Small interfering peptide (siP) for in vivo examination of the developing lung interactonome

Interfering peptides targeting protein-protein interactions: the next generation of drugs?

Protein-protein interactions (PPIs) are well recognized as promising therapeutic targets. Consequently, interfering peptides (IPs) - natural or synthetic peptides capable of interfering with PPIs - are receiving increasing attention. Given their physicochemical characteristics, IPs seem better suited than small molecules to interfere with the large surfaces implicated in PPIs. Progress on peptide administration, stability, biodelivery and safety are also encouraging the interest in peptide drug development. The concept of IPs has been validated for several PPIs, generating great expectations for their therapeutic potential. Here, we describe approaches and methods useful for IPs identification and in silico, physicochemical and biological-based strategies for their design and optimization. Selected promising in-vivo-validated examples are described and advantages, limitations and potential of IPs as therapeutic tools are discussed.

CFTR: a hub for kinases and crosstalk of cAMP and Ca2+

Cystic fibrosis (CF) is caused by mutations in the CF transmembrane conductance regulator (CFTR). The resulting disease is pleiotropic consistent with the idea that CFTR acts as a node within a network of signalling proteins. CFTR is not only a regulator of multiple transport proteins and controlled by numerous kinases but also participates in many signalling pathways that are disrupted after expression of its commonest mutant (F508del-CFTR). It operates in membrane compartments creating a scaffold for cytoskeletal elements, surface receptors, kinases and phosphodiesterases. CFTR is exposed to membrane-local second messengers such that a CFTR-interacting, low cellular energy sensor kinase (AMP- and ADP-activated kinase, AMPK) signals through a high energy phosphohistidine protein kinase (nucleoside diphosphate kinase, NDPK). CFTR also translocates a Ca(2+)-dependent adenylate cyclase to its proximity so that a rigid separation between cAMP-dependent and Ca(2+)-dependent regulation of Cl(-) transport becomes obsolete. In the presence of wild-type CFTR, parallel activation of CFTR and outwardly rectifying anoctamin 6 Cl(-) channels is observed, while the Ca(2+)-activated anoctamin 1 Cl(-) channel is inhibited. In contrast, in CF cells, CFTR is missing/mislocalized and the outwardly rectifying chloride channel is attenuated while Ca(2+)-dependent Cl(-) secretion (anoctamin 1) appears upregulated. Additionally, we consider the idea that F508del-CFTR when trapped in the endoplasmic reticulum augments IP3-mediated Ca(2+) release by providing a shunt pathway for Cl(-). CFTR and the IP3 receptor share the characteristic that they both assemble their partner proteins to increase the plasticity of their hub responses. In CF, the CFTR hub fails to form at the plasma membrane, with widespread detrimental consequences for cell signalling.

CFTR mutations altering CFTR fragmentation

Most CF (cystic fibrosis) results from deletion of a phenylalanine (F⁵⁰⁸) in the CFTR {CF transmembrane-conductance regulator; ABCC7 [ABC (ATP-binding cassette) sub-family C member 7]} which causes ER (endoplasmic reticulum) degradation of the mutant. Using stably CFTR-expressing BHK (baby-hamster kidney) cell lines we demonstrated that wild-type CTFR and the F508delCFTR mutant are cleaved into differently sized N- and C-terminal-bearing fragments, with each hemi-CFTR carrying its nearest NBD (nucleotide-binding domain), reflecting differential cleavage through the central CFTR R-domain. Similar NBD1-bearing fragments are present in the natively expressing HBE (human bronchial epithelial) cell line. We also observe multiple smaller fragments of different sizes in BHK cells, particularly after F508del mutation (ladder pattern). Trapping wild-type CFTR in the ER did not generate a F508del fragmentation fingerprint. Fragments change their size/pattern again post-mutation at sites involved in CFTR's in vitro interaction with the pleiotropic protein kinase CK2 (S511A in NBD1). The F508del and S511A mutations generate different fragmentation fingerprints that are each unlike the wild-type; yet, both mutants generate new N-terminal-bearing CFTR fragments that are not observed with other CK2-related mutations (S511D, S422A/D and T1471A/D). We conclude that the F508delCFTR mutant is not degraded completely and there exists a relationship between CFTR's fragmentation fingerprint and the CFTR sequence through putative CK2-interactive sites that lie near F508.

Strategies for identifying synthetic peptides to act as inhibitors of NADPH oxidases, or "all that you did and did not want to know about Nox inhibitory peptides"

Phagocytes utilize reactive oxygen species (ROS) to kill pathogenic microorganisms. The source of ROS is an enzymatic complex (the NADPH oxidase), comprising a membrane-associated heterodimer (flavocytochrome b (558)), consisting of subunits Nox2 and p22(phox), and four cytosolic components (p47(phox), p67(phox), p40(phox), and Rac). The primordial ROS (superoxide) is generated by the reduction of molecular oxygen by NADPH via redox centers located on Nox2. This process is activated by the translocation of the cytosolic components to the membrane and their assembly with Nox2. Membrane translocation is preceded by interactions among cytosolic components. A number of proteins structurally and functionally related to Nox2 have been discovered in many cells (the Nox family) and these have pleiotropic functions related to the production of ROS. An intense search is underway to design therapeutic means to modulate Nox-dependent overproduction of ROS, associated with diseases. Among drug candidates, a central position is held by synthetic peptides reflecting domains in oxidase components involved in NADPH oxidase assembly. Peptides, corresponding to domains in Nox2, p22(phox), p47(phox), and Rac, found to be oxidase activation inhibitory in vitro, are reviewed. Usually, peptides are inhibitory only when added preceding assembly of the complex. Although competition with intact components seems most likely, less obvious mechanisms are, sometimes, at work. The use of peptides as inhibitory drugs in vivo requires the development of methods to assure cell penetration, resistance to degradation, and avoidance of toxicity, and modest successes have been achieved. The greatest challenge remains the discovery of peptide inhibitors acting specifically on individual Nox isoforms.

Cystic fibrosis transmembrane regulator fragments with the Phe508 deletion exert a dual allosteric control over the master kinase CK2

Cystic fibrosis mostly follows a single Phe508 deletion in CFTR (cystic fibrosis transmembrane regulator) (CFTRDeltaF508), thereby causing premature fragmentation of the nascent protein with concomitant alterations of diverse cellular functions. We show that CK2, the most pleiotropic protein kinase, undergoes allosteric control of its different cellular forms in the presence of short CFTR peptides encompassing the Phe508 deletion: these CFTRDeltaF508 peptides drastically inhibit the isolated catalytic subunit (alpha) of the kinase and yet up-regulate the holoenzyme, composed of two catalytic and two non-catalytic (beta) subunits. Remarkable agreement between in silico docking and our biochemical data point to different sites for the CFTRDeltaF508 peptide binding on isolated CK2alpha and on CK2beta assembled into the holoenzyme, suggesting that CK2 targeting may be perturbed in cells expressing CFTRDeltaF508; this could shed light on some pleiotropic aspects of cystic fibrosis disease.

Epithelial IgG and its relationship to the loss of F508 in the common mutant form of the cystic fibrosis transmembrane conductance regulator

The most debilitating feature of cystic fibrosis (CF) disease is uncontrolled inflammation of respiratory epithelium. The relationship between the commonest mutated form of CFTR (F508del or DeltaF508) and inflammation has not yet been elucidated. Here, we present a new paradigm suggesting that CFTR can interact with intra-epithelial IgG, establishing a direct link between normal CFTR and the immune system. Further, our data show that the amino-acid sequence local to F508 can bind IgG with high affinity, dependent on F508, such that loss of F508 abolishes this link both in vitro and in the intact cell.