Formation of the murine endoderm: lessons from the mouse, frog, fish, and chick

Gallbladder microbiota in early vertebrates provides evolutionary insights into mucosal homeostasis

The gallbladder (GB) microbiota plays critical roles in mammalian metabolism and immune homeostasis, and its relationship with human disease has been extensively studied over the past decade. However, very little is known about the interplay between GB microbiota and the immune functions of teleost fish, the earliest bony vertebrate with a GB. Therefore, this study sought to investigate the composition of the teleost GB microbiota and the potential mechanisms through which it affects mucosal immunity. In our results, we found that the GB mucosa (GM) and bile bacterial community shared a similar microbiological composition with that of the gut mucosa in naïve individuals. IHNV infection induced a profound GB inflammation and disrupted their microbial homeostasis followed by a strong anti-bacterial response. Interestingly, beneficial bacteria from the Lactobacillales order showed a significant increase in the abundance of the bile microbial community, whereas the structure of the Mycoplasmatales order in the gut microbial community was markedly changed. All in all, our study characterized the structure of the GB microbial ecosystem in teleost fish, and the fish GB microbiome shared a high similarity with the gut microbiota. More importantly, our findings offer solid evidence that the teleost GB evolved immune functions to preserve its mucosal microbial homeostasis, suggesting that both the microbiota and mucosal immunity of the GB might have co-evolved in early vertebrates.

A Stem Cell Approach to Cure Type 1 Diabetes

Treatment of type 1 diabetes with insulin injection is expensive, complicated, and insufficient. While cadaveric islet transplantations coupled with immunosuppressants can cure diabetes, the scarcity of acceptable islets is problematic. Developmental research on pancreas formation has informed in vitro differentiation of human pluripotent stem cells into functional islets. Although generating β cells from stem cells offers a potential cure for type 1 diabetes, several challenges remain, including protecting the cells from the immune system.

The endoderm: a divergent cell lineage with many commonalities

The endoderm is a progenitor tissue that, in humans, gives rise to the majority of internal organs. Over the past few decades, genetic studies have identified many of the upstream signals specifying endoderm identity in different model systems, revealing them to be divergent from invertebrates to vertebrates. However, more recent studies of the cell behaviours driving endodermal morphogenesis have revealed a surprising number of shared features, including cells undergoing epithelial-to-mesenchymal transitions (EMTs), collective cell migration, and mesenchymal-to-epithelial transitions (METs). In this Review, we highlight how cross-organismal studies of endoderm morphogenesis provide a useful perspective that can move our understanding of this fascinating tissue forward.

The emergent landscape of the mouse gut endoderm at single-cell resolution

Here we delineate the ontogeny of the mammalian endoderm by generating 112,217 single-cell transcriptomes, which represent all endoderm populations within the mouse embryo until midgestation. We use graph-based approaches to model differentiating cells, which provides a spatio-temporal characterization of developmental trajectories and defines the transcriptional architecture that accompanies the emergence of the first (primitive or extra-embryonic) endodermal population and its sister pluripotent (embryonic) epiblast lineage. We uncover a relationship between descendants of these two lineages, in which epiblast cells differentiate into endoderm at two distinct time points-before and during gastrulation. Trajectories of endoderm cells were mapped as they acquired embryonic versus extra-embryonic fates and as they spatially converged within the nascent gut endoderm, which revealed these cells to be globally similar but retain aspects of their lineage history. We observed the regionalized identity of cells along the anterior-posterior axis of the emergent gut tube, which reflects their embryonic or extra-embryonic origin, and the coordinated patterning of these cells into organ-specific territories.

Functional and phenotypic distinction of the first two trophoblast subdivisions and identification of the border between them during early postimplantation: A prerequisite for understanding early patterning during placentogenesis

The early stages of mouse placentogenesis (placenta formation) involve poorly understood patterning events within polar trophectoderm-derived trophoblast, the progenitor of all placental trophoblast cell types. By early postimplantation [embryonic day 5.5 (E5.5)], this patterning causes early trophoblast to become subdivided into extraembryonic ectoderm (ExE) and ectoplacental cone (EPC). A prerequisite to understanding this patterning requires knowing the location of ExE-EPC border and being able to distinguish the entire ExE from EPC at E5.5/E6.5, a time when the proamnioitic cavity within ExE is not fully established. However, these issues are unknown, as they have not been directly addressed. Here, we directly addressed these using trophoblast explant culture to functionally test for the location of ExE-EPC border, combined with phenotypic characterization of trophoblast proximal and distal to it. We show for the first time that the proximal-distal level of ExE-EPC border within E5.5/E6.5 trophoblast coincides with where Reichert's membrane (outermost basement membrane of conceptus) inserts into early trophoblast and with the proximal limit of extraembryonic visceral endoderm (primitive endoderm derivative covering part of early trophoblast). Based on these novel findings, we discovered that (a) the entire E5.5/E6.5 ExE can be distinguished from EPC because it is epithelial and specifically expresses Erf and Claudin4 and (b) at E5.5/E6.5, the entire EPC differs from ExE in that it is not epithelial and specifically expresses Snail. This work is expected to contribute to understanding the cellular and molecular basis of early trophoblast patterning during placentogenesis.

Daphnia magna and Xenopus laevis as in vivo models to probe toxicity and uptake of quantum dots functionalized with gH625

The use of quantum dots (QDs) for nanomedicine is hampered by their potential toxicologic effects and difficulties with delivery into the cell interior. We accomplished an in vivo study exploiting Daphnia magna and Xenopus laevis to evaluate both toxicity and uptake of QDs coated with the membranotropic peptide gH625 derived from the glycoprotein H of herpes simplex virus and widely used for drug delivery studies. We evaluated and compared the effects of QDs and gH625-QDs on the survival, uptake, induction of several responsive pathways and genotoxicity in D. magna, and we found that QDs coating plays a key role. Moreover, studies on X. laevis embryos allowed to better understand their cell/tissue localization and delivery efficacy. X. laevis embryos raised in Frog Embryo Teratogenesis Assay-Xenopus containing QDs or gH625-QDs showed that both nanoparticles localized in the gills, lung and intestine, but they showed different distributions, indicating that the uptake of gH625-QDs was enhanced; the functionalized QDs had a significantly lower toxic effect on embryos’ survival and phenotypes. We observed that D. magna and X. laevis are useful in vivo models for toxicity and drug delivery studies.

Co-expression of Foxa.a, Foxd and Fgf9/16/20 defines a transient mesendoderm regulatory state in ascidian embryos

In many bilaterian embryos, nuclear β-catenin (nβ-catenin) promotes mesendoderm over ectoderm lineages. Although this is likely to represent an evolutionary ancient developmental process, the regulatory architecture of nβ-catenin-induced mesendoderm remains elusive in the majority of animals. Here, we show that, in ascidian embryos, three nβ-catenin transcriptional targets, Foxa.a, Foxd and Fgf9/16/20, are each required for the correct initiation of both the mesoderm and endoderm gene regulatory networks. Conversely, these three factors are sufficient, in combination, to produce a mesendoderm ground state that can be further programmed into mesoderm or endoderm lineages. Importantly, we show that the combinatorial activity of these three factors is sufficient to reprogramme developing ectoderm cells to mesendoderm. We conclude that in ascidian embryos, the transient mesendoderm regulatory state is defined by co-expression of Foxa.a, Foxd and Fgf9/16/20.

DOI:http://dx.doi.org/10.7554/eLife.14692.001

Roles of FGFs As Paracrine or Endocrine Signals in Liver Development, Health, and Disease

The liver plays important roles in multiple processes including metabolism, the immune system, and detoxification and also has a unique capacity for regeneration. FGFs are growth factors that have diverse functions in development, health, and disease. The FGF family now comprises 22 members. Several FGFs have been shown to play roles as paracrine signals in liver development, health, and disease. FGF8 and FGF10 are involved in embryonic liver development, FGF7 and FGF9 in repair in response to liver injury, and FGF5, FGF8, FGF9, FGF17, and FGF18 in the development and progression of hepatocellular carcinoma. In contrast, FGF15/19 and FGF21 are endocrine signals. FGF15/19, which is produced in the ileum, is a negative regulator of bile acid metabolism and a stimulator of gallbladder filling. FGF15/19 is a postprandial, insulin-independent activator of hepatic protein and glycogen synthesis. It is also required for hepatocellular carcinoma and liver regeneration. FGF21 is a hepatokine produced in the liver. FGF21 regulates glucose and lipid metabolism in white adipose tissue. Serum FGF21 levels are elevated in non-alcoholic fatty liver. FGF21 also protects against non-alcoholic fatty liver. These findings provide new insights into the roles of FGFs in the liver and potential therapeutic strategies for hepatic disorders.

Gutsy moves in mice: cellular and molecular dynamics of endoderm morphogenesis

Despite the importance of the gut and its accessory organs, our understanding of early endoderm development is still incomplete. Traditionally, endoderm has been difficult to study because of its small size and relative fragility. However, recent advances in live cell imaging technologies have dramatically expanded our understanding of this tissue, adding a new appreciation for the complex molecular and morphogenetic processes that mediate gut formation. Several spatially and molecularly distinct subpopulations have been shown to exist within the endoderm before the onset of gastrulation. Here, we review findings that have uncovered complex cell movements within the endodermal layer, before and during gastrulation, leading to the conclusion that cells from primitive endoderm contribute descendants directly to gut.

Pancreatic Differentiation from Murine Embryonic Stem Cells

Pluripotent stem cells are considered as a cell source for replacement therapies for pancreatic beta cells and other organs.We identified tetrabenazine (TBZ), vesicular monoamine transporter 2 (VMAT2) inhibitor as a promoter of late-stage differentiation of Pdx1-positive pancreatic progenitor cells into Ngn3-positive endocrine progenitor cells. A cell-permeable cAMP analog, dBu-cAMP promotes beta cell maturation in late stage of differentiation. The induced beta cells can secrete insulin in a glucose-dependent manner.Our protocol consists of a three -step differentiation process. ES cell recapitulate embryonic developmental processes in vitro. Therefore, the ES cell differentiation system is a useful model for the understanding of molecular mechanism of beta-cell differentiation and are useful for application for future regenerative medicine.

How to make a functional β-cell

Insulin-secreting pancreatic β-cells are essential regulators of mammalian metabolism. The absence of functional β-cells leads to hyperglycemia and diabetes, making patients dependent on exogenously supplied insulin. Recent insights into β-cell development, combined with the discovery of pluripotent stem cells, have led to an unprecedented opportunity to generate new β-cells for transplantation therapy and drug screening. Progress has also been made in converting terminally differentiated cell types into β-cells using transcriptional regulators identified as key players in normal development, and in identifying conditions that induce β-cell replication in vivo and in vitro. Here, we summarize what is currently known about how these strategies could be utilized to generate new β-cells and highlight how further study into the mechanisms governing later stages of differentiation and the acquisition of functional capabilities could inform this effort.

Generating β-cells in vitro: progress towards a Holy Grail

PURPOSE OF REVIEW

Diabetes is a debilitating disease characterized by a chronic inability to normalize blood glucose levels. Transplanting cadaveric pancreata or isolated pancreatic islets can restore glucose homeostasis, but organ demand outstrips supply. Consequently, there is significant interest in alternative tissue sources. This review summarizes state-of-the-art efforts to generate scalable, functional β-cells to treat diabetes.

RECENT FINDINGS

Applying knowledge gleaned from developmental biology, human pluripotent stem cells can be treated stepwise with combinations of small molecules, developmentally relevant growth factors, and morphogens to generate pancreatic progenitor cells (PPCs) in vitro. Transplanted PPCs can then further mature in vivo into functional islet-like tissues containing all of the endocrine hormone cells present in adult islets and can reverse hyperglycemia in a diabetic animal model. Recent publications demonstrate that skin, liver, and other cell lineages may also be reprogrammed to functional β-like cells.

SUMMARY

Although generation of fully functional β-cells in vitro has not yet been achieved, possible intermediate approaches to treat diabetes include using PPCs or reprogramming adult cells to β-like cells. A cell therapy with either approach will require isolation from the host immune response. Ongoing efforts are addressing this need through the use of immune-isolation devices to avoid immunosuppressive drugs.

Deconstructing pancreas developmental biology

The relentless nature and increasing prevalence of human pancreatic diseases, in particular, diabetes mellitus and adenocarcinoma, has motivated further understanding of pancreas organogenesis. The pancreas is a multifunctional organ whose epithelial cells govern a diversity of physiologically vital endocrine and exocrine functions. The mechanisms governing the birth, differentiation, morphogenesis, growth, maturation, and maintenance of the endocrine and exocrine components in the pancreas have been discovered recently with increasing tempo. This includes recent studies unveiling mechanisms permitting unexpected flexibility in the developmental potential of immature and mature pancreatic cell subsets, including the ability to interconvert fates. In this article, we describe how classical cell biology, genetic analysis, lineage tracing, and embryological investigations are being complemented by powerful modern methods including epigenetic analysis, time-lapse imaging, and flow cytometry-based cell purification to dissect fundamental processes of pancreas development.