Promoting ectopic pancreatic fates: pancreas development and future diabetes therapies.

Diabetes is a disease that could be treated more effectively with a better understanding of pancreas development. This review examines the role of master regulator genes driving crucial steps in pancreas development, from foregut specification to differentiation of the five endocrine cell types. The roles of Pdx1, Ptf1a, and Ngn3 are particularly examined as they are both necessary and sufficient for promoting pancreatic cell fates (Pdx1, Ptf1a) and endocrine cell development (Ngn3). The roles of Arx and Pax4 are studied as they compose part of the regulatory mechanism balancing development of different types of endocrine cells within the iselts and promote the development of alpha/PP and beta/delta cell progenitors, respectively. The roles of the aforementioned genes, and the consequences of misexpression of them for functionality of the pancreas, are examined through recent studies in model organisms, particularly Xenopus and zebrafish. Recent developments in cell replacement therapy research are also covered, concentrating on stem cell research (coaxing both adult and embryonic stem cells toward a beta cell fate) and transdifferentiation (generating beta cells from other differentiated cell types).

Endoderm specification and differentiation in Xenopus embryos.

It is known from work with amniote embryos that regional specification of the gut requires cell-cell signalling between the mesoderm and the endoderm. In recent years, much of the interest in Xenopus endoderm development has focused on events that occur before gastrulation and this work has led to a different model whereby regional specification of the endoderm is autonomous. In this paper, we examine the specification and differentiation of the endoderm in Xenopus using neurula and tail-bud-stage embryos and we show that the current hypothesis of stable autonomous regional specification is not correct. When the endoderm is isolated alone from neurula and tail bud stages, it remains fully viable but will not express markers of regional specification or differentiation. If mesoderm is present, regional markers are expressed. If recombinations are made between mesoderm and endoderm, then the endodermal markers expressed have the regional character of the mesoderm. Previous results with vegetal explants had shown that endodermal differentiation occurs cell-autonomously, in the absence of mesoderm. We have repeated these experiments and have found that the explants do in fact show some expression of mesoderm markers associated with lateral plate derivatives. We believe that the formation of mesoderm cells by the vegetal explants accounts for the apparent autonomous development of the endoderm. Since the fate map of the Xenopus gut shows that the mesoderm and endoderm of each level do not come together until tail bud stages, we conclude that stable regional specification of the endoderm must occur quite late, and as a result of inductive signals from the mesoderm.

Patterning the endoderm: the importance of neighbours.

The endodermal germ layer gives rise to the inner epithelial lining of the gastrointestinal tract, while that of the mesoderm gives rise to the outer smooth muscle layer. Much of the work in chick shows that the mesoderm plays an important role in endodermal differentiation, and recent results in Xenopus have begun to elucidate the factors involved in establishing endodermal cell fate. However, little is know about the signals responsible for the initial specification and pattern of the endoderm. In a recent paper, Wells and Melton have investigated the importance of early mesectodermal-endodermal interactions in the initial specification of the early mouse endoderm.(1) They demonstrate that the initial specification and differentiation of the endoderm does not occur cell-autonomously, but requires signals released from the mesectoderm.

Tbx5 is essential for heart development.

Mutations in the Tbx5 transcription factor cause heart septal defects found in human Holt-Oram Syndrome. The complete extent to which Tbx5 functions in heart development, however, has not been established. Here we show that, in Xenopus embryos, Tbx5 is expressed in the early heart field, posterior to the cardiac homeobox transcription factor, Nkx2.5. During morphogenesis, Tbx5 is expressed throughout the heart tube except the anterior portion, the bulbus cordis. When Tbx5 activity is antagonized with a hormone-inducible, dominant negative version of the protein, the heart fails to develop. These results suggest that, in addition to its function in heart septation, Tbx5 has a more global role in cardiac specification and heart development in vertebrate embryos.

A Xenopus homologue of aml-1 reveals unexpected patterning mechanisms leading to the formation of embryonic blood.

The Runt domain gene AML1 is essential for definitive hematopoiesis during murine embryogenesis. We have isolated Xaml, a Xenopus AML1 homologue in order to investigate the patterning mechanisms responsible for the generation of hematopoietic precursors. Xaml is expressed early in the developing ventral blood island in a pattern that anticipates that of later globin. Analysis of globin and Xaml expression in explants, in embryos with perturbed dorsal ventral patterning, and by lineage tracing indicates that the formation of the ventral blood island is more complex than previously thought and involves contributions from both dorsal and ventral tissues. A truncated Xaml protein interferes with primitive hematopoiesis. Based on these results, we propose that Runt domain proteins function in the specification of hematopoietic stem cells in vertebrate embryos.

A vegetally localized T-box transcription factor in Xenopus eggs specifies mesoderm and endoderm and is essential for embryonic mesoderm formation.

Pattern formation in early embryogenesis is guided by maternal, localized determinants and by inductive interactions between cells. In Xenopus eggs, localized molecules have been identified and some, such as Vg1 and Xwnt-11, can specify cell fates by functioning as inducers or patterning agents. We have used differential screening to identify new Xenopus genes that regulate mesodermal patterning, and we have isolated a new member of the T-box family of transcription factors. This gene, named Brat, is expressed maternally and its transcripts are localized to the vegetal hemisphere of the egg. During early embryonic cleavage, Brat mRNA becomes partitioned primarily within vegetal cells that are fated to form the endoderm. Zygotic expression of Brat begins at the onset of gastrulation within the presumptive mesoderm of the marginal zone. Consistent with its zygotic expression pattern, Brat induces, in a dose-dependent manner, a full spectrum of mesodermal genes that mark tissues across the dorsal-ventral axis, from the blood through the Spemann organizer. Brat also induces endoderm, consistent with its vegetal localization, making Brat a good candidate for a maternal determinant of the endoderm. We tested whether endogenous Brat is required for mesoderm formation by expressing a dominant-negative, transcriptional repressor form of Brat in embryos. This treatment inhibited mesoderm formation and severely disrupted normal development, thereby establishing that Brat plays a critical role in embryonic mesoderm formation and body patterning.