Shintaro Naganos

Fasting Launches CRTC to Facilitate Long-Term Memory Formation in Drosophila

Hunger and Memory During starvation, are all brain functions slowed down, or are specific functions disabled to save energy? Plaçais and Preat (p. 440) investigated how the brain of Drosophila deals with severe resource limitation. The brain cut selected expenses to reduce the threat to survival and switched off the formation of aversive long-term memory that depends on costly protein synthesis. However, Hirano et al. (p. 443) focused on mild food-deprivation, which actually enhanced long-term memory formation. Presumably, improved memory should enhance survival when competing for limited food. After longer food deprivation, enhancement of aversive long-term memory decreased, while that of appetitive long-term memory remained high: Presumably, as starvation nears, it becomes more important to pursue food at all costs, and so appetitive memory takes precedence over aversive memories. Different types of memory interact in fed versus starved states to promote survival-oriented behavior. Canonical aversive long-term memory (LTM) formation in Drosophila requires multiple spaced trainings, whereas appetitive LTM can be formed after a single training. Appetitive LTM requires fasting prior to training, which increases motivation for food intake. However, we found that fasting facilitated LTM formation in general; aversive LTM formation also occurred after single-cycle training when mild fasting was applied before training. Both fasting-dependent LTM (fLTM) and spaced training–dependent LTM (spLTM) required protein synthesis and cyclic adenosine monophosphate response element–binding protein (CREB) activity. However, spLTM required CREB activity in two neural populations—mushroom body and DAL neurons—whereas fLTM required CREB activity only in mushroom body neurons. fLTM uses the CREB coactivator CRTC, whereas spLTM uses the coactivator CBP. Thus, flies use distinct LTM machinery depending on their hunger state.

Protein kinase A inhibits a consolidated form of memory in Drosophila

Increasing activity of the cAMP/protein kinase A (PKA) pathway has often been proposed as an approach to improve memory in various organisms. However, here we demonstrate that single-point mutations, which decrease PKA activity, dramatically improve aversive olfactory memory in Drosophila. These mutations do not affect formation of early memory phases or of protein synthesis-dependent long-term memory but do cause a significant increase in a specific consolidated form of memory, anesthesia-resistant memory. Significantly, heterozygotes of null mutations in PKA are sufficient to cause this memory increase. Expressing a PKA transgene in the mushroom bodies, brain structures critical for memory formation in Drosophila, reduces memory back to wild-type levels. These results indicate that although PKA is critical for formation of several memory phases, it also functions to inhibit at least one memory phase.

Glial Dysfunction Causes Age-Related Memory Impairment in Drosophila

Several aging phenotypes, including age-related memory impairment (AMI), are thought to be caused by cumulative oxidative damage. In Drosophila, age-related impairments in 1 hr memory can be suppressed by reducing activity of protein kinase A (PKA). However, the mechanism for this effect has been unclear. Here we show that decreasing PKA suppresses AMI by reducing activity of pyruvate carboxylase (PC), a glial metabolic enzyme whose amounts increase upon aging. Increased PC activity causes AMI through a mechanism independent of oxidative damage. Instead, increased PC activity is associated with decreases in D-serine, a glia-derived neuromodulator that regulates NMDA receptor activity. D-serine feeding suppresses both AMI and memory impairment caused by glial overexpression of dPC, indicating that an oxidative stress-independent dysregulation of glial modulation of neuronal activity contributes to AMI in Drosophila.

Mutations in the Drosophila insulin receptor substrate, CHICO, impair olfactory associative learning

CHICO, the Drosophila homolog of vertebrate insulin receptor substrate (IRS), mediates insulin/insulin-like growth factor signaling (IIS), and reductions in chico severely disrupt cell growth and proliferation. We found extensive expression of chico in various Drosophila brain regions including the mushroom bodies (MBs), critical neural structures for olfactory learning. chico null mutants have significantly reduced brain sizes and perform poorly in an olfactory associative learning task, although their sensitivity to the odors and electric shocks used in this learning paradigm are normal. When initial memory is normalized by training for different amounts of time (short-duration training protocols), memory retention and retrieval in chico flies are indistinguishable from that of wild-type flies, demonstrating that chico mutants are defective specifically for memory formation. Inducing expression of a chico(+) transgene in neurons throughout development restores normal learning in a chico background, while inducing chico(+) specifically at the adult stage does not, suggesting that chico is required for development of a brain region required for forming olfactory associations. Significantly, expressing chico(+) in the MBs restores the number of MB neurons to wild-type amounts and also rescues chico learning defects. Our results suggest that chico-dependent growth of the MBs is essential for development of learning ability.

Long‐term enhancement of synaptic transmission between antennal lobe and mushroom body in cultured Drosophila brain

•  During olfactory aversive conditioning in Drosophila, odour and shock information are delivered to the mushroom bodies (MBs) through projection neurons in the antennal lobes (ALs) and ascending fibres of the ventral nerve cord (AFV), respectively. •  Using an isolated cultured brain expressing a Ca2+ indicator in the MBs, we demonstrated that the simultaneous stimulation of the ALs and AFV establishes long‐term enhancement (LTE) in AL‐induced Ca2+ responses. •  The physiological properties of LTE, including associativity, input specificity and persistence, are highly reminiscent of those of olfactory memory. •  Similar to olfactory aversive memory, LTE requires the activation of nicotinic acetylcholine receptors that mediate the AL‐evoked Ca2+ response, NMDA receptors that mediate the AFV‐induced Ca2+ response, and D1 dopamine receptors during the simultaneous stimulation of the ALs and AFV. •  Considering the physiological and genetic analogies, we propose that LTE at the AL–MB synapse can be a relevant cellular model for olfactory memory.

Coincident postsynaptic activity gates presynaptic dopamine release to induce plasticity in Drosophila mushroom bodies

Simultaneous stimulation of the antennal lobes (ALs) and the ascending fibers of the ventral nerve cord (AFV), two sensory inputs to the mushroom bodies (MBs), induces long-term enhancement (LTE) of subsequent AL-evoked MB responses. LTE induction requires activation of at least three signaling pathways to the MBs, mediated by nicotinic acetylcholine receptors (nAChRs), NMDA receptors (NRs), and D1 dopamine receptors (D1Rs). Here, we demonstrate that inputs from the AL are transmitted to the MBs through nAChRs, and inputs from the AFV are transmitted by NRs. Dopamine signaling occurs downstream of both nAChR and NR activation, and requires simultaneous stimulation of both pathways. Dopamine release requires the activity of the rutabaga adenylyl cyclase in postsynaptic MB neurons, and release is restricted to MB neurons that receive coincident stimulation. Our results indicate that postsynaptic activity can gate presynaptic dopamine release to regulate plasticity. DOI: http://dx.doi.org/10.7554/eLife.21076.001

Learning defects in Drosophila growth restricted chico mutants are caused by attenuated adenylyl cyclase activity

BackgroundReduced insulin/insulin-like growth factor signaling (IIS) is a major cause of symmetrical intrauterine growth retardation (IUGR), an impairment in cell proliferation during prenatal development that results in global growth defects and mental retardation. In Drosophila, chico encodes the only insulin receptor substrate. Similar to other animal models of IUGR, chico mutants have defects in global growth and associative learning. However, the physiological and molecular bases of learning defects caused by chico mutations, and by symmetrical IUGR, are not clear.ResultsIn this study, we found that chico mutations impair memory-associated synaptic plasticity in the mushroom bodies (MBs), neural centers for olfactory learning. Mutations in chico reduce expression of the rutabaga-type adenylyl cyclase (rut), leading to decreased cAMP synthesis in the MBs. Expressing a rut+ transgene in the MBs restores memory-associated plasticity and olfactory associative learning in chico mutants, without affecting growth. Thus chico mutations disrupt olfactory learning, at least in part, by reducing cAMP signaling in the MBs.ConclusionsOur results suggest that some cognitive defects associated with reduced IIS may occur, independently of developmental defects, from acute reductions in cAMP signaling.

Carbon monoxide, a retrograde messenger generated in post-synaptic mushroom body neurons evokes local dopamine release

Dopaminergic neurons innervate extensive areas of the brain and release dopamine (DA) onto a wide range of target neurons. However, DA release is also precisely regulated, and in Drosophila, DA is released specifically onto mushroom body (MB) neurons, which have been coincidentally activated by cholinergic and glutamatergic inputs. The mechanism for this precise release has been unclear. Here we found that coincidentally activated MB neurons generate carbon monoxide (CO) which functions as a retrograde signal evoking local DA release from presynaptic terminals. CO production depends on activity of heme oxygenase in post-synaptic MB neurons, and CO-evoked DA release requires Ca2+ efflux through ryanodine receptors in DA terminals. CO is only produced in MB areas receiving coincident activation, and removal of CO using scavengers blocks DA release. We propose that DA neurons utilize two distinct modes of transmission to produce global and local DA signaling. SIGNIFICANCE STATEMENT Dopamine (DA) is needed for various higher brain functions including memory formation. However, DA neurons form extensive synaptic connections, while memory formation requires highly specific and localized DA release. Here we identify a mechanism through which DA release from presynaptic terminals is controlled by postsynaptic activity. Postsynaptic neurons activated by cholinergic and glutamatergic inputs generate carbon monoxide, which acts as a retrograde messenger inducing presynaptic DA release. Released DA is required for memory-associated plasticity. Our work identifies a novel mechanism that restricts DA release to the specific postsynaptic sites that require DA during memory formation.