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

Suprachiasmatic nucleus VIPergic fibers show a circadian rhythm of expansion and retraction

In animals, overt circadian rhythms of physiology and behavior are centrally regulated by a circadian clock located in specific brain regions. In the fruit fly Drosophila and in mammals, these clocks rely on single-cell oscillators, but critical for their function as central circadian pacemakers are network properties that change dynamically throughout the circadian cycle as well as in response to environmental stimuli.1,2,3 In the fly, this plasticity involves circadian rhythms of expansion and retraction of clock neuron fibers.4,5,6,7,8,9,10,11,12,13,14 Whether these drastic structural changes are a universal property of central neuronal pacemakers is unknown. To address this question, we studied neurons of the mouse suprachiasmatic nucleus (SCN) that express vasoactive intestinal polypeptide (VIP), which are critical for the SCN to function as a central circadian pacemaker. By targeting the expression of the fluorescent protein tdTomato to these neurons and using tissue clearing techniques to visualize all SCN VIPergic neurons and their fibers, we show that, similar to clock neurons in the fly, VIPergic fibers undergo a daily rhythm of expansion and retraction, with maximal branching during the day. This rhythm is circadian, as it persists under constant environmental conditions and is present in both males and females. We propose that circadian structural remodeling of clock neurons represents a key feature of central circadian pacemakers that is likely critical to regulate network properties, the response to environmental stimuli, and the regulation of circadian outputs.

Does glial lipid dysregulation alter sleep in Alzheimer's and Parkinson's disease?

In this opinion article, we discuss potential connections between sleep disturbances observed in Alzheimer's disease (AD) and Parkinson's disease (PD) and the dysregulation of lipids in the brain. Research using Drosophila has highlighted the role of glial-mediated lipid metabolism in sleep and diurnal rhythms. Relevant to AD, the formation of lipid droplets in glia, which occurs in response to elevated neuronal reactive oxygen species (ROS), is required for sleep. In disease models, this process is disrupted, arguing a connection to sleep dysregulation. Relevant to PD, the degradation of neuronally synthesized glucosylceramides by glia requires glucocerebrosidase (GBA, a PD-associated risk factor) and this regulates sleep. Loss of GBA in glia causes an accumulation of glucosylceramides and neurodegeneration. Overall, research primarily using Drosophila has highlighted how dysregulation of glial lipid metabolism may underlie sleep disturbances in neurodegenerative diseases.

Glia-derived secretory fatty acid binding protein Obp44a regulates lipid storage and efflux in the developing Drosophila brain

Glia derived secretory factors play diverse roles in supporting the development, physiology, and stress responses of the central nervous system (CNS). Through transcriptomics and imaging analyses, we have identified Obp44a as one of the most abundantly produced secretory proteins from Drosophila CNS glia. Protein structure homology modeling and Nuclear Magnetic Resonance (NMR) experiments reveal Obp44a as a fatty acid binding protein (FABP) with a high affinity towards long-chain fatty acids in both native and oxidized forms. Further analyses demonstrate that Obp44a effectively infiltrates the neuropil, traffics between neuron and glia, and is secreted into hemolymph, acting as a lipid chaperone and scavenger to regulate lipid and redox homeostasis in the developing brain. In agreement with this essential role, deficiency of Obp44a leads to anatomical and behavioral deficits in adult animals and elevated oxidized lipid levels. Collectively, our findings unveil the crucial involvement of a noncanonical lipid chaperone to shuttle fatty acids within and outside the brain, as needed to maintain a healthy brain lipid environment. These findings could inspire the design of novel approaches to restore lipid homeostasis that is dysregulated in CNS diseases.

Glucosylceramide accumulation in microglia triggers STING-dependent neuroinflammation and neurodegeneration in mice

Mutations in the gene encoding the lysosomal enzyme glucocerebrosidase (GCase) are responsible for Gaucher disease (GD) and are considered the strongest genetic risk factor for Parkinson's disease (PD) and Lewy body dementia (LBD). GCase deficiency leads to extensive accumulation of glucosylceramides (GCs) in cells and contributes to the neuropathology of GD, PD, and LBD by triggering chronic neuroinflammation. Here, we investigated the mechanisms by which GC accumulation induces neuroinflammation. We found that GC accumulation within microglia induced by pharmacological inhibition of GCase triggered STING-dependent inflammation, which contributed to neuronal loss both in vitro and in vivo. GC accumulation in microglia induced mitochondrial DNA (mtDNA) leakage to the cytosol to trigger STING-dependent inflammation. Rapamycin, a compound that promotes lysosomal activity, improved mitochondrial function, thereby decreasing STING signaling. Furthermore, lysosomal damage caused by GC accumulation led to defects in the degradation of activated STING, further exacerbating inflammation mediated by microglia. Thus, limiting STING activity may be a strategy to suppress neuroinflammation caused by GCase deficiency.

Circadian Biology and the Neurovascular Unit

Mammalian physiology and cellular function are subject to significant oscillations over the course of every 24-hour day. It is likely that these daily rhythms will affect function as well as mechanisms of disease in the central nervous system. In this review, we attempt to survey and synthesize emerging studies that investigate how circadian biology may influence the neurovascular unit. We examine how circadian clocks may operate in neural, glial, and vascular compartments, review how circadian mechanisms regulate cell-cell signaling, assess interactions with aging and vascular comorbidities, and finally ask whether and how circadian effects and disruptions in rhythms may influence the risk and progression of pathophysiology in cerebrovascular disease. Overcoming identified challenges and leveraging opportunities for future research might support the development of novel circadian-based treatments for stroke.

A Near-Infrared Fluorogenic Probe for Rapid, Specific, and Ultrasensitive Detection of Sphingosine in Living Cells and In Vivo

Sphingosine (Sph) plays important roles in various complex biological processes. Abnormalities in Sph metabolism can result in various diseases, including neurodegenerative disorders. However, due to the lack of rapid and accurate detection methods, understanding sph metabolic in related diseases is limited. Herein, a series of near-infrared fluorogenic probes DMS-X (X = 2F, F, Cl, Br, and I) are designed and synthesized. The fast oxazolidinone ring formation enables the DMS-2F to detect Sph selectively and ultrasensitively, and the detection limit reaches 9.33 ± 0.41 nm. Moreover, it is demonstrated that DMS-2F exhibited a dose- and time-dependent response to Sph and can detect sph in living cells. Importantly, for the first time, the changes in Sph levels induced by Aβ42 oligomers and H2 O2 are assessed through a fluorescent imaging approach, and further validated the physiological processes by which Aβ42 oligomers and reactive oxygen species (ROS)-induce changes in intracellular Sph levels. Additionally, the distribution of Sph in living zebrafish is successfully mapped by in vivo imaging of a zebrafish model. This work provides a simple and efficient method for probing Sph in living cells and in vivo, which will facilitate investigation into the metabolic process of Sph and the connection between Sph and disease pathologies.

Glucocerebrosidase deficiency leads to neuropathology via cellular immune activation

Mutations in GBA (glucosylceramidase beta), which encodes the lysosomal enzyme glucocerebrosidase (GCase), are the strongest genetic risk factor for the neurodegenerative disorders Parkinson's disease (PD) and Lewy body dementia. Recent work has suggested that neuroinflammation may be an important factor in the risk conferred by GBA mutations. We therefore systematically tested the contributions of immune-related genes to neuropathology in a Drosophila model of GCase deficiency. We identified target immune factors via RNA-Seq and proteomics on heads from GCase-deficient flies, which revealed both increased abundance of humoral factors and increased macrophage activation. We then manipulated the identified immune factors and measured their effect on head protein aggregates, a hallmark of neurodegenerative disease. Genetic ablation of humoral (secreted) immune factors did not suppress the development of protein aggregation. By contrast, re-expressing Gba1b in activated macrophages suppressed head protein aggregation in Gba1b mutants and rescued their lifespan and behavioral deficits. Moreover, reducing the GCase substrate glucosylceramide in activated macrophages also ameliorated Gba1b mutant phenotypes. Taken together, our findings show that glucosylceramide accumulation due to GCase deficiency leads to macrophage activation, which in turn promotes the development of neuropathology.

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.

Plasma essential fatty acid on hospital admission is a marker of COVID-19 disease severity

It is important for allocation of resources to predict those COVID patients at high risk of dying or organ failure. Early signals to initiate cellular events of host immunity can be derived from essential fatty acid metabolites preceding the cascade of proinflammatory signals. Much research has focused on understanding later proinflammatory responses. We assessed if remodelling of plasma phospholipid content of essential fatty acids by the COVID-19 virus provides early markers for potential death and disease severity. Here we show that, at hospital admission, COVID-19 infected subjects who survive exhibit higher proportions of C20:4n-6 in plasma phospholipids concurrent with marked proinflammatory cytokine elevation in plasma compared to healthy subjects. In contrast, more than half of subjects who die of this virus exhibit very low C18:2n-6 and C20:4n-6 content in plasma phospholipids on hospital admission compared with healthy control subjects. Moreover, in these subjects who die, the low level of primary inflammatory signals indicates limited or aberrant stimulation of host immunity. We conclude that COVID-19 infection results in early fundamental remodelling of essential fatty acid metabolism. In subjects with high mortality, it appears that plasma n-6 fatty acid content is too low to stimulate cellular events of host immunity.

Multi-omics profiling reveals rhythmic liver function shaped by meal timing

Post-translational modifications (PTMs) couple feed-fast cycles to diurnal rhythms. However, it remains largely uncharacterized whether and how meal timing organizes diurnal rhythms beyond the transcriptome. Here, we systematically profile the daily rhythms of the proteome, four PTMs (phosphorylation, ubiquitylation, succinylation and N-glycosylation) and the lipidome in the liver from young female mice subjected to either day/sleep time-restricted feeding (DRF) or night/wake time-restricted feeding (NRF). We detect robust daily rhythms among different layers of omics with phosphorylation the most nutrient-responsive and succinylation the least. Integrative analyses reveal that clock regulation of fatty acid metabolism represents a key diurnal feature that is reset by meal timing, as indicated by the rhythmic phosphorylation of the circadian repressor PERIOD2 at Ser971 (PER2-pSer971). We confirm that PER2-pSer971 is activated by nutrient availability in vivo. Together, this dataset represents a comprehensive resource detailing the proteomic and lipidomic responses by the liver to alterations in meal timing.

Sleep Promotion by 3-Hydroxy-4-Iminobutyric Acid in Walnut Diaphragma juglandis Fructus

Insufficient sleep can produce a multitude of deleterious repercussions on various domains of human well-being. Concomitantly, the walnut (Juglans mandshurica) confers numerous salutary biological activities pertaining to sleep. Nevertheless, the sedative and hypnotic capacities of walnut's functional constituents remain obscure. In this investigation, we analyzed the sedative and hypnotic components of the walnut Diaphragma juglandis fructus and innovatively discovered a compound, defined as 3-hydroxy-4-iminobutyric acid (HIBA), which disrupts motor activity and enhances sleep duration by regulating the neurotransmitters (GABA, DA, etc.) within the brain and serum of mice. Subsequently, a metabolomics approach of the serum, basal ganglia, hypothalamus, and hippocampus as well as the gut microbiota was undertaken to unravel the underlying molecular mechanisms of sleep promotion. Our data reveal that HIBA can regulate the metabolism of basal ganglia (sphingolipids, acylcarnitines, etc.), possibly in relation to HIBA's influence on the gut microbiome (Muribaculum, Bacteroides, Lactobacillus, etc.). Therefore, we introduce a novel natural product, HIBA, and explicate the modulation of sleep promotion in mice based on the microbiota-gut-brain axis. This study contributes fresh insights toward natural product-based sleep research.

From seconds to days: Neural plasticity viewed through a lipid lens

Many adult neurons are dynamically remodeled across timescales ranging from the rapid addition and removal of specific synaptic connections, to largescale structural plasticity events that reconfigure circuits over hours, days, and months. Membrane lipids, including brain-enriched sphingolipids, play crucial roles in these processes. In this review, we summarize progress at the intersection of neuronal activity, lipids, and structural remodeling. We highlight how brain activity modulates lipid metabolism to enable adaptive structural plasticity, and showcase glia as key players in membrane remodeling. These studies reveal that lipids act as critical signaling molecules that instruct the dynamic architecture of the brain.