A key role of Pox meso in somatic myogenesis of Drosophila.

The Pax gene Pox meso (Poxm) was the first and so far only gene whose initial expression was shown to occur specifically in the anlage of the somatic mesoderm, yet its role in somatic myogenesis remained unknown. Here we show that it is one of the crucial genes regulating the development of the larval body wall muscles in Drosophila. It has two distinct functions expressed during different phases of myogenesis. The early function, partially redundant with the function of lethal of scute [l(1)sc], demarcates the ;Poxm competence domain', a domain of competence for ventral and lateral muscle development and for the determination of at least some adult muscle precursor cells. The late function is a muscle identity function, required for the specification of muscles DT1, VA1, VA2 and VA3. Our results led us to reinterpret the roles of l(1)sc and twist in myogenesis and to propose a solution of the 'l(1)sc conundrum'.

Liprin-alpha has LAR-independent functions in R7 photoreceptor axon targeting.

In the Drosophila visual system, the color-sensing photoreceptors R7 and R8 project their axons to two distinct layers in the medulla. Loss of the receptor tyrosine phosphatase LAR from R7 photoreceptors causes their axons to terminate prematurely in the R8 layer. Here we identify a null mutation in the Liprin-alpha gene based on a similar R7 projection defect. Liprin-alpha physically interacts with the inactive D2 phosphatase domain of LAR, and this domain is also essential for R7 targeting. However, another LAR-dependent function, egg elongation, requires neither Liprin-alpha nor the LAR D2 domain. Although human and Caenorhabditis elegans Liprin-alpha proteins have been reported to control the localization of LAR, we find that LAR localizes to focal adhesions in Drosophila S2R+ cells and to photoreceptor growth cones in vivo independently of Liprin-alpha. In addition, Liprin-alpha overexpression or loss of function can affect R7 targeting in the complete absence of LAR. We conclude that Liprin-alpha does not simply act by regulating LAR localization but also has LAR-independent functions.

Target recognition and synaptogenesis by motor axons: responses to the sidestep protein.

Sidestep (Side) is a pivotal molecular player in embryonic motor axon pathfinding. But questions about its functional repertoire remain: (i) can Side permanently overturn targeting preferences? (ii) does it promote synaptogenesis, and (iii) can Side facilitate synaptic stabilization? To address these questions, Side was temporally and spatially misexpressed and the visible consequences for neuromuscular junction morphology were assessed. When Side was misexpressed either broadly or selectively in muscles during targeting in a wildtype background motor axon targeting preferences were permanently overturned. However the misexpression of Side in all muscles post-targeting neither changed synapse morphology, nor compensated for a lack of the synapse-stabilizing protein Fasciclin II (FasII). Rather Side appears to be dependent on FasII, instead of on intrinsic ability, for sustaining targeting changes. We propose that Side helps to bring motor axons to their correct muscle targets and promotes synaptogenesis, then FasII serves to stabilize the synaptic contacts.

Necessity and redundancy of guidepost cells in the embryonic Drosophila CNS.

Guidepost cells are specific cellular cues in the embryonic environment utilized by axonal growth cones in pathfinding decisions. In the embryonic Drosophila CNS the RP motor axons make stereotypic pathways choices involving distinct cellular contacts: (i) extension across the midline via contact with the axon and cell body of the homologous contralateral RP motoneuron, (ii) extension down the contralateral longitudinal connective (CLC) through contact with connective axons and longitudinal glia, and (iii) growth into the intersegmental nerve (ISN) through contact with ISN axons and the segmental boundary glial cell (SBC). We have now ablated putative guidepost cells in each of the CNS pathway subsections and uncovered their impact on subsequent RP motor axon pathfinding. Removal of the longitudinal glia or the SBC did not adversely affect pathfinding. This suggests that the motor axons either utilized the alternative axonal substrates, or could still make filopodial contact with the next pathway section's cues. In contrast, RP motor axons did require contact with the axon and soma of their contralateral RP homologue. Absence of this neuronal substrate frequently impeded RP axon outgrowth, suggesting that the next cues were beyond filopodial reach. Together these are the first direct ablations of putative guidepost cells in the CNS of this model system, and have uncovered both pathfinding robustness and susceptibility by RP axons in the absence of specific contacts.

Development of a polar morphology by identified embryonic motoneurons.

Motoneuron morphology arises through the coordinated growth of the motor axon and dendrites. In the Drosophila embryo the RP motoneurons have a contralaterally-extended motor axon, ipsilateral dendrites that extend a short distance in the ipsilateral connective, and a tuft of short dendrites in the contralateral connective. In the present study mechanical and genetic manipulations were utilized to test if (i) the ipsilateral dendrites can develop an axon morphology, (ii) the presence of the contralateral motor axon suppresses the development of an axon-like morphology by the ipsilateral dendrites and (iii) whether establishment of a contralateral motor axon can be genetically suppressed. It was found that an ipsilateral motor axon could develop-but only at the expense of the contralateral motor axon. Axotomy could overturn the normal polarity of the RP motoneurons in favor of the development of an ipsilateral motor axon, and this reversed morphology was also observed when the motor axon could not extend across the midline in the commissureless mutant. These findings show that the RP motoneurons have the plasticity for an alternative polarity, but that the extension of an ipsilateral axon is normally suppressed by the presence of the contralateral axon. The RP motoneurons now represent a genetically amenable in vivo system for analyzing the basis of polarity formation in neurons.

Myoblast fusion in Drosophila.

Somatic muscle formation is an unusual process as it requires the cells involved, the myoblasts, to relinquish their individual state and fuse with one another to form a syncitial muscle fiber. The potential use of myoblast fusion therapies to rebuild damaged muscles has generated continuing interest in elucidating the molecular basis of the fusion process. Yet, until recently, few of the molecular players involved in this process had been identified. Now, however, it has been possible to couple a detailed understanding of the cellular basis of the fusion process with powerful classical and molecular genetic strategies in the Drosophila embryo. We review the cellular studies, and the recent genetic and biochemical analyses that uncovered interacting extracellular molecules present on fusing myoblasts and the intracellular effectors that facilitate fusion. With the conservation of proteins and protein functions across species, it is likely that these findings in Drosophila will benefit understanding of the myoblast fusion process in higher organisms.