EB1 contributes to proper front-to-back polarity in neutrophil-like HL-60 cells

Regulation and functions of the RhoA regulatory guanine nucleotide exchange factor GEF-H1

Since the discovery by Madaule and Axel in 1985 of the first Ras homologue (Rho) protein in Aplysia and its human orthologue RhoB, membership in the Rho GTPase family has grown to 20 proteins, with representatives in all eukaryotic species. These GTPases are molecular switches that cycle between active (GTP bound) and inactivate (GDP bound) states. The exchange of GDP for GTP on Rho GTPases is facilitated by guanine exchange factors (GEFs). Approximately 80 Rho GEFs have been identified to date, and only a few GEFs associate with microtubules. The guanine nucleotide exchange factor H1, GEF-H1, is a unique GEF that associates with microtubules and is regulated by the polymerization state of microtubule networks. This review summarizes the regulation and functions of GEF-H1 and discusses the roles of GEF-H1 in human diseases.

A mechano-signalling network linking microtubules, myosin IIA filaments and integrin-based adhesions

The interrelationship between microtubules and the actin cytoskeleton in mechanoregulation of integrin-mediated adhesions is poorly understood. Here, we show that the effects of microtubules on two major types of cell-matrix adhesion, focal adhesions and podosomes, are mediated by KANK family proteins connecting the adhesion protein talin with microtubule tips. Both total microtubule disruption and microtubule uncoupling from adhesions by manipulations with KANKs trigger a massive assembly of myosin IIA filaments, augmenting focal adhesions and disrupting podosomes. Myosin IIA filaments are indispensable effectors in the microtubule-driven regulation of integrin-mediated adhesions. Myosin IIA filament assembly depends on Rho activation by the RhoGEF GEF-H1, which is trapped by microtubules when they are connected with integrin-mediated adhesions via KANK proteins but released after their disconnection. Thus, microtubule capture by integrin-mediated adhesions modulates the GEF-H1-dependent effect of microtubules on the assembly of myosin IIA filaments. Subsequent actomyosin reorganization then remodels the focal adhesions and podosomes, closing the regulatory loop.

How neutrophils resist shear stress at blood vessel walls: molecular mechanisms, subcellular structures, and cell-cell interactions

Neutrophils are the first cells arriving at sites of tissue injury or infection to combat invading pathogens. Successful neutrophil recruitment to sites of inflammation highly depends on specific molecular mechanisms, fine-tuning the received information into signaling pathways and converting them into well-described recruitment steps. This review highlights the impact of vascular flow conditions on neutrophil recruitment and the multitude of mechanisms developed to enable this sophisticated process under wall shear stress conditions. The recruitment process underlies a complex interplay between adhesion and signaling molecules, as well as chemokines, in which neutrophils developed specific mechanisms to travel to sites of lesion in low and high shear stress conditions. Rolling, as the first step in the recruitment process, highly depends on endothelial selectins and their ligands on neutrophils, inducting of intracellular signaling and subsequently activating β₂ integrins, enabling adhesion and postadhesion events. In addition, subcellular structures, such as microvilli, tethers, and slings allow the cell to arrest, even under high wall shear stress. Thereby, microvilli that are pulled out from the cell body form tethers that develop into slings upon their detachment from the substrate. In addition to the above-described primary capture, secondary capture of neutrophils via neutrophil-neutrophil or neutrophil-platelet interaction promotes the process of neutrophil recruitment to sites of lesion. Thus, precise mechanisms based on a complex molecular interplay, subcellular structures, and cell-cell interactions turn the delicate process of neutrophil trafficking during flow into a robust response allowing effective neutrophil accumulation at sites of injury.