The alternatively-included 11a sequence modifies the effects of Mena on actin cytoskeletal organization and cell behavior.

During tumor progression, alternative splicing gives rise to different Mena protein isoforms. We analyzed how Mena11a, an isoform enriched in epithelia and epithelial-like cells, affects Mena-dependent regulation of actin dynamics and cell behavior. While other Mena isoforms promote actin polymerization and drive membrane protrusion, we find that Mena11a decreases actin polymerization and growth factor-stimulated membrane protrusion at lamellipodia. Ectopic Mena11a expression slows mesenchymal-like cell motility, while isoform-specific depletion of endogenous Mena11a in epithelial-like tumor cells perturbs cell:cell junctions and increases membrane protrusion and overall cell motility. Mena11a can dampen membrane protrusion and reduce actin polymerization in the absence of other Mena isoforms, indicating that it is not simply an inactive Mena isoform. We identify a phosphorylation site within 11a that is required for some Mena11a-specific functions. RNA-seq data analysis from patient cohorts demonstrates that the difference between mRNAs encoding constitutive Mena sequences and those containing the 11a exon correlates with metastasis in colorectal cancer, suggesting that 11a exon exclusion contributes to invasive phenotypes and leads to poor clinical outcomes.

Actin foci facilitate activation of the phospholipase C-γ in primary T lymphocytes via the WASP pathway.

Wiscott Aldrich Syndrome protein (WASP) deficiency results in defects in calcium ion signaling, cytoskeletal regulation, gene transcription and overall T cell activation. The activation of WASP constitutes a key pathway for actin filament nucleation. Yet, when WASP function is eliminated there is negligible effect on actin polymerization at the immunological synapse, leading to gaps in our understanding of the events connecting WASP and calcium ion signaling. Here, we identify a fraction of total synaptic F-actin selectively generated by WASP in the form of distinct F-actin 'foci'. These foci are polymerized de novo as a result of the T cell receptor (TCR) proximal tyrosine kinase cascade, and facilitate distal signaling events including PLCγ1 activation and subsequent cytoplasmic calcium ion elevation. We conclude that WASP generates a dynamic F-actin architecture in the context of the immunological synapse, which then amplifies the downstream signals required for an optimal immune response.

Brag2 differentially regulates ß1- and ß3-integrin-dependent adhesion in endothelial cells and is involved in developmental and pathological angiogenesis.

ß1-Integrins are essential for angiogenesis. The mechanisms regulating integrin function in endothelial cells (EC) and their contribution to angiogenesis remain elusive. Brag2 is a guanine nucleotide exchange factor for the small Arf-GTPases Arf5 and Arf6. The role of Brag2 in EC and angiogenesis and the underlying molecular mechanisms remain unclear. siRNA-mediated Brag2-silencing reduced EC angiogenic sprouting and migration. Brag2-siRNA transfection differentially affected α5ß1- and αVß3-integrin function: specifically, Brag2-silencing increased focal/fibrillar adhesions and adhesion on ß1-integrin ligands (fibronectin and collagen), while reducing the adhesion on the αVß3-integrin ligand, vitronectin. Consistent with these results, Brag2-silencing enhanced surface expression of α5ß1-integrin, while reducing surface expression of αVß3-integrin. Mechanistically, Brag2-mediated αVß3-integrin-recycling and ß1-integrin endocytosis and specifically of the active/matrix-bound α5ß1-integrin present in fibrillar/focal adhesions (FA), suggesting that Brag2 contributes to the disassembly of FA via ß1-integrin endocytosis. Arf5 and Arf6 are promoting downstream of Brag2 angiogenic sprouting, ß1-integrin endocytosis and the regulation of FA. In vivo silencing of the Brag2-orthologues in zebrafish embryos using morpholinos perturbed vascular development. Furthermore, in vivo intravitreal injection of plasmids containing Brag2-shRNA reduced pathological ischemia-induced retinal and choroidal neovascularization. These data reveal that Brag2 is essential for developmental and pathological angiogenesis by promoting EC sprouting through regulation of adhesion by mediating ß1-integrin internalization and link for the first time the process of ß1-integrin endocytosis with angiogenesis.

MicroRNA-34a regulates cardiac ageing and function.

Ageing is the predominant risk factor for cardiovascular diseases and contributes to a significantly worse outcome in patients with acute myocardial infarction. MicroRNAs (miRNAs) have emerged as crucial regulators of cardiovascular function and some miRNAs have key roles in ageing. We propose that altered expression of miRNAs in the heart during ageing contributes to the age-dependent decline in cardiac function. Here we show that miR-34a is induced in the ageing heart and that in vivo silencing or genetic deletion of miR-34a reduces age-associated cardiomyocyte cell death. Moreover, miR-34a inhibition reduces cell death and fibrosis following acute myocardial infarction and improves recovery of myocardial function. Mechanistically, we identified PNUTS (also known as PPP1R10) as a novel direct miR-34a target, which reduces telomere shortening, DNA damage responses and cardiomyocyte apoptosis, and improves functional recovery after acute myocardial infarction. Together, these results identify age-induced expression of miR-34a and inhibition of its target PNUTS as a key mechanism that regulates cardiac contractile function during ageing and after acute myocardial infarction, by inducing DNA damage responses and telomere attrition.

Protocadherin-12 cleavage is a regulated process mediated by ADAM10 protein: evidence of shedding up-regulation in pre-eclampsia.

Protocadherins are a group of transmembrane proteins with homophilic binding activity, members of the cadherin superfamily. Apart from their role in adhesion, the cellular functions of protocadherins are essentially unknown. Protocadherin (PCDH)12 was previously identified in invasive trophoblasts and endothelial and mesangial cells in the mouse. Invalidation studies revealed that the protein was required for optimal placental development. In this article, we show that its human homolog is abundantly expressed in various trophoblast subtypes of the human placenta and at lower levels in endothelial cells. We demonstrate that PCDH12 is shed at high rates in vitro. The shedding mechanism depends on ADAM10 and results in reduced cellular adhesion in a cell migration assay. PCDH12 is subsequently cleaved by the γ-secretase complex, and its cytoplasmic domain is rapidly degraded by the proteasome. PCDH12 shedding is regulated by interlinked intracellular pathways, including those involving protein kinase C, PI3K, and cAMP, that either increase or inhibit cleavage. In endothelial cells, VEGF, prostaglandin E(2), or histamine regulates PCDH12 shedding. The extracellular domain of PCDH12 was also detected in human serum and urine, thus providing evidence of PCDH12 shedding in vivo. Importantly, we observed an increase in circulating PCDH12 in pregnant women who later developed a pre-eclampsia, a frequent pregnancy syndrome and a major cause of maternal and fetal morbidity and mortality. In conclusion, we speculate that, like in mice, PCDH12 may play an important role in human placental development and that proteolytic cleavage in response to external factors, such as cytokines and pathological settings, regulates its activity.

MicroRNA-92a controls angiogenesis and functional recovery of ischemic tissues in mice.

MicroRNAs (miRs) are small noncoding RNAs that regulate gene expression by binding to target messenger RNAs (mRNAs), leading to translational repression or degradation. Here, we show that the miR-17approximately92 cluster is highly expressed in human endothelial cells and that miR-92a, a component of this cluster, controls the growth of new blood vessels (angiogenesis). Forced overexpression of miR-92a in endothelial cells blocked angiogenesis in vitro and in vivo. In mouse models of limb ischemia and myocardial infarction, systemic administration of an antagomir designed to inhibit miR-92a led to enhanced blood vessel growth and functional recovery of damaged tissue. MiR-92a appears to target mRNAs corresponding to several proangiogenic proteins, including the integrin subunit alpha5. Thus, miR-92a may serve as a valuable therapeutic target in the setting of ischemic disease.

Caspase-8 is involved in neovascularization-promoting progenitor cell functions.

OBJECTIVE: Endothelial progenitor cells (EPCs) comprise a heterogeneous population of cells, which improve therapeutic neovascularization after ischemia. The neovascularization-promoting potential of progenitor cells depends on survival and retention of the infused cells to the tissue. Caspases mediate apoptosis but are also involved in other critical biological processes. Therefore, we aimed to address the role of caspases in proangiogenic cells. METHODS AND RESULTS: The caspase-8 inhibitor zIETD abrogated the ex vivo formation of EPCs, inhibited EPC adhesion and migration, and reduced their capacity to improve neovascularization in vivo. Consistently, cells isolated from caspase-8-deficient mice exhibited a reduced capacity for enhancing neovascularization when transplanted into mice after hindlimb ischemia. Because inhibition of Caspase-8 reduced the adhesion and homing functions of EPCs, we further determined the surface expression of integrins and receptors involved in cell recruitment to ischemic tissues. Pharmacological inhibition of caspase-8 and genetic depletion of caspase-8 reduced the expression of the fibronectin receptor subunits alpha5 and beta1 and the SDF-1 receptor CXCR4. Moreover, we identified the E3 ubiquitin ligase Cbl-b, which negatively regulates integrin and receptor-mediated signaling, as a potential Caspase-8 substrate. CONCLUSIONS: In summary, our data demonstrate a novel apoptosis-unrelated role of caspase-8 in proangiogenic cells.

Role of the small GTPase Rap1 for integrin activity regulation in endothelial cells and angiogenesis.

Ras-associated protein 1 (Rap1), a small GTPase, attracted attention because of its involvement in several aspects of cell adhesion, including integrin- and cadherin-mediated adhesion. Yet, the role of Rap1 genes and of Rap1 effectors for angiogenesis has not been investigated. Human umbilical vein endothelial cells (HUVECs) express Rap1a and Rap1b mRNA. To determine the contribution of Rap1 activity for angiogenesis, we overexpressed Rap1GAP1, a GTPase-activating protein that inhibits Rap1 activity. Overexpression of Rap1GAP1 significantly blocked angiogenic sprouting and tube-forming activity of HUVECs as well as migration and integrin-dependent adhesion. Silencing of Rap1a, Rap1b, or both significantly blocked HUVECs sprouting under basal and basic fibroblast growth factor-stimulated conditions and reduced HUVEC migration and integrin-dependent adhesion. We found that Rap1a and Rap1b are essential for the conformational activation of beta(1)-integrins in endothelial cells. Furthermore, silencing of Rap1a and Rap1b prevented phosphorylation of tyrosine 397 in focal adhesion kinase (FAK) and vascular endothelial growth factor-induced Akt1-activation. Rap1a(-/-)-deficient and Rap1a(+/-) heterozygote mice displayed reduced neovascularization after hind limb ischemia compared with wild-type mice. Silencing of RAPL significantly blocked the Rap1-induced sprouting of HUVECs, suggesting that the angiogenic activity of Rap1 is partly mediated by RAPL. Our data demonstrate a critical role of Rap1 in the regulation of beta(1)-integrin affinity, adhesion, and migration in endothelial cells and in postnatal neovascularization.

Sustained persistence of transplanted proangiogenic cells contributes to neovascularization and cardiac function after ischemia.

Circulating blood-derived vasculogenic cells improve neovascularization of ischemic tissue by a broad repertoire of potential therapeutic actions. Whereas initial studies documented that the cells incorporate and differentiate to cardiovascular cells, other studies suggested that short-time paracrine mechanisms mediate the beneficial effects. The question remains to what extent a physical incorporation is contributing to the beneficial effects of cell therapy. By using the inducible suicide gene thymidine kinase to deplete transplanted cells, we determined the contribution of physical incorporation in 3 animal models. After acute myocardial infarction, depletion of cells 14 days after infusion resulted in a reduction of capillary density and a substantial deterioration of heart function. Likewise, neovascularization of Matrigel plugs and ischemic limbs was significantly suppressed when infused cells were depleted 7 days after infusion. Induction of cell death in the previously transplanted cells reduced perfusion and led to vascular leakage as evidenced by Evans blue extravasation. These results indicate that physical incorporation and persistence of cells contribute to cell-mediated improvement of neovascularization and cardiac function. Long-term paracrine activities and/or cell intrinsic mechanisms may have contributed to the maintenance of functional improvement.

Phosphatidylinositol-3-kinase-gamma is integral to homing functions of progenitor cells.

Endothelial progenitor cells (EPCs) and hematopoietic progenitor cells are recruited to ischemic regions, improving neovascularization. beta1 and beta2 integrins play a crucial role for progenitor cell homing to ischemic tissues. Integrin activity is regulated by chemokines and their respective G protein-coupled receptors. The phosphatidylinositol-3-kinase catalytic subunit gamma (PI3Kgamma) is the PI3K isoform that selectively transduces signals from G protein-coupled receptors. Here, we investigated the role of PI3Kgamma as a signaling intermediate in the chemokine-induced integrin-dependent homing functions of progenitor cells. A pharmacological PI3Kgamma inhibitor significantly reduced chemokine-induced chemotaxis and stromal cell-derived factor (SDF)1alpha-induced transmigration of human EPCs. Moreover, the PI3Kgamma inhibitor significantly reduced SDF1alpha-induced adhesion of EPCs to intercellular adhesion molecule-1 and human umbilical vein endothelial cell monolayers. These findings were corroborated with Lin(-) bone marrow-derived progenitor cells from PI3Kgamma-deficient mice that displayed reduced SDF1alpha-induced migration and intercellular adhesion molecule-1 adhesion as compared with wild-type cells. Pharmacological inhibition or genetic ablation of PI3Kgamma reduced SDF1alpha-induced integrin activation in human EPCs and in murine Lin(-) BM-derived progenitor cells, respectively. In vivo, the homing of PI3Kgamma-deficient Lin(-) progenitor cells to ischemic muscles after intravenous infusion in the model of hindlimb ischemia and their neovascularization-promoting capacity was reduced as compared with wild-type cells. In conclusion, PI3Kgamma is integral to the integrin-dependent homing of progenitor cells.

Activation of Epac stimulates integrin-dependent homing of progenitor cells.

Cell therapy is a novel promising option for treatment of ischemic diseases. Administered endothelial progenitor cells (EPCs) are recruited to ischemic regions and improve neovascularization. However, the number of cells that home to ischemic tissues is restricted. The GTPase Rap1 plays an important role in the regulation of adhesion and chemotaxis. We investigated whether pharmacologic activation of Epac1, a nucleotide exchange protein for Rap1, which is directly activated by cAMP, can improve the adhesive and migratory capacity of distinct progenitor cell populations. Stimulation of Epac by a cAMP-analog increased Rap1 activity and stimulated the adhesion of human EPCs, CD34(+) hematopoietic progenitor cells, and mesenchymal stem cells (MSCs). Specifically, short-term stimulation with a specific Epac activator increased the beta2-integrin-dependent adhesion of EPCs to endothelial cell monolayers, and of EPC and CD34(+) cells to ICAM-1. Furthermore, the Epac activator enhanced the beta1-integrin-dependent adhesion of EPCs and MSCs to the matrix protein fibronectin. In addition, Epac1 activation induced the beta1- and beta2-integrin-dependent migration of EPCs on fibronectin and fibrinogen. Interestingly, activation of Epac rapidly increased lateral mobility of beta1- and beta2-integrins, thereby inducing integrin polarization, and stimulated beta1-integrin affinity, whereas the beta2-integrin affinity was not increased. Furthermore, prestimulation of EPCs with the Epac activator increased homing to ischemic muscles and neovascularization-promoting capacity of intravenously injected EPCs in the model of hind limb ischemia. These data demonstrate that activation of Epac1 increases integrin activity and integrin-dependent homing functions of progenitor cells and enhances their in vivo therapeutic potential. These results may provide a platform for the development of novel therapeutic approaches to improve progenitor cell homing.

High-mobility group box 1 activates integrin-dependent homing of endothelial progenitor cells.

Endothelial progenitor cells (EPCs) are recruited to ischemic regions and improve neovascularization. Integrins contribute to EPC homing. High-mobility group box 1 (HMGB1) is a nuclear protein that is released extracellularly on cell necrosis and tissue damage, eliciting a proinflammatory response and stimulating tissue repair. In the present study, we investigated the effects of HMGB1 on EPC homing. EPCs express the HMGB1 receptors RAGE (receptor for advanced glycation end products) and TLR2 (Toll-like receptor 2). EPC migration was stimulated by HMGB1 in a RAGE-dependent manner. In addition, the HMGB1-induced migration of EPCs on fibronectin and fibrinogen was significantly inhibited by antibodies against beta1 and beta2 integrins, respectively. Short-term prestimulation of EPCs with HMGB1 also increased EPC adhesion to endothelial cell monolayers, and this effect was blocked by antibodies to beta2 integrins or RAGE. HMGB1 increased EPC adhesion to the immobilized integrin ligands intercellular adhesion molecule-1 and fibronectin in a RAGE-dependent manner. Strikingly, HMGB1 rapidly increased integrin affinity and induced integrin polarization. Using intravital microscopy in a tumor model of neovascularization, prestimulation of EPCs with HMGB1 enhanced the initial in vivo adhesion of EPCs to microvessels and the recruitment of EPCs in the tumor tissue. In addition, prestimulation of EPCs with HMGB1 increased the homing of EPCs to ischemic muscles. In conclusion, these data represent a link between HMGB1 and integrin functions of EPCs and demonstrate that HMGB1 stimulates EPC homing to ischemic tissues. These results may provide a platform for the development of novel therapeutic approaches to improve EPC homing.