In mammals, complement C3a receptor (C3aR) is a member of the G protein-coupled receptor family (GPCRs) (Crass T et al. 1996). C3aR is activated upon binding of complement fragments C3a. C3aR transduces signals through multiple pathways, leading to the inhibition of adenylyl cyclase, activation of MAP kinase, phosphoinositol accumulation and calcium mobilization (Sayah S et al. 2003; Ricklin D et al. 2010).
Upon complement activation chicken C3 is cleaved into fragments that resemble mammalian C3a and C3b (Laursen I and Koch C 1989). Chicken C3 and its cleavage products were detected in liver, plasma and different eye tissues (Haynes T et al. 2013). In addition to C3 activation fragments, chicken C3aR mRNA and protein were found in eye tissues at E4 (embryo's stage at which retinectomies are performed). Histological analysis in combination with immuno-histochemical and western blotting procedures using post-retinectomy samples revealed that the treatment with chicken C3a induced chick retina regeneration via MAPK-STAT3 signaling in a C3aR-dependent manner (Haynes T et al. 2013).