Dynamics of N6-methyladenosine modification during Alzheimer's disease development.

N6-methyladenosine (m6A) modification is a common RNA modification in the central nervous system and has been linked to various neurological disorders, including Alzheimer's disease (AD). However, the dynamic of mRNA m6A modification and m6A enzymes during the development of AD are not well understood. Therefore, this study examined the expression profiles of m6A and its enzymes in the development of AD. The results showed that changes in the expression levels of m6A regulatory factors occur in the early stages of AD, indicating a potential role for m6A modification in the onset of the disease. Additionally, the analysis of mRNA m6A expression profiles using m6A-seq revealed significant differences in m6A modification between AD and control brains. The genes with differential methylation were found to be enriched in GO and KEGG terms related to processes such as inflammation response, immune system processes. And the differently expressed genes (DEGs) are negatively lryassociated with genes involved in microglia hemostasis, but positively associated with genes related to "disease-associated microglia" (DAM) associated genes. These findings suggest that dysregulation of mRNA m6A modification may contribute to the development of AD by affecting the function and gene expression of microglia.

Microglia facilitate and stabilize the response to general anesthesia via modulating the neuronal network in a brain region-specific manner.

General anesthesia leads to a loss of consciousness and an unrousable state in patients. Although general anesthetics are widely used in clinical practice, their underlying mechanisms remain elusive. The potential involvement of nonneuronal cells is unknown. Microglia are important immune cells in the central nervous system (CNS) that play critical roles in CNS function and dysfunction. We unintentionally observed delayed anesthesia induction and early anesthesia emergence in microglia-depleted mice. We found that microglial depletion differentially regulates neuronal activities by suppressing the neuronal network of anesthesia-activated brain regions and activating emergence-activated brain regions. Thus, microglia facilitate and stabilize the anesthesia status. This influence is not mediated by dendritic spine plasticity. Instead, it relies on the activation of microglial P2Y12 and subsequent calcium influx, which facilitates the general anesthesia response. Together, we elucidate the regulatory role of microglia in general anesthesia, extending our knowledge of how nonneuronal cells modulate neuronal activities.

Transcriptional and epigenetic decoding of the microglial aging process.

As important immune cells, microglia undergo a series of alterations during aging that increase the susceptibility to brain dysfunctions. However, the longitudinal characteristics of microglia remain poorly understood. In this study, we mapped the transcriptional and epigenetic profiles of microglia from 3- to 24-month-old mice. We first discovered unexpected sex differences and identified age-dependent microglia (ADEM) genes during the aging process. We then compared the features of aging and reactivity in female microglia at single-cell resolution and epigenetic level. To dissect functions of aged microglia excluding the influence from other aged brain cells, we established an accelerated microglial turnover model without directly affecting other brain cells. By this model, we achieved aged-like microglia in non-aged brains and confirmed that aged-like microglia per se contribute to cognitive decline. Collectively, our work provides a comprehensive resource for decoding the aging process of microglia, shedding light on how microglia maintain brain functions.

Microglial debris is cleared by astrocytes via C4b-facilitated phagocytosis and degraded via RUBICON-dependent noncanonical autophagy in mice.

Microglia are important immune cells in the central nervous system (CNS) that undergo turnover throughout the lifespan. If microglial debris is not removed in a timely manner, accumulated debris may influence CNS function. Clearance of microglial debris is crucial for CNS homeostasis. However, underlying mechanisms remain obscure. We here investigate how dead microglia are removed. We find that although microglia can phagocytose microglial debris in vitro, the territory-dependent competition hinders the microglia-to-microglial debris engulfment in vivo. In contrast, microglial debris is mainly phagocytosed by astrocytes in the brain, facilitated by C4b opsonization. The engulfed microglial fragments are then degraded in astrocytes via RUBICON-dependent LC3-associated phagocytosis (LAP), a form of noncanonical autophagy. Interference with C4b-mediated engulfment and subsequent LAP disrupt the removal and degradation of microglial debris, respectively. Together, we elucidate the cellular and molecular mechanisms of microglial debris removal in mice, extending the knowledge on the maintenance of CNS homeostasis.

Deep brain stimulation of fornix for memory improvement in Alzheimer's disease: A critical review.

Memory reflects the brain function in encoding, storage and retrieval of the data or information, which is a fundamental ability for any live organism. The development of approaches to improve memory attracts much attention due to the underlying mechanistic insight and therapeutic potential to treat neurodegenerative diseases with memory loss, such as Alzheimer's disease (AD). Deep brain stimulation (DBS), a reversible, adjustable, and non-ablative therapy, has been shown to be safe and effective in many clinical trials for neurodegenerative and neuropsychiatric disorders. Among all potential regions with access to invasive electrodes, fornix is considered as it is the major afferent and efferent connection of the hippocampus known to be closely associated with learning and memory. Indeed, clinical trials have demonstrated that fornix DBS globally improved cognitive function in a subset of patients with AD, indicating fornix can serve as a potential target for neurosurgical intervention in treating memory impairment in AD. The present review aims to provide a better understanding of recent progresses in the application of fornix DBS for ameliorating memory impairments in AD patients.

Failure of observing NeuroD1-induced microglia-to-neuron conversion in vitro is not attributed to the low NeuroD1 expression level.

NeuroD1-induced microglia-to-neuron conversion is hotly debated. Recently, we published a paper in Neuron demonstrating that NeuroD1 cannot induce microglia-to-neuron cross-lineage conversion. In the same issue of Neuron, Matsuda et al., who observed the "NeuroD1-induced microglia-to-neuron conversion" phenotype, responded to our study. They claimed that we failed to observe NeuroD1-induced microglia-to-neuron conversion in vitro due to the low NeuroD1 expression efficiency in our experiment. They argued that the NeuroD1 upregulation in our study was around 200-fold (vs. control), whereas the upregulation in Nakashima lab was 3000-fold, 15 times higher than ours. In fact, this is not true. We compared the expression level from the original paper and found that our NeuroD1 expression level was comparable to that of Matsuda et al. (Neuron 101:472-485.e477, 2019), or even higher. Therefore, the failure of observing NeuroD1-induced microglia-to-neuron conversion cannot be attributable to the low expression level.

NeuroD1 induces microglial apoptosis and cannot induce microglia-to-neuron cross-lineage reprogramming.

The regenerative capacity of neurons is limited in the central nervous system (CNS), with irreversible neuronal loss upon insult. In contrast, microglia exhibit extraordinary capacity for repopulation. Matsuda et al. (2019) recently reported NeuroD1-induced microglia-to-neuron conversion, aiming to provide an "unlimited" source to regenerate neurons. However, the extent to which NeuroD1 can exert cross-lineage reprogramming of microglia (myeloid lineage) to neurons (neuroectodermal lineage) is unclear. In this study, we unexpectedly found that NeuroD1 cannot convert microglia to neurons in mice. Instead, NeuroD1 expression induces microglial cell death. Moreover, lineage tracing reveals non-specific leakage of similar lentiviruses as previously used for microglia-to-neuron conversion, which confounds the microglia-to-neuron observation. In summary, we demonstrated that NeuroD1 cannot induce microglia-to-neuron cross-lineage reprogramming. We here propose rigid principles for verifying glia-to-neuron conversion. This Matters Arising paper is in response to Matsuda et al. (2019), published in Neuron.

Microglia replacement by bone marrow transplantation (Mr BMT) in the central nervous system of adult mice.

Microglia are important immune cells in the central nervous system (CNS). Mutations in microglia may cause CNS disorders. Replacement of dysfunctional microglia with allogeneic wild-type microglia can correct the gene deficiency, thus treating the neurogenic diseases. However, traditional approaches cannot efficiently replace microglia at the adulthood. Here, we introduce a potentially clinical-feasible strategy named microglia replacement by bone marrow transplantation that achieves efficient microglia replacement at the whole CNS scale, including the brain, spinal cord, and retina in adult mice. For complete details on the use and execution of this protocol, please refer to Xu et al. (2020). The original abbreviation of this microglia replacement strategy is mrBMT. We hereby change the name to Mr BMT.

Microglia replacement by microglia transplantation (Mr MT) in the adult mouse brain.

Mutations in microglia may cause brain disorders. Replacement of dysfunctional microglia by allogeneic wild-type microglia from bone marrow transplantation (Mr BMT) or peripheral blood can correct the gene deficiency at the brain-wide scale but cannot achieve precise replacement at specific brain regions. Here, we introduce a strategy with potential clinical relevance-microglia replacement by microglia transplantation (Mr MT), combining tamoxifen-induced ablation of Mr BMT cells and intracranial injection of microglia to mouse brain, to achieve region-sepcific microglia replacement. The original abbreviation of this microglia replacement strategy is mrMT. We hereby change the name to Mr MT. For complete details on the use and execution of this protocol, please refer to Xu et al. (2020).

Protocol for microglia replacement by peripheral blood (Mr PB).

Microglia are important immune cells in the central nervous system. Replacement of mutated microglia by wild-type cells through microglia replacement by bone marrow transplantation can correct gene deficiencies. However, the limited availability of bone marrow cells may restrict its potential of becoming a widely used clinical treatment. Here, we introduce a potentially clinical-feasible strategy achieving efficient microglia replacement by peripheral blood cells in mice, boosting the donor cell availability. We named it microglia replacement by peripheral blood (Mr PB). For complete details on the use and execution of this protocol, please refer to Xu et al. (2020). The original abbreviation of this microglia replacement strategy is mrPB. We hereby change the name to Mr PB.

Primary Microglia Isolation from Postnatal Mouse Brains.

Microglia are the mononuclear phagocytes in the central nervous system (CNS), which play key roles in maintaining homeostasis and regulating the inflammatory process in the CNS. To study the microglial biology in vitro, primary microglia show great advantages compared to immortalized microglial cell lines. However, microglia isolation from the postnatal mouse brain is relatively less efficient and time-consuming. In this protocol, we provide a quick and easy-to-follow method to isolate primary microglia from the neonatal mouse brain. The overall steps of this protocol include brain dissection, primary brain cell culture, and microglia isolation. Using this approach, researchers can obtain primary microglia with high purity. In addition, the harvested primary microglia were able to respond to the lipopolysaccharides challenge, indicating they retained their immune function. Collectively, we developed a simplified approach to efficiently isolate primary microglia with high purity, which facilitates a wide range of microglial biology investigations in vitro.

Common cellular and molecular mechanisms and interactions between microglial activation and aberrant neuroplasticity in depression.

It has been suggested that inflammation is involved in the pathophysiology of depression. As tissue-specific macrophages in the central nervous system (CNS), microglia play an important role in neuroinflammation. Resident microglia become activated towards the pro-inflammatory (M1) phenotype or the anti-inflammatory (M2) phenotype during neuroinflammation. In the CNS, neurons report to microglia regarding their statuses and can regulate microglial activation, while microglia also modulate neuronal activities, including neuroplasticity. The molecular mechanisms underlying the communication between microglia and neurons, which include intracellular and extracellular signalling pathways, might be complex and of great importance for new research on the pathogenesis of depression. The present review aims to discuss the common cellular and molecular mechanisms for microglial activation and aberrant neuroplasticity in depression and the role of these processes in the pathogenesis of depression.

Efficient Strategies for Microglia Replacement in the Central Nervous System.

Microglia are important immune cells in the central nervous system (CNS). Dysfunctions of gene-deficient microglia contribute to the development and progression of multiple CNS diseases. Microglia replacement by nonself cells has been proposed to treat microglia-associated disorders. However, some attempts have failed due to low replacement efficiency, such as with the traditional bone marrow transplantation approach. In this study, we develop three efficient strategies for microglia replacement: microglia replacement by bone marrow transplantation (mrBMT), microglia replacement by peripheral blood (mrPB), and microglia replacement by microglia transplantation (mrMT). mrBMT and mrPB allow microglia-like cells to efficiently replace resident microglia in the whole CNS. On the other hand, mrMT achieves microglia replacement in brain regions of interest. In summary, the present study offers effective tactics for microglia replacement with diverse application scenarios, which potentially opens up a window on treating microglia-associated CNS disorders.

Cis-control of Six1 expression in neural crest cells during craniofacial development.

BACKGROUND: Six1 is a transcriptional factor that plays an important role in embryonic development. Mouse and chick embryos deficient for Six1 have multiple craniofacial anomalies in the facial bones and cartilages. Multiple Six1 enhancers have been identified, but none of them has been reported to be active in the maxillary and mandibular process. RESULTS: We studied two Six1 enhancers in the chick neural crest tissues during craniofacial development. We showed that two evolutionarily conserved enhancers, Six1E1 and Six1E2, act synergistically. Neither Six1E1 nor Six1E2 alone can drive enhancer reporter signal in the maxillary or mandibular processes. However, their combination, Six1E, showed robust enhancer activity in these tissues. Similar reporter signal can also be driven by the mouse homolog of Six1E. Mutations of multiple conserved transcriptional factor binding sites altered the enhancer activity of Six1E, especially mutation of the LIM homeobox binding site, dramatically reduced the enhancer activity, implying that the Lhx protein family be an important regulator of Six1 expression. CONCLUSION: This study, for the first time, described the synergistic activation of two Six1 enhancers in the maxillary and mandibular processes and will facilitate more detailed studies of the regulation of Six1 in craniofacial development.

Dual extra-retinal origins of microglia in the model of retinal microglia repopulation.

Elucidating the origin of microglia is crucial for understanding their functions and homeostasis. Previous study has indicated that Nestin-positive progenitor cells differentiate into microglia and replenish the brain after depleting most brain microglia. Microglia have also shown the capacity to repopulate the retina after eliminating all retinal microglia. However, the origin(s) of repopulated retinal microglia is/are unknown. In this study, we aim to investigate the origins of repopulated microglia in the retina. Interestingly, we find that repopulated retinal microglia are not derived from Nestin-positive progenitor cells. Instead, they have two origins: the center-emerging microglia are derived from residual microglia in the optic nerve and the periphery-emerging microglia are derived from macrophages in the ciliary body/iris. Therefore, we have for the first time identified the extra-retinal origins of microglia in the adult mammalian retina by using a model of microglial repopulation, which may shed light on the target exploration of therapeutic interventions for retinal degenerative disorders.

Repopulated microglia are solely derived from the proliferation of residual microglia after acute depletion.

Newborn microglia rapidly replenish the whole brain after selective elimination of most microglia (>99%) in adult mice. Previous studies reported that repopulated microglia were largely derived from microglial progenitor cells expressing nestin in the brain. However, the origin of these repopulated microglia has been hotly debated. In this study, we investigated the origin of repopulated microglia by a series of fate-mapping approaches. We first excluded the blood origin of repopulated microglia via parabiosis. With different transgenic mouse lines, we then demonstrated that all repopulated microglia were derived from the proliferation of the few surviving microglia (<1%). Despite a transient pattern of nestin expression in newly forming microglia, none of repopulated microglia were derived from nestin-positive non-microglial cells. In summary, we conclude that repopulated microglia are solely derived from residual microglia rather than de novo progenitors, suggesting the absence of microglial progenitor cells in the adult brain.

Asymmetric localization of DLC1 defines avian trunk neural crest polarity for directional delamination and migration.

Following epithelial-mesenchymal transition, acquisition of avian trunk neural crest cell (NCC) polarity is prerequisite for directional delamination and migration, which in turn is essential for peripheral nervous system development. However, how this cell polarization is established and regulated remains unknown. Here we demonstrate that, using the RHOA biosensor in vivo and in vitro, the initiation of NCC polarization is accompanied by highly activated RHOA in the cytoplasm at the cell rear and its fluctuating activity at the front edge. This differential RHOA activity determines polarized NC morphology and motility, and is regulated by the asymmetrically localized RhoGAP Deleted in liver cancer (DLC1) in the cytoplasm at the cell front. Importantly, the association of DLC1 with NEDD9 is crucial for its asymmetric localization and differential RHOA activity. Moreover, NC specifiers, SOX9 and SOX10, regulate NEDD9 and DLC1 expression, respectively. These results present a SOX9/SOX10-NEDD9/DLC1-RHOA regulatory axis to govern NCC migratory polarization.