Directing exocrine secretory vesicles to the apical membrane by actin cables generated by the formin mDia1

Abstract

The final stage in exocrine secretion involves translocation of vesicles from their storage areas to the apical membrane. We show that actin-coated secretory vesicles of the exocrine pancreas travel this distance over bundles of specialized actin cables emanating from the apical plasma membrane. These bundles are stable structures that require constant G-actin incorporation and are distinct from the actin web that surrounds the exocrine lumen. The murine mammalian Diaphanous-related formin 1 (mDia1) was identified as a generator of these cables. The active form of mDia1 localized to the apical membrane, and introduction of an active form of mDia1 led to a marked increase in bundle density along the lumen perimeter. Compromising formation of the cables does not prevent secretion, but results in disorganized trafficking and fusion between secretory vesicles. Similar apical secretory tracks were also found in the submandibular salivary glands. Together with previous results that identified a role for Diaphanous in apical secretion in tubular organs of Drosophila, the role of Diaphanous formins at the final stages of secretion appears to be highly conserved.actin polymerization | secretion tracks | pancreatic acini

Spatial integration of E-cadherin adhesion, signalling and the epithelial cytoskeleton

“…FMNL2 recruitment to cadherin receptors is driven by Rac1 activation [24], but mDia1 localization at junctions [23] requires active RhoA [22]. mDia1 also modulates F-actin cables to facilitate the release of exocrine secretory vesicles in pancreatic cells [25] and regulates E-cadherin transport downstream of RhoB [26].…”

Assessing the Secretory Capacity of Pancreatic Acinar Cells

“…Incubated pancreatic acini at 37ºC in a 5% CO 2 humidified air. Allow acini to recover in oxygenated resuspension medium for 30 min before subsequent examination 3,8 .…”

“…The functional unit of the exocrine pancreas, the acinus, is a cluster of epithelial cells which forms and shares a joint lumen. Upon hormonal or neuronal stimulation, vesicles filled with enzymes are transported to the apical cell surface, fuse with it and expel their content into the lumen [1][2][3] . The secreted enzymes are then drained by a coalescing set of ducts into the small intestine, where they catalyze the breakdown of food into nutrients.…”

“…This setup was originally developed in the late 1970's and was since extended for the study of several exocrine tissues such as the salivary and mammary glands [3][4][5][6][7] . Notably, it allows the study of secretion with minimal interference to the complex cellular organizations of these polarized epithelia.…”

Protein Composition and Biogenesis of the Pancreatic Zymogen Granules

“…By analogy, Rab27B, slps and myosin Vc could form a complex on ZGs to regulate the tethering of ZGs at the apical membrane. In exocrine pancreas, targeting of ZGs to the apical cell surface requires an intact microtubule system and is then transferred to actin (25). The minus end-driven microtubular transport is thought mediated by molecular motor, dynein rather than kinesin.…”