Aberrant Localization of Intracellular Organelles, Ca2+ Signaling, and Exocytosis in Mist1 Null Mice

Abstract

Ca2؉ signaling and exocytosis are highly polarized functions of pancreatic acinar cells. ؊/؊ cells. Deletion of Mist1 also led to mislocalization of the Golgi apparatus and markedly reduced digestive enzyme content. The combination of aberrant Ca 2؉ signaling and reduced digestive enzyme content resulted in poor secretion of digestive enzymes. Yet, food consumption and growth of Mist1 ؊/؊ mice were normal for at least 32 weeks. These findings reveal that Mist1 is critical to normal organelle localization in exocrine cells and highlight the critical importance of maintaining cellular architecture and polarized localization of cellular organelles in generating a propagating apical-to-basal Ca 2؉ wave. The studies also reveal the spare capacity of the exocrine pancreas that allows normal growth and development in the face of compromised exocrine pancreatic function.

Murine Embryonic Stem Cell–Derived Pancreatic Acinar Cells Recapitulate Features of Early Pancreatic Differentiation

“…Ela-pur p48-Mist1 cells (Fig. 7A, right) displayed a rapid and transient increase in Ca 2+ concentration, similar to that described for primary isolated mouse acinar cells (38). In contrast, Ela-pur C cells (left) showed a smaller increase in Ca 2+ concentration lacking the typical transientness of the normal response.…”

“…It is intriguing that this figure is similar to the proportion of cells that could be efficiently transduced by adenoviral transgene delivery during selection. Because Ela-pur C cells which spontaneously differentiated exhibited an aberrant pattern of Ca 2+ response, similar to that described in acinar cells of Mist-1 deficient mice (38), it is tempting to speculate that efficient expression of Mist1 renders the Ca 2+ signalling machinery more mature in elastase-producing cells, as has been proposed in vivo. Also, the percentage of cells exhibiting zymogen granule-like vesicules was higher in Ela-pur p48-Mist1 cells than in any other experimental condition, in agreement with the fact that the number of zymogen granules is reduced in Mist1 knock-out mice (19).…”

MIST1 Links Secretion and Stress as both Target and Regulator of the Unfolded Protein Response

“…Pancreatic acinar cells lacking MIST1 lose nearly all ability to secrete digestive enzymes and have significantly increased basal cellular autophagy, characteristics that are found under ER stress conditions as well as in cells lacking Xbp1 (12,21,24,49,50). The observed XBP1-mediated induction of Mist1 during ER stress, coupled with the marked reduction in secretion associated with Mist1 KO acinar cells (12,21), prompted us to investigate if MIST1 might regulate a previously uncharacterized set of gene targets needed for both basal and ER stress-induced maintenance of the secretory machinery. For these studies, chromatin from WT mouse pancreata was isolated using two independently derived MIST1 antibodies and subsequently analyzed via ChIP-Seq strategies.…”

“…Recently, the basic helix-loop-helix (bHLH) transcription factor MIST1 (encoded by Bhlha15) has emerged as a potent regulator of a wide variety of secretory cell functions and responses to stress (9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19). Secretory cells lacking MIST1 exhibit structural and functional defects, including altered cytoskeletal organization, a change in cell polarity, and improper cell-to-cell communication (12,17,(19)(20)(21). Interestingly, the most severe phenotype observed in Mist1 knockout (Mist1 KO ) cells is a nearly complete loss of secretory capacity (21,22), a tantalizing finding that hints at a much more extensive role for MIST1 and its target genes beyond mere regulation of cargo-cytoskeletal interactions.…”

“…Secretory cells lacking MIST1 exhibit structural and functional defects, including altered cytoskeletal organization, a change in cell polarity, and improper cell-to-cell communication (12,17,(19)(20)(21). Interestingly, the most severe phenotype observed in Mist1 knockout (Mist1 KO ) cells is a nearly complete loss of secretory capacity (21,22), a tantalizing finding that hints at a much more extensive role for MIST1 and its target genes beyond mere regulation of cargo-cytoskeletal interactions. While several studies have investigated small numbers of MIST1 gene targets (12-14, 18, 19, 23), no comprehensive analysis of MIST1 genes has been performed that fully explains the severe secretory dysfunction observed in the absence of MIST1.…”