Heterozygous TREM2 mutations in frontotemporal dementia

Microglia rescue neurons from aggregate-induced neuronal dysfunction and death through tunneling nanotubes

Microglia are crucial for maintaining brain health and neuron function. Here, we report that microglia establish connections with neurons using tunneling nanotubes (TNTs) in both physiological and pathological conditions. These TNTs facilitate the rapid exchange of organelles, vesicles, and proteins. In neurodegenerative diseases like Parkinson's and Alzheimer's disease, toxic aggregates of alpha-synuclein (α-syn) and tau accumulate within neurons. Our research demonstrates that microglia use TNTs to extract neurons from these aggregates, restoring neuronal health. Additionally, microglia share their healthy mitochondria with burdened neurons, reducing oxidative stress and normalizing gene expression. Disrupting mitochondrial function with antimycin A before TNT formation eliminates this neuroprotection. Moreover, co-culturing neurons with microglia and promoting TNT formation rescues suppressed neuronal activity caused by α-syn or tau aggregates. Notably, TNT-mediated aggregate transfer is compromised in microglia carrying Lrrk22(Gly2019Ser) or Trem2(T66M) and (R47H) mutations, suggesting a role in the pathology of these gene variants in neurodegenerative diseases.

Current understanding on TREM-2 molecular biology and physiopathological functions

Triggering receptor expressed on myeloid cells 2 (TREM-2), a glycosylated receptor belonging to the immunoglobin superfamily and especially expressed in the myeloid cell lineage, is frequently explained as a reminiscent receptor for both adaptive and innate immunity regulation. TREM-2 is also acknowledged to influence NK cell differentiation via the PI3K and PLCγ signaling pathways, as well as the partial activation or direct inhibition of T cells. Additionally, TREM-2 overexpression is substantially linked to cell-specific functions, such as enhanced phagocytosis, reduced toll-like receptor (TLR)-mediated inflammatory cytokine production, increased transcription of anti-inflammatory cytokines, and reshaped T cell function. Whereas TREM-2-deficient cells exhibit diminished phagocytic function and enhanced proinflammatory cytokines production, proceeding to inflammatory injuries and an immunosuppressive environment for disease progression. Despite the growing literature supporting TREM-2+ cells in various diseases, such as neurodegenerative disorders and cancer, substantial facets of TREM-2-mediated signaling remain inadequately understood relevant to pathophysiology conditions. In this direction, herein, we have summarized the current knowledge on TREM-2 biology and cell-specific TREM-2 expression, particularly in the modulation of pivotal TREM-2-dependent functions under physiopathological conditions. Furthermore, molecular regulation and generic biological relevance of TREM-2 are also discussed, which might provide an alternative approach for preventing or reducing TREM-2-associated deformities. At last, we discussed the TREM-2 function in supporting an immunosuppressive cancer environment and as a potential drug target for cancer immunotherapy. Hence, summarized knowledge of TREM-2 might provide a window to overcome challenges in clinically effective therapies for TREM-2-induced diseases in humans.

Homozygous TREM2 c.549del; p.(Leu184Serfs*5) variant causing Nasu-Hakola disease in three siblings in a consanguineous Iraqi family: Case report and review of literature

BACKGROUND

The Triggering Receptor Expressed on Myeloid Cells 2 protein (TREM2) plays a crucial role in various biological processes, including osteoclast differentiation, and disease-associated microglia (DAM) activation to regulate neuroinflammation, and phagocytosis in the brain. Genetic variations in TREM2 are implicated in neurodegenerative disorders, such as Nasu-hakola disease (NHD), characterized by bone lesions, neuropsychiatric disorders, and early-onset dementia.

METHODS

We studied 3 siblings with suspected NHD. Whole-exome sequencing was conducted on the proband to identify the possible genetic cause(s) and by Sanger sequencing to validate the identified variants in the two other affected siblings, a healthy sister, and the parents.

RESULTS

We identified a novel homozygous deletion (c.549del; p.(Leu184Serfs*5)) in TREM2. Our literature review reveals 16 TREM2 mutations causing early-onset dementia and bone lesions.

CONCLUSION

These findings, alongside previous research, elucidate the clinical spectrum of TREM2-related diseases, aiding accurate diagnosis and patient care. This knowledge is vital for understanding TREM2-dependent DAM and its involvement in the pathogenesis of neurodevelopmental disorders which can help to develop targeted therapies and improve outcomes for TREM2-affected individuals.

Computational saturation mutagenesis to explore the effect of pathogenic mutations on extra-cellular domains of TREM2 associated with Alzheimer's and Nasu-Hakola disease

CONTEXT

The specialised family of triggering receptors expressed on myeloid cells (TREMs) plays a pivotal role in causing neurodegenerative disorders and activating microglial anti-inflammatory responses. Nasu-Hakola disease (NHD), a rare autosomal recessive disorder, has been associated with mutations in TREM2, which is also responsible for raising the risk of Alzheimer's disease (AD). Herein, we have made an endeavour to differentiate the confirmed pathogenic variants in TREM2 extra-cellular domain (ECD) linked with NHD and AD using mutation-induced fold stability change (∆∆G), with the computation of 12distinct structure-based methods through saturation mutagenesis. Correlation analysis between relative solvent accessibility (RSA) and ∆∆G expresses the discrete distributive behaviour of mutants associated with TREM2 in AD (R2 = 0.061) and NHD (R2 = 0.601). Our findings put an emphasis on W50 and V126 as major players in maintaining V-like domain in TREM2. Interestingly, we discern that both of them interact with a common residue Y108, which is dissolved upon mutation. This Y108 could have structural or functional role for TREM2 which can be an ideal candidate for further study. Furthermore, the residual interaction network highlights the importance of R47 and R62 in maintaining the CDR loops that are crucial for ligand binding. Future studies using biophysical characterisation of ligand interactions in TREM2-ECD would be helpful for the development of novel therapeutics for AD and NHD.

METHODS

ConSurf algorithm and ENDscript were used to determine the position and conservation of each residue in the wild-type ECD of TREM2. The mutation-induced fold stability change (∆∆G) of confirmed pathogenic mutants associated with NHD and AD was estimated using 12 state-of-the-art structure-based protein stability tools. Furthermore, we also computed the effect of random mutation on these sites using computational saturation mutagenesis. Linear regression analysis was performed using mutants ∆∆G and RSA through GraphPad software. In addition, a comprehensive non-bonded residual interaction network (RIN) of wild type and its mutants of TREM2-ECD was enumerated using RING3.0.

sTREM2 is associated with attenuated tau aggregate accumulation in the presence of amyloid-β pathology

Triggering Receptor Expressed on Myeloid Cell 2 (TREM2) plays a crucial role in the transition of microglia from a state of homeostasis to a state associated with the disease. Mutations in TREM2 are strongly linked with a higher risk of developing neurodegenerative diseases, including Alzheimer's disease. There have been contradictory findings regarding the potential detrimental or protective effects of microglial activation and TREM2-related microglial responses in Alzheimer's disease. Although previous studies reported increased CSF soluble TREM2 (sTREM2) in different clinical stages of Alzheimer's disease, the exact association between Alzheimer's disease hallmarks such as amyloid-beta and tau pathology remains unclear. In the present study, I aimed to investigate the association between TREM2-related microglial responses and tau accumulation in the presence and absence of amyloid-beta pathology in order to give a better view of the role of microglial activation in Alzheimer's disease development. Imaging data of 178 non-demented participants including 107 amyloid-beta-negative participants, 71 amyloid-beta-positive were recruited from Alzheimer's disease Neuroimaging Initiative. The CSF sTREM2 was used as an in vivo indicator of microglial responses associated with TREM2. Furthermore, I used longitudinal tau-PET and resting-state functional MRI connectomes in order to investigate the association of TREM2-related microglial activation and tau spreading through functional connections. A higher level of sTREM2 was associated with slower tau aggregate accumulation in non-demented amyloid-beta-positive. Furthermore, measuring the tau spreading through inter-connected regions using functional MRI connectomes confirms that the TREM2-related microglial activity might be a protective factor against tau pathology in brain tissue. These findings demonstrate that in individuals with initial amyloid-beta abnormalities, TREM2-related microglial activation is linked to reduced regional accumulation of tau aggregates and also, spreading across inter-connected brain regions, as evaluated through functional MRI connectomes during the early stages of Alzheimer's disease.

Reappraisal of the Concept of Accelerated Aging in Neurodegeneration and Beyond

BACKGROUND

Genetic and epigenetic changes, oxidative stress and inflammation influence the rate of aging, which diseases, lifestyle and environmental factors can further accelerate. In accelerated aging (AA), the biological age exceeds the chronological age.

OBJECTIVE

The objective of this study is to reappraise the AA concept critically, considering its weaknesses and limitations.

METHODS

We reviewed more than 300 recent articles dealing with the physiology of brain aging and neurodegeneration pathophysiology.

RESULTS

(1) Application of the AA concept to individual organs outside the brain is challenging as organs of different systems age at different rates. (2) There is a need to consider the deceleration of aging due to the potential use of the individual structure-functional reserves. The latter can be restored by pharmacological and/or cognitive therapy, environment, etc. (3) The AA concept lacks both standardised terminology and methodology. (4) Changes in specific molecular biomarkers (MBM) reflect aging-related processes; however, numerous MBM candidates should be validated to consolidate the AA theory. (5) The exact nature of many potential causal factors, biological outcomes and interactions between the former and the latter remain largely unclear.

CONCLUSIONS

Although AA is commonly recognised as a perspective theory, it still suffers from a number of gaps and limitations that assume the necessity for an updated AA concept.

TREM2-Deficient Microglia Attenuate Tau Spreading In Vivo

The role of TREM2 in Alzheimer's disease (AD) is not fully understood. Previous studies investigating the effect of TREM2 deletion on tauopathy mouse models without the contribution of b-amyloid have focused only on tau overexpression models. Herein, we investigated the effects of TREM2 deficiency on tau spreading using a mouse model in which endogenous tau is seeded to produce AD-like tau features. We found that Trem2-/- mice exhibit attenuated tau pathology in multiple brain regions concomitant with a decreased microglial density. The neuroinflammatory profile in TREM2-deficient mice did not induce an activated inflammatory response to tau pathology. These findings suggest that reduced TREM2 signaling may alter the response of microglia to pathological tau aggregates, impairing their activation and decreasing their capacity to contribute to tau spreading. However, caution should be exercised when targeting TREM2 as a therapeutic entry point for AD until its involvement in tau aggregation and propagation is better understood.

Defects in lysosomal function and lipid metabolism in human microglia harboring a TREM2 loss of function mutation

TREM2 is an innate immune receptor expressed by microglia in the adult brain. Genetic variation in the TREM2 gene has been implicated in risk for Alzheimer's disease and frontotemporal dementia, while homozygous TREM2 mutations cause a rare leukodystrophy, Nasu-Hakola disease (NHD). Despite extensive investigation, the role of TREM2 in NHD pathogenesis remains poorly understood. Here, we investigate the mechanisms by which a homozygous stop-gain TREM2 mutation (p.Q33X) contributes to NHD. Induced pluripotent stem cell (iPSC)-derived microglia (iMGLs) were generated from two NHD families: three homozygous TREM2 p.Q33X mutation carriers (termed NHD), two heterozygous mutation carriers, one related non-carrier, and two unrelated non-carriers. Transcriptomic and biochemical analyses revealed that iMGLs from NHD patients exhibited lysosomal dysfunction, downregulation of cholesterol genes, and reduced lipid droplets compared to controls. Also, NHD iMGLs displayed defective activation and HLA antigen presentation. This defective activation and lipid droplet content were restored by enhancing lysosomal biogenesis through mTOR-dependent and independent pathways. Alteration in lysosomal gene expression, such as decreased expression of genes implicated in lysosomal acidification (ATP6AP2) and chaperone mediated autophagy (LAMP2), together with reduction in lipid droplets were also observed in post-mortem brain tissues from NHD patients, thus closely recapitulating in vivo the phenotype observed in iMGLs in vitro. Our study provides the first cellular and molecular evidence that the TREM2 p.Q33X mutation in microglia leads to defects in lysosomal function and that compounds targeting lysosomal biogenesis restore a number of NHD microglial defects. A better understanding of how microglial lipid metabolism and lysosomal machinery are altered in NHD and how these defects impact microglia activation may provide new insights into mechanisms underlying NHD and other neurodegenerative diseases.

Association between Cerebrospinal Fluid Soluble TREM2, Alzheimer's Disease and Other Neurodegenerative Diseases

BACKGROUND

Cerebrospinal fluid (CSF) soluble triggering receptor expressed on myeloid cells 2 (sTREM2) is a potential biomarker and therapy target for neurodegenerative diseases (NDDs). The purpose of this meta-analysis was to investigate the association between CSF sTREM2 level and NDDs, and to reveal the dynamic changes in CSF sTREM2 level in Alzheimer's disease (AD) continuum.

METHODS

We systematically searched PubMed, Embase, Web of Science, and Cochrane Library databases for observational studies, which compared the levels of CSF sTREM2 between NDDs and controls. Sources of heterogeneity were analyzed using sensitivity analysis, subgroup analysis and meta-regression. We assessed pooled data using a random-effects model.

RESULTS

Twenty-two observational studies which included 5716 participates were identified. Compared with the controls, the whole AD continuum group showed a significant increase in CSF sTREM2 level (standardized mean difference [SMD]: 0.41, 95% confidence intervals [CI]: 0.24, 0.58, p < 0.001). The mild cognitive impairment (MCI) group displayed the largest effect size (SMD, 0.49 [95% CI: 0.10, 0.88], p = 0.014), followed by the AD cohort (SMD, 0.40 [95% CI: 0.18, 0.63], p < 0.001). The increase in sTREM2 in the preclinical stage of AD (pre-AD) group was the lowest (SMD, 0.29 [95% CI: 0.03, 0.55], p = 0.031). Other NDDs also showed an increase in the CSF sTREM2 levels compared with control groups (SMD, 0.77 [95% CI: 0.37, 1.16], p < 0.001).

CONCLUSIONS

The pooled data confirmed that NDDs are associated with increased CSF sTREM2 level, thereby suggesting the CSF sTREM2 as a potential dynamic biomarker and therapy target for NDDs.

Neurocognitive patterns across genetic levels in behavioral variant frontotemporal dementia: a multiple single cases study

BACKGROUND

Behavioral variant frontotemporal dementia (bvFTD) has been related to different genetic factors. Identifying multimodal phenotypic heterogeneity triggered by various genetic influences is critical for improving diagnosis, prognosis, and treatments. However, the specific impact of different genetic levels (mutations vs. risk variants vs. sporadic presentations) on clinical and neurocognitive phenotypes is not entirely understood, specially in patites from underrepresented regions such as Colombia.

METHODS

Here, in a multiple single cases study, we provide systematic comparisons regarding cognitive, neuropsychiatric, brain atrophy, and gene expression-atrophy overlap in a novel cohort of FTD patients (n = 42) from Colombia with different genetic levels, including patients with known genetic influences (G-FTD) such as those with genetic mutations (GR1) in particular genes (MAPT, TARDBP, and TREM2); patients with risk variants (GR2) in genes associated with FTD (tau Haplotypes H1 and H2 and APOE variants including ε2, ε3, ε4); and sporadic FTD patients (S-FTD (GR3)).

RESULTS

We found that patients from GR1 and GR2 exhibited earlier disease onset, pervasive cognitive impairments (cognitive screening, executive functioning, ToM), and increased brain atrophy (prefrontal areas, cingulated cortices, basal ganglia, and inferior temporal gyrus) than S-FTD patients (GR3). No differences in disease duration were observed across groups. Additionally, significant neuropsychiatric symptoms were observed in the GR1. The GR1 also presented more clinical and neurocognitive compromise than GR2 patients; these groups, however, did not display differences in disease onset or duration. APOE and tau patients showed more neuropsychiatric symptoms and primary atrophy in parietal and temporal cortices than GR1 patients. The gene-atrophy overlap analysis revealed atrophy in regions with specific genetic overexpression in all G-FTD patients. A differential family presentation did not explain the results.

CONCLUSIONS

Our results support the existence of genetic levels affecting the clinical, neurocognitive, and, to a lesser extent, neuropsychiatric presentation of bvFTD in the present underrepresented sample. These results support tailored assessments characterization based on the parallels of genetic levels and neurocognitive profiles in bvFTD.

Novel potent liposome agonists of triggering receptor expressed on myeloid cells 2 phenocopy antibody treatment in cells

The receptor Triggering Receptor Expressed on Myeloid cells 2 (TREM2) is associated with several neurodegenerative diseases including Alzheimer's Disease and TREM2 stimulation represents a novel therapeutic opportunity. TREM2 can be activated by antibodies targeting the stalk region, most likely through receptor dimerization. Endogenous ligands of TREM2 are suggested to be negatively charged apoptotic bodies, mimicked by phosphatidylserine incorporated in liposomes and other polyanionic molecules likely binding to TREM2 IgV fold. However, there has been much discrepancy in the literature on the nature of phospholipids (PLs) that can activate TREM2 and on the stability of the corresponding liposomes over time. We describe optimized liposomes as robust agonists selective for TREM2 over TREM1 in cellular system. The detailed structure/activity relationship studies of lipid polar heads indicate that negatively charged lipid heads are required for activity and we identified the shortest maximally active PL sidechain. Optimized liposomes are active on both TREM2 common variant and TREM2 R47H mutant. Activity and selectivity were further confirmed in different native TREM2 expressing cell types including on integrated cellular responses such as stimulation of phagocytic activity. Such tool agonists will be useful in further studies of TREM2 biology in cellular systems alongside antibodies, and in the design of small molecule synthetic TREM2 agonists.

Triggering receptor expressed on myeloid cells 2 deficiency exacerbates injury-induced inflammation in a mouse model of tauopathy

Traumatic brain injury (TBI) promotes several Alzheimer's disease-like pathological features, including microtubule-associated protein tau (MAPT) accumulation within neurons. Macrophage activation in the injured hTau mouse model of tauopathy raises the question whether there is a relationship between MAPT pathology and alterations in macrophage activation following TBI. Triggering receptor expressed on myeloid cells 2 (TREM2) is a critical regulator of microglia and macrophage phenotype, but its mechanisms on TBI remain unclear. To address the association with TREM2 in TBI and MAPT pathology, we studied TREM2 deficiency in hTau mice (hTau;Trem2^-/- ) 3 (acute phase) and 120 (chronic phase) days after experimental TBI. At three days following injury, hTau;Trem2^-/- mice exhibited reduced macrophage activation both in the cortex and hippocampus. However, to our surprise, hTau;Trem2^-/- mice exposed to TBI augments macrophage accumulation in the corpus callosum and white matter near the site of tissue damage in a chronic phase, which results in exacerbated axonal injury, tau aggregation, and impaired neurogenesis. We further demonstrate that TREM2 deficiency in hTau injured mice promotes neuronal dystrophy in the white matter due to impaired phagocytosis of apoptotic cells. Remarkably, hTau;Trem2^-/- exposed to TBI failed to restore blood-brain barrier integrity. These findings imply that TREM2 deficiency accelerates inflammation and neurodegeneration, accompanied by attenuated microglial phagocytosis and continuous blood-brain barrier (BBB) leakage, thus exacerbating tauopathy in hTau TBI mice.

Microglial TYROBP/DAP12 in Alzheimer's disease: Transduction of physiological and pathological signals across TREM2

TYROBP (also known as DAP12 or KARAP) is a transmembrane adaptor protein initially described as a receptor-activating subunit component of natural killer (NK) cells. TYROBP is expressed in numerous cell types, including peripheral blood monocytes, macrophages, dendritic cells, and osteoclasts, but a key point of recent interest is related to the critical role played by TYROBP in the function of many receptors expressed on the plasma membrane of microglia. TYROBP is the downstream adaptor and putative signaling partner for several receptors implicated in Alzheimer's disease (AD), including SIRP1β, CD33, CR3, and TREM2. TYROBP has received much of its current notoriety because of its importance in brain homeostasis by signal transduction across those receptors. In this review, we provide an overview of evidence indicating that the biology of TYROBP extends beyond its interaction with these four ligand-binding ectodomain-intramembranous domain molecules. In addition to reviewing the structure and localization of TYROBP, we discuss our recent progress using mouse models of either cerebral amyloidosis or tauopathy that were engineered to be TYROBP-deficient or TYROBP-overexpressing. Remarkably, constitutively TYROBP-deficient mice provided a model of genetic resilience to either of the defining proteinopathies of AD. Learning behavior and synaptic electrophysiological function were preserved at normal physiological levels even in the face of robust cerebral amyloidosis (in APP/PSEN1;Tyrobp-/- mice) or tauopathy (in MAPTP301S;Tyrobp-/- mice). A fundamental underpinning of the functional synaptic dysfunction associated with each proteotype was an accumulation of complement C1q. TYROBP deficiency prevented C1q accumulation associated with either proteinopathy. Based on these data, we speculate that TYROBP plays a key role in the microglial sensome and the emergence of the disease-associated microglia (DAM) phenotype. TYROBP may also play a key role in the loss of markers of synaptic integrity (e.g., synaptophysin-like immunoreactivity) that has long been held to be the feature of human AD molecular neuropathology that most closely correlates with concurrent clinical cognitive function.

TREM2 Gene Compound Heterozygosity in Neurodegenerative Disorders

BACKGROUND

Homozygous variants of the TREM2 and TYROBP genes have been shown to be causative for multiple bone cysts and neurodegeneration leading to progressive dementia (NHD, Nasu-Hakola disease).

OBJECTIVE

To determine if biallelic variants of these genes and/or oligogenic inheritance could be responsible for a wider spectrum of neurodegenerative conditions.

METHODS

We analyzed 52 genes associated with neurodegenerative disorders using targeted next generation sequencing in a selected group of 29 patients (n = 14 Alzheimer's disease, n = 8 frontotemporal dementia, n = 7 amyotrophic lateral sclerosis) carrying diverse already determined rare variants in exon 2 of TREM2. Molecular modeling was used to get an insight into the potential effects of the mutation.

RESULTS

We identified a novel mutation c.401_406delinsTCTAT; p.(Asp134Valfs*55) in exon 3 of TREM2 in an Alzheimer's disease patient also carrying the p.Arg62His TREM2 variant. Molecular modeling revealed that the identified mutation prevents anchoring of the TREM2 protein in the membrane, leaving the core of the Ig-like domain intact.

CONCLUSION

Our results expand the spectrum of neurodegenerative diseases, where the carriers of biallelic mutations in TREM2 have been described for Alzheimer's disease, and highlight the impact of variant burden in other genes on phenotypic heterogeneity.

Mitochondrial Damage-Associated Molecular Patterns Content in Extracellular Vesicles Promotes Early Inflammation in Neurodegenerative Disorders

Neuroinflammation is a common hallmark in different neurodegenerative conditions that share neuronal dysfunction and a progressive loss of a selectively vulnerable brain cell population. Alongside ageing and genetics, inflammation, oxidative stress and mitochondrial dysfunction are considered key risk factors. Microglia are considered immune sentinels of the central nervous system capable of initiating an innate and adaptive immune response. Nevertheless, the pathological mechanisms underlying the initiation and spread of inflammation in the brain are still poorly described. Recently, a new mechanism of intercellular signalling mediated by small extracellular vesicles (EVs) has been identified. EVs are nanosized particles (30-150 nm) with a bilipid membrane that carries cell-specific bioactive cargos that participate in physiological or pathological processes. Damage-associated molecular patterns (DAMPs) are cellular components recognised by the immune receptors of microglia, inducing or aggravating neuroinflammation in neurodegenerative disorders. Diverse evidence links mitochondrial dysfunction and inflammation mediated by mitochondrial-DAMPs (mtDAMPs) such as mitochondrial DNA, mitochondrial transcription factor A (TFAM) and cardiolipin, among others. Mitochondrial-derived vesicles (MDVs) are a subtype of EVs produced after mild damage to mitochondria and, upon fusion with multivesicular bodies are released as EVs to the extracellular space. MDVs are particularly enriched in mtDAMPs which can induce an immune response and the release of pro-inflammatory cytokines. Importantly, growing evidence supports the association between mitochondrial dysfunction, EV release and inflammation. Here, we describe the role of extracellular vesicles-associated mtDAMPS in physiological conditions and as neuroinflammation activators contributing to neurodegenerative disorders.

Dynamic insights into the effects of nonsynonymous polymorphisms (nsSNPs) on loss of TREM2 function

Single nucleotide variations in Triggering Receptor Expressed on Myeloid Cells 2 (TREM2) are associated with many neurodegenerative diseases, including Nasu-Hakola disease (NHD), frontotemporal dementia (FTD), and late-onset Alzheimer's disease because they disrupt ligand binding to the extracellular domain of TREM2. However, the effects of nonsynonymous single nucleotide polymorphisms (nsSNPs) in TREM2 on disease progression remain unknown. In this study, we identified several high-risk nsSNPs in the TREM2 gene using various deleterious SNP predicting algorithms and analyzed their destabilizing effects on the ligand recognizing region of the TREM2 immunoglobulin (Ig) domain by molecular dynamics (MD) simulation. Cumulative prediction by all tools employed suggested the three most deleterious nsSNPs involved in loss of TREM2 function are rs549402254 (W50S), rs749358844 (R52C), and rs1409131974 (D104G). MD simulation showed that these three variants cause substantial structural alterations and conformational remodeling of the apical loops of the TREM2 Ig domain, which is responsible for ligand recognition. Detailed analysis revealed that these variants substantially increased distances between apical loops and induced conformation remodeling by changing inter-loop nonbonded contacts. Moreover, all nsSNPs changed the electrostatic potentials near the putative ligand-interacting region (PLIR), which suggested they might reduce specificity or loss of binding affinity for TREM2 ligands. Overall, this study identifies three potential high-risk nsSNPs in the TREM2 gene. We propose further studies on the molecular mechanisms responsible for loss of TREM2 function and the associations between TREM2 nsSNPs and neurodegenerative diseases.

Clinical Characterization of Atypical Primary Progressive Aphasia in a 3-Year Longitudinal Study: A Case Report

The logopenic variant of primary progressive aphasia (lvPPA) is the most recent variant of primary progressive aphasia (PPA) to be identified; thus far, it has been poorly investigated. Despite being typically associated with Alzheimer disease (AD), lvPPA has recently been linked to frontotemporal lobe degeneration (FTLD), with distinctive cognitive and neural features that are worthy of further investigation. Here, we describe the neuropsychological and linguistic profile, as well as cerebral abnormalities, of an individual exhibiting PPA and carrying a pathogenetic variant in the GRN gene, from a 3-year longitudinal perspective. The individual's initial profile resembled lvPPA because it was characterized by word-finding difficulties and phonological errors in spontaneous speech in addition to sentence repetition and phonological short-term memory impairments. The individual's structural and metabolic imaging data demonstrated left temporal and bilateral frontal atrophy and hypometabolism, respectively. On follow-up, as the pathology progressed, dysprosody, stereotypical speech patterns, agrammatism, and orofacial apraxia appeared, suggesting an overlap with the nonfluent variant of PPA (nfvPPA). Severe sentence comprehension impairment also became evident. Our longitudinal and multidisciplinary diagnostic approach allowed us to better characterize the progression of a GRN-positive lvPPA profile, providing neuropsychological and imaging indicators that might be helpful to improve classification between different PPA variants and to address a nosological issue. Finally, we discuss the importance of early diagnosis of PPA given the possible overlap between different PPA variants during the progression of the pathology.

Glial cells and adaptive immunity in frontotemporal dementia with tau pathology

Neuroinflammation is involved in the aetiology of many neurodegenerative disorders including Alzheimer's disease, Parkinson's disease and motor neuron disease. Whether neuroinflammation also plays an important role in the pathophysiology of frontotemporal dementia is less well known. Frontotemporal dementia is a heterogeneous classification that covers many subtypes, with the main pathology known as frontotemporal lobar degeneration. The disease can be categorized with respect to the identity of the protein that causes the frontotemporal lobar degeneration in the brain. The most common subgroup describes diseases caused by frontotemporal lobar degeneration associated with tau aggregation, also known as primary tauopathies. Evidence suggests that neuroinflammation may play a role in primary tauopathies with genome-wide association studies finding enrichment of genetic variants associated with specific inflammation-related gene loci. These loci are related to both the innate immune system, including brain resident microglia, and the adaptive immune system through possible peripheral T-cell involvement. This review discusses the genetic evidence and relates it to findings in animal models expressing pathogenic tau as well as to post-mortem and PET studies in human disease. Across experimental paradigms, there seems to be a consensus regarding the involvement of innate immunity in primary tauopathies, with increased microglia and astrocyte density and/or activation, as well as increases in pro-inflammatory markers. Whilst it is less clear as to whether inflammation precedes tau aggregation or vice versa; there is strong evidence to support a microglial contribution to the propagation of hyperphosphorylated in tau frontotemporal lobar degeneration associated with tau aggregation. Experimental evidence-albeit limited-also corroborates genetic data pointing to the involvement of cellular adaptive immunity in primary tauopathies. However, it is still unclear whether brain recruitment of peripheral immune cells is an aberrant result of pathological changes or a physiological aspect of the neuroinflammatory response to the tau pathology.

Microglia Phenotypes Converge in Aging and Neurodegenerative Disease

Microglia, the primary immune cells of the central nervous system, hold a multitude of tasks in order to ensure brain homeostasis and are one of the best predictors of biological age on a cellular level. We and others have shown that these long-lived cells undergo an aging process that impedes their ability to perform some of the most vital homeostatic functions such as immune surveillance, acute injury response, and clearance of debris. Microglia have been described as gradually transitioning from a homeostatic state to an activated state in response to various insults, as well as aging. However, microglia show diverse responses to presented stimuli in the form of acute injury or chronic disease. This complexity is potentially further compounded by the distinct alterations that globally occur in the aging process. In this review, we discuss factors that may contribute to microglial aging, as well as transcriptional microglia alterations that occur in old age. We then compare these distinct phenotypic changes with microglial phenotype in neurodegenerative disease.

The Role of Microglia in Inherited White-Matter Disorders and Connections to Frontotemporal Dementia

Microglia play a critical but poorly understood role in promoting white-matter homeostasis. In this review, we leverage advances in human genetics and mouse models of leukodystrophies to delineate our current knowledge and identify outstanding questions regarding the impact of microglia on central nervous system white matter. We first focus on the role of pathogenic mutations in genes, such as TREM2, TYROBP, and CSF1R, that cause leukodystrophies in which the primary deficit is thought to originate in microglia. We next discuss recent advances in disorders such as adrenoleukodystrophy and Krabbe disease, in which microglia play an increasingly recognized role. We conclude by reviewing the roles of GRN and related genes, such as TMEM106B, PSAP, and SORT1, that affect microglial biology and associate with several types of disease, including multiple leukodystrophies as well as forms of frontotemporal dementia (FTD) presenting with white-matter abnormalities. Taken together, mouse and human data support the notion that loss of microglia-facilitated white-matter homeostasis plays an important role in the development of leukodystrophies and suggest novel mechanisms contributing to FTD.

TREM2/PLCγ2 signalling in immune cells: function, structural insight, and potential therapeutic modulation

The central role of the resident innate immune cells of the brain (microglia) in neurodegeneration has become clear over the past few years largely through genome-wide association studies (GWAS), and has rapidly become an active area of research. However, a mechanistic understanding (gene to function) has lagged behind. That is now beginning to change, as exemplified by a number of recent exciting and important reports that provide insight into the function of two key gene products – TREM2 (Triggering Receptor Expressed On Myeloid Cells 2) and PLCγ2 (Phospholipase C gamma2) – in microglia, and their role in neurodegenerative disorders. In this review we explore and discuss these recent advances and the opportunities that they may provide for the development of new therapies.

The Role of White Matter Dysfunction and Leukoencephalopathy/Leukodystrophy Genes in the Aetiology of Frontotemporal Dementias: Implications for Novel Approaches to Therapeutics

Frontotemporal dementia (FTD) is a common cause of presenile dementia and is characterized by behavioural and/or language changes and progressive cognitive deficits. Genetics is an important component in the aetiology of FTD, with positive family history of dementia reported for 40% of cases. This review synthesizes current knowledge of the known major FTD genes, including C9orf72 (chromosome 9 open reading frame 72), MAPT (microtubule-associated protein tau) and GRN (granulin), and their impact on neuronal and glial pathology. Further, evidence for white matter dysfunction in the aetiology of FTD and the clinical, neuroimaging and genetic overlap between FTD and leukodystrophy/leukoencephalopathy are discussed. The review highlights the role of common variants and mutations in genes such as CSF1R (colony-stimulating factor 1 receptor), CYP27A1 (cytochrome P450 family 27 subfamily A member 1), TREM2 (triggering receptor expressed on myeloid cells 2) and TMEM106B (transmembrane protein 106B) that play an integral role in microglia and oligodendrocyte function. Finally, pharmacological and non-pharmacological approaches for enhancing remyelination are discussed in terms of future treatments of FTD.

TREM2, microglia, and Alzheimer's disease

Triggering receptor expressed on myeloid cells 2 (TREM2) has been suggested to play a crucial role in Alzheimer's disease (AD) pathogenesis, as revealed by genome-wide association studies (GWAS). Since then, rapidly increasing literature related to TREM2 has focused on elucidating its role in AD pathology. In this review, we summarize our understanding of TREM2 biology, explore TREM2 functions in microglia, address the multiple mechanisms of TREM2 in AD, and raise key questions for further investigations to elucidate the detailed roles and molecular mechanisms of TREM2 in microglial responses. A major breakthrough in our understanding of TREM2 is based on our hypothesis suggesting that TREM2 may act as a multifaceted player in microglial functions in AD brain homeostasis. We conclude that TREM2 can not only influence microglial functions in amyloid and tau pathologies but also participate in inflammatory responses and metabolism, acting alone or with other molecules, such as apolipoprotein E (APOE). This review provides novel insight into the broad role of TREM2 in microglial function in AD and enables us to develop new strategies aimed at the immune system to treat AD pathogenesis.

From beta amyloid to altered proteostasis in Alzheimer's disease

Alzheimer's disease (AD) is an age related neurodegenerative disorder causing severe disability and important socio-economic burden, but with no cure available to date. To disentangle this puzzling disease genetic studies represented an important way for the comprehension of pathogenic mechanisms. Abnormal processing and accumulation of amyloid-β peptide (Aβ) has been considered the main cause and trigger factor of the disease. The amyloid cascade theory has fallen into crisis because the failure of several anti-amyloid drugs trials and because of the simple equation AD = abnormal Aβ deposition is not always the case. We now know that multiple neurodegenerative diseases share common pathogenic mechanisms leading to accumulation of misfolded protein species. Genome Wide Association studies (GWAS) led to the identification of large numbers of DNA common variants (SNPs) distributed on different chromosomes and modulating the Alzheimer's risk. GWAS genes fall into several common pathways such as immune system and neuroinflammation, lipid metabolism, synaptic dysfunction and endocytosis, all of them addressing to novel routes for different pathogenic mechanisms. Other hints could be derived from epidemiological and experimental studies showing some lifestyles may have a major role in the pathogenesis of many age-associated diseases by modifying cell metabolism, proteostasis and microglia mediated neuroinflammation.

Danger-Sensing/Patten Recognition Receptors and Neuroinflammation in Alzheimer's Disease

Fibrillar aggregates and soluble oligomers of both Amyloid-β peptides (Aβs) and hyperphosphorylated Tau proteins (p-Tau-es), as well as a chronic neuroinflammation are the main drivers causing progressive neuronal losses and dementia in Alzheimer’s disease (AD). However, the underlying pathogenetic mechanisms are still much disputed. Several endogenous neurotoxic ligands, including Aβs, and/or p-Tau-es activate innate immunity-related danger-sensing/pattern recognition receptors (PPRs) thereby advancing AD’s neuroinflammation and progression. The major PRR families involved include scavenger, Toll-like, NOD-like, AIM2-like, RIG-like, and CLEC-2 receptors, plus the calcium-sensing receptor (CaSR). This quite intricate picture stresses the need to identify the pathogenetically topmost Aβ-activated PRR, whose signaling would trigger AD’s three main drivers and their intra-brain spread. In theory, the candidate might belong to any PRR family. However, results of preclinical studies using in vitro nontumorigenic human cortical neurons and astrocytes and in vivo AD-model animals have started converging on the CaSR as the pathogenetically upmost PRR candidate. In fact, the CaSR binds both Ca²⁺ and Aβs and promotes the spread of both Ca²⁺ dyshomeostasis and AD’s three main drivers, causing a progressive neurons’ death. Since CaSR’s negative allosteric modulators block all these effects, CaSR’s candidacy for topmost pathogenetic PRR has assumed a growing therapeutic potential worth clinical testing.

Gene expression and functional deficits underlie TREM2-knockout microglia responses in human models of Alzheimer's disease

The discovery of TREM2 as a myeloid-specific Alzheimer's disease (AD) risk gene has accelerated research into the role of microglia in AD. While TREM2 mouse models have provided critical insight, the normal and disease-associated functions of TREM2 in human microglia remain unclear. To examine this question, we profile microglia differentiated from isogenic, CRISPR-modified TREM2-knockout induced pluripotent stem cell (iPSC) lines. By combining transcriptomic and functional analyses with a chimeric AD mouse model, we find that TREM2 deletion reduces microglial survival, impairs phagocytosis of key substrates including APOE, and inhibits SDF-1α/CXCR4-mediated chemotaxis, culminating in an impaired response to beta-amyloid plaques in vivo. Single-cell sequencing of xenotransplanted human microglia further highlights a loss of disease-associated microglial (DAM) responses in human TREM2 knockout microglia that we validate by flow cytometry and immunohistochemistry. Taken together, these studies reveal both conserved and novel aspects of human TREM2 biology that likely play critical roles in the development and progression of AD.

Neuroimaging in genetic frontotemporal dementia and amyotrophic lateral sclerosis

Frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS) have a strong clinical, genetic and pathological overlap. This review focuses on the current understanding of structural, functional and molecular neuroimaging signatures of genetic FTD and ALS. We overview quantitative neuroimaging studies on the most common genes associated with FTD (MAPT, GRN), ALS (SOD1), and both (C9orf72), and summarize visual observations of images reported in the rarer genes (CHMP2B, TARDBP, FUS, OPTN, VCP, UBQLN2, SQSTM1, TREM2, CHCHD10, TBK1).

TREM2 Variants and Neurodegenerative Diseases: A Systematic Review and Meta-Analysis

TREM2 (triggering receptor expressed on myeloid cells 2) gene variants were reported to increase the risk of Alzheimer's disease (AD) and even other neurodegenerative diseases (frontotemporal dementia (FTD), Parkinson's disease (PD) and amyotrophic lateral sclerosis (ALS)), but so far, no definite conclusion has been drawn. The aim of our systematic review and meta-analysis was to investigate the role of TREM2 variants in neurodegenerative diseases. A total of 39 papers (including 26 case-control studies and 13 case reports) were retrieved from PubMed, MEDLINE, EMBASE, and the Cochrane library in this study. A fixed effect model was used to pool results in the analysis. Three variants in TREM2 (rs75932628 (R47H), rs2234255 (H157Y), and rs143332484 (R62H)) were significantly associated with AD risk, but the similar associations between rs104894002 (Q33X), rs2234253 (T96K), rs142232675 (D87N), rs2234256 (L211P), and AD were not proven. Rs75932628 also increased risk of PD in North Americans and FTD, but not PD in Europeans or ALS. In the systematic review, 12 biallelic TREM2 mutations (e.g., rs104894002, rs201258663 (T66M), and rs386834144, etc.) have been described to cause Polycystic Lipomembranous Osteodysplasia with Sclerosing Leukoencephalopathy (PLOSL) in 14 families. And homozygous mutations also have been reported to cause FTD without typical bone phenotypes in 7 families. This study demonstrates that multiple variants in TREM2 have association with the onset of AD, FTD, and PD in North Americans and also play a key role in the phenotypes of the rare familial genetic disorder.

Review: Neuropathology of non-tau frontotemporal lobar degeneration

Frontotemporal dementia (FTD) is a heterogeneous clinical syndrome associated with frontotemporal lobar degeneration (FTLD) as a relatively consistent neuropathological hallmark feature. However, the discoveries in the past decade of many of the relevant pathological proteins aggregating in human FTD brains in addition to several new FTD causing gene mutations underlined that FTD is a diverse condition on the neuropathological and genetic basis. This resulted in a novel molecular classification of these conditions based on the predominant protein abnormality and allows most cases of FTD to be placed now into one of three broad molecular subgroups; FTLD with tau, TAR DNA-binding protein 43 or FET protein accumulation (FTLD-tau, FTLD-TDP and FTLD-FET respectively). This review will provide an overview of the molecular neuropathology of non-tau FTLD, insights into disease mechanisms gained from the study of human post mortem tissue as well as discussion of current controversies in the field.

Control of Innate Immunity by Sialic Acids in the Nervous Tissue

Sialic acids (Sias) are the most abundant terminal sugar residues of glycoproteins and glycolipids on the surface of mammalian cells. The nervous tissue is the organ with the highest expression level of Sias. The ‘sialylation’ of glycoconjugates is performed via sialyltransferases, whereas ‘desialylation’ is done by sialidases or is a possible consequence of oxidative damage. Sialic acid residues on the neural cell surfaces inhibit complement and microglial activation, as well as phagocytosis of the underlying structures, via binding to (i) complement factor H (CFH) or (ii) sialic acid-binding immunoglobulin-like lectin (SIGLEC) receptors. In contrast, activated microglial cells show sialidase activity that desialylates both microglia and neurons, and further stimulates innate immunity via microglia and complement activation. The desialylation conveys neurons to become susceptible to phagocytosis, as well as triggers a microglial phagocytosis-associated oxidative burst and inflammation. Dysfunctions of the ‘Sia–SIGLEC’ and/or ‘Sia–complement’ axes often lead to neurological diseases. Thus, Sias on glycoconjugates of the intact glycocalyx and its desialylation are major regulators of neuroinflammation.

Activation of FAK/Rac1/Cdc42-GTPase signaling ameliorates impaired microglial migration response to Aβ42 in triggering receptor expressed on myeloid cells 2 loss-of-function murine models

Mutation of Triggering receptor expressed on myeloid cells 2 (TREM2) impairs the response of microglia to amyloid-β (Aβ) pathology in Alzheimer's disease (AD), although the mechanism governing TREM2-regulated microglia recruitment to Aβ plaques remains unresolved. Here, we confirm that TREM2 mutation attenuates microglial migration. Then, using Trem2^-/- mice and an R47H variant mouse model for AD generated for this study, we show that TREM2 deficiency or the AD-associated R47H mutation results in inhibition of FAK and Rac1/Cdc42-GTPase signaling critical for cell migration. Intriguingly, treatment with CN04, a Rac1/Cdc42-GTPase activator, partially enhances microglial migration in response to oligomeric Aβ₄₂ in Trem2^-/- or R47H microglia both in vitro and in vivo. Our study shows that the dysfunction of microglial migration in the AD-associated TREM2 R47H variant is caused by FAK/Rac1/Cdc42 signaling disruption, and that activation of this signaling ameliorates impaired microglial migration response to Aβ₄₂ , suggesting a therapeutic target for R47H-bearing patients with high risk of AD.

Opposite microglial activation stages upon loss of PGRN or TREM2 result in reduced cerebral glucose metabolism

Microglia adopt numerous fates with homeostatic microglia (HM) and a microglial neurodegenerative phenotype (MGnD) representing two opposite ends. A number of variants in genes selectively expressed in microglia are associated with an increased risk for neurodegenerative diseases such as Alzheimer's disease (AD) and frontotemporal lobar degeneration (FTLD). Among these genes are progranulin (GRN) and the triggering receptor expressed on myeloid cells 2 (TREM2). Both cause neurodegeneration by mechanisms involving loss of function. We have now isolated microglia from Grn^−/− mice and compared their transcriptomes to those of Trem2^−/−mice. Surprisingly, while loss of Trem2 enhances the expression of genes associated with a homeostatic state, microglia derived from Grn^−/− mice showed a reciprocal activation of the MGnD molecular signature and suppression of gene characteristic for HM. The opposite mRNA expression profiles are associated with divergent functional phenotypes. Although loss of TREM2 and progranulin resulted in opposite activation states and functional phenotypes of microglia, FDG (fluoro‐2‐deoxy‐d‐glucose)‐μPET of brain revealed reduced glucose metabolism in both conditions, suggesting that opposite microglial phenotypes result in similar wide spread brain dysfunction.

Assessment of TREM2 rs75932628 variant's association with Parkinson's disease in a Greek population and Meta-analysis of current data

BACKGROUND

Α number of genetic variants are considered to confer susceptibility to Parkinson's disease (PD). Rs75392628 (R47H), a rare variant of TREM2 gene, has been linked to PD, although its role on PD remains conflicting.

OBJECTIVE

Detection of a possible contribution of rs75392628 variant of TREM2 gene to PD risk.

METHODS

A total of 358 PD patients and 358 healthy controls genotyped for rs75392628. In addition, a meta-analysis was performed by merging our results with those from previous studies.

RESULTS

The rare variant of rs75932628 (47H) of TREM2 gene was not detected on cohort. Meta-analysis of a total of 9271 PD cases and 9777 controls across 14 independent PD data sets from 9 studies, including the present study, did not show any statistically significant effect of rs75392628 on PD risk (OR_FE:1.54 95% CI:0.87-2.73. OR_RE: 1.54, 95%CI: 0.71-3.32).

CONCLUSIONS

Rs75392628 TREM2 variant is rather unlikely to be a major genetic risk contributor of PD.

Enhancing protective microglial activities with a dual function TREM2 antibody to the stalk region

Triggering receptor expressed on myeloid cells 2 (TREM2) is essential for the transition of homeostatic microglia to a disease-associated microglial state. To enhance TREM2 activity, we sought to selectively increase the full-length protein on the cell surface via reducing its proteolytic shedding by A Disintegrin And Metalloproteinase (i.e., α-secretase) 10/17. We screened a panel of monoclonal antibodies against TREM2, with the aim to selectively compete for α-secretase-mediated shedding. Monoclonal antibody 4D9, which has a stalk region epitope close to the cleavage site, demonstrated dual mechanisms of action by stabilizing TREM2 on the cell surface and reducing its shedding, and concomitantly activating phospho-SYK signaling. 4D9 stimulated survival of macrophages and increased microglial uptake of myelin debris and amyloid β-peptide in vitro. In vivo target engagement was demonstrated in cerebrospinal fluid, where nearly all soluble TREM2 was 4D9-bound. Moreover, in a mouse model for Alzheimer's disease-related pathology, 4D9 reduced amyloidogenesis, enhanced microglial TREM2 expression, and reduced a homeostatic marker, suggesting a protective function by driving microglia toward a disease-associated state.

Increased oxidative stress, hyperphosphorylation of tau, and dystrophic microglia in the hippocampus of aged Tupaia belangeri

Aging is a major risk factor for the development of neurodegenerative diseases. Alzheimer's disease and other neurodegenerative diseases are characterized by abnormal and prominent protein aggregation in the brain, partially due to deficiency in protein clearance. It has been proposed that alterations in microglia phagocytosis and debris clearance hasten the onset of neurodegeneration. Dystrophic microglia are abundant in aged humans, and it has been associated with the onset of disease. Furthermore, alterations in microglia containing ferritin are associated with neurodegenerative conditions. To further understand the process of microglia dysfunction during the aging process, we used hippocampal sections from Tupaia belangeri (tree shrews). Adult (mean age 3.8 years), old (mean age 6 years), and aged (mean age 7.5 years) tree shrews were used for histochemical and immunostaining techniques to determine ferritin and Iba1 positive microglia, iron tissue content, tau hyperphosphorylation and oxidized-RNA in dentate gyrus, subiculum, and CA1-CA3 hippocampal regions. Our results indicated that aged tree shrews presented an increased number of activated microglia containing ferritin, but microglia labeled with Iba1 with a dystrophic phenotype was more abundant in aged individuals. With aging, oxidative damage to RNA (8OHG) increased significantly in all hippocampal regions, while tau hyperphosphorylation (AT100) was enhanced in DG, CA3, and SUB in aged animals. Phagocytic inclusions of 8OHG- and AT100-damaged cells were observed in activated M2 microglia in old and aged animals. These data indicate that aged tree shrew may be a suitable model for translational research to study brain and microglia alterations during the aging process.

Analysis of Cerebrospinal Fluid Soluble TREM2 and Polymorphisms in Sporadic Parkinson's Disease in a Chinese Population

BACKGROUND

Triggering receptor expressed on myeloid cells 2 (TREM2) is a microglial surface receptor that mediates the degradation disorder of amyloid β (Aβ) in Alzheimer's disease. However, the role of TREM2 in Parkinson's disease (PD) and α-Synclein (α-Syn) degradation is largely unknown.

METHODS

In this case-control study on Chinese population, we sequenced for polymorphisms in exon 2 of the TREM2 gene in 1,292 individuals, PD cases (n = 612), healthy controls (n = 680) by Sanger sequence, and compared the distribution of allelic frequencies between the two groups by the Fisher's exact test. Additionally, we developed and used the enzyme-linked immunosorbent assay to evaluated soluble TREM2 (sTREM2) levels in the cerebrospinal fluid (CSF), and plasma in partial of sequenced groups (55 PD and 40 healthy controls) analyzed their relationship with total a-syn (t-a-Syn).

RESULTS

Two novel variants were detected in exon 2 of the TREM2 gene, namely, p.S81 N, p.G58D; however, these were not significantly associated with PD (612 PD and 680 healthy controls). sTREM2 in CSF was significantly upregulated in PD patients compared to healthy controls (433.1 ± 24.7 pg/mL vs. 275.2 ± 17.9 pg/mL, p < 0.0001), but not in plasma (281.7 ± 29.3 pg/mL vs. 257.8 ± 16.5 pg/mL, p = 0.805). In PD patients, sTREM2 was positively correlated with t-α-syn (r = 0.62, p = 0.0001) in CSF, but not in plasma (r = 0.02, p = 0.89).

CONCLUSIONS

Although it may not indicate that exon 2 polymorphisms of TREM2 play a role in the pathogenesis of PD in the Chinese population, our findings described above highlight the relevance of CSF sTREM2 as a promising biomarker and are extremely possible to the therapeutic target for PD in the future.

Plasma levels of soluble TREM2 and neurofilament light chain in TREM2 rare variant carriers

Background

Results from recent clinical studies suggest that cerebrospinal fluid (CSF) biomarkers that are indicative of Alzheimer’s disease (AD) can be replicated in blood, e.g. amyloid-beta peptides (Aβ₄₂ and Aβ₄₀) and neurofilament light chain (NFL). Such data proposes that blood is a rich source of potential biomarkers reflecting central nervous system pathophysiology and should be fully explored for biomarkers that show promise in CSF. Recently, soluble fragments of the triggering receptor expressed on myeloid cells 2 (sTREM2) protein in CSF have been reported to be increased in prodromal AD and also in individuals with TREM2 rare genetic variants that increase the likelihood of developing dementia.

Methods

In this study, we measured the levels of plasma sTREM2 and plasma NFL using the MesoScale Discovery and single molecule array platforms, respectively, in 48 confirmed TREM2 rare variant carriers and 49 non-carriers.

Results

Our results indicate that there are no changes in plasma sTREM2 and NFL concentrations between TREM2 rare variant carriers and non-carriers. Furthermore, plasma sTREM2 is not different between healthy controls, mild cognitive impairment (MCI) or AD.

Conclusion

Concentrations of plasma sTREM2 do not mimic the recent changes found in CSF sTREM2.

Neurodegenerative Disease-Associated Variants in TREM2 Destabilize the Apical Ligand-Binding Region of the Immunoglobulin Domain

Single nucleotide variations in Triggering Receptor Expressed on Myeloid Cells 2 (TREM2) have been linked to both late-onset Alzheimer's disease and behavioral variant frontotemporal dementia (FTD), the latter presenting either in isolation or with cystic bone lesions in a condition called Nasu-Hakola disease. Models of the extracellular domain of TREM2 show that Nasu-Hakola disease-associated mutations are grossly inactivating by truncation, frameshift, or unfolding, that Alzheimer's disease (AD)-associated variants localize to a putative ligand-interacting region (PLIR) on the extracellular surface, and that FTD-associated variants are found in the hydrophobic core. However, while these disease-associated residues are predicted to play some role in disrupting ligand binding to the extracellular domain of TREM2, how they ultimately lead to disease remains unknown. Here, we used in silico molecular modeling to investigate all-atom models of TREM2 and characterize the effects on conformation and dynamical motion of AD-associated R47H and R62H as well as FTD-associated T96K, D86V, and T66M variants compared to the benign N68K variant and the common variant. Our model, which is based on a published 2.2 Å resolution crystal structure of the TREM2 extracellular domain, finds that both AD- and FTD-associated variants cause localized instability in three loops adjacent to the PLIR that correspond to the complementarity-determining regions (CDRs) of antibodies. This instability ultimately disrupts tethering between these CDRs and the core of the immunoglobulin domain, exposing a group of otherwise-buried, negatively charged residues. This instability and exposure of negatively charged residues is most severe following introduction of the T66M variant that has been described as causing FTD even in the heterozygous state and is less severe following introduction of variants that are less strongly tied to FTD or of those associated with AD. Thus, our results provide further evidence that the proposed loss-of-function caused by neurodegenerative disease-associated variants may be driven by altered conformational stability of the ligand-interacting CDR and, ultimately, loss of affinity or specificity for TREM2 ligands.

The p.R47H Variant of TREM2 Gene is Associated With Late-onset Alzheimer Disease in Colombian Population

OBJECTIVE

We evaluated the association of several single-nucleotide polymorphisms in the triggering receptor expressed on myeloid cells 2 (TREM2) gene in a Colombian sample of late-onset Alzheimer disease (LOAD).

METHODS

The p.Q33* (rs104894002), p.R47H (rs75932628), p.R62H (rs143332484), and p.D87N (rs142232675) variants of TREM2 gene were directly genotyped using KASPar technology in 358 cases and 329 healthy controls. Sanger sequencing was used to validate >10% of KASPar's results. The Fisher exact test was used to compare the distribution of allelic and genotype frequency between cases and controls, and the Bonferroni correction was set at P<0.05.

RESULTS

The minor allele frequency of rs75932628-T was 0.009 in cases and was not found in any healthy controls which suggests a significant association between rs75932628-T and LOAD risk in our sample (P=0.010). The rs143332484-T variant did not exhibit a significant association (P=0.160), whereas rs104894002 and rs142232675 were not found.

CONCLUSIONS

Our findings suggest that the rs75932628-T variant of TREM2 is an important risk factor for LOAD in the Colombian population.

Microglial inflammation and phagocytosis in Alzheimer's disease: Potential therapeutic targets

One of the largest unmet medical needs is a disease-modifying treatment for Alzheimer's disease (AD). Recently, the role of microglia in disease, particularly AD, has gained great interest, following the identification of several disease risk-associated genes that are highly expressed in microglia. Microglia play a critical homeostatic role in the brain, with neuroinflammatory and phagocytic mechanisms being of particular importance. Here, we review the role of NLRP3, the complement system, and the triggering receptor expressed in myeloid cells 2 (TREM2) in modulating microglial functions. We have reviewed the targets, their molecular pathways and the therapeutic interventions aimed at modulating these targets, in the hope of discovering a novel therapeutic approach for the treatment of AD. LINKED ARTICLES: This article is part of a themed section on Therapeutics for Dementia and Alzheimer's Disease: New Directions for Precision Medicine. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.18/issuetoc.

Frontotemporal Dementia and Chorea Associated with a Compound Heterozygous TREM2 Mutation

Frontotemporal dementia (FTD) is clinically characterized by behavioral changes, language impairment, and executive dysfunction. FTD usually belongs to the frontotemporal lobar degeneration (FTLD) disease group, and its familial forms are dominantly inherited and linked to a group of genes relevant to frontal and temporal brain pathology, such as MAPT, GRN, C9ORF72, TARDBP, CHMP2B, VCP, and FUS. However, FTD can also be associated with different clinical or pathological phenotypes caused by mutations in other genes, whose heredity can be dominant or recessive. In this work we report on a familial case of FTD characterized by behavioral changes and aphasia, very early onset and very long duration, choreic movements, and white matter lesions at magnetic resonance imaging. We performed a wide-range genetic analysis, using a next generation sequencing approach, to evaluate a number of genes involved in neurodegeneration. We found a previously unreported compound heterozygous mutation in TREM2, that is commonly associated with the recessively inherited Nasu-Hakola disease. We discuss the differential diagnosis to be taken into account in cases of FTD presenting with atypical features.

A nonsynonymous mutation in PLCG2 reduces the risk of Alzheimer's disease, dementia with Lewy bodies and frontotemporal dementia, and increases the likelihood of longevity

The genetic variant rs72824905-G (minor allele) in the PLCG2 gene was previously associated with a reduced Alzheimer's disease risk (AD). The role of PLCG2 in immune system signaling suggests it may also protect against other neurodegenerative diseases and possibly associates with longevity. We studied the effect of the rs72824905-G on seven neurodegenerative diseases and longevity, using 53,627 patients, 3,516 long-lived individuals and 149,290 study-matched controls. We replicated the association of rs72824905-G with reduced AD risk and we found an association with reduced risk of dementia with Lewy bodies (DLB) and frontotemporal dementia (FTD). We did not find evidence for an effect on Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS) and multiple sclerosis (MS) risks, despite adequate sample sizes. Conversely, the rs72824905-G allele was associated with increased likelihood of longevity. By-proxy analyses in the UK Biobank supported the associations with both dementia and longevity. Concluding, rs72824905-G has a protective effect against multiple neurodegenerative diseases indicating shared aspects of disease etiology. Our findings merit studying the PLCγ2 pathway as drug-target.

TREM2 R47H (rs75932628) variant is unlikely to contribute to Multiple Sclerosis susceptibility and severity in a large Greek MS cohort

BACKGROUND

Multiple Sclerosis is a multifactorial autoimmune disease of the central nervous system, characterized by focal inflammation, demyelination and secondary axonal injury. TREM2 is a signaling protein which participates in the innate immune system by implication to inflammation, proliferation and phagocytosis. The R47H (rs75392628) rare variant of the TREM2 gene has been related to various neurological diseases and leads to impaired signaling, lipoprotein binding, lipoprotein uptake and surface uptake.

AIM

To assess the role of TREM2 rs75932628 on MS risk through a genetic candidate gene association case-control study in a Greek population.

METHODS

1246 MS cases and 398 controls were genotyped for this variant.

RESULTS

No MS or healthy subjects carried the variant.

CONCLUSION

This variant does not seem to play a determining role in the pathogenesis of MS, although further studies examining the presence of TREM2 mutations in other, phylogenetically different populations and the epigenetic regulation of this gene are needed in order to thoroughly investigate its role in MS.

The Transcriptional Landscape of Microglial Genes in Aging and Neurodegenerative Disease

Microglia, the brain-resident myeloid cells, are strongly implicated in Alzheimer's disease (AD) pathogenesis by human genetics. However, the mechanisms by which microglial gene expression is regulated in a region-specific manner over the course of normal aging and in neurodegenerative disease are only beginning to be deciphered. Herein, we used a specific marker of microglia (TMEM119) and a cell-type expression profiling tool (CellMapper) to identify a human microglial gene expression module. Surprisingly, we found that microglial module genes are robustly expressed in several healthy human brain regions known to be vulnerable in AD, in addition to other regions affected only later in disease or spared in AD. Surveying the microglial gene set for differential expression over the lifespan in mouse models of AD and a related tauopathy revealed that the majority of microglial module genes were significantly upregulated in cortex and hippocampus as a function of age and transgene status. Extending these results, we also observed significant upregulation of microglial module genes in several AD-affected brain regions in addition to other regions using postmortem brain tissue from human AD samples. In pathologically confirmed AD cases, we found preliminary evidence that microglial genes may be dysregulated in a sex-specific manner. Finally, we identified specific and significant overlap between the described microglial gene set—identified by unbiased co-expression analysis—and genes known to impart risk for AD. Our findings suggest that microglial genes show enriched expression in AD-vulnerable brain regions, are upregulated during aging and neurodegeneration in mice, and are upregulated in pathologically affected brain regions in AD. Taken together, our data-driven findings from multiple publicly accessible datasets reemphasize the importance of microglial gene expression alterations in AD and, more importantly, suggest that regional and sex-specific variation in microglial gene expression may be implicated in risk for and progression of neurodegenerative disease.

Microglia-neuron crosstalk: Signaling mechanism and control of synaptic transmission

The continuous crosstalk between microglia and neurons is required for microglia housekeeping functions and contributes to brain homeostasis. Through these exchanges, microglia take part in crucial brain functions, including development and plasticity. The alteration of neuron-microglia communication contributes to brain disease states with consequences, ranging from synaptic function to neuronal survival. This review focuses on the signaling pathways responsible for neuron-microglia crosstalk, highlighting their physiological roles and their alteration or specific involvement in disease. In particular, we discuss studies, establishing how these signaling allow microglial cells to control relevant physiological functions during brain development, including synaptic formation and circuit refinement. In addition, we highlight how microglia and neurons interact functionally to regulate highly dynamical synaptic functions. Microglia are able to release several signaling molecules involved in the regulation of synaptic activity and plasticity. On the other side, molecules of neuronal origin control microglial processes motility in an activity-dependent manner. Indeed, the continuous crosstalk between microglia and neurons is required for the sensing and housekeeping functions of microglia and contributes to the maintenance of brain homeostasis and, particularly, to the sculpting of neuronal connections during development. These interactions lay on the delicate edge between physiological processes and homeostasis alteration in pathology and are themselves altered during neuroinflammation. The full description of these processes could be fundamental for understanding brain functioning in health and disease.

Aminophospholipids are signal-transducing TREM2 ligands on apoptotic cells

Variants of triggering receptor expressed on myeloid cells 2 (TREM2) are associated with an increased incidence of Alzheimer’s disease, as well as other neurodegenerative disorders. Using a newly developed, highly sensitive reporter cell model, consisting of Jurkat T cells stably overexpressing a reporter gene and a gene encoding TREM2DAP12 fusion protein, we show here that TREM2-dependent signal transduction in response to apoptotic Neuro2a cells is mediated by aminophospholipid ligands, phosphatidylserine and phosphatidylethanolamine, which are not exposed on the intact cell surface, but become exposed upon apoptosis. We also show that signal-transducing TREM2 ligands different from aminophospholipids, which appear to be derived from neurons, might be present in membrane fractions of mouse cerebral cortex. These results may suggest that TREM2 regulates microglial function by transducing intracellular signals from aminophospholipids on apoptotic cells, as well as unidentified ligands in the membranes of the cerebral cortex.

Genetic screen in a large series of patients with primary progressive aphasia

INTRODUCTION

Primary progressive aphasia (PPA) is a neurological syndrome, associated with both frontotemporal dementia and Alzheimer's disease, in which progressive language impairment emerges as the most salient clinical feature during the initial stages of disease.

METHODS

We screened the main genes associated with Alzheimer's disease and frontotemporal dementia for pathogenic and risk variants in a cohort of 403 PPA cases.

RESULTS

In this case series study, 14 (3.5%) cases carried (likely) pathogenic variants: four C9orf72 expansions, nine GRN, and one TARDBP mutation. Rare risk variants, TREM2 R47H and MAPT A152T, were associated with a three- to seven-fold increase in risk for PPA.

DISCUSSION

Our results show that while pathogenic variants within the most common dementia genes were rarely associated with PPA, these were found almost exclusively in GRN and C9orf72, suggesting that PPA is more TDP43- than tau-related in our series. This is consistent with the finding that PPA frequency in dominantly inherited dementias is the highest in kindreds with GRN variants.

Frontotemporal dementia spectrum: first genetic screen in a Greek cohort

Frontotemporal dementia (FTD) is a heterogeneous group of neurodegenerative syndromes associated with several causative and susceptibility genes. Herein, we aimed to determine the incidence of the most common causative dementia genes in a cohort of 118 unrelated Greek FTD spectrum patients. We also screened for novel possible disease-associated variants in additional 21 genes associated with FTD or amyotrophic lateral sclerosis. Pathogenic or likely pathogenic variants were identified in 16 cases (13.6%). These included repeat expansions in C9orf72 and loss-of-function GRN variants, and likely pathogenic variants in TARDBP, MAPT, and PSEN1. We also identified 14 variants of unknown significance in other rarer FTD or amyotrophic lateral sclerosis genes that require further segregation and functional analysis. Our genetic screen revealed a high genetic burden in familial Greek FTD cases (30.4%), whereas only two of the sporadic cases (3.5%) carried a likely pathogenic variant. A substantial number of familial cases still remain without an obvious causal variant, suggesting the existence of other FTD genetic causes besides those currently screened in clinical routine.