Phosphorylation of α-synuclein at T64 results in distinct oligomers and exerts toxicity in models of Parkinson's disease.

α-Synuclein accumulates in Lewy bodies, and this accumulation is a pathological hallmark of Parkinson's disease (PD). Previous studies have indicated a causal role of α-synuclein in the pathogenesis of PD. However, the molecular and cellular mechanisms of α-synuclein toxicity remain elusive. Here, we describe a novel phosphorylation site of α-synuclein at T64 and the detailed characteristics of this post-translational modification. T64 phosphorylation was enhanced in both PD models and human PD brains. T64D phosphomimetic mutation led to distinct oligomer formation, and the structure of the oligomer was similar to that of α-synuclein oligomer with A53T mutation. Such phosphomimetic mutation induced mitochondrial dysfunction, lysosomal disorder, and cell death in cells and neurodegeneration in vivo, indicating a pathogenic role of α-synuclein phosphorylation at T64 in PD.

Galectin-3 promotes the adipogenic differentiation of PDGFRα+ cells and ectopic fat formation in regenerating muscle.

Worldwide prevalence of obesity is associated with the increase of lifestyle-related diseases. The accumulation of intermuscular adipose tissue (IMAT) is considered a major problem whereby obesity leads to sarcopenia and metabolic disorders and thus is a promising target for treating these pathological conditions. However, whereas obesity-associated IMAT is suggested to originate from PDGFRα+ mesenchymal progenitors, the processes underlying this adipogenesis remain largely unexplored. Here, we comprehensively investigated intra- and extracellular changes associated with these processes using single-cell RNA sequencing and mass spectrometry. Our single-cell RNA sequencing analysis identified a small PDGFRα+ cell population in obese mice directed strongly toward adipogenesis. Proteomic analysis showed that the appearance of this cell population is accompanied by an increase in galectin-3 in interstitial environments, which was found to activate adipogenic PPARγ signals in PDGFRα+ cells. Moreover, IMAT formation during muscle regeneration was significantly suppressed in galectin-3 knockout mice. Our findings, together with these multi-omics datasets, could unravel microenvironmental networks during muscle regeneration highlighting possible therapeutic targets against IMAT formation in obesity.

Phosphoregulation of the transcription factor Mxr1 plays a crucial role in the concentration-regulated methanol induction in Komagataella phaffii.

Methylotrophic yeasts can utilize methanol as the sole carbon and energy source, and the expression of their methanol-induced genes is regulated based on the environmental methanol concentration. Our understanding of the function of transcription factors and Wsc family of proteins in methanol-induced gene expression and methanol sensing is expanding, but the methanol signal transduction mechanism remains undetermined. Our study has revealed that the transcription factor KpMxr1 is involved in the concentration-regulated methanol induction (CRMI) in Komagataella phaffii (Pichia pastoris) and that the phosphorylation state of KpMxr1 changes based on methanol concentration. We identified the functional regions of KpMxr1 and determined its multiple phosphorylation sites. Non-phosphorylatable substitution mutations of these newly identified phosphorylated threonine and serine residues resulted in significant defects in CRMI. We revealed that KpMxr1 receives the methanol signal from Wsc family proteins via KpPkc1 independent of the mitogen-activated protein kinase (MAPK) cascade and speculate that the activity of KpPkc1 influences KpMxr1 phosphorylation state. We propose that the CRMI pathway from Wsc to KpMxr1 diverges from KpPkc1 and that phosphoregulation of KpMxr1 plays a crucial role in CRMI.

A pilot study assessing sphingolipids and glycolipids dysmetabolism in idiopathic normal pressure hydrocephalus.

Idiopathic normal pressure hydrocephalus usually exhibits triad of symptoms including gait disturbance, urinary incontinence, and dementia with ventriculomegaly. Currently, its pathogenesis remains to be fully elucidated. To provide a better understanding of this order, we examined whether dysmetabolism of sphingolipids as major lipid components in the brain present in cerebrospinal fluid (CSF) of the patients. Here, we measured various sphingolipidsincluding ceramide and sphingomyelin and glycolipids by electrospray ionization-tandem mass spectrometry in the cerebrospinal fluid of 19 consecutive idiopathic normal pressure hydrocephalus patients, 49 Parkinson's disease patients, and 17 neurologically normal controls. The data showed that there was a significant and specific reduction of all galactosylceramide subspecies levels in idiopathic normal pressure hydrocephalus patients compared with other groups, whereas ceramide and sphingomyelin levels as well as other neutral glycolipids such as glucosylceramide and lactosylceramide were similar in both disease states. Multiple regression analysis of sex and age did not show any correlation with galactosylceramide levels. We also examined whether MMSE scores are correlated with sphingolipid levels in iNPH patients. A specific subspecies of sphingomyelin (d18:1/18:0) only exhibited a statistically significant negative correlation (p = 0.0473, R = -0.4604) with MMSE scores but no other sphingolipids in iNPH patients. These data strongly suggest that myelin-rich galactosylceramide metabolism is severely impaired in idiopathic normal pressure hydrocephalus patients and might serve as the basis of biomarker for this disorder.

Proteome analysis of CD5-positive diffuse large B cell lymphoma FFPE tissue reveals downregulation of DDX3X, DNAJB1, and B cell receptor signaling pathway proteins including BTK and Immunoglobulins.

BACKGROUND: The molecular pathology of diffuse large B cell lymphoma (DLBCL) has been extensively studied. Among DLBCL subtypes, the prognosis of CD5-positive DLBCL is worse than that of CD5-negative DLBCL, considering the central nervous system relapse and poor response to R-CHOP therapy. However, the molecular mechanisms underlying the tumorigenesis and progression of CD5-positive DLBCL remain unknown. METHODS: To identify molecular markers that can be targeted for treating DLBCL, a proteomic study was performed using liquid chromatography-mass spectrometry with chemically pretreated formalin-fixed paraffin-embedded specimens from CD5-positive (n = 5) and CD5-negative DLBCL patients (n = 6). RESULTS: Twenty-one proteins showed significant downregulation in CD5-positive DLBCL compared to CD5-negative DLBCL. Principal component analysis of protein expression profiling in CD5-positive and CD5-negative DLBCL revealed that DNAJB1, DDX3X, and BTK, which is one of the B cell phenotypic proteins, were the most significantly downregulated proteins and served as biomarkers that distinguished both groups. Additionally, a set of immunoglobulins, including IgG4, exhibited significant downregulation. Immunohistochemistry analysis for BTK demonstrated reduced staining in CD5-positive DLBCL compared to CD5-negative DLBCL. CONCLUSIONS: In conclusion, DNAJB1 and DDX3X, BTK, and a set of immunoglobulins are promising biomarkers. Probably, the suppression of BCR signaling is the unique phenotype of CD5-positive DLBCL. This formalin-fixed paraffin-embedded (FFPE)-based profiling may help to develop novel therapeutic molecularly targeted drugs for treating DLBCL.

Clinical predictors of cardiac syncope in patients with unexplained syncope after the implantation of an insertable cardiac monitor.

Syncope prognosis is related to both its etiology and comorbidities, with cardiac syncope (CS) having higher risks for mortality and cardiovascular events than syncope of non-cardiac causes. Although a novel insertable cardiac monitor (ICM) is an effective diagnostic tool for unexplained syncope, decision regarding ICM implantation with a high pre-test likelihood of CS should contribute to economic cost reduction and avoidance of unnecessary complications. This study aimed to investigate clinical factors associated with CS after ICM implantation in patients with unexplained syncope. This retrospective observational study included 31 consecutive patients with ICM implantation for syncope between September 2016 and August 2021. The initial examinations for syncope included a detailed history, physical examination, blood tests, 12-lead electrocardiograms, and transthoracic echocardiography. Of the 31 patients, 13 (41.9%) experienced recurrent CS during follow-up (676 ± 469 days). Among several clinical factors, syncope-related minor injuries (p = 0.017) and higher brain natriuretic peptide (BNP; p = 0.043) levels were significantly associated with CS. Moreover, multivariable analysis showed that both syncope-related minor injuries (odds ratio, 11.2; 95% confidence interval, 1.4-88.4; p = 0.022) and BNP higher than 64.0 pg/mL (odds ratio, 7.0; 95% confidence interval, 1.1-44.2; p = 0.038) were independent predictors of CS after ICM implantation. In conclusion, a history of minor injury secondary to syncope and higher BNP levels were independent CS predictors in patients receiving ICM for syncope. These results emphasized the utility of ICM implantation early in the diagnostic journey of patients presenting with CS predictors requiring specific treatments.

Limitations and benefits of FDG-PET/CT in NF1 patients with nerve sheath tumors: A cross-sectional/longitudinal study.

The purposes of this study were to re-confirm the usefulness of PET/CT in the differentiation of benignity/malignancy of neurogenic tumors in NF1 patients, and to analyze the natural course of plexiform neurofibroma (pNF) and clarify whether PET/CT is also useful for detecting tumors other than neurogenic tumors. PET/CT was prospectively imaged in 36 NF1 patients. There were 14 malignant peripheral nerve sheath tumors (MPNSTs) in 14 patients, and 54 pNFs in 30 patients. Nine patients had both MPNST and pNF. Maximal standardized uptake value (SUVmax) was significantly higher in MPNST (median 7.6: range 4.1-10.4) (P < .001) compared with that of pNF (median 3.7: range 1.6-9.3). The cut-off value of 5.8 resulted in a sensitivity of 78.6% and specificity of 88.9%. Median age was 29 y, and median maximum tumor diameter was 82 mm in 14 MPNST patients. The 5-y overall survival rate was 46.8%. Three patients with low-grade MPNST were alive without disease at the time of this report. In 9 patients in which pNF and MPNST co-existed, 2 showed a higher SUVmax of pNF than that of MPNST. Natural history analysis of pNF (n = 43) revealed that no factors significantly correlated with increased tumor size. Nine lesions other than neurogenic tumors were detected by PET/CT including 5 thyroid lesions and 3 malignant neoplasms. This study revealed the usefulness and limitation of PET/CT for NF1 patients. In the future, it will be necessary to study how to detect over time the malignant transformation of pNF to MPNST, via an intermediate tumor.

Lewy pathology of the esophagus correlates with the progression of Lewy body disease: a Japanese cohort study of autopsy cases.

Lewy body disease (LBD) is a spectrum of progressive neurodegenerative disorders characterized by the wide distribution of Lewy bodies and neurites in the central and peripheral nervous system (CNS, PNS). Clinical diagnoses include Parkinson's disease (PD), dementia with Lewy bodies, or pure autonomic failure. All types of LBD are accompanied by non-motor symptoms (NMSs) including gastrointestinal dysfunctions such as constipation. Its relationship to Lewy body-related α-synucleinopathy (Lewy pathology) of the enteric nervous system (ENS) is attracting attention because it can precede the motor symptoms. To clarify the role of ENS Lewy pathology in disease progression, we performed a clinicopathological study using the Brain Bank for Aging Research in Japan. Five-hundred and eighteen cases were enrolled in the study. Lewy pathology of the CNS and PNS, including the lower esophagus as a representative of the ENS, was examined via autopsy findings. Results showed that one-third of older people (178 cases, 34%) exhibited Lewy pathology, of which 78 cases (43.8%) exhibited the pathology in the esophagus. In the esophageal wall, Auerbach's plexus (41.6%) was most susceptible to the pathology, followed by the adventitia (33.1%) and Meissner's plexus (14.6%). Lewy pathology of the esophagus was significantly associated with autonomic failures such as constipation (p < 0.0001) and among PNS regions, correlated the most with LBD progression (r = 0.95, p < 0.05). These findings suggest that the propagation of esophageal Lewy pathology is a predictive factor of LBD.

Clioquinol kills astrocyte-derived KT-5 cells by the impairment of the autophagy-lysosome pathway.

Clioquinol has been implicated as a causative agent for subacute myelo-optico-neuropathy (SMON) in humans, although the mechanism remains to be elucidated. In this study, we utilized astrocyte-derived cell line, KT-5 cells to explore its potential cytotoxicity on glial cells. KT-5 cells were exposed in vitro to a maximum of 50 µM clioquinol for up to 24 h. 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenylte trazolium bromide (MTT) assay of the cells revealed that clioquinol induced significant cell damage and death. We also found that clioquinol caused accumulation of microtubule-associated protein light chain-3 (LC3)-II and sequestosome-1 (p62) in a dose- and time-dependent manner, suggesting the abnormality of autophagy-lysosome pathway. Consistent with these findings, an exposure of 20 µM clioquinol induced the accumulation of cellular autophagic vacuoles. Moreover, an exposure of 20 µM clioquinol provoked a statistically significant reduction of intracellular lysosomal acid hydrolases activities but no change in lysosomal pH. It also resulted in a significant decline of intracellular ATP levels, enhanced cellular levels of reactive oxygen species, and eventually cell death. This cell death at least did not appear to occur via apoptosis. 10 µM Chloroquine, lysosomal inhibitor, blocked the autophagic degradation and augmented clioquinol-cytotoxicity, whereas rapamycin, an inducer of autophagy, rescued clioquinol-induced cytotoxicity. Thus, our present results strongly suggest clioquinol acts as a potentially cytotoxic agent to glial cells. For future clinical application of clioquinol on the treatment of neurological and cancer disorders, we should take account of this type of cell death mechanism.

DNA Alkylation of the RUNX-Binding Sequence by CBI-PI Polyamide Conjugates*.

Many types of molecular targeted drugs that inhibit cancer growth by acting on specific molecules have been developed. The runt-related transcription factor (RUNX) family, which induces cancer development by binding to a specific DNA sequence, has attracted attention as a new target for cancer treatment. We have developed Chb-M', which targets the RUNX-binding sequence. Chb-M' was developed by conjugating pyrrole-imidazole (PI) polyamides and chlorambucil as an anticancer agent. It was recently reported that Chb-M' had a remarkable anticancer effect in vivo. In this study, to explore the possibility of an alternative structure, we designed a new series of CBI-PI polyamides, in which seco-CBI was applied as a DNA-alkylating agent. We examined the characteristics of the CBI-PI polyamides targeting the RUNX-binding sequence and found that these conjugates have great potential for cancer treatment.

The high-resolution crystal structure of lobster hemocyanin shows its enzymatic capability as a phenoloxidase.

Hemocyanin (Hc) and phenoloxidase (PO) are members of the type 3 copper protein family. Although arthropod Hc and PO exhibit similar three-dimensional structures of the copper-containing active site, Hc functions as an oxygen transport protein, showing minimal or no phenoloxidase activity. Here, we present the crystal structure of the oxy form of Hc from Panulirus japonicus (PjHc) at 1.58 Å resolution. The structure of the di-copper active site of PjHc was found to be almost identical to that of PO. Although conserved amino acids and the water molecule crucial for the enzymatic activity were observed in PjHc at almost the same positions as those in PO, PjHc showed no enzymatic activity under our experimental conditions. One striking difference between PjHc and arthropod PO was the presence of a "blocker residue" near the binuclear copper site of PjHc. This blocker residue comprised a phenylalanine residue tightly stacked with an imidazole ring of a CuA coordinated histidine and hindered substrates from accessing the active site. Our results suggest that the blocker residue is also a determining factor of the catalytic activity of type 3 copper proteins.

Anti-fumarase antibody promotes the dropout of photoreceptor inner and outer segments in diabetic macular oedema.

AIMS/HYPOTHESIS: In diabetic macular oedema (DMO), blood components passing through the disrupted blood-retinal barrier cause neuroinflammation, but the mechanism by which autoantibodies induce neuroglial dysfunction is unknown. The aim of this study was to identify a novel autoantibody and to evaluate its pathological effects on clinically relevant photoreceptor injuries. METHODS: Biochemical purification and subsequent peptide fingerprinting were applied to identify autoantigens. The titres of autoantibodies in DMO sera were quantified and their associations with clinical variables were evaluated. Two animal models (i.e. passive transfer of autoantibodies and active immunisation) were characterised with respect to autoimmune mechanisms underlying photoreceptor injuries. RESULTS: After screening serum IgG from individuals with DMO, fumarase, a Krebs cycle enzyme expressed in inner segments, was identified as an autoantigen. Serum levels of anti-fumarase IgG in participants with DMO were higher than those in diabetic participants without DMO (p < 0.001) and were related to photoreceptor damage and visual dysfunction. Passively transferred fumarase IgG from DMO sera in concert with complement impaired the function and structure of rodent photoreceptors. This was consistent with complement activation in the damaged photoreceptors of mice immunised with fumarase. Fumarase was recruited to the cell surface by complement and reacted to this autoantibody. Subsequently, combined administration of anti-fumarase antibody and complement elicited mitochondrial disruption and caspase-3 activation. CONCLUSIONS/INTERPRETATION: This study has identified anti-fumarase antibody as a serum biomarker and demonstrates that the generation of this autoantibody might be a pathological mechanism of autoimmune photoreceptor injuries in DMO.

ER-resident protein 46 (ERp46) triggers the mannose-trimming activity of ER degradation-enhancing α-mannosidase-like protein 3 (EDEM3).

Protein folding in the cell is regulated by several quality-control mechanisms. Correct folding of glycoproteins in the endoplasmic reticulum (ER) is tightly monitored by the recognition of glycan signals by lectins in the ER-associated degradation (ERAD) pathway. In mammals, mannose trimming from N-glycans is crucial for disposal of misfolded glycoproteins. The mannosidases responsible for this process are ER mannosidase I and ER degradation-enhancing α-mannosidase-like proteins (EDEMs). However, the molecular mechanism of mannose removal by EDEMs remains unclear, partly owing to the difficulty of reconstituting mannosidase activity in vitro Here, our analysis of EDEM3-mediated mannose-trimming activity on a misfolded glycoprotein revealed that ERp46, an ER-resident oxidoreductase, associates stably with EDEM3. This interaction, which depended on the redox activity of ERp46, involved formation of a disulfide bond between the cysteine residues of the ERp46 redox-active sites and the EDEM3 α-mannosidase domain. In a defined in vitro system consisting of recombinant proteins purified from HEK293 cells, the mannose-trimming activity of EDEM3 toward the model misfolded substrate, the glycoprotein T-cell receptor α locus (TCRα), was reconstituted only when ERp46 had established a covalent interaction with EDEM3. On the basis of these findings, we propose that disposal of misfolded glycoproteins through mannose trimming is tightly connected to redox-mediated regulation in the ER.

Sal-like 4 protein levels in breast cancer cells are post-translationally down-regulated by tripartite motif-containing 21.

Sal-like 4 (SALL4) is a transcription factor that enhances proliferation and migration in breast cancer cells. SALL4 expression therefore has the potential to promote cancer malignancy. However, the regulatory mechanisms involved in SALL4 protein expression have not been thoroughly elucidated. In this study, we observed that treating MCF-7 and SUM159 breast cancer cell lines with a proteasome inhibitor increases SALL4 protein levels, suggesting that SALL4 is degraded by the ubiquitin-proteasome system. Using immunoprecipitation to uncover SALL4-binding proteins, we identified an E3 ubiquitin-protein ligase, tripartite motif-containing 21 (TRIM21). Using an EGFP reporter probe of the major SALL4 isoform SALL4B, we observed that shRNA-mediated knockdown of TRIM21 increases cellular SALL4B levels. Immunostaining experiments revealed that TRIM21 localizes to the nucleus, and a K64R substitution in the nuclear localization motif in SALL4B increased SALL4B levels in the cytoplasm. These results suggested that TRIM21 is involved in nuclear SALL4 degradation. To identify the amino acid residue that is targeted by TRIM21, we fragmented the SALL4B sequence, fused it to EGFP, and identified Lys-190 in SALL4B as TRIM21's target residue. Amino acid sequence alignments of SALL family members indicated that the region around SALL4 Lys-190 is conserved in both SALL1 and SALL3. Because SALL1 and SALL4 have similar functions, we constructed a SALL1-EGFP probe and found that the TRIM21 knockdown increases SALL1 levels, indicating that TRIM21 degrades both SALL1 and SALL4. Our findings extend our understanding of SALL4 and SALL1 regulation and may contribute to the development of SALL4-targeting therapies.

Thioredoxin interacting protein (Txnip) forms redox sensitive high molecular weight nucleoprotein complexes.

Thioredoxin interacting protein (Txnip) is an α-arrestin protein that regulates pleiotropic biological responses. Txnip acts as a cancer suppressor and is a critical regulator of energy metabolism. To investigate molecular mechanisms involving Txnip, we searched for its protein binding partners using tandem affinity purification and proteomics analyses and identified several viable candidates, including HSP90, HSP70, and Prp31. We showed, by native PAGE, that Txnip is involved in the formation of high molecular weight complexes (1000-1300 kDa) in the nuclear fraction of cells treated with glucose and bortezomib. DTT treatment partly dissolved these high molecular weight complexes, suggesting that Txnip forms redox sensitive high-order nucleoprotein complexes. RNAse treatment slightly decreased the complex and RNA-seq showed differential expression of RNAs in the complex between Txnip protein overexpressing and control cells, indicating the involvement of RNAs in the complex. These results collectively provide a model whereby Txnip exerts its functions through multiple binding partners, forming transient higher-order complexes to regulate other signaling molecules.

Electron injection from mitochondrial transcription factor A to DNA associated with thymine dimer photo repair.

Electron transfer through π-stacked arrays of double-stranded DNA contributes to the redox chemistry of bases, including guanine oxidation and thymine-thymine dimer repair by photolyase. 5-Bromouracil is an attractive photoreactive thymine analogue that can be used to investigate electron transfer in DNA, and is a useful probe for protein-DNA interaction analysis. In the present study using BrU we found that UV irradiation facilitated electron injection from mitochondrial transcription factor A into DNA. We also observed that this electron injection could lead to repair of a thymine-thymine dimer.

Combined hepatocellular-cholangiocarcinoma with angiosarcomatoid change: A case report with immunohistochemical study.

Sarcomatoid combined hepatocellular-cholangiocarcinoma (cHCC-CCA) is a rare condition, with only 16 cases reported to date; however, there have been no reports of hepatic sarcomatoid carcinoma with angiosarcomatous features. Here, we report a rare case of cHCC-CCA with angiosarcomatoid changes in a 77-year-old man. The tumor was biphasic with malignant epithelial and mesenchymal components. Histologically, the epithelial component was concordant with classical type cHCC-CCA. The mesenchymal component included angiosarcomatoid cells growing in a vasoformative structure and undifferentiated spindle cells. Immunohistochemical analyses showed that angiosarcomatoid cells were positive for CD31, factor VIII-related antigen, and other angiosarcoma markers. Characteristically diffuse and strong expression of p53 protein was observed in the nuclei of angiosarcomatoid cells but not in carcinoma cells, suggesting that TP53 gene mutations commonly occur in these cells. Transitional zones were observed between HCC and spindle cells and between HCC and angiosarcomatoid cells. A small portion of undifferentiated spindle cells expressed pan-cytokeratin. These findings suggested that this extremely rare tumor developed from dedifferentiation or metaplastic changes of cHCC-CCA.

Biomimetic Artificial Epigenetic Code for Targeted Acetylation of Histones.

While the central role of locus-specific acetylation of histone proteins in eukaryotic gene expression is well established, the availability of designer tools to regulate acetylation at particular nucleosome sites remains limited. Here, we develop a unique strategy to introduce acetylation by constructing a bifunctional molecule designated Bi-PIP. Bi-PIP has a P300/CBP-selective bromodomain inhibitor (Bi) as a P300/CBP recruiter and a pyrrole-imidazole polyamide (PIP) as a sequence-selective DNA binder. Biochemical assays verified that Bi-PIPs recruit P300 to the nucleosomes having their target DNA sequences and extensively accelerate acetylation. Bi-PIPs also activated transcription of genes that have corresponding cognate DNA sequences inside living cells. Our results demonstrate that Bi-PIPs could act as a synthetic programmable histone code of acetylation, which emulates the bromodomain-mediated natural propagation system of histone acetylation to activate gene expression in a sequence-selective manner.

Watt-level 193 nm source generation based on compact collinear cascaded sum frequency mixing configuration.

A compact, dual-stage, collinear-cascaded sum-frequency mixing configuration is presented for generating 193 nm sources. Due to the less-introduced, deep-ultraviolet optical components, the system is less prone to damage. In our proof-of-concept experiments, a 1030 nm laser and a 1553 nm laser synchronized to each other were used as drivers and an average power of ~0.7 W was obtained. For comparison, the noncollinear configuration gave an average power of 0.77 W. The difference of 0.07 W is attributed to the spatial walk-off inside the cesium lithium borate (CLBO) crystal, confirmed by indirect visualization. A possible way to overcome the small gap of 0.07 W is proposed for future work.

Carbamylated albumin is one of the target antigens of anti-carbamylated protein antibodies.

Objectives: Anti-carbamylated protein (anti-CarP) antibodies are detected in RA patients. Fetal calf serum is used as an antigen source in anti-CarP ELISA, and the precise target antigens have not been found. We aimed to identify the target antigens of anti-CarP antibodies. Methods: Western blotting of anti-CarP antibodies was conducted. Anti-carbamylated human albumin (CarALB) antibody was detected by in-house ELISA for 493 RA patients and 144 healthy controls (HCs). An inhibition ELISA of anti-CarP antibodies by CarALB and citrullinated albumin (citALB) was performed using eight RA patients' sera. Serum CarALB was detected by liquid chromatography-tandem mass spectroscopy (LC/MS/MS), and the serum MPO concentration was measured by ELISA. Results: We focused on carbamylated albumin because it corresponded to the size of the thickest band detected by western blotting of anti-CarP antibodies. Anti-CarALB antibody was detected in 31.4% of RA patients, and the correlation of the titres between anti-CarALB and anti-CarP was much closer than that between anti-citALB and anti-CCP antibodies (ρ = 0.59 and ρ = 0.16, respectively). The inhibition ELISA showed that anti-CarP antibodies were inhibited by CarALB, but not by citALB. CarALB was detected in sera from RA patients by LC/MS/MS. The serum MPO concentration was correlated with disease activity and was higher in RA patients with anti-CarALB antibody than in those without. Conclusion: We found that carbamylated albumin is a novel target antigen of anti-CarP antibodies, and it is the first reported target antigen that has not been reported as the target of ACPA.