Pancreatic acinar cells exhibit a polarity that is characterized
by the localization of secretory granules at the apical
membrane. However, the factors that regulate cellular
polarity in these cells are not well understood. In this study,
we investigated the effect of Mist1, a basic helix-loop-helix
transcription factor, on the cellular architecture of pancreatic
acinar cells. Mist1-null mice displayed secretory granules
that were diffuse throughout the pancreatic acinar cells, from
the apical to basolateral membranes, whereas Mist1
heterozygote mice showed apical localization of secretory
granules. Deletion of the Mist1 gene decreased the expression
of type 3 inositol 1,4,5-triphosphate receptors (IP3R) but did
not affect apical localization and expression of IP3R2. Mist1-
null mice also displayed an increase in luminal areas and an
increase in the expression of zymogen granules in pancreatic
acinar cells. These results suggest that Mist1 plays a critical
role in polar localization of cellular organelles and in
maintaining cellular architecture in mouse pancreatic acinar
cells.