The adenomatous polyposis coli-associated guanine nucleotide exchange factor Asef is involved in angiogenesis

Circulating Blood Prognostic Biomarker Signatures for Hemorrhagic Cerebral Cavernous Malformations (CCMs)

Cerebral cavernous malformations (CCMs) are a neurological disorder characterized by enlarged intracranial capillaries in the brain, increasing the susceptibility to hemorrhagic strokes, a major cause of death and disability worldwide. The limited treatment options for CCMs underscore the importance of prognostic biomarkers to predict the likelihood of hemorrhagic events, aiding in treatment decisions and identifying potential pharmacological targets. This study aimed to identify blood biomarkers capable of diagnosing and predicting the risk of hemorrhage in CCM1 patients, establishing an initial set of circulating biomarker signatures. By analyzing proteomic profiles from both human and mouse CCM models and conducting pathway enrichment analyses, we compared groups to identify potential blood biomarkers with statistical significance. Specific candidate biomarkers primarily associated with metabolism and blood clotting pathways were identified. These biomarkers show promise as prognostic indicators for CCM1 deficiency and the risk of hemorrhagic stroke, strongly correlating with the likelihood of hemorrhagic cerebral cavernous malformations (CCMs). This lays the groundwork for further investigation into blood biomarkers to assess the risk of hemorrhagic CCMs.

Deciphering potential vascularization factors of on-chip co-cultured hiPSC-derived cerebral organoids

The lack of functional vascular system in stem cell-derived cerebral organoids (COs) limits their utility in modeling developmental processes and disease pathologies. Unlike other organs, brain vascularization is poorly understood, which makes it particularly difficult to mimic in vitro. Although several attempts have been made to vascularize COs, complete vascularization leading to functional capillary network development has only been achieved via transplantation into a mouse brain. Understanding the cues governing neurovascular communication is therefore imperative for establishing an efficient in vitro system for vascularized cerebral organoids that can emulate human brain development. Here, we used a multidisciplinary approach combining microfluidics, organoids, and transcriptomics to identify molecular changes in angiogenic programs that impede the successful in vitro vascularization of human induced pluripotent stem cell (iPSC)-derived COs. First, we established a microfluidic cerebral organoid (CO)-vascular bed (VB) co-culture system and conducted transcriptome analysis on the outermost cell layer of COs cultured on the preformed VB. Results revealed coordinated regulation of multiple pro-angiogenic factors and their downstream targets. The VEGF-HIF1A-AKT network was identified as a central pathway involved in the angiogenic response of cerebral organoids to the preformed VB. Among the 324 regulated genes associated with angiogenesis, six transcripts represented significantly regulated growth factors with the capacity to influence angiogenic activity during co-culture. Subsequent on-chip experiments demonstrated the angiogenic and vasculogenic potential of cysteine-rich angiogenic inducer 61 (CYR61) and hepatoma-derived growth factor (HDGF) as potential enhancers of organoid vascularization. Our study provides the first global analysis of cerebral organoid response to three-dimensional microvasculature for in vitro vascularization.

Rational design of a sensitivity-enhanced tracer for discovering efficient APC-Asef inhibitors

The adenomatous polyposis coli (APC)–Rho guanine nucleotide exchange factor 4 (Asef) protein–protein interaction (PPI) is essential for colorectal cancer metastasis, making it a promising drug target. Herein, we obtain a sensitivity-enhanced tracer (tracer 7) with a high binding affinity (K_d = 0.078 μM) and wide signal dynamic range (span = 251 mp). By using tracer 7 in fluorescence-polarization assays for APC–Asef inhibitor screening, we discover a best-in-class inhibitor, MAI-516, with an IC₅₀ of 0.041 ± 0.004 μM and a conjugated transcriptional transactivating sequence for generating cell-permeable MAIT-516. MAIT-516 inhibits CRC cell migration by specifically hindering the APC–Asef PPI. Furthermore, MAIT-516 exhibits no cytotoxic effects on normal intestinal epithelial cell and colorectal cancer cell growth. Overall, we develop a sensitivity-enhanced tracer for fluorescence polarization assays, which is used for the precise quantification of high-activity APC–Asef inhibitors, thereby providing insight into PPI drug development.

The adenomatous polyposis coli (APC)–Asef protein interaction is essential for colorectal cancer metastasis. Here, the authors present the rational design of a sensitivity-enhanced tracer for fluorescence polarization assays, enabling them to discover more efficient APC–Asef interaction inhibitors.

Microtubules as Major Regulators of Endothelial Function: Implication for Lung Injury

Endothelial dysfunction has been attributed as one of the major complications in COVID-19 patients, a global pandemic that has already caused over 4 million deaths worldwide. The dysfunction of endothelial barrier is characterized by an increase in endothelial permeability and inflammatory responses, and has even broader implications in the pathogenesis of acute respiratory syndromes such as ARDS, sepsis and chronic illnesses represented by pulmonary arterial hypertension and interstitial lung disease. The structural integrity of endothelial barrier is maintained by cytoskeleton elements, cell-substrate focal adhesion and adhesive cell junctions. Agonist-mediated changes in endothelial permeability are directly associated with reorganization of actomyosin cytoskeleton leading to cell contraction and opening of intercellular gaps or enhancement of cortical actin cytoskeleton associated with strengthening of endothelial barrier. The role of actin cytoskeleton remodeling in endothelial barrier regulation has taken the central stage, but the impact of microtubules in this process remains less explored and under-appreciated. This review will summarize the current knowledge on the crosstalk between microtubules dynamics and actin cytoskeleton remodeling, describe the signaling mechanisms mediating this crosstalk, discuss epigenetic regulation of microtubules stability and its nexus with endothelial barrier maintenance, and overview a role of microtubules in targeted delivery of signaling molecules regulating endothelial permeability and inflammation.

Activity of Arhgef4 is modulated through Staufen1 in neurons

The role of Arhgef4, also known as adenomatous polyposis coli (APC)-stimulated guanine nucleotide exchange factor 1 (Asef1), has been identified in colorectal cancers. Interestingly, Arhgef4 is more highly expressed in brain regions than intestinal regions, suggesting a role in neurons. In our previous study, we reported that Arhgef4 negatively regulates the level of PSD-95 in excitatory post-synaptic regions by binding with Staufen1. However, modulation of Arhgef4 guanine nucleotide exchange factor (GEF) activity in neurons has not been reported. We examined the configuration of protein interactions when Arhgef4 binds to APC and/or Staufen1. Arhgef4 simultaneously binds to Staufen1 with APC. Staufen1 overexpression blocked the GEF activity of Arhgef4. Consistent with this, Staufen1 overexpression blocked the Arhgef4-induced increase in dendritic protrusions in cultured neurons. Taken together, our data suggest that the GEF activity of Arhgef4 could be negatively modulated by Staufen1 binding.

A perspective on medicinal chemistry approaches towards adenomatous polyposis coli and Wnt signal based colorectal cancer inhibitors

Colorectal cancer (CRC) is one of the major causes of carcinogenic mortality in numbers only after lung and breast cancers. The mutations in adenomatous polyposis coli (APC) gene leads to formation of colorectal polyps in the colonic region and which develop as a malignant tumour upon coalition with patient related risk factors. The protein-protein interaction (PPI) of APC with Asef (A Rac specific guanine nucleotide exchange factor) overwhelms the patient's conditions by rapidly spreading in the entire colorectal region. Most mutations in APC gene occur in mutated cluster region (MCR), where it specifically binds with the cytosolic β-catenin to regulate the Wnt signalling pathway required for CRC cell adhesion, invasion, progression, differentiation and stemness in initial cell cycle phages. The current broad spectrum perspective is attempted to elaborate the sources of identification, development of selective APC inhibitors by targeting emopamil-binding protein (EBP) & dehydrocholesterol reductase-7 & 24 (DHCR-7 & 24); APC-Asef, β-catenin/APC, Wnt/β-catenin, β-catenin/TCF4 PPI inhibitors with other vital Wnt signal cellular proteins and APC/Pol-β interface of colorectal cancer. In this context, this perspective would serve as a platform for design of new medicinal agents by targeting cellular essential components which could accelerate anti-colorectal potential candidates.

Wnt-Independent and Wnt-Dependent Effects of APC Loss on the Chemotherapeutic Response

Resistance to chemotherapy occurs through mechanisms within the epithelial tumor cells or through interactions with components of the tumor microenvironment (TME). Chemoresistance and the development of recurrent tumors are two of the leading factors of cancer-related deaths. The Adenomatous Polyposis Coli (APC) tumor suppressor is lost in many different cancers, including colorectal, breast, and prostate cancer, and its loss correlates with a decreased overall survival in cancer patients. While APC is commonly known for its role as a negative regulator of the WNT pathway, APC has numerous binding partners and functional roles. Through APC's interactions with DNA repair proteins, DNA replication proteins, tubulin, and other components, recent evidence has shown that APC regulates the chemotherapy response in cancer cells. In this review article, we provide an overview of some of the cellular processes in which APC participates and how they impact chemoresistance through both epithelial- and TME-derived mechanisms.

Mutations in SPATA13/ASEF2 cause primary angle closure glaucoma

Current estimates suggest 50% of glaucoma blindness worldwide is caused by primary angle-closure glaucoma (PACG) but the causative gene is not known. We used genetic linkage and whole genome sequencing to identify Spermatogenesis Associated Protein 13, SPATA13 (NM_001166271; NP_001159743, SPATA13 isoform I), also known as ASEF2 (Adenomatous polyposis coli-stimulated guanine nucleotide exchange factor 2), as the causal gene for PACG in a large seven-generation white British family showing variable expression and incomplete penetrance. The 9 bp deletion, c.1432_1440del; p.478_480del was present in all affected individuals with angle-closure disease. We show ubiquitous expression of this transcript in cell lines derived from human tissues and in iris, retina, retinal pigment and ciliary epithelia, cornea and lens. We also identified eight additional mutations in SPATA13 in a cohort of 189 unrelated PACS/PAC/PACG samples. This gene encodes a 1277 residue protein which localises to the nucleus with partial co-localisation with nuclear speckles. In cells undergoing mitosis SPATA13 isoform I becomes part of the kinetochore complex co-localising with two kinetochore markers, polo like kinase 1 (PLK-1) and centrosome-associated protein E (CENP-E). The 9 bp deletion reported in this study increases the RAC1-dependent guanine nucleotide exchange factors (GEF) activity. The increase in GEF activity was also observed in three other variants identified in this study. Taken together, our data suggest that SPATA13 is involved in the regulation of mitosis and the mutations dysregulate GEF activity affecting homeostasis in tissues where it is highly expressed, influencing PACG pathogenesis.

Discovery of novel pyrazoline derivatives containing methyl-1H-indole moiety as potential inhibitors for blocking APC-Asef interactions

A series of novel pyrazoline derivatives containing methyl-1H-indole moiety were discovered as potential inhibitors for blocking APC-Asef interactions. The top hit Q19 suggested potency of inhibiting APC-Asef interactions and attractive preference for human-sourced colorectal cells. It was already comparable with the previous representative and the positive control Regorafenib before further pharmacokinetic optimization. The introduction of methyl-1H-indole moiety realized the Mitochondrial affection thus might connect the impact on the protein-interaction level with the apoptosis events. The molecular docking simulation inferred that bringing trifluoromethyl groups seemed a promising approach for causing more key interactions such as H-bonds. This work raised referable information for further discovery of inhibitors for blocking APC-Asef interactions.

Design, synthesis and biological evaluation of 2-H pyrazole derivatives containing morpholine moieties as highly potent small molecule inhibitors of APC-Asef interaction

Mutated adenomatous polyposis coli (APC) selectively combining with Asef has been reported to be implicated in promoting colon cancer proliferation, invasion and metastasis in several cancer biotherapy studies. However, there were universally resistance and harsh terms in disrupting APC-Asef interaction in biotherapy. Under the circumstances small-molecule inhibitors as the new APC interface could resolve the problems. In this research, a series of novel dihydropyrazole derivatives containing morpholine as high potent interaction inhibitors between APC and Asef were first synthesized after selection by means of docking simulation and virtual screening. Afterwards they were evaluated interaction inhibition of APC-Asef and pharmacological efficiency both in vitro and in vivo utilizing orthotopic transplantation model with multi-angle of view. Among them, compound 7g exhibited most excellent anti-proliferation activities against HCT116 cells with IC₅₀ of 0.10 ± 0.01 μM than Regorafenib (IC₅₀ = 0.16 ± 0.04 μM). The results favored our rational design intention and provides a new class of small-molecule inhibitors available for the development of colon tumor therapeutics targeting APC-Asef interaction inhibitions.

Adenomatous polyposis coli-stimulated GEF 1 (Asef1) is a negative regulator of excitatory synaptic function

Guanine nucleotide exchange factors (GEFs) play important roles in many cellular processes, including regulation of the structural plasticity of dendritic spines. A GEF protein, adenomatous polyposis coli-stimulated GEF 1 (Asef1, ARHGEF4) is highly expressed in the nervous system. However, the function of Asef1 has not been investigated in neurons. Here, we present evidence showing that Asef1 negatively regulates the synaptic localization of postsynaptic density protein 95 (PSD-95) in the excitatory synapse by inhibiting Staufen-mediated synaptic localization of PSD-95. Accordingly, Asef1 expression impairs synaptic transmission in hippocampal cultured neurons. In addition, neuronal activity facilitates the dissociation of Asef1 from Staufen in a phosphoinositide 3 kinase (PI3K)-dependent manner. Taken together, our data reveal Asef1 functions as a negative regulator of synaptic localization of PSD-95 and synaptic transmission.

ARHGEF4 predicts poor prognosis and promotes cell invasion by influencing ERK1/2 and GSK-3α/β signaling in pancreatic cancer

Rho guanine nucleotide exchange factor 4 (ARHGEF4) is a guanine nucleotide exchange factor that is specific for Rac1 and Cdc42. The aim of the present study was to investigate the role of ARHGEF4 in the motility and invasiveness of pancreatic cancer cells. Evaluation of an immunohistochemical staining of 102 resected pancreatic cancer samples demonstrated that high ARHGEF4 expression was correlated with an independent predictor of worse overall survival in univariate and multivariate analyses. Immunofluorescence analyses and Matrigel invasion assays demonstrated that suppression of ARHGEF4 inhibited the formation of membrane protrusions, and in turn inhibited cell motility and invasion. A phosphoprotein array analysis demonstrated that knockdown of ARHGEF4 decreased phosphorylated extracellular signal-regulated kinase (ERK)1/2 and glycogen synthase kinase-3 (GSK-3)α/β in pancreatic cancer cells, and ERK1/2 and GSK-3α/β were associated with ARHGEF4-related motility and invasiveness through an increase in cell protrusions. These results suggested that ARHGEF4 stimulates ERK1/2 and GSK-3α/β, and provided evidence that ARHGEF4 promotes cell motility and invasiveness. Inhibition of ARHGEF4 may be a novel approach to a targeted molecular therapy, as any such therapy would limit the motility and invasiveness of pancreatic cancer cells.

Microtubules-associated Rac regulation of endothelial barrier: a role of Asef in acute lung injury

The endothelial barrier function regulated by the cytoskeletal reorganizations has been implicated in the pathogenesis of multiple lung diseases including asthma, sepsis, edema, and acute respiratory distress syndrome. The extensive studies have established that activation of small GTPase Rac is a key mechanism in endothelial barrier protection but the role of microtubules-associated Rac in the endothelial functions remains poorly understood. With the emerging evidences that microtubules disassembly also plays a critical role in actin cytoskeleton remodeling leading to endothelial permeability, the knowledge on microtubules-mediated regulation of endothelial barrier is imperative to better understand the etiology of lung injuries as well as to develop novel therapeutics against these disorders. In this regard, our recent studies have revealed some novel aspects of microtubules-mediated regulation of endothelial barrier functions and unraveled a putative role of Rac-specific guanine nucleotide exchange factor Asef in mediating the barrier protective effects of hepatocyte growth factor. In this review, we will discuss the role of this novel Rac activator Asef in endothelial barrier protection and its regulation by microtubules.

Peptidomimetic inhibitors of APC-Asef interaction block colorectal cancer migration

The binding of adenomatous polyposis coli (APC) to its receptor Asef relieves the negative intramolecular regulation of Asef and leads to aberrant cell migration in human colorectal cancer. Because of its crucial role in metastatic dissemination, the interaction between APC and Asef is an attractive target for anti-colorectal-cancer therapy. We rationally designed a series of peptidomimetics that act as potent inhibitors of the APC interface. Crystal structures and biochemical and cellular assays showed that the peptidomimetics in the APC pocket inhibited the migration of colorectal cells by disrupting APC-Asef interaction. By using the peptidomimetic inhibitor as a chemical probe, we found that CDC42 was the downstream GTPase involved in APC-stimulated Asef activation in colorectal cancer cells. Our work demonstrates the feasibility of exploiting APC-Asef interaction to regulate the migration of colorectal cancer cells, and provides what to our knowledge is the first class of protein-protein interaction inhibitors available for the development of cancer therapeutics targeting APC-Asef signaling.

Rho GAPs and GEFs: controling switches in endothelial cell adhesion

Within blood vessels, endothelial cell–cell and cell–matrix adhesions are crucial to preserve barrier function, and these adhesions are tightly controlled during vascular development, angiogenesis, and transendothelial migration of inflammatory cells. Endothelial cellular signaling that occurs via the family of Rho GTPases coordinates these cell adhesion structures through cytoskeletal remodelling. In turn, Rho GTPases are regulated by GTPase-activating proteins (GAPs) and guanine nucleotide exchange factors (GEFs). To understand how endothelial cells initiate changes in the activity of Rho GTPases, and thereby regulate cell adhesion, we will discuss the role of Rho GAPs and GEFs in vascular biology. Many potentially important Rho regulators have not been studied in detail in endothelial cells. We therefore will first overview which GAPs and GEFs are highly expressed in endothelium, based on comparative gene expression analysis of human endothelial cells compared with other tissue cell types. Subsequently, we discuss the relevance of Rho GAPs and GEFs for endothelial cell adhesion in vascular homeostasis and disease.

Asef mediates HGF protective effects against LPS-induced lung injury and endothelial barrier dysfunction

Increased vascular endothelial permeability and inflammation are major pathological mechanisms of pulmonary edema and its life-threatening complication, the acute respiratory distress syndrome (ARDS). We have previously described potent protective effects of hepatocyte growth factor (HGF) against thrombin-induced hyperpermeability and identified the Rac pathway as a key mechanism of HGF-mediated endothelial barrier protection. However, anti-inflammatory effects of HGF are less understood. This study examined effects of HGF on the pulmonary endothelial cell (EC) inflammatory activation and barrier dysfunction caused by the gram-negative bacterial pathogen lipopolysaccharide (LPS). We tested involvement of the novel Rac-specific guanine nucleotide exchange factor Asef in the HGF anti-inflammatory effects. HGF protected the pulmonary EC monolayer against LPS-induced hyperpermeability, disruption of monolayer integrity, activation of NF-kB signaling, expression of adhesion molecules intercellular adhesion molecule-1 and vascular cell adhesion molecule-1, and production of IL-8. These effects were critically dependent on Asef. Small-interfering RNA-induced downregulation of Asef attenuated HGF protective effects against LPS-induced EC barrier failure. Protective effects of HGF against LPS-induced lung inflammation and vascular leak were also diminished in Asef knockout mice. Taken together, these results demonstrate potent anti-inflammatory effects by HGF and delineate a key role of Asef in the mediation of the HGF barrier protective and anti-inflammatory effects. Modulation of Asef activity may have important implications in therapeutic strategies aimed at the treatment of sepsis and acute lung injury/ARDS-induced gram-negative bacterial pathogens.

Asef controls vascular endothelial permeability and barrier recovery in the lung

This is the first report of Asef involvement in the regulation of endothelial vascular permeability in vitro and in vivo. Asef activation in endothelial cells by hepatocyte growth factor suppressed the Rho-dependent pathway of agonist-induced endothelial permeability and promoted Rac1-dependent endothelial barrier recovery.

Increased levels of hepatocyte growth factor (HGF) in injured lungs may reflect a compensatory response to diminish acute lung injury (ALI). HGF-induced activation of Rac1 GTPase stimulates endothelial barrier protective mechanisms. This study tested the involvement of Rac-specific guanine nucleotide exchange factor Asef in HGF-induced endothelial cell (EC) cytoskeletal dynamics and barrier protection in vitro and in a two-hit model of ALI. HGF induced membrane translocation of Asef and stimulated Asef Rac1-specific nucleotide exchange activity. Expression of constitutively activated Asef mutant mimicked HGF-induced peripheral actin cytoskeleton enhancement. In contrast, siRNA-induced Asef knockdown or expression of dominant-negative Asef attenuated HGF-induced Rac1 activation evaluated by Rac-GTP pull down and FRET assay with Rac1 biosensor. Molecular inhibition of Asef attenuated HGF-induced peripheral accumulation of cortactin, formation of lamellipodia-like structures, and enhancement of VE-cadherin adherens junctions and compromised HGF-protective effect against thrombin-induced RhoA GTPase activation, Rho-dependent cytoskeleton remodeling, and EC permeability. Intravenous HGF injection attenuated lung inflammation and vascular leak in the two-hit model of ALI induced by excessive mechanical ventilation and thrombin signaling peptide TRAP6. This effect was lost in Asef^−/− mice. This study shows for the first time the role of Asef in HGF-mediated protection against endothelial hyperpermeability and lung injury.

RhoGEFs in cell motility: novel links between Rgnef and focal adhesion kinase

Rho guanine exchange factors (GEFs) are a large, diverse family of proteins defined by their ability to catalyze the exchange of GDP for GTP on small GTPase proteins such as Rho family members. GEFs act as integrators from varied intra- and extracellular sources to promote spatiotemporal activity of Rho GTPases that control signaling pathways regulating cell proliferation and movement. Here we review recent studies elucidating roles of RhoGEF proteins in cell motility. Emphasis is placed on Dbl-family GEFs and connections to development, integrin signaling to Rho GTPases regulating cell adhesion and movement, and how these signals may enhance tumor progression. Moreover, RhoGEFs have additional domains that confer distinctive functions or specificity. We will focus on a unique interaction between Rgnef (also termed Arhgef28 or p190RhoGEF) and focal adhesion kinase (FAK), a non-receptor tyrosine kinase that controls migration properties of normal and tumor cells. This Rgnef-FAK interaction activates canonical GEF-dependent RhoA GTPase activity to govern contractility and also functions as a scaffold in a GEF-independent manner to enhance FAK activation. Recent studies have also brought to light the importance of specific regions within the Rgnef pleckstrin homology (PH) domain for targeting the membrane. As revealed by ongoing Rgnef-FAK investigations, exploring GEF roles in cancer will yield fundamental new information on the molecular mechanisms promoting tumor spread and metastasis.

MAPs: cellular navigators for microtubule array orientations in Arabidopsis

Microtubules are subcellular nanotubes composed of α- and β-tubulin that arise from microtubule nucleation sites, mainly composed of γ-tubulin complexes [corrected]. Cell wall encased plant cells have evolved four distinct microtubule arrays that regulate cell division and expansion. Microtubule-associated proteins, the so called MAPs, construct, destruct and reorganize microtubule arrays thus regulating their spatiotemporal transitions during the cell cycle. By physically binding to microtubules and/or modulating their functions, MAPs control microtubule dynamic instability and/or interfilament cross talk. We survey the recent analyses of Arabidopsis MAPs such as MAP65, MOR1, CLASP, katanin, TON1, FASS, TRM, TAN1 and kinesins in terms of their effects on microtubule array organizations and plant development.

The miR-1-NOTCH3-Asef pathway is important for colorectal tumor cell migration

The tumor suppressor adenomatous polyposis coli (APC) is mutated in sporadic and familial colorectal tumors. APC stimulates the activity of the Cdc42- and Rac1-specific guanine nucleotide exchange factor Asef and promotes the migration and invasion of colorectal tumor cells. Furthermore, Asef is overexpressed in colorectal tumors and is required for colorectal tumorigenesis. It is also known that NOTCH signaling plays critical roles in colorectal tumorigenesis and fate determination of intestinal progenitor cells. Here we show that NOTCH3 up-regulates Asef expression by activating the Asef promoter in colorectal tumor cells. Moreover, we demonstrate that microRNA-1 (miR-1) is down-regulated in colorectal tumors and that miR-1 has the potential to suppress NOTCH3 expression through direct binding to its 3’-UTR region. These results suggest that the miR-1-NOTCH3-Asef pathway is important for colorectal tumor cell migration and may be a promising molecular target for the treatment of colorectal tumors.

Rho guanine nucleotide exchange factors: regulators of Rho GTPase activity in development and disease

The aberrant activity of Ras homologous (Rho) family small GTPases (20 human members) has been implicated in cancer and other human diseases. However, in contrast to the direct mutational activation of Ras found in cancer and developmental disorders, Rho GTPases are activated most commonly in disease by indirect mechanisms. One prevalent mechanism involves aberrant Rho activation via the deregulated expression and/or activity of Rho family guanine nucleotide exchange factors (RhoGEFs). RhoGEFs promote formation of the active GTP-bound state of Rho GTPases. The largest family of RhoGEFs is comprised of the Dbl family RhoGEFs with 70 human members. The multitude of RhoGEFs that activate a single Rho GTPase reflects the very specific role of each RhoGEF in controlling distinct signaling mechanisms involved in Rho activation. In this review, we summarize the role of Dbl RhoGEFs in development and disease, with a focus on Ect2 (epithelial cell transforming squence 2), Tiam1 (T-cell lymphoma invasion and metastasis 1), Vav and P-Rex1/2 (PtdIns(3,4,5)P3 (phosphatidylinositol (3,4,5)-triphosphate)-dependent Rac exchanger).

Mechanisms of RAS/β-catenin interactions

Signaling through the WNT/β-catenin and the RAS (rat sarcoma)/MAPK (mitogen-activated protein kinase) pathways plays a key role in the regulation of various physiological cellular processes including proliferation, differentiation, and cell death. Aberrant mutational activation of these signaling pathways is closely linked to the development of cancer in many organs, in humans as well as in laboratory animals. Over the past years, more and more evidence for a close linkage of the two oncogenic signaling cascades has accumulated. Using different experimental approaches, model systems, and experimental conditions, a variety of molecular mechanisms have been identified by which signal transduction through WNT/β-catenin and RAS interact, either in a synergistic or an antagonistic manner. Mechanisms of interaction comprise an upstream crosstalk at the level of pathway-activating ligands and their receptors, interrelations of cytosolic kinases involved in either pathways, as well as interaction in the nucleus related to the joint regulation of target gene transcription. Here, we present a comprehensive review of the current knowledge on the interaction of RAS/MAPK- and WNT/β-catenin-driven signal transduction in mammalian cells.

Rho guanine exchange factors in blood vessels: fine-tuners of angiogenesis and vascular function

The angiogenic cascade is a multi-step process essential for embryogenesis and other physiological and pathological processes. Rho family GTPases are binary molecular switches and serve as master regulators of various basic cellular processes. Rho GTPases are known to exert important functions in angiogenesis and vascular physiology. These functions demand a tight and context-specific control of cellular processes requiring superordinate control by a multitude of guanine nucleotide exchange factors (GEFs). GEFs display various features enabling them to fine-tune the actions of Rho GTPases in the vasculature: (1) GEFs regulate specific steps of the angiogenic cascade; (2) GEFs show a spatio-temporally specific expression pattern; (3) GEFs differentially regulate endothelial function depending on their subcellular location; (4) GEFs mediate crosstalk between complex signaling cascades and (5) GEFs themselves are regulated by another layer of interacting proteins. The aim of this review is to provide an overview about the role of GEFs in regulating angiogenesis and vascular function and to point out current limitations as well as clinical perspectives.

Structural basis for the recognition of Asef by adenomatous polyposis coli

Adenomatous polyposis coli (APC) regulates cell-cell adhesion and cell migration through activating the APC-stimulated guanine nucleotide-exchange factor (GEF; Asef), which is usually autoinhibited through the binding between its Src homology 3 (SH3) and Dbl homology (DH) domains. The APC-activated Asef stimulates the small GTPase Cdc42, which leads to decreased cell-cell adherence and enhanced cell migration. In colorectal cancers, truncated APC constitutively activates Asef and promotes cancer cell migration and angiogenesis. Here, we report crystal structures of the human APC/Asef complex. We find that the armadillo repeat domain of APC uses a highly conserved surface groove to recognize the APC-binding region (ABR) of Asef, conformation of which changes dramatically upon binding to APC. Key residues on APC and Asef for the complex formation were mutated and their importance was demonstrated by binding and activity assays. Structural superimposition of the APC/Asef complex with autoinhibited Asef suggests that the binding between APC and Asef might create a steric clash between Asef-DH domain and APC, which possibly leads to a conformational change in Asef that stimulates its GEF activity. Our structures thus elucidate the molecular mechanism of Asef recognition by APC, as well as provide a potential target for pharmaceutical intervention against cancers.