C. Argyriou et al. / Peroxisome biogenesis disorders Peroxisome biochemistryThere are several hundred peroxisomes in all mammalian cells, each containing more than 50 matrix enzymes required for multiple vital metabolic pathways, including the catabolism and synthesis of important cellular lipids [reviewed in [8]]. The best characterized peroxisomal pathways include ␤oxidation of very long chain fatty acids (VLCFA), ␣-oxidation of methyl-branched phytanic acid and the biosynthesis of ether phospholipid (plasmalogen) (see Table 1). The metabolites of these pathways are routinely screened when a peroxisome biogenesis defect is suspected.Peroxisomal ␤-oxidation utilizes a variety of different fatty acid substrates in catabolic and synthetic processes: very long (≥ C22) straight chain saturated and unsaturated fatty acids (VLCFA), (2 S) methyl-branched chain pristanic acid and (25 S) di-and tri-hydroxycholestanoic acids (D/THCA). Docosahexanoic acid (DHA), a critical polyunsaturated omega-3 fatty acid, is synthesized in the peroxisome after ␤-oxidation of its precursor [9][10][11]. D/THCA are C27 intermediary bile acids that undergo one step of ␤-oxidation to form the primary C24 bile acids, cholic and chenodeoxycholic acid. Peroxisomal ␤-oxidation also participates in the regulation of other complex lipids, including pro-inflammatory molecules such as eicosanoids, leukotrienes, and prostaglandins [12-14] [reviewed in [8]]. Furthermore, peroxisomes are the site of ␤-oxidation of dicarboxylic acids [15]. The initial oxidation step is performed in peroxisomes by acyl-CoA oxidase 1 (ACOX1 for VLCFA, or ACOX2 for pristanic and D/THCA), generating hydrogen peroxide (hence the name, peroxisome) as a byproduct that is detoxified by peroxisomal catalase [16-19] [reviewed in [20]]. D-bifunctional protein (DBP or HSD17B4) catalyzes the second (hydration) and third (dehydration) step of peroxisomal ␤-oxidation [21][22][23]. The final step is catalyzed by thiolase (either 3-oxoacyl-CoA thiolase (ACAA1) or SCPX in humans) [reviewed in [8]]. The peroxisomal fatty acid oxidation system is generally not able to degrade fatty acids to completion, so C16 or shorter fatty acids are transported as an acylcarnitine derivative to mitochondria for complete oxidation [11,[24][25][26][27].3-methyl branched fatty acids (predominantly phytanic acid), because of the position of the methyl group, require a preliminary ␣-oxidation step in order to undergo ␤-oxidation [28,29]. This requires the peroxisomal enzymes, phytanyl-CoA hydroxylase (PHYH), phytanyl-CoA lyase, and pristanal dehydrogenase, to generate pristanic acid [30-33] [reviewed in [8]]. Both pristanic acid and D/THCA are generated as (R) and (S) enantiomers, which require conversion to the (S) form by peroxisomal alpha-methyl-CoA racemase (AMACR) before ␤-oxidation [34]. Phytanic acid is exclusively dietary in origin and obtained from meats and dairy products of ruminant animals and certain fatty fishes [35,36]. It accumulates in patients with ZSD and RCDP1, as well as in patients wit...

Peroxisomal biogenesis disorders (PBDs) are genetic disorders of peroxisome biogenesis and metabolism that are characterized by profound developmental and neurological phenotypes. The most severe class of PBDs-Zellweger spectrum disorder (ZSD)-is caused by mutations in peroxin genes that result in both nonfunctional peroxisomes and mitochondrial dysfunction. It is unclear, however, how defective peroxisomes contribute to mitochondrial impairment. In order to understand the molecular basis of this inter-organellar relationship, we investigated the fate of peroxisomal mRNAs and proteins in ZSD model systems. We found that peroxins were still expressed and a subset of them accumulated on the mitochondrial membrane, which resulted in gross mitochondrial abnormalities and impaired mitochondrial metabolic function. We showed that overexpression of ATAD1, a mitochondrial quality control factor, was sufficient to rescue several aspects of mitochondrial function in human ZSD fibroblasts. Together, these data suggest that aberrant peroxisomal protein localization is necessary and sufficient for the devastating mitochondrial morphological and metabolic phenotypes in ZSDs.

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Zellweger spectrum disorder (ZSD) results from biallelic mutations in PEX genes required for peroxisome biogenesis. PEX1-G843D is a common hypomorphic allele in the patient population that is associated with milder disease. In prior work using a PEX1-G843D/null patient fibroblast line expressing a green fluorescent protein (GFP) reporter with a peroxisome-targeting signal (GFP-PTS1), we demonstrated that treatments with the chemical chaperone betaine and flavonoid acacetin diacetate recovered peroxisome functions. To identify more effective compounds for preclinical investigation, we evaluated 54 flavonoids using this cell-based phenotype assay. Diosmetin showed the most promising combination of potency and efficacy (EC50 2.5 µM). All active 5′,7′-dihydroxyflavones showed greater average efficacy than their corresponding flavonols, whereas the corresponding flavanones, isoflavones, and chalcones tested were inactive. Additional treatment with the proteostasis regulator bortezomib increased the percentage of importrescued cells over treatment with flavonoids alone. Cotreatments of diosmetin and betaine showed the most robust additive effects, as confirmed by three independent functional assays in primary PEX1-G843D patient cells, but neither agent was active alone or in combination in patient cells homozygous for the PEX1 c.2097_2098insT null allele. Moreover, diosmetin treatment increased PEX1, PEX6, and PEX5 protein levels in PEX1-G843D patient cells, but none of these proteins increased in PEX1 null cells. We propose that diosmetin acts as a pharmacological chaperone that improves the stability, conformation, and functions of PEX1/PEX6 exportomer complexes required for peroxisome assembly. We suggest that diosmetin, in clinical use for chronic venous disease, and related flavonoids warrant further preclinical investigation for the treatment of PEX1-G843Dassociated ZSD. K E Y W O R D SAAA ATPase, betaine, chaperone therapy, diosmetin, peroxisome biogenesis disorder, PEX1 J Cell Biochem. 2019;120:3243-3258.wileyonlinelibrary.com/journal/jcb

The immune response is essential for survival by destroying microorganisms and pre-cancerous cells. However, inflammation, one aspect of this response, can result in short- and long-term deleterious side-effects. Mclk1 +/− mutant mice can be long-lived despite displaying a hair-trigger inflammatory response and chronically activated macrophages as a result of high mitochondrial ROS generation. Here we ask whether this phenotype is beneficial or simply tolerated. We used models of infection by Salmonella serovars and found that Mclk1 +/− mutants mount a stronger immune response, control bacterial proliferation better, and are resistant to cell and tissue damage resulting from the response, including fibrosis and types of oxidative damage that are considered to be biomarkers of aging. Moreover, these same types of tissue damage were found to be low in untreated 23 months-old mutants. We also examined the initiation of tumour growth after transplantation of mouse LLC1 carcinoma cells into Mclk1 +/− mutants, as well as during spontaneous tumorigenesis in Mclk1 +/− Trp53 +/− double mutants. Tumour latency was increased by the Mclk1 +/− genotype in both models. Furthermore, we used the transplantation model to show that splenic CD8+ T lymphocytes from Mclk1 +/− graft recipients show enhanced cytotoxicity against LLC1 cells in vitro. Mclk1 +/− mutants thus display an association of an enhanced immune response with partial protection from age-dependent processes and from pathologies similar to those that are found with increased frequency during the aging process. This suggests that the immune phenotype of these mutants might contribute to their longevity. We discuss how these findings suggest a broader view of how the immune response might impact the aging process.