KIT-D816V oncogenic activity is controlled by the juxtamembrane docking site Y568-Y570.

Mutation of KIT receptor tyrosine kinase at residue D816 results in ligand-independent constitutive kinase activity. This mutation occurs in most patients with mastocytosis, a myeloproliferative neoplasm, and is detected at lower frequencies in acute myeloid leukemia and in germ cell tumors. Other KIT mutations occur in gastrointestinal stromal tumors (GIST) and mucosal melanoma. KIT is considered as a bona fide therapeutic target as c-kit mutations are driving oncogenes in these pathologies. However, several evidences suggest that KIT-D816V mutant is not as aggressive as other KIT mutants. Here, we show that an intracellular docking site in the juxtamembrane region of KIT maintains a negative regulation on KIT-D816V transforming potential. Sixteen signaling proteins were shown to interact with this motif. We further demonstrate that mutation of this site results in signaling modifications, altered gene expression profile and increased transforming activity of KIT-D816V mutant. This result was unexpected as mutations of the homologous sites on wild-type (WT) KIT, or on the related oncogenic FLT3-ITD receptor, impair their function. Our results support the hypothesis that, KIT-D816V mutation is a mild oncogenic event that is sufficient to confer partial transforming properties, but requires additional mutations to acquire its full transforming potential.

Junctional adhesion molecule C (JAM-C) distinguishes CD27+ germinal center B lymphocytes from non-germinal center cells and constitutes a new diagnostic tool for B-cell malignancies.

Differentiation of naïve B cells into plasma cells or memory cells occurs in the germinal centers (GCs) of lymph follicles or alternatively via a GC- and T-cell-independent pathway. It is currently assumed that B-cell lymphomas correlate to normal B-cell differentiation stages, but the precise correlation of several B-cell lymphomas to these two pathways remains controversial. In the present report, we describe the junctional adhesion molecule C (JAM-C), currently identified at the cell-cell border of endothelial cells, as a new B-cell marker with a tightly regulated expression during B-cell differentiation. Expression of JAM-C in tonsils allows distinction between two CD27+ B-cell subpopulations: JAM-C- GC B cells and JAM-C+ non-germinal B cells. The expression of JAM-C in different B-cell lymphomas reveals a disease-specific pattern and allows a clear distinction between JAM-C- lymphoproliferative syndromes (chronic lymphocytic leukemia, mantle cell lymphoma and follicular lymphoma) and JAM-C+ ones (hairy cell leukemia, marginal zone B-cell lymphoma). Therefore, we propose JAM-C as a new identification tool in B-cell lymphoma diagnosis.

Pulmonary dysfunction and impaired granulocyte homeostasis result in poor survival of Jam-C-deficient mice.

Jam-C(-/-) mice exhibit growth retardation and multilobular pneumonia concomitant with poor survival of the mice under conventional housing conditions. The deficient mice present a mega-oesophagus and have altered airway responsiveness. In addition, the number of circulating granulocytes is increased in Jam-C(-/-) mice as compared to control animals. These phenotypes probably reflect the different functions of JAM-C expressed by endothelial and mesenchymal cells. Indeed, the deregulation in the number of circulating granulocytes is caused by the lack of JAM-C expression on endothelial cells since rescuing endothelial expression of the protein in the Jam-C(-/-) mice is sufficient to restore homeostasis. More importantly, the rescue of vascular JAM-C expression is accompanied by better survival of deficient mice, suggesting that endothelial expression of JAM-C is mandatory for animal survival from opportunistic infections and fatal pneumonia.

The role of junctional adhesion molecule C (JAM-C) in acute pancreatitis.

The recruitment of inflammatory cells contributes significantly to tissue injury in acute pancreatitis. This process implies several molecular interactions between circulating and endothelial cells. The adhesion molecule junctional adhesion molecule C (JAM-C) is involved in leukocyte transendothelial migration and it can form homophilic (JAM-C/JAM-C) and heterophilic interactions with the leukocyte integrin alpha(M)beta(2). In this study, the effect of early administration of monoclonal antibodies directed against JAM-C in cerulein-induced acute pancreatitis was assessed. This reagent significantly blocked influx of leukocytes, release of serum amylase, secretion of inflammatory cytokines, and acinar cell necrosis. These effects were rapid and protected against tissue injury throughout the duration of the model. Conversely, cerulein-induced acute pancreatitis was more severe in transgenic mice overexpressing JAM-C on endothelial cells under the control of the Tie2 promoter. It is proposed that JAM-C expressed by endothelial cells contributes to the pathophysiology of acute pancreatitis and could be considered a target for clinical applications.

Heterogeneity of endothelial junctions is reflected by differential expression and specific subcellular localization of the three JAM family members.

Endothelial cells are linked to each other through intercellular junctional complexes that regulate the barrier and fence function of the vascular wall. The nature of these intercellular contacts varies with the need for permeability: For example, in brain the impervious blood-brain barrier is maintained by "tight" contacts between endothelial cells. By contrast, in high endothelial venules (HEVs), where lymphocytes continuously exit the bloodstream, the contacts are generally leaky. The precise molecular components that define the type of junction remain to be characterized. An immunoglobulin superfamily molecule named JAM-2, specifically expressed in lymphatic endothelial cells and HEVs, was recently identified. JAM-3 was cloned and characterized in the current study, and JAM-1, -2, and -3 were shown to form a novel protein family belonging to the larger cortical thymocyte Xenopus (CTX) molecular family. Using antibodies specific for each of the 3 family members, their specific participation in different types of cell-cell contact in vivo and their specific and differential localization in lateral contacts or tight junctions were demonstrated. Furthermore, it was shown that JAM-1 and JAM-2 differentially regulate paracellular permeability, suggesting that the presence of JAM-1, -2, or -3 in vascular junctions may play a role in regulating vascular function in vivo.

JAM-2, a novel immunoglobulin superfamily molecule, expressed by endothelial and lymphatic cells.

Cell-cell contacts are essential for morphogenesis and tissue function and play a vital role in mediating endothelial cohesion within the vascular system during vessel growth and organization. We identified a novel junctional adhesion molecule, named JAM-2, by a selective RNA display method, which allowed identification of transcripts encoding immunoglobulin superfamily molecules regulated during coculture of endothelial cells with tumor cells. The JAM-2 transcript is highly expressed during embryogenesis and is detected in lymph node and Peyer's patches RNA of adult mice. Accordingly, antibodies specific for JAM-2 stain high endothelial venules and lymphatic vessels in lymphoid organs, and vascular structures in the kidney. Using real time video microscopy, we show that JAM-2 is localized within minutes to the newly formed cell-cell contact. The role of the protein in the sealing of cell-cell contact is further suggested by the reduced paracellular permeability of cell monolayer transfected with JAM-2 cDNA, and by the localization of JAM-2 to tight junctional complexes of polarized cells. Taken together, our results suggest that JAM-2 is a novel vascular molecule, which participates in interendothelial junctional complexes.

The parting of the endothelium: miracle, or simply a junctional affair?

Leukocyte extravasation from the blood across the endothelium is vital for the functioning of the immune system. Our understanding of the early steps of this process has developed rapidly. However, it is still unclear how leukocytes undergo the final step, migrating through the junctions that mediate adhesion between adjacent endothelial cells, while preserving the barrier function of the endothelium. The first stage of transmigration - tethering and rolling - is mediated by interactions between selectins on the surface of leukocytes and glycosylated proteins such as GlyCAM-1 on the surface of endothelial cells. Stimulation of the leukocyte by chemokines then induces tight adhesion, which involves binding of activated leukocyte integrins to endothelial ICAM-1/VCAM-1 molecules. Passage of the leukocyte across the endothelium appears to require delocalization of certain endothelial cell molecules and proteolytic degradation of junctional complexes.

Special communicationthe critical role of mechanical forces in blood vessel development, physiology and pathology

The following extended abstracts were presented at the Research Initiatives in Vascular Disease Conference, Movers and Shakers in the Vascular Tree-Hemodynamic and Biomechanical Factors in Blood Vessel Pathology, sponsored by The Lifeline Foundation and the Cardiovascular & Interventional Radiology Research and Educational Foundation; jointly sponsored by the International Society for Cardiovascular Surgery, North American Chapter, The Society for Vascular Surgery, and The Society of Cardiovascular and Interventional Radiology; in cooperation with the National Institutes of Health-National Heart, Lung &Blood Institute on Mar 11-12, 1999, in Bethesda, Md.

Two human genes related to murine vanin-1 are located on the long arm of human chromosome 6.

We report here the identification of two distinct human cDNAs, called VNN1 and VNN2, related to the recently described mouse Vanin-1 molecule involved in lymphocyte migration (M. Aurrand-Lions et al., 1996, Immunity 5: 391-405). Tissue distribution of the expression of these two human Vanin-like genes is differential. Since Vanin-1 shares significant homologies with human biotinidase (BTD), we describe here a new family of related genes including at least four members: mouse Vanin-1, VNN1, VNN2, and BTD. We have mapped the murine locus encompassing the Vanin-1 gene on mouse chromosome 10 in position A2B1. The two human Vanin-like genes are closely linked, since they were found on the same YAC clone and colocalized on human chromosome 6q23-q24 known to contain several genetic alterations linked to the progression of metastatic human cancers.

A novel immunoglobulin superfamily junctional molecule expressed by antigen presenting cells, endothelial cells and platelets.

The architecture of lymphoid microenvironments depends upon complex interactions between several stromal cell types. We describe in this report the cloning of a cDNA which encodes a novel membrane molecule containing two external Ig-like domains. It is expressed at the junction between endothelial cells including HEV. It is also expressed by platelets and MHC class II+ antigen presenting cells in thymic medulla and T-cell areas in peripheral lymphoid organs. These cells which lack in RelB-deficient mice include tissue-derived dendritic, epithelial cells and macrophages. Thus, this molecule might contribute to the organization of cell junctions in different microenvironments.

Thymocytes and RelB-dependent medullary epithelial cells provide growth-promoting and organization signals, respectively, to thymic medullary stromal cells.

The thymic medulla is composed of distinct epithelial cell subsets, defined in this report by the reactivity of two novel antibodies, 95 and 29, raised against mouse thymic epithelial cell lines. These antibodies were used to probe the development of medulla in wild-type or mutant thymuses. In CD3 epsilon-deficient mice where thymocyte maturation is arrested at the CD4- CD8- stage, few scattered 95+ and 29+ epithelial cells are found. When few mature thymocytes develop as in CD3- zeta/eta mice, expansion and organization of 95+ but not 29+ cells, becomes detectable. In RelB-deficient mice, T cell maturation proceeds normally but negative selection is inefficient due to the lack of thymic medulla and dendritic cells. Strikingly, 29+ epithelial cells are absent and 95+ medullary epithelial cells are scattered throughout the thymus, intermingling with CDR1+ cortical epithelium. In chimeric mice lacking only dendritic cells, the corticomedullary junction persists and both 95+ and 29+ epithelial cells are localized in the medulla. These results suggest that two types of signals are required for development of thymic medulla. A growth signal depends upon the presence of maturing thymocytes, but organization of the thymic medulla requires the presence of activated 29+ medullary epithelial cells.

Thy-3, a developmentally regulated T-cell glycoprotein associated to Thy-1 in detergent-resistant membrane microdomains.

The function of the Thy-1 molecule, a major T-cell glycoprotein, is still obscure. Its functional properties might be due to the anchoring via a glycosylphosphatidylinositol group which favors gathering of molecules in functional membrane subregions called microdomains. Using novel monoclonal antibodies, we describe a 53-kDa Thy-1-associated glycoprotein called Thy-3. Thy-3 expression is restricted to T lymphocytes, becomes detectable on double-positive thymocytes, and depends on that of Thy-1. Anti-Thy-3 antibodies immunoprecipitate Thy-1 and Thy-3 or Thy-3 alone in detergents which preserve or disrupt microdomains, respectively. These antibodies induce thymocyte aggregation and interfere with adhesion of thymocytes to a thymic epithelial cell line as previously shown with anti-Thy-1 antibodies. Thus, Thy-3 is a T lineage-specific glycoprotein associated to Thy-1 in membrane microdomains and might contribute to the function of Thy-1 in T-cell differentiation.

Altered cellular proliferation and mesoderm patterning in Polycomb-M33-deficient mice.

In Drosophila, the trithorax-group and the Polycomb-group genes are necessary to maintain the expression of the homeobox genes in the appropriate segments. Loss-of-function mutations in those groups of genes lead to misexpression of the homeotic genes resulting in segmental homeotic transformations. Recently, mouse homologues of the Polycomb-group genes were identified including M33, the murine counterpart of Polycomb. In this report, M33 was targeted in mice by homologous recombination in embryonic stem (ES) cells to assess its function during development. Homozygous M33 (-/-) mice show greatly retarded growth, homeotic transformations of the axial skeleton, sternal and limb malformations and a failure to expand in vitro of several cell types including lymphocytes and fibroblasts. In addition, M33 null mutant mice show an aggravation of the skeletal malformations when treated to RA at embryonic day 7.5, leading to the hypothesis that, during development, the M33 gene might play a role in defining access to retinoic acid response elements localised in the regulatory regions of several Hox genes.

Vanin-1, a novel GPI-linked perivascular molecule involved in thymus homing.

Migration of hematopoietic precursor cells to the thymus is shown to depend upon a novel molecule called Vanin-1 expressed by perivascular thymic stromal cells. An anti-Vanin-1 antibody blocks the binding of pro-T cells to thymic sections in vitro, the in vivo accumulation of bone marrow cells around cortical thymic vessels, and long-term thymic regeneration. Thus, it interferes with the entry, and not the differentiation, of hematopoietic precursor cells. The Vanin-1 gene codes for a GPI-anchored 70 kDa protein that shows homology only with human biotinidase. Transfection of thymic stromal cells with the Vanin-1 cDNA enhances thymocyte adhesion in vitro. These data suggest that Vanin-1 regulates late adhesion steps of thymus homing under physiological, noninflammatory conditions.