Nuclear RNase P of Trypanosoma brucei: a single protein in place of the multicomponent RNA-protein complex

RNase P is the endonuclease that removes 5′ extensions from tRNA precursors. In its best-known form, the enzyme is composed of a catalytic RNA and a protein moiety variable in number and mass. This ribonucleoprotein enzyme is widely considered ubiquitous and apparently reached its highest complexity in the eukaryal nucleus, where it is typically composed of at least ten subunits. Here, we show that in the protist Trypanosoma brucei, two proteins are the sole forms of RNase P. They localize to the nucleus and the mitochondrion, respectively, and have RNase P activity each on their own. The protein-RNase P is, moreover, capable of replacing nuclear RNase P in yeast cells. This shows that complex ribonucleoprotein structures and RNA catalysis are not necessarily required to support tRNA 5′ end formation in eukaryal cells.

Of P and Z: mitochondrial tRNA processing enzymes

Mitochondrial tRNAs are generally synthesized as part of polycistronic transcripts. Release of tRNAs from these precursors is thus not only required to produce functional adaptors for translation, but also responsible for the maturation of other mitochondrial RNA species. Cleavage of mitochondrial tRNAs appears to be exclusively accomplished by endonucleases. 5'-end maturation in the mitochondria of different Eukarya is achieved by various kinds of RNase P, representing the full range of diversity found in this enzyme family. While ribonucleoprotein enzymes with RNA components of bacterial-like appearance are found in a few unrelated protists, algae, and fungi, highly degenerate RNAs of dramatic size variability are found in the mitochondria of many fungi. The majority of mitochondrial RNase P enzymes, however, appear to be pure protein enzymes. Human mitochondrial RNase P, the first to be identified and possibly the prototype of all animal mitochondrial RNases P, is composed of three proteins. Homologs of its nuclease subunit MRPP3/PRORP, are also found in plants, algae and several protists, where they are apparently responsible for RNase P activity in mitochondria (and beyond) without the help of extra subunits. The diversity of RNase P enzymes is contrasted by the uniformity of mitochondrial RNases Z, which are responsible for 3'-end processing. Only the long form of RNase Z, which is restricted to eukarya, is found in mitochondria, even when an additional short form is present in the same organism. Mitochondrial tRNA processing thus appears dominated by new, eukaryal inventions rather than bacterial heritage. This article is part of a Special Issue entitled: Mitochondrial Gene Expression.

Maternally inherited essential hypertension is associated with the novel 4263A>G mutation in the mitochondrial tRNAIle gene in a large Han Chinese family

RATIONALE

Despite maternal transmission of hypertension in some pedigrees, pathophysiology of maternally inherited hypertension remains poorly understood.

OBJECTIVE

To establish a causative link between mitochondrial dysfunction and essential hypertension.

METHOD AND RESULTS

A total of 106 subjects from a large Chinese family underwent clinical, genetic, molecular, and biochemical evaluations. Fifteen of 24 adult matrilineal relatives exhibited a wide range of severity in essential hypertension, whereas none of the offspring of affected fathers had hypertension. The age at onset of hypertension in the maternal kindred varied from 20 years to 69 years, with an average of 44 years. Mutational analysis of their mitochondrial genomes identified a novel homoplasmic 4263A>G mutation located at the processing site for the tRNA(Ile) 5'-end precursor. An in vitro processing analysis showed that the 4263A>G mutation reduced the efficiency of the tRNA(Ile) precursor 5'-end cleavage catalyzed by RNase P. tRNA Northern analysis revealed that the 4263A>G mutation caused ≈46% reduction in the steady-state level of tRNA(Ile). An in vivo protein-labeling analysis showed ≈32% reduction in the rate of mitochondrial translation in cells carrying the 4263A>G mutation. Impaired mitochondrial translation is apparently a primary contributor to the reductions in the rate of overall respiratory capacity, malate/glutamate-promoted respiration, succinate/glycerol-3-phosphate-promoted respiration, or N,N,N',N'-tetramethyl-p-phenylenediamine/ascorbate-promoted respiration and the increasing level of reactive oxygen species in cells carrying the 4263A>G mutation.

CONCLUSIONS

These data provide direct evidence that mitochondrial dysfunction caused by mitochondrial tRNA(Ile) 4263A>G mutation is involved in essential hypertension. Our findings may provide new insights into pathophysiology of maternally transmitted hypertension.

A single Arabidopsis organellar protein has RNase P activity

The ubiquitous endonuclease RNase P is responsible for the 5' maturation of tRNA precursors. Until the discovery of human mitochondrial RNase P, these enzymes had typically been found to be ribonucleoproteins, the catalytic activity of which is associated with the RNA component. Here we show that, in Arabidopsis thaliana mitochondria and plastids, a single protein called 'proteinaceous RNase P' (PRORP1) can perform the endonucleolytic maturation of tRNA precursors that defines RNase P activity. In addition, PRORP1 is able to cleave tRNA-like structures involved in the maturation of plant mitochondrial mRNAs. Finally, we show that Arabidopsis PRORP1 can replace the bacterial ribonucleoprotein RNase P in Escherichia coli cells. PRORP2 and PRORP3, two paralogs of PRORP1, are both localized in the nucleus.

Processing mitochondrial (t)RNAs: new enzyme, old job

While the principal mode of synthesis of the different human mitochondrial RNA species was recognized almost three decades ago, the constituents of one of the key players of the postulated RNA processing machinery were identified only recently. Human mitochondrial RNase P, the endonuclease responsible for tRNA 5' end maturation, turned out to be unlike any of its previously characterized cousins. It is not only devoid of the RNA moiety thought to be diagnostic of this type of enzymes so far, but it is instead built like a patchwork of multifunctional proteins coming together to moonlight in tRNA 5' end cleavage.

Isolated cytochrome c oxidase deficiency as a cause of MELAS

Deletion of a single nucleotide (7630delT) within MT-CO2, the gene of subunit II of cytochrome c oxidase (COX), was identified in a clinically typical MELAS case. The deletion-induced frameshift results in a stop codon close to the 5' end of the reading frame. The lack of subunit II (COII) precludes the assembly of COX and leads to the degradation of unassembled subunits, even those not directly affected by the mutation. Despite mitochondrial proliferation and transcriptional upregulation of nuclear and mtDNA-encoded COX genes (including MT-CO2), a severe COX deficiency was found with all investigations of the muscle biopsy (histochemistry, biochemistry, immunoblotting). The 7630delT mutation in MT-CO2 leads to a lack of COII with subsequent misassembly and degradation of respiratory complex IV despite transcriptional upregulation of its subunits. The genetic and pathobiochemical heterogeneity of MELAS appears to be greater than previously appreciated.

RNase P without RNA: identification and functional reconstitution of the human mitochondrial tRNA processing enzyme

tRNAs are synthesized as immature precursors, and on their way to functional maturity, extra nucleotides at their 5' ends are removed by an endonuclease called RNase P. All RNase P enzymes characterized so far are composed of an RNA plus one or more proteins, and tRNA 5' end maturation is considered a universal ribozyme-catalyzed process. Using a combinatorial purification/proteomics approach, we identified the components of human mitochondrial RNase P and reconstituted the enzymatic activity from three recombinant proteins. We thereby demonstrate that human mitochondrial RNase P is a protein enzyme that does not require a trans-acting RNA component for catalysis. Moreover, the mitochondrial enzyme turns out to be an unexpected type of patchwork enzyme, composed of a tRNA methyltransferase, a short-chain dehydrogenase/reductase-family member, and a protein of hitherto unknown functional and evolutionary origin, possibly representing the enzyme's metallonuclease moiety. Apparently, animal mitochondria lost the seemingly ubiquitous RNA world remnant after reinventing RNase P from preexisting components.

Salmonella diagnosis in pig production: methodological problems in monitoring the prevalence in pigs and pork

Salmonellosis is an important foodborne infection in industrialized and developing countries. Especially for human Salmonellosis caused by Salmonella enterica serovar Typhimurium, pigs and pork are the major sources of infection. Mitigation and control strategies that result from surveillance programs attempt to reduce or even eradicate Salmonella in pork to lower consumers' risks. The methodology for Salmonella screening in pigs is generally based on antibody detection at slaughter with meat juice as the sample matrix. The instructions to most commercial enzyme-linked immunosorbent assay (ELISA) kits for the detection of Salmonella antibodies state that their product is suitable for antibody detection in meat juice and sera. In the present study, we show that it is essential to recalculate the percent optical density (OD%) data obtained from meat juice by the ELISA (IDEXX HerdCheck swine Salmonella) by the following regression equation: OD%sera = -70.5587 + 128.1490/ {1 + exp[(-18.8969 - OD%meatjuice)/27.6032]}(1.1771), r = 0.87, to compare results with those obtained from sera. By this regression equation, we were able to compare the Salmonella antibody levels (classified as <10, 10 to <20, 20 to <40, and > or =40 OD%) for sows, growers, and slaughter pigs. We identified significantly higher numbers of growers with lower OD% levels than for sows and slaughter pigs. Without a recalculation of the meat juice results, the higher fraction of samples with low OD% values led to an underestimation of the actual seroprevalence.

Control of BVDV-infection on common grassland--the key for successful BVDV-eradication in Lower Austria

A bovine viral diarrhea/mucosal disease (BVD/MD) control and eradication program was introduced in Lower Austria in 1996, according to the Swedish model. At present 9800 out of 17,000 herds are part of this program. An important risk factor for BVDV-transmission under local conditions is communal grazing. Approximately 3-4% of livestock share common pastures, in which susceptible pregnant cattle may be mixed with unrecognised persistently infected (PI) animals. Rules and regulations were defined to allow only herds free from BVDV-infection on to common grassland. At the moment, 5067 herds are certified free from BVDV. The percentage of BVDV-free herds in regions with intensive pasture utilisation is higher (57.3%) than in the other regions (43.0%) of Lower Austria. With a reliable system for identification of PI-animals and a high certainty of prevention of PI-animals on common grassland, the main transmission of BVDV infection can be stopped, even if the animals are derived from infected herds and transiently infected animals cannot be excluded.

Accumulation of very long-chain fatty acids does not affect mitochondrial function in adrenoleukodystrophy protein deficiency

X-linked adrenoleukodystrophy (X-ALD, OMIM 300100) is a severe inherited neurodegenerative disease, associated with the accumulation of very long-chain fatty acids (VLCFA). The recent unexpected observation that the accumulation of VLCFA in tissues of the Abcd1-deficient mouse model for X-ALD is not due to a deficiency in VLCFA degradation, led to the hypothesis that mitochondrial abnormalities might contribute to X-ALD pathology. Here, we report that in spite of substantial accumulation of VLCFA in whole muscle homogenates, normal VLCFA levels were detected in mitochondria obtained by organellar fractionation. Polarographic analyses of the respiratory chain as well as enzymatic assays of isolated muscle mitochondria revealed no differences between X-ALD and control mice. Moreover, analysis by electron microscopy, revealed normal size, structure and localization of mitochondria in muscle of both groups. Similar to the results obtained in skeletal muscle, the mitochondrial enzyme activities in brain homogenates of Abcd1-deficient and wild-type animals also did not differ. Finally, studies on mitochondrial oxidative phosphorylation in permeabilized human skin fibroblasts of X-ALD patients and controls revealed no abnormalities. Thus, we conclude that the accumulation of VLCFA per se does not cause mitochondrial abnormalities and vice versa-mitochondrial abnormalities are not responsible for the accumulation of VLCFA in X-ALD mice.

Mitochondrial genotype and risk for Alzheimer's disease: cross-sectional data from the Vienna-Transdanube-Aging "VITA" study

The Vienna Transdanube Aging (VITA) study searches for early markers of Alzheimer's disease (AD) by examining the mental status in a community-based cohort of 606, 75-years old volunteers that are then related to various clinical and genetic analyses. To determine whether mutations in mtDNA are involved in expression of AD, the mtDNA of 79 "control" participants is screened for alterations by sequencing of "hot-spot-regions". This study on mtDNA mutations has eliminated the influence of aging on the occurrence of mtDNA alterations by sequencing samples from persons at the age of exactly 75 years. Thus, our cohort reveals a snap-shot of mitochondrial sequences of elderly persons. So far, a high percentage (56%) of persons with known or unknown mutations in the fragments analyzed were found. These data will be compared in due time to a cohort of participants with proven late-onset AD.

The expanding mutational spectrum of MERRF substitution G8361A in the mitochondrial tRNALys gene

In a case of childhood-onset myoclonus epilepsy with "ragged-red fibers" (MERRF), a hitherto unreported mutation within the mitochondrial tRNA(Lys) gene was identified as the cause of the disease. Substitution G8361A was maternally inherited, heteroplasmic in all tissues tested, and correlated with mitochondrial dysfunction in individual muscle fibers. The growing number of MERRF-associated mutations within the tRNA(Lys) gene affirms the specific role of this mitochondrial tRNA in the pathogenesis of the disease.

Expression of activins C and E induces apoptosis in human and rat hepatoma cells

Activins C and E (homodimers of the betaC and betaE subunits), which are almost exclusively expressed in the liver, are members of the transforming growth factor beta (TGFbeta) superfamily of growth factors. We examined their expression in three different hepatoma cell lines and found that, compared with normal liver or primary hepatocytes, human hepatoblastoma (HepG2), human hepatocellular carcinoma (Hep3B) and rat hepatoma (H4IIEC3) cells have either completely lost or drastically reduced the expression of activins C and E. In order to elucidate the biological function of these proteins we transiently transfected HepG2, Hep3B and H4IIEC3 cell lines with rat activin betaC or betaE cDNA to study the consequences of restoring activin expression in hepatoma cells. Transfection with activin betaA, a known inhibitor of hepatic DNA synthesis and inducer of apoptosis, served as a positive control. We found that transfection of the three cell lines with activin betaC or betaE, as well as with activin betaA, reduced the increase in cell number by up to 40% compared with cells transfected with a control plasmid. Co-culture with a CHO cell clone secreting activin C also inhibited HepG2 cell multiplication. Furthermore, the three hepatoma cell lines studied showed an enhanced rate of apoptosis and elevated levels of active caspases in response to activin transfection. These results indicate that activins C and E share the potential to induce apoptosis in liver derived cell lines with activin A and TGFbeta1.

Overexpression of activin beta(C) or activin beta(E) in the mouse liver inhibits regenerative deoxyribonucleic acid synthesis of hepatic cells

Activins are dimeric growth factors composed of beta-subunits, four of which have been isolated so far. Whereas activin beta(A) and beta(B) are expressed in many tissues, the expression of activin beta(C) and beta(E) is confined to the liver. To date no biological role or activity has been assigned to activins formed from beta(C) or beta(E) subunits (activin C and E). Because activin A (beta(A)beta(A)), among its various functions in other tissues, appears to be a negative regulator of liver growth, we hypothesized a similar role for activin C and E. Using a nonviral gene transfer system we specifically delivered genes encoding activin beta(C), beta(E), or beta(A) to the mouse liver. The mRNA analysis and reporter gene coexpression both indicated a reproducible temporal and spatial transgene expression pattern. The effects of activin overexpression were studied in the context of a regenerative proliferation of hepatic cells, a result of the tissue damage associated with the hydrodynamics based gene transfer procedure. Activin beta(C), beta(E), or beta(A) expression, all temporarily inhibited regenerative DNA synthesis of hepatocytes and nonparenchymal cells, though to a varying degree. This first report of a biological activity of activin C and E supports an involvement in liver tissue homeostasis and further emphasizes the role of the growing activin family in liver physiology.

Localization and quantification of Cd- and Cu-specific metallothionein isoform mRNA in cells and organs of the terrestrial gastropod Helix pomatia

A quantitative assay based on real-time detection polymerase chain reaction (rtdPCR) was applied to analyze basal and metal-induced mRNA levels of two metallothionein (MT) isoforms (Cd-MT and Cu-MT) in organs of the terrestrial gastropod Helix pomatia. The results show that specific Cd-MT mRNA levels increase with Cd tissue burden, identifying hepatopancreas and gut as the main organs of Cd accumulation and, accordingly, the predominant organs of Cd-MT mRNA expression. In situ hybridization localized this isoform in epithelial cells of hepatopancreas, gut, and kidney. In contrast to the observed Cd-dependent inducibility of the Cd-binding MT isoform, gene expression of the Cu-binding MT could not be induced by either Cd or Cu exposure. Only very low mRNA amounts of the Cu-MT isoform were found in snail hepatopancreas and kidney, whereas the mantle exhibited high basal mRNA levels of this isoform. In situ localization revealed that the Cu-MT gene expression was restricted to one cell type, the so-called rhogocytes, which are present to various extents in the different organs examined. These results suggest a metal-specific sharing of functions between the two MT isoforms. The Cd-MT isoform apparently plays a crucial role in Cd detoxification, as demonstrated by the inducibility of this isoform, as well as its specific localization in the main metabolic and Cd storing organs. The predominant presence of Cu-MT in rhogocytes of snail mantle strengthens the hypothesis that this isoform may regulate Cu availability in hemocyanin synthesis.

Estrogen receptor-alpha and estrogen receptor-beta are present in the human growth plate in childhood and adolescence, in identical distribution

OBJECTIVE

To localize estrogen receptor-alpha (ER-alpha) and estrogen receptor-beta (ER-beta) within the growth plate and adjacent bony tissue of children in the prepubertal and pubertal age period.

METHODS

Tissue was taken during orthopedic surgery (epiphysiodesis) for correction of congenital or traumatic leg length difference in 2 prepubertal females and 2 adolescent males. Immunohistochemistry was performed on paraffin-embedded or cryostat sections by using commercially available rabbit polyclonal antibodies for ER-alpha and ER-beta.

RESULTS

Both ER-alpha and ER-beta were detected within the growth plate in all sections investigated. Immunostaining was restricted to hypertrophic chondrocytes. In the bony tissue adjacent to the growth plate, osteoblasts stained positive for both ER-alpha and ER-beta, whereas osteocytes and osteoclasts were negative. Staining with ER-alpha was mainly nuclear but some cells also showed cytoplasmic signals, while ER-beta staining was predominantly cytoplasmic, only few nuclei stained positive. There was no difference in the local distribution of both ERs between tissue from prepubertal and pubertal patients.

CONCLUSION

Our findings indicate that the hypertrophic chondrocyte is the main target cell for estrogen action within the growth plate. The presence of ER in prepubertal children suggests that estrogens play a role in skeletal maturation under physiological conditions also in this age-group.

Cellular gene dose and kinetics of gene expression in mouse livers transfected by high-volume tail-vein injection of naked DNA

Direct gene transfer to mammalian tissues has significant potential for biomedical research and gene therapy. Recently, the efficient transfer of naked plasmid DNA to the mouse liver by a rapid high-volume tail-vein injection was reported. We carried out a systematic analysis of the dose and time dependence of the expression of the Escherichia coli beta-galactosidase gene transferred by this technique. Surprisingly, the DNA concentration of the administered solution determined primarily the cellular gene dose and, hence, the expression of the transgene in individual hepatocytes, while the number of transfected cells was largely independent of the supplied plasmid mass. Transgene expression was transient: after a rapid onset and a peak at 8 h past injection, it gradually declined and was no longer detectable 4 weeks later. Although gene transfer was accompanied by tissue damage and subsequent regenerative proliferation, the decline in transgene expression was not due to increased hepatocyte turnover or to promoter downregulation, but instead cells apparently lost the plasmid DNA. Furthermore, we show that "nakedness" of the injected DNA is indeed a prerequisite for efficient transfer by the hydrodynamics-based procedure. Our data provide important clues for the successful use of this gene transfer technique, and may point directions for studies on the underlying mechanisms.

Follistatin overexpression in rodent liver tumors: a possible mechanism to overcome activin growth control

The activin-follistatin system is a potent growth regulatory system of liver tissue homeostasis. Activin A inhibits hepatocellular DNA synthesis and induces cell death. Follistatin binds activin and sequesters it from the signaling pathway. Consistently, follistatin has been reported to act as an inducer of DNA synthesis in the liver. Using RNase protection analysis, we studied the expression of follistatin in rat and mouse liver tumors as a possible mechanism to overcome activin growth control. Approximately 40% of the tumors (nine of 24 each), most of them hepatocellular carcinomas, displayed increased levels of follistatin mRNA when compared to tumor-surrounding liver tissue. The degree of overexpression was highly variable but independent of the carcinogen treatment that animals had received. It was also independent from the histological stage of malignancy and further found in rat liver adenomas. Follistatin expression was also observed in cell lines derived from human hepatocellular carcinomas. Overexpression of follistatin may represent a unique strategy of hepatic tumors to overcome the inhibitory action of a growth factor, activin, by decreasing its local bioavailability.

A novel mechanism for mitogenic signaling via pro-transforming growth factor alpha within hepatocyte nuclei

Transforming growth factor (TGF) alpha, an important mediator of growth stimulation, is known to act via epidermal growth factor receptor (EGF-R) binding in the cell membrane. Here we show by immunohistology, 2-dimensional immunoblotting, and mass spectrometry of nuclear fractions that the pro-protein of wild-type TGF-alpha occurs in hepatocyte nuclei of human, rat, and mouse liver. Several findings show a close association between nuclear pro-TGF-alpha and DNA synthesis. (1) The number of pro-TGF-alpha+ nuclei was low in resting liver and increased dramatically after partial hepatectomy and after application of hepatotoxic chemicals or the primary mitogen cyproterone acetate (CPA); in any case, S phase occurred almost exclusively in pro-TGF-alpha+ nuclei. The same was found in human cirrhotic liver. (2) In primary culture, 7% of hepatocytes synthesized pro-TGF-alpha, which then translocated to the nucleus; 70% of these nuclei subsequently entered DNA replication, whereas only 2% of pro-TGF-alpha- hepatocytes were in S phase. (3) The frequency of hepatocytes coexpressing pro-TGF-alpha and DNA synthesis was increased by the hepatomitogens CPA or prostaglandin E(2) and was decreased by the growth inhibitor TGF-beta1. (4) Treatment with mature TGF-alpha increased DNA synthesis exclusively in pro-TGF-alpha- hepatocytes, which was abrogated by the EGF-R tyrosine kinase inhibitor tyrphostin A25. In conclusion, TGF-alpha gene products may exert mitogenic effects in hepatocytes via 2 different signaling mechanisms: (1) the "classic" pathway of mature TGF-alpha via EGF-R in the membrane and (2) a novel pathway involving the presence of pro-TGF-alpha in the nucleus.

Expression and dimerization of the rat activin subunits betaC and betaE: evidence for the ormation of novel activin dimers

Activins are cytokines of the transforming growth factor beta family, which plays a central role in the determination of cell fate and the regulation of tissue balance. Family members are composed of two subunits and this dimerization is critical for liganding their cognate receptors and execution of proper functions. In the current study we focused on the localization of activin betaA, betaB, betaC and betaE subunits in the adult rat and analyzed the composition of putative activin beta dimers. By dissecting tissue distribution of various activins, we found that the liver, in particular the hepatocytes, is the major source for activin betaC and betaE transcripts, since other tissues almost failed to express these isoforms. In sharp contrast, the emergence of activin betaA and betaB appeared ubiquitous. Using a highly selective proteome approach, we were able to identify homo- as well as heterodimers of individual activin subunits, indicating a high redundancy of ligand composition. Certainly, this broad potential to homo- and heterodimerize has to be considered in future studies on activin function.