Barbara Corneo

Artemis, a novel DNA double-strand break repair/V(D)J recombination protein, is mutated in human severe combined immune deficiency.

The V(D)J recombination process insures the somatic diversification of immunoglobulin and antigen T cell receptor encoding genes. This reaction is initiated by a DNA double-strand break (dsb), which is resolved by the ubiquitously expressed DNA repair machinery. Human T-B-severe combined immunodeficiency associated with increased cellular radiosensitivity (RS-SCID) is characterized by a defect in the V(D)J recombination leading to an early arrest of both B and T cell maturation. We previously mapped the disease-related locus to the short arm of chromosome 10. We herein describe the cloning of the gene encoding a novel protein involved in V(D)J recombination/DNA repair, Artemis, whose mutations cause human RS-SCID. Protein sequence analysis strongly suggests that Artemis belongs to the metallo-beta-lactamase superfamily.

Stage-specific signaling through TGFβ family members and WNT regulates patterning and pancreatic specification of human pluripotent stem cells.

The generation of insulin-producing β-cells from human pluripotent stem cells is dependent on efficient endoderm induction and appropriate patterning and specification of this germ layer to a pancreatic fate. In this study, we elucidated the temporal requirements for TGFβ family members and canonical WNT signaling at these developmental stages and show that the duration of nodal/activin A signaling plays a pivotal role in establishing an appropriate definitive endoderm population for specification to the pancreatic lineage. WNT signaling was found to induce a posterior endoderm fate and at optimal concentrations enhanced the development of pancreatic lineage cells. Inhibition of the BMP signaling pathway at specific stages was essential for the generation of insulin-expressing cells and the extent of BMP inhibition required varied widely among the cell lines tested. Optimal stage-specific manipulation of these pathways resulted in a striking 250-fold increase in the levels of insulin expression and yielded populations containing up to 25% C-peptide+ cells.

Rag mutations reveal robust alternative end joining

Mammalian cells repair DNA double-strand breaks (DSBs) through either homologous recombination or non-homologous end joining (NHEJ). V(D)J recombination, a cut-and-paste mechanism for generating diversity in antigen receptors, relies on NHEJ for repairing DSBs introduced by the Rag1–Rag2 protein complex. Animals lacking any of the seven known NHEJ factors are therefore immunodeficient. Nevertheless, DSB repair is not eliminated entirely in these animals: evidence of a third mechanism, ‘alternative NHEJ’, appears in the form of extremely rare V(D)J junctions and a higher rate of chromosomal translocations. The paucity of these V(D)J events has suggested that alternative NHEJ contributes little to a cell’s overall repair capacity, being operative only (and inefficiently) when classical NHEJ fails. Here we find that removing certain portions of murine Rag proteins reveals robust alternative NHEJ activity in NHEJ-deficient cells and some alternative joining activity even in wild-type cells. We propose a two-tier model in which the Rag proteins collaborate with NHEJ factors to preserve genomic integrity during V(D)J recombination.

Adult Human RPE Can Be Activated into a Multipotent Stem Cell that Produces Mesenchymal Derivatives

The retinal pigment epithelium (RPE) is a monolayer of cells underlying and supporting the neural retina. It begins as a plastic tissue, capable, in some species, of generating lens and retina, but differentiates early in development and remains normally nonproliferative throughout life. Here we show that a subpopulation of adult human RPE cells can be activated in vitro to a self-renewing cell, the retinal pigment epithelial stem cell (RPESC) that loses RPE markers, proliferates extensively, and can redifferentiate into stable cobblestone RPE monolayers. Clonal studies demonstrate that RPESCs are multipotent and in defined conditions can generate both neural and mesenchymal progeny. This plasticity may explain human pathologies in which mesenchymal fates are seen in the eye, for example in proliferative vitroretinopathy (PVR) and phthisis bulbi. This study establishes the RPESC as an accessible, human CNS-derived multipotent stem cell, useful for the study of fate choice, replacement therapy, and disease modeling.

Human RPE stem cells grown into polarized RPE monolayers on a polyester matrix are maintained after grafting into rabbit subretinal space.

Summary Transplantation of the retinal pigment epithelium (RPE) is being developed as a cell-replacement therapy for age-related macular degeneration. Human embryonic stem cell (hESC) and induced pluripotent stem cell (iPSC)-derived RPE are currently translating toward clinic. We introduce the adult human RPE stem cell (hRPESC) as an alternative RPE source. Polarized monolayers of adult hRPESC-derived RPE grown on polyester (PET) membranes had near-native characteristics. Trephined pieces of RPE monolayers on PET were transplanted subretinally in the rabbit, a large-eyed animal model. After 4 days, retinal edema was observed above the implant, detected by spectral domain optical coherence tomography (SD-OCT) and fundoscopy. At 1 week, retinal atrophy overlying the fetal or adult transplant was observed, remaining stable thereafter. Histology obtained 4 weeks after implantation confirmed a continuous polarized human RPE monolayer on PET. Taken together, the xeno-RPE survived with retained characteristics in the subretinal space. These experiments support that adult hRPESC-derived RPE are a potential source for transplantation therapies.

CORTECON: A Temporal Transcriptome Analysis of In Vitro Human Cerebral Cortex Development from Human Embryonic Stem Cells

Many neurological and psychiatric disorders affect the cerebral cortex, and a clearer understanding of the molecular processes underlying human corticogenesis will provide greater insight into such pathologies. To date, knowledge of gene expression changes accompanying corticogenesis is largely based on murine data. Here we present a searchable, comprehensive, temporal gene expression data set encompassing cerebral cortical development from human embryonic stem cells (hESCs). Using a modified differentiation protocol that yields neurons suggestive of prefrontal cortex, we identified sets of genes and long noncoding RNAs that significantly change during corticogenesis and those enriched for disease-associations. Numerous alternatively spliced genes with varying temporal patterns of expression are revealed, including TGIF1, involved in holoprosencephaly, and MARK1, involved in autism. We have created a database (http://cortecon.neuralsci.org/) that provides online, query-based access to changes in RNA expression and alternatively spliced transcripts during human cortical development.

Epstein-Barr virus-associated post-transplant lympho-proliferative disease of donor origin in liver transplant recipients

BACKGROUND/AIMS Post-transplant lymphoproliferative disease, a potential complication of solid organ transplantation, occurs in about 3% of orthotopic liver transplant recipients. We report the genetic and virological characterization of two cases of post-transplant lymphoproliferative disease that occurred early (4 and 6 months) after orthotopic liver transplant as large-cell non-Hodgkins lymphomas located at the hepatic hilum. METHODS Lymphomatous tissues were analyzed for clonality and presence of Epstein-Barr virus (EBV) sequences by Southern blot, polymerase chain reaction, and in situ hybridization techniques. RESULTS The tumors in both cases were sustained by a clonal proliferation of B lymphocytes containing type A EBV DNA. Moreover, in situ hybridization with a digoxigenin-labeled EBV-specific probe evidenced a strong nuclear signal in most of the neoplastic cells. DNA microsatellite analysis at three different loci detected alleles of donor origin in both tumor samples, suggesting that the neoplastic B cells were of donor origin. CONCLUSIONS EBV-infected donor B lymphocytes might be responsible for intragraft post-transplant lymphoproliferative disease in orthotopic liver transplant recipients. As 20 to 30% of post-transplant lymphomas involve the graft itself, donor-derived post-transplant lymphoproliferative disease might be more frequent than presently appreciated. Prospective studies are needed to assess its real incidence and identify possible risk factors.

A Multiplex High-Throughput Gene Expression Assay to Simultaneously Detect Disease and Functional Markers in Induced Pluripotent Stem Cell-Derived Retinal Pigment Epithelium

There is continuing interest in the development of lineage‐specific cells from induced pluripotent stem (iPS) cells for use in cell therapies and drug discovery. Although in most cases differentiated cells show features of the desired lineage, they retain fetal gene expression and do not fully mature into “adult‐like” cells. Such cells may not serve as an effective therapy because, once implanted, immature cells pose the risk of uncontrolled growth. Therefore, there is a need to optimize lineage‐specific stem cell differentiation protocols to produce cells that no longer express fetal genes and have attained “adult‐like” phenotypes. Toward that goal, it is critical to develop assays that simultaneously measure cell function and disease markers in high‐throughput format. Here, we use a multiplex high‐throughput gene expression assay that simultaneously detects endogenous expression of multiple developmental, functional, and disease markers in iPS cell‐derived retinal pigment epithelium (RPE). We optimized protocols to differentiate iPS cell‐derived RPE that was then grown in 96‐ and 384‐well plates. As a proof of principle, we demonstrate differential expression of eight genes in iPS cells, iPS cell‐derived RPE at two different differentiation stages, and primary human RPE using this multiplex assay. The data obtained from the multiplex gene expression assay are significantly correlated with standard quantitative reverse transcription‐polymerase chain reaction‐based measurements, confirming the ability of this high‐throughput assay to measure relevant gene expression changes. This assay provides the basis to screen for compounds that improve RPE function and maturation and target disease pathways, thus providing the basis for effective treatments of several retinal degenerative diseases.

KDR identifies a conserved human and murine hepatic progenitor and instructs early liver development.

Understanding the fetal hepatic niche is essential for optimizing the generation of functional hepatocyte-like cells (hepatic cells) from human embryonic stem cells (hESCs). Here, we show that KDR (VEGFR2/FLK-1), previously assumed to be mostly restricted to mesodermal lineages, marks a hESC-derived hepatic progenitor. hESC-derived endoderm cells do not express KDR but, when cultured in media supporting hepatic differentiation, generate KDR+ hepatic progenitors and KDR- hepatic cells. KDR+ progenitors require active KDR signaling both to instruct their own differentiation into hepatic cells and to non-cell-autonomously support the functional maturation of cocultured KDR- hepatic cells. Analysis of human fetal livers suggests that similar progenitors are present in human livers. Lineage tracing in mice provides in vivo evidence of a KDR+ hepatic progenitor for fetal hepatoblasts, adult hepatocytes, and adult cholangiocytes. Altogether, our findings reveal that KDR is a conserved marker for endoderm-derived hepatic progenitors and a functional receptor instructing early liver development.

Sustained Levels of FGF2 Maintain Undifferentiated Stem Cell Cultures with Biweekly Feeding

An essential aspect of stem cell culture is the successful maintenance of the undifferentiated state. Many types of stem cells are FGF2 dependent, and pluripotent stem cells are maintained by replacing FGF2-containing media daily, while tissue-specific stem cells are typically fed every 3rd day. Frequent feeding, however, results in significant variation in growth factor levels due to FGF2 instability, which limits effective maintenance due to spontaneous differentiation. We report that stabilization of FGF2 levels using controlled release PLGA microspheres improves expression of stem cell markers, increases stem cell numbers and decreases spontaneous differentiation. The controlled release FGF2 additive reduces the frequency of media changes needed to maintain stem cell cultures, so that human embryonic stem cells and induced pluripotent stem cells can be maintained successfully with biweekly feedings.