Structure of the Hir histone chaperone complex.

The evolutionarily conserved HIRA/Hir histone chaperone complex and ASF1a/Asf1 co-chaperone cooperate to deposit histone (H3/H4)2 tetramers on DNA for replication-independent chromatin assembly. The molecular architecture of the HIRA/Hir complex and its mode of histone deposition have remained unknown. Here, we report the cryo-EM structure of the S. cerevisiae Hir complex with Asf1/H3/H4 at 2.9-6.8 Å resolution. We find that the Hir complex forms an arc-shaped dimer with a Hir1/Hir2/Hir3/Hpc2 stoichiometry of 2/4/2/4. The core of the complex containing two Hir1/Hir2/Hir2 trimers and N-terminal segments of Hir3 forms a central cavity containing two copies of Hpc2, with one engaged by Asf1/H3/H4, in a suitable position to accommodate a histone (H3/H4)2 tetramer, while the C-terminal segments of Hir3 harbor nucleic acid binding activity to wrap DNA around the Hpc2-assisted histone tetramer. The structure suggests a model for how the Hir/Asf1 complex promotes the formation of histone tetramers for their subsequent deposition onto DNA.

Structural basis of a transcription pre-initiation complex on a divergent promoter.

Most eukaryotic promoter regions are divergently transcribed. As the RNA polymerase II pre-initiation complex (PIC) is intrinsically asymmetric and responsible for transcription in a single direction, it is unknown how divergent transcription arises. Here, the Saccharomyces cerevisiae Mediator complexed with a PIC (Med-PIC) was assembled on a divergent promoter and analyzed by cryoelectron microscopy. The structure reveals two distinct Med-PICs forming a dimer through the Mediator tail module, induced by a homodimeric activator protein localized near the dimerization interface. The tail dimer is associated with ∼80-bp upstream DNA, such that two flanking core promoter regions are positioned and oriented in a suitable form for PIC assembly in opposite directions. Also, cryoelectron tomography visualized the progress of the PIC assembly on the two core promoter regions, providing direct evidence for the role of the Med-PIC dimer in divergent transcription.

Structural visualization of de novo transcription initiation by Saccharomyces cerevisiae RNA polymerase II.

Previous structural studies of the initiation-elongation transition of RNA polymerase II (pol II) transcription have relied on the use of synthetic oligonucleotides, often artificially discontinuous to capture pol II in the initiating state. Here, we report multiple structures of initiation complexes converted de novo from a 33-subunit yeast pre-initiation complex (PIC) through catalytic activities and subsequently stalled at different template positions. We determine that PICs in the initially transcribing complex (ITC) can synthesize a transcript of ∼26 nucleotides before transitioning to an elongation complex (EC) as determined by the loss of general transcription factors (GTFs). Unexpectedly, transition to an EC was greatly accelerated when an ITC encountered a downstream EC stalled at promoter proximal regions and resulted in a collided head-to-end dimeric EC complex. Our structural analysis reveals a dynamic state of TFIIH, the largest of GTFs, in PIC/ITC with distinct functional consequences at multiple steps on the pathway to elongation.

Expanding the Epitranscriptomic RNA Sequencing and Modification Mapping Mass Spectrometry Toolbox with Field Asymmetric Waveform Ion Mobility and Electrochemical Elution Liquid Chromatography.

Post-transcriptional modifications of RNA strongly influence the RNA structure and function. Recent advances in RNA sequencing and mass spectrometry (MS) methods have identified over 140 of these modifications on a wide variety of RNA species. Most next-generation sequencing approaches can only map one RNA modification at a time, and while MS can assign multiple modifications simultaneously in an unbiased manner, MS cannot accurately catalog and assign RNA modifications in complex biological samples due to limitations in the fragment length and coverage depth. Thus, a facile method to identify novel RNA modifications while simultaneously locating them in the context of their RNA sequences is still lacking. We combined two orthogonal modes of RNA ion separation before MS identification: high-field asymmetric ion mobility separation (FAIMS) and electrochemically modulated liquid chromatography (EMLC). FAIMS RNA MS increases both coverage and throughput, while EMLC LC-MS orthogonally separates RNA molecules of different lengths and charges. The combination of the two methods offers a broadly applicable platform to improve the length and depth of MS-based RNA sequencing while providing contextual access to the analysis of RNA modifications.

Cryo-EM Structure and Functional Studies of EBNA1 Binding to the Family of Repeats and Dyad Symmetry Elements of Epstein-Barr Virus oriP.

Epstein-Barr nuclear antigen 1 (EBNA1) is a multifunctional viral-encoded DNA-binding protein essential for Epstein-Barr virus (EBV) DNA replication and episome maintenance. EBNA1 binds to two functionally distinct elements at the viral origin of plasmid replication (oriP), termed the dyad symmetry (DS) element, required for replication initiation and the family of repeats (FR) required for episome maintenance. Here, we determined the cryo-electron microscopy (cryo-EM) structure of the EBNA1 DNA binding domain (DBD) from amino acids (aa) 459 to 614 and its interaction with two tandem sites at the DS and FR. We found that EBNA1 induces a strong DNA bending angle in the DS, while the FR is more linear. The N-terminal arm of the DBD (aa 444 to 468) makes extensive contact with DNA as it wraps around the minor groove, with some conformational variation among EBNA1 monomers. Mutation of variable-contact residues K460 and K461 had only minor effects on DNA binding but had abrogated oriP-dependent DNA replication. We also observed that the AT-rich intervening DNA between EBNA1 binding sites in the FR can be occupied by the EBNA1 AT hook, N-terminal domain (NTD) aa 1 to 90 to form a Zn-dependent stable complex with EBNA1 DBD on a 2×FR DNA template. We propose a model showing EBNA1 DBD and NTD cobinding at the FR and suggest that this may contribute to the oligomerization of viral episomes important for maintenance during latent infection. IMPORTANCE EBV latent infection is causally linked to diverse cancers and autoimmune disorders. EBNA1 is the viral-encoded DNA binding protein required for episomal maintenance during latent infection and is consistently expressed in all EBV tumors. The interaction of EBNA1 with different genetic elements confers different viral functions, such as replication initiation at DS and chromosome tethering at FR. Here, we used cryo-EM to determine the structure of the EBNA1 DNA-binding domain (DBD) bound to two tandem sites at the DS and at the FR. We also show that the NTD of EBNA1 can interact with the AT-rich DNA sequence between tandem EBNA1 DBD binding sites in the FR. These results provide new information on the mechanism of EBNA1 DNA binding at DS and FR and suggest a higher-order oligomeric structure of EBNA1 bound to FR. Our findings have implications for targeting EBNA1 in EBV-associated disease.

scAAV2-Mediated Expression of Thioredoxin 2 and C3 Transferase Prevents Retinal Ganglion Cell Death and Lowers Intraocular Pressure in a Mouse Model of Glaucoma.

Elevated intraocular pressure (IOP) in glaucoma causes retinal ganglion cell (RGC) loss and damage to the optic nerve. Although IOP is controlled pharmacologically, no treatment is available to restore retinal and optic nerve function. In this paper, we aimed to develop a novel gene therapy for glaucoma using an AAV2-based thioredoxin 2 (Trx2)-exoenzyme C3 transferase (C3) fusion protein expression vector (scAAV2-Trx2-C3). We evaluated the therapeutic effects of this vector in vitro and in vivo using dexamethasone (DEX)-induced glaucoma models. We found that scAAV2-Trx2-C3-treated HeLa cells had significantly reduced GTP-bound active RhoA and increased phosphor-cofilin Ser3 protein expression levels. scAAV2-Trx2-C3 was also shown to inhibit oxidative stress, fibronectin expression, and alpha-SMA expression in DEX-treated HeLa cells. NeuN immunostaining and TUNEL assay in mouse retinal tissues was performed to evaluate its neuroprotective effect upon RGCs, whereas changes in mouse IOP were monitored via rebound tonometer. The present study showed that scAAV2-Trx2-C3 can protect RGCs from degeneration and reduce IOP in a DEX-induced mouse model of glaucoma, while immunohistochemistry revealed that the expression of fibronectin and alpha-SMA was decreased after the transduction of scAAV2-Trx2-C3 in murine eye tissues. Our results suggest that AAV2-Trx2-C3 modulates the outflow resistance of the trabecular meshwork, protects retinal and other ocular tissues from oxidative damage, and may lead to the development of a gene therapeutic for glaucoma.

One minute analysis of 200 histone posttranslational modifications by direct injection mass spectrometry.

DNA and histone proteins define the structure and composition of chromatin. Histone posttranslational modifications (PTMs) are covalent chemical groups capable of modeling chromatin accessibility, mostly due to their ability in recruiting enzymes responsible for DNA readout and remodeling. Mass spectrometry (MS)-based proteomics is the methodology of choice for large-scale identification and quantification of protein PTMs, including histones. High sensitivity proteomics requires online MS coupling with relatively low throughput and poorly robust nano-liquid chromatography (nanoLC) and, for histone proteins, a 2-d sample preparation that includes histone purification, derivatization, and digestion. We present a new protocol that achieves quantitative data on about 200 histone PTMs from tissue or cell lines in 7 h from start to finish. This protocol includes 4 h of histone extraction, 3 h of derivatization and digestion, and only 1 min of MS analysis via direct injection (DI-MS). We demonstrate that this sample preparation can be parallelized for 384 samples by using multichannel pipettes and 96-well plates. We also engineered the sequence of a synthetic "histone-like" peptide to spike into the sample, of which derivatization and digestion benchmarks the quality of the sample preparation. We ensure that DI-MS does not introduce biases in histone peptide ionization as compared to nanoLC-MS/MS by producing and analyzing a library of synthetically modified histone peptides mixed in equal molarity. Finally, we introduce EpiProfileLite for comprehensive analysis of this new data type. Altogether, our workflow is suitable for high-throughput screening of >1000 samples per day using a single mass spectrometer.

Two Cases of Mange Mite (Sarcoptes scabiei) Infestation in Long-Tailed Goral (Naemorhedus caudatus) in Republic of Korea.

The long-tailed goral, Naemorhedus caudatus (Mammalia: Bovidae), is one of the endangered animals in the Republic of Korea (Korea). Sarcoptic mange mites infested in diverse species of mammals, including humans, but no case has been reported in long-tailed gorals. We report 2 cases of mange mite, Sarcoptes scabiei, infestation in long-tailed gorals. Mange mites were sampled in the skin legions of the 2 long-tailed gorals, which were rescued in 2 different regions, Uljin-gun, Gyeongsangbuk-do and Cheorwon-gun, Gangwon-do, Korea. Our results showed that the ectoparasite was the itch mite that burrowed into skin and caused scabies on the morphological inspection and placed within the phylogenetic relations of the species. The present study confirmed for the first time in Korea that mange mites are pathogenic scabies of long-tailed goral. Closer surveillance of this pathogenic ectoparasite in zoonotic and infectious ecosystems is warranted.

Metal-free class Ie ribonucleotide reductase from pathogens initiates catalysis with a tyrosine-derived dihydroxyphenylalanine radical.

All cells obtain 2'-deoxyribonucleotides for DNA synthesis through the activity of a ribonucleotide reductase (RNR). The class I RNRs found in humans and pathogenic bacteria differ in (i) use of Fe(II), Mn(II), or both for activation of the dinuclear-metallocofactor subunit, ß; (ii) reaction of the reduced dimetal center with dioxygen or superoxide for this activation; (iii) requirement (or lack thereof) for a flavoprotein activase, NrdI, to provide the superoxide from O2; and (iv) use of either a stable tyrosyl radical or a high-valent dimetal cluster to initiate each turnover by oxidizing a cysteine residue in the α subunit to a radical (Cys•). The use of manganese by bacterial class I, subclass b-d RNRs, which contrasts with the exclusive use of iron by the eukaryotic Ia enzymes, appears to be a countermeasure of certain pathogens against iron deprivation imposed by their hosts. Here, we report a metal-free type of class I RNR (subclass e) from two human pathogens. The Cys• in its α subunit is generated by a stable, tyrosine-derived dihydroxyphenylalanine radical (DOPA•) in ß. The three-electron oxidation producing DOPA• occurs in Escherichia coli only if the ß is coexpressed with the NrdI activase encoded adjacently in the pathogen genome. The independence of this new RNR from transition metals, or the requirement for a single metal ion only transiently for activation, may afford the pathogens an even more potent countermeasure against transition metal-directed innate immunity.

Sequential Windowed Acquisition of Reporter Masses for Quantitation-First Proteomics.

The standard approach for proteomic data acquisition of isobaric-tagged samples by mass spectrometry is data-dependent acquisition. This semistochastic, identification-first paradigm generates a wealth of peptide-level data without regard to relative abundance. We introduce a data acquisition concept called sequential windowed acquisition of reporter masses (SWARM). This approach performs quantitation first, thereby allowing subsequent acquisition decisions to be predicated on user-defined patterns of reporter ion intensities. The efficacy of this approach is validated through experiments with both synthetic mixtures of Escherichia coli ribosomes spiked into human cell lysates at known ratios and the quantitative evaluation of the human proteome's response to the inhibition of cullin-based protein ubiquitination via the small molecule MLN4924. We find that SWARM-informed parallel reaction monitoring acquisitions display effective acquisition biasing toward analytes displaying quantitative characteristics of interest, resulting in an improvement in the detection of differentially abundant analytes. The SWARM concept provides a flexible platform for the further development of new acquisition methods.

Chemical Derivatization of Affinity Matrices Provides Protection from Tryptic Proteolysis.

The enrichment of biotinylated proteins using immobilized streptavidin has become a staple methodology for affinity purification-based proteomics. Many of these workflows rely upon tryptic digestion to elute streptavidin-captured moieties from the beads. The concurrent release of high amounts of streptavidin-derived peptides into the digested sample, however, can significantly hamper the effectiveness of downstream proteomic analyses by increasing the complexity and dynamic range of the mixture. Here, we describe a strategy for the chemical derivatization of streptavidin that renders it largely resistant to proteolysis by trypsin and thereby dramatically reduces the amount of streptavidin contamination in the sample. This rapid and robust approach improves the effectiveness of mass spectrometry-based characterization of streptavidin-purified samples making it broadly useful for a wide variety of applications. In addition, we show that this chemical protection strategy can also be applied to other affinity matrices including immobilized antibodies against HA epitopes.

KCTD7 deficiency defines a distinct neurodegenerative disorder with a conserved autophagy-lysosome defect.

OBJECTIVE: Several small case series identified KCTD7 mutations in patients with a rare autosomal recessive disorder designated progressive myoclonic epilepsy (EPM3) and neuronal ceroid lipofuscinosis (CLN14). Despite the name KCTD (potassium channel tetramerization domain), KCTD protein family members lack predicted channel domains. We sought to translate insight gained from yeast studies to uncover disease mechanisms associated with deficiencies in KCTD7 of unknown function. METHODS: Novel KCTD7 variants in new and published patients were assessed for disease causality using genetic analyses, cell-based functional assays of patient fibroblasts and knockout yeast, and electron microscopy of patient samples. RESULTS: Patients with KCTD7 mutations can exhibit movement disorders or developmental regression before seizure onset, and are distinguished from similar disorders by an earlier age of onset. Although most published KCTD7 patient variants were excluded from a genome sequence database of normal human variations, most newly identified patient variants are present in this database, potentially challenging disease causality. However, genetic analysis and impaired biochemical interactions with cullin 3 support a causal role for patient KCTD7 variants, suggesting deleterious alleles of KCTD7 and other rare disease variants may be underestimated. Both patient-derived fibroblasts and yeast lacking Whi2 with sequence similarity to KCTD7 have impaired autophagy consistent with brain pathology. INTERPRETATION: Biallelic KCTD7 mutations define a neurodegenerative disorder with lipofuscin and lipid droplet accumulation but without defining features of neuronal ceroid lipofuscinosis or lysosomal storage disorders. KCTD7 deficiency appears to cause an underlying autophagy-lysosome defect conserved in yeast, thereby assigning a biological role for KCTD7. Ann Neurol 2018;84:774-788.

Copper-zinc superoxide dismutase is activated through a sulfenic acid intermediate at a copper ion entry site.

Metallochaperones are a diverse family of trafficking molecules that provide metal ions to protein targets for use as cofactors. The copper chaperone for superoxide dismutase (Ccs1) activates immature copper-zinc superoxide dismutase (Sod1) by delivering copper and facilitating the oxidation of the Sod1 intramolecular disulfide bond. Here, we present structural, spectroscopic, and cell-based data supporting a novel copper-induced mechanism for Sod1 activation. Ccs1 binding exposes an electropositive cavity and proposed "entry site" for copper ion delivery on immature Sod1. Copper-mediated sulfenylation leads to a sulfenic acid intermediate that eventually resolves to form the Sod1 disulfide bond with concomitant release of copper into the Sod1 active site. Sod1 is the predominant disulfide bond-requiring enzyme in the cytoplasm, and this copper-induced mechanism of disulfide bond formation obviates the need for a thiol/disulfide oxidoreductase in that compartment.

Serological evidence of H5-subtype influenza A virus infection in indigenous avian and mammalian species in Korea.

In Korea, H5-subtype highly pathogenic avian influenza (HPAI) has caused huge economic losses in poultry farms through outbreaks of H5N1 since 2003, H5N8 since 2013 and H5N6 since 2016. Although it was reported that long-distance migratory birds may play a major role in the global spread of avian influenza viruses (AIVs), transmission from such birds to poultry has not been confirmed. Intermediate hosts in the wild also may be a potential factor in viral transmission. Therefore, a total of 367 serum samples from wild animals were collected near major migratory bird habitats from 2011 to 2016 and tested by AIV-specific blocking ELISA and hemagglutination inhibition (HI) test. Two mammalian and eight avian species were seropositive according to the ELISA test. Among these, two mammalian (Hydropotes inermis and Prionailurus bengalensis) and three avian (Aegypius monachus, Cygnus cygnus, and Bubo bubo) species showed high HI titres (> 1,280) against one or two H5-subtype AIVs. As H. inermis (water deer), P. bengalensis (leopard cat), and B. bubo (Eurasian eagle owl) are indigenous animals in Korea, evidence of H5-subtype AIV in these animals implies that continuous monitoring of indigenous animals should be followed to understand interspecies transmission ecology of H5-subtype influenza viruses.

Role of HIF-1α in response of tumors to a combination of hyperthermia and radiation in vivo.

PURPOSE: Mild temperature hyperthermia (MTH) increases blood flow and oxygenation in tumours. On the other hand, high-dose-per-fraction irradiation damages blood vessels, decreases blood flow and increases hypoxia in tumours. The radiation-induced hypoxia in tumours activates hypoxia-inducible factor-1α (HIF-1α) and its target genes, such as vascular endothelial growth factor (VEGF), promoting revascularization and recurrence. In the present study, we examined the hypothesis that MTH inhibits radiation-induced upregulation of HIF-1α and its target genes by increasing tumour oxygenation. MATERIALS AND METHODS: FSaII fibrosarcoma tumours grown subcutaneously in the legs of C3H mice were used. Tumours were irradiated with 15 Gy using a 60Co irradiator or heated at 41 °C for 30 min using an Oncothermia heating unit. Blood perfusion and hypoxia in tumours were assessed with Hoechst 33342 and pimonidazole staining, respectively. Expression levels of HIF-1α and VEGF were determined using immunohistochemical techniques. Apoptosis of tumour cells was quantitated via TUNEL staining and the effects of treatments on tumour growth rate were assessed by measuring tumour diameters. RESULTS: Irradiation of FSaII tumours with a single dose of 15 Gy led to significantly decreased blood perfusion, increased hypoxia and upregulation of HIF-1α and VEGF. On the other hand, MTH at 41 °C for 30 min increased blood perfusion and tumour oxygenation, thereby suppressing radiation-induced HIF-1α and VEGF in tumours, leading to enhanced apoptosis of tumour cells and tumour growth delay. CONCLUSION: MTH enhances the anti-tumour effect of high-dose irradiation, at least partly by inhibiting radiation-induced upregulation of HIF-1α.

Serological and Molecular Detection of Toxoplasma gondii and Babesia microti in the Blood of Rescued Wild Animals in Gangwon-do (Province), Korea.

Infections of Toxoplasma gondii and Babesia microti are reported in many wild animals worldwide, but information on their incidence and molecular detection in Korean wild fields is limited. In this study, the prevalence of T. gondii and B. microti infection in blood samples of 5 animal species (37 Chinese water deer, 23 raccoon dogs, 6 roe deer, 1 wild boar, and 3 Eurasian badgers) was examined during 2008-2009 in Gangwon-do (Province), the Republic of Korea (=Korea) by using serological and molecular tests. The overall seropositivity of T. gondii was 8.6% (6/70); 10.8% in Chinese water deer, 4.3% in raccoon dogs, and 16.7% in roe deer. PCR revealed only 1 case of T. gondii infection in Chinese water deer, and phylogenic analysis showed that the positive isolate was practically identical to the highly pathogenetic strain type I. In B. microti PCR, the positive rate was 5.7% (4/70), including 2 Chinese water deer and 2 Eurasian badgers. Phylogenetic analysis results of 18S rRNA and the ß-tubulin gene showed that all positive isolates were US-type B. microti. To our knowledge, this is the first report of B. microti detected in Chinese water deer and Eurasian badger from Korea. These results indicate a potentially high prevalence of T. gondii and B. microti in wild animals of Gangwon-do, Korea. Furthermore, Chinese water deer might act as a reservoir for parasite infections of domestic animals.

Successful repair of coracoid and femur fractures in a Eurasian eagle owl (Bubo bubo) and its post-release survival.

To our knowledge, this is the first case of concurrent diaphyseal left coracoid and left femoral fractures in a Eurasian eagle owl and its post-release survival in Korea. The femur was surgically repaired using an external skeletal fixator-intramedullary (IM) pin tie-in method, and the coracoid was repaired solely with an IM pin on day 6 after femur surgery. The eagle owl underwent a gradual rehabilitation process. The bird was successfully rehabilitated and released 101 d after initial presentation. The bird was monitored using a wildlife tracking device and was confirmed to have survived for over 5 mon in the wild.

A case of rickets in an artificially raised white-tailed eagle (Haliaeetus albicilla) chick at a zoo.

A two-week-old white-tailed eagle presented with an inability to stand and flex its limbs. Despite hatching naturally and owing to lack of parental attention, the bird was raised indoors by zookeepers with no access to sunlight. Palpation and radiographic examination of the bilateral tibiotarsus and femur bone revealed pronounced deformation and curvature, and bilateral decreased bone densities, respectively. The reduced calcium concentration in the blood was treated with calcium gluconate injections and calcium-supplemented feeds. Chopped mouse tails were fed directly, and whole pink-skinned nude mice were fed weekly. The zookeeper also gently massaged the bird and dressed it with a bandage. Sunlight exposure was provided daily. Saliva containing chicken feed was obtained from the mother. The bird could stand properly after four weeks of treatment, and the blood calcium concentration was restored to normal levels.

Application of 3D-printed prosthetic lower beak in an Oriental stork (Ciconia boyciana).

To our knowledge, ours is the first case of applying a 3D-printed prosthetic beak to an Oriental stork (Ciconia boyciana). A stork in captivity underwent several surgeries for beak fractures, but the lower-mandible fractures failed to be repaired. Therefore, we applied a patient-specific beak prosthesis of titanium alloy and nylon. Because the prosthetic beak could not be maintained due to mandible and soft-tissue inflammation, the stork was euthanized. Still, we confirmed typical behavior and feeding for ~3 months after surgery. This report highlights some of the challenges we encountered and identifies process improvements required for a more successful surgery.

Identification of a Putative α-synuclein Radioligand Using an in silico Similarity Search.

PURPOSE: Previous studies from our lab utilized an ultra-high throughput screening method to identify compound 1 as a small molecule that binds to alpha-synuclein (α-synuclein) fibrils. The goal of the current study was to conduct a similarity search of 1 to identify structural analogs having improved in vitro binding properties for this target that could be labeled with radionuclides for both in vitro and in vivo studies for measuring α-synuclein aggregates. METHODS: Using 1 as a lead compound in a similarity search, isoxazole derivative 15 was identified to bind to α-synuclein fibrils with high affinity in competition binding assays. A photocrosslinkable version was used to confirm binding site preference. Derivative 21, the iodo-analog of 15, was synthesized, and subsequently radiolabeled isotopologs [125I]21 and [11C]21 were successfully synthesized for use in in vitro and in vivo studies, respectively. [125I]21 was used in radioligand binding studies in post-mortem Parkinson's disease (PD) and Alzheimer's disease (AD) brain homogenates. In vivo imaging of an α-synuclein mouse model and non-human primates was performed with [11C]21. RESULTS: In silico molecular docking and molecular dynamic simulation studies for a panel of compounds identified through a similarity search, were shown to correlate with Ki values obtained from in vitro binding studies. Improved affinity of isoxazole derivative 15 for α-synuclein binding site 9 was indicated by photocrosslinking studies with CLX10. Design and successful (radio)synthesis of iodo-analog 21 of isoxazole derivative 15 enabled further in vitro and in vivo evaluation. Kd values obtained in vitro with [125I]21 for α-synuclein and Aß42 fibrils were 0.48 ± 0.08 nM and 2.47 ± 1.30 nM, respectively. [125I]21 showed higher binding in human postmortem PD brain tissue compared with AD tissue, and low binding in control brain tissue. Lastly, in vivo preclinical PET imaging showed elevated retention of [11C]21 in PFF-injected mouse brain. However, in PBS-injected control mouse brain, slow washout of the tracer indicates high non-specific binding. [11C]21 showed high initial brain uptake in a healthy non-human primate, followed by fast washout that may be caused by rapid metabolic rate (21% intact [11C]21 in blood at 5 min p.i.). CONCLUSION: Through a relatively simple ligand-based similarity search, we identified a new radioligand that binds with high affinity (<10 nM) to α-synuclein fibrils and PD tissue. Although the radioligand has suboptimal selectivity for α-synuclein towards Aß and high non-specific binding, we show here that a simple in silico approach is a promising strategy to identify novel ligands for target proteins in the CNS with the potential to be radiolabeled for PET neuroimaging studies.