Summary.  The glycoprotein VI (GPVI)–FcR γ‐chain complex initiates powerful activation of platelets by the subendothelial matrix proteins collagen and laminin through an immunoreceptor tyrosine‐based activation motif (ITAM)‐regulated signaling pathway. ITAMs are characterized by two YxxL sequences separated by 6–12 amino acids and are found associated with several classes of immunoglobulin (Ig) and C‐type lectin receptors in hematopoietic cells, including Fc receptors. Cross‐linking of the Ig GPVI leads to phosphorylation of two conserved tyrosines in the FcR γ‐chain ITAM by Src family tyrosine kinases, followed by binding and activation of the tandem SH2 domain‐containing Syk tyrosine kinase and stimulation of a downstream signaling cascade that culminates in activation of phospholipase Cγ2 (PLCγ2). In contrast, the C‐type lectin receptor CLEC‐2 mediates powerful platelet activation through Src and Syk kinases, but regulates Syk through a novel dimerization mechanism via a single YxxL motif known as a hemITAM. CLEC‐2 is a receptor for podoplanin, which is expressed at high levels in several tissues, including type 1 lung alveolar cells, lymphatic endothelial cells, kidney podocytes and some tumors, but is absent from vascular endothelial cells and platelets. In this article, we compare the mechanism of platelet activation by GPVI and CLEC‐2 and consider their functional roles in hemostasis and other vascular processes, including maintenance of vascular integrity, angiogenesis and lymphogenesis.

The platelet surface is poorly characterized due to the low abundance of many membrane proteins and the lack of specialist tools for their investigation. In this study we identified novel human platelet and mouse megakaryocyte membrane proteins using specialist proteomics and genomics approaches. Three separate methods were used to enrich platelet surface proteins prior to identification by liquid chromatography and tandem mass spectrometry: lectin affinity chromatography, biotin/NeutrAvidin affinity chromatography, and free flow electrophoresis. Many known, abundant platelet surface transmembrane proteins and several novel proteins were identified using each receptor enrichment strategy. In total, two or more unique peptides were identified for 46, 68, and 22 surface membrane, intracellular membrane, and membrane proteins of unknown subcellular localization, respectively. The majority of these were single transmembrane proteins. To complement the proteomics studies, we analyzed the transcriptome of a highly purified preparation of mature primary mouse megakaryocytes using serial analysis of gene expression in view of the increasing impor-

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Angiogenesis describes the development of new blood vessels from pre-existing vessels. The hijacking of this physiological process by tumours allows them to develop their own supplies of nutrients and oxygen, enabling their growth and metastasis. A large body of literature has accumulated over the last 20 years relating to angiogenesis, including signalling pathways involved in this process. One such pathway uses Slit-Roundabout proteins that are implicated in the development of cancers and tumour angiogenesis. The Roundabout family of receptors are large, single-pass transmembrane cell surface receptors involved in directing cell migration in response to their cognate Slit ligands. Although best known for their role in neuronal development, Slits and Roundabouts have now been implicated in myogenesis, leukocyte chemotaxis and tumour angiogenesis, confirming that the Robo signalling pathway functions across multiple cell types. We review here the evidence for a role for Slits and Roundabouts in cancer. In particular, we focus on the role of Robo1 and Robo4 in tumour angiogenesis and discuss the signalling pathways downstream of these proteins mediating endothelial cell migration.