Rescue of a Drosophila NF1 mutant phenotype by protein kinase A

Growth in North American white children with neurofibromatosis 1 (NF1)

OBJECTIVE

To analyse the distributions of and generate growth charts for stature and occipitofrontal circumference (OFC) in neurofibromatosis 1 (NF1) patients.

DESIGN

Cross sectional database survey.

SETTING

The National Neurofibromatosis Foundation International Database (NFDB) includes clinical information on NF1 patients from 14 participating centres in North America.

SUBJECTS

A total of 569 white, North American, NF1 patients, 55% female and 45% male.

MAIN OUTCOME MEASURES

Stature and OFC measurements of NF1 patients were compared to age and sex matched population norms using z score standardisation and centile curves.

RESULTS

The distributions of stature and OFC are shifted and unimodal among NF1 patients; 13% of patients have short stature (>/=2 standard deviations below the population mean) and 24% have macrocephaly (OFC >/=2 standard deviations above the population mean).

CONCLUSIONS

Alterations of stature and OFC are not limited to NF1 patients with frank short stature or macrocephaly.

Insights into the pathogenesis of neurofibromatosis 1 vasculopathy

Intrinsic lesions of arterial walls are an important manifestation of neurofibromatosis 1 (NF1). Neurofibromin is expressed in blood vessel endothelial and smooth muscle cells, and NF1 vasculopathy may result from an alteration of neurofibromin function in these cells. Elucidation of the role of neurofibromin in the maintenance and repair of blood vessels may lead to novel approaches to the treatment of NF1 vasculopathy and vascular disease in general.

Genetic control of cell size

Over the past 25 years, the genetic control of cell size has mainly been addressed in yeast, a single-celled organism. Recent insights from Drosophila have shed light on the signalling pathways responsible for adjusting and maintaining cell size in metazoans. Evidence is emerging for a signalling cascade conserved in evolution that links external nutrient sources to cell size.

Single cell Ras-GTP analysis reveals altered Ras activity in a subpopulation of neurofibroma Schwann cells but not fibroblasts

Neurofibromatosis type 1 (NF1) is a common genetic disorder characterized by multiple neurofibromas, peripheral nerve tumors containing mainly Schwann cells and fibroblasts. The NF1 gene encodes neurofibromin, a tumor suppressor postulated to function in part as a Ras GTPase-activating protein. The roles of different cell types and of elevated Ras-GTP in neurofibroma formation are unclear. To determine which neurofibroma cell type has altered Ras-GTP regulation, we developed an immunocytochemical assay for active, GTP-bound Ras. In NIH 3T3 cells, the assay detected overexpressed, constitutively activated K-, N-, and Ha-Ras and insulin-induced endogenous Ras-GTP. In dissociated neurofibroma cells from NF1 patients, Ras-GTP was elevated in Schwann cells but not fibroblasts. Twelve to 62% of tumor Schwann cells showed elevated Ras-GTP, unexpectedly revealing neurofibroma Schwann cell heterogeneity. Increased basal Ras-GTP did not correlate with increased cell proliferation. Normal human Schwann cells, however, did not demonstrate elevated basal Ras activity. Furthermore, compared with cells from wild type littermates, Ras-GTP was elevated in all mouse Nf1(-/-) Schwann cells but never in Nf1(-/-) mouse fibroblasts. Our results indicate that Ras activity is detectably increased in only some neurofibroma Schwann cells and suggest that neurofibromin is not an essential regulator of Ras activity in fibroblasts.

Genetic control of size in Drosophila

During the past ten years, significant progress has been made in understanding the basic mechanisms of the development of multicellular organisms. Genetic analysis of the development of Caenorhabditis elegans and Drosophila has unearthed a fruitful number of genes involved in establishing the basic body plan, patterning of limbs, specification of cell fate and regulation of programmed cell death. The genes involved in these developmental processes have been conserved throughout evolution and homologous genes are involved in the patterning of insect and human limbs. Despite these important discoveries, we have learned astonishingly little about one of the most obvious distinctions between animals: their difference in body size. The mass of the smallest mammal, the bumble-bee bat, is 2 g while that of the largest mammal, the blue whale, is 150 t or 150 million grams. Remarkably, even though they are in the same class, body size can vary up to 75-million-fold. Furthermore, this body growth can be finite in the case of most vertebrates or it can occur continuously throughout life, as for trees, molluscs and large crustaceans. Currently, we know comparatively little about the genetic control of body size. In this article we will review recent evidence from vertebrates and particularly from Drosophila that implicates insulin/insulin-like growth factor-I and other growth pathways in the control of cell, organ and body size.

Spatially restricted factors cooperate with notch in the regulation of Enhancer of split genes

Expression of the Drosophila Enhancer of split [E(spl)] genes, and their homologues in other species, is dependent on Notch activation. The seven E(spl) genes are clustered in a single complex and their functions overlap significantly; however, the individual genes have distinct patterns of expression. To investigate how this regulation is achieved and to find out whether there is shared or cross regulation between E(spl) genes, we have analysed the enhancer activity of sequences from the adjacent E(spl)mbeta, E(spl)mgamma and E(spl)mdelta genes and made comparisons to E(spl)m8. We find that although regulatory elements can be shared, most aspects of the expression of each individual gene are recapitulated by small (400-500 bp) evolutionarily conserved enhancers. Activated Notch or a Suppressor of Hairless-VP16 fusion are only sufficient to elicit transcription from the E(spl) enhancers in a subset of locations, indicating a requirement for other factors. In tissue culture cells, proneural proteins synergise with Suppressor of Hairless and Notch to promote expression from E(spl)mgamma and E(spl)m8, but this synergy is only observed in vivo with E(spl)m8. We conclude that additional factors besides the proneural proteins limit the response of E(spl)mgamma in vivo. In contrast to the other genes, E(spl)mbeta exhibits little response to proneural proteins and its high level of activity in the wing imaginal disc suggests that wing-specific factors cooperate with Notch to activate the E(spl)mbeta enhancer. These results demonstrate that Notch activity must be integrated with other transcriptional regulators and, since the activation of target genes is critical in determining the developmental consequences of Notch activity, provide a framework for understanding Notch function in different developmental contexts.

Neurofibromin negatively regulates neurotrophin signaling through p21ras in embryonic sensory neurons

Embryonic sensory and sympathetic neurons that lack neurofibromin, the protein product of the neurofibromatosis type 1 (Nfl) gene, survive and extend neurites in the absence of neurotrophins. To determine whether neurofibromin negatively regulates neurotrophin signaling through its interaction with p21ras, we used Fab antibody fragments to block Ras function in DRG, trigeminal, nodose, and SCG neurons isolated from Nfl(-/-) and wild-type mouse embryos. We show that introduction of anti-Ras Fab fragments significantly reduces the ability of neurofibromin-deficient neurons to survive in the absence of neurotrophins. Moreover, addition of H-ras protein enhances the survival of Nfl(-/-), but not wild-type, DRG neurons. Our results are consistent with a major role for neurofibromin in modulating Trk signaling through p21ras during neuronal development.

Comparative genomics of the eukaryotes

A comparative analysis of the genomes of Drosophila melanogaster, Caenorhabditis elegans, and Saccharomyces cerevisiae-and the proteins they are predicted to encode-was undertaken in the context of cellular, developmental, and evolutionary processes. The nonredundant protein sets of flies and worms are similar in size and are only twice that of yeast, but different gene families are expanded in each genome, and the multidomain proteins and signaling pathways of the fly and worm are far more complex than those of yeast. The fly has orthologs to 177 of the 289 human disease genes examined and provides the foundation for rapid analysis of some of the basic processes involved in human disease.

Genetic suppression of polyglutamine toxicity in Drosophila

A Drosophila model for Huntington's and other polyglutamine diseases was used to screen for genetic factors modifying the degeneration caused by expression of polyglutamine in the eye. Among 7000 P-element insertions, several suppressor strains were isolated, two of which led to the discovery of the suppressor genes described here. The predicted product of one, dHDJ1, is homologous to human heat shock protein 40/HDJ1. That of the second, dTPR2, is homologous to the human tetratricopeptide repeat protein 2. Each of these molecules contains a chaperone-related J domain. Their suppression of polyglutamine toxicity was verified in transgenic flies.

New function for NF1 tumor suppressor

The expression and subcellular localization of neurofibromatosis type 1 tumor suppressor was studied in keratinocytes induced to differentiate by increased Ca2+ concentration of the culture medium. Differentiating keratinocytes became intensely immunoreactive for neurofibromatosis type 1 protein, which was apparently associated with cellular fibrils. Double immunolabeling with antibodies to cytokeratin 14 and neurofibromatosis type 1 protein suggested an association of intermediate type cytoskeleton and neurofibromatosis type 1 protein. The presence of neurofibromatosis type 1 protein in cell preparations treated with cytoskeletal buffer indicated a high affinity interaction between intermediate filaments and neurofibromatosis type 1 protein. Further studies utilizing double immunolabelings revealed that the intense neurofibromatosis type 1 tumor suppressor signal on intermediate filaments was temporally limited to the period in keratinocyte differentiation in which the formation of desmosomes takes place. Keratinocytes were also cultured from nine patients with type 1 neurofibromatosis and were studied with respect to cell morphology, and association of neurofibromatosis type 1 protein with intermediate cytoskeleton. The results showed that keratinocytes cultured from patients with neurofibromatosis type 1 displayed a highly variable cell size and morphology compared to controls. The latter findings represent predicted alterations in a situation where cytoskeletal organization is disturbed. Furthermore, differentiating neurofibromatosis type 1 keratinocytes were characterized by a reduced number of cytokeratin bundles that were decorated neurofibromatosis type 1 protein. The results of this study suggest that neurofibromatosis type 1 tumor suppressor exerts its effects in part by controlling organization of cytoskeleton during the formation of cellular contacts.

Minor lesion mutational spectrum of the entire NF1 gene does not explain its high mutability but points to a functional domain upstream of the GAP-related domain

More than 500 unrelated patients with neurofibromatosis type 1 (NF1) were screened for mutations in the NF1 gene. For each patient, the whole coding sequence and all splice sites were studied for aberrations, either by the protein truncation test (PTT), temperature-gradient gel electrophoresis (TGGE) of genomic PCR products, or, most often, by direct genomic sequencing (DGS) of all individual exons. A total of 301 sequence variants, including 278 bona fide pathogenic mutations, were identified. As many as 216 or 183 of the genuine mutations, comprising 179 or 161 different ones, can be considered novel when compared to the recent findings of Upadhyaya and Cooper, or to the NNFF mutation database. Mutation-detection efficiencies of the various screening methods were similar: 47.1% for PTT, 53.7% for TGGE, and 54.9% for DGS. Some 224 mutations (80.2%) yielded directly or indirectly premature termination codons. These mutations showed even distribution over the whole gene from exon 1 to exon 47. Of all sequence variants determined in our study, <20% represent C-->T or G-->A transitions within a CpG dinucleotide, and only six different mutations also occur in NF1 pseudogenes, with five being typical C-->T transitions in a CpG. Thus, neither frequent deamination of 5-methylcytosines nor interchromosomal gene conversion may account for the high mutation rate of the NF1 gene. As opposed to the truncating mutations, the 28 (10.1%) missense or single-amino-acid-deletion mutations identified clustered in two distinct regions, the GAP-related domain (GRD) and an upstream gene segment comprising exons 11-17. The latter forms a so-called cysteine/serine-rich domain with three cysteine pairs suggestive of ATP binding, as well as three potential cAMP-dependent protein kinase (PKA) recognition sites obviously phosphorylated by PKA. Coincidence of mutated amino acids and those conserved between human and Drosophila strongly suggest significant functional relevance of this region, with major roles played by exons 12a and 15 and part of exon 16.

A neurofibromatosis-1-regulated pathway is required for learning in Drosophila

The tumour-suppressor gene Neurofibromatosis 1 (Nf1) encodes a Ras-specific GTPase activating protein (Ras-GAP). In addition to being involved in tumour formation, NF1 has been reported to cause learning defects in humans and Nf1 knockout mice. However, it remains to be determined whether the observed learning defect is secondary to abnormal development. The Drosophila NF1 protein is highly conserved, showing 60% identity of its 2,803 amino acids with human NF1 (ref. 12). Previous studies have suggested that Drosophila NF1 acts not only as a Ras-GAP but also as a possible regulator of the cAMP pathway that involves the rutabaga (rut)-encoded adenylyl cyclase. Because rut was isolated as a learning and short-term memory mutant, we have pursued the hypothesis that NF1 may affect learning through its control of the Rut-adenylyl cyclase/cAMP pathway. Here we show that NF1 affects learning and short-term memory independently of its developmental effects. We show that G-protein-activated adenylyl cyclase activity consists of NF1-independent and NF1-dependent components, and that the mechanism of the NF1-dependent activation of the Rut-adenylyl cyclase pathway is essential for mediating Drosophila learning and memory.

Quantitative morphology of the corpus callosum in children with neurofibromatosis and attention-deficit hyperactivity disorder

Neurofibromatosis-1 is a common autosomal-dominant genetic disorder associated with numerous physical anomalies and an increased incidence of attention-deficit hyperactivity disorder (ADHD). Studies of children with idiopathic ADHD have suggested a link between corpus callosum size and symptom severity. This study examines the contribution of corpus callosum morphology to symptoms of ADHD in children with neurofibromatosis. Eighteen control subjects and 36 children with neurofibromatosis underwent magnetic resonance imaging of the brain. Twelve subjects with neurofibromatosis had evidence of ADHD and 24 did not. Subjects with neurofibromatosis had significantly larger total corpus callosum area and significantly larger regional measurements in three of seven areas. However, there were no differences between the neurofibromatosis alone and neurofibromatosis plus ADHD groups. Increased severity of attention problems was associated with smaller total callosal areas. These results suggest that some features of ADHD in children with neurofibromatosis could be linked to quantifiable differences in brain morphology, but the nature of the genetic mutation in neurofibromatosis suggests that neurochemical effects also could be important.

Molecular and cellular mechanisms of cognitive function: implications for psychiatric disorders

Recent studies on the molecular and cellular basis of learning and memory have brought us closer than ever to understanding the mechanisms of synaptic plasticity and their relevance to memory formation. Genetic approaches have played a central role in these new findings because the same mutant mice can be studied with molecular, cellular, circuit, and behavioral tools. Therefore, the results can be used to construct models that cut across levels of analytical complexity, forging connections from the biochemistry of the modified protein to the behavior of the mutant mice. These findings are not only improving our understanding of learning and memory, they are also enriching our understanding of cognitive disorders, such as neurofibromatosis type I. Mechanisms underlying long-term changes in synaptic function are likely to be at the heart of many cognitive and emotional processes in humans. Therefore, molecular and cellular insights into learning and memory undoubtedly will have a profound impact on the understanding and treatment of psychiatric disorders.

Functional interaction between Galpha(z) and Rap1GAP suggests a novel form of cellular cross-talk

G(z) is a member of the G(i) family of trimeric G proteins whose primary role in cell physiology is still unknown. In an ongoing effort to elucidate the cellular functions of G(z), the yeast two-hybrid system was employed to identify proteins that specifically interact with a mutationally activated form of Galpha(z). One of the molecules uncovered in this screen was Rap1GAP, a previously identified protein that specifically stimulates GTP hydrolytic activity of the monomeric G protein Rap1 and thus is believed to function as a down-regulator of Rap1 signaling. Like G(z), the precise role of Rap1 in cell physiology is poorly understood. Biochemical analysis using purified recombinant proteins revealed that the physical interaction between Galpha(z) and Rap1GAP blocks the ability of RGSs (regulators of G protein signaling) to stimulate GTP hydrolysis of the alpha subunit, and also attenuates the ability of activated Galpha(z) to inhibit adenylyl cyclase. Structure-function analyses indicate that the first 74 amino-terminal residues of Rap1GAP, a region distinct from the catalytic core domain responsible for the GAP activity toward Rap1, is required for this interaction. Co-precipitation assays revealed that Galpha(z), Rap1GAP, and Rap1 can form a stable complex. These data suggest that Rap1GAP acts as a signal integrator to somehow coordinate and/or integrate G(z) signaling and Rap1 signaling in cells.

Characterization of Saccharomyces cerevisiae strains expressing ira1 mutant alleles modeled after disease-causing mutations in NF1

The 2818 amino acids of neurofibromin, the product of the human NF1 gene, include a 230 amino acid Ras-GAP related domain (GRD). Functions which may be associated with the rest of the protein remain unknown. However, many NF1 mutations in neurofibromatosis 1 patients are found downstream of the GRD, suggesting that the C-terminal region of the protein is also functionally important. Since the C-terminal region of neurofibromin encompassing these mutations is homologous with the corresponding regions in the two Saccharomyces cerevisiae Ras-GAPs, Ira1p and Ira2p, we chose yeast as a model system for functional exploration of this region (Ira-C region). Three missense mutations that affect the Ira-C region of NF1 were used as a model for the mutagenesis of IRA1. The yeast phenotypes of heat shock sensitivity, iodine staining, sporulation efficiency, pseudohyphae formation, and GAP activity were scored. Even though none of the mutations directly affected the Ira1p-GRD, mutations at two of the three sites resulted in a decrease in the GAP activity present in ira1 cells. The third mutation appeared to disassociate the phenotypes of sporulation ability and GAP activity. This and other evidence suggest an effector function for Ira1p.

Cell-autonomous and non-autonomous growth-defective mutants of Drosophila melanogaster

During animal development, growth of the various tissues and organs that make up the body must be coordinated. Despite recent progress in understanding growth control within the cell unit, the mechanisms that coordinate growth at the organismal level are still poorly understood. To study this problem, we performed a genetic screen for larval growth-defective mutants in Drosophila melanogaster. Characterization of these mutants revealed distinct types of larval growth defects. An allelic series for the translation initiation factor, Eif4A, showed different growth rates and suggests that Eif4A could be used as a dose-dependent growth regulator. Two mutants that fail to exit cellular quiescence at larval hatching (milou and eif4(1006)) have a DNA replication block that can be bypassed by overexpression of the E2F transcription factor. A mutation (bonsai) in a homolog of the prokaryotic ribosomal protein, RPS15, causes a growth defect that is non-cell-autonomous. Our results emphasize the importance of translational regulation for the exit from quiescence. They suggest that the level of protein synthesis required for cell cycle progression varies according to tissue type. The isolation of non-cell-autonomous larval growth-defective mutants suggests that specialized organs coordinate growth throughout the animal and provides new tools for studies of organismal growth regulation.

Transgenic inhibitors identify two roles for protein kinase A in Drosophila development

We have initiated an analysis of protein kinase A (PKA) in Drosophila using transgenic techniques to modulate PKA activity in specific tissues during development. We have constructed GAL4/UAS-regulated transgenes in active and mutant forms that encode PKAc, the catalytic subunit of PKA, and PKI(1-31), a competitive inhibitor of PKAc. We present evidence that the wild-type transgenes are active and summarize the phenotypes produced by a number of GAL4 enhancer-detector strains. We compare the effects of transgenes encoding PKI(1-31) with those encoding PKAr*, a mutant regulatory subunit that constitutively inhibits PKAc because of its inability to bind cyclic AMP. Both inhibitors block larval growth, but only PKAr* alters pattern formation by activating the Hedgehog signaling pathway. Therefore, transgenic PKI(1-31) should provide a tool to investigate the role of PKAc in larval growth regulation without concomitant changes in pattern formation. The different effects of PKI(1-31) and PKAr* suggest two distinct roles, cytoplasmic and nuclear, for PKAc in Hedgehog signal transduction. Alternatively, PKAr* may target proteins other than PKAc, suggesting a role for free PKAr in signal transduction, a role inhibited by PKAc in reversal of the classical relationship of these subunits.

Control of growth and differentiation by Drosophila RasGAP, a homolog of p120 Ras-GTPase-activating protein

Mammalian Ras GTPase-activating protein (GAP), p120 Ras-GAP, has been implicated as both a downregulator and effector of Ras proteins, but its precise role in Ras-mediated signal transduction pathways is unclear. To begin a genetic analysis of the role of p120 Ras-GAP we identified a homolog from the fruit fly Drosophila melanogaster through its ability to complement the sterility of a Schizosaccharomyces pombe (fission yeast) gap1 mutant strain. Like its mammalian homolog, Drosophila RasGAP stimulated the intrinsic GTPase activity of normal mammalian H-Ras but not that of the oncogenic Val12 mutant. RasGAP was tyrosine phosphorylated in embryos and its Src homology 2 (SH2) domains could bind in vitro to a small number of tyrosine-phosphorylated proteins expressed at various developmental stages. Ectopic expression of RasGAP in the wing imaginal disc reduced the size of the adult wing by up to 45% and suppressed ectopic wing vein formation caused by expression of activated forms of Breathless and Heartless, two Drosophila receptor tyrosine kinases of the fibroblast growth factor receptor family. The in vivo effects of RasGAP overexpression required intact SH2 domains, indicating that intracellular localization of RasGAP through SH2-phosphotyrosine interactions is important for its activity. These results show that RasGAP can function as an inhibitor of signaling pathways mediated by Ras and receptor tyrosine kinases in vivo. Genetic interactions, however, suggested a Ras-independent role for RasGAP in the regulation of growth. The system described here should enable genetic screens to be performed to identify regulators and effectors of p120 Ras-GAP.

p21 ras and phosphatidylinositol-3 kinase are required for survival of wild-type and NF1 mutant sensory neurons

Nerve growth factor (NGF) is a required differentiation and survival factor for sympathetic and a majority of neural crest-derived sensory neurons in the developing vertebrate peripheral nervous system. Although much is known about the function of NGF, the intracellular signaling cascade that it uses continues to be a subject of intense study. p21 ras signaling is considered necessary for sensory neuron survival. How additional intermediates downstream or in parallel may function has not been fully understood yet. Two intracellular signaling cascades, extra cellular regulated kinase (erk) and phosphatidylinositol-3 (PI 3) kinase, transduce NGF signaling in the pheochromocytoma cell line PC12. To elucidate the role these cascades play in survival and differentiation, we used a combination of recombinant adenoviruses and chemical inhibitors to perturb these pathways in sensory neurons from wild-type mice and mice deficient for neurofibromin in which the survival and differentiation pathway is constitutively active. We demonstrate that ras activity is both necessary and sufficient for the survival of embryonic sensory neurons. Downstream of ras, however, the erk cascade is neither required nor sufficient for neuron survival or overall differentiation. Instead, the activity of PI 3 kinase is necessary for the survival of the wild-type and neurofibromin-deficient neurons. Therefore, we conclude that in sensory neurons, NGF acts via a signaling pathway, which includes both ras and PI 3 kinase.

Frizzled regulation of Notch signalling polarizes cell fate in the Drosophila eye

The Drosophila eye, a paradigm for epithelial organization, is highly polarized with mirror-image symmetry about the equator. The R3 and R4 photoreceptors in each ommatidium are vital in this polarity; they adopt asymmetrical positions in adult ommatidia and are the site of action for several essential genes. Two such genes are frizzled (fz) and dishevelled (dsh), the products of which are components of a signalling pathway required in R3, and which are thought to be activated by a diffusible signal. Here we show that the transmembrane receptor Notch is required downstream of dsh in R3/R4 for them to adopt distinct fates. By using an enhancer for the Notch target gene Enhancer of split mdelta, we show that Notch becomes activated specifically in R4. We propose that Fz/Dsh promotes activity of the Notch ligand Delta and inhibits Notch receptor activity in R3, creating a difference in Notch signalling capacity between R3 and R4. Subsequent feedback in the Notch pathway ensures that this difference becomes amplified. This interplay between Fz/Dsh and Notch indicates that polarity is established through local comparisons between two cells and explains how a signal from one position (for example, the equator in the eye) could be interpreted by all ommatidia in the field.

Neurofibromin modulation of ras activity is required for normal endocardial-mesenchymal transformation in the developing heart

Endocardial cushions are the precursors of the cardiac valves and form by a process of epithelial-mesenchymal transformation. Secreted growth factors from myocardium induce endocardial cells to transform into mesenchyme and invade the overlying extracellular matrix. Here, we show that the product of the Nf1 neurofibromatosis gene is required to regulate this event. In the absence of neurofibromin, mouse embryo hearts develop overabundant endocardial cushions due to hyperproliferation and lack of normal apoptosis. Neurofibromin deficiency in explant cultures is reproduced by activation of ras signaling pathways, and the Nf1(−/−) mutant phenotype is prevented by inhibiting ras in vitro. These results indicate that neurofibromin normally acts to modulate epithelial-mesenchymal transformation and proliferation in the developing heart by down regulating ras activity.

Genomic characterization of the Neurofibromatosis Type 1 gene of Fugu rubripes

The genomic structure of the Neurofibromatosis Type1 (NF1) gene of Fugu rubripes was investigated by sequence analysis of two overlapping cosmids. The Fugu NF1 gene spans 27 kb and is 13 times smaller than the human counterpart owing primarily to reduced intron size. The predicted amino acid sequence is highly related to that of human neurofibromin, exhibiting an overall similarity of 91.5%. Nearly all exons described for the human NF1 gene could be identified, except exon 12b and the alternatively spliced exons 9br and 48a. With the exception of the splice acceptor site in front of exon 16, all splice sites are in identical positions to those found in the human gene. Intron 1, which is 100-140 kb long in humans, spans 2575 bp in the Fugu NF1 gene. Another large intron of the human NF1 gene, intron 27b (45-50 kb), is 3942 bp of size in Fugu. Sequences related to the OMgp gene (Oligodendrocyte-Myelin-glycoprotein) or the EVI2A gene (ecotropic viral integration site), which are inserted into human NF1 intron 27b, were not detected in the corresponding Fugu intron. However, a single exon gene with similarity to the human EVI2B gene has been found on the reverse strand of Fugu intron 27b. This suggests that the human EVI2B gene and the Fugu gene in intron 27b have a common ancestor. We found the expression of this inserted gene in liver and kidney, but not in brain tissue of Fugu rubripes.

Structural analysis of the GAP-related domain from neurofibromin and its implications

Neurofibromin is the product of the NF1 gene, whose alteration is responsible for the pathogenesis of neurofibromatosis type 1 (NF1), one of the most frequent genetic disorders in man. It acts as a GTPase activating protein (GAP) on Ras; based on homology to p120GAP, a segment spanning 250-400 aa and termed GAP-related domain (NF1GRD; 25-40 kDa) has been shown to be responsible for GAP activity and represents the only functionally defined segment of neurofibromin. Missense mutations found in NF1 patients map to NF1GRD, underscoring its importance for pathogenesis. X-ray crystallographic analysis of a proteolytically treated catalytic fragment of NF1GRD comprising residues 1198-1530 (NF1-333) of human neurofibromin reveals NF1GRD as a helical protein that resembles the corresponding fragment derived from p120GAP (GAP-334). A central domain (NF1c) containing all residues conserved among RasGAPs is coupled to an extra domain (NF1ex), which despite very limited sequence homology is surprisingly similar to the corresponding part of GAP-334. Numerous point mutations found in NF1 patients or derived from genetic screening protocols can be analysed on the basis of the three-dimensional structural model, which also allows identification of the site where structural changes in a differentially spliced isoform are to be expected. Based on the structure of the complex between Ras and GAP-334 described earlier, a model of the NF1GRD-Ras complex is proposed which is used to discuss the strikingly different properties of the Ras-p120GAP and Ras-neurofibromin interactions.

A subset of notch functions during Drosophila eye development require Su(H) and the E(spl) gene complex

The Notch signalling pathway is involved in many processes where cell fate is decided. Previous work showed that Notch is required at successive steps during R8 specification in the Drosophila eye. Initially, Notch enhances atonal expression and promotes atonal function. After atonal autoregulation has been established, Notch signalling represses atonal expression during lateral specification. In this paper we investigate which known components of the Notch pathway are involved in each signalling process. Using clonal analysis we show that a ligand of Notch, Delta, is required along with Notch for both proneural enhancement and lateral specification, while the downstream components Suppressor-of-Hairless and Enhancer-of-Split are involved only in lateral specification. Our data point to a distinct signal transduction pathway during proneural enhancement by Notch. Using misexpression experiments we also show that particular Enhancer-of-split bHLH genes can differ greatly in their contribution to lateral specification.

Gene discovery in Drosophila: new insights for learning and memory

Genetic approaches have been used to investigate increasingly complex biological systems. Here we review the current state of genetic analysis of learning and memory in the fruitfly, Drosophila melanogaster. Emerging findings support two main themes. First, discovery and manipulation of genes involved with behavioral plasticity in genetically accessible systems such as D. melanogaster enables dissection of the biochemical, cellular, anatomical, and behavioral pathways of learning and memory. Second, because core cellular mechanisms of simple forms of learning are evolutionarily conserved, biological pathways discovered in invertebrates are likely to be conserved in vertebrate systems as well.

Biological characterization of Drosophila Rapgap1, a GTPase activating protein for Rap1

The activity of Ras family proteins is modulated in vivo by the function of GTPase activating proteins, which increase their intrinsic rate of GTP hydrolysis. We have isolated cDNAs encoding a GAP for the Drosophila Rap1 GTPase. Drosophila Rapgap1 encodes an 850-amino acid protein with a central region that displays substantial sequence similarity to human RapGAP. This domain, when expressed in Escherichia coli, potently stimulates Rap1 GTPase activity in vitro. Unlike Rap1, which is ubiquitously expressed, Rapgap1 expression is highly restricted. Rapgap1 is expressed at high levels in the developing photoreceptor cells and in the optic lobe. Rapgap1 mRNA is also localized in the pole plasm in an oskar-dependent manner. Although mutations that completely abolish Rapgap1 function display no obvious phenotypic abnormalities, overexpression of Rapgap1 induces a rough eye phenotype that is exacerbated by reducing Rap1 gene dosage. Thus, Rapgap1 can function as a negative regulator of Rap1-mediated signaling in vivo.

Requirement of Drosophila NF1 for activation of adenylyl cyclase by PACAP38-like neuropeptides

The human neurofibromatosis type 1 (NF1) tumor suppressor protein functions as a Ras-specific guanosine triphosphatase-activating protein, but the identity of Ras- mediated pathways modulated by NF1 remains unknown. A study of Drosophila NF1 mutants revealed that NF1 is essential for the cellular response to the neuropeptide PACAP38 (pituitary adenylyl cyclase-activating polypeptide) at the neuromuscular junction. The peptide induced a 100-fold enhancement of potassium currents by activating the Ras-Raf and adenylyl cyclase-adenosine 3',5'-monophosphate (cAMP) pathways. This response was eliminated in NF1 mutants. NF1 appears to regulate the rutabaga-encoded adenylyl cyclase rather than the Ras-Raf pathway. Moreover, the NF1 defect was rescued by the exposure of cells to pharmacological treatment that increased concentrations of cAMP.