PINK1 Inhibits Multimeric Aggregation and Signaling of MAVS and MAVS-Dependent Lung Pathology

Mitochondria have emerged as important signaling organelles where intracellular perturbations are integrated and, consequently, intracellular signaling pathways are modulated to execute appropriate cellular functions. MAVS (mitochondrial antiviral signaling protein) represents such an example that functions as a platform molecule to mediate mitochondrial innate immune signaling. Recently, multimeric aggregation of MAVS has been identified as a key molecular process for its signaling. The underlying mechanisms to regulate this, however, are still incompletely understood. We hypothesized that PINK1 (PTEN-induced kinase 1) plays an important role in the regulation of multimeric MAVS aggregation and its consequent pathobiology. To test whether PINK1 interacts with MAVS, bimolecular fluorescence complementation analysis and IP were performed. RLH (RIG-I-like helicase) and NLRP3 inflammasome signaling were evaluated by in vitro assay. In vivo functional significance of PINK1 in the regulation of MAVS signaling was evaluated from both murine modeling of influenza viral infection and bleomycin-induced experimental pulmonary fibrosis, wherein MAVS plays important roles. Multimeric MAVS aggregation was induced by mitochondria dysfunction, and, during this event, the stabilized PINK1 interacted physically with MAVS and antagonized multimeric MAVS aggregation. Accordingly, the MAVS-mediated antiviral innate immune and NLRP3 inflammasome signaling were enhanced in PINK1 deficiency. In addition, in vivo studies revealed that MAVS-mediated pulmonary antiviral innate immune responses and fibrotic responses after bleomycin injury were enhanced in PINK1 deficiency. In conclusion, these results establish a new role of PINK1 in the regulation of MAVS signaling and the consequent pulmonary pathobiology.

Peroxisomes and Innate Immunity: Antiviral Response and Beyond

Peroxisomes are ubiquitous organelles with well-defined functions in lipid and reactive oxygen species metabolism, having a significant impact on a large number of important diseases. Growing evidence points to them, in concert with mitochondria, as important players within the antiviral response. In this review we summarize and discuss the recent findings concerning the relevance of peroxisomes within innate immunity. We not only emphasize their importance as platforms for cellular antiviral signaling but also review the current information concerning their role in the control of bacterial infections. We furthermore review the recent data that pinpoints peroxisomes as regulators of inflammatory processes.

Diverse intracellular pathogens activate type III interferon expression from peroxisomes

Type I interferon responses are considered the primary means by which viral infections are controlled in mammals. Despite this view, several pathogens activate antiviral responses in the absence of type I interferons. The mechanisms controlling type I interferon-independent responses are undefined. We found that RIG-I like receptors (RLRs) induce type III interferon expression in a variety of human cell types, and identified factors that differentially regulate expression of type I and type III interferons. We identified peroxisomes as a primary site of initiation of type III interferon expression, and revealed that the process of intestinal epithelial cell differentiation upregulates peroxisome biogenesis and promotes robust type III interferon responses in human cells. These findings highlight the importance of different intracellular organelles in specific innate immune responses.

OCTN3 is a mammalian peroxisomal membrane carnitine transporter

Carnitine is a zwitterion essential for the beta-oxidation of fatty acids. The role of the carnitine system is to maintain homeostasis in the acyl-CoA pools of the cell, keeping the acyl-CoA/CoA pool constant even under conditions of very high acyl-CoA turnover, thereby providing cells with a critical source of free CoA. Carnitine derivatives can be moved across intracellular barriers providing a shuttle mechanism between mitochondria, peroxisomes, and microsomes. We now demonstrate expression and colocalization of mOctn3, the intermediate-affinity carnitine transporter (Km 20 microM), and catalase in murine liver peroxisomes by TEM using immunogold labelled anti-mOctn3 and anti-catalase antibodies. We further demonstrate expression of hOCTN3 in control human cultured skin fibroblasts both by Western blotting and immunostaining analysis using our specific anti-mOctn3 antibody. In contrast with two peroxisomal biogenesis disorders, we show reduced expression of hOCTN3 in human PEX 1 deficient Zellweger fibroblasts in which the uptake of peroxisomal matrix enzymes is impaired but the biosynthesis of peroxisomal membrane proteins is normal, versus a complete absence of hOCTN3 in human PEX 19 deficient Zellweger fibroblasts in which both the uptake of peroxisomal matrix enzymes as well as peroxisomal membranes are deficient. This supports the localization of hOCTN3 to the peroxisomal membrane. Given the impermeability of the peroxisomal membrane and the key role of carnitine in the transport of different chain-shortened products out of peroxisomes, there appears to be a critical need for the intermediate-affinity carnitine/organic cation transporter, OCTN3, on peroxisomal membranes now shown to be expressed in both human and murine peroxisomes. This Octn3 localization is in keeping with the essential role of carnitine in peroxisomal lipid metabolism.

Limkain b1, a novel human autoantigen localized to a subset of ABCD3 and PXF marked peroxisomes

Detection of self-reactive antibodies has an established role in the diagnosis and monitoring of many human autoimmune diseases. Autoantibodies with restricted reactivity to cytoplasmic compartments and structures are an occasional incidental finding following routine examination of serum for antinuclear antibody reactivity. A prerequisite for rational exploitation of self-reactive antibodies, in either clinical or research settings, is the establishment of the molecular identity of the target autoantigen(s). Here we report on the identification of a novel autoantigen that co-localizes with a subset of cytoplasmic microbodies marked by ABCD3 (PMP-70) and/or PXF (PEX19). Immunoscreening a HeLa cell cDNA expression library with a human autoimmune serum identified two clones that encode fragments of limkain b1 (LKAP). We demonstrate that mouse polyclonal antibodies raised against a bacterially expressed fragment of limkain b1 mark the same cytoplasmic structures as human serum, as does an EGFP:LKAPCT429 fusion protein expressed in HeLa cells. An immunoblot screen against a bacterially expressed MBP:LKAPCT429 fusion protein substrate, using a cohort of 16 additional human sera that display Hep 2 cell cytoplasmic staining patterns similar to the prototype serum, identified three additional sera reactive to limkain b1. This is the first report establishing the molecular identity of a peroxisomal autoantigen. Preliminary results suggest that limkain b1 may be a relatively common target of human autoantibodies reactive to cytoplasmic vesicle-like structures.

Hemozoin (malarial pigment) inhibits differentiation and maturation of human monocyte-derived dendritic cells: a peroxisome proliferator-activated receptor-gamma-mediated effect

Acute and chronic Plasmodium falciparum malaria are accompanied by severe immunodepression possibly related to subversion of dendritic cells (DC) functionality. Phagocytosed hemozoin (malarial pigment) was shown to inhibit monocyte functions related to immunity. Hemozoin-loaded monocytes, frequently found in circulation and adherent to endothelia in malaria, may interfere with DC development and play a role in immunodepression. Hemozoin-loaded and unloaded human monocytes were differentiated in vitro to immature DC (iDC) by treatment with GM-CSF and IL-4, and to mature DC (mDC) by LPS challenge. In a second setting, hemozoin was fed to iDC further cultured to give mDC. In both settings, cells ingested large amounts of hemozoin undegraded during DC maturation. Hemozoin-fed monocytes did not apoptose but their differentiation and maturation to DC was severely impaired as shown by blunted expression of MHC class II and costimulatory molecules CD83, CD80, CD54, CD40, CD1a, and lower levels of CD83-specific mRNA in hemozoin-loaded iDC and mDC compared with unfed or latex-loaded DC. Further studies indicated activation of peroxisome proliferator-activated receptor-gamma (PPAR-gamma) in hemozoin-loaded iDC and mDC, associated with increased expression of PPAR-gamma mRNA, without apparent involvement of NF-kappaB. Moreover, expression of PPAR-gamma was induced and up-regulation of CD83 was inhibited by supplementing iDC and mDC with plausible concentrations of 15(S)-hydroxyeicosatetraenoic acid, a PPAR-gamma ligand abundantly produced by hemozoin via heme-catalyzed lipoperoxidation.

Immunological detection of alkaline-diaminobenzidine-negativeperoxisomes of the nematode Caenorhabditis elegans purification and unique pH optima of peroxisomal catalase

We purified catalase-2 of the nematode Caenorhabditis elegans and identified peroxisomes in this organism. The peroxisomes of C. elegans were not detectable by cytochemical staining using 3, 3'-diaminobenzidine, a commonly used method depending on the peroxidase activity of peroxisomal catalase at pH 9 in which genuine peroxidases are inactive. The cDNA sequences of C. elegans predict two catalases very similar to each other throughout the molecule, except for the short C-terminal sequence; catalase-2 (500 residues long) carries a peroxisomal targeting signal 1-like sequence (Ser-His-Ile), whereas catalase-1 does not. The catalase purified to near homogeneity from the homogenate of C. elegans cells consisted of a subunit of 57 kDa and was specifically recognized by anti-(catalase-2) serum but not by anti-(catalase-1) serum. Subcellular fractionation and indirect immunoelectron microscopy of the nematode detected catalase-2 inside vesicles judged to be peroxisomes using morphological criteria. The purified enzyme (220 kDa) was tetrameric, similar to many catalases from various sources, but exhibited unique pH optima for catalase (pH 6) and peroxidase (pH 4) activities; the latter value is unusually low and explains why the peroxidase activity was undetectable using the standard alkaline diaminobenzidine-staining method. These results indicate that catalase-2 is peroxisomal and verify that it can be used as a marker enzyme for C. elegans peroxisomes.

Different recognition by peroxisome proliferator structures in rat peroxisomal induction: application of sandwich ELISA using monoclonal antibody against rat peroxisomes

A novel assay for a peroxisomal beta-oxidation enzyme by sandwich ELISA using a monoclonal antibody (RPX-5) against purified rat liver peroxisomes was developed. Immunoblot analysis revealed that RPX-5 recognized a 78 Kd protein, which is a peroxisomal bifunctional enzyme (PBE) in the beta-oxidation pathway. Immunoprecipitation by RPX-5 and the resulting reduction of PBE activity were dependent on RPX-5 concentrations. Sandwich ELISA using RPX-5 could be used to assay PBE in the range of 30 to 2000 ng protein/ml. In rat hepatocyte cultures, the PBE amount by this assay correlated well with PBE activity, with correlation coefficients of 0.965. Studying the mechanisms of peroxisomal induction, patterns of peroxisomal induction were examined by co-treatment of rat hepatocytes with various peroxisome proliferators (PxPs). Treatment with clofibrate and bezafibrate resulted in neither an additive nor synergistic effect on PBE level. On the other hand, co-treatment with either bezafibrate-Wy-14,643 or clofibrate-MEHP(mono(2-ethylhexyl)phthalate) both resulted in an additive effect. From these results, it is suggested that PxPs of the fibrate group may exert their functions via a common process, and non-fibrate PxPs via a different process in hepatocytes. The cognition site for peroxisome proliferators, therefore, might not involve a single site for inducing peroxisomal enzymes.

Antibodies against pex14p block ATP-independent binding of matrix proteins to peroxisomes in vitro

The membrane protein Pex14p is a key component of the protein import machinery of peroxisomes. Antibodies raised against human Pex14p recognise a 66 kDa protein in sunflower glyoxysomes (HaPex14p) and immunoprecipitate in vitro-translated Arabidopsis Pex14p (AtPex14p). These antibodies inhibit the ATP-independent binding to sunflower peroxisome membranes of peroxisome targeting signal type (PTS) 1- and PTS2-targeted matrix proteins, but not an integral membrane protein. These results suggest that Pex14p functions before the ATP-dependent step of peroxisome assembly.