N-Homocysteinylation impairs collagen cross-linking in cystathionine β-synthase-deficient mice: a novel mechanism of connective tissue abnormalities

Komrower Memorial Lecture 2023. Molecular basis of phenotype expression in homocystinuria: Where are we 30 years later?

This review summarises progress in the research of homocystinuria (HCU) in the past three decades. HCU due to cystathionine β-synthase (CBS) was discovered in 1962, and Prof. Jan Peter Kraus summarised developments in the field in the first-ever Komrower lecture in 1993. In the past three decades, significant advancements have been achieved in the biology of CBS, including gene organisation, tissue expression, 3D structures, and regulatory mechanisms. Renewed interest in CBS arose in the late 1990s when this enzyme was implicated in biogenesis of H2S. Advancements in genetic and biochemical techniques enabled the identification of several hundreds of pathogenic CBS variants and the misfolding of missense mutations as a common mechanism. Several cellular, invertebrate and murine HCU models allowed us to gain insights into functional and metabolic pathophysiology of the disease. Establishing the E-HOD consortium and patient networks, HCU Network Australia and HCU Network America, offered new possibilities for acquiring clinical data in registries and data on patients´ quality of life. A recent analysis of data from the E-HOD registry showed that the clinical variability of HCU is broad, extending from severe childhood disease to milder (late) adulthood forms, which typically respond to pyridoxine. Pyridoxine responsiveness appears to be the key factor determining the clinical course of HCU. Increased awareness about HCU played a role in developing novel therapies, such as gene therapy, correction of misfolding by chaperones, removal of methionine from the gut and enzyme therapies that decrease homocysteine or methionine in the circulation.

Homocysteine metabolites inhibit autophagy, elevate amyloid beta, and induce neuropathy by impairing Phf8/H4K20me1-dependent epigenetic regulation of mTOR in cystathionine β-synthase-deficient mice

The loss of cystathionine β-synthase (CBS), an important homocysteine (Hcy)-metabolizing enzyme or the loss of PHF8, an important histone demethylase participating in epigenetic regulation, causes severe intellectual disability in humans. Similar neuropathies were also observed in Cbs-/- and Phf8-/- mice. How CBS or PHF8 depletion can cause neuropathy was unknown. To answer this question, we examined a possible interaction between PHF8 and CBS using Cbs-/- mouse and neuroblastoma cell models. We quantified gene expression by RT-qPCR and western blotting, mTOR-bound H4K20me1 by chromatin immunoprecipitation (CHIP) assay, and amyloid β (Aβ) by confocal fluorescence microscopy using anti-Aβ antibody. We found significantly reduced expression of Phf8, increased H4K20me1, increased mTOR expression and phosphorylation, and increased App, both on protein and mRNA levels in brains of Cbs-/- mice versus Cbs+/- sibling controls. Autophagy-related Becn1, Atg5, and Atg7 were downregulated while p62, Nfl, and Gfap were upregulated on protein and mRNA levels, suggesting reduced autophagy and increased neurodegeneration in Cbs-/- brains. In mouse neuroblastoma N2a or N2a-APPswe cells, treatments with Hcy-thiolactone, N-Hcy-protein or Hcy, or Cbs gene silencing by RNA interference significantly reduced Phf8 expression and increased total H4K20me1 as well as mTOR promoter-bound H4K20me1. This led to transcriptional mTOR upregulation, autophagy downregulation, and significantly increased APP and Aβ levels. The Phf8 gene silencing increased Aβ, but not APP, levels. Taken together, our findings identify Phf8 as a regulator of Aβ synthesis and suggest that neuropathy of Cbs deficiency is mediated by Hcy metabolites, which transcriptionally dysregulate the Phf8 → H4K20me1 → mTOR → autophagy pathway thereby increasing Aβ accumulation.

Hyperhomocysteinemia and Accelerated Aging: The Pathogenic Role of Increased Homocysteine in Atherosclerosis, Osteoporosis, and Neurodegeneration

Cardiovascular diseases and osteoporosis, seemingly unrelated disorders that occur with advanced age, share major pathogenetic mechanisms contributing to accelerated atherosclerosis and bone loss. Hyperhomocysteinemia (hHcy) is among these mechanisms that can cause both vascular and bone disease. In its more severe form, hHcy can present early in life as homocystinuria, an inborn error of metabolic pathways of the sulfur-containing amino acid methionine. In its milder forms, hHcy may go undiagnosed and untreated into adulthood. As such, hHcy may serve as a potential therapeutic target for cardiovascular disease, osteoporosis, thrombophilia, and neurodegeneration, collectively representing accelerated aging. Multiple trials to lower cardiovascular risk and improve bone density with homocysteine-lowering agents, yet none has proven to be clinically meaningful. To understand this unmet clinical need, this review will provide mechanistic insight into the pathogenesis of vascular and bone disease in hHcy, using homocystinuria as a model for accelerated atherosclerosis and bone density loss, a model for accelerated aging.

Reviewing the Regulators of COL1A1

The collagen family contains 28 proteins, predominantly expressed in the extracellular matrix (ECM) and characterized by a triple-helix structure. Collagens undergo several maturation steps, including post-translational modifications (PTMs) and cross-linking. These proteins are associated with multiple diseases, the most pronounced of which are fibrosis and bone diseases. This review focuses on the most abundant ECM protein highly implicated in disease, type I collagen (collagen I), in particular on its predominant chain collagen type I alpha 1 (COLα1 (I)). An overview of the regulators of COLα1 (I) and COLα1 (I) interactors is presented. Manuscripts were retrieved searching PubMed, using specific keywords related to COLα1 (I). COL1A1 regulators at the epigenetic, transcriptional, post-transcriptional and post-translational levels include DNA Methyl Transferases (DNMTs), Tumour Growth Factor β (TGFβ), Terminal Nucleotidyltransferase 5A (TENT5A) and Bone Morphogenic Protein 1 (BMP1), respectively. COLα1 (I) interacts with a variety of cell receptors including integrinβ, Endo180 and Discoidin Domain Receptors (DDRs). Collectively, even though multiple factors have been identified in association to COLα1 (I) function, the implicated pathways frequently remain unclear, underscoring the need for a more spherical analysis considering all molecular levels simultaneously.

Recent therapeutic approaches to cystathionine beta-synthase-deficient homocystinuria

Cystathionine beta-synthase (CBS)-deficient homocystinuria (HCU) is the most common inborn error of sulfur amino acid metabolism. The pyridoxine non-responsive form of the disease manifests itself by massively increasing plasma and tissue concentrations of homocysteine, a toxic intermediate of methionine metabolism that is thought to be the major cause of clinical complications including skeletal deformities, connective tissue defects, thromboembolism and cognitive impairment. The current standard of care involves significant dietary interventions that, despite being effective, often adversely affect quality of life of HCU patients, leading to poor adherence to therapy and inadequate biochemical control with clinical complications. In recent years, the unmet need for better therapeutic options has resulted in development of novel enzyme and gene therapies and exploration of pharmacological approaches to rescue CBS folding defects caused by missense pathogenic mutations. Here, we review scientific evidence and current state of affairs in development of recent approaches to treat HCU.

Cystathionine β-synthase gene inactivation dysregulates major urinary protein biogenesis and impairs sexual signaling in mice

Reproductive success in mice depends on sexually dimorphic major urinary proteins (Mup) that facilitate interactions between females and males. Deletion of cystathionine β-synthase (Cbs) gene, a metabolic gene important for homeostasis of one-carbon metabolism, impairs reproduction by causing female infertility in mice. Here, we examined Mup biogenesis and sexual signaling in Cbs^-/- versus Cbs^+/- mice. We found significantly reduced levels of total urinary Mup protein in male and female Cbs^-/- versus Cbs^+/- mice. SDS-PAGE/Western blot, ESI-MS, and RT-qPCR analyses of the liver, plasma, and urinary proteins identified a male-specific Mup20 in Cbs^-/- , but not in Cbs^+/- females. The 18 893 Da Mup20 became the most abundant in urine of Cbs^-/- females and males. Effects of Cbs genotype on 18 645 Da, 18 693 Da, and 18 709 Da Mup species abundance were Mup- and sex-specific. Cbs genotype-dependent changes in hepatic Mups and Mup20 expression were similar at the protein and mRNA level. Changes in Mups, but not in Mup20, can be explained by downregulation of hepatic Zhx2 and Ghr receptors in Cbs^-/- mice. Behavioral testing showed that Cbs^+/- females ignored Cbs^-/- male urine but were attracted to Cbs^+/- male urine. Cbs^+/- males ignored urine of Cbs^-/- males but countermarked urine of other Cbs^+/- males and were attracted to urines of Cbs^-/- as well as Cbs^+/- females. Cbs^-/- males did not countermark urine of Cbs^+/- males but were attracted to urines of Cbs^+/- females. Taken together, these findings show that Cbs, a metabolic gene, interacts with the processes involved in Mup biogenesis that are essential for the maintenance of sexual dimorphism and signaling and suggest that dysregulation of these interactions impairs reproductive fitness in mice.

Homocysteinylation and Sulfhydration in Diseases

Homocysteine (Hcy) is an important intermediate in methionine metabolism and generation of one-carbon units, and its dysfunction is associated with many pathological states. Although Hcy is a non-protein amino acid, many studies have demonstrated protein-related homocysteine metabolism and possible mechanisms underlying homocysteinylation. Homocysteinylated proteins lose their original biological function and have a negative effect on the various disease phenotypes. Hydrogen sulfide (H₂S) has been recognized as an important gaseous signaling molecule with mounting physiological properties. H₂S modifies small molecules and proteins via sulfhydration, which is supposed to be essential in the regulation of biological functions and signal transduction in human health and disorders. This review briefly introduces Hcy and H₂S, further discusses pathophysiological consequences of homocysteine modification and sulfhydryl modification, and ultimately makes a prediction that H₂S might exert a protective effect on the toxicity of homocysteinylation of target protein via sulfhydration. The highlighted information here yields new insights into the role of protein modification by Hcy and H₂S in diseases.

The association of homocysteine, folate, vitamin B12, and vitamin B6 with fracture incidence in older adults: a systematic review and meta-analysis

Background

Diverse conclusions have been drawn regarding the association of homocysteine (HCY) deficiency and supplements of B vitamins with fracture incidence in older adults. The aim of this meta-analysis was to investigate the association of HCY and B vitamins (folate, vitamin B12, and B6) with fracture incidence in older adults and whether supplements of B vitamins reduce the risk of fracture.

Methods

The PubMed, Embase, and Cochrane library databases were systematically searched from their inception dates to 1 July 2019 to identify relevant published articles. Meta-analysis was performed to pool hazard ratios (HRs) or risk ratios (RRs) and 95% confidence intervals (CIs) using a random effects model.

Results

A total of 28 studies fulfilled the inclusion criteria. High serum HCY was an independent risk factor for fractures in older persons (HR =1.25, 95% CI: 1.12 to 1.40), but only at the highest quartile level (>15 µmol/L) (HR =1.71, 95% CI: 1.37 to 2.12), rather than the second and third quartile. Multiple sensitivity and subgroup analyses supported the consistency and stability of this result. A severe deficiency of folate, instead of vitamin B12 and B6, was found to increase the risk of fracture in older adults (HR =1.46, 95% CI: 1.06 to 2.02; 1.24, 95% CI: 0.79 to 1.95; 1.36, 95% CI: 0.90 to 2.06, respectively). For the interventional effect, there was no significant association of combined folate and vitamin B12, combined folate, vitamin B12 and B6, or single vitamin B6 supplementation with the decrease of fracture risk.

Discussion

This meta-analysis revealed that significantly elevated serum level of HCY is positively associated with fracture incidence in older adults, yet the necessity and threshold for intervention by B vitamins require further large-scale high-quality clinical trials to validate.

PROSPERO identifier

CRD42019122586.

Hyperhomocysteinemia and its relations to conventional risk factors for cardiovascular diseases in adult Nigerians: the REMAH study

BACKGROUND

Evidence linking homocysteine (Hcy) with cardiovascular diseases (CVD) or its risk factors are limited in a sub-Saharan black population.

OBJECTIVE

We set out to evaluate the association between Hcy and hypertension and other CVD risk factors in a population of adult Nigerians.

METHODS

Data of 156 adults aged 18-70 years was accessed from the North Central study site of the REmoving the MAsk on Hypertension (REMAH) study. Homocysteine, blood glucose and lipid profile in whole blood/serum were measured using standard laboratory methods. Hypertension was diagnosed if average of 5 consecutive blood pressure (BP) measurements obtained using a mercury sphygmomanometer was equal to or higher than 140 systolic and/or 90 mmHg diastolic or the individual is on antihypertensive medication. Hyperhomocysteinemia (HHcy) was defined as Hcy > 10 µmol/L.

RESULTS

Of the 156 participants, 72 (43.5%) were hypertensive, of whom 18 had HHcy. Subjects with HHcy were significantly (p < 0.05) older (41.5 vs. 40.6yrs), had lower HDL-cholesterol (0.6 vs. 0.8 mmol/L) and higher systolic (145.5 vs. 126.0 mmHg) and diastolic BP (92.9 vs. 79.6 mmHg), compared to those without HHcy. Intake of alcohol and a 1 yr increase in age were respectively and significantly (p < 0.05) associated with a 1.54 and 0.10 µmol/L increase in Hcy. In a multivariable model adjusted for age, sex and body mass index, a 1 µmol/L increase in Hcy, was associated with a 1.69 mmHg and 1.34 mmHg increase in systolic and diastolic pressure (p < 0.0001) respectively; and a 0.01 mmol/L decrease in HDL-cholesterol (p < 0.05).

CONCLUSION

HHcy occurs among hypertensive Nigerians and it is independently associated with age, HDL-cholesterol, systolic and diastolic BP.

Proteomic exploration of cystathionine β-synthase deficiency: implications for the clinic

Introduction: Homocystinuria due to cystathionine β-synthase (CBS) deficiency, the most frequent inborn error of sulfur amino acid metabolism, is characterized biochemically by severely elevated homocysteine (Hcy) and related metabolites, such as Hcy-thiolactone and N-Hcy-protein. CBS deficiency reduces life span and causes pathological abnormalities affecting most organ systems in the human body, including the cardiovascular (thrombosis, stroke), skeletal/connective tissue (osteoporosis, thin/non-elastic skin, thin hair), and central nervous systems (mental retardation, seizures), as well as the liver (fatty changes), and the eye (ectopia lentis, myopia). Molecular basis of these abnormalities were largely unknown and available treatments remain ineffective. Areas covered: Proteomic and transcriptomic studies over the past decade or so, have significantly contributed to our understanding of mechanisms by which the CBS enzyme deficiency leads to clinical manifestations associated with it. Expert opinion: Recent findings, discussed in this review, highlight the involvement of dysregulated proteostasis in pathologies associated with CBS deficiency, including thromboembolism, stroke, neurologic impairment, connective tissue/collagen abnormalities, hair defects, and hepatic toxicity. To ameliorate these pathologies, pharmacological, enzyme replacement, and gene transfer therapies are being developed.

Cardiovascular findings in classic homocystinuria

Objective

describe cardiovascular findings from echocardiograms and electrocardiograms in patients with Classic Homocystinuria.

Methods

this retrospective exploratory study evaluated fourteen subjects with Classic Homocystinuria (median age = 27.3 years; male n = 8, B6-non-responsive n = 9 patients), recruited by convenience sampling from patients seen Hospital de Clínicas de Porto Alegre (Brazil), between January 1997 and July 2020. Data on clinical findings, echocardiogram and electrocardiogram were retrieved from medical records.

Results

Eight patients presented some abnormalities on echocardiogram (n = 6) or electrocardiogram (n = 5). The most frequent finding was mild tricuspid regurgitation (n = 3), followed by mitral valve prolapse, mild mitral regurgitation, enlarged left atrium and aortic valve sclerosis (n = 2 patients each). Aortic root ectasia was found in one patient. Venous thrombosis was reported in six patients: deep vein thrombosis of lower limbs (n = 3), ischaemic stroke (n = 1), cerebral venous sinus thrombosis (n = 1) and pulmonary vein thrombosis (n = 1).

Conclusion

mild valvulopathies seen to be common in patients with Classic Homocystinuria, but more studies regarding echocardiogram and electrocardiogram in this population are needed to draw absolute conclusions.

The role of cystatin C as a proteasome inhibitor in multiple myeloma

Objectives: Bone destruction and renal impairment are two frequent complications of multiple myeloma (MM). Cystatin C, an extracellular cysteine proteinase inhibitor, is encoded by the housekeeping gene CST3 and associated with human tumors. The role of cystatin C in multiple myeloma has been revealed recently. The purpose of this study was to explore the role of cystatin C as a proteasome inhibitor in multiple myeloma. Methods : A comprehensive literature review was conducted through Pubmed to summarize the published evidence on cystatin C in multiple myeloma. English literature sources since 1999 were searched, using the terms cystatin C, multiple myeloma. Results: cystatin C is a sensitive indicator for the diagnosis of myeloma nephropathy and has a dual role in myeloma bone disease. Also, cystatin C reflects tumor burden and is strongly associated with prognosis in patients with multiple myeloma. Conclusion: Cystatin C have great diagnostic and prognostic value in multiple myeloma. It can provide a new treatment direction for MM by designing and searching for antagonists of cystatin C or cysteine protease agonists using cystatin C as a therapeutic target.

Lessons Learned from Inherited Metabolic Disorders of Sulfur-Containing Amino Acids Metabolism

The metabolism of sulfur-containing amino acids (SAAs) requires an orchestrated interplay among several dozen enzymes and transporters, and an adequate dietary intake of methionine (Met), cysteine (Cys), and B vitamins. Known human genetic disorders are due to defects in Met demethylation, homocysteine (Hcy) remethylation, or cobalamin and folate metabolism, in Hcy transsulfuration, and Cys and hydrogen sulfide (H2S) catabolism. These disorders may manifest between the newborn period and late adulthood by a combination of neuropsychiatric abnormalities, thromboembolism, megaloblastic anemia, hepatopathy, myopathy, and bone and connective tissue abnormalities. Biochemical features include metabolite deficiencies (e.g. Met, S-adenosylmethionine (AdoMet), intermediates in 1-carbon metabolism, Cys, or glutathione) and/or their accumulation (e.g. S-adenosylhomocysteine, Hcy, H2S, or sulfite). Treatment should be started as early as possible and may include a low-protein/low-Met diet with Cys-enriched amino acid supplements, pharmacological doses of B vitamins, betaine to stimulate Hcy remethylation, the provision of N-acetylcysteine or AdoMet, or experimental approaches such as liver transplantation or enzyme replacement therapy. In several disorders, patients are exposed to long-term markedly elevated Met concentrations. Although these conditions may inform on Met toxicity, interpretation is difficult due to the presence of additional metabolic changes. Two disorders seem to exhibit Met-associated toxicity in the brain. An increased risk of demyelination in patients with Met adenosyltransferase I/III (MATI/III) deficiency due to biallelic mutations in the MATIA gene has been attributed to very high blood Met concentrations (typically >800 μmol/L) and possibly also to decreased liver AdoMet synthesis. An excessively high Met concentration in some patients with cystathionine β-synthase deficiency has been associated with encephalopathy and brain edema, and direct toxicity of Met has been postulated. In summary, studies in patients with various disorders of SAA metabolism showed complex metabolic changes with distant cellular consequences, most of which are not attributable to direct Met toxicity.

Cystathionine β-synthase deficiency: different changes in proteomes of thrombosis-resistant Cbs-/- mice and thrombosis-prone CBS-/- humans

Cystathionine β-synthase (CBS)-deficient patients are prone to vascular thrombosis. In contrast, Cbs^-/- mice show no abnormalities in blood coagulation. To identify molecular basis underlying these disparately different thrombotic phenotypes, we analyzed plasma proteomes of Cbs^-/- vs. Cbs^+/+ mice (8-month-old, 12/group, sex-matched) and CBS^-/- vs. CBS^+/+ humans (37 ± 7-year-old, 10-14/group, sex-matched) using label-free mass spectrometry. We identified 117 and 41 differentiating plasma proteins in Cbs^-/- mice and CBS^-/- humans, respectively. Twenty-one proteins were shared between CBS^-/- humans and Cbs^-/- mice, with sixteen changed in the opposite direction. Proteins involved in blood coagulation and complement/coagulation cascades represented a greater fraction of the differentiating proteins in CBS^-/- patients (51%) than in Cbs^-/- mice (21%). Top canonical pathways, identified by Ingenuity Pathways Analysis, such as LXR/RXR, FXR/RXR activation (- log[P-value] = 30-31) and atherosclerosis signaling (- log[P-value] = 10-11) were similarly affected in Cbs^-/- mice and CBS^-/- humans. The Coagulation System was affected stronger in CBS^-/- humans than in Cbs^-/- mice (- log[P-value] = 15 vs. 10, respectively) while acute phase response and complement system were affected stronger in Cbs^-/- mice (- log[P-value] = 33 and 22, respectively) than in humans (- log[P-value] = 22 and 6, respectively). Other pathways, including IL-7 signaling and B cell development were affected only in Cbs^-/- mice. Taken together, our findings suggest that differences in these processes, in particular in the Coagulation System, could account for the thrombotic phenotype in CBS^-/- patients and the absence of thrombosis in Cbs^-/- mice. Overall, our findings suggest that Cbs^-/- mice have a better adaptive response to protect from prothrombotic effects of hyperhomocysteinemia than CBS^-/- humans.

The Cbs Locus Affects the Expression of Senescence Markers and mtDNA Copy Number, but not Telomere Dynamics in Mice

Cystathionine β-synthase (CBS) is a housekeeping enzyme that catalyzes the first step of the homocysteine to cysteine transsulfuration pathway. Homozygous deletion of the Cbs gene in mice causes severe hyperhomocysteinemia and reduces life span. Here, we examined a possible involvement of senescence, mitochondrial DNA, and telomeres in the reduced life span of Cbs^-/- mice. We found that senescence-related p21, Pai-1, Mcp1, and Il-6 mRNAs were significantly upregulated (2-10-fold) in liver, while p21 was upregulated in the brain of Cbs^-/- mice (n = 20) compared with control Cbs^+/- siblings (n = 20) in a sex- and age-dependent manner. Telomere length in blood (n = 80), liver (n = 40), and brain (n = 40) was not affected by the Cbs^-/- genotype, but varied with sex and/or age. Levels of mitochondrial DNA tended to be reduced in livers, but not brains and blood, of Cbs^-/- females (n = 20-40). The Cbs^-/- genotype significantly reduced Tert mRNA expression in brain, but not liver, in a sex- and age-dependent manner. Multiple regression analysis showed that the senescence-related liver (but not brain) mRNAs and liver (but not brain or blood) mitochondrial DNA were associated with the Cbs genotype. In contrast, telomere length in blood, brain, and liver was not associated with the Cbs genotype or hyperhomocysteinemia, but was associated with sex (in brain and liver) and age (in brain and blood). Taken together, these findings suggest that the changes in senescence marker expression and mtDNA levels, but not telomere shortening, could account for the reduced life span of Cbs^-/- mice.

Mechanisms of homocysteine-induced damage to the endothelial, medial and adventitial layers of the arterial wall

Homocysteine (Hcy) is a non-protein forming amino acid which is the direct metabolic precursor of methionine. Increased concentration of serum Hcy is considered a risk factor for cardiovascular disease and is specifically linked to various diseases of the vasculature. Serum Hcy is associated with atherosclerosis, hypertension and aneurysms of the aorta in humans, though the precise mechanisms by which Hcy contributes to these conditions remain elusive. Results from clinical trials that successfully lowered serum Hcy without reducing features of vascular disease in cardiovascular patients have cast doubt on whether or not Hcy directly impacts the vasculature. However, studies in animals and in cell culture suggest that Hcy has a vast array of toxic effects on the vasculature, with demonstrated roles in endothelial dysfunction, medial remodeling and adventitial inflammation. It is hypothesized that rather than serum Hcy, tissue-bound Hcy and the incorporation of Hcy into proteins could underlie the toxic effects of Hcy on the vasculature. In this review, we present evidence for Hcy-associated vascular disease in humans, and we critically examine the possible mechanisms by which Hcy specifically impacts the endothelial, medial and adventitial layers of the arterial wall. Deciphering the mechanisms by which Hcy interacts with proteins in the arterial wall will allow for a better understanding of the pathomechanisms of hyperhomocysteinemia and will help to define a better means of prevention at the appropriate window of life.

Memantine Protects From Exacerbation of Ischemic Stroke and Blood Brain Barrier Disruption in Mild But Not Severe Hyperhomocysteinemia

Background Hyperhomocysteinemia is a risk factor for ischemic stroke; however, a targeted treatment strategy is lacking partly because of limited understanding of the causal role of homocysteine in cerebrovascular pathogenesis. Methods and Results In a genetic model of cystathionine beta synthase (CBS) deficiency, we tested the hypothesis that elevation in plasma total homocysteine exacerbates cerebrovascular injury and that memantine, a N-methyl-D-aspartate receptor antagonist, is protective. Mild or severe elevation in plasma total homocysteine was observed in Cbs+/- (6.1±0.3 μmol/L) or Cbs-/- (309±18 μmol/L) mice versus Cbs+/+ (3.1±0.6 μmol/L) mice. Surprisingly, Cbs-/- and Cbs+/- mice exhibited similar increases in cerebral infarct size following middle cerebral artery ischemia/reperfusion injury, despite the much higher total homocysteine levels in Cbs-/- mice. Likewise, disruption of the blood brain barrier was observed in both Cbs+/- and Cbs-/- mice. Administration of the N-methyl-D-aspartate receptor antagonist memantine protected Cbs+/- but not Cbs-/- mice from cerebral infarction and blood brain barrier disruption. Our data suggest that the differential effect of memantine in Cbs+/- versus Cbs-/- mice may be related to changes in expression of N-methyl-D-aspartate receptor subunits. Cbs-/-, but not Cbs+/- mice had increased expression of NR2B subunit, which is known to be relatively insensitive to homocysteine. Conclusions These data provide experimental evidence that even a mild increase in plasma total homocysteine can exacerbate cerebrovascular injury and suggest that N-methyl-D-aspartate receptor antagonism may represent a strategy to prevent reperfusion injury after acute ischemic stroke in patients with mild hyperhomocysteinemia.

Short tandem repeats and methylation in the promoter region affect expression of cystathionine beta-synthase gene in the laying hen

Homocysteine can be converted to cysteine via the transsulfuration pathway where cystathionine beta-synthase (CBS) is a rate-limiting enzyme. Homocysteine is thought to play a role in bone remodelling and strength. Previous results indicated that some of the difference in bone strength of end-of-lay hens may be associated with CBS expression level. To investigate if differences in the promoter region of the CBS gene might be responsible for observed differences in gene expression between CBS alleles post mortem- and in-vitro expression studies have been undertaken. Transfection of the DF-1 avian cell line with a series of deletion fragments of the 5' promoter, or constructs containing three CBS allele sequences, with a luciferase reporter revealed that a core region of -155 to +131 bp in the CBS promoter was essential for mRNA expression. We found that a variable number of short tandem repeats (7 nucleotide motif and 6 nucleotide repeats) in the core region of the promoter affecting the transcriptional activity and a strong effect for gene expression. However, methylation of the 6 nucleotide repeats varied between allelic variants and these maybe responsible for differences in promoter activity. Our findings indicated variable short tandem repeats and the differentially methylated sites in the promoter region may be responsible for CBS expression differences in the bone of laying hens.

Sex affects N-homocysteinylation at lysine residue 212 of albumin in mice

The modification of protein lysine residues by the thioester homocysteine (Hcy)-thiolactone has been implicated in cardiovascular and neurodegenerative diseases. However, only a handful of proteins carrying Hcy on specific lysine residues have been identified and quantified in humans or animals. In the present work, we developed a liquid chromatography/mass spectrometry targeted assay, based on multiple reaction monitoring, for quantification of N-Hcy-Lys212 (K212Hcy) and N-Hcy-Lys525 (K525Hcy) sites in serum albumin in mice. Using this assay, we found that female (n = 20) and male (n = 13) Cbs^−/− mice had significantly elevated levels of K212Hcy and K525Hcy modifications in serum albumin relative to their female (n = 19) and male (n = 17) Cbs^+/− littermates. There was significantly more K212Hcy modification in Cbs^−/− males than in Cbs^−/− females (5.78 ± 4.21 vs. 3.15 ± 1.38 units, P = 0.023). Higher K212Hcy levels in males than in females were observed also in Cbs^+/− mice (2.72 ± 0.81 vs. 1.89 ± 1.07 units, P = 0.008). In contrast, levels of the K525Hcy albumin modification were similar between males and females, both in Cbs^−/− and Cbs^+/− mice. These findings suggest that the sex-specific K212Hcy modification in albumin might have an important biological function in mice that is not affected by the Cbs genotype.

Homocysteine Modification in Protein Structure/Function and Human Disease

Epidemiological studies established that elevated homocysteine, an important intermediate in folate, vitamin B₁₂, and one carbon metabolism, is associated with poor health, including heart and brain diseases. Earlier studies show that patients with severe hyperhomocysteinemia, first identified in the 1960s, exhibit neurological and cardiovascular abnormalities and premature death due to vascular complications. Although homocysteine is considered to be a nonprotein amino acid, studies over the past 2 decades have led to discoveries of protein-related homocysteine metabolism and mechanisms by which homocysteine can become a component of proteins. Homocysteine-containing proteins lose their biological function and acquire cytotoxic, proinflammatory, proatherothrombotic, and proneuropathic properties, which can account for the various disease phenotypes associated with hyperhomocysteinemia. This review describes mechanisms by which hyperhomocysteinemia affects cellular proteostasis, provides a comprehensive account of the biological chemistry of homocysteine-containing proteins, and discusses pathophysiological consequences and clinical implications of their formation.

Urinary excretion of homocysteine thiolactone and the risk of acute myocardial infarction in coronary artery disease patients: the WENBIT trial

OBJECTIVES

No individual homocysteine (Hcy) metabolite has been studied as a risk marker for coronary artery disease (CAD). Our objective was to examine Hcy-thiolactone, a chemically reactive metabolite generated by methionyl-tRNA synthetase and cleared by the kidney, as a risk predictor of incident acute myocardial infarction (AMI) in the Western Norway B-Vitamin Intervention Trial.

DESIGN

Single centre, prospective double-blind clinical intervention study, randomized in a 2 × 2 factorial design.

SUBJECTS AND METHODS

Patients with suspected CAD (n = 2049, 69.8% men; 61.2-year-old) were randomized to groups receiving daily (i) folic acid (0.8 mg)/vitamin B₁₂ (0.4 mg)/vitamin B₆ (40 mg); (ii) folic acid/vitamin B₁₂ ; (iii) vitamin B₆ or (iv) placebo. Urinary Hcy-thiolactone was quantified at baseline, 12 and 38 months.

RESULTS

Baseline urinary Hcy-thiolactone/creatinine was significantly associated with plasma tHcy, ApoA1, glomerular filtration rate, potassium and pyridoxal 5'-phosphate (positively) and with age, hypertension, smoking, urinary creatinine, plasma bilirubin and kynurenine (negatively). During median 4.7-years, 183 patients (8.9%) suffered an AMI. In Cox regression analysis, Hcy-thiolactone/creatinine was associated with AMI risk (hazard ratio = 1.58, 95% confidence interval = 1.10-2.26, P = 0.012 for trend; adjusted for age, gender, tHcy). This association was confined to patients with pyridoxic acid below median (adjusted HR = 2.72, 95% CI = 1.47-5.03, P = 0.0001; P_interaction = 0.020). B-vitamin/folate treatments did not affect Hcy-thiolactone/creatinine and its AMI risk association.

CONCLUSIONS

Hcy-thiolactone/creatinine ratio is a novel AMI risk predictor in patients with suspected CAD, independent of traditional risk factors and tHcy, but modified by vitamin B₆ catabolism. These findings lend a support to the hypothesis that Hcy-thiolactone is mechanistically involved in cardiovascular disease.

Identification of suitable reference genes for real-time qPCR in homocysteine-treated human umbilical vein endothelial cells

The imbalance in homocysteine (Hcy) metabolism has been implicated in the pathogenesis of human diseases, including cardiovascular and neurodegenerative disorders. When attempting to identify gene expression profiles using quantitative real-time reverse transcription polymerase chain reaction (RT-qPCR), the selection of suitable reference genes is important. Here, the expression levels of 10 commonly used reference genes were assessed for normalization of RT-qPCR in Hcy-treated human umbilical vein endothelial cells (HUVECs) and control cells. The suitability of eight selected candidate genes was comparatively analyzed across the tested samples and separately ranked by four programs, geNorm, NormFinder, BestKeeper, and the ΔCt method. Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was the most stable gene in the final ranking using the RankAggreg package. Surprisingly, the β-actin (ACTB) levels decreased significantly in Hcy-treated HUVECs compared with control HUVECs (P<0.05), and further study indicated that Hcy suppressed the expression of ACTB by upregulating the miR-145-5p level in Hcy-treated HUVECs. Our data suggest that GAPDH can be used as a reliable reference gene, while ACTB cannot; normalization of gene expression in RT-qPCR experiments in Hcy-treated HUVECs. The data, which identifies a suitable reference gene in Hcy-treated HUVECs, will contribute to the design of an effective and accurate method for quantitation of gene expression.

Hydrogen Sulfide Promotes Bone Homeostasis by Balancing Inflammatory Cytokine Signaling in CBS-Deficient Mice through an Epigenetic Mechanism

Previously, we have shown hyperhomocysteinemia (HHcy) to have a detrimental effect on bone remodeling, which is associated with osteoporosis. During transsulfuration, Hcy is metabolized into hydrogen sulfide (H₂S), a gasotransmitter molecule known to regulate bone formation. Therefore, in the present study, we examined whether H₂S ameliorates HHcy induced epigenetic and molecular alterations leading to osteoporotic bone loss. To test this mechanism, we employed cystathionine-beta-synthase heterozygote knockout mice, fed with a methionine rich diet (CBS^+/− +Met), supplemented with H₂S-donor NaHS for 8 weeks. Treatment with NaHS, normalizes plasma H₂S, and completely prevents trabecular bone loss in CBS^+/− mice. Our data showed that HHcy caused inhibition of HDAC3 activity and subsequent inflammation by imbalancing redox homeostasis. The mechanistic study revealed that inflammatory cytokines (IL-6, TNF-α) are transcriptionally activated by an acetylated lysine residue in histone (H3K27ac) of chromatin by binding to its promoter and subsequently regulating gene expression. A blockade of HDAC3 inhibition in CBS^+/− mice by HDAC activator ITSA-1, led to the remodeling of histone landscapes in the genome and thereby attenuated histone acetylation-dependent inflammatory signaling. We also confirmed that RUNX2 was sulfhydrated by administration of NaHS. Collectively, restoration of H₂S may provide a novel treatment for CBS-deficiency induced metabolic osteoporosis.

Mutations in Homocysteine Metabolism Genes Increase Keratin N-Homocysteinylation and Damage in Mice

Genetic or nutritional deficiencies in homocysteine (Hcy) metabolism increase Hcy-thiolactone, which causes protein damage by forming isopetide bonds with lysine residues, generating N-Hcy-protein. In the present work, we studied the prevalence and genetic determinants of keratin damage caused by homocysteinylation. We found that in mammals and birds, 35 to 98% of Hcy was bound to hair keratin via amide or isopeptide bond (Hcy-keratin), while 2 to 65% was S-Hcy-keratin. A major fraction of hair Hcy-keratin (56% to 93%), significantly higher in birds than in mammals, was sodium dodecyl sulfate-insoluble. Genetic hyperhomocysteinemia significantly increased N-Hcy-keratin levels in the mouse pelage. N-Hcy-keratin was elevated 3.5-, 6.3-, and 11.7-fold in hair from Mthfr -/-, Cse -/-, or Cbs -/- mice, respectively. The accumulation of N-Hcy in hair keratin led to a progressive reduction of N-Hcy-keratin solubility in sodium dodecyl sulfate, from 0.39 ± 0.04 in wild-type mice to 0.19 ± 0.03, 0.14 ± 0.01, and 0.07 ± 0.03 in Mthfr -/-, Cse -/-, or Cbs -/-animals, respectively. N-Hcy-keratin accelerated aggregation of unmodified keratin in Cbs -/- mouse hair. Keratin methionine, copper, and iron levels in mouse hair were not affected by hyperhomocysteinemia. These findings provide evidence that pelage keratin is N-homocysteinylated in vivo in mammals and birds, and that this process causes keratin damage, manifested by a reduced solubility.

Demethylation of methionine and keratin damage in human hair

Growing human head hair contains a history of keratin and provides a unique model for studies of protein damage. Here, we examined mechanism of homocysteine (Hcy) accumulation and keratin damage in human hair. We found that the content of Hcy-keratin increased along the hair fiber, with levels 5-10-fold higher levels in older sections at the hair's tip than in younger sections at hair's base. The accumulation of Hcy led to a complete loss of keratin solubility in sodium dodecyl sulfate. The increase in Hcy-keratin was accompanied by a decrease in methionine-keratin. Levels of Hcy-keratin were correlated with hair copper and iron in older hair. These relationships were recapitulated in model experiments in vitro, in which Hcy generation from Met exhibited a similar dependence on copper or iron. Taken together, these findings suggest that Hcy-keratin accumulation is due to copper/iron-catalyzed demethylation of methionine residues and contributes to keratin damage in human hair.

Enzyme Replacement Therapy Ameliorates Multiple Symptoms of Murine Homocystinuria

Classical homocystinuria (HCU) is the most common inherited disorder of sulfur amino acid metabolism caused by deficiency in cystathionine beta-synthase (CBS) activity and characterized by severe elevation of homocysteine in blood and tissues. Treatment with dietary methionine restriction is not optimal, and poor compliance leads to serious complications. We developed an enzyme replacement therapy (ERT) and studied its efficacy in a severe form of HCU in mouse (the I278T model). Treatment was initiated before or after the onset of clinical symptoms in an effort to prevent or reverse the phenotype. ERT substantially reduced and sustained plasma homocysteine concentration at around 100 μM and normalized plasma cysteine for up to 9 months of treatment. Biochemical balance was also restored in the liver, kidney, and brain. Furthermore, ERT corrected liver glucose and lipid metabolism. The treatment prevented or reversed facial alopecia, fragile and lean phenotype, and low bone mass. In addition, structurally defective ciliary zonules in the eyes of I278T mice contained low density and/or broken fibers, while administration of ERT from birth partially rescued the ocular phenotype. In conclusion, ERT maintained an improved metabolic pattern and ameliorated many of the clinical complications in the I278T mouse model of HCU.

Enzyme replacement therapy prevents loss of bone and fat mass in murine homocystinuria

Skeletal and connective tissue defects are the most striking symptoms in patients suffering from classical homocystinuria (HCU). Here, we determined body composition and bone mass in three mouse models of HCU and assessed whether a long-term administration of enzyme replacement therapy (ERT) corrected the phenotype. The mouse models of HCU were analyzed using dual-energy X-ray absorptiometry and the data were complemented by plasma biochemical profiles. Both the mouse model lacking CBS (KO) and the one expressing human CBS mutant transgene on a mouse CBS null background (I278T) showed marked bone loss and decreased weight mostly due to a lower fat content compared with negative controls. In contrast, the HO mouse expressing the human CBS WT transgene on a mouse CBS null background showed no such phenotype despite similar plasma biochemical profile to the KO and I278T mice. More importantly, administration of ERT rescued bone mass and changes in body composition in the KO mice treated since birth and reversed bone loss and improved fat content in the I278T mice injected after the development of clinical symptoms. Our study suggests that ERT for HCU may represent an effective way of preventing the skeletal problems in patients without a restricted dietary regime.

The cystathionine β-synthase/hydrogen sulfide pathway contributes to microglia-mediated neuroinflammation following cerebral ischemia

The mechanisms underlying neuroinflammation following cerebral ischemia remain unclear. Hydrogen sulfide (H₂S), a newly identified gasotransmitter, has been reported to regulate inflammation. In the current study, we investigated whether the endogenous H₂S production pathway contributed to microglia-mediated neuroinflammation following stroke. We used a mouse middle cerebral artery occlusion (MCAO) model and an in vitro cellular model to mimic ischemia-induced microglial neuroinflammation. Expression of the H₂S synthase cystathionine β-synthase (CBS) and H₂S synthetic activity were rapidly decreased in the ischemic brain tissue following MCAO. Consistently, when cultured microglia were polarized toward a pro-inflammatory phenotype with conditioned medium collected from neurons that had been subjected to oxygen-glucose deprivation (OGD neuron CM), they displayed reduced CBS expression and H₂S production. Enhancing H₂S bioavailability either by overexpressing CBS or by supplementing with exogenous H₂S donors promoted a shift in microglial polarization from ischemia-induced pro-inflammatory phenotypes toward anti-inflammatory phenotypes. Mechanistically, microglia that were exposed to OGD neuron CM displayed reduced activation of AMP-activated protein kinase (AMPK), which was rescued by overexpressing CBS or by supplementing with H₂S donors. Moreover, the promoting effects of H₂S donors on microglial anti-inflammatory polarization were abolished by an AMPK inhibitor or CaMKKβ inhibitor. Our results suggested that reduced CBS-H₂S-AMPK cascade activity contributed to microglia-mediated neuroinflammation following stroke. Targeting the CBS-H₂S pathway is a promising therapeutic approach for ischemic stroke.

Vitamin B12 and Semen Quality

Various studies have revealed the effects of vitamin B₁₂, also named cobalamin, on semen quality and sperm physiology; however, these studies collectively are still unsummarized. Here, we systematically discuss and summarize the currently understood role of vitamin B₁₂ on semen quality and sperm physiology. We searched the Web of Science, PubMed, and Scopus databases for only English language articles or abstracts from September 1961 to March 2017 (inclusive) using the key words “vitamin B₁₂” and “cobalamin” versus “sperm”. Certain relevant references were included to support the empirical as well as the mechanistic discussions. In conclusion, the mainstream published work demonstrates the positive effects of vitamin B₁₂ on semen quality: first, by increasing sperm count, and by enhancing sperm motility and reducing sperm DNA damage, though there are a few in vivo system studies that have deliberated some adverse effects. The beneficial effects of vitamin B₁₂ on semen quality may be due to increased functionality of reproductive organs, decreased homocysteine toxicity, reduced amounts of generated nitric oxide, decreased levels of oxidative damage to sperm, reduced amount of energy produced by spermatozoa, decreased inflammation-induced semen impairment, and control of nuclear factor-κB activation. However, additional research, mainly clinical, is still needed to confirm these positive effects.