Inside-out signaling promotes dynamic changes in the carcinoembryonic antigen-related cellular adhesion molecule 1 (CEACAM1) oligomeric state to control its cell adhesion properties

Structural aspects of CEACAM1 interactions

Carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) is a membrane protein that plays an important role in a variety of immune and non-immune functions. Such functions are regulated by its activity as a homophilic ligand but also through its ability to interact as a heterophilic ligand with various host proteins. These include CEACAM5, T cell immunoglobulin-mucin like protein-3 (TIM-3) and, potentially, protein death protein 1 (PD-1). Furthermore, CEACAM1 is targeted by various pathogens to allow them to invade a host and bypass an effective immune response. Clinically, CEACAM1 plays an important role in infectious diseases, autoimmunity and cancer. In this review, we describe the structural basis for CEACAM1 interactions as a homophilic and heterophilic ligand. We discuss the regulation of its monomeric, dimeric and oligomeric states in cis and trans binding as well as the consequences for eliciting downstream signalling activities. Furthermore, we explore the potential role of avidity in determining CEACAM1's activities.

Mechanism and function of CEACAM1 splice isoforms

BACKGROUND

Alternative splicing is a fundamental mechanism in the post-transcriptional regulation of genes. The multifunctional transmembrane glycoprotein receptor carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) undergoes extensive alternative splicing to allow for tunable functions in cell signalling, adhesion and modulation of immune and metabolic responses. Splice isoforms that differ in their ectodomain and short or long cytoplasmic tail (CEACAM1-S/CEACAM1-L) have distinct functional roles. The mechanisms that regulate CEACAM1 RNA splicing remain elusive.

METHODS

This narrative review summarizes the current knowledge of the mechanism and function of CEACAM1 splice isoforms. Historical perspectives address the biological significance of the glycosylated Ig domains, the variable exon 7, and phosphorylation events that dictate its signal transduction pathways. The use of small antisense molecules to target mis-spliced variable exon 7 is discussed.

RESULTS

The Ig variable-like N domain mediates cell adhesion and immune checkpoint inhibitory functions. Gly and Tyr residues in the transmembrane (TM) domain are essential for dimerization. Calmodulin, Calcium/Calmodulin-dependent protein kinase II delta (CamK2D), Actin and Annexin A2 are binding partners of CEACAM1-S. Homology studies of the muCEACAM1-S and huCEACAM1-S TM predict differences in their signal transduction pathways. Hypoxia-inducible factor 1-α (HIF-1-α) induces alternative splicing to produce CEACAM1-S under limited oxygen conditions. Antisense small molecules directed to exon 7 may correct faulty expression of the short and long cytoplasmic tail splicing isoforms.

CONCLUSION

More pre-clinical and clinical studies are needed to elucidate the precise mechanisms by which CEACAM1 RNA splicing may be exploited to develop targeted interventions towards novel therapeutic strategies.

CEACAM1 in vascular homeostasis and inflammation

INTRODUCTION

The glycoprotein Carcinoembryonic Antigen-related Cell Adhesion Molecule 1 (CEACAM1), also known as CD66a, is a member of the immunoglobulin superfamily. It is expressed in a variety of tissues including epithelial, immune, as well as endothelial cells, and is crucial to diverse physiological and pathological mechanisms. This review aims to provide a comprehensive understanding of CEACAM1's multifaceted roles in vascular biology and inflammatory processes.

METHODS

Directed literature research was conducted using databases, such as PubMed, and relevant studies were categorized based on the physiological effects of CEACAM1.

RESULTS

CEACAM1 plays a pivotal role in vascular homeostasis, particularly influencing the formation, maturation, and aging of blood vessels, as well as the endothelial barrier function. It supports endothelium-dependent vasodilation and nitric oxide formation, thus promoting vascular integrity and regulating blood pressure. Additionally, CEACAM1 is of emerging importance to vascular inflammation and its potential clinical consequences.

CONCLUSION

CEACAM1 is a crucial regulator of vascular homeostasis and inflammation with significant implications for cardiovascular health. Despite the lack of understanding of tissue-specific modulation and isoform-dependent mechanisms, CEACAM1 could be a promising therapeutic target for the prevention of cardiovascular disease in the future.

CEACAM1-engineered MSCs have a broad spectrum of immunomodulatory functions and therapeutic potential via cell-to-cell interaction

Mesenchymal stem cells (MSCs) have garnered attention for their regenerative and immunomodulatory capabilities in clinical trials for various diseases. However, the effectiveness of MSC-based therapies, especially for conditions like graft-versus-host disease (GvHD), remains uncertain. The cytokine interferon (IFN)-γ has been known to enhance the immunosuppressive properties of MSCs through cell-to-cell interactions and soluble factors. In this study, we observed that IFN-γ-treated MSCs upregulated the expression of carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1), associated with immune evasion through the inhibition of natural killer (NK) cell cytotoxicity. To co-opt this immunomodulatory function, we generated MSCs overexpressing CEACAM1 and found that CEACAM1-engineered MSCs significantly reduced NK cell activation and cytotoxicity via cell-to-cell interaction, independent of NKG2D ligand regulation. Furthermore, CEACAM1-engineered MSCs effectively inhibited the proliferation and activation of T cells along with the inflammatory responses of monocytes. In a humanized GvHD mouse model, CEACAM1-MSCs, particularly CEACAM1-4S-MSCs, demonstrated therapeutic potential by improving survival and alleviating symptoms. These findings suggest that CEACAM1 expression on MSCs contributes to MSC-mediated regulation of immune responses and that CEACAM1-engineered MSC could have therapeutic potential in conditions involving immune dysregulation.

Inhibition of CEACAM1 expression in cytokine-activated neutrophils using JAK inhibitors

OBJECTIVES

Carcinoembryonic-antigen-related cell-adhesion molecule 1 (CEACAM1) is an adhesion molecule that acts as a coinhibitory receptor in the immune system. We previously demonstrated that CEACAM1 is predominantly expressed on peripheral blood neutrophils in patients with RA. The aim of the present study was to investigate the effects of Janus kinase inhibitors (JAKi) on cytokine-activated human neutrophils and CEACAM1 expression.

METHODS

Peripheral blood neutrophils were obtained from healthy subjects. Isolated neutrophils were stimulated with tumor necrosis factor-alpha (TNF-α) or granulocyte-macrophage colony-stimulating factor (GM-CSF) in the presence or absence of JAKi. The expression of CEACAM1 in peripheral blood neutrophils was analyzed by flow cytometry. Protein phosphorylation of signal transducer and activator of transcription (STAT)1, STAT3, and STAT5 was assessed by western blot using phospho-specific antibodies.

RESULTS

We found that TNF-α-induced CEACAM1 expression was marginally suppressed after pretreatment with pan-JAK inhibitor, tofacitinib. Moreover, TNF-α induced STAT1 and STAT3 phosphorylation at the late stimulation phase (4 to 16 h). The expressions of CEACAM1 on neutrophils were markedly up-regulated by GM-CSF not by interleukin (IL)-6 stimulation. All JAKi inhibited GM-CSF-induced CEACAM1 expressions on neutrophils, however, the inhibitory effects of baricitinib were larger compared to those of tofacitinib or filgotinib. Moreover, CEACAM1 was marginally upregulated in interferon (IFN)-γ stimulated neutrophils. Similarly, JAKi inhibited IFN-γ-induced CEACAM1 expressions on neutrophils.

CONCLUSIONS

We demonstrated that JAKi prevent GM-CSF-induced CEACAM1 expression in neutrophils, and JAKi-induced inhibition depends on their selectivity against JAK isoforms. These findings suggest that JAKi can modulate the expression of CEACAM1 in cytokine-activated neutrophils, thereby limiting their activation.

Cell4D: a general purpose spatial stochastic simulator for cellular pathways

BACKGROUND

With the generation of vast compendia of biological datasets, the challenge is how best to interpret 'omics data alongside biochemical and other small-scale experiments to gain meaningful biological insights. Key to this challenge are computational methods that enable domain-users to generate novel hypotheses that can be used to guide future experiments. Of particular interest are flexible modeling platforms, capable of simulating a diverse range of biological systems with low barriers of adoption to those with limited computational expertise.

RESULTS

We introduce Cell4D, a spatial-temporal modeling platform combining a robust simulation engine with integrated graphics visualization, a model design editor, and an underlying XML data model capable of capturing a variety of cellular functions. Cell4D provides an interactive visualization mode, allowing intuitive feedback on model behavior and exploration of novel hypotheses, together with a non-graphics mode, compatible with high performance cloud compute solutions, to facilitate generation of statistical data. To demonstrate the flexibility and effectiveness of Cell4D, we investigate the dynamics of CEACAM1 localization in T-cell activation. We confirm the importance of Ca2+ microdomains in activating calmodulin and highlight a key role of activated calmodulin on the surface expression of CEACAM1. We further show how lymphocyte-specific protein tyrosine kinase can help regulate this cell surface expression and exploit spatial modeling features of Cell4D to test the hypothesis that lipid rafts regulate clustering of CEACAM1 to promote trans-binding to neighbouring cells.

CONCLUSIONS

Through demonstrating its ability to test and generate hypotheses, Cell4D represents an effective tool to help integrate knowledge across diverse, large and small-scale datasets.

T-cell immunoglobulin and mucin-domain containing-3 (TIM-3): Solving a key puzzle in autoimmune diseases

Dysfunctional immune cells participate in the pathogenesis of a variety of autoimmune diseases, although the specific mechanisms remain elusive and effective clinical interventions are lacking. Recent research on immune checkpoint molecules has revealed significant expression of T cell immunoglobulin and mucin domain-containing protein 3 (TIM-3) on the surfaces of various immune cells. These include different subsets of T cells, macrophages, dendritic cells, natural killer cells, and mast cells. Further investigation into its protein structure, ligands, and intracellular signaling pathway activation mechanisms has found that TIM-3, by binding with different ligands, is involved in the regulation of crucial biological processes such as proliferation, apoptosis, phenotypic transformation, effector protein synthesis, and cellular interactions of various immune cells. The TIM-3-ligand axis plays a pivotal role in the pathogenesis of numerous conditions, including autoimmune diseases, infections, cancers, transplant rejection, and chronic inflammation. This article primarily focuses on the research findings of TIM-3 in the field of autoimmune diseases, with a special emphasis on the structure and signaling pathways of TIM-3, its types of ligands, and the potential mechanisms implicated in systemic lupus erythematosus, multiple sclerosis, rheumatoid arthritis, as well as other autoimmune diseases and chronic inflammation. The latest research results in the field of immunology suggest that TIM-3 dysfunction affects various immune cells and participates in the pathogenesis of diseases. Monitoring the activity of its receptor-ligand axis can serve as a novel biological marker for disease clinical diagnosis and prognosis evaluation. More importantly, the TIM-3-ligand axis and the downstream signaling pathway molecules may become key targets for targeted intervention treatment of autoimmune-related diseases.

Activation of CEACAM1 with an agonistic monoclonal antibody results in inhibition of melanoma cells

Inhibitory receptors (IRs), such as the carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1), are cell surface molecules expressed on both normal epithelial, endothelial, and hematopoietic cells and on neoplastic cells. IRs are usually used by cancer cells to inhibit immune cell functions. Thus, CEACAM1 positive tumor cells can interact homophilically with CEACAM1 expressed on T and NK cells to inhibit their antibody-dependent cell-mediated cytotoxicity (ADCC). In this study, we investigated the effect of agonistic/activating anti-CEACAM1 monoclonal antibody (mAb) on melanoma cell lines in vitro and in vivo, following our hypothesis that activation of CEACAM1 on melanoma cells by distinct mAbs may induce inhibition of cancer cell proliferation and/or their death. To address this, we established an activating anti-CEACAM1 mAb (CCM5.01) and characterized its binding to the CEACAM1 receptor. Using this mAb, we assessed the expression of CEACAM1 on four different human melanoma cell lines by western blot and flow cytometry and determined its effect on cell viability in vitro by MTT assay. Furthermore, we evaluated the mAb mechanism of action and found that binding of CEACAM1 with CCM5.01 induced SHP1 phosphorylation and p53 activation resulting in melanoma cell apoptosis. For in vivo studies, a xenograft model of melanoma was performed by injection of Mel-14 cells subcutaneously (s.c.) in SCID/Beige mice followed by intraperitoneal (i.p.) injection of CCM5.01 or of IgG1 isotype control every other day. CCM5.01 treated mice showed a slight but not significant decrease in tumor weight in comparison to the control group. Based on the obtained data, we suggest that activating CEACAM1 on melanoma cells might be a promising novel approach to fight cancers expressing this IR.

Structural analysis of human CEACAM1 oligomerization

The human (h) CEACAM1 GFCC’ face serves as a binding site for homophilic and heterophilic interactions with various microbial and host ligands. hCEACAM1 has also been observed to form oligomers and micro-clusters on the cell surface which are thought to regulate hCEACAM1-mediated signaling. However, the structural basis for hCEACAM1 higher-order oligomerization is currently unknown. To understand this, we report a hCEACAM1 IgV oligomer crystal structure which shows how GFCC’ face-mediated homodimerization enables highly flexible ABED face interactions to arise. Structural modeling and nuclear magnetic resonance (NMR) studies predict that such oligomerization is not impeded by the presence of carbohydrate side-chain modifications. In addition, using UV spectroscopy and NMR studies, we show that oligomerization is further facilitated by the presence of a conserved metal ion (Zn⁺⁺ or Ni⁺⁺) binding site on the G strand of the FG loop. Together these studies provide biophysical insights on how GFCC’ and ABED face interactions together with metal ion binding may facilitate hCEACAM1 oligomerization beyond dimerization.

The crystal structure of human CEACAM1 IgV oligomer and structural analyses provide insight into higher-order oligomerization involving GFCC’ face-mediated homodimerization, flexible ABED interfaces, and dynamic metal-ion bridging.

Correlated STORM-homoFRET imaging reveals highly heterogeneous membrane receptor structures

Mapping the self-organization and spatial distribution of membrane proteins is key to understanding their function. Developing methods that can provide insight into correlations between membrane protein co-localization and interactions is challenging. We report here on a correlated stochastic optical reconstruction microscopy (STORM)/homoFRET imaging approach for resolving the nanoscale distribution and oligomeric state of membrane proteins. Using live cell homoFRET imaging of carcinoembryonic antigen-related cellular adhesion molecule (CEACAM1), a cell-surface receptor known to exist in a complex equilibrium between monomer and dimer/oligomer states, we revealed highly heterogeneous diffraction-limited structures on the surface of HeLa cells. Furthermore, correlated super-resolved STORM imaging showed that these structures comprised a complex mixture and spatial distribution of self-associated CEACAM1 molecules. In conclusion, this correlated approach provides a compelling strategy for addressing challenging questions about the interplay between membrane protein concentration, distribution, interaction, clustering, and function.

Human CEACAM1 N-domain dimerization is independent from glycan modifications

Carcinoembryonic cellular adhesion molecules (CEACAMs) serve diverse roles in cell signaling, proliferation, and survival and are made up of one or several immunoglobulin (Ig)-like ectodomains glycosylated in vivo. The physiological oligomeric state and how it contributes to protein function are central to understanding CEACAMs. Two putative dimer conformations involving different CEACAM1 N-terminal Ig-like domain (CCM1) protein faces (ABED and GFCC'C″) were identified from crystal structures. GFCC'C″ was identified as the dominant CCM1 solution dimer, but ambiguity regarding the effect of glycosylation on dimer formation calls its physiological relevance into question. We present the first crystal structure of minimally glycosylated CCM1 in the GFCC'C″ dimer conformation and characterization in solution by continuous-wave and double electron-electron resonance electron paramagnetic resonance spectroscopy. Our results suggest the GFCC'C″ dimer is dominant in solution with different levels of glycosylation, and structural conservation and co-evolved residues support that the GFCC'C″ dimer is conserved across CEACAMs.

Structural basis of the dynamic human CEACAM1 monomer-dimer equilibrium

Human (h) carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) function depends upon IgV-mediated homodimerization or heterodimerization with host ligands, including hCEACAM5, hTIM-3, PD-1, and a variety of microbial pathogens. However, there is little structural information available on how hCEACAM1 transitions between monomeric and dimeric states which in the latter case is critical for initiating hCEACAM1 activities. We therefore mutated residues within the hCEACAM1 IgV GFCC' face including V39, I91, N97, and E99 and examined hCEACAM1 IgV monomer-homodimer exchange using differential scanning fluorimetry, multi-angle light scattering, X-ray crystallography and/or nuclear magnetic resonance. From these studies, we describe hCEACAM1 homodimeric, monomeric and transition states at atomic resolution and its conformational behavior in solution through NMR assignment of the wildtype (WT) hCEACAM1 IgV dimer and N97A mutant monomer. These studies reveal the flexibility of the GFCC' face and its important role in governing the formation of hCEACAM1 dimers and selective heterodimers.

Structural characterization of a dimeric complex between the short cytoplasmic domain of CEACAM1 and the pseudo tetramer of S100A10-Annexin A2 using NMR and molecular dynamics

AIIt, a heterotetramer of S100A10 (P11) and Annexin A2, plays a key role in calcium dependent, membrane associations with a variety of proteins. We previously showed that AIIt interacts with the short cytoplasmic domain (12 amino acids) of CEACAM1 (CEACAM1-SF). Since the cytoplasmic domains of CEACAM1 help regulate the formation of cis- or trans-dimers at the cell membrane, we investigated the possible role of their association with AIIt in this process. Using NMR and molecular dynamics, we show that AIIt and its pseudoheterodimer interacts with two molecules of short cytoplasmic domain isoform peptides, and that interaction depends on the binding motif 454-Phe-Gly-Lys-Thr-457 where Phe-454 binds in a hydrophobic pocket of AIIt, the null mutation Phe454Ala reduces binding by 2.5 fold, and the pseudophosphorylation mutant Thr457Glu reduces binding by three fold. Since these two residues in CEACAM1-SF were also found to play a role in the binding of calmodulin and G-actin at the membrane, we hypothesize a sequential set of three interactions are responsible for regulation of cis- to trans-dimerization of CEACAM1. The hydrophobic binding pocket in AIIt corresponds to a previously identified binding pocket for a peptide found in SMARCA3 and AHNAK, suggesting a conserved functional motif in AIIt allowing multiple proteins to reversibly interact with integral membrane proteins in a calcium dependent manner.

The current landscape of coronavirus-host protein-protein interactions

In less than 20 years, three deadly coronaviruses, SARS-CoV, MERS-CoV and SARS-CoV-2, have emerged in human population causing hundreds to hundreds of thousands of deaths. Other coronaviruses are causing epizootic representing a significant threat for both domestic and wild animals. Members of this viral family have the longest genome of all RNA viruses, and express up to 29 proteins establishing complex interactions with the host proteome. Deciphering these interactions is essential to identify cellular pathways hijacked by these viruses to replicate and escape innate immunity. Virus-host interactions also provide key information to select targets for antiviral drug development. Here, we have manually curated the literature to assemble a unique dataset of 1311 coronavirus-host protein–protein interactions. Functional enrichment and network-based analyses showed coronavirus connections to RNA processing and translation, DNA damage and pathogen sensing, interferon production, and metabolic pathways. In particular, this global analysis pinpointed overlooked interactions with translation modulators (GIGYF2-EIF4E2), components of the nuclear pore, proteins involved in mitochondria homeostasis (PHB, PHB2, STOML2), and methylation pathways (MAT2A/B). Finally, interactome data provided a rational for the antiviral activity of some drugs inhibiting coronaviruses replication. Altogether, this work describing the current landscape of coronavirus-host interactions provides valuable hints for understanding the pathophysiology of coronavirus infections and developing effective antiviral therapies.

CEACAM1 structure and function in immunity and its therapeutic implications

The type I membrane protein receptor carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) distinctively exhibits significant alternative splicing that allows for tunable functions upon homophilic binding. CEACAM1 is highly expressed in the tumor environment and is strictly regulated on lymphocytes such that its expression is restricted to activated cells where it is now recognized to function in tolerance pathways. CEACAM1 is also an important target for microbes which have co-opted these attributes of CEACAM1 for the purposes of invading the host and evading the immune system. These properties, among others, have focused attention on CEACAM1 as a unique target for immunotherapy in autoimmunity and cancer. This review examines recent structural information derived from the characterization of CEACAM1:CEACAM1 interactions and heterophilic modes of binding especially to microbes and how this relates to CEACAM1 function. Through this, we aim to provide insights into targeting CEACAM1 for therapeutic intervention.

NMR analysis of free and lipid nanodisc anchored CEACAM1 membrane proximal peptides with Ca2+/CaM

CEACAM1, a homotypic transmembrane receptor with 12 or 72 amino acid cytosolic domain isoforms, is converted from inactive cis-dimers to active trans-dimers by calcium-calmodulin (Ca²⁺/CaM). Previously, the weak binding of Ca²⁺/CaM to the human 12 AA cytosolic domain was studied using C-terminal anchored peptides. We now show the binding of ¹⁵N labeled Phe-454 cytosolic domain peptides in solution or membrane anchored using NMR demonstrates a significant role for the lipid bilayer. Although binding is increased by the mutation Phe454Ala, this mutation was previously shown to abrogate actin binding. On the other hand, Ca²⁺/CaM binding is abrogated by phosphorylation of nearby Thr-457, a post-translation modification required for actin binding and subsequent in vitro lumen formation. Binding of Ca²⁺/CaM to a membrane proximal peptide from the long 72 AA cytosolic domain anchored to lipid nanodiscs was very weak compared to lipid free conditions, suggesting membrane specific effects between the two isoforms. NMR analysis of ¹⁵N labeled Ca²⁺/CaM with unlabeled peptides showed the C-lobe of Ca²⁺/CaM is involved in peptide interactions, and hydrophobic residues such as Met-109, Val-142 and Met-144 play important roles in binding peptide. This information was incorporated into transmembrane models of CEACAM1 binding to Ca²⁺/CaM. The lack of Ca²⁺/CaM binding to phosphorylated Thr-457, a residue we have previously shown to be phosphorylated by CaMK2D, also dependent on Ca²⁺/CaM, suggests stepwise binding of the cytosolic domain first to Ca²⁺/CaM and then to actin.

Serum-derived carcinoembryonic antigen (CEA) activates fibroblasts to induce a local re-modeling of the extracellular matrix that favors the engraftment of CEA-expressing tumor cells

Elevated levels of the carcinoembryonic antigen (CEA; CEACAM5) in the serum of colorectal cancer (CRC) patients represent a clinical biomarker that correlates with disease recurrence. However, a mechanistic role for soluble CEA (sCEA) in tumor progression and metastasis remains to be established. In our study, we report that sCEA acts as a paracrine factor, activating human fibroblasts by signaling through both the STAT3 and AKT1-mTORC1 pathways, promoting their transition to a cancer-associated fibroblast (CaF) phenotype. sCEA-activated fibroblasts express and secrete higher levels of fibronectin, including cellular EDA⁺ -fibronectin (Fn-EDA) that selectively promote the implantation and adherence of CEA-expressing cancer cells. Immunohistochemical analyses of liver tissues derived from CRC patients with elevated levels of sCEA reveal that the expression of cellular Fn-EDA co-registers with CEA-expressing liver metastases. Taken together, these findings indicate a direct role for sCEA as a human fibroblast activation factor, in priming target tissues for the engraftment of CEA-expressing cancer cells, through the differentiation of tissue-resident fibroblasts, resulting in a local change in composition of the extracellular matrix.

CEACAM1 promotes CD8+ T cell responses and improves control of a chronic viral infection

Dysfunction of CD8⁺ T cells can lead to the development of chronic viral infection. Identifying mechanisms responsible for such T cell dysfunction is therefore of great importance to understand how to prevent persistent viral infection. Here we show using lymphocytic choriomeningitis virus (LCMV) infection that carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) is fundamental for recruiting lymphocyte-specific protein kinase (Lck) into the T cell receptor complex to form an efficient immunological synapse. CEACAM1 is essential for activation of CD8⁺ T cells, and the absence of CEACAM1 on virus-specific CD8⁺ T cells limits the antiviral CD8⁺ T cell response. Treatment with anti-CEACAM1 antibody stabilizes Lck in the immunological synapse, prevents CD8⁺ T cell exhaustion, and improves control of virus infection in vivo. Treatment of human virus-specific CD8⁺ T cells with anti-CEACAM1 antibody similarly enhances their proliferation. We conclude that CEACAM1 is an important regulator of virus-specific CD8⁺ T cell functions in mice and humans and represents a promising therapeutic target for modulating CD8⁺ T cells.

Chronic viral infections are frequently associated with the dysfunction of CD8⁺ T cells which includes loss of function and results in CD8⁺ T cell exhaustion. Here the authors show a role of CEACAM1 in promoting responsive CD8⁺ T cells in the context of a chronic lymphocytic choriomeningitis virus (LCMV) infection model.

Structural templating of J-aggregates: Visualizing bis(monoacylglycero)phosphate domains in live cells

Identifying the key structural and dynamical determinants that drive the association of biomolecules, whether in solution, or perhaps more importantly in a membrane environment, has critical implications for our understanding of cellular dynamics, processes, and signaling. With recent advances in high-resolution imaging techniques, from the development of new molecular labels to technical advances in imaging methodologies and platforms, researchers are now reaping the benefits of being able to directly characterize and quantify local dynamics, structures, and conformations in live cells and tissues. These capabilities are providing unique insights into association stoichiometries, interactions, and structures on sub-micron length scales. We previously examined the role of lipid headgroup chemistry and phase state in guiding the formation of pseudoisocyanine (PIC) dye J-aggregates on supported planar bilayers [Langmuir, 25, 10719]. We describe here how these same J-aggregates can report on the in situ formation of organellar membrane domains in live cells. Live cell hyperspectral confocal microscopy using GFP-conjugated GTPase markers of early (Rab5) and late (Rab7) endosomes revealed that the PIC J-aggregates were confined to domains on either the limiting membrane or intralumenal vesicles (ILV) of late endosomes, known to be enriched in the anionic lipid bis(monoacylglycero)phosphate (BMP). Correlated confocal fluorescence - atomic force microscopy performed on endosomal membrane-mimetic supported planar lipid bilayers confirmed BMP-specific templating of the PIC J-aggregates. These data provide strong evidence for the formation of BMP-rich lipid domains during multivesicular body formation and portend the application of structured dye aggregates as markers of cellular membrane domain structure, size, and formation.

Up-regulation of carcinoembryonic antigen-related cell adhesion molecule 1 in gastrointestinal cancer and its clinical relevance

Serum carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) is dysregulated in various malignant tumors and has been associated with tumor progression. However, the expression and regulatory mechanisms of serum CEACAM1 in gastrointestinal cancer are still unclear. The expression ratio of the CEACAM1-L and CEACAM1-S isoforms has seldom been investigated in gastrointestinal cancer. In this study, we intended to explore the expression and diagnostic value of CEACAM1 in gastrointestinal cancer. Serum CEACAM1 levels were measured by enzyme-linked immunosorbent assay. The protein expression and distribution of CEACAM1 in tumors were examined by immunohistochemical staining. The expression patterns and ratio of CEACAM1-L/S were analyzed by reverse transcription-polymerase chain reaction. The results showed that serum CEACAM1 levels were significantly higher in cancer patients than in healthy controls. CEACAM1 was found in secreted forms within the neoplastic glands, and its expression was more intense at the tumor invasion front. The CEACAM1-L/S (L:S) ratios were up-regulated during tumorigenesis. Our data suggest that the serum level of CEACAM1 may be used to discriminate gastrointestinal cancer patients from health controls.

Neural immunoglobulin superfamily interaction networks

The immunoglobulin superfamily (IgSF) encompasses hundreds of cell surface proteins containing multiple immunoglobulin-like (Ig) domains. Among these are neural IgCAMs, which are cell adhesion molecules that mediate interactions between cells in the nervous system. IgCAMs in some vertebrate IgSF subfamilies bind to each other homophilically and heterophilically, forming small interaction networks. In Drosophila, a global 'interactome' screen identified two larger networks in which proteins in one IgSF subfamily selectively interact with proteins in a different subfamily. One of these networks, the 'Dpr-ome', includes 30 IgSF proteins, each of which is expressed in a unique subset of neurons. Recent evidence shows that one interacting protein pair within the Dpr-ome network is required for development of the brain and neuromuscular system.

High Density or Urban Sprawl: What Works Best in Biology?

With new approaches in imaging-from new tools or reagents to processing algorithms-come unique opportunities and challenges to our understanding of biological processes, structures, and dynamics. Although innovations in super-resolution imaging are affording novel perspectives into how molecules structurally associate and localize in response to, or in order to initiate, specific signaling events in the cell, questions arise as to how to interpret these observations in the context of biological function. Just as each neighborhood in a city has its own unique vibe, culture, and indeed density, recent work has shown that membrane receptor behavior and action is governed by their localization and association state. There is tremendous potential in developing strategies for tracking how the populations of these molecular neighborhoods change dynamically.

Glycosylation Alters Dimerization Properties of a Cell-surface Signaling Protein, Carcinoembryonic Antigen-related Cell Adhesion Molecule 1 (CEACAM1)

Human carcinoembryonic antigen-related cell adhesion molecule 1 (C?/Au: EACAM1) is a cell-surface signaling molecule involved in cell adhesion, proliferation, and immune response. It is also implicated in cancer angiogenesis, progression, and metastasis. This diverse set of effects likely arises as a result of the numerous homophilic and heterophilic interactions that CEACAM1 can have with itself and other molecules. Its N-terminal Ig variable (IgV) domain has been suggested to be a principal player in these interactions. Previous crystal structures of the β-sandwich-like IgV domain have been produced using Escherichia coli-expressed material, which lacks native glycosylation. These have led to distinctly different proposals for dimer interfaces, one involving interactions of ABED β-strands and the other involving GFCC'C″ β-strands, with the former burying one prominent glycosylation site. These structures raise questions as to which form may exist in solution and what the effect of glycosylation may have on this form. Here, we use NMR cross-correlation measurements to examine the effect of glycosylation on CEACAM1-IgV dimerization and use residual dipolar coupling (RDC) measurements to characterize the solution structure of the non-glycosylated form. Our findings demonstrate that even addition of a single N-linked GlcNAc at potential glycosylation sites inhibits dimer formation. Surprisingly, RDC data collected on E. coli expressed material in solution indicate that a dimer using the non-glycosylated GFCC'C″ interface is preferred even in the absence of glycosylation. The results open new questions about what other factors may facilitate dimerization of CEACAM1 in vivo, and what roles glycosylation may play in heterophylic interactions.

Diverse oligomeric states of CEACAM IgV domains

Carcinoembryonic antigen-related cell adhesion molecules (CEACAMs) comprise a large family of cell surface adhesion molecules that bind to themselves and other family members to carry out numerous cellular functions, including proliferation, signaling, differentiation, tumor suppression, and survival. They also play diverse and significant roles in immunity and infection. The formation of CEACAM oligomers is caused predominantly by interactions between their N-terminal IgV domains. Although X-ray crystal structures of CEACAM IgV domain homodimers have been described, how CEACAMs form heterodimers or remain monomers is poorly understood. To address this key aspect of CEACAM function, we determined the crystal structures of IgV domains that form a homodimeric CEACAM6 complex, monomeric CEACAM8, and a heterodimeric CEACAM6-CEACAM8 complex. To confirm and quantify these interactions in solution, we used analytical ultracentrifugation to measure the dimerization constants of CEACAM homodimers and isothermal titration calorimetry to determine the thermodynamic parameters and binding affinities of CEACAM heterodimers. We found the CEACAM6-CEACAM8 heterodimeric state to be substantially favored energetically relative to the CEACAM6 homodimer. Our data provide a molecular basis for the adoption of the diverse oligomeric states known to exist for CEACAMs and suggest ways in which CEACAM6 and CEACAM8 regulate the biological functions of one another, as well as of additional CEACAMs with which they interact, both in cis and in trans.

Role of GxxxG Motifs in Transmembrane Domain Interactions

Transmembrane (TM) helices of integral membrane proteins can facilitate strong and specific noncovalent protein-protein interactions. Mutagenesis and structural analyses have revealed numerous examples in which the interaction between TM helices of single-pass membrane proteins is dependent on a GxxxG or (small)xxx(small) motif. It is therefore tempting to use the presence of these simple motifs as an indicator of TM helix interactions. In this Current Topic review, we point out that these motifs are quite common, with more than 50% of single-pass TM domains containing a (small)xxx(small) motif. However, the actual interaction strength of motif-containing helices depends strongly on sequence context and membrane properties. In addition, recent studies have revealed several GxxxG-containing TM domains that interact via alternative interfaces involving hydrophobic, polar, aromatic, or even ionizable residues that do not form recognizable motifs. In multipass membrane proteins, GxxxG motifs can be important for protein folding, and not just oligomerization. Our current knowledge thus suggests that the presence of a GxxxG motif alone is a weak predictor of protein dimerization in the membrane.

Expression of Opacity Proteins Interferes with the Transmigration of Neisseria gonorrhoeae across Polarized Epithelial Cells

Neisseria gonorrhoeae (GC) establishes infection at the mucosal surface of the human genital tract, most of which is lined with polarized epithelial cells. GC can cause localized as well as disseminated infections, leading to various complications. GC constantly change their surface structures via phase and antigenic variation, which has been implicated as a means for GC to establish infection at various anatomic locations of male and female genital tracks. However, the exact contribution of each surface molecule to bacterial infectivity remains elusive due to their phase variation. Using a GC derivative that is genetically devoid of all opa genes (MS11∆Opa), this study shows that Opa expression interferes with GC transmigration across polarized human epithelial cells. MS11∆Opa transmigrates across polarized epithelial cells much faster and to a greater extent than MS11Opa+, while adhering at a similar level as MS11Opa+. When MS11Opa+, able to phase vary Opa expression, was inoculated, only those bacteria that turn off Opa expression transmigrate across the polarized epithelial monolayer. Similar to bacteria alone or co-cultured with non-polarized epithelial cells, MS11∆Opa fails to form large microcolonies at the apical surface of polarized epithelial cells. Apical inoculation of MS11Opa+, but not MS11∆Opa, induces the recruitment of the Opa host-cell receptor carcinoembryonic antigen–related cell adhesion molecules (CEACAMs) to the apical junction and the vicinity of bacterial adherent sites. Our results suggest that Opa expression limits gonococcal ability to invade into subepithelial tissues by forming tight interactions with neighboring bacteria and by inducing CEACAM redistribution to cell junctions.

CEACAM1 regulates TIM-3-mediated tolerance and exhaustion

T-cell immunoglobulin domain and mucin domain-3 (TIM-3, also known as HAVCR2) is an activation-induced inhibitory molecule involved in tolerance and shown to induce T-cell exhaustion in chronic viral infection and cancers. Under some conditions, TIM-3 expression has also been shown to be stimulatory. Considering that TIM-3, like cytotoxic T lymphocyte antigen 4 (CTLA-4) and programmed death 1 (PD-1), is being targeted for cancer immunotherapy, it is important to identify the circumstances under which TIM-3 can inhibit and activate T-cell responses. Here we show that TIM-3 is co-expressed and forms a heterodimer with carcinoembryonic antigen cell adhesion molecule 1 (CEACAM1), another well-known molecule expressed on activated T cells and involved in T-cell inhibition. Biochemical, biophysical and X-ray crystallography studies show that the membrane-distal immunoglobulin-variable (IgV)-like amino-terminal domain of each is crucial to these interactions. The presence of CEACAM1 endows TIM-3 with inhibitory function. CEACAM1 facilitates the maturation and cell surface expression of TIM-3 by forming a heterodimeric interaction in cis through the highly related membrane-distal N-terminal domains of each molecule. CEACAM1 and TIM-3 also bind in trans through their N-terminal domains. Both cis and trans interactions between CEACAM1 and TIM-3 determine the tolerance-inducing function of TIM-3. In a mouse adoptive transfer colitis model, CEACAM1-deficient T cells are hyper-inflammatory with reduced cell surface expression of TIM-3 and regulatory cytokines, and this is restored by T-cell-specific CEACAM1 expression. During chronic viral infection and in a tumour environment, CEACAM1 and TIM-3 mark exhausted T cells. Co-blockade of CEACAM1 and TIM-3 leads to enhancement of anti-tumour immune responses with improved elimination of tumours in mouse colorectal cancer models. Thus, CEACAM1 serves as a heterophilic ligand for TIM-3 that is required for its ability to mediate T-cell inhibition, and this interaction has a crucial role in regulating autoimmunity and anti-tumour immunity.

Signaling by epithelial members of the CEACAM family - mucosal docking sites for pathogenic bacteria

Carcinoembryonic antigen-related cell adhesion molecules (CEACAMs) comprise a group of immunoglobulin-related vertebrate glycoproteins. Several family members, including CEACAM1, CEA, and CEACAM6, are found on epithelial tissues throughout the human body. As they modulate diverse cellular functions, their signaling capacity is in the focus of current research. In this review we will summarize the knowledge about common signaling processes initiated by epithelial CEACAMs and suggest a model of signal transduction by CEACAM family members lacking significant cytoplasmic domains. As pathogenic and non-pathogenic bacteria exploit these receptors during mucosal colonization, we try to highlight the connection between CEACAMs, microbes, and cellular responses. Special emphasis in this context is placed on the functional interplay between CEACAMs and integrins that influences matrix adhesion of epithelial cells. The cooperation between these two receptor families provides an intriguing example of the fine tuning of cellular responses and their manipulation by specialized microorganisms.

T cell receptor signaling can directly enhance the avidity of CD28 ligand binding

T cell activation takes place in the context of a spatial and kinetic reorganization of cell surface proteins and signaling molecules at the contact site with an antigen presenting cell, termed the immunological synapse. Coordination of the activation, recruitment, and signaling from T cell receptor (TCR) in conjunction with adhesion and costimulatory receptors regulates both the initiation and duration of signaling that is required for T cell activation. The costimulatory receptor, CD28, is an essential signaling molecule that determines the quality and quantity of T cell immune responses. Although the functional consequences of CD28 engagement are well described, the molecular mechanisms that regulate CD28 function are largely unknown. Using a micropipet adhesion frequency assay, we show that TCR signaling enhances the direct binding between CD28 and its ligand, CD80. Although CD28 is expressed as a homodimer, soluble recombinant CD28 can only bind ligand monovalently. Our data suggest that the increase in CD28-CD28 binding is mediated through a change in CD28 valency. Molecular dynamic simulations and in vitro mutagenesis indicate that mutations at the base of the CD28 homodimer interface, distal to the ligand-binding site, can induce a change in the orientation of the dimer that allows for bivalent ligand binding. When expressed in T cells, this mutation allows for high avidity CD28–CD80 interactions without TCR signaling. Molecular dynamic simulations also suggest that wild type CD28 can stably adopt a bivalent conformation. These results support a model whereby inside-out signaling from the TCR can enhance CD28 ligand interactions by inducing a change in the CD28 dimer interface to allow for bivalent ligand binding and ultimately the transduction of CD28 costimulatory signals that are required for T cell activation.

Phosphorylation of CEACAM1 molecule by calmodulin kinase IID in a three-dimensional model of mammary gland lumen formation

Carcinoembryonic antigen-related cell adhesion molecule-1 (CEACAM1), a transmembrane protein, expressed on normal breast epithelial cells is down-regulated in breast cancer. Phosphorylation of Thr-457 on the short cytoplasmic domain isoform (CEACAM1-SF) that is predominant in normal epithelial cells is required for lumen formation in a three-dimensional model that involves apoptosis of the central acinar cells. Calmodulin kinase IID (CaMKIID) was selected as a candidate for the kinase required for Thr-457 phosphorylation from a gene chip analysis comparing genes up-regulated in MCF7 cells expressing wild type CEACAM1-SF compared with the T457A-mutated gene (Chen, C. J., Kirshner, J., Sherman, M. A., Hu, W., Nguyen, T., and Shively, J. E. (2007) J. Biol. Chem. 282, 5749-5760). Up-regulation of CaMKIID during lumen formation was confirmed by analysis of mRNA and protein levels. CaMKIID was able to phosphorylate a synthetic peptide corresponding to the cytoplasmic domain of CEACAM1-SF and was covalently bound to biotinylated and T457C-modified peptide in the presence of a kinase trap previously described by Shokat and co-workers (Maly, D. J., Allen, J. A., and Shokat, K. M. (2004) J. Am. Chem. Soc. 126, 9160-9161). When cell lysates from wild type-transfected MCF7 cells undergoing lumen formation were incubated with the peptide and kinase trap, a cross-linked band corresponding to CaMKIID was observed. When these cells were treated with an RNAi that inhibits CaMKIID expression, lumen formation was blocked by over 90%. We conclude that CaMKIID specifically phosphorylates Thr-457 on CEACAM1-SF, which in turn regulates the process of lumen formation via apoptosis of the central acinar cells.