Association Studies of Specific Cholesterol Related Genes (APOE, LPL, and CETP) with Lipid Profile and Memory Function: A Correlative Study Among Rural and Tribal Population of Dharmapuri District, India

Resveratrol Mediated Regulation of Hippocampal Neuroregenerative Plasticity via SIRT1 Pathway in Synergy with Wnt Signaling: Neurotherapeutic Implications to Mitigate Memory Loss in Alzheimer's Disease

Alzheimer's disease (AD) is a major form of dementia. Abnormal amyloidogenic event-mediated degeneration of cholinergic neurons in the cognitive centers of the brain has been attributed to neuropathological sequelae and behavioral deficits in AD. Besides, impaired adult neurogenesis in the hippocampus has experimentally been realized as an underlying cause of dementia regardless of neurodegeneration. Therefore, nourishing the neurogenic process in the hippocampus has been considered an effective therapeutic strategy to mitigate memory loss. In the physiological state, the Wnt pathway has been identified as a potent mitogenic generator in the hippocampal stem cell niche. However, downstream components of Wnt signaling have been noticed to be downregulated in AD brains. Resveratrol (RSV) is a potent Sirtuin1 (SIRT1) enhancer that facilitates neuroprotection and promotes neurogenesis in the hippocampus of the adult brain. While SIRT1 is an important positive regulator of Wnt signaling, ample reports indicate that RSV treatment strongly mediates the fate determination of stem cells through Wnt signaling. However, the possible therapeutic roles of RSV-mediated SIRT1 enhancement on the regulation of hippocampal neurogenesis and reversal of memory loss through the Wnt signaling pathway have not been addressed yet. Taken together, this review describes RSV-mediated effects on the regulation of hippocampal neurogenesis via the activation of SIRT1 in synergy with the Wnt signaling. Further, the article emphasizes a hypothesis that RSV treatment can provoke the activation of quiescent neural stem cells and prime their neurogenic capacity in the hippocampus via Wnt signaling in AD.

Involvement of Lipids in Alzheimer's Disease Pathology and Potential Therapies

Lipids constitute the bulk of the dry mass of the brain and have been associated with healthy function as well as the most common pathological conditions of the brain. Demographic factors, genetics, and lifestyles are the major factors that influence lipid metabolism and are also the key components of lipid disruption in Alzheimer's disease (AD). Additionally, the most common genetic risk factor of AD, APOE ϵ4 genotype, is involved in lipid transport and metabolism. We propose that lipids are at the center of Alzheimer's disease pathology based on their involvement in the blood-brain barrier function, amyloid precursor protein (APP) processing, myelination, membrane remodeling, receptor signaling, inflammation, oxidation, and energy balance. Under healthy conditions, lipid homeostasis bestows a balanced cellular environment that enables the proper functioning of brain cells. However, under pathological conditions, dyshomeostasis of brain lipid composition can result in disturbed BBB, abnormal processing of APP, dysfunction in endocytosis/exocytosis/autophagocytosis, altered myelination, disturbed signaling, unbalanced energy metabolism, and enhanced inflammation. These lipid disturbances may contribute to abnormalities in brain function that are the hallmark of AD. The wide variance of lipid disturbances associated with brain function suggest that AD pathology may present as a complex interaction between several metabolic pathways that are augmented by risk factors such as age, genetics, and lifestyles. Herewith, we examine factors that influence brain lipid composition, review the association of lipids with all known facets of AD pathology, and offer pointers for potential therapies that target lipid pathways.

Causal association of circulating cholesterol levels with dementia: a mendelian randomization meta-analysis

Prospective studies have shown that abnormally circulating cholesterol is associated with the risk of dementia. However, whether the association is causal or not remains unclear. We attempt to infer the causal association in a MR meta-analysis by using ApoE gene polymorphisms as instrument variables. Studies with dementia risk (27 studies) or circulating lipid levels (7 studies) were included, with totally 3136 dementia patients and 3103 healthy controls. The analyses showed that carriers of ε2 allele significantly were of decreased risk of AD (OR = 0.70; 95% CI: 0.58-0.84; P < 0.01), whereas carriers of ε4 allele were of increased risk of AD (OR = 3.62; 95% CI: 3.03-4.32; P < 0.05), compared to these of ε3 allele. Circulating TC was significantly reduced in carriers of ε2 allele (WMD = - 0.29 mmol/L; 95% CI: -0.54 to -0.03; P < 0.05) and increased in carriers of ε4 allele (WMD = 0.42 mmol/l; 95% CI: 0.001-0.84; P < 0.05). In addition, carriers of ε4 allele had reduction in circulating HDL-C (WMD = - 0.04 mmol/L; 95% CI: - 0.07 to -0.001; P < 0.05). In comparing allele ε2 with ε3, the predicted OR of having AD for 1 mg/dL increment in circulating TC was 0.97 (95% CI: 0.86-0.98; P < 0.05). Comparing allele ε4 with ε3, the predicted OR for a 1 mg/dL increment in TC was 1.08 (95% CI: 1.05-17.58; P < 0.05), and reduction in HDL-C was 2.30 (95% CI: 1.51-43.99; P < 0.05). Our findings demonstrate that high circulating TC and reduced HDL-C levels might be potential risk factors of the development of AD.

Possible Existence of the Hypothalamic-Pituitary-Hippocampal (HPH) Axis: A Reciprocal Relationship Between Hippocampal Specific Neuroestradiol Synthesis and Neuroblastosis in Ageing Brains with Special Reference to Menopause and Neurocognitive Disorders

The hippocampus-derived neuroestradiol plays a major role in neuroplasticity, independent of circulating estradiol that originates from gonads. The response of hypothalamus-pituitary regions towards the synthesis of neuroestradiol in the hippocampus is an emerging scientific concept in cognitive neuroscience. Hippocampal plasticity has been proposed to be regulated via neuroblasts, a major cellular determinant of functional neurogenesis in the adult brain. Defects in differentiation, integration and survival of neuroblasts in the hippocampus appear to be an underlying cause of neurocognitive disorders. Gonadotropin receptors and steroidogenic enzymes have been found to be expressed in neuroblasts in the hippocampus of the brain. However, the reciprocal relationship between hippocampal-specific neuroestradiol synthesis along neuroblastosis and response of pituitary based feedback regulation towards regulation of estradiol level in the hippocampus have not completely been ascertained. Therefore, this conceptual article revisits (1) the cellular basis of neuroestradiol synthesis (2) a potential relationship between neuroestradiol synthesis and neuroblastosis in the hippocampus (3) the possible involvement of aberrant neuroestradiol production with mitochondrial dysfunctions and dyslipidemia in menopause and adult-onset neurodegenerative disorders and (4) provides a hypothesis for the possible existence of the hypothalamic-pituitary-hippocampal (HPH) axis in the adult brain. Eventually, understanding the regulation of hippocampal neurogenesis by abnormal levels of neuroestradiol concentration in association with the feedback regulation of HPH axis might provide additional cues to establish a neuroregenerative therapeutic management for mood swings, depression and cognitive decline in menopause and neurocognitive disorders.

Genetic reprogramming of somatic cells into neuroblasts through a co-induction of the doublecortin gene along the Yamanaka factors: A promising approach to model neuroregenerative disorders

Neural stem cell (NSC) mediated adult neurogenesis represents the regenerative plasticity of the brain. The functionality of the neurogenic process appears to be operated by neuroblasts, the multipotent immature neuronal population of the adult brain. While neuroblasts have been realized to play a major role in synaptic remodeling and immunogenicity, neurodegenerative disorders have been characterized by failure in the terminal differentiation, maturation, integration and survival of newborn neuroblasts. Advancement in understanding the impaired neuroregenerative process along the neuropathological conditions has currently been limited by lack of an appropriate experimental model of neuroblasts. The genetic reprogramming of somatic cells into pluripotent state offers a potential strategy for the experimental modeling of brain disorders. Thus, the induced pluripotent stem cell (iPSC) based direct reprogramming of somatic cells into neuroblasts would represent a potential tool to understand the regenerative biology of the adult brain. Therefore, this concise article discusses the significance of iPSCs, the functional roles of neuroblasts in the adult brain and provides a research hypothesis for the direct reprogramming of somatic cells into neuroblasts through the co-induction of a potential proneurogenic marker, the doublecortin (DCX) gene along with the Yamanaka factors. The proposed cellular model of adult neurogenesis may provide us with further insights into neuropathogenesis of many neurodegenerative disorders and will provide a potential experimental platform for diagnostic, drug discovery and regenerative therapeutic strategies.

Time-dependent effect of oligomeric amyloid-β (1-42)-induced hippocampal neurodegeneration in rat model of Alzheimer's disease

OBJECTIVE

Alzheimer's disease (AD) is characterized with an abnormal deposition of insoluble amyloid-beta (Aβ) peptide plaques, tangles formation and synaptic dysfunction. These result in impaired functioning of neuronal circuits and alter the behavioral response owing to activation of neurotransmitter receptors. Recently, it has been implicated that Aβ influences N-methyl d-aspartate (NMDA) receptor activation in AD; however, the molecular mechanism underlying remains unclear. Thus, emerged specific aim to study the time-course effect of oligomeric Aβ_(1-42) (oAβ_1-42) on the mRNA expression of genes encoding NMDA and acetylcholine receptors in the rat model of AD.

METHODS

 Aggregated forms of synthetic Aβ peptides were injected bilaterally into the intrahippocampal region of rat brain using stereotaxic surgery. Behavioral analysis was performed using eight-arm Radial Arm Maze task at the end of experimental period. Euthanized rat brain hippocampal tissue was used to study the mRNA expression of glutamatergic and cholinergic receptor using semiquantitative reverse transcription-polymerase chain reaction.

RESULTS

oAβ_1-42 decreased the gene expression level of α7-nicotinic acetylcholine receptor and increased the mRNA expression of NMDA receptor 2A, and -2B subunits. In particular, oAβ_1-42 aggregates increased the retention time and altered the behavioral response in rats after 15 days of injection. Further, amyloid-β_1-42 are highly expressed in 15 days after postinjection in hippocampus of adult rats.

CONCLUSION

Acute exposure of oAβ_1-42 modulated differential gene expression of glutamatergic and cholinergic receptors in hippocampus of adult rats and is duration dependent reflecting changes in hippocampal circuitry system underlying learning and memory impairments.

ABBREVIATIONS

AD: Alzheimer's disease, Aβ: amyloid-β; oAβ_1-42: oligomeric amyloid-β 1-42 full length peptide; CAM: calmodulin; CNS: central nervous system; CR: Congo red; DG: dentate gyrus; EC: entorhinal cortex; HFIP: 1,1,1,3,3,3-hexafluoro-2-propanol; IBO: ibotenic acid; NMDA: N-methyl d-aspartate; NMDAR: N-methyl d-aspartate receptor; NR2A: N-methyl d-aspartate receptor 2A; NR2B: N-methyl d-aspartate receptor 2B; ACh: acetylcholine; α7-nAChR: α7-nicotinic acetylcholine receptor; PBS: phosphate buffered saline; RAM: Radial Arm Maze; ThT: thioflavin T.