Increased generation of cyclopentenone prostaglandins after brain ischemia and their role in aggregation of ubiquitinated proteins in neurons

Blood Markers in Relation to a History of Traumatic Brain Injury Across Stages of Cognitive Impairment in a Diverse Cohort

BACKGROUND

Traumatic brain injury (TBI) has been linked to multiple pathophysiological processes that could increase risk for Alzheimer's disease and related dementias (ADRD). However, the impact of prior TBI on blood biomarkers for ADRD remains unknown.

OBJECTIVE

Using cross-sectional data, we assessed whether a history of TBI influences serum biomarkers in a diverse cohort (approximately 50% Hispanic) with normal cognition, mild cognitive impairment, or dementia.

METHODS

Levels of glial fibrillary acidic protein (GFAP), neurofilament light (NFL), total tau (T-tau), and ubiquitin carboxy-terminal hydrolase-L1 (UCHL1) were measured for participants across the cognitive spectrum. Participants were categorized based on presence and absence of a history of TBI with loss of consciousness, and study samples were derived through case-control matching. Multivariable general linear models compared concentrations of biomarkers in relation to a history of TBI and smoothing splines modelled biomarkers non-linearly in the cognitively impaired groups as a function of time since symptom onset.

RESULTS

Each biomarker was higher across stages of cognitive impairment, characterized by clinical diagnosis and Mini-Mental State Examination performance, but these associations were not influenced by a history of TBI. However, modelling biomarkers in relation to duration of cognitive symptoms for ADRD showed differences by history of TBI, with only GFAP and UCHL1 being elevated.

CONCLUSIONS

Serum GFAP, NFL, T-tau, and UCHL1 were higher across stages of cognitive impairment in this diverse clinical cohort, regardless of TBI history, though longitudinal investigation of the timing, order, and trajectory of the biomarkers in relation to prior TBI is warranted.

Role of UCHL1 in the pathogenesis of neurodegenerative diseases and brain injury

UCHL1 is a multifunctional protein expressed at high concentrations in neurons in the brain and spinal cord. UCHL1 plays important roles in regulating the level of cellular free ubiquitin and redox state as well as the degradation of select proteins. This review focuses on the potential role of UCHL1 in the pathogenesis of neurodegenerative diseases and brain injury and recovery. Subjects addressed in the review include 1) Normal physiological functions of UCHL1. 2) Posttranslational modification sites and splice variants that alter the function of UCHL1 and mouse models with mutations and deletions of UCHL1. 3) The hypothesized role and pathogenic mechanisms of UCHL1 in neurodegenerative diseases and brain injury. 4) Potential therapeutic strategies targeting UCHL1 in these disorders.

Chlorogenic acid modulates the ubiquitin-proteasome system in stroke animal model

BACKGROUND

Chlorogenic acid, a phenolic compound, has potent antioxidant and neuroprotective properties. The ubiquitin-proteasome system is an important regulators of neurodevelopment and modulators of neuronal function. This system is associated with neurodevelopment and neurotransmission through degradation and removal of damaged proteins. Activation of the ubiquitin-proteasome system is a critical factor in preventing cell death. We have previously reported a decrease in the activity of the ubiquitin-proteasome system during cerebral ischemia. This study investigated whether chlorogenic acid regulates the ubiquitin-proteasome system in an animal stroke model. In adult rats, middle cerebral artery occlusion (MCAO) surgery was performed to induce focal cerebral ischemia. Chlorogenic acid (30 mg/kg) or normal saline was injected into the abdominal cavity 2 h after MCAO surgery, and cerebral cortex tissues were collected 24 h after MCAO damage.

RESULTS

Chlorogenic acid attenuated neurobehavioral disorders and histopathological changes caused by MCAO damage. We identified the decreases in ubiquitin C-terminal hydrolase L1, ubiquitin thioesterase OTUB1, proteasome subunit α type 1, proteasome subunit α type 3, and proteasome subunit β type 4 expression using a proteomics approach in MCAO animals. The decrease in these proteins was alleviated by chlorogenic acid. In addition, the results of reverse transcription-polymerase chain reaction confirmed these changes. The identified proteins were markedly reduced in MCAO damage, while chlorogenic acid prevented these reductions induced by MCAO. The decrease of ubiquitin-proteasome system proteins in ischemic damage was associated with neuronal apoptosis.

CONCLUSIONS

Our results showed that chlorogenic acid regulates ubiquitin-proteasome system proteins and protects cortical neurons from neuronal damage. These results provide evidence that chlorogenic acid has neuroprotective effects and maintains the ubiquitin-proteasome system in ischemic brain injury.

Timapiprant, a prostaglandin D2 receptor antagonist, ameliorates pathology in a rat Alzheimer's model

We investigated the relevance of the prostaglandin D2 pathway in Alzheimer's disease, because prostaglandin D2 is a major prostaglandin in the brain. Thus, its contribution to Alzheimer's disease merits attention, given the known impact of the prostaglandin E2 pathway in Alzheimer's disease. We used the TgF344-AD transgenic rat model because it exhibits age-dependent and progressive Alzheimer's disease pathology. Prostaglandin D2 levels in hippocampi of TgF344-AD and wild-type littermates were significantly higher than prostaglandin E2. Prostaglandin D2 signals through DP1 and DP2 receptors. Microglial DP1 receptors were more abundant and neuronal DP2 receptors were fewer in TgF344-AD than in wild-type rats. Expression of the major brain prostaglandin D2 synthase (lipocalin-type PGDS) was the highest among 33 genes involved in the prostaglandin D2 and prostaglandin E2 pathways. We treated a subset of rats (wild-type and TgF344-AD males) with timapiprant, a potent highly selective DP2 antagonist in development for allergic inflammation treatment. Timapiprant significantly mitigated Alzheimer's disease pathology and cognitive deficits in TgF344-AD males. Thus, selective DP2 antagonists have potential as therapeutics to treat Alzheimer's disease.

Mutation of a Ubiquitin Carboxy Terminal Hydrolase L1 Lipid Binding Site Alleviates Cell Death, Axonal Injury, and Behavioral Deficits After Traumatic Brain Injury in Mice

Ubiquitin carboxy terminal hydrolase L1 (UCHL1) is a protein highly expressed in neurons that may play important roles in the ubiquitin proteasome pathway (UPP) in neurons, axonal integrity, and motor function after traumatic brain injury (TBI). Binding of reactive lipid species to cysteine 152 of UCHL1 results in unfolding, aggregation, and inactivation of the enzyme. To test the role of this mechanism in TBI, mice bearing a cysteine to alanine mutation at site 152 (C152A mice) that renders UCHL1 resistant to inactivation by reactive lipids were subjected to the controlled cortical impact model (CCI) of TBI and compared to wild type (WT) controls. Alterations in protein ubiquitination and activation of autophagy pathway markers in traumatized brain were detected by immunoblotting. Cell death and axonal injury were determined by histological assessment and anti-amyloid precursor protein (APP) immunohistochemistry. Behavioral outcomes were determined using the beam balance and Morris water maze tests. C152A mice had reduced accumulation of ubiquitinated proteins, decreased activation of the autophagy markers Beclin-1 and LC3B, a decreased number of abnormal axons, decreased CA1 cell death, and improved motor and cognitive function compared to WT controls after CCI; no significant change in spared tissue volume was observed. These results suggest that binding of lipid substrates to cysteine 152 of UCHL1 is important in the pathogenesis of injury and recovery after TBI and may be a novel target for future therapeutic approaches.

Thorough overview of ubiquitin C-terminal hydrolase-L1 and glial fibrillary acidic protein as tandem biomarkers recently cleared by US Food and Drug Administration for the evaluation of intracranial injuries among patients with traumatic brain injury

Traumatic brain injury (TBI) is a major cause of mortality and morbidity affecting all ages. It remains to be a diagnostic and therapeutic challenge, in which, to date, there is no Food and Drug Administration‐approved drug for treating patients suffering from TBI. The heterogeneity of the disease and the associated complex pathophysiology make it difficult to assess the level of the trauma and to predict the clinical outcome. Current injury severity assessment relies primarily on the Glasgow Coma Scale score or through neuroimaging, including magnetic resonance imaging and computed tomography scans. Nevertheless, such approaches have certain limitations when it comes to accuracy and cost efficiency, as well as exposing patients to unnecessary radiation. Consequently, extensive research work has been carried out to improve the diagnostic accuracy of TBI, especially in mild injuries, because they are often difficult to diagnose. The need for accurate and objective diagnostic measures led to the discovery of biomarkers significantly associated with TBI. Among the most well‐characterized biomarkers are ubiquitin C‐terminal hydrolase‐L1 and glial fibrillary acidic protein. The current review presents an overview regarding the structure and function of these distinctive protein biomarkers, along with their clinical significance that led to their approval by the US Food and Drug Administration to evaluate mild TBI in patients.

The Role of Ubiquitin-Proteasome Pathway and Autophagy-Lysosome Pathway in Cerebral Ischemia

The ubiquitin-proteasome pathway and autophagy-lysosome pathway are two major routes for clearance of aberrant cellular components to maintain protein homeostasis and normal cellular functions. Accumulating evidence shows that these two pathways are impaired during cerebral ischemia, which contributes to ischemic-induced neuronal necrosis and apoptosis. This review aims to critically discuss current knowledge and controversies on these two pathways in response to cerebral ischemic stress. We also discuss molecular mechanisms underlying the impairments of these protein degradation pathways and how such impairments lead to neuronal damage after cerebral ischemia. Further, we review the recent advance on the understanding of the involvement of these two pathways in the pathological process during many therapeutic approaches against cerebral ischemia. Despite recent advances, the exact role and molecular mechanisms of these two pathways following cerebral ischemia are complex and not completely understood, of which better understanding will provide avenues to develop novel therapeutic strategies for ischemic stroke.

The chemistry, biology and pharmacology of the cyclopentenone prostaglandins

The cyclopentenone prostaglandins (CyPGs) are a small group compounds that are a subset of the eicosanoid superfamily, which are metabolites of arachidonic acid as well as other polyunsaturated fatty acids. The CyPGs are defined by a structural feature, namely, a five-membered carbocyclic ring containing an alfa-beta unsaturated keto group. The two most studied members are PGA₂ and 15d-PGJ₂ (15-deoxy-Δ12,14-prostaglandin J₂); other less studied members are PGA₁, Δ¹²-PGJ₂, and PGJ₂. They are involved in a number of biological activities including the ability to resolve chronic inflammation and the growth and survival of cells, particularly those of cancerous or neurological origin. Also, they can activate the prostaglandin DP2 receptor as well as the ligand-dependent transcription factor PPAR-gamma. Their ability to promote the resolution of chronic inflammation makes it of particular interest to have a good understanding of their actions. Since their discovery, the literature on the CyPGs has greatly expanded both in size and in scope; these reports are covered in the current review.

PACAP27 mitigates an age-dependent hippocampal vulnerability to PGJ2-induced spatial learning deficits and neuroinflammation in mice

BACKGROUND

Inflammation in the brain is mediated by the cyclooxygenase pathway, which leads to the production of prostaglandins. Prostaglandin (PG) D2, the most abundant PG in the brain, increases under pathological conditions and is spontaneously metabolized to PGJ2. PGJ2 is highly neurotoxic, with the potential to transition neuroinflammation into a chronic state and contribute to neurodegeneration as seen in many neurological diseases. Conversely, PACAP27 is a lipophilic peptide that raises intracellular cAMP and is an anti-inflammatory agent. The aim of our study was to investigate the therapeutic potential of PACAP27 to counter the behavioral and neurotoxic effects of PGJ2 observed in aged subjects.

METHODS

PGJ2 was injected bilaterally into the hippocampal CA1 region of 53-week-old and 12-week-old C57BL/6N male mice, once per week over 3 weeks (three total infusions) and included co-infusions of PACAP27 within respective treatment groups. Our behavioral assessments looked at spatial learning and memory performance on the 8-arm radial maze, followed by histological analyses of fixed hippocampal tissue using Fluoro-Jade C and fluorescent immunohistochemistry focused on IBA-1 microglia.

RESULTS

Aged mice treated with PGJ2 exhibited spatial learning and long-term memory deficits, as well as neurodegeneration in CA3 pyramidal neurons. Aged mice that received co-infusions of PACAP27 exhibited remediated learning and memory performance and decreased neurodegeneration in CA3 pyramidal neurons. Moreover, microglial activation in the CA3 region was also reduced in aged mice cotreated with PACAP27.

CONCLUSIONS

Our data show that PGJ2 can produce a retrograde spread of damage not observed in PGJ2-treated young mice, leading to age-dependent neurodegeneration of hippocampal neurons producing learning and memory deficits. PACAP27 can remediate the behavioral and neurodegenerative effects that PGJ2 produces in aged subjects. Targeting specific neurotoxic prostaglandins, such as PGJ2, offers great promise as a new therapeutic strategy downstream of cyclooxygenases, to combat the neuronal deficits induced by chronic inflammation.

Mitochondrial and calcium perturbations in rat CNS neurons induce calpain-cleavage of Parkin: Phosphatase inhibition stabilizes pSer65Parkin reducing its calpain-cleavage

Mitochondrial impairment and calcium (Ca⁺⁺) dyshomeostasis are associated with Parkinson's disease (PD). When intracellular ATP levels are lowered, Ca⁺⁺-ATPase pumps are impaired causing cytoplasmic Ca⁺⁺ to be elevated and calpain activation. Little is known about the effect of calpain activation on Parkin integrity. To address this gap, we examined the effects of mitochondrial inhibitors [oligomycin (Oligo), antimycin and rotenone] on endogenous Parkin integrity in rat midbrain and cerebral cortical cultures. All drugs induced calpain-cleavage of Parkin to ~36.9/43.6 kDa fragments. In contrast, treatment with the proinflammatory prostaglandin J2 (PGJ2) and the proteasome inhibitor epoxomicin induced caspase-cleavage of Parkin to fragments of a different size, previously shown by others to be triggered by apoptosis. Calpain-cleaved Parkin was enriched in neuronal mitochondrial fractions. Pre-treatment with the phosphatase inhibitor okadaic acid prior to Oligo-treatment, stabilized full-length Parkin phosphorylated at Ser⁶⁵, and reduced calpain-cleavage of Parkin. Treatment with the Ca⁺⁺ ionophore A23187, which facilitates Ca⁺⁺ transport across the plasma membrane, mimicked the effect of Oligo by inducing calpain-cleavage of Parkin. Removing extracellular Ca⁺⁺ from the media prevented oligomycin- and ionophore-induced calpain-cleavage of Parkin. Computational analysis predicted that calpain-cleavage of Parkin liberates its UbL domain. The phosphagen cyclocreatine moderately mitigated Parkin cleavage by calpain. Moreover, the pituitary adenylate cyclase activating peptide (PACAP27), which stimulates cAMP production, prevented caspase but not calpain-cleavage of Parkin. Overall, our data support a link between Parkin phosphorylation and its cleavage by calpain. This mechanism reflects the impact of mitochondrial impairment and Ca⁺⁺-dyshomeostasis on Parkin integrity and could influence PD pathogenesis.

Role of UCHL1 in axonal injury and functional recovery after cerebral ischemia

Many neuroprotective strategies have failed to translate to clinical trials, perhaps because of a failure to preserve white matter function. Ubiquitin C-terminal hydrolase L1 (UCHL1), a neuron-specific protein essential for axonal function, is deactivated by reactive lipids produced after cerebral ischemia. Mutation of the cysteine residue 152-reactive lipid-binding site of UCHL1 decreased axonal injury after hypoxia and ischemia in vitro and in vivo, preserved axonal conductance and synaptic function, and improved motor behavior after ischemia in mice. These results suggest that UCHL1 may play an important role in maintaining axonal function after cerebral ischemia. Restoration of UCHL1 activity or prevention of degradation of UCHL1 activity by preventing binding of substrates to cysteine residue 152 could be useful approaches for treatment of stroke.

Ubiquitin C-terminal hydrolase L1 (UCHL1) is a unique brain-specific deubiquitinating enzyme. Mutations in and aberrant function of UCHL1 have been linked to many neurological disorders. UCHL1 activity protects neurons from hypoxic injury, and binding of stroke-induced reactive lipid species to the cysteine 152 (C152) of UCHL1 unfolds the protein and disrupts its function. To investigate the role of UCHL1 and its adduction by reactive lipids in inhibiting repair and recovery of function following ischemic injury, a knock-in (KI) mouse expressing the UCHL1 C152A mutation was generated. Neurons derived from KI mice had less cell death and neurite injury after hypoxia. UCHL1 C152A KI and WT mice underwent middle cerebral artery occlusion (MCAO) or sham surgery. White matter injury was significantly decreased in KI compared with WT mice 7 d after MCAO. Histological analysis revealed decreased tissue loss at 21 d after injury in KI mice. There was also significantly improved sensorimotor recovery in postischemic KI mice. K63- and K48-linked polyubiquitinated proteins were increased in penumbra of WT mouse brains but not in KI mouse brains at 24 h post MCAO. The UCHL1 C152A mutation preserved excitatory synaptic drive to pyramidal neurons and their excitability in the periinfarct zone; axonal conduction velocity recovered by 21 d post MCAO in KI mice in corpus callosum. These results demonstrate that UCHL1 activity is an important determinant of function after ischemia and further demonstrate that the C152 site of UCHL1 plays a significant role in functional recovery after stroke.

15-Deoxy-Δ12,14-prostaglandin J2 promotes phosphorylation of eukaryotic initiation factor 2α and activates the integrated stress response

Stress granules (SGs) are cytoplasmic RNA–protein aggregates formed in response to inhibition of translation initiation. SGs contribute to the stress response and are implicated in a variety of diseases, including cancer and some forms of neurodegeneration. Neurodegenerative diseases often involve chronic phosphorylation of eukaryotic initiation factor 2α (eIF2α), with deletions of eIF2α kinases or treatment with eIF2α kinase inhibitors being protective in some animal models of disease. However, how and why the integrated stress response (ISR) is activated in different forms of neurodegeneration remains unclear. Because neuroinflammation is common to many neurodegenerative diseases, we hypothesized that inflammatory factors contribute to ISR activation in a cell-nonautonomous manner. Using fluorescence microscopy and immunoblotting, we show here that the endogenously produced product of inflammation, 15-deoxy-Δ^12,14-prostaglandin J2 (15-d-PGJ2), triggers eIF2α phosphorylation, thereby activating the ISR, repressing bulk translation, and triggering SG formation. Our findings define a mechanism by which inflammation activates the ISR in a cell-nonautonomous manner and suggest that inhibition of 15-d-PGJ2 production might be a useful therapeutic strategy in some neuroinflammatory contexts.

Prostaglandin J2 promotes O-GlcNAcylation raising APP processing by α- and β-secretases: relevance to Alzheimer's disease

Regulation of the amyloid precursor protein (APP) processing by α- and β-secretases is of special interest to Alzheimer's disease (AD), as these proteases prevent or mediate amyloid beta formation, respectively. Neuroinflammation is also implicated in AD. Our data demonstrate that the endogenous mediator of inflammation prostaglandin J2 (PGJ2) promotes full-length APP (FL-APP) processing by α- and β-secretases. The decrease in FL-APP was independent of proteasomal, lysosomal, calpain, caspase, and γ-secretase activities. Moreover, PGJ2-treatment promoted cleavage of secreted APP, specifically sAPPα and sAPPβ, generated by α and β-secretase, respectively. Notably, PGJ2-treatment induced caspase-dependent cleavage of sAPPβ. Mechanistically, PGJ2-treatment selectively diminished mature (O- and N-glycosylated) but not immature (N-glycosylated only) FL-APP. PGJ2-treatment also increased the overall levels of protein O-GlcNAcylation, which occurs within the nucleocytoplasmic compartment. It is known that APP undergoes O-GlcNAcylation and that the latter protects proteins from proteasomal degradation. Our results suggest that by increasing protein O-GlcNAcylation levels, PGJ2 renders mature APP less prone to proteasomal degradation, thus shunting APP toward processing by α- and β-secretases.

Neurotoxic mechanisms by which the USP14 inhibitor IU1 depletes ubiquitinated proteins and Tau in rat cerebral cortical neurons: Relevance to Alzheimer's disease

In Alzheimer's disease proteasome activity is reportedly downregulated, thus increasing it could be therapeutically beneficial. The proteasome-associated deubiquitinase USP14 disassembles polyubiquitin-chains, potentially delaying proteasome-dependent protein degradation. We assessed the protective efficacy of inhibiting or downregulating USP14 in rat and mouse (Usp14^axJ) neuronal cultures treated with prostaglandin J2 (PGJ2). IU1 concentrations (_HIU1>25μM) reported by others to inhibit USP14 and be protective in non-neuronal cells, reduced PGJ2-induced Ub-protein accumulation in neurons. However, _HIU1 alone or with PGJ2 is neurotoxic, induces calpain-dependent Tau cleavage, and decreases E1~Ub thioester levels and 26S proteasome assembly, which are energy-dependent processes. We attribute the two latter _HIU1 effects to ATP-deficits and mitochondrial Complex I inhibition, as shown herein. These _HIU1 effects mimic those of mitochondrial inhibitors in general, thus supporting that ATP-depletion is a major mediator of _HIU1-actions. In contrast, low IU1 concentrations (_LIU1≤25μM) or USP14 knockdown by siRNA in rat cortical cultures or loss of USP14 in cortical cultures from ataxia (Usp14^axJ) mice, failed to prevent PGJ2-induced Ub-protein accumulation. PGJ2 alone induces Ub-protein accumulation and decreases E1~Ub thioester levels. This seemingly paradoxical result may be attributed to PGJ2 inhibiting some deubiquitinases (such as UCH-L1 but not USP14), thus triggering Ub-protein stabilization. Overall, IU1-concentrations that reduce PGJ2-induced accumulation of Ub-proteins are neurotoxic, trigger calpain-mediated Tau cleavage, lower ATP, E1~Ub thioester and E1 protein levels, and reduce proteasome activity. In conclusion, pharmacologically inhibiting (with low or high IU1 concentrations) or genetically down-regulating USP14 fail to enhance proteasomal degradation of Ub-proteins or Tau in neurons.

Life and death in the trash heap: The ubiquitin proteasome pathway and UCHL1 in brain aging, neurodegenerative disease and cerebral Ischemia

The ubiquitin proteasome pathway (UPP) is essential for removing abnormal proteins and preventing accumulation of potentially toxic proteins within the neuron. UPP dysfunction occurs with normal aging and is associated with abnormal accumulation of protein aggregates within neurons in neurodegenerative diseases. Ischemia disrupts UPP function and thus may contribute to UPP dysfunction seen in the aging brain and in neurodegenerative diseases. Ubiquitin carboxy-terminal hydrolase L1 (UCHL1), an important component of the UPP in the neuron, is covalently modified and its activity inhibited by reactive lipids produced after ischemia. As a result, degradation of toxic proteins is impaired which may exacerbate neuronal function and cell death in stroke and neurodegenerative diseases. Preserving or restoring UCHL1 activity may be an effective therapeutic strategy in stroke and neurodegenerative diseases.

Prostaglandin J2: a potential target for halting inflammation-induced neurodegeneration

Prostaglandins (PGs) are produced via cyclooxygenases, which are enzymes that play a major role in neuroinflammation. Epidemiological studies show that chronic treatment with low levels of cyclooxygenase inhibitors (nonsteroidal anti-inflammatory drugs (NSAIDs)) lowers the risk for Alzheimer's disease (AD) and Parkinson's disease (PD) by as much as 50%. Unfortunately, inhibiting cyclooxygenases with NSAIDs blocks the synthesis of downstream neuroprotective and neurotoxic PGs, thus producing adverse side effects. We focus on prostaglandin J2 (PGJ2) because it is highly neurotoxic compared to PGA1, D2, and E2. Unlike other PGs, PGJ2 and its metabolites have a cyclopentenone ring with reactive α,β-unsaturated carbonyl groups that form covalent Michael adducts with key cysteines in proteins and GSH. Cysteine-binding electrophiles such as PGJ2 are considered to play an important role in determining whether neurons will live or die. We discuss in vitro and in vivo studies showing that PGJ2 induces pathological processes relevant to neurodegenerative disorders such as AD and PD. Further, we discuss our work showing that increasing intracellular cAMP with the lipophilic peptide PACAP27 counteracts some of the PGJ2-induced detrimental effects. New therapeutic strategies that neutralize the effects of specific neurotoxic PGs downstream from cyclooxygenases could have a significant impact on the treatment of chronic neurodegenerative disorders with fewer adverse side effects.

The point mutation UCH-L1 C152A protects primary neurons against cyclopentenone prostaglandin-induced cytotoxicity: implications for post-ischemic neuronal injury

Cyclopentenone prostaglandins (CyPGs), such as 15-deoxy-Δ^12,14-prostaglandin J₂ (15dPGJ2), are reactive prostaglandin metabolites exerting a variety of biological effects. CyPGs are produced in ischemic brain and disrupt the ubiquitin-proteasome system (UPS). Ubiquitin-C-terminal hydrolase L1 (UCH-L1) is a brain-specific deubiquitinating enzyme that has been linked to neurodegenerative diseases. Using tandem mass spectrometry (MS) analyses, we found that the C152 site of UCH-L1 is adducted by CyPGs. Mutation of C152 to alanine (C152A) inhibited CyPG modification and conserved recombinant UCH-L1 protein hydrolase activity after 15dPGJ2 treatment. A knock-in (KI) mouse expressing the UCH-L1 C152A mutation was constructed with the bacterial artificial chromosome (BAC) technique. Brain expression and distribution of UCH-L1 in the KI mouse was similar to that of wild type (WT) as determined by western blotting. Primary cortical neurons derived from KI mice were resistant to 15dPGJ2 cytotoxicity compared with neurons from WT mice as detected by the WST-1 cell viability assay and caspase-3 and poly ADP ribose polymerase (PARP) cleavage. This protective effect was accompanied with significantly less ubiquitinated protein accumulation and aggregation as well as less UCH-L1 aggregation in C152A KI primary neurons after 15dPGJ2 treatment. Additionally, 15dPGJ2-induced axonal injury was also significantly attenuated in KI neurons as compared with WT. Taken together, these studies indicate that UCH-L1 function is important in hypoxic neuronal death, and the C152 site of UCH-L1 has a significant role in neuronal survival after hypoxic/ischemic injury.

Identification of a cytotoxic molecule in heat-modified citrus pectin

Modified forms of citrus pectin possess anticancer properties. However, their mechanism of action and the structural features involved remain unclear. Here, we showed that citrus pectin modified by heat treatment displayed cytotoxic effects in cancer cells. A fractionation approach was used aiming to identify active molecules. Dialysis and ethanol precipitation followed by HPLC analysis evidenced that most of the activity was related to molecules with molecular weight corresponding to low degree of polymerization oligogalacturonic acid. Heat-treatment of galacturonic acid also generated cytotoxic molecules. Furthermore, heat-modified galacturonic acid and heat-fragmented pectin contained the same molecule that induced cell death when isolated by HPLC separation. Mass spectrometry analyses revealed that 4,5-dihydroxy-2-cyclopenten-1-one was one cytotoxic molecule present in heat-treated pectin. Finally, we synthesized the enantiopure (4R,5R)-4,5-dihydroxy-2-cyclopenten-1-one and demonstrated that this molecule was cytotoxic and induced a similar pattern of apoptotic-like features than heat-modified pectin.

Protein disulfide isomerase as a novel target for cyclopentenone prostaglandins: implications for hypoxic ischemic injury

Cyclooxygenase-2 (COX-2) is an important contributor to ischemic brain injury. Identification of the downstream mediators of COX-2 toxicity may allow the development of targeted therapies. Of particular interest is the cyclopentenone family of prostaglandin metabolites. Cyclopentenone prostaglandins (CyPGs) are highly reactive molecules that form covalent bonds with cellular thiols. Protein disulfide isomerase (PDI) is an important molecule for the restoration of denatured proteins following ischemia. Because PDI has several thiols, including thiols within the active thioredoxin-like domain, we hypothesized that PDI is a target of CyPGs and that CyPG binding of PDI is detrimental. CyPG-PDI binding was detected in vitro via immunoprecipitation and MS. CyPG-PDI binding decreased PDI enzymatic activity in recombinant PDI treated with CyPG, and PDI immunoprecipitated from neuronal culture treated with CyPG or anoxia. Toxic effects of binding were demonstrated in experiments showing that: (a) pharmacologic inhibition of PDI increased cell death in anoxic neurons, (b) PDI overexpression protected neurons exposed to anoxia and SH-SY5Y cells exposed to CyPG, and (c) PDI overexpression in SH-SY5Y cells attenuated ubiquitination of proteins and decreased activation of pro-apoptotic caspases. In conclusion, CyPG production and subsequent binding of PDI is a novel and potentially important mechanism of ischemic brain injury. We show that CyPGs bind to PDI, cyclopentenones inhibit PDI activity, and CyPG-PDI binding is associated with increased neuronal susceptibility to anoxia. Additional studies are necessary to determine the relative role of CyPG-dependent inhibition of PDI activity in ischemia and other neurodegenerative disorders.

A20 suppresses vascular inflammation by recruiting proinflammatory signaling molecules to intracellular aggresomes

A20 protects against pathologic vascular remodeling by inhibiting the inflammatory transcription factor NF-κB. A20's function has been attributed to ubiquitin editing of receptor-interacting protein 1 (RIP1) to influence activity/stability. The validity of this mechanism was tested using a murine model of transplant vasculopathy and human cells. Mouse C57BL/6 aortae transduced with adenoviruses containing A20 (or β-galactosidase as a control) were allografted into major histocompatibility complex-mismatched BALB/c mice. Primary endothelial cells, smooth muscle cells, or transformed epithelial cells (all human) were transfected with wild-type A20 or with catalytically inactive mutants as a control. NF-κB activity and intracellular localization of RIP1 was monitored by reporter gene assay, immunofluorescent staining, and Western blotting. Native and catalytically inactive versions of A20 had similar inhibitory effects on NF-κB activity (-70% vs. -76%; P > 0.05). A20 promoted localization of RIP1 to insoluble aggresomes in murine vascular allografts and in human cells (53% vs. 0%) without altering RIP1 expression, and this process was increased by the assembly of polyubiquitin chains (87% vs. 28%; P < 0.05). A20 captures polyubiquitinated signaling intermediaries in insoluble aggresomes, thus reducing their bioavailability for downstream NF-κB signaling. This novel mechanism contributes to protection from vasculopathy in transplanted organs treated with exogenous A20.

Neuroinflammation and J2 prostaglandins: linking impairment of the ubiquitin-proteasome pathway and mitochondria to neurodegeneration

The immune response of the CNS is a defense mechanism activated upon injury to initiate repair mechanisms while chronic over-activation of the CNS immune system (termed neuroinflammation) may exacerbate injury. The latter is implicated in a variety of neurological and neurodegenerative disorders such as Alzheimer and Parkinson diseases, amyotrophic lateral sclerosis, multiple sclerosis, traumatic brain injury, HIV dementia, and prion diseases. Cyclooxygenases (COX-1 and COX-2), which are key enzymes in the conversion of arachidonic acid into bioactive prostanoids, play a central role in the inflammatory cascade. J2 prostaglandins are endogenous toxic products of cyclooxygenases, and because their levels are significantly increased upon brain injury, they are actively involved in neuronal dysfunction induced by pro-inflammatory stimuli. In this review, we highlight the mechanisms by which J2 prostaglandins (1) exert their actions, (2) potentially contribute to the transition from acute to chronic inflammation and to the spreading of neuropathology, (3) disturb the ubiquitin-proteasome pathway and mitochondrial function, and (4) contribute to neurodegenerative disorders such as Alzheimer and Parkinson diseases, and amyotrophic lateral sclerosis, as well as stroke, traumatic brain injury (TBI), and demyelination in Krabbe disease. We conclude by discussing the therapeutic potential of targeting the J2 prostaglandin pathway to prevent/delay neurodegeneration associated with neuroinflammation. In this context, we suggest a shift from the traditional view that cyclooxygenases are the most appropriate targets to treat neuroinflammation, to the notion that J2 prostaglandin pathways and other neurotoxic prostaglandins downstream from cyclooxygenases, would offer significant benefits as more effective therapeutic targets to treat chronic neurodegenerative diseases, while minimizing adverse side effects.

PACAP27 prevents Parkinson-like neuronal loss and motor deficits but not microglia activation induced by prostaglandin J2

Neuroinflammation is a major risk factor in Parkinson's disease (PD). Alternative approaches are needed to treat inflammation, as anti-inflammatory drugs such as NSAIDs that inhibit cyclooxygenase-2 (COX-2) can produce devastating side effects, including heart attack and stroke. New therapeutic strategies that target factors downstream of COX-2, such as prostaglandin J2 (PGJ2), hold tremendous promise because they will not alter the homeostatic balance offered by COX-2 derived prostanoids. In the current studies, we report that repeated microinfusion of PGJ2 into the substantia nigra of non-transgenic mice, induces three stages of pathology that mimic the slow-onset cellular and behavioral pathology of PD: mild (one injection) when only motor deficits are detectable, intermediate (two injections) when neuronal and motor deficits as well as microglia activation are detectable, and severe (four injections) when dopaminergic neuronal loss is massive accompanied by microglia activation and motor deficits. Microglia activation was evaluated in vivo by positron emission tomography (PET) with [(11)C](R)PK11195 to provide a regional estimation of brain inflammation. PACAP27 reduced dopaminergic neuronal loss and motor deficits induced by PGJ2, without preventing microglia activation. The latter could be problematic in that persistent microglia activation can exert long-term deleterious effects on neurons and behavior. In conclusion, this PGJ2-induced mouse model that mimics in part chronic inflammation, exhibits slow-onset PD-like pathology and is optimal for testing diagnostic tools such as PET, as well as therapies designed to target the integrated signaling across neurons and microglia, to fully benefit patients with PD.

A mitochondrial pathway for biosynthesis of lipid mediators

The central role of mitochondria in metabolic pathways and in cell-death mechanisms requires sophisticated signalling systems. Essential in this signalling process is an array of lipid mediators derived from polyunsaturated fatty acids. However, the molecular machinery for the production of oxygenated polyunsaturated fatty acids is localized in the cytosol and their biosynthesis has not been identified in mitochondria. Here we report that a range of diversified polyunsaturated molecular species derived from a mitochondria-specific phospholipid, cardiolipin (CL), is oxidized by the intermembrane-space haemoprotein, cytochrome c. We show that a number of oxygenated CL species undergo phospholipase A2-catalysed hydrolysis and thus generate multiple oxygenated fatty acids, including well-known lipid mediators. This represents a new biosynthetic pathway for lipid mediators. We demonstrate that this pathway, which includes the oxidation of polyunsaturated CLs and accumulation of their hydrolysis products (oxygenated linoleic, arachidonic acids and monolysocardiolipins), is activated in vivo after acute tissue injury.

A review of analytical methods for eicosanoids in brain tissue

Eicosanoids are potent lipid mediators of inflammation and are known to play an important role in numerous pathophysiological processes. Furthermore, inflammation has been proven to be a mediator of diseases such as hypertension, atherosclerosis, Alzheimer's disease, cancer and rheumatoid arthritis. Hence, these lipid mediators have gained significant attention in recent years. This review focuses on chromatographic and mass spectrometric methods that have been used to analyze arachidonic acid and its metabolites in brain tissue. Recently published analytical methods such as LC-MS/MS and GC-MS/MS are discussed and compared in terms of limit of quantitation and sample preparation procedures, including solid phase extraction and derivatization. Analytical challenges are also highlighted.

Rapid and simultaneous quantitation of prostanoids by UPLC-MS/MS in rat brain

The metabolites of arachidonic acid (AA) produced from the cyclooxygenase (COX) pathway, collectively termed as prostanoids, and from the CYP 450 pathway, eicosanoids, have been implicated in various neuro-degenerative and neuroinflammatory diseases. This study developed a quantitative UPLC-MS/MS method to simultaneously measure 11 prostanoids including prostaglandins and cyclopentenone metabolites in the rat brain cortical tissue. Linear calibration curves ranging from 0.104 to 33.3ng/ml were validated. The inter-day and intra-day variance for all metabolites was less than 15%. The extraction recovery efficiency and matrix (deionized water) effects measured at 12.5ng/ml (750pg on column) ranged from 88 to 100% and 3 to 14%, respectively, with CV% values below 20%. Additionally, applying the processing and extraction conditions of this method to our previous CYP450 eicosanoids method resulted in overall improvement in extraction recovery and reduction in matrix effects at low (0.417ng/ml) and high (8.33ng/ml) concentrations. In rat brain cortical tissue samples, concentrations of prostanoids ranged from 10.2 to 937pmol/g wet tissue and concentration of eicosanoids ranged from 2.23 to 793pmol/g wet tissue. These data demonstrate that the successive measurement of prostanoids and eicosanoids from a single extracted sample of rat brain tissue can be achieved with a UPLC-MS/MS system and that this method is necessary for evaluation of these metabolites to delineate their role in various neuroinflammatory and cerebrovascular disorders.

Role of the ubiquitin-proteasome system in brain ischemia: friend or foe?

The ubiquitin-proteasome system (UPS) is a catalytic machinery that targets numerous cellular proteins for degradation, thus being essential to control a wide range of basic cellular processes and cell survival. Degradation of intracellular proteins via the UPS is a tightly regulated process initiated by tagging a target protein with a specific ubiquitin chain. Neurons are particularly vulnerable to any change in protein composition, and therefore the UPS is a key regulator of neuronal physiology. Alterations in UPS activity may induce pathological responses, ultimately leading to neuronal cell death. Brain ischemia triggers a complex series of biochemical and molecular mechanisms, such as an inflammatory response, an exacerbated production of misfolded and oxidized proteins, due to oxidative stress, and the breakdown of cellular integrity mainly mediated by excitotoxic glutamatergic signaling. Brain ischemia also damages protein degradation pathways which, together with the overproduction of damaged proteins and consequent upregulation of ubiquitin-conjugated proteins, contribute to the accumulation of ubiquitin-containing proteinaceous deposits. Despite recent advances, the factors leading to deposition of such aggregates after cerebral ischemic injury remain poorly understood. This review discusses the current knowledge on the role of the UPS in brain function and the molecular mechanisms contributing to UPS dysfunction in brain ischemia with consequent accumulation of ubiquitin-containing proteins. Chemical inhibitors of the proteasome and small molecule inhibitors of deubiquitinating enzymes, which promote the degradation of proteins by the proteasome, were both shown to provide neuroprotection in brain ischemia, and this apparent contradiction is also discussed in this review.

Prostaglandin D2 toxicity in primary neurons is mediated through its bioactive cyclopentenone metabolites

Prostaglandin D2 (PGD2) is the most abundant prostaglandin in brain but its effect on neuronal cell death is complex and not completely understood. PGD2 may modulate neuronal cell death via activation of DP receptors or its metabolism to the cyclopentenone prostaglandins (CyPGs) PGJ2, Δ(12)-PGJ2 and 15-deoxy-Δ(12,14)-PGJ2, inducing cell death independently of prostaglandin receptors. This study aims to elucidate the effect of PGD2 on neuronal cell death and its underlying mechanisms. PGD2 dose-dependently induced cell death in rat primary neuron-enriched cultures in concentrations of ≥10μM, and this effect was not reversed by treatment with either DP1 or DP2 receptor antagonists. Antioxidants N-acetylcysteine (NAC) and glutathione which contain sulfhydryl groups that can bind to CyPGs, but not ascorbate or tocopherol, attenuated PGD2-induced cell death. Conversion of PGD2 to CyPGs was detected in neuronal culture medium; treatment with these CyPG metabolites alone exhibited effects similar to those of PGD2, including apoptotic neuronal cell death and accumulation of ubiquitinated proteins. Disruption of lipocalin-type prostaglandin D synthase (L-PGDS) protected neurons against hypoxia. These results support the hypothesis that PGD2 elicits its cytotoxic effects through its bioactive CyPG metabolites rather than DP receptor activation in primary neuronal culture.