Constitutively active presynaptic release factor Unc13A prevents homeostatic sleep rebound in Drosophila

One of the major characteristics of sleep is homeostatic sleep rebound following sleep loss. While the molecular mechanisms of baseline sleep regulation have been intensively studied, a specific molecular understanding of sleep rebound remains elusive. Here, we show that a constitutively active form of the Munc13-family presynaptic release factor Unc13A, which lacks the inhibitory Ca²⁺/calmodulin interaction domain (Unc13A^WRWR), dominantly suppressed homeostatic sleep rebound following acute sleep deprivation, leading to a near-complete elimination of recovery sleep ("Reboundless"). In contrast, baseline sleep remained largely normal. Through a genetic modifier screen, we identified partial loss of αSnap, a cofactor of N-ethylmaleimide-sensitive factor (NSF) required for disassembly and recycling of post-fusion cis-SNARE complex, as a potent suppressor of the reboundless phenotype of Unc13A^WRWR. Given that Unc13A promotes the formation of the fusion-competent trans-SNARE complex, these findings implicate that sleep rebound critically depends on a delicate balance between SNARE complex assembly and recycling. Since Unc13A^WRWR has been shown to promote presynaptic release probability (P_r), we speculate that active Unc13A suppresses rebound sleep likely by increasing P_r and subsequently enhancing synaptic transmission. Consistently, an elevated dependence on trans-SNARE formation and cis-SNARE recycling seems to explain the functional and behavioral phenotypes of Unc13A^WRWR. Taken together, our data suggest a fundamental role of P_r mediated by Unc13A and SNARE dynamics in sleep homeostasis.