Emerging treatments in Neurogastroenterology: Perspectives of guanylyl cyclase C agonists use in functional gastrointestinal disorders and inflammatory bowel diseases.

BACKGROUND: Functional gastrointestinal disorders (FGID) and inflammatory bowel diseases (IBD) are the most frequent pathologic conditions affecting the gastrointestinal (GI) tract and both significantly reduce patients' quality of life. Recent studies suggest that guanylyl cyclase C (GC-C) expressed in the GI tract constitutes a novel pharmacological target in the treatment of FGID and IBD. Endogenous GC-C agonists - guanylin peptides: guanylin and uroguanylin, by the regulation of water and electrolyte transport, are involved in the maintenance of homeostasis in the intestines and integrity of the intestinal mucosa. Linaclotide, a synthetic agonist of GC-C was approved by Food and Drug Administration and European Medicines Agency as a therapeutic in constipation-predominant irritable bowel syndrome (IBS-C) and chronic idiopathic constipation (CIC). Lately, several preclinical and clinical trials focused on assessment of therapeutic properties of synthetic agonists of uroguanylin, plecanatide, and SP-333. Plecanatide is currently tested as a potential therapeutic in diseases related to constipation and SP-333 is a promising drug in ulcerative colitis treatment. PURPOSE: Here, we discuss the most recent findings and future trends on the development of GC-C agonists and their use in clinical trials.

Identification of GRY-RBP as an apolipoprotein B RNA-binding protein that interacts with both apobec-1 and apobec-1 complementation factor to modulate C to U editing.

C to U editing of apolipoprotein B (apoB) mRNA involves the interaction of a multicomponent editing enzyme complex with a requisite RNA sequence embedded within an AU-rich context. This enzyme complex includes apobec-1, an RNA-specific cytidine deaminase, and apobec-1 complementation factor (ACF), a novel 65-kDa RNA-binding protein, that together represent the minimal core of the editing enzyme complex. The precise composition of the holo-enzyme, however, remains unknown. We have previously isolated an enriched fraction of S100 extracts, prepared from chicken intestinal cells, that displays apoB RNA binding and which, following supplementation with apobec-1, permits efficient C to U editing. Peptide sequencing of this most active fraction reveals the presence of ACF as well as GRY-RBP, an RNA-binding protein with approximately 50% homology to ACF. GRY-RBP was independently isolated from a two-hybrid screen of chicken intestinal cDNA. GRY-RBP binds to ACF, to apobec-1, and also binds apoB RNA. Experiments using recombinant proteins demonstrate that GRY-RBP binds to ACF and inhibits both the binding of ACF to apoB RNA and C to U RNA editing. This competitive inhibition is rescued by addition of ACF, suggesting that GRY-RBP binds to and sequesters ACF. As further evidence of the role of GRY-RBP, rat hepatoma cells treated with an antisense oligonucleotide to GRY-RBP demonstrated an increase in C to U editing of endogenous apoB RNA. ACF and GRY-RBP colocalize in the nucleus of transfected cells and, in cotransfection experiments with apobec-1, each appears to colocalize in a predominantly nuclear distribution. Taken together, the results indicate that GRY-RBP is a member of the ACF gene family that may function to modulate C to U RNA editing through binding either to ACF or to apobec-1 or, alternatively, to the target RNA itself.

Intracellular localization of human cytidine deaminase. Identification of a functional nuclear localization signal.

The cytidine deaminases belong to the family of multisubunit enzymes that catalyze the hydrolytic deamination of their substrate to a corresponding uracil product. They play a major role in pyrimidine nucleoside and nucleotide salvage. The intracellular distribution of cytidine deaminase and related enzymes has previously been considered to be cytosolic. Here we show that human cytidine deaminase (HCDA) is present in the nucleus. A highly specific, affinity purified polyclonal antibody against HCDA was used to analyze the intracellular localization of native HCDA in a variety of mammalian cells by in situ immunochemistry. Native HCDA was found to be present in the nucleus as well as the cytoplasm in several cell types. Indirect immunofluorescence microscopy indicated a predominantly nuclear localization of FLAG-tagged HCDA overexpressed in these cells. We have identified an amino-terminal bipartite nuclear localization signal that is both necessary and sufficient to direct HCDA and a non-nuclear reporter protein to the nucleus. We also show HCDA binding to the nuclear import receptor, importin alpha. Similar putative bipartite nuclear localization sequences are found in other cytidine/deoxycytidylate deaminases. The results presented here suggest that the pyrimidine nucleotide salvage pathway may operate in the nucleus. This localization may have implications in the regulation of nucleoside and nucleotide metabolism and nucleic acid biosynthesis.

C-->U editing of apolipoprotein B mRNA in marsupials: identification and characterisation of APOBEC-1 from the American opossum Monodelphus domestica.

The C->U editing of RNA is widely found in plant and animal species. In mammals it is a discrete process confined to the editing of apolipoprotein B (apoB) mRNA in eutherians and the editing of the mitochondrial tRNA for glycine in marsupials. Here we have identified and characterised apoB mRNA editing in the American opossum Monodelphus domestica. The apoB mRNA editing site is highly conserved in the opossum and undergoes complete editing in the small intestine, but not in the liver or other tissues. Opossum APOBEC-1 cDNA was cloned, sequenced and expressed. The encoded protein is similar to APOBEC-1 of eutherians. Motifs previously identified as involved in zinc binding, RNA binding and catalysis, nuclear localisation and a C-terminal leucine-rich domain are all conserved. Opossum APOBEC-1 contains a seven amino acid C-terminal extension also found in humans and rabbits, but not present in rodents. The opossum APOBEC-1 gene has the same intron/exon organisation in the coding sequence as the eutherian gene. Northern blot and RT-PCR analyses and an editing assay indicate that no APOBEC-1 was expressed in the liver. Thus the far upstream promoter responsible for hepatic expression in rodents does not operate in the opossum. An APOBEC-1-like enzyme such as might be involved in C->U RNA editing of tRNA in marsupial mitochondria was not demonstrated. The activity of opossum APOBEC-1 in the presence of both chicken and rodent auxiliary editing proteins was comparable to that of other mammals. These studies extend the origins of APOBEC-1 back 170 000 000 years to marsupials and help bridge the gap in the origins of this RNA editing process between birds and eutherian mammals.

Escherichia coli cytidine deaminase provides a molecular model for ApoB RNA editing and a mechanism for RNA substrate recognition.

ApoB RNA-editing enzyme (APOBEC-1) is a cytidine deaminase. Molecular modeling and mutagenesis show that APOBEC-1 is related in quaternary and tertiary structure to Escherichia coli cytidine deaminase (ECCDA). Both enzymes form a homodimer with composite active sites constructed with contributions from each monomer. Significant gaps are present in the APOBEC-1 sequence, compared to ECCDA. The combined mass of the gaps (10 kDa) matches that for the minimal RNA substrate. Their location in ECCDA suggests how APOBEC-1 can be reshaped to accommodate an RNA substrate. In this model, the asymmetrical binding to one active site of a downstream U (equivalent to the deamination product) helps target the other active site for deamination of the upstream C substrate.

Evolutionary origins of apoB mRNA editing: catalysis by a cytidine deaminase that has acquired a novel RNA-binding motif at its active site.

The site-specific C to U editing of apolipoprotein B100 (apoB100) mRNA requires a 27 kDa protein (p27) with homology to cytidine deaminase. Here, we show that p27 is a zinc-containing deaminase, which operates catalytically like the E. coli enzyme that acts on monomeric substrate. In contrast with the bacterial enzyme that does not bind RNA, p27 interacts with its polymeric apoB mRNA substrate at AU sequences adjacent to the editing site. This interaction is necessary for editing. RNA binding is mediated through amino acid residues involved in zinc coordination, in proton shuttling, and in forming the alpha beta alpha structure that encompasses the active site. However, certain mutations that inactivate the enzyme do not affect RNA binding. Thus, RNA binding does not require a catalytically active site. The acquisition of polymeric substrate binding provides a route for the evolution of this editing enzyme from one that acts on monomeric substrates.

Abetalipoproteinemia is caused by defects of the gene encoding the 97 kDa subunit of a microsomal triglyceride transfer protein.

Abetalipoproteinemia is an inherited disorder of lipoprotein metabolism. Affected individuals produce virtually no circulating apolipoprotein B-containing lipoproteins (chylomicrons, very low density lipoprotein, low density lipoprotein and lipoprotein (a)). Malabsorption of the antioxidant vitamin E occurs, leading to spinocerebellar and retinal degeneration. Biochemical and genetic studies show that abetalipoproteinemia is not a defect of lipid biosynthesis or of the apolipoprotein B gene. Instead a microsomal triglyceride transfer protein, which exists as a complex with protein disulphide isomerase in the endoplasmic reticulum, has been implicated. We have cloned and sequenced the human cDNA encoding microsomal triglyceride transfer protein. The predicted amino acid sequence shows extensive homology to vitellogenin, the precursor of the lipovitellin complex, which has been shown by X-ray crystallography to contain a large lipid storage cavity. Microsomal triglyceride transfer protein is expressed in ovary, testis and kidney, in addition to liver and small intestine. A homozygous mutation that disrupts splicing has been identified in affected siblings with classical abetalipoproteinemia. These results elucidate a key process in the packaging of apolipoprotein B with lipid, and should increase our understanding of the processes regulating the production of atherogenic lipoproteins.

Characterization of genetic markers in the 5'flanking region of the apo A1 gene.

Genetic variation of apo A1/C3/A4 is associated with hyperlipidaemia and coronary heart disease. We report the polymerase chain reaction (PCR) conditions for determining three polymorphic sites in the 5'flanking region of apoA1 using DNA prepared from small aliquots of whole blood. These polymorphisms identify six haplotypes that will be of value in genetic studies.