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 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.
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 and 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.
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.
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.
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.
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:
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.
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.
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.
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.
TREML1 participates in several molecular interactions relevant to neurodegeneration:
| Partner | Interaction Type | Functional Significance |
|---|---|---|
| DAP12 (TYROBP) | Direct binding | ITAM-mediated signal transduction |
| SYK | Kinase interaction | Downstream signaling activation |
| TREM2 | Homology/cluster | Potential co-regulation of microglial function |
| HLA-DR | Potential interaction | MHC class II involvement in antigen presentation |
| Integrins | Cell adhesion | Platelet activation and aggregation |
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.
Key questions remain regarding TREML1 in neurodegeneration:
TREML1 shares significant structural and functional homology with TREM2, but important differences exist that may underlie their distinct roles in disease pathogenesis:
| Feature | TREML1 | TREM2 |
|---|---|---|
| Chromosomal Location | 6p21.1 (MHC) | 6p21.1 (MHC) |
| Primary Expression | Platelets, neutrophils, monocytes | Microglia, macrophages |
| AD Genetic Evidence | Weak | Strong (R47H, R62H) |
| Ligand Specificity | Broader, includes HSP70 | Amyloid, lipids, ApoE |
| DAP12 Coupling | Yes | Yes |
| Soluble Form | Yes (sTREML1) | Yes (sTREM2) |
| Cell Type Specificity | Peripheral > CNS | CNS > Peripheral |
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.
Given the structural similarity between TREML1 and TREM2, potential functional redundancy may exist:
Several TREML1 variants have been characterized, though their functional significance in neurodegeneration requires further investigation:
| Variant | Type | Position | Population Frequency | Predicted Effect |
|---|---|---|---|---|
| rs3803800 | SNP | 5'UTR | ~15% | Altered expression |
| rs6919088 | SNP | Intron | ~20% | Splicing modifier |
| rs3742704 | SNP | Coding (A98V) | ~5% | Possible functional |
| rs213071 | SNP | 3'UTR | ~25% | miRNA binding |
While TREML1 has not shown strong independent GWAS signals for AD, several factors complicate genetic analysis:
TREML1 expression is modulated by genetic variants:
TREML1 participates in complex molecular interactions that influence its function:
| Interactor | Interaction Type | Functional Consequence |
|---|---|---|
| DAP12 (TYROBP) | Direct binding | ITAM signaling, SYK activation |
| SYK | Kinase substrate | Downstream cascade activation |
| PI3K (p85) | Signaling cascade | AKT pathway activation |
| GRB2 | Adaptor | MAPK pathway integration |
| PLCγ | Enzyme activation | Calcium signaling |
The DAP12-SYK axis activates multiple downstream pathways:
Bioinformatic analyses reveal TREML1 co-expression with:
Like other TREM family members, TREML1 can be shed from the cell surface to generate a soluble form:
sTREML1 levels may correlate with disease states:
| Condition | sTREML1 Change | Sample Size | Reference |
|---|---|---|---|
| Sepsis | Elevated | n=200 | [@zhang2003] |
| Cardiovascular disease | Elevated | n=150 | [@haspel2003] |
| Inflammatory disorders | Variable | n=100 | [@mendoza2007] |
| Neurodegeneration | Not well studied | - | - |
Several factors complicate sTREML1 as a biomarker:
sTREM2 has been more extensively studied as a biomarker:
While TREML1 is not currently used clinically for neurodegeneration diagnosis:
Targeting TREML1 for therapeutic benefit faces several challenges:
| Strategy | Approach | Challenges | Status |
|---|---|---|---|
| Receptor agonism | Activate TREML1 signaling | Ligand identification | Discovery |
| Receptor antagonism | Block excessive inflammation | Specificity | Research |
| Soluble receptor | Decoy receptor | BBB penetration | Preclinical |
| Downstream blockade | SYK, PI3K inhibitors | Specificity, toxicity | Research |
Given the complexity of neuroinflammation, combination strategies may be beneficial:
Several mouse models have been developed to study TREML1:
| Model | Description | Key Findings |
|---|---|---|
| Treml1 KO | Constitutive knockout | Altered platelet function |
| Treml1 Tg | Transgenic overexpression | Enhanced inflammation |
| Trem1/2 DKO | Double knockout | Compensatory mechanisms |
Cellular models for studying TREML1:
Key methods for TREML1 research:
Key questions remain regarding TREML1 in neurodegeneration: