Biomedical subjects

Sheilendra Mehta

Publications and source records attributed to Sheilendra Mehta.

4 recordsLinked to original sources

PubMed2005

Cross-talk between bone morphogenetic protein and transforming growth factor-beta signaling is essential for exendin-4-induced insulin-positive differentiation of AR42J cells.

A key goal of cellular engineering is to manipulate progenitor cells to become beta-cells, allowing cell replacement therapy to cure diabetes mellitus. As a paradigm for cell engineering, we have studied the molecular mechanisms by which AR42J cells become beta-cells. Bone morphogenetic proteins (BMPs), implicated in a myriad of developmental pathways, have not been well studied in insulin-positive differentiation. We found that the canonical intracellular mediators of BMP signaling, Smad-1 and Smad-8, were significantly elevated in AR42J cells undergoing insulin-positive differentiation in response to exendin-4 treatment, suggesting a role for BMP signaling in beta-cell formation. Similarly, endogenous BMP-2 ligand and ALK-1 receptor (activin receptor-like kinase-1; known to activate Smads 1 and 8) mRNAs were specifically up-regulated in exendin-4-treated AR42J cells. Surprisingly, Smad-1 and Smad-8 levels were suppressed by the addition of BMP-soluble receptor inhibition of BMP ligand binding to its receptor. Here, insulin-positive differentiation was also ablated. BMP-2 ligand antisense also strongly inhibited Smad-1 and Smad-8 expression, again with the abolition of insulin-positive differentiation. These results demonstrate a previously unrecognized key role for BMP signaling in mediating insulin-positive differentiation through the intracellular Smad signaling pathway. In short, BMP signaling may represent a novel downstream target of exendin-4 (glucagon-like peptide 1) signaling and potentially serve as an upstream regulator of transforming growth factor-beta isoform signaling to differentiate the acinar-like AR42J cells into insulin-secreting cells.

Animals↗

PubMed2005

Pancreatic differentiation.

Over the last decade, with the advent of new techniques and technologies in modern molecular biology, our understanding of the underlying mechanisms responsible for organ differentiation has developed rapidly. Despite this, our knowledge of these signaling pathways is still far from complete. Some of these advances, such as the creation of transgenic mouse models, have given us new tools to help us understand the interactions of the various transcription factors that are responsible for the creation of various cell types from a single cell type during embryogenesis. This knowledge then gives rise to the concept of creating new ways to manipulate stem cells in order to correct the deficiencies present in various disease processes. Here, we present work that focuses specifically on pancreatic development. The ultimate goal of our research in studying the mechanisms of the basic differentiation of pancreatic precursor cells is to gain the knowledge necessary to be able to engineer stem cells specifically into beta-cells in the treatment of diabetes.

Cell Differentiation↗

PubMed2005

Synergistic endocrine induction by GLP-1 and TGF-beta in the developing pancreas.

OBJECTIVES: Glucagon-like peptide-1 (GLP-1) is known to stimulate glucose-dependent insulin production and secretion by pancreatic beta-cells. Preliminary evidence suggests that GLP-1 may also influence endocrine differentiation from pancreatic progenitor cells. Additionally, TGF-beta signaling can also control endocrine differentiation by both inhibiting proliferation and enhancing differentiation of endocrine progenitor cells to become mature beta-cells. Here we document synergy of these two signaling pathways in the differentiation of endocrine cells in the developing pancreas. METHODS: Embryonic pancreas was harvested from mice at day 11.5 and cultured for six days with GLP-1 agonist, exendin-4, and/or TGF-beta1 ligand. Also, a pan-neutralizing TGF-beta isoform antibody was used alone or with exendin-4 to study TGF-beta inhibition in this system. Pancreatic cultures were processed for immunohistochemistry. RESULTS: Exogenous TGF-beta1 and exendin-4 each individually enhanced both insulin and glucagon differentiation dose-dependently. However, when combined there was an additive effect to a 4.5-fold increase in insulin-positive differentiation. We also saw suppression of amylase-positive differentiation. Surprisingly, TGF-beta pan-neutralizing antibody also gave an augmentation of endocrine differentiation by 1.5 to 2-fold, but no synergistic effect was seen with exendin-4. CONCLUSION: We conclude that TGF-beta isoforms have a specific synergistic role with GLP-1 pathway signaling in early pancreatic development, toward endocrine differentiation and away from acinar differentiation.

Animals↗

PubMed2003

Defective sonic hedgehog signaling in esophageal atresia with tracheoesophageal fistula.

BACKGROUND: The pathogenesis of esophageal atresia and tracheoesophageal fistula (EA/TEF) remains unknown. We have found previously that an initial esophageal atresia, followed by an abnormal (absent) branching pattern of the middle branch of a trifurcation of the lung/tracheal bud, leads to the neonatal finding of TEF. Mice null mutant for hedgehog signaling can experience the development of EA/TEF, but the mechanism for this development is also unknown. Given that EA/TEF in humans appears not to be due to genetic defects, a hedgehog mutation cause seems very unlikely. However, defective hedgehog signaling that is caused by environmental effects in the human embryo likely could be implicated. We studied a teratogen-induced model of EA/TEF to determine the mechanism by which defective hedgehog signaling may lead to EA/TEF. METHODS: We injected Adriamycin into pregnant rats to induce EA/TEF in rat embryos. We first quantified sonic hedgehog (Shh) signaling pathway molecule expression using real-time, semiquantitative reverse-transcriptase polymerase chain reaction for Shh, Shh receptors (patched and smoothened), and downstream intracellular targets of those receptors (Gli family members). On the basis of these findings, we then developed an in vitro culture system for the day-12 embryonic TEF and manipulated Shh signaling using either exogenous Shh or Shh inhibitors. RESULTS: By reverse transcriptase-polymerase chain reaction, a unique difference between the fistula tract and control tissues was that Gli-2 (downstream signaling molecule of Shh) messenger RNA levels were much lower in the fistula tract than in the adjacent esophagus (P =.002). Surprisingly, in the culture experiments, the fistula tract was induced to branch by exogenous Shh. Such branching of the fistula was unexpected and further supports the presumed respiratory origin of the fistula tract because the normal lung, but not normal esophagus, branched in response to Shh. The Shh inhibitor had no effect, which indicated that defective signaling, rather than hyperfunctioning Shh, is critical to the nonbranching phenotype of the fistula tract in TEF. CONCLUSIONS: The recapitulation of respiratory developmental morphogenesis by the fistula tract of TEF in the presence of exogenous Shh, together with the quantitative reduction in normal, endogenous levels of Gli-2, strongly suggests that 1 mechanism for the formation of the fistula tract is the lack of proper Shh signaling because of Gli-2 deficiency, with subsequent straight, nonbranching caudal growth of the fistula tract. This deficiency can be rescued by excess exogenous Shh, thus reestablishing respiratory morphogenesis.

Animals↗