Summary Decreasing glucagon action lowers the blood glucose and may be useful therapeutically for diabetes. However, interrupted glucagon signaling leads to α-cell proliferation. To identify postulated hepatic-derived, circulating factor(s) responsible for α-cell proliferation, we used transcriptomics/proteomics/metabolomics in three models of interrupted glucagon signaling and found that proliferation of mouse, zebrafish, and human α-cells was mTOR- and FoxP transcription factor-dependent. Changes in hepatic amino acid (AA) catabolism gene expression predicted the observed increase in circulating AA. Mimicking these AA levels stimulated α-cell proliferation in a newly developed in vitro assay with L-glutamine being a critical AA. α-cell expression of the AA transporter Slc38a5 was markedly increased in mice with interrupted glucagon signaling and played a role in α-cell proliferation. These results indicate a hepatic-α-islet cell axis where glucagon regulates serum AA availability and AA, especially L-glutamine, regulates α-cell proliferation and mass via mTOR-dependent nutrient sensing.

Glucagon is a critical regulator of glucose homeostasis; however, mechanisms regulating glucagon action and α-cell function and number are incompletely understood. To elucidate the role of the hepatic glucagon receptor (Gcgr) in glucagon action, we generated mice with hepatocyte-specific deletion of the glucagon receptor. GcgrHep−/− mice exhibited reductions in fasting blood glucose and improvements in insulin sensitivity and glucose tolerance compared with wild-type controls, similar in magnitude to changes observed in Gcgr−/− mice. Despite preservation of islet Gcgr signaling, GcgrHep−/− mice developed hyperglucagonemia and α-cell hyperplasia. To investigate mechanisms by which signaling through the Gcgr regulates α-cell mass, wild-type islets were transplanted into Gcgr−/− or GcgrHep−/− mice. Wild-type islets beneath the renal capsule of Gcgr−/− or GcgrHep−/− mice exhibited an increased rate of α-cell proliferation and expansion of α-cell area, consistent with changes exhibited by endogenous α-cells in Gcgr−/− and GcgrHep−/− pancreata. These results suggest that a circulating factor generated after disruption of hepatic Gcgr signaling can increase α-cell proliferation independent of direct pancreatic input. Identification of novel factors regulating α-cell proliferation and mass may facilitate the generation and expansion of α-cells for transdifferentiation into β-cells and the treatment of diabetes.

Exposure to pesticides has been suggested to increase the risk of Parkinson's disease (PD), but the mechanisms responsible for this association are not clear. Here, we report that perinatal exposure of mice during gestation and lactation to low levels of dieldrin (0.3, 1, or 3 mg/kg every 3 days) alters dopaminergic neurochemistry in their offspring and exacerbates MPTP toxicity. At 12 wk of age, protein and mRNA levels of the dopamine transporter (DAT) and vesicular monoamine transporter 2 (VMAT2) were increased by perinatal dieldrin exposure in a dose-related manner. We then administered MPTP (2 x 10 mg/kg s.c) at 12 wk of age and observed a greater reduction of striatal dopamine in dieldrin-exposed offspring, which was associated with a greater DAT:VMAT2 ratio. Additionally, dieldrin exposure during development potentiated the increase in GFAP and alpha-synuclein levels induced by MPTP, indicating increased neurotoxicity. In all cases there were greater effects observed in the male offspring than the female, similar to that observed in human cases of PD. These data suggest that developmental exposure to dieldrin leads to persistent alterations of the developing dopaminergic system and that these alterations induce a "silent" state of dopamine dysfunction, thereby rendering dopamine neurons more vulnerable later in life.

Highlights d NTPDase3 is a marker highly expressed in adult human pancreatic b cells d NTPDase3 expression is maintained in b cells from individuals with diabetes d Use of an NTPDase3 antibody enables isolation of live b cells from human islets d NTPDase3 antibody also allows detection of human b cells by in vivo imaging

Glucagon antagonism is a potential treatment for diabetes. One potential side effect is α-cell hyperplasia, which has been noted in several approaches to antagonize glucagon action. To investigate the molecular mechanism of the α-cell hyperplasia and to identify the responsible factor, we created a zebrafish model in which glucagon receptor (gcgr) signaling has been interrupted. The genetically and chemically tractable zebrafish, which provides a robust discovery platform, has two glucagon receptor genes (gcgra and gcgrb) in its genome. Sequence, phylogenetic, and synteny analyses suggest that these are co-orthologs of the human GCGR. Similar to its mammalian counterparts, gcgra and gcgrb are mainly expressed in the liver. We inactivated the zebrafish gcgra and gcgrb using TALEN (Transcription activator-like effector nuclease) first individually and then both genes, and assessed the number of α-cells using an α-cell reporter line, Tg(gcga:GFP). Compared to wild-type fish at 7 days postfertilization, there were more α-cells in gcgra−/−, gcgrb−/−, and gcgra−/−;gcgrb−/− fish and there was an increased rate of α-cell proliferation in the gcgra−/−; gcgrb−/− fish. Glucagon levels were higher but free glucose levels were lower in gcgra−/−, gcgrb−/−, and gcgra−/−;gcgrb−/− fish, similar to Gcgr−/− mice. These results indicate that the compensatory α-cell hyperplasia in response to interruption of glucagon signaling is conserved in zebrafish. The robust α-cell hyperplasia in gcgra−/−;gcgrb−/− larvae provides a platform to screen for chemical and genetic suppressors, and ultimately to identify the stimulus of α-cell hyperplasia and its signaling mechanism.

Parkinson's disease (PD) is primarily thought of as a disease of aging. However recent evidence points to the potential for exposure to xenobiotics during development to increase risk of PD. Here, we report that developmental exposure to the organochlorine pesticide heptachlor alters the dopamine system and increases neurotoxicity in an animal model of PD. Exposure of pregnant mice to heptachlor led to increased levels of the dopamine transporter (DAT) and vesicular monoamine transporter 2 (VMAT2) levels at both the protein and mRNA level in their offspring. Increased DAT and VMAT2 levels were accompanied by alterations of mRNA levels of nuclear transcription factors that control dopamine neuron development and regulate DAT and VMAT2 levels in adulthood. At 12 weeks of age, control and heptachlor-exposed offspring were administered a moderate dose (2×10 mg/kg) of the parkinsonism-inducing agent MPTP. Greater neurotoxicity as evidenced by a greater loss of striatal dopamine and potentiation of increased levels of glial fibrillary acidic protein and α-synuclein was observed in heptachlor-exposed offspring. The neurotoxicity observed was greater in the male offspring than the female offspring, suggesting that males are more susceptible to the longterm effects of developmental heptachlor exposure. These data suggest that developmental heptachlor exposure causes long-term alterations of the dopamine system thereby rendering it more susceptible to dopaminergic damage in adulthood. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Although most cases of PD are observed later in life, there is evidence that the disease process begins long before the disease has progressed to the point at which it is diagnosed (Calne and Langston, 1983;Fearnley and Lees, 1991;Di Paola and Uitti, 1996). The presence of a lengthy preclinical phase and the linkage of environmental exposures to increased risk of PD has led to speculation that early life exposures to environmental toxicants may enhance dopaminergic neurodegeneration or increase vulnerability of the dopamine system in adulthood leading to an increased risk of PD (Carvey et al., 2003;Cory-Slechta et al., 2005;Richardson et al., 2006;Barlow et al., 2007). NIH Public AccessPesticide exposure has been on of the most studied environmental risk factors for PD (Semchuk et al., 1992;Tanner and Goldman, 1996;Le Couteur et al., 1999;Priyadarshi et al., 2000; Asherio et al., 2006). However, the possibility that developmental pesticide exposure contributes to PD has only recently begun to be explored. Early postnatal exposure of mice to the ...

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Glucagon and its partner insulin are dually linked in both their secretion from islet cells and their action in the liver. Glucagon signaling increases hepatic glucose output, and hyperglucagonemia is partly responsible for the hyperglycemia in diabetes, making glucagon an attractive target for therapeutic intervention. Interrupting glucagon signaling lowers blood glucose but also results in hyperglucagonemia and α-cell hyperplasia. Investigation of the mechanism for α-cell proliferation led to the description of a conserved liver–α-cell axis where glucagon is a critical regulator of amino acid homeostasis. In return, amino acids regulate α-cell function and proliferation. New evidence suggests that dysfunction of the axis in humans may result in the hyperglucagonemia observed in diabetes. This discussion outlines important but often overlooked roles for glucagon that extend beyond glycemia and supports a new role for α-cells as amino acid sensors.