Neuromusculin, a Drosophila gene expressed in peripheral neuronal precursors and muscles, encodes a cell adhesion molecule

A single-cell atlas of the sexually dimorphic Drosophila foreleg and its sensory organs during development

To respond to the world around them, animals rely on the input of a network of sensory organs distributed throughout the body. Distinct classes of sensory organs are specialized for the detection of specific stimuli such as strain, pressure, or taste. The features that underlie this specialization relate both to the neurons that innervate sensory organs and the accessory cells they comprise. To understand the genetic basis of this diversity of cell types, both within and between sensory organs, we performed single-cell RNA sequencing on the first tarsal segment of the male Drosophila melanogaster foreleg during pupal development. This tissue displays a wide variety of functionally and structurally distinct sensory organs, including campaniform sensilla, mechanosensory bristles, and chemosensory taste bristles, as well as the sex comb, a recently evolved male-specific structure. In this study, we characterize the cellular landscape in which the sensory organs reside, identify a novel cell type that contributes to the construction of the neural lamella, and resolve the transcriptomic differences among support cells within and between sensory organs. We identify the genes that distinguish between mechanosensory and chemosensory neurons, resolve a combinatorial transcription factor code that defines 4 distinct classes of gustatory neurons and several types of mechanosensory neurons, and match the expression of sensory receptor genes to specific neuron classes. Collectively, our work identifies core genetic features of a variety of sensory organs and provides a rich, annotated resource for studying their development and function.

Genetic and Molecular Analysis of Essential Genes in Centromeric Heterochromatin of the Left Arm of Chromosome 3 in Drosophila melanogaster

A large portion of the Drosophila melanogaster genome is contained within heterochromatic regions of chromosomes, predominantly at centromeres and telomeres. The remaining euchromatic portions of the genome have been extensively characterized with respect to gene organization, function and regulation. However, it has been difficult to derive similar data for sequences within centromeric (centric) heterochromatin because these regions have not been as amenable to analysis by standard genetic and molecular tools. Here we present an updated genetic and molecular analysis of chromosome 3L centric heterochromatin (3L Het). We have generated and characterized a number of new, overlapping deficiencies (Dfs) which remove regions of 3L Het. These Dfs were critically important reagents in our subsequent genetic analysis for the isolation and characterization of lethal point mutations in the region. The assignment of these mutations to genetically-defined essential loci was followed by matching them to gene models derived from genome sequence data: this was done by using molecular mapping plus sequence analysis of mutant alleles, thereby aligning genetic and physical maps of the region. We also identified putative essential gene sequences in 3L Het by using RNA interference to target candidate gene sequences. We report that at least 25, or just under 2/3 of loci in 3L Het, are essential for viability and/or fertility. This work contributes to the functional annotation of centric heterochromatin in Drosophila, and the genetic and molecular tools generated should help to provide important insights into the organization and functions of gene sequences in 3L Het.

Single cell RNA-sequencing identifies a metabolic aspect of apoptosis in Rbf mutant

The function of Retinoblastoma tumor suppressor (pRB) is greatly influenced by the cellular context, therefore the consequences of pRB inactivation are cell-type-specific. Here we employ single cell RNA-sequencing (scRNA-seq) to profile the impact of an Rbf mutation during Drosophila eye development. First, we build a catalogue of 11,500 wild type eye disc cells containing major known cell types. We find a transcriptional switch occurring in differentiating photoreceptors at the time of axonogenesis. Next, we map a cell landscape of Rbf mutant and identify a mutant-specific cell population that shows intracellular acidification due to increase in glycolytic activity. Genetic experiments demonstrate that such metabolic changes, restricted to this unique Rbf mutant population, sensitize cells to apoptosis and define the pattern of cell death in Rbf mutant eye disc. Thus, these results illustrate how scRNA-seq can be applied to dissect mutant phenotypes.

The function of the Retinoblastoma (Rb) protein is regulated by its cellular environment. Here, the authors perform single cell RNA-sequencing during Drosophila eye development and identify the impact of an Rbf mutation, which sensitises specific cells to apoptosis by changing metabolism.

Tip cells act as dynamic cellular anchors in the morphogenesis of looped renal tubules in Drosophila

Tissue morphogenesis involves both the sculpting of tissue shape and the positioning of tissues relative to one another in the body. Using the renal tubules of Drosophila, we show that a specific distal tubule cell regulates both tissue architecture and position in the body cavity. Focusing on the anterior tubules, we demonstrate that tip cells make transient contacts with alary muscles at abdominal segment boundaries, moving progressively forward as convergent extension movements lengthen the tubule. Tip cell anchorage antagonizes forward-directed, TGF-β-guided tubule elongation, thereby ensuring the looped morphology characteristic of renal tubules from worms to humans. Distinctive tip cell exploratory behavior, adhesion, and basement membrane clearing underlie target recognition and dynamic interactions. Defects in these features obliterate tip cell anchorage, producing misshapen and misplaced tubules with impaired physiological function.

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Tip cells stabilize elongating renal tubules by binding to specific muscle targets

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Tip cell anchorage antagonizes migration, producing excretory tubule looping

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Tip cell exploration and adhesion dynamics underpin regulated tubule morphogenesis

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Lack of continued anchorage results in deformed tubules with impaired function

A genetic and molecular profile of third chromosome centric heterochromatin in Drosophila melanogaster

In this review, we combine the results of our published and unpublished work with the published results of other laboratories to provide an updated map of the centromeric heterochromatin of chromosome 3 in Drosophila melanogaster. To date, we can identify more than 20 genes (defined DNA sequences with well-characterized functions and (or) defined genetic complementation groups), including at least 16 essential loci. With the ongoing emergence of data from genetic, cytological, and genome sequencing studies, we anticipate continued, substantial progress towards understanding the function, structure, and evolution of centric heterochromatin.

Genetic Programs Activated by Proneural Proteins in the Developing Drosophila PNS

Neurogenesis depends on a family of proneural transcriptional activator proteins, but the "proneural" function of these factors is poorly understood, in part because the ensemble of genes they activate, directly or indirectly, has not been identified systematically. We have taken a direct approach to this problem in Drosophila. Fluorescence-activated cell sorting was used to recover a purified population of the cells that comprise the "proneural clusters" from which sensory organ precursors of the peripheral nervous system (PNS) arise. Whole-genome microarray analysis and in situ hybridization was then used to identify and verify a set of genes that are preferentially expressed in proneural cluster cells. Genes in this set encode proteins with a diverse array of implied functions, and loss-of-function analysis of two candidate genes shows that they are indeed required for normal PNS development. Bioinformatic and reporter gene studies further illuminate the cis-regulatory codes that direct expression in proneural clusters.

Cell and molecular biology of myoblast fusion

In organisms from Drosophila to mammals, the musculature is comprised of an elaborate array of distinct fibers that are generated by the fusion of committed myoblasts. These muscle fibers differ from each other in features that include location, pattern of innervation, site of attachment, and size. The sizes of the newly formed muscles of an embryo are controlled in large part by the number of cells that form the syncitial fiber. Over the past few decades, an extensive body of literature has described the process of myoblast fusion in vertebrates, relying primarily on the strengths of tissue culture model systems. More recently, genetic studies in Drosophila embryos have provided new insights into the process. Together, these studies define the steps necessary for myoblast differentiation, the acquisition of fusion competence, the recognition and adhesion between myoblasts, and the fusion of two lipid bilayers into one. In this review, we have attempted to combine insights from both Drosophila and vertebrate studies to trace the processes and molecules involved in myoblast fusion. Implicit in this approach is the assumption that fundamental aspects of myoblast fusion will be similar, independent of the organism in which it is occurring.

Gliogenesis inDrosophila: genome-wide analysis of downstream genes ofglial cells missingin the embryonic nervous system

In Drosophila, the glial cells missing (gcm) gene encodes a transcription factor that controls the determination of glial versus neuronal fate. In gcm mutants, presumptive glial cells are transformed into neurons and, conversely, when gcm is ectopically misexpressed, presumptive neurons become glia. Although gcm is thought to initiate glial cell development through its action on downstream genes that execute the glial differentiation program, little is known about the identity of these genes. To identify gcm downstream genes in a comprehensive manner, we used genome-wide oligonucleotide arrays to analyze differential gene expression in wild-type embryos versus embryos in which gcm is misexpressed throughout the neuroectoderm. Transcripts were analyzed at two defined temporal windows during embryogenesis. During the first period of initial gcm action on determination of glial cell precursors, over 400 genes were differentially regulated. Among these are numerous genes that encode other transcription factors, which underscores the master regulatory role of gcm in gliogenesis. During a second later period, when glial cells had already differentiated, over 1200 genes were differentially regulated. Most of these genes, including many genes for chromatin remodeling factors and cell cycle regulators, were not differentially expressed at the early stage, indicating that the genetic control of glial fate determination is largely different from that involved in maintenance of differentiated cells. At both stages, glial-specific genes were upregulated and neuron-specific genes were downregulated, supporting a model whereby gcm promotes glial development by activating glial genes, while simultaneously repressing neuronal genes. In addition, at both stages, numerous genes that were not previously known to be involved in glial development were differentially regulated and, thus, identified as potential new downstream targets of gcm. For a subset of the differentially regulated genes, tissue-specific in vivo expression data were obtained that confirmed the transcript profiling results. This first genome-wide analysis of gene expression events downstream of a key developmental transcription factor presents a novel level of insight into the repertoire of genes that initiate and maintain cell fate choices in CNS development.

Forked end: a novel transmembrane protein involved in neuromuscular specificity in drosophila identified by gain-of-function screening

The Drosophila neuromuscular connectivity provides an excellent model system for studies on target recognition and selective synapse formation. To identify molecules involved in neuromuscular recognition, we conducted gain-of-function screening for genes whose forced expression in all muscles alters the target specificity. We report here the identification of a novel transmembrane protein, Forked end (FEND), encoded by the fend gene, by the said screening. When the FEND expression was induced in all muscles, motoneurons that normally innervate muscle 12 formed ectopic synapses on a neighboring muscle 13. The target specificity of these motoneurons was also altered in the loss-of-function mutant of fend. During embryonic development, fend mRNA was detected in a subset of cells in the central nervous system and in the periphery. These results suggest that FEND is a novel axon guidance molecule involved in neuromuscular specificity.

Pathfinding by sensory axons in Drosophila: substrates and choice points in early lch5 axon outgrowth

We have examined the pattern of axon growth from the lateral chordotonal (lch5) neurons in the body wall of the Drosophila embryo and identified cellular substrates and choice points involved in early axon pathfinding by these sensory neurons. At the first choice point (TP1), the lch5 growth cones contact the most distal cells of the spiracular branch (SB) of the trachea. The SB provides a substrate along which the axons extend internally to the level of the intersegmental nerve (ISN). In the absence of the SB, the lch5 axons often stall near TP1 or follow aberrant routes towards the CNS. At the second choice point (TP2), the lch5 growth cones make their first contact with other axons and turn ventrally toward the CNS, fasciculating specifically with the motor axons of the ISN.

sidestep encodes a target-derived attractant essential for motor axon guidance in Drosophila

At specific choice points in the periphery, subsets of motor axons defasciculate from other axons in the motor nerves and steer into their muscle target regions. Using a large-scale genetic screen in Drosophila, we identified the sidestep (side) gene as essential for motor axons to leave the motor nerves and enter their muscle targets. side encodes a target-derived transmembrane protein (Side) that is a novel member of the immunoglobulin superfamily (IgSF). Side is expressed on embryonic muscles during the period when motor axons leave their nerves and extend onto these muscles. In side mutant embryos, motor axons fail to extend onto muscles and instead continue to extend along their motor nerves. Ectopic expression of Side results in extensive and prolonged motor axon contact with inappropriate tissues expressing Side.

Senseless, a Zn finger transcription factor, is necessary and sufficient for sensory organ development in Drosophila

The senseless (sens) gene is required for proper development of most cell types of the embryonic and adult peripheral nervous system (PNS) of Drosophila. Sens is a nuclear protein with four Zn fingers that is expressed and required in the sensory organ precursors (SOP) for proper proneural gene expression. Ectopic expression of Sens in many ectodermal cells causes induction of PNS external sensory organ formation and is able to recreate an ectopic proneural field. Hence, sens is both necessary and sufficient for PNS development. Our data indicate that proneural genes activate sens expression. Sens is then in turn required to further activate and maintain proneural gene expression. This feedback mechanism is essential for selective enhancement and maintenance of proneural gene expression in the SOPs.

Ectopic expression of NCAM in skeletal muscle of transgenic mice results in terminal sprouting at the neuromuscular junction and altered structure but not function

The neuromuscular system provides an excellent model for the analysis of molecular interactions involved in the development and plasticity of synaptic contacts. The neural cell adhesion molecule (NCAM) is believed to be involved in the development and plasticity of the neuromuscular junction, in particular the axonal sprouting response observed in paralyzed and denervated muscle. In order to explore the role of myofiber NCAM in modulating the differentiation of motor neurons, we generated transgenic mice expressing a GPI-anchored NCAM isoform that is normally found in developing and denervated muscle, under the control of a skeletal muscle-specific promoter. This results in the constitutive expression of NCAM at postnatal ages, a time when the endogenous mouse NCAM is absent from the myofiber. We found that a significant number of neuromuscular junctions in adult transgenic animals displayed terminal sprouting (>20%) reminiscent of that elicited in response to cessation of neuromuscular activity. Additionally, a significant increase in the size and complexity of neuromuscular synapses as a result of extensive intraterminal sprouting was detected. Electrophysiological studies, however, revealed no significant alterations of neuromuscular transmission at this highly efficient synapse. Sprouting in response to paralysis or following nerve crush was also significantly enhanced in transgenic animals. These results suggest that in this ectopic expression model NCAM can directly modulate synaptic structure and motor neuron-muscle interactions. The results contrast with knockout experiments of the NCAM gene, where very limited changes in the neuromuscular system were observed.

Multiple signalling pathways establish cell fate and cell number in Drosophila malpighian tubules

A unique cell, the tip mother cell, arises in the primordium of each Drosophila Malpighian tubule by lateral inhibition within a cluster of achaete-expressing cells. This cell maintains achaete expression and divides to produce daughters of equivalent potential, of which only one, the tip cell, adopts the primary fate and continues to express achaete, while in the other, the sibling cell, achaete expression is lost (M. Hoch et al., 1994, Development 120, 3439-3450). In this paper we chart the mechanisms by which achaete expression is differentially maintained in the tip cell lineage to stabilise cell fate. First, wingless is required to maintain the expression of achaete in the tubule primordium so that wingless mutants lack tip cells. Conversely, increasing wingless expression results in the persistence of achaete expression in the cell cluster. Second, Notch signalling is restricted by the asymmetric segregation of Numb, as the tip mother cell divides, so that achaete expression is maintained only in the tip cell. In embryos mutant for Notch tip cells segregate at the expense of sibling cells, whereas in numb neither daughter cell adopts the tip cell fate resulting in tubules with two sibling cells. Conversely, when numb is overexpressed two tip cells segregate and tubules have no sibling cells. Analysis of cell proliferation in the developing tubules of embryos lacking Wingless after the critical period for tip cell allocation reveals an additional requirement for wingless for the promotion of cell division. In contrast, alteration in the expression of numb has no effect on the final tubule cell number.

Distinct localization of SAPK isoforms in neurons of adult mouse brain implies multiple signaling modes of SAPK pathway

Various cellular and environmental stresses lead to the activation of stress-activated protein kinase (SAPK), which is also referred to as c-Jun N-terminal kinase (JNK). In mammals, multiple SAPK isoforms, encoded by three independent genes, were identified. To gain insight into the roles of SAPK pathway in adult mouse brain, detailed expression patterns of three SAPK isoforms in brain were examined by using immunohistochemical and cell biological analyses. SAPKbeta was heavily expressed in almost all regions of brain as previously reported. Interestingly, SAPKgamma was also widely expressed at high levels. SAPKgamma expression was generally overlapped with SAPKbeta although there were some exceptions such as in hippocampus, where SAPKgamma was restricted to CA3 and CA4 regions while SAPKbeta was evenly expressed. SAPKalpha was widely expressed, but at low levels. It is particularly intriguing to note the differential subcellular localization of SAPK isoforms in neurons. In brain of normally reared mice, SAPKbeta was identified in nucleus as well as in cytoplasm of neurons, while SAPKgamma was detected mainly in cytoplasm and dendrites. Biochemical and immunological analyses revealed extraordinarily high basal activities of all SAPK isoforms in brain compared to peripheral organs, indicating that SAPK pathway may play a role in normal brain physiology. In addition, differential regional and subcellular localizations of SAPK isoforms allow us to speculate multiple signaling modes for SAPK activation in brain.

Muscle pattern diversification in Drosophila: the story of imaginal myogenesis

There are two phases of somatic muscle formation in Drosophila. During embryonic development, one phase of myogenesis generates larval muscle elements that mediate the relatively simple behavioural repertoire of the larva. During pupal metamorphosis, a diverse pattern of muscle fibres are assembled, and these facilitate the more elaborate behavioural patterns of the adult fly. In this review, we discuss the current status of understanding of the cellular, genetic, and molecular mechanisms of pattern formation during the second phase, imaginal muscle development. We briefly compare aspects of embryonic and adult myogenesis in Drosophila and muscle development in vertebrates and highlight conserved themes and disparities between these diverse myogenic programmes.

Molecular cloning of multiple splicing variants of JIP-1 preferentially expressed in brain

Stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) is activated by a variety of cellular or environmental stresses. Proper regulation of the SAPK/JNK pathway may be critical for cell survival or death under various conditions. In this study, we report the molecular cloning of novel isoforms of JIP-1, which harbor a putative phosphotyrosine interaction domain and a helix-loop-helix domain, as well as an SH3 homologous region in the C terminus. Northern analysis indicates that transcription variant jip-1 is expressed in brain and kidney and transcription variants jip-2 and jip-3 are specifically expressed in brain. In situ hybridization data showed that the hybridized jip messages were heavily concentrated in adult brain, and were particularly enriched in the cerebral cortex and hippocampus, the brain regions vulnerable to pathological states such as hypoxia-ischemia, epilepsy, and Alzheimer's disease. All the deduced protein products of the jip transcription variants appear to have a similar property in that they inhibit the SAPK/JNK stimulation when overexpressed. Inhibition of SAPK activation by overexpression of the novel isoform JIP-2a resulted in suppression of etoposide-induced cell death in a neuroglioma cell line, N18TG. These findings suggest that JIP may play an important role in regulation of the SAPK pathway that is involved in stress-induced cellular responses.

M-spondin, a novel ECM protein highly homologous to vertebrate F-spondin, is localized at the muscle attachment sites in the Drosophila embryo

The muscle attachment site (MAS) in Drosophila provides a unique and excellent model system to study the mechanism of cell-matrix adhesion in developing organisms. Here, we report on the isolation and characterization of a novel extracellular matrix (ECM) molecule localized at the MAS, encoded by the M-spondin (mspo) gene. M-spondin protein contains a thrombospondin type I repeat (TSR) previously found in a variety of ECM molecules. Furthermore, it shares two conserved domains with F-spondin, a vertebrate ECM molecule with TSRs. The presence of TSR(s) and the two homologous domains thus defines a novel gene family of ECM molecules. The mspo mRNA was expressed by a large subset of muscles in the embryonic body wall. Secreted M-spondin protein diffused and eventually became immobilized at the MAS in late embryos. When expressed in S2 cells, the protein was secreted and became concentrated in the matrix on the surface of the culture dish. Genetic analysis revealed that both deletion mutants and misexpression mutants suffered no obvious developmental defects. We propose that M-spondin, although its function is redundant, is a component of the ECM and mediates mechanical linkage between the muscles and apodemes.

klingon, a novel member of the Drosophila immunoglobulin superfamily, is required for the development of the R7 photoreceptor neuron

klingon is a member of the Immunoglobulin superfamily and is expressed in a restricted pattern of neurons during embryonic neurogenesis and in the R7 photoreceptor precursor throughout its development. Starting from the H214 enhancer trap line, we identified a transcription unit, klingon, that encodes a putative protein of 528 amino acids and contains three C2-type Immunoglobulin-like domains followed by one fibronectin type III repeat. When Klingon is expressed in S2 tissue culture cells, it is associated with the cell membrane by a glycosyl-phosphatidylinositol linkage and can mediate homophilic adhesion. Genetic analysis has revealed that klingon is an essential gene that participates in the development of the R7 neuron. Ectopic expression of klingon in all neurons in a sevenless background can alter the position of the R8 rhabdomere.

Structural features of a close homologue of L1 (CHL1) in the mouse: a new member of the L1 family of neural recognition molecules

We have identified a close homologue of L1 (CHL1) in the mouse. CHL1 comprises an N-terminal signal sequence, six immunoglobulin (Ig)-like domains, 4.5 fibronectin type III (FN)-like repeats, a transmembrane domain and a C-terminal, most likely intracellular domain of approximately 100 amino acids. CHL1 is most similar in its extracellular domain to chicken Ng-CAM (approximately 40% amino acid identity), followed by mouse L1, chicken neurofascin, chicken Nr-CAM, Drosophila neuroglian and zebrafish L1.1 (37-28% amino acid identity), and mouse F3, rat TAG-1 and rat BIG-1 (approximately 27% amino acid identity). The similarity with other members of the Ig superfamily [e.g. neural cell adhesion molecule (N-CAM), DCC, HLAR, rse] is 16-11%. The intracellular domain is most similar to mouse and chicken Nr-CAM, mouse and rat neurofascin (approximately 60% amino acid identity) followed by chicken neurofascin and Ng-CAM, Drosophila neuroglian and zebrafish L1.1 and L1.2 (approximately 40% amino acid identity). Besides the high overall homology and conserved modular structure among previously recognized members of the L1 family (mouse/human L1/rat NILE; chicken Ng-CAM; chicken/mouse Nr-CAM; Drosophila neuroglian; zebrafish L1.1 and L1.2; chicken/mouse neurofascin/rat ankyrin-binding glycoprotein), criteria characteristic of L1 were identified with regard to the number of amino acids between positions of conserved amino acid residues defining distances within and between two adjacent Ig-like domains and FN-like repeats. These show a collinearity in the six Ig-like domains and four adjacent FN-like repeats that is remarkably conserved between L1 and molecules containing these modules (designated the L1 family cassette), including the GPI-linked forms of the F3 subgroup (mouse F3/chicken F11/human CNTN1; rat BIG-1/mouse PANG; rat TAG-1/mouse TAX-1/chicken axonin-1). The colorectal cancer molecule (DCC), previously introduced as an N-CAM-like molecule, conforms to the L1 family cassette. Other structural features of CHL 1 shared between members of the L1 family are a high degree of N-glycosidically linked carbohydrates (approximately 20% of its molecular mass), which include the HNK-1 carbohydrate structure, and a pattern of protein fragments comprising a major 185 kDa band and smaller fragments of 165 and 125 kDa. As for the other L1 family members, predominant expression of CHL1 is observed in the nervous system and at later developmental stages. In the central nervous system CHL1 is expressed by neurons, but, in contrast to L1, also by glial cells. Our findings suggest a common ancestral L1-like molecule which evolved via gene duplication to generate a diversity of structurally and functionally distinct yet similar molecules.

Hikaru genki protein is secreted into synaptic clefts from an early stage of synapse formation in Drosophila

The development of neural circuits is regulated by a large number of factors that are localized at distinct neural sites. We report here the localization of one of these factors, hikaru genki (hig) protein, at synaptic clefts in the pupal and adult nervous systems of Drosophila. In hig mutants, unusually frequent bursting activity of the muscles and abnormal motor behavior during the adult stage suggest the misfunction of neuromuscular circuitry. Our immunohistochemical analyses revealed that hig protein, produced by neurons, is secreted from the presynaptic terminals into the spaces between the presynaptic and postsynaptic terminals. In addition, we have found that the localization of this protein in the synaptic spaces temporally correlates with its functional requirement during a critical period that occurs in the middle stage of pupal formation, a period when a number of dendrite and axon growth cones meet to form synapses. These findings indicate that hig protein functions in the formation of functional neural circuits from the early stages of synapse formation.

Regulatory interactions during early neurogenesis in Drosophila

During neurogenesis in Drosophila, ectodermal cells are endowed with the capacity to become neuronal precursors. Following their selection, these cells initiate neuronal lineage development and differentiation. The processes of neuronal precursor specification and neuronal lineage development require the activities of several groups of genes functioning in a complex, hierarchical regulatory network. Whereas the proneural genes promote neurogenic potential, neurogenic genes restrict the acquisition of this identity to a subset of ectodermal cells. Following their selection, these cells express the pan neural neuronal precursor genes and a set of neuronal lineage identity genes. While lineage identity genes allow the various lineages to acquire specific identities, neuronal precursor genes presumably regulate functional and developmental characteristics common to all neuronal precursor cells.

Expression and functional analysis of a novel isoform of gicerin, an immunoglobulin superfamily cell adhesion molecule

We have cloned a novel cDNA of gicerin, a cell adhesion molecule belonging to the immunoglobulin superfamily. Both gicerin isoforms share the same extracellular domain, which has five immunoglobulin-like loop structures and a transmembrane domain as s-gicerin, but differ in the cytoplasmic tail domain. As the newly identified form has a larger cytoplasmic domain than the previously reported form, we refer to them as l-gicerin and s-gicerin, respectively. l-gicerin is transcribed from a distinct mRNA containing an inserted sequence not found in s-gicerin mRNA which caused a frameshift for the coding region for a cytoplasmic domain. Previous studies demonstrated that gicerin showed a doublet band of 82 and 90 kDa in chicken gizzard smooth muscle. We report that the 82-kDa protein corresponds to s-gicerin and the 90-kDa protein to l-gicerin. We also found that the two gicerin isoforms are expressed differentially in the developing nervous system. Functional analysis of these gicerin isoforms in stable transfectants revealed that they had differ in their homophilic adhesion properties, as well as in heterophilic cell adhesion assayed with neurite outgrowth factor. In addition, these isoforms have neurite-promoting activity by their homophilic adhesion, but differ in their ability to promote neurite outgrowth.

Heavy chain dimers as well as complete antibodies are efficiently formed and secreted from Drosophila via a BiP-mediated pathway

We have constructed a stable Drosophila cell line co-expressing heavy chain (HC) and light chain (LC) immunoglobulins of a humanized monoclonal antibody (mAb) that recognizes the F antigen of respiratory syncytial virus (Tempest, P. R., Bremmer, P., Lambert, M., Taylor, G., Furze, J. M., Carr, F. J., and Harris, W. J. (1991) Bio/Technology 9, 266-271. These cells efficiently secrete antibody with substrate binding activity indistinguishable from that produced from vertebrate cell lines. Significantly, the Drosophila homologue of the immunoglobulin binding chaperone protein (BiP), hsc72, was found to interact specifically with the immunoglobulin HC in an ATP-dependent fashion, similar to the BiP-HC interaction known to occur in vertebrate cells. This is, in fact, the first substrate ever shown to interact specifically with Drosophila hsc72. Most surprisingly, expression of heavy chains in the absence of LC led to the efficient secretion of heavy chain dimers. Moreover, this secretion occurred in association with hsc72. This dramatically contrasts with what is seen in vertebrate cells where in the absence of LC, HC remains sequestered inside the cell in stable association with BiP. Our results clearly suggest that Drosophila BiP can substitute for its mammalian counterpart and chaperone the secretion of active IgG. However, the finding that Drosophila BiP can also uniquely chaperone heavy chain dimers indicates mechanistic differences that may relate to the evolved need for retaining immature IgGs in vertebrates.

Fasciclin III as a synaptic target recognition molecule in Drosophila

Fasciclin III, a cell adhesion molecule of the immunoglobulin superfamily, is expressed by motor neuron RP3 and its synaptic targets (muscle cells 6 and 7) during embryonic neuromuscular development of Drosophila. We report here that RP3 often incorrectly innervates neighbouring non-target muscle cells when these cells misexpress fasciclin III, but still innervates normal targets in the fasciclin III null mutant. Fasciclin III manipulations do not influence target selections by other motor neurons, including fasciclin III-expressing RP1. We propose that fasciclin III acts as a synaptic target recognition molecule for motor neuron RP3, and also that its absence can be compensated for by other molecule(s).

Mutations affecting the pattern of the PNS in Drosophila reveal novel aspects of neuronal development

Through a systematic genetic screen, we have identified 55 mutations that affect the development of the PNS of Drosophila embryos. These mutations specify 13 novel and 5 previously characterized genes and define new phenotypes for 2 other known genes. Five classes of mutant phenotypes were identified in the screen: gain of neurons, loss of neurons, abnormal position of chordotonal neurons, aberrant neuronal trajectories, and abnormal morphology of neurons. Phenotypic analyses of mutations identified in this study revealed three novel aspects of PNS development. First, we have identified a novel gene that may be required to define glial versus neuronal cell identity. Second, our data indicate that neuronal migration plays an important role in pattern formation in the embryonic PNS. Third, we have identified mutations that cause a lack of sensory organs, but unlike mutations in proneural genes, do not affect the formation of sensory organ precursors. These genes may be required for key aspects of neuronal differentiation. Our studies suggest that approximately 70 essential genes are required for proper PNS development in Drosophila embryos.

The Drosophila 18 wheeler is required for morphogenesis and has striking similarities to Toll

We have isolated and characterized a novel gene, named 18 wheeler (18w) for its unique segmental expression pattern in Drosophila embryos and expression in cells that migrate extensively. 18 wheeler transcripts accumulate in embryos in a pattern reminiscent of segment polarity genes. Mutations in 18w cause death during larval development and early adulthood. Escaping mutant adults often display leg, antenna, and wing deformities, presumably resulting from improper eversion of imaginal discs. Sequence analysis indicates that 18w encodes a transmembrane protein with an extracellular moiety containing many leucine rich repeats and cysteine motifs, and an intracellular domain bearing homology to the cytoplasmic portion of the interleukin-1-receptor. Expression of 18W protein in non-adhesive Schneider 2 cells promotes rapid and robust aggregation of cells. Analysis of the expression of 18w in different mutant backgrounds shows that it is under control of segment polarity and homeotic genes. The data suggest that the 18W protein participates in the developmental program specified by segmentation and homeotic genes as a cell adhesion or receptor molecule that facilitates cell movements.

Genetic and molecular dissection of axon pathfinding in the Drosophila nervous system

Over the past year, several systematic genetic screens designed to identify mutations that specifically disrupt axon pathfinding processes in the Drosophila nervous system have been described. The analysis of mutations isolated in these screens, in concert with ongoing cellular studies and the ever increasing number of identified proteins expressed in the Drosophila nervous system, is providing further insights into the molecular mechanisms of growth cone guidance.