title: Comprehensive alpha, beta and delta cell transcriptomes reveal that ghrelin selectively activates delta cells and promotes somatostatin release from pancreatic islets.
abstract: OBJECTIVE
  Complex local crosstalk amongst endocrine cells within the islet ensures tight coordination of their endocrine output. This is illustrated by the recent demonstration that the negative feedback control by delta cells within pancreatic islets determines the homeostatic set-point for plasma glucose during mouse postnatal development. However, the close association of islet endocrine cells that facilitates paracrine crosstalk also complicates the distinction between effects mediated directly on beta cells from indirect effects mediated via local intermediates, such as somatostatin from delta cells.
  
  
  METHODS
  To resolve this problem, we generated reporter mice that allow collection of pure pancreatic delta cells along with alpha and beta cells from the same islets and generated comprehensive transcriptomes for each islet endocrine cell type. These transcriptomes afford an unparalleled view of the receptors expressed by delta, alpha and beta cells, and allow the prediction of which signal targets which endocrine cell type with great accuracy.
  
  
  RESULTS
  From these transcriptomes, we discovered that the ghrelin receptor is expressed exclusively by delta cells within the islet, which was confirmed by fluorescent in situ hybridization and qPCR. Indeed, ghrelin increases intracellular calcium in delta cells in intact mouse islets, measured by GCaMP6 and robustly potentiates glucose-stimulated somatostatin secretion on mouse and human islets in both static and perfusion assays. In contrast, des-acyl-ghrelin at the same dose had no effect on somatostatin secretion and did not block the actions of ghrelin.
  
  
  CONCLUSIONS
  These results offer a straightforward explanation for the well-known insulinostatic actions of ghrelin. Rather than engaging beta cells directly, ghrelin engages delta cells to promote local inhibitory feedback that attenuates insulin release. These findings illustrate the power of our approach to resolve some of the long-standing conundrums with regard to the rich feedback that occurs within the islet that is integral to islet physiology and therefore highly relevant to diabetes.
authors.affiliation: Department of Neurobiology, Physiology & Behavior, College of Biological Sciences, University of California, One Shields Avenue, 196 Briggs Hall, Davis, CA, USA. Electronic address: mrdigruccio@ucdavis.edu.
authors.forename: Michael R
authors.initials: MR
authors.lastname: DiGruccio
authors.affiliation: Department of Neurobiology, Physiology & Behavior, College of Biological Sciences, University of California, One Shields Avenue, 196 Briggs Hall, Davis, CA, USA. Electronic address: ammawla@ucdavis.edu.
authors.forename: Alex M
authors.initials: AM
authors.lastname: Mawla
authors.affiliation: Clayton Foundation Laboratories for Peptide Biology, The Salk Institute for Biological Studies, 10010 North Torrey Pines road, La Jolla, CA, USA. Electronic address: donaldson@salk.edu.
authors.forename: Cynthia J
authors.initials: CJ
authors.lastname: Donaldson
authors.affiliation: Department of Neurobiology, Physiology & Behavior, College of Biological Sciences, University of California, One Shields Avenue, 196 Briggs Hall, Davis, CA, USA. Electronic address: gmnoguchi@ucdavis.edu.
authors.forename: Glyn M
authors.initials: GM
authors.lastname: Noguchi
authors.affiliation: Clayton Foundation Laboratories for Peptide Biology, The Salk Institute for Biological Studies, 10010 North Torrey Pines road, La Jolla, CA, USA. Electronic address: vaughan@salk.edu.
authors.forename: Joan
authors.initials: J
authors.lastname: Vaughan
authors.affiliation: Department of Neurobiology, Physiology & Behavior, College of Biological Sciences, University of California, One Shields Avenue, 196 Briggs Hall, Davis, CA, USA.
authors.forename: Christopher
authors.initials: C
authors.lastname: Cowing-Zitron
authors.affiliation: Department of Neurobiology, Physiology & Behavior, College of Biological Sciences, University of California, One Shields Avenue, 196 Briggs Hall, Davis, CA, USA. Electronic address: tvandermeulen@ucdavis.edu.
authors.forename: Talitha
authors.initials: T
authors.lastname: van der Meulen
authors.affiliation: Department of Neurobiology, Physiology & Behavior, College of Biological Sciences, University of California, One Shields Avenue, 196 Briggs Hall, Davis, CA, USA. Electronic address: mhuising@ucdavis.edu.
authors.forename: Mark O
authors.initials: MO
authors.lastname: Huising
keywords: Alpha cell; Beta cell; Crhr2, Corticotropin-releasing hormone receptor type 2; Delta cell; FISH, Fluorescent in situ hybridization; GSSS, Glucose-stimulated somatostatin secretion; Ghrelin; Ghsr, Growth hormone secretagogue receptor; Iapp, Islet amyloid polypeptide; RPKM, Reads per kilobase gene model per million reads sequenced; Somatostatin release; Transcriptome; Trpm2, Transient receptor potential melastatin 2; Ucn3, Urocortin 3; YFP, Yellow fluorescent protein