Ceramide and Related-Sphingolipid Levels Are Not Altered in Disease-Associated Brain Regions of APP and APP/PS1 Mouse Models of Alzheimer's Disease: Relationship with the Lack of Neurodegeneration?

Application of sphingolipid-based nanocarriers in drug delivery: an overview

Sphingolipids (SL) are well recognized for their cell signaling through extracellular and intracellular pathways. Based on chemistry different types of SL are biosynthesized in mammalian cells and have specific function in cellular activity. SL has an ampiphilic structure with have hydrophobic body attached to the polar head enables their use as a drug delivery agent in the form of nanocarriers. SL-based liposomes can improve the solubility of lipophilic drugs through host and drug complexes and are more stable than conventional liposomal formulations. Preclinical studies of SL nanocarriers are reported on topical delivery, oral delivery, ocular delivery, chemotherapeutic delivery, cardiovascular delivery and Alzheimer's disease. The commercial challenges and patents related to SL nanoformulations are highlighted in this article.

The relevance between abnormally elevated serum ceramide and cognitive impairment in Alzheimer's disease model mice and its mechanism

RATIONALE

The plasma ceramide levels in Alzheimer's disease (AD) patients are found abnormally elevated, which is related to cognitive decline.

OBJECTIVES

This research was aimed to investigate the mechanisms of aberrant elevated ceramides in the pathogenesis of AD.

RESULTS

The ICR mice intracerebroventricularly injected with Aβ1-42 and APP/PS1 transgenic mice were employed as AD mice. The cognitive deficiency, impaired episodic and spatial memory were observed without altered spontaneous ability. The serum levels of p-tau and ceramide were evidently elevated. The modified expressions and activities of glycogen synthase kinase-3β (GSK-3β) and protein phosphatase 2A (PP2A) influenced the serum content of p-tau. The levels of ceramide synthesis-related genes including sptlc1, sptlc2, cers2, and cers6 in the liver of AD mice were increased, while the ceramide degradation-related gene asah2 did not significantly change. The regulations of these genes were conducted by activated nuclear factor kappa-B (NF-κB) signaling. NF-κB, promoted by free fatty acid (FFA), also increased the hepatic concentrations of proinflammatory cytokines. The FFA amount was modulated by fatty acid synthesis-related genes acc1 and srebp-1c. Besides, the decreased levels of pre-proopiomelanocortin (pomc) mRNA and increased agouti-related protein (agrp) mRNA were found in the hypothalamus without significant alteration of melanocortin receptor 4 (MC4R) mRNA. The bioinformatic analyses proved the results using GEO datasets and AlzData.

CONCLUSIONS

Ceramide was positively related to the increased p-tau and impaired cognitive function. The increased generation of ceramide and endoplasmic reticulum stress in the hypothalamus was positively related to fatty acid synthesis and NF-κB signaling via brain-liver axis.

Apoptotic Pathways and Alzheimer's Disease: Probing Therapeutic Potential

Apoptosis is an intrinsic biochemical, cellular process that regulates cell death and is crucial for cell survival, cellular homeostasis, and maintaining the optimum functional status. Apoptosis in a predetermined and programmed manner regulates several molecular events, including cell turnover, embryonic development, and immune system functions but may be the exclusive contributor to several disorders, including neurodegenerative manifestations, when it functions in an aberrant and disorganized manner. Alzheimer's disease (AD) is a fatal, chronic neurodegenerative disorder where apoptosis has a compelling and divergent role. The well-characterized pathological features of AD, including extracellular plaques of amyloid-beta, intracellular hyperphosphorylated tangles of tau protein (NFTs), inflammation, mitochondrial dysfunction, oxidative stress, and excitotoxic cell death, also instigate an abnormal apoptotic cascade in susceptible brain regions (cerebral cortex, hippocampus). The apoptotic players in these regions affect cellular organelles (mitochondria and endoplasmic reticulum), interact with trophic factors, and several pathways, including PI3K/AKT, JNK, MAPK, mTOR signalling. This dysregulated apoptotic cascade end with an abnormal neuronal loss which is a primary event that may precede the other events of AD progression and correlates well with the degree of dementia. The present review provides insight into the diverse and versatile apoptotic mechanisms that are indispensable for neuronal survival and constitute an integral part of the pathological progression of AD. Identification of potential targets (restoring apoptotic and antiapoptotic balance, caspases, TRADD, RIPK1, FADD, TNFα, etc.) may be valuable and advantageous to decide the fate of neurons and to develop potential therapeutics for treatment of AD.

CERTL reduces C16 ceramide, amyloid-β levels, and inflammation in a model of Alzheimer's disease

BACKGROUND

Dysregulation of ceramide and sphingomyelin levels have been suggested to contribute to the pathogenesis of Alzheimer's disease (AD). Ceramide transfer proteins (CERTs) are ceramide carriers which are crucial for ceramide and sphingomyelin balance in cells. Extracellular forms of CERTs co-localize with amyloid-β (Aβ) plaques in AD brains. To date, the significance of these observations for the pathophysiology of AD remains uncertain.

METHODS

A plasmid expressing CERTL, the long isoform of CERTs, was used to study the interaction of CERTL with amyloid precursor protein (APP) by co-immunoprecipitation and immunofluorescence in HEK cells. The recombinant CERTL protein was employed to study interaction of CERTL with amyloid-β (Aβ), Aβ aggregation process in presence of CERTL, and the resulting changes in Aβ toxicity in neuroblastoma cells. CERTL was overexpressed in neurons by adeno-associated virus (AAV) in a mouse model of familial AD (5xFAD). Ten weeks after transduction, animals were challenged with behavior tests for memory, anxiety, and locomotion. At week 12, brains were investigated for sphingolipid levels by mass spectrometry, plaques, and neuroinflammation by immunohistochemistry, gene expression, and/or immunoassay.

RESULTS

Here, we report that CERTL binds to APP, modifies Aβ aggregation, and reduces Aβ neurotoxicity in vitro. Furthermore, we show that intracortical injection of AAV, mediating the expression of CERTL, decreases levels of ceramide d18:1/16:0 and increases sphingomyelin levels in the brain of male 5xFAD mice. CERTL in vivo over-expression has a mild effect on animal locomotion, decreases Aβ formation, and modulates microglia by decreasing their pro-inflammatory phenotype.

CONCLUSION

Our results demonstrate a crucial role of CERTL in regulating ceramide levels in the brain, in amyloid plaque formation and neuroinflammation, thereby opening research avenues for therapeutic targets of AD and other neurodegenerative diseases.

Altered Brain Metabolome Is Associated with Memory Impairment in the rTg4510 Mouse Model of Tauopathy

Alzheimer’s disease (AD) is characterized, amongst other features, by the pathologic accumulation of abnormally phosphorylated tau filaments in neurons that lead to neurofibrillary tangles. However, the molecular mechanisms by which the abnormal processing of tau leads to neurodegeneration and cognitive impairment remain unknown. Metabolomic techniques can comprehensively assess disturbances in metabolic pathways that reflect changes downstream from genomic, transcriptomic and proteomic systems. In the present study, we undertook a targeted metabolomic approach to determine a total of 187 prenominated metabolites in brain cortex tissue from wild type and rTg4510 animals (a mice model of tauopathy), in order to establish the association of metabolic pathways with cognitive impairment. This targeted metabolomic approach revealed significant differences in metabolite concentrations of transgenic mice. Brain glutamine, serotonin and sphingomyelin C18:0 were found to be predictors of memory impairment. These findings provide informative data for future research on AD, since some of them agree with pathological alterations observed in diseased humans.

Suppression of Angiotensin-(1-7) on the Disruption of Blood-Brain Barrier in Rat of Brain Glioma

Glioblastoma multiforme (GBM) is the most primary brain tumor, specially characterized with the damage of blood-brain barrier (BBB). The Ang-(1-7) was proven to have an inhibitory effect on glioblastoma growth. However, its role on blood-brain barrier (BBB) and the underlying molecular mechanism remains unclear. In this study, Ang-(1-7) significantly relieved the damage of blood-brain barrier in rats with intracranial U87 gliomas as evaluated by magnetic resonance imaging (MRI). Furthermore, its treatment attenuated BBB permeability, tumor growth and edema formation. Similarly, Ang-(1-7) also decreased U87 glioma cells barrier permeability in vitro. Further analysis showed that Ang-(1-7) could effectively restore tight junction protein (claudin-5 and ZO-1) expression levels both in rats and U87 glioma cells by affecting the activation of JNK pathway. SP600125, an inhibitor of JNK, significantly enhanced the expression of Claudin-5 and ZO-1, and decreased the disruption of BBB and enhanced the efficiency of Ang-(1-7) in glioma rats. Taken together, this study demonstrated a protective role of Ang-(1-7) in glioma-induced blood-brain barrier damage by regulating tight junction protein expression. Accordingly, Ang-(1-7) may become a promising therapeutic agent against glioma.

Comprehensive identification of age-related lipidome changes in rat amygdala during normal aging

Brain lipids are integral components of brain structure and function. However, only recent advancements of chromatographic techniques together with mass spectrometry allow comprehensive identification of lipid species in complex brain tissue. Lipid composition varies between the individual areas and the majority of previous reports was focusing on individual lipids rather than a lipidome. Herein, a mass spectrometry-based approach was used to evaluate age-related changes in the lipidome of the rat amygdala obtained from young (3 months) and old (20 months) males of the Sprague-Dawley rat strain. A total number of 70 lipid species with significantly changed levels between the two animal groups were identified spanning four main lipid classes, i.e. glycerolipids, glycerophospholipids, sphingolipids and sterol lipids. These included phospholipids with pleiotropic brain function, such as derivatives of phosphatidylcholine, phosphatidylserine, and phosphatidylethanolamine. The analysis also revealed significant level changes of phosphatidic acid, diacylglycerol, sphingomyelin and ceramide that directly represent lipid signaling and affect amygdala neuronal activity. The amygdala is a crucial brain region for cognitive functions and former studies on rats and humans showed that this region changes its activity during normal aging. As the information on amygdala lipidome is very limited the results obtained in the present study represent a significant novelty and may contribute to further studies on the role of lipid molecules in age-associated changes of amygdala function.

The impact of cholesterol, DHA, and sphingolipids on Alzheimer's disease

Alzheimer's disease (AD) is a devastating neurodegenerative disorder currently affecting over 35 million people worldwide. Pathological hallmarks of AD are massive amyloidosis, extracellular senile plaques, and intracellular neurofibrillary tangles accompanied by an excessive loss of synapses. Major constituents of senile plaques are 40–42 amino acid long peptides termed β-amyloid (Aβ). Aβ is produced by sequential proteolytic processing of the amyloid precursor protein (APP). APP processing and Aβ production have been one of the central scopes in AD research in the past. In the last years, lipids and lipid-related issues are more frequently discussed to contribute to the AD pathogenesis. This review summarizes lipid alterations found in AD postmortem brains, AD transgenic mouse models, and the current understanding of how lipids influence the molecular mechanisms leading to AD and Aβ generation, focusing especially on cholesterol, docosahexaenoic acid (DHA), and sphingolipids/glycosphingolipids.

Evidence for premature lipid raft aging in APP/PS1 double-transgenic mice, a model of familial Alzheimer disease

Altered lipid raft homeostasis has been considered to contribute to cellular deregulation, leading to neuronal loss in Alzheimer disease. Alterations in these microdomains affect amyloid precursor protein (APP) processing, resulting in neurotoxic conditions, but modifications of the molecular structure of lipid rafts in Alzheimer disease model mice have not been characterized. Using a lipidomic approach, we investigated frontal cortex lipid rafts inwild-type mice and in double-transgenic APP/presenilin 1 (PS1) mice. Lipid rafts in wild-type mice undergo age-dependent modifications, that is, decreased cholesterol and sterol esters levels, augmented sphingomyelin and saturated fatty acid content, and increased phospholipids/cholesterol ratio. These age-dependent changes were more dramatic and occurred earlier in APP/PS1 mice; other lipid classes (e.g. sulfatides) and essential long-chain polyunsaturated fatty acids (including docosahexaenoic and arachidonic acids) were also affected. Steady state anisotropy measurements demonstrated that APP/PS1 animals exhibit more viscous (membrane-ordered) lipid rafts and that this is mainly attributable to reduced unsaturation of phospholipids and increased sphingomyelin levels rather than to changes in cholesterol. In summary, we demonstrate that aging is accompanied by alteration of the physicochemical structure of lipid raft microdomains. This "lipid raft aging," a metaphenomenon, is considerably exacerbated by the induced amyloid burden in APP/PS1 genotype.

Could plasma sphingolipids be diagnostic or prognostic biomarkers for Alzheimer's disease?

Understanding the etiopathological processes of Alzheimer's disease (AD) in the preclinical and early clinical stages will be important in developing new therapeutic targets and biomarkers. There is growing consensus that nonamyloid targets will be necessary to reverse or slow AD progression. Lipidomic, metabolomic and targeted approaches have identified pathways and products of sphingolipid metabolism that are altered early in the course of AD and contribute to the neuropathological alterations associated with AD, including amyloid-β production, tau formation and neurodegeneration. In this article, we briefly review the current literature on the role of sphingolipids in the underlying pathophysiology of AD, and then discuss the current state of translating these findings to clinical populations and the potential utility of plasma sphingolipids as diagnostic and/or prognostic indicators of AD.