GEF at work: Vav in protruding filopodia.

The Dbl family proto-oncogene vav is a nucleotide exchange factor for Rho family GTPases and is involved in triggering cytoskeletal changes contributing to the alterations of cell shape and motility, as well as in the induction of gene expression. In vitro and in vivo Vav is regulated by multiple tyrosine phosphorylation and binding to phosphatidylinositol phosphates. Although recruitment of Vav to the plasma membrane appears important for the activation of Vav function, there is little information on the precise subcellular localization of Vav in living cells. Employing live video fluorescence and immunoelectron microscopy, we show that GFP-tagged full-length Vav, and several mutants in which the N-terminal regulatory calponin homology (CH) domain has been deleted, specifically localize to the tips of filopodia. This localization was congruent with a high content of tyrosine phosphorylation in these regions. Consistent with earlier observations, mutants lacking the C-terminal SH domain region were unable to translocate to the filopodia tips. The enrichment in filopodial tips persisted despite their lateral movement but was dependent on forward growth. Upon retraction, the signal was rapidly lost, indicating that Vav undergoes a specific and transient translocation in response to actin-based, protrusive events in filopodia.

Use of genetically modified glial cells overexpressing laminin alpha1-chain peptides in neurite outgrowth studies.

1. C6 glioma cells were transfected with two constructs carrying C-terminal laminin alpha1-chain sequences of 117 and 114 bp length, respectively. These sequences are specifically known to code for peptides which have neurite-promoting activity. 2. The stable expression and secretion of the two peptides was detected by Northern and Western blot analysis. 3. Primary neuronal cultures derived from embryonic mouse forebrain were cocultured with these transfected cells and exhibited a substantial increase in neurite outgrowth and in survival time. Conditioned media from the transfected cells generated similar effects. 4. Organotypic cultures from embryonic mouse brain were used as a second system as being closer to the in vivo situation. Again, coculture of brain slices with transfected cells or treatment with laminin peptide-containing media increased neuronal outgrowth.

Live dynamics of GFP-calponin: isoform-specific modulation of the actin cytoskeleton and autoregulation by C-terminal sequences.

The calponin family of F-actin-, tropomyosin- and calmodulin-binding proteins currently comprises three genetic variants. Their functional roles implicated from in vitro studies include the regulation of actomyosin interactions in smooth muscle cells (h1 calponin), cytoskeletal organisation in non-muscle cells (h2 calponin) and the control of neurite outgrowth (acidic calponin). We have now investigated the effects of calponin (CaP) isoforms and their C-terminal deletion mutants on the actin cytoskeleton by time lapse video microscopy of GFP fusion proteins in living smooth muscle cells and fibroblasts. It is shown that h1 CaP associates with the actin stress fibers in the more central part of the cell, whereas h2 CaP localizes to the ends of stress fibres and in the motile lamellipodial protrusions of spreading cells. Cells expressing h2 CaP spread more efficiently than those expressing h1 CaP and expression of GFP h1 CaP resulted in reduced cell motility in wound healing experiments. Notably, expression of GFP h1 CaP, but not GFP h2 CaP, conferred increased resistance of the actin cytoskeleton to the actin polymerization antagonists cytochalasin B and latrunculin B, as well as to the protein kinase inhibitors H7-dihydrochloride and rho-kinase inhibitor Y-27632. These data point towards a dual role of CaP in the stabilization and regulation of the actin cytoskeleton in vivo. Deletion studies further identify an autoregulatory role for the unique C-terminal tail sequences in the respective CaP isoforms.

Astrocytes and neurons express the tight junction-specific protein occludin in vitro.

The expression of occludin, an integral plasma membrane protein specifically located at tight junctions, was studied in various epithelial and nonepithelial tissues by means of RT-PCR, Western blotting, and immunofluorescent staining. Besides detection in epithelial and endothelial tissue, expression of occludin was found in primary and secondary cultures of neurons and astrocytes. Differentiation of astrocytes in vitro led to a marked decrease in occludin expression. Extractability of occludin from plasma membranes differed considerably between epithelial and nonepithelial cells. Following treatment with Triton X-100, occludin was completely extracted from astrocytic membranes but not from membranes derived from MDCK cells, suggesting a difference in the cytoplasmic and/or plasma membrane anchoring of occludin between these cell types.