FMRP-PKA Activity Negative Feedback Regulates RNA Binding-Dependent Fibrillation in Brain Learning and Memory Circuitry

Experience-dependent MAPK/ERK signaling in glia regulates critical period remodeling of synaptic glomeruli

Early-life critical periods allow initial sensory experience to remodel brain circuitry so that synaptic connectivity can be optimized to environmental input. In the Drosophila juvenile brain, olfactory sensory neuron (OSN) synaptic glomeruli are pruned by glial phagocytosis in dose-dependent response to early odor experience during a well-defined critical period. Extracellular signal-regulated kinase (ERK) separation of phases-based activity reporter of kinase (SPARK) biosensors reveal experience-dependent signaling in glia during this critical period. Glial ERK-SPARK signaling is depressed by removal of Draper receptors orchestrating glial phagocytosis. Cell-targeted genetic knockdown of glial ERK signaling reduces olfactory experience-dependent glial pruning of the OSN synaptic glomeruli in a dose-dependent mechanism. Noonan Syndrome is caused by gain-of-function mutations in protein tyrosine phosphatase non-receptor type 11 (PTPN11) inhibiting ERK signaling, and a glial-targeted patient-derived mutation increases experience-dependent glial ERK signaling and impairs experience-dependent glial pruning of the OSN synaptic glomeruli. We conclude that critical period experience drives glial ERK signaling that is required for dose-dependent pruning of brain synaptic glomeruli, and that altered glial ERK signaling impairs this critical period mechanism in a Noonan Syndrome disease model.

Modeling neurodegenerative and neurodevelopmental disorders in the Drosophila mushroom body

The common fruit fly Drosophila melanogaster provides a powerful platform to investigate the genetic, molecular, cellular, and neural circuit mechanisms of behavior. Research in this model system has shed light on multiple aspects of brain physiology and behavior, from fundamental neuronal function to complex behaviors. A major anatomical region that modulates complex behaviors is the mushroom body (MB). The MB integrates multimodal sensory information and is involved in behaviors ranging from sensory processing/responses to learning and memory. Many genes that underlie brain disorders are conserved, from flies to humans, and studies in Drosophila have contributed significantly to our understanding of the mechanisms of brain disorders. Genetic mutations that mimic human diseases-such as Fragile X syndrome, neurofibromatosis type 1, Parkinson's disease, and Alzheimer's disease-affect MB structure and function, altering behavior. Studies dissecting the effects of disease-causing mutations in the MB have identified key pathological mechanisms, and the development of a complete connectome promises to add a comprehensive anatomical framework for disease modeling. Here, we review Drosophila models of human neurodevelopmental and neurodegenerative disorders via the effects of their underlying mutations on MB structure, function, and the resulting behavioral alterations.

Use-Dependent, Untapped Dual Kinase Signaling Localized in Brain Learning Circuitry

Imaging brain learning and memory circuit kinase signaling is a monumental challenge. The separation of phases-based activity reporter of kinase (SPARK) biosensors allow circuit-localized studies of multiple interactive kinases in vivo, including protein kinase A (PKA) and extracellular signal-regulated kinase (ERK) signaling. In the precisely-mapped Drosophila brain learning/memory circuit, we find PKA and ERK signaling differentially enriched in distinct Kenyon cell connectivity nodes. We discover that potentiating normal circuit activity induces circuit-localized PKA and ERK signaling, expanding kinase function within new presynaptic and postsynaptic domains. Activity-induced PKA signaling shows extensive overlap with previously selective ERK signaling nodes, while activity-induced ERK signaling arises in new connectivity nodes. We find targeted synaptic transmission blockade in Kenyon cells elevates circuit-localized ERK induction in Kenyon cells with normally high baseline ERK signaling, suggesting lateral and feedback inhibition. We discover overexpression of the pathway-linking Meng-Po (human SBK1) serine/threonine kinase to improve learning acquisition and memory consolidation results in dramatically heightened PKA and ERK signaling in separable Kenyon cell circuit connectivity nodes, revealing both synchronized and untapped signaling potential. Finally, we find that a mechanically-induced epileptic seizure model (easily shocked "bang-sensitive" mutants) has strongly elevated, circuit-localized PKA and ERK signaling. Both sexes were used in all experiments, except for the hemizygous male-only seizure model. Hyperexcitable, learning-enhanced, and epileptic seizure models have comparably elevated interactive kinase signaling, suggesting a common basis of use-dependent induction. We conclude that PKA and ERK signaling modulation is locally coordinated in use-dependent spatial circuit dynamics underlying seizure susceptibility linked to learning/memory potential.

Cholesterol is required for activity-dependent synaptic growth

Changes in cholesterol content of neuronal membranes occur during development and brain aging. Little is known about whether synaptic activity regulates cholesterol levels in neuronal membranes and whether these changes affect neuronal development and function. We generated transgenic flies that express the cholesterol-binding D4H domain of perfringolysin O toxin and found increased levels of cholesterol in presynaptic terminals of Drosophila larval neuromuscular junctions following increased synaptic activity. Reduced cholesterol impaired synaptic growth and largely prevented activity-dependent synaptic growth. Presynaptic knockdown of adenylyl cyclase phenocopied the impaired synaptic growth caused by reducing cholesterol. Furthermore, the effects of knocking down adenylyl cyclase and reducing cholesterol were not additive, suggesting that they function in the same pathway. Increasing cAMP levels using a dunce mutant with reduced phosphodiesterase activity failed to rescue this impaired synaptic growth, suggesting that cholesterol functions downstream of cAMP. We used a protein kinase A (PKA) sensor to show that reducing cholesterol levels reduced presynaptic PKA activity. Collectively, our results demonstrate that enhanced synaptic activity increased cholesterol levels in presynaptic terminals and that these changes likely activate the cAMP-PKA pathway during activity-dependent growth.

Wolbachia-Driven Memory Loss in a Parasitic Wasp Increases Superparasitism to Enhance Horizontal Transmission

Horizontal transmission of the endosymbiont, Wolbachia, may occur during superparasitism when parasitoid females deposit a second clutch of eggs on a host. Wolbachia may increase the superparasitism tendency of Trichogramma wasps by depriving their memory. To test this hypothesis, we investigated the effects of conditioning experience and memory inhibitors (actinomycin D [ACD] and anisomycin [ANI]) on memory capacity, and expressions of memory-related genes (CREB1 and PKA), and superparasitism frequency of Wolbachia-infected (TDW) and uninfected (TD) lines of Trichogramma dendrolimi after conditioning with lemon or peppermint odor. We detected the presence of Wolbachia in eggs, larvae, pre-pupae, pupae, and adults of Trichogramma by using fluorescence in situ hybridization. The results showed that TDW females had a more reduced memory capacity than TD females after conditioning. Compared with TD females, TDW females showed a higher proportion of superparasitism and a downregulation of CREB1 and PKA genes after conditioning. TD females fed ACD or ANI showed a higher tendency for superparasitism and a downregulation of CREB1 and PKA, along with memory loss after conditioning than TD females fed honey solution only. The presence of Wolbachia was detected in the anterior region of the larva, pre-pupa, and pupa, but was not found in the head of the adult. The results provide evidence of host behavioral manipulation of Wolbachia by depriving memory of host Trichogramma wasps based on Poulin' s criteria. These host behavioral changes led by Wolbachia may be caused by the virulence of Wolbachia on the nervous system of the host. IMPORTANCE The endosymbiotic bacteria, Wolbachia, live widely within cells of arthropods. Wolbachia are not only transmitted vertically from host mother to offspring, but are also transmitted horizontally among host individuals. Horizontal transmission is expected to occur during superparasitism when host parasitoid females deposit a clutch of eggs on a host previously parasitized by the same parasitoid species. Thus, a question is proposed regarding whether superparasitism behavior is a behavior modification induced by the symbiont to favor symbiont transmission. This study highlights behavioral mechanisms of Wolbachia-induced superparasitism in Trichogramma wasps and the manipulation of symbionts on host parasitoids.

RNA-Binding Proteins as Epigenetic Regulators of Brain Functions and Their Involvement in Neurodegeneration

A central aspect of nervous system development and function is the post-transcriptional regulation of mRNA fate, which implies time- and site-dependent translation, in response to cues originating from cell-to-cell crosstalk. Such events are fundamental for the establishment of brain cell asymmetry, as well as of long-lasting modifications of synapses (long-term potentiation: LTP), responsible for learning, memory, and higher cognitive functions. Post-transcriptional regulation is in turn dependent on RNA-binding proteins that, by recognizing and binding brief RNA sequences, base modifications, or secondary/tertiary structures, are able to control maturation, localization, stability, and translation of the transcripts. Notably, most RBPs contain intrinsically disordered regions (IDRs) that are thought to be involved in the formation of membrane-less structures, probably due to liquid-liquid phase separation (LLPS). Such structures are evidenced as a variety of granules that contain proteins and different classes of RNAs. The other side of the peculiar properties of IDRs is, however, that, under altered cellular conditions, they are also prone to form aggregates, as observed in neurodegeneration. Interestingly, RBPs, as part of both normal and aggregated complexes, are also able to enter extracellular vesicles (EVs), and in doing so, they can also reach cells other than those that produced them.

RNP components condense into repressive RNP granules in the aging brain

Cytoplasmic RNP condensates enriched in mRNAs and proteins are found in various cell types and associated with both buffering and regulatory functions. While a clear link has been established between accumulation of aberrant RNP aggregates and progression of aging-related neurodegenerative diseases, the impact of physiological aging on neuronal RNP condensates has never been explored. Through high-resolution imaging, we uncover that RNP components progressively cluster into large yet dynamic granules in the aging Drosophila brain. We further show that age-dependent clustering is caused by an increase in the stoichiometry of the conserved helicase Me31B/DDX6, and requires PKA kinase activity. Finally, our functional analysis reveals that mRNA species recruited to RNP condensates upon aging exhibit age-dependent translational repression, indicating that co-clustering of selected mRNAs and translation regulators into repressive condensates may contribute to the specific post-transcriptional changes in gene expression observed in the course of aging.

Aberrant RNA condensates are a hallmark of age-related neurodegenerative diseases. Here, the authors show that RNA condensation increases in aging Drosophila brains, triggering translation repression.

Towards Kinase Inhibitor Therapies for Fragile X Syndrome: Tweaking Twists in the Autism Spectrum Kinase Signaling Network

Absence of the Fragile X Mental Retardation Protein (FMRP) causes autism spectrum disorders and intellectual disability, commonly referred to as the Fragile X syndrome. FMRP is a negative regulator of protein translation and is essential for neuronal development and synapse formation. FMRP is a target for several post-translational modifications (PTMs) such as phosphorylation and methylation, which tightly regulate its cellular functions. Studies have indicated the involvement of FMRP in a multitude of cellular pathways, and an absence of FMRP was shown to affect several neurotransmitter receptors, for example, the GABA receptor and intracellular signaling molecules such as Akt, ERK, mTOR, and GSK3. Interestingly, many of these molecules function as protein kinases or phosphatases and thus are potentially amendable by pharmacological treatment. Several treatments acting on these kinase-phosphatase systems have been shown to be successful in preclinical models; however, they have failed to convincingly show any improvements in clinical trials. In this review, we highlight the different protein kinase and phosphatase studies that have been performed in the Fragile X syndrome. In our opinion, some of the paradoxical study conclusions are potentially due to the lack of insight into integrative kinase signaling networks in the disease. Quantitative proteome analyses have been performed in several models for the FXS to determine global molecular processes in FXS. However, only one phosphoproteomics study has been carried out in Fmr1 knock-out mouse embryonic fibroblasts, and it showed dysfunctional protein kinase and phosphatase signaling hubs in the brain. This suggests that the further use of phosphoproteomics approaches in Fragile X syndrome holds promise for identifying novel targets for kinase inhibitor therapies.

Temporally and Spatially Localized PKA Activity within Learning and Memory Circuitry Regulated by Network Feedback

Dynamic functional connectivity within brain circuits requires coordination of intercellular signaling and intracellular signal transduction. Critical roles for cAMP-dependent protein kinase A (PKA) signaling are well established in the Drosophila mushroom body (MB) learning and memory circuitry, but local PKA activity within this well-mapped neuronal network is uncharacterized. Here, we use an in vivo PKA activity sensor (PKA-SPARK) to test spatiotemporal regulatory requirements in the MB axon lobes. We find immature animals have little detectable PKA activity, whereas postcritical period adults show high field-selective activation primarily in just 3/16 defined output regions. In addition to the age-dependent PKA activity in distinct α'/β' lobe nodes, females show sex-dependent elevation compared with males in these same restricted regions. Loss of neural cell body Fragile X mental retardation protein (FMRP) and Rugose [human Neurobeachin (NBEA)] suppresses localized PKA activity, whereas overexpression (OE) of MB lobe PKA-synergist Meng-Po (human SBK1) promotes PKA activity. Elevated Meng-Po subverts the PKA age-dependence, with elevated activity in immature animals, and spatial-restriction, with striking γ lobe activity. Testing circuit signaling requirements with temperature-sensitive shibire (human Dynamin) blockade, we find broadly expanded PKA activity within the MB lobes. Using transgenic tetanus toxin to block MB synaptic output, we find greatly heightened PKA activity in virtually all MB lobe fields, although the age-dependence is maintained. We conclude spatiotemporally restricted PKA activity signaling within this well-mapped learning/memory circuit is age-dependent and sex-dependent, driven by FMRP-Rugose pathway activation, temporally promoted by Meng-Po kinase function, and restricted by output neurotransmission providing network feedback.