Pancreatic acinar cell dysfunction in CFTR(-/-) mice is associated with impairments in luminal pH and endocytosis

Abnormal Paneth cell granule dissolution and compromised resistance to bacterial colonization in the intestine of CF mice

Paneth cells of intestinal crypts contribute to host defense by producing antimicrobial peptides that are packaged as granules for secretion into the crypt lumen. Here, we provide evidence using light and electron microscopy that postsecretory Paneth cell granules undergo limited dissolution and accumulate within the intestinal crypts of cystic fibrosis (CF) mice. On the basis of this finding, we evaluated bacterial colonization and expression of two major constituents of Paneth cells, i.e., alpha-defensins (cryptdins) and lysozyme, in CF murine intestine. Paneth cell granules accumulated in intestinal crypt lumens in both untreated CF mice with impending intestinal obstruction and in CF mice treated with an osmotic laxative that prevented overt clinical symptoms and mucus accretion. Ultrastructure studies indicated little change in granule morphology within mucus casts, whereas granules in laxative-treated mice appear to undergo limited dissolution. Protein extracts from CF intestine had increased levels of processed cryptdins compared with those from wild-type (WT) littermates. Nonetheless, colonization with aerobic bacteria species was not diminished in the CF intestine and oral challenge with a cryptdin-sensitive enteric pathogen, Salmonella typhimurium, resulted in greater colonization of CF compared with WT intestine. Modest downregulation of cryptdin and lysozyme mRNA in CF intestine was shown by microarray analysis, real-time quantitative PCR, and Northern blot analysis. Based on these findings, we conclude that antimicrobial peptide activity in CF mouse intestine is compromised by inadequate dissolution of Paneth cell granules within the crypt lumens.

Mutational and functional analysis of SLC4A4 in a patient with proximal renal tubular acidosis

Permanent isolated proximal renal tubular acidosis (pRTA) with ocular abnormalities is a systemic disease with isolated pRTA, short stature and ocular abnormalities. We identified a novel homozygous deletion of nucleotide 2,311 adenine in the kidney type Na+/HCO3- cotransporter (kNBC1) cDNA in a patient with permanent isolated pRTA. This mutation is predicted to result in a frame shift at codon 721 forming a stop codon after 29 amino acids anomalously transcribed from the SLC4A4 gene. Cosegregation of this mutation with the disease was supported by heterozygosity in the parents of the affected patient. The absence of this mutation in 156 alleles of 78 normal individuals indicates that this mutation is related to the disease and is not a common DNA sequence polymorphism. When injected into Xenopus oocytes, the mutant cRNA failed to induce electrogenic transport activity. In addition, immunofluorescence and Western blot analysis failed to detect the expression of the full-length protein in mutant-injected oocytes. Our results expand the spectrum of kNBC1 mutations in permanent isolated pRTA with ocular abnormalities and increase our understanding of the renal tubular mechanism that is essential for acid-base homeostasis.

Localization of Na+-HCO-3 cotransporter (NBC-1) variants in rat and human pancreas

Mutations in Na(+)-HCO(3)(-) cotransporter (NBC-1) cause proximal renal tubular acidosis (pRTA) associated with ocular abnormalities. One pRTA patient had increased serum amylase, suggesting possible evidence of pancreatitis. To further delineate a link between NBC-1 inactivation and pancreatic dysfunction, immunohistochemical analysis was performed on rat and human pancreas using antibodies against kidney-type (kNBC-1) and pancreatic-type (pNBC-1) transporters. In rat pancreas, the anti-pNBC-1 antibody labeled acinar cells and both apical and basolateral membranes of medium and large duct cells. In human pancreas, on the other hand, the anti-pNBC-1 antibody did not label acinar cells, although it did label the basolateral membranes of the entire duct system. The labeling by anti-kNBC-1 antibody was detected in only a limited number of rat pancreatic duct cells. To examine the effects of pRTA-related mutations, R342S and R554H, on pNBC-1 function, we performed functional analysis and found that both mutants had reduced transport activities compared with the wild-type pNBC-1. These results indicate that pNBC-1 is the predominant variant that mediates basolateral HCO(3)(-) uptake into duct cells in both rat and human pancreas. The loss of pNBC-1 function is predicted to have significant impact on overall ductal HCO(3)(-) secretion, which could potentially lead to pancreatic dysfunction.

Cystic fibrosis and related diseases of the pancreas

The discovery of the gene for cystic fibrosis (CF), the cystic fibrosis transmembrane conductance regulator (CFTR), brought about a new era in the study of this disease. Identification of the molecular target has yielded a flood of data that add to our understanding of the pathogenesis, diagnosis and treatment of CF. The CFTR protein is a cAMP-regulated Cl(-) channel with multiple functions in epithelial cells. In the exocrine pancreas the CFTR plays a key role in the apical Cl(-), HCO(3)(-), and water transport in duct cells. The severe loss of functions, caused by mutations of the CFTR gene, leads to pathological lesions of the pancreas. Over 1200 CFTR mutations and polymorphisms have been identified and their diversity may explain the high level of heterogeneity in the CF phenotype. Mutation analyses of the CFTR gene have revealed a spectrum of CFTR-related diseases that do not fit the classical CF picture but are associated with dysfunction of CFTR, such as chronic pancreatitis.