Byungki Jang

Accumulation of citrullinated proteins by up-regulated peptidylarginine deiminase 2 in brains of scrapie-infected mice: a possible role in pathogenesis.

Peptidylarginine deiminases (PADs), which are a group of posttranslational modification enzymes, are involved in protein citrullination (deimination) by the conversion of peptidylarginine to peptidylcitrulline in a calcium concentration-dependent manner. Among the PADs, PAD2 is widely distributed in various tissues and is the only type that is expressed in brain. To elucidate the involvement of protein citrullination by PAD2 in the pathogenesis of brain-specific prion diseases, we examined the profiles of citrullinated proteins using the brains of scrapie-infected mice as a prion disease model. We found that, compared with controls, increased levels of citrullinated proteins of various molecular weights were detected in different brain sections of scrapie-infected mice. In support of this data, expression levels of PAD2 protein as well as its enzyme activity were significantly increased in brain sections of scrapie-infected mice, including hippocampus, brain stem, and striatum. Additionally, the expression levels of PAD2 mRNA were increased during scrapie infection. Moreover, PAD2 immunoreactivity was increased in scrapie-infected brains, with staining detected primarily in reactive astrocytes. Using two-dimensional electrophoresis and matrix-assisted laser desorption/ionization-time of flight mass spectrometry, various citrullinated proteins were identified in the brains of scrapie-infected mice, including glial fibrillary acidic protein, myelin basic protein, enolases, and aldolases. This study suggests that accumulated citrullinated proteins and abnormal activation of PAD2 may function in the pathogenesis of prion diseases and serve as potential therapeutic targets.

Involvement of peptidylarginine deiminase-mediated post-translational citrullination in pathogenesis of sporadic Creutzfeldt-Jakob disease

Peptidylarginine deiminases (PADs)-mediated post-translational citrullination processes play key roles in protein functions and structural stability through the conversion of arginine to citrulline in the presence of excessive calcium concentrations. In brain, PAD2 is abundantly expressed and can be involved in citrullination in disease. Recently, we have reported pathological characterization of PAD2 and citrullinated proteins in scrapie-infected mice, but the implication of protein citrullination in the pathophysiology in human prion disease is not clear. In the present study, we explored the molecular and biological involvement of PAD2 and the pathogenesis of citrullinated proteins in frontal cortex of patients with sporadic Creutzfeldt-Jakob disease (sCJD). We found increased expression of PAD2 in reactive astrocytes that also contained increased levels of citrullinated proteins. In addition, PAD activity was significantly elevated in patients with sCJD compared to controls. From two-dimensional gel electrophoresis and MALDI-TOF mass analysis, we found various citrullinated candidates, including cytoskeletal and energy metabolism-associated proteins such as vimentin, glial fibrillary acidic protein, enolase, and phosphoglycerate kinase. Based on these findings, our investigations suggest that PAD2 activation and aberrant citrullinated proteins could play a role in pathogenesis and have value as a marker for the postmortem classification of neurodegenerative diseases.

Subcellular Localization of Peptidylarginine Deiminase 2 and Citrullinated Proteins in Brains of Scrapie-Infected Mice: Nuclear Localization of PAD2 and Membrane Fraction-Enriched Citrullinated Proteins

Peptidylarginine deiminase (PAD) and citrullinated proteins have emerged as key molecules in various human diseases, but detailed subcellular localizations of PAD2 and citrullinated proteins are poorly mapped in brain under normal and pathologic conditions. We performed subcellular fractionation and electron microscopic analysis using brains of normal and scrapie-infected mice. Peptidylarginine deiminase 2 was abundantly present in cytosol and weakly in microsomal and mitochondrial fractions and expression in these fractions was higher in brains of scrapie-infected mice. Despite relatively low PAD2 expression, in microsomal and mitochondrial fractions, citrullinated proteins were present at high levels in these fractions in scrapie-infected brains. Surprisingly, increased PAD2 expression and accumulated citrullinated proteins were also found in nuclear fractions in scrapie-infected brains. By electron microscopy, PAD2 and citrullinated proteins in scrapie-infected brains were widely distributed in most cellular compartments including mitochondria, endoplasmic reticulum, glial filaments, nuclei, and Golgi apparatus in astrocytes and hippocampal neurons. Taken together, we report for the first time the nuclear localization of PAD2 and the detailed subcellular localization of PAD2 and of citrullinated proteins in scrapie-infected brains. Our findings suggest that different subcellular compartmentalization of PAD2 and citrullinated proteins may have different physiological roles in normal and neurodegenerative conditions.

Peptidylarginine deiminase modulates the physiological roles of enolase via citrullination: links between altered multifunction of enolase and neurodegenerative diseases

The citrullination of enolase by PAD (peptidylarginine deiminase) has emerged as an important post-translational modification in human disorders; however, the physiological function of citrullination remains unknown. In the present study, we report that citrullination diversely regulates the biological functions of ENO1 (α-enolase) and NSE (neuron-specific enolase). We developed three mouse IgG1 monoclonal antibodies with specificity to the following: (i) citrullination of Arg9 of ENO1 [ENO1Cit9; anti-CE1 (citrullinated enolase 1) antibody]; (ii) citrullination of Arg9 in ENO1 and NSE (ENO1Cit9/NSECit9; anti-CE1/2 antibody); and (iii) citrullination of Arg429 of NSE (NSECit429; anti-CE2 antibody). Regardless of the total protein expression level, the levels of ENO1Cit9 and NSECit429 were elevated, and their immunoreactivities were also increased in cortical neuronal cells or around blood vessels in the frontal cortex of patients with sporadic Creutzfeldt-Jakob disease and Alzheimers disease compared with controls. In a time- and dose-dependent manner, PAD negatively regulated enolase activity via citrullination, and enolase in diseased patients was more inactive than in controls. Interestingly, the citrullination of enolase effectively promoted its proteolytic degradation by Ca2+-dependent calpain-1, and leupeptin (calpain inhibitor I) abrogated this degradation. Surprisingly, using an affinity assay, the citrullination of enolase enhanced its plasminogen-binding affinity, which was blocked by the lysine analogue ϵ-aminocaproic acid. These findings suggest that PAD-mediated citrullination regulates the diverse physiological activities of enolase and that CE may be a candidate diagnostic/prognostic factor for degenerative diseases.

Peptidylarginine deiminase and protein citrullination in prion diseases: Strong evidence of neurodegeneration

The post-translational citrullination (deimination) process is mediated by peptidylarginine deiminases (PADs), which convert peptidylarginine into peptidylcitrulline in the presence of high calcium concentrations. Over the past decade, PADs and protein citrullination have been commonly implicated as abnormal pathological features in neurodegeneration and inflammatory responses associated with diseases such as multiple sclerosis, Alzheimer disease and rheumatoid arthritis. Based on this evidence, we investigated the roles of PADs and citrullination in the pathogenesis of prion diseases. Prion diseases (also known as transmissible spongiform encephalopathies) are fatal neurodegenerative diseases that are pathologically well characterized as the accumulation of disease-associated misfolded prion proteins, spongiform changes, glial cell activation and neuronal loss. We previously demonstrated that the upregulation of PAD2, mainly found in reactive astrocytes of infected brains, leads to excessive citrullination, which is correlated with disease progression. Further, we demonstrated that various cytoskeletal and energy metabolism-associated proteins are particularly vulnerable to citrullination. Our recent in vivo and in vitro studies elicited altered functions of enolase as the result of citrullination; these altered functions included reduced enzyme activity, increased protease sensitivity and enhanced plasminogen-binding affinity. These findings suggest that PAD2 and citrullinated proteins may play a key role in the brain pathology of prion diseases. By extension, we believe that abnormal increases in protein citrullination may be strong evidence of neurodegeneration.

Mass spectrometric identification of citrullination sites and immunohistochemical detection of citrullinated glial fibrillary acidic protein in Alzheimer's disease brains

Peptidylarginine deiminases (PADs) are posttranslational modification enzymes that convert protein arginine to citrulline residues in a calcium ion‐dependent manner. Previously, we reported the abnormal accumulation of citrullinated proteins and the increase in the amount of PAD2 in hippocampi from Alzheimers disease (AD) patients. Moreover, glial fibrillary acidic protein (GFAP), an astrocyte‐specific marker protein, and vimentin were identified as citrullinated proteins by using two‐dimensional gel electrophoresis and MALDI‐TOF mass spectrometry. To clarify the substrate specificity of PADs against GFAP, we prepared recombinant human (rh)PAD1, rhPAD2, rhPAD3, rhPAD4, and rhGFAP. After incubation of rhGFAP with rhPAD1, rhPAD2, rhPAD3, and rhPAD4, citrullinated (cit‐)rhGFAP was detected by Western blotting. The citrullination of rhGFAP by rhPAD2 was unique, specific, and time dependent; additionally, rhPAD1 slightly citrullinated rhGFAP. We then generated eight anti‐cit‐rhGFAP monoclonal antibodies, CTGF‐125, −128, −129, −1212, −1213, −1221, −122R, and −1224R, which reacted specifically with cit‐rhGFAP. Two of those eight monoclonal antibodies, CTGF‐122R and −1224R, reacted with both cit‐rhGFAP and rhGFAP in Western blots. By using the CTGF‐1221 antibody and a tandem mass spectrometer, we identified the two independent citrullination sites (R270Cit and R416Cit) of cit‐rhGFAP. Immunohistochemical analysis with CTGF‐1221 antibody revealed cit‐GFAP staining in the hippocampus of AD brain, and the cit‐GFAP‐positive cells appeared to be astrocyte‐like cells. These collective results strongly suggest that PAD2 is responsible for the citrullination of GFAP in the progression of AD and that the monoclonal antibody CTGF‐1221, reacting with cit‐GFAP at R270Cit and R416Cit, is useful for immunohistochemical investigation of AD brains.

Calsenilin regulates presenilin 1/γ-secretase-mediated N-cadherin ε-cleavage and β-catenin signaling

Presenilin 1 (PS1) is a component of the γ‐secretase complex that cleaves a variety of type I membrane proteins, including the β‐amyloid precursor protein (β‐APP), Notch, and neuronal (N)‐ and epithelial (E)‐cadherins. N‐cadherin is an essential adhesion molecule that forms a complex with, and is cleaved by, PS1/γ‐secretase and β‐catenin in the plasma membrane. The purpose of this study was to determine whether calsenilin, a presenilin‐interacting protein, has a functional role in PS1/γ‐secretase‐mediated N‐cadherin ∊‐cleavage using Western blot analysis, RT‐PCR, immunoprecipitation, subcellular fractionation, biotinylation, and a luciferase reporter assay in SH‐SY5Y neuroblastoma cells. Here, we demonstrate that the expression of calsenilin leads to a disruption of PS1/γ‐secretase‐mediated ∊‐cleavage of N‐cadherin, which results in the significant accumulation of N‐cadherin C‐terminal fragment 1 (Ncad/CTF1), the reduction of cytoplasmic Ncad/CTF2 release, and a deceleration of PS1‐CTF delivery to the cell surface. Interestingly, we also found that the expression of calsenilin is associated with the redistribution of β‐catenin from the cell surface to a cytoplasmic pool, as well as with the negative regulation of genes that are targets of T‐cell factor/β‐catenin nuclear signaling. Taken together, our findings suggest that calsenilin is a novel negative regulator of N‐cadherin processing that plays an important role in β‐catenin signaling.—Jang, C., Choi, J.‐K., Na, Y.‐J., Jang, B., Wasco, W., Buxbaum, J. D., Kim, Y.‐S., Choi, E.‐K. Calsenilin regulates presenilin 1/γ‐secretase‐mediated N‐cadherin ∊‐cleavage and β‐catenin signaling. FASEB J. 25, 4174–4183 (2011). www.fasebj.org

Induction of peptidylarginine deiminase 2 and 3 by dibutyryl cAMP via cAMP‐PKA signaling in human astrocytoma U‐251MG cells

Peptidylarginine deiminases (PADs) are posttranslational modification enzymes that citrullinate (deiminate) protein arginine residues in a calcium‐dependent manner, yielding citrulline residues. Enzymatic citrullination abolishes positive charges of native protein molecules, inevitably causing significant alterations in their structure and function. Previously, we reported the abnormal accumulation of citrullinated proteins and an increase of PAD2 content in hippocampi of patients with Alzheimer disease. In this study, we investigated PAD expression by using dibutyryl cAMP (dbcAMP) in human astrocytoma U‐251MG cells. Under normal culture conditions, PAD2 and PAD3 mRNA expression is detectable with quantitative PCR in U‐251MG cells. The addition of dbcAMP in a dose‐dependent manner significantly increased this mRNA expression and protein levels. Moreover, PAD enzyme activity also increased significantly and dose‐dependently. Furthermore, the expression of PAD2 and PAD3 mRNA was inhibited by the cAMP‐dependent PKA inhibitor KT5720, suggesting that such expression of dbcAMP‐induced PAD2 and PAD3 mRNA is mediated by the cAMP‐PKA signaling pathway in U‐251MG cells. This is the first report to document the PAD2 and PAD3 mRNA expression induced by dbcAMP and to attribute the induction of these genes to mediation by the cAMP‐PKA signaling pathway in U‐251MG cells.

Phosphatidylinositol-Glycan-Phospholipase D Is Involved in Neurodegeneration in Prion Disease

PrPSc is formed from a normal glycosylphosphatidylinositol (GPI)-anchored prion protein (PrPC) by a posttranslational modification. Most GPI-anchored proteins have been shown to be cleaved by GPI phospholipases. Recently, GPI-phospholipase D (GPI-PLD) was shown to be a strictly specific enzyme for GPI anchors. To investigate the involvement of GPI-PLD in the processes of neurodegeneration in prion diseases, we examined the mRNA and protein expression levels of GPI-PLD in the brains of a prion animal model (scrapie), and in both the brains and cerebrospinal fluids (CSF) of sporadic and familial Creutzfeldt-Jakob disease (CJD) patients. We found that compared with controls, the expression of GPI-PLD was dramatically down-regulated in the brains of scrapie-infected mice, especially in the caveolin-enriched membrane fractions. Interestingly, the observed decrease in GPI-PLD expression levels began at the same time that PrPSc began to accumulate in the infected brains and this decrease was also observed in both the brain and CSF of CJD patients; however, no differences in expression were observed in either the brains or CSF specimens from Alzheimer’s disease patients. Taken together, these results suggest that the down-regulation of GPI-PLD protein may be involved in prion propagation in the brains of prion diseases.

Upregulation of Connexin 43 Expression Via C-Jun N-Terminal Kinase Signaling in Prion Disease

Prion infection leads to neuronal cell death, glial cell activation, and the accumulation of misfolded prion proteins. However, the altered cellular environments in animals with prion diseases are poorly understood. In the central nervous system, cells connect the cytoplasm of adjacent cells via connexin (Cx)-assembled gap junction channels to allow the direct exchange of small molecules, including ions, neurotransmitters, and signaling molecules, which regulate the activities of the connected cells. Here, we investigate the role of Cx43 in the pathogenesis of prion diseases. Upregulated Cx43 expression, which was dependent on c-Jun N-Terminal Kinase (JNK)/c-Jun signaling cascades, was found in prion-affected brain tissues and hippocampal neuronal cells. Scrapie infection-induced Cx43 formed aggregated plaques within the cytoplasmic compartments at the cell-cell interfaces. The ethidium bromide (EtBr) uptake assay and scrape-loading dye transfer assay demonstrated that increased Cx43 has functional consequences for the activity of Cx43 hemichannels. Interestingly, blockade of PrPSc accumulation reduced Cx43 expression through the inhibition of JNK signaling, indicating that PrPSc accumulation may be directly involved in JNK activation-mediated Cx43 upregulation. Overall, our findings describe a scrapie infection-mediated novel regulatory signaling pathway of Cx43 expression and may suggest a role for Cx43 in the pathogenesis of prion diseases.