Carcinoembryonic antigen-related cell adhesion molecules (CEACAMs) belong to a group of mammalian immunoglobulinrelated glycoproteins. They are involved in cell-cell recognition and modulate cellular processes that range from the shaping of tissue architecture and neovascularization to the regulation of insulin homeostasis and T-cell proliferation. CEACAMs have also been identified as receptors for host-specific viruses and bacteria in mice and humans, respectively, making these proteins an interesting example of pathogen-host co-evolution. Forward and reverse genetics in the mouse now provide powerful novel models to elucidate the action of CEACAM family members in vivo. IntroductionIn multicellular organisms, cell-cell adhesion is vital to guide cells to their proper location during embryonic development and to mediate the integration of single cells into functional tissues and organs. Members of the immunoglobulin superfamily of cell adhesion molecules (IgCAMs) constitute a large group of cell surface glycoproteins with ancient roots in the animal kingdom that specialize in cell-cell adhesion. All IgCAMs posses at least one immunoglobulin (Ig)-like domain, a compact structure $85-110 amino acids long characterized by two b-sheets packed against each other [1]. The Ig fold comes in minor variations that allow subdivision into Ig variable (IgV) and Ig constant (IgC)-1 and -2 domains. In evolutionary terms, the Ig-fold is a success story that has made Ig-domain-encoding sequences the most abundant genes in the human genome. This does not come as a surprise, as the Ig domain seems to be perfectly suited to provide proteins with a universal interface that can be fine-tuned for almost every binding task. In the case of IgCAMs, the binding specificity is often directed towards molecules of their own class or towards themselves, a feature that has been well studied on the genetic, biochemical and structural levels for several neuronal IgCAMs, in particular N-CAM [2,3].Another prominent member of the IgCAM superfamily is the carcinoembryonic antigen (CEA), which is involved in homotypic and heterotypic interactions with other closely related IgCAMs and which constitutes a clinically relevant diagnostic marker in the surveillance of colon tumors. Together with its paralogues, CEA has been grouped in the CEA-related cell adhesion molecule (CEACAM) family, a subdivision of IgCAMs so far only known from mammals [4]. The overall domain organization of human CEACAMs, the number of known splice variants and the distribution of orthologues in other mammalian species is presented in Figure 1. It is important to note that over the years members of the CEACAM family have been going through a number of name changes; for example, the original biliary glycoprotein (Bgp), later classified as the CD66a antigen, has now become CEACAM1 (for current and historic nomenclature of the CEACAM family see also Figure 1). Though CEACAMs have been studied for decades with regard to their tumor-associated features [5], it appears from recent st...

SummarySeveral bacterial pathogens exploit carcinoembryonic antigen-related cell adhesion molecules (CEACAMs) to promote attachment and uptake into eukaryotic host cells. The widely expressed isoform CEACAM1 is involved in cell-cell adhesion, regulation of cell proliferation, insulin homeostasis, and neo-angiogenesis, processes that depend on the cytoplasmic domain of CEACAM1. By analysing the molecular requirements for CEACAM1-mediated internalization of bacteria, we surprisingly find that the CEACAM1 cytoplasmic domain is completely obsolete for bacterial uptake. Accordingly, CEACAM1-4L as well as a CEACAM1 mutant with a complete deletion of the cytoplasmic domain (CEACAM1 DCT) promote equivalent internalization of several human pathogens. CEACAM1-4L-and CEACAM1 DCT-mediated uptake proceeds in the presence of inhibitors of actin microfilament dynamics, which is in contrast to CEACAM3-mediated internalization. Bacteria-engaged CEACAM1-4L and CEACAM1 DCT, but not CEACAM3, localize to a gangliosid GM1-and GPI-anchored protein-containing portion of the plasma membrane. In addition, interference with cholesterol-rich membrane microdomains severely blocks bacterial uptake via CEACAM1-4L and CEACAM1 DCT, but not CEACAM3. Similar to GPIanchored CEACAM6, both CEACAM1-4L as well as CEACAM1 DCT partition into a low-density, Tritoninsoluble membrane fraction upon receptor clustering, whereas CEACAM3 is not detected in this fraction. Bacterial uptake by truncated CEACAM1 or chimeric CEACAM1/CEACAM3 molecules reveals that the transmembrane domain of CEACAM1 is responsible for its association with membrane microdomains. Together, these data argue for a functional role of lipid rafts in CEACAM1-mediated endocytosis that is promoted by the transmembrane domain of the receptor and that might be relevant for CEACAM1 function in physiologic settings.

Several Gram-negative human pathogens recognize members of the carcinoembryonic antigen-related cell adhesion molecule (CEACAM) family. Pathogenic Neisseriae employ distinct isoforms of the colony opacity-associated proteins (Opa CEA proteins) to bind to the amino-terminal domains of CEACAMs. Here we present a novel approach to rapidly determine the CEACAM-binding properties of single bacteria. Expression of the isolated amino-terminal domains of various CEACAMs in eukaryotic cells yields soluble probes that selectively recognize Opa CEA -expressing bacteria in a pull-down assay format. Furthermore, by expressing soluble CEACAMs as fusions to green-fluorescent protein (CEACAM-N-GFP), CEACAM-binding bacteria can be decorated with a fluorescent label and analysed by flow cytometry allowing the specific detection of receptor binding events on the level of single bacteria. Besides its potential for rapid and quantitative analysis of pathogen-receptor interactions, this novel approach allows the detection of receptor recognition in heterogeneous bacterial populations and might represent a valuable tool for profiling the host binding capabilities of various microorganisms.

BackgroundSeveral human-restricted Gram-negative bacteria exploit carcinoembryonic antigen-related cell adhesion molecules (CEACAMs) for host colonization. For example, Neisseria meningitidis engages these human receptors via outer membrane proteins of the colony opacity-associated (Opa) protein family triggering internalization into non-phagocytic cells.Principal FindingsWe report that a non-opaque strain of N. meningitidis selectively interacts with CEACAM1, but not other CEACAM family members. Using functional assays of bacterial adhesion and internalisation, microscopic analysis, and a panel of CEACAM1 deletion mutants we demonstrate that the engagement of CEACAM1 by non-opaque meningococci occurs in a manner distinct from Opa protein-mediated association. In particular, the amino-terminal domain of CEACAM1 is necessary, but not sufficient for Opa protein-independent binding, which requires multiple extracellular domains of the human receptor in a cellular context. Knock-down of CEACAM1 interferes with binding to lung epithelial cells, whereas chemical or pharmacological disruption of host protein glycosylation does not abrogate CEACAM1 recognition by non-opaque meningococci. The previously characterized meningococcal invasins NadA or Opc do not operate in a CEACAM1-dependent manner.ConclusionsThe results demonstrate a mechanistically distinct, Opa protein-independent interaction between N. meningitidis and human CEACAM1. Our functional investigations suggest the presence of a second CEACAM1-binding invasin on the meningococcal surface that associates with the protein backbone and not the carbohydrate structures of CEACAM1. The redundancy in meningococcal CEACAM1-binding factors further highlights the important role of CEACAM recognition in the biology of this human-adapted pathogen.

Human granulocytes express several glycoproteins of the CEACAM family. One family member, CEACAM3, operates as a single-chain phagocytic receptor, initiating the detection, internalization, and destruction of a limited set of gram-negative bacteria. In contrast, the function of CEACAM4, a closely related protein, is completely unknown. This is mainly a result of a lack of a specific ligand for CEACAM4. By generating chimeric proteins containing the extracellular bacteria-binding domain of CEACAM3 and the transmembrane and cytoplasmic part of CEACAM4 (CEACAM3/4) we demonstrate that this chimeric receptor can trigger efficient phagocytosis of attached particles. Uptake of CEACAM3/4-bound bacteria requires the intact ITAM of CEACAM4, and this motif is phosphorylated by Src family PTKs upon receptor clustering. Furthermore, SH2 domains derived from Src PTKs, PI3K, and the adapter molecule Nck are recruited and associate directly with the phosphorylated CEACAM4 ITAM. Deletion of this sequence motif or inhibition of Src PTKs blocks CEACAM4-mediated uptake. Together, our results suggest that this orphan receptor of the CEACAM family has phagocytic function and prompt efforts to identify CEACAM4 ligands.