**1.4. Metabolic regulation and the central nervous system: the central role of mitochondria**

Mitochondria provide the energy required for neuron function because they transform the potential chemical energy stored in covalent bonds of glucose or fatty acids into chemical energy stored in the covalent bonds between ATP phosphates. This last form of potential chemical energy is easily usable by the cell and has been selected throughout evolution as the mechanism by means of which all cellular processes that require the use of energy readily dispose of it [10, 17]. The body must maintain a balance between the needs of cells and the availability of fuel, which is called metabolic homeostasis. The constant availability of fuel in the blood is called caloric homeostasis, whereby the blood level of fuel (in ATP equivalents) does not decrease below certain limits regardless of whether the individual is in a state of good nutrition or fasting. The maintenance of metabolic homeostasis is achieved through the integration of three main factors: (1) the concentration of nutrients in the blood, which affects the speed with which these are used and stored in different tissues, (2) hormone levels in blood (first messengers) that transmit information to specific tissues on the state of the organism and the contribution or demand of nutrients, and (3) the central nervous system (CNS) that by way of neural signals controls the metabolism directly or through the release of hormones [9, 18]. Despite its essential role in the energy metabolism of the brain and other tissues, the amount of circulating glucose is limited. To ensure its continued provision, the body stores metabolic fuels to provide glucose or energy in case of need. Within the homeostatic mechanisms that allow regulation of the availability of combustible molecules, hormonal control is one of the most important. Insulin and glucagon are the main hormones that regulate the storage and use of fuels. Insulin is an anabolic hormone that promotes the storage, while glucagon is the hormone that stimulates the mobilization of the combustible molecules [17, 18]. Other hormones, such as adrenaline, are released as a CNS response to hypoglycemia, exercise, and other types of physiological stress. Along with other stress hormones (glucocorticoids), adrenaline increases the availability of fuels. One of the requirements to maintain and perpetuate life is the preservation of homeostasis, that is, the constancy of the internal environment (blood levels of ions, lipids, and carbohydrates) within narrow limits. These conditions must be maintained even in varied situations such as rest, exercise, satiety, or fasting. How is our body harmonized to survive in different metabolic situations? In mammals, the coordination of metabolism is achieved through the neuroendocrine system. The main hormones involved in the regulation of intermediate metabolism are insulin, glucagon, catecholamines, and cortisol [19–21].

**1.3. High mutation speed**

158 Mitochondrial DNA - New Insights

**mitochondria**

The mtDNA has a spontaneous mutation rate 10 times higher than that of nuclear DNA. Continuous mitochondrial production of oxygen radicals by the final oxidation of carbon compounds probably damages the unprotected mtDNA (e.g., mtDNA reminds bacterial genomes as it lacks histones). Therefore, the within-species individual sequence variation is large, up to about 70 nucleotides. Within single individuals, low heterogeneity levels in the mtDNA will be generated throughout life. It has been proposed that the decrease in respiratory capacity of the tissues, which takes place during aging, may be due to an accumulation of such mitochondrial damages. This theory was first evidenced in a study by the Attardi Group, who documented that the mitochondria deteriorate with age as a result of the accumulation of mutations [13]. Mitochondrial dysfunction is characterized by a deficient production of energy, a failure in calcium homeostasis, an activation of proteases and phospholipases, activation of nitric oxide synthase, and an abundant generation of free radicals [14–16]. Mitochondria, besides being the main source of free radicals, are also very susceptible to oxidative stress, which is made evident by a massive induction of lipid peroxidation, protein oxidation, and mutations in mtDNA. Oxidative stress also induces apoptotic death, and the mitochondria play a central role in this phenomenon since there is cyto-

**Figure 3.** Mitochondrial homoplasmy and heteroplasmy. The expression of mitochondrial diseases is variable; a single cell may receive a uniform collection of mtDNA (homoplasmy) or a mixture of mutant and wild-type mtDNA (heteroplasmy). The proportion of mutant mtDNA molecules determines the penetrance and severity of expression.

chrome c release to the cytoplasm and opening of the permeability transition pore [16].

Mitochondria provide the energy required for neuron function because they transform the potential chemical energy stored in covalent bonds of glucose or fatty acids into chemical

**1.4. Metabolic regulation and the central nervous system: the central role of** 

The brain must generate large amounts of ATP to maintain the membrane potential, which is essential for the transmission of nerve impulses. Under normal conditions the brain only uses glucose as fuel, oxidizing it through aerobic glycolysis. It does not use fatty acids. In fact, 60% of the total glucose consumed by the body is used by the brain. The metabolism of the brain is totally aerobic, consuming 20% of the total oxygen consumed by the body. It does not have appreciable reserves of glycogen or other fuels so it requires the constant supply of oxygen and glucose that cross the blood–brain barrier with ease [19, 22].

Practically, until adulthood, we are well protected against damage to mitochondria since the body is able to produce antioxidant systems that defend us from it [23]. But as we get older, changes occur inside our cells that determine the progressive destruction of mitochondria and, therefore, bring about aging and disease [24].

The sequence variations existing between different individuals have been very useful for anthropological, ethnological, and forensic studies and are the basis for the hypothesis that all existing humans descend from a woman who lived in Africa about 250,000 years ago (**Figure 4**) [25–27].

**c.** The cytoplasmic pattern recognition receptors (CPRRs), which detect and initiate the immune response against intracellular pathogens, recognize the molecules of the mitochon-

Mitochondrial Aging and Metabolism: The Importance of a Good Relationship in the Central…

http://dx.doi.org/10.5772/intechopen.76652

161

**d.** After detecting potential threat, the CPRRs form a complex called inflammasome that captures the inflammatory cytokine interleukin-1β, which then recruits components of the

These four steps represent a simplified diagram of mitochondrial dysfunction that leads to cell destruction; however, free radicals are not the only inducers of cell death by inflamma-

Circulating carbohydrates, mainly glucose and fructose, also participate in aging processes. When these blood sugars come into contact with proteins and lipids, a harmful reaction occurs that forms compounds called advanced glycation end products (AGEs). The AGEs bind to a receptor on the surface of the cells called the PFG receptor (receptor for advanced glycation end products (RAGE)). After activation, the RAGEs induce the movement of the nuclear mediator factor kappa-B (NF-kB) to the nucleus where numerous inflammatory genes are activated. AGEs are formed mainly in vivo, and glycation is exacerbated by elevated blood

Hence, molecules that protect and revitalize mitochondria could recreate a "juvenile" state of

Mitochondria in the elderly are, for the most part, are dysfunctional, unlike young individuals in whom no mitochondrial damage is observed. So much so is the mitochondrial dysfunction caused by oxidative damage due to free radicals that are already a marker of aging and

The mitochondrial theory, which proposes that mitochondrial defects associated with age are controlled by the accumulation of mutations in mitochondrial DNA. There is, however, a growing body of contradictory evidence that has raised questions about the validity of this theory. It has been suggested that the mitochondrial defects associated with age are not controlled by the accumulation of mutations in mitochondrial DNA but by another form of genetic regulation. Contrary to the mitochondrial theory of aging, the epigenetic regulation of

Damage to mitochondrial DNA causes changes or mutations in the DNA sequence. The accumulation of these changes is associated with a reduced life expectancy and the early onset of characteristics related to aging such as weight and hair loss, the curvature of the spine, and osteoporosis [38]. There is, however, a growing body of contradictory evidence that has raised questions about the validity of this theory. Tsukuba's team, in particular, has made a compelling research that has led them to propose that the mitochondrial defects associated with age are not controlled by the accumulation of mitochondrial DNA mutations but by another form of genetic regulation [2, 39, 40]. The researchers compared mitochondrial respiration and the amount of DNA damage in the mitochondria, expecting that respiration would decrease and DNA damage would increase in the cells of the elderly group. The elderly group had lower respiration as the accepted theory indicates; however, there was no difference in amounts of

glucose levels. Dietary AGE also contributes to inflammation [17, 18, 35].

drial discharge as potential threats.

tion [33, 34].

immune system to destroy the altered cell.

protection against free radicals (**Figure 5**) [36].

respiration controls defects associated with age [37].

pathologies associated with age [37].

**Figure 4.** Mitochondrial aging theory. The production of intracellular free radicals, and its effects on cellular components, is a determinant of lifespan. Mitochondria is the prime target for oxidative damage; accumulation of reactive oxygen species (ROS) leads to changes of the inner membrane permeability causing release of intramitochondrial content and activating a cytoplasmic immune response leading to apoptotic cell death.

Mitochondria in the elderly are, for the most part, dysfunctional, unlike young individuals in whom little mitochondrial damage is observed: with the passage of time, devastating changes occur inside our cells that lead to the destruction of mitochondria and consequently trigger aging and disease [15, 28]. Production rates of superoxide anions and hydrogen peroxide (free radicals) increase significantly, specifically deteriorating the mitochondria. At the same time, the levels of endogenous antioxidants (which would contribute to diminishing the harmful effects of free radicals) decrease. There is also a significant reduction of molecules capable of capturing free radicals before they can attack other molecules. Both factors decrease mitochondrial defenses which then become more vulnerable [29, 30]. Oxidative damage accumulated in mitochondrial DNA and other components of the mitochondria (as well as in the cell as a whole) leads to the deterioration of mitochondria, and as a consequence of that deterioration, more free radicals are produced [24].

According to the mitochondrial theory of aging, this growing spiral of deterioration is a process of aging in itself; the number and functional state of the mitochondria determine, in a very specific way, the biologically determined lifespan of individuals. The recent research identifies this mitochondrial aberration associated with age as one of the main mechanisms in chronic inflammation [Ref]. Specifically, mitochondrial dysfunction acts as a mechanism of inflammation in the following manner [31, 32]:


These four steps represent a simplified diagram of mitochondrial dysfunction that leads to cell destruction; however, free radicals are not the only inducers of cell death by inflammation [33, 34].

Circulating carbohydrates, mainly glucose and fructose, also participate in aging processes. When these blood sugars come into contact with proteins and lipids, a harmful reaction occurs that forms compounds called advanced glycation end products (AGEs). The AGEs bind to a receptor on the surface of the cells called the PFG receptor (receptor for advanced glycation end products (RAGE)). After activation, the RAGEs induce the movement of the nuclear mediator factor kappa-B (NF-kB) to the nucleus where numerous inflammatory genes are activated. AGEs are formed mainly in vivo, and glycation is exacerbated by elevated blood glucose levels. Dietary AGE also contributes to inflammation [17, 18, 35].

Hence, molecules that protect and revitalize mitochondria could recreate a "juvenile" state of protection against free radicals (**Figure 5**) [36].

Mitochondria in the elderly are, for the most part, are dysfunctional, unlike young individuals in whom no mitochondrial damage is observed. So much so is the mitochondrial dysfunction caused by oxidative damage due to free radicals that are already a marker of aging and pathologies associated with age [37].

Mitochondria in the elderly are, for the most part, dysfunctional, unlike young individuals in whom little mitochondrial damage is observed: with the passage of time, devastating changes occur inside our cells that lead to the destruction of mitochondria and consequently trigger aging and disease [15, 28]. Production rates of superoxide anions and hydrogen peroxide (free radicals) increase significantly, specifically deteriorating the mitochondria. At the same time, the levels of endogenous antioxidants (which would contribute to diminishing the harmful effects of free radicals) decrease. There is also a significant reduction of molecules capable of capturing free radicals before they can attack other molecules. Both factors decrease mitochondrial defenses which then become more vulnerable [29, 30]. Oxidative damage accumulated in mitochondrial DNA and other components of the mitochondria (as well as in the cell as a whole) leads to the deterioration of mitochondria, and as a consequence of that deterioration, more free radicals are produced [24]. According to the mitochondrial theory of aging, this growing spiral of deterioration is a process of aging in itself; the number and functional state of the mitochondria determine, in a very specific way, the biologically determined lifespan of individuals. The recent research identifies this mitochondrial aberration associated with age as one of the main mechanisms in chronic inflammation [Ref]. Specifically, mitochondrial dysfunction acts as a mechanism of

**Figure 4.** Mitochondrial aging theory. The production of intracellular free radicals, and its effects on cellular components, is a determinant of lifespan. Mitochondria is the prime target for oxidative damage; accumulation of reactive oxygen species (ROS) leads to changes of the inner membrane permeability causing release of intramitochondrial content and

**a.** The accumulation of free radicals induces a greater permeability in the membrane of the

**b.** The molecular components normally contained within the mitochondria pass into the cell

inflammation in the following manner [31, 32]:

activating a cytoplasmic immune response leading to apoptotic cell death.

mitochondria.

160 Mitochondrial DNA - New Insights

cytoplasm.

The mitochondrial theory, which proposes that mitochondrial defects associated with age are controlled by the accumulation of mutations in mitochondrial DNA. There is, however, a growing body of contradictory evidence that has raised questions about the validity of this theory. It has been suggested that the mitochondrial defects associated with age are not controlled by the accumulation of mutations in mitochondrial DNA but by another form of genetic regulation. Contrary to the mitochondrial theory of aging, the epigenetic regulation of respiration controls defects associated with age [37].

Damage to mitochondrial DNA causes changes or mutations in the DNA sequence. The accumulation of these changes is associated with a reduced life expectancy and the early onset of characteristics related to aging such as weight and hair loss, the curvature of the spine, and osteoporosis [38]. There is, however, a growing body of contradictory evidence that has raised questions about the validity of this theory. Tsukuba's team, in particular, has made a compelling research that has led them to propose that the mitochondrial defects associated with age are not controlled by the accumulation of mitochondrial DNA mutations but by another form of genetic regulation [2, 39, 40]. The researchers compared mitochondrial respiration and the amount of DNA damage in the mitochondria, expecting that respiration would decrease and DNA damage would increase in the cells of the elderly group. The elderly group had lower respiration as the accepted theory indicates; however, there was no difference in amounts of

Incredibly, the defects associated with age had reversed: all fibroblasts had respiration rates comparable to those of the fetal fibroblast cell line, regardless of whether they were derived from the young or the elderly. This indicates that the aging process in the mitochondria is

Mitochondrial Aging and Metabolism: The Importance of a Good Relationship in the Central…

http://dx.doi.org/10.5772/intechopen.76652

163

Mitochondria are the easiest target for damage by free radicals due to two reasons:

**2.** They lack the antioxidant defenses that are present in other parts of the cell [43, 44].

There is strong evidence that the accumulated DNA damage of mitochondria is directly related to aging metabolic disorders and diseases [45]. The difference between mitochondria and other intracellular compartments is that the mitochondria have their own DNA. The production of free radicals (including superoxide anions and hydrogen peroxide) in mitochondria is a corollary to energy production (**Figure 6**). The accumulation of these by-products inside mitochondria damages their structure and their DNA. This damage is similar to that produced by ionizing radiation, and today there is an important scientific consensus that considers it as one of the main factors of aging [46]; so much so, that mitochondrial dysfunction caused by oxidative damage due to free radicals is already a marker of aging and the pathologies associated with aging, like in Alzheimer's disease, Parkinson's disease, and can-

The energy metabolism intrinsic to the maintenance of the organism and environmental factors (pollution, smoking) determine the continuous generation of oxygen radicals. These radicals produce oxidative damage to lipids, proteins, and DNA, and damaged molecules accumulate during aging [44, 46]. The deterioration secondary to aging is observed more clearly in postmitotic cells, which, when damaged, cannot be replaced by new cells, as is the case of the neuron. Although it has not been possible to demonstrate with certainty what is the role of this damage in senescence, oxidative stress would be one of the mechanisms pos-

Oxidative stress can increase with aging, both due to increased generation of oxygen radicals and by the decrease in the ability to eliminate these radicals (antioxidant mechanisms) [51]. There is still discussion regarding the apparent decrease in antioxidant mechanisms during aging [51, 52]. However, the available evidence, with respect to the maximum lifespan of individuals, suggests that the mechanisms of defense against oxidation would not be very relevant [52, 53]. The levels of antioxidant enzymes and the low molecular weight antioxidants show an inverse correlation with the maximum longevity of the animals, which indicates that pro-oxidative activity as such is the most relevant one [54]. Nor has it been found that supplementation with antioxidants (or the opposite effect, the elimination of antioxidant mechanisms) significantly modifies the maximum lifespan of an animal. In contrast, studies

controlled by epigenetic regulation, not by mutations [37].

**1.** They are exactly where free radicals are produced.

sibly involved in neurodegenerative diseases [50].

**2. Oxidative stress and aging**

cer [47–49].

**Figure 5.** AGEs. The activation of inflammatory genes is triggered by the formation of AGEs. AGEs activate NF-kB through a cascade of reactions. This image is a modification of QIAGEN's original [Torres-Sánchez ED].

DNA damage. This epigenetic regulation may be responsible for the effects associated with age that is seen in mitochondria [7, 41, 42].

To test this theory, a research reprogrammed human fibroblast cell lines derived from the young and from the old to a state similar to that of embryonic stem cells. Then, they returned these cells back to their fibroblast form, and their mitochondrial respiratory function was examined; the researchers looked for genes that could be controlled epigenetically causing these mitochondrial defects associated with age and found two that regulate the production of glycine in the mitochondria, CGAT, and SHMT2 and showed that by changing the regulation of these genes, they can produce defects or restore mitochondrial function in fibroblast cell lines. The addition of glycine for 10 days in the culture medium of the fibroblast cell line of the 97-year-old people restored its respiratory function. This suggests that glycine treatment can reverse the breathing defects associated with aging in elderly human fibroblasts. Incredibly, the defects associated with age had reversed: all fibroblasts had respiration rates comparable to those of the fetal fibroblast cell line, regardless of whether they were derived from the young or the elderly. This indicates that the aging process in the mitochondria is controlled by epigenetic regulation, not by mutations [37].
