There is a paucity of reports on CXCR function

There is a paucity of reports on CXCR5 function in fish. In terms of expression, CXCR5 is highly expressed in lymphoid tissues such as kidney and spleen in grass carp and can be modulated by a range of immune stimulants including peptidoglycan, LPS, polyI:C and phytohaemagglutinin (Xu et al., 2010). In zebrafish, CXCR5 is detected in glial cells when they differentiate into neurons in the adult zebrafish brain. It regulates the regenerative neurogenesis response and promotes ventricular cell proliferation when injury occurs in the telencephalon (Kizil et al., 2012). Overexpression of dominant negative CXCR5 variants significantly reduces the number of newborn neurons in lesioned telencephalon in the zebrafish Vacuolin-1 (Kizil et al., 2012). These findings demonstrate that CXCR5’s roles in neuron development in the brain and migration of neural cells is likely to be conserved in fish.
CXCR6 CXCR6, originally referred to as BONZO, is the sole receptor interacting with CXCL16, which is an unusual CXC chemokine in being produced as both membrane bound and soluble forms (Matloubian et al., 2000, Wilbanks et al., 2001). The membrane bound CXCL16 consists of an N-terminal chemokine domain, a highly glycosylated mucin-like stalk, a transmembrane domain, and a short cytoplasmic tail (Matloubian et al., 2000), and functions as a scavenger receptor for oxidised low-density lipoprotein and bacteria as well as an adhesion mediator for leucocytes (Nakayama et al., 2003, Shimaoka et al., 2000, Shimaoka et al., 2004). The soluble form of CXCL16 is generated by proteolytic cleavage of the membrane molecule, involving disintegrin and metalloproteinases (Abel et al., 2004) and coordinates migration of CXCR6+ cells (Borst et al., 2012). CXCL16 is produced by macrophages, dendritic cells, Th1 cells, platelets and cancer cells (Borst et al., 2012, Darash-Yahana et al., 2009, Galkina et al., 2007, Kim et al., 2001, Zernecke et al., 2008). Its expression can be up-regulated by IFN-γ and TNF-α (Abel et al., 2004) and a high level of expression is usually associated with the progression of human diseases such as rheumatoid arthritis (Nanki et al., 2005), atherosclerosis (Galkina et al., 2007), coronary artery disease (Abel et al., 2004), and liver injury (Xu et al., 2005). CXCL16 promotes growth of CXCR6-expressing cancer cells and primary CD4+ T cells in inflammation-associated cancers (Darash-Yahana et al., 2009). Curiously, CXCL16 shares some degree of structural similarities with CX3CL1 (fractalkine) (Wang et al., 2008a). CXCL16 mediated cell migration is facilitated by the interaction of soluble CXCL16 with CXCR6, expressed predominantly on the cell surface of CD4+ Th1 cells, cancer infiltrating lymphocytes and platelets (Borst et al., 2012, Kim et al., 2001, Liao et al., 1997, Wilbanks et al., 2001). CXCR6 is involved in the recruitment and homing of inflammatory cells and proliferation and invasion of tumour cells (Galkina et al., 2007). It also participates in adhesion and chemotaxis of platelets (Borst et al., 2012), and serves as a co-receptor for HIV (Deng et al., 1997, Matloubian et al., 2000). Activation of CXCR6 by CXCL16 triggers cellular responses involving phosphatidylinositide 3-kinase (PI3K) and its downstream effector Akt (protein kinase B) (Wang et al., 2008a).