Specification ofDrosophilamotoneuron identity by the combinatorial action of POU and LIM-HD factors

• DOI: 10.1242/dev.01418
• Publication Date: 2004-10-07T00:13:55Z
• Authors: Sarah J. Certel; Stefan Thor
• Keywords: 0301 basic medicine; Elegans Homeoproteins Unc-86; Heterozygote; 571; LIM-Homeodomain Proteins; Immunoglobulin Superfamily; Nerve Tissue Proteins; Domain Transcription Factor; 03 medical and health sciences; Animals; Drosophila Proteins; Motor Neurons; 0303 health sciences; Insulin-Gene Enhancer; Muscles; Motor-Neuron; Molecular-Mechanisms; Gene Expression Regulation, Developmental; Axons; DNA-Binding Proteins; Drosophila melanogaster; POU Domain Factors; Caenorhabditis-Elegans; Binding Protein Ldb1; Ventral Nerve Cord; Serotonergic Neurons; Transcription Factors

In both vertebrates and invertebrates, members of the LIM-homeodomain(LIM-HD) family of transcription factors act in combinatorial codes to specify motoneuron subclass identities. In the developing Drosophila embryo,the LIM-HD factors Islet (Tailup) and Lim3, specify the set of motoneuron subclasses that innervate ventral muscle targets. However, as several subclasses express both Islet and Lim3, this combinatorial code alone cannot explain how these motoneuron groups are further differentiated. To identify additional factors that may act to refine this LIM-HD code, we have analyzed the expression of POU genes in the Drosophila embryonic nerve cord. We find that the class III POU protein, Drifter (Ventral veinless), is co-expressed with Islet and Lim3 specifically in the ISNb motoneuron subclass. Loss-of-function and misexpression studies demonstrate that the LIM-HD combinatorial code requires Drifter to confer target specificity between the ISNb and TN motoneuron subclasses. To begin to elucidate molecules downstream of the LIM-HD code, we examined the involvement of the Beaten path (Beat)family of immunoglobulin-containing cell-adhesion molecules. We find that beat Ic genetically interacts with islet and Lim3in the TN motoneuron subclass and can also rescue the TN fasciculation defects observed in islet and Lim3 mutants. These results suggest that in the TN motoneuron context, Islet and Lim3 may specify axon target selection through the actions of IgSF call-adhesion molecules.