Biliary glycoprotein (Bgp, C-CAM, or CD66a) is an immunoglobulin-like cell adhesion molecule and functions as a tumor suppressor protein. We have previously shown that the Bgp1 isoform responsible for inhibition of colonic, liver, prostate, and breast tumor cell growth contains within its cytoplasmic domain two tyrosine residues positioned in immunoreceptor tyrosine-based inhibition motif (ITIM) consensus sequences. Moreover, we determined that these residues, upon phosphorylation, associate with the protein-tyrosine phosphatase SHP-1. In this report, we have further evaluated the structural bases of the association of Bgp1 with Tyr phosphatases. First, we demonstrate that Bgp1 also associates with the SHP-2 Tyr phosphatase, but not with an unrelated Tyr phosphatase, PTP-PEST. Association of Bgp1 and SHP-2 involves the Tyr residues within the Bgp1 ITIM sequences, Val at position ؉3 relative to the second Tyr (Tyr-515), and the SHP-2 N-terminal SH2 domain. In addition, our results indicate that residues ؉4, ؉5, and ؉6 relative to Tyr-515 in the Bgp1 cytoplasmic domain play a significant role in these interactions, as their deletion reduced Bgp1 Tyr phosphorylation and association with SHP-1 and SHP-2 by as much as 80%. Together, these results indicate that both SHP-1 and SHP-2 interact with the Bgp1 cytoplasmic domain via ITIM-like sequences. Furthermore, they reveal that the C-terminal amino acids of Bgp1 are critical for these interactions.
Apoptosis plays a crucial role in brain development by ensuring that only appropriately growing, migrating, and synapse-forming neurons and their associated glial cells survive. This process involves an intimate relationship between cell-cell interactions and developmental cues and is further impacted by environmental stress during neurogenesis and disease. Oligodendrocytes (OLs), the major myelin-forming cells in the central nervous system, largely form after this wave of neurogenesis but also show a selective vulnerability to cell death stimuli depending on their stage of development. This can affect not only embryonic and early postnatal brain formation but also the response to demyelinating pathologies. In the present review, we discuss the stagespecific sensitivity of OL lineage cells to damage-induced death and how this might impact myelin survival and regeneration during injury or disease. Apoptosis is a major form of programmed cell death required for the normal development of metazoan tissues, including the brain. 1 During embryonic development of the brain, many more cells than ultimately needed are generated and then selection occurs, resulting in the apoptotic depletion of the surplus cells. The importance of the apoptotic pathway in early brain development has been demonstrated by the targeted deletion in mice of the death-specific cysteine proteases caspase-3 or caspase-9, or of the co-activator Apaf-1, all of which cause severe brain overgrowth and perinatal death. 2,3 In the adult brain, 90% of the cells belong to the glial lineage, which includes oligodendrocytes (OLs), astrocytes and microglia. The glia ensures proper development, function and repair of the neuronal network. This is possible through continuous cross-talk between the glia and neurons mediated by neurotransmitters, cytokines, growth and trophic factor secretion and signaling in a reciprocal manner. [4][5][6][7] In the central nervous system (CNS), OLs are responsible for axon myelination, which insulates the electrical signals transmitted between neurons. OL and neuron development is tightly regulated and the myelin sheath is constructed only when OLs reach maturity and neurons have grown appropriately. In response to injury or during the course of neurological diseases, the neuron-glia network can be replenished to some extent but the degree of repair is dependent on the developmental stage of the OL. This complexity is compounded by the differential sensitivity of OL lineage cells to apoptotic stimuli. In the present review, we will first briefly examine the stages of differentiation of OL cells and then discuss several diseases that are impacted by OL apoptosis, noting how the stage of cell differentiation governs the sensitivity to apoptosis. Defined Stages of OL DevelopmentOligodendrocyte development can be divided into four distinct stages according to the temporal expression of cell surface markers and morphology (Table 1). 8 In the first stage, OL progenitor cells (OPCs) originate from the neuroepithelium of the ventricul...
Hippi functions as an adapter protein that mediates pro-apoptotic signaling from poly-glutamine-expanded huntingtin, an established cause of Huntington disease, to the extrinsic cell death pathway. To explore other functions of Hippi we generated Hippi knock-out mice. This deletion causes randomization of the embryo turning process and heart looping, which are hallmarks of defective left-right (LR) axis patterning. We report that motile monocilia normally present at the surface of the embryonic node, and proposed to initiate the break in LR symmetry, are absent on Hippi-/- embryos. Furthermore, defects in central nervous system development are observed. The Sonic hedgehog (Shh) pathway is downregulated in the neural tube in the absence of Hippi, which results in failure to establish ventral neural cell fate. Together, these findings demonstrate a dual role for Hippi in cilia assembly and Shh signaling during development, in addition to its proposed role in apoptosis signal transduction in the adult brain under pathogenically stressful conditions.
CEACAM1, also known as C-CAM, BGP and CD66a, is a member of the carcinoembryonic antigen (CEA) family which is itself part of the immunoglobulin supergene family. CEACAM1 is involved in intercellular adhesion, signal transduction and tumor cell growth regulation. CEACAM1 is down-regulated in colon and prostate carcinomas, as well as in endometrial, bladder and hepatic tumors, and 30% of breast cancers. We have shown in a mouse colon tumor model that CEACAM1 with a long cytoplasmic domain inhibited the development of tumors whereas a splice variant lacking the cytoplasmic domain did not. In this study, we de®ne the subregions of the long cytoplasmic domain participating in the tumor inhibition phenotype of CEACAM1. We show that a single point mutation of Tyr488, conforming to an Immuno-receptor Tyrosine Inhibition Motif (ITIM), was sucient to reverse the in vivo tumor cell growth inhibition. Substitution or deletion of residues in the C-terminal region of the CEACAM1 cytoplasmic domain also led to reversal of tumor cell growth inhibition. This result is in agreement with our previous studies demonstrating the C-terminal region of the cytoplasmic domain in¯uences the levels of CEACAM1 Tyr phosphorylation and its association with the protein Tyr phosphatases SHP-1 and SHP-2. Furthermore, removal of the N-terminal domain of CEACAM1, essential for intercellular adhesion, did not impair the tumor inhibitory eect. These results suggest that Tyr phosphorylation or dephosphorylation of the CEACAM1 cytoplasmic domain represents a crucial step in the control of epithelial cell proliferation.
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