of the senescent cell-containing ST spots shows that presence of senescent cells is associated with a gene expression signature suggestive of neuroinflammation. GO enrichment analysis of differentially expressed genes in the outer region of senescent cell-containing ST spots ("neighboring ST spots") also identified functions related to microglia activation and neuroinflammation. In conclusion, senescent cells accumulate with age in the white matter, the hippocampi and cortical grey matter and likely contribute to the genesis of inflammatory foci in a paracrine manner.

The Specific Role of Reactive Astrocytes in Stroke

Astrocytes are essential in maintaining normal brain functions such as blood brain barrier (BBB) homeostasis and synapse formation as the most abundant cell type in the central nervous system (CNS). After the stroke, astrocytes are known as reactive astrocytes (RAs) because they are stimulated by various damage-associated molecular patterns (DAMPs) and cytokines, resulting in significant changes in their reactivity, gene expression, and functional characteristics. RAs perform multiple functions after stroke. The inflammatory response of RAs may aggravate neuro-inflammation and release toxic factors to exert neurological damage. However, RAs also reduce excitotoxicity and release neurotrophies to promote neuroprotection. Furthermore, RAs contribute to angiogenesis and axonal remodeling to promote neurological recovery. Therefore, RAs’ biphasic roles and mechanisms make them an effective target for functional recovery after the stroke. In this review, we summarized the dynamic functional changes and internal molecular mechanisms of RAs, as well as their therapeutic potential and strategies, in order to comprehensively understand the role of RAs in the outcome of stroke disease and provide a new direction for the clinical treatment of stroke.

Influence of age and sex on microRNA response and recovery in the hippocampus following sepsis

Sepsis, defined as a dysregulated host immune response to infection, is a common and dangerous clinical syndrome. The excessive host inflammatory response can induce immediate and persistent cognitive decline, which can be worse in older individuals. Sex-specific differences in the outcome of infectious diseases and sepsis appear to favor females. We employed a murine model to examine the influence of age and sex on the brain's microRNA (miR) response following sepsis. Young and old mice of both sexes underwent cecal ligation and puncture (CLP) with daily restraint stress. Expression of hippocampal miR was examined in age- and sex-matched controls at 1 and 4 days post-CLP. Few miR were modified in a similar manner across age or sex and these few miR were generally associated with neuroprotection against inflammation. Similar to previous work examining transcription, young females exhibited a better recovery of the miR profile from day 1 to day 4, relative to young males and old females. For young males and all female groups, the initial response mainly involved a decrease in miR expression. In contrast, old males exhibited only upregulated miR on day 1 and day 4 and many of the miR upregulated on day 1 and day 4 were linked to neurodegeneration, increased neuroinflammation, and cognitive impairment. The results emphasize age and sex differences in epigenetic mechanisms that likely contribute to susceptibility or resilience to cognitive impairment due to sepsis.

Targeting Microglial Disturbances to Protect the Brain From Neurodevelopmental Disorders Associated With Prematurity

Microglial activation during critical phases of brain development can result in short- and long-term consequences for neurological and psychiatric health. Several studies in humans and rodents have shown that microglial activation, leading to a transition from the homeostatic state toward a proinflammatory phenotype, has adverse effects on the developing brain and neurodevelopmental disorders. Targeting proinflammatory microglia may be an effective strategy for protecting the brain and attenuating neurodevelopmental disorders induced by inflammation. In this review we focus on the role of inflammation and the activation of immature microglia (pre-microglia) soon after birth in prematurity-associated neurodevelopmental disorders, and the specific features of pre-microglia during development. We also highlight the relevance of immunomodulatory strategies for regulating activated microglia in a rodent model of perinatal brain injury. An original neuroprotective approach involving a nanoparticle-based therapy and targeting microglia, with the aim of improving myelination and protecting the developing brain, is also addressed.

Neuronal ApoE upregulates MHC-I expression to drive selective neurodegeneration in Alzheimer's disease

Selective neurodegeneration is a critical causal factor in Alzheimer's disease (AD); however, the mechanisms that lead some neurons to perish, whereas others remain resilient, are unknown. We sought potential drivers of this selective vulnerability using single-nucleus RNA sequencing and discovered that ApoE expression level is a substantial driver of neuronal variability. Strikingly, neuronal expression of ApoE-which has a robust genetic linkage to AD-correlated strongly, on a cell-by-cell basis, with immune response pathways in neurons in the brains of wild-type mice, human ApoE knock-in mice and humans with or without AD. Elimination or over-expression of neuronal ApoE revealed a causal relationship among ApoE expression, neuronal MHC-I expression, tau pathology and neurodegeneration. Functional reduction of MHC-I ameliorated tau pathology in ApoE4-expressing primary neurons and in mouse hippocampi expressing pathological tau. These findings suggest a mechanism linking neuronal ApoE expression to MHC-I expression and, subsequently, to tau pathology and selective neurodegeneration.

Histological and Immunohistochemical Characterization of Vomeronasal Organ Aging in Mice

Simple Summary

Chemical communication has been intensely studied and the importance of its role in animal life has been ascertained. Located in the nasal cavity, the vomeronasal organ is one of the main actors in charge of chemical reception. Alterations of this organ have proven to modify behavioral responses to semiochemical expositions. For all the other organs, a well-known origin of alteration is aging. The objective of this study was to analyze this effect on the vomeronasal organ condition and to determine the nature of these potential changes. This study demonstrates that this organ is significantly impacted by aging. In particular, old mice present strong signs of neuronal degeneration compared to adults.

Abstract

The vomeronasal organ (VNO) plays a crucial role in animal behavior since it is responsible for semiochemical detection and, thus, for intra- and interspecific chemical communication, through the vomeronasal sensory epithelium (VNSE), composed of bipolar sensory neurons. This study aimed to explore a well-recognized cause of neuronal degeneration, only rarely explored in this organ: aging. Murine VNOs were evaluated according to 3 age groups (3, 10, and 24 months) by histology to assess VNSE changes such as cellular degeneration or glycogen accumulation and by immunohistochemistry to explore nervous configuration, proliferation capability, and apoptosis with the expression of olfactory marker protein (OMP), Gαi2, Gαo, Ki-67, and cleaved caspase-3 proteins. These markers were quantified as percentages of positive signal in the VNSE and statistical analyses were performed. Cellular degeneration increased with age (p < 0.0001) as well as glycogen accumulation (p < 0.0001), Gαo expression (p < 0.0001), and the number of cleaved-caspase3 positive cells (p = 0.0425), while OMP and Gαi2 expressions decreased with age (p = 0.0436 and p < 0.0001, respectively). Ki67-positive cells were reduced, even if this difference was not statistically significant (p = 0.9105). Due to the crucial role of VNO in animal life, this study opens the door to interesting perspectives about chemical communication efficiency in aging animals.

Genome-wide DNA methylation analysis of cognitive function in middle and old-aged Chinese monozygotic twins

Cognitive ability plays an important role in mental and physical well-beings in the increasingly ageing populations. Here, based on a sample of 30 cognitive function-discordant monozygotic twin pairs, we aimed to detect specific epigenetic variants potentially related to cognitive function by conducting an epigenome-wide association study (EWAS). Association between methylation level of single CpG site with cognitive function score was tested by linear mixed effect model. Functions of cis-regulatory regions and ontology enrichments were predicted by Genomic Regions Enrichment of Annotations Tool (GREAT). Differentially methylated regions (DMRs) were detected by comb-p python library. A list of 28 CpG sites were identified to reach the level of P < 1 × 10^-4, and the strongest association (cor = 0.138, P = 2.549 × 10^-6) was detected for DNA CpG site (Chr17: 40,700,490 bp) located at HSD17B1P1. The identified 14,065 genomic CpG sites (P < 0.05) were mapped to 2646 genes, especially HSD17B1P1, CUL4A, INTS8, GFI1B, ZNF467, CDH15, and PSMA1. GREAT ontology enrichments mainly highlighted nicotine pharmacodynamics pathway, GABA-B receptor II/nicotinic acetylcholine receptor/hedgehog/endothelin/Wnt signaling pathways, Parkinson disease, Huntington disease, glycolysis, neuronal system, and toll-like receptor binding. We detected 15 DMRs located at/near 16 genes, especially LINC01551, LINC02282, and FAM32A. And 32 cognitive function-associated differentially methylated genes could be replicated, such as SHANK2, ABCA2, PRDM16, NCOR2, and INPP5A. Our EWAS in monozygotic twins identify specific epigenetic variations which are significantly involved in functional genes, biological function and pathways that mediate cognitive function. The findings provide clues to further identify new diagnostic biomarkers and therapeutic targets for cognitive dysfunction.

Implications for microglial sex differences in tau-related neurodegenerative diseases

Tauopathies are a group of neurodegenerative diseases that involve pathological changes to the tau protein. Neuroinflammation is a commonly reported feature of tauopathies that has been demonstrated to exacerbate tau pathology and, hence, neurodegeneration. Microglia can mediate the inflammatory response in order to maintain brain homeostasis. In the aged brain, microglia are reported to undergo morphological and functional changes, adopting a pro-inflammatory profile and loss of homeostatic functions. Dystrophic and dysfunctional microglia are associated with tau pathology in the healthy and diseased brain which is proposed to contribute to disease development and progression. Microglia have also been recently demonstrated to possess sexually dimorphic roles in the developing, adult and aged brain. The sex differences in microglial functionality suggest that microglia may contribute to tauopathies which may differ between sexes. This review highlights the detrimental loop between age-related microglial changes and tau pathology with implications for microglial sexual dichotomy.

Aging and pathological aging signatures of the brain: through the focusing lens of SIRT6

Brain-specific SIRT6-KO mice present increased DNA damage, learning impairments, and neurodegenerative phenotypes, placing SIRT6 as a key protein in preventing neurodegeneration. In the aging brain, SIRT6 levels/activity decline, which is accentuated in Alzheimer's patients. To understand SIRT6 roles in transcript pattern changes, we analyzed transcriptomes of young WT, old WT and young SIRT6-KO mice brains, and found changes in gene expression related to healthy and pathological aging. In addition, we traced these differences in human and mouse samples of Alzheimer's and Parkinson's diseases, healthy aging and calorie restriction (CR). Our results define four gene expression categories that change with age in a pathological or non-pathological manner, which are either reversed or not by CR. We found that each of these gene expression categories is associated with specific transcription factors, thus serving as potential candidates for their category-specific regulation. One of these candidates is YY1, which we found to act together with SIRT6 regulating specific processes. We thus argue that SIRT6 has a pivotal role in preventing age-related transcriptional changes in brains. Therefore, reduced SIRT6 activity may drive pathological age-related gene expression signatures in the brain.

The effect of aged microglia on synaptic impairment and its relevance in neurodegenerative diseases

Microglia serve key functions in the central nervous system (CNS), participating in the establishment and regulation of synapses and the neuronal network, and regulating activity-dependent plastic changes. As the neuroimmune system, they respond to endogenous and exogenous signals to protect the CNS. In aging, one of the main changes is the establishment of inflamm-aging, a mild chronic inflammation that reduces microglial response to stressors. Neuroinflammation depends mainly on the increased activation of microglia. Microglia over-activation may result in a reduced capacity for performing normal functions related to migration, clearance, and the adoption of an anti-inflammatory state, contributing to an increased susceptibility for neurodegeneration. Oxidative stress contributes both to aging and to the progression of neurodegenerative diseases. Increased production of reactive oxygen species (ROS) and neuroinflammation associated with age- and disease-dependent mechanisms affect synaptic activity and neurotransmission, leading to cognitive dysfunction. Astrocytes prevent microglial cell cytotoxicity by mechanisms mediated by transforming growth factor β1 (TGFβ1). However, TGFβ1-Smad3 pathway is impaired in aging, and the age-related impairment of TGFβ signaling can reduce protective activation while facilitating cytotoxic activation of microglia. A critical analysis on the effect of aging microglia on neuronal function is relevant for the understanding of age-related changes on neuronal function. Here, we present evidence in the context of the "microglial dysregulation hypothesis", which leads to the reduction of the protective functions and increased cytotoxicity of microglia, to discuss the mechanisms involved in neurodegenerative changes and Alzheimer's disease.

Cholinergic Modulation of Glial Function During Aging and Chronic Neuroinflammation

Aging is a complex biological process that increases the risk of age-related cognitive degenerative diseases such as dementia, including Alzheimer’s disease (AD), Lewy Body Dementia (LBD), and mild cognitive impairment (MCI). Even non-pathological aging of the brain can involve chronic oxidative and inflammatory stress, which disrupts the communication and balance between the brain and the immune system. There has been an increasingly strong connection found between chronic neuroinflammation and impaired memory, especially in AD. While microglia and astrocytes, the resident immune cells of the central nervous system (CNS), exerting beneficial effects during the acute inflammatory phase, during chronic neuroinflammation they can become more detrimental. Central cholinergic circuits are involved in maintaining normal cognitive function and regulating signaling within the entire cerebral cortex. While neuronal-glial cholinergic signaling is anti-inflammatory and anti-oxidative, central cholinergic neuronal degeneration is implicated in impaired learning, memory sleep regulation, and attention. Although there is evidence of cholinergic involvement in memory, fewer studies have linked the cholinergic anti-inflammatory and anti-oxidant pathways to memory processes during development, normal aging, and disease states. This review will summarize the current knowledge of cholinergic effects on microglia and astroglia, and their role in both anti-inflammatory and anti-oxidant mechanisms, concerning normal aging and chronic neuroinflammation. We provided details on how stimulation of α7 nicotinic acetylcholine (α7nACh) receptors can be neuroprotective by increasing amyloid-β phagocytosis, decreasing inflammation and reducing oxidative stress by promoting the nuclear factor erythroid 2-related factor 2 (Nrf2) pathways and decreasing the release of pro-inflammatory cytokines. There is also evidence for astroglial α7nACh receptor stimulation mediating anti-inflammatory and antioxidant effects by inhibiting the nuclear factor-κB (NF-κB) pathway and activating the Nrf2 pathway respectively. We conclude that targeting cholinergic glial interactions between neurons and glial cells via α7nACh receptors could regulate neuroinflammation and oxidative stress, relevant to the treatment of several neurodegenerative diseases.

Persistent Infection with Herpes Simplex Virus 1 and Alzheimer's Disease-A Call to Study How Variability in Both Virus and Host may Impact Disease

Increasing attention has focused on the contributions of persistent microbial infections with the manifestation of disease later in life, including neurodegenerative conditions such as Alzheimer’s disease (AD). Current data has shown the presence of herpes simplex virus 1 (HSV-1) in regions of the brain that are impacted by AD in elderly individuals. Additionally, neuronal infection with HSV-1 triggers the accumulation of amyloid beta deposits and hyperphosphorylated tau, and results in oxidative stress and synaptic dysfunction. All of these factors are implicated in the development of AD. These data highlight the fact that persistent viral infection is likely a contributing factor, rather than a sole cause of disease. Details of the correlations between HSV-1 infection and AD development are still just beginning to emerge. Future research should investigate the relative impacts of virus strain- and host-specific factors on the induction of neurodegenerative processes over time, using models such as infected neurons in vitro, and animal models in vivo, to begin to understand their relationship with cognitive dysfunction.

Tamoxifen induction of Cre recombinase does not cause long-lasting or sexually divergent responses in the CNS epigenome or transcriptome: implications for the design of aging studies

The systemic delivery of tamoxifen (Tam) to activate inducible CreERT2-loxP transgenic mouse systems is now widely used in neuroscience studies. This critical technological advancement allows temporal control of DNA-cre recombination, avoidance of embryonically lethal phenotypes, and minimization of residual cell labeling encountered in constitutively active drivers. Despite its advantages, the use of Tam has the potential to cause long-lasting, uncharacterized side effects on the transcriptome and epigenome in the CNS, given its mixed estrogen receptor (ER) agonist/antagonist actions. With the welcome focus on including both sexes in biomedical studies and efforts to understand sex differences, Tam administration could also cause sexually divergent responses that would confound studies. To examine these issues, epigenetic and transcriptomic profiles were compared in C57BL/6 J female and male hippocampus, cortex, and retina 1 month after a 5-day Tam treatment typical for cre induction, or vehicle control (sunflower seed oil). Cytosine methylation and hydroxymethylation levels, in both CG and non-CG contexts, were unchanged as determined by oxidative bisulfite sequencing. Long-lasting Tam transcriptomic effects were also not evident/minimal. Furthermore, there is no evidence of sexually divergent responses with Tam administration and Tam did not alter sex differences evident in controls. Combined with recently reported data that Tam alone does not cause long-lasting changes in behavior and neurogenesis, our findings provide confidence that Tam can be used as a cre-recombinase inducer without introducing significant confounds in transcriptomic and epigenomic neuroscience studies, particularly those focused on genomic and transcriptomic aspects of the aging brain.

Aging Neurovascular Unit and Potential Role of DNA Damage and Repair in Combating Vascular and Neurodegenerative Disorders

Progressive neurological deterioration poses enormous burden on the aging population with ischemic stroke and neurodegenerative disease patients, such as Alzheimers’ disease and Parkinson’s disease. The past two decades have witnessed remarkable advances in the research of neurovascular unit dysfunction, which is emerging as an important pathological feature that underlies these neurological disorders. Dysfunction of the unit allows penetration of blood-derived toxic proteins or leukocytes into the brain and contributes to white matter injury, disturbed neurovascular coupling and neuroinflammation, which all eventually lead to cognitive dysfunction. Recent evidences suggest that aging-related oxidative stress, accumulated DNA damage and impaired DNA repair capacities compromises the genome integrity not only in neurons, but also in other cell types of the neurovascular unit, such as endothelial cells, astrocytes and pericytes. Combating DNA damage or enhancing DNA repair capacities in the neurovascular unit represents a promising therapeutic strategy for vascular and neurodegenerative disorders. In this review, we focus on aging related mechanisms that underlie DNA damage and repair in the neurovascular unit and introduce several novel strategies that target the genome integrity in the neurovascular unit to combat the vascular and neurodegenerative disorders in the aging brain.

Precision Aging: Applying Precision Medicine to the Field of Cognitive Aging

The current "one size fits all" approach to our cognitive aging population is not adequate to close the gap between cognitive health span and lifespan. In this review article, we present a novel model for understanding, preventing, and treating age-related cognitive impairment (ARCI) based on concepts borrowed from precision medicine. We will discuss how multiple risk factors can be classified into risk categories because of their interrelatedness in real life, the genetic variants that increase sensitivity to, or ameliorate, risk for ARCI, and the brain drivers or common mechanisms mediating brain aging. Rather than providing a definitive model of risk for ARCI and cognitive decline, the Precision Aging model is meant as a starting point to guide future research. To that end, after briefly discussing key risk categories, genetic risks, and brain drivers, we conclude with a discussion of steps that must be taken to move the field forward.

Dietary Restriction and Neuroinflammation: A Potential Mechanistic Link

Chronic neuroinflammation is a common feature of the aged brain, and its association with the major neurodegenerative changes involved in cognitive impairment and motor dysfunction is well established. One of the most potent antiaging interventions tested so far is dietary restriction (DR), which extends the lifespan in various organisms. Microglia and astrocytes are two major types of glial cells involved in the regulation of neuroinflammation. Accumulating evidence suggests that the age-related proinflammatory activation of astrocytes and microglia is attenuated under DR. However, the molecular mechanisms underlying DR-mediated regulation of neuroinflammation are not well understood. Here, we review the current understanding of the effects of DR on neuroinflammation and suggest an underlying mechanistic link between DR and neuroinflammation that may provide novel insights into the role of DR in aging and age-associated brain disorders.

Astrocytes and Aging

By 2050, the aging population is predicted to expand by over 100%. Considering this rapid growth, and the additional strain it will place on healthcare resources because of age-related impairments, it is vital that researchers gain a deeper understanding of the cellular interactions that occur with normal aging. A variety of mammalian cell types have been shown to become compromised with age, each with a unique potential to contribute to disease formation in the aging body. Astrocytes represent the largest group of glial cells and are responsible for a variety of essential functions in the healthy central nervous system (CNS). Like other cell types, aging can cause a loss of normal function in astrocytes which reduces their ability to properly maintain a healthy CNS environment, negatively alters their interactions with neighboring cells, and contribute to the heightened inflammatory state characteristic of aging. The goal of this review article is to consolidate the knowledge and research to date regarding the role of astrocytes in aging. In specific, this review article will focus on the morphology and molecular profile of aged astrocytes, the consequence of astrocyte dysfunction on homeostatic functions during aging, and the role of astrocytes in age-related neurodegenerative diseases.

Exposure to environmental enrichment attenuates addiction-like behavior and alters molecular effects of heroin self-administration in rats

Environmental factors profoundly affect the addictive potential of drugs of abuse and may also modulate the neuro-anatomical/neuro-chemical impacts of uncontrolled drug use and relapse propensity. This study examined the impact of environmental enrichment on heroin self-administration, addiction-related behaviors, and molecular processes proposed to underlie these behaviors. Male Sprague-Dawley rats in standard and enriched housing conditions intravenously self-administered similar amounts of heroin over 14 days. However, environmental enrichment attenuated progressive ratio, extinction, and reinstatement session responding after 14 days of enforced abstinence. Molecular mechanisms, namely DNA methylation and gene expression, are proposed to underlie abstinence-persistent behaviors. A global reduction in methylation is reported to coincide with addiction, but no differences in total genomic methylation or repeat element methylation were observed in CpG or non-CpG (CH) contexts across the mesolimbic circuitry as assessed by multiple methods including whole genome bisulfite sequencing. Immediate early gene expression associated with drug seeking, taking, and abstinence also were examined. EGR1 and EGR2 were suppressed in mesolimbic regions with heroin-taking and environmental enrichment. Site-specific methylation analysis of EGR1 and EGR2 promoter regions using bisulfite amplicon sequencing (BSAS) revealed hypo-methylation in the EGR2 promoter region and EGR1 intragenic CpG sites with heroin-taking and environmental enrichment that was associated with decreased mRNA expression. Taken together, these findings illuminate the impact of drug taking and environment on the epigenome in a locus and gene-specific manner and highlight the need for positive, alternative rewards in the treatment and prevention of drug addiction.

Dopaminergic neurons show increased low-molecular-mass protein 7 activity induced by 6-hydroxydopamine in vitro and in vivo

Background

Abnormal expression of major histocompatibility complex class I (MHC-I) is increased in dopaminergic (DA) neurons in the substantia nigra (SN) in Parkinson’s disease (PD). Low-molecular-mass protein 7 (β5i) is a proteolytic subunit of the immunoproteasome that regulates protein degradation and the MHC pathway in immune cells.

Methods

In this study, we investigated the role of β5i in DA neurons using a 6-hydroxydopamine (6-OHDA) model in vitro and vivo.

Results

We showed that 6-OHDA upregulated β5i expression in DA neurons in a concentration- and time-dependent manner. Inhibition and downregulation of β5i induced the expression of glucose-regulated protein (Bip) and exacerbated 6-OHDA neurotoxicity in DA neurons. The inhibition of β5i further promoted the activation of Caspase 3-related pathways induced by 6-OHDA. β5i also activated transporter associated with antigen processing 1 (TAP1) and promoted MHC-I expression on DA neurons.

Conclusion

Taken together, our data suggest that β5i is activated in DA neurons under 6-OHDA treatment and may play a neuroprotective role in PD.

Electronic supplementary material

The online version of this article (10.1186/s40035-018-0125-9) contains supplementary material, which is available to authorized users.

Epigenetic control of gene regulation during development and disease: A view from the retina

Complex biological processes, such as organogenesis and homeostasis, are stringently regulated by genetic programs that are fine-tuned by epigenetic factors to establish cell fates and/or to respond to the microenvironment. Gene regulatory networks that guide cell differentiation and function are modulated and stabilized by modifications to DNA, RNA and proteins. In this review, we focus on two key epigenetic changes - DNA methylation and histone modifications - and discuss their contribution to retinal development, aging and disease, especially in the context of age-related macular degeneration (AMD) and diabetic retinopathy. We highlight less-studied roles of DNA methylation and provide the RNA expression profiles of epigenetic enzymes in human and mouse retina in comparison to other tissues. We also review computational tools and emergent technologies to profile, analyze and integrate epigenetic information. We suggest implementation of editing tools and single-cell technologies to trace and perturb the epigenome for delineating its role in transcriptional regulation. Finally, we present our thoughts on exciting avenues for exploring epigenome in retinal metabolism, disease modeling, and regeneration.

Aging in the Brain: New Roles of Epigenetics in Cognitive Decline

Gene expression in the aging brain depends on transcription signals generated by senescent physiology, interacting with genetic and epigenetic programs. In turn, environmental factors influence epigenetic mechanisms, such that an epigenetic-environmental link may contribute to the accumulation of cellular damage, susceptibility or resilience to stressors, and variability in the trajectory of age-related cognitive decline. Epigenetic mechanisms, DNA methylation and histone modifications, alter chromatin structure and the accessibility of DNA. Furthermore, small non-coding RNA, termed microRNA (miRNA) bind to messenger RNA (mRNA) to regulate translation. In this review, we examine key questions concerning epigenetic mechanisms in regulating the expression of genes associated with brain aging and age-related cognitive decline. In addition, we highlight the interaction of epigenetics with senescent physiology and environmental factors in regulating transcription.

Expression of the purine biosynthetic enzyme phosphoribosyl formylglycinamidine synthase in neurons

Purines are metabolic building blocks essential for all living organisms on earth. De novo purine biosynthesis occurs in the brain and appears to play important roles in neural development. Phosphoribosyl formylglycinamidine synthase (FGAMS, also known as PFAS or FGARAT), a core enzyme involved in the de novo synthesis of purines, may play alternative roles in viral pathogenesis. To date, no thorough investigation of the endogenous expression and localization of de novo purine biosynthetic enzymes has been conducted in human neurons or in virally infected cells. In this study, we characterized expression of FGAMS using multiple neuronal models. In differentiated human SH-SY5Y neuroblastoma cells, primary rat hippocampal neurons, and in whole-mouse brain sections, FGAMS immunoreactivity was distributed within the neuronal cytoplasm. FGAMS immunolabeling in vitro demonstrated extensive distribution throughout neuronal processes. To investigate potential changes in FGAMS expression and localization following viral infection, we infected cells with the human pathogen herpes simplex virus 1. In infected fibroblasts, FGAMS immunolabeling shifted from a diffuse cytoplasmic location to a mainly perinuclear localization by 12 h post-infection. In contrast, in infected neurons, FGAMS localization showed no discernable changes in the localization of FGAMS immunoreactivity. There were no changes in total FGAMS protein levels in either cell type. Together, these data provide insight into potential purine biosynthetic mechanisms utilized within neurons during homeostasis as well as viral infection. Cover Image for this Issue: doi: 10.1111/jnc.14169.

Caloric restriction mitigates age-associated hippocampal differential CG and non-CG methylation

Brain aging is marked by cognitive decline and susceptibility to neurodegeneration. Calorie restriction (CR) increases neurogenesis, improves memory function, and protects from age-associated neurological disorders. Epigenetic mechanisms, including DNA methylation, are vital to normal central nervous system cellular and memory functions and are dysregulated with aging. The beneficial effects of CR have been proposed to work through epigenetic processes, but this is largely unexplored. We therefore tested whether life long CR prevents age-related hippocampal DNA methylation changes. Hippocampal DNA from young (3 months) and old (24 months) male mice fed ad libitum and 24-month-old mice fed a 40% calorie-restricted diet from 3 months of age were examined by genome-wide bisulfite sequencing to measure methylation with base specificity. Over 27 million CG and CH (non-CG) sites were examined. Of the ∼40,000 differentially methylated CG and ∼80,000 CH sites with aging, >1/3 were prevented by CR and were found across genomic regulatory regions and gene pathways. CR also caused alterations to CG and CH methylation at sites not differentially methylated with aging, and these CR-specific changes demonstrated a different pattern of regulatory element and gene pathway enrichment than those affected by aging. CR-specific DNA methyltransferase 1 and Tet methylcytosine dioxygenase 3 promoter hypermethylation corresponded to reduced gene expression. These findings demonstrate that CR attenuates age-related CG and CH hippocampal methylation changes, in combination with CR-specific methylation that may also contribute to the neuroprotective effects of CR. The prevention of age-related methylation alterations is also consistent with the prolongevity effects of CR working through an epigenetic mechanism.

Analysis of DNA modifications in aging research

As geroscience research extends into the role of epigenetics in aging and age-related disease, researchers are being confronted with unfamiliar molecular techniques and data analysis methods that can be difficult to integrate into their work. In this review, we focus on the analysis of DNA modifications, namely cytosine methylation and hydroxymethylation, through next-generation sequencing methods. While older techniques for modification analysis performed relative quantitation across regions of the genome or examined average genome levels, these analyses lack the desired specificity, rigor, and genomic coverage to firmly establish the nature of genomic methylation patterns and their response to aging. With recent methodological advances, such as whole genome bisulfite sequencing (WGBS), bisulfite oligonucleotide capture sequencing (BOCS), and bisulfite amplicon sequencing (BSAS), cytosine modifications can now be readily analyzed with base-specific, absolute quantitation at both cytosine-guanine dinucleotide (CG) and non-CG sites throughout the genome or within specific regions of interest by next-generation sequencing. Additional advances, such as oxidative bisulfite conversion to differentiate methylation from hydroxymethylation and analysis of limited input/single-cells, have great promise for continuing to expand epigenomic capabilities. This review provides a background on DNA modifications, the current state-of-the-art for sequencing methods, bioinformatics tools for converting these large data sets into biological insights, and perspectives on future directions for the field.

Does the Interplay Between Aging and Neuroinflammation Modulate Alzheimer's Disease Clinical Phenotypes? A Clinico-Pathological Perspective

Alzheimer's disease (AD) is a chronic neurodegenerative disorder and is the most common cause of dementia worldwide. Cumulative data suggests that neuroinflammation plays a prominent and early role in AD, and there is compelling data from different research groups of age-associated dysregulation of the neuroimmune system. From the clinical point of view, despite clinical resemblance and neuropathological findings, there are important differences between the group of patients with sporadic early-onset (<65 years old) and late-onset AD (>65 years old). Thus, it seems important to understand the age-dependent relationship between neuroinflammation and the underlying biology of AD in order to identify potential explanations for clinical heterogeneity, interpret biomarkers, and promote the best treatment to different clinical AD phenotypes. The study of the delicate balance between pro-inflammatory or anti-inflammatory sides of immune players in the different ages of onset of AD would be important to understand treatment efficacy in clinical trials and eventually, not only direct treatment to early disease stages, but also the possibility of establishing different treatment approaches depending on the age of the patient. In this review, we would like to summarize what is currently known about the interplay between "normal" age associated inflammatory changes and AD pathological mechanisms, and also the potential differences between early-onset and late-onset AD taking into account the age-related neuroimmune background at disease onset.

Sexually divergent DNA methylation patterns with hippocampal aging

DNA methylation is a central regulator of genome function, and altered methylation patterns are indicative of biological aging and mortality. Age‐related cellular, biochemical, and molecular changes in the hippocampus lead to cognitive impairments and greater vulnerability to neurodegenerative disease that varies between the sexes. The role of hippocampal epigenomic changes with aging in these processes is unknown as no genome‐wide analyses of age‐related methylation changes have considered the factor of sex in a controlled animal model. High‐depth, genome‐wide bisulfite sequencing of young (3 month) and old (24 month) male and female mouse hippocampus revealed that while total genomic methylation amounts did not change with aging, specific sites in CG and non‐CG (CH) contexts demonstrated age‐related increases or decreases in methylation that were predominantly sexually divergent. Differential methylation with age for both CG and CH sites was enriched in intergenic and intronic regions and under‐represented in promoters, CG islands, and specific enhancer regions in both sexes, suggesting that certain genomic elements are especially labile with aging, even if the exact genomic loci altered are predominantly sex‐specific. Lifelong sex differences in autosomal methylation at CG and CH sites were also observed. The lack of genome‐wide hypomethylation, sexually divergent aging response, and autosomal sex differences at CG sites was confirmed in human data. These data reveal sex as a previously unappreciated central factor of hippocampal epigenomic changes with aging. In total, these data demonstrate an intricate regulation of DNA methylation with aging by sex, cytosine context, genomic location, and methylation level.

Sex differences in the phagocytic and migratory activity of microglia and their impairment by palmitic acid

Sex differences in the incidence, clinical manifestation, disease course, and prognosis of neurological diseases, such as autism spectrum disorders or Alzheimer's disease, have been reported. Obesity has been postulated as a risk factor for cognitive decline and Alzheimer's disease and, during pregnancy, increases the risk of autism spectrum disorders in the offspring. Obesity is associated with increased serum and brain levels of free fatty acids, such as palmitic acid, which activate microglial cells triggering a potent inflammatory cascade. In this study, we have determined the effect of palmitic acid in the inflammatory profile, motility, and phagocytosis of primary male and female microglia, both in basal conditions and in the presence of a pro-inflammatory stimulus (interferon-γ). Male microglia in vitro showed higher migration than female microglia under basal and stimulated conditions. In contrast, female microglia had higher basal and stimulated phagocytic activity than male microglia. Palmitic acid did not affect basal migration or phagocytosis, but abolished the migration and phagocytic activity of male and female microglia in response to interferon-γ. These findings extend previous observations of sex differences in microglia and suggest that palmitic acid impairs the protective responses of these cells.

Retinal gene expression responses to aging are sexually divergent

PURPOSE

Sex and age are critical factors in a variety of retinal diseases but have garnered little attention in preclinical models. The current lack of knowledge impairs informed decision making regarding inclusion and design of studies that incorporate both sexes and/or the effects of aging. The goal of this study was to examine normative mouse retina gene expression in both sexes and with advancing age.

METHODS

Retinal gene expression in female and male C57BL/6JN mice at 3 months and 24 months of age were compared for sex differences and aging responses through whole transcriptome microarray analysis. Sex differences and age-related changes were examined in the context of cellular pathways and processes, regulatory patterns, and cellular origin, as well as for overlap with described changes in retinal disease models. Selected age and sex differences were confirmed with quantitative PCR.

RESULTS

Age-related gene expression changes demonstrated commonalities and sexually divergent responses. Several cellular pathways and processes, especially inflammation-related, are affected and were over-represented in fibroblast, microglial, and ganglion cell-specific genes. Lifelong, and age-dependent, sex differences were observed and were over-represented in fibroblast-specific genes. Age and sex differences were also observed to be regulated in models of diabetic retinopathy, glaucoma, and other diseases.

CONCLUSIONS

These findings demonstrate that most age-related changes in retinal gene expression are sexually divergent and that there are significant sex differences in gene expression throughout the lifespan. These data serve as a resource for vision researchers seeking to include sex and age as factors in their preclinical studies.

Sexually divergent induction of microglial-associated neuroinflammation with hippocampal aging

Background

The necessity of including both males and females in molecular neuroscience research is now well understood. However, there is relatively limited basic biological data on brain sex differences across the lifespan despite the differences in age-related neurological dysfunction and disease between males and females.

Methods

Whole genome gene expression of young (3 months), adult (12 months), and old (24 months) male and female C57BL6 mice hippocampus was analyzed. Subsequent bioinformatic analyses and confirmations of age-related changes and sex differences in hippocampal gene and protein expression were performed.

Results

Males and females demonstrate both common expression changes with aging and marked sex differences in the nature and magnitude of the aging responses. Age-related hippocampal induction of neuroinflammatory gene expression was sexually divergent and enriched for microglia-specific genes such as complement pathway components. Sexually divergent C1q protein expression was confirmed by immunoblotting and immunohistochemistry. Similar patterns of cortical sexually divergent gene expression were also evident. Additionally, inter-animal gene expression variability increased with aging in males, but not females.

Conclusions

These findings demonstrate sexually divergent neuroinflammation with aging that may contribute to sex differences in age-related neurological diseases such as stroke and Alzheimer’s, specifically in the complement system. The increased expression variability in males suggests a loss of fidelity in gene expression regulation with aging. These findings reveal a central role of sex in the transcriptomic response of the hippocampus to aging that warrants further, in depth, investigations.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-017-0920-8) contains supplementary material, which is available to authorized users.