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Abstract
Protein-tyrosyl phosphorylation, regulated by protein tyrosine kinases and protein
tyrosine phosphatases (PTPs), is a key cellular control mechanism. Until recently,
little was known about PTPs. However, the past two years have witnessed an explosion
of information about PTP structure, regulation and function. Crystal structures of
several PTPs have provided insights into enzymatic mechanisms and regulation and suggested
the design of 'substrate-trapping' mutants. Candidate homophilic and heterophilic
ligands for transmembrane PTPs have been identified, and roles for transmembrane PTPs
in regulating cell-cell interactions have been suggested. Finally, progress has been
made in understanding signaling by Src homology 2 domain containing PTPs and PTPs
controlling yeast osmoregulatory pathways.
A variety of cell adhesion mechanisms underlie the way that cells are organized in tissues. Stable cell interactions are needed to maintain the structural integrity of tissues, and dynamic changes in cell adhesion participate in the morphogenesis of developing tissues. Stable interactions actually require active adhesion mechanisms that are very similar to those involved in tissue dynamics. Adhesion mechanisms are highly regulated during tissue morphogenesis and are intimately related to the processes of cell motility and cell migration. In particular, the cadherins and the integrins have been implicated in the control of cell movement. Cadherin mediated cell compaction and cellular rearrangements may be analogous to integrin-mediated cell spreading and motility on the ECM. Regulation of cell adhesion can occur at several levels, including affinity modulation, clustering, and coordinated interactions with the actin cytoskeleton. Structural studies have begun to provide a picture of how the binding properties of adhesion receptors themselves might be regulated. However, regulation of tissue morphogenesis requires complex interactions between the adhesion receptors, the cytoskeleton, and networks of signaling pathways. Signals generated locally by the adhesion receptors themselves are involved in the regulation of cell adhesion. These regulatory pathways are also influenced by extrinsic signals arising from the classic growth factor receptors. Furthermore, signals generated locally be adhesion junctions can interact with classic signal transduction pathways to help control cell growth and differentiation. This coupling between physical adhesion and developmental signaling provides a mechanism to tightly integrate physical aspects of tissue morphogenesis with cell growth and differentiation, a coordination that is essential to achieve the intricate patterns of cells in tissues.
We used the enhancer detection/GAL4 system in Drosophila to direct increased levels of Fasciclin II (Fas II) expression on motoneuron growth cones and axons and to direct ectopic Fas II expression on other cells they encounter. Four classes of abnormal phenotypes are observed: "bypass" phenotypes, in which axons fail to defasciculate at the choice point where they would normally enter their muscle target region and instead extend past their target; "detour" phenotypes, in which these bypass growth cones enter their muscle target region at a different location; "stall" phenotypes, in which axons that enter their muscle target region fail to defasciculate from one another to probe their muscle targets; and "misroute" phenotypes, in which growth cones are diverted onto abnormal pathways by contact with Fas II-positive cells. These phenotypes show that changes in the pattern and level of Fas II expression can alter growth cone guidance, apparently in part by modulating the ability of these growth cones to respond to other guidance cues.
Immune complexes are potent activators of inflammatory cells, triggering effector responses through the crosslinking of Fc receptors (FcRs) such as Fc(epsilon)RI or Fc(gamma)RIII. On B cells and mast cells, immune complexes are also negative regulators of activation triggered by antigen and Fc receptors, a consequence of coligation of the B-cell antigen receptor or Fc(epsilon)RI, respectively, and the inhibitory receptor Fc(gamma)RIIB. Here we show that inhibitory signalling by Fc(gamma)RIIB does not require the SH2-domain-containing protein tyrosine phosphatase, SHP-1, in mast cells and results in the recruitment of the SH2-domain-containing inositol polyphosphate 5-phosphatase, SHIP, to the tyrosine-phosphorylated 13-amino-acid inhibitory motif of Fc(gamma)RIIB in both B cells and mast cells. SHIP, by hydrolysing the 5-phosphate of phosphatidylinositol(3,4,5)P3 and inositol(1,3,4,5)P4, suggests a mechanism by which Fc(gamma)RIIB can inhibit calcium influx and downstream responses triggered by immune receptors.
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