Abnormal Amino Acid Profiles of Blood and Cerebrospinal Fluid from Cystathionine β-Synthase-Deficient Mice, an Animal Model of Homocystinuria

A Novel Finding of Increased ß-Aminoisobutyric Acid Levels in Classic Homocystinuria With Homocysteine-Lowering Treatment

Hyperhomocysteinemia is an independent risk factor for cardiovascular disease. Although commonly seen as a milder elevation of homocysteine levels in adult patients, on rare occasions, the internist may face extremely elevated homocysteine levels (>100 µmol/L). In such rare cases, the search for a monogenic disease is warranted. In this report, we present a patient with classical homocystinuria, where the diagnosis was delayed due to various factors. The patient experienced a constellation of symptoms over an extended period, including visual problems, recurrent thrombosis, and neurodevelopmental delay. Delayed diagnosis of genetic diseases is problematic, as patients may grow from pediatric care to adult internal medicine, where knowledge and exposure to such a rare genetic disorder are limited. A diagnosis was finally confirmed with amino acid profiling, revealing extremely elevated homocysteine levels, which were reduced with sequential treatment modalities, including folate, vitamin B12, vitamin B6, methionine restriction, and betaine. We also present derangements in other amino acids, namely, methionine, taurine, serine, and urea cycle products. With treatment, a progressive increase in body weight is noticed. Furthermore, we present a novel finding of increased levels of ß-aminoisobutyric acid with homocysteine-lowering treatment. ß-aminisobutyric acid is a myokine that potentiates some of the metabolic benefits of exercising muscle such as improved insulin resistance and browning of white adipose tissue.

A High-Methionine Diet for One-Week Induces a High Accumulation of Methionine in the Cerebrospinal Fluid and Confers Bipolar Disorder-like Behavior in Mice

Methionine (Met) is considered the most toxic amino acid in mammals. Here, we investigated biochemical and behavioral impacts of ad libitum one-week feeding of high-Met diets on mice. Adult male mice were fed the standard rodent diet that contained 0.44% Met (1×) or a diet containing 16 graded Met doses (1.2×–13×). High-Met diets for one-week induced a dose-dependent decrease in body weight and an increase in serum Met levels with a 2.55 mM peak (versus basal 53 µM) on the 12×Met diet. Total homocysteine (Hcy) levels were also upregulated while concentrations of other amino acids were almost maintained in serum. Similarly, levels of Met and Hcy (but not the other amino acids) were highly elevated in the cerebrospinal fluids of mice on the 10×Met diet; the Met levels were much higher than Hcy and the others. In a series of behavioral tests, mice on the 10×Met diet displayed increased anxiety and decreased traveled distances in an open-field test, increased activity to escape from water soaking and tail hanging, and normal learning/memory activity in a Y-maze test, which were reflections of negative/positive symptoms and normal cognitive function, respectively. These results indicate that high-Met ad libitum feeding even for a week can induce bipolar disorder-like disease models in mice.

Hyperhomocysteinemia: Metabolic Role and Animal Studies with a Focus on Cognitive Performance and Decline-A Review

Disturbances in the one-carbon metabolism are often indicated by altered levels of the endogenous amino acid homocysteine (HCys), which is additionally discussed to causally contribute to diverse pathologies. In the first part of the present review, we profoundly and critically discuss the metabolic role and pathomechanisms of HCys, as well as its potential impact on different human disorders. The use of adequate animal models can aid in unravelling the complex pathological processes underlying the role of hyperhomocysteinemia (HHCys). Therefore, in the second part, we systematically searched PubMed/Medline for animal studies regarding HHCys and focused on the potential impact on cognitive performance and decline. The majority of reviewed studies reported a significant effect of HHCys on the investigated behavioral outcomes. Despite of persistent controversial discussions about equivocal findings, especially in clinical studies, the present evaluation of preclinical evidence indicates a causal link between HHCys and cognition-related- especially dementia-like disorders, and points out the further urge for large-scale, well-designed clinical studies in order to elucidate the normalization of HCys levels as a potential preventative or therapeutic approach in human pathologies.

Homocysteine Hypothesis on the Impaired Peripheral but Not Central Nervous System Oxytocin Responses in Cystathionine γ-Lyase-Deficient Dam Mice

An elevated plasma homocysteine level is an independent risk factor for cardiovascular diseases, neurological disorders, and pregnancy complications. We recently demonstrated partial lactation failure in cystathionine γ-lyase-deficient (Cth^-/-) dam mice and their defective oxytocin responses in peripheral tissues: uterine (ex vivo) and mammary gland (in vivo). We reasoned that elevated levels of circulatory homocysteine in Cth^-/- dam mice counteract with oxytocin-dependent milk ejection from the mammary gland. Based on our observation that those mice displayed normal maternal behaviors against their pups and adult Cth^-/- male mice exhibited normal social behaviors against adult wild-type female mice, both of which are regulated by oxytocin in the central nervous system (CNS), we conducted the present study to investigate the amino acid profiles, including total homocysteine, in both blood and cerebrospinal fluid (CSF) of wild-type and Cth^-/- female mice before pregnancy and at day 1 of lactation (L1). Serum levels of total homocysteine in wild-type and Cth^-/- L1 dam mice were 9.44 and 188 µmol/L, respectively, whereas their CSF levels were below 0.21 (limit of quantification) and 3.62 µmol/L, respectively. Their CSF/serum level ratio was the lowest (1/51.9) among all 20 proteinogenic amino acids, sulfur-containing amino acids, and citrulline/ornithine in Cth^-/- mice. Therefore, we hypothesize that the blood-brain barrier protects the CNS from high levels of circulatory homocysteine in Cth^-/- dam mice, thereby conferring normal oxytocin-dependent maternal behaviors.

Prenatal Stress in Maternal Hyperhomocysteinemia: Impairments in the Fetal Nervous System Development and Placental Function

The article presents current views on maternal hyperhomocysteinemia (HHcy) as an important factor causing prenatal stress and impaired nervous system development in fetuses and newborns in early ontogenesis, as well as complications in adulthood. Experimental data demonstrate that prenatal HHcy (PHHcy) affects the morphological maturation of the brain and activity of its neurotransmitter systems. Cognitive deficit observed in the offspring subjected to PHHcy in experimental studies can presumably cause the predisposition to various neurodegenerative diseases, as the role of maternal HHcy in the pathogenesis such diseases has been proven in clinical studies. The review also discusses molecular mechanisms of the HHcy neurotoxic action on the nervous system development in the prenatal and early postnatal periods, which include oxidative stress, apoptosis activation, changes in the DNA methylation patterns and microRNA levels, altered expression and processing of neurotrophins, and neuroinflammation induced by an increased production of pro-inflammatory cytokines. Special attention is given to the maternal HHcy impact on the placenta function and its possible contribution to the brain function impairments in the offspring. Published data suggest that some effects of PHHcy on the developing fetal brain can be due to the disturbances in the transport functions of the placenta resulting in an insufficient supply of nutrients necessary for the proper formation and functioning of brain structures.

Neuroprotective Roles of the Reverse Transsulfuration Pathway in Alzheimer's Disease

The reverse transsulfuration pathway has emerged as a central hub that integrates the metabolism of sulfur-containing amino acids and redox homeostasis. Transsulfuration involves the transfer of sulfur from homocysteine to cysteine. Cysteine serves as the precursor for several sulfur-containing molecules, which play diverse roles in cellular processes. Recent evidence shows that disruption of the flux through the pathway has deleterious consequences. In this review article, I will discuss the actions and regulation of the reverse transsulfuration pathway and its links to other metabolic pathways, which are disrupted in Alzheimer’s disease (AD). The potential nodes of therapeutic intervention are also discussed, which may pave the way for the development of novel treatments.

Preeclampsia-Like Features and Partial Lactation Failure in Mice Lacking Cystathionine γ-Lyase-An Animal Model of Cystathioninuria

Elevated plasma homocysteine levels are considered as a risk factor for cardiovascular diseases as well as preeclampsia—a pregnancy disorder characterized by hypertension and proteinuria. We previously generated mice lacking cystathionine γ-lyase (Cth) as cystathioninuria models and found them to be with cystathioninemia/homocysteinemia. We investigated whether Cth-deficient (Cth^−/−) pregnant mice display any features of preeclampsia. Cth^−/− females developed normally but showed mild hypertension (~10 mmHg systolic blood pressure elevation) in late pregnancy and mild proteinuria throughout development/pregnancy. Cth^−/− dams had normal numbers of pups and exhibited normal maternal behavior except slightly lower breastfeeding activity. However, half of them could not raise their pups owing to defective lactation; they could produce/store the first milk in their mammary glands but not often provide milk to their pups after the first ejection. The serum oxytocin levels and oxytocin receptor expression in the mammary glands were comparable between wild-type and Cth^−/− dams, but the contraction responses of mammary gland myoepithelial cells to oxytocin were significantly lower in Cth^−/− dams. The contraction responses to oxytocin were lower in uteruses isolated from Cth^−/− mice. Our results suggest that elevated homocysteine or other unknown factors in preeclampsia-like Cth^−/− dams interfere with oxytocin that regulates milk ejection reflex.