The WAVE complex associates with sites of saddle membrane curvature

How local interactions of actin regulators yield large-scale organization of cell shape and movement is not well understood. Here we investigate how the WAVE complex organizes sheet-like lamellipodia. Using super-resolution microscopy, we find that the WAVE complex forms actin-independent 230-nm-wide rings that localize to regions of saddle membrane curvature. This pattern of enrichment could explain several emergent cell behaviors, such as expanding and self-straightening lamellipodia and the ability of endothelial cells to recognize and seal transcellular holes. The WAVE complex recruits IRSp53 to sites of saddle curvature but does not depend on IRSp53 for its own localization. Although the WAVE complex stimulates actin nucleation via the Arp2/3 complex, sheet-like protrusions are still observed in ARP2-null, but not WAVE complex-null, cells. Therefore, the WAVE complex has additional roles in cell morphogenesis beyond Arp2/3 complex activation. Our work defines organizing principles of the WAVE complex lamellipodial template and suggests how feedback between cell shape and actin regulators instructs cell morphogenesis.     

Light-based feedback for controlling intracellular signaling dynamics

The ability to apply precise inputs to signaling species in live cells would be transformative for interrogating and understanding complex cell-signaling systems. Here we report an 'optogenetic' method for applying custom signaling inputs using feedback control of a light-gated protein-protein interaction. We applied this strategy to perturb protein localization and phosphoinositide 3-kinase activity, generating time-varying signals and clamping signals to buffer against cell-to-cell variability or changes in pathway activity.     

Membrane tension maintains cell polarity by confining signals to the leading edge during neutrophil migration

Little is known about how neutrophils and other cells establish a single zone of actin assembly during migration. A widespread assumption is that the leading edge prevents formation of additional fronts by generating long-range diffusible inhibitors or by sequestering essential polarity components. We use morphological perturbations, cell-severing experiments, and computational simulations to show that diffusion-based mechanisms are not sufficient for long-range inhibition by the pseudopod. Instead, plasma membrane tension could serve as a long-range inhibitor in neutrophils. We find that membrane tension doubles during leading-edge protrusion, and increasing tension is sufficient for long-range inhibition of actin assembly and Rac activation. Furthermore, reducing membrane tension causes uniform actin assembly. We suggest that tension, rather than diffusible molecules generated or sequestered at the leading edge, is the dominant source of long-range inhibition that constrains the spread of the existing front and prevents the formation of secondary fronts.     

Central oxyntomodulin causes anorexigenic effects associated with the hypothalamus and alimentary canal in chicks (Gallus gallus)

Several peptides that are derived from proglucagon including glucagon, glucagon-like peptide-1 (GLP-1), and oxyntomodulin (OXM) cause satiety in mammals. Glucagon and GLP-1 also cause satiety in the avian, but the effects of OXM on avian appetite-related processes are not reported. Thus, this study was conducted to elucidate whether OXM induces satiety in chicks and to determine its mechanism of induction. Intracerebroventricular (ICV) OXM, in a linear-dose dependent manner, potently decreased feed and water intake. However, we found that the effect on water intake was secondary to a reduction in feed intake. Chicks treated with ICV OXM had decreased c-Fos immunoreactivity in the regio lateralis hypothalami, but the nucleus infundibuli hypothalami (homologue to the mammalian arcuate nucleus) had increased c-Fos immunoreactivity. ICV OXM also caused total alimentary canal transit time to be decreased. We conclude that changes in the hypothalamus and gut may contribute to anorexigenic effects after ICV OXM in chicks. Through divergent evolution of birds and mammals, the central anorexigenic effects of OXM may have been conserved.     

A PtdInsP(3)- and Rho GTPase-mediated positive feedback loop regulates neutrophil polarity

When presented with a gradient of chemoattractant, many eukaryotic cells respond with polarized accumulation of the phospholipid PtdIns(3,4,5)P(3). This lipid asymmetry is one of the earliest readouts of polarity in neutrophils, Dictyostelium discoideum and fibroblasts. However, the mechanisms that regulate PtdInsP(3) polarization are not well understood. Using a cationic lipid shuttling system, we have delivered exogenous PtdInsP(3) to neutrophils. Exogenous PtdInsP(3) elicits accumulation of endogenous PtdInsP(3) in a positive feedback loop that requires endogenous phosphatidylinositol-3-OH kinases (PI(3)Ks) and Rho family GTPases. This feedback loop is important for establishing PtdInsP(3) polarity in response to both chemoattractant and to exogenous PtdInsP(3); it may function through a self-organizing pattern formation system. Emergent properties of positive and negative regulatory links between PtdInsP(3) and Rho family GTPases may constitute a broadly conserved module for the establishment of cell polarity during eukaryotic chemotaxis.     

WASP integrates substrate topology and cell polarity to guide neutrophil migration

To control their movement, cells need to coordinate actin assembly with the geometric features of their substrate. Here, we uncover a role for the actin regulator WASP in the 3D migration of neutrophils. We show that WASP responds to substrate topology by enriching to sites of inward, substrate-induced membrane deformation. Superresolution imaging reveals that WASP preferentially enriches to the necks of these substrate-induced invaginations, a distribution that could support substrate pinching. WASP facilitates recruitment of the Arp2/3 complex to these sites, stimulating local actin assembly that couples substrate features with the cytoskeleton. Surprisingly, WASP only enriches to membrane deformations in the front half of the cell, within a permissive zone set by WASP’s front-biased regulator Cdc42. While WASP KO cells exhibit relatively normal migration on flat substrates, they are defective at topology-directed migration. Our data suggest that WASP integrates substrate topology with cell polarity by selectively polymerizing actin around substrate-induced membrane deformations in the front half of the cell.     

Homer3 regulates the establishment of neutrophil polarity

Most chemoattractants rely on activation of the heterotrimeric G-protein Gαi to regulate directional cell migration, but few links from Gαi to chemotactic effectors are known. Through affinity chromatography using primary neutrophil lysate, we identify Homer3 as a novel Gαi2-binding protein. RNA interference–mediated knockdown of Homer3 in neutrophil-like HL-60 cells impairs chemotaxis and the establishment of polarity of phosphatidylinositol 3,4,5-triphosphate (PIP₃) and the actin cytoskeleton, as well as the persistence of the WAVE2 complex. Most previously characterized proteins that are required for cell polarity are needed for actin assembly or activation of core chemotactic effectors such as the Rac GTPase. In contrast, Homer3-knockdown cells show normal magnitude and kinetics of chemoattractant-induced activation of phosphoinositide 3-kinase and Rac effectors. Chemoattractant-stimulated Homer3-knockdown cells also exhibit a normal initial magnitude of actin polymerization but fail to polarize actin assembly and intracellular PIP₃ and are defective in the initiation of cell polarity and motility. Our data suggest that Homer3 acts as a scaffold that spatially organizes actin assembly to support neutrophil polarity and motility downstream of GPCR activation.     

Taking evolution seriously in political science

In this essay, we explore the epistemological and ontological assumptions that have been made to make political science “scientific.” We show how political science has generally adopted an ontologically reductionist philosophy of science derived from Newtonian physics and mechanics. This mechanical framework has encountered problems and constraints on its explanatory power, because an emphasis on equilibrium analysis is ill-suited for the study of political change. We outline the primary differences between an evolutionary ontology of social science and the physics-based philosophy commonly employed. Finally, we show how evolutionary thinking adds insight into the study of political phenomena and research questions that are of central importance to the field, such as preference formation.     

Hem-1 complexes are essential for Rac activation, actin polymerization, and myosin regulation during neutrophil chemotaxis

Migrating cells need to make different actin assemblies at the cell's leading and trailing edges and to maintain physical separation of signals for these assemblies. This asymmetric control of activities represents one important form of cell polarity. There are significant gaps in our understanding of the components involved in generating and maintaining polarity during chemotaxis. Here we characterize a family of complexes (which we term leading edge complexes), scaffolded by hematopoietic protein 1 (Hem-1), that organize the neutrophil's leading edge. The Wiskott-Aldrich syndrome protein family Verprolin-homologous protein (WAVE)2 complex, which mediates activation of actin polymerization by Rac, is only one member of this family. A subset of these leading edge complexes are biochemically separable from the WAVE2 complex and contain a diverse set of potential polarity-regulating proteins. RNA interference–mediated knockdown of Hem-1–containing complexes in neutrophil-like cells: (a) dramatically impairs attractant-induced actin polymerization, polarity, and chemotaxis; (b) substantially weakens Rac activation and phosphatidylinositol-(3,4,5)-tris-phosphate production, disrupting the (phosphatidylinositol-(3,4,5)-tris-phosphate)/Rac/F-actin–mediated feedback circuit that organizes the leading edge; and (c) prevents exclusion of activated myosin from the leading edge, perhaps by misregulating leading edge complexes that contain inhibitors of the Rho-actomyosin pathway. Taken together, these observations show that versatile Hem-1–containing complexes coordinate diverse regulatory signals at the leading edge of polarized neutrophils, including but not confined to those involving WAVE2-dependent actin polymerization.