Isolation of hepatoblasts based on the expression of Dlk/Pref-1

Flow cytometric isolation and clonal identification of self-renewing bipotent hepatic progenitor cells in adult mouse liver

The adult liver progenitor cells appear in response to several types of pathological liver injury, especially when hepatocyte replication is blocked. These cells are histologically identified as cells that express cholangiocyte markers and proliferate in the portal area of the hepatic lobule. Although these cells play an important role in liver regeneration, the precise characterization that determines these cells as self-renewing bipotent primitive hepatic cells remains to be shown. Here we attempted to isolate cells that express a cholangiocyte marker from the adult mouse liver and perform single cell-based analysis to examine precisely bilineage differentiation potential and self-renewing capability of these cells. Based on the results of microarray analysis and immunohistochemistry, we used an antibody against CD133 and isolate CD133(+) cells via flow cytometry. We then cultured and propagated isolated cells in a single cell culture condition and examined their potential for proliferation and differentiation in vitro and in vivo. Isolated cells that could form large colonies (LCs) in culture gave rise to both hepatocytes and cholangiocytes as descendants, while maintaining undifferentiated cells by self-renewing cell divisions. The clonogenic progeny of an LC-forming cell is capable of reconstituting hepatic tissues in vivo by differentiating into fully functional hepatocytes. Moreover, the deletion of p53 in isolated LC-forming cells resulted in the formation of tumors with some characteristics of hepatocellular carcinoma and cholangiocarcinoma upon subcutaneous injection into immunodeficient mutant mice. These data provide evidence for the stem cell-like capacity of isolated and clonally cultured CD133(+) LC-forming cells.

CONCLUSION

Our method for prospectively isolating hepatic progenitor cells from the adult mouse liver will facilitate study of their roles in liver regeneration and carcinogenesis.

MicroRNA expression patterns and function in endodermal differentiation of human embryonic stem cells

BACKGROUND/AIMS

microRNAs (miRNAs) are small noncoding RNAs that regulate cognate mRNAs post-transcriptionally. Human embryonic stem cells (hESC), which exhibit the characteristics of pluripotency and self-renewal, may serve as a model to study the role of miRNAs in early human development. We aimed to determine whether endodermally-differentiated hESC demonstrate a unique miRNA expression pattern, and whether overexpression of endoderm-specific miRNA may affect hESC differentiation.

METHODS

miRNA expression was profiled in undifferentiated and NaButyrate-induced differentiated hESC of two lines, using microarray and quantitative RT-PCR. Then, the effect of lentiviral-based overexpression of liver-specific miR-122 on hESC differentiation was analyzed, using genomewide gene microarrays.

RESULTS

The miRNA profiling revealed expression of three novel miRNAs in undifferentiated and differentiated hESC. Upon NaButyrate induction, two of the most upregulated miRNAs common to both cell lines were miR-24 and miR-10a, whose target genes have been shown to inhibit endodermal differentiation. Furthermore, induction of several liver-enriched miRNAs, including miR-122 and miR-192, was observed in parallel to induction of endodermal gene expression. Stable overexpression of miR-122 in hESC was unable to direct spontaneous differentiation towards a clear endodermal fate, but rather, delayed general differentiation of these cells.

CONCLUSIONS

Our results demonstrate that expression of specific miRNAs correlates with that of specific genes upon differentiation, and highlight the potential role of miRNAs in endodermal differentiation of hESC.

Hematopoietic progenitors from early murine fetal liver possess hepatic differentiation potential

Bipotential hepatoblasts differentiate into hepatocytes and cholangiocytes during liver development. It is believed that hepatoblasts originate from endodermal tissue. Here, we provide evidence for the presence of hepatic progenitor cells in the hematopoietic compartment at an early stage of liver development. Flow cytometric analysis showed that at early stages of liver development, approximately 13% of CD45(+) cells express Delta-like protein-1, a marker of hepatoblasts. Furthermore, reverse transcriptase-PCR data suggest that many hepatic genes are expressed in these cells. Cell culture experiments confirmed the hepatic differentiation potential of these cells with the loss of the CD45 marker. We observed that both hematopoietic activity in Delta-like protein-1(+) cells and hepatic activity in CD45(+) cells were high at embryonic day 10.5 and declined thereafter. Clonal analysis revealed that the hematopoietic fraction of fetal liver cells at embryonic day 10.5 gave rise to both hepatic and hematopoietic colonies. The above results suggest a common source of these two functionally distinct cell lineages. In utero transplantation experiments confirmed these results, as green fluorescent protein-expressing CD45(+) cells at the same stage of development yielded functional hepatocytes and hematopoietic reconstitution. Since these cells were unable to differentiate into cytokeratin-19-expressing cholangiocytes, we distinguished them from hepatoblasts. This preliminary study provides hope to correct many liver diseases during prenatal development via transplantation of fetal liver hematopoietic cells.

Characterization of hepatic progenitors from human fetal liver during second trimester

AIM: To enrich hepatic progenitors using epithelial cell adhesion molecule (EpCAM) as a marker from human fetal liver and investigate the expression of human leukocyte antigen (HLA) and their markers associated with hepatic progenitor cells.

METHODS: EpCAM +ve cells were isolated using magnetic cell sorting (MACS) from human fetuses (n = 10) at 15-25 wk gestation. Expression of markers for hepatic progenitors such as albumin, alpha-fetoprotein (AFP), CD29 (integrin β1), CD49f (integrin α6) and CD90 (Thy 1) was studied by using flow cytometry, immunocytochemistry and RT-PCR; HLA class I (A, B, C) and class II (DR) expression was studied by flow cytometry only.

RESULTS: FACS analysis indicated that EpCAM +ve cells were positive for CD29, CD49f, CD90, CD34, HLA class I, albumin and AFP but negative for HLA class II (DR) and CD45. RT PCR showed that EpCAM +ve cells expressed liver epithelial markers (CK18), biliary specific marker (CK19) and hepatic markers (albumin, AFP). On immunocytochemical staining, EpCAM +ve cells were shown positive signals for CK18 and albumin.

CONCLUSION: Our study suggests that these EpCAM +ve cells can be used as hepatic progenitors for cell transplantation with a minimum risk of alloreactivity and these cells may serve as a potential source for enrichment of hepatic progenitor.

Dlk1 influences differentiation and function of B lymphocytes

The Dlk1 (delta-like-1) gene is a member of the epidermal growth factor (EGF)-like homeotic gene family. It influences cell-cell interactions between stromal cells and pro-B cells in vitro. To define the in vivo role of the dlk protein in B cell development, we established a Dlk1-/- mouse model. In spleens of Dlk1-/- mice, transitional B cell numbers were increased and the ratio between transitional B cell subsets was altered. Numbers of follicular B cells decreased, while the number of marginal zone B cells and the size of the marginal zone were increased. Loss of dlk resulted in increased immunoglobulin G1 (IgG1) and IgG3 in preimmune sera. Furthermore, there was an exaggerated primary T-dependent antigen-specific humoral immune response. In bone marrow, the lack of dlk led to increased numbers of the earliest B lineage cells in young mice without affecting numbers of later B lineage cells. In vitro experiments showed that lack of dlk on either stromal cells or pro-B cells caused changes in differentiation and proliferation of pro-B cells, suggesting that lack of dlk leads to changes in cell-cell interactions in the bone marrow microenvironment. These results show that dlk expression is essential for normal B cell development.

The spatial and temporal expression of delta-like protein 1 in the rat pituitary gland during development

An analysis of secreted proteins by the signal sequence trap method using a cDNA library of the rat pituitary anlage at embryonic days (E) 13.5 revealed the abundant expression of delta-like protein 1 (Dlk1) in the pituitary gland. Dlk1, an epidermal growth factor-like repeat protein in preadipocytes, functions in maintaining the preadipose state. Expression of Dlk1 mRNA in the pituitary at E13.5 and in the adult pituitary was confirmed by in situ hybridization. The expression pattern of Dlk1 during pituitary development was also studied by immunohistochemistry. Dlk1 protein first appeared in Rathke's pouch and the infundibulum at E11.5; as development proceeded, expression became restricted to the pars distalis and pars tuberalis (PT). Dlk1 was expressed in most ACTH cells during the embryonic stages, but its expression was limited to only a few ACTH cells in the adult pituitary. It was also expressed in a small population of TSH, GTH, and PRL cells throughout development, whereas it was present in the cytoplasm of most GH cells at all developmental stages. Similarly, Dlk1 was localized in the cytoplasm of PT cells during development. These findings provide new insights into the mechanism of Dlk1 regarding its regulation of pituitary hormone-secreting cells during development.

The serum levels of the EGF-like homeotic protein dlk1 correlate with different metabolic parameters in two hormonally different children populations in Spain

BACKGROUND

The Dlk1 gene encodes for dlk1, a transmembrane protein belonging to the EGF-like repeat-containing family. Dlk1 has been shown to act as a regulator of adipogenesis. Fc-dlk1 transgenic mice show a decrease in adipose tissue and glucose tolerance, hypertriglyceridaemia and lower insulin sensitivity. Dlk1-deficient mice show growth retardation, increased serum lipid metabolites and develop obesity. These data advocate for a role of dlk1 in the maintenance of lipid homeostasis, and suggest that dlk1 levels may influence the development of cardiovascular disease.

AIM AND METHODS

In this study, we analysed whether dlk1 serum levels could be indicative of the different hormonal or metabolic status shown by two Spanish children populations (6-8 years-old), Orense and Murcia. We determined dlk1 serum levels by ELISA assay, using an antibody raised against the recombinant protein, and performed a correlation analysis against measurements of several hormonal and biochemical parameters in samples from 494 subjects.

RESULTS

We found a statistically significant positive correlation between serum levels of dlk1 and those of glucose (P < 0.05), total cholesterol (P < 0.01) and high-density lipoprotein-cholesterol (HDL-C) (P < 0.01) in children from Murcia, but not from Orense's population, where dehydroepiandrosterone-sulphate (DHEA-S) levels were significantly higher (P < 0.01) and dlk1 correlated positively with insulin (P < 0.01), homeostasis model assessment (HOMA) (P < 0.01) and free fatty acids (FFA) (P < 0.05).

CONCLUSIONS

dlk1 serum levels appear related to the anabolic status of the children in association with changes in the levels of DHEA-S, which have been associated with hyperinsulinaemia and diabetes. Monitoring dlk1 levels may be important to evaluate the metabolic and hormonal stage of child development.

p75 Neurotrophin receptor is a marker for precursors of stellate cells and portal fibroblasts in mouse fetal liver

BACKGROUND & AIMS

Hepatic stellate cells (HSCs) and portal fibroblasts (PFs) are 2 distinct mesenchymal cells in adult liver. HSCs in sinusoids accumulate lipids and express p75 neurotrophin receptor (p75NTR). HSCs and PFs play pivotal roles in liver regeneration and fibrosis. However, the roles of mesenchymal cells in fetal liver remain poorly understood. In this study, we aimed to characterize mesenchymal cells in mouse fetal liver.

METHODS

We prepared an anti-p75NTR monoclonal antibody applicable for flow cytometry and immunohistochemistry. p75NTR(+) cells isolated from fetal liver by flow cytometry were characterized by reverse-transcription polymerase chain reaction, immunohistochemistry, and cell cultivation. Lipid-containing cells were visualized by Oil-red O staining.

RESULTS

p75NTR(+) cells in fetal liver were clearly distinct from endothelial cells and showed characteristics of mesenchymal cells. At embryonic day (E) 10.5, p75NTR(+) cells were present at the periphery of the liver bud in close contact with endothelial cells, and spread over the liver at E11.5. With the formation of the liver architecture, they began to localize to 2 distinct areas, parenchymal and portal areas, and lipid-containing p75NTR(+) cells increased accordingly. p75NTR(+) cells around portal veins were adjacent to cholangiocytes and expressed Jagged1, a crucial factor for the commitment of hepatoblasts to cholangiocytes. By cultivation, p75NTR(+) cells showed features of adult HSCs with markedly increased expression of glial fibrillary acidic protein and alpha-smooth muscle actin.

CONCLUSIONS

p75NTR(+) mesenchymal cells in fetal liver include progenitors for HSCs and PFs, and the anti-p75NTR monoclonal antibody is useful for their isolation.

Prospero-related homeobox 1 and liver receptor homolog 1 coordinately regulate long-term proliferation of murine fetal hepatoblasts

During early to late-fetal liver development, bipotential hepatoblasts proliferate and differentiate into hepatocytes and cholangiocytes. The prospero-related homeobox 1 gene (Prox1) is expressed in hepatoblasts, and the inactivation of Prox1 causes defective early liver development, in particular, faulty migration of fetal hepatoblasts. Prox1 binds to another hepatocyte-enriched transcription factor, liver receptor homolog 1 (Lrh1), and suppresses its transcriptional activity. However, the molecular mechanism by which Prox1 and Lrh1 regulate the characteristics of fetal hepatic cells remains unknown. We investigated the contribution of Prox1 and Lrh1 in early liver development. Embryonic day 13 liver-derived CD45-Ter119-Dlk+ cells were purified as fetal hepatic stem/progenitor cells, and formation of colonies derived from single cells was detected under low-density culture conditions. We found that overexpression of Prox1 using retrovirus infection induced migration and proliferation of fetal hepatic stem/progenitor cells. In contrast, overexpression of Lrh1 suppressed colony formation. Prox1 induced the long-term proliferation of fetal hepatic stem/progenitor cells, which exhibited both high proliferative activity and bipotency for differentiation. Prox1 up-regulated expression of cyclins D2, E1, and E2, whereas it suppressed expression of p16(ink4a), the cdk inhibitor. In addition, overexpression of Prox1 significantly inhibited the proximal promoter activity of p16(ink4a).

CONCLUSION

These results suggested that Prox1 and Lrh1 coordinately regulate development of hepatic stem/progenitor cells and that Prox1 induces fetal hepatocytic proliferation through the suppression of the promoter activity of p16(ink4a).

Scalable selection of hepatocyte- and hepatocyte precursor-like cells from culture of differentiating transgenically modified murine embryonic stem cells

Potential therapeutic applications of embryonic stem cell (ESC)-derived hepatocytes are limited by their relatively low output in differentiating ESC cultures, as well as by the danger of contamination with tumorigenic undifferentiated ESCs. To address these problems, we developed transgenic murine ESC clones possessing bicistronic expression vector that contains the alpha-fetoprotein gene promoter driving a cassette for the enhanced green "live" fluorescent reporter protein (eGFP) and a puromycin resistance gene. Under established culture conditions these clones allowed for both monitoring of differentiation and for puromycin selection of hepatocyte-committed cells in a suspension mass culture of transgenic ESC aggregates ("embryoid bodies" [EBs]). When plated on fibronectin, the selected eGFP-positive cells formed colonies, in which intensely proliferating hepatocyte precursor-like cells gave rise to morphologically differentiated cells expressing alpha-1-antitrypsin, alpha-fetoprotein, and albumin. A number of cells synthesized glycogen and in some of the cells cytokeratin 18 microfilaments were detected. Major hepatocyte marker genes were expressed in the culture, along with the gene and protein expression of stem/progenitor markers, suggesting the features of both hepatocyte precursors and more advanced differentiated cells. When cultured in suspension, the EB-derived puromycin-selected cells formed spheroids capable of outgrowing on an adhesive substrate, resembling the behavior of fetal mouse hepatic progenitor cells. The established system based on the highly efficient selection/purification procedure could be suitable for scalable generation of ESC-derived hepatocyte- and hepatocyte precursor-like cells and offers a potential in vitro source of cells for transplantation therapy of liver diseases, tissue engineering, and drug and toxicology screening.

Promoter-defined isolation and identification of hepatic progenitor cells from the human fetal liver

Hepatoblasts, which are considered one type of hepatic progenitor cell, reside in the fetal liver. To selectively identify these cells, we transfected primary cultured human fetal liver cells (FLCs) with a pGL3 vector bearing the gene for the enhanced green fluorescence protein (EGFP) under the control of the alpha-fetoprotein (AFP) promoter expressed in hepatoblasts. The FLCs were then sorted by fluorescence-activated cell sorting (FACS) on the basis of AFP promoter-driven EGFP expression. The EGFP-positive cells expressed AFP, albumin, and cytokeratin 19, and could be expanded in vitro. Thus, the AFP promoter-EGFP reporter system is highly useful for identification and isolation of hepatic progenitor cells.

Progenitor gene DLK1 might be an independent prognostic factor of liver cancer

BACKGROUND

Delta-like 1 homolog (DLK1) is a marker for progenitor cells of the liver. The gene encoding DLK1 is expressed early during embryonic development but, importantly, it is also expressed in some human liver cancers. However, the prognostic value of the DLK1 gene has not been investigated.

OBJECTIVES

To examine the association between the DLK1 gene and survival time and whether high levels of expression of DLK1 are a prognostic factor for liver cancer.

METHODS

We evaluated 60 cases of primary liver cancer, and investigated the link between the expression of DLK1 and patient survival. Clinical characteristics of the cases used for our study, such as tumor size, differentiation and staging, are statistically evenly distributed. Using RT-PCR, western blotting and immunohistochemistry, we analyzed the expression of DLK1 in the tumor samples and evaluated the results statistically.

RESULTS

DLK1 was expressed in 22 of the 60 cases (36.7%), and analysis of the survival of the patients revealed that DLK1-positive patients had a shorter survival time than DLK1-negative patients. Cox regression analysis also showed that DLK1 is a risk factor. However, DLK1 expression does not seem to correlate with other classic prognostic factors such as alpha-fetoprotein (AFP), tumor-node-metastasis (TNM) and vascular invasion, which implies that it is an independent prognostic factor.

Human serum levels of fetal antigen 1 (FA1/Dlk1) increase with obesity, are negatively associated with insulin sensitivity and modulate inflammation in vitro

OBJECTIVE

To investigate fetal antigen 1 (FA1) protein within the context of human obesity and its relation with insulin sensitivity.

SUBJECTS

Cross-sectional study that analyses circulating levels of FA1 in two selected human cohorts: n=127 men for the study of FA1 circulating levels in the context of obesity and insulin sensitivity (S(i)); and n=61 severely obese women before and after bariatric surgery. The response in vitro to FA1 protein on human cell lines of monocytes, preadipocytes and mature adipocytes was studied.

MEASUREMENTS

Anthropometrical parameters: body mass index, waist-to-hip ratio, waist circumference, fat-free mass and fat mass. Clinical parameters: lipid profile (high-density lipoprotein (HDL) cholesterol, low-density lipoprotein (LDL) cholesterol, total cholesterol, triglycerides), glycemic profile (fasting glucose, insulin, S(i), HOMA-IR (Homeostasis Model Assessment of Insulin Resistance), cytokines (sIL-6), adipokines (adiponectin) and circulating soluble fractions of tumor necrosis factor-alpha receptors 1 and 2 (sTNFR1 and sTNFR2).

RESULTS

IN the obesity study, levels of FA1 in serum were found to increase with obesity. The S(i) index was negatively dependent on FA1 levels. In severe obesity, serum levels of FA1 decreased 1.4-fold 6 months after bariatric surgery. In vitro assays with FA1 protein on human monocytes and adipocytes cell lines modified the expression of pro-inflammatory cytokines and adipokines (tumor necrosis factor-alpha (TNFalpha), monocyte chemoattractant protein-1 (MCP-1), IL-6 (interleukin-6) and adiponectin).

CONCLUSION

FA1 serum levels were increased in obese subjects and might influence S(i). The stimulatory effect of FA1 protein on pro-inflammatory cytokines on both immune and adipose cell types could contribute to worsening the inflammatory environment observed in obesity.

Hepatic progenitor cells in the mouse extrahepatic bile duct after a bile duct ligation

The intrahepatic bile duct has been suggested to be a source of hepatic progenitor cells in the severely damaged liver. In contrast, little attention has been paid to the question of whether hepatic progenitor cells exist in the extrahepatic bile duct (EHBD). In the present study, we examined the phenotypic changes of the mouse EHBD following bile duct ligation. After bile duct ligation, the number of c-Kit-positive epithelial cells increased in the EHBD. The ligated EHBD expressed mRNA for hepatic progenitor cell markers, including c-Kit and Thy-1. Hepatocyte markers such as albumin and cytochrome P450 7a1 were also transiently detected in the EHBD after a bile duct ligation. In a culture of EHBD cells, we detected hepatic progenitor cells that were positive for both staining with anti-albumin antibodies and Dolichos biflorus agglutinin, a biliary epithelial cell-specific lectin. Furthermore, hepatic progenitor cells positive for both c-Kit and albumin were found in the cultured EHBD population. Additionally EHBD-derived hepatocyte-like cells were also observed in the culture. A transplantation study revealed that EHBD cells integrate into the parenchyma and are albumin positive. These data suggest that hepatic progenitor cells emerge in the EHBD following bile duct ligation, that subsequently give rise to hepatocyte-like cells. We also observed that the gall bladder transiently expressed hepatocyte markers after bile duct ligation. Our results suggest a potential of the EHBD and gall bladder as useful transplantable sources for liver injury.

Purification of fetal liver stem/progenitor cells containing all the repopulation potential for normal adult rat liver

BACKGROUND & AIMS

Previously, we showed high-level, long-term liver replacement after transplantation of unfractionated embryonic day (ED) 14 fetal liver stem/progenitor cells (FLSPC). However, for clinical applications, it will be essential to transplant highly enriched cells, while maintaining high repopulation potential.

METHODS

Dlk-1, a member of the delta-like family of cell surface transmembrane proteins, is highly expressed in human and rodent fetal liver. Dlk-1(+) cells, isolated from ED14 fetal liver using immunomagnetic beads, were examined for their hepatic gene expression profile and characteristic properties in vitro and their proliferative and differentiation potential in vivo after transplantation into normal adult rat liver.

RESULTS

Rat ED14 FLSPC were purified to 95% homogeneity and exhibited cell culture and gene expression characteristics expected for hepatic stem/progenitor cells. Rat ED14 FLSPC are alpha-fetoprotein(+)/cytokeratin-19(+) or alpha-fetoprotein(+)/cytokeratin-19(-) and contain all of the normal liver repopulation capacity found in fetal liver. Hematopoietic stem cells, a major component in crude fetal liver cell preparations that engraft in other organs, such as bone marrow, spleen, and lung, are totally removed by Dlk-1 selection, and Dlk-1 purified FLSPC repopulate only the liver.

CONCLUSIONS

This is the first study reporting purification of hepatic stem/progenitor cells from fetal liver that are fully capable of repopulating the normal adult liver. This represents a major advance toward developing protocols that will be essential for clinical application of liver cell transplantation therapy.

Differentially expressed genes in a porcine adult hepatic stem-like cell line and their expression in developing and regenerating liver

To identify differentially expressed genes in adult hepatic stem cells, we performed suppression-subtractive hybridization (SSH) between adult porcine hepatic stem-like cells (HSLCs) and hepatocytes, and the expression of selected genes was assessed in porcine fetal livers and regenerating liver in an 80% hepatectomy model. SSH and subsequent differential screening selected 39 clones that were expressed differentially in HSLCs, including six known genes, 10 unknown genes, one unidentified gene and some chimeric fragments. Four of these genes showed significantly higher expression in HSLCs than in mature hepatocytes: anti-leukoproteinase, matrix Gla protein, amyloid-beta precursor protein (APP) and dickkopf-3 (DKK-3). Among them, the mRNA expression of APP and DKK-3 was significantly higher in fifth GW fetal liver than in seventh and thirteenth GW fetal and adult livers, unlike the expression patterns of alpha-fetoprotein (AFP) or albumin. These mRNAs were detected in the parenchyma of fifth GW fetal liver, whereas in normal adult liver possible expression was limited to the periportal area. On the other hand, immunohistochemistry, Masson's trichrome staining and silver impregnation demonstrated APP and DKK-3 proteins in fifth GW fetal liver in which intralobular bile ducts and hepatic plates had not completely developed. DKK-3 and AFP mRNAs were upregulated on the seventh day (7D) after 80% hepatectomy. In the liver tissue, DKK-3 and AFP proteins were detected in mesenchymal cells in the periportal area and parenchyma, respectively. These data for DKK-3 expression in adult livers suggest the possible presence of adult HSLCs in the periportal area. The pattern of histological staining suggested that 7D liver was in the process of regeneration, showing a character similar to the fifth GW fetal liver. It is speculated that DKK-3 is upregulated in immature and developing livers, and has possible involvement in hepatic differentiation and liver regeneration.

New primary culture systems to study the differentiation and proliferation of mouse fetal hepatoblasts

Hepatoblasts have the potential to differentiate into both hepatocytes and biliary epithelial cells through a differentiation program that has not been fully elucidated. With the aim to better define the mechanism of differentiation of hepatoblasts, we isolated hepatoblasts and established new culture systems. We isolated hepatoblasts from E12.5 fetal mouse liver by using E-cadherin. The E-cadherin+ cells expressed alpha-fetoprotein (AFP) and albumin (Alb) but not cytokeratin 19 (CK19). Transplantation of the E-cadherin+ cells into mice that had been subjected to liver injury or biliary epithelial injury led to differentiation of the cells into hepatocytes or biliary epithelial cells, respectively. In a low-cell-density culture system in the absence of additional growth factors, E-cadherin+ cells formed colonies of various sizes, largely comprising Alb-positive cells. Supplementation of the culture medium with hepatocyte growth factor and epidermal growth factor promoted proliferation of the cells. Thus the low-cell-density culture system should be useful to identify inductive factors that regulate the proliferation and differentiation of hepatoblasts. In a high-cell-density system in the presence of oncostatin M+dexamethasone, E14.5, but not E12.5, E-cadherin+ cells differentiated into mature hepatocytes, suggesting that unidentified factors are involved in hepatic maturation. Culture of E-cadherin+ cells derived from E12.5 or E14.5 liver under high-cell-density conditions should allow elucidation of the mechanism of hepatic differentiation in greater detail. These new culture systems should be of use to identify growth factors that induce hepatoblasts to proliferate or differentiate into hepatocytes and biliary epithelial cells.

Signaling networks in hepatic oval cell activation

Oval cells are hypothesized to be the progeny of intrahepatic stem cells, also referred to as adult liver stem cells. The mechanisms by which these cells are activated to proliferate and differentiate during liver regeneration is important for the development of new therapies to treat liver disease. Oval cell activation is the first step in progenitor-dependent liver regeneration in response to certain types of injury. This review describes what is currently known about the factors involved in oval cell activation, proliferation, migration, and differentiation.

Stem cells, cell transplantation and liver repopulation

Liver transplantation is currently the only therapeutic option for patients with end-stage chronic liver disease and for severe acute liver failure. Because of limited donor availability, attention has been focused on the possibility to restore liver mass and function through cell transplantation. Stem cells are a promising source for liver repopulation after cell transplantation, but whether or not the adult mammalian liver contains hepatic stem cells is highly controversial. Part of the problem is that proliferation of mature adult hepatocytes is sufficient to regenerate the liver after two-thirds partial hepatectomy or acute toxic liver injury and participation of stem cells is not required. However, under conditions in which hepatocyte proliferation is blocked, undifferentiated epithelial cells in the periportal areas, called "oval cells", proliferate, differentiate into hepatocytes and restore liver mass. These cells are referred to as facultative liver stem cells, but they do not repopulate the normal liver after their transplantation. In contrast, epithelial cells isolated from the early fetal liver can effectively repopulate the normal liver, but they are already traversing the hepatic lineage and may not be true stem cells. Mesenchymal stem cells and embryonic stem cells can be induced to differentiate along the hepatic lineage in culture, but at present these cells are inefficient in repopulating the liver. This review will characterize these various cell types and compare the properties of these cells and the conditions under which they do or do not repopulate the liver following their transplantation.

Delta-like 1 participates in the specification of ventral midbrain progenitor derived dopaminergic neurons

Delta-like 1 (Dlk1), a member of the Delta/Notch protein family, is expressed in the mouse ventral midbrain (VM) as early as embryonic day 11.5 (E11.5) followed by exclusive expression in tyrosine 3-monooxygenase (TH) positive neurons from E12.5 onwards. To further elucidate the yet unknown function of Dlk1 in VM neuron development, we investigated the effect of soluble Dlk1 protein as well as the intrinsic Dlk1 function in the course of VM progenitor expansion and dopaminergic (DA) neuron differentiation in vitro. Dlk1 treatment during expansion increased DA progenitor proliferation and the proportion of NR4A2+ neurons expressing TH after differentiation, whereas Dlk1 treatment during the course of DA precursor differentiation did not alter TH+ neuron counts. In contrast, silencing of endogenously expressed Dlk1 prior to DA precursor differentiation partially prevented the expression of DA neuron markers, which was not accompanied with alteration of overall or local proliferation. Due to the latter finding in combination with the absence of Dlk1 negative DA neurons in differentiated cultures, we suggest that Dlk1 expression might have a permissive effect on DA neuron differentiation in vitro. The study presented here is the first publication identifying Dlk1 effects on ventral midbrain-derived DA precursor differentiation.

Application of liver stem cells for cell therapy

The worldwide shortage of donor livers to transplant end stage liver disease patients has prompted the search for alternative cell therapies for intractable liver disease. Embryonic stem cells can be readily differentiated into hepatocytes, and their transplantation into animals has improved liver function in the absence of teratoma formation: their use in bioartificial liver support is an obvious application. In animal models of liver disease, adopting strategies to provide a selective advantage for transplanted foetal or adult hepatocytes have proved highly effective in repopulating recipient livers, but the poor success of today's hepatocyte transplants can be attributed to the lack of a clinically applicable procedure to force a similar repopulation of the human liver. The activation of bipotential hepatic progenitor cells is clearly vital for survival in many cases of acute liver failure, but surprisingly little progress has been made with these cells in terms of transplantation. Finally there is the controversial subject of autologous bone marrow, and while the contribution of these indigenous cells to liver turnover seems at best, trivial, results from a small number of phase 1 studies of transplantation of bone marrow to cirrhotic patients have been moderately encouraging.

Analysis of the callipyge phenotype through skeletal muscle development; association of Dlk1 with muscle precursor cells

The callipyge mutation in sheep in the form of the paternal heterozygote results in skeletal muscle hypertrophy, which is most pronounced in the hindquarters. Overexpression of one of the genes in the region of the causative single-nucleotide polymorphism, Dlk1, is postulated to be a primary cause of the muscle hypertrophy although the mechanism is not clear. This study examined the expression of Dlk1 mRNA and its encoded protein in skeletal muscles of callipyge and wild-type sheep. The muscles examined included those that demonstrate hypertrophy in callipyge sheep as well as an unaffected muscle. The expression pattern of Dlk1 protein in these muscles was also measured over a developmental time course ranging from 80 days of gestation to 12 weeks after birth. Quantitative reverse transcription-polymerase chain reaction demonstrated that Dlk1 mRNA was significantly increased in affected, but not unaffected, muscles from callipyge sheep at 120 days of gestation through to 12 weeks of age. Immuno-localization of Dlk1 was pronounced in the interstitial connective tissue of fetal muscle but was less intense at later ages. No clear difference in Dlk1 immuno-localization was noted between genotypes in the fetal samples. Strong myofiber-specific Dlk1 immuno-localization was observed in hypertrophied callipyge muscles at 12 weeks of age. This staining was exclusively associated with fast type II myofibers and these had a significantly larger mean cross-sectional area, compared with fast type II myofibers in control sheep that did not overexpress Dlk1. In addition, Dlk1 immuno-localization was associated with a sub-population of Pax7-positive mononucleated cells in all skeletal muscles examined during fetal development and at birth, but this was not apparent at 12 weeks. There were no genotype-dependent alterations in the mRNA expression patterns of a number of promyogenic transcription factors indicating that the callipyge mutation was not affecting muscle cell differentiation per se. We postulate that Dlk1 is implicated in the commitment and/or proliferation of fetal myoblasts as well as in the maintenance of hypertrophy in fully differentiated myofibers.

New cell surface markers for murine fetal hepatic stem cells identified through high density complementary DNA microarrays

Isolation of hepatic stem cells from the adult liver (AL) has not yet been achieved due to the lack of specific cell surface markers. To identify new surface markers for hepatic stem cells, we analyzed differences in the gene expression profile of embryonic day (ED) 13.5 fetal liver stem/progenitor cells (FLSPC) versus AL by complementary DNA (cDNA) microarray technology. Using FLSPC purified to >90% by immunomagnetic selection for E-cadherin and high density (27k) mouse cDNA microarrays, we identified 474 genes that are more strongly expressed in FLSPC (FLSPC-up genes) and 818 genes that are more strongly expressed in AL (AL-up genes). The most highly overrepresented gene ontology (GO) categories for FLSPC-up genes are nucleus, cellular proliferation, and cell cycle control. AL-up genes are overrepresented for genes in metabolic pathways for specific hepatic functions. We identified 24 FLSPC-up gene surface markers and 69 AL-up gene surface markers. Western blot studies confirmed the expression of the FLSPC-up gene neighbor of Punc E11 (Nope) in fetal liver, but expression was not detectable in AL. Immunohistochemistry, confocal microscopy, and fluorescence-activated cell sorting (FACS) analysis of fetal liver demonstrated that Nope is specifically expressed on the surface of FLSPC within the fetal liver.

CONCLUSION

This is the first microarray study to analyze the specific gene expression profile of purified murine FLSPC. Our analysis identified 24 new/potential cell surface markers for murine fetal hepatic stem cells, of which Nope may be particularly useful in future studies to identify, characterize and isolate hepatic stem cells from the AL.

Human hepatic stem cells from fetal and postnatal donors

Human hepatic stem cells (hHpSCs), which are pluripotent precursors of hepatoblasts and thence of hepatocytic and biliary epithelia, are located in ductal plates in fetal livers and in Canals of Hering in adult livers. They can be isolated by immunoselection for epithelial cell adhesion molecule–positive (EpCAM+) cells, and they constitute ∼0.5–2.5% of liver parenchyma of all donor ages. The self-renewal capacity of hHpSCs is indicated by phenotypic stability after expansion for >150 population doublings in a serum-free, defined medium and with a doubling time of ∼36 h. Survival and proliferation of hHpSCs require paracrine signaling by hepatic stellate cells and/or angioblasts that coisolate with them. The hHpSCs are ∼9 μm in diameter, express cytokeratins 8, 18, and 19, CD133/1, telomerase, CD44H, claudin 3, and albumin (weakly). They are negative for α-fetoprotein (AFP), intercellular adhesion molecule (ICAM) 1, and for markers of adult liver cells (cytochrome P450s), hemopoietic cells (CD45), and mesenchymal cells (vascular endothelial growth factor receptor and desmin). If transferred to STO feeders, hHpSCs give rise to hepatoblasts, which are recognizable by cordlike colony morphology and up-regulation of AFP, P4503A7, and ICAM1. Transplantation of freshly isolated EpCAM+ cells or of hHpSCs expanded in culture into NOD/SCID mice results in mature liver tissue expressing human-specific proteins. The hHpSCs are candidates for liver cell therapies.

Evaluation of gene expression related to hepatic cell maturation and differentiation in a chemically induced mouse hepatoblastoma cell line

The MHB-2 cell line, established from a mouse hepatoblastoma (HB), was subjected to the reverse transcriptase-polymerase chain reaction (RT-PCR) for evaluation of gene expression related to cell differentiation. RNAs for c-kit, CD34, thy-1, albumin, cytokeratin (CK) 8, 18 and 19 could be detected, but expression of alpha-fetoprotein, glucose-6-phosphatase, tyrosine aminotransferase and CK7 was not observed. MHB-2 cells were positive for CK8/18 but negative for c-kit, CD34, thy-1 and albumin on protein level. Immunohistochemical staining of the HB in vivo revealed diffusely expressed c-kit. Thy-1-positive HB cells were sparsely observed, but the tumor was negative for CD34 and rarely positive for CK8/18. By in situ hybridization, the HB was positive for CK18 but negative for CK19. Slight expression of albumin, but the lack of immature hepatocytic marker suggested some heterogeneous hepatocyte or an undifferentiated cell from other origin. Furthermore, positive expression of CK19 as well as CK8 and CK18 in culture strongly suggested the differentiation into a biliary lineage or the bidirectional state. In conclusion, the present study indicated the mouse HB to have de-differentiated, bipotent, or biliary-like cell characteristics, and considering the histological difference between HB and biliary tumors, it suggests the mouse HB cells are closely like some sort of hepatic undifferentiated cells.

Remarkable heterogeneity displayed by oval cells in rat and mouse models of stem cell-mediated liver regeneration

The experimental protocols used in the investigation of stem cell-mediated liver regeneration in rodents are characterized by activation of the hepatic stem cell compartment in the canals of Hering followed by transit amplification of oval cells and their subsequent differentiation along hepatic lineages. Although the protocols are numerous and often used interchangeably across species, a thorough comparative phenotypic analysis of oval cells in rats and mice using well-established and generally acknowledged molecular markers has not been provided. In the present study, we evaluated and compared the molecular phenotypes of oval cells in several of the most commonly used protocols of stem cell-mediated liver regeneration-namely, treatment with 2-acetylaminofluorene and partial (70%) hepatectomy (AAF/PHx); a choline-deficient, ethionine-supplemented (CDE) diet; a 3,5-diethoxycarbonyl-1,4-dihydro-collidin (DDC) diet; and N-acetyl-paraaminophen (APAP). Reproducibly, oval cells showing reactivity for cytokeratins (CKs), muscle pyruvate kinase (MPK), the adenosine triphosphate-binding cassette transporter ABCG2/BCRP1 (ABCG2), alpha-fetoprotein (AFP), and delta-like protein 1/preadipocyte factor 1 (Dlk/Pref-1) were induced in rat liver treated according to the AAF/PHx and CDE but not the DDC protocol. In mouse liver, the CDE, DDC, and APAP protocols all induced CKs and ABCG2-positive oval cells. However, AFP and Dlk/Pref-1 expression was rarely detected in oval cells.

CONCLUSION

Our results delineate remarkable phenotypic discrepancies exhibited by oval cells in stem cell-mediated liver regeneration between rats and mice and underline the importance of careful extrapolation between individual species.

Tim2 is expressed in mouse fetal hepatocytes and regulates their differentiation

Liver development is regulated by various extracellular molecules such as cytokines and cell surface proteins. Although several such regulators have been identified, additional molecules are likely to be involved in liver development. To identify such molecules, we employed the signal sequence trap (SST) method to screen cDNAs encoding a secreted or membrane protein from fetal liver and obtained a number of clones. Among them, we found that T cell immunoglobulin and mucin domain 2 (Tim2) was expressed specifically on immature hepatocytes in the fetal liver. Tim2 has been shown to regulate immune responses, but its role in liver development had not been studied. We have examined the possible role of Tim2 in hepatocyte differentiation. At first, we prepared a soluble Tim2 fusion protein consisting of its extracellular domain and the Fc domain of human IgG (Tim2-hFc) and found that it bound to fetal and adult hepatocytes, suggesting that there are Tim2-binding molecules on hepatocytes. Second, Tim2-hFc inhibited the differentiation of hepatocytes in fetal liver primary culture, i.e., the expression of mature hepatic enzymes and accumulation of glycogen were severely reduced. Third, Tim2-hFc also inhibited proliferation of fetal hepatocytes. Fourth, down-regulation of Tim2 expression by small interfering RNA (siRNA) enhanced the expression of liver differentiation marker genes.

CONCLUSION

It is strongly suggested that Tim2 is involved in the differentiation of fetal hepatocytes.

The EGF-like Protein dlk1 Inhibits Notch Signaling and Potentiates Adipogenesis of Mesenchymal Cells

The EGF-like homeotic gene Dlk1 appears to function as an inhibitor of adipogenesis. Overexpression of Dlk1 prevents adipogenesis of 3T3-L1 cells. Dlk1-deficient mice are obese; however, adipose tissue still develops in Fc-dlk1 transgenic mice, suggesting that Dlk1 is not a strict inhibitor of adipogenesis. To clarify the role of Dlk1 in adipogenesis, we studied whether Dlk1 could act differently on this process depending upon the differentiation state of the precursor cells. We found that Dlk1 is a potentiator of adipogenesis for mesenchymal C3H10T1/2 cells. This potentiating effect can be triggered by overexpressing the entire protein or the extracellular EGF-like-containing region, but not by overexpressing the intracellular dlk1 sequence. In addition, coculture of C3H10T1/2 cells with other cells expressing Dlk1, but not with cells lacking Dlk1 expression, enhances their adipogenic response. Potentiation of adipogenesis by Dlk1 was associated with changes in the activation of ERK1/2 after IGFI/insulin induction. Finally, as reported with other cells, dlk1 functioned as a Notch signaling inhibitor in C3H10T1/2 cells, but inhibition of Notch1 expression prevented the potentiating effects of Dlk1 in adipogenesis. These data suggest that Dlk1 may potentiate or inhibit adipogenesis depending upon the cellular context, and that Notch1 expression and activation are important factors in this context.

The Novel Gene EGFL9/Dlk2, Highly Homologous to Dlk1, Functions as a Modulator of Adipogenesis

The Dlk1 gene appears to function as a regulator of adipogenesis. Adult Dlk1-deficient mice are obese, but adipose tissue still develops in transgenic mice overexpressing an Fc-dlk1 fusion protein, and neither type of genetically modified mice displays serious abnormalities. It was therefore possible that one yet unidentified gene might either compensate or antagonize for the absence or for overexpression, respectively, of Dlk1 in those animals. In database searches, we found a novel gene, EGFL9, encoding for a protein whose structural features are virtually identical to those of dlk1, suggesting it may function in a similar way. As dlk1 does, the protein encoded by EGFL9/Dlk2 affects adipogenesis of 3T3-L1 preadipocytes and mesenchymal C3H10T1/2 cells; however, it does so in an opposite way to that of dlk1. In addition, expression levels of both genes appear to be inversely correlated in both cell lines. Moreover, enforced changes in the expression of one gene affect the expression levels of the other. Our data suggest that adipogenesis may be modulated by the coordinated expression of Dlk1 and EGFL9/Dlk2.

Restricted co-expression of Dlk1 and the reciprocally imprinted non-coding RNA, Gtl2: implications for cis-acting control

Dlk1 and Gtl2 are reciprocally imprinted neighboring genes located within a 1 Mb imprinted domain on murine distal chromosome 12. The two genes are expressed and developmentally regulated during mammalian embryogenesis. Dlk1/Pref1 encodes a transmembrane protein with homology to members of the Notch/Delta developmental signaling pathway and Gtl2 generates alternatively spliced poly-adenylated transcripts lacking a conserved open reading frame. An intergenic differentially methylated region (IG-DMR) located 13 kb upstream of Gtl2 has been shown to regulate imprinting throughout the domain by an as yet unknown mechanism. In order to gain insights into regulation at this domain and to compare it with imprinting control at other loci, we compared the expression profile of Dlk1 with Gtl2 during mouse embryogenesis in normal conceptuses and in those with uniparental disomy for chromosome 12. The expression profile of these genes suggests a causative role for Dlk1 and Gtl2 in the pathologies found in uniparental disomy animals, characterized by defects in skeletal muscle maturation, bone formation, placenta size and organization and prenatal lethality. Here, we show restricted overlap in cellular expression of these two genes throughout development. Dlk1 is imprinted and expressed in cell types within the lung, liver and placenta where Gtl2 is not expressed. Gtl2 is highly expressed in the central nervous system (CNS), whereas Dlk1 is found localized to specific regions such as the hypothalamus. Co-expression is observed in most of the mesodermal-derived tissues, notably the skeletal muscle where both genes are strongly co-expressed. In this tissue, Dlk1 shows a relaxation of imprinting with some expression from the maternal allele. These findings indicate that the general mechanism of imprinting at the stages analyzed is not through the co-ordinate non-coding RNA or insulator mechanisms observed for other imprinted domains, and suggest that the two genes have independent tissue-specific functions.

In vitro differentiation of hepatic progenitor cells from mouse embryonic stem cells induced by sodium butyrate

Recently it was shown that embryonic stem (ES) cells could differentiate into hepatocytes both in vitro and in vivo, however, prospective hepatic progenitor cells have not yet been isolated and characterized from ES cells. Here we presented a novel 4-step procedure for the differentiation of mouse ES cells into hepatic progenitor cells and then hepatocytes. The differentiated hepatocytes were identified by morphological, biochemical, and functional analyses. The hepatic progenitor cells were isolated from the cultures after the withdrawal of sodium butyrate, which was characterized by scant cytoplasm, ovoid nuclei, the ability of rapid proliferation, expression of a series of hepatic progenitor cell markers, and the potential of differentiation into hepatocytes and bile duct-like cells under the proper conditions that favor hepatocyte and bile epithelial differentiation. The differentiation of hepatocytes from hepatic progenitor cells was characterized by a number of hepatic cell markers including albumin secretion, upregulated transcription of glucose-6-phosphatase and tyrosine aminotransferase, and functional phenotypes such as glycogen storage. The results from our experiments demonstrated that ES cells could differentiate into a novel bipotential hepatic progenitor cell and mature into hepatocytes with typical morphological, phenotypic and functional characteristics, which provides an useful model for the studies of key events during early liver development and a potential source of transplantable cells for cell-replacement therapies.

Embryonic stem cells develop into hepatocytes after intrasplenic transplantation in CCl4-treated mice

AIM: To transplant undifferentiated embryonic stem (ES) cells into the spleens of carbon tetrachloride (CCl₄)-treated mice to determine their ability to differentiate into hepatocytes in the liver.

METHODS: CCl₄, 0.5 mL/kg body weight, was injected into the peritoneum of C57BL/6 mice twice a week for 5 wk. In group 1 (n = 12), 1 x 10⁵ undifferentiated ES cells (0.1 mL of 1 x 10⁶/mL solution), genetically labeled with GFP, were transplanted into the spleens 1 d after the second injection. Group 2 mice (n = 12) were injected with 0.2 mL of saline twice a week, instead of CCl₄, and the same amount of ES cells was transplanted into the spleens. Group 3 mice (n = 6) were treated with CCl₄ and injected with 0.1 mL of saline into the spleen, instead of ES cells. Histochemical analyses of the livers were performed on post-transplantation d (PD) 10, 20, and 30.

RESULTS: Considerable numbers of GFP-immunopositive cells were found in the periportal regions in group 1 mice (CCl₄-treated) on PD 10, however, not in those untreated with CCl₄ (group 2). The GFP-positive cells were also immunopositive for albumin (ALB), alpha-1 antitrypsin, cytokeratin 18, and hepatocyte nuclear factor 4 alpha on PD 20. Interestingly, most of the GFP-positive cells were immunopositive for DLK, a hepatoblast marker, on PD 10. Although very few ES-derived cells were demonstrated immunohistologically in the livers of group 1 mice on PD 30, improvements in liver fibrosis were observed. Unexpectedly, liver tumor formation was not observed in any of the mice that received ES cell transplantation during the experimental period.

CONCLUSION: Undifferentiated ES cells developed into hepatocyte-like cells with appropriate integration into tissue, without uncontrolled cell growth.

Antibodies to Stem Cell Marker Antigens Reduce Engraftment of Hematopoietic Stem Cells

Hematopoietic stem cells (HSCs) have enormous potential for use in transplantation and gene therapy. However, the frequency of repopulating HSCs is often very low; thus, highly effective techniques for cell enrichment and maintenance are required to obtain sufficient cell numbers for therapeutic use and for studies of HSC physiology. Common methods of HSC enrichment use antibodies recognizing HSC surface marker antigens. Because antibodies are known to alter the physiology of other cell types, we investigated the effect of such enrichment strategies on the physiology and lineage commitment of HSCs. We sorted HSCs using a method that does not require antibodies: exclusion of Hoechst 33342 to isolate side population (SP) cells. To elucidate the effect of antibody binding on this HSC population, we compared untreated SP cells with SP cells treated with the Sca-1(+)c-Kit(+)Lin(-) (SKL) antibody cocktail prior to SP sorting. Our findings revealed that HSCs incubated with the antibody cocktail had decreased expression of the stem cell-associated genes c-Kit, Cd34, Tal-1, and Slamf1 relative to untreated SP cells or to cells treated with polyclonal isotype control antibodies. Moreover, SKL antibodies induced cycling in SP cells and diminished their ability to confer long-term hematopoietic engraftment in lethally irradiated mice. Taken together, these data suggest that antibody-based stem cell isolation procedures can have negative effects on HSC physiology.

Two populations of Thy1-positive mesenchymal cells regulate in vitro maturation of hepatic progenitor cells

We previously reported that the in vitro maturation of CD49f(+)Thy1(-)CD45(-) (CD49f positive) fetal hepatic progenitor cells (HPCs) is supported by Thy1-positive mesenchymal cells derived from the fetal liver. These mesenchymal cell preparations contain two populations, one of a cuboidal shape and the other spindle shaped in morphology. In this study, we determined that the mucin-type transmembrane glycoprotein gp38 could distinguish cuboidal cells from spindle cells by immunocytochemistry. RT-PCR analysis revealed differences between isolated CD49f(+/-)Thy1(+)gp38(+)CD45(-) (gp38 positive) cells and CD49f(+/-)Thy1(+)gp38(-)CD45(-) (gp38 negative) cells, whereas both cells expressed mesenchymal cell markers. The coculture with gp38-positive cells promoted the maturation of CD49f-positive HPCs, which was estimated by positivity for periodic acid-Schiff (PAS) staining, whereas the coculture with gp38-negative cells maintained CD49f-positive HPCs negative for PAS staining. The expression of mature hepatocyte markers, such as tyrosine aminotransferase, tryptophan-2,3-dioxygenase, and glucose-6-phosphatase, were upregulated on HPCs by coculture with gp38-positive cells. Furthermore, transmission electron microscopy revealed the acquisition of mature hepatocyte features by HPCs cocultured with gp38-positive cells. This effect on maturation of HPCs was inhibited by the addition of conditioned medium derived from gp38-negative cells. By contrast, the upregulation of bromodeoxyuridine incorporation by HPCs demonstrated the proliferative effect of coculture with gp38-negative cells. In conclusion, these results suggest that in vitro maturation of HPCs promoted by gp38-positive cells may be opposed by an inhibitory effect of gp38-negative cells, which likely maintain the immature, proliferative state of HPCs.

Comparative proteomic analysis of primary mouse liver c-Kit−(CD45/TER119)− stem/progenitor cells

Liver stem/progenitor cells play a key role in liver development and maybe also in liver cancer development. In our previous study a population of c-Kit-(CD45/TER119)- liver stem/progenitor cells in mouse fetal liver, was successfully sorted with large amount (10(6)-10(7)) by using immuno-magnetic microbeads. In this study, the sorted liver stem/progenitor cells were used for proteomic study. Proteins of the sorted liver stem/progenitor cells and unsorted fetal liver cells were investigated using two-dimensional electrophoresis. A two-dimensional proteome map of liver stem/progenitor cells was obtained for the first time. Proteins that exhibited significantly upregulation in liver stem/progenitor cells were identified by peptide mass fingerprinting and peptide sequencing. Nineteen protein spots corresponding to 12 different proteins were identified as showing significant upregulation in liver stem/progenitor cells and seem to play important roles in such cells in cell metabolism, cell cycle regulation, and stress. An interesting finding is that most of the upregulated proteins were overexpressed in various cancers (11 of 12, including 6 in human hepatocellular carcinoma (HCC)) and involved in cancer development as reported in previous studies. Some of the identified proteins were validated by real-time PCR, Western blotting, and immunostaining. Taken together, the data presented provide a significant new protein-level insight into the biology of liver stem/progenitor cells, a key population of cells that might be also involved in liver cancer development.

Development of murine hepatic sinusoidal endothelial cells characterized by the expression of hyaluronan receptors

Endothelial cells (ECs) display distinct structural and functional characteristics depending on the tissue and developmental stage; however, the development of tissue-specific ECs remains poorly understood. Here, we describe the development of hepatic sinusoids in mice based on the expression of hyaluronan receptors Stab2 and Lyve-1. Flk-1(+) cells in and around the liver bud begin to express Stab2 at embryonic day (E) 9.5, before the formation of vascular lumen. Hepatic sinusoidal endothelial cells (HSECs) begin to express Lyve-1 at E10.5, and both markers continue to be expressed in HSECs thereafter. Although HSECs and lymphatic ECs (LECs) are known to share functional and phenotypic characteristics, we clearly show that HSECs can be distinguished from LECs by the expression of molecular markers and higher endocytotic activity. Our results provide new insight into the development of tissue-specific ECs and phenotypic criteria to distinguish HSECs from other types of ECs, including LECs.

Molecular Mechanism of Liver Development and Regeneration

The liver is the central organ for metabolism and has strong regenerative capability. Although the liver has been studied mostly biochemically and histopathologically, genetic studies using gene-targeting technology have identified a number of cytokines, intracellular signaling molecules, and transcription factors involved in liver development and regeneration. In addition, various in vitro systems such as fetal liver explant culture and primary culture of fetal liver cells have been established, and the combination of genetic and in vitro studies has accelerated investigation of liver development. Identification of the cell-surface molecules of liver progenitors has made it possible to identify and isolate liver progenitors, making the liver a unique model for stem cell biology. In this review, we summarize progresses in understanding liver development and regeneration.

Notch signaling reveals developmental plasticity of Pax4(+) pancreatic endocrine progenitors and shunts them to a duct fate

Relatively little is known about the developmental signals that specify the types and numbers of pancreatic cells. Previous studies suggested that Notch signaling in the pancreas inhibits differentiation and promotes the maintenance of progenitor cells, but it remains unclear whether Notch also controls cell fate choices as it does in other tissues. To study the impact of Notch in progenitors of the beta cell lineage, we generated mice that express Cre-recombinase under control of the Pax4 promoter. Lineage analysis of Pax4(+) cells demonstrates they are specified endocrine progenitors that contribute equally to four islet cell fates, contrary to expectations raised by the dispensable role of Pax4 in the specification of the alpha and PP subtypes. In addition, we show that activation of Notch in Pax4(+) progenitors inhibits their differentiation into alpha and beta endocrine cells and shunts them instead toward a duct fate. These observations reveal an unappreciated degree of developmental plasticity among early endocrine progenitors and raise the possibility that a bipotent duct-endocrine progenitor exists during development. Furthermore, the redirection of Pax4(+) cells from alpha and beta endocrine fates toward a duct cell type suggests a positive role for Notch signaling in duct specification and is consistent with the more widely defined role for Notch in cell fate determination.

Cholangiocyte marker-positive and -negative fetal liver cells differ significantly in their ability to regenerate the livers of adult rats exposed to retrorsine

We have used monoclonal antibodies against cell-surface developmental epitopes in combination with micromagnetic beads to isolate phenotypically defined subpopulations of cholangiocyte marker-positive fetal liver epithelial cells (CMP-FLEC). Differentiation potential was evaluated by injecting cell isolates from dipeptidyl peptidase IV (DPPIV) positive (DPPIV+) Fischer donor rats into the spleen of partially hepatectomized, DPPIV negative (DPPIV-)Fischer host rats exposed to retrorsine. At various time points, liver tissue was harvested and cells in DPPIV+ colonies were phenotyped by immunofluorescence and histochemical protocols. Functional differentiation and liver replacement were determined by comparing donor and host hepatocyte protein expression patterns and DPPIV enzyme activity in extracts from livers of host rats receiving CMP-FLEC. Our results showed that bipotentiality was retained during differentiation and maturation of CMP-FLEC, indicating that the acquisition of ductal morphology and phenotype were not indicative of lineage commitment. CMP-FLEC transplanted into the adult rat liver lost ductal and gained hepatocyte markers, and acquired protein expression patterns in 2D gels with a close similarity (&gt;75% spot match) to host hepatocytes but differing significantly from the transplanted CMP-FLEC cell isolate (&lt;25%spot match). The average size of donor hepatocyte colonies increased with time so that by 1 year, up to 70% of the host rat liver was replaced by CMP-FLEC derived DPPIV+ hepatocytes. Depletion of CMP-FLEC from fetal liver isolates resulted in a marked decrease in adult liver colonization, suggesting that a high percentage of the hepatocyte colonies in animals receiving total fetal liver isolates are derived from CMP-FLEC.

Multiple imprinted and stemness genes provide a link between normal and tumor progenitor cells of the developing human kidney

Wilms' tumor (WT), the embryonic kidney malignancy, is suggested to evolve from a progenitor cell population of uninduced metanephric blastema, which typically gives rise to nephrons. However, apart from blastema, WT specimens frequently contain cells that have differentiated into renal tubular or stromal phenotypes, complicating their analysis. We aimed to define tumor-progenitor genes that function in normal kidney development using WT xenografts (WISH-WT), in which the blastema accumulates with serial passages at the expense of differentiated cells. Herein, we did transcriptional profiling using oligonucleotide microarrays of WISH-WT, WT source, human fetal and adult kidneys, and primary and metastatic renal cell carcinoma. Among the most significantly up-regulated genes in WISH-WT, we identified a surprising number of paternally expressed genes (PEG1/MEST, PEG3, PEG5/NNAT, PEG10, IGF2, and DLK1), as well as Meis homeobox genes [myeloid ecotropic viral integration site 1 homologue 1 (MEIS1) and MEIS2], which suppress cell differentiation and maintain self-renewal. A comparison between independent WISH-WT and WT samples by real-time PCR showed most of these genes to be highly overexpressed in the xenografts. Concomitantly, they were significantly induced in human fetal kidneys, strictly developmentally regulated throughout mouse nephrogenesis and overexpressed in the normal rat metanephric blastema. Furthermore, in vitro differentiation of the uninduced blastema leads to rapid down-regulation of PEG3, DLK1, and MEIS1. Interestingly, ischemic/reperfusion injury to adult mouse kidneys reinduced the expression of PEG3, PEG10, DLK1, and MEIS1, hence simulating embryogenesis. Thus, multiple imprinted and stemness genes that function to expand the renal progenitor cell population may lead to evolution and maintenance of WT.

Hepatic Stem Cells: In Search of

The field of stem cell biology has exploded with the study of a wide range of cellular populations involving endodermal, mesenchymal, and ectodermal organs. One area of extensive study has included the identification of hepatic stem and progenitor cell subpopulations. Liver stem cells provide insights into the potential pathways involving liver regeneration that are independent of mature hepatocytes. Hepatic progenitor cells are either bipotent or multipotent and capable of multiple rounds of replication. They have been identified in fetal as well as adult liver. Various injury models have been used to expand this cellular compartment. The nomenclature, origin, and function of the hepatic progenitor cell populations are areas of ongoing debate. In this review, we will discuss the different definitions and functions of hepatic progenitor cells as well as the current research efforts examining their therapeutic potential.

Ectodomain shedding of preadipocyte factor 1 (Pref-1) by tumor necrosis factor alpha converting enzyme (TACE) and inhibition of adipocyte differentiation

Preadipocyte factor 1 (Pref-1), an epidermal growth factor repeat containing transmembrane protein found in the preadipocytes, inhibits adipocyte differentiation in vitro and in vivo. Here, we examined the processing of membrane form of Pref-1A to release the 50-kDa soluble form that inhibits adipocyte differentiation. The ectodomain cleavage of Pref-1 is markedly enhanced by phorbol 12-myristate 13-acetate in a dose- and time-dependent manner. The basal and stimulated cleavage is inhibited by the broad metalloproteinase inhibitor GM6001, a fact that suggests that cleavage of membrane Pref-1A is dependent on a metalloproteinase. Next, we showed that release of soluble Pref-1A is inhibited by TAPI-0 and by a tissue inhibitor of metalloproteinase-3, TIMP-3, that can inhibit tumor necrosis factor alpha converting enzyme (TACE), but not by TIMP-1 or TIMP-2. On the other hand, overexpression of TACE increases Pref-1 cleavage to produce the 50-kDa soluble form. Furthermore, this cleavage was not detected in cells with TACE mutation or with TACE small interfering RNA. TACE-mediated shedding of Pref-1 ectodomain inhibits adipocyte differentiation of 3T3-L1 cells and in Pref-1-null mouse embryo fibroblasts transduced with Pref-1A. Identification of TACE as the major protease responsible for conversion of membrane-bound Pref-1 to the biologically active diffusible form provides a new insight into Pref-1 function in adipocyte differentiation.

Isolation, characterization and culture of Thy1-positive cells from fetal rat livers

AIM: To investigate whether Thy1 recognizes oval cells in the fetal liver and to characterize the cultured Thy1- selected cells from E14 rat livers.

METHODS: Thy1 populations were analyzed by fluorescence activated cell sorter analysis. Thy1 positive cells were isolated using magnetic beads. Hepatic markers were detected by Western blotting, immunocytochemistry and RT-PCR.

RESULTS: The percentage of Thy1-positive cells decreased during early development of fetal rat liver (E13-E16). E14 fetal livers contained 7.8% Thy1 positive cells, of which 61% were positive for α-fetoprotein (AFP) and 25% expressed albumin. The Thy1+ population expressed oval cell markers c-Kit and CXCR4, liver enriched-transcription factors HNF1α and HNF6, hepatocytic markers albumin, AFP and cytokeratin 18, and biliary marker cytokeratin 19. Thy1- selected cells formed only mesenchymal colonies when plated on collagen and in serum-containing media. Thy1 selected cells were able to form hepatic colonies positive for HNF1α, HNF6, albumin, AFP, cytokeratin 18, cytokeratin 19 and glycogen, when grown on STO feeder layers in serum free-media.

CONCLUSION: Oval cells positive for Thy1 are present in early liver embryonic stages.

Isolation of multipotent progenitor cells from human fetal liver capable of differentiating into liver and mesenchymal lineages

Little is known about the differentiation capabilities of nonhematopoietic cells of the human fetal liver. We report the isolation and characterization of a human fetal liver multipotent progenitor cell (hFLMPC) population capable of differentiating into liver and mesenchymal cell lineages. Human fetal livers (74-108 days of gestation) were dissociated and maintained in culture. We treated the colonies with geneticin and mechanically isolated hFLMPCs, which were kept in an undifferentiated state by culturing on feeder layers. We derived daughter colonies by serial dilution, verifying monoclonality using the Humara assay. hFLMPCs, which have been maintained in culture for up to 100 population doublings, have a high self-renewal capability with a doubling time of 46 h. The immunophenotype is: CD34+, CD90+, c-kit+, EPCAM+, c-met+, SSEA-4+, CK18+, CK19+, albumin-, alpha-fetoprotein-, CD44h+, and vimentin+. Passage 1 (P1) and P10 cells have identical morphology, immunophenotype, telomere length, and differentiation capacity. Placed in appropriate media, hFLMPCs differentiate into hepatocytes and bile duct cells, as well as into fat, bone, cartilage, and endothelial cells. Our results suggest that hFLMPCs are mesenchymal-epithelial transitional cells, probably derived from mesendoderm. hFLMPCs survive and differentiate into functional hepatocytes in vivo when transplanted into animal models of liver disease. hFLMPCs are a valuable tool for the study of human liver development, liver injury, and hepatic repopulation.

An efficient method of sorting liver stem cells by using immuno-magnetic microbeads

AIM: To develop a method to isolate liver stem cells fast and efficiently.

METHODS: Fetal mouse liver cells were characterized by cell surface antigens (c-Kit and CD45/TER119) using flow cytometry. The candidate liver stem cells were sorted by using immuno-magnetic microbeads and identified by clone-forming culture, RT-PCR and immunofluorescence assays.

RESULTS: The c-Kit^–(CD45/TER119)^– cell population with 97.9% of purity were purified by immuno-magnetic microbeads at one time. The yield of this separation was about 6% of the total sorting cells and the cell viability was above 98%. When cultured in vitro these cells had high clone-forming and self-renewing ability and expressed markers of hepatocytes and bile duct cells. Functionally mature hepatocytes were observed after 21 d of culture.

CONCLUSION: This method offers an excellent tool for the enrichment of liver stem cells with high purity and viability, which could be used for further studies. It is fast, efficient, simple and not expensive.

The phenotypes of pluripotent human hepatic progenitors

Human livers contain two pluripotent hepatic progenitors, hepatic stem cells and hepatoblasts, with size, morphology, and gene expression profiles distinct from that of mature hepatocytes. Hepatic stem cells, the precursors to hepatoblasts, persist in stable numbers throughout life, and those isolated from the livers of all age donors from fetal to adult are essentially identical in their gene and protein expression profiles. The gene expression profile of hepatic stem cells throughout life consists of high levels of expression of cytokeratin 19 (CK19), neuronal cell adhesion molecule (NCAM), epithelial cell adhesion molecule (EpCAM), and claudin-3 (CLDN-3); low levels of albumin; and a complete absence of expression of alpha-fetoprotein (AFP) and adult liver-specific proteins. By contrast, hepatoblasts, the dominant cell population in fetal and neonatal livers, decline in numbers with age and are found as <0.1% of normal adult livers. They express high levels of AFP, elevated levels of albumin, low levels of expression of adult liver-specific proteins, low levels of CK19, and a loss of NCAM and CLDN-3. Mature hepatocytes lack expression altogether of EpCAM, NCAM, AFP, CLDN-3, cytokeratin 19, and have acquired the well-known adult-specific profile that includes expression of high levels of albumin, cytochrome P4503A4, connexins, phosphoenolpyruvate carboxykinase, and transferrin. Thus, hepatic stem cells have a unique stem cell phenotype, whereas hepatoblasts have low levels of expression of both stem cell genes and genes expressed in high levels in mature hepatocytes.