The visual process in the vertebrate eye requires high amounts of metabolic energy and thus oxygen. Oxygen supply of the avian
retina is a challenging task because birds have large eyes, thick retinae, and high metabolic rates but neither deep retinal
nor superficial capillaries. Respiratory proteins such as myoglobin may enhance oxygen supply to certain tissues, and thus
the mammalian retina harbors high amounts of neuroglobin. Globin E (GbE) was recently identified as an eye-specific globin
of chicken (Gallus gallus). Orthologous GbE genes were found in zebra finch and turkey genomes but appear to be absent in non-avian vertebrate classes. Analyses of globin
phylogeny and gene synteny showed an ancient origin of GbE but did not help to assign it to any specific globin type. We show
that the photoreceptor cells of the chicken retina have a high level of GbE protein, which accumulates to ∼10 μm in the total eye. Quantitative real-time RT-PCR revealed an ∼50,000-fold higher level of GbE mRNA in the eye than in the
brain. Spectroscopic analysis and ligand binding kinetics of recombinant chicken GbE reveal a penta-coordinated globin with
an oxygen affinity of P50 = 5.8 torrs at 25 °C and 15 torrs at 41 °C. Together these data suggest that GbE helps to sustain oxygen supply to the avian
"5H and I). Studies of zebrafish and Xenopus documented predominant expression of GbX in the nervous system (Fuchs et al. 2006; Blank, Wollberg, et al. 2011). However, those studies did not examine gene expression in the gonads, which suggests that the main "
"This is especially true for the eye-specific globin E gene (GbE) which is only transcribed in the eye of adult birds (Table 2). Thus, the protein is suggested to be involved in the oxygen supply of this metabolically highly active tissue (Blank et al. 2011a). "
"Changes in the size and membership composition of the a-and b-globin gene families may produce changes in the developmental regulation of Hb synthesis and may therefore constrain or potentiate functional divergence between Hb isoforms that incorporate the products of paralogous globin genes (Berenbrink et al. 2005; Berenbrink 2007; Opazo et al. 2008a, 2008b; Runck et al. 2009; Hoffmann, Storz, et al. 2010; Storz, Hoffmann, et al. 2011; Storz, Opazo, et al. 2011; Grispo et al. 2012; Damsgaard et al. 2013; Opazo et al. 2013; Storz et al. 2013). Indeed, over a deeper timescale of animal evolution, the co-option and functional divergence of more ancient members of the globin gene superfamily have played a well-documented role in evolutionary innovation (Hoffmann, Opazo, et al. 2010; Blank et al. 2011; Hoffmann, Opazo, Hoogewijs, et al. 2012; Hoffmann, Opazo, Storz, 2012; Hoogewijs et al. 2012; Schwarze and Burmester 2013; Schwarze et al. 2014). All gnathostome taxa that have been examined to date express structurally and functionally distinct Hb isoforms during different stages of prenatal development (Hardison 2001), and some tetrapod groups coexpress different Hb isoforms during postnatal life. "
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