Rac1-null mouse embryonic fibroblasts are motile and respond to platelet-derived growth factor.

Previous studies of Rac1 in fibroblasts have used dominant negative constructs, which may have nonspecific effects. We used a conditional Rac1 allele to critically examine Rac1 function in mouse fibroblasts. Lack of Rac1 had dramatic effects on nonconfluent cells, which were elongated and had extensive blebbing, but no lamellipodia or ruffle formation. However, Rac1-null fibroblasts translocated using pseudopodia-like protrusions without lamellipodia, migrating toward a platelet-derived growth factor (PDGF) gradient as efficiently as their wild-type counterparts. Rac1-null fibroblasts closed wounds in vitro and spread on a fibronectin substrate, although at a slower rate than wild-type cells. However, Rac1-null cells were markedly impaired in proliferation, with a defect in G1 to S transition, although they were capable of surviving in culture for more than 2 wk. These results refine our understanding of the functions of Rac1, indicate that lamellipodia formation is not required for cell motility, and show that PDGF-induced chemotaxis can occur in the absence of both lamellipodia and Rac1.

Loss of Tsc1/Tsc2 activates mTOR and disrupts PI3K-Akt signaling through downregulation of PDGFR.

Tuberous sclerosis (TSC) is a familial tumor syndrome due to mutations in TSC1 or TSC2, in which progression to malignancy is rare. Primary Tsc2(-/-) murine embryo fibroblast cultures display early senescence with overexpression of p21CIP1/WAF1 that is rescued by loss of TP53. Tsc2(-/-)TP53(-/-) cells, as well as tumors from Tsc2(+/-) mice, display an mTOR-activation signature with constitutive activation of S6K, which is reverted by treatment with rapamycin. Rapamycin also reverts a growth advantage of Tsc2(-/-)TP53(-/-) cells. Tsc1/Tsc2 does not bind directly to mTOR, however, nor does it directly influence mTOR kinase activity or cellular phosphatase activity. There is a marked reduction in Akt activation in Tsc2(-/-)TP53(-/-) and Tsc1(-/-) cells in response to serum and PDGF, along with a reduction in cell ruffling. PDGFRalpha and PDGFRbeta expression is markedly reduced in both the cell lines and Tsc mouse renal cystadenomas, and ectopic expression of PDGFRbeta in Tsc2-null cells restores Akt phosphorylation in response to serum, PDGF, EGF, and insulin. This activation of mTOR along with downregulation of PDGFR PI3K-Akt signaling in cells lacking Tsc1 or Tsc2 may explain why these genes are rarely involved in human cancer. This is in contrast to PTEN, which is a negative upstream regulator of this pathway.

Time-resolved structural studies with serial crystallography: A new light on retinal proteins.

Structural information of the different conformational states of the two prototypical light-sensitive membrane proteins, bacteriorhodopsin and rhodopsin, has been obtained in the past by X-ray cryo-crystallography and cryo-electron microscopy. However, these methods do not allow for the structure determination of most intermediate conformations. Recently, the potential of X-Ray Free Electron Lasers (X-FELs) for tracking the dynamics of light-triggered processes by pump-probe serial femtosecond crystallography has been demonstrated using 3D-micron-sized crystals. In addition, X-FELs provide new opportunities for protein 2D-crystal diffraction, which would allow to observe the course of conformational changes of membrane proteins in a close-to-physiological lipid bilayer environment. Here, we describe the strategies towards structural dynamic studies of retinal proteins at room temperature, using injector or fixed-target based serial femtosecond crystallography at X-FELs. Thanks to recent progress especially in sample delivery methods, serial crystallography is now also feasible at synchrotron X-ray sources, thus expanding the possibilities for time-resolved structure determination.

Mechanisms of gelsolin-dependent and -independent EGF-stimulated cell motility in a human lung epithelial cell line.

Acquisition of motility is an important step in malignant progression of tumor cells and involves dynamic changes in actin filament architecture orchestrated by many actin binding proteins. A role for the actin-binding protein gelsolin has been demonstrated in fibroblast motility. In this report, we investigated the role of gelsolin in bronchial epithelial cell motility. The non-tumorigenic bronchial epithelial cell line, NL20 migrated towards EGF in a modified Boyden chamber cell motility assay. However, the tumorigenic NL20-TA cell line derived from the NL20 cells and lacking gelsolin, did not migrate towards EGF. Ectopic expression of gelsolin in NL20-TA cells restored the EGF response, while motility of NL20-TA derived cells towards serum, PDGF, and fibronectin was independent of gelsolin expression. PI3-kinase inhibition failed to block EGF-stimulated motility in gelsolin transfected NL20-TA cells. Furthermore, EGF stimulated a motility response in cells lacking gelsolin in the presence of fibronectin or fibrinogen that was blocked with PI3-kinase inhibition. Thus, EGF-stimulated motility in NL20 cells and its derivatives are gelsolin dependent and PI3-kinase independent, while fibronectin and fibrinogen enhances EGF-stimulated motility through a pathway independent of gelsolin and dependent upon PI3-kinase.

Arp2/3 complex-deficient mouse fibroblasts are viable and have normal leading-edge actin structure and function.

RNA interference silencing of up to 90% of Arp3 protein expression, a major subunit of the Arp2/3 complex, proportionately decreases the intracellular motility of Listeria monocytogenes and actin nucleation activity ascribable to the Arp2/3 complex in mouse embryonic fibroblasts. However, the Arp2/3-deficient cells exhibit unimpaired lamellipodial actin network structure, translational locomotion, spreading, actin assembly, and ruffling responses. In addition, Arp3-silenced cells expressing neural Wiskott-Aldrich syndrome protein-derived peptides that inhibit Arp2/3 complex function in wild-type cells retained normal PDGF-induced ruffling. The Arp2/3 complex can be dispensable for leading-edge actin remodeling.

A ratiometric expressible FRET sensor for phosphoinositides displays a signal change in highly dynamic membrane structures in fibroblasts.

Phosphoinositides are important signal transduction intermediates in cell growth, survival, and motility. We have invented a fluorescence sensor for polyphosphorylated phosphoinositides based on a peptide derived from the Listeria protein ActA that undergoes a random coil to helix transition upon lipid binding. The sensor, termed CAY, is a fusion protein of cyan and yellow fluorescent proteins flanking the peptide at its N- and C-termini, respectively. CAY displays fluorescence resonance energy transfer in vitro in the absence of phosphorylated phosphoinositides, and this energy transfer is lost upon interaction with these phospholipids. These results demonstrate that a short peptide undergoing a coil to helix transition can be sufficient for the engineering of a FRET-based biosensor. CAY is predominantly localized to the cytoplasm in fibroblasts expressing the sensor but shows loss of fluorescence resonance energy transfer in regions of active actin dynamics such as ruffles that have previously been demonstrated to contain high levels of phosphoinositides.

Chemotactic signaling pathways in neutrophils: from receptor to actin assembly.

In this review, we present an overview of the signaling elements between neutrophil chemotactic receptors and the actin cytoskeleton that drives cell motility. From receptor-ligand interactions, activation of heterotrimeric G-proteins, their downstream effectors PLC and PI-3 kinase, the activation of small GTPases of the Rho family, and their regulation of particular cytoskeletal regulatory proteins, we describe pathways specific to the chemotaxing neutrophil and elements documented to be important for neutrophil function.

Visualization of retroviral replication in living cells reveals budding into multivesicular bodies.

Retroviral assembly and budding is driven by the Gag polyprotein and requires the host-derived vacuolar protein sorting (vps) machinery. With the exception of human immunodeficiency virus (HIV)-infected macrophages, current models predict that the vps machinery is recruited by Gag to viral budding sites at the cell surface. However, here we demonstrate that HIV Gag and murine leukemia virus (MLV) Gag also drive assembly intracellularly in cell types including 293 and HeLa cells, previously believed to exclusively support budding from the plasma membrane. Using live confocal microscopy in conjunction with electron microscopy of cells generating fluorescently labeled virions or virus-like particles, we observed that these retroviruses utilize late endosomal membranes/multivesicular bodies as assembly sites, implying an endosome-based pathway for viral egress. These data suggest that retroviruses can interact with the vps sorting machinery in a more traditional sense, directly linked to the mechanism by which cellular proteins are sorted into multivesicular endosomes.