AN EARLY PHASE OF EMBRYONIC DLX5 EXPRESSION DEFINES THE ROSTRAL BOUNDARY OF THE NEURAL PLATE

Relatively little is known about the molecular events that specify the rostrocaudal axis of the neural plate. Here we show that a member of the Distal-less (Dlx) homeobox gene family, Dlx5, is one of the earlie st known markers for the most rostral ectoderm, before the formation o f an overt neural plate. During late gastrulation Dlx5 expression beco mes localized to the anterior neural ridge, which defines the rostral boundary of the neural plate, and also extends caudolaterally, marking the region of the presumptive neural crest. Subsequently, Dlx5 is exp ressed in tissues (olfactory epithelium, ventral cephalic epithelium) that are believed to derive from the anterior neural ridge, based on t he avian fate map. The early phase of Dlx5 expression in the anterior neural ridge and its derivatives is distinct from a later phase of exp ression in the ventral telencephalon and diencephalon and also appears to be unique for Dlx5 among members of the Dir family. Another distin ctive feature of Dlx5 expression is the occurrence of an alternative t ranscript (delta Dlx5), which encodes a truncated protein lacking the homeodomain, and represents a significant fraction of total Dlx5 trans cripts at all embryonic stages that were examined. In contrast with fu ll-length DLX5, the delta DLX5 truncated protein is deficient in DNA-b inding activity and does not interact with the homeoprotein partner MS X1. Taken together, our findings suggest that Dlx5 activity may be reg ulated via the expression of an alternative transcript and demonstrate that Dlx5 marks the anterior boundary of the neural plate.