MIST1 and PTF1 Collaborate in Feed-Forward Regulatory Loops That Maintain the Pancreatic Acinar Phenotype in Adult Mice

Abstract

bMuch remains unknown regarding the regulatory networks formed by transcription factors in mature, differentiated mammalian cells in vivo, despite many studies of individual DNA-binding transcription factors. We report a constellation of feed-forward loops formed by the pancreatic transcription factors MIST1 and PTF1 that govern the differentiated phenotype of the adult pancreatic acinar cell. PTF1 is an atypical basic helix-loop-helix transcription factor complex of pancreatic acinar cells and is critical to acinar cell fate specification and differentiation. MIST1, also a basic helix-loop-helix transcription factor, enhances the formation and maintenance of the specialized phenotype of professional secretory cells. The MIST1 and PTF1 collaboration controls a wide range of specialized cellular processes, including secretory protein synthesis and processing, exocytosis, and homeostasis of the endoplasmic reticulum. PTF1 drives Mist1 transcription, and MIST1 and PTF1 bind and drive the transcription of over 100 downstream acinar genes. PTF1 binds two canonical bipartite sites within a 0.7-kb transcriptional enhancer upstream of Mist1 that are essential for the activity of the enhancer in vivo. MIST1 and PTF1 coregulate target genes synergistically or additively, depending on the target transcriptional enhancer. The frequent close binding proximity of PTF1 and MIST1 in pancreatic acinar cell chromatin implies extensive collaboration although the collaboration is not dependent on a stable physical interaction.T he mammalian pancreas is a mixed exocrine and endocrine gland consisting of acini, ducts, and islets. Approximately 90% of the mass of the adult pancreas is exocrine acinar tissue. Pancreatic acinar cells are highly specialized for the synthesis and secretion of digestive enzymes that are flushed via ducts to the intestine for digestion of complex nutrients (1). Abundant rough endoplasmic reticulum (ER) supports an extraordinary level of secretory protein synthesis (2). Maintenance of ER homeostasis without the accumulation of misfolded and unfolded proteins is especially important for acinar function and viability (3). Acinar cells are polarized, with basal rough ER and an extensive supranuclear Golgi apparatus for sorting and condensing newly synthesized secretory proteins into secretory vesicles (zymogen granules) that fill the apical cellular domain nearest the luminal plasma membrane. Secretion is regulated to ensure an appropriate surge of digestive enzyme release in response to feeding (1).Several transcription factors, including PTF1 and MIST1, are known to play crucial roles in the specification, differentiation, and maturation of pancreatic acinar cells (4-6). PTF1 is a complex of three tightly associated DNA-binding subunits: the cell-typerestricted basic helix-loop-helix (bHLH) protein PTF1A, one of the common bHLH E proteins (e.g., E47) (7), and RBPJ or its paralog RBPJL (8, 9). All three subunits contribute to the recognition of an extended bipartite binding sequence consisting of an E box boun...

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

“…This observation is consistent with previous studies showing that gastric chief cells also induce Mist1 during ER stress (19), and it supports the hypothesis that transient ER stress and Xbp1s expression may be the developmental trigger that activates Mist1 within regenerating stomach crypts. This is in contrast to pancreatic acinar cells, in which developmental expression of Mist1 is achieved by the acinar transcription factor complex PTF1 (27) while the ER stress-induced enhancement of MIST1 reported here is fully dependent on XBP1. Interestingly, a modest 2-fold increase in Ptf1a expression is also observed during thapsigargininduced ER stress (unpublished results), suggesting that the PTF1 complex might play a minor role in augmenting Mist1 gene expression in ER-stressed pancreatic acinar cells.…”

“…Thus, in the context of ER stress, the scaling effect of MIST1 is achieved by amplifying an initial transcriptional activation achieved by XBP1. This model is supported by additional findings showing that MIST1 similarly collaborates with PTF1 to jointly regulate developmental gene targets via proximal DNA binding (27). The many observations that MIST1's transcriptional activity typically produces moderate changes in gene expression levels rather than functioning as a more customary binary on/off switch support this supposition.…”

“…3B). To further investigate these pathways, we utilized RNA-Seq to evaluate global gene expression differences in adult pancreata isolated from WT and Mist1 KO mice (27). As shown in Fig.…”

“…A complete description of the ChIP-Seq and analyses procedures can be found in Jiang et al (27). Briefly, ChIPSeq was conducted on whole murine pancreatic lysates (total, n ϭ 4) using two separate anti-MIST1 antibodies to minimize the effects of spurious, nonspecific antibody binding.…”

Transcriptional Maintenance of Pancreatic Acinar Identity, Differentiation, and Homeostasis by PTF1A

“…Thus, PTF1A and MIST1 also collaborate at target genes. Indeed, we have recently shown that PTF1A and MIST1 collaborate extensively through reiterated feed-forward regulatory loops controlling acinar genes (102). The extensive regulatory relationships show that PTF1A collaborates with the other network core factors in feed-forward control strategies at large numbers of acinar genes.…”