Novel roles for Pdx1 in the endocrine pancreas

Inadequate β-cell function and numbers contribute to the progression of all major forms of diabetes, a highly prevalent disorder with significant morbidity and mortality, resulting in intense focus in recent years on developing alternate sources of β-cells for cell replacement therapies. Recent advances in generating β-like cells from stem/precursor cells or mature related cell-types have been directly guided by our understanding of transcription factors and signaling pathways that regulate embryonic development of insulin-producing β-cells. Pancreatic duodenal homeobox 1 (Pdx1) is a homeodomain transcription factor that governs both early pancreatic organogenesis and the later differentiation of endocrine β-cells. Homozygous mutations in humans and mice lead to pancreatic agenesis. In contrast, Pdx1 heterozygous mutations lead to glucose intolerance with age due to defects in β-cell function and survival. Similarly, heterozygous mutations in humans result in autosomal dominant Maturity Onset Diabetes of the Young (MODY) as well as type-2 diabetes. Human Pdx1 mutations are located throughout protein: in the N-terminal transactivation domain, the central DNA-binding homeodomain and the evolutionary conserved but poorly understood C-terminal region. To determine the role of C-terminal domain in vivo as well as to gain new insights into specific roles for Pdx1, I characterized a Pdx1 hypomorphic allele, Pdx1ΔC/ΔC , that prevents translation of the C-terminus. Pdx1 ΔC/ΔC animals display a global reduction in all endocrine lineages during development resulting from decreased numbers of Neurogenin 3 (Ngn3)-expressing endocrine progenitors. Pdx1 occupies a conserved Ngn3 enhancer at embryonic day 13.5 and physically interacts with the one-cut transcription factor Hnf6 to directly regulate Ngn3 expression. Moreover, the absence of the C-terminus impairs Pdx1 transactivation of Ngn3. Pdx1ΔC/ΔC mice also have reduced Hnf6, Hnf1β, Foxa2, and Sox9 transcripts, suggesting that Pdx1 contributes to specification of Ngn3+ endocrine progenitors by directly regulating Ngn3 expression and by participating in a recently described transcription factor co-regulatory network regulating the endocrine progenitor. Postnatally, Pdx1ΔC/ΔC mice develop overt diabetes by 3-4 weeks of age, due to defects in both β-cell mass and function. Further studies will determine the mechanisms whereby Pdx1 regulates postnatal function, which may reveal additional insight into how Pdx1 mutations lead to diabetes

Novel roles for Pdx1 in the endocrine pancreas

Inadequate β-cell function and numbers contribute to the progression of all major forms of diabetes, a highly prevalent disorder with significant morbidity and mortality, resulting in intense focus in recent years on developing alternate sources of β-cells for cell replacement therapies. Recent advances in generating β-like cells from stem/precursor cells or mature related cell-types have been directly guided by our understanding of transcription factors and signaling pathways that regulate embryonic development of insulin-producing β-cells. Pancreatic duodenal homeobox 1 (Pdx1) is a homeodomain transcription factor that governs both early pancreatic organogenesis and the later differentiation of endocrine β-cells. Homozygous mutations in humans and mice lead to pancreatic agenesis. In contrast, Pdx1 heterozygous mutations lead to glucose intolerance with age due to defects in β-cell function and survival. Similarly, heterozygous mutations in humans result in autosomal dominant Maturity Onset Diabetes of the Young (MODY) as well as type-2 diabetes. Human Pdx1 mutations are located throughout protein: in the N-terminal transactivation domain, the central DNA-binding homeodomain and the evolutionary conserved but poorly understood C-terminal region. To determine the role of C-terminal domain in vivo as well as to gain new insights into specific roles for Pdx1, I characterized a Pdx1 hypomorphic allele, Pdx1ΔC/ΔC , that prevents translation of the C-terminus. Pdx1 ΔC/ΔC animals display a global reduction in all endocrine lineages during development resulting from decreased numbers of Neurogenin 3 (Ngn3)-expressing endocrine progenitors. Pdx1 occupies a conserved Ngn3 enhancer at embryonic day 13.5 and physically interacts with the one-cut transcription factor Hnf6 to directly regulate Ngn3 expression. Moreover, the absence of the C-terminus impairs Pdx1 transactivation of Ngn3. Pdx1ΔC/ΔC mice also have reduced Hnf6, Hnf1β, Foxa2, and Sox9 transcripts, suggesting that Pdx1 contributes to specification of Ngn3+ endocrine progenitors by directly regulating Ngn3 expression and by participating in a recently described transcription factor co-regulatory network regulating the endocrine progenitor. Postnatally, Pdx1ΔC/ΔC mice develop overt diabetes by 3-4 weeks of age, due to defects in both β-cell mass and function. Further studies will determine the mechanisms whereby Pdx1 regulates postnatal function, which may reveal additional insight into how Pdx1 mutations lead to diabetes

On the origin of the β cell

The major forms of diabetes are characterized by pancreatic islet β-cell dysfunction and decreased β-cell numbers, raising hope for cell replacement therapy. Although human islet transplantation is a cell-based therapy under clinical investigation for the treatment of type 1 diabetes, the limited availability of human cadaveric islets for transplantation will preclude its widespread therapeutic application. The result has been an intense focus on the development of alternate sources of β cells, such as through the guided differentiation of stem or precursor cell populations or the transdifferentiation of more plentiful mature cell populations. Realizing the potential for cell-based therapies, however, requires a thorough understanding of pancreas development and β-cell formation. Pancreas development is coordinated by a complex interplay of signaling pathways and transcription factors that determine early pancreatic specification as well as the later differentiation of exocrine and endocrine lineages. This review describes the current knowledge of these factors as they relate specifically to the emergence of endocrine β cells from pancreatic endoderm. Current therapeutic efforts to generate insulin-producing β-like cells from embryonic stem cells have already capitalized on recent advances in our understanding of the embryonic signals and transcription factors that dictate lineage specification and will most certainly be further enhanced by a continuing emphasis on the identification of novel factors and regulatory relationships

Loss-of-function of Endothelin receptor type A results in Oro-Oto-Cardiac syndrome

© 2020 Wiley Periodicals, Inc. Craniofacial morphogenesis is regulated in part by signaling from the Endothelin receptor type A (EDNRA). Pathogenic variants in EDNRA signaling pathway components EDNRA, GNAI3, PCLB4, and EDN1 cause Mandibulofacial Dysostosis with Alopecia (MFDA), Auriculocondylar syndrome (ARCND) 1, 2, and 3, respectively. However, cardiovascular development is normal in MFDA and ARCND individuals, unlike Ednra knockout mice. One explanation may be that partial EDNRA signaling remains in MFDA and ARCND, as mice with reduced, but not absent, EDNRA signaling also lack a cardiovascular phenotype. Here we report an individual with craniofacial and cardiovascular malformations mimicking the Ednra−/− mouse phenotype, including a distinctive micrognathia with microstomia and a hypoplastic aortic arch. Exome sequencing found a novel homozygous missense variant in EDNRA (c.1142A\u3eC; p.Q381P). Bioluminescence resonance energy transfer assays revealed that this amino acid substitution in helix 8 of EDNRA prevents recruitment of G proteins to the receptor, abrogating subsequent receptor activation by its ligand, Endothelin-1. This homozygous variant is thus the first reported loss-of-function EDNRA allele, resulting in a syndrome we have named Oro-Oto-Cardiac Syndrome. Further, our results illustrate that EDNRA signaling is required for both normal human craniofacial and cardiovascular development, and that limited EDNRA signaling is likely retained in ARCND and MFDA individuals. This work illustrates a straightforward approach to identifying the functional consequence of novel genetic variants in signaling molecules associated with malformation syndromes

The diabetes gene Pdx1 regulates the transcriptional network of pancreatic endocrine progenitor cells in mice

Heterozygous mutations in the gene encoding the pancreatic homeodomain transcription factor pancreatic duodenal homeobox 1 (PDX1) are associated with maturity onset diabetes of the young, type 4 (MODY4) and type 2 diabetes. Pdx1 governs the early embryonic development of the pancreas and the later differentiation of the insulin-producing islet β cells of the endocrine compartment. We derived a Pdx1 hypomorphic allele that reveals a role for Pdx1 in the specification of endocrine progenitors. Mice homozygous for this allele displayed a selective reduction in endocrine lineages associated with decreased numbers of endocrine progenitors and a marked reduction in levels of mRNA encoding the proendocrine transcription factor neurogenin 3 (Ngn3). During development, Pdx1 occupies an evolutionarily conserved enhancer region of Ngn3 and interacts with the transcription factor one cut homeobox 1 (Hnf6) to activate this enhancer. Furthermore, mRNA levels of all 4 members of the transcription factor network that regulates Ngn3 expression, SRY-box containing gene 9 (Sox9), Hnf6, Hnf1b, and forkhead box A2 (Foxa2), were decreased in homozygous mice. Pdx1 also occupied regulatory sequences in Foxa2 and Hnf1b. Thus, Pdx1 contributes to specification of endocrine progenitors both by regulating expression of Ngn3 directly and by participating in a cross-regulatory transcription factor network during early pancreas development. These results provide insights that may be applicable to β cell replacement strategies involving the guided differentiation of ES cells or other progenitor cell types into the β cell lineage, and they suggest a molecular mechanism whereby human PDX1 mutations cause diabetes

Preexisting pancreatic acinar cells contribute to acinar cell, but not islet β cell, regeneration

It has been suggested that pancreatic acinar cells can serve as progenitors for pancreatic islets, a concept with substantial implications for therapeutic efforts to increase insulin-producing β cell mass in patients with diabetes. We report what we believe to be the first in vivo lineage tracing approach to determine the plasticity potential of pancreatic acinar cells. We developed an acinar cell–specific inducible Cre recombinase transgenic mouse, which, when mated with a reporter strain and pulsed with tamoxifen, resulted in permanent and specific labeling of acinar cells and their progeny. During various time periods of observation and using several models to provoke injury, we failed to observe any chase of the labeled cells into the endocrine compartment, indicating that acinar cells do not normally transdifferentiate into islet β cells in vivo in adult mice. In contrast, we observed a substantial role for replication of preexisting acinar cells in the regeneration of new acinar cells after partial pancreatectomy. These results indicate that mature acinar cells harbor a facultative acinar but not endocrine progenitor capacity