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Activity-dependent synaptic plasticity at the Drosophila larval NMJ. (A) Calcium accumulation, along with neuronal activity, regulates the calcium/calmodulin-dependent adenylate cyclase, rutabaga. Dunce and rutabaga regulate the level of cAMP. Cell adhesion molecule Fas II and cAMP response element-binding proteins (CREB) are downstream of the cAMP pathway and responsible for increasing sprouting and pre-synaptic transmitter release. PKA and PP1 are also regulated by increased cAMP levels, and they, in turn, regulate CaMKII. The PKA-PP1-CaMKII interaction controls the withdrawal/retraction process through Plexin B receptor and Semaphorin-2a (Sema2a) chemorepulsion. The synaptic localization of Discs large (DLG) regulated by CaMKII controls the clustering of synaptic molecules, such as Fas II or Shaker. (B) The secretion of Wg proteins is enhanced in a calcium-dependent manner. The activation of a bidirectional Wg signaling pathway causes the nuclear import of the C-terminal fragment of Dfrizzled-2 (DFz2, receptor of Wg) at the post-synaptic terminal of NMJ and the rearrangement of pre-synaptic terminal structures involving the Shaggy/GSK-3β kinase, which controls the organization of the cytoskeleton and number of boutons through Futsch at the pre-synaptic terminal of NMJ. Moreover, activated Wnt pathway at the post-synaptic terminal also regulates the localization of glutamate receptors (GluRs) around synaptic boutons.
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The Drosophila nervous system is a valuable model to examine the mechanisms of activity-dependent synaptic modification (plasticity) owing to its relatively simple organization and the availability of powerful genetic tools. The larval neuromuscular junction (NMJ) in particular is an accessible model for the study of synaptic development and plasti...
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... This increase in Wnt secretion provokes structural alterations in dendritic arborizations (Yu and Malenka, 2003;Wayman et al., 2006;Ferrari et al., 2018) and spines (Ciani et al., 2011;Tabatadze et al., 2014;McLeod et al., 2018), promoting changes in synaptic strength and plasticity. Drosophila served as a model of choice to explore the instrumental role of Wg in directing ADSP (Budnik and Salinas, 2011;Bai and Suzuki, 2020). ADSP can be elicited at the NMJ when the preparation is submitted to a repeated stimulation protocol (Ataman et al., 2008;Piccioli and Littleton, 2014;Vasin et al., 2014Vasin et al., , 2019Maldonado-Díaz et al., 2021), similar to the one used on hippocampal neurons to elicit LTP (Wu et al., 2001). ...
From fly to man, the Wingless (Wg)/Wnt signaling molecule is essential for both the stability and plasticity of the nervous system. The Drosophila neuromuscular junction (NMJ) has proven to be a useful system for deciphering the role of Wg in directing activity-dependent synaptic plasticity (ADSP), which, in the motoneuron, has been shown to be dependent on both the canonical and the noncanonical calcium Wg pathways. Here we show that the noncanonical planar cell polarity (PCP) pathway is an essential component of the Wg signaling system controlling plasticity at the motoneuron synapse. We present evidence that disturbing the PCP pathway leads to a perturbation in ADSP. We first show that a PCP-specific allele of disheveled (dsh) affects the de novo synaptic structures produced during ADSP. We then show that the Rho GTPases downstream of Dsh in the PCP pathway are also involved in regulating the morphological changes that take place after repeated stimulation. Finally, we show that Jun kinase is essential for this phenomenon, whereas we found no indication of the involvement of the transcription factor complex AP1 (Jun/Fos). This work shows the involvement of the neuronal PCP signaling pathway in supporting ADSP. Because we find that AP1 mutants can perform ADSP adequately, we hypothesize that, upon Wg activation, the Rho GTPases and Jun kinase are involved locally at the synapse, in instructing cytoskeletal dynamics responsible for the appearance of the morphological changes occurring during ADSP.
... As a result of exposure, pre-synaptic neurons (PrSN) release specific neurotransmitters into the synaptic cleft (SC). Released neurotransmitters bind to specific receptors of post-synaptic neurons (PoSN) and trigger the activation of cyclic adenosine monophosphate (cAMP) (Bai and Suzuki 2020;Lin et al. 2021). Further upregulated cAMP activates protein kinase A (PKA) and enzyme-regulated kinase 1/2 (ERK-1/2). ...
In the central nervous system, bidirectional communication between the brain and gut results in memory formation due to synaptic plasticity changes. During a healthy state, oral balanced microflora plays a pivotal role in memory formation by inhibiting the enterotoxin level produced by infectious pathogens. In disease conditions, beneficial microbial dysbiosis may result in excess enterotoxin production. Further, excess enterotoxin secretion prevents beneficial bacteria's proliferation and impairs neurotransmitter precursor compounds' transport to the brain. Blockade of neurotransmitter precursor compounds may result in the development of memory loss. The present study stated the role of Lactobacillus acidophilus in recovering memory loss. Reversal of cognitive impairment is shown with the help of a three-step behavioural analysis, which consists of one pre-infusive behavioural analysis and two post-infusive behavioural analyses (phase 1 and 2). The pre-infusive analysis showed no cognitive impairment in an assimilated environment without any infusions. After oral microbial infusions, phase 1 of post-infusive behavioural analysis showed the presence of cognitive impairment in the experimental groups who received oral infusions. Formed cognitive impairment is reverted with the help of L. acidophilus oral infusion in phase 2 of post-infusive analysis. Comparative three-step behavioural analysis proved that Pseudomonas aeuroginosa induced cognitive impairment may revert to normal conditions with the help of L. acidophilus. The outcome of the present study proves that cognitive impairment developed due to poor oral hygiene can be treated with the help of probiotic microorganisms.
... To examine the presence and study the physiological mechanisms underlying the functioning of engram cells, scientists created a class of tools known as activity-dependent tools. These are based on the definition of engram cells and the molecular mechanisms contributing to neural activity (Bai and Suzuki, 2020;Colecraft, 2020;Demchuk et al., 2020;Nectow and Nestler, 2020;Wei et al., 2021;Kasatkina and Verkhusha, 2022;Liu et al., 2022;Robison and Nestler, 2022;Soutschek and Schratt, 2023). A sensory experience triggers a sensory-evoked activity, which drives a synaptic input onto the neurons and initiates membrane depolarization and calcium influx into the cytoplasm (Cohen and Greenberg, 2008). ...
The theory of engrams, proposed several years ago, is highly crucial to understanding the progress of memory. Although it significantly contributes to identifying new treatments for cognitive disorders, it is limited by a lack of technology. Several scientists have attempted to validate this theory but failed. With the increasing availability of activity-dependent tools, several researchers have found traces of engram cells. Activity-dependent tools are based on the mechanisms underlying neuronal activity and use a combination of emerging molecular biological and genetic technology. Scientists have used these tools to tag and manipulate engram neurons and identified numerous internal connections between engram neurons and memory. In this review, we provide the background, principles, and selected examples of applications of existing activity-dependent tools. Using a combination of traditional definitions and concepts of engram cells, we discuss the applications and limitations of these tools and propose certain developmental directions to further explore the functions of engram cells.
... The Drosophila melanogaster larval neuromuscular junction (NMJ) -a synapse formed between motor neurons (MNs) and skeletal muscle fibers, critical for the control of muscle contraction, has been widely used as an in vivo model in which to study the machinery that regulates bouton formation 10,11 . During larval growth, synaptic boutons are added to the neuronal arbor through developmental and activitydependent processes, although it is not entirely understood whether these processes are regulated by common mechanisms. ...
... However, perhaps due to the small size of boutons and limitations of optical microscopy, little is known about activity-dependent bouton formation in mammalian systems. In contrast, Drosophila NMJs allow clear identification of single boutons, providing a good model for such studies 10,11 , which resulted in the identification of several conserved signaling pathways important for activity-dependent bouton outgrowth, including Wnt-, BMP-, and synapsin/PKA-dependent pathways [13][14][15]22 . Additionally, live imaging of NMJs has shown that in response to elevated activity new boutons form within minutes, mostly during muscle contraction, and emerge by budding off the membrane from the presynaptic arbor 14 , distinct from the embryonic GC. ...
... Also, there is evidence that this type of bouton growth is coupled to synaptic transmission, as it is associated with actin dynamics 14,23 and requires proteins that regulate the SV machinery 13,15 . Moreover, recent studies have indicated that NMJ plasticity is regulated by trans-synaptic signaling and requires factors involved in physical interactions between motor neurons (MNs), muscle and glial cells [11][12][13][14][24][25][26] . However, few of these have addressed the mechanisms used by neurons to (1) coordinate activity with the cytoskeleton to mediate the morphologic changes that initiate bouton addition and (2) synchronize these changes with the intercellular microenvironment. ...
Wired neurons form new presynaptic boutons in response to increased synaptic activity, however the mechanism(s) by which this occurs remains uncertain. Drosophila motor neurons (MNs) have clearly discernible boutons that display robust structural plasticity, being therefore an ideal system in which to study activity-dependent bouton genesis. Here, we show that in response to depolarization and in resting conditions, MNs form new boutons by membrane blebbing, a pressure-driven mechanism that occurs in 3-D cell migration, but to our knowledge not previously described to occur in neurons. Accordingly, F-actin is decreased in boutons during outgrowth, and non-muscle myosin-II is dynamically recruited to newly formed boutons. Furthermore, muscle contraction plays a mechanical role, which we hypothesize promotes bouton addition by increasing MN confinement. Overall, we identified a mechanism by which established circuits form new boutons allowing their structural expansion and plasticity, using trans-synaptic physical forces as the main driving force.
... An environmental change experienced by all organisms is the cycle between night and day. Disruptions of the circadian pattern of light input impose additional challenges on the Drosophila visual system [4][5][6] . In response to continuous light, photoreceptors adapt by downregulating the number of their synapses 4 . ...
... Adaptive responses in rhabdomere structures were revealed by mutations interfering with the Unfolded protein response (UPR) due to loss of BiP AMPylation 5 or by interference with endolysosomal trafficking 7 . The adaptive changes of the visual system are regulated by a feedback mechanism that receives input from the circadian system and relies on cellular stress pathways for maintaining homeostasis during its execution [4][5][6][7] . Conditions that dysregulate stress pathways may thus cause vulnerabilities during adaptative challenges, similar to observations of compromised proteostasis enhancing the progression of neurodegenerative diseases [8][9][10] . ...
In nervous systems, retrograde signals are key for organizing circuit activity and maintaining neuronal homeostasis. We identify the conserved Allnighter (Aln) pseudokinase as a cell non-autonomous regulator of proteostasis responses necessary for normal sleep and structural plasticity of Drosophila photoreceptors. In aln mutants exposed to extended ambient light, proteostasis is dysregulated and photoreceptors develop striking, but reversible, dysmorphology. The aln gene is widely expressed in different neurons, but not photoreceptors. However, secreted Aln protein is retrogradely endocytosed by photoreceptors. Inhibition of photoreceptor synaptic release reduces Aln levels in lamina neurons, consistent with secreted Aln acting in a feedback loop. In addition, aln mutants exhibit reduced night time sleep, providing a molecular link between dysregulated proteostasis and sleep, two characteristics of ageing and neurodegenerative diseases.
... In general, the list is enriched in genes specific for neuroblasts [54][55][56][57][58] and distinct neural subtypes [59][60][61][62][63][64][65], important for neurogenesis [66][67][68][69] and nervous system development [70][71][72][73]. There are also genes that are important for synaptogenesis [74]; synapse assembly, structure, and function [75][76][77][78]; and synaptic plasticity [79] in the list. Genes required for neuron-dendrite morphogenesis [80][81][82], neuronal wiring [83], axon guidance [84,85], axon and dendrite pruning [86,87], and neuronal remodeling [88][89][90][91] are enriched in the list. ...
Activation of local translation in neurites in response to stimulation is an important step in the formation of long-term memory (LTM). CPEB proteins are a family of translation factors involved in LTM formation. The Drosophila CPEB protein Orb2 plays an important role in the development and function of the nervous system. Mutations of the coding region of the orb2 gene have previously been shown to impair LTM formation. We found that a deletion of the 3’UTR of the orb2 gene similarly results in loss of LTM in Drosophila. As a result of the deletion, the content of the Orb2 protein remained the same in the neuron soma, but significantly decreased in synapses. Using RNA immunoprecipitation followed by high-throughput sequencing, we detected more than 6000 potential Orb2 mRNA targets expressed in the Drosophila brain. Importantly, deletion of the 3′UTR of orb2 mRNA also affected the localization of the Csp, Pyd, and Eya proteins, which are encoded by putative mRNA targets of Orb2. Therefore, the 3′UTR of the orb2 mRNA is important for the proper localization of Orb2 and other proteins in synapses of neurons and the brain as a whole, providing a molecular basis for LTM formation.
... Specifically, both glutamatergic and GABAergic signaling are involved in the regulation of neuronal proliferation, migration, and differentiation, as well as formation of synaptic contacts, neural plasticity and the refinement of neuronal circuits later in life (Behuet et al., 2019), where estimates suggest that around 90% of synapses in the adult mammalian brain utilize glutamate (Andersen et al., 2021). The evolutionary relevance and importance of glutamate in synaptic formation, development and plasticity is emphasized by experiments involving Drosophila melanogaster and C. Elegans, where these functions are conserved (Petersen et al., 1997;Featherstone et al., 2005;Bozorgmehr et al., 2013;Lau et al., 2013;Aponte-Santiago et al., 2020;Bai and Suzuki, 2020). Also conserved is GABAergic signaling that provides inhibitory inputs to neural circuits, suggesting that these mechanisms emerged early in evolution (Das et al., 2011;Behuet et al., 2019;Bai and Suzuki, 2020;Cuentas-Condori et al., 2020). ...
... The evolutionary relevance and importance of glutamate in synaptic formation, development and plasticity is emphasized by experiments involving Drosophila melanogaster and C. Elegans, where these functions are conserved (Petersen et al., 1997;Featherstone et al., 2005;Bozorgmehr et al., 2013;Lau et al., 2013;Aponte-Santiago et al., 2020;Bai and Suzuki, 2020). Also conserved is GABAergic signaling that provides inhibitory inputs to neural circuits, suggesting that these mechanisms emerged early in evolution (Das et al., 2011;Behuet et al., 2019;Bai and Suzuki, 2020;Cuentas-Condori et al., 2020). If the carotid body is architected with organized networks of cells forming circuit-like arrangements that are responsive to experience, then, given the importance of glutamate and GABA in synapse and circuit formation in the brain, coupled with evidence of the existence of both glutamate and GABA systems from published RNAseq data (Pauza et al., 2022), we will explore evidence for the existence of glutamate and GABA in the carotid body. ...
The carotid body is the primary peripheral chemoreceptor in the body, and critical for respiration and cardiovascular adjustments during hypoxia. Yet considerable evidence now implicates the carotid body as a multimodal sensor, mediating the chemoreflexes of a wide range of physiological responses, including pH, temperature, and acidosis as well as hormonal, glucose and immune regulation. How does the carotid body detect and initiate appropriate physiological responses for these diverse stimuli? The answer to this may lie in the structure of the carotid body itself. We suggest that at an organ-level the carotid body is comparable to a miniature brain with compartmentalized discrete regions of clustered glomus cells defined by their neurotransmitter expression and receptor profiles, and with connectivity to defined reflex arcs that play a key role in initiating distinct physiological responses, similar in many ways to a switchboard that connects specific inputs to selective outputs. Similarly, within the central nervous system, specific physiological outcomes are co-ordinated, through signaling via distinct neuronal connectivity. As with the brain, we propose that highly organized cellular connectivity is critical for mediating co-ordinated outputs from the carotid body to a given stimulus. Moreover, it appears that the rudimentary components for synaptic plasticity, and learning and memory are conserved in the carotid body including the presence of glutamate and GABAergic systems, where evidence pinpoints that pathophysiology of common diseases of the carotid body may be linked to deviations in these processes. Several decades of research have contributed to our understanding of the central nervous system in health and disease, and we discuss that understanding the key processes involved in neuronal dysfunction and synaptic activity may be translated to the carotid body, offering new insights and avenues for therapeutic innovation.
... We exposed flies to a light intensity of ;10,000 lux corresponding to light measured in the shade of a summer day (Schlichting et al., 2019). Photoreceptors implement multiple protective mechanisms to keep their activation level and their output to downstream neurons within a working range (Stavenga, 1995;Juusola and Hardie, 2001;Sugie et al., 2015Sugie et al., , 2018Bai and Suzuki, 2020). We found that R7 axons are highly resilient in wild-type flies as R7 axon loss could be observed only starting after 3 weeks of continuous ;10,000 lux light exposure (data not shown). ...
In human neurodegenerative diseases, neurons undergo axonal degeneration months to years before they die. Here, we developed a system modelling early degenerative events in Drosophila adult photoreceptor cells. Thanks to the stereotypy of their axonal projections, this system delivers quantitative data on sporadic and progressive axonal degeneration of photoreceptor cells. Using this method, we show that exposure of adult female flies to a constant light stimulation for several days overcomes the intrinsic resilience of R7 photoreceptors and leads to progressive axonal degeneration. This was not associated with apoptosis. We furthermore provide evidence that loss of synaptic integrity between R7 and a postsynaptic partner preceded axonal degeneration, thus recapitulating features of human neurodegenerative diseases. Finally, our experiments uncovered a role of postsynaptic partners of R7 to initiate degeneration, suggesting that postsynaptic cells signal back to the photoreceptor to maintain axonal structure. This model can be used to dissect cellular and circuit mechanisms involved in the early events of axonal degeneration, allowing for a better understanding of how neurons cope with stress and lose their resilience capacities.SIGNIFICANCE STATEMENT:Neurons can be active and functional for several years. In the course of ageing and in disease conditions leading to neurodegeneration, subsets of neurons lose their resilience and start dying. What initiates this turning point at the cellular level is not clear. Here, we developed a model allowing to systematically describe this phase. The loss of synapses and axons represents an early and functionally relevant event towards degeneration. Utilizing the ordered distribution of Drosophila photoreceptors axon terminals, we assembled a system to study sporadic initiation of axon loss and delineated a role for non-cell-autonomous activity regulation in the initiation of axon degeneration. This work will help shedding light on key steps in the etiology of non-familial cases of neurodegenerative diseases.
... To dissect the molecular mechanisms underlying activity-dependent synaptic plasticity, research has turned towards studying this phenomenon using a variety of model systems. Indeed, in addition to the in vivo work carried out in rodents (for review [10]) research also turned to hippocampal neurons in culture [11][12][13][14][15], Caenorhabditis elegans sensory system and neuromuscular junction [16][17][18][19], and the Drosophila melanogaster glutamatergic neuromuscular junction (NMJ) [20][21][22][23]. Methods to elicit activity-dependent synaptic plasticity at the fruit fly Drosophila melanogaster NMJ have been numerous. ...
The Drosophila NMJ is a system of choice for investigating the mechanisms underlying the structural and functional modifications evoked during activity-dependent synaptic plasticity. Because fly genetics allows considerable versatility, many strategies can be employed to elicit this activity. Here, we compare three different stimulation methods for eliciting activity-dependent changes in structure and function at the Drosophila NMJ. We find that the method using patterned stimulations driven by a K+-rich solution creates robust structural modifications but reduces muscle viability, as assessed by resting potential and membrane resistance. We argue that, using this method, electrophysiological studies that consider the frequency of events, rather than their amplitude, are the only reliable studies. We contrast these results with the expression of CsChrimson channels and red-light stimulation at the NMJ, as well as with the expression of TRPA channels and temperature stimulation. With both these methods we observed reliable modifications of synaptic structures and consistent changes in electrophysiological properties. Indeed, we observed a rapid appearance of immature boutons that lack postsynaptic differentiation, and a potentiation of spontaneous neurotransmission frequency. Surprisingly, a patterned application of temperature changes alone is sufficient to provoke both structural and functional plasticity. In this context, temperature-dependent TRPA channel activation induces additional structural plasticity but no further increase in the frequency of spontaneous neurotransmission, suggesting an uncoupling of these mechanisms.
... This result indicates that HR downregulation in L2 is activity-dependent. According to previous studies, the Drosophila neuromuscular junction and Drosophila olfactory system require calmodulin kinase II (CaMKII) for activity-dependent synaptic plasticity (Bai and Suzuki, 2020). Moreover, studies of the rodent hippocampus have demonstrated the involvement of the CaMKII-related pathway and the transcriptional factor cAMP response element-binding (CREB) protein in activity-dependent postsynaptic modification. ...
Activity-dependent synaptic plasticity is crucial for responses to the environment. Although the plasticity mechanisms of presynaptic photoreceptor neurons in the Drosophila visual system have been well studied, postsynaptic modifications remain elusive. In addition, further studies on the adaption of the visual system to different light experiences at a circuitry scale are required. Using the modified transcriptional reporter of intracellular Ca2+ method, we describe a way to visualize circuitry changes according to different light experiences. We found enhanced postsynaptic neuronal activity responses in lamina monopolar neuron L2 after prolonged light treatment. Although L1 also has connections with photoreceptors, there were no enhanced activity responses in L1. We also report in this study that activity-dependent transcriptional downregulation of inhibitory histamine receptors (HRs) occurs in postsynaptic neuron L2, but not in L1, during continuous light conditions. We expressed exogenous HR proteins in L2 neurons and found that it attenuated the enhanced activity response caused by constant light exposure. These findings, together with the fact that histamine is the main inhibitory neurotransmitter released by photoreceptors in the Drosophila visual system, confirmed our hypothesis that the activity-dependent transcriptional downregulation of HRs is responsible for the constant light exposure-induced circuitry response changes in L2. The results successfully demonstrated the selective circuit change after synaptic remodeling evoked by long-term activation and provided in vivo evidence of circuitry plasticity upon long-term environmental stimulation.
