Toll, a muscle cell surface molecule, locally inhibits synaptic initiation of the RP3 motoneuron growth cone in Drosophila

Conservation and novelty in the evolution of cell adhesion and extracellular matrix genes

New proteins and modules have been invented throughout evolution. Gene "birth dates" in Caenorhabditis elegans range from the origins of cellular life through adaptation to a soil habitat. Possibly half are "metazoan" genes, having arisen sometime between the yeast-metazoan and nematode-chordate separations. These include basement membrane and cell adhesion molecules implicated in tissue organization. By contrast, epithelial surfaces facing the environment have specialized components invented within the nematode lineage. Moreover, interstitial matrices were likely elaborated within the vertebrate lineage. A strategy for concerted evolution of new gene families, as well as conservation of adaptive genes, may underlie the differences between heterochromatin and euchromatin.

Functional dissection of drosophila capricious: its novel roles in neuronal pathfinding and selective synapse formation

Drosophila Capricious (CAPS) is a transmembrane protein with leucine-rich repeat (LRR) motifs, expressed on small subsets of neurons and muscles, including muscle 12 and the motoneurons that innervate it (muscle 12 MNs). Panmuscle ectopic expression of CAPS alters the target specificity of muscle 12 MNs, indicating that CAPS can function in muscles as a target recognition molecule. In this study, we first examined the effect of ectopic panneural expression of CAPS on the motoneuronal circuit. We found that panneural expression of CAPS alters the pathfinding of muscle 12 MNs. The defect appeared to be caused by changes in the steering behavior of muscle 12 MNs at a specific choice point along their pathway to the target muscle. These results revealed a novel function of CAPS in axon pathfinding. We then performed deletion analyses of CAPS. We expressed CAPS lacking the intracellular domain in all neurons or in all muscles, and studied their ability to induce the pathfinding and targeting phenotypes. We found that the function of muscularly expressed CAPS in target recognition is intracellular domain dependent, whereas the function of neurally expressed CAPS in pathfinding is not, suggesting that CAPS may function in neurons and muscles in a different manner. The requirement of the intracellular domain for the function of muscularly expressed CAPS suggests the presence of a signaling event within muscle cells that is essential for selective synapse formation.

Control of development and immunity by rel transcription factors in Drosophila

The Drosophila Rel/NF-kappaB transcription factors - Dorsal, Dif, and Relish - control several biological processes, including embryonic pattern formation, muscle development, immunity, and hematopoiesis. Molecular-genetic analysis of 12 mutations that cause embryonic dorsal/ventral patterning defects has defined the steps that control the formation of this axis. Regulated activation of the Toll receptor leads to the establishment of a gradient of nuclear Dorsal protein, which in turn governs the subdivision of the axis and specification of ventral, lateral and dorsal fates. Phenotypic analysis of dorsal-ventral embryonic mutants and the characterization of the two other fly Rel proteins, Dif and Relish, have shown that the intracellular portion of the Toll to Cactus pathway also controls the innate immune response in Drosophila. Innate immunity and hematopoiesis are regulated by analogous Rel/NF-kappaB-family pathways in mammals. The elucidation of the complex regulation and diverse functions of Drosophila Rel proteins underscores the relevance of basic studies in Drosophila.

LOLA has the properties of a master regulator of axon-target interaction for SNb motor axons of Drosophila

The proper pathfinding and target recognition of an axon requires the precisely choreographed expression of a multitude of guidance factors: instructive and permissive, positive and negative, and secreted and membrane bound. We show here that the transcription factor LOLA is required for pathfinding and targeting of the SNb motor nerve in Drosophila. We also show that lola is a dose-dependent regulator of SNb development: by varying the expression of one lola isoform we can progressively titrate the extent of interaction of SNb motor axons with their target muscles, from no interaction at all, through wild-type patterning, to apparent hyperinnervation. The phenotypes we observe from altered expression of LOLA suggest that this protein may help orchestrate the coordinated expression of the genes required for faithful SNb development.

The gene mod(mdg4) affects synapse specificity and structure in Drosophila

The mechanisms by which synapse assembly and maturation are orchestrated during development are largely unknown. We used P-element mutagenesis and a larval anatomical screen to isolate mutants in which synapse structure was altered. Here, we describe a mutation isolated with this screen, branch point disrupted (bpd), in which both synapse specificity and synapse morphology were altered. Synaptic terminals in bpd mutants developed abnormally, forming multiple branch points, overgrowing to inappropriate neighboring muscles, and establishing aberrant folding of postsynaptic membranes. Ultrastructural characterization of synaptic boutons in bpd demonstrated abnormal layering of the postsynaptic specialization or subsynaptic reticulum (SSR). Genetic and molecular analyses revealed that bpd is an allele of mod(mdg4), a gene coding for a protein with many similarities to transcription factors, which has been implicated in the regulation of chromatin insulation. Our results suggest that mod(mdg4) may regulate a gene(s) essential to normal synapse formation.

zfNLRR, a novel leucine-rich repeat protein is preferentially expressed during regeneration in zebrafish

zfNLRR is a novel transmembrane protein that is most prominently expressed during regeneration of the zebrafish central nervous system. Retinal ganglion cells and descending spinal cord neurons strongly increase zfNLRR mRNA levels after axotomy in the adult. In contrast, during development expression is hardly detectable and is restricted to a few sensory systems. In the adult brain, zfNLRR mRNA is found at low levels in several motor and premotor systems. Sequence analysis reveals that zfNLRR contains in its extracellular region 12 leucine-rich repeats, 1 immunoglobulin-like domain and 1 fibronectin type III-like domain. The same protein binding motifs were identified in transmembrane proteins from frog, mouse, and human. Together, they constitute a novel family of vertebrate neuronal leucine-rich repeat proteins. Three distinct isoforms are identified so far. On the basis of its structural features and expression pattern, we propose that zfNLRR functions as a neuronal-specific adhesion molecule or soluble ligand binding receptor, primarily during restoration of the nervous system after injury.

Control of muscle fibre and motoneuron diversification

Recent studies have elucidated both the mechanism of early formation of diverse muscle fibre types and the matching of diverse populations of motoneurons to their appropriate muscle targets. Highlights include the demonstration that distinct signals are necessary for the formation of several distinct myoblast populations in the vertebrate somite, the identification of motoneuron subtypes, studies of how motoneurons target appropriate muscles, and rapid progress on the Drosophila neuromuscular system. We propose a model in which four classes of decision control the patterning of both motoneurons and muscles.

Muscle structure and innervation are affected by loss of Dorsal in the fruit fly, Drosophila melanogaster

In Drosophila, the Rel-protein Dorsal and its inhibitor, Cactus, act in signal transduction pathways that control the establishment of dorsoventral polarity during embryogenesis and the immune response during postembryonic life. Here we present data indicating that Dorsal is also involved in the control of development and maintenance of innervation in somatic muscles. Dorsal and Cactus are colocalized in all somatic muscles during postembryonic development. In larvae and adults, these proteins are distributed at low levels in the cytoplasm and nuclei and at much higher levels in the postsynaptic component of glutamatergic neuromuscular junctions. Absence of Dorsal, in homozygous dorsal mutant larvae results in muscle misinsertions, duplications, nuclear hypotrophy, disorganization of actin bundles, and altered subcellular distribution of Cactus. Some muscles show very abnormal neuromuscular junctions, and some motor axon terminals are transformed into growth cone-like structures embedded in synaptotagmin-enriched vesicles. The detailed phenotype suggests a role of Dorsal signalling in the maintenance and plasticity of the NMJ.

Genetic and molecular analyses of motoneuron development

Motoneurons have distinct identities and muscle targets. Recent classical and molecular genetic studies in flies and vertebrates have begun to elucidate how motoneuron identities and target specificities are established. Many of the same molecules participate in the guidance of both vertebrate and fly motor axons. It is less clear, however, whether the same molecular mechanisms establish vertebrate and fly motoneuron identities.

Commissureless endocytosis is correlated with initiation of neuromuscular synaptogenesis

We show that the Commissureless (COMM) transmembrane protein is required at neuromuscular synaptogenesis. All muscles in the Drosophila embryo express COMM during the period of motoneuron-muscle interaction. It is endocytosed into muscles before synaptogenesis. In comm loss-of-function mutants, motoneuron growth cones fail to initiate synaptogenesis at target muscles. This stall phenotype is rescued by supplying wild-type COMM to the muscles. Cytoplasmically truncated COMM protein fails to internalize. Expressing this mutant protein in muscles phenocopies the synaptogenesis defects of comm mutants. Thus, synaptogenesis initiation is positively correlated with endocytosis of COMM in postsynaptic muscle cells. We propose that COMM is an essential part of the dynamic cell surface remodeling needed by postsynaptic cells in coordinating synaptogenesis initiation.

Cytoplasmic polyadenylation of Toll mRNA is required for dorsal-ventral patterning in Drosophila embryogenesis

Toll encodes a receptor that is critical for dorsal-ventral patterning in the early Drosophila embryo. Previous data have suggested that the accumulation of Toll protein in the embryo temporally correlates with elongation of the poly (A) tail of the message. Here, we demonstrate that Toll mRNA is translationally activated by regulated cytoplasmic polyadenylation. We also identify a 192 nucleotide regulatory element in the Toll 3′ UTR that is necessary for robust translational activation of Toll mRNA and also regulates polyadenylation. UV crosslinking analyses suggest that two proteins bind specifically to the 192 nucleotide element. One or both of these proteins may be factors that are required for translational regulation or cytoplasmic polyadenylation. These studies demonstrate that regulated polyadenylation plays a critical role in the Drosophila dorsal-ventral patterning system.

Drosophila synapse formation: regulation by transmembrane protein with Leu-rich repeats, CAPRICIOUS

Upon reaching the target region, neuronal growth cones transiently search through potential targets and form synaptic connections with only a subset of these. The capricious (caps) gene may regulate these processes in Drosophila. caps encodes a transmembrane protein with leucine-rich repeats (LRRs). During the formation of neuromuscular synapses, caps is expressed in a small number of synaptic partners, including muscle 12 and the motorneurons that innervate it. Loss-of-function and ectopic expression of caps alter the target specificity of muscle 12 motorneurons, indicating a role for caps in selective synapse formation.

Homophilic synaptic target recognition mediated by immunoglobulin-like cell adhesion molecule Fasciclin III

We demonstrate that the cell adhesion molecule Fasciclin III (FAS3) mediates synaptic target recognition through homophilic interaction. FAS3 is expressed by the RP3 motoneuron and its target muscles during synaptic target recognition. The RP3 growth cone can form synapses on muscles that ectopically express FAS3. This mistargeting is dependent on FAS3 expression in the motoneurons. In addition, when the FAS3-negative aCC and SNa motoneuron growth cones ectopically express FAS3, they gain the ability to recognize FAS3-expressing muscles as alternative targets. We propose that homophilic synaptic target recognition serves as a basic mechanism of neural network formation.