Liver organogenesis promoted by endothelial cells prior to vascular function

Tissue-specific endothelial cells: a promising approach for augmentation of soft tissue repair in orthopedics

Biologics are playing an increasingly significant role in the practice of modern medicine and surgery in general and orthopedics in particular. Cell-based approaches are among the most important and widely used modalities in orthopedic biologics, with mesenchymal stem cells and other multi/pluripotent cells undergoing evaluation in numerous preclinical and clinical studies. On the other hand, fully differentiated endothelial cells (ECs) have been found to perform critical roles in homeostasis of visceral tissues through production of an adaptive panel of so-called "angiocrine factors." This newly discovered function of ECs renders them excellent candidates for novel approaches in cell-based biologics. Here, we present a review of the role of ECs and angiocrine factors in some visceral tissues, followed by an overview of current cell-based approaches and a discussion of the potential applications of ECs in soft tissue repair.

International consensus guidelines for scoring the histopathological growth patterns of liver metastasis

BACKGROUND

Liver metastases present with distinct histopathological growth patterns (HGPs), including the desmoplastic, pushing and replacement HGPs and two rarer HGPs. The HGPs are defined owing to the distinct interface between the cancer cells and the adjacent normal liver parenchyma that is present in each pattern and can be scored from standard haematoxylin-and-eosin-stained (H&E) tissue sections. The current study provides consensus guidelines for scoring these HGPs.

METHODS

Guidelines for defining the HGPs were established by a large international team. To assess the validity of these guidelines, 12 independent observers scored a set of 159 liver metastases and interobserver variability was measured. In an independent cohort of 374 patients with colorectal liver metastases (CRCLM), the impact of HGPs on overall survival after hepatectomy was determined.

RESULTS

Good-to-excellent correlations (intraclass correlation coefficient >0.5) with the gold standard were obtained for the assessment of the replacement HGP and desmoplastic HGP. Overall survival was significantly superior in the desmoplastic HGP subgroup compared with the replacement or pushing HGP subgroup (P=0.006).

CONCLUSIONS

The current guidelines allow for reproducible determination of liver metastasis HGPs. As HGPs impact overall survival after surgery for CRCLM, they may serve as a novel biomarker for individualised therapies.

Endothelial cell regulation of salivary gland epithelial patterning

Perfusion-independent regulation of epithelial pattern formation by the vasculature during organ development and regeneration is of considerable interest for application in restoring organ function. During murine submandibular salivary gland development, the vasculature co-develops with the epithelium during branching morphogenesis; however, it is not known whether the vasculature has instructive effects on the epithelium. Using pharmacological inhibitors and siRNA knockdown in embryonic organ explants, we determined that VEGFR2-dependent signaling is required for salivary gland epithelial patterning. To test directly for a requirement for endothelial cells in instructive epithelial patterning, we developed a novel ex vivo cell fractionation/reconstitution assay. Immuno-depletion of CD31⁺ endothelial cells in this assay confirmed a requirement for endothelial cells in epithelial patterning of the gland. Depletion of endothelial cells or inhibition of VEGFR2 signaling in organ explants caused an aberrant increase in cells expressing the ductal proteins K19 and K7, with a reduction in Kit⁺ progenitor cells in the endbuds of reconstituted glands. Addition of exogenous endothelial cells to reconstituted glands restored epithelial patterning, as did supplementation with the endothelial cell-regulated mesenchymal factors IGFBP2 and IGFBP3. Our results demonstrate that endothelial cells promote expansion of Kit⁺ progenitor cells and suppress premature ductal differentiation in early developing embryonic submandibular salivary gland buds.

Activated ovarian endothelial cells promote early follicular development and survival

Background

New data suggests that endothelial cells (ECs) elaborate essential “angiocrine factors”. The aim of this study is to investigate the role of activated ovarian endothelial cells in early in-vitro follicular development.

Methods

Mouse ovarian ECs were isolated using magnetic cell sorting or by FACS and cultured in serum free media. After a constitutive activation of the Akt pathway was initiated, early follicles (50–150 um) were mechanically isolated from 8-day-old mice and co-cultured with these activated ovarian endothelial cells (AOEC) (n = 32), gel (n = 24) or within matrigel (n = 27) in serum free media for 14 days. Follicular growth, survival and function were assessed.

Results

After 6 passages, flow cytometry showed 93% of cells grown in serum-free culture were VE-cadherin positive, CD-31 positive and CD 45 negative, matching the known EC profile. Beginning on day 4 of culture, we observed significantly higher follicular and oocyte growth rates in follicles co-cultured with AOECs compared with follicles on gel or matrigel. After 14 days of culture, 73% of primary follicles and 83% of secondary follicles co-cultured with AOEC survived, whereas the majority of follicles cultured on gel or matrigel underwent atresia.

Conclusions

This is the first report of successful isolation and culture of ovarian ECs. We suggest that co-culture with activated ovarian ECs promotes early follicular development and survival. This model is a novel platform for the in vitro maturation of early follicles and for the future exploration of endothelial-follicular communication.

Capsule

In vitro development of early follicles necessitates a complex interplay of growth factors and signals required for development. Endothelial cells (ECs) may elaborate essential “angiocrine factors” involved in organ regeneration. We demonstrate that co-culture with ovarian ECs enables culture of primary and early secondary mouse ovarian follicles.

Pluripotent stem cell-derived organoids: using principles of developmental biology to grow human tissues in a dish

Pluripotent stem cell (PSC)-derived organoids are miniature, three-dimensional human tissues generated by the application of developmental biological principles to PSCs in vitro The approach to generate organoids uses a combination of directed differentiation, morphogenetic processes, and the intrinsically driven self-assembly of cells that mimics organogenesis in the developing embryo. The resulting organoids have remarkable cell type complexity, architecture and function similar to their in vivo counterparts. In the past five years, human PSC-derived organoids with components of all three germ layers have been generated, resulting in the establishment of a new human model system. Here, and in the accompanying poster, we provide an overview of how principles of developmental biology have been essential for generating human organoids in vitro, and how organoids are now being used as a primary research tool to investigate human developmental biology.

Paracrine signals regulate human liver organoid maturation from induced pluripotent stem cells

A self-organizing organoid model provides a new approach to study the mechanism of human liver organogenesis. Previous animal models documented that simultaneous paracrine signaling and cell-to-cell surface contact regulate hepatocyte differentiation. To dissect the relative contributions of the paracrine effects, we first established a liver organoid using human induced pluripotent stem cells (iPSCs), mesenchymal stem cells (MSCs) and human umbilical vein endothelial cells (HUVECs) as previously reported. Time-lapse imaging showed that hepatic-specified endoderm iPSCs (HE-iPSCs) self-assembled into three-dimensional organoids, resulting in hepatic gene induction. Progressive differentiation was demonstrated by hepatic protein production after in vivo organoid transplantation. To assess the paracrine contributions, we employed a Transwell system in which HE-iPSCs were separately co-cultured with MSCs and/or HUVECs. Although the three-dimensional structure did not form, their soluble factors induced a hepatocyte-like phenotype in HE-iPSCs, resulting in the expression of bile salt export pump. In conclusion, the mesoderm-derived paracrine signals promote hepatocyte maturation in liver organoids, but organoid self-organization requires cell-to-cell surface contact. Our in vitro model demonstrates a novel approach to identify developmental paracrine signals regulating the differentiation of human hepatocytes.

Human bone marrow mesenchymal stem cells for retinal vascular injury

PURPOSE

To examine the potential of intravitreally implanted human bone marrow-derived mesenchymal stem cells (BMSCs) to affect vascular repair and the blood-retina barrier in mice and rats with oxygen-induced retinopathy, diabetic retinopathy or retinal ischaemia-reperfusion damage.

METHODS

Three study groups (oxygen-induced retinopathy group: 18 C57BL/6J mice; diabetic retinopathy group: 15 rats; retinal ischaemia-reperfusion model: 18 rats) received BMSCs injected intravitreally. Control groups (oxygen-induced retinopathy group: 12 C57BL/6J mice; diabetic retinopathy group: 15 rats; retinal ischaemia-reperfusion model: 18 rats) received an intravitreal injection of phosphate-buffered saline. We applied immunohistological techniques to measure retinal vascularization, spectroscopic measurements of intraretinally extravasated fluorescein-conjugated dextran to quantify the blood-retina barrier breakdown, and histomorphometry to assess retinal thickness and retinal ganglion cell count.

RESULTS

In the oxygen-induced retinopathy model, the study group with intravitreally injected BMSCs as compared with the control group showed a significantly (p = 0.001) smaller area of retinal neovascularization. In the diabetic retinopathy model, study group and control group did not differ significantly in the amount of intraretinally extravasated dextran. In the retinal ischaemia-reperfusion model, on the 7th day after retina injury, the retina was significantly thicker in the study group than in the control group (p = 0.02), with no significant difference in the retinal ganglion cell count (p = 0.36).

CONCLUSIONS

Intravitreally implanted human BMSCs were associated with a reduced retinal neovascularization in the oxygen-induced retinopathy model and with a potentially cell preserving effect in the retinal ischaemia-reperfusion model. Intravitreal BMSCs may be of potential interest for the therapy of retinal vascular disorders.

Retinal vascular injuries and intravitreal human embryonic stem cell-derived haemangioblasts

OBJECTIVE

To investigate whether intravitreally applied haemangioblasts (HB) derived from human embryonic stem cells (hESCs) are helpful for the repair of vascular damage caused in animals by an oxygen-induced retinopathy (OIR), by an induced diabetic retinopathy (DR) or by an induced retinal ischaemia with subsequent reperfusion.

METHODS

Human embryonic stem cell-derived HBs were transplanted intravitreally into C57BL/6J mice (OIR model), into male Wistar rats with an induced DR and into male Wistar rats undergoing induced retinal ischaemia with subsequent reperfusion. Control groups of animals received an intravitreal injection of endothelial cells (ECs) or phosphate-buffered saline (PBS). We examined the vasculature integrity in the mice with OIR, the blood-retina barrier in the rats with induced DR, and retinal thickness and retinal ganglion cell density in retina flat mounts of the rats with the retinal ischaemic-reperfusion retinopathy.

RESULTS

In the OIR model, the study group versus control groups showed a significantly (p < 0.001) smaller retinal avascular area [5.1 ± 2.7%;n = 18 animals versus 12.2 ± 2.8% (PBS group; n = 10 animals) and versus 11.8 ± 3.7% (EC group; n = 8 animals)] and less retinal neovascularization [6.3 ± 2.5%;n = 18 versus 15.2 ± 6.3% (n = 10; PBS group) and versus 15.8 ± 3.3% (n = 8; EC group)]. On retinal flat mounts, hESC-HBs were integrated into damaged retinal vessels and stained positive for PECAM (CD31) as EC marker. In the DR model, the study group versus the EC control group showed a significantly (p = 0.001) better blood-retina barrier function as measured at 2 days after the intravitreal injections [study group: 20.2 ± 12.8 μl/(g × hr); n = 6; versus EC control group: 52.9 ± 9.9 μl/(g × hr; n = 6)]. In the retinal ischaemia-reperfusion model, the groups did not differ significantly in retinal thickness and retinal ganglion cell density at 2, 5 and 7 days after baseline.

CONCLUSION

By integrating into damaged retinal vessels and differentiating into ECs, intravitreally administered hESC-HBs may have partially repaired a retinal vascular injury caused by OIR model and DR.

Cell-based approaches for augmentation of tendon repair

Cell-based approaches are among the principal interventions in orthobiologics to improve tendon and ligament healing and to combat degenerative processes. The number of options available for investigation are expanding rapidly and investigators have an increasing number of cell types to choose from for research purposes. However, in part due to the current regulatory environment, the list of available cells at clinicians' disposal for therapeutic purposes is still rather limited. In this review, we present an overview of the main cellular categories in current use. Notable recent developments in cell-based approaches include the introduction of diverse sources of mesenchymal stem cells, pluripotent cells of extra-embryonic origin, and the emerging popularity of fully differentiated cells such as tenocytes and endothelial cells. Delivery strategies are discussed and a succinct discussion of the current regulatory environment in the United States is presented.

Vascular endothelium plays a key role in directing pulmonary epithelial cell differentiation

J. Yao et al. demonstrate that loss of MGP, a BMP inhibitor, causes abnormal hepatic differentiation in lungs. They find that interactions between endothelium and epithelium separate pulmonary from hepatic differentiation during development. Lack of MGP triggers hepatic differentiation in the pulmonary epithelium, as regulated by the endothelium.

The vascular endothelium is critical for induction of appropriate lineage differentiation in organogenesis. In this study, we report that dysfunctional pulmonary endothelium, resulting from the loss of matrix Gla protein (MGP), causes ectopic hepatic differentiation in the pulmonary epithelium. We demonstrate uncontrolled induction of the hepatic growth factor (HGF) caused by dysregulated cross talk between pulmonary endothelium and epithelium in Mgp-null lungs. Elevated HGF induced hepatocyte nuclear factor 4 α (Hnf4a), which competed with NK2 homeobox 1 (Nkx2.1) for binding to forkhead box A2 (Foxa2) to drive hepatic differentiation in Mgp-null airway progenitor cells. Limiting endothelial HGF reduced Hnf4a, abolished interference of Hnf4a with Foxa2, and reduced hepatic differentiation in Mgp-null lungs. Together, our results suggest that endothelial–epithelial interactions, maintained by MGP, are essential in pulmonary cell differentiation.

The fate of the vitelline and umbilical veins during the development of the human liver

Differentiation of endodermal cells into hepatoblasts is well studied, but the remodeling of the vitelline and umbilical veins during liver development is less well understood. We compared human embryos between 3 and 10 weeks of development with pig and mouse embryos at comparable stages, and used Amira 3D reconstruction and Cinema 4D remodeling software for visualization. The vitelline and umbilical veins enter the systemic venous sinus on each side via a common entrance, the hepatocardiac channel. During expansion into the transverse septum at Carnegie Stage (CS)12 the liver bud develops as two dorsolateral lobes or 'wings' and a single ventromedial lobe, with the liver hilum at the intersection of these lobes. The dorsolateral lobes each engulf a vitelline vein during CS13 and the ventromedial lobe both umbilical veins during CS14, but both venous systems remain temporarily identifiable inside the liver. The dominance of the left-sided umbilical vein and the rightward repositioning of the sinuatrial junction cause de novo development of left-to-right shunts between the left umbilical vein in the liver hilum and the right hepatocardiac channel (venous duct) and the right vitelline vein (portal sinus), respectively. Once these shunts have formed, portal branches develop from the intrahepatic portions of the portal vein on the right side and the umbilical vein on the left side. The gall bladder is a reliable marker for this hepatic vascular midline. We found no evidence for large-scale fragmentation of embryonic veins as claimed by the 'vestigial' theory. Instead and in agreement with the 'lineage' theory, the vitelline and umbilical veins remained temporally identifiable inside the liver after being engulfed by hepatoblasts. In agreement with the 'hemodynamic' theory, the left-right shunts develop de novo.

Effects of Co-Culture Media on Hepatic Differentiation of hiPSC with or without HUVEC Co-Culture

The derivation of hepatocytes from human induced pluripotent stem cells (hiPSC) is of great interest for applications in pharmacological research. However, full maturation of hiPSC-derived hepatocytes has not yet been achieved in vitro. To improve hepatic differentiation, co-cultivation of hiPSC with human umbilical vein endothelial cells (HUVEC) during hepatic differentiation was investigated in this study. In the first step, different culture media variations based on hepatocyte culture medium (HCM) were tested in HUVEC mono-cultures to establish a suitable culture medium for co-culture experiments. Based on the results, two media variants were selected to differentiate hiPSC-derived definitive endodermal (DE) cells into mature hepatocytes with or without HUVEC addition. DE cells differentiated in mono-cultures in the presence of those media variants showed a significant increase (p < 0.05) in secretion of α-fetoprotein and in activities of cytochrome P450 (CYP) isoenzymes CYP2B6 and CYP3A4 as compared with cells differentiated in unmodified HCM used as control. Co-cultivation with HUVEC did not further improve the differentiation outcome. Thus, it can be concluded that the effect of the used medium outweighed the effect of HUVEC co-culture, emphasizing the importance of the culture medium composition for hiPSC differentiation.

Epithelial Morphogenesis during Liver Development

Tissue stem/progenitor cells supply multiple types of epithelial cells that eventually acquire specialized functions during organ development. In addition, three-dimensional (3D) tissue structures need to be established for organs to perform their physiological functions. The liver contains two types of epithelial cells, namely, hepatocytes and cholangiocytes, which are derived from hepatoblasts, fetal liver stem/progenitor cells (LPCs), in mid-gestation. Hepatocytes performing many metabolic reactions form cord-like structures, whereas cholangiocytes, biliary epithelial cells, form tubular structures called intrahepatic bile ducts. Analyses for human genetic diseases and mutant mice have identified crucial molecules for liver organogenesis. Functions of those molecules can be examined in in vitro culture systems where LPCs are induced to differentiate into hepatocytes or cholangiocytes. Recent technical advances have revealed 3D epithelial morphogenesis during liver organogenesis. Therefore, the liver is a good model to understand how tissue stem/progenitor cells differentiate and establish 3D tissue architectures during organ development.

An In Vitro Human Liver Model by iPSC-Derived Parenchymal and Non-parenchymal Cells

During liver development, hepatoblasts and liver non-parenchymal cells (NPCs) such as liver sinusoidal endothelial cells (LSECs) and hepatic stellate cells (HSCs) constitute the liver bud where they proliferate and differentiate. Accordingly, we reasoned that liver NPCs would support the maturation of hepatocytes derived from human induced pluripotent stem cells (hiPSCs), which usually exhibit limited functions. We found that the transforming growth factor β and Rho signaling pathways, respectively, regulated the proliferation and maturation of LSEC and HSC progenitors isolated from mouse fetal livers. Based on these results, we have established culture systems to generate LSECs and HSCs from hiPSCs. These hiPSC-derived NPCs exhibited distinctive phenotypes and promoted self-renewal of hiPSC-derived liver progenitor cells (LPCs) over the long term in the two-dimensional culture system without exogenous cytokines and hepatic maturation of hiPSC-derived LPCs. Thus, a functional human liver model can be constructed in vitro from the LPCs, LSECs, and HSCs derived from hiPSCs.

Endothelial Progenitor Cells for the Vascularization of Engineered Tissues

Self-assembled microvasculature from cocultures of endothelial cells (ECs) and stromal cells has significantly advanced efforts to vascularize engineered tissues by enhancing perfusion rates in vivo and producing investigative platforms for microvascular morphogenesis in vitro. However, to clinically translate prevascularized constructs, the issue of EC source must be resolved. Endothelial progenitor cells (EPCs) can be noninvasively supplied from the recipient through adult peripheral and umbilical cord blood, as well as derived from induced pluripotent stem cells, alleviating antigenicity issues. EPCs can also differentiate into all tissue endothelium, and have demonstrated potential for therapeutic vascularization. Yet, EPCs are not the standard EC choice to vascularize tissue constructs in vitro. Possible reasons include unresolved issues with EPC identity and characterization, as well as uncertainty in the selection of coculture, scaffold, and culture media combinations that promote EPC microvessel formation. This review addresses these issues through a summary of EPC vascular biology and the effects of tissue engineering design parameters upon EPC microvessel formation. Also included are perspectives to integrate EPCs with emerging technologies to produce functional, organotypic vascularized tissues.

Implantation of healthy matrix-embedded endothelial cells rescues dysfunctional endothelium and ischaemic tissue in liver engraftment

OBJECTIVE

Liver transplantation is limited by ischaemic injury which promotes endothelial cell and hepatocyte dysfunction and eventually organ failure. We sought to understand how endothelial state determines liver recovery after hepatectomy and engraftment.

DESIGN

Matrix-embedded endothelial cells (MEECs) with retained healthy phenotype or control acellular matrices were implanted in direct contact with the remaining median lobe of donor mice undergoing partial hepatectomy (70%), or in the interface between the remaining median lobe and an autograft or isograft from the left lobe in hepatectomised recipient mice. Hepatic vascular architecture, DNA fragmentation and apoptosis in the median lobe and grafts, serum markers of liver damage and phenotype of macrophage and lymphocyte subsets in the liver after engraftment were analysed 7 days post-op.

RESULTS

Healthy MEECs create a functional vascular splice in donor and recipient liver after 70% hepatectomy in mouse protecting these livers from ischaemic injury, hepatic congestion and inflammation. Macrophages recruited adjacent to the vascular nodes into the implants switched to an anti-inflammatory and regenerative profile M2. MEECs improved liver function and the rate of liver regeneration and prevented apoptosis in donor liver lobes, autologous grafts and syngeneic engraftment.

CONCLUSIONS

Implants with healthy endothelial cells rescue liver donor and recipient endothelium and parenchyma from ischaemic injury after major hepatectomy and engraftment. This study highlights endothelial-hepatocyte crosstalk in hepatic repair and provides a promising new approach to improve regenerative medicine outcomes and liver transplantation.

Endothelial cells are not required for specification of respiratory progenitors

Crosstalk between mesenchymal and epithelial cells influences organogenesis in multiple tissues, such as lung, pancreas, liver, and the nervous system. Lung mesenchyme comprises multiple cell types, however, and precise identification of the mesenchymal cell type(s) that drives early events in lung development remains unknown. Endothelial cells have been shown to be required for some aspects of lung epithelial patterning, lung stem cell differentiation, and regeneration after injury. Furthermore, endothelial cells are involved in early liver and pancreas development. From these observations we hypothesized that endothelial cells might also be required for early specification of the respiratory field and subsequent lung bud initiation. We first blocked VEGF signaling in E8.5 cultured foreguts with small molecule VEGFR inhibitors and found that lung specification and bud formation were unaltered. However, when we examined E9.5 mouse embryos carrying a mutation in the VEGFR Flk-1, which do not develop endothelial cells, we found that respiratory progenitor specification was impeded. Because the E9.5 embryos were substantially smaller than control littermates, suggesting the possibility of developmental delay, we isolated and cultured foreguts from mutant and control embryos on E8.5, when no size differences were apparent. We found that both specification of the respiratory field and lung bud formation occurred in mutant and control explants. These observations were unaffected by the presence or absence of serum. We also observed that hepatic specification and initiation occurred in the absence of endothelial cells, and that expansion of the liver epithelium in culture did not differ between mutant and control explants. Consistent with previously published results, we also found that pancreatic buds were not maintained in cultured foreguts when endothelial cells were absent. Our observations support the conclusion that endothelial cells are not required for early specification of lung progenitors and bud initiation, and that the diminished lung specification seen in E9.5 Flk-/- embryos is likely due to developmental delay resulting from the insufficient delivery of oxygen, nutrients, and other factors in the absence of a vasculature.

Multilineage communication regulates human liver bud development from pluripotency

Conventional two-dimensional differentiation from pluripotency fails to recapitulate cell interactions occurring during organogenesis. Three-dimensional organoids generate complex organ-like tissues; however, it is unclear how heterotypic interactions affect lineage identity. Here we use single-cell RNA sequencing to reconstruct hepatocyte-like lineage progression from pluripotency in two-dimensional culture. We then derive three-dimensional liver bud organoids by reconstituting hepatic, stromal, and endothelial interactions, and deconstruct heterogeneity during liver bud development. We find that liver bud hepatoblasts diverge from the two-dimensional lineage, and express epithelial migration signatures characteristic of organ budding. We benchmark three-dimensional liver buds against fetal and adult human liver single-cell RNA sequencing data, and find a striking correspondence between the three-dimensional liver bud and fetal liver cells. We use a receptor-ligand pairing analysis and a high-throughput inhibitor assay to interrogate signalling in liver buds, and show that vascular endothelial growth factor (VEGF) crosstalk potentiates endothelial network formation and hepatoblast differentiation. Our molecular dissection reveals interlineage communication regulating organoid development, and illuminates previously inaccessible aspects of human liver development.

Endothelial cell-derived matrix promotes the metabolic functional maturation of hepatocyte via integrin-Src signalling

The extracellular matrix (ECM) microenvironment is involved in the regulation of hepatocyte phenotype and function. Recently, the cell‐derived extracellular matrix has been proposed to represent the bioactive and biocompatible materials of the native ECM. Here, we show that the endothelial cell‐derived matrix (EC matrix) promotes the metabolic maturation of human adipose stem cell‐derived hepatocyte‐like cells (hASC‐HLCs) through the activation of the transcription factor forkhead box protein A2 (FOXA2) and the nuclear receptors hepatocyte nuclear factor 4 alpha (HNF4α) and pregnane X receptor (PXR). Reducing the fibronectin content in the EC matrix or silencing the expression of α5 integrin in the hASC‐HLCs inhibited the effect of the EC matrix on Src phosphorylation and hepatocyte maturation. The inhibition of Src phosphorylation using the inhibitor PP2 or silencing the expression of Src in hASC‐HLCs also attenuated the up‐regulation of the metabolic function of hASC‐HLCs in a nuclear receptor‐dependent manner. These data elucidate integrin‐Src signalling linking the extrinsic EC matrix signals and metabolic functional maturation of hepatocyte. This study provides a model for studying the interaction between hepatocytes and non‐parenchymal cell‐derived matrix.

Hypertrophic scar regression is linked to the occurrence of endothelial dysfunction

Most microvessels have been shown to become stenosed or completely occluded during hypertrophic scar progression. Here, we examined the morphology of capillary endothelial cells (ECs) and fibroblasts using immunofluorescence staining for CD31 and alpha-smooth muscle actin (α-SMA) and electron microscopy. In addition, ECs and fibroblasts were isolated from scar tissues, and the levels of transforming growth factor beta 1 (TGF-β1), platelet-derived growth factor (PDGF), endothelin 1 (ET-1), vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) were assayed using ELISAs. Furthermore, we assessed cell viability, total collagen production, and cell apoptosis in hypertrophic scar-derived fibroblasts cultured with EC-conditioned medium. Then, anti-TGF-β1, anti-PDGF, anti-ET-1, anti-VEGF, and anti-bFGF neutralising antibodies were individually added to the EC medium to identify which growth factor plays a more important role in inhibiting fibroblasts biology. Our results showed microvessel lumen occlusion and EC atrophy during scar development, particularly in regressive scars (RSs). Additionally, EC growth factor secretion decreased and reached the lowest levels in RSs. Furthermore, based on the culture results, RS EC medium inhibited fibroblast viability and collagen production and induced apoptosis. Moreover, TGF-β1, PDGF, and bFGF played more important roles in these processes than VEGF and ET-1. The endothelial dysfunction occurring in hypertrophic scars contributes to fibroblast inhibition and scar regression, and reduced TGF-β1, PDGF, and bFGF levels play key roles during this process.

Nutritional modulation of mouse and human liver bud growth through a branched-chain amino acid metabolism

Liver bud progenitors experience a transient amplification during the early organ growth phase, yet the mechanism responsible is not fully understood. Collective evidence highlights the specific requirements in stem cell metabolism for expanding organ progenitors during organogenesis and regeneration. Here, transcriptome analyses show that progenitors of the mouse and human liver bud growth stage specifically express the gene branched chain aminotransferase 1, encoding a known breakdown enzyme of branched-chain amino acids (BCAAs) for energy generation. Global metabolome analysis confirmed the active consumption of BCAAs in the growing liver bud, but not in the later fetal or adult liver. Consistently, maternal dietary restriction of BCAAs during pregnancy significantly abrogated the conceptus liver bud growth capability through a striking defect in hepatic progenitor expansion. Under defined conditions, the supplementation of L-valine specifically among the BCAAs promoted rigorous growth of the human liver bud organoid in culture by selectively amplifying self-renewing bi-potent hepatic progenitor cells. These results highlight a previously underappreciated role of branched-chain amino acid metabolism in regulating mouse and human liver bud growth that can be modulated by maternal nutrition in vivo or cultural supplement in vitro.

Endothelial Notch1 Activity Facilitates Metastasis

Endothelial cells (ECs) provide angiocrine factors orchestrating tumor progression. Here, we show that activated Notch1 receptors (N1ICD) are frequently observed in ECs of human carcinomas and melanoma, and in ECs of the pre-metastatic niche in mice. EC N1ICD expression in melanoma correlated with shorter progression-free survival. Sustained N1ICD activity induced EC senescence, expression of chemokines and the adhesion molecule VCAM1. This promoted neutrophil infiltration, tumor cell (TC) adhesion to the endothelium, intravasation, lung colonization, and postsurgical metastasis. Thus, sustained vascular Notch signaling facilitates metastasis by generating a senescent, pro-inflammatory endothelium. Consequently, treatment with Notch1 or VCAM1-blocking antibodies prevented Notch-driven metastasis, and genetic ablation of EC Notch signaling inhibited peritoneal neutrophil infiltration in an ovarian carcinoma mouse model.

Directional Fluid Transport across Organ-Blood Barriers: Physiology and Cell Biology

Directional fluid flow is an essential process for embryo development as well as for organ and organism homeostasis. Here, we review the diverse structure of various organ-blood barriers, the driving forces, transporters, and polarity mechanisms that regulate fluid transport across them, focusing on kidney-, eye-, and brain-blood barriers. We end by discussing how cross talk between barrier epithelial and endothelial cells, perivascular cells, and basement membrane signaling contribute to generate and maintain organ-blood barriers.

GATA4-dependent organ-specific endothelial differentiation controls liver development and embryonic hematopoiesis

Microvascular endothelial cells (ECs) are increasingly recognized as organ-specific gatekeepers of their microenvironment. Microvascular ECs instruct neighboring cells in their organ-specific vascular niches through angiocrine factors, which include secreted growth factors (angiokines), extracellular matrix molecules, and transmembrane proteins. However, the molecular regulators that drive organ-specific microvascular transcriptional programs and thereby regulate angiodiversity are largely elusive. In contrast to other ECs, which form a continuous cell layer, liver sinusoidal ECs (LSECs) constitute discontinuous, permeable microvessels. Here, we have shown that the transcription factor GATA4 controls murine LSEC specification and function. LSEC-restricted deletion of Gata4 caused transformation of discontinuous liver sinusoids into continuous capillaries. Capillarization was characterized by ectopic basement membrane deposition, formation of a continuous EC layer, and increased expression of VE-cadherin. Correspondingly, ectopic expression of GATA4 in cultured continuous ECs mediated the downregulation of continuous EC-associated transcripts and upregulation of LSEC-associated genes. The switch from discontinuous LSECs to continuous ECs during embryogenesis caused liver hypoplasia, fibrosis, and impaired colonization by hematopoietic progenitor cells, resulting in anemia and embryonic lethality. Thus, GATA4 acts as master regulator of hepatic microvascular specification and acquisition of organ-specific vascular competence, which are indispensable for liver development. The data also establish an essential role of the hepatic microvasculature in embryonic hematopoiesis.

Platelets prime hematopoietic and vascular niche to drive angiocrine-mediated liver regeneration

In mammals, the livers regenerate after chemical injury or resection of hepatic lobe by hepatectomy. How liver regeneration is initiated after mass loss remains to be defined. Here, we report that following liver injury, activated platelets deploy SDF-1 and VEGF-A to stimulate CXCR7⁺ liver sinusoidal endothelial cell (LSEC) and VEGFR1⁺ myeloid cell, orchestrating hepatic regeneration. After carbon tetrachloride (CCl₄) injection or hepatectomy, platelets and CD11b⁺VEGFR1⁺ myeloid cells were recruited LSEC, and liver regeneration in both models was impaired in thrombopoietin-deficient (Thpo^-/-) mice lacking circulating platelets. This impeded regeneration phenotype was recapitulated in mice with either conditional ablation of Cxcr7 in LSEC (Cxcr7^iΔ/iΔ) or Vegfr1 in myeloid cell (Vegfr1^lysM/lysM). Both Vegfr1^lysM/lysM and Cxcr7^iΔ/iΔ mice exhibited suppressed expression of hepatocyte growth factor and Wnt2, two crucial trophogenic angiocrine factors instigating hepatocyte propagation. Of note, administration of recombinant thrombopoietin restored the prohibited liver regeneration in the tested genetic models. As such, our data suggest that platelets and myeloid cells jointly activate the vascular niche to produce pro-regenerative endothelial paracrine/angiocrine factors. Modulating this "hematopoietic-vascular niche" might help to develop regenerative therapy strategy for hepatic disorders.

Making It New Again: Insight Into Liver Development, Regeneration, and Disease From Zebrafish Research

The adult liver of most vertebrates is predominantly comprised of hepatocytes. However, these cells must work in concert with biliary, stellate, vascular, and immune cells to accomplish the vast array of hepatic functions required for physiological homeostasis. Our understanding of liver development was accelerated as zebrafish emerged as an ideal vertebrate system to study embryogenesis. Through work in zebrafish and other models, it is now clear that the cells in the liver develop in a coordinated fashion during embryogenesis through a complex yet incompletely understood set of molecular guidelines. Zebrafish research has uncovered many key players that govern the acquisition of hepatic potential, cell fate, and plasticity. Although rare, some hepatobiliary diseases-especially biliary atresia-are caused by developmental defects; we discuss how research using zebrafish to study liver development has informed our understanding of and approaches to liver disease. The liver can be injured in response to an array of stressors including viral, mechanical/surgical, toxin-induced, immune-mediated, or inborn defects in metabolism. The liver has thus evolved the capacity to efficiently repair and regenerate. We discuss the emerging field of using zebrafish to study liver regeneration and highlight recent advances where zebrafish genetics and imaging approaches have provided novel insights into how cell plasticity contributes to liver regeneration.

Recellularization via the bile duct supports functional allogenic and xenogenic cell growth on a decellularized rat liver scaffold

Recent years have seen a proliferation of methods leading to successful organ decellularization. In this experiment we examine the feasibility of a decellularized liver construct to support growth of functional multilineage cells. Bio-chamber systems were used to perfuse adult rat livers with 0.1% SDS for 24 hours yielding decellularized liver scaffolds. Initially, we recellularized liver scaffolds using a human tumor cell line (HepG2, introduced via the bile duct). Subsequent studies were performed using either human tumor cells co-cultured with human umbilical vein endothelial cells (HUVECs, introduced via the portal vein) or rat neonatal cell slurry (introduced via the bile duct). Bio-chambers were used to circulate oxygenated growth medium via the portal vein at 37C for 5-7 days. Human HepG2 cells grew readily on the scaffold (n = 20). HepG2 cells co-cultured with HUVECs demonstrated viable human endothelial lining with concurrent hepatocyte growth (n = 10). In the series of neonatal cell slurry infusion (n = 10), distinct foci of neonatal hepatocytes were observed to repopulate the parenchyma of the scaffold. The presence of cholangiocytes was verified by CK-7 positivity. Quantitative albumin measurement from the grafts showed increasing albumin levels after seven days of perfusion. Graft albumin production was higher than that observed in traditional cell culture. This data shows that rat liver scaffolds support human cell ingrowth. The scaffold likewise supported the engraftment and survival of neonatal rat liver cell slurry. Recellularization of liver scaffolds thus presents a promising model for functional liver engineering.

HDL activation of endothelial sphingosine-1-phosphate receptor-1 (S1P1) promotes regeneration and suppresses fibrosis in the liver

Regeneration of hepatic sinusoidal vasculature is essential for non-fibrotic liver regrowth and restoration of its metabolic capacity. However, little is known about how this specialized vascular niche is regenerated. Here we show that activation of endothelial sphingosine-1-phosphate receptor-1 (S1P₁) by its natural ligand bound to HDL (HDL-S1P) induces liver regeneration and curtails fibrosis. In mice lacking HDL-S1P, liver regeneration after partial hepatectomy was impeded and associated with aberrant vascular remodeling, thrombosis and peri-sinusoidal fibrosis. Notably, this "maladaptive repair" phenotype was recapitulated in mice that lack S1P₁ in the endothelium. Reciprocally, enhanced plasma levels of HDL-S1P or administration of SEW2871, a pharmacological agonist specific for S1P₁ enhanced regeneration of metabolically functional vasculature and alleviated fibrosis in mouse chronic injury and cholestasis models. This study shows that natural and pharmacological ligands modulate endothelial S1P₁ to stimulate liver regeneration and inhibit fibrosis, suggesting that activation of this pathway may be a novel therapeutic strategy for liver fibrosis.

Complex Tissue and Disease Modeling using hiPSCs

Defined genetic models based on human pluripotent stem cells have opened new avenues for understanding disease mechanisms and drug screening. Many of these models assume cell-autonomous mechanisms of disease but it is possible that disease phenotypes or drug responses will only be evident if all cellular and extracellular components of a tissue are present and functionally mature. To derive optimal benefit from such models, complex multicellular structures with vascular components that mimic tissue niches will thus likely be necessary. Here we consider emerging research creating human tissue mimics and provide some recommendations for moving the field forward.

Gene regulatory networks in differentiation and direct reprogramming of hepatic cells

Liver development proceeds by sequential steps during which gene regulatory networks (GRNs) determine differentiation and maturation of hepatic cells. Characterizing the architecture and dynamics of these networks is essential for understanding how cell fate decisions are made during development, and for recapitulating these processes during in vitro production of liver cells for toxicology studies, disease modelling and regenerative therapy. Here we review the GRNs that control key steps of liver development and lead to differentiation of hepatocytes and cholangiocytes in mammals. We focus on GRNs determining cell fate decisions and analyse subcircuitry motifs that may confer specific dynamic properties to the networks. Finally, we put our analysis in the perspective of recent attempts to directly reprogram cells to hepatocytes by forced expression of transcription factors.

Effective endothelial cell and human pluripotent stem cell interactions generate functional insulin-producing beta cells

AIMS/HYPOTHESIS

Endothelial cells (ECs) play an essential role in pancreatic organogenesis. We hypothesise that effective in vitro interactions between human microvascular endothelial cells (HMECs) and human pluripotent stem cells (hPSCs) results in the generation of functional pancreatic beta cells.

METHODS

Embryoid bodies (EBs) derived from hPSCs were cultured alone (controls) or with ECs in collagen gels. Subsequently, cells were analysed for pancreatic beta cell markers, and then isolated and expanded. Insulin secretion in response to glucose was evaluated in vitro by static and dynamic (perifusion) assays, and in vivo by EB transplantation into immunodeficient mice.

RESULTS

Co-cultured EBs had a higher expression of mature beta cells markers and enhanced insulin secretion in vitro, compared with controls. In mice, transplanted EBs had higher levels of human C-peptide secretion with a significant reduction in hyperglycaemia after the selective destruction of native pancreatic beta cells. In addition, there was significant in vitro upregulation of bone morphogenetic proteins 2 and 4 (BMP-2, 4) in co-cultured cells, compared with controls.

CONCLUSIONS/INTERPRETATION

ECs provide essential signalling in vitro, such as activation of the BMP pathway, for derivation of functional insulin-producing beta cells from hPSCs.

Vascular ontogeny within selected thoracoabdominal organs and the limbs

The cardiovascular system is fundamental to life. Its vessels are the conduits for delivery of nutrients and oxygen to organs and the removal of wastes. During embryonic development, the vascular system is instrumental in the formation of organs. It contributes to the form and pattern of organs as diverse as the limbs and the gonads. Recent advances in molecular biology and genomics have afforded great insight to the control of vascular development at subcellular levels of organization. Nevertheless, there is little assembled information concerning the vascular development of the organ systems of the body. This paper begins by reviewing the modes of formation of embryonic blood vessels. This is followed by summaries of the ontogeny of the vasculature that supplies selected major thoracic and abdominal organs (heart, gut, liver, gonads, and kidney). The paper concludes with a description of the arterial development of the upper and lower extremities.

Characterization of a Fetal Liver Cell Population Endowed with Long-Term Multiorgan Endothelial Reconstitution Potential

Stable reconstitution of vascular endothelial beds upon transplantation of progenitor cells represents an important challenge due to the paucity and generally limited integration/expansion potential of most identified vascular related cell subsets. We previously showed that mouse fetal liver (FL) hemato/vascular cells from day 12 of gestation (E12), expressing the Stem Cell Leukaemia (SCL) gene enhancer transgene (SCL‐PLAP⁺ cells), had robust endothelial engraftment potential when transferred to the blood stream of newborns or adult conditioned recipients, compared to the scarce vascular contribution of adult bone marrow cells. However, the specific SCL‐PLAP⁺ hematopoietic or endothelial cell subset responsible for the long‐term reconstituting endothelial cell (LTR‐EC) activity and its confinement to FL developmental stages remained unknown. Using a busulfan‐treated newborn transplantation model, we show that LTR‐EC activity is restricted to the SCL‐PLAP⁺VE‐cadherin⁺CD45⁻ cell population, devoid of hematopoietic reconstitution activity and largely composed by Lyve1⁺ endothelial‐committed cells. SCL‐PLAP⁺ Ve‐cadherin⁺CD45⁻ cells contributed to the liver sinusoidal endothelium and also to the heart, kidney and lung microvasculature. LTR‐EC activity was detected at different stages of FL development, yet marginal activity was identified in the adult liver, revealing unknown functional differences between fetal and adult liver endothelial/endothelial progenitors. Importantly, the observations that expanding donor‐derived vascular grafts colocalize with proliferating hepatocyte‐like cells and participate in the systemic circulation, support their functional integration into young livers. These findings offer new insights into the engraftment, phonotypical, and developmental characterization of a novel endothelial/endothelial progenitor cell subtype with multiorgan LTR‐EC activity, potentially instrumental for the treatment/genetic correction of vascular diseases. Stem Cells2017;35:507–521

The hyaloid vasculature facilitates basement membrane breakdown during choroid fissure closure in the zebrafish eye

A critical aspect of vertebrate eye development is closure of the choroid fissure (CF). Defects in CF closure result in colobomas, which are a significant cause of childhood blindness worldwide. Despite the growing number of mutated loci associated with colobomas, we have a limited understanding of the cell biological underpinnings of CF closure. Here, we utilize the zebrafish embryo to identify key phases of CF closure and regulators of the process. Utilizing Laminin-111 as a marker for the basement membrane (BM) lining the CF, we determine the spatial and temporal patterns of BM breakdown in the CF, a prerequisite for CF closure. Similarly, utilizing a combination of in vivo time-lapse imaging, β-catenin immunohistochemistry and F-actin staining, we determine that tissue fusion, which serves to close the fissure, follows BM breakdown closely. Periocular mesenchyme (POM)-derived endothelial cells, which migrate through the CF to give rise to the hyaloid vasculature, possess distinct actin foci that correlate with regions of BM breakdown. Disruption of talin1, which encodes a regulator of the actin cytoskeleton, results in colobomas and these correlate with structural defects in the hyaloid vasculature and defects in BM breakdown. cloche mutants, which entirely lack a hyaloid vasculature, also possess defects in BM breakdown in the CF. Taken together, these data support a model in which the hyaloid vasculature and/or the POM-derived endothelial cells that give rise to the hyaloid vasculature contribute to BM breakdown during CF closure.

Regulation of Hematopoiesis and Osteogenesis by Blood Vessel-Derived Signals

In addition to their conventional role as a versatile transport system, blood vessels provide signals controlling organ development, regeneration, and stem cell behavior. In the skeletal system, certain capillaries support perivascular osteoprogenitor cells and thereby control bone formation. Blood vessels are also a critical component of niche microenvironments for hematopoietic stem cells. Here we discuss key pathways and factors controlling endothelial cell behavior in bone, the role of vessels in osteogenesis, and the nature of vascular stem cell niches in bone marrow.

The endoderm indirectly influences morphogenetic movements of the zebrafish head kidney through the posterior cardinal vein and VegfC

Integration of blood vessels and organ primordia determines organ shape and function. The head kidney in the zebrafish interacts with the dorsal aorta (DA) and the posterior cardinal vein (PCV) to achieve glomerular filtration and definitive hematopoiesis, respectively. How the head kidney co-develops with both the axial artery and vein remains unclear. We found that in endodermless sox32-deficient embryos, the head kidney associated with the PCV but not the DA. Disrupted convergent migration of the PCV and the head kidney in sox32-deficient embryos was rescued in a highly coordinated fashion through the restoration of endodermal cells. Moreover, grafted endodermal cells abutted the host PCV endothelium in the transplantation assay. Interestingly, the severely-disrupted head kidney convergence in the sox32-deficient embryo was suppressed by both the cloche mutation and the knockdown of endothelial genes, indicating that an interaction between the endoderm and the PCV restricts the migration of the head kidney. Furthermore, knockdown of either vegfC or its receptor vegfr3 suppressed the head kidney convergence defect in endodermless embryos and perturbed the head kidney-PCV association in wild-type embryos. Our findings thus underscore a role for PCV and VegfC in patterning the head kidney prior to organ assembly and function.

Transplantation of Endothelial Cells to Mitigate Acute and Chronic Radiation Injury to Vital Organs

Current therapeutic approaches for treatment of exposure to radiation involve the use of antioxidants, chelating agents, recombinant growth factors and transplantation of stem cells (e.g., hematopoietic stem cell transplantation). However, exposure to high-dose radiation is associated with severe damage to the vasculature of vital organs, often leading to impaired healing, tissue necrosis, thrombosis and defective regeneration caused by aberrant fibrosis. It is very unlikely that infusion of protective chemicals will reverse severe damage to the vascular endothelial cells (ECs). The role of irradiated vasculature in mediating acute and chronic radiation syndromes has not been fully appreciated or well studied. New approaches are necessary to replace and reconstitute ECs in organs that are irreversibly damaged by radiation. We have set forth the novel concept that ECs provide paracrine signals, also known as angiocrine signals, which not only promote healing of irradiated tissue but also direct organ regeneration without provoking fibrosis. We have developed innovative technologies that enable manufacturing and banking of human GMP-grade ECs. These ECs can be transplanted intravenously to home to and engraft to injured tissues where they augment organ repair, while preventing maladaptive fibrosis. In the past, therapeutic transplantation of ECs was not possible due to a shortage of availability of suitable donor cell sources and preclinical models, a lack of understanding of the immune privilege of ECs, and inadequate methodologies for expansion and banking of engraftable ECs. Recent advances made by our group as well as other laboratories have breached the most significant of these obstacles with the development of technologies to manufacture clinical-scale quantities of GMP-grade and human ECs in culture, including genetically diverse reprogrammed human amniotic cells into vascular ECs (rAC-VECs) or human pluripotent stem cells into vascular ECs (iVECs). This approach provides a path to therapeutic EC transplantation that can be infused concomitantly or sequentially with hematopoietic stem cell transplantation more than 24 h after irradiation to support multi-organ regeneration, thereby improving immediate and long-term survival, while limiting long-term morbidity resulting from nonregenerative damage repair pathways.

Reverse engineering liver buds through self-driven condensation and organization towards medical application

The self-organizing tissue-based approach coupled with induced pluripotent stem (iPS) cell technology is evolving as a promising field for designing organoids in culture and is expected to achieve valuable practical outcomes in regenerative medicine and drug development. Organoids show properties of functional organs and represent an alternative to cell models in conventional two-dimensional differentiation platforms; moreover, organoids can be used to investigate mechanisms of development and disease, drug discovery and toxicity assessment. Towards a more complex and advanced organoid model, it is essential to incorporate multiple cell lineages including developing vessels. Using a self-condensation method, we recently demonstrated self-organizing "organ buds" of diverse systems together with human mesenchymal and endothelial progenitors, proposing a new reverse engineering method to generate a more complex organoid structure. In this section, we review characters of organ bud technology based on two important principles: self-condensation and self-organization focusing on liver bud as an example, and discuss their practicality in regenerative medicine and potential as research tools for developmental biology and drug discovery.

Organoids from adult liver and pancreas: Stem cell biology and biomedical utility

The liver and pancreas are critical organs maintaining whole body metabolism. Historically, the expansion of adult-derived cells from these organs in vitro has proven challenging and this in turn has hampered studies of liver and pancreas stem cell biology, as well as being a roadblock to disease modelling and cell replacement therapies for pathologies in these organs. Recently, defined culture conditions have been described which allow the in vitro culture and manipulation of adult-derived liver and pancreatic material. Here we review these systems and assess their physiological relevance, as well as their potential utility in biomedicine.

Blood vessel crosstalk during organogenesis-focus on pancreas and endothelial cells

Blood vessels form a highly branched, interconnected, and largely stereotyped network of tubes that sustains every organ and tissue in vertebrates. How vessels come to take on their particular architecture, or how they are 'patterned,' and in turn, how they influence surrounding tissues are fundamental questions of organogenesis. Decades of work have begun to elucidate how endothelial progenitors arise and home to precise locations within tissues, integrating attractive and repulsive cues to build vessels where they are needed. Conversely, more recent findings have revealed an exciting facet of blood vessel interaction with tissues, where vascular cells provide signals to developing organs and progenitors therein. Here, we discuss the exchange of reciprocal signals between endothelial cells and neighboring tissues during embryogenesis, with a special focus on the developing pancreas. Understanding the mechanisms driving both sides of these interactions will be crucial to the development of therapies, from improving organ regeneration to efficient production of cell based therapies. Specifically, elucidating the interface of the vasculature with pancreatic lineages, including endocrine cells, will instruct approaches such as generation of replacement beta cells for Type I diabetes. WIREs Dev Biol 2016, 5:598-617. doi: 10.1002/wdev.240 For further resources related to this article, please visit the WIREs website.

Stage-specific regulation of the WNT/β-catenin pathway enhances differentiation of hESCs into hepatocytes

BACKGROUND & AIMS

Hepatocytes differentiated from human embryonic stem cells (hESCs) have the potential to overcome the shortage of primary hepatocytes for clinical use and drug development. Many strategies for this process have been reported, but the functionality of the resulting cells is incomplete. We hypothesize that the functionality of hPSC-derived hepatocytes might be improved by making the differentiation method more similar to normal in vivo hepatic development.

METHODS

We tested combinations of growth factors and small molecules targeting candidate signaling pathways culled from the literature to identify optimal conditions for differentiation of hESCs to hepatocytes, using qRT-PCR for stage-specific markers to identify the best conditions. Immunocytochemistry was then used to validate the selected conditions. Finally, induction of expression of metabolic enzymes in terminally differentiated cells was used to assess the functionality of the hESC-derived hepatocytes.

RESULTS

Optimal differentiation of hESCs was attained using a 5-stage protocol. After initial induction of definitive endoderm (stage 1), we showed that inhibition of the WNT/β-catenin pathway during the 2nd and 3rd stages of differentiation was required to specify first posterior foregut, and then hepatic gut cells. In contrast, during the 4th stage of differentiation, we found that activation of the WNT/β-catenin pathway allowed generation of proliferative bipotent hepatoblasts, which then were efficiently differentiated into hepatocytes in the 5th stage by dual inhibition of TGF-β and NOTCH signaling.

CONCLUSION

Here, we show that stage-specific regulation of the WNT/β-catenin pathway results in improved differentiation of hESCs to functional hepatocytes.

Angiocrine functions of organ-specific endothelial cells

Endothelial cells that line capillaries are not just passive conduits for delivering blood. Tissue-specific endothelium establishes specialized vascular niches that deploy sets of growth factors, known as angiocrine factors. These cues participate actively in the induction, specification, patterning and guidance of organ regeneration, as well as in the maintainance of homeostasis and metabolism. When upregulated following injury, they orchestrate self-renewal and differentiation of tissue-specific resident stem and progenitor cells into functional organs. Uncovering the mechanisms by which organotypic endothelium distributes physiological levels of angiocrine factors both spatially and temporally will lay the foundation for clinical trials that promote organ repair without scarring.