Expression and function of ATP-dependent potassium channels in zebrafish islet ß-cells.

ATP-sensitive potassium channels (KATP channels) are critical nutrient sensors in many mammalian tissues. In the pancreas, KATP channels are essential for coupling glucose metabolism to insulin secretion. While orthologous genes for many components of metabolism-secretion coupling in mammals are present in lower vertebrates, their expression, functionality and ultimate impact on body glucose homeostasis are unclear. In this paper, we demonstrate that zebrafish islet ß-cells express functional KATP channels of similar subunit composition, structure and metabolic sensitivity to their mammalian counterparts. We further show that pharmacological activation of native zebrafish KATP using diazoxide, a specific KATP channel opener, is sufficient to disturb glucose tolerance in adult zebrafish. That ß-cell KATP channel expression and function are conserved between zebrafish and mammals illustrates the evolutionary conservation of islet metabolic sensing from fish to humans, and lends relevance to the use of zebrafish to model islet glucose sensing and diseases of membrane excitability such as neonatal diabetes.

Genetic risk analysis of a patient with fulminant autoimmune type 1 diabetes mellitus secondary to combination ipilimumab and nivolumab immunotherapy.

BACKGROUND: Checkpoint inhibitor immunotherapy is becoming an effective treatment modality for an increasing number of malignancies. As a result, autoinflammatory side-effects are also being observed more commonly in the clinic. We are currently unable to predict which patients will develop more severe toxicities associated with these treatment regimens. CASE PRESENTATION: We present a patient with stage IV melanoma that developed rapid onset autoimmune type 1 diabetes (T1D) in response to combination ipilimumab and nivolumab immunotherapy. At the time of the patient's presentation with diabetes ketoacidosis, a confirmed anti-GAD antibody seroconversion was noted. Longer-term follow-up of this patient has demonstrated a durable complete response based on PET CT imaging along with a persistently undetectable C-peptide level. Single nucleotide polymorphism gene sequencing and HLA risk allele analysis has revealed the patient to lack any established genetic predisposition to the development of autoimmune T1D. CONCLUSIONS: While larger studies are necessary to better understand the role of genetic risk factors for the development of autoimmune toxicities in those patients undergoing checkpoint inhibitor immunotherapy, these results suggest that pre-screening patients for known T1D risk alleles may not be indicated. Additional investigation is needed to determine whether an approach such as T cell receptor clonotypic analysis to identify the presence of autoreactive T cell clones may be an effective approach for predicting which patients are at risk for the development of autoinflammatory toxicities while undergoing checkpoint inhibitor immunotherapy.

Treatment-related neuroendocrine prostate cancer resulting in Cushing's syndrome.

Here we present, to the best of our knowledge, the first case of a paraneoplastic Cushing's syndrome (hypercortisolism) resulting from treatment-related neuroendocrine prostate cancer - a highly aggressive and difficult disease to treat. A 51-year-old man was started on androgen deprivation therapy after presenting with metastatic prostate cancer, characterized by diffuse osseous metastasis. Shortly thereafter, he developed progressive disease with biopsy proven neuroendocrine prostate cancer as well as symptoms of increased skin pigmentation, hypokalemia, hypertension, hyperglycemia and profound weakness, consistent with ectopic Cushing's syndrome. Molecular analysis of the patient's tumor through RNA sequencing showed high expression of several genes including CHGA, ASCL1, CALCA, HES6, PCSK1, CALCB and INSM1 confirming his neuroendocrine phenotype; elevated POMC expression was found, supporting the diagnosis of ectopic Cushing's syndrome.

A Pdx-1-Regulated Soluble Factor Activates Rat and Human Islet Cell Proliferation.

The homeodomain transcription factor Pdx-1 has important roles in pancreas and islet development as well as in ß-cell function and survival. We previously reported that Pdx-1 overexpression stimulates islet cell proliferation, but the mechanism remains unclear. Here, we demonstrate that overexpression of Pdx-1 triggers proliferation largely by a non-cell-autonomous mechanism mediated by soluble factors. Consistent with this idea, overexpression of Pdx-1 under the control of a ß-cell-specific promoter (rat insulin promoter [RIP]) stimulates proliferation of both α and ß cells, and overexpression of Pdx-1 in islets separated by a Transwell membrane from islets lacking Pdx-1 overexpression activates proliferation in the untreated islets. Microarray and gene ontology (GO) analysis identified inhibin beta-B (Inhbb), an activin subunit and member of the transforming growth factor ß (TGF-ß) superfamily, as a Pdx-1-responsive gene. Overexpression of Inhbb or addition of activin B stimulates rat islet cell and ß-cell proliferation, and the activin receptors RIIA and RIIB are required for the full proliferative effects of Pdx-1 in rat islets. In human islets, Inhbb overexpression stimulates total islet cell proliferation and potentiates Pdx-1-stimulated proliferation of total islet cells and ß cells. In sum, this study identifies a mechanism by which Pdx-1 induces a soluble factor that is sufficient to stimulate both rat and human islet cell proliferation.

Nkx6.1-mediated insulin secretion and ß-cell proliferation is dependent on upregulation of c-Fos.

Understanding the molecular pathways that enhance ß-cell proliferation, survival, and insulin secretion may be useful to improve treatments for diabetes. Nkx6.1 induces proliferation through the Nr4a nuclear receptors, and improves insulin secretion and survival through the peptide hormone VGF. Here we demonstrate that Nkx6.1-mediated upregulation of Nr4a1, Nr4a3, and VGF is dependent on c-Fos expression. c-Fos overexpression results in activation of Nkx6.1 responsive genes and increases ß-cell proliferation, insulin secretion, and cellular survival. c-Fos knockdown impedes Nkx6.1-mediated ß-cell proliferation and insulin secretion. These data demonstrate that c-Fos is critical for Nkx6.1-mediated expansion of functional ß-cell mass.

Assessment of Toxicological Perturbations and Variants of Pancreatic Islet Development in the Zebrafish Model.

The pancreatic islets, largely comprised of insulin-producing beta cells, play a critical role in endocrine signaling and glucose homeostasis. Because they have low levels of antioxidant defenses and a high perfusion rate, the endocrine islets may be a highly susceptible target tissue of chemical exposures. However, this endpoint, as well as the integrity of the surrounding exocrine pancreas, is often overlooked in studies of developmental toxicology. Disruption of development by toxicants can alter cell fate and migration, resulting in structural alterations that are difficult to detect in mammalian embryo systems, but that are easily observed in the zebrafish embryo model (Danio rerio). Using endogenously expressed fluorescent protein markers for developing zebrafish beta cells and exocrine pancreas tissue, we documented differences in islet area and incidence rates of islet morphological variants in zebrafish embryos between 48 and 96 h post fertilization (hpf), raised under control conditions commonly used in embryotoxicity assays. We identified critical windows for chemical exposures during which increased incidences of endocrine pancreas abnormalities were observed following exposure to cyclopamine (2-12 hpf), Mono-2-ethylhexyl phthalate (MEHP) (3-48 hpf), and Perfluorooctanesulfonic acid (PFOS) (3-48 hpf). Both islet area and length of the exocrine pancreas were sensitive to oxidative stress from exposure to the oxidant tert-butyl hydroperoxide during a highly proliferative critical window (72 hpf). Finally, pancreatic dysmorphogenesis following developmental exposures is discussed with respect to human disease.

Nkx6.1 regulates islet ß-cell proliferation via Nr4a1 and Nr4a3 nuclear receptors.

Loss of functional ß-cell mass is a hallmark of type 1 and type 2 diabetes, and methods for restoring these cells are needed. We have previously reported that overexpression of the homeodomain transcription factor NK6 homeobox 1 (Nkx6.1) in rat pancreatic islets induces ß-cell proliferation and enhances glucose-stimulated insulin secretion, but the pathway by which Nkx6.1 activates ß-cell expansion has not been defined. Here, we demonstrate that Nkx6.1 induces expression of the nuclear receptor subfamily 4, group A, members 1 and 3 (Nr4a1 and Nr4a3) orphan nuclear receptors, and that these factors are both necessary and sufficient for Nkx6.1-mediated ß-cell proliferation. Consistent with this finding, global knockout of Nr4a1 results in a decrease in ß-cell area in neonatal and young mice. Overexpression of Nkx6.1 and the Nr4a receptors results in increased expression of key cell cycle inducers E2F transcription factor 1 and cyclin E1. Furthermore, Nkx6.1 and Nr4a receptors induce components of the anaphase-promoting complex, including ubiquitin-conjugating enzyme E2C, resulting in degradation of the cell cycle inhibitor p21. These studies identify a unique bipartite pathway for activation of ß-cell proliferation, suggesting several unique targets for expansion of functional ß-cell mass.

Detection of autofluorescent Mycobacterium chelonae in living zebrafish.

Mycobacterium chelonae is widespread in aquatic environments and can cause mycobacteriosis with low virulence in zebrafish. The risk of infection in zebrafish is exacerbated in closed-recirculating aquatic systems where rapidly growing mycobacteria can live on biofilms, as well as in zebrafish tissues. We have discovered a method of identifying and visualizing M. chelonae infections in living zebrafish using endogenous autofluorescence. Infected larvae are easily identified and can be excluded from experimental results. Because infection may reduce fertility in zebrafish, the visualization of active infection in contaminated eggs of transparent casper females simplifies screening. Transparent fish are also particularly useful as sentinels that can be examined periodically for the presence of autofluorescence, which can then be tested directly for M. chelonae.

Pdx-1 activates islet α- and ß-cell proliferation via a mechanism regulated by transient receptor potential cation channels 3 and 6 and extracellular signal-regulated kinases 1 and 2.

The homeodomain transcription factor Pdx-1 has important roles in pancreatic development and ß-cell function and survival. In the present study, we demonstrate that adenovirus-mediated overexpression of Pdx-1 in rat or human islets also stimulates cell replication. Moreover, cooverexpression of Pdx-1 with another homeodomain transcription factor, Nkx6.1, has an additive effect on proliferation compared to either factor alone, implying discrete activating mechanisms. Consistent with this, Nkx6.1 stimulates mainly ß-cell proliferation, whereas Pdx-1 stimulates both α- and ß-cell proliferation. Furthermore, cyclins D1/D2 are upregulated by Pdx-1 but not by Nkx6.1, and inhibition of cdk4 blocks Pdx-1-stimulated but not Nkx6.1-stimulated islet cell proliferation. Genes regulated by Pdx-1 but not Nkx6.1 were identified by microarray analysis. Two members of the transient receptor potential cation (TRPC) channel family, TRPC3 and TRPC6, are upregulated by Pdx-1 overexpression, and small interfering RNA (siRNA)-mediated knockdown of TRPC3/6 or TRPC6 alone inhibits Pdx-1-induced but not Nkx6.1-induced islet cell proliferation. Pdx-1 also stimulates extracellular signal-regulated kinase 1 and 2 (ERK1/2) phosphorylation, an effect partially blocked by knockdown of TRPC3/6, and blockade of ERK1/2 activation with a MEK1/2 inhibitor partially impairs Pdx-1-stimulated proliferation. These studies define a pathway by which overexpression of Pdx-1 activates islet cell proliferation that is distinct from and additive to a pathway activated by Nkx6.1.

Imaging beta cell regeneration and interactions with islet vasculature in transparent adult zebrafish.

Blood vessel networks provide nutrients and gaseous exchange that are essential for functions. Pancreatic islet capillaries deliver oxygen to endocrine cells while transporting hormones to organs and peripheral locations throughout the body. We have developed a zebrafish diabetes model in which adult islets can be followed in vivo during beta cell regeneration while calibrating changes in beta cell mass and fasting blood glucose levels. After genetic ablation, beta cells are initially dysfunctional or dying, and blood glucose levels increase fourfold. During a 2-week period, hyperglycemia eventually normalizes as beta cell mass regenerates. We show that mCherry-fluorescent, insulin-positive beta cells re-emerge in close contact with the vascular endothelium. Alterations in the dense vascular network of zebrafish islets were visualized by the expression of green fluorescent protein (GFP) in endothelial cells derived from the Fli transcription factor promoter. The rapid destruction and regeneration of beta cell mass was evaluated in the same animal over time, providing a functional model for investigating the interactions of islet cell types with vascular cells as well as the consequences of hyperglycemia on other tissues. Regenerating adult zebrafish can be utilized as vertebrate, metabolically active models for generating new insights into treatments for type 2 diabetes.

Creating new ß cells: cellular transmutation by genomic alchemy.

To address insulin insufficiency, diabetes research has long focused on techniques for replacing insulin-producing ß cells. Studies in mice have suggested that, under some conditions, α cells possess the capacity to transdifferentiate into ß cells, although the mechanisms that drive this conversion are unclear. In this issue, Bramswig et al. analyzed the methylation states of purified human α, ß, and acinar cells and found α cells exhibit intrinsic phenotypic plasticity associated with specific histone methylation profiles. In addition to expanding our understanding of this potential source of ß cells, this compendium of carefully generated human gene expression and epigenomic data in islet cell subtypes constitutes a truly valuable resource for the field.

Regeneration of the pancreas in adult zebrafish.

OBJECTIVE: Regenerating organs in diverse biological systems have provided clues to processes that can be harnessed to repair damaged tissue. Adult mammalian beta-cells have a limited capacity to regenerate, resulting in diabetes and lifelong reliance on insulin. Zebrafish have been used as a model for the regeneration of many organs. We demonstrate the regeneration of adult zebrafish pancreatic beta-cells. This nonmammalian model can be used to define pathways for islet-cell regeneration in humans. RESEARCH DESIGN AND METHODS: Adult transgenic zebrafish were injected with a single high dose of streptozotocin or metronidazole and anesthetized at 3, 7, or 14 days or pancreatectomized. Blood glucose measurements were determined and gut sections were analyzed using specific endocrine, exocrine, and duct cell markers as well as markers for dividing cells. RESULTS: Zebrafish recovered rapidly without the need for insulin injections, and normoglycemia was attained within 2 weeks. Although few proliferating cells were present in vehicles, ablation caused islet destruction and a striking increase of proliferating cells, some of which were Pdx1 positive. Dividing cells were primarily associated with affected islets and ducts but, with the exception of surgical partial pancreatectomy, were not extensively beta-cells. CONCLUSIONS: The ability of the zebrafish to regenerate a functional pancreas using chemical, genetic, and surgical approaches enabled us to identify patterns of cell proliferation in islets and ducts. Further study of the origin and contribution of proliferating cells in reestablishing islet function could provide strategies for treating human diseases.

The liver--a potential new player in islet regeneration?

Pancreatic islet beta cell mass expands in response to certain physiological conditions such as pregnancy and obesity, but the signaling pathways involved are not well understood. Possible insights come from a newly described regulatory circuit through which obesity-enhanced kinase signaling in the liver triggers expansion of islet mass and enhanced insulin secretion.

The zebrafish down syndrome cell adhesion molecule is involved in cell movement during embryogenesis.

The Down syndrome cell adhesion molecule (Dscam) is a protein overexpressed in the brains of Down syndrome patients and implicated in mental retardation. Dscam is involved in axon guidance and branching in Drosophila, but cellular roles in vertebrates have yet to be elucidated. To understand its role in vertebrate development, we cloned the zebrafish homolog of Dscam and showed that it shares high amino acid identity and structure with the mammalian homologs. Zebrafish dscam is highly expressed in developing neurons, similar to what has been described in Drosophila and mouse. When dscam expression is diminished by morpholino injection, embryos display few neurons and their axons do not enter stereotyped pathways. Zebrafish dscam is also present at early embryonic stages including blastulation and gastrulation. Its loss results in early morphogenetic defects. dscam knockdown results in impaired cell movement during epiboly as well as in subsequent stages. We propose that migrating cells utilize dscam to remodel the developing embryo.

Identification of beta-cell-specific insulin gene transcription factor RIPE3b1 as mammalian MafA.

Of the three critical enhancer elements that mediate beta-cell-specific and glucose-responsive expression of the insulin gene, only the identity of the transcription factor binding to the RIPE3b element (RIPE3b1) has remained elusive. Using a biochemical purification approach, we have identified the RIPE3b1 factor as a mammalian homologue of avian MafA/L-Maf (mMafA). The avian MafA is a cell-type determination factor that expressed ectopically can trigger lens differentiation program, but no mammalian homologue of avian MafA has previously been identified. Here, we report cloning of the human mafA (hMafA) and demonstrate that it can specifically bind the insulin enhancer element RIPE3b and activate insulin-gene expression. In addition, mMafA has a very restrictive cellular distribution and is selectively expressed in pancreatic beta but not in alpha cells. We suggest that mMafA has an essential role in the function and differentiation of beta-cells and thus may be associated with the pathophysiological origins of diabetes.

Sonic hedgehog is required early in pancreatic islet development.

Pancreatic organogenesis relies on a complex interplay of cell-autonomous and extracellular signals. We demonstrate that the morphogen sonic hedgehog (Shh) is required for pancreatic development in zebrafish. Genetic mutants of Shh and its signaling pathway establish this dependence as specific to endocrine, but not exocrine, pancreas. Using cyclopamine to inhibit hedgehog signaling, we show that transient Shh signaling is necessary during gastrulation for subsequent differentiation of endoderm into islet tissue. A second hedgehog-dependent activity occurring later in development was also identified and may be analogous to the known action of Shh in gut endoderm to direct localization of pancreatic development. The early action of Shh may be part of a more general process allowing neuroendocrine cells to originate in nonneuroectodermally derived tissues.