Myelination generates aberrant ultrastructure that is resolved by microglia

Dynamic development of microglia and macrophages after spinal cord injury

JOURNAL/nrgr/04.03/01300535-202512000-00029/figure1/v/2025-01-31T122243Z/r/image-tiff Secondary injury following spinal cord injury is primarily characterized by a complex inflammatory response, with resident microglia and infiltrating macrophages playing pivotal roles. While previous studies have grouped these two cell types together based on similarities in structure and function, an increasing number of studies have demonstrated that microglia and macrophages exhibit differences in structure and function and have different effects on disease processes. In this study, we used single-cell RNA sequencing and spatial transcriptomics to identify the distinct evolutionary paths of microglia and macrophages following spinal cord injury. Our results showed that microglia were activated to a pro-inflammatory phenotype immediately after spinal cord injury, gradually transforming to an anti-inflammatory steady state phenotype as the disease progressed. Regarding macrophages, our findings highlighted abundant communication with other cells, including fibroblasts and neurons. Both pro-inflammatory and neuroprotective effects of macrophages were also identified; the pro-inflammatory effect may be related to integrin β2 ( Itgb2 ) and the neuroprotective effect may be related to the oncostatin M pathway. These findings were validated by in vivo experiments. This research underscores differences in the cellular dynamics of microglia and macrophages following spinal cord injury, and may offer new perspectives on inflammatory mechanisms and potential therapeutic targets.

Dicer deficiency affects microglial function during demyelination and impairs remyelination

Microglia are essential regulators of central nervous system (CNS) homeostasis, playing key roles in demyelination and remyelination. Dysregulated microglial activity contributes to pathological inflammation and impaired repair processes in demyelinating diseases. Here, we investigate the role of Dicer1, a critical enzyme in microRNA biogenesis, in affecting microglial function, demyelination, and remyelination. Loss of Dicer1 in microglia resulted in amplified inflammatory responses, defective myelin debris clearance, and disruption of metabolic homeostasis, leading to exacerbated demyelination and delayed remyelination. Transcriptomic analysis revealed significant upregulation of inflammatory pathways, including interferon signaling and JAK/STAT activation, alongside a loss of homeostatic microglial gene expression. Protein-level validation confirmed sustained secretion of pro-inflammatory cytokines such as IFN-γ, IL-16, and CXCL12, creating a chronic inflammatory environment that impaired remyelination. Furthermore, Dicer1-deficient microglia failed to support oligodendrocyte progenitor cells (OPCs) differentiation/maturation, with increased apoptosis of mature oligodendrocytes (OLs), contributing to remyelination failure. These findings identify Dicer1 as a critical regulator of microglial homeostasis and inflammation resolution, highlighting its potential as a therapeutic target to mitigate inflammation and promote repair in demyelinating diseases.

Disruption of Oligodendroglial Autophagy Leads to Myelin Morphological Deficits, Neuronal Apoptosis, and Cognitive Decline in Aged Mice

The aging central nervous system (CNS) is often marked by myelin degeneration, yet the underlying mechanisms remain elusive. This study delves into the previously unexplored role of autophagy in maintaining CNS myelin during aging. We generated the transgenic mouse line plpCreERT2; atg5f/f, enabling selective deletion of the core autophagic component Atg5 in oligodendrocytes (OLs) following tamoxifen administration in adulthood, while analysis was conducted on aged mice. Our findings reveal that oligodendroglial autophagy inactivation leads to significant alterations in myelin protein levels. Moreover, the ultrastructural analysis revealed pronounced myelin deficits and increased degeneration of axons, accompanied by apoptosis, as confirmed by immunohistochemistry. Behaviorally, aged knockout (cKO) mice exhibited marked deficits in learning and memory tasks, indicative of cognitive impairment. Additionally, we observed increased activation of microglia, suggesting an inflammatory response linked to the absence of autophagic activity in OLs. These results underscore the critical role of autophagy in OLs for the preservation of CNS myelin and axonal integrity during aging. Our study highlights autophagy as a vital mechanism for neural maintenance, offering potential therapeutic avenues for combating age-related neurodegenerative diseases.

The crosstalk between CNS resident glial cells and peripheral immune cells is critical for age-dependent demyelination and subsequent remyelination

White-matter diseases like multiple sclerosis begin in young adulthood. Aging, being a risk factor, contributes to the progression of these diseases and makes neurological disabilities worsen. Aging causes white matter alteration due to myelin loss, axonal degeneration, and hyperintensities, resulting in cognitive impairment and neurological disorders. Aging also negatively affects central nervous system resident glial cells and peripheral immune cells, contributing to myelin degeneration and diminished myelin renewal process. Restoration of myelin failure with aging accelerates the progression of cognitive decline. This review will mainly focus on how age-related altered functions of glial and peripheral cells will affect myelin sheath alteration and myelin restoration. This understanding can give us insights into the underlying mechanisms of demyelination and failure of remyelination with aging concerning altered glial and peripheral immune cell function and their crosstalk. Also, we will explain the therapeutic strategies to enhance the remyelination process of an aging brain to improve the cognitive health of an aging person.

Extrinsic Apoptosis and Necroptosis in Telencephalic Development: A Single-Cell Mass Cytometry Study

Regulated cell death is integral to sculpting the developing brain, yet the relative contributions of extrinsic apoptosis and necroptosis remain unclear. Here, we leverage single-cell mass cytometry (CyTOF) to characterize the cellular landscape of the mouse telencephalon in wild-type (WT), RIPK3 knockout (RIPK3 KO), and RIPK3/Caspase-8 double knockout (DKO) mice. Strikingly, combined deletion of RIPK3 and Caspase-8 leads to a 12.6% increase in total cell count, challenging the prevailing notion that intrinsic apoptosis exclusively governs developmental cell elimination. Detailed subpopulation analysis reveals that DKO mice display selective enrichment of Tbr2⁺ intermediate progenitors and endothelial cells, underscoring distinct, cell type-specific roles for extrinsic apoptotic and necroptotic pathways. These findings provide a revised framework for understanding the coordinated regulation of cell number during telencephalic development and suggest potential mechanistic links to neurodevelopmental disorders characterized by aberrant cell death.

Oligodendrocytes in Alzheimer's disease pathophysiology

Our understanding of Alzheimer's disease (AD) has transformed from a purely neuronal perspective to one that acknowledges the involvement of glial cells. Despite remarkable progress in unraveling the biology of microglia, astrocytes and vascular elements, the exploration of oligodendrocytes in AD is still in its early stages. Contrary to the traditional notion of oligodendrocytes as passive bystanders in AD pathology, emerging evidence indicates their active participation in and reaction to amyloid and tau pathology. Oligodendrocytes undergo a functional transition to a disease-associated state, engaging in immune modulation, stress responses and cellular survival. Far from being inert players, they appear to serve a dual role in AD pathogenesis, potentially offering defense mechanisms against pathology while also contributing to disease progression. This Review explores recent advancements in understanding the roles of oligodendrocytes and their myelin sheaths in the context of AD, shedding light on their complex interactions within the disease pathology.

White matter aging and its impact on brain function

Aging has a detrimental impact on white matter, resulting in reduced volume, compromised structural integrity of myelinated axons, and an increase in white matter hyperintensities. These changes are closely linked to cognitive decline and neurological disabilities. The deterioration of myelin and its diminished ability to regenerate as we age further contribute to the progression of neurodegenerative disorders. Understanding these changes is crucial for devising effective disease prevention strategies. Here, we will discuss the structural alterations in white matter that occur with aging and examine the cellular and molecular mechanisms driving these aging-related transformations. We highlight how the progressive disruption of white matter may initiate a self-perpetuating cycle of inflammation and neural damage.

Fish microglia: Beyond the resident macrophages of the central nervous system - A review of their morphofunctional characteristics

From classical to modern literature on microglia, the importance of the potential and variability of these immune cells in vertebrates has been pointed out. Recent aspects such as relationships and interactions between microglia and neurons in both normal and injured neural tissues, as well as their nexus with other organs and with the microbiota, or how these cells are modulated during development and adulthood are current topics of major interest. State-of-the-art research methodologies, including microscopy and potent in vivo imaging techniques, genomic and proteomic methods, current culture conditions together with the easy maintenance and manipulation of some fish embryos and adult specimens such as zebrafish (Danio rerio), have emerged and adapted to the phylogenetic position of some fish species. Furthermore, these advancements have facilitated the development of successful protocols aimed at addressing significant hypotheses and unresolved questions regarding vertebrate glia. The present review aims to analyse the available information on fish microglia, mainly the most recent one concerning teleosts, to establish an overview of their structural and immune functional features as a basis for their potentialities, heterogeneity, diversification, involvement, and relationships with neurons under normal and pathological conditions.

Glial Cell Development and Function in the Zebrafish Central Nervous System

Over the past decades the zebrafish has emerged as an excellent model organism with which to study the biology of all glial cell types in nervous system development, plasticity, and regeneration. In this review, which builds on the earlier work by Lyons and Talbot in 2015, we will summarize how the relative ease to manipulate the zebrafish genome and its suitability for intravital imaging have helped understand principles of glial cell biology with a focus on oligodendrocytes, microglia, and astrocytes. We will highlight recent findings on the diverse properties and functions of these glial cell types in the central nervous system and discuss open questions and future directions of the field.

Ultrastructural features of psychological stress resilience in the brain: a microglial perspective

Psychological stress is the major risk factor for major depressive disorder. Sustained stress causes changes in behaviour, brain connectivity and in its cells and organelles. Resilience to stress is understood as the ability to recover from stress in a positive way or the resistance to the negative effects of psychological stress. Microglia, the resident immune cells of the brain, are known players of stress susceptibility, but less is known about their role in stress resilience and the cellular changes involved. Ultrastructural analysis has been a useful tool in the study of microglia and their function across contexts of health and disease. Despite increased access to electron microscopy, the interpretation of electron micrographs remains much less accessible. In this review, we will first present microglia and the concepts of psychological stress susceptibility and resilience. Afterwards, we will describe ultrastructural analysis, notably of microglia, as a readout to study the mechanisms underlying psychological stress resilience. Lastly, we will cover nutritional ketosis as a therapeutic intervention that was shown to be effective in promoting psychological stress resilience as well as modifying microglial function and ultrastructure.

Exploring the potential of MFG-E8 in neurodegenerative diseases

Milk fat globule-epidermal growth factor 8 (MFG-E8) is a multifunctional glycoprotein regulating intercellular interactions in various biological and pathological processes. This review summarizes the effects and mechanisms of MFG-E8 in neurodegenerative diseases (NDDs), emphasizing its roles in inflammation, apoptosis, and oxidative stress. In this review, will also explore the potential of MFG-E8 as a diagnostic biomarker and its therapeutic applications in neurodegenerative disorders. Recent studies have revealed intriguing characteristics of using MFG-E8 as a potential drug for treating various brain disorders. While the discovery, origin, expression, and physiological functions of MFG-E8 in various organs and tissues are well defined, its role in the brain remains less understood. This is particularly true for NDDs, indicating unmet medical needs. Elucidating its role in the brain could position MFG-E8 as a potential treatment for NDDs.

Loss of CLN3 in microglia leads to impaired lipid metabolism and myelin turnover

Loss-of-function mutations in CLN3 cause juvenile Batten disease, featuring neurodegeneration and early-stage neuroinflammation. How loss of CLN3 function leads to early neuroinflammation is not yet understood. Here, we have comprehensively studied microglia from Cln3∆ex7/8 mice, a genetically accurate disease model. Loss of CLN3 function in microglia leads to lysosomal storage material accumulation and abnormal morphology of subcellular organelles. Moreover, pathological proteomic signatures are indicative of defects in lysosomal function and abnormal lipid metabolism. Consistent with these findings, CLN3-deficient microglia are unable to efficiently turnover myelin and metabolize the associated lipids, showing defects in lipid droplet formation and cholesterol accumulation. Accordingly, we also observe impaired myelin integrity in aged Cln3∆ex7/8 mouse brain. Autophagy inducers and cholesterol-lowering drugs correct the observed microglial phenotypes. Taken together, these data implicate a cell-autonomous defect in CLN3-deficient microglia that impacts their ability to support neuronal cell health, suggesting microglial targeted therapies should be considered for CLN3 disease.

Monitoring Myelin Lipid Composition and the Structure of Myelinated Fibers Reveals a Maturation Delay in CMT1A

Findings accumulated over time show that neurophysiological, neuropathological, and molecular alterations are present in CMT1A and support the dysmyelinating rather than demyelinating nature of this neuropathy. Moreover, uniform slowing of nerve conduction velocity is already manifest in CMT1A children and does not improve throughout their life. This evidence and our previous studies displaying aberrant myelin composition and structure in adult CMT1A rats prompt us to hypothesize a myelin and axon developmental defect in the CMT1A peripheral nervous system. Peripheral myelination begins during the early stages of development in mammals and, during this process, chemical and structural features of myelinated fibers (MFs) evolve towards a mature phenotype; deficiencies within this self-modulating circuit can cause its blockage. Therefore, to shed light on pathophysiological mechanisms that occur during development, and to investigate the relationship among axonal, myelin, and lipidome deficiencies in CMT1A, we extensively analyzed the evolution of both myelin lipid profile and MF structure in WT and CMT1A rats. Lipidomic analysis revealed a delayed maturation of CMT1A myelin already detectable at P10 characterized by a deprivation of sphingolipid species such as hexosylceramides and long-chain sphingomyelins, whose concentration physiologically increases in WT, and an increase in lipids typical of unspecialized plasma membranes, including phosphatidylcholines and phosphatidylethanolamines. Consistently, advanced morphometric analysis on more than 130,000 MFs revealed a delay in the evolution of CMT1A axon and myelin geometric parameters, appearing concomitantly with lipid impairment. We here demonstrate that, during normal development, MFs undergo a continuous maturation process in both chemical composition and physical structure, but these processes are delayed in CMT1A.

Astrocytes and microglia in the coordination of CNS development and homeostasis

Glia have emerged as important architects of central nervous system (CNS) development and maintenance. While traditionally glial contributions to CNS development and maintenance have been studied independently, there is growing evidence that either suggests or documents that glia may act in coordinated manners to effect developmental patterning and homeostatic functions in the CNS. In this review, we focus on astrocytes, the most abundant glia in the CNS, and microglia, the earliest glia to colonize the CNS highlighting research that documents either suggestive or established coordinated actions by these glial cells in various CNS processes including cell and/or debris clearance, neuronal survival and morphogenesis, synaptic maturation, and circuit function, angio-/vasculogenesis, myelination, and neurotransmission. Some molecular mechanisms underlying these processes that have been identified are also described. Throughout, we categorize the available evidence as either suggestive or established interactions between microglia and astrocytes in the regulation of the respective process and raise possible avenues for further research. We conclude indicating that a better understanding of coordinated astrocyte-microglial interactions in the developing and mature brain holds promise for developing effective therapies for brain pathologies where these processes are perturbed.

CNS macrophage contributions to myelin health

Myelin is the membrane surrounding neuronal axons in the central nervous system (CNS), produced by oligodendrocytes to provide insulation for electrical impulse conduction and trophic/metabolic support. CNS dysfunction occurs following poor development of myelin in infancy, myelin damage in neurological diseases, and impaired regeneration of myelin with disease progression in aging. The lack of approved therapies aimed at supporting myelin health highlights the critical need to identify the cellular and molecular influences on oligodendrocytes. CNS macrophages have been shown to influence the development, maintenance, damage and regeneration of myelin, revealing critical interactions with oligodendrocyte lineage cells. CNS macrophages are comprised of distinct populations, including CNS-resident microglia and cells associated with CNS border regions (the meninges, vasculature, and choroid plexus), in addition to macrophages derived from monocytes infiltrating from the blood. Importantly, the distinct contribution of these macrophage populations to oligodendrocyte lineage responses and myelin health are only just beginning to be uncovered, with the advent of new tools to specifically identify, track, and target macrophage subsets. Here, we summarize the current state of knowledge on the roles of CNS macrophages in myelin health, and recent developments in distinguishing the roles of macrophage populations in development, homeostasis, and disease.

The role of microglial TREM2 in development: A path toward neurodegeneration?

The nervous and the immune systems undergo a continuous cross talk, starting from early development and continuing throughout adulthood and aging. Defects in this cross talk contribute to neurodevelopmental and neurodegenerative diseases. Microglia are the resident immune cells in the brain that are primarily involved in this bidirectional communication. Among the microglial genes, trem2 is a key player, controlling the functional state of microglia and being at the forefront of many processes that require interaction between microglia and other brain components, such as neurons and oligodendrocytes. The present review focuses on the early developmental window, describing the early brain processes in which TREM2 is primarily involved, including the modulation of synapse formation and elimination, the control of neuronal bioenergetic states as well as the contribution to myelination processes and neuronal circuit formation. By causing imbalances during these early maturation phases, dysfunctional TREM2 may have a striking impact on the adult brain, making it a more sensitive target for insults occurring during adulthood and aging.

From Blur to Brilliance: The Ascendance of Advanced Microscopy in Neuronal Cell Biology

The intricate network of the brain's neurons and synapses poses unparalleled challenges for research, distinct from other biological studies. This is particularly true when dissecting how neurons and their functional units work at a cell biological level. While traditional microscopy has been foundational, it was unable to reveal the deeper complexities of neural interactions. However, an imaging renaissance has transformed our capabilities. Advancements in light and electron microscopy, combined with correlative imaging, now achieve unprecedented resolutions, uncovering the most nuanced neural structures. Maximizing these tools requires more than just technical proficiency. It is crucial to align research aims, allocate resources wisely, and analyze data effectively. At the heart of this evolution is interdisciplinary collaboration, where various experts come together to translate detailed imagery into significant biological insights. This review navigates the latest developments in microscopy, underscoring both the promise of and prerequisites for bending this powerful tool set to understanding neuronal cell biology.

Dynamics of mature myelin

Myelin, which is produced by oligodendrocytes, insulates axons to facilitate rapid and efficient action potential propagation in the central nervous system. Traditionally viewed as a stable structure, myelin is now known to undergo dynamic modulation throughout life. This Review examines these dynamics, focusing on two key aspects: (1) the turnover of myelin, involving not only the renewal of constituents but the continuous wholesale replacement of myelin membranes; and (2) the structural remodeling of pre-existing, mature myelin, a newly discovered form of neural plasticity that can be stimulated by external factors, including neuronal activity, behavioral experience and injury. We explore the mechanisms regulating these dynamics and speculate that myelin remodeling could be driven by an asymmetry in myelin turnover or reactivation of pathways involved in myelin formation. Finally, we outline how myelin remodeling could have profound impacts on neural function, serving as an integral component of behavioral adaptation.

Reflective imaging of myelin integrity in the human and mouse central nervous systems

The structural integrity of myelin sheaths in the central nervous system (CNS) is crucial for the maintenance of its function. Electron microscopy (EM) is the gold standard for visualizing individual myelin sheaths. However, the tissue processing involved can induce artifacts such as shearing of myelin, which can be difficult to distinguish from true myelin abnormalities. Spectral confocal reflectance (SCoRe) microscopy is an imaging technique that leverages the differential refractive indices of compacted CNS myelin in comparison to surrounding parenchyma to detect individual compact myelin internodes with reflected light, positioning SCoRe as a possible complementary method to EM to assess myelin integrity. Whether SCoRe is sensitive enough to detect losses in myelin compaction when myelin quantity is otherwise unaffected has not yet been directly tested. Here, we assess the capacity of SCoRe to detect differences in myelin compaction in two mouse models that exhibit a loss of myelin compaction without demyelination: microglia-deficient mice (Csf1r-FIRE Δ/Δ) and wild-type mice fed with the CSF1R inhibitor PLX5622. In addition, we compare the ability to detect compact myelin sheaths using SCoRe in fixed-frozen versus paraffin-embedded mouse tissue. Finally, we show that SCoRe can successfully detect individual sheaths in aged human paraffin-embedded samples of deep white matter regions. As such, we find SCoRe to be an attractive technique to investigate myelin integrity, with sufficient sensitivity to detect myelin ultrastructural abnormalities and the ability to perform equally well in tissue preserved using different methods.

Translating Molecular Approaches to Oligodendrocyte-Mediated Neurological Circuit Modulation

The central nervous system (CNS) exhibits remarkable adaptability throughout life, enabled by intricate interactions between neurons and glial cells, in particular, oligodendrocytes (OLs) and oligodendrocyte precursor cells (OPCs). This adaptability is pivotal for learning and memory, with OLs and OPCs playing a crucial role in neural circuit development, synaptic modulation, and myelination dynamics. Myelination by OLs not only supports axonal conduction but also undergoes adaptive modifications in response to neuronal activity, which is vital for cognitive processing and memory functions. This review discusses how these cellular interactions and myelin dynamics are implicated in various neurocircuit diseases and disorders such as epilepsy, gliomas, and psychiatric conditions, focusing on how maladaptive changes contribute to disease pathology and influence clinical outcomes. It also covers the potential for new diagnostics and therapeutic approaches, including pharmacological strategies and emerging biomarkers in oligodendrocyte functions and myelination processes. The evidence supports a fundamental role for myelin plasticity and oligodendrocyte functionality in synchronizing neural activity and high-level cognitive functions, offering promising avenues for targeted interventions in CNS disorders.

An inducible genetic tool to track and manipulate specific microglial states reveals their plasticity and roles in remyelination

Recent single-cell RNA sequencing studies have revealed distinct microglial states in development and disease. These include proliferative-region-associated microglia (PAMs) in developing white matter and disease-associated microglia (DAMs) prevalent in various neurodegenerative conditions. PAMs and DAMs share a similar core gene signature. However, the extent of the dynamism and plasticity of these microglial states, as well as their functional significance, remains elusive, partly due to the lack of specific tools. Here, we generated an inducible Cre driver line, Clec7a-CreERT2, that targets PAMs and DAMs in the brain parenchyma. Utilizing this tool, we profiled labeled cells during development and in several disease models, uncovering convergence and context-dependent differences in PAM and DAM gene expression. Through long-term tracking, we demonstrated microglial state plasticity. Lastly, we specifically depleted DAMs in demyelination, revealing their roles in disease recovery. Together, we provide a versatile genetic tool to characterize microglial states in CNS development and disease.

Neuro-immune interactions in health and disease: Insights from FENS-Hertie 2022 Winter School

In a great partnership, the Federation of European Neuroscience Societies (FENS) and the Hertie Foundation organized the FENS-Hertie 2022 Winter School on 'Neuro-immune interactions in health and disease'. The school selected 27 PhD students and 13 postdoctoral fellows from 20 countries and involved 14 faculty members experts in the field. The Winter School focused on a rising field of research, the interactions between the nervous and both innate and adaptive immune systems under pathological and physiological conditions. A fine-tuned neuro-immune crosstalk is fundamental for healthy development, while disrupted neuro-immune communication might play a role in neurodegeneration, neuroinflammation and aging. However, much is yet to be understood about the underlying mechanisms of these neuro-immune interactions in the healthy brain and under pathological scenarios. In addition to new findings in this emerging field, novel methodologies and animal models were presented to foment research on neuro-immunology. The FENS-Hertie 2022 Winter School provided an insightful knowledge exchange between students and faculty focusing on the latest discoveries in the biology of neuro-immune interactions while fostering great academic and professional opportunities for early-career neuroscientists from around the world.

Oligodendrocyte calcium signaling promotes actin-dependent myelin sheath extension

Myelin is essential for rapid nerve signaling and is increasingly found to play important roles in learning and in diverse diseases of the CNS. Morphological parameters of myelin such as sheath length are thought to precisely tune conduction velocity, but the mechanisms controlling sheath morphology are poorly understood. Local calcium signaling has been observed in nascent myelin sheaths and can be modulated by neuronal activity. However, the role of calcium signaling in sheath formation remains incompletely understood. Here, we use genetic tools to attenuate oligodendrocyte calcium signaling during myelination in the developing mouse CNS. Surprisingly, genetic calcium attenuation does not grossly affect the number of myelinated axons or myelin thickness. Instead, calcium attenuation causes myelination defects resulting in shorter, dysmorphic sheaths. Mechanistically, calcium attenuation reduces actin filaments in oligodendrocytes, and an intact actin cytoskeleton is necessary and sufficient to achieve accurate myelin morphology. Together, our work reveals a cellular mechanism required for accurate CNS myelin formation and may provide mechanistic insight into how oligodendrocytes respond to neuronal activity to sculpt and refine myelin sheaths.

Microglia regulation of central nervous system myelin health and regeneration

Microglia are resident macrophages of the central nervous system that have key functions in its development, homeostasis and response to damage and infection. Although microglia have been increasingly implicated in contributing to the pathology that underpins neurological dysfunction and disease, they also have crucial roles in neurological homeostasis and regeneration. This includes regulation of the maintenance and regeneration of myelin, the membrane that surrounds neuronal axons, which is required for axonal health and function. Myelin is damaged with normal ageing and in several neurodegenerative diseases, such as multiple sclerosis and Alzheimer disease. Given the lack of approved therapies targeting myelin maintenance or regeneration, it is imperative to understand the mechanisms by which microglia support and restore myelin health to identify potential therapeutic approaches. However, the mechanisms by which microglia regulate myelin loss or integrity are still being uncovered. In this Review, we discuss recent work that reveals the changes in white matter with ageing and neurodegenerative disease, how this relates to microglia dynamics during myelin damage and regeneration, and factors that influence the regenerative functions of microglia.

An inducible genetic tool for tracking and manipulating specific microglial states in development and disease

Recent single-cell RNA sequencing studies have revealed distinct microglial states in development and disease. These include proliferative region-associated microglia (PAM) in developing white matter and disease-associated microglia (DAM) prevalent in various neurodegenerative conditions. PAM and DAM share a similar core gene signature and other functional properties. However, the extent of the dynamism and plasticity of these microglial states, as well as their functional significance, remains elusive, partly due to the lack of specific tools. Here, we report the generation of an inducible Cre driver line, Clec7a-CreERT2, designed to target PAM and DAM in the brain parenchyma. Utilizing this tool, we profile labeled cells during development and in several disease models, uncovering convergence and context-dependent differences in PAM/DAM gene expression. Through long-term tracking, we demonstrate surprising levels of plasticity in these microglial states. Lastly, we specifically depleted DAM in cuprizone-induced demyelination, revealing their roles in disease progression and recovery.

The Roles of Caloric Restriction Mimetics in Central Nervous System Demyelination and Remyelination

The dysfunction of myelinating glial cells, the oligodendrocytes, within the central nervous system (CNS) can result in the disruption of myelin, the lipid-rich multi-layered membrane structure that surrounds most vertebrate axons. This leads to axonal degeneration and motor/cognitive impairments. In response to demyelination in the CNS, the formation of new myelin sheaths occurs through the homeostatic process of remyelination, facilitated by the differentiation of newly formed oligodendrocytes. Apart from oligodendrocytes, the two other main glial cell types of the CNS, microglia and astrocytes, play a pivotal role in remyelination. Following a demyelination insult, microglia can phagocytose myelin debris, thus permitting remyelination, while the developing neuroinflammation in the demyelinated region triggers the activation of astrocytes. Modulating the profile of glial cells can enhance the likelihood of successful remyelination. In this context, recent studies have implicated autophagy as a pivotal pathway in glial cells, playing a significant role in both their maturation and the maintenance of myelin. In this Review, we examine the role of substances capable of modulating the autophagic machinery within the myelinating glial cells of the CNS. Such substances, called caloric restriction mimetics, have been shown to decelerate the aging process by mitigating age-related ailments, with their mechanisms of action intricately linked to the induction of autophagic processes.

Mertk-expressing microglia influence oligodendrogenesis and myelin modelling in the CNS

BACKGROUND

Microglia, an immune cell found exclusively within the CNS, initially develop from haematopoietic stem cell precursors in the yolk sac and colonise all regions of the CNS early in development. Microglia have been demonstrated to play an important role in the development of oligodendrocytes, the myelin producing cells in the CNS, as well as in myelination. Mertk is a receptor expressed on microglia that mediates immunoregulatory functions, including myelin efferocytosis.

FINDINGS

Here we demonstrate an unexpected role for Mertk-expressing microglia in both oligodendrogenesis and myelination. The selective depletion of Mertk from microglia resulted in reduced oligodendrocyte production in early development and the generation of pathological myelin. During demyelination, mice deficient in microglial Mertk had thinner myelin and showed signs of impaired OPC differentiation. We established that Mertk signalling inhibition impairs oligodendrocyte repopulation in Xenopus tadpoles following demyelination.

CONCLUSION

These data highlight the importance of microglia in myelination and are the first to identify Mertk as a regulator of oligodendrogenesis and myelin ultrastructure.

Lipofuscin-like autofluorescence within microglia and its impact on studying microglial engulfment

Engulfment of cellular material and proteins is a key function for microglia, a resident macrophage of the central nervous system (CNS). Among the techniques used to measure microglial engulfment, confocal light microscopy has been used the most extensively. Here, we show that autofluorescence (AF) likely due to lipofuscin (lipo-AF) and typically associated with aging, can also be detected within microglial lysosomes in the young mouse brain by light microscopy. This lipo-AF signal accumulates first within microglia and it occurs earliest in white versus gray matter. Importantly, in gray matter, lipo-AF signal can confound the interpretation of antibody-labeled synaptic material within microglia in young adult mice. We further show that there is an age-dependent accumulation of lipo-AF inside and outside of microglia, which is not affected by amyloid plaques. We finally implement a robust and cost-effective strategy to quench AF in mouse, marmoset, and human brain tissue.

Dicer deficiency in microglia leads to accelerated demyelination and failed remyelination

Microglia are the resident immune cells of the central nervous system (CNS) and are important regulators of normal brain functions. In CNS demyelinating diseases like multiple sclerosis (MS), the functions of these cells are of particular interest. Here we probed the impact of microRNA (miRNA)-mediated post-transcriptional gene regulation using a mouse model lacking microglia/macrophage-specific Dicer expression during demyelination and remyelination. Conditional Dicer ablation and loss of miRNAs in adult microglia led to extensive demyelination and impaired myelin processing. Interestingly, demyelination was accompanied by increased apoptosis of mature oligodendrocytes (OLs) and arresting OL progenitor cells (OPCs) in the precursor stage. At the transcriptional level, Dicer -deficient microglia led to downregulation of microglial homeostatic genes, increased cell proliferation, and a shift towards a disease-associated phenotype. Loss of remyelination efficiency in these mice was accompanied by stalling of OPCs in the precursor stage. Collectively, these results highlight a new role of microglial miRNAs in promoting a pro-regenerative phenotype in addition to promoting OPC maturation and differentiation during demyelination and remyelination.

The Involvement of Neuroinflammation in the Onset and Progression of Parkinson's Disease

Parkinson's disease is a neurodegenerative disease exhibiting the fastest growth in incidence in recent years. As with most neurodegenerative diseases, the pathophysiology is incompletely elucidated, but compelling evidence implicates inflammation, both in the central nervous system and in the periphery, in the initiation and progression of the disease, although it is not yet clear what triggers this inflammatory response and where it begins. Gut dysbiosis seems to be a likely candidate for the initiation of the systemic inflammation. The therapies in current use provide only symptomatic relief, but do not interfere with the disease progression. Nonetheless, animal models have shown promising results with therapies that target various vicious neuroinflammatory cascades. Translating these therapeutic strategies into clinical trials is still in its infancy, and a series of issues, such as the exact timing, identifying biomarkers able to identify Parkinson's disease in early and pre-symptomatic stages, or the proper indications of genetic testing in the population at large, will need to be settled in future guidelines.

Contribution of Oligodendrocytes, Microglia, and Astrocytes to Myelin Debris Uptake in an Explant Model of Inflammatory Demyelination in Rats

The internalization and degradation of myelin in glia contributes to the resolution of neuroinflammation and influences disease progression. The identification of a three-dimensional experimental model to study myelin processing under neuroinflammation will offer a novel approach for studying treatment strategies favoring inflammation resolution and neuroprotection. Here, by using a model of neuroinflammation in hippocampal explants, we show that myelin debris accumulated immediately after insult and declined at 3 days, a time point at which tentative repair processes were observed. Olig2+ oligodendrocytes upregulated the LRP1 receptor and progressively increased MBP immunoreactivity both at peri-membrane sites and within the cytosol. Oligodendrocyte NG2+ precursors increased in number and immunoreactivity one day after insult, and moderately internalized MBP particles. Three days after insult MBP was intensely coexpressed by microglia and, to a much lesser extent, by astrocytes. The engulfment of both MBP+ debris and whole MBP+ cells contributed to the greatest microglia response. In addition to improving our understanding of the spatial-temporal contribution of glial scarring to myelin uptake under neuroinflammation, our findings suggest that the exposure of hippocampal explants to LPS + IFN-γ-induced neuroinflammation may represent a valuable demyelination model for studying both the extrinsic and intrinsic myelin processing by glia under neuroinflammation.

Nicotinamide enhances myelin production after demyelination through reduction of astrogliosis and microgliosis

Caloric restriction is the chronic reduction of total caloric intake without malnutrition and has attracted a lot of attention as, among multiple other effects, it attenuates demyelination and stimulates remyelination. In this study we have evaluated the effect of nicotinamide (NAM), a well-known caloric restriction mimetic, on myelin production upon demyelinating conditions. NAM is the derivative of nicotinic acid (vitamin B3) and a precursor of nicotinamide adenine dinucleotide (NAD+), a ubiquitous metabolic cofactor. Here, we use cortical slices ex vivo subjected to demyelination or cultured upon normal conditions, a lysolecithin (LPC)-induced focal demyelination mouse model as well as primary glial cultures. Our data show that NAM enhances both myelination and remyelination ex vivo, while it also induces myelin production after LPC-induced focal demyelination ex vivo and in vivo. The increased myelin production is accompanied by reduction in both astrogliosis and microgliosis in vivo. There is no direct effect of NAM on the oligodendrocyte lineage, as no differences are observed in oligodendrocyte precursor cell proliferation or differentiation or in the number of mature oligodendrocytes. On the other hand, NAM affects both microglia and astrocytes as it decreases the population of M1-activated microglia, while reducing the pro-inflammatory phenotype of astrocytes as assayed by the reduction of TNF-α. Overall, we show that the increased myelin production that follows NAM treatment in vivo is accompanied by a decrease in both astrocyte and microglia accumulation at the lesion site. Our data indicate that NAM influences astrocytes and microglia directly, in favor of the remyelination process by promoting a less inflammatory environment.

Wobbly hedgehog syndrome- a progressive neurodegenerative disease

Wobbly hedgehog syndrome (WHS) has been long considered to be a myelin disease primarily affecting the four-toed hedgehog. In this study, we have shown for the first time that demyelination is accompanied by extensive remyelination in WHS. However, remyelination is not enough to compensate for the axonal degeneration and neuronal loss, resulting in a progressive neurodegenerative disease reminiscent of progressive forms of multiple sclerosis (MS) in humans. Thus, understanding the pathological features of WHS may shed light on the disease progression in progressive MS and ultimately help to develop therapeutic strategies for both diseases.

Highlights

Wobbly hedgehog syndrome (WHS) is a progressive neurodegenerative disease.Spongy degeneration of the brain and spinal cord is the diagnostic feature of WHS.WHS affected brain and spinal cord show extensive demyelination and remyelination.Axonal degeneration is accompanied by loss of neurons in WHS.

Oligodendrocyte calcium signaling sculpts myelin sheath morphology

Myelin is essential for rapid nerve signaling and is increasingly found to play important roles in learning and in diverse diseases of the CNS. Morphological parameters of myelin such as sheath length and thickness are regulated by neuronal activity and can precisely tune conduction velocity, but the mechanisms controlling sheath morphology are poorly understood. Local calcium signaling has been observed in nascent myelin sheaths and can be modulated by neuronal activity. However, the role of calcium signaling in sheath formation and remodeling is unknown. Here, we used genetic tools to attenuate oligodendrocyte calcium signaling during active myelination in the developing mouse CNS. Surprisingly, we found that genetic calcium attenuation did not grossly affect the number of myelinated axons or myelin thickness. Instead, calcium attenuation caused striking myelination defects resulting in shorter, dysmorphic sheaths. Mechanistically, calcium attenuation reduced actin filaments in oligodendrocytes, and an intact actin cytoskeleton was necessary and sufficient to achieve accurate myelin morphology. Together, our work reveals a novel cellular mechanism required for accurate CNS myelin formation and provides mechanistic insight into how oligodendrocytes may respond to neuronal activity to sculpt myelin sheaths throughout the nervous system.

Enhancing axonal myelination in seniors: A review exploring the potential impact cannabis has on myelination in the aged brain

Consumption of cannabis is on the rise as public opinion trends toward acceptance and its consequent legalization. Specifically, the senior population is one of the demographics increasing their use of cannabis the fastest, but research aimed at understanding cannabis' impact on the aged brain is still scarce. Aging is characterized by many brain changes that slowly alter cognitive ability. One process that is greatly impacted during aging is axonal myelination. The slow degradation and loss of myelin (i.e., demyelination) in the brain with age has been shown to associate with cognitive decline and, furthermore, is a common characteristic of numerous neurological diseases experienced in aging. It is currently not known what causes this age-dependent degradation, but it is likely due to numerous confounding factors (i.e., heightened inflammation, reduced blood flow, cellular senescence) that impact the many cells responsible for maintaining overall homeostasis and myelin integrity. Importantly, animal studies using non-human primates and rodents have also revealed demyelination with age, providing a reliable model for researchers to try and understand the cellular mechanisms at play. In rodents, cannabis was recently shown to modulate the myelination process. Furthermore, studies looking at the direct modulatory impact cannabis has on microglia, astrocytes and oligodendrocyte lineage cells hint at potential mechanisms to prevent some of the more damaging activities performed by these cells that contribute to demyelination in aging. However, research focusing on how cannabis impacts myelination in the aged brain is lacking. Therefore, this review will explore the evidence thus far accumulated to show how cannabis impacts myelination and will extrapolate what this knowledge may mean for the aged brain.