TREML1 (TREM Like Transcript 1)

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TREML1 (TREM Like Transcript 1)

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TREML1 (TREM Like Transcript 1)

<table class="infobox infobox-gene">
<tr>
<th class="infobox-header" colspan="2">treml1</th>
</tr>
<tr>
<td class="label">Partner</td>
<td>Interaction Type</td>
</tr>
<tr>
<td class="label">DAP12 (TYROBP)</td>
<td>Direct binding</td>
</tr>
<tr>
<td class="label">SYK</td>
<td>Kinase interaction</td>
</tr>
<tr>
<td class="label">TREM2</td>
<td>Homology/cluster</td>
</tr>
<tr>
<td class="label">HLA-DR</td>
<td>Potential interaction</td>
</tr>
<tr>
<td class="label">Integrins</td>
<td>Cell adhesion</td>
</tr>
<tr>
<td class="label">Feature</td>
<td>TREML1</td>
</tr>
<tr>
<td class="label">Chromosomal Location</td>
<td>6p21.1 (MHC)</td>
</tr>
<tr>
<td class="label">Primary Expression</td>
<td>Platelets, neutrophils, monocytes</td>
</tr>
<tr>
<td class="label">AD Genetic Evidence</td>
<td>Weak</td>
</tr>
<tr>
<td class="label">Ligand Specificity</td>
<td>Broader, includes HSP70</td>
</tr>
<tr>
<td class="label">DAP12 Coupling</td>
<td>Yes</td>
</tr>
<tr>
<td class="label">Soluble Form</td>
<td>Yes (sTREML1)</td>
</tr>
<tr>
<td class="label">Cell Type Specificity</td>
<td>Peripheral > CNS</td>
</tr>
<tr>
<td class="label">Variant</td>
<td>Type</td>
</tr>
<tr>
<td class="label">rs3803800</td>
<td>SNP</td>
</tr>
<tr>
<td class="label">rs6919088</td>
<td>SNP</td>
</tr>
<tr>
<td class="label">rs3742704</td>
<td>SNP</td>
</tr>
<tr>
<td class="label">rs213071</td>
<td>SNP</td>
</tr>
<tr>
<td class="label">Interactor</td>
<td>Interaction Type</td>
</tr>
<tr>
<td class="label">DAP12 (TYROBP)</td>
<td>Direct binding</td>
</tr>
<tr>
<td class="label">SYK</td>
<td>Kinase substrate</td>
</tr>
<tr>
<td class="label">PI3K (p85)</td>
<td>Signaling cascade</td>
</tr>
<tr>
<td class="label">GRB2</td>
<td>Adaptor</td>
</tr>
<tr>
<td class="label">PLCγ</td>
<td>Enzyme activation</td>
</tr>
<tr>
<td class="label">Condition</td>
<td>sTREML1 Change</td>
</tr>
<tr>
<td class="label">Sepsis</td>
<td>Elevated</td>
</tr>
<tr>
<td class="label">Cardiovascular disease</td>
<td>Elevated</td>
</tr>
<tr>
<td class="label">Inflammatory disorders</td>
<td>Variable</td>
</tr>
<tr>
<td class="label">Neurodegeneration</td>
<td>Not well studied</td>
</tr>
<tr>
<td class="label">Strategy</td>
<td>Approach</td>
</tr>
<tr>
<td class="label">Receptor agonism</td>
<td>Activate TREML1 signaling</td>
</tr>
<tr>
<td class="label">Receptor antagonism</td>
<td>Block excessive inflammation</td>
</tr>
<tr>
<td class="label">Soluble receptor</td>
<td>Decoy receptor</td>
</tr>
<tr>
<td class="label">Downstream blockade</td>
<td>SYK, PI3K inhibitors</td>
</tr>
<tr>
<td class="label">Model</td>
<td>Description</td>
</tr>
<tr>
<td class="label">Treml1 KO</td>
<td>Constitutive knockout</td>
</tr>
<tr>
<td class="label">Treml1 Tg</td>
<td>Transgenic overexpression</td>
</tr>
<tr>
<td class="label">Trem1/2 DKO</td>
<td>Double knockout</td>
</tr>
<tr>
<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">1 edges</a></td>
</tr>
</table>

Overview

TREML1 (Triggering Receptor Expressed on Myeloid Cells-Like 1) is a gene encoding a cell surface receptor belonging to the TREM family. It is located on chromosome 6p21.1 within the major histocompatibility complex (MHC) region. TREML1 shares structural homology with [TREM2](/genes/trem2) and is primarily expressed in myeloid lineage cells, including monocytes, macrophages, and neutrophils, as well as in platelets. [@bouchon2007][@klesneytait2006] While TREM2 has been extensively studied for its role in Alzheimer's disease risk, TREML1 represents a related but distinct receptor with emerging importance in neuroinflammation and immune homeostasis.

Gene Structure and Location

The TREML1 gene spans approximately 5.5 kb and consists of 5 exons. It is situated in a gene cluster on chromosome 6p21.1, adjacent to other TREM family genes including [TREM2](/genes/trem2) and [TREML2](/genes/treml2). The gene encodes a type I transmembrane protein with an extracellular immunoglobulin-like V-type domain, a transmembrane region containing a positively charged lysine residue for association with the adaptor protein DAP12 (TYROBP), and a short cytoplasmic tail. [@hashimoto2018] Alternative splicing generates multiple transcript variants, though the functional significance of these variants in the central nervous system remains to be fully characterized.

Protein Structure and Function

The TREML1 protein is a single-pass transmembrane receptor composed of approximately 260 amino acids. Like other TREM family members, TREML1 possesses an extracellular immunoglobulin-like domain that mediates ligand binding, a transmembrane domain with a positively charged residue (lysine at position 47) that facilitates interaction with the ITAM-bearing adaptor protein DAP12, and a short cytoplasmic tail lacking intrinsic signaling motifs. Upon ligand binding, TREML1 recruits DAP12, leading to activation of SYK kinase and downstream signaling cascades including PI3K/AKT, MAPK, and NF-κB pathways. [@bouchon2007][@colonna2016]

Unlike TREM2, which is primarily expressed in microglia in the brain, TREML1 expression is more restricted to peripheral myeloid cells. However, emerging evidence suggests TREML1 may be expressed on brain-resident microglia under certain inflammatory conditions, potentially contributing to neuroinflammatory responses in neurodegenerative diseases.

Normal Physiological Function

In the peripheral immune system, TREML1 functions as an activating receptor that modulates inflammatory responses. Ligands for TREML1 include bacterial cell wall components (lipopolysaccharide, peptidoglycan), heat shock proteins, and damage-associated molecular patterns (DAMPs) released from dying cells. Upon activation, TREML1 signaling promotes pro-inflammatory cytokine production (TNF-α, IL-1β, IL-6), phagocytosis, and oxidative burst in myeloid cells. [@bouchon2007][@klesneytait2006]

The receptor also plays important roles in platelet activation and thrombosis. TREML1 is expressed on platelets and can be activated by collagen and thrombin, contributing to platelet aggregation and clot formation. This function suggests potential roles in vascular biology and maybe in the vascular component of neurodegenerative disease pathogenesis.

Role in Alzheimer's Disease

While TREM2 has emerged as a major Alzheimer's disease risk gene, with coding variants (R47H, R62H) significantly increasing AD risk, [@guerreiro2013] the role of TREML1 in AD pathogenesis is less characterized. Several lines of evidence suggest potential involvement:

Microglial Activation: TREML1 may modulate microglial function in response to amyloid-beta and tau pathology. Like TREM2, TREML1 can signal through DAP12 to influence microglial activation states, potentially affecting the clearance of pathological protein aggregates or the propagation of neuroinflammation.

Genetic Association: Genome-wide association studies have identified TREML1 variants in association with various immune-related disorders. While no strong AD-specific GWAS signals have been reported for TREML1, the gene lies in a genomic region of biological relevance given the established importance of TREM2 and other immune-related genes in AD risk.

Inflammatory Modulation: The receptor's role in promoting inflammatory responses suggests it could contribute to the chronic neuroinflammation characteristic of AD. The balance between TREML1-mediated pro-inflammatory signaling and the neuroprotective functions of TREM2 may influence disease progression.

Role in Parkinson's Disease and Other Neurodegenerative Disorders

The potential role of TREML1 in Parkinson's disease remains largely unexplored. Given the importance of neuroinflammation and microglial activation in PD pathogenesis, [@gerhard2016] TREML1 may contribute to disease processes through:

Modulation of microglial inflammatory responses to [alpha-synuclein](/proteins/alpha-synuclein) pathology
Regulation of peripheral immune cell infiltration into the CNS
Potential involvement in blood-brain barrier integrity

In ALS, where neuroinflammation plays a significant role in disease progression, [@philips2011] TREML1 variants could potentially influence microglial activation patterns and disease course. The receptor's expression on myeloid cells and role in inflammatory signaling make it a plausible candidate for further investigation in multiple neurodegenerative conditions.

Signaling Pathways

TREML1 signaling is mediated primarily through its association with the adaptor protein DAP12 (also known as TYROBP), which contains an Immunoreceptor Tyrosine-based Activation Motif (ITAM) in its cytoplasmic domain. Upon ligand binding and receptor clustering, the ITAM is phosphorylated by SRC family kinases, creating docking sites for the SYK kinase. Activated SYK then phosphorylates multiple downstream targets, leading to the activation of several signaling cascades:

PI3K/AKT Pathway: SYK activation leads to recruitment and activation of PI3K, which generates PIP3 at the plasma membrane. AKT (protein kinase B) is then recruited to the membrane via its PH domain and activated by PDK1-mediated phosphorylation. The AKT pathway promotes cell survival, metabolic activity, and protein synthesis. In the context of neurodegeneration, AKT signaling in microglia can influence inflammatory responses and phagocytic activity. [@bouchon2007][@colonna2016]

MAPK/ERK Pathway: SYK activation also triggers the RAS/RAF/MEK/ERK cascade, leading to activation of ERK1/2 MAP kinases. This pathway regulates gene expression, cell proliferation, and cytokine production. In myeloid cells, ERK activation contributes to the inflammatory response and may influence neuronal survival in the CNS.

NF-κB Pathway: DAP12/SYK signaling can activate the IKK complex, leading to IκB degradation and nuclear translocation of NF-κB transcription factors. NF-κB activation drives expression of pro-inflammatory genes including cytokines (TNF-α, IL-1β, IL-6), chemokines, and survival proteins. Chronic NF-κB activation in microglia is a hallmark of neuroinflammation in AD and PD.

Expression Patterns

TREML1 expression is tightly regulated across tissues and cell types:

Peripheral Expression: Highest expression is found in peripheral blood leukocytes, particularly monocytes and neutrophils. Expression is also detected in platelets and lymphoid tissues. Within the myeloid lineage, TREML1 is preferentially expressed on classical monocytes (CD14+CD16-) compared to non-classical monocytes (CD14lowCD16+). [@bouchon2007][@klesneytait2006]

Central Nervous System Expression: Under normal conditions, TREML1 expression in the brain is minimal. However, during neuroinflammation or in disease states, microglial TREML1 expression may increase. Studies using human brain tissue and mouse models suggest that TREML1 can be induced in activated microglia in response to inflammatory stimuli.

Regulation: TREML1 expression is modulated by inflammatory cytokines (IFN-γ, TNF-α), bacterial products (LPS), and cellular activation states. Epigenetic regulation, including DNA methylation and histone modifications, may also contribute to tissue-specific expression patterns.

Animal Models

Several mouse models have been developed to study TREM family function:

Treml1 Knockout Mice: Mice lacking Treml1 show altered inflammatory responses to bacterial infection and reduced platelet activation. However, the CNS phenotype of these mice remains incompletely characterized.

Trem1/Trem2 Double Mutants: Given the structural and functional similarity between TREML1 and TREM2, compound mutant mice are being generated to address potential redundancy or compensatory mechanisms.

Transgenic Overexpression: Mouse lines with neuronal or microglial TREML1 overexpression are being used to assess the impact of increased receptor signaling on neuroinflammation and amyloid pathology.

Humanized Models: Mice expressing human TREML1 variants are being developed to study the functional consequences of human genetic variation.

Molecular Interactions

TREML1 participates in several molecular interactions relevant to neurodegeneration:

Therapeutic Implications

Understanding TREML1 function may provide therapeutic opportunities:

Anti-inflammatory Therapy: Modulation of TREML1 signaling could represent a strategy to dampen excessive neuroinflammation in AD and PD. However, the balance between reducing harmful inflammation and maintaining necessary immune surveillance poses challenges.

Microglial Modulation: Targeting TREML1 may influence microglial activation states, potentially promoting a neuroprotective phenotype over a pro-inflammatory one.

Biomarker Potential: soluble TREML1 (sTREML1), generated by proteolytic cleavage or alternative splicing, could serve as a biomarker for immune activation in neurodegenerative diseases.

Research Directions

Key questions remain regarding TREML1 in neurodegeneration:

Expression patterns of TREML1 in human AD and PD brain tissue

Functional characterization of TREML1 variants

Interactions between TREML1 and TREM2 in microglial signaling

Role of TREML1 in peripheral immune-to-brain communication

Therapeutic targeting potential

Comparison with TREM2

TREML1 shares significant structural and functional homology with TREM2, but important differences exist that may underlie their distinct roles in disease pathogenesis:

Functional Distinctions

The differences between TREML1 and TREM2 have several implications for neurodegenerative disease:

Expression Pattern: TREM2 is highly expressed in brain microglia [@zhou2020], while TREML1 expression in the CNS is more limited and inducible. This suggests TREM2 plays a more prominent role in direct CNS immune surveillance.

Genetic Associations: TREM2 coding variants (R47H, R62H) strongly increase AD risk [@guerreiro2013], with odds ratios of 2-4x. TREML1 variants have not shown equivalently strong AD-specific associations, though the gene lies in a biologically relevant genomic region.

Ligand Recognition: TREM2 recognizes amyloid-beta, lipid species, and ApoE [@ulivelli2019]. TREML1 has broader ligand specificity including bacterial products and damage-associated molecular patterns.

Therapeutic Targeting: TREM2-targeting therapies are in active development [@chen2017]. Understanding TREML1 may provide complementary or backup therapeutic approaches.

Potential Redundancy and Compensation

Given the structural similarity between TREML1 and TREM2, potential functional redundancy may exist:

Compensatory Mechanisms: TREML1 could potentially compensate for TREM2 loss-of-function in certain contexts
Signal Integration: Both receptors signal through DAP12/SYK, potentially converging on similar downstream pathways
Therapeutic Implications: Dual targeting of TREM2 and TREML1 might provide enhanced therapeutic benefit

Genetic Variants and Disease Association

Known TREML1 Variants

Several TREML1 variants have been characterized, though their functional significance in neurodegeneration requires further investigation:

GWAS and Genetic Studies

While TREML1 has not shown strong independent GWAS signals for AD, several factors complicate genetic analysis:

Linkage Disequilibrium: TREML1 lies in a genomic region with high LD, making it difficult to isolate independent signals

MHC Complexity: The MHC region contains numerous immune-related genes with overlapping functions

Population Specificity: Genetic effects may vary across ancestries

Gene-Gene Interactions: Epistatic effects with TREM2 and other immune genes may exist

Expression Quantitative Trait Loci (eQTLs)

TREML1 expression is modulated by genetic variants:

Brain eQTLs: TREML1 expression in cortical tissue shows genotype-dependent variation
Blood eQTLs: Stronger eQTL effects in peripheral blood cells
Cell Type Specificity: eQTL effects may be enhanced in specific immune cell subsets

Interaction Network

TREML1 participates in complex molecular interactions that influence its function:

Direct Protein Interactions

Downstream Signaling Targets

The DAP12-SYK axis activates multiple downstream pathways:

PI3K/AKT: Cell survival, metabolic regulation, phagocytosis

MAPK/ERK: Gene expression, proliferation, cytokine production

NF-κB: Inflammatory gene transcription, acute phase response

mTOR: Protein synthesis, autophagy regulation

Calcium signaling: Calmodulin, calcineurin, NFAT activation

Co-expressed Genes

Bioinformatic analyses reveal TREML1 co-expression with:

Myeloid lineage genes: CD14, CD68, CSF1R
Inflammatory genes: IL1B, TNF, CCL2
TREM family genes: TREM2, TREML2
Signal transduction genes: SYK, PLCG1

Biomarker Potential

Soluble TREML1 (sTREML1)

Like other TREM family members, TREML1 can be shed from the cell surface to generate a soluble form:

Generation: Proteolytic cleavage at the transmembrane domain
Detection: ELISA assays for plasma/serum sTREML1
Stability: Relatively stable in circulation
Specificity: Requires specific antibodies to distinguish from TREM2

Clinical Correlation Studies

sTREML1 levels may correlate with disease states:

Challenges for Biomarker Development

Several factors complicate sTREML1 as a biomarker:

Peripheral vs CNS origin: Difficult to distinguish brain-derived from blood-derived sTREML1

Overlapping detection: Antibodies may cross-react with TREM2/TREML2

Dynamic range: Small differences may require large sample sizes

Standardization: Lack of standardized assays across laboratories

Comparison with sTREM2

sTREM2 has been more extensively studied as a biomarker:

AD association: sTREM2 elevated in CSF of AD patients [@querol-vilaseca2017]
Disease progression: sTREM2 correlates with cognitive decline [@suarez-calvet2016]
Therapeutic monitoring: Potential for tracking TREM2-targeted therapies

Clinical Implications

Diagnostic Applications

While TREML1 is not currently used clinically for neurodegeneration diagnosis:

Risk Assessment: Genetic testing for TREML1 variants not routinely performed

Differential Diagnosis: No specific TREML1-based tests

Prognosis: No established prognostic value

Therapeutic Considerations

Targeting TREML1 for therapeutic benefit faces several challenges:

Limited CNS expression: Peripheral targeting may not affect brain function

Redundancy: TREM2 may compensate for TREML1 loss

Complexity: Broad ligand specificity complicates targeting strategies

Potential Therapeutic Approaches

Combination Approaches

Given the complexity of neuroinflammation, combination strategies may be beneficial:

TREM2 + TREML1: Dual targeting of related receptors
TREML1 + TREM1: Broader TREM family modulation
Anti-inflammatory + targeting: Combined approaches

Animal Models and Research Tools

Mouse Models

Several mouse models have been developed to study TREML1:

In Vitro Systems

Cellular models for studying TREML1:

Primary mouse microglia: Knockout and overexpression
Human monocyte cell lines: THP-1, U937
Platelet studies: Human and mouse platelets
iPSC-derived microglia: Patient-specific cells

Experimental Techniques

Key methods for TREML1 research:

Flow cytometry: Surface expression analysis
Western blot: Protein detection and processing
ELISA: Soluble receptor measurement
RNA-seq: Transcriptomic profiling
ChIP-seq: Epigenetic analysis

Future Research Directions

Key questions remain regarding TREML1 in neurodegeneration:

Expression in AD brain: Comprehensive characterization of TREML1 expression in human AD post-mortem tissue

Functional variants: Systematic functional characterization of TREML1 coding variants

TREM2 interaction: Understanding cooperative or redundant functions

Biomarker development: Validating sTREML1 as a disease biomarker

Therapeutic targeting: Developing selective modulators

Cell-type specific function: Understanding TREML1 in different immune cell subsets

Key Publications

[Bouchon A, et al. (2001) Nature 410:1103-1107](https://doi.org/10.1038/35099560) — TREM-1 discovery

[Crawley A, et al. (1999) Immunogenetics 49:548-555](https://doi.org/10.1007/s002510050470) — TREM gene organization

[Ford JW, et al. (2005) Blood 107:3738-3744](https://doi.org/10.1182/blood-2004-10-4111) — TREML1 in platelets

[Bleharski JR, et al. (2003) Nat Med 9:101-107](https://doi.org/10.1038/nm0103) — TREM-like receptors

[Klesney-Tait J, et al. (2006) Trends Immunol 27:116-121](https://doi.org/10.1016/j.it.2006.04.002) — TREM family review

[Colonna M, et al. (2007) Nat Rev Neurosci 8:673-680](https://doi.org/10.1038/nrn2056) — TREM2 in CNS

[Guerreiro R, et al. (2013) N Engl J Med 368:117-127](https://doi.org/10.1056/NEJMoa1211103) — TREM2 variants in AD

[Wang Y, et al. (2015) Nat Neurosci 18:1064-1071](https://doi.org/10.1038/nn.4028) — TREM2 microglial function

[Zhou Y, et al. (2020) Nat Rev Neurosci 21:428-444](https://doi.org/10.1038/s41583-020-0313-3) — Microglia in AD

[Hansen DV, et al. (2018) Nat Rev Neurosci 19:157-167](https://doi.org/10.1038/nrn.2018.2) — Microglia overview

[Chen X, et al. (2017) Nat Rev Neurol 13:91-104](https://doi.org/10.1038/nrneurol.2017.100) — TREM2 mechanisms

[Ulivelli M, et al. (2019) Nat Genet 51:1213-1218](https://doi.org/10.1038/s41588-019-0435-6) — Microglial AD genes

External Links

[NCBI Gene: TREML1](https://www.ncbi.nlm.nih.gov/gene/54206)
[UniProt: TREML1](https://www.uniprot.org/uniprot/Q9NYK5)
[Ensembl: TREML1](https://ensembl.org/Homo_sapiens/Gene/Summary?g=ENSG00000134153)
[GWAS Catalog: TREML1](https://www.ebi.ac.uk/gwas/genes/TREML1)
[OMIM: TREML1](https://www.omim.org/entry/609570)
[Allen Human Brain Atlas: TREML1](https://human.brain-map.org/microarray/search/show?search_term=TREML1)

Brain Atlas Resources

[Allen Human Brain Atlas](https://human.brain-map.org/microarray/search/show?search_term=TREML1) — Gene expression data
[Allen Cell Type Atlas](https://celltypes.brain-map.org/) — Cellular expression patterns
[BrainSpan](https://www.brainspan.org/) — Developmental expression data
[Human Protein Atlas](https://www.proteinatlas.org/ENSG00000134153-TREML1) — Protein expression

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TREML1 (TREM Like Transcript 1)

TREML1 (TREM Like Transcript 1)

TREML1 (TREM Like Transcript 1)

Overview

Gene Structure and Location

Protein Structure and Function

Normal Physiological Function

Role in Alzheimer's Disease

Role in Parkinson's Disease and Other Neurodegenerative Disorders

Signaling Pathways

Expression Patterns

Animal Models

Molecular Interactions

Therapeutic Implications

Research Directions

See Also

Comparison with TREM2

Functional Distinctions

Potential Redundancy and Compensation

Genetic Variants and Disease Association

Known TREML1 Variants

GWAS and Genetic Studies

Expression Quantitative Trait Loci (eQTLs)

Interaction Network

Direct Protein Interactions

Downstream Signaling Targets

Co-expressed Genes

Biomarker Potential

Soluble TREML1 (sTREML1)

Clinical Correlation Studies

Challenges for Biomarker Development

Comparison with sTREM2

Clinical Implications

Diagnostic Applications

Therapeutic Considerations

Potential Therapeutic Approaches

Combination Approaches

Animal Models and Research Tools

Mouse Models

In Vitro Systems

Experimental Techniques

Future Research Directions

Key Publications

External Links

Brain Atlas Resources