**2. Aging and vessels**

Cellular senescence is the state in which normal cells cease to divide. It can be thought of as a type of programmed cell cycle arrest. The senescent cells go through changes in gene expression and secretion of soluble factors in response to excessive stresses [25]. The secreted soluble factors from senescent cells are referred to as the SASP [26–28]. The SASP includes interleukins (IL) and chemokines such as IL-1, IL-6, IL-8, monocyte chemoattractant protein (MCP)-2, and macrophage inflammatory protein (MIP)-1 [29]. In addition, nitric oxide, growth factors, and several matrix metalloproteinases have also been identified in the SASP [14]. Several studies have reported that SASP factors have roles in inducing normal cells to acquire a senescent phenotype and can act in a paracrine manner to affect the activity of other nearby cells [30]. This might suggest that SASP factors do not merely arise as a result of cell senescence but could rather act to promote the senescence phenotype in normal cells.

Although it still remains equivocal whether neurodegenerative diseases arise as a result of, or from, aging-related changes, it is clear, however, that the prevalence of neurodegenerative diseases that show a cognitive decline is positively associated with aging. Increases in oxidative stress and inflammation in response to aging trigger cellular senescence and the appropriate downstream responses. A series of these insults can aggravate age-related or neurodegenerative pathogenesis with several vascular diseases [31].

#### **2.1 Endothelium**

The endothelium is one of the vessel constituents and is important in vascular structure because of its high versatility. The endothelium has multifunctional

**5**

*Aging, Cerebrovascular Burden, and Cognitive Decline DOI: http://dx.doi.org/10.5772/intechopen.89854*

endothelial NO synthase (eNOS) [35, 36].

in immune homeostasis.

**2.2 Atherosclerosis**

roles in maintaining blood fluidity and metabolic homeostasis. The features are important in regulating the delivery of water and nutrients through the whole body [32]. The endothelium also plays an important role in the resolution of inflammatory responses. Morphological and functional changes in the endothelium are also involved in the development of numerous pathological disorders [33, 34]. Endothelial cells are located in the inner layer of the vasculature and can come into contact with blood-containing macrophages. Therefore, endothelial cells are the first target of cytokines circulating in the blood. Simultaneously, endothelial cells also secrete pro-inflammatory cytokines, such as IL-1, IL-6, IL-8, IL-15, MCP-1, and tumor necrosis factor (TNF)-α [29]. In fact, endothelial cell-derived pro-inflammatory cytokines and chemokines are important in recruiting immune cells to the site and play a role in wound healing, angiogenesis, and inflammatory diseases. In addition to their inflammatory function, endothelial cells also produce pro-thrombotic mediators and cellular adhesion molecules (ICAM-1, intercellular adhesion molecule-1; PAI-1, plasminogen activator inhibitor-1) and enhance the adhesion and transmigration of immune cells into underlying tissues. It has also been reported that senescent endothelial cells have reduced expression levels of nitric oxide (NO) as well as reduced expression levels and phosphorylation of

It is well accepted that senescence in vascular endothelial cells is a common factor in various age-related diseases. Associated with aging, endothelial cells dynamically change the inflammatory phenotype to SASP [21]. A recent study has demonstrated that alterations in toll-like receptor (TLR) and TLR ligands during aging might determine the state of the inflammatory process [29]. Since endothelial cells are not immune cells, they cannot recognize foreign antigens via TLRs or present antigens through MHC II molecules to T and B cells [37]. On the other hand, endothelial cells have been shown to upregulate the mRNA levels of TLR-2, TLR-4, TLR-7, TLR-8, TLR-9, and TLR-10 during aging [38]. Since aging is a chronic inflammatory condition in the lower level, nonimmune endothelial cells could play a role in controlling and maintaining immune homeostasis. Taken these reasons, endothelial cells could be a therapeutic target to allow for recovery of a disruption

Endothelial cells derived from patients with severe coronary artery disease have also been shown to be mostly senescent endothelial cells with reduced levels of telomeric DNA-binding factor 1 (TRF1) and increased telomere oxidation [20]. When TRF1 was overexpressed in a cellular aging model, namely, human umbilical vein endothelial cells' (HUVECs) passage for a long period of time, the telomere-associated DNA damage foci and the SASP were decreased [39, 40]. These results suggest that telomere dysfunction and the SASP occur prior to cellular senescence and induce vascular dysfunction following cardiovascular disease (CVD) development.

Atherosclerosis and its associated clinical outcomes such as vascular stiffness are initiated and progressed through dysfunction in senescent endothelial cells [29]. During oxidation in the vessel, there are changes in its physicochemical properties including lipid charge, size, and content. Oxidized low-density lipid (oxLDL) becomes different from natural LDL. The oxLDL stimulates endothelial cells to induce the expression of adhesion molecules such as E-selectin and vascular cell adhesion molecule-1 (VCAM-1) on the surface of the artery [41]. Numerous senescent endothelial cells are detected in the aorta of the human with atherosclerotic plaques [42, 43]. The formation of atherosclerotic plaques is initiated by macrophages that infiltrate into the arterial intima in response to oxLDL present in

#### *Aging, Cerebrovascular Burden, and Cognitive Decline DOI: http://dx.doi.org/10.5772/intechopen.89854*

*New Insight into Cerebrovascular Diseases - An Updated Comprehensive Review*

both a current and a future social and economic burden.

lar damage-derived cognitive impairment.

**2. Aging and vessels**

in normal cells.

vascular diseases [31].

**2.1 Endothelium**

are increasing. These diseases have become important social concerns and represent

Cellular senescence is the state in which normal cells cease to divide. It can be thought of as a type of programmed cell cycle arrest. The senescent cells go through changes in gene expression and secretion of soluble factors in response to excessive stresses [25]. The secreted soluble factors from senescent cells are referred to as the SASP [26–28]. The SASP includes interleukins (IL) and chemokines such as IL-1, IL-6, IL-8, monocyte chemoattractant protein (MCP)-2, and macrophage inflammatory protein (MIP)-1 [29]. In addition, nitric oxide, growth factors, and several matrix metalloproteinases have also been identified in the SASP [14]. Several studies have reported that SASP factors have roles in inducing normal cells to acquire a senescent phenotype and can act in a paracrine manner to affect the activity of other nearby cells [30]. This might suggest that SASP factors do not merely arise as a result of cell senescence but could rather act to promote the senescence phenotype

Although it still remains equivocal whether neurodegenerative diseases arise as a result of, or from, aging-related changes, it is clear, however, that the prevalence of neurodegenerative diseases that show a cognitive decline is positively associated with aging. Increases in oxidative stress and inflammation in response to aging trigger cellular senescence and the appropriate downstream responses. A series of these insults can aggravate age-related or neurodegenerative pathogenesis with several

The endothelium is one of the vessel constituents and is important in vascular structure because of its high versatility. The endothelium has multifunctional

In the aging process, age-related cerebrovascular dysfunction results from multiple pathophysiological changes. The first of them is oxidative stress and inflammation. Excessive oxidative stress is well known to contribute to vascular aging in both animals [4, 7–10] and humans [11, 12]. It is an overproduction of reactive oxygen species (ROS) rather than accumulation of ROS. In cellular senescence, inflammatory mediators such as chemokines and cytokines are secreted in an autocrine or paracrine manner. This is often referred to as the senescence-associated secretory phenotype (SASP) [13–15]. The second is the narrowing of the vascular lumen caused by atherosclerotic plaques, which is referred to as atherosclerosis in large vessels or arteriosclerosis in small vessels [16–18]. Technically a further factor is endothelial cell senescence induced by the SASP in the aging process. This arises as a result of the action of pro-inflammatory cytokines such as interleukin (IL)-1 and IL-6 [19]. Under aging, atherosclerotic plaques are prone to arise in the human aorta and coronary arteries, which contain senescent endothelial cells [20]. The senescent phenotypes of endothelial cells can be physiologically classified as having either an anti-inflammatory phenotype or a pro-inflammatory senescent phenotype [21]. Recently, numerous studies have focused on the status of the immune system during aging [22–24]. This chapter introduces the current knowledge about cognitive decline according to cerebrovascular aging relevant to cellular senescence and the changes in the SASPs. It also provides approaches on how senescent vessels exposed to the SASP enhance age-related cerebrovascular degeneration and vascu-

**4**

roles in maintaining blood fluidity and metabolic homeostasis. The features are important in regulating the delivery of water and nutrients through the whole body [32]. The endothelium also plays an important role in the resolution of inflammatory responses. Morphological and functional changes in the endothelium are also involved in the development of numerous pathological disorders [33, 34]. Endothelial cells are located in the inner layer of the vasculature and can come into contact with blood-containing macrophages. Therefore, endothelial cells are the first target of cytokines circulating in the blood. Simultaneously, endothelial cells also secrete pro-inflammatory cytokines, such as IL-1, IL-6, IL-8, IL-15, MCP-1, and tumor necrosis factor (TNF)-α [29]. In fact, endothelial cell-derived pro-inflammatory cytokines and chemokines are important in recruiting immune cells to the site and play a role in wound healing, angiogenesis, and inflammatory diseases. In addition to their inflammatory function, endothelial cells also produce pro-thrombotic mediators and cellular adhesion molecules (ICAM-1, intercellular adhesion molecule-1; PAI-1, plasminogen activator inhibitor-1) and enhance the adhesion and transmigration of immune cells into underlying tissues. It has also been reported that senescent endothelial cells have reduced expression levels of nitric oxide (NO) as well as reduced expression levels and phosphorylation of endothelial NO synthase (eNOS) [35, 36].

It is well accepted that senescence in vascular endothelial cells is a common factor in various age-related diseases. Associated with aging, endothelial cells dynamically change the inflammatory phenotype to SASP [21]. A recent study has demonstrated that alterations in toll-like receptor (TLR) and TLR ligands during aging might determine the state of the inflammatory process [29]. Since endothelial cells are not immune cells, they cannot recognize foreign antigens via TLRs or present antigens through MHC II molecules to T and B cells [37]. On the other hand, endothelial cells have been shown to upregulate the mRNA levels of TLR-2, TLR-4, TLR-7, TLR-8, TLR-9, and TLR-10 during aging [38]. Since aging is a chronic inflammatory condition in the lower level, nonimmune endothelial cells could play a role in controlling and maintaining immune homeostasis. Taken these reasons, endothelial cells could be a therapeutic target to allow for recovery of a disruption in immune homeostasis.

Endothelial cells derived from patients with severe coronary artery disease have also been shown to be mostly senescent endothelial cells with reduced levels of telomeric DNA-binding factor 1 (TRF1) and increased telomere oxidation [20]. When TRF1 was overexpressed in a cellular aging model, namely, human umbilical vein endothelial cells' (HUVECs) passage for a long period of time, the telomere-associated DNA damage foci and the SASP were decreased [39, 40]. These results suggest that telomere dysfunction and the SASP occur prior to cellular senescence and induce vascular dysfunction following cardiovascular disease (CVD) development.

## **2.2 Atherosclerosis**

Atherosclerosis and its associated clinical outcomes such as vascular stiffness are initiated and progressed through dysfunction in senescent endothelial cells [29]. During oxidation in the vessel, there are changes in its physicochemical properties including lipid charge, size, and content. Oxidized low-density lipid (oxLDL) becomes different from natural LDL. The oxLDL stimulates endothelial cells to induce the expression of adhesion molecules such as E-selectin and vascular cell adhesion molecule-1 (VCAM-1) on the surface of the artery [41]. Numerous senescent endothelial cells are detected in the aorta of the human with atherosclerotic plaques [42, 43]. The formation of atherosclerotic plaques is initiated by macrophages that infiltrate into the arterial intima in response to oxLDL present in

the vessel [69]. Lipid-laden foam cells become pro-inflammatory, and macrophages secrete pro-inflammatory cytokines. The recruited macrophages move into the artery, and consequently, the atherosclerotic plaque size and complexity are getting larger [44].

Atherosclerosis occurs following chronic exposure to cellular stressors. It has been reported that SASP can be restrained simply by inhibiting TNF-α without any transmission of senescence signals in an autocrine or paracrine manner [45]. Anti-TNF-α treatment has been demonstrated to physiologically reduce the noncell autonomous effects of SASP. In patients treated with adalimumab (Humira, a human monoclonal antibody against TNF-α), epigenetic modifications were triggered. The molecular mechanisms are identified as TNF-α signaling in senescent endothelial cells, which raises the possibility of therapeutic approaches for ageassociated diseases. In senescent human umbilical vein endothelial cells (HUVECs), the levels of miR-146a-5p and miR-126-3p show higher than younger HUVECs. Additionally, the levels of miR-146a-5p are increased in both senescent and young HUVECs following lipopolysaccharide (LPS) exposure, whereas the level of miR-126-3p is decreased only in senescent HUVECs and is unchanged in young HUVECs [45]. This study also showed that changes in the levels of several microRNAs (miR-NAs) do not arise as a result of just treatment with LPS or an anti-TNF-α antibody. These data suggest that some miRNAs work in an age-dependent manner in vessels and endothelial cells.

#### **2.3 Vascular aging**

From a classical perspective, the aged vasculature is viewed as having worsened vasodilation, arterial stiffness, remodeling of the extracellular matrix, intimal thickening, and endothelial cell dysfunction [46]. The effects of vascular aging have been explored extensively and have been attributed to the number of different causes with genome instability and mTOR being two of the major causes. Studies with mTOR-inhibiting drugs in vessels have shown that such drugs have a deleterious effect on endothelial function in patients who have advanced arterial aging and inserted coronary stents [47–49]. Compared to human studies, cultured endothelial cells have augmented anti-inflammatory effects to mTOR inhibitors with a dosedependent manner and have increased cytostatic effect [50]. It also showed that rapamycin, an mTOR inhibitor, induces the expression of PAI-1 in mice as well as in cultured endothelial cells [51]. When the Atg7 gene, one of the mediators of the autophagic process, is deleted in mice, vascular aging is accelerated [52]. In a mouse model of Hutchinson-Gilford progeria, autophagy is activated by AMPK activation and inhibition of mTOR [53]. These results showed that the mTOR–AMPK signaling pathway might be a link to the regulation of autophagy in age-related diseases. Based on the results described above, it is hard to firmly establish that rapamycin plays a preventive role on cellular senescence.

Telomere shortening is known to be a hallmark of cellular senescence. Telomeres are significantly shorter in several endothelial cells and vascular smooth muscle cells (VSMCs), which is a clear marker of SASP in aged vasculature. The shortened telomeres in mouse vascular tissue have been shown to be sufficient to induce endothelial dysfunction [54], whereas human VSMCs still have a normal phenotype sustaining plaque stability regardless of telomere length [55]. Although telomere shortening is common in vascular aging and CVD, it remains unclear whether telomere shortening is sufficient to lead to cellular senescence and vascular degeneration in aged vessels [56].

In the patients with severe coronary artery disease, most of endothelial cells demonstrated the features of senescent endothelial cells that reduced levels of

**7**

*Aging, Cerebrovascular Burden, and Cognitive Decline DOI: http://dx.doi.org/10.5772/intechopen.89854*

type 2 diabetes, and obesity [59–62].

sues during the aging process [63].

**3. Aging and cerebrovascular function**

pericytes, and the astrocyte end feet [67].

the following subsections.

**3.1 Cerebral vasculature**

CVD development.

marized as follows:

telomeric DNA-binding factor 1 (TRF1) and increased telomere oxidation as mentioned above [20, 39, 40]. These results suggest that telomere dysfunction and the SASP occur prior to cellular senescence and induce vascular dysfunction following

The causes of vascular aging and vascular aging-related phenotypes can be sum-

i.Oxidative DNA damage: during aging, the onset of CVD led by changes in vascular endothelial cells results in an impairment of endotheliumdependent vasodilation, an overproduction of pro-inflammatory and prothrombotic factors, and an increase in oxidative stress [57]. In the human population, age is an important and independent risk factor for CVD [58].

ii.Telomere shortening: an association between telomere shortening and CVD risk factors has been found in atherosclerosis, arterial stiffness, type 1 and

iii.Genome instability: a failure in DNA damage repair occurs in cells and tis-

As aging progresses, cerebrovascular function declines which can increase the possibility for ischemic stroke, intracerebral hemorrhages (ICHs), microbleeds, and cognitive decline [64]. Healthy functional cerebral vessels can coordinate with CBF and appropriately supply blood to the brain [65]. However, cerebrovascular aging has the following features, which can lead to age-related cerebrovascular diseases: (i) endothelial senescence, (ii) oxidative stress and inflammation, (iii) microvascular rarefaction, (iv) arterial stiffness, (v) vascular lumen narrowness, and (vi) CBF reduction. Each feature according to cerebrovascular aging is described in detail in

Cerebral vessels play a critical role in mediating between the whole body and the brain by transporting molecules between the blood and brain [5]. The brain vasculature that supplies blood to the brain tissue consists of two blood supply systems. One is the internal carotid artery system. This system is responsible for approximately 70% of the total CBF. The other system is the vertebral artery system, which is responsible for approximately 30% of the total CBF. These two major blood systems converge at Willis' circle, which allows communication between the left and right brain hemispheres, and branch out into the whole brain through cerebral arteries [66]. The most common structural feature of cerebral vessels is the blood–brain barrier (BBB), which comprises tight junctions and adherens junctions [5]. These tight junctions exist between endothelial cells, the basal membrane,

The BBB is important because it prevents harmful molecules from entering the brain tissue from the systemic circulation. Accordingly, a malfunction in BBB permeability has been reported in neurodegenerative disorders and cognitive decline that leads to dementia [68–70]. CBF is regulated in response to blood pressure through cerebral autoregulation. It was demonstrated that there were minor differences between CBF and blood pressure (within 10 mmHg of blood pressure)

#### *Aging, Cerebrovascular Burden, and Cognitive Decline DOI: http://dx.doi.org/10.5772/intechopen.89854*

*New Insight into Cerebrovascular Diseases - An Updated Comprehensive Review*

larger [44].

and endothelial cells.

**2.3 Vascular aging**

plays a preventive role on cellular senescence.

eration in aged vessels [56].

the vessel [69]. Lipid-laden foam cells become pro-inflammatory, and macrophages secrete pro-inflammatory cytokines. The recruited macrophages move into the artery, and consequently, the atherosclerotic plaque size and complexity are getting

Atherosclerosis occurs following chronic exposure to cellular stressors. It has been reported that SASP can be restrained simply by inhibiting TNF-α without any transmission of senescence signals in an autocrine or paracrine manner [45]. Anti-TNF-α treatment has been demonstrated to physiologically reduce the noncell autonomous effects of SASP. In patients treated with adalimumab (Humira, a human monoclonal antibody against TNF-α), epigenetic modifications were triggered. The molecular mechanisms are identified as TNF-α signaling in senescent endothelial cells, which raises the possibility of therapeutic approaches for ageassociated diseases. In senescent human umbilical vein endothelial cells (HUVECs), the levels of miR-146a-5p and miR-126-3p show higher than younger HUVECs. Additionally, the levels of miR-146a-5p are increased in both senescent and young HUVECs following lipopolysaccharide (LPS) exposure, whereas the level of miR-126-3p is decreased only in senescent HUVECs and is unchanged in young HUVECs [45]. This study also showed that changes in the levels of several microRNAs (miR-NAs) do not arise as a result of just treatment with LPS or an anti-TNF-α antibody. These data suggest that some miRNAs work in an age-dependent manner in vessels

From a classical perspective, the aged vasculature is viewed as having worsened

Telomere shortening is known to be a hallmark of cellular senescence. Telomeres

are significantly shorter in several endothelial cells and vascular smooth muscle cells (VSMCs), which is a clear marker of SASP in aged vasculature. The shortened telomeres in mouse vascular tissue have been shown to be sufficient to induce endothelial dysfunction [54], whereas human VSMCs still have a normal phenotype sustaining plaque stability regardless of telomere length [55]. Although telomere shortening is common in vascular aging and CVD, it remains unclear whether telomere shortening is sufficient to lead to cellular senescence and vascular degen-

In the patients with severe coronary artery disease, most of endothelial cells demonstrated the features of senescent endothelial cells that reduced levels of

vasodilation, arterial stiffness, remodeling of the extracellular matrix, intimal thickening, and endothelial cell dysfunction [46]. The effects of vascular aging have been explored extensively and have been attributed to the number of different causes with genome instability and mTOR being two of the major causes. Studies with mTOR-inhibiting drugs in vessels have shown that such drugs have a deleterious effect on endothelial function in patients who have advanced arterial aging and inserted coronary stents [47–49]. Compared to human studies, cultured endothelial cells have augmented anti-inflammatory effects to mTOR inhibitors with a dosedependent manner and have increased cytostatic effect [50]. It also showed that rapamycin, an mTOR inhibitor, induces the expression of PAI-1 in mice as well as in cultured endothelial cells [51]. When the Atg7 gene, one of the mediators of the autophagic process, is deleted in mice, vascular aging is accelerated [52]. In a mouse model of Hutchinson-Gilford progeria, autophagy is activated by AMPK activation and inhibition of mTOR [53]. These results showed that the mTOR–AMPK signaling pathway might be a link to the regulation of autophagy in age-related diseases. Based on the results described above, it is hard to firmly establish that rapamycin

**6**

telomeric DNA-binding factor 1 (TRF1) and increased telomere oxidation as mentioned above [20, 39, 40]. These results suggest that telomere dysfunction and the SASP occur prior to cellular senescence and induce vascular dysfunction following CVD development.

The causes of vascular aging and vascular aging-related phenotypes can be summarized as follows:

