Alexandra Matzke

A CD44v6 peptide reveals a role of CD44 in VEGFR-2 signaling and angiogenesis

A specific splice variant of the CD44 cell- surface protein family, CD44v6, has been shown to act as a coreceptor for the receptor tyrosine kinase c-Met on epithelial cells. Here we show that also on endothelial cells (ECs), the activity of c-Met is dependent on CD44v6. Furthermore, another receptor tyrosine kinase, VEGFR-2, is also regulated by CD44v6. The CD44v6 ectodomain and a small peptide mimicking a specific extracellular motif of CD44v6 or a CD44v6-specific antibody prevent CD44v6-mediated receptor activation. This indicates that the extracellular part of CD44v6 is required for interaction with c-Met or VEGFR-2. In the cytoplasm, signaling by activated c-Met and VEGFR-2 requires association of the CD44 carboxy-terminus with ezrin that couples CD44v6 to the cytoskeleton. CD44v6 controls EC migration, sprouting, and tubule formation induced by hepatocyte growth factor (HGF) or VEGF-A. In vivo the development of blood vessels from grafted EC spheroids and angiogenesis in tumors is impaired by CD44v6 blocking reagents, suggesting that the coreceptor function of CD44v6 for c-Met and VEGFR-2 is a promising target to block angiogenesis in pathologic conditions.

A five-amino-acid peptide blocks Met- and Ron-dependent cell migration.

Various human cancers express elevated levels of the receptor tyrosine kinases Met or Ron and v6-containing isoforms of CD44. The activation of Met and Ron requires the presence of such CD44 v6-containing isoforms that act as coreceptors. Three amino acids within the v6 sequence were identified by mutational analysis to be essential for the coreceptor function: EWQ in the rat sequence and RWH in human. Peptides comprising these three amino acids (the smallest containing only five amino acids) efficiently act as competitors and block ligand-dependent activation of Met or Ron and subsequent cell migration.

Haploinsufficiency of c-Met in cd44−/− Mice Identifies a Collaboration of CD44 and c-Met In Vivo

ABSTRACT Recent evidence has shown that the activation of receptor tyrosine kinases is not only dependent on binding of their ligands but in addition requires adhesion molecules as coreceptors. We have identified CD44v6 as a coreceptor for c-Met in several tumor and primary cells. The CD44v6 ectodomain is required for c-Met activation, whereas the cytoplasmic tail recruits ERM proteins and the cytoskeleton into a signalosome complex. Here we demonstrate that c-Met (and hepatocyte growth factor and Gab1) is haploinsufficient in a cd44−/− background, as the cd44−/−; met+/− (and cd44−/−; hgf+/− and cd44−/−; gab1+/−) mice die at birth. They have impaired synaptic transmission in the respiratory rhythm-generating network and alterations in the phrenic nerve. These results are the first genetic data showing that CD44 and c-Met collaborate in vivo and that they are involved in synaptogenesis and axon myelination in the central and peripheral nervous systems.

c-Met recruits ICAM-1 as a coreceptor to compensate for the loss of CD44 in Cd44 null mice.

CD44v6 acts as a coreceptor for the receptor tyrosine kinases c-Met and VEGFR-2. It is shown that ICAM-1 can act as a new coreceptor in CD44v6-negative tumor cells. Furthermore, ICAM-1 can substitute for CD44v6 as a coreceptor for c-Met in primary hepatocytes and in liver regeneration in Cd44 null mice.

Internalization of Met Requires the Co-Receptor CD44v6 and Its Link to ERM Proteins

Receptor Tyrosine Kinases (RTKs) are involved in many cellular processes and play a major role in the control of cell fate. For these reasons, RTK activation is maintained under tight control. Met is an essential RTK that induces proliferation, differentiation, migration, survival and branching morphogenesis. Deregulation of Met by overexpression, amplification or lack of effective degradation leads to cancer and metastasis. We have shown that Met relies on CD44v6 for its activation and for signaling in several cancer cell lines and also in primary cells. In this paper, we show that internalization of Met is dependent on CD44v6 and the binding of Ezrin to the CD44v6 cytoplasmic domain. Both CD44v6 and Met are co-internalized upon Hepatocyte Growth Factor induction suggesting that Met-induced signaling from the endosomes relies on its collaboration with CD44v6 and the link to the cytoskeleton provided by ERM proteins.

Endocytosis of Fgf8 Is a Double-Stage Process and Regulates Spreading and Signaling

Tightly controlled concentration gradients of morphogens provide positional information and thus regulate tissue differentiation and morphogenesis in multicellular organisms. However, how such morphogenetic fields are formed and maintained remains debated. Here we show that fibroblast growth factor 8 (Fgf8) morphogen gradients in zebrafish embryos are established and maintained by two essential mechanisms. Firstly, Fgf8 is taken up into the cell by clathrin-mediated endocytosis. The speed of the uptake rate defines the range of the morphogenetic gradient of Fgf8. Secondly, our data demonstrate that after endocytosis the routing of Fgf8 from the early endosome to the late endosome shuts down signaling. Therefore, intracellular endocytic transport regulates the intensity and duration of Fgf8 signaling. We show that internalization of Fgf8 into the early endosome and subsequent transport towards the late endosome are two independent processes. Therefore, we hypothesize that Fgf8 receiving cells control both, the propagation width and the signal strength of the morphogen.