Polarized Cytokine Release Triggered by P2X7 Receptor from Retinal Pigmented Epithelial Cells Dependent on Calcium Influx.

Cytokine release from non-inflammatory cells is a key step in innate immunity, and agonists triggering cytokine release are central in coordinating responses. P2X7 receptor (P2X7R) stimulation by extracellular ATP is best known to active the NLRP3 inflammasome and release IL-1ß, but stimulation also leads to release of other cytokines. As cytokine signaling by retinal pigmented epithelial (RPE) cells is implicated in retinal neurodegeneration, the role of P2X7R in release of cytokine IL-6 from RPE cells was investigated. P2X7R stimulation triggered IL-6 release from primary mouse RPE, human iPS-RPE and human ARPE-19 cells. IL-6 release was polarized, with predominant rise across apical membranes. IL-6 release was inhibited by P2X7R antagonists A438079, A839977, and AZ10606120, but not the NRTI lamivudine (3TC), P2X1R antagonist NF279, or P2Y1R antagonist MRS2179. P2X7R-mediated IL-6 release required extracellular Ca2+ and was blocked by Ca2+ chelator BAPTA. IL-6 release and Ca2+ elevation occurred rapidly, consistent with vesicular IL-6 staining in unstimulated cells. P2X7R stimulation did not trigger IL-1ß release in these unprimed cells. P2X7R-mediated IL-6 release was enhanced in RPE cells from the ABCA4-/- mouse model of retinal degeneration. In summary, P2X7R stimulation triggers rapid Ca2+-dependent IL-6 release across the apical membrane of RPE cells.

Generation of human control iPSC line CHOPi004-A from juvenile foreskin fibroblast cells.

The CHOPWT4 iPSC line was generated as a control for applications such as differentiation analyses to the three germ layers and derivative tissues. Human foreskin fibroblasts were reprogrammed using the non-integrating Sendai virus expressing Oct3/4, Sox2, c-myc, and Klf4.

CRISPR Activation Enhances In Vitro Potency of AAV Vectors Driven by Tissue-Specific Promoters.

Validation of gene transfer vectors containing tissue-specific promoters in cell-based functional assays poses a formidable challenge for gene therapy product development. Here, we describe a novel approach based on CRISPR/dCas9 transcriptional activation to achieve robust transgene expression from transgene cassettes containing tissue or cell type-specific promoters after infection with AAV vectors in cell-based systems. Guide RNA sequences targeting two promoters that are highly active within mammalian photoreceptors were screened in a novel promoter activation assay. Using this screen, we generated and characterized stable cell lines that co-express dCas9.VPR and top-performing guide RNA candidates. These cells exhibit potent activation of proviral plasmids after transfection or after infection with AAV vectors delivering transgene cassettes carrying photoreceptor-specific promoters. In addition, we interrogated mechanisms to optimize this platform through the addition of multiple guide RNA sequences and co-expression of the universal adeno-associated virus receptor (AAVR). Collectively, this investigation identifies a rapid and broadly applicable strategy to enhance in vitro expression and to evaluate potency of AAV vectors that rely upon cell or tissue-specific regulatory elements.

Comparative AAV-eGFP Transgene Expression Using Vector Serotypes 1-9, 7m8, and 8b in Human Pluripotent Stem Cells, RPEs, and Human and Rat Cortical Neurons.

Recombinant adeno-associated virus (rAAV), produced from a nonpathogenic parvovirus, has become an increasing popular vector for gene therapy applications in human clinical trials. However, transduction and transgene expression of rAAVs can differ across in vitro and ex vivo cellular transduction strategies. This study compared 11 rAAV serotypes, carrying one reporter transgene cassette containing a cytomegalovirus immediate-early enhancer (eCMV) and chicken beta actin (CBA) promoter driving the expression of an enhanced green-fluorescent protein (eGFP) gene, which was transduced into four different cell types: human iPSC, iPSC-derived RPE, iPSC-derived cortical, and dissociated embryonic day 18 rat cortical neurons. Each cell type was exposed to three multiplicity of infections (MOI: 1E4, 1E5, and 1E6 vg/cell). After 24, 48, 72, and 96 h posttransduction, GFP-expressing cells were examined and compared across dosage, time, and cell type. Retinal pigmented epithelium showed highest AAV-eGFP expression and iPSC cortical the lowest. At an MOI of 1E6 vg/cell, all serotypes show measurable levels of AAV-eGFP expression; moreover, AAV7m8 and AAV6 perform best across MOI and cell type. We conclude that serotype tropism is not only capsid dependent but also cell type plays a significant role in transgene expression dynamics.

Functional Cortical Axon Tracts Generated from Human Stem Cell-Derived Neurons.

IMPACT STATEMENT: Axon regeneration is negligible in the adult mammalian brain, and thus, white matter damage often leads to permanent neurological deficits. A novel approach for axon repair is the generation of axon tracts in the laboratory setting followed by transplantation of these constructs. This article details a human substrate for this repair strategy. Using the technique of axon stretch growth, functional cortical axon tracts are generated from human pluripotent stem cells at rates of up to 1 mm/day. These results form the basis of a potential patient-specific protocol for cerebral axon transplantation after injury.

Use of induced pluripotent stem cell models to probe the pathogenesis of Choroideremia and to develop a potential treatment.

Choroideremia (CHM) is a rare monogenic, X-linked recessive inherited retinal degeneration resulting from mutations in the Rab Escort Protein-1 (REP1) encoding CHM gene. The primary retinal cell type leading to CHM is unknown. In this study, we explored the utility of induced pluripotent stem cell-derived models of retinal pigmented epithelium (iPSC-RPE) to study disease pathogenesis and a potential gene-based intervention in four different genetically distinct forms of CHM. A number of abnormal cell biologic, biochemical, and physiologic functions were identified in the CHM mutant cells. We then identified a recombinant adeno-associated virus (AAV) serotype, AAV7m8, that is optimal for both delivering transgenes to iPSC-RPEs as well as to appropriate target cells (RPE cells and rod photoreceptors) in the primate retina. To establish the proof of concept of AAV7m8 mediated CHM gene therapy, we developed AAV7m8.hCHM, which delivers the human CHM cDNA under control of CMV-enhanced chicken ß-actin promoter (CßA). Delivery of AAV7m8.hCHM to CHM iPSC-RPEs restored protein prenylation, trafficking and phagocytosis. The results confirm that AAV-mediated delivery of the REP1-encoding gene can rescue defects in CHM iPSC-RPE regardless of the type of disease-causing mutation. The results also extend our understanding of mechanisms involved in the pathophysiology of choroideremia.

Generation of Hermansky-Pudlak Syndrome Type 1 (HPS1) induced pluripotent stem cells (iPSCs).

Hermansky-Pudlak syndrome (HPS) is a rare autosomal recessive disorder characterized by deficiencies in lysosome-related organelles such as melanosomes and platelet-dense granules. The disorder is classified into nine different subtypes (HPS1-HPS9) based on genetic mutations in 9 unique genes. Here we describe the generation of an HPS1 iPSC line (CHOPHPS1) using a Cre-excisable polycistronic STEMCCA lentivirus. This line was derived from human fibroblasts isolated from a patient carrying a duplicative mutation in the HPS1 gene. The patient presented with oculocutaneous albinism, early pulmonary fibrosis, and hemorrhagic diathesis.

Generation of Hermansky Pudlak syndrome type 2 (HPS2) induced pluripotent stem cells (iPSCs).

Hermansky-Pudlak syndrome type 2 (HPS2) is a rare autosomal recessive disorder resulting from functional mutations in the adaptor-related protein complex 3, beta 1 subunit (AP3B1) gene. This gene plays a role in organelle biogenesis associated with melanosomes, platelet dense granules, and lysosomes. Here we describe the generation of an HPS2 iPS cell line (CHOPHPS2) using a Cre-excisable polycistronic STEMCCA lentivirus. This line was derived from human fibroblasts isolated from a patient carrying two mutations in the AP3B1 gene. The patient presented with severe neutropenia, ocular albinism, interstitial pulmonary fibrosis, hemorrhagic diathesis, and an absence of platelet-dense granules.

Dysregulation of the Transforming Growth Factor ß Pathway in Induced Pluripotent Stem Cells Generated from Patients with Diamond Blackfan Anemia.

Diamond Blackfan Anemia (DBA) is an inherited bone marrow failure syndrome with clinical features of red cell aplasia and variable developmental abnormalities. Most affected patients have heterozygous loss of function mutations in ribosomal protein genes but the pathogenic mechanism is still unknown. We generated induced pluripotent stem cells from DBA patients carrying RPS19 or RPL5 mutations. Transcriptome analysis revealed the striking dysregulation of the transforming growth factor ß (TGFß) signaling pathway in DBA lines. Expression of TGFß target genes, such as TGFBI, BAMBI, COL3A1 and SERPINE1 was significantly increased in the DBA iPSCs. We quantified intermediates in canonical and non-canonical TGFß pathways and observed a significant increase in the levels of the non-canonical pathway mediator p-JNK in the DBA iPSCs. Moreover, when the mutant cells were corrected by ectopic expression of WT RPS19 or RPL5, levels of p-JNK returned to normal. Surprisingly, nuclear levels of SMAD4, a mediator of canonical TGFß signaling, were decreased in DBA cells due to increased proteolytic turnover. We also observed the up-regulation of TGFß1R, TGFß2, CDKN1A and SERPINE1 mRNA, and the significant decrease of GATA1 mRNA in the primitive multilineage progenitors. In summary our observations identify for the first time a dysregulation of the TGFß pathway in the pathobiology of DBA.

Level of RUNX1 activity is critical for leukemic predisposition but not for thrombocytopenia.

To explore how RUNX1 mutations predispose to leukemia, we generated induced pluripotent stem cells (iPSCs) from 2 pedigrees with germline RUNX1 mutations. The first, carrying a missense R174Q mutation, which acts as a dominant-negative mutant, is associated with thrombocytopenia and leukemia, and the second, carrying a monoallelic gene deletion inducing a haploinsufficiency, presents only as thrombocytopenia. Hematopoietic differentiation of these iPSC clones demonstrated profound defects in erythropoiesis and megakaryopoiesis and deregulated expression of RUNX1 targets. iPSC clones from patients with the R174Q mutation specifically generated an increased amount of granulomonocytes, a phenotype reproduced by an 80% RUNX1 knockdown in the H9 human embryonic stem cell line, and a genomic instability. This phenotype, found only with a lower dosage of RUNX1, may account for development of leukemia in patients. Altogether, RUNX1 dosage could explain the differential phenotype according to RUNX1 mutations, with a haploinsufficiency leading to thrombocytopenia alone in a majority of cases whereas a more complete gene deletion predisposes to leukemia.

Hematopoietic differentiation of pluripotent stem cells in culture.

This chapter describes a two-dimensional "monolayer" system for differentiating human pluripotent stem cells (PSCs) into "primitive" hematopoietic progenitor cells (HPCs) resembling those produced in vivo by the early embryonic yolk sac. This experimental system utilizes defined conditions without serum or feeder cells. Cytokines are added sequentially to stimulate the formation of mesoderm and its subsequent patterning to hematopoietic progenitors. The HPCs produced by this protocol have multi-lineage potential (erythroid, megakaryocyte, and myeloid) and can be isolated as a homogeneous population for use in standard hematopoietic studies including liquid expansion to mature lineages and colony assays. In addition, the HPCs can be cryopreserved for distribution or analysis at later times. The HPCs generated by this protocol have been used successfully to better define intrinsic variation in hematopoietic potential between different PSC lines and to model human hematopoietic diseases using patient-derived induced pluripotent stem cells.

Utilization of the AAVS1 safe harbor locus for hematopoietic specific transgene expression and gene knockdown in human ES cells.

Human pluripotent stem cells offer a powerful system to study human biology and disease. Here, we report a system to both express transgenes specifically in ES cell derived hematopoietic cells and knockdown gene expression stably throughout the differentiation of ES cells. We characterize a CD43 promoter construct that when inserted into the AAVS1 "safe harbor" locus utilizing a zinc finger nuclease specifically drives GFP expression in hematopoietic cells derived from a transgenic ES cell line and faithfully recapitulates endogenous CD43 expression. In addition, using the same gene targeting strategy we demonstrate that constitutive expression of short hairpin RNAs within a microRNA backbone can suppress expression of PU.1, an important regulator of myeloid cell development. We show that PU.1 knockdown cell lines display an inhibition in myeloid cell formation and skewing towards erythroid development. Overall, we have generated a powerful system to track hematopoietic development from pluripotent stem cells and study gene function through hematopoietic specific gene expression and constitutive gene knockdown.

The negative impact of Wnt signaling on megakaryocyte and primitive erythroid progenitors derived from human embryonic stem cells.

The Wnt gene family consists of structurally related genes encoding secreted signaling molecules that have been implicated in many developmental processes, including regulation of cell fate and patterning during embryogenesis. Previously, we found that Wnt signaling is required for primitive or yolk sac-derived-erythropoiesis using the murine embryonic stem cell (ESC) system. Here, we examine the effect of Wnt signaling on the formation of early hematopoietic progenitors derived from human ESCs. The first hematopoietic progenitor cells in the human ESC system express the pan-hematopoietic marker CD41 and the erythrocyte marker, glycophorin A or CD235. We have developed a novel serum-free, feeder-free, adherent differentiation system that can efficiently generate large numbers of CD41+CD235+ cells. We demonstrate that this cell population contains progenitors not just for primitive erythroid and megakaryocyte cells but for the myeloid lineage as well and term this population the primitive common myeloid progenitor (CMP). Treatment of mesoderm-specified cells with Wnt3a led to a loss of hematopoietic colony-forming ability while the inhibition of canonical Wnt signaling with DKK1 led to an increase in the number of primitive CMPs. Canonical Wnt signaling also inhibits the expansion and/or survival of primitive erythrocytes and megakaryocytes, but not myeloid cells, derived from this progenitor population. These findings are in contrast to the role of Wnt signaling during mouse ESC differentiation and demonstrate the importance of the human ESC system in studying species-specific differences in development.

Clonal genetic and hematopoietic heterogeneity among human-induced pluripotent stem cell lines.

Induced pluripotent stem cells (iPSCs) hold great promise for modeling human hematopoietic diseases. However, intrinsic variability in the capacities of different iPSC lines for hematopoietic development complicates comparative studies and is currently unexplained. We created and analyzed 3 separate iPSC clones from fibroblasts of 3 different normal individuals using a standardized approach that included excision of integrated reprogramming genes by Cre-Lox mediated recombination. Gene expression profiling and hematopoietic differentiation assays showed that independent lines from the same individual were generally more similar to one another than those from different individuals. However, one iPSC line (WT2.1) exhibited a distinctly different gene expression, proliferation rate, and hematopoietic developmental potential relative to all other iPSC lines. This "outlier" clone also acquired extensive copy number variations (CNVs) during reprogramming, which may be responsible for its divergent properties. Our data indicate how inherent and acquired genetic differences can influence iPSC properties, including hematopoietic potential.

Ribosomal and hematopoietic defects in induced pluripotent stem cells derived from Diamond Blackfan anemia patients.

Diamond Blackfan anemia (DBA) is a congenital disorder with erythroid (Ery) hypoplasia and tissue morphogenic abnormalities. Most DBA cases are caused by heterozygous null mutations in genes encoding ribosomal proteins. Understanding how haploinsufficiency of these ubiquitous proteins causes DBA is hampered by limited availability of tissues from affected patients. We generated induced pluripotent stem cells (iPSCs) from fibroblasts of DBA patients carrying mutations in RPS19 and RPL5. Compared with controls, DBA fibroblasts formed iPSCs inefficiently, although we obtained 1 stable clone from each fibroblast line. RPS19-mutated iPSCs exhibited defects in 40S (small) ribosomal subunit assembly and production of 18S ribosomal RNA (rRNA). Upon induced differentiation, the mutant clone exhibited globally impaired hematopoiesis, with the Ery lineage affected most profoundly. RPL5-mutated iPSCs exhibited defective 60S (large) ribosomal subunit assembly, accumulation of 12S pre-rRNA, and impaired erythropoiesis. In both mutant iPSC lines, genetic correction of ribosomal protein deficiency via complementary DNA transfer into the "safe harbor" AAVS1 locus alleviated abnormalities in ribosome biogenesis and hematopoiesis. Our studies show that pathological features of DBA are recapitulated by iPSCs, provide a renewable source of cells to model various tissue defects, and demonstrate proof of principle for genetic correction strategies in patient stem cells.

AAV-mediated gene therapy for choroideremia: preclinical studies in personalized models.

Choroideremia (CHM) is an X- linked retinal degeneration that is symptomatic in the 1(st) or 2(nd) decade of life causing nyctalopia and loss of peripheral vision. The disease progresses through mid-life, when most patients become blind. CHM is a favorable target for gene augmentation therapy, as the disease is due to loss of function of a protein necessary for retinal cell health, Rab Escort Protein 1 (REP1).The CHM cDNA can be packaged in recombinant adeno-associated virus (rAAV), which has an established track record in human gene therapy studies, and, in addition, there are sensitive and quantitative assays to document REP1 activity. An animal model that accurately reflects the human condition is not available. In this study, we tested the ability to restore REP1 function in personalized in vitro models of CHM: lymphoblasts and induced pluripotent stems cells (iPSCs) from human patients. The initial step of evaluating safety of the treatment was carried out by evaluating for acute retinal histopathologic effects in normal-sighted mice and no obvious toxicity was identified. Delivery of the CHM cDNA to affected cells restores REP1 enzymatic activity and also restores proper protein trafficking. The gene transfer is efficient and the preliminary safety data are encouraging. These studies pave the way for a human clinical trial of gene therapy for CHM.

Generation of human induced pluripotent stem cells from peripheral blood using the STEMCCA lentiviral vector.

Through the ectopic expression of four transcription factors, Oct4, Klf4, Sox2 and cMyc, human somatic cells can be converted to a pluripotent state, generating so-called induced pluripotent stem cells (iPSCs)(1-4). Patient-specific iPSCs lack the ethical concerns that surround embryonic stem cells (ESCs) and would bypass possible immune rejection. Thus, iPSCs have attracted considerable attention for disease modeling studies, the screening of pharmacological compounds, and regenerative therapies(5). We have shown the generation of transgene-free human iPSCs from patients with different lung diseases using a single excisable polycistronic lentiviral Stem Cell Cassette (STEMCCA) encoding the Yamanaka factors(6). These iPSC lines were generated from skin fibroblasts, the most common cell type used for reprogramming. Normally, obtaining fibroblasts requires a skin punch biopsy followed by expansion of the cells in culture for a few passages. Importantly, a number of groups have reported the reprogramming of human peripheral blood cells into iPSCs(7-9). In one study, a Tet inducible version of the STEMCCA vector was employed(9), which required the blood cells to be simultaneously infected with a constitutively active lentivirus encoding the reverse tetracycline transactivator. In contrast to fibroblasts, peripheral blood cells can be collected via minimally invasive procedures, greatly reducing the discomfort and distress of the patient. A simple and effective protocol for reprogramming blood cells using a constitutive single excisable vector may accelerate the application of iPSC technology by making it accessible to a broader research community. Furthermore, reprogramming of peripheral blood cells allows for the generation of iPSCs from individuals in which skin biopsies should be avoided (i.e. aberrant scarring) or due to pre-existing disease conditions preventing access to punch biopsies. Here we demonstrate a protocol for the generation of human iPSCs from peripheral blood mononuclear cells (PBMCs) using a single floxed-excisable lentiviral vector constitutively expressing the 4 factors. Freshly collected or thawed PBMCs are expanded for 9 days as described(10,11) in medium containing ascorbic acid, SCF, IGF-1, IL-3 and EPO before being transduced with the STEMCCA lentivirus. Cells are then plated onto MEFs and ESC-like colonies can be visualized two weeks after infection. Finally, selected clones are expanded and tested for the expression of the pluripotency markers SSEA-4, Tra-1-60 and Tra-1-81. This protocol is simple, robust and highly consistent, providing a reliable methodology for the generation of human iPSCs from readily accessible 4 ml of blood.

Trisomy 21-associated defects in human primitive hematopoiesis revealed through induced pluripotent stem cells.

Patients with Down syndrome (trisomy 21, T21) have hematologic abnormalities throughout life. Newborns frequently exhibit abnormal blood counts and a clonal preleukemia. Human T21 fetal livers contain expanded erythro-megakaryocytic precursors with enhanced proliferative capacity. The impact of T21 on the earliest stages of embryonic hematopoiesis is unknown and nearly impossible to examine in human subjects. We modeled T21 yolk sac hematopoiesis using human induced pluripotent stem cells (iPSCs). Blood progenitor populations generated from T21 iPSCs were present at normal frequency and proliferated normally. However, their developmental potential was altered with enhanced erythropoiesis and reduced myelopoiesis, but normal megakaryocyte production. These abnormalities overlap with those of T21 fetal livers, but also reflect important differences. Our studies show that T21 confers distinct developmental stage- and species-specific hematopoietic defects. More generally, we illustrate how iPSCs can provide insight into early stages of normal and pathological human development.

Self-renewing endodermal progenitor lines generated from human pluripotent stem cells.

The use of human pluripotent stem cells for laboratory studies and cell-based therapies is hampered by their tumor-forming potential and limited ability to generate pure populations of differentiated cell types in vitro. To address these issues, we established endodermal progenitor (EP) cell lines from human embryonic and induced pluripotent stem cells. Optimized growth conditions were established that allow near unlimited (>10(16)) EP cell self-renewal in which they display a morphology and gene expression pattern characteristic of definitive endoderm. Upon manipulation of their culture conditions in vitro or transplantation into mice, clonally derived EP cells differentiate into numerous endodermal lineages, including monohormonal glucose-responsive pancreatic ß-cells, hepatocytes, and intestinal epithelia. Importantly, EP cells are nontumorigenic in vivo. Thus, EP cells represent a powerful tool to study endoderm specification and offer a potentially safe source of endodermal-derived tissues for transplantation therapies.

Generation of transgene-free lung disease-specific human induced pluripotent stem cells using a single excisable lentiviral stem cell cassette.

The development of methods to achieve efficient reprogramming of human cells while avoiding the permanent presence of reprogramming transgenes represents a critical step toward the use of induced pluripotent stem cells (iPSC) for clinical purposes, such as disease modeling or reconstituting therapies. Although several methods exist for generating iPSC free of reprogramming transgenes from mouse cells or neonatal normal human tissues, a sufficiently efficient reprogramming system is still needed to achieve the widespread derivation of disease-specific iPSC from humans with inherited or degenerative diseases. Here, we report the use of a humanized version of a single lentiviral "stem cell cassette" vector to accomplish efficient reprogramming of normal or diseased skin fibroblasts obtained from humans of virtually any age. Simultaneous transfer of either three or four reprogramming factors into human target cells using this single vector allows derivation of human iPSC containing a single excisable viral integration that on removal generates human iPSC free of integrated transgenes. As a proof of principle, here we apply this strategy to generate >100 lung disease-specific iPSC lines from individuals with a variety of diseases affecting the epithelial, endothelial, or interstitial compartments of the lung, including cystic fibrosis, α-1 antitrypsin deficiency-related emphysema, scleroderma, and sickle-cell disease. Moreover, we demonstrate that human iPSC generated with this approach have the ability to robustly differentiate into definitive endoderm in vitro, the developmental precursor tissue of lung epithelia.