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

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.

Characterization of the rat catechol-O-methyltransferase gene proximal promoter: identification of a nuclear protein-DNA interaction that contributes to the tissue-specific regulation

The methylating enzyme catechol-O-methyltransferase (COMT) is an important inactivator of substrates containing catechol-structure, such as catechol neurotransmitters and hormones. In previous studies, the rat COMT gene has been cloned and characterized, and it has been shown that the two COMT polypeptides, S- and MB-COMT, are expressed from one gene by cooperation of two separate promoters. One promoter, P2, functions constitutively, whereas the other, the proximal P1 promoter, is regulated in a tissue-specific manner. In this report, a more detailed analysis of the rat P1 promoter is presented. By using reporter gene constructs, it is shown that upstream sequences of the P1 promoter contain several regions that modulate the expression either positively or negatively. These experiments also show that the region between the MB- and S-ATG translation initiation codons is indispensable for the activity of this promoter. Analysis of this region by DNase I footprinting and gel retardation assays identified the presence of several DNA elements with SP1 and NF1 recognition site homologies that bound both liver and brain nuclear proteins. However, one 11-nucleotide-long DNA region containing an overlapping consensus binding sequence for CREB and C/EBP-like factors reacted only with the liver nuclear lysate. Supershift experiments suggest that the transcription factor C/EBPalpha mediates the tissue-specific expression of the rat COMT P1 promoter.

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.

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.

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.

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.