Mechanisms underlying regulated CFTR trafficking

Controlling subcellular delivery to optimize therapeutic effect

This article focuses on drug targeting to specific cellular organelles for therapeutic purposes. Drugs can be delivered to all major organelles of the cell (cytosol, endosome/lysosome, nucleus, nucleolus, mitochondria, endoplasmic reticulum, Golgi apparatus, peroxisomes and proteasomes) where they exert specific effects in those particular subcellular compartments. Delivery can be achieved by chemical (e.g., polymeric) or biological (e.g., signal sequences) means. Unidirectional targeting to individual organelles has proven to be immensely successful for drug therapy. Newer technologies that accommodate multiple signals (e.g., protein switch and virus-like delivery systems) mimic nature and allow for a more sophisticated approach to drug delivery. Harnessing different methods of targeting multiple organelles in a cell will lead to better drug delivery and improvements in disease therapy.

Protein kinase A regulates ATP hydrolysis and dimerization by a CFTR (cystic fibrosis transmembrane conductance regulator) domain

Gating of the CFTR Cl- channel is associated with ATP hydrolysis at the nucleotide-binding domains (NBD1, NBD2) and requires PKA (protein kinase A) phosphorylation of the R domain. The manner in which the NBD1, NBD2 and R domains of CFTR (cystic fibrosis transmembrane conductance regulator) interact to achieve a properly regulated ion channel is largely unknown. In this study we used bacterially expressed recombinant proteins to examine interactions between these soluble domains of CFTR in vitro. PKA phosphorylated a fusion protein containing NBD1 and R (NBD1-R-GST) on CFTR residues Ser-660, Ser-700, Ser-712, Ser-737, Ser-768, Ser-795 and Ser-813. Phosphorylation of these serine residues regulated ATP hydrolysis by NBD1-R-GST by increasing the apparent K(m) for ATP (from 70 to 250 microM) and the Hill coefficient (from 1 to 1.7) without changing the V(max). When fusion proteins were photolabelled with 8-azido-[alpha-32P]ATP, PKA phosphorylation increased the apparent k(d) for nucleotide binding and it caused binding to become co-operative. PKA phosphorylation also resulted in dimerization of NBD1-R-GST but not of R-GST, a related fusion protein lacking the NBD1 domain. Finally, an MBP (maltose-binding protein) fusion protein containing the NBD2 domain (NBD2-MBP) associated with and regulated the ATPase activity of PKA-phosphorylated NBD1-R-GST. Thus when the R domain in NBD1-R-GST is phosphorylated by PKA, ATP binding and hydrolysis becomes co-operative and NBD dimerization occurs. These findings suggest that during the activation of native CFTR, phosphorylation of the R domain by PKA can control the ability of the NBD1 domain to hydrolyse ATP and to interact with other NBD domains.

The adenosine 2b receptor is recruited to the plasma membrane and associates with E3KARP and Ezrin upon agonist stimulation

We have previously shown that adenosine is formed in the intestinal lumen during active inflammation from neutrophil-derived 5'-AMP. Acting through the adenosine A2b receptor (A2bR), the luminally derived adenosine induces vectorial chloride secretion and a polarized secretion of interleukin-6 to the intestinal lumen. Although some G protein-coupled receptors interact with anchoring or signaling molecules, not much is known in this critical area for the A2bR. We used the model intestinal epithelial cell line, T84, and Caco2-BBE cells stably transfected with GFP-A2b receptor to study the intestinal A2bR. The A2bR is present in both the apical and basolateral membranes of intestinal epithelia. Apical or basolateral stimulation of the A2bR induces recruitment of the receptor to the plasma membrane and caveolar fractions. The A2bR co-immunoprecipitates with E3KARP and ezrin upon agonist stimulation. Ezrin interacts with E3KARP and PKA and the interaction between ezrin and E3KARP is enhanced by agonist stimulation. Our data suggest that the A2bR is recruited to the plasma membrane upon apical or basolateral agonist stimulation and interacts with E3KARP and ezrin. We speculate that such an interaction may not only anchor the A2bR to the plasma membrane but may also function to stabilize the receptor in a signaling complex in the plasma membrane.

Enteroinvasive bacteria alter barrier and transport properties of human intestinal epithelium: role of iNOS and COX-2

BACKGROUND & AIMS

Various invasive pathogens cause diarrhea, but the mechanism(s) are poorly understood. We hypothesized that nitric oxide and prostaglandins might modulate chloride secretory and barrier properties of the infected intestinal epithelium and that diarrhea is caused, in part, by altered expression of inducible NO synthase (iNOS) and cyclooxygenase 2 (COX-2).

METHODS

Studies were conducted in human intestinal epithelial cell lines (HT29/cl.19A, Caco-2, and T84). Cells were infected with enteroinvasive Escherichia coli (EIEC 029:NM) or Salmonella dublin (SD), or nonpathogenic, noninvasive bacteria (Streptococcus thermophilus [ST] and Lactobacillus acidophilus [LA]). Infected cells and controls were tested for transepithelial resistance, chloride secretion, prostaglandin E2, guanosine 3',5'-cyclic monophosphate and adenosine 3',5'-cyclic monophosphate, and protein expression.

RESULTS

Cells infected with EIEC or SD, but not uninfected controls or ST/LA-exposed monolayers, showed a progressive reduction in transepithelial resistance starting at 6-12 hours. Infected HT29/cl.19A and Caco-2 cells, but not T84 cells, also showed an increase in total nitrite. Expression of iNOS, and consequently COX-2, was also increased, followed by increased production of prostaglandins and cyclic nucleotides. Furthermore, basal and stimulated chloride secretory responses to various agonists were enhanced in HT29/cl.19A and Caco-2 cells after infection with enteroinvasive bacteria, and this effect was reversed for some agonists by iNOS or COX-2 inhibitors. Increased expression of cystic fibrosis transmembrane conductance regulator and NKCC1 was also observed in EIEC or SD-infected cells vs. controls, secondary to NO synthase activity.

CONCLUSIONS

Up-regulation of iNOS and COX-2 by enteroinvasive bacteria can modulate chloride secretion and barrier function in intestinal epithelial cells. Thus, these enzymes represent possible therapeutic targets in infectious diarrhea.

Traffic jam: a compendium of human diseases that affect intracellular transport processes

As sequencing of the human genome nears completion, the genes that cause many human diseases are being identified and functionally described. This has revealed that many human diseases are due to defects of intracellular trafficking. This 'Toolbox' catalogs and briefly describes these diseases.