SIRT3 Deacetylates Ceramide Synthases: IMPLICATIONS FOR MITOCHONDRIAL DYSFUNCTION AND BRAIN INJURY

Gastrodin regulates the expression of renin-angiotensin system-SIRT3 and proinflammatory mediators in reactive astrocytes via activated microglia

Gastrodin, an anti-inflammatory herbal agent, is known to suppress microglia activation. Here, we investigated whether it would exert a similar effect in reactive astrocytes and whether it might act through the renin-angiotensin system (RAS) and sirtuin 3 (SIRT3). Angiotensinogen (ATO), angiotensin-converting enzyme (ACE), angiotensin II type 1 (AT1) and type 2 (AT2) receptor and SIRT3 expression was detected in TNC-1 astrocytes treated with BV-2 microglia conditioned medium (CM) with or without gastrodin and lipopolysaccharide (LPS) pre-treatment by RT-PCR, immunofluorescence and western blotting analysis. Expression of C3 (A1 astrocyte marker), S100A10 (A2 astrocyte marker), proinflammatory cytokines and neurotrophic factors was then evaluated. The results showed a significant increase of ATO, ACE, AT1, SIRT3, C3, proinflammatory cytokines and neurotrophic factors expression in TNC-1 astrocytes incubated in CM + LPS when compared with cells incubated in the CM, but AT2 and S100A10 expression was reduced. TNC-1 astrocytes responded vigorously to BV-2 CM treated with gastrodin + LPS as compared with the control. This was evident by the decreased expression of the abovementioned protein markers, except for AT2 and S100A10. Interestingly, SIRT3, IGF-1 and BDNF expression was enhanced, suggesting that gastrodin inhibited the expression of RAS and proinflammatory mediators but promoted the expression of neurotrophic factors. And gastrodin regulated the phenotypic changes of astrocytes through AT1. Additionally, azilsartan (a specific inhibitor of AT1) inhibited the expression of C3 and S100A10, which remained unaffected in gastrodin and azilsartan combination treatment. These findings provide evidence that gastrodin may have a therapeutic effect via regulating RAS-SIRT3.

Lipidomic analysis identifies long-chain acylcarnitine as a target for ischemic stroke

INTRODUCTION

Lipid metabolism dysfunction is widely involved in the pathological process of acute ischemic stroke (AIS). The coordination of lipid metabolism between neurons and astrocytes is of great significance. However, the full scope of lipid dynamic changes and the function of key lipids during AIS remain unknown. Hence, identifying lipid alterations and characterizing their key roles in AIS is of great importance.

METHODS

Untargeted and targeted lipidomic analyses were applied to profile lipid changes in the ischemic penumbra and peripheral blood of transient middle cerebral artery occlusion (tMCAO) mice as well as the peripheral blood of AIS patients. Infarct volume and neurological deficits were assessed after tMCAO. The cell viability and dendritic complexity of primary neurons were evaluated by CCK8 assay and Sholl analysis. Seahorse, MitoTracker Green, tetramethyl rhodamine methyl ester (TMRM), 2',7'-dichlorodihydrofluorescein diacetate (DCFH-DA) and MitoSOX were used as markers of mitochondrial health. Fluorescent and isotopic free fatty acid (FFA) pulse-chase assays were used to track FFA flux in astrocytes.

RESULTS

Long-chain acylcarnitines (LCACs) were the lipids with the most dramatic changes in the ischemic penumbra and peripheral blood of tMCAO mice. LCACs were significantly elevated on admission in AIS patients and associated with poor outcomes in AIS patients. Increasing LCACs through a bolus administration of palmitoylcarnitine amplified stroke injury, while decreasing LCACs by overexpressing carnitine palmitoyltransferase 2 (CPT2) ameliorated stroke injury. Palmitoylcarnitine aggravated astrocytic mitochondrial damage after OGD/R, while CPT2 overexpression in astrocytes ameliorated cocultured neuron viability. Further study revealed that astrocytes stimulated by OGD/R liberated FFAs from lipid droplets into mitochondria to form LCACs, resulting in mitochondrial damage and lowered astrocytic metabolic support and thereby aggravated neuronal damage.

CONCLUSION

LCACs could accumulate and damage neurons by inducing astrocytic mitochondrial dysfunction in AIS. LCACs play a crucial role in the pathology of AIS and are novel promising diagnostic and prognostic biomarkers for AIS.

4'-O-methylbavachalcone alleviates ischemic stroke injury by inhibiting parthanatos and promoting SIRT3

Cerebral ischemia-reperfusion injury (CIRI) can induce massive death of ischemic penumbra neurons via oxygen burst, exacerbating brain damage. Parthanatos is a form of caspase-independent cell death involving excessive activation of PARP-1, closely associated with intense oxidative stress following CIRI. 4'-O-methylbavachalcone (MeBavaC), an isoprenylated chalcone component in Fructus Psoraleae, has potential neuroprotective effects. This study primarily investigates whether MeBavaC can act on SIRT3 to alleviate parthanatos of ischemic penumbra neurons induced by CIRI. MeBavaC was oral gavaged to the middle cerebral artery occlusion-reperfusion (MCAO/R) rats after occlusion. The effects of MeBavaC on cerebral injury were detected by the neurological deficit score and cerebral infarct volume. In vitro, PC-12 cells were subjected to oxygen and glucose deprivation/reoxygenation (OGD/R), and assessed cell viability and cell injury. Also, the levels of ROS, mitochondrial membrane potential (MMP), and intracellular Ca2+ levels were detected to reflect mitochondrial function. We conducted western blotting analyses of proteins involved in parthanatos and related signaling pathways. Finally, the exact mechanism between the neuroprotection of MeBavaC and parthanatos was explored. Our results indicate that MeBavaC reduces the cerebral infarct volume and neurological deficit scores in MCAO/R rats, and inhibits the decreased viability of PC-12 cells induced by OGD/R. MeBavaC also downregulates the expression of parthanatos-related death proteins PARP-1, PAR, and AIF. However, this inhibitory effect is weakened after the use of a SIRT3 inhibitor. In conclusion, the protective effect of MeBavaC against CIRI may be achieved by inhibiting parthanatos of ischemic penumbra neurons through the SIRT3-PARP-1 axis.

Emerging roles and therapeutic potentials of sphingolipids in pathophysiology: emphasis on fatty acyl heterogeneity

Sphingolipids not only exert structural roles in cellular membranes, but also act as signaling molecules in various physiological and pathological processes. A myriad of studies have shown that abnormal levels of sphingolipids and their metabolic enzymes are associated with a variety of human diseases. Moreover, blood sphingolipids can also be used as biomarkers for disease diagnosis. This review summarizes the biosynthesis, metabolism, and pathological roles of sphingolipids, with emphasis on the biosynthesis of ceramide, the precursor for the biosynthesis of complex sphingolipids with different fatty acyl chains. The possibility of using sphingolipids for disease prediction, diagnosis, and treatment is also discussed. Targeting endogenous ceramides and complex sphingolipids along with their specific fatty acyl chain to promote future drug development will also be discussed.

Retinal ischemia-reperfusion injury induces intense lipid synthesis and remodeling

The retina is a high-metabolism tissue composed of various cell types with complex functions that relies heavily on the blood supply to maintain homeostasis. Retinal ischemia-reperfusion injury is a critical pathogenic mechanism in glaucoma, and changes in lipid molecules may lead to retinal tissue damage. However, retinal lipid profile alterations caused by this mechanism remain unclear. Thus, this study employed a retinal ischemia-reperfusion model to analyze changes in the lipid profile between sham-operated and ischemia-reperfusion groups. We discovered that ischemia-reperfusion injury-induced alterations in 338 lipid molecules, which potentially caused lipid droplet formation and mitochondrial damage. Notably, we identified characteristic changes in various lipids, including cholesterol esters, cardiolipin, and ceramide, which may serve as potential biomarkers for assessing the severity of retinal injury and therapeutic interventions. The ischemia-reperfusion-specific features identified in this study provide a more comprehensive understanding of the pathophysiological mechanisms underlying this condition.

Sphingolipids in mitochondria-from function to disease

Sphingolipids are not only structural components of cellular membranes but also play vital roles in cell signaling and modulation of cellular processes. Within mitochondria, sphingolipids exert diverse effects on mitochondrial dynamics, energy metabolism, oxidative stress, and cell death pathways. In this review, we summarize literature addressing the crucial role of sphingolipids in mitochondria, highlighting their impact on mitochondrial dynamics, cellular bioenergetics, and important cell processes including apoptosis and mitophagy.

The heat shock protein Hsp27 controls mitochondrial function by modulating ceramide generation

Sphingolipids have key functions in membrane structure and cellular signaling. Ceramide is the central molecule of the sphingolipid metabolism and is generated by ceramide synthases (CerS) in the de novo pathway. Despite their critical function, mechanisms regulating CerS remain largely unknown. Using an unbiased proteomics approach, we find that the small heat shock protein 27 (Hsp27) interacts specifically with CerS1 but not other CerS. Functionally, our data show that Hsp27 acts as an endogenous inhibitor of CerS1. Wild-type Hsp27, but not a mutant deficient in CerS1 binding, inhibits CerS1 activity. Additionally, silencing of Hsp27 enhances CerS1-generated ceramide accumulation in cells. Moreover, phosphorylation of Hsp27 modulates Hsp27-CerS1 interaction and CerS1 activity in acute stress-response conditions. Biologically, we show that Hsp27 knockdown impedes mitochondrial function and induces lethal mitophagy in a CerS1-dependent manner. Overall, we identify an important mode of CerS1 regulation and CerS1-mediated mitophagy through protein-protein interaction with Hsp27.

Lipidomics in pathogenesis, progression and treatment of nonalcoholic steatohepatitis (NASH): Recent advances

Nonalcoholic fatty liver disease (NAFLD) is a chronic liver disease affecting up to 30% of the general adult population. NAFLD encompasses a histological spectrum ranging from pure steatosis to non-alcoholic steatohepatitis (NASH). NASH can progress to cirrhosis and is becoming the most common indication for liver transplantation, as a result of increasing disease prevalence and of the absence of approved treatments. Lipidomic readouts of liver blood and urine samples from experimental models and from NASH patients disclosed an abnormal lipid composition and metabolism. Collectively, these changes impair organelle function and promote cell damage, necro-inflammation and fibrosis, a condition termed lipotoxicity. We will discuss the lipid species and metabolic pathways leading to NASH development and progression to cirrhosis, as well as and those species that can contribute to inflammation resolution and fibrosis regression. We will also focus on emerging lipid-based therapeutic opportunities, including specialized proresolving lipid molecules and macrovesicles contributing to cell-to-cell communication and NASH pathophysiology.

Ceramide in cerebrovascular diseases

Ceramide, a bioactive sphingolipid, serves as an important second messenger in cell signal transduction. Under stressful conditions, it can be generated from de novo synthesis, sphingomyelin hydrolysis, and/or the salvage pathway. The brain is rich in lipids, and abnormal lipid levels are associated with a variety of brain disorders. Cerebrovascular diseases, which are mainly caused by abnormal cerebral blood flow and secondary neurological injury, are the leading causes of death and disability worldwide. There is a growing body of evidence for a close connection between elevated ceramide levels and cerebrovascular diseases, especially stroke and cerebral small vessel disease (CSVD). The increased ceramide has broad effects on different types of brain cells, including endothelial cells, microglia, and neurons. Therefore, strategies that reduce ceramide synthesis, such as modifying sphingomyelinase activity or the rate-limiting enzyme of the de novo synthesis pathway, serine palmitoyltransferase, may represent novel and promising therapeutic approaches to prevent or treat cerebrovascular injury-related diseases.

Role of SIRT3 in mitochondrial biology and its therapeutic implications in neurodegenerative disorders

Sirtuin 3 (SIRT3), a mitochondrial deacetylase expressed preferentially in high-metabolic-demand tissues including the brain, requires NAD⁺ as a cofactor for catalytic activity. It regulates various processes such as energy homeostasis, redox balance, mitochondrial quality control, mitochondrial unfolded protein response (UPRmt), biogenesis, dynamics and mitophagy by altering protein acetylation status. Reduced SIRT3 expression or activity causes hyperacetylation of hundreds of mitochondrial proteins, which has been linked with neurological abnormalities, neuro-excitotoxicity and neuronal cell death. A body of evidence has suggested, SIRT3 activation as a potential therapeutic modality for age-related brain abnormalities and neurodegenerative disorders.

Sirtuin-3: A potential target for treating several types of brain injury

Sirtuin-3 (SIRT3) is responsible for maintaining mitochondrial homeostasis by deacetylating substrates in an NAD+-dependent manner. SIRT3, the primary deacetylase located in the mitochondria, controls cellular energy metabolism and the synthesis of essential biomolecules for cell survival. In recent years, increasing evidence has shown that SIRT3 is involved in several types of acute brain injury. In ischaemic stroke, subarachnoid haemorrhage, traumatic brain injury, and intracerebral haemorrhage, SIRT3 is closely related to mitochondrial homeostasis and with the mechanisms of pathophysiological processes such as neuroinflammation, oxidative stress, autophagy, and programmed cell death. As SIRT3 is the driver and regulator of a variety of pathophysiological processes, its molecular regulation is significant. In this paper, we review the role of SIRT3 in various types of brain injury and summarise SIRT3 molecular regulation. Numerous studies have demonstrated that SIRT3 plays a protective role in various types of brain injury. Here, we present the current research available on SIRT3 as a target for treating ischaemic stroke, subarachnoid haemorrhage, traumatic brain injury, thus highlighting the therapeutic potential of SIRT3 as a potent mediator of catastrophic brain injury. In addition, we have summarised the therapeutic drugs, compounds, natural extracts, peptides, physical stimuli, and other small molecules that may regulate SIRT3 to uncover additional brain-protective mechanisms of SIRT3, conduct further research, and provide more evidence for clinical transformation and drug development.

Role of SIRT3 in neurological diseases and rehabilitation training

Sirtuin3 (SIRT3) is a deacetylase that plays an important role in normal physiological activities by regulating a variety of substrates. Considerable evidence has shown that the content and activity of SIRT3 are altered in neurological diseases. Furthermore, SIRT3 affects the occurrence and development of neurological diseases. In most cases, SIRT3 can inhibit clinical manifestations of neurological diseases by promoting autophagy, energy production, and stabilization of mitochondrial dynamics, and by inhibiting neuroinflammation, apoptosis, and oxidative stress (OS). However, SIRT3 may sometimes have the opposite effect. SIRT3 can promote the transfer of microglia. Microglia in some cases promote ischemic brain injury, and in some cases inhibit ischemic brain injury. Moreover, SIRT3 can promote the accumulation of ceramide, which can worsen the damage caused by cerebral ischemia-reperfusion (I/R). This review comprehensively summarizes the different roles and related mechanisms of SIRT3 in neurological diseases. Moreover, to provide more ideas for the prognosis of neurological diseases, we summarize several SIRT3-mediated rehabilitation training methods.

[Effects of Long-Term Ethanol Consumption on Learning-Memory Functions in Mice and the Mechanisms Involved]

Objective

To investigate the effect of long-term ethanol consumption on learning-memory functions in mice and the mechanisms involved.

Methods

Thirty male C57BL6/J mice were randomly assigned to 3 groups, with 10 mice in each group. The three groups included a control group in which the mice were given water ad libitum for 30 days, a long-term ethanol consumption group, or the EtOH group, in which the mice were given 6% (volume fraction) ethanol ad libitum for 30 days, and a long-term alcoholism group, or the EtOH+G group, in which the mice were given 5% (volume fraction) ethanol ad libitum for 30 days plus intermittent intragastric gavage of 20% ethanol at 3.5 g per kilogram body mass once every three days. After 30 days, the learning-memory functions of the mice were evaluated. At the conclusion of the experiment, the brain tissue of the mice was collected in order to examine the oxygen consumption rate (OCR) of mitochondria, the levels of pan-acetylation and protein oxidative stress in the hippocampal tissue, and the expression of sirtuin-3 (SIRT3) in hippocampus.

Results

Morris water maze test showed that, compared with those of the control group, the times of crossing the platform and the percentage of platform time in the EtOH group and the EtOH+G group were both lower, and the EtOH+G group had the lowest results ( P<0.05). Western blot results showed that long-term ethanol intake increased the levels of protein oxidative stress and pan-acetylation in the hippocampal tissue and down-regulated SIRT3 expression of hippocampal mitochondria. The results of mitochondrial complex Ⅱ respiration showed that the brain mitochondrial 3-state respiration in the EtOH group and the EtOH+G group was lower than that in the control group ( P<0.05). Compared that with the control group, the mitochondrial maximum respiration in EtOH+G group was decreased ( P<0.05).

Conclusions

Both long-term ethanol consumption and long-term alcoholism can reduce learning-memory functions and long-term alcoholism has the greater impact of the two. The potential mechanism may involve the down-regulation of the expression of SIRT3 protein in the hippocampus, which results in an increased level of pan-acetylation and enhanced expression of oxidative stress protein in the hippocampus, affects the mitochondrial functions of the brain, inhibits the oxidative phosphorylation capacity of mitochondrial complex Ⅱ, reduces the ATP energy supply of the brain tissue, and thus affects the learning-memory function.

Role of NAD+ and FAD in Ischemic Stroke Pathophysiology: An Epigenetic Nexus and Expanding Therapeutic Repertoire

The redox coenzymes viz., oxidized β-nicotinamide adenine dinucleotide (NAD⁺) and flavin adenine dinucleotide (FAD) by way of generation of optimal reducing power and cellular energy currency (ATP), control a staggering array of metabolic reactions. The prominent cellular contenders for NAD⁺ utilization, inter alia, are sirtuins (SIRTs) and poly(ADP-ribose) polymerase (PARP-1), which have been significantly implicated in ischemic stroke (IS) pathogenesis. NAD⁺ and FAD are also two crucial epigenetic enzyme-required metabolites mediating histone deacetylation and poly(ADP-ribosyl)ation through SIRTs and PARP-1 respectively, and demethylation through FAD-mediated lysine specific demethylase activity. These enzymes and post-translational modifications impinge on the components of neurovascular unit, primarily neurons, and elicit diverse functional upshots in an ischemic brain. These could be circumstantially linked with attendant cognitive deficits and behavioral outcomes in post-stroke epoch. Parsing out the contribution of NAD⁺/FAD-synthesizing and utilizing enzymes towards epigenetic remodeling in IS setting, together with their cognitive and behavioral associations, combined with possible therapeutic implications will form the crux of this review.

Lipidomics Analysis Reveals a Protective Effect of Myriocin on Cerebral Ischemia/Reperfusion Model Rats

Ceramide accumulation has been associated with ischemic stroke. Myriocin is an effective serine palmitoyltransferase (SPT) inhibitor that reduces ceramide levels by inhibiting the de novo synthesis pathway. However, the role of myriocin in cerebral ischemia/reperfusion (I/R) injury and its underlying mechanism remain unknown. The present study established an experimental rat model of middle cerebral artery occlusion (MCAO). We employed ultra-performance liquid chromatograph quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF/MS)-based lipidomic analysis to identify the disordered lipid metabolites and the effects of myriocin in cerebral cortical tissues of rats. In this study, we found 15 characterized lipid metabolites involved in sphingolipid and glycerophospholipid metabolism in cerebral I/R-injured rats, and these alterations were significantly alleviated by myriocin. Specifically, the mRNA expression of metabolism-related enzyme genes was detected by real-time quantitative polymerase chain reaction (RT-qPCR). We demonstrated that myriocin could regulate the mRNA expression of ASMase, NSMase, SGMS1, SGMS2, ASAH1, ACER2, and ACER3, which are involved in sphingolipid metabolism and PLA2, which is involved in glycerophospholipid metabolism. Moreover, TUNEL and Western blot assays showed that myriocin plays a key role in regulating neuronal cell apoptosis. In summary, the present work provides a new perspective for the systematic study of metabolic changes in ischemic stroke and the therapeutic applications of myriocin.

Involvement of Ceramide Metabolism in Cerebral Ischemia

Ischemic stroke, caused by the interruption of blood flow to the brain and subsequent neuronal death, represents one of the main causes of disability in worldwide. Although reperfusion therapies have shown efficacy in a limited number of patients with acute ischemic stroke, neuroprotective drugs and recovery strategies have been widely assessed, but none of them have been successful in clinical practice. Therefore, the search for new therapeutic approaches is still necessary. Sphingolipids consist of a family of lipidic molecules with both structural and cell signaling functions. Regulation of sphingolipid metabolism is crucial for cell fate and homeostasis in the body. Different works have emphasized the implication of its metabolism in different pathologies, such as diabetes, cancer, neurodegeneration, or atherosclerosis. Other studies have shown its implication in the risk of suffering a stroke and its progression. This review will highlight the implications of sphingolipid metabolism enzymes in acute ischemic stroke.

Sirtuin 3 and mitochondrial permeability transition pore (mPTP): A systematic review

Mitochondrial permeability transition pore (mPTP) is a channel that opens at the inner mitochondrial membrane under conditions of stress. Sirtuin 3 (Sirt3) is a mitochondrial deacetylase known to play a major role in stress resistance and a regulatory role in cell death. This systematic review aims to elucidate the role of Sirt3 in mPTP inhibition. Electronic databases, including PubMed, EMBASE, Web of Science and Cochrane Library were searched up to May 2020. Original studies that investigated the relationship between Sirt3 and mPTP were included. Two reviewers independently extracted data on study characteristics, methods and outcomes. A total of 194 articles were found. Twenty-nine articles, which met criteria were included in the systematic review. Twenty-three studies provided evidence of the inhibitory effect of Sirt3 on the mPTP aperture. This review summarizes up-to-date evidence of the protective and inhibitory role of Sirt3 through deacetylating Cyclophilin D (CypD) on the mPTP aperture. Furthermore, we discuss the implications of this effect in disease.

Oxygen-Glucose Deprivation/Reperfusion-Induced Sirt3 Reduction Facilitated Neuronal Injuries in an Apoptosis-Dependent Manner During Prolonged Reperfusion

Cerebral ischemia is a major cause of morbidity and permanent disability. To date, no treatments for cerebral ischemia/reperfusion injury can be effectively administered beyond 4-6 h after the ischemic insult. Our study aimed to clarify the significance of Sirt3 during acute cerebral ischemia and explore Sirt3-targeted therapy for ischemic injuries. Upon establishing the oxygen-glucose deprivation/reperfusion (OGD/R) cell model, changes of Sirt3 protein levels and the effects of Sirt3 overexpression on primary hippocampal neurons were detected at indicated time points. Moreover, mitochondrial damage was observed in neurons upon OGD/R injury. The results showed that compared with the normoxia group, Sirt3 protein was significantly decreased in hippocampal neurons exposed to 1 h of OGD followed by 12 h of reperfusion. In addition, the reduction of Sirt3 protein levels contributed to OGD/R-induced neuronal injuries, a higher ratio of neuronal apoptosis, and extensive production of reactive oxygen species (ROS). However, all neuronal injuries were partly rescued by Sirt3 overexpression induced by lentivirus transfection. Mitochondrial morphologies were significantly impaired after OGD/R, but partly salvaged by Sirt3 overexpression. We further explored whether pharmacologically activating Sirt3 is protective for neurons, and found that treatment with honokiol (a Sirt3 agonist) after OGD exposure activated Sirt3 during reperfusion and significantly alleviated OGD/R-induced neuronal injuries. Because mitochondrial functions are essential for neuronal survival, the current results indicate that Sirt3 may be an efficient target to suppress ischemic injuries via maintenance of mitochondrial homeostasis. Our current findings shed light on a novel therapeutic strategy against subacute ischemic injuries.

Mitochondrial sirtuins, metabolism, and aging

Maintaining metabolic homeostasis is essential for cellular and organismal health throughout life. Of the multiple signaling pathways that regulate metabolism, such as PI3K/AKT, mTOR, AMPK, and sirtuins, mammalian sirtuins also play unique roles in aging. By understanding how sirtuins regulate metabolic processes, we can start to understand how they slow down or accelerate biological aging. Here, we review the biology of SIRT3, SIRT4, and SIRT5, known as the mitochondrial sirtuins due to their localization in the mitochondrial matrix. First, we will focus on canonical pathways that regulate metabolism more broadly and how these are integrated with aging regulation. Then, we will summarize the current knowledge about functional differences between SIRT3, SIRT4, and SIRT5 in metabolic control and integration in signaling networks. Finally, we will discuss how mitochondrial sirtuins regulate processes associated with aging and oxidative stress, calorie restriction and disease.

Alterations in mitochondrial dynamics with age-related Sirtuin1/Sirtuin3 deficiency impair cardiomyocyte contractility

Sirtuin1 (SIRT1) and Sirtuin3 (SIRT3) protects cardiac function against ischemia/reperfusion (I/R) injury. Mitochondria are critical in response to myocardial I/R injury as disturbance of mitochondrial dynamics contributes to cardiac dysfunction. It is hypothesized that SIRT1 and SIRT3 are critical components to maintaining mitochondria homeostasis especially mitochondrial dynamics to exert cardioprotective actions under I/R stress. The results demonstrated that deficiency of SIRT1 and SIRT3 in aged (24–26 months) mice hearts led to the exacerbated cardiac dysfunction in terms of cardiac systolic dysfunction, cardiomyocytes contractile defection, and abnormal cardiomyocyte calcium flux during I/R stress. Moreover, the deletion of SIRT1 or SIRT3 in young (4–6 months) mice hearts impair cardiomyocyte contractility and shows aging‐like cardiac dysfunction upon I/R stress, indicating the crucial role of SIRT1 and SIRT3 in protecting myocardial contractility from I/R injury. The biochemical and seahorse analysis showed that the deficiency of SIRT1/SIRT3 leads to the inactivation of AMPK and alterations in mitochondrial oxidative phosphorylation (OXPHOS) that causes impaired mitochondrial respiration in response to I/R stress. Furthermore, the remodeling of the mitochondria network goes together with hypoxic stress, and mitochondria undergo the processes of fusion with the increasing elongated branches during hypoxia. The transmission electron microscope data showed that cardiac SIRT1/SIRT3 deficiency in aging alters mitochondrial morphology characterized by the impairment of mitochondria fusion under I/R stress. Thus, the age‐related deficiency of SIRT1/SIRT3 in the heart affects mitochondrial dynamics and respiration function that resulting in the impaired contractile function of cardiomyocytes in response to I/R.

Plasma ceramides are associated with outcomes in acute ischemic stroke patients

BACKGROUND/PURPOSE

Sphingolipids are major constituents of eukaryotic cell membranes and play key roles in cellular regulatory processes. Our recent results in an experimental stroke animal model demonstrated changes in sphingolipids in response to acute ischemic brain injury. This study aimed to investigate the plasma levels of sphingosine-1-phosphate (S1P) and ceramides in acute ischemic stroke (AIS) patients and their associations with functional outcomes.

METHODS

Plasma samples were collected from patients with AIS at <48 and 48-72 h post stroke and from nonstroke controls. The levels of S1P and ceramides with different fatty acyl chain lengths were measured by the ultra-high-pressure liquid chromatography-electrospray ionization tandem mass spectrometry (UHPLC-ESI-MS/MS). A poor functional outcome was defined as a modified Rankin Scale (mRS) score ≥2 at 3 months after AIS.

RESULTS

The results showed that S1P and very-long-chain ceramides were significantly decreased in AIS patients (n = 87; poor outcome, 56.3%) compared to nonstroke controls (n = 30). In contrast, long-chain ceramides were significantly increased in AIS patients. More importantly, higher levels of Cer(d18:1/18:0), Cer(d18:1/20:0), and Cer(d18:1/22:0) at 48-72 h were significantly associated with poor functional outcomes after adjusting for potential clinical confounders, including age, sex, hypertension, and National Institutes of Health Stroke Scale score at admission.

CONCLUSION

Our study supported the dynamic metabolism of sphingolipids after the occurrence of AIS. Ceramides could be potential prognostic markers for patients with AIS.

Regulation of SIRT3 on mitochondrial functions and oxidative stress in Parkinson's disease

Sirtuin-3 (SIRT3) is a NAD⁺-dependent protein deacetylase that is located in mitochondria, regulating mitochondrial proteins and maintaining cellular antioxidant status. Increasing evidence demonstrates that SIRT3 plays a role in degenerative disorders including Parkinson's disease (PD), which is a devastating nervous system disease currently with no effective treatments available. Although the etiology of PD is still largely ambiguous, substantial evidence indicates that mitochondrial dysfunction and oxidative stress play major roles in the pathogenesis of PD. The imbalance of reactive oxygen species (ROS) production and detoxification leads to oxidative stress that can accelerate the progression of PD. By causing conformational changes in the deacetylated proteins SIRT3 modulates the activities and biological functions of a variety of proteins involved in mitochondrial antioxidant defense and various mitochondrial functions. Increasingly more studies have suggested that upregulation of SIRT3 confers beneficial effect on neuroprotection in various PD models. This review discusses the mechanism by which SIRT3 regulates intracellular oxidative status and mitochondrial function with an emphasis in discussing in detail the regulation of SIRT3 on each component of the five complexes of the mitochondrial respiratory chain and mitochondrial antioxidant defense, as well as the pharmacological regulation of SIRT3 in light of therapeutic strategies for PD.

Sphingolipids in Type 1 Diabetes: Focus on Beta-Cells

Type 1 diabetes (T1DM) is a chronic autoimmune disease, with a strong genetic background, leading to a gradual loss of pancreatic beta-cells, which secrete insulin and control glucose homeostasis. Patients with T1DM require life-long substitution with insulin and are at high risk for development of severe secondary complications. The incidence of T1DM has been continuously growing in the last decades, indicating an important contribution of environmental factors. Accumulating data indicates that sphingolipids may be crucially involved in T1DM development. The serum lipidome of T1DM patients is characterized by significantly altered sphingolipid composition compared to nondiabetic, healthy probands. Recently, several polymorphisms in the genes encoding the enzymatic machinery for sphingolipid production have been identified in T1DM individuals. Evidence gained from studies in rodent islets and beta-cells exposed to cytokines indicates dysregulation of the sphingolipid biosynthetic pathway and impaired function of several sphingolipids. Moreover, a number of glycosphingolipids have been suggested to act as beta-cell autoantigens. Studies in animal models of autoimmune diabetes, such as the Non Obese Diabetic (NOD) mouse and the LEW.1AR1-iddm (IDDM) rat, indicate a crucial role of sphingolipids in immune cell trafficking, islet infiltration and diabetes development. In this review, the up-to-date status on the findings about sphingolipids in T1DM will be provided, the under-investigated research areas will be identified and perspectives for future studies will be given.

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.

The interplay between oxidative stress and bioenergetic failure in neuropsychiatric illnesses: can we explain it and can we treat it?

Nitro-oxidative stress and lowered antioxidant defences play a key role in neuropsychiatric disorders such as major depression, bipolar disorder and schizophrenia. The first part of this paper details mitochondrial antioxidant mechanisms and their importance in reactive oxygen species (ROS) detoxification, including details of NO networks, the roles of H₂O₂ and the thioredoxin/peroxiredoxin system, and the relationship between mitochondrial respiration and NADPH production. The second part highlights and identifies the causes of the multiple pathological sequelae arising from self-amplifying increases in mitochondrial ROS production and bioenergetic failure. Particular attention is paid to NAD⁺ depletion as a core cause of pathology; detrimental effects of raised ROS and reactive nitrogen species on ATP and NADPH generation; detrimental effects of oxidative and nitrosative stress on the glutathione and thioredoxin systems; and the NAD⁺-induced signalling cascade, including the roles of SIRT1, SIRT3, PGC-1α, the FOXO family of transcription factors, Nrf1 and Nrf2. The third part discusses proposed therapeutic interventions aimed at mitigating such pathology, including the use of the NAD⁺ precursors nicotinamide mononucleotide and nicotinamide riboside, both of which rapidly elevate levels of NAD⁺ in the brain and periphery following oral administration; coenzyme Q₁₀ which, when given with the aim of improving mitochondrial function and reducing nitro-oxidative stress in the brain, may be administered via the use of mitoquinone, which is in essence ubiquinone with an attached triphenylphosphonium cation; and N-acetylcysteine, which is associated with improved mitochondrial function in the brain and produces significant decreases in oxidative and nitrosative stress in a dose-dependent manner.

Sirt3 Protects Against Ischemic Stroke Injury by Regulating HIF-1α/VEGF Signaling and Blood-Brain Barrier Integrity

Sirtuin 3 (Sirt3) is a member of the Sirtuin family proteins and known to regulate multiple physiological processes such as metabolism and aging. As stroke is an aging-related disease, in this work, we attempt to examine the role and potential mechanism of Sirt3 in regulating ischemic stroke by using a permanent middle cerebral artery occlusion (pMCAO) model in wild type (WT) and Sirt3 knockout (KO) mice, coupled with oxygen glucose deprivation (OGD) experiments in cultured primary astrocytes. Sirt3 deficiency aggravated neuronal cell apoptosis and neurological deficits after brain ischemia. In addition, Sirt3 KO mice showed more severe blood-brain barrier (BBB) disruption and inflammatory responses compared with WT group in the acute phase. Furthermore, specific overexpression of Sirt3 in astrocytes by injecting glial fibrillary acidic protein (GFAP)::Sirt3 virus in ischemic region showed protective effect against stroke-induced damage. Mechanistically, Sirt3 could regulate vascular endothelial growth factor (VEGF) expression by inhibiting hypoxia inducible factor-1α (HIF-1α) signaling after ischemia (OGD). Our results have shown that Sirt3 plays a protective role in ischemic stroke via regulating HIF-1α/VEGF signaling in astrocytes, and reversal of the Sirt3 expression at the acute phase could be a worthy direction for stroke therapy.

Critical Shifts in Cerebral White Matter Lipid Profiles After Ischemic-Reperfusion Brain Injury in Fetal Sheep as Demonstrated by the Positive Ion Mode MALDI-Mass Spectrometry

Ischemic-reperfusion (I/R) injury to cerebral white matter during the perinatal period leads to long-term cognitive and motor disabilities in children. Immature white matter oligodendrocytes are especially vulnerable to metabolic insults such as those caused by hypoxic, ischemic, and reperfusion injury. Consequences include an impaired capacity of oligodendrocytes to generate and maintain mature lipid-rich myelin needed for efficient neuronal conductivity. Further research is needed to increase an understanding of the early, possibly reversible myelin-associated pathologies that accompany I/R white matter injury. This experiment characterized I/R time-dependent alterations in cerebral white matter lipid profiles in an established fetal sheep model. Fetal sheep (127 days gestation) were subjected to 30 min of bilateral carotid artery occlusion followed by 4 h (n = 5), 24 h (n = 7), 48 h (n = 3), or 72 h (n = 5) of reperfusion, or sham treatment (n = 5). Supraventricular cerebral white matter lipids were analyzed using the positive ionization mode matrix-assisted laser desorption/ionization mass spectrometry. Striking I/R-associated shifts in phospholipid (PL) and sphingolipid expression with a prominent upregulation of cardiolipin, phosphatidylcholine, phosphatidylinositol monomannoside, sphingomyelin, sulfatide, and ambiguous or unidentified lipids were observed to occur mainly at I/R-48 and normalized or suppressed responses at I/R-72. In fetal sheep, cerebral I/R caused major shifts in white matter myelin lipid composition favoring the upregulated expression of diverse PLs and sphingolipids which are needed to support neuronal membrane, synaptic, metabolic, and cell signaling functions.

The yin and yang faces of the mitochondrial deacetylase sirtuin 3 in age-related disorders

Aging, the most important risk factor for many of the chronic diseases affecting Western society, is associated with a decline in mitochondrial function and dynamics. Sirtuin 3 (SIRT3) is a mitochondrial deacetylase that has emerged as a key regulator of fundamental processes which are frequently dysregulated in aging and related disorders. This review highlights recent advances and controversies regarding the yin and yang functions of SIRT3 in metabolic, cardiovascular and neurodegenerative diseases, as well as the use of SIRT3 modulators as a therapeutic strategy against those disorders. Although most studies point to a protective role upon SIRT3 activation, there are conflicting findings that need a better elucidation. The discovery of novel SIRT3 modulators with higher selectivity together with the assessment of the relative importance of different SIRT3 enzymatic activities and the relevance of crosstalk between distinct sirtuin isoforms will be pivotal to validate SIRT3 as a useful drug target for the prevention and treatment of age-related diseases.

SIRT3 Regulation of Mitochondrial Quality Control in Neurodegenerative Diseases

Neurodegenerative diseases are disorders that are characterized by a progressive decline of motor and/or cognitive functions caused by the selective degeneration and loss of neurons within the central nervous system. The most common neurodegenerative diseases are Alzheimer's disease (AD), Parkinson's disease (PD), and Huntington's disease (HD). Neurons have high energy demands, and dysregulation of mitochondrial quality and function is an important cause of neuronal degeneration. Mitochondrial quality control plays an important role in maintaining mitochondrial integrity and ensuring normal mitochondrial function; thus, defects in mitochondrial quality control are also significant causes of neurodegenerative diseases. The mitochondrial deacetylase SIRT3 has been found to have a large effect on mitochondrial function. Recent studies have also shown that SIRT3 has a role in mitochondrial quality control, including in the refolding or degradation of misfolded/unfolded proteins, mitochondrial dynamics, mitophagy, and mitochondrial biogenesis, all of which are affected in neurodegenerative diseases.

Dichotomous Sirtuins: Implications for Drug Discovery in Neurodegenerative and Cardiometabolic Diseases

Sirtuins (SIRT1-7), a class of NAD⁺-dependent deacylases, are central regulators of metabolic homeostasis and stress responses. While numerous salutary effects associated with sirtuin activation, especially SIRT1, are well documented, other reports show health benefits resulting from sirtuin inhibition. Furthermore, conflicting findings have been obtained regarding the pathophysiological role of specific sirtuin isoforms, suggesting that sirtuins act as 'double-edged swords'. Here, we provide an integrated overview of the different findings on the role of mammalian sirtuins in neurodegenerative and cardiometabolic disorders and attempt to dissect the reasons behind these different effects. Finally, we discuss how addressing these obstacles may provide a better understanding of the complex sirtuin biology and improve the likelihood of identifying effective and selective drug targets for a variety of human disorders.

Melatonin ameliorates cerebral ischemia/reperfusion injury through SIRT3 activation

AIMS

Previous literature has shown that melatonin plays a critical role in protecting against cerebral ischemia/reperfusion (I/R) injury. Sirtuin3(SIRT3), as one member of the sirtuin family, protects against oxidative stress-related diseases. However, the association between melatonin and SIRT3 in cerebral I/R injury is not well understood. Our experiment was planned to investigate whether melatonin protects against cerebral I/R injury through SIRT3 activation.

MAIN METHODS

We selected transient middle cerebral artery occlusion (tMCAO) mice as the model of cerebral I/R injury. Male C57/BL6 mice were pre-treated with or without a selective SIRT3 inhibitor and then subjected to tMCAO surgery. Melatonin (20 mg/kg) was given to mice by intraperitoneal injection after ischemia and before reperfusion. Then, we observed the changes in the SIRT3 and downstream relative proteins, infarction volume, neurological score, Nissl, H&E and TUNEL staining, and the expression of apoptosis proteins after tMCAO.

KEY FINDINGS

Melatonin upregulated the expression of SIRT3 after tMCAO, and alleviated the neurological dysfunction and cell apoptosis through SIRT3 activation.

SIGNIFICANCE

Our research proved that melatonin promoted SIRT3 expression after tMCAO and alleviated cerebral I/R injury by activating the SIRT3 signaling pathway. This study provides novel therapeutic targets and mechanisms for the treatment of ischemic stroke in the clinic, especially during cerebrovascular reperfusion.

The Role of Mitochondrial and Endoplasmic Reticulum Reactive Oxygen Species Production in Models of Perinatal Brain Injury

Significance: Perinatal brain injury is caused by hypoxia-ischemia (HI) in term neonates, perinatal arterial stroke, and infection/inflammation leading to devastating long-term neurodevelopmental deficits. Therapeutic hypothermia is the only currently available treatment but is not successful in more than 50% of term neonates suffering from hypoxic-ischemic encephalopathy. Thus, there is an urgent unmet need for alternative or adjunct therapies. Reactive oxygen species (ROS) are important for physiological signaling, however, their overproduction/accumulation from mitochondria and endoplasmic reticulum (ER) during HI aggravate cell death. Recent Advances and Critical Issues: Mechanisms underlying ER stress-associated ROS production have been primarily elucidated using either non-neuronal cells or adult neurodegenerative experimental models. Findings from mature brain cannot be simply transferred to the immature brain. Therefore, age-specific studies investigating ER stress modulators may help investigate ER stress-associated ROS pathways in the immature brain. New therapeutics such as mitochondrial site-specific ROS inhibitors that selectively inhibit superoxide (O2•-)/hydrogen peroxide (H2O2) production are currently being developed. Future Directions: Because ER stress and oxidative stress accentuate each other, a combinatorial therapy utilizing both antioxidants and ER stress inhibitors may prove to be more protective against perinatal brain injury. Moreover, multiple relevant targets need to be identified for targeting ROS before they are formed. The role of organelle-specific ROS in brain repair needs investigation. Antioxid. Redox Signal. 31, 643-663.

Mitochondrial protein import is regulated by p17/PERMIT to mediate lipid metabolism and cellular stress

How lipid metabolism is regulated at the outer mitochondrial membrane (OMM) for transducing stress signaling remains largely unknown. We show here that this process is controlled by trafficking of ceramide synthase 1 (CerS1) from the endoplasmic reticulum (ER) to the OMM by a previously uncharacterized p17, which is now renamed protein that mediates ER-mitochondria trafficking (PERMIT). Data revealed that p17/PERMIT associates with newly translated CerS1 on the ER surface to mediate its trafficking to the OMM. Cellular stress induces Drp1 nitrosylation/activation, releasing p17/PERMIT to retrieve CerS1 for its OMM trafficking, resulting in mitochondrial ceramide generation, mitophagy and cell death. In vivo, CRISPR-Cas9-dependent genetic ablation of p17/PERMIT prevents acute stress-mediated CerS1 trafficking to OMM, attenuating mitophagy in p17/PERMIT^-/- mice, compared to controls, in various metabolically active tissues, including brain, muscle, and pancreas. Thus, these data have implications in diseases associated with accumulation of damaged mitochondria such as cancer and/or neurodegeneration.

Annexin A1 Bioactive Peptide Promotes Resolution of Neuroinflammation in a Rat Model of Exsanguinating Cardiac Arrest Treated by Emergency Preservation and Resuscitation

Neuroinflammation initiated by damage-associated molecular patterns, including high mobility group box 1 protein (HMGB1), has been implicated in adverse neurological outcomes following lethal hemorrhagic shock and polytrauma. Emergency preservation and resuscitation (EPR) is a novel method of resuscitation for victims of exsanguinating cardiac arrest, shown in preclinical studies to improve survival with acceptable neurological recovery. Sirtuin 3 (SIRT3), the primary mitochondrial deacetylase, has emerged as a key regulator of metabolic and energy stress response pathways in the brain and a pharmacological target to induce a neuronal pro-survival phenotype. This study aims to examine whether systemic administration of an Annexin-A1 bioactive peptide (ANXA1sp) could resolve neuroinflammation and induce sirtuin-3 regulated cytoprotective pathways in a novel rat model of exsanguinating cardiac arrest and EPR. Adult male rats underwent hemorrhagic shock and ventricular fibrillation, induction of profound hypothermia, followed by resuscitation and rewarming using cardiopulmonary bypass (EPR). Animals randomly received ANXA1sp (3 mg/kg, in divided doses) or vehicle. Neuroinflammation (HMGB1, TNFα, IL-6, and IL-10 levels), cerebral cell death (TUNEL, caspase-3, pro and antiapoptotic protein levels), and neurologic scores were assessed to evaluate the inflammation resolving effects of ANXA1sp following EPR. Furthermore, western blot analysis and immunohistochemistry were used to interrogate the mechanisms involved. Compared to vehicle controls, ANXA1sp effectively reduced expression of cerebral HMGB1, IL-6, and TNFα and increased IL-10 expression, which were associated with improved neurological scores. ANXA1sp reversed EPR-induced increases in expression of proapoptotic protein Bax and reduction in antiapoptotic protein Bcl-2, with a corresponding decrease in cerebral levels of cleaved caspase-3. Furthermore, ANXA1sp induced autophagic flux (increased LC3II and reduced p62 expression) in the brain. Mechanistically, these findings were accompanied by upregulation of the mitochondrial protein deacetylase Sirtuin-3, and its downstream targets FOXO3a and MnSOD in ANXA1sp-treated animals. Our data provide new evidence that engaging pro-resolving pharmacological strategies such as Annexin-A1 biomimetic peptides can effectively attenuate neuroinflammation and enhance the neuroprotective effects of EPR after exsanguinating cardiac arrest.

Measuring Respiration in Isolated Murine Brain Mitochondria: Implications for Mechanistic Stroke Studies

Measuring mitochondrial respiration in brain tissue is very critical in understanding the physiology and pathology of the central nervous system. Particularly, measurement of respiration in isolated mitochondria provides the advantage over the whole cells or tissues as the changes in respiratory function are intrinsic to mitochondrial structures rather than the cellular signaling that regulates mitochondria. Moreover, a high-throughput technique for measuring mitochondrial respiration minimizes the experimental time and the sample-to-sample variation. Here, we provide a detailed protocol for measuring respiration in isolated brain non-synaptosomal mitochondria using Agilent Seahorse XFe24 Analyzer. We optimized the protocol for the amount of mitochondria and concentrations of ADP, oligomycin, and trifluoromethoxy carbonylcyanide phenylhydrazone (FCCP) for measuring respiratory parameters for complex I-mediated respiration. In addition, we measured complex II-mediated respiratory parameters. We observed that 10 µg of mitochondrial protein per well, ADP concentrations ranging between 2.5 and 10 mmol/L along with 5 µmol/L of oligomycin, and 5 µmol/L of FCCP are ideal for measuring the complex I-mediated respiration in isolated mouse brain mitochondria. Furthermore, we determined that 2.5 µg of mitochondrial protein per well is ideal for measuring complex II-mediated respiration. Notably, we provide a discussion of logical analysis of data and how the assay could be utilized to design mechanistic studies for experimental stroke. In conclusion, we provide detailed experimental design for measurement of various respiratory parameters in isolated brain mitochondria utilizing a novel high-throughput technique along with interpretation and analysis of data.

The Cross-Talk Between Sphingolipids and Insulin-Like Growth Factor Signaling: Significance for Aging and Neurodegeneration

Bioactive sphingolipids: sphingosine, sphingosine-1-phosphate (S1P), ceramide, and ceramide-1-phosphate (C1P) are increasingly implicated in cell survival, proliferation, differentiation, and in multiple aspects of stress response in the nervous system. The opposite roles of closely related sphingolipid species in cell survival/death signaling is reflected in the concept of tightly controlled sphingolipid rheostat. Aging has a complex influence on sphingolipid metabolism, disturbing signaling pathways and the properties of lipid membranes. A metabolic signature of stress resistance-associated sphingolipids correlates with longevity in humans. Moreover, accumulating evidence suggests extensive links between sphingolipid signaling and the insulin-like growth factor I (IGF-I)-Akt-mTOR pathway (IIS), which is involved in the modulation of aging process and longevity. IIS integrates a wide array of metabolic signals, cross-talks with p53, nuclear factor κB (NF-κB), or reactive oxygen species (ROS) and influences gene expression to shape the cellular metabolic profile and stress resistance. The multiple connections between sphingolipids and IIS signaling suggest possible engagement of these compounds in the aging process itself, which creates a vulnerable background for the majority of neurodegenerative disorders.

Astragaloside IV Protects Primary Cerebral Cortical Neurons from Oxygen and Glucose Deprivation/Reoxygenation by Activating the PKA/CREB Pathway

Stroke is one of the major leading causes of death and disability worldwide, and post-stroke cognitive impairment is a major contributor to this disability. Astragaloside IV (AST-IV) is a primary bioactive compound of Radix Astragali, which is widely used in traditional Chinese medicine to treat stroke. AST-IV was found to possess cognition-enhancing properties against ischemic stroke; however, the mechanisms underlying this effect remain largely elusive. Mitochondrial health is critical to cell viability after ischemic injury. Cyclic AMP response element-binding protein (CREB) is a transcription factor that can be activated by protein kinase A (PKA) to preserve mitochondria, regulate memory and cognitive functions. We used an in vitro model of ischemic injury via oxygen and glucose deprivation (OGD) of cultured neurons, which led to PKA inactivation and decreased CREB phosphorylation, reduced cell viability, and increased neuronal apoptosis. We hypothesized that AST-IV could protect OGD-exposed cerebral cortical neurons by modulating the PKA/CREB signaling pathway and preserving mitochondrial function. We found that the mitochondrial and cellular injuries induced by OGD were reversed following treatment with AST-IV. The activity of neuronal mitochondria was evaluated by measuring the mitochondrial potential and the levels of reactive oxygen species (ROS) and adenosine triphosphate (ATP). AST-IV significantly enhanced PKA and CREB phosphorylation and prevented OGD-induced mitochondrial dysfunction, thereby protecting neurons exposed to OGD from injury and death. Furthermore, the effects of AST-IV were partially blocked by a PKA inhibitor. Collectively, these data elucidated the molecular mechanisms underlying the protective effects of AST-IV against ischemic injury in cortical neurons.

Acid sphingomyelinase deficiency protects mitochondria and improves function recovery after brain injury

Traumatic brain injury (TBI) is one of the leading causes of disability worldwide and a prominent risk factor for neurodegenerative diseases. The expansion of nervous tissue damage after the initial trauma involves a multifactorial cascade of events, including excitotoxicity, oxidative stress, inflammation, and deregulation of sphingolipid metabolism that further mitochondrial dysfunction and secondary brain damage. Here, we show that a posttranscriptional activation of an acid sphingomyelinase (ASM), a key enzyme of the sphingolipid recycling pathway, resulted in a selective increase of sphingosine in mitochondria during the first week post-TBI that was accompanied by reduced activity of mitochondrial cytochrome oxidase and activation of the Nod-like receptor protein 3 inflammasome. TBI-induced mitochondrial abnormalities were rescued in the brains of ASM KO mice, which demonstrated improved behavioral deficit recovery compared with WT mice. Furthermore, an elevated autophagy in an ASM-deficient brain at the baseline and during the development of secondary brain injury seems to foster the preservation of mitochondria and brain function after TBI. Of note, ASM deficiency attenuated the early stages of reactive astrogliosis progression in an injured brain. These findings highlight the crucial role of ASM in governing mitochondrial dysfunction and brain-function impairment, emphasizing the importance of sphingolipids in the neuroinflammatory response to TBI.

SIRT3 Regulation Under Cellular Stress: Making Sense of the Ups and Downs

Sirtuin 3 (SIRT3) is an NAD+ dependent deacetylase that resides primarily in mitochondria and functions to maintain mitochondrial homeostasis under stress. SIRT3 expression has been observed to change under a number of different stresses in multiple tissues and model systems. Inconsistencies in the literature with regards to how and when SIRT3 protein levels change indicates that the mechanism of SIRT3 regulation is multi-faceted. Alterations in SIRT3 have been observed in experimental models of cellular stress, however, the effect these changes have on mitochondrial health remain unknown. Neurons are highly dependent on proper mitochondrial function for their survival. SIRT3 dynamics and function have been studied using models of genotoxic, metabolic, and oxidative stresses, although it remains unclear how SIRT3 is being regulated under these conditions. A closer look into SIRT3 regulation under stress conditions in various model systems will help incorporate the many SIRT3 regulatory mechanisms at play in disease states. In this review, we describe the observations that have been made about SIRT3 protein modulation under basic stress conditions. We then point out consistencies and contradictions in these observations and what they mean. Lastly, we present the observations made in the complicated neuronal stress of stroke. We hope that this review will help consolidate the ambiguous SIRT3 literature and provide a framework for investigation of SIRT3 regulation during stress response.

Ceramide synthase-6 confers resistance to chemotherapy by binding to CD95/Fas in T-cell acute lymphoblastic leukemia

Ceramide synthases (CERS) produce ceramides which are key intermediators in the biosynthesis of complex sphingolipids and play an important role in cell proliferation, differentiation, apoptosis and senescence. CERS6 is an isoform of ceramide synthases known to generate ceramides with C16 acyl chain (C₁₆-Cer). CERS6 and C₁₆-Cer levels were significantly higher in acute lymphoblastic leukemia (ALL) cells in comparison to peripheral blood mononuclear cells and T lymphocytes derived from healthy human volunteers. We investigated the role of CERS6 in chemo-resistance in T-ALL cell lines. Stable knockdown of CERS6 in CCRF-CEM and MOLT-4 cells resulted in increased sensitivity to ABT-737, a pan-BCL-2 inhibitor, while CCRF-CEM cells with exogenous CERS6 expression showed resistance to ABT-737 relative to the vector control. The cytotoxic activity of ABT-737 in CERS6 knockdown cells was significantly reduced by the addition of a caspase-8 inhibitor Z-IETD, suggesting that CERS6 alters the cytotoxicity via extrinsic pathway of apoptosis. By co-immunoprecipitation of CERS6 in CCRF-CEM cells, we identified CD95/Fas, a mediator of extrinsic apoptotic pathway, as a novel CERS6 binding partner. In Fas pull-down samples, FADD (Fas-associated protein with death domain) was detected at higher levels in cells with CERS6 knockdown compared with control cells when treated with ABT-737, and this was reversed by the overexpression of CERS6, demonstrating that CERS6 interferes with Fas–FADD DISC assembly. CERS6 may serve as a biomarker in determining the effectiveness of anticancer agents acting via the extrinsic pathway in T-ALL.

Bioactive Lipid Species and Metabolic Pathways in Progression and Resolution of Nonalcoholic Steatohepatitis

The prevalence of nonalcoholic steatohepatitis (NASH) is increasing worldwide, yet there are no effective treatments. A decade has passed since the initial lipidomics analyses of liver tissues from patients with nonalcoholic fatty liver disease. We have learned that liver cells from patients with NASH have an abnormal lipid composition and that the accumulation of lipids leads to organelle dysfunction, cell injury and death, and chronic inflammation, called lipotoxicity. We review the lipid species and metabolic pathways that contribute to the pathogenesis of NASH and potential therapeutic targets, including enzymes involved in fatty acid and triglyceride synthesis, bioactive sphingolipids and polyunsaturated-derived eicosanoids, and specialized pro-resolving lipid mediators. We discuss the concept that NASH is a disease that can resolve and the roles of lipid molecules in the resolution of inflammation and regression of fibrosis.

The Role of SIRT3 in the Brain Under Physiological and Pathological Conditions

Sirtuin enzymes are a family of highly seven conserved protein deacetylases, namely SIRT1 through SIRT7, whose enzymatic activities require the cofactor nicotinamide adenine dinucleotide (NAD⁺). Sirtuins reside in different compartments within cells, and their activities have been shown to regulate a number of cellular pathways involved in but not limited to stress management, apoptosis and inflammatory responses. Given the importance of mitochondrial functional state in neurodegenerative conditions, the mitochondrial SIRT3 sirtuin, which is the primary deacetylase within mitochondria, has garnered considerable recent attention. It is now clear that SIRT3 plays a major role in regulating a host of mitochondrial molecular cascades that can contribute to both normal and pathophysiological processes. However, most of the currently available knowledge on SIRT3 stems from studies in non-neuronal cells, and the consequences of the interactions between SIRT3 and its targets in the CNS are only beginning to be elucidated. In this review, we will summarize current advances relating to SIRT3, and explore how its known functions could influence brain physiology.

The NAD+-Dependent Family of Sirtuins in Cerebral Ischemia and Preconditioning

SIGNIFICANCE

Sirtuins are an evolutionarily conserved family of NAD+-dependent lysine deacylases and ADP ribosylases. Their requirement for NAD+ as a cosubstrate allows them to act as metabolic sensors that couple changes in the energy status of the cell to changes in cellular physiological processes. NAD+ levels are affected by several NAD+-producing and NAD+-consuming pathways as well as by cellular respiration. Thus their intracellular levels are highly dynamic and are misregulated in a spectrum of metabolic disorders including cerebral ischemia. This, in turn, compromises several NAD+-dependent processes that may ultimately lead to cell death. Recent Advances: A number of efforts have been made to replenish NAD+ in cerebral ischemic injuries as well as to understand the functions of one its important mediators, the sirtuin family of proteins through the use of pharmacological modulators or genetic manipulation approaches either before or after the insult. Critical Issues and Future Directions: The results of these studies have regarded the sirtuins as promising therapeutic targets for cerebral ischemia. Yet, additional efforts are needed to understand the role of some of the less characterized members and to address the sex-specific effects observed with some members. Sirtuins also exhibit cell-type-specific expression in the brain as well as distinct subcellular and regional localizations. As such, they are involved in diverse and sometimes opposing cellular processes that can either promote neuroprotection or further contribute to the injury; which also stresses the need for the development and use of sirtuin-specific pharmacological modulators. Antioxid. Redox Signal. 28, 691-710.

Sphingolipids and their metabolism in physiology and disease

Studies of bioactive lipids in general and sphingolipids in particular have intensified over the past several years, revealing an unprecedented and unanticipated complexity of the lipidome and its many functions, which rivals, if not exceeds, that of the genome or proteome. These results highlight critical roles for bioactive sphingolipids in most, if not all, major cell biological responses, including all major cell signalling pathways, and they link sphingolipid metabolism to key human diseases. Nevertheless, the fairly nascent field of bioactive sphingolipids still faces challenges in its biochemical and molecular underpinnings, including defining the molecular mechanisms of pathway and enzyme regulation, the study of lipid-protein interactions and the development of cellular probes, suitable biomarkers and therapeutic approaches.

Ceramide and Ischemia/Reperfusion Injury

Ceramide, a bioactive membrane sphingolipid, functions as an important second messenger in apoptosis and cell signaling. In response to stresses, it may be generated by de novo synthesis, sphingomyelin hydrolysis, and/or recycling of complex sphingolipids. It is cleared from cells through the activity of ceramidases, phosphorylation to ceramide-1-phosphate, or resynthesis into more complex sphingolipids. Ischemia/reperfusion (IR) injury occurs when oxygen/nutrition is rapidly reintroduced into ischemic tissue, resulting in cell death and tissue damage, and is a major concern in diverse clinical settings, including organ resection and transplantation. Numerous reports show that ceramide levels are markedly elevated during IR. Mitochondria are major sites of reactive oxygen species (ROS) production and play a key role in IR-induced and ceramide-mediated cell death and tissue damage. During the development of IR injury, the initial response of ROS and TNF-alpha production activates two major ceramide generating pathways (sphingomyelin hydrolysis and de novo ceramide synthesis). The increased ceramide has broad effects depending on the IR phases, including both pro- and antiapoptotic effects. Therefore, strategies that reduce the levels of ceramide, for example, by modulation of ceramidase and/or sphingomyelinases activities, may represent novel and promising therapeutic approaches to prevent or treat IR injury in diverse clinical settings.

Acid sphingomyelinase promotes mitochondrial dysfunction due to glutamate-induced regulated necrosis

Inhibiting the glutamate/cystine antiporter system x_c^-, a key antioxidant defense machinery in the CNS, could trigger a novel form of regulated necrotic cell death, ferroptosis. The underlying mechanisms of system x_c^--dependent cell demise were elucidated using primary oligodendrocytes (OLs) treated with glutamate to block system x_c^- function. Pharmacological analysis revealed ferroptosis as a major contributing factor to glutamate-initiated OL death. A sphingolipid profile showed elevations of ceramide species and sphingosine that were preventable by inhibiting of an acid sphingomyelinase (ASM) activity. OL survival was enhanced by both downregulating ASM expression and blocking ASM activity. Glutamate-induced ASM activation seems to involve posttranscriptional mechanisms and was associated with a decreased GSH level. Further investigation of the mechanisms of OL response to glutamate revealed enhanced reactive oxygen species production, augmented lipid peroxidation, and opening of the mitochondrial permeability transition pore that were attenuated by hindering ASM. Of note, knocking down sirtuin 3, a deacetylase governing the mitochondrial antioxidant system, reduced OL survival. The data highlight the importance of the mitochondrial compartment in regulated necrotic cell death and accentuate the novel role of ASM in disturbing mitochondrial functions during OL response to glutamate toxicity, which is essential for pathobiology in stroke and traumatic brain injury.