Visual neurotransmission in Drosophila requires expression of Fic in glial capitate projections

Abstract

Fic domains can catalyze the addition of adenosine monophosphate to target proteins. To date, the function of Fic domain proteins in eukaryotic physiology remains unknown. We generated genetic models of the single Drosophila Fic domain–containing protein, Fic. Flies lacking Fic were viable and fertile, but blind. Photoreceptor cells depolarized normally following light stimulation, but failed to activate postsynaptic neurons, as indicated by the loss of ON transients in electroretinograms, consistent with a neurotransmission defect. Functional rescue of neurotransmission required expression of enzymatically active Fic on capitate projections of glia cells, but not neurons, supporting a role in the recycling of the visual neurotransmitter histamine. Histamine levels were reduced in the lamina of Fic null flies, and dietary histamine partially restored ON transients. These findings establish a previously unknown regulatory mechanism in visual neurotransmission and provide, to the best of our knowledge, the first evidence for a role of glial capitate projections in neurotransmitter recycling.

Unfolded Protein Response-regulated Drosophila Fic (dFic) Protein Reversibly AMPylates BiP Chaperone during Endoplasmic Reticulum Homeostasis

“…In an attempt to understand the physiological function of AMPylation in eukaryotes, we knocked out the gene encoding the Drosophila FicD protein (dfic). We found that dFic enzymatic activity is required in glial cells for visual neurotransmission in flies (15). In addition, this study demonstrated that the catalytic domain of dFic resides in the lumen of the ER, where it is glycosylated.…”

“…Previous studies have shown that flies without functional dFic in glial cells have impaired visual neurotransmission (15). This suggests that dFic substrate could be a component of visual signaling or a transporter of neurotransmitters.…”

HypE-specific Nanobodies as Tools to Modulate HypE-mediated Target AMPylation

“…Eukaryotes, including Caenorhabditis elegans, Drosophila melanogaster, Mus musculus, and Homo sapiens, encode at least one Fic protein each, with the exception of most fungi, which contain no known AMPylases (16,17). The Drosophila Fic protein, referred to as CG9523 or dFic, possesses auto-AMPylation activity in vitro; this activity is diminished upon mutation of the conserved histidine residue within the Fic motif (18). Mutant flies that lack dFic are blind, but otherwise viable.…”

Global Profiling of Huntingtin-associated protein E (HYPE)-Mediated AMPylation through a Chemical Proteomic Approach

“…Indeed, the E234G mutation substantially boosts HYPE's activity as demonstrated by the elevated autoAMPylation of HYPE itself (5, 9) and a few of its recently reported substrates, including the ER chaperone BiP in vivo (14,15) and several histone proteins in vitro (16,17). HYPE activity was initially implicated in visual neurotransmission in flies (18) and later in regulation of the unfolded protein response (UPR) in transfected cells, although there is limited consensus over the mechanism (14,15). Most recently, it has been proposed that HYPE activity might have a role in regulation of gene expression; however, the mechanistic details remain to be elucidated (17).…”