Up-regulation of semaphorin 4A expression in human retinal pigment epithelial cells by PACAP released from cocultured neural cells.

Development and homeostasis of multicellular organisms require interactions between neighbouring cells. We recently established an in vitro model of cell-cell interaction based on a collagen vitrigel membrane. We have now examined the role of neural cells in retinal homeostasis by coculture of human retinal pigment epithelial (RPE) cells and neural cells on opposite sides of such a membrane. The neural cells (differentiated PC12 cells) induced up-regulation of semaphorin 4A (Sema4A), a member of the semaphorin family of neural guidance proteins, in RPE (ARPE19) cells. This effect of the neural cells was mimicked by the neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) and was abolished by the PACAP antagonist PACAP(6-38). Coculture with neural cells or stimulation with PACAP also induced the phosphorylation of extracellular-signal-regulated kinase in ARPE19 cells, and this effect of the neural cells was inhibited by PACAP(6-38). Finally, among various cytokines examined, only the amount of interleukin-6 released by cocultures of ARPE19 and neural cells differed from that released by ARPE19 cells cultured alone. Interleukin-6 was not detected in culture supernatants of neural cells, and the reduction in the amount of interleukin-6 released by the cocultures compared with that released by ARPE19 cells alone was prevented by PACAP(6-38). Our findings suggest that PACAP released from retinal neural cells (photoreceptors or optic nerve cells) may regulate Sema4A expression in RPE cells and thereby contribute to the maintenance of retinal structure and function. Development and homeostasis of multicellular organisms require interactions between neighbouring cells. With the use of a coculture system based on a collagen vitrigel membrane, we have now shown that neural cells induce up-regulation of the neural guidance protein Sema4A in RPE cells. This effect of neural cells appears to be mediated by the neuropeptide PACAP. PACAP released from retinal neural cells (photoreceptors or optic nerve cells) may thus regulate Sema4A expression in RPE cells and thereby contribute to the maintenance of retinal structure and function.