**1. Introduction: history of vascular dysfunction in Alzheimer's disease**

Alzheimer's disease (AD) presents itself as a progressive neurological disorder, which is the major cause of dementia leading to death in the elderly. It affects thinking, orientation and memory, causing impairmentincognition, social behaviour andmotivation[1].Approximately 47.5 million people worldwide have dementia, of which the most common contributor is AD with 60–70% [1]. In 2010, the total global societal costs were estimated to be US \$604 billion corresponding to 1.0% of the worldwide gross domestic product [1].

In 1906, Dr. Alois Alzheimer [2] noted two microscopic neuropathological findings, which were further characterized and eventually established as the hallmarks of AD: senile neuritic plaques, which are aggregates that are primarily composed of beta‐amyloid (Aβ) peptides; [3, 4] and neurofibrillary tangles, which are primarily composed of intra‐neuronal hyperphos‐ phorylated tau aggregates [5]. Aβ, a 4 kDa peptide, is a proteolytic cleavage product of the amyloid precursor protein (APP) by the action of α and γ secretase enzymes [6, 7]. Mutations either in the *APP* gene or in the secretase enzyme complex lead to a β secretase cleavage, forming a pathogenic Aβ species (Aβ1‐42). These Aβ molecules aggregate to form oligomers, which multimerize into protofibrils, followed by the formation of dense core amyloid plaques [8–10].

## **1.1. Initial clinical observations linking AD and vascular disease**

Post‐mortem analysis has established that 50–84% of the brains of persons, who die aged 80– 90+ years, show appreciable cerebrovascular lesions and although there is a debate around their impact on AD pathology, it is suggested that the independent dementia caused by vascular and AD‐type pathologies may have additive or synergistic effect on cognitive impairment [11]. Vascular pathologies that have been seen in the aged human brain include: cerebral amyloid angiopathy (CAA), cerebral atherosclerosis, small vessel disease in most cases caused by hypertensive vasculopathy or microvascular degeneration, blood‐brain barrier (BBB) dysfunction causing white matter lesions, microinfarctions, lacunar infarcts and microbleeds [11]. Studies in post‐mortem of human brains also found evidence of increased angiogenesis in the hippocampus, midfrontal cortex, substantia nigra pars compacta, and locus coeruleus of AD brains compared to control brains suggesting that vascular dysfunction is an inherent part of AD pathology [12, 13].

## **1.2. Genetic risk factors linking AD and vascular disease**

Epidemiological studies have identified risk factors for AD that are similar to those for cardiovascular disease (CVD) [14] such as hypertension during midlife, diabetes mellitus, smoking, apolipoprotein E (APOE) 4 isoforms, hypercholesterolemia, homocysteinemia and, in particular, age [1]. Familial AD is caused most commonly by presenilin 1 (*PSEN1*) or presenilin 2 (*PSEN2*) mutations. It is also seen that the presenilins are expressed in the heart and are critical to cardiac development. The work by Li *et al.* indicated that *PSEN1* and *PSEN2* mutations are associated with dilated cardiomyopathy (DCM) and heart failure and implicate novel mechanisms of myocardial disease [15]. Amyloid is a known vasculotrope and an increased amyloid aggregation in AD brains is seen to be in interaction with the angiogenic and CAA positive vessels [14]. Apolipoprotein 3 (APOE3) is responsible for normal lipid metabolism; however, the APOE4 isoforms is strongly associated with the late onset of AD [13]. Carriers of this isoform show a decreased cerebral blood flow and have also been linked to disorders associated with elevated cholesterol levels or lipid derangements (*i.e.* hyperlipopro‐ teinemia type III, coronary heart disease, strokes, peripheral artery disease and diabetes mellitus) [15]. These overlapping genetic risk factors might give us a direction for understand‐ ing the mechanisms of the disease‐related pathways.

**1. Introduction: history of vascular dysfunction in Alzheimer's disease**

corresponding to 1.0% of the worldwide gross domestic product [1].

94 Physiologic and Pathologic Angiogenesis - Signaling Mechanisms and Targeted Therapy

**1.1. Initial clinical observations linking AD and vascular disease**

inherent part of AD pathology [12, 13].

**1.2. Genetic risk factors linking AD and vascular disease**

[8–10].

Alzheimer's disease (AD) presents itself as a progressive neurological disorder, which is the major cause of dementia leading to death in the elderly. It affects thinking, orientation and memory, causing impairmentincognition, social behaviour andmotivation[1].Approximately 47.5 million people worldwide have dementia, of which the most common contributor is AD with 60–70% [1]. In 2010, the total global societal costs were estimated to be US \$604 billion

In 1906, Dr. Alois Alzheimer [2] noted two microscopic neuropathological findings, which were further characterized and eventually established as the hallmarks of AD: senile neuritic plaques, which are aggregates that are primarily composed of beta‐amyloid (Aβ) peptides; [3, 4] and neurofibrillary tangles, which are primarily composed of intra‐neuronal hyperphos‐ phorylated tau aggregates [5]. Aβ, a 4 kDa peptide, is a proteolytic cleavage product of the amyloid precursor protein (APP) by the action of α and γ secretase enzymes [6, 7]. Mutations either in the *APP* gene or in the secretase enzyme complex lead to a β secretase cleavage, forming a pathogenic Aβ species (Aβ1‐42). These Aβ molecules aggregate to form oligomers, which multimerize into protofibrils, followed by the formation of dense core amyloid plaques

Post‐mortem analysis has established that 50–84% of the brains of persons, who die aged 80– 90+ years, show appreciable cerebrovascular lesions and although there is a debate around their impact on AD pathology, it is suggested that the independent dementia caused by vascular and AD‐type pathologies may have additive or synergistic effect on cognitive impairment [11]. Vascular pathologies that have been seen in the aged human brain include: cerebral amyloid angiopathy (CAA), cerebral atherosclerosis, small vessel disease in most cases caused by hypertensive vasculopathy or microvascular degeneration, blood‐brain barrier (BBB) dysfunction causing white matter lesions, microinfarctions, lacunar infarcts and microbleeds [11]. Studies in post‐mortem of human brains also found evidence of increased angiogenesis in the hippocampus, midfrontal cortex, substantia nigra pars compacta, and locus coeruleus of AD brains compared to control brains suggesting that vascular dysfunction is an

Epidemiological studies have identified risk factors for AD that are similar to those for cardiovascular disease (CVD) [14] such as hypertension during midlife, diabetes mellitus, smoking, apolipoprotein E (APOE) 4 isoforms, hypercholesterolemia, homocysteinemia and, in particular, age [1]. Familial AD is caused most commonly by presenilin 1 (*PSEN1*) or presenilin 2 (*PSEN2*) mutations. It is also seen that the presenilins are expressed in the heart and are critical to cardiac development. The work by Li *et al.* indicated that *PSEN1* and *PSEN2* mutations are associated with dilated cardiomyopathy (DCM) and heart failure and implicate novel mechanisms of myocardial disease [15]. Amyloid is a known vasculotrope and an

#### **1.3. Factors linked to AD and increased angiogenesis: melanotransferrin (p97), VEGF, transglutaminases (factor XIIIa and tTG)**

Melanotransferrin (also known as p97 or melanoma tumour antigen) is a member of the transferrin family and is responsible for the cellular uptake of iron. P97 was shown to be present in the capillary endothelium in a normal brain, in contrast to the brain from patients with AD, where it is found to be localized in microglia cells, associated with senile plaques [16, 17]. Serum normally contains very low levels of p97; however, it is reported to increase by five‐ and six‐fold in patients with AD [18, 19]. From this observation, it was proposed that serum p97 could be a potential biochemical marker for this disease. It was further demonstrated that melanotransferrin exerts an angiogenic response quantitatively similar to that elicited by fibroblast growth factor 2 [20], and hypervascularity has been shown to be a feature in the brains of AD patients [12]. Overexpression of vascular endothelial growth factor (VEGF) receptor 2 was observed in newly formed vessels, suggesting that the angiogenic activity of melanotransferrin may depend on activation of endogenous VEGF [20]. VEGF is the major player in pathological/dysfunctional blood vessel formation. It is shown that VEGF is highly up‐regulated in AD brains via the inflammatory pathway and also that VEGF co‐aggregates with Aβ in AD brains [13]. The role of transglutaminases in AD is highly debated; however, it is shown that the activity of these enzymes might contribute to both angiogenesis and in the formation of protein aggregates in the AD brain [21, 22].
