The receptor tyrosine kinase QEK5 mRNA is expressed in a gradient within the neural retina and the tectum

In the retinotectal system, positional information has long been postulated to take the form of molecular gradients within both the retina and the tectum. Recent reports have implicated Mek4, a member of the Eph (also named class V) family of tyrosine kinase receptors (RTKs), and two ligands, RAGS and ELF-1, in this process. Here, we report the cloning and distribution pattern of QEK5, another member of the Eph family of RTKs, isolated from a quail cDNA library. During retinal differentiation, QEK5 transcripts accumulate in a ventral to dorsal gradient within the retinal neuroepithelium, where its expression becomes restricted to the ganglion and bipolar cell layers. Within the tectum, QEK5 transcripts are detectable in a posterior to anterior gradient in the ventricular layer and newly formed superficial layers. The pattern of QEK5 expression in the retina and tectum is distinct from that of Mek4, suggesting that complex patterns of Eph RTKs and their ligands may play a role in cell-cell interactions involved in retinotectal projections and differentiation of the central nervous system.