Drosophila photoreceptors express cysteine peptidase tan

Abstract

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.

Development and evolution of insect pigmentation: Genetic mechanisms and the potential consequences of pleiotropy

“…These genes are expressed in epidermal cells, where they are used to synthesize pigments (Fig. 3), as well as in neurons and neuron-associated cells, where they putatively influence behavior [55][56][57]. The Tan protein catalyzes the conversion of N-beta-analyl-dopamine to dopamine during melanin production, and of carcinine to histamine during neurotransmission [58].…”

“…The Tan protein catalyzes the conversion of N-beta-analyl-dopamine to dopamine during melanin production, and of carcinine to histamine during neurotransmission [58]. Ebony catalyzes the reverse hydrolysis reaction, suggesting it is playing a complementary role to Tan in the cycling of neurotransmitters [57]. The enzyme Dopa-decarboxylase functions upstream of Ebony and Tan in dopamine synthesis, and is required for the production of both histamine and serotonin neurotransmitters [59].…”

Pigmentation and behavior: potential association through pleiotropic genes in <i>Drosophila</i>

“…It is suggested that Ebony functions as an enzyme which conjugates histamine to beta-alanine in the glia close to histamine release sites of photoreceptor cells in the lamina (Borycz et al, 2002;Richardt et al, 2002Richardt et al, , 2003True et al, 2005;Wagner et al, 2007;Gavin et al, 2007). This reaction terminates the action of the histamine released into the synaptic cleft in response to light by removing it by glial cells, where it is converted into carcinine (beta-alanyl histamine) by Ebony (see Fig.…”

Natural Variation of ebony Gene Controlling Thoracic Pigmentation in Drosophila melanogaster

“…ebony mutants are also known to have defects in neuronal activities at photoreceptor terminals (Hotta and Benzer 1969;Heisenberg 1971;Borycz et al 2002), arrhythmic locomotor activity (Newby and Jackson 1991), and unusual sexual behavior (Kyriacou et al 1978;Kyriacou 1981). These pleiotropic effects are likely to stem from the EBONY expression in the nervous systems (Hovemann et al 1998;Richardt et al 2002Richardt et al , 2003Wagner et al 2007); thus, variation of this gene is likely to have some impact on individual fitness under natural conditions.…”

“…Immunocytochemical and reporter gene assays have shown that EBONY is expressed in multiple tissues including the epidermal cells of thorax segment (Hovemann et al 1998;Richardt et al 2002Richardt et al , 2003Wittkopp et al 2002;Gibert et al 2007;Wagner et al 2007), in which the highest level of the EBONY expression is found in cells that produce the trident pigmentation in ebony mutants (Wittkopp et al 2002). Also, e-lacZ transformants showed high expression of the ebony transcripts at the trident producing cells (Gibert et al 2007).…”