A reference human induced pluripotent stem cell line for large-scale collaborative studies

Human induced pluripotent stem cell (iPSC) lines are a powerful tool for studying development and disease, but the considerable phenotypic variation between lines makes it challenging to replicate key findings and integrate data across research groups. To address this issue, we sub-cloned candidate human iPSC lines and deeply characterized their genetic properties using whole genome sequencing, their genomic stability upon CRISPR-Cas9-based gene editing, and their phenotypic properties including differentiation to commonly used cell types. These studies identified KOLF2.1J as an all-around well-performing iPSC line. We then shared KOLF2.1J with groups around the world who tested its performance in head-to-head comparisons with their own preferred iPSC lines across a diverse range of differentiation protocols and functional assays. On the strength of these findings, we have made KOLF2.1J and its gene-edited derivative clones readily accessible to promote the standardization required for large-scale collaborative science in the stem cell field.

Biofunctionalised bacterial cellulose scaffold supports the patterning and expansion of human embryonic stem cell-derived dopaminergic progenitor cells

BACKGROUND

Stem cell-based therapies for neurodegenerative diseases like Parkinson's disease are a promising approach in regenerative medicine and are now moving towards early stage clinical trials. However, a number of challenges remain including the ability to grow stem cells in vitro on a 3-dimensional scaffold, as well as their loss, by leakage or cell death, post-implantation. These issues could, however, be helped through the use of scaffolds that support the growth and differentiation of stem cells both in vitro and in vivo. The present study focuses on the use of bacterial cellulose as an in vitro scaffold to promote the growth of different stem cell-derived cell types. Bacterial cellulose was used because of its remarkable properties such as its wettability, ability to retain water and low stiffness, all of which is similar to that found in brain tissue.

METHODS

We cultured human embryonic stem cell-derived progenitor cells on bacterial cellulose with growth factors that were covalently functionalised to the surface via silanisation. Epifluorescence microscopy and immunofluorescence were used to detect the differentiation of stem cells into dopaminergic ventral midbrain progenitor cells. We then quantified the proportion of cells that differentiated into progenitor cells and compared the effect of growing cells on biofunctionalised cellulose versus standard cellulose.

RESULTS

We show that the covalent functionalisation of bacterial cellulose sheets with bioactive peptides improves the growth and differentiation of human pluripotent stem cells into dopaminergic neuronal progenitors.

CONCLUSIONS

This study suggests that the biocompatible material, bacterial cellulose, has potential applications in cell therapy approaches as a means to repair damage to the central nervous system, such as in Parkinson's disease but also in tissue engineering.

Assessing Human Embryonic Stem Cell-Derived Dopaminergic Neuron Progenitor Transplants Using Non-invasive Imaging Techniques

PURPOSE

Human pluripotent stem cell (hPSC)-derived dopaminergic neuron progenitor cells (DAPCs) are a potential therapy for Parkinson's disease (PD). However, their intracranial administration raises safety concerns including uncontrolled proliferation, migration and inflammation. Here, we apply a bimodal imaging approach to investigate the fate of DAPC transplants in the rat striatum.

PROCEDURES

DAPCs co-expressing luciferase and ZsGreen or labelled with micron-sized particles of iron oxide (MPIOs) were transplanted in the striatum of RNU rats (n = 6 per group). DAPCs were tracked in vivo using bioluminescence and magnetic resonance (MR) imaging modalities.

RESULTS

Transgene silencing in differentiating DAPCs accompanied with signal attenuation due to animal growth rendered the bioluminescence undetectable by week 2 post intrastriatal transplantation. However, MR imaging of MPIO-labelled DAPCs showed that transplanted cells remained at the site of injection for over 120 days. Post-mortem histological analysis of DAPC transplants demonstrated that labelling with either luciferase/ZsGreen or MPIOs did not affect the ability of cells to differentiate into mature dopaminergic neurons. Importantly, labelled cells did not elicit increased glial reactivity compared to non-labelled cells.

CONCLUSIONS

In summary, our findings support the transplantation of hPSC-derived DAPCs as a safe treatment for PD.

A blueprint for translational regenerative medicine

The past few decades have produced a large number of proof-of-concept studies in regenerative medicine. However, the route to clinical adoption is fraught with technical and translational obstacles that frequently consign promising academic solutions to the so-called "valley of death." Here, we present a proposed blueprint for translational regenerative medicine. We offer principles to help guide the selection of cells and materials, present key in vivo imaging modalities, and argue that the host immune response should be considered throughout design and development. Last, we suggest a pathway to navigate the often complex regulatory and manufacturing landscape of translational regenerative medicine.

Stem cell culture conditions and stability: a joint workshop of the PluriMes Consortium and Pluripotent Stem Cell Platform

Human stem cells have the potential to transform medicine. However, hurdles remain to ensure that manufacturing processes produce safe and effective products. A thorough understanding of the biological processes occurring during manufacture is fundamental to assuring these qualities and thus, their acceptability to regulators and clinicians. Leaders in both human pluripotent and somatic stem cells, were brought together with experts in clinical translation, biomanufacturing and regulation, to discuss key issues in assuring appropriate manufacturing conditions for delivery of effective and safe products from these cell types. This report summarizes the key issues discussed and records consensus reached by delegates and emphasizes the need for accurate language and nomenclature in the scientific discourse around stem cells.

Loss of hSef promotes metastasis through upregulation of EMT in prostate cancer

We have previously reported that the negative signaling regulator Similar Expression to FGF (hSef) is downregulated in prostate cancer and its loss is associated with clinical metastasis. Here, we explored the mechanistic basis of this finding. We first confirmed our clinical observation by testing hSef manipulation in an in vivo metastasis model. hSef stable expressing cells (PC3M-hSef) or empty vector controls (PC3M-EV) were injected subcutaneously into the lateral thoracic walls of NOD-SCID gamma mice and lungs were harvested at autopsy. In this model, 6/7 PC3M-EV xenografts had definitive lung micro-metastasis whilst only 1/6 PC3M-hSef xenografts exhibited metastasis recapitulating the clinical scenario (p = 0.03). Gene expression studies revealed key perturbations in genes involved in cell motility and epithelial to mesenchymal transition (EMT) along with alterations in cognate signaling pathways. These results were validated in an EMT specific PCR array whereby hSef over-expression and silencing reciprocally altered E-Cadherin expression (p = <0.001) amongst other EMT markers. Immunohistochemistry of excised tumors from the xenografts also confirmed the effect of hSef in suppressing E-Cadherin expression at the protein level. Phosphokinase arrays further demonstrated a role for hSef in attenuating signaling of not only ERK-MAPK but also the JNK and p38 pathways as well. Taken together, these data suggest evidence that loss of hSef may be a critical event facilitating tumor dissemination of prostate cancer through alteration of EMT. Detection of downregulated hSef, along with other negative regulators, may therefore be a useful biomarker heralding a transition to a metastatic phenotype and warrants further exploration in this context.

Abstract A24: A cascade of masked nuclear export signals regulates the BRCA2 tumor suppressor pathway

Inherited germline mutations in a single copy of the BRCA2 tumor suppressor predispose individuals to breast, ovarian, pancreatic and other cancers. BRCA2 exerts its tumor suppressive function in the cell nucleus through its role in the repair of DNA breaks by homologous recombination, where it controls the accumulation of the RAD51 recombinase enzyme at sites of DNA breakage. Cancer associated in-frame point mutations in BRCA2 have been shown to result in cytoplasmic localization of the protein, but the mechanism underlying this is unclear.

Here, we identify an unrecognized mechanism controlling the nuclear localization of BRCA2 and its cargo RAD51, which is disrupted by the common cancer-associated mis-sense mutation BRCA2D2723H. A nuclear export signal (NES) in BRCA2 is masked by its interaction with a partner protein, DSS1, such that point mutations impairing BRCA2-DSS1 binding render BRCA2 cytoplasmic. In turn, cytoplasmic mis-localization of mutant BRCA2 inhibits the nuclear retention of RAD51, by exposing a similar NES in RAD51 that is usually obscured by the BRCA2-RAD51 interaction. We demonstrate the functionality of these export sequences by cellular localization assays and in vitro binding analyses to the exportin CRM1.

We note two key implications of our work. Firstly, CRM1 mediated nuclear export is relevant to the localization of tumor suppressors such as p53 and BRCA1. However, this is the first report to our knowledge, of a ‘cascade’ of NES-masking interactions mediating nuclear localization for a tumor suppressor pathway. Our findings suggest the exploration of analogous mechanisms in other cancer-relevant cellular networks. Secondly, we also note that BRCA2D2723H decreases RAD51 nuclear retention even when wildtype BRCA2 is present. This is consistent with a transdominant effect for at least a fraction of the heterozygous disease-associated mutations noted in BRCA2. We find an increase in endogenous DNA damage in cells heterozygous for the D2723H mutation, suggesting that BRCA2D2723H heterozygosity causes a cumulative effect on genome stability in patients, which acts over years to promote carcinogenesis. Transdominance may also explain the paradox of tumorigenesis occurring in certain cases without a somatic second hit to BRCA2.

Together, our results link the nucleo-cytoplasmic translocation of the BRCA2 tumor suppressor to the cellular mechanisms of disease following its germline inactivation.

Citation Format: Anand D. Jeyasekharan, Yang Liu, Hiroyoshi Hattori, Venkat Pisupati, Eeson Rajendra, Asta Bjork Jonsdottir, Miyoung Lee, Elayanambi Sundaramoorthy, Simon Schlachter, Ashok Venkitaraman. A cascade of masked nuclear export signals regulates the BRCA2 tumor suppressor pathway. [abstract]. In: Proceedings of the Third AACR International Conference on Frontiers in Basic Cancer Research; Sep 18-22, 2013; National Harbor, MD. Philadelphia (PA): AACR; Cancer Res 2013;73(19 Suppl):Abstract nr A24.

A cancer-associated BRCA2 mutation reveals masked nuclear export signals controlling localization

Germline missense mutations affecting a single BRCA2 allele predispose humans to cancer. Here we identify a protein-targeting mechanism that is disrupted by the cancer-associated mutation, BRCA2(D2723H), and that controls the nuclear localization of BRCA2 and its cargo, the recombination enzyme RAD51. A nuclear export signal (NES) in BRCA2 is masked by its interaction with a partner protein, DSS1, such that point mutations impairing BRCA2-DSS1 binding render BRCA2 cytoplasmic. In turn, cytoplasmic mislocalization of mutant BRCA2 inhibits the nuclear retention of RAD51 by exposing a similar NES in RAD51 that is usually obscured by the BRCA2-RAD51 interaction. Thus, a series of NES-masking interactions localizes BRCA2 and RAD51 in the nucleus. Notably, BRCA2(D2723H) decreases RAD51 nuclear retention even when wild-type BRCA2 is also present. Our findings suggest a mechanism for the regulation of the nucleocytoplasmic distribution of BRCA2 and RAD51 and its impairment by a heterozygous disease-associated mutation.

Kruppel-associated Box (KRAB)-associated co-repressor (KAP-1) Ser-473 phosphorylation regulates heterochromatin protein 1β (HP1-β) mobilization and DNA repair in heterochromatin

The DNA damage response encompasses a complex series of signaling pathways that function to regulate and facilitate the repair of damaged DNA. Recent studies have shown that the repair of transcriptionally inactive chromatin, named heterochromatin, is dependent upon the phosphorylation of the co-repressor, Krüppel-associated box (KRAB) domain-associated protein (KAP-1), by the ataxia telangiectasia-mutated (ATM) kinase. Co-repressors, such as KAP-1, function to regulate the rigid structure of heterochromatin by recruiting histone-modifying enzymes, such HDAC1/2, SETDB1, and nucleosome-remodeling complexes such as CHD3. Here, we have characterized a phosphorylation site in the HP1-binding domain of KAP-1, Ser-473, which is phosphorylated by the cell cycle checkpoint kinase Chk2. Expression of a nonphosphorylatable S473A mutant conferred cellular sensitivity to DNA-damaging agents and led to defective repair of DNA double-strand breaks in heterochromatin. In addition, cells expressing S473A also displayed defective mobilization of the HP1-β chromodomain protein. The DNA repair defect observed in cells expressing S473A was alleviated by depletion of HP1-β, suggesting that phosphorylation of KAP-1 on Ser-473 promotes the mobilization of HP1-β from heterochromatin and subsequent DNA repair. These results suggest a novel mechanism of KAP-1-mediated chromatin restructuring via Chk2-regulated HP1-β exchange from heterochromatin, promoting DNA repair.

The GDI-like solubilizing factor PDEδ sustains the spatial organization and signalling of Ras family proteins

We identify a role for the GDI-like solubilizing factor (GSF) PDEδ in modulating signalling through Ras family G proteins by sustaining their dynamic distribution in cellular membranes. We show that the GDI-like pocket of PDEδ binds and solubilizes farnesylated Ras proteins, thereby enhancing their diffusion in the cytoplasm. This mechanism allows more effective trapping of depalmitoylated Ras proteins at the Golgi and polycationic Ras proteins at the plasma membrane to counter the entropic tendency to distribute these proteins over all intracellular membranes. Thus, PDEδ activity augments K/Hras signalling by enriching Ras at the plasma membrane; conversely, PDEδ down-modulation randomizes Ras distributions to all membranes in the cell and suppresses regulated signalling through wild-type Ras and also constitutive oncogenic Ras signalling in cancer cells. Our findings link the activity of PDEδ in determining Ras protein topography to Ras-dependent signalling.

Germline Brca2 heterozygosity promotes Kras(G12D) -driven carcinogenesis in a murine model of familial pancreatic cancer

Inherited heterozygous BRCA2 mutations predispose carriers to tissue-specific cancers, but somatic deletion of the wild-type allele is considered essential for carcinogenesis. We find in a murine model of familial pancreatic cancer that germline heterozygosity for a pathogenic Brca2 truncation suffices to promote pancreatic ductal adenocarcinomas (PDACs) driven by Kras(G12D), irrespective of Trp53 status. Unexpectedly, tumor cells retain a functional Brca2 allele. Correspondingly, three out of four PDACs from patients inheriting BRCA2(999del5) did not exhibit loss-of-heterozygosity (LOH). Three tumors from these patients displaying LOH were acinar carcinomas, which also developed only in mice with biallelic Brca2 inactivation. We suggest a revised model for tumor suppression by BRCA2 with implications for the therapeutic strategy targeting BRCA2 mutant cancer cells.

Specific Association of Mouse MDC1/NFBD1 with NBS1 at Sites of DNA-Damage

Human MDC1/NFBD1 has been found to interact with key players of the DNA-damage response machinery. Here, we identify and describe a functional homologue of MDC1/ NFBD1 in Mus musculus. The mouse homologue, mMDC1, retains the key motifs identified in the human protein and in response to ionizing radiation forms foci that co-localize with the MRE11-RAD50-NBS1 (MRN) complex and factors such as gammaH2AX and 53BP1. In addition, mMDC1 is associated with DNA damage sites generated during meiotic recombination as well as the X and Y chromosomes during the late stages of meiotic prophase I. Finally, whereas MDC1 shows strong colocalization with the MRN complex in response to DNA damage it does not co-localize with the MRN complex on replicating chromatin. These data suggest that mMDC1 is a marker for both exogenously and endogenously generated DNA double-stranded breaks and that its interaction with the MRN complex is initiated exclusively by DNA damage.