Expression analysis by RT-qPCR showed that and were expressed during development and in adult zebrafish [3] whereas was not detected at any timepoint [55]. 3.1. is usually well described in mammals and it is relatively recently that zebrafish became a chief model to study the mechanisms of Rho GTPases function in vertebrates in vivo. Thirty-two Rho genes have been identified in zebrafish that represent homology to 17 human genes [3], however, overall research has so far focused on three members, Rac1, Cdc42 and RhoA. In this review, we will mainly spotlight the many different functions of zebrafish Rac1, Cdc42 and RhoA that have been studied so far. We will also summarize the latest advances in imaging, genetic and pharmacological tools to investigate their function during development and disease in zebrafish. 2. Rac1 in Zebrafish Development CAY10603 As introduced above, Rac1 is one of the most studied small Rho GTPases and a plethora of data has revealed an essential role for zebrafish Rac1 in cell cytoskeletal rearrangement and motility in different cell types including mesodermal cells, endothelial cells and neurons. In this way, Rac1 controls fundamental morphogenetic processes during development that rely on substantial cell movement and cellular reorganization. 2.1. Rac1 and Cell Motility in Different Cell Types During the early development in fish and amphibians, the embryonic body is shaped through gastrulation. This involves extensive cell movements including epiboly, cell internalization and convergence-extension (CE). The first elegant demonstration of a role for Rac1 in the dorsal migration of lateral cells during zebrafish gastrulation was described by Hammerschmidt laboratory [4]. Cell transplantation experiments, that generate chimeric larvae, have shown an autonomous role for Rac1 in promoting lamellipodia formation in these migrating cells downstream of the hyaluronan synthetizing enzyme 2 (Has2). Cellular extensions were visualized using membrane localized GFP. Yu-Long Li et al. have also proposed a strong link between Rac1 signaling and F-actin business, downstream of PI3K, that coordinates cell movements during epiboly progression [5]. Other studies highlighted a role for Rac1 in the extension of the embryonic dorsal axis and migration of the presomitic mesoderm in zebrafish downstream of p120 Catenin [6]. Further novel and exciting data revealed that this axial mesendoderm follows a true collective process of migration that is mediated by E-cadherin, Wnt-PCP signaling and Rac1 [7]. Drawing on four-dimensional imaging with detailed cell morphology analysis and delicate modification CAY10603 of cellular CAY10603 environment, Dumortier et al. showed a requirement for Rac1, as an intrinsic directionality signal, in collective mesendoderm migration. CAY10603 It has also been shown that Rac1 acts downstream of the TORC2 (Target of Rapamycin Complex 2) component, Sin1, to ensure the migration of the anterior most mesoderm [8]. Endoderm is one of the three germ layers that also needs to internalize and expand over the entire embryo. Live analysis of endodermal cells led by Nicolas David and his colleagues, revealed an active, oriented and actin-based migration that drives these cells to their inner most position. This process was Cd55 also dependent on Rac1 activity [9]. This followed an original work published by Stainiers laboratory showing a requirement for Nodal signaling in endodermal cell motility and actin dynamics via Rac1. In this study, the authors generated a that labels actin-based structures in endodermal cells in order to monitor their behavior. Furthermore, the authors were able to measure Rac1 activity specifically in these cells thanks to a fluorescent Rac1 probe, RFP-PBD, adding to the originality of this work [10]. Studies in mouse embryos showed an important role for Rac1 in mediating intracellular signals required for early gastrulation such as PI3K-Akt and Nap1/WAVE complex [11], and zebrafish embryos revealed precise features of cell dynamics during early morphogenesis, as well as useful information regarding.