CRISPR screening uncovers a central requirement for HHEX in pancreatic lineage commitment and plasticity restriction

The pancreas and liver arise from a common pool of progenitors. However, the underlying mechanisms that drive their lineage diversification from the foregut endoderm are not fully understood. To tackle this question, we undertook a multifactorial approach that integrated human pluripotent-stem-cell-guided differentiation, genome-scale CRISPR-Cas9 screening, single-cell analysis, genomics and proteomics. We discovered that HHEX, a transcription factor (TF) widely recognized as a key regulator of liver development, acts as a gatekeeper of pancreatic lineage specification. HHEX deletion impaired pancreatic commitment and unleashed an unexpected degree of cellular plasticity towards the liver and duodenum fates. Mechanistically, HHEX cooperates with the pioneer TFs FOXA1, FOXA2 and GATA4, shared by both pancreas and liver differentiation programmes, to promote pancreas commitment, and this cooperation restrains the shared TFs from activating alternative lineages. These findings provide a generalizable model for how gatekeeper TFs like HHEX orchestrate lineage commitment and plasticity restriction in broad developmental contexts.

Specific Temporal Requirement of Prox1 Activity During Pancreatic Acinar Cell Development

BACKGROUND AND AIMS

An interactive regulatory network assembled through the induction and downregulation of distinct transcription factors governs acinar cell maturation. Understanding how this network is built is relevant for protocols of directed pancreatic acinar differentiation. The murine transcription factor Prox1 is highly expressed in multipotent pancreatic progenitors and in various mature pancreatic cell types except for acinar cells. In this study, we investigated when is Prox1 expression terminated in developing acinar cells and the potential involvement of its activity in acinar cell specification/differentiation. We also investigated the effects of sustained Prox1 expression in acinar maturation and maintenance.

METHODS

Prox1 acinar expression was analyzed by immunofluorescence and confocal microscopy. Prox1-null embryos (Prox1GFPCre/Δ), Prox1AcOE transgenic mice, histologic and immunostaining methods, transmission electron microscopy, functional assays, and quantitative RNA and RNA-sequencing methods were used to investigate the effects of Prox1 functional deficiency and sustained Prox1 expression in acinar maturation and homeostasis.

RESULTS

Immunostaining results reveal transient Prox1 expression in newly committed embryonic acinar cells. RNA-sequencing demonstrate precocious expression of multiple "late" acinar genes in the pancreas of Prox1GFPCre/Δ embryos. Prox1AcOE transgenic mice carrying sustained Prox1 acinar expression have relatively normal pancreas development. In contrast, Prox1AcOE adult mice have severe pancreatic alterations involving reduced acinar gene expression, abnormal acinar secretory granules, acinar atrophy, increased endoplasmic reticulum stress, and mild chronic inflammation.

CONCLUSION

Prox1 transient expression in early acinar cells is necessary for correct sequential gene expression. Prox1 expression is terminated in developing acinar cells to complete maturation and to preserve homeostasis.

Differential Cell Susceptibilities to KrasG12D in the Setting of Obstructive Chronic Pancreatitis

BACKGROUND & AIMS

Activating mutation of the KRAS gene is common in some cancers, such as pancreatic cancer, but rare in other cancers. Chronic pancreatitis is a predisposing condition for pancreatic ductal adenocarcinoma (PDAC), but how it synergizes with KRAS mutation is not known.

METHODS

We used a mouse model to express an activating mutation of Kras in conjunction with obstruction of the main pancreatic duct to recapitulate a common etiology of human chronic pancreatitis. Because the cell of origin of PDAC is not clear, Kras mutation was introduced into either duct cells or acinar cells.

RESULTS

Although KrasG12D expression in both cell types was protective against damage-associated cell death, chronic pancreatitis induced p53, p21, and growth arrest only in acinar-derived cells. Mutant duct cells did not elevate p53 or p21 expression and exhibited increased proliferation driving the appearance of PDAC over time.

CONCLUSIONS

One mechanism by which tissues may be susceptible or resistant to KRASG12D-initiated tumorigenesis is whether they undergo a p53-mediated damage response. In summary, we have uncovered a mechanism by which inflammation and intrinsic cellular programming synergize for the development of PDAC.

Imaging mass spectrometry enables molecular profiling of mouse and human pancreatic tissue

AIMS/HYPOTHESIS

The molecular response and function of pancreatic islet cells during metabolic stress is a complex process. The anatomical location and small size of pancreatic islets coupled with current methodological limitations have prevented the achievement of a complete, coherent picture of the role that lipids and proteins play in cellular processes under normal conditions and in diseased states. Herein, we describe the development of untargeted tissue imaging mass spectrometry (IMS) technologies for the study of in situ protein and, more specifically, lipid distributions in murine and human pancreases.

METHODS

We developed matrix-assisted laser desorption/ionisation (MALDI) IMS protocols to study metabolite, lipid and protein distributions in mouse (wild-type and ob/ob mouse models) and human pancreases. IMS allows for the facile discrimination of chemically similar lipid and metabolite isoforms that cannot be distinguished using standard immunohistochemical techniques. Co-registration of MS images with immunofluorescence images acquired from serial tissue sections allowed accurate cross-registration of cell types. By acquiring immunofluorescence images first, this serial section approach guides targeted high spatial resolution IMS analyses (down to 15 μm) of regions of interest and leads to reduced time requirements for data acquisition.

RESULTS

MALDI IMS enabled the molecular identification of specific phospholipid and glycolipid isoforms in pancreatic islets with intra-islet spatial resolution. This technology shows that subtle differences in the chemical structure of phospholipids can dramatically affect their distribution patterns and, presumably, cellular function within the islet and exocrine compartments of the pancreas (e.g. 18:1 vs 18:2 fatty acyl groups in phosphatidylcholine lipids). We also observed the localisation of specific GM3 ganglioside lipids [GM3(d34:1), GM3(d36:1), GM3(d38:1) and GM3(d40:1)] within murine islet cells that were correlated with a higher level of GM3 synthase as verified by immunostaining. However, in human pancreas, GM3 gangliosides were equally distributed in both the endocrine and exocrine tissue, with only one GM3 isoform showing islet-specific localisation.

CONCLUSIONS/INTERPRETATION

The development of more complete molecular profiles of pancreatic tissue will provide important insight into the molecular state of the pancreas during islet development, normal function, and diseased states. For example, this study demonstrates that these results can provide novel insight into the potential signalling mechanisms involving phospholipids and glycolipids that would be difficult to detect by targeted methods, and can help raise new hypotheses about the types of physiological control exerted on endocrine hormone-producing cells in islets. Importantly, the in situ measurements afforded by IMS do not require a priori knowledge of molecules of interest and are not susceptible to the limitations of immunohistochemistry, providing the opportunity for novel biomarker discovery. Notably, the presence of multiple GM3 isoforms in mouse islets and the differential localisation of lipids in human tissue underscore the important role these molecules play in regulating insulin modulation and suggest species, organ, and cell specificity. This approach demonstrates the importance of both high spatial resolution and high molecular specificity to accurately survey the molecular composition of complex, multi-functional tissues such as the pancreas.

The BRG1/SOX9 axis is critical for acinar cell-derived pancreatic tumorigenesis

Chromatin remodeler Brahma related gene 1 (BRG1) is silenced in approximately 10% of human pancreatic ductal adenocarcinomas (PDAs). We previously showed that BRG1 inhibits the formation of intraductal pancreatic mucinous neoplasm (IPMN) and that IPMN-derived PDA originated from ductal cells. However, the role of BRG1 in pancreatic intraepithelial neoplasia-derived (PanIN-derived) PDA that originated from acinar cells remains elusive. Here, we found that exclusive elimination of Brg1 in acinar cells of Ptf1a-CreER; KrasG12D; Brg1fl/fl mice impaired the formation of acinar-to-ductal metaplasia (ADM) and PanIN independently of p53 mutation, while PDA formation was inhibited in the presence of p53 mutation. BRG1 bound to regions of the Sox9 promoter to regulate its expression and was critical for recruitment of upstream regulators, including PDX1, to the Sox9 promoter and enhancer in acinar cells. SOX9 expression was downregulated in BRG1-depleted ADMs/PanINs. Notably, Sox9 overexpression canceled this PanIN-attenuated phenotype in KBC mice. Furthermore, Brg1 deletion in established PanIN by using a dual recombinase system resulted in regression of the lesions in mice. Finally, BRG1 expression correlated with SOX9 expression in human PDAs. In summary, BRG1 is critical for PanIN initiation and progression through positive regulation of SOX9. Thus, the BRG1/SOX9 axis is a potential target for PanIN-derived PDA.

ARID1A Maintains Differentiation of Pancreatic Ductal Cells and Inhibits Development of Pancreatic Ductal Adenocarcinoma in Mice

BACKGROUND & AIMS

The ARID1A gene encodes a protein that is part of the large adenosine triphosphate (ATP)-dependent chromatin remodeling complex SWI/SNF and is frequently mutated in human pancreatic ductal adenocarcinomas (PDACs). We investigated the functions of ARID1A during formation of PDACs in mice.

METHODS

We performed studies with Ptf1a-Cre;Kras^G12D mice, which express activated Kras in the pancreas and develop pancreatic intraepithelial neoplasias (PanINs), as well as those with disruption of Aird1a (Ptf1a-Cre;Kras^G12D;Arid1a^f/f mice) or disruption of Brg1 (encodes a catalytic ATPase of the SWI/SNF complex) (Ptf1a-Cre;Kras^G12D; Brg1^f/fmice). Pancreatic ductal cells (PDCs) were isolated from Arid1a^f/f mice and from Arid1a^f/f;SOX9OE mice, which overexpress human SOX9 upon infection with an adenovirus-expressing Cre recombinase. Pancreatic tissues were collected from all mice and analyzed by histology and immunohistochemistry; cells were isolated and grown in 2-dimensional and 3-dimensional cultures. We performed microarray analyses to compare gene expression patterns in intraductal papillary mucinous neoplasms (IPMNs) from the different strains of mice. We obtained 58 samples of IPMNs and 44 samples of PDACs from patients who underwent pancreatectomy in Japan and analyzed them by immunohistochemistry.

RESULTS

Ptf1a-Cre;Kras^G12D mice developed PanINs, whereas Ptf1a-Cre;Kras^G12D;Arid1a^f/f mice developed IPMNs and PDACs; IPMNs originated from PDCs. ARID1A-deficient IPMNs did not express SOX9. ARID1A-deficient PDCs had reduced expression of SOX9 and dedifferentiated in culture. Overexpression of SOX9 in these cells allowed them to differentiate and prevented dilation of ducts. Among mice with pancreatic expression of activated Kras, those with disruption of Arid1a developed fewer PDACs from IPMNs than mice with disruption of Brg1. ARID1A-deficient IPMNs had reduced activity of the mTOR pathway. Human IPMN and PDAC specimens had reduced levels of ARID1A, SOX9, and phosphorylated S6 (a marker of mTOR pathway activation). Levels of ARID1A correlated with levels of SOX9 and phosphorylated S6.

CONCLUSIONS

ARID1A regulates expression of SOX9, activation of the mTOR pathway, and differentiation of PDCs. ARID1A inhibits formation of PDACs from IPMNs in mice with pancreatic expression of activated KRAS and is down-regulated in IPMN and PDAC tissues from patients.

Formation of retinal direction-selective circuitry initiated by starburst amacrine cell homotypic contact

A common strategy by which developing neurons locate their synaptic partners is through projections to circuit-specific neuropil sublayers. Once established, sublayers serve as a substrate for selective synapse formation, but how sublayers arise during neurodevelopment remains unknown. Here, we identify the earliest events that initiate formation of the direction-selective circuit in the inner plexiform layer of mouse retina. We demonstrate that radially migrating newborn starburst amacrine cells establish homotypic contacts on arrival at the inner retina. These contacts, mediated by the cell-surface protein MEGF10, trigger neuropil innervation resulting in generation of two sublayers comprising starburst-cell dendrites. This dendritic scaffold then recruits projections from circuit partners. Abolishing MEGF10-mediated contacts profoundly delays and ultimately disrupts sublayer formation, leading to broader direction tuning and weaker direction-selectivity in retinal ganglion cells. Our findings reveal a mechanism by which differentiating neurons transition from migratory to mature morphology, and highlight this mechanism’s importance in forming circuit-specific sublayers.

Our experience of the world relies on circuits spanning the sense organs and the brain that process information received through our senses. These circuits are made up of many different types of nerve cells that form connections with each other while the brain is developing. For these circuits to be set up properly, nerve cells have to be selective about how they connect with each other. However, researchers know little about how exactly nerve cells form the right connections, or about which genes and proteins are involved.

One of the better understood circuits in the body is known as the ‘direction-selective circuit’. Found in the retina at the back of the eye of all backboned animals, this circuit’s task is to detect the direction that objects are moving. In the case of mice, scientists have identified all of the cells that make up the circuit, and know how they are all supposed to be connected together. This is a useful starting point for researchers to look in more detail at how nerve cells make the right connections during development to set up a working circuit.

Ray et al. looked at how the direction-selective circuit forms in the retinas of young mice by genetically engineering cells to carry fluorescent proteins, or staining them with chemicals. This allowed the cells to be examined under a microscope at different points in their development. It turns out that one type of cell, known as the ‘starburst amacrine cell’ because of its firework-like shape, coordinates the formation of the whole direction-selective circuit. First, starburst cells branch out and touch each other. Next, they build a scaffold for the circuit with their branch-like extensions. Finally, other cell types follow this scaffold to form connections and complete the circuit.

Ray et al. identified a protein called MEGF10 on the surface of starburst cells that tells the cells when they have made contact with each other. When starburst cells had MEGF10 taken away, or were prevented from contacting each other, they did not build a scaffold properly, and the circuit was less effective at detecting movement.

It is possible that cells in other brain circuits use a similar method to form connections. Understanding more about how nerve cells form circuits will help researchers to work out what goes wrong in developmental disorders that affect vision, memory and learning. This knowledge would be helpful for designing new treatments for these conditions.

c-Myc downregulation is required for preacinar to acinar maturation and pancreatic homeostasis

BACKGROUND AND AIMS

c-Myc is highly expressed in pancreatic multipotent progenitor cells (MPC) and in pancreatic cancer. The transition from MPC to unipotent acinar progenitors is associated with c-Myc downregulation; a role for c-Myc in this process, and its possible relationship to a role in cancer, has not been established.

DESIGN

Using coimmunoprecipitation assays, we demonstrate that c-Myc and Ptf1a interact. Using reverse transcriptase qPCR, western blot and immunofluorescence, we show the erosion of the acinar programme. To analyse the genomic distribution of c-Myc and Ptf1a and the global transcriptomic profile, we used ChIP-seq and RNA-seq, respectively; validation was performed with ChIP-qPCR and RT-qPCR. Lineage-tracing experiments were used to follow the effect of c-Myc overexpression in preacinar cells on acinar differentiation.

RESULTS

c-Myc binds and represses the transcriptional activity of Ptf1a. c-Myc overexpression in preacinar cells leads to a massive erosion of differentiation. In adult Ela1-Myc mice: (1) c-Myc binds to Ptf1a, and Tcf3 is downregulated; (2) Ptf1a and c-Myc display partially overlapping chromatin occupancy but do not bind the same E-boxes; (3) at the proximal promoter of genes coding for digestive enzymes, we find reduced PTF1 binding and increased levels of repressive chromatin marks and PRC2 complex components. Lineage tracing of committed acinar precursors reveals that c-Myc overexpression does not restore multipotency but allows the persistence of a preacinar-like cell population. In addition, mutant KRas can lead to c-Myc overexpression and acinar dysregulation.

CONCLUSIONS

c-Myc repression during development is crucial for the maturation of preacinar cells, and c-Myc overexpression can contribute to pancreatic carcinogenesis through the induction of a dedifferentiated state.

Abstract B19: Kras mutation imparts neoplastic potential on duct cells but not acinar cells in a mouse model of obstructive chronic pancreatitis

Understanding the progression of human pancreatic cancer is difficult due to the late stage of diagnosis of this deadly disease. We must therefore rely upon robust model systems to understand how pancreatic cancer arises, how its acute and chronic phases of progression are regulated, and how best to identify and treat it, hopefully still in the early stage. Models of pancreatic ductal adenocarcinoma (PDAC) that accurately reflect human disease are still being developed. To mimic human disease, PDAC must arise from mutations occurring in adult animals and with etiologies relevant to humans. We developed a new mouse model that incorporates obstructive chronic pancreatitis with tissue-specific, adult-onset expression of mutant Kras. We found that KrasG12D expression in duct cells but not in acinar cells led to progression of metaplastic ducts to eventual dysplasia and cancer. In early-stage disease, Kras mutation in acinar cells led to increased acinar-to-ductal metaplasia but did not downregulate p53, thus leading to reduced cell survival. Ducts naturally express much lower p53 levels, however, and there was increased survival of KrasG12D-mediated, duct-derived metaplastic cells. Furthermore, acinar cells upregulated Pdx1 during acinar-to-ductal metaplasia while duct cells did not do so during their metaplastic transition. Without chronic pancreatitis, Kras mutation in acinar cells causes abundant Pancreatic Intraepithelial Neoplasm (PanIN)-like lesions. However, overexpression of Pdx1 in acinar cells in this context permitted acinar-to-ductal metaplasia but prevented PanIN-like lesion formation, suggesting that Pdx1 can repress neoplastic progression. In summary, in the setting of obstructive chronic pancreatitis, ducts are the principal source of cancer development via reduced Pdx1 and p53 levels and increased cell survival.

Citation Format: Fong C. Pan, Jessica N. Kim, Chanjuan Shi, Mary K. Washington, Maike Sander, Maureen Gannon, Robert D. Beauchamp, Christopher V. Wright, Anna L. Means.{Authors}. Kras mutation imparts neoplastic potential on duct cells but not acinar cells in a mouse model of obstructive chronic pancreatitis. [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Advances in Science and Clinical Care; 2016 May 12-15; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2016;76(24 Suppl):Abstract nr B19.

Temporal identity transition from Purkinje cell progenitors to GABAergic interneuron progenitors in the cerebellum

In the cerebellum, all GABAergic neurons are generated from the Ptf1a-expressing ventricular zone (Ptf1a domain). However, the machinery to produce different types of GABAergic neurons remains elusive. Here we show temporal regulation of distinct GABAergic neuron progenitors in the cerebellum. Within the Ptf1a domain at early stages, we find two subpopulations; dorsally and ventrally located progenitors that express Olig2 and Gsx1, respectively. Lineage tracing reveals the former are exclusively Purkinje cell progenitors (PCPs) and the latter Pax2-positive interneuron progenitors (PIPs). As development proceeds, PCPs gradually become PIPs starting from ventral to dorsal. In gain- and loss-of-function mutants for Gsx1 and Olig1/2, we observe abnormal transitioning from PCPs to PIPs at inappropriate developmental stages. Our findings suggest that the temporal identity transition of cerebellar GABAergic neuron progenitors from PCPs to PIPs is negatively regulated by Olig2 and positively by Gsx1, and contributes to understanding temporal control of neuronal progenitor identities.

Abstract PR01: Downregulation of PTF1A is a crucial and rate-limiting step in pancreatic cancer initiation

The goal of this study is to understand the endogenous mechanisms limiting the ability of oncogenic KRAS to initiate pancreatic tumorigenesis. Recently, our lab and others discovered that although pancreatic ductal adenocarcinoma (PDAC) shares phenotypic characteristics with normal pancreatic duct cells, it actually originates from mature exocrine acinar cells. In analyzing the initiating events of PDAC, we have identified the acinar cell transcription factor, PTF1A, as a critical factor inhibiting KRAS from reprogramming acinar cells to ductal tumor precursors. Binding sites for PTF1A are found upstream of essentially all acinar differentiation products and, importantly, PTF1A maintains its own expression through a positive autoregulatory loop. Given the central role of PTF1A in regulating acinar-specific gene expression, we wanted to determine if loss of this transcription factor plays a role in pancreatic intraepithelial neoplasia (PanIN) formation. Importantly, we find that PTF1A is downregulated in early (stage 1) PanINs of humans and mouse models, suggesting that loss of PTF1A expression could be a rate-limiting step in pancreatic cancer initiation.

To test whether PTF1A loss is a functionally important step in PanIN development, we deleted the Ptf1a gene in mice using an acinar-specific inducible Cre (Ptf1a cKO). These studies confirm that PTF1A downregulation is an essential step in PanIN formation, as Cre-mediated deletion of acinar cell Ptf1a in the presence of oncogenic KRAS (KrasG12D) acutely and dramatically accelerates PanIN formation. Widespread PanINs are observed within two weeks of combined KRAS activation/Ptf1a deletion, by which time KRAS alone has induced few or no PanINs. At six weeks post-recombination, the combination of KRAS/Ptf1a cKO induces >15-fold more PanINs than KRAS alone. Additionally, we have found that Ptf1a heterozygosity sensitizes pancreatic cells to KRAS-mediated PanIN formation, indicating that transformation requires reduction of PTF1A activity below a critical autoregulatory threshold needed to maintain acinar identity. Based on these current studies, we hypothesize the existence of mutual antagonism between acinar differentiation, which is maintained through PTF1A, and transformation driven by KRAS.

KRAS normally transforms acinar cells only weakly, but both endogenous and mutant KRAS activity can be increased by inflammation, such as that stimulated by caerulein-induced pancreatitis. As KRAS is potentiated by Ptf1a deletion, we asked whether inflammation would be sufficient to transform Ptf1a-deficient acinar cells even without mutant KRAS. In the absence of pancreatitis, Ptf1a deletion alone has relatively little short-term effect: Ptf1a-deficient acinar cells eventually lose their differentiated phenotype and express ductal markers, but they do not undergo hyperplasia or dysplasia. After subjecting Ptf1a cKO mice to caerulein-induced pancreatitis, however, we observed widespread acinar-to-ductal metaplasia, histological structures that resemble PanINs and stain positively with Alcian blue, and loss of amylase staining throughout the pancreas. Our current data indicate that PTF1A has a critical role in guarding against genetic (oncogenic KRAS) and environmental (pancreatitis) insults, and demonstrate that loss of Ptf1a expression is the critical event of acinar cell transformation. Going forward, we will use gene expression profiling and ChIP-seq analysis to identify PTF1A target genes responsible for inhibiting KRAS, and to characterize the epigenetic processes by which KRAS and inflammation reprogram acinar cells to a PanIN phenotype.

This abstract is also presented as Poster A3.

Citation Format: Nathan Michael Krah, Mary P. Bronner, Christopher V. Wright, L. Charles Murtaugh. Downregulation of PTF1A is a crucial and rate-limiting step in pancreatic cancer initiation. [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Innovations in Research and Treatment; May 18-21, 2014; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2015;75(13 Suppl):Abstract nr PR01.

Cooperative transcriptional regulation of the essential pancreatic islet gene NeuroD1 (beta2) by Nkx2.2 and neurogenin 3

Nkx2.2 and NeuroD1 are two critical regulators of pancreatic beta cell development. Nkx2.2 is a homeodomain transcription factor that is essential for islet cell type specification and mature beta cell function. NeuroD1 is a basic helix-loop-helix transcription factor that is critical for islet beta cell maturation and maintenance. Although both proteins influence beta cell development directly downstream of the endocrine progenitor factor, neurogenin3 (Ngn3), a connection between the two proteins in the regulation of beta cell fate and function has yet to be established. In this study, we demonstrate that Nkx2.2 transcriptional activity is required to facilitate the activation of NeuroD1 by Ngn3. Furthermore, Nkx2.2 is necessary to maintain high levels of NeuroD1 expression in developing mouse and zebrafish islets and in mature beta cells. Interestingly, Nkx2.2 regulates NeuroD1 through two independent promoter elements, one that is bound and activated directly by Nkx2.2 and one that appears to be regulated by Nkx2.2 through an indirect mechanism. Together, these findings suggest that Nkx2.2 coordinately activates NeuroD1 with Ngn3 within the endocrine progenitor cell and also plays a role in the maintenance of NeuroD1 expression to regulate beta cell function in the mature islet. Collectively, these findings further define the conserved regulatory networks involved in islet beta cell formation and function.

Nodal signaling and the evolution of deuterostome gastrulation

Chordates, including vertebrates, evolved within a group of animals called the deuterostomes. All holoblastic deuterostomes gastrulate at the vegetal pole and the blastopore becomes the anus, while a mouth is formed at the anterior or to the oral side. Nodal is a member of the TGF-beta superfamily of signaling molecules that are important in signaling between cells during many embryonic processes in vertebrate embryos. Nodal has also been found in other invertebrate deuterostomes, such as ascidians and sea urchins, but, so far, is missing in protostomes. Nodal has been shown to be particularly important in determining left-right asymmetries in vertebrate embryos, but less information is available for its developmental role in the invertebrate deuterostomes. We review gastrulation in the deuterostomes, then examine nodal expression early during mesoderm formation and later during the establishment of asymmetries in both vertebrates and invertebrates. Nodal is expressed asymmetrically on the left side in chordates and on the presumptive oral side of the embryo in echinoid echinoderms. The expression of nodal is in different germ layers in embryos of different phyla. Expression is in the ectoderm in most of the invertebrate deuterostomes, and in the mesoderm in vertebrates. We summarize the work that has been published to date, especially nodal expression in the invertebrate deuterostomes, and suggest future experiments to better understand the evolution of nodal signaling and deuterostome gastrulation.

Pancreatic cancer in mice and man: the Penn Workshop 2004

A three-day conference cosponsored by the National Cancer Institute Mouse Models of Human Cancer Consortium and the Abramson Cancer Center of the University of Pennsylvania was convened on December 1, 2004, in Philadelphia, Pennsylvania. The purpose of the conference was to compare the histopathologic changes in murine models of exocrine pancreatic cancer to human disease and to discuss potential preclinical applications of these models. The participants of this international meeting included over 100 physicians and scientists with expertise in pancreatic cancer pathology, therapy, detection, and biology, and they were organized accordingly into working groups. The format of the meeting was a series of short presentations by individual participants followed by working group breakout sessions. The working groups presented their reports on the final day of the conference, and highlights of selected individual presentations and working group recommendations are summarized here and in an accompanying pathology consensus report.

Evidence for repatterning of the gastric fundic epithelium associated with Ménétrier's disease and TGFalpha overexpression

BACKGROUND & AIMS

Increase of intramucosal transforming growth factor alpha (TGFalpha) levels in the gastric fundus leads to oxyntic atrophy and massive foveolar hyperplasia in both metallothionein (MT)-TGFalpha mice and patients with Ménétrier's disease. We have evaluated the hypothesis that increased levels of TGFalpha in the fundus induces an antral pattern of cell differentiation in fundic glands by studying Pdx1, a transcription factor whose expression normally is confined to the gastric antrum.

METHODS

Induction of Pdx1 expression was evaluated in Pdx1(lacZ/+)/MT-TGFalpha bigenic mice treated with zinc. The distribution of Pdx1 in MT-TGFalpha mice and Ménétrier's disease patients was evaluated with anti-Pdx1 antibodies. Transcript levels were evaluated by quantitative polymerase chain reaction in mouse and human tissues and AGS cells.

RESULTS

In Pdx1(lacZ/+) mice, Pdx1 was expressed in antral mucosal cells including gastrin cells and TFF2-expressing deep glandular mucous cells. Zinc treatment for 2 to 8 weeks in Pdx1(lacZ/+)/MT-TGFalpha transgenic mice resulted in expression of Pdx1 throughout the fundus. No ectopic fundic Pdx1 expression was observed in either H. felis-infected or DMP777-treated mice. In MT-TGFalpha mice, 8 weeks of zinc treatment elicited nuclear Pdx1 staining throughout the fundic mucosa. TGFalpha treatment in AGS cells led to increases in Pdx1 and gastrin messenger RNA expression. Fundic sections from Ménétrier's disease patients showed nuclear Pdx1 staining throughout the fundic glands. Treatment of a Ménétrier's disease patient with an anti-epidermal growth factor receptor monoclonal antibody reduced fundic expression of both Pdx1 and gastrin.

CONCLUSIONS

Overexpression of TGFalpha in MT-TGFalpha mice and Ménétrier's disease patients elicits ectopic expression in the fundus of Pdx1, consistent with the phenotype of antralization.

Cell-specific expression of glucose-dependent-insulinotropic polypeptide is regulated by the transcription factor PDX-1

Glucose-dependent insulinotropic polypeptide (GIP) is a potent stimulator of insulin secretion and comprises an important component of the enteroinsular axis. GIP is synthesized in enteroendocrine K-cells located principally in the upper small intestine. The homeobox-containing gene PDX-1 is also expressed in the small intestine and plays a critical role in pancreatic development and in the expression of pancreatic-specific genes. Previous studies determined that the transcription factors GATA-4 and ISL-1 are important for GIP expression. In this study, we demonstrate that PDX-1 is also involved in regulating GIP expression in K-cells. Using immunohistochemistry, we verified the expression of PDX-1 protein in the nucleus of GIP-expressing mouse K-cells and evaluated the expression of PDX-1, serotonin, and GIP in wild-type and PDX-1(-/-) mice at 18.5 d after conception. Although we demonstrated a 97.8% reduction in the number of GIP-expressing cells in PDX-1(-/-) mice; there was no statistical difference in the number of serotonin-positive cells. Additionally, PDX-1 transcripts and protein were detected in a GIP-expressing neuroendocrine cell line, STC-1. Electromobility shift assays using STC-1 nuclear extracts demonstrated the specific binding of PDX-1 protein to a specific regulatory region in the GIP promoter. Using chromatin immunoprecipitation analysis, we demonstrated binding of PDX-1 to this same region of the GIP promoter in intact cells. Lastly, overexpression of PDX-1 in transient transfection assays led to a specific increase in the activity of GIP/Luc reporter constructs. The results of these studies indicate that the transcription factor PDX-1 plays a critical role in the cell-specific expression of the GIP gene.

Regulatory regions driving developmental and tissue-specific expression of the essential pancreatic gene pdx1

pdx1 (pancreatic and duodenal homeobox gene-1), which is expressed broadly in the embryonic pancreas and, later, in a more restricted manner in the mature beta cells in the islets of Langerhans, is essential both for organ formation and beta cell gene expression and function. We carried out a transgenic reporter gene analysis to identify region- and cell type-specific regulatory regions in pdx1. A 14.5-kb pdx1 genomic fragment corrected the glucose intolerance of pdx1(+/-) animals but, moreover, fully rescued the severe gut and pancreas defects in pdx1(-/-) embryos. Sequences sufficient to direct reporter expression to the entire endogenous pdx1 expression domain lie within 4.3 kb of 5' flanking DNA. In this region, we identified two distinct fragments that drive reporter gene expression to different sets of islet neuroendocrine cells. One shows pan-endocrine cell specificity, the other is selectively activated in insulin-producing beta cells. The endocrine-specific regulatory regions overlap a localized region of 5' flanking DNA that is remarkably conserved in sequence between vertebrate pdx1 genes, and which has been associated with beta cell-selective expression in cultured cell lines. This region contains potential binding sites for several transcription factors implicated in endodermal development and the pathogenesis of some forms of type-2 diabetes. These results are consistent with our previous proposal that conserved upstream pdx1 sequences exert control over pdx1 during embryonic organogenesis and islet endocrine cell differentiation. We propose that mutations affecting the expression and/or activity of transcription factors operating via these sequences may predispose towards diabetes, at least in part by direct effects on endocrine pdx1 expression.

Wnt8 is required in lateral mesendodermal precursors for neural posteriorization in vivo

The dorsal ectoderm of the vertebrate gastrula was proposed by Nieuwkoop to be specified towards an anterior neural fate by an activation signal, with its subsequent regionalization along the anteroposterior (AP) axis regulated by a graded transforming activity, leading to a properly patterned forebrain, midbrain, hindbrain and spinal cord. The activation phase involves inhibition of BMP signals by dorsal antagonists, but the later caudalization process is much more poorly characterized. Explant and overexpression studies in chick, Xenopus, mouse and zebrafish implicate lateral/paraxial mesoderm in supplying the transforming influence, which is largely speculated to be a Wnt family member.

We have analyzed the requirement for the specific ventrolaterally expressed Wnt8 ligand in the posteriorization of neural tissue in zebrafish wild-type and Nodal-deficient embryos (Antivin overexpressing or cyclops;squint double mutants), which show extensive AP brain patterning in the absence of dorsal mesoderm. In different genetic situations that vary the extent of mesodermal precursor formation, the presence of lateral wnt8-expressing cells correlates with the establishment of AP brain pattern. Cell tracing experiments show that the neuroectoderm of Nodal-deficient embryos undergoes a rapid anterior-to-posterior transformation in vivo during a short period at the end of the gastrula stage. Moreover, in both wild-type and Nodal-deficient embryos, inactivation of Wnt8 function by morpholino (MOwnt8) translational interference dose-dependently abrogates formation of spinal cord and posterior brain fates, without blocking ventrolateral mesoderm formation. MOwnt8 also suppresses the forebrain deficiency in bozozok mutants, in which inactivation of a homeobox gene causes ectopic wnt8 expression. In addition, the bozozok forebrain reduction is suppressed in bozozok;squint;cyclops triple mutants, and is associated with reduced wnt8 expression, as seen in cyclops;squint mutants. Hence, whereas boz and Nodal signaling largely cooperate in gastrula organizer formation, they have opposing roles in regulating wnt8 expression and forebrain specification. Our findings provide strong support for a model of neural transformation in which a planar gastrula-stage Wnt8 signal, promoted by Nodal signaling and dorsally limited by Bozozok, acts on anterior neuroectoderm from the lateral mesoderm to produce the AP regional patterning of the CNS.

Mechanisms of left-right asymmetry: what's right and what's left?

A recent meeting at the Juan March Foundation in Madrid, Spain, covered current understanding of the pathways and mechanisms involved in generating left-right asymmetry.

Head and trunk in zebrafish arise via coinhibition of BMP signaling by bozozok and chordino

Spatial variations in the levels of bone morphogenetic protein (BMP) signaling are a critical determinant of dorsoanterior-ventroposterior pattern in vertebrate embryos. Whereas BMP overexpression abolishes both head and trunk development, known single and double loss-of-function mutations in BMP inhibitors have less dramatic effects. We report that combining mutations in the zebrafish genes bozozok and chordino causes a synergistic loss of head and trunk, whereas most cells express ventro-posterior markers and develop into a tail. Genetic inactivation of BMP signaling fully suppresses these defects. Thus, a remarkably simple genetic mechanism, involving a coinhibition of BMP function by the partially overlapping bozozok and chordino pathways is used to specify vertebrate head and trunk.

Persistent expression of HNF6 in islet endocrine cells causes disrupted islet architecture and loss of beta cell function

We used transgenesis to explore the requirement for downregulation of hepatocyte nuclear factor 6 (HNF6) expression in the assembly, differentiation, and function of pancreatic islets. In vivo, HNF6 expression becomes downregulated in pancreatic endocrine cells at 18. 5 days post coitum (d.p.c.), when definitive islets first begin to organize. We used an islet-specific regulatory element (pdx1(PB)) from pancreatic/duodenal homeobox (pdx1) gene to maintain HNF6 expression in endocrine cells beyond 18.5 d.p.c. Transgenic animals were diabetic. HNF6-overexpressing islets were hyperplastic and remained very close to the pancreatic ducts. Strikingly, alpha, delta, and PP cells were increased in number and abnormally intermingled with islet beta cells. Although several mature beta cell markers were expressed in beta cells of transgenic islets, the glucose transporter GLUT2 was absent or severely reduced. As glucose uptake/metabolism is essential for insulin secretion, decreased GLUT2 may contribute to the etiology of diabetes in pdx1(PB)-HNF6 transgenics. Concordantly, blood insulin was not raised by glucose challenge, suggesting profound beta cell dysfunction. Thus, we have shown that HNF6 downregulation during islet ontogeny is critical to normal pancreas formation and function: continued expression impairs the clustering of endocrine cells and their separation from the ductal epithelium, disrupts the spatial organization of endocrine cell types within the islet, and severely compromises beta cell physiology, leading to overt diabetes.

Activin/nodal responsiveness and asymmetric expression of a Xenopus nodal-related gene converge on a FAST-regulated module in intron 1

Vertebrate Nodal-related factors play central roles in mesendoderm induction and left-right axis specification, but the mechanisms regulating their expression are largely unknown. We identify an element in Xnr1 intron 1 that is activated by activin and Vg1, autoactivated by Xnrs, and suppressed by ventral inducers like BMP4. Intron 1 contains three FAST binding sites on which FAST/Smad transcriptional complexes can assemble; these sites are differentially involved in intron 1-mediated reporter gene expression. Interference with FAST function abolishes intron 1 activity, and transcriptional activation of Xnrs by activin in embryonic tissue explant assays, identifying FAST as an essential mediator of Xnr autoregulation and/or ‘signal relay’ from activin-like molecules. Furthermore, the mapping of endogenous activators of the Xnr1 intronic enhancer within Xenopus embryos agrees well with the pattern of Xnr1 transcription during embryogenesis. In transgenic mice, Xnr1 intron 1 mimics a similarly located enhancer in the mouse nodal gene, and directs FAST site-dependent expression in the primitive streak during gastrulation, and unilateral expression during early somitogenesis. The FAST cassette is similar in an ascidian nodal-related gene, suggesting an ancient origin for this regulatory module. Thus, an evolutionarily conserved intronic enhancer in Xnr1 is involved in both mesendoderm induction and asymmetric expression during left-right axis formation.

The lefty-related factor Xatv acts as a feedback inhibitor of nodal signaling in mesoderm induction and L-R axis development in xenopus

In mouse, lefty genes play critical roles in the left-right (L-R) axis determination pathway. Here, we characterize the Xenopus lefty-related factor antivin (Xatv). Xatv expression is first observed in the marginal zone early during gastrulation, later becoming restricted to axial tissues. During tailbud stages, axial expression resolves to the neural tube floorplate, hypochord, and (transiently) the notochord anlage, and is joined by dynamic expression in the left lateral plate mesoderm (LPM) and left dorsal endoderm. An emerging paradigm in embryonic patterning is that secreted antagonists regulate the activity of intercellular signaling factors, thereby modulating cell fate specification. Xatv expression is rapidly induced by dorsoanterior-type mesoderm inducers such as activin or Xnr2. Xatv is not an inducer itself, but antagonizes both Xnr2 and activin. Together with its expression pattern, this suggests that Xatv functions during gastrulation in a negative feedback loop with Xnrs to affect the amount and/or character of mesoderm induced. Our data also provide insights into the way that lefty/nodal signals interact in the initiation of differential L-R morphogenesis. Right-sided misexpression of Xnr1 (endogenously expressed in the left LPM) induces bilateral Xatv expression. Left-sided Xatv overexpression suppresses Xnr1/XPitx2 expression in the left LPM, and leads to severely disturbed visceral asymmetry, suggesting that active ‘left’ signals are critical for L-R axis determination in frog embryos. We propose that the induction of lefty/Xatv in the left LPM by nodal/Xnr1 provides an efficient self-regulating mechanism to downregulate nodal/Xnr1 expression and ensure a transient ‘left’ signal within the embryo.

Pancreatic beta cell-specific transcription of the pdx-1 gene. The role of conserved upstream control regions and their hepatic nuclear factor 3beta sites

To identify potential transactivators of pdx-1, we sequenced approximately 4.5 kilobases of the 5' promoter region of the human and chicken homologs, assuming that sequences conserved with the mouse gene would contain critical cis-regulatory elements. The sequences associated with hypersensitive site 1 (HSS1) represented the principal area of homology within which three conserved subdomains were apparent: area I (-2694 to -2561 base pairs (bp)), area II (-2139 to -1958 bp), and area III (-1879 to -1799 bp). The identities between the mouse and chicken/human genes are very high, ranging from 78 to 89%, although only areas I and III are present within this region in chicken. Pancreatic beta cell-selective expression was shown to be controlled by mouse and human area I or area II, but not area III, from an analysis of pdx-1-driven reporter activity in transfected beta- and non-beta cells. Mutational and functional analyses of conserved hepatic nuclear factor 3 (HNF3)-like sites located within area I and area II demonstrated that activation by these regions was mediated by HNF3beta. To determine if a similar regulatory relationship might exist within the context of the endogenous gene, pdx-1 expression was measured in embryonic stem cells in which one or both alleles of HNF3beta were inactivated. pdx-1 mRNA levels induced upon differentiation to embryoid bodies were down-regulated in homozygous null HNF3beta cells. Together, these results suggest that the conserved sequences represented by areas I and II define the binding sites for factors such as HNF3beta, which control islet beta cell-selective expression of the pdx-1 gene.

Activin- and Nodal-related factors control antero-posterior patterning of the zebrafish embryo

Definition of cell fates along the dorso-ventral axis depends on an antagonistic relationship between ventralizing transforming growth factor-beta superfamily members, the bone morphogenetic proteins and factors secreted from the dorsal organizer, such as Noggin and Chordin. The extracellular binding of the last group to the bone morphogenetic proteins prevents them from activating their receptors, and the relative ventralizer:antagonist ratio is thought to specify different dorso-ventral cell fates. Here, by taking advantage of a non-genetic interference method using a specific competitive inhibitor, the Lefty-related gene product Antivin, we provide evidence that cell fate along the antero-posterior axis of the zebrafish embryo is controlled by the morphogenetic activity of another transforming growth factor-beta superfamily subgroup--the Activin and Nodal-related factors. Increasing antivin doses progressively deleted posterior fates within the ectoderm, eventually resulting in the removal of all fates except forebrain and eyes. In contrast, overexpression of activin or nodal-related factors converted ectoderm that was fated to be forebrain into more posterior ectodermal or mesendodermal fates. We propose that modulation of intercellular signalling by Antivin/Activin and Nodal-related factors provides a mechanism for the graded establishment of cell fates along the antero-posterior axis of the zebrafish embryo.

Expansion of Pdx1-expressing pancreatic epithelium and islet neogenesis in transgenic mice overexpressing transforming growth factor alpha

BACKGROUND & AIMS

The progenitor cells responsible for transforming growth factor (TGF)-alpha-induced pancreatic ductal metaplasia and neoplasia remain uncharacterized. During pancreatic development, differentiated cell types arise from ductal progenitor cells expressing the Pdx1 homeodomain transcription factor. The aims of this study were, first, to evaluate the role of Pdx1-expressing stem cells in MT-TGFalpha transgenic mice, and second, to further characterize cell proliferation and differentiation in this model.

METHODS

To assess Pdx1 gene expression in normal and metaplastic epithelium, we performed in vivo reporter gene analysis using heterozygous Pdx1(lacZ/+) and bigenic Pdx1(lacZ/+)/MT-TGFalpha mice.

RESULTS

Pdx1(lacZ/+)/MT-TGFalpha bigenics showed up-regulated Pdx1 expression in premalignant metaplastic ductal epithelium. In addition to Pdx1 gene activation, TGF-alpha-induced metaplastic epithelium demonstrated a pluripotent differentiation capacity, as evidenced by focal expression of Pax6 and initiation of islet cell neogenesis. The majority of Pdx1-positive epithelial cells showed no expression of insulin, similar to the pattern observed during embryonic development.

CONCLUSIONS

Overexpression of TGF-alpha induces expansion of a Pdx1-expressing epithelium characterized by focal expression of Pax6 and initiation of islet neogenesis. These findings suggest that premalignant events induced by TGF-alpha in mouse pancreas may recapitulate a developmental program active during embryogenesis.

Xenopus nodal-related signaling is essential for mesendodermal patterning during early embryogenesis

Previously, we showed that Xenopus nodal-related factors (Xnrs) can act as mesoderm inducers, and that activin induces Xnr transcription, suggesting that Xnrs relay or maintain induction processes initiated by activin-like molecules. We used a dominant negative cleavage mutant Xnr2 (cmXnr2) to carry out loss-of-function experiments to explore the requirement for Xnr signaling in early amphibian embryogenesis, and the relationship between activin and Xnrs. cmXnr2 blocked mesoderm induction caused by Xnr, but not activin, RNA. In contrast, cmXnr2 did suppress mesoderm and endoderm induction by activin protein, while Xnr transcript induction was unaffected by cmXnr2, consistent with an interference with the function of Xnr peptides that were induced by activin protein treatment. The severe hyperdorsalization and gastrulation defects caused by Xnr2 in whole embryos were rescued by cmXnr2, establishing a specific antagonistic relationship between the normal and cleavage mutant proteins. Expression of cmXnr2 resulted in delayed dorsal lip formation and a range of anterior truncations that were associated with delayed and suppressed expression of markers for dorsoanterior endoderm, in which the recently recognized head organizer activity resides. Reciprocally, Xnr2 induced dorsoanterior endodermal markers, such as cerberus, Xhex-1 and Frzb, in animal cap ectoderm. The migratory behavior of head mesendoderm explanted from cmXnr2 RNA-injected embryos was drastically reduced. These results indicate that Xnrs play crucial roles in initiating gastrulation, probably by acting downstream of an activin-like signaling pathway that leads to dorsal mesendodermal specification, including setting up the head organizer.

Mox2 is a component of the genetic hierarchy controlling limb muscle development

The skeletal muscles of the limbs develop from myogenic progenitors that originate in the paraxial mesoderm and migrate into the limb-bud mesenchyme. Among the genes known to be important for muscle development in mammalian embryos are those encoding the basic helix-loop-helix (bHLH) myogenic regulatory factors (MRFs; MyoD, Myf5, myogenin and MRF4) and Pax3, a paired-type homeobox gene that is critical for the development of limb musculature. Mox1 and Mox2 are closely related homeobox genes that are expressed in overlapping patterns in the paraxial mesoderm and its derivatives. Here we show that mice homozygous for a null mutation of Mox2 have a developmental defect of the limb musculature, characterized by an overall reduction in muscle mass and elimination of specific muscles. Mox2 is not needed for the migration of myogenic precursors into the limb bud, but it is essential for normal appendicular muscle formation and for the normal regulation of myogenic genes, as demonstrated by the downregulation of Pax3 and Myf5 but not MyoD in Mox2-deficient limb buds. Our findings show that the MOX2 homeoprotein is an important regulator of vertebrate limb myogenesis.

Bmp4 is required for the generation of primordial germ cells in the mouse embryo

In many organisms the allocation of primordial germ cells (PGCs) is determined by the inheritance of maternal factors deposited in the egg. However, in mammals, inductive cell interactions are required around gastrulation to establish the germ line. Here, we show that Bmp4 homozygous null embryos contain no PGCs. They also lack an allantois, an extraembryonic mesodermal tissue derived, like the PGCs, from precursors in the proximal epiblast. Heterozygotes have fewer PGCs than normal, due to a reduction in the size of the founding population and not to an effect on its subsequent expansion. Analysis of beta-galactosidase activity in Bmp4(lacZneo) embryos reveals that prior to gastrulation, Bmp4 is expressed in the extraembryonic ectoderm. Later, Bmp4 is expressed in the extraembryonic mesoderm, but not in PGCs. Chimera analysis indicates that it is the Bmp4 expression in the extraembryonic ectoderm that regulates the formation of allantois and primordial germ cell precursors, and the size of the founding population of PGCs. The initiation of the germ line in the mouse therefore depends on a secreted signal from the previously segregated, extraembryonic, trophectoderm lineage.

Zebrafish nodal-related 2 encodes an early mesendodermal inducer signaling from the extraembryonic yolk syncytial layer

Nodal-related factors have been implicated in mesodermal and neural patterning, and left-right asymmetry, in mouse, frog, and chicken embryos. We describe the isolation and characterization of zebrafish nodal-related 2 (znr2). znr2 is expressed at low levels maternally, and zygotic transcripts localize to dorsal blastomeres at MBT. Slightly later, znr2 is also expressed dorsally in the extraembryonic yolk syncytial layer (YSL). During early gastrulation, znr2 expression expands to include deep and superficial cells in the entire marginal zone and YSL. During shield stages, expression is primarily localized to superficial noninvoluting cells of the organizer called dorsal forerunners. Znr2 misexpression in whole fish embryos expands or duplicates dorsoanterior and axial cell fates. Furthermore, Znr2 overexpression exclusively in the YSL, a region implicated in endogenous mesodermal induction, causes broadened or duplicated gsc expression in the overlying blastoderm. Functional comparison of Znr2 and another recently identified zebrafish nodal-related factor, Znr1/Cyclops, reveals distinct inductive properties of each ligand. Znr2 efficiently induces organizer-type dorsoanterior mesodermal and endodermal markers, but only weakly, if at all, neural markers. In contrast, while Znr1/Cyclops reproducibly induces mesodermal and neural markers, it is an inefficient inducer of organizer-type mesoderm. Our results suggest that znr2 encodes a robust mesendodermal inducer that signals nonautonomously during the earliest stages of embryonic patterning, and that part of this activity arises from within the YSL.

Induction of the zebrafish ventral brain and floorplate requires cyclops/nodal signalling

Zebrafish cyclops (cyc) mutations cause deficiencies in the dorsal mesendoderm and ventral neural tube, leading to neural defects and cyclopia. Here we report that cyc encodes a transforming growth factor-beta (TGF-beta)-related intercellular signalling molecule that is similar to mouse nodal. cyc is expressed in dorsal mesendoderm at gastrulation and in the prechordal plate until early somitogenesis. Expression reappears transiently in the left lateral-plate mesoderm, and in an unprecedented asymmetric pattern in the left forebrain. Injection of cyc RNA non-autonomously restores sonic hedgehog-expressing cells of the ventral brain and floorplate that are absent in cyc mutants, whereas inducing activities are abolished by cyc, a mutation of a conserved cysteine in the mature ligand. Our results indicate that cyc provides an essential non-cell-autonomous signal at gastrulation, leading to induction of the floorplate and ventral brain.

An endocrine-exocrine switch in the activity of the pancreatic homeodomain protein PDX1 through formation of a trimeric complex with PBX1b and MRG1 (MEIS2)

HOX proteins and some orphan homeodomain proteins form complexes with either PBX or MEIS subclasses of homeodomain proteins. This interaction can increase the binding specificity and transcriptional effectiveness of the HOX partner. Here we show that specific members of both PBX and MEIS subclasses form a multimeric complex with the pancreatic homeodomain protein PDX1 and switch the nature of its transcriptional activity. The two activities of PDX1 are exhibited through the 10-bp B element of the transcriptional enhancer of the pancreatic elastase I gene (ELA1). In pancreatic acinar cells the activity of the B element requires other elements of the ELA1 enhancer; in beta-cells the B element can activate a promoter in the absence of other enhancer elements. In acinar cell lines the activity is mediated by a complex comprising PDX1, PBX1b, and MRG1 (MEIS2). In contrast, beta-cell lines are devoid of PBX1b and MRG1, so that a trimeric complex does not form, and the beta-cell-type activity is mediated by PDX1 without PBX1b and MRG1. The presence of specific nuclear isoforms of PBX and MEIS is precisely regulated in a cell-type-specific manner. The beta-cell-type activity can be detected in acinar cells if the B element is altered to retain binding of PDX1 but prevent binding of the PDX1-PBX1b-MRG1 complex. These observations suggest that association with PBX and MEIS partners controls the nature of the transcriptional activity of the organ-specific PDX1 transcription factor in exocrine versus endocrine cells.

Targeted oncogenesis of hormone-negative pancreatic islet progenitor cells

Transgenic mice containing an upstream glucokinase (betaGK) promoter- simian virus 40 T antigen (Tag) fusion gene develop neuroendocrine tumors primarily in the pancreas, gut, and pituitary. Pancreatic tumors from a line with delayed tumorigenesis were of two different types: insulinomas and noninsulinomas. The noninsulinomas are often periductal in location, express none of the four major islet peptide hormones, Glut-2, Pdx1, tyrosine hydroxylase, Pax4, Pax6, or Nkx6.1, but do express glucokinase, Sur1, Isl1, Hnf3beta, Hnf6, Beta2/NeuroD, and Nkx2.2. Cells from two different noninsulinoma tumors, when adapted to culture, began to express either insulin, glucagon, or somatostatin. Given the partial gene expression repertoire of the noninsulinoma tumors, their apparent periductal origin, and the ability of these cells to partially cytodifferentiate in culture, we suggest that these tumors are derived from islet progenitor cells. Thus, betaGK-Tag transgenic mice provide a new model system for studying the events that occur during both islet cell neogenesis and normal embryonic development.

XBMPRII, a novel Xenopus type II receptor mediating BMP signaling in embryonic tissues

Bone Morphogenetic Proteins (BMPs) are potent regulators of embryonic cell fate that are presumed to initiate signal transduction in recipient cells through multimeric, transmembrane, serine/threonine kinase complexes made up of type I and type II receptors. BMPRII was identified previously in mammals as the only type II receptor that binds BMPs, but not activin or TGFbeta, in vitro. We report the cloning and functional analysis in vivo of its Xenopus homolog, XBMPRII. XBMPRII is expressed maternally and zygotically in an initially unrestricted manner. Strikingly, XBMPRII transcripts then become restricted to the mesodermal precursors during gastrulation. Expression is lower in the dorsal organizer region, potentially providing a mechanism to suppress the actions of BMP4 on dorsally fated tissues. Similar to the results seen for a truncated type I BMP receptor (tBR), a dominant-negative form of XBMPRII (tBRII) can dorsalize ventral mesoderm, induce extensive secondary body axes, block mesoderm induction by BMP4 and directly neuralize ectoderm, strongly suggesting that XBMPRII mediates BMP signals in vivo. However, although both tBRII and tBR can induce partial secondary axes, marker analysis shows that tBRII-induced axes are more anteriorly extended. Additionally, coinjection of tBRII and tBR synergistically increases the incidence of secondary axis formation. A truncated activin type II receptor (deltaXAR1) is known to block both activin and BMP signaling in vivo. Here we show that such crossreactivity does not occur for tBRII, in that it does not affect activin signaling. Furthermore, our studies indicate that the full-length activin type II receptor (XAR1) overcomes a block in BMP4 signaling imposed by tBRII, implicating XAR1 as a common component of BMP and activin signaling pathways in vivo. These data implicate XBMPRII as a type II receptor with high selectivity for BMP signaling, and therefore as a critical mediator of the effects of BMPs as mesodermal patterning agents and suppressors of neural fate during embryogenesis.

Hepatocyte nuclear factor 3beta is involved in pancreatic beta-cell-specific transcription of the pdx-1 gene

The mammalian homeobox gene pdx-1 is expressed in pluripotent precursor cells in the dorsal and ventral pancreatic bud and duodenal endoderm, which will produce the pancreas and the rostral duodenum. In the adult, pdr-1 is expressed principally within insulin-secreting pancreatic islet beta cells and cells of the duodenal epithelium. Our objective in this study was to localize sequences within the mouse pdx-1 gene mediating selective expression within the islet. Studies of transgenic mice in which a genomic fragment of the mouse pdx-1 gene from kb -4.5 to +8.2 was used to drive a beta-galactosidase reporter showed that the control sequences sufficient for appropriate developmental and adult specific expression were contained within this region. Three nuclease-hypersensitive sites, located between bp -2560 and -1880 (site 1), bp -1330 and -800 (site 2), and bp -260 and +180 (site 3), were identified within the 5'-flanking region of the endogenous pdx-1 gene. Pancreatic beta-cell-specific expression was shown to be controlled by sequences within site 1 from an analysis of the expression pattern of various pdr-1-herpes simplex virus thymidine kinase promoter expression constructs in transfected beta-cell and non-beta-cell lines. Furthermore, we also established that this region was important in vivo by demonstrating that expression from a site 1-driven beta-galactosidase reporter construct was directed to islet beta-cells in transgenic mice. The activity of the site 1-driven constructs was reduced substantially in beta-cell lines by mutating a hepatocyte nuclear factor 3 (HNF3)-like site located between nucleotides -2007 and -1996. Gel shift analysis indicated that HNF3beta present in islet beta cells binds to this element. Immunohistochemical studies revealed that HNF3beta was present within the nuclei of almost all islet beta cells and subsets of pancreatic acinar cells. Together, these results suggest that HNF3beta, a key regulator of endodermal cell lineage development, plays an essential role in the cell-type-specific transcription of the pdx-1 gene in the pancreas.

Functional differences among Xenopus nodal-related genes in left-right axis determination

An association has been noted previously in chick, mouse and frog embryos between asymmetric nodal-related gene expression and embryonic situs, implying an evolutionarily conserved role in left-right specification. Of the four Xenopus nodal-related genes expressed during gastrulation, only Xnr-1 is re-expressed unilaterally in the left lateral plate mesoderm at neurula/tailbud stages. Here, we show that the asymmetric expression of Xnr-1 can be made bilaterally symmetric by right-sided microinjection of RNA encoding active Xenopus hedgehog proteins. Moreover, we provide the first evidence that Xnr-1 expression per se is a causal factor in left-right axis determination. When plasmids expressing Xnr-1 were delivered unilaterally to the right side of Xenopus embryos, a reversed laterality of both the heart and gut (homotaxic reversal) was induced in 40% of surviving embryos, while an additional 10–20% showed reversal of the heart or gut alone (heterotaxia). This effect on laterality was specific to Xnr-1, since neither Xnr-2 nor Xnr-3 plasmids had this activity. In addition, we find that Xnr-1 and Xnr-2, which have both been defined as mesoderm inducers from overexpression studies, show quantitative differences in their ability to induce dorsal mesoderm. Together, these findings suggest that the various Xnrs perform substantially different functions during Xenopus embryogenesis. Moreover, they strongly support the hypothesis that left lateral plate expression of nodal-related genes is a causative factor in the determination of asymmetry in vertebrate embryos.

Functional characterization of the transactivation properties of the PDX-1 homeodomain protein

Pancreas formation is prevented in mice carrying a null mutation in the PDX-1 homeoprotein, demonstrating a key role for this factor in development. PDX-1 can also bind to and activate transcription from cis-acting regulatory sequences in the insulin and somatostatin genes, which are expressed in pancreatic islet beta and delta cells, respectively. In this study, we compared the functional properties of PDX-1 with those of the closely related Xenopus homeoprotein XIHbox8. Analysis of chimeras between PDX-1, XIHbox8, and the DNA-binding domain of the Saccharomyces cerevisiae transcription factor GAL4 revealed that their transactivation domain was contained within the N-terminal region (amino acids 1 to 79). Detailed mutagenesis of this region indicated that transactivation is mediated by three highly conserved sequences, spanning amino acids 13 to 22 (subdomain A), 32 to 38 (subdomain B), and 60 to 73 (subdomain C). These sequences were also required by PDX-1 to synergistically activate insulin enhancer-mediated transcription with another key insulin gene activator, the E2A-encoded basic helix-loop-helix E2-5 and E47 proteins. These results indicated that N-terminal sequences conserved between the mammalian PDX-1 and Xenopus XIHbox8 proteins are important in transcriptional activation. Stable expression of the PDX-1 deltaABC mutant in the insulin- and PDX-1-expressing betaTC3 cell line resulted in a threefold reduction in the rate of endogenous insulin gene transcription. Strikingly, the level of the endogenous PDX-1 protein was reduced to very low levels in these cells. These results suggest that PDX-1 is not absolutely essential for insulin gene expression in betaTC3 cells. We discuss the possible significance of these findings for insulin gene transcription in islet beta cells.

Differentiation of new insulin-producing cells is induced by injury in adult pancreatic islets

The ability of the adult pancreas to generate new insulin (beta) cells has been controversial because of difficulties in unequivocally identifying the precursor population. We recently determined that beta cells were generated during development from precursors that expressed the homeodomain-containing transcription factor pancreas duodenum homeobox gene-1 (PDX-1). To investigate whether PDX-1+ stem cells are present in adult pancreas, we examined two animal models of diabetes. One model was produced by injecting adult mice with streptozotocin (SZ), a toxin that produces hyperglycemia due to rapid and massive beta cell death. After SZ-mediated elimination of existing IN+/PDX-1+ cells, a population of somatostatin (SOM)+/PDX-1+ cells, a cell type thought to represent an embryonic islet precursor cell, appeared in islets. The appearance of SOM+/PDX-1+ cells was followed in time by the differentiation to SOM+/IN+/PDX-1+ cells. SOM+/PDX-1+ cells also appeared in islets of nonobese diabetic mice, a strain of mice in which beta cell destruction is immune-mediated. Our findings establish the existence of PDX-1+ beta cell precursors in the adult pancreas and indicate that their differentiation is induced by islet injury.

Differential localization of Mox-1 and Mox-2 proteins indicates distinct roles during development

Transcript localizations for Mox genes have implicated this homeobox gene subfamily in the early steps of mesoderm formation. We have extended these studies by determining the protein expression profile of Mox-1 and Mox-2 during mouse development. The time of onset of Mox protein expression has been accurately obtained to provide clues as to their roles during gastrulation. Expression of Mox-1 protein is first detected in the newly formed mesoderm of primitive streak stage mouse embryos (7.5 days post-coitum, d.p.c.). In contrast, Mox-2 protein is first detected at 9.0 d.p.c. in thr already formed somites. Additionally, immunostaining reveals new and distinct areas of Mox expression in the branchial arches and limbs that were not reported in our previous mRNA localization analysis. Mouse Mox-2 antibodies cross-react specifically in similar embryonic tissues in chick indicating the conservation of function of Mox genes in vertebrates. These expression data suggest that the Mox genes function transiently in the formation of mesodermal and mesenchymal derivatives, after their initial specification, but before their overt differentiation. Furthermore, while there appears to be some overlap in protein expression between Mox-1 and Mox-2 during somitogenesis, unique areas of expression indicate several distinct roles for the Mox genes during development.

Conserved left-right asymmetry of nodal expression and alterations in murine situs inversus

Vertebrates have characteristic and conserved left-right (L-R) visceral asymmetries, for example the left-sided heart. In humans, alterations of L-R development can have serious clinical implications, including cardiac defects. Although little is known about how the embryonic L-R axis is established, a recent study in the chick embryo revealed L-R asymmetric expression of several previously cloned genes, including Cnr-1 (for chicken nodal-related-1), and indicated how this L-R molecular asymmetry might be important for subsequent visceral morphogenesis. Here we show that nodal is asymmetrically expressed in mice at similar stages, as is Xnr-1 (for Xenopus nodal related-1) in frogs. We also examine nodal expression in two mouse mutations that perturb L-R development, namely situs inversus viscerum (iv), in which assignment of L-R asymmetry is apparently random and individuals develop either normally or are mirror-image-reversed (situs inversus), and inversion of embryonic turning (inv), in which all individuals develop with situs inversus. In both, nodal expression is strikingly affected, being reversed or converted to symmetry. These results further support a key role for nodal and nodal-related genes in interpreting and relaying L-R patterning information in vertebrates. To our knowledge, our results provide the first direct evidence that iv and inv normally function well before the appearance of morphological L-R asymmetry.

Bone morphogenetic protein-4 (BMP-4) acts during gastrula stages to cause ventralization of Xenopus embryos

Injection of RNA encoding BMP-4 into the early Xenopus embryo suppresses formation of dorsal and anterior cell types. To understand this phenomenon, it is necessary to know the stage at which BMP-4 acts. In this paper, we present three lines of evidence showing that BMP-4 misexpression has no effect on the initial steps of mesoderm induction, either dorsal or ventral, but instead causes ventralization during gastrulation. Firstly, activation of organizer-specific genes such as goosecoid, Xnot, pintallavis and noggin occurs normally in embryos injected with BMP-4 RNA, but transcript levels are then rapidly down-regulated as gastrulation proceeds. Similarly, BMP-4 does not affect the initial activation of goosecoid by activin in animal caps, but expression then declines precipitously. Secondly, embryos made ventral by injection with BMP-4 RNA cannot be rescued by grafts of Spemann's organizer at gastrula stages. Such embryos therefore differ from those made ventral by UV-irradiation, where the defect occurs early and rescue can be effected by the organizer. Finally, the dorsalizing effects of the organizer, and of the candidate dorsalizing signal noggin, both of which exert their effects during gastrulation, can be counteracted by BMP-4. Together, these experiments demonstrate that BMP-4 can act during gastrulation both to promote ventral mesoderm differentiation and to attenuate dorsalizing signals derived from the organizer.

PDX-1 is required for pancreatic outgrowth and differentiation of the rostral duodenum

It has been proposed that the Xenopus homeobox gene, XlHbox8, is involved in endodermal differentiation during pancreatic and duodenal development (Wright, C.V.E., Schnegelsberg, P. and De Robertis, E.M. (1988). Development 105, 787–794). To test this hypothesis directly, gene targeting was used to make two different null mutations in the mouse XlHbox8 homolog, pdx-1. In the first, the second pdx-1 exon, including the homeobox, was replaced by a neomycin resistance cassette. In the second, a lacZ reporter was fused in-frame with the N terminus of PDX-1, replacing most of the homeodomain. Neonatal pdx-1 −/− mice are apancreatic, in confirmation of previous reports (Jonsson, J., Carlsson, L., Edlund, T. and Edlund, H. (1994). Nature 371, 606–609). However, the pancreatic buds do form in homozygous mutants, and the dorsal bud undergoes limited proliferation and outgrowth to form a small, irregularly branched, ductular tree. This outgrowth does not contain insulin or amylase-positive cells, but glucagon-expressing cells are found. The rostral duodenum shows a local absence of the normal columnar epithelial lining, villi, and Brunner's glands, which are replaced by a GLUT2-positive cuboidal epithelium resembling the bile duct lining. Just distal of the abnormal epithelium, the numbers of enteroendocrine cells in the villi are greatly reduced. The PDX-1/beta-galactosidase fusion allele is expressed in pancreatic and duodenal cells in the absence of functional PDX-1, with expression continuing into perinatal stages with similar boundaries and expression levels. These results offer additional insight into the role of pdx-1 in the determination and differentiation of the posterior foregut, particularly regarding the proliferation and differentiation of the pancreatic progenitors.

Nodal-related signals induce axial mesoderm and dorsalize mesoderm during gastrulation

Mouse embryos homozygous for a null mutation in nodal arrest development at early gastrulation and contain little or no embryonic mesoderm. Here, two Xenopus nodal-related genes (Xnr-1 and Xnr-2) are identified and shown to be expressed transiently during embryogenesis, first within the vegetal region of late blastulae and later in the marginal zone during gastrulation, with enrichment in the dorsal lip. Xnrs and mouse nodal function as dose-dependent dorsoanterior and ventral mesoderm inducers in whole embryos and explanted animal caps. Using a plasmid vector to produce Xnr proteins during gastrulation, we show that, in contrast to activin and other TGF beta-like molecules, Xnr-1 and Xnr-2 can dorsalize ventral marginal zone explants and induce muscle differentiation. Xnr signalling also rescues a complete embryonic axis in UV-ventralized embryos. The patterns of Xnr expression, the activities of the proteins and the phenotype of mouse nodal mutants, all argue strongly that a signaling pathway involving nodal, or nodal-related peptides, is an essential conserved element in mesoderm differentiation associated with vertebrate gastrulation and axial patterning.

Reduction of insulin gene transcription in HIT-T15 beta cells chronically exposed to a supraphysiologic glucose concentration is associated with loss of STF-1 transcription factor expression

Chronic exposure of HIT-T15 beta cells to elevated glucose concentrations leads to decreased insulin gene transcription. The reduction in expression is accompanied by diminished binding of a glucose-sensitive transcription factor (termed GSTF) that interacts with two (A+T)-rich elements within the 5' flanking control region of the insulin gene. In this study we examined whether GSTF corresponds to the recently cloned insulin gene transcription factor STF-1, a homeodomain protein whose expression is restricted to the nucleus of endodermal cells of the duodenum and pancreas. We found that an affinity-purified antibody recognizing STF-1 supershifted the GSTF activator complex formed from HIT-T15 extracts. In addition, we demonstrated a reduction in STF-1 mRNA and protein levels that closely correlated with the change in GSTF binding in HIT-T15 cells chronically cultured under supraphysiologic glucose concentrations. The reduction in STF-1 expression in these cells could be accounted for by a change in the rate of STF-1 gene transcription, suggesting a posttranscriptional control mechanism. In support of this hypothesis, no STF-1 mRNA accumulated in HIT-T15 cells passaged in 11.1 mM glucose. The only RNA species detected was a 6.4-kb STF-1 RNA species that hybridized with 5' and 3' STF-1-specific cDNA probes. We suggest that the 6.4-kb RNA represents an STF-1 mRNA precursor and that splicing of this RNA is defective in these cells. Overall, this study suggests that reduced expression of a key transcriptional regulatory factor, STF-1, contributes to the decrease in insulin gene transcription in HIT-T15 cells chronically cultured in supraphysiologic glucose concentration.

Autonomous endodermal determination in Xenopus: regulation of expression of the pancreatic gene XlHbox 8

In neural plate stage Xenopus embryos, XlHbox 8 expression marks anterior endodermal cells fated to develop into pancreas/duodenum, and expression continues in adult pancreas in exocrine duct, acinar, and islet cells. Here, XlHbox 8 is used as a marker in experiments addressing the mechanisms of early endodermal patterning, particularly with respect to the role of specific polypeptide growth factors. When mesoderm-free vegetal explants (VEs) from early blastula stage embryos are cultured in isolation, XlHbox 8 expression develops autonomously in the dorsal region, strongly suggesting that endodermal region-specific determination occurs before MBT. Data from microinjection experiments using RNA encoding the activin and FGF dominant negative receptors and growth factor treatments of isolated VEs suggest that activin positively regulates XlHbox 8 expression, whereas bFGF is a potent negative regulator. Moreover, bFGF induces mesodermal marker expression in VEs. This suggests that the early endodermal determination state is plastic and that elevated levels of bFGF may convert vegetal (endodermal) cells into mesoderm. We propose a model for XlHbox 8 regulation in which an early signal from the Nieuwkoop center (whose eventual fate is endoderm) predisposes dorsovegetal cells for autonomous XlHbox 8 expression, in an area of high local activin (or activin-like) ligand concentration, and low relative concentrations of bFGF.

Cloning of a Xenopus laevis cDNA encoding focal adhesion kinase (FAK) and expression during early development

Focal adhesion kinase (FAK) is a widely produced nonreceptor protein-tyrosine kinase thought to participate in signalling pathways activated in response to cell interaction with the extracellular matrix. Fibronectin-dependent cell adhesion mediated by integrin receptors plays a critical role in mesodermal cell migration during amphibian gastrulation in early development. As a first step toward understanding the role of FAK in Xenopus laevis (Xl) early development, we isolated cDNAs encoding Xl FAK and deduced the entire amino acid (aa) sequence. Xl FAK has 89-91% overall identity to the homologs previously described from mouse, human and chicken sources. Within the catalytic domain, the aa identity is about 97%. Northern blot analysis revealed that abundant maternal FAK transcript is present in Xl eggs, with levels decreasing slightly through cleavage and early blastula stages. At early gastrulation, the FAK mRNA level becomes modestly elevated, followed by a steady decline through late gastrulation. The mRNA level undergoes a further drop at the neurula stage, then begins a steady increase through the tailbud and tadpole stages. These data indicate that the steady-state level of FAK mRNA is regulated during Xl early development, and are consistent with a proposed role for FAK in the process of gastrulation.

The expression pattern of Xenopus Mox-2 implies a role in initial mesodermal differentiation

We have isolated a Xenopus homolog of the murine Mox-2 gene. As is the case for the mouse homolog, mesoderm specific expression of Xenopus Mox-2 (X. Mox-2) expression begins during gastrulation. Using whole mount in situ hybridization, we show that X. Mox-2 is expressed in undifferentiated dorsal, lateral and ventral mesoderm in the posterior of neurula/tailbud embryos, with expression more anteriorly detected in the dermatomes. In the tailbud tadpole, X. Mox-2 is expressed in tissues of the tailbud itself that represent a site of continued gastrulation-like processes resulting in mesoderm formation. X. Mox-2 is not expressed in the marginal zone of blastula, nor in the dorsal lip of gastrula, nor midline tissues (i.e. prospective notochord). Treatments that affect mesodermal patterning during embryonic development, including LiCl and ultraviolet light, and injection of mRNAs encoding BMP-4, or dominant negative activin and FGF receptors, produce changes in X. Mox-2 expression consistent with the types of tissues affected by these manipulations. X. Mox-2 expression is induced more in animal caps treated with FGF than those treated with activin. Together with the fact that X. Mox-2 activation in animal caps requires protein synthesis, our data suggest that X. Mox-2 is involved in initial mesodermal differentiation, downstream of molecules affecting mesoderm induction and determination such as Brachyury and goosecoid, and upstream of factors controlling terminal differentiation such as MyoD and myf5. X. Mox-2, therefore, is another useful marker for understanding the formation of mesoderm in amphibian development.