Neurogenin 3 is regulated by neurotrophic tyrosine kinase receptor type 2 (TRKB) signaling in the adult human exocrine pancreas.

Michael J Shamblott,Marci L O’Driscoll,Dustin L Mcguire,Danielle L Gomez

doi:10.1186/s12964-016-0146-x

Michael J Shamblott, Marci L O’Driscoll + Show 2 more

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https://doi.org/10.1186/s12964-016-0146-x

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Abstract

BackgroundReports of exocrine-to-endocrine reprogramming through expression or stabilization of the transcription factor neurogenin 3 (NGN3) have generated renewed interest in harnessing pancreatic plasticity for therapeutic applications. NGN3 is expressed by a population of endocrine progenitor cells that give rise exclusively to hormone-secreting cells within pancreatic islets and is necessary and sufficient for endocrine differentiation during development. In the adult human pancreas, NGN3 is expressed by dedifferentiating exocrine cells with a phenotype resembling endocrine progenitor cells and the capacity for endocrine differentiation in vitro. Neurotrophic tyrosine kinase receptor type 2 (TRKB), which regulates neuronal cell survival, differentiation and plasticity, was identified as highly overexpressed in the NGN3 positive cell transcriptome compared to NGN3 negative exocrine cells. This study was designed to determine if NGN3 is regulated by TRKB signaling in the adult human exocrine pancreas.MethodsTranscriptome analysis, quantitative reverse transcriptase polymerase chain reaction (RTPCR) and immunochemistry were used to identify TRKB isoform expression in primary cultures of human islet-depleted exocrine tissue and human cadaveric pancreas biopsies. The effects of pharmacological modulation of TRKB signaling on the expression of NGN3 were assessed by Student’s t-test and ANOVA.ResultsApproximately 30 % of cultured exocrine cells and 95 % of NGN3+ cells express TRKB on their cell surface. Transcriptome-based exon splicing analyses, isoform-specific quantitative RTPCR and immunochemical staining demonstrate that TRKB-T1, which lacks a tyrosine kinase domain, is the predominant isoform expressed in cultured exocrine tissue and is expressed in histologically normal cadaveric pancreas biopsies. Pharmacological inhibition of TRKB significantly decreased the percentage of NGN3+ cells, while a TRKB agonist significantly increased this percentage. Inhibition of protein kinase B (AKT) blocked the effect of the TRKB agonist, while inhibition of tyrosine kinase had no effect. Modulation of TRKB and AKT signaling did not significantly affect the level of NGN3 mRNA.ConclusionsIn the adult human exocrine pancreas, TRKB-T1 positively regulates NGN3 independent of effects on NGN3 transcription. Targeting mechanisms controlling the NGN3+ cell population size and endocrine cell fate commitment represent a potential new approach to understand pancreas pathobiology and means whereby cell populations could be expanded for therapeutic purposes.Electronic supplementary materialThe online version of this article (doi:10.1186/s12964-016-0146-x) contains supplementary material, which is available to authorized users.

Highlights

Reports of exocrine-to-endocrine reprogramming through expression or stabilization of the transcription factor neurogenin 3 (NGN3) have generated renewed interest in harnessing pancreatic plasticity for therapeutic applications
Neurogenin 3 (NGN3) is expressed by dedifferentiated cells in the adult human exocrine pancreas that have begun to take on characteristics of endocrine progenitor cells during development
Neurotrophic tyrosine kinase receptor type 2 (TRKB), a cytokine receptor that promotes functional plasticity in cells of the developing and adult nervous system, was identified as being overexpressed by the NGN3+ cell population compared to NGN3- exocrine cells

Summary

Introduction

Reports of exocrine-to-endocrine reprogramming through expression or stabilization of the transcription factor neurogenin 3 (NGN3) have generated renewed interest in harnessing pancreatic plasticity for therapeutic applications. NGN3 is expressed by a population of endocrine progenitor cells that give rise exclusively to hormone-secreting cells within pancreatic islets and is necessary and sufficient for endocrine differentiation during development. In the adult human pancreas, NGN3 is expressed by dedifferentiating exocrine cells with a phenotype resembling endocrine progenitor cells and the capacity for endocrine differentiation in vitro. Neurotrophic tyrosine kinase receptor type 2 (TRKB), which regulates neuronal cell survival, differentiation and plasticity, was identified as highly overexpressed in the NGN3 positive cell transcriptome compared to NGN3 negative exocrine cells. NGN3+ cells isolated by coexpression of the cell surface glycoprotein CD133 from adult human exocrine tissue have a transcriptome consistent with exocrine-cell dedifferentiation, a phenotype that resembles endocrine progenitor cells during development and a capacity for endocrine differentiation in vitro [5]. The TRKB-T1 isoform, which lacks a tyrosine kinase domain [23,24,25,26], can form heterodimers with TRKB-FL to reduce tyrosine kinase activity [27] or positively signal through the p75 low-affinity nerve growth factor receptor [28], AKT [29] or Rho guanine dissociation inhibitor and Rho/Rac GTPases [30, 31]

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