Uptake of precursor and synthesis of transmitter in a histaminergic photoreceptor

Downregulation of DmMANF in Glial Cells Results in Neurodegeneration and Affects Sleep and Lifespan in Drosophila melanogaster

In Drosophila melanogaster, mesencephalic astrocyte-derived neurotrophic factor (DmMANF) is an evolutionarily conserved ortholog of mammalian MANF and cerebral dopamine neurotrophic factor (CDNF), which have been shown to promote the survival of dopaminergic neurons in the brain. We observed especially high levels of DmMANF in the visual system of Drosophila, particularly in the first optic neuropil (lamina). In the lamina, DmMANF was found in glial cells (surface and epithelial glia), photoreceptors and interneurons. Interestingly, silencing of DmMANF in all neurons or specifically in photoreceptors or L2 interneurons had no impact on the structure of the visual system. However, downregulation of DmMANF in glial cells induced degeneration of the lamina. Remarkably, this degeneration in the form of holes and/or tightly packed membranes was observed only in the lamina epithelial glial cells. Those membranes seem to originate from the endoplasmic reticulum, which forms autophagosome membranes. Moreover, capitate projections, the epithelial glia invaginations into photoreceptor terminals that are involved in recycling of the photoreceptor neurotransmitter histamine, were less numerous after DmMANF silencing either in neurons or glial cells. The distribution of the alpha subunit of Na+/K+-ATPase protein in the lamina cell membranes was also changed. At the behavioral level, silencing of DmMANF either in neurons or glial cells affected the daily activity/sleep pattern, and flies showed less activity during the day but higher activity during the night than did controls. In the case of silencing in glia, the lifespan of flies was also shortened. The obtained results showed that DmMANF regulates many functions in the brain, particularly those dependent on glial cells.

Activity and coexpression of Drosophila black with ebony in fly optic lobes reveals putative cooperative tasks in vision that evade electroretinographic detection

Drosophila mutants black and ebony show pigmentation defects in the adult cuticle, which disclose their cooperative activity in β-alanyl-dopamine formation. In visual signal transduction, Ebony conjugates β-alanine to histamine, forming β-alanyl-histamine or carcinine. Mutation of ebony disrupts signal transduction and reveals an electroretinogram (ERG) phenotype. In contrast to the corresponding cuticle phenotype of black and ebony, there is no ERG phenotype observed when black expression is disrupted. This discrepancy calls into question the longstanding assumption of Black and Ebony interaction. The purpose of this study was to investigate the role of Black and Ebony in fly optic lobes. We excluded a presynaptic histamine uptake pathway and confirmed histamine recycling via carcinine formation in glia. β-Alanine supply for this pathway is independent of enzymatic synthesis by Black and β-alanine synthase Pyd3. Two versions of Black are expressed in vivo. Black is a specific aspartate decarboxylase with no activity on glutamate. RNA in situ hybridization and anti-Black antisera localized Black expression in the head. Immunolabeling revealed expression in lamina glia, in large medulla glia, in glia of the ocellar ganglion, and in astrocyte-like glia below the ocellar ganglion. In these glia types, Black expression is strictly accompanied by Ebony expression. Activity, localization, and strict coexpression with Ebony strongly indicate a specific mode of functional interaction that, however, evades ERG detection.

The evolutionary origins of glia

The evolutionary origins of glia are lost in time, as soft tissues rarely leave behind fossil footprints, and any molecular footprints they might have been left we have yet to decipher. Nevertheless, because of the growing realization of the importance glia plays in the development and functioning of the nervous system, lessons we can draw about commonalities among different taxa (including vertebrates) brought about either from a common origin, or from common adaptational pressures, shed light on the roles glia play in all nervous systems. The Acoelomorpha, primitive interstitial flatworms with very simple cellular organization and currently at the base of the bilaterian phylogeny, possess glia-like cells. If they indeed represent the ancestors of all other Bilateria, then it is possible that all glias derive from a common ancestor. However, basal taxa lacking convincing glia are found in most major phyletic lines: urochordates, hemichordates, bryozoans, rotifers, and basal platyhelminths. With deep phylogenies currently in flux, it is equally possible that glia in several lines had different origins. If developmental patterns are any indication, glia evolved from ectodermal cells, possibly from a mobile lineage, and even possibly independently in different regions of the body. As to what functions might have brought about the evolution of glia, by-product removal, structural support, phagocytic needs, developmental programming, and circuit modulation may be the more likely. Explaining possible cases of glial loss is more difficult, as once evolved, glia appears to keep inventing new functions, giving it continued value even after the original generative need becomes obsolete. Among all the uncertainties regarding the origin of glia, one thing is certain: that our ideas about those origins will change with every rearrangement in deep phylogeny and with continued advances in invertebrate molecular and developmental areas.

Alternative tasks of Drosophila tan in neurotransmitter recycling versus cuticle sclerotization disclosed by kinetic properties

Upon a stimulus of light, histamine is released from Drosophila photoreceptor axonal endings. It is taken up into glia where Ebony converts it into beta-alanyl-histamine (carcinine). Carcinine moves into photoreceptor cells and is there cleaved into beta-alanine and histamine by Tan activity. Tan thus provides a key function in the recycling pathway of the neurotransmitter histamine. It is also involved in the process of cuticle formation. There, it cleaves beta-alanyl-dopamine, a major component in cuticle sclerotization. Active Tan enzyme is generated by a self-processing proteolytic cleavage from a pre-protein at a conserved Gly-Cys sequence motif. We confirmed the dependence on the Gly-Cys motif by in vitro mutagenesis. Processing time delays the rise to full Tan activity up to 3 h behind its putative circadian RNA expression in head. To investigate its pleiotropic functions, we have expressed Tan as a His(6) fusion protein in Escherichia coli and have purified it to homogeneity. We found wild type and mutant His(6)-Tan protein co-migrating in size exclusion chromatography with a molecular weight compatible with homodimer formation. We conclude that dimer formation is preceding pre-protein processing. Drosophila tan(1) null mutant analysis revealed that amino acid Arg(217) is absolutely required for processing. Substitution of Met(256) in tan(5), on the contrary, does not affect processing extensively but renders it prone to degradation. This also leads to a strong tan phenotype although His(6)-Tan(5) retains activity. Kinetic parameters of Tan reveal characteristic differences in K(m) and k(cat) values of carcinine and beta-alanyl-dopamine cleavage, which conclusively illustrate the divergent tasks met by Tan.

The dynamics of signaling at the histaminergic photoreceptor synapse of arthropods

Histamine, a ubiquitous aminergic messenger throughout the body, also serves as a neurotransmitter in both vertebrates and invertebrates. In particular, the photoreceptors of adult arthropods use histamine, modulating its release to signal increases and decreases in light intensity. Strong evidence from various arthropod species indicates that histamine is synthesized and stored in photoreceptors, undergoes Ca-dependent release, inhibits postsynaptic interneurons by gating Cl channels, and is then recycled. In Drosophila, the synthetic enzyme, histidine decarboxylase, and the subunits of the histamine-gated chloride channel have been cloned. Possible histamine transporters at synaptic vesicles and for reuptake remain elusive. Indeed, the mechanisms that remove histamine from the synaptic cleft, and that help terminate histamine's action, are unexpectedly complex, their details remaining unresolved. A major pathway in Drosophila, and possibly other arthropod species, is by conjugation of histamine to beta-alanine to form carcinine in adjacent glia. This conjugate then returns to the photoreceptors where it is hydrolysed to liberate histamine, which is then loaded into synaptic vesicles. Evidence from other species suggests that direct reuptake of histamine into the photoreceptors may also occur. Light depolarizes the photoreceptors, causing histamine release and postsynaptic inhibition; dimming hyperpolarizes the photoreceptors, causing a decrease in histamine release and an "off" response in the postsynaptic cell. Further pursuit of histamine's action at these highly specialized synapses should lead to an understanding of how they signal minute changes in presynaptic membrane potential, how they reliably extract signals from noise, and how they adapt to a wide range of presynaptic membrane potentials.

Drosophila photoreceptors express cysteine peptidase tan

The Drosophila mutant tan (t) shows reciprocal pigmentation defects compared with the ebony (e) mutant. Visual phenotypes, however, are similar in both flies: Electroretinogram (ERG) recordings lack "on" and "off" transients, an indication of impaired synaptic transmission to postsynaptic cells L1 and L2. Cloning of tan revealed transcription of the gene in the retina, apparently in photoreceptor cells. We expressed Tan in Escherichia coli and confirmed by Western blotting and mass spectroscopic analyses that Tan is expressed as preprotein, followed by proteolytic cleavage into two subunits at a conserved --Gly--Cys-- motif like its fungal ortholog isopenicillin-N N-acyltransferase (IAT). Tan thus belongs to the large family of cysteine peptidases. To discriminate expression of Tan and Ebony in retina and optic neuropils, we raised antisera against specific Tan peptides. Testing for colocalization with GMR-driven n-Syb-GFP labeling revealed that Tan expression is confined to the photoreceptor cells R1-R8. A close proximity of Tan and Ebony expression is evident in lamina cartridges, where three epithelial glia cells envelop the six photoreceptor terminals R1-R6. In the medulla, R7/R8 axonal terminals appeared lined up side by side with glial extensions. This local proximity supports a model for Drosophila visual synaptic transmission in which Tan and Ebony interact biochemically in a putative histamine inactivation and recycling pathway in Drosophila.

Ebony, a novel nonribosomal peptide synthetase for beta-alanine conjugation with biogenic amines in Drosophila

Using Ebony protein either expressed in Escherichia coli or in Schneider S2 cells, we provide evidence for its substrate specificity and reaction mechanism. Ebony activates beta-alanine to aminoacyladenylate by an adenylation domain and covalently attaches it as a thioester to a thiolation domain in a nonribosomal peptide synthetase (NRPS) related mechanism. In a second reaction, biogenic amines act as external nucleophiles on beta-alanyl-S-pantetheine-Ebony, thereby releasing in a fast reaction the dipeptide (peptidoamine) in a process that is novel in higher eucaryotes. Therefore, we define Ebony as a beta-alanyl-biogenic amine synthetase. Insight into the reaction mechanism stems from mutational analysis of an invariant serine that disclosed Ebony as a multienzyme with functional analogy to the starting modules of NRPSs. In light of a putative biogenic amine-deactivating capacity, Ebony function in the nervous system must be reconsidered. We propose that in the Drosophila eye Ebony is involved in the transmission process by inactivation of histamine through beta-alanyl conjugation.

Mitochondria are redistributed in Drosophila photoreceptors lacking milton, a kinesin-associated protein

Photoreceptors are richly supplied with mitochondria, where they are required to meet the energetic demands, in the soma, of phototransduction and, in the terminal, of neurotransmitter release. Compromising the latter, we have made photoreceptors R1-R6 in Drosophila ommatidia homozygous for either of two alleles, milt(186) and milt(92), of milton in whole-eye mosaics. Such mutant photoreceptors fail to target mitochondria to their terminals. We show from quantitative electron microscopy (EM) that mitochondria are totally lacking at the terminal but nevertheless abundant and present throughout the soma, where their distribution differs from that of control ommatidia, however, being more heavily concentrated in the nuclear region. Mitochondria are sparse at the basalmost level of mutant ommatidia, and are lacking beneath the basement membrane, in the axons and terminals of these cells. The absence of mitochondria from R1-R6 terminals and concommitant reductions in synaptic vesicle packing density, previously reported, we show here are accompanied by reduced immunoreactivity to the photoreceptor transmitter histamine but not by any change in total head histamine content, as determined by high-performance liquid chromatography. Mutant terminals also contain vesicle profiles with a wider range of sizes. These two phenotypes suggest that the reduced availability of ATP when mutant terminals lack a mitochondrial supply compromises their ability to pump histamine into synaptic vesicles and perturbs membrane distribution within the terminal. In addition, a band of somata in the lamina cortex, at least some of which are postsynaptic neurons not homozygous for milton, also shows altered mitochondrial targeting, with abnormal clusters of mitochondria, as visualized by immunolabeling with anti-hsp and by serial EM. Within the lamina, terminals of mutant photoreceptors are penetrated by neighboring cells with invaginations that frequently contain mitochondria, suggesting that a mechanism exists for intercellular metabolic support. Our findings indicate the direct and compensatory responses in a population of neurons when mitochondria are not correctly targeted to their synaptic terminals.

tan and ebony genes regulate a novel pathway for transmitter metabolism at fly photoreceptor terminals

In Drosophila melanogaster, ebony and tan, two cuticle melanizing mutants, regulate the conjugation (ebony) of beta-alanine to dopamine or hydrolysis (tan) of the beta-alanyl conjugate to liberate dopamine. beta-alanine biosynthesis is regulated by black. ebony and tan also exert unexplained reciprocal defects in the electroretinogram, at ON and OFF transients attributable to impaired transmission at photoreceptor synapses, which liberate histamine. Compatible with this impairment, we show that both mutants have reduced histamine contents in the head, as measured by HPLC, and have correspondingly reduced numbers of synaptic vesicles in their photoreceptor terminals. Thus, the histamine phenotype is associated with sites of synaptic transmission at photoreceptors. We demonstrate that when they receive microinjections into the head, wild-type Sarcophaga bullata (in whose larger head such injections are routinely possible) rapidly (<5 sec) convert exogenous [3H]histamine into its beta-alanine conjugate, carcinine, a novel metabolite. Drosophila tan has an increased quantity of [3H]carcinine, the hydrolysis of which is blocked; ebony lacks [3H]carcinine, which it cannot synthesize. Confirming these actions, carcinine rescues the histamine phenotype of ebony, whereas beta-alanine rescues the carcinine phenotype of black;tan double mutants. The equilibrium ratio between [3H]carcinine and [3H]histamine after microinjecting wild-type Sarcophaga favors carcinine hydrolysis, increasing to only 0.5 after 30 min. Our findings help resolve a longstanding conundrum of the involvement of tan and ebony in photoreceptor function. We suggest that reversible synthesis of carcinine occurs in surrounding glia, serving to trap histamine after its release at photoreceptor synapses; subsequent hydrolysis liberates histamine for reuptake.

Ebony protein in the Drosophila nervous system: optic neuropile expression in glial cells

The Drosophila ebony mutation (Bridges and Morgan, [1923] Publs Carnegie Inst Wash 327:50) reveals a pleiotropic phenotype with cuticular and behavioral defects. To understand Ebony function in the nervous system, particularly in transmission of the visual signal, it is essential to know the cell type and temporal characteristics of its expression throughout development. Therefore, we raised an antiserum against an Ebony peptide to detect the protein in whole-mount and slice preparations of Drosophila. Attention was focused on ebony expression in the adult optic neuropiles of the fly. Colocalization of Ebony with neuronal or glial cell markers in frozen sections showed non-neuronal expression of ebony in the lamina and medulla neuropiles. Furthermore, colocalization with glial cell markers demonstrated glial expression of ebony in epithelial glia of the lamina and neuropile glia of the distal medulla. This finding was confirmed for the lamina epithelial glia by electron microscopic examination of immunolabeling by using the diaminobenzidine method. These glia have in common that they match the two sites of histamine release from the compound eye's photoreceptors. Possible ways in which the biochemical activity of Ebony might function with respect to histamine release are considered.

Histamine metabolism in the visual system of the horseshoe crab Limulus polyphemus

There is now strong evidence that arthropod photoreceptors use histamine as a neurotransmitter. The synthesis, storage and release of histamine from arthropod photoreceptors have been demonstrated, and the postsynaptic effects of histamine and the endogenous neurotransmitter are similar. However, a full understanding of these photoreceptor synapses also requires knowledge of histamine inactivation and metabolism. Relatively little is known about histamine metabolism in the nervous system of arthropods, and mechanisms appear to differ with the species. This study focuses on histamine metabolism in visual tissues of the horseshoe crab Limulus polyphemus, a chelicerate. We present two major findings: (1) histamine is metabolized to imidazole acetic acid and to gamma-glutamyl histamine. (2) relatively low levels of histamine metabolites accumulate in Limulus visual tissues.