DOI 10.1515/bmc-2013-0004 BioMol Concepts 2013; 4(4): 335–347 Review Y u t a k a K o y a m a * Endothelin systems in the brain: involvement in pathophysiological responses of damaged nerve tissues Abstract: In addition to their potent vasoconstriction Introduction effects, endothelins (ETs) show multiple actions in vari- ous tissues including the brain. The brain contains high Since their discovery as a novel peptide family (1) , the levels of ETs, and their production is stimulated in many functions of endothelins (ETs) have been intensively inves- brain disorders. Accumulating evidence indicates that tigated in the circulatory system, because of their potent activation of brain ET receptors is involved in several vasoconstriction effects. However, soon after their dis- pathophysiological responses in damaged brains. In this covery, it was shown that ET ligands and ET receptors are article, the roles of brain ET systems in relation to brain present in various tissues, and multiple functions for ET disorders are reviewed. In the acute phase of stroke, pro- systems were postulated (2) . Accumulating evidence indi- longed vasospasm of cerebral arteries and brain edema cates regulatory roles for ETs in functions other than vascu- occur, both of which aggravate brain damage. Studies lar tone, such as hypertrophy, fibrosis, inflammation, and using ET antagonists show that activation of ET receptors A various other physiological and pathological functions. in the brain vascular smooth muscle induces vasospasm Several ET receptor agonists and antagonists have been after stroke. Brain edema is induced by increased activity developed (Table 1 ). At present, there are many selective of vascular permeability factors, such as vascular endothe- ET agonists and antagonists, some of which are now used lial growth factor and matrix metalloproteinases. Activa- clinically (3, 4) . Since the discovery of ETs, ET ligands and tion of ET receptors stimulates astrocytic production of B their receptors have been known to be highly expressed in these permeability factors. Increases in reactive astrocytes the brain (5, 6) . Therefore, specialized roles for ET systems are observed in neurodegenerative diseases and in the in the nervous system have been postulated. In addition to chronic phase of stroke, where they facilitate the repair of roles in neurotransmission and embryonic development, damaged nerve tissues by releasing neurotrophic factors. investigations of brain ET systems have shown their sig- ETs promote the induction of reactive astrocytes through nificance in brain disorders including Alzheimer ’ s disease ET receptors. ETs also stimulate the production of astro- B (AD) and stroke (7 – 9) . Development of effective treatments cytic neurotrophic factors. Recent studies have shown for brain dysfunctions in neurodegenerative diseases and high expression of ET receptors in neural progenitors. B stroke is still greatly needed. Many neuroscientists are Activation of ET receptors in neural progenitors promotes B engaged in searches for novel targets of neuroprotective their proliferation and migration, suggesting roles for ET B drugs. In this article, recent studies examining the pos- receptors in neurogenesis. Much effort has been invested sible roles of brain ET systems in the pathophysiological in the pursuit of novel drugs to induce protection or repair responses of damaged brains are reviewed. of damaged nerve tissues. From these studies, the phar- macological significance of brain ET systems as a possible target of neuroprotective drugs is anticipated. ET ligands Keywords: brain edema; endothelin; neurodegenerative The ET peptide family consists of three isopeptides: ET-1, disease; neuroprotection; stroke; vasospasm. ET-2, and ET-3 (Figure 1 ). These endogenous ET ligands have a structural similarity; they comprise 21 amino *Corresponding author: Yutaka Koyama, Laboratory of Pharmacology, Faculty of Pharmacy, Osaka Ohtani University, acids with two disulfide bonds. In humans, ET-1, ET-2, 3-11-1 Nishikiori-Kita, Tonda-bayashi, Osaka 584-8540, Japan, and ET-3 are encoded as large precursor proteins, prepro- e-mail: [email protected] ETs, by distinct genes. The biological actions of ETs are 336 Y. Koyama: Roles of brain endothelins Table 1 Agonists and antagonists for ET receptors. mediated by two types of receptors: ETA and ETB receptors. Because ET-1 is the most abundant isopeptide in many Agonists Antagonists tissues, many investigations of ET ligands have focused ET non-selective ET-1 SB209670, bosentan, on ET-1. ET-1 activates both ETA and ETB receptors, and TAK-044, tezosentan induces biological effects including vasoconstriction and ET selective Sarafotoxin 6b BQ123, darusentan, A proliferation (2 – 4) . ET-2 differs from ET-1 by two amino ambrisentan, acids, but has similar receptor selectivity to ET-1. ET-2 is sitaxsentan, highly expressed in intestine, ovary, and pituitary glands clazosentan, S-0139, SB234551, Ro-61- (10 – 12) . Owing to similar receptor selectivity, the biologi- 1790 cal actions of ET-2 were originally thought to overlap with ETB selective Sarafotoxin BQ788, IRL-2500, ET-1. However, recent studies have shown that ET-2 has 6c, IRL-1620, distinct actions from ET-1 (13, 14) . ET-3, which differs from BQ3020, A-192621, RES-701-1 ET-1 by six amino acids, is abundant in the intestine, lung, Ala 1,3,11,15 -ET-1 and brain (2, 7) . ET-3 has high affinity for ETB receptors, Dibasic pair-specific endopeptidase Dibasic pair-specific endopeptidase Lys-Arg Arg-Arg Arg-Arg Arg-Arg Prepro-ET-1 Prepro-ET-2 1517391 212 1476987178 ECEs ECEs Trp-Val Trp-Val NH COOH NH COOH Big-ET-1 2 Big-ET-2 2 12138 12138 Ser Leu Ser Ser Cys Ser Cys NH2 Tyr Ser Met Cys Ser Cys Leu NH2 ET-1 Asp ET-2 Asp Lys Glu Cys Val Tyr PheCys His Leu Asp Ile Ile Trp COOH Lys Glu Cys Val Tyr PheCys His Leu Asp Ile Ile Trp COOH Dibasic pair-specific endopeptidase Arg Arg Arg-Gly Prepro-ET-3 1 96 117 137 238 ECEs Trp-Ile Big-ET-3 NH2 COOH 12141 Tyr Tyr Phe Cys Tyr Cys Lys NH2 ET-3 Asp Lys Glu Cys Val Tyr Tyr Cys His Leu Asp Ile Ile Trp COOH Figure 1 Biosynthesis of human ETs from prepro-ETs. ET-1 is translated as an inactive precursor protein called prepro-ET-1. Prepro-ET-1 is cleaved by dibasic pair-specific endopeptidases and converted to big-ET-1. Specific processing of big-ET-1 by endothelin-converting enzymes (ECEs) results in production of mature ET-1. There are three distinct ET family peptides, ET-1, ET-2, and ET-3, all of which consist of 16 amino acids and two intramolecular disulfide bonds and are produced by a similar process to ET-1. Y. Koyama: Roles of brain endothelins 337 but not ETA receptors, indicating that it is an endogenous mRNA is also regulated by alterations of mRNA stability. ETB ligand. Gene knockout of either ET-3 or ETB receptors Human prepro-ET-1 mRNA has an AU-rich element (ARE) in mice results in a similar impairment in enteric neuron in the 3 ′ -untranslated region (22) , which is required for development (15, 16) . These mouse phenotypes resem- ARE-binding proteins to degenerate transcripts. ARE- ble Hirschsprung ’ s disease, and in accordance with this, binding proteins, such as AU-binding factor 1 (AUF1) and patients with Hirschsprung ’ s disease have a mutated ETB glyceraldehyde-3-phosphate dehydrogenase (G3PDH), receptor gene (17 – 19) . induce a rapid degradation of prepro-ET-1 mRNA (31, 32) . As occurs for many peptide hormones, the active Regulation by AUF1 and G3PDH are suggested to be forms of ETs are produced by cleavage of their precursor involved in ET-1 production induced by heat shock and peptides (Figure 1 ). For human ET-1, the 212 amino acid oxidative stress, respectively (31, 32) . Recent studies inactive precursor protein (prepro-ET-1) is translated from proposed that expression of prepro-ET-1 mRNA is also its mRNA. Prepro-ET-1 is cleaved by a dibasic pair-specific regulated by microRNAs (miRNAs). Analysis of the endopeptidase at Lys 51 -Arg 52 and Arg 92 -Arg 93 to produce a 3 ′ -untranslated region of prepro-ET-1 mRNA revealed 38 amino acid precursor, big-ET-1. The active form of ET-1 the presence of complementary binding sequences for is produced from big ET-1 after processing at Trp 21 -Val 22 by miRNAs. Yeligar et al. found that overexpression of specific proteases called endothelin converting enzymes miR199 and miR155 in endothelial cells reduced expres- (ECEs). Although the three ET isopeptides are encoded by sion of prepro-ET-1 mRNA (33) . distinct genes, production of ET-2 and ET-3 is mediated by a similar process to ET-1 (Figure 1 ) (20, 21) . ET receptor subtypes Regulation of ET production Receptors for ETs are classified into ETA and ETB types (Figure 3 ). These ET receptors differ in their selectivity In vascular endothelial cells, mature ET-1 is continu- for ET ligands. ETA receptors have ligand preferences = > > ously released through a constitutive pathway. The of ET-1 ET-2 ET-3, whereas ET B receptors show equal rate-limiting step of ET production is therefore thought selectivity for the three ET ligands. High expression of to be at transcription of the prepro-ET-1 gene. Human ETA receptors is observed in the vascular smooth muscle prepro-ET-1 genes have five exons and a 5 ′ -flanking region, and cardiocytes (4) . Activation of vascular ET A receptors spanning approximately 6.8 kb of DNA (Figure 2 A) (22) . is responsible for the potent vasoconstriction caused by Examination of regulation of ET-1 expression showed that ETs. ETB receptors are abundantly expressed in the vas- transcription of prepro-ET-1 mRNA is regulated by various cular endothelium, kidney, lung, and brain.