Rho-guanine nucleotide exchange factors during development: Force is nothing without control

Blastopore gating mechanism to regulate extracellular fluid excretion

Fluid uptake and efflux play roles in early embryogenesis as well as in adult homeostasis. Multicellular organisms have two main pathways for fluid movement: cellular-level, such as transcellular and paracellular pathways, and tissue-level, involving muscle contraction. Interestingly, early Xenopus embryos with immature functional muscles excrete archenteron fluid via a tissue-level mechanism that opens the blastopore through a gating mechanism that is unclear. Using microelectrodes, we show that the archenteron has a constant fluid pressure and as development progress the blastopore pressure resistance decreases. Combining physical perturbations and imaging analyses, we found that the pushing force exerted by the circumblastoporal collars (CBCs) at the slit periphery regulates pressure resistance. We show that apical constriction at the blastopore dorsoventral ends contributes to this pushing force, and relaxation of ventral constriction causes fluid excretion. These results indicate that actomyosin contraction mediates temporal control of tissue-level blastopore opening and fluid excretion in early Xenopus embryos.

Spatiotemporal recruitment of RhoGTPase protein GRAF inhibits actomyosin ring constriction in Drosophila cellularization

Actomyosin contractility is regulated by Rho-GTP in cell migration, cytokinesis and morphogenesis in embryo development. Whereas Rho activation by Rho-GTP exchange factor (GEF), RhoGEF2, is well known in actomyosin contractility during cytokinesis at the base of invaginating membranes in Drosophila cellularization, Rho inhibition by RhoGTPase-activating proteins (GAPs) remains to be studied. We have found that the RhoGAP, GRAF, inhibits actomyosin contractility during cellularization. GRAF is enriched at the cleavage furrow tip during actomyosin assembly and initiation of ring constriction. Graf depletion shows increased Rho-GTP, increased Myosin II and ring hyper constriction dependent upon the loss of the RhoGTPase domain. GRAF and RhoGEF2 are present in a balance for appropriate activation of actomyosin ring constriction. RhoGEF2 depletion and abrogation of Myosin II activation in Rho kinase mutants suppress the Graf hyper constriction defect. Therefore, GRAF recruitment restricts Rho-GTP levels in a spatiotemporal manner for inhibiting actomyosin contractility during cellularization.

Integrated single cell data analysis reveals cell specific networks and novel coactivation markers

BACKGROUND

Large scale single cell transcriptome profiling has exploded in recent years and has enabled unprecedented insight into the behavior of individual cells. Identifying genes with high levels of expression using data from single cell RNA sequencing can be useful to characterize very active genes and cells in which this occurs. In particular single cell RNA-Seq allows for cell-specific characterization of high gene expression, as well as gene coexpression.

RESULTS

We offer a versatile modeling framework to identify transcriptional states as well as structures of coactivation for different neuronal cell types across multiple datasets. We employed a gamma-normal mixture model to identify active gene expression across cells, and used these to characterize markers for olfactory sensory neuron cell maturity, and to build cell-specific coactivation networks. We found that combined analysis of multiple datasets results in more known maturity markers being identified, as well as pointing towards some novel genes that may be involved in neuronal maturation. We also observed that the cell-specific coactivation networks of mature neurons tended to have a higher centralization network measure than immature neurons.

CONCLUSION

Integration of multiple datasets promises to bring about more statistical power to identify genes and patterns of interest. We found that transforming the data into active and inactive gene states allowed for more direct comparison of datasets, leading to identification of maturity marker genes and cell-specific network observations, taking into account the unique characteristics of single cell transcriptomics data.

Identification of novel MYO18A interaction partners required for myoblast adhesion and muscle integrity

The unconventional myosin MYO18A that contains a PDZ domain is required for muscle integrity during zebrafish development. However, the mechanism by which it functions in myofibers is not clear. The presence of a PDZ domain suggests that MYO18A may interact with other partners to perform muscle-specific functions. Here we performed double-hybrid screening and co-immunoprecipitation to identify MYO18A-interacting proteins, and have identified p190RhoGEF and Golgin45 as novel partners for the MYO18A PDZ domain. We have also identified Lurap1, which was previously shown to bind MYO18A. Functional analyses indicate that, similarly as myo18a, knockdown of lurap1, p190RhoGEF and Golgin45 by morpholino oligonucleotides disrupts dystrophin localization at the sarcolemma and produces muscle lesions. Simultaneous knockdown of myo18a with either of these genes severely disrupts myofiber integrity and dystrophin localization, suggesting that they may function similarly to maintain myofiber integrity. We further show that MYO18A and its interaction partners are required for adhesion of myoblasts to extracellular matrix, and for the formation of the Golgi apparatus and organization of F-actin bundles in myoblast cells. These findings suggest that MYO18A has the potential to form a multiprotein complex that links the Golgi apparatus to F-actin, which regulates muscle integrity and function during early development.

Regulation of cell adhesion and migration in cortical neurons: Not only Rho but also Rab family small GTPases

Accumulating evidence indicate that Rho family small GTPases, including RhoA, Rac1 and Cdc42, control cytoskeletal organization and cell adhesion, and thereby cell migration in vitro and in vivo. Recently, the involvement of other small GTPases, such as Rab and Arf family proteins in cell migration has also been evaluated. Rab5, Rab11 and Rab7, which regulate endocytosis, recycling and lysosomal degradation pathways, respectively, are shown to have essential roles in the migration of immature neurons during the development of cerebral cortex in vivo. These Rab proteins control distinct steps of neuronal migration through the regulation of N-cadherin-mediated cell adhesion. In this extra view paper, I will discuss the functions of Rho and Rab family small GTP ases in cell migration with particular focus on the migrating neurons in the developing cerebral cortex.

Cellular and physical mechanisms of branching morphogenesis

Branching morphogenesis is the developmental program that builds the ramified epithelial trees of various organs, including the airways of the lung, the collecting ducts of the kidney, and the ducts of the mammary and salivary glands. Even though the final geometries of epithelial trees are distinct, the molecular signaling pathways that control branching morphogenesis appear to be conserved across organs and species. However, despite this molecular homology, recent advances in cell lineage analysis and real-time imaging have uncovered surprising differences in the mechanisms that build these diverse tissues. Here, we review these studies and discuss the cellular and physical mechanisms that can contribute to branching morphogenesis.

Ibuprofen protects ventilator-induced lung injury by downregulating Rho-kinase activity in rats

Background. Ventilator-induced lung injury-(VILI-) induced endothelial permeability is regulated through the Rho-dependent signaling pathway. Ibuprofen inhibits Rho activation in animal models of spinal-cord injury and Alzheimer's disease. The study aims to investigate ibuprofen effects on high tidal volume associated VILI. Methods. Twenty-eight adult male Sprague-Dawley rats were randomized to receive a ventilation strategy with three different interventions for 2 h: (1) a high-volume zero-positive end-expiratory pressure (PEEP) (HVZP) group; (2) an HVZP + ibuprofen 15 mg/kg group; and (3) an HVZP + ibuprofen 30 mg/kg group. A fourth group without ventilation served as the control group. Rho-kinase activity was determined by ratio of phosphorylated ezrin, radixin, and moesin (p-ERM), substrates of Rho-kinase, to total ERM. VILI was characterized by increased pulmonary protein leak, wet-to-dry weight ratio, cytokines level, and Rho guanine nucleotide exchange factor (GEF-H1), RhoA activity, p-ERM/total ERM, and p-myosin light chain (MLC) protein expression. Results. Ibuprofen pretreatment significantly reduced the HVZP ventilation-induced increase in pulmonary protein leak, wet-to-dry weight ratio, bronchoalveolar lavage fluid interleukin-6 and RANTES levels, and lung GEF-H1, RhoA activity, p-ERM/total ERM, and p-MLC protein expression. Conclusion. Ibuprofen attenuated high tidal volume induced pulmonary endothelial hyperpermeability. This protective effect was associated with a reduced Rho-kinase activity.

Early targets of lithium in rat kidney inner medullary collecting duct include p38 and ERK1/2

Almost half of patients receiving lithium salts have nephrogenic diabetes insipidus. Chronic lithium exposure induces AQP2 downregulation and changes in the cellular composition of the collecting duct. In order to understand these pathophysiological events, we determined the earliest lithium targets in rat inner medullary collecting duct (IMCD) by examining changes in the IMCD phosphoproteome after acute lithium administration. IMCDs were isolated 9 h after lithium exposure, a time when urinary concentrating impairment was evident. We found 1093 unique phosphopeptides corresponding to 492 phosphoproteins identified and quantified by mass spectrometry. Label-free quantification identified 152 upregulated and 56 downregulated phosphopeptides in response to lithium. Bioinformatic analysis highlighted several signaling proteins including MAP kinases and cell-junction proteins. The majority of the upregulated phosphopeptides contained a proline-directed motif, a known target of MAPK. Four hours after lithium exposure, phosphorylation sites in the activation loops of ERK1/2 and p38 were upregulated. Increased expression of phospho-Ser261-AQP2 (proline-directed motif) was concomitant with the increase in urine output. Pretreatment with MAPK inhibitors reversed the increased Ser261-AQP2 phosphorylation. Thus, in IMCD, ERK1/2 and p38 are early targets of lithium and may play a role in the onset of lithium-induced polyuria.

A novel role for p115RhoGEF in regulation of epithelial plasticity

Epithelial plasticity plays a critical role during physiological processes, such as wound healing and tissue regeneration, and dysregulation of epithelial plasticity can lead to pathological conditions, such as cancer. Cell-cell junctions are a critical feature of epithelial cells and loss of junctions is associated with acquisition of mesenchymal features, such as enhanced protrusion and migration. Although Rho has been implicated in regulation of junctions in epithelial cells, the role of Rho signaling in the regulation of epithelial plasticity has not been understood. We show that members of the RGS RhoGEFs family play a critical role in regulation of epithelial cell-cell junctions in breast epithelial cells. We identify a novel role for p115RhoGEF in regulation of epithelial plasticity. Loss of p115RhoGEF leads to decreased junctional E-cadherin and enhanced protrusiveness and migration. Conversely, overexpression of p115RhoGEF enhanced junctional E-cadherin and inhibited cell protrusion and migration. siRNA screen of 23 Rho effectors showed that members of the Diaphanous-Related Formin (DRF) family are required for p115RhoGEF-mediated changes in epithelial plasticity. Thus, our data indicates a novel role for p115RhoGEF in regulation of epithelial plasticity, which is dependent on Rho-DRF signaling module.

PDZ-RhoGEF is essential for CXCR4-driven breast tumor cell motility through spatial regulation of RhoA

The CXCL12-CXCR4 chemokine signaling pathway is a well-established driver of cancer progression. One key process promoted by CXCR4 stimulation is tumor cell motility; however, the specific signaling pathways leading to migration remain poorly understood. Previously, we have shown that CXCL12 stimulation of migration depends on temporal regulation of RhoA. However, the specific RhoGEF that translates CXCR4 signaling into RhoA activity and cell motility is unknown. We screened the three regulator of G-protein signaling RhoGEFs (LSC, LARG and PRG) and found that PRG selectively regulated the migration and invasion of CXCR4-overexpressing breast tumor cells. Interestingly, we found that PDZ-RhoGEF (PRG) was required for spatial organization of F-actin structures in the center, but not periphery of the cells. The effects on the cytoskeleton were mirrored by the spatial effects on RhoA activity that were dependent upon PRG. Loss of PRG also enhanced adherens junctions in the epithelial-like MCF7-CXCR4 cell line, and inhibited directional persistence and polarity in the more mesenchymal MDA-MB-231 cell line. Thus, PRG is essential for CXCR4-driven tumor cell migration through spatial regulation of RhoA and the subsequent organization of the cytoskeletal structures that support motility. Furthermore, immunohistochemical analysis of human breast tumor tissues shows a significant increase of PRG expression in the invasive areas of the tumors, suggesting that this RhoGEF is associated with breast tumor invasion in vivo.

Formin-mediated actin polymerization cooperates with Mushroom body defect (Mud)-Dynein during Frizzled-Dishevelled spindle orientation

To position the mitotic spindle, cytoskeletal components must be coordinated to generate cortical forces on astral microtubules. Although the dynein motor is common to many spindle orientation systems, 'accessory pathways' are often also required. In this work, we identified an accessory spindle orientation pathway in Drosophila that functions with Dynein during planar cell polarity, downstream of the Frizzled (Fz) effector Dishevelled (Dsh). Dsh contains a PDZ ligand and a Dynein-recruiting DEP domain that are both required for spindle orientation. The Dsh PDZ ligand recruits Canoe/Afadin and ultimately leads to Rho GTPase signaling mediated through RhoGEF2. The formin Diaphanous (Dia) functions as the Rho effector in this pathway, inducing F-actin enrichment at sites of cortical Dsh. Chimeric protein experiments show that the Dia-actin accessory pathway can be replaced by an independent kinesin (Khc73) accessory pathway for Dsh-mediated spindle orientation. Our results define two 'modular' spindle orientation pathways and show an essential role for actin regulation in Dsh-mediated spindle orientation.

Deciphering the molecular and functional basis of Dbl family proteins: a novel systematic approach toward classification of selective activation of the Rho family proteins

The diffuse B-cell lymphoma (Dbl) family of the guanine nucleotide exchange factors is a direct activator of the Rho family proteins. The Rho family proteins are involved in almost every cellular process that ranges from fundamental (e.g. the establishment of cell polarity) to highly specialized processes (e.g. the contraction of vascular smooth muscle cells). Abnormal activation of the Rho proteins is known to play a crucial role in cancer, infectious and cognitive disorders, and cardiovascular diseases. However, the existence of 74 Dbl proteins and 25 Rho-related proteins in humans, which are largely uncharacterized, has led to increasing complexity in identifying specific upstream pathways. Thus, we comprehensively investigated sequence-structure-function-property relationships of 21 representatives of the Dbl protein family regarding their specificities and activities toward 12 Rho family proteins. The meta-analysis approach provides an unprecedented opportunity to broadly profile functional properties of Dbl family proteins, including catalytic efficiency, substrate selectivity, and signaling specificity. Our analysis has provided novel insights into the following: (i) understanding of the relative differences of various Rho protein members in nucleotide exchange; (ii) comparing and defining individual and overall guanine nucleotide exchange factor activities of a large representative set of the Dbl proteins toward 12 Rho proteins; (iii) grouping the Dbl family into functionally distinct categories based on both their catalytic efficiencies and their sequence-structural relationships; (iv) identifying conserved amino acids as fingerprints of the Dbl and Rho protein interaction; and (v) defining amino acid sequences conserved within, but not between, Dbl subfamilies. Therefore, the characteristics of such specificity-determining residues identified the regions or clusters conserved within the Dbl subfamilies.

Intersectin 2 nucleotide exchange factor regulates Cdc42 activity during Xenopus early development

Intersectin 2 (ITSN2) is an evolutionarily conserved scaffold protein involved in endocytic internalization, regulation of actin cytoskeleton and epithelial morphogenesis. Recent studies of different Itsn-deficient organisms revealed that this gene is essential for the functioning of the nervous system and for organism viability. Here we report investigations on a possible role of the ITSN2 long isoform in the early embryonic development of Xenopus laevis. In vertebrates, alternative splicing generates several alternatively spliced isoforms of ITSN2. To date the long splice variant of ITSN2 (ITSN2-L) has been reported only for mammals. We show that transcripts of ITSN2-L can be detected in Xenopus embryos from the first cleavage onwards. Overexpression of functional domains of ITSN2-L in embryos resulted in aberrant phenotypes. The strongest phenotype was produced by the C-terminal extension of ITSN2-L. Embryos displayed hyperpigmentation and gastrulation failure that were incompatible with survival. The C-terminus of ITSN2-L includes the DH-PH tandem, a nucleotide exchange factor for the small GTPase Cdc42 and the C2 domain. Further investigations revealed that the DH-PH tandem was responsible for the development of the phenotype affecting the actin cytoskeleton in embryos. Observed developmental defects depended on Cdc42. The effect of expression of the constitutively active GTPase strongly resembled that of the DH-PH tandem. The dominant negative Cdc42 partially rescued developmental defects induced by the expression of the DH-PH tandem. Thus, our data indicate that the ITSN2 exchange factor regulates the activity of Cdc42 during embryo development affecting actin cytoskeleton in Xenopus embryos.