Purpurin expression in the zebrafish retina during early development and after optic nerve lesion in adults

Purpurin, a retina-specific protein, is known to play a role in cell adhesion during development of the chicken retina. Although purpurin has been significantly detected in adult chicken retina, its function in the matured retina is not well understood. Therefore, to determine the expression pattern of purpurin in the retina, we simultaneously investigated expression patterns of purpurin in the zebrafish retina during development in larvae and optic nerve regeneration after nerve transection in adults. In early development, levels of purpurin suddenly increased in the zebrafish retina 3 to 5 days after fertilization, and purpurin-positive immunoreactivity was diffusely located in all retinal layers. In contrast, levels of purpurin mRNA rapidly increased in the adult retina 1-3 days after optic nerve transection, and rapidly declined by 10 days after injury. Signal for purpurin mRNA was seen only in photoreceptors. Immunohistochemistry showed that levels of purpurin protein were also increased in the retina 1-3 days after nerve injury, but positive staining was located in photoreceptors and ganglion cells, and the staining in ganglion cells was stronger than that in photoreceptors. Thus, the transient expression of purpurin protein was greatly different during development and optic nerve regeneration. In the former, purpurin may be required in all retinal layers, whereas in the latter, purpurin may be required for injured ganglion cells.