Definition
Intermedin (IMD) is a novel member of the calcitonin gene-related peptide (CGRP) family which also includes adrenomedullin (AM).
Discovery
In early 2004, a novel peptide was independently identified by two separate groups. Roh and colleagues initially identified human IMD from an expressed sequence tag and used phylogenetic profiling to identify the peptide in other mammals and nonvertebrates. Immunohistochemical analysis of peptide expression revealed that high levels of the peptide and/or preprohormone were present in cells within the intermediate lobe of the pituitary; hence, the group called the peptide intermedin1. At the same time, Takei et al. were examining a rat cDNA library for potential mammalian homologs of the five identified puffer fish adrenomedullin (AM) family members. They identified a 148-amino acid preprohormone that was processed into a 47-amino acid, mature peptide with 33% sequence homology to AM and 71% homology to puffer fish AM2 and thus referred to the peptide as mammalian AM22.
Structural Characteristics
The human IMD gene is believed to encode a prepropeptide of 148 amino acids with a signal peptide for secretion at the N terminus, which may generate a 47-amino acid mature peptide (IMD1–47) i.e. IMDL and a shorter 40-amino acid one (IMD8–47) i.e. IMDS by proteolytic cleavage at the N-terminal proximate basic residues followed by an amidated C terminus. Intermedins has a stretch of 47 residues at the C terminus is flanked by dibasic proteolytic cleavage sites at the N terminus and an a-amidation donor residue at the C terminus. The putative mature region of intermedin shares 28% sequence identity with ADM and 20% with CGRP. Importantly, Intermedins adopts an N-terminal disulfide- bonded loop leading into an a-helix, followed by a disordered structure that is shared by all calcitonin/CGRP family peptides. Analysis of orthologous intermedins indicate that the positions of N-terminal dibasic cleavage sites vary by a few amino acids among different species, whereas an arginine residue seven amino acids downstream of the dibasic cleavage motif of human intermedin is conserved in all species, suggesting that the mature intermedin from human and other species could be a 40-amino acid peptide3.
Mode of action
Intermedin activates the cAMP-dependent Pathway via the CGRP Receptor— A study shows that treatment of human neuroblastoma SK-N-MC cells and rat L6 skeletal myoblast cells, known to express different levels of calcitonin receptor-like receptor (CRLR) and receptor activity-modifying protein (RAMPs), with synthetic intermedin peptides resulted in increased cAMP production, whereas treatment with the truncated amidated intermedin fragment (intermedin (IMD)- (17–47)), or a 31-amino acid peptide from the preproregion of human intermedin (prointermedin-(55–85)) had no effect in either cell line. This implied that a-amidation and residues 8–16 of intermedin are important for intermedin bioactivity. Further it was also shown that the stimulatory effect of intermedin was suppressed by cotreatment with a calcitonin gene-related peptide (CGRP) receptor antagonist (CGRP-(8–37)) in L6 cells3.
Functions
Intermedin/Adrenomedullin-2 elevate blood pressure and inhibit food and water intake - Intermedin is homologous to adrenomedullin (AM) and can activate both the AM and calcitonin gene-related peptide (CGRP) receptors. It has been reported that administration of IMD into the lateral cerebroventricle (i.c.v.) of rats caused significant, long-lasting elevations in mean arterial pressure and heart rate. These elevations are similar to the effects of CGRP and significantly greater than the effects of AM. IMD-induced elevations in mean arterial pressure were inhibited by i.c.v. administration of phentolamine indicating that IMD activates the sympathetic nervous system. I.c.v administration of IMD also inhibited food and water intake. The effects on feeding are likely related to activation of the CGRP receptor and are independent of the effects on water intake, which are likely through the AM receptor. A study indicate that IMD has potent actions within the CNS that may be a result of the combined activation of both the AM and CGRP receptors4.
Expression of the Counter-Regulatory Peptide Intermedin is Augmented in the Presence of Oxidative Stress in Hypertrophied Cardiomyocytes - A study was done using the spontaneously hypertensive rat (SHR) and normotensive Wistar Kyoto (WKY), to examine the presence of myocardial oxidative stress, concentric hypertrophy expression of IMD, AM and receptor components. It was found that in left and right ventricular cardiomyocytes from SHR vs. WKY cell width (26% left, 15% right) and mRNA expression of hypertrophic markers ANP (2.7 fold left, 2.7 fold right) and BNP (2.2 fold left, 2.0 fold right) were enhanced. In left ventricular cardiomyocytes only oxidative stress was indicated by increased membrane protein carbonyl content (71%) and augmented production of O2- anion (64%) and IMD (6.8 fold), RAMP1 (2.5 fold) and RAMP3 (2.0 fold) mRNA was increased while AM and RAMP2 mRNA was not altered and abundance of RAMP1 (by 48%), RAMP2 (by 41%) and RAMP3 (by 90%) monomers in cell membranes was decreased. This implied that robust augmentation of IMD expression in hypertrophied left ventricular cardiomyocytes indicates a prominent role for this counter regulatory peptide in the adaptation of the SHR myocardium to the stresses imposed by chronic hypertension5.
References
1. Roh J, Chang CL, Bhalla A, Klein C, and Hsu SYT (2004). Intermedin is a calcitonin/calcitonin gene-related peptide family peptide acting through the calcitonin receptor-like receptor/receptor activity-modifying protein receptor complex. J. Biol. Chem., 279, 7264–7274.
2. Takei Y, Inoue K, Ogoshi M, Kawahara T, Bannai H, and Miyano S (2004). Novel fish-derived adrenomedullin in mammals: structure and possible function. Peptides, 25, 1643- 1656.
3. Jaesook Roh, Chia Lin Chang, Alka Bhalla, Cynthia Klein, and Sheau Yu Teddy Hsu (2004). Intermedin Is a Calcitonin/Calcitonin Gene-related Peptide Family Peptide Acting through the Calcitonin Receptor-liken Receptor/Receptor Activity-modifying Protein Receptor Complexes. J. Bio. Chem., 279 (20), 7264–7274.
4. Taylor MM, Bagley SL, Samson WK (2004). Intermedin/Adrenomedullin-2 acts within the central nervous system to elevate blood pressure and inhibit food and water intake. Am. J Physiol. Regul. Integr. Comp. Physiol., 288, R919-R927.
5. David Bell, Y Zhao, Francis PG. McCoy, Adrian Devine and Barbara J. McDermott (2008). Expression of the Counter-Regulatory Peptide Intermedin is augmented in the Presence of Oxidative Stress in Hypertrophied Cardiomyocytes. Cell. Physiol. Biochem., 21, 409-420.