Myosin 1E localizes to actin polymerization sites in lamellipodia, affecting actin dynamics and adhesion formation

Because the actin network in active lamellipodia is continuously assembling at the edge, moving inward and disassembling, there is a question as to how actin-binding proteins and other components are transported to the leading edge and how nascent adhesions are stabilized. Active transport could play a significant role in these functions but the components involved are unknown. We show here that Myosin 1E (a long tailed Myosin 1 isoform) rapidly moves to the tips of active lamellipodia and to actin-rich early adhesions, unlike Myosin 1G, 1B or 1C (short tailed isoforms). Myosin 1E co-localizes with CARMIL, FHOD1, Arp3 and β3-integrin in those early adhesions. But these structures precede stable paxillin-rich adhesions. Myosin 1E movement depends upon actin-binding domains and the presence of an SH3 oligomerization domain. Overexpression of a Myosin 1E deletion mutant without the extreme C-terminal interacting (SH3) domain (Myosin 1EΔSH3) increases edge fluctuations and decreases stable adhesion lifetimes. In contrast, overexpression of Myosin 1E full tail domain (TH1+TH2+TH3/SH3) decreases edge fluctuation. In Myosin 1E knockdown cells, and more prominently in cells treated with Myosin 1 inhibitor, cell-matrix adhesions are also short-lived and fail to mature. We suggest that, by moving to actin polymerization sites and early adhesion sites in active lamellipodia, Myosin 1E might play important roles in transporting not only important polymerizing proteins but also proteins involved in adhesion stabilization.

Role of a conserved residue R780 in Escherichia coli multidrug transporter AcrB

Multidrug efflux pumps play important roles in bacteria drug resistance. A major multidrug efflux system in Gram-negative bacteria is composed of the inner membrane transporter AcrB, outer membrane protein channel TolC, and membrane fusion protein AcrA. These three proteins form a large complex that spans both layers of cell membranes and the periplasmic space. AcrB exists and functions as a homotrimer. To identify residues at the trimer interface that play important roles in AcrB function, we conducted site directed mutagenesis and discovered a key residue, R780. Although R780K was partially functional, all other R780 mutants tested were completely nonfunctional. Replacement of R780 by other residues disrupted trimer association. However, a decrease of trimer stability was not the lone cause for the observed loss of activity, because the activity loss could not be restored by strengthening trimer interaction. Using both heat and chemical denaturation methods, we found that the mutation decreased protein stability. Finally, we identified a repressor mutation, M774K, through random mutagenesis. It restored the activity of AcrBR780A to a level close to that of the wild-type protein. To examine the mechanism of activity restoration, we monitored denaturation of AcrBR780A/M774K and found that the repressor mutation improved protein stability. These results suggest that R780 is critical for AcrB stability. When R780 was replaced by Ala, the protein retained the overall structure, still trimerized in the cell membrane, and interacted with AcrA. However, local structural rearrangement might have occurred and lead to the decrease of protein stability and loss of substrate efflux activity.

Modularity and functional plasticity of scaffold proteins as p(l)acemakers in cell signaling

Cells coordinate and integrate various functional modules that control their dynamics, intracellular trafficking, metabolism and gene expression. Such capacity is mediated by specific scaffold proteins that tether multiple components of signaling pathways at plasma membrane, Golgi apparatus, mitochondria, endoplasmic reticulum, nucleus and in more specialized subcellular structures such as focal adhesions, cell-cell junctions, endosomes, vesicles and synapses. Scaffold proteins act as "pacemakers" as well as "placemakers" that regulate the temporal, spatial and kinetic aspects of protein complex assembly by modulating the local concentrations, proximity, subcellular dispositions and biochemical properties of the target proteins through the intricate use of their modular protein domains. These regulatory mechanisms allow them to gate the specificity, integration and crosstalk of different signaling modules. In addition to acting as physical platforms for protein assembly, many professional scaffold proteins can also directly modify the properties of their targets while they themselves can be regulated by post-translational modifications and/or mechanical forces. Furthermore, multiple scaffold proteins can form alliances of higher-order regulatory networks. Here, we highlight the emerging themes of scaffold proteins by analyzing their common and distinctive mechanisms of action and regulation, which underlie their functional plasticity in cell signaling. Understanding these mechanisms in the context of space, time and force should have ramifications for human physiology and for developing new therapeutic approaches to control pathological states and diseases.

Mechanotransduction in vivo by repeated talin stretch-relaxation events depends upon vinculin

The focal adhesion protein talin undergoes cycles of stretching and relaxation in living cells, suggesting a role in the transduction of mechanical into biochemical signals.

Mechanotransduction is a critical function for cells, in terms of cell viability, shaping of tissues, and cellular behavior. In vitro, cellular level forces can stretch adhesion proteins that link extracellular matrix to the actin cytoskeleton exposing hidden binding sites. However, there is no evidence that in vivo forces produce significant in vivo stretching to cause domain unfolding. We now report that the adhesion protein, talin, is repeatedly stretched by 100–350 nm in vivo by myosin contraction of actin filaments. Using a functional EGFP-N-Talin1-C-mCherry to measure the length of single talin molecules, we observed that the C-terminal mCherry was normally displaced in the direction of actin flow by 90 to >250 nm from N-EGFP but only by 50–60 nm (talin's length in vitro) after myosin inhibition. Individual talin molecules transiently stretched and relaxed. Peripheral, multimolecular adhesions had green outside and red proximal edges. They also exhibited transient, myosin-dependent stretching of 50–350 nm for 6–16 s; however, expression of the talin-binding head of vinculin increased stretching to about 400 nm and suppressed dynamics. We suggest that rearward moving actin filaments bind, stretch, and release talin in multiple, stochastic stick-slip cycles and that multiple vinculin binding and release cycles integrate pulling on matrices into biochemical signals.

How are mechanical forces that act on the surface of a cell transformed into biochemical signals within the cell? Studies of isolated proteins suggest that some of them can stretch, but whether this also happens in living cells remains unclear. In this study, we have been able to measure the stretching of single molecules of a cellular adhesion protein called talin in vivo by tagging each end of the protein with a different fluorescent marker and observing changes in the distance between the two markers with a new microscopic method. Talin is a large cellular protein that concentrates at sites where the cell attaches to the substratum and links integrins in the cell membrane to the actin filament network in the cell. In our study, a green tag at the integrin-binding site was close to the cell surface, whereas a red tag at the actin-binding site was displaced inward by actin flow. We observed repeated protein stretching to 5–8 times the native protein length and relaxation linked to the transduction process in living cells in culture. Individual molecules stretched for 6–16 seconds over ranges of 50–350 nm. Cell adhesion sites, where hundreds of talin molecules were displaced in concert, had similar dynamics. These cycles of stretching and relaxation required the contractile protein myosin. The head domain of vinculin—an adhesion site protein that binds strongly to the stretched talin—kept the adhesions stretched and blocked large oscillations in length. These observations indicate that there is repeated stretching of talin, and that adhesion proteins play a role in the transduction of mechanical signals into biochemical signals through binding and release of vinculin and possibly other focal adhesion proteins.

Physical presence during gamma stereotactic radiosurgery

During an invited visit to the University of Pittsburgh Medical Center (UPMC) gamma knife facility, officials from the Nuclear Regulatory Commission (NRC) observed what they considered as an apparent violation of the physical presence requirements specified in 10 CFR 35.615(f)(3). This event initiated an inspection and two different but related investigations by the NRC Office of Investigations (OI). Based on the NRC inspection and investigations, the NRC identified three apparent violations that were under consideration for escalated enforcement. The University of Pittsburgh (licensee) requested an Alternative Dispute Resolution (ADR) session with the NRC to resolve issues related to whether a violation occurred, the appropriate enforcement action, and the appropriate corrective action. As a result of the ADR mediation session, the licensee and NRC agreed to final disposition of this matter by way of a single violation of the regulatory requirement in 10 CFR 35.24(b), whereby the licensee's Radiation Safety Officer failed to ensure that the physical presence requirements of 10 CFR 35.615(f)(3) were consistently met and failed to ensure that written directives were consistently signed by the Authorized User in accordance with 10 CFR 35.32. In addition to corrective actions the licensee had already taken to prevent recurrence, it also agreed to inform other licensees in the industry of this event, so that they may learn from this incident and take appropriate actions to assure that these types of violations do not occur at their institutions.

Energy-dependent nucleolar localization of p53 in vitro requires two discrete regions within the p53 carboxyl terminus

The p53 tumor suppressor is a nucleocytoplasmic shuttling protein that is found predominantly in the nucleus of cells. In addition to mutation, abnormal p53 cellular localization is one of the mechanisms that inactivate p53 function. To further understand features of p53 that contribute to the regulation of its trafficking within the cell, we analysed the subnuclear localization of wild-type and mutant p53 in human cells that were either permeabilized with detergent or treated with the proteasome inhibitor MG132. We, here, show that either endogenously expressed or exogenously added p53 protein localizes to the nucleolus in detergent-permeabilized cells in a concentration- and ATP hydrolysis-dependent manner. Two discrete regions within the carboxyl terminus of p53 are essential for nucleolar localization in permeabilized cells. Similarly, localization of p53 to the nucleolus after proteasome inhibition in unpermeabilized cells requires sequences within the carboxyl terminus of p53. Interestingly, genotoxic stress markedly decreases the association of p53 with the nucleolus, and phosphorylation of p53 at S392, a site that is modified by such stress, partially impairs its nucleolar localization. The possible significance of these findings is discussed.

Magnetic tweezers in cell biology

We discuss herein the theory as well as some design considerations of magnetic tweezers. This method of generating force on magnetic particles bound to biological entities is shown to have a number of advantages over other techniques: forces are exerted in noncontact mode, they can be large in magnitude (order of 10 nanonewtons), and adjustable in direction, static or oscillatory. One apparatus built in our laboratory is described in detail, along with examples of experimental applications and results.

Local force and geometry sensing regulate cell functions

The shapes of eukaryotic cells and ultimately the organisms that they form are defined by cycles of mechanosensing, mechanotransduction and mechanoresponse. Local sensing of force or geometry is transduced into biochemical signals that result in cell responses even for complex mechanical parameters such as substrate rigidity and cell-level form. These responses regulate cell growth, differentiation, shape changes and cell death. Recent tissue scaffolds that have been engineered at the micro- and nanoscale level now enable better dissection of the mechanosensing, transduction and response mechanisms.

Transformation proteins and DNA uptake localize to the cell poles in Bacillus subtilis

The Gram-positive, rod-forming bacterium Bacillus subtilis efficiently binds and internalizes transforming DNA. The localization of several competence proteins, required for DNA uptake, has been studied using fluorescence microscopy. At least three proteins (ComGA, ComFA, and YwpH) are preferentially associated with the cell poles and appear to colocalize. This association is dynamic; the proteins accumulate at the poles as transformability develops and then delocalize as transformability wanes. DNA binding and uptake also occur preferentially at the cell poles, as shown using fluorescent DNA and in single-molecule experiments with laser tweezers. In addition to the prominent polar sites, the competence proteins also localize as foci in association with the lateral cell membrane, but this distribution does not exhibit the same temporal changes as the polar accumulation. The results suggest the regulated assembly and disassembly of a DNA-uptake machine at the cell poles.

Rap1 is involved in cell stretching modulation of p38 but not ERK or JNK MAP kinase

Mechanical force or mechanical stress modulates intracellular signal pathways, including the mitogen-activated protein kinase (MAP kinase) cascades. In our system, cell stretching activated and cell contraction inactivated all three MAP kinase pathways (MKK1/2-extracellular signal-regulated kinase (ERK), MKK4 (SEK1)-cJun N-terminal kinase (JNK) and MKK3/6-p38 pathways). However, little is known about the molecular mechanisms that link the mechanical force to the MAP kinase cascades. To test whether Ras and Rap1 are possible components in the stretch-activated MAP kinase pathways, we examined if Ras and Rap1 were activated by cell stretching and if inhibition of their activity decreased the stretch-enhanced MAP kinase activity. Rap1 was activated by cell stretching and inactivated by cell contraction, whereas Ras was inactivated by cell stretching and activated by cell contraction. Rap1GapII and SPA-1, downregulators of Rap1 activity, decreased the stretch-enhanced p38 activity, whereas a dominant-negative mutant of Ras (RasN17) did not inhibit the stretch-initiated activation of MAP kinases. Furthermore, overexpression of Rap1 enhanced p38 activity but not ERK or JNK activity. These results indicate that Rap1 is involved in transducing the stretch-initiated signal to the MKK3/6-p38 pathway, but not to the MEK1/2-ERK or the MKK4 (SEK1)/MKK7-JNK pathway. Thus, Rap1 plays a unique role in force-initiated signal transduction.

First one in, last one out: the role of gabaergic transmission in generation and degeneration

This paper is the result of discussions between scientists working in widely separated areas, united by an interest in the hippocampus. The discussions focused on the possible role of GABA in the development and maturation of the hippocampus and in neurodegeneration in Alzheimer's disease (AD). GABA neurons are among the first to differentiate in the hippocampus and the properties of GABA neurotransmission in the developing hippocampus are distinct from those in the adult. GABAergic transmission may play a role in the clustering and maturation of GABA receptors, as well as of receptors for other neurotransmitters. The development and maturation of synaptic connections involves changes in the organization of the cytoskeleton, and mechanical force generation is probably required to establish appropriate points of contact. This generation of force may require coupling of specific receptors to the cytoskeleton through specialized proteins. In AD, much of the developmental process is progressively unraveled in the hippocampus, as afferent fibers, most notably from entorhinal excitatory neurons and from basal forebrain cholinergic cells, degenerate. This denervation undoubtedly has consequences for receptor systems, dendritic morphology and the underlying cytoskeleton. GABA neurons remain in the AD hippocampus, and may actually contribute to abnormal firing and degeneration of remaining pyramidal neurons. This attempt to bring together data from different areas of research has allowed the development of a scheme which identifies significant specific gaps in our knowledge, which could be readily filled by focused experimental work.

Fast axonal transport is required for growth cone advance

Growth cones are capable of advancing despite linkage to a stationary axonal cytoskeleton in chick and murine dorsal root ganglion neurites. Several lines of evidence point to the growth cone as the site of cytoskeletal elongation. Fast axonal transport is probably the means by which cytoskeletal elements or cofactors are rapidly moved through the axon. We report that direct, but reversible, inhibition of fast axonal transport with laser optical tweezers inhibits growth cone motility if cytoskeletal attachment to the cell body is maintained. Advancement ceases after a distance-dependent lag period which correlates with the rate of fast axonal transport. But severing the axonal cytoskeleton with the laser tweezers allows growth cones to advance considerably further. We suggest that axon elongation requires fast axonal transport but growth cone motility does not.

Reduced chronic hemolysis during high-dose vitamin E administration in Mediterranean-type glucose-6-phosphate dehydrogenase deficiency

The observation that high-dose oral vitamin E supplementation (800 IU per day) improved red-cell survival in two rare disorders associated with increased red-cell susceptibility to oxidative stress prompted a similar trial in 23 patients with Mediterranean glucose-6-phosphate dehydrogenase (G6PD) deficiency. Three months of vitamin E administration resulted in decreased chronic hemolysis as evidenced by improved red-cell life span (P less than 0.025), with an improvement in red-cell half-life from 22.9 +/- 0.7 days to 25.1 +/- 0.6 days (mean +/- S.E.M.), increased hemoglobin concentration (P less than 0.001), and decreased reticulocytosis (P less than 0.001) as compared with base-line values. Evaluation after one year of vitamin E administration demonstrated sustained improvement in all these indexes. Controlled clinical trials of vitamin E supplementation may be warranted to examine its efficacy in ameliorating acute hemolytic crises or in reducing morbidity from neonatal jaundice in this relatively common genetic disorder.