Exon recognition and nucleocytoplasmic partitioning determine AMPD1 alternative transcript production

AMP deaminase in rat brain: localization in neurons and ependymal cells

The purine nucleotide cycle enzyme AMP deaminase (AMPD) catalyzes the irreversible hydrolytic deamination of AMP. The physiological function of the purine nucleotide cycle in the brain is unknown. In situ hybridization and immunocytochemical studies were performed to identify the regional and cellular expression of AMPD in rat brain with the goal of elucidating the neural function of the purine nucleotide cycle. AMPD messenger RNA was detected in ventricular ependymal cells and cells of the choroid plexus and in neurons of distinct brain areas. Although only low antibody titers were obtained by immunization with the purified sheep brain AMPD, immunization of mice with synthetic lipopeptide vaccines containing oligopeptides derived from a known partial complementary DNA sequence of the enzyme yielded an antiserum suitable for immunocytochemistry. Immunostaining of cells in culture showed that neurons but not astroglial cells express appreciable amounts of the enzyme. Results of immunocytochemical staining performed on rat brain slices were in accord with the localization of AMPD messenger RNA, thus confirming the expression of AMPD in neurons of the brain stem, hippocampus, cerebellar nuclei and mesencephalic nuclei, as well as in ventricular ependymal cells and their cilia.

A novel bipartite intronic splicing enhancer promotes the inclusion of a mini-exon in the AMP deaminase 1 gene

Alternative splicing of the 12-base exon 2 of the adenosine monophosphate deaminase (AMPD) gene is subject to regulation by both cis- and trans-regulatory signals. The extent of exon 2 inclusion is stage- and cell type-specific and is subject to the physiological state of the cell. In adult skeletal muscle, a cell type that regulates the activity of this allosteric enzyme at several levels, the exon 2-plus form of AMPD, predominates. We have performed a systematic analysis of the cis-acting regulatory sequences that reside in the intron immediately downstream of this mini-exon. A complex element comprising sequences that enhance exon 2 inclusion and sequences that counteract this effect resides in the middle of this intron. We demonstrate that the enhancing component is bipartite, with more than a kilobase of sequence separating the two functional sites. The presence of even minimal levels the mini-exon in the fully processed AMPD mRNA requires both of these sites, neither of which appears in any other published splicing enhancer. An RNA binding activity derived from a muscle cell line requires both of the enhancing sites. Mutations in either of the sites that eliminate exon 2 inclusion abrogate this binding activity.

Positive and negative elements mediate control of alternative splicing in the AMPD1 gene

The second exon of the AMP deaminase (AMPD) 1 gene is alternatively spliced in response to stage-specific signals elaborated during myocyte differentiation. Since inheritance of the mutation in exon 2 of the AMPD1 gene has been recently shown to be associated with a better prognosis of congestive heart failure and the alternative splicing of exon 2 modulates the residual activity of AMPD1 in individuals with this mutant allele, the regulatory mechanism of alternative splicing in the AMPD1 gene is clinically intriguing. Retention or exclusion of exon 2 results from the interplay between negative and positive elements in the primary transcript. Exon 2 is intrinsically defective and difficult to recognize. Herein, we show that this property of exon 2 is the consequence of three defects; a suboptimal 3' splice acceptor site, a suboptimal 5' splice donor site and the small size of the exon. An improvement in any one of these defects relieves the masking of this exon. Further, this defective exon can only be identified in the presence of the adjacent downstream intron.

The in vivo minigene approach to analyze tissue-specific splicing

The exact mechanisms leading to alternative splice site selection are still poorly understood. However, recently cotransfection studies in eukaryotic cells were successfully used to decipher contributions of RNA elements (cis-factors), their interacting protein components (trans-factors) or the cell type to alternative pre-mRNA splicing. Splice factors often work in a concentration dependent manner, resulting in a gradual change of alternative splicing patterns of a minigene when the amount of a trans-acting protein is increased by cotransfections. Here, we give a detailed description of this technique that allows analysis of large gene fragments (up to 10-12 kb) under in vivo condition. Furthermore, we provide a summary of 44 genes currently investigated to demonstrate the general feasibility of this technique.

Clinical heterogeneity and molecular mechanisms in inborn muscle AMP deaminase deficiency

Lack of the muscle-specific isoform of AMP deaminase (myoadenylate deaminase deficiency) can cause a metabolic myopathy, with exercise-induced muscle symptoms such as early fatigue, cramps and/or myalgia. It is the most common muscle enzyme defect in man, found in about 2-3% of all muscle biopsies. The genetic basis of the inherited defect is the nonsense mutation C34-T in the AMPD1 gene encoding myoadenylate deaminase. The mutation results in a premature stop of the enzyme synthesis. In a healthy German population, the frequency of the mutant allele was 0.1, and 1% of this population is expected to be homozygous for the mutation. In people with muscle symptoms, the allele frequency was significantly higher (0.145). The correlation between allele frequency and muscle symptoms underscores the clinical significance of this defect. However, the vast majority of homozygous subjects do not develop a metabolic myopathy. This clinical heterogeneity may be due to molecular genetic factors such as alternative splicing of the exon harbouring the mutation, or due to metabolic conditions such as pathways compensating for the defect. The real basis for the high percentage of asymptomatic homozygous subjects remains to be revealed.

Cloning and expression of cDNA encoding heart-type isoform of AMP deaminase

Mouse cDNA for the heart-type (H) isoform of AMP deaminase (EC 3.5.4.6., AMPD) has been isolated and characterized. The cDNA for the Ampd gene expressed in heart is predicted to encode a peptide of 766 amino acids with a molecular mass of 88 kDa. The coding region of this cDNA was quite homologous to the human AMPD3 gene which encodes the erythrocyte-type (E) isoform of AMPD, although the H-isoform of rodent AMPD was reported to be immunologically distinct from the E-isoform of human AMPD. The non-coding region of the isolated cDNA is not homologous to human AMPD3, while rodent Ampd1 has similarity to human AMPD1 in the non-coding region as well as in the coding region. The transcripts for the cloned cDNA were expressed in heart, slow-twitch skeletal muscle and non-muscle tissues. Prokaryotic expression showed that this cDNA encodes a catalytically active enzyme which reacts to the specific antibody raised to the H-isoform of AMPD.

Molecular biology of AMP deaminase deficiency

In man, there are at least four isoforms of adenosine monophosphate deaminase (AMPD): myoadenylate deaminase in skeletal muscle, the L isoform in liver, and the E1 and E2 isoforms in erythrocytes. Myoadenylate deaminase is encoded by the AMPD1 gene located on chromosome 1 p13-p21, the L isoform by the AMPD2 gene, and both isoforms in erythrocytes by the AMPD3 gene. Myoadenylate deaminase deficiency is found in 2-3% of all muscle biopsies. The inborn type of myoadenylate deaminase deficiency is caused by a single mutant allele harbouring two mutations: C34-->T (Gln-->Stop) and C143-->T (Pro-48-->Leu). Population studies revealed a frequency of the mutant allele of 0.12 in Caucasian Americans and Germans. The C34-->T mutation is located in exon 2, which is alternatively spliced in part of the AMPD1 transcript in human muscle. Since the second mutation does not affect enzyme function, alternatively spliced mRNA encodes a catalytically active enzyme. Only one patient with a disorder linked to liver AMPD has been described so far. In this patient the decreased inhibition of this enzyme by GTP resulted in uric acid overproduction and gout. A complete lack of erythroyte AMPD activity is found in asymptomatic subjects. The molecular basis of both disorders is not yet known.

Control of nucleocytoplasmic transport

The movement of macromolecules between the nucleus and cytoplasm is tightly controlled. In the past few years it has become increasingly apparent that nuclear traffic is regulated not only by recognition of specific signals on proteins and RNAs, but also by cellular factors that modulate the efficacy with which these signals are recognized.