Mass Spectrometric Imaging of Ceramide Biomarkers Tracks Therapeutic Response in Traumatic Brain Injury

Alzheimer's-Associated Upregulation of Mitochondria-Associated ER Membranes After Traumatic Brain Injury

Traumatic brain injury (TBI) can lead to neurodegenerative diseases such as Alzheimer's disease (AD) through mechanisms that remain incompletely characterized. Similar to AD, TBI models present with cellular metabolic alterations and modulated cleavage of amyloid precursor protein (APP). Specifically, AD and TBI tissues display increases in amyloid-β as well as its precursor, the APP C-terminal fragment of 99 a.a. (C99). Our recent data in cell models of AD indicate that C99, due to its affinity for cholesterol, induces the formation of transient lipid raft domains in the ER known as mitochondria-associated endoplasmic reticulum (ER) membranes ("MAM" domains). The formation of these domains recruits and activates specific lipid metabolic enzymes that regulate cellular cholesterol trafficking and sphingolipid turnover. Increased C99 levels in AD cell models promote MAM formation and significantly modulate cellular lipid homeostasis. Here, these phenotypes were recapitulated in the controlled cortical impact (CCI) model of TBI in adult mice. Specifically, the injured cortex and hippocampus displayed significant increases in C99 and MAM activity, as measured by phospholipid synthesis, sphingomyelinase activity and cholesterol turnover. In addition, our cell type-specific lipidomics analyses revealed significant changes in microglial lipid composition that are consistent with the observed alterations in MAM-resident enzymes. Altogether, we propose that alterations in the regulation of MAM and relevant lipid metabolic pathways could contribute to the epidemiological connection between TBI and AD.

Potential Biomarkers in Experimental Animal Models for Traumatic Brain Injury

Traumatic brain injury (TBI) is a complex and multifaceted disorder that has become a significant public health concern worldwide due to its contribution to mortality and morbidity. This condition encompasses a spectrum of injuries, including axonal damage, contusions, edema, and hemorrhage. Unfortunately, specific effective therapeutic interventions to improve patient outcomes following TBI are currently lacking. Various experimental animal models have been developed to mimic TBI and evaluate potential therapeutic agents to address this issue. These models are designed to recapitulate different biomarkers and mechanisms involved in TBI. However, due to the heterogeneous nature of clinical TBI, no single experimental animal model can effectively mimic all aspects of human TBI. Accurate emulation of clinical TBI mechanisms is also tricky due to ethical considerations. Therefore, the continued study of TBI mechanisms and biomarkers, of the duration and severity of brain injury, treatment strategies, and animal model optimization is necessary. This review focuses on the pathophysiology of TBI, available experimental TBI animal models, and the range of biomarkers and detection methods for TBI. Overall, this review highlights the need for further research to improve patient outcomes and reduce the global burden of TBI.

The ERAD system is restricted by elevated ceramides

Misfolded proteins in the endoplasmic reticulum (ER) are removed through a process known as ER-associated degradation (ERAD). ERAD occurs through an integral membrane protein quality control system that recognizes substrates, retrotranslocates the substrates across the membrane, and ubiquitinates and extracts the substrates from the membrane for degradation at the cytosolic proteasome. While ERAD systems are known to regulate lipid biosynthetic enzymes, the regulation of ERAD systems by the lipid composition of cellular membranes remains unexplored. Here, we report that the ER membrane composition influences ERAD function by incapacitating substrate extraction. Unbiased lipidomic profiling revealed that elevation of specific very-long-chain ceramides leads to a marked increase in the level of ubiquitinated substrates in the ER membrane and concomitantly reduces extracted substrates in the cytoplasm. This work reveals a previously unrecognized mechanism in which ER membrane lipid remodeling changes the activity of ERAD.

Neurotrauma investigation through spatial omics guided by mass spectrometry imaging: Target identification and clinical applications

Traumatic brain injury (TBI) represents one of the major public health concerns worldwide due to the increase in TBI incidence as a result of injuries from daily life accidents such as sports and motor vehicle transportation as well as military-related practices. This type of central nervous system trauma is known to predispose patients to several neurological disorders such as Parkinson's disease, Alzheimer's disease, chronic trauamatic encephalopathy, and age-related Dementia. Recently, several proteomic and lipidomic platforms have been applied on different TBI studies to investigate TBI-related mechanisms that have broadened our understanding of its distinct neuropathological complications. In this study, we provide an updated comprehensive overview of the current knowledge and novel perspectives of the spatially resolved microproteomics and microlipidomics approaches guided by mass spectrometry imaging used in TBI studies and its applications in the neurotrauma field. In this regard, we will discuss the use of the spatially resolved microproteomics and assess the different microproteomic sampling methods such as laser capture microdissection, parafilm assisted microdissection, and liquid microjunction extraction as accurate and precise techniques in the field of neuroproteomics. Additionally, we will highlight lipid profiling applications and their prospective potentials in characterizing molecular processes involved in the field of TBI. Specifically, we will discuss the phospholipid metabolism acting as a precursor for proinflammatory molecules such as eicosanoids. Finally, we will survey the current state of spatial neuroproteomics and microproteomics applications and present the various studies highlighting their findings in these fields.

Acid Sphingomyelinase Inhibitor, Imipramine, Reduces Hippocampal Neuronal Death after Traumatic Brain Injury

Traumatic brain injury (TBI) broadly degrades the normal function of the brain after a bump, blow, or jolt to the head. TBI leads to the aggravation of pre-existing brain dysfunction and promotes neurotoxic cascades that involve processes such as oxidative stress, loss of dendritic arborization, and zinc accumulation. Acid sphingomyelinase (ASMase) is an enzyme that hydrolyzes sphingomyelin to ceramide in cells. Under normal conditions, ceramide plays an important role in various physiological functions, such as differentiation and apoptosis. However, under pathological conditions, excessive ceramide production is toxic and activates the neuronal-death pathway. Therefore, we hypothesized that the inhibition of ASMase activity by imipramine would reduce ceramide formation and thus prevent TBI-induced neuronal death. To test our hypothesis, an ASMase inhibitor, imipramine (10 mg/kg, i.p.), was administrated to rats immediately after TBI. Based on the results of this study, we confirmed that imipramine significantly reduced ceramide formation, dendritic loss, oxidative stress, and neuronal death in the TBI-imipramine group compared with the TBI-vehicle group. Additionally, we validated that imipramine prevented TBI-induced cognitive dysfunction and the modified neurological severity score. Consequently, we suggest that ASMase inhibition may be a promising therapeutic strategy to reduce hippocampal neuronal death after TBI.

MALDI mass spectrometry imaging workflow for the aquatic model organisms Danio rerio and Daphnia magna

Lipids play various essential roles in the physiology of animals. They are also highly dependent on cellular metabolism or status. It is therefore crucial to understand to which extent animals can stabilize their lipid composition in the presence of external stressors, such as chemicals that are released into the environment. We developed a MALDI MS imaging workflow for two important aquatic model organisms, the zebrafish (Danio rerio) and water flea (Daphnia magna). Owing to the heterogeneous structure of these organisms, developing a suitable sample preparation workflow is a highly non-trivial but crucial part of this work and needs to be established first. Relevant parameters and practical considerations in order to preserve tissue structure and composition in tissue sections are discussed for each application. All measurements were based on high mass accuracy enabling reliable identification of imaged compounds. In zebrafish we demonstrate that a detailed mapping between histology and simultaneously determined lipid composition is possible at various scales, from extended structures such as the brain or gills down to subcellular structures such as a single axon in the central nervous system. For D. magna we present for the first time a MALDI MSI workflow, that demonstrably maintains tissue integrity during cryosectioning of non-preserved samples, and allows the mapping of lipids in the entire body and the brood chamber inside the carapace. In conclusion, the lipid signatures that we were able to detect with our method provide an ideal basis to analyze changes caused by pollutants in two key aquatic model organisms.

Lipidome Alterations following Mild Traumatic Brain Injury in the Rat

Traumatic brain injury (TBI) poses a major health challenge, with tens of millions of new cases reported globally every year. Brain damage resulting from TBI can vary significantly due to factors including injury severity, injury mechanism and exposure to repeated injury events. Therefore, there is need for robust blood biomarkers. Serum from Sprague Dawley rats was collected at several timepoints within 24 h of mild single or repeat closed head impacts. Serum samples were analyzed via ultra-high-performance liquid chromatography-mass spectrometry (UHPLC-MS) in positive and negative ion modes. Known lipid species were identified through matching to in-house tandem MS databases. Lipid biomarkers have a unique potential to serve as objective molecular measures of injury response as they may be liberated to circulation more readily than larger protein markers. Machine learning and feature selection approaches were used to construct lipid panels capable of distinguishing serum from injured and uninjured rats. The best multivariate lipid panels had over 90% cross-validated sensitivity, selectivity, and accuracy. These mapped onto sphingolipid signaling, autophagy, necroptosis and glycerophospholipid metabolism pathways, with Benjamini adjusted p-values less than 0.05. The novel lipid biomarker candidates identified provide insight into the metabolic pathways altered within 24 h of mild TBI.

Classification of target tissues of Eisenia fetida using sequential multimodal chemical analysis and machine learning

Acquiring comprehensive knowledge about the uptake of pollutants, impact on tissue integrity and the effects at the molecular level in organisms is of increasing interest due to the environmental exposure to numerous contaminants. The analysis of tissues can be performed by histological examination, which is still time-consuming and restricted to target-specific staining methods. The histological approaches can be complemented with chemical imaging analysis. Chemical imaging of tissue sections is typically performed using a single imaging approach. However, for toxicological testing of environmental pollutants, a multimodal approach combined with improved data acquisition and evaluation is desirable, since it may allow for more rapid tissue characterization and give further information on ecotoxicological effects at the tissue level. Therefore, using the soil model organism Eisenia fetida as a model, we developed a sequential workflow combining Fourier transform infrared spectroscopy (FTIR) and matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) for chemical analysis of the same tissue sections. Data analysis of the FTIR spectra via random decision forest (RDF) classification enabled the rapid identification of target tissues (e.g., digestive tissue), which are relevant from an ecotoxicological point of view. MALDI imaging analysis provided specific lipid species which are sensitive to metabolic changes and environmental stressors. Taken together, our approach provides a fast and reproducible workflow for label-free histochemical tissue analyses in E. fetida, which can be applied to other model organisms as well.

Traumatic Brain Injury: An Age-Dependent View of Post-Traumatic Neuroinflammation and Its Treatment

Traumatic brain injury (TBI) is a leading cause of death and disability all over the world. TBI leads to (1) an inflammatory response, (2) white matter injuries and (3) neurodegenerative pathologies in the long term. In humans, TBI occurs most often in children and adolescents or in the elderly, and it is well known that immune responses and the neuroregenerative capacities of the brain, among other factors, vary over a lifetime. Thus, age-at-injury can influence the consequences of TBI. Furthermore, age-at-injury also influences the pharmacological effects of drugs. However, the post-TBI inflammatory, neuronal and functional consequences have been mostly studied in experimental young adult animal models. The specificity and the mechanisms underlying the consequences of TBI and pharmacological responses are poorly understood in extreme ages. In this review, we detail the variations of these age-dependent inflammatory responses and consequences after TBI, from an experimental point of view. We investigate the evolution of microglial, astrocyte and other immune cells responses, and the consequences in terms of neuronal death and functional deficits in neonates, juvenile, adolescent and aged male animals, following a single TBI. We also describe the pharmacological responses to anti-inflammatory or neuroprotective agents, highlighting the need for an age-specific approach to the development of therapies of TBI.

Chemical analysis of the human brain by imaging mass spectrometry

Analysis of the chemical makeup of the brain enables a deeper understanding of several neurological processes. Molecular imaging that deciphers the spatial distribution of neurochemicals with high specificity and sensitivity is an exciting avenue in this aspect. The past two decades have witnessed a significant surge of mass spectrometry imaging (MSI) that can simultaneously map the distribution of hundreds to thousands of biomolecules in the tissue specimen at a fairly high resolution, which is otherwise beyond the scope of other molecular imaging techniques. In this review, we have documented the evolution of MSI technologies in imaging the anatomical distribution of neurochemicals in the human brain in the context of several neuro diseases. This review also addresses the potential of MSI to be a next-generation molecular imaging technique with its promising applications in neuropathology.

Diacylglycerol Lipase-β Knockout Mice Display a Sex-Dependent Attenuation of Traumatic Brain Injury-Induced Mortality with No Impact on Memory or Other Functional Consequences

Background: The endogenous cannabinoid system modulates inflammatory signaling in a variety of pathological states, including traumatic brain injury (TBI). The selective expression of diacylglycerol lipase-β (DAGL-β), the 2-arachidonylglycerol biosynthetic enzyme, on resident immune cells of the brain (microglia) and the role of this pathway in neuroinflammation, suggest that this enzyme may contribute to TBI-induced neuroinflammation. Accordingly, we tested whether DAGL-β^-/- mice would show a protective phenotype from the deleterious consequences of TBI on cognitive and neurological motor functions. Materials and Methods: DAGL-β^-/- and -β^+/+ mice were subjected to the lateral fluid percussion model of TBI and assessed for learning and memory in the Morris water maze (MWM) Fixed Platform (reference memory) and Reversal (cognitive flexibility) tasks, as well as in a cued MWM task to infer potential sensorimotor/motivational deficits. In addition, subjects were assessed for motor behavior (Rotarod and the Neurological Severity Score assays) and in the light/dark box and the elevated plus maze to infer whether these manipulations affected anxiety-like behavior. Finally, we also examined whether brain injury disrupts the ceramide/sphingolipid lipid signaling system and if DAGL-β deletion offers protection. Results: TBI disrupted all measures of neurological motor function and reduced body weight, but did not affect body temperature or performance in common assays used to infer anxiety. TBI also impaired performance in MWM Fixed Platform and Reversal tasks, but did not affect cued MWM performance. Although no differences were found between DAGL-β^-/- and -β^+/+ mice in any of these measures, male DAGL-β^-/- mice displayed an unexpected survival-protective phenotype, which persisted at increased injury severities. In contrast, TBI did not elicit mortality in female mice regardless of genotype. TBI also produced significant changes in sphingolipid profiles (a family of lipids, members of which have been linked to both apoptotic and antiapoptotic pathways), in which DAGL-β deletion modestly altered levels of select species. Conclusions: These findings indicate that although DAGL-β does not play a necessary role in TBI-induced cognitive and neurological function, it appears to contribute to the increased vulnerability of male mice to TBI-induced mortality, whereas female mice show high survival rates irrespective of DAGL-β expression.

Recent Advances in Fluorescence Imaging of Traumatic Brain Injury in Animal Models

Traumatic brain injury (TBI) is one of the top three specific neurological disorders, requiring reliable, rapid, and sensitive imaging of brain vessels, tissues, and cells for effective diagnosis and treatment. Although the use of medical imaging such as computed tomography (CT) and magnetic resonance imaging (MRI) for the TBI detection is well established, the exploration of novel TBI imaging techniques is of great interest. In this review, recent advances in fluorescence imaging for the diagnosis and evaluation of TBI are summarized and discussed in three sections: imaging of cerebral vessels, imaging of brain tissues and cells, and imaging of TBI-related biomarkers. Design strategies for probes and labels used in TBI fluorescence imaging are also described in detail to inspire broader applications. Moreover, the multimodal TBI imaging platforms combining MRI and fluorescence imaging are also briefly introduced. It is hoped that this review will promote more studies on TBI fluorescence imaging, and enable its use for clinical diagnosis as early as possible, helping TBI patients get better treatment and rehabilitation.

Quantitative Mass Spectrometry Imaging of Metabolomes and Lipidomes for Tracking Changes and Therapeutic Response in Traumatic Brain Injury Surrounding Injured Area at Chronic Phase

Traumatic brain injury (TBI) is a complex disease process that may contribute to temporary or permanent disability. Tracking spatial changes of lipids and metabolites in the brain helps unveil the underlying mechanisms of the disease procession and therapeutic response. Here, the liquid microjunction surface sampling technique was used for mass spectrometry imaging of both lipids and metabolites in rat models of controlled cortical impact with and without XueFu ZhuYu decoction treatment, and the work was focused on the diffuse changes outside the injured area at chronic phase (14 days after injury). Quantitative information was provided for phosphotidylcholines and cerebrosides by adding internal standards in the sampling solvent. With principal component analysis for the imaging data, the midbrain was found to be the region with the largest diffuse changes following TBI outside the injured area. In detail, several phosphatidylcholines, phosphatidylethanolamines, phosphatidic acids, and diacylglycerols were found to be significantly up-regulated particularly in midbrain and thalamus after TBI and XFZY treatment. It is associated with the reported "self-repair" mechanisms at the chronic phase of TBI activated by neuroinflammation. Several glycosphingolipids were found to be increased in most of brain regions after TBI, which was inferred to be associated with neuroinflammation and oxidative stress triggered by TBI. Moreover, different classes of small matabolites were significantly changed after TBI, including fatty acids, amino acids, and purines. All these compounds were involved in 10 metabolic pathway networks, and 6 target proteins of XFZY were found related to the impacted pathways. These results shed light on the molecular mechanisms of TBI pathologic processes and therapeutic response.

Glycosphingolipids

Glycosphingolipids are amphiphilic plasma membrane components formed by a glycan linked to a specific lipid moiety. In this chapter we report on these compounds, on their role played in our cells to maintain the correct cell biology.In detail, we report on their structure, on their metabolic processes, on their interaction with proteins and from this, their property to modulate positively in health and negatively in disease, the cell signaling and cell biology.

Mechanosensation in traumatic brain injury

Traumatic brain injury (TBI) is distinct from other neurological disorders because it is induced by a discrete event that applies extreme mechanical forces to the brain. This review describes how the brain senses, integrates, and responds to forces under both normal conditions and during injury. The response to forces is influenced by the unique mechanical properties of brain tissue, which differ by region, cell type, and sub-cellular structure. Elements such as the extracellular matrix, plasma membrane, transmembrane receptors, and cytoskeleton influence its properties. These same components also act as force-sensors, allowing neurons and glia to respond to their physical environment and maintain homeostasis. However, when applied forces become too large, as in TBI, these components may respond in an aberrant manner or structurally fail, resulting in unique pathological sequelae. This so-called "pathological mechanosensation" represents a spectrum of cellular responses, which vary depending on the overall biomechanical parameters of the injury and may be compounded by repetitive injuries. Such aberrant physical responses and/or damage to cells along with the resulting secondary injury cascades can ultimately lead to long-term cellular dysfunction and degeneration, often resulting in persistent deficits. Indeed, pathological mechanosensation not only directly initiates secondary injury cascades, but this post-physical damage environment provides the context in which these cascades unfold. Collectively, these points underscore the need to use experimental models that accurately replicate the biomechanics of TBI in humans. Understanding cellular responses in context with injury biomechanics may uncover therapeutic targets addressing various facets of trauma-specific sequelae.

Will Sirtuins Be Promising Therapeutic Targets for TBI and Associated Neurodegenerative Diseases?

Traumatic brain injury (TBI), a leading cause of morbidity worldwide, induces mechanical, persistent structural, and metabolic abnormalities in neurons and other brain-resident cells. The key pathological features of TBI include neuroinflammation, oxidative stress, excitotoxicity, and mitochondrial dysfunction. These pathological processes persist for a period of time after TBIs. Sirtuins are evolutionarily conserved nicotinamide-adenine dinucleotide (NAD+)-dependent deacetylases and mono-ADP-ribosyl transferases. The mammalian sirtuin family has seven members, referred to as Sirtuin (SIRT) 1-7. Accumulating evidence suggests that SIRT1 and SIRT3 play a neuroprotective role in TBI. Although the evidence is scant, considering the involvement of SIRT2, 4-7 in other brain injury models, they may also intervene in similar pathophysiology in TBI. Neurodegenerative diseases are generally accepted sequelae of TBI. It was found that TBI and neurodegenerative diseases have many similarities and overlaps in pathological features. Besides, sirtuins play some unique roles in some neurodegenerative diseases. Therefore, we propose that sirtuins might be a promising therapeutic target for both TBI and associated neurodegenerative diseases. In this paper, we review the neuroprotective effects of sirtuins on TBI as well as related neurodegeneration and discuss the therapeutic potential of sirtuin modulators.

Acid Sphingomyelinase Inhibition Mitigates Histopathological and Behavioral Changes in a Murine Model of Traumatic Brain Injury

Traumatic brain injury (TBI) can lead to the development of chronic traumatic encephalopathy as a result of neuronal phosphorylated tau (p-tau) protein aggregation and neuroinflammation. Acid sphingomyelinase (Asm) may also contribute to post-TBI neurodegenerative disorders. We hypothesized that Asm inhibition would ameliorate p-tau aggregation, neuroinflammation, and behavioral changes after TBI in a murine model. TBI was generated using a weight-drop method. Asm inhibition in wild-type mice was achieved with a single injection of amitriptyline 1 h after TBI. Genetic Asm ablation was achieved using Asm-deficient mice (Asm^-/-). Thirty days after TBI, mice underwent behavioral testing with the forced swim test for symptoms of depression or were euthanized for neurohistological analysis. Neuroinflammation was quantified using the microglial markers, ionized calcium-binding adaptor molecule 1 and transmembrane protein 119. Compared to sham mice, TBI mice demonstrated increased hippocampal p-tau. Mice that received amitriptyline after TBI demonstrated decreased p-tau compared to mice that received a saline control. Further, post-TBI Asm^-/- mice demonstrated lower levels of p-tau compared to wild-type mice. Though a decrease in neuroinflammation was observed at 1 month post-TBI, no change was demonstrated with mice treated with amitriptyline. Similarly, TBI mice were more likely to show depression compared to mice that received amitriptyline after TBI. Utilizing a weight-drop method to induce moderate TBI, we have shown that genetic deficiency or pharmacological inhibition of Asm prevented hippocampal p-tau aggregation 1 month after injury as well as decreased symptoms of depression. These findings highlight an opportunity to potentially reduce the long-term consequences of TBI.

MeV-SIMS TOF Imaging of Organic Tissue with Continuous Primary Beam

MeV-SIMS is an emerging mass spectrometry imaging method, which utilizes fast, heavy ions to desorb secondary molecules. High yields and low fragmentation rates of large molecules, associated with the electronic sputtering process, make it particularly useful in biomedical research, where insight into distribution of organic molecules is needed. Since the implementation of MeV-SIMS in to the micro-beam line at the tandem accelerator of Jožef Stefan Institute, MeV-SIMS provided some valuable observations on the distribution of biomolecules in plant tissue, as discussed by Jenčič et al. (Nucl. Inst. Methods Phys. Res. B. 371, 205-210, 2016; Nucl. Inst. Methods Phys. Res. B. 404, 140-145, 2017). However, limited focusing ability of the chlorine ion beam only allowed imaging at the tissue level. In order to surpass shortcomings of the existing method, we introduced a new approach, where we employ a continuous, low-current primary beam. In this mode, we bombard thin samples with a steady chlorine ion flux of approx. 5000 ions/s. After desorbing molecules, chlorine ions penetrate through the thinly cut sample and trigger the time-of-flight "start" signal on a continuous electron multiplier detector, positioned behind the sample. Such bombardment is more effective than previously used pulsing-beam mode, which demanded several orders of magnitude higher primary ion beam currents. Sub-micrometer focusing of low-current primary ion beam allows imaging of biological tissue on a subcellular scale. Simultaneously, new time-of-flight acquisition approach also improves mass resolution by a factor of 5. Within the article, we compare the performance of both methods and demonstrate the application of continuous mode on biological tissue. We also describe the thin sample preparation protocol, necessary for measurements with low primary ion currents.

A window into the brain: Tools to assess pre-clinical efficacy of biomaterials-based therapies on central nervous system disorders

Therapeutic conveyance into the brain is a cardinal requirement for treatment of diverse central nervous system (CNS) disorders and associated pathophysiology. Effectual shielding of the brain by the blood-brain barrier (BBB) sieves out major proportion of therapeutics with the exception of small lipophilic molecules. Various nano-delivery systems (NDS) provide an effective solution around this obstacle owing to their small size and targeting properties. To date, these systems have been used for several pre-clinical disease models including glioma, neurodegenerative diseases and psychotic disorders. An efficacy screen for these systems involves a test battery designed to probe into the multiple facets of therapeutic delivery. Despite their wide application in redressing various disease targets, the efficacy evaluation strategies for all can be broadly grouped into four modalities, namely: histological, bio-imaging, molecular and behavioural. This review presents a comprehensive insight into all of these modalities along with their strengths and weaknesses as well as perspectives on an ideal design for a panel of tests to screen brain nano-delivery systems.

"Redox lipidomics technology: Looking for a needle in a haystack"

Aerobic life is based on numerous metabolic oxidation reactions as well as biosynthesis of oxygenated signaling compounds. Among the latter are the myriads of oxygenated lipids including a well-studied group of polyunsaturated fatty acids (PUFA) - octadecanoids, eicosanoids, and docosanoids. During the last two decades, remarkable progress in liquid-chromatography-mass spectrometry has led to significant progress in the characterization of oxygenated PUFA-containing phospholipids, thus designating the emergence of a new field of lipidomics, redox lipidomics. Although non-enzymatic free radical reactions of lipid peroxidation have been mostly associated with the aberrant metabolism typical of acute injury or chronic degenerative processes, newly accumulated evidence suggests that enzymatically catalyzed (phospho)lipid oxygenation reactions are essential mechanisms of many physiological pathways. In this review, we discuss a variety of contemporary protocols applicable for identification and quantitative characterization of different classes of peroxidized (phospho)lipids. We describe applications of different types of LCMS for analysis of peroxidized (phospho)lipids, particularly cardiolipins and phosphatidylethanolalmines, in two important types of programmed cell death - apoptosis and ferroptosis. We discuss the role of peroxidized phosphatidylserines in phagocytotic signaling. We exemplify the participation of peroxidized neutral lipids, particularly tri-acylglycerides, in immuno-suppressive signaling in cancer. We also consider new approaches to exploring the spatial distribution of phospholipids in the context of their oxidizability by MS imaging, including the latest achievements in high resolution imaging techniques. We present innovative approaches to the interpretation of LC-MS data, including audio-representation analysis. Overall, we emphasize the role of redox lipidomics as a communication language, unprecedented in diversity and richness, through the analysis of peroxidized (phospho)lipids.

Imaging with mass spectrometry, the next frontier in sphingolipid research? A discussion on where we stand and the possibilities ahead

In the last ten years, mass spectrometry (MS) has become the favored analytical technique for sphingolipid (SPL) analysis and measurements. Indeed MS has the unique ability to both acquire sensitive and quantitative measurements and to resolve the molecular complexity characteristic of SPL molecules, both across the different SPL families and within the same SPL family. Currently, two complementary MS-based approaches are used for lipid research: analysis of lipid extracts, mainly by infusion electrospray ionization (ESI), and mass spectrometry imaging (MSI) from a sample surface (i.e. intact tissue sections, cells, model membranes, thin layer chromatography plates) (Fig. 1). The first allows for sensitive and quantitative information about total lipid molecular species from a given specimen from which lipids have been extracted and chromatographically separated prior to the analysis; the second, albeit generally less quantitative and less specific in the identification of molecular species due to the complexity of the sample, allows for spatial information of lipid molecules from biological specimens. In the field of SPL research, MS analysis of lipid extracts from biological samples has been commonly utilized to implicate the role of these lipids in specific biological functions. On the other hand, the utilization of MSI in SPL research represents a more recent development that has started to provide interesting descriptive observations regarding the distribution of specific classes of SPLs within tissues. Thus, it is the aim of this review to discuss how MSI technology has been employed to extend the study of SPL metabolism and the type of information that has been obtained from model membranes, single cells and tissues. We envision this discussion as a complementary compendium to the excellent technical reviews recently published about the specifics of MSI technologies, including their application to SPL analysis (Fuchs et al., 2010; Berry et al., 2011; Ellis et al., 2013; Eberlin et al., 2011; Kraft and Klitzing, 2014).

Sphingolipids in neurodegeneration (with focus on ceramide and S1P)

For many decades, research on sphingolipids associated with neurodegenerative disease focused on alterations in glycosphingolipids, particularly glycosylceramides (cerebrosides), sulfatides, and gangliosides. This seemed quite natural since many of these glycolipids are constituents of myelin and accumulated in lipid storage diseases (sphingolipidoses) resulting from enzyme deficiencies in glycolipid metabolism. With the advent of recognizing ceramide and its derivative, sphingosine-1-phosphate (S1P), as key players in lipid cell signaling and regulation of cell death and survival, research focus shifted toward these two sphingolipids. Ceramide and S1P are invoked in a plethora of cell biological processes participating in neurodegeneration such as ER stress, autophagy, dysregulation of protein and lipid transport, exosome secretion and neurotoxic protein spreading, neuroinflammation, and mitochondrial dysfunction. Hence, it is timely to discuss various functions of ceramide and S1P in neurodegenerative disease and to define sphingolipid metabolism and cell signaling pathways as potential targets for therapy.

Spatial and molecular changes of mouse brain metabolism in response to immunomodulatory treatment with teriflunomide as visualized by MALDI-MSI

Multiple sclerosis (MS) is an immune-mediated neurodegenerative disease of the central nervous system (CNS). One of the most promising recent medications for MS is teriflunomide. Its primary mechanism of action is linked to effects on the peripheral immune system by inhibiting dihydroorotate dehydrogenase (DHODH)-catalyzed de novo pyrimidine synthesis and reducing the expansion of lymphocytes in the peripheral immune system. Some in vitro studies suggested, however, that it can also have a direct effect on the CNS compartment. This potential alternative mode of action depends on the drug's capacity to traverse the blood-brain barrier (BBB) and to exert an effect on the complex network of brain biochemical pathways. In this paper, we demonstrate the application of high-resolution/high-accuracy matrix-assisted laser desorption/ionization Fourier-transform ion cyclotron resonance mass spectrometry for molecular imaging of the mouse brain coronal sections from animals treated with teriflunomide. Specifically, in order to assess the effect of teriflunomide on the mouse CNS compartment, we investigated the feasibility of teriflunomide to traverse the BBB. Secondly, we systematically evaluated the spatial and semi-quantitative brain metabolic profiles of 24 different endogenous compounds after 4-day teriflunomide administration. Even though the drug was not detected in the examined cerebral sections (despite the high detection sensitivity of the developed method), in-depth study of the endogenous metabolic compartment revealed noticeable alterations as a result of teriflunomide administration compared to the control animals. The observed differences, particularly for purine and pyrimidine nucleotides as well as for glutathione and carbohydrate metabolism intermediates, shed some light on the potential impact of teriflunomide on the mouse brain metabolic networks. Graphical Abstract.

Lipidomics biomarker studies: Errors, limitations, and the future

Lipidomics is an ever-expanding subfield of metabolomics that surveys 3000 to 5000 individual lipids across more than 56 lipid subclasses, including lipid peroxidation products. Unfortunately, there exists a large number of publications with poor quality data obtained with unit mass resolution leading to many lipid misidentifications. This is further complicated by poor scientific oversight with regard to recognition of isobar issues, sample collection, and sample storage issues that inexplicably requires more detailed attention. Inadvertent or intentional obfuscation of relative quantification data represented as absolute quantification is a subtle but profound difference that may readers outside of the field may not realize, therefore, instigating disservice and unnecessary distrust in the scientific community. These issues need to be addressed aggressively as high quality data are essential for the translation of biomarker research to clinical practice.