Homocysteine (Hcy) is a toxic, sulfur-containing intermediate of methionine metabolism. Hyperhomocysteinemia (hHcy), as a consequence of impaired Hcy metabolism or defects in crucial co-factors that participate in its recycling, is assumed as an independent human stroke risk factor. Neural cells are sensitive to prolonged hHcy treatment, because Hcy cannot be metabolized either by the transsulfuration pathway or by the folate/vitamin B12 independent remethylation pathway. Its detrimental effect after ischemia-induced damage includes accumulation of reactive oxygen species (ROS) and posttranslational modifications of proteins via homocysteinylation and thiolation. Ischemic preconditioning (IPC) is an adaptive response of the CNS to sub-lethal ischemia, which elevates tissues tolerance to subsequent ischemia. The main focus of this review is on the recent data on homocysteine metabolism and mechanisms of its neurotoxicity. In this context, the review documents an increased oxidative stress and functional modification of enzymes involved in redox balance in experimentally induced hyperhomocysteinemia. It also gives an interpretation whether hyperhomocysteinemia alone or in combination with IPC affects the ischemia-induced neurodegenerative changes as well as intracellular signaling. Studies document that hHcy alone significantly increased Fluoro-Jade C- and TUNEL-positive cell neurodegeneration in the rat hippocampus as well as in the cortex. IPC, even if combined with hHcy, could still preserve the neuronal tissue from the lethal ischemic effects. This review also describes the changes in the mitogen-activated protein kinase (MAPK) protein pathways following ischemic injury and IPC. These studies provide evidence for the interplay and tight integration between ERK and p38 MAPK signaling mechanisms in response to the hHcy and also in association of hHcy with ischemia/IPC challenge in the rat brain. Further investigations of the protective factors leading to ischemic tolerance and recognition of the co-morbid risk factors would result in development of new avenues for exploration of novel therapeutics against ischemia and stroke.

Alzheimer's disease (AD) is a progressive and irreversible neurodegenerative disorder that results in massive hippocampal and neocortical neuronal loss leading to dementia and eventual death. The exact cause of Alzheimer's disease is not fully explored, although a number of risk factors have been recognized, including high plasma concentration of homocysteine (Hcy). Hyperhomocysteinemia (hHcy) is considered a strong, independent risk factor for stroke and dementia. However, the molecular background underlying these mechanisms linked with hHcy and ischemic stroke is not fully understood. Paper describes rat model of global forebrain ischemia combined with the experimentally induced hHcy. Global ischemia-reperfusion injury (IRI) was developed by 4-vessels occlusion lasting for 15 min followed by reperfusion period of 72 h. hHcy was induced by subcutaneous injection of 0.45 µmol/g of Hcy in duration of 14 days. The results showed remarkable neural cell death induced by hHcy in the brain cortex and neurodegeneration is further aggravated by global IRI. We demonstrated degeneration of cortical neurons, alterations in number and morphology of tissue astrocytes and dysregulation of oxidative balance with increased membrane protein oxidation. Complementary to, an immunohistochemical analysis of tau protein and β-amyloid peptide showed that combination of hHcy with the IRI might lead to the progression of AD-like pathological features. Conclusively, these findings suggest that combination of risk factor hHcy with IRI aggravates neurodegeneration processes and leads to development of AD-like pathology in cerebral cortex.

Hyperhomocysteinemia (hHCy) is recognized as a co-morbid risk factor of human stroke. It also aggravates the ischemia-induced injury by increased production of reactive oxygen species, and by the homocysteinylation and thiolation of functional proteins. Ischemic preconditioning represents adaptation of the CNS to sub-lethal ischemia, resulting in increased brain tolerance to subsequent ischemia. We present here an overview of recent data on the homocysteine (Hcy) metabolism and on the genetic and metabolic causes of hHCy-related neuropathologies in humans. In this context, the review documents for an increased oxidative stress and for the functional modifications of enzymes involved in the redox balance in experimentally induced hHCy. Hcy metabolism leads also to the redox imbalance and increased oxidative stress resulting in elevated lipoperoxidation and protein oxidation, the products known to be included in the neuronal degeneration. Additionally, we examine the effect of the experimental hHCy in combination with ischemic insult, and/or with the preischemic challenge on the extent of neuronal degeneration as well as the intracellular signaling and the regulation of DNA methylation. The review also highlights that identification of the effects of co-morbid factors in the mechanisms of ischemic tolerance mechanisms would lead to improved therapeutics, especially the brain tissue.

Hyperhomocysteinemia (hHcy) is regarded as an independent and strong risk factor for cerebrovascular diseases, stroke, and dementias. The hippocampus has a crucial role in spatial navigation and memory processes and is being constantly studied for neurodegenerative disorders. We used a moderate methionine (Met) diet at a dose of 2 g/kg of animal weight/day in duration of four weeks to induce mild hHcy in adult male Wistar rats. A novel approach has been used to explore the hippocampal metabolic changes using proton magnetic resonance spectroscopy (1H MRS), involving a 7T MR scanner in combination with histochemical and immunofluorescence analysis. We found alterations in the metabolic profile, as well as remarkable histo-morphological changes such as an increase of hippocampal volume, alterations in number and morphology of astrocytes, neurons, and their processes in the selective vulnerable brain area of animals treated with a Met-enriched diet. Results of both methodologies suggest that the mild hHcy induced by Met-enriched diet alters volume, histo-morphological pattern, and metabolic profile of hippocampal brain area, which might eventually endorse the neurodegenerative processes.

Epigenetic regulations play an important role in both normal and pathological conditions of an organism, and are influenced by various exogenous and endogenous factors. Hyperhomocysteinemia (hHcy), as a risk factor for several pathological conditions affecting the central nervous system, is supposed to alter the epigenetic signature of the given tissue, which therefore worsens the subsequent damage. To investigate the effect of hHcy in combination with ischemia-reperfusion injury (IRI) and histone acetylation, we used the hHcy animal model of global forebrain ischemia in rats. Cresyl violet staining showed massive neural disintegration in the M1 (primary motor cortex) region as well as in the CA1 (cornu ammonis 1) area of the hippocampus induced by IRI. Neural loss was significantly higher in the group with induced hHcy. Moreover, immunohistochemistry and Western blot analysis of the brain cortex showed prominent changes in the acetylation of histones H3 and H4, at lysine 9 and 12, respectively, as a result of IRI and induced hHcy. It seems that the differences in histone acetylation patterns in the cortical region have a preferred role in pathological processes induced by IRI associated with hHcy and could be considered in therapeutic strategies.