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

Abstract

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. © 2013 Wiley Periodicals, Inc.

Drosophila Ebony: a novel type of nonribosomal peptide synthetase related enzyme with unusually fast peptide bond formation kinetics

“…After cleavage by Tan, balanine is thought to return into epithelial glia via pigment cells. As previously proposed [20], transport of carcinine into and of released b-alanine out of photoreceptor cells might depend on an antiporter shuttle system. The high concentration of b-alanine detected in head [36] and shown to accumulate in retinal and lamina glia [38] could push the equilibrium of the Ebony loading reaction towards the pre-activated form and prime it for histamine inactivation.…”

“…In the Drosophila eye Ebony is expressed in glia [18][19][20]. In the lamina, it is preferentially found in epithelial glia, which unsheathes the synaptic cleft of photoreceptor axonal endings.…”

“…In the lamina, it is preferentially found in epithelial glia, which unsheathes the synaptic cleft of photoreceptor axonal endings. Weak cooperative binding of released histamine to postsynaptic chloride channels and exclusion of a direct re-uptake into the photoreceptor axon [20] calls for fast transport of histamine into this glia. There, it is inactivated by Ebony-catalyzed conjugation with b-alanine [21].…”