Attenuation of Ischemic Stroke-Caused Brain Injury by a Monoamine Oxidase Inhibitor Involves Improved Proteostasis and Reduced Neuroinflammation

UBQLN1 links proteostasis and mitochondria function to telomere maintenance in human embryonic stem cells

BACKGROUND

Telomeres consist of repetitive DNA sequences at the chromosome ends to protect chromosomal stability, and primarily maintained by telomerase or occasionally by alternative telomere lengthening of telomeres (ALT) through recombination-based mechanisms. Additional mechanisms that may regulate telomere maintenance remain to be explored. Simultaneous measurement of telomere length and transcriptome in the same human embryonic stem cell (hESC) revealed that mRNA expression levels of UBQLN1 exhibit linear relationship with telomere length.

METHODS

In this study, we first generated UBQLN1-deficient hESCs and compared with the wild-type (WT) hESCs the telomere length and molecular change at RNA and protein level by RNA-seq and proteomics. Then we identified the potential interacting proteins with UBQLN1 using immunoprecipitation-mass spectrometry (IP-MS). Furthermore, the potential mechanisms underlying the shortened telomeres in UBQLN1-deficient hESCs were analyzed.

RESULTS

We show that Ubiquilin1 (UBQLN1) is critical for telomere maintenance in human embryonic stem cells (hESCs) via promoting mitochondrial function. UBQLN1 deficiency leads to oxidative stress, loss of proteostasis, mitochondria dysfunction, DNA damage, and telomere attrition. Reducing oxidative damage and promoting mitochondria function by culture under hypoxia condition or supplementation with N-acetylcysteine partly attenuate the telomere attrition induced by UBQLN1 deficiency. Moreover, UBQLN1 deficiency/telomere shortening downregulates genes for neuro-ectoderm lineage differentiation.

CONCLUSIONS

Altogether, UBQLN1 functions to scavenge ubiquitinated proteins, preventing their overloading mitochondria and elevated mitophagy. UBQLN1 maintains mitochondria and telomeres by regulating proteostasis and plays critical role in neuro-ectoderm differentiation.

Advances in the study of the glymphatic system and aging

The glymphatic system is cerebrospinal fluid-brain tissue fluid exchange flow mediated by aquaporin-4 (AQP4) on the end feet of astrocytes for a system, which is capable of rapidly removing brain metabolites and thus maintaining brain homeostasis, and is known as the central immune system. Dysfunction of the glymphatic system causes accumulation of misfolded and highly phosphorylated proteins (amyloid-β and Tau proteins), which destabilizes the proteins, and the body's neuroinflammatory factors are altered causing aging of the immune system and leading to neurodegenerative diseases. Damage to the glymphatic system and aging share common manifestations, as well as unstudied biological mechanisms that are also linked, such as mitochondria, oxidative stress, chronic inflammation, and sleep. In this paper, we first summarize the structure, function, and research methods of the glymphatic system and the relationship between the glymphatic system and the peripheral immune system, and second, sort out and summarize the factors of the glymphatic system in removing metabolites and resolving aging-related diseases and factors affecting aging, to explore its related biological mechanisms, and moreover, to provide a new way of thinking for treating or intervening aging-related diseases.

Fisetin alleviates cerebral ischemia/reperfusion injury by regulating Sirt1/Foxc1/Ubqln1 pathway-mediated proteostasis

BACKGROUND

Cerebral ischemia/reperfusion injury (IRI) is pathologically associated with protein damage. The flavonoid fisetin has good therapeutic effects on cerebral IRI. However, the role of fisetin in regulating protein damage during cerebral IRI development remains unclear. This study investigated the pharmacological effects of fisetin on protein damage during cerebral IRI progression and defined the underlying mechanism of action.

METHODS

In vivo and in vitro models of cerebral IRI were established by middle cerebral artery occlusion/reperfusion (MACO/R) and oxygen-glucose deprivation/reperfusion (OGD/R) treatment, respectively. Triphenyl tetrazolium chloride staining was performed to detect cerebral infarct size, and the modified neurologic severity score was used to examine neurological deficits. LDH activity and protein damage were assessed using kits. HT22 cell vitality and apoptosis were examined using CCK-8 assay and TUNEL staining, respectively. Interactions between Foxc1, Ubqln1, Sirt1, and Ezh2 were analyzed using CoIP, ChIP and/or dual-luciferase reporter gene assays.

RESULTS

Fisetin alleviated protein damage and ubiquitinated protein aggregation and neuronal death caused by MCAO/R and OGD/R. Ubqln1 knockdown abrogated the inhibitory effect of fisetin on OGD/R-induced protein damage, ubiquitinated protein aggregation, and neuronal death in HT22 cells. Further experiments demonstrated that Foxc1 functions as a transcriptional activator of Ubqln1 and that Sirt1 promotes Foxc1 expression by deacetylating Ezh2 and inhibiting its activity. Furthermore, Sirt1 knockdown abrogated fisetin-mediated biological effects on OGD/R-treated HT22 cells.

CONCLUSION

Fisetin improved proteostasis during cerebral IRI by regulating the Sirt1/Foxc1/Ubqln1 signaling axis. Our findings strongly suggest that fisetin-mediated inhibition of protein damage after ischemic stroke is a part of the mechanism through which fisetin is neuroprotective in cerebral IRI.

Cardioprotective Effect of 2-Ethyl-3-Hydroxy-6-Methylpyridinium 2-Nitroxysuccinate Against Adrenaline/Hydrocortisone-Induced Myocardial Ischemia in Mice: Modulation of Free-Radical Processes in Biomembranes and Monoamine Oxidase A Activity

Oxidative stress (OS) plays a key role in the development of cardiovascular diseases (CVD) in three major ways: reactive oxygen species (ROS)-induced reduction of nitric oxide (NO) bioavailability, ROS-induced inflammation and ROS-induced mitochondrial dysfunction. Oxidation of lipid molecules under the action of ROS leads to damage to membrane structures, changes the functioning of membrane-bound enzymes, and impairs membrane permeability and stability. An increase in OS results in the occurrence of endothelial dysfunction and drug tolerance, side effects, requiring discontinuation of drugs. All of these are significant problems of cardiotherapy. Therefore, the search for new alternative NO donors continues. The present research was aimed at studying the protective effect of 2-ethyl-3-hydroxy-6-methylpyridinium 2-nitroxysuccinate (NS) on the cardiovascular system on mouse myocardial ischemia (MI) model. The NS hybrid molecule includes a synthetic vitamin B6 analog 2-ethyl-3-hydroxy-6-methylpyridine (an antioxidant) and 2-nitroxysuccinic acid (a source of nitric oxide). Using the electron paramagnetic resonance (EPR) method and biochemical methods, we showed that the pronounced ability of NS to release NO is favorably combines with the capacity to prevent OS due to mechanisms such as suppression of the lipid peroxidation (LPO) process, antiradical activity and inhibition of the mitochondrial membrane-bound monoamine oxidase A (MAO-A). Using histological methods, we established that the administration of NS (10 mg/kg, i.p.) reduces the number of ischemic fibers and protects cardiomyocytes against ischemia injury. Thus, the complex protective effect allows us to consider NS as an alternative NO donor and a candidate for the development of a new pharmaceutical agent for the treatment of CVD.

Inhibition of monoamine oxidase-a increases respiration in isolated mouse cortical mitochondria

Monoamine oxidase (MAO) is an enzyme located on the outer mitochondrial membrane that metabolizes amine substrates like serotonin, norepinephrine and dopamine. MAO inhibitors (MAOIs) are frequently utilized to treat disorders such as major depression or Parkinson's disease (PD), though their effects on brain mitochondrial bioenergetics are unclear. These studies measured bioenergetic activity in mitochondria isolated from the mouse cortex in the presence of inhibitors of either MAO-A, MAO-B, or both isoforms. We found that only 10 μM clorgyline, the selective inhibitor of MAO-A and not MAO-B, increased mitochondrial oxygen consumption rate in State V(CI) respiration compared to vehicle treatment. We then assessed mitochondrial bioenergetics, reactive oxygen species (ROS) production, and Electron Transport Chain (ETC) complex function in the presence of 0, 5, 10, 20, 40, or 80 μM of clorgyline to determine if this change was dose-dependent. The results showed increased oxygen consumption rates across the majority of respiration states in mitochondria treated with 5, 10, or 20 μM with significant bioenergetic inhibition at 80 μM clorgyline. Next, we assessed mitochondrial ROS production in the presence of the same concentrations of clorgyline in two different states: high mitochondrial membrane potential (ΔΨm) induced by oligomycin and low ΔΨm induced by carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone (FCCP). There were no changes in ROS production in the presence of 5, 10, 20, or 40 μM clorgyline compared to vehicle after the addition of oligomycin or FCCP. There was a significant increase in mitochondrial ROS in the presence of 80 μM clorgyline after FCCP addition, as well as reduced Complex I and Complex II activities, which are consistent with inhibition of bioenergetics seen at this dose. There were no changes in Complex I, II, or IV activities in mitochondria treated with low doses of clorgyline. These studies shed light on the direct effect of MAO-A inhibition on brain mitochondrial bioenergetic function, which may be a beneficial outcome for those taking these medications.

Inflammation, Lifestyle Factors, and the Microbiome-Gut-Brain Axis: Relevance to Depression and Antidepressant Action

Depression is considered a major public health concern, where existing pharmacological treatments are not equally effective across all patients. The pathogenesis of depression involves the interaction of complex biological components, such as the immune system and the microbiota-gut-brain axis. Adjunctive lifestyle-oriented approaches for depression, including physical exercise and special diets are promising therapeutic options when combined with traditional antidepressants. However, the mechanisms of action of these strategies are incompletely understood. Accumulating evidence suggests that physical exercise and specific dietary regimens can modulate both the immune system and gut microbiota composition. Here, we review the current information about the strategies to alleviate depression and their crosstalk with both inflammatory mechanisms and the gut microbiome. We further discuss the role of the microbiota-gut-brain axis as a possible mediator for the adjunctive therapies for depression through inflammatory mechanisms. Finally, we review existing and future adjunctive strategies to manipulate the gut microbiota with potential use for depression, including physical exercise, dietary interventions, prebiotics/probiotics, and fecal microbiota transplantation.

Glymphatic Dysfunction Induced Oxidative Stress and Neuro-Inflammation in Major Depression Disorders

Major Depression disorder (MDD) is a potentially life-threatening mental illness, however, many patients have a poor response to current treatments. Recent studies have suggested that stress- or trauma-induced oxidative stress and inflammation could be important factors involved in the development of MDD, but the mechanisms remain unclear. We showed that the glymphatic system is a recently discovered structure in the brain that may be involved in the clearance of large molecular and cell debris in extracellular space. In addition, the glymphatic system can help with the removal of reactive oxygen species (ROS) and cytokines such as IL-1β and HIF-1α. Glymphatic impairment can lead to ROS accumulation in the microenvironment, inducing cellular injury signaling and activating NLRP3 in microglia to induce inflammation and, thus, many brain diseases, including psychiatric disorders. Therefore, trauma-induced glymphatic impairment could induce oxidative stress and inflammation, and thus MDD. This paper will review recent advances with regard to stress-induced glymphatic system impairment and ROS-mediated inflammation in MDD.

A monoamine oxidase B inhibitor ethyl ferulate suppresses microglia-mediated neuroinflammation and alleviates ischemic brain injury

Microglia are the resident macrophages in the brain, which play a critical role in post-stroke neuroinflammation. Accordingly, targeting neuroinflammation could be a promising strategy to improve ischemic stroke outcomes. Ethyl ferulate (EF) has been confirmed to possess anti-inflammatory properties in several disease models, including acute lung injury, retinal damage and diabetes-associated renal injury. However, the effects of EF on microglial activation and the resolution of post-stroke neuroinflammation remains unknown. Here, we found that EF suppressed pro-inflammatory response triggered by lipopolysaccharide (LPS) stimulation in primary microglia and BV2 cell lines, as well as post-stroke neuroinflammation in an in vivo transient middle cerebral artery occlusion (tMCAO) stroke model in C57BL/6 mice, consequently ameliorating ischemic brain injury. Furthermore, EF could directly bind and inhibit the activity of monoamine oxidase B (MAO-B) to reduce pro-inflammatory response. Taken together, our study identified a MAO-B inhibitor, Ethyl ferulate, as an active compound with promising potentials for suppressing post-stroke neuroinflammation.

UBQLN proteins in health and disease with a focus on UBQLN2 in ALS/FTD

Ubiquilin (UBQLN) proteins are a dynamic and versatile family of proteins found in all eukaryotes that function in the regulation of proteostasis. Besides their canonical function as shuttle factors in delivering misfolded proteins to the proteasome and autophagy systems for degradation, there is emerging evidence that UBQLN proteins play broader roles in proteostasis. New information suggests the proteins function as chaperones in protein folding, protecting proteins prior to membrane insertion, and as guardians for mitochondrial protein import. In this review, we describe the evidence for these different roles, highlighting how different domains of the proteins impart these functions. We also describe how changes in the structure and phase separation properties of UBQLNs may regulate their activity and function. Finally, we discuss the pathogenic mechanisms by which mutations in UBQLN2 cause amyotrophic lateral sclerosis and frontotemporal dementia. We describe the animal model systems made for different UBQLN2 mutations and how lessons learnt from these systems provide fundamental insight into the molecular mechanisms by which UBQLN2 mutations drive disease pathogenesis through disturbances in proteostasis.

Monoamine Neurotransmitters Control Basic Emotions and Affect Major Depressive Disorders

Major depressive disorder (MDD) is a common and complex mental disorder, that adversely impacts an individual’s quality of life, but its diagnosis and treatment are not accurately executed and a symptom-based approach is utilized in most cases, due to the lack of precise knowledge regarding the pathophysiology. So far, the first-line treatments are still based on monoamine neurotransmitters. Even though there is a lot of progress in this field, the mechanisms seem to get more and more confusing, and the treatment is also getting more and more controversial. In this study, we try to review the broad advances of monoamine neurotransmitters in the field of MDD, and update its effects in many advanced neuroscience studies. We still propose the monoamine hypothesis but paid special attention to their effects on the new pathways for MDD, such as inflammation, oxidative stress, neurotrophins, and neurogenesis, especially in the glial cells, which have recently been found to play an important role in many neurodegenerative disorders, including MDD. In addition, we will extend the monoamine hypothesis to basic emotions; as suggested in our previous reports, the three monoamine neurotransmitters play different roles in emotions: dopamine—joy, norepinephrine—fear (anger), serotonins—disgust (sadness). Above all, this paper tries to give a full picture of the relationship between the MDD and the monoamine neurotransmitters such as DA, NE, and 5-HT, as well as their contributions to the Three Primary Color Model of Basic Emotions (joy, fear, and disgust). This is done by explaining the contribution of the monoamine from many sides for MDD, such the digestive tract, astrocytes, microglial, and others, and very briefly addressing the potential of monoamine neurotransmitters as a therapeutic approach for MDD patients and also the reasons for its limited clinical efficacy, side effects, and delayed onset of action. We hope this review might offer new pharmacological management of MDD.

Stroke-Heart Syndrome: Recent Advances and Challenges

After ischemic stroke, there is a significant burden of cardiovascular complications, both in the acute and chronic phase. Severe adverse cardiac events occur in 10% to 20% of patients within the first few days after stroke and comprise a continuum of cardiac changes ranging from acute myocardial injury and coronary syndromes to heart failure or arrhythmia. Recently, the term stroke–heart syndrome was introduced to provide an integrated conceptual framework that summarizes neurocardiogenic mechanisms that lead to these cardiac events after stroke. New findings from experimental and clinical studies have further refined our understanding of the clinical manifestations, pathophysiology, and potential long‐term consequences of the stroke–heart syndrome. Local cerebral and systemic mediators, which mainly involve autonomic dysfunction and increased inflammation, may lead to altered cardiomyocyte metabolism, dysregulation of (tissue‐resident) leukocyte populations, and (micro‐) vascular changes. However, at the individual patient level, it remains challenging to differentiate between comorbid cardiovascular conditions and stroke‐induced heart injury. Therefore, further research activities led by joint teams of basic and clinical researchers with backgrounds in both cardiology and neurology are needed to identify the most relevant therapeutic targets that can be tested in clinical trials.

Impact of Cardiovascular Diseases on Ischemic Stroke Outcomes

Stroke induces complex pathological cascades in the affected brain area, leading to brain injury and functional disability. To fight against cerebral ischemia/reperfusion-induced neuronal death, numerous neuroprotective strategies and reagents have been studied. However, translation of these neuroprotective drugs to clinical trials has been unsuccessful. To date, the tissue plasminogen activator is still the only FDA-approved drug for treating ischemic stroke. Thus, it is obligatory to identify and validate additional therapeutic strategies for stroke. A stroke rarely occurs without any other pathophysiological condition; but instead, it often has multi-morbidity conditions, one of which is cardiac disease. Indeed, up to half of the stroke cases are associated with cardiac and large artery diseases. As an adequate blood supply is essential for the brain to maintain its normal function, any pathophysiological alterations in the heart are frequently implicated in stroke outcomes. In this review, we summarize some of the cardiovascular factors that influence stroke outcomes and propose that considering these factors in designing stroke therapies should enhance success in clinical trials. We also highlight the recent advances regarding the potential effect of protein aggregates in a peripheral organ, such as in the heart, on ischemic stroke-caused brain injury and functional recovery. Including these and other comorbidity factors in the future therapeutic strategy designs should facilitate translational success toward developing effective combinational therapies for the disorder.

GABAA Receptor-Stabilizing Protein Ubqln1 Affects Hyperexcitability and Epileptogenesis after Traumatic Brain Injury and in a Model of In Vitro Epilepsy in Mice

Posttraumatic epilepsy (PTE) is a major public health concern and strongly contributes to human epilepsy cases worldwide. However, an effective treatment and prevention remains a matter of intense research. The present study provides new insights into the gamma aminobutyric acid A (GABA_A)-stabilizing protein ubiquilin-1 (ubqln1) and its regulation in mouse models of traumatic brain injury (TBI) and in vitro epilepsy. We performed label-free quantification on isolated cortical GABAergic interneurons from GAD67-GFP mice that received unilateral TBI and discovered reduced expression of ubqln1 24 h post-TBI. To investigate the link between this regulation and the development of epileptiform activity, we further studied ubqln1 expression in hippocampal and cortical slices. Epileptiform events were evoked pharmacologically in acute brain slices by administration of picrotoxin (PTX, 50 μM) and kainic acid (KA, 500 nM) and recorded in the hippocampal CA1 subfield using Multi-electrode Arrays (MEA). Interestingly, quantitative Western blots revealed significant decreases in ubqln1 expression 1–7 h after seizure induction that could be restored by application of the non-selective monoamine oxidase inhibitor nialamide (NM, 10 μM). In picrotoxin-dependent dose–response relationships, NM administration alleviated the frequency and peak amplitude of seizure-like events (SLEs). These findings indicate a role of the monoamine transmitter systems and ubqln1 for cortical network activity during posttraumatic epileptogenesis.

Coexpression of Gene Transcripts with Monoamine Oxidase A Quantified by Human In Vivo Positron Emission Tomography

The monoamine oxidase A (MAO-A) is integral to monoamine metabolism and is thus relevant to the pathophysiology of various neuropsychiatric disorders; however, associated gene-enzyme relations are not well understood. This study aimed to unveil genes coexpressed with MAO-A. Therefore, 18 179 mRNA expression maps (based on the Allen Human Brain Atlas) were correlated with the cerebral distribution volume (VT) of MAO-A assessed in 36 healthy subjects (mean age ± standard deviation: 32.9 ± 8.8 years, 18 female) using [11C]harmine positron emission tomography scans. Coexpression analysis was based on Spearman's ρ, over-representation tests on Fisher's exact test with false discovery rate (FDR) correction. The analysis revealed 35 genes in cortex (including B-cell translocation gene family, member 3, implicated in neuroinflammation) and 247 genes in subcortex (including kallikrein-related peptidase 10, implicated in Alzheimer's disease). Significantly over-represented Gene Ontology terms included "neuron development", "neuron differentiation", and "cell-cell signaling" as well as "axon" and "neuron projection". In vivo MAO-A enzyme distribution and MAOA expression did not correlate in cortical areas (ρ = 0.08) while correlation was found in subcortical areas (ρ = 0.52), suggesting influences of region-specific post-transcriptional and -translational modifications. The herein reported information could contribute to guide future genetic studies, deepen the understanding of associated pathomechanisms and assist in the pursuit of novel therapeutic targets.

The Repertoire of Small-Molecule PET Probes for Neuroinflammation Imaging: Challenges and Opportunities beyond TSPO

Neuroinflammation is an adaptive response of the central nervous system to diverse potentially injurious stimuli, which is closely associated with neurodegeneration and typically characterized by activation of microglia and astrocytes. As a noninvasive and translational molecular imaging tool, positron emission tomography (PET) could provide a better understanding of neuroinflammation and its role in neurodegenerative diseases. Ligands to translator protein (TSPO), a putative marker of neuroinflammation, have been the most commonly studied in this context, but they suffer from serious limitations. Herein we present a repertoire of different structural chemotypes and novel PET ligand design for classical and emerging neuroinflammatory targets beyond TSPO. We believe that this Perspective will support multidisciplinary collaborations in academic and industrial institutions working on neuroinflammation and facilitate the progress of neuroinflammation PET probe development for clinical use.

Monoamine Oxidase-B Inhibitor Reduction in Pro-Inflammatory Cytokines Mediated by Inhibition of cAMP-PKA/EPAC Signaling

Mucosal epithelial cell integrity is an important component of innate immunity and it protects the host from an environment rich in microorganisms. Virulence factors from Gram-negative bacteria [e.g. lipopolysaccharide (LPS)] induce significant pro-inflammatory cytokine expression. Monoamine oxidase (MAO) inhibitors reduce cytokine expression in a variety of inflammatory models and may therefore have therapeutic potential for a number of inflammatory diseases. We tested the anti-inflammatory therapeutic potential of a recently developed reversible MAO-B inhibitor (RG0216) with reduced transport across the blood–brain barrier. In an epithelial cell culture model, RG0216 significantly decreased LPS-induced interleukin (IL)-6 and IL-1β gene and protein expression and was as effective as equimolar concentrations of deprenyl (an existing irreversible MAO-B inhibitor). Hydrogen peroxide and modulating dopamine receptor signaling had no effect on cytokine expression. We showed that LPS-induced expression of IL-6 and IL-1β was cAMP dependent, that IL-6 and IL-1β expression were induced by direct cAMP activation (forskolin) and that RG0216 and deprenyl effectively reduced cAMP-mediated cytokine expression. Targeted protein kinase A (PKA) and Exchange Protein Activated by cAMP (EPAC) activation regulated IL-6 and IL-1β expression, albeit in different ways, but both cytokines were effectively decreased with RG0216. RG0216 reduction of LPS-induced cytokine expression occurred by acting downstream of the cAMP-PKA/EPAC signaling cascade. This represents a novel mechanism by which MAO-B selective inhibitors regulate LPS-induced IL-6 and IL-1β expression.

Insights into the Mechanism of the Therapeutic Potential of Herbal Monoamine Oxidase Inhibitors in Neurological Diseases

Monoamine oxidase (MAO) is an enzyme that catalyzes the deamination of monoamines and other proteins. MAO's hyperactivation results in the massive generation of reactive oxygen species, which leads to a variety of neurological diseases such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, Huntington's disease, and depression-like disorders. Although synthetic MAO inhibitors are clinically available, they are associated with side effects such as hepatotoxicity, cheese reaction, hypertensive crisis, and so on, necessitating the investigation of alternative MAO inhibitors from a natural source with a safe profile. Herbal medications have a significant impact on the prevention of many diseases; additionally, they have fewer side effects and serve as a precursor for drug development. This review discusses the potential of herbal MAO inhibitors as well as their associated mechanism of action, with an aim to foster future research on herbal MAO inhibitors as potential treatment for neurological diseases.

Monoamine Oxidase Inhibitors: A Review of Their Anti-Inflammatory Therapeutic Potential and Mechanisms of Action

Chronic inflammatory diseases are debilitating, affect patients' quality of life, and are a significant financial burden on health care. Inflammation is regulated by pro-inflammatory cytokines and chemokines that are expressed by immune and non-immune cells, and their expression is highly controlled, both spatially and temporally. Their dysregulation is a hallmark of chronic inflammatory and autoimmune diseases. Significant evidence supports that monoamine oxidase (MAO) inhibitor drugs have anti-inflammatory effects. MAO inhibitors are principally prescribed for the management of a variety of central nervous system (CNS)-associated diseases such as depression, Alzheimer's, and Parkinson's; however, they also have anti-inflammatory effects in the CNS and a variety of non-CNS tissues. To bolster support for their development as anti-inflammatories, it is critical to elucidate their mechanism(s) of action. MAO inhibitors decrease the generation of end products such as hydrogen peroxide, aldehyde, and ammonium. They also inhibit biogenic amine degradation, and this increases cellular and pericellular catecholamines in a variety of immune and some non-immune cells. This decrease in end product metabolites and increase in catecholamines can play a significant role in the anti-inflammatory effects of MAO inhibitors. This review examines MAO inhibitor effects on inflammation in a variety of in vitro and in vivo CNS and non-CNS disease models, as well as their anti-inflammatory mechanism(s) of action.

Peripherally misfolded proteins exacerbate ischemic stroke-induced neuroinflammation and brain injury

BACKGROUND

Protein aggregates can be found in peripheral organs, such as the heart, kidney, and pancreas, but little is known about the impact of peripherally misfolded proteins on neuroinflammation and brain functional recovery following ischemic stroke.

METHODS

Here, we studied the ischemia/reperfusion (I/R) induced brain injury in mice with cardiomyocyte-restricted overexpression of a missense (R120G) mutant small heat shock protein, αB-crystallin (CryAB^R120G), by examining neuroinflammation and brain functional recovery following I/R in comparison to their non-transgenic (Ntg) littermates. To understand how peripherally misfolded proteins influence brain functionality, exosomes were isolated from CryAB^R120G and Ntg mouse blood and were used to treat wild-type (WT) mice and primary cortical neuron-glia mix cultures. Additionally, isolated protein aggregates from the brain following I/R were isolated and subjected to mass-spectrometric analysis to assess whether the aggregates contained the mutant protein, CryAB^R120G. To determine whether the CryAB^R120G misfolding can self-propagate, a misfolded protein seeding assay was performed in cell cultures.

RESULTS

Our results showed that CryAB^R120G mice exhibited dramatically increased infarct volume, delayed brain functional recovery, and enhanced neuroinflammation and protein aggregation in the brain following I/R when compared to the Ntg mice. Intriguingly, mass-spectrometric analysis of the protein aggregates isolated from CryAB^R120G mouse brains confirmed presence of the mutant CryAB^R120G protein in the brain. Importantly, intravenous administration of WT mice with the exosomes isolated from CryAB^R120G mouse blood exacerbated I/R-induced cerebral injury in WT mice. Moreover, incubation of the CryAB^R120G mouse exosomes with primary neuronal cultures induced pronounced protein aggregation. Transduction of CryAB^R120G aggregate seeds into cell cultures caused normal CryAB proteins to undergo dramatic aggregation and form large aggregates, suggesting self-propagation of CryAB^R120G misfolding in cells.

CONCLUSIONS

These results suggest that peripherally misfolded proteins in the heart remotely enhance neuroinflammation and exacerbate brain injury following I/R likely through exosomes, which may represent an underappreciated mechanism underlying heart-brain crosstalk.

Monoamine Oxidase Inhibitors: From Classic to New Clinical Approaches

Monoamine oxidases (MAOs) are involved in the oxidative deamination of different amines and neurotransmitters. This pointed them as potential targets for several disorders and along the last 70 years a wide variety of MAO inhibitors have been developed as successful drugs for the treatment of complex diseases, being the first drugs approved for depression in the late 1950s. The discovery of two MAO isozymes (MAO-A and B) with different substrate selectivity and tissue expression patterns led to novel therapeutic approaches and to the development of new classes of inhibitors, such as selective irreversible and reversible MAO-B inhibitors and reversible MAO-A inhibitors. Significantly, MAO-B inhibitors constitute a widely studied group of compounds, some of them approved for the treatment of Parkinson's disease. Further applications are under development for the treatment of Alzheimer's disease, amyotrophic lateral sclerosis, and cardiovascular diseases, among others. This review summarizes the most important aspects regarding the development and clinical use of MAO inhibitors, going through mechanistic and structural details, new indications, and future perspectives. Monoamine oxidases (MAOs) catalyze the oxidative deamination of different amines and neurotransmitters. The two different isozymes, MAO-A and MAO-B, are located at the outer mitochondrial membrane in different tissues. The enzymatic reaction involves formation of the corresponding aldehyde and releasing hydrogen peroxide (H₂O₂) and ammonia or a substituted amine depending on the substrate. MAO's role in neurotransmitter metabolism made them targets for major depression and Parkinson's disease, among other neurodegenerative diseases. Currently, these compounds are being studied for other diseases such as cardiovascular ones.

Click-Reaction-Triggered SERS Signals for Specific Detection of Monoamine Oxidase B Activity

Human monoamine oxidases (MAOs) play important roles in maintaining the homeostasis of biogenic amines. One of its isoforms, monoamine oxidase B (MAOB), is thought to be involved in several neurodegenerative diseases, which make the selective detection of MAOB activity essential. In this work, a novel surface-enhanced Raman scattering (SERS) sensor was fabricated and the MAOB activity was specifically determined by detecting the SERS signals of an enzyme-catalyzed reaction product via an amine-aldehyde click reaction. This process was simply achieved by coating core-shell gold-silver nanoparticles (Au@Ag NPs) on 3-aminopropyl aminopropyl triethoxysilane (APTES)-modified glass, and then, a monolayer of cysteamine (CA) was attached to the nanoparticle surface as a linker through Ag-S bonds. Using phenethylamine (PA) as a specific substrate of MAOB, the enzyme product phenylacetaldehyde (PAA) will produce significant Raman signals via the amine-aldehyde click reaction with CA, while other molecules, such as MAOB and PA, have no signal output because they cannot form close interaction with nanoparticles due to the existence of a CA layer. This sensor was further used for the specific determination of MAOB activity in clinical blood samples and the MAOB inhibitor assessment successfully. Meanwhile, by changing the click reaction types and taking advantage of the SERS fingerprint peaks for the specific click reaction products, this strategy offers huge potential to detect multiple enzyme activities simultaneously and can be used for new click reaction screening, enzyme-related disease diagnosis, drug screening, and clinical diagnosis.

Synthesis and identification of a novel derivative of salidroside as a selective, competitive inhibitor of monoamine oxidase B with enhanced neuroprotective properties

Salidroside [(2R,3S,4S,5R,6R)-2-(hydroxymethyl)-6-(4-hydroxyphenethoxy)tetrahy-dro-2H-pyran-3,4,5-triol] is an antioxidant, anti-inflammatory and neuroprotective agent, but its drug-like properties are unoptimized and its mechanism of actions is uncertain. We synthesized twenty-six novel derivatives of salidroside and examined them in CoCl₂-treated PC12 cells using MTT assay. pOBz, synthesized by esterifying the phenolic hydroxyl group of salidroside with benzoyl chloride, was one of five derivatives that were more cytoprotective than salidroside, with an EC₅₀ of 0.038 μM versus 0.30 μM for salidroside. pOBz was also more lipophilic, with log P of 1.44 versus -0.89 for salidroside. Reverse virtual docking predicted that pOBz would bind strongly with monoamine oxidase (MAO) B by occupying its entrance and substrate cavities, and by interacting with the inter-cavity gating residue Ile199 and Tyr435 of the substrate cavity. Enzymatic studies confirmed that pOBz competitively inhibited the activity of purified human MAO-B (K_i = 0.041 μM versus K_i = 0.92 μM for salidroside), and pOBz was highly selective for MAO-B over MAO-A. In vivo, pOBz inhibited cerebral MAO activity after middle cerebral artery occlusion with reperfusion in rats, and it reduced cerebral infarct volume, improved neurological function and NeuN expression, and inhibited complement C3 expression and apoptosis. Our results suggest that pOBz is a structurally novel type of competitive and selective MAO-B inhibitor, with potent neuroprotective properties after cerebral ischemia-reperfusion injury in rats.

A Role for Endoplasmic Reticulum Stress in Intracerebral Hemorrhage

The endoplasmic reticulum (ER) is an intracellular organelle that performs multiple functions, such as lipid biosynthesis, protein folding, and maintaining intracellular calcium homeostasis. Thus, conditions wherein the ER is unable to fold proteins is defined as ER stress, and an inbuilt quality control mechanism, called the unfolded protein response (UPR), is activated during ER stress, which serves as a recovery system that inhibits protein synthesis. Further, based on the severity of ER stress, the response could involve both proapoptotic and antiapoptotic phases. Intracerebral hemorrhage (ICH) is the second most common subtype of cerebral stroke and many lines of evidence have suggested a role for the ER in major neurological disorders. The injury mechanism during ICH includes hematoma formation, which in turn leads to inflammation, elevated intracranial pressure, and edema. A proper understanding of the injury mechanism(s) is required to effectively treat ICH and closing the gap between our current understanding of ER stress mechanisms and ICH injury can lead to valuable advances in the clinical management of ICH.