In several pathological conditions, epithelial cells demonstrate a breakdown of barrier function and acquire an invasive phenotype. Endothelial cells in particular are maintained in a mesenchymal state during the cell invasion phase of angiogenesis. We show here that tyrosine phosphorylation of the adherens junction protein VE-cadherin at two critical tyrosines, Tyr-658 and Tyr-731, via tyrosine kinase activation or phosphatase inactivation was sufficient to prevent the binding of p120-and ␤-catenin, respectively, to the cytoplasmic tail of VE-cadherin. In fact, phosphorylation at either site led to the inhibition of cell barrier function. Cells expressing wild-type VE-cadherin showed decreased cell migration compared with cells lacking VE-cadherin, whereas expression of VE-cadherin with a simple phosphomimetic tyrosine-to-glutamic acid mutation of Y658E or Y731E was sufficient to restore the migratory response. These findings demonstrate that a single phosphorylation event within the VE-cadherin cytoplasmic tail is sufficient to maintain cells in a mesenchymal state.

No abstract

Chemokines represent key factors in the outburst of the immune response, by activating and directing the leukocyte traffic, both in lymphopoiesis and in immune surveillance. Neurobiologists took little interest in chemokines for many years, until their link to acquired immune deficiency syndromeassociated dementia became established, and thus their importance in this field has been neglected. Nevertheless, the body of data on their expression and role in the CNS has grown in the past few years, along with a new vision of brain as an immunologically competent and active organ. A large number of chemokines and chemokine receptors are expressed in neurons, astrocytes, microglia and oligodendrocytes, either constitutively or induced by inflammatory mediators. They are involved in many neuropathological processes in which an inflammatory state persists, as well as in brain tumor progression and metastasis. Moreover, there is evidence for a crucial role of CNS chemokines under physiological conditions, similar to well known functions in the immune system, such as proliferation and developmental patterning, but also peculiar to the CNS, such as regulation of neural transmission, plasticity and survival. Keywords: acquired immune deficiency syndrome dementia complex, Alzheimer's disease, brain tumors, chemokines, multiple sclerosis, stroke. Since their discovery in 1987, when a new factor with chemotactic activity for neutrophils (later named interleukin IL 8, now called CXCL8) was identified, the chemokine family has grown and more than 50 chemokines that interact with at least 20 receptors have been identified to date (Walz et al. 1987;Yoshimura et al. 1987). Chemokines are a group of structurally and functionally related proteins that exert their biological activity by binding to cell surface receptors in many instances through interaction with sulfated proteins and proteoglycans. Chemokines have been highly conserved during evolution, as indicated by their expression in numerous different species of mammals, birds and fish, and by the amino acid similarities across species. These proteins are produced by many cellular sources and their actions cover a wide range of functions.Chemokines were initially recognized to play a role in leukocyte communication and migration, both in physiological and pathological contexts. These molecules control immune cell trafficking and recirculation of the leukocyte population between the blood vessels, lymph, lymphoid organs and tissues, a process important in host immune

Stromal cell-derived factor-1 (SDF-1), the ligand of the CXCR4 receptor, is a chemokine involved in chemotaxis and brain development that also acts as co-receptor for HIV-1 infection. We previously demonstrated that CXCR4 and SDF-1a are expressed in cultured type-I cortical rat astrocytes, cortical neurones and cerebellar granule cells. Here, we investigated the possible functions of CXCR4 expressed in rat type-I cortical astrocytes and demonstrated that SDF-1a stimulated the proliferation of these cells in vitro. The proliferative activity induced by SDF-1a in astrocytes was reduced by PD98059, indicating the involvement of extracellular signal-regulated kinases (ERK1/2) in the astrocyte proliferation induced by CXCR4 stimulation. This observation was further con®rmed showing that SDF-1a treatment selectively activated ERK1/2, but not p38 or stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK). Moreover, both astrocyte proliferation and ERK1/2 phosphorylation, induced by SDF-1a, were inhibited by pertussis toxin (PTX) and wortmannin treatment indicating the involvement of a PTX sensitive G-protein and of phosphatidyl inositol-3 kinase in the signalling of SDF-1a. In addition, Pyk2 activation represent an upstream components for the CXCR4 signalling to ERK1/2 in astrocytes. To our knowledge, this is the ®rst report demonstrating a proliferative effect for SDF-1a in primary cultures of rat type-I astrocytes, and showing that the activation of ERK1/2 is responsible for this effect. These data suggest that CXCR4/ SDF-1 should play an important role in physiological and pathological glial proliferation, such as brain development, reactive gliosis and brain tumour formation.

Abstract:Chemokines are a family of proteins that chemoattract and activate cells by interacting with specific receptors on the surface of their targets. The chemokine stromal cell-derived factor 1, (SDF1), binds to the seventransmembrane G protein-coupled CXCR4 receptor and acts to modulate cell migration, differentiation, and proliferation. CXCR4 and SDF1 are reported to be expressed in various tissues including brain. Here we show that SDF1 and CXCR4 are expressed in cultured cortical type I rat astrocytes, cortical neurons, and cerebellar granule cells. In cortical astrocytes, prolonged treatment with lipopolysaccharide induced an increase of SDF1 expression and a down-regulation of CXCR4, whereas treatment with phorbol esters did not affect SDF1 expression and down-modulated CXCR4 receptor expression. We also demonstrated the ability of human SDF1␣ (hSDF1␣) to increase the intracellular calcium level in cultured astrocytes and cortical neurons, whereas in the same conditions, cerebellar granule cells did not modify their intracellular calcium concentration. Furthermore, in cortical astrocytes, the simultaneous treatment of hSDF1␣ with the HIV-1 capside glycoprotein gp120 inhibits the cyclic AMP formation induced by forskolin treatment.