Inhibitory effect of homocysteine on rat neural stem cell growth in vitro is associated with reduced protein levels and enzymatic activities of aconitase and respiratory complex III

The impact of amino acid metabolism on adult neurogenesis

Adult neurogenesis is a multistage process during which newborn neurons are generated through the activation and proliferation of neural stem cells (NSCs) and integrated into existing neural networks. Impaired adult neurogenesis has been observed in various neurological and psychiatric disorders, suggesting its critical role in cognitive function, brain homeostasis, and neural repair. Over the past decades, mounting evidence has identified a strong association between metabolic status and adult neurogenesis. Here, we aim to summarize how amino acids and their neuroactive metabolites affect adult neurogenesis. Furthermore, we discuss the causal link between amino acid metabolism, adult neurogenesis, and neurological diseases. Finally, we propose that systematic elucidation of how amino acid metabolism regulates adult neurogenesis has profound implications not only for understanding the biological underpinnings of brain development and neurological diseases, but also for providing potential therapeutic strategies to intervene in disease progression.

Homocysteine restrains hippocampal neurogenesis in focal ischemic rat brain by inhibiting DNA methylation

Ischemic stroke represents a major cause of mortality worldwide. An elevated level of homocysteine (Hcy) is recognized as a powerful risk factor of ischemic stroke. We previously reported that Hcy induces cytotoxicity and proliferation inhibition in neural stem cells (NSCs) derived from the neonatal rat hippocampus in vitro. However, the toxic potential of Hcy on NSCs and its underlying mechanisms are not entirely clear in ischemic brain. Since DNA methylation is critical for establishing the diverse cell fates in the central nervous system, we hypothesized that negative effect of Hcy (an intermediate in the one-carbon metabolism) on neurogenesis might be link to DNA methylation in ischemic stroke. In our study, the rats in Hcy intervention group were intraperitoneally injected with 2% Hcy solution (5 mL/kg/d) for 7 consecutive days before MCAO surgery until they were sacrificed. Our study indicated that Hcy inhibited NSCs self-renewal capacity, which was exhibited by lowering the number of DCX+/BrdU+ and NeuN+/BrdU+ in ischemic brain hippocampus. A reduction in the activity of the DNA methyltransferases (DNMTs), total methylation level and the number of 5mC+/NeuN+ and DCX+/5mC+ cells was observed in Hcy-treated ischemic brains. Additionally, Hcy also induced an increase in S-adenosylhomocysteine (SAH), and a decrease in the ratio of S-adenosylmethionine (SAM) to SAH. These results suggest that the alterations in DNA methylation may be an important mechanism by which Hcy inhibits neurogenesis after stroke. Hcy-induced DNA hypomethylation may be mainly caused by a reduction in the DNMT activity which is regulated by the concentrations of SAM and SAH. Maintaining normal DNA methylation by lowering Hcy level may possess therapeutic potential for promoting neurological recovery and reconstruction after stroke.

Homocysteine and Mitochondria in Cardiovascular and Cerebrovascular Systems

Elevated concentration of homocysteine (Hcy) in the blood plasma, hyperhomocysteinemia (HHcy), has been implicated in various disorders, including cardiovascular and neurodegenerative diseases. Accumulating evidence indicates that pathophysiology of these diseases is linked with mitochondrial dysfunction. In this review, we discuss the current knowledge concerning the effects of HHcy on mitochondrial homeostasis, including energy metabolism, mitochondrial apoptotic pathway, and mitochondrial dynamics. The recent studies suggest that the interaction between Hcy and mitochondria is complex, and reactive oxygen species (ROS) are possible mediators of Hcy effects. We focus on mechanisms contributing to HHcy-associated oxidative stress, such as sources of ROS generation and alterations in antioxidant defense resulting from altered gene expression and post-translational modifications of proteins. Moreover, we discuss some recent findings suggesting that HHcy may have beneficial effects on mitochondrial ROS homeostasis and antioxidant defense. A better understanding of complex mechanisms through which Hcy affects mitochondrial functions could contribute to the development of more specific therapeutic strategies targeted at HHcy-associated disorders.

Homocysteine aggravates DNA damage by impairing the FA/Brca1 Pathway in NE4C murine neural stem cells

There is existing evidence that elevated homocysteine (Hcy) levels are risk factors for some neurodegenerative disorders. The pathogenesis of neurological diseases could be contributed to excessive cell dysfunction and death caused by defective DNA damage response (DDR) and accumulated DNA damage. Hcy is a neurotoxic amino acid and acts as a DNA damage inducer. However, it is not clear whether Hcy participates in the DDR. To investigate the effects of Hcy on DNA damage and the DDR, we employed mitomycin C (MMC) to cause DNA damage in NE4C murine neural stem cells (NSCs). Compared to treatment with MMC alone, we found that co-treatment with MMC and Hcy worsened DNA damage and increased death in NE4C cells. Intriguingly, in this DNA damage model mimicked by MMC, immunoblotting results showed that the monoubiquitination levels of Fanconi anemia complementation group I (Fanci) and Fanconi anemia complementation group D2 (Fancd2) were decreased to about 60.3% and 55.7% by supplementing cell culture medium with Hcy, indicating Hcy inactivates the function of Fanci and Fancd2 in DNA damage conditions. Given Breast Cancer 1 (BRCA1) is an important downstream of FANCD2, we next detected the interaction between Fancd2 and Brca1 in NE4C cells. Compared to treatment with MMC alone, the Fancd2-Brca1 interaction and the amount of Brca1 on chromatin were decreased when cells were co-exposed to MMC and Hcy, suggesting Hcy could impair the Fanconi anemia (FA)/Brca1 pathway. Taken together, our study demonstrates that Hcy may enhance cell death, which contributes to the accumulation of DNA damage and promotion of hypersensitivity to cytotoxicity by impairing the FA/Brca1 pathway in murine NSCs in the presence of DNA damage.

Hydrogen sulfide attenuates hyperhomocysteinemia-induced mitochondrial dysfunctions in brain

Hyperhomocysteinemia (HHcy) has been implicated in the development of neurodegenerative conditions and mild cognitive disorders. Mitochondrial dysfunctions are the major mechanisms involved in homocysteine (Hcy)-induced neurotoxicity. Although, hydrogen sulfide has been reported as potent antioxidant, its effects on Hcy-induced mitochondrial dysfunctions have not been studied. Therefore, the present study has been designed to evaluate the protective effect of NaHS on Hcy-induced mitochondrial dysfunctions in brain. NaHS supplementation decreased reactive oxygen species and nitrite levels in the cortex and hippocampus of animals with HHcy. NaHS supplementation increased the activity of mitochondrial electron transport chain components; NADH dehydrogenase, cytochrome c oxidase and F0-F1 ATPase in the cortex and hippocampus of HHcy animals along with in-gel activity of complex I - complex V in the mitochondria isolated from the cortex and hippocampus of HHcy animals. Moreover, NaHS supplementation also increased the mitochondrial complex I, II and IV mediated oxygen consumption rate in Hcy treated mitochondria. NaHS administration prevented the Hcy-induced mitochondrial damage as suggested by the decreased mitochondrial swelling in the cortex and hippocampus of HHcy animals. NaHS supplementation decreased the activity of lactate dehydrogenase isozymes (1-5) in the brain regions of HHcy animals. The expression of protein kinase c δ was also decreased in the brain regions of HHcy animals following NaHS supplementation. This was accompanied by reduced activity of caspase-3 indicating anti-apoptotic effect of H₂S. Taken together, the findings suggest that H₂S supplementation ameliorates Hcy-induced oxidative stress and mitochondrial dysfunctions suggesting H₂S releasing drugs may be a novel therapeutic approach to treat HHcy associated neurological disorders.

Homocysteine enhances neural stem cell autophagy in in vivo and in vitro model of ischemic stroke

The elevated level of the amino acid metabolite homocysteine (Hcy) is known as a risk factor for ischemic stroke. The molecular mechanisms responsible for neurotoxicity of Hcy remain largely unknown in ischemic brains. The previous studies have shown that Hcy decreases the proliferation and viability of neural stem cells (NSCs) in vivo and in vitro. Autophagy is required for the maintenance of NSCs homeostasis. In the current study, we hypothesized that the toxic effect of Hcy on NSCs may involve the changes in autophagy level following cerebral ischemia/reperfusion injury. The results showed that Hcy reduced cell viability, increased LDH release, and induced nonapoptotic cell death in primary NSCs exposed to oxygen–glucose deprivation)/reoxygenation (OGD/R). Treatment with autophagy inhibitor 3-methyladenine (3MA) partly reversed the decrease in the viability and prevented LDH release triggered by Hcy combined with OGD/R. Increased punctate LC3 dots co-localizing with Nestin-stained NSCs were also observed in the subventricular zone of Hcy-treated MCAO animals, which were partially blocked by 3MA. In vitro studies further revealed that Hcy induced the formation of autophagosomes, markedly increased the expression of the autophagic markers and decreased p-ERK, p-PI3K, p-AKT, and p-mTOR levels. In addition, MHY1485, an activator of mTOR, reduced Hcy-induced increase in LC3 and Beclin 1 protein levels, meanwhile ERK and PI3K activators (TPA, curcumin for ERK and IGF-1 for PI3K, respectively) enhanced Hcy-triggered mTOR inhibition in OGD/R NSCs. Our findings suggest that Hcy may cause excessive autophagy by downregulation of both PI3K-AKT- and ERK- dependent mTOR signaling, thereby facilitates the toxicity of Hcy on NSCs in ischemic brains.

The Cardiovascular Effect of Systemic Homocysteine Is Associated with Oxidative Stress in the Rostral Ventrolateral Medulla

It has been demonstrated that homocysteine (HCY) is a significant risk factor of hypertension, which is characterized by overactivity of sympathetic tone. Excessive oxidative stress in the rostral ventrolateral medulla (RVLM), a key region for control of sympathetic outflow, contributes to sympathetic hyperactivity in hypertension. Therefore, the goal of the present study is to determine the effect of systemic HCY on production of reactive oxygen species (ROS) in the RVLM. In the rat model of the diet-induced hyperhomocysteinemia (L-methionine, 1 g/kg/day, 8 weeks), we found that the HCY resulted in a significant increase (≈3.7-fold, P < 0.05) in ROS production in the RVLM, which was paralleled with enhanced sympathetic tone and blood pressure (BP). Compared to the vehicle group, levels of BP and basal renal sympathetic nerve activity in the HCY group were significantly (P < 0.05, n = 5) increased by an average of 27 mmHg and 31%, respectively. Furthermore, the rats treated with L-methionine (1 g/kg/day, 8 weeks) showed an upregulation of NADPHase (NOX4) protein expression and a downregulation of superoxide dismutase protein expression in the RVLM. The current data suggest that central oxidative stress induced by systemic HCY plays an important role in hypertension-associated sympathetic overactivity.