Genetic analysis of the Shaker gene complex of Drosophila melanogaster

The role of Evi/Wntless in exporting Wnt proteins

Intercellular communication by Wnt proteins governs many essential processes during development, tissue homeostasis and disease in all metazoans. Many context-dependent effects are initiated in the Wnt-producing cells and depend on the export of lipidated Wnt proteins. Although much focus has been on understanding intracellular Wnt signal transduction, the cellular machinery responsible for Wnt secretion became better understood only recently. After lipid modification by the acyl-transferase Porcupine, Wnt proteins bind their dedicated cargo protein Evi/Wntless for transport and secretion. Evi/Wntless and Porcupine are conserved transmembrane proteins, and their 3D structures were recently determined. In this Review, we summarise studies and structural data highlighting how Wnts are transported from the ER to the plasma membrane, and the role of SNX3-retromer during the recycling of its cargo receptor Evi/Wntless. We also describe the regulation of Wnt export through a post-translational mechanism and review the importance of Wnt secretion for organ development and cancer, and as a future biomarker.

Mnb/Dyrk1A orchestrates a transcriptional network at the transition from self-renewing neurogenic progenitors to postmitotic neuronal precursors

The Down syndrome and microcephaly related gene Mnb/Dyrk1A encodes an evolutionary conserved protein kinase subfamily that plays important roles in neurodevelopment. minibrain (mnb) mutants of Drosophila melanogaster (Dm) exhibit reduced adult brains due to neuronal deficits generated during larval development. These deficits are the consequence of the apoptotic cell death of numerous neuronal precursors that fail to properly exit the cell cycle and differentiate. We have recently found that in both the Dm larval brain and the embryonic vertebrate central nervous system (CNS), a transient expression of Mnb/Dyrk1A promotes the cell cycle exit of newborn neuronal precursors by upregulating the expression of the cyclin-dependent kinase inhibitor p27kip1 (called Dacapo in Dm). In the larval brain, Mnb performs this action by regulating the expression of three transcription factors, Asense (Ase), Deadpan (Dpn) and Prospero (Pros), which are key regulators of the self-renewal, proliferation, and terminal differentiation of neural progenitor cells. We have here studied in detail the cellular/temporal expression pattern of Ase, Dpn, Pros and Mnb, and have analyzed possible regulatory effects among them at the transitions from neurogenic progenitors to postmitotic neuronal precursors in the Dm larval brain. The emerging picture of this analysis reveals an intricate regulatory network in which Mnb appears to play a pivotal role helping to delineate the dynamics of the expression patterns of Ase, Dpn and Pros, as well as their specific functions in the aforementioned transitions. Our results also show that Ase, Dpn and Pros perform several cross-regulatory actions and contribute to shape the precise cellular/temporal expression pattern of Mnb. We propose that Mnb/Dyrk1A plays a central role in CNS neurogenesis by integrating molecular mechanisms that regulate progenitor self-renewal, cell cycle progression and neuronal differentiation.

Troponin-I enhances and is required for oncogenic overgrowth

Human tumors of various tissue origins show an intriguing over-expression of genes not considered oncogenes, such as that encoding Troponin-I (TnI), a well-known muscle protein. Out of the three TnI genes known in humans, the slow form, TNNI1, is affected the most. Drosophila has only one TnI gene, wupA. Here, we studied excess- and loss-of function of wupA in Drosophila, and assayed TNNI1 down regulation in human tumors growing in mice. Drosophila TnI excess-of-function increases proliferation and potentiates oncogenic mutations in Ras, Notch and Lgl genes. By contrast, TnI loss-of-function reduces proliferation and antagonizes the overgrowth due to these oncogenic mutations. Troponin-I defective cells undergo Flower- and Sparc-dependent cell competition. TnI can localize to the nucleus and its excess elicits transcriptional up-regulation of InR, Rap1 and Dilp8, which is consistent with the increased cell proliferation. Human tumor cell lines treated with a human Troponin-I peptide arrest in G₀/G_1. In addition, proliferation of non-small-cell lung carcinoma xenografts in mice is restrained by TNNI1 down-regulation. Thus, Troponin-I reveals a novel function in cell proliferation that may be of therapeutic interest in certain types of cancer.

A cis-regulatory mutation in troponin-I of Drosophila reveals the importance of proper stoichiometry of structural proteins during muscle assembly

Rapid and high wing-beat frequencies achieved during insect flight are powered by the indirect flight muscles, the largest group of muscles present in the thorax. Any anomaly during the assembly and/or structural impairment of the indirect flight muscles gives rise to a flightless phenotype. Multiple mutagenesis screens in Drosophila melanogaster for defective flight behavior have led to the isolation and characterization of mutations that have been instrumental in the identification of many proteins and residues that are important for muscle assembly, function, and disease. In this article, we present a molecular-genetic characterization of a flightless mutation, flightless-H (fliH), originally designated as heldup-a (hdp-a). We show that fliH is a cis-regulatory mutation of the wings up A (wupA) gene, which codes for the troponin-I protein, one of the troponin complex proteins, involved in regulation of muscle contraction. The mutation leads to reduced levels of troponin-I transcript and protein. In addition to this, there is also coordinated reduction in transcript and protein levels of other structural protein isoforms that are part of the troponin complex. The altered transcript and protein stoichiometry ultimately culminates in unregulated acto-myosin interactions and a hypercontraction muscle phenotype. Our results shed new insights into the importance of maintaining the stoichiometry of structural proteins during muscle assembly for proper function with implications for the identification of mutations and disease phenotypes in other species, including humans.

Transcriptional adaptor ADA3 of Drosophila melanogaster is required for histone modification, position effect variegation, and transcription

The Drosophila melanogaster gene diskette (also known as dik or dAda3) encodes a protein 29% identical to human ADA3, a subunit of GCN5-containing histone acetyltransferase (HAT) complexes. The fly dADA3 is a major contributor to oogenesis, and it is also required for somatic cell viability. dADA3 localizes to chromosomes, and it is significantly reduced in dGcn5 and dAda2a, but not in dAda2b, mutant backgrounds. In dAda3 mutants, acetylation at histone H3 K9 and K14, but not K18, and at histone H4 K12, but not K5, K8, and K16, is significantly reduced. Also, phosphorylation at H3 S10 is reduced in dAda3 and dGcn5 mutants. Variegation for white (w(m4)) and scute (Hw(v)) genes, caused by rearrangements of X chromosome heterochromatin, is modified in a dAda3(+) gene-dosage-dependent manner. The effect is not observed with rearrangements involving Y heterochromatin (bw(D)), euchromatin (Scutoid), or transvection effects on chromosomal pairing (white and zeste interaction). Activity of scute gene enhancers, targets for Iroquoi transcription factors, is abolished in dAda3 mutants. Also, Iroquoi-associated phenotypes are sensitive to dAda3(+) gene dosage. We conclude that dADA3 plays a role in HAT complexes which acetylate H3 and H4 at specific residues. In turn, this acetylation results in chromatin structure effects of certain rearrangements and transcription of specific genes.

The Drm-Bowl-Lin relief-of-repression hierarchy controls fore- and hindgut patterning and morphogenesis

The elucidation of pathways linking patterning to morphogenesis is a problem of great interest. We show here that, in addition to their roles in patterning and morphogenesis of the hindgut, the Drosophila genes drumstick (drm) and bowl are required in the foregut for spatially localized gene expression and the morphogenetic processes that form the proventriculus. drm and bowl belong to a family of genes encoding C(2)H(2) zinc finger proteins; the other two members of this family are odd-skipped (odd) and sob. In both the fore- and hindgut, drm acts upstream of lines (lin), which encodes a putative transcriptional regulator, and relieves its repressive function. In spite of its phenotypic similarities with drm, bowl was found in both foregut and hindgut to act downstream, rather than upstream, of lin. These results support a hierarchy in which Drm relieves the repressive effect of Lin on Bowl, and Bowl then acts to promote spatially localized expression of genes (particularly the JAK/STAT pathway ligand encoded by upd) that control fore- and hindgut morphogenesis. Since the odd-family and lin are conserved in mosquito, mouse, and humans, we propose that the odd-family genes and lin may also interact to control patterning and morphogenesis in other insects and in vertebrates.

A tale of two tails: cytosolic termini and K(+) channel function

The enormous variety of neuronal action potential waveforms can be ascribed, in large part, to the sculpting of their falling phases by currents through voltage-gated potassium channels. These proteins play several additional roles in other tissues such as the regulation of heartbeat and of insulin release from pancreatic cells as well as auditory signal processing in the cochlea. The functional channel is a tetramer with either six or two transmembrane segments per monomer. Selectivity filters, voltage sensors and gating elements have been mapped to residues within the transmembrane region. Cytoplasmic residues, which are accessible targets for signal transduction cascades and provide attractive means of regulation of channel activity, are now seen to be capable of modulating various aspects of channel function. Here we review structural studies on segments of the cytoplasmic tails of K(+) channels, as well as the range of modulatory activities of these tails.

Localized JAK/STAT signaling is required for oriented cell rearrangement in a tubular epithelium

Rearrangement of cells constrained within an epithelium is a key process that contributes to tubular morphogenesis. We show that activation in a gradient of the highly conserved JAK/STAT pathway is essential for orienting the cell rearrangement that drives elongation of a genetically tractable model. Using loss-of-function and gain-of-function experiments, we show that the components of the pathway from ligand to the activated transcriptional regulator STAT are required for cell rearrangement in the Drosophila embryonic hindgut. The difference in effect between localized expression of ligand (Unpaired) and dominant active JAK (Hopscotch) demonstrates that the ligand plays a cell non-autonomous role in hindgut cell rearrangement. Taken together with the appearance of STAT92E in a gradient in the hindgut epithelium, these results support a model in which an anteroposterior gradient of ligand results in a gradient of activated STAT. These results provide the first example in which JAK/STAT signaling plays a required role in orienting cell rearrangement that elongates an epithelium.

DLG differentially localizes Shaker K+-channels in the central nervous system and retina of Drosophila

Subcellular localization of ion channels is crucial for the transmission of electrical signals in the nervous system. Here we show that Discs-Large (DLG), a member of the MAGUK (membrane-associated guanylate kinases) family in Drosophila, co-localizes with Shaker potassium channels (Sh Kch) in most synaptic areas of the adult brain and in the outer membrane of photoreceptors. However, DLG is absent from axonal tracts in which Sh channels are concentrated. Truncation of the C-terminal of Sh (including the PDZ binding site) disturbs its pattern of distribution in both CNS and retina, while truncation of the guanylate kinase/C-terminal domain of DLG induces ectopic localization of these channels to neuronal somata in the CNS, but does not alter the distribution of channels in photoreceptors. Immunocytochemical, membrane fractionation and detergent solubilization analysis indicate that the C-terminal of Sh Kch is required for proper trafficking to its final destination. Thus, several major conclusions emerge from this study. First, DLG plays a major role in the localization of Sh channels in the CNS and retina. Second, localization of DLG in photoreceptors but not in the CNS seems to depend on its interaction with Sh. Third, the guanylate kinase/C-terminal domain of DLG is involved in the trafficking of Shaker channels but not of DLG in the CNS. Fourth, different mechanisms for the localization of Sh Kch operate in different cell types.

Ariadne-1: a vital Drosophila gene is required in development and defines a new conserved family of ring-finger proteins

We report the identification and functional characterization of ariadne-1 (ari-1), a novel and vital Drosophila gene required for the correct differentiation of most cell types in the adult organism. Also, we identify a sequence-related gene, ari-2, and the corresponding mouse and human homologues of both genes. All these sequences define a new protein family by the Acid-rich, RING finger, B-box, RING finger, coiled-coil (ARBRCC) motif string. In Drosophila, ari-1 is expressed throughout development in all tissues. The mutant phenotypes are most noticeable in cells that undergo a large and rapid membrane deposition, such as rewiring neurons during metamorphosis, large tubular muscles during adult myogenesis, and photoreceptors. Occasional survivors of null alleles exhibit reduced life span, motor impairments, and short and thin bristles. Single substitutions at key cysteines in each RING finger cause lethality with no survivors and a drastic reduction of rough endoplasmic reticulum that can be observed in the photoreceptors of mosaic eyes. In yeast two-hybrid assays, the protein ARI-1 interacts with a novel ubiquitin-conjugating enzyme, UbcD10, whose sequence is also reported here. The N-terminal RING-finger motif is necessary and sufficient to mediate this interaction. Mouse and fly homologues of both ARI proteins and the Ubc can substitute for each other in the yeast two-hybrid assay, indicating that ARI represents a conserved novel mechanism in development. In addition to ARI homologues, the RBR signature is also found in the Parkinson-disease-related protein Parkin adjacent to an ubiquitin-like domain, suggesting that the study of this mechanism could be relevant for human pathology.

An extracellular activator of the Drosophila JAK/STAT pathway is a sex-determination signal element

Metazoans use diverse and rapidly evolving mechanisms to determine sex. In Drosophila melanogaster an X-chromosome-counting mechanism determines the sex of an individual by regulating the master switch gene, Sex-lethal (Sxl). The X-chromosome dose is communicated to Sxl by a set of X-linked signal elements (XSEs), which activate transcription of Sxl through its 'establishment' promoter, SxlPe. Here we describe a new XSE called sisterlessC (sisC) whose mode of action differs from that of previously characterized XSEs, all of which encode transcription factors that activate SxlPe directly. In contrast, sisC encodes a secreted ligand for the Drosophila Janus kinase (JAK) and 'signal transducer and activator of transcription' (STAT) signal transduction pathway and is allelic to outstretched (os, also called unpaired). We conclude that sisC works indirectly on Sxl through this signalling pathway because mutations in sisC or in the genes encoding Drosophila JAK or STAT reduce expression of SxlPe similarly. The involvement of os in sex determination confirms that secreted ligands can function in cell-autonomous processes. Unlike sex signals for other organisms, sisC has acquired its sex-specific function while maintaining non-sex-specific roles in development, a characteristic that it shares with all other Drosophila XSEs.

The haplolethal region at the 16F gene cluster of Drosophila melanogaster: structure and function

Extensive aneuploid analyses had shown the existence of a few haplolethal (HL) regions and one triplolethal region in the genome of Drosophila melanogaster. Since then, only two haplolethals, 22F1-2 and 16F, have been directly linked to identified genes, dpp and wupA, respectively. However, with the possible exception of dpp, the actual bases for this dosage sensitivity remain unknown. We have generated and characterized dominant-lethal mutations and chromosomal rearrangements in 16F and studied them in relation to the genes in the region. This region extends along 100 kb and includes at least 14 genes. The normal HL function depends on the integrity of a critical 4-kb window of mostly noncoding sequences within the wupA transcription unit that encodes the muscle protein troponin I (TNI). All dominant lethals are breakpoints within that window, which prevent the functional expression of TNI and other adjacent genes in the proximal direction. However, independent mutations in these genes result in recessive lethal phenotypes only. We propose that the HL at 16F represents a long-range cis regulatory region that acts upon a number of functionally related genes whose combined haploidy would yield the dominant-lethal effect.

Drosophila unpaired encodes a secreted protein that activates the JAK signaling pathway

In vertebrates, many cytokines and growth factors have been identified as activators of the JAK/STAT signaling pathway. In Drosophila, JAK and STAT molecules have been isolated, but no ligands or receptors capable of activating the pathway have been described. We have characterized the unpaired (upd) gene, which displays the same distinctive embryonic mutant defects as mutations in the Drosophila JAK (hopscotch) and STAT (stat92E) genes. Upd is a secreted protein, associated with the extracellular matrix, that activates the JAK pathway. We propose that Upd is a ligand that relies on JAK signaling to stimulate transcription of pair-rule genes in a segmentally restricted manner in the early Drosophila embryo.

scully, an essential gene of Drosophila, is homologous to mammalian mitochondrial type II L-3-hydroxyacyl-CoA dehydrogenase/amyloid-beta peptide-binding protein

The characterization of scully, an essential gene of Drosophila with phenocritical phases at embryonic and pupal stages, shows its extensive homology with vertebrate type II L-3-hydroxyacyl-CoA dehydrogenase/ERAB. Genomic rescue demonstrates that four different lethal mutations are scu alleles, the molecular nature of which has been established. One of them, scu3127, generates a nonfunctional truncated product. scu4058 also produces a truncated protein, but it contains most of the known functional domains of the enzyme. The other two mutations, scu174 and scuS152, correspond to single amino acid changes. The expression of scully mRNA is general to many tissues including the CNS; however, it is highest in both embryonic gonadal primordia and mature ovaries and testes. Consistent with this pattern, the phenotypic analysis suggests a role for scully in germ line formation: mutant testis are reduced in size and devoid of maturing sperm, and mutant ovarioles are not able to produce viable eggs. Ultrastructural analysis of mutant spermatocytes reveals the presence of cytoplasmic lipid inclusions and scarce mitochondria. In addition, mutant photoreceptors contain morphologically aberrant mitochondria and large multilayered accumulations of membranous material. Some of these phenotypes are very similar to those present in human pathologies caused by beta-oxidation disorders.

Enhanced neurotransmitter release is associated with reduction of neuronal branching in a Drosophila mutant overexpressing frequenin

Frequenin is a Drosophila Ca2+ binding protein whose overexpression causes a chronic facilitation of transmitter release at the larval neuromuscular junction and multiple firing of action potentials. These functional abnormalities are similar to those found in other hyperexcitable mutants (Shaker, ether-a-gogo, Hyperkinetic) which, in turn, exhibit increased branching at the motor nerve endings. We report here that mutants which overexpress frequenin have motor nerve terminals with reduced number and length of branches as well as number of synaptic boutons. Similar defects are observed in transgenic flies which have additional copies of the frequenin gene indicating that the phenotype can be adscribed to the overexpression of the protein. The ultrastructure of boutons, however, appears indistinguishable from wild type. In addition, we show here that frequenin overexpression leads also to a down regulation of Shaker proteins expression. The contrast between the observations in frequenin and the other hyperexcitable mutants indicates that nerve terminal morphology and enhanced transmitter release do not have a direct causal relationship.

Presynaptic recordings from Drosophila: correlation of macroscopic and single-channel K+ currents

We have performed direct electrophysiological recordings from Drosophila peptidergic synaptic boutons in situ, taking advantage of a mutation, ecdysone, which causes an increase in size of these terminals. Using patch-clamp techniques, we have analyzed voltage-dependent potassium currents at the macroscopic and single-channel level. The synaptic membrane contained at least two distinct voltage-activated potassium currents with different kinetics and voltage sensitivity: an IA-like current with fast activation and inactivation kinetics and voltage-dependent steady-state inactivation; a complex delayed current that includes a slowly inactivating component, resembling the IK described in other preparations; and a noninactivating component. The IA-like current in these peptidergic boutons is not encoded by the gene Shaker, because it is not affected by null mutations at this locus. Rather, synaptic IA has properties similar to those of the Shal-encoded IA. Single-channel recordings revealed the presence in synaptic membranes of three different potassium channel types (A2, KD, KL), with biophysical properties that could account for the macroscopic currents and resemble those of the Shal, Shab, and Shaw channels described in heterologous expression systems and Drosophila neuronal somata. A2 channels (6-9 pS) have brief open times, and like the macroscopic IA they exhibited voltage-dependent steady-state inactivation and a rapidly inactivating ensemble average current profile. KD channels (13-16 pS) had longer open times, activate and inactivate with much slower kinetics, and may account for the slowly inactivating component of the macroscopic current. KL (44-54 pS) channels produced a noninactivating ensemble average and may contribute to the delayed macroscopic current observed.

Presynaptic recordings from Drosophila: correlation of macroscopic and single-channel K+ currents

We have performed direct electrophysiological recordings from Drosophila peptidergic synaptic boutons in situ, taking advantage of a mutation, ecdysone, which causes an increase in size of these terminals. Using patch-clamp techniques, we have analyzed voltage-dependent potassium currents at the macroscopic and single-channel level. The synaptic membrane contained at least two distinct voltage-activated potassium currents with different kinetics and voltage sensitivity: an IA-like current with fast activation and inactivation kinetics and voltage-dependent steady-state inactivation; a complex delayed current that includes a slowly inactivating component, resembling the IK described in other preparations; and a noninactivating component. The IA-like current in these peptidergic boutons is not encoded by the gene Shaker, because it is not affected by null mutations at this locus. Rather, synaptic IA has properties similar to those of the Shal-encoded IA. Single-channel recordings revealed the presence in synaptic membranes of three different potassium channel types (A2, KD, KL), with biophysical properties that could account for the macroscopic currents and resemble those of the Shal, Shab, and Shaw channels described in heterologous expression systems and Drosophila neuronal somata. A2 channels (6-9 pS) have brief open times, and like the macroscopic IA they exhibited voltage-dependent steady-state inactivation and a rapidly inactivating ensemble average current profile. KD channels (13-16 pS) had longer open times, activate and inactivate with much slower kinetics, and may account for the slowly inactivating component of the macroscopic current. KL (44-54 pS) channels produced a noninactivating ensemble average and may contribute to the delayed macroscopic current observed.

Isolation of mutations in the Drosophila homologues of the human Neurofibromatosis 2 and yeast CDC42 genes using a simple and efficient reverse-genetic method

Reverse genetic analysis in Drosophila has been greatly aided by a growing collection of lethal P transposable element insertions that provide molecular tags for the identification of essential genetic loci. However, because the screens performed to date primarily have generated autosomal P-element insertions, this collection has not been as useful for performing reverse genetic analysis of X-linked genes. We have designed a reverse genetic screen that takes advantage of the hemizygosity of the X chromosome in males together with a cosmid-based transgene that serves as an autosomally linked duplication of a small region of the X chromosome. The efficacy and efficiency of this method is demonstrated by the isolation of mutations in Drosophila homologues of two well-studied genes, the human Neurofibromatosis 2 tumor suppressor and the yeast CDC42 gene. The method we describe should be of general utility for the isolation of mutations in other X-linked genes, and should also provide an efficient method for the isolation of new allcles of existing X-linked or autosomal mutations in Drosophila.

Essential role for dlg in synaptic clustering of Shaker K+ channels in vivo

The assemblage of specific ion channels and receptors at synaptic sites is crucial for signaling between pre- and postsynaptic cells. However, the mechanisms by which proteins are targeted to and clustered at synapses are poorly understood. Here we show that the product of the Drosophila discs-large gene, DLG, is colocalized with Shaker K+ channels, which are clustered at glutamatergic synapses at the larval neuromuscular junction. In heterologous cells, DLG can cluster Shaker-type K+ channels, and, in the yeast two-hybrid system, the DLG PDZ1-2 domains bind directly to the C-terminal tail of Shaker proteins. We also demonstrate that DLG-Shaker interactions are required in vivo for Shaker clustering at the neuromuscular junction. Synaptic clustering of Shaker channels is abolished not only by mutations in dlg but also by a mutation in Shaker that deletes its C-terminal DLG binding motif. Analyses of various dlg mutant alleles suggest that channel clustering and synaptic targeting functions depend on distinct DLG domains. These studies demonstrate for the first time that DLG plays an important role in synaptic organization in vivo that correlates with its ability to bind directly to specific membrane proteins of the synapse.

The segment polarity gene porcupine encodes a putative multitransmembrane protein involved in Wingless processing

The Wnt protein Wingless (Wg) functions as a signal in patterning of both the Drosophila embryo and imaginal discs. Lack of porcupine (porc) activity is associated with mutant phenotypes similar to those of wg mutations. In porc mutant embryos, Wg protein is confined to the cells that produce it, suggesting that Porc plays a role in processing or secretion of Wg. porc encodes a novel transmembrane protein that appears to be concentrated at the endoplasmic reticulum. We present both genetic and in vitro evidence demonstrating that porc is involved specifically in the processing of Wg. We identified a human sequence related to Porc suggesting the existence of a family of proteins involved in processing of Wnts.

Molecular characterization of neurally expressing genes in the para sodium channel gene cluster of drosophila

To elucidate the mechanisms regulating expression of para, which encodes the major class of sodium channels in the Drosophila nervous system, we have tried to locate upstream cis-acting regulatory elements by mapping the transcriptional start site and analyzing the region immediately upstream of para in region 14D of the polytene chromosomes. From these studies, we have discovered that the region contains a cluster of neurally expressing genes. Here we report the molecular characterization of the genomic organization of the 14D region and the genes within this region, which are: calnexin (Cnx), actin related protein 14D (Arp 14D), calcineurin A 14D (CnnA14D), and chromosome associated protein (Cap). The tight clustering of these genes, their neuronal expression patterns, and their potential functions related to expression, modulation, or regulation of sodium channels raise the possibility that these genes represent a functionally related group sharing some coordinate regulatory mechanism.

Immunochemical characterization and developmental expression of Shaker potassium channels from the nervous system of Drosophila

We have raised antisera against recombinant peptides expressed from cDNAs fragments common to all splicing variants generated at the Shaker locus of Drosophila and used them as a tool to biochemically characterize these channel proteins. This antisera succeeded in detecting the expression of multiple Shaker potassium channels (Sh Kch), proteins with variable molecular mass (65-85 kDa) and pI (5.5-7). Additionally, for first time, specific Sh proteins of 40-45 kDa most probably corresponding to some of the so-called short Sh cDNAs previously isolated by others have been identified. Using genetic criteria, it has been determined that at least a good part of this variety of proteins is generated by alternative splicing. Developmental experiments show a double wave of Sh Kch channel expression with a first pick at the third instar larvae stage, a minimum at the beginning of puparation, and the highest plateau 36 h after hatching of adult flies. The pattern of Sh splice variants changes dramatically throughout development. A detergent-resistant fraction with about 50% of Sh Kch which seems to be anchored to submembranous structures has been found. Finally, other biochemical properties of Sh Kch, like membrane fractionation and glycosylation, are also described.

minibrain: a new protein kinase family involved in postembryonic neurogenesis in Drosophila

The development of the adult central nervous system of Drosophila requires a precise and reproducible pattern of neuroblast proliferation during postembryonic neurogenesis. We show here that mutations in the minibrain (mnb) gene cause an abnormal spacing of neuroblasts in the outer proliferation center (opc) of larval brain, with the implication that mnb opc neuroblasts produce less neuronal progeny than do wild type. As a consequence, the adult mnb brain exhibits a specific and marked size reduction of the optic lobes and central brain hemispheres. The insufficient number of distinct neurons in mnb brains is correlated with specific abnormalities in visual and olfactory behavior. The mnb gene encodes a novel, cell type-specific serine-threonine protein kinase family that is expressed and required in distinct neuroblast proliferation centers during postembryonic neurogenesis. The mnb kinases share extensive sequence similarities with kinases involved in the regulation of cell division.

Functional relationships between genes of the Shaker gene complex of Drosophila

Different mutations belonging to the HLI and HLII complementation groups of the haplolethal (HL) region of the Shaker complex (ShC) are described. The HLI complementation group includes viable (hdp), recessive lethals [l(1)1614], semidominant lethals [l(1)8384] and dominant lethals [l(1)5051, l(1)9916, l(1)13193], lack-of-function alleles that affect nervous system, cuticle and muscle development. The HLI complementation group encodes troponin I. HLII lack-of-function mutations [l(1)174 and l(1)4058] affect nervous system development. The semidominant lethal HLI mutation l(1)8384 shows differential complementation with other mutations in the ME and HL regions of ShC. Thus, heterozygous combinations of l(1)8384 with ME mutations l(1)162 and l(1)387 are poorly viable. The same phenomenon is observed for heterozygotes of l(1)8384 with HL mutations l(1)1199, l(1)2288 and l(1)3014. These specific interactions indicate the existence of functional relationships among the genetic elements of ShC. The implications for the understanding of the functional organization of ShC are discussed.

Functional interactions between the gene tetanic and the Shaker gene complex of Drosophila

Different phenotypes associated with the tetanic (tta) mutation such as appendage contraction, maternal effect and low viability and fertility are enhanced by one extra dose of the Shaker gene complex (ShC). The tta mutation is lethal with two extra doses of ShC. In addition, tta embryos have a defective nervous system. In this paper, I analyse the interaction between tta and ShC to gain insight into their relationship. Aneuploid analysis suggests that the lethality is due to an interaction of the tta mutation with the maternal effect (ME) region of this gene complex. Mutations in the ME region of ShC partially suppress this interaction. Trans-heterozygous combinations of MEI[l(1)305] and MEIII [l(1)459] mutations causes dominant lethality in a tta background. Trans-heterozygous combinations of an MEII[l(1)1359] mutation with the cited MEI and MEIII mutations are lethal in a tta background. Double mutant combinations and gene dosage experiments, suggest that tta also interacts with the viable (V) region of ShC. These specific genetic interactions indicate that tta and the ME and V regions of ShC are functionally related. These results, together with the previous electrophysiological, molecular and biochemical studies on these mutants suggest an interaction at the protein level. Thus, in the case of the V region, the tta gene product may modulate the activity of the K+ channels encoded in this region. Furthermore, the extreme dosage sensitivity of the interaction between tta and ShC suggests a stoichiometric requirement for the different gene products involved, which might be physically associated and form heteromultimers.

Frequenin--a novel calcium-binding protein that modulates synaptic efficacy in the Drosophila nervous system

The T(X;Y)V7 rearrangement in Drosophila has originally been recognized as a Shaker-like mutant because of its behavioral and electrophysiological phenotype. The gene whose expression is altered by the V7 rearrangement has been characterized. It encodes a novel Ca(2+)-binding protein named frequenin, which is related to recoverin and visinin. In vitro, the frequenin protein functions like recoverin as a Ca(2+)-sensitive guanylyl cyclase activator. Anti-frequenin antibodies stain the central and peripheral nervous system in Drosophila embryos and in larval and adult tissue sections. Frequenin appears to be particularly enriched in synapses, such as the motor nerve endings at neuromuscular junctions. Neuromuscular junctions of transgenic flies, which overexpress frequenin upon heat shock, exhibit an extraordinarily enhanced, frequency-dependent facilitation of neurotransmitter release, with properties identical to those observed in V7 junctions. We propose that frequenin represents a new element for the Ca(2+)-dependent modulation of synaptic efficacy.

Molecules and cognition: the latterday lessons of levels, language, and lac. Evolutionary overview of brain structure and function in some vertebrates and invertebrates

The characteristics of the nervous systems of a number of organisms in different phyla are examined at the recombinant DNA, protein, neuroanatomic, neurophysiological, and cognitive levels. Among the invertebrates, special attention is paid to the advantages as well as the shortcomings of the fly Drosophila melanogaster, the worm Caenorhabditis elegans, the honey bee Apis mellifera, the sea hare Aplysia californica, the octopus Octopus vulgaris, and the squid Loligo pealei. Among vertebrates, the focus is on Homo sapiens, the mouse Mus musculus, the rat Rattus norvegicus, the cat Felis catus, the macaque monkey Macaca fascicularis, the barn owl Tyto alba, and the zebrafish Brachydanio rerio. Vertebrate nervous systems have also been compared in fossil vs. extant organisms. I conclude that complex nervous systems arose in the Early Cambrian via a big bang that was underpinned by a modular method of construction involving massive pleiotropy of gene circuits. This rapidity of construction had enormous implications for the degrees of freedom that were subsequently available to evolving nervous systems. I also conclude that at the level of neuronal populations and interactions of neuropiles there is no model system between phyla except at the basic macromolecular level. Further, I argue that to achieve a significant understanding of the functions of extant nervous systems we need to concentrate on fewer organisms in greater depth and manipulate genomes via transgenic technologies to understand the behavioral outputs that are possible from an organism. Finally, I analyze the concepts of "perceptual categorization" and "information processing" and the difficulties involved in the extrapolation of computer analogies to sophisticated nervous systems.

Plasticity of motor nerve terminals in Drosophila T (X,Y)V7 mutant: effect of deregulation of the novel calcium-binding protein frequenin

The Drosophila T(X,Y)V7 mutant is characterized by abnormally large motor responses that build up upon repetitive stimulation. Genetically it is characterized by a chromosomal breakpoint located at the proximal end of the Shaker gene complex. This mutation affects a gene which encodes a novel calcium-binding protein: the frequenin. Since neuronal activity is known to affect neurite elongation we looked for the geometry of motor terminal arborization in this mutant. Our results show a significant reduction in number and length of motor terminal branches in mutants as compared to wild type. This observation is opposite to the effect of other hyperexcitable mutations such as Shaker or ether-a-gogo or Hyperkinetic. Thus the V7 phenotype cannot be interpreted as a result of changes in motoneuron firing pattern. According to results obtained on transformed larvae in which frequenin cDNA expression was under the control of a heat shock promoter, it appears that the morphological phenotype of V7 may be due to specific effects of deregulation of this calcium-binding protein.

Abnormal muscle development in the heldup3 mutant of Drosophila melanogaster is caused by a splicing defect affecting selected troponin I isoforms

The troponin I (TnI) gene of Drosophila melanogaster encodes a family of 10 isoforms resulting from the differential splicing of 13 exons. Four of these exons (6a1, 6a2, 6b1, and 6b2) are mutually exclusive and very similar in sequence. TnI isoforms show qualitative specificity whereby each muscle expresses a selected repertoire of them. In addition, TnI isoforms show quantitative specificity whereby each muscle expresses characteristic amounts of each isoform. In the mutant heldup3, the development of the thoracic muscles DLM, DVM, and TDT is aborted. The mutation consists of a one-nucleotide displacement of the 3' AG splice site at the intron preceding exon 6b1, resulting in the failure to produce all exon 6b1-containing TnI isoforms. These molecular changes in a constituent of the thin filaments cause the selective failure to develop the DLM, DVM, and TDT muscles while having no visible effect on other muscles wherein exon 6b1 expression is minor.

Abnormal muscle development in the heldup3 mutant of Drosophila melanogaster is caused by a splicing defect affecting selected troponin I isoforms

The troponin I (TnI) gene of Drosophila melanogaster encodes a family of 10 isoforms resulting from the differential splicing of 13 exons. Four of these exons (6a1, 6a2, 6b1, and 6b2) are mutually exclusive and very similar in sequence. TnI isoforms show qualitative specificity whereby each muscle expresses a selected repertoire of them. In addition, TnI isoforms show quantitative specificity whereby each muscle expresses characteristic amounts of each isoform. In the mutant heldup3, the development of the thoracic muscles DLM, DVM, and TDT is aborted. The mutation consists of a one-nucleotide displacement of the 3' AG splice site at the intron preceding exon 6b1, resulting in the failure to produce all exon 6b1-containing TnI isoforms. These molecular changes in a constituent of the thin filaments cause the selective failure to develop the DLM, DVM, and TDT muscles while having no visible effect on other muscles wherein exon 6b1 expression is minor.

Genetic and developmental analysis of polytene section 17 of the X chromosome of Drosophila melanogaster

Polytene section 17 of the X chromosome of Drosophila melanogaster, previously known to contain six putative lethal complementation groups important in oogenesis and embryogenesis, has here been further characterized genetically and developmentally. We constructed fcl+Y, a duplication of this region, which allowed us to conduct mutagenesis screens specific for the region and to perform complementation analyses (previously not possible). We recovered 67 new lethal mutations which defined 15 complementation groups within Df(1)N19 which deletes most of polytene section 17. The zygotic lethal phenotypes of these and preexisting mutations within polytene section 17 were examined, and their maternal requirements were analysed in homozygous germline clones using the dominant female sterile technique. We present evidence that an additional gene, which produces two developmentally regulated transcripts, is located in this region and is involved in embryogenesis, although no mutations in this gene were identified. In this interval of 37 to 43 polytene chromosome bands we have defined 17 genes, 12 (71%) of which are of significance to oogenesis or embryogenesis.

Genetic and molecular analysis of new female-specific lethal mutations at the gene Sxl of Drosophila melanogaster

We have isolated three female-specific lethal mutations at the gene Sex-lethal (Sxl): Sxlfb, Sxlfc and Sxlfd. We have carried out the complementation analysis between these mutations and other previously reported Sxlf mutations. It is possible to classify the alleles tested in this report into two complementation groups: the bc group defined by Sxlfb, and Sxlfc, and the LS group defined by SxlfLS. The other alleles tested affect both complementation groups albeit with different degrees. Contrary to what happens with mutations at the LS group, mutations at the bc group do not affect sex determination, nor late dosage compensation nor oogenesis. Both Sxlfb and Sxlfc present a DNA insertion of at least 5 kb between position -10 and -11 on the molecular map, within the fourth intron. On the contrary, Sxlfd, a strong mutation affecting all Sxl functions, is not associated to any detectable DNA alteration in Southern blots, so that it seems to be a "point" mutation. In agreement with their phenotypes, both Sxlfc/SxlfLS and Sxlfc homozygous female larvae express only the late Sxl transcripts characteristic of females, while females homozygous for SxlfLS express only the late Sxl transcripts characteristic of males. Moreover, Sxlfc presents a lethal synergistic interaction with mutations at either da or the X:A ratio, two signals that define the initial activity state of Sxl, while SxlfLS do not. These data suggest that the two complementation groups are related to the two sets of early and late Sxl transcripts, which are responsible for the early and late Sxl functions, respectively: Sxlfb and Sxlfc would affect the early functions and SxlfLS would affect the late Sxl functions.

Mutational analysis of the Drosophila miniature-dusky (m-dy) locus: effects on cell size and circadian rhythms

A mutational analysis has been performed to explore the function of the Drosophila melanogaster miniature-dusky (m-dy) locus. Mutations at this locus affect wing development, fertility and behavior. The genetic characterization of 13 different mutations suggests that m and dy variants are alleles of a single complex gene. All of these mutations alter wing size, apparently by reducing the volume of individual epidermal cells of the developing wing. In m mutants, epidermal cell boundaries persist in the mature wing, whereas they normally degenerate 1-2 hr after eclosion in wild-type or dy flies. This has permitted the direct visualization of cell size differences among several m mutants. Mutations at the m-dy locus also affect behavioral processes. Three out of nine dy alleles (dyn1, dyn3 and dyn4) lengthen the circadian period of the activity and eclosion rhythms by approximately 1.5 hr. In contrast, m mutants have normal circadian periods, but an abnormally large percentage of individuals express aperiodic bouts of activity. These behavior genetic studies also indicate that an existing "rhythm" mutation known as Andante is an allele of the m-dy locus. The differential effects of certain m-dy mutations on wing and behavioral phenotypes suggest that separable domains of function exist within this locus.

Nerve terminal excitability and neuromuscular transmission in T(X;Y)V7 and Shaker mutants of Drosophila melanogaster

We investigated the neuromuscular transmission in relation with genetic neuronal excitability changes in mutants T(X;Y)V7 and ShK,S133 of Drosophila. These mutations affect two different genes belonging to the Shaker gene complex which encode different yet functionally related proteins. Experiments were performed on neuromuscular junctions from Drosophila larvae by recording pre- and postsynaptic membrane currents using external electrodes. It was found that the neuromuscular electrophysiological phenotype of T(X;Y)V7 is caused by presynaptic membrane hyperexcitability probably in relation with a Ca2(+)-dependent down regulation of voltage dependent K channels. By contrast, the ShKS133 phenotype can be explained solely by action potential widening due to the absence of type-A K channels.

Troponin I is encoded in the haplolethal region of the Shaker gene complex of Drosophila

We have analyzed one of the nine complementation groups that constitute the haplolethal (HL) region of the Shaker gene complex (ShC). Five mutations, including a dominant lethal, define this complementation group: HL I. Mutant phenotypes show abnormal embryogenesis with structural defects in the nervous system and aberrant degeneration of specific adult muscles in addition to altered action potentials. HL I encodes a family of proteins with extensive homology to invertebrate troponin I (TnI). Members of this family are brought about by two alternative and two mutually exclusive exons in conjunction with two differential polyadenylation sites. Transcription analysis indicates that some isoforms are adult specific and others are synthesized throughout development, except during early metamorphosis. Certain isoforms of Drosophila TnI are expressed in specific muscles. The specificity of mutant phenotypes suggests a functional role of particular TnI isoforms in the development and the mature activity of muscle and nervous systems.