Proteoglycans support proper granule formation in pancreatic acinar cells

Zymogen granules (ZG) are specialized organelles in the exocrine pancreas which allow digestive enzyme storage and regulated secretion. The molecular mechanisms of their biogenesis and the sorting of zymogens are still incompletely understood. Here, we investigated the role of proteoglycans in granule formation and secretion of zymogens in pancreatic AR42J cells, an acinar model system. Cupromeronic Blue cytochemistry and biochemical studies revealed an association of proteoglycans primarily with the granule membrane. Removal of proteoglycans by carbonate treatment led to a loss of membrane curvature indicating a supportive role in the maintenance of membrane shape and stability. Chemical inhibition of proteoglycan synthesis impaired the formation of normal electron-dense granules in AR42J cells and resulted in the formation of unusually small granule structures. These structures still contained the zymogen carboxypeptidase, a cargo molecule of secretory granules, but migrated to lighter fractions after density gradient centrifugation. Furthermore, the basal secretion of amylase was increased in AR42J cells after inhibitor treatment. In addition, irregular-shaped granules appeared in pancreatic lobules. We conclude that the assembly of a proteoglycan scaffold at the ZG membrane is supporting efficient packaging of zymogens and the proper formation of stimulus-competent storage granules in acinar cells of the pancreas.

Structured RhoGEF recruitment drives myosin II organization on large exocytic vesicles

The Rho family of GTPases plays a crucial role in cellular mechanics by regulating actomyosin contractility through the parallel induction of actin and myosin assembly and function. Using exocytosis of large vesicles in the Drosophila larval salivary gland as a model, we followed the spatiotemporal regulation of Rho1, which in turn creates distinct organization patterns of actin and myosin. After vesicle fusion, low levels of activated Rho1 reach the vesicle membrane and drive actin nucleation in an uneven, spread-out pattern. Subsequently, the Rho1 activator RhoGEF2 distributes as an irregular meshwork on the vesicle membrane, activating Rho1 in a corresponding punctate pattern and driving local myosin II recruitment, resulting in vesicle constriction. Vesicle membrane buckling and subsequent crumpling occur at local sites of high myosin II concentrations. These findings indicate that distinct thresholds for activated Rho1 create a biphasic mode of actomyosin assembly, inducing anisotropic membrane crumpling during exocrine secretion.

Dysfunction of pancreatic exocrine secretion after experimental spinal cord injury

Pancreatic exocrine dysfunction is an underdiagnosed comorbidity in individuals living with spinal cord injury (SCI) who often present cholestasis, acute pancreatitis or high levels of serum pancreatic enzymes. Parasympathetic control of pancreatic exocrine secretion (PES) is mediated in the medullary dorsal vagal complex in part through cholecystokinin (CCK) release. Our previous reports indicate high thoracic (T3-) SCI reduces vagal afferent sensitivity to GI regulatory peptides, like CCK and thyrotropin releasing hormone (TRH). To date, the effects of experimental SCI on PES are unknown. Here we investigated the modulation of PES following T3-SCI in rats. We measured PES volume and amylase concentration in control and T3-SCI rats (3-days or 3-weeks after injury) following: (i) intra-duodenal administration of a mixed-nutrient liquid meal (Ensure® ™) or (ii) central TRH injection (100 pmol) in the dorsal motor nucleus of the vagus. In a separate cohort of overnight-fasted rats, basal serum amylase levels were measured. The baseline volume of PES secretion was lower in 3-week rats destined to receive Ensure® or TRH following T3-SCI surgery compared to control. PES protein concentration was significantly reduced at baseline in 3-week T3-SCI and elevated in 3-day and 3-week T3-SCI rats postprandially but only elevated in 3-day rats following TRH microinjection. Serum amylase activity levels were elevated in 3-day T3-SCI rats and remained at similar levels post 3-weeks T3-SCI. Our data suggest that vagally-mediated regulation of multiple visceral organs is disrupted in the days and weeks following experimental SCI.