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EMBO Rep. 2019 Mar;20(3). pii: e46975. doi: 10.15252/embr.201846975. Epub 2019 Jan 28.

The E3 ligase Highwire promotes synaptic transmission by targeting the NAD-synthesizing enzyme dNmnat.

Author information

1
Department of Developmental Biology, Washington University in St. Louis School of Medicine, St. Louis, MO, USA.
2
Department of Neurobiology, University of Southern California, Los Angeles, CA, USA.
3
Oak Crest Institute of Science, Monrovia, CA, USA.
4
Department of Developmental Biology, Washington University in St. Louis School of Medicine, St. Louis, MO, USA diantonio@wustl.edu.

Abstract

The ubiquitin ligase Highwire restrains synaptic growth and promotes evoked neurotransmission at NMJ synapses in Drosophila Highwire regulates synaptic morphology by downregulating the MAP3K Wallenda, but excess Wallenda signaling does not account for the decreased presynaptic release observed in highwire mutants. Hence, Highwire likely has a second substrate that inhibits neurotransmission. Highwire targets the NAD+ biosynthetic and axoprotective enzyme dNmnat to regulate axonal injury responses. dNmnat localizes to synapses and interacts with the active zone protein Bruchpilot, leading us to hypothesize that Highwire promotes evoked release by downregulating dNmnat. Here, we show that excess dNmnat is necessary in highwire mutants and sufficient in wild-type larvae to reduce quantal content, likely via disruption of active zone ultrastructure. Catalytically active dNmnat is required to drive defects in evoked release, and depletion of a second NAD+ synthesizing enzyme is sufficient to suppress these defects in highwire mutants, suggesting that excess NAD+ biosynthesis is the mechanism inhibiting neurotransmission. Thus, Highwire downregulates dNmnat to promote evoked synaptic release, suggesting that Highwire balances the axoprotective and synapse-inhibitory functions of dNmnat.

KEYWORDS:

DLK; NAD+ biosynthesis; Nmnat; synaptic development; synaptic transmission

PMID:
30692130
PMCID:
PMC6399608
[Available on 2020-03-01]
DOI:
10.15252/embr.201846975

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