**2. The implications of metabolic syndrome on brain mitochondria and its association with the development of AD:** *in vivo* **studies and clinical studies**

### **2.1. MetS condition from a high-fat diet-induced obese-insulin-resistant model**

resistance is the main contributor toward MetS. Insulin resistance is a pathological condition, in which target tissues cannot take up glucose into the cells at the physiological insulin level. It is characterized by hyperinsulinemia with euglycemia. MetS is often represented by an obese-insulin-resistant condition. It can lead to the development of not only cardiovascular diseases but also stroke [4] and neurodegeneration [5]. In addition, data from clinical trials have indicated that hyperinsulinemia during insulin resistance is related to cognitive decline in elderly adults [6, 7]. MetS has been induced in several animal models to enable the investigation of the mechanisms responsible for the adverse effects of the MetS condition on cognitive impairment. MetS has been induced in animal models by using high-fat/high-calorie diet consumption. Interestingly, previous studies have investigated the effects of long-term high-fat diet (HFD) consumption on metabolic and brain dysfunction [8, 9]. Those data demonstrated that the consumption of a HFD for 8 weeks caused obese-insulin resistance or MetS, as indicated by central obesity, hyperinsulinemia, dyslipidemia, and raised blood pressure [8, 9]; however, cognitive impairment and brain insulin resistance were observed later at the end of 12 weeks of HFD consumption [8, 10]. Those findings suggest that the metabolic dis-

Pre-Alzheimer's disease or mild cognitive impairment (MCI) is a condition of memory decline but does not significantly affect the normal function of a person's life [11]; however, Alzheimer's disease (AD) is an irreversible chronic neurodegenerative disease and it is the most common type of dementia [12]. The presence of neurofibrillary tangles and amyloid beta deposition in the brain is hallmarks of AD [12]. Recent studies have shown that the incidence of AD has increased in MetS subjects [13–15]. Those findings suggest that there is a possible connection in the pathogenesis between MetS and AD. Data from a clinical study suggest that oxidative

Mitochondria are known as the major source of oxidative stress [16]. Brain mitochondrial dysfunction was observed in several pathological conditions, including MetS and AD [17–22]. That dysfunction causes increased oxidative stress [10] and leads to the activation of several stress kinases [19]. Subsequently, a raised oxidative stress impaired brain insulin receptor function [23], inhibited insulin-degrading enzymes and increased beta-secretase activity [23, 24], resulting in increased hyperphosphorylated tau and amyloid beta deposition in the brain [19]. Therefore, brain mitochondrial dysfunction could be an important feature in AD pathogenesis in the MetS condition. Furthermore, the elevation of oxidative stress caused the imbalance of brain mitochondrial dynamics [25]. Mitochondrial dynamics are a key process for the maintenance of cell life and death through the balancing of mitochondrial fission and fusion [26]. In the physiological status of the brain, mitochondrial dynamics enables mitochondria to recruit subcellular components, exchange substrates between mitochondria, and control mitochondrial shape [26]. Recently, it has been proposed that brain mitochondrial dynamic imbalance is another mechanism that is involved in the brain pathogenesis of MetS and AD [27, 28]. Examples from the recent research are as follows: (1) several studies have reported that levels of Dynamin-related protein 1 (Drp1) and mitochondrial fission 1 (Fis1), markers of mitochondrial fission, were increased in the brains of MetS and AD animals [29, 30], leading to neuronal apoptosis [29]; (2) mitochondrial fusion protein levels were decreased in the brains of both MetS and AD animals [29, 30]. Therefore, a mitochondrial dynamic imbalance

stress is a key component that regulates the development of AD in MetS subjects [15]

may play an important role in cognitive dysfunction in MetS and AD [26, 30]

turbance preceded cognitive dysfunction in induced MetS

60 Alzheimer's Disease - The 21st Century Challenge

Obese-insulin resistance is characterized by body weight gain and peripheral insulin insensitivity [20–22, 31–34]. These characteristics are similar to those seen in the MetS condition in humans. In addition to peripheral insulin resistance, brain insulin resistance has also been reported in the obese condition in rats [20–22, 31, 34]. A diet containing 60% E from fat is considered to be a high-fat diet (HFD), and it has been widely used to induce obese-insulin resistance in rodents [20–22, 31–33]. In some studies, it has been found that HFD consumption increased plasma cholesterol and free fatty acid levels [20–22, 31, 32, 34]. However, the plasma glucose level was not increased, but hyperinsulinemia was observed following HFD consumption even after long-term consumption of a HFD (12 months), indicating a pre-diabetic state [20, 31–34].

HFD consumption between 16 weeks and 12 months caused brain mitochondrial damage, including an increased mitochondrial ROS production [20–22, 31, 34], a reduced mitochondrial membrane potential [19, 31, 34–36], and an impaired mitochondrial morphology as indicated by an increased mitochondrial swelling [20–22, 31, 33]. Furthermore, HFD reduced adenosine triphosphate (ATP) production [34]. Although several studies suggested that HFD caused brain mitochondrial dysfunction, Jorgensen et al. reported that HFD did not impair brain mitochondrial function even when the rats were given a HFD for 12 months. Therefore, the effects of a HFD on brain mitochondrial function still need to be elucidated.

There are several studies which have shown that brain mitochondrial damage could impair cognitive function and synaptic plasticity [20–22, 31, 33, 34]. Various cognitive tests have been used such as the Morris water maze (MWM), novel object recognition (NOR), novel object smelling (NOS), and Y-maze test. The MWM and Y-maze are tests for hippocampal-dependent learning process, including the acquisition of spatial memory and long-term spatial memory [36]. NOR and NOS are used to assess non-force driving and spontaneous memory [35, 37].

Rats and mice fed on a HFD for 16–20 weeks had an increased time to reach the platform and a decreased time in the target quadrant and crossing target number, compared with normal diet (ND)-fed animals, when cognitive function was assessed using the MWM [20–22, 31, 34]. Furthermore, recognition index was decreased in HFD-fed mice, compared to ND-fed mice [34]. Mice fed on a HFD for 12 months did not indicate an impaired discrimination index following the NOS test, but there were decreased percentage correction alterations in the Y-maze test [33]. These accumulative data suggested that the consumption of a HFD caused obese-insulin resistance, brain mitochondrial dysfunction, and synaptic dysplasticity, possibly leading to cognitive dysfunction. However, no study has demonstrated brain mitochondrial dysfunction with elevated AD markers such as Aβ levels and hyperphosphorylated tau in HFD-fed animals. This suggests that obese-insulin resistance can lead to the development of brain mitochondrial dysfunction and cognitive impairment or MCI or pre-AD without AD symptoms. Data regarding the effects of HFD-induced obese-insulin resistance on brain mitochondria and its association with the development of AD are shown in **Table 1** and are summarized in **Figure 1**.


**Table 1.** Implications of obese-insulin resistance on brain mitochondria and its association with the development of Alzheimer's disease.

**2.2. Type 2 diabetes mellitus model**

of Alzheimer's disease in non-AD and AD models.

caused hyperglycemia in rodents [38

an increased brain oxidative stress.

insulin resistance [38

Type 2 diabetes mellitus (T2DM) is diagnosed when hyperglycemia is observed along with

**Figure 1.** The effects of insulin resistance and T2DM on brain mitochondria and their association with the development

Beside the effects of T2DM on oxidative stress and mitochondrial membrane depolarization, nuclear respiratory factor 2 (NRF2) levels were reduced in the brains of T2DM mice [38]. NRF2 acts as an antioxidant and detoxifying enzyme and helps to reduce oxidative stress in mitochondria [42]. Therefore, a decreased level of NRF2 directly impairs the brain mitochon

drial redox system, which leads to the reduction of brain mitochondrial antioxidant capacity. In addition, a previous study showed a decrease in brain mitochondrial numbers in T2DM mice [41]. The possible explanation may be due to a decrease in NRF2 in the brain of T2DM mice, in which NRF2 regulates brain mitochondrial biogenesis [43]. These data indicate that T2DM caused brain mitochondrial damage and brain mitochondrial dysfunction, resulting in

Consistent with the findings from obese-insulin-resistant animals, T2DM animals also devel

oped brain mitochondrial dysfunction with cognitive impairment, quantified using the MWM test, as indicated by an increased escape latency and time in target quadrant and a decreased crossing target number [39, 41]. Also, T2DM rats had decreased percentage cor

rection alterations and total distance, when the abilities of these animals were investigated using the Y-maze test [39]. T2DM also affected brain synaptic plasticity proteins, as indicated by reducing postsynaptic density protein 95 (PSD95) and synaptosomal-associated protein 25 (SNAP25) levels [38]. However, T2DM did not affect synaptophysin protein levels [38].

with low-dose streptozotocin, and a high-calorie diet were used [38

animals are also found to develop brain mitochondrial damage [38

–41]. In order to create a T2DM animal model, a combination of a HFD

–41]. Similar to obese-insulin-resistant models, T2DM

Mitochondrial Link Between Metabolic Syndrome and Pre-Alzheimer's Disease

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63

–41].

–41]. Both regimens




Mitochondrial Link Between Metabolic Syndrome and Pre-Alzheimer's Disease http://dx.doi.org/10.5772/intechopen.75306 63

**Figure 1.** The effects of insulin resistance and T2DM on brain mitochondria and their association with the development of Alzheimer's disease in non-AD and AD models.

#### **2.2. Type 2 diabetes mellitus model**

**Study model** **Animal/diet/duration**

Wistar rats/HFD (60%

•

↑ BW, insulin, HOMA,

• •

↓ MMP

•

↑Time to reach platform

↑ ROS

*MWM*

cholesterol

• •

↓Peripheral insulin

○

↑Swelling

•

↓Time in target quadrant

*Synaptic plasticity*

•

↓LTP

sensitivity

•

C57BL/6 mice/HFD or

•

↑BW, insulin, HOMA,

• • •

↓ATP

*MWM*

• •

↓Crossing target number

↓Time in target quadrant

↓MMP

•

↓Recognition index

↑ROS

*NOR*

N/A

HFD-induced obese-insulin

[34]

resistance leads to brain

mitochondrial dysfunction and

cognitive dysfunction.

FA, cholesterol

•

↓Peripheral insulin

sensitivity

•

C57BL/6 mice/HFD (60%

• •

↔Glucose

↑ BW, insulin

•

↑Swelling

*NOS*

N/A

HFD-induced obese-insulin

[47]

resistance leads to synaptic

dysplasticity, brain mitochondrial

dysfunction and cognitive

dysfunction.

(elongated

• *Y-maze test*

•

↓% Correct alterations

*Synaptic plasticity*

•

Wistar rats/HFD (60%

• FA

•

↔Glucose

•

↔ RCR

Abbreviations: BW, body weight; HOMA, homeostasis model assessment; HFD, high-fat diet; ND, normal diet; ROS, reactive oxygen species; MMP, mitochondrial

membrane potential; RCR, respiratory control ratio; MWM, Morris water maze; LTP, long-term potentiation; NOR, novel object recognition; NOS, novel smell recognition;

Implications of obese-insulin resistance on brain mitochondria and its association with the development of Alzheimer's disease.

↑BW, insulin, HOMA,

•

↔ State 3, 4

N/A

N/A

HFD-induced obese-insulin

[32]

resistance does not impair brain

mitochondrial function.

respiration

E fat) or ND (13% E

fat)/12 months

N/A, not assessed.

**Table 1.**

↓Synaptic density

↔Discrimination index

mitochondria)

E fat) or ND (12% E

fat)/12 months

↓Brain insulin signaling

ND/20 weeks

↓Brain insulin signaling

↔Glucose

E fat) or ND (20% E

fat)/16 weeks

**Major findings**

**Metabolic parameters**

**Mitochondrial** 

**Cognitive function**

**AD** 

**Interpretation**

**marker**

N/A

HFD-induced obese-insulin

[20–22,

31]

62 Alzheimer's Disease - The 21st Century Challenge

resistance leads to synaptic

dysplasticity and brain

mitochondrial dysfunction and

finally results in cognitive decline.

**parameters**

**Refs**

Type 2 diabetes mellitus (T2DM) is diagnosed when hyperglycemia is observed along with insulin resistance [38–41]. In order to create a T2DM animal model, a combination of a HFD with low-dose streptozotocin, and a high-calorie diet were used [38–41]. Both regimens caused hyperglycemia in rodents [38–41]. Similar to obese-insulin-resistant models, T2DM animals are also found to develop brain mitochondrial damage [38–41].

Beside the effects of T2DM on oxidative stress and mitochondrial membrane depolarization, nuclear respiratory factor 2 (NRF2) levels were reduced in the brains of T2DM mice [38]. NRF2 acts as an antioxidant and detoxifying enzyme and helps to reduce oxidative stress in mitochondria [42]. Therefore, a decreased level of NRF2 directly impairs the brain mitochondrial redox system, which leads to the reduction of brain mitochondrial antioxidant capacity. In addition, a previous study showed a decrease in brain mitochondrial numbers in T2DM mice [41]. The possible explanation may be due to a decrease in NRF2 in the brain of T2DM mice, in which NRF2 regulates brain mitochondrial biogenesis [43]. These data indicate that T2DM caused brain mitochondrial damage and brain mitochondrial dysfunction, resulting in an increased brain oxidative stress.

Consistent with the findings from obese-insulin-resistant animals, T2DM animals also developed brain mitochondrial dysfunction with cognitive impairment, quantified using the MWM test, as indicated by an increased escape latency and time in target quadrant and a decreased crossing target number [39, 41]. Also, T2DM rats had decreased percentage correction alterations and total distance, when the abilities of these animals were investigated using the Y-maze test [39]. T2DM also affected brain synaptic plasticity proteins, as indicated by reducing postsynaptic density protein 95 (PSD95) and synaptosomal-associated protein 25 (SNAP25) levels [38]. However, T2DM did not affect synaptophysin protein levels [38].

In the T2DM model, brain mitochondrial markers were evaluated along with the changes in AD markers. It is interesting that T2DM rats developed AD signs, specifically that the levels of AD markers, including Aβ42 and hyperphosphorylated tau, were significantly increased in T2DM rats, when compared with non-T2DM rats [39, 41]. In addition, acetylcholine esterase enzyme activity was increased, and ACh levels were decreased in the brains of T2DM mice [39]. These data suggested that T2DM rats had impaired brain mitochondrial dysfunction and synaptic plasticity, leading to cognitive dysfunction and showed increased AD markers. Interestingly, those findings indicated that AD was developing in the T2DM condition. Contrary to the findings from animal studies, when Loo et al. investigated the effect of T2DM on mitochondrial function in human mononuclear cells, their data showed that T2DM did not affect mitochondrial function [40]. Their findings showed that T2DM affected regional mitochondria, but not systemic mitochondria. Data regarding the effects of T2DM on brain mitochondria and its association with the development of AD are shown in **Table 2** and are

**3. The implications of high-calorie diet consumption on brain** 

**Mitochondrial parameters**

• ↔Mito number • ↔Mito morphology

• ↓PGC1α • ↔NRF1,2 • TFAM

• ↓PGC-1α • ↓NRF1,2

*NOR*

• ↓Discrimination index

**mitochondrial function and brain function in an AD model:** *in vivo*

Two AD animal models, including 3xTg AD mice and APPswe/PS1dE9 mice, have been used to investigate the implications of high-calorie diet consumption on brain mitochondrial function

**Study model Major findings Refs**

• ↔Discrimination index *Y-maze test* • ↔ % Correct alterations *Synaptic plasticity* • ↔ Synaptic number

N/A • ↑APP

*NOR*

**Cognitive function AD marker Interpretation**

Mitochondrial Link Between Metabolic Syndrome and Pre-Alzheimer's Disease

http://dx.doi.org/10.5772/intechopen.75306

• ↓ADAM10 • ↓IDE • ↔BACE1 • ↑Cortical soluble Aβ40, Aβ42 insoluble Aβ42

• ↔APP p-Tau/Tau • ↓IDE • ↑Cortical insoluble Aβ42

• ↔Aβ42 Obesity did not alter

model.

brain mitochondria and AD markers AD

Obesity increased AD markers, but did not alter brain mitochondrial biogenesis in AD model.

Obesity increased AD markers, impaired brain mitochondria biogenesis, and cognitive function in AD model.

[33]

65

[17]

[18]

summarized in **Figure 1**.

**studies**

**Animal/diet/ duration**

APPswe/PS1 dE9 mice/HFD (45% E fat) or ND/12 weeks

APPswe/PS1 dE9 mice/HFD (45% E fat) or ND/24 weeks

3xTgAD mice/HFD (60% E fat) or ND (12% E fat)/12 months **Metabolic parameters**


• ↑BW • ↔Brain insulin signaling

• ↑BW, insulin, glucose • ↓Peripheral insulin sensitivity • ↓Brain insulin signaling


Abbreviations: BW, body weight; STZ, streptozotocin; T2DM, type 2 diabetes mellitus; HFD, high-fat diet; ND, normal diet; ROS, reactive oxygen species; MMP, mitochondrial membrane potential; Ach, acetylcholine; AChE, acetylcholine esterase; ATP, adenosine triphosphate; RCR, respiratory control ratio; NRF, nuclear respiratory factor; MWM, Morris water maze; N/A, not assessed.

**Table 2.** Implications of type 2 diabetes mellitus (T2DM) on brain mitochondria and its association with the development of Alzheimer's disease.

not affect mitochondrial function [40]. Their findings showed that T2DM affected regional mitochondria, but not systemic mitochondria. Data regarding the effects of T2DM on brain mitochondria and its association with the development of AD are shown in **Table 2** and are summarized in **Figure 1**.
