br CXCR The CXC receptor
CXCR1/2 The CXC receptor 1 and 2 (CXCR1 and CXCR2) have been well characterised in vertebrates. In mammals and birds, the two receptors are shared by the ELR+ CXC ligands, namely the CXCL8 family members that have proinflammatory roles in recruitment of neutrophils, monocytes and macrophages to sites of infection. Activation of CXCR1 and CXCR2 leads to a cascade of cellular events responsible for migration, rac inhibitor and adhesion of the target cells. CXCR1 primarily binds with CXCL6–8 whilst CXCR2 is less ligand specific, interacting with almost all of the ELR+ CXC chemokines including CXCL1–3 and CXCL5–8 (Fan et al., 2007, Stillie et al., 2009). Chemokine receptors and other GPCRs can be recycled to the cell surface after internalization with their ligands (Fan et al., 2003), a process that is mediated by cellular pathways involving multiple signalling molecules such as β-arrestins, Activating Protein (AP) 2, G protein-coupled Receptor Kinases (GRKs) and dynamin (Barlic et al., 1999). Recent studies have shown that CXCR1 and CXCR2 recruit GRK2 and GRK6 respectively, to regulate leucocyte functions (Raghuwanshi et al., 2012). This can lead to functional differences. For example, CXCR2 but not CXCR1 is involved in CXCL8 mediated angiogenesis and cancer growth (Heidemann et al., 2003). The first fish CXCR1/2 homologue was identified in peritoneal leucocytes of carp (Cyprinus carpio), induced by treatment with sodium alginate, using suppression subtractive hybridization (Fujiki et al., 1999). Subsequently CXCR1 and CXCR2 have been found in a wide range of teleost fish species including rainbow trout (Oncorhynchus mykiss) (Xu et al., 2014a, Zhang et al., 2002), fugu (Takifugu rubripes) (Huising et al., 2003a, Huising et al., 2003b); zebrafish (Danio rerio) (Deng et al., 2013, Oehlers et al., 2010); Chinese perch (Siniperca chuatsi) (Chen et al., 2009), miiuy croaker (Miichthys miiuy) (Xu et al., 2014b) and several elasmobranch species, including lesser spotted catshark (Scyliorhinus caniculus), basking shark (Cetorhinus maximus), great white shark (Carcharodon carcharias), cuckoo ray (Raja naevus) (Goostrey et al., 2005) and elephant shark (Callorhinchus milii) (Venkatesh et al., 2014), and more recently in the coelacanth (Latimeria chalumnae) (Xu et al., 2014a) (Fig. 1 and Supplementary file 1). Comparative analysis of CXCR protein sequences reveals that C. milii CXCR2 possesses the conserved motif (I[L]L[I]XL[I]L) and PDZ-like ligand domain (STTIL[I]) in the C-terminal region, which have been shown to be important for cellular signalling of human CXCR2 (Fig. 2) (Baugher and Richmond, 2008, Fan et al., 2001, Marchese, 2014). In general, the CXCR1 and CXCR2 genes are tandemly linked in the vertebrate genomes, although in teleost fish there are usually two loci for CXCR1 (i.e. encoding for CXCR1a and CXCR1b), and only one is linked to CXCR2. CXCR1/2 have not been reported in jawless fish but two predicted proteins with moderate sequence homology to CXCR1 and CXCR2 are identifiable in the sea lamprey (Petromyzon marinus) genome (Ensembl Acc. No.: ENSPMAP00000011202, annotated as CXCR3 in the Ensembl database) and could be the putative receptor for the CXCL8 identified in this species (Najakshin et al., 1999). In the phylogenetic tree (Supplementary file 2), this protein together with another putative lamprey receptor (Ensembl Acc. No.: ENSPMAP00000011160) is located in the branch containing CXCR1–5. Although most jawed vertebrates typically possess both CXCR1 and CXCR2, teleost CXCR1 and CXCR2 are not 1:1 orthologues to their tetrapod counterparts (Saha et al., 2007). Several hypotheses have been proposed regarding how these receptors have evolved during vertebrate evolution. It is believed that CXCR1/R2 like genes were present before the divergence of the bony fish and tetrapods (Xu et al., 2014a), and this notion is supported by the high conservation of gene synteny of the CXCR1/2 genes in the genomes of different vertebrates; as seen in spotted gar (Lepisosteus oculatus), an extant species of Holostei. CXCR1 and CXCR2 are also tandemly linked and flanked by GPAR1 and TNS1, an arrangement seen in reptiles and mammals (Xu et al., 2014a) (unpublished data). The elephant shark genome contains two copies of putative tandemly clustered CXCR1/R2 genes (Venkatesh et al., 2014), one of which is a partial sequence. The phylogenetic relationship of one of the elephant shark CXCR1/2 like proteins with CXCR1 and CXCR2 is ambiguous since the NJ tree placed it in the CXCR2 clade whilst the Bayesian posterior probability and maximum likelihood bootstrap scores support the orthologous relationship of the elephant shark CXCR1/2 with teleost and coelacanth CXCR1 (Fig. 1 and Supplementary file 2). However, phylogenetic analysis alone cannot establish the evolutionary relationship of the two receptors between fish and tetrapods. It is possible that events such as gene conversion and lineage specific gene gain and loss combined with selection pressure from the ligands could contribute to the divergence of the two receptors in different vertebrate lineages (Shields, 2000;Xu et al., 2014a).