Rab13-dependent trafficking of RhoA is required for directional migration and angiogenesis

Role of angiomotin family members in human diseases (Review)

Angiomotin (Amot) family members, including Amot, Amot-like protein 1 (Amotl1) and Amot-like protein 2 (Amotl2), have been found to interact with angiostatins. In addition, Amot family members are involved in various physiological and pathological functions such as embryonic development, angiogenesis and tumorigenesis. Some studies have also demonstrated its regulation in signaling pathways such as the Hippo signaling pathway, AMPK signaling pathway and mTOR signaling pathways. Amot family members play an important role in neural stem cell differentiation, dendritic formation and synaptic maturation. In addition, an increasing number of studies have focused on their function in promoting and/or suppressing cancer, but the underlying mechanisms remain to be elucidated. The present review integrated relevant studies on upstream regulation and downstream signals of Amot family members, as well as the latest progress in physiological and pathological functions and clinical applications, hoping to offer important ideas for further research.

Angiomotin family proteins in the Hippo signaling pathway

The Motin family proteins (Motins) are a class of scaffolding proteins consisting of Angiomotin (AMOT), AMOT-like protein 1 (AMOTL1), and AMOT-like protein 2 (AMOTL2). Motins play a pivotal role in angiogenesis, tumorigenesis, and neurogenesis by modulating multiple cellular signaling pathways. Recent findings indicate that Motins are components of the Hippo pathway, a signaling cascade involved in development and cancer. This review discusses how Motins are integrated into the Hippo signaling network, as either upstream regulators or downstream effectors, to modulate cell proliferation and migration. The repression of YAP/TAZ by Motins contributes to growth inhibition, whereas subcellular localization of Motins and their interactions with actin fibers are critical in regulating cell migration. The net effect of Motins on cell proliferation and migration may contribute to their diverse biological functions.

Single-cell sequencing analysis reveals development and differentiation trajectory of Schwann cells manipulated by M. leprae

BACKGROUND

M. leprae preferentially infects Schwann cells (SCs) in the peripheral nerves leading to nerve damage and irreversible disability. Knowledge of how M. leprae infects and interacts with host SCs is essential for understanding mechanisms of nerve damage and revealing potential new therapeutic strategies.

METHODOLOGY/PRINCIPAL FINDINGS

We performed a time-course single-cell sequencing analysis of SCs infected with M. leprae at different time points, further analyzed the heterogeneity of SCs, subpopulations associated with M. leprae infection, developmental trajectory of SCs and validated by Western blot or flow cytometry. Different subpopulations of SCs exhibiting distinct genetic features and functional enrichments were present. We observed two subpopulations associated with M. leprae infection, a stem cell-like cell subpopulation increased significantly at 24 h but declined by 72 h after M. leprae infection, and an adipocyte-like cell subpopulation, emerged at 72 h post-infection. The results were validated and confirmed that a stem cell-like cell subpopulation was in the early stage of differentiation and could differentiate into an adipocyte-like cell subpopulation.

CONCLUSIONS/SIGNIFICANCE

Our results present a systematic time-course analysis of SC heterogeneity after infection by M. leprae at single-cell resolution, provide valuable information to understand the critical biological processes underlying reprogramming and lipid metabolism during M. leprae infection of SCs, and increase understanding of the disease-causing mechanisms at play in leprosy patients as well as revealing potential new therapeutic strategies.

Proximity labelling identifies pro-migratory endocytic recycling cargo and machinery of the Rab4 and Rab11 families

Endocytic recycling controls the return of internalised cargoes to the plasma membrane to coordinate their positioning, availability and downstream signalling. The Rab4 and Rab11 small GTPase families regulate distinct recycling routes, broadly classified as fast recycling from early endosomes (Rab4) and slow recycling from perinuclear recycling endosomes (Rab11), and both routes handle a broad range of overlapping cargoes to regulate cell behaviour. We adopted a proximity labelling approach, BioID, to identify and compare the protein complexes recruited by Rab4a, Rab11a and Rab25 (a Rab11 family member implicated in cancer aggressiveness), revealing statistically robust protein-protein interaction networks of both new and well-characterised cargoes and trafficking machinery in migratory cancer cells. Gene ontological analysis of these interconnected networks revealed that these endocytic recycling pathways are intrinsically connected to cell motility and cell adhesion. Using a knock-sideways relocalisation approach, we were further able to confirm novel links between Rab11, Rab25 and the ESCPE-1 and retromer multiprotein sorting complexes, and identify new endocytic recycling machinery associated with Rab4, Rab11 and Rab25 that regulates cancer cell migration in the 3D matrix.

Exosomal proteomics identifies RAB13 as a potential regulator of metastasis for HCC

BACKGROUND

Exosomal proteins from cancer cells are becoming new biomarkers for cancer monitoring and efficacy evaluation. However, their biological function and molecular mechanism underlying tumor metastasis are largely unknown.

METHODS

Bioinformatic methods such as bulk gene expression analysis, single-cell RNA sequencing data analysis, and gene set enrichment analysis were employed to identify metastasis-associated proteins. The in vitro and in vivo experiments were used to investigate the function of RAB13 in HCC metastasis.

RESULTS

We identified RAB13 as one of the critical regulators of metastasis in HCC-derived exosomes for the first time. In vitro, the invasiveness of HCC cell lines could be attenuated by RAB13 silence. In vivo, tumor size and proportion of high-grade lung metastatic nodule could be reduced in the mice with orthotopic transplantation of tumors and intravenously injected with exosomes derived from MHCC97H cell with RAB13 silence (si-RAB13-Exo), as compared with those without RAB13 silence (si-NC-Exo). Moreover, in si-RAB13-Exo group, circulating tumor cell counts were decreased at the third, fourth, and fifth weeks after orthotopic transplantation of tumors, and MMP2 (matrix metalloproteinase 2)/TIMP2 (tissue inhibitor of metalloproteinases 2) ratio was also significantly decreased. In addition, RAB13 expression was also associated with VEGF levels, microvessel density, and tube formation of vascular endothelial cells by both in vitro and in vivo models, indicating that RAB13 was associated with angiogenesis in HCC.

CONCLUSIONS

We have demonstrated exosomal RAB13 as a potential regulator of metastasis for HCC by in silico, in vitro, and in vivo methods, which greatly improve our understanding of the functional impact of exosomal proteins on HCC metastasis.

Characterization and computational simulation of human Syx, a RhoGEF implicated in glioblastoma

Structural discovery of guanine nucleotide exchange factor (GEF) protein complexes is likely to become increasingly relevant with the development of new therapeutics targeting small GTPases and development of new classes of small molecules that inhibit protein‐protein interactions. Syx (also known as PLEKHG5 in humans) is a RhoA GEF implicated in the pathology of glioblastoma (GBM). Here we investigated protein expression and purification of ten different human Syx constructs and performed biophysical characterizations and computational studies that provide insights into why expression of this protein was previously intractable. We show that human Syx can be expressed and isolated and Syx is folded as observed by circular dichroism (CD) spectroscopy and actively binds to RhoA as determined by co‐elution during size exclusion chromatography (SEC). This characterization may provide critical insights into the expression and purification of other recalcitrant members of the large class of oncogenic—Diffuse B‐cell lymphoma (Dbl) homology GEF proteins. In addition, we performed detailed homology modeling and molecular dynamics simulations on the surface of a physiologically realistic membrane. These simulations reveal novel insights into GEF activity and allosteric modulation by the plekstrin homology (PH) domain. These newly revealed interactions between the GEF PH domain and the membrane embedded region of RhoA support previously unexplained experimental findings regarding the allosteric effects of the PH domain from numerous activity studies of Dbl homology GEF proteins. This work establishes new hypotheses for structural interactivity and allosteric signal modulation in Dbl homology RhoGEFs.

An emerging role of KRAS in biogenesis, cargo sorting and uptake of cancer-derived extracellular vesicles

Extracellular vesicles (EVs) are nanovesicles secreted for intercellular communication with endosomal network regulating secretion of small EVs (or exosomes) that play roles in cancer progression. As an essential oncoprotein, Kirsten rat sarcoma virus (KRAS) is tightly regulated by its endosomal trafficking for membrane attachment. However, the crosstalk between KRAS and EVs has been scarcely discussed despite its endocytic association. An overview of the oncogenic role of KRAS focusing on its correlation with cancer-associated EVs should provide important clues for disease prognosis and inspire novel therapeutic approaches for treating KRAS mutant cancers. Therefore, this review summarizes the relevant studies that provide substantial evidence linking KRAS mutation to EVs and discusses the oncogenic implication from the aspects of biogenesis, cargo sorting, and release and uptake of the EVs.

Small GTPases and Their Regulators: A Leading Road toward Blood Vessel Development in Zebrafish

Members of the Ras superfamily have been found to perform several functions leading to the development of eukaryotes. These small GTPases are divided into five major subfamilies, and their regulators can “turn on” and “turn off” signals. Recent studies have shown that this superfamily of proteins has various roles in the process of vascular development, such as vasculogenesis and angiogenesis. Here, we discuss the role of these subfamilies in the development of the vascular system in zebrafish.

Rho GTPases in Retinal Vascular Diseases

The Rho family of small GTPases (Rho GTPases) act as molecular switches that transduce extrinsic stimuli into cytoskeletal rearrangements. In vascular endothelial cells (ECs), Cdc42, Rac1, and RhoA control cell migration and cell–cell junctions downstream of angiogenic and inflammatory cytokines, thereby regulating vascular formation and permeability. While these Rho GTPases are broadly expressed in various types of cells, RhoJ is enriched in angiogenic ECs. Semaphorin 3E (Sema3E) releases RhoJ from the intracellular domain of PlexinD1, by which RhoJ induces actin depolymerization through competition with Cdc42 for their common effector proteins. RhoJ further mediates the Sema3E-induced association of PlexinD1 with vascular endothelial growth factor receptor (VEGFR) 2 and the activation of p38. Upon stimulation with VEGF-A, RhoJ facilitates the formation of a holoreceptor complex comprising VEGFR2, PlexinD1, and neuropilin-1, leading to the prevention of VEGFR2 degradation and the maintenance of intracellular signal transduction. These pleiotropic roles of RhoJ are required for directional EC migration in retinal angiogenesis. This review highlights the latest insights regarding Rho GTPases in the field of vascular biology, as it will be informative to consider their potential as targets for the treatment of aberrant angiogenesis and hyperpermeability in retinal vascular diseases.

RNA localization and co-translational interactions control RAB13 GTPase function and cell migration

Numerous RNAs exhibit specific distribution patterns in mammalian cells. However, the functional and mechanistic consequences are relatively unknown. Here, we investigate the functional role of RNA localization at cellular protrusions of migrating mesenchymal cells, using as a model the RAB13 RNA, which encodes a GTPase important for vesicle‐mediated membrane trafficking. While RAB13 RNA is enriched at peripheral protrusions, the expressed protein is concentrated perinuclearly. By specifically preventing RAB13 RNA localization, we show that peripheral RAB13 translation is not important for the overall distribution of the RAB13 protein or its ability to associate with membranes, but is required for full activation of the GTPase and for efficient cell migration. RAB13 translation leads to a co‐translational association of nascent RAB13 with the exchange factor RABIF. Our results indicate that RAB13‐RABIF association at the periphery is required for directing RAB13 GTPase activity to promote cell migration. Thus, translation of RAB13 in specific subcellular environments imparts the protein with distinct properties and highlights a means of controlling protein function through local RNA translation.

RAB13 mRNA compartmentalisation spatially orients tissue morphogenesis

Polarised targeting of diverse mRNAs to cellular protrusions is a hallmark of cell migration. Although a widespread phenomenon, definitive functions for endogenous targeted mRNAs and their relevance to modulation of in vivo tissue dynamics remain elusive. Here, using single-molecule analysis, gene editing and zebrafish live-cell imaging, we report that mRNA polarisation acts as a molecular compass that orients motile cell polarity and spatially directs tissue movement. Clustering of protrusion-derived RNAseq datasets defined a core 192-nt localisation element underpinning precise mRNA targeting to sites of filopodia formation. Such targeting of the small GTPase RAB13 generated tight spatial coupling of mRNA localisation, translation and protein activity, achieving precise subcellular compartmentalisation of RAB13 protein function to create a polarised domain of filopodia extension. Consequently, genomic excision of this localisation element and perturbation of RAB13 mRNA targeting-but not translation-depolarised filopodia dynamics in motile endothelial cells and induced mispatterning of blood vessels in zebrafish. Hence, mRNA polarisation, not expression, is the primary determinant of the site of RAB13 action, preventing ectopic functionality at inappropriate subcellular loci and orienting tissue morphogenesis.

BNIP-2 retards breast cancer cell migration by coupling microtubule-mediated GEF-H1 and RhoA activation

Microtubules display dynamic turnover during cell migration, leading to cell contractility and focal adhesion maturation regulated by Rho guanosine triphosphatase activity. This interplay between microtubules and actomyosin is mediated by guanine nucleotide exchange factor (GEF)-H1 released after microtubule depolymerization or microtubule disconnection from focal adhesions. However, how GEF-H1 activates Rho upon microtubule disassembly remains elusive. Here, we found that BNIP-2, a BCH domain-containing protein that binds both RhoA and GEF-H1 and traffics with kinesin-1 on microtubules, is important for GEF-H1-driven RhoA activation upon microtubule disassembly. Depletion of BNIP-2 in MDA-MB-231 breast cancer cells decreases RhoA activity and promotes cell migration. Upon nocodazole-induced microtubule disassembly, the interaction between BNIP-2 and GEF-H1 increases, while knockdown of BNIP-2 reduces RhoA activation and cell rounding via uncoupling RhoA-GEF-H1 interaction. Together, these findings revealed that BNIP-2 couples microtubules and focal adhesions via scaffolding GEF-H1 and RhoA, fine-tuning RhoA activity and cell migration.

Rabs in Signaling and Embryonic Development

Rab GTPases play key roles in various cellular processes. They are essential, among other roles, to membrane trafficking and intracellular signaling events. Both trafficking and signaling events are crucial for proper embryonic development. Indeed, embryogenesis is a complex process in which cells respond to various signals and undergo dramatic changes in their shape, position, and function. Over the last few decades, cellular studies have highlighted the novel signaling roles played by Rab GTPases, while numerous studies have shed light on the important requirements of Rab proteins at various steps of embryonic development. In this review, we aimed to generate an overview of Rab contributions during animal embryogenesis. We first briefly summarize the involvement of Rabs in signaling events. We then extensively highlight the contribution of Rabs in shaping metazoan development and conclude with new approaches that will allow investigation of Rab functions in vivo.

Regulation of VEGFR2 trafficking and signaling by Rab GTPase-activating proteins

Vascular endothelial growth factor receptor-2 (VEGFR2) and its ligands (VEGFs) are crucial players in vasculogenesis and angiogenesis. General blocking of this signaling system with antibodies or small molecule inhibitors is an established strategy to treat cancer and age-related macular degeneration. Nevertheless, the activated receptor can signal to discrete downstream signaling pathways and the equilibrium between these pathways is modulated by coreceptors and distinct isoforms of VEGF. Here we investigated the influence of Rab GTPase activating proteins (RabGAPs) on VEGFR2 signaling, tube formation, and migration of endothelial cells. We demonstrate that members of the TBC1D10 subfamily of RabGAPs have opposite effects. Whereas TBC1D10A leads to increased Erk1/2 signaling, TBC1D10B lowered Erk1/2 and p38 signaling and reduced tube formation in vitro. TBC1D10A is a RabGAP acting on RAB13 that was shown before to play a role in angiogenesis and we could indeed show colocalization of these two proteins with VEGFR2 in activated cells. In addition, we observed that cells expressing TBC1D10B show lower expression of VEGFR2 and NRP1 on filopodia of activated cells. Taken together, our systematic analysis of influence of RabGAPs on VEGFR2 signaling identifies the TBC1D10 subfamily members as modulators of angiogenesis.

Building Blood Vessels-One Rho GTPase at a Time

Blood vessels are required for the survival of any organism larger than the oxygen diffusion limit. Blood vessel formation is a tightly regulated event and vessel growth or changes in permeability are linked to a number of diseases. Elucidating the cell biology of endothelial cells (ECs), which are the building blocks of blood vessels, is thus critical to our understanding of vascular biology and to the development of vascular-targeted disease treatments. Small GTPases of the Rho GTPase family are known to regulate several processes critical for EC growth and maintenance. In fact, many of the 21 Rho GTPases in mammals are known to regulate EC junctional remodeling, cell shape changes, and other processes. Rho GTPases are thus an attractive target for disease treatments, as they often have unique functions in specific vascular cell types. In fact, some Rho GTPases are even expressed with relative specificity in diseased vessels. Interestingly, many Rho GTPases are understudied in ECs, despite their known expression in either developing or mature vessels, suggesting an even greater wealth of knowledge yet to be gleaned from these complex signaling pathways. This review aims to provide an overview of Rho GTPase signaling contributions to EC vasculogenesis, angiogenesis, and mature vessel barrier function. A particular emphasis is placed on so-called "alternative" Rho GTPases, as they are largely understudied despite their likely important contributions to EC biology.

Vesicle trafficking pathways that direct cell migration in 3D matrices and in vivo

Cell migration is a vital process in development and disease, and while the mechanisms that control motility are relatively well understood on two-dimensional surfaces, the control of cell migration in three dimensions (3D) and in vivo has only recently begun to be understood. Vesicle trafficking pathways have emerged as a key regulatory element in migration and invasion, with the endocytosis and recycling of cell surface cargos, including growth factor and chemokine receptors, adhesion receptors and membrane-associated proteases, being of major importance. We highlight recent advances in our understanding of how endocytic trafficking controls the availability and local activity of these cargoes to influence the movement of cells in 3D matrix and in developing organisms. In particular, we discuss how endocytic trafficking of different receptor classes spatially restricts signals and activity, usually to the leading edge of invasive cells.

Hold Me, but Not Too Tight-Endothelial Cell-Cell Junctions in Angiogenesis

Endothelial cell-cell junctions must perform seemingly incompatible tasks during vascular development-providing stable connections that prevent leakage, while allowing dynamic cellular rearrangements during sprouting, anastomosis, lumen formation, and functional remodeling of the vascular network. This review aims to highlight recent insights into the molecular mechanisms governing endothelial cell-cell adhesion in the context of vascular development.

The physiological role of Motin family and its dysregulation in tumorigenesis

Members in Motin family, or Angiomotins (AMOTs), are adaptor proteins that localize in the membranous, cytoplasmic or nuclear fraction in a cell context-dependent manner. They control the bioprocesses such as migration, tight junction formation, cell polarity, and angiogenesis. Emerging evidences have demonstrated that AMOTs participate in cancer initiation and progression. Many of the previous studies have focused on the involvement of AMOTs in Hippo-YAP1 pathway. However, it has been controversial for years that AMOTs serve as either positive or negative growth regulators in different cancer types because of the various cellular origins. The molecular mechanisms of these opposite roles of AMOTs remain elusive. This review comprehensively summarized how AMOTs function physiologically and how their dysregulation promotes or inhibits tumorigenesis. Better understanding the functional roles of AMOTs in cancers may lead to an improvement of clinical interventions as well as development of novel therapeutic strategies for cancer patients.

Loss of Mpdz impairs ependymal cell integrity leading to perinatal-onset hydrocephalus in mice

Hydrocephalus is a common congenital anomaly. LCAM1 and MPDZ (MUPP1) are the only known human gene loci associated with non‐syndromic hydrocephalus. To investigate functions of the tight junction‐associated protein Mpdz, we generated mouse models. Global Mpdz gene deletion or conditional inactivation in Nestin‐positive cells led to formation of supratentorial hydrocephalus in the early postnatal period. Blood vessels, epithelial cells of the choroid plexus, and cilia on ependymal cells, which line the ventricular system, remained morphologically intact in Mpdz‐deficient brains. However, flow of cerebrospinal fluid through the cerebral aqueduct was blocked from postnatal day 3 onward. Silencing of Mpdz expression in cultured epithelial cells impaired barrier integrity, and loss of Mpdz in astrocytes increased RhoA activity. In Mpdz‐deficient mice, ependymal cells had morphologically normal tight junctions, but expression of the interacting planar cell polarity protein Pals1 was diminished and barrier integrity got progressively lost. Ependymal denudation was accompanied by reactive astrogliosis leading to aqueductal stenosis. This work provides a relevant hydrocephalus mouse model and demonstrates that Mpdz is essential to maintain integrity of the ependyma.

Intersectin-s interaction with DENND2B facilitates recycling of epidermal growth factor receptor

Epidermal growth factor (EGF) activates the EGF receptor (EGFR) and stimulates its internalization and trafficking to lysosomes for degradation. However, a percentage of EGFR undergoes ligand-independent endocytosis and is rapidly recycled back to the plasma membrane. Importantly, alterations in EGFR recycling are a common hallmark of cancer, and yet, our understanding of the machineries controlling the fate of endocytosed EGFR is incomplete. Intersectin-s is a multi-domain adaptor protein that is required for internalization of EGFR Here, we discover that intersectin-s binds DENND2B, a guanine nucleotide exchange factor for the exocytic GTPase Rab13, and this interaction promotes recycling of ligand-free EGFR to the cell surface. Intriguingly, upon EGF treatment, DENND2B is phosphorylated by protein kinase D and dissociates from intersectin-s, allowing for receptor targeting to degradation. Our study thus reveals a novel mechanism controlling the fate of internalized EGFR with important implications for cancer.

The role of small GTPases and EPAC-Rap signaling in the regulation of the blood-brain and blood-retinal barriers

Maintenance and regulation of the vascular endothelial cell junctional complex is critical for proper barrier function of the blood-brain barrier (BBB) and the highly related blood-retinal barrier (BRB) that help maintain proper neuronal environment. Recent research has demonstrated that the junctional complex is actively maintained and can be dynamically regulated. Studies focusing on the mechanisms of barrier formation, maintenance, and barrier disruption have been of interest to understanding development of the BBB and BRB and identifying a means for therapeutic intervention for diseases ranging from brain tumors and dementia to blinding eye diseases. Research has increasingly revealed that small GTPases play a critical role in both barrier formation and disruption mechanisms. This review will summarize the current data on small GTPases in barrier regulation with an emphasis on the EPAC-Rap1 signaling pathway to Rho in endothelial barriers, as well as explore its potential involvement in paracellular flux and transcytosis regulation.

Traffic jams and the complex role of α-Synuclein aggregation in Parkinson disease

A common pathological event among various neurodegenerative disorders (NDs) is the misfolding and aggregation of different proteins in the brain. This is thought to potentiate aberrant protein-protein interactions that culminate in the disruption of several biological processes and, ultimately, in neuronal cell loss. Although protein aggregates are a common hallmark in several disorders, the molecular pathways leading to their generation remain unclear. The misfolding and aggregation of α-Synuclein (aSyn) is the pathological hallmark of Parkinson disease (PD), the second most common age related ND. It has been postulated that oligomeric species of aSyn, rather than more mature aggregated forms of the protein, are the causative agents of cytotoxicity. In recent years, we have been investigating the molecular mechanisms underlying the initial steps of aSyn accumulation in living cells. Using an unbiased genome-wide lentiviral RNAi screen we identified trafficking and kinase genes as modulators of aSyn oligomerization, aggregation, and toxicity. Among those, Rab8b, Rab11a, Rab13 and Slp5 were found to promote the clearance of aSyn inclusions and reduce aSyn toxicity. Moreover, we found that endocytic recycling and secretion of aSyn was enhanced upon expression of Rab11a or Rab13 in cells accumulating aSyn inclusions. Altogether, our findings suggest specific trafficking steps may prove beneficial as targets for therapeutic intervention in synucleinopathies, and should be further investigated in other models.

Tight junctions as regulators of tissue remodelling

Formation of tissue barriers by epithelial and endothelial cells requires neighbouring cells to interact via intercellular junctions, which includes tight junctions. Tight junctions form a semipermeable paracellular diffusion barrier and act as signalling hubs that guide cell behaviour and differentiation. Components of tight junctions are also expressed in cell types not forming tight junctions, such as cardiomyocytes, where they associate with facia adherens and/or gap junctions. This review will focus on tight junction proteins and their importance in tissue homeostasis and remodelling with a particular emphasis on what we have learned from animal models and human diseases.

Regulation of Cancer Cell Behavior by the Small GTPase Rab13

The members of the Rab family of GTPases are master regulators of cellular membrane trafficking. With ∼70 members in humans, Rabs have been implicated in all steps of membrane trafficking ranging from vesicle formation and transport to vesicle docking/tethering and fusion. Vesicle trafficking controls the localization and levels of a myriad of proteins, thus regulating cellular functions including proliferation, metabolism, cell-cell adhesion, and cell migration. It is therefore not surprising that impairment of Rab pathways is associated with diseases including cancer. In this review, we highlight evidence supporting the role of Rab13 as a potent driver of cancer progression.

Rab13 Traffics on Vesicles Independent of Prenylation

Rab GTPases are critical regulators of membrane trafficking. The canonical view is that Rabs are soluble in their inactive GDP-bound form, and only upon activation and conversion to their GTP-bound state are they anchored to membranes through membrane insertion of a C-terminal prenyl group. Here we demonstrate that C-terminal prenylation is not required for Rab13 to associate with and traffic on vesicles. Instead, inactive Rab13 appears to associate with vesicles via protein-protein interactions. Only following activation does Rab13 associate with the plasma membrane, presumably with insertion of the C-terminal prenyl group into the membrane.

Sequential recruitment of Rab GTPases during early stages of phagocytosis

The phagocytosis and destruction of pathogens and dead cells by macrophages is important for innate immunity and tissue maintenance. Multiple Rab family GTPases engage effector molecules to coordinate the early stages of phagocytosis, which include rapid changes in actin polymerization, membrane phospholipids, trafficking and the activation of receptors. Defining the spatiotemporal, sequential recruitment of these Rabs is critical for insights into how phagocytosis is initiated and coordinated. Here, we screened GFP-tagged Rabs expressed in fixed and live cells to identify and stratify those recruited to early phagocytic membranes at stages defined by phospholipid transitions. We propose a sequence of Rabs 35, 13, 8a, 8b, 27a, 10, and 31 that precedes and accompanies phagocytic cup closure, followed after closure by recruitment of endosomal Rabs 5a, 5b, 5c, 14, and 11. Reducing the expression of individual Rabs by siRNA knockdown, notably Rabs 35 and 13, disrupts phagocytosis prior to phagocytic cup closure, confirming a known role for Rab35 and revealing anew the involvement of Rab13. The results enhance our understanding of innate immune responses in macrophages by revealing the sequence of Rabs that initiates phagocytosis.

Angiomotin is a novel component of cadherin-11/β-catenin/p120 complex and is critical for cadherin-11-mediated cell migration

Loss of E-cadherin and up-regulation of mesenchymal cadherins, a hallmark of the epithelial-mesenchymal transition, contributes to migration and dissemination of cancer cells. Expression of human cadherin-11 (Cad11), also known as osteoblast cadherin, in prostate cancer increases the migration of prostate cancer cells. How Cad11 mediates cell migration is unknown. Using the human Cad11 cytoplasmic domain in pulldown assays, we identified human angiomotin (Amot), known to be involved in cell polarity, migration, and Hippo pathway, as a component of the Cad11 protein complex. Deletion analysis showed that the last C-terminal 10 amino acids in Cad11 cytoplasmic domain are required for Amot binding. Further, Cad11 preferentially interacts with Amot-p80 than Amot-p130 isoform and binds directly to the middle domain of Amot-p80. Cad11-Amot interaction affects Cad11-mediated cell migration, but not homophilic adhesion, as deletion of Amot binding motif of Cad11 (Cad11-ΔAmot) did not abolish Cad11-mediated cell-cell adhesion of mouse L cells, but significantly reduced Cad11-mediated cell migration of human C4-2B4 and PC3-mm2 prostate cancer cells and human HEK293T cells. Together, our studies identified Amot-p80 as a novel component of the Cad11 complex and demonstrated that Amot-p80 is critical for Cad11-mediated cell migration.

Nucleoside diphosphate kinase B regulates angiogenesis through modulation of vascular endothelial growth factor receptor type 2 and endothelial adherens junction proteins

OBJECTIVE

Nucleoside diphosphate kinase B (NDPKB) participates in the activation of heterotrimeric and monomeric G proteins, which are pivotal mediators in angiogenic signaling. The role of NDPKB in angiogenesis has to date not been defined. Therefore, we analyzed the contribution of NDPKB to angiogenesis and its underlying mechanisms in well-characterized in vivo and in vitro models.

APPROACH AND RESULTS

Zebrafish embryos were depleted of NDPKB by morpholino-mediated knockdown. These larvae displayed severe malformations specifically in vessels formed by angiogenesis. NDPKB-deficient (NDPKB(-/-)) mice were subjected to oxygen-induced retinopathy. In this model, the number of preretinal neovascularizations in NDPKB(-/-) mice was strongly reduced in comparison with wild-type littermates. In accordance, a delayed blood flow recovery was detected in the NDPKB(-/-) mice after hindlimb ligation. In in vitro studies, a small interfering RNA-mediated knockdown of NDPKB was performed in human umbilical endothelial cells. NDPKB depletion impaired vascular endothelial growth factor (VEGF)-induced sprouting and hampered the VEGF-induced spatial redistributions of the VEGF receptor type 2 and VE-cadherin at the plasma membrane. Concomitantly, NDPKB depletion increased the permeability of the human umbilical endothelial cell monolayer.

CONCLUSIONS

This is the first report to show that NDPKB is required for VEGF-induced angiogenesis and contributes to the correct localization of VEGF receptor type 2 and VE-cadherin at the endothelial adherens junctions. Therefore, our data identify NDPKB as a novel molecular target to modulate VEGF-dependent angiogenesis.

The Angiomotins--from discovery to function

Angiomotins were originally identified as angiostatin binding proteins and implicated in the regulation of endothelial cell migration. Recent studies have shed light on the role of Angiomotins and other members of the Motin protein family in epithelial cells and in pathways directly linked to the pathogenesis of cancer. In particular, Motins have been shown to play a role in signaling pathways regulated by small G-proteins and the Hippo-YAP pathway. In this review the role of the Motin protein family in these signaling pathways will be described and open questions will be discussed.

Inversin/Nephrocystin-2 is required for fibroblast polarity and directional cell migration

Inversin is a ciliary protein that critically regulates developmental processes and tissue homeostasis in vertebrates, partly through the degradation of Dishevelled (Dvl) proteins to coordinate Wnt signaling in planar cell polarity (PCP). Here, we investigated the role of Inversin in coordinating cell migration, which highly depends on polarity processes at the single-cell level, including the spatial and temporal organization of the cytoskeleton as well as expression and cellular localization of proteins in leading edge formation of migrating cells. Using cultures of mouse embryonic fibroblasts (MEFs) derived from inv(-/-) and inv(+/+) animals, we confirmed that both inv(-/-) and inv(+/+) MEFs form primary cilia, and that Inversin localizes to the primary cilium in inv(+/+) MEFs. In wound healing assays, inv(-/-) MEFs were severely compromised in their migratory ability and exhibited cytoskeletal rearrangements, including distorted lamellipodia formation and cilia orientation. Transcriptome analysis revealed dysregulation of Wnt signaling and of pathways regulating actin organization and focal adhesions in inv(-/-) MEFs as compared to inv(+/+) MEFs. Further, Dvl-1 and Dvl-3 localized to MEF primary cilia, and β-catenin/Wnt signaling was elevated in inv(-/-) MEFs, which moreover showed reduced ciliary localization of Dvl-3. Finally, inv(-/-) MEFs displayed dramatically altered activity and localization of RhoA, Rac1, and Cdc42 GTPases, and aberrant expression and targeting of the Na(+)/H(+) exchanger NHE1 and ezrin/radixin/moesin (ERM) proteins to the edge of cells facing the wound. Phosphorylation of β-catenin at the ciliary base and formation of well-defined lamellipodia with localization and activation of ERM to the leading edge of migrating cells were restored in inv(-/-) MEFs expressing Inv-GFP. Collectively, our findings point to the significance of Inversin in controlling cell migration processes, at least in part through transcriptional regulation of genes involved in Wnt signaling and pathways that control cytoskeletal organization and ion transport.

Coordination of VEGF receptor trafficking and signaling by coreceptors

During development, regeneration and in certain pathological settings, the vasculature is expanded and remodeled substantially. Proper morphogenesis and function of blood vessels are essential in multicellular organisms. Upon stimulation with growth factors including vascular endothelial growth factors (VEGFs), the activation, internalization and sorting of receptor tyrosine kinases (RTKs) orchestrate developmental processes and the homeostatic maintenance of all organs including the vasculature. Previously, RTK signaling was thought to occur exclusively at the plasma membrane, a process that was subsequently terminated by endocytosis and receptor degradation. However, this model turned out to be an oversimplification and there is now a substantial amount of reports indicating that receptor internalization and trafficking to intracellular compartments depends on coreceptors leading to the activation of specific signaling pathways. Here we review the latest findings concerning endocytosis and intracellular trafficking of VEGFRs. The body of evidence is compelling that VEGF receptor trafficking is coordinated with other proteins such as Neuropilin-1, ephrin-B2, VE-cadherin and protein phosphatases.

Novel interaction of Rab13 and Rab8 with endospanins

Rab GTPases regulate vesicular traffic in eukaryotic cells by cycling between the active GTP-bound and inactive GDP-bound states. Their functions are modulated by the diverse selection of effector proteins that bind to specific Rabs in their activated state. We previously described the expression of Rab13 in bone cells. To search for novel Rab13 interaction partners, we screened a newborn rat bone marrow cDNA library for Rab13 effectors with a bacterial two-hybrid system. We found that Rab13 binds to the C-terminus of Endospanin-2, a small transmembrane protein. In addition to Rab13 also Rab8 bound to Endospanin-2, while no binding of Rab7, Rab10, Rab11 or Rab32 was observed. Rab13 and Rab8 also interacted with Endospanin-1, a close homolog of Endospanin-2. Rab13 and Endospanin-2 colocalised in perinuclear vesicular structures in Cos1 cells suggesting direct binding also in vivo. Endospanin-2 is implicated in the regulation of the cell surface growth hormone receptor (GHR), but the inhibition of Rab13 expression did not affect GHR cell surface expression. This suggests that the Rab13–Endospanin-2 interaction may have functions other than GHR regulation. In conclusion, we have identified a novel interaction for Rab13 and Rab8 with Endospanin-2 and Endospanin-1. The role of this interaction in cell physiology, however, remains to be elucidated.

The myosin motor Myo1c is required for VEGFR2 delivery to the cell surface and for angiogenic signaling

Vascular endothelial growth factor receptor-2 (VEGFR2) is a receptor tyrosine kinase that is expressed in endothelial cells and regulates angiogenic signal transduction under both physiological and pathological conditions. VEGFR2 turnover at the plasma membrane (PM) is regulated by its transport through endocytic and secretory transport pathways. Short-range cargo trafficking along actin filaments is commonly regulated by motor proteins of myosin superfamily. In the current study, performed in primary human endothelial cells, we demonstrate that unconventional myosin 1c (Myo1c; class I family member) regulates the localization of VEGFR2 at the PM. We further demonstrate that the recruitment of VEGFR2 to the PM and its colocalization with Myo1c and caveolin-1 occur in response to VEGF-A (VEGF) stimulation. In addition, VEGF-induced delivery of VEGFR2 to the cell surface requires Myo1c; surface VEGFR2 levels are reduced in the absence of Myo1c and, more importantly, are restored by the overexpression of wild-type but not mutant Myo1c. Subcellular density gradient fractionation revealed that partitioning of VEGFR2 into caveolin-1- and Myo1c-enriched membrane fractions is dependent on VEGF stimulation. Myo1c depletion resulted in increased VEGF-induced VEGFR2 transport to the lysosomes for degradation and was rescued by applying either brefeldin A, which blocks trafficking between the endoplasmic reticulum and the Golgi complex, or dynasore, an inhibitor of dynamin-mediated endocytosis. Myo1c depletion also reduced VEGF-induced VEGFR2 phosphorylation at Y1175 and phosphorylation-dependent activation of ERK1/2 and c-Src kinase, leading to reduced cell proliferation and cell migration. This is the first report demonstrating that Myo1c is an important mediator of VEGF-induced VEGFR2 delivery to the cell surface and plays a role in angiogenic signaling.

Membrane traffic as a coordinator of cell migration and junction remodeling

The change in the overall shape of developing organs is a consequence of the cumulative movement, reshaping, and proliferation of the individual mural cells that make up the walls of these organs. Recent observations suggest that the shape and the position of endothelial cells (ECs) in growing blood vessels are highly dynamic, implying that these cells remodel their junctions extensively and do not preserve their initial relative positions. In order to determine the mechanisms that confer the dynamic behavior of mural ECs, we tracked the trafficking of a cell junction protein complex that consists of the RhoA-specific guanine exchange factor (GEF) Syx, the scaffold protein Mupp1, and the phospholipid binding protein Amot.1 We found that RhoA co-trafficked with this complex on the same endocytic vesicles, and that its cellular activity pattern was determined by Rab13-dependent trafficking. The vesicles were targeted by a Rab13-associated protein complex to Tyr(1175)-phosphorylated VEGFR2 at the leading edge of ECs migrating under a VEGF gradient. These results indicate that the dynamic behavior of ECs in sprouting vessels is conferred by using the same protein complex for the regulation of both cell junctions and cell motility. Together with previous studies that demonstrated regulation of Rac signaling by Rab5-dependent trafficking,(2) it appears now that membrane traffic is tightly coupled to the regulation of Rho GTPases, and, consequently, to the regulation of the actin cytoskeleton, cell junctions, and cell migration.

Early life stage trimethyltin exposure induces ADP-ribosylation factor expression and perturbs the vascular system in zebrafish

Trimethyltin chloride (TMT) is an organotin contaminant, widely detected in aqueous environments, posing potential human and environmental risks. In this study, we utilized the zebrafish model to investigate the impact of transient TMT exposure on developmental progression, angiogenesis, and cardiovascular development. Embryos were waterborne exposed to a wide TMT concentration range from 8 to 96 h post fertilization (hpf). The TMT concentration that led to mortality in 50% of the embryos (LC(50)) at 96 hpf was 8.2 μM; malformations in 50% of the embryos (EC(50)) was 2.8 μM. The predominant response observed in surviving embryos was pericardial edema. Additionally, using the Tg (fli1a: EGFP) y1 transgenic zebrafish line to non-invasively monitor vascular development, TMT exposure led to distinct disarrangements in the vascular system. The most susceptible developmental stage to TMT exposure was between 48 and 72 hpf. High density whole genome microarrays were used to identify the early transcriptional changes following TMT exposure from 48 to 60 hpf or 72 hpf. In total, 459 transcripts were differentially expressed at least 2-fold (P<0.05) by TMT compared to control. Using Ingenuity Pathway Analysis (IPA) tools, it was revealed that the transcripts misregulated by TMT exposure were clustered in numerous categories including metabolic and cardiovascular disease, cellular function, cell death, molecular transport, and physiological development. In situ localization of highly elevated transcripts revealed intense staining of ADP-ribosylation factors arf3 and arf5 in the head, trunk, and tail regions. When arf5 expression was blocked by morpholinos, the zebrafish did not display the prototypical TMT-induced vascular deficits, indicating that the induction of arf5 was necessary for TMT-induced vascular toxicity.

Regulation of VEGF signaling by membrane traffic

Recent findings have drawn attention to the role of membrane traffic in the signaling of vascular endothelial growth factor (VEGF). The significance of this development stems from the pivotal function of VEGF in vasculogenesis and angiogenesis. The outline of the regulation of VEGF receptor (VEGFR) signaling by membrane traffic is similar to that of the epidermal growth factor receptor (EGFR), a prototype of the intertwining between membrane traffic and signaling. There are, however, unique features in VEGFR signaling that are conferred in part by the involvement of the co-receptor neuropilin (Nrp). Nrp1 and VEGFR2 are integrated into membrane traffic through the adaptor protein synectin, which recruits myosin VI, a molecular motor that drives inward trafficking [17,21,64]. The recent detection of only mild vascular defects in a knockin mouse model that expresses Nrp1 lacking a cytoplasmic domain [104], questions the co-receptor's role in VEGF signaling and membrane traffic. The regulation of endocytosis by ephrin-B2 is another feature unique to VEGR2/3 [18,19], but it awaits a mechanistic explanation. Current models do not fully explain how membrane traffic bridges between VEGFR and the downstream effectors that produce its functional outcome, such as cell migration. VEGF-A appears to accomplish this task in part by recruiting endocytic vesicles carrying RhoA to internalized active VEGFR2 [58].

RHGF-2 is an essential Rho-1 specific RhoGEF that binds to the multi-PDZ domain scaffold protein MPZ-1 in Caenorhabditis elegans

RhoGEF proteins activate the Rho family of small GTPases and thus play a key role in regulating fundamental cellular processes such as cell morphology and polarity, cell cycle progression and gene transcription. We identified a Caenorhabditis elegans RhoGEF protein, RHGF-2, as a binding partner of the C. elegans multi-PDZ domain scaffold protein MPZ-1 (MUPP1 in mammals). RHGF-2 exhibits significant identity to the mammalian RhoGEFs PLEKHG5/Tech/Syx and contains a class I C-terminal PDZ binding motif (SDV) that interacts most strongly to MPZ-1 PDZ domain eight. RHGF-2 RhoGEF activity is specific to the C. elegans RhoA homolog RHO-1 as determined by direct binding, GDP/GTP exchange and serum response element-driven reporter activity. rhgf-2 is an essential gene since rhgf-2 deletion mutants do not elongate during embryogenesis and hatch as short immobile animals that arrest development. Interestingly, the expression of a functional rhgf-2::gfp transgene appears to be exclusively neuronal and rhgf-2 overexpression results in loopy movement with exaggerated body bends. Transient expression of RHGF-2 in N1E-115 neuroblastoma cells prevents neurite outgrowth similar to constitutive RhoA activation in these cells. Together, these observations indicate neuronally expressed RHGF-2 is an essential RHO-1 specific RhoGEF that binds most strongly to MPZ-1 PDZ domain eight and is required for wild-type C. elegans morphology and growth.

Cell surface dynamics – how Rho GTPases orchestrate the interplay between the plasma membrane and the cortical cytoskeleton

Small GTPases are known to regulate hundreds of cell functions. In particular, Rho family GTPases are master regulators of the cytoskeleton. By regulating actin nucleation complexes, Rho GTPases control changes in cell shape, including the extension and/or retraction of surface protrusions and invaginations. Protrusion and invagination of the plasma membrane also involves the interaction between the plasma membrane and the cortical cytoskeleton. This interplay between membranes and the cytoskeleton can lead to an increase or decrease in the plasma membrane surface area and its tension as a result of the fusion (exocytosis) or internalization (endocytosis) of membranous compartments, respectively. For a long time, the cytoskeleton and plasma membrane dynamics were investigated separately. However, studies from many laboratories have now revealed that Rho GTPases, their modulation of the cytoskeleton, and membrane traffic are closely connected during the dynamic remodeling of the cell surface. Arf- and Rab-dependent exocytosis of specific vesicles contributes to the targeting of Rho GTPases and their regulatory factors to discrete sites of the plasma membrane. Rho GTPases regulate the tethering of exocytic vesicles and modulate their subsequent fusion. They also have crucial roles in the different forms of endocytosis, where they participate in the sorting of membrane domains as well as the sculpting and sealing of membrane flasks and cups. Here, we discuss how cell surface dynamics depend on the orchestration of the cytoskeleton and the plasma membrane by Rho GTPases.

Endothelial development taking shape

Blood vessel development is a vital process during embryonic development, during tissue growth, regeneration and disease processes in the adult. In the past decade researchers have begun to unravel basic molecular mechanisms that regulate the formation of vascular lumen, sprouting angiogenesis, fusion of vessels, and pruning of the vascular plexus. The understanding of the biology of these angiogenic processes is increasingly driven through studies on vascular development at the cellular resolution. Single cell analysis in vivo, advanced genetic tools and the widespread use of powerful animal models combined with improved imaging possibilities are delivering new insights into endothelial cell form, function and behavior angiogenesis. Moreover, the combination of in silico modeling and experimentation including dynamic imaging promotes insights into higher level cooperative behavior leading to functional patterning of vascular networks. Here we summarize recent concepts and advances in the field of vascular development, focusing in detail on the endothelial cell.

Molecular control of endothelial cell behaviour during blood vessel morphogenesis

The vertebrate vasculature forms an extensive branched network of blood vessels that supplies tissues with nutrients and oxygen. During vascular development, coordinated control of endothelial cell behaviour at the levels of cell migration, proliferation, polarity, differentiation and cell-cell communication is critical for functional blood vessel morphogenesis. Recent data uncover elaborate transcriptional, post-transcriptional and post-translational mechanisms that fine-tune key signalling pathways (such as the vascular endothelial growth factor and Notch pathways) to control endothelial cell behaviour during blood vessel sprouting (angiogenesis). These emerging frameworks controlling angiogenesis provide unique insights into fundamental biological processes common to other systems, such as tissue branching morphogenesis, mechanotransduction and tubulogenesis.