The autoimmune regulator: a key toward understanding the molecular pathogenesis of autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy

Autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED) is an autoimmune disease with autosomal recessive inheritance. APECED is characterized by the breakdown of tolerance to several organ-specific selfantigens. The symptoms of APECED fall into three main categories: autoimmune polyendocrinopathies, chronic mucocutaneous candidiasis, and ectodermal dystrophies. The gene defective in APECED, AIRE, has been cloned and numerous mutations in this gene have been found in patients with APECED. AIRE is predicted to encode a 545-amino-acid protein containing structural domains characteristic for transcription regulators. The protein has been shown to act as a transcriptional activator in vitro. The AIRE protein is mainly localized to the nucleus, where it can be detected as speckles resembling nuclear bodies. In humans, the expression of AIRE has been observed predominantly in immunologically relevant tissues, especially the thymus. Recently, we have shown in the mouse that Aire is also expressed in various tissues and cell types outside the immune system.

Genotypes of catechol-O-methyltransferase and response to levodopa treatment in patients with Parkinson's disease

A single nucleotide polymorphism at the nucleotide 1947 in the catechol-O-methyltransferase (COMT) gene encodes the high and low activity forms of the enzyme. We investigated COMT genotypes of 73 Korean patients with Parkinson's disease (PD), 29 with multiple system atrophy (MSA), and 49 controls, and analyzed the response to levodopa challenge in the PD patients. We found no significant difference in the distribution of the COMT genotypes among the three groups. The frequencies of the G- and A-alleles in the total population were 75 and 25%, respectively. The levodopa response was determined by a single oral levodopa challenge test with Sinemet (25/250 mg) in the patients with PD. The motor response evaluated by the time to peak response, the duration and magnitude of the response in the motor part of the Unified Parkinson's Disease Rating Scale; tapping or walking times showed no significant difference between the genotypes. Thus, pharmacokinetic or pharmacodynamic factors other than the investigated genetic variant of the COMT enzyme seem to determine the response to levodopa in PD.

Subcellular location and expression pattern of autoimmune regulator (Aire), the mouse orthologue for human gene defective in autoimmune polyendocrinopathy candidiasis ectodermal dystrophy (APECED)

Autoimmune polyendocrinopathy candidiasis ectodermal dystrophy (APECED), also known as autoimmune polyglandular syndrome Type I (APS1), is an autosomal recessive autoimmune disease caused by mutations in a gene designated as AIRE (autoimmune regulator). Here we have studied the expression of Aire in transfected cell lines and in adult mouse tissues. Our results show that Aire has a dual subcellular location and that it is expressed in multiple immunologically relevant tissues such as the thymus, spleen, lymph nodes, and bone marrow. In addition, Aire expression was detected in various other tissues such as kidney, testis, adrenal glands, liver, and ovary. These findings suggest that APECED protein might also have a function(s) outside the immune system.(J Histochem Cytochem 49:197-208, 2001)

Molecular mechanisms controlling the rate and specificity of catechol O-methylation by human soluble catechol O-methyltransferase

Molecular mechanisms determining the turn-over rate and specificity of catechol O-methylation were studied by combining enzyme kinetic measurements, computational modeling of substrate properties and fitting ligands in a 3D model of the active site of the enzyme. Enzyme kinetic measurements were carried out for 46 compounds, including most clinically used catechol drugs, by using recombinant human soluble catechol O-methyltransferase (COMT). The most important mechanism decreasing the turnover rate and increasing affinity was the electron withdrawing effect of substituents. Several other mechanisms by which substituents affected reactivity and affinity were identified. Highest turnover rates were determined for unsubstituted catechol and pyrogallol. Pyrogallol derivatives generally seemed to be more specific substrates than catechols. Catecholestrogens were the most specific endogenous substrates, whereas catecholamines were rather poor substrates. Among the catechol drugs used in the L-DOPA treatment of Parkinson's disease, the COMT inhibitors entacapone and tolcapone were not methylated, whereas the DOPA decarboxylase inhibitor benserazide was 15 times more specific substrate than L-DOPA, the target of COMT inhibition. The structure-activity relationships found allow the prediction of reactivity, affinity, and specificity with useful accuracy for catechols with a wide range of structures and properties. The knowledge can be used in the evaluation of metabolic interactions of endogenous catechols, drugs and dietary catechols, and in the designing of drugs with the catechol pharmacophore.

Pharmacogenetics of catechol-O-methyltransferase: frequency of low activity allele in a Ghanaian population

Catechol-O-methyltransferase (COMT) catalyses the O-methylation of neurotransmitters, catechol hormones and drugs such as levodopa and methyldopa. Ethnic differences in COMT activity have been observed in several populations. Previous studies suggest that the g1947G>A low activity allele is less common in individuals of African origin. COMT genotyping was performed using a mini-sequencing method in 195 healthy Ghanaians with a frequency of the homozygous g1947G>A of 6%. This study provides confirmation that the low activity COMT allele is less common in individuals of African origin. This finding may be important clinically with regards to the treatment of many neuropsychiatric disorders and in the pathophysiology of various human disorders including estrogen-induced cancers, Parkinson's disease, depression and hypertension.

Production of human UDP-glucuronosyltransferases 1A6 and 1A9 using the Semliki Forest virus expression system

Human UDP-glucuronosyltransferases (UGTs) 1A6 and 1A9 were expressed using Semliki Forest virus (SFV) vectors. Infection of chinese hamster lung fibroblast V79 cells with recombinant SFV-UGT viruses resulted in efficient protein expression as detected by metabolic labeling, Western blot analyses and immunofluorescence microscopy. The expression of UGT 1A6 and UGT1A9 in the SFV-infected cells was approximately two fold higher than in a stable V79 cell line. No UGT signal was detected in noninfected cells. In addition, SFV-UGT viruses also efficiently infected other mammalian cells, such as baby hamster kidney (BHK), chinese hamster ovary (CHO) and human lung (WI-26 VA4) cells leading to high production of recombinant enzyme. The measurement of enzyme activities and kinetic parameters using p-nitrophenol and nitrocatechol (entacapone) as substrates for UGT1A6 and UGT1A9, respectively, showed that the overall kinetic properties of the enzymes produced by the two systems were similar. We conclude that the SFV expression system represents an efficient, fast and versatile method for production of metabolic enzymes for in vitro assays.

Radiochemical high-performance liquid chromatographic assay for the determination of catechol O-methyltransferase activity towards various substrates

A new chromatographic catechol O-methyltransferase (COMT) assay based on S-adenosyl-L-[methyl-14C]methionine and on-line radioactivity detection was developed. With minor modifications in the mobile phase composition the methylation velocities for 30 structurally diverse compounds including simple catechols, neurotransmitters, catecholestrogens and catecholic drugs could be measured using human and rat recombinant soluble COMT. The enzymes showed very similar substrate selectivities. The radiochemical method was validated using 3,4-dihydroxybenzoic acid as a model substrate and it was shown that accurate and reproducible methylation velocity values could be achieved for both of the catecholic hydroxyls. The method proved to be suited for determining the enzyme kinetic parameters and can probably be further used for gathering enzyme kinetic data on differentially substituted catechols in order to construct proper structure-activity relationships for COMT.

Soluble and membrane-bound catechol-O-methyltransferase in normal and malignant mammary gland

The levels of 26 kDa-soluble (S) and 30 kDa-membrane-bound (MB) catechol-O-methyltransferase (COMT) polypeptides were determined in paired samples from normal and neoplastic breast tissue of 32 patients with breast cancer. Immunohistochemical staining showed that the COMT reaction in normal mammary tissue was restricted to the epithelial cells in the ducti and lobuli, whereas in the tumors a strong reaction was also seen in the malignant cells. The amounts of COMT proteins in tumors could not be correlated with various clinical or pathological parameters. Quantitative immunoblotting analysis revealed that the total amount of COMT proteins in tumors was more than 50% higher than in respective normal samples in 26 out of 32 patients. Five cases showed less than a 50% difference and in one case less COMT was detected in the tumor. In most cases the amount of both S- and MB-COMT forms was increased. The average amount of total COMT was 178 +/- 57 pg/microg total protein in normal tissue and 566 +/- 94 pg/microg total protein in tumor. Respective values for S-COMT were 137 +/- 52 pg/microg total protein in normal tissue and 369 +/- 62 pg/microg total protein in tumor and for MB-COMT 41 +/- 10 and 197 +/- 41 pg/microg total protein, respectively. Analysis of COMT-specific transcripts suggested that the COMT enzyme level in tumors is determined in some cases by transcriptional and in some cases by post-transcriptional mechanisms.

Functional consequences of amyloidosis mutation for gelsolin polypeptide -- analysis of gelsolin-actin interaction and gelsolin processing in gelsolin knock-out fibroblasts

Gelsolin, an actin-modulating protein, derived from a single gene exists in intracellular and secreted forms. A point mutation at position 187 of both forms of gelsolin causes familial amyloidosis of the Finnish type (FAF). Here, we expressed both isoforms of the wild-type and FAF mutant gelsolin in mouse embryonic gelsolin-null fibroblasts. We demonstrate that the FAF mutation does not interfere with the normal actin-modulating function of intracellular gelsolin, and that aberrant processing of secreted FAF gelsolin to FAF amyloid precursor takes place in the gelsolin-negative background. These results suggest that, in patients with FAF, symptoms are caused by the accumulation in their tissues of amyloid derived from plasma gelsolin and are not due to functional differences in cytoplasmic gelsolin.

Localization of the APECED protein in distinct nuclear structures

Autoimmune-polyendocrinopathy-candidiasis-ecto-dermaldystrophy (APECED) is the only systemic autoimmune disease with a monogenic background known so far revealing no association with the major histocompatibility complex region. We have recently isolated the gene defective in this syndrome and characterized several different mutations in individuals with the disorder. The novel gene, AIRE, contains a putative bipartite nuclear targeting signal predicting a nuclear location of the corresponding protein. The presence of two PHD-type zinc finger domains as well as the newly described putative DNA-binding domain, SAND, in the amino acid sequence of the APECED protein implies that it may be involved in the regulation of gene expression. Using transient expression of AIRE cDNA in mammalian cells we demonstrate here the nuclear location of the APECED protein. Immunohistochemical staining of transfected cells revealed that most of the recombinant 58 kDa APECED protein is present in the form of nuclear dots. By double immuno-fluorescence labelling we further show that these APECED-containing structures and the previously described PML nuclear bodies are largely non-overlapping. The AIRE protein was also visualized in multiple human tissues: a subset of the cells in thymus, in spleen and in lymph node showed nuclear staining with APECED antiserum. Immunofluorescence labelling of peripheral blood mononuclear leukocytes also revealed a nuclear body-like staining pattern in a fraction of these cells. These data from both in vitro and ex vivo systems, together with the predicted structural features of the APECED protein, suggest that this protein is most probably involved in the regulation of gene expression.

Induction of UDP-glycosyltransferase family 1 genes in rat liver: different patterns of mRNA expression with two inducers, 3-methylcholanthrene and beta-naphthoflavone

Uridine diphosphate (UDP)-glucuronosyltransferases (UGTs), presently called UDP-glycosyltransferases, catalyse the detoxification of many toxic and carcinogenic compounds. Glucuronidation is also a major metabolic pathway for numerous drugs. The UGT1A6 gene (formerly known as UGT1*06 and UGT1A1) has been suggested to belong to the aryl hydrocarbon (Ah) gene battery, which consists of several genes encoding for drug-metabolising enzymes regulated by dioxin and other ligands of the Ah receptor. In this study, we analysed the localisation of UGT1A6 expression in rat liver by in situ hybridisation to mRNA. Two different RNA probes were used, one which was specific to UGT1A6 and the other against the C terminal sequence shared by all UGT1 genes. In this study, no UGT1A6 mRNA was detected in the control animals. However, other gene(s) of the UGT1 family were expressed in the perivenous region surrounding the central veins as detected by hybridisation with the probe against the common region of the UGT1 genes. Treatment with the lower dose (5 mg/kg) of 3-methylcholanthrene (3MC) induced expression of UGT1A6 perivenously. Treatment with the higher dose (25 mg/kg) of 3-Methylcholanthrene resulted in a more panacinar expression pattern. In contrast to the perivenous induction observed with 3-methylcholanthrene, treatment with 15 mg/kg of beta-naphthoflavone (BNF) resulted in strong induction in the periportal region. The results reveal an inducer-specific pattern of UGT1A6 expression similar to that demonstrated earlier for other Ah battery genes, namely CYP1A1, CYP1A2, GSTYalpha and ALDH3. The finding further supports the notion that common factors regulate the regional hepatic expression of Ah battery genes.

Solution structure and main chain dynamics of the regulatory domain (Residues 1-91) of human cardiac troponin C

The three-dimensional structure of calcium-loaded regulatory, i.e. N-terminal, domain (1-91) of human cardiac troponin C (cNTnC) was determined by NMR in water/trifluoroethanol (91:9 v/v) solution. The single-calcium-loaded cardiac regulatory domain is in a "closed" conformation with comparatively little exposed hydrophobic surface. Difference distance matrices computed from the families of Ca2+-cNTnC, the apo and two-calcium forms of the skeletal TnC (sNTnC) structures reveal similar relative orientations for the N, A, and D helices. The B and C helices are closer to the NAD framework in Ca2+-cNTnC and in apo-sNTnC than in 2.Ca2+-sNTnC. However, there is an indication of a conformational exchange based on broad 15N resonances for several amino acids measured at several temperatures. A majority of the amides in the alpha-helices and in the calcium binding loop exhibit very fast motions with comparatively small amplitudes according to the Lipari-Szabo model. A few residues at the N and C termini are flexible. Data were recorded from nonlabeled and 15N-labeled samples, and backbone dynamics was investigated by 15N T1, T2, and heteronuclear nuclear Overhauser effect as well as by relaxation interference measurements.

Ethnic differences in catechol O-methyltransferase pharmacogenetics: frequency of the codon 108/158 low activity allele is lower in Kenyan than Caucasian or South-west Asian individuals

Catechol O-methyltransferase (COMT) inactivates neurotransmitters, hormones and drugs such as levodopa. COMT activity is inherited in an autosomal recessive manner and individuals with low activity have thermolabile COMT protein. A low activity allele has been demonstrated at codon 108/158 of the soluble and membrane bound COMT protein, respectively, whereby a G to A transition results in a valine to methionine substitution, rendering the protein more thermolabile. As ethnic differences in erythrocyte COMT activity have been previously demonstrated, the frequency of low activity alleles were investigated in 265 British Caucasian, 99 British South-west Asian and 102 Kenyan individuals. Genotyping of COMT codon 108/158 was performed using a minisequencing method. Erythrocyte COMT activity was measured in 60 British Caucasian individuals by radiochemical assay. The frequency of low activity alleles was 0.54 in Caucasians, 0.49 in South-west Asians, and 0.32 in Kenyans. There was a much lower frequency of individuals with homozygous low activity allele in the Kenyan population (9%) than in Caucasians (31%) or South-west Asians (27%). Erythrocyte COMT activity was lower and less thermostable in individuals with homozygous low activity alleles. The data provide molecular evidence that low COMT is less common in African individuals than the Caucasian population.

Genetic polymorphism of catechol-O-methyltransferase (COMT): correlation of genotype with individual variation of S-COMT activity and comparison of the allele frequencies in the normal population and parkinsonian patients in Finland

The catechol-O-methyltransferase (COMT) gene occurs as two polymorphic alleles, which code for a high activity thermostable and low activity thermolabile form of the enzyme. We devised a fast solid-phase minisequencing assay for genotyping the COMT gene at nucleotide position 544 encoding amino acid residue 158. The method was applied to correlate the genotype of the COMT gene with the biological activity of the COMT enzyme. In red blood cells from individuals homozygous for G at nucleotide position 544 coding for Val-158, the activity of COMT ranged from 0.55-1.03 pmol min-1 mg-1 protein, and in individuals homozygous for A at position 544 coding for Met-158, the activity ranged from 0.21-0.43 pmol min-1 mg-1. Heterozygotes showed intermediate activities of 0.20-0.88 pmol min-1 mg-1. The thermostability (heated/unheated) at 48 degrees C of the high activity form was shown to be about two-fold compared to that of the low activity form of the enzyme. By analysing 76 individual samples and three pooled samples representing altogether 3140 individuals using the solid-phase minisequencing method, the two COMT alleles were shown to be equally distributed in the Finnish population. No statistically significant difference in the frequencies of the COMT alleles was found when comparing the normal population with a sample of 158 Finnish patients with Parkinson's disease.

Expression and intracellular localization of catechol O-methyltransferase in transfected mammalian cells

The intracellular localization of soluble and membrane-bound isoforms of rat and human catechol O-methyltransferase (COMT) was studied by expressing the recombinant COMT proteins either separately or together in mammalian cell lines (HeLa and COS-7 cells) and in rat primary neurons. The distribution of soluble and membrane-bound COMT enzyme was visualized by immunocytochemistry. For comparison, the localization of native COMT was studied in rat C6 glioma cells by immunoelectron microscopy. Staining of cells expressing membrane-bound COMT with a COMT-specific antiserum revealed an immunofluorescence signal in intracellular reticular structures and in the nuclear membrane. Double-staining of the cells with antisera against proteins specific for the rough endoplasmic reticulum indicated that they colocalized with membrane-bound COMT, suggesting that it resided in the endoplasmic reticulum. Notably, no COMT-specific fluorescence of plasma membranes was detected. The signal in the endoplasmic reticulum was also evident in the cells expressing both recombinant COMT forms. Intracellular native COMT reaction was detected by immunoelectron microscopy in rat C6 glioma cells and an intense cytoplasmic signal was seen in the primary neurons infected with the recombinant Semliki Forest virus. The cells expressing recombinant soluble COMT revealed intense nuclear staining together with diffuse cytoplasmic immunoreactivity, suggesting that a part of soluble COMT is transported to nuclei. Western blotting from rat liver and brain revealed soluble COMT in the nuclei. Enzyme activity measurements from liver cytoplasmic and nuclear fractions suggested that about 5% of the soluble COMT resided in nuclei. The intracellular localization of both COMT forms implies that COMT acts in the cytoplasm and possibly also in the nuclear compartment, and that the physiological substrates of COMT enzymes may have to be internalized before their methylation by COMT.

Purification methods of mammalian catechol-O-methyltransferases

The protein purification strategies used for obtaining homogeneous rat and human soluble catechol-O-methyltransferase (S-COMT) polypeptides are reviewed. Expression and purification of recombinant rat and human S-COMT in Escherichia coli and for human S-COMT in baculevirus-infected insect cells made it possible to elucidate the S-COMT polypeptides in more detail. The application of these purification methods has allowed the crystallization of the rat S-COMT protein and the analysis of the kinetic properties of the enzyme in great detail. The availability of the pure S-COMT protein together with the structural data has also greatly enhanced the development of more potent COMT inhibitors.

Catechol-O-methyltransferase in rat sensory ganglia and spinal cord

The localization of catechol-O-methyltransferase immunoreactivity in rat dorsal root ganglia and in the spinal cord and its co-existence with substance P, calcitonin gene-related peptide and fluoride-resistant acid phosphatase in dorsal root ganglion cells was examined with immunohistochemical and histochemical double-staining methods. Analysis of dorsal of dorsal root ganglia at both cervical and lumbar levels revealed catechol-O-methyltransferase immunoreactivity in numerous dorsal root ganglion cells. Double-staining studies showed that catechol-O-methyltransferase and substance P immunoreactivities were located in different cells with a few exceptions, whereas both catechol-O-methyltransferase and calcitonin gene-related peptide immunoreactivities were detected in about 10% of all labeled cells positive for one of the two markers at both levels studied. The great majority of fluoride-resistant alkaline phosphatase-positive cells were also immunoreactive for catechol-O-methyltransferase. Again, no difference was found between cervical and lumbar levels. Catechol-O-methyltransferase immunoreactivity was also found in the neuropil of the dorsal horn of the spinal cord. The staining was most intense in the superficial laminae (I-III) and overlapped partly with substance P and calcitonin gene-related peptide immunoreactivity. Western blotting analysis revealed that soluble catechol-O-methyltransferase was the clearly dominating form of the enzyme in dorsal root ganglia. The distribution pattern of catechol-O-methyltransferase in dorsal horn and sensory neurons suggests that the enzyme may modulate sensory neurotransmission.

Neuronal and non-neuronal catechol-O-methyltransferase in primary cultures of rat brain cells

Previous biochemical and histochemical studies have suggested that catechol-O-methyltransferase (COMT) is a predominantly glial enzyme in the brain. The aim of this work was to study its localization and molecular forms in primary cultures, where cell types can be easily distinguished with specific markers, COMT immunoreactivity was studied in primary astrocytic cultures from newborn rat cerebral cortex, and in neuronal cultures from rat brain from 18-day-old rat embryos using antisera against rat recombinant COMT made in guinea pig. Double-staining studies with specific cell markers to distinguish astrocytes, neurons and oligodendrocytes were performed. COMT immunoreactivity colocalized with a specific oligodendrocyte marker galactocerebroside in cells displaying oligodendrocyte morphology, flat cells displaying type-1 astrocyte morphology and glial fibrillary acidic protein, in branched cells displaying type-2 astrocyte morphology and in cell bodies of neurons, the processes of which displayed neurofilament immunoreactivity. Western blots detected both soluble 24 kDa and membrane-bound 28-kDa COMT proteins in neuronal and astrocyte cultures. The results suggest that COMT is synthesized by cultured astrocytes, oligodendrocytes and neurons.

Kinetics of human soluble and membrane-bound catechol O-methyltransferase: a revised mechanism and description of the thermolabile variant of the enzyme

Human soluble (S) and membrane-bound (MB) catechol O-methyltransferase (COMT, EC 2.1.1.6) enzymes have been expressed at sufficiently high levels in Escherichia coli and in baculovirus-infected insect cells to allow kinetic characterization of the enzyme forms. The use of tight-binding inhibitors such as entacapone enabled the estimation of actual enzyme concentrations and, thereby, comparison of velocity parameters, substrate selectivity, and regioselectivity of the methylation of both enzyme forms. Kinetics of the methylation reaction of dopamine, (-)-noradrenaline, L-dopa, and 3,4-dihydroxybenzoic acid was studied in detail. Here, the catalytic number (Vmax) of S-COMT was somewhat higher than that of MB-COMT for all four substrates. The Km values varied considerably, depending on both substrate and enzyme form. S-COMT showed about 15 times higher Km values for catecholamines than MB-COMT. The distinctive difference between the enzyme forms was also the higher affinity of MB-COMT for the coenzyme S-adenosyl-L-methionine (AdoMet). The average dissociation constants Ks were 3.4 and 20.2 microM for MB-COMT and S-COMT, respectively. Comparison between the kinetic results and the atomic structure of S-COMT is presented, and a revised mechanism for the reaction cycle is discussed. Two recently published human COMT cDNA sequences differed in the position of S-COMT amino acid 108, the residue being either Val-108 [Lundström et al. (1991) DNA Cell. Biol. 10, 181-189] or Met-108 [Bertocci et al. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 1416-1420].(ABSTRACT TRUNCATED AT 250 WORDS)

Two different allelic mutations in a Finnish family with lecithin:cholesterol acyltransferase deficiency

Lecithin:cholesterol acyltransferase (LCAT) deficiency is a genetic disorder associated with low levels of serum HDL cholesterol. The proband of the Finnish LCAT-deficient family had corneal opacities, proteinuria, anemia with stomatocytosis, low serum HDL cholesterol (0.27 mmol/L), and low LCAT activity. Sequence analysis of his LCAT gene revealed compound heterozygosity for two different mutations: a C insertion in exon 1 between nucleotides 932 and 937 and a C-to-T point mutation in exon 6 at position 4976. The C insertion in exon 1 is predicted to result in premature termination and a truncated polypeptide containing only 16 amino acids. The C-to-T point mutation in exon 6 substitutes cysteine for arginine at residue 399. The functional significance of the Arg399-->Cys mutation was examined by expressing the mutated and wild-type LCAT cDNAs in COS cells. COS cells transfected with mutated and wild-type cDNAs showed comparable levels of mature LCAT mRNA. However, LCAT activity in the cell media of COS cells transfected with the mutant LCAT cDNA was significantly lower than that of COS cells transfected with the wild-type cDNA (1.4% versus 12.0% cholesterol esterified, respectively). A polymerase chain reaction-based duplex assay, in which both mutations can be detected simultaneously, was used for preliminary screening of Finnish subjects with serum HDL levels below 0.9 mmol/L; two additional individuals heterozygous for the Arg399-->Cys mutation were identified.(ABSTRACT TRUNCATED AT 250 WORDS)

Catechol-O-methyltransferase (COMT) in rat brain: immunoelectron microscopic study with an antiserum against rat recombinant COMT protein

Localization of catechol-O-methyltransferase (COMT) in rat cerebral cortex, neostriatum and cerebellar cortex was studied with preembedding immunoelectron microscopy using a specific antiserum raised against rat recombinant COMT protein. In all areas, immunoreactivity was found both in astrocytes and in neuronal processes. Reaction product was seen in the cytoplasm and in association with tubular structures of dendritic processes. Immunoreactivity was also located postsynaptically in dendritic spines and associated with the postsynaptic membrane. Strong immunoreaction was also seen in the cytoplasm of ependymal cells lining the ventricles, and in tanycytes in median eminence. The results suggest that postsynaptic dendritic spines and astrocytic processes may be the sites of catecholamine inactivation by COMT in rat brain.

O-methylation of L-dopa in melanin metabolism and the presence of catechol-O-methyltransferase in melanocytes

O-Methylation of L-dopa was investigated as a possible regulatory mechanism in melanin metabolism. The methylation product of L-dopa, 3-O-methoxytyrosine was detected in extracts of cultured human melanocytes. The enzyme catechol-O-methyltransferase is responsible for this O-methylation and that of the dihydroxyindolic intermediates of melanogenesis. The enzyme is present in melanocytes in its soluble and membrane-bound isoforms. Immuno-electron microscopy suggests the presence of the membrane-bound enzyme in the endoplasmic reticulum. This localization may indicate a role of catechol-O-methyltransferase in protecting the melanocyte against reactive dihydroxyphenolic intermediates of melanogenesis leaking from the melanogenic compartments. On the other hand, the O-methylation of L-dopa may serve as a regulatory point in melanogenesis during early stage of tyrosinase processing in the endoplasmic reticulum.

Binding of a new Ca2+ sensitizer, levosimendan, to recombinant human cardiac troponin C. A molecular modelling, fluorescence probe, and proton nuclear magnetic resonance study

The binding of a new calcium sensitizer, levosimendan, to human cardiac troponin C (cTnC) is described. Fluorescence studies done on dansylated recombinant human cTnC and a site-directed mutant showed that levosimendan modulated the calcium-induced conformational change in cTnC, and revealed the role of Asp-88 in the binding of the drug to the NH2-terminal domain of cTnC. Furthermore, NMR studies performed on the NH2-terminal fragment of cTnC showed a spatial proximity between levosimendan and Met81, Met85, and Phe77 in the drug-protein complex. These data were used to build an optimized model of the drug-protein complex, in which levosimendan binds cTnC at the hydrophobic pocket of the NH2-terminal domain. The role of the binding of levosimendan to cTnC in the pharmacological action of this drug in vivo is discussed.

Genomic organization of the human catechol O-methyltransferase gene and its expression from two distinct promoters

Human genomic DNA fragments containing catechol O-methyltransferase (COMT) sequences were isolated and the exon-intron structure analysed by sequencing, PCR and comparing to the human COMT cDNA sequences. The gene contains six exons, of which exons 1 and 2 are noncoding. MB-ATG and S-ATG codons, responsible for the initiation of translation of the membrane-bound (MB) and soluble (S) forms of the enzyme, are located in exon 3. Two distinct COMT-specific transcripts, 1.3 kb and 1.5 kb, were detected in various human tissues and cell lines. Different quantities of the shorter COMT-specific mRNA in the tissues studied suggest a tissue-specific regulation of the COMT gene at transcriptional level. Mapping of the 5' ends of the COMT mRNAs showed that transcription initiates at multiple sites in two separate DNA regions, which are preceded by functional promoter sequences. The proximal promoter (P1), located between the two translation initiation codons and extending approximately 200 bp upstream of the MB-ATG initiation codon, apparently gives rise to the 1.3-kb S-COMT mRNA (S-mRNA). The distal promoter (P2) is located in a DNA fragment in front of and partly overlapping the transcription-start region of the 1.5-kb transcript, suggesting that it controls the expression of this MB-mRNA. Similarities between the rat and human COMT gene promoters are analyzed.

Distribution of catechol-O-methyltransferase enzyme in rat tissues

In the present study we show the distribution of catechol-O-methyltransferase (COMT) in various rat tissues with a highly specific antiserum prepared against recombinant rat COMT. Immunoprecipitation and immunocytochemical controls confirmed the COMT-specificity of the antibodies. The antiserum detected both the 24 KD soluble and the 28 KD membrane-bound forms of the enzyme. By immunohistochemical staining the COMT enzyme was found in most rat tissues. Staining was most intense in the liver and in the kidney, in agreement with previous studies and our immunoblotting results. In the gastrointestinal tract, epithelial cells of the stomach, duodenum, and ileum were immunoreactive for COMT. In pancreas, COMT immunoreactivity was found in insulin-producing beta-cells and somatostatin-producing D-cells but not in glucagon-producing alpha-cells of the islets of Langerhans. In pituitary, COMT immunoreactivity was found in cleft cells, in pituicytes of the posterior lobe, and in the anterior lobe, partly in the same cells containing luteinizing hormone (LH). In other endocrine organs, COMT immunoreactivity was found in epithelial cells of the thyroid gland and in zona glomerulosa of the adrenal cortex. In the brain, brightest immunofluorescence was seen in ependymal cells of the cerebral ventricles and choroid plexus. Weak to moderate immunofluorescence was found in the neuropil of several brain areas, including striatum and cortex. Scattered small neurons in spinal sensory ganglia were also COMT immunoreactive. Previous immunocytochemical studies, enzyme activity determinations, and distribution of the COMT mRNA are in general agreement with the results presented here. The wide distribution of COMT in different tissues suggests an important role for this protein in inactivation of catechol compounds.

A novel amino acid substitution (His183-->Gln) in exon 5 of the lipoprotein lipase gene results in loss of catalytic activity: phenotypic expression of the mutant gene in a heterozygous state

We have identified a hitherto unrecognized mutation of the lipoprotein lipase gene (LPL) in a Finnish family with Russian and Swiss ancestors. A single base pair substitution of a guanine for cytosine in codon 183 of exon 5 of the LPL gene results in a change of histidine to glutamine in the mature enzyme protein. Expression of a mutant cDNA construct in COS cells resulted in secretion of inactive LPL enzyme protein confirming the functional significance of the mutation. The proband, a 50-year-old female and her two daughters were all heterozygous for the His183-->Gln mutation. Clinically, the proband was characterized by variable and occasionally severe hypertriglyceridemia, obesity, hypertension, coronary heart disease and non-insulin-dependent diabetes mellitus. The daughters, aged 24 and 19 years, were also obese but had milder hypertriglyceridemia. In conclusion, we have identified a novel LPL mutation that results in the synthesis of an inactive enzyme protein. Although the assessment of a causative link between the mutation and hyperlipidemia awaits further studies, our data suggest that heterozygosity for a functional defect of LPL should be considered in patients presenting with the metabolic dyslipidemic syndrome, "syndrome-X."

Production of rat soluble and membrane-bound catechol O-methyltransferase forms from bifunctional mRNAs

In the rat, the catechol O-methyltransferase (COMT) gene has been found to contain two promoters, P1 and P2. This organization enables the gene to produce a soluble (S-COMT) and a membrane-associated (MB-COMT) protein by using two in-frame ATG initiation codons (S- and MB-ATG). The P1 promoter expresses a 1.6 kb transcript (S-mRNA) which codes for the S-COMT polypeptide only. Here we demonstrate that the P2 promoter controls the expression of alternatively spliced 1.9 kb transcripts (MB-mRNA) which differ by a 27-nucleotide region immediately upstream of the MB-AUG codon. The presence of the 27-base sequence alters the nucleotide at position -3 from G to C, thereby changing the translation initiation context of the MB-AUG codon. Expression experiments in COS-7 cells using full-length COMT cDNAs showed that this alteration affected the initiation of the translation of the MB-AUG and consequently changed the relative amounts of MB- and S-COMT polypeptides produced. No proteolytic cleavage of the MB-COMT form to S-COMT was detected in in vitro or in vivo pulse-chase experiments. We conclude that the bifunctional 1.9 kb mRNAs are able to produce both S-COMT and MB-COMT polypeptide by the leaky scanning mechanism of translation initiation.

Structure of the rat catechol-O-methyltransferase gene: separate promoters are used to produce mRNAs for soluble and membrane-bound forms of the enzyme

The enzyme catechol-O-methyltransferase (COMT) catalyzes the inactivation of catechol-containing molecules by methylation. The cDNAs for the rat and human COMT have recently been cloned and recombinant proteins expressed in prokaryotic and eukaryotic cells. We describe here the structure of the rat COMT gene and its 5'-flanking sequences. The gene spans at least 13 kb and is composed of 5 exons, the first one noncoding. The two ATG codons for the initiation of translation of the membrane-bound (MB-COMT) and soluble (S-COMT) forms of the enzyme reside in the second exon. The gene expresses two mRNA species of 1.6 kb and 1.9 kb that have different tissue distributions. The expression of the transcripts is regulated by at least two promoters, P1 and P2. The P1 promoter expresses the shorter transcript in a tissue-specific manner and is located between the ATG codons in the coding region of the longer transcript. The P2 promoter is constitutive and responsible for the expression of the longer transcript. The shorter 1.6-kb mRNA (S-mRNA) produces only the S-COMT polypeptide, whereas the longer 1.9-kb mRNA (MB-mRNA) is able to direct synthesis of both forms of the COMT enzyme.

Expression of recombinant soluble and membrane-bound catechol O-methyltransferase in eukaryotic cells and identification of the respective enzymes in rat brain

The rat and human recombinant soluble and membrane-bound catechol O-methyltransferase (S- and MB-COMT, respectively) were expressed using mammalian and baculovirus vectors. Low levels of rat and human S-COMT polypeptides were detected by immunoprecipitation in K-562 cell lines transfected with the S-COMT vectors. From K-562 cells transfected with the rat MB-COMT construct, both S- and MB-COMT recombinant proteins were detected by a rat COMT-specific anti-serum. Infection of lepidopteran Spodoptera frugiperda cells with recombinant S- or MB-COMT baculovirus constructs yielded high amounts of enzymically active and immunoreactive S- or MB-COMT proteins, respectively. Pulse/chase experiments with [35S]methionine-labelled insect cells infected with the MB-COMT baculovirus showed that the 30-kDa recombinant human MB-COMT polypeptide was not processed into the 25-kDa S-COMT form. Subcellular fractionations of insect cells, followed by immunoblotting with COMT antiserum, showed that recombinant S-COMT was found only in the soluble, cytoplasmic fraction, whereas MB-COMT resided both in soluble and membrane fractions. The recombinant MB-COMT sedimented in Percoll gradients at the density of 1.042 g/ml cosedimenting with the plasma-membrane marker. Fractionation and immunoblotting experiments on homogenized total rat brains indicated that the rat S-COMT (24 kDa) and some of the rat MB-COMT (28 kDa) was recovered in soluble fractions, whereas the microsomal material having COMT activity contained the MB-COMT polypeptide. The rat brain microsomal MB-COMT had a density of 1.042 g/ml in Percoll gradients, cosedimenting with the plasma-membrane and rough-endoplasmic-reticulum marker enzymes. The meta/para methylation ratio of dihydroxybenzoic-acid substrate by different recombinant and rat brain COMT-containing subcellular fractions was analysed.

Deletion of the 3'-untranslated region of aspartylglucosaminidase mRNA results in a lysosomal accumulation disease

Aspartylglucosaminuria (AGU) is a lysosomal storage disease due to mutations in the aspartylglucosaminidase (AGA) gene. The deficient enzyme activity in patients' cells blocks one of the final steps in the degradation of N-linked glycoproteins. All the AGU mutations identified so far affect the coding region of the AGA gene. Here we report a homozygous 876-base pair deletion, which removes the 3'-noncoding area but leaves the coding region of the AGA mRNA intact. This deletion does not prevent transcription termination or polyadenylation of the patient's truncated mRNA, and the steady state level of the mRNA is comparable with the control. However, the quantity of AGA polypeptide chains in the patient's fibroblasts is negligible. This suggests that the deletion interferes with the translational efficiency in vivo and provides a unique model to pursue the biological significance of untranslated regions of human mRNAs.

Expression of enzymatically active rat liver and human placental catechol-O-methyltransferase in Escherichia coli; purification and partial characterization of the enzyme

To produce sufficient amounts of recombinant catechol-O-methyltransferase (COMT) for structural and functional studies the coding regions of the rat liver and human placental COMT genes have been introduced into a bacterial expression vector pKEX14. Recombinant COMT was produced in Escherichia coli up to 10% of total bacterial protein after the induction of the T7 RNA polymerase gene with isopropyl-beta-D-thiogalactopyranoside. Both the rat and human enzymes were enzymatically active, soluble and reacted with anti-COMT antiserum in Western blotting. Both enzymes were purified from E. coli cells and partially characterized by determining their specific activity, apparent molecular weight and pI.

The human catechol-O-methyltransferase (COMT) gene maps to band q11.2 of chromosome 22 and shows a frequent RFLP with BglI

We have been able to assign the human catechol-O-methyltransferase gene (COMT) to chromosome 22q11.2 by using Southern blot analysis of panels of somatic cell hybrids and chromosomal in situ hybridization. Furthermore, Southern blot analysis of DNA from blood and bone marrow samples of a patient with chronic myeloid leukemia (CML), having an extra Philadelphia chromosome (Ph1) in addition to the one produced by the reciprocal translocation between chromosomes 9 and 22, showed increased COMT and BCR gene dosage as compared to DNAs originating from CML patients with only one Ph1 chromosome or from chromosomally normal individuals. Control hybridizations of the same blot with TCRG- and TCRA-specific probes showed corresponding signal intensities in all samples. A relatively frequent two-allele COMT gene RFLP (PIC = 0.37) was recognized in DNAs digested with BglI. Our gene mapping result is in concordance with that previously reported by Brahe et al. (1986), who used an autoradiozymogram assay on different somatic cell hybrids to map this gene to chromosome 22.

Cell-free synthesis of rat and human catechol O-methyltransferase. Insertion of the membrane-bound form into microsomal membranes in vitro

The protein-coding capacities of rat and human catechol O-methyltransferase (COMT) DNA clones were analysed by in vitro transcription and translation using bacteriophage RNA polymerase and rabbit reticulocyte lysate. Two types of clones corresponding to the structures of human placental cDNA clones were used. The shorter clones, containing the 663-residue open reading frame for the soluble COMT (S-COMT), produced 24-kDa (rat) and 26-kDa (human) polypeptides. Translation of the longer clones, containing 43 (rat) or 50 (human) amino acid amino-terminal extensions to the S-COMT polypeptides, yielded 28-kDa (rat) and 30-kDa (human) putative membrane-bound COMT (MB-COMT) polypeptides as the main products. These clones also yielded low amounts of the S-COMT polypeptides. Labelling time or ionic conditions during translation did not eliminate the shorter products, suggesting translation initiation from the second S-COMT AUG codon. In accordance with this postulation, the relative amount of S-COMT could be affected by changing the translation initiation contexts preceding the first AUG codon. The 28-kDa and 30-kDa products, but not the 24-kDa and 26-kDa products, associated with microsomal membranes cotranslationally, indicating that the amino-terminal extensions were functional signal sequences. However, the presence of membranes did not affect the mobilities of the proteins in SDS/polyacrylamide gels. The MB-COMT polypeptides could not be released from the microsomes by treatments with phospholipase C or alkali and were not protected by the microsomes against proteinase K digestion. These results indicate that MB-COMT synthesized in vitro is an integral membrane protein having an amino-terminal signal-anchor sequence.

In vitro mutagenesis helps to unravel the biological consequences of aspartylglucosaminuria mutation

Aspartylglucosaminuria (AGU) is a lysosomal storage disease resulting in severe mental retardation. We have recently reported that mutations in the aspartylglucosaminidase (AGA) locus are responsible for this disease. About 90% of reported AGU cases are found in Finland, and we have shown that the vast majority (98%) of AGU alleles in this isolated population contain two point mutations located 5 bp apart. We expressed these Arg161----Gln and Cys163----Ser mutations separately in vitro and demonstrated that deficient enzyme activity is caused by the Cys163----Ser mutation, whereas the Arg161----Gln substitution represents a rare polymorphism. Further analyses of in vitro expressed AGA proteins and the enzyme purified from an AGU patient revealed that Cys163 participates in and S-S bridge. The absence of this covalent cross-link in the mutated protein most probably results in disturbed folding of the polypeptide chain and a consequent decrease in its intracellular stability.

Expression of recombinant calf prochymosin in mammalian cell culture

The calf preprochymosin cDNA was cloned into an extrachromosomal mammalian cell expression vector containing Epstein-Barr virus sequences using polymerase chain reaction. Transfection of HeLa cells yielded Hygromycin B resistant cell clones, expressing immunoreactive prochymosin, which was quantitatively secreted into the culture medium. Based on Western blotting we estimated that selected cell clones produced about 10-20 mg prochymosin per liter in 20 h. The biological activity of the secreted chymosin was confirmed by milk clotting assay.

Cloning and characterization of human placental catechol-O-methyltransferase cDNA

Catechol-O-methyltransferase (COMT) cDNA clones were isolated from a human placental cDNA library using synthetic oligonucleotides as probes. All four positive clones isolated contained an open reading frame, which potentially coded for a 24.4-kD polypeptide, presumably corresponding to the cytoplasmic form of the COMT (S-COMT). In addition to the S-COMT sequences, two of the clones carried extensions in the 5' end, which potentially coded for a 50-amino-acid peptide extending the S-COMT reading frame. This sequence contained a stretch of signal sequence-like hydrophobic amino acids in its amino terminus. The deduced human COMT polypeptide had 80% similarity with the previously characterized rat COMT. Expression of one of the cDNA clones in human K-562 cells resulted in cell clones with 3- to 10-fold increased COMT activity. Cell-free translation of transcripts synthesized in vitro from one of the short cDNAs yielded a 26-kD product, similar in size to human S-COMT. Translation of transcripts from one of the long cDNAs gave 30-kD and 26-kD polypeptides, suggesting translation initiation from two different AUG initiation codons. The 30-kD protein, but not the 25-kD protein, associated with microsomal membranes in translation lysates. A potential polyadenylation signal AATTAA was detected in the 3' ends of two of the clones 265 nucleotides downstream from the COMT translation termination codon. RNA blotting on human placental RNA revealed a 1.5-kb-long COMT-specific transcript. DNA analysis suggested that human, as well as rat, canine and monkey cells have one gene for COMT.

Molecular cloning and characterization of rat liver catechol-O-methyltransferase

The coding sequence of rat liver catechol-O-methyl-transferase (COMT; EC 2.1.1.6) was determined from rat cDNA and genomic libraries were screened with DNA probes and specific antiserum. The open reading frame consisted of 663 nucleotides coding for a 221-amino acid (aa) polypeptide with a deduced Mr of 24,747. No obvious hydrophobic signal sequence, membrane-spanning domains, or potential N-glycosylation sites were found in this sequence. The identity of the clone and the accuracy of the sequence was verified by direct aa sequencing of the tryptic peptides derived from the purified rat liver enzyme. Primer extension analysis showed that the transcription start point of the rat liver COMT mRNA was 450 bp upstream from the translation start codon. A putative polyadenylation signal (ATTAAA) was found in the 3'-noncoding region. The predicted size of the COMT transcript was 1.8-2.0 kb, which could be confirmed from Northern hybridization analyses of the isolated rat liver mRNA. One polypeptide of 25 kDa, could be immunoprecipitated with anti-COMT antibody from in vitro translation of rat liver mRNA. Employing the DNA blot analysis only one COMT-encoding gene was found in the rat genome.

Efficient synthesis of influenza virus hemagglutinin in mammalian cells with an extrachromosomal Epstein-Barr virus vector

The capability of an Epstein-Barr virus hybrid vector (EBV-CMV), containing the cytomegalovirus (CMV) immediate early enhancer and simian virus 40 promoter, to produce large amounts of authentic mammalian proteins was studied. The cDNA of influenza virus hemagglutinin (HA), a cell surface glycoprotein, was inserted into this vector and the EBV-CMV-HA plasmid was transfected into two human and two monkey cell lines. Southern-blot analysis revealed that the EBV-CMV-HA plasmid was maintained in extrachromosomal state and the recombinant cell clones contained on the average three copies (range 1-24) of the transfected DNA. The recombinant HA polypeptides from different cell clones, selected either randomly or by fluorescence-activated cell sorter, were analysed using immunological techniques. Three of the four cell lines expressed recombinant HA on the cell surface in glycosylated form. The highest production levels, 11.5 micrograms/10(6) cells, were obtained in HeLa cells containing only two copies of EBV-CMV-HA DNA per cell. The protein levels correlated with the mRNA levels in Northern-blot analysis. A corresponding vector, containing the same regulatory signals for HA expression, but lacking the EBV sequences, yielded clones with significantly lower expression levels. The results confirm that the extrachromosomal EBV-CMV vector is very useful in the production of apparently authentic mammalian glycoproteins.

Comparison of mammalian cell expression vectors with and without an EBV-replicon

We have characterized the properties of an Epstein-Barr virus vector (EBV-CMV) and compared its expression potential with a respective integrating vector (CMV). These vectors were used to express chloramphenicol acetyltransferase (CAT) gene in human HeLa, 293, monkey CV-1, dog MDCK, and hamster R 1610 cells. The EBV-CMV-cat DNA replicates extrachromosomally in HeLa, 293 and CV-1 cells, where also high expression of CAT gene was observed. The EBV-CMV vector integrated in MDCK and R 1610 cells and the CMV vector integrated in all cells tested. Integration yielded mostly clones with low CAT expression. In all cell lines, except HeLa cells, the existence of the extrachromosomal but not the integrated vector DNA is strictly dependent on the Hygromycin B selection pressure. The extrachromosomal state of the EBV vector is a prerequisite for good expression particularly in human and monkey cells.

An EBV-based mammalian cell expression vector for efficient expression of cloned coding sequences

The construction of a mammalian cell expression vector using human cytomegalovirus immediate early gene enhancer to initiate transcription of inserted coding sequences is described. The vector also carries Epstein-Barr virus EBNA-1 nuclear antigen gene, ori-P sequences and hygromycin B resistance gene hph from E. coli. The expression capacity of this construct was tested by inserting the chloramphenicol acetyltransferase (CAT) gene into the vector. The EBV-CAT construct was transfected into various cell lines and high levels of CAT activity were obtained in human and monkey cells. In these cells, the vector DNA also replicates as an extrachromosomal element having 1 to 20 copies per cell. In most cases, the vector copy number and the expression level of inserted gene was in positive correlation in different cell clones.

Dose effects of transfected c-Ha-rasVal 12 oncogene in transformed cell clones

We have examined the expression of the transformed phenotype in a series of clonal lines of NIH/3T3 cells transfected with the human c-Ha-rasVal 12 oncogene and the neomycin phosphotransferase gene. Cells from individual transformed foci were cloned and subjected to detailed analyses of the ras sequences. Three clones were found that expressed approximately one, 2-4, or 4-8 copies of the human c-ras oncogene, respectively. A fourth clone had multiple copies of the transfected sequences, and expressed abundant c-Ha-ras RNA. Analysis of the transformed phenotype of various clones indicated that cells expressing low levels of mutant c-Ha-ras had lost some of their extracellular fibronectin network, and were barely altered in their cytoskeleton. In contrast, cells expressing abundant c-Ha-ras had lost both their actin and fibronectin networks and showed an increase in plasminogen activator activity. Cells with amplified c-Ha-rasVal 12 grew better in low serum, formed large colonies in soft agar and showed enhanced activity of ornithine decarboxylase, the rate-controlling enzyme in polyamine biosynthesis. These results show that the dosage level of the mutant oncogene makes a significant contribution to the transformed phenotype of c-Ha-ras oncogene-transformed cells.

Isolation and characterization of a 2.2-kb operon preceding the alpha-amylase gene of Bacillus amyloliquefaciens

A DNA region of 2.8 X 10(3) base pairs (2.8 kb) upstream of the Bacillus amyloliquefaciens alpha-amylase gene has been isolated. This DNA gave rise to a 2.2-kb transcript. The 3' end of the transcript was mapped with S1 nuclease and shown to terminate 49 base pairs upstream of the -35 region of the alpha-amylase promoter. In B. subtilis minicells this 2.2-kb transcript coded for three different polypeptides, thus indicating a polycistronic operon-type structure. The location and the order of the polypeptides were established using DNA deletions. The joining of the 2.2-kb operon to the downstream alpha-amylase gene in the plasmid pUB110 did not have any significant effect on the level of expression of the alpha-amylase.