Neuroglobin: From structure to function in health and disease

Abstract

In 2000, the third member of the globin family was discovered in human and mouse brain and named neuroglobin (Ngb). Ngb is a monomeric 3/3 globin structurally similar to myoglobin and to the α- and β-chains of hemoglobin, however it displays a bis-histidyl six-coordinate heme-Fe atom. Therefore, ligand binding to the Ngb metal center is limited from the dissociation of the distal His(E7)64-Fe bond. From its discovery, more than 500 papers on Ngb structure, expression, reactivity, and localization have been published to highlight its biochemical properties and its role(s) in health and disease. In vivo experiments have shown that increased levels of Ngb significantly protect both heart and brain from hypoxic/ischemic and oxidative stress-related insults, whereas decreased Ngb levels lead to an exacerbation of tissue injuries. Although some contradictory data emerged, human Ngb overexpression has been hypothesized to protect neurons from mitochondrial dysfunctions and neurodegenerative disorders such as Alzheimer's disease, and to play a shielding role in cancer cells. Recently, the recognition of Ngb interactors and inducers enlarges the functions of this stress-inducible globin, opening new therapeutic approaches to prevent neuronal cell death. Here, structural and functional aspects of human Ngb are examined critically to highlight its roles in health and disease.

Introduction

Globins are present in all kingdoms of living organisms where they display several functions, including ligand (e.g., O₂, CO, and NO) sensing, transport, as well as storage, and heme-based catalysis (Burmester, Hankeln, 2014, Vinogradov, Moens, 2008, Wajcman et al, 2009).

From the structural viewpoint, globins display the 3/3 or 2/2 fold, built by six or four α-helices, respectively, that form a sandwich around the heme (Ascenzi, Brunori, 2016, Perutz, 1979, Pesce et al, 2000). The 3/3 globin fold is present in: (i) vertebrate and non-vertebrate myoglobins (Mb) and hemoglobins (Hb) (≥150 amino acids) (Bolognesi et al, 1997, Burmester, Hankeln, 1999, Ebner et al, 2003, Hoogewijs et al, 2004, Perutz, 1979); (ii) bacterial, fungal and yeast chimeric flavo-hemoglobins (flavoHb) (~400 amino acids) (Ermler et al., 1995); (iii) bacterial single-domain flavo-hemoglobins (flavoHbs; ~160 amino acids) (Tarricone et al., 1997); (iv) plant symbiotic and non-symbiotic Hbs (~160 amino acids) (Vazquez-Limon et al., 2012); (v) metazoan neuroglobin (Ngb; ~150 amino acids) (Brunori, Vallone, 2006, Brunori, Vallone, 2007, Burmester et al, 2000, Pesce et al, 2002); (vi) vertebrate cytoglobin (Cygb; ~190 amino acids) (Oleksiewicz et al., 2011); (vii) globin E (GbE), which is present in several organs and tissues of birds, turtles, and of the coelacanth Latimeria chalumnae (~150 amino acids) (Burmester, Hankeln, 2014, Kugelstadt et al, 2004); (viii) the membrane-bound globin X (GbX) present in Metazoa but not in birds and mammals (~200 amino acids) (Roesner et al., 2005); and (ix) globin Y (GbY) that has been found in Xenopus, platypus, lizard, elephant shark, coelacanth, and turtles (~160 amino acids) (Burmester, Hankeln, 2014, Fuchs et al, 2006). Moreover, the N-terminal 3/3 globin fold has been found in bacterial and archeal globin-coupled sensor proteins (i.e., heme-based aerostatic transducer), protoglobins, and two-domain gene regulators (>200 amino acids) (Freitas et al, 2003, Pesce et al, 2013a, Pesce et al, 2013b, Zhang, Phillips, 2003). Recently, the internal circular permuted globin domain, showing the 3/3 fold, has been identified in the metazoan chimeric globin named androglobin (Adgb; ~1550 amino acids) (Hoogewijs et al., 2012). The 2/2 globin topology has been found in protozoa, cyanobacteria, nemertea and bacteria (<130 amino acids) as well as in algae and plants (>160 amino acids). The 2/2 globins have been ordered in three phylogenetic groups (I or N, II or O, and III or P), which are orthologous within each group and paralogous across the different groups (Ascenzi et al, 2007, Pesce et al, 2013a, Wittenberg et al, 2002). Lastly, the nemertean worm Cerebratulus lacteus displays the smallest naturally occurring globin, named miniHb (109 amino acids), which appears almost equally distant from globins showing the 3/3 and 2/2 folds and represents the third member of the globin superfamily (Pesce et al, 2002, Vandergon, Riggs, 2008, Vandergon et al, 1998). Interestingly, some microorganisms synthesize 3/3 and 2/2 globins (Vinogradov et al, 2013a, Vinogradov et al, 2013b), opening the unanswered question(s) of their different roles.

Nerve globins are widespread within non-vertebrates and vertebrates (Vinogradov et al., 2006), the first nerve globin having been recognized in the nerve cord of the polychaete annelid Aphrodite aculeata in 1872 (Lankester, 1872). Progressively, Adgb, Cygb, GbE, GbX, GbY, Hb, and Mb have been reported to be expressed in the nervous system of most living organisms; however, GbE, GbX, and GbY have not been found in mammals (Ascenzi et al, 2013, Avivi et al, 2010, Biagioli et al, 2009, Blank et al, 2011, Burmester, Hankeln, 2014, Burmester et al, 2002, Corti et al, 2016a, Emara et al, 2010, Emara et al, 2014a, Emara et al, 2014b, Ferrer et al, 2011, Fuchs et al, 2006, Hardison, 1996, Hardison, 1998, Hoogewijs et al, 2012, Kawada et al, 2001, Kugelstadt et al, 2004, Oleksiewicz et al, 2011, Richter et al, 2009, Roesner et al, 2005, Russo et al, 2013, Schelshorn et al, 2009, Schneuer et al, 2012, Schwarze, Burmester, 2013, Schwarze et al, 2014, Vinogradov et al, 2013a, Vinogradov et al, 2013b, Wakasugi et al, 2011, Weber, Vinogradov, 2001).

The co-expression of multiple nerve globins in the nervous system has been hypothesized to play specific functions (Ascenzi et al., 2014). Only in 2000, the first vertebrate nerve globin, named Ngb, has been identified in neuronal tissues of mice and humans (Burmester et al., 2000). Then, Ngb has been found in other invertebrates and vertebrates including the nematode Caenorhabditis elegans (Tilleman et al., 2011), the zebrafish Danio rerio (Awenius et al., 2001), the pufferfish Tetraodon nigroviridis (Awenius et al., 2001), and the Antarctic icefish Chaenocephalus aceratus (Cheng et al., 2009a) with the exception of lampreys and ray-finned fishes (Burmester and Hankeln, 2014) (see Section 2).

From its discovery, more than 500 papers on Ngb structure, expression, reactivity, and localization are present in the PubMed database to highlight structure–function relationship and its roles in health and disease. In vivo experiments have shown that Ngb significantly protects brain from hypoxic/ischemic and oxidative stress-related insults. Although some contradictory data emerged, Ngb over-expression has been postulated to protect neurons from mitochondrial dysfunctions, neurodegenerative disorders such as Alzheimer's disease (AD), and to play a shielding role on cancer cell survival. Recently, the recognition of Ngb interactors and inducers has enlarged the functions of this stress-inducible globin, opening new therapeutic approaches to prevent cell apoptosis. The aim of this review is to dissect critically the structure–function relationships of Ngb to highlight its roles in health and disease.