A new mouse model of post-traumatic joint injury allows to identify the contribution of Gli1+ mesenchymal progenitors in arthrofibrosis and acquired heterotopic endochondral ossification

Complex injury and open reconstructive surgeries of the knee often lead to joint dysfunction that may alter the normal biomechanics of the joint. Two major complications that often arise are excessive deposition of fibrotic tissue and acquired heterotopic endochondral ossification. Knee arthrofibrosis is a fibrotic joint disorder where aberrant buildup of scar tissue and adhesions develop around the joint. Heterotopic ossification is ectopic bone formation around the periarticular tissues. Even though arthrofibrosis and heterotopic ossification pose an immense clinical problem, limited studies focus on their cellular and molecular mechanisms. Effective cell-targeted therapeutics are needed, but the cellular origin of both knee disorders remains elusive. Moreover, all the current animal models of knee arthrofibrosis and stiffness are developed in rats and rabbits, limiting genetic experiments that would allow us to explore the contribution of specific cellular targets to these knee pathologies. Here, we present a novel mouse model where surgically induced injury and hyperextension of the knee lead to excessive deposition of disorganized collagen in the meniscus, synovium, and joint capsule in addition to formation of extra-skeletal bone in muscle and soft tissues within the joint capsule. As a functional outcome, arthrofibrosis and acquired heterotopic endochondral ossification coupled with a significant increase in total joint stiffness were observed. By employing this injury model and genetic lineage tracing, we also demonstrate that Gli1+ mesenchymal progenitors proliferate after joint injury and contribute to the pool of fibrotic cells in the synovium and ectopic osteoblasts within the joint capsule. These findings demonstrate that Gli1+ cells are a major cellular contributor to knee arthrofibrosis and acquired heterotopic ossification that manifest after knee injury. Our data demonstrate that genetic manipulation of Gli1+ cells in mice may offer a platform for identification of novel therapeutic targets to prevent knee joint dysfunction after chronic injury.

Local Drug-Induced Modulation of gp130 Receptor Signaling Delays Disease Progression in a Pig Model of Temporo-Mandibular Joint Osteoarthritis

Temporomandibular joint disorders (TMJs) are a multifaceted group of chronic disorders characterized by stiffness in the jaw, limited jaw mobility and pain when opening or closing the mouth. TMJs are relatively common, with incidence rates in the range of 5–12%, with nearly twice as many women as men being affected. One of the primary causes of TMJs is a degenerative disease of joints, such as osteoarthritis (OA), characterized by progressive loss of cartilage which causes stiffness, swelling, and pain. Currently, there are no disease-modifying agents on the market for OA. We have recently discovered a small molecule, R805 acting as a modulator of glycoprotein 130 (gp130) receptor for IL-6 family of cytokines. R805 enables regenerative outputs of endogenous joint stem and progenitor cells through immunomodulation in the joint microenvironment by reducing the levels of destructive cytokines and supporting chondrocyte survival and anabolism. Extensive testing has shown R805 to be safe at doses far above the therapeutic level. Here, we have conducted a pivotal efficacy study in our newly-established pig model of TMJ post-traumatic OA. IA injection of R805 has shown a highly significant reduction of articular cartilage degeneration, reduced synovitis and degenerative changes in subchondral bone in the mandibular condyle compared to the vehicle-treated group. These data will support additional pre-clinical development of R805 as a first-in-class injectable therapeutic for TMJ osteoarthritis.

gp130/STAT3 signaling is required for homeostatic proliferation and anabolism in postnatal growth plate and articular chondrocytes

Growth of long bones and vertebrae is maintained postnatally by a long-lasting pool of progenitor cells. Little is known about the molecular mechanisms that regulate the output and maintenance of the cells that give rise to mature cartilage. Here we demonstrate that postnatal chondrocyte-specific deletion of a transcription factor Stat3 results in severely reduced proliferation coupled with increased hypertrophy, growth plate fusion, stunting and signs of progressive dysfunction of the articular cartilage. This effect is dimorphic, with females more strongly affected than males. Chondrocyte-specific deletion of the IL-6 family cytokine receptor gp130, which activates Stat3, phenocopied Stat3-deletion; deletion of Lifr, one of many co-receptors that signals through gp130, resulted in a milder phenotype. These data define a molecular circuit that regulates chondrogenic cell maintenance and output and reveals a pivotal positive function of IL-6 family cytokines in the skeletal system with direct implications for skeletal development and regeneration.

A Single-Cell Culture System for Dissecting Microenvironmental Signaling in Development and Disease of Cartilage Tissue

Cartilage tissue is comprised of extracellular matrix and chondrocytes, a cell type with very low cellular turnover in adults, providing limited capacity for regeneration. However, in development a significant number of chondrocytes actively proliferate and remodel the surrounding matrix. Uncoupling the microenvironmental influences that determine the balance between clonogenic potential and terminal differentiation of these cells is essential for the development of novel approaches for cartilage regeneration. Unfortunately, most of the existing methods are not applicable for the analysis of functional properties of chondrocytes at a single cell resolution. Here we demonstrate that a novel 3D culture method provides a long-term and permissive in vitro niche that selects for highly clonogenic, colony-forming chondrocytes which maintain cartilage-specific matrix production, thus recapitulating the in vivo niche. As a proof of concept, clonogenicity of Sox9^IRES–EGFP mouse chondrocytes is almost exclusively found in the highest GFP⁺ fraction known to be enriched for chondrocyte progenitor cells. Although clonogenic chondrocytes are very rare in adult cartilage, we have optimized this system to support large, single cell-derived chondrogenic organoids with complex zonal architecture and robust chondrogenic phenotype from adult pig and human articular chondrocytes. Moreover, we have demonstrated that growth trajectory and matrix biosynthesis in these organoids respond to a pro-inflammatory environment. This culture method offers a robust, defined and controllable system that can be further used to interrogate the effects of various microenvironmental signals on chondrocytes, providing a high throughput platform to assess genetic and environmental factors in development and disease.

Environmental factors influence somatic cell reprogramming to cardiomyocyte-like cells

Direct cardiac reprogramming, which refers to somatic cell (i.e. fibroblast) fate conversion to cardiomyocyte-like cell without transitioning through an intermediate pluripotent state, provides a novel therapeutic strategy for heart regeneration by converting resident cardiac fibroblasts to cardiomyocytes in situ. However, several limitations need to be addressed prior to clinical translation of this technology. They include low efficiency of reprogramming, heterogeneity of starting fibroblasts, functional immaturity of induced cardiomyocytes (iCMs), virus immunogenicity and toxicity, incomplete understanding of changes in the epigenetic landscape as fibroblasts undergo reprogramming, and the environmental factors that influence fate conversion. Several studies have demonstrated that a combination of enforced expression of cardiac transcription factors along with certain cytokines and growth factors in the presence of favorable environmental cues (including extracellular matrix, topography, and mechanical properties) enhance the efficiency and quality of direct reprogramming. This paper reviews the literature on the influence of the microenvironment on direct cardiac reprogramming in vitro and in vivo.

Symmetry breaking of tissue mechanics in wound induced hair follicle regeneration of laboratory and spiny mice

Tissue regeneration is a process that recapitulates and restores organ structure and function. Although previous studies have demonstrated wound-induced hair neogenesis (WIHN) in laboratory mice (Mus), the regeneration is limited to the center of the wound unlike those observed in African spiny (Acomys) mice. Tissue mechanics have been implicated as an integral part of tissue morphogenesis. Here, we use the WIHN model to investigate the mechanical and molecular responses of laboratory and African spiny mice, and report these models demonstrate opposing trends in spatiotemporal morphogenetic field formation with association to wound stiffness landscapes. Transcriptome analysis and K14-Cre-Twist1 transgenic mice show the Twist1 pathway acts as a mediator for both epidermal-dermal interactions and a competence factor for periodic patterning, differing from those used in development. We propose a Turing model based on tissue stiffness that supports a two-scale tissue mechanics process: (1) establishing a morphogenetic field within the wound bed (mm scale) and (2) symmetry breaking of the epidermis and forming periodically arranged hair primordia within the morphogenetic field (μm scale). Thus, we delineate distinct chemo-mechanical events in building a Turing morphogenesis-competent field during WIHN of laboratory and African spiny mice and identify its evo-devo advantages with perspectives for regenerative medicine.

A Human Skeletal Muscle Atlas Identifies the Trajectories of Stem and Progenitor Cells across Development and from Human Pluripotent Stem Cells

The developmental trajectory of human skeletal myogenesis and the transition between progenitor and stem cell states are unclear. We used single-cell RNA sequencing to profile human skeletal muscle tissues from embryonic, fetal, and postnatal stages. In silico, we identified myogenic as well as other cell types and constructed a "roadmap" of human skeletal muscle ontogeny across development. In a similar fashion, we also profiled the heterogeneous cell cultures generated from multiple human pluripotent stem cell (hPSC) myogenic differentiation protocols and mapped hPSC-derived myogenic progenitors to an embryonic-to-fetal transition period. We found differentially enriched biological processes and discovered co-regulated gene networks and transcription factors present at distinct myogenic stages. This work serves as a resource for advancing our knowledge of human myogenesis. It also provides a tool for a better understanding of hPSC-derived myogenic progenitors for translational applications in skeletal muscle-based regenerative medicine.

In vitro behavior of tendon stem/progenitor cells on bioactive electrospun nanofiber membranes for tendon-bone tissue engineering applications

Purpose

In order to accelerate the tendon-bone healing processes and achieve the efficient osteointegration between the tendon graft and bone tunnel, we aim to design bioactive electrospun nanofiber membranes combined with tendon stem/progenitor cells (TSPCs) to promote osteogenic regeneration of the tendon and bone interface.

Methods

In this study, nanofiber membranes of polycaprolactone (PCL), PCL/collagen I (COL-1) hybrid nanofiber membranes, poly(dopamine) (PDA)-coated PCL nanofiber membranes and PDA-coated PCL/COL-1 hybrid nanofiber membranes were successfully fabricated by electrospinning. The biochemical characteristics and nanofibrous morphology of the membranes, as well as the characterization of rat TSPCs, were defined in vitro. After co-culture with different types of electrospun nanofiber membranes in vitro, cell proliferation, viability, adhesion and osteogenic differentiation of TSPCs were evaluated at different time points.

Results

Among all the membranes, the performance of the PCL/COL-1 (volume ratio: 2:1 v/v) group was superior in terms of its ability to support the adhesion, proliferation, and osteogenic differentiation of TSPCs. No benefit was found in this study to include PDA coating on cell adhesion, proliferation and osteogenic differentiation of TSPCs.

Conclusion

The PCL/COL-1 hybrid electrospun nanofiber membranes are biocompatible, biomimetic, easily fabricated, and are capable of supporting cell adhesion, proliferation, and osteogenic differentiation of TSPCs. These bioactive electrospun nanofiber membranes may act as a suitable functional biomimetic scaffold in tendon-bone tissue engineering applications to enhance tendon-bone healing abilities.

Mapping molecular landmarks of human skeletal ontogeny and pluripotent stem cell-derived articular chondrocytes

Tissue-specific gene expression defines cellular identity and function, but knowledge of early human development is limited, hampering application of cell-based therapies. Here we profiled 5 distinct cell types at a single fetal stage, as well as chondrocytes at 4 stages in vivo and 2 stages during in vitro differentiation. Network analysis delineated five tissue-specific gene modules; these modules and chromatin state analysis defined broad similarities in gene expression during cartilage specification and maturation in vitro and in vivo, including early expression and progressive silencing of muscle- and bone-specific genes. Finally, ontogenetic analysis of freshly isolated and pluripotent stem cell-derived articular chondrocytes identified that integrin alpha 4 defines 2 subsets of functionally and molecularly distinct chondrocytes characterized by their gene expression, osteochondral potential in vitro and proliferative signature in vivo. These analyses provide new insight into human musculoskeletal development and provide an essential comparative resource for disease modeling and regenerative medicine.

Hepatic Leukemia Factor Maintains Quiescence of Hematopoietic Stem Cells and Protects the Stem Cell Pool during Regeneration

The transcription factor hepatic leukemia factor (HLF) is strongly expressed in hematopoietic stem cells (HSCs) and is thought to influence both HSC self-renewal and leukemogenesis. However, the physiological role of HLF in hematopoiesis and HSC function is unclear. Here, we report that mice lacking Hlf are viable with essentially normal hematopoietic parameters, including an intact HSC pool during steady-state hematopoiesis. In contrast, when challenged through transplantation, Hlf-deficient HSCs showed an impaired ability to reconstitute hematopoiesis and became gradually exhausted upon serial transplantation. Transcriptional profiling of Hlf-deficient HSCs revealed changes associated with enhanced cellular activation, and cell-cycle analysis demonstrated a significant reduction of quiescent HSCs. Accordingly, toxic insults targeting dividing cells completely eradicated the HSC pool in Hlf-deficient mice. In summary, our findings point to HLF as a critical regulator of HSC quiescence and as an essential factor for maintaining the HSC pool during regeneration.

Drug-induced modulation of gp130 signalling prevents articular cartilage degeneration and promotes repair

OBJECTIVE

Human adult articular cartilage (AC) has little capacity for repair, and joint surface injuries often result in osteoarthritis (OA), characterised by loss of matrix, hypertrophy and chondrocyte apoptosis. Inflammation mediated by interleukin (IL)-6 family cytokines has been identified as a critical driver of proarthritic changes in mouse and human joints, resulting in a feed-forward process driving expression of matrix degrading enzymes and IL-6 itself. Here we show that signalling through glycoprotein 130 (gp130), the common receptor for IL-6 family cytokines, can have both context-specific and cytokine-specific effects on articular chondrocytes and that a small molecule gp130 modulator can bias signalling towards anti-inflammatory and antidegenerative outputs.

METHODS

High throughput screening of 170 000 compounds identified a small molecule gp130 modulator termed regulator of cartilage growth and differentiation (RCGD 423) that promotes atypical homodimeric signalling in the absence of cytokine ligands, driving transient increases in MYC and pSTAT3 while suppressing oncostatin M- and IL-6-mediated activation of ERK and NF-κB via direct competition for gp130 occupancy.

RESULTS

This small molecule increased proliferation while reducing apoptosis and hypertrophic responses in adult chondrocytes in vitro. In a rat partial meniscectomy model, RCGD 423 greatly reduced chondrocyte hypertrophy, loss and degeneration while increasing chondrocyte proliferation beyond that observed in response to injury. Moreover, RCGD 423 improved cartilage healing in a rat full-thickness osteochondral defect model, increasing proliferation of mesenchymal cells in the defect and also inhibiting breakdown of cartilage matrix in de novo generated cartilage.

CONCLUSION

These results identify a novel strategy for AC remediation via small molecule-mediated modulation of gp130 signalling.

Whole genome sequencing and comparative transcriptome analysis of a novel seawater adapted, salt-resistant rice cultivar - sea rice 86

BACKGROUND

Rice (Oryza sativa) is critical for human nutrition worldwide. Due to a growing population, cultivars that produce high yields in high salinity soil are of major importance. Here we describe the discovery and molecular characterization of a novel sea water adapted rice strain, Sea Rice 86 (SR86).

RESULTS

SR86 can produce nutritious grains when grown in high salinity soil. Compared to a salt resistant rice cultivar, Yanfen 47 (YF47), SR86 grows in environments with up to 3X the salt content, and produces grains with significantly higher nutrient content in 12 measured components, including 2.9X calcium and 20X dietary fiber. Whole genome sequencing demonstrated that SR86 is a relatively ancient indica subspecies, phylogenetically close to the divergence point of the major rice varietals. SR86 has 12 chromosomes with a total genome size of 373,130,791 bps, slightly smaller than other sequenced rice genomes. Via comparison with 3000 rice genomes, we identified 42,359 putative unique, high impact variants in SR86. Transcriptome analysis of SR86 grown under normal and high saline conditions identified a large number of differentially expressed and salt-induced genes. Many of those genes fall into several gene families that have established or suggested roles in salt tolerance, while others represent potentially novel mediators of salt adaptation.

CONCLUSIONS

Whole genome sequencing and transcriptome analysis of SR86 has laid a foundation for further molecular characterization of several desirable traits in this novel rice cultivar. A number of candidate genes related to salt adaptation identified in this study will be valuable for further functional investigation.

Frequency of mononuclear diploid cardiomyocytes underlies natural variation in heart regeneration

Adult mammalian cardiomyocyte regeneration after injury is thought to be minimal. Mononuclear diploid cardiomyocytes (MNDCMs), a relatively small subpopulation in the adult heart, may account for the observed degree of regeneration, but this has not been tested. We surveyed 120 inbred mouse strains and found that the frequency of adult mononuclear cardiomyocytes was surprisingly variable (>7-fold). Cardiomyocyte proliferation and heart functional recovery after coronary artery ligation both correlated with pre-injury MNDCM content. Using genome-wide association, we identified Tnni3k as one gene that influences variation in this composition and demonstrated that Tnni3k knockout resulted in elevated MNDCM content and increased cardiomyocyte proliferation after injury. Reciprocally, overexpression of Tnni3k in zebrafish promoted cardiomyocyte polyploidization and compromised heart regeneration. Our results corroborate the relevance of MNDCMs in heart regeneration. Moreover, they imply that intrinsic heart regeneration is not limited nor uniform in all individuals, but rather is a variable trait influenced by multiple genes.

Abstract A07: A novel patient-derived xenograft model to define the role of TSLP-induced CRLF2 signals and identify therapies for Ph-like B-ALL

A subset of high-risk B cell acute lymphoblastic leukemia (ALL) shows a gene expression profile similar to Philadelphia chromosome positive (Ph+) ALL and has been described as Ph-like ALL. Approximately 50% of Ph-like B-ALL is characterized by genetic alterations leading to overexpression of CRLF2 (CRLF2 B-ALL). CRLF2 B-ALL occurs 5 times more often in Hispanic and Native American children than others and is prevalent in adolescents and young adults. Biologically, CRLF2 acts as a receptor component for the cytokine, TSLP, which induces JAK2-STAT5 and PI3/AKT/mTOR pathway activation downstream of binding to CRLF2. While activating JAK mutations are associated with CRLF2 B-ALL, over half of CRLF2 B-ALL lack such mutations. Our data show that primary human bone marrow (BM) stromal cells express TSLP. Thus TSLP is present in the tumor microenvironment to provide TSLP-induced CRLF2 signals that could play a role in the initiation, maintenance and/or progression of CRLF2 B-ALL. Consistent with this, TSLP has been reported to increase in vitro production of human fetal B cell precursors. However studies of TSLP in B lymphopoiesis have been conducted almost exclusively in mice which show low homology (~40%) with respect to human TSLP and CRLF2. Further, phospho flow cytometry assays show that human, but not mouse TSLP activates CRLF2 signals in primary human CRLF2 B-ALL cells and cell lines as indicated by increased pSTAT5, pAKT and pS6. These data indicate that the mouse TSLP present in classic patient derived xenograft models (PDX) does not produce the TSLP-induced CRLF2 signals present in the patient. To address this challenge we engineered PDX mice to produce human TSLP (hTSLP) by transplanting them with stromal cells transduced to express hTSLP (+T mice). Control (T) mice were produced by transplantation with stroma transduced with a control vector. Supernatant from engineered +T stroma, but not T stroma, induced JAK/STAT5 and PI3K/AKT/mTOR pathway activation in human CRLF2 B-ALL cells. ELISA assays showed that serum levels of hTSLP in mice was proportional to numbers of stromal cells injected at weekly time points. Normal human serum levels of hTSLP (12-32 pg/ml) could be achieved in +T mice, while hTSLP was undetectable in T mice. Because TSLP has been shown to increase in vitro production of human B cell precursors, we evaluated the in vivo functionality of our model by comparing the production of normal B cell precursors in the BM of +T and T PDX mice generated with human umbilical cord blood CD34+ cells. Data from 3 different cord blood donors showed that production of B cell precursors is 3-5 fold increased in +T as compared to T mice. TSLP-induced increases were specific to B lineage cells, initiated in the earliest CD19+ B cell precursors, and maintained through later stages of B cell development. Next we evaluate the in vivo functionality of our model using primary CRLF2 B-ALL leukemia cells. Human CRLF2 B-ALL cells were isolated from the BM of PDX mice and whole genome microarray was performed. Evaluation of microarray data by Gene Set Enrichment Analysis (GSEA) and Ingenuity Pathway Analysis showed that genes downstream of mTOR pathway activation were upregulated in +T as compared to T PDX mice, confirming hTSLP activity in the +T PDX mice. To determine whether +T PDX mice provide a preclinical model of B-ALL that more closely mirrors patients than T PDX mice, we compared RNAseq gene expression profiles of leukemia cells from +T and T PDX mice to that from original patient sample. The gene expression pattern in +T mice was significantly closer to primary patient sample than that from T mice. The +T and T PDX mice described here provide a novel preclinical model for studying the role of TSLP in the initiation, progression and maintenance of CRLF2 B-ALL and for evaluating drug efficacy in an in vivo model that more closely mirrors the in vivo environment present in patients.

Citation Format: Olivia L. Francis, Terry-Ann Milford, Ineavely Baez, Jacqueline S. Coats, Christopher L. Morris, Ross Fisher, Ben Van Handel, Ruijun Su, Batul Suterwala, Muhammad Kamal, Shadi Farzin Gohar, Sinisa Dovat, Kimberly J. Payne. A novel patient-derived xenograft model to define the role of TSLP-induced CRLF2 signals and identify therapies for Ph-like B-ALL. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Pediatric Cancer Research: From Mechanisms and Models to Treatment and Survivorship; 2015 Nov 9-12; Fort Lauderdale, FL. Philadelphia (PA): AACR; Cancer Res 2016;76(5 Suppl):Abstract nr A07.

Genetic Variability Overrides the Impact of Parental Cell Type and Determines iPSC Differentiation Potential

Reports on the retention of somatic cell memory in induced pluripotent stem cells (iPSCs) have complicated the selection of the optimal cell type for the generation of iPSC biobanks. To address this issue we compared transcriptomic, epigenetic, and differentiation propensities of genetically matched human iPSCs derived from fibroblasts and blood, two tissues of the most practical relevance for biobanking. Our results show that iPSC lines derived from the same donor are highly similar to each other. However, genetic variation imparts a donor-specific expression and methylation profile in reprogrammed cells that leads to variable functional capacities of iPSC lines. Our results suggest that integration-free, bona fide iPSC lines from fibroblasts and blood can be combined in repositories to form biobanks. Due to the impact of genetic variation on iPSC differentiation, biobanks should contain cells from large numbers of donors.

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Isogenic iPSC from fibroblasts and blood have similar differentiation propensities

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Donor-dependent variability affects molecular and differentiation propensities of iPSCs

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Impact of donor variability exceeds source-cell-specific differences in iPSC lines

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Bona fide iPSC lines from different tissues can be combined in the repositories

Endocardial-to-mesenchymal transformation and mesenchymal cell colonization at the onset of human cardiac valve development

The elucidation of mechanisms in semilunar valve development might enable the development of new therapies for congenital heart disorders. Here, we found differences in proliferation-associated genes and genes repressed by VEGF between human semilunar valve leaflets from first and second trimester hearts. The proliferation of valve interstitial cells and ventricular valve endothelial cells (VECs) and cellular density declined from the first to the second trimester. Cytoplasmic expression of NFATC1 was detected in VECs (4 weeks) and, later, cells in the leaflet/annulus junction mesenchyme expressing inactive NFATC1 (5.5-9 weeks) were detected, indicative of endocardial-to-mesenchymal transformation (EndMT) in valvulogenesis. At this leaflet/annulus junction, CD44(+) cells clustered during elongation (11 weeks), extending toward the tip along the fibrosal layer in second trimester leaflets. Differing patterns of maturation in the fibrosa and ventricularis were detected via increased fibrosal periostin content, which tracked the presence of the CD44(+) cells in the second trimester. We revealed that spatiotemporal NFATC1 expression actively regulates EndMT during human valvulogenesis, as early as 4 weeks. Additionally, CD44(+) cells play a role in leaflet maturation toward the trilaminar structure, possibly via migration of VECs undergoing EndMT, which subsequently ascend from the leaflet/annulus junction.

A novel xenograft model to study the role of TSLP-induced CRLF2 signals in normal and malignant human B lymphopoiesis

Thymic stromal lymphopoietin (TSLP) stimulates in-vitro proliferation of human fetal B-cell precursors. However, its in-vivo role during normal human B lymphopoiesis is unknown. Genetic alterations that cause overexpression of its receptor component, cytokine receptor-like factor 2 (CRLF2), lead to high-risk B-cell acute lymphoblastic leukemia implicating this signaling pathway in leukemogenesis. We show that mouse thymic stromal lymphopoietin does not stimulate the downstream pathways (JAK/STAT5 and PI3K/AKT/mTOR) activated by the human cytokine in primary high-risk leukemia with overexpression of the receptor component. Thus, the utility of classic patient-derived xenografts for in-vivo studies of this pathway is limited. We engineered xenograft mice to produce human thymic stromal lymphopoietin (+T mice) by injection with stromal cells transduced to express the cytokine. Control (-T) mice were produced using stroma transduced with control vector. Normal levels of human thymic stromal lymphopoietin were achieved in sera of +T mice, but were undetectable in -T mice. Patient-derived xenografts generated from +T as compared to -T mice showed a 3-6-fold increase in normal human B-cell precursors that was maintained through later stages of B-cell development. Gene expression profiles in high-risk B-cell acute lymphoblastic leukemia expanded in +T mice indicate increased mTOR pathway activation and are more similar to the original patient sample than those from -T mice. +T/-T xenografts provide a novel pre-clinical model for understanding this pathway in B lymphopoiesis and identifying treatments for high-risk B-cell acute lymphoblastic leukemia with overexpression of cytokine-like factor receptor 2.

Scl binds to primed enhancers in mesoderm to regulate hematopoietic and cardiac fate divergence

Scl/Tal1 confers hemogenic competence and prevents ectopic cardiomyogenesis in embryonic endothelium by unknown mechanisms. We discovered that Scl binds to hematopoietic and cardiac enhancers that become epigenetically primed in multipotent cardiovascular mesoderm, to regulate the divergence of hematopoietic and cardiac lineages. Scl does not act as a pioneer factor but rather exploits a pre-established epigenetic landscape. As the blood lineage emerges, Scl binding and active epigenetic modifications are sustained in hematopoietic enhancers, whereas cardiac enhancers are decommissioned by removal of active epigenetic marks. Our data suggest that, rather than recruiting corepressors to enhancers, Scl prevents ectopic cardiogenesis by occupying enhancers that cardiac factors, such as Gata4 and Hand1, use for gene activation. Although hematopoietic Gata factors bind with Scl to both activated and repressed genes, they are dispensable for cardiac repression, but necessary for activating genes that enable hematopoietic stem/progenitor cell development. These results suggest that a unique subset of enhancers in lineage-specific genes that are accessible for regulators of opposing fates during the time of the fate decision provide a platform where the divergence of mutually exclusive fates is orchestrated.

Abstract 11: Xenogen-free In Vitro Differentiation and Expansion of Human Pluripotent Stem Cell-Derived Cardiovascular Progenitor Cells

Background: Human pluripotent stem cell-derived cardiovascular progenitor cells (hPSC-CPCs) represent a tractable option for cell-based therapy for heart disease. However, to be clinically relevant, these cells must be derived under good manufacturing practices (GMP)-compatible conditions and produced in great enough quantities to treat adult patients. Here we sought to demonstrate for the first time the generation and expansion of clinically relevant numbers of hPSC-CPCs in xenogen-free protocol.

Methods and Results: GMP-grade human induced pluripotent stem cells (GMP-hiPSCs) and human embryonic stem cells (H1 and H9) were dissociated into single cells and cultured in low attachment dishes to differentiate into CPCs in StemPro medium including small molecules and human cytokines with high efficiency of 86%, 80% and 66% for GMP-hiPSCs, H1 and H9, respectively (Figure 1). All hPSC-CPCs possessed trilineage differentiation potentials, as shown by differentiation into endothelial and smooth muscle cells and functional cardiomyocytes (Figure 2). Moreover, sorted hPSC-CPCs expanded >5 fold in 10 days in xenogen-free conditions while still maintaining trilineage differentiation potential and an efficiency of ~70% (Figure 3).

Conclusions: Here we demonstrate a xenogeny-free CPC derivation and expansion protocol that can generate clinically relevant numbers of GMP-grade cardiovascular progenitors that could be used in a clinical setting.

Human developmental chondrogenesis as a basis for engineering chondrocytes from pluripotent stem cells

Joint injury and osteoarthritis affect millions of people worldwide, but attempts to generate articular cartilage using adult stem/progenitor cells have been unsuccessful. We hypothesized that recapitulation of the human developmental chondrogenic program using pluripotent stem cells (PSCs) may represent a superior approach for cartilage restoration. Using laser-capture microdissection followed by microarray analysis, we first defined a surface phenotype (CD166^low/negCD146^low/negCD73⁺CD44^lowBMPR1B⁺) distinguishing the earliest cartilage committed cells (prechondrocytes) at 5–6 weeks of development. Functional studies confirmed these cells are chondrocyte progenitors. From 12 weeks, only the superficial layers of articular cartilage were enriched in cells with this progenitor phenotype. Isolation of cells with a similar immunophenotype from differentiating human PSCs revealed a population of CD166^low/negBMPR1B⁺ putative cartilage-committed progenitors. Taken as a whole, these data define a developmental approach for the generation of highly purified functional human chondrocytes from PSCs that could enable substantial progress in cartilage tissue engineering.

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BMPR1B and LIFR mark immature primary chondrocytes throughout ontogeny

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LIF is highly expressed by synovial cells

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LIF inhibits chondrocyte maturation and hypertrophy

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Human development dictates how to generate chondrocyte-enriched progenitors from PSCs

Abstract 356: Scl Represses Cardiogenesis via Distant Enhancers during Hemogenic Endothelium Specification

Understanding the mechanisms directing mesoderm specification holds a great potential to advance the development of cell-based therapies for cardiovascular and blood disorders. The bHLH transcription factor Scl is known as the master regulator of the hematopoietic fate. We recently discovered that, in addition to its critical function in promoting the establishment of hemogenic endothelium during hematopoietic stem/progenitor cell (HS/PC) development, Scl is also required to repress cardiomyogenesis in endothelium in hematopoietic tissues and endocardium in the heart. However, the mechanisms for the cardiac repression have remained unknown.

Using ChIP-sequencing and microarray analysis of Flk+ mesoderm differentiated from mouse ES cells, we show that Scl both directly activates a broad gene regulatory network required for hemogenic endothelium and HS/PC development (e.g. Runx1, cMyb, Lyl1, Mef2C, Sox7 etc.), and directly represses transcriptional regulators required for cardiogenesis (e.g. Gata4, Gata6, Myocd, etc.) and mesoderm development (Eomes, Mixl1, Etv2, etc.). Repression of cardiac and mesodermal programs occurs during a short developmental window through Scl binding to distant enhancers, while binding to hematopoietic regulators extends throughout HS/PC and red blood cell development and encompasses both distant and proximal binding sites. We also discovered that, surprisingly, Scl complex partners Gata 1 and 2 are dispensable for hematopoietic vs. cardiac specification and Scl binding to majority of its target genes. Nevertheless, Gata factors co-operate with Scl to activate selected transcription factors that facilitate HS/PC emergence from hemogenic endothelium. These results denote Scl as a true master regulator of hematopoietic vs. cardiac fate choice and suggest a mechanism by which lineage-specific bHLH factors direct the divergence of competing fates.

Abstract 136: Distinct Temporal Requirements for Scl to Repress Cardiomyogenesis in Endothelial-derived Cells in Hemogenic Tissues and the Heart

Identification of precursors with the capacity to generate cardiomyocytes is critical for advancing cardiac regenerative medicine. By analyzing knockout embryos for the bHLH factor Scl, we demonstrated that endothelial cells in hematopoietic tissues and the heart possess latent cardiomyogenic capacity. Furthermore, analysis of tamoxifen-inducible Rosa26-Cre ERT2 Scl fl/fl embryos suggested that the time window during which Scl is required for cardiac repression extends later in the heart versus the yolk sac. However, the cell types in which Scl acts remained elusive. We then deleted Scl in a cell-type specific manner in early mesoderm using Mesp1-Cre and in endothelial cells using Tie2-Cre. Lineage tracing in Mesp1-Cre Rosa26-YFP embryos demonstrated that at E9.5, a large majority of hematopoietic and endothelial cells in the yolk sac and heart were labeled. Moreover, deletion of Scl in Mesp1-Cre Scl fl/fl embryos phenocopied the germline knockout, essentially abrogating hematopoiesis and promoting the emergence of CD31 + PDGFRα + cardiomyogenic precursors and ectopic expression of the cardiomyocyte genes Myl7 and Tnnt2 in yolk sac vasculature. In contrast, deletion of Scl after endothelium had been specified in Tie2-Cre Scl fl/fl embryos did not grossly affect yolk sac hematopoiesis, nor did it induce ectopic cardiomyogenesis in hemogenic tissues. However, endothelial-derived cells in the hearts of Tie2-Cre Scl fl/fl embryos evidenced profound expansion of CD31 + PDGFRα + cardiogenic precursors at E11.5 and E13.5, as well as displayed dramatic upregulation of Myl7 and Tnnt2 , showing that the requirement for Scl to repress the cardiomyogenic program extends longer in endothelial derivatives in the heart than in the yolk sac. These data demonstrate that endocardial-derived cells in the heart retain latent cardiomyogenic potential until mid-gestation and nominate Scl as a critical regulator of endocardial fate.

Expansion on stromal cells preserves the undifferentiated state of human hematopoietic stem cells despite compromised reconstitution ability

Lack of HLA-matched hematopoietic stem cells (HSC) limits the number of patients with life-threatening blood disorders that can be treated by HSC transplantation. So far, insufficient understanding of the regulatory mechanisms governing human HSC has precluded the development of effective protocols for culturing HSC for therapeutic use and molecular studies. We defined a culture system using OP9M2 mesenchymal stem cell (MSC) stroma that protects human hematopoietic stem/progenitor cells (HSPC) from differentiation and apoptosis. In addition, it facilitates a dramatic expansion of multipotent progenitors that retain the immunophenotype (CD34+CD38-CD90+) characteristic of human HSPC and proliferative potential over several weeks in culture. In contrast, transplantable HSC could be maintained, but not significantly expanded, during 2-week culture. Temporal analysis of the transcriptome of the ex vivo expanded CD34+CD38-CD90+ cells documented remarkable stability of most transcriptional regulators known to govern the undifferentiated HSC state. Nevertheless, it revealed dynamic fluctuations in transcriptional programs that associate with HSC behavior and may compromise HSC function, such as dysregulation of PBX1 regulated genetic networks. This culture system serves now as a platform for modeling human multilineage hematopoietic stem/progenitor cell hierarchy and studying the complex regulation of HSC identity and function required for successful ex vivo expansion of transplantable HSC.

Scl represses cardiomyogenesis in prospective hemogenic endothelium and endocardium

Endothelium in embryonic hematopoietic tissues generates hematopoietic stem/progenitor cells; however, it is unknown how its unique potential is specified. We show that transcription factor Scl/Tal1 is essential for both establishing the hematopoietic transcriptional program in hemogenic endothelium and preventing its misspecification to a cardiomyogenic fate. Scl(-/-) embryos activated a cardiac transcriptional program in yolk sac endothelium, leading to the emergence of CD31+Pdgfrα+ cardiogenic precursors that generated spontaneously beating cardiomyocytes. Ectopic cardiogenesis was also observed in Scl(-/-) hearts, where the disorganized endocardium precociously differentiated into cardiomyocytes. Induction of mosaic deletion of Scl in Scl(fl/fl)Rosa26Cre-ER(T2) embryos revealed a cell-intrinsic, temporal requirement for Scl to prevent cardiomyogenesis from endothelium. Scl(-/-) endothelium also upregulated the expression of Wnt antagonists, which promoted rapid cardiomyocyte differentiation of ectopic cardiogenic cells. These results reveal unexpected plasticity in embryonic endothelium such that loss of a single master regulator can induce ectopic cardiomyogenesis from endothelial cells.

Characterization and therapeutic potential of induced pluripotent stem cell-derived cardiovascular progenitor cells

BACKGROUND

Cardiovascular progenitor cells (CPCs) have been identified within the developing mouse heart and differentiating pluripotent stem cells by intracellular transcription factors Nkx2.5 and Islet 1 (Isl1). Study of endogenous and induced pluripotent stem cell (iPSC)-derived CPCs has been limited due to the lack of specific cell surface markers to isolate them and conditions for their in vitro expansion that maintain their multipotency.

METHODOLOGY/PRINCIPAL FINDINGS

We sought to identify specific cell surface markers that label endogenous embryonic CPCs and validated these markers in iPSC-derived Isl1(+)/Nkx2.5(+) CPCs. We developed conditions that allow propagation and characterization of endogenous and iPSC-derived Isl1(+)/Nkx2.5(+) CPCs and protocols for their clonal expansion in vitro and transplantation in vivo. Transcriptome analysis of CPCs from differentiating mouse embryonic stem cells identified a panel of surface markers. Comparison of these markers as well as previously described surface markers revealed the combination of Flt1(+)/Flt4(+) best identified and facilitated enrichment for Isl1(+)/Nkx2.5(+) CPCs from embryonic hearts and differentiating iPSCs. Endogenous mouse and iPSC-derived Flt1(+)/Flt4(+) CPCs differentiated into all three cardiovascular lineages in vitro. Flt1(+)/Flt4(+) CPCs transplanted into left ventricles demonstrated robust engraftment and differentiation into mature cardiomyocytes (CMs).

CONCLUSION/SIGNIFICANCE

The cell surface marker combination of Flt1 and Flt4 specifically identify and enrich for an endogenous and iPSC-derived Isl1(+)/Nkx2.5(+) CPC with trilineage cardiovascular potential in vitro and robust ability for engraftment and differentiation into morphologically and electrophysiologically mature adult CMs in vivo post transplantation into adult hearts.

Combinatorial assembly of developmental stage-specific enhancers controls gene expression programs during human erythropoiesis

Gene-distal enhancers are critical for tissue-specific gene expression, but their genomic determinants within a specific lineage at different stages of development are unknown. Here we profile chromatin state maps, transcription factor occupancy, and gene expression profiles during human erythroid development at fetal and adult stages. Comparative analyses of human erythropoiesis identify developmental stage-specific enhancers as primary determinants of stage-specific gene expression programs. We find that erythroid master regulators GATA1 and TAL1 act cooperatively within active enhancers but confer little predictive value for stage specificity. Instead, a set of stage-specific coregulators collaborates with master regulators and contributes to differential gene expression. We further identify and validate IRF2, IRF6, and MYB as effectors of an adult-stage expression program. Thus, the combinatorial assembly of lineage-specific master regulators and transcriptional coregulators within developmental stage-specific enhancers determines gene expression programs and temporal regulation of transcriptional networks in a mammalian genome.

Trophoblasts regulate the placental hematopoietic niche through PDGF-B signaling

The placenta is a hematopoietic organ that supports hematopoietic stem/progenitor cell (HSPC) generation and expansion without promoting differentiation. We identified PDGF-B signaling in trophoblasts as a key component of the unique placental hematopoietic microenvironment that protects HSPCs from premature differentiation. Loss of PDGF-B or its receptor, PDGFRβ, induced definitive erythropoiesis in placental labyrinth vasculature. This was evidenced by accumulation of CFU-Es and actively proliferating definitive erythroblasts that clustered around central macrophages, highly reminiscent of erythropoiesis in the fetal liver. Ectopic erythropoiesis was not due to a requirement of PDGF-B signaling in hematopoietic cells but rather in placental trophoblasts, which upregulated Epo in the absence of PDGF-B signaling. Furthermore, overexpression of hEPO specifically in the trophoblasts in vivo was sufficient to convert the placenta into an erythropoietic organ. These data provide genetic evidence of a signaling pathway that is required to restrict erythroid differentiation to specific anatomical niches during development.

Defining the nature of human pluripotent stem cell progeny

While it is clear that human pluripotent stem cells (hPSCs) can differentiate to generate a panoply of various cell types, it is unknown how closely in vitro development mirrors that which occurs in vivo. To determine whether human embryonic stem cells (hESCs) and human-induced pluripotent stem cells (hiPSCs) make equivalent progeny, and whether either makes cells that are analogous to tissue-derived cells, we performed comprehensive transcriptome profiling of purified PSC derivatives and their tissue-derived counterparts. Expression profiling demonstrated that hESCs and hiPSCs make nearly identical progeny for the neural, hepatic, and mesenchymal lineages, and an absence of re-expression from exogenous reprogramming factors in hiPSC progeny. However, when compared to a tissue-derived counterpart, the progeny of both hESCs and hiPSCs maintained expression of a subset of genes normally associated with early mammalian development, regardless of the type of cell generated. While pluripotent genes (OCT4, SOX2, REX1, and NANOG) appeared to be silenced immediately upon differentiation from hPSCs, genes normally unique to early embryos (LIN28A, LIN28B, DPPA4, and others) were not fully silenced in hPSC derivatives. These data and evidence from expression patterns in early human fetal tissue (3-16 weeks of development) suggest that the differentiated progeny of hPSCs are reflective of very early human development (< 6 weeks). These findings provide support for the idea that hPSCs can serve as useful in vitro models of early human development, but also raise important issues for disease modeling and the clinical application of hPSC derivatives.

Hematopoietic stem cell development in the placenta

The placenta is a highly vascularized organ that mediates fetal-maternal exchange during pregnancy and is thereby vital for the survival and growth of the developing embryo. In addition to having this well-established role in supporting pregnancy, the placenta was recently shown to function as a hematopoietic organ. The placenta is unique among other fetal hematopoietic organs, as it is capable of both generating multipotential hematopoietic cells de novo and establishing a major hematopoietic stem cell (HSC) pool in the conceptus, while protecting HSCs from premature differentiation. The mouse placenta contains two distinct vascular regions that support hematopoiesis: the large vessels in the chorionic plate where HSCs/progenitors are thought to emerge and the labyrinth vasculature where nascent HSCs/progenitors may colonize for expansion and possible functional maturation. Defining how this cytokine- and growth factor rich organ supports HSC generation, maturation and expansion may ultimately help to establish culture protocols for HSC expansion or de novo generation from pluripotent cells.

Recapitulation of the embryonic cardiovascular progenitor cell niche

Stem or progenitor cell populations are often established in unique niche microenvironments that regulate cell fate decisions. Although niches have been shown to be critical for the normal development of several tissues, their role in the cardiovascular system is poorly understood. In this study, we characterized the cardiovascular progenitor cell (CPC) niche in developing human and mouse hearts, identifying signaling pathways and extracellular matrix (ECM) proteins that are crucial for CPC maintenance and expansion. We demonstrate that collagen IV (ColIV) and β-catenin-dependent signaling are essential for maintaining and expanding undifferentiated CPCs. Since niches are three-dimensional (3D) structures, we investigated the impact of a 3D microenvironment that mimics the in vivo niche ECM. Employing electrospinning technologies, 3D in vitro niche substrates were bioengineered to serve as culture inserts. The three-dimensionality of these structures increased mouse embryonic stem cell differentiation into CPCs when compared to 2D control cultures, which was further enhanced by incorporation of ColIV into the substrates. Inhibiting p300-dependent β-catenin signals with the small molecule IQ1 facilitated further expansion of CPCs. Our study represents an innovative approach to bioengineer cardiac niches that can serve as unique 3D in vitro systems to facilitate CPC expansion and study CPC biology.

Human Fetal Hemoglobin Expression Is Regulated by the Developmental Stage-Specific Repressor BCL11A

Differences in the amount of fetal hemoglobin (HbF) that persists into adulthood affect the severity of sickle cell disease and the β-thalassemia syndromes. Genetic association studies have identified sequence variants in the gene BCL11A that influence HbF levels. Here, we examine BCL11A as a potential regulator of HbF expression. The high-HbF BCL11A genotype is associated with reduced BCL11A expression. Moreover, abundant expression of full-length forms of BCL11A is developmentally restricted to adult erythroid cells. Down-regulation of BCL11A expression in primary adult erythroid cells leads to robust HbF expression. Consistent with a direct role of BCL11A in globin gene regulation, we find that BCL11A occupies several discrete sites in the β-globin gene cluster. BCL11A emerges as a therapeutic target for reactivation of HbF in β-hemoglobin disorders.