Pex13, the mouse ortholog of the human peroxisome biogenesis disorder PEX13 gene: gene structure, tissue expression, and localization of the protein to peroxisomes

Mitochondrial changes and oxidative stress in a mouse model of Zellweger syndrome neuropathogenesis

Zellweger syndrome (ZS) is a peroxisome biogenesis disorder that involves significant neuropathology, the molecular basis of which is still poorly understood. Using a mouse model of ZS with brain-restricted deficiency of the peroxisome biogenesis protein PEX13, we demonstrated an expanded and morphologically modified brain mitochondrial population. Cultured fibroblasts from PEX13-deficient mouse embryo displayed similar changes, as well as increased levels of mitochondrial superoxide and membrane depolarization; this phenotype was rescued by antioxidant treatment. Significant oxidative damage to neurons in brain was indicated by products of lipid and DNA oxidation. Similar overall changes were observed for glial cells. In toto, these findings suggest that mitochondrial oxidative stress and aberrant mitochondrial dynamics are associated with the neuropathology arising from PEX13 deficiency.

Comparison of the peroxisomal matrix protein import system of different organisms. Exploration of possibilities for developing inhibitors of the import system of trypanosomatids for anti-parasite chemotherapy

In recent decades, research on peroxisome biogenesis has been particularly boosted since the role of these organelles in metabolism became unraveled. Indeed in plants, yeasts and fungi, peroxisomes play an important role in the adaptation of metabolism during developmental processes and/or altered environmental conditions. In mammals their importance is illustrated by the fact that several severe human inherited diseases have been identified as peroxisome biogenesis disorders (PBD). Particularly interesting are the glycosomes - peroxisome-like organelles in trypanosomatids where the major part of the glycolytic pathway is sequestered - because it was demonstrated that proper compartmentalization of matrix proteins inside glycosomes is essential for the parasite. Although the overall process of peroxisome biogenesis seems well conserved between species, careful study of the literature reveals nonetheless many differences at various steps. In this review, we present a comparison of the first two steps of peroxisome biogenesis - receptor loading and docking at the peroxisomal membrane - in yeasts, mammals, plants and trypanosomatids and highlight major differences in the import process between species despite the conservation of (some of) the proteins involved. Some of the unique features of the process as it occurs in trypanosomatids will be discussed with regard to the possibilities for exploiting them for the development of compounds that could specifically disturb interactions between trypanosomatid peroxins. This strategy could eventually lead to the discovery of drugs against the diseases caused by these parasites.

Identification, characterization and essentiality of the unusual peroxin 13 from Trypanosoma brucei

Peroxin 13 (PEX13) is one of the components of a peroxisomal membrane complex involved in import of proteins into the matrix of the organelles and has previously been characterized in a variety of organisms. Trypanosomatids (Trypanosoma, Leishmania), protozoan parasites having peroxisome-like organelles designated glycosomes, possess an unusual PEX13 which shares very low sequence identity with others and lacks some typical PEX13 characteristics. It was identified in the databases through its multiple YGx motifs present in a glycine-rich N-terminal region of low sequence complexity. Like other PEX13s, it contains predicted transmembrane segments and a SH3 domain in its C-terminal half. The localization of T. brucei PEX13 in the glycosomal membrane was confirmed by expression of a fusion construct with Green Fluorescent Protein, and western blot analysis of purified organelles and membranes. The C-terminal half of the protein was shown to interact with the third of three pentapeptide repeats of the previously characterized PEX5, the receptor of glycosomal proteins with a type 1 peroxisome-targeting signal, and with PEX14, another component of the same peroxisomal protein import complex in the membrane. PEX13 is essential for the parasite; depletion by RNA interference results in mislocalization of glycosomal proteins and death of the parasites.

Differential expression of peroxisomal matrix and membrane proteins during postnatal development of mouse brain

In peroxisomal biogenesis disorders, serious neurological abnormalities can be observed in the patients and the respective knockout mouse models. As a prerequisite for a better understanding of the relationship between the absence of peroxisomes and the observed neuropathology, knowledge of the regional and cell-type specific distribution of peroxisomal proteins in mouse brain is necessary. Therefore, we investigated the expression of distinct peroxins, peroxisomal membrane and matrix proteins (e.g. Pex5p, Pex14p, Pex13p, PMP70, catalase, peroxisomal thiolase, Acox1, "SKL"-PTS1 proteins) by indirect immunofluorescence 1) in primary cultures of the medial neocortex, hippocampus, and cerebellum of newborn mice and 2) in paraffin sections of mouse brain of different ages (newborn-adult). Quantitative analysis revealed a comparable abundance (number/microm(2)) of peroxisomes in cultured neurons and astrocytes of all three brain regions. In contrast, catalase immunoreactivity was higher in cultured astrocytes than in neurons. In mouse brain tissue, the abundance of peroxisomes decreased by half during postnatal development, also exhibiting prominent differences between distinct brain regions and cell types. Catalase protein levels in neuronal peroxisomes, however, decreased much more strongly in the neocortex, CA1-3 areas of the hippocampus, dentate gyrus, cerebellar nuclei, and cerebellar cortex but remained high in Bergmann glia and other astrocytes, epithelial cells of the choroid plexus, and ependyma. Similar age-dependent changes were found for thiolase and Acox1 protein levels. Developmental changes were confirmed by Western blot analysis using enriched peroxisomal and cytosolic fractions of the brain tissue as well as by measurement of catalase activity.

Failure of microtubule-mediated peroxisome division and trafficking in disorders with reduced peroxisome abundance

In contrast to peroxisomes in normal cells, remnant peroxisomes in cultured skin fibroblasts from a subset of the clinically severe peroxisomal disorders that includes the biogenesis disorder Zellweger syndrome and the single-enzyme defect D-bifunctional protein (D-BP) deficiency, are enlarged and significantly less abundant. We tested whether these features could be related to the known role of microtubules in peroxisome trafficking in mammalian cells. We found that remnant peroxisomes in fibroblasts from patients with PEX1-null Zellweger syndrome or D-BP deficiency exhibited clustering and loss of alignment along peripheral microtubules. Similar effects were observed for both cultured embryonic fibroblasts and brain neurons from a PEX13-null mouse with a Zellweger-syndrome-like phenotype, and a less-pronounced effect was observed for fibroblasts from an infantile Refsum patient who was homozygous for a milder PEX1 mutation. By contrast, such changes were not seen for patients with peroxisomal disorders characterized by normal peroxisome abundance and size. Stable overexpression of PEX11beta to induce peroxisome proliferation largely re-established the alignment of peroxisomal structures along peripheral microtubules in both PEX1-null and D-BP-deficient cells. In D-BP-deficient cells, peroxisome division was apparently driven to completion, as induced peroxisomal structures were similar to the spherical parental structures. By contrast, in PEX1-null cells the majority of induced peroxisomal structures were elongated and tubular. These structures were apparently blocked at the division step, despite having recruited DLP1, a protein necessary for peroxisome fission. These findings indicate that the increased size, reduced abundance, and disturbed cytoplasmic distribution of peroxisomal structures in PEX1-null and D-BP-deficient cells reflect defects at different stages in peroxisome proliferation and division, processes that require association of these structures with, and dispersal along, microtubules.

Pex13 inactivation in the mouse disrupts peroxisome biogenesis and leads to a Zellweger syndrome phenotype

Zellweger syndrome is the archetypical peroxisome biogenesis disorder and is characterized by defective import of proteins into the peroxisome, leading to peroxisomal metabolic dysfunction and widespread tissue pathology. In humans, mutations in the PEX13 gene, which encodes a peroxisomal membrane protein necessary for peroxisomal protein import, can lead to a Zellweger phenotype. To develop mouse models for this disorder, we have generated a targeted mouse with a loxP-modified Pex13 gene to enable conditional Cre recombinase-mediated inactivation of Pex13. In the studies reported here, we crossed these mice with transgenic mice that express Cre recombinase in all cells to generate progeny with ubiquitous disruption of Pex13. The mutant pups exhibited many of the clinical features of Zellweger syndrome patients, including intrauterine growth retardation, severe hypotonia, failure to feed, and neonatal death. These animals lacked morphologically intact peroxisomes and showed deficient import of matrix proteins containing either type 1 or type 2 targeting signals. Biochemical analyses of tissue and cultured skin fibroblasts from these animals indicated severe impairment of peroxisomal fatty acid oxidation and plasmalogen synthesis. The brains of these animals showed disordered lamination in the cerebral cortex, consistent with a neuronal migration defect. Thus, Pex13(-/-) mice reproduce many of the features of Zellweger syndrome and PEX13 deficiency in humans.