The linker for activation of B cells (LAB)/non-T cell activation linker (NTAL) regulates triggering receptor expressed on myeloid cells (TREM)-2 signaling and macrophage inflammatory responses independently of the linker for activation of T cells

Untangling the Role of TREM2 in Conjugation with Microglia in Neuronal Dysfunction: A Hypothesis on a Novel Pathway in the Pathophysiology of Alzheimer's Disease

Alzheimer's disease (AD) is a complex neurodegenerative disorder involving heterogenous pathophysiological characteristics, which has become a challenge to therapeutics. The major pathophysiology of AD comprises amyloid-β (Aβ), tau, oxidative stress, and apoptosis. Recent studies indicate the significance of Triggering receptor expressed on myeloid cells 2 (TREM2) and its mutant variants in AD. TREM2 are the transmembrane receptors of microglial cells that performs a broad range of physiological cell processes. Phagocytosis of Aβ is one of the physiological roles of TREM2, which plays a pivotal role in AD progression. R47H, a mutant variant of TREM2, increases the risk of AD by impairing TREM2-Aβ binding. Inconclusive evidence regarding the TREM2 signaling cascade mechanism of Aβ phagocytosis motivates the current review to propose a new hypothesis. The review systematically assesses the cross talk between TREM2 and other AD pathological domains and the influence of TREM2 on amyloid and tau seeding. Disease associated microglia (DAM), a novel state of microglia with unique transcriptional and functional signatures reported in neurodegenerative conditions, also depend on the TREM2 pathway for its differentiation. DAM is suggested to have a neuroprotective role. We hypothesize that TREM2, along with its signaling adaptors and endogenous proteins, play a key role in ameliorating Aβ clearance. We indicate that TREM2 has the potential to ameliorate the Aβ burden, though with differential clearance ability and may act as a potential therapeutic target.

Immune Dysregulation in Monogenic Inborn Errors of Immunity in Oman: Over A Decade of Experience From a Single Tertiary Center

Background

Inborn errors of immunity (IEIs) are being recognized as an important cause of morbidity and mortality in communities with a high frequency of consanguinity, such as Oman, and thus recessively inherited conditions. Various monogenic causes of IEI have been recently discovered; however, the disease phenotype may be variable and does not always include infection at presentation, leading to a delay in diagnosis and a poor outcome. It is now well recognized that immune dysregulation manifestations are observed in a significant proportion of patients with IEI and occasionally precede infection.

Methods

Here, we retrospectively report the epidemiological, clinical, immunological, and molecular findings and outcomes from 239 patients with IEI who were diagnosed and managed at the Royal Hospital, Oman, from January 2010 to October 2021.

Results

The estimated annual cumulative mean incidence of IEI was 25.5 per 100,000 Omani live births with a total prevalence of 15.5 per 100,000 Omani population. Both the high incidence and prevalence are attributed to the high rate of consanguinity (78.2%). Defects affecting cellular and humoral immunity including severe combined immunodeficiency (SCID), combined immunodeficiency (CID), and CID with syndromic features were the predominant defects in IEI (36%). Immune dysregulation was a prominent manifestation and occurred in approximately a third of all patients with IEI (32%), with a mean age of onset of 81 months and a mean diagnostic delay of 50.8 months. The largest percentage of patients who showed such clinical signs were in the category of diseases of immune dysregulation (41%), followed by predominantly antibody deficiency (18%). The overall mortality rate in our cohort was 25.1%, with higher death rates seen in CID including SCID and diseases of immune dysregulation.

Conclusion

Immune dysregulation is a frequent manifestation of Omani patients with IEI. Early detection through raising awareness of signs of IEI including those of immune dysregulation and implementation of newborn screening programs will result in early intervention and improved overall outcome.

Research Progress on the Role of Microglia Membrane Proteins or Receptors in Neuroinflammation and Degeneration

Microglia are intrinsic immune cells of the central nervous system and play a dual role (pro-inflammatory and anti-inflammatory) in the homeostasis of the nervous system. Neuroinflammation mediated by microglia serves as an important stage of ischemic hypoxic brain injury, cerebral hemorrhage disease, neurodegeneration and neurotumor of the nervous system and is present through the whole course of these diseases. Microglial membrane protein or receptor is the basis of mediating microglia to play the inflammatory role and they have been found to be upregulated by recognizing associated ligands or sensing changes in the nervous system microenvironment. They can then allosterically activate the downstream signal transduction and produce a series of complex cascade reactions that can activate microglia, promote microglia chemotactic migration and stimulate the release of proinflammatory factor such as TNF-α, IL-β to effectively damage the nervous system and cause apoptosis of neurons. In this paper, several representative membrane proteins or receptors present on the surface of microglia are systematically reviewed and information about their structures, functions and specific roles in one or more neurological diseases. And on this basis, some prospects for the treatment of novel coronavirus neurological complications are presented.

Targeting TREM2 for Parkinson's Disease: Where to Go?

Parkinson’s disease (PD) is one of most common neurodegenerative disorders caused by a combination of environmental and genetic risk factors. Currently, numerous population genetic studies have shown that polymorphisms in myeloid cell-triggered receptor II (TREM2) are associated with a variety of neurodegenerative disorders. Recently, TREM2 has been verified to represent a promising candidate gene for PD susceptibility and progression. For example, the expression of TREM2 was apparently increased in the prefrontal cortex of PD patients. Moreover, the rare missense mutations in TREM2 (rs75932628, p.R47H) was confirmed to be a risk factor of PD. In addition, overexpression of TREM2 reduced dopaminergic neurodegeneration in the 1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine mouse model of PD. Due to the complex pathogenesis of PD, there is still no effective drug treatment. Thus, TREM2 has received increasing widespread attention as a potential therapeutic target. This review focused on the variation of TREM2 in PD and roles of TREM2 in PD pathogenesis, such as excessive-immune inflammatory response, α-Synuclein aggregation and oxidative stress, to further provide evidence for new immune-related biomarkers and therapies for PD.

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.

Systematic Pan-Cancer Analysis Identifies TREM2 as an Immunological and Prognostic Biomarker

Triggering receptor expressed on myeloid cells-2 (TREM2) is a transmembrane receptor of the immunoglobulin superfamily and a crucial signaling hub for multiple pathological pathways that mediate immunity. Although increasing evidence supports a vital role for TREM2 in tumorigenesis of some cancers, no systematic pan-cancer analysis of TREM2 is available. Thus, we aimed to explore the prognostic value, and investigate the potential immunological functions, of TREM2 across 33 cancer types. Based on datasets from The Cancer Genome Atlas, and the Cancer Cell Line Encyclopedia, Genotype Tissue-Expression, cBioPortal, and Human Protein Atlas, we employed an array of bioinformatics methods to explore the potential oncogenic roles of TREM2, including analyzing the relationship between TREM2 and prognosis, tumor mutational burden (TMB), microsatellite instability (MSI), DNA methylation, and immune cell infiltration of different tumors. The results show that TREM2 is highly expressed in most cancers, but present at low levels in lung cancer. Further, TREM2 is positively or negatively associated with prognosis in different cancers. Additionally, TREM2 expression was associated with TMB and MSI in 12 cancer types, while in 20 types of cancer, there was a correlation between TREM2 expression and DNA methylation. Six tumors, including breast invasive carcinoma, cervical squamous cell carcinoma and endocervical adenocarcinoma, kidney renal clear cell carcinoma, lung squamous cell carcinoma, skin cutaneous melanoma, and stomach adenocarcinoma, were screened out for further study, which demonstrated that TREM2 gene expression was negatively correlated with infiltration levels of most immune cells, but positively correlated with infiltration levels of M1 and M2 macrophages. Moreover, correlation with TREM2 expression differed according to T cell subtype. Our study reveals that TREM2 can function as a prognostic marker in various malignant tumors because of its role in tumorigenesis and tumor immunity.

Imbalance of Microglial TLR4/TREM2 in LPS-Treated APP/PS1 Transgenic Mice: A Potential Link Between Alzheimer's Disease and Systemic Inflammation

Clinically, superimposed systemic inflammation generally has significant deleterious effects on the Alzheimer's disease (AD) progression. However, the related molecular mechanisms remain poorly understood. Microglial toll-like receptor 4 (TLR4) and triggering receptor expressed on myeloid cells 2 (TREM2) are two key regulators of inflammation that may play an essential role in this complex pathophysiological process. In this study, intraperitoneal injection of lipopolysaccharide (LPS) into APP/PS1 transgenic AD model was used to mimic systemic inflammation in the development of AD. Initial results from the cortex showed that compared with wild-type mice, APP/PS1 mice exhibited elevated gene and protein expression levels of both TLR4 and TREM2 with different degree. Interestingly, after LPS treatment, TLR4 expression was persistently up-regulated, while TREM2 expression was significantly down-regulated in APP/PS1 mice, suggesting that the negative regulatory effect of TREM2 on inflammation might be suppressed by LPS-induced hyperactive TLR4. This imbalance of TLR4/TREM2 contributed to microglial over-activation, followed by increased neuronal apoptosis in the cortex of APP/PS1 mice; these changes did not alter the expression level of Aβ_1-42. Similar alterations were observed in our in vitro experiment with β-amyloid_1-42 (Aβ_1-42)-treated N9 microglia. Further, Morris water maze (MWM) testing data indicated that LPS administration acutely aggravated cognitive impairment in APP/PS1 mice, suggesting that the addition of systemic inflammation can potentially accelerate the progression of AD. Collectively, we conclude that an imbalance of TLR4/TREM2 may be a potential link between AD and systemic inflammation. TREM2 can serve as a potential therapeutic target for treating systemic inflammation in AD progression.

Comparative proteomic analysis of cat eye syndrome critical region protein 1- function in tumor-associated macrophages and immune response regulation of glial tumors

Introduction

Tumor associated macrophages (TAMs) promote tumor development, angiogenesis and distal metastasis. In previous studies, we showed that Cat Eye Syndrome Critical Region Protein 1 (CECR1) is expressed by M2-like TAMs in human glioma samples. CECR1 promoted M2 TAMs differentiation and affected glioma cell proliferation and migration. Here we investigated the proteomic profile of TAMs expressing CECR1 in absence or presence of glioma cells.

Results

CECR1 siRNA transfection upregulated 67 proteins in THP-1-derived Macrophages (MQs). Pathway annotation mapped this set to 3 major pathways relevant for MQ function, including 'MHC-I antigen presentation', 'phagosome maturation' and 'endocytosis'. Co-culture of siCECR1 THP-1-derived MQs with U87 glioma cells attenuated the changes observed on protein and mRNA level in response to MQ CECR1 silencing. SiCECR1 in U87 co-cultured MQs was associated with an IL-10^low, IL-12^high M1-like phenotype. In U87 co-culture conditions, SiCECR1 also downregulated S20 proteasome complex proteins PSMA5, PSMA7, PSMC6 and PSMD8. This protein profile was linked to a low proliferation rate of siCECR1 MQs. Overlap analysis identified S100A9 and PLAU as CECR1-related proteins that were significantly correlated with expression of CECR1 and macrophage lineage markers in three large public GBM datasets.

Conclusion

This study reports the molecular pathways and key molecules that are mediated by CECR1 function in THP- 1-derived MQs and TAMs in glioma.

Methods

PMA-treated THP-1 cells (MQs) were siRNA transfected for CECR1 in vitro, with or without stimulation of the primary glioma cell line U87. Lysates were analyzed by (nano)LC-MS. Significant altered protein levels were identified (P < 0.05), followed by pathway annotation.

TREM-2 serves as a negative immune regulator through Syk pathway in an IL-10 dependent manner in lung cancer

During infection, triggering receptor expressed on myeloid cells-2 (TREM-2) restrains dendritic cells (DCs) and macrophages (MΦs) phagocytosis, as well as reduces pro-inflammatory cytokines release through DNAX-activation protein 12 (DAP12) signaling. However, the role of TREM-2 signaling in cancer has never been elucidated. In the current study, we found that TREM-2 was up-regulated on peripheral blood monocytes in tumor-bearing host. More TREM-2⁺DCs were detected in the lung of 3LL tumor-bearing mice. On the other hand, the level of TREM-2 on pulmonary MΦs positively correlated with the pathological staging of lung cancer. However, surgical or chemotherapeutic reduction of tumor burden led to the obvious decline of TREM-2. In vitro, TREM-2 expression of bone marrow (BM)-derived DCs and MΦs was induced by conditional medium (CM) containing the supernatant of 3LL cells. TREM-2⁺DCs from CM and/or tumor-bearing mice held altered phenotypes (CD80^LowCD86^LowMHCII^Low) and impaired functions, such as, reduced interleukin (IL)-12 secretion, increased IL-10 production, and weakened ovalbumin (OVA)-endocytic capacity; also developed potent inhibitory effect on T cell proliferation that could be partially reversed by TREM-2 blockage. Moreover, spleen tyrosine kinase (Syk) inhibitor restrained IL-10 production of TREM-2⁺DC. Remarkably, IL-10 neutralizing antibody and Syk inhibitor both lowered the suppressive potential of TREM-2⁺DCs in T cell proliferation. Also, adoptive transfer of this TREM-2⁺DCs accelerated the tumor growth rather than jeopardized survival in lung cancer-bearing mice. In conclusion, these results indicate that TREM-2 might act as a negative immuno-regulatory molecule through Syk pathway in an IL-10 dependent manner and partially predicts prognosis in lung cancer patients.

TREM2 in Neurodegenerative Diseases

TREM2 variants have been identified as risk factors for Alzheimer's disease (AD) and other neurodegenerative diseases (NDDs). Because TREM2 encodes a receptor exclusively expressed on immune cells, identification of these variants conclusively demonstrates that the immune response can play an active role in the pathogenesis of NDDs. These TREM2 variants also confer the highest risk for developing Alzheimer's disease of any risk factor identified in nearly two decades, suggesting that understanding more about TREM2 function could provide key insights into NDD pathology and provide avenues for novel immune-related NDD biomarkers and therapeutics. The expression, signaling and function of TREM2 in NDDs have been extensively investigated in an effort to understand the role of immune function in disease pathogenesis and progression. We provide a comprehensive review of our current understanding of TREM2 biology, including new insights into the regulation of TREM2 expression, and TREM2 signaling and function across NDDs. While many open questions remain, the current body of literature provides clarity on several issues. While it is still often cited that TREM2 expression is decreased by pro-inflammatory stimuli, it is now clear that this is true in vitro, but inflammatory stimuli in vivo almost universally increase TREM2 expression. Likewise, while TREM2 function is classically described as promoting an anti-inflammatory phenotype, more than half of published studies demonstrate a pro-inflammatory role for TREM2, suggesting that its role in inflammation is much more complex. Finally, these components of TREM2 biology are applied to a discussion of how TREM2 impacts NDD pathologies and the latest assessment of how these findings might be applied to immune-directed clinical biomarkers and therapeutics.

TREM2-Ligand Interactions in Health and Disease

The protein triggering receptor expressed on myeloid cells-2 (TREM2) is an immunomodulatory receptor with a central role in myeloid cell activation and survival. In recent years, the importance of TREM2 has been highlighted by the identification of coding variants that increase risk for Alzheimer's disease and other neurodegenerative diseases. Animal studies have further shown the importance of TREM2 in neurodegenerative and other inflammatory disease models including chronic obstructive pulmonary disease, multiple sclerosis, and stroke. A mechanistic understanding of TREM2 function remains elusive, however, due in part to the absence of conclusive information regarding the identity of endogenous TREM2 ligands. While many TREM2 ligands have been proposed, their physiological role and mechanism of engagement remain to be determined. In this review, we highlight the suggested roles of TREM2 in these diseases and the recent advances in our understanding of TREM2 and discuss putative TREM2-ligand interactions and their potential roles in signaling during health and disease. We develop a model based on the TREM2 structure to explain how different TREM2 ligands might interact with the receptor and how disease risk variants may alter ligand interactions. Finally, we propose future experimental directions to establish the role and importance of these different interactions on TREM2 function.

SCIMP is a transmembrane non-TIR TLR adaptor that promotes proinflammatory cytokine production from macrophages

Danger signals activate Toll-like receptors (TLRs), thereby initiating inflammatory responses. Canonical TLR signalling, via Toll/Interleukin-1 receptor domain (TIR)-containing adaptors and proinflammatory transcription factors such as NF-κB, occurs in many cell types; however, additional mechanisms are required for specificity of inflammatory responses in innate immune cells. Here we show that SCIMP, an immune-restricted, transmembrane adaptor protein (TRAP), promotes selective proinflammatory cytokine responses by direct modulation of TLR4. SCIMP is a non-TIR-containing adaptor, binding directly to the TLR4-TIR domain in response to lipopolysaccharide. In macrophages, SCIMP is constitutively associated with the Lyn tyrosine kinase, is required for tyrosine phosphorylation of TLR4, and facilitates TLR-inducible production of the proinflammatory cytokines IL-6 and IL-12p40. Point mutations in SCIMP abrogating TLR4 binding also prevent SCIMP-mediated cytokine production. SCIMP is, therefore, an immune-specific TLR adaptor that shapes host defence and inflammation.

Toll-like receptors engage TIR domain-containing adaptors to control proinflammatory gene expression in response to pathogens and tissue damage. Here the authors show that the non-TIR domain-containing transmembrane protein SCIMP is a previously unrecognized TLR adaptor expressed by macrophages.

Depletion of the triggering receptor expressed on myeloid cells 2 inhibits progression of renal cell carcinoma via regulating related protein expression and PTEN-PI3K/Akt pathway

The triggering receptor expressed on myeloid cells 2 (TREM-2) is suggested to be involved in the development of certain human malignancies. However, the functions of TREM-2 in renal cell carcinoma (RCC) are still less known. To reveal the effects of TREM-2 on the RCC progression, we examined the TREM-2 expression in RCC tumor tissues. Then, we analyzed the cell proliferation, cell apoptosis, cell cycle and expression of the relative factors in two selected RCC cell lines post RNA interference. We also analyzed the functions of TREM-2 in an in vivo nude mouse model. We found that, the expression of TREM-2 was abnormally elevated in RCC tumor tissues. Silencing TREM-2 inhibited cell growth, induced G1 phase arrest of cell cycle and cell apoptosis in RCC cells. In vivo, the results showed that depletion of TREM-2 significantly inhibited the ACHN tumor growth in the nude mouse model. The analysis of relative protein factors suggested that silencing TREM-2 downregulated the expression levels of Bcl2 and PCNA, and upregulated the expression levels of Bax and caspase-3 in RCC cell lines. Depletion of TREM-2 inactivated PI3K/Akt pathway through increasing the expression of PTEN. Taken together, TREM-2 acts as an oncogene in the development of RCC and can be considered as a novel therapeutic factor in the treatment of RCC.

Autocrine IL-10 functions as a rheostat for M1 macrophage glycolytic commitment by tuning nitric oxide production

Inflammatory maturation of M1 macrophages by proinflammatory stimuli such as toll like receptor ligands results in profound metabolic reprogramming resulting in commitment to aerobic glycolysis as evidenced by repression of mitochondrial oxidative phosphorylation (OXPHOS) and enhanced glucose utilization. In contrast, "alternatively activated" macrophages adopt a metabolic program dominated by fatty acid-fueled OXPHOS. Despite the known importance of these developmental stages on the qualitative aspects of an inflammatory response, relatively little is know regarding the regulation of these metabolic adjustments. Here we provide evidence that the immunosuppressive cytokine IL-10 defines a metabolic regulatory loop. Our data show for the first time that lipopolysaccharide (LPS)-induced glycolytic flux controls IL-10-production via regulation of mammalian target of rapamycin (mTOR) and that autocrine IL-10 in turn regulates macrophage nitric oxide (NO) production. Genetic and pharmacological manipulation of IL-10 and nitric oxide (NO) establish that metabolically regulated autocrine IL-10 controls glycolytic commitment by limiting NO-mediated suppression of OXPHOS. Together these data support a model where autocine IL-10 production is controlled by glycolytic flux in turn regulating glycolytic commitment by preserving OXPHOS via suppression of NO. We propose that this IL-10-driven metabolic rheostat maintains metabolic equilibrium during M1 macrophage differentiation and that perturbation of this regulatory loop, either directly by exogenous cellular sources of IL-10 or indirectly via limitations in glucose availability, skews the cellular metabolic program altering the balance between inflammatory and immunosuppressive phenotypes.

The triggering receptor expressed on myeloid cells (TREM) in inflammatory bowel disease pathogenesis

The Triggering Receptors Expressed on Myeloid cells (TREM) are a family of cell-surface molecules that control inflammation, bone homeostasis, neurological development and blood coagulation. TREM-1 and TREM-2, the best-characterized receptors so far, play divergent roles in several infectious diseases. In the intestine, TREM-1 is highly expressed by macrophages, contributing to inflammatory bowel disease (IBD) pathogenesis. Contrary to current understanding, TREM-2 also promotes inflammation in IBD by fueling dendritic cell functions. This review will focus specifically on recent insights into the role of TREM proteins in IBD development, and discuss opportunities for novel treatment approaches.

What happens to microglial TREM2 in Alzheimer's disease: Immunoregulatory turned into immunopathogenic?

Microglia play major roles in initiation, coordination and execution of innate immunity in the brain. In the adult brain, these include maintenance of homeostasis, neuron and tissue repair, and eliminating infectious agents, apoptotic cells, and misfolded proteins. Some of these activities are accompanied by inflammatory reactions; and others are performed with no inflammatory effects. Under normal conditions, triggering receptor expressed on myeloid cells 2 (TREM2) belongs to the second category. It pairs with the adaptor protein DNAX-activating protein of 12kDa (DAP12) to induce phagocytosis of apoptotic neurons without inflammatory responses, and to regulate Toll-like receptor-mediated inflammatory responses, and microglial activation. Although ligands for TREM2 are largely unknown, the mitochondrial heat shock protein 60, expressed on cell surface of apoptotic neurons, is a specific ligand that activates TREM2-mediated phagocytosis by microglia. TREM2 also phagocytoses amyloid beta peptide in cultured cells. Several TREM2 mutations have been identified recently that increase the risk of Alzheimer's disease, Frontotemporal dementia, Parkinson's disease, and amyotrophic lateral sclerosis. Some of these mutations cause impaired proteolysis of full-length TREM2 at the plasma membrane to different degrees. The defects in the intramembrane cleavage result in dysfunction of phagocytosis signaling. The association of TREM2 mutations with neurodegenerative disease also calls for the understanding of the biology and pathological role of non-mutated TREM2 on human brains and microglia. This review provides a summary of current literature in TREM2 and DAP12 from several aspects, and proposes a theory that loss of TREM2 functions might contribute to the immunopathogenic role of microglia in Alzheimer's disease.

Environmental factors determine DAP12 deficiency to either enhance or suppress immunopathogenic processes

DNAX-activation protein 12 (DAP12), a transmembrane adapter, plays a major role in transducing activation signals in natural killer cells and various myeloid cells. Quantitative RT-PCR detected in normal mouse eyes considerable levels of DAP12 and multiple DAP12-coupled receptors, in particular TREM-1, Clec5a and SIRPb1. The role of DAP12 and its receptors in experimental autoimmune diseases has been controversial. Here, we analysed the effect of DAP12 deficiency on the capacity of mice to mount immunopathogenic cellular responses to the uveitogenic ocular antigen and interphotoreceptor retinoid-binding protein (IRBP), and to develop experimental autoimmune uveitis (EAU). Surprisingly, sequential analysis of EAU in mice deficient in DAP12 in two different animal facilities at first revealed enhanced disease as compared with wild-type mice, but when these mice were re-derived into a second, cleaner, animal facility, the response of control mice was essentially unchanged, whereas the DAP12 null mice were markedly hyporesponsive relative to controls in the new facility. Accordingly, when stimulated in vitro with IRBP, lymphocytes from the DAP12-deficient mice housed in the two facilities proliferated and produced opposite profiles of pro-inflammatory and anti-inflammatory cytokines, compared with their controls. These findings therefore demonstrate that the effects of DAP12 deficiency on development of autoimmune disease are dramatically affected by environmental factors.

Palmitoylated transmembrane adaptor proteins in leukocyte signaling

Transmembrane adaptor proteins (TRAPs) are structurally related proteins that have no enzymatic function, but enable inducible recruitment of effector molecules to the plasma membrane, usually in a phosphorylation dependent manner. Numerous surface receptors employ TRAPs for either propagation or negative regulation of the signal transduction. Several TRAPs (LAT, NTAL, PAG, LIME, PRR7, SCIMP, LST1/A, and putatively GAPT) are known to be palmitoylated that could facilitate their localization in lipid rafts or tetraspanin enriched microdomains. This review summarizes expression patterns, binding partners, signaling pathways, and biological functions of particular palmitoylated TRAPs with an emphasis on the three most recently discovered members, PRR7, SCIMP, and LST1/A. Moreover, we discuss in silico methodology used for discovery of new family members, nature of their binding partners, and microdomain localization.

LAB/NTAL facilitates fungal/PAMP-induced IL-12 and IFN-γ production by repressing β-catenin activation in dendritic cells

Fungal pathogens elicit cytokine responses downstream of immunoreceptor tyrosine-based activation motif (ITAM)-coupled or hemiITAM-containing receptors and TLRs. The Linker for Activation of B cells/Non-T cell Activating Linker (LAB/NTAL) encoded by Lat2, is a known regulator of ITAM-coupled receptors and TLR-associated cytokine responses. Here we demonstrate that LAB is involved in anti-fungal immunity. We show that Lat2-/- mice are more susceptible to C. albicans infection than wild type (WT) mice. Dendritic cells (DCs) express LAB and we show that it is basally phosphorylated by the growth factor M-CSF or following engagement of Dectin-2, but not Dectin-1. Our data revealed a unique mechanism whereby LAB controls basal and fungal/pathogen-associated molecular patterns (PAMP)-induced nuclear β-catenin levels. This in turn is important for controlling fungal/PAMP-induced cytokine production in DCs. C. albicans- and LPS-induced IL-12 and IL-23 production was blunted in Lat2-/- DCs. Accordingly, Lat2-/- DCs directed reduced Th1 polarization in vitro and Lat2-/- mice displayed reduced Natural Killer (NK) and T cell-mediated IFN-γ production in vivo/ex vivo. Thus our data define a novel link between LAB and β-catenin nuclear accumulation in DCs that facilitates IFN-γ responses during anti-fungal immunity. In addition, these findings are likely to be relevant to other infectious diseases that require IL-12 family cytokines and an IFN-γ response for pathogen clearance.

Tyrosine phosphorylation-independent regulation of lipopolysaccharide-mediated response by the transmembrane adaptor protein LAB

Linker for activation of B cells (LAB)/non-T cell activation linker is a transmembrane adaptor protein that functions in immunoreceptor-mediated signaling. Published studies have shown that LAB has both positive and negative roles in regulating TCR and high-affinity Fc receptor-mediated signaling and cellular function. In this study, we showed that LAB was also expressed in dendritic cells and that LAB deficiency affected LPS-mediated signaling and cytokine production. LPS-mediated MAPK activation was enhanced in LAB(-/-) bone marrow-derived dendritic cells. These bone marrow-derived dendritic cells also produced more TNF-α, IL-6, and IL-10 than wild-type cells. Moreover, LAB(-/-) mice were hyperresponsive to LPS-induced septic shock. These data indicated that LAB has a negative role in LPS-mediated responses. By using LAB knockin mice, which harbor mutations at five membrane-distal tyrosines, we further showed that, in contrast to its role in immunoreceptor-mediated signaling, LAB function in LPS-mediated signaling pathway did not depend on its tyrosine phosphorylation. Our study suggested a novel mechanism by which LAB functions in the regulation of innate immunity.

Mammary gland morphological and gene expression changes underlying pregnancy protection of breast cancer tumorigenesis

A full-term pregnancy early in life reduces lifetime risk of developing breast cancer, and the effect can be mimicked in rodents by full-term pregnancy or short-term treatment with exogenous estrogen and progesterone. To gain insight into the protective mechanism, 15 3-mo-old postpubertal virgin Lewis rats were randomly assigned to three groups: control (C), pregnancy (P), or hormone (H). The P group animals underwent a full-term pregnancy, and H group animals were implanted subcutaneously with silastic capsules filled with ethynyl estradiol and megesterol acetate for 21 days. C and P animals were implanted with sham capsules. On day 21 capsules were removed, which was followed by a 49-day involution period, euthanasia, and mammary tissue collection. Global gene expression was measured using Rat Genome 230.2 Arrays. Histological analysis revealed that P and H treatments induced sustained morphological changes in the mammary gland with significantly increased percentages of mammary parenchyma and stromal tissues and higher ratio of stroma to parenchyma. Transcriptome analysis showed that P and H treatments induced sustained global changes in gene expression in the mammary gland. Analysis of commonly up- and downregulated genes in P and H relative to C treatment showed increased expression of three matrix metallopeptidases (Mmp3, 8, and 12), more differentiated mammary phenotype, enhanced innate and adaptive immunity, and reduced cell proliferation and angiogenic signatures. The sustained morphological and global gene expression changes in mammary tissue after pregnancy and hormone treatment may function together to provide the protective effect against breast cancer.

A tale of two TRAPs: LAT and LAB in the regulation of lymphocyte development, activation, and autoimmunity

Transmembrane adaptor proteins (TRAPs) link antigen receptor engagement to downstream cellular processes. Although these proteins typically lack intrinsic enzymatic activity, they are phosphorylated on multiple tyrosine residues following lymphocyte activation, allowing them to function as scaffolds for the assembly of multi-molecular signaling complexes. Among the many TRAPs that have been discovered in recent years, the LAT (linker for activation of T cells) family of adaptor proteins plays an important role in the positive and negative regulation of lymphocyte maturation, activation, and differentiation. Of the two members in this family, LAT is an indispensable component controlling T cell and mast cell activation and function; LAB (linker for activation of B cells), also called NTAL, is necessary to fine-tune lymphocyte activation and may be a key regulator of innate immune responses. Here, we review recent advances on the function of LAT and LAB in the regulation of development and activation of immune cells.

TLR-mediated signaling pathways circumvent the requirement for DAP12 in mast cells for the induction of inflammatory mediator release

TLR, expressed on the surface of mast cells, respond to a variety of bacterial and viral components to induce and enhance high-affinity IgE receptor-mediated cytokine production. Recent reports have indicated that specific TLR-dependent responses in macrophages and dendritic cells are regulated by the ITAM-containing molecule, DAP12. When phosphorylated, DAP12 recruits Syk, which is a critical molecule for mast cell activation. We therefore examined whether DAP12 similarly regulates TLR-mediated responses in mast cells. DAP12 was confirmed to be expressed in both human and mouse mast cells and, upon phosphorylation, to recruit Syk. However, although TLR agonists induced cytokine production, and synergistically enhanced high-affinity IgE receptor-mediated cytokine production, surprisingly, they failed to increase DAP12 phosphorylation in mouse bone marrow-derived mast cells (BMMC). Furthermore, normal TLR-mediated responses were observed in DAP12(-/-) BMMC. However, DAP12 phosphorylation and subsequent Syk recruitment were observed in BMMC following Con A-induced aggregation of mannose-glycosylated receptors, and these responses, together with Con A-induced degranulation, were substantially reduced in the DAP12(-/-) BMMC. These data demonstrate that TLR have differential requirements for DAP12 for their function in different cell types and that the inability of TLR to influence mast cell degranulation may be linked to their inability to utilize DAP12 to recruit Syk.

LAB/NTAL/Lat2: a force to be reckoned with in all leukocytes?

LAB/NTAL/Lat2 is a transmembrane adaptor protein closely related to LAT. It is expressed in various myeloid and lymphoid cells, many of which also express LAT. Phosphorylation of LAB occurs following engagement of various ITAM- and non-ITAM-linked receptors and can play positive and negative roles following receptor engagement. LAT binds PLCγ directly, resulting in efficient Ca²+ flux and degranulation. However, LAB does not contain a PLCγ-binding motif and only binds PLCγ indirectly, possibly via Grb2, thereby resulting in suboptimal signaling. As LAT can signal more efficiently than LAB, competition between the 2 for space/substrates in the lipid rafts can attenuate signaling. This competition model requires coexpression of LAT; however, LAB is repressive, even in cells lacking substantial LAT expression such as macrophages and mature B cells. The reported interaction between LAB and the ubiquitin E3-ligase c-Cbl suggests 1 possible mechanism for LAT-independent inhibition by LAB, but such a model requires further investigation. Given the wide-reaching expression pattern of LAB, LAB has the ability to modulate signaling in virtually every type of leukocyte. Regardless of its ultimate mode of action, the potent regulatory capability of LAB proves this protein to be a complex adaptor that warrants continued, substantial scrutiny by biochemists and immunologists alike.