posted on 2016-02-22, 05:36authored byMichael
E. Madonna, Jennifer Schurdak, Ying-kui Yang, Stephen Benoit, Glenn L. Millhauser
The agouti-related protein (AgRP) plays a central role
in energy
balance by reducing signaling through the hypothalamic melanocortin
receptors (McRs) 3 and 4, in turn stimulating feeding and decreasing
energy expenditure. Mature AgRP(83-132), produced by endoproteolytic
processing, contains a central region that folds as an inhibitor cystine
knot (ICK) stabilized by a network of disulfide bonds; this domain
alone carries the molecular features for high affinity McR binding
and inverse agonism. Outside of the ICK domain are two polypeptide
segments, an N-terminal extension and a C-terminal loop, both completely
conserved but of unknown function. Here we examine the physiological
roles of these non-ICK segments by developing a panel of modified
AgRPs that were administered to rats through intracerebroventricular
(ICV) injection. Analysis of food consumption demonstrates that basic
(positively charged) residues are essential for potent short- and
long-term AgRP stimulated feeding. Moreover, we demonstrate an approximate
linear relationship between protein charge density and 24 h food
intake. Next, we developed artificial AgRP(83-132) analogues with
increased positive charge and found that these species were substantially
more potent than wild type. A single dose of one protein, designated
AgRP-4K, results in enhanced feeding for well over a week and weight
gain that is nearly double that of AgRP(83-132). These studies suggest
new strategies for the development of potent orexigenic species and
may serve as leads for the development of therapeutics for treating
wasting conditions such as cachexia.