TREM2 Human

What is the cellular expression pattern of TREM2 in the human brain?

Contrary to previous assumptions based on animal models, human TREM2 expression appears to follow a distinct pattern. Studies using post-mortem resources from the Medical Research Council Cognitive Function and Ageing Studies (MRC-CFAS) revealed that TREM2 predominantly labels monocytes within vascular lumens rather than resident microglia or perivascular macrophages in most cases (284/299 participants) . This finding challenges the widely held belief that TREM2 is primarily expressed by microglia in humans.

Specifically, when comparing immunostaining patterns of TREM2 with established macrophage/microglial markers Iba1 and CD68, researchers observed that while Iba1 and CD68 consistently labeled microglia and perivascular macrophages as expected, TREM2 antibody (Sigma HPA010917) primarily identified monocytes within blood vessels . Interestingly, in acute infarct cases (5 out of 6), TREM2-positive cells were identified within brain parenchyma, suggesting TREM2 may mark recruited monocytes rather than resident microglia .

What are the known alternative splicing variants of human TREM2?

The human TREM2 gene undergoes alternative splicing, producing multiple transcript variants. The canonical transcript encompasses 5 exons, but recent research has identified a novel splice variant lacking exon 2 (Δe2) . This Δe2 isoform constitutes a significant fraction of TREM2 transcripts in the human brain, with highly variable inter-individual expression ranging from 3.7% to 35% across different brain regions .

The protein encoded by the Δe2 splice variant has a significant structural difference: it lacks the V-set immunoglobulin domain from its extracellular portion but retains its transmembrane and cytoplasmic domains . Functional studies show that while full-length TREM2 can restore phagocytic capacity and promote interferon type I response in knockout cells, the Δe2 isoform fails to rescue these functions, suggesting the V-set domain is critical for TREM2's primary functions .

How do peripheral and central expression patterns of TREM2 differ?

TREM2 exhibits tissue-specific expression patterns that vary considerably between central nervous system and peripheral tissues. According to GTEx database analysis, TREM2 is predominantly expressed in brain, lung, and nerve tissues . Adipose tissues (subcutaneous and visceral) rank 12th and 20th respectively among 54 tissue types in terms of normalized expression levels .

In peripheral tissues, particularly adipose tissue, TREM2 expression has been correlated with BMI in a gender-specific manner, showing significant association in males but not females . This suggests differential regulation and potentially distinct functions of TREM2 across different tissue environments. The variation in expression patterns likely reflects specialized roles of TREM2 in tissue-specific immune responses and metabolic functions.

How does TREM2 influence Alzheimer's disease pathology?

TREM2 appears to play a protective role in Alzheimer's disease pathology through several mechanisms. Experimental studies using AD mouse models demonstrate that overexpression of human TREM2 reprograms microglia into a more phagocytic and less inflammatory state . This reprogramming results in significant pathological improvements, including:

• Reduced amyloid plaque formation and accumulation

• Decreased dystrophic neurite development around plaques

• Preserved learning and memory functions compared to AD controls

At the molecular level, TREM2 directly binds Aβ peptides, triggering microglial activation, cytokine secretion, and enhanced degradation of internalized Aβ . This binding mechanism provides a direct pathway through which TREM2 mediates clearance of pathological proteins in AD.

The protective role of TREM2 is further supported by the observation that TREM2 variants, particularly the R47H mutation, are among the strongest genetic risk factors for late-onset Alzheimer's disease . These variants likely compromise TREM2's ability to bind Aβ and facilitate microglial phagocytosis, thereby reducing clearance of pathological proteins and promoting disease progression.

What methodological approaches are most effective for studying TREM2's role in human neurodegeneration?

A comprehensive investigation of TREM2 in human neurodegeneration requires multiple complementary methodological approaches:

When studying human brain tissue specifically, researchers should pay careful attention to post-mortem interval (ideally under 6 hours) and tissue preservation methods . For isoform-specific quantification, TaqMan qRT-PCR assays with probes spanning specific exon junctions (e.g., exon 2/3 for full-length and exon 1/3 for Δe2 isoform) provide accurate measurements when validated against plasmid standards .

How does TREM2 function change during disease progression?

TREM2 function undergoes dynamic changes throughout neurodegenerative disease progression, reflecting its role in the evolving immune response. In early disease stages, TREM2 expression may increase as microglia and/or recruited monocytes respond to initial pathological changes. This is supported by observations in acute infarct cases where TREM2-positive cells appear within brain parenchyma .

As disease progresses, the effectiveness of TREM2-mediated responses may diminish, either due to overwhelming pathology or altered microglial/monocyte function. In chronic lesions, such as old infarcts, phagocytic foamy macrophages are CD68-positive but TREM2-negative, suggesting a temporal regulation of TREM2 expression during the evolution of inflammatory responses .

The differences in TREM2 expression between acute and chronic pathological states highlight its specific role in early immune responses, particularly monocyte recruitment, rather than in long-term phagocytic activity of tissue macrophages .

How do specific TREM2 mutations and variants impact protein function?

TREM2 variants exhibit diverse functional consequences depending on the affected domain and the nature of the mutation:

The functional consequences of these variants provide insight into domain-specific functions of TREM2. The V-set immunoglobulin domain, absent in the Δe2 isoform and affected by the R47H variant, appears critical for ligand binding and phagocytic function. Overexpression studies in knockout cell models demonstrate that full-length TREM2, but not the Δe2 isoform, can restore phagocytic capacity and interferon type I response .

What experimental approaches can differentiate the functions of different TREM2 isoforms?

Differentiating functions between TREM2 isoforms requires specialized experimental approaches:

• Genetic Manipulation Technologies:

  • CRISPR/Cas9 disruption of specific exons (e.g., exon 2 for studying Δe2)

  • Lentiviral expression systems for controlled introduction of specific isoforms

  • Inducible expression systems (e.g., doxycycline-regulated) for temporal control

• Functional Readouts:

  • Phagocytosis assays using fluorescent substrates

  • Cytokine profiling to assess inflammatory responses

  • Transcriptomic analysis to identify isoform-specific downstream effects

• Binding Studies:

  • Surface plasmon resonance to measure binding affinities

  • Immunoprecipitation to identify isoform-specific binding partners

  • In vitro binding assays with purified proteins

A particularly effective approach combines CRISPR/Cas9 knockout of endogenous TREM2 followed by "add-back" experiments using lentiviral constructs expressing specific isoforms. Such methods have demonstrated that full-length TREM2, but not Δe2, restores phagocytic capacity and interferon responses in knockout cells .

How does inter-individual variation in TREM2 splicing impact disease susceptibility?

The inter-individual variation in TREM2 splicing, particularly the relative abundance of the Δe2 isoform, may contribute to differential disease susceptibility. The Δe2 isoform shows remarkable variability between individuals, ranging from 3.7% to 35% of total TREM2 transcripts in the human brain . This variability appears tissue-specific, with different patterns observed in frontal cortex versus hippocampus .

This hypothesis aligns with observations that TREM2 haploinsufficiency increases risk for neurodegenerative diseases, while complete loss-of-function causes more severe conditions like Nasu-Hakola disease .

What evidence supports TREM2's role in human metabolic disorders?

Computational analysis using public databases provides compelling evidence for TREM2's involvement in human metabolic regulation, particularly obesity:

• Expression Correlation:

  • Significant correlation between TREM2 expression levels and BMI in adipose tissues from GTEx database samples

  • Gender-specific association, evident in males but not females

• Genetic Association:

  • Identification of a coding SNP in TREM2 (rs2234256, L211P) significantly associated with BMI in UK Biobank cohort

  • This SNP shows particularly strong association with BMI compared to previously identified obesity-related SNPs

  • Individuals carrying this SNP as heterozygous show significantly higher BMI values, with an even stronger effect in homozygous carriers

These findings establish TREM2 as a significant factor in human obesity, particularly in males, and suggest potential gender-specific regulatory mechanisms that warrant further investigation .

What methodologies are most appropriate for studying TREM2 in metabolic tissues?

Studying TREM2 in metabolic tissues requires specialized approaches that address tissue-specific expression patterns and regulatory mechanisms:

• Tissue-Specific Expression Analysis:

  • RNA isolation protocols optimized for adipose tissue

  • Careful selection of cell-specific markers to identify TREM2-expressing populations

  • Consideration of depot-specific differences (visceral vs. subcutaneous adipose)

• Genetic Association Studies:

  • Large cohorts with detailed metabolic phenotyping

  • Gender-stratified analysis to capture sex-specific effects

  • Integration of multiple '-omics' datasets (genomics, transcriptomics, metabolomics)

• Functional Studies:

  • Primary adipose tissue culture systems

  • Differentiation protocols for adipocytes and adipose tissue macrophages

  • Metabolic flux analysis in TREM2-modulated cells

• Translational Approaches:

  • Correlation between TREM2 variants and clinical metabolic parameters

  • Integration of mouse model findings with human genetic data

  • Therapeutic targeting strategies based on tissue-specific expression

These methodologies should be applied with consideration of the gender-specific effects observed in human studies, where TREM2's association with BMI is evident in males but not females .

How does TREM2 function differ between adipose tissue and brain?

TREM2 exhibits tissue-specific functions that reflect its distinct roles in adipose tissue versus brain:

These differences suggest that while TREM2 maintains its basic function as a myeloid cell receptor across tissues, its specific ligands, signaling pathways, and physiological outcomes are tailored to tissue-specific requirements. In the brain, TREM2 appears primarily involved in monocyte recruitment and acute responses to injury , while in adipose tissue, it functions in metabolic regulation with strong gender-specific effects .

What are the most reliable approaches for quantifying different TREM2 isoforms?

Accurate quantification of TREM2 isoforms requires specialized techniques that can distinguish between structurally similar transcripts:

• TaqMan qRT-PCR with Junction-Spanning Probes:

  • Probes designed to span exon junctions (e.g., exon 2/3 for full-length, exon 1/3 for Δe2)

  • Absolute quantification using plasmid standards containing cloned isoform sequences

  • Controls for assay specificity testing against each isolated isoform

• Digital PCR:

  • Provides absolute quantification without reliance on standard curves

  • Particularly useful for low-abundance transcripts

  • Higher precision for detecting small differences in isoform ratios

• RNA Sequencing:

  • Junction reads analysis for comprehensive isoform detection

  • Long-read sequencing (Oxford Nanopore, PacBio) for unambiguous isoform identification

  • Computational approaches to distinguish isoforms from sequencing data

For optimal results, researchers should combine multiple approaches and include appropriate controls. Studies have successfully used TaqMan qRT-PCR assays with junction-spanning probes validated against plasmid standards to determine that the Δe2 isoform accounts for approximately 10% of total TREM2 transcripts in human brain, with considerable inter-individual variability .

What are the key considerations for validating antibodies against human TREM2?

Antibody validation for human TREM2 research requires rigorous testing due to several challenges:

• Specificity Testing:

  • Validation using CRISPR/Cas9-generated TREM2 knockout cells as negative controls

  • Testing against cells expressing only specific TREM2 isoforms

  • Cross-reactivity assessment with other TREM family proteins

• Domain-Specific Validation:

  • Confirmation of epitope location relative to splice junctions

  • Ability to distinguish full-length versus Δe2 isoforms

  • Testing across multiple applications (WB, IHC, flow cytometry)

• Application-Specific Optimization:

  • For immunohistochemistry, optimization of fixation and antigen retrieval protocols

  • For flow cytometry, verification of surface versus intracellular staining

  • For Western blotting, consideration of glycosylation states affecting migration

• Reproducibility Assessment:

  • Batch-to-batch consistency evaluation

  • Performance in different tissue types and preparation methods

  • Comparison with alternative antibodies targeting different epitopes

Previous successful validation has been demonstrated with the Sigma antibody HPA010917 for immunostaining TREM2 in human brain sections, with parallel staining of established markers (Iba1, CD68) serving as controls for cell-type specificity .

What experimental design best addresses the gender-specific effects of TREM2?

Given the observed gender-specific effects of TREM2, particularly in metabolic contexts , experimental designs must specifically account for these differences:

• Sample Stratification:

  • A priori separation of male and female samples in all analyses

  • Adequate statistical power for gender-stratified analyses

  • Matching for age and other relevant variables within gender groups

• Hormonal Considerations:

  • Assessment of hormonal status (particularly estrogen levels)

  • Consideration of menstrual/estrous cycle in female subjects

  • Hormone manipulation studies in experimental models

• Multi-tissue Analysis:

  • Parallel examination of TREM2 function across tissues

  • Investigation of tissue-specific gender dimorphism

  • Integration of brain and peripheral tissue findings

• Genetic Background Effects:

  • Analysis of gender-specific effects of TREM2 variants

  • Consideration of X-chromosome gene regulation

  • Evaluation of gender-specific genetic modifiers

This approach has successfully revealed that the correlation between TREM2 expression levels and BMI in adipose tissues is significant in males but not in females , demonstrating the importance of gender-stratified analysis in TREM2 research.

What therapeutic approaches targeting TREM2 show the most promise?

Based on current understanding of TREM2 biology, several therapeutic approaches show promise:

• TREM2 Enhancing Strategies:

  • Agonistic antibodies that mimic ligand binding

  • Small molecules that enhance TREM2 signaling

  • Gene therapy approaches to increase TREM2 expression

• Isoform-Specific Targeting:

  • Promotion of full-length over Δe2 isoform expression

  • Antisense oligonucleotides to modulate splicing

  • Stabilization of full-length TREM2 protein

• Pathway-Focused Approaches:

  • Targeting downstream effectors of TREM2 signaling

  • Enhancing the phagocytic capacity of myeloid cells

  • Modulating inflammatory responses in TREM2-dependent pathways

Studies in AD mouse models demonstrate that overexpressing human TREM2 reprograms microglia, reduces amyloid plaques and dystrophic neurites, and maintains normal learning and memory . These findings suggest that boosting TREM2 function could ameliorate AD pathology and potentially other neurodegenerative conditions.

How might inter-individual variation in TREM2 splicing impact personalized medicine approaches?

The high inter-individual variability in TREM2 isoform expression, particularly the Δe2 variant (ranging from 3.7-35% of total TREM2 transcripts) , has significant implications for personalized medicine:

• Diagnostic Applications:

  • TREM2 isoform ratios as potential biomarkers for disease susceptibility

  • Integration of isoform profiles with genetic variant status

  • Development of blood-based assays to reflect central TREM2 status

• Therapeutic Stratification:

  • Identification of patients most likely to benefit from TREM2-targeted therapies

  • Dose adjustment based on functional TREM2 levels

  • Combination strategies for patients with suboptimal TREM2 function

• Monitoring Approaches:

  • Longitudinal assessment of TREM2 isoform ratios during disease progression

  • Evaluation of therapeutic response based on TREM2 functional restoration

  • Development of imaging biomarkers for TREM2-expressing cell populations

This personalized approach would address the observation that the Δe2 isoform lacks the critical V-set immunoglobulin domain and fails to restore phagocytic capacity or promote interferon responses in experimental models .

What are the most critical unanswered questions regarding human TREM2 biology?

Despite significant advances, several critical questions about human TREM2 biology remain unanswered:

• Cell Type Specificity:

  • Reconciliation of apparent species differences in cellular expression patterns

  • Definitive identification of TREM2-expressing cells in human brain under various conditions

  • Mechanisms regulating cell-type specific expression patterns

• Isoform Regulation:

  • Factors controlling alternative splicing of TREM2

  • Mechanisms behind inter-individual variability in isoform ratios

  • Physiological significance of the Δe2 isoform

• Gender Dimorphism:

  • Molecular basis for gender-specific effects in metabolic regulation

  • Hormonal influences on TREM2 expression and function

  • Therapeutic implications of gender differences

• Disease Progression Effects:

  • Temporal changes in TREM2 function during disease evolution

  • Transition points between protective and potentially detrimental roles

  • Integration of TREM2 function with other disease pathways

Addressing these questions will require integrated approaches combining human tissue studies, advanced cellular models, and innovative methodologies designed to overcome the specific challenges of studying TREM2 in complex human diseases.