Plexin signaling hampers integrin-based adhesion, leading to Rho-kinase independent cell rounding, and inhibiting lamellipodia extension and cell motility.

Plexins encode receptors for semaphorins, molecular signals guiding cell migration, and axon pathfinding. The mechanisms mediating plexin function are poorly understood. Plexin activation in adhering cells rapidly leads to retraction of cellular processes and cell rounding "cell collapse"). Here we show that, unexpectedly, this response does not require the activity of Rho-dependent kinase (ROCK) nor the contraction of F-actin cables. Interestingly, integrin-based focal adhesive structures are disassembled within minutes upon plexin activation; this is followed by actin depolymerization and, eventually, by cellular collapse. We also show that plexin activation hinders cell attachment to adhesive substrates, blocks the extension of lamellipodia, and thereby inhibits cell migration. We conclude that plexin signaling uncouples cell substrate-adhesion from cytoskeletal dynamics required for cell migration and axon extension.

Different point mutations in the met oncogene elicit distinct biological properties.

The MET proto-oncogene, encoding the tyrosine kinase receptor for HGF, controls genetic programs leading to cell growth, invasiveness, and protection from apoptosis. Recently, MET mutations have been identified in hereditary and sporadic forms of papillary renal carcinoma (PRC). Introduction of different naturally occurring mutations into the MET cDNA results in the acquisition of distinct biochemical and biological properties of transfected cells. Some mutations result in a high increase in tyrosine kinase activity and confer transforming ability in focus forming assays. These mutants hyperactivate the Ras signaling pathway. Other mutations are devoid of transforming potential but are effective in inducing protection from apoptosis and sustaining anchorage-independent growth. These Met(PRC) receptors interact more efficiently with the intracellular transducer Pi3Kinase. The reported results show that MET(PRC) mutations can be responsible for malignant transformation through different mechanisms, either by increasing the growth ability of cells or by protecting cells from apoptosis and allowing accumulation of other genetic lesions.-Giordano, S., Maffe, A., Williams, T. A., Artigiani, S., Gual, P., Bardelli, A., Basilico, C., Michieli, P., Comoglio, P. M. Different point mutations in the met oncogene elicit distinct biological properties.

Plexins are a large family of receptors for transmembrane, secreted, and GPI-anchored semaphorins in vertebrates.

In Drosophila, plexin A is a functional receptor for semaphorin-1a. Here we show that the human plexin gene family comprises at least nine members in four subfamilies. Plexin-B1 is a receptor for the transmembrane semaphorin Sema4D (CD100), and plexin-C1 is a receptor for the GPI-anchored semaphorin Sema7A (Sema-K1). Secreted (class 3) semaphorins do not bind directly to plexins, but rather plexins associate with neuropilins, coreceptors for these semaphorins. Plexins are widely expressed: in neurons, the expression of a truncated plexin-A1 protein blocks axon repulsion by Sema3A. The cytoplasmic domain of plexins associates with a tyrosine kinase activity. Plexins may also act as ligands mediating repulsion in epithelial cells in vitro. We conclude that plexins are receptors for multiple (and perhaps all) classes of semaphorins, either alone or in combination with neuropilins, and trigger a novel signal transduction pathway controlling cell repulsion.

Plexins, semaphorins, and scatter factor receptors: a common root for cell guidance signals?

Semaphorins, the plexin family of semaphorin receptors, and scatter factor receptors share evolutionarily conserved protein modules, such as the semaphorin domain and Met Related Sequences (MRS). All these proteins also have in common a role in mediating cell guidance cues. During development, scatter factor receptors control cell migration, epithelial tubulogenesis, and neurite extension. Semaphorins and their receptors are known signals for axon guidance; they are also suspected to regulate developmental processes involving cell migration and morphogenesis, and have been implicated in immune function and tumor progression. Scatter factors and secreted semaphorins are diffusible ligands, whereas membrane-bound semaphorins signal by cell-cell interaction. Cell guidance control by semaphorins requires plexins, alone or in a receptor complex with neuropilins. Semaphorins, besides their role in axon guidance, are expected to have multiple functions in morphogenesis and tissue remodeling by mediating cell-repelling cues through plexin receptors.