Agouti Related Peptides

Definition
Agouti-related peptide (AgRP) is a neuropeptide produced in the brain in the arcuate nucleus of the hypothalamus. It is an endogenous antagonist of melanocortin receptors (MC3-R and MC4-R) which plays a critical role in energy balance.

Discovery
It was identified independently by two teams based on sequence similarity with Agouti signalling peptide, a protein synthesized in the skin that controls coat color1,2.

Classification
There are 3 synthetic AgRP fragments in humans, viz AgRP (25-51), AgRP (54-82) and AgRP (83-132). Amino-terminal fragments AgRP (25-51) and (54-82) were devoid of significant antagonist activity, whereas the amidated carboxyl-terminal AGRP fragment (83-132)-NH2 is potently active3.

Structural characteristics
The human AgRP is 132 amino acids in length, and is about 25 percent identical to agouti polypeptide. It contains 11 cysteines, the majority of which are located at the carboxyl terminal end of the polypeptide, and form 5 disulfide bridges4. The C-terminal portion of the peptide (87-132) is believed to be necessary for optimal binding and contains a five fingered spider-toxin motif with an eight amino acid portion mimicking alpha -MSH.

Mode of action
AgRP binds specifically to MC3-R and MC4-R as an inverse agonist. This inverse agonism antagonizes the action of alpha-MSH and also brings down the level of cAMP (secondary messanger) produced in the affected cells.

Functions
The major function of AgRP is to increase appetite and decrease metabolism. It is one of the most potent and longest appetite stimulator. It can used in treatment of obesity and is particularly beneficial in the prevention and treatment of type 2 diabetes5.

References
1.Shutter JR, Graham M, Kinsey AC, Scully S, Lüthy R, Stark KL (March 1997). "Hypothalamic expression of ART, a novel gene related to agouti, is up-regulated in obese and diabetic mutant mice". Genes & Development 11 (5): 593–602.
2.Ollmann MM, Wilson BD, Yang YK, Kerns JA, Chen Y, Gantz I, Barsh GS (October 1997). "Antagonism of central melanocortin receptors in vitro and in vivo by agouti-related protein". Science (New York, N.Y.) 278 (5335): 135–8.
3.Quillan JM, Sadée W, Wei ET, Jimenez C, Ji L, Chang JK (1998). A synthetic human Agouti-related protein-(83-132)-NH2 fragment is a potent inhibitor of melanocortin receptor function. FEBS Lett, 428(1-2):59-62.
4.Bures EJ, Hui JO, Young Y, Chow DT, Katta V, Rohde MF, Zeni L, Rosenfeld RD, Stark KL, Haniu M (1998). Determination of disulfide structure in agouti-related protein (AGRP) by stepwise reduction and alkylation. Biochemistry, 37(35):12172-12177.
5.Wilding JP (2002). Neuropeptides and appetite control. Diabet Med, 19(8):619-627