Relaxin: new peptides, receptors and novel actions Ross A.D. Bathgate, Chrishan S. Samuel, Tanya C.D. Burazin, Andrew L. Gundlach and Geoffrey W. Tregear Howard Florey Institute, The University of Melbourne, VIC 3010, Australia Relaxin has long been known as a hormone of preg- nancy. Until recently, little was known of potential roles for relaxin in non-pregnant females and males. The identification of a new gene encoding relaxin-3 (RLN3), the discovery of the elusive relaxin receptor and a novel role for relaxin-1 in regulating the normal turnover of collagen has provided us with unique insights into potential new roles for this peptide family. The Rln3 gene appears to be predominantly expressed in the brain, and mapping studies indicate a highly developed network of Rln3, Rln1 and relaxin receptor-expressing cells in the brain, suggesting that relaxin peptides might have important roles in the central nervous sys- tem. Rln1-knockout mice show progressive tissue fibro- sis as they age, and this fibrosis leads to functional changes in both the heart and lungs. Hence, the biologi- cal significance of this enigmatic peptide family is expanding, as are its potential clinical uses. First identified 75 years ago, relaxin has long been regarded as a hormone of pregnancy. In most mammals, relaxin is produced in the corpus luteum and/or placenta, with actions on the connective tissue of the reproductive tract to assist pregnancy maintenance, facilitate delivery and prepare the mammary gland for lactation (for review see [1]). Relaxin is a peptide, structurally similar to insulin, comprising two chains (A and B; Fig. 1) linked by disulfide bonds. A puzzling feature of relaxin biology is the major differences among species, not only in the primary amino acid sequence of the peptides (Fig. 1), but also in their apparent physiological roles. The actions of relaxin on various tissues, cells and models of induced fibrosis to inhibit collagen biosynthesis and promote collagen breakdown are well established [1–6], and have led to clinical trials of human relaxin in promoting cervical ripening in women with delayed delivery [7] and in the treatment of scleroderma [8]. Although in both these clinical trials relaxin was found to be safe and well tolerated, it failed to meet the primary efficacy endpoints, high- lighting the fact that we still know very little about its exact physiological role and actions in humans. Here, we focus on exciting new developments in the relaxin field. The discovery of a new gene encoding relaxin- 3(RLN3), predominantly expressed in the brain, novel actions for relaxin discovered in the Rln1-knockout (KO) mouse, together with the recent discovery of two orphan leucine-rich repeat-containing G-protein-coupled recep- tors (LGRs), LGR7 and LGR8, which respond to relaxin stimulation, has refocused attention on this unique peptide family. Discovery of relaxin-3 Until recently, only one relaxin encoding gene (RLN1) had been characterized in most mammals, with the exception Fig. 1. Comparisons of relaxin-3 A and B chain peptide sequences with other relaxin peptides. The sequence homology between relaxin-3 peptides is remark- able. By contrast, mouse relaxin-1 and human relaxin-2 have relatively low hom- ology to their relaxin-3 counterparts. Cons H1,2,3: consensus sequence between human relaxin-1 [9], -2 [10] and -3 [13]. Cons 3: consensus sequence between human, mouse [13], rat [14] and zebrafish [12] relaxin-3. Cons mouse: consensus sequence between mouse relaxin-1 [19] and relaxin-3. ‘ þ ’ Denotes a conservative substitution, ‘·’ denotes no homology. Conserved residues involved in relaxin receptor binding R, R, I [15] are highlighted in blue. Cysteine residues involved in the disulfide linkages essential for the structure of the peptide are highlighted in red. Unique glycine residues that are conserved in virtually all known relaxin pep- tides and are postulated to be essential for the secondary structure of relaxin are highlighted in green. TRENDS in Endocrinology & Metabolism B chain aligns 1 5 10 15 20 25 Human 1 KWKDDVIKL C G RELV RAQ IAI C GMSTWS Human 2 DSWMEEVIKL C G RELV RAQ IAI C GMSTWS ......++L C G R E.+ R A. I .. C G .S.W .. Human 3 RAAPYGVRL C G REFI RAV IFT C GGSRW Cons 3 .... YGV+L C G R EFI R AV I FT C G GSRW. Zebrafish 3 GP-SYGVKL C G REFI RAV IFT C GGSRW Rat 3 RPAPYGVKL C G REFI RAV IFT C GGSRW Mouse 3 RPAPYGVKL C G REFI RAV IFT C GGSRW Cons mouse ...... +++ C G R E+. R .+ I .. C G .S... Mouse 1 RVSEEWMDGFIRM C G REYA REL IKI C GASVGRLAL A chain aligns 1 5 10 15 20 Human 1 RPYVALFEK CCLI G CTKRSLAKY C Human 2 QLYSALANK CCHV G CTKRSLARF C ...+.L... CC .. G C +K .. ++ .. C . Human 3 DVLAGLSSS CCKW G CSKSEISSL C Cons 3 DV +.GLS.+ CC +W G C SK.+ISSL C . Zebrafish 3 DVVVGLSNA CCKW G CSKGEISSL C Rat 3 DVLAGLSSS CCEW G CSKSQISSL C Mouse 3 DVLAGLSSS CCEW G CSKSQISSL C Cons mouse + ....+ S.. CC .. G C S+ .. I ..L- C . Mouse 1 ESGGLMSQQ CCHV G CSRRSIAKLY C Cons H1,2,3 Cons H1,2,3 Corresponding author: R.A.D. Bathgate (r.bathgate@hfi.unimelb.edu.au). Review TRENDS in Endocrinology and Metabolism Vol.14 No.5 July 2003 207 http://tem.trends.com 1043-2760/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S1043-2760(03)00081-X