Betaine-homocysteine methyltransferase is a developmentally regulated enzyme crystallin in rhesus monkey lens

Mild Choline Deficiency and MTHFD1 Synthetase Deficiency Interact to Increase Incidence of Developmental Delays and Defects in Mice

Folate and choline are interconnected metabolically. The MTHFD1 R653Q SNP is a risk factor for birth defects and there are concerns that choline deficiency may interact with this SNP and exacerbate health risks. 80–90% of women do not meet the Adequate Intake (AI) for choline. The objective of this study was to assess the effects of choline deficiency on maternal one-carbon metabolism and reproductive outcomes in the MTHFD1-synthetase deficient mouse (Mthfd1S), a model for MTHFD1 R653Q. Mthfd1S^+/+ and Mthfd1S^+/− females were fed control (CD) or choline-deficient diets (ChDD; 1/3 the amount of choline) before mating and during pregnancy. Embryos were evaluated for delays and defects at 10.5 days gestation. Choline metabolites were measured in the maternal liver, and total folate measured in maternal plasma and liver. ChDD significantly decreased choline, betaine, phosphocholine, and dimethylglycine in maternal liver (p < 0.05, ANOVA), and altered phosphatidylcholine metabolism. Maternal and embryonic genotype, and diet-genotype interactions had significant effects on defect incidence. Mild choline deficiency and Mthfd1S^+/− genotype alter maternal one-carbon metabolism and increase incidence of developmental defects. Further study is required to determine if low choline intakes contribute to developmental defects in humans, particularly in 653QQ women.

Anticataractogenic effect of betaine in chick embryo hydrocortisone-induced cataract model

Background & objectives:

Cataract is one of the leading causes of blindness in the world. The aim of the present study was to investigate anticataractogenic effect of betaine in chick embryo hydrocortisone (HC)-induced cataract model.

Methods:

The study included 60 fertilized eggs divided into six groups each having 10 eggs: one group treated with only HC (HC group); three treated with both HC and different doses of betaine (HC/B 1.00, HC/B 0.50 and HC/B 0.25 groups) and two non-HC groups treated with only phosphate-buffered saline (PBS group) or betaine (B group). After the injections, lenses of the embryos were removed and classified into five stages according to the lens opacification. The amounts of reduced glutathione (GSH) in the removed lenses were measured.

Results:

All the lenses in non-HC-treated groups were clear, whereas in the HC-treated group, 90 per cent of the lenses had cataract (stages 4 and 5). The mean score of lens opacity was significantly lower in all HC/B groups compared to HC group (2.4-3.5 vs. 4.4, P<0.05). Among HC/B groups, the HC/B 0.25 group had significantly lower mean score of lens opacity compared to remaining HC/B groups treated with higher doses of betaine. In addition, the mean reduced GSH level was significantly higher in HC/B 0.25 group compared to HC, HC/B 1.00 and HC/B 0.50 groups (P<0.001).

Interpretation & conclusions:

The present results show beneficial anti-cataract and anti-oxidant effects of 0.25 μmol/egg betaine on HC-induced cataract in the chick embryo.

Identification of hepatic protein-protein interaction targets for betaine homocysteine S-methyltransferase

Protein-protein interactions are an important mechanism for the regulation of enzyme function allowing metabolite channeling, crosstalk between pathways or the introduction of post-translational modifications. Therefore, a number of high-throughput studies have been carried out to shed light on the protein networks established under different pathophysiological settings. Surprisingly, this type of information is quite limited for enzymes of intermediary metabolism such as betaine homocysteine S-methyltransferase, despite its high hepatic abundancy and its role in homocysteine metabolism. Here, we have taken advantage of two approaches, affinity purification combined with mass spectrometry and yeast two-hybrid, to further uncover the array of interactions of betaine homocysteine S-methyltransferase in normal liver of Rattus norvegicus. A total of 131 non-redundant putative interaction targets were identified, out of which 20 were selected for further validation by coimmunoprecipitation. Interaction targets validated by two different methods include: S-methylmethionine homocysteine methyltransferase or betaine homocysteine methyltransferase 2, methionine adenosyltransferases α1 and α2, cAMP-dependent protein kinase catalytic subunit alpha, 4-hydroxyphenylpyruvic acid dioxygenase and aldolase b. Network analysis identified 122 nodes and 165 edges, as well as a limited number of KEGG pathways that comprise: the biosynthesis of amino acids, cysteine and methionine metabolism, the spliceosome and metabolic pathways. These results further expand the connections within the hepatic methionine cycle and suggest putative cross-talks with additional metabolic pathways that deserve additional research.

Betaine homocysteine S-methyltransferase emerges as a new player of the nuclear methionine cycle

The paradigm of a cytoplasmic methionine cycle synthesizing/eliminating metabolites that are transported into/out of the nucleus as required has been challenged by detection of significant nuclear levels of several enzymes of this pathway. Here, we show betaine homocysteine S-methyltransferase (BHMT), an enzyme that exerts a dual function in maintenance of methionine levels and osmoregulation, as a new component of the nuclear branch of the cycle. In most tissues, low expression of Bhmt coincides with a preferential nuclear localization of the protein. Conversely, the liver, with very high Bhmt expression levels, presents a main cytoplasmic localization. Nuclear BHMT is an active homotetramer in normal liver, although the total enzyme activity in this fraction is markedly lower than in the cytosol. N-terminal basic residues play a role in cytoplasmic retention and the ratio of glutathione species regulates nucleocytoplasmic distribution. The oxidative stress associated with d-galactosamine (Gal) or buthionine sulfoximine (BSO) treatments induces BHMT nuclear translocation, an effect that is prevented by administration of N-acetylcysteine (NAC) and glutathione ethyl ester (EGSH), respectively. Unexpectedly, the hepatic nuclear accumulation induced by Gal associates with reduced nuclear BHMT activity and a trend towards increased protein homocysteinylation. Overall, our results support the involvement of BHMT in nuclear homocysteine remethylation, although moonlighting roles unrelated to its enzymatic activity in this compartment cannot be excluded.

Nonhuman Primate Ocular Biometry

PURPOSE

To examine ocular growth in nonhuman primates (NHPs) from measurements on ex vivo eyes.

METHODS

We obtained NHP eyes from animals that had been killed as part of other studies or because of health-related issues. Digital calipers were used to measure the horizontal, vertical, and anteroposterior globe diameters as well as corneal horizontal and vertical diameters of excised globes from 98 hamadryas baboons, 551 cynomolgus monkeys, and 112 rhesus monkeys, at ages ranging from 23 to 360 months. Isolated lens sagittal thickness and equatorial diameter were measured by shadowphotogrammetry. Wet and fixed dry weights were obtained for lenses.

RESULTS

Nonhuman primate globe growth continues throughout life, slowing toward an asymptotic maximum. The final globe size scales with negative allometry to adult body size. Corneal growth ceases at around 20 months. Lens diameter increases but thickness decreases with increasing age. Nonhuman primate lens wet and dry weight accumulation is monophasic, continuing throughout life toward asymptotic maxima. The dry/wet weight ratio reaches a maximum of 0.33.

CONCLUSIONS

Nonhuman primate ocular globe and lens growth differ in several respects from those in humans. Although age-related losses of lens power and accommodative amplitude are similar, lens growth and properties are different indicating care should be taken in extrapolating NHP observations to the study of human accommodation.

Central role of betaine-homocysteine S-methyltransferase 3 in chondral ossification and evidence for sub-functionalization in neoteleost fish

BACKGROUND

To better understand the complex mechanisms of bone formation it is fundamental that genes central to signaling/regulatory pathways and matrix formation are identified. Cell systems were used to analyze genes differentially expressed during extracellular matrix mineralization and bhmt3, coding for a betaine-homocysteine S-methyltransferase, was shown to be down-regulated in mineralizing gilthead seabream cells.

METHODS

Levels and sites of bhmt3 expression were determined by qPCR and in situ hybridization throughout seabream development and in adult tissues. Transcriptional regulation of bhmt3 was assessed from the activity of promoter constructs controlling luciferase gene expression. Molecular phylogeny of vertebrate BHMT was determined from maximum likelihood analysis of available sequences.

RESULTS

bhmt3 transcript is abundant in calcified tissues and localized in cartilaginous structures undergoing endo/perichondral ossification. Promoter activity is regulated by transcription factors involved in bone and cartilage development, further demonstrating the central role of Bhmt3 in chondrogenesis and/or osteogenesis. Molecular phylogeny revealed the explosive diversity of bhmt genes in neoteleost fish, while tissue distribution of bhmt genes in seabream suggested that neoteleostean Bhmt may have undergone several steps of sub-functionalization.

CONCLUSIONS

Data on bhmt3 gene expression and promoter activity evidences a novel function for betaine-homocysteine S-methyltransferase in bone and cartilage development, while phylogenetic analysis provides new insights into the evolution of vertebrate BHMTs and suggests that multiple gene duplication events occurred in neoteleost fish lineage.

GENERAL SIGNIFICANCE

High and specific expression of Bhmt3 in gilthead seabream calcified tissues suggests that bone-specific betaine-homocysteine S-methyltransferases could represent a suitable marker of chondral ossification.

Evolutionary Analyses and Natural Selection of Betaine-Homocysteine S-Methyltransferase (BHMT) and BHMT2 Genes

Betaine-homocysteine S-methyltransferase (BHMT) and BHMT2 convert homocysteine to methionine using betaine and S-methylmethionine, respectively, as methyl donor substrates. Increased levels of homocysteine in blood are associated with cardiovascular disease. Given their role in human health and nutrition, we identified BHMT and BHMT2 genes and proteins from 38 species of deuterostomes including human and non-human primates. We aligned the genes to look for signatures of selection, to infer evolutionary rates and events across lineages, and to identify the evolutionary timing of a gene duplication event that gave rise to two genes, BHMT and BHMT2. We found that BHMT was present in the genomes of the sea urchin, amphibians, reptiles, birds and mammals; BHMT2 was present only across mammals. BHMT and BHMT2 were present in tandem in the genomes of all monotreme, marsupial and placental species examined. Evolutionary rates were accelerated for BHMT2 relative to BHMT. Selective pressure varied across lineages, with the highest dN/dS ratios for BHMT and BHMT2 occurring immediately following the gene duplication event, as determined using GA Branch analysis. Nine codons were found to display signatures suggestive of positive selection; these contribute to the enzymatic or oligomerization domains, suggesting involvement in enzyme function. Gene duplication likely occurred after the divergence of mammals from other vertebrates but prior to the divergence of extant mammalian subclasses, followed by two deletions in BHMT2 that affect oligomerization and methyl donor specificity. The faster evolutionary rate of BHMT2 overall suggests that selective constraints were reduced relative to BHMT. The dN/dS ratios in both BHMT and BHMT2 was highest following the gene duplication, suggesting that purifying selection played a lesser role as the two paralogs diverged in function.

Quantitative proteomics analysis by iTRAQ in human nuclear cataracts of different ages and normal lens nuclei

PURPOSE

The goal of this study was to quantitatively identify the differentially expressed proteins in nuclear cataracts of different ages and normal lens nuclei in humans.

EXPERIMENTAL DESIGN

Forty-eight human lens nucleus samples with hardness grades III, IV were obtained during cataract surgery by extracapsular cataract extraction. Seven normal transparent human lens nuclei were obtained from fresh normal cadaver eyes during corneal transplantation surgery. Lens nuclei were divided into seven groups according to age and optic axis: Group A (average age 80.8 ± 1.2 years), Group B (average age 57.0 ± 4.0 years), Group C average age 80.3 ± 4.5 years), Group D (average age 56.9 ± 4.2 years), Group E (average age 78.1 ± 2.5 years), Group F (average age 57.6 ± 3.3 years) and Group G (seven normal transparent human lenses from normal cadaver eyes, average age 34.7 ± 4.2 years). Water-soluble, water-insoluble, and water-insoluble-urea-soluble protein fractions were extracted from samples. The three-part protein fractions from the individual lenses were combined to form the total proteins of each sample. The proteomic profiles of each group were further analyzed using 8-plex iTRAQ labeling combined with 2D-LC-MS/MS. The data were analyzed with the ProteinPilot software for peptide matching, protein identification, and quantification. Differentially expressed proteins were validated by Western blotting.

RESULTS

We employed biological and technical replicates and selected the intersection of the two results, which included 80 proteins. Nine proteins were differentially expressed among the 80 proteins identified using proteomic techniques. In age-related nuclear cataracts (ARNC), the expression levels of fatty acid-binding protein and pterin-4-alpha-carbinolamine dehydratase were upregulated, whereas the levels of alpha-crystallin B chain (CRYAB), GSH synthetase, phakinin, gamma-crystallin C, phosphoglycerate kinase 1, betaine-homocysteine S-methyltransferase 1 (BHMT1), and spectrin beta chain were downregulated. These proteins may be associated with abnormal protein aggregation and oxidative stress. GSH synthetase and CRYAB expression levels in the nuclear cataract decreased with age. The mass spectrometric analysis results were consistent with the Western blot validation.

CONCLUSION AND CLINICAL RELEVANCE

The results indicate that CRYAB and GSH synthetase may be involved in ARNC pathogenesis. iTRAQ combined with 2D-LC-MS/MS provides new methods for future studies of pathological mechanisms and protective drug development for ARNC.

Transcriptome profiling of the dynamic life cycle of the scypohozoan jellyfish Aurelia aurita

Background

The moon jellyfish Aurelia aurita is a widespread scyphozoan species that forms large seasonal blooms. Here we provide the first comprehensive view of the entire complex life of the Aurelia Red Sea strain by employing transcriptomic profiling of each stage from planula to mature medusa.

Results

A de novo transcriptome was assembled from Illumina RNA-Seq data generated from six stages throughout the Aurelia life cycle. Transcript expression profiling yielded clusters of annotated transcripts with functions related to each specific life-cycle stage. Free-swimming planulae were found highly enriched for functions related to cilia and microtubules, and the drastic morphogenetic process undergone by the planula while establishing the future body of the polyp may be mediated by specifically expressed Wnt ligands. Specific transcripts related to sensory functions were found in the strobila and the ephyra, whereas extracellular matrix functions were enriched in the medusa due to high expression of transcripts such as collagen, fibrillin and laminin, presumably involved in mesoglea development. The CL390-like gene, suggested to act as a strobilation hormone, was also highly expressed in the advanced strobila of the Red Sea species, and in the medusa stage we identified betaine-homocysteine methyltransferase, an enzyme that may play an important part in maintaining equilibrium of the medusa’s bell. Finally, we identified the transcription factors participating in the Aurelia life-cycle and found that 70% of these 487 identified transcription factors were expressed in a developmental-stage-specific manner.

Conclusions

This study provides the first scyphozoan transcriptome covering the entire developmental trajectory of the life cycle of Aurelia. It highlights the importance of numerous stage-specific transcription factors in driving morphological and functional changes throughout this complex metamorphosis, and is expected to be a valuable resource to the community.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1320-z) contains supplementary material, which is available to authorized users.

Molecular characterization of betaine-homocysteine methyltransferase 1 from the liver, and effects of aestivation on its expressions and homocysteine concentrations in the liver, kidney and muscle, of the African lungfish, Protopterus annectens

Homocysteine accumulation has numerous deleterious effects, and betaine-homocysteine S-methyltransferase (BHMT) catalyses the synthesis of methionine from homocysteine and betaine. This study aimed to determine homocysteine concentrations, and mRNA expression levels and protein abundances of bhmt1/Bhmt1 in the liver, kidney and muscle of the African lungfish, Protopterus annectens, during the induction (6 days), maintenance (6 months) or arousal (3 days after arousal) phase of aestivation. The homocysteine concentration decreased significantly in the liver of P. annectens after 6 days or 6 months of aestivation, but it returned to the control level upon arousal. By contrast, homocysteine concentrations in the kidney and muscle remained unchanged during the three phases of aestivation. The complete coding cDNA sequence of bhmt1 from P. annectens consisted of 1236 bp, coding for 412 amino acids. The Bhmt1 from P. annectens had a close phylogenetic relationship with those from tetrapods and Callorhinchus milii. The expression of bhmt1 was detected in multiple organs/tissues of P. annectens, and this is the first report on the expression of bhmt1/Bhmt1 in animal skeletal muscle. The mRNA and protein expression levels of bhmt1/Bhmt1 were up-regulated in the liver of P. annectens during the induction and maintenance phases of aestivation, possibly to regulate the hepatic homocysteine concentration. The significant increase in hepatic Bhmt1 protein abundance during the arousal phase could be a response to increased cellular methylation for the purpose of tissue reconstruction. Unlike the liver, Bhmt1 expression in the kidney and muscle of P. annectens was regulated translationally, and its up-regulation could be crucial to prevent homocysteine accumulation.

Solution properties of γ-crystallins: hydration of fish and mammal γ-crystallins

Lens γ crystallins are found at the highest protein concentration of any tissue, ranging from 300 mg/mL in some mammals to over 1000 mg/mL in fish. Such high concentrations are necessary for the refraction of light, but impose extreme requirements for protein stability and solubility. γ-crystallins, small stable monomeric proteins, are particularly associated with the lowest hydration regions of the lens. Here, we examine the solvation of selected γ-crystallins from mammals (human γD and mouse γS) and fish (zebrafish γM2b and γM7). The thermodynamic water binding coefficient B₁ could be probed by sucrose expulsion, and the hydrodynamic hydration shell of tightly bound water was probed by translational diffusion and structure-based hydrodynamic boundary element modeling. While the amount of tightly bound water of human γD was consistent with that of average proteins, the water binding of mouse γS was found to be relatively low. γM2b and γM7 crystallins were found to exhibit extremely low degrees hydration, consistent with their role in the fish lens. γM crystallins have a very high methionine content, in some species up to 15%. Structure-based modeling of hydration in γM7 crystallin suggests low hydration is associated with the large number of surface methionine residues, likely in adaptation to the extremely high concentration and low hydration environment in fish lenses. Overall, the degree of hydration appears to balance stability and tissue density requirements required to produce and maintain the optical properties of the lens in different vertebrate species.

Evolution of crystallins for a role in the vertebrate eye lens

The camera eye lens of vertebrates is a classic example of the re-engineering of existing protein components to fashion a new device. The bulk of the lens is formed from proteins belonging to two superfamilies, the α-crystallins and the βγ-crystallins. Tracing their ancestry may throw light on the origin of the optics of the lens. The α-crystallins belong to the ubiquitous small heat shock proteins family that plays a protective role in cellular homeostasis. They form enormous polydisperse oligomers that challenge modern biophysical methods to uncover the molecular basis of their assembly structure and chaperone-like protein binding function. It is argued that a molecular phenotype of a dynamic assembly suits a chaperone function as well as a structural role in the eye lens where the constraint of preventing protein condensation is paramount. The main cellular partners of α-crystallins, the β- and γ-crystallins, have largely been lost from the animal kingdom but the superfamily is hugely expanded in the vertebrate eye lens. Their structures show how a simple Greek key motif can evolve rapidly to form a complex array of monomers and oligomers. Apart from remaining transparent, a major role of the partnership of α-crystallins with β- and γ-crystallins in the lens is to form a refractive index gradient. Here, we show some of the structural and genetic features of these two protein superfamilies that enable the rapid creation of different assembly states, to match the rapidly changing optical needs among the various vertebrates.

The human crystallin gene families

Crystallins are the abundant, long-lived proteins of the eye lens. The major human crystallins belong to two different superfamilies: the small heat-shock proteins (α-crystallins) and the βγ-crystallins. During evolution, other proteins have sometimes been recruited as crystallins to modify the properties of the lens. In the developing human lens, the enzyme betaine-homocysteine methyltransferase serves such a role. Evolutionary modification has also resulted in loss of expression of some human crystallin genes or of specific splice forms. Crystallin organization is essential for lens transparency and mutations; even minor changes to surface residues can cause cataract and loss of vision.

Is protein methylation in the human lens a result of non-enzymatic methylation by S-adenosylmethionine?

Since crystallins in the human lens do not turnover, they are susceptible to modification by reactive molecules over time. Methylation is a major post-translational lens modification, however the source of the methyl group is not known and the extent of modification across all crystallins has yet to be determined. Sites of methylation in human lens proteins were determined using HPLC/mass spectrometry following digestion with trypsin. The overall extent of protein methylation increased with age, and there was little difference in the extent of modification between soluble and insoluble crystallins. Several different cysteine and histidine residues in crystallins from adult lenses were found to be methylated with one cysteine (Cys 110 in γD crystallin) at a level approaching 70%, however, methylation of crystallins was not detected in fetal or newborn lenses. S-adenosylmethionine (SAM) was quantified at significant (10-50 μM) levels in lenses, and in model experiments SAM reacted readily with N-α-tBoc-cysteine and N-α-tBoc-histidine, as well as βA3-crystallin. The pattern of lens protein methylation seen in the human lens was consistent with non-enzymatic alkylation. The in vitro data shows that SAM can act directly to methylate lens proteins and SAM was present in significant concentrations in human lens. Thus, non-enzymatic methylation of crystallins by SAM offers a possible explanation for this major human lens modification.

Betaine-homocysteine methyltransferase: human liver genotype-phenotype correlation

Betaine-homocysteine methyltransferase (BHMT) catalyzes the remethylation of homocysteine. BHMT is highly expressed in the human liver. In the liver, BHMT catalyzes up to 50% of homocysteine metabolism. Understanding the relationship between BHMT genetic polymorphisms and function might increase our understanding of the role of this reaction in homocysteine remethylation and in S-adenosylmethionine-dependent methylation. To help achieve those goals, we measured levels of BHMT enzyme activity and immunoreactive protein in 268 human hepatic surgical biopsy samples from adult subjects as well as 73 fetal hepatic tissue samples obtained at different gestational ages. BHMT protein levels were correlated significantly (p<0.001) with levels of enzyme activity in both fetal and adult tissues, but both were decreased in fetal tissue when compared with levels in the adult hepatic biopsies. To determine possible genotype-phenotype correlations, 12 tag SNPs for BHMT and the closely related BHMT2 gene were selected from SNPs observed during our own gene resequencing studies as well as from HapMap. These SNPs data were used to genotype DNA from the adult hepatic surgical biopsy samples, and genotype-phenotype association analysis was performed. Three SNPs (rs41272270, rs16876512, and rs6875201), located 28kb upstream, in the 5'-UTR and in intron 1 of BHMT, respectively, were significantly correlated with both BHMT activity (p=3.41E-8, 2.55E-9 and 2.46E-10, respectively) and protein levels (p=5.78E-5, 1.08E-5 and 6.92E-6, respectively). We also imputed 230 additional SNPs across the BHMT and BHMT2 genes, identifying an additional imputed SNP, rs7700790, that was also highly associated with hepatic BHMT enzyme activity and protein. However, none of the 3 genotyped or one imputed SNPs displayed a "shift" during electrophoretic mobility shift assays. These observations may help us to understand individual variation in the regulation of BHMT in the human liver and its possible relationship to variation in methylation.

Pre- and postnatal health: evidence of increased choline needs

Choline, a micronutrient found in food, serves as the starting material for several important metabolites that play key roles in fetal development, particularly the brain. Although human beings' requirement for choline is unknown, an Adequate Intake level of 425 mg/day was established for women with upward adjustments to 450 and 550 mg/day during pregnancy and lactation, respectively. The importance of choline in human development is supported by observations that a human fetus receives a large supply of choline during gestation; pregnancy causes depletion of hepatic choline pools in rats consuming a normal diet; human neonates are born with blood levels that are three times higher than maternal blood concentrations; and large amounts of choline are present in human milk. The development of the central nervous system is particularly sensitive to choline availability with evidence of effects on neural tube closure and cognition. Existing data show that the majority of pregnant (and presumably lactating) women are not achieving the target intake levels and that certain common genetic variants may increase requirements for choline beyond current recommendations. Because choline is not found in most varieties of prenatal vitamins (or regular multivitamins), increased consumption of choline-rich foods may be needed to meet the high pre- and postnatal demands for choline.

Presbyopia and cataract: a question of heat and time

Not only are human lenses different in many ways from those of non-primates, they also undergo dramatic changes with age. These age-dependent alterations lead to perturbations in the properties of older lenses, and ultimately to disturbances in visual function, which typically become apparent at middle age. Recent data suggest that many, if not all, of these age-dependent features can be traced to the lack of macromolecular turnover in the lens and to the inexorable modifications to proteins and membrane components over a period of decades. Exposure of lenses to heat can reproduce many of these alterations, suggesting that long-term incubation at body temperature may be an important factor in aging the human lens. Two conclusions flow from this. Firstly, the human lens may be an ideal tissue for studying macromolecular aging in man. Secondly, it will be extremely challenging to examine the origin of human age-related conditions, such as presbyopia and nuclear cataract, using traditional laboratory animals. Characterising the unfolding and decomposition of long-lived macromolecules appears to provide the key to understanding the two most common human lens disorders: presbyopia and age-related nuclear cataract.

Effects of diabetes and insulin on betaine-homocysteine S-methyltransferase expression in rat liver

Elevation of plasma homocysteine levels has been recognized as an independent risk factor for the development of cardiovascular disease, a major complication of diabetes. Plasma homocysteine reflects a balance between its synthesis via S-adenosyl-L-methionine-dependent methylation reactions and its removal through the transmethylation and the transsulfuration pathways. Betaine-homocysteine methyltransferase (BHMT, EC 2.1.1.5) is one of the enzymes involved in the remethylation pathway. BHMT, a major zinc metalloenzyme in the liver, catalyzes the transfer of methyl groups from betaine to homocysteine to form dimethylglycine and methionine. We have previously shown that plasma homocysteine levels and the transsulfuration pathway are affected by diabetes. In the present study, we found increased BHMT activity and mRNA levels in livers from streptozotocin-diabetic rats. In the rat hepatoma cell line (H4IIE cells), glucocorticoids (triamcinolone) increased the level and rate of BHMT mRNA synthesis. In the same cell line, insulin decreased the abundance of BHMT mRNA and the rate of de novo mRNA transcription of the gene. Thus the decreased plasma homocysteine in various models of diabetes could be due to enhanced homocysteine removal brought about by a combination of increased transsulfuration of homocysteine to cysteine and increased remethylation of homocysteine to methionine by BHMT.

Rat liver betaine-homocysteine S-methyltransferase equilibrium unfolding: insights into intermediate structure through tryptophan substitutions

Equilibrium folding of rat liver BHMT (betaine-homocysteine methyltransferase), a TIM (triosephosphate isomerase)-barrel tetrameric protein, has been studied using urea as denaturant. A combination of activity measurements, tryptophan fluorescence, CD and sedimentation-velocity studies suggested a multiphasic process including two intermediates, a tetramer (I4) and a monomer (J). Analysis of denaturation curves for single- and six-tryptophan mutants indicated that the main changes leading to the tetrameric intermediate are related to alterations in the helix alpha4 of the barrel, as well as in the dimerization arm. Further dissociation to intermediate J included changes in the loop connecting the C-terminal alpha-helix of contact between dimers, disruption of helix alpha4, and initial alterations in helix alpha7 of the barrel, as well as in the dimerization arm. Evolution of the monomeric intermediate continued through additional perturbations in helix alpha7 of the barrel and the C-terminal loop. Our data highlight the essential role of the C-terminal helix in dimer-dimer binding through its contribution to the increased stability shown by BHMT as compared with other TIM barrel proteins. The results are discussed in the light of the high sequence conservation shown by betaine-homocysteine methyltransferases and the knowledge available for other TIM-barrel proteins.

Time dependency of metabolic changes in rat lens after in vivo UVB irradiation analysed by HR-MAS 1H NMR spectroscopy

The lens ability to protect against, and repair ultraviolet radiation (UVR) induced damages, is of crucial importance to avoid cataract development. The influence of UVR-induced damage and repair processes on the lens metabolites are not fully understood. Observation of short- and long-term changes in light scattering and the metabolic profile of pigmented rat lenses after threshold UVR exposure might serve to better understand the protective mechanisms in the lens. By using high resolution magic angle spinning (HR-MAS) 1H NMR spectroscopy it was possible to investigate the metabolites of intact rat lenses. Brown-Norway rats were exposed to 15 kJm(-2) UVB irradiation. One eye was exposed and the contralateral served as control. The rats were sacrificed 5, 25, 125, and 625 hr post-exposure and the lenses were removed. The degree of cataract was quantified by measurement of lens forward light scattering. Thereafter, proton NMR spectra from intact lenses were obtained and relative changes in metabolite concentrations were determined. The light scattering in the lens peaked at 25 hr post-exposure and decreased thereafter. The lowest level of light scattering was measured 625 hr after exposure. No significant changes in concentration were observed for the metabolites 5 and 25 hr post-exposure except the total amount of adenosine tri- and diphosphate (ATP/ADP) that showed a significant decrease already 5 hr after exposure. At 125 hr the lens concentrations of lactate, succinate, phospho-choline, taurine, betaine, myo-inositol, and ATP/ADP showed a significant decrease (p<0.05). Phenylalanine was the only metabolite that revealed a significant increase 125 hr post-exposure. At 625 hr most of the metabolic changes seemed to normalise back to control levels. However, the concentration of betaine and phospho-choline were still showing a significant decrease 625 hr after UVB irradiation. The impact of UVB irradiation on the metabolic profile did not follow the same time dependency as the development of cataract. While the light scattering peaked at 25 hr post-exposure, significant changes in the endogenous metabolites were observed after 125 hr. Both the metabolic changes and the light scattering seemed to average back to normal within a month after exposure. Significant decrease in osmolytes like taurine, myo-inositol and betaine indicated osmotic stress and loss of homeostasis. This study also demonstrated that HR-MAS 1H NMR spectroscopy provides high quality spectra of intact lenses. These spectra contain a variety of information that might contribute to a better understanding of the metabolic response to drugs or endogenous stimuli like UVB irradiation.

The NEIBank project for ocular genomics: data-mining gene expression in human and rodent eye tissues

NEIBank is a project to gather and organize genomic resources for eye research. The first phase of this project covers the construction and sequence analysis of cDNA libraries from human and animal model eye tissues to develop an overview of the repertoire of genes expressed in the eye and a resource of cDNA clones for further studies. The sequence data are grouped and identified using the tools of bioinformatics and the results are displayed through a web site where they can be interrogated by keyword search, chromosome location, by Blast (sequence comparison) or by alignment on completed genomes. Many novel proteins and novel splice forms of known genes have already emerged from analysis of the accumulating data. This review provides an overview of the current state of the database for human eye tissues, with specific comparisons to some parallel data from mouse and rat, and with illustrative examples of the kinds of insights and discoveries these data can produce. One of the major themes that emerges is that at the molecular level human eye tissues have significant differences from those of rodents, encompassing species specific genes, alternative splice forms and great variation in levels of gene expression. These point to specific adaptations and mechanisms in the human eye and emphasize that care needs to be taken in the application of appropriate animal model systems.

The presence of a transsulfuration pathway in the lens: a new oxidative stress defense system

The finding that a lens under oxidative stress accumulated free and protein-bound cysteine (protein-S-S-cysteine) in the fiber cells prompted us to examine if there is an alternative source for cysteine pools besides the active cysteine transport system in the lens, namely, the transsulfuration pathway of homocysteine-cystathionine-cysteine, which utilises methionine through transmethylation. We examined the presence of the gene for cystathionine-beta-synthase (CBS), the rate limiting enzyme that converts homocysteine to cystathionine in the transsulfuration pathway, in human lens epithelial (HLE) B3 cells using PCR with primers designed based on the sequence of human liver CBS (Forward 5'-CCA CAC TGC CCC GGC AAA AT-3'; Reverse 5'-CTG GCA ATG CCC GTG ATG GT-3'). The purified DNA fragment (586 bp) from PCR analysis was sequenced and confirmed the homology with CBS gene from other human tissues. The CBS protein band (67 kDa) was present in the HLE cells, which reacted positively with the human liver anti-CBS antibody. The enzyme protein was detected in the pig and human lenses with the highest intensity in the epithelial layer, lower but equal quantities of CBS was present in the cortical and nuclear regions. Human nuclear CBS increased while epithelial CBS decreased with aging. Oxidative stress transiently upregulated the gene expression of CBS both in HLE cells (0.1 mMH2O2) and in pig lens cultured in TC 199 medium (0.5 mMH2O2). The catalytic activity for CBS, which was assayed by measuring the production of C14-cystathionine from C14-serine in the presence of homocysteine, S-adenosyl-methionine and pyridoxal phosphate, was detectable in the HLE cells and transiently activated with H2O2. Free cystathionine accumulated when HLE B3 cells were treated with propargylglycine (PGG), an inhibitor of cystathionase, the downstream enzyme that converts cystathionine to cysteine. More cystathionine accumulation occurred when the cells were simultaneously exposed to PGG and 0.1 mMH2O2. We have shown that oxidative stress of H2O2 could increase the flux of this transsulfuration pathway by committing more homocysteine to cysteine and glutathione production as H2O2 (0.1 mM) inhibited the remethylation enzyme of methionine synthase while concurrently activating the CBS enzyme. This is the first evidence that a transsulfuration pathway is present in the lens, and that it can be upregulated under oxidative stress to provide additional redox potential for the cells.

Betaine in human nutrition

Betaine is distributed widely in animals, plants, and microorganisms, and rich dietary sources include seafood, especially marine invertebrates ( approximately 1%); wheat germ or bran ( approximately 1%); and spinach ( approximately 0.7%). The principal physiologic role of betaine is as an osmolyte and methyl donor (transmethylation). As an osmolyte, betaine protects cells, proteins, and enzymes from environmental stress (eg, low water, high salinity, or extreme temperature). As a methyl donor, betaine participates in the methionine cycle-primarily in the human liver and kidneys. Inadequate dietary intake of methyl groups leads to hypomethylation in many important pathways, including 1) disturbed hepatic protein (methionine) metabolism as determined by elevated plasma homocysteine concentrations and decreased S-adenosylmethionine concentrations, and 2) inadequate hepatic fat metabolism, which leads to steatosis (fatty accumulation) and subsequent plasma dyslipidemia. This alteration in liver metabolism may contribute to various diseases, including coronary, cerebral, hepatic, and vascular diseases. Betaine has been shown to protect internal organs, improve vascular risk factors, and enhance performance. Databases of betaine content in food are being developed for correlation with population health studies. The growing body of evidence shows that betaine is an important nutrient for the prevention of chronic disease.

High sodium chloride intake decreases betaine-homocysteine S-methyltransferase expression in guinea pig liver and kidney

Betaine-homocysteine S-methyltransferase (BHMT) is the only enzyme known to catabolize betaine. In addition to being a substrate for BHMT, betaine also functions as an osmoprotectant that accumulates in the kidney medulla under conditions of high extracellular osmolarity. The mechanisms that regulate the partitioning of betaine between its use as a methyl donor and its accumulation as an osmoprotectant are not completely understood. The aim of this study was to determine whether BHMT expression is regulated by salt intake. This report shows that guinea pigs express BHMT in the liver, kidney, and pancreas and that the steady-state levels of BHMT mRNA in kidney and liver decrease 68% and 93% in guinea pigs consuming tap water containing high levels of salt compared with animals provided untreated tap water. The animals consuming the salt water also had approximately 50% less BHMT activity in the liver and kidney, and steady-state protein levels decreased approximately 30% in both organs. Pancreatic BHMT activity and protein levels were unaffected by the high salt treatment. The complex mechanisms involved in the downregulation of hepatic and renal BHMT expression in guinea pigs drinking salt water remain to be clarified, but the physiological significance of this downregulation may be to expedite the transport and accumulation of betaine into the kidney medulla under conditions of high extracellular osmolarity.

Crystal Structure of Rat Liver Betaine Homocysteine S-Methyltransferase Reveals New Oligomerization Features and Conformational Changes Upon Substrate Binding

Betaine homocysteine S-methyltransferase (BHMT) is one of the two enzymes known to methylate homocysteine to generate methionine in the liver. It presents a Zn(2+) atom linked to three essential Cys residues. The crystal structure of rat liver BHMT has been solved at 2.5A resolution, using crystals with P2(1) symmetry and 45% solvent content in the cell. The asymmetric unit contains the whole functional tetramer showing point symmetry 222. The overall fold of the subunit consists mostly of a (alpha/beta)(8) barrel, as for human BHMT. From the end of the barrel, the polypeptide chain extends away and makes many interactions with a different subunit, forming tight dimers. The most remarkable structural feature of rat liver BHMT is the presence of a helix including residues 381-407, at the C terminus of the chain, which bind together the dimers AB to CD. A strong ion-pair and more than 60 hydrophobic interactions keep this helix stacked to the segment 316-349 from the opposite subunit. Moreover, the crystal structure of free rat liver BHMT clearly shows that Tyr160 is the fourth ligand coordinated to Zn, which is replaced by Hcy upon binding. Two residues essential for substrate recognition, Phe76 and Tyr77, are provided by a conformational change in a partially disordered loop (L2). The crucial role of these residues is highlighted by site-directed mutagenesis.

Methylation and carbamylation of human gamma-crystallins

Accessible sulfhydryls of cysteine residues are likely sites of reaction in long-lived proteins such as human lens crystallins. Disulfide bonding between cysteines is a major contributor to intermolecular cross-linking and aggregation of crystallins. A recently reported modification of gammaS-crystallins, S-methylation of cysteine residues, can prevent disulfide formation. The aim of this study was to determine whether cysteines in gammaC-, gammaD-, and gammaB-crystallins are also S-methylated. Our data show that all the gamma-crystallins are S-methylated, but only at specific cysteines. In gammaD-crystallin, methylation is exclusively at Cys 110, whereas in gammaC- and gammaB-crystallins, the principal methylation site is Cys 22 with minor methylation at Cys 79. gammaD-crystallin is the most heavily methylated gamma-crystallin. gammaD-Crystallins from adult lenses are 37%-70% methylated, whereas gammaC and gammaB are approximately 12% methylated. The specificity of gamma-crystallin methylation and its occurrence in young clear lenses supports the idea that inhibition of disulfide bonding by S-methylation may play a protective role against cataract. Another modification, not reported previously, is carbamylation of the N termini of gammaB-, gammaC-, gammaD-crystallins. N-terminal carbamylation is likely a developmentally related modification that does not negatively impact crystallin function.

Investigations of a common genetic variant in betaine-homocysteine methyltransferase (BHMT) in coronary artery disease

Hyperhomocysteinemia, a risk factor for cardiovascular disease, can be caused by genetic mutations in enzymes of homocysteine metabolism. Homocysteine remethylation to methionine is catalyzed by folate-dependent methionine synthase, or by betaine-homocysteine methyltransferase (BHMT), which utilizes betaine as the methyl donor. Since genetic variants in folate-dependent remethylation have been reported to increase risk for cardiovascular disease and other common disorders, we screened BHMT for sequence changes that might alter risk for coronary artery disease (CAD). A variant in exon 6-R239Q-was identified. The frequency of this change was examined in 504 individuals who had undergone coronary angiography and were stratified into controls (those with no or mild disease) and cases (those with significant [>50% reduction in luminal diameter stenosis] 1-, 2-, 3-vessel disease). Although this variant did not affect plasma homocysteine, the QQ genotype was present in higher frequency in those with no or mild disease, compared with those with significant disease (11 vs. 6%), suggesting that it may decrease risk of CAD; a statistically-significant decrease was seen in the older subjects (13 vs. 7%). Multivariate analysis for the entire group revealed an odds ratio of 0.48 (95% CI: 0.21-1.06) for the QQ genotype; this association was similar in the younger (OR=0.36; 95% CI: 0.09-1.41) and older subjects (OR=0.42; 95% CI: 0.15-1.18). Our study suggests that the Q allele of the R239Q mutation may decrease the risk of CAD and that this variant warrants additional investigation of its relationship with the development of CAD as well as other homocysteine-dependent disorders.

Active-site-mutagenesis study of rat liver betaine-homocysteine S-methyltransferase

A site-directed-mutagenesis study of putative active-site residues in rat liver betaine-homocysteine S-methyltransferase has been carried out. Identification of these amino acids was based on data derived from a structural model of the enzyme. No alterations in the CD spectra or the gel-filtration chromatography elution pattern were observed with the mutants, thus suggesting no modification in the secondary structure content or in the association state of the proteins. All the mutants obtained showed a reduction of the enzyme activity, the most dramatic effect being that of Glu(159), followed by Tyr(77) and Asp(26). Changes in affinity for either of the substrates, homocysteine or betaine, were detected when substitutions were performed of Glu(21), Asp(26), Phe(74) and Cys(186). Interestingly, Asp(26), postulated to be involved in homocysteine binding, has a strong effect on affinity for betaine. The relevance of these results is discussed in the light of very recent structural data obtained for the human enzyme.

Combining combinatorial chemistry and affinity chromatography: highly selective inhibitors of human betaine: homocysteine S-methyltransferase

A new method to find novel protein targets for ligands of interest is proposed. The principle of this approach is based on affinity chromatography and combinatorial chemistry. The proteins within a crude rat liver homogenate were allowed to interact with a combinatorial library of phosphinic pseudopeptides immobilized on affinity columns. Betaine: homocysteine S-methyltransferase (BHMT) was one of the proteins that was retained and subsequently eluted from these supports. The phosphinic pseudopeptides, which served as immobilized ligands for the isolation of rat BHMT, were then tested for their ability to inhibit human recombinant BHMT in solution. The most potent inhibitor also behaved as a selective ligand for the affinity purification of BHMT from a complex media. Further optimization uncovered Val-Phe-psi[PO(2-)-CH(2)]-Leu-His-NH(2) as a potent BHMT inhibitor that has an IC(50) of about 1 microM.

Immunohistochemical detection of betaine-homocysteine S-methyltransferase in human, pig, and rat liver and kidney

Betaine-homocysteine S-methyltransferase (BHMT) has been shown to be expressed at high levels in the livers of all vertebrate species tested. It has also been shown to be abundant in primate and pig kidney but notably very low in rat kidney and essentially absent from the other major organs of monogastric animals. We recently showed by enzyme activity and Western analysis that pig kidney BHMT was only expressed in the cortex and was absent from the medulla. Using immunohistochemical detection, we report here that in human, pig, and rat kidney, BHMT is expressed in the proximal tubules of the cortex. Immunohistochemical staining for BHMT in human, pig, and rat liver indicate high expression in hepatocytes. The staining patterns are consistent with cytosolic expression in both organs.

Betaine-homocysteine methyltransferase-2: cDNA cloning, gene sequence, physical mapping, and expression of the human and mouse genes

Anomalies in folate and homocysteine metabolism can result in homocysteinemia and are implicated in disorders ranging from vascular disease to neural tube defects. Two enzymes are known to methylate homocysteine, vitamin B(12)-dependent methionine synthase (MTR) and betaine-homocysteine methyltransferase (BHMT). BHMT uses betaine, an intermediate of choline oxidation, as a methyl donor and is expressed primarily in the liver and kidney. We report the discovery of a novel betaine-homocysteine methyltransferase gene in humans and mice. The human BHMT2 gene is predicted to encode a 363-amino-acid protein (40.3 kDa) that shows 73% amino acid identity to BHMT. The BHMT2 transcript in humans is most abundant in adult liver and kidney and is found at reduced levels in the brain, heart, and skeletal muscle. The mouse Bhmt2 gene shows 69% amino acid identity and 79% similarity to the mouse Bhmt gene and 82% amino acid identity and 87% similarity to the human BHMT2 gene. Bhmt2 is expressed in fetal heart, lung, liver, kidney and eye. The discovery of a third gene with putative homocysteine methyltransferase activity is important for understanding the biochemical balance in using methyltetrahydrofolate and betaine as methyl donors as well as the metabolic flux between folate and choline metabolism in health and disease.

Isolation and characterization of a mouse betaine-homocysteine S-methyltransferase gene and pseudogene

Betaine-homocysteine S-methyltransferase (BHMT) is one of the enzymes involved in the branch point metabolism of homocysteine. Elevated levels of plasma homocysteine may be a risk factor for the development of vascular disease; however, whether BHMT has a significant role in the regulation of plasma levels of homocysteine remains to be determined. As a prelude to creating a mouse strain deficient in BHMT activity, we screened a lambda library containing mouse SvJ 129 genomic DNA for the mouse BHMT gene using random probes made from the human cDNA. One genomic isolate was completely sequenced and found to encode an intronless BHMT pseudogene (mBHMT-ps). mBHMT-ps was then used as a template for the generation of random probes that were used to screen a BAC library containing mouse 129 Sv/Ev genomic DNA. In order to discriminate between pseudogenes and the authentic BHMT gene, a secondary PCR-based screen was employed which used primers designed from the pseudogene sequence that would predictably amplify across introns. Using this strategy, we isolated six mouse genomic clones that tested positive for the presence of all seven introns characteristic of the human gene, and the BHMT gene of one clone was completely sequenced. Like the human BHMT gene, the mouse gene spans 21kb and is encoded by eight exons interrupted by seven introns. The structure of the mouse BHMT gene is described herein as well as the 5'-flanking region of the gene adjacent to exon 1, which we demonstrate is capable of conferring basal promoter activity in Chinese Hamster Ovary cells.

Antithrombin III, a serpin family protease inhibitor, is a major heparin binding protein in porcine aqueous humor

Our hypothesis is that the proteins in aqueous humor may be involved in the regulation of outflow facility through the trabecular meshwork and uveoscleral meshwork. In this study, we analyzed the profile of heparin-binding proteins present in porcine aqueous humor to identify and characterize secretory proteins with a binding affinity for heparin. A single step involving heparin-sepharose affinity chromatography of porcine aqueous humor yielded a approximately 60 kDa protein as the major heparin-binding species. This protein was specifically eluted from the column by heparin. The N-terminal sequence and immunological cross reactivity of this protein confirmed its identity as antithrombin III. Aqueous humor from different species, as well as cells from human trabecular meshwork, Schlemm's canal, and lens epithelium, contained detectable amounts of antithrombin III. Based on its known anticoagulative function in endothelial cells and effects on the production of prostacyclin, it is reasonable to speculate that antithrombin III present in aqueous humor might influence the physiology of the trabecular and uveoscleral meshwork and thereby regulate intraocular pressure.

Interaction between dietary methionine and methyl donor intake on rat liver betaine-homocysteine methyltransferase gene expression and organization of the human gene

We previously showed that rat liver betaine-homocysteine methyltransferase (BHMT) mRNA content and activity increased 4-fold when rats were fed a methionine-deficient diet containing adequate choline, compared with rats fed the same diet with control levels of methionine (Park, E. I., Renduchintala, M. S., and Garrow, T. A. (1997) J. Nutr. Biochem. 8, 541-545). A further 2-fold increase was observed in rats fed the methionine-deficient diet with supplemental betaine. The nutrition studies reported here were designed to determine whether other methyl donors would induce rat liver BHMT gene expression when added to a methionine-deficient diet and to define the relationship between the degree of methionine restriction and level of methyl donor intake on BHMT expression. Therefore, rats were fed amino acid-defined diets varying in methionine and methyl donor composition. The effect of diet on BHMT expression was evaluated using Northern, Western, and enzyme activity analyses. Similar to when betaine was added to a methionine-deficient diet, choline or sulfonium analogs of betaine induced BHMT expression. The diet-induced induction of hepatic BHMT activity was mediated by increases in the steady-state level of its mRNA and immunodetectable protein. Using methyl donor-free diets, we found that methionine restriction was required but alone not sufficient for the high induction of BHMT expression. Concomitant with methionine restriction, dietary methyl groups were required for high levels of BHMT induction, and a dose-dependent relationship was observed between methyl donor intake and BHMT induction. Furthermore, the severity of methionine restriction influenced the magnitude of BHMT induction. To study the molecular mechanisms that regulate the expression of BHMT, we have cloned the human BHMT gene. This gene spans about 20 kilobases of DNA and contains 8 exons and 7 introns. Using RNA isolated from human liver and hepatoma cells, a major transcriptional start site has been mapped using the 5' rapid amplification of cDNA ends technique, and this start site is 26 nucleotides downstream from a putative TATA box.