Alzheimer’s disease (AD), the leading cause of dementia, is a progressive neurodegenerative disease ^[1] that is clinically characterized by memory loss, cognitive impairment, and behavioral disturbances [2]. Since its discovery in 1906, AD has emerged as one of the most costly, fatal, and burdensome diseases of this century [3]. The pathogenesis of AD involves various biological processes [4] involving the abnormal deposition of amyloid beta peptide (Aβ) [5], the accumulation of neurofibrillary tangles (NFTs) [6], neuroinflammation [7], neuronal apoptosis [8], neurotransmitter abnormities [9], and oxidative stress [10]. Despite considerable efforts, drug discovery for the treatment of AD has been slow, with only acetylcholinesterase (AChE)/butyrylcholinesterase (BChE) inhibitors [11] such as galantamine, donepezil, tacrine, and rivastigmine currently available as therapies [12]. However, these treatments only delay the onset of symptoms and cannot halt disease progression and are often associated with significant side effects [13]. Therefore, the development of new therapeutic drugs is urgently needed. Saponins, a type of natural compound, have been extensively studied for their various pharmacological properties [14]. Of particular interest is their potential to enhance learning and memory in individuals with AD [15].

Saponins are naturally occurring compounds that are widely distributed in various plants [16], and they can be divided into two major groups based on their chemical structure: triterpenoid saponins and steroidal saponins [17]. Triterpenoid saponins are further subdivided into tetracyclic triterpenes and pentacyclic triterpenes and are mainly found in plants such as Pentaaceae, Leguminosae, Poria, and Platycodonaceae [18]. The main saponin skeletons of the triterpenoid saponins include dammarane, oleanane, ursane, and lupane (Figure 1Figure 1 A–, B, C, D) [19]. Steroidal saponins, on the other hand, are mainly found in plants such as Dioscoreaceae, Liliaceae, and Scrophulariaceae [20]. The main saponin metaskeletons of steroidal saponins include spirostane, furostane, cholestane, and cardenolide (Figure 1Figure 1 E–, F, G, H) [21]. Saponins possess multiple bioactivities, such as reduction of amyloid beta (Aβ) deposition [22], inhibition of tau protein phosphorylation [23], antioxidation [24], antiapoptosis [25], and anti-inflammation [26]. These properties make saponins promising therapeutic candidates for AD and other neurological disorders [27]. Meanwhile, the diversity of saponins found in different plants [28] makes them a valuable source of potential drugs for the treatment of AD. However, there is a current lack of comprehensive and systematic review in this field. To address this gap, we thoroughly searched the relevant literature in the major databases, including PubMed and Web of Science, using keywords “Alzheimer’s disease” or “AD” and “saponin”. By browsing all the relevant studies from May 2007 to May 2023, excluding review articles, 63 references were selected, involving 40 saponins extracted from different herbs. The selected references are known to be effective in the treatment of AD, present clear mechanisms, authentic and reliable information, and contain the latest findings in the field. The studies highlighted that saponins have exhibited various beneficial effects such as reducing Aβ levels, reducing NETs, exerting antioxidative, antiapoptotic, anti-inflammatory, and increased neurotransmitter-enhancing effects [25]. The underlying mechanisms behind these effects include reducing amyloid precursor protein (APP) production [29], improving tau protein phosphorylation [30], and reducing reactive oxygen species (ROS) generation [31], inhibiting the apoptotic and inflammatory signaling pathways [31,32], and increasing neurotransmitter expression [33]. This comprehensive review sheds light on the potential saponins as therapeutic agents for AD and related neurological disorders.

 

Figure 1. Representative triterpenoid saponins include (A) Oleanane, (B) Ursane, (C) Lupane, (D) Dammarane and steroidal saponins include (E) Cholestane, (F) Furostane, (G) Spirostane, (H) Cardenolide.

 

Aβ is a peptide that is a major component of the senile plaques found in the brains of people with AD ^[29]. Its accumulation is highly neurotoxic and is considered a hallmark of AD ^[30], which can result in impaired cognitive function, including spatial memory ^[31]. Aβ is generated from the APP and is typically enclosed by microglia and dystrophic synapses that aggregate around neurons ^[32][33]. APP is a transmembrane protein located in the synapse of neurons, which can be cleaved by both amyloid and nonamyloid pathways ^[34]. In the nonamyloid pathway, APP is sequentially cleaved by α-secretase (mainly ADAM10) and γ-secretase ^[35], resulting in P3 peptide (P3), C83 carboxy-terminal fragment (C83), APP intracellular domain (AICD), and soluble amyloid precursor protein-α (sAPPα) with beneficial neurotrophic effects ^[36]. Conversely, in the amyloid pathway, APP is first cleaved by β-secretase and sAPPβ is secreted ^[37]. Subsequently, γ-secretase cleaves the C-terminal fragment (C99) of the residual APP and eventually leads to the release of peptides of different lengths ^[38]. The most prevalent of them are Aβ1-40 and Aβ1-42 ^[39], which are neurotoxic fragments capable of oligomerization, aggregation, and subsequent plaque formation ^[40]. In vivo, Aβ is degraded by a variety of proteases, most notably insulin-degrading enzymes (IDE) and neprilysin (NEP) ^[41]. However, the production or activity of these clearance enzymes may decrease with age, leading to a failure to clear Aβ in a timely manner ^[42]. This phenomenon has been linked to the reduced activity of the peroxisome proliferator-activated receptor γ (PPARγ) ^[43], which is a transcription factor that regulates the expression of IDE and Bace1, reduces Aβ production ^[44], promotes Aβ clearance ^[45], and exerts neuroprotective effects ^[46]. An imbalance between Aβ production and clearance is responsible for the accumulation of Aβ in the brain ^[47]. In order to develop novel treatments for AD, it may be beneficial to inhibit Aβ production, enhance its clearance, or directly combat its neurotoxicity ^[48].

Aβ is a peptide that is a major component of the senile plaques found in the brains of people with AD [34]. Its accumulation is highly neurotoxic and is considered a hallmark of AD [35], which can result in impaired cognitive function, including spatial memory [36]. Aβ is generated from the APP and is typically enclosed by microglia and dystrophic synapses that aggregate around neurons [37,38]. APP is a transmembrane protein located in the synapse of neurons, which can be cleaved by both amyloid and nonamyloid pathways [39]. In the nonamyloid pathway, APP is sequentially cleaved by α-secretase (mainly ADAM10) and γ-secretase [40], resulting in P3 peptide (P3), C83 carboxy-terminal fragment (C83), APP intracellular domain (AICD), and soluble amyloid precursor protein-α (sAPPα) with beneficial neurotrophic effects [41]. Conversely, in the amyloid pathway, APP is first cleaved by β-secretase and sAPPβ is secreted [42]. Subsequently, γ-secretase cleaves the C-terminal fragment (C99) of the residual APP and eventually leads to the release of peptides of different lengths [43]. The most prevalent of them are Aβ1-40 and Aβ1-42 [44], which are neurotoxic fragments capable of oligomerization, aggregation, and subsequent plaque formation [45]. , Aβ is degraded by a variety of proteases, most notably insulin-degrading enzymes (IDE) and neprilysin (NEP) [46]. However, the production or activity of these clearance enzymes may decrease with age, leading to a failure to clear Aβ in a timely manner [47]. This phenomenon has been linked to the reduced activity of the peroxisome proliferator-activated receptor γ (PPARγ) [48], which is a transcription factor that regulates the expression of IDE and Bace1, reduces Aβ production [49], promotes Aβ clearance [50], and exerts neuroprotective effects [51]. An imbalance between Aβ production and clearance is responsible for the accumulation of Aβ in the brain [52]. In order to develop novel treatments for AD, it may be beneficial to inhibit Aβ production, enhance its clearance, or directly combat its neurotoxicity [53]

Natural saponins have shown potential in affecting Aβ metabolism through different pathways ^[49]. For example, some natural saponins are able to inhibit the formation of Aβ by reducing APP production. Ginsenoside Rg1, a tetracyclic triterpenoid saponin extracted from ginseng ^[50], has been shown to reduce Aβ deposition in APP/presenilin 1(PS1) double transgenic AD model mice by lowering APP levels ^[51]. Pharmacokinetic studies have also shown that ginsenoside Rg1 can cross the blood–brain barrier (BBB) and the blood–cerebrospinal fluid barrier (BCSFB), which provides theoretical support for its ability to improve learning and memory abilities ^[52]. Xanthoceraside, another tetracyclic triterpenoid saponin, extracted from the bark of the Xanthoceras sorbifolia Bunge commonly used in traditional Chinese medicine (TCM) to treat rheumatism ^[53]

Natural saponins have shown potential in affecting Aβ metabolism through different pathways [54]. For example, some natural saponins are able to inhibit the formation of Aβ by reducing APP production. Ginsenoside Rg1, a tetracyclic triterpenoid saponin extracted from ginseng [55], has been shown to reduce Aβ deposition in APP/presenilin 1(PS1) double transgenic AD model mice by lowering APP levels [56]. Pharmacokinetic studies have also shown that ginsenoside Rg1 can cross the blood–brain barrier (BBB) and the blood–cerebrospinal fluid barrier (BCSFB), which provides theoretical support for its ability to improve learning and memory abilities [57]. Xanthoceraside, another tetracyclic triterpenoid saponin, extracted from the bark of the Xanthoceras sorbifolia Bunge commonly used in traditional Chinese medicine (TCM) to treat rheumatism [58]

_, has been shown to reduce APP protein levels and Aβ deposition in the cerebral cortex and hippocampus, thereby ameliorating cognitive function dysfunction in an AD mouse model induced by Aβ intracerebroventricular (ICV) injection ^[54]. In addition, ginsenoside Rh2, another ginseng derivative used in TCM, has been reported to regulate APP expression by reducing cholesterol and lipid raft levels. This ultimately led to an increase in sAPPα levels and a decrease in Aβ concentrations ^[55].

has been shown to reduce APP protein levels and Aβ deposition in the cerebral cortex and hippocampus, thereby ameliorating cognitive function dysfunction in an AD mouse model induced by Aβ intracerebroventricular (ICV) injection [59]. In addition, ginsenoside Rh2, another ginseng derivative used in TCM, has been reported to regulate APP expression by reducing cholesterol and lipid raft levels. This ultimately led to an increase in sAPPα levels and a decrease in Aβ concentrations [60].

Most saponins have been shown to inhibit Aβ deposition by modulating the activity of APP-processing enzymes, such as ginsenoside Rg1 [61], RAPO-1-3 [62], onjisaponin B [62], pseudoginsenoside-F11 (PF11) [63], theasaponin E1 [64], anginsenoside (20S)-Rg3 [65], and ginsenoside C-K (CK) [66]. Ginsenoside Rg1 reduces the γ-secretase responsible for Aβ production by attenuating the Aβ-mediated inhibition of cAMP response element-binding protein (CREB) phosphorylation and protein kinase A (PKA) activity [61]. It also upregulates ADAM10 while reducing BACE1 in Wistar rat models of AD induced by ovariectomy (OVX) and D-galactose (D-gal) [67]. In sAPPα-transfected HT22 cells and neuroblastoma (SH-SY5Y) cells, ginsenoside Rg1 has been found to increase the levels of sAPPα and estrogen receptor (ER) α, elevate α-secretase activity, and decrease extracellular release of Aβ. Further studies have shown that these effects are mediated by the upregulation of phosphorylated extracellular signal-regulated kinase 1/2 (pERK1/2) and phosphorylated protein kinase B (pAkt) [68] but can be reversed by ER antagonists and potentially attenuated by inhibitors of protein kinase C (PKC), MAPK, and phosphatidyl 3-kinase (PI3K) [68]. Yuan Zhi (RAPO) is a TCM formulation that is commonly used to treat dementia due to its neuroprotective effects [69]. Its active ingredient, RAPO-1-3, and onjisaponin B, an acyl saponin with a similar constituent to RAPO-1-3, have been found to reduce Aβ production by promoting the degradation of APP protein through interference with the PS1/BACE1 interaction [62]. Another compound, PF11, which is a pentacyclic triterpene abundant in ginseng [70], has also demonstrated efficacy in inhibiting APP amyloidogenic processing. By reducing the expression of BACE1, PF11 reduced Aβ deposition and ameliorated cognitive impairment and synaptic dysfunction in SAMP8 mice [63]. Theasaponin E1, a pentacyclic triterpene extracted from green tea seeds [71], has been shown to reduce the production of Aβ by increasing the activity of NEP and ADAM10 in APP (SweAPP N2a) cells while inhibiting the expression of BACE1, APP, and PS1 [64]. Furthermore, CK, which is produced through the degradation of protopanaxadiol saponins by the gut microbiota [72], has been studied for its neuroprotective properties. Currently, it is primarily obtained by glycosyl hydrolysis of proto-ginsenoside diol-type saponins [73]. In scopolamine-induced ICR mice, CK was found to reduce the expression of BACE1 and PS1, increase IDE activity, reduce Aβ expression, and improve memory function [66]. In addition, ginsenoside (20S)-Rg3, a component of heat-processed ginseng, has been found to reduce Aβ levels by increasing phosphatidylinositol 4-kinase IIα(PI4KIIα) activity and ultimately decrease the expression of γ-secretase by decreasing the association of PS1 fragments and lipid rafts in cultured primary neurons and in the brains of an AD mouse model [65].

Most saponins have been shown to inhibit Aβ deposition by modulating the activity of APP-processing enzymes, such as ginsenoside Rg1 ^[56], RAPO-1-3 ^[57], onjisaponin B ^[57], pseudoginsenoside-F11 (PF11) ^[58], theasaponin E1 ^[59], anginsenoside (20S)-Rg3 ^[60], and ginsenoside C-K (CK) ^[61]. Ginsenoside Rg1 reduces the γ-secretase responsible for Aβ production by attenuating the Aβ-mediated inhibition of cAMP response element-binding protein (CREB) phosphorylation and protein kinase A (PKA) activity ^[56]. It also upregulates ADAM10 while reducing BACE1 in Wistar rat models of AD induced by ovariectomy (OVX) and D-galactose (D-gal) ^[62]. In sAPPα-transfected HT22 cells and neuroblastoma (SH-SY5Y) cells, ginsenoside Rg1 has been found to increase the levels of sAPPα and estrogen receptor (ER) α, elevate α-secretase activity, and decrease extracellular release of Aβ. Further studies have shown that these effects are mediated by the upregulation of phosphorylated extracellular signal-regulated kinase 1/2 (pERK1/2) and phosphorylated protein kinase B (pAkt) ^[63] but can be reversed by ER antagonists and potentially attenuated by inhibitors of protein kinase C (PKC), MAPK, and phosphatidyl 3-kinase (PI3K) ^[63]. Yuan Zhi (RAPO) is a TCM formulation that is commonly used to treat dementia due to its neuroprotective effects ^[64]. Its active ingredient, RAPO-1-3, and onjisaponin B, an acyl saponin with a similar constituent to RAPO-1-3, have been found to reduce Aβ production by promoting the degradation of APP protein through interference with the PS1/BACE1 interaction ^[57]. Another compound, PF11, which is a pentacyclic triterpene abundant in ginseng ^[65], has also demonstrated efficacy in inhibiting APP amyloidogenic processing. By reducing the expression of BACE1, PF11 reduced Aβ deposition and ameliorated cognitive impairment and synaptic dysfunction in SAMP8 mice ^[58]. Theasaponin E1, a pentacyclic triterpene extracted from green tea seeds ^[66], has been shown to reduce the production of Aβ by increasing the activity of NEP and ADAM10 in APP (SweAPP N2a) cells while inhibiting the expression of BACE1, APP, and PS1 ^[59]. Furthermore, CK, which is produced through the degradation of protopanaxadiol saponins by the gut microbiota ^[67], has been studied for its neuroprotective properties. Currently, it is primarily obtained by glycosyl hydrolysis of proto-ginsenoside diol-type saponins ^[68]. In scopolamine-induced ICR mice, CK was found to reduce the expression of BACE1 and PS1, increase IDE activity, reduce Aβ expression, and improve memory function ^[61]. In addition, ginsenoside (20S)-Rg3, a component of heat-processed ginseng, has been found to reduce Aβ levels by increasing phosphatidylinositol 4-kinase IIα(PI4KIIα) activity and ultimately decrease the expression of γ-secretase by decreasing the association of PS1 fragments and lipid rafts in cultured primary neurons and in the brains of an AD mouse model ^[60].

Aside from inhibiting Aβ production, certain saponins have been found to promote the clearance of Aβ. For example, minor ginsenoside F1, a trace ginsenoside derived from Panax ginseng ^[69], has been shown to effectively reduce Aβ plaques and alleviate cognitive impairment in APP/PS1 mice by enhancing the expression of pCREB ^[70]. Similarly, Bacopaside I (BS-I), a tetracyclic triterpene and standardized extract of Bacopa monnieri ^[71], has been shown to be neuroprotective and improve cognitive function ^[72]. The aglycones of Bacopa monnieri and its derivatives have good intestinal absorption and BBB permeability ^[73]. Recent studies suggest that BS-I induces sufficient innate immune stimulation and phagocytosis to promote amyloid clearance, thereby reducing amyloid plaque burden in APP/PS1 mice ^[74]. In addition, Aβ deposition in the brain is linked to the dysfunction of the endosomal–lysosomal system dysfunction ^[75], which is regulated by the transcription factor EB (TFEB) ^[76]. PF11 has been observed to increase Aβ clearance by promoting the mammalian target of rapamycin (mTOR)-dependent TFEB-mediated lysosome biogenesis and alleviating endosomal–lysosomal system dysfunction through the conversion of Rab5 to Rab7 ^[77].

Aside from inhibiting Aβ production, certain saponins have been found to promote the clearance of Aβ. For example, minor ginsenoside F1, a trace ginsenoside derived from Panax ginseng [74], has been shown to effectively reduce Aβ plaques and alleviate cognitive impairment in APP/PS1 mice by enhancing the expression of pCREB [75]. Similarly, Bacopaside I (BS-I), a tetracyclic triterpene and standardized extract of Bacopa monnieri [76], has been shown to be neuroprotective and improve cognitive function [77]. The aglycones of Bacopa monnieri and its derivatives have good intestinal absorption and BBB permeability [78]. Recent studies suggest that BS-I induces sufficient innate immune stimulation and phagocytosis to promote amyloid clearance, thereby reducing amyloid plaque burden in APP/PS1 mice [79]. In addition, Aβ deposition in the brain is linked to the dysfunction of the endosomal–lysosomal system dysfunction [80], which is regulated by the transcription factor EB (TFEB) [81]. PF11 has been observed to increase Aβ clearance by promoting the mammalian target of rapamycin (mTOR)-dependent TFEB-mediated lysosome biogenesis and alleviating endosomal–lysosomal system dysfunction through the conversion of Rab5 to Rab7 [82].

2.2. Inhibiting Aberrant Tau Protein Phosphorylation

2.2. Inhibiting Aberrant Tau Protein Phosphorylation

NFTs, a well-known pathological hallmark of AD [104], are composed of excessively phosphorylated proteins linked by helical filaments known as tubulin-associated unit (tau) protein [105]. Tau protein, mainly found in neuronal axons, is a microtubule-associated protein involved in various functions such as axonal microtubule maintenance, cytoplasmic transport, and regulation of neuronal signaling [106,107]. Its optimal function depends on the low levels of normal phosphorylation [108]. However, overphosphorylation of tau protein leads to its detachment from microtubules, resulting in microtubule depolymerization, aggregation into NFTs, and ultimately neurodegeneration and cell death [109]. The correlation between NFT density and cognitive decline suggests that tau protein plays a central role in AD [110]. Furthermore, the prevalence of tau protein pathology positively correlates with cognitive impairment in AD, where a person with AD has increased levels of tau protein, hyperphosphorylated tau protein, and aggregated tau in their brain [111]. Abnormal tau protein phosphorylation plays a greater role in AD than the total tau protein [112]. As such, it is considered a critical pathogenic step in the development of NFTs and, subsequently, AD [113].

Several saponins have been found to inhibit the hyperphosphorylation of tau proteins, protect synaptic structures, and reduce neuronal damage. An octillol-type saponin, PF11, has been found to increase the activity of phosphatase-2A (PP2A) by enhancing leucine carboxyl methyltransferase-1 (LCMT-1) and improving cognitive impairment in SAMP/8 mice by inhibiting the hyperphosphorylation of tau protein at serine 396 and tyrosine 205 in the brain [63]. Streptozotocin (STZ) is a glucosamine–nitrosourea compound commonly used in AD studies to induce tau proteins’ hyperphosphorylation and cognitive impairment [114]. Tau protein phosphorylation occurs at multiple sites, usually associated with glycogen synthase kinase (GSK) 3 and cyclin-dependent kinase 5 (CDK5). PF11 was also shown to attenuate STZ-induced tau hyperphosphorylation in the brain of male Wistar rats by correcting the dysregulation of the insulin receptor substrate 1/PI3K/Akt/GSK-3β and calpain I/CDK5 signaling pathways [70]. Additionally, PF11 ameliorated STZ-induced learning and memory deficits, synaptic damage, and neuronal death [70]. Another active compound, Ginsenoside Rb1, found in ginseng [115], exhibits neuroprotective properties against different neurotoxins [116]. Research has shown that ginsenoside Rb1 counters cognitive impairment induced by aluminum oxide in ICR mice while reducing Ser396 tau phosphorylation by increasing PP2A levels and decreasing p-GSK levels [111,117]. Ginsenoside Rd, another compound found in ginseng [118], has been shown to have potential therapeutic applications in neurodegenerative diseases [119]. It has been demonstrated to reduce tau proteins’ hyperphosphorylation and neurotoxicity induced by okadaic acid (OA) by increasing PP2A activity in both rat and cortical neuronal models of AD [120].

NFTs, a well-known pathological hallmark of AD ^[78], are composed of excessively phosphorylated proteins linked by helical filaments known as tubulin-associated unit (tau) protein ^[79]. Tau protein, mainly found in neuronal axons, is a microtubule-associated protein involved in various functions such as axonal microtubule maintenance, cytoplasmic transport, and regulation of neuronal signaling ^[80][81]. Its optimal function depends on the low levels of normal phosphorylation ^[82]. However, overphosphorylation of tau protein leads to its detachment from microtubules, resulting in microtubule depolymerization, aggregation into NFTs, and ultimately neurodegeneration and cell death ^[83]. The correlation between NFT density and cognitive decline suggests that tau protein plays a central role in AD ^[84]. Furthermore, the prevalence of tau protein pathology positively correlates with cognitive impairment in AD, where a person with AD has increased levels of tau protein, hyperphosphorylated tau protein, and aggregated tau in their brain ^[85]. Abnormal tau protein phosphorylation plays a greater role in AD than the total tau protein ^[86]. As such, it is considered a critical pathogenic step in the development of NFTs and, subsequently, AD ^[87].

Theasaponin E1, a vital compound found in green tea seeds, shows remarkable potential in reducing p-tau protein levels and inhibiting AD-promoting genes while activating AD-remitting genes [121]. This potential has been observed in AD models constructed from SHY5Y and glioblastoma (HTB2) cells. Notably, theasaponin E1 successfully reduced p-tau protein by inhibiting several enzymes, including GSK-3β, CDK5, c-Jun NH2-terminal kinase (JNK), MAPK, ERK1/MARK, and calmodulin-dependent protein kinase II alpha [121]. Similarly, Xanthoceraside, a triterpene saponin extracted from the shell of Xanthoceras sorbifolia Bunge, has been shown to exert neuroprotective effects by inhibiting the phosphorylation of tau protein [122]. In a study conducted on mice with AD, the administration of Xanthoceraside significantly decreased the expression of pGSK-3β and tau protein at Ser396 and Ser404, thereby effectively inhibiting the phosphorylation of tau protein [59].

Several saponins have been found to inhibit the hyperphosphorylation of tau proteins, protect synaptic structures, and reduce neuronal damage. An octillol-type saponin, PF11, has been found to increase the activity of phosphatase-2A (PP2A) by enhancing leucine carboxyl methyltransferase-1 (LCMT-1) and improving cognitive impairment in SAMP/8 mice by inhibiting the hyperphosphorylation of tau protein at serine 396 and tyrosine 205 in the brain ^[58]. Streptozotocin (STZ) is a glucosamine–nitrosourea compound commonly used in AD studies to induce tau proteins’ hyperphosphorylation and cognitive impairment ^[88]. Tau protein phosphorylation occurs at multiple sites, usually associated with glycogen synthase kinase (GSK) 3 and cyclin-dependent kinase 5 (CDK5). PF11 was also shown to attenuate STZ-induced tau hyperphosphorylation in the brain of male Wistar rats by correcting the dysregulation of the insulin receptor substrate 1/PI3K/Akt/GSK-3β and calpain I/CDK5 signaling pathways ^[65]. Additionally, PF11 ameliorated STZ-induced learning and memory deficits, synaptic damage, and neuronal death ^[65]. Another active compound, Ginsenoside Rb1, found in ginseng ^[89], exhibits neuroprotective properties against different neurotoxins ^[90]. Research has shown that ginsenoside Rb1 counters cognitive impairment induced by aluminum oxide in ICR mice while reducing Ser396 tau phosphorylation by increasing PP2A levels and decreasing p-GSK levels ^[85][91]. Ginsenoside Rd, another compound found in ginseng ^[92], has been shown to have potential therapeutic applications in neurodegenerative diseases ^[93]. It has been demonstrated to reduce tau proteins’ hyperphosphorylation and neurotoxicity induced by okadaic acid (OA) by increasing PP2A activity in both rat and cortical neuronal models of AD ^[94]. Theasaponin E1, a vital compound found in green tea seeds, shows remarkable potential in reducing p-tau protein levels and inhibiting AD-promoting genes while activating AD-remitting genes ^[95]. This potential has been observed in AD models constructed from SHY5Y and glioblastoma (HTB2) cells. Notably, theasaponin E1 successfully reduced p-tau protein by inhibiting several enzymes, including GSK-3β, CDK5, c-Jun NH2-terminal kinase (JNK), MAPK, ERK1/MARK, and calmodulin-dependent protein kinase II alpha ^[95]. Similarly, Xanthoceraside, a triterpene saponin extracted from the shell of Xanthoceras sorbifolia Bunge, has been shown to exert neuroprotective effects by inhibiting the phosphorylation of tau protein ^[96]. In a study conducted on mice with AD, the administration of Xanthoceraside significantly decreased the expression of pGSK-3β and tau protein at Ser396 and Ser404, thereby effectively inhibiting the phosphorylation of tau protein ^[54].

2.3. Anti-Inflammatory Effect

2.3. Anti-Inflammatory Effect

In recent years, neuroinflammation has become recognized as a third neuropathological hallmark of AD, in addition to Aβ and NFTs [123]. Neuroinflammation is an inflammatory response to pathological insults that occur within the central nervous system (CNS), such as infection, trauma, ischemia, and toxin accumulation [124]. This response is characterized by the proliferation and activation of microglia and astrocytes in the brain [125]. Such activation is usually accompanied by the secretion of inflammatory cytokines such as interleukin-1β (IL-1β), interleukin 6 (IL-6), and tumor necrosis factor α (TNF-α) [126]. Microglia are innate immune cells that reside within the CNS and are thought to be important regulators in the inflammatory response within the brain [127]. In the early stages of AD, activated microglia can exert protective effects by phagocytosing and degrading Aβ [128,129]. However, when Aβ accumulates excessively, it binds various receptors on the surface of microglia such as cluster of differentiation (CD) 14, CD36 [130], Toll-like receptor (TLR) 4, and TLR2, to promote the release of inflammatory factors [131]. This response leads to neuroinflammation and further impairment of cognitive function. Similarly, astrocyte activation is also associated with Aβ [132]. Activated astrocytes produce various inflammatory mediators [133] and secrete small amounts of Aβ [134], resulting in a chronic vicious cycle [133]. These reactions can exacerbate neuroinflammation and damage neurons.

In recent years, neuroinflammation has become recognized as a third neuropathological hallmark of AD, in addition to Aβ and NFTs ^[97]. Neuroinflammation is an inflammatory response to pathological insults that occur within the central nervous system (CNS), such as infection, trauma, ischemia, and toxin accumulation ^[98]. This response is characterized by the proliferation and activation of microglia and astrocytes in the brain ^[99]. Such activation is usually accompanied by the secretion of inflammatory cytokines such as interleukin-1β (IL-1β), interleukin 6 (IL-6), and tumor necrosis factor α (TNF-α) ^[100]. Microglia are innate immune cells that reside within the CNS and are thought to be important regulators in the inflammatory response within the brain ^[101]. In the early stages of AD, activated microglia can exert protective effects by phagocytosing and degrading Aβ ^[102][103]. However, when Aβ accumulates excessively, it binds various receptors on the surface of microglia such as cluster of differentiation (CD) 14, CD36 ^[104], Toll-like receptor (TLR) 4, and TLR2, to promote the release of inflammatory factors ^[105]. This response leads to neuroinflammation and further impairment of cognitive function. Similarly, astrocyte activation is also associated with Aβ ^[106]. Activated astrocytes produce various inflammatory mediators ^[107] and secrete small amounts of Aβ ^[108], resulting in a chronic vicious cycle ^[107]. These reactions can exacerbate neuroinflammation and damage neurons. Multiple pathways are involved in microglia- and astrocyte-mediated neuroinflammation ^[109], one of which is the nuclear factor kappa-B (NF-κB) pathway ^[110]. This pathway activates the transcription of TNF-α, IL-1β, and IL-6, which exacerbates inflammation ^[111]. Microglial activation also depends on other signaling pathways such as the PI3K/Akt/mTOR pathway ^[112] which can promote the expression of the inflammatory mediators NO synthase (NOS) and cyclooxygenase-2 (Cox-2) ^[113][114]. Furthermore, MAPK/p38 ^[115], Janus kinase (JAK)/signal transducer, and activator of transcription (STAT) and other pathways also contribute to neuroinflammation ^[116]. Therefore, modulating inflammatory signaling pathways and inhibiting the production of proinflammatory factors may offer a promising approach to the treatment of AD.

Multiple pathways are involved in microglia- and astrocyte-mediated neuroinflammation [135], one of which is the nuclear factor kappa-B (NF-κB) pathway [136]. This pathway activates the transcription of TNF-α, IL-1β, and IL-6, which exacerbates inflammation [137]. Microglial activation also depends on other signaling pathways such as the PI3K/Akt/mTOR pathway [138]which can promote the expression of the inflammatory mediators NO synthase (NOS) and cyclooxygenase-2 (Cox-2) [139,140]. Furthermore, MAPK/p38 [141], Janus kinase (JAK)/signal transducer, and activator of transcription (STAT) and other pathways also contribute to neuroinflammation [142]. Therefore, modulating inflammatory signaling pathways and inhibiting the production of proinflammatory factors may offer a promising approach to the treatment of AD.

2.4. Improvement in Mitochondrial Function and Antioxidative Stress

Oxidative stress refers to an imbalance between oxidation and antioxidation in the body ^[117][174], which is mainly characterized by the increased production of reactive oxygen species and reduced ability of antioxidants to combat them ^[118][175]. The brain is susceptible to oxidative damage due to its high lipid content and lacks effective antioxidant defense mechanisms, despite high oxygen consumption ^[119][120][176,177]. It is now well established that the level of oxidation in the brain also increases with age ^[121][178]. Interestingly, extensive oxidative damage can be observed in the stage of mild cognitive impairment that precedes the typical clinical manifestations of AD, suggesting that oxidative stress may be a central mechanism in driving the disease ^[122][179].

Several natural saponins may act as exogenous antioxidants to combat AD by managing oxidative stress ^[123][124][192,193]. Ginsenoside Rh2, a rare ginsenoside with few reports on its neuroprotective effects compared to other similar molecules ^[125][194]_, has shown significant neuroprotective effects in a scopolamine-induced model of memory impairment in ICR mice. Ginsenoside Rh2 effectively managed MDA levels while increasing glutathione (GSH) levels and SOD activity in the brain, effectively inhibiting oxidative stress ^[126][195]. These findings suggest that ginsenoside Rh2 has great potential for the treatment of AD by inhibiting oxidative stress in the brain. Crude saponins from BF showed potent antioxidant effects in addition to anti-inflammatory effects. In addition, saponins from BF alleviated oxidative stress, reduced ROS, and MDA levels by restoring GSH-Px, GSH, and CAT levels in the Aβ ICV-induced BALB/c AD mouse model ^[127][161]. Similarly, ginsenoside Rd was found to reduce protein carbonyls and 4-hydroxy-2-nonenal (4-HNE) levels by decreasing the glutathione disulfide (GSSG)/GSH ratio, thereby counteracting oxidative stress in the brain of Aβ-induced AD mouse models ^[128][170]. Ginsenoside Rg3, a tetracyclic triterpenoid saponin, is the major bioactive component of ginseng and is widely used for the treatment of tumors ^[129][196]. Results from in vivo studies have shown that ginsenoside Rg3 can improve mitochondrial dysfunction and enhance ROS scavenging ability by increasing SOD, CAT, and GSH-Px levels, thus ameliorating oxidative stress in the brain of D-gal-induced AD rats ^[130][197]. PF11 was found to inhibit the production of both APP and Aβ in the cortex and hippocampus, while restoring the activity of SOD and GSH-Px in the brains of AD mice and reducing MDA levels in the cortex ^[131][198]. Ginsenoside Rg1 has also been found to protect primary cortical neurons against Aβ-induced increases in ROS and H₂O₂ levels by inhibiting oxidative stress ^[132]. This results in a decrease in mitochondrial membrane potential and an increase in cytochrome 通过抑制氧化应激来降低水平[199]。这导致线粒体膜电位降低，细胞色素C (（Cyt-c) oxidase activity, ultimately leading to a decrease in Cyt-c release and indicating improved mitochondrial function ^[132]. Another newly discovered saponin compound, huangjingsterol ）氧化酶活性增加，最终导致Cyt-c释放减少，表明线粒体功能得到改善[199]。另一种新发现的皂苷化合物，来自虎杖的黄晶甾醇B from Polygonatum cyrtonema Hua, showed significant antioxidant activity ^[133] by reducing oxidative stress and protecting cells from Aβ-induced damage through modulating the activity of ，通过调节GSH-Px and 和SOD ^[133]. 的活性来减少氧化应激并保护细胞免受Aβ诱导的损伤，显示出显着的抗氧化活性[200]。同样，研究人员发现，来自Similarly, researchers have found that a saponin from the fruit of Solanum anguivi can protect the P2 region of rat brain synaptosomes from oxidative damage induced by ferrous iron and sodium nitroprusside, reduce ROS generation, restore total thiol levels, and protect mitochondrial function ^[123]. This suggests that saponins from Solanum anguivi fruits may have antioxidant properties that benefit mitochondrial function ^[134]. 果实的皂苷可以保护大鼠脑突触体的P200区域免受亚铁和硝普钠诱导的氧化损伤，减少ROS生成，恢复总硫醇水平，并保护线粒体功能[2]。这表明来自茄果实的皂苷可能具有抗氧化特性，有益于线粒体功能[192]。PGS has a similar effect with its ability to upregulate intracellular heme oxygenase具有类似的效果，能够上调细胞内血红素加氧酶-1 (HO-1), SOD, CAT, and 201（HO-1）、SOD、CAT和GSH-Px to inhibit Aβ-induced ROS production and reduce oxidative damage in AD brains ^[135]，以抑制Aβ诱导的ROS产生并减少AD大脑中的氧化损伤[1]。尽管人参具有神经保护作用，但其抗氧化特性的机制仍然知之甚少. Despite the demonstrated neuroprotective effects of Panax ginseng, the mechanisms underlying its antioxidant properties remain poorly understood. However, a recent study has shown that the primary bioactive compound in Panax ginseng, 然而，最近的一项研究表明，人参中的主要生物活性化合物NTR1, is able to prevent the loss of mitochondrial membrane potential and reduce the accumulation of ROS induced by Aβ ^[136]. This suggests that 53能够防止线粒体膜电位的丧失，并减少Aβ诱导的ROS积累[1]。这表明NTR1 may exert neuroprotective effects by inhibiting oxidative stress. Furthermore, most previous experiments have focused on saponins extracted from Panax ginseng roots, and the pharmacological effects of other parts, such as saponins in P. notoginseng leaves (202可能通过抑制氧化应激来发挥神经保护作用。此外，以前的大多数实验都集中在从人参根中提取的皂苷上，其他部分的药理作用，如三七叶（LPNS), are largely unknown ^[137]. ）中的皂苷，在很大程度上是未知的[1]。LPNS, which mainly include the ginsenosides 主要包括人参皂苷Rb203、Rb1, Rb2, Rb3, and Rc,、Rb2和Rc，已被证明具有有效的神经保护特性[3]。在 have been shown to have potent neuroprotective properties ^[137]. In H₂O₂- or oxygen and glucose deprivation (OGD)-induced astrocytes from the cortex of 或来自SD rats and 大鼠和SH-SY5Y cells, LPNS increased the levels of nuclear factor erythroid2-related factor 2 (Nrf2) and its downstream HO-1 and glutathione S-transferase pi 1 (GSTP1) in response to increased ROS, as well as significantly reducing lactate dehydrogenase (LDH) expression and increasing the cellular activity of astrocytes ^[137].细胞皮层的氧和葡萄糖剥夺（OGD）诱导的星形胶质细胞，LPNS增加了核因子红细胞2相关因子2（Nrf2）及其下游HO-1和谷胱甘肽S-转移酶pi 1（GSTP1）的水平，以响应ROS升高，以及显着降低乳酸脱氢酶（LDH）表达并增加星形胶质细胞的细胞活性[203]。

2.5. Antiapoptotic Effect

2.5. 抗凋亡作用

It is widely accepted that neuronal apoptosis contributes significantly to the pathogenesis of 人们普遍认为，神经元细胞凋亡对AD ^[138] and is even a pathological hallmark of the disease ^[139]. Neuronal apoptosis has been found to be abundant in 的发病机制有显著贡献[213]，甚至是该疾病的病理标志[214]。AD autopsy samples ^[140] and several 尸检样本中发现神经元凋亡很多[215]，一些AD models have successfully reproduced this apoptosis ^[141]. The biochemical signature of apoptosis is mainly the activation of cysteinase triggered by intrinsic or extrinsic apoptotic pathways ^[142]. Apoptosis triggered by external cellular signals is called the 模型已经成功地再现了这种细胞凋亡[216]。细胞凋亡的生化特征主要是由内在或外在凋亡途径触发的半胱氨酸酶的活化[217]。由外部细胞信号触发的细胞凋亡称为“death receptor“ pathway, which involves the activation of caspase死亡受体”途径，它涉及半胱天冬酶-8 followed by cleavage of caspase-3, leading to apoptosis ^[143]. In contrast, the intrinsic apoptotic pathway is driven by the mitochondria ^[144]. Mitochondrial damage induces the release of 的激活，然后是半胱天冬酶-3的切割，导致细胞凋亡[218]。相反，内在凋亡途径由线粒体驱动[219]。线粒体损伤诱导Cyt-c and other apoptotic factors into the cytoplasm, which then activate caspase和其他凋亡因子释放到细胞质中，进而激活半胱天冬酶-9 and caspase-3, resulting in apoptosis ^[145]. The 和半胱天冬酶-3，导致细胞凋亡[220]。B细胞淋巴瘤-cell lymphoma-2 (Bcl-2) protein family includes proapoptotic proteins, BCL2-associated X (BAX), Bcl-2 antagonist killer 1 (BAK), Bcl-2-like protein 11 (BIM), p53 upregulated modulator of apoptosis (PUMA), and Bcl2 modifying factor (BMF), and antiapoptotic proteins, such as Bcl-2, lymphoma-extra-large (2（Bcl-2）蛋白家族包括促凋亡蛋白，BCL2相关X（BAX），Bcl-2拮抗剂杀手1（BAK），Bcl-2样蛋白11（BIM），p53上调细胞凋亡调节剂（PUMA）和Bcl2修饰因子（BMF），以及抗凋亡蛋白，如Bcl-2，淋巴瘤超大（Bcl-xL), Bcl-2 like 2 (Bcl-w), and myeloid cell leukemia-1 (Mcl-1), which tightly regulates this apoptotic process by controlling the ratio of proapoptotic and antiapoptotic proteins ^[146][147], particularly the ），Bcl-2样2（Bcl-w）和骨髓细胞白血病-1（Mcl-1），它们通过控制促凋亡蛋白和抗凋亡蛋白的比例来严格调节这种凋亡过程[221，222]，特别是Bax/Bcl-2 ratio ^[148]. In 比值[223]。在AD patients, 患者中，Bcl-2 expression is reduced in the hippocampus ^[149], while 在海马体中的表达减少[224]，而Bax accumulates near senile plaques and tau protein fiber tangles in an 在AD animal model, suggesting that mitochondria-mediated apoptosis is closely linked to the pathology of AD ^[150].动物模型中积聚在老年斑块和tau蛋白纤维缠结附近，提示线粒体介导的细胞凋亡与AD的病理密切相关[225]。 Numerous saponins, including astragaloside (许多皂苷，包括黄芪甲苷（AST), NTR2, PGS, AS-IV, NTR1, Ginsenoside Rg2, and Ginsenoside Rg1, have shown antiapoptotic effects in AD-associated models. AST, a small molecule saponin derived from ），NTR2，PGS，AS-IV，NTR1，人参皂苷Rg2和人参皂苷Rg1，已在AD相关模型中显示出抗凋亡作用。AST是一种来源于膜黄芪的小分子皂苷，在中国已被广泛用于慢性病的治疗[226]。在体内和体外实验中，Astragalus membranaceus, has been widely used in China for the treatment of chronic diseases ^[151]. In in vivo and in vitro experiments, AST can prevent Aβ-induced apoptosis in cortical neurons by modulating the 可以通过调节PI3K/Akt and MAPK (or ERK) pathway ^[152]. This effect is reversed by the 和MAPK（或ERK）途径来预防Aβ诱导的皮质神经元凋亡[227]。PI3K inhibitor 抑制剂LY294002 and mimicked by the inhibition of ERK and enhanced by its inhibitor U0126, suggesting that the neuroprotective effect of AST may also be related to the inhibition of the ERK pathways ^[152]. Similarly, 逆转了这种作用，并通过抑制ERK和抑制剂U0126增强，表明AST的神经保护作用也可能与ERK通路的抑制有关[227]。同样，NTR2 can reverse the Aβ-induced apoptosis of pri可以通过mary cortical neurons via miiR-27a/SOX8/axis and ameliorate cognitive dysfunction in AD mice ^[153]. 轴逆转Aβ诱导的原代皮质神经元凋亡，并改善AD小鼠的认知功能障碍[159]。PGS may provide neuroprotection not only by reducing inflammation but also by inhibiting apoptosis. 不仅可以通过减少炎症来提供神经保护，还可以通过抑制细胞凋亡来提供神经保护。PGS increased the expression of Bcl-2 family proteins, resulting in a reduction in Cyt-c release and caspase-9 and -3 expression, thereby inhibiting Aβ-induced apoptosis in HT22 cells. PGS achieved this by inhibiting p38, ERK, and JNK activation, as evidenced by decreased levels of 增加了Bcl-2家族蛋白的表达，导致Cyt-c释放和半胱天冬酶-9和-3表达减少，从而抑制Aβ诱导的HT22细胞凋亡。PGS通过抑制p38、ERK和JNK激活来实现这一点，p-p38/p38, 、p-ERK/ERK, and 和p-JNK/JNK, and by downregulating MAPK signaling ^[135]. In vitro and in vivo experiments constructed in 水平降低，以及下调MAPK信号传导[153]。在HT-22 cells and C细胞和淀粉样蛋白β蛋白片段57BL/6 induced by amyloid beta protein fragment 1-42 oligomers (AβO) have shown that AS-IV reduced the Aβ-induced inhibition of the brain-derived neurotrophic factor (BDNF)–tyrosine kinase receptor B (TrkB) signaling pathway by regulating PPARγ ^[154]. This mechanism lea-6寡聚物（AβO）诱导的C1BL/42中构建的体外和体内实验表明，AS-IV通过调节PPARγ降低了Aβ诱导的对脑源性神经营养因子（BDNF）-酪氨酸激酶受体B（TrkB）信号通路的抑制[228]。这种机制导致末端脱氧核苷酸转移酶ds to the reduction in the number and fluorescence density of terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL)-positive cells and caspase-3 levels, ultimately alleviating hippocampal neuronal apoptosis and memory impairment ^[154]. In 切口末端标记（TUNEL）阳性细胞的数量和荧光密度降低和半胱天冬酶-3水平，最终缓解海马神经元凋亡和记忆障碍[228]。在Aβ-induced 诱导的PC12 cells, NTR1 has also been shown to exert an antiapoptotic effect, reducing the percentage of TUNEL-positive cells and the levels of caspase-3, Bax, and the 细胞中，NTR1也被证明具有抗凋亡作用，降低TUNEL阳性细胞的百分比以及半胱天冬酶-3、Bax和Bax/Bcl-2 ratio, and exerted an antiapoptotic effect by suppressing the MAPK pathway ^[136]. 比值的水平，并通过抑制MAPK途径发挥抗凋亡作用[202]。