The Drosophila dopamine 2-like receptor D2R (Dop2R) is required in the blood brain barrier for male courtship

The blood brain barrier (BBB) has the essential function to protect the brain from potentially hazardous molecules while also enabling controlled selective uptake. How these processes and signaling inside BBB cells control neuronal function is an intense area of interest. Signaling in the adult Drosophila BBB is required for normal male courtship behavior and relies on male-specific molecules in the BBB. Here we show that the dopamine receptor D2R is expressed in the BBB and is required in mature males for normal mating behavior. Conditional adult male knockdown of D2R in BBB cells causes courtship defects. The courtship defects observed in genetic D2R mutants can be rescued by expression of normal D2R specifically in the BBB of adult males. Drosophila BBB cells are glial cells. Our findings thus identify a specific glial function for the DR2 receptor and dopamine signaling in the regulation of a complex behavior.

Mate Choice: Should I Mate or Should I Go?

A mature virgin female fruit fly will initially resist copulation, while she assesses the desirability of her suitor. A new study identifies a neural circuit that controls rejection and shows how it changes from rejection to acceptance and copulation.

High functional conservation of takeout family members in a courtship model system

takeout (to) is one of the male-specific genes expressed in the fat body that regulate male courtship behavior, and has been shown to act as a secreted protein in conjunction with courtship circuits. There are 23 takeout family members in Drosophila melanogaster, and homologues of this family are distributed across insect species. Sequence conservation among family members is low. Here we test the functional conservation of takeout family members by examining whether they can rescue the takeout courtship defect. We find that despite their sequence divergence takeout members from Aedes aegypti and Epiphas postvittana, as well as family members from D. melanogaster can substitute for takeout in courtship, demonstrating their functional conservation. Making use of the known E. postvittana Takeout structure, we used homology modeling and amphipathic helix analysis and found high overall structural conservation, including high conservation of the structure and amphipathic lining of an internal cavity that has been shown to accommodate hydrophobic ligands. Together these data suggest a high degree of structural conservation that likely underlies functional conservation in courtship. In addition, we have identified a role for a conserved exposed protein motif important for the protein’s role in courtship.

Juvenile Hormone Is Required in Adult Males for Drosophila Courtship

Juvenile Hormone (JH) has a prominent role in the regulation of insect development. Much less is known about its roles in adults, although functions in reproductive maturation have been described. In adult females, JH has been shown to regulate egg maturation and mating. To examine a role for JH in male reproductive behavior we created males with reduced levels of Juvenile Hormone Acid O-Methyl Transferase (JHAMT) and tested them for courtship. JHAMT regulates the last step of JH biosynthesis in the Corpora Allata (CA), the organ of JH synthesis. Males with reduced levels of JHAMT showed a reduction in courtship that could be rescued by application of Methoprene, a JH analog, shortly before the courtship assays were performed. In agreement with this, reducing JHAMT conditionally in mature flies led to courtship defects that were rescuable by Methoprene. The same result was also observed when the CA were conditionally ablated by the expression of a cellular toxin. Our findings demonstrate that JH plays an important physiological role in the regulation of male mating behavior.

Multiple pathways mediate the sex-peptide-regulated switch in female Drosophila reproductive behaviours

Male-derived sex-peptide (SP) induces profound changes in the behaviour of Drosophila females, resulting in decreased receptivity to further mating and increased egg laying. SP can mediate the switch in female reproductive behaviours via a G protein-coupled receptor, SPR, in neurons expressing fruitless, doublesex and pickpocket. Whether SPR is the sole receptor and whether SP induces the postmating switch in a single pathway has not, to our knowledge been tested. Here we report that the SP response can be induced in the absence of SPR when SP is ectopically expressed in neurons or when SP, transferred by mating, can access neurons through a leaky blood brain barrier. Membrane-tethered SP can induce oviposition via doublesex, but not fruitless and pickpocket neurons in SPR mutant females. Although pickpocket and doublesex neurons rely on G(o) signalling to reduce receptivity and induce oviposition, G(o) signalling in fruitless neurons is required only to induce oviposition, but not to reduce receptivity. Our results show that SP's action in reducing receptivity and inducing oviposition can be separated in fruitless and doublesex neurons. Hence, the SP-induced postmating switch incorporates shared, but also distinct circuitry of fruitless, doublesex and pickpocket neurons and additional receptors.

Sex-specific signaling in the blood-brain barrier is required for male courtship in Drosophila

Soluble circulating proteins play an important role in the regulation of mating behavior in Drosophila melanogaster. However, how these factors signal through the blood–brain barrier (bbb) to interact with the sex-specific brain circuits that control courtship is unknown. Here we show that male identity of the blood–brain barrier is necessary and that male-specific factors in the bbb are physiologically required for normal male courtship behavior. Feminization of the bbb of adult males significantly reduces male courtship. We show that the bbb–specific G-protein coupled receptor moody and bbb–specific Go signaling in adult males are necessary for normal courtship. These data identify sex-specific factors and signaling processes in the bbb as important regulators of male mating behavior.

Complex behaviors such as mating behavior are controlled by the brain. Ensembles of brain cells work in networks to ensure proper behavior at the right time. While the state of these cells plays an important role in whether and how the behavior is displayed, information from outside the brain is also required. Often, this information is provided by hormones that are present in the circulating fluid (such as the blood). However, the brain is protected by a layer of very tight cells, the so-called blood–brain barrier, that keeps unwanted molecules out. So how then do hormones and other regulatory factors “talk” to the brain? We are studying this question by examining the mating behavior of males of a model organism, the fruit fly Drosophila melanogaster. We have found that the blood–brain barrier cells themselves contain male-specific molecules that play an important role. When they are absent, courtship behavior is compromised. We have also identified how outside factors talk to the brain: by using a cellular signaling protein and a particular signaling pathway. Together they are well suited to pass on outside information to the brain network that regulates mating behavior.

Systems behavior: of male courtship, the nervous system and beyond in Drosophila

Male courtship in fruit flies is regulated by the same major regulatory genes that also determine general sexual differentiation of the animal. Elaborate genetics has given us insight into the roles of these master genes. These findings have suggested two separate and independent pathways for the regulation of sexual behavior and other aspects of sexual differentiation. Only recently have molecular studies started to look at the downstream effector genes and how they might control sex-specific behavior. These studies have confirmed the essential role of the previously identified male specific products of the fruitless gene in the neuronal circuits in which it is expressed. But there is increasing evidence that a number of non-neuronal tissues and pathways play a pivotal role in modulating this circuit and assuring efficient courtship.

The roles of fruitless and doublesex in the control of male courtship

Male courtship in Drosophila melanogaster is a robust innate behavior that is shaped by sensory input and experience. It is regulated by the general sex-determination pathway through the sex-specific forms of fruitless and doublesex. Recent findings have shown that both fruitless and doublesex are required for courtship. This chapter reviews the role of these proteins and the neurons that express them in the regulation of courtship behavior. In particular it discusses how doublesex and fruitless contribute to the generation of sexually dimorphic neurons, the role of cell death, and the emerging information about circuits that underlie the behavior.

A functional genomics strategy reveals clockwork orange as a transcriptional regulator in the Drosophila circadian clock

The Drosophila circadian clock consists of integrated autoregulatory feedback loops, making the clock difficult to elucidate without comprehensively identifying the network components in vivo. Previous studies have adopted genome-wide screening for clock-controlled genes using high-density oligonucleotide arrays that identified hundreds of clock-controlled genes. In an attempt to identify the core clock genes among these candidates, we applied genome-wide functional screening using an RNA interference (RNAi) system in vivo. Here we report the identification of novel clock gene candidates including clockwork orange (cwo), a transcriptional repressor belonging to the basic helix-loop-helix ORANGE family. cwo is rhythmically expressed and directly regulated by CLK-CYC through canonical E-box sequences. A genome-wide search for its target genes using the Drosophila genome tiling array revealed that cwo forms its own negative feedback loop and directly suppresses the expression of other clock genes through the E-box sequence. Furthermore, this negative transcriptional feedback loop contributes to sustaining a high-amplitude circadian oscillation in vivo. Based on these results, we propose that the competition between cyclic CLK-CYC activity and the adjustable threshold imposed by CWO keeps E-box-mediated transcription within the controllable range of its activity, thereby rendering a Drosophila circadian clock capable of generating high-amplitude oscillation.

A role for the adult fat body in Drosophila male courtship behavior

Mating behavior in Drosophila depends critically on the sexual identity of specific regions in the brain, but several studies have identified courtship genes that express products only outside the nervous system. Although these genes are each active in a variety of non-neuronal cell types, they are all prominently expressed in the adult fat body, suggesting an important role for this tissue in behavior. To test its role in male courtship, fat body was feminized using the highly specific Larval serum protein promoter. We report here that the specific feminization of this tissue strongly reduces the competence of males to perform courtship. This effect is limited to the fat body of sexually mature adults as the feminization of larval fat body that normally persists in young adults does not affect mating. We propose that feminization of fat body affects the synthesis of male-specific secreted circulating proteins that influence the central nervous system. In support of this idea, we demonstrate that Takeout, a protein known to influence mating, is present in the hemolymph of adult males but not females and acts as a secreted protein.

PER-dependent rhythms in CLK phosphorylation and E-box binding regulate circadian transcription

Transcriptional activation by CLOCK-CYCLE (CLK-CYC) heterodimers and repression by PERIOD-TIMELESS (PER-TIM) heterodimers are essential for circadian oscillator function in Drosophila. PER-TIM was previously found to interact with CLK-CYC to repress transcription, and here we show that this interaction inhibits binding of CLK-CYC to E-box regulatory elements in vivo. Coincident with the interaction between PER-TIM and CLK-CYC is the hyperphosphorylation of CLK. This hyperphosphorylation occurs in parallel with the PER-dependent entry of DOUBLE-TIME (DBT) kinase into a complex with CLK-CYC, where DBT destabilizes both CLK and PER. Once PER and CLK are degraded, a novel hypophosphorylated form of CLK accumulates in parallel with E-box binding and transcriptional activation. These studies suggest that PER-dependent rhythms in CLK phosphorylation control rhythms in E-box-dependent transcription and CLK stability, thus linking PER and CLK function during the circadian cycle and distinguishing the transcriptional feedback mechanism in flies from that in mammals.

The Drosophila takeout gene is regulated by the somatic sex-determination pathway and affects male courtship behavior

The Drosophila somatic sex-determination regulatory pathway has been well studied, but little is known about the target genes that it ultimately controls. In a differential screen for sex-specific transcripts expressed in fly heads, we identified a highly male-enriched transcript encoding Takeout, a protein related to a superfamily of factors that bind small lipophilic molecules. We show that sex-specific takeout transcripts derive from fat body tissue closely associated with the adult brain and are dependent on the sex determination genes doublesex (dsx) and fruitless (fru). The male-specific Doublesex and Fruitless proteins together activate Takeout expression, whereas the female-specific Doublesex protein represses takeout independently of Fru. When cells that normally express takeout are feminized by expression of the Transformer-F protein, male courtship behavior is dramatically reduced, suggesting that male identity in these cells is necessary for behavior. A loss-of-function mutation in the takeout gene reduces male courtship and synergizes with fruitless mutations, suggesting that takeout plays a redundant role with other fru-dependent factors involved in male mating behavior. Comparison of Takeout sequences to the Drosophila genome reveals a family of 20 related secreted factors. Expression analysis of a subset of these genes suggests that the takeout gene family encodes multiple factors with sex-specific functions.

Protein phosphorylation plays an essential role in the regulation of alternative splicing and sex determination in Drosophila

Alternative mRNA splicing directed by SR proteins and the splicing regulators TRA and TRA2 is an essential feature of Drosophila sex determination. These factors are highly phosphorylated, but the role of their phosphorylation in vivo is unclear. We show that mutations in the Drosophila LAMMER kinase, Doa, alter sexual differentiation and interact synergistically with tra and tra2 mutations. Doa mutations disrupt sex-specific splicing of doublesex pre-mRNA, a key regulator of sex determination, by affecting the phosphorylation of one or more proteins in the female-specific splicing enhancer complex. Examination of pre-mRNAs regulated similarly to dsx shows that the requirement for Doa is substrate specific. These results demonstrate that a SR protein kinase plays a specific role in developmentally regulated alternative splicing.

Analysis of the functional specificity of RS domains in vivo

A number of splicing factors contain extensive regions that are rich in arginine and serine (RS domains). These domains are thought to facilitate protein-protein interactions that are critical in the regulation of alternative splicing. Using a domain swap strategy, we have tested the ability of RS domains from several proteins to substitute in vivo for an essential RS domain in the Drosophila splicing regulator TRA-2. By several criteria, RS domains were found to vary significantly in their ability to support the splicing regulation functions of TRA-2. The RS domain of dU2AF50 functioned efficiently, while that of the dSRp55 protein did not. Moreover, we find similar differences in the ability of RS domains to direct fusion proteins to discrete subnuclear sites at which TRA-2 associates with spermatocyte chromosomes. These results indicate that RS domains are not all functionally equivalent in vivo.

Autoregulation of transformer-2 alternative splicing is necessary for normal male fertility in Drosophila

In the male germline of Drosophila the transformer-2 protein is required for differential splicing of pre-mRNAs from the exuperantia and att genes and autoregulates alternative splicing of its own pre-mRNA. Autoregulation of TRA-2 splicing results in production of two mRNAs that differ by the splicing/retention of the M1 intron and encode functionally distinct protein isoforms. Splicing of the intron produces an mRNA encoding TRA-2(226), which is necessary and sufficient for both male fertility and regulation of downstream target RNAs. When the intron is retained, an mRNA is produced encoding TRA-2(179), a protein with no known function. We have previously shown that repression of M1 splicing is dependent on TRA-2(226), suggesting that this protein quantitatively limits its own expression through a negative feedback mechanism at the level of splicing. Here we examine this idea, by testing the effect that variations in the level of tra-2 expression have on the splicing of M1 and on male fertility. Consistent with our hypothesis, we observe that as tra-2 gene dosage is increased, smaller proportions of TRA-2(226) mRNA are produced, limiting expression of this isoform. Feedback regulation is critical for male fertility, since it is significantly decreased by a transgene in which repression of M1 splicing cannot occur and TRA-2(226) mRNA is constitutively produced. The effect of this transgene becomes more severe as its dosage is increased, indicating that fertility is sensitive to an excess of TRA-2(226). Our results suggest that autoregulation of TRA-2(226) expression in male germ cells is necessary for normal spermatogenesis.

A human homologue of the Drosophila sex determination factor transformer-2 has conserved splicing regulatory functions

Regulation of gene expression through alternative pre-mRNA splicing appears to occur in all metazoans, but most of our knowledge about splicing regulators derives from studies on genetically identified factors from Drosophila. Among the best studied of these is the transformer-2 (TRA-2) protein which, in combination with the transformer (TRA) protein, directs sex-specific splicing of pre-mRNA from the sex determination gene doublesex (dsx). Here we report the identification of htra-2 alpha, a human homologue of tra-2. Two alternative types of htra-2 alpha cDNA clones were identified that encode different protein isoforms with striking organizational similarity to Drosophila tra-2 proteins. When expressed in flies, one hTRA-2 alpha isoform partially replaces the function of Drosophila TRA-2, affecting both female sexual differentiation and alternative splicing of dsx pre-mRNA. Like Drosophila TRA-2, the ability of hTRA-2 alpha to regulate dsx is female-specific and depends on the presence of the dsx splicing enhancer. These results demonstrate that htra-2 alpha has conserved a striking degree of functional specificity during evolution and leads us to suggest that, although they are likely to serve different roles in development, the tra-2 products of flies and humans have similar molecular functions.

The cyclic AMP system and Drosophila learning

The cyclic AMP (cAMP) system plays a critical role in olfactory learning in the fruit fly, Drosophila melanogaster, as evidenced by the following: [1] The dunce gene encodes a form of cAMP phosphodiesterase (PDE). Flies carrying mutations at this gene show reduced PDE activity, high cAMP levels, and deficits in olfactory learning and memory [2]. The rutabaga gene encodes one type of adenylyl cyclase (AC) similar in properties to the Type I AC characterized from vertebrate brain. This enzyme is activated by G-protein and Ca++ and has been postulated to be a molecular coincidence detector, capable of integrating information from two independent sources such as the conditioned stimulus (CS) and the unconditioned stimulus (US) delivered to animals during Pavlovian conditioning. Rutabaga mutant flies are deficient in AC activity and show behavioral defects similar to those exhibited by dunce mutants [3]. Flies carrying mutations in the gene (DC0) that encodes the catalytic subunit of protein kinase A (PKA), the major mediator of cAMP actions, show alterations in learning performance and a loss in PKA activity. All three genes are expressed preferentially in mushroom bodies, neuroanatomical sites that mediate olfactory learning. Interestingly, the PDE and the catalytic subunit of PKA are found primarily in axonal and dendritic compartments of the mushroom body cells, whereas the AC is found primarily in the axonal compartment. The reason for this differential compartmentalization is unclear, although the hypothetical role of AC as coincidence detector would predict that CS and US stimuli are integrated in the axonal compartment. These observations suggest that cAMP is a dominant second messenger utilized by mushroom body cells to modulate their physiology while the animal is learning and consolidating memory. However, many other types of molecules are likely involved in the physiological alterations that occur in these cells during learning, including cell surface proteins, transcription factors, and synaptic proteins.

Conditional rescue of the dunce learning/memory and female fertility defects with Drosophila or rat transgenes

Mutations in the Drosophila dunce gene, the structural gene for a cAMP-specific phosphodiesterase (PDE), disrupt normal learning and memory and lead to female sterility. Our experiments address a long-standing question in the genetic dissection of learning and memory of whether dnc is involved in physiological processes underlying learning. Conditional expression of dunce transgenes in dnc adults shortly before training significantly improves learning over nontransgenic controls. Remarkably, behavioral rescue was also observed after induction of a transgene carrying a rat counterpart of dunce. Induction of the transgenes in adult dnc females confers partial rescue of the female sterility phenotype. These data are consistent with a major physiological requirement for the gene's activity in the learning process and show that a rat counterpart can substitute functionally for the Drosophila gene.

The Drosophila dunce locus: learning and memory genes in the fly

The dunce gene, one of several genes critical for normal learning and memory in Drosophila, is organized in a complex and bizarre way, with enormous introns containing several other unrelated genes. Recent studies have focused on the spatial expression pattern of the product, cAMP phosphodiesterase, and have provisionally identified the mushroom bodies as important sites of action of dunce within adult brain. In addition, the recent cloning and characterization of dunce counterparts from mammals has revealed that these too may participate in animal behavior and, in particular, in the regulation of mood.