Biomolecular Markers of Brain Aging

Characterized by the gradual loss of physiological integrity, impaired function, and increased susceptibility to death, aging is considered the primary risk factor for major human diseases, such as cancer, diabetes, cardiovascular disorders, and neurodegenerative diseases. The time-dependent accumulation of cellular damage is widely considered the general cause of aging. While the mechanism of normal aging is still unresolved, researchers have identified different markers of aging, including genomic instability, telomere attrition, epigenetic alterations, loss of proteostasis, deregulated nutrient-sensing, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, and altered intercellular communication. Theories of aging can be divided into two categories: (1) aging is a genetically programmed process, and (2) aging is a random process caused by gradual damage to the organism over time as a result of its vital activities. Aging affects the entire human body, and aging of the brain is undoubtedly different from all other organs, as neurons are highly differentiated postmitotic cells, and the lifespan of most neurons in the postnatal period is equal to the lifespan of the brain. In this chapter, we discuss the conserved mechanisms of aging that may underlie the changes observed in the aging brain, with a focus on mitochondrial function and oxidative stress, autophagy and protein turnover, insulin/IGF signaling, target of rapamycin (TOR) signaling, and sirtuin function.

Neurogenesis and brain aging

Human aging affects the entire organism, but aging of the brain must undoubtedly be different from that of all other organs, as neurons are highly differentiated postmitotic cells, for the majority of which the lifespan in the postnatal period is equal to the lifespan of the entire organism. In this work, we examine the distinctive features of brain aging and neurogenesis during normal aging, pathological aging (Alzheimer's disease), and accelerated aging (Hutchinson-Gilford progeria syndrome and Werner syndrome).

A Generative Network Model of the Human Brain Normal Aging Process

The human brain is approximately a symmetric structure, although the functional brain does not exhibit symmetry. Functional brain aging process modelling is essential for the understanding of hypothesized generative mechanisms for human brain networks throughout one’s lifespan. We present a novel generative network model of the human functional brain network, which is the hybrid of the local naïve Bayes model and the anatomical similarity correction (LNBE). We use LNBE, as well as published generative network models to simulate the aging process of the functional brain network, to construct artificial brain networks and to reveal the generative mechanisms and evolutionary patterns of human functional brain across human lifespans. It is suggested that the idea of classifying common neighbours while considering anatomical distances during network formation can provide a much more similar generative mechanism of the human fMRI brain aging process as well as a more practical generative network model of it. We hold that studies on brain normal aging process modelling have the potential to improve the way in which early warnings for latent injury or disease are practised today and advance healthcare.

Accelerated aging and aging process in the brain

One of the approaches to the research of the problem of aging is the study of genetic pathologies leading to accelerated aging, such as the Hutchinson-Gilford progeria syndrome, Werner syndrome, and Down syndrome. Probably, this approach can be used in an attempt to understand the neuronal mechanisms underlying normal and pathological brain aging. The analysis of the current state of scientific knowledge about these pathologies shows that in the Hutchinson-Gilford progeria and Werner syndrome, the rate of brain aging is significantly lower than the rate of whole body aging, whereas in Down syndrome, the brain ages faster than other organs due to amyloid-beta accumulation and chronic oxidative stress in the brain tissue. The main point of a previously proposed hypothesis is that the aging of higher animals and humans is associated with an increased level of reactive oxygen species in mitochondria with age, which activates apoptosis, thus reducing the number of functioning cells.

Antioxidant mechanism of mitochondria-targeted plastoquinone SkQ1 is suppressed in aglycemic HepG2 cells dependent on oxidative phosphorylation

Previously suggested antioxidant mechanisms for mitochondria-targeted plastoquinone SkQ1 included: i) ion-pairing of cationic SkQ1⁺ with free fatty acid anions resulting in uncoupling; ii) SkQ1H₂ ability to interact with lipoperoxyl radical; iii) interference with electron flow at the inner ubiquinone (Q) binding site of Complex III (Q_i), involving the reduction of SkQ1 to SkQ1H₂ by ubiquinol. We elucidated SkQ1 antioxidant properties by confocal fluorescence semi-quantification of mitochondrial superoxide (J_m) and cytosolic H₂O₂ (J_c) release rates in HepG2 cells. Only in glycolytic cells, SkQ1 prevented the rotenone-induced enhancement of J_m and J_c but not basal releases without rotenone. The effect ceased in glutaminolytic aglycemic cells, in which the redox parameter NAD(P)H/FAD increased after rotenone in contrast to its decrease in glycolytic cells. Autofluorescence decay indicated decreased NADPH/NADH ratios with rotenone in both metabolic modes. SkQ1 did not increase cell respiration and diminished J_m established high by antimycin or myxothiazol but not by stigmatellin. The revealed SkQ1 antioxidant modes reflect its reduction to SkQ1H₂ at Complex I I_Q or Complex III Q_i site. Both reductions diminish electron diversions to oxygen thus attenuating superoxide formation. Resulting SkQ1H₂ oxidizes back to SkQ1at the second (flavin) Complex I site, previously indicated for MitoQ₁₀. Regeneration proceeds only at lower NAD(P)H/FAD in glycolytic cells. In contrast, cyclic SkQ1 reduction/SkQ1H₂ oxidation does not substantiate antioxidant activity in intact cells in the absence of oxidative stress (neither pro-oxidant activity, representing a great advantage). A targeted delivery to oxidative-stressed tissues is suggested for the effective antioxidant therapy based on SkQ1.

Role of Nerve Growth Factor in Plasticity of Forebrain Cholinergic Neurons

Neuronal plastic rearrangements during the development and functioning of neurons are largely regulated by trophic factors, including nerve growth factor (NGF). NGF is also involved in the pathogenesis of Alzheimer's disease. In the brain, NGF is produced in structures innervated by basal forebrain cholinergic neurons and retrogradely transported along the axons to the bodies of cholinergic neurons. NGF is essential for normal development and functioning of the basal forebrain; it affects formation of the dendritic tree and modulates the activities of choline acetyltransferase and acetylcholinesterase in basal forebrain neurons. The trophic effect of NGF is mediated through its interactions with TrkA and p75 receptors. Experimental and clinical studies have shown that brain levels of NGF are altered in various pathologies. However, the therapeutic use of NGF is limited by its poor ability to penetrate the blood-brain barrier, adverse side effects that are due to the pleiotropic action of this factor, and the possibility of immune response to NGF. For this reason, the development of gene therapy methods for treating NGF deficit-associated pathologies is of particular interest. Another approach is creation of low molecular weight NGF mimetics that would interact with the corresponding receptors and display high biological activity but be free of the unfavorable effects of NGF.

Alzheimer's Disease: An Exacerbation of Senile Phenoptosis

Alzheimer's disease is characterized by progressive memory loss and cognitive decline accompanied by degeneration of neuronal synapses, massive loss of neurons in the brain, eventually resulting in complete degradation of personality and death. Currently, the cause of the disease is not fully understood, but it is believed that the person's age is the major risk factor for development of Alzheimer's disease. People who have survived after cerebral stroke or traumatic brain injury have substantially increased risk of developing Alzheimer's disease. Social exclusion, low social activity, physical inactivity, poor mental performance, and low level of education are among risk factors for development of this neurodegenerative disease, which is consistent with the concept of phenoptosis (Skulachev, V. P., et al. (1999) Biochemistry (Moscow), 64, 1418-1426; Skulachev, M. V., and Skulachev, V. P. (2014) Biochemistry (Moscow), 79, 977-993) stating that rate of aging is related to psychological and social aspects in human behavior. Here we assumed that Alzheimer's disease might be considered as an exacerbation of senile phenoptosis. If so, then development of this disease could be slowed using mitochondria-targeted antioxidants due to the accumulated data demonstrating a link between mitochondrial dysfunction and oxidative stress both with normal aging and Alzheimer's disease.

Mitochondria-Targeted Plastoquinone Antioxidant SkQR1 Has Positive Effect on Memory of Rats

A single intraperitoneal injection to rats of the mitochondria-targeted plastoquinone antioxidant SkQR1 at dose 1 µmol/kg significantly improved reproduction by the rats of the passive avoidance conditional reflex. In vitro experiments on hippocampal slices showed that a single intraperitoneal injection of SkQR1 24 h before the preparation of the slice significantly increases the synaptic transmission efficiency of the pyramidal neurons of the CA1 field. The findings indicate that SkQR1 has a positive effect on memory processes.

Intracellular redox state as target for anti-influenza therapy: are antioxidants always effective?

Influenza virus infections represent a big issue for public health since effective treatments are still lacking. In particular, the emergence of strains resistant to drugs limits the effectiveness of anti-influenza agents. For this reason, many efforts have been dedicated to the identification of new therapeutic strategies aimed at targeting the virus-host cell interactions. Oxidative stress is a characteristic of some viral infections including influenza. Because antioxidants defend cells from damage caused by reactive oxygen species induced by different stimuli including pathogens, they represent interesting molecules to fight infectious diseases. However, most of the available studies have found that these would-be panaceas could actually exacerbate the diseases they claim to prevent, and have thus revealed "the dark side" of these molecules. This review article discusses the latest opportunities and drawbacks of the antioxidants used in anti-influenza therapy and new perspectives.