Regulation of specific protein synthesis in cytodifferentiation

The role of noncoding RNAs in pancreatic birth defects

Congenital defects in the pancreas can cause severe health issues such as pancreatic cancer and diabetes which require lifelong treatment. Regenerating healthy pancreatic cells to replace malfunctioning cells has been considered a promising cure for pancreatic diseases including birth defects. However, such therapies are currently unavailable in the clinic. The developmental gene regulatory network underlying pancreatic development must be reactivated for in vivo regeneration and recapitulated in vitro for cell replacement therapy. Thus, understanding the mechanisms driving pancreatic development will pave the way for regenerative therapies. Pancreatic progenitor cells are the precursors of all pancreatic cells which use epigenetic changes to control gene expression during differentiation to generate all of the distinct pancreatic cell types. Epigenetic changes involving DNA methylation and histone modifications can be controlled by noncoding RNAs (ncRNAs). Indeed, increasing evidence suggests that ncRNAs are indispensable for proper organogenesis. Here, we summarize recent insight into the role of ncRNAs in the epigenetic regulation of pancreatic development. We further discuss how disruptions in ncRNA biogenesis and expression lead to developmental defects and diseases. This review summarizes in vivo data from animal models and in vitro studies using stem cell differentiation as a model for pancreatic development.

Mesozooplankton size structure in the Canary Current System

Changes in plankton composition influences the dynamics of marine food webs and carbon sinking rates. Understanding the core structure and function of the plankton distribution is of paramount importance to know their role in trophic transfer and efficiency. Here, we studied the zooplankton distribution, abundance, composition, and size spectra for the characterization of the community under different oceanographic conditions in the Canaries-African Transition Zone (C-ATZ). This region is a transition zone between the coastal upwelling and the open ocean showing a high variability because of the physical, chemical, and biological changes between eutrophic and oligotrophic conditions through the annual cycle. During the late winter bloom (LWB), chlorophyll a and primary production were higher compared to that of the stratified season (SS), especially in the upwelling influenced area. Abundance distribution analysis clustered stations into two main groups according to the season (productive versus stratified season), and one group sampled in the upwelling influenced area. Size-spectra analysis showed steeper slopes during daytime in the SS, suggesting a less structured community and a higher trophic efficiency during the LWB due to the favorable oceanographic conditions. We also observed a significant difference between day and nighttime size spectra due to community change during diel vertical migration. Cladocera were the key taxa differentiating an Upwelling-group, from a LWB- and SS-group. These two latter groups were differentiated by Salpidae and Appendicularia mainly. Data obtained in this study suggested that abundance composition might be useful when describing community taxonomic changes, while size-spectra gives an idea of the ecosystem structure, predatory interactions with higher trophic levels and shifts in size structure.

Recent advances in tissue stem cells

Stem cells are undifferentiated cells capable of self-renewal and differentiation, giving rise to specialized functional cells. Stem cells are of pivotal importance for organ and tissue development, homeostasis, and injury and disease repair. Tissue-specific stem cells are a rare population residing in specific tissues and present powerful potential for regeneration when required. They are usually named based on the resident tissue, such as hematopoietic stem cells and germline stem cells. This review discusses the recent advances in stem cells of various tissues, including neural stem cells, muscle stem cells, liver progenitors, pancreatic islet stem/progenitor cells, intestinal stem cells, and prostate stem cells, and the future perspectives for tissue stem cell research.

p120 Catenin is required for normal tubulogenesis but not epithelial integrity in developing mouse pancreas

The intracellular protein p120 catenin aids in maintenance of cell-cell adhesion by regulating E-cadherin stability in epithelial cells. In an effort to understand the biology of p120 catenin in pancreas development, we ablated p120 catenin in mouse pancreatic progenitor cells, which resulted in deletion of p120 catenin in all epithelial lineages of the developing mouse pancreas: islet, acinar, centroacinar, and ductal. Loss of p120 catenin resulted in formation of dilated epithelial tubules, expansion of ductal epithelia, loss of acinar cells, and the induction of pancreatic inflammation. Aberrant branching morphogenesis and tubulogenesis were also observed. Throughout development, the phenotype became more severe, ultimately resulting in an abnormal pancreas comprised primarily of duct-like epithelium expressing early progenitor markers. In pancreatic tissue lacking p120 catenin, overall epithelial architecture remained intact; however, actin cytoskeleton organization was disrupted, an observation associated with increased cytoplasmic PKCζ. Although we observed reduced expression of adherens junction proteins E-cadherin, β-catenin, and α-catenin, p120 catenin family members p0071, ARVCF, and δ-catenin remained present at cell membranes in homozygous p120(f/f) pancreases, potentially providing stability for maintenance of epithelial integrity during development. Adult mice homozygous for deletion of p120 catenin displayed dilated main pancreatic ducts, chronic pancreatitis, acinar to ductal metaplasia (ADM), and mucinous metaplasia that resembles PanIN1a. Taken together, our data demonstrate an essential role for p120 catenin in pancreas development.

Maturation of stem cell-derived beta-cells guided by the expression of urocortin 3

Type 1 diabetes (T1D) is a devastating disease precipitated by an autoimmune response directed at the insulin-producing beta-cells of the pancreas for which no cure exists. Stem cell-derived beta-cells show great promise for a cure as they have the potential to supply unlimited numbers of cells that could be derived from a patient's own cells, thus eliminating the need for immunosuppression. Current in vitro protocols for the differentiation of stem cell-derived beta-cells can successfully generate pancreatic endoderm cells. In diabetic rodents, such cells can differentiate further along the beta-cell lineage until they are eventually capable of restoring normoglycemia. While these observations demonstrate that stem cell-derived pancreatic endoderm has the potential to differentiate into mature, glucose-responsive beta-cells, the signals that direct differentiation and maturation from pancreatic endoderm onwards remain poorly understood. In this review, we analyze the sequence of events that culminates in the formation of beta-cells during embryonic development. and summarize how current protocols to generate beta-cells have sought to capitalize on this ontogenic template. We place particular emphasis on the current challenges and opportunities which occur in the later stages of beta-cell differentiation and maturation of transplantable stem cell-derived beta-cells. Another focus is on the question how the use of recently identified maturation markers such as urocortin 3 can be instrumental in guiding these efforts.

Exocrine ontogenies: on the development of pancreatic acinar, ductal and centroacinar cells

This review summarizes our current understanding of exocrine pancreas development, including the formation of acinar, ductal and centroacinar cells. We discuss the transcription factors associated with various stages of exocrine differentiation, from multipotent progenitor cells to fully differentiated acinar and ductal cells. Within the branching epithelial tree of the embryonic pancreas, this involves the progressive restriction of multipotent pancreatic progenitor cells to either a central "trunk" domain giving rise to the islet and ductal lineages, or a peripheral "tip" domain giving rise to acinar cells. This review also discusses the soluble morphogens and other signaling pathways that influence these events. Finally, we examine centroacinar cells as an enigmatic pancreatic cell type whose lineage remains uncertain, and whose possible progenitor capacities continue to be explored.

MafA and MafB activity in pancreatic β cells

Analyses in mouse models have revealed crucial roles for MafA (musculoaponeurotic fibrosarcoma oncogene family A) and MafB in islet β cells, with MafB being required during development and MafA in adults. These two closely related transcription factors regulate many genes essential for glucose sensing and insulin secretion in a cooperative and sequential manner. Significantly, the switch from MafB to MafA expression also appears to be vital for functional maturation of β cells produced by human embryonic stem (hES) cell differentiation. This review summarizes the discovery, distribution, and function of MafA and MafB in rodent pancreatic β cells, and describes some key questions regarding their importance to β cells.

Replacement of Rbpj with Rbpjl in the PTF1 complex controls the final maturation of pancreatic acinar cells

BACKGROUND & AIMS

The mature pancreatic acinar cell is dedicated to the production of very large amounts of digestive enzymes. The early stages of pancreatic development require the Rbpj form of the trimeric Pancreas Transcription Factor 1 complex (PTF1-J). As acinar development commences, Rbpjl gradually replaces Rbpj; in the mature pancreas, PTF1 contains Rbpjl (PTF1-L). We investigated whether PTF1-L controls the expression of genes that complete the final stage of acinar differentiation.

METHODS

We analyzed acinar development and transcription in mice with disrupted Rbpjl (Rbpjl(ko/ko) mice). We performed comprehensive analyses of the messenger RNA population and PTF1 target genes in pancreatic acinar cells from these and wild-type mice.

RESULTS

In Rbpjl(ko/ko) mice, acinar differentiation was incomplete and characterized by decreased expression (as much as 99%) of genes that encode digestive enzymes or proteins of regulated exocytosis and mitochondrial metabolism. Whereas PTF1-L bound regulatory sites of genes in normal adult pancreatic cells, the embryonic form (PTF1-J) persisted in the absence of Rbpjl and replaced PTF1-L; the extent of replacement determined gene expression levels. Loss of PTF1-L reduced expression (>2-fold) of only about 50 genes, 90% of which were direct targets of PTF1-L. The magnitude of the effects on individual digestive enzyme genes correlated with the developmental timing of gene activation. Absence of Rbpjl increased pancreatic expression of liver-restricted messenger RNA.

CONCLUSIONS

Replacement of Rbpj by Rbpjl in the PTF1 complex drives acinar differentiation by maximizing secretory protein synthesis, stimulating mitochondrial metabolism and cytoplasmic creatine-phosphate energy stores, completing the packaging and secretory apparatus, and maintaining acinar-cell homeostasis.

[Neuroendocrine system of the pancreas and gastrointestinal tract: origin and development]

Gastroenteropancreatic neuroendocrine tumours (GEP NETs) originate from the neuroendocrine cells through the gastrointestinal tract and endocrine pancreas. The embryologic development of the pancreas is a complex process that begins with the "stem cell" that come from the endodermus. These cells go through two phases: in the first transition the "stem cell" differentiates in exocrine and endocrine cells. This process is regulated by transcription factors such as Pdx1 ("insulin promoter factor 1"), Hlxb6 and SOX9. In the second transition the neuroendocrine cell differentiates in the 5 cell types (alpha, beta, delta, PP y epsilon.). This process is regulated through the balance between factors favoring differentiation (mainly neurogenin 3) and inhibitor factors which depend on Notch signals. The existence of a third transition in postnatal pancreas is hypothesized. The "stem cell" from pancreatic ducts would become adult beta cells, through autoduplication and neogenesis. In the small gut of the adult the stem cell are placed in the intestinal crypts and develop to villi in secretor lines (enterocytes, globet and Paneths cells) or neuroendocrine cells from which at least 10 cell types depend. This process is regulated by transcription factors: Math1, neurogenina 3 and NeuroD.

Stem cells to pancreatic beta-cells: new sources for diabetes cell therapy

The number of patients worldwide suffering from the chronic disease diabetes mellitus is growing at an alarming rate. Insulin-secreting beta-cells in the islet of Langerhans are damaged to different extents in diabetic patients, either through an autoimmune reaction present in type 1 diabetic patients or through inherent changes within beta-cells that affect their function in patients suffering from type 2 diabetes. Cell replacement strategies via islet transplantation offer potential therapeutic options for diabetic patients. However, the discrepancy between the limited number of donor islets and the high number of patients who could benefit from such a treatment reflects the dire need for renewable sources of high-quality beta-cells. Human embryonic stem cells (hESCs) are capable of self-renewal and can differentiate into components of all three germ layers, including all pancreatic lineages. The ability to differentiate hESCs into beta-cells highlights a promising strategy to meet the shortage of beta-cells. Here, we review the different approaches that have been used to direct differentiation of hESCs into pancreatic and beta-cells. We will focus on recent progress in the understanding of signaling pathways and transcription factors during embryonic pancreas development and how this knowledge has helped to improve the methodology for high-efficiency beta-cell differentiation in vitro.

An illustrated review of early pancreas development in the mouse

Pancreas morphogenesis and cell differentiation are highly conserved among vertebrates during fetal development. The pancreas develops through simple budlike structures on the primitive gut tube to a highly branched organ containing many specialized cell types. This review presents an overview of key molecular components and important signaling sources illustrated by an extensive three-dimensional (3D) imaging of the developing mouse pancreas at single cell resolution. The 3D documentation covers the time window between embryonic days 8.5 and 14.5 in which all the pancreatic cell types become specified and therefore includes gene expression patterns of pancreatic endocrine hormones, exocrine gene products, and essential transcription factors. The 3D perspective provides valuable insight into how a complex organ like the pancreas is formed and a perception of ventral and dorsal pancreatic growth that is otherwise difficult to uncover. We further discuss how this global analysis of the developing pancreas confirms and extends previous studies, and we envisage that this type of analysis can be instrumental for evaluating mutant phenotypes in the future.

Minireview: transcriptional regulation in pancreatic development

Considerable progress has been made in the understanding of the sequential activation of signal transduction pathways and the expression of transcription factors during pancreas development. Much of this understanding has been obtained by analyses of the phenotypes of mice in which the expression of key genes has been disrupted (knockout mice). Knockout of the genes for Pdx1, Hlxb9, Isl1, or Hex results in an arrest of pancreas development at a very early stage (embryonic d 8-9). Disruption of genes encoding components of the Notch signaling pathway, e.g. Hes1 or neurogenin-3, abrogates development of the endocrine pancreas (islets of Langerhans). Disruption of transcription factor genes expressed more downstream in the developmental cascade (Beta2/NeuroD, Pax4, NKx2.2, and Nkx6.1) curtails the formation of insulin-producing beta-cells. An understanding of the importance of transcription factor genes during pancreas development has provided insights into the pathogenesis of diabetes, in which the mass of insulin-producing beta-cells is reduced.

Post-natal maturation of acinar cells of the guinea pig pancreas: an ultrastructural morphometric study

The morphological maturation of the acinar cells of the guinea pig pancreas during post-natal development was characterized morphometrically by determining the intracytoplasmic accumulation of rough endoplasmic reticulum (RER) and zymogen granules. The following results were obtained for the period analysed, i.e., from 2 to 70 days of post-natal life: (a) the acinar cell volume increased by 210% (P < 0.01); (b) the mostly cisternal RER occupied more than 30% of the cytoplasm at any age studied and their total volume and surface in the cell were increased by 300 and 534% (P < 0.01), respectively; (c) maturation in the morphological pattern of the RER was observed; (d) the mean number of zymogen granules per cell increased from 261 at 2 days to 422 at 70 days (P < 0.01), while their mean diameter increased from 0.52 to 0.94 micron (P < 0.01) during the same period; (e) these increases in granule number and size were responsible for a 500% (P < 0.01) increase in total volume from 2 to 70 days and for a 304% increase (P < 0.01) in total surface from 2 to 35 days; (f) the RER and the zymogen granules together occupied 44, 54, 55 and 57% of the cytoplasm at 2, 14, 35 and 70 days of age, respectively. We conclude that although the pancreatic acinar cells of the guinea pig are morphologically well differentiated at 2 days of age, with the cytoplasm already showing a large amount of RER and zymogen granules, they are still immature. Morphological maturation of the acinar cell occurs during the first months of post-natal life and is characterized by a substantial gain in cell volume and intracytoplasmic accumulation of RER and zymogen granules, which significantly increase of both their absolute volume and total surface, with a higher growth rate being observed during the period from 2 to 14 days of post-natal life.

Skeletal and cardiac alpha-actin isoforms exhibit unanticipated temporal and tissue-specific gene expression patterns in developing avian limbs and embryos

The initial expression of skeletal muscle structural genes typically occurs after myogenic determination factor gene expression. We investigated this temporal relationship via a reverse transcription-polymerase chain reaction (RT-PCR) analysis of skeletal and cardiac alpha-actin (s- and c-actin) mRNA during chick limb development. c-actin transcripts were first detected at the beginning of muscle cytodifferentiation in stage 24/25 limbs, shortly after the initial appearance of MyoD and myogenin mRNAs, and were not detected in nonmyogenic tissues. In contrast, s-actin mRNA was detected in limbs at stage 15-16, periods when myf5 and MRF4 but not MyoD and myogenin transcripts are present (Lin-Jones, J., and Hauschka, S. D., Dev. Biol. 174, 407-422, 1996). While s-actin mRNA was not detected in E7 neural retina and was at variable levels in stage 9-15 neural tube, significant levels were consistently detected in mesodermal tissues which contribute nonmyogenic cells to the limb: stage 9-12 lateral plate and distal portions of stage 25/26 limbs. s-actin transcripts detected in the earliest limbs could thus be in myogenic and/or nonmyogenic cells. These data indicate that while c-actin expression is activated at the onset of limb muscle cytodifferentiation, s-actin expression occurs much earlier, as well as in some nonmyogenic tissues. Whether the precocious expression of s-actin plays a functional role in muscle development remains to be determined.

Pancreatic adenocarcinoma cell line, MDAPanc-28, with features of both acinar and ductal cells

CONCLUSION

We established a new human pancreatic adenocarcinoma cell line, MDAPanc-28. Studies on this new line indicate that it expressed both acinar and ductal gene products suggesting that the patterns of gene expression in the pancreatic adenocarcinoma from which this cell line arose have features similar to those of the protodifferentiated cells hypothesized by Rutter and his colleagues for the developing pancreas (1,2).

BACKGROUND

The cell line arose from a tumor that, like most pancreatic adenocarcinomas, was ductal on the basis of its histological appearance.

METHODS

Once the cell line was established in culture, they were subjected to cytogenetic analysis and tested for their ability to grow in nude mice. RNA from the cells was analyzed by Northern blot analysis and PCR of reverse transcribed cDNA for the expression of both acinar and duct cell gene products. DNA was analyzed for the presence of mutated K-ras at codon 12.

RESULTS

The cell line expressed trypsin and ribonuclease RNA, which are considered to be acinar cell markers, and carbonic anhydrase II (CAII), which is considered to be a duct-cell markers. The histological appearance of xenografts in nude mice was similar to that of the tumor from which the cell line was established. The chromosome number varied between 46 and 60.

Urothelial transformation into functional glandular tissue in situ by instructive mesenchymal induction

It is generally believed that adult tissue is terminally differentiated. The ureter is derived from the metanephric diverticulum which, along with the derivatives of the metanephric mesoderm, forms the kidney. In our experiments, the left ureters of adult male athymic mouse hosts were severed below the kidney, and mesenchyme from neonatal rat seminal vesicles (SVM) was grafted to the cut end of the ureter, thus bringing adult mouse ureter epithelium (URE) in contact with neonatal rat SVM. After four to eight weeks, the in situ tissue recombinants were harvested, and the epithelial secretory proteins recovered. In 5 of 11 cases, an induction occurred, resulting in an in situ transformation of the non-glandular transitional epithelium of the adult mouse ureter into the simple columnar epithelium of the seminal vesicle (SV). Functional cytodifferentiation was examined in these neonatal rat SVM + adult mouse URE tissue recombinants using antibodies against SV-specific secretory (SVS) proteins of the mouse and rat. From the cut end of the ureter, the adult URE was induced to undergo SV morphogenesis, to express SV cytodifferentiation, and to produce the complete spectrum of major SVS proteins characteristic of the mouse. The induced seminal vesicle epithelium (SVE) also expressed androgen receptors (AR) which are not seen in urothelial tissue. Staining with Hoechst dye 33258, which can distinguish cells of mouse and rat origin, further demonstrated that the induced SVE was indeed of mouse origin and not a contaminant of the inducing rat SVM. in addition, neonatal mouse vaginal mesenchyme was grafted in situ beneath the bladder mucosa of adult male mice, and the host animals were killed after three months. The vaginal mesenchyme implanted into the bladders induced prostate-like acini, indicating that the above reprogramming of adult organs in situ is not an isolate occurrence. These results set a precedent for the "recreation" of new vital organs, such as the kidney, in situ by demonstrating that adult epithelial cells retain a developmental plasticity equivalent to their undifferentiated fetal counterparts and are capable of being reprogrammed in situ to express a completely new morphological, biochemical, and functional phenotype.

The establishment of the hepatic architecture is a prerequisite for the development of a lobular pattern of gene expression

We have studied the expression patterns of ammonia-metabolising enzymes and serum proteins in intrasplenically transplanted embryonic rat hepatocytes by in situ hybridisation and immunohistochemical analysis. The enzymic phenotype of individually settled hepatocytes was compared with that of hepatocytes being organised into a three-dimensional hepatic structure. Our results demonstrate that development towards the terminally differentiated state with zonal differences in enzyme content requires the incorporation of hepatocytes into lobular structures. Outside such an architectural context, phenotypic maturation becomes arrested and hepatocytes linger in the protodifferentiated state. These features identify the foetal period as a crucial time for normal liver development and show that the establishment of the terminally differentiated hepatocellular phenotype, beginning with the differentiation of hepatocytes from the embryonic foregut, is realised via a multistep process.

An endocrine-specific element is an integral component of an exocrine-specific pancreatic enhancer

We have analyzed the function of individual elements of the elastase I transcriptional enhancer in transgenic animals. This pancreas-specific enhancer comprises three functional elements, one of which (the B element) plays a dual role. Within the context of the enhancer, the B element contributes to appropriate acinar cell expression. However, when separated from the other enhancer components, the B element selectively directs transcription in islet cells of transgenic animals. This islet-specific activity is normally suppressed by an upstream repressor domain. The B element binds a novel islet-specific factor, and similar B-like elements are present in other pancreatic genes, both exocrine and endocrine specific. We suggest that a principal role of this transcriptional element and its associated factors is to activate many pancreatic genes as part of the program of pancreatic determination prior to the divergence of the acinar and islet cell lineages.

Stromal regulation of epithelial function

Stromal influences upon epithelia are part of a continuum of cellular interactions that begins at fertilization and extends into adulthood. In parenchymal organs, the most thoroughly characterized interactions have been those that occur during development between mesenchyme, embryonic stroma, and epithelium. Mesenchyme is essential for epithelial proliferation, morphogenesis, and differentiation. Hormones affect stromal-epithelial interactions, and in some cases, steroid hormones may produce their effects on the epithelium indirectly, acting via the mesenchyme. In many adult organs the epithelia continually proliferate and differentiate and consequently may be considered developing systems within the mature organism. This is especially true in organs with a rapidly renewing epithelium, such as the intestine, and in organs that have cycles of functional activity, such as those of the female reproductive system. The mechanisms by which stroma affects epithelial structure and function are not well understood. Current models of how signaling may be accomplished include transmission via diffusible substances, via the extracellular matrix (ECM), and via direct cell-cell contact. Growth factors and organ-specific paracrine factors are candidates for stromal cues that affect the epithelium in some systems. Components of the ECM appear to play a role in permissive interactions and may affect epithelial function by changing cell shape or by binding ECM to the cell surface integrin receptors. Signaling via direct stromal-epithelial contact may be accomplished via interactions between complimentary cell surface adhesion molecules. The importance of stromal-epithelial interactions is reemphasized by several models of carcinogenesis that suggest that perturbations in these interactions may be involved in tumor progression.

Fetotoxic alterations in the normal ontogenies of rat microsomal and lysosomal enzymes

The activity patterns during development for acid phosphatase (Ac-P), alkaline phosphatase (A1-P), beta-glucuronidase (beta G), and UDP-glucuronyltransferase (UDPGT) have been determined in various tissues of the rat for corn oil and distilled water controls as well as in animals prenatally exposed to four fetotoxic chemicals. Postnatal assays were performed on both sexes separately. In control animals, tissue-specific differences between male and female activity levels were found for UDPGT. In the liver of mature offspring, enzyme activity was greater in males than in females. Although no sex difference was observed in the intestine, the kidneys of females exhibited higher values than those of males. An original computer-assisted methodology is presented, designed (a) to permit a mathematical description for the complex curves exhibited by these ontogeny profiles, and (b) to assess the statistical significance of chemical-induced alterations in these complex developmental patterns, specifically, to target sensitive periods and subtle changes near the fetotoxic threshold. Oral administration (days 6-18 of gestation) of 3,3',4,4'-tetrachlorobiphenyl (4CB) to pregnant females resulted in an induction of liver UDPGT activity in offspring postnatally, and some alterations in the perinatal pattern of beta G in the same tissue. This treatment also produced differences in the intestinal patterns of Ac-P and male UDPGT. No significant changes were observed in offspring exposed to diethylstilbestrol (DES). Treatment with zeranol (ZN) caused reductions in activity over the entire postnatal period for beta G in liver, brain, intestine, and kidney, for A1-P in brain, and for Ac-P in the intestine. Cadmium-treated dams gave birth to offspring that exhibited slightly altered ontogenies only in intestine for UDPGT and AcP. The alterations in these developmental profiles indicate periods of increased sensitivity, and may be useful in directing more specific studies into the fetotoxic mechanisms of these compounds.

Coexpression of the c-myc protooncogene with alpha-fetoprotein and albumin in fetal mouse liver

The level of mRNAs for the c-myc protooncogene and the serum proteins alpha-fetoprotein (AFP) and albumin in liver, visceral yolk sac and gut between day 9 and day 19 of mouse gestation was studied by in situ hybridization employing single-stranded RNA probes. In the prehepatocyte population, c-myc was coexpressed with albumin and AFP. No heterogeneity was noted within this cell population with respect to the expression of these mRNAs up to day 15. AFP expression was high in the liver primordium and rose further until day 15. Albumin mRNA was expressed weakly but distinctly in the hepatic bud and increased throughout fetal life. C-myc expression in prehepatocytes exhibited a maximum around day 13 and a dramatic decline after day 15, but was much lower in other cell types of the fetal liver. In the visceral yolk sac, AFP was strongly expressed, with albumin expression first becoming detectable at day 13, while c-myc mRNA was detected up to day 9. In the endodermal gut epithelium, c-myc expression was high, albumin mRNA was not detected and AFP message was restricted to individual loops of the gut. These results suggest that a period of high c-myc expression in the developing liver may allow rapid expansion of the prehepatocyte population at a specific stage of differentiation.

Specific regulation of the gene expression of some pancreatic enzymes during postnatal development and weaning in the calf

The construction of cDNA library from calf pancreas allowed us to examine the mRNA levels of four pancreatic hydrolases (chymotrypsin, lipase, trypsin and amylase) during postnatal development in preruminant and ruminant animals. The lack of parallel variations in the levels of the enzyme specific activities suggested that protein synthesis was not coordinately regulated. In preruminant calves, the change in chymotrypsin and lipase mRNA concentrations (0-28 day period) and in trypsin mRNA concentrations (0-119 day period) was opposite to that in the corresponding specific activities. In contrast, both the activity and mRNA profiles of amylase during the latter period, on the one hand, and those of chymotrypsin and lipase during the 28-119 day period, on the other hand, were comparable. However, the extent to which the specific activity and mRNA concentration of each enzyme were increased did not necessarily coincide. The observed changes in mRNA levels probably resulted from some transcriptional control of the gene expression and/or variation in mRNA stability. Moreover, a translational regulation of the messengers could explain the existence of non-parallel mRNA and specific activity profiles. In sharp contrast with the multiple control of protein synthesis during postnatal development in preruminant calves, weaning was found to induce the same increase in enzyme activity and corresponding mRNA for each of the four pancreatic enzymes, suggesting that pretranslational modulation of gene expression was mainly, if not exclusively, concerned.

In developing brown adipose tissue c-myc protooncogene expression is restricted to early differentiation stages

The expression pattern of the c-myc protooncogene during fetal development of the mouse was studied by RNA in situ hybridization to tissue sections. In day 15.5 post coitum (p.c.) and day 16.5 p.c. embryos, distinct c-myc expression was observed at the site of brown adipose tissue development. High c-myc expression at this site coincides with expression of the NADH-dependent, cytosolic form of glycerol-3-phosphate dehydrogenase (GPDH). Fat deposition in the form of single small droplets occurs as early as day 15.5 p.c. At earlier stages (day 14.5 p.c. and earlier) the mesenchymal precursor cells at this site show neither c-myc nor GPDH expression. During fetal development the brown adipose tissue increases enormously in size. GPDH expression increases concomitantly as does multilocular fat deposition. However, c-myc expression is not detectable in brown adipose tissue later than day 16.5 p.c.

Accumulation of carbamoylphosphate-synthetase and phosphoenolpyruvate-carboxykinase mRNA in embryonic rat hepatocytes. Evidence for translational control during the initial phases of hepatocyte-specific gene expression in vitro

The aim of this study was to establish whether the initial accumulation of hepatocyte-specific proteins after hormone induction is regulated at the pretranslational and/or the translational level. To this end, mRNA molar concentrations were determined and compared with rates of protein synthesis from previous studies [van Roon, M.A., Charles, R. & Lamers, W.H. (1987) Eur. J. Biochem. 165, 229-234]. In vivo, carbamoylphosphate-synthetase mRNA starts to accumulate at day 17 of pregnancy. Phosphoenolpyruvate-carboxykinase mRNA starts to accumulate only just prior to birth. Embryonic day 14 (i.e. 8 days before the expected day of birth), livers were chosen to study the regulation of the initiation of hepatocyte-specific mRNA accumulation in vitro. Accumulation of carbamoylphosphate-synthetase and phosphoenolpyruvate-carboxykinase mRNA is regulated by the same hormones as accumulation of the respective proteins. The rate at which carbamoylphosphate-synthetase and phosphoenolpyruvate-carboxykinase mRNA molecules accumulate in cultured embryonic hepatocytes is relatively low, compared to that of postnatal hepatocytes. However, the increase of the rate of synthesis of carbamoylphosphate-synthetase and phosphoenolpyruvate-carboxykinase protein is even 3-6-fold slower than that of mRNA. This shows that initially mRNAs accumulate intracellularly to a relatively high concentration without being efficiently translated or translatable. Only after the mRNA concentration reaches a plateau of 72 h and 48 h respectively, the cellular capacity to synthesize the respective proteins increases. Therefore, the translational efficiency is certainly one of the major rate-limiting factors of the initial phases of expression of the hepatocyte-specific genes for carbamoylphosphate synthetase and phosphoenolpyruvate carboxykinase.

The rat elastase I regulatory element is an enhancer that directs correct cell specificity and developmental onset of expression in transgenic mice

A total of 134 base pairs of the 5' flanking sequence of the elastase I gene is sufficient and necessary to direct expression of the passive human growth hormone gene (hGH) to the exocrine pancreas. We demonstrate that this elastase I regulatory region contains a transcriptional enhancer which directs acinar cell-specific expression in transgenic animals. The elastase I enhancer specifies correct expression of the linked hGH gene in an orientation- and position-independent manner and can activate a heterologous promoter. The enhancer also directs the appropriate temporal activation of the hGH gene in the developing pancreas. Transcription is initiated correctly for the elastase I or hGH promoter, and the transcripts are correctly processed regardless of the enhancer position within or outside the fusion gene. The elastase I enhancer generates coincident DNase I-hypersensitive sites in pancreatic chromatin when moved 3 kilobases upstream or within the first intron of the hGH gene and when associated with the hGH promoter.

The rat elastase I regulatory element is an enhancer that directs correct cell specificity and developmental onset of expression in transgenic mice

A total of 134 base pairs of the 5' flanking sequence of the elastase I gene is sufficient and necessary to direct expression of the passive human growth hormone gene (hGH) to the exocrine pancreas. We demonstrate that this elastase I regulatory region contains a transcriptional enhancer which directs acinar cell-specific expression in transgenic animals. The elastase I enhancer specifies correct expression of the linked hGH gene in an orientation- and position-independent manner and can activate a heterologous promoter. The enhancer also directs the appropriate temporal activation of the hGH gene in the developing pancreas. Transcription is initiated correctly for the elastase I or hGH promoter, and the transcripts are correctly processed regardless of the enhancer position within or outside the fusion gene. The elastase I enhancer generates coincident DNase I-hypersensitive sites in pancreatic chromatin when moved 3 kilobases upstream or within the first intron of the hGH gene and when associated with the hGH promoter.

Amino acid environment determines expression of carbamoylphosphate synthetase and phosphoenolpyruvate carboxykinase in embryonic rat hepatocytes

A completely defined medium (EHM-1), which reflects the amino acid composition of fetal rat serum and contains albumin as the sole proteinaceous compound, allows the accumulation of carbamoylphosphate synthetase and phosphoenolpyruvate carboxykinase in the presence of dexamethasone, dibutyryl cyclic AMP, and triiodothyronine to approximately twice the level attained in a standard culture medium (RPMI 1640) supplemented with 10% fetal bovine serum (and hormones). Using the EHM-1 medium we could show that the capacity of hepatocytes to synthesize phosphoenolpyruvate carboxykinase in the presence of hormones is manifest as soon as the cells differentiate from the embryonic foregut (embryonic Day 11). Furthermore we could show that embryonic hepatocytes can become binuclear or polyploid when cultured in the presence of thyroid hormone.

Large particles associated with gap junctions of pancreatic exocrine cells during embryonic and neonatal development

The formation of gap junctions was studied in pancreatic exocrine cells of rats and mice during late embryonic and neonatal development by the freeze-fracture replica method. Small gap junctions were present in association with tight junctional strands near the cell apex during embryonic development. Independently of tight junctions, small gap junctions were sometimes seen more basally on day 13 to 15 of gestation. The gap junctions increased in number and were rapidly enlarged by day 18 to 20 of gestation. Large particles 12-13 nm in diameter were frequently associated with the gap junction, which consisted of 10 nm particles. The large particles were either irregularly distributed or arranged in hexagonal patterns. The number of large particles decreased with time, so that they sparsely rimmed the gap junction in postnatal animals. This suggests that large particles are precursors of typical gap junctional particles, and that they participate in rapid growth of the gap junction during late embryonic development. It may be also possible that large particles represent functionally different gap junctions.

Perinatal development of the liver in rat and spiny mouse. Its relation to altricial and precocial timing of birth

Rat (Rattus norvegicus) and spiny mouse (Acomys cahirinus) are closely related murinoid species that mainly differ in the developmental timing of birth. A comparison between the developmental profiles of some characteristic enzymes of the liver of both species was carried out to elucidate the question to what extent are these enzymic profiles and hence the maturation of the liver related to the timing of birth? It was found that these organotypic enzymes first become detectable at the same developmental stage in both species. Likewise, the weaning phase of the enzymic profiles occurs at the same developmental time point in both species. It is argued that both the first appearance and the weaning increase in enzyme activity levels occur at endogenously programmed timepoints with only superimposed effects of hormones. In contrast, the perinatal phase of the enzymic profile is completely dependent on the developmental timing of birth and therefore appears not to be anchored to a particular developmental timepoint, but rather to be dependent on birth-associated (hormonal) adaptation. In accordance with this hypothesis it was found that the morphological development of the liver proceeded independent of the timing of birth. Furthermore, the hormonal regulation of the investigated enzymes was found to be the same in both species. Despite the more advanced state of morphological development of the liver in the spiny mouse at birth, it was found that the inducibility of organotypic gene expression by hormones in spiny mouse fetuses was as limited as in rat fetuses. This observation therefore suggests that the intra-uterine environment is responsible for the limited inducibility of enzymes before birth.

The level of expression of the rat growth hormone gene in liver tumor cells is at least eight orders of magnitude less than that in anterior pituitary cells

Rat liver hepatoma cells (HTC) which express liver-specific gene products were assayed for the expression of the rat growth hormone (rGH) gene, which is normally expressed in anterior pituitary somatotrophs. The combination of immunoprecipitation and two-dimensional gel electrophoresis provided a highly sensitive assay for rGH synthesis at levels as low as one part in 10(9) of cell protein synthesis (or four molecules of rGH per cell). No rGH expression was detected at this level. The lack of expression in HTC cells did not derive from a deletion of the rGH gene, as shown by Southern hybridization analysis of genomic DNA. Because the gene is expressed at greater than 30% of anterior pituitary protein synthesis, differentiation regulated rGH expression by over 10(8)-fold between the two cell types. Additionally, DNA-excess solution hybridization was used to measure the level of rGH mRNA sequences. A novel and general method for preparing single-strand probes from recombinant plasmids was developed. Hybridization analyses with a sensitivity of detection of 1 part in 10(8) failed to detect any rGH RNA sequences in either the nucleus or cytoplasm of HTC cells. It is concluded, therefore, that the restriction in rGH expression in the liver tumor cells is likely to occur at the level of the transcription of the gene, and that for all practical purposes, the rGH gene is completely shut off in the hepatoma cells.

Increase in zymogen granule volume accounts for increase in volume density during prenatal development of pancreas

The sudden increase in volume density of zymogen granules in acinar cells of the fetal rat pancreas was examined with particular attention to the respective roles of granule size and number in this event. Volume density increased some twelvefold, from about 3% of cytoplasmic volume at 17 days to about 45% at 20 days, following a sigmoidal pattern in which the greatest rate of increase occurred during day 18. This increase in volume density was primarily the result of an increase in granule volume. Zymogen granule diameter increased from 0.55 micron at 17 days to 1.20 micron at 20 days, an order of magnitude increase in average granule volume. The total number of granules in the tissue increased in proportion to the increase in organ weight (cell number and size), but changes in the number of granules per unit cytoplasmic volume were minor (+ 40%) in comparison to the increase in volume density. The distribution of granule diameter was roughly normal and unimodal at each time interval, and the increase in average diameter over time was marked by an increase in the upper limit of the size distribution and an increased percentage of large granules. The size of condensing vacuoles also increased during this period, and their distributions were roughly coextensive with those seen for zymogen granules at the same time. The potential origins of changes in granule size are discussed, as well as the important effect that size has on the number of granules observed in "two-dimensional" tissue sections viewed in the electron microscope. If size is not considered in our estimates, then we underestimate the numerical density in cells with small granules compared to those with large granules. The results indicate the central role of granule size, as opposed to number, in determining granule volume density in the embryonic pancreas.

The level of expression of the rat growth hormone gene in liver tumor cells is at least eight orders of magnitude less than that in anterior pituitary cells

Rat liver hepatoma cells (HTC) which express liver-specific gene products were assayed for the expression of the rat growth hormone (rGH) gene, which is normally expressed in anterior pituitary somatotrophs. The combination of immunoprecipitation and two-dimensional gel electrophoresis provided a highly sensitive assay for rGH synthesis at levels as low as one part in 10(9) of cell protein synthesis (or four molecules of rGH per cell). No rGH expression was detected at this level. The lack of expression in HTC cells did not derive from a deletion of the rGH gene, as shown by Southern hybridization analysis of genomic DNA. Because the gene is expressed at greater than 30% of anterior pituitary protein synthesis, differentiation regulated rGH expression by over 10(8)-fold between the two cell types. Additionally, DNA-excess solution hybridization was used to measure the level of rGH mRNA sequences. A novel and general method for preparing single-strand probes from recombinant plasmids was developed. Hybridization analyses with a sensitivity of detection of 1 part in 10(8) failed to detect any rGH RNA sequences in either the nucleus or cytoplasm of HTC cells. It is concluded, therefore, that the restriction in rGH expression in the liver tumor cells is likely to occur at the level of the transcription of the gene, and that for all practical purposes, the rGH gene is completely shut off in the hepatoma cells.

Effects of dexamethasone and 5-bromodeoxyuridine on protein synthesis and secretion during in vitro pancreatic development

Protein synthesis and secretion during in vitro pancreatic development and after treatment with the glucocorticoid dexamethasone and the thymidine analog 5-bromodeoxyuridine (BrdU) was monitored using two-dimensional gel electrophoresis. At 14 days gestation, the synthesis of more than 200 proteins and the secretion of a complex set of proteins was detected. The relative rate of synthesis and secretion of the majority of this set of proteins decreased dramatically during development; after 6 days of culture most were no longer detected. In contrast, the synthesis and secretion of pancreas-specific exocrine proteins amylase, a Sepharose binding protein (protein 2), and chymotrypsinogen first detected after one day in culture, increased throughout the 6-day culture period. Other pancreatic digestive (pro)enzymes normally found in the adult such as the basic form of chymotrypsinogen, lipase, ribonuclease, and trypsinogen were not detected during the culture period. Thus at least two distinct regulatory events are involved in the expression of the exocrine genes during development. Dexamethasone treatment during the 6-day culture period selectively increased the synthesis of amylase and several other minor secretory proteins. BrdU treatment caused major changes in the protein synthetic and secretory patterns of the pancreas as well as in morphogenesis. BrdU treated pancreases showed greatly reduced synthesis of amylase, protein 2, and chymotrypsinogen and prolonged synthesis of many proteins normally detected only at early stages of pancreatic development. BrdU treatment also stimulated the secretion of a set of proteins ostensibly associated with duct cells. Thus, BrdU specifically alters the developmental program of the pancreas.

Control of the changes in rat-liver carbamoyl-phosphate synthase (ammonia) protein levels during ontogenesis: evidence for a perinatal change in immunoreactivity of the enzyme

To analyze the changes in rat-liver carbamoyl-phosphate synthase (Cpase) protein levels during ontogenesis, these levels were determined by means of two independent methods, i.e. radioimmunoassay and densitometric assay. During normal development the changes in catalytic activity of Cpase are accompanied by equivalent changes in the quantities of enzyme protein. We have obtained evidence for the existence of a perinatal Cpase which is immunochemically different from adult Cpase as immunoreactivity of Cpase decreases in the perinatal period and remains constant thereafter.

Morphologic and biochemical characteristics of isolated and cultured pancreatic ducts

Rat and hamster pancreatic ducts were isolated by digestion with collagenase plus chymotrypsin and were cultured for eight weeks in an agarose matrix. Freshly isolated and cultured ducts were characterized morphologically and biochemically. The in vivo morphology of the ducts was maintained in vitro, although certain differences were noted. Both interlobular and intralobular ducts could be identified. gamma-Glutamyltranspeptidase and Mg-ATPase were stable enzymatic activities of the ducts of both species; alkaline phosphatase persisted only in the hamster ducts. Carbonic anhydrase and (Na + K)ATPase were minor activities of the rat ducts. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the rat ducts suggested that actin was the major duct peptide and that the major zymogens were greatly diminished. These results demonstrate that pancreatic ducts can be maintained in vitro and can be used for biochemical studies of this minor pancreatic tissue type.

Insulin gene expression during development of the fetal bovine pancreas

Poly(A+) RNA was isolated from the bovine pancreas at three stages of fetal development. Approximately 1% of the total RNA from first, second, and third trimester fetuses was polyadenylated, and the mean chain length of each RNA population was 1350 nucleotides. In cell-free protein synthesis experiments the concentration of insulin-immunoreactive translation products was 10.2%, 11.3%, and 9.7% for first, second, and third trimesters, respectively. Insulin mRNA sequences were estimated by transcription of insulin mRNA to [3H]cDNA and hybridization of cDNA with plasmid pI19 DNA containing rat proinsulin I sequences. Hybridization experiments gave insulin mRNA concentrations of 7.6%, 12.9%, and 3.9% for first, second, and third trimesters, respectively. These results show that insulin mRNA levels vary during development and become proportionally lower in third trimester, when the exocrine tissue is rapidly increasing in mass.

Multihormonal control of enzyme clusters in rat liver ontogenesis. II. Role of glucocorticosteroid and thyroid hormone and of glucagon and insulin

The role of glucocorticosteroid and thyroid hormone and of glucagon and insulin in the pre- and postnatal developmental formation of carbamoyl-phosphate synthase, ornithine transcarbamoylase, arginase, glutamate dehydrogenase, tyrosine aminotransferase, glucose-6-phosphatase, hexokinase and glucokinase activities in rat liver was investigated. Glucocorticosteroids and a low insulin/glucagon ratio always stimulate formation of carbamoyl-phosphate synthase, ornithine transcarbamoylase, arginase, glutamate dehydrogenase, tyrosine aminotransferase and glucose-6-phosphatase, while glucocorticosteroids and a high insulin/glucagon ratio stimulate formation of glucokinase. Thyroid hormone stimulates the formation of carbamoyl-phosphate synthase, arginase and tyrosine aminotransferase only before birth, whereas it stimulates the formation of glutamate dehydrogenase and glucose-6-phosphatase both before and after birth. Ornithine transcarbamoylase activity is depressed after thyroid-hormone treatment before and after birth. DNA content is always decreased by glucocorticosteroids and increased by thyroid hormone. The effect of these hormones on hexokinase is complex, probably due to different responses of the constitutive isozymes. With the exception of the effects of thyroid hormone on carbamoyl-phosphate synthase, arginase and tyrosine aminotransferase before birth, which may be indirect, the responses of enzyme activities and DNA content to treatment with glucocorticosteroid hormones, glucagon, insulin and thyroid hormone are qualitatively the same in fetuses, neonates, sucklings, weanlings and adults. Thus, the developmental profiles of the enzyme clusters reflect the changing levels of the relevant hormones. The enzymes that are stimulated by glucocorticosteroids and the insulin/glucagon ratio show increases in enzyme activity perinatally and around weaning, and relatively low activities in between, while those enzymes that are additionally stimulated by thyroid hormone differ in exhibiting relatively high activities between birth and weaning.

Rna synthesis and processing during cytodifferentiation in fetal rat pancreas

In fetal rat pancreas cytodifferentiation occurs between day 14 and day 20 of gestation and is accompanied by an exponential increase in the cellular accumulation of tissue specific proteins and an elaboration of the cellular organelles associated with their synthesis and secretion. Evaluation of RNA synthesis by [3H] uridine incorporation into trichloroacetic acid precipitable material showed that during this period the apparent rate of RNA synthesis increased 7.5 fold from 2 x 10(3) dpm/micrograms DNA/h on day 15 to 1.5 x 10(4) dpm/micrograms DNA/h on day 19; [3H] leucine uptake showed that the rate of protein synthesis increased about the same extent with the major difference being that the maximum rate of protein synthesis occurred on day 19, one day after the maximum rate of RNA synthesis. The soluble pyrimidine nucleotide pools decreased from 122 pmol/micrograms DNA on day 14 to 15 pmol/micrograms DNA on day 16 followed by an increase to 104 pmol/micrograms DNA on day 19; the purine nucleotide pools decreased from 367 pmol/micrograms DNA on day 14 to 286 pmol/micrograms DNA on day 16 and then increased to 635 pmol/micrograms DNA on day 19. These values roughly paralleled the transitions observed in the rates of RNA and protein synthesis. Agarose-acrylamide slab gel electrophoresis showed an increase in RNA synthesis and an increase in ribosomal RNA synthesis and processing with cytodifferentiation.

Proteins synthesized and secreted during rat pancreatic development

The synthesis and secretion of proteins during development of the pancreas was analyzed using two-dimensional gel electrophoresis. The pattern of synthesis of the total proteins of the pancreas was found to change very little from 14 to 18 d gestation. In addition, the protein synthetic pattern of the embryonic pancreas was very similar to the protein patterns of several other embryonic tissues (gut, lung, and mesenchyme). Between 18 d gestation and the adult stage, the synthesis of the majority of protein species fades as the synthesis of the secretory (pro)enzymes becomes dominant. Thus, the terminal differentiation of the pancreas appears to involve the dominant expression of a limited set of genes (coding, in part, for the digestive [pro]enzymes) while the pattern of expression of the remaining domain remains relatively unchanged. Many of the secretory (pro)enzymes were identified and their synthesis during development was monitored. The synthesis of several secretory proteins was detected between 15 and 18 d gestation (e.g., amylase and chymotrypsinogen), whereas the synthesis of others was not detected until after 18 d gestation (i.e., trypsinogen, ribonuclease, proelastase, and lipase). Between 18 d gestation and the adult stage, the synthesis of the digestive (pro)enzymes increases to > 90% of pancreatic protein synthesis. The secretion of digestive (pro)enzymes was detected as early as 15 d gestation. The selective release of a second set of proteins was detected in the early embryo. These proteins are not detected in the adult pancreas or in zymogen granules but are also released by several other embryonic tissues. The function of this set of proteins is unknown.