Most data regarding fish CXCR functions have accumulated

Most data regarding fish CXCR4 functions have accumulated from work in zebrafish where a range of genetic tools and imaging technologies have been developed. As mentioned above, in teleost fish the two CXCR4s (CXCR4a and 4b) are assumed to interact with two ligands (CXCL12a and 12b) and this different pairing of ligands/receptors appears to effect different functions in regulating development of organs. In zebrafish, interaction between CXCL12a and CXCR4b is essential to guide migration of germ cells during embryogenesis, whilst the CXCL12b–CXCR4a axis controls directional migration of the endoderm during zebrafish gastrulation (Mizoguchi et al., 2008, Nair and Schilling, 2008). Depletion of CXCR4a expression causes separation of the endoderm from the mesoderm. CXCR4b signalling also mediates migration of primordial germ cells toward the location of the prospective gonadal primordium in zebrafish and medaka (Doitsidou et al., 2002, Herpin et al., 2008). Activation of CXCR4 requires phosphorylation of the intracellular C terminal region of the receptor, resulting in recruitment of regulatory proteins (Roland et al., 2003, Ueda et al., 2006). Interestingly, it has been shown recently that a Hox homeobox gene (Hoxb8a), best known for its role in axial patterning during development, plays a role in the modulation of CXCR4b and CXCR7b expression (Breau et al., 2013). CXCR4 is widely expressed in the central nervous system and its roles in neuron development and function have been well documented in mammals and, more recently, in bony fish. Most CXCR4-expressing ganglia progenitors differentiate to form sensory neurons and CXCL12a/SDF-1a–CXCR4b signalling is critical for directing neurons to reach the site of ganglion assembly and the development of the lateral line sensory system in zebrafish (Li et al., 2004). CXCR4 influences the migration of microglia to sites of α-Cyperone injury in zebrafish and aids the wound healing process (Chuang et al., 2013). Knockdown of CXCR4b expression in zebrafish leads to abnormal GnRH3 axonal projections and cell migration (Palevitch et al., 2010). In addition, CXCR4 affects regeneration and heart functions (Itou et al., 2012), retinal growth (Li et al., 2005) and muscle myogenesis (Chong et al., 2007). It is highly expressed in human cancer cells and is often used as a biomarker in cancer diagnosis (Cabioglu et al., 2005). The teleost CXCL12-CXCR4 system serves as an ideal model to study the subfunctionalization of duplicated genes (He and Zhang, 2005). The two zebrafish CXCL12 genes have diversified their functions in concert with the two receptors, to generate relative specificity of ligand/receptor binding (CXCL12a–CXCR4b, CXCL12b–CXCR4a). In non-teleost vertebrates, both CXCL12 and CXCR4 exist as single copy genes, and so what drives the subfunctionalization of the CXCL12 and CXCR4 genes in teleosts is an interesting question. It is generally thought that chemokine functions are regulated mainly at two different levels, via change in gene expression and through ligand and receptor binding specificity (Boldajipour et al., 2011); for example, a mutation of N33S in zebrafish CXCL12a significantly reduced its binding affinity with CXCR4b (Boldajipour et al., 2011). Many other genes in teleosts have also been duplicated, by either the 3rd whole genome duplication in basal teleosts and/or random duplication, and in terms of CXCRs, at least two copies of CXCR1, CXCR3, CXCR4 and ACKR3/CXCR7 genes are present (Fig. 1 and Supplementary file 4). Their subfunctionalization is expected to be complex. In addition to their primary roles in development, CXCR4s regulate the homeostasis of immune cells in vertebrates. Fish CXCR4s are detected in immune tissues and lymphocytes (Daniels et al., 1999, Jia and Zhang, 2009, Lin et al., 2012, Yeh and Klesius, 2010, Zhang et al., 2013). In rainbow trout, VHSV infection up-regulates CXCR4 expression in the gills and CD8+ cells (Aquilino et al., 2014). Interestingly, in orange spotted grouper (Epinephelus coioides), CXCR4 expression is significantly up-regulated in the eye and brain where nervous necrosis virus infection and replication take place (Lin et al., 2012), suggesting it may be involved in neuron pathology in these tissues. Stress can also induce CXCR4b expression, as seen in Wuchang bream (Megalobrama amblycephala) exposed to high concentrations of nitrate (Zhang et al., 2013).