ESCRT-III Membrane Trafficking Misregulation Contributes To Fragile X Syndrome Synaptic Defects

Advances in understanding the roles of actin scaffolding and membrane trafficking in dendrite development

Dendritic morphology is typically highly branched, and the branching and synaptic abundance of dendrites can enhance the receptive range of neurons and the diversity of information received, thus providing the basis for information processing in the nervous system. Once dendritic development is aberrantly compromised or damaged, it may lead to abnormal connectivity of the neural network, affecting the function and stability of the nervous system and ultimately triggering a series of neurological disorders. Research on the regulation of dendritic developmental processes has flourished, and much progress is now being made in its regulatory mechanisms. Noteworthily, dendrites are characterized by an extremely complex dendritic arborization that cannot be attributed to individual protein functions alone, requiring a systematic analysis of the intrinsic and extrinsic signals and the coordinated roles among them. Actin cytoskeleton organization and membrane vesicle trafficking are required during dendrite development, with actin providing tracks for vesicles and vesicle trafficking in turn providing material for actin assembly. In this review, we focus on these two basic biological processes and discuss the molecular mechanisms and their synergistic effects underlying the morphogenesis of neuronal dendrites. We also offer insights and discuss strategies for the potential preventive and therapeutic treatment of neuropsychiatric disorders.

Secreted C-type lectin regulation of neuromuscular junction synaptic vesicle dynamics modulates coordinated movement

The synaptic cleft manifests enriched glycosylation, with structured glycans coordinating signaling between presynaptic and postsynaptic cells. Glycosylated signaling ligands orchestrating communication are tightly regulated by secreted glycan-binding lectins. Using the Drosophila neuromuscular junction (NMJ) as a model glutamatergic synapse, we identify a new Ca2+-binding (C-type) lectin, Lectin-galC1 (LGC1), which modulates presynaptic function and neurotransmission strength. We find that LGC1 is enriched in motoneuron presynaptic boutons and secreted into the NMJ extracellular synaptomatrix. We show that LGC1 limits locomotor peristalsis and coordinated movement speed, with a specific requirement for synaptic function, but not NMJ architecture. LGC1 controls neurotransmission strength by limiting presynaptic active zone (AZ) and postsynaptic glutamate receptor (GluR) aligned synapse number, reducing both spontaneous and stimulation-evoked synaptic vesicle (SV) release, and capping SV cycling rate. During high-frequency stimulation (HFS), mutants have faster synaptic depression and impaired recovery while replenishing depleted SV pools. Although LGC1 removal increases the number of glutamatergic synapses, we find that LGC1-null mutants exhibit decreased SV density within presynaptic boutons, particularly SV pools at presynaptic active zones. Thus, LGC1 regulates NMJ neurotransmission to modulate coordinated movement.

shrub is required for spermatogenesis of Drosophila melanogaster

Shrub (CG8055) encodes the vps32/snf7 protein, a filament-forming subunit of the ESCRT (endosomal sorting complexes required for transport)-III complex involved in inward membrane budding. It was reported that shrub was required for abscission in female germline stem cells. In this study, we showed that the expression level of shrub in the testis was significantly higher than that in the ovary of 1-day-old Drosophila melanogaster, suggesting a role in male reproduction. Then we used nosGal4 driver to knockdown shrub specifically in the fly testis and found that this resulted in a significantly lower paternal effect egg hatch rate relative to the control group. Immunofluorescence staining showed that shrub knockdown in fly testes caused an accumulation of early-stage germ cells and lack of spectrin caps. In the late stages (spermiogenesis), the control testis contained multiple compacted spermatid bundles and individualization complexes (ICs) consisting of actin cones, whereas there were scattered spermatid nuclei and only a few ICs with disorganized actin cones in the shrub knockdown testis. Finally, the control seminal vesicle was full of mature sperms with needle-like heads, but in shrub knockdown testis 75% of seminal vesicles had no mature sperms. We also found that knockdown of shrub in fly testes led to upregulated expression of several cytoskeleton-associated genes, and an accumulation of ubiquitylated proteins. These results suggest that knockdown of shrub in fly testes might damage spermatogenesis by affecting transportability.

Neuronal fragile X mental retardation protein activates glial insulin receptor mediated PDF-Tri neuron developmental clearance

Glia engulf and phagocytose neurons during neural circuit developmental remodeling. Disrupting this pruning process contributes to Fragile X syndrome (FXS), a leading cause of intellectual disability and autism spectrum disorder in mammals. Utilizing a Drosophila FXS model central brain circuit, we identify two glial classes responsible for Draper-dependent elimination of developmentally transient PDF-Tri neurons. We find that neuronal Fragile X Mental Retardation Protein (FMRP) drives insulin receptor activation in glia, promotes glial Draper engulfment receptor expression, and negatively regulates membrane-molding ESCRT-III Shrub function during PDF-Tri neuron clearance during neurodevelopment in Drosophila. In this context, we demonstrate genetic interactions between FMRP and insulin receptor signaling, FMRP and Draper, and FMRP and Shrub in PDF-Tri neuron elimination. We show that FMRP is required within neurons, not glia, for glial engulfment, indicating FMRP-dependent neuron-to-glia signaling mediates neuronal clearance. We conclude neuronal FMRP drives glial insulin receptor activation to facilitate Draper- and Shrub-dependent neuronal clearance during neurodevelopment in Drosophila.

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

Fragile X mental retardation protein (FMRP) promotes cyclic AMP (cAMP) signaling. Using an in vivo protein kinase A activity sensor (PKA-SPARK), we find that Drosophila FMRP (dFMRP) and human FMRP (hFMRP) enhance PKA activity in a central brain learning and memory center. Increasing neuronal PKA activity suppresses FMRP in Kenyon cells, demonstrating an FMRP-PKA negative feedback loop. A patient-derived R140Q FMRP point mutation mislocalizes PKA-SPARK activity, whereas deletion of the RNA-binding argi-nine-glycine-glycine (RGG) box (hFMRP-ΔRGG) produces fibrillar PKA-SPARK assemblies colocalizing with ribonucleoprotein (RNP) and aggregation (thioflavin T) markers, demonstrating fibrillar partitioning of cytosolic protein aggregates. hFMRP-ΔRGG reduces dFMRP levels, indicating RGG-independent regulation. Short-term hFMRP-ΔRGG induction produces activated PKA-SPARK puncta, whereas long induction drives fibrillar assembly. Elevated temperature disassociates hFMRP-ΔRGG aggregates and blocks activated PKA-SPARK localization. These results suggest that FMRP regulates compartmentalized signaling via complex assembly, directing PKA activity localization, with FMRP RGG box RNA binding restricting separation via low-complexity interactions.

FMRP is required for brain cAMP induction and cAMP-dependent PKA activation, but the FMRP mechanism is uncharacterized. Sears and Broadie test FXS patient-derived and FMRP domain-deficient mutants to reveal conserved FMRP functions regulating PKA activation, subcellular localization, and reversible partitioning into elongated fibrillar assemblies in brain learning/ memory circuit neurons.

Genetic background mutations drive neural circuit hyperconnectivity in a fragile X syndrome model

BACKGROUND

Neural circuits are initially assembled during development when neurons synapse with potential partners and later refined as appropriate connections stabilize into mature synapses while inappropriate contacts are eliminated. Disruptions to this synaptogenic process impair connectivity optimization and can cause neurodevelopmental disorders. Intellectual disability (ID) and autism spectrum disorder (ASD) are often characterized by synaptic overgrowth, with the maintenance of immature or inappropriate synapses. Such synaptogenic defects can occur through mutation of a single gene, such as fragile X mental retardation protein (FMRP) loss causing the neurodevelopmental disorder fragile X syndrome (FXS). FXS represents the leading heritable cause of ID and ASD, but many other genes that play roles in ID and ASD have yet to be identified.

RESULTS

In a Drosophila FXS disease model, one dfmr150M null mutant stock exhibits previously unreported axonal overgrowths at developmental and mature stages in the giant fiber (GF) escape circuit. These excess axon projections contain both chemical and electrical synapse markers, indicating mixed synaptic connections. Extensive analyses show these supernumerary synapses connect known GF circuit neurons, rather than new, inappropriate partners, indicating hyperconnectivity within the circuit. Despite the striking similarities to well-characterized FXS synaptic defects, this new GF circuit hyperconnectivity phenotype is driven by genetic background mutations in this dfmr150M stock. Similar GF circuit synaptic overgrowth is not observed in independent dfmr1 null alleles. Bulked segregant analysis (BSA) was combined with whole genome sequencing (WGS) to identify the quantitative trait loci (QTL) linked to neural circuit hyperconnectivity. The results reveal 8 QTL associated with inappropriate synapse formation and maintenance in the dfmr150M mutant background.

CONCLUSIONS

Synaptogenesis is a complex, precisely orchestrated neurodevelopmental process with a large cohort of gene products coordinating the connectivity, synaptic strength, and excitatory/inhibitory balance between neuronal partners. This work identifies a number of genetic regions that contain mutations disrupting proper synaptogenesis within a particularly well-mapped neural circuit. These QTL regions contain potential new genes involved in synapse formation and refinement. Given the similarity of the synaptic overgrowth phenotype to known ID and ASD inherited conditions, identifying these genes should increase our understanding of these devastating neurodevelopmental disease states.

Expression of mutant CHMP2B linked to neurodegeneration in humans disrupts circadian rhythms in Drosophila

Mutations in CHMP2B, an ESCRT-III (endosomal sorting complexes required for transport) component, are associated with frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS). Neurodegenerative disorders including FTD are also associated with a disruption in circadian rhythms, but the mechanism underlying this defect is not well understood. Here, we ectopically expressed the human CHMP2B variant associated with FTD (CHMP2BIntron5) in flies using the GMR-GAL4 driver (GMR>CHMP2BIntron5) and analyzed their circadian rhythms at behavioral, cellular, and biochemical level. In GMR>CHMP2BIntron5 flies, we observed disrupted eclosion rhythms, shortened free-running circadian locomotor period, and reduced levels of timeless (tim) mRNA-a circadian pacemaker gene. We also observed that the GMR-GAL4 driver, primarily known for its expression in the retina, drives expression in a subset of tim expressing neurons in the optic lobe of the brain. The patterning of these GMR- and tim-positive neurons in the optic lobe, which appears distinct from the putative clusters of circadian pacemaker neurons in the fly brain, was disrupted in GMR>CHMP2BIntron5 flies. These results demonstrate that CHMP2BIntron5 can disrupt the normal function of the circadian clock in Drosophila.

Structural and Functional Abnormalities in the Olfactory System of Fragile X Syndrome Models

Fragile X Syndrome (FXS) is the most common inherited form of intellectual disability. It is produced by mutation of the Fmr1 gene that encodes for the Fragile Mental Retardation Protein (FMRP), an important RNA-binding protein that regulates the expression of multiple proteins located in neuronal synapses. Individuals with FXS exhibit abnormal sensory information processing frequently leading to hypersensitivity across sensory modalities and consequently a wide array of behavioral symptoms. Insects and mammals engage primarily their sense of smell to create proper representations of the external world and guide adequate decision-making processes. This feature in combination with the exquisitely organized neuronal circuits found throughout the olfactory system (OS) and the wide expression of FMRP in brain regions that process olfactory information makes it an ideal model to study sensory alterations in FXS models. In the last decade several groups have taken advantage of these features and have used the OS of fruit fly and rodents to understand neuronal alteration giving rise to sensory perception issues. In this review article, we will discuss molecular, morphological and physiological aspects of the olfactory information processing in FXS models. We will highlight the decreased inhibitory/excitatory synaptic balance and the diminished synaptic plasticity found in this system resulting in behavioral alteration of individuals in the presence of odorant stimuli.

Activity-Dependent Remodeling of Drosophila Olfactory Sensory Neuron Brain Innervation during an Early-Life Critical Period

Critical periods are windows of development when the environment has a pronounced effect on brain circuitry. Models of neurodevelopmental disorders, including autism spectrum disorders, intellectual disabilities, and schizophrenia, are linked to disruption of critical period remodeling. Critical periods open with the onset of sensory experience; however, it remains unclear exactly how sensory input modifies brain circuits. Here, we examine olfactory sensory neuron (OSN) innervation of the Drosophila antennal lobe of both sexes as a genetic model of this question. We find that olfactory sensory experience during an early-use critical period drives loss of OSN innervation of antennal lobe glomeruli and subsequent axon retraction in a dose-dependent mechanism. This remodeling does not result from olfactory receptor loss or OSN degeneration, but rather from rapid synapse elimination and axon pruning in the target olfactory glomerulus. Removal of the odorant stimulus only during the critical period leads to OSN reinnervation, demonstrating that remodeling is transiently reversible. We find that this synaptic refinement requires the OSN-specific olfactory receptor and downstream activity. Conversely, blocking OSN synaptic output elevates glomeruli remodeling. We find that GABAergic neurotransmission has no detectable role, but that glutamatergic signaling via NMDA receptors is required for OSN synaptic refinement. Together, these results demonstrate that OSN inputs into the brain manifest robust, experience-dependent remodeling during an early-life critical period, which requires olfactory reception, OSN activity, and NMDA receptor signaling. This work reveals a pathway linking initial olfactory sensory experience to glutamatergic neurotransmission in the activity-dependent remodeling of brain neural circuitry in an early-use critical period.SIGNIFICANCE STATEMENT Neurodevelopmental disorders manifest symptoms at specific developmental milestones that suggest an intersection between early sensory experience and brain neural circuit remodeling. One classic example is Fragile X syndrome caused by loss of an RNA-binding translation regulator of activity-dependent synaptic refinement. As a model, Drosophila olfactory circuitry is well characterized, genetically tractable, and rapidly developing, and thus ideally suited to probe underlying mechanisms. Here, we find olfactory sensory neurons are dramatically remodeled by heightened sensory experience during an early-life critical period. We demonstrate removing the olfactory stimulus during the critical period can reverse the connectivity changes. We find that this remodeling requires neural activity and NMDA receptor-mediated glutamatergic transmission. This improved understanding may help us design treatments for neurodevelopmental disorders.

Fragile X Mental Retardation Protein positively regulates PKA anchor Rugose and PKA activity to control actin assembly in learning/memory circuitry

Recent work shows Fragile X Mental Retardation Protein (FMRP) drives the translation of very large proteins (>2000 aa) mediating neurodevelopment. Loss of function results in Fragile X syndrome (FXS), the leading heritable cause of intellectual disability (ID) and autism spectrum disorder (ASD). Using the Drosophila FXS disease model, we discover FMRP positively regulates the translation of the very large A-Kinase Anchor Protein (AKAP) Rugose (>3000 aa), homolog of ASD-associated human Neurobeachin (NBEA). In the central brain Mushroom Body (MB) circuit, where Protein Kinase A (PKA) signaling is necessary for learning/memory, FMRP loss reduces Rugose levels and targeted FMRP overexpression elevates Rugose levels. Using a new in vivo transgenic PKA activity reporter (PKA-SPARK), we find FMRP loss reduces PKA activity in MB Kenyon cells whereas FMRP overexpression elevates PKA activity. Consistently, loss of Rugose reduces PKA activity, but Rugose overexpression has no independent effect. A well-established PKA output is regulation of F-actin cytoskeleton dynamics. In the FXS disease model, F-actin is aberrantly accumulated in MB lobes and single MB Kenyon cells. Consistently, Rugose loss results in similar F-actin accumulation. Moreover, targeted FMRP, Rugose and PKA overexpression all result in increased F-actin accumulation in the MB circuit. These findings uncover a FMRP-Rugose-PKA mechanism regulating actin cytoskeleton. This study reveals a novel FMRP mechanism controlling neuronal PKA activity, and demonstrates a shared mechanistic connection between FXS and NBEA associated ASD disease states, with a common link to PKA and F-actin misregulation in brain neural circuits. SIGNIFICANCE STATEMENT: Autism spectrum disorder (ASD) arises from a wide array of genetic lesions, and it is therefore critical to identify common underlying molecular mechanisms. Here, we link two ASD states; Neurobeachin (NBEA) associated ASD and Fragile X syndrome (FXS), the most common inherited ASD. Using established Drosophila disease models, we find Fragile X Mental Retardation Protein (FMRP) positively regulates translation of NBEA homolog Rugose, consistent with a recent advance showing FMRP promotes translation of very large proteins associated with ASD. FXS exhibits reduced cAMP induction, a potent activator of PKA, and Rugose/NBEA is a PKA anchor. Consistently, we find brain PKA activity strikingly reduced in both ASD models. We discover this pathway regulation controls actin cytoskeleton dynamics in brain neural circuits.

Fragile X Mental Retardation Protein Regulates Activity-Dependent Membrane Trafficking and Trans-Synaptic Signaling Mediating Synaptic Remodeling

Fragile X syndrome (FXS) is the leading monogenic cause of autism and intellectual disability. The disease arises through loss of fragile X mental retardation protein (FMRP), which normally exhibits peak expression levels in early-use critical periods, and is required for activity-dependent synaptic remodeling during this transient developmental window. FMRP canonically binds mRNA to repress protein translation, with targets that regulate cytoskeleton dynamics, membrane trafficking, and trans-synaptic signaling. We focus here on recent advances emerging in these three areas from the Drosophila disease model. In the well-characterized central brain mushroom body (MB) olfactory learning/memory circuit, FMRP is required for activity-dependent synaptic remodeling of projection neurons innervating the MB calyx, with function tightly restricted to an early-use critical period. FMRP loss is phenocopied by conditional removal of FMRP only during this critical period, and rescued by FMRP conditional expression only during this critical period. Consistent with FXS hyperexcitation, FMRP loss defects are phenocopied by heightened sensory experience and targeted optogenetic hyperexcitation during this critical period. FMRP binds mRNA encoding Drosophila ESCRTIII core component Shrub (human CHMP4 homolog) to restrict Shrub translation in an activity-dependent mechanism only during this same critical period. Shrub mediates endosomal membrane trafficking, and perturbing Shrub expression is known to interfere with neuronal process pruning. Consistently, FMRP loss and Shrub overexpression targeted to projection neurons similarly causes endosomal membrane trafficking defects within synaptic boutons, and genetic reduction of Shrub strikingly rescues Drosophila FXS model defects. In parallel work on the well-characterized giant fiber (GF) circuit, FMRP limits iontophoretic dye loading into central interneurons, demonstrating an FMRP role controlling core neuronal properties through the activity-dependent repression of translation. In the well-characterized Drosophila neuromuscular junction (NMJ) model, developmental synaptogenesis and activity-dependent synaptic remodeling both require extracellular matrix metalloproteinase (MMP) enzymes interacting with the heparan sulfate proteoglycan (HSPG) glypican dally-like protein (Dlp) to restrict trans-synaptic Wnt signaling, with FXS synaptogenic defects alleviated by both MMP and HSPG reduction. This new mechanistic axis spanning from activity to FMRP to HSPG-dependent MMP regulation modulates activity-dependent synaptogenesis. We discuss future directions for these mechanisms, and intersecting research priorities for FMRP in glial and signaling interactions.