Background
Alzheimer’s disease (AD) is the most common degenerative brain disease. AD causes dementia, whose progressive onset leads to a gradual worsening of cognitive functioning, including memory. In the initial stages of AD, problems with memory related to recent events occur with alterations in cognitive function such as language ability and judgment, which eventually lead to the complete loss of motor function. AD accounts for > 70% of all dementia cases [
1]. AD is associated with various cellular changes in the brain, including synaptic alterations, mitochondrial structural and functional changes, abnormal inflammatory responses, extracellular amyloid beta (Aβ) accumulation, and intracellular neurofibrillary tangles [
2‐
4]. In particular, it is known that Aβ is directly correlated with AD pathology and that longer isoforms with 42–43 residues (the smaller isoform has 40 residues) cause neurodegeneration and cognitive dysfunction, eventually progressing to dementia [
5,
6]. Reduced Aβ clearance or Aβ overproduction may cause Aβ accumulation in subcellular compartments, including synapses and mitochondria, and may impair organelle and ultimately neuronal function [
7‐
9].
AD is caused by hippocampal atrophy, which is involved in memory and learning, the presence of senile plaques, and the accumulation of hyperphosphorylated aggregates of tau protein [
10,
11]. Overexpression or hyperphosphorylation of tau protein due to AD impairs axonal migration of organelles including the mitochondria [
12,
13]. There is substantial evidence suggesting that mitochondrial dysfunction is associated with aging and neurodegenerative diseases. Reduced mitochondrial function has been demonstrated using AD transgenic mouse models as well as post-mortem brain tissue of patients with AD [
14,
15], fibroblasts, and blood cells [
16‐
18]. Mitochondrial dysfunction participates in the progression of AD and is present in all stages of the disease. It has been suggested that AD is not limited to the brain but could be a systemic disease [
16].
In the brain, gamma oscillation occurs between 25 and 145 Hz, affecting various behavioral functions such as attention and memory [
19]. This is disrupted in animal models and clinical studies of AD and other disorders [
20‐
22]. Specifically, in the brains of AD patients, gamma rhythms are known to be interrupted and Aβ has been suggested as the potential cause, particularly in the hippocampus [
23]. Aβ accumulation affects memory through the inhibition of electrical signaling such as gamma oscillations, which play an important role in cognitive functioning and sensory response [
24‐
26]. The wavelength, duration, and intensity of light exposure regulate the cognitive tasks that the brain responds to, and these light responses have been observed in subcortical areas, such as the hypothalamus, the brain stem, and the thalamus, as well as in limbic areas including the amygdala and the hippocampus [
27]. According to Naeser et al. [
28], cognitive function is improved through light-emitting diode (LED) therapy in patients with chronic traumatic brain injury. In addition, previous studies have consistently reported that exercise is beneficial to brain functioning and is a primary method of preventing and treating AD in combination with drugs. However, unlike certain drugs that target localized causes, the positive effects of exercise mitigate or delay multiple aspects related to AD. In animal studies using familial AD gene mutations, exercise was reported to minimize neurotoxicity caused by AD neuropathy and stimulate neuronal regeneration, contributing to an improvement in cognitive functioning through a reduction in beta-secretase activity [
29], decreased accumulation of amyloid plaques and soluble Aβ [
30,
31], and decreased pTau [
32]. Therefore, this study aimed to examine the effects of exercise training in a 40-Hz light flicker environment on Aβ accumulation in the hippocampus, Akt/tau, mitochondrial function, neuroplasticity, and cognitive functioning in an AD animal model.
Discussion
AD is an aging and neurodegenerative disorder characterized by deficits in learning, memory, and communication. AD accounts for 60 to 80% of dementia worldwide. The 3xTg AD animal model resembles the progression of cognitive and behavioral deficiencies seen in AD patients and is well suited for clinical studies of AD pathology. In this study, deficiencies in spatial learning, memory, and long-term memory were evaluated in the 3xTg AD group using the Morris Water maze and step through tests. In previous studies, 3xTg AD mice have shown loss of cognitive function and dementia-like behavioral and psychological symptoms along with in vivo long-term potentiation disorders, which are more severe in the older population [
33]. This is particularly evident in the vulnerable areas of the brain, and the hippocampus is one of the most rapidly affected areas [
34]. Signals that create our thoughts and memories are transmitted through nerve cells and the synapses between them. Aβ, an important protein for nerve cell growth and repair, is usually produced under physiological conditions and is excreted through the urine but when brain damage occurs, it is produced excessively and adheres to nerve cells and forms aggregates, blocking synapses, impeding the transmission of nerve signals, and causing inflammation and damage in neurons. When this occurs, memory dysfunction is a major symptom as the hippocampus and the temporal lobe are the first to develop abnormalities, which gradually spread to other areas of the brain. Altered cognitive function, including learning and memory deficits, are closely related to Aβ accumulation in the hippocampus. Aβ aggregation can cause synaptic dysfunction and neurodegeneration, which can impair cognitive function including spatial memory [
35,
36]. In addition, Aβ stimulates glycogen kinase 3 (GSK3) to induce tau protein phosphorylation [
37], and tau overexpression also causes an increased Aβ plaque accumulation [
38]. GSK3, which promotes tau overexpression, is inhibited by phosphorylation of Ser21 by GSKα or Ser9 by GSK3β [
39] and is increased in the absence of Akt activity [
40]. Previous studies have shown that p-GSK3β (ser9) and p-Akt (ser473) were decreased while the expression of p-tau and Aβ increased simultaneously in the 3xTg-AD mouse [
41,
42]. Aβ is closely associated with mitochondrial dysfunction, as evidenced by the accumulation of mitochondrial damage in the brains of AD patients [
43], which may induce neuronal apoptosis [
44], impair the movement of mitochondria, and cause synaptic degeneration by reducing mitochondrial length [
45]. In the present study, the 3xTg-AD group with high Aβ expression also showed decreased mitochondrial function in the hippocampus such as decreased Ca
2+ retention in hippocampal mitochondria and increased H
2O
2 production, a marker of reactive oxygen species (ROS). This deterioration led to an increase in cell death due to an increase in apoptosis and the expression of pro-apoptotic factors including Bax, cytochrome c, and caspase-3, as well as a decrease in the anti-apoptotic factor, Bcl-2. Cellular proliferation, neurogenesis, and synaptic markers BDNF, PSD95, and synaptophysin were also reduced in the hippocampus. Therefore, overexpression of Aβ and tau may decrease mitochondrial function, increase cell death, decrease neuron production, and reduce neuronal plasticity in the hippocampus. In previous studies, an increase in Aβ concentration resulted in increased secretion of H
2O
2, dysregulation of the cytosolic and mitochondrial Ca
2+ homeostasis, and cytochrome c in the mitochondria [
46,
47]. Particularly, mitochondrial Ca
2+ overloading results in an increase of ROS production in mitochondria [
48], the oxidative stress leads to abnormalities in the cell calcium storage, and the ability to control oxidative stress and respond to metabolic disorder links the AD-causing gene mutations to the disease process [
49], while cell death was increased as were Bax levels with a concomitant decrease in Bcl-2 in the hippocampus of the 3xTg AD mouse, showing deficiencies in cognitive functions due to a decrease in BDNF, PSD95, and synaptophysin expression [
50‐
52]. Aβ is an important pathological factor in AD. It is a neurotoxin, which can pathologically affect various brain cells. It seems that the decrease in cognitive function occurs due to inhibition of neuroplasticity following an alteration in mitochondrial function caused by excessive accumulation of Aβ and overexpression of tau in the hippocampus.
A new treatment method using LED has been recently suggested as a non-invasive therapy to treat AD. In particular, 40-Hz light flicker (LED flickers 40 times per second) is effective in stimulating the brain and restoring gamma rhythms. In an animal AD model, 40-Hz light flicker reduced p-tau and Aβ, which resolved memory abnormalities [
53,
54]. Long-term visual stimulation using 40-Hz light flickering entrained gamma oscillations in the visual cortex, CA1 of the hippocampus, and the prefrontal cortex in Tau P301S and CK-p25 mice; as a result, spatial learning and memory and protein levels of various synaptic signaling and synaptic plasticity markers were improved [
55]. Duan et al. [
56] reported that the light from the LED inhibited apoptosis, which induces Aβ. The visual stimulation of 40-Hz light flickering has been shown to improve cognitive function by reducing neuronal and synaptic loss as well as amyloid plaques and tau phosphorylation in various AD mouse models (5XFAD, APP/PS1, P301S, and CK-p25) [
50‐
52,
57]. In the present study, gamma oscillation was not measured, but in previous studies, it has been shown that the gamma oscillation in the hippocampus was altered with respect to time and concentration of Aβ [
23], while the 3xTg-AD model showed synchronization of abnormal beta and gamma frequencies [
58]. In the present study, the 40-Hz light flickering treatment group showed a reduction in tau phosphorylation and Aβ in the hippocampus and an improvement in spatial learning, memory, long-term memory, mitochondrial function, and neuroplasticity. This may suggest that the expression of tau and Aβ in the hippocampus was reduced by rescued gamma oscillations, which then led to an improvement in cognitive function. This may be due to the enhancement of mitochondrial function such as Ca
2+ retention and ROS stabilization in the hippocampus, increased neuroplasticity such as the increase in proteins related to neurogenesis and synapses, and the inhibition of apoptosis.
Another non-invasive method to treat AD is exercise. Physical activity is known to promote brain health and improve cognitive functioning in the elderly; it has also been shown to increase hippocampal size and increase BDNF levels and neurogenesis [
59,
60]. Exercise plays an important role in protecting against cognitive disorders due to dementia [
61], and studies have reported low plasma Aβ and brain amyloid levels in people with low levels of physical activity [
62]. In addition, in various AD animal models, exercise has been shown to delay or protect against the progression of AD by reducing Aβ and hyperphosphorylated tau protein [
63‐
66], activating p-Akt and p-GSK3β, and reducing hyperphosphorylated tau levels [
67]. As shown, exercise can play an important role in AD treatment including regulation of tau and Aβ in many of the previous studies. In addition, exercise-induced neuronal activity in the hippocampus requires increased mitochondrial capacity to produce ATP from oxidative phosphorylation of glucose. As a result, ROS may accumulate, but exercise may activate mitochondrial function and mitigate ROS-induced neurotoxins, and the protective effect of exercise against ROS production may be important in the hippocampus of patients with AD [
68]. Exercise also improves hippocampal function by alleviating ROS such as H
2O
2 and inducing Ca
2+ retention in hippocampal mitochondria under various neurotoxic conditions, and that mitochondrial function activated by exercise decreases apoptosis [
69]. Furthermore, Aβ-dependent cell death is significantly suppressed after exercise in the hippocampus of an AD model [
70]. In addition, exercise increases synaptic plasticity in the hippocampus, and Revilla et al. [
71] found increased synaptophysin and PSD95 protein expression in the hippocampus of the 3xTg AD model.
In the present study, mitochondrial function and neuroplasticity were improved by Aβ and tau overexpression in an AD animal model through exercise as in the previous study. However, the therapeutic benefit of this in patients with AD remains controversial. Our study showed that exercise along with a non-invasive approach such as 40-Hz light flickering led to a significant improvement in AD patients, which was an important conclusion. Although much research on 40-Hz light flickering is still needed, Aβ and tau protein levels were suppressed, and the improvement in Aβ and tau expression caused by 40-Hz light flickering may have induced various positive cellular effects. Thus, under these circumstances, exercise may have a positive effect on this AD animal model as a complimentary therapy to 40-Hz light flickering.
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.