Function of the Drosophila receptor guanylyl cyclase Gyc76C in PlexA-mediated motor axon guidance

Two distinct mechanisms of Plexin A function in Drosophila optic lobe lamination and morphogenesis

Visual circuit development is characterized by subdivision of neuropils into layers that house distinct sets of synaptic connections. We find that, in the Drosophila medulla, this layered organization depends on the axon guidance regulator Plexin A. In Plexin A null mutants, synaptic layers of the medulla neuropil and arborizations of individual neurons are wider and less distinct than in controls. Analysis of semaphorin function indicates that Semaphorin 1a, acting in a subset of medulla neurons, is the primary partner for Plexin A in medulla lamination. Removal of the cytoplasmic domain of endogenous Plexin A has little effect on the formation of medulla layers; however, both null and cytoplasmic domain deletion mutations of Plexin A result in an altered overall shape of the medulla neuropil. These data suggest that Plexin A acts as a receptor to mediate morphogenesis of the medulla neuropil, and as a ligand for Semaphorin 1a to subdivide it into layers. Its two independent functions illustrate how a few guidance molecules can organize complex brain structures by each playing multiple roles.

Two distinct mechanisms of Plexin A function in Drosophila optic lobe lamination and morphogenesis

Visual circuit development is characterized by subdivision of neuropils into layers that house distinct sets of synaptic connections. We find that in the Drosophila medulla, this layered organization depends on the axon guidance regulator Plexin A. In plexin A null mutants, synaptic layers of the medulla neuropil and arborizations of individual neurons are wider and less distinct than in controls. Analysis of Semaphorin function indicates that Semaphorin 1a, provided by cells that include Tm5 neurons, is the primary partner for Plexin A in medulla lamination. Removal of the cytoplasmic domain of endogenous Plexin A does not disrupt the formation of medulla layers; however, both null and cytoplasmic domain deletion mutations of plexin A result in an altered overall shape of the medulla neuropil. These data suggest that Plexin A acts as a receptor to mediate morphogenesis of the medulla neuropil, and as a ligand for Semaphorin 1a to subdivide it into layers. Its two independent functions illustrate how a few guidance molecules can organize complex brain structures by each playing multiple roles.

Summary statement

The axon guidance molecule Plexin A has two functions in Drosophila medulla development; morphogenesis of the neuropil requires its cytoplasmic domain, but establishing synaptic layers through Semaphorin 1a does not.

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

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

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

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

•

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

•

Gyc76C modulates NF-κB signaling through downstream Toll receptor components

•

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

•

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

A Receptor Guanylate Cyclase, Gyc76C, Mediates Humoral, and Cellular Responses in Distinct Ways in Drosophila Immunity

Innate immunity is an evolutionarily conserved host defense system against infections. The fruit fly Drosophila relies solely on innate immunity for infection defense, and the conservation of innate immunity makes Drosophila an ideal model for understanding the principles of innate immunity, which comprises both humoral and cellular responses. The mechanisms underlying the coordination of humoral and cellular responses, however, has remained unclear. Previously, we identified Gyc76C, a receptor-type guanylate cyclase that produces cyclic guanosine monophosphate (cGMP), as an immune receptor in Drosophila. Gyc76C mediates the induction of antimicrobial peptides for humoral responses by a novel cGMP pathway including a membrane-localized cGMP-dependent protein kinase, DG2, through downstream components of the Toll receptor such as dMyD88. Here we show that Gyc76C is also required for the proliferation of blood cells (hemocytes) for cellular responses to bacterial infections. In contrast to Gyc76C-dependent antimicrobial peptide induction, Gyc76C-dependent hemocyte proliferation is meditated by a small GTPase, Ras85D, and not by DG2 or dMyD88, indicating that Gyc76C mediates the cellular and humoral immune responses in distinct ways.

GIPC proteins negatively modulate Plexind1 signaling during vascular development

Semaphorins (SEMAs) and their Plexin (PLXN) receptors are central regulators of metazoan cellular communication. SEMA-PLXND1 signaling plays important roles in cardiovascular, nervous, and immune system development, and cancer biology. However, little is known about the molecular mechanisms that modulate SEMA-PLXND1 signaling. As PLXND1 associates with GIPC family endocytic adaptors, we evaluated the requirement for the molecular determinants of their association and PLXND1's vascular role. Zebrafish that endogenously express a Plxnd1 receptor with a predicted impairment in GIPC binding exhibit low penetrance angiogenesis deficits and antiangiogenic drug hypersensitivity. Moreover, gipc mutant fish show angiogenic impairments that are ameliorated by reducing Plxnd1 signaling. Finally, GIPC depletion potentiates SEMA-PLXND1 signaling in cultured endothelial cells. These findings expand the vascular roles of GIPCs beyond those of the Vascular Endothelial Growth Factor (VEGF)-dependent, proangiogenic GIPC1-Neuropilin 1 complex, recasting GIPCs as negative modulators of antiangiogenic PLXND1 signaling and suggest that PLXND1 trafficking shapes vascular development.

Semaphorins and their Signaling Mechanisms

Semaphorins are extracellular signaling proteins that are essential for the development and maintenance of many organs and tissues. The more than 20-member semaphorin protein family includes secreted, transmembrane and cell surface-attached proteins with diverse structures, each characterized by a single cysteine-rich extracellular sema domain, the defining feature of the family. Early studies revealed that semaphorins function as axon guidance molecules, but it is now understood that semaphorins are key regulators of morphology and motility in many different cell types including those that make up the nervous, cardiovascular, immune, endocrine, hepatic, renal, reproductive, respiratory and musculoskeletal systems, as well as in cancer cells. Semaphorin signaling occurs predominantly through Plexin receptors and results in changes to the cytoskeletal and adhesive machinery that regulate cellular morphology. While much remains to be learned about the mechanisms underlying the effects of semaphorins, exciting work has begun to reveal how semaphorin signaling is fine-tuned through different receptor complexes and other mechanisms to achieve specific outcomes in various cellular contexts and physiological systems. These and future studies will lead to a more complete understanding of semaphorin-mediated development and to a greater understanding of how these proteins function in human disease.

Motor axon guidance in Drosophila

The Drosophila motor system starts to assemble during embryonic development. It is composed of 30 muscles per abdominal hemisegment and 36 motor neurons assembling into nerve branches to exit the CNS, navigate within the muscle field and finally establish specific connections with their target muscles. Several families of guidance molecules that play a role controlling this process as well as transcriptional regulators that program the behavior of specific motor neuron have been identified. In this review we summarize the role of both groups of molecules in the motor system as well as their relationship where known. It is apparent that partially redundant guidance protein families and membrane molecules with different functional output direct guidance decisions cooperatively. Some distinct transcriptional regulators seem to control guidance of specific nerve branches globally directing the expression of groups of pathfinding molecules in all motor neurons within the same motor branch.

Structure analyses reveal a regulated oligomerization mechanism of the PlexinD1/GIPC/myosin VI complex

The GIPC family adaptor proteins mediate endocytosis by tethering cargo proteins to the myosin VI motor. The structural mechanisms for the GIPC/cargo and GIPC/myosin VI interactions remained unclear. PlexinD1, a transmembrane receptor that regulates neuronal and cardiovascular development, is a cargo of GIPCs. GIPC-mediated endocytic trafficking regulates PlexinD1 signaling. Here, we unravel the mechanisms of the interactions among PlexinD1, GIPCs and myosin VI by a series of crystal structures of these proteins in apo or bound states. GIPC1 forms a domain-swapped dimer in an autoinhibited conformation that hinders binding of both PlexinD1 and myosin VI. PlexinD1 binding to GIPC1 releases the autoinhibition, promoting its interaction with myosin VI. GIPCs and myosin VI interact through two distinct interfaces and form an open-ended alternating array. Our data support that this alternating array underlies the oligomerization of the GIPC/Myosin VI complexes in solution and cells.

DOI:http://dx.doi.org/10.7554/eLife.27322.001

Post-endocytic sorting of Plexin-D1 controls signal transduction and development of axonal and vascular circuits

Local endocytic events involving receptors for axon guidance cues play a central role in controlling growth cone behaviour. Yet, little is known about the fate of internalized receptors, and whether the sorting events directing them to distinct endosomal pathways control guidance decisions. Here, we show that the receptor Plexin-D1 contains a sorting motif that interacts with the adaptor protein GIPC1 to facilitate transport to recycling endosomes. This sorting process promotes colocalization of Plexin-D1 with vesicular pools of active R-ras, leading to its inactivation. In the absence of interaction with GIPC1, missorting of Plexin-D1 results in loss of signalling activity. Consequently, Gipc1 mutant mice show specific defects in axonal projections, as well as vascular structures, that rely on Plexin-D1 signalling for their development. Thus, intracellular sorting steps that occur after receptor internalization by endocytosis provide a critical level of control of cellular responses to guidance signals.

Molecular mechanisms controlling axonal growth cone behaviour are only partially understood. Here the authors reveal a role of an adaptor protein GIPC1 in Plexin-D1 receptor recycling, and show that this process is required for axon track formation and vascular patterning in mice.

Glia Get Neurons in Shape

Glial cells are essential components of the nervous system. In this issue, Singhvi et al. uncover cellular and molecular mechanisms through which C. elegans glia shape sensory neuron terminals and thus control animal thermosensing behaviors.

Receptor-Type Guanylyl Cyclase at 76C (Gyc76C) Regulates De Novo Lumen Formation during Drosophila Tracheal Development

Lumen formation and maintenance are important for the development and function of essential organs such as the lung, kidney and vasculature. In the Drosophila embryonic trachea, lumena form de novo to connect the different tracheal branches into an interconnected network of tubes. Here, we identify a novel role for the receptor type guanylyl cyclase at 76C (Gyc76C) in de novo lumen formation in the Drosophila trachea. We show that in embryos mutant for gyc76C or its downsteam effector protein kinase G (PKG) 1, tracheal lumena are disconnected. Dorsal trunk (DT) cells of gyc76C mutant embryos migrate to contact each other and complete the initial steps of lumen formation, such as the accumulation of E-cadherin (E-cad) and formation of an actin track at the site of lumen formation. However, the actin track and E-cad contact site of gyc76C mutant embryos did not mature to become a new lumen and DT lumena did not fuse. We also observed failure of the luminal protein Vermiform to be secreted into the site of new lumen formation in gyc76C mutant trachea. These DT lumen formation defects were accompanied by altered localization of the Arf-like 3 GTPase (Arl3), a known regulator of vesicle-vesicle and vesicle-membrane fusion. In addition to the DT lumen defect, lumena of gyc76C mutant terminal cells were shorter compared to wild-type cells. These studies show that Gyc76C and downstream PKG-dependent signaling regulate de novo lumen formation in the tracheal DT and terminal cells, most likely by affecting Arl3-mediated luminal secretion.

cGMP-Dependent Protein Kinase Encoded by foraging Regulates Motor Axon Guidance in Drosophila by Suppressing Lola Function

Correct pathfinding and target recognition of a developing axon are exquisitely regulated processes that require multiple guidance factors. Among these factors, the second messengers, cAMP and cGMP, are known to be involved in establishing the guidance cues for axon growth through different intracellular signaling pathways. However, whether and how cGMP-dependent protein kinase (PKG) regulates axon guidance remains poorly understood. Here, we show that the motor axons of intersegmental nerve b (ISNb) in the Drosophila embryo display targeting defects during axon development in the absence of foraging(for), a gene encoding PKG.In vivo tag expression revealed PKG to be present in the ventral nerve code at late embryonic stages, supporting its function in embryonic axon guidance. Mechanistic studies showed that the transcription factor longitudinal lacking(lola) genetically interacts with for.PKG physically associates with the LolaT isoform via the C-terminal zinc-finger-containing domain. Overexpression of PKG leads to the cytoplasmic retention of LolaT in S2 cells, suggesting a role for PKG in mediating the nucleocytoplasmic trafficking of Lola. Together, these findings reveal a novel function of PKG in regulating the establishment of neuronal connectivity by sequestering Lola in the cytoplasm.

SIGNIFICANCE STATEMENT

Axon pathfinding and target recognition are important processes in the formation of specific neuronal connectivity, which rely upon precise coordinated deployment of multiple guidance factors. This paper reveals the role of cGMP-dependent protein kinase (PKG) in regulating the pathfinding and targeting of the developing axons in Drosophila Moreover, our study indicates that PKG regulates the cytoplasmic-nuclear trafficking of the transcription factor LolaT, suggesting a mechanism of PKG in directing motor axon guidance. These findings highlight a new function of PKG in axon guidance by suppressing a transcription factor.

The Gyc76C Receptor Guanylyl Cyclase and the Foraging cGMP-Dependent Kinase Regulate Extracellular Matrix Organization and BMP Signaling in the Developing Wing of Drosophila melanogaster

The developing crossveins of the wing of Drosophila melanogaster are specified by long-range BMP signaling and are especially sensitive to loss of extracellular modulators of BMP signaling such as the Chordin homolog Short gastrulation (Sog). However, the role of the extracellular matrix in BMP signaling and Sog activity in the crossveins has been poorly explored. Using a genetic mosaic screen for mutations that disrupt BMP signaling and posterior crossvein development, we identify Gyc76C, a member of the receptor guanylyl cyclase family that includes mammalian natriuretic peptide receptors. We show that Gyc76C and the soluble cGMP-dependent kinase Foraging, likely linked by cGMP, are necessary for normal refinement and maintenance of long-range BMP signaling in the posterior crossvein. This does not occur through cell-autonomous crosstalk between cGMP and BMP signal transduction, but likely through altered extracellular activity of Sog. We identify a novel pathway leading from Gyc76C to the organization of the wing extracellular matrix by matrix metalloproteinases, and show that both the extracellular matrix and BMP signaling effects are largely mediated by changes in the activity of matrix metalloproteinases. We discuss parallels and differences between this pathway and other examples of cGMP activity in both Drosophila melanogaster and mammalian cells and tissues.

Signaling between cells regulates many processes, including the choices cells make between different fates during development and regeneration, and misregulation of such signaling underlies many human pathologies. To understand how such signals control developmental decisions, it is necessary to elucidate both how cells regulate and respond to different levels of signaling, and how different types of signals combine and regulate each other. We have used genetic screening in the fruitfly Drosophila melanogaster to identify mutations that reduce or eliminate signals carried by Bone Morphogenetic Proteins (BMPs), and show that BMP signaling is sensitive Gyc76C, a peptide receptor that stimulates the production of cGMP in cells. We identify downstream intracellular effectors of this cGMP activity, but provide evidence that the effects on the BMP pathway are not mediated at the intracellular level, but rather through cGMP’s effects upon the extracellular matrix and matrix-remodeling proteinases, which in turn affects the activity of extracellular BMP-binding proteins. We discuss differences and parallels with other examples of cGMP activity in Drosophila melanogaster and mammals.