Thesis #267

In Drosophila, a set of evolutionarily conserved transcription factors are required for the specification of neuronal identity along the anteroposterior (AP) and dorsoventral (DV) axes, such as the Hox genes for AP, or the columnar genes for DV axis patterning. The results presented in this thesis analyse the expression and function of the Hox genes and the columnar gene ventral nervous system defective (vnd) during embryonic brain development of Drosophila. These results provide evidence that the Hox gene labial (lab) is required for the regionalized specification of the tritocerebral neuromere. Misexpression of posterior Hox genes in the embryonic neuroectoderm results in a lab loss-of function phenotype and a corresponding lack of Labial protein expression in the tritocerebrum. This is due to repression of labial gene transcription operating on a 3.65kb brain-specific lab-enhancer element. A functional analysis of Antennapedia and Ultrabithorax protein domains shows that the transcriptional repression of labial requires homeodomain-DNA interactions but is not dependent on a functional hexapeptide. The repressive activity of a Hox protein on labial expression in the tritocerebrum can, however, be abolished by concomitant misexpression of a Hox protein and the co-factors Homothorax (HTH) and nuclear-targeted Extradenticle (EXD) , suggesting that specification of tritocerebral neuronal identity requires equilibrated levels of a Hox protein and Hth and n-Exd cofactors. Moreover, evidence is presented that mutational inactivation of the columnar gene vnd results in regionalized axonal patterning defects which are similar to the brain phenotype caused by mutation of the Hox gene lab. However, in contrast to lab, vnd is required for precursor cell development and neuronal progeny maintenance during tritocerebral neuromere formation. In vnd mutant embryos, a subset of identified tritocerebral neuroblasts which normally express lab do not form. During later stages, programmed cell death leads to reduced or absent neuronal tissue which is normally specified by lab. The resulting vnd mutant brain phenotype is characterized by the lack of the tritocerebral neuromere, which can be rescued by targeted inactivation of the apoptotic program. Thus, in contrast to its DV patterning function in the VNC, vnd is required for AP patterning during embryonic brain development of Drosophila. These results indicate that the activity of the columnar gene vnd is integrated into pattern formation along the anteroposterior neuraxis by generating and maintaining cells which subsequently become specified by the activity of the Hox gene lab.