Paxillin-ARF GAP signaling and the cytoskeleton

Single-cell mapping of regulatory DNA:Protein interactions

Gene expression is coordinated by a multitude of transcription factors (TFs), whose binding to the genome is directed through multiple interconnected epigenetic signals, including chromatin accessibility and histone modifications. These complex networks have been shown to be disrupted during aging, disease, and cancer. However, profiling these networks across diverse cell types and states has been limited due to the technical constraints of existing methods for mapping DNA:Protein interactions in single cells. As a result, a critical gap remains in understanding where TFs or other chromatin remodelers bind to DNA and how these interactions are perturbed in pathological contexts. To address this challenge, we developed a transformative single-cell immuno-tethering DNA:Protein mapping technology. By coupling a species-specific antibody-binding nanobody to a cytosine base editing enzyme, this approach enables profiling of even weak or transient factor binding to DNA, a task that was previously unachievable in single cells. Thus, our Docking & Deamination followed by sequencing (D&D-seq) technique induces cytosine-to-uracil edits in genomic regions bound by the target protein, offering a novel means to capture DNA:Protein interactions with unprecedented resolution. Importantly, this technique can be seamlessly incorporated into common single-cell multiomics workflows, enabling multimodal analysis of gene regulation in single cells. We tested the ability of D&D-seq to record TF binding both in bulk and at the single-cell level by profiling CTCF and GATA family members, obtaining high specificity and efficiency, with clear identification of TF footprint and signal retention in the targeted cell subpopulations. Furthermore, the deamination reaction showed minimal off-target activity, with high concordance to bulk ChIP-seq reference data. Applied to primary human peripheral blood mononuclear cells (PBMCs), D&D-seq successfully identified CTCF binding sites and enabled integration with advanced machine-learning algorithms for predicting 3D chromatin structure. Furthermore, we integrated D&D-seq with single-cell genotyping to assess the impact of IDH2 mutations on CTCF binding in a human clonal hematopoiesis sample, uncovering altered binding and chromatin co-accessibility patterns in mutant cells. Altogether, D&D-seq represents an important technological advance enabling the direct mapping of TF or chromatin remodeler binding to the DNA in primary human samples, opening new avenues for understanding chromatin and transcriptional regulation in health and disease.

The small molecule inhibitor NAV-2729 has a complex target profile including multiple ADP-ribosylation factor regulatory proteins

The ADP-ribosylation factor (Arf) GTPases and their regulatory proteins are implicated in cancer progression. NAV-2729 was previously identified as a specific inhibitor of Arf6 that reduced progression of uveal melanoma in an orthotopic xenograft. Here, our goal was to assess the inhibitory effects of NAV-2729 on the proliferation of additional cell types. We found NAV-2729 inhibited proliferation of multiple cell lines, but Arf6 expression did not correlate with NAV-2729 sensitivity, and knockdown of Arf6 affected neither cell viability nor sensitivity to NAV-2729. Furthermore, binding to native Arf6 was not detected; however, we determined that NAV-2729 inhibited both Arf exchange factors and Arf GTPase-activating proteins. ASAP1, a GTPase-activating protein linked to cancer progression, was further investigated. We demonstrated that NAV-2729 bound to the PH domain of ASAP1 and changed ASAP1 cellular distribution. However, ASAP1 knockdown did not fully recapitulate the cytoskeletal effects of NAV-2729 nor affect cell proliferation. Finally, our screens identified 48 other possible targets of NAV-2729. These results illustrate the complexities of defining targets of small molecules and identify NAV-2729 as a model PH domain-binding inhibitor.

[A comparison of DNA methylation profiles of blood mononuclear cells in patients with multiple sclerosis in remission and relapse]

OBJECTIVE

To study the whole-genome DNA methylation profiles of peripheral blood mononuclear blood cells (PBMCs) of patients with relapsing-remitting multiple sclerosis (RRMS) in remission and relapse in order to assess the contribution of this epigenetic mechanism of gene expression regulation to the activity of the pathological process.

MATERIAL AND METHODS

Eight patients with RRMS in remission and 6 patients in relapse were included in the study. Methylation levels of DNA CpG sites in PBMCs were analyzed using Infinium HumanMethylation450 BeadChip DNA microarrays.

RESULTS

Seven differentially methylated positions (DMPs) were identified, of which 3 were hypermethylated (cg02981003, cg18486102, cg19533582) and 4 were hypomethylated (cg16814680, cg1964802, cg18584440, cg08291996) during RRMS relapse. Five DMPs are located in protein-coding genes (GPR123, FAIM2, BTNL2, ZNF8, ASAP2), one in microRNA gene (MIR548N), and one in an intergenic region. For all identified DMPs, we observed a change in DNA methylation levels of more than 20% (range 20.2-57.5%). Hierarchical clustering of DNA samples on the heatmap shows their clear aggregation into separate clusters corresponding to RRMS patients in the stages of relapse and remission.

CONCLUSION

For the first time it was shown that during relapse and remission of RRMS there are differences in the DNA methylation profile that allow discrimination between these clinical stages. These data indicate the involvement of the epigenetic mechanism of DNA methylation in the activation of the pathological process in RRMS.

Programmed death ligand 1 intracellular interactions with STAT3 and focal adhesion protein Paxillin facilitate lymphatic endothelial cell remodeling

Lymphatic endothelial cells (LECs) comprise lymphatic capillaries and vessels that guide immune cells to lymph nodes (LNs) and form the subcapsular sinus and cortical and medullary lymphatic structures of the LN. During an active immune response, the lymphatics remodel to accommodate the influx of immune cells from the tissue, but factors involved in remodeling are unclear. Here, we determined that a TSS motif within the cytoplasmic domain of programmed death ligand 1 (PD-L1), expressed by LECs in the LN, participates in lymphatic remodeling. Mutation of the TSS motif to AAA does not affect surface expression of PD-L1, but instead causes defects in LN cortical and medullary lymphatic organization following immunostimulant, Poly I:C, administration in vivo. Supporting this observation, in vitro treatment of the LEC cell line, SVEC4-10, with cytokines TNFα and IFNα significantly impeded SVEC4-10 movement in the presence of the TSS-AAA cytoplasmic mutation. The cellular movement defects coincided with reduced F-actin polymerization, consistent with differences previously found in dendritic cells. Here, in addition to loss of actin polymerization, we define STAT3 and Paxillin as important PD-L1 binding partners. STAT3 and Paxillin were previously demonstrated to be important at focal adhesions for cellular motility. We further demonstrate the PD-L1 TSS-AAA motif mutation reduced the amount of pSTAT3 and Paxillin bound to PD-L1 both before and after exposure to TNFα and IFNα. Together, these findings highlight PD-L1 as an important component of a membrane complex that is involved in cellular motility, which leads to defects in lymphatic organization.

The Arf-GAP Proteins AoGcs1 and AoGts1 Regulate Mycelial Development, Endocytosis, and Pathogenicity in Arthrobotrys oligospora

Small GTPases from the ADP-ribosylation factor (Arf) family and their activating proteins (Arf-GAPs) regulate mycelial development, endocytosis, and virulence in fungi. Here, we identified two orthologous Arf-GAP proteins, AoGcs1 and AoGts1, in a typical nematode-trapping fungus Arthrobotrys oligospora. The transcription of Aogcs1 and Aogts1 was highly expressed in the sporulation stage. The deletion of Aogcs1 and Aogts1 caused defects in DNA damage, endocytosis, scavenging of reactive oxygen species, lipid droplet storage, mitochondrial activity, autophagy, serine protease activity, and the response to endoplasmic reticulum stress. The combined effects resulted in slow growth, decreased sporulation capacity, increased susceptibility to chemical stressors and heat shock, and decreased pathogenicity of the mutants compared with the wild-type (WT) strain. Although deletion of Aogcs1 and Aogts1 produced similar phenotfypic traits, their roles varied in conidiation and proteolytic activity. The ΔAogts1 mutant showed a remarkable reduction in conidial yield compared with the WT strain but not in proteolytic activity; in contrast, the ΔAogcs1 mutant showed an increase in proteolytic activity but not in sporulation. In addition, the growth of ΔAogcs1 and ΔAogts1 mutants was promoted by rapamycin, and the ΔAogts1 mutant was sensitive to H-89. Collectively, the ΔAogts1 mutant showed a more remarkable difference compared with the WT strain than the ΔAogcs1 mutant. Our study further illustrates the importance of Arf-GAPs in the growth, development, and pathogenicity of nematode-trapping fungi.

PMP2/FABP8 induces PI(4,5)P2-dependent transbilayer reorganization of sphingomyelin in the plasma membrane

Sphingomyelin (SM) is a mammalian lipid mainly distributed in the outer leaflet of the plasma membrane (PM). We show that peripheral myelin protein 2 (PMP2), a member of the fatty-acid-binding protein (FABP) family, can localize at the PM and controls the transbilayer distribution of SM. Genetic screening with genome-wide small hairpin RNA libraries identifies PMP2 as a protein involved in the transbilayer movement of SM. A biochemical assay demonstrates that PMP2 is a phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂)-binding protein. PMP2 induces the tubulation of model membranes in a PI(4,5)P₂-dependent manner, accompanied by the modification of the transbilayer membrane distribution of lipids. In the PM of PMP2-overexpressing cells, inner-leaflet SM is increased whereas outer-leaflet SM is reduced. PMP2 is a causative protein of Charcot-Marie-Tooth disease (CMT). A mutation in PMP2 associated with CMT increases its affinity for PI(4,5)P₂, inducing membrane tubulation and the subsequent transbilayer movement of lipids.

MiR-216b regulates the tumorigenesis of gastric cancer by targeting PXN

BACKGROUND

Accumulating evidence has demonstrated that microRNAs (miRNAs) are associated with tumorigenesis. miR-216b can play a vital role in the genesis and development of gastric cancer (GC), and its molecular mechanisms require further elucidation.

METHODS

The biological effects of miR-216b in GC cells were investigated by MTT, transwell assays, and cell cycle. Western blot and luciferase assay were performed to demonstrate the direct binding of miR-216b on PXN 3'UTR. Furthermore, MTT, colony formation assays, transwell assays, and flow cytometry analysis, as well as xenograft mice model, were used to measure the effects of miR-216b-PXN on GC cell proliferation, migration, and invasion indicated by in vitro and in vivo.

RESULTS

Our results showed that miR-216b acted as a tumor suppressor in GC progression. miR-216b overexpression suppressed GC cell proliferation, migration, and invasion in vitro. Luciferase reporter assays identified paxillin (PXN) as a novel target gene of miR-216b. PXN overexpression could partially rescue miR-216b-induced the inhibitory effects in GC cells. Besides, overexpression of miR-216b contributed to the activation of PI3K/AKT signaling via partly regulating PXN in GC cells.

CONCLUSIONS

The above results showed that miR-216b could offer a novel therapeutic avenue by targeting PXN in GC.

On the adhesion-velocity relation and length adaptation of motile cells on stepped fibronectin lanes

The biphasic adhesion-velocity relation is a universal observation in mesenchymal cell motility. It has been explained by adhesion-promoted forces pushing the front and resisting motion at the rear. Yet, there is little quantitative understanding of how these forces control cell velocity. We study motion of MDA-MB-231 cells on microlanes with fields of alternating Fibronectin densities to address this topic and derive a mathematical model from the leading-edge force balance and the force-dependent polymerization rate. It reproduces quantitatively our measured adhesion-velocity relation and results with keratocytes, PtK1 cells, and CHO cells. Our results confirm that the force pushing the leading-edge membrane drives lamellipodial retrograde flow. Forces resisting motion originate along the whole cell length. All motion-related forces are controlled by adhesion and velocity, which allows motion, even with higher Fibronectin density at the rear than at the front. We find the pathway from Fibronectin density to adhesion structures to involve strong positive feedbacks. Suppressing myosin activity reduces the positive feedback. At transitions between different Fibronectin densities, steady motion is perturbed and leads to changes of cell length and front and rear velocity. Cells exhibit an intrinsic length set by adhesion strength, which, together with the length dynamics, suggests a spring-like front-rear interaction force. We provide a quantitative mechanistic picture of the adhesion-velocity relation and cell response to adhesion changes integrating force-dependent polymerization, retrograde flow, positive feedback from integrin to adhesion structures, and spring-like front-rear interaction.

Quantitative proteomics revealed modulation of macrophages by MetQ gene of Streptococcus suis serotype 2

Streptococcus suis serotype 2 (SS2) is a serious zoonotic pathogen; it can lead to symptoms of streptococcal toxic shock syndrome (STSS) in humans and sepsis in pigs, and poses a great threat to public health. The SS2 MetQ gene deletion strain has attenuated antiphagocytosis, although the mechanism of antiphagocytosis and pathogenesis of MetQ in SS2 has remained unclear. In this study, stable isotope labeling by amino acids in cell culture (SILAC) based liquid chromatography–mass spectrometry (LC–MS) and subsequent bioinformatics analysis was used to determine differentially expressed proteins of RAW264.7 cells infected with △MetQ and ZY05719. Proteomic results were verified by quantitative real-time polymerase chain reaction (qRT-PCR) and Western blotting for selected proteins. Further research was focused mainly on immune system processes related to downregulated proteins, such as Src and Ccl9, and actin cytoskeleton and endocytosis related upregulated proteins, like Pstpip1 and Ppp1r9b. The proteomic results in this study shed light on the mechanism of antiphagocytosis and innate immunity of macrophages infected with △MetQ and ZY05719, which might provide novel targets to prevent or control the infection of SS2.

A Begomovirus Nuclear Shuttle Protein-Interacting Immune Hub: Hijacking Host Transport Activities and Suppressing Incompatible Functions

Begomoviruses (Geminiviridae family) represent a severe constraint to agriculture worldwide. As ssDNA viruses that replicate in the nuclei of infected cells, the nascent viral DNA has to move to the cytoplasm and then to the adjacent cell to cause disease. The begomovirus nuclear shuttle protein (NSP) assists the intracellular transport of viral DNA from the nucleus to the cytoplasm and cooperates with the movement protein (MP) for the cell-to-cell translocation of viral DNA to uninfected cells. As a facilitator of intra- and intercellular transport of viral DNA, NSP is predicted to associate with host proteins from the nuclear export machinery, the intracytoplasmic active transport system, and the cell-to-cell transport complex. Furthermore, NSP functions as a virulence factor that suppresses antiviral immunity against begomoviruses. In this review, we focus on the protein-protein network that converges on NSP with a high degree of centrality and forms an immune hub against begomoviruses. We also describe the compatible host functions hijacked by NSP to promote the nucleocytoplasmic and intracytoplasmic movement of viral DNA. Finally, we discuss the NSP virulence function as a suppressor of the recently described NSP-interacting kinase 1 (NIK1)-mediated antiviral immunity. Understanding the NSP-host protein-protein interaction (PPI) network will probably pave the way for strategies to generate more durable resistance against begomoviruses.

ARF GTPases and their GEFs and GAPs: concepts and challenges

Detailed structural, biochemical, cell biological, and genetic studies of any gene/protein are required to develop models of its actions in cells. Studying a protein family in the aggregate yields additional information, as one can include analyses of their coevolution, acquisition or loss of functionalities, structural pliability, and the emergence of shared or variations in molecular mechanisms. An even richer understanding of cell biology can be achieved through evaluating functionally linked protein families. In this review, we summarize current knowledge of three protein families: the ARF GTPases, the guanine nucleotide exchange factors (ARF GEFs) that activate them, and the GTPase-activating proteins (ARF GAPs) that have the ability to both propagate and terminate signaling. However, despite decades of scrutiny, our understanding of how these essential proteins function in cells remains fragmentary. We believe that the inherent complexity of ARF signaling and its regulation by GEFs and GAPs will require the concerted effort of many laboratories working together, ideally within a consortium to optimally pool information and resources. The collaborative study of these three functionally connected families (≥70 mammalian genes) will yield transformative insights into regulation of cell signaling.

The overexpression of PXN promotes tumor progression and leads to radioresistance in cervical cancer

AIM

We aim to investigate the functions of PXN in cervical cancer.

MATERIALS & METHODS

PXN protein was investigated by immunohistochemistry in a panel of cervical cancer. A series of in vitro and in vivo assays were used to explore the efficacy of PXN.

RESULTS

PXN was significantly upregulated in cervical cancer, which associated with tumor stage, poor differentiation, lymphovascular space invasion and lymphatic metastasis. Knockdown of PXN notably impaired cellular growth and colony formation by suppressing Bcl-2 and inducing marked apoptosis. Moreover, PXN led to resistance to radiation, and downregulation of PXN resensitized C33A cells to radiation.

CONCLUSION

PXN was frequently upregulated and acted as an oncogene via regulating Bcl-2 in cervical cancer, which supports PXN as a potent therapeutic target.

Molecular Regulation of Sprouting Angiogenesis

The term angiogenesis arose in the 18th century. Several studies over the next 100 years laid the groundwork for initial studies performed by the Folkman laboratory, which were at first met with some opposition. Once overcome, the angiogenesis field has flourished due to studies on tumor angiogenesis and various developmental models that can be genetically manipulated, including mice and zebrafish. In addition, new discoveries have been aided by the ability to isolate primary endothelial cells, which has allowed dissection of various steps within angiogenesis. This review will summarize the molecular events that control angiogenesis downstream of biochemical factors such as growth factors, cytokines, chemokines, hypoxia-inducible factors (HIFs), and lipids. These and other stimuli have been linked to regulation of junctional molecules and cell surface receptors. In addition, the contribution of cytoskeletal elements and regulatory proteins has revealed an intricate role for mobilization of actin, microtubules, and intermediate filaments in response to cues that activate the endothelium. Activating stimuli also affect various focal adhesion proteins, scaffold proteins, intracellular kinases, and second messengers. Finally, metalloproteinases, which facilitate matrix degradation and the formation of new blood vessels, are discussed, along with our knowledge of crosstalk between the various subclasses of these molecules throughout the text. Compr Physiol 8:153-235, 2018.

Arf GAPs and molecular motors

Arf GTPase-activating proteins (Arf GAPs) were first identified as regulators of the small GTP-binding proteins ADP-ribosylation factors (Arfs). The Arf GAPs are a large family of proteins in metazoans, outnumbering the Arfs that they regulate. The members of the Arf GAP family have complex domain structures and some have been implicated in particular cellular functions, such as cell migration, or with particular pathologies, such as tumor invasion and metastasis. The specific effects of Arfs sometimes depend on the Arf GAP involved in their regulation. These observations have led to speculation that the Arf GAPs themselves may affect cellular activities in capacities beyond the regulation of Arfs. Recently, 2 Arf GAPs, ASAP1 and AGAP1, have been found to bind directly to and influence the activity of myosins and kinesins, motor proteins associated with filamentous actin and microtubules, respectively. The Arf GAP-motor protein interaction is critical for cellular behaviors involving the actin cytoskeleton and microtubules, such as cell migration and other cell movements. Arfs, then, may function with molecular motors through Arf GAPs to regulate microtubule and actin remodeling.

Recombinant Slit2 Reduces Surgical Brain Injury Induced Blood Brain Barrier Disruption via Robo4 Dependent Rac1 Activation in a Rodent Model

Brain tissue surrounding surgical resection site can be injured inadvertently due to procedures such as incision, retractor stretch, and electrocauterization when performing neurosurgical procedures, which is termed as surgical brain injury (SBI). Blood brain barrier (BBB) disruption due to SBI can exacerbate brain edema in the post-operative period. Previous studies showed that Slit2 exhibited vascular anti-permeability effects outside the brain. However, BBB protective effects of Slit2 following SBI has not been evaluated. The objective of this study was to evaluate whether recombinant Slit2 via its receptor roundabout4 (Robo4) and the adaptor protein, Paxillin were involved in reducing BBB permeability in SBI rat model. Our results showed that endogenous Slit2 increased in the surrounding peri-resection brain tissue post-SBI, Robo4 remained unchanged and Paxillin showed a decreasing trend. Recombinant Slit2 administered 1 h before injury increased BBB junction proteins, reduced BBB permeability, and decreased neurodeficits 24 h post-SBI. Furthermore, recombinant Slit2 administration increased Rac1 activity which was reversed by Robo4 and Paxillin siRNA. Our findings suggest that recombinant Slit2 reduced SBI-induced BBB permeability, possibly by stabilizing BBB tight junction via Robo4 mediated Rac1 activation. Slit2 may be beneficial for BBB protection during elective neurosurgeries.

Arf GAPs: A family of proteins with disparate functions that converge on a common structure, the integrin adhesion complex

ADP-ribosylation factors (Arfs) are members of the Ras GTPase superfamily. The function of Arfs is dependent on GTPase-activating proteins (GAPs) and guanine nucleotide exchange factors (GEFs), which allow Arfs to cycle between the GDP-bound and GTP-bound forms. Arf GAPs have been shown to be present in integrin adhesion complexes, which include focal adhesions. Integrin adhesion complexes are composed of integrins, scaffolding proteins and signaling proteins and regulate cell proliferation, survival, differentiation and migration. Understanding the role of Arf GAPs in the regulation of integrin adhesion complexes is relevant to understanding normal physiology and cancer. In this review, we will discuss the contribution of the Arf GAP family members to the regulation of integrin adhesion complexes, examining the diverse mechanisms by which they control integrin adhesion complex formation, maturation and dissolution. GIT1 and ARAP2 serve as GAPs for Arf6, regulating Rac1 and other effectors by mechanisms still being defined. In contrast, GIT2 regulates Rac1 independent of Arf6. AGAP2 binds to and regulates focal adhesion kinase (FAK). ARAP2 and ACAP1, both Arf6 GAPs, regulate membrane trafficking of integrins through different endocytic pathways, exerting opposite effects on focal adhesions. ASAP1 not only regulates actin cytoskeleton remodeling through its interaction with nonmuscle myosin 2A, but is also important in integrin recycling. These examples illustrate the diversity and versatility of Arf GAPs as regulators of integrin adhesion complex structure and function.

Multiparametric Analysis of Cell Shape Demonstrates that β-PIX Directly Couples YAP Activation to Extracellular Matrix Adhesion

Mechanical signals from the extracellular matrix (ECM) and cellular geometry regulate the nuclear translocation of transcriptional regulators such as Yes-associated protein (YAP). Elucidating how physical signals control the activity of mechanosensitive proteins poses a technical challenge, because perturbations that affect cell shape may also affect protein localization indirectly. Here, we present an approach that mitigates confounding effects of cell-shape changes, allowing us to identify direct regulators of YAP localization. This method uses single-cell image analysis and statistical models that exploit the naturally occurring heterogeneity of cellular populations. Through systematic depletion of all human kinases, Rho family GTPases, GEFs, and GTPase activating proteins (GAPs), together with targeted chemical perturbations, we found that β-PIX, a Rac1/Ccd42 GEF, and PAK2, a Rac1/Cdc42 effector, drive both YAP activation and cell-ECM adhesion turnover during cell spreading. Our observations suggest that coupling YAP to adhesion dynamics acts as a mechano-timer, allowing cells to rapidly tune gene expression in response to physical signals.

GIT1 enhances neurite outgrowth by stimulating microtubule assembly

GIT1, a G-protein-coupled receptor kinase interacting protein, has been reported to be involved in neurite outgrowth. However, the neurobiological functions of the protein remain unclear. In this study, we found that GIT1 was highly expressed in the nervous system, and its expression was maintained throughout all stages of neuritogenesis in the brain. In primary cultured mouse hippocampal neurons from GIT1 knockout mice, there was a significant reduction in total neurite length per neuron, as well as in the average length of axon-like structures, which could not be prevented by nerve growth factor treatment. Overexpression of GIT1 significantly promoted axon growth and fully rescued the axon outgrowth defect in the primary hippocampal neuron cultures from GIT1 knockout mice. The GIT1 N terminal region, including the ADP ribosylation factor-GTPase activating protein domain, the ankyrin domains and the Spa2 homology domain, were sufficient to enhance axonal extension. Importantly, GIT1 bound to many tubulin proteins and microtubule-associated proteins, and it accelerated microtubule assembly in vitro. Collectively, our findings suggest that GIT1 promotes neurite outgrowth, at least partially by stimulating microtubule assembly. This study provides new insight into the cellular and molecular pathogenesis of GIT1-associated neurological diseases.

Geometry sensing through POR1 regulates Rac1 activity controlling early osteoblast differentiation in response to nanofiber diameter

Bone grafting procedures in the United States rely heavily upon autografts and allografts, which are donor-dependent, cause donor site pain, and can transmit disease. Synthetic bone grafts can reduce these risks; however, synthetics lack the bone differentiating (osteoinductive) abilities of auto- and allografts. Achieving innate osteoinductive properties of synthetics through surface modifications is currently under investigation. This study focuses on nanofibers, with emphasis on how fiber diameter and the potential curvature sensor POR1 affect the activation of the signaling molecules Rac1 and Arf1, and leading to expression of alkaline phosphatase (ALP), an osteoinductive marker. Diameters of 0.1, 0.3, and 1.0 μm were compared against a flat control. The highest level of Rac1 activation was achieved on the smallest fibers (0.1 μm), a trend that was lost in POR1 knockdowns. This supports the hypothesis that on small nanofibers, POR1 favorably binds to highly curved cell membranes, which allows Rac1 to subsequently dissociate and activate. When the curvature is insufficient to bind POR1, POR1 binds to inactive Rac1 and competitively inhibits its activation. Arf1 activation followed an opposite trend, with the largest nanofibers exhibiting the highest activity. This trend reinforces the known interaction between Rac1 and Arf1 through the GIT-PIX complex, an Arf1 GAP and Rac1 GEF, respectively. Large, (1.0 μm), nanofibers demonstrated the highest ALP activity, indicating that ALP expression is inversely dependent on Rac1 activation. Knockdown of POR1 resulted in increased ALP activity across the substrates but without regard to the curvature sensing trend seen previously. Thus, POR1 senses curvature and increases Rac1 activity, which negatively regulates bone differentiation.

Signaling networks that regulate cell migration

SUMMARY

Stimuli that promote cell migration, such as chemokines, cytokines, and growth factors in metazoans and cyclic AMP in Dictyostelium, activate signaling pathways that control organization of the actin cytoskeleton and adhesion complexes. The Rho-family GTPases are a key convergence point of these pathways. Their effectors include actin regulators such as formins, members of the WASP/WAVE family and the Arp2/3 complex, and the myosin II motor protein. Pathways that link to the Rho GTPases include Ras GTPases, TorC2, and PI3K. Many of the molecules involved form gradients within cells, which define the front and rear of migrating cells, and are also established in related cellular behaviors such as neuronal growth cone extension and cytokinesis. The signaling molecules that regulate migration can be integrated to provide a model of network function. The network displays biochemical excitability seen as spontaneous waves of activation that propagate along the cell cortex. These events coordinate cell movement and can be biased by external cues to bring about directed migration.

How to find a leucine in a haystack? Structure, ligand recognition and regulation of leucine-aspartic acid (LD) motifs

LD motifs (leucine-aspartic acid motifs) are short helical protein-protein interaction motifs that have emerged as key players in connecting cell adhesion with cell motility and survival. LD motifs are required for embryogenesis, wound healing and the evolution of multicellularity. LD motifs also play roles in disease, such as in cancer metastasis or viral infection. First described in the paxillin family of scaffolding proteins, LD motifs and similar acidic LXXLL interaction motifs have been discovered in several other proteins, whereas 16 proteins have been reported to contain LDBDs (LD motif-binding domains). Collectively, structural and functional analyses have revealed a surprising multivalency in LD motif interactions and a wide diversity in LDBD architectures. In the present review, we summarize the molecular basis for function, regulation and selectivity of LD motif interactions that has emerged from more than a decade of research. This overview highlights the intricate multi-level regulation and the inherently noisy and heterogeneous nature of signalling through short protein-protein interaction motifs.

ASAP3 expression in non-small cell lung cancer: association with cancer development and patients' clinical outcome

ASAP3 belongs to Arf-specific GTPase-activating proteins which regulate Arfs by stimulating their intrinsic GTP hydrolysis. ASAP3 expression and the clinical significance in malignant tumors are largely unknown. In this study, we examined ASAP3 expression in non-small cell lung cancer (NSCLC) to find out its clinicopathological significance. Immunohistochemistry shows elevated expression of ASAP3 in cancer tissues (54.8 % (57/104)) compared to normal lung tissues (18.0 % (9/50)) (p < 0.05). Increased ASAP3 expression was associated with poor differentiation, lymph node metastasis, and advanced TNM stages in NSCLC (p < 0.05). Survival analysis reveals that ASAP3 expression contributes to patients' poor clinical outcome (p < 0.05). We also examined ASAP3 expression in several lung cancer cell lines using Western blotting. We downregulated ASAP3 expression in LTE cell which has a relative high level of ASAP3 expression using siRNA and found that reduced ASAP3 leads to significant inhibition of cancer cell invasion (p < 0.05). These data indicate that ASAP3 is elevated in NSCLC and may contribute to cancer development and patients' poor clinical outcome, which is possibly due to its critical roles in regulating cancer invasion.

Preconditioning effects of physiological cyclic stretch on pathologically mechanical stretch-induced alveolar epithelial cell apoptosis and barrier dysfunction

BACKGROUND

We aim to investigate the effects of preconditioning of physiological cyclic stretch on the alveolar epithelial cell apoptosis induced by pathologically mechanical stretch and barrier dysfunction and how these effects are linked to differential expression of small GTPases Rac and Rho mRNA.

METHODS

Pulmonary alveolar epithelial cells were subjected to different treatments of cyclic stretch (CS) at 5% and 20% elongation, respectively. Cells maintained in normal cell culture were used as negative control. On the other hand, cell apoptosis and Rac/Rho activities in cells with or without preconditioning of physiologically relevant magnitudes of CS (5% CS) with different durations (0, 15, 30, 60 and 120 min) in prior to 6-h treatment with pathological CS stimulation (20% CS) were compared and measured.

RESULTS

Pathological CS could cause a significant increase in apoptosis rate, which is considered to be associated with the repression of Rac mRNA and activation of Rho mRNA. In contrast, physiological 5%-CS preconditioning suppressed cell apoptosis and induced nearly complete monolayer recovery with fewer actin stress fibers and paracellular gap formation. Consistent with differential effects on cell apoptosis and epithelial cell integrity, physiological CS preconditioning enhanced expression of Rac mRNA but inhibited Rho activation.

CONCLUSIONS

Physiological CS preconditioning has an inhibitory effect on cell apoptosis while exerts a stimulatory impact on epithelial cell recovery via regulation of Rac and Rho activities.

Filamin A regulates neuronal migration through brefeldin A-inhibited guanine exchange factor 2-dependent Arf1 activation

Periventricular heterotopias is a malformation of cortical development, characterized by ectopic neuronal nodules around ventricle lining and caused by an initial migration defect during early brain development. Human mutations in the Filamin A (FLNA) and ADP-ribosylation factor guanine exchange factor 2 [ARFGEF2; encoding brefeldin-A-inhibited guanine exchange factor-2 (BIG2)] genes give rise to this disorder. Previously, we have reported that Big2 inhibition impairs neuronal migration and binds to FlnA, and its loss promotes FlnA phosphorylation. FlnA phosphorylation dictates FlnA-actin binding affinity and consequently alters focal adhesion size and number to effect neuronal migration. Here we show that FlnA loss similarly impairs migration, reciprocally enhances Big2 expression, but also alters Big2 subcellular localization in both null and conditional FlnA mice. FlnA phosphorylation promotes relocalization of Big2 from the Golgi toward the lipid ruffles, thereby activating Big2-dependent Arf1 at the cell membrane. Loss of FlnA phosphorylation or Big2 function impairs Arf1-dependent vesicle trafficking at the periphery, and Arf1 is required for maintenance of cell-cell junction connectivity and focal adhesion assembly. Loss of Arf1 activity disrupts neuronal migration and cell adhesion. Collectively, these studies demonstrate a potential mechanism whereby coordinated interactions between actin (through FlnA) and vesicle trafficking (through Big2-Arf) direct the assembly and disassembly of membrane protein complexes required for neuronal migration and neuroependymal integrity.

Barrier enhancing signals in pulmonary edema

Increased endothelial permeability and reduction of alveolar liquid clearance capacity are two leading pathogenic mechanisms of pulmonary edema, which is a major complication of acute lung injury, severe pneumonia, and acute respiratory distress syndrome, the pathologies characterized by unacceptably high rates of morbidity and mortality. Besides the success in protective ventilation strategies, no efficient pharmacological approaches exist to treat this devastating condition. Understanding of fundamental mechanisms involved in regulation of endothelial permeability is essential for development of barrier protective therapeutic strategies. Ongoing studies characterized specific barrier protective mechanisms and identified intracellular targets directly involved in regulation of endothelial permeability. Growing evidence suggests that, although each protective agonist triggers a unique pattern of signaling pathways, selected common mechanisms contributing to endothelial barrier protection may be shared by different barrier protective agents. Therefore, understanding of basic barrier protective mechanisms in pulmonary endothelium is essential for selection of optimal treatment of pulmonary edema of different etiology. This article focuses on mechanisms of lung vascular permeability, reviews major intracellular signaling cascades involved in endothelial monolayer barrier preservation and summarizes a current knowledge regarding recently identified compounds which either reduce pulmonary endothelial barrier disruption and hyperpermeability, or reverse preexisting lung vascular barrier compromise induced by pathologic insults.

Quantitative phosphoproteome analysis unveils LAT as a modulator of CD3ζ and ZAP-70 tyrosine phosphorylation

Signaling through the T cell receptor (TCR) initiates adaptive immunity and its perturbation may results in autoimmunity. The plasma membrane scaffolding protein LAT acts as a central organizer of the TCR signaling machinery to activate many functional pathways. LAT-deficient mice develop an autoimmune syndrome but the mechanism of this pathology is unknown. In this work we have compared global dynamics of TCR signaling by MS-based quantitative phosphoproteomics in LAT-sufficient and LAT-defective Jurkat T cells. Surprisingly, we found that many TCR-induced phosphorylation events persist in the absence of LAT, despite ERK and PLCγ1 phosphorylation being repressed. Most importantly, the absence of LAT resulted in augmented and persistent tyrosine phosphorylation of CD3ζ and ZAP70. This indicates that LAT signaling hub is also implicated in negative feedback signals to modulate upstream phosphorylation events. Phosphorylation kinetics data resulting from this investigation is documented in a database (phosphoTCR) accessible online. The MS data have been deposited to the ProteomeXchange with identifier PXD000341.

Identification of paxillin domains interacting with β-catenin

Barrier-protective agonists induce association of focal adhesions (FA) and adherens junctions (AJ) in endothelial cells. Here we identified specific domains of FA protein paxillin interacting with AJ protein and examined regulation of paxillin domain interactions with β-catenin by Rac GTPase. Co-expression of paxillin LD-1,2; LD-3,4; LIM-1,2; and LIM-3,4 domains with β-catenin showed exclusive interaction of LIM-1,2 and LIM-3,4 with β-catenin, which was enhanced by agonist-induced Rac activation or expression of activated Rac mutant. These results demonstrate a novel function of paxillin LIM domains in targeting β-catenin in a Rac-dependent manner, which may play a role in Rac-dependent control of FA-AJ interactions and monolayer integrity.

Heterotrimeric G proteins, focal adhesion kinase, and endothelial barrier function

Ligands by binding to G protein coupled receptors (GPCRs) stimulate dissociation of heterotrimeric G proteins into Gα and Gβγ subunits. Released Gα and Gβγ subunits induce discrete signaling cues that differentially regulate focal adhesion kinase (FAK) activity and endothelial barrier function. Activation of G proteins downstream of receptors such as protease activated receptor 1 (PAR1) and histamine receptors rapidly increases endothelial permeability which reverses naturally within the following 1-2 h. However, activation of G proteins coupled to the sphingosine-1-phosphate receptor 1 (S1P1) signal cues that enhance basal barrier endothelial function and restore endothelial barrier function following the increase in endothelial permeability by edemagenic agents. Intriguingly, both PAR1 and S1P1 activation stimulates FAK activity, which associates with alteration in endothelial barrier function by these agonists. In this review, we focus on the role of the G protein subunits downstream of PAR1 and S1P1 in regulating FAK activity and endothelial barrier function.

Role of FAK in S1P-regulated endothelial permeability

The vascular endothelium serves as a semi-selective barrier between the circulating contents of the blood and the tissues through which they flow. Disruption of this barrier results in significant organ dysfunction during devastating inflammatory syndromes such as sepsis and acute lung injury (ALI). Sphingosine 1-phosphate (S1P) is an endogenous lipid regulator of endothelial permeability that produces potent barrier enhancement via actin and junctional protein rearrangement and resultant cytoskeletal changes. A key effector protein in this S1P response is focal adhesion kinase (FAK), a highly conserved cytoplasmic tyrosine kinase involved in the engagement of integrins and assembly of focal adhesions (FA) through the catalysis of multiple downstream signals. After stimulation by S1P, endothelial FAK undergoes specific tyrosine phosphorylation that results in activation of the kinase and dynamic interactions with other effector molecules to improve the endothelial barrier. FAK participates in peripheral actin cytoskeletal rearrangement as well as cell-matrix (FA) and cell-cell (adherens junction) junctional complex strengthening that combine to decrease vascular permeability. This review summarizes the current knowledge of the role of FAK in mediating enhanced endothelial barrier function by S1P.

High-Throughput Fluorescence Polarization Assay for the Enzymatic Activity of GTPase-Activating Protein of ADP-Ribosylation Factor (ARFGAP)

GTPase-activating proteins of ADP-ribosylation factors (ARFGAPs) play key cellular roles in vesicle production and trafficking, adhesion, migration, and development. Dysfunctional regulation of ARFGAPs has been implicated in various diseases, including cancer, Alzheimer disease, and autism. Unfortunately, there are few mechanistic details describing how ARFGAPs contribute to disease states. In this regard, it would be extremely helpful to have a set of small molecules that selectively and directly modulate specific ARFGAPs as probes to dissect ARFGAP-regulated cell signaling under various conditions. Currently, such probes are lacking, and their identification is hampered by the lack of a suitable high-throughput assay to monitor ARFGAP activity. Here, the authors describe and validate a robust high-throughput assay using fluorescence polarization to monitor the ability of diverse ARFGAPs to enhance the capacity of ARF1 to hydrolyze guanosine triphosphate.

Paxillin and phospholipase D interact to regulate actin-based processes in Dictyostelium discoideum

The actin cytoskeleton forms a membrane-associated network whose proper regulation is essential for numerous processes, including cell differentiation, proliferation, adhesion, chemotaxis, endocytosis, exocytosis, and multicellular development. In this report, we show that in Dictyostelium discoideum, paxillin (PaxB) and phospholipase D (PldB) colocalize and coimmunoprecipitate, suggesting that they interact physically. Additionally, the phenotypes observed during development, cell sorting, and several actin-required processes, including cyclic AMP (cAMP) chemotaxis, cell-substrate adhesion, actin polymerization, phagocytosis, and exocytosis, reveal a genetic interaction between paxB and pldB, suggesting a functional interaction between their gene products. Taken together, our data point to PldB being a required binding partner of PaxB during processes involving actin reorganization.

HIV immune evasion disruption of antigen presentation by the HIV Nef protein

The Human Immunodeficiency Virus (HIV) Nef protein is necessary for high viral loads and for timely progression to AIDS. Nef plays a number of roles, but its effect on antigen presentation and immune evasion are among the best characterized. Cytotoxic T lymphocytes (CTLs) recognize and lyse virally infected cells by detecting viral antigens in complex with host major histocompatibility complex class I (MHC-I) molecules on the infected cell surface. The HIV Nef protein disrupts antigen presentation at the cell surface by interfering with the normal trafficking pathway of MHC-I and thus reduces CTL recognition and lysis of infected cells. The molecular mechanism by which Nef causes MHC-I downmodulation is becoming more clear, but some questions remain. A better understanding of how Nef disrupts antigen presentation may lead to the development of drugs that enhance the ability of the anti-HIV CTLs to control HIV disease.

Systems microscopy approaches to understand cancer cell migration and metastasis

Cell migration is essential in a number of processes, including wound healing, angiogenesis and cancer metastasis. Especially, invasion of cancer cells in the surrounding tissue is a crucial step that requires increased cell motility. Cell migration is a well-orchestrated process that involves the continuous formation and disassembly of matrix adhesions. Those structural anchor points interact with the extra-cellular matrix and also participate in adhesion-dependent signalling. Although these processes are essential for cancer metastasis, little is known about the molecular mechanisms that regulate adhesion dynamics during tumour cell migration. In this review, we provide an overview of recent advanced imaging strategies together with quantitative image analysis that can be implemented to understand the dynamics of matrix adhesions and its molecular components in relation to tumour cell migration. This dynamic cell imaging together with multiparametric image analysis will help in understanding the molecular mechanisms that define cancer cell migration.

The WW-HECT protein Smurf2 interacts with the Docking Protein NEDD9/HEF1 for Aurora A activation

The multi-functional adaptor protein NEDD9/HEF1/Cas-L regulates cell motility, invasion and cell cycle progression, and plays key roles in cancer progression and metastasis. NEDD9 is localized to the centrosome and is required for activation of Aurora A kinase in mitosis. Here we demonstrate that the HECT-WW protein Smurf2 physically associates with NEDD9 and is required for the stability of NEDD9 protein. Smurf2 depletion results in a marked decrease in NEDD9 protein levels, by facilitating polyubiquitination and proteasomal degradation of NEDD9. Conversely, forced overexpression of Smurf2 results in upregulation of endogenous NEDD9 protein, confirming the role for Smurf2 in NEDD9 stability. Cells with Smurf2 depletion fail to activate Aurora A at the G₂/M boundary, leading to a marked delay in mitotic entry. These observations suggest that the stable complex of Smurf2 and NEDD9 is required for timely entry into mitosis via Aurora A activation.

Thrombospondin type I domain containing 7A (THSD7A) mediates endothelial cell migration and tube formation

Angiogenesis is a highly organized process controlled by a series of molecular events. While much effort has been devoted to identifying angiogenic factors and their reciprocal receptors, far less information is available on the molecular mechanisms underlying directed endothelial cell migration. To search for novel proteins that participate in this process, we used the serial analysis of gene expression (SAGE) transcript profiling approach to identify genes that are selectively expressed in endothelial cells (ECs). Two EC SAGE libraries were constructed from human umbilical vein and artery ECs to enable data-mining against other non-ECs. A novel endothelial protein, Thrombospondin Type I Domain Containing 7A (THSD7A), with preferential expression in placenta vasculature and in human umbilical vein endothelial cells (HUVECs) was identified and targeted for further characterization. Overexpression of a THSD7A carboxyl-terminal fragment in HUVECs inhibited cell migration and disrupted tube formation, while suppression of THSD7A expression enhanced HUVEC migration and tube formation. Immunohistological analysis revealed that THSD7A was expressed at the leading edge of migrating HUVECs, and it co-localized with alpha(V)beta(3) integrin and paxillin. This distribution was dispersed from focal adhesions after disruption of the actin cytoskeleton, suggesting the involvement of THSD7A in cytoskeletal organization. Our results show that THSD7A is a novel placenta endothelial protein that mediates EC migration and tube formation, and they highlight its potential as a new target for anti-angiogenic therapy.

Enhanced interaction between focal adhesion and adherens junction proteins: involvement in sphingosine 1-phosphate-induced endothelial barrier enhancement

Sphingosine 1-phosphate (S1P) is an important vascular barrier regulatory agonist which enhances the junctional integrity of human lung endothelial cell monolayers. We have now demonstrated that S1P induced cortical actin ring formation and redistribution of focal adhesion kinase (FAK) and paxillin to the cell periphery suggesting the critical role of cell-cell adhesion in endothelial barrier enhancement. Co-immunoprecipitation studies revealed increased association of VE-cadherin with FAK and paxillin in S1P-challenged human pulmonary artery endothelial cell (HPAEC) monolayers. Furthermore, S1P-induced enhancement of VE-cadherin interaction with alpha-catenin and beta-catenin was associated with the increased formation of FAK-beta-catenin protein complexes. Depletion of beta-catenin (siRNA) resulted in loss of S1P-mediated VE-cadherin association with FAK and paxillin rearrangement. Furthermore, transendothelial electrical resistance (an index of barrier function) demonstrated that beta-catenin siRNA significantly attenuated S1P-induced barrier enhancement. These results demonstrate a mechanism of S1P-induced endothelial barrier enhancement via beta-catenin-linked adherens junction and focal adhesion interaction.

Cortical deficiency of laminin gamma1 impairs the AKT/GSK-3beta signaling pathway and leads to defects in neurite outgrowth and neuronal migration

Laminins have dramatic and varied actions on neurons in vitro. However, their in vivo function in brain development is not clear. Here we show that knockout of laminin gamma1 in the cerebral cortex leads to defects in neuritogenesis and neuronal migration. In the mutant mice, cortical layer structures were disrupted, and axonal pathfinding was impaired. During development, loss of laminin expression impaired phosphorylation of FAK and paxillin, indicating defects in integrin signaling pathways. Moreover, both phosphorylation and protein levels of GSK-3beta were significantly decreased, but only phosphorylation of AKT was affected in the mutant cortex. Knockout of laminin gamma1 expression in vitro, dramatically inhibited neurite growth. These results indicate that laminin regulates neurite growth and neuronal migration via integrin signaling through the AKT/GSK-3beta pathway, and thus reveal a novel mechanism of laminin function in brain development.

Nck adapter proteins: functional versatility in T cells

Nck is a ubiquitously expressed adapter protein that is almost exclusively built of one SH2 domain and three SH3 domains. The two isoproteins of Nck are functionally redundant in many aspects and differ in only few amino acids that are mostly located in the linker regions between the interaction modules. Nck proteins connect receptor and non-receptor tyrosine kinases to the machinery of actin reorganisation. Thereby, Nck regulates activation-dependent processes during cell polarisation and migration and plays a crucial role in the signal transduction of a variety of receptors including for instance PDGF-, HGF-, VEGF- and Ephrin receptors. In most cases, the SH2 domain mediates binding to the phosphorylated receptor or associated phosphoproteins, while SH3 domain interactions lead to the formation of larger protein complexes. In T lymphocytes, Nck plays a pivotal role in the T cell receptor (TCR)-induced reorganisation of the actin cytoskeleton and the formation of the immunological synapse. However, in this context, two different mechanisms and adapter complexes are discussed. In the first scenario, dependent on an activation-induced conformational change in the CD3epsilon subunits, a direct binding of Nck to components of the TCR/CD3 complex was shown. In the second scenario, Nck is recruited to the TCR complex via phosphorylated Slp76, another central constituent of the membrane proximal activation complex. Over the past years, a large number of putative Nck interactors have been identified in different cellular systems that point to diverse additional functions of the adapter protein, e.g. in the control of gene expression and proliferation.

Paxillin nuclear-cytoplasmic localization is regulated by phosphorylation of the LD4 motif: evidence that nuclear paxillin promotes cell proliferation

Paxillin, a major focal-adhesion complex component belongs to the subfamily of LIM domain proteins and participates in cell adhesion-mediated signal transduction. It is implicated in cell-motility responses upon activation of cell-surface receptors and can recruit, among others, the GIT1 [GRK (G-protein-coupled-receptor kinase)-interacting ARF (ADP-ribosylation factor) GAP (GTPase-activating protein)]–PIX [PAK (p21-activated kinase)-interacting exchange factor]–PAK1 complex. Several adhesion proteins including zyxin, Hic5 and Trip6 are also nuclear and can exert transcriptional effects. In the present study we show that endogenous paxillin shuttles between the cytoplasm and nucleus, and we have used a variety of tagged paxillin constructs to map the nuclear export signal. This region overlaps an important LD4 motif that binds GIT1 and FAK1 (focal-adhesion kinase 1). We provide evidence that phosphorylation of Ser272 within LD4 blocks nuclear export, and we show that this modification also reduces GIT1, but not FAK1, binding; however, Ser272 phosphorylation does not appear to be mediated by PAK1 as previously suggested. Expression of nuclear-localized paxillin LIM domains stimulate DNA synthesis and cell proliferation. By real-time PCR analysis we have established that overexpression of either full-length paxillin or a truncated nuclear form suppresses expression of the parental imprinted gene H19, and modulation of this locus probably affects the rate of NIH-3T3 cell proliferation.

Small GTPases in mechanosensitive regulation of endothelial barrier

Alterations in vascular permeability are defining feature of diverse processes including atherosclerosis, inflammation, ischemia/reperfusion injury, and ventilator-induced lung injury. Clinical observations and experimental studies support an essential role of mechanical forces in pathophysiologic regulation of lung barrier. Accumulating data demonstrate that decreased levels of blood flow and increased cyclic stretch of lung tissues associated with lung mechanical ventilation at high tidal volumes increase vascular permeability, activate inflammatory cytokine production, alveolar flooding, leukocyte infiltration, and hypoxemia, and increase morbidity and mortality. Potential synergism between pathologic mechanical stimulation and inflammatory molecules resulting in vascular leak and lung injury becomes increasingly recognized. This review will discuss a role of Rho family of small GTPases in the mechanochemical regulation of pulmonary endothelial permeability associated with ventilator induced lung injury.

Cell and molecular biology of invadopodia

The controlled degradation of the extracellular matrix is crucial in physiological and pathological cell invasion alike. In vitro, degradation occurs at specific sites where invasive cells make contact with the extracellular matrix via specialized plasma membrane protrusions termed invadopodia. Considerable progress has been made in recent years toward understanding the basic molecular components and their ultrastructural features; generating substantial interest in invadopodia as a paradigm to study the complex interactions between the intracellular trafficking, signal transduction, and cytoskeleton regulation machineries. The next level will be to understand whether they may also represent valid biological targets to help advance the anticancer drug discovery process. Current knowledge will be reviewed here together with some of the most important open questions in invadopodia biology.

The Drosophila homologue of Arf-GAP GIT1, dGIT, is required for proper muscle morphogenesis and guidance during embryogenesis

GIT1-like proteins are GTPase-activating proteins (GAPs) for Arfs and interact with a variety of signaling molecules to function as integrators of pathways controlling cytoskeletal organization and cell motility. In this report, we describe the characterization of a Drosophila homologue of GIT1, dGIT, and show that it is required for proper muscle morphogenesis and myotube guidance in the fly embryo. The dGIT protein is concentrated at the termini of growing myotubes and localizes to muscle attachment sites in late stage embryos. dgit mutant embryos show muscle patterning defects and aberrant targeting in subsets of their muscles. dgit mutant muscles fail to localize the p21-activated kinase, dPak, to their termini. dPak and dGIT form a complex in the presence of dPIX and dpak mutant embryos show similar muscle morphogenesis and targeting phenotypes to that of dgit. We propose that dGIT and dPak are part of a complex that promotes proper muscle morphogenesis and myotube targeting during embryogenesis.

A novel nucleocytoplasmic traffic GTPase identified as a functional target of the bipartite geminivirus nuclear shuttle protein

SUMMARY

In contrast to the accumulated data on nuclear transport mechanisms of macromolecules, little is known concerning the regulated release of nuclear-exported complexes and their subsequent trans-cytoplasmic movement. The bipartite begomovirus nuclear shuttle protein (NSP) facilitates the nuclear export of viral DNA and cooperates with the movement protein (MP) to transport viral DNA across the plant cell wall. Here, we identified a cellular NSP-interacting GTPase (NIG) with biochemical properties consistent with a nucleocytoplasmic transport role. We show that NIG is a cytosolic GTP-binding protein that accumulates around the nuclear envelope and possesses intrinsic GTPase activity. NIG interacts with NSP in vitro and in vivo (under transient expression), and redirects the viral protein from the nucleus to the cytoplasm. We propose that NIG acts as a positive contributor to geminivirus infection by modulating NSP nucleocytoplasmic shuttling and hence facilitating MP-NSP interaction in the cortical cytoplasm. In support of this, overexpression of NIG in Arabidopsis enhances susceptibility to geminivirus infection. In addition to highlighting the relevance of NIG as a cellular co-factor for NSP function, our findings also have implications for general nucleocytoplasmic trafficking of cellular macromolecules.

Ral-regulated interaction between Sec5 and paxillin targets Exocyst to focal complexes during cell migration

Changes in cellular behavior that cause epithelial cells to lose adhesiveness, acquire a motile invasive phenotype and metastasize to secondary sites are complex and poorly understood. Molecules that normally function to integrate adhesive spatial information with cytoskeleton dynamics and membrane trafficking probably serve important functions in cellular transformation. One such complex is the Exocyst, which is essential for targeted delivery of membrane and secretory proteins to specific plasma membrane sites to maintain epithelial cell polarity. Upon loss of cadherin-mediated adhesion in Dunning R3327-5'A prostate tumor cells, Exocyst localization shifts from lateral membranes to tips of protrusive membrane extensions. Here, it colocalizes and co-purifies with focal complex proteins that regulate membrane trafficking and cytoskeleton dynamics. These sites are the preferred destination of post-Golgi transport vesicles ferrying biosynthetic cargo, such as alpha(5)-integrin, which mediates adhesion of cells to the substratum, a process essential to cell motility. Interference with Exocyst activity impairs integrin delivery to plasma membrane and inhibits tumor cell motility and matrix invasiveness. Localization of Exocyst and, by extension, targeting of Exocyst-dependent cargo, is dependent on Ral GTPases, which control association between Sec5 and paxillin. Overexpression of Ral-uncoupled Sec5 mutants inhibited Exocyst interaction with paxillin in 5'A cells, as did RNAi-mediated reduction of either RalA or RalB. Reduction of neither GTPase significantly altered steady-state levels of assembled Exocyst in these cells, but did change the observed localization of Exocyst proteins.

Cross talk between paxillin and Rac is critical for mediation of barrier-protective effects by oxidized phospholipids

We previously reported that the barrier-protective effects of oxidized 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine (OxPAPC) on pulmonary endothelial cells (ECs) delineate the role of Rac- and Cdc42-dependent mechanisms and described the involvement of the focal adhesion (FA) protein paxillin in enhancement of the EC barrier upon OxPAPC challenge. This study examined a potential role of paxillin in the feedback mechanism of Rac regulation by FAs in OxPAPC-stimulated ECs. Our results demonstrate that OxPAPC induced Rac-dependent, Rho-independent peripheral accumulation of paxillin-containing FAs and time-dependent paxillin phosphorylation. Molecular inhibition of Rac decreased association of paxillin with the Rac-specific guanine nucleotide exchange factor beta-PIX. Molecular inhibition of paxillin also attenuated OxPAPC-induced enhancement of adherens junctions critical for the EC barrier-protective response, accumulation of vascular endothelial cadherin in the membrane fractions, and decreased activation of Rac and its effector p21-activated kinase (PAK1). Expression of paxillin with a mutated PAK1-dependent phosphorylation site (S273A) attenuated OxPAPC-induced PAK1 activation and the EC barrier-protective response. These results suggest that PAK1-specific paxillin phosphorylation at Ser(273) is critically involved in the positive-feedback regulation of the Rac-PAK1 pathway and may contribute to sustained enhancement of the EC barrier caused by oxidized phospholipids.

ASAP3 is a focal adhesion-associated Arf GAP that functions in cell migration and invasion

ASAP3, an Arf GTPase-activating protein previously called DDEFL1 and ACAP4, has been implicated in the pathogenesis of hepatocellular carcinoma. We have examined in vitro and in vivo functions of ASAP3 and compared it to the related Arf GAP ASAP1 that has also been implicated in oncogenesis. ASAP3 was biochemically similar to ASAP1: the pleckstrin homology domain affected function of the catalytic domain by more than 100-fold; catalysis was stimulated by phosphatidylinositol 4,5-bisphosphate; and Arf1, Arf5, and Arf6 were used as substrates in vitro. Like ASAP1, ASAP3 associated with focal adhesions and circular dorsal ruffles. Different than ASAP1, ASAP3 did not localize to invadopodia or podosomes. Cells, derived from a mammary carcinoma and from a glioblastoma, with reduced ASAP3 expression had fewer actin stress fiber, reduced levels of phosphomyosin, and migrated more slowly than control cells. Reducing ASAP3 expression also slowed invasion of mammary carcinoma cells. In contrast, reduction of ASAP1 expression had no effect on migration or invasion. We propose that ASAP3 functions nonredundantly with ASAP1 to control cell movement and may have a role in cancer cell invasion. In comparing ASAP1 and ASAP3, we also found that invadopodia are dispensable for the invasive behavior of cells derived from a mammary carcinoma.

Arf GAPs as regulators of the actin cytoskeleton

The Arf (ADP-ribosylation factor) GAPs (GTPase-activating proteins) are a family of proteins with a common catalytic domain that induces hydrolysis of GTP bound to Arf GTP-binding proteins. At least three groups of multidomain Arf GAPs affect the actin cytoskeleton and cellular activities, such as migration and movement, that depend on the cytoskeleton. One role of the Arf GAPs is to regulate membrane remodelling that accompanies actin polymerization. Regulation of membrane remodelling is mediated in part by the regulation of Arf proteins. However, Arf GAPs also regulate actin independently of effects on membranes or Arf. These functions include acting as upstream regulators of Rho family proteins and providing a scaffold for Rho effectors and exchange factors. With multiple functional elements, the Arf GAPs could integrate signals and biochemical activities that result in co-ordinated changes in actin and membranes necessary for a wide range of cellular functions.

Contribution of AZAP-Type Arf GAPs to cancer cell migration and invasion

Arf GAPs are a family of proteins with a common catalytic domain that induces hydrolysis of GTP bound to the small GTP-binding protein Arf. The proteins are otherwise structurally diverse. Several subtypes of Arf GAPs have been found to be targets of oncogenes and to control cell proliferation and cell migration. The latter effects are thought to be mediated by coordinating changes in actin remodeling and membrane traffic. In this chapter, we discuss Arf GAPs that have been linked to oncogenesis and the molecular mechanisms underlying the effects of these proteins in cancer cells. We also discuss the enzymology of the Arf GAPs related to possible targeted inhibition of specific subtypes of Arf GAPs.