Rho GTPases are molecular switches that regulate many essential cellular processes, including actin dynamics, gene transcription, cell-cycle progression and cell adhesion. About 30 potential effector proteins have been identified that interact with members of the Rho family, but it is still unclear which of these are responsible for the diverse biological effects of Rho GTPases. This review will discuss how Rho GTPases physically interact with, and regulate the activity of, multiple effector proteins and how specific effector proteins contribute to cellular responses. To date most progress has been made in the cytoskeleton field, and several biochemical links have now been established between GTPases and the assembly of filamentous actin. The main focus of this review will be Rho, Rac and Cdc42, the three best characterized mammalian Rho GTPases, though the genetic analysis of Rho GTPases in lower eukaryotes is making increasingly important contributions to this field.

Rho GTPases and their effector proteins.

Existing knowledge of genetic variants affecting risk of coronary artery disease (CAD) is largely based on genome-wide association study (GWAS) analysis of common SNPs. Leveraging phased haplotypes from the 1000 Genomes Project, we report a GWAS meta-analysis of ∼185,000 CAD cases and controls, interrogating 6.7 million common (minor allele frequency (MAF) > 0.05) and 2.7 million low-frequency (0.005 < MAF < 0.05) variants. In addition to confirming most known CAD-associated loci, we identified ten new loci (eight additive and two recessive) that contain candidate causal genes newly implicating biological processes in vessel walls. We observed intralocus allelic heterogeneity but little evidence of low-frequency variants with larger effects and no evidence of synthetic association. Our analysis provides a comprehensive survey of the fine genetic architecture of CAD, showing that genetic susceptibility to this common disease is largely determined by common SNPs of small effect size.

/pdf/a-comprehensive-1000-genomes-based-genome-wide-association-4p4zlig8vc.pdf

A comprehensive 1000 Genomes–based genome-wide association meta-analysis of coronary artery disease

Filopodia are thin, actin-rich plasma-membrane protrusions that function as antennae for cells to probe their environment. Consequently, filopodia have an important role in cell migration, neurite outgrowth and wound healing and serve as precursors for dendritic spines in neurons. The initiation and elongation of filopodia depend on the precisely regulated polymerization, convergence and crosslinking of actin filaments. The increased understanding of the functions of various actin-associated proteins during the initiation and elongation of filopodia has provided new information on the mechanisms of filopodia formation in distinct cell types.

Filopodia: molecular architecture and cellular functions

https://www.seas.upenn.edu/%7Edischer/pdfs/Cell_on_Gel-BJ.pdf

Substrate Compliance versus Ligand Density in Cell on Gel Responses

Cell migration involves dynamic and spatially regulated changes to the cytoskeleton and cell adhesion. The Rho GTPases play key roles in coordinating the cellular responses required for cell migration. Recent research has revealed new molecular links between Rho family proteins and the actin cytoskeleton, showing that they act to regulate actin polymerization, depolymerization and the activity of actin-associated myosins. In addition, studies on integrin signalling suggest that the substratum continuously feeds signals to Rho proteins in migrating cells to influence migration rate. There is also increasing evidence that Rho proteins affect the organization of the microtubule and intermediate filament networks and that this is important for cell migration.

Rho GTPases and cell migration.

Dendritic spines are small protrusions along dendrites where the postsynaptic components of most excitatory synapses reside in the mature brain. Morphological changes in these actin-rich structures are associated with learning and memory formation. Despite the pivotal role of the actin cytoskeleton in spine morphogenesis, little is known about the mechanisms regulating actin filament polymerization and depolymerization in dendritic spines. We show that the filopodia-like precursors of dendritic spines elongate through actin polymerization at both the filopodia tip and root. The small GTPase Rif and its effector mDia2 formin play a central role in regulating actin dynamics during filopodia elongation. Actin filament nucleation through the Arp2/3 complex subsequently promotes spine head expansion, and ADF/cofilin-induced actin filament disassembly is required to maintain proper spine length and morphology. Finally, we show that perturbation of these key steps in actin dynamics results in altered synaptic transmission.

/pdf/defining-mechanisms-of-actin-polymerization-and-3aj9gpaq26.pdf

Defining mechanisms of actin polymerization and depolymerization during dendritic spine morphogenesis

The making of filopodia

https://www.cell.com/developmental-cell/pdf/S1534-5807(06)00209-7.pdf

Staying in Shape with Formins

Cell migration is initiated by lamellipodia—membrane-enclosed sheets of cytoplasm containing densely packed actin filament networks. Although the molecular details of network turnover remain obscure, recent work points towards key roles in filament nucleation for Arp2/3 complex and its activator WAVE complex. Here, we combine fluorescence recovery after photobleaching (FRAP) of different lamellipodial components with a new method of data analysis to shed light on the dynamics of actin assembly/disassembly. We show that Arp2/3 complex is incorporated into the network exclusively at the lamellipodium tip, like actin, at sites coincident with WAVE complex accumulation. Capping protein likewise showed a turnover similar to actin and Arp2/3 complex, but was confined to the tip. In contrast, cortactin—another prominent Arp2/3 complex regulator—and ADF/cofilin—previously implicated in driving both filament nucleation and disassembly—were rapidly exchanged throughout the lamellipodium. These results suggest that Arp2/3- and WAVE complex-driven actin filament nucleation at the lamellipodium tip is uncoupled from the activities of both cortactin and cofilin. Network turnover is additionally regulated by the spatially segregated activities of capping protein at the tip and cofilin throughout the mesh.

/pdf/arp2-3-complex-interactions-and-actin-network-turnover-in-6viymvtv87.pdf

Arp2/3 complex interactions and actin network turnover in lamellipodia

Formins have important roles in the nucleation of actin and the formation of linear actin filaments, but their role in filopodium formation has remained elusive. Dictyostelium discoideum Diaphanous-related formin dDia2 is enriched at the tips of filopodia and interacts with profilin II and Rac1. An FH1FH2 fragment of dDia2 nucleated actin polymerization and removed capping protein from capped filament ends. Genetic studies showed that dDia2 is important for cell migration as well as the formation, elongation and maintenance of filopodia. Here we provide evidence that dDia2 specifically controls filopodial dynamics by regulating actin turnover at the barbed ends of actin filaments.

/pdf/the-diaphanous-related-formin-ddia2-is-required-for-the-5bsvjgxi6a.pdf

Jan Faix

Papers

Defining mechanisms of actin polymerization and depolymerization during dendritic spine morphogenesis

The making of filopodia

Staying in Shape with Formins

Arp2/3 complex interactions and actin network turnover in lamellipodia

The Diaphanous-related formin dDia2 is required for the formation and maintenance of filopodia