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Secretagogin protects Pdx1 from proteasomal degradation to control a transcriptional program required for ? cell specification.

ABSTRACT: OBJECTIVE:Specification of endocrine cell lineages in the developing pancreas relies on extrinsic signals from non-pancreatic tissues, which initiate a cell-autonomous sequence of transcription factor activation and repression switches. The steps in this pathway share reliance on activity-dependent Ca2+ signals. However, the mechanisms by which phasic Ca2+ surges become converted into a dynamic, cell-state-specific and physiologically meaningful code made up by transcription factors constellations remain essentially unknown. METHODS:We used high-resolution histochemistry to explore the coincident expression of secretagogin and transcription factors driving ? cell differentiation. Secretagogin promoter activity was tested in response to genetically manipulating Pax6 and Pax4 expression. Secretagogin null mice were produced with their pancreatic islets morphologically and functionally characterized during fetal development. A proteomic approach was utilized to identify the Ca2+-dependent interaction of secretagogin with subunits of the 26S proteasome and verified in vitro by focusing on Pdx1 retention. RESULTS:Here, we show that secretagogin, a Ca2+ sensor protein that controls ? and ? cell turnover in adult, is in fact expressed in endocrine pancreas from the inception of lineage segregation in a Pax4-and Pax6-dependent fashion. By genetically and pharmacologically manipulating secretagogin expression and interactome engagement in vitro, we find secretagogin to gate excitation-driven Ca2+ signals for ? cell differentiation and insulin production. Accordingly, secretagogin-/- fetuses retain a non-committed pool of endocrine progenitors that co-express both insulin and glucagon. We identify the Ca2+-dependent interaction of secretagogin with subunits of the 26S proteasome complex to prevent Pdx1 degradation through proteasome inactivation. This coincides with retained Nkx6.1, Pax4 and insulin transcription in prospective ? cells. CONCLUSIONS:In sum, secretagogin scales the temporal availability of a Ca2+-dependent transcription factor network to define ? cell identity.

SUBMITTER: Malenczyk K

PROVIDER: S-EPMC6034064 | biostudies-literature | 2018 Aug

REPOSITORIES: biostudies-literature

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Secretagogin protects Pdx1 from proteasomal degradation to control a transcriptional program required for β cell specification.

Malenczyk Katarzyna K Szodorai Edit E Schnell Robert R Lubec Gert G Szabó Gábor G Hökfelt Tomas T Harkany Tibor T

Molecular metabolism 20180605

<h4>Objective</h4>Specification of endocrine cell lineages in the developing pancreas relies on extrinsic signals from non-pancreatic tissues, which initiate a cell-autonomous sequence of transcription factor activation and repression switches. The steps in this pathway share reliance on activity-dependent Ca<sup>2+</sup> signals. However, the mechanisms by which phasic Ca<sup>2+</sup> surges become converted into a dynamic, cell-state-specific and physiologically meaningful code made up by tran ...[more]

PMID: 29910119

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Project description:BackgroundInsulin-producing beta cells emerge during pancreas development in two sequential waves. Recently described later-forming beta cells in zebrafish show high similarity to second wave mammalian beta cells in developmental capacity. Loss-of-function studies in mouse and zebrafish demonstrated that the homeobox transcription factors Pdx1 and Hb9 are both critical for pancreas and beta cell development and discrete stage-specific requirements for these genes have been uncovered. Previously, exocrine and endocrine cell recovery was shown to follow loss of pdx1 in zebrafish, but the progenitor cells and molecular mechanisms responsible have not been clearly defined. In addition, interactions of pdx1 and hb9 in beta cell formation have not been addressed.ResultsTo learn more about endocrine progenitor specification, we examined beta cell formation following morpholino-mediated depletion of pdx1 and hb9. We find that after early beta cell reduction, recovery occurs following loss of either pdx1 or hb9 function. Unexpectedly, simultaneous knockdown of both hb9 and pdx1 leads to virtually complete and persistent beta cell deficiency. We used a NeuroD:EGFP transgenic line to examine endocrine cell behavior in vivo and developed a novel live-imaging technique to document emergence and migration of late-forming endocrine precursors in real time. Our data show that Notch-responsive progenitors for late-arising endocrine cells are predominantly post mitotic and depend on pdx1. By contrast, early-arising endocrine cells are specified and differentiate independent of pdx1.ConclusionsThe nearly complete beta cell deficiency after combined loss of hb9 and pdx1 suggests functional cooperation, which we clarify as distinct roles in early and late endocrine cell formation. A novel imaging approach permitted visualization of the emergence of late endocrine cells within developing embryos for the first time. We demonstrate a pdx1-dependent progenitor population essential for the formation of duct-associated, second wave endocrine cells. We further reveal an unexpectedly low mitotic activity in these progenitor cells, indicating that they are set aside early in development.

Dataset Information

Secretagogin protects Pdx1 from proteasomal degradation to control a transcriptional program required for ? cell specification.

Publications

Secretagogin protects Pdx1 from proteasomal degradation to control a transcriptional program required for β cell specification.

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