According to the amyloid hypothesis, which explains why synaptic dysfunction and neurodegeneration are brought on by the aggregation of the Aβ-peptide (Van Dyck, 2018). The main contributing factor to AD is errors in the mechanisms directing Aβ formation, accumulation, or elimination. Aβ aggregation stages impair cell-to-cell communication and stimulate the immune system, which then causes inflammation and eventually kills brain cells.

The APP is processed in two distinct pathways as shown in Figure 1. Nonamyloidogenic pathway: The α-secretase enzyme first cleaves APP within the Aβ domain, and then γ-secretase cleaves at the C-terminus. Amyloidogenic pathway: Instead of α-secretase, β-secretase (BACE1) cleaves APP first at the N-terminus of the Aβ domain, and γ-secretase then cleaves it at the C-terminus. The Aβ amylogenic peptides are produced by this chain of events, which then assemble into oligomers to create extracellular neurotoxic plaques in the brain. A similar APP intracellular C-terminal domain (AICD) is released from both pathways (Thinakaran and Koo, 2008). When compared to other fragments, Aβ is chemically “stickier” than those formed by APP proteolytic processes. Small clusters (oligomers) are formed by the fragments initially, followed by chains of clusters (fibrils), and finally “mats” of fibrils (beta-sheets). The final stage is the forming of plaques which contain clusters of beta-sheets and other chemicals (Jung et al., 2010). The amyloid cascade hypothesis (ACH) explains AD pathogenesis from the outcome of two significant facts: (i) Identification of Aβ as a key component of senile plaques (SPs). (ii) Mutations of APP genes and the presenilin 1 and 2 genes (PSEN1 and PSEN2) which are typically detected at the early stage of AD. As a result, it is believed that the emergence of Aβ within SPs is caused by these mutations, which also cause dementia and neuronal cell death (Reitz, 2012).

The aggregation of Aβ causes the formation of neurofibrillary tangles (NFTs) from hyperphosphorylation of tau and its accumulation into tangles is another pathological cause of AD (McGleenon et al., 2009). In normal conditions, tau supports neuronal structures and functions in the brain (Kolarova et al., 2012). However, under pathological circumstances, tau became excessively hyperphosphorylated and aggregated into fibrils known as neurofibrillary tangles. The accumulation of abnormal tau and tangles in neurons leads to neurotoxicity and neuronal degeneration (Gómez-Isla et al., 1997). In addition to the formation of NFTs, Tau phosphorylation impairs tau’s ability to bind microtubules, which impacts neuronal activities such as axonal transport and mitochondrial respiration (Ittner and Götz, 2011). Microtubule depolymerization, self-aggregation, and detachment caused by tau hyperphosphorylation result in neuronal cell death (Suganthy et al., 2016).

Strong neurotoxic candidates that alter Aβ and tau aggregation include metal dyshomeostasis (Figure 4). Metal ions’ effects on the aggregation of Aβ and tau have been elucidated. Metals like Zn²⁺, Cu²⁺, Fe³⁺, Mn²⁺, Pb²⁺, Cd²⁺, Hg²⁺, and Al³⁺ stimulate amyloidogenic pathways and Aβ aggregation. [red arrow] (O’brien and Wong, 2011). The neurotoxic Aβ-peptide produced by the cleavage of the APP by β- and γ-secretase is secreted into the extracellular space where it spontaneously changes into amyloid plaques. On the other hand, as seen in Figure 4A, the presence of Mg²⁺, Fe²⁺, and Li²⁺ inhibits the production of Aβ [blue arrow] (O’brien and Wong, 2011).

Tau hyperphosphorylation and aggregation are promoted by metal ions like Zn²⁺, Cu²⁺, Fe³⁺, Mg²⁺, Mn²⁺, Pb²⁺, Cd²⁺, Hg²⁺, and Al³⁺ [red arrow]. Numerous kinases, including glycogen synthase kinase-3 beta (GSK-3β; Rankin et al., 2007), cyclin-dependent kinase 5 (CDK-5), and others, strongly phosphorylate tau (Kimura et al., 2014). If protein phosphatase 2A (PP2A) is not activated, the hyperphosphorylation of tau may persist (Goedert, 1993). Tau that has been hyperphosphorylated forms NFTs. As depicted in Figure 4B, metal ions like Fe²⁺, and Li²⁺, however, lessen tau hyperphosphorylation [blue arrow].

The polymeric forms of Aβ cause alterations in biochemical components and brain cell activities that lead to neuropathology associated with AD symptoms. According to reports, one of the earliest clinical manifestations of AD is increased oxidative stress. Hydrogen peroxide (H₂O₂) created due to the reduction of metal ions by Aβ-peptides served as a mediator of the oxidative stress as shown in Figure 5 (Huang et al., 1999; Atwood et al., 2003). Aβ-peptides act as powerful oxidation catalysts and can capture transition metal ions like Cu, Fe, and Zn (Miura et al., 2000). In addition, it was shown that Aβ was toxic to neuronal cultures, and Cu²⁺ ions made it more toxic (Cuajungco et al., 2000). Reactive oxygen species can be produced by the Aβ/Cu(Fe) complexes as a toxin mediator (Huang et al., 1999). Furthermore, AD brains have an extracellular and intracellular accumulation of metal ions with high concentrations of Aβ plaques (Lovell et al., 1998; Religa et al., 2006), which produced free radicals. Because of lipid peroxidation and oxidative protein modification, several biomolecules in the AD brain experience conformational and structural changes that impair their ability to function, which, in turn, affects a variety of cellular processes (Qi et al., 2005). By upregulating the expression of the BACE1 gene, increasing oxidative stress enhances APP processing and ultimately increases Aβ generation (Tong et al., 2005; Coma et al., 2008; Quiroz-Baez et al., 2009). This causes oxidative stress and endoplasmic reticulum (ER) stress by increasing ROS and the accompanying rise in abnormal APP and phosphorylated tau. The ER function can be severely damaged by long-term ER stress, which also causes apoptotic signaling (Ogata et al., 2006; Kouroku et al., 2007). Aβ promotes Ca²⁺ release from neurons’ ER Ca²⁺ pools, increasing intracellular free Ca²⁺ (Zhou et al., 2020). Increased expression of the NR2B subunit of NMDAR causes a rise in Ca²⁺ ion concentration in extrasynaptic regions (Jusko et al., 2008), which, then increases the level of intracellular endoplasmic reticulum Ca²⁺ production (Yin et al., 1994). Studies revealed that Ca²⁺ overload could increase ER stress and facilitate mitochondrial Ca²⁺ uptake by suppressing the expression of the anti-apoptotic protein B-cell lymphoma 2 (Bcl2) and increasing the phosphorylation of extracellular regulated protein kinases (Erk) protein, which would ultimately lead to cytotoxicity and cellular apoptosis (Hajnóczky et al., 2003; Zieg et al., 2008; Zhang et al., 2018).

The expression of pro-inflammatory cytokines was increased in response to neuropathological insults induced by Aβ and its interaction with vascular RAGE (Deane et al., 2008). Microglia enhance the clearance of Aβ, but a constant generation of Aβ causes the microglia to become chronically activated, which promotes more amyloid deposition (Hickman et al., 2018). According to Kim and Choi (2015), exposure to Aβ results in microglial activation, which, in turn, causes the generation of reactive oxygen species and neurotoxic pro-inflammatory cytokines. Tau hyperphosphorylation is a result of ROS-activating p38 mitogen-activated protein kinases (p38 MAPK; Giraldo et al., 2014). p38 MAPK has been linked to neuroinflammation and AD due to its ability to activate nuclear factor-B (NF-κB; Kheiri et al., 2018), a master regulator of neuroinflammation gene transcription in the brains of AD patients (Chen et al., 2012; Liao et al., 2016; Olajide and Sarker, 2020). But data indicate that nuclear factor E2-related factor 2 (Nrf2) is negatively regulated by NF-κB (Liu et al., 2008; Kim and Vaziri, 2010; Yu et al., 2011). Substantial evidence connects the activation of the Nrf2 protection mechanism to NF-κB-mediated inflammatory actions (Nair et al., 2008; Sandberg et al., 2014). To uphold the aforementioned finding, Rojo et al. (2010) showed that cyclooxygenase-2 (COX-2), inducible nitric oxide synthases (iNOS), IL-6, and TNF-α levels are elevated when microglia are activated in Nrf2-deficient rats. Ramsey et al. (2007) first noticed this, reporting that the hippocampus of AD patients’ brains had lower amounts of Nrf2 than normal. According to Lee and Kim (2017), through the activation of p38 MAPK, Aβ plaques cause neuronal impairments such as mitochondrial dysfunction, apoptosis, tau phosphorylation, and synaptic dysfunction; the primary cause of neuroinflammation in AD is increased microglial p38 MAPK signaling brought on by Aβ, which results in the production of pro-inflammatory mediators such interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α), cyclooxygenase-2 (COX-2), and inducible nitric oxide synthase (iNOS); the pathophysiology of the AD brain is worsened by the production of IL-1β from microglia, which increases p38 MAPK activation in neurons and astrocytes; Aβ plaques and IL-1β generated an increase in P38 MAPK activation in astrocytes. By releasing iNOS, COX-2, and TNF-α, this activation accelerates neuroinflammation (Figure 6).

As a neurotransmitter, acetylcholine (Ach) aids in the communication between nerve cells and is essential for memory and learning processes (Kihara and Shimohama, 2004; Francis, 2005). A report revealed that Alzheimer’s patients have reduced amounts of Ach in their brains (Kihara and Shimohama, 2004). Ach is decreased because oxidative stress is induced and inflammatory cytokines are produced by Aβ (Esposito et al., 2006). Free radicals produced due to amyloid peptides have been shown to lower the concentration of Ach by causing cholinergic neurons in the hippocampus to degenerate (Vinod et al., 2009). Additionally, acetylcholinesterase (AChE) activity increases and deactivates acetylcholine in synaptic clefts in the vicinity of amyloid plaques (Mordn et al., 1993; Sberna et al., 1997). According to another study, the amyloid peptide inhibits the production of acetylcholine (ACh) by causing choline to seep through cell membranes (Ehrenstein et al., 1997). Ach deficiency caused cognitive impairment and ultimately AD (Parent et al., 2013; Deture and Dickson, 2019) as shown in Figure 7.

According to Yiannopoulou and Papageorgiou (2013), the formation of amyloid oligomers, which mediates the amyloid cascade, is primarily responsible for neurotoxicity. The main pathophysiologic pillars are oxidation, inflammation, excessive glutamate, and tau hyperphosphorylation. Anti-amyloid disease-modifying treatments (DMTs) have therefore concentrated on three main mechanisms of action (MOAs), including reducing the formation of Aβ₄₂, reducing the burden of Aβ-plaque, and promoting Aβ clearance (Yiannopoulou and Papageorgiou, 2020). Hence, inhibiting the formation of A𝛽-peptide accumulation and tau hyperphosphorylation may be part of the treatment for AD (Mendiola-Precoma et al., 2016). Physical exercise, a healthy diet, and mental stimulation are further AD prevention strategies (Nelson and Tabet, 2015).

Acetylcholinesterase inhibitors (AChEIs), such as rivastigmine, donepezil, and galantamine, are clinically effective in increasing the availability of acetylcholine at synapses and thereby inhibiting cognitive decline in AD (Andrieu et al., 2015; Hampel et al., 2018; Cummings et al., 2019). Nevertheless, diarrhea, nausea, and vomiting are some of the typical negative effects of AChEIs on the digestive system (Yiannopoulou and Papageorgiou, 2020). Memantine, which was approved in 2003, selectively binds to open calcium channels that are controlled by NMDA receptors, inhibiting NMDA-mediated ion flux and reducing pathologically excessive glutamate levels (Yiannopoulou and Papageorgiou, 2013; Matsunaga et al., 2015; Cummings et al., 2019). Memantine also reduces the activity of glycogen synthase kinase 3𝛽 (GSK-3𝛽), which, in turn, reduces tau phosphorylation (Prentice et al., 2015; Folch et al., 2016).

Despite extensive and expensive trials, the Food and Drug Administration (FDA) has not approved any DMTs or new medications for AD since 2003 (Anderson et al., 2017; Hukins et al., 2019). The β-secretase (BACE) inhibitors, lanabecestat (Burki, 2018), verubecestat (Egan et al., 2019), and atabecestat (Henley et al., 2019), as well as the anti-amyloid agents such as semagacestat (Doody et al., 2013), bapineuzumab (Vandenberghe et al., 2016), and solanezumab (Neurology, 2016), failed in recent phase 3 clinical trials. The acknowledged explanations for the numerous failures include inadequate understanding of the pathophysiology, inappropriate drug doses, late therapies in disease progression, and wrong therapeutic targets (Gauthier et al., 2016).