Peroxisome targeting signal type 1 (PTS1) receptor is involved in import of both PTS1 and PTS2: studies with PEX5-defective CHO cell mutants

To investigate the mechanisms of peroxisome assembly and the molecular basis of peroxisome assembly disorders, we isolated and characterized a peroxisome-deficient CHO cell mutant, ZP139, which was found to belong to human complementation group II, the same group as that of our earlier mutant, ZP105. These mutants had a phenotypic deficiency in the import of peroxisomal targeting signal type 1 (PTS1) proteins. Amino-terminal extension signal (PTS2)-mediated transport, including that of 3-ketoacyl coenzyme A thiolase, was also defective in ZP105 but not in ZP139. PEX5 cDNA, encoding the PTS1 receptor (PTS1R), was isolated from wild-type CHO-K1 cells. PTS1R's deduced primary sequence comprised 595 amino acids, 7 amino acids less than the human homolog, and contained seven tetratricopeptide repeat (TPR) motifs at the C-terminal region. Chinese hamster PTS1R showed 94, 28, and 24% amino acid identity with PTS1Rs from humans, Pichia pastoris, and Saccharomyces cerevisiae, respectively. A PTS1R isoform (PTS1RL) with 632 amino acid residues was identified in CHO cells; for PTS1R, 37 amino acids were inserted between residues at positions 215 and 216 of a shorter isoform (PTS1RS). Southern blot analysis of CHO cell genomic DNA suggested that these two isoforms are derived from a single gene. Both types of PEX5 complemented impaired import of PTS1 in mutants ZP105 and ZP139. PTS2 import in ZP105 was rescued only by PTS1RL. This finding strongly suggests that PTS1RL is also involved in the transport of PTS2. Mutations in PEX5 were determined by reverse transcription-PCR: a G-to-A transition resulted in one amino acid substitution: Gly298Glu of PTS1RS (G335E of PTS1RL) in ZP105 and Gly485Glu of PTS1RS (G522E of PTS1RL) in ZP139. Both mutations were in the TPR domains (TPR1 and TPR6), suggesting the functional consequence of these domains in protein translocation. The implications of these mutations are discussed.

Face immersion increases vagal activity as assessed by heart rate variability

We examined whether the diving reflex without breath-holding (face immersion alone) increases vagal activity, as determined by heart rate variability. A group of 15 men [mean age 20 (SD 3) years, height 172 (SD 5) cm, body mass 68 (SD 9) kg] performed 12 trials at various breathing frequencies (5, 10, 15, 20, 30 breaths x min(-1) and uncontrolled breath) with or without face immersion. The R-R intervals of the ECG and gas exchange variables were recorded during the 2 min of each trial. The subjects immersed their faces in 8 10 degrees C water while breathing through a short snorkel. The subject sat in the same position either with or without face immersion. The mean R-R interval (RRmean), standard deviations (SD[RR]) and coefficient of variance (CV[RR]) of the R-R interval were calculated from the R-R intervals during 30-120 s. The face immersion significantly increased SD(RR) and CV(RR) (P < 0.05), and increased RRmean (P < 0.05) at 20 breaths x min(-1). Face immersion itself had no effect on oxygen uptake, tidal volume, end-tidal O2 and CO2 partial pressures. The diving reflex without breath-holding increased the heart rate variability, indicating that face immersion alone increases vagal activity.

Effect of attenuated Erysipelothrix rhusiopathiae vaccine in pigs infected with porcine reproductive respiratory syndrome virus

Twenty 2nd specific pathogen-free pigs were divided into 4 groups: Group A were infected with porcine reproductive and respiratory syndrome (PRRS) virus at 6 weeks of age and treated with available swine erysipelas and swine fever combined vaccine (vaccinated) at 7 weeks of age; Group B were vaccinated at 7 weeks of age and infected with PRRS virus at 8 weeks of age; Group C were vaccinated at 7 weeks of age: Group D were neither vaccinated nor infected with PRRS virus. All pigs were challenged to Erysipelothrix rhusiopathiae C42 strain at 10 weeks of age. No clinical signs appeared after vaccination of group A and B pigs, thus confirming that the safety of the vaccine was not influenced by infection with PRRS virus. None of the pigs in Groups A and C developed erysipelas after challenge exposure to E. rhusiopathiae. In contrast, fever and/or urticaria appeared transiently in all pigs of Group B after challenge exposure. At the time of challenge exposure to E. rhusiopathiae, the PRRS virus titer was high in sera of Group B, but was low in those from Group A. However, vaccination of pigs with attenuated E. rhusiopathiae was effective in dual infection with PRRS virus and E. rhusiopathiae, because the clinical signs were milder and the E. rhusiopathiae strain was less recovered from these pigs compared to pigs of group D.

Newly identified Chinese hamster ovary cell mutants defective in peroxisome biogenesis represent two novel complementation groups in mammals

We isolated peroxisome biogenesis mutants from Chinese hamster ovary (CHO) cells, using the 9-(1'-pyrene)nonanol/ultraviolet (P9OH/ UV) method and wild-type CHO-K1 cells that had been stably transfected with cDNA encoding Pex2p (formerly peroxisome assembly factor-1, PAF-1). Three mutant cell clones, ZP110, ZP111, and ZP114, showed cytosolic localization of catalase, thereby indicating a defect in peroxisome biogenesis, whereas ZP112 and ZP113 contained fewer but larger catalase-positive particles. Mutant ZP115 displayed an aberrant, tubular structure immunoreactive to anti-catalase antibody. Mutants lacking morphologically recognizable peroxisomes also showed the typical peroxisome assembly-defective phenotype such as severe loss of catalase latency and resistance to 12-(1'-pyrene)dodecanoic acid (P12)/UV treatment. ZP110 and ZP111, and ZP114 were found to belong to two novel complementation groups, respectively, by complementation group analysis with cDNA transfection and cell fusion. Cell fusion with fibroblasts from patients with peroxisome biogenesis disorders such as Zellweger syndrome revealed that ZP110 and ZP114 could not be classified to any of human complementation groups. Thus, ZP110/ZP111 and ZP114 are the first, two peroxisome-deficient cell mutants of newly identified complementation groups distinct from the ten mammalian groups previously characterized.

Isolation and characterization of peroxisome-deficient Chinese hamster ovary cell mutants representing human complementation group III

We made use of the 9-(1'-pyrene)nonanol/ultraviolet (P9OH/UV) method and isolated peroxisome-deficient mutant cells. TKa cells, the wild-type Chinese hamster ovary (CHO) cells, CHO-K1, that had been stably transfected with cDNA encoding Pex2p (formerly peroxisome assembly factor-1, PAF-1) were used to avoid frequent isolation of the Z65-type, PEX2-defective mutants. P9OH/UV-resistant cell colonies were examined for the intracellular location of catalase, a peroxisomal matrix enzyme, by immunofluorescence microscopy and using anti-catalase antibody. As six mutant cell clones showed cytosolic catalase, there was likely to be a deficiency in peroxisome assembly. These mutants also showed the typical peroxisome assembly-defective phenotype, including significant decrease of dihydroxyacetonephosphate acyltransferase, the first step key enzyme in plasmalogen synthesis, and loss of resistance to 12-(1'-pyrene)dodecanoic acid/UV treatment. By transfection of Pex2p and Pex6p (formerly PAF-2) cDNAs and cell fusion analysis between the CHO cell mutants, two mutants, ZP104 and ZP109, were found to belong to a novel complementation group. Further complementation analysis using fibroblasts from patients with peroxisome biogenesis disorders revealed that the mutants belonged to human complementation group III. Taken together, ZP104 and ZP109 are in a newly identified fifth complementation group in CHO mutants reported to date and represent the human complementation group III.

Identification of an extended half-site motif required for the function of peroxisome proliferator-activated receptor alpha

BACKGROUND

Peroxisome proliferator-activated receptors (PPARs) belong to the nuclear hormone receptor superfamily and regulate many genes of the proteins involved in lipid metabolism, including peroxisomal acyl-CoA oxidase (AOX). Through heterodimerization with retinoid X receptors (RXRs), PPAR was believed to recognize the sequence elements consisting of two directly repeating 6-bp half-sites spaced by one nucleotide (DR-1), located in the regulatory regions of these genes.

RESULTS

Employing the peroxisome proliferator-responsive enhancer of the rat AOX gene, we analysed the minimal sequence requirements for enhancer activity and PPARalpha/RXRalpha binding. We found that the sequence just downstream of the DR-1 motif is indispensable for both functions. By a direct selection procedure of high-affinity binding sites from a random sequence pool, we identified a consensus sequence at the four positions next to DR-1. We also suggest that PPARalpha binds to the downstream half-site, whereas RXRalpha binds to the upstream half-site of the AOX DR-1.

CONCLUSIONS

An extended half-site of 10-bp, but not a simple 6-bp half-site, is required for the PPARalpha binding, upon heterodimer formation with RXRalpha. The binding polarity of PPARalpha/RXRalpha seems to be opposite to that of other RXR-involving heterodimers.

Isolation of a new peroxisome-deficient CHO cell mutant defective in peroxisome targeting signal-1 receptor

For the study of mechanism of peroxisome biogenesis, we attempted to isolate CHO cell mutants deficient in peroxisome biogenesis. We used as the parent strain a stable CHO transformant of rat PEX2 (formerly named peroxisome assembly factor-1) cDNA, to avoid unusually frequent isolation of Pex2 mutants. Among the three peroxisome-deficient mutants obtained, ZP102 was a new CHO mutant of complementation group 2, and was restored for peroxisome assembly by the transfection of human PEX5 (formerly called PXR1 or PTS1R) cDNA. This approach would facilitate the isolation of new complementation gorups of peroxisome-deficient CHO mutants and the identification of essential genes for peroxisome biogenesis.

An orphan nuclear receptor lacking a zinc-finger DNA-binding domain: interaction with several nuclear receptors

The yeast two-hybrid screening was applied to cloning cDNAs of proteins that interact with peroxisome proliferator-activated receptor alpha (PPAR alpha). We obtained from a rat liver cDNA library a clone encoding a protein related to the ligand-binding domain of the members of nuclear hormone receptor superfamily, whereas apparently lacking the zinc-finger DNA-binding domain. This protein interacted with the activated forms of several nuclear receptors, and thus is a novel type of heterodimer-forming nuclear receptor.

Human peroxisome assembly factor-2 (PAF-2): a gene responsible for group C peroxisome biogenesis disorder in humans

Peroxisome-biogenesis disorders (PBD) are genetically heterogeneous and can be classified into at least ten complementation groups. We recently isolated the cDNA for rat peroxisome assembly factor-2 (PAF-2) by functional complementation using the peroxisome-deficient Chinese-hamster-ovary cell mutant, ZP92. To clarify the novel pathogenic gene of PBD, we cloned the full-length human PAF-2 cDNA that morphologically and biochemically restores peroxisomes of group C Zellweger fibroblasts (the same as group 4 in the Kennedy-Krieger Institute) and identified two pathogenic mutations in the PAF-2 gene in two patients with group C Zellweger syndrome. The 2,940-bp open reading frame of the human PAF-2 cDNA encodes a 980-amino-acid protein that shows 87.1% identity with rat PAF-2 and also restored the peroxisome assembly after gene transfer to fibroblasts of group C patients. Direct sequencing of the PAF-2 gene revealed a homozygous 1-bp insertion at nucleotide 511 (511 insT) in one patient with group C Zellweger syndrome (ZS), which introduces a premature termination codon in the PAF-2 gene, and, in the second patient, revealed a splice-site mutation in intron 3 (IVS3+1G-->A), which skipped exon 3, an event that leads to peroxisome deficiency. Chromosome mapping utilizing FISH indicates that PAF-2 is located on chromosome 6p21.1. These results confirm that human PAF-2 cDNA restores peroxisome of group C cells and that defects in the PAF-2 produce peroxisome deficiency of group C PBD.

Simple and rapid quantitative assay of 13C-labelled urea in human serum using liquid chromatography-atmospheric pressure chemical ionization mass spectrometry

A simple and rapid quantitative method for 13C-labelled urea ([13C]urea) in human serum was developed by using high-performance liquid chromatography-atmospheric pressure chemical ionization mass spectrometry (HPLC-APCI-MS). This method is used to establish and normalize the [13C]urea breath test, which is considered as an effective diagnostic method for Helicobacter pylori infection. HPLC-APCI-MS, involving a simple pretreatment process such as diluting serum with water, was shown to be able to discriminate the extrinsic [13C]urea from intrinsic urea present at high concentration in serum. In addition, a 13C nuclear magnetic resonance spectroscopic quantitative method for [13C]urea in human urine is also described. The precision and accuracy of measured concentrations in these two methods were found to be within the acceptable limit. An application of these methods to investigate the pharmacokinetic profile of orally administered [13C]urea in human serum and urine is also presented.

Local gastric and serum concentrations of rebamipide following oral administration to patients with chronic gastritis

The potential anti-ulcer action of rebamipide (CAS 11911-87-6, OPC-12759) on experimental and clinical injury to the gastric mucosa depends on its presence in sufficient concentration in the gastric mucosa. To investigate its local penetration, rebamipide levels in the gastric mucosa and serum were measured. A dose of 100 mg of rebamipide was given orally (tablet) to patients with chronic gastritis (n = 32). Gastroscopy was performed between 22 and 353 min after drug ingestion. Venous blood was sampled for determination of serum concentrations. Samples were analyzed by high-performance liquid chromatography (HPLC). The maximal concentration in the gastric mucosa was reached after 30-60 min, and the concentration remained elevated for about 120 min thereafter. The mean mucosal concentration of rebamipide between 30 and 120 min after ingestion was 60.0 +/- 109.8 micrograms/g of tissue, which was higher than 0.1 mmol/l (37 micrograms/g of tissue). The serum concentration of rebamipide peaked about 30-60 min after drug ingestion and remained elevated for about 5 h thereafter. The mean serum concentration of rebamipide between 30 and 120 min after ingestion was 0.25 +/- 0.23 microgram/ml, being below the level of 1.0 mumol/l (0.37 microgram/ml). Data indicate that the concentration of rebamipide in the gastric mucosa resulted from local penetration, and suggest that the blood level and systemic distribution of rebamipide have little effect on its antioxidative and anti-neutrophilic activities.

Correction by gene expression of biochemical abnormalities in fibroblasts from Zellweger patients

Zellweger syndrome is a prototype of peroxisomal biogenesis disorders and a fatal autosomal recessive disease with no effective therapy. We identified nine genetic complementation groups of these disorders, and mutations in peroxisome assembly factor-1 (PAF-1) and the 70-kD peroxisomal membrane protein (PMP70) genes have been detected by our group F and Roscher's group 1, respectively. We now describe permanent recovery from generalized peroxisomal abnormalities in fibroblasts of a Zellweger patient from group F, such as biochemical defects of peroxisomal beta-oxidation, plasmalogen biosynthesis, and morphologic absence of peroxisomes, by stable transfection of human cDNA encoding PAF-1. In the light of these observations, we designed a gene expression system using fibroblasts from patients with peroxisomal biogenesis disorders. In Zellweger fibroblasts obtained from Roscher's group 1 and transfected with human cDNA encoding PMP70, peroxisomes were not morphologically identifiable, and peroxisomal function did not normalize.

Insertion of the 70-kDa peroxisomal membrane protein into peroxisomal membranes in vivo and in vitro

Biosynthesis and intracellular transport of 70-kda peroxisomal membrane protein (pmp70) has been studied in rat hepatoma, h-4-ii-e cells. Pulse-chase analysis showed that a newly synthesized 35S-PMP70 first appeared in the cytosolic fraction and was then transported into the peroxisomal fraction. The half-life of 35S-PMP70 in the cytosolic fraction was approximately 3 min. Integration of 35S-PMP70 into membranes occurred in the peroxisomal fraction and was completed within 30 min. No proteolytic processing of 35S-PMP70 was observed. An in vitro import system was reconstituted to characterize the insertion mechanism of PMP70 into peroxisomes. Peroxisomes isolated from rat liver were incubated at 26 degrees C with [35S]methionine-labeled in vitro translation products of PMP70 mRNA in the presence of the cytosolic fraction. The peroxisomes were reisolated and insertion of 35S-PMP70 into the membrane was analyzed using a Na2CO3 procedure. The binding and insertion of 35S-PMP70 were dependent on temperature and incubation time and was specific for peroxisomes. Pretreatment of peroxisomes with trypsin and chymotrypsin almost abolished the binding and insertion of 35S-PMP70. The translation products contained several truncated 35S-PMP70s. The NH2 terminally truncated 35S-PMP70s, with a molecular mass greater than 50 kDa, bound to and inserted into peroxisomal membranes, whereas truncated 35S-PMP70s smaller than 45 kDa did not. These results suggest that PMP70 is post-translationally transported to peroxisomes without processing and inserted into peroxisomal membranes by a specific mechanism in which a proteinaceous receptor and a certain internal sequence of PMP70 are involved.

Peroxisome assembly factor-2, a putative ATPase cloned by functional complementation on a peroxisome-deficient mammalian cell mutant

Rat peroxisome assembly factor-2 (PAF-2) cDNA was isolated by functional complementation of peroxisome deficiency of a mutant CHO cell line, ZP92, using transient transfection assay. This cDNA encodes a 978-amino acid protein with two putative ATP-binding sites. PAF-2 is a member of a putative ATPase family, including two yeast gene products essential for peroxisome assembly. A stable transformant of ZP92 with the cDNA was morphologically and biochemically restored for peroxisome biogenesis. Fibroblasts derived from patients deficient in peroxisome biogenesis (complementation group C) were also complemented with PAF-2 cDNA, indicating that PAF-2 is a strong candidate for the pathogenic gene of group C peroxisome deficiency.

Cloning, expression, and characterization of cDNAs encoding Arabidopsis thaliana squalene synthase

We have isolated and characterized two overlapping cDNA clones for Arabidopsis thaliana squalene synthase. Their nucleotide sequences contained an open reading frame for a 410-amino acid polypeptide (calculated molecular mass, 47 kDa). The deduced amino acid sequence of the Arabidopsis polypeptide was significantly homologous (42-44% identical) to the sequences of known squalene synthases of several species, from yeast to man, but it was much less homologous to that of tomato phytoene synthase. To express the Arabidopsis enzyme in Escherichia coli, the entire coding region was subcloned into an expression vector. A cell-free extract of E. coli transformed with the recombinant plasmid, in the presence of NADPH and Mg2+, efficiently converted [14C]farnesyl diphosphate into squalene. On the other hand, in the absence of NADPH and the presence of Mn2+, the cell-free extract formed dehydrosqualene as a secondary product. Another E. coli extract expressing mouse squalene synthase showed the same activity as the Arabidopsis enzyme. Therefore, both the structure and reaction mechanism of squalene synthases are markedly conserved in taxonomically remote eukaryotes.

cDNA cloning of a putative G protein-coupled receptor from brain

Using degenerate oligonucleotide primers corresponding to conserved regions of the G-protein coupled receptor superfamily, we carried out a low-stringency polymerase chain reaction and obtained two novel partial-length clones from a rat brain cDNA library. We used one of these clones for conventional library screening and isolated a longer cDNA clone, designated as RBU-15, from another rat brain library. Although RBU-15 was truncated at its 5' end, Northern blot analysis revealed that the gene was expressed in the brain and spleen. Next, we isolated a full-length cDNA clone, designated as HB-954, from a human fetal brain library, using RBU-15 as a probe. The deduced amino acid sequence of HB-954 contained four putative glycosylation sites in the N-terminal part, seven transmembrane domains, and a large cytosolic domain in the C-terminal part. The protein products of RBU-15 and HB-954 likely belong to a distinctive subfamily, because no receptors in the superfamily were found to be closely related to them.

Nucleotide sequence of a cDNA for mouse squalene epoxidase

A full-length cDNA encoding mouse squalene epoxidase was isolated by screening a mouse liver cDNA library with the rat squalene epoxidase gene as a probe. The cDNA had an open reading frame for a 572 amino acid polypeptide with a calculated molecular mass of 63.8 kDa. The predicted amino acid sequence of the mouse enzyme contained an FAD-binding motif, and was 93% identical to that of the rat enzyme. The former is one amino acid shorter than the latter. Blotting analyses showed that the mRNA is 2.8 kb in size and that a single copy of the gene is present in the mouse genome.

Identification of two splice isoforms of mRNA for mouse hepatocyte nuclear factor 4 (HNF-4)

Hepatocyte nuclear factor 4 (HNF-4) is a liver-enriched transcription factor involved in the expression of many liver-specific genes. In the preceding communication (Hata, S., Tsukamoto, T. and Osumi, T. (1992) Biochim. Biophys. Acta 1131, 211-213), we reported the presence of two isoforms of mRNA for HNF-4 in rat liver and kidney. The longer isoform contained a segment of 30 bases which was not present in the shorter one. As an initial step to determine whether or not other mammals have these mRNA isoforms, we isolated a cDNA for mouse HNF-4 using the rat HNF-4 gene as a probe. The cDNA had an open reading frame for a 465 amino acid polypeptide. The deduced amino acid sequence was remarkably conserved between mouse HNF-4 and rat HNF-4 (99.6% identical). Moreover, like the cDNA for the larger rat isoform, the mouse cDNA contained an extra segment of 30 bp in the coding region near the C-terminus. Blotting analyses showed that the mRNA is about 3.7 kb in size and that a single copy of the gene is present in the mouse genome. Next we carried out the polymerase chain reaction (PCR) using primers located just upstream and downstream of the extra segment. Two PCR products were amplified from a mouse liver cDNA library. Determination of their nucleotide sequences proved that they exactly corresponded to the two rat isoforms. Finally, we amplified a DNA fragment (1.1 kb in size) from mouse genomic DNA using the same PCR primers as above. Its nucleotide sequence unequivocally confirmed that different splice donor sites were used to generate the two isoforms.

Molecular cloning and functional expression of a cDNA for mouse squalene synthase

Using a probe obtained by PCR amplification, a full-length cDNA encoding squalene synthase was isolated from a mouse liver cDNA library. Its nucleotide sequence had an open reading frame fro a 416 amino acid polypeptide (calculated molecular mass, 48 kDa). In vitro transcription of the cDNA followed by in vitro translation produced a protein of the expected size. The deduced amino acid sequence was 93%, 88% and 46% identical to those of the rat, human and budding yeast squalene synthases, respectively. Blotting analyses showed that the mRNA is 1.6 kb in size and that less than two copies of the gene are present in the mouse genome. To establish the enzyme activity, the entire coding region was subcloned into an expression plasmid so that it was in frame with the N-terminal region of beta-galactosidase. Escherichia coli, which was transformed with the recombinant plasmid, expressed high activity of converting farnesyl diphosphate into squalene.

Post-translational import of 3-ketoacyl-CoA thiolase into rat liver peroxisomes in vitro

Cell-free translation products of hepatic free polysomal RNA from a clofibrate-treated rat were incubated at 26 degrees C for 0-60 min with a post-heavy mitochondrial supernatant fraction from normal rat liver. Exogenously added proteinase K-resistant precursor and mature forms of peroxisomal 3-ketoacyl-CoA thiolase were recovered in a particulate fraction and increased with time. Both forms of thiolase cosedimented with peroxisomes, when the proteinase K-treated import reaction mixture was centrifuged in a sucrose density gradient. The in vitro import and processing of thiolase precursors, types A and B, was likewise reproduced with highly purified peroxisomes. These results strongly suggest that the precursor form of 3-ketoacyl-CoA thiolase is translocated into peroxisomes, apparently without tight coupling with proteolytic processing to the mature protein.