Long-distance mechanism of neurotransmitter recycling mediated by glial network facilitates visual function in Drosophila

Abstract

Neurons rely on glia to recycle neurotransmitters such as glutamate and histamine for sustained signaling. Both mammalian and insect glia form intercellular gap-junction networks, but their functional significance underlying neurotransmitter recycling is unknown. Using the Drosophila visual system as a genetic model, here we show that a multicellular glial network transports neurotransmitter metabolites between perisynaptic glia and neuronal cell bodies to mediate long-distance recycling of neurotransmitter. In the first visual neuropil (lamina), which contains a multilayer glial network, photoreceptor axons release histamine to hyperpolarize secondary sensory neurons. Subsequently, the released histamine is taken up by perisynaptic epithelial glia and converted into inactive carcinine through conjugation with β-alanine for transport. In contrast to a previous assumption that epithelial glia deliver carcinine directly back to photoreceptor axons for histamine regeneration within the lamina, we detected both carcinine and β-alanine in the fly retina, where they are found in photoreceptor cell bodies and surrounding pigment glial cells. Downregulating Inx2 gap junctions within the laminar glial network causes β-alanine accumulation in retinal pigment cells and impairs carcinine synthesis, leading to reduced histamine levels and photoreceptor synaptic vesicles. Consequently, visual transmission is impaired and the fly is less responsive in a visual alert analysis compared with wild type. Our results suggest that a gap junction-dependent laminar and retinal glial network transports histamine metabolites between perisynaptic glia and photoreceptor cell bodies to mediate a novel, long-distance mechanism of neurotransmitter recycling, highlighting the importance of glial networks in the regulation of neuronal functions.

Cryptochrome 2 mediates directional magnetoreception in cockroaches

“…Colocalization of Cry2 with antisera raised against the alpha subunit of Na/K-ATPase (32) (SI Appendix, Fig. S10 A-C) resulted in a pattern similar in structure to Drosophila epithelial glial cells (33,34), suggesting these cells are glia localized underneath the retina between the two basement membranes (35). Higher magnification revealed an orderly columnar alignment of the Cry2-positive cells (Fig.…”

Drosophila Vision Depends on Carcinine Uptake by an Organic Cation Transporter

“…The continuation of visual transmission depends on the recycling of histamine, in which the visual glia play a pivotal role. The released histamine is taken up by epithelial glia (Edwards and Meinertzhagen, 2010) , converted into carcinine through Ebony (Richardt et al, 2002; Ziegler et al, 2013) , and delivered back to photoreceptors either directly or indirectly through the laminar glia network (Chaturvedi et al, 2014; Romero-Calderón et al, 2008). In the photoreceptor, carcinine is cleaved by Tan to regenerate histamine (Borycz et al, 2002; Gavin et al, 2007) .…”

“…The VAR assay was performed as described (Chaturvedi et al, 2014) . An overnight starved fly was placed into a vertical white chamber (5.5 × 2 × 0.5 cm) covered with a thin glass slide from one side.…”

“…Carcinine can be released into the laminar neuropil region and directly transported back into photoreceptor axons. Alternatively, it can also be transported to the retina via the gap-junctional glial network (Saint Marie and Carlson, 1985) and delivered into the cell bodies of photoreceptors (Chaturvedi et al, 2014) . In the photoreceptor, carcinine is cleaved by the peptidase Tan to regenerate histamine and β-alanine (Borycz et al, 2002; Gavin et al, 2007) .…”

Quantification of Histamine and Carcinine in Drosophila melanogaster Tissues

“…6 Glial cells typically uptake histamine and then convert it to carcinine, which is β-alanyl histamine. 7 The enzyme for this conversion, N-β-alanyl-dopamine synthase (also called Ebony), also converts dopamine, and several other biogenic amines, into a β-alanyl derivative. 8 Conversion of dopamine to N-β-alanyl dopamine is important for cuticle scleritization and pigmentation; mutants deficient in N-β-alanyl-dopamine synthase appear dark ( ebony ) and also exhibit an altered visual processing phenotype.…”