The putative residue apelin proprotein proapelin or apelin i

The putative 55-residue apelin proprotein (proapelin or apelin-55) is processed from a 77-residue preproprotein through removal of a 22-residue signal peptide (Table 1). Apelin-55 is further processed into bioactive isoforms of 36, 17, or 13 residues, likely through protease-mediated mechanisms. To date, the only processing mechanism shown is conversion of the 55-residue proprotein directly and preferentially to apelin-13 by proprotein convertase subtilisin kexin subtype 3 (PCSK3, or furin) [3]. Apelin-55 was not detected when apelin was originally discovered in 1998 [4], leading to the attribution of this isoform as either being non-existent or an inactive proprotein [5]. Correspondingly, apelin processing was theorized to occur intracellularly [6], leading to a release of only the shorter 36 to 13 residue bioactive isoforms. In Cilostazol to these hypotheses, intact apelin-55 was more recently detected in colostrum and milk [7], providing direct evidence for extracellularly localized proprotein. The presence of extracellular apelin-55 implies the potential that processing of apelin-55 to shorter isoforms may occur extracellularly, putting into question mechanisms relying exclusively on intracellular processing. This also leads to the question of whether apelin-55 can directly activate the AR or whether proteolytic processing is essential for bioactivity. The C-terminal residues of apelin are required for receptor binding [8]. Apelin-55 is therefore theoretically capable of binding to the AR and activating downstream signalling pathways. Interestingly, studies have indicated that apelin isoforms differ in their binding affinities for the AR, where shorter isoforms (e.g., apelin-13) bind weakly and longer isoforms (e.g., apelin-36) more tightly [9]. These differing affinities, in turn, may explain the observed isoform-dependent differences in potencies [4], rates of receptor recycling [10], and signalling biases [11]. The other ligand of the AR, apela (also known as ELABELA [12] or Toddler [13]), also exhibits isoform-dependent differences in signalling bias and potencies [14]. Much less is known about the apela family of peptides, or even which isoform(s) are present in vivo. In terms of apelinergic system function, the majority of published work to date has focused on the shortest of the apelin ligands: apelin-13. The physiological roles of longer isoforms remain relatively little studied, but Composite transposons may behave profoundly differently.
Materials and methods
Results and discussion
Conclusion
Transparency document
Acknowledgements Thanks to Dr. Younes Anini for informed discussions; Bruce Stewart for technical support; Dr. David Waisman for CD spectropolarimeter access; Dr. Eileen Denovan-Wright for microplate scanner access; and, Dr. Mike Lumsden (NMR3) and Ian Burton (NRC) for NMR instrument support. This work was supported by a Canadian Institutes of Health Research (CIHR) Operating Grant (MOP-111138 to JKR); a Nova Scotia Health Research Foundation (NSHRF) Scotia Support Grant (MED-SSG-2015-10041 to JKR); Strategic Cooperative Education Initiative funding from Nova Scotia Economic and Rural Development and Tourism (to JKR); and, a Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant (RGPIN-355310-2013 to DJD). Key infrastructure was provided through grants from the Canadian Foundation for Innovation, NSERC, and the Dalhousie Medical Research Foundation. The TCI probe for the 16.4T NMR spectrometer at the NRC-BMRF was provided by Dalhousie University through an Atlantic Canada Opportunities Agency Grant. KS is supported by an NSERC Alexander Graham Bell Canadian Graduate Scholarship, SKH was supported by an NSERC Undergraduate Student Research Award, AP was supported by a trainee award from the Beatrice Hunter Cancer Research Institute with funds provided by the Canadian Imperial Bank of Commerce and the Harvey Graham Cancer Research Fund as part of The Terry Fox Strategic Health Research Training Program in Cancer Research at CIHR, and JKR is supported by a CIHR New Investigator Award.