The ability to differentiate human pluripotent stem cells into endothelial cells with properties of cord-blood endothelial colony–forming cells (CB-ECFCs) may enable the derivation of clinically relevant numbers of highly proliferative blood vessel–forming cells to restore endothelial function in patients with vascular disease. We describe a protocol to convert human induced pluripotent stem cells (hiPSCs) or embryonic stem cells (hESCs) into cells similar to CB-ECFCs at an efficiency of >108 ECFCs produced from each starting pluripotent stem cell. The CB-ECFC-like cells display a stable endothelial phenotype with high clonal proliferative potential and the capacity to form human vessels in mice and to repair the ischemic mouse retina and limb, and they lack teratoma formation potential. We identify Neuropilin-1 (NRP-1)-mediated activation of KDR signaling through VEGF165 as a critical mechanism for the emergence and maintenance of CB-ECFC-like cells.

Endothelium forms a semi-permeable barrier that separates blood from the underlying tissue. Barrier function is largely determined by cell-cell and cell-matrix adhesions that define the limits of cell borders. Yet, such cell-cell and cell-matrix tethering is critically reliant upon the nature of adherence within the cell itself. Indeed, the actin cytoskeleton fulfills this essential function, to provide a strong, dynamic intracellular scaffold that organizes integral membrane proteins with the cell's interior, and responds to environmental cues to orchestrate appropriate cell shape. The actin cytoskeleton is comprised of three distinct, but interrelated structures, including actin cross-linking of spectrin within the membrane skeleton, the cortical actin rim, and actomyosin-based stress fibers. This review addresses each of these actin-based structures, and discusses cellular signals that control the disposition of actin in different endothelial cell phenotypes.

IntroductionThe first cord blood (CB) transplantation saved the life of a young patient with Fanconi anemia using HLA-matched sibling CB cells, 1 a procedure made possible by identification and cryopreservation of transplantable hematopoietic progenitor cells (HPCs) and hematopoietic stem cells (HSCs) in CB. 2 More than 20 000 CB transplantations have treated the same malignant and nonmalignant disorders as bone marrow (BM). 3-8 CB transplantation is possible because of CB banks, and how long CB can be stored in a cryopreserved state with efficient recovery of HSCs and HPCs is critical for CB banking. We reported highly efficient recovery of CB HPCs after 5, 9 10, 10 and 15 11 years, and recovery of HSCs after 15 years. 11 We now report efficient recovery of functional HPCs up to 21-23.5 years, with more in depth studies on CB HSC engraftment in immune deficient mice, recovery of responsive T cells, generation of induced pluripotent stem (iPS) cells, 12-14 and detection of endothelial colony forming cells (ECFCs). 15 MethodsCB cells were scheduled for discard. 2 The study was approved by the Institutional Review Board of Indiana University (IU). Cryopreservation, thawing, and plating were as reported. 2,9-11 CB was assessed within 36 hours of collection. Cells were either separated into a mononuclear (MNC) fraction (Ficoll-Hypaque; Pharmacia) and aliquoted into cryotubes (Nalge Nunc) or left unseparated and aliquoted into cryo-freezer bags, 2,16,17 in 10% Dimethylsulfoxide and 10% autologous plasma for eventual analysis of HPC recovery. Percent recovery from MNC or unseparated cryopreserved cells was based on total prefreeze cells per volume of the exact same CB unit. 2,9-11 After thaw of unseparated cells, CD34 ϩ cells were magnetic-bead separated 11 for HSC engraftment and iPS cell generation studies. CD4 ϩ and CD8 ϩ T lymphocytes were separated from the CD34 ϩ -depleted cells and stimulated on plates precoated with anti-CD3 (OKT3, 0.5 g/mL) and anti-CD28 (clone CD28.2, 1 g/mL) with 10% FBS, 50M 2ME and 10ng/mL IL-15 as described. 18 Immune-deficient mouse assay for human CB donor chimerism was as reported, 11 except that recipients were NOD/SCID/IL2Rg null (NSG). 19 iPS cell generationAt IU, CD34 ϩ cells isolated from thawed, unseparated cells were grown with 10% FBS, 10 ng/mL human (h) SCF, 10 ng h Flt3-ligand, and 10 ng h Thrombopoietin/mL for 3 days. At day 4, cells were spin-infected (2200 rpm; 45 minutes) with concentrated lentiviral vectors Sox2-Oct4-EGFP and cMyc-Klaf4 (pc DNA-HIV-CS-CGW, provided by Dr P. Zoltick, Children's Hospital, Philadelphia; supplemental Figure 1, available on the Blood Web site; see the Supplemental Materials link at the top of the online article) in ␣-MEM medium with polybrene (Sigma-Aldrich). Medium was replaced at 6 days with the cytokines noted in this paragraph. At day 7, cells were transferred to mitotically inactivated murine embryonic fibroblasts (MEFs) and cultured as for human embryonic stem cells (hESCs). 20 iPS cells were also generated at Johns Hopkins using retro...

Exoenzyme Y (ExoY) is a Pseudomonas aeruginosa toxin that is introduced into host cells through the type 3 secretion system (T3SS). Once inside the host cell cytoplasm, ExoY generates cyclic nucleotides that cause tau phosphorylation and microtubule breakdown. Microtubule breakdown causes interendothelial cell gap formation and tissue edema. Although ExoY transiently induces interendothelial cell gap formation, it remains unclear whether ExoY prevents repair of the endothelial cell barrier. Here, we test the hypothesis that ExoY intoxication impairs recovery of the endothelial cell barrier following gap formation, decreasing migration, proliferation, and lung repair. Pulmonary microvascular endothelial cells (PMVECs) were infected with P. aeruginosa strains for 6 h, including one possessing an active ExoY (PA103 exoUexoT::Tc pUCPexoY; ExoY(+)), one with an inactive ExoY (PA103ΔexoUexoT::Tc pUCPexoY(K81M); ExoY(K81M)), and one that lacks PcrV required for a functional T3SS (ΔPcrV). ExoY(+) induced interendothelial cell gaps, whereas ExoY(K81M) and ΔPcrV did not promote gap formation. Following gap formation, bacteria were removed and endothelial cell repair was examined. PMVECs were unable to repair gaps even 3-5 days after infection. Serum-stimulated growth was greatly diminished following ExoY intoxication. Intratracheal inoculation of ExoY(+) and ExoY(K81M) caused severe pneumonia and acute lung injury. However, whereas the pulmonary endothelial cell barrier was functionally improved 1 wk following ExoY(K81M) infection, pulmonary endothelium was unable to restrict the hyperpermeability response to elevated hydrostatic pressure following ExoY(+) infection. In conclusion, ExoY is an edema factor that chronically impairs endothelial cell barrier integrity following lung injury.

While most somatic cells undergoing induced pluripotent stem (iPS) cell reprogramming with Yamanaka factors accumulate at stable partially reprogrammed stages, the molecular mechanisms required to achieve full reprogramming are unknown. MicroRNAs (miRNAs) fine-tune mRNA translation and are implicated in reprogramming, but miRNA functional targets critical for complete iPS cell reprogramming remain elusive. We identified methyl-DNA binding domain protein 2 (MBD2) as an epigenetic suppressor, blocking full reprogramming of somatic to iPS cells through direct binding to NANOG promoter elements preventing transcriptional activation. When we overexpressed miR-302 cluster we observed a significant increase in conversion of partial to fully reprogrammed iPS cells by suppressing MBD2 expression, thereby increasing NANOG expression. Thus, expression of exogenous miR-302 cluster (without miR-367) is efficient in attaining a fully reprogrammed iPS state in partially reprogrammed cells by relieving MBD2-mediated inhibition of NANOG expression. Our studies provide a direct molecular mechanism involved in generating complete human iPS cell reprogramming to study disease pathogenesis, drug screening, and for potential cell-based therapies.

Pseudomonas aeruginosa uses a type III secretion system to introduce the adenylyl and guanylyl cyclase exotoxin Y (ExoY) into the cytoplasm of endothelial cells. ExoY induces Tau hyperphosphorylation and insolubility, microtubule breakdown, barrier disruption and edema, although the mechanism(s) responsible for microtubule breakdown remain poorly understood. Here we investigated both microtubule behavior and centrosome activity to test the hypothesis that ExoY disrupts microtubule dynamics. Fluorescence microscopy determined that infected pulmonary microvascular endothelial cells contained fewer microtubules than control cells, and further studies demonstrated that the microtubule-associated protein Tau was hyperphosphorylated following infection and dissociated from microtubules. Disassembly/reassembly studies determined that microtubule assembly was disrupted in infected cells, with no detectable effects on either microtubule disassembly or microtubule nucleation by centrosomes. This effect of ExoY on microtubules was abolished when the cAMP-dependent kinase phosphorylation site (Ser-214) on Tau was mutated to a non-phosphorylatable form. These studies identify Tau in microvascular endothelial cells as the target of ExoY in control of microtubule architecture following pulmonary infection by Pseudomonas aeruginosa and demonstrate that phosphorylation of tau following infection decreases microtubule assembly.

Recently a protocol is established to obtain large quantities of human induced pluripotent stem cells (iPSC)‐derived endothelial progenitors, called endothelial colony forming cells (ECFC), and of candidate smooth‐muscle forming cells (SMFC). Here, the suitability for assembling in spheroids, and in larger 3D cell constructs is tested. iPSC‐derived ECFC and SMFC are labeled with tdTomato and eGFP, respectively. Spheroids are formed in ultra‐low adhesive wells, and their dynamic proprieties are studied by time‐lapse microscopy, or by confocal microscopy. Spheroids are also tested for fusion ability either in the wells, or assembled on the Regenova 3D bioprinter which laces them in stainless steel micro‐needles (the “Kenzan” method). It is found that both ECFC and SMFC formed spheroids in about 24 h. Fluorescence monitoring indicated a continuous compaction of ECFC spheroids, but stabilization in those prepared from SMFC. In mixed spheroids, the cell distribution changed continuously, with ECFC relocating to the core, and showing pre‐vascular organization. All spheroids have the ability of in‐well fusion, but only those containing SMFC are robust enough to sustain assembling in tubular structures. In these constructs a layered distribution of alpha smooth muscle actin‐positive cells and extracellular matrix deposition is found. In conclusion, iPSC‐derived vascular cell spheroids represent a promising new cellular material for scaffold‐free biofabrication.

Soluble adenylyl cyclase toxins, such as Pseudomonas aeruginosa exoY, generate a cAMP pool that retracts cell borders. However, the cytoskeletal basis by which this cAMP signal retracts cell borders is not known. We sought to determine whether activation of chimeric, soluble adenylyl cyclase I/II (sACI/II) reorganizes either microtubules or peripheral actin. Endothelial cells were stably transfected with either green fluorescent protein-labeled alpha-tubulin or beta-actin, and then infected with adenovirus to express sACI/II. Forskolin, which stimulates both the endogenously expressed transmembrane adenylyl cyclases and sACI/II, induced cell retraction accompanied by the reorganization of peripheral microtubules. However, cortical filamentous-actin (f-actin) did not reorganize into stress fibers, and myosin light-chain-20 phosphorylation was decreased. Isoproterenol, which activates endogenous adenylyl cyclases but does not activate sACI/II, did not induce endothelial cell gaps and did not influence microtubule or f-actin architecture. Thus, sACI/II generates a cAMP signal that reorganizes microtubules and induces cell retraction, without inducing f-actin stress fibers. These findings illustrate that endothelial cell gap formation can proceed without f-actin stress fiber formation, and provide mechanistic insight how bacterial adenylyl cyclase toxins reorganize the cytoskeleton to induce cell rounding.