Aberrant Oligodendrogenesis in Down Syndrome: Shift in Gliogenesis?

Single-nucleus RNA sequencing reveals cell type-specific transcriptome alterations of Down syndrome hippocampus using the Dp16 mouse model

BACKGROUND

Down syndrome (DS), the most frequently occurring human chromosomal disorder, is caused by trisomy 21. The exact molecular effects of trisomy on certain cell populations in the brain remain poorly understood.

OBJECTIVE

The purpose of this study was to investigate the effects of trisomy on the transcriptomes of various types of neurons and nonneuronal cells in the hippocampus.

METHODS

A total of 8993 nuclei from the WT and 6445 nuclei from the Dp16 hippocampus were analyzed by single-nucleus RNA sequencing (snRNA-seq). Cell clustering was achieved by the Seurat program.

RESULTS

Hippocampal cells were grouped into multiple neuronal and nonneuronal populations. Only a limited number of trisomic genes were upregulated (q < 0.001) over 1.25-fold in a specific type of hippocampal cell. Specifically, deregulation of genes associated with synaptic signaling and organization was observed in multiple cell populations, including excitatory neurons, oligodendrocytes, and microglia. This observation suggests the potential importance of synapse deficits in DS. Interestingly, GO annotation of the upregulated genes suggested potential activation of the immune system by hippocampal excitatory neurons. Fewer trisomic genes were altered in nonneuronal cells than in neurons. Notably, microglial transcriptome analysis revealed significantly (q < 0.001) increased expression of C1qb and C1qc, which suggested potential involvement of complement-mediated synapse loss mediated by microglia in DS.

CONCLUSION

The trisomy-related hippocampal deficits should be driven by a small amount, not all, of the trisomic genes in a specific type of cell. Our work may help to narrow down both the molecular and cellular targets for future gene therapies in DS.

Single-Nucleus Profiling Identifies Accelerated Oligodendrocyte Precursor Cell Senescence in a Mouse Model of Down Syndrome

Down syndrome (DS), the most common genetic cause of intellectual disability, is associated with lifelong cognitive deficits. However, the mechanisms by which triplication of chromosome 21 genes drive neuroinflammation and cognitive dysfunction are poorly understood. Here, using the Ts65Dn mouse model of DS, we performed an integrated single-nucleus ATAC and RNA-sequencing (snATAC-seq and snRNA-seq) analysis of the adult cortex. We identified cell type-specific transcriptional and chromatin-associated changes in the Ts65Dn cortex, including regulators of neuroinflammation, transcription and translation, myelination, and mitochondrial function. We discovered enrichment of a senescence-associated transcriptional signature in Ts65Dn oligodendrocyte (OL) precursor cells (OPCs) and epigenetic changes consistent with a loss of heterochromatin. We found that senescence is restricted to a subset of OPCs concentrated in deep cortical layers. Treatment of Ts65Dn mice with a senescence-reducing flavonoid rescued cortical OPC proliferation, restored microglial homeostasis, and improved contextual fear memory. Together, these findings suggest that cortical OPC senescence may be an important driver of neuropathology in DS.

Comprehensive volumetric phenotyping of the neonatal brain in Down syndrome

Down syndrome (DS) is the most common genetic cause of intellectual disability with a wide range of neurodevelopmental outcomes. To date, there have been very few in vivo neuroimaging studies of the neonatal brain in DS. In this study we used a cross-sectional sample of 493 preterm- to term-born control neonates from the developing Human Connectome Project to perform normative modeling of regional brain tissue volumes from 32 to 46 weeks postmenstrual age, accounting for sex and age variables. Deviation from the normative mean was quantified in 25 neonates with DS with postnatally confirmed karyotypes from the Early Brain Imaging in DS study. Here, we provide the first comprehensive volumetric phenotyping of the neonatal brain in DS, which is characterized by significantly reduced whole brain, cerebral white matter, and cerebellar volumes; reduced relative frontal and occipital lobar volumes, in contrast with enlarged relative temporal and parietal lobar volumes; enlarged relative deep gray matter volume (particularly the lentiform nuclei); and enlargement of the lateral ventricles, amongst other features. In future, the ability to assess phenotypic severity at the neonatal stage may help guide early interventions and, ultimately, help improve neurodevelopmental outcomes in children with DS.

Down Syndrome Altered Cell Composition in Blood, Brain, and Buccal Swab Samples Profiled by DNA-Methylation-Based Cell-Type Deconvolution

Down syndrome (DS) is a genetic disorder caused by an extra copy of chromosome 21 that presents developmental dysfunction and intellectual disability. To better understand the cellular changes associated with DS, we investigated the cell composition in blood, brain, and buccal swab samples from DS patients and controls using DNA methylation-based cell-type deconvolution. We used genome-scale DNA methylation data from Illumina HumanMethylation450k and HumanMethylationEPIC arrays to profile cell composition and trace fetal lineage cells in blood samples (DS N = 46; control N = 1469), brain samples from various regions (DS N = 71; control N = 101), and buccal swab samples (DS N = 10; control N = 10). In early development, the number of cells from the fetal lineage in the blood is drastically lower in DS patients (Δ = 17.5%), indicating an epigenetically dysregulated maturation process for DS patients. Across sample types, we observed significant alterations in relative cell-type proportions for DS subjects compared with the controls. Cell-type proportion alterations were present in samples from early development and adulthood. Our findings provide insight into DS cellular biology and suggest potential cellular interventional targets for DS.

Extracellular Vesicle Treatment Alleviates Neurodevelopmental and Neurodegenerative Pathology in Cortical Spheroid Model of Down Syndrome

Down syndrome (DS), or trisomy 21, is manifested in a variety of anatomical and cellular abnormalities resulting in intellectual deficits and early onset of Alzheimer's disease (AD) with no effective treatments available to alleviate the pathologies associated with the disorder. The therapeutic potential of extracellular vesicles (EVs) has emerged recently in relation to various neurological conditions. We have previously demonstrated the therapeutic efficacy of mesenchymal stromal cell-derived EVs (MSC-EVs) in cellular and functional recovery in a rhesus monkey model of cortical injury. In the current study, we evaluated the therapeutic effect of MSC-EVs in a cortical spheroid (CS) model of DS generated from patient-derived induced pluripotent stem cells (iPSCs). Compared to euploid controls, trisomic CS display smaller size, deficient neurogenesis, and AD-related pathological features, such as enhanced cell death and depositions of amyloid beta (Aβ) and hyperphosphorylated tau (p-tau). EV-treated trisomic CS demonstrated preserved size, partial rescue in the production of neurons, significantly decreased levels of Aβ and p-tau, and a reduction in the extent of cell death as compared to the untreated trisomic CS. Together, these results show the efficacy of EVs in mitigating DS and AD-related cellular phenotypes and pathological depositions in human CS.

Shaking up the silence: consequences of HMGN1 antagonizing PRC2 in the Down syndrome brain

Intellectual disability is a well-known hallmark of Down Syndrome (DS) that results from the triplication of the critical region of human chromosome 21 (HSA21). Major studies were conducted in recent years to gain an understanding about the contribution of individual triplicated genes to DS-related brain pathology. Global transcriptomic alterations and widespread changes in the establishment of neural lineages, as well as their differentiation and functional maturity, suggest genome-wide chromatin organization alterations in trisomy. High Mobility Group Nucleosome Binding Domain 1 (HMGN1), expressed from HSA21, is a chromatin remodeling protein that facilitates chromatin decompaction and is associated with acetylated lysine 27 on histone H3 (H3K27ac), a mark correlated with active transcription. Recent studies causatively linked overexpression of HMGN1 in trisomy and the development of DS-associated B cell acute lymphoblastic leukemia (B-ALL). HMGN1 has been shown to antagonize the activity of the Polycomb Repressive Complex 2 (PRC2) and prevent the deposition of histone H3 lysine 27 trimethylation mark (H3K27me3), which is associated with transcriptional repression and gene silencing. However, the possible ramifications of the increased levels of HMGN1 through the derepression of PRC2 target genes on brain cell pathology have not gained attention. In this review, we discuss the functional significance of HMGN1 in brain development and summarize accumulating reports about the essential role of PRC2 in the development of the neural system. Mechanistic understanding of how overexpression of HMGN1 may contribute to aberrant brain cell phenotypes in DS, such as altered proliferation of neural progenitors, abnormal cortical architecture, diminished myelination, neurodegeneration, and Alzheimer's disease-related pathology in trisomy 21, will facilitate the development of DS therapeutic approaches targeting chromatin.

The innate immune system stimulating cytokine GM-CSF improves learning/memory and interneuron and astrocyte brain pathology in Dp16 Down syndrome mice and improves learning/memory in wild-type mice

Down syndrome (DS) is characterized by chronic neuroinflammation, peripheral inflammation, astrogliosis, imbalanced excitatory/inhibitory neuronal function, and cognitive deficits in both humans and mouse models. Suppression of inflammation has been proposed as a therapeutic approach to treating DS co-morbidities, including intellectual disability (DS/ID). Conversely, we discovered previously that treatment with the innate immune system stimulating cytokine granulocyte-macrophage colony-stimulating factor (GM-CSF), which has both pro- and anti-inflammatory activities, improved cognition and reduced brain pathology in a mouse model of Alzheimer's disease (AD), another inflammatory disorder, and improved cognition and reduced biomarkers of brain pathology in a phase II trial of humans with mild-to-moderate AD. To investigate the effects of GM-CSF treatment on DS/ID in the absence of AD, we assessed behavior and brain pathology in 12-14 month-old DS mice (Dp[16]1Yey) and their wild-type (WT) littermates, neither of which develop amyloid, and found that subcutaneous GM-CSF treatment (5 μg/day, five days/week, for five weeks) improved performance in the radial arm water maze in both Dp16 and WT mice compared to placebo. Dp16 mice also showed abnormal astrocyte morphology, increased percent area of GFAP staining in the hippocampus, clustering of astrocytes in the hippocampus, and reduced numbers of calretinin-positive interneurons in the entorhinal cortex and subiculum, and all of these brain pathologies were improved by GM-CSF treatment. These findings suggest that stimulating and/or modulating inflammation and the innate immune system with GM-CSF treatment may enhance cognition in both people with DS/ID and in the typical aging population.

Prenatal and Postnatal Pharmacotherapy in Down Syndrome: The Search to Prevent or Ameliorate Neurodevelopmental and Neurodegenerative Disorders

Those with Down syndrome (DS)-trisomy for chromosome 21-are routinely impacted by cognitive dysfunction and behavioral challenges in children and adults and Alzheimer's disease in older adults. No proven treatments specifically address these cognitive or behavioral changes. However, advances in the establishment of rodent models and human cell models promise to support development of such treatments. A research agenda that emphasizes the identification of overexpressed genes that contribute demonstrably to abnormalities in cognition and behavior in model systems constitutes a rational next step. Normalizing expression of such genes may usher in an era of successful treatments applicable across the life span for those with DS.

Curcumin Ameliorates White Matter Injury after Ischemic Stroke by Inhibiting Microglia/Macrophage Pyroptosis through NF-κB Suppression and NLRP3 Inflammasome Inhibition

NLRP3 inflammasome-mediated pyroptosis is a proinflammatory programmed cell death pathway, which plays a vital role in functional outcomes after stroke. We previously described the beneficial effects of curcumin against stroke-induced neuronal damage through modulating microglial polarization. However, the impact of curcumin on microglial pyroptosis remains unknown. Here, stroke was modeled in mice by middle cerebral artery occlusion (MCAO) for 60 minutes and treated with curcumin (150 mg/kg) intraperitoneally immediately after reperfusion, followed by daily administrations for 7 days. Curcumin ameliorated white matter (WM) lesions and brain tissue loss 21 days poststroke and improved sensorimotor function 3, 10, and 21 days after stroke. Furthermore, curcumin significantly reduced the number of gasdermin D⁺ (GSDMD⁺) Iba1⁺ and caspase-1⁺Iba1⁺ microglia/macrophage 21 days after stroke. In vitro, lipopolysaccharide (LPS) with ATP treatment was used to induce pyroptosis in primary microglia. Western blot revealed a decrease in pyroptosis-related proteins, e.g., GSDMD-N, cleaved caspase-1, NLRP3, IL-1β, and IL-18, following in vitro or in vivo curcumin treatment. Mechanistically, both in vivo and in vitro studies confirmed that curcumin inhibited the activation of the NF-κB pathway. NLRP3 knocked down by siRNA transfection markedly increased the inhibitory effects of curcumin on microglial pyroptosis and proinflammatory responses, both in vitro and in vivo. Furthermore, stereotaxic microinjection of AAV-based NLRP3 shRNA significantly improved sensorimotor function and reduced WM lesion following curcumin treatment in MCAO mice. Our study suggested that curcumin reduced stroke-induced WM damage, improved functional outcomes, and attenuated microglial pyroptosis, at least partially, through suppression of the NF-κB/NLRP3 signaling pathway, further supporting curcumin as a potential therapeutic drug for stroke.

Unraveling Human Brain Development and Evolution Using Organoid Models

Brain organoids are proving to be physiologically relevant models for studying human brain development in terms of temporal transcriptional signature recapitulation, dynamic cytoarchitectural development, and functional electrophysiological maturation. Several studies have employed brain organoid technologies to elucidate human-specific processes of brain development, gene expression, and cellular maturation by comparing human-derived brain organoids to those of non-human primates (NHPs). Brain organoids have been established from a variety of NHP pluripotent stem cell (PSC) lines and many protocols are now available for generating brain organoids capable of reproducibly representing specific brain region identities. Innumerous combinations of brain region specific organoids derived from different human and NHP PSCs, with CRISPR-Cas9 gene editing techniques and strategies to promote advanced stages of maturation, will successfully establish complex brain model systems for the accurate representation and elucidation of human brain development. Identified human-specific processes of brain development are likely vulnerable to dysregulation and could result in the identification of therapeutic targets or disease prevention strategies. Here, we discuss the potential of brain organoids to successfully model human-specific processes of brain development and explore current strategies for pinpointing these differences.

New Insights Into the Roles of Microglial Regulation in Brain Plasticity-Dependent Stroke Recovery

Stroke remains the leading cause of long-term disability worldwide with significant long-term sequelae. However, there is no highly effective treatment to enhance post-stroke recovery despite extensive efforts in exploring rehabilitative therapies. Neurorehabilitation is recognized as the cornerstone of functional restoration therapy in stroke, where treatments are focused on neuroplastic regulation to reverse neural structural disruption and improve neurofunctional networks. Post-stroke neuroplasticity changes begin within hours of symptom onset and reaches a plateau by 3 to 4 weeks within the global brain in animal studies. It plays a determining role in spontaneous stroke recovery. Microglia are immediately activated following cerebral ischemia, which has been found both proximal to the primary ischemic injury and at the remote brain regions which have functional connections to the primary injury area. Microglia exhibit different activation profiles based on the microenvironment and adaptively switch their phenotypes in a spatiotemporal manner in response to brain injuries. Microglial activation coincides with neuroplasticity after stroke, which provides the fundamental base for the microglia-mediated inflammatory responses involved in the entire neural network rewiring and brain repair. Microglial activation exerts important effects on spontaneous recovery after stroke, including structural and functional reestablishment of neurovascular networks, neurogenesis, axonal remodeling, and blood vessel regeneration. In this review, we focus on the crosstalk between microglial activation and endogenous neuroplasticity, with a special focus on the plastic alterations in the whole brain network and their implications for structural and functional restoration after stroke. We then summarize recent advances in the impacts of microglial phenotype polarization on brain plasticity, trying to discuss the potential efficacy of microglia-based extrinsic restorative interventions in promoting post-stroke recovery.

Rapid and Efficient Generation of Myelinating Human Oligodendrocytes in Organoids

Human stem cell derived brain organoids are increasingly gaining attention as an ideal model system for investigating neurological diseases, particularly those that involve myelination defects. However, current protocols for generating brain organoids with sufficiently mature oligodendrocytes that deposit myelin on endogenously produced neurons are lengthy and complicated. Taking advantage of a human pluripotent stem cell line that reports on SOX10 expression, we developed a protocol that involves a 42 day exposure of neuroectoderm-derived organoids to a cocktail of growth factors and small molecules that collectively foster oligodendrocyte specification and survival. Importantly, the resulting day 42 brain organoids contain both myelinating oligodendrocytes, cortical neuronal cells and astrocytes. These oligodendrocyte brain organoids therefore constitute a valuable and tractable platform for functional neurogenomics and drug screening for white matter diseases.

C21orf91 Regulates Oligodendroglial Precursor Cell Fate-A Switch in the Glial Lineage?

Neuropathological diseases of the central nervous system (CNS) are frequently associated with impaired differentiation of the oligodendroglial cell lineage and subsequent alterations in white matter structure and dynamics. Down syndrome (DS), or trisomy 21, is the most common genetic cause for cognitive impairments and intellectual disability (ID) and is associated with a reduction in the number of neurons and oligodendrocytes, as well as with hypomyelination and astrogliosis. Recent studies mainly focused on neuronal development in DS and underestimated the role of glial cells as pathogenic players. This also relates to C21ORF91, a protein considered a key modulator of aberrant CNS development in DS. We investigated the role of C21orf91 ortholog in terms of oligodendrogenesis and myelination using database information as well as through cultured primary oligodendroglial precursor cells (OPCs). Upon modulation of C21orf91 gene expression, we found this factor to be important for accurate oligodendroglial differentiation, influencing their capacity to mature and to myelinate axons. Interestingly, C21orf91 overexpression initiates a cell population coexpressing astroglial- and oligodendroglial markers indicating that elevated C21orf91 expression levels induce a gliogenic shift towards the astrocytic lineage reflecting non-equilibrated glial cell populations in DS brains.

Klotho Pathways, Myelination Disorders, Neurodegenerative Diseases, and Epigenetic Drugs

In this review we outline a rationale for identifying neuroprotectants aimed at inducing endogenous Klotho activity and expression, which is epigenetic action, by definition. Such an approach should promote remyelination and/or stimulate myelin repair by acting on mitochondrial function, thereby heralding a life-saving path forward for patients suffering from neuroinflammatory diseases. Disorders of myelin in the nervous system damage the transmission of signals, resulting in loss of vision, motion, sensation, and other functions depending on the affected nerves, currently with no effective treatment. Klotho genes and their single-pass transmembrane Klotho proteins are powerful governors of the threads of life and death, true to the origin of their name, Fates, in Greek mythology. Among its many important functions, Klotho is an obligatory co-receptor that binds, activates, and/or potentiates critical fibroblast growth factor activity. Since the discovery of Klotho a little over two decades ago, it has become ever more apparent that when Klotho pathways go awry, oxidative stress and mitochondrial dysfunction take over, and age-related chronic disorders are likely to follow. The physiological consequences can be wide ranging, potentially wreaking havoc on the brain, eye, kidney, muscle, and more. Central nervous system disorders, neurodegenerative in nature, and especially those affecting the myelin sheath, represent worthy targets for advancing therapies that act upon Klotho pathways. Current drugs for these diseases, even therapeutics that are disease modifying rather than treating only the symptoms, leave much room for improvement. It is thus no wonder that this topic has caught the attention of biomedical researchers around the world.