TREM2 activation on microglia promotes myelin debris clearance and remyelination in a model of multiple sclerosis

NINJ1: A new player in multiple sclerosis pathogenesis and potential therapeutic target

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS) characterized by demyelination. Current treatment options for MS focus on immunosuppression, but their efficacy can be limited. Recent studies suggest a potential role for nerve injury-induced protein 1 (NINJ1) in MS pathogenesis. NINJ1, a protein involved in cell death and inflammation, may contribute to the infiltration and activation of inflammatory cells in the CNS, potentially through enhanced blood-brain barrier crossing; enhancing plasma membrane rupture during cell death, leading to the release of inflammatory mediators and further tissue damage. This review explores the emerging evidence for NINJ1's involvement in MS. It discusses how NINJ1 might mediate the migration of immune cells across the blood-brain barrier, exacerbate neuroinflammation, and participate in plasma membrane rupture-related damage. Finally, the review examines potential therapeutic strategies targeting NINJ1 for improved MS management. Abbreviations: MS, Multiple sclerosis; CNS, Central nervous system; BBB, Blood-brain barrier; GSDMD, Gasdermin-D; EAE, Experimental autoimmune encephalitis; HMGB-1, High mobility group box-1 protein; LDH, Lactate dehydrogenase; PMR, Plasma membrane rupture; DMF, Dimethyl fumarate; DUSP1, Dual-specificity phosphatase 1; PAMPs, Pathogen-associated molecular patterns; DAMPs, Danger-associated molecular patterns; PRRs, Pattern recognition receptors; GM-CSF, Granulocyte-macrophage colony stimulating factor; IFN-γ, Interferon gamma; TNF, Tumor necrosis factor; APCs, Antigen-presenting cells; ECs, Endothelial cells; TGF-β, Transforming growth factor-β; PBMCs, Peripheral blood mononuclear cells; FACS, Fluorescence-activated cell sorting; MCP-1, Monocyte chemoattractant protein-1; NLRP3, Pyrin domain-containing 3; TCR, T cell receptor; ROS, Reactive oxygen species; AP-1, Activator protein-1; ANG1, Angiopoietin 1; BMDMs, Bone marrow-derived macrophages; Arp2/3, actin-related protein 2/3; EMT, epithelial-mesenchymal transition; FAK, focal adhesion kinase; LIMK1, LIM domain kinase 1; PAK1, p21-activated kinases 1; Rac1, Ras-related C3 botulinum toxin substrate 1; β-cat, β-caten; MyD88, myeloid differentiation primary response gene 88; TIRAP, Toll/interleukin-1 receptor domain-containing adapter protein; TLR4, Toll-like receptor 4; IRAKs, interleukin-1 receptor-associated kinases; TRAF6, TNF receptor associated factor 6; TAB2/3, TAK1 binding protein 2/3; TAK1, transforming growth factor-β-activated kinase 1; JNK, c-Jun N-terminal kinase; ERK1/2, Extracellular Signal Regulated Kinase 1/2; IKK, inhibitor of kappa B kinase; IκB, inhibitor of NF-κB; NF-κB, nuclear factor kappa-B; AP-1, activator protein-1; ASC, Apoptosis-associated Speck-like protein containing a CARD; NEK7, NIMA-related kinase 7; NLRP3, Pyrin domain-containing 3; CREB, cAMP response element-binding protein.

Galectin-3 alleviates demyelination by modulating microglial anti-inflammatory polarization through PPARγ-CD36 axis

Demyelination is characterized by disruption of myelin sheath and disorders in myelin formation. Currently, there are no effective therapeutic treatments available. Microglia, especially anti-inflammatory phenotype microglia are critical for remyelination. Galectin-3 (Gal-3), which is known to modulate microglia activation, is correlated with myelination. In this study, we aimed to elucidate the roles of Gal-3 during myelin formation and explore the efficiency and mechanism of rGal-3 administration in remyelination. We enrolled Gal-3 knockout (Lgals3 KO) mice and demonstrated Lgals3 KO causes demyelination during spontaneous myelinogenesis. We performed a cuprizone (CPZ) intoxication model and found Lgals3 KO aggravates demyelinated lesions and favors microglial pro-inflammatory phenotype polarization. Recombinant Gal-3 (rGal-3) administration alleviates CPZ intoxication and drives microglial towards anti-inflammatory phenotype. Additionally, RNA sequencing results reveal the correlation between Gal-3 and the PPARγ-CD36 axis. Thus, we performed SSO and GW9662 administration to inhibit the activation of the PPARγ-CD36 axis and found that rGal-3 administration modulates microglial phenotype polarization by regulating the PPARγ-CD36 axis. Together, our findings highlight the importance of Gal-3 in myelination and provide insights into rGal-3 administration for modulating microglial anti-inflammatory phenotype polarization through the PPARγ-CD36 axis.

CX3CR1+/UCHL1+ microglial extracellular vesicles in blood: a potential biomarker for multiple sclerosis

In neuroinflammation, distinguishing microglia from macrophages and identifying microglial-specific biomarkers in peripheral blood pose significant challenges. This study comprehensively profiled the extracellular vesicles (EVs) of microglia and macrophages, respectively, revealing co-expressed EVs with UCHL1 and CX3CR1 as EVs derived specifically from microglia in human blood. After extensive validation, using optimized nano flow cytometry, we evaluated plasma CX3CR1+/UCHL1+ EVs across clinical cohorts [multiple sclerosis (MS), HTLV-1 associated myelopathy (HAM), Alzheimer's disease (AD), and Parkinson's disease (PD)], along with established neurodegenerative markers (NMDAR2A and NFL). The findings discovered a notable rise in CX3CR1+/UCHL1+ EVs in MS, particularly heightened in HAM, in contrast to controls. Conversely, AD and PD exhibited unaltered or diminished levels of microglial EVs. An integrated model of CX3CR1+/UCHL1+, NMDAR2A+, and NFL+ EVs demonstrated promising diagnostic potential for distinguishing MS from controls and HAM. As to the disease duration, CX3CR1+/UCHL1+ EVs increased in the initial five years of MS, stabilizing thereafter, whereas NMDAR2A+ and NFL+ EVs remained stable initially but increased significantly in the subsequent five years, suggesting their correlation with disease duration. This study uncovers unique blood microglial EVs with potential as biomarkers for MS diagnosis, differentiation from HAM, and correlation with disease duration.

Microglia and TREM2

TREM2 is a membrane receptor solely expressed on microglia in normal brain. In this review we outline recent advances in TREM2 biology and its implications for microglial function, with particular emphasis on findings from iPSC-derived microglia (iMG) expressing TREM2 loss-of-function mutations. Alterations in receptor proximal and distal signalling underlie TREM2 risk variants linked to neurodegenerative disease, principally NH-linked FTD, and late-onset AD, but emerging data suggest roles for TREM2 in PD, MS and ALS. TREM2 downstream functions include phagocytosis of myelin debris, amyloid beta peptides, and phosphatidylserine-expressing cells (resulting from damage or stress). Microglial survival, migration, DAMP signalling, inflammasome activation, and intercellular signalling including tau spreading via exosomes, as well as roles for sTREM2 in protection and as a biomarker are discussed. The role of TREM2 in metabolic homeostasis, and immunometabolic switching are discussed regarding microglial responses to damage and protection. The use of iPSC models to investigate the role of TREM2 in AD, PD, MS, ALS, and other neurodegenerative diseases could prove invaluable due to their ability to recapitulate human pathology, allowing a full understanding of TREM2 and microglial involvement in the underlying disease mechanisms and progression. This article is part of the Special Issue on "Microglia".

Mononuclear phagocytes in autoimmune neuroinflammation

A healthy mammalian central nervous system (CNS) harbors a diverse population of leukocytes including members of the mononuclear phagocyte system (MPS). Exerting their specific functions, CNS tissue-resident macrophages as well as associated monocytes and dendritic cells (DCs) maintain CNS homeostasis. Under neuroinflammatory conditions, leukocytes from the systemic immune compartment invade the CNS. This review focuses on the newly discovered roles of the MPS in autoimmune neuroinflammation elicited by encephalitogenic T cells. We propose that CNS-associated DCs act as gatekeepers and antigen-presenting cells that guide the adaptive immune response while bone marrow (BM)-derived monocytes contribute to immunopathology and tissue damage. By contrast, CNS-resident macrophages primarily support tissue function and promote the repair and maintenance of CNS functions.

Inflammation in Metal-Induced Neurological Disorders and Neurodegenerative Diseases

Essential metals play critical roles in maintaining human health as they participate in various physiological activities. Nonetheless, both excessive accumulation and deficiency of these metals may result in neurotoxicity secondary to neuroinflammation and the activation of microglia and astrocytes. Activation of these cells can promote the release of pro-inflammatory cytokines. It is well known that neuroinflammation plays a critical role in metal-induced neurotoxicity as well as the development of neurological disorders, such as Alzheimer's disease (AD), Parkinson's disease (PD), and multiple sclerosis (MS). Initially seen as a defense mechanism, persistent inflammatory responses are now considered harmful. Astrocytes and microglia are key regulators of neuroinflammation in the central nervous system, and their excessive activation may induce sustained neuroinflammation. Therefore, in this review, we aim to emphasize the important role and molecular mechanisms underlying metal-induced neurotoxicity. Our objective is to raise the awareness on metal-induced neuroinflammation in neurological disorders. However, it is not only just neuroinflammation that different metals could induce; they can also cause harm to the nervous system through oxidative stress, apoptosis, and autophagy, to name a few. The primary pathophysiological mechanism by which these metals induce neurological disorders remains to be determined. In addition, given the various pathways through which individuals are exposed to metals, it is necessary to also consider the effects of co-exposure to multiple metals on neurological disorders.

Microglial TREM2 promotes phagocytic clearance of damaged neurons after status epilepticus

In the central nervous system, triggering receptor expressed on myeloid cells 2 (TREM2) is exclusively expressed by microglia and is critical for microglial proliferation, migration, and phagocytosis. Microglial TREM2 plays an important role in neurodegenerative diseases, such as Alzheimer's disease and amyotrophic lateral sclerosis. However, little is known about how TREM2 affects microglial function within epileptogenesis. To investigate this, we utilized male TREM2 knockout (KO) mice within the intra-amygdala kainic acid seizure model. Electroencephalographic analysis, immunocytochemistry, and RNA sequencing revealed that TREM2 deficiency significantly promoted seizure-induced pathology. We found that TREM2 KO increased both the severity of acute status epilepticus and the number of spontaneous recurrent seizures characteristic of chronic focal epilepsy. Phagocytic clearance of damaged neurons by microglia was also impaired by TREM2 KO and reduced phagocytic activity correlated with increased spontaneous seizures. Analysis of human tissue from patients who underwent surgical resection for drug resistant temporal lobe epilepsy also showed a negative correlation between expression of the microglial phagocytic marker CD68 and focal to bilateral tonic-clonic generalized seizure history. These results indicate that microglial TREM2 and phagocytic activity are important to epileptogenic pathology.

Trem2-deficiency aggravates and accelerates age-related myelin degeneration

Aging is the greatest known risk factor for most neurodegenerative diseases. Myelin degeneration is an early pathological indicator of these diseases and a normal part of aging; albeit, to a lesser extent. Despite this, little is known about the contribution of age-related myelin degeneration on neurodegenerative disease. Microglia participate in modulating white matter events from demyelination to remyelination, including regulation of (de)myelination by the microglial innate immune receptor triggering receptor expressed on myeloid cells 2 (TREM2). Here, we demonstrate Trem2-deficiency aggravates and accelerates age-related myelin degeneration in the striatum. We show TREM2 is necessary for remyelination by recruiting reparative glia and mediating signaling that promotes OPC differentiation/maturation. In response to demyelination, TREM2 is required for phagocytosis of large volumes of myelin debris. In addition to lysosomal regulation, we show TREM2 can modify the ER stress response, even prior to overt myelin debris, that prevents lipid accumulation and microglial dysfunction. These data support a role for Trem2-dependent interactions in age-related myelin degeneration and suggest a basis for how early dysfunctional microglia could contribute to disease pathology through insufficent repair, defective phagocytosis, and the ER stress response.

Repeated trans-spinal magnetic stimulation promotes microglial phagocytosis of myelin debris after spinal cord injury through LRP-1

Spinal cord injury (SCI) is a serious trauma of the central nervous system. The clearance of myelin debris is a critical step in the functional recovery following spinal cord injury (SCI). Recent studies have begun to reveal critical roles for professional phagocytes in the central nervous system, microglia, and their receptors in the control of myelin debris in neurodegenerative disease. Repeated trans-spinal magnetic stimulation (rTSMS) has been demonstrated as a noninvasive SCI treatment that enhances tissue repair and functional recovery. In this study, we investigated the role and molecular mechanism of rTSMS on microglial phagocytosis of myelin debris in a rat SCI model. In our studies, we found that rTSMS significantly promoted the motor function recovery of SCI rats associated with the inhibition the neuroinflammation and glia scar formation. Immunofluorescence results further showed that the rTSMS promotes the clearance of myelin debris by microglia in vivo and in vitro. Additionally, receptor-associated protein (RAP), a Low-density lipoprotein receptor-related protein-1 (LRP-1) inhibitor, could cancel the accelerated microglial phagocytosis of myelin debris after rTSMS in vitro experiments. Simultaneously, Elisa's results and western blotting respectively showed that rTSMS significantly decreased the levels of soluble LRP-1(sLRP-1) and the LRP-1 splicing enzyme of ADAM17. In conclusion, rTSMS could promote the clearance of myelin debris by microglia through LRP-1 to improve the functional recovery of SCI rats.

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.

Montelukast improves disease outcome in SOD1G93A female mice by counteracting oligodendrocyte dysfunction and aberrant glial reactivity

BACKGROUND AND PURPOSE

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder characterized by progressive motor neuron (MN) loss and consequent muscle atrophy, for which no effective therapies are available. Recent findings reveal that disease progression is fuelled by early aberrant neuroinflammation and the loss of oligodendrocytes with neuroprotective and remyelinating properties. On this basis, pharmacological interventions capable of restoring a pro-regenerative local milieu and re-establish proper oligodendrocyte functions may be beneficial.

EXPERIMENTAL APPROACH

Here, we evaluated the in vivo therapeutic effects of montelukast (MTK), an antagonist of the oligodendroglial G protein-coupled receptor 17 (GPR17) and of cysteinyl-leukotriene receptor 1 (CysLT1R) receptors on microglia and astrocytes, in the SOD1G93A ALS mouse model. We chronically treated SOD1G93A mice with MTK, starting from the early symptomatic disease stage. Disease progression was assessed by behavioural and immunohistochemical approaches.

KEY RESULTS

Oral MTK treatment significantly extended survival probability, delayed body weight loss and ameliorated motor functionalityonly in female SOD1G93A mice. Noteworthy, MTK significantly restored oligodendrocyte maturation and induced significant changes in the reactive phenotype and morphological features of microglia/macrophages and astrocytes in the spinal cord of female SOD1G93A mice, suggesting enhanced pro-regenerative functions. Importantly, concomitant MN preservation has been detected after MTK administration. No beneficial effects were observed in male mice, highlighting a sex-based difference in the protective activity of MTK.

CONCLUSIONS AND IMPLICATIONS

Our results provide the first preclinical evidence indicating that repurposing of MTK, a safe and marketed anti-asthmatic drug, may be a promising sex-specific strategy for personalized ALS treatment.

Neurotropic murine coronavirus mediated demyelination: Factors dampening pathogenesis

Virus infections and autoimmune responses are implicated as primary triggers of demyelinating diseases. Specifically, the association of Epstein-Barr virus (EBV) infection with development of multiple sclerosis (MS) has re-ignited an interest in virus induced autoimmune responses to CNS antigens. Nevertheless, demyelination may also be caused by immune mediated bystander pathology in an attempt to control direct infection in the CNS. Tissue damage as a result of anti-viral responses or low level viral persistence may lead to immune activation manifesting in demyelinating lesions, axonal damage and clinical symptoms. This review focuses on the neurotropic mouse coronavirus induced demyelination model to highlight how immune responses activated during the acute phase pave the way to dampen pathology and promote repair. We specifically discuss the role of immune dampening factors programmed cell death ligand 1 (PD-L1) and interleukin (IL)-10, as well as microglia and triggering receptor expressed on myeloid cells 2 (Trem2), in limiting demyelination independent of viral persistence.

Differential Expression of PACAP/VIP Receptors in the Post-Mortem CNS White Matter of Multiple Sclerosis Donors

Pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal peptide (VIP) are two neuroprotective and anti-inflammatory molecules of the central nervous system (CNS). Both bind to three G protein-coupled receptors, namely PAC1, VPAC1 and VPAC2, to elicit their beneficial effects in various CNS diseases, including multiple sclerosis (MS). In this study, we assessed the expression and distribution of PACAP/VIP receptors in the normal-appearing white matter (NAWM) of MS donors with a clinical history of either relapsing-remitting MS (RRMS), primary MS (PPMS), secondary progressive MS (SPMS) or in aged-matched non-MS controls. Gene expression studies revealed MS-subtype specific changes in PACAP and VIP and in the receptors' levels in the NAWM, which were partly corroborated by immunohistochemical analyses. Most PAC1 immunoreactivity was restricted to myelin-producing cells, whereas VPAC1 reactivity was diffused within the neuropil and in axonal bundles, and VPAC2 in small vessel walls. Within and around lesioned areas, glial cells were the predominant populations showing reactivity for the different PACAP/VIP receptors, with distinctive patterns across MS subtypes. Together, these data identify the differential expression patterns of PACAP/VIP receptors among the different MS clinical entities. These results may offer opportunities for the development of personalized therapeutic approaches to treating MS and/or other demyelinating disorders.

Interplay between microglia and environmental risk factors in Alzheimer's disease

Alzheimer's disease, among the most common neurodegenerative disorders, is characterized by progressive cognitive impairment. At present, the Alzheimer's disease main risk remains genetic risks, but major environmental factors are increasingly shown to impact Alzheimer's disease development and progression. Microglia, the most important brain immune cells, play a central role in Alzheimer's disease pathogenesis and are considered environmental and lifestyle "sensors." Factors like environmental pollution and modern lifestyles (e.g., chronic stress, poor dietary habits, sleep, and circadian rhythm disorders) can cause neuroinflammatory responses that lead to cognitive impairment via microglial functioning and phenotypic regulation. However, the specific mechanisms underlying interactions among these factors and microglia in Alzheimer's disease are unclear. Herein, we: discuss the biological effects of air pollution, chronic stress, gut microbiota, sleep patterns, physical exercise, cigarette smoking, and caffeine consumption on microglia; consider how unhealthy lifestyle factors influence individual susceptibility to Alzheimer's disease; and present the neuroprotective effects of a healthy lifestyle. Toward intervening and controlling these environmental risk factors at an early Alzheimer's disease stage, understanding the role of microglia in Alzheimer's disease development, and targeting strategies to target microglia, could be essential to future Alzheimer's disease treatments.

V-ATPase B2 promotes microglial phagocytosis of myelin debris by inactivating the MAPK signaling pathway

Microglial phagocytosis of myelin debris is a crucial process for promoting myelin regeneration in conditions such as multiple sclerosis (MS). Vacuolar-ATPase B2 (V-ATPase B2) has been implicated in various cellular processes, but its role in microglial phagocytosis and its potential impact on MS-related responses remain unclear. In this study, we employed BV-2 murine microglial cells to investigate the influence of V-ATPase B2 on the phagocytosis of myelin debris by microglia. The results revealed that V-ATPase B2 expression increased in response to myelin debris exposure. Overexpression of V-ATPase B2 significantly enhanced BV-2 phagocytosis of myelin debris. Additionally, V-ATPase B2 overexpression shifted microglial polarization towards an anti-inflammatory M2 phenotype, coupled with decreased lysosomal pH and enhanced lysosome degradation capacity. Moreover, endoplasmic reticulum (ER) stress inhibitor, 4-PBA, reversed the effects of V-ATPase B2 silencing on ER stress, M2 polarization, and lysosomal degradation of BV-2 cells. The MAPK pathway was inhibited upon V-ATPase B2 overexpression, contributing to heightened myelin debris clearance by BV-2 cells. Notably, MAPK pathway inhibition partially attenuated the inhibitory effects of V-ATPase B2 knockdown on myelin debris clearance. In conclusion, our findings reveal a pivotal role for V-ATPase B2 in promoting microglial phagocytosis of myelin debris by regulating microglial polarization and lysosomal function via the MAPK signaling pathway, suggesting that targeting V-ATPase B2 may hold therapeutic potential for enhancing myelin debris clearance and modulating microglial responses in MS and related neuroinflammatory disorders.

Microglia and Astrocytes in Alzheimer's Disease: Significance and Summary of Recent Advances

Alzheimer's disease, one of the most common forms of dementia, is characterized by a slow progression of cognitive impairment and neuronal loss. Currently, approved treatments for AD are hindered by various side effects and limited efficacy. Despite considerable research, practical treatments for AD have not been developed. Increasing evidence shows that glial cells, especially microglia and astrocytes, are essential in the initiation and progression of AD. During AD progression, activated resident microglia increases the ability of resting astrocytes to transform into reactive astrocytes, promoting neurodegeneration. Extensive clinical and molecular studies show the involvement of microglia and astrocyte-mediated neuroinflammation in AD pathology, indicating that microglia and astrocytes may be potential therapeutic targets for AD. This review will summarize the significant and recent advances of microglia and astrocytes in the pathogenesis of AD in three parts. First, we will review the typical pathological changes of AD and discuss microglia and astrocytes in terms of function and phenotypic changes. Second, we will describe microglia and astrocytes' physiological and pathological role in AD. These roles include the inflammatory response, "eat me" and "don't eat me" signals, Aβ seeding, propagation, clearance, synapse loss, synaptic pruning, remyelination, and demyelination. Last, we will review the pharmacological and non-pharmacological therapies targeting microglia and astrocytes in AD. We conclude that microglia and astrocytes are essential in the initiation and development of AD. Therefore, understanding the new role of microglia and astrocytes in AD progression is critical for future AD studies and clinical trials. Moreover, pharmacological, and non-pharmacological therapies targeting microglia and astrocytes, with specific studies investigating microglia and astrocyte-mediated neuronal damage and repair, may be a promising research direction for future studies regarding AD treatment and prevention.

Myelin debris phagocytosis in demyelinating disease

Demyelinating diseases are often caused by a variety of triggers, including immune responses, viral infections, malnutrition, hypoxia, or genetic factors, all of which result in the loss of myelin in the nervous system. The accumulation of myelin debris at the lesion site leads to neuroinflammation and inhibits remyelination; therefore, it is crucial to promptly remove the myelin debris. Initially, Fc and complement receptors on cellular surfaces were the primary clearance receptors responsible for removing myelin debris. However, subsequent studies have unveiled the involvement of additional receptors, including Mac-2, TAM receptors, and the low-density lipoprotein receptor-related protein 1, in facilitating the removal process. In addition to microglia and macrophages, which serve as the primary effector cells in the disease phase, a variety of other cell types such as astrocytes, Schwann cells, and vascular endothelial cells have been demonstrated to engage in the phagocytosis of myelin debris. Furthermore, we have concluded that oligodendrocyte precursor cells, as myelination precursor cells, also exhibit this phagocytic capability. Moreover, our research group has innovatively identified the low-density lipoprotein receptor as a potential phagocytic receptor for myelin debris. In this article, we discuss the functional processes of various phagocytes in demyelinating diseases. We also highlight the alterations in signaling pathways triggered by phagocytosis, and provide a comprehensive overview of the various phagocytic receptors involved. Such insights are invaluable for pinpointing potential therapeutic strategies for the treatment of demyelinating diseases by targeting phagocytosis.

Neutral or Detrimental Effects of TREM2 Agonist Antibodies in Preclinical Models of Alzheimer's Disease and Multiple Sclerosis

Human genetics and preclinical studies have identified key contributions of TREM2 to several neurodegenerative conditions, inspiring efforts to modulate TREM2 therapeutically. Here, we characterize the activities of three TREM2 agonist antibodies in multiple mixed-sex mouse models of Alzheimer's disease (AD) pathology and remyelination. Receptor activation and downstream signaling are explored in vitro, and active dose ranges are determined in vivo based on pharmacodynamic responses from microglia. For mice bearing amyloid-β (Aβ) pathology (PS2APP) or combined Aβ and tau pathology (TauPS2APP), chronic TREM2 agonist antibody treatment had limited impact on microglia engagement with pathology, overall pathology burden, or downstream neuronal damage. For mice with demyelinating injuries triggered acutely with lysolecithin, TREM2 agonist antibodies unexpectedly disrupted injury resolution. Likewise, TREM2 agonist antibodies limited myelin recovery for mice experiencing chronic demyelination from cuprizone. We highlight the contributions of dose timing and frequency across models. These results introduce important considerations for future TREM2-targeting approaches.

Intrinsic organization of the corpus callosum

The corpus callosum-the largest commissural fiber system connecting the two cerebral hemispheres-is considered essential for bilateral sensory integration and higher cognitive functions. Most studies exploring the corpus callosum have examined either the anatomical, physiological, and neurochemical organization of callosal projections or the functional and/or behavioral aspects of the callosal connections after complete/partial callosotomy or callosal lesion. There are no works that address the intrinsic organization of the corpus callosum. We review the existing information on the activities that take place in the commissure in three sections: I) the topographical and neurochemical organization of the intracallosal fibers, II) the role of glia in the corpus callosum, and III) the role of the intracallosal neurons.

High-fat diet consumption promotes adolescent neurobehavioral abnormalities and hippocampal structural alterations via microglial overactivation accompanied by an elevated serum free fatty acid concentration

Mounting evidence suggests that high-fat diet (HFD) consumption increases the risk for depression, but the neurophysiological mechanisms involved remain to be elucidated. Here, we demonstrated that HFD feeding of C57BL/6J mice during the adolescent period (from 4 to 8 weeks of age) resulted in increased depression- and anxiety-like behaviors concurrent with changes in neuronal and myelin structure in the hippocampus. Additionally, we showed that hippocampal microglia in HFD-fed mice assumed a hyperactive state concomitant with increased PSD95-positive and myelin basic protein (MBP)-positive inclusions, implicating microglia in hippocampal structural alterations induced by HFD consumption. Along with increased levels of serum free fatty acids (FFAs), abnormal deposition of lipid droplets and increased levels of HIF-1α protein (a transcription factor that has been reported to facilitate cellular lipid accumulation) within hippocampal microglia were observed in HFD-fed mice. The use of minocycline, a pharmacological suppressor of microglial overactivation, effectively attenuated neurobehavioral abnormalities and hippocampal structural alterations but barely altered lipid droplet accumulation in the hippocampal microglia of HFD-fed mice. Coadministration of triacsin C abolished the increases in lipid droplet formation, phagocytic activity, and ROS levels in primary microglia treated with serum from HFD-fed mice. In conclusion, our studies demonstrate that the adverse influence of early-life HFD consumption on behavior and hippocampal structure is attributed at least in part to microglial overactivation that is accompanied by an elevated serum FFA concentration and microglial aberrations represent a potential preventive and therapeutic target for HFD-related emotional disorders.

Blocking cGAS-STING pathway promotes post-stroke functional recovery in an extended treatment window via facilitating remyelination

BACKGROUND

Ischemic stroke is a major cause of worldwide death and disability, with recombinant tissue plasminogen activator being the sole effective treatment, albeit with a limited treatment window. The cyclic GMP-AMP synthase (cGAS) and stimulator of interferon genes (STING) pathway is emerging as the major DNA-sensing pathway to invoke immune responses in neuroinflammatory disorders.

METHODS

By performing a series of neurobehavioral assessments, electrophysiological analysis, high-throughput sequencing, and cell-based assays based on the transient middle cerebral artery occlusion (tMCAO) mouse stroke model, we examined the effects and underlying mechanisms of genetic and pharmacological inhibition of the cGAS-STING pathway on long-term post-stroke neurological functional outcomes.

FINDINGS

Blocking the cGAS-STING pathway, even 3 days after tMCAO, significantly promoted functional recovery in terms of white matter structural and functional integrity as well as sensorimotor and cognitive functions. Mechanistically, the neuroprotective effects via inhibiting the cGAS-STING pathway were contributed not only by inflammation repression at the early stage of tMCAO but also by modifying the cell state of phagocytes to facilitate remyelination at the sub-acute phase. The activation of the cGAS-STING pathway significantly impeded post-stroke remyelination through restraining myelin debris uptake and degradation and hindering oligodendrocyte differentiation and maturation.

CONCLUSIONS

Manipulating the cGAS-STING pathway has an extended treatment window in promoting long-term post-stroke functional recovery via facilitating remyelination in a mouse stroke model. Our results highlight the roles of the cGAS-STING pathway in aggregating stroke pathology and propose a new way for improving functional recovery after ischemic stroke.

FUNDING

This work was primarily funded by the National Key R&D Program of China.

Decoding sTREM2: its impact on Alzheimer's disease - a comprehensive review of mechanisms and implications

Soluble Triggering Receptor Expressed on Myeloid Cells 2 (sTREM2) plays a crucial role in the pathogenesis of Alzheimer's disease (AD). This review comprehensively examines sTREM2's involvement in AD, focusing on its regulatory functions in microglial responses, neuroinflammation, and interactions with key pathological processes. We discuss the dynamic changes in sTREM2 levels in cerebrospinal fluid and plasma throughout AD progression, highlighting its potential as a therapeutic target. Furthermore, we explore the impact of genetic variants on sTREM2 expression and its interplay with other AD risk genes. The evidence presented in this review suggests that modulating sTREM2 activity could influence AD trajectory, making it a promising avenue for future research and drug development. By providing a holistic understanding of sTREM2's multifaceted role in AD, this review aims to guide future studies and inspire novel therapeutic strategies.

TREM2 deficiency aggravates renal injury by promoting macrophage apoptosis and polarization via the JAK-STAT pathway in mice

The triggering receptor expressed on myeloid cells 2 (TREM2) is an immune receptor that affects cellular phenotypes by modulating phagocytosis and metabolism, promoting cell survival, and counteracting inflammation. Its role in renal injury, in particular, unilateral ureteral obstruction (UUO) or ischemia-reperfusion injury (IRI)-induced renal injury remains unclear. In our study, WT and Trem2-/- mice were employed to evaluate the role of TREM2 in renal macrophage infiltration and tissue injury after UUO. Bone marrow-derived macrophages (BMDM) from both mouse genotypes were cultured and polarized for in vitro experiments. Next, the effects of TREM2 on renal injury and macrophage polarization in IRI mice were also explored. We found that TREM2 expression was upregulated in the obstructed kidneys. TREM2 deficiency exacerbated renal inflammation and fibrosis 3 and 7 days after UUO, in association with reduced macrophage infiltration. Trem2-/- BMDM exhibited increased apoptosis and poorer survival compared with WT BMDM. Meanwhile, TREM2 deficiency augmented M1 and M2 polarization after UUO. Consistent with the in vivo observations, TREM2 deficiency led to increased polarization of BMDM towards the M1 proinflammatory phenotype. Mechanistically, TREM2 deficiency promoted M1 and M2 polarization via the JAK-STAT pathway in the presence of TGF-β1, thereby affecting cell survival by regulating mTOR signaling. Furthermore, cyclocreatine supplementation alleviated cell death caused by TREM2 deficiency. Additionally, we found that TREM2 deficiency promoted renal injury, fibrosis, and macrophage polarization in IRI mice. The current data suggest that TREM2 deficiency aggravates renal injury by promoting macrophage apoptosis and polarization via the JAK-STAT pathway. These findings have implications for the role of TREM2 in the regulation of renal injury that justify further evaluation.

Microglia phagocytic mechanisms: Development informing disease

Microglia are tissue-resident macrophages and professional phagocytes of the central nervous system (CNS). In development, microglia-mediated phagocytosis is important for sculpting the cellular architecture. This includes the engulfment of dead/dying cells, pruning extranumerary synapses and axons, and phagocytosing fragments of myelin sheaths. Intriguingly, these developmental phagocytic mechanisms by which microglia sculpt the CNS are now appreciated as important for eliminating synapses, myelin, and proteins during neurodegeneration. Here, we discuss parallels between neurodevelopment and neurodegeneration, which highlights how development is informing disease. We further discuss recent advances and challenges towards therapeutically targeting these phagocytic pathways and how we can leverage development to overcome these challenges.

Biomaterial strategies for regulating the neuroinflammatory response

Injury and disease in the central nervous system (CNS) can result in a dysregulated inflammatory environment that inhibits the repair of functional tissue. Biomaterials present a promising approach to tackle this complex inhibitory environment and modulate the mechanisms involved in neuroinflammation to halt the progression of secondary injury and promote the repair of functional tissue. In this review, we will cover recent advances in biomaterial strategies, including nanoparticles, hydrogels, implantable scaffolds, and neural probe coatings, that have been used to modulate the innate immune response to injury and disease within the CNS. The stages of inflammation following CNS injury and the main inflammatory contributors involved in common neurodegenerative diseases will be discussed, as understanding the inflammatory response to injury and disease is critical for identifying therapeutic targets and designing effective biomaterial-based treatment strategies. Biomaterials and novel composites will then be discussed with an emphasis on strategies that deliver immunomodulatory agents or utilize cell-material interactions to modulate inflammation and promote functional tissue repair. We will explore the application of these biomaterial-based strategies in the context of nanoparticle- and hydrogel-mediated delivery of small molecule drugs and therapeutic proteins to inflamed nervous tissue, implantation of hydrogels and scaffolds to modulate immune cell behavior and guide axon elongation, and neural probe coatings to mitigate glial scarring and enhance signaling at the tissue-device interface. Finally, we will present a future outlook on the growing role of biomaterial-based strategies for immunomodulation in regenerative medicine and neuroengineering applications in the CNS.

Microglia, Trem2, and Neurodegeneration

Microglia are a specialized type of neuroimmune cells that undergo morphological and molecular changes through multiple signaling pathways in response to pathological protein aggregates, neuronal death, tissue injury, or infections. Microglia express Trem2, which serves as a receptor for a multitude of ligands enhancing their phagocytic activity. Trem2 has emerged as a critical modulator of microglial activity, especially in many neurodegenerative disorders. Human TREM2 mutations are associated with an increased risk of developing Alzheimer disease (AD) and other neurodegenerative diseases. Trem2 plays dual roles in neuroinflammation and more specifically in disease-associated microglia. Most recent developments on the molecular mechanisms of Trem2, emphasizing its role in uptake and clearance of amyloid β (Aβ) aggregates and other tissue debris to help protect and preserve the brain, are encouraging. Although Trem2 normally stimulates defense mechanisms, its dysregulation can intensify inflammation, which poses major therapeutic challenges. Recent therapeutic approaches targeting Trem2 via agonistic antibodies and gene therapy methodologies present possible avenues for reducing the burden of neurodegenerative diseases. This review highlights the promise of Trem2 as a therapeutic target, especially for Aβ-associated AD, and calls for more mechanistic investigations to understand the context-specific role of microglial Trem2 in developing effective therapies against neurodegenerative diseases.

Interleukin-9 protects from microglia- and TNF-mediated synaptotoxicity in experimental multiple sclerosis

BACKGROUND

Multiple sclerosis (MS) is a progressive neurodegenerative disease of the central nervous system characterized by inflammation-driven synaptic abnormalities. Interleukin-9 (IL-9) is emerging as a pleiotropic cytokine involved in MS pathophysiology.

METHODS

Through biochemical, immunohistochemical, and electrophysiological experiments, we investigated the effects of both peripheral and central administration of IL-9 on C57/BL6 female mice with experimental autoimmune encephalomyelitis (EAE), a model of MS.

RESULTS

We demonstrated that both systemic and local administration of IL-9 significantly improved clinical disability, reduced neuroinflammation, and mitigated synaptic damage in EAE. The results unveil an unrecognized central effect of IL-9 against microglia- and TNF-mediated neuronal excitotoxicity. Two main mechanisms emerged: first, IL-9 modulated microglial inflammatory activity by enhancing the expression of the triggering receptor expressed on myeloid cells-2 (TREM2) and reducing TNF release. Second, IL-9 suppressed neuronal TNF signaling, thereby blocking its synaptotoxic effects.

CONCLUSIONS

The data presented in this work highlight IL-9 as a critical neuroprotective molecule capable of interfering with inflammatory synaptopathy in EAE. These findings open new avenues for treatments targeting the neurodegenerative damage associated with MS, as well as other inflammatory and neurodegenerative disorders of the central nervous system.

Early adversity causes sex-specific deficits in perforant pathway connectivity and contextual memory in adolescent mice

BACKGROUND

Early life adversity impairs hippocampal development and function across diverse species. While initial evidence indicated potential variations between males and females, further research is required to validate these observations and better understand the underlying mechanisms contributing to these sex differences. Furthermore, most of the preclinical work in rodents was performed in adult males, with only few studies examining sex differences during adolescence when such differences appear more pronounced. To address these concerns, we investigated the impact of limited bedding (LB), a mouse model of early adversity, on hippocampal development in prepubescent and adolescent male and female mice.

METHODS

RNA sequencing, confocal microscopy, and electron microscopy were used to evaluate the impact of LB and sex on hippocampal development in prepubescent postnatal day 17 (P17) mice. Additional studies were conducted on adolescent mice aged P29-36, which included contextual fear conditioning, retrograde tracing, and ex vivo diffusion magnetic resonance imaging (dMRI).

RESULTS

More severe deficits in axonal innervation and myelination were found in the perforant pathway of prepubescent and adolescent LB males compared to LB female littermates. These sex differences were due to a failure of reelin-positive neurons located in the lateral entorhinal cortex (LEC) to innervate the dorsal hippocampus via the perforant pathway in males, but not LB females, and were strongly correlated with deficits in contextual fear conditioning.

CONCLUSIONS

LB impairs the capacity of reelin-positive cells located in the LEC to project and innervate the dorsal hippocampus in LB males but not female LB littermates. Given the critical role that these projections play in supporting normal hippocampal function, a failure to establish proper connectivity between the LEC and the dorsal hippocampus provides a compelling and novel mechanism to explain the more severe deficits in myelination and contextual freezing found in adolescent LB males.

Inhibition of ADORA3 promotes microglial phagocytosis and alleviates chronic ischemic white matter injury

BACKGROUND

Adenosine A3 receptor (ADORA3) belongs to the adenosine receptor families and the role of ADORA3 in vascular dementia (VaD) is largely unexplored. The present study sought to determine the therapeutic role of ADORA3 antagonist in a mouse model of VaD.

METHODS

The GSE122063 dataset was selected to screen the differential expression genes and pathways between VaD patients and controls. A mouse model of bilateral carotid artery stenosis (BCAS) was established. The cognitive functions were examined by the novel object recognition test, Y maze test, and fear of conditioning test. The white matter injury (WMI) was examined by 9.4 T MRI, western blot, and immunofluorescence staining. The mechanisms of ADORA3-regulated phagocytosis by microglia were examined using qPCR, western blot, dual immunofluorescence staining, and flow cytometry.

RESULTS

The expression of ADORA3 was elevated in brain tissues of VaD patients and ADORA3 was indicated as a key gene for VaD in the GSE122063. In BCAS mice, the expression of ADORA3 was predominantly elevated in microglia in the corpus callosum. ADORA3 antagonist promotes microglial phagocytosis to myelin debris by facilitating cAMP/PKA/p-CREB pathway and thereby ameliorates WMI and cognitive impairment in BCAS mice. The therapeutic effect of ADORA3 antagonist was partially reversed by the inhibition of the cAMP/PKA pathway.

CONCLUSIONS

ADORA3 antagonist alleviates chronic ischemic WMI by modulating myelin clearance of microglia, which may be a potential therapeutic target for the treatment of VaD.

CNS Resident Innate Immune Cells: Guardians of CNS Homeostasis

Although the CNS has been considered for a long time an immune-privileged organ, it is now well known that both the parenchyma and non-parenchymal tissue (meninges, perivascular space, and choroid plexus) are richly populated in resident immune cells. The advent of more powerful tools for multiplex immunophenotyping, such as single-cell RNA sequencing technique and upscale multiparametric flow and mass spectrometry, helped in discriminating between resident and infiltrating cells and, above all, the different spectrum of phenotypes distinguishing border-associated macrophages. Here, we focus our attention on resident innate immune players and their primary role in both CNS homeostasis and pathological neuroinflammation and neurodegeneration, two key interconnected aspects of the immunopathology of multiple sclerosis.

Prolonged myelin deficits contribute to neuron loss and functional impairments after ischaemic stroke

Ischaemic stroke causes neuron loss and long-term functional deficits. Unfortunately, effective approaches to preserving neurons and promoting functional recovery remain unavailable. Oligodendrocytes, the myelinating cells in the CNS, are susceptible to oxygen and nutrition deprivation and undergo degeneration after ischaemic stroke. Technically, new oligodendrocytes and myelin can be generated by the differentiation of oligodendrocyte precursor cells (OPCs). However, myelin dynamics and their functional significance after ischaemic stroke remain poorly understood. Here, we report numerous denuded axons accompanied by decreased neuron density in sections from ischaemic stroke lesions in human brain, suggesting that neuron loss correlates with myelin deficits in these lesions. To investigate the longitudinal changes in myelin dynamics after stroke, we labelled and traced pre-existing and newly-formed myelin, respectively, using cell-specific genetic approaches. Our results indicated massive oligodendrocyte death and myelin loss 2 weeks after stroke in the transient middle cerebral artery occlusion (tMCAO) mouse model. In contrast, myelin regeneration remained insufficient 4 and 8 weeks post-stroke. Notably, neuronal loss and functional impairments worsened in aged brains, and new myelin generation was diminished. To analyse the causal relationship between remyelination and neuron survival, we manipulated myelinogenesis by conditional deletion of Olig2 (a positive regulator) or muscarinic receptor 1 (M1R, a negative regulator) in OPCs. Deleting Olig2 inhibited remyelination, reducing neuron survival and functional recovery after tMCAO. Conversely, enhancing remyelination by M1R conditional knockout or treatment with the pro-myelination drug clemastine after tMCAO preserved white matter integrity and neuronal survival, accelerating functional recovery. Together, our findings demonstrate that enhancing myelinogenesis is a promising strategy to preserve neurons and promote functional recovery after ischaemic stroke.

Human Glial Cells as Innovative Targets for the Therapy of Central Nervous System Pathologies

In vitro and preclinical in vivo research in the last 35 years has clearly highlighted the crucial physiopathological role of glial cells, namely astrocytes/microglia/oligodendrocytes and satellite glial cells/Schwann cells in the central and peripheral nervous system, respectively. Several possible pharmacological targets to various neurodegenerative disorders and painful conditions have therefore been successfully identified, including receptors and enzymes, and mediators of neuroinflammation. However, the translation of these promising data to a clinical setting is often hampered by both technical and biological difficulties, making it necessary to perform experiments on human cells and models of the various diseases. In this review we will, therefore, summarize the most relevant data on the contribution of glial cells to human pathologies and on their possible pharmacological modulation based on data obtained in post-mortem tissues and in iPSC-derived human brain cells and organoids. The possibility of an in vivo visualization of glia reaction to neuroinflammation in patients will be also discussed.

Rat primary cortical cell tri-culture to study effects of amyloid-beta on microglia function

Introduction

The etiology and progression of sporadic Alzheimer's Disease (AD) have been studied for decades. One proposed mechanism is that amyloid-beta (Aβ) proteins induce neuroinflammation, synapse loss, and neuronal cell death. Microglia play an especially important role in Aβ clearance, and alterations in microglial function due to aging or disease may result in Aβ accumulation and deleterious effects on neuronal function. However, studying these complex factors in vivo , where numerous confounding processes exist, is challenging, and until recently, in vitro models have not allowed sustained culture of microglia, astrocytes and neurons in the same culture. Here, we employ a tri-culture model of rat primary neurons, astrocytes, and microglia and compare it to co-culture (neurons and astrocytes) and mono-culture enriched for microglia to study microglial function (i.e., motility and Aβ clearance) and proteomic response to exogenous Aβ.

Methods

We established cortical co-culture (neurons and astrocytes), tri-culture (neurons, astrocytes, and microglia), and mono-culture (microglia) from perinatal rat pups. On days in vitro (DIV) 7 - 14, the cultures were exposed to fluorescently-labeled Aβ (FITC-Aβ) particles for varying durations. Images were analyzed to determine the number of FITC-Aβ particles after specific lengths of exposure. A group of cells were stained for βIII-tubulin, GFAP, and Iba1 for morphological analysis via quantitative fluorescence microscopy. Cytokine profiles from conditioned media were obtained. Live-cell imaging with images acquired every 5 minutes for 4 hours was employed to extract microglia motility parameters (e.g., Euclidean distance, migration speed, directionality ratio).

Results and discussion

FITC-Aβ particles were more effectively cleared in the tri-culture compared to the co-culture. This was attributed to microglia engulfing FITC-Aβ particles, as confirmed via epifluorescence and confocal microscopy. Adding FITC-Aβ significantly increased the size of microglia, but had no significant effect on neuronal surface coverage or astrocyte size. Analysis of the cytokine profile upon FITC-Aβ addition revealed a significant increase in proinflammatory cytokines (TNF-α, IL-1α, IL-1β, IL-6) in tri-culture, but not co-culture. In addition, Aβ addition altered microglia motility marked by swarming-like motion with decreased Euclidean distance yet unaltered speed. These results highlight the importance of cell-cell communication in microglia function (e.g., motility and Aβ clearance) and the utility of the tri-culture model to further investigate microglia dysfunction in AD.

TREM2 deficiency impairs the energy metabolism of Schwann cells and exacerbates peripheral neurological deficits

Triggering receptor expressed on myeloid cells-2 (TREM2) has been implicated in susceptibility to neurodegenerative disease. Schwann cells (SCs), the predominant glial cell type in the peripheral nervous system (PNS), play a crucial role in myelination, providing trophic support for neurons and nerve regeneration. However, the function of TREM2 in SCs has not been fully elucidated. Here, we found that TREM2 is expressed in SCs but not in neurons in the PNS. TREM2 deficiency leads to disruption of glycolytic flux and oxidative metabolism in SCs, impairing cell proliferation. The energy crisis caused by TREM2 deficiency triggers mitochondrial damage and autophagy by activating AMPK and impairing PI3K-AKT-mTOR signaling. Combined metabolomic analysis demonstrated that energic substrates and energy metabolic pathways were significantly impaired in TREM2-deficient SCs. Moreover, TREM2 deficiency impairs energy metabolism and axonal growth in sciatic nerve, accompanied by exacerbation of neurological deficits and suppression of nerve regeneration in a mouse model of acute motor axonal neuropathy. These results indicate that TREM2 is a critical regulator of energy metabolism in SCs and exerts neuroprotective effects on peripheral neuropathy. TREM2 deficiency impairs glycolysis and oxidative metabolism in Schwann cells, resulting in compromised cell proliferation. The energy crisis caused by TREM2 deficiency induces mitochondrial damage and autophagy by activating AMPK and impairing PI3K-AKT-mTOR signaling. Moreover, TREM2 deficiency disrupts the energy metabolism of the sciatic nerve and impairs support for axonal regeneration, accompanied by exacerbation of neurological deficits and suppression of nerve regeneration in a mouse model of acute motor axonal neuropathy (by FigDraw).

Boosting Microglial Lipid Metabolism via TREM2 Signaling by Biomimetic Nanoparticles to Attenuate the Sevoflurane-Induced Developmental Neurotoxicity

Lipid metabolism has been considered as a potential therapeutic target in sevoflurane-induced neurotoxicity that can potentially affect the learning and memory function in the developmental brain. Recently, triggering receptor expressed on myeloid cells 2 (TREM2) is identified as a crucial step in regulating lipid metabolism and associated with the pathogenesis of neurodegenerative diseases. Herein, it is reported that quercetin modified Cu2- x Se (abbreviated as CSPQ) nanoparticles can ameliorate sevoflurane-induced neurotoxicity by tuning the microglial lipid metabolism and promoting microglial M2-like polarization via TREM2 signaling pathway, in which the apolipoprotein E (ApoE), and adenosine triphosphate-binding cassette transporters (ABCA1 and ABCG1) levels are upregulated. Furthermore, the protective effects of CSPQ nanoparticles against sevoflurane-induced neurotoxicity via TREM2 are further demonstrated by the small interfering RNA (siRNA)-TREM2 transfected BV2 cells, which are obviously not influenced by CSPQ nanoparticles. The cell membrane coated CSPQ (referred as CSPQ@CM) nanoparticles can significantly reduce sevoflurane-induced learning and memory deficits, improve lipid metabolism dysfunction, and promote the remyelination in the hippocampus of mice. The study shows great potential of targeting microglial lipid metabolism in promoting remyelination of neurons for treatment of neurotoxicity and neurodegenerative diseases.

Remyelination in the Central Nervous System

The inability of the mammalian central nervous system (CNS) to undergo spontaneous regeneration has long been regarded as a central tenet of neurobiology. However, while this is largely true of the neuronal elements of the adult mammalian CNS, save for discrete populations of granule neurons, the same is not true of its glial elements. In particular, the loss of oligodendrocytes, which results in demyelination, triggers a spontaneous and often highly efficient regenerative response, remyelination, in which new oligodendrocytes are generated and myelin sheaths are restored to denuded axons. Yet remyelination in humans is not without limitation, and a variety of demyelinating conditions are associated with sustained and disabling myelin loss. In this work, we will (1) review the biology of remyelination, including the cells and signals involved; (2) describe when remyelination occurs and when and why it fails, including the consequences of its failure; and (3) discuss approaches for therapeutically enhancing remyelination in demyelinating diseases of both children and adults, both by stimulating endogenous oligodendrocyte progenitor cells and by transplanting these cells into demyelinated brain.

TREM2 improves microglia function and synaptic development in autism spectrum disorders by regulating P38 MAPK signaling pathway

BACKGROUND

Autism spectrum disorder (ASD) encompasses a diverse range of neurodevelopmental disorders, but the precise underlying pathogenesis remains elusive. This study aim to explore the potential mechanism of TREM2 in regulating microglia function in ASD.

MATERIALS AND METHODS

The offspring rat model of ASD was established through prenatal exposure to valproic acid (VPA), and the behavioral symptoms of the ASD model were observed. On postnatal day (PND) 7 and PND 28, the effects of prenatally exposure to VPA on synaptic development and microglia phenotype of offspring rats were observed. Primary microglia were cultured in vitro. Lentivirus and adenovirus were utilized to interfere with TREM2 and overexpress TREM2.

RESULTS

Prenatally VPA exposure induced offspring rats to show typical ASD core symptoms, which led to abnormal expression of synapse-related proteins in the prefrontal cortex of offspring rats, changed the phenotype of microglia in offspring rats, promoted the polarization of microglia to pro-inflammatory type, and increased inflammatory response. The experimental results in vitro showed that overexpression of TREM2 could increase the expression of Gephyrin, decrease the content of CD86 protein and increase the content of CD206 protein. In addition, after the expression of TREM2 was interfered, the content of p-P38 MAPK protein increased and the content of p-ELK-1 protein decreased.

CONCLUSION

The protective influence of TREM2 on the VPA-induced ASD model is attributed to its inhibition of the P38 MAPK pathway, this protective effect may be achieved by promoting the polarization of microglia to anti-inflammatory phenotype and improving the neuronal synaptic development.

Single-cell transcriptomics analysis of cellular heterogeneity and immune mechanisms in neurodegenerative diseases

Single-cell transcriptomics analysis is an advanced technology that can describe the intracellular transcriptome in complex tissues. It profiles and analyses datasets by single-cell RNA sequencing. Neurodegenerative diseases are identified by the abnormal apoptosis of neurons in the brain with few or no effective therapy strategies at present, which has been a growing healthcare concern and brought a great burden to society. The transcriptome of individual cells provides deep insights into previously unforeseen cellular heterogeneity and gene expression differences in neurodegenerative disorders. It detects multiple cell subsets and functional changes during pathological progression, which deepens the understanding of the molecular underpinnings and cellular basis of neurodegenerative diseases. Furthermore, the transcriptome analysis of immune cells shows the regulation of immune response. Different subtypes of immune cells and their interaction are found to contribute to disease progression. This finding enables the discovery of novel targets and biomarkers for early diagnosis. In this review, we emphasize the principles of the technology, and its recent progress in the study of cellular heterogeneity and immune mechanisms in neurodegenerative diseases. The application of single-cell transcriptomics analysis in neurodegenerative disorders would help explore the pathogenesis of these diseases and develop novel therapeutic methods.

Effect of triggering receptor expressed on myeloid cells 2-associated alterations on lipid metabolism in macrophages in the development of non-alcoholic fatty liver disease

BACKGROUND AND AIM

Triggering receptor expressed on myeloid cells 2 (TREM2) plays crucial roles in metabolic homeostasis and inflammatory response. Altered metabolic function in macrophages could modulate their activation and immune phenotype. The present study aimed to investigate the expression of TREM2 in non-alcoholic fatty liver disease (NAFLD) and to clarify the underlying mechanism of TREM2 on macrophages lipid metabolism and oxidative stress.

METHODS

Hepatic TREM2 expression and its relationship with NAFLD progression were analyzed in patients with NAFLD and mice fed a high-fat diet. Lipid metabolism and oxidative stress were investigated in macrophages from NAFLD mice or stimulated with saturated fatty acids. Knockdown and overexpression of TREM2 were further explored.

RESULTS

Triggering receptor expressed on myeloid cells 2+ macrophages were increased along with NAFLD development, characterized by aggravated steatosis and liver damage in humans and mice. TREM2 expression was upregulated and lipid metabolism was changed in macrophages from NAFLD mice or metabolically activated by saturated fatty acid in vitro, as demonstrated by increased lipid uptake and catabolism, but reduced de novo synthesis of fatty acids (FAs). Regulation of TREM2 expression in lipid-laden macrophages reprogrammed lipid metabolism, especially the fatty acid oxidation capacity of mitochondria. TREM2 knockdown promoted oxidative stress by aggravating FAs deposition in mitochondria. Intervention of mitochondrial FAs transport in lipid-laden macrophages alleviated FA deposition and reactive oxygen species production induced by TREM2 knockdown.

CONCLUSIONS

Triggering receptor expressed on myeloid cells 2 expression was associated with the lipid metabolic profile and reactive oxygen species production in macrophages. High expression of TREM2 in macrophages may protect the liver from oxidative stress in NAFLD.

Pyrolae herba alleviates cognitive impairment via hippocampal TREM2 signaling modulating neuroinflammation and neurogenesis in lipopolysaccharide-treated mice

ETHNOPHARMACOLOGICAL RELEVANC

Pyrolae herba (PH), a kind of Chinese herb, has been identified to have an anti-inflammatory effect, while the potential for treating cognitive impairment (CI), as well as the underlying mechanisms, is unclear. Currently, the interaction between neuroinflammation and neural function play a critical role in pathophysiology of CI.

AIM OF THE STUDY

To elucidate therapeutic effect of PH for CI as well as its underlying mechanisms with LPS-treated mice model.

METHODS AND MATERIALS

In this study, male C57BL6/J mice received lipopolysaccharide (LPS) injection for 10 days to establish CI model and were administrated with PH for 14 days. We used piracetam as a positive control. Memory and spatial function was tested by Morris water maze (MWM). The level of inflammation-related cytokines (TNF-α, IL-1β, IL-10, IL-6) were determined by enzyme-linked immunosorbent assay (ELISA) in serum and western blot in hippocampus. Immunofluorescence (IF) was used to measure the levels of ionized calcium binding linker molecule 1 (IBA-1), glial fibrillary acidic protein (GFAP), BrdU, Ki67 and doublecortin (DCX) in hippocampus. The mRNA sequencing was used to screen the potential target of PH with therapeutic CI. Reverse transcription-polymerase chain reaction (RT-PCR) was used to determine the gene alteration of triggering receptor expressed on myeloid cells 2 (TREM2) in hippocampus. We used western blot to determine protein expressions of TREM2 and its related signaling, as well as synaptic proteins in hippocampus.

RESULTS

The results revealed that LPS contributed to CI, and PH or piracetam treatment significantly ameliorated CI in MWM test. LPS contributed to increasing expressions of TNF-α and IL-1β in serum and hippocampus, which both reversed by PH or piracetam. PH or piracetam could inhibit the activation of glial cells including microglia and astrocyte in the hippocampus in LPS-induced CI model. The mRNA sequencing and RT-PCR results showed that LPS significantly increased the gene expression of TREM2, which was reversed by PH. The alteration of TREM2 expression was the most significant among the 10 genes (TREM2, Slc24a2, Ptch2, Gck, Il1rapl1, Cadps2, Btbd11, Secisbp2l, Tenm3 and Prepl) in hippocampus. Protein results showed that LPS upregulated the expressions of TREM2 and its related proteins including DAP12, spleen tyrosine kinase (SYK) phosphorylation and ADAM 10, which were all reversed by PH or piracetam in hippocampus. Furthermore, LPS was capable of reducing the expression of BrdU and DCX co-labeled positive cells in hippocampal dentate gyrus (DG), which was reversed only by PH. Moreover, PH or piracetam treatment significantly increased the expression of Ki67 and DCX co-labeled positive cells in hippocampal DG. The expression of synapsin1 was obviously decreased by LPS and was significantly reversed by PH or piracetam.

CONCLUSIONS

PH could alleviate CI by suppressing the secretion of pro-inflammatory cytokines and mitigating astrocyte activity by restraining microglia's activation in hippocampus, further facilitating neurogenesis and proliferation, thereby enhancing pre-synaptic protein. This study highlighted on the clinical application of PH, which might promote the use of phytomedicine in CI patients.

sTREM2 Differentially Affects Cytokine Expression in Myeloid-Derived Cell Models via MAPK-JNK Signaling Pathway

TREM2 is a critical innate immune receptor primarily expressed on myeloid-derived cells, such as microglia and macrophages. Mutations in TREM2 are linked to several neurodegenerative diseases including Alzheimer's disease (AD). TREM2 can be cleaved from the cell membrane and released as soluble TREM2 (sTREM2). sTREM2 levels are shown to peak prior to AD, with its levels fluctuating throughout disease progression. However, the mechanism by which sTREM2 may affect innate immune responses is largely uncharacterized. In this study, we investigated whether sTREM2 can induce inflammatory response in myeloid-derived THP-1 monocytes and macrophages and characterized the signaling mechanisms involved. Our results show that sTREM2 was capable of stimulating the expression of several inflammatory cytokines in THP-1 cells throughout the time course of 2 h to 8 h but inducing anti-inflammatory cytokine expression at later time points. A TREM2 antibody was capable of inhibiting the expression of some cytokines induced by sTREM2 but enhancing others. The complex of sTREM2/TREM2 antibody was shown to enhance IL-1β expression, which was partially blocked by an NLRP3 specific inhibitor, indicating that the complex activated the NRLP3 inflammasome pathway. sTREM2 was also shown to have differential effects on cytokine expression in M0, M1, and M2 macrophages differentiated from THP-1 cells. sTREM2 has a more stimulating effect on cytokine expression in M0 macrophages, less of an effect on M2 macrophages, and some inhibitory effects on cytokine expression in M1 macrophages at early time points. Analyses of several signaling pathways revealed that sTREM2-induced expression of cytokines occurs mainly through MAPK-JNK signaling. Our work reveals differential effects of sTREM2 on cytokine expression profiles of THP-1 cells and macrophages and demonstrates that the MAPK-JNK signaling pathway is mainly responsible for sTREM2-induced cytokine expression.

The Contribution of Microglia and Brain-Infiltrating Macrophages to the Pathogenesis of Neuroinflammatory and Neurodegenerative Diseases during TMEV Infection of the Central Nervous System

The infection of the central nervous system (CNS) with neurotropic viruses induces neuroinflammation and is associated with the development of neuroinflammatory and neurodegenerative diseases, including multiple sclerosis and epilepsy. The activation of the innate and adaptive immune response, including microglial, macrophages, and T and B cells, while required for efficient viral control within the CNS, is also associated with neuropathology. Under healthy conditions, resident microglia play a pivotal role in maintaining CNS homeostasis. However, during pathological events, such as CNS viral infection, microglia become reactive, and immune cells from the periphery infiltrate into the brain, disrupting CNS homeostasis and contributing to disease development. Theiler's murine encephalomyelitis virus (TMEV), a neurotropic picornavirus, is used in two distinct mouse models: TMEV-induced demyelination disease (TMEV-IDD) and TMEV-induced seizures, representing mouse models of multiple sclerosis and epilepsy, respectively. These murine models have contributed substantially to our understanding of the pathophysiology of MS and seizures/epilepsy following viral infection, serving as critical tools for identifying pharmacological targetable pathways to modulate disease development. This review aims to discuss the host-pathogen interaction during a neurotropic picornavirus infection and to shed light on our current understanding of the multifaceted roles played by microglia and macrophages in the context of these two complexes viral-induced disease.

The aging mouse CNS is protected by an autophagy-dependent microglia population promoted by IL-34

Microglia harness an unutilized health-promoting potential in age-related neurodegenerative and neuroinflammatory diseases, conditions like progressive multiple sclerosis (MS). Our research unveils an microglia population emerging in the cortical brain regions of aging mice, marked by ERK1/2, Akt, and AMPK phosphorylation patterns and a transcriptome indicative of activated autophagy - a process critical for cellular adaptability. By deleting the core autophagy gene Ulk1 in microglia, we reduce this population in the central nervous system of aged mice. Notably, this population is found dependent on IL-34, rather than CSF1, although both are ligands for CSF1R. When aging mice are exposed to autoimmune neuroinflammation, the loss of autophagy-dependent microglia leads to neural and glial cell death and increased mortality. Conversely, microglial expansion mediated by IL-34 exhibits a protective effect. These findings shed light on an autophagy-dependent neuroprotective microglia population as a potential target for treating age-related neuroinflammatory conditions, including progressive MS.

Trem2 expression in microglia is required to maintain normal neuronal bioenergetics during development

Triggering receptor expressed on myeloid cells 2 (Trem2) is a myeloid cell-specific gene expressed in brain microglia, with variants that are associated with neurodegenerative diseases, including Alzheimer's disease. Trem2 is essential for microglia-mediated synaptic refinement, but whether Trem2 contributes to shaping neuronal development remains unclear. Here, we demonstrate that Trem2 plays a key role in controlling the bioenergetic profile of pyramidal neurons during development. In the absence of Trem2, developing neurons in the hippocampal cornus ammonis (CA)1 but not in CA3 subfield displayed compromised energetic metabolism, accompanied by reduced mitochondrial mass and abnormal organelle ultrastructure. This was paralleled by the transcriptional rearrangement of hippocampal pyramidal neurons at birth, with a pervasive alteration of metabolic, oxidative phosphorylation, and mitochondrial gene signatures, accompanied by a delay in the maturation of CA1 neurons. Our results unveil a role of Trem2 in controlling neuronal development by regulating the metabolic fitness of neurons in a region-specific manner.

Soluble TREM2 triggers microglial dysfunction in neuromyelitis optica spectrum disorders

Microglia-mediated neuroinflammation contributes to acute demyelination in neuromyelitis optica spectrum disorders (NMOSD). Soluble triggering receptor expressed on myeloid cells 2 (sTREM2) in the CSF has been associated with microglial activation in several neurodegenerative diseases. However, the basis for this immune-mediated attack and the pathophysiological role of sTREM2 in NMOSD remain to be elucidated. Here, we performed Mendelian randomization analysis and identified a genetic association between increased CSF sTREM2 and NMOSD risk. CSF sTREM2 was elevated in patients with NMOSD and was positively correlated with neural injury and other neuroinflammation markers. Single-cell RNA sequencing of human macrophage/microglia-like cells in CSF, a proxy for microglia, showed that increased CSF sTREM2 was positively associated with microglial dysfunction in patients with NMOSD. Furthermore, we demonstrated that sTREM2 is a reliable biomarker of microglial activation in a mouse model of NMOSD. Using unbiased transcriptomic and lipidomic screens, we identified that excessive activation, overwhelmed phagocytosis of myelin debris, suppressed lipid metabolism and enhanced glycolysis underlie sTREM2-mediated microglial dysfunction, possibly through the nuclear factor kappa B (NF-κB) signalling pathway. These molecular and cellular findings provide a mechanistic explanation for the genetic association between CSF sTREM2 and NMOSD risk and indicate that sTREM2 could be a potential biomarker of NMOSD progression and a therapeutic target for microglia-mediated neuroinflammation.

Impaired microglial phagocytosis promotes seizure development

In the central nervous system, triggering receptor expressed on myeloid cells 2 (TREM2) is exclusively expressed by microglia and is critical for microglial proliferation, migration, and phagocytosis. TREM2 plays an important role in neurodegenerative diseases, such as Alzheimer's disease and amyotrophic lateral sclerosis. However, little is known about the role TREM2 plays in epileptogenesis. To investigate this, we utilized TREM2 knockout (KO) mice within the murine intra-amygdala kainic acid seizure model. Electroencephalographic analysis, immunocytochemistry, and RNA sequencing revealed that TREM2 deficiency significantly promoted seizure-induced pathology. We found that TREM2 KO increased both acute status epilepticus and spontaneous recurrent seizures characteristic of chronic focal epilepsy. Mechanistically, phagocytic clearance of damaged neurons by microglia was impaired in TREM2 KO mice and the reduced phagocytic capacity correlated with increased spontaneous seizures. Analysis of human tissue from patients who underwent surgical resection for drug resistant temporal lobe epilepsy also showed a negative correlation between microglial phagocytic activity and focal to bilateral tonic-clonic generalized seizure history. These results indicate that microglial TREM2 and phagocytic activity may be important to epileptogenesis and the progression of focal temporal lobe epilepsy.

TREM2 regulates microglial lipid droplet formation and represses post-ischemic brain injury

Triggering receptor expressed on myeloid cells 2 (TREM2) is a transmembrane receptor protein predominantly expressed in microglia within the central nervous system (CNS). TREM2 regulates multiple microglial functions, including lipid metabolism, immune reaction, inflammation, and microglial phagocytosis. Recent studies have found that TREM2 is highly expressed in activated microglia after ischemic stroke. However, the role of TREM2 in the pathologic response after stroke remains unclear. Herein, TREM2-deficient microglia exhibit an impaired phagocytosis rate and cholesteryl ester (CE) accumulation, leading to lipid droplet formation and upregulation of Perilipin-2 (PLIN2) expression after hypoxia. Knockdown of TREM2 results in increased lipid synthesis (PLIN2, SOAT1) and decreased cholesterol clearance and lipid hydrolysis (LIPA, ApoE, ABCA1, NECH1, and NPC2), further impacting microglial phenotypes. In these lipid droplet-rich microglia, the TGF-β1/Smad2/3 signaling pathway is downregulated, driving microglia towards a pro-inflammatory phenotype. Meanwhile, in a neuron-microglia co-culture system under hypoxic conditions, we found that microglia lost their protective effect against neuronal injury and apoptosis when TREM2 was knocked down. Under in vivo conditions, TREM2 knockdown mice express lower TGF-β1 expression levels and a lower number of anti-inflammatory M2 phenotype microglia, resulting in increased cerebral infarct size, exacerbated neuronal apoptosis, and aggravated neuronal impairment. Our work suggests that TREM2 attenuates stroke-induced neuroinflammation by modulating the TGF-β1/Smad2/3 signaling pathway. TREM2 may play a direct role in the regulation of inflammation and also exert an influence on the post-ischemic inflammation and the stroke pathology progression via regulation of lipid metabolism processes. Thus, underscoring the therapeutic potential of TREM2 agonists in ischemic stroke and making TREM2 an attractive new clinical target for the treatment of ischemic stroke and other inflammation-related diseases.

Function and mechanism of TREM2 in bacterial infection

Triggering receptor expressed on myeloid cells 2 (TREM2), which is a lipid sensing and phagocytosis receptor, plays a key role in immunity and inflammation in response to pathogens. Here, we review the function and signaling of TREM2 in microbial binding, engulfment and removal, and describe TREM2-mediated inhibition of inflammation by negatively regulating the Toll-like receptor (TLR) response. We further illustrate the role of TREM2 in restoring organ homeostasis in sepsis and soluble TREM2 (sTREM2) as a diagnostic marker for sepsis-associated encephalopathy (SAE). Finally, we discuss the prospect of TREM2 as an interesting therapeutic target for sepsis.

Microglia and Systemic Immunity

Microglia are specialized immune cells that reside in the central nervous system (CNS) and play a crucial role in maintaining the homeostasis of the brain microenvironment. While traditionally regarded as a part of the innate immune system, recent research has highlighted their role in adaptive immunity. The CNS is no longer considered an immune-privileged organ, and increasing evidence suggests bidirectional communication between the immune system and the CNS. Microglia are sensitive to systemic immune signals and can respond to systemic inflammation by producing various inflammatory cytokines and chemokines. This response is mediated by activating pattern recognition receptors (PRRs), which recognize pathogen- and danger-associated molecular patterns in the systemic circulation. The microglial response to systemic inflammation has been implicated in several neurological conditions, including depression, anxiety, and cognitive impairment. Understanding the complex interplay between microglia and systemic immunity is crucial for developing therapeutic interventions to modulate immune responses in the CNS.