Exocrine specific expression of Connexin32 is dependent on the basic helix-loop-helix transcription factor Mist1.

J Michael Rukstalis,Agnes Kowalik,Christopher L Pin,Liqin Zhu,Darcy Lidington,Stephen F Konieczny

doi:10.1242/jcs.00631

J Michael Rukstalis, Agnes Kowalik + Show 4 more

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https://doi.org/10.1242/jcs.00631

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Abstract

Gap junctions are intercellular channels that provide direct passage of small molecules between adjacent cells. In pancreatic acini, the connexin26 (Cx26) and connexin32 (Cx32) proteins form functional channels that coordinate the secretion of digestive enzymes. Although the function of Cx26/Cx32 gap junctions are well characterized, the regulatory circuits that control the spatial and temporal expression patterns of these connexin genes are not known. In an effort to identify the molecular pathways that regulate connexin gene expression, we examined Cx26 and Cx32 gene activities in mice lacking the basic helix-loop-helix transcription factor Mist1 (Mist1KO). Mist1, Cx26 and Cx32 are co-expressed in most exocrine cell types, and acinar cells from Mist1KO mice exhibit a highly disorganized cellular architecture and an altered pattern of expression for several genes involved in regulated exocytosis. Analysis of Mist1KO mice revealed a dramatic decrease in both connexin proteins, albeit through different molecular mechanisms. Cx32 gene transcription was greatly reduced in all Mist1KO exocrine cells, while Cx26 gene expression remained unaffected. However, in the absence of Cx32 protein, Cx26 did not participate in gap junction formation, leading to a complete lack of intercellular communication among Mist1KO acinar cells. Additional studies testing Mist1 gene constructs in pancreatic exocrine cells confirmed that Mist1 transcriptionally regulates expression of the Cx32 gene. We conclude that Mist1 functions as a positive regulator of Cx32 gene expression and, in its absence, acinar cell gap junctions and intercellular communication pathways become disrupted.

Highlights

Pancreatic acini consist of exocrine cells whose main function is to produce and deliver the enzymes necessary for digestion
Expression of Cx32 is significantly reduced in Mist1KO pancreatic acini The unusual morphology of the Mist1KO pancreatic acini, coupled with the altered expression of proteins involved in regulated exocytosis, suggests that cell junction complexes may be disrupted (Pin et al, 2001)
Connexin32 (Cx32) is one of two connexin proteins expressed in serous exocrine cells and increases in gap junctions are readily detected during acinar cell development at the stage coinciding with cell organization (Bock et al, 1997; Yamamoto and Kataoka, 1985)

Summary

Introduction

Pancreatic acini consist of exocrine cells whose main function is to produce and deliver the enzymes necessary for digestion. Binding of CCK to G-protein coupled receptors initiates a signaling cascade that culminates in the release of intracellular calcium, which in turn promotes movement of the zymogen granules (ZG) toward the apical cell surface and exocytosis of enzymes into pancreatic ducts (Joseph, 1996; McNiven and Marlowe, 1999). Improper signaling leads to impaired exocytosis and premature enzyme activation (Saluja et al, 1989). Owing to this specialized function, acinar cells exhibit extensive extra- and intracellular organization, with the granules aligned to the apical border adjacent to a centrally located duct while the receptors for secretagogue binding are located on the basal aspect of the cell

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