It has been 35 years since the isolation from pig spinal cord by Minamino et al. of numerous small neuropeptides whose common feature was the stimulation of smooth muscle contraction. These were named neuromedins, and neuromedin U (NMU) is one of them. NMU exerts a potent contractile effect on the muscles of the rat uterus, hence the derivation of its name (U - uterus).
In the 1990s, the structure of NMUs in various species was recognized and their synthesis was elucidated. NMU is a highly conserved neuropeptide present in many species, existing as multiple isoforms. Moreover, specific binding of the neuropeptide to various organs was demonstrated. The presence of NMU-like immunoreactivity was also reported (RIA and immunohistochemistry) in a number of human organs and various animal species. Major physiological effects in which NMU is involved have also been identified. These include smooth muscle contraction, increased blood pressure, gastric emptying and modification of intestinal ion transport and motility.
With the introduction of modern molecular biology techniques, at the turn of the 20th and 21st century, specific NMU receptors (NMUR1 and NMUR2) were identified and characterized. Their distribution in the body and intracellular signaling pathways regulated by these receptors has been recognized. In parallel, NMU was shown to be involved in the regulation of body metabolism, exhibiting anorexigenic effects. This last observation has greatly increased the interest of numerous research centers in the physiological role of this neuromedin.
Another significant discovery related to NMU was the isolation of neuromedin S (NMS) from rat brain. Both NMU and NMS are endogenous ligands for NMUR1 and NMUR2 receptors. Since then, many studies have focused on comparing the physiological effects of both neuropeptides in different cells and organisms. The precursors of NMU and NMS (preproNMU - ppNMU and preproNMS - ppNMS) share high structural similarity, and as shown by Mori et al. and Ensho et al., other peptides may also originate from them, which they named “neuromedin U precursor related peptide” (NURP) and “neuromedin S precursor related peptide” (NSRP). In this context, it should be noted that already in 2009 Bechtold et al. showed that mice proNMU104-136, actually named NURP33, is involved in the regulation of energy homeostasis. Currently, several groups are undertaking research on the role of these peptides in the regulation of various biological processes.
Several review papers have been published in the literature on NMU, NMS and their receptors as well as their physiological role. In this context, the contributions by Brighton et al., Budhiraja and Chugh, Mori et al., Mitchell et al., Malendowicz et al., Martinez and O’Driscoll, and Alhosaini et al. deserve to be mentioned. On the other hand, the aim of the current review is to provide readers with the recent data on the physiological role of NMU and NMS action, with a particular focus on the differences in the action of these neuromedins, at both cellular and organ levels. In this review, our attention will focus mainly on the human, rat and mouse data.
In humans both ppNMU and ppNMS are composed of 10 exons and 9 introns. According to the ENSEMBL genome database, the primary mRNA for ppNMU undergoes alternative splicing, leading to the six alternative splice variants of mature ppNMU mRNA, including: ppNMU var.1 - 816 nt, (174 aa - primary ppNMU form encoding mature NMU25 peptide an NURP), ppNMU var.2 - 693 nt, (147 aa, encoding mature NMU25 and NURP fragment), ppNMU var.3 - 658 nt, (149 aa, encoding 21 amino acids of NMU and NURP fragment), ppNMU var.4 - 283 nt (transcript variant does not contain translated open reading frame), ppNMU var.5 - 756 nt (158 aa - encoding mature NMU25 peptide an NURP), ppNMU var.6 - 616 nt. (transcript variant does not contain translated open reading frame). It should be emphasized that ppNMU and ppNMS genes are located on different chromosomes (ppNMU on 4q12 and ppNMS on 2q11.2) (ENSEMBL).
In the rat, the ppNmu gene comprises 10 exons positioned on chromosome 14. There are discrepancies between the number of alternative splice variants reported in the ENSEMBL and NCBI databases. According to NCBI database there are four alternative splice variants of rat ppNmu described as: “neuromedin U (Nmu), mRNA” (708 nt, 174 aa- primary rat ppNmu form), “transcript variant X1” (1107 nt, non-protein coding), “transcript variant X2 “ (966 nt, 164 aa),”transcript variant X3” (1077 nt, non-protein coding). In ENSEMBL database only one splice variant is presented that correspond to “neuromedin U (Nmu), mRNA” - primary rat ppNmu form (NCBI). As in humans, the ppNms gene contains 10 exons and has a single transcript whose length is 496 nt and 152 aa. This gene is localized on chromosome 9.
In the mouse, the ppNmu gene also contains 10 exons, but is located on chromosome 5. This gene has a single splice variant whose length is 828 nt and 174 aa. In the mouse, the NMS gene is also composed of 10 exons. As reported in the NCBI database this gene has 2 alternative splice variants: “transcript variant 1” - 1019 nt, 153 aa; “transcript variant 2” - 995 nt and 145 aa. These two transcript variants are also found in the ENSEMBL database.
The information given above about the structure of genes encoding ppNMU and ppNMS and their transcripts indicates significant species differences in ppNMU and ppNMS gene structure. This is reflected in the large number of NMU isoforms described in various animal species. In contrast to NMU, the structure of the NMS gene and the number of its transcripts are less variable across species.
In human ppNMU is cleaved into the following 3 chains: neuromedin U25 (NMU25), neuromedin U precursor-related peptide 36 (NURP36) or neuromedin U precursor-related peptide 33 (NURP33). On the other hand ppNMS is cleaved into: neuromedin S33 (NMS33), neuromedin S precursor related peptide 37(NSRP37) and neuromedin S precursor related peptide 34 (NSRP34). In the rat, the NMU is composed of 23 amino acid residues and the NMS of 36. In the mouse, the corresponding numbers are 20 and 36 amino acid residues.
In the mammalian NMUs a common C-terminal sequence – Phe-Leu-Phe-Arg-Pro-Arg-Asn-NH2 –contains the active site of the neuropeptide, which is formed by the amino acid residues between positions 2 and 8. Available data indicate that in most species studied, the five amino acids at the C-terminus of the NMUs are totally conserved, suggesting that this region is of major importance for biological activity. NMU and NMS are structurally related neuropeptides. They share the same amidated C-terminal heptapeptide and bind to the same receptors NMUR1 and NMUR2.
Other peptides derived from ppNMU and ppNMS, namely NURP36, NURP33, NSRP37 and NSRP34 exert some biological effects, however, they do not act through either NMUR1 or NMUR2. Their biological effects will be presented in more detail in subsequent sections.
The first report of the presence of a highly specific 125I-NMU binding site in cell membranes obtained from rat uterus suggested that it was a single class of binding site with a Kd of 0.35 nM. Soon, two NMU receptors were identified using modern molecular biology techniques. Through the efforts of many research groups, the two genes were quickly characterized and their expression in different species and different tissues and organs was identified. After several years, these receptors were named NMUR1 and NMUR2. NMUR1 expression is mainly observed in peripheral tissues and organs, while the highest expression of NMUR2 occurs in the central nervous system (CNS) [for review see]. Both receptors are typical GPCRs, with characteristic seven transmembrane domains.
In humans NMUR1 gene consists of 3 exons and 2 introns, the size of encoded mRNA is 3274 bp and the receptor is composed of 426 aa residues. Current data from NCBI contain information on the presence of a total of six alternative splice variants of this gene (neuromedin U receptor 1 and transcript variants X1-X5 - encoding 403, 376, 370, 333, 313 aa length, respectively) while the Ensembl database contains information on one isoform. NMUR2 gene, on the other hand, consists of 4 exons and 3 introns. The size of its mRNA is 2067 bp and the receptor is composed of 415 aa residues. In the NCBI there are only 2 splice variants for this gene (neuromedin U receptor 2 and transcript variant X1 encoding 321 aa). NMUR1 gene is located on human chromosome 2 - position q37.1 and NMUR2 gene on chromosome 5 - position q33.1. There are two splice variants of the NMUR1 gene and single transcript of the NMUR2 gene in the rat and mouse.
The high number of NMUR1 and NMUR2 alternative splice varia
