To gain insight into the developmental role of onecut, its expression pattern was examined to determine where it might function. For this purpose, mouse polyclonal antibodies were raised against two non-overlapping peptide regions at the N-terminus of Onecut. The earliest expression of Onecut protein is detected during early embryogenesis within the central nervous system (CNS) at the time correlated with neuroblast formation (stage 8-9 embryos). In late stage 15 embryos, Onecut is expressed in both the CNS and the peripheral nervous system (PNS). During larval development, onecut is expressed in photoreceptor (R) cells of the third instar imaginal eye disc. An antibody to a neural-specific protein Elav was used to confirm this expression pattern. Double-antibody staining with anti-Elav shows that the onset of Onecut expression follows that of Elav. This suggests that the R cell precursors have already been committed to a neural developmental pathway before Onecut is expressed, which would imply a role for Onecut in subsequent stages of neural cell differentiation and that it does not function in the determination of the R cell fate (Nguyen, 2000).
During pupal development, the expression of Onecut is also restricted to neuronal cells. This includes the 8 R cells of the ommatidial clusters and a single cell that is most likely the neuronal cell of the nerve bristle group, which is shared by three surrounding ommatidial clusters. In the adult head, Onecut is expressed in all differentiated photoreceptors as well as in the nuclei of brain neurons in the lamina, medulla, and many cells of the lobula complex. Interestingly, studies on the Drosophila Eph receptor tyrosine kinase (dek) gene, which lies upstream of onecut, reveal a neural-specific enhancer within a 520 bp EcoRI- HindIII fragment located 300 bp from the start site of onecut and 2.2 kb from the dek start site (Scully, 1999). This enhancer may indeed be responsible for both onecut and dek expression in the nervous system. Similar to prokaryotes, multi-gene regulation by a common regulatory unit does exist in eukaryotes, particularly for genes that are functionally related. In this case, onecut and dek may function in some aspects of neuronal development. Consistent with this idea, genetic analysis of the mammalian Eph receptors points to a role in axon pathfinding, while dek is expressed in growth cones and axons of embryonic interneurons, and in larval R cells (Scully, 1999).
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date revised: 10 April 2010
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