The sleep gene insomniac ubiquitinates targets at postsynaptic densities and is required for retrograde homeostatic signaling

Abstract

27The nervous system confronts challenges during development and experience that can 28 destabilize information processing. To adapt to these perturbations, synapses homeostatically 29 adjust synaptic strength, a process referred to as homeostatic synaptic plasticity. At the 30 Drosophila neuromuscular junction, inhibition of postsynaptic glutamate receptors activates 31 retrograde signaling that precisely increases presynaptic neurotransmitter release to restore 32 baseline synaptic strength. However, the nature of the underlying postsynaptic induction 33 process remains enigmatic. Here, we designed a forward genetic screen to identify factors 34 necessary in the postsynaptic compartment to generate retrograde homeostatic signaling. This 35 approach identified insomniac (inc), a gene that encodes a putative adaptor for the Cullin-3 36 ubiquitin ligase complex and is essential for normal sleep regulation. Intriguingly, we find that 37Inc rapidly traffics to postsynaptic densities and is required for increased ubiquitination following 38 acute receptor inhibition. Our study suggests that Inc-dependent ubiquitination, 39 compartmentalized at postsynaptic densities, gates retrograde signaling and provides an 40 intriguing molecular link between the control of sleep behavior and homeostatic plasticity at 41 synapses. 42 43 homeostatically enhance neurotransmitter release during PHP (Kiragasi et al., 2017; Li et al., 70 2018c;Weyhersmuller et al., 2011). Furthermore, candidate molecules 71 involved in retrograde signaling have been proposed (Orr et al., 2017;Wang et al., 2014). 72However, despite these significant insights, forward genetic screens have failed to shed light on 73 the postsynaptic mechanisms that induce retrograde signaling, a process that remains 74 enigmatic Goel et al., 2017;Hauswirth et al., 2018). 75 Little is known about the signal transduction system in the postsynaptic compartment 76 that initiates retrograde homeostatic communication. It is clear that pharmacological blockade or 77 genetic loss of GluRIIA-containing receptors initiates retrograde PHP signaling. Perturbation of 78 these receptors lead to reduced levels of active (phosphorylated) Ca 2+ /calmodulin-dependent 79 protein kinase II (CaMKII) (Goel et al., 2017; Haghighi et al., 2003; Li et al., 2018c; Newman et 80 al., 2017). However, inhibition of postsynaptic CaMKII activity alone is not sufficient to induce 81 PHP expression (Haghighi et al., 2003), suggesting that additional signaling in the postsynaptic 82 compartment is required to generate retrograde communication. Furthermore, rapid PHP 83 signaling induced by pharmacological receptor blockade does not require new protein synthesis 84 (Frank et al., 2006; Goel et al., 2017). Finally, CaMKII signaling is compartmentalized at 85 postsynaptic densities, where PHP can be expressed with specificity at synapses with 86 diminished receptor function (Li et al., 2018b; Newman et al., 2017), suggesting that retrograde 87 communication happens locally between individual pre-and post-s...

Homeostatic scaling of active zone scaffolds maintains global synaptic strength

“…A recent study also found that AZ remodeling is blocked in brp mutants but that rapid PHP is expressed (Böhme et al, 2019). This implies other mechanisms, such as phosphorylation or ubiquitination, are necessary to express rapid PHP (Hauswirth et al, 2018; Kikuma et al, 2018 Preprint ; Wentzel et al, 2018). Over chronic timescales, however, the sustained expression of PHP requires both arl8 and brp (Fig.…”

“…We have presented evidence for a mirrored process across the presynaptic terminals of individual neurons, where the abundance of AZ material is adaptively scaled to maintain stable synaptic strength. These pre- and postsynaptic processes could work independently or in conjunction to adapt to altered innervation (Davis and Goodman, 1998; Branco et al, 2008; Tokuoka and Goda, 2008), excitability (Murthy et al, 2001; Paradis et al, 2001), activity (Butz et al, 2009; Holtmaat and Svoboda, 2009), injury (Goel and Dickman, 2018), and even sleep (Bushey et al, 2011; Hengen et al, 2016; Diering et al, 2017; Kikuma et al, 2018 Preprint ). The complex interplay of Hebbian and homeostatic regulation operating locally and globally, over distinct spatiotemporal scales and at both pre- and postsynaptic compartments, endows synapses with this astonishing combination of flexibility and stability.…”