Vertebrate nonmuscle cells express two actin isoforms: cytoplasmic ␤-and ␥-actin. Because of the presence and localized translation of ␤-actin at the leading edge, this isoform is generally accepted to specifically generate protrusive forces for cell migration. Recent evidence also implicates ␤-actin in gene regulation. Cell migration without ␤-actin has remained unstudied until recently and it is unclear whether other actin isoforms can compensate for this cytoplasmic function and/or for its nuclear role. Primary mouse embryonic fibroblasts lacking ␤-actin display compensatory expression of other actin isoforms. Consistent with this preservation of polymerization capacity, ␤-actin knockout cells have unchanged lamellipodial protrusion rates despite a severe migration defect. To solve this paradox we applied quantitative proteomics revealing a broad genetic reprogramming of ␤-actin knockout cells. This also explains why reintroducing ␤-actin in knockout cells does not restore the affected cell migration. Pathway analysis suggested increased Rho-ROCK signaling, consistent with observed phenotypic changes. We therefore developed and tested a model explaining the phenotypes in ␤-actin knockout cells based on increased Rho-ROCK signaling and increased TGF␤ production resulting in increased adhesion and contractility in the knockout cells. Inhibiting ROCK or myosin restores migration of ␤-actin knockout cells indicating that other actins compensate for ␤-actin in this process. Consequently, isoactins act redundantly in providing propulsive forces for cell migration, but ␤-actin has a unique nuclear function, regulating expression on transcriptional and post-translational levels, thereby preventing myogenic differentiation.

The dynamic actin cytoskeleton, consisting of six actin isoforms in mammals and a variety of actin binding proteins is essential for all developmental processes and for the viability of the adult organism. Actin isoform specific functions have been proposed for muscle contraction, cell migration, endo-and exocytosis and maintaining cell shape. However, these specific functions for each of the actin isoforms during development are not well understood. Based on transgenic mouse models, we will discuss the expression patterns of the six conventional actin isoforms in mammals during development and adult life. Ablation of actin genes usually leads to lethality and affects expression of other actin isoforms at the cell or tissue level. A good knowledge of their expression and functions will contribute to fully understand severe phenotypes or diseases caused by mutations in actin isoforms. Cell Motil. Cytoskeleton 66: 798-815, 2009. '

Advances in genomics and other -omic fields in the last decade have resulted in unprecedented volumes of complex data now being available. These data can enable physicians to provide their patients with care that is more personalized, predictive, preventive and participatory. The expertise required to manage and understand this data is to be found in fields outside of medical science, thus multidisciplinary collaboration coupled to a systems approach is key to unlocking its potential, with concomitant new ways of working. Systems medicine can build on the successes in the field of systems biology, recognizing the human body as the multidimensional network of networks that it is. While systems medicine can provide a conceptual and theoretical framework, its practical goal is to provide physicians the tools necessary for harnessing the rapid advances in basic biomedical science into their routine clinical arsenal.

Cancer cell migration enables metastatic spread causing most cancer deaths. Rho-family GTPases control cell migration, but being embedded in a highly interconnected feedback network, the control of their dynamical behavior during cell migration remains elusive. To address this question, we reconstructed the Rho-family GTPases signaling network involved in cell migration, and developed a Boolean network model to analyze the different states and emergent rewiring of the Rho-family GTPases signaling network at protrusions and during extracellular matrix-dependent cell migration. Extensive simulations and experimental validations revealed that the bursts of RhoA activity induced at protrusions by EGF are regulated by a negative-feedback module composed of Src, FAK, and CSK. Interestingly, perturbing this module interfered with cyclic Rho activation and extracellular matrix-dependent migration, suggesting that CSK inhibition can be a novel and effective intervention strategy for blocking extracellular matrix-dependent cancer cell migration, while Src inhibition might fail, depending on the genetic background of cells. Thus, this study provides new insights into the mechanisms that regulate the intricate activation states of Rho-family GTPases during extracellular matrix-dependent migration, revealing potential new targets for interfering with extracellular matrix-dependent cancer cell migration.

Biological systems are known to be both robust and evolvable to internal and external perturbations, but what causes these apparently contradictory properties? We used Boolean network modeling and attractor landscape analysis to investigate the evolvability and robustness of the human signaling network. Our results show that the human signaling network can be divided into an evolvable core where perturbations change the attractor landscape in state space, and a robust neighbor where perturbations have no effect on the attractor landscape. Using chemical inhibition and overexpression of nodes, we validated that perturbations affect the evolvable core more strongly than the robust neighbor. We also found that the evolvable core has a distinct network structure, which is enriched in feedback loops, and features a higher degree of scale-freeness and longer path lengths connecting the nodes. In addition, the genes with high evolvability scores are associated with evolvability-related properties such as rapid evolvability, low species broadness, and immunity whereas the genes with high robustness scores are associated with robustness-related properties such as slow evolvability, high species broadness, and oncogenes. Intriguingly, US Food and Drug Administration-approved drug targets have high evolvability scores whereas experimental drug targets have high robustness scores.

A-Raf belongs to the family of oncogenic Raf kinases that are involved in mitogenic signaling by activating the MEK-ERK pathway. Low kinase activity of A-Raf toward MEK suggested that A-Raf might have alternative functions. We recently identified A-Raf as a potent inhibitor of the proapoptotic mammalian sterile 20-like kinase (MST2) tumor suppressor pathway in several cancer entities including head and neck, colon, and breast. Independent of kinase activity, A-Raf binds to MST2 thereby efficiently inhibiting apoptosis. Here, we show that the interaction of A-Raf with the MST2 pathway is regulated by subcellular compartmentalization. Although in proliferating normal cells and tumor cells A-Raf localizes to the mitochondria, differentiated non-carcinogenic cells of head and neck epithelia, which express A-Raf at the plasma membrane. The constitutive or induced re-localization of A-Raf to the plasma membrane compromises its ability to efficiently sequester and inactivate MST2, thus rendering cells susceptible to apoptosis. Physiologically, A-Raf re-localizes to the plasma membrane upon epithelial differentiation in vivo. This re-distribution is regulated by the scaffold protein kinase suppressor of Ras 2 (KSR2). Downregulation of KSR2 during mammary epithelial cell differentiation or siRNA-mediated knockdown re-localizes A-Raf to the plasma membrane causing the release of MST2. By using the MCF7 cell differentiation system, we could demonstrate that overexpression of A-Raf in MCF7 cells, which induces differentiation. Our findings offer a new paradigm to understand how differential localization of Raf complexes affects diverse signaling functions in normal cells and carcinomas. A-Raf is a member of the Raf family of serine-threonine protein kinases, which comprises A-Raf, B-Raf, and Raf-1. Raf kinases are at the apex of the three-tiered Raf/MEK/ extracellular signal-regulated kinase (ERK) (mitogen-activated protein kinase (MAPK)) pathway regulating fundamental cellular functions, including differentiation, transformation, apoptosis, proliferation, and metabolism. 1,2 Activation of Ras GTPases at the cell membrane initiates Raf kinase activation and sequential phosphorylation and activation of the serinethreonine kinases MEK1/2 and ERK1/2. 3,4 In comparison to Raf-1 and B-Raf, A-Raf is only weakly activated by oncogenic H-Ras and Src 5 and is a poor MEK kinase, 5-8 which is due to unique non-conserved amino acid substitutions in the N-region. 9 Only recently, the first somatic oncogenic mutations of A-Raf were identified in lung adenocarcinomas 10 and Langerhans cell histiocytosis. 11,12 We recently showed that A-Raf and Raf-1, independent of their kinase activity, bind the proapoptotic mammalian sterile 20-like kinase (MST2) thereby suppressing MST2 activation and MST2-induced apoptosis. 13-17 MST2 employs several mechanisms to induce apoptosis including the transcriptional induction of PUMA, 14 a BH3 domain protein that causes mitochondrial depolarization and subsequent cell death. 18 Although Raf-1 counteracts MS...

The Raf kinase inhibitor protein 1 (RKIP-1) was the first reported endogenous inhibitor of Raf-1-MEK-ERK/MAPK cascade, by interfering with the phosphorylation of MEK by Raf-1. However, RKIP's functions related to the MAPK signaling are far more complex. Newer data indicate that by modulating different protein-protein interactions, RKIP is involved in fine-tuning cell signaling, modulating ERK dynamics, and regulating cross talk between different pathways. Here, we describe the molecular mechanisms by which RKIP controls MAPK signaling at different levels and vice versa and its regulation via feedback phosphorylation. We also focus on several discrepancies and questions that remain, such as the RKIP binding regulation by Raf-1 N-region phosphorylation, the possible B-Raf inhibition, and the effects of RKIP-lipid binding. We also describe how RKIP's role as key signaling modulator of many cell fate decisions leads to the fact that fine control of RKIP activity and regulation is crucial to avoid pathological processes, such as metastasis, pulmonary arterial hypertension, and heart failure.

The pharmaceutical industry is faced with a range of challenges with the ever‐escalating costs of drug development and a drying out of drug pipelines. By harnessing advances in ‐omics technologies and moving away from the standard, reductionist model of drug discovery, there is significant potential to reduce costs and improve efficacy. Embedding systems biology approaches in drug discovery, which seek to investigate underlying molecular mechanisms of potential drug targets in a network context, will reduce attrition rates by earlier target validation and the introduction of novel targets into the currently stagnant market. Systems biology approaches also have the potential to assist in the design of multidrug treatments and repositioning of existing drugs, while stratifying patients to give a greater personalization of medical treatment. WIREs Syst Biol Med 2014, 6:1–11. doi: 10.1002/wsbm.1253 This article is categorized under: Translational, Genomic, and Systems Medicine > Therapeutic Methods Translational, Genomic, and Systems Medicine > Translational Medicine