Hypersecretion of zymogen granules in the pathogenesis of cystic fibrosis

The ontogeny of saliva secretion in infants and esophagoprotection

BACKGROUND

Several studies have reported that severe reflux esophagitis is rare in infants despite the well known high occurrence of regurgitation in early infancy. There is evidence of the importance of saliva for the pre-epithelial protection of the esophageal mucosa.

RESULTS

A longitudinal study conducted on healthy infants indicated that the stimulated capacity of saliva secretion (saliva output per kg of body weight) was significantly higher during their first year of age compared to older children and adults. In addition, this secretion pattern was also observed in low weight newborns during the first weeks of life and persisted in infants with severe protein-calorie malnutrition (marasmus).

CONCLUSION

The greater ability to secrete saliva is an important physiological condition that may protect the infant from acid/pepsin aggression to the esophagus during early stages of development.

CFTR activity and mitochondrial function

Cystic Fibrosis (CF) is a frequent and lethal autosomal recessive disease, caused by mutations in the gene encoding the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR). Before the discovery of the CFTR gene, several hypotheses attempted to explain the etiology of this disease, including the possible role of a chloride channel, diverse alterations in mitochondrial functions, the overexpression of the lysosomal enzyme α-glucosidase and a deficiency in the cytosolic enzyme glucose 6-phosphate dehydrogenase. Because of the diverse mitochondrial changes found, some authors proposed that the affected gene should codify for a mitochondrial protein. Later, the CFTR cloning and the demonstration of its chloride channel activity turned the mitochondrial, lysosomal and cytosolic hypotheses obsolete. However, in recent years, using new approaches, several investigators reported similar or new alterations of mitochondrial functions in Cystic Fibrosis, thus rediscovering a possible role of mitochondria in this disease. Here, we review these CFTR-driven mitochondrial defects, including differential gene expression, alterations in oxidative phosphorylation, calcium homeostasis, oxidative stress, apoptosis and innate immune response, which might explain some characteristics of the complex CF phenotype and reveals potential new targets for therapy.

Intracellular CFTR: localization and function

Intracellular CFTR: Localization and Function. Physiol. Rev. 79, Suppl.: S175-S191, 1999. - There is considerable evidence that CFTR can function as a chloride-selective anion channel. Moreover, this function has been localized to the apical membrane of chloride secretory epithelial cells. However, because cystic fibrosis transmembrane conductance regulator (CFTR) is an integral membrane protein, it will also be present, to some degree, in a variety of other membrane compartments (including endoplasmic reticulum, Golgi stacks, endosomes, and lysosomes). An incomplete understanding of the molecular mechanisms by which alterations in an apical membrane chloride conductance could give rise to the various clinical manifestations of cystic fibrosis has prompted the suggestion that CFTR may also play a role in the normal function of certain intracellular compartments. A variety of intracellular functions have been attributed to CFTR, including regulation of membrane vesicle trafficking and fusion, acidification of organelles, and transport of small anions. This paper aims to review the evidence for localization of CFTR in intracellular organelles and the potential physiological consequences of that localization.

Mislocalization of delta F508 CFTR in cystic fibrosis sweat gland

Misprocessing and mislocalization of the cystic fibrosis (CF) transmembrane conductance regulator (CFTR) has been described for the major CF-causing mutation (delta F508) in heterologous expression systems in vitro. We have generated monoclonal antibodies (mAbs) to CFTR with the aim of localizing the protein and its CF variants in vivo. Of the tissues where CFTR was observed, only the sweat gland is readily available and does not undergo secondary changes due to CF disease pathology. Sweat ducts from CF patients homozygous for delta F508 did not show the typical apical membrane staining seen in control biopsies. This demonstrates that the biosynthetic arrest and intracellular retention of delta F508 CFTR initially observed in vitro does occur in vivo and emphasizes the need to focus efforts on understanding the mislocalization.

Clinical applications of electrophoresis of human salivary proteins

Human salivary proteins have been studied by electrophoresis in denaturing and non-denaturing polyacrylamide gel electrophoresis (PAGE) as well as by isoelectric focusing (IEF) and two-dimensional procedures, and the clinical applications of this have been reviewed. Whilst non-denaturing PAGE is useful in studying polymorphisms, sodium dodecylsulphate PAGE appears to be otherwise preferable. Immobilized pH gradients containing carrier ampholytes (CAs) give better resolution than CA-based IEF and overcome the problems of cathode drift and loss of basic material. Proline-rich proteins stain poorly with conventional procedures and special techniques are necessary. In clinical studies, findings must be viewed over and above the large number of polymorphisms which occur normally. Studies relating salivary protein and peptide profiles to dental caries susceptibility are encouraging. Specific protein abnormalities have been associated with connective tissue disorders and could form the basis of new non-invasive diagnostic procedures. Protein differences associated with cystic fibrosis and diabetes mellitus, however, merit reinvestigation with the new procedures now available. Detection of HIV antigens in saliva is a new area of research. In the light of new techniques available and new information which has arisen from DNA studies, future prospects for the clinical applications of electrophoresis of saliva look good.

Salivary gland changes in disease

Ultrastructural alterations occurring in human salivary glands as a result of a variety of diseases are described. Major changes in these organs in cases of cystic fibrosis are probably the result of duct blockage, as indicated by study of chronically inflamed salivary glands. A new disease of salivary glands is reported in which parotid serous granules are distorted by bundles of fine filaments, which may be due to the untoward polymerization of a secretory protein, perhaps proline-rich protein. Finally, a clear-cell adenocarcinoma of palatine gland origin is described in which the nuclei display complex pseudoinclusions and the cytoplasm contains numerous herpes-type viruses.

Defective beta-adrenergic secretory responses in submandibular acinar cells from cystic fibrosis patients

The in-vitro investigation of secretory responses of submandibular tissues from three cystic fibrosis (CF) patients and four control subjects showed that responses to a beta-adrenergic stimulus (isoproterenol) were much poorer in CF cells than in control cells. The beta-adrenergic secretory responses of the CF cells (as measured by amylase and mucin secretion) were increased in the presence of 3-isobutyl-l-methyl xanthine, a cyclic nucleotide phosphodiesterase inhibitor. Perhaps an alteration in a regulator of cyclic adenosine monophosphate and Ca2+ metabolism in CF cells is responsible for the decrease in beta-adrenergic function. This proposal would account for the defective regulation of protein secretion, Cl- transport, and Ca2+ homoeostasis in CF exocrine cells and thus might be directly related to the genetic defect.

Cystic fibrosis: a casualty of "detoxification"?

It is proposed that excessive and/or aberrant function of cytochromes P450, due to a combination of genetic and environmental influences, is the basic defect in cystic fibrosis. Organs that are involved in oxidative detoxification reactions in foetal life are thus at risk: tissue damage is initiated by excessive production of oxygen free radicals which deplete cellular antioxidants and provoke the secretion of mucus.

Effect of chronic treatment with cystic fibrosis fibroblast medium on rat submandibular gland acinar cells

The effect of chronic treatment with cystic fibrosis (CF) fibroblast medium on rat submandibular gland and pancreas was investigated. Rats were injected for 8 days with conditioned medium from normal or CF fibroblasts. The elemental content of the acinar cells was measured by X-ray microanalysis of cryosections. A significant increase in cellular calcium, and a decrease in cellular sodium concentrations were found after treatment with CF medium. The ultrastructure of the submandibular acinar cells was not affected by the conditioned CF fibroblast culture medium. No effect of treatment with CF medium on ultrastructure and elemental content of pancreatic acinar cells could be demonstrated. The response to alpha-adrenergic, beta-adrenergic, cholinergic, and peptidergic stimulation in submandibular gland acinar cells of rats injected with normal or CF medium was investigated in vitro. With regard to changes in elemental composition after stimulation, no significant differences in response between the two groups could be found. Apparently, a factor in conditioned medium from cultured CF fibroblasts induces a net increase in calcium content of rat submandibular gland acinar cells. Possibly, this factor acts in a similar way in CF patients and may cause elevated calcium levels in CF cells.

Adrenergic secretory responses of submandibular tissues from control subjects and cystic fibrosis patients

Adrenergic secretory responses of submandibular glands from control subjects and cystic fibrosis patients have been studied in vitro. In control tissues, isoproterenol (10 mumol/l) and noradrenaline (10 mumol/l) increased release of mucins and amylase to a similar extent (approximately 3-fold) and their actions were mediated by stimulation of beta-adrenergic receptors. In cystic fibrosis tissues, isoproterenol did not significantly increase release of mucins or amylase above the basal rate during 40 min incubation, whereas secretion in response to noradrenaline was not significantly different from that in control tissues. In the presence of a phosphodiesterase inhibitor, secretion of mucins and amylase in response to isoproterenol (10 mumol/l) in cystic fibrosis tissues was increased to the same level as that of noradrenaline (10 mumol/l); giving the same pattern of adrenergic responses in cystic fibrosis tissues as in control. The results suggest that overactivity of phosphodiesterase in cystic fibrosis cells might be the cause of the observed decreased secretion in response to a beta-adrenergic agonist.

Cystic fibrosis serum stimulates mucin secretion but not calcium efflux from rat submandibular acini

Cystic fibrosis serum stimulated release of mucins from rat submandibular acini to a significantly greater degree than did control serum. This action was not due to non-specific cell lysis, as monitored by lactate dehydrogenase release and was not accompanied by increases in 45Ca2+-efflux. The actions of cystic fibrosis serum and control serum in stimulating amylase release from rat exocrine pancreatic acini could not be distinguished. Thus, a specific action of cystic fibrosis serum in increasing mucin secretion, but not calcium permeability, in rat exocrine acini has been demonstrated.

The effects of exercise on protein and electrolyte secretion in parotid saliva

1. Ten subjects collected ten 1 min samples of parotid saliva at a constant flow rate 1-2 hr before exercise, immediately after running 3-8 miles and 3 hr after exercise. 2. Exercise had no significant effects on the concentration of sodium, potassium, calcium, magnesium, chloride, bicarbonate or inorganic phosphate. 3. Exercise caused a marked elevation in total protein concentration and in seven of the subjects the initial samples after exercise were cloudy due to protein precipitation. The precipitated protein could be dissolved in sodium EDTA. 4. The saliva samples collected immediately after exercise from the seven subjects in which precipitation occurred contained higher concentrations of protein, calcium and phosphate than in those of the other three subjects. 5. Electrophoretic analysis revealed that all proteins appeared to show a proportional increase in the cloudy saliva, as compared with the clear saliva but differential precipitation of certain proteins occurred in the former which was largely restricted to proteins having a high affinity for calcium phosphate. 6. Electronmicroscopic examination of centrifuged cloudy saliva produced after exercise revealed the presence of rounded droplets of homogeneous structure in contrast to the fine granular deposits produced in clear parotid saliva by in vitro procedures causing precipitation of calcium phosphate crystals on which protein adsorbed.

Serum immunoreactive trypsin in cystic fibrosis

Serum immunoreactive trypsin (IRT) concentrations were measured blind in sera from 33 patients with cystic fibrosis and in 32 age-matched controls. Thirty-two patients had abnormal trypsin concentrations. In 30 of these patients the level of IRT was very low whereas in the other two (the youngest of the group) it was extremely high. All 30 patients with low IRT concentrations had clinical or biochemical evidence of malabsorption. Serum IRT concentration may thus be a useful diagnostic test for cystic fibrosis. In view of the recent observation that neonates with cystic fibrosis consistently have raised IRT concentrations, and the fact that the youngest two patients in our series also had markedly increased IRT, it would appear that the initial elevation of IRT reflects early damage to the pancreas with release of enzyme/zymogen; this is thereafter superseded by a progressive destruction of the pancreas resulting in low IRT concentrations in adults with cystic fibrosis.

Serum isoamylase activities in diabetes mellitus

Serum pancreatic isoamylase activities were used to assess exocrine pancreatic function in 39 patients with diabetes mellitus (21 on insulin, 12 on sulphonylureas, and six on biguanides or diet), and the results were compared with serum immunoreactive trypsin concentrations. Thirteen patients had decreased pancreatic isoamylase activity, the insulin-dependent diabetics showing the highest incidence of abnormality. This incidence of abnormality is similar to that previously described for serum immunoreactive trypsin, and the two procedures gave excellent overall correlation (r = 0.9). Our observations offer further evidence that pancreatic exocrine function is impaired in diabetes mellitus. Serum isoamylase determination provides a convenient, inexpensive, and rapid procedure for its detection.

Abnormal levels of 3':5'-cyclic AMP in isoproterenol-stimulated fibroblasts from patients with cystic fibrosis

To determine if the abnormalities of exocrine secretion characteristic of cystic fibrosis could be investigated in vitro, I studied the synthesis of 3':5'-cyclic AMP after isoproterenol stimulation in skin fibroblasts derived from patients with cystic fibrosis and from normal individuals. Comparison of normal and cystic fibrosis cells showed that the latter had 2- to 5-fold greater levels of intracellular cyclic AMP after stimulation with isoproterenol. The difference between the strains was observed at every stage of the culture cycle and was specific to stimulation by beta-adrenergic agonists. It could not be accounted for by different dose- or time-response curves nor by leakage of cyclic AMP into the medium. The increased sensitivity to catecholamines may reflect an intrinsic genetic property of cystic fibrosis cells, and it may be feasible to use this system to study the biochemical basis of the genetic defect in cystic fibrosis.

Effects of cystic fibrosis serum on the rat parotid gland

Injections of serum from human patients with cystic fibrosis into adult rats caused pronounced structural modifications and increased mitotic rate in the parotid gland. Mitotic rate was increased from a low level of 0.02/1,000 acinar cells in parotid glands of adult rats to 6.5/1,000 acinar cells after 2 or 3 days of serum injection. At the light and electron microscopic levels, significant acinar cell atrophy and degranulation were observed. Cellular necrosis, and increases in quantity of lysosome-like dense bodies, mast cells, and macrophages were also detected. These changes are suggestive of tissue response to injurious foreign protein. Furthermore, the fact that normal sera pronounced the same kind of effects (but greatly reduced in extent) strengthens the view that these effects result from the immunologic response of the host organ to foreign antigen. Since, however, the responses of the rat parotid to cystic fibrosis serum were considerably more marked than those elicited by normal serum, the rat parotid may thus have potential usefulness in assaying for the presence of human cystic fibrosis factor.