The Hox gene Abdominal B prevents reaper-mediated apoptosis of the dMP2 neurons in posterior segments of the embryonic CNS. I. Miguel-Aliaga, S. Thor. Department of Neurobiology, Harvard Medical School, Boston, MA.

   The Drosophila ventral nerve cord (VNC) is segmentally repeated, with the majority of cell types and structures present in each segment. However, increasingly detailed studies of neuroblast lineages and neuronal subtypes reveal a more complex picture. For instance, several types of peptidergic neurons often display a segmental pattern that traverses the traditional VNC subdivisions. We previously identified a pair of peptidergic neurons in the A6-8 segments, which we named "paired Vap" (ventral abdominal posterior) neurons. We have found that these Vap neurons are actually the well-studied dMP2 interneurons. Surprisingly, the dMP2 neurons are proctolin-expressing motor neurons that exit the VNC in the posterior nerve to innervate the hindgut. We have observed that, as previously described, the dMP2 neurons are initially born and extend their axons in all segments of the VNC. However, at stage 17, they undergo programmed cell death (PCD) in all segments anterior to A6. This kind of PCD is rare in invertebrates, since it occurs in mature neurons that have already extended their axons. We have determined that dMP2 death is mediated by the cell death gene reaper (rpr), and have found that rpr is indeed only expressed in the anterior dMP2s. Hox genes regulate morphogenesis along the antero-posterior body axis of animals, but the biological processes and target genes they control are poorly understood. Recent studies have shown that Hox genes can activate rpr in certain contexts, thus indicating that they exert their tissue-modeling function in part by directly regulating key PCD genes. However, their possible role as modulators of PCD in the Drosophila VNC has not been investigated. We have found that the Hox gene Abdominal B (AbdB) is required for the suppression of dMP2 PCD in posterior segments in a cell-autonomous, post-mitotic fashion. Our results reveal a novel function for a Hox gene in an identified, differentiated neuron in the Drosophila CNS.