Noc2 is essential in normal regulation of exocytosis in endocrine and exocrine cells

Abstract

Rab3 is a subfamily of the small GTP-binding protein Rab family and plays an important role in exocytosis. Several potential effectors of Rab3, including rabphilin3 and Rims (Rim1 and Rim2), have been isolated and characterized. Noc2 was identified originally in endocrine pancreas as a molecule homologous to rabphilin3, but its role in exocytosis is unclear. To clarify the physiological function of Noc2 directly, we have generated Noc2 knockout (Noc2 ؊͞؊ ) mice. Glucose intolerance with impaired insulin secretion was induced in vivo by acute stress in Noc2 ؊͞؊ mice, but not in wild-type (Noc2 ؉͞؉ ) mice. Ca 2؉ -triggered insulin secretion from pancreatic isles of Noc2 ؊͞؊ mice was markedly impaired, but was completely restored by treatment with pertussis toxin, which inhibits inhibitory G protein Gi͞o signaling. In addition, the inhibitory effect of clonidine, an ␣2-adrenoreceptor agonist, on insulin secretion was significantly greater in Noc2 ؊/؊ islets than in Noc2 ؉͞؉ islets. Impaired Ca 2؉ -triggered insulin secretion was rescued by adenovirus gene transfer of wild-type Noc2 but not by that of mutant Noc2, which does not bind to Rab3. Accordingly, Noc2 positively regulates insulin secretion from endocrine pancreas by inhibiting Gi͞o signaling, and the interaction of Noc2 and Rab3 is required for the effect. Interestingly, we also found a marked accumulation of secretory granules in various exocrine cells of Noc2 ؊͞؊ mice, especially in exocrine pancreas with no amylase response to stimuli. Thus, Noc2, a critical effector of Rab3, is essential in normal regulation of exocytosis in both endocrine and exocrine cells.

Cellular expression of Noc2, a Rab effector protein, in endocrine and exocrine tissues in the mouse

“…To manifest action, the active GTP-bound Rab protein utilizes specific effector proteins possessing a Rab-interacting domain linked to the granular/vesicular membrane or cytoskeleton. Accumulating evidence suggests the direct involvement of Rab effector proteins, such as rabphilin3, granuphilin and Noc2, in the exocytosis of cytoplasmic granules in amine/peptide-secreting endocrine cells (Yi et al 2002;Fukuda et al 2004;Matsumoto et al 2004). Noc2 is unique among the Rab partners in its being small in molecular size and devoid of domains with predictable functions, apart from the Rab-binding motif (Cheviet et al 2004b).…”

“…The phenotype of Noc2-knockout mice is characterized by the remarkable accumulation of secretory granules in exocrine cells rather than in endocrine cells (Matsumoto et al 2004). A marked accumulation of secretory granules was found in the exocrine pancreas, salivary gland, gastric chief cells, intestinal Paneth cells, and duodenal…”

“…Noc2 -/-mice exhibit glucose intolerance with impaired insulin secretion when exposed to stress (Matsumoto et al 2004). Under normal conditions, however, no significant morphological changes indicating the impaired secretion of secretory granules were found in insulin-secreting β cells or in other endocrine cells of the knockout mice.…”

“…Noc2-deficient mice (Matsumoto et al 2004) were used to confirm the specificity of the immunoreaction with the Noc2 antibody. After 24 h starvation the animals were deeply anesthetized by an intraperitoneal injection of pentobarbital sodium, then perfused through the left ventricle of the heart with physiological saline and subsequently with 4% paraformaldehyde in a 0.1 M phosphate buffer, pH 7.4.…”

“…In accord with the morphological changes, there was no amylase secretion from the exocrine pancreas in response to adequate stimuli (Matsumoto et al 2004). Although conventional Northern blotting showed an endocrine tissue-specific expression of Noc2 mRNA 5 (Kotake et al 1997), another northern blot analysis with longer exposure time found a further broad expression of Noc2 mRNA in non-endocrine tissues including the liver, lung, kidney, heart and testis (Manabe et al 2004).…”