Implication of ZOG protein (zona glomerulosa-specific protein) in zone development of the adrenal cortex

Molecular Mechanisms of Stem/Progenitor Cell Maintenance in the Adrenal Cortex

The adrenal cortex is characterized by three histologically and functionally distinct zones: the outermost zona glomerulosa (zG), the intermediate zona fasciculata, and the innermost zona reticularis. Important aspects of the physiology and maintenance of the adrenocortical stem/progenitor cells have emerged in the last few years. Studies have shown that the adrenocortical cells descend from a pool of progenitors that are localized in the subcapsular region of the zG. These cells continually undergo a process of centripetal displacement and differentiation, which is orchestrated by several paracrine and endocrine cues, including the pituitary-derived adrenocorticotrophic hormone, and angiotensin II. However, while several roles of the endocrine axes on adrenocortical function are well established, the mechanisms coordinating the maintenance of an undifferentiated progenitor cell pool with self-renewal capacity are poorly understood. Local factors, such as the composition of the extracellular matrix (ECM) with embedded signaling molecules, and the activity of major paracrine effectors, including ligands of the sonic hedgehog and Wnt signaling pathways, are thought to play a major role. Particularly, the composition of the ECM, which exhibits substantial differences within each of the three histologically distinct concentric zones, has been shown to influence the differentiation status of adrenocortical cells. New data from other organ systems and different experimental paradigms strongly support the conclusion that the interactions of ECM components with cell-surface receptors and secreted factors are key determinants of cell fate. In this review, we summarize established and emerging data on the paracrine and autocrine regulatory loops that regulate the biology of the progenitor cell niche and propose a role for bioengineered ECM models in further elucidating this biology in the adrenal.

Conditional mutagenesis of Gata6 in SF1-positive cells causes gonadal-like differentiation in the adrenal cortex of mice

Transcription factor GATA6 is expressed in the fetal and adult adrenal cortex and has been implicated in steroidogenesis. To characterize the role of transcription factor GATA6 in adrenocortical development and function, we generated mice in which Gata6 was conditionally deleted using Cre-LoxP recombination with Sf1-cre. The adrenal glands of adult Gata6 conditional knockout (cKO) mice were small and had a thin cortex. Cytomegalic changes were evident in fetal and adult cKO adrenal glands, and chromaffin cells were ectopically located at the periphery of the glands. Corticosterone secretion in response to exogenous ACTH was blunted in cKO mice. Spindle-shaped cells expressing Gata4, a marker of gonadal stroma, accumulated in the adrenal subcapsule of Gata6 cKO mice. RNA analysis demonstrated the concomitant upregulation of other gonadal-like markers, including Amhr2, in the cKO adrenal glands, suggesting that GATA6 inhibits the spontaneous differentiation of adrenocortical stem/progenitor cells into gonadal-like cells. Lhcgr and Cyp17 were overexpressed in the adrenal glands of gonadectomized cKO vs control mice, implying that GATA6 also limits sex steroidogenic cell differentiation in response to the hormonal changes that accompany gonadectomy. Nulliparous female and orchiectomized male Gata6 cKO mice lacked an adrenal X-zone. Microarray hybridization identified Pik3c2g as a novel X-zone marker that is downregulated in the adrenal glands of these mice. Our findings offer genetic proof that GATA6 regulates the differentiation of steroidogenic progenitors into adrenocortical cells.

Localizing transcriptional regulatory elements at the mouse Dlk1 locus

Much effort has focused recently on determining the mechanisms that control the allele-specific expression of genes subject to genomic imprinting, yet imprinting regulation is only one aspect of configuring appropriate expression of these genes. Imprinting control mechanisms must interact with those regulating the tissue-specific expression pattern of each imprinted gene in a cluster. Proper expression of the imprinted Delta-like 1 (Dlk1)-Maternally expressed gene 3 (Meg3) gene pair is required for normal fetal development in mammals, yet the mechanisms that control tissue-specific expression of these genes are unknown. We have used a combination of in vivo and in vitro expression assays to localize cis-regulatory elements that may regulate Dlk1 expression in the mouse embryo. A bacterial artificial chromosome transgene encompassing the Dlk1 gene and 77 kb of flanking sequence conferred expression in most endogenous Dlk1-expressing tissues. In combination with previous transgenic data, these experiments localize the majority of Dlk1 cis-regulatory elements to a 41 kb region upstream of the gene. Cross-species sequence conservation was used to further define potential regulatory elements, several of which functioned as enhancers in a luciferase expression assay. Two of these elements were able to drive expression of a lacZ reporter transgene in Dlk1-expressing tissues in the mouse embryo. The sequence proximal to Dlk1 therefore contains at least two discrete regions that may regulate tissue-specificity of Dlk1 expression.

DLK2 is a transcriptional target of KLF4 in the early stages of adipogenesis

The epidermal growth factor-like protein DLK2, highly homologous to DLK1, has been identified as a modulator of adipogenesis in vitro. Knocking down Dlk2 expression prevents adipogenesis of 3T3-L1 cells but enhances that of the mesenchymal cell line C3H10T1/2. The expression of Dlk2 shows two peaks along this differentiation process: the first one, in response to 3-isobutyl-1-methylxanthine (IBMX) and dexamethasone (Dex), and the second, shortly after exposure to insulin. Nothing is known about the transcriptional regulation of Dlk2 during adipogenesis. Here, we report that, during early adipogenesis of 3T3-L1 cells, Dlk2 expression is controlled independently by IBMX and Dex. We also show that KLF4, a transcription factor critical for the control of early adipogenesis, binds directly to the Dlk2 promoter and increases Dlk2 expression in response to IBMX. Overexpression of KLF4 leads to an increase in DLK2 expression levels, whereas KLF4 knockdown downregulates the transcriptional activity of the Dlk2 promoter. Finally, we demonstrate that KLF4 regulates the basal expression of Dlk2 in C3H10T1/2 cells, and it is required for the adipogenic differentiation of those cells. These results indicate that KLF4 mediates the transcriptional regulation of Dlk2 in response to IBMX during the early stages of adipogenesis.

Characterization of a proximal Sp1 response element in the mouse Dlk2 gene promoter

BACKGROUND

DLK2 is an EGF-like membrane protein, closely related to DLK1, which is involved in adipogenesis. Both proteins interact with the NOTCH1 receptor and are able to modulate its activation. The expression of the gene Dlk2 is coordinated with that of Dlk1 in several tissues and cell lines. Unlike Dlk1, the mouse Dlk2 gene and its locus at chromosome 17 are not fully characterized.

RESULTS

The goal of this work was the characterization of Dlk2 mRNA, as well as the analysis of the mechanisms that control its basal transcription. First, we analyzed the Dlk2 transcripts expressed by several mouse cells lines and tissues, and mapped the transcription start site by 5' Rapid Amplification of cDNA Ends. In silico analysis revealed that Dlk2 possesses a TATA-less promoter containing minimal promoter elements associated with a CpG island, and sequences for Inr and DPE elements. Besides, it possesses six GC-boxes, considered as consensus sites for the transcription factor Sp1. Indeed, we report that Sp1 directly binds to the Dlk2 promoter, activates its transcription, and regulates its level of expression.

CONCLUSIONS

Our results provide the first characterization of Dlk2 transcripts, map the location of the Dlk2 core promoter, and show the role of Sp1 as a key regulator of Dlk2 transcription, providing new insights into the molecular mechanisms that contribute to the expression of the Dlk2 gene.

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.

dlk acts as a negative regulator of Notch1 activation through interactions with specific EGF-like repeats

The protein dlk, encoded by the Dlk1 gene, belongs to the Notch epidermal growth factor (EGF)-like family of receptors and ligands, which participate in cell fate decisions during development. The molecular mechanisms by which dlk regulates cell differentiation remain unknown. By using the yeast two-hybrid system, we found that dlk interacts with Notch1 in a specific manner. Moreover, by using luciferase as a reporter gene under the control of a CSL/RBP-Jk/CBF-1-dependent promoter in the dlk-negative, Notch1-positive Balb/c 14 cell line, we found that addition of synthetic dlk EGF-like peptides to the culture medium or forced expression of dlk decreases endogenous Notch activity. Furthermore, the expression of the gene Hes-1, a target for Notch1 activation, diminishes in confluent Balb/c14 cells transfected with an expression construct encoding for the extracellular EGF-like region of dlk. The expression of Dlk1 and Notch1 increases in 3T3-L1 cells maintained in a confluent state for several days, which is associated with a concomitant decrease in Hes-1 expression. On the other hand, the decrease of Dlk1 expression in 3T3-L1 cells by antisense cDNA transfection is associated with an increase in Hes-1 expression. These results suggest that dlk functionally interacts in vivo with Notch1, which may lead to the regulation of differentiation processes modulated by Notch1 activation and signaling, including adipogenesis.

Expression of PEG11 and PEG11AS transcripts in normal and callipyge sheep

BACKGROUND

The callipyge mutation is located within an imprinted gene cluster on ovine chromosome 18. The callipyge trait exhibits polar overdominant inheritance due to the fact that only heterozygotes inheriting a mutant paternal allele (paternal heterozygotes) have a phenotype of muscle hypertrophy, reduced fat and a more compact skeleton. The mutation is a single A to G transition in an intergenic region that results in the increased expression of several genes within the imprinted cluster without changing their parent-of-origin allele-specific expression.

RESULTS

There was a significant effect of genotype (p < 0.0001) on the transcript abundance of DLK1, PEG11, and MEG8 in the muscles of lambs with the callipyge allele. DLK1 and PEG11 transcript levels were elevated in the hypertrophied muscles of paternal heterozygous animals relative to animals of the other three genotypes. The PEG11 locus produces a single 6.5 kb transcript and two smaller antisense strand transcripts, referred to as PEG11AS, in skeletal muscle. PEG11AS transcripts were detectable over a 5.5 kb region beginning 1.2 kb upstream of the PEG11 start codon and spanning the entire open reading frame. Analysis of PEG11 expression by quantitative PCR shows a 200-fold induction in the hypertrophied muscles of paternal heterozygous animals and a 13-fold induction in homozygous callipyge animals. PEG11 transcripts were 14-fold more abundant than PEG11AS transcripts in the gluteus medius of paternal heterozygous animals. PEG11AS transcripts were expressed at higher levels than PEG11 transcripts in the gluteus medius of animals of the other three genotypes.

CONCLUSIONS

The effect of the callipyge mutation has been to alter the expression of DLK1, GTL2, PEG11 and MEG8 in the hypertrophied skeletal muscles. Transcript abundance of DLK1 and PEG11 was highest in paternal heterozygous animals and exhibited polar overdominant gene expression patterns; therefore, both genes are candidates for causing skeletal muscle hypertrophy. There was unique relationship of PEG11 and PEG11AS transcript abundance in the paternal heterozygous animals that suggests a RNA interference mechanism may have a role in PEG11 gene regulation and polar overdominance in callipyge sheep.

Neonatal salt-wasting and 11 β-hydroxylase deficiency in a child carrying a homozygous deletion hybrid CYP11B2 (aldosterone synthase)-CYP11B1 (11 β-hydroxylase)

This article reports the case of a boy diagnosed at 1.8 years of age with congenital adrenal hyperplasia due to 11 beta-hydroxylase deficiency. The patient showed salt-wasting episodes during the neonatal period. On molecular analysis, a homozygous deletion hybrid (CYP11B2-CYP11B1) involving the CYP11B locus at 8q24.3 was found. Southern blot analysis showed the break point of the chimera gene to be located before intron 5; sequence analysis identified it at exon 4 between codons 202 and 248. This CYP11B2(5')/B1(3') hybrid should lack aldosterone synthase activity (due to the CYP11B1 residues at exons 5 and 6), and the enzyme it codes for should not be promoted by adrenocorticotropic hormone (ACTH) (CYP11B2 promoter sequences). The patient phenotype - neonatal salt-wasting and 11 beta-hydroxylase deficiency - is in agreement with this hybrid structure. This is the first time a homozygous deletion hybrid generated by unequal crossover has been described in exon 4. This genetic lesion appears to be the reciprocal product from the recombination event that causes glucocorticoid-remediable aldosteronism, a duplication dominant allele (CYP11B2-CYP11B1/B2-CYP11B1) coding for additional aldosterone synthase activity regulated by ACTH. The clinical presentation of the condition in this patient contributes to the in vivo understanding of the regulation of this complex locus in which two 'duplicated' genes have evolved different regulatory and enzymatic activities involved in mineralocorticoid and glucocorticoid synthesis in the adrenal glands. The fact that this allele was first predicted and has now been documented clinically and molecularly in vivo is particularly noteworthy.

Runx2 deficiency in chondrocytes causes adipogenic changes in vitro

Runx2 (runt-related transcription factor 2) is an important transcription factor for chondrocyte differentiation as well as for osteoblast differentiation. To investigate the function of Runx2 in chondrocytes, we isolated chondrocytes from the rib cartilage of Runx2-deficient (Runx2-/-) mice and examined the effect of Runx2 deficiency on chondrocyte function and behavior in culture for up to 12 days. At the beginning of the culture, Runx2-/- chondrocytes actively proliferated, had a polygonal shape and expressed type II collagen; these are all characteristics of chondrocytes. However, they gradually accumulated lipid droplets that stained with oil red O and resembled adipocytes. Northern blot analysis revealed that the expression of adipocyte-related differentiation marker genes including PPAR gamma (peroxisome proliferator-activated receptor gamma), aP2 and Glut4 increased over time in culture, whereas expression of type II collagen decreased. Furthermore, the expression of Pref-1, an important inhibitory gene of adipogenesis, was remarkably decreased. Adenoviral introduction of Runx2 or treatment with transforming growth factor-beta, retinoic acid, interleukin-1 beta, basic fibroblast growth factor, platelet-derived growth factor or parathyroid hormone inhibited the adipogenic changes in Runx2-/- chondrocytes. Runx2 and transforming growth factor-beta synergistically upregulated interleukin-11 expression, and the addition of interleukin-11 to the culture medium reduced adipogenesis in Runx2-/- chondrocytes. These findings indicate that depletion of Runx2 resulted in the loss of the differentiated phenotype in chondrocytes and induced adipogenic differentiation in vitro, and show that Runx2 plays important roles in maintaining the chondrocyte phenotype and in inhibiting adipogenesis. Our findings suggest that these Runx2-dependent functions are mediated, at least in part, by interleukin-11.

Regulation of human skeletal stem cells differentiation by Dlk1/Pref-1

Dlk-1/Pref-1 was identified as a novel regulator of human skeletal stem cell differentiation. Dlk1/Pref-1 is expressed in bone and cultured osteoblasts, and its constitutive overexpression led to inhibition of osteoblast and adipocyte differentiation of human marrow stromal cells.

INTRODUCTION

Molecular control of human mesenchymal stem cell (hMSC) differentiation into osteoblasts and adipocytes is not known. In this study, we examined the role of delta-like 1/preadipocyte factor-1 (Dlk1/Pref-1) in regulating the differentiation of hMSCs.

MATERIALS AND METHODS

As a model for hMSCs, we have stably transduced telomerase-immortalized hMSC (hMSC-TERT) with the full length of human Dlk1/Pref-1 cDNA and tested its effect on hMSC growth and differentiation into osteoblasts or adipocytes as assessed by cytochemical staining, FACS analysis, and real time PCR. Ex vivo calvaria organ cultures assay was used to confirm the in vitro effect of Dlk/Pref-1 on bone formation.

RESULTS

Dlk1/Pref-1 was found to be expressed in fetal and adult bone, hMSCs, and some osteoblastic cell lines. A retroviral vector containing the human Dlk1/Pref-1 cDNA was used to create a cell line (hMSC-dlk1) expressing high levels of Dlk1/Pref-1 protein. Overexpression of Dlk1/Pref-1 did not affect the proliferation rate of hMSC, but the ability to form mature adipocytes, mineralized matrix in vitro, and new bone formation in neonatal murine calvariae organ cultures was reduced. These effects were associated with inhibition of gene expression markers of late stages of adipocyte (adipocyte fatty acid-binding protein [aP2], peroxisome proliferator-activated receptor-gamma2 [PPARgamma2], and adiponectin [APM1]) and osteoblast differentiation (alkaline phosphatase [ALP], collagen type I [Col1], and osteocalcin [OC]). Lineage commitment markers for adipocytes (adipocyte determination and differentiation factor -1 [ADD1]) and osteoblasts (core binding factor/runt-related binding factor 2 [Cbfa1/Runx2]) were not affected.

CONCLUSION

During hMSC differentiation, Dlk1/Pref-1 maintains the size of the bipotential progenitor cell pool by inhibiting the formation of mature osteoblasts and adipocytes.

DAX1 and its network partners: exploring complexity in development

DAX1 encoded by NR0B1, when mutated, is responsible for X-linked adrenal hypoplasia congenita (AHC). AHC is due to failure of the adrenal cortex to develop normally and is fatal if untreated. When duplicated, this gene is associated with an XY sex-reversed phenotype. DAX1 expression is present during development of the steroidogenic hypothalamic-pituitary-adrenal-gonadal (HPAG) axis and persists into adult life. Despite recognition of the crucial role for DAX1, its function remains largely undefined. The phenotypes of patients and animal models are complex and not always in agreement. Investigations using cell lines have proved difficult to interpret, possibly reflecting cell line choices and their limited characterization. We will review the efforts of our group and others to identify appropriate cell lines for optimizing ex vivo analysis of NR0B1 function throughout development. We will examine the role of DAX1 and its network partners in development of the hypothalamic-pituitary-adrenal/gonadal axis (HPAG) using a variety of different types of investigations, including those in model organisms. This network analysis will help us to understand normal and abnormal development of the HPAG. In addition, these studies permit identification of candidate genes for human inborn errors of HPAG development.

An inverse correlation between expression of a preprocathepsin B-related protein with cysteine-rich sequences and steroid 11beta -hydroxylase in adrenocortical cells

A cDNA encoding a secretory protein hitherto unknown was cloned from mouse adrenocortical cells by subtractive hybridization between the cells without and with expressing steroid 11beta-hydroxylase (Cyp11b-1), a marker for the functional differentiation of cells in the zonae fasciculata reticularis (zFR). The deduced protein consisting of 466 amino acids contained a secretory signal, epidermal growth factor-like repeats, and a proteolytically inactive cathepsin B-related sequence. The amino acid sequence was 89% identical with that of human tubulointerstitial nephritis antigen-related protein. Among the mouse organs examined, adrenal glands prominently expressed its mRNA. The mRNA and its encoded protein were detected in the outer adrenocortical zones that do not express Cyp11b-1, i.e. the zona glomerulosa and the undifferentiated cell zone, while being undetectable in zFR that express Cyp11b-1. The new protein was designated as adrenocortical zonation factor 1 (AZ-1). Clonal lines with different levels of AZ-1 expression were established from Y-1 adrenocortical cells that originally express Cyp11b-1 but little AZ-1. Analyses of the clonal lines revealed that Cyp11b-1 is detected in the clonal lines maintaining little AZ-1 expression and becomes undetectable in those expressing AZ-1. On the other hand, irrespective of the AZ-1 expression, all clones expressed cholesterol side-chain cleavage enzyme, which occurs throughout the cortical zones. These results demonstrated that adrenocortical cells expressing AZ-1 do not express Cyp11b-1, whereas those with little AZ-1 express this zFR marker in vitro and in vivo, implying a putative role of AZ-1 in determining the zonal differentiation of adrenocortical cells.

Recent insights into organogenesis of the adrenal cortex

The primary endocrine organs responsible for steroid hormone biosynthesis--the adrenal cortex and gonads--are derived from the urogenital ridge. Several recent discoveries in human and mouse genetics have begun to unravel the complex genetic cascade that dictates adrenocortical cell lineage, proliferation and differentiation. The factors that regulate adrenocortical organogenesis and the maintenance of growth promote or block a cascade of transcription factors that differentially coordinate the proliferation and differentiation of the gland. Here, we outline the developmental milestones of the adrenal cortex with recent contributions to the field, focusing on factors that have been shown to play a role in vivo in humans and mice.

Characterization of a proximal element in the rat preadipocyte factor-1 (Pref-1) gene promoter

Preadipocyte factor-1 (Pref-1) was shown to negatively regulate adipocyte differentiation. We recently reported that ZOG, a rat homolog of Pref-1, was specifically expressed in the adrenal zona glomerulosa. Results of the investigation of Pref-1 expression in preadipocyte and in undifferentiated adrenal cortex suggested that down-regulation of Pref-1 gene was closely correlated with the differentiation process. In this study we demonstrate that an upstream region (from -76 to -47) of the rat Pref-1 gene was essential for its expression in adrenocortical carcinoma-derived H295R cells. A nucleotide sequence found in this region, GCGTGGGCGTGGGCGGGGG (Egr/GC-box), seemed to contain three elements, two early growth response (Egr) elements and one GC-box, overlapping each other. Mutations of four or five nucleotides in a 7-nucleotides-stretch in the midst of the Egr/GC-box eliminated the binding of Sp1/3, abolished the activation by Egr-factor(s) and diminished the Pref-1 promoter activity. When mutations were introduced into the outside of the middle portion, the binding of Sp1/3 to the Egr/GC-box was abolished similarly. However, the decrease in the promoter activity was less than that found with the construct mutated at the middle. These results indicated that an element present at the 7-nucleotides-stretch in the midst of the Egr/GC-box might be important for the Pref-1 promoter activity, and this proximal element was possibly activated by a still-unidentified nuclear factor(s). This element would function as the promoter of the Pref-1 gene in H295R cells, but not in HeLa cells.

Midkine is expressed early in rat fetal adrenal development

Adrenal gland development is complex and poorly understood at the molecular level. Only a subset of patients with adrenal hypoplasia congenita (AHC) carry mutations in DAX1, a member of the nuclear hormone receptor superfamily. Therefore we set out to identify other candidate genes responsible for AHC by characterizing genes involved in fetal adrenal development. To identify these genes, we studied the differential expression of genes in fetal rat adrenals comparing tissues at 14 and 15 days postcoitum (dpc) since this period encompasses major morphological change in rat adrenal development. Fetal rat adrenals were dissected, cDNAs were prepared, and suppressive subtractive hybridization was performed. We isolated 126 clones of putatively differentially expressed clones and approximately 250 bp of each of the clones was sequenced. The most interesting putative developmental genes were examined. One member of the extracellular PTN/MDK (pleiotrophin/midkine) heparin-binding protein family involved in regulation of growth and differentiation was selected for initial study. We obtained full-length transcript by 3' rapid amplification of cDNA ends and performed Northern analysis on rat adrenal RNA from fetuses at 13, 14, 15, 17, and 19 dpc and newborns. Results from those analyses demonstrated the highest Mdk expression at days 13 and 14 followed by a moderate decrease of expression during the fetal stages thereafter. In the newborn, Mdk expression is nearly undetectable. Our results indicate that Mdk has a very specific pattern of fetal expression in the adrenals. We conclude that Mdk is involved early in fetal development of the rat adrenal. Therefore, MDK is a candidate gene for AHC not due to DAX1 mutations.

Novel imprinted DLK1/GTL2 domain on human chromosome 14 contains motifs that mimic those implicated in IGF2/H19 regulation

The evolution of genomic imprinting in mammals occurred more than 100 million years ago, and resulted in the formation of genes that are functionally haploid because of parent-of-origin-dependent expression. Despite ample evidence from studies in a number of species suggesting the presence of imprinted genes on human chromosome 14, their identity has remained elusive. Here we report the identification of two reciprocally imprinted genes, GTL2 and DLK1, which together define a novel imprinting cluster on human chromosome 14q32. The maternally expressed GTL2 (gene trap locus 2) gene encodes for a nontranslated RNA. DLK1 (delta, Drosophila, homolog-like 1) is a paternally expressed gene that encodes for a transmembrane protein containing six epidermal growth factor (EGF) repeat motifs closely related to those present in the delta/notch/serrate family of signaling molecules. The paternal expression, chromosomal localization, and biological function of DLK1 also make it a likely candidate gene for the callipyge phenotype in sheep. Many of the predicted structural and regulatory features of the DLK1/GTL2 domain are highly analogous to those implicated in IGF2/H19 imprint regulation, including two hemimethylated consensus binding sites for the vertebrate enhancer blocking protein, CTCF. These results provide evidence that a common mechanism and domain organization may be used for juxtapositioned, reciprocally imprinted genes.

Does fetal antigen 1 (FA1) identify cells with regenerative, endocrine and neuroendocrine potentials? A study of FA1 in embryonic, fetal, and placental tissue and in maternal circulation

Fetal antigen 1 (FA1) is a circulating EGF multidomain glycoprotein. FA1 and its membrane-associated precursor is defined by the mRNAs referred to as delta-like (dlk), preadipocyte factor 1 (pref-1) or zona glomerulosa-specific factor (ZOG). Using a polyclonal antibody recognising both forms, the localisation of FA1/dlk was analysed in embryonic and fetal tissues between week 5 to 25 of gestation and related to germinal origin and development. FA1 was observed in endodermally derived hepatocytes, glandular cells of the pancreas anlage, and in respiratory epithelial cells. FA1 was also present in mesodermally derived cells of the renal proximal tubules, adrenal cortex, Leydig and Hilus cells of the testes and ovaries, fetal chondroblasts, and skeletal myotubes. Ectodermally derived neuro- and adenohypophysial cells, cells in the floor of the 3rd ventricle and plexus choroideus were also FA1 positive. The number of cells expressing FA1 decreased during fetal development where the expression became restricted to specific functional cells. Epidermis, gut epithelium, gall bladder, blood cells, spleen, thyroid gland, salivary glands, and smooth muscle cells were FA1 negative. Analysis of extra-embryonic tissues from normal and pathological pregnancies revealed FA1 in stromal cells surrounding the blood islands of the yolk sac as well as in placental fibroblasts where the expression was most pronounced in diploid, androgenic complete hydatidiform moles. However, as measured by ELISA, the circulating maternal FA1 levels in complete moles were not different from normal pregnancies. The results presented suggest that FA1 is a growth and/or differentiation factor extensively expressed in immature cells and down-regulated during fetal development. FA1 down-regulation was associated with a shift in the subcellular localisation indicating differential post-translational/post-transcriptional modifications during fetal development. FA1 may be a new marker of cellular subtypes with a regenerative potential and of specific cells with endocrine or neuroendocrine functions.

Genomic organization and genetic mapping of the bovine PREF-1 gene

As a potential regulator of nutrient partitioning in beef cattle, we have cloned and genetically mapped the bovine PREF-1 gene. A full-length PREF-1 cDNA was isolated by iterative purification from a mixed-tissue cDNA library to which adipose contributed mRNA. Analysis of partial cDNAs from this library revealed that the 3'-terminal exon of the bovine PREF-1 mRNA is spliced in a manner analogous to its murine ortholog. However, we have also detected a PREF-1 splice form apparently unique to cattle. Aside from this alternative selection of a splice donor in the bovine fifth exon, the exon/intron junctions of the bovine PREF-1 gene recapitulate those observed for mice. The sequences proximal to the bovine PREF-1 transcription start site are homologous to the mouse PREF-1 promoter. Importantly, the sequence experimentally identified as critical to PREF-1 "suppression in adipocyte differentiation" is conserved in the bovine gene. The bovine PREF-1 gene was mapped to the telomeric end of BTA 21 by virtue of a physically linked microsatellite with seven alleles and 285 informative meiosis.