Protein 7B2 is essential for the targeting and activation of PC2 into the regulated secretory pathway of rMTC 6-23 cells

Inside the Insulin Secretory Granule

The pancreatic β-cell is purpose-built for the production and secretion of insulin, the only hormone that can remove glucose from the bloodstream. Insulin is kept inside miniature membrane-bound storage compartments known as secretory granules (SGs), and these specialized organelles can readily fuse with the plasma membrane upon cellular stimulation to release insulin. Insulin is synthesized in the endoplasmic reticulum (ER) as a biologically inactive precursor, proinsulin, along with several other proteins that will also become members of the insulin SG. Their coordinated synthesis enables synchronized transit through the ER and Golgi apparatus for congregation at the trans-Golgi network, the initiating site of SG biogenesis. Here, proinsulin and its constituents enter the SG where conditions are optimized for proinsulin processing into insulin and subsequent insulin storage. A healthy β-cell is continually generating SGs to supply insulin in vast excess to what is secreted. Conversely, in type 2 diabetes (T2D), the inability of failing β-cells to secrete may be due to the limited biosynthesis of new insulin. Factors that drive the formation and maturation of SGs and thus the production of insulin are therefore critical for systemic glucose control. Here, we detail the formative hours of the insulin SG from the luminal perspective. We do this by mapping the journey of individual members of the SG as they contribute to its genesis.

V-ATPase-mediated granular acidification is regulated by the V-ATPase accessory subunit Ac45 in POMC-producing cells

The regulation of the V-ATPase, the proton pump mediating intraorganellar acidification, is still elusive. We find that excess of the neuroendocrine V-ATPase accessory subunit Ac45 reduces the intragranular pH and consequently disturbs prohormone convertase activation and prohormone processing. Thus, Ac45 represents the first V-ATPase regulator.

The vacuolar (H⁺)-ATPase (V-ATPase) is an important proton pump, and multiple critical cell-biological processes depend on the proton gradient provided by the pump. Yet, the mechanism underlying the control of the V-ATPase is still elusive but has been hypothesized to involve an accessory subunit of the pump. Here we studied as a candidate V-ATPase regulator the neuroendocrine V-ATPase accessory subunit Ac45. We transgenically manipulated the expression levels of the Ac45 protein specifically in Xenopus intermediate pituitary melanotrope cells and analyzed in detail the functioning of the transgenic cells. We found in the transgenic melanotrope cells the following: i) significantly increased granular acidification; ii) reduced sensitivity for a V-ATPase-specific inhibitor; iii) enhanced early processing of proopiomelanocortin (POMC) by prohormone convertase PC1; iv) reduced, neutral pH–dependent cleavage of the PC2 chaperone 7B2; v) reduced 7B2-proPC2 dissociation and consequently reduced proPC2 maturation; vi) decreased levels of mature PC2 and consequently reduced late POMC processing. Together, our results show that the V-ATPase accessory subunit Ac45 represents the first regulator of the proton pump and controls V-ATPase-mediated granular acidification that is necessary for efficient prohormone processing.

Strain-specific steroidal control of pituitary function

We have previously shown that 7B2 null mice on the 129/SvEvTac (129) genetic background die at 5 weeks of age with hypercorticosteronemia due to a Cushing's-like disease unless they are rescued by adrenalectomy; however, 7B2 nulls on the C57BL/6NTac (B6) background remain healthy, with normal steroid levels. Since background exerts such a profound influence on the phenotype of this mutation, we have evaluated whether these two different mouse strains respond differently to high circulating steroids by chronically treating wild-type 129 and B6 mice with the synthetic steroid dexamethasone (Dex). Dex treatment decreased the dopamine content of the neurointermediate lobes (NIL) of 129 mice, leading to NIL enlargement and increased total D(2)R mRNA in the 129, but not the B6, NIL. Despite the decrease in this inhibitory transmitter, Dex-treated 129 mice exhibited reduced circulating alpha-melanocyte-stimulating hormone (alpha-MSH) along with reduced POMC-derived peptides compared with controls, possibly due to reduced POMC content in the NIL. In contrast, Dex-treated B6 mice showed lowered cellular ACTH, unchanged alpha-MSH and beta-endorphin, and increased circulating alpha-MSH, most likely due to increased cleavage of NIL ACTH by increased PC2. Dex-treated 129 mice exhibited hyperinsulinemia and lowered blood glucose, whereas Dex-treated B6 mice showed slightly increased glucose levels despite their considerably increased insulin levels. Taken together, our results suggest that the endocrinological response of 129 mice to chronic Dex treatment is very different from that of B6 mice. These strain-dependent differences in steroid sensitivity must be taken into account when comparing different lines of transgenic or knockout mice.

The Arg617-Arg618 cleavage site in the C-terminal domain of PC1 plays a major role in the processing and targeting of the enzyme within the regulated secretory pathway

The C-terminal domain of the prohormone convertase PC1 is involved in targeting of the enzyme to secretory granules in neuroendocrine cells and is subsequently processed in this compartment at an Arg617-Arg618 site. Three other dibasics are found in the C-terminal domain of mouse PC1. Here, we examined the role of the four dibasics in targeting PC1 to secretory granules. All 15 possible combinations of dibasic mutations were performed. Wild-type (WT) and mutant PC1 were stably expressed in neuroendocrine PC12 cells that lacked endogenous PC1. Processing, secretion and intracellular localization of PC1 and its mutants were analyzed. Leaving intact Arg617-Arg618 and mutating any combination of the three other dibasics yielded proteins that were stored and processed in secretory granules, similarly to WT PC1. Mutating Arg617-Arg618 alone or with any one of the three remaining dibasics generated proteins that were efficiently stored in secretory granules but were not processed further. Mutating Arg617-Arg618 with more than one of the remaining dibasics produced proteins that reached the TGN but were not stored in secretory granules and exited the cells through the constitutive secretory pathway. These data demonstrate that the Arg617-Arg618 plays a prominent role in targeting PC1 to secretory granules.

ACTH secretion by mouse corticotroph AtT20 cells is negatively modulated by the intracellular level of 7B2

7B2 is a pan-neuroendocrine protein known to facilitate the trafficking and activation of the prohormone proprotein convertase-2 (PC2). 7B2-null mice not only lack PC2 activity, but they also develop an adrenocorticotropic hormone (ACTH) hypersecretion syndrome, suggesting that 7B2 may regulate hormone secretion. To verify this possibility, we introduced into mouse corticotroph AtT20 cells a retroviral vector carrying either a sense or an antisense 7B2 transgene to induce higher and lower 7B2 expression, respectively. Relative to control AtT20 cells, 7B2-overexpressing cells released less ACTH following KCl-induced membrane depolarization, whereas cells expressing lower levels of 7B2 released relatively more, suggesting that 7B2-related peptides modulate regulated secretion in neuroendocrine cells.

Neuroendocrine secretory protein 7B2: structure, expression and functions

7B2 is an acidic protein residing in the secretory granules of neuroendocrine cells. Its sequence has been elucidated in many phyla and species. It shows high similarity among mammals. A Pro-Pro-Asn-Pro-Cys-Pro polyproline motif is its most conserved feature, being carried by both vertebrate and invertebrate sequences. It is biosynthesized as a precursor protein that is cleaved into an N-terminal fragment and a C-terminal peptide. In neuroendocrine cells, 7B2 functions as a specific chaperone for the proprotein convertase (PC) 2. Through the sequence around its Pro-Pro-Asn-Pro-Cys-Pro motif, it binds to an inactive proPC2 and facilitates its transport from the endoplasmic reticulum to later compartments of the secretory pathway where the zymogen is proteolytically matured and activated. Its C-terminal peptide can inhibit PC2 in vitro and may contribute to keep the enzyme transiently inactive in vivo. The PC2-7B2 model defines a new neuroendocrine paradigm whereby proteolytic activation of prohormones and proneuropeptides in the secretory pathway is spatially and temporally regulated by the dynamics of interactions between converting enzymes and their binding proteins. Interestingly, unlike PC2-null mice, which are viable, 7B2-null mutants die early in life from Cushing's disease due to corticotropin ('ACTH') hypersecretion by the neurointermediate lobe, suggesting a possible involvement of 7B2 in secretory granule formation and in secretion regulation. The mechanism of this regulation is yet to be elucidated. 7B2 has been shown to be a good marker of several neuroendocrine cell dysfunctions in humans. The possibility that anomalies in its structure and expression could be aetiological causes of some of these dysfunctions warrants investigation.

Translational regulation of proinsulin biosynthesis and proinsulin conversion in the pancreatic beta-cell

Insulin secretion from the pancreatic beta -cell can be initiated in minutes, vary as much as 50-100-fold, and be sustained for several hours without need for changes in insulin gene transcription. Remarkably, the cellular content of the hormone and its molecular composition do not vary appreciably in the face of changes of insulin granule exocytosis. Minimal morphological changes are apparent, further indicating that the movement of lipids and membrane proteins between the granule storage pool, the plasma membrane, and Golgi are likewise tightly controlled. Such homeostasis is achieved by an interplay of signaling pathways originating from the metabolism of glucose with downstream targets at the level of translation of dense-core granule proteins, granule biogenesis, and membrane trafficking. Our scant knowledge in this area is confined mostly to a descriptive account of the fate of the major secreted components, principally insulin and the enzymes PC1, PC2, and CPH involved in the proteolytic conversion of proinsulin to insulin. A common theme seems to be the role of intracellular energy homeostasis in integrating the stimulus-secretion and stimulus-biosynthetic responses of this cell.

Interaction of Drosophila melanogaster prohormone convertase 2 and 7B2. Insect cell-specific processing and secretion

The prohormone convertases (PCs) are an evolutionarily ancient group of proteases required for the maturation of neuropeptide and peptide hormone precursors. In Drosophila melanogaster, the homolog of prohormone convertase 2, dPC2 (amontillado), is required for normal hatching behavior, and immunoblotting data indicate that flies express 80- and 75-kDa forms of this protein. Because mouse PC2 (mPC2) requires 7B2, a helper protein for productive maturation, we searched the fly data base for the 7B2 signature motif PPNPCP and identified an expressed sequence tag clone encoding the entire open reading frame for this protein. dPC2 and d7B2 cDNAs were subcloned into expression vectors for transfection into HEK-293 cells; mPC2 and rat 7B2 were used as controls. Although active mPC2 was detected in medium in the presence of either d7B2 or r7B2, dPC2 showed no proteolytic activity upon coexpression of either d7B2 or r7B2. Labeling experiments showed that dPC2 was synthesized but not secreted from HEK-293 cells. However, when dPC2 and either d7B2 or r7B2 were coexpressed in Drosophila S2 cells, abundant immunoreactive dPC2 was secreted into the medium, coincident with the appearance of PC2 activity. Expression and secretion of dPC2 enzyme activity thus appears to require insect cell-specific posttranslational processing events. The significant differences in the cell biology of the insect and mammalian enzymes, with 7B2 absolutely required for secretion of dPC2 and zymogen conversion occurring intracellularly in the case of dPC2 but not mPC2, support the idea that the Drosophila enzyme has specific requirements for maturation and secretion that can be met only in insect cells.

Mutations in the catalytic domain of prohormone convertase 2 result in decreased binding to 7B2 and loss of inhibition with 7B2 C-terminal peptide

Prohormone convertases 1 (PC1) and 2 (PC2) are members of a family of subtilisin-like proprotein convertases responsible for proteolytic maturation of a number of different prohormones and proneuropeptides. Although sharing more than 50% homology in their catalytic domains, PC1 and PC2 exhibit differences in substrate specificity and susceptibility to inhibitors. In addition to these differences, PC2, unlike PC1 and other members of the family, specifically binds the neuroendocrine protein 7B2. In order to identify determinants responsible for the specific properties of the PC2 catalytic domain, we compared its primary sequence with that of other PCs. This allowed us to distinguish a PC2-specific sequence at positions 242-248. We constructed two PC2 mutants in which residues 242 and 243 and residues 242-248 were replaced with the corresponding residues of PC1. Studies of in vivo cleavage of proenkephalin, in vivo production of alpha-MSH from proopiomelanocortin, and in vitro cleavage of a PC2-specific artificial substrate by mutant PC2s did not reveal profound alterations. On the other hand, both mutant pro-PC2s exhibited a considerably reduced ability to bind to 21-kDa 7B2. In addition, inhibition of mutant PC2-(242-248) by the potent natural inhibitor 7B2 CT peptide was almost completely abolished. Taken together, our results show that residues 242-248 do not play a significant role in defining the substrate specificity of PC2 but do contribute greatly to binding 7B2 and are critical for inhibition with the 7B2 CT peptide.

Coordinate regulation of neuroendocrine convertase PC2 and peptide 7B2 in P19 neurons

Convertases are proteases responsible for the bioactivation of many proteins and peptides having a potential role in ontogenesis. As a model to study regulation of convertases in embryo, we use the P19 embryonal carcinoma cell line, which can differentiate into various cell types. The expression of convertase PC2 and its specific binding peptide 7B2 are co-induced during neuronal differentiation of P19 cells. We investigated the possibility that expression of both proteins may be coregulated by T3 and dexamethasone, activators of nuclear receptors, isobutylmethylxanthine, and dibutyryl cAMP, activators of protein kinase A, and phorbol 12-myristate 13-acetate, an activator of protein kinase C. Western blotting results show that expression of PC2 and 7B2 can be upregulated by modulators of the protein kinases, and upregulation needs not be strictly stoichiometric.