Constitutive hippocampal cholesterol loss underlies poor cognition in old rodents

Lysosomal cholesterol accumulation in aged astrocytes impairs cholesterol delivery to neurons and can be rescued by cannabinoids

Cholesterol is crucial for the proper functioning of eukaryotic cells, especially neurons, which rely on cholesterol to maintain their complex structure and facilitate synaptic transmission. However, brain cells are isolated from peripheral cholesterol by the blood-brain barrier and mature neurons primarily uptake the cholesterol synthesized by astrocytes for proper function. This study aimed to investigate the effect of aging on cholesterol trafficking in astrocytes and its delivery to neurons. We found that aged astrocytes accumulated high levels of cholesterol in the lysosomal compartment, and this cholesterol buildup can be attributed to the simultaneous occurrence of two events: decreased levels of the ABCA1 transporter, which impairs ApoE-cholesterol export from astrocytes, and reduced expression of NPC1, which hinders cholesterol release from lysosomes. We show that these two events are accompanied by increased microR-33 in aged astrocytes, which targets ABCA1 and NPC1. In addition, we demonstrate that the microR-33 increase is triggered by oxidative stress, one of the hallmarks of aging. By coculture experiments, we show that cholesterol accumulation in astrocytes impairs the cholesterol delivery from astrocytes to neurons. Remarkably, we found that this altered transport of cholesterol could be alleviated through treatment with endocannabinoids as well as cannabidiol or CBD. Finally, according to data demonstrating that aged astrocytes develop an A1 phenotype, we found that cholesterol buildup is also observed in reactive C3+ astrocytes. Given that reduced neuronal cholesterol affects synaptic plasticity, the ability of cannabinoids to restore cholesterol transport from aged astrocytes to neurons holds significant implications in aging and inflammation.

Cholesterol Metabolism in Neurodegenerative Diseases

SIGNIFICANCE

Cholesterol plays a crucial role in brain, where it is highly concentrated and tightly regulated to support normal brain functions. It serves as a vital component of cell membranes, ensuring their integrity, and acts as a key regulator of various brain processes. Dysregulation of cholesterol metabolism in the brain has been linked to impaired brain function and the onset of neurodegenerative diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), and Huntington's disease (HD).

RECENT ADVANCES

A significant advancement has been the identification of astrocyte-derived ApoE as a key regulator of de novo cholesterol biosynthesis in neurons, providing insights into how extracellular signals influence neuronal cholesterol levels. Additionally, the development of antibody-based therapies, particularly for AD, presents promising opportunities for therapeutic interventions.

CRITICAL ISSUES

Despite significant research, the association between cholesterol and neurodegenerative diseases remains inconclusive. It is crucial to distinguish between plasma cholesterol and brain cholesterol, as these pools are relatively independent. This differentiation should be considered when evaluating statin-based treatment approaches. Assessing not only the total cholesterol content in the brain but also its distribution among different types of brain cells is essential.

FUTURE DIRECTION

Establishing a causal link between changes in brain/plasma cholesterol levels and the onset of brain dysfunction/neurodegenerative diseases remains a key objective. Additionally, conducting cell-specific analyses of cholesterol homeostasis in various types of brain cells under pathological conditions will enhance our understanding of cholesterol metabolism in neurodegenerative diseases. Manipulating cholesterol levels to restore homeostasis may represent a novel approach for alleviating neurological symptom.

APOB100 transgenic mice exemplify how the systemic circulation content may affect the retina without altering retinal cholesterol input

Apolipoprotein B (APOB) is a constituent of unique lipoprotein particles (LPPs) produced in the retinal pigment epithelium (RPE), which separates the neural retina from Bruch's membrane (BrM) and choroidal circulation. These LPPs accumulate with age in BrM and contribute to the development of age-related macular degeneration, a major blinding disease. The APOB100 transgenic expression in mice, which unlike humans lack the full-length APOB100, leads to lipid deposits in BrM. Herein, we further characterized APOB100 transgenic mice. We imaged mouse retina in vivo and assessed chorioretinal lipid distribution, retinal sterol levels, retinal cholesterol input, and serum content as well as tracked indocyanine green-bound LPPs in mouse plasma and retina after an intraperitoneal injection. Retinal function and differentially expressed proteins were also investigated. APOB100 transgenic mice had increased serum LDL content and an additional higher density HDL subpopulation; their retinal cholesterol levels (initially decreased) became normal with age. The LPP cycling between the RPE and choroidal circulation was increased. Yet, LPP trafficking from the RPE to the neural retina was limited, and total retinal cholesterol input did not change. There were lipid deposits in the RPE and BrM, and retinal function was impaired. Retinal proteomics provided mechanistic insights. Collectively, our data suggested that the serum LDL/HDL ratio may not affect retinal pathways of cholesterol input as serum LPP load is mainly handled by the RPE, which offloads LPP excess to the choroidal circulation rather than neural retina. Different HDL subpopulations should be considered in studies linking serum LPPs and age-related macular degeneration.

Reexamining the Causes and Effects of Cholesterol Deposition in the Brains of Patients with Alzheimer's Disease

Alzheimer's disease (AD) is a degenerative disease of the central nervous system. Numerous studies have shown that imbalances in cholesterol homeostasis in the brains of AD patients precede the onset of clinical symptoms. In addition, cholesterol deposition has been observed in the brains of AD patients even though peripheral cholesterol does not enter the brain through the blood‒brain barrier (BBB). Studies have demonstrated that cholesterol metabolism in the brain is associated with many pathological conditions, such as amyloid beta (Aβ) production, Tau protein phosphorylation, oxidative stress, and inflammation. In 2022, some scholars put forward a new hypothesis of AD: the disease involves lipid invasion and its exacerbation of the abnormal metabolism of cholesterol in the brain. In this review, by discussing the latest research progress, the causes and effects of cholesterol retention in the brains of AD patients are analyzed and discussed. Additionally, the possible mechanism through which AD may be improved by targeting cholesterol is described. Finally, we propose that improving the impairments in cholesterol removal observed in the brains of AD patients, instead of further reducing the already impaired cholesterol synthesis in the brain, may be the key to preventing cholesterol deposition and improving the corresponding pathological symptoms.

The translational potential of cholesterol-based therapies for neurological disease

Cholesterol is an important metabolite and membrane component and is enriched in the brain owing to its role in neuronal maturation and function. In the adult brain, cholesterol is produced locally, predominantly by astrocytes. When cholesterol has been used, recycled and catabolized, the derivatives are excreted across the blood-brain barrier. Abnormalities in any of these steps can lead to neurological dysfunction. Here, we examine how precise interactions between cholesterol production and its use and catabolism in neurons ensures cholesterol homeostasis to support brain function. As an example of a neurological disease associated with cholesterol dyshomeostasis, we summarize evidence from animal models of Huntington disease (HD), which demonstrate a marked reduction in cholesterol biosynthesis with clinically relevant consequences for synaptic activity and cognition. In addition, we examine the relationship between cholesterol loss in the brain and cognitive decline in ageing. We then present emerging therapeutic strategies to restore cholesterol homeostasis, focusing on evidence from HD mouse models.

Phytosterols reverse antiretroviral-induced hearing loss, with potential implications for cochlear aging

Cholesterol contributes to neuronal membrane integrity, supports membrane protein clustering and function, and facilitates proper signal transduction. Extensive evidence has shown that cholesterol imbalances in the central nervous system occur in aging and in the development of neurodegenerative diseases. In this work, we characterize cholesterol homeostasis in the inner ear of young and aged mice as a new unexplored possibility for the prevention and treatment of hearing loss. Our results show that cholesterol levels in the inner ear are reduced during aging, an effect that is associated with an increased expression of the cholesterol 24-hydroxylase (CYP46A1), the main enzyme responsible for cholesterol turnover in the brain. In addition, we show that pharmacological activation of CYP46A1 with the antiretroviral drug efavirenz reduces the cholesterol content in outer hair cells (OHCs), leading to a decrease in prestin immunolabeling and resulting in an increase in the distortion product otoacoustic emissions (DPOAEs) thresholds. Moreover, dietary supplementation with phytosterols, plant sterols with structure and function similar to cholesterol, was able to rescue the effect of efavirenz administration on the auditory function. Altogether, our findings point towards the importance of cholesterol homeostasis in the inner ear as an innovative therapeutic strategy in preventing and/or delaying hearing loss.

The normalizing effects of the CYP46A1 activator efavirenz on retinal sterol levels and risk factors for glaucoma in Apoj-/- mice

Apolipoprotein J (APOJ) is a multifunctional protein with genetic evidence suggesting an association between APOJ polymorphisms and Alzheimer's disease as well as exfoliation glaucoma. Herein we conducted ocular characterizations of Apoj^-/- mice and found that their retinal cholesterol levels were decreased and that this genotype had several risk factors for glaucoma: increased intraocular pressure and cup-to-disk ratio and impaired retinal ganglion cell (RGC) function. The latter was not due to RGC degeneration or activation of retinal Muller cells and microglia/macrophages. There was also a decrease in retinal levels of 24-hydroxycholesterol, a suggested neuroprotectant under glaucomatous conditions and a positive allosteric modulator of N-methyl-D-aspartate receptors mediating the light-evoked response of the RGC. Therefore, Apoj^-/- mice were treated with low-dose efavirenz, an allosteric activator of CYP46A1 which converts cholesterol into 24-hydroxycholesterol. Efavirenz treatment increased retinal cholesterol and 24-hydroxycholesterol levels, normalized intraocular pressure and cup-to-disk ratio, and rescued in part RGC function. Retinal expression of Abcg1 (a cholesterol efflux transporter), Apoa1 (a constituent of lipoprotein particles), and Scarb1 (a lipoprotein particle receptor) was increased in EVF-treated Apoj^-/- mice, indicating increased retinal cholesterol transport on lipoprotein particles. Ocular characterizations of Cyp46a1^-/- mice supported the beneficial efavirenz treatment effects via CYP46A1 activation. The data obtained demonstrate an important APOJ role in retinal cholesterol homeostasis and link this apolipoprotein to the glaucoma risk factors and retinal 24-hydroxycholesterol production by CYP46A1. As the CYP46A1 activator efavirenz is an FDA-approved anti-HIV drug, our studies suggest a new therapeutic approach for treatment of glaucomatous conditions.

Effects of food-borne cholesterol supplementation on lead-induced neurodevelopmental impairments of rats based on BDNF signaling pathway and cholesterol metabolism

Despite the ubiquity and prevalence of lead (Pb) in the environment and industry, the mechanism of lead-induced neurotoxicity in the brain remains unclear, let alone its prevention and treatment. In this study, we hypothesized that exogenous cholesterol supplementation acts as an effective remedy for lead-induced neurodevelopmental impairments caused by lead. Forty 21-day-old male rats were randomly divided into four groups and administered 0.1 % lead water and/or 2 % cholesterol-containing feed for 30 d. Ultimately, rats in the lead group lost weight, accompanied by spatial learning and memory impairments as verified by the Morris water maze test, in which the escape latency of rats was prolonged, and the number of crossings in the target platform and the residence time in the target quadrant were significantly diminished compared to the control group. Hematoxylin-Eosin (H&E) staining and Nissl staining illustrated that typical pathological morphology occurred in the brain tissue of the lead group, where the tissue structure was loose, the number of hippocampal neurons and granulosa cells decreased significantly and were arranged loosely, along with enlarged intercellular space, light matrix staining, and decline in Nissl bodies. In addition, inflammatory response and oxidative stress were significantly induced by lead. Immunofluorescence experiments showed apparent activation of astrocytes and microglia, followed by the enhancement of TNF-α and IL-β levels. Moreover, the MDA content in the lead group was elevated dramatically, whereas the activities of SOD and GSH were significantly inhibited. As for the mechanism, western blot and qRT-PCR experiments were performed, where lead could significantly inhibit the BDNF-TrkB signaling pathway, lowering the protein expression of BDNF and TrkB. Cholesterol metabolism was also affected by lead exposure, in which cholesterol metabolism-related protein expression and gene transcription, including SREBP2, HMGCR, and LDLR, were downregulated. However, cholesterol supplementation efficiently detoxified the negative effects of lead-induced neurotoxicity, reversing the inflammatory response, oxidative stress, inactivation of the BDNF signaling pathway, and imbalance of cholesterol metabolism, thus improving the learning and memory ability of rats. In brief, our study demonstrated that cholesterol supplementation could ameliorate the deficiency of learning and memory induced by lead, which is closely associated with the initiation of the BDNF/TrkB signaling pathway and regulation of cholesterol metabolism.

Requirement of Cholesterol for Calcium-Dependent Vesicle Fusion by Strengthening Synaptotagmin-1-Induced Membrane Bending

Cholesterol is essential for neuronal activity and function. Cholesterol depletion in the plasma membrane impairs synaptic transmission. However, the molecular mechanisms by which cholesterol deficiency leads to defects in vesicle fusion remain poorly understood. Here, it is shown that cholesterol is required for Ca²⁺ -dependent native vesicle fusion using the in vitro reconstitution of fusion and amperometry to monitor exocytosis in chromaffin cells. Purified native vesicles are crucial for the reconstitution of physiological Ca²⁺ -dependent fusion, because vesicle-mimicking liposomes fail to reproduce the cholesterol effect. Intriguingly, cholesterol has no effect on the membrane binding of synaptotagmin-1, a Ca²⁺ sensor for ultrafast fusion. Cholesterol strengthens local membrane deformation and bending induced by synaptotagmin-1, thereby lowering the energy barrier for Ca²⁺ -dependent fusion to occur. The data provide evidence that cholesterol depletion abolishes Ca²⁺ -dependent vesicle fusion by disrupting synaptotagmin-1-induced membrane bending, and suggests that cholesterol is an essential lipid regulator for Ca²⁺ -dependent fusion.

Cholesterol-24-hydroxylase (CYP46) in the old brain: Analysis of positive populations and factors triggering its expression in astrocytes

Cholesterol-24-hydroxylase (CYP46), a member of the cytochrome P450 superfamily of enzymes, is selectively expressed in the brain and is mainly responsible for cholesterol turnover in the central nervous system. Although increased cyp46A1 gene expression has been linked to cognitive alterations in aging and observed in neurodegenerative diseases and after traumatic brain injury, a detailed characterization of the brain regions and cell types in which CYP46 is expressed in old individuals has not been performed. Using immunohistochemistry and immunofluorescence, we investigated the specific regions and cell populations in the brain, in which cyp46A1 is expressed in 24-month-old mice. We found that CYP46 is localized in the same neuronal populations in young and old brains, mainly in the hippocampus, in cortical layers, and in Purkinje neurons of the cerebellum. No increase in CYP46 levels was found in astrocytes in old mice brains, in primary astrocyte-neuron cocultures aged in vitro, or in primary cultures of senescent astrocytes. However, interleukin-6 treatment strongly induced cyp46A1 expression in reactive astrocytes characterized by high GFAP levels but had no effect in nonactivated astrocytes. Our data suggest that cholesterol-24-hydroxylase expression is triggered in reactive astrocytes in response to proinflammatory signals, probably as part of a response mechanism to injury.

c-Abl kinase at the crossroads of healthy synaptic remodeling and synaptic dysfunction in neurodegenerative diseases

Our ability to learn and remember depends on the active formation, remodeling, and elimination of synapses. Thus, the development and growth of synapses as well as their weakening and elimination are essential for neuronal rewiring. The structural reorganization of synaptic complexes, changes in actin cytoskeleton and organelle dynamics, as well as modulation of gene expression, determine synaptic plasticity. It has been proposed that dysregulation of these key synaptic homeostatic processes underlies the synaptic dysfunction observed in many neurodegenerative diseases. Much is known about downstream signaling of activated N-methyl-D-aspartate and α-amino-3-hydroxy-5-methyl-4-isoazolepropionate receptors; however, other signaling pathways can also contribute to synaptic plasticity and long-lasting changes in learning and memory. The non-receptor tyrosine kinase c-Abl (ABL1) is a key signal transducer of intra and extracellular signals, and it shuttles between the cytoplasm and the nucleus. This review focuses on c-Abl and its synaptic and neuronal functions. Here, we discuss the evidence showing that the activation of c-Abl can be detrimental to neurons, promoting the development of neurodegenerative diseases. Nevertheless, c-Abl activity seems to be in a pivotal balance between healthy synaptic plasticity, regulating dendritic spines remodeling and gene expression after cognitive training, and synaptic dysfunction and loss in neurodegenerative diseases. Thus, c-Abl genetic ablation not only improves learning and memory and modulates the brain genetic program of trained mice, but its absence provides dendritic spines resiliency against damage. Therefore, the present review has been designed to elucidate the common links between c-Abl regulation of structural changes that involve the actin cytoskeleton and organelles dynamics, and the transcriptional program activated during synaptic plasticity. By summarizing the recent discoveries on c-Abl functions, we aim to provide an overview of how its inhibition could be a potentially fruitful treatment to improve degenerative outcomes and delay memory loss.

The synaptic lipidome in health and disease

Adequate homeostasis of lipid, protein and carbohydrate metabolism is essential for cells to perform highly specific tasks in our organism, and the brain, with its uniquely high energetic requirements, posesses singular characteristics. Some of these are related to its extraordinary dotation of synapses, the specialized subcelluar structures where signal transmission between neurons occurs in the central nervous system. The post-synaptic compartment of excitatory synapses, the dendritic spine, harbors key molecules involved in neurotransmission tightly packed within a minute volume of a few femtoliters. The spine is further compartmentalized into nanodomains that facilitate the execution of temporo-spatially separate functions in the synapse. Lipids play important roles in this structural and functional compartmentalization and in mechanisms that impact on synaptic transmission. This review analyzes the structural and dynamic processes involving lipids at the synapse, highlighting the importance of their homeostatic balance for the physiology of this complex and highly specialized structure, and underscoring the pathologies associated with disbalances of lipid metabolism, particularly in the perinatal and late adulthood periods of life. Although small variations of the lipid profile in the brain take place throughout the adult lifespan, the pathophysiological consequences are clinically manifested mostly during late adulthood. Disturbances in lipid homeostasis in the perinatal period leads to alterations during nervous system development, while in late adulthood they favor the occurrence of neurodegenerative diseases.

Plasma membrane and brain dysfunction of the old: Do we age from our membranes?

One of the characteristics of aging is a gradual hypo-responsiveness of cells to extrinsic stimuli, mainly evident in the pathways that are under hormone control, both in the brain and in peripheral tissues. Age-related resistance, i.e., reduced response of receptors to their ligands, has been shown to Insulin and also to leptin, thyroid hormones and glucocorticoids. In addition, lower activity has been reported in aging for ß-adrenergic receptors, adenosine A2B receptor, and several other G-protein-coupled receptors. One of the mechanisms proposed to explain the loss of sensitivity to hormones and neurotransmitters with age is the loss of receptors, which has been observed in several tissues. Another mechanism that is finding more and more experimental support is related to the changes that occur with age in the lipid composition of the neuronal plasma membrane, which are responsible for changes in the receptors’ coupling efficiency to ligands, signal attenuation and pathway desensitization. In fact, recent works have shown that altered membrane composition—as occurs during neuronal aging—underlies reduced response to glutamate, to the neurotrophin BDNF, and to insulin, all these leading to cognition decay and epigenetic alterations in the old. In this review we present evidence that altered functions of membrane receptors due to altered plasma membrane properties may be a triggering factor in physiological decline, decreased brain function, and increased vulnerability to neuropathology in aging.

Interactions between the Nicotinic and Endocannabinoid Receptors at the Plasma Membrane

Compartmentalization, together with transbilayer and lateral asymmetries, provide the structural foundation for functional specializations at the cell surface, including the active role of the lipid microenvironment in the modulation of membrane-bound proteins. The chemical synapse, the site where neurotransmitter-coded signals are decoded by neurotransmitter receptors, adds another layer of complexity to the plasma membrane architectural intricacy, mainly due to the need to accommodate a sizeable number of molecules in a minute subcellular compartment with dimensions barely reaching the micrometer. In this review, we discuss how nature has developed suitable adjustments to accommodate different types of membrane-bound receptors and scaffolding proteins via membrane microdomains, and how this "effort-sharing" mechanism has evolved to optimize crosstalk, separation, or coupling, where/when appropriate. We focus on a fast ligand-gated neurotransmitter receptor, the nicotinic acetylcholine receptor, and a second-messenger G-protein coupled receptor, the cannabinoid receptor, as a paradigmatic example.

MicroRNA-206 down-regulated human umbilical cord mesenchymal stem cells alleviate cognitive decline in D-galactose-induced aging mice

Background

Non-pathological cognitive decline is a neurodegenerative condition associated with brain aging owing to epigenetic changes, telomere shortening, stem cells exhaustion, or altered differentiation. Human umbilical cord mesenchymal stem cells (hUCMSCs) have shown excellent therapeutic prospects on the hallmarks of aging. In this study, we aimed to elucidate the role of hUCMSCs with down-regulated miRNA-206 (hUCMSCs anti-miR-206) on cognitive decline and the underlying mechanism.

Methods

After daily subcutaneous injection of D-gal (500 mg/kg/d) for 8 weeks, 17-week-old male C57BL/6 J mice were stem cells transplanted by lateral ventricular localization injection. During the 10-day rest period, were tested the behavioral experiments applied to cognitive behavior in the hippocampus. And then, the mice were sacrificed for sampling to complete the molecular and morphological experiments.

Results

Our behavioral experiments of open field test (OFT), new object recognition test (NOR), and Y-maze revealed that D-galactose (D-gal)-induced aging mice treated with hUCMSCs anti-miR-206 had no obvious spontaneous activity disorder and had recovery in learning and spatial memory ability compared with the PBS-treated group. The hUCMSCs anti-miR-206 reconstituted neuronal physiological function in the hippocampal regions of the aging mice with an increase of Nissl bodies and the overexpression of Egr-1, BDNF, and PSD-95.

Conclusion

This study first reports that hUCMSCs anti-miR-206 could provide a novel stem cell-based antiaging therapeutic approach.

Altered Cholesterol Homeostasis in Huntington’s Disease

Huntington’s disease (HD) is an autosomal dominant genetic disorder caused by an expansion of the CAG repeat in the first exon of Huntingtin’s gene. The associated neurodegeneration mainly affects the striatum and the cortex at early stages and progressively spreads to other brain structures. Targeting HD at its earlier stages is under intense investigation. Numerous drugs were tested, with a rate of success of only 3.5% approved molecules used as symptomatic treatment. The restoration of cholesterol metabolism, which is central to the brain homeostasis and strongly altered in HD, could be an interesting disease-modifying strategy. Cholesterol is an essential membrane component in the central nervous system (CNS); alterations of its homeostasis have deleterious consequences on neuronal functions. The levels of several sterols, upstream of cholesterol, are markedly decreased within the striatum of HD mouse model. Transcription of cholesterol biosynthetic genes is reduced in HD cell and mouse models as well as post-mortem striatal and cortical tissues from HD patients. Since the dynamic of brain cholesterol metabolism is complex, it is essential to establish the best method to target it in HD. Cholesterol, which does not cross the blood-brain-barrier, is locally synthesized and renewed within the brain. All cell types in the CNS synthesize cholesterol during development but as they progress through adulthood, neurons down-regulate their cholesterol synthesis and turn to astrocytes for their full supply. Cellular levels of cholesterol reflect the dynamic balance between synthesis, uptake and export, all integrated into the context of the cross talk between neurons and glial cells. In this review, we describe the latest advances regarding the role of cholesterol deregulation in neuronal functions and how this could be a determinant factor in neuronal degeneration and HD progression. The pathways and major mechanisms by which cholesterol and sterols are regulated in the CNS will be described. From this overview, we discuss the main clinical strategies for manipulating cholesterol metabolism in the CNS, and how to reinstate a proper balance in HD.

The role of DHCR24 in the pathogenesis of AD: re-cognition of the relationship between cholesterol and AD pathogenesis

Previous studies show that 3β-hydroxysterol-Δ24 reductase (DHCR24) has a remarked decline in the brain of AD patients. In brain cholesterol synthetic metabolism, DHCR24 is known as the heavily key synthetase in cholesterol synthesis. Moreover, mutations of DHCR24 gene result in inhibition of the enzymatic activity of DHCR24, causing brain cholesterol deficiency and desmosterol accumulation. Furthermore, in vitro studies also demonstrated that DHCR24 knockdown lead to the inhibition of cholesterol synthesis, and the decrease of plasma membrane cholesterol and intracellular cholesterol level. Obviously, DHCR24 could play a crucial role in maintaining cholesterol homeostasis via the control of cholesterol synthesis. Over the past two decades, accumulating data suggests that DHCR24 activity is downregulated by major risk factors for AD, suggesting a potential link between DHCR24 downregulation and AD pathogenesis. Thus, the brain cholesterol loss seems to be induced by the major risk factors for AD, suggesting a possible causative link between brain cholesterol loss and AD. According to previous data and our study, we further found that the reduced cholesterol level in plasma membrane and intracellular compartments by the deficiency of DHCR24 activity obviously was involved in β-amyloid generation, tau hyperphosphorylation, apoptosis. Importantly, increasing evidences reveal that the brain cholesterol loss and lipid raft disorganization are obviously linked to neuropathological impairments which are associated with AD pathogenesis. Therefore, based on previous data and research on DHCR24, we suppose that the brain cholesterol deficiency/loss might be involved in the pathogenesis of AD.

Contribution of Membrane Lipids to Postsynaptic Protein Organization

The precise subsynaptic organization of proteins at the postsynaptic membrane controls synaptic transmission. In particular, postsynaptic receptor complexes are concentrated in distinct membrane nanodomains to optimize synaptic signaling. However, despite the clear functional relevance of subsynaptic receptor organization to synaptic transmission and plasticity, the mechanisms that underlie the nanoscale organization of the postsynaptic membrane remain elusive. Over the last decades, the field has predominantly focused on the role of protein-protein interactions in receptor trafficking and positioning in the synaptic membrane. In contrast, the contribution of lipids, the principal constituents of the membrane, to receptor positioning at the synapse remains poorly understood. Nevertheless, there is compelling evidence that the synaptic membrane is enriched in specific lipid species and that deregulation of lipid homeostasis in neurons severely affects synaptic functioning. In this review we focus on how lipids are organized at the synaptic membrane, with special emphasis on how current models of membrane organization could contribute to protein distribution at the synapse and synaptic transmission. Finally, we will present an outlook on how novel technical developments could be applied to study the dynamic interplay between lipids and proteins at the postsynaptic membrane.

Principles of brain aging: Status and challenges of modeling human molecular changes in mice

Due to the extension of human life expectancy, the prevalence of cognitive impairment is rising in the older portion of society. Developing new strategies to delay or attenuate cognitive decline is vital. For this purpose, it is imperative to understand the cellular and molecular events at the basis of brain aging. While several organs are directly accessible to molecular analysis through biopsies, the brain constitutes a notable exception. Most of the molecular studies are performed on postmortem tissues, where cell death and tissue damage have already occurred. Hence, the study of the molecular aspects of cognitive decline largely relies on animal models and in particular on small mammals such as mice. What have we learned from these models? Do these animals recapitulate the changes observed in humans? What should we expect from future mouse studies? In this review we answer these questions by summarizing the state of the research that has addressed cognitive decline in mice from several perspectives, including genetic manipulation and omics strategies. We conclude that, while extremely valuable, mouse models have limitations that can be addressed by the optimal design of future studies and by ensuring that results are cross-validated in the human context.

Cholesterol Dysmetabolism in Alzheimer's Disease: A Starring Role for Astrocytes?

In recent decades, the impairment of cholesterol metabolism in the pathogenesis of Alzheimer’s disease (AD) has been intensively investigated, and it has been recognized to affect amyloid β (Aβ) production and clearance, tau phosphorylation, neuroinflammation and degeneration. In particular, the key role of cholesterol oxidation products, named oxysterols, has emerged. Brain cholesterol metabolism is independent from that of peripheral tissues and it must be preserved in order to guarantee cerebral functions. Among the cells that help maintain brain cholesterol homeostasis, astrocytes play a starring role since they deliver de novo synthesized cholesterol to neurons. In addition, other physiological roles of astrocytes are to modulate synaptic transmission and plasticity and support neurons providing energy. In the AD brain, astrocytes undergo significant morphological and functional changes that contribute to AD onset and development. However, the extent of this contribution and the role played by oxysterols are still unclear. Here we review the current understanding of the physiological role exerted by astrocytes in the brain and their contribution to AD pathogenesis. In particular, we focus on the impact of cholesterol dysmetabolism on astrocyte functions suggesting new potential approaches to develop therapeutic strategies aimed at counteracting AD development.

Nanoscale Sub-Compartmentalization of the Dendritic Spine Compartment

Compartmentalization of the membrane is essential for cells to perform highly specific tasks and spatially constrained biochemical functions in topographically defined areas. These membrane lateral heterogeneities range from nanoscopic dimensions, often involving only a few molecular constituents, to micron-sized mesoscopic domains resulting from the coalescence of nanodomains. Short-lived domains lasting for a few milliseconds coexist with more stable platforms lasting from minutes to days. This panoply of lateral domains subserves the great variety of demands of cell physiology, particularly high for those implicated in signaling. The dendritic spine, a subcellular structure of neurons at the receiving (postsynaptic) end of central nervous system excitatory synapses, exploits this compartmentalization principle. In its most frequent adult morphology, the mushroom-shaped spine harbors neurotransmitter receptors, enzymes, and scaffolding proteins tightly packed in a volume of a few femtoliters. In addition to constituting a mesoscopic lateral heterogeneity of the dendritic arborization, the dendritic spine postsynaptic membrane is further compartmentalized into spatially delimited nanodomains that execute separate functions in the synapse. This review discusses the functional relevance of compartmentalization and nanodomain organization in synaptic transmission and plasticity and exemplifies the importance of this parcelization in various neurotransmitter signaling systems operating at dendritic spines, using two fast ligand-gated ionotropic receptors, the nicotinic acetylcholine receptor and the glutamatergic receptor, and a second-messenger G-protein coupled receptor, the cannabinoid receptor, as paradigmatic examples.

Dendritic spine membrane proteome and its alterations in autistic spectrum disorder

Dendritic spines are small protrusions stemming from the dendritic shaft that constitute the primary specialization for receiving and processing excitatory neurotransmission in brain synapses. The disruption of dendritic spine function in several neurological and neuropsychiatric diseases leads to severe information-processing deficits with impairments in neuronal connectivity and plasticity. Spine dysregulation is usually accompanied by morphological alterations to spine shape, size and/or number that may occur at early pathophysiological stages and not necessarily be reflected in clinical manifestations. Autism spectrum disorder (ASD) is one such group of diseases involving changes in neuronal connectivity and abnormal morphology of dendritic spines on postsynaptic neurons. These alterations at the subcellular level correlate with molecular changes in the spine proteome, with alterations in the copy number, topography, or in severe cases in the phenotype of the molecular components, predominantly of those proteins involved in spine recognition and adhesion, reflected in abnormally short lifetimes of the synapse and compensatory increases in synaptic connections. Since cholinergic neurotransmission participates in the regulation of cognitive function (attention, memory, learning processes, cognitive flexibility, social interactions) brain acetylcholine receptors are likely to play an important role in the dysfunctional synapses in ASD, either directly or indirectly via the modulatory functions exerted on other neurotransmitter receptor proteins and spine-resident proteins.

Multiomics of synaptic junctions reveals altered lipid metabolism and signaling following environmental enrichment

Membrane lipids and their metabolism have key functions in neurotransmission. Here we provide a quantitative lipid inventory of mouse and rat synaptic junctions. To this end, we developed a multiomics extraction and analysis workflow to probe the interplay of proteins and lipids in synaptic signal transduction from the same sample. Based on this workflow, we generate hypotheses about novel mechanisms underlying complex changes in synaptic connectivity elicited by environmental stimuli. As a proof of principle, this approach reveals that in mice exposed to an enriched environment, reduced endocannabinoid synthesis and signaling is linked to increased surface expression of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) in a subset of Cannabinoid-receptor 1 positive synapses. This mechanism regulates synaptic strength in an input-specific manner. Thus, we establish a compartment-specific multiomics workflow that is suitable to extract information from complex lipid and protein networks involved in synaptic function and plasticity.

Increased exosome secretion in neurons aging in vitro by NPC1-mediated endosomal cholesterol buildup

The present study unveils the molecular mechanism through which neurons in vitro compensate age-associated proteostasis defects by increasing exosome release.

As neurons age, they show a decrease in their ability to degrade proteins and membranes. Because undegraded material is a source of toxic products, defects in degradation are associated with reduced cell function and survival. However, there are very few dead neurons in the aging brain, suggesting the action of compensatory mechanisms. We show in this work that ageing neurons in culture show large multivesicular bodies (MVBs) filled with intralumenal vesicles (ILVs) and secrete more small extracellular vesicles than younger neurons. We also show that the high number of ILVs is the consequence of the accumulation of cholesterol in MVBs, which in turn is due to decreased levels of the cholesterol extruding protein NPC1. NPC1 down-regulation is the consequence of a combination of upregulation of the NPC1 repressor microRNA 33, and increased degradation, due to Akt-mTOR targeting of NPC1 to the phagosome. Although releasing more exosomes can be beneficial to old neurons, other cells, neighbouring and distant, can be negatively affected by the waste material they contain.

Stimulation of mGluR1/5 Improves Defective Internalization of AMPA Receptors in NPC1 Mutant Mouse

Niemann-Pick type C1 (NPC1) disease is characterized by neurodegeneration caused by cholesterol accumulation in the late endosome/lysosome. In this study, a defective basal and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-stimulated internalization of GluR2-containing AMPA receptors in NPC1-/- cortical neurons was detected. Our results show that the amount of cholesterol and group I metabotropic glutamate receptors (mGluR1/5) in lipid rafts of NPC1-/- cortical tissue and neurons are decreased and their downstream signals of p-ERK are defective, which are restored by a rebalance of cholesterol homeostasis through β-cyclodextrin (β-CD) treatment. Application of 3,5-dihydroxyphenylglycine (DHPG)-a mGluR1/5 agonist-and β-CD markedly increases the internalization of AMPA receptors and decreases over-influx of calcium in NPC1-/- neurons, respectively. Furthermore, the defective phosphorylated GluR2 and protein kinase C signals are ameliorated by the treatment with DHPG and β-CD, respectively, suggesting an involvement of them in internalization dysfunction. Taken together, our data imply that abnormal internalization of AMPA receptors is a critical mechanism for neuronal dysfunction and the correction of dysfunctional mGluR1/5 is a potential therapeutic strategy for NPC1 disease.

Cholesterol-recognition motifs in the transmembrane domain of the tyrosine kinase receptor family: The case of TRKB

Cholesterol is an essential constituent of cell membranes. The discovery of cholesterol-recognition amino acid consensus (CRAC) motif in proteins indicated a putative direct, non-covalent interaction between cholesterol and proteins. In the present study, we evaluated the presence of a CRAC motif and its inverted version (CARC) in the transmembrane region (TMR) of the tyrosine kinase receptor family (RTK) in several species using in silico methods. CRAC motifs were found across all species analyzed, while CARC was found only in vertebrates. The tropomyosin-related kinase B (TRKB), a member of the RTK family, through interaction with its endogenous ligand brain-derived neurotrophic factor (BDNF) is a core participant in the neuronal plasticity process and exhibits a CARC motif in its TMR. Upon identifying the conserved CARC motif in the TRKB, we performed molecular dynamics simulations of the mouse TRKB.TMR. The simulations indicated that cholesterol interaction with the TRKB CARC motif occurs mainly at the central Y433 residue. Our binding assay suggested a bell-shaped effect of cholesterol on BDNF interaction with TRKB receptors, and our results suggest that CARC/CRAC motifs may play a role in the function of the RTK family TMR.

DHCR24 Knock-Down Induced Tau Hyperphosphorylation at Thr181, Ser199, Thr231, Ser262, Ser396 Epitopes and Inhibition of Autophagy by Overactivation of GSK3β/mTOR Signaling

Accumulating evidences supported that knock-down of DHCR24 is linked to the pathological risk factors of AD, suggesting a potential role of DHCR24 in AD pathogenesis. However, the molecular mechanism link between DHCR24 and tauopathy remains unknown. Here, in order to elucidate the relationship between DHCR24 and tauopathy, we will focus on the effect of DHCR24 on the tau hyperphosphorylation at some toxic sites. In present study, we found that DHCR24 knock-down significantly lead to the hyperphosphorylation of tau sites at Thr181, Ser199, Thr231, Ser262, Ser396. Moreover, DHCR24 knock-down also increase the accumulation of p62 protein, simultaneously decreased the ratio of LC3-II/LC3-I and the number of autophagosome compared to the control groups, suggesting the inhibition of autophagy activity. In contrast, DHCR24 knock-in obviously abolished the effect of DHCR24 knock-down on tau hyperphosphrylation and autophagy. In addition, to elucidate the association between DHCR24 and tauopathy, we further showed that the level of plasma membrane cholesterol, lipid raft-anchored protein caveolin-1, and concomitantly total I class PI3-K (p110α), phospho-Akt (Thr308 and Ser473) were significantly decreased, resulting in the disruption of lipid raft/caveola and inhibition of PI3-K/Akt signaling in silencing DHCR24 SH-SY5Y cells compared to control groups. At the same time, DHCR24 knock-down simultaneously decreased the level of phosphorylated GSK3β at Ser9 (inactive form) and increased the level of phosphorylated mTOR at Ser2448 (active form), leading to overactivation of GSK3β and mTOR signaling. On the contrary, DHCR24 knock-in largely increased the level of membrane cholesterol and caveolin-1, suggesting the enhancement of lipid raft/caveola. And synchronously DHCR24 knock-in also abolished the effect of DHCR24 knock-down on the inhibition of PI3-K/Akt signaling as well as the overactivation of GSK3β and mTOR signaling. Collectively, our data strongly supported DHCR24 knock-down lead to tau hyperphosphorylation and the inhibition of autophagy by a lipid raft-dependent PI3-K/Akt-mediated GSK3β and mTOR signaling. Taking together, our results firstly demonstrated that the decrease of plasma membrane cholesterol mediated by DHCR24 deficiency might contribute to the tauopathy in AD and other tauopathies.

Disposition of Proteins and Lipids in Synaptic Membrane Compartments Is Altered in Q175/Q7 Huntington's Disease Mouse Striatum

Dysfunction at synapses is thought to be an early change contributing to cognitive, psychiatric and motor disturbances in Huntington's disease (HD). In neurons, mutant Huntingtin collects in aggregates and distributes to the same sites as wild-type Huntingtin including on membranes and in synapses. In this study, we investigated the biochemical integrity of synapses in HD mouse striatum. We performed subcellular fractionation of striatal tissue from 2 and 6-month old knock-in Q175/Q7 HD and Q7/Q7 mice. Compared to striata of Q7/Q7 mice, proteins including GLUT3, Na+/K+ ATPase, NMDAR 2b, PSD95, and VGLUT1 had altered distribution in Q175/Q7 HD striata of 6-month old mice but not 2-month old mice. These proteins are found on plasma membranes and pre- and postsynaptic membranes supporting hypotheses that functional changes at synapses contribute to cognitive and behavioral symptoms of HD. Lipidomic analysis of mouse fractions indicated that compared to those of wild-type, fractions 1 and 2 of 6 months Q175/Q7 HD had altered levels of two species of PIP2, a phospholipid involved in synaptic signaling, increased levels of cholesterol ester and decreased cardiolipin species. At 2 months, increased levels of species of acylcarnitine, phosphatidic acid and sphingomyelin were measured. EM analysis showed that the contents of fractions 1 and 2 of Q7/Q7 and Q175/Q7 HD striata had a mix of isolated synaptic vesicles, vesicle filled axon terminals singly or in clusters, and ER and endosome-like membranes. However, those of Q175/Q7 striata contained significantly fewer and larger clumps of particles compared to those of Q7/Q7. Human HD postmortem putamen showed differences from control putamen in subcellular distribution of two proteins (Calnexin and GLUT3). Our biochemical, lipidomic and EM analysis show that the presence of the HD mutation conferred age dependent disruption of localization of synaptic proteins and lipids important for synaptic function. Our data demonstrate concrete biochemical changes suggesting altered integrity of synaptic compartments in HD mice that may mirror changes in HD patients and presage cognitive and psychiatric changes that occur in premanifest HD.

Simvastatin impairs hippocampal synaptic plasticity and cognitive function in mice

Lipophilic statins which are blood brain barrier (BBB) permeable are speculated to affect the cholesterol synthesis and neural functions in the central nervous system. However, whether these statins can affect cholesterol levels and synaptic plasticity in hippocampus and the in vivo consequence remain unclear. Here, we report that long-term subcutaneous treatments of simvastatin significantly impair mouse hippocampal synaptic plasticity, reflected by the attenuated long-term potentiation of field excitatory postsynaptic potentials. The simvastatin administration causes a deficiency in recognition and spatial memory but fails to affect motor ability and anxiety behaviors in the mice. Mass spectrometry imaging indicates a significant decrease in cholesterol intensity in hippocampus of the mice receiving chronic simvastatin treatments. Such effects of simvastatin are transient because drug discontinuation can restore the hippocampal cholesterol level and synaptic plasticity and the memory function. These findings may provide further clues to elucidate the mechanisms of neurological side effects, especially the brain cognitive function impairment, caused by long-term usage of BBB-permeable statins.

Antidepressant drugs act by directly binding to TRKB neurotrophin receptors

It is unclear how binding of antidepressant drugs to their targets gives rise to the clinical antidepressant effect. We discovered that the transmembrane domain of tyrosine kinase receptor 2 (TRKB), the brain-derived neurotrophic factor (BDNF) receptor that promotes neuronal plasticity and antidepressant responses, has a cholesterol-sensing function that mediates synaptic effects of cholesterol. We then found that both typical and fast-acting antidepressants directly bind to TRKB, thereby facilitating synaptic localization of TRKB and its activation by BDNF. Extensive computational approaches including atomistic molecular dynamics simulations revealed a binding site at the transmembrane region of TRKB dimers. Mutation of the TRKB antidepressant-binding motif impaired cellular, behavioral, and plasticity-promoting responses to antidepressants in vitro and in vivo. We suggest that binding to TRKB and allosteric facilitation of BDNF signaling is the common mechanism for antidepressant action, which may explain why typical antidepressants act slowly and how molecular effects of antidepressants are translated into clinical mood recovery.

Cholesterol Metabolism in Neurodegenerative Diseases: Molecular Mechanisms and Therapeutic Targets

Cholesterol is an indispensable component of the cell membrane and plays vital roles in critical physiological processes. Brain cholesterol accounts for a large portion of total cholesterol in the human body, and its content must be tightly regulated to ensure normal brain function. Disorders of cholesterol metabolism in the brain are linked to neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), and other atypical cognitive deficits that arise at old age. However, the specific role of cholesterol metabolism disorder in the pathogenesis of neurodegenerative diseases has not been fully elucidated. Statins that are a class of lipid-lowering drugs have been reported to have a positive effect on neurodegenerative diseases. Herein, we reviewed the physiological and pathological conditions of cholesterol metabolism and discussed the possible mechanisms of cholesterol metabolism and statin therapy in neurodegenerative diseases.

Lovastatin Differentially Regulates α7 and α4 Neuronal Nicotinic Acetylcholine Receptor Levels in Rat Hippocampal Neurons

Neuronal α7 and α4β2 are the predominant nicotinic acetylcholine receptor (nAChR) subtypes found in the brain, particularly in the hippocampus. The effects of lovastatin, an inhibitor of cholesterol biosynthesis, on these two nAChRs endogenously expressed in rat hippocampal neuronal cells were evaluated in the 0.01-1 µM range. Chronic (14 days) lovastatin treatment augmented cell-surface levels of α7 and α4 nAChRs, as measured by fluorescence microscopy and radioactive ligand binding assays. This was accompanied in both cases by an increase in total protein receptor levels as determined by Western blots. At low lovastatin concentrations (10-100 nM), the increase in α4 nAChR in neurites was higher than in neuronal cell somata; the opposite occurred at higher (0.5-1 µM) lovastatin concentrations. In contrast, neurite α7 nAChRs raised more than somatic α7 nAChRs at all lovastatin concentrations tested. These results indicate that cholesterol levels homeostatically regulate α7 and α4 nAChR levels in a differential manner through mechanisms that depend on statin concentration and receptor localization. The neuroprotective pleomorphic effects of statins may act by reestablishing the homeostatic equilibrium.

Striatal infusion of cholesterol promotes dose-dependent behavioral benefits and exerts disease-modifying effects in Huntington's disease mice

A variety of pathophysiological mechanisms are implicated in Huntington's disease (HD). Among them, reduced cholesterol biosynthesis has been detected in the HD mouse brain from pre-symptomatic stages, leading to diminished cholesterol synthesis, particularly in the striatum. In addition, systemic injection of cholesterol-loaded brain-permeable nanoparticles ameliorates synaptic and cognitive function in a transgenic mouse model of HD. To identify an appropriate treatment regimen and gain mechanistic insights into the beneficial activity of exogenous cholesterol in the HD brain, we employed osmotic mini-pumps to infuse three escalating doses of cholesterol directly into the striatum of HD mice in a continuous and rate-controlled manner. All tested doses prevented cognitive decline, while amelioration of disease-related motor defects was dose-dependent. In parallel, we found morphological and functional recovery of synaptic transmission involving both excitatory and inhibitory synapses of striatal medium spiny neurons. The treatment also enhanced endogenous cholesterol biosynthesis and clearance of mutant Huntingtin aggregates. These results indicate that cholesterol infusion to the striatum can exert a dose-dependent, disease-modifying effect and may be therapeutically relevant in HD.

Novel therapeutic strategies for Alzheimer's disease targeting brain cholesterol homeostasis

Alzheimer's disease (AD) is a progressive neurodegenerative disorder and the most common cause of dementia. Aβ plaques and tauopathy are two major concerns associated with AD. Moreover, excessive Aβ accumulation can lead to other nonspecific metabolic brain abnormalities. There are various genetic, environmental, and other risk factors associated with AD. Identification of risk factors and its mechanisms by which these factors impart role in AD pathology would be helpful for the prevention of AD progression. Altered cholesterol homeostasis could be considered as a risk factor for AD progression. Brain cholesterol dysmetabolism is recognized as one of the crucial attributes for AD that affect major hallmarks of AD including neurodegeneration. To fill the gap between altered cholesterol levels in the brain and AD, the researchers started focusing on statins as re-purposing drugs for AD treatment. The various other hypothesis has been suggested due to a lack of beneficial results of statins in clinical trials, such as reduced brain cholesterol could underlie poor cognition. Unfortunately, it is still unclear, whether an increase or decrease in brain cholesterol levels responsible for Alzheimer's disease or not. Presently, scientists believed that managing the level of cholesterol in the brain may help as an alternative treatment strategy for AD. In this review, we focused on the therapeutic strategies for AD by targeting brain cholesterol levels.

Impact of Sex and Age on the Mevalonate Pathway in the Brain: A Focus on Effects Induced by Maternal Exposure to Exogenous Compounds

The mevalonate pathway produces cholesterol and other compounds crucial for numerous cellular processes. It is well known that age and sex modulate this pathway in the liver. Recently, similar effects were also noted in different brain areas, suggesting that alterations of the mevalonate pathway are at the root of marked sex-specific disparities in some neurodevelopmental disorders related to disturbed cholesterol homeostasis. Here, we show how the mevalonate pathway is modulated in a sex-, age- and region-specific manner, and how maternal exposure to exogenous compounds can disturb the regulation of this pathway in the brain, possibly inducing functional alterations.

Syndapin I Loss-of-Function in Mice Leads to Schizophrenia-Like Symptoms

Schizophrenia is associated with cognitive and behavioral dysfunctions thought to reflect imbalances in neurotransmission systems. Recent screenings suggested that lack of (functional) syndapin I (PACSIN1) may be linked to schizophrenia. We therefore studied syndapin I KO mice to address the suggested causal relationship to schizophrenia and to analyze associated molecular, cellular, and neurophysiological defects. Syndapin I knockout (KO) mice developed schizophrenia-related behaviors, such as hyperactivity, reduced anxiety, reduced response to social novelty, and an exaggerated novel object response and exhibited defects in dendritic arborization in the cortex. Neuromorphogenic deficits were also observed for a schizophrenia-associated syndapin I mutant in cultured neurons and coincided with a lack of syndapin I–mediated membrane recruitment of cytoskeletal effectors. Syndapin I KO furthermore caused glutamatergic hypofunctions. Syndapin I regulated both AMPAR and NMDAR availabilities at synapses during basal synaptic activity and during synaptic plasticity—particularly striking were a complete lack of long-term potentiation and defects in long-term depression in syndapin I KO mice. These synaptic plasticity defects coincided with alterations of postsynaptic actin dynamics, synaptic GluA1 clustering, and GluA1 mobility. Both GluA1 and GluA2 were not appropriately internalized. Summarized, syndapin I KO led to schizophrenia-like behavior, and our analyses uncovered associated molecular and cellular mechanisms.

Mitochondrial Cholesterol in Alzheimer's Disease and Niemann-Pick Type C Disease

Mitochondrial dysfunction has been recognized as a key player in neurodegenerative diseases, including Alzheimer's disease (AD) and Niemann–Pick type C (NPC) disease. While the pathogenesis of both diseases is different, disruption of intracellular cholesterol trafficking has emerged as a common feature of both AD and NPC disease. Nutritional or genetic mitochondrial cholesterol accumulation sensitizes neurons to Aβ-mediated neurotoxicity in vitro and promotes cognitive decline in AD models. In addition to the primary accumulation of cholesterol and sphingolipids in lysosomes, NPC disease is also characterized by an increase in mitochondrial cholesterol levels in affected organs, predominantly in brain and liver. In both diseases, mitochondrial cholesterol accumulation disrupts membrane physical properties and restricts the transport of glutathione into mitochondrial matrix, thus impairing the mitochondrial antioxidant defense strategy. The underlying mechanisms leading to mitochondrial cholesterol accumulation in AD and NPC diseases are not fully understood. In the present manuscript, we discuss evidence for the potential role of StARD1 in promoting the trafficking of cholesterol to mitochondria in AD and NPC, whose upregulation involves an endoplasmic reticulum stress and a decrease in acid ceramidase expression, respectively. These findings imply that targeting StARD1 or boosting the mitochondrial antioxidant defense may emerge as a promising approach for both AD and NPC disease.

Linking Nanoscale Dynamics of AMPA Receptor Organization to Plasticity of Excitatory Synapses and Learning

The spatiotemporal organization of neurotransmitter receptors in the postsynaptic membrane is a fundamental determinant of synaptic transmission and thus of information processing by the brain. The ionotropic AMPA subtype of glutamate receptors (AMPARs) mediate fast excitatory synaptic transmission in the CNS. The number of AMPARs located en face presynaptic glutamate release sites sets the efficacy of synaptic transmission. Understanding how this number is set and regulated has been the topic of intense research in the last two decades. We showed that AMPARs are not stable in the synapse as initially thought. They continuously enter and exit the postsynaptic density by lateral diffusion, and they exchange between the neuronal surface and intracellular compartments by endocytosis and exocytosis at extrasynaptic sites. Regulation of these various trafficking pathways has emerged as a key mechanism for activity-dependent plasticity of synaptic transmission, a process important for learning and memory. I here present my view of these findings. In particular, the advent of super-resolution microscopy and single-molecule tracking has helped to uncover the intricacy of AMPARs' dynamic organization at the nanoscale. In addition, AMPAR surface diffusion is highly regulated by a variety of factors, including neuronal activity, stress hormones, and neurodegeneration, suggesting that AMPAR diffusion-trapping may play a central role in synapse function. Using innovative tools to understand further the link between receptor dynamics and synapse plasticity is now unveiling new molecular mechanisms of learning. Modifying AMPAR dynamics may emerge as a new target to correct synapse dysfunction in the diseased brain.

Nrg1 Intracellular Signaling Is Neuroprotective upon Stroke

The schizophrenia risk gene NRG1 controls the formation of excitatory and inhibitory synapses in cortical circuits. While the expression of different NRG1 isoforms occurs during development, adult neurons primarily express the CRD-NRG1 isoform characterized by a highly conserved intracellular domain (NRG1-ICD). We and others have demonstrated that Nrg1 intracellular signaling promotes dendrite elongation and excitatory connections during neuronal development. However, the role of Nrg1 intracellular signaling in adult neurons and pathological conditions remains largely unaddressed. Here, we investigated the role of Nrg1 intracellular signaling in neuroprotection and stroke. Our bioinformatic analysis revealed the evolutionary conservation of the NRG1-ICD and a decrease in NRG1 expression with age in the human frontal cortex. Hence, we first evaluated whether Nrg1 signaling may affect pathological hallmarks in an in vitro model of neuronal senescence; however, our data failed to reveal a role for Nrg1 in the activation of the stress-related pathway p38 MAPK and DNA damage. Previous studies demonstrated that the soluble EGF domain of Nrg1 alleviated brain ischemia, a pathological process involving the generation of free radicals, reactive oxygen species (ROS), and excitotoxicity. Hence, we tested the hypothesis that Nrg1 intracellular signaling could be neuroprotective in stroke. We discovered that Nrg1 expression significantly increased neuronal survival upon oxygen-glucose deprivation (OGD), an established in vitro model for stroke. Notably, the specific activation of Nrg1 intracellular signaling by expression of the Nrg1-ICD protected neurons from OGD. Additionally, time-lapse experiments confirmed that Nrg1 intracellular signaling increased the survival of neurons exposed to OGD. Finally, we investigated the relevance of Nrg1 intracellular signaling in stroke in vivo. Using viral vectors, we expressed the Nrg1-ICD in cortical neurons and subsequently challenged them by a focal hemorrhagic stroke; our data indicated that Nrg1 intracellular signaling improved neuronal survival in the infarcted area. Altogether, these data highlight Nrg1 intracellular signaling as neuroprotective upon ischemic lesion both in vitro and in vivo. Given the complexity of the neurotoxic effects of stroke and the involvement of various mechanisms, such as the generation of ROS, excitotoxicity, and inflammation, further studies are required to determine the molecular bases of the neuroprotective effect of Nrg1 intracellular signaling. In conclusion, our research highlights the stimulation of Nrg1 intracellular signaling as a promising target for cortical stroke treatment.

Aging Increases Hippocampal DUSP2 by a Membrane Cholesterol Loss-Mediated RTK/p38MAPK Activation Mechanism

Numerous studies suggest that the increased activity of p38MAPK plays an important role in the abnormal immune and inflammatory response observed in the course of neurodegenerative diseases such as Alzheimer's disease. On the other hand, high levels of p38MAPK are present in the brain during normal aging, suggesting the existence of mechanisms that keep the p38MAPK-regulated pro-inflammatory activity within physiological limits. In this study, we show that high p38MAPK activity in the hippocampus of old mice is in part due to the reduction in membrane cholesterol that constitutively occurs in the aging brain. Mechanistically, membrane cholesterol reduction increases p38MAPK activity through the stimulation of a subset of tyrosine kinase receptors (RTKs). In turn, activated p38MAPK increases the expression and activity of the phosphatase DUSP2, which is known to reduce the activity of different MAPKs, including p38MAPK. These results suggest that the loss of membrane cholesterol that constitutively occurs with age takes part in a negative-feedback loop that keeps p38MAPK activity levels within physiological range. Thus, conditions that increase p38MAPK activity such as cellular stressors or that inhibit DUSP2 will amplify inflammatory activity with its consequent deleterious functional changes.

Alterations in cholesterol metabolism as a risk factor for developing Alzheimer's disease: Potential novel targets for treatment

Alzheimer's disease (AD) is the most common form of dementia and it is characterized by the deposition of amyloid-β (Aβ) plaques and neurofibrillary tangles in the brain. However, the complete pathogenesis of the disease is still unknown. High level of serum cholesterol has been found to positively correlate with an increased risk of dementia and some studies have reported a decreased prevalence of AD in patients taking cholesterol-lowering drugs. Years of research have shown a strong correlation between blood hypercholesterolemia and AD, however cholesterol is not able to cross the Blood Brain Barrier (BBB) into the brain. Cholesterol lowering therapies have shown mixed results in cognitive performance in AD patients, raising questions of whether brain cholesterol metabolism in the brain should be studied separately from peripheral cholesterol metabolism and what their relationship is. Unlike cholesterol, oxidized cholesterol metabolites known as oxysterols are able to cross the BBB from the circulation into the brain and vice-versa. The main oxysterols present in the circulation are 24S-hydroxycholesterol and 27-hydroxycholesterol. These oxysterols and their catalysing enzymes have been found to be altered in AD brains and there is evidence indicating their influence in the progression of the disease. This review gives a broad perspective on the relationship between hypercholesterolemia and AD, cholesterol lowering therapies for AD patients and the role of oxysterols in pathological and non-pathological conditions. Also, we propose cholesterol metabolites as valuable targets for prevention and alternative AD treatments.

Age-associated cholesterol reduction triggers brain insulin resistance by facilitating ligand-independent receptor activation and pathway desensitization

In the brain, insulin plays an important role in cognitive processes. During aging, these faculties decline, as does insulin signaling. The mechanism behind this last phenomenon is unclear. In recent studies, we reported that the mild and gradual loss of cholesterol in the synaptic fraction of hippocampal neurons during aging leads to a decrease in synaptic plasticity evoked by glutamate receptor activation and also by receptor tyrosine kinase (RTK) signaling. As insulin and insulin growth factor activity are dependent on tyrosine kinase receptors, we investigated whether the constitutive loss of brain cholesterol is also involved in the decay of insulin function with age. Using long‐term depression (LTD) induced by application of insulin to hippocampal slices as a read‐out, we found that the decline in insulin function during aging could be monitored as a progressive impairment of insulin‐LTD. The application of a cholesterol inclusion complex, which donates cholesterol to the membrane and increases membrane cholesterol levels, rescued the insulin signaling deficit and insulin‐LTD. In contrast, extraction of cholesterol from hippocampal neurons of adult mice produced the opposite effect. Furthermore, in vivo inhibition of Cyp46A1, an enzyme involved in brain cholesterol loss with age, improved insulin signaling. Fluorescence resonance energy transfer (FRET) experiments pointed to a change in receptor conformation by reduced membrane cholesterol, favoring ligand‐independent autophosphorylation. Together, these results indicate that changes in membrane fluidity of brain cells during aging play a key role in the decay of synaptic plasticity and cognition that occurs at this late stage of life.

NLRP3 inflammasome activation in inflammaging

The process of aging is associated with the appearance of low-grade subclinical inflammation, termed inflammaging, that can accelerate age-related diseases. In Western societies the age-related inflammatory response can additionally be aggravated by an inflammatory response related to modern lifestyles and excess calorie consumption, a pathophysiologic inflammatory response that was coined metaflammation. Here, we summarize the current knowledge of mechanisms that drive both of these processes and focus our discussion the emerging concept that a key innate immune pathway, the NLRP3 inflammasome, is centrally involved in the recognition of triggers that appear during physiological aging and during metabolic stress. We further discuss how these processes are involved in the pathogenesis of common age-related pathologies and highlight potential strategies by which the detrimental inflammatory responses could be pharmacologically addressed.

Pro- and Anti-inflammatory Effects of High Cholesterol Diet on Aged Brain

Both hypercholesterolemia and aging are related to cognitive decline or Alzheimer's disease. However, their interactive influence on the neurodegenerative progress remains unclear. To address this issue, 6-month-old and 16-month-old female mice were fed a 3% cholesterol diet for 8 weeks, followed by hippocampus-related functional, pathological, biochemical and molecular analyses. The high cholesterol diet did not exacerbate age-dependent cognitive decline and hippocampal neuronal death, and even greatly mitigated decreases of synaptophysin and growth associated protein 43 expression in the hippocampus of aged mice. Compared with young controls, aged mice fed normal diet showed mild activation of hippocampal microglia with increased expression of CD68, a marker of the microglial M1 phenotype, and decreased expression of CD206, a marker of the microglial M2 phenotype. More interestingly, the high cholesterol diet not only improved NLRP3 inflammasome activation and IL-1β expression, but also increased levels of anti-inflammatory cytokines IL-4 and IL-6 in the hippocampus of old mice, suggesting playing pro- and anti-neuroinflammatory effects. In addition, the cholesterol rich diet resulted in a defect of the blood-brain barrier of aged hippocampus, as revealed by increased brain albumin content. These results have revealed both harmful and protective effects of high cholesterol diet on aged brain, which helps us to understand that hypercholesterolemia in the aged population is not associated with dementia and cognitive impairment.

Aging Triggers a Repressive Chromatin State at Bdnf Promoters in Hippocampal Neurons

Cognitive capacities decline with age, an event accompanied by the altered transcription of synaptic plasticity genes. Here, we show that the transcriptional induction of Bdnf by a mnemonic stimulus is impaired in aged hippocampal neurons. Mechanistically, this defect is due to reduced NMDA receptor (NMDAR)-mediated activation of CaMKII. Decreased NMDAR signaling prevents changes associated with activation at specific Bdnf promoters, including displacement of histone deacetylase 4, recruitment of the histone acetyltransferase CBP, increased H3K27 acetylation, and reduced H3K27 trimethylation. The decrease in NMDA-CaMKII signaling arises from constitutive reduction of synaptic cholesterol that occurs with normal aging. Increasing the levels of neuronal cholesterol in aged neurons in vitro, ex vivo, and in vivo restored NMDA-induced Bdnf expression and chromatin remodeling. Furthermore, pharmacological prevention of age-associated cholesterol reduction rescued signaling and cognitive deficits of aged mice. Thus, reducing hippocampal cholesterol loss may represent a therapeutic approach to reverse cognitive decline during aging.

DHCR24 exerts neuroprotection upon inflammation-induced neuronal death

BACKGROUND

DHCR24, involved in the de novo synthesis of cholesterol and protection of neuronal cells against different stress conditions, has been shown to be selectively downregulated in neurons of the affected brain areas in Alzheimer's disease.

METHODS

Here, we investigated whether the overexpression of DHCR24 protects neurons against inflammation-induced neuronal death using co-cultures of mouse embryonic primary cortical neurons and BV2 microglial cells upon acute neuroinflammation. Moreover, the effects of DHCR24 overexpression on dendritic spine density and morphology in cultured mature mouse hippocampal neurons and on the outcome measures of ischemia-induced brain damage in vivo in mice were assessed.

RESULTS

Overexpression of DHCR24 reduced the loss of neurons under inflammation elicited by LPS and IFN-γ treatment in co-cultures of mouse neurons and BV2 microglial cells but did not affect the production of neuroinflammatory mediators, total cellular cholesterol levels, or the activity of proteins linked with neuroprotective signaling. Conversely, the levels of post-synaptic cell adhesion protein neuroligin-1 were significantly increased upon the overexpression of DHCR24 in basal growth conditions. Augmentation of DHCR24 also increased the total number of dendritic spines and the proportion of mushroom spines in mature mouse hippocampal neurons. In vivo, overexpression of DHCR24 in striatum reduced the lesion size measured by MRI in a mouse model of transient focal ischemia.

CONCLUSIONS

These results suggest that the augmentation of DHCR24 levels provides neuroprotection in acute stress conditions, which lead to neuronal loss in vitro and in vivo.