Neuronal phenotype in the mature nervous system is maintained by persistent retrograde bone morphogenetic protein signaling

Mayday sustains trans-synaptic BMP signaling required for synaptic maintenance with age

Maintaining synaptic structure and function over time is vital for overall nervous system function and survival. The processes that underly synaptic development are well understood. However, the mechanisms responsible for sustaining synapses throughout the lifespan of an organism are poorly understood. Here, we demonstrate that a previously uncharacterized gene, CG31475, regulates synaptic maintenance in adult Drosophila NMJs. We named CG31475 mayday due to the progressive loss of flight ability and synapse architecture with age. Mayday is functionally homologous to the human protein Cab45, which sorts secretory cargo from the Trans Golgi Network (TGN). We find that Mayday is required to maintain trans-synaptic BMP signaling at adult NMJs in order to sustain proper synaptic structure and function. Finally, we show that mutations in mayday result in the loss of both presynaptic motor neurons as well as postsynaptic muscles, highlighting the importance of maintaining synaptic integrity for cell viability.

A low affinity cis-regulatory BMP response element restricts target gene activation to subsets of Drosophila neurons

Retrograde BMP signaling and canonical pMad/Medea-mediated transcription regulate diverse target genes across subsets of Drosophila efferent neurons, to differentiate neuropeptidergic neurons and promote motor neuron terminal maturation. How a common BMP signal regulates diverse target genes across many neuronal subsets remains largely unresolved, although available evidence implicates subset-specific transcription factor codes rather than differences in BMP signaling. Here we examine the cis-regulatory mechanisms restricting BMP-induced FMRFa neuropeptide expression to Tv4-neurons. We find that pMad/Medea bind at an atypical, low affinity motif in the FMRFa enhancer. Converting this motif to high affinity caused ectopic enhancer activity and eliminated Tv4-neuron expression. In silico searches identified additional motif instances functional in other efferent neurons, implicating broader functions for this motif in BMP-dependent enhancer activity. Thus, differential interpretation of a common BMP signal, conferred by low affinity pMad/Medea binding motifs, can contribute to the specification of BMP target genes in efferent neuron subsets.

Pancreatic islet-autonomous insulin and smoothened-mediated signalling modulate identity changes of glucagon+ α-cells

The mechanisms that restrict regeneration and maintain cell identity following injury are poorly characterized in higher vertebrates. Following β-cell loss, 1-2% of the glucagon-producing α-cells spontaneously engage in insulin production in mice. Here we explore the mechanisms inhibiting α-cell plasticity. We show that adaptive α-cell identity changes are constrained by intra-islet insulin- and Smoothened-mediated signalling, among others. The combination of β-cell loss or insulin-signalling inhibition, with Smoothened inactivation in α- or δ-cells, stimulates insulin production in more α-cells. These findings suggest that the removal of constitutive 'brake signals' is crucial to neutralize the refractoriness to adaptive cell-fate changes. It appears that the maintenance of cell identity is an active process mediated by repressive signals, which are released by neighbouring cells and curb an intrinsic trend of differentiated cells to change.

Drosophila female-specific Ilp7 motoneurons are generated by Fruitless-dependent cell death in males and by a double-assurance survival role for Transformer in females

Female-specific Ilp7 neuropeptide-expressing motoneurons (FS-Ilp7 motoneurons) are required in Drosophila for oviduct function in egg laying. Here, we uncover cellular and genetic mechanisms underlying their female-specific generation. We demonstrate that programmed cell death (PCD) eliminates FS-Ilp7 motoneurons in males, and that this requires male-specific splicing of the sex-determination gene fruitless (fru) into the Fru^MC isoform. However, in females, fru alleles that only generate Fru^M isoforms failed to kill FS-Ilp7 motoneurons. This blockade of Fru^M-dependent PCD was not attributable to doublesex gene function but to a non-canonical role for transformer (tra), a gene encoding the RNA splicing activator that regulates female-specific splicing of fru and dsx transcripts. In both sexes, we show that Tra prevents PCD even when the Fru^M isoform is expressed. In addition, we found that Fru^MC eliminated FS-Ilp7 motoneurons in both sexes, but only when Tra was absent. Thus, Fru^MC-dependent PCD eliminates female-specific neurons in males, and Tra plays a double-assurance function in females to establish and reinforce the decision to generate female-specific neurons.

Gene Regulatory Mechanisms Underlying the Spatial and Temporal Regulation of Target-Dependent Gene Expression in Drosophila Neurons

Neuronal differentiation often requires target-derived signals from the cells they innervate. These signals typically activate neural subtype-specific genes, but the gene regulatory mechanisms remain largely unknown. Highly restricted expression of the FMRFa neuropeptide in Drosophila Tv4 neurons requires target-derived BMP signaling and a transcription factor code that includes Apterous. Using integrase transgenesis of enhancer reporters, we functionally dissected the Tv4-enhancer of FMRFa within its native cellular context. We identified two essential but discrete cis-elements, a BMP-response element (BMP-RE) that binds BMP-activated pMad, and a homeodomain-response element (HD-RE) that binds Apterous. These cis-elements have low activity and must be combined for Tv4-enhancer activity. Such combinatorial activity is often a mechanism for restricting expression to the intersection of cis-element spatiotemporal activities. However, concatemers of the HD-RE and BMP-RE cis-elements were found to independently generate the same spatiotemporal expression as the Tv4-enhancer. Thus, the Tv4-enhancer atypically combines two low-activity cis-elements that confer the same output from distinct inputs. The activation of target-dependent genes is assumed to 'wait' for target contact. We tested this directly, and unexpectedly found that premature BMP activity could not induce early FMRFa expression; also, we show that the BMP-insensitive HD-RE cis-element is activated at the time of target contact. This led us to uncover a role for the nuclear receptor, seven up (svp), as a repressor of FMRFa induction prior to target contact. Svp is normally downregulated immediately prior to target contact, and we found that maintaining Svp expression prevents cis-element activation, whereas reducing svp gene dosage prematurely activates cis-element activity. We conclude that the target-dependent FMRFa gene is repressed prior to target contact, and that target-derived BMP signaling directly activates FMRFa gene expression through an atypical gene regulatory mechanism.

The Crossroads of Synaptic Growth Signaling, Membrane Traffic and Neurological Disease: Insights from Drosophila

Neurons require target-derived autocrine and paracrine growth factors to maintain proper identity, innervation, homeostasis and survival. Neuronal growth factor signaling is highly dependent on membrane traffic, both for the packaging and release of the growth factors themselves, and for regulation of intracellular signaling by their transmembrane receptors. Here, we review recent findings from the Drosophila larval neuromuscular junction (NMJ) that illustrate how specific steps of intracellular traffic and inter-organelle interactions impinge on signaling, particularly in the bone morphogenic protein, Wingless and c-Jun-activated kinase pathways, regulating elaboration and stability of NMJ arbors, construction of synapses and synaptic transmission and homeostasis. These membrane trafficking and signaling pathways have been implicated in human motor neuron diseases including amyotrophic lateral sclerosis and hereditary spastic paraplegia, highlighting their importance for neuronal health and survival.

Retrograde BMP signaling at the synapse: a permissive signal for synapse maturation and activity-dependent plasticity

At the Drosophila neuromuscular junction (NMJ), the loss of retrograde, trans-synaptic BMP signaling causes motoneuron terminals to have fewer synaptic boutons, whereas increased neuronal activity results in a larger synapse with more boutons. Here, we show that an early and transient BMP signal is necessary and sufficient for NMJ growth as well as for activity-dependent synaptic plasticity. This early critical period was revealed by the temporally controlled suppression of Mad, the SMAD1 transcriptional regulator. Similar results were found by genetic rescue tests involving the BMP4/5/6 ligand Glass bottom boat (Gbb) in muscle, and alternatively the type II BMP receptor Wishful Thinking (Wit) in the motoneuron. These observations support a model where the muscle signals back to the innervating motoneuron's nucleus to activate presynaptic programs necessary for synaptic growth and activity-dependent plasticity. Molecular genetic gain- and loss-of-function studies show that genes involved in NMJ growth and plasticity, including the adenylyl cyclase Rutabaga, the Ig-CAM Fasciclin II, the transcription factor AP-1 (Fos/Jun), and the adhesion protein Neurexin, all depend critically on the canonical BMP pathway for their effects. By contrast, elevated expression of Lar, a receptor protein tyrosine phosphatase found to be necessary for activity-dependent plasticity, rescued the phenotypes associated with the loss of Mad signaling. We also find that synaptic structure and function develop using genetically separable, BMP-dependent mechanisms. Although synaptic growth depended on Lar and the early, transient BMP signal, the maturation of neurotransmitter release was independent of Lar and required later, ongoing BMP signaling.

Circadian period integrates network information through activation of the BMP signaling pathway

Circadian pacemaker neurons in the Drosophila brain gather network information through the highly conserved BMP signaling pathway to establish the daily period of locomotor behavior.

Living organisms use biological clocks to maintain their internal temporal order and anticipate daily environmental changes. In Drosophila, circadian regulation of locomotor behavior is controlled by ∼150 neurons; among them, neurons expressing the PIGMENT DISPERSING FACTOR (PDF) set the period of locomotor behavior under free-running conditions. To date, it remains unclear how individual circadian clusters integrate their activity to assemble a distinctive behavioral output. Here we show that the BONE MORPHOGENETIC PROTEIN (BMP) signaling pathway plays a crucial role in setting the circadian period in PDF neurons in the adult brain. Acute deregulation of BMP signaling causes period lengthening through regulation of dClock transcription, providing evidence for a novel function of this pathway in the adult brain. We propose that coherence in the circadian network arises from integration in PDF neurons of both the pace of the cell-autonomous molecular clock and information derived from circadian-relevant neurons through release of BMP ligands.

The circadian clock controls rhythms in behavior, physiology, and metabolism in all living organisms. The molecular components as well as the neuronal network required to keep this clock running have been identified in several species. In the Drosophila brain this neuronal network is represented by an ensemble of 150 neurons, and among them, those expressing the Pigment Dispersing Factor (PDF) neuropeptide encompass the “central oscillator”—also called master clock as it ensures 24-hour periods—of the fly brain. In this study we show that the widely conserved Bone Morphogenetic Protein (BMP) signaling pathway is present in PDF neurons, and upon adult-specific activation it lengthens the endogenous period of locomotor behavior. We find that period lengthening correlates with delayed accumulation of nuclear PERIOD, a core component of the molecular clock. We also identified a putative DNA binding motif for the BMP pathway nuclear components within the regulatory region of the Clock (Clk) promoter, another core component of the circadian machinery. Interestingly, upon BMP pathway activation endogenous CLK levels are downregulated, thus accounting for the lengthening of the endogenous period. We propose that the endogenous period is a network property commanded by PDF neurons that results from integration of information from both the autonomous molecular clock and the nonautonomous BMP signaling pathway.

Chordin expression in the adult rat brain

Bone morphogenetic proteins (BMPs) exert its biological functions by interacting with membrane bound receptors. However, functions of BMPs are also regulated in the extracellular space by secreted antagonistic regulators. Chordin is an extracellular BMP antagonist that binds BMP-2, 4, and 7 with high affinity and thus interferes with binding to BMP receptors. Although chordin expression has been well described in the early development of the CNS, little information is available for its expression in the adult CNS. We, thus, investigated chordin expression in the adult rat CNS using immunohistochemistry. Chordin was intensely expressed in most neurons, and their dendrites and axons. In addition, abundant chordin expression was also observed in the neuropil of the gray matters where high plasticity is reported, such as the molecular layer of the cerebellum and the superficial layer of the superior colliculus. Furthermore, we found that astrocytes and ependymal cells also express chordin protein. These data indicate that chordin is more widely expressed throughout the adult CNS than previously reported, and its continued abundant expression in the adult brain strongly supports the idea that chordin plays pivotal roles also in the adult brain.

Female-biased dimorphism underlies a female-specific role for post-embryonic Ilp7 neurons in Drosophila fertility

In Drosophila melanogaster, much of our understanding of sexually dimorphic neuronal development and function comes from the study of male behavior, leaving female behavior less well understood. Here, we identify a post-embryonic population of Insulin-like peptide 7 (Ilp7)-expressing neurons in the posterior ventral nerve cord that innervate the reproductive tracts and exhibit a female bias in their function. They form two distinct dorsal and ventral subsets in females, but only a single dorsal subset in males, signifying a rare example of a female-specific neuronal subset. Female post-embryonic Ilp7 neurons are glutamatergic motoneurons innervating the oviduct and are required for female fertility. In males, they are serotonergic/glutamatergic neuromodulatory neurons innervating the seminal vesicle but are not required for male fertility. In both sexes, these neurons express the sex-differentially spliced fruitless-P1 transcript but not doublesex. The male fruitless-P1 isoform (fruM) was necessary and sufficient for serotonin expression in the shared dorsal Ilp7 subset, but although it was necessary for eliminating female-specific Ilp7 neurons in males, it was not sufficient for their elimination in females. By contrast, sex-specific RNA-splicing by female-specific transformer is necessary for female-type Ilp7 neurons in females and is sufficient for their induction in males. Thus, the emergence of female-biased post-embryonic Ilp7 neurons is mediated in a subset-specific manner by a tra- and fru-dependent mechanism in the shared dorsal subset, and a tra-dependent, fru-independent mechanism in the female-specific subset. These studies provide an important counterpoint to studies of the development and function of male-biased neuronal dimorphism in Drosophila.

Klumpfuss controls FMRFamide expression by enabling BMP signaling within the NB5-6 lineage

A number of transcription factors that are expressed within most, if not all, embryonic neuroblast (NB) lineages participate in neural subtype specification. Some have been extensively studied in several NB lineages (e.g. components of the temporal gene cascade) whereas others only within specific NB lineages. To what extent they function in other lineages remains unknown. Klumpfuss (Klu), the Drosophila ortholog of the mammalian Wilms tumor 1 (WT1) protein, is one such transcription factor. Studies in the NB4-2 lineage have suggested that Klu functions to ensure that the two ganglion mother cells (GMCs) in this embryonic NB lineage acquire different fates. Owing to limited lineage marker availability, these observations were made only for the NB4-2 lineage. Recent findings reveal that Klu is necessary for larval neuroblast growth and self-renewal. We have extended the study of Klu to the well-known embryonic NB5-6T lineage and describe a novel role for Klu in the Drosophila embryonic CNS. Our results demonstrate that Klu is expressed specifically in the postmitotic Ap4/FMRFa neuron, promoting its differentiation through the initiation of BMP signaling. Our findings indicate a pleiotropic function of Klu in Ap cluster specification in general and particularly in Ap4 neuron differentiation, indicating that Klu is a multitasking transcription factor. Finally, our studies indicate that a transitory downregulation of klu is crucial for the specification of the Ap4/FMRFa neuron. Similar to WT1, klu seems to have either self-renewal or differentiation-promoting functions, depending on the developmental context.

Establishing and maintaining gene expression patterns: insights from sensory receptor patterning

In visual and olfactory sensory systems with high discriminatory power, each sensory neuron typically expresses one, or very few, sensory receptor genes, excluding all others. Recent studies have provided insights into the mechanisms that generate and maintain sensory receptor expression patterns. Here, we review how this is achieved in the fly retina and compare it with the mechanisms controlling sensory receptor expression patterns in the mouse retina and in the mouse and fly olfactory systems.

Maintaining differentiated cellular identity

Various studies have demonstrated that somatic differentiated cells can be reprogrammed into other differentiated states or into pluripotency, thus showing that the differentiated cellular state is not irreversible. These findings have generated intense interest in the process of reprogramming and in mechanisms that govern the pluripotent state. However, the realization that differentiated cells can be triggered to switch to considerably different lineages also emphasizes that we need to understand how the identity of mature cells is normally maintained. Here we review recent studies on how the differentiated state is controlled at the transcriptional level and discuss how new insights have begun to elucidate mechanisms underlying the stable maintenance of mature cell identities.

Temporally tuned neuronal differentiation supports the functional remodeling of a neuronal network in Drosophila

During insect metamorphosis, neuronal networks undergo extensive remodeling by restructuring their connectivity and recruiting newborn neurons from postembryonic lineages. The neuronal network that directs the essential behavior, ecdysis, generates a distinct behavioral sequence at each developmental transition. Larval ecdysis replaces the cuticle between larval stages, and pupal ecdysis externalizes and expands the head and appendages to their adult position. However, the network changes that support these differences are unknown. Crustacean cardioactive peptide (CCAP) neurons and the peptide hormones they secrete are critical for ecdysis; their targeted ablation alters larval ecdysis progression and results in a failure of pupal ecdysis. In this study, we demonstrate that the CCAP neuron network is remodeled immediately before pupal ecdysis by the emergence of 12 late CCAP neurons. All 12 are CCAP efferents that exit the central nervous system. Importantly, these late CCAP neurons were found to be entirely sufficient for wild-type pupal ecdysis, even after targeted ablation of all other 42 CCAP neurons. Our evidence indicates that late CCAP neurons are derived from early, likely embryonic, lineages. However, they do not differentiate to express their peptide hormone battery, nor do they project an axon via lateral nerve trunks until pupariation, both of which are believed to be critical for the function of CCAP efferent neurons in ecdysis. Further analysis implicated ecdysone signaling via ecdysone receptors A/B1 and the nuclear receptor ftz-f1 as the differentiation trigger. These results demonstrate the utility of temporally tuned neuronal differentiation as a hard-wired developmental mechanism to remodel a neuronal network to generate a scheduled change in behavior.

Developmental transcriptional networks are required to maintain neuronal subtype identity in the mature nervous system

During neurogenesis, transcription factors combinatorially specify neuronal fates and then differentiate subtype identities by inducing subtype-specific gene expression profiles. But how is neuronal subtype identity maintained in mature neurons? Modeling this question in two Drosophila neuronal subtypes (Tv1 and Tv4), we test whether the subtype transcription factor networks that direct differentiation during development are required persistently for long-term maintenance of subtype identity. By conditional transcription factor knockdown in adult Tv neurons after normal development, we find that most transcription factors within the Tv1/Tv4 subtype transcription networks are indeed required to maintain Tv1/Tv4 subtype-specific gene expression in adults. Thus, gene expression profiles are not simply “locked-in,” but must be actively maintained by persistent developmental transcription factor networks. We also examined the cross-regulatory relationships between all transcription factors that persisted in adult Tv1/Tv4 neurons. We show that certain critical cross-regulatory relationships that had existed between these transcription factors during development were no longer present in the mature adult neuron. This points to key differences between developmental and maintenance transcriptional regulatory networks in individual neurons. Together, our results provide novel insight showing that the maintenance of subtype identity is an active process underpinned by persistently active, combinatorially-acting, developmental transcription factors. These findings have implications for understanding the maintenance of all long-lived cell types and the functional degeneration of neurons in the aging brain.

For neurons to function properly, they must establish and then maintain their unique, subtype-specific gene expression profiles. These unique gene expression profiles are established during development by networks of DNA–binding proteins, termed transcription factors (TFs). However, how neurons maintain their unique gene expression profiles in the mature and aging brain is largely unknown. Recent advances in inducible genetic technologies now allow us to manipulate gene expression in adult neurons, after normal development. Applying such techniques, we examined the effect of knocking down TF expression in two adult neuronal subtypes. We show that the TF networks that establish unique gene expression profiles during development are then required to maintain them thereafter. Thus, gene expression profiles are not simply “locked-in,” but must be actively maintained by persistent developmental TF networks. However, we found that critical cross-regulatory relationships that had existed between TFs during development were not present in the adult, even between persisting TFs. This highlights important differences between developmental and maintenance transcriptional networks in individual neurons. The dependence of subtype gene expression on active mechanisms represents a potential Achilles heel for long-lived cells, as deterioration of those active mechanisms could lead to functional degeneration of neurons with advancing age.

Retrograde BMP signaling controls Drosophila behavior through regulation of a peptide hormone battery

Retrograde BMP signaling in neurons plays conserved roles in synaptic efficacy and subtype-specific gene expression. However, a role for retrograde BMP signaling in the behavioral output of neuronal networks has not been established. Insect development proceeds through a series of stages punctuated by ecdysis, a complex patterned behavior coordinated by a dedicated neuronal network. In Drosophila, larval ecdysis sheds the old cuticle between larval stages, and pupal ecdysis everts the head and appendages to their adult external position during metamorphosis. Here, we found that mutants of the type II BMP receptor wit exhibited a defect in the timing of larval ecdysis and in the completion of pupal ecdysis. These phenotypes largely recapitulate those previously observed upon ablation of CCAP neurons, an integral subset of the ecdysis neuronal network. Here, we establish that retrograde BMP signaling in only the efferent subset of CCAP neurons (CCAP-ENs) is required to cell-autonomously upregulate expression of the peptide hormones CCAP, Mip and Bursicon β. In wit mutants, restoration of wit exclusively in CCAP neurons significantly rescued peptide hormone expression and ecdysis phenotypes. Moreover, combinatorial restoration of peptide hormone expression in CCAP neurons in wit mutants also significantly rescued wit ecdysis phenotypes. Collectively, our data demonstrate a novel role for retrograde BMP signaling in maintaining the behavioral output of a neuronal network and uncover the underlying cellular and gene regulatory substrates.

BMP2 expression in the adult rat brain

Bone morphogenetic protein-2 (BMP2) is a member of the transforming growth factor-β (TGF-β) superfamily and plays important roles in multiple biological events. Although BMP2 expression has been well described in the early development of the central nervous system (CNS), little information is available on its expression in the adult CNS. We thus investigated BMP2 expression in the adult rat CNS by using immunohistochemistry. Here we show that BMP2 is widely expressed throughout the adult CNS. In addition, besides intense BMP2 expression in almost all neurons, we found BMP2 expression in astrocytes and ependymal cells. Interestingly, we found that the axons of olfactory sensory neurons express BMP2. In addition, in the glomerular layer, BMP2 was very strongly expressed in some glomeruli, whereas the other glomeruli were weakly stained, suggesting that the variations in BMP2 expression level in each glomerus might be cues for each axon to find its adequate target and to keep its identity. Furthermore, we compared the expression patterns of BMP2 and BMP4. Interestingly, BMP4 was preferentially expressed in the dendrites of several neurons, whereas BMP2 was basically not expressed in the dendrites; however, it was detected in the axons. This means that in a single neuron the localizations of BMP2 and BMP4 are differentially regulated. These data indicate that BMP2 is more widely expressed throughout the adult CNS than previously reported, and its continued abundant expression in the adult brain strongly supports the idea that BMP2 also plays pivotal roles in the adult brain.

Bone morphogenetic protein receptor expressions in the adult rat brain

Bone morphogenetic proteins (BMP) are members of the transforming growth factor β (TGF-β) superfamily. BMPs exert its biological functions by interacting with membrane bound receptors belonging to the serine/threonine kinase family including bone morphogenetic protein receptor I (BMPRIA, BMPRIB) and type II (BMPRII). Although BMPR expressions have been well described in the early development of the CNS, little information is available for their expressions in the adult CNS. We, thus, investigated BMPR expressions in the adult rat CNS using immunohistochemistry. Here, we show that BMPRIA, IB and II proteins are widely expressed throughout the adult CNS. Interestingly, we observed that BMPRIA, IB and II proteins are abundantly expressed in many kinds of axons. In addition, we found that BAMRIB-IR was preferentially expressed in dendrites of many neurons throughout the CNS, while BMPRIA was mainly expressed in cell bodies, showing that BMPRIA and BMPRIB are differentially targeted in a single neuron. In addition, besides abundant BMPR expressions in neurons, we exhibited BMPR expressions in astrocytes and ependymal cells. These data indicate that BMPRs are more widely expressed throughout the adult CNS than previously reported, and their continued abundant expressions in the adult brain strongly support the idea that BMPRs play pivotal roles also in the adult brain.

Peptide-induced modulation of synaptic transmission and escape response in Drosophila requires two G-protein-coupled receptors

Neuropeptides are found in both mammals and invertebrates and can modulate neural function through activation of G-protein-coupled receptors (GPCRS). The precise mechanisms by which many of these GPCRs modulate specific signaling cascades to regulate neural function are not well defined. We used Drosophila melanogaster as a model to examine both the cellular and behavioral effects of DPKQDFMRFamide, the most abundant peptide encoded by the dFMRF gene. We show that DPKQDFMRFamide enhanced synaptic transmission through activation of two G-protein-coupled receptors, Fmrf Receptor (FR) and Dromyosupressin Receptor-2 (DmsR-2). The peptide increased both the presynaptic Ca(2+) response and the quantal content of released transmitter. Peptide-induced modulation of synaptic function could be abrogated by depleting intracellular Ca(2+) stores or by interfering with Ca(2+) release from the endoplasmic reticulum through disruption of either the ryanodine receptor or the inositol 1,4,5-trisphosphate receptor. The peptide also altered behavior. Exogenous DPKQDFMRFamide enhanced fictive locomotion; this required both the FR and DmsR-2. Likewise, both receptors were required for an escape response to intense light exposure. Thus, coincident detection of a peptide by two GPCRs modulates synaptic function through effects of Ca(2+)-induced Ca(2+) release, and we hypothesize that these mechanisms are involved in behavioral responses to environmental stress.

A genetic mosaic approach for neural circuit mapping in Drosophila

Transgenic manipulation of subsets of brain cells is increasingly used for studying behaviors and their underlying neural circuits. In Drosophila, the GAL4-upstream activating sequence (UAS) binary system is powerful for gene manipulation, but GAL4 expression is often too broad for fine mapping of neural circuits. Here, we describe the development of unique molecular genetic tools to restrict GAL4 expression patterns. Building on the GAL4-UAS system, our method adds two components: a collection of enhancer-trap recombinase, Flippase (ET-FLP), transgenic lines that provide inheritable, reproducible, and tissue-specific FLP and an FRT-dependent GAL80 "flip-in" construct that converts FLP expression into tissue-specific repression of GAL4 by GAL80. By including a UAS-encoded fluorescent protein, circuit morphology can be simultaneously marked while the circuit function is assessed using another UAS transgene. In a proof-of-principle analysis, we applied this ET-FLP-induced intersectional GAL80/GAL4 repression (FINGR) method to map the neural circuitry underlying fly wing inflation. The FINGR system is versatile and powerful in combination with the vast collection of GAL4 lines for neural circuit mapping as well as for clonal analysis based on the infusion of the yeast-derived FRT/FLP system of mitotic recombination into Drosophila. The strategies and tactics underlying our FINGR system are also applicable to other genetically amenable organisms in which transgenes including the GAL4, UAS, GAL80, and FLP factors can be applied.

Retrograde neural circuit specification by target-derived neurotrophins and growth factors

Neural circuit assembly during development involves a series of highly regulated steps. While genetically pre-determined programs play key roles in the early steps including neurogenesis, migration, and initial growth and guidance of axons; increasing evidence indicates that as the axons reach their targets, the late steps of neuronal differentiation and connectivity formation may be influenced or even specified by target-derived signals. Here we attempt to provide a brief synthesized review on the roles of retrograde neurotrophin and growth factor signaling in regulating the final stages of neural circuit specificity such as axonal projection, dendritic patterning, neurotransmitter phenotype acquisition, and synapse formation.