The protein content and morphogenesis of zymogen granules

Structural Basis of the Pancreatitis-Associated Autoproteolytic Failsafe Mechanism in Human Anionic Trypsin

Objective

Methods

Results

Conclusion

Regulation of secretory granule size by the precise generation and fusion of unit granules

Morphometric evidence derived from studies of mast cells, pancreatic acinar cells and other cell types supports a model in which the post-Golgi processes that generate mature secretory granules can be resolved into three steps: (1) fusion of small, Golgi-derived progranules to produce immature secretory granules which have a highly constrained volume; (2) transformation of such immature granules into mature secretory granules, a process often associated with a reduction in the maturing granule's volume, as well as changes in the appearance of its content and (3) fusion of secretory granules of the smallest size, termed 'unit granules', forming granules whose volumes are multiples of the unit granule's volume. Mutations which perturb this process can cause significant pathology. For example, Chediak-Higashi syndrome / lysosomal trafficking regulator (CHS)/(Lyst) mutations result in giant secretory granules in a number of cell types in human beings with the Chediak-Higashi syndrome and in 'beige' (Lyst(bg)/Lyst(bg)) mice. Analysis of the secretory granules of mast cells and pancreatic acinar cells in Lyst-deficient beige mice suggests that beige mouse secretory granules retain the ability to fuse randomly with other secretory granules no matter what the size of the fusion partners. By contrast, in normal mice, the pattern of granule-granule fusion occurs exclusively by the addition of unit granules, either to each other or to larger granules. The normal pattern of fusion is termed unit addition and the fusion evident in cells with CHS/Lyst mutations is called random addition. The proposed model of secretory granule formation has several implications. For example, in neurosecretory cells, the secretion of small amounts of cargo in granules constrained to a very narrow size increases the precision of the information conveyed by secretion. By contrast, in pancreatic acinar cells and mast cells, large granules composed of multiple unit granules permit the cells to store large amounts of material without requiring the amount of membrane necessary to package the same amount of cargo into small granules. In addition, the formation of mature secretory granules that are multimers of unit granules provides a mechanism for mixing in large granules the contents of unit granules which differ in their content of cargo.

The incoherence of the vesicle theory of protein secretion

The rates at which cells secrete peptides and proteins must on average equal their rate of synthesis. This basic equality has unanticipated and seemingly categorical negative consequences for the vesicle theory of protein secretion. This is because the transport mechanisms it proposes, such as the budding and fusion of small vesicles and secretion by exocytosis, are not capable of balancing forces. What follows is an account of the analysis that leads to this conclusion.

Human cationic trypsinogen. Arg(117) is the reactive site of an inhibitory surface loop that controls spontaneous zymogen activation

Mutation of Arg(117), an autocatalytic cleavage site, is the most frequent amino acid change found in the cationic trypsinogen (Tg) of patients with hereditary pancreatitis. In the present study, the role of Arg(117) was investigated in wild-type cationic Tg and in the activation-resistant Lys(15) --> Gln mutant (K15Q-Tg), in which Tg-specific properties of Arg(117) can be examined selectively. We found that trypsinolytic cleavage of the Arg(117)-Val(118) bond did not proceed to completion, but due to trypsin-catalyzed re-synthesis an equilibrium was established between intact Tg and its cleaved, two-chain form. In the absence of Ca(2+), at pH 8.0, the hydrolysis equilibrium (K(hyd) = [cleaved Tg]/[intact Tg]) was 5.4, whereas 5 mm Ca(2+) reduced the rate of cleavage at Arg(117) at least 20-fold, and shifted K(hyd) to 0.7. These observations indicate that the Arg(117)-Val(118) bond exhibits properties analogous to the reactive site bond of canonical trypsin inhibitors and suggest that this surface loop might serve as a low affinity inhibitor of zymogen activation. Consistent with this notion, autoactivation of cationic Tg was inhibited by the cleaved form of K15Q-Tg, with an estimated K(i) of 80 microm, while no inhibition was observed with K15Q-Tg carrying the Arg(117) --> His mutation. Finally, zymogen breakdown due to other trypsinolytic pathways was shown to proceed almost 2000-fold slower than cleavage at Arg(117). Taken together, the findings suggest two independent, successively functional trypsin-mediated mechanisms against pathological Tg activation in the pancreas. At low trypsin concentrations, cleavage at Arg(117) results in inhibition of trypsin, whereas high trypsin concentrations degrade Tg, thus limiting further zymogen activation. Loss of Arg(117)-dependent trypsin inhibition can contribute to the development of hereditary pancreatitis associated with the Arg(117) --> His mutation.

Hydrocortisone induces an increase of amylase content in individual zymogen granules from rat pancreas

This study was performed to evaluate the effects of different doses of hydrocortisone (1, 10 and 25 mg/kg/day) administered for 1, 3 and 8 days on pancreatic enzyme storage in rats. The enzyme content in both pancreas homogenates and in individual isolated zymogen granules (ZGs) was measured using standard biochemical assays and flow cytometry, respectively. Hydrocortisone did not alter the total amount of pancreatic DNA but increased the pancreas enzyme content in a time-dose-dependent way. Amylase activity was significantly increased after hydrocortisone administration at day +8 when 10 mg/kg/day was used, and from the first day of treatment when 25 mg/kg/day was administered. A significant increase in trypsin activity was also observed in response to 25 mg/kg/day of hydrocortisone but only from the third day of treatment onwards. As compared with control rats, chronic administration of either 1 or 10 mg/kg/day of hydrocortisone did not alter significantly either the size or the percentage of the two ZG subpopulations (Z1 and Z2) identified in the pancreas by flow cytometry; in addition, no significant changes were observed in the mean amylase content per individual granule, although its mean concentration increased in rats treated with 10 mg/kg/day for 3 and 8 days. Nevertheless, when 25 mg/kg/day of hydrocortisone were administered for 1 and 3 days, a significant increase in the proportion of Z1 ZGs was observed, which may be related to the formation of new and smaller ZGs. When a very high dose of hydrocortisone (25 mg/kg/day) was used, an overall increase in the pancreatic enzyme content related to an increase in the mean amylase content per individual ZG was observed; this effect was apparent from the first day of treatment in the Z1 subset of ZGs and from day +3 in the Z2 subpopulation. Only a high concentration of hydrocortisone was able to alter the enzyme storage process in individual zymogen granules, but they maintain a normal enzyme load at lower hydrocortisone doses.

Enzyme changes in zymogen granules and in pancreatic secretion throughout long-term CCK treatment

Pancreatic enzyme storage and secretion were studied in rats treated twice daily with s.c. injections (5 micrograms/kg) of CCK-8 for 3, 7, and 15 days. Isolated zymogen granules were analyzed by flow cytometry to determine their FSC (forward scatter), SSC (side scatter), and amylase and trypsinogen contents. DNA content, pancreatic weight, and both basal and stimulated pancreatic secretion under i.v. CCK infusion (1.25 micrograms/kg/h) were also studies. Two subsets of zymogen granules were identified by flow cytometry in both control and CCK-treated rats on the basis of FSC and SSC parameters: Z1 (smaller and less complex) and Z2. Both subsets displayed a high degree of heterogeneity with respect to their enzyme content per zymogen granule. During the first 7 days of CCK treatment, hyperplasia and hypertrophy developed in the rats together with changes in the zymogen granules, reflected by a significantly decreased FSC, and increased SSC, and an increase in the mean trypsinogen/amylase ratio per granule. A rise in pancreatic enzyme secretion, especially of trypsin, was observed. After 15 days of CCK administration, a simultaneous decrease in amylase content and increase in trypsinogen content per zymogen granule was observed. A desensitization of the pancreas to CCK happened after 15 days of CCK administration, reflected by a reduction of all the pancreatic functions that had been increased at shorter CCK administration periods. Nevertheless, trypsinogen appeared resistant to desensitization because its secretion significantly increased in response to an i.v. infusion of CCK. CCK treatment displayed a differential packaging of the enzymes in individual zymogen granules; the trypsinogen/amylase ration was significantly higher in Z2 zymogen granules than in Z1 subset throughout the treatment.

Exocytosis from rat submandibular granular tubules during cyclocytidine stimulation shows unusual features, including changes in the granule membrane

Sequential secretory changes in granular tubule cells caused by the secretagogue cyclocytidine (75 mg/kg i.p.) were studied at the ultrastructural level, in perfusion (n = 5 animals) and immersion (n = 8 animals) fixed rat submandibular glands, using the periodic acid-thiocarbohydrazide-silver proteinate technique (PA-TCH-SP). The onset of secretion varied from 45 to 75 minutes after administering the cyclocytidine. During the initial stages of overt secretion, structural changes occurred irregularly in a progressive fashion with: (1) an increase in granule membrane staining with PA-TCH-SP and a parallel alignment of the secretory granules with the adjacent apical plasma membrane, which developed a honeycomb-like appearance; (2) docking of these secretory granules to the apical plasma membrane; (3) early secretion of some secretory granules in a semiclassical exocytotic fashion (but this was rarely witnessed). During stages (1) and (2), the cytochemical characteristics of the membrane of the secretory granules, as well as of the plasma membrane, suggest a priming process is occurring. After these initial preparatory phases, further structural changes occurred in the granule membranes with a gradually progressive formation of microvesicles and granule fusions; secretion continued in an explosive manner with proteinaceous material being transferred to lumina in at least three different ways: (1) by typical exocytosis (but it was infrequent); (2) from granules fused intracellularly into aggregates (compound exocytosis); and (3) some apocrine-type of secretion through bleb formation. The formation of these intracellular aggregations was associated with the microvesicles in the granule membranes and some aggregates became very large. Secretion of their contents into lumina occurred through elongated membrane channels. The material secreted included microvesicular forms that had become interiorised in the granular aggregates, and any cytoplasm that may also have been entrapped.

Soft X-ray microscopes and their biological applications

In this review we propose to address the question: for the life-science researcher, what does X-ray microscopy have to offer that is not otherwise easily available?

We will see that the answer depends on a combination of resolution, penetrating power, analytical sensitivity, compatibility with wet specimens, and the ease of image interpretation.