Specification of neuropeptide cell identity by the integration of retrograde BMP signaling and a combinatorial transcription factor code

Development of Drosophila motoneurons: specification and morphology

In the Drosophila ventral nerve cord, segmentally repeated sets of approximately 80 motoneurons are generated during embryogenesis. Within each hemisegment, each motoneuron is characterised by its axonal projection and innervation of a particular target muscle as well as its dendritic tree in the central nervous system. Codes of transcriptional regulators appear to specify in a hierarchical fashion the cell type, motoneuron sub-types and eventually unique cellular identities. Recent studies show that patterns of connectivity in the periphery are mirrored by patterns of dendritic arborisation centrally thereby providing a neuronal correlate of connectivity to the anatomy of the motor system in the periphery. While the principal mechanisms that underlie the development of the peripheral neuromuscular system have been studied in some detail, much less is known about how the dendrites and their patterns of connections develop in the CNS.

Requirement of Abdominal-A and Abdominal-B in the developing genitalia of Drosophila breaks the posterior downregulation rule

The genitalia of Drosophila derive from the genital disc and require the activity of the Abdominal-B (Abd-B) Hox gene. This gene encodes two different proteins, Abd-B M and Abd-B R. We show here that the embryonic genital disc, like the larval genital disc, is formed by cells from the eighth (A8), ninth (A9) and tenth (A10) abdominal segments, which most likely express the Abd-B M, Abd-B R and Caudal products, respectively. Abd-B m is needed for the development of A8 derivatives such as the external and internal female genitalia, the latter also requiring abdominal-A (abd-A), whereas Abd-B r shapes male genitalia (A9 in males). Although Abd-B r represses Abd-B m in the embryo, in at least part of the male A9 such regulation does not occur. In the male A9, some Abd-B m(-)r(-) or Abd-B r(-) clones activate Distal-less and transform part of the genitalia into leg or antenna. In the female A8, many Abd-B m(-)r(-) mutant clones produce similar effects, and also downregulate or eliminate abdominal-A expression. By contrast, although Abd-B m is the main or only Abd-B transcript present in the female A8, Abd-B m(-) clones induced in this primordium do not alter Distal-less or abd-A expression, and transform the A8 segment into the A4. The relationship between Abd-B and abd-A in the female genital disc is opposite to that of the embryonic epidermis, and contravenes the rule that posteriorly expressed Hox genes downregulate more anterior ones.

LIM Kinase1 controls synaptic stability downstream of the type II BMP receptor

Here, we demonstrate that the BMP receptor Wishful Thinking (Wit) is required for synapse stabilization. In the absence of BMP signaling, synapse disassembly and retraction ensue. Remarkably, downstream Smad-mediated signaling cannot fully account for the stabilizing activity of the BMP receptor. We identify LIM Kinase1 (DLIMK1)-dependent signaling as a second, parallel pathway that confers the added synapse-stabilizing activity of the BMP receptor. We show that DLIMK1 binds a region of the Wit receptor that is necessary for synaptic stability but is dispensable for Smad-mediated synaptic growth. A genetic analysis demonstrates that DLIMK1 is necessary, presynaptically, for synapse stabilization, but is not necessary for normal synaptic growth or function. Furthermore, presynaptic expression of DLIMK1 in a wit or mad mutant significantly rescues synaptic stability, growth, and function. DLIMK1 localizes near synaptic microtubules and functions independently of ADF/cofilin, highlighting a novel requirement for DLIMK1 during synapse stabilization rather than actin-dependent axon outgrowth.

Morphogens and synaptogenesis in Drosophila

During development and adult life synapses are remodeled in response to genetic programs and environmental cues. This synaptic plasticity is thought to be the basis of learning and memory. The larval neuromuscular junction of Drosophila is established during embryogenesis and grows during larval development to accommodate muscle growth and maintain synaptic homeostasis. This growth is dependent on bidirectional communication between the motoneuron and the muscle fiber. The best-characterized retrograde signaling pathway is defined by Glass bottom boat (Gbb), a morphogen of the transforming growth factor-beta (TGF-beta) superfamily. Gbb acts as a muscle-derived retrograde signal that activates the TGF-beta pathway presynaptically. This pathway includes the type II receptor Wishful thinking, type I receptors Thick veins and Saxophone, and the second messenger Smads Mothers against dpp (Mad) and Medea. Mutations that block this pathway result in small synapses that are morphologically aberrant and severely impaired functionally. An emerging anterograde signaling pathway is defined by Wingless, a morphogen of the Wnt family that acts as a motoneuron-derived anterograde signal required for both pre- and postsynaptic development. In the absence of Wingless the neuronal microtubule cytoskeleton regulator Futsch is down-regulated and synaptic growth impaired. Some of these morphogens have conserved roles in mammalian synaptogenesis, and genetic analysis suggests that additional signaling molecules are required for synaptic growth at the Drosophila neuromuscular junction.

TheDrosophilatype II receptor, Wishful thinking, binds BMP and myoglianin to activate multiple TGFβ family signaling pathways

Wishful thinking (Wit) is a Drosophila transforming growth factor-beta (TGFbeta) superfamily type II receptor most related to the mammalian bone morphogenetic protein (BMP) type II receptor, BMPRII. To better understand its function, we undertook a biochemical approach to establish the ligand binding repertoire and downstream signaling pathway. We observed that BMP4 and BMP7, bound to receptor complexes comprised of Wit and the type I receptor thickveins and saxophone to activate a BMP-like signaling pathway. Further we demonstrated that both myoglianin and its most closely related mammalian ligand, myostatin, interacted with a Wit and Baboon (Babo) type II-type I receptor complex to activate TGFbeta/activin-like signaling pathways. These results thereby demonstrate that Wit binds multiple ligands to activate both BMP and TGFbeta-like signaling pathways. Given that myoglianin is expressed in muscle and glial-derived cells, these results also suggest that Wit may mediate myoglianin-dependent signals in the nervous system.

A developmental switch in the response of DRG neurons to ETS transcription factor signaling

Two ETS transcription factors of the Pea3 subfamily are induced in subpopulations of dorsal root ganglion (DRG) sensory and spinal motor neurons by target-derived factors. Their expression controls late aspects of neuronal differentiation such as target invasion and branching. Here, we show that the late onset of ETS gene expression is an essential requirement for normal sensory neuron differentiation. We provide genetic evidence in the mouse that precocious ETS expression in DRG sensory neurons perturbs axonal projections, the acquisition of terminal differentiation markers, and their dependence on neurotrophic support. Together, our findings indicate that DRG sensory neurons exhibit a temporal developmental switch that can be revealed by distinct responses to ETS transcription factor signaling at sequential steps of neuronal maturation.

By expressing ETS transcription factors at different developmental stages of dorsal root ganglion development, the authors show that late onset of ETS expression is essential for normal sensory neuron differentiation

Crystal structure of BMP-9 and functional interactions with pro-region and receptors

Bone morphogenetic proteins (BMPs), a subset of the transforming growth factor (TGF)-beta superfamily, regulate a diverse array of cellular functions during development and in the adult. BMP-9 (also known as growth and differentiation factor (GDF)-2) potently induces osteogenesis and chondrogenesis, has been implicated in the differentiation of cholinergic neurons, and may help regulate glucose metabolism. We have determined the structure of BMP-9 to 2.3 A and examined the differences between our model and existing crystal structures of other BMPs, both in isolation and in complex with their receptors. TGF-beta ligands are translated as precursors, with pro-regions that generally dissociate after cleavage from the ligand, but in some cases (including GDF-8 and TGF-beta1, -2, and -3), the pro-region remains associated after secretion from the cell and inhibits binding of the ligand to its receptor. Although the proregion of BMP-9 remains tightly associated after secretion, we find, in several cell-based assays, that the activities of BMP-9 and BMP-9.pro-region complex were equivalent. Activin receptor-like kinase 1 (ALK-1), an orphan receptor in the TGF-beta family, was also identified as a potential receptor for BMP-9 based on surface plasmon resonance studies (BIAcore) and the ability of soluble ALK-1 to block the activity of BMP-9.pro-region complex in cell-based assays.

Regulators acting in combinatorial codes also act independently in single differentiating neurons

In the Drosophila ventral nerve cord, a small number of neurons express the LIM-homeodomain gene apterous (ap). These ap neurons can be subdivided based upon axon pathfinding and their expression of neuropeptidergic markers. ap, the zinc finger gene squeeze, the bHLH gene dimmed, and the BMP pathway are all required for proper specification of these cells. Here, using several ap neuron terminal differentiation markers, we have resolved how each of these factors contributes to ap neuron diversity. We find that these factors interact genetically and biochemically in subtype-specific combinatorial codes to determine certain defining aspects of ap neuron subtype identity. However, we also find that ap, dimmed, and squeeze additionally act independently of one another to specify certain other defining aspects of ap neuron subtype identity. Therefore, within single neurons, we show that single regulators acting in numerous molecular contexts differentially specify multiple subtype-specific traits.

Synaptic strengthening mediated by bone morphogenetic protein-dependent retrograde signaling in the Drosophila CNS

Retrograde signaling is an essential component of synaptic development and physiology. Previous studies show that bone morphogenetic protein (BMP)-dependent retrograde signaling is required for the proper development of the neuromuscular junction (NMJ) in Drosophila. These studies, moreover, raised the significant possibility that the development of central motor circuitry might similarly be reliant on such signaling. To test this hypothesis, retrograde signaling between postsynaptic motoneurons and their presynaptic interneurons is examined. Postsynaptic expression of an adenylate cyclase encoded by rutabaga (rut), is sufficient to strengthen synaptic transmission at these identified central synapses. Results are presented to show that the underlying mechanism is dependent on BMP retrograde signaling. Thus, presynaptic expression of an activated TGF-beta receptor, thickvien (tkv), or postsynaptic expression of a TGF-beta ligand, glass-bottom boat (gbb), is sufficient to phenocopy strengthening of synaptic transmission. In the absence of gbb, endogenous synaptic transmission is significantly weakened and, moreover, postsynaptic overexpression of rut is unable to potentiate synaptic function. Potentiation of presynaptic neurotransmitter release, mediated by increased postsynaptic expression of gbb, is dependent on normal cholinergic activity, indicative that either the secretion of this retrograde signal, or its transduction, is activity dependent. Thus, in addition to the development of the NMJ and expression of myoactive FMRFamide-like peptides in specific central neurons, the results of the present study indicate that this retrograde signaling cascade also integrates the development and function of central motor circuitry that controls movement in Drosophila larvae.

Impaired spermatogenesis and male fertility defects in CIZ/Nmp4-disrupted mice

CIZ (Cas interacting zinc finger protein), also called Nmp4 (nuclear matrix protein 4), is a nucleo-cytoplasmic shuttling transcription factor that regulates the expression of collagen and matrix metalloproteinases. CIZ/Nmp4 was originally cloned by its binding to p130(Cas), a focal adhesion protein, and was recently shown to suppress BMP2 (bone mophogenetic protein 2) signalling. To explore the physiological role of CIZ/Nmp4, we disrupted CIZ/Nmp4-gene by inserting beta-galactosidase and neomycin resistance genes into the 2nd exon of CIZ/Nmp4-gene, which is utilized by all the sequenced alternative forms. CIZ-/- mice were born and grew to adulthood. Although they tend to be smaller than wild-type mice, no pathological abnormality was observed except in the testis. Histological analysis of the testes revealed variable degrees of spermatogenic cell degeneration within the seminiferous tubules of CIZ-/- mice, resembling the histology of the 'Germinal-cell aplasia with focal spermatogenesis'. Some of the CIZ-/- male mice developed infertility. TUNEL assay on testis sections revealed an increased occurrence of apoptosis of spermatogenic cells in the testes of CIZ-/- mice. CIZ/Nmp4 was co-localized with Smad1 in the testis, suggesting that a disregulation of BMP signalling could cause these phenotypes. These results suggest that CIZ/Nmp4 plays roles in the progress and the maintenance of spermatogenesis.

Poliovirus type 1 infection of murinePRNP-knockout neuronal cells

Transfection of the prion protein gene (Prnp) into prion-deficient mouse cells was shown to reduce the replication of coxsackievirus B3, an enterovirus. Because mice can be susceptible to poliovirus infection by parenteral routes, the authors tested the susceptibility to poliovirus-1 (PV-1) of a panel of murine neuronal cell lines differing in their ability to express Prnp. The investigated cell lines (prionless HpL3.4 cells, HpL3.4 cells transfected with a Prnp vector, HpL3.4 cells transfected with a void vector, wild-type Hw3.5 Prnp(+/+) cells) expressed the murine homologue (Tage4) of human poliovirus receptor (CD155/hPVR). PV-1 infection of Prnp(-/-) HpL3.4 cells resulted in the production of high viral titers, though viral antigens could be detected in only 0.5% to 2% of cells. Wild-type Prnp(+/+) cells and prionless cells transfected with the Prnp gene were not permissive to PV-1. Results of viral titration and immunofluorescence were confirmed by conventional polymerase chain reaction (PCR) and quantitative real-time PCR. Exposure to PV-1 had no influence on the gene expression profile of Prnp(+/+) cells. In contrast, PV-1 infection was associated with upregulation of several genes in permissive Prnp(-/-) cell cultures: type I interferon (IFN) genes, IFN-related developmental regulator 1 (IFNRD1), tumor necrosis factor superfamily member 13b (TNFSF13b), interleukin (IL) - 7, granulocyte/macrophage colony-stimulating factors (CSFs), hepatocyte growth factor (HGF), vascular endothelial growth factor-A, transforming growth factors beta1 and beta3 (TGFb1, TGFb3), as well as a variety of bone morphogenetic proteins endowed with neuroprotective activity. Distinction of permissive from nonpermissive neuronal cells on the basis of Prnp expression suggests that prion-deficient mice could represent an extraordinarily sensitive animal model for poliovirus infection.

Bone morphogenetic proteins regulate ionotropic glutamate receptors in human retina

Bone morphogenetic proteins (BMPs) are required for the development of retina, but their role in the mature eye is unknown. We therefore examined the expression of BMP-7 in adult human retina and assessed its effects on horizontal cells cultured from adult human retina. BMP-7 expression was detected in all retinal layers, with high levels of expression being present in the inner and outer nuclear layers. Human horizontal cells, found in the inner nuclear layer, possess both AMPA and kainate receptors, and glutamatergic agonists that activate these receptors induce prominent inward currents. Exposure to BMP-7 suppresses the kainate receptor current but enhances the AMPA receptor current. BMP-6, activin, and cartilage-derived morphogenic protein-2 (CDMP-2) have similar effects to BMP-7 and act just as rapidly (< 1 s). In contrast BMP-2 and transforming growth factor-beta2 are inactive. The actions of BMP-7 on both AMPA and kainate receptors were blocked by the nonselective kinase inhibitor, staurosporine. In contrast, the serine/threonine kinase inhibitors blocked only the effects of BMP-7 on the AMPA current. Thus, BMPs rapidly and differentially regulate two ionotropic glutamate receptors through distinct pathways, neither of which involves nuclear regulatory activity. These observations suggest that BMPs might modify synaptic function in the mature nervous system.

Drosophila serpin 4 functions as a neuroserpin-like inhibitor of subtilisin-like proprotein convertases

The proteolytic processing of neuropeptide precursors is believed to be regulated by serine proteinase inhibitors, or serpins. Here we describe the molecular cloning and functional expression of a novel member of the serpin family, Serine protease inhibitor 4 (Spn4), that we propose is involved in the regulation of peptide maturation in Drosophila. The Spn4 gene encodes at least two different serpin proteins, generated by alternate splicing of the last coding exon. The closest vertebrate homolog to Spn4 is neuroserpin. Like neuroserpin, one of the Spn4 proteins (Spn4.1) features a unique C-terminal extension, reminiscent of an endoplasmic reticulum (ER) retention signal; however, Spn4.1 and neuroserpin have divergent reactive site loops, with Spn4.1 showing a generic recognition site for furin/SPC1, the founding member of the intracellularly active family of subtilisin-like proprotein convertases (SPCs). In vitro, Spn4.1 forms SDS-stable complexes with the SPC furin and directly inhibits it. When Spn4.1 is overexpressed in specific peptidergic cells of Drosophila larvae, the animals exhibit a phenotype consistent with disrupted neuropeptide processing. This observation, together with the unique combination of an ER-retention signal, a target sequence for SPCs in the reactive site loop, and the in vitro inhibitory activity against furin, strongly suggests that Spn4.1 is an intracellular regulator of SPCs.

Segment-specific prevention of pioneer neuron apoptosis by cell-autonomous, postmitotic Hox gene activity

In vertebrates, neurons often undergo apoptosis after differentiating and extending their axons. By contrast, in the developing nervous system of invertebrate embryos apoptosis typically occurs soon after cells are generated. Here, we show that the Drosophila dMP2 and MP1 pioneer neurons undergo segment-specific apoptosis at late embryonic stages, long after they have extended their axons and have performed their pioneering role in guiding follower axons. This segmental specificity is achieved by differential expression of the Hox gene Abdominal B, which in posterior segments prevents pioneer neuron death postmitotically and cell-autonomously by repressing the RHG-motif cell death activators reaper and grim. Our results identify the first clear case of a cell-autonomous and anti-apoptotic role for a Hox gene in vivo. In addition, they provide a novel mechanism linking Hox positional information to differences in neuronal architecture along the anteroposterior axis by the selective elimination of mature neurons.

Independent roles of the dachshund and eyes absent genes in BMP signaling, axon pathfinding and neuronal specification

In the Drosophila nerve cord, a subset of neurons expresses the neuropeptide FMRFamide related (Fmrf). Fmrf expression is controlled by a combinatorial code of intrinsic factors and an extrinsic BMP signal. However, this previously identified code does not fully explain the regulation of Fmrf. We have found that the Dachshund (Dac) and Eyes Absent (Eya) transcription co-factors participate in this combinatorial code. Previous studies have revealed an intimate link between Dac and Eya during eye development. Here, by analyzing their function in neurons with multiple phenotypic markers, we demonstrate that they play independent roles in neuronal specification, even within single cells. dac is required for high-level Fmrf expression, and acts potently together with apterous and BMP signaling to trigger Fmrf expression ectopically, even in motoneurons. By contrast, eya regulates Fmrf expression by controlling both axon pathfinding and BMP signaling, but cannot trigger Fmrf ectopically. Thus, we show that dac and eya perform entirely different functions in a single cell type to ultimately regulate a single phenotypic outcome.

Control of neuronal phenotype: what targets tell the cell bodies

Assembly of neuronal circuits is controlled by the sequential acquisition of neuronal subpopulation-specific identities at progressive developmental steps. Whereas neuronal features involved in initial phases of differentiation are already established at cell-cycle exit, recent findings, based mainly on work in the peripheral nervous system, suggest that the timely integration of signals encountered en route to targets and from the target region itself is essential to control late steps in connectivity. As neurons project towards their targets they require target-derived signals to establish mature axonal projections and acquire neuronal traits such as the expression of distinct combinations of neurotransmitters. Recent evidence presented in this review shows that this principle, of a signaling interplay between target-derived signals and neuronal cell bodies, is often mediated through transcriptional events and is evolutionarily conserved.

Molecular motors in neuronal development, intracellular transport and diseases

Molecular motors such as kinesin superfamily proteins (KIFs), dynein superfamily proteins and myosin superfamily proteins have diverse and fundamental roles in many cellular processes, including neuronal development and the pathogenesis of neuronal diseases. During neuronal development, KIFs take significant roles in the regulation of axon-collateral branch extension, which is essential for brain wiring. Cytoplasmic dynein together with LIS1 takes pivotal roles in neocortical layer formation. In axons, anterograde transport is mediated by KIFs, whereas retrograde transport is mediated mainly by cytoplasmic dynein, and dysfunction of motors results in neurodegenerative diseases. In dendrites, the transport of NMDA and AMPA receptors is mediated by KIFs, and the motor has been shown to play a significant part in establishing learning and memory.

Transcriptional control of synaptic differentiation by retrograde signals

Synaptic differentiation during development is a multi-step process, which requires reciprocal communication between pre- and postsynaptic cells. Cell surface interactions can induce the assembly of synaptic specializations but maintenance and growth of synapses depend on transcriptional regulation. Transcriptional responses associated with synaptic differentiation are observed in central and peripheral neurons and depend on retrograde signals coming from the target region. Although the identity of most of the retrograde signaling pathways remains to be identified, the TGFbeta family of growth factors have emerged as one crucial signal at the neuromuscular junction. Here, we discuss evidence for transcriptional control during synaptic differentiation and the signaling pathways mediating retrograde TGFbeta signaling.

Drosophila short neuropeptide F regulates food intake and body size

Neuropeptides regulate a wide range of animal behavior including food consumption, circadian rhythms, and anxiety. Recently, Drosophila neuropeptide F, which is the homolog of the vertebrate neuropeptide Y, was cloned, and the function of Drosophila neuropeptide F in feeding behaviors was well characterized. However, the function of the structurally related short neuropeptide F (sNPF) was unknown. Here, we report the cloning, RNA, and peptide localizations, and functional characterizations of the Drosophila sNPF gene. The sNPF gene encodes the preprotein containing putative RLRF amide peptides and was expressed in the nervous system of late stage embryos and larvae. The embryonic and larval localization of the sNPF peptide in the nervous systems revealed the larval central nervous system neural circuit from the neurons in the brain to thoracic axons and to connective axons in the ventral ganglion. In the adult brain, the sNPF peptide was localized in the medulla and the mushroom body. However, the sNPF peptide was not detected in the gut. The sNPF mRNA and the peptide were expressed during all developmental stages from embryo to adult. From the feeding assay, the gain-of-function sNPF mutants expressed in nervous systems promoted food intake, whereas the loss-of-function mutants suppressed food intake. Also, sNPF overexpression in nervous systems produced bigger and heavier flies. These findings indicate that the sNPF is expressed in the nervous systems to control food intake and regulate body size in Drosophila melanogaster.

A flexible activin explains the membrane-dependent cooperative assembly of TGF-beta family receptors

A new crystal structure of activin in complex with the extracellular domain of its type II receptor (ActRIIb-ECD) shows that the ligand exhibits an unexpected flexibility. The motion in the activin dimer disrupts its type I receptor interface, which may account for the disparity in its affinity for type I versus type II receptors. We have measured the affinities of activin and its antagonist inhibin for ActRIIb-ECD and found that the affinity of the 2-fold symmetric homodimer activin for ActRIIb-ECD depends on the availability of two spatially coupled ActRIIb-ECD molecules, whereas the affinity of the heterodimer inhibin does not. Our results indicate that activin's affinity for its two receptor types is greatly influenced by their membrane-restricted setting. We propose that activin affinity is modulated by the ligand flexibility and that cooperativity is achieved by binding to two ActRII chains that immobilize activin in a type I binding-competent orientation.

Wounds increase activin in skin and a vasoactive neuropeptide in sensory ganglia

Successful healing of skin wounds requires sensory innervation and the release of vasoactive neuropeptides that dilate blood vessels and deliver serum proteins to the wound, and that cause pain that protects from further injury. Activin has been proposed as a target-derived regulator of sensory neuropeptides during development, but its role in the mature nervous system is unknown. While adult skin contains a low level of activin, protein levels in skin adjacent to a wound increase rapidly after an excision. Neurons containing the neuropeptide calcitonin gene-related peptide (CGRP) increased in sensory ganglia that projected to the wounded skin, but not in ganglia that projected to unwounded skin, suggesting that neurons respond to a local skin signal. Indeed, many adult sensory neurons respond with increased CGRP expression to the application of activin in vitro and utilize a smad-mediated signal transduction pathway in this response. A second skin-derived factor nerve growth factor (NGF) also increased in wounded skin and increased CGRP in cultured adult dorsal root ganglia (DRG) neurons but with lower efficacy. Together, these data support the hypothesis that activin made by skin cells regulates changes in sensory neuropeptides following skin injury, thereby promoting vasodilation and wound healing.

Unique accumulation of neuropeptides in an insect: FMRFamide-related peptides in the cockroach, Periplaneta americana

FMRFamides belong to the most extensively studied neuropeptides in invertebrates and exhibit diverse physiological effects on different target organs, such as muscles, intestine and the nervous system. This study on the American cockroach confirms for the first time that extended FMRFamides occur in non-dipteran insects. By means of tandem mass spectrometry, these neuropeptides were structurally elucidated, and sequence information was used for subsequent cloning of the cockroach FMRFamide gene. This precursor gene encodes for 24 putative peptides and shows sufficient similarity with the Drosophila FMRFamide gene. Of the 24 peptides, 23 were detected by mass spectrometric methods; it is the highest number of neuropeptide forms shown to be expressed from a single precursor in any insect. The expression was traced back to single neurons in the thoracic ganglia. The unique accumulation of these FMRFamide-related peptides in thoracic perisympathetic organs provides the definite evidence for a tagma-specific distribution of peptidergic neurohormones in neurohaemal release sites of the insect CNS. Excitatory effects of the cockroach FMRFamides were observed on antenna-heart preparations. In addition, the newly described FMRFamides reduce the spike frequency of dorsal-unpaired median neurons and reduce the intracellular calcium concentration, which may affect the peripheral release of the biogenic amine octopamine.

Computational identification of developmental enhancers: conservation and function of transcription factor binding-site clusters in Drosophila melanogaster and Drosophila pseudoobscura

27 predicted gene-regulatory regions in the Drosophila melanogaster genome were analyzed in vivo, confirming 15 active enhancer regions. A comparison with Drosophila pseudoobscura sequences revealed that conservation of binding-site clusters accurately discriminates functional regions from non-functional ones.

Background

The identification of sequences that control transcription in metazoans is a major goal of genome analysis. In a previous study, we demonstrated that searching for clusters of predicted transcription factor binding sites could discover active regulatory sequences, and identified 37 regions of the Drosophila melanogaster genome with high densities of predicted binding sites for five transcription factors involved in anterior-posterior embryonic patterning. Nine of these clusters overlapped known enhancers. Here, we report the results of in vivo functional analysis of 27 remaining clusters.

Results

We generated transgenic flies carrying each cluster attached to a basal promoter and reporter gene, and assayed embryos for reporter gene expression. Six clusters are enhancers of adjacent genes: giant, fushi tarazu, odd-skipped, nubbin, squeeze and pdm2; three drive expression in patterns unrelated to those of neighboring genes; the remaining 18 do not appear to have enhancer activity. We used the Drosophila pseudoobscura genome to compare patterns of evolution in and around the 15 positive and 18 false-positive predictions. Although conservation of primary sequence cannot distinguish true from false positives, conservation of binding-site clustering accurately discriminates functional binding-site clusters from those with no function. We incorporated conservation of binding-site clustering into a new genome-wide enhancer screen, and predict several hundred new regulatory sequences, including 85 adjacent to genes with embryonic patterns.

Conclusions

Measuring conservation of sequence features closely linked to function - such as binding-site clustering - makes better use of comparative sequence data than commonly used methods that examine only sequence identity.

A targeted gain of function screen in the embryonic CNS of Drosophila

In order to identify genes involved in the development of the central nervous system (CNS) we have undertaken a gain of function screen in the embryonic CNS of Drosophila. Transposable P-elements and the UAS/GAL4 system were used to initiate transcription of genes in a pan-neural pattern using scaGAL4. Over 4100 individual P-element insertion lines were screened with monoclonal antibodies BP102 and 1D4 to visualize axon pathways. Twenty-five P-element insertions corresponding to 18 genes resulted in aberrant CNS axon pathfinding when misexpressed with scaGAL4. Genes involved in axon guidance, embryonic patterning, and cell cycle regulation were isolated. In addition, we identified several zinc finger transcription factors not previously implicated in axon guidance or CNS development. This group includes Squeeze, Kruppel homolog-1, Hepatocyte nuclear factor 4, and two uncharacterized genes, CG11966 and CG9650. Calnexin99A, a putative molecular chaperone, was isolated as well.

Drosophila uses two distinct neuropeptide amidating enzymes, dPAL1 and dPAL2

Neuropeptide alpha-amidation is a common C-terminal modification of secretory peptides, frequently required for biological activity. In mammals, amidation is catalyzed by the sequential actions of two enzymes [peptidylglycine-alpha-hydroxylating monooxygenase (PHM) and peptidyl-alpha-hydroxyglycine alpha-amidating lyase (PAL)] that are co-synthesized within a single bifunctional precursor. The Drosophila genome predicts expression of one monofunctional PHM gene and two monofunctional PAL genes. Drosophila PHM encodes an active enzyme that is required for peptide amidation in vivo. Here we initiate studies of the two Drosophila PAL genes. dPAL1 has two predicted transmembrane domains, whereas dPAL2 is predicted to be soluble and secreted. dPAL2 expressed in heterologous cells is secreted readily and co-localized with hormone. In contrast, dPAL1 is secreted poorly, even when expressed with a cleaved signal replacing the predicted transmembrane domains; the majority of dPAL1 stays in the endoplasmic reticulum. Both proteins display PAL enzymatic activity. Compared to the catalytic core of rat PAL, the two Drosophila lyases have higher K(m) values, higher pH optima and similarly broad divalent metal ion requirements. Antibodies to dPAL1 and dPAL2 reveal co-expression in many identified neuroendocrine neurons. Although dPAL1 is broadly expressed, dPAL2 is found in only a limited subset of neurons. dPAL1 expression is highly correlated with the non-amidated peptide proctolin. Tissue immunostaining demonstrates that dPAL1 is largely localized to the cell soma, whereas dPAL2 is distributed throughout neuronal processes.

Axial patterning of C. elegans male sensilla identities by selector genes

The fan and rays of the C. elegans male tail constitute a compound sensory organ essential for mating. Within this organ, the individual sensilla, known as rays, have unique identities. We show that ray identities are patterned by a selector gene mechanism in a manner similar to other serially homologous axial structures. One selector gene that promotes the identities of a subset of the rays is the Hox gene egl-5. Within EGL-5-expressing rays, further patterning is provided by a Pax-6 homolog and a signal of the TGFbeta family. These genes and pathway coordinately specify multiple ray properties affecting all three terminal ray cell types. These properties include complex patterns of FMRFamide-like (FaRP) neuropeptides, serotonin (5HT) and dopamine expression, and ray morphology. Differences in these differentiated characteristics give each sensillum a unique identity and potentially endow the compound ray organ with a higher-order information gathering capacity.

Retrograde regulation in the CNS; neuron-specific interpretations of TGF-beta signaling

Retrograde signals influence neuronal survival, differentiation, synaptogenesis, and plasticity. Several recent papers describe novel roles for the well-studied TGF-beta pathway in retrograde synaptic signaling. While each dissects spatial and molecular aspects of TGF-beta signaling in a specific synaptic context, together these studies demonstrate that a specific retrograde signal may be interpreted in diverse, neuron-specific ways. Thus, a neuron's intrinsic properties and its other extrinsic signaling inputs determine its cellular and genomic response to TGF-beta.

Orchestrating development and function: retrograde BMP signaling in the Drosophila nervous system

Recent work has shown that bone morphogenetic protein (BMP) growth factors regulate development of the larval neuromuscular junction (NMJ) of Drosophila. Intriguingly, the same BMP growth factors also influence the expression of circulating hormones that modulate the physiological properties of NMJs. Together, the results suggest that retrograde growth factor signaling by BMPs integrates neuromuscular development and function at both local and global levels in the animal.

Ap-let neurons--a peptidergic circuit potentially controlling ecdysial behavior in Drosophila

Here we describe a novel set of peptidergic neurons conserved throughout all developmental stages in the Drosophila central nervous system (CNS). We show that a small complement of 28 apterous-expressing cells (Ap-let neurons) in the ventral nerve cord (VNC) of Drosophila larvae co-express numerous gene products. The products include the neuroendocrine-specific bHLH regulator called Dimmed (Dimm), four neuropeptide biosynthetic enzymes (PC2, Fur1, PAL2, and PHM), and a specific dopamine receptor subtype (dDA1). For the PC2, Fur1, and PAL2 enzymes, and for the dDA1 receptor, this neuronal pattern represents the vast majority of their total expression in the VNC. In addition, while Dimm and PHM are present in the peritracheal Inka cells in larvae, pupae, and adults, Ap, PC2, Fur1, PAL2, and dDA1 are not. PC2, PAL2, and DA1 receptor expression were all controlled by both dimm and ap. Previous genetic analysis of animals deficient in PC2 revealed an abnormal larval ecdysis phenotype. Together, these data support the hypothesis that the small cohort of Ap-let interneurons regulates larval ecdysis behavior by secretion of an unidentified amidated peptide(s). This hypothesis further predicts that the production of the Ap-let neuropeptide(s) is dependent on each of four specific enzymes, and that a certain aspect(s) of its production and/or release is regulated by dopamine input.

Specification of chemosensory neuron subtype identities in Caenorhabditis elegans

Cellular diversity in the nervous system arises from the presence of multiple neuronal subtypes, each of which is specialized to perform a unique function. Work in Caenorhabditis elegans has begun to reveal the pathways that are essential for the specification of identities of neuronal subtypes in its chemosensory system. The functions of each chemosensory neuron subtype are specified by distinct developmental cascades, using molecules from well-conserved transcription factor families. Additional cellular complexity is generated by novel mechanisms that further diversify the identities of the left and right members of a bilateral sensory neuron pair.

DAF-5 is a Ski oncoprotein homolog that functions in a neuronal TGF beta pathway to regulate C. elegans dauer development

An unconventional TGF beta superfamily pathway plays a crucial role in the decision between dauer diapause and reproductive growth. We have studied the daf-5 gene, which, along with the daf-3 Smad gene, is antagonized by upstream receptors and receptor-regulated Smads. We show that DAF-5 is a novel member of the Sno/Ski superfamily that binds to DAF-3 Smad, suggesting that DAF-5, like Sno/Ski, is a regulator of transcription in a TGF beta superfamily signaling pathway. However, we present evidence that DAF-5 is an unconventional Sno/Ski protein, because DAF-5 acts as a co-factor, rather than an antagonist, of a Smad protein. We show that expressing DAF-5 in the nervous system rescues a daf-5 mutant, whereas muscle or hypodermal expression does not. Previous work suggested that DAF-5 and DAF-3 function in pharyngeal muscle to regulate gene expression, but our analysis of regulation of a pharynx specific promoter suggests otherwise. We present a model in which DAF-5 and DAF-3 control the production or release of a hormone from the nervous system by either regulating the expression of biosynthetic genes or by altering the connectivity or the differentiated state of neurons.

Mechanisms of Synapse Assembly and Disassembly

The mechanisms that govern synapse formation and elimination are fundamental to our understanding of neural development and plasticity. The wiring of neural circuitry requires that vast numbers of synapses be formed in a relatively short time. The subsequent refinement of neural circuitry involves the formation of additional synapses coincident with the disassembly of previously functional synapses. There is increasing evidence that activity-dependent plasticity also involves the formation and disassembly of synapses. While we are gaining insight into the mechanisms of both synapse assembly and disassembly, we understand very little about how these phenomena are related to each other and how they might be coordinately controlled to achieve the precise patterns of synaptic connectivity in the nervous system. Here, we review our current understanding of both synapse assembly and disassembly in an effort to unravel the relationship between these fundamental developmental processes.

Retrograde Gbb signaling through the Bmp type 2 receptor wishful thinking regulates systemic FMRFa expression in Drosophila

Amidated neuropeptides of the FMRFamide class regulate numerous physiological processes including synaptic efficacy at the Drosophila neuromuscular junction (NMJ). We demonstrate here that mutations in wishful thinking (wit) a gene encoding a Drosophila Bmp type 2 receptor that is required for proper neurotransmitter release at the neuromuscular junction, also eliminates expression of FMRFa in that subset of neuroendocrine cells (Tv neurons) which provide the systemic supply of FMRFa peptides. We show that Gbb, a Bmp ligand expressed in the neurohemal organ provides a retrograde signal that helps specify the peptidergic phenotype of the Tv neurons. Finally, we show that supplying FMRFa in neurosecretory cells partially rescues the wit lethal phenotype without rescuing the primary morphological or electrophysiological defects of wit mutants. We propose that Wit and Gbb globally regulate NMJ function by controlling both the growth and transmitter release properties of the synapse as well as the expression of systemic modulators of NMJ synaptic activity.

Direct signaling by the BMP type II receptor via the cytoskeletal regulator LIMK1

Bone morphogenetic proteins (BMPs) regulate multiple cellular processes, including cell differentiation and migration. Their signals are transduced by the kinase receptors BMPR-I and BMPR-II, leading to Smad transcription factor activation via BMPR-I. LIM kinase (LIMK) 1 is a key regulator of actin dynamics as it phosphorylates and inactivates cofilin, an actin depolymerizing factor. During a search for LIMK1-interacting proteins, we isolated clones encompassing the tail region of BMPR-II. Although the BMPR-II tail is not involved in BMP signaling via Smad proteins, mutations truncating this domain are present in patients with primary pulmonary hypertension (PPH). Further analysis revealed that the interaction between LIMK1 and BMPR-II inhibited LIMK1's ability to phosphorylate cofilin, which could then be alleviated by addition of BMP4. A BMPR-II mutant containing the smallest COOH-terminal truncation described in PPH failed to bind or inhibit LIMK1. This study identifies the first function of the BMPR-II tail domain and suggests that the deregulation of actin dynamics may contribute to the etiology of PPH.