Targeted mutagenesis and genetic analysis of a Drosophila receptor-linked protein tyrosine phosphatase gene

Adult expression of Semaphorins and Plexins is essential for motor neuron survival

Axon guidance cues direct the growth and steering of neuronal growth cones, thus guiding the axons to their targets during development. Nonetheless, after axons have reached their targets and established functional circuits, many mature neurons continue to express these developmental cues. The role of axon guidance cues in the adult nervous system has not been fully elucidated. Using the expression pattern data available on FlyBase, we found that more than 96% of the guidance genes that are expressed in the Drosophila melanogaster embryo continue to be expressed in adults. We utilized the GeneSwitch and TARGET systems to spatiotemporally knockdown the expression of these guidance genes selectively in the adult neurons, once the development was completed. We performed an RNA interference (RNAi) screen against 44 guidance genes in the adult Drosophila nervous system and identified 14 genes that are required for adult survival and normal motility. Additionally, we show that adult expression of Semaphorins and Plexins in motor neurons is necessary for neuronal survival, indicating that guidance genes have critical functions in the mature nervous system.

Complex protein interactions mediate Drosophila Lar function in muscle tissue

The type IIa family of receptor protein tyrosine phosphatases (RPTPs), including Lar, RPTPσ and RPTPδ, are well-studied in coordinating actin cytoskeletal rearrangements during axon guidance and synaptogenesis. To determine whether this regulation is conserved in other tissues, interdisciplinary approaches were utilized to study Lar-RPTPs in the Drosophila musculature. Here we find that the single fly ortholog, Drosophila Lar (Dlar), is localized to the muscle costamere and that a decrease in Dlar causes aberrant sarcomeric patterning, deficits in larval locomotion, and integrin mislocalization. Sequence analysis uncovered an evolutionarily conserved Lys-Gly-Asp (KGD) signature in the extracellular region of Dlar. Since this tripeptide sequence is similar to the integrin-binding Arg-Gly-Asp (RGD) motif, we tested the hypothesis that Dlar directly interacts with integrin proteins. However, structural analyses of the fibronectin type III domains of Dlar and two vertebrate orthologs that include this conserved motif indicate that this KGD tripeptide is not accessible and thus unlikely to mediate physical interactions with integrins. These results, together with the proteomics identification of basement membrane (BM) proteins as potential ligands for type IIa RPTPs, suggest a complex network of protein interactions in the extracellular space that may mediate Lar function and/or signaling in muscle tissue.

Molecular Mechanisms Underlying Motor Axon Guidance in Drosophila

Decoding the molecular mechanisms underlying axon guidance is key to precise understanding of how complex neural circuits form during neural development. Although substantial progress has been made over the last three decades in identifying numerous axon guidance molecules and their functional roles, little is known about how these guidance molecules collaborate to steer growth cones to their correct targets. Recent studies in Drosophila point to the importance of the combinatorial action of guidance molecules, and further show that selective fasciculation and defasciculation at specific choice points serve as a fundamental strategy for motor axon guidance. Here, I discuss how attractive and repulsive guidance cues cooperate to ensure the recognition of specific choice points that are inextricably linked to selective fasciculation and defasciculation, and correct pathfinding decision-making.

Receptor Tyrosine Kinases and Phosphatases in Neuronal Wiring: Insights From Drosophila

Tyrosine phosphorylation is at the crossroads of many signaling pathways. Brain wiring is not an exception, and several receptor tyrosine kinases (RTKs) and tyrosine receptor phosphates (RPTPs) have been involved in this process. Considerable work has been done on RTKs, and for many of them, detailed molecular mechanisms and functions in several systems have been characterized. In contrast, RPTPs have been studied considerably less and little is known about their ligands and substrates. In both families, we find redundancy between different members to accomplish particular wiring patterns. Strikingly, some RTKs and RPTPs have lost their catalytic activity during evolution, but not their importance in biological processes. In this regard, we have to keep in mind that these proteins have multiple domains and some of their functions are independent of tyrosine phosphorylation/dephosphorylation. Since RTKs and RPTPs are enzymes involved not only in early stages of axon and dendrite pathfinding but also in synapse formation and physiology, they have a potential as drug targets. Drosophila has been a key model organism in the search of a better understanding of brain wiring, and its sophisticated toolbox is very suitable for studying the function of genes with pleiotropic functions such as RTKs and RPTPs, from wiring to synaptic formation and function. In these review, we mainly cover findings from this model organism and complement them with discoveries in vertebrate systems.

Receptor tyrosine phosphatases regulate birth order-dependent axonal fasciculation and midline repulsion during development of the Drosophila mushroom body

Receptor tyrosine phosphatases (RPTPs) are required for axon guidance during embryonic development in Drosophila. Here we examine the roles of four RPTPs during development of the larval mushroom body (MB). MB neurons extend axons into parallel tracts known as the peduncle and lobes. The temporal order of neuronal birth is reflected in the organization of axons within these tracts. Axons of the youngest neurons, known as core fibers, extend within a single bundle at the center, while those of older neurons fill the outer layers. RPTPs are selectively expressed on the core fibers of the MB. Ptp10D and Ptp69D regulate segregation of the young axons into a single core bundle. Ptp69D signaling is required for axonal extension beyond the peduncle. Lar and Ptp69D are necessary for the axonal branching decisions that create the lobes. Avoidance of the brain midline by extending medial lobe axons involves signaling through Lar.

Regulation of CNS and motor axon guidance in Drosophila by the receptor tyrosine phosphatase DPTP52F

Receptor-linked protein tyrosine phosphatases (RPTPs) regulate axon guidance and synaptogenesis in Drosophila embryos and larvae. We describe DPTP52F, the sixth RPTP to be discovered in Drosophila. Our genomic analysis indicates that there are likely to be no additional RPTPs encoded in the fly genome. Five of the six Drosophila RPTPs have C. elegans counterparts, and three of the six are also orthologous to human RPTP subfamilies. DPTP52F, however, has no clear orthologs in other organisms. The DPTP52F extracellular domain contains five fibronectin type III repeats and it has a single phosphatase domain. DPTP52F is selectively expressed in the CNS of late embryos, as are DPTP10D, DLAR, DPTP69D and DPTP99A. To define developmental roles of DPTP52F, we used RNA interference (RNAi)-induced phenotypes as a guide to identify Ptp52F alleles among a collection of EMS-induced lethal mutations. Ptp52F single mutant embryos have axon guidance phenotypes that affect CNS longitudinal tracts. This phenotype is suppressed in Dlar Ptp52F double mutants, indicating that DPTP52F and DLAR interact competitively in regulating CNS axon guidance decisions. Ptp52F single mutations also cause motor axon phenotypes that selectively affect the SNa nerve. DPTP52F, DPTP10D and DPTP69D have partially redundant roles in regulation of guidance decisions made by axons within the ISN and ISNb motor nerves.

Dominant Drop mutants are gain-of-function alleles of the muscle segment homeobox gene (msh) whose overexpression leads to the arrest of eye development

Dominant Drop (Dr) mutations are nearly eyeless and have additional recessive phenotypes including lethality and patterning defects in eye and sensory bristles due to cis-regulatory lesions in the cell cycle regulator string (stg). Genetic analysis demonstrates that the dominant small eye phenotype is the result of separate gain-of-function mutations in the closely linked muscle segment homeobox (msh) gene, encoding a homeodomain transcription factor required for patterning of muscle and nervous system. Reversion of the Dr(Mio) allele was coincident with the generation of lethal loss-of-function mutations in msh in cis, suggesting that the dominant eye phenotype is the result of ectopic expression. Molecular genetic analysis revealed that two dominant Dr alleles contain lesions upstream of the msh transcription start site. In the Dr(Mio) mutant, a 3S18 retrotransposon insertion is the target of second-site mutations (P-element insertions or deletions) which suppress the dominant eye phenotype following reversion. The pattern of 3S18 expression and the absence of msh in eye imaginal discs suggest that transcriptional activation of the msh promoter accounts for ectopic expression. Dr dominant mutations arrest eye development by blocking the progression of the morphogenetic furrow leading to photoreceptor cell loss via apoptosis. Gal4-mediated ubiquitous expression of msh in third-instar larvae was sufficient to arrest the morphogenetic furrow in the eye imaginal disc and resulted in lethality prior to eclosion. Dominant mutations in the human msx2 gene, one of the vertebrate homologs of msh, are associated with craniosynostosis, a disease affecting cranial development. The Dr mutations are the first example of gain-of-function mutations in the msh/msx gene family identified in a genetically tractible model organism and may serve as a useful tool to identify additional genes that regulate this class of homeodomain proteins.

Complex genetic interactions among four receptor tyrosine phosphatases regulate axon guidance in Drosophila

Four receptor-linked protein tyrosine phosphatases are selectively expressed on central nervous system axons in the Drosophila embryo. Published data show that three of these (DLAR, DPTP69D, DPTP99A) regulate motor axon guidance decisions during embryonic development. Here we examine the role of the fourth neural phosphatase, DPTP10D, by analyzing double-, triple-, and quadruple-mutant embryos lacking all possible combinations of the phosphatases. This analysis shows that all four phosphatases participate in guidance of interneuronal axons within the longitudinal tracts of the central nervous system. In the neuromuscular system, DPTP10D works together with the other three phosphatases to facilitate outgrowth and bifurcation of the SNa nerve, but acts in opposition to the others in regulating extension of ISN motor axons past intermediate targets. Our results provide evidence for three kinds of genetic interactions among the neural tyrosine phosphatases: partial redundancy, competition, and collaboration.

Inflated wings, tissue autolysis and early death in tissue inhibitor of metalloproteinases mutants of Drosophila

In vertebrates, tissue inhibitors of metalloproteinases (TIMPs) play key roles in extracellular matrix (ECM) homeostasis and growth control. Deletion of the recently cloned Timp gene of Drosophila results in a subviable phenotype. Adult flies display inflated wings similar to integrin mutants, suffer from a bloated gut and progressive dissolution of internal tissues, and die prematurely. Our results demonstrate that the Timp gene product controls selective aspects of ECM function in Drosophila, and suggest that it is involved in cell adhesion/cell signaling pathways. Hence, Drosophila Timp mutants may prove useful as a model system for a wide variety of pathological conditions related to ECM dysregulation.

Pattern formation in the absence of cell proliferation: tissue-specific regulation of cell cycle progression by string (stg) during Drosophila eye development

During Drosophila eye development, the posterior-to-anterior movement of the morphogenetic furrow coordinates cell cycle progression with the early events of pattern formation. The cdc25 phosphatase string (stg) has been proposed to contribute to the synchronization of retinal precursors anterior to the furrow by driving cells in G(2) through mitosis and into a subsequent G(1). Genetic and molecular analysis of Drop (Dr) mutations suggests that they represent novel cis-regulatory alleles of stg that inactivate expression in eye. Retinal precursors anterior to the furrow lacking stg arrest in G(2) and fail to enter mitosis, while cells within the furrow accumulate high levels of cyclins A and B. Although G(2)-arrested cells initiate normal pattern formation, the absence of stg results in retinal patterning defects due to the recruitment of extra photoreceptor cells. These results demonstrate a requirement for stg in cell cycle regulation and cell fate determination during eye development.

Molecular screening for P-element insertions in a large genomic region of Drosophila melanogaster using polymerase chain reaction mediated by the vectorette

As an alternative to existing methods for the detection of new insertions during a transposon mutagenesis, we adapted the method of vectorette ligation to genomic restriction fragments followed by PCR to obtain genomic sequences flanking the transposon. By combining flies containing a defined genomic transposon with an excess of flies containing unrelated insertion sites, we demonstrate the specificity and sensitivity of the procedure in the detection of integration events. This method was applied in a transposon-tagging screen for BJ1, the Drosophila homolog of the vertebrate gene Regulator of Chromosome Condensation (RCCI). Genetic mobilization of a single genomic P element was used to generate preferentially new local insertions from which integrations into a genomic region surrounding the BJ1 gene were screened. Flies harboring new insertions were phenotypically selected on the basis of the zeste1-dependent transvection of white. We detected a single transposition to a 13-kb region close to the BJ1 gene among 6650 progeny that were analyzed. Southern analysis of the homozygous line confirmed the integration 3 kb downstream of BJ1.

Neurotactin functions in concert with other identified CAMs in growth cone guidance in Drosophila

We have isolated and characterized mutations in Drosophila neurotactin, a gene that encodes a cell adhesion protein widely expressed during neural development. Analysis of both loss and gain of gene function conditions during embryonic and postembryonic development revealed specific requirements for neurotactin during axon outgrowth, fasciculation, and guidance. Furthermore, embryos of some double mutant combinations of neurotactin and other genes encoding adhesion/signaling molecules, including neuroglian, derailed, and kekkon1, displayed phenotypic synergy. This result provides evidence for functional cooperativity in vivo between the adhesion and signaling pathways controlled by neurotactin and the other three genes.

Receptor-like protein tyrosine phosphatases: alike and yet so different

Reversible phosphorylation on tyrosine residues is an extremely rapid and powerful posttranslational modification that is used in signalling pathways for the regulation of cell growth and differentiation. Over the past several years an impressive number of receptor-like protein tyrosine phosphatase (RPTPase) family members have been identified by molecular cloning, and undoubtedly many more will follow. This review provides an overview of the molecular data that are available for the currently identified RPTPases and discusses their possible biological implications.

Tyrosine phosphorylation and axon guidance: of mice and flies

Recent genetic evidence suggests that tyrosine kinases and tyrosine phosphatases can control the guidance of specific growth cones. Within a family of related phosphatases or kinases, individual members can have partially redundant functions. Receptor phosphatases can work together at one guidance choice point, but in opposition at another. The specific combination of kinases and phosphatases active in a growth cone may be an important determinant of pathway choice. One mechanism by which these proteins could control guidance decisions is through regulation of adhesion between growth cones and axons.

Axon guidance: motor-way madness

Recent results have revealed for the first time that receptor-like protein tyrosine phosphatases help to control the navigation of motor axons in the Drosophila nervous system.

Receptor tyrosine phosphatases are required for motor axon guidance in the Drosophila embryo

The receptor tyrosine phosphatases DPTP69D and DPTP99A are expressed on motor axons in Drosophila embryos. In mutant embryos lacking DPTP69D protein, motor neuron growth cones stop growing before reaching their muscle targets, or follow incorrect pathways that bypass these muscles. Mutant embryos lacking DPTP99A are indistinguishable from wild type. Motor axon defects in dptp69D dptp99A double mutant embryos, however, are much more severe than in embryos lacking only DPTP69D. Our results demonstrate that DPTP69D and DPTP99A are required for motor axon guidance and that they have partially redundant functions during development of the neuro-muscular system.

Expression of protein tyrosine phosphatase genes during oogenesis in Drosophila melanogaster

The spatial and temporal expression of seven Drosophila protein tyrosine phosphatase genes during oogenesis was examined by whole mount in-situ hybridization of antisense RNA probes to ovaries. Our observations indicate diverse expression patterns consistent with multiple roles for protein tyrosine phosphatases in the ovary. DPTP99A and corkscrew transcripts are expressed in follicle cells, consistent with possible roles in the EGF receptor signaling pathway. Transcripts from corkscrew and DPTP10D are detected in the germline during oogenesis and localized to the oocyte during egg chamber development. Localization of the two transcripts is disrupted by mutations in egalitarian and Bicaudal D. DLAR and DPTP4E transcripts are found in the germline during the same developmental stages as DPTP10D transcripts, but their transcripts are not localized to the oocyte. DPTP61F transcription is detected only after stage 6 of oogenesis. After stage 10B these transcripts are transported to the oocyte; thus ovarian transcription of DPTP61F may reflect a maternal contribution of the mRNA for later use during embryogenesis. DPTP69D transcripts are sequestered in the nucleus from stage 7 to stage 10, and then released to the cytoplasm. Our observations suggest that the export of DPTP69D mRNA from the nucleus is temporally regulated during oogenesis.