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We describe an active polymer network in which processive molecular motors control network elasticity. This system consists of actin filaments cross-linked by filamin A (FLNa) and contracted by bipolar filaments of muscle myosin II. The myosin motors stiffen the network by more than two orders of magnitude by pulling on actin filaments anchored in the network by FLNa cross-links, thereby generating internal stress. The stiffening response closely mimics the effects of external stress applied by mechanical shear. Both internal and external stresses can drive the network into a highly nonlinear, stiffened regime. The active stress reaches values that are equivalent to an external stress of 14 Pa, consistent with a 1-pN force per myosin head. This active network mimics many mechanical properties of cells and suggests that adherent cells exert mechanical control by operating in a nonlinear regime where cell stiffness is sensitive to changes in motor activity. This design principle may be applicable to engineering novel biologically inspired, active materials that adjust their own stiffness by internal catalytic control.

Although simple and low-cost interventions for sickle cell disease (SCD) exist in many developing countries, child mortality associated with SCD remains high, in part, because of the lack of access to diagnostic tests for SCD. A density-based test using aqueous multiphase systems (SCD-AMPS) is a candidate for a low-cost, point-of-care diagnostic for SCD. In this paper, the field evaluation of SCD-AMPS in a large (n = 505) case-control study in Zambia is described. Of the two variations of the SCD-AMPS used, the best system (SCD-AMPS-2) demonstrated a sensitivity of 86% (82-90%) and a specificity of 60% (53-67%). Subsequent analysis identified potential sources of false positives that include clotting, variation between batches of SCD-AMPS, and shipping conditions. Importantly, SCD-AMPS-2 was 84% (62-94%) sensitive in detecting SCD in children between 6 months and 1 year old. In addition to an evaluation of performance, an assessment of end-user operability was done with health workers in rural clinics in Zambia. These health workers rated the SCD-AMPS tests to be as simple to use as lateral flow tests for malaria and HIV.

α-Actinin-4 is a widely expressed protein that employs an actin-binding site with two calponin homology domains to crosslink actin filaments (F-actin) in a Ca²⁺-sensitive manner in vitro. An inherited, late-onset form of kidney failure is caused by point mutations in the α-actinin-4 actin-binding domain. Here we show that α-actinin-4/F-actin aggregates, observed in vivo in podocytes of humans and mice with disease, likely form as a direct result of the increased actin-binding affinity of the protein. We document that exposure of a buried actin-binding site 1 in mutant α-actinin-4 causes an increase in its actin-binding affinity, abolishes its Ca²⁺ regulation in vitro, and diverts its normal localization from actin stress fibers and focal adhesions in vivo. Inactivation of this buried actin-binding site returns the affinity of the mutant to that of the WT protein and abolishes aggregate formation in cells. In vitro, actin filaments crosslinked by the mutant α-actinin-4 exhibit profound changes of structural and biomechanical properties compared with WT α-actinin-4. On a molecular level, our findings elucidate the physiological importance of a dynamic interaction of α-actinin with F-actin in podocytes in vivo. We propose that a conformational change with full exposure of actin-binding site 1 could function as a switch mechanism to regulate the actin-binding affinity of α-actinin and possibly other calponin homology domain proteins under physiological conditions.

Cooling of blood platelets clusters the von Willebrand factor receptor complex. Macrophage$\\alpha_M\\beta_2$integrins bind to the$GPIb\\alpha$subunit of the clustered complex, resulting in rapid clearance of transfused, cooled platelets. This precludes refrigeration of platelets for transfusion, but the current practice of room temperature storage has major drawbacks. We document that$\\alpha_M\\beta_2$is a lectin that recognizes exposed$\\beta-N-acetylglucosamine$residues of N-linked glycans on$GPIb\\alpha$. Enzymatic galactosylation of chilled platelets blocks$\\alpha_M\\beta_2$recognition, prolonging the circulation of functional cooled platelets. Platelet-associated galactosyltransferase produces efficient galactosylation when uridine phosphate-galactose is added, affording a potentially simple method for storing platelets in the cold.

The serine/threonine kinase p21-activated kinase 1 (Pak1) controls the actin cytoskeletal and ruffle formation through mechanisms that are independent of GTPase activity. Here we identify filamin FLNa as a Pak1-interacting protein through a yeast two-hybrid screen using the amino terminus of Pak1 as a bait. FLNa is stimulated by physiological signalling molecules to undergo phosphorylation by Pak1 and to interact and colocalize with endogenous Pak1 in membrane ruffles. The ruffle-forming activity of Pak1 is functional in FLNa-expressing cells but not in FLNa-deficient cells. In FLNa, the Pak1-binding site involves tandem repeat 23 in the carboxyl terminus and phosphorylation takes place on serine 2152. The FLNa-binding site in Pak1 is localized between amino acids 52 and 132 in the conserved Cdc42/Rac-interacting (CRIB) domain; accordingly, FLNa binding to the CRIB domain stimulates Pak1 kinase activity. Our results indicate that FLNa may be essential for Pak1-induced cytoskeletal reorganization and that the two-way regulatory interaction between Pak1 and FLNa may contribute to the local stimulation of Pak1 activity and its targets in cytoskeletal structures.

Depletion of the circulating actin-binding protein, plasma gelsolin (pGSN) has been described in septic patients and animals. We hypothesized that the extent of pGSN reduction correlates with outcomes of septic patients and that circulating actin is a manifestation of sepsis. We assayed pGSN in plasma samples from non-surgical septic patients identified from a pre-existing database which prospectively enrolled patients admitted to adult intensive care units at an academic hospital. We identified 21 non-surgical septic patients for the study. Actinemia was detected in 17 of the 21 patients, suggesting actin released into circulation from injured tissues is a manifestation of sepsis. Furthermore, we documented the depletion of pGSN in human clinical sepsis, and that the survivors had significantly higher pGSN levels than the non-survivors (163+/-47 mg/L vs. 89+/-48 mg/L, p = 0.01). pGSN levels were more strongly predictive of 28-day mortality than APACHE III scores. For every quartile reduction in pGSN, the odds of death increased 3.4-fold. We conclude that circulating actin and pGSN deficiency are associated with early sepsis. The degree of pGSN deficiency correlates with sepsis mortality. Reversing pGSN deficiency may be an effective treatment for sepsis.

FilGAP is a newly recognized filamin A (FLNa)-binding RhoGTPase-activating protein. The GTPase-activating protein (GAP) activity of FilGAP is specific for Rac and FLNa binding targets FilGAP to sites of membrane protrusion, where it antagonizes Rac in vivo . Dominant-negative FilGAP constructs lacking GAP activity or knockdown of endogenous FilGAP by small interference RNA (siRNA) induce spontaneous lamellae formation and stimulate cell spreading on fibronectin. Knockdown of endogenous FilGAP abrogates ROCK-dependent suppression of lamellae. Conversely, forced expression of FilGAP induces numerous blebs around the cell periphery and a ROCK-specific inhibitor suppresses bleb formation. ROCK phosphorylates FilGAP, and this phosphorylation stimulates its RacGAP activity and is a requirement for FilGAP-mediated bleb formation. FilGAP is, therefore, a mediator of the well-established antagonism of Rac by RhoA that suppresses leading edge protrusion and promotes cell retraction to achieve cellular polarity.

Mutations in filamin A (FLNa), an essential cytoskeletal protein with multiple binding partners, cause developmental anomalies in humans. We determined the structure of the 23rd Ig repeat of FLNa (IgFLNa23) that interacts with FilGAP, a Rac-specific GTPase-activating protein and regulator of cell polarity and movement, and the effect of the three disease-related mutations on this interaction. A combination of NMR structural analysis and in silico modeling revealed the structural interface details between the C and D beta-strands of the IgFLNa23 and the C-terminal 32 residues of FilGAP. Mutagenesis of the predicted key interface residues confirmed the binding constraints between the two proteins. Specific loss-of-function FLNa constructs were generated and used to analyze the importance of the FLNa-FilGAP interaction in vivo. Point mutagenesis revealed that disruption of the FLNa-FilGAP interface perturbs cell spreading. FilGAP does not bind FLNa homologs FLNb or FLNc establishing the importance of this interaction to the human FLNa mutations. Tight complex formation requires dimerization of both partners and the correct alignment of the binding surfaces, which is promoted by a flexible hinge domain between repeats 23 and 24 of FLNa. FLNa mutations associated with human developmental anomalies disrupt the binding interaction and weaken the elasticity of FLNa/F-actin network under high mechanical stress. Mutational analysis informed by structure can generate reagents for probing specific cellular interactions of FLNa. Disease-related FLNa mutations have demonstrable effects on FLNa function.

Intersectin-s is a modular scaffolding protein regulating the formation of clathrin-coated vesicles 1 , 2 . In addition to the Eps15 homology (EH) and Src homology 3 (SH3) domains of intersectin-s, the neuronal variant (intersectin-l) also has Dbl homology (DH), pleckstrin homology (PH) and C2 domains 1 , 3 , 4 , 5 , 6 , 7 . We now show that intersectin-l functions through its DH domain as a guanine nucleotide exchange factor (GEF) for Cdc42. In cultured cells, expression of DH-domain-containing constructs cause actin rearrangements specific for Cdc42 activation. Moreover, in vivo studies reveal that stimulation of Cdc42 by intersectin-l accelerates actin assembly via N-WASP and the Arp2/3 complex. N-WASP binds directly to intersectin-l and upregulates its GEF activity, thereby generating GTP-bound Cdc42, a critical activator of N-WASP. These studies reveal a role for intersectin-l in a novel mechanism of N-WASP activation and in regulation of the actin cytoskeleton.

For millennia, human survival depended on our innate abilities to fight pathogens and repair injuries. Only recently has medical science prolonged longevity and improved quality of life. Physicians and academic researchers contribute to such progress, but the principal contributor is private industry that produces the tools - drugs and medical devices - enabling doctors to prevent and cure disease. Heavy regulation and biology's complexity and unpredictability make medical innovation extremely difficult and expensive. Pharmaphobia describes how an ideological crusade, stretching over the last quarter century, has used distortion and flawed logic to make medical innovation even harder in a misguided pursuit of theoretical professional purity. Bureaucrats, reporters, politicians, and predatory lawyers have built careers attacking the medical products industry, belittling its critical contributions to medical innovation and accusing it of non-existent malfeasance: overselling product value, flaunting safety and corrupting physicians and academics who partner with it. The mania has imposed \"conflict-of-interest\" regulations limiting or banning valuable interactions between industry and physicians and researchers and diverting scarce resources from innovation to compliance. The victims are patients suffering from cancer, dementia, and other serious diseases for which new treatments are delayed, reduced, or eliminated as a result of these pointless regulations. With breathtaking detail, Thomas Stossel shows how this attack on doctors who work with industry limits medical innovation and inhibits the process of bringing new products into medical care.