Sphingolipids regulate neuromuscular synapse structure and function in Drosophila

Differential functions of phosphatidylinositol 4-kinases in neurotransmission and synaptic development

Phosphoinositides, such as PI(4,5)P2, are known to function as structural components of membranes, signalling molecules, markers of membrane identity, mediators of protein recruitment and regulators of neurotransmission and synaptic development. Phosphatidylinositol 4-kinases (PI4Ks) synthesize PI4P, which are precursors for PI(4,5)P2, but may also have independent functions. The roles of PI4Ks in neurotransmission and synaptic development have not been studied in detail. Previous studies on PI4KII and PI4KIIIβ at the Drosophila larval neuromuscular junction have suggested that PI4KII and PI4KIIIβ enzymes may serve redundant roles, where single PI4K mutants yielded mild or no synaptic phenotypes. However, the precise synaptic functions (neurotransmission and synaptic growth) of these PI4Ks have not been thoroughly studied. Here, we used PI4KII and PI4KIIIβ null mutants and presynaptic-specific knockdowns of these PI4Ks to investigate their roles in neurotransmission and synaptic growth. We found that PI4KII and PI4KIIIβ appear to have non-overlapping functions. Specifically, glial PI4KII functions to restrain synaptic growth, whereas presynaptic PI4KIIIβ promotes synaptic growth. Furthermore, loss of PI4KIIIβ or presynaptic PI4KII impairs neurotransmission. The data presented in this study uncover new roles for PI4K enzymes in neurotransmission and synaptic growth.

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.

Glycosphingolipids are linked to elevated neurotransmission and neurodegeneration in a Drosophila model of Niemann Pick type C

The lipid storage disease Niemann Pick type C (NPC) causes neurodegeneration owing primarily to loss of NPC1. Here, we employed a Drosophila model to test links between glycosphingolipids, neurotransmission and neurodegeneration. We found that Npc1a nulls had elevated neurotransmission at the glutamatergic neuromuscular junction (NMJ), which was phenocopied in brainiac (brn) mutants, impairing mannosyl glucosylceramide (MacCer) glycosylation. Npc1a; brn double mutants had the same elevated synaptic transmission, suggesting that Npc1a and brn function within the same pathway. Glucosylceramide (GlcCer) synthase inhibition with miglustat prevented elevated neurotransmission in Npc1a and brn mutants, further suggesting epistasis. Synaptic MacCer did not accumulate in the NPC model, but GlcCer levels were increased, suggesting that GlcCer is responsible for the elevated synaptic transmission. Null Npc1a mutants had heightened neurodegeneration, but no significant motor neuron or glial cell death, indicating that dying cells are interneurons and that elevated neurotransmission precedes neurodegeneration. Glycosphingolipid synthesis mutants also had greatly heightened neurodegeneration, with similar neurodegeneration in Npc1a; brn double mutants, again suggesting that Npc1a and brn function in the same pathway. These findings indicate causal links between glycosphingolipid-dependent neurotransmission and neurodegeneration in this NPC disease model.

Early Alterations in Structural and Functional Properties in the Neuromuscular Junctions of Mutant FUS Mice

Amyotrophic lateral sclerosis (ALS) is manifested as skeletal muscle denervation, loss of motor neurons and finally severe respiratory failure. Mutations of RNA-binding protein FUS are one of the common genetic reasons of ALS accompanied by a 'dying back' type of degeneration. Using fluorescent approaches and microelectrode recordings, the early structural and functional alterations in diaphragm neuromuscular junctions (NMJs) were studied in mutant FUS mice at the pre-onset stage. Lipid peroxidation and decreased staining with a lipid raft marker were found in the mutant mice. Despite the preservation of the end-plate structure, immunolabeling revealed an increase in levels of presynaptic proteins, SNAP-25 and synapsin 1. The latter can restrain Ca2+-dependent synaptic vesicle mobilization. Indeed, neurotransmitter release upon intense nerve stimulation and its recovery after tetanus and compensatory synaptic vesicle endocytosis were markedly depressed in FUS mice. There was a trend to attenuation of axonal [Ca2+]in increase upon nerve stimulation at 20 Hz. However, no changes in neurotransmitter release and the intraterminal Ca2+ transient in response to low frequency stimulation or in quantal content and the synchrony of neurotransmitter release at low levels of external Ca2+ were detected. At a later stage, shrinking and fragmentation of end plates together with a decrease in presynaptic protein expression and disturbance of the neurotransmitter release timing occurred. Overall, suppression of synaptic vesicle exo-endocytosis upon intense activity probably due to alterations in membrane properties, synapsin 1 levels and Ca2+ kinetics could be an early sign of nascent NMJ pathology, which leads to neuromuscular contact disorganization.

Sphingomyelinase modulates synaptic vesicle mobilization at the mice neuromuscular junctions

AIMS

Sphingomyelin is an abundant component of the presynaptic membrane and an organizer of lipid rafts. In several pathological conditions, sphingomyelin is hydrolyzed due to an upregulation and release of secretory sphingomyelinases (SMases). Herein, the effects of SMase on exocytotic neurotransmitter release were studied in the diaphragm neuromuscular junctions of mice.

MAIN METHODS

Microelectrode recordings of postsynaptic potentials and styryl (FM) dyes were used to estimate neuromuscular transmission. Membrane properties were assessed with fluorescent techniques.

KEY FINDINGS

Application of SMase at a low concentration (0.01 U ml^-1) led to a disruption of lipid-packing in the synaptic membranes. Neither spontaneous exocytosis nor evoked neurotransmitter release (in response to single stimuli) were affected by SMase treatment. However, SMase significantly increased neurotransmitter release and the rate of fluorescent FM-dye loss from the synaptic vesicles at 10, 20 and 70 Hz stimulation of the motor nerve. In addition, SMase treatment prevented a shift of the exocytotic mode from "full-collapse" fusion to "kiss-and-run" during high-frequency (70 Hz) activity. The potentiating effects of SMase on neurotransmitter release and FM-dye unloading were suppressed when synaptic vesicle membranes were also exposed to this enzyme (i.e., stimulation occurred during SMase treatment).

SIGNIFICANCE

Thus, hydrolysis of the plasma membrane sphingomyelin can enhance mobilization of synaptic vesicles and facilitate full fusion mode of exocytosis, but SMase acting on vesicular membrane had a depressant effect on the neurotransmission. Partially, the effects of SMase can be related with the changes in synaptic membrane properties and intracellular signaling.

Longitudinal spatial mapping of lipid metabolites reveals pre-symptomatic changes in the hippocampi of Huntington's disease transgenic mice

In Huntington's disease (HD), a key pathological feature includes the development of inclusion-bodies of fragments of the mutant huntingtin protein in the neurons of the striatum and hippocampus. To examine the molecular changes associated with inclusion-body formation, we applied MALDI-mass spectrometry imaging and deuterium pulse labelling to determine lipid levels and synthesis rates in the hippocampus of a transgenic mouse model of HD (R6/1 line). The R6/1 HD mice lacked inclusions in the hippocampus at 6 weeks of age (pre-symptomatic), whereas inclusions were pervasive by 16 weeks of age (symptomatic). Hippocampal subfields (CA1, CA3 and DG), which formed the highest density of inclusion formation in the mouse brain showed a reduction in the relative abundance of neuron-enriched lipids that have roles in neurotransmission, synaptic plasticity, neurogenesis, and ER-stress protection. Lipids involved in the adaptive response to ER stress (phosphatidylinositol, phosphatidic acid, and ganglioside classes) displayed increased rates of synthesis in HD mice relative to WT mice at all the ages examined, including prior to the formation of the inclusion bodies. Our findings, therefore, support a role for ER stress occurring pre-symptomatically and potentially contributing to pathological mechanisms underlying HD.

Brain Lipids and Lipid Droplet Dysregulation in Alzheimer's Disease and Neuropsychiatric Disorders

Background

Lipids are essential components of the structure and for the function of brain cells. The intricate balance of lipids, including phospholipids, glycolipids, cholesterol, cholesterol ester, and triglycerides, is crucial for maintaining normal brain function. The roles of lipids and lipid droplets and their relevance to neurodegenerative and neuropsychiatric disorders (NPDs) remain largely unknown.

Summary

Here, we reviewed the basic role of lipid components as well as a specific lipid organelle, lipid droplets, in brain function, highlighting the potential impact of altered lipid metabolism in the pathogenesis of Alzheimer's disease (AD) and NDPs.

Key Messages

Brain lipid dysregulation plays a pivotal role in the pathogenesis and progression of neurodegenerative and NPDs including AD and schizophrenia. Understanding the cell type-specific mechanisms of lipid dysregulation in these diseases is crucial for identifying better diagnostic biomarkers and for developing therapeutic strategies aiming at restoring lipid homeostasis.

Glial control of sphingolipid levels sculpts diurnal remodeling in a circadian circuit

Structural plasticity in the brain often necessitates dramatic remodeling of neuronal processes, with attendant reorganization of the cytoskeleton and membranes. Although cytoskeletal restructuring has been studied extensively, how lipids might orchestrate structural plasticity remains unclear. We show that specific glial cells in Drosophila produce glucocerebrosidase (GBA) to locally catabolize sphingolipids. Sphingolipid accumulation drives lysosomal dysfunction, causing gba1b mutants to harbor protein aggregates that cycle across circadian time and are regulated by neural activity, the circadian clock, and sleep. Although the vast majority of membrane lipids are stable across the day, a specific subset that is highly enriched in sphingolipids cycles daily in a gba1b-dependent fashion. Remarkably, both sphingolipid biosynthesis and degradation are required for the diurnal remodeling of circadian clock neurites, which grow and shrink across the day. Thus, dynamic sphingolipid regulation by glia enables diurnal circuit remodeling and proper circadian behavior.

The Putative Drosophila TMEM184B Ortholog Tmep Ensures Proper Locomotion by Restraining Ectopic Firing at the Neuromuscular Junction

TMEM184B is a putative seven-pass membrane protein that promotes axon degeneration after injury. TMEM184B mutation causes aberrant neuromuscular architecture and sensory and motor behavioral defects in mice. The mechanism through which TMEM184B causes neuromuscular defects is unknown. We employed Drosophila melanogaster to investigate the function of the closely related gene, Tmep (CG12004), at the neuromuscular junction. We show that Tmep is required for full adult viability and efficient larval locomotion. Tmep mutant larvae have a reduced body contraction rate compared to controls, with stronger deficits in females. In recordings from body wall muscles, Tmep mutants show substantial hyperexcitability, with many postsynaptic potentials fired in response to a single stimulation, consistent with a role for Tmep in restraining synaptic excitability. Additional branches and satellite boutons at Tmep mutant neuromuscular junctions are consistent with an activity-dependent synaptic overgrowth. Tmep is expressed in endosomes and synaptic vesicles within motor neurons, suggesting a possible role in synaptic membrane trafficking. Using RNAi knockdown, we show that Tmep is required in motor neurons for proper larval locomotion and excitability, and that its reduction increases levels of presynaptic calcium. Locomotor defects can be rescued by presynaptic knockdown of endoplasmic reticulum calcium channels or by reducing evoked release probability, further suggesting that excess synaptic activity drives behavioral deficiencies. Our work establishes a critical function for Tmep in the regulation of synaptic transmission and locomotor behavior.

Modeling C9orf72-Related Frontotemporal Dementia and Amyotrophic Lateral Sclerosis in Drosophila

An intronic hexanucleotide (GGGGCC) expansion in the C9orf72 gene is the most common genetic cause of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS). In the decade following its discovery, much progress has been made in enhancing our understanding of how it precipitates disease. Both loss of function caused by reduced C9orf72 transcript levels, and gain of function mechanisms, triggered by the production of repetitive sense and antisense RNA and dipeptide repeat proteins, are thought to contribute to the toxicity. Drosophila models, with their unrivaled genetic tractability and short lifespan, have played a key role in developing our understanding of C9orf72-related FTD/ALS. There is no C9orf72 homolog in fly, and although this precludes investigations into loss of function toxicity, it is useful for elucidating mechanisms underpinning gain of function toxicity. To date there are a range of Drosophila C9orf72 models, encompassing different aspects of gain of function toxicity. In addition to pure repeat transgenes, which produce both repeat RNA and dipeptide repeat proteins (DPRs), RNA only models and DPR models have been generated to unpick the individual contributions of RNA and each dipeptide repeat protein to C9orf72 toxicity. In this review, we discuss how Drosophila models have shaped our understanding of C9orf72 gain of function toxicity, and address opportunities to utilize these models for further research.

Neutral Ceramidase Is Required for the Reproduction of Brown Planthopper, Nilaparvata lugens (Stål)

Ceramides are bioactive sphingolipids that have been implicated in insect development; however, their role in insect reproduction remains poorly understood. Here, we report the pivotal role of neutral ceramidase (NCER) in the female reproduction of the brown planthopper (BPH), Nilaparvata lugens (Stål), a significant pest in rice cultivation in Asia. LC-MS/MS demonstrated that, among different developmental stages of BPH, the levels of ceramides were highest in 1st instar nymphs and lowest in adults. The transcription of NCER was negatively correlated with the levels of ceramides at different developmental stages of BPH, in that the transcript levels of NCER were the highest, whereas ceramides levels were the lowest in BPH adults. Knocking down NCER through RNA interference (RNAi) increased the levels of ceramides in BPH females and ovaries, which resulted in a delay in oocyte maturation, a reduction in oviposition and egg hatching rate, as well as the production of vulnerable offspring. Transmission electron microscopy (TEM) analysis and TdT-mediated dUTP Nick-End Labeling (TUNEL) assays showed mitochondrial deficiency and apoptosis in NCER-deficient oocytes. Taken together, these results suggest that NCER plays a crucial role in female reproduction in BPH, likely by regulating the levels of ceramides.

Co-expression of C9orf72 related dipeptide-repeats over 1000 repeat units reveals age- and combination-specific phenotypic profiles in Drosophila

A large intronic hexanucleotide repeat expansion (GGGGCC) within the C9orf72 (C9orf72-SMCR8 Complex Subunit) locus is the most prevalent genetic cause of both Frontotemporal Dementia (FTD) and Motor Neuron Disease (MND). In patients this expansion is typically hundreds to thousands of repeat units in length. Repeat associated non-AUG translation of the expansion leads to the formation of toxic, pathological Dipeptide-Repeat Proteins (DPRs). To date there remains a lack of in vivo models expressing C9orf72 related DPRs with a repeat length of more than a few hundred repeats. As such our understanding of how physiologically relevant repeat length DPRs effect the nervous system in an ageing in vivo system remains limited. In this study we generated Drosophila models expressing DPRs over 1000 repeat units in length, a known pathological length in humans. Using these models, we demonstrate each DPR exhibits a unique, age-dependent, phenotypic and pathological profile. Furthermore, we show co-expression of specific DPR combinations leads to distinct, age-dependent, phenotypes not observed through expression of single DPRs. We propose these models represent a unique, in vivo, tool for dissecting the molecular mechanisms implicated in disease pathology, opening up new avenues in the study of both MND and FTD.

Lipid metabolism in astrocytic structure and function

Astrocytes are the most abundant glial cell in the central nervous system and are involved in multiple processes including metabolic homeostasis, blood brain barrier regulation and neuronal crosstalk. Astrocytes are the main storage point of glycogen in the brain and it is well established that astrocyte uptake of glutamate and release of lactate prevents neuronal excitability and supports neuronal metabolic function. However, the role of lipid metabolism in astrocytes in relation to neuronal support has been until recently, unclear. Lipids play a fundamental role in astrocyte function, including energy generation, membrane fluidity and cell to cell signaling. There is now emerging evidence that astrocyte storage of lipids in droplets has a crucial physiological and protective role in the central nervous system. This pathway links β-oxidation in astrocytes to inflammation, signalling, oxidative stress and mitochondrial energy generation in neurons. Disruption in lipid metabolism, structure and signalling in astrocytes can lead to pathogenic mechanisms associated with a range of neurological disorders.

Sphingolipids regulate neuromuscular synapse structure and function in Drosophila

Sphingolipids are found in abundance at synapses and have been implicated in regulation of synapse structure, function, and degeneration. Their precise role in these processes, however, remains obscure. Serine Palmitoyl-transferase (SPT) is the first enzymatic step for synthesis of sphingolipids. Analysis of the Drosophila larval neuromuscular junction (NMJ) revealed mutations in the SPT enzyme subunit, lace/SPTLC2 resulted in deficits in synaptic structure and function. Although NMJ length is normal in lace mutants, the number of boutons per NMJ is reduced to ∼50% of the wild type number. Synaptic boutons in lace mutants are much larger but show little perturbation to the general ultrastructure. Electrophysiological analysis of lace mutant synapses revealed strong synaptic transmission coupled with predominance of depression over facilitation. The structural and functional phenotypes of lace mirrored aspects of Basigin (Bsg), a small Ig-domain adhesion molecule also known to regulate synaptic structure and function. Mutant combinations of lace and Bsg generated large synaptic boutons, while lace mutants showed abnormal accumulation of Bsg at synapses, suggesting that Bsg requires sphingolipid to regulate structure of the synapse. In support of this, we found Bsg to be enriched in lipid rafts. Our data points to a role for sphingolipids in the regulation and fine-tuning of synaptic structure and function while sphingolipid regulation of synaptic structure may be mediated via the activity of Bsg.

Impact of Drosophila Models in the Study and Treatment of Friedreich's Ataxia

Drosophila melanogaster has been for over a century the model of choice of several neurobiologists to decipher the formation and development of the nervous system as well as to mirror the pathophysiological conditions of many human neurodegenerative diseases. The rare disease Friedreich’s ataxia (FRDA) is not an exception. Since the isolation of the responsible gene more than two decades ago, the analysis of the fly orthologue has proven to be an excellent avenue to understand the development and progression of the disease, to unravel pivotal mechanisms underpinning the pathology and to identify genes and molecules that might well be either disease biomarkers or promising targets for therapeutic interventions. In this review, we aim to summarize the collection of findings provided by the Drosophila models but also to go one step beyond and propose the implications of these discoveries for the study and cure of this disorder. We will present the physiological, cellular and molecular phenotypes described in the fly, highlighting those that have given insight into the pathology and we will show how the ability of Drosophila to perform genetic and pharmacological screens has provided valuable information that is not easily within reach of other cellular or mammalian models.