Analysis of Drosophila cGMP-dependent Protein Kinases and Assessment of Their in Vivo Roles by Targeted Expression in a Renal Transporting Epithelium

Characterizing the Protein Isoforms of foraging (for), the PKGI Ortholog in Drosophila melanogaster

The foraging (for) gene of Drosophila melanogaster encodes a cGMP-dependent protein kinase (PKG), which is a major effector of the cGMP signaling pathway involved in the regulation of behaviour and metabolic traits. Despite being well studied at the transcript level, little is known about the for gene at the protein level. Here, we provide a detailed characterization of the for gene protein (FOR) products and present new tools for their study, including five isoform-specific antibodies and a transgenic strain that carries an HA-labelled for allele (for^BAC::HA). Our results showed that multiple FOR isoforms were expressed in the larval and adult stages of D. melanogaster and that the majority of whole-body FOR expression arises from three (P1, P1α, and P3) of eight putative protein isoforms. We found that FOR expression differed between the larval and adult stages and between the dissected larval organs we analyzed, which included the central nervous system (CNS), fat body, carcass, and intestine. Moreover, we showed that the FOR expression differed between two allelic variants of the for gene, namely, for^s (sitter) and for^R (rover), that are known to differ in many food-related traits. Together, our in vivo identification of FOR isoforms and the existence of temporal, spatial, and genetic differences in their expression lay the groundwork for determining their functional significance.

Modulation of the NO-cGMP pathway has no effect on olfactory responses in the Drosophila antenna

Olfaction is a crucial sensory modality in insects and is underpinned by odor-sensitive sensory neurons expressing odorant receptors that function in the dendrites as odorant-gated ion channels. Along with expression, trafficking, and receptor complexing, the regulation of odorant receptor function is paramount to ensure the extraordinary sensory abilities of insects. However, the full extent of regulation of sensory neuron activity remains to be elucidated. For instance, our understanding of the intracellular effectors that mediate signaling pathways within antennal cells is incomplete within the context of olfaction in vivo. Here, with the use of optical and electrophysiological techniques in live antennal tissue, we investigate whether nitric oxide signaling occurs in the sensory periphery of Drosophila. To answer this, we first query antennal transcriptomic datasets to demonstrate the presence of nitric oxide signaling machinery in antennal tissue. Next, by applying various modulators of the NO-cGMP pathway in open antennal preparations, we show that olfactory responses are unaffected by a wide panel of NO-cGMP pathway inhibitors and activators over short and long timescales. We further examine the action of cAMP and cGMP, cyclic nucleotides previously linked to olfactory processes as intracellular potentiators of receptor functioning, and find that both long-term and short-term applications or microinjections of cGMP have no effect on olfactory responses in vivo as measured by calcium imaging and single sensillum recording. The absence of the effect of cGMP is shown in contrast to cAMP, which elicits increased responses when perfused shortly before olfactory responses in OSNs. Taken together, the apparent absence of nitric oxide signaling in olfactory neurons indicates that this gaseous messenger may play no role as a regulator of olfactory transduction in insects, though may play other physiological roles at the sensory periphery of the antenna.

cGMP signaling pathway that modulates NF-κB activation in innate immune responses

The nuclear factor-kappa B (NF-κB) pathway is an evolutionarily conserved signaling pathway that plays a central role in immune responses and inflammation. Here, we show that Drosophila NF-κB signaling is activated via a pathway in parallel with the Toll receptor by receptor-type guanylate cyclase, Gyc76C. Gyc76C produces cyclic guanosine monophosphate (cGMP) and modulates NF-κB signaling through the downstream Tollreceptor components dMyd88, Pelle, Tube, and Dif/Dorsal (NF-κB). The cGMP signaling pathway comprises a membrane-localized cGMP-dependent protein kinase (cGK) called DG2 and protein phosphatase 2A (PP2A) and is crucial for host survival against Gram-positive bacterial infections in Drosophila. A membrane-bound cGK, PRKG2, also modulates NF-κB activation via PP2A in human cells, indicating that modulation of NF-κB activation in innate immunity by the cGMP signaling pathway is evolutionarily conserved.

•

Drosophila NF-κB signaling is activated by Gyc76C in parallel with the Toll receptor

•

Gyc76C modulates NF-κB signaling through downstream Toll receptor components

•

In Drosophila, the pathway comprises a cGMP-dependent protein kinase (cGK) and PP2A

•

In human cells, a membrane-bound cGK, PRKG2, also modulates NF-κB signaling via PP2A

Expression of the foraging gene in adult Drosophila melanogaster

The foraging gene in Drosophila melanogaster, which encodes a cGMP-dependent protein kinase, is a highly conserved, complex gene with multiple pleiotropic behavioral and physiological functions in both the larval and adult fly. Adult foraging expression is less well characterized than in the larva. We characterized foraging expression in the brain, gastric system, and reproductive systems using a T2A-Gal4 gene-trap allele. In the brain, foraging expression appears to be restricted to multiple sub-types of glia. This glial-specific cellular localization of foraging was supported by single-cell transcriptomic atlases of the adult brain. foraging is extensively expressed in most cell types in the gastric and reproductive systems. We then mapped multiple cis-regulatory elements responsible for parts of the observed expression patterns by a nested cloned promoter-Gal4 analysis. The mapped cis-regulatory elements were consistently modular when comparing the larval and adult expression patterns. These new data using the T2A-Gal4 gene-trap and cloned foraging promoter fusion GAL4's are discussed with respect to previous work using an anti-FOR antibody, which we show here to be non-specific. Future studies of foraging's function will consider roles for glial subtypes and peripheral tissues (gastric and reproductive systems) in foraging's pleiotropic behavioral and physiological effects.

Genome of the pincer wasp Gonatopus flavifemur reveals unique venom evolution and a dual adaptation to parasitism and predation

Background

Hymenoptera comprise extremely diverse insect species with extensive variation in their life histories. The Dryinidae, a family of solitary wasps of Hymenoptera, have evolved innovations that allow them to hunt using venom and a pair of chelae developed from the fore legs that can grasp prey. Dryinidae larvae are also parasitoids of Auchenorrhyncha, a group including common pests such as planthoppers and leafhoppers. Both of these traits make them effective and valuable for pest control, but little is yet known about the genetic basis of its dual adaptation to parasitism and predation.

Results

We sequenced and assembled a high-quality genome of the dryinid wasp Gonatopus flavifemur, which at 636.5 Mb is larger than most hymenopterans. The expansion of transposable elements, especially DNA transposons, is a major contributor to the genome size enlargement. Our genome-wide screens reveal a number of positively selected genes and rapidly evolving proteins involved in energy production and motor activity, which may contribute to the predatory adaptation of dryinid wasp. We further show that three female-biased, reproductive-associated yellow genes, in response to the prey feeding behavior, are significantly elevated in adult females, which may facilitate the egg production. Venom is a powerful weapon for dryinid wasp during parasitism and predation. We therefore analyze the transcriptomes of venom glands and describe specific expansions in venom Idgf-like genes and neprilysin-like genes. Furthermore, we find the LWS2-opsin gene is exclusively expressed in male G. flavifemur, which may contribute to partner searching and mating.

Conclusions

Our results provide new insights into the genome evolution, predatory adaptation, venom evolution, and sex-biased genes in G. flavifemur, and present genomic resources for future in-depth comparative analyses of hymenopterans that may benefit pest control.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01081-6.

The Drosophila melanogaster foraging gene affects social networks

Drosophila melanogaster displays social behaviors including courtship, mating, aggression, and group foraging. Recent studies employed social network analyses (SNAs) to show that D. melanogaster strains differ in their group behavior, suggesting that genes influence social network phenotypes. Aside from genes associated with sensory function, few studies address the genetic underpinnings of these networks. The foraging gene (for) is a well-established example of a pleiotropic gene that regulates multiple behavioral phenotypes and their plasticity. In D. melanogaster, there are two naturally occurring alleles of for called rover and sitter that differ in their larval and adult food-search behavior as well as other behavioral phenotypes. Here, we hypothesize that for affects behavioral elements required to form social networks and the social networks themselves. These effects are evident when we manipulate gene dosage. We found that flies of the rover and sitter strains exhibit differences in duration, frequency, and reciprocity of pairwise interactions, and they form social networks with differences in assortativity and global efficiency. Consistent with other adult phenotypes influenced by for, rover-sitter heterozygotes show intermediate patterns of dominance in many of these characteristics. Multiple generations of backcrossing a rover allele into a sitter strain showed that many but not all of these rover-sitter differences may be attributed to allelic variation at for. Our findings reveal the significant role that for plays in affecting social network properties and their behavioral elements in Drosophila melanogaster.

The foraging Gene and Its Behavioral Effects: Pleiotropy and Plasticity

The Drosophila melanogaster foraging (for) gene is a well-established example of a gene with major effects on behavior and natural variation. This gene is best known for underlying the behavioral strategies of rover and sitter foraging larvae, having been mapped and named for this phenotype. Nevertheless, in the last three decades an extensive array of studies describing for's role as a modifier of behavior in a wide range of phenotypes, in both Drosophila and other organisms, has emerged. Furthermore, recent work reveals new insights into the genetic and molecular underpinnings of how for affects these phenotypes. In this article, we discuss the history of the for gene and its role in natural variation in behavior, plasticity, and behavioral pleiotropy, with special attention to recent findings on the molecular structure and transcriptional regulation of this gene.

The Mechanosensitive Ion Channel Piezo Inhibits Axon Regeneration

Neurons exhibit a limited ability of repair. Given that mechanical forces affect neuronal outgrowth, it is important to investigate whether mechanosensitive ion channels may regulate axon regeneration. Here, we show that DmPiezo, a Ca²⁺-permeable non-selective cation channel, functions as an intrinsic inhibitor for axon regeneration in Drosophila. DmPiezo activation during axon regeneration induces local Ca²⁺ transients at the growth cone, leading to activation of nitric oxide synthase and the downstream cGMP kinase Foraging or PKG to restrict axon regrowth. Loss of DmPiezo enhances axon regeneration of sensory neurons in the peripheral and CNS. Conditional knockout of its mammalian homolog Piezo1 in vivo accelerates regeneration, while its pharmacological activation in vitro modestly reduces regeneration, suggesting the role of Piezo in inhibiting regeneration may be evolutionarily conserved. These findings provide a precedent for the involvement of mechanosensitive channels in axon regeneration and add a potential target for modulating nervous system repair.

Epithelial Function in the Drosophila Malpighian Tubule: An In Vivo Renal Model

The insect renal (Malpighian) tubule has long been a model system for the study of fluid secretion and its neurohormonal control, as well as studies on ion transport mechanisms. To extend these studies beyond the boundaries of classical physiology, a molecular genetic approach together with the 'omics technologies is required. To achieve this in any vertebrate transporting epithelium remains a daunting task, as the genetic tools available are still relatively unsophisticated. Drosophila melanogaster, however, is an outstanding model organism for molecular genetics. Here we describe a technique for fluid secretion assays in the D. melanogaster equivalent of the kidney nephron. The development of this first physiological assay for a Drosophila epithelium, allowing combined approaches of integrative physiology and functional genomics, has now provided biologists with an entirely new model system, the Drosophila Malpighian tubule, which is utilized in multiple fields as diverse as kidney disease research and development of new modes of pest insect control.

Pleiotropy of the Drosophila melanogaster foraging gene on larval feeding-related traits

Little is known about the molecular underpinning of behavioral pleiotropy. The Drosophila melanogaster foraging gene is highly pleiotropic, affecting many independent larval and adult phenotypes. Included in foraging's multiple phenotypes are larval foraging path length, triglyceride levels, and food intake. foraging has a complex structure with four promoters and 21 transcripts that encode nine protein isoforms of a cGMP dependent protein kinase (PKG). We examined if foraging's complex molecular structure underlies the behavioral pleiotropy associated with this gene. Using a promotor analysis strategy, we cloned DNA fragments upstream of each of foraging's transcription start sites and generated four separate forpr-Gal4s. Supporting our hypothesis of modular function, they had discrete, restricted expression patterns throughout the larva. In the CNS, forpr1-Gal4 and forpr4-Gal4 were expressed in neurons while forpr2-Gal4 and forpr3-Gal4 were expressed in glia cells. In the gastric system, forpr1-Gal4 and forpr3-Gal4 were expressed in enteroendocrine cells of the midgut while forpr2-Gal4 was expressed in the stem cells of the midgut. forpr3-Gal4 was expressed in the midgut enterocytes, and midgut and hindgut visceral muscle. forpr4-Gal4's gastric system expression was restricted to the hindgut. We also found promoter specific expression in the larval fat body, salivary glands, and body muscle. The modularity of foraging's molecular structure was also apparent in the phenotypic rescues. We rescued larval path length, triglyceride levels (bordered on significance), and food intake of for0 null larvae using different forpr-Gal4s to drive UAS-forcDNA. In a foraging null genetic background, forpr1-Gal4 was the only promoter driven Gal4 to rescue larval path length, forpr3-Gal4 altered triglyceride levels, and forpr4-Gal4 rescued food intake. Our results refine the spatial expression responsible for foraging's associated phenotypes, as well as the sub-regions of the locus responsible for their expression. foraging's pleiotropy arises at least in part from the individual contributions of its four promoters.

An ant-plant mutualism through the lens of cGMP-dependent kinase genes

In plant-animal mutualisms, how an animal forages often determines how much benefit its plant partner receives. In many animals, foraging behaviour changes in response to foraging gene expression or activation of the cGMP-dependent protein kinase (PKG) that foraging encodes. Here, we show that this highly conserved molecular mechanism affects the outcome of a plant-animal mutualism. We studied the two PKG genes of Allomerus octoarticulatus, an Amazonian ant that defends the ant-plant Cordia nodosa against herbivores. Some ant colonies are better 'bodyguards' than others. Working in the field in Peru, we found that colonies fed with a PKG activator recruited more workers to attack herbivores than control colonies. This resulted in less herbivore damage. PKG gene expression in ant workers correlated with whether an ant colony discovered an herbivore and how much damage herbivores inflicted on leaves in a complex way; natural variation in expression levels of the two genes had significant interaction effects on ant behaviour and herbivory. Our results suggest a molecular basis for ant protection of plants in this mutualism.

The corticotropin-releasing factor-like diuretic hormone 44 (DH44) and kinin neuropeptides modulate desiccation and starvation tolerance in Drosophila melanogaster

Malpighian tubules are critical organs for epithelial fluid transport and stress tolerance in insects, and are under neuroendocrine control by multiple neuropeptides secreted by identified neurons. Here, we demonstrate roles for CRF-like diuretic hormone 44 (DH44) and Drosophila melanogaster kinin (Drome-kinin, DK) in desiccation and starvation tolerance. Gene expression and labelled DH44 ligand binding data, as well as highly selective knockdowns and/or neuronal ablations of DH44 in neurons of the pars intercerebralis and DH44 receptor (DH44-R2) in Malpighian tubule principal cells, indicate that suppression of DH44 signalling improves desiccation tolerance of the intact fly. Drome-kinin receptor, encoded by the leucokinin receptor gene, LKR, is expressed in DH44 neurons as well as in stellate cells of the Malpighian tubules. LKR knockdown in DH44-expressing neurons reduces Malpighian tubule-specific LKR, suggesting interactions between DH44 and LK signalling pathways. Finally, although a role for DK in desiccation tolerance was not defined, we demonstrate a novel role for Malpighian tubule cell-specific LKR in starvation tolerance. Starvation increases gene expression of epithelial LKR. Also, Malpighian tubule stellate cell-specific knockdown of LKR significantly reduced starvation tolerance, demonstrating a role for neuropeptide signalling during starvation stress.

Testing candidate genes for attention-deficit/hyperactivity disorder in fruit flies using a high throughput assay for complex behavior

Fruit flies are important model organisms for functional testing of candidate genes in multiple disciplines, including the study of human diseases. Here we use a high-throughput locomotor activity assay to test the response on activity behavior of gene disruption in Drosophila melanogaster. The aim was to investigate the impact of disruption of 14 candidate genes for human attention-deficit/hyperactivity disorder (ADHD) on fly behavior. By obtaining a range of correlated measures describing the space of variables for behavioral activity we show, that some mutants display similar phenotypic responses, and furthermore, that the genes disrupted in those mutants had common molecular functions; namely processes related to cGMP activity, cation channels and serotonin receptors. All but one of the candidate genes resulted in aberrant behavioral activity, suggesting involvement of these genes in behavioral activity in fruit flies. Results provide additional support for the investigated genes being risk candidate genes for ADHD in humans.

Cell signalling mechanisms for insect stress tolerance

Insects successfully occupy most environmental niches and this success depends on surviving a broad range of environmental stressors including temperature, desiccation, xenobiotic, osmotic and infection stress. Epithelial tissues play key roles as barriers between the external and internal environments and therefore maintain homeostasis and organismal tolerance to multiple stressors. As such, the crucial role of epithelia in organismal stress tolerance cannot be underestimated. At a molecular level, multiple cell-specific signalling pathways including cyclic cAMP, cyclic cGMP and calcium modulate tissue, and hence, organismal responses to stress. Thus, epithelial cell-specific signal transduction can be usefully studied to determine the molecular mechanisms of organismal stress tolerance in vivo. This review will explore cell signalling modulation of stress tolerance in insects by focusing on cell signalling in a fluid transporting epithelium--the Malpighian tubule. Manipulation of specific genes and signalling pathways in only defined tubule cell types can influence the survival outcome in response to multiple environmental stressors including desiccation, immune, salt (ionic) and oxidative stress, suggesting that studies in the genetic model Drosophila melanogaster may reveal novel pathways required for stress tolerance.

Examining the role of foraging and malvolio in host-finding behavior in the honey bee parasite, Varroa destructor (Anderson & Trueman)

When a female varroa mite, Varroa destructor (Anderson & Trueman), invades a honey bee brood cell, the physiology rapidly changes from feeding phoretic to reproductive. Changes in foraging and malvolio transcript levels in the brain have been associated with modulated intra-specific food searching behaviors in insects and other invertebrates. Transcription profiles for both genes were examined during and immediately following brood cell invasion to assess their role as potential control elements. Vdfor and Vdmvl transcripts were found in all organs of varroa mites with the highest Vdfor transcript levels in ovary-lyrate organs and the highest Vdmvl in Malpighian tubules. Changes in transcript levels of Vdfor and Vdmvl in synganglia were not associated with the cell invasion process, remaining comparable between early reproductive mites (collected from the pre-capping brood cells) and phoretic mites. However, Vdfor and Vdmvl transcript levels were lowered by 37 and 53%, respectively, in synganglia from reproductive mites compared to early reproductive mites, but not significantly different to levels in synganglia from phoretic mites. On the other hand, in whole body preparations the Vdfor and Vdmvl had significantly higher levels of transcript in reproductive mites compared to phoretic and early reproductive, mainly due to the presence of both transcripts accumulating in the eggs carried by the ovipositing mite. Varroa mites are a critical component for honey bee population decline and finding varroa mite genes associated with brood cell invasion, reproduction, ion balance and other physiological processes will facilitate development of novel control avenues for this honey bee parasite.

Data-mining the FlyAtlas online resource to identify core functional motifs across transporting epithelia

Background

Comparative analysis of tissue-specific transcriptomes is a powerful technique to uncover tissue functions. Our FlyAtlas.org provides authoritative gene expression levels for multiple tissues of Drosophila melanogaster (1). Although the main use of such resources is single gene lookup, there is the potential for powerful meta-analysis to address questions that could not easily be framed otherwise. Here, we illustrate the power of data-mining of FlyAtlas data by comparing epithelial transcriptomes to identify a core set of highly-expressed genes, across the four major epithelial tissues (salivary glands, Malpighian tubules, midgut and hindgut) of both adults and larvae.

Method

Parallel hypothesis-led and hypothesis-free approaches were adopted to identify core genes that underpin insect epithelial function. In the former, gene lists were created from transport processes identified in the literature, and their expression profiles mapped from the flyatlas.org online dataset. In the latter, gene enrichment lists were prepared for each epithelium, and genes (both transport related and unrelated) consistently enriched in transporting epithelia identified.

Results

A key set of transport genes, comprising V-ATPases, cation exchangers, aquaporins, potassium and chloride channels, and carbonic anhydrase, was found to be highly enriched across the epithelial tissues, compared with the whole fly. Additionally, a further set of genes that had not been predicted to have epithelial roles, were co-expressed with the core transporters, extending our view of what makes a transporting epithelium work. Further insights were obtained by studying the genes uniquely overexpressed in each epithelium; for example, the salivary gland expresses lipases, the midgut organic solute transporters, the tubules specialize for purine metabolism and the hindgut overexpresses still unknown genes.

Conclusion

Taken together, these data provide a unique insight into epithelial function in this key model insect, and a framework for comparison with other species. They also provide a methodology for function-led datamining of FlyAtlas.org and other multi-tissue expression datasets.

Receptor guanylyl cyclase Gyc76C is required for invagination, collective migration and lumen shape in the Drosophila embryonic salivary gland

The Drosophila embryonic salivary gland is formed by the invagination and collective migration of cells. Here, we report on a novel developmental role for receptor-type guanylyl cyclase at 76C, Gyc76C, in morphogenesis of the salivary gland. We demonstrate that Gyc76C and downstream cGMP-dependent protein kinase 1 (DG1) function in the gland and surrounding mesoderm to control invagination, collective migration and lumen shape. Loss of gyc76C resulted in glands that failed to invaginate, complete posterior migration and had branched lumens. Salivary gland migration defects of gyc76C mutant embryos were rescued by expression of wild-type gyc76C specifically in the gland or surrounding mesoderm, whereas invagination defects were rescued primarily by expression in the gland. In migrating salivary glands of gyc76C mutant embryos, integrin subunits localized normally to gland-mesoderm contact sites but talin localization in the surrounding circular visceral mesoderm and fat body was altered. The extracellular matrix protein, laminin, also failed to accumulate around the migrating salivary gland of gyc76C mutant embryos, and gyc76C and laminin genetically interacted in gland migration. Our studies suggest that gyc76C controls salivary gland invagination, collective migration and lumen shape, in part by regulating the localization of talin and the laminin matrix.

Signaling by Drosophila capa neuropeptides

The capa peptide family, originally identified in the tobacco hawk moth, Manduca sexta, is now known to be present in many insect families, with increasing publications on capa neuropeptides each year. The physiological actions of capa peptides vary depending on the insect species but capa peptides have key myomodulatory and osmoregulatory functions, depending on insect lifestyle, and life stage. Capa peptide signaling is thus critical for fluid homeostasis and survival, making study of this neuropeptide family attractive for novel routes for insect control. In Dipteran species, including the genetically tractable Drosophila melanogaster, capa peptide action is diuretic; via elevation of nitric oxide, cGMP and calcium in the principal cells of the Malpighian tubules. The identification of the capa receptor (capaR) in several insect species has shown this to be a canonical GPCR. In D. melanogaster, ligand-activated capaR activity occurs in a dose-dependent manner between 10(-6) and 10(-12)M. Lower concentrations of capa peptide do not activate capaR, either in adult or larval Malpighian tubules. Use of transgenic flies in which capaR is knocked-down in only Malpighian tubule principal cells demonstrates that capaR modulates tubule fluid secretion rates and in doing so, sets the organismal response to desiccation. Thus, capa regulates a desiccation-responsive pathway in D. melanogaster, linking its role in osmoregulation and fluid homeostasis to environmental response and survival. The conservation of capa action between some Dipteran species suggests that capa's role in desiccation tolerance may not be confined to D. melanogaster.

Mosquito protein kinase G phosphorylates flavivirus NS5 and alters flight behavior in Aedes aegypti and Anopheles gambiae

Many arboviral proteins are phosphorylated in infected mammalian cells, but it is unknown if the same phosphorylation events occur when insects are similarly infected. One of the mammalian kinases responsible for phosphorylation, protein kinase G (PKG), has been implicated in the behavior of multiple nonvector insects, but is unstudied in mosquitoes. PKG from Aedes aegypti was cloned, and phosphorylation of specific viral sites was monitored by mass spectrometry from biochemical and cell culture experiments. PKG from Aedes mosquitoes is able to phosphorylate dengue nonstructural protein 5 (NS5) at specific sites in cell culture and cell-free systems and autophosphorylates its own regulatory domain in a cell-free system. Injecting Aedes aegypti and Anopheles gambiae mosquitoes with a pharmacological PKG activator resulted in increased Aedes wing activity during periods of their natural diurnal/crepuscular activity and increased Anopheles nocturnal locomotor/flight activity. Thus, perturbation of the PKG signaling pathway in mosquitoes alters flight behavior. The demonstrated effect of PKG alterations is consistent with a viral PKG substrate triggering increased PKG activity. This increased PKG activity could be the mechanism by which dengue virus increases flight behavior and possibly facilitates transmission. Whether or not PKG is part of the mechanism by which dengue increases flight behavior, this report is the first to show PKG can modulate behavior in hematophagous disease vectors.

Inhibition of diuretic stimulation of an insect secretory epithelium by a cGMP-dependent protein kinase

The rate of urine secretion by insect Malpighian tubules (MTs) is regulated by multiple diuretic and antidiuretic hormones, often working either synergistically or antagonistically. In the Drosophila melanogaster MT, only diuretic factors have been reported. Two such agents are the biogenic amine tyramine (TA) and the peptide drosokinin (DK), both of which act on the stellate cells of the tubule to increase transepithelial chloride conductance. In the current study, TA and DK signaling was quantified by microelectrode recording of the transepithelial potential in isolated Drosophila MTs. Treatment of tubules with cGMP caused a significant reduction in the depolarizing responses to both TA and DK, while cAMP had no effect on these responses. To determine whether a specific cGMP-dependent protein kinase (PKG) was mediating this inhibition, PKG expression was knocked down by RNAi in either the principal cells or the stellate cells. Knockdown of Pkg21D in the stellate cells eliminated the modulation of TA and DK signaling. Knockdown of Pkg21D with a second RNAi construct also reduced the modulation of TA signaling. In contrast, knockdown of the expression of foraging or CG4839, which encodes a known and a putative PKG, respectively, had no effect. These data indicate that cGMP, acting through the Pkg21D gene product in the stellate cells, can inhibit signaling by the diuretic agents TA and DK. This represents a novel function for cGMP and PKG in the Drosophila MT and suggests the existence of an antidiuretic hormone in Drosophila.

Receptor-type guanylyl cyclase Gyc76C is required for development of the Drosophila embryonic somatic muscle

Guanylyl cyclases mediate a number of physiological processes, including smooth muscle function and axonal guidance. Here, we report a novel role for Drosophila receptor-type guanylyl cyclase at 76C, Gyc76C, in development of the embryonic somatic muscle. In embryos lacking function of Gyc76C or the downstream cGMP-dependent protein kinase (cGK), DG1, patterning of the somatic body wall muscles was abnormal with ventral and lateral muscle groups showing the most severe defects. In contrast, specification and elongation of the dorsal oblique and dorsal acute muscles of gyc76C mutant embryos was normal, and instead, these muscles showed defects in proper formation of the myotendinous junctions (MTJs). During MTJ formation in gyc76C and pkg21D mutant embryos, the βPS integrin subunit failed to localize to the MTJs and instead was found in discrete puncta within the myotubes. Tissue-specific rescue experiments showed that gyc76C function is required in the muscle for proper patterning and βPS integrin localization at the MTJ. These studies provide the first evidence for a requirement for Gyc76C and DG1 in Drosophila somatic muscle development, and suggest a role in transport and/or retention of integrin receptor subunits at the developing MTJs.

Immune and stress response 'cross-talk' in the Drosophila Malpighian tubule

The success of insects is in large part due to their ability to survive environmental stress, including heat, cold, and dehydration. Insects are also exposed to infection, osmotic or oxidative stress, and to xenobiotics or toxins. The molecular mechanisms of stress sensing and response have been widely investigated in mammalian cell lines, and the area of stress research is now so vast to be beyond the scope of a single review article. However, the mechanisms by which stress inputs to the organism are sensed and integrated at the tissue and cellular level are less well understood. Increasingly, common molecular events between immune and other stress responses are observed in vivo; and much of this work stems of efforts in insect molecular science and physiology. We describe here the current knowledge in the area of immune and stress signalling and response at the level of the organism, tissue and cell, focussing on a key epithelial tissue in insects, the Malpighian tubule, and drawing together the known pathways that modulate responses to different stress insults. The tubules are critical for insect survival and are increasingly implicated in responses to multiple and distinct stress inputs. Importantly, as tubule function is central to survival, they are potentially key targets for insect control, which will be facilitated by increased understanding of the complexities of stress signalling in the organism.

The nitric oxide-cyclic GMP pathway regulates FoxO and alters dopaminergic neuron survival in Drosophila

Activation of the forkhead box transcription factor FoxO is suggested to be involved in dopaminergic (DA) neurodegeneration in a Drosophila model of Parkinson's disease (PD), in which a PD gene product LRRK2 activates FoxO through phosphorylation. In the current study that combines Drosophila genetics and biochemical analysis, we show that cyclic guanosine monophosphate (cGMP)-dependent kinase II (cGKII) also phosphorylates FoxO at the same residue as LRRK2, and Drosophila orthologues of cGKII and LRRK2, DG2/For and dLRRK, respectively, enhance the neurotoxic activity of FoxO in an additive manner. Biochemical assays using mammalian cGKII and FoxO1 reveal that cGKII enhances the transcriptional activity of FoxO1 through phosphorylation of the FoxO1 S319 site in the same manner as LRRK2. A Drosophila FoxO mutant resistant to phosphorylation by DG2 and dLRRK (dFoxO S259A corresponding to human FoxO1 S319A) suppressed the neurotoxicity and improved motor dysfunction caused by co-expression of FoxO and DG2. Nitric oxide synthase (NOS) and soluble guanylyl cyclase (sGC) also increased FoxO's activity, whereas the administration of a NOS inhibitor L-NAME suppressed the loss of DA neurons in aged flies co-expressing FoxO and DG2. These results strongly suggest that the NO-FoxO axis contributes to DA neurodegeneration in LRRK2-linked PD.

Biochemical activity and multiple locations of particulate guanylate cyclase in Rhyacophila dorsalis acutidens (Insecta: Trichoptera) provide insights into the cGMP signalling pathway in Malpighian tubules

In insect renal physiology, cGMP and cAMP have important regulatory roles. In Drosophila melanogaster, considered a good model for molecular physiology studies, and in other insects, cGMP and cAMP act as signalling molecules in the Malpighian tubules (MTs). However, many questions related to cyclic nucleotide functions are unsolved in principal cells (PC) and stellate cells (SC), the two cell types that compose the MT. In PC, despite the large body of information available on soluble guanylate cyclase (sGC) in the cGMP pathway, the functional circuit of particulate guanylate cyclase (pGC) remains obscure. In SC, on the other side, the synthesis and physiological role of the cGMP are still unknown. Our biochemical data regarding the presence of cyclic nucleotides in the MTs of Rhyacophila dorsalis acutidens revealed a cGMP level above the 50%, in comparison with the cAMP. The specific activity values for the membrane-bound guanylate cyclase were also recorded, implying that, besides the sGC, pGC is a physiologically relevant source of cGMP in MTs. Cytochemical studies showed ultrastructurally that there was a great deal of pGC on the basolateral membranes of both the principal and stellate cells. In addition, pGC was also detected in the contact zone between the two cell types and in the apical microvillar region of the stellate cells bordering the tubule lumen. The pGC signal is so well represented in PC and, unexpectedly in SC of MTs, that it is possible to hypothesize the existence of still uncharacterized physiological processes regulated by the pGC-cGMP system.

cGK substrates

Signalling of cGK (cGMP-dependent protein kinases) are mediated through phosphorylation of specific substrates. Several substrates of cGKI and cGKII were identified meanwhile. Some cGKI substrates are specifically regulated by the cGKIalpha or the cGKIbeta isozyme. In various cells and tissues, different cGK substrates exist that are essential for the regulation of diverse functions comprising tissue contractility, cell motility, cell contact, cellular secretion, cell proliferation, and cell differentiation. On the molecular level, cGKI substrates fulfill various cellular functions regulating e.g. the intracellular calcium and potassium concentration, the calcium sensitivity, and the organisation of the intracellular cytoskeleton. cGKII substrates are involved e.g. in chloride transport, sodium/proton transport and transcriptional regulation. The understanding of cGK signalling and function depends strongly on the identification of further specific substrates. In the last years, diverse approaches ranging from biochemistry to genetic deletion lead to the identification and establishment of several substrates, which raised new insights in the molecular mechanisms of cGK functions and elucidated new cellular cGK functions. However, the analysis of the dynamic signalling of cGK in tissues and cells will be necessary to discover new signalling pathways and substrates.

Regulation of a Drosophila melanogaster cGMP-specific phosphodiesterase by prenylation and interaction with a prenyl-binding protein

Post-translational modification by isoprenylation is a pivotal process for the correct functioning of many signalling proteins. The Drosophila melanogaster cGMP-PDE (cGMP-specific phosphodiesterase) DmPDE5/6 possesses a CaaX-box prenylation signal motif, as do several novel cGMP-PDEs from insect and echinoid species (in CaaX, C is cysteine, a is an aliphatic amino acid and X is 'any' amino acid). DmPDE5/6 is prenylated in vivo at Cys(1128) and is localized to the plasma membrane when expressed in Drosophila S2 cells. Site-directed mutagenesis of the prenylated cysteine residue (C1128S-DmPDE5/6), pharmacological inhibition of prenylation or co-expression of DmPrBP (Drosophila prenyl-binding protein)/delta each alters the subcellular localization of DmPDE5/6. Thus prenylation constitutes a critical post-translational modification of DmPDE5/6 for membrane targeting. Co-immunoprecipitation and subcellular-fractionation experiments have shown that DmPDE5/6 interacts with DmPrBP/delta in Drosophila S2 cells. Transgenic lines allow targeted expression of tagged prenylation-deficient C1128S-DmPDE5/6 in Type I (principal) cells in Drosophila Malpighian tubules, an in vivo model for DmPDE5/6 function. In contrast with wild-type DmPDE5/6, which was exclusively associated with the apical membrane, the C1128S-DmPDE5/6 mutant form was located primarily in the cytosol, although some residual association occurred at the apical membrane. Despite the profound change in intracellular localization of C1128S-DmPDE5/6, active transport of cGMP is affected in the same way as it is by DmPDE5/6. This suggests that, in addition to prenylation and interaction with DmPrBP/delta, further functional membrane-targeting signals exist within DmPDE5/6.

Increased expression of a cGMP-dependent protein kinase in rotation-adapted western corn rootworm (Diabrotica virgifera virgifera L.)

A new 'variant' behavior in western corn rootworm (WCR) has resulted in egg-laying into non-cornfields, compared to 'normal' deposition of eggs in cornfields, allowing these insects to circumvent crop rotation. No morphological or genetic characteristics have been defined to differentiate between the normal and variant biotypes. Cyclic GMP-dependent protein kinases (PKG) have been implicated in the regulation of behaviors in vertebrates, insects, and nematodes, including foraging behavior in Drosophila. A cDNA with homology to the Drosophila melanogaster foraging gene (called Dvfor1) was cloned from WCR. The deduced DvFOR1 protein is approximately 70% similar to FOR proteins in Drosophila, silkworm (Bombyx mori) and honeybee (Apis mellifera). It contains a coiled-coil region, two tandem cyclic nucleotide-binding domains, a serine/threonine kinase catalytic domain, and a serine/threonine kinase catalytic domain extension, which are all characteristically found in PKG proteins. Real-time PCR assays of foraging transcript levels in heads of normal and rotation adapted females of WCR obtained from lab-reared insect colonies indicated that the variants had higher levels (25%) of PKG expression than normals. The magnitude of this increase is similar to that observed in Drosophila rover phenotypes compared to sitter phenotypes. However, Diabrotica contains at least two different foraging gene transcripts, which complicates establishing a direct link between the level of gene expression and insect behavior.

A new role for a classical gene: white transports cyclic GMP

Guanosine 3'-5' cyclic monophosphate (cGMP) and adenosine 3'-5' cyclic monophosphate (cAMP) are important regulators of cell and tissue function. However, cGMP and cAMP transport have received relatively limited attention, especially in model organisms where such studies can be conducted in vivo. The Drosophila Malpighian (renal) tubule transports cGMP and cAMP and utilises these as signalling molecules. We show here via substrate competition and drug inhibition studies that cAMP transport - but not cGMP transport - requires the presence of di- or tri-carboxylates; and that transport of both cyclic nucleotides occurs via ATP binding cassette sub-family G2 (ABCG2), but not via ABC sub-family C (ABCC), transporters. In Drosophila, the white (w) gene is known for the classic eye colour mutation. However, gene expression data show that of all adult tissues, w is most highly expressed in Malpighian tubules. Furthermore, as White is a member of the ABCG2 transporter class, it is a potential candidate for a tubule cGMP transporter. Assay of cGMP transport in w(-) (mutant) tubules shows that w is required for cGMP transport but not cAMP transport. Targeted over-expression of w in w(-) tubule principal cells significantly increases cGMP transport compared with that in w(-) controls. Conversely, treatment of wild-type tubules with cGMP increases w mRNA expression levels, implying that cGMP is a physiologically relevant substrate for White. Immunocytochemical localisation reveals that White is expressed in intracellular vesicles in tubule principal cells, suggesting that White participates in vesicular transepithelial transport of cGMP.

Integrative physiology, functional genomics and the phenotype gap: a guide for comparative physiologists

Classical, curiosity-led comparative physiology finds itself at a crossroads. Major funding for classical physiology is becoming harder to find, as grant agencies focus on more molecular approaches or on science with more immediate strategic value to their respective countries. In turn, this shift in funding places Zoology and Animal Science departments under enormous stress: student numbers are buoyant, but how can research funding be maintained at high levels? Our research group has argued for the redefinition of integrative physiology as the investigation of gene function in an organotypic context in the intact animal. Implicit in this definition is the use of transgenics and reverse genetics to manipulate gene function in a cell-specific manner; this in turn implies the use of a genetically tractable 'model organism'. The significance of this definition is that it aligns integrative physiology with functional genomics. Again, functional genomics draws heavily on reverse genetics to elucidate the function of novel genes. The phenotype gap (the mismatch between what a genetic model organism's genome encodes and the reasons that it has historically been studied) emphasises the need to attract and empower functional biologists: can all 13,500 genes in Drosophila really be explained in terms of developmental biology? So, by embracing the integrative physiology manifesto, comparative physiologists can not only accelerate their own research, but their functional skills can make them indispensable in the post-genomic endeavour.

Theforaging gene ofDrosophila melanogaster: Spatial-expression analysis and sucrose responsiveness

The ability to identify and respond to food is essential for survival, yet little is known about the neural substrates that regulate natural variation in food-related traits. The foraging (for) gene in Drosophila melanogaster encodes a cGMP-dependent protein kinase (PKG) and has been shown to function in food-related traits. To investigate the tissue distribution of FOR protein, we generated an antibody against a common region of the FOR isoforms. In the adult brain we localized FOR to neuronal clusters and projections including neurons that project to the central complex, a cluster within the dorsoposterior region of the brain hemispheres, a separate cluster medial to optic lobes and lateral to brain hemispheres, a broadly distributed frontal-brain cluster, axon bundles of the antennal nerve and of certain subesophageal-ganglion nerves, and the medulla optic lobe. These newly described tissue distribution patterns of FOR protein provide candidate neural clusters and brain regions for investigation of neural networks that govern foraging-related traits. To determine whether FOR has a behavioral function in neurons we expressed UAS-for in neurons using an elav-gal4 driver and measured the effect on adult sucrose responsiveness (SR), known to be higher in rovers than sitters, the two natural variants of foraging. We found that pan-neuronal expression of for caused an increase in the SR of sitters, demonstrating a neural function for PKG in this food-related behavior.

cGMP signalling in a transporting epithelium

The biochemical aspects of cGMP signalling are well known, although in vivo roles of cGMP have only been recently discovered through work in genetic model organisms. The Drosophila melanogaster Malpighian (renal) tubule has been used to address the roles of cGMP in epithelial function. Here, we describe some of this work and outline recent progress in understanding the organotypic function of novel phosphodiesterases encoded by the D. melanogaster genome.

Novel subcellular locations and functions for secretory pathway Ca2+/Mn2+-ATPases

Secretory pathway Ca2+/Mn2+-ATPases (SPCAs) are important for maintenance of cellular Ca2+and Mn2+homeostasis, and, to date, all SPCAs have been found to localize to the Golgi apparatus. The single Drosophila SPCA gene ( SPoCk) was identified by an in silico screen for novel Ca2+-ATPases. It encoded three SPoCk isoforms with novel, distinct subcellular specificities in the endoplasmic reticulum (ER) and peroxisomes in addition to the Golgi. Furthermore, expression of the peroxisome-associated SPoCk isoform was sexually dimorphic. Overexpression of organelle-specific SPoCk isoforms impacted on cytosolic Ca2+handling in both cultured Drosophila cells and a transporting epithelium, the Drosophila Malpighian (renal) tubule. Specifically, the ER isoform impacted on inositol ( 1 , 4 , 5 )-trisphosphate-mediated Ca2+signaling and the Golgi isoform impacted on diuresis, whereas the peroxisome isoform colocalized with Ca2+“spherites” and impacted on calcium storage and transport. Interfering RNA directed against the common exons of the three SPoCk isoforms resulted in aberrant Ca2+signaling and abolished neuropeptide-stimulated diuresis by the tubule. SPoCk thus contributed to both of the contrasting requirements for Ca2+in transporting epithelia: to transport or store Ca2+in bulk without compromising its use as a signal.

A novel role for a Drosophila homologue of cGMP-specific phosphodiesterase in the active transport of cGMP

cGMP was first discovered in urine, demonstrating that kidney cells extrude this cyclic nucleotide. Drosophila Malpighian tubules provide a model renal system in which a homologue of mammalian PDE (phosphodiesterase) 6 is expressed. In humans, this cG-PDE (cGMP-specific PDE) is specifically expressed in the retinal system, where it controls visual signal transduction. In order to gain insight into the functional role of DmPDE6 (Drosophila PDE6-like enzyme) in epithelial function, we generated transgenic animals with targeted expression of DmPDE6 to tubule Type I (principal) cells. This revealed localization of DmPDE6 primarily at the apical membranes. As expected, overexpression of DmPDE6 resulted in elevated cG-PDE activity and decreased tubule cGMP content. However, such targeted overexpression of DmPDE6 creates a novel phenotype that manifests itself in inhibition of the active transport and efflux of cGMP by tubules. This effect is specific to DmPDE6 action, as no effect on cGMP transport is observed in tubules from a bovine PDE5 transgenic line which display reduced rates of fluid secretion, an effect not seen in DmPDE6 transgenic animals. Specific ablation of DmPDE6 in tubule principal cells, via expression of a targeted DmPDE6 RNAi (RNA interference) transgene, conferred increased active transport of cGMP, confirming a direct role for DmPDE6 in regulating cGMP transport in tubule principal cells. Pharmacological inhibition of DmPDE6 in wild-type tubules using the cG-PDE inhibitor, zaprinast, similarly results in stimulated cGMP transport. We provide the first demonstration of a novel role for a cG-PDE in modulating cGMP transport and efflux.

Function of cGMP-Dependent Protein Kinases as Revealed by Gene Deletion

Over the past few years, a wealth of biochemical and functional data have been gathered on mammalian cGMP-dependent protein kinases (cGKs). In mammals, three different kinases are encoded by two genes. Mutant and chimeric cGK proteins generated by molecular biology techniques yielded important biochemical knowledge, such as the function of the NH2-terminal domains of cGKI and cGKII, the identity of the cGMP-binding sites of cGKI, and the substrate specificity of the enzymes. Genetic approaches have proven especially useful for the analysis of the biological functions of cGKs. Recently, some of the in vivo targets and mechanisms leading to changes in neuronal adaptation, smooth muscle relaxation and growth, intestinal water secretion, bone growth, renin secretion, and other important functions have been identified. These data show that cGKs are signaling molecules involved in many biological functions.

Multigene family encoding 3',5'-cyclic-GMP-dependent protein kinases in Paramecium tetraurelia cells

In the ciliate Paramecium tetraurelia, 3',5'-cyclic GMP (cGMP) is one of the second messengers involved in several signal transduction pathways. The enzymes for its production and degradation are well established for these cells, whereas less is known about the potential effector proteins. On the basis of a current Paramecium genome project, we have identified a multigene family with at least 35 members, all of which encode cGMP-dependent protein kinases (PKGs). They can be classified into 16 subfamilies with several members each. Two of the genes, PKG1-1 and PKG2-1, were analyzed in more detail after molecular cloning. They encode monomeric enzymes of 770 and 819 amino acids, respectively, whose overall domain organization resembles that in higher eukaryotes. The enzymes contain a regulatory domain of two tandem cyclic nucleotide-binding sites flanked by an amino-terminal region for intracellular localization and a catalytic domain with highly conserved regions for ATP binding and catalysis. However, some Paramecium PKGs show a different structure. In Western blots, PKGs are detected both as cytosolic and as structure-bound forms. Immunofluorescence labeling shows enrichment in the cell cortex, notably around the dense-core secretory vesicles (trichocysts), as well as in cilia. Immunogold electron microscopy analysis reveals consistent labeling of ciliary membranes, of the membrane complex composed of cell membrane and cortical Ca2+ stores, and of regions adjacent to ciliary basal bodies, trichocysts, and trafficking vesicles. Since PKGs (re)phosphorylate the exocytosis-sensitive phosphoprotein pp63/pf upon stimulation, the role of PKGs during stimulated exocytosis is discussed, in addition to a role in ciliary beat regulation.

Signalling via cGMP: lessons from Drosophila

Guanosine 3', 5'-cyclic monophosphate (cGMP) signalling has received increasing attention over the last decade, since the discovery of the gaseous signalling molecule, nitric oxide, which activates cGMP synthesis. Furthermore, research into cGMP signalling has also been stimulated by the development of Viagra and pharmacologically active related compounds, which act to prevent cGMP breakdown. While much is known about the biochemical aspects of components of the cGMP signalling pathway, the precise in vivo roles of such components have only recently come to light through work in model organisms. This review outlines recent work utilising the genetic model organism Drosophila melanogaster in studies of organotypic cGMP signalling. While organisms such as Drosophila may not be the obvious choice for such studies, use of this model has proved that unique and detailed insights for cGMP signalling can be achieved.

Cyclic nucleotide phosphodiesterases in Drosophila melanogaster

Cyclic nucleotide PDEs (phosphodiesterases) are important enzymes that regulate intracellular levels of cAMP and cGMP. In the present study, we identify and characterize novel PDEs in the genetic model, Drosophila melanogaster. The Drosophila genome encodes five novel PDE genes in addition to dunce. Predicted PDE sequences of Drosophila show highly conserved critical domains when compared with human PDEs. Thus PDE-encoding genes of D. melanogaster are CG14940-PDE1C, CG8279-PDE6beta, CG5411-PDE8A, CG32648-PDE9 and CG10231-PDE11. Reverse transcriptase-PCRs of adult tissues reveal widespread expression of PDE genes. Drosophila Malpighian (renal) tubules express all the six PDEs: Drosophila PDE1, dunce (PDE4), PDE6, PDE8, PDE9 and PDE11. Antipeptide antibodies were raised against PDE1, PDE6, PDE9 and PDE11. Verification of antibody specificity by Western blotting of cloned and expressed PDE constructs allowed the immunoprecipitation studies of adult Drosophila lysates. Biochemical characterization of immunoprecipitated endogenous PDEs showed that PDE1 is a dual-specificity PDE (Michaelis constant Km for cGMP: 15.3+/-1 microM; Km cAMP: 20.5+/-1.5 microM), PDE6 is a cGMP-specific PDE (Km cGMP: 37+/-13 microM) and PDE11 is a dual-specificity PDE (Km cGMP: 6+/-2 microM; Km cAMP: 18.5+/-5.5 microM). Drosophila PDE1, PDE6 and PDE11 display sensitivity to vertebrate PDE inhibitors, zaprinast (IC50 was 71+/-39 microM for PDE1, 0.65+/-0.015 microM for PDE6 and 1.6+/-0.5 microM for PDE11) and sildenafil (IC50 was 1.3+/-0.9 microM for PDE1, 0.025+/-0.005 microM for PDE6 and 0.12+/-0.06 microM for PDE11). We provide the first characterization of a cGMP-specific PDE and two dual-specificity PDEs in Drosophila, and show a high degree of similarity in structure and function between human and Drosophila PDEs.

Function-informed transcriptome analysis of Drosophila renal tubule

Analysis of the transcriptome of the Drosophila melanogaster Malpighian (renal) tubule gives a radically new view of the function of the tubule, emphasising solute transport rather than fluid secretion.

Background

Comprehensive, tissue-specific, microarray analysis is a potent tool for the identification of tightly defined expression patterns that might be missed in whole-organism scans. We applied such an analysis to Drosophila melanogaster Malpighian (renal) tubule, a defined differentiated tissue.

Results

The transcriptome of the D. melanogaster Malpighian tubule is highly reproducible and significantly different from that obtained from whole-organism arrays. More than 200 genes are more than 10-fold enriched and over 1,000 are significantly enriched. Of the top 200 genes, only 18 have previously been named, and only 45% have even estimates of function. In addition, 30 transcription factors, not previously implicated in tubule development, are shown to be enriched in adult tubule, and their expression patterns respect precisely the domains and cell types previously identified by enhancer trapping. Of Drosophila genes with close human disease homologs, 50 are enriched threefold or more, and eight enriched 10-fold or more, in tubule. Intriguingly, several of these diseases have human renal phenotypes, implying close conservation of renal function across 400 million years of divergent evolution.

Conclusions

From those genes that are identifiable, a radically new view of the function of the tubule, emphasizing solute transport rather than fluid secretion, can be obtained. The results illustrate the phenotype gap: historically, the effort expended on a model organism has tended to concentrate on a relatively small set of processes, rather than on the spread of genes in the genome.