Pancytopenia and thrombosis defects in zebrafish mutants of Fanconi anemia genes

Progressive pancytopenia is a common feature observed in DNA crosslink repair deficiency disorder, Fanconi anemia (FA). However, this phenotype has not been recapitulated in single FA gene knockout animal models. In this study, we analyzed hematological characteristics in zebrafish null mutants for two FA genes, fanca and fanco. In adult mutants, we demonstrate age-associated reduction in blood cell counts for all lineages, resembling progressive pancytopenia in FA patients. In larval mutants, we demonstrate vascular injury-induced thrombosis defects, particularly upon treatment with crosslinking agent diepoxybutane (DEB), indicating DNA damage induced inefficiency of thrombocytes.

Multiplexed CRISPR/Cas9-mediated knockout of 19 Fanconi anemia pathway genes in zebrafish revealed their roles in growth, sexual development and fertility

Fanconi Anemia (FA) is a genomic instability syndrome resulting in aplastic anemia, developmental abnormalities, and predisposition to hematological and other solid organ malignancies. Mutations in genes that encode proteins of the FA pathway fail to orchestrate the repair of DNA damage caused by DNA interstrand crosslinks. Zebrafish harbor homologs for nearly all known FA genes. We used multiplexed CRISPR/Cas9-mediated mutagenesis to generate loss-of-function mutants for 17 FA genes: fanca, fancb, fancc, fancd1/brca2, fancd2, fance, fancf, fancg, fanci, fancj/brip1, fancl, fancm, fancn/palb2, fanco/rad51c, fancp/slx4, fancq/ercc4, fanct/ube2t, and two genes encoding FA-associated proteins: faap100 and faap24. We selected two indel mutations predicted to cause premature truncations for all but two of the genes, and a total of 36 mutant lines were generated for 19 genes. Generating two independent mutant lines for each gene was important to validate their phenotypic consequences. RT-PCR from homozygous mutant fish confirmed the presence of transcripts with indels in all genes. Interestingly, 4 of the indel mutations led to aberrant splicing, which may produce a different protein than predicted from the genomic sequence. Analysis of RNA is thus critical in proper evaluation of the consequences of the mutations introduced in zebrafish genome. We used fluorescent reporter assay, and western blots to confirm loss-of-function for several mutants. Additionally, we developed a DEB treatment assay by evaluating morphological changes in embryos and confirmed that homozygous mutants from all the FA genes that could be tested (11/17), displayed hypersensitivity and thus were indeed null alleles. Our multiplexing strategy helped us to evaluate 11 multiple gene knockout combinations without additional breeding. Homozygous zebrafish for all 19 single and 11 multi-gene knockouts were adult viable, indicating FA genes in zebrafish are generally not essential for early development. None of the mutant fish displayed gross developmental abnormalities except for fancp^-/- fish, which were significantly smaller in length than their wildtype clutch mates. Complete female-to-male sex reversal was observed in knockouts for 12/17 FA genes, while partial sex reversal was seen for the other five gene knockouts. All adult females were fertile, and among the adult males, all were fertile except for the fancd1 mutants and one of the fancj mutants. We report here generation and characterization of zebrafish knockout mutants for 17 FA disease-causing genes, providing an integral resource for understanding the pathophysiology associated with the disrupted FA pathway.

Deficiencies in repair of DNA damage can cause diseases such as Fanconi anemia (FA), which is characterized by birth defects, bone marrow failure, anemia, leukemia and other cancers. A set of proteins constitute the FA pathway and together orchestrate the DNA repair process. Inactivation of one or more gene(s) encoding the proteins of the DNA repair pathway in an animal model would enable us to study the functions of these proteins in maintenance of normal cellular functions and the overall health of an individual in the absence of function. We systematically targeted the FA pathway in zebrafish using CRISPR/Cas9. We generated 36 fish lines with loss-of-function mutations in 19 FA pathway genes and showed that all survive to adulthood. We did not notice obvious morphological changes except in fancp gene-inactivated fish, which were smaller in length. However, all mutant fish were either exclusively or in majority male. Unlike reduced fertility among FA patients, all adult mutant fish were fertile, except for the fancd1 and fancj knockout males. These mutant zebrafish will serve as a huge resource for the scientific community to study the role of FA proteins in fish development, DNA repair, and as models for FA disease.

Common and overlapping oncogenic pathways contribute to the evolution of acute myeloid leukemias

Fusion oncogenes in acute myeloid leukemia (AML) promote self-renewal from committed progenitors, thereby linking transformation and self-renewal pathways. Like most cancers, AML is a genetically and biologically heterogeneous disease, but it is unclear whether transformation results from common or overlapping genetic programs acting downstream of multiple mutations or by the engagement of unique genetic programs acting cooperatively downstream of individual mutations. This distinction is important, because the involvement of common programs would imply the existence of common molecular targets to treat AML, no matter which oncogenes are involved. Here we show that the ability to promote self-renewal is a generalized property of leukemia-associated oncogenes. Disparate oncogenes initiated overlapping transformation and self-renewal gene expression programs, the common elements of which were defined in established leukemic stem cells from an animal model as well as from a large cohort of patients with differing AML subtypes, where they strongly predicted pathobiological character. Notably, individual genes commonly activated in these programs could partially phenocopy the self-renewal function of leukemia-associated oncogenes in committed murine progenitors. Furthermore, they could generate AML following expression in murine bone marrow. In summary, our findings reveal the operation of common programs of self-renewal and transformation downstream of leukemia-associated oncogenes, suggesting that mechanistically common therapeutic approaches to AML are likely to be possible, regardless of the identity of the driver oncogene involved.

Germ line gain of function with SOS1 mutation in hereditary gingival fibromatosis

Mutation of human SOS1 is responsible for hereditary gingival fibromatosis type 1, a benign overgrowth condition of the gingiva. Here, we investigated molecular mechanisms responsible for the increased rate of cell proliferation in gingival fibroblasts caused by mutant SOS1 in vitro. Using ectopic expression of wild-type and mutant SOS1 constructs, we found that truncated SOS1 could localize to the plasma membrane, without growth factor stimuli, leading to sustained activation of Ras/MAPK signaling. Additionally, we observed an increase in the magnitude and duration of ERK signaling in hereditary gingival fibromatosis gingival fibroblasts that was associated with phosphorylation of retinoblastoma tumor suppressor protein and the up-regulation of cell cycle regulators, including cyclins C, D, and E and the E2F/DP transcription factors. These factors promote cell cycle progression from G(1) to S phase, and their up-regulation may underlie the increased gingival fibroblast proliferation observed. Selective depletion of wild-type and mutant SOS1 through small interfering RNA demonstrates the link between mutation of SOS1, ERK signaling, cell proliferation rate, and the expression levels of Egr-1 and proliferating cell nuclear antigen. These findings elucidate the mechanisms for gingival overgrowth mediated by SOS1 gene mutation in humans.

Exclusion of known gene for enamel development in two Brazilian families with amelogenesis imperfecta

Amelogenesis imperfecta (AI) is a genetically heterogeneous group of diseases that result in defective development of tooth enamel. Mutations in several enamel proteins and proteinases have been associated with AI. The object of this study was to evaluate evidence of etiology for the six major candidate gene loci in two Brazilian families with AI. Genomic DNA was obtained from family members and all exons and exon-intron boundaries of the ENAM, AMBN, AMELX, MMP20, KLK4 and Amelotin gene were amplified and sequenced. Each family was also evaluated for linkage to chromosome regions known to contain genes important in enamel development. The present study indicates that the AI in these two families is not caused by any of the known loci for AI or any of the major candidate genes proposed in the literature. These findings indicate extensive genetic heterogeneity for non-syndromic AI.

Analysis of conditional paralytic mutants in Drosophila sarco-endoplasmic reticulum calcium ATPase reveals novel mechanisms for regulating membrane excitability

Individual contributions made by different calcium release and sequestration mechanisms to various aspects of excitable cell physiology are incompletely understood. SERCA, a sarco-endoplasmic reticulum calcium ATPase, being the main agent for calcium uptake into the ER, plays a central role in this process. By isolation and extensive characterization of conditional mutations in the Drosophila SERCA gene, we describe novel roles of this key protein in neuromuscular physiology and enable a genetic analysis of SERCA function. At motor nerve terminals, SERCA inhibition retards calcium sequestration and reduces the amplitude of evoked excitatory junctional currents. This suggests a direct contribution of store-derived calcium in determining the quantal content of evoked release. Conditional paralysis of SERCA mutants is also marked by prolonged neural activity-driven muscle contraction, thus reflecting the phylogenetically conserved role of SERCA in terminating contraction. Further analysis of ionic currents from mutants uncovers SERCA-dependent mechanisms regulating voltage-gated calcium channels and calcium-activated potassium channels that together control muscle excitability. Finally, our identification of dominant loss-of-function mutations in SERCA indicates novel intra- and intermolecular interactions for SERCA in vivo, overlooked by current structural models.

Acute induction of conserved synaptic signaling pathways in Drosophila melanogaster

Analyses of early molecular and cellular events associated with long-term plasticity remain hampered in Drosophila by the lack of an acute procedure to activate signal transduction pathways, gene expression patterns, and other early cellular events associated with long-term synaptic change. Here we describe the development and first use of such a technique. Bursts of neural activity induced in Drosophila comatosets and CaP60A Kumts mutants, with conditional defects in N-ethylmaleimide-sensitive fusion factor 1 and sarco-endoplasmic reticulum Ca2+ ATPase, respectively, result in persistent (>4 hr) activation of neuronal extracellular signal-regulated kinase (ERK). ERK activation at the larval neuromuscular junction coincides with rapid reduction of synaptic Fasciclin II; in soma, nuclear translocation of activated ERK occurs together with increased transcription of the immediate-early genes Fos and c/EBP (CCAAT element binding protein). The effect of "seizure-stimulation" on ERK activation requires neural activity and is mediated through activation of MEK (MAPK/erk kinase), the MAPKK (mitogen-activated protein kinase kinase) that functions upstream of ERK. Our results (1) provide direct proof for the conservation of synaptic signaling pathways in arthropods, (2) demonstrate the utility of a new genetic tool for analysis of synaptic plasticity in Drosophila, and (3) potentially enable new proteomic and genomic analyses of activity-regulated molecules in an important model organism.

Endocytosis in Drosophila: progress, possibilities, prognostications

By many outside the field, endocytosis is often perceived as a "house-keeping" function performed via identical mechanisms in yeast and man. Recent discoveries have done much to reduce this misperception. (1) Endocytosis occurs via different mechanisms and different pathways in different cellular contexts. (2) Molecular mechanisms that regulate homologous pathways in unicellular and multicellular organisms show considerable variance. (3) Temporally controlled endocytosis of specific regulatory molecules underlies several important and intricate biological processes including synapse formation, synaptic plasticity, cell fate determination, and morphogen gradient formation. Interactions between endocytosis and cytoskeletal and signaling pathways have been particularly revealing. In this intellectual context, Drosophila has become invaluable as a metazoan genetic model in which to understand the many faces of endocytosis. This review discusses two aspects of work in Drosophila: (a) its contributions toward understanding fundamental mechanisms that underlie the operation of endocytic pathways; (b) how analyses in Drosophila provide insights into varied biological processes regulated by endocytosis. In addition, while offering our commentary on merits and limitations of Drosophila work, we speculate on likely areas for contributions and future research on endocytosis in Drosophila.

Drosophila stoned proteins regulate the rate and fidelity of synaptic vesicle internalization

At an initial step during synaptic vesicle recycling, dynamin and adaptor proteins mediate the endocytosis of synaptic vesicle components from the plasma membrane. StonedA and stonedB, novel synaptic proteins encoded by a single Drosophila gene, have predicted structural similarities to adaptors and other proteins implicated in endocytosis. Here, we test possible roles of the stoned proteins in synaptic vesicle internalization via analyses of third instar larval neuromuscular synapses in two Drosophila stoned (stn) mutants, stn(ts) and stn(8P1). Both mutations reduce presynaptic levels of stonedA and stonedB, although stn(ts) has relatively weak effects. The mutations cause retention of synaptic vesicle proteins on the presynaptic plasma membrane but do not alter the levels or distribution of endocytosis proteins, dynamin, alpha-adaptin, and clathrin. In addition, stn(8P1) mutants exhibit depletion and enlargement of synaptic vesicles. To determine whether these defects arise from altered synaptic vesicle endocytosis or from defects in synaptic vesicle biogenesis, we implemented new methods to assess directly the efficiency of synaptic vesicle recycling and membrane internalization at Drosophila nerve terminals. Behavioral and electrophysiological analyses indicate that stn(ts), an allele with normal evoked release and synaptic vesicle number, enhances defects in synaptic vesicle recycling shown by Drosophila shi(ts) mutants. A dye uptake assay demonstrates that slow synaptic vesicle recycling in stn(ts) is accompanied by a reduced rate of synaptic vesicle internalization after exocytosis. These observations are consistent with a model in which stonedA and stonedB act to facilitate the internalization of synaptic vesicle components from the plasma membrane.

Nucleoside diphosphate kinase, a source of GTP, is required for dynamin-dependent synaptic vesicle recycling

Nucleoside diphosphate kinase (NDK), an enzyme encoded by the Drosophila abnormal wing discs (awd) or human nm23 tumor suppressor genes, generates nucleoside triphosphates from respective diphosphates. We demonstrate that NDK regulates synaptic vesicle internalization at the stage where function of the dynamin GTPase is required. awd mutations lower the temperature at which behavioral paralysis, synaptic failure, and blocked membrane internalization occur at dynamin-deficient, shi(ts), mutant nerve terminals. Hypomorphic awd alleles display shi(ts)-like defects. NDK is present at synapses and its enzymatic activity is essential for normal presynaptic function. We suggest a model in which dynamin activity in nerve terminals is highly dependent on NDK-mediated supply of GTP. This connection between NDK and membrane internalization further strengthens an emerging hypothesis that endocytosis, probably of activated growth factor receptors, is an important tumor suppressor activity in vivo.

The products of the Drosophila stoned locus interact with synaptic vesicles via synaptotagmin

The stoned locus of Drosophila melanogaster encodes two novel proteins, stonedA (STNA) and stonedB (STNB), both of which are expressed in the nervous system. Flies with defects at the stoned locus have abnormal behavior and altered synaptic transmission. Genetic interactions, in particular with the shibire (dynamin) mutation, indicated a presynaptic function for stoned and suggested an involvement in vesicle cycling. Immunological studies revealed colocalization of the stoned proteins at the neuromuscular junction with the integral synaptic vesicle protein synaptotagmin (SYT). We show here that stoned interacts genetically with synaptotagmin to produce a lethal phenotype. The STNB protein is found by co-immunoprecipitation to be associated with synaptic vesicles, and glutathione S-transferase pull-downs demonstrate an in vitro interaction between the micro2-homology domain of STNB and the C2B domain of the SYTI isoform. The STNA protein is also found in association with vesicles, and it too exhibits an in vitro association with SYTI. However, we find that the bulk of STNA is in a nonmembranous fraction. By using the shibire mutant to block endocytosis, STNB is shown to be present on some synaptic vesicles before exocytosis. However, STNB is not associated with all synaptic vesicles. We hypothesize that STNB specifies a subset of synaptic vesicles with a role in the synaptic vesicle cycle that is yet to be determined.

Drosophila endosomal proteins hook and deep orange regulate synapse size but not synaptic vesicle recycling

To study the function of endosomes at synapses we analyzed the localization and function of two Drosophila endosomal proteins, Hook and Deep orange (Dor), at the larval neuromuscular junction. Hook, a negative regulator of endocytic trafficking, and Dor, a positive regulator of endocytic trafficking, are highly enriched at synapses, especially close to postsynaptic membranes. Mutations in hook (hk) and dor do not affect synaptic vesicle recycling, as assessed by electrophysiological analysis of synaptic transmission and behavioral studies of double mutants with shi(ts) mutations that alter vesicle recycling. However, hk and dor mutations alter the number of presynaptic varicosities (synapse size) in opposing ways. Synapse size is increased in hk(11) mutants and is decreased in dor(4) mutants. Double mutants for dor and hk show a dor-like phenotype. These effects on synapse size parallel known functions of Hook and Dor in endocytosis and strongly indicate a role for endocytic trafficking in the regulation of synapse size in vivo. Our observations suggest a model in which Hook and Dor function in later stages of endocytosis is essential for regulating synaptic plasma membrane composition but not synaptic vesicle recycling.

nalyot, a mutation of the Drosophila myb-related Adf1 transcription factor, disrupts synapse formation and olfactory memory

nalyot (nal) is a novel olfactory memory mutant of Drosophila, encoding Adf1, a myb-related transcription factor. Following extended training sessions, Adf1 mutants show normal early memory but defective longterm memory. Adf1 shows widespread spatiotemporal expression, yet mutant alleles reveal no discernible disruptions in gross morphology of the nervous system. Studies at the larval neuromuscular junction, however, reveal a role for Adf1 in the modulation of synaptic growth-in contrast to the role established for dCREB2 in the control of synaptic function (Davis et al., 1996). These findings suggest that Adf1 and dCREB2 regulate distinct transcriptional cascades involved in terminal stages of synapse maturation. More generally, Adf1 provides a novel link between molecular mechanisms of developmental and behavioral plasticity.

Synaptic localization and restricted diffusion of a Drosophila neuronal synaptobrevin--green fluorescent protein chimera in vivo

Fluorescent markers for subcellular compartments in Drosophila neurons should allow one to combine genetic mutant analysis with visualization of subcellular structures in vivo. Here we describe an analysis of two markers which may be used to observe different compartments of live Drosophila synapses. Soluble jellyfish green fluorescent protein (GFP) expressed at high levels in neurons diffuses freely in the neuronal cytosol as evidenced by confocal microscopy and fluorescence recovery from photobleaching experiments. Thus, the distribution pattern of soluble GFP in motor axons and larval motor terminals indicates the expected distribution for diffusible presynaptic molecules. In contrast to GFP, a neurally expressed neuronal synaptobrevin-GFP chimera (n-syb GFP) is transported down axons and specifically localized to nerve terminals. We demonstrate that n-syb GFP labels synaptic-vesicle membrane at larval motor terminals by documenting its restriction to presynaptic varicosities, its colocalization with synaptic vesicle antigens, and its redistribution in Drosophila shits1 mutant nerve terminals transiently depleted of synaptic vesicles. Surprisingly, n-syb GFP expressed in muscle is concentrated at the subsynaptic reticulum (SSR), postsynaptic infoldings of muscle plasma membrane. We suggest, using different membrane markers, that this apparent postsynaptic enrichment simply reflects a concentration of plasma membrane in the SSR, rather than a selective targeting of n-syb GFP to postsynaptic sites. Utilities and implications of these studies are demonstrated or discussed.

Functional analysis of dynamin isoforms in Drosophila melanogaster

Dynamin and dynamin-like proteins are required for endocytosis, synaptic vesicle recycling and membrane trafficking. From the shibire locus in Drosophila melanogaster, six different isoforms of dynamin are generated by alternative splicing. However, the roles of the individual isoforms in cellular processes are unknown. To investigate functional differences among the dynamin isoforms, transgenic lines were generated that individually expressed each of 3 different isoforms under UASGAL4 control. The expression of the isoforms was controlled by neural promoter (elav)-driven GAL4, or by a shibire-promoter driven GAL4 transgene. Reporter gene expression indicated that the shi promoter is active during embryogenesis, and in larvae, pupae, and adults in a pattern consistent with normal dynamin expression. To assay for the ability of dynamin isoforms to function in vivo, the isoforms expressed via these GAL4 drivers were tested for the ability to rescue shibire phenotypes. When expressed at very high levels all individual isoforms tested rescued the temperature-sensitive paralytic phenotype of shi(ts2) flies; however, this rescue was partial, suggesting that no single tested isoform is sufficient for synaptic vesicle recycling in vivo. When tested for ability to rescue lethality induced by heat-pulsing larvae during development, shi- promoter driven expression of individual isoforms conferred significant resistance to heat treatment during larval development. However, all 3 isoforms were unable to rescue the lethality of shi12-12B mutants which are severely hypomorphic (or null) for shibire function. Taken together, these observations suggest that individual shibire isoforms have specific molecular activities in vivo.

A product of the Drosophila stoned locus regulates neurotransmitter release

The Drosophila stoned locus encodes two novel gene products termed stonedA and stonedB, which possess sequence motifs shared by proteins involved in intracellular vesicle traffic. A specific requirement for stoned in the synaptic vesicle cycle has been suggested by synthetic genetic interactions between stoned and shibire, a gene essential for synaptic vesicle recycling (Petrovich et al., 1993). A synaptic role of stoned gene products also is suggested by altered synaptic transients in electroretinograms recorded from stoned mutant eyes (Petrovich et al., 1993). We show here that the stonedA protein is highly enriched at Drosophila nerve terminals. Mutant alleles that affect stonedA disrupt the normal regulation of synaptic vesicle exocytosis at neuromuscular synapses of Drosophila. Spontaneous neurotransmitter release is enhanced dramatically, and evoked release is reduced substantially in such stoned mutants. Ultrastructural studies reveal no evidence of major disorganization at stoned mutant nerve terminals. Thus, our data indicate a direct role for stonedA in regulating synaptic vesicle exocytosis. However, genetic and morphological observations suggest additional, subtle effects of stoned mutations on synaptic vesicle recycling. Remarkably, almost all phenotypes of stoned mutants are similar to those previously described for mutants of synaptotagmin, a protein postulated to regulate both exocytosis and the recycling of synaptic vesicles. We propose a model in which stonedA functions together with synaptotagmin to regulate synaptic vesicle cycling.

Not just pretty eyes: Drosophila eye-colour mutations and lysosomal delivery

Analysis of Drosophila eye-colour mutations has made seminal contributions to the fields of genetics and biochemistry. Recent findings suggest that a subset of eye-colour genes is crucial for vesicular transport of proteins to pigment granules, specialized lysosomes of eye-pigment cells. Thus, classical work describing more than 85 eye-colour mutations and their genetic interactions offers a remarkable, untapped resource for the genetic analysis of protein delivery to lysosomes.

Probable mechanisms underlying interallelic complementation and temperature-sensitivity of mutations at the shibire locus of Drosophila melanogaster

The shibire locus of Drosophila melanogaster encodes dynamin, a GTPase required for the fission of endocytic vesicles from plasma membrane. Biochemical studies indicate that mammalian dynamin is part of a complex containing multiple dynamin subunits and other polypeptides. To gain insight into sequences of dynamin critical for its function, we have characterized in detail a collection of conditional and lethal shi alleles. We describe a probable null allele of shi and show that its properties are distinct from those of two classes of lethal alleles (termed I and II) that show intergroup, interallelic complementation. Sequenced class I alleles, which display dominant properties, carry missense mutations in conserved residues in the GTPase domain of dynamin. In contrast, the sequenced class II alleles, which appear completely recessive, carry missense mutations in conserved residues of a previously uncharacterized "middle domain" that lies adjacent to the GTPase region. These data suggest that critical interactions mediated by this middle domain are severely affected by the class II lethal mutations; thus, the mutant sequences should be very useful for confirming the in vivo relevance of interactions observed in vitro. Viable heteroallelic combinations of shi lethals show rapid and reversible temperature-sensitive paralytic phenotypes hitherto only described for the ts alleles of shi. When taken together with the molecular analysis of shi mutations, these observations suggest that the GTPase domain of dynamin carries an intrinsically temperature-sensitive activity: hypomorphic mutations that reduce this activity at low temperatures result in conditional temperature-sensitive phenotype. These observations explain why screens for conditional paralytic mutants in Drosophila inevitably recover ts alleles of shi at high frequencies.

The organization of extrinsic neurons and their implications in the functional roles of the mushroom bodies in Drosophila melanogaster Meigen

Although the importance of the Drosophila mushroom body in olfactory learning and memory has been stressed, virtually nothing is known about the brain regions to which it is connected. Using Golgi and GAL4-UAS techniques, we performed the first systematic attempt to reveal the anatomy of its extrinsic neurons. A novel presynaptic reporter construct, UAS-neuronal synaptobrevin-green fluorescent protein (n-syb-GFP), was used to reveal the direction of information in the GAL4-labeled neurons. Our results showed that the main target of the output neurons from the mushroom body lobes is the anterior part of the inferior medial, superior medial, and superior lateral protocerebrum. The lobes also receive afferents from these neuropils. The lack of major output projections directly to the deutocerebrum's premotor pathways discourages the view that the role of the mushroom body may be that of an immediate modifier of behavior. Our data, as well as a critical evaluation of the literature, suggest that the mushroom body may not by itself be a "center" for learning and memory, but that it can equally be considered as a preprocessor of olfactory signals en route to "higher" protocerebral regions.

A genetic and mosaic analysis of a locus involved in the anesthesia response of Drosophila melanogaster

We describe a genetic and behavioral analysis of several alleles of har38, a mutant with altered sensitivity to the general anesthetic halothane. We obtained a P-element-induced allele of har38 and generated several excision alleles by remobilizing the P element. The mutants narrow abdomen (na) and har85 are confirmed to be allelic to har 38. Besides a decreased sensitivity to halothane, all mutant alleles of this locus cause a characteristic walking behavior in the absence of anesthetics. We have quantified this behavior using a geotaxis apparatus. Responses of the mutant alleles to different inhalational anesthetics were tested. The results strongly favor a multipathway model for the onset of anesthesia. Mosaic flies were tested for their response to halothane and checked for their abnormal walking behavior. The analysis suggests that both the behaviors are exhibited only by such mosaics as have the entire head of mutant origin. It is likely that this focus represents an element of a common pathway in the anesthetic response to several inhalational anesthetics but not all. This result is the first demonstration of regional specificity in the CNS of any animal for general anesthetic action.

Alleviation of the temperature-sensitive paralytic phenotype of shibire(ts) mutants in Drosophila by sub-anesthetic concentrations of carbon dioxide

Cellular mechanisms involved in general anesthesia are unknown. We report here that sub-anesthetic concentrations of carbon dioxide specifically suppress the temperature-sensitive paralytic phenotype of Drosophila shibire(ts) mutants that have a conditional block in synaptic vesicle recycling. Carbon dioxide not only suppresses the onset of temperature-sensitive paralysis, but also rapidly reverses paralysis induced at the restrictive temperature. This effect of CO2 is most pronounced at about 35% in air, and depends on the absolute concentration of available carbon dioxide rather than on the ratio of oxygen to CO2. Other general anesthetics, halothane, N2 or argon do not suppress the paralytic phenotype of shibire significantly at concentrations we tested. Paralysis of the other temperature sensitive paralytic mutants in our collection is not suppressed by carbon dioxide. These behavioral observations are discussed in the light of possible mechanisms underlying paralysis of shi(ts) flies. We suggest that spontaneous seizures induced in shi(ts) flies held at their restrictive temperatures cause vesicle depletion at critical synapses and consequent behavioral paralysis. The effect of subanesthetic concentrations of CO2 may be to depress spontaneous CNS activity, thus raising the threshold temperature at which synaptic vesicle depletion occurs. In support of this model, we show that the threshold temperature for paralysis is reduced in shi(ts) flies when CNS activity is increased by pharmacological or genetic manipulations, and that subanesthetic concentrations of CO2 aggravate, rather than alleviate, the t.s. paralytic phenotype of hypoactive parats flies defective in axonal voltage-gated sodium channels.

Traffic of dynamin within individual Drosophila synaptic boutons relative to compartment-specific markers

Presynaptic terminals contain several specialized compartments, which have been described by electron microscopy. We show in an identified Drosophila neuromuscular synapse that several of these compartments-synaptic vesicle clusters, presynaptic plasma membrane, presynaptic cytosol, and axonal cytoskeleton-labeled by specific reagents may be resolved from one another by laser scanning confocal microscopy. Using a panel of compartment-specific markers and Drosophila shibire(ts1) mutants to trap an intermediate stage in synaptic vesicle recycling, we have examined the localization and redistribution of dynamin within single synaptic varicosities at the larval neuromuscular junction. Our results suggest that dynamin is not a freely diffusible molecule in resting nerve terminals; rather, it appears localized to synaptic sites by association with yet uncharacterized presynaptic components. In shi(ts1) nerve terminals depleted of synaptic vesicles, dynamin is quantitatively redistributed to the plasma membrane. It is not, however, distributed uniformly over presynaptic plasmalemma; instead, fluorescence images show "hot spots" of dynamin on the plasma membrane of vesicle-depleted nerve terminals. We suggest that these dynamin-rich domains may mark the active zones for synaptic vesicle endocytosis first described at the frog neuromuscular junction.

Distinct roles for N-ethylmaleimide-sensitive fusion protein (NSF) suggested by the identification of a second Drosophila NSF homolog

The N-ethylmaleimide-sensitive fusion protein (NSF) is a cytoplasmic protein implicated in the fusion of intracellular transport vesicles with their target membranes. NSF is thought to function in the fusion of essentially all types of vesicles, including endoplasmic reticulum, Golgi, and endocytic vesicles, as well as secretory vesicles undergoing regulated fusion (for review see Rothman, J.E. (1994) Nature 372, 55-63). However, little experimental evidence exists to address the possibility that organisms might have multiple NSF proteins serving distinct functions in the same or different cells. We previously cloned a neurally expressed Drosophila homolog, dNSF-1 (Ordway, R.W., Pallanck, L., and Ganetzky, B. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 5715-5719), and have subsequently identified mutations in this gene that confer an apparent failure of synaptic transmission at elevated temperature (Pallanck, L., Ordway, R.W., and Ganetzky, B. (1995) Nature, 376, 25; Siddiqi, O., and Benzer, S. (1976) Proc. Natl. Acad. Sci. U.S.A. 73, 3253-3257). Here we report that 1) Drosophila contains a second NSF homolog, termed dNSF-2, that exhibits 84% amino acid identity to dNSF-1, 2) dNSF-1 and dNSF-2 display overlapping but different temporal expression, and 3) multiple transcripts are derived from the dNSF-2 gene. These findings raise the possibility that different NSF gene products serve distinct or overlapping functions with the organism.

Redistribution of synaptic vesicles and their proteins in temperature-sensitive shibire(ts1) mutant Drosophila

From an extract of Drosophila melanogaster head homogenates, a membrane fraction can be isolated that has the same sedimentation properties as vertebrate synaptic vesicles and contains Drosophila synaptotagmin. The fraction disappears from homogenates of temperature-sensitive (ts) mutant shibire(ts1) (shi(ts1)) flies paralyzed by exposure to non-permissive temperatures, and reappears on return to permissive temperatures. Since reversible, temperature-dependent depletion of synaptic vesicles is known to occur in shibire(ts1) flies, we conclude that the fraction we have identified contains synaptic vesicles. We have examined the fate of synaptic vesicle membrane proteins in shibire flies at nonpermissive temperatures and found that all of these vesicle antigens are transferred to rapidly sedimenting membranes and codistribute with a plasma membrane marker by both glycerol velocity and metrizamide density sedimentation and by confocal microscopy. Three criteria were used to establish that other neuron-specific antigens--neuronal synaptobrevin and cysteine-string proteins--are legitimate components of synaptic vesicles: cosedimentation with Drosophila synaptotagmin, immunoadsorption, and disappearance of these antigens from the vesicle fractions in paralyzed shibire flies.

The Drosophila easily shocked gene: a mutation in a phospholipid synthetic pathway causes seizure, neuronal failure, and paralysis

We have characterized easily shocked (eas), a Drosophila "band-sensitive" paralytic mutant. Electrophysiological recordings from flight muscles in the giant fiber pathway of adult eas flies reveal that induction of paralysis with electrical stimulation results in a brief seizure, followed by a failure of the muscles to respond to giant fiber stimulation. Molecular cloning, germline transformation, and biochemical experiments show that eas mutants are defective in the gene for ethanolamine kinase, which is required for a pathway of phosphatidylethanolamine synthesis. Assays of phospholipid composition reveal that total phosphatidylethanolamine is decreased in eas mutants. The data suggest that eas bang sensitivity is due to an excitability defect caused by altered membrane phospholipid composition.

Facile formation of heteromultimeric potassium channels by expression of cloned human cDNAs

Complementary DNAs representing three voltage-gated potassium channels of human origin have previously been expressed in Xenopus laevis oocytes by injecting RNA transcribed in vitro [Ramaswami, M., Gautam, M., Kamb, A., Rudy, B., Tanouye, M. A. & Mathew, M. K. (1990) Mol. Cell. Nueorsci 1, 214-223]. We have coinjected RNAs for pairs of K(+)-channel genes into Xenopus oocytes. Analysis of the kinetics of the evoked currents, their voltage dependence and pharmacological sensitivities demonstrate that channels formed on coinjection of RNA pairs have properties distinct from those evoked by either channel type alone. We conclude that these currents arise from heteromultimeric aggregates of the subunits encoded by the individual RNAs. Quantitative analysis of the currents indicate that at least 60% of the current seen can be ascribed to heteromultimeric channels demonstrating their facile formation. Given that there are a large number of primary transcripts present in the nervous system, the demonstration of pharmacologically distinct heteromultimers may complicate the extension of studies on single, cloned K(+)-channels in heterologous systems to neuronal cells.

Intermediates in synaptic vesicle recycling revealed by optical imaging of Drosophila neuromuscular junctions

We show that uptake and release of the styryl dye FM1-43 may be used to monitor synaptic vesicle exocytosis and recycling at Drosophila larval neuromuscular junctions. At Drosophila nerve terminals, FM1-43 specifically labels subsynaptic domains enriched in synaptotagmin, in a manner that requires Ca2+, membrane depolarization, and shibire (shi) function. Endocytosis rates, very low in unstimulated synapses, are induced severalfold by the exocytosis of synaptic vesicles. Using shi(ts)1 mutant synapses to separate synaptic vesicle fusion and recycling temporally, we show that recycling events subsequent to the shi block do not require extracellular Ca2+. We suggest that two distinct intermediate stages in vesicle recycling may be trapped and analyzed at Drosophila neuromuscular junctions.

Genetic studies on dynamin function in Drosophila

The shibire(ts2) mutation of Drosophila melanogaster causes a temperature sensitive inhibition of endocytosis; this in turn leads to synaptic-vesicle depletion and consequent paralysis. Heat-pulses delivered during development of shibire(ts2) individuals affect the morphology of a number of adult structures. A simple screening protocol has been used to isolate several mutations that partially suppress the temperature-sensitive paralytic phenotype of shibire(ts2) mutant animals. All of these mutations very tightly linked to shibire and are likely to be second site intragenic mutations that restore partial activity to the shibire(ts2) product. The mutations suppress both behavioral, and easily-scored developmental phenotypes of shibire(ts2) characterized in this paper. Our results suggest that defects in endocytosis, and not in microtubule interactions, are responsible for all of the phenotypes of shibire(ts2) mutant Drosophila examined in this study.

A role for hydrophobic residues in the voltage-dependent gating of Shaker K+ channels

A leucine heptad repeat is well conserved in voltage-dependent ion channels. Herein we examine the role of the repeat region in Shaker K+ channels through substitution of the leucines in the repeat and through coexpression of normal and truncated products. In contrast to leucine-zipper DNA-binding proteins, we find that the subunit assembly of Shaker does not depend on the leucine heptad repeat. Instead, we report that substitutions of the leucines in the repeat produce large effects on the observed voltage dependence of conductance voltage and prepulse inactivation curves. Our results suggest that the leucines mediate interactions that play an important role in the transduction of charge movement into channel opening and closing.

Two sodium-channel genes in Drosophila: implications for channel diversity

We describe two Drosophila melanogaster transcription units that are highly homologous to a rat Na+-channel cDNA. They appear to encode the major subunits of two distinct Na+-channel proteins. One of these maps to the second chromosome and is identical to a Na+-channel gene whose partial sequence has been previously reported [Salkoff, L., Butler, A., Wei, A., Scavarda, N., Giffen, K., Ifune, K., Goodman, R. & Mandel, G. (1987) Science 237, 744-749]. The other transcription unit maps to position 14C/D, on the X chromosome, close to the paralyzed (para) gene. Mutations in para affect membrane excitability in Drosophila neurons [Ganetzky, B. & Wu, C.F. (1986) Annu. Rev. Genet. 20, 13-44]. Sequence comparisons suggest that two Na+-channel genes arose early in evolution, before the divergence of vertebrate and invertebrate lines.