Transcriptional regulation in endoderm development: characterization of an enhancer controlling Hnf3g expression by transgenesis and targeted mutagenesis

Transcriptional, epigenetic and microRNA regulation of growth plate

Endochondral ossification is a critical event in bone formation, particularly in long shaft bones. Many cellular differentiation processes work in concert to facilitate the generation of cartilage primordium to formation of trabecular structures, all of which occur within the growth plate. Previous studies have revealed that the growth plate is tightly regulated by various transcription factors, epigenetic systems, and microRNAs. Hence, understanding these mechanisms that regulate the growth plate is crucial to furthering the current understanding on skeletal diseases, and in formulating effective treatment strategies. In this review, we focus on describing the function and mechanisms of the transcription factors, epigenetic systems, and microRNAs known to regulate the growth plate.

The molecular functions of hepatocyte nuclear factors - In and beyond the liver

The hepatocyte nuclear factors (HNFs) namely HNF1α/β, FOXA1/2/3, HNF4α/γ and ONECUT1/2 are expressed in a variety of tissues and organs, including the liver, pancreas and kidney. The spatial and temporal manner of HNF expression regulates embryonic development and subsequently the development of multiple tissues during adulthood. Though the HNFs were initially identified individually based on their roles in the liver, numerous studies have now revealed that the HNFs cross-regulate one another and exhibit synergistic relationships in the regulation of tissue development and function. The complex HNF transcriptional regulatory networks have largely been elucidated in rodent models, but less so in human biological systems. Several heterozygous mutations in these HNFs were found to cause diseases in humans but not in rodents, suggesting clear species-specific differences in mutational mechanisms that remain to be uncovered. In this review, we compare and contrast the expression patterns of the HNFs, the HNF cross-regulatory networks and how these liver-enriched transcription factors serve multiple functions in the liver and beyond, extending our focus to the pancreas and kidney. We also summarise the insights gained from both human and rodent studies of mutations in several HNFs that are known to lead to different disease conditions.

EGFR-mediated macrophage activation promotes colitis-associated tumorigenesis

Epidermal growth factor receptor (EGFR) signaling is a known mediator of colorectal carcinogenesis. Studies have focused on the role of EGFR signaling in epithelial cells, although the exact nature of the role of EGFR in colorectal carcinogenesis remains a topic of debate. Here, we present evidence that EGFR signaling in myeloid cells, specifically macrophages, is critical for colon tumorigenesis in the azoxymethane-dextran sodium sulfate (AOM-DSS) model of colitis-associated carcinogenesis (CAC). In a human tissue microarray, colonic macrophages demonstrated robust EGFR activation in the pre-cancerous stages of colitis and dysplasia. Utilizing the AOM-DSS model, mice with a myeloid-specific deletion of Egfr had significantly decreased tumor multiplicity and burden, protection from high-grade dysplasia and significantly reduced colitis. Intriguingly, mice with gastrointestinal epithelial cell-specific Egfr deletion demonstrated no differences in tumorigenesis in the AOM-DSS model. The alterations in tumorigenesis in myeloid-specific Egfr knockout mice were accompanied by decreased macrophage, neutrophil and T-cell infiltration. Pro-tumorigenic M2 macrophage activation was diminished in myeloid-specific Egfr-deficient mice, as marked by decreased Arg1 and Il10 mRNA expression and decreased interleukin (IL)-4, IL10 and IL-13 protein levels. Surprisingly, diminished M1 macrophage activation was also detectable, as marked by significantly reduced Nos2 and Il1b mRNA levels and decreased interferon (IFN)-γ, tumor necrosis factor (TNF)-α and IL-1β protein levels. The alterations in M1 and M2 macrophage activation were confirmed in bone marrow-derived macrophages from mice with the myeloid-specific Egfr knockout. The combined effect of restrained M1 and M2 macrophage activation resulted in decreased production of pro-angiogenic factors, CXCL1 and vascular endothelial growth factor (VEGF), and reduced CD31+ blood vessels, which likely contributed to protection from tumorigenesis. These data reveal that EGFR signaling in macrophages, but not in colonic epithelial cells, has a significant role in CAC. EGFR signaling in macrophages may prove to be an effective biomarker of CAC or target for chemoprevention in patients with inflammatory bowel disease.

FOXA3 Is Expressed in Multiple Cell Lineages in the Mouse Testis and Regulates Pdgfra Expression in Leydig Cells

The three FOXA transcription factors are mainly known for their roles in the liver. However, Foxa3-deficient mice become progressively sub/infertile due to germ cell loss. Because no data were available regarding the localization of the FOXA3 protein in the testis, immunohistochemistry was performed on mouse testis sections. In the fetal testis, a weak but consistent staining for FOXA3 is detected in the nucleus of Sertoli cells. In prepubertal and adult life, FOXA3 remains present in Sertoli cells of some but not all seminiferous tubules. FOXA3 is also detected in the nucleus of some peritubular cells. From postnatal day 20 onward, FOXA3 is strongly expressed in the nucleus of Leydig cells. To identify FOXA3 target genes in Leydig cells, MLTC-1 Leydig cells were transfected with a series of Leydig cell gene reporters in the presence of a FOXA3 expression vector. The platelet-derived growth factor receptor α (Pdgfra) promoter was significantly activated by FOXA3. The Pdgfra promoter contains three potential FOX elements and progressive 5' deletions and site-directed mutagenesis revealed that the most proximal element at -78 bp was sufficient to confer FOXA3 responsiveness. FOXA3 from Leydig cells could bind to this element in vitro (electrophoretic mobility shift assay) and was recruited to the proximal Pdgfra promoter in vivo (chromatin immunoprecipitation). Finally, endogenous Pdgfra messenger RNA levels were reduced in FOXA3-deficient MLTC-1 Leydig cells. Taken together, our data identify FOXA3 as a marker of the Sertoli cell lineage and of the adult Leydig cell population, and as a regulator of Pdgfra transcription in Leydig cells.

Chondrocyte hypertrophy in skeletal development, growth, and disease

Most of our bones form through the process of endochondral ossification, which is tightly regulated by the activity of the cartilage growth plate. Chondrocyte maturation through the various stages of growth plate physiology ultimately results in hypertrophy. Chondrocyte hypertrophy is an essential contributor to longitudinal bone growth, but recent data suggest that these cells also play fundamental roles in signaling to other skeletal cells, thus coordinating endochondral ossification. On the other hand, ectopic hypertrophy of articular chondrocytes has been implicated in the pathogenesis of osteoarthritis. Thus, a better understanding of the processes that control chondrocyte hypertrophy in the growth plate as well as in articular cartilage is required for improved management of both skeletal growth disorders and osteoarthritis. This review summarizes recent findings on the regulation of hypertrophic chondrocyte differentiation, the cellular mechanisms involved in hypertrophy, and the role of chondrocyte hypertrophy in skeletal physiology and pathophysiology.

Gene targeting study reveals unexpected expression of brain-expressed X-linked 2 in endocrine and tissue stem/progenitor cells in mice

Identification of genes specifically expressed in stem/progenitor cells is an important issue in developmental and stem cell biology. Genome-wide gene expression analyses in liver cells performed in this study have revealed a strong expression of X-linked genes that include members of the brain-expressed X-linked (Bex) gene family in stem/progenitor cells. Bex family genes are expressed abundantly in the neural cells and have been suggested to play important roles in the development of nervous tissues. However, the physiological role of its individual members and the precise expression pattern outside the nervous system remain largely unknown. Here, we focused on Bex2 and examined its role and expression pattern by generating knock-in mice; the enhanced green fluorescence protein (EGFP) was inserted into the Bex2 locus. Bex2-deficient mice were viable and fertile under laboratory growth conditions showing no obvious phenotypic abnormalities. Through an immunohistochemical analysis and flow cytometry-based approach, we observed unique EGFP reporter expression patterns in endocrine and stem/progenitor cells of the liver, pyloric stomach, and hematopoietic system. Although Bex2 seems to play redundant roles in vivo, these results suggest the significance and potential applications of Bex2 in studies of endocrine and stem/progenitor cells.

FoxA family members are crucial regulators of the hypertrophic chondrocyte differentiation program

During endochondral ossification, small, immature chondrocytes enlarge to form hypertrophic chondrocytes, which express collagen X. In this work, we demonstrate that FoxA factors are induced during chondrogenesis, bind to conserved binding sites in the collagen X enhancer, and can promote the expression of a collagen X-luciferase reporter in both chondrocytes and fibroblasts. In addition, we demonstrate by both gain- and loss-of-function analyses that FoxA factors play a crucial role in driving the expression of both endogenous collagen X and other hypertrophic chondrocyte-specific genes. Mice engineered to lack expression of both FoxA2 and FoxA3 in their chondrocytes display defects in chondrocyte hypertrophy, alkaline phosphatase expression, and mineralization in their sternebrae and, in addition, exhibit postnatal dwarfism that is coupled to significantly decreased expression of both collagen X and MMP13 in their growth plates. Our findings indicate that FoxA family members are crucial regulators of the hypertrophic chondrocyte differentiation program.

Foxa1 and Foxa2 control the differentiation of goblet and enteroendocrine L- and D-cells in mice

BACKGROUND & AIMS

The winged helix transcription factors Foxa1 and Foxa2 are expressed in all epithelia of the gastrointestinal tract from its embryonic origin into adulthood. In vitro studies have shown that Foxa1/a2 can transactivate the promoters of Mucin 2 (Muc2), which is expressed in goblet cells, and of preproglucagon, which is expressed in enteroendocrine cells. These findings suggest Foxa1/a2 as critical factors in the differentiation of gut epithelial cells.

METHODS

Mice with intestine-specific simultaneous deletion of Foxa1 and Foxa2 were derived using the Cre-loxP system and analyzed using histologic and molecular means.

RESULTS

Both Foxa1 and Foxa2 were deleted successfully in the epithelia of the small intestine and colon using Villin-Cre mice. Immunohistochemical staining showed that Foxa1/a2 mutants lack glucagon-like peptide-1- and peptide-2-expressing cells (L-cells), and have reduced numbers of somatostatin (D-cells) and peptide YY-expressing cells (L-cells). Preproglucagon, somatostatin, and peptide YY messenger RNA (mRNA) levels also were reduced significantly in Foxa1/a2 mutants. Thus, Foxa1 and Foxa2 are essential regulators of these enteroendocrine lineages in vivo. The mRNA levels of transcription factors Islet-1 and Pax6 were reduced significantly in the small intestine, showing that Foxa1 and Foxa2 impact on a transcription factor network in the enteroendocrine lineage. In addition, deletion of Foxa1/a2 caused a reduction in goblet cell number with altered expression of the secretory mucins Muc2, Mucin5b, Mucin5ac, and Mucin 6.

CONCLUSIONS

The winged helix factors Foxa1 and Foxa2 are essential members of the transcription factor network that govern secretory cell differentiation in the mammalian gastrointestinal tract.

SPDEF is required for mouse pulmonary goblet cell differentiation and regulates a network of genes associated with mucus production

Various acute and chronic inflammatory stimuli increase the number and activity of pulmonary mucus-producing goblet cells, and goblet cell hyperplasia and excess mucus production are central to the pathogenesis of chronic pulmonary diseases. However, little is known about the transcriptional programs that regulate goblet cell differentiation. Here, we show that SAM-pointed domain-containing Ets-like factor (SPDEF) controls a transcriptional program critical for pulmonary goblet cell differentiation in mice. Initial cell-lineage-tracing analysis identified nonciliated secretory epithelial cells, known as Clara cells, as the progenitors of goblet cells induced by pulmonary allergen exposure in vivo. Furthermore, in vivo expression of SPDEF in Clara cells caused rapid and reversible goblet cell differentiation in the absence of cell proliferation. This was associated with enhanced expression of genes regulating goblet cell differentiation and protein glycosylation, including forkhead box A3 (Foxa3), anterior gradient 2 (Agr2), and glucosaminyl (N-acetyl) transferase 3, mucin type (Gcnt3). Consistent with these findings, levels of SPDEF and FOXA3 were increased in mouse goblet cells after sensitization with pulmonary allergen, and the proteins were colocalized in goblet cells lining the airways of patients with chronic lung diseases. Deletion of the mouse Spdef gene resulted in the absence of goblet cells in tracheal/laryngeal submucosal glands and in the conducting airway epithelium after pulmonary allergen exposure in vivo. These data show that SPDEF plays a critical role in regulating a transcriptional network mediating the goblet cell differentiation and mucus hyperproduction associated with chronic pulmonary disorders.

Protein or amino acid deprivation differentially regulates the hepatic forkhead box protein A (FOXA) genes through an activating transcription factor-4-independent pathway

The FOXA (forkhead box A) proteins (FOXA1, FOXA2, and FOXA3) play a critical role in the development of the liver, and they also regulate metabolism in adult hepatic tissue. The liver responds to changes in nutrient availability by initiating a number of stress signaling pathways. The present studies demonstrated that in mouse dams fed a low-protein diet hepatic expression of FOXA2 and FOXA3 messenger RNA, but not FOXA1, was induced. Conversely, fetal liver did not exhibit this regulation. Amino acid deprivation of HepG2 hepatoma cells also enhanced transcription from the FOXA2 and FOXA3 genes. In contrast, endoplasmic reticulum stress inhibited the expression of FOXA1, only slightly induced FOXA2, and had no effect on FOXA3. The FOXA2 and FOXA3 messenger RNA induction by amino acid deprivation did not require activating transcription factor 4, a critical component of the conventional amino acid response (AAR) pathway, but their induction was partially dependent on CCAAT/enhancer-binding protein beta. Simultaneous knockdown of both FOXA2 and FOXA3 by small interfering RNA did not affect the activation of other amino acid responsive genes, suggesting that the FOXA proteins are not required for the known AAR pathway. Collectively, the results document that the hepatic FOXA family of genes are differentially regulated by amino acid availability.

Identification of DNA regions and a set of transcriptional regulatory factors involved in transcriptional regulation of several human liver-enriched transcription factor genes

Mammalian tissue- and/or time-specific transcription is primarily regulated in a combinatorial fashion through interactions between a specific set of transcriptional regulatory factors (TRFs) and their cognate cis-regulatory elements located in the regulatory regions. In exploring the DNA regions and TRFs involved in combinatorial transcriptional regulation, we noted that individual knockdown of a set of human liver-enriched TRFs such as HNF1A, HNF3A, HNF3B, HNF3G and HNF4A resulted in perturbation of the expression of several single TRF genes, such as HNF1A, HNF3G and CEBPA genes. We thus searched the potential binding sites for these five TRFs in the highly conserved genomic regions around these three TRF genes and found several putative combinatorial regulatory regions. Chromatin immunoprecipitation analysis revealed that almost all of the putative regulatory DNA regions were bound by the TRFs as well as two coactivators (CBP and p300). The strong transcription-enhancing activity of the putative combinatorial regulatory region located downstream of the CEBPA gene was confirmed. EMSA demonstrated specific bindings of these HNFs to the target DNA region. Finally, co-transfection reporter assays with various combinations of expression vectors for these HNF genes demonstrated the transcriptional activation of the CEBPA gene in a combinatorial manner by these TRFs.

On the origin of the beta cell

The major forms of diabetes are characterized by pancreatic islet beta-cell dysfunction and decreased beta-cell numbers, raising hope for cell replacement therapy. Although human islet transplantation is a cell-based therapy under clinical investigation for the treatment of type 1 diabetes, the limited availability of human cadaveric islets for transplantation will preclude its widespread therapeutic application. The result has been an intense focus on the development of alternate sources of beta cells, such as through the guided differentiation of stem or precursor cell populations or the transdifferentiation of more plentiful mature cell populations. Realizing the potential for cell-based therapies, however, requires a thorough understanding of pancreas development and beta-cell formation. Pancreas development is coordinated by a complex interplay of signaling pathways and transcription factors that determine early pancreatic specification as well as the later differentiation of exocrine and endocrine lineages. This review describes the current knowledge of these factors as they relate specifically to the emergence of endocrine beta cells from pancreatic endoderm. Current therapeutic efforts to generate insulin-producing beta-like cells from embryonic stem cells have already capitalized on recent advances in our understanding of the embryonic signals and transcription factors that dictate lineage specification and will most certainly be further enhanced by a continuing emphasis on the identification of novel factors and regulatory relationships.

Crucial role of vHNF1 in vertebrate hepatic specification

Mouse liver induction occurs via the acquisition of ventral endoderm competence to respond to inductive signals from adjacent mesoderm, followed by hepatic specification. Little is known about the regulatory circuit involved in these processes. Through the analysis of vHnf1 (Hnf1b)-deficient embryos, generated by tetraploid embryo complementation, we demonstrate that lack of vHNF1 leads to defective hepatic bud formation and abnormal gut regionalization. Thickening of the ventral hepatic endoderm and expression of known hepatic genes do not occur. At earlier stages, hepatic specification of vHnf1-/- ventral endoderm is disrupted. More importantly, mutant ventral endoderm cultured in vitro loses its responsiveness to inductive FGF signals and fails to induce the hepatic-specification genes albumin and transthyretin. Analysis of liver induction in zebrafish indicates a conserved role of vHNF1 in vertebrates. Our results reveal the crucial role of vHNF1 at the earliest steps of liver induction: the acquisition of endoderm competence and the hepatic specification.

Impaired male fertility and atrophy of seminiferous tubules caused by haploinsufficiency for Foxa3

Foxa1, 2 and 3 (formerly HNF-3alpha, -beta and -gamma) constitute a sub-family of winged helix transcription factors with multiple roles in mammalian organ development. While all three Foxa mRNAs are present in endoderm derivatives including liver and pancreas, only Foxa3 is expressed in the testis. Here we demonstrate by genetic lineage tracing that Foxa3 is expressed in postmeiotic germ and interstitial Leydig cells. The germinal epithelium of Foxa3-deficient testes is characterized by a loss of germ cells secondary to an increase in germ cell apoptosis that ultimately leads to a Sertoli cell-only syndrome. Remarkably, not only the Foxa3(-/-) mice but also Foxa3(+/-) mice exhibited loss of germ cells. This cellular phenotype caused significantly reduced fertility and testis weight of both Foxa3(-/-) and Foxa3(+/-) mice. Using microarray analysis, we found a dramatic downregulation of the zinc finger protein 93 and the testicular tumor-associated paraneoplastic Ma antigen (PNMA) and increased expression of a number of genes including zinc finger protein 94 and several kallikrein 1-related peptidases which could account for at least part of the observed phenotype. In summary, we have identified Foxa3 as a transcriptional regulator with a dominant phenotype in germ cell maintenance and suggest FOXA3 as a potential candidate gene for subfertility in man.

Foxa2 is required for the differentiation of pancreatic alpha-cells

The differentiation of insulin-producing beta-cells has been investigated in great detail; however, little is known about the factors that delineate the second-most abundant endocrine lineage, the glucagon-producing alpha-cell. Here we utilize a novel YAC-based Foxa3Cre transgene to delete the winged helix transcription factor Foxa2 (formerly HNF-3beta) in the pancreatic primordium during midgestation. The resulting Foxa2(loxP/loxP); Foxa3Cre mice are severely hypoglycemic and die within the first week of life. Mutant mice are hypoglucagonemic secondary to a 90% reduction of glucagon expression. While the number of mature glucagon-positive alpha-cells is dramatically reduced, specification of alpha-cell progenitors is not affected by Foxa2 deficiency. By marker gene analysis, we show that the expression of the alpha-cell transcription factors Arx, Pax6, and Brn4 does not require Foxa2 in the transcriptional hierarchy governing alpha-cell differentiation.

Loss of Klf4 in mice causes altered proliferation and differentiation and precancerous changes in the adult stomach

BACKGROUND & AIMS

The epithelial zinc-finger transcription factor Klf4 (formerly GKLF) regulates cellular proliferation and differentiation in vitro. Klf4 null mice die by postnatal day 1 and show changes in epithelial differentiation of skin and colon.

METHODS

We used tissue-specific gene ablation to generate mice lacking Klf4 in their gastric epithelia. Klf4 mutant mice and controls were killed for histology, immunohistochemistry, quantitative real-time polymerase chain reaction (qPCR), and serum gastrin levels. Klf4 messenger RNA (mRNA) levels were analyzed in Foxa3-Cdx2 transgenic mice and controls. Human gastric cancers and matched normal tissue were used for qPCR and immunohistochemistry for KLF4.

RESULTS

Klf4 mutant mice survive to adulthood and show increased proliferation and altered differentiation of their gastric epithelia. Klf4 mutants also display aberrant expression of acidic mucins and TFF2/SP-positive cells, findings characteristic of premalignant conditions, but no inflammation, intestinal metaplasia, dysplasia, or cancer up to 1 year of age. Expression of KLF4 is nearly absent in human gastric cancer, suggesting that failure to activate KLF4 during normal cellular differentiation may be a common feature of gastric cancers. p21 WAF1/CIP1 is an in vivo target of Klf4, but Klf4 is not a mediator of Cdx2.

CONCLUSIONS

Loss of a single genetic factor, Klf4, leads to dramatic changes in the gastric epithelia of mice, and Klf4 is part of a regulatory pathway involving p21 WAF1/CIP1 but not Cdx2. Thus, Klf4 is critical for normal gastric epithelial homeostasis.

The vitamin K-dependent gamma-glutamyl carboxylase gene contains a TATA-less promoter with a novel upstream regulatory element

The expression of the vitamin K-dependent gamma-glutamyl carboxylase gene in liver is developmentally regulated. Since the gene product catalyzes an essential post-translational modification of the vitamin K-dependent blood coagulation proteins, the regulation of carboxylase expression is critical for hemostasis. We analyzed the activity of the rat carboxylase gene 5'-regulatory DNA sequences in rat hepatoma cell lines at different states of differentiation. These studies demonstrated that the 2.6-kb 5'-flanking sequence has differentiation-dependent transcriptional activity. Transient gene expression assays, examining the effects of nested deletions and site-directed mutagenesis of putative regulatory sequences, together with electrophoretic mobility shift assays (EMSAs) were used to identify sequences critical for the developmentally regulated transcription of the rat carboxylase gene. We identified a DNA sequence (-76 to -65; GTTCCGGCCTTC) not known to bind to transcription factors, yet which functions as an upstream promoter element. In vivo genomic DNA footprinting confirms the presence of nuclear protein-DNA interactions at this site in the endogenous carboxylase gene in differentiated hepatoma cells. Therefore, this DNA sequence has specific nuclear protein-binding activity and functional properties consistent with a regulatory element that plays a critical role in the developmental expression of the carboxylase gene, and hence the regulation of vitamin K-dependent blood coagulation protein synthesis.

Cdx2 ectopic expression induces gastric intestinal metaplasia in transgenic mice

BACKGROUND & AIMS

Intestinal-type gastric cancer is often preceded by intestinal metaplasia in humans. The genetic events responsible for the transdifferentiation that occurs in intestinal metaplasia are not well understood. Cdx2, a transcription factor whose expression is normally limited to the intestine, has been detected in gastric intestinal metaplasia. Cdx2 induces differentiation of intestinal epithelial cells in vitro; therefore, we sought to establish whether a causal relationship exists between Cdx2 activation and intestinal metaplasia.

METHODS

Cdx2 expression was directed to the gastric mucosa in transgenic mice using cis-regulatory elements of Foxa3 (Hnf3gamma). Transgenic mice were analyzed for histologic and gene expression changes.

RESULTS

Histologic examination of the gastric mucosa of the Foxa3/Cdx2 mice revealed the presence of alcian blue-positive intestinal-type goblet cells, a hallmark of intestinal metaplasia. In addition, Cdx2 induced the expression of intestine-specific genes.

CONCLUSIONS

Gastric expression of Cdx2 alone was sufficient to induce intestinal metaplasia in mice. These mice represent a powerful tool to investigate the molecular mechanisms that promote intestinal metaplasia. Moreover, as gastric cancer in humans is often preceded by intestinal metaplasia, the phenotype described here strongly suggests involvement of Cdx2 in the initiation of the process leading to intestinal neoplasia of the gastric mucosa.

A transcription factor regulatory circuit in differentiated pancreatic cells

Mutations in the human genes encoding hepatocyte nuclear factors (HNF) 1alpha, 1beta, 4alpha, and IPF1(PDX1/IDX1/STF1) result in pancreatic beta cell dysfunction and diabetes mellitus. In hepatocytes, hnf4alpha controls the transcription of hnf1alpha, suggesting that this same interaction may operate in beta cells and thus account for the common diabetic phenotype. We show that, in pancreatic islet and exocrine cells, hnf4alpha expression unexpectedly depends on hnf1alpha. This effect is tissue-specific and mediated through direct occupation by hnf1alpha of an alternate promoter located 45.6 kb from the previously characterized hnf4alpha promoter. Hnf1alpha also exerts direct control of pancreatic-specific expression of hnf4gamma and hnf3gamma. Hnf1alpha dependence of hnf4alpha, hnf4gamma, hnf3gamma, and two previously characterized distal targets (glut2 and pklr) is established only after differentiated cells arise during pancreatic embryonic development. These studies define an unexpected hierarchical regulatory relationship between two genes involved in human monogenic diabetes in the cells, which are relevant to its pathophysiology. Furthermore, they indicate that hnf1alpha is an essential component of a transcription factor circuit whose role may be to maintain differentiated functions of pancreatic cells.

Size matters: use of YACs, BACs and PACs in transgenic animals

In 1993, several groups, working independently, reported the successful generation of transgenic mice with yeast artificial chromosomes (YACs) using standard techniques. The transfer of these large fragments of cloned genomic DNA correlated with optimal expression levels of the transgenes, irrespective of their location in the host genome. Thereafter, other groups confirmed the advantages of YAC transgenesis and position-independent and copy number-dependent transgene expression were demonstrated in most cases. The transfer of YACs to the germ line of mice has become popular in many transgenic facilities to guarantee faithful expression of transgenes. This technique was rapidly exported to livestock and soon transgenic rabbits, pigs and other mammals were produced with YACs. Transgenic animals were also produced with bacterial or P1-derived artificial chromosomes (BACs/PACs) with similar success. The use of YACs, BACs and PACs in transgenesis has allowed the discovery of new genes by complementation of mutations, the identification of key regulatory sequences within genomic loci that are crucial for the proper expression of genes and the design of improved animal models of human genetic diseases. Transgenesis with artificial chromosomes has proven useful in a variety of biological, medical and biotechnological applications and is considered a major breakthrough in the generation of transgenic animals. In this report, we will review the recent history of YAC/BAC/PAC-transgenic animals indicating their benefits and the potential problems associated with them. In this new era of genomics, the generation and analysis of transgenic animals carrying artificial chromosome-type transgenes will be fundamental to functionally identify and understand the role of new genes, included within large pieces of genomes, by direct complementation of mutations or by observation of their phenotypic consequences.

Initiation and early patterning of the endoderm

We review the early stages of endoderm formation in the major animal models. In Amphibia maternal molecules are important in initiating endoderm formation. This is followed by successive signaling events that establish and then pattern the endoderm. In other organisms there are differences in endodermal development, particularly in the initial, prephylotypic stages. Later many of the same key families of transcription factors and signaling cassettes are used in all animals, but more work will be needed to establish exact evolutionary homologies.

Mice with an increased glucocorticoid receptor gene dosage show enhanced resistance to stress and endotoxic shock

Targeted mutagenesis of the glucocorticoid receptor has revealed an essential function for survival and the regulation of multiple physiological processes. To investigate the effects of an increased gene dosage of the receptor, we have generated transgenic mice carrying two additional copies of the glucocorticoid receptor gene by using a yeast artificial chromosome. Interestingly, overexpression of the glucocorticoid receptor alters the basal regulation of the hypothalamo-pituitary-adrenal axis, resulting in reduced expression of corticotropin-releasing hormone and adrenocorticotrope hormone and a fourfold reduction in the level of circulating glucocorticoids. In addition, primary thymocytes obtained from transgenic mice show an enhanced sensitivity to glucocorticoid-induced apoptosis. Finally, analysis of these mice under challenge conditions revealed that expression of the glucocorticoid receptor above wild-type levels leads to a weaker response to restraint stress and a strongly increased resistance to lipopolysaccharide-induced endotoxic shock. These results underscore the importance of tight regulation of glucocorticoid receptor expression for the control of physiological and pathological processes. Furthermore, they may explain differences in the susceptibility of humans to inflammatory diseases and stress, depending on individual prenatal and postnatal experiences known to influence the expression of the glucocorticoid receptor.

Identification of a mouse germ cell-less homologue with conserved activity in Drosophila

Drosophila Germ cell-less (Gcl) has previously been shown to be important in early events during the formation of pole cells, which are the germ cell precursors in the fly. We have isolated a 524 amino acid mouse gene with 32% identity and 49% similarity to Drosophila gcl, termed mgcl-1. Like Drosophila Gcl, mGcl-1 localizes to the nuclear envelope. Ectopic expression of mgcl-1 in Drosophila rescues the gcl-null phenotype, indicating that mGcl-1 is a functional homologue of Gcl. mgcl-1 maps to chromosome 6 at 47.3 cM, and is expressed at low levels at all embryonic stages examined from 8.5 to 18.5 d.p.c. as well as in many adult tissues. Different from Drosophila gcl, mgcl-1 is not highly expressed at the time the primordial germ cells appear in the mouse, but high mgcl-1 expression is found in selected mouse adult male germ cells. The differences in these expression patterns in light of conserved activity between the two genes is discussed.

Modular regulation of cGATA-5 gene expression in the developing heart and gut

The evolutionarily conserved GATA-5 transcription factor is an early and persistent marker of heart and gut development in diverse vertebrate species. To search for control regions that might regulate the chicken GATA-5 (cGATA-5) gene, we assayed a set of cGATA-5/lacZ constructs in transgenic mice and found evidence for two functionally conserved control regions that regulate different facets of cGATA-5 gene expression. The more distal control region is activated in embryonic endoderm at the head-fold stage, whereas the other control region contains a regulatory module that is activated in a restricted region of endoderm following closure of the gut tube. Remarkably, the latter control region also contains a complex regulatory module that is activated in the cardiac crescent at the head-fold stage and subsequently functions in several mesodermal components of the developing heart, including the outer (epicardial) layer. We discuss these results in terms of possible contributions of epicardial-derived cells to the formation of heart valves, conduction tissue, and compact myocardium. These transgenes thus reveal, and provide a means to further analyze, transcriptional programs for several facets of heart morphogenesis and gut development.

The use of yeast artificial chromosomes in transgenic animals: expression studies of the tyrosinase gene in transgenic mice

Variegation and inherited somatic mosaicism has been observed in transgenic mice carrying yeast artificial chromosomes (YACs) in which a DNAse I hypersensitive site (HS) located -12 kb upstream of the mouse tyrosinase gene had been deleted. At present, we are generating new transgenic animals with minor deletions of the HS.

Variant hepatocyte nuclear factor 1 is required for visceral endoderm specification

Genetic and molecular evidence indicates that visceral endoderm, an extraembryonic cell lineage, is required for gastrulation, early anterior neural patterning, cell death and specification of posterior mesodermal cell fates. We show that variant Hepatocyte Nuclear Factor 1 (vHNF1), a homeodomain-containing transcription factor first expressed in the primitive endoderm, is required for the specification of visceral endoderm. vHnf1-deficient mouse embryos develop normally to the blastocyst stage, start implantation, but die soon afterwards, with abnormal or absent extraembryonic region, poorly organised ectoderm and no discernible visceral or parietal endoderm. However, immunostaining analysis of E5.5 nullizygous mutant embryos revealed the presence of parietal endoderm-like cells lying on an abnormal basal membrane. Homozygous mutant blastocyst outgrowths or differentiated embryonic stem cells do not express early or late visceral endoderm markers. In addition, in vHnf1 null embryoid bodies there is no activation of the transcription factors HNF-4alpha1, HNF1alpha and HNF-3gamma. Aggregation of vHnf1-deficient embryonic stem cells with wild-type tetraploid embryos, which contribute exclusively to extraembryonic tissues, rescues periimplantation lethality and allows development to progress to early organogenesis. Our results place vHNF1 in a preeminent position in the regulatory network that specifies the visceral endoderm and highlight the importance of this cell lineage for proper growth and differentiation of primitive ectoderm in pregastrulating embryos.

Molecular mechanisms controlling lung morphogenesis

The complex process of lung formation is determined by the action of numerous genes that influence cell commitment, differentiation, and proliferation. This review summarizes current knowledge of various factors involved in lung morphogenesis correlating their temporal and spatial expression with their proposed functions at various times during the developmental process. Rapid progress in understanding the pathways involved in lung morphogenesis will likely provide the framework with which to elucidate the mechanisms contributing to lung malformations and the pathogenesis of genetic and acquired lung diseases.

Targeted disruption of the gene encoding hepatocyte nuclear factor 3gamma results in reduced transcription of hepatocyte-specific genes

The winged helix transcription factor hepatocyte nuclear factor 3gamma (HNF3gamma) is expressed in embryonic endoderm and its derivatives liver, pancreas, stomach, and intestine, as well as in testis and ovary. We have generated mice carrying an Hnf3g-lacZ fusion which deletes most of the HNF3gamma coding sequence as well as 5.5 kb of 3' flanking region. Mice homozygous for the mutation are fertile, develop normally, and show no morphological defects. The mild phenotype change of the Hnf3g-/- mice can be explained in part by an upregulation of HNF3alpha and HNF3beta in the liver of the mutant animals. Analysis of steady-state mRNA levels as well as transcription rates showed that levels of expression of several HNF3 target genes (phosphoenolpyruvate carboxykinase, transferrin, tyrosine aminotransferase) were reduced by 50 to 70%, indicating that HNF3gamma is an important activator of these genes in vivo.