Mushroom body subsets encode CREB2-dependent water-reward long-term memory in Drosophila

Abstract

Long-term memory (LTM) formation depends on the conversed cAMP response elementbinding protein (CREB)-dependent gene transcription followed by de novo protein synthesis. Thirsty fruit flies can be trained to associate an odor with water reward to form waterreward LTM (wLTM), which can last for over 24 hours without a significant decline. The role of de novo protein synthesis and CREB-regulated gene expression changes in neural circuits that contribute to wLTM remains unclear. Here, we show that acute inhibition of protein synthesis in the mushroom body (MB) αβ or γ neurons during memory formation using a cold-sensitive ribosome-inactivating toxin disrupts wLTM. Furthermore, adult stage-specific expression of dCREB2b in αβ or γ neurons also disrupts wLTM. The MB αβ and γ neurons can be further classified into five different neuronal subsets including αβ core, αβ surface, αβ posterior, γ main, and γ dorsal. We observed that the neurotransmission from αβ surface and γ dorsal neuron subsets is required for wLTM retrieval, whereas the αβ core, αβ posterior, and γ main are dispensable. Adult stage-specific expression of dCREB2b in αβ surface and γ dorsal neurons inhibits wLTM formation. In vivo calcium imaging revealed that αβ surface and γ dorsal neurons form wLTM traces with different dynamic properties, and these memory traces are abolished by dCREB2b expression. Our results suggest that a small population of neurons within the MB circuits support long-term storage of water-reward memory in Drosophila.

Serotonin Signals Modulate Mushroom Body Output Neurons for Sustaining Water-Reward Long-Term Memory in Drosophila

“…It remains unclear why these distinct α′β′-related MBONs act at different periods during wLTM consolidation. We speculate that MBON-γ3α′1, MBON-α′1α′3, and MBON-α′2 are indirectly connected to the γ-dorsal and αβ-surface MB subset neurons, which are required for CREB2- and protein synthesis-dependent wLTM ( Lee et al, 2020 ).…”

“…Leucokinin controls water memory expression by inhibiting two classes of dopaminergic neurons in the fly brain to permit thirsty-dependent water memory expression (Senapati et al, 2019). wLTM formation in Drosophila requires the expression of several genes associated with learning and memory within the brain, as observed in mammals (Scharf et al, 2002;Cepeda et al, 2006;Shyu et al, 2017;Lee et al, 2020).…”

“…The motivational state of fruit flies, such as the thirst for water, induces a more efficient, strong, and robust long-lasting memory compared to the shock-punitive aversive memory ( Tully et al, 1994 ; Shyu et al, 2017 ). One-session training for pairing of an odor with water induces not only water-reward short-term memory in a few minutes to hours, but also a water-reward LTM (wLTM) that can persist for approximately 2 days ( Lin et al, 2014 ; Shyu et al, 2017 ; Lee et al, 2020 ). It has been shown that thirsty-dependent water memory is mediated by neuropeptide leucokinin, which is released from a pair of neurons in the fly brain.…”

“…After memory acquisition, neurotransmission from MB α′β′ neurons is required for wLTM consolidation. Finally, the fly requires neurotransmission from specific subsets of αβ neurons, γ neurons, and their MBONs to recall the wLTM ( Shyu et al, 2017 ; Lee et al, 2020 ). It remains unclear how wLTM is transferred from α′β′ neurons to αβ neurons and γ neuron subsets.…”

DataSheet1.docx

“…It remains unclear why these distinct α′β′-related MBONs act at different periods during wLTM consolidation. We speculate that MBON-γ3α′1, MBON-α′1α′3, and MBON-α′2 are indirectly connected to the γ-dorsal and αβ-surface MB subset neurons, which are required for CREB2-and protein synthesisdependent wLTM (Lee et al, 2020).…”