Structures of the Human PGD2 Receptor CRTH2 Reveal Novel Mechanisms for Ligand Recognition.

The signaling of prostaglandin D2 (PGD2) through G-protein-coupled receptor (GPCR) CRTH2 is a major pathway in type 2 inflammation. Compelling evidence suggests the therapeutic benefits of blocking CRTH2 signaling in many inflammatory disorders. Currently, a number of CRTH2 antagonists are under clinical investigation, and one compound, fevipiprant, has advanced to phase 3 clinical trials for asthma. Here, we present the crystal structures of human CRTH2 with two antagonists, fevipiprant and CAY10471. The structures, together with docking and ligand-binding data, reveal a semi-occluded pocket covered by a well-structured amino terminus and different binding modes of chemically diverse CRTH2 antagonists. Structural analysis suggests a ligand entry port and a binding process that is facilitated by opposite charge attraction for PGD2, which differs significantly from the binding pose and binding environment of lysophospholipids and endocannabinoids, revealing a new mechanism for lipid recognition by GPCRs.

Molecular basis for lipid recognition by the prostaglandin D2 receptor CRTH2.

Prostaglandin D2 (PGD2) signals through the G protein-coupled receptor (GPCR) CRTH2 to mediate various inflammatory responses. CRTH2 is the only member of the prostanoid receptor family that is phylogenetically distant from others, implying a nonconserved mechanism of lipid action on CRTH2. Here, we report a crystal structure of human CRTH2 bound to a PGD2 derivative, 15R-methyl-PGD2 (15mPGD2), by serial femtosecond crystallography. The structure revealed a "polar group in"-binding mode of 15mPGD2 contrasting the "polar group out"-binding mode of PGE2 in its receptor EP3. Structural comparison analysis suggested that these two lipid-binding modes, associated with distinct charge distributions of ligand-binding pockets, may apply to other lipid GPCRs. Molecular dynamics simulations together with mutagenesis studies also identified charged residues at the ligand entry port that function to capture lipid ligands of CRTH2 from the lipid bilayer. Together, our studies suggest critical roles of charge environment in lipid recognition by GPCRs.

Substrate-induced product-release mechanism of lipocalin-type prostaglandin D synthase.

Prostaglandin D2 (PGD2), an endogenous somnogen, is a unique PG that is secreted into the cerebrospinal fluid. PGD2 is a relatively fragile molecule and should be transported to receptors localized in the basal forebrain without degradation. However, it remains unclear how PGD2 is stably carried to such remote receptors. Here, we demonstrate that the PGD2-synthesizing enzyme, Lipocalin-type prostaglandin D synthase (L-PGDS), binds not only its substrate PGH2 but also its product PGD2 at two distinct binding sites for both ligands. This behaviour implys its PGD2 carrier function. Nevertheless, since the high affinity (Kd = âˆ¼0.6 µM) of PGD2 in the catalytic binding site is comparable to that of PGH2, it may act as a competitive inhibitor, while our binding assay exhibits only weak inhibition (Ki = 189 µM) of the catalytic reaction. To clarify this enigmatic behavior, we determined the solution structure of L-PGDS bound to one substrate analog by NMR and compared it with the two structures: one in the apo form and the other in substrate analogue complex with 1:2 stoichiometry. The structural comparisons showed clearly that open or closed forms of loops at the entrance of ligand binding cavity are regulated by substrate binding to two sites, and that the binding to a second non-catalytic binding site, which apparently substrate concentration dependent, induces opening of the cavity that releases the product. From these results, we propose that L-PGDS is a unique enzyme having a carrier function and a substrate-induced product-release mechanism.

Short Total Synthesis of Δ12-Prostaglandin J2 and Related Prostaglandins. Design, Synthesis, and Biological Evaluation of Macrocyclic Δ12-Prostaglandin J2 Analogues.

Comprised of a large collection of structurally diverse molecules, the prostaglandins exhibit a wide range of biological properties. Among them are Δ12-prostaglandin J2 (Δ12-PGJ2) and Δ12-prostaglandin J3 (Δ12-PGJ3), whose unusual structural motifs and potent cytotoxicities present unique opportunities for chemical and biological investigations. Herein, we report a short olefin-metathesis-based total synthesis of Δ12-PGJ2 and its application to the construction of a series of designed analogues possessing monomeric, dimeric, trimeric, and tetrameric macrocyclic lactones consisting of units of this prostaglandin. Biological evaluation of these analogues led to interesting structure-activity relationships and trends and the discovery of a number of more potent antitumor agents than their parent naturally occurring molecules.

Prostaglandin D2-ethanolamide induces skin cancer apoptosis by suppressing the activity of cellular antioxidants.

The combined incidence of melanoma and non-melanoma skin cancer (NMSC) is greater than the incidence of all other malignancies in the US. Previously, we demonstrated that the endocannabinoid, arachidonoyl-ethanolamide (AEA), was a potent inducer of apoptosis in NMSC. The metabolism of AEA to the prostaglandin, PGD2-EA, was a prerequisite for AEA cytotoxicity. However, the mechanism of PGD2-EA cell death has not been clearly defined. In the present study, we report that PGD2-EA causes apoptosis in melanoma and NMSC cells. Mass spectrometry analysis revealed that PGD2-EA was dehydrated to three J-series prostaglandins; PGJ2-EA, Δ12PGJ2-EA, and 15deoxy,Δ12,14 PGJ2-EA. PGD2-EA inhibited the antioxidant activity of glutathione and thioredoxin which then caused oxidative stress. This increase in oxidative stress was accompanied by the activation of endoplasmic reticulum (ER) stress and apoptosis. The effect of PGD2-EA was independent of DP1, DP2, and PPARγ receptors suggesting that PGD2-EA cytotoxicity was mediated by its metabolic product, 15dPGJ2-EA.

Astrocytes synthesize primary and cyclopentenone prostaglandins that are negative regulators of their proliferation.

Recently, the modulation of cellular inflammatory responses via endogenous regulators became a major focus of medically relevant investigations. Prostaglandins (PGs) are attractive regulatory molecules, but their synthesis and mechanisms of action in brain cells are still unclear. Astrocytes are involved in manifestation of neuropathology and their proliferation is an important part of astrogliosis, a cellular neuroinflammatory response. The aims of our study were to measure synthesis of PGs by astrocytes, and evaluate their influence on proliferation in combination with addition of inflammatory pathway inhibitors. With UPLC-MS/MS analysis we detected primary PGs (1410 ±â€¯36 pg/mg PGE2, 344 ±â€¯24 PGD2) and cyclopentenone PGs (cyPGs) (87 ±â€¯17 15d-PGJ2, 308 ±â€¯23 PGA2) in the extracellular medium after 24-h lipopolysaccharide (LPS) stimulation of astrocytes. PGs reduced astrocytic proliferation with the following order of potencies (measured as inhibition at 20 µM): most potent 15d-PGJ2 (90%) and PGA2 (80%), > PGD2 (40%) > 15d-PGA2 (20%) > PGE2 (5%), the least potent. However, PGF2α and 2-cyclopenten-1-one, and ciglitazone and rosiglitazone (synthetic agonists of PPARγ) had no effect. Combinations of cyPGs with SC-560 or NS-398 (specific anti-inflammatory inhibitors of cyclooxygenase-1 and -2, respectively) were not effective; while GW9662 (PPARγ antagonist) or MK-741 (inhibitor of multidrug resistance protein-1, MRP1, and CysLT1 receptors) amplified the inhibitory effect of PGA2 and 15d-PGJ2. Although concentrations of individual PGs and cyPGs are low, all of them, as well as primary PGs suppress proliferation. Thus, the effects are potentially additive, and activated PGs synthesis suppresses proliferation in astrocytes.

Comparison of pro-adipogenic effects between prostaglandin (PG) D2 and its stable, isosteric analogue, 11-deoxy-11-methylene-PGD2, during the maturation phase of cultured adipocytes.

Prostaglandin (PG) D2 is relatively unstable and dehydrated non-enzymatically into PGJ2 derivatives, which are known to serve as pro-adipogenic factors by activating peroxisome proliferator-activated receptor (PPAR) γ, a master regulator of adipogenesis. 11-Deoxy-11-methylene-PGD2 (11d-11m-PGD2) is a novel, chemically stable, isosteric analogue of PGD2 in which the 11-keto group is replaced by an exocyclic methylene. Here we attempted to investigate pro-adipogenic effects of PGD2 and 11d-11m-PGD2 and to compare the difference in their ways during the maturation phase of cultured adipocytes. The dose-dependent study showed that 11d-11m-PGD2 was significantly more potent than natural PGD2 to stimulate the storage of fats suppressed in the presence of indomethacin, a cyclooxygenase inhibitor. These pro-adipogenic effects were caused by the up-regulation of adipogenesis as evident with higher gene expression levels of adipogenesis markers. Analysis of transcript levels revealed the enhanced gene expression of two subtypes of cell-surface membrane receptors for PGD2, namely the prostanoid DP1 and DP2 (chemoattractant receptor-homologous molecule expressed on Th2 cells (CRTH2)) receptors together with lipocalin-type PGD synthase during the maturation phase. Specific agonists for DP1, CRTH2, and PPARγ were appreciably effective to rescue adipogenesis attenuated by indomethacin. The action of PGD2 was attenuated by specific antagonists for DP1 and PPARγ. By contrast, the effect of 11d-11m-PGD2 was more potently interfered by a selective antagonist for CRTH2 than that for DP1 while PPARγ antagonist GW9662 had almost no inhibitory effects. These results suggest that PGD2 exerts its pro-adipogenic effect principally through the mediation of DP1 and PPARγ, whereas the stimulatory effect of 11d-11m-PGD2 on adipogenesis occurs preferentially by the interaction with CRTH2.

A chemoproteomic platform to quantitatively map targets of lipid-derived electrophiles.

Cells produce electrophilic products with the potential to modify and affect the function of proteins. Chemoproteomic methods have provided a means to qualitatively inventory proteins targeted by endogenous electrophiles; however, ascertaining the potency and specificity of these reactions to identify the sites in the proteome that are most sensitive to electrophilic modification requires more quantitative methods. Here we describe a competitive activity-based profiling method for quantifying the reactivity of electrophilic compounds against >1,000 cysteines in parallel in the human proteome. Using this approach, we identified a select set of proteins that constitute 'hot spots' for modification by various lipid-derived electrophiles, including the oxidative stress product 4-hydroxy-2-nonenal (HNE). We show that one of these proteins, ZAK kinase, is labeled by HNE on a conserved, active site-proximal cysteine and that the resulting enzyme inhibition creates a negative feedback mechanism that can suppress the activation of JNK pathways normally induced by oxidative stress.

The cyclopentenone prostaglandin 15d-PGJ2 inhibits the NLRP1 and NLRP3 inflammasomes.

Inflammasomes are cytosolic protein complexes that respond to diverse danger signals by activating caspase-1. The sensor components of the inflammasome, often proteins of the nucleotide-binding oligomerization domain-like receptor (NLR) family, detect stress, danger stimuli, and pathogen-associated molecular patterns. We report that the eicosanoid 15-deoxy-Δ(12,14)-PGJ2 (15d-PGJ2) and related cyclopentenone PGs inhibit caspase-1 activation by the NLR family leucine-rich repeat protein (NLRP)1 and NLRP3 inflammasomes. This inhibition was independent of the well-characterized role of 15d-PGJ2 as a peroxisome proliferator receptor-γ agonist, its activation of NF erythroid 2-related factor 2, or its anti-inflammatory function as an inhibitor of NF-κB. Instead, 15d-PGJ2 prevents the autoproteolytic activation of caspase-1 and the maturation of IL-1ß through induction of a cellular state inhibitory to caspase-1 proteolytic function. The eicosanoid does not directly modify or inactivate the caspase-1 enzyme. Rather, inhibition is dependent on de novo protein synthesis. In a mouse peritonitis model of gout, using monosodium urate crystals to activate NLRP3, 15d-PGJ2 caused a significant inhibition of cell recruitment and associated IL-1ß release. Furthermore, in a murine anthrax infection model, 15d-PGJ2 reversed anthrax lethal toxin-mediated NLRP1-dependent resistance. The findings reported in this study suggest a novel mechanism for the anti-inflammatory properties of the cyclopentenone PGs through inhibition of caspase-1 and the inflammasome.

Total Synthesis Confirms the Molecular Structure Proposed for Oxidized Levuglandin D2.

Levuglandins (LG)D2 and LGE2 are γ-ketoaldehyde levulinaldehyde derivatives with prostanoid side chains produced by spontaneous rearrangement of the endoperoxide intermediate PGH2 in the biosynthesis of prostaglandins. Covalent adduction of LGs with the amyloid peptide Aß1-42 promotes formation of the type of oligomers that have been associated with neurotoxicity and are a pathologic hallmark of Alzheimer's disease. Within 1 min of their generation during the production of PGH2 by cyclooxygenation of arachidonic acid, LGs are sequestered by covalent adduction to proteins. In view of this high proclivity for covalent adduction, it is understandable that free LGs have never been detected in vivo. Recently a catabolite, believed to be an oxidized derivative of LGD2 (ox-LGD2), a levulinic acid hydroxylactone with prostanoid side chains, was isolated from the red alga Gracilaria edulis and detected in mouse tissues and in the lysate of phorbol-12-myristate-13-acetate-treated THP-1 cells incubated with arachidonic acid. Such oxidative catabolism of LGD2 is remarkable because it must be outstandingly efficient to prevail over adduction with proteins and because it requires a unique dehydrogenation. We now report a concise total synthesis that confirms the molecular structure proposed for ox-LGD2. The synthesis also produces ox-LGE2, which readily undergoes allylic rearrangement to Δ6-ox-LGE2.

Synthesis and Biological Investigation of Δ(12)-Prostaglandin J3 (Δ(12)-PGJ3) Analogues and Related Compounds.

A series of Δ(12)-prostaglandin J3 (Δ(12)-PGJ3) analogues and derivatives were synthesized employing an array of synthetic strategies developed specifically to render them readily available for biological investigations. The synthesized compounds were evaluated for their cytotoxicity against a number of cancer cell lines, revealing nanomolar potencies for a number of them against certain cancer cell lines. Four analogues (2, 11, 21, and 27) demonstrated inhibition of nuclear export through a covalent addition at Cys528 of the export receptor Crm1. One of these compounds (i.e., 11) is currently under evaluation as a potential drug candidate for the treatment of certain types of cancer. These studies culminated in useful and path-pointing structure-activity relationships (SARs) that provide guidance for further improvements in the biological/pharmacological profiles of compounds within this class.

Identification and Structural Analysis of New Nrf2 Activators by Mechanism-Based Chemical Transformation of 15-Deoxy-Δ12, 14 -PGJ2.

Mechanism-based chemical transformation of 15-deoxy-Δ12, 14 -PGJ2 (15d-PGJ2 ) resulted in a series of new NF-E2-related factor-2 (Nrf2) activators and detailed elucidation of the function of each electrophilic binding site. In addition, HO-1 expression resulting from Nrf2 activation through enhanced dissociation of the Keap1-Nrf2 complex by the new activators was proved.

ROSics: chemistry and proteomics of cysteine modifications in redox biology.

Post-translational modifications (PTMs) occurring in proteins determine their functions and regulations. Proteomic tools are available to identify PTMs and have proved invaluable to expanding the inventory of these tools of nature that hold the keys to biological processes. Cysteine (Cys), the least abundant (1-2%) of amino acid residues, are unique in that they play key roles in maintaining stability of protein structure, participating in active sites of enzymes, regulating protein function and binding to metals, among others. Cys residues are major targets of reactive oxygen species (ROS), which are important mediators and modulators of various biological processes. It is therefore necessary to identify the Cys-containing ROS target proteins, as well as the sites and species of their PTMs. Cutting edge proteomic tools which have helped identify the PTMs at reactive Cys residues, have also revealed that Cys residues are modified in numerous ways. These modifications include formation of disulfide, thiosulfinate and thiosulfonate, oxidation to sulfenic, sulfinic, sulfonic acids and thiosulfonic acid, transformation to dehydroalanine (DHA) and serine, palmitoylation and farnesylation, formation of chemical adducts with glutathione, 4-hydroxynonenal and 15-deoxy PGJ2, and various other chemicals. We present here, a review of relevant ROS biology, possible chemical reactions of Cys residues and details of the proteomic strategies employed for rapid, efficient and sensitive identification of diverse and novel PTMs involving reactive Cys residues of redox-sensitive proteins. We propose a new name, "ROSics," for the science which describes the principles of mode of action of ROS at molecular levels.

Implication of the anti-inflammatory bioactive lipid prostaglandin D2-glycerol ester in the control of macrophage activation and inflammation by ABHD6.

Proinflammatory macrophages are key mediators in several pathologies; thus, controlling their activation is necessary. The endocannabinoid system is implicated in various inflammatory processes. Here we show that in macrophages, the newly characterized enzyme α/ß-hydrolase domain 6 (ABHD6) controls 2-arachidonoylglycerol (2-AG) levels and thus its pharmacological effects. Furthermore, we characterize a unique pathway mediating the effects of 2-AG through its oxygenation by cyclooxygenase-2 to give rise to the anti-inflammatory prostaglandin D2-glycerol ester (PGD2-G). Pharmacological blockade of cyclooxygenase-2 or of prostaglandin D synthase prevented the effects of increasing 2-AG levels by ABHD6 inhibition in vitro, as well as the 2-AG-induced increase in PGD2-G levels. Together, our data demonstrate the physiological relevance of the interaction between the endocannabinoid and prostanoid systems. Moreover, we show that ABHD6 inhibition in vivo allows for fine-tuning of 2-AG levels in mice, therefore reducing lipopolysaccharide-induced inflammation, without the characteristic central side effects of strong increases in 2-AG levels obtained following monoacylglycerol lipase inhibition. In addition, administration of PGD2-G reduces lipopolysaccharide-induced inflammation in mice, thus confirming the biological relevance of this 2-AG metabolite. This points to ABHD6 as an interesting therapeutic target that should be relevant in treating inflammation-related conditions, and proposes PGD2-G as a bioactive lipid with potential anti-inflammatory properties in vivo.

15-Deoxy-Δ¹²,¹4-prostaglandin J2 as an electrophilic mediator.

Lipid-derived electrophilic molecules are endogenously generated and are causally involved in many pathophysiological effects. Prostaglandin D2, a major cyclooxygenase product in a variety of tissues and cells, readily undergoes dehydration to yield the J-series PGs such as 15-deoxy-Δ(12,14)-PGJ2 (15d-PGJ2). Because of the electrophilic α,ß-unsaturated ketone moiety present in its cyclopentenone ring, 15d-PGJ2 acts as an endogenous electrophile. 15d-PGJ2 can covalently react via the Michael addition reaction with critical cellular nucleophiles, such as the free cysteine residues of proteins that play a key role in the regulation of the intracellular signaling pathways. Covalent modification of cellular proteins by 15d-PGJ2 may be one of the most important mechanisms by which 15d-PGJ2 induces many biological responses involved in the pathophysiological effects associated with inflammation. This current review is intended to provide a comprehensive summary of 15d-PGJ2 as an endogenous electrophilic mediator of biological activities.

Inhibition of transglutaminase 2, a novel target for pulmonary fibrosis, by two small electrophilic molecules.

Idiopathic pulmonary fibrosis (IPF) is characterized by progressive fibrotic destruction of normal lung architecture. Due to a lack of effective treatment options, new treatment approaches are needed. We previously identified transglutaminase (TG)2, a multifunctional protein expressed by human lung fibroblasts (HLFs), as a positive driver of fibrosis. TG2 catalyzes crosslinking of extracellular matrix proteins, enhances cell binding to fibronectin and integrin, and promotes fibronectin expression. We investigated whether the small electrophilic molecules 2-cyano-3,12-dioxoolean-1,9-dien-28-oic acid (CDDO) and 15-deoxy-delta-12,14-prostaglandin J2 (15d-PGJ2) inhibit the expression and profibrotic functions of TG2. CDDO and 15d-PGJ2 reduced expression of TG2 mRNA and protein in primary HLFs from control donors and donors with IPF. CDDO and 15d-PGJ2 also decreased the in vitro profibrotic effector functions of HLFs including collagen gel contraction and cell migration. The decrease in TG2 expression did not occur through activation of the peroxisome proliferator activated receptor γ or generation of reactive oxidative species. CDDO and 15d-PGJ2 inhibited the extracellular signal-regulated kinase pathway, resulting in the suppression of TG2 expression. This is the first study to show that small electrophilic compounds inhibit the expression and profibrotic effector functions of TG2, a key promoter of fibrosis. These studies identify new and important antifibrotic activities of these two small molecules, which could lead to new treatments for fibrotic lung disease.

Levuglandin forms adducts with histone h4 in a cyclooxygenase-2-dependent manner, altering its interaction with DNA.

Inflammation and subsequent cyclooxygenase-2 (COX-2) activity has long been linked with the development of cancer, although little is known about any epigenetic effects of COX-2. A product of COX-2 activation, levuglandin (LG) quickly forms covalent bonds with nearby primary amines, such as those in lysine, which leads to LG-protein adducts. Here, we demonstrate that COX-2 activity causes LG-histone adducts in cultured cells and liver tissue, detectable through LC-MS, with the highest incidence in histone H4. Adduction is blocked by a γ-ketoaldehyde scavenger, which has no effect on COX-2 activity as measured by PGE2 production. Formation of the LG-histone adduct is associated with an increased histone solubility in NaCl, indicating destabilization of the nucleosome structure; this is also reversed with scavenger treatment. These data demonstrate that COX-2 activity can cause histone adduction and loosening of the nucleosome complex, which could lead to altered transcription and contribute to carcinogenesis.

Proteomic identification of the candidate target proteins of 15-deoxy-delta12,14-prostaglandin J2.

15-Deoxy-delta12, 14-prostaglandin J(2) (15d-PGJ(2)) is an endogenous anti-inflammatory lipid derived from PGD(2). One potential mechanism for its activity is the covalent modification of cellular proteins, via a reactive α,ß-unsaturated carbonyl group in its cyclopentenone ring, which in turn alters protein function. In order to identify the candidate target proteins covalently modified by 15d-PGJ(2) in human aortic endothelial cell (EC), EC was treated with biotinylated-15d-PGJ(2), the modified proteins extracted by Neutravidin affinity-purification and the proteins identified by LTQ Orbitrap mass spectrometer. Classification of the 358 identified proteins was performed using PANTHER classification system (www.pantherdb.org), showing that the proteins mapped to metabolic process, cellular process, and transport activity. This protein data set highlights the potential for 15d-PGJ(2) to covalently modify cellular proteins and provides a source of data that will aid further studies on the mechanism of action of this endogenous regulator of inflammation.

The antagonistic actions of endogenous interleukin-1ß and 15-deoxy-Δ12,14-prostaglandin J2 regulate the temporal synthesis of matrix metalloproteinase-9 in sheared chondrocytes.

Mechanical overloading of articular cartilage producing hydrostatic stress, tensile strain, and fluid flow results in irreversible cartilage erosion and osteoarthritis (OA). Application of high fluid shear to chondrocytes recapitulates the earmarks of OA as evidenced by the induction of proinflammatory cytokines and prostaglandins, which are capable of inducing the expression of matrix-degrading enzymes. Matrix metalloproteinase-9 (MMP-9) synthesis is detected at early but not late stages of OA. However, the underlying mechanism(s) of the MMP-9 temporal regulation remains unknown. Using the T/C-28a2 chondrocyte cell line as a model system, we demonstrated that high fluid shear induces a marked increase in MMP-9 expression at short shear exposure times (3-6 h), which falls below basal levels after prolonged shear exposure (12-48 h). High fluid shear stress induced the rapid and sustained synthesis of IL-1ß, activating PI3K, ERK1/2, and JNK, which are in turn responsible for MMP-9 expression. Prolonged shear exposure (>12 h) induced 15-deoxy-Δ(12,14)-prostaglandin J(2) (15d-PGJ(2)) synthesis, which exerted an antagonistic effect on IL-1ß-mediated PI3K-, ERK1/2-, and JNK-dependent NF-κB activation, thereby suppressing MMP-9 expression in human chondrocytes. Reconstructing the signaling network that regulates shear-mediated MMP-9 expression in human chondrocytes may provide insights for developing strategies to treat arthritic disorders.

Cyclopentenone prostaglandin-induced unfolding and aggregation of the Parkinson disease-associated UCH-L1.

Ubiquitin carboxyl-terminal hydrolase L1 (UCH-L1) has been implicated in Parkinson's disease (PD) and is present in neurofibrillary tangles or Lewy bodies. However, the molecular basis for UCH-L1s involvement in proteinacious fibril formation is still elusive, especially in regard to the pathogenicity of the I93M mutation. Here we show that modification of UCH-L1 by cyclopentenone prostaglandins causes unfolding and aggregation. A single thiol group on Cys152 reacts with the alpha,beta-unsaturated carbonyl center in the cyclopentenone ring of prostaglandins, resulting in a covalent adduct. We also show that the PD-associated I93M mutant of UCH-L1 is well-folded, structurally similar to the wild-type protein, and aggregates upon conjugation by cyclopentenone prostaglandins. Our findings suggest a possible mechanistic link between UCH-L1 modification by cyclopentenone prostaglandins and the etiology of neurodegeneration.