Localisation of metallothionein isoform mRNAs in rat hepatoma (H4) cells

An eleven nucleotide section of the 3'-untranslated region is required for perinuclear localization of rat metallothionein-1 mRNA

Localization of mRNAs provides a novel mechanism for synthesis of proteins close to their site of function. MT1 (metallothionein-1) is a small, metal-binding protein that is largely cytoplasmic but which can be found in the nucleus. The localization of rat MT1 requires the perinuclear localization of its mRNA by a mechanism dependent on the 3'-UTR (3'-untranslated region). The present study investigates the nature of this mRNA localization signal using Chinese-hamster ovary cells transfected with gene constructs in which either MT1 or the globin coding region is linked to different sequences from the MT1 3'-UTR. Deletion, mutagenesis and antisense oligonucleotide approaches indicate that nt 45-76 of the 3'-UTR, in particular nt 66-76, are required for the localization of either MT1 mRNA or chimaeric transcripts in which a beta-globin coding region is linked to sequences from the MT1 3'-UTR. This section of the 3'-UTR contains a CACC repeat. Two mutations that are predicted to alter the secondary structure of this region also impair localization. Our hypothesis is that the perinuclear localization signal in MT1 mRNA is formed by a combination of the CACC repeat and its structural context.

Direct visualization of mRNA colocalization with mitochondria in living cells using molecular beacons

The intracellular localization and specific organelle association of mRNA may reflect essential functions, stages, and stability of mRNA. We report the direct visualization of subcellular localization of K-ras and glyceraldehyde 3-phosphate dehydrogenase (GAPDH) mRNAs in live HDF cells using molecular beacons together with membrane-permeabilization and peptide-based delivery. Unexpectedly, we found that both K-ras and GAPDH mRNAs colocalize with mitochondria. Extensive control studies are performed, including the use of fluorescence in-situ hybridization (FISH), negative-control beacons, and the detection of colocalization of 28S ribosomal RNA with the rough endoplasmic reticulum (ER), suggesting that the mRNA localization and colocalization patterns observed in our study are true and specific. Our observation reveals intriguing subcellular associations of mRNA with organelles such as mitochondria, which may provide new insight into the transport, dynamics, and functions of mRNA and mRNA-protein interactions.

Intracellular trafficking of micronutrients: from gene regulation to nutrient requirements

The intracellular distribution of micronutrients, as well as their uptake, is important for cell function. In some cases the distribution of micronutrients or their related proteins is determined by gene expression mechanisms. The 3' untranslated region (3'UTR) of metallothionein-1 mRNA determines localisation of the mRNA, and in turn intracellular trafficking of the protein product. Using transfected cells we have evidence for the trafficking of metallothionein-1 into the nucleus and for its involvement in protection from oxidative stress and DNA damage. When nutritional supply of Se is limited, selenoprotein expression is altered, but not all selenoproteins are affected equally; the available Se is prioritised for synthesis of particular selenoproteins. The prioritisation involves differences in mRNA translation and stability due to 3'UTR sequences. Potentially, genetic variation in these regulatory mechanisms may affect nutrient requirements. Genetic polymorphisms in the 3'UTR from two selenoprotein genes have been observed; one polymorphism affects selenoprotein synthesis. These examples illustrate how molecular approaches can contribute at several levels to an increased understanding of nutrient metabolism and requirements. First, they provide the tools to investigate regulatory features in genes and their products. Second, understanding these processes can provide model systems to investigate nutrient metabolism at the cellular level. Third, once key features have been identified, the availability of human genome sequence information and single nucleotide polymorphism databases present possibilities to define the extent of genetic variation in genes of nutritional relevance. Ultimately, the functionality of any variations can be defined and subgroups of the population with subtly different nutrient requirements identified.

Distinct regions in the 3' untranslated region are responsible for targeting and stabilizing utrophin transcripts in skeletal muscle cells

In this study, we have sought to determine whether utrophin transcripts are targeted to a distinct subcellular compartment in skeletal muscle cells, and have examined the role of the 3' untranslated region (UTR) in regulating the stability and localization of utrophin transcripts. Our results show that utrophin transcripts associate preferentially with cytoskeleton-bound polysomes via actin microfilaments. Because this association is not evident in myoblasts, our findings also indicate that the localization of utrophin transcripts with cytoskeleton-bound polysomes is under developmental influences. Transfection of LacZ reporter constructs containing the utrophin 3'UTR showed that this region is critical for targeting chimeric mRNAs to cytoskeleton-bound polysomes and controlling transcript stability. Deletion studies resulted in the identification of distinct regions within the 3'UTR responsible for targeting and stabilizing utrophin mRNAs. Together, these results illustrate the contribution of posttranscriptional events in the regulation of utrophin in skeletal muscle. Accordingly, these findings provide novel targets, in addition to transcriptional events, for which pharmacological interventions may be envisaged to ultimately increase the endogenous levels of utrophin in skeletal muscle fibers from Duchenne muscular dystrophy (DMD) patients.

Influence of metallothionein-1 localization on its function

Metallothioneins (MTs) have a major role to play in metal metabolism, and may also protect DNA against oxidative damage. MT protein has been found localized in the nucleus during S-phase. The mRNA encoding the MT-1 isoform has a perinuclear localization, and is associated with the cytoskeleton; this targeting, due to signals within the 3'-untranslated region (3'-UTR), facilitates nuclear localization of MT-1 during S-phase [Levadoux, Mahon, Beattie, Wallace and Hesketh (1999) J. Biol. Chem. 274, 34961-34966]. Using cells transfected with MT gene constructs differing in their 3'-UTRs, the role of MT protein in the nucleus has been studied. Chinese hamster ovary cells were transfected with either the full MT gene (MTMT cells) or with the MT 5'-UTR and coding region linked to the 3'-UTR of glutathione peroxidase (MTGSH cells). Cell survival following exposure to oxidative stress and chemical agents was higher in cells expressing the native MT gene than in cells where MT localization was disrupted, or in untransfected cells. Also, MTMT cells showed less DNA damage than MTGSH cells in response to either hydrogen peroxide or mutagen. After exposure to UV light or mutagen, MTMT cells showed less apoptosis than MTGSH cells, as assessed by DNA fragmentation and flow cytometry. The data indicate that the perinuclear localization of MT mRNA is important for the function of MT in a protective role against DNA damage and apoptosis induced by external stress.

Signal transduction pathways, and nuclear translocation of zinc and metallothionein during differentiation of myoblasts

The changes in subcellular localization of metallothionein during differentiation were studied in two myoblast cell lines, L6 and H9C2. Addition of insulin like growth factor-I or lowering foetal bovine serum to 1% can induce differentiation of myoblasts to myotubes. Metallothionein and zinc were localized mainly in the cytoplasm in myoblasts but were translocated into the nucleus of newly formed myotubes during early differentiation. In fully differentiated myotubes, metallothionein content was decreased with a cytoplasmic localization. Addition of an inhibitor of mitogen-activated protein kinase, PD 98059, did not affect differentiation but blocked nuclear translocation of metallothionein. LY 294092, an inhibitor of PI3 kinase, and rapamycin, an inhibitor of p70S6 serine/threonine kinase, abolished insulin-like growth factor-I induced differentiation of myoblasts, retained metallothionein in the cytoplasm, and decreased metallothionein content. These results demonstrate that the cytoplasmic-nuclear translocation of metallothionein occurs during the early stage of differentiation of myoblasts to myotubes and can be blocked by inhibition of certain signal transduction pathways. The transient nuclear localization of metallothionein and zinc may be related to a high requirement for zinc for metabolic activities during the early stage of differentiation.

Nuclear import of metallothionein requires its mRNA to be associated with the perinuclear cytoskeleton

The influence of mRNA localization on metallothionein-1 protein distribution was studied by immunocytochemistry. We used Chinese hamster ovary cells that had been transfected with either a native metallothionein-1 gene construct or metallothionein-1 5'-untranslated region and coding sequences linked to the 3'-untranslated region from glutathione peroxidase. The change in the 3'-untranslated region caused the delocalization of the mRNA with a loss of the perinuclear localization and association with the cytoskeleton. Clones were selected which expressed similar levels of metallothionein-1 protein, as assessed by radioimmunoassay. The results showed that loss of metallothionein-1 mRNA localization was associated with a loss of metallothionein-1 protein localization, most notably with a lack of metallothionein-1 protein in the nucleus of synchronized cells which were beginning to synthesize DNA. This indicates that the association of metallothionein-1 mRNA with the cytoskeleton around the nucleus is essential for efficient shuttling of the protein into the nucleus during the G(1) to S phase transition. This is the first demonstration of a physiological role for perinuclear mRNA localization and we propose that such localization may be important for a wide range of nuclear proteins, including those that shuttle between nucleus and cytoplasm in a cell cycle dependent manner.

Cloning of the mating type loci from Pyrenopeziza brassicae reveals the presence of a novel mating type gene within a discomycete MAT 1-2 locus encoding a putative metallothionein-like protein

The mating type loci were cloned from Pyrenopeziza brassicae by chromosome walking from a mating type-linked polymerase chain reaction (PCR) fragment and shown to be idiomorphic by sequence analysis. The MAT 1-1 locus is approximately 3.2 kb and contains a single gene encoding a putative high-mobility group (HMG) domain protein. The MAT 1-2 locus is approximately 3.9 kb with three open reading frames (ORFs) encoding a putative HMG domain, an alpha-1 domain and metallothionein-like proteins. The putative alpha-1 domain ORF on MAT 1-2 is transcribed in the opposite orientation to the other two transcripts and extends into non-idiomorphic sequence. This is the first report of sequence analysis of the mating type loci from a discomycete fungus, which has revealed an interesting mating type infrastructure within the MAT 1-2 locus. Although metallothionein-like proteins have been implicated in a number of processes in animals and plants, they have not to date been implicated in the mating process of filamentous fungi. Possible roles for metallothionein-like proteins in the mating process are discussed.

Regulatory signals in messenger RNA: determinants of nutrient–gene interaction and metabolic compartmentation

Nutrition has marked influences on gene expression and an understanding of the interaction between nutrients and gene expression is important in order to provide a basis for determining the nutritional requirements on an individual basis. The effects of nutrition can be exerted at many stages between transcription of the genetic sequence and production of a functional protein. This review focuses on the role of post-transcriptional control, particularly mRNA stability, translation and localization, in the interactions of nutrients with gene expression. The effects of both macronutrients and micronutrients on regulation of gene expression by post-transcriptional mechanisms are presented and the post-transcriptional regulation of specific genes of nutritional relevance (glucose transporters, transferrin, selenoenzymes, metallothionein, lipoproteins) is described in detail. The function of the regulatory signals in the untranslated regions of the mRNA is highlighted in relation to control of mRNA stability, translation and localization and the importance of these mRNA regions to regulation by nutrients is illustrated by reference to specific examples. The localization of mRNA by signals in the untranslated regions and its function in the spatial organization of protein synthesis is described; the potential of such mechanisms to play a key part in nutrient channelling and metabolic compartmentation is discussed. It is concluded that nutrients can influence gene expression through control of the regulatory signals in these untranslated regions and that the post-transcriptional regulation of gene expression by these mechanisms may influence nutritional requirements. It is emphasized that in studies of nutritional control of gene expression it is important not to focus only on regulation through gene promoters but also to consider the possibility of post-transcriptional control.

Trace elements and apoptosis

It is known that apoptosis is considered to be responsible for selective deletion of cells during embryogenesis, the homeostasis of cell populations in continuously renewing tissues (i.e., serving as a counterbalance to mitosis), and tissue involution in response to chemical or physical stimuli. There are many publications on these questions. On the other hand, the intracellular processes that contribute to apoptosis are incompletely understood. Therefore, the role of apoptosis in the intracellular accumulation and outflow of minerals is of considerable importance in light of both their essential functions and toxic effects.

The 3' untranslated region plays a role in the targeting of metallothionein-I mRNA to the perinuclear cytoplasm and cytoskeletal-bound polysomes

The mechanism of localisation of metallothionein-I (MT-I) mRNA was studied in transfected cells by in situ hybridisation and cell fractionation. Hepatoma cells were transfected with the 5'-untranslated region and coding region of the beta-globin gene alone or linked to either the beta-globin 3'-untranslated region (3'-UTR) or the MT-I 3'-UTR. The wild-type beta-globin mRNA and the beta-globin mRNA lacking its native 3'-UTR were present in free and cytoskeletal-bound polysomes to a similar extent and showed no localisation. Chimaeric globin-metallothionein transcripts were significantly enriched in cytoskeletal-bound polysomes and were localised in the perinuclear cytoplasm. Chimaeric globin-metallothionein and wild-type globin transcripts were of similar stability. Chinese Hamster Ovary cells were transfected with constructs in which the MT-I 5'-untranslated region and coding sequences were linked to either the endogenous 3'-UTR or the glutathione peroxidase 3'-UTR. Wild-type MT-I transcripts were localised in the perinuclear cytoplasm but the chimaeric MT-I-glutathione peroxidase transcripts showed no distinct localisation. The results indicate that the 3'-UTR of MT-I mRNA contains a localisation signal which promotes both the association of the mRNA with the cytoskeleton and its perinuclear localisation.

The compartmentalization of protein synthesis: importance of cytoskeleton and role in mRNA targeting

Following the synthesis of mRNA molecules in eukaryotic cells, the transcripts are processed in the nucleus and subsequently transported through the nuclear membrane into the cytoplasm before being sequestered into polysomes where the information contained in the RNA molecule is translated into an amino acid sequence. Recent evidence suggests that an association of mRNAs with the cytoskeleton might be important in targeting mechanisms and, furthermore, in the transport of mRNA from the nucleus to its correct location in the cytoplasm. Until recently, polysomes have been considered to exist in two classes, namely free or membrane-bound. There is now compelling evidence, however, that ribosomes, in addition to being associated with endoplasmic reticulum membranes, also are associated with components of the cytoskeleton. Thus, a large number of morphological and biochemical studies have shown that mRNA, polysomes and translational factors are associated with cytoskeletal structures. Although the actual nature and significance of the interaction between components of the translational apparatus and the cytoskeleton is not yet understood in detail, it would seem evident that such interactions are important in both the spatial organization and control of protein synthesis. Recent work has shown that a subcellular fraction, enriched in cytoskeletal components, contains polysomes and these (cytoskeletal-bound) polysomes have been shown to contain specific mRNA species. Thus, a population of cytoskeletal-bound polysomes may provide a specialized mechanism for the sorting, targeting and topographical segregation of mRNAs. In this review, current knowledge of the subcellular compartmentalization of mRNAs is discussed.