Advances and Therapeutic Potential of Anthraquinone Compounds in Neurodegenerative Diseases: A Comprehensive Review

Introduction

The worldwide socioeconomic burden of neurodegenerative diseases (NDDs), such as Alzheimer’s disease (AD), Parkinson’s disease (PD), and amyotrophic lateral sclerosis (ALS), is growing significantly as life expectancy rises.¹ Due to its irreversibility and lack of effective treatment, NDDs are becoming highly prevalent.² Numerous NDDs advance to dementia,³ which is anticipated to impact around 150 million individuals worldwide by 2050, incurring an economic burden of $10 trillion and incalculable costs to patients and their families.^4,5

Despite the wide variability in the pathological presentations of NDDs, they exhibit comparable pathological characteristics, including the progressive deterioration of neuronal structure and function, along with analogous clinical symptoms, such as cognitive, motor, and behavioral deficits.^6–8 NDDs are intricately linked to cerebrovascular conditions, including cerebral ischemia-reperfusion injury (CIRI), traumatic brain injury, intracerebral hemorrhage (ICH), and ischemic stroke (IS), as well as neurological disorders such as epilepsy (EPI). Clinical and pathological evidence suggests that they share pathogenic mechanisms, including neuroinflammation, oxidative stress (OS), pyroptosis, and mitochondrial dysfunction.^9–13 Moreover, converging evidence indicates that vascular dysfunction is a key pathological component of NDDs. Vascular alterations occur in the majority of NDDs, particularly in capillaries. Recent investigations have demonstrated that these modifications induce vascular dysfunction and facilitate the course of NDDs.^14–17 In addition, there is a close clinical correlation among these diseases. Patients with Vascular Cognitive Impairment Disorder (VCID) frequently exhibit episodic memory impairments, challenges in word retrieval, temporal disorientation, and mild executive dysfunctions. Consequently, distinguishing AD from VCID can be challenging, as they are often indistinguishable during clinical evaluation.¹⁸ Cerebral ischemia can induce the aggregation of proteins linked to neurodegenerative diseases.¹⁹ Stroke-induced secondary neurodegeneration shares a number of striking similarities to other NDDs, such as AD, especially with the substantial accumulation of the neurotoxic protein amyloid-β (Aβ).²⁰ There is growing evidence indicating the presence of clinical and sub-clinical EPI in AD, Lewy body dementia, PD, and other less prevalent NDDs. For instance, Epidemiological studies have shown that unprovoked EPI occurs in 10–22% of patients with AD. Furthermore, biomarkers related to NDDs have been detected in the cerebrospinal fluid or blood of patients with EPI, such as Aβ, Tau, and α-synuclein (α-syn).^21,22 Some studies have grouped these diseases with typical NDDs.^23,24 Therefore, these diseases were adopted for our discussion.

In the fight against NDDs, extensive research has sought to elucidate mechanisms and potential therapeutic targets.⁸ However, current therapeutic approaches offer no permanent solution and only provide symptomatic relief.⁶ Natural products, particularly those derived from medicinal plants, have been utilized for centuries in disease therapy, supported by their inherent multi-component, multi-target, and multi-pathway synergistic mechanisms.²⁵ Their intrinsic ability to cross the blood-brain barrier (BBB) and act on intracellular targets has made small-molecule compounds highly attractive for neurotherapeutic development.²⁶

The Biological Activity and Mechanism of Anthraquinones

Anthraquinones, primarily derived from rhubarb (Rheum officinale Baill., Rheum tanguticum Maxim. ex Balf., Rheum palmatum L)., comprise emodin (EM), aloe-emodin (AE), chrysophanol (CHR), rhein (RHE), and physcion (PHY). Chemically, these compounds share a planar anthraquinone core with diverse substituent patterns. CHR is 1,8-dihydroxy-3-methylanthraquinone,²⁷ EM is 1,3,8-trihydroxy-6-methylanthraquinone,²⁸ and RHE is 4,5-dihydroxyanthraquinone-2-carboxylic acid.²⁹ AE and PHY share a similar skeleton, with variations in hydroxyl, methyl, and carboxyl groups,³⁰ as shown in Figure 1. Their structural arrangement facilitates reversible oxidation-reduction processes.³¹

Figure 1 The chemical structure of anthraquinones.

Functionally, these anthraquinones provide diverse neuroprotection via antioxidant and mitochondrial-supporting mechanisms, particularly by activating the nuclear factor erythroid-2-related factor 2 (Nrf2) and sirtuin1/peroxisome proliferator-activated receptor-γ coactivator-1α (SIRT1/PGC-1α) pathways. They effectively modulate neuroinflammation by inhibiting NLRP3 inflammasome activation, suppressing pro-inflammatory cytokines, and regulating microglial polarization towards an anti-inflammatory M2 phenotype. Additionally, anthraquinones mitigate neuronal loss by modulating intricate signaling pathways that regulate cell death processes, including apoptosis, pyroptosis, and ferroptosis. By suppressing Aβ aggregation and tau hyperphosphorylation, these compounds directly engage the central molecular pathology of AD. Beyond AD, they also exert neuroprotective effects in PD and Huntington’s disease (HD) models, attenuating motor deficits and proteinopathy. The therapeutic effects and molecular mechanisms of anthraquinones on NDDs have been summarized in Tables 1, 2 and 3.

Table 1 Summary of Preclinical Information on the Therapeutic Effects and Molecular Mechanisms of Emodin on NDDs

Table 2 Summary of Preclinical Information on the Therapeutic Effects and Molecular Mechanisms of CHR on NDDs

Table 3 Summary of Preclinical Information on the Therapeutic Effects and Molecular Mechanisms of RHE, PHY, and AE on NDDs

Nevertheless, no summary of contemporary pharmacological research in this area focuses on action targets and pathways. Consequently, emerging evidence has delineated the molecular targets and signaling pathways by which anthraquinones modulate NDDs pathology. The pathway regulation related to AD is plotted in Figure 2, whereas Figures 3 and 4 present the corresponding pathways for PD and IS, respectively. A comprehensive review of the included literature aims to provide valuable references for future research and accelerate the clinical application of anthraquinones.

Figure 2 The molecular mechanism of anthraquinone-based treatment for AD. Anthraquinones exert anti-AD effects primarily by regulating OS and mitochondrial homeostasis, suppressing neuroinflammation, and inhibiting programmed cell death. Specifically, these pathways involve activation of antioxidant signaling and mitochondrial biogenesis, leading to reduced ROS and restored mitochondrial function; inhibition of inflammasome-related inflammatory signaling and microglial activation, resulting in decreased pro-inflammatory cytokine production; and modulation of apoptosis and ferroptosis through the regulation of caspase activation and LPO. In addition, pathways associated with Aβ and Tau pathology, neurotrophic support, calcium signaling, and metal homeostasis are also involved. Collectively, these multitarget actions converge to mitigate AD-associated neurodegeneration.

Figure 3 The molecular mechanism of anthraquinone-based treatment for PD. Anthraquinones exert anti-PD effects primarily by modulating redox balance, inflammatory signaling, and mitochondrial integrity. Regulation of antioxidant responses limits excessive ROS accumulation and mitigates OS, while coordinated modulation of MAPK- and PI3K-associated signaling dampens inflammatory cascades and reduces pro-inflammatory cytokine release. In parallel, restoration of mitochondrial function and energy metabolism contributes to the restraint of apoptotic and ferroptotic cell death. Additional regulatory processes, including α-syn aggregation, metal ion homeostasis, and neurotrophic factor support, further participate in the overall neuroprotective outcome in PD.

Figure 4 The molecular mechanism of anthraquinone-based treatment for IS. Anthraquinones exert anti-IS effects primarily by suppressing inflammation, OS, and programmed cell death. ERK1/2, Nrf2, and NLRP3 signaling pathways were involved. Meanwhile, promotion of the BBB function contributes to overall neuroprotection in IS.

Molecular Mechanisms of Neuroprotection by Anthraquinones

Anti-Oxidative Stress

Due to its high lipid content and substantial oxygen demand, the brain is particularly susceptible to OS.⁶⁷ Excessive generation of reactive oxygen species (ROS) leads to OS, a central pathological hallmark of NDDs that drives neuronal dysfunction and cell death.^68–70 Most NDDs, including AD, PD, HD, and ALS, are characterized by elevated ROS levels and compromised antioxidant defense mechanisms.⁶⁸

Nrf2

Nuclear factor erythroid 2-related factor 2 (Nrf2) serves as the master regulator of the cellular antioxidant defense system, orchestrating redox homeostasis and protecting against oxidative injury.^71–73 Nrf2 regulates the expression of numerous antioxidant enzymes, including heme oxygenase-1 (HO-1), glutathione reductase, superoxide dismutase-1 (SOD-1), and catalase (CAT), all of which can scavenge endogenous ROS.^74,75 Under resting conditions, Nrf2 is primarily localized in the cytoplasm, where it interacts with Kelch-like ECH-associated protein 1.⁷⁶ During the occurrence and development of OS, Nrf2 translocates to the nucleus and binds to antioxidant response elements (ARE), triggering the activation of several downstream proteins and detoxifying enzymes. This process regulates the expression of many antioxidant genes and maintains a proper redox balance.^77–79

CHR

CHR activated the Nrf2/HO-1 pathway in the hippocampus and cortex of PTZ-induced epileptic mice, decreasing OS markers (malondialdehyde [MDA] and nitric oxide [NO]) and increasing antioxidant enzymes (glutathione S-transferase and CAT). Molecular docking confirmed that CHR directly interacted with Nrf2.⁴⁵

Emodin

In PC12 cells exposed to Aβ25-35, the translocation of Nrf2 from the cytoplasm to the nucleus was inhibited, whereas emodin reversed this effect. Emodin treatment groups at varying doses significantly reduced OS and enhanced the expression levels of cystine/glutamate antiporter and glutathione peroxidase-4 (GPX4).³² Similarly, emodin upregulated Nrf2 and its downstream effectors (HO-1, SOD-1, and CAT) to reduce ROS and preserved mitochondrial membrane potential (MMP) in AD models.²⁸

RHE

According to Cai et al, RHE treatment significantly increased glutathione (GSH) levels and GPX4 expression in the brain, while markedly reducing MDA content compared with the middle cerebral artery occlusion (MCAO) group, suggesting a reduction in OS. Additionally, total Nrf2 expression increased distinctly in the H-RHE group, and the protein bands in the RHE-treated group showed higher intensity than those in the MCAO group. Evaluations of GPX4 and nuclear Nrf2 yielded similar findings.²⁹

PHY

Ahmad et al confirmed that PHY elevated Nrf2 and HO-1 expression, and reversed the effects of LPS-induced OS. The concentration of GSH in the cortex and hippocampus decreased after LPS treatment but was significantly increased following PHY treatment, while MDA levels were markedly reduced.⁶⁴

ERK1/2

Extracellular signal-regulated protein kinases 1/2 (ERK1/2) belong to the mitogen-activated protein kinase (MAPK) family and play crucial roles in neuronal differentiation, survival, structural plasticity, long-term potentiation, and memory formation.⁸⁰ Transient ERK activation promotes neuronal survival,⁸¹ whereas sustained ERK hyperphosphorylation drives premature senescence, apoptosis, and dysregulated autophagy.⁸² An abnormal accumulation of activated ERK1/2 in neurons has been documented in AD brains.⁸³ In vivo and in vitro, phosphorylated ERK expression was markedly elevated in the PD model group.⁸⁴

SH-SY5Y cells have been utilized in numerous models of NDDs, including PD and AD.^85,86 Chen et al exposed SH-SY5Y 26 cells to zinc sulfate and different concentrations of emodin, followed by assessment of ERK1/2 expression, OS, and mitochondrial function. Emodin pretreatment rectified zinc-induced synaptic dysfunction, diminished OS by inhibiting phospho-ERK1/2 signaling, and curtailed mitochondrial dissipation.³⁸ In the rat MCAO model, emodin reduced infarct volume and cell death, and additionally restored PC12 cells’ viability while decreasing ROS generation and glutamate release under ischemic and hypoxic conditions, possibly via activation of the ERK1/2 signaling pathway.³⁹

PI3K/mTOR

The phosphatidylinositol 3-kinase (PI3K)/mammalian target of rapamycin (mTOR) signaling pathway is a critical regulator of cellular survival, proliferation, and metabolism.⁸⁷ Growth hormone, oxygen, amino acid availability, and cellular energy status all influence mTORC1 activity.⁸⁸

PI3K/AKT/mTOR

PI3K phosphorylates protein kinase B (AKT), triggering downstream effects that stimulate mTOR. AKT is essential for neuronal proliferation and synapse formation.⁸⁹ Numerous NDDs, such as AD,⁹⁰ multiple sclerosis (MS),⁹¹ HD,⁹² and PD,⁹³ have been linked to the pathophysiology of the PI3K/AKT/mTOR signaling pathway.

In ICH-associated neurological defects in rats, CHR was neuroprotective by reversing neurobehavioral changes and dramatically lowering the increased levels of PI3K, AKT, and mTOR proteins. The treatment doses of CHR at 10 mg/kg and 20 mg/kg resulted in significant increases in GSH, CAT, SOD, and GPX levels compared with the vehicle and sham control groups.⁴⁶

PI3K/mTOR/GSK3β

Glycogen synthase kinase-3 (GSK-3) is an evolutionarily conserved serine/threonine kinase prevalent in mammalian eukaryotic cells.⁴¹ GSK3β, a GSK-3 isoform, plays a critical role in neuronal cell death, and its pharmacological inhibition has been shown to reduce α-syn accumulation in PD.⁹⁴

In H2O2-induced SH-SY5Y cells, p-PI3K, p-mTOR, and p-GSK3β expression were significantly elevated, whereas emodin significantly reversed the above expression. Emodin could prevent the activation of the PI3K/mTOR/GSK3β pathway induced by H2O2, thereby exerting neuroprotective effects.⁴¹

ROS/TXNIP

Thioredoxin-interacting protein (TXNIP), also known as thioredoxin (TRX) binding protein-2,⁹⁵ possesses an α-arrestin domain that engages with and diminishes the activity of cytosolic TRX and mitochondrial TRX (activated form), hence regulating cellular redox signaling.⁹⁶ TRX1 and TRX2 modulate ROS levels in the cytoplasm and mitochondria, respectively. However, TXNIP inhibits TRX’s antioxidant function. Consequently, TXNIP acts as a pro-oxidant, augmenting ROS production and leading to OS.⁹⁵

In the d-galactose- and Aβ25-35-induced AD model, CHR significantly improved the survival rate and attenuated OS. Furthermore, CHR markedly decreased MDA and lactate dehydrogenase (LDH) levels, concomitantly enhancing the activities of T-SOD, CAT, and GSH. Mechanistically, CHR simultaneously downregulated TXNIP at the protein and mRNA levels and upregulated TRX expression, likely via modulation of the ROS/TXNIP signaling pathway.⁴⁷

Mitochondrial Protection

Within the central nervous system (CNS), mitochondria are essential for multiple cellular processes, including neurotransmitter metabolism, ROS generation, calcium homeostasis, and regulation of cell death.⁹⁷ Mitochondria are the principal source of intracellular ROS and serve as critical targets of Aβ toxicity during AD pathogenesis.⁹⁸ Mitochondrial impairment is characterized by diminished metabolic activity, elevated ROS generation, membrane permeability transition, and the release of mitochondrial proteins into the cytoplasm.⁷⁴ Anthraquinones exert mitochondrial protection via multiple pathways.

Promotion of Mitochondrial Biogenesis

Mitochondrial biogenesis is the process by which new mitochondria are generated from pre-existing mitochondria.⁹⁹ Mitochondrial dysfunction is considered a pivotal factor in the pathogenesis of various neurological illnesses, and adaptive mitochondrial biogenesis has been observed in the nervous system.¹⁰⁰ Moreover, mitochondrial biogenesis, an intrinsic antioxidant defense mechanism, is actively upregulated in pathological conditions to counteract intracellular OS.^101,102

SIRT1/PCG-1α

SIRT1 is an NAD⁺-dependent protein deacetylase that belongs to the SIRT family and plays a vital role in aging, cell death, mitochondrial biogenesis, inflammation, and energy regulation by deacetylating histone and a variety of non-histone proteins.^103–105 SIRT1 deacetylates and activates the mitochondrial regulator PGC-1α.⁵⁹ Activated PGC-1α targets and enhances the expression of downstream mitochondrial transcription factor A (TFAM). TFAM stimulates mitochondrial DNA transcription and replication, producing new, healthy mitochondria.^60,106,107

Yin et al demonstrated that RHE alleviated mitochondrial OS in Aβ oligomer-induced primary neurons by enhancing SIRT1/PGC-1α-mediated mitochondrial biogenesis.⁵⁹ In the Aβ precursor protein/presenilin 1 (APP/PS1) transgenic mouse model of AD, RHE considerably improved cognitive and memory deficits. SIRT1, PGC-1α, and nuclear respiratory factor 1 (NRF1) expression were significantly lower in the APP/PS1 group than in the WT (age-matched wild-type mice with the same background) group, indicating compromised mitochondrial biogenesis. However, RHE therapy significantly raised SIRT1, PGC-1α, and NRF1 expression. These results suggested that RHE enhanced SIRT1/PGC-1α-mediated mitochondrial biogenesis.⁶⁰ Moreover, RHE-loaded nanoparticles (K8@Fe−Rh/Pda NPs) activated SIRT1/PGC-1α pathway, thereby correcting mitochondrial dynamics and increasing cytochrome c oxidase activity in Aβ42 oligomer-burdened neurons, helping repair damage triggered by Aβ aggregation.¹⁰⁸

Facilitation of Mitophagy

Cellular mitophagy refers to the selective encapsulation and degradation of damaged mitochondria by cells via an autophagic process, thereby preserving mitochondrial integrity.¹⁰⁹ Mitophagy plays a vital part in energy availability, neuronal survival, mitochondrial and metabolic balance, and general health.¹¹⁰ OS, nutritional deficiencies, and aging can cause mitochondrial depolarization, leading to mitochondrial dysfunction and disruption of cellular homeostasis, which are closely associated with NDDs.^65,111–113 Four primary steps make up mitophagy: 1) Damaged mitochondria lose their membrane potential and depolarize, 2) Autophagosomes wrap mitochondria to produce mitochondrial autophagosomes, 3) Mitochondrial autophagosomes merge with lysosomes, and 4) Lysosomes break down mitochondrial contents.¹¹⁴ Several studies have shown that AD patients’ brains exhibit aberrant mitophagy, which impairs the normal clearance of damaged mitochondria.¹⁰⁹

In the APP/PS1 double transgenic mouse model, AE reduced hippocampal neuronal damage and ameliorated cognitive dysfunction. AE treatment markedly decreased sequestosome 1 (p62), Beclin-1, microtubule-associated protein 1 light chain 3 beta (LC3B), PGC-1α, p-AMPK, and SIRT3, suggesting that AE induced mitophagy via the AMPK/PGC-1α/SIRT3 pathway.⁶⁵

Maintenance of Mitochondrial Integrity

Structure determines function. Maintaining mitochondrial structure integrity is another crucial way to exert mitochondrial function.

CHR

In Aβ25-35-induced PC12 cells, transmission electron microscopy revealed that the model group showed substantial mitochondrial morphological damage, including reduced cristae and fractured mitochondrial membranes. The CHR-treated model group displayed markedly improved mitochondrial morphology, as evidenced by restoration of mitochondrial cristae and double-membrane structures.⁴⁹

Emodin

Xia et al demonstrated that emodin reversed MMP impairment in Aβ25-35-treated PC12 cells.³² Ubiquinol-cytochrome c reductase core protein 1 (UQCRC1), a crucial part of mitochondrial complex III, sustains bioenergetic homeostasis via mitochondrial preservation and apoptotic control. In the MPP⁺-induced PD model, emodin could enhance UQCRC1 expression, stabilize mitochondrial function, and prevent MMP collapse.⁴⁰

Anti-Inflammatory Effects

Neuroinflammation, a core CNS response to acute injury and chronic NDDs, plays a central role in neurodegeneration and disease progression.^115,116 Anthraquinones exert anti-neuroinflammatory effects mainly through the following pathways.

Regulation of Inflammatory Signaling Pathways

NLRP3 Inflammasome

The NLRP3 inflammasome is a complex that orchestrates the innate immune response by detecting diverse damage and pathogen-associated molecular patterns.¹¹⁷ Three major components of the NLRP3 inflammasome are NLRP3, the adaptor protein apoptosis-associated speck-like protein containing a caspase activation and recruitment domain (ASC), and pro-caspase-1.¹¹⁸ Upon receiving a danger signal, NLRP3 activates ASC, which attracts pro-caspase-1 and cleaves it into active caspase-1, which facilitates the maturation of pro-interleukin-1β (IL-1β) and induces inflammation.¹¹⁹ Enhanced activation of NLRP3 has been associated with various chronic conditions, including neurodegenerative disorders.¹²⁰

Inhibition of Inflammation

In Aβ25-35-induced PC12 cells, the expression of key inflammatory signaling molecules, including Toll-like receptor-4 (TLR4), phosphorylated nuclear factor kappa-B (p-NF-κB), NLRP3, as well as the proinflammatory cytokines IL-1β and tumor necrosis factor-alpha (TNF-α), were markedly increased. Emodin effectively attenuated signaling protein levels and cytokine levels. These findings provide compelling evidence that emodin downregulated the TLR4/p-NF-κB/NLRP3 pathway and exerted anti-inflammatory effects in AD.³² Experimental autoimmune encephalomyelitis (EAE) is presently the predominant animal model for MS.¹²¹ In this model, microglia and the NLRP3 inflammasome intensified neuroinflammation, which was crucial for the onset of MS. Emodin suppressed NLRP3 inflammasome signaling by upregulating SIRT1 and PGC-1α, thereby attenuating demyelination and neuroinflammatory responses and ultimately ameliorating behavioral deficits and neuropathological alterations.⁴²

Zhang et al reported that cerebral ischemia/reperfusion (I/R) induced brain tissue injury and activated the NLRP3 inflammasome. CHR could prevent NLRP3 activation and provide neural protection against IS.⁵² In the Aβ25-35-induced AD model, CHR markedly enhanced learning and memory deficits, mitigated hippocampus neuronal injury, and substantially decreased the protein and mRNA expression levels of NLRP3, IL-1β, and IL-18, potentially through the ROS/TXNIP/NLRP3 signaling pathway.⁴⁷

AE exerted neuroprotective effects in brain I/R models by preventing NLRP3 inflammasome activation, thereby regulating microglial polarization and pyroptosis.¹¹

In another study on quinones as preventive agents in AD, Chen et al reported that quinones in Traditional Chinese Medicine (TCM) and Chinese Medicinal Formulas were promising prophylactic agents due to their ability to regulate the NLRP3 pathway. Furthermore, anthraquinones exhibited the most potent effect among quinones, especially AE, CHR, and RHE.¹¹⁵

Inhibition of Pyroptosis

Pyroptosis, an inflammatory form of programmed cell death that occurs in both microglia and neurons, is initiated by various inflammasomes (such as NLRP3 inflammasome) and executed by caspases and the gasdermin (GSDM) family.^122,123 Activated caspase-1 cleaves gasdermin D (GSDMD) to generate the N-terminal fragment (GSDMD-NT), which oligomerizes at the plasma membrane to form pores, leading to cell swelling, membrane rupture, and the subsequent release of intracellular contents, thereby inducing cell death.^124,125 A variety of NDDs, including AD, PD, ALS, and MS, are linked to pyroptosis.^125–129

In the MCAO/R models, CHR suppressed GSDMD transcription by activating the Nrf2/ARE signaling pathway, consequently diminishing pyroptosis in hippocampal neurons.⁵⁴ In a similar study, Xia et al observed that CHR postconditioning protected against CIRI by inhibiting NLRP3-mediated pyroptosis.⁵⁵

Cui et al demonstrated that emodin significantly reduced pyroptosis in EAE rats by reducing the molecular level of the NLRP3 but had no noticeable effect in healthy rats, providing a solid experimental foundation for emodin’s therapeutic application in the clinical treatment of MS patients.⁴² Besides, Jiang et al found that emodin mitigated LPS/ATP-induced pyroptosis in BV2 cells by inhibiting NLRP3 inflammasome activity and the cleavage of the pyroptotic effector protein GSDMD, thereby exerting neuroprotective properties.³⁶

PI3K/Akt

PI3K modulates numerous critical processes in the inflammatory response to infection and injury. By attracting and activating innate immune cells, such as macrophages, the PI3K/Akt axis can modulate inflammatory responses.¹³⁰ Previous studies have shown that targeting the PI3K/AKT signaling pathway and its downstream mediators exerts neuroprotective effects by reducing neuronal inflammation.^131,132

PI3K/Akt/mTOR

Integrative metabolomics and network pharmacology analyses were conducted to elucidate the metabolic mechanisms and therapeutic potential of Didang decoction (DDD, a traditional TCM decoction) in CIRI, revealing that emodin is a major bioactive constituent and AKT1 is a core molecular target. Molecular docking further predicted a stable interaction between emodin and AKT1, suggesting a direct regulatory effect on the pathway. DDD treatment significantly enhanced phosphorylation of PI3K, Akt, and mTOR. PI3K/Akt/mTOR activation was implicated in suppressing neuroinflammation by phosphorylating NF-κB, downregulating NLRP3, and lowering the systemic production of pro-inflammatory cytokines.¹³³

A recent study demonstrated that CHR ameliorated neurobehavioral deficits by downregulating the expression of PI3K, AKT, and mTOR proteins. Additionally, CHR significantly inhibited the production of inflammatory cytokines, including IL-1β, IL-6, and TNF-α. The findings suggested that in rats with ICH-induced neurological dysfunction, CHR might be a promising therapeutic strategy for repairing neuronal damage driven by upregulation of the PI3K/AKT/mTOR signaling pathway.⁴⁶

Myd88/PI3K/Akt/ NF-κB

Myeloid differentiation factor 88 (MyD88) is a key adapter protein in innate immunity, plays a critical role in TLR-mediated signaling transduction and downstream signaling cascades, including PI3K/Akt.^43,134 Upon phosphorylation, the PI3K/Akt pathway activates NF-κB, thereby initiating an inflammatory cascade.¹³⁵

In EAE mice, emodin reduced IκB (NF-κB inhibitor) kinase activation and the consequent IκB degradation by suppressing MyD88 and preventing PI3K and Akt phosphorylation. Consequently, NF-κB nuclear translocation was reduced, thereby suppressing the production of proinflammatory cytokines and preventing microglial overactivation.⁴³

MAPK

MAPK signaling is a recognized prototypical pro-inflammatory signaling pathway. The MAPK family encompasses ERK1/2, JNK, and p38 signaling pathways.¹³⁶ Upon activation of the MAPK cascade, IκB undergoes ubiquitination and proteasome-mediated degradation, thereby releasing NF-κB and initiating an inflammatory cascade.^137–139 Several studies have confirmed that inhibition of MAPK signaling pathways can reduce neuroinflammation and ameliorate the neurodegenerative process.^140,141

In MPTP-induced PD mice, RHE improved motor symptoms and reduced dopaminergic neuron damage by suppressing activation of the MAPK/IκB signaling pathway and reducing levels of pro-inflammatory cytokines such as IL-1β, IL-6, and TNF-α.⁶²

TLR4-NFκB

The NF-κB family of transcription factors is crucial for regulating various biological processes, including inflammation, immune responses, cell survival, and cellular differentiation.¹⁴² TLRs are crucial membrane proteins involved in signal transduction and the innate immune system. The quintessential route involves the interaction of lipopolysaccharide with TLR4 on microglial surfaces, thereby activating multiple signaling transduction pathways, including PI3K/AKT, MAPK, and mTOR, ultimately leading to the activation of NF-κB.^139,143

RHE

In PTZ-induced epileptic mice, RHE exerted anticonvulsant and neuroprotective effects by inhibiting TLR4-NFκB signaling and reducing the secretion of inflammatory cytokines, including TNF-α, IL-6, IL-1β, and IL-18.⁹

PHY

Liu et al reported that PHY treatment significantly reduced the expression of NF-κB, TNF-α, and IL-1β in both the cortex and hippocampus in an LPS-induced gliosis model. Moreover, PHY could reduce the relative protein abundance of TLR4 and the responsiveness of glial cells in the mouse brain. These results might provide a modern theoretical framework for clinical research that examines the pathogenic mechanisms and treatment modalities for AD and neuroinflammation.⁶⁴

CHR

The fundamental processes of ethanol-induced neurodegeneration are intricate and involve several cellular functions, including inflammation, OS, and apoptosis.⁵⁶ It has been demonstrated that ethanol exposure activates the NF-κB pathway in several brain regions, particularly the cerebral cortex and hippocampus.¹⁴⁴ In ethanol-induced neurodegeneration mice, administration of CHR decreased the expression of TLR4, NFκB, IL-1β, TNF-α, and Caspase-3 in the cortex and hippocampal regions.⁵⁶ In addition, CHR ameliorated memory loss and reduced histopathological changes in neurons in stress-intensified PTZ-induced EPI in mice. Meanwhile, CHR significantly reduced the mRNA expression levels of TLR-4, NF-κB, TNF-α, IL-1β, and Caspase-3.⁴⁵

Suppression of Pro-Inflammatory Cytokines

Suppression of pro-inflammatory cytokines is another critical anti-inflammatory mechanism. It has been suggested that the basal inflammatory state and aging play a significant role in the rising prevalence of chronic illnesses, such as metabolic, neurodegenerative, and cardiovascular disorders.¹⁴⁵ Proinflammatory cytokines, such as IL-1β, IL-6, IL-12, IL-18, and TNF, together with anti-inflammatory cytokines including IL-4, IL-10, IL-13, and IL-19, regulate immune responses through interactions with immune cells.^146,147 Relevant studies showed that patients with AD or PD have higher levels of these pro-inflammatory cytokines in their serum, cerebrospinal fluid (CSF), and post-mortem brain tissue.^148–150

Emodin

Zheng et al investigated the effects of emodin in EAE models. Treatment with emodin significantly downregulated the mRNA expression of IL-6 and IL-17A, indicating that emodin alleviated inflammatory injury by suppressing the transcription of proinflammatory cytokines.⁴³ An additional study concluded that emodin reduced levels of IL-18, IL-1β, and TNF-α in LPS/ATP-induced BV2 cells.³⁶

RHE

An analogous study observed that in the MPTP-induced PD model, RHE decreased levels of pro-inflammatory cytokines (IL-1β, IL-6, and TNF-α) and microglial M1 polarization markers. Consequently, PD movement abnormalities improved while neurodegeneration and α-syn deposition decreased.⁶²

CHR

In diabetic encephalopathy (DE), sustained hyperglycemia and advanced glycation end products induce chronic neuroinflammation, thereby compromising neuronal function and synaptic plasticity. These changes were accompanied by elevated levels of inflammatory cytokines, ultimately leading to cognitive deficits.^151–153 In the streptozotocin (STZ)-induced DE model, the levels of pro-inflammatory cytokines (IL-1β, IL-4, IL-6, TNF-α) in the hippocampus were markedly increased. However, CHR alleviated learning and memory impairments by preventing the overexpression of pro-inflammatory cytokines in the hippocampal regions of DE mice.⁵⁷

Modulation of Microglial Polarization

Microglia, the indigenous macrophages of the CNS, employ their own receptor repertoire to surveil the brain’s surroundings dynamically.¹⁵⁴ In a homeostatic brain, microglia perform functions in synapse pruning, damage repair, maintenance of homeostasis, phagocytosis, support of other glial cells, and intercellular communication.¹⁵⁵ Microglia are generally classified into two phenotypes based on their activation state. Upon M1 activation, microglia secrete pro-inflammatory chemotactic molecules and cytokines that have neurotoxic effects.¹⁵⁶ Whereas M2 microglia release anti-inflammatory cytokines and brain-derived neurotrophic factors (BDNF), which perform neuroprotective benefits by inhibiting inflammatory responses and promoting regeneration and repair.¹⁵⁷ Aberrant activation of microglia and the overproduction of proinflammatory mediators are pivotal contributors to the pathogenesis of NDDs, as well as to neuronal injury secondary to traumatic brain injury and stroke.^158,159

CHR

By arresting microglia toward a pro-inflammatory phenotype and downregulating pro-inflammatory cytokine expression, CHR demonstrated neuroprotection and improved the brain plasticity milieu of stroke rats in the chronic phase.¹³ In LPS-induced BV2 cells, treatment with CHR reduced the release of proinflammatory mediators, including inducible nitric oxide synthase, prostaglandin E2, TNF-α, IL-6, and IL-1β. Furthermore, CHR exerted its anti-inflammatory effects by suppressing the activation of p38, ERK1/2, and JNK signaling pathways in microglia.⁵⁸

Emodin

Iba-1 and CD68 indicate the extent of microglial aggregation and the level of M1 phenotypic activation, respectively.¹⁶⁰ In the EAE models, emodin therapy mitigated Iba-1 and CD68 levels.⁴² In a similar study, emodin reduced the expression of phosphorylated PI3K, Akt, NF-κB, and MyD88, as well as the levels of markers CD86 and CD206 in microglia.⁴³ These findings demonstrated the neuroprotective and anti-inflammatory effects of emodin mediated by regulating microglial polarization.

RHE

Chi et al demonstrated that activated microglia drive neuroinflammation by regulating NO production through the glutamine-aspartate-arginine pathway. Additionally, RHE suppressed microglia-mediated neuroinflammation by suppressing NO production in microglia.⁶¹

AE

In the CIRI model, AE decreased microglial NLRP3 inflammasome activation, induced a M1 to M2 phenotypic change, and inhibited microglial pyroptosis, thus performing a neuroprotective effect.¹¹

Inhibition of Apoptosis

Apoptosis is a form of programmed cell death in which BCL-2-associated X protein (Bax), Bcl-2, and Caspase-3 play crucial regulatory roles.¹⁶¹ Several investigations have demonstrated that increased neuronal death is a factor behind NDDs.^162,163

CHR

Apoptotic and necrotic signaling pathways are activated as a result of nitrosative/oxidative stress in cerebral I/R animals. CHR inhibited nitrosative/oxidative stress by reducing 3-NT levels, inhibiting caspase-3 activation, and suppressing NOx generation, thereby preventing NO-mediated neuronal cell death.⁵³ CHR reduced endoplasmic reticulum stress-mediated apoptosis in AD mice by downregulating endoplasmic reticulum stress markers (GRP78, CHOP) and apoptotic caspases (3, 9, 12), an effect mediated through inhibition of the PERK signaling pathway.⁴⁸

Emodin

Emodin, another prominent anthraquinone, reduced glutamate release via presynaptic adenosine A1 receptors, thereby mitigating excitotoxicity-induced apoptosis.¹⁶⁴ In Aβ1-42-induced U251 cells, emodin activated the Nrf2 signaling pathway, leading to upregulation of Bcl-2, downregulation of Bax, and suppression of caspase-3, −8, and −9 activation, ultimately reducing apoptosis.²⁸ Moreover, emodin regulated the PKM2/Nrf2/ARE pathway in OS-induced PC12 cells, in which PKM2 dissociation promoted Nrf2 activation, leading to upregulation of antioxidant genes (Gclc, Gclm) and restoration of GSH levels, thereby diminishing caspase-3 activation.³⁴ Emodin additionally upregulated activin A through the activin A/Smads pathway to downregulate caspase-3 in neurons deprived of oxygen and glucose,³⁵ and regulated multiple survival pathways, including PI3K/Akt, AMPK/GSK-3β, and PI3K/mTOR/GSK3β.^41,165,166

RHE

RHE protected Aβ1-42-oligomer-burdened neurons from apoptosis by activating the SIRT1/PGC-1α pathway, which enhanced mitochondrial biogenesis, suppressed cytochrome c release, and inhibited caspase-3 activation.⁵⁹

Inhibition of Amyloid-Beta Aggregation and Tau Pathology

Characterized by behavioral impairment and cognitive dysfunction,¹⁶⁷ AD is the most common NDD. Aβ peptides aggregate to form senile plaques, whereas hyperphosphorylated tau assembles into helical filaments. Together, these structures form neurofibrillary tangles (NFTs), which are key clinical features of AD and contribute to chronic inflammation, synapse loss, and neuronal death.^168–170 Furthermore, tau was identified as the secondary agent in mixed pathology forms of PD,¹⁷¹ MS,¹⁷² and HD.¹⁷³

Inhibition of Aβ Aggregation

Emodin

A vital factor in the pathophysiology of AD is the impairment of APP metabolism. By controlling the breakdown of amyloid linked to AD, the protein kinase C (PKC) pathway exhibits neuroprotective benefits. After 4 hours of emodin treatment, cell membrane PKC activity increased by almost 52%.³³ As reported by Wang et al, cognitive impairment in AD transgenic mice was reduced by 60–70%, and Aβ deposition levels decreased by 50–70% when a high dose of emodin was administered to 8-month-old B6C3-Tg mice for two months.²⁶

CHR

BACE1 (beta-site APP cleaving enzyme 1) is a type I transmembrane protein. In AD, APP is sequentially cleaved by BACE1 and γ-secretase, leading to the production and release of the Aβ peptide in the brain.¹⁷⁴ Razieh Amini et al evaluated the inhibitory effects of 79 plant-derived compounds on BACE1 through virtual screening and molecular docking. Among them, CHR showed nanomolar-range BACE1 inhibitory activity and demonstrated stability in molecular dynamics simulations, suggesting its therapeutic potential against AD and PD.¹⁷⁵

AE

By reducing beta-sheet-rich aggregates, encouraging alpha-helical structures, and lowering oligomeric formation, AE suppressed hemoglobin aggregation—a mechanism related to amyloid-like misfolding, and might provide a therapeutic basis for diseases caused by protein aggregation, such as AD.¹⁷⁶

Inhibition of Tau Phosphorylation

CaM/CaMKIV

In AD, Aβ accumulation could oversaturate neurons with calcium, trigger the calmodulin (CaM)/calcium/calmodulin-dependent protein kinase IV (CaMKIV) signaling cascade, and subsequently hyperphosphorylate tau.^50,177

Ye et al conducted a series of studies demonstrating the neuroprotective effects of CHR in AD. CHR prevented ultrastructural alterations in hippocampal CA1 neurons, alleviated hippocampal neuronal damage, and decreased the levels of CaM, p-CaMKIV, and p-tau in the hippocampus of AD rats. These findings provide compelling evidence that CHR exerts neuroprotective effects by downregulating the CaM/CaMKIV signaling pathway and reducing p-tau expression.⁵⁰ The same results were observed in the cell model.⁵¹

Ca²⁺/EGFR-PLCγ

Cellular calcium ion (Ca²⁺) homeostasis is essential for regulating neuronal physiology, encompassing development and differentiation, synaptic plasticity, and cognitive functions such as learning and memory.²⁷ Dysregulation of Ca²⁺ homeostasis occurs in AD, suggesting its involvement in the onset of this disease.¹⁷⁸ Elevated intracellular Ca²⁺ promotes tau phosphorylation,¹⁷⁹ while the epidermal growth factor receptor (EGFR)/phospholipase C gamma (PLCγ)/CaMK pathway facilitates the Ca²⁺ release.¹⁸⁰

Compared with Aβ25-35-treated PC12 cells, cells in the CHR group exhibited reduced cellular damage and tighter intercellular junctions. Moreover, treatment with CHR markedly decreased tau mRNA and phosphorylated tau protein levels, as well as mRNA and protein levels of EGFR, PLCγ, and inositol 1,4,5-trisphosphate receptor (a Ca²⁺ channel protein). The results indicated that CHR exerted neuroprotective effects by alleviating calcium overload, preventing Ca²⁺ from binding to CaM, and attenuating tau hyperphosphorylation, likely through inhibition of the EGFR-PLCγ signaling pathway.²⁷

Previous studies had shown that emodin simultaneously suppressed Aβ accumulation and tau phosphorylation. In the APP/PS1 mice, emodin improved AD-like behavior, and emodin-treated animals, especially those given a dose of 20 mg/kg, showed reduced expression levels of Aβ1-42 and p-tau when compared to untreated APP/PS1 mice.²⁸ Hyperhomocysteinemia (HHcy) is an autonomous risk factor for dementia.¹⁸¹ HHcy-induced dementia rats were characterized by excessive Aβ production, tau hyperphosphorylation, and depletion of neuronal and synaptic proteins. Administration of emodin effectively reversed these pathological changes by reducing BACE1 levels and increasing protein phosphatase 2A activity.¹⁸²

Inhibition of Acetylcholinesterase Activity

Acetylcholine (ACh) is a neurotransmitter in the nervous system that controls how cholinergic neurons work physically and behaviorally. It was regulated and degraded by acetylcholinesterase (AChE) in the CNS.^183,184 AChE facilitates the aggregation of Aβ into oligomers and fibrils in the brain, enhancing their neurotoxicity and thereby promoting neuroinflammation, OS, and synaptic dysfunction.^185,186 To date, one of the primary therapeutic medications used to treat AD is acetylcholinesterase inhibitors (AChEIs).

AE

In a scopolamine-induced amnesia animal model, AE significantly ameliorated cognitive deficits (>70%). In vitro, AE demonstrated dose-dependent inhibition of AChE activity.⁶⁶

Emodin

A recent study reported that emodin directly inhibited AChE activity and exerted neuroprotective effects against Aβ25-35-induced cytotoxicity in PC12 cells.³² Systems pharmacology combined with molecular simulation studies revealed that emodin exhibited high-affinity binding to AChE, resulting in potent enzyme inhibition. It occupied the active site via hydrogen bonds and hydrophobic interactions, thereby preventing ACh degradation and enhancing cholinergic nerve transmission. Molecular dynamics simulations further verified the structural stability of the emodin-AChE complex. Its mechanism of action is similar to that of the clinical drug Donepezil, suggesting that emodin could serve as a potential natural AChE inhibitor.¹⁸⁷ Polygoni Multiflori Radix Praeparata combined with Acori Tatarinowii Rhizoma is widely employed in TCM for the prevention and treatment of AD. Xie et al reported that the combination decoction improved cognitive performance and significantly decreased AChE expression, an effect likely mediated by bioactive compounds, including emodin,¹⁸⁸ but further investigation is warranted.

Focusing on the investigation of new AChEIs, Duran assessed the effects of selected quinones on AChE in vitro by comparing them with the reference drug Tacrine. The findings indicated that anthraquinones could associate with either the unbound enzyme or the enzyme-substrate complex, consequently resulting in enzyme inhibition. It provides a new basis for exploring alternative inhibitors of traditional AChEIs.¹⁸³

Regulation of the Metallic Equilibrium State

A variety of physiological processes in the CNS, such as energy metabolism, DNA replication, protein synthesis, and myelin and neurotransmitter synthesis, all rely critically on metal ions. Dysregulation of metal ions leads to neuronal death via OS, ferroptosis, copper excision, cellular senescence, or neuroinflammation.¹⁸⁹ Several amyloidogenic proteins associated with NDDs, such as prion protein, α-syn, and Aβ, can bind metal ions and influence metal homeostasis.¹⁹⁰ Therefore, agents capable of normalizing metal ion homeostasis may represent potential therapeutic strategies for NDDs.

Ferroptosis

Ferroptosis, a regulated form of cell death driven by iron overload, inactivation of GPX4, and subsequent lipid peroxidation (LPO),^191,192 has been widely reported in the pathologies of AD and PD.¹⁹³ Meanwhile, a variety of neurological disorders have been linked to iron buildup in defective brain regions.^194–198

Emodin

Fe²⁺ accumulation induced cytotoxicity in Aβ25-35-incubated PC12 cells, while emodin significantly inhibited Fe²⁺ accumulation and conferred cellular protection by upregulating GPX4 via Nrf2 to suppress LPO.³² A study of the mechanisms by which emodin attenuated dopaminergic neuron loss revealed that emodin reduced erastin-induced ferroptotic death by upregulating UQCRC1 and preventing MPP⁺-induced ferroptotic cytotoxicity. This provided a clinical potential for emodin’s intervention in ferroptosis-related neurodegeneration.⁴⁰

CHR

In Aβ25-35-treated rat and PC12 cells, CHR attenuated ferroptosis by upregulating GPX4 expression, GPX activity, and GSH levels, while concomitantly decreasing ROS and LPO accumulation.⁴⁹

RHE

Moreover, in CIRI models, Liu et al reported that RHE suppressed the expression of ferroptosis-related proteins both in vitro and in vivo, through a mechanism involving the Nrf2/SLC7A11/GPX4 pathway.²⁹

Inhibition of Zinc-Induced Neurotoxicity

Bush et al proposed the “Metal Hypothesis of Alzheimer’s Disease”, which pointed out that zinc and copper are critical for Aβ aggregation in AD brain.^199,200 In vitro, higher exogenously applied zinc concentrations (>300 μM) can cause neuronal loss.²⁰¹ Meanwhile, Noxious stimuli that raise intracellular zinc ions can trigger widespread neuronal death.²⁰² Moreover, some studies have confirmed that aberrant zinc release from metallothionein (MT)-3 may result in neuronal loss in the thalamus and the hippocampal CA1 area.²⁰³

The entry of Zn²⁺ into neurons is considered a critical step in Zn²⁺-induced neurotoxicity.²⁰⁴ Emodin prevented zinc entry into neuronal cells, thereby reducing intracellular zinc levels and subsequent ATP and NAD⁺ depletion. This process suppressed ROS accumulation and endoplasmic reticulum stress, inactivated the AMPK/ACC signaling pathway, and ultimately protected SH-SY5Y neuroblastoma cells from apoptosis.³⁷

Prolongation of Metallothionein Induction

MT, an endogenous metal-detoxifying protein, regulates metal ion homeostasis by forming metal-sulfhydryl bonds, thereby ensuring proper metal distribution and exerting neuroprotective effects.²⁰⁵

By prolonging MT release, emodin was effective against Aβ and α-syn toxicity in aged transgenic Caenorhabditis elegans, demonstrating a promising lead in AD and PD research.⁴⁴

Activation of the Estrogen Receptor

Estrogen, the female sex hormone, is attributed with significant neuroprotective qualities.²⁰⁶ An increased incidence of AD in women during and after menopause has been linked to a drop in estrogen levels.²⁰⁷ Several studies have reported that women exhibit more pronounced Aβ-related neuropathology and cognitive decline than men.^208,209 The estrogen receptor (ER) is a significant class of ligand-activated transcription factors that play vital functions in modulating the effects of estrogens within a cellular context, including ERα and ERβ.^207,210 ERs in the brain regulate neuronal differentiation, proliferation, neuroinflammation, cholesterol metabolism, synaptic plasticity, and behavior.²⁰⁶ In AD, ER has been reported to co-localize with NFTs,²¹¹ inhibit tau phosphorylation,²¹² and serve as an early biomarker in prodromal AD.²¹³

Systemic pharmacology and molecular modeling studies showed that emodin could bind to and activate ERα, with binding stability comparable to that of estradiol. Through ERα, emodin regulated key signaling pathways, including MAPK, PI3K/AKT, and EGFR. It enhanced synaptic plasticity, promoted neuronal survival, and inhibited inflammatory responses and OS, thereby exerting a neuroprotective effect. This mechanism provided a molecular basis for the application of emodin in AD, especially in female patients with estrogen deficiency.¹⁸⁷

Regulation of the Gut Microbiota

Throughout human history, microbes have exerted a pivotal influence on human health and survival. Growing evidences indicate that the gut microbiome and the CNS communicate bidirectionally, forming the so-called “microbiota-gut-brain axis”.^214,215 Diverse microorganisms in the gastrointestinal tract can significantly influence the pathophysiology of CNS diseases through neurological, endocrine, and immune pathways associated with the microbiota-gut-brain axis, particularly in PD and AD.^216–218

Wang et al demonstrated that chronic RHE treatment improved recognition memory deficits in high-fat (HF) diet-induced obese male mice. Moreover, RHE suppressed the elevation of plasma LPS levels and the accumulation of proinflammatory macrophages in the colon. Additionally, it altered the gut microbiota by increasing the abundance of Bifidobacterium and Lactobacillus species, while decreasing that of the Bacteroides-Prevotella group and Desulfovibrio species. Therefore, as a preventive strategy against chronic HF diet-induced cognitive decline, microbiome dysregulation, and neuroinflammation, supplementation with RHE-enriched foods or herbal medicines may be beneficial.⁶³ Nonetheless, given the limited available evidence, this process requires further investigation.

Upregulation of BDNF Levels

BDNF, a pivotal neurotrophic factor in the CNS, plays a crucial role in neuronal differentiation, maturation, and survival.^219,220 In addition, neurotrophins and their receptors, particularly BDNF, are thought to be molecular mediators of functional and morphological synaptic plasticity since they are expressed in brain regions with high levels of plasticity, such as the cerebral cortex and hippocampus.²²¹ Furthermore, reduced BDNF levels have been observed in the development of NDDs, including AD,²²² PD,²²³ MS,²²⁴ and HD.²²⁵

In the HF diet-induced obesity model, long-term RHE treatment improved recognition memory by raising BDNF in the perirhinal cortex.⁶³

Inhibition of α-Syn

The overexpression and abnormal aggregation of α-syn are key contributors to PD pathogenesis.^226–228 Mechanistic studies show that chronic exposure to preformed α-syn fibrils or α-syn overexpression induces dopaminergic neuron dysfunction and subsequent cell death.^229,230

RHE

In a mouse model of PD, RHE ameliorated motor deficits and reduced dopaminergic neuronal damage. Furthermore, RHE reduced the production of α-syn by suppressing neuroinflammation mediated by MAPK/IκB.⁶² However, the specific mechanism by which RHE decreased α-syn accumulation still needs to be explored.

Emodin

Garcia’s work assessed the stability of primary metabolites in the roots of R. palmatum against key protein targets relevant to PD, as well as their absorption, distribution, metabolism, excretion, and toxicity properties. The results indicated that emodin exhibited notable inhibitory potential against α-syn aggregation.²³¹

Amelioration of Blood-Brain Barrier Permeability in NDDs and Related Cerebrovascular Diseases

BBB dysfunction, characterized by increased permeability, allows the infiltration of toxins and immune cells into the CNS.²³² BBB disruption contributes to the pathology of various neurological disorders, including NDDs, stroke, and EPI.²³³

CHR

Zhang et al assessed BBB permeability using the Evans blue assay to explore the protective effects of CHR against CIRI. Animals from the MCAO and vehicle groups exhibited marked BBB breakdown, while treatment with CHR-H or CHR-M significantly reduced Evans blue extravasation.⁵²

RHE

In a rat model of CIRI, RHE significantly reduced cerebral infarction volume and BBB disruption, and exhibited a dose-dependent neuroprotective effect.²⁹

Complementary Effects and Multi-Target Therapeutic Strategies

Combination therapies based on anthraquinones and other bioactive compounds have shown considerable potential to ameliorate the multifactorial pathogenesis of NDDs by collectively modulating multiple targets. For instance, hybrid molecules integrating anthraquinone scaffolds with other pharmacophores further extend multi-target capabilities: the RHE-based hybrid RHE-HUP (an inhibitor of AChE) simultaneously inhibited AChE and BACE1, reduced APP levels, increased ADAM10 (non-amyloidogenic pathway) and insulin-degrading enzyme (Aβ clearance), and suppressed protein tyrosine phosphatase 1B to alleviate neuroinflammation, ultimately restoring dendritic spines and cognitive function in APP/PS1 mice.²³⁴ Similarly, in I/R injury, the anthraquinone emodin combined with ginsenoside Rb1 exerted multi-pathway neuroprotection by simultaneously downregulating Connexin 43 (gap junction communication) and Aquaporin 4 (cerebral edema), resulting in a reduced infarction area, improved BBB integrity, and recovery of neurological function compared to monotherapies, exemplifying TCM “component compatibility” for multi-pathway regulation.¹²

In addition, combinations of multiple anthraquinones have also demonstrated considerable potential. An optimized formulation of rhubarb aglycones (including AE, RHE, emodin, CHR, and PHY) regulated metabolic disturbances (lipid, energy, and amino acid metabolism) in I/R rats, collectively reducing cerebral infarction and neuronal apoptosis through coordinated modulation of plasma/urine metabolites such as lactate, taurine, and phosphocreatine.²³⁵ In the MCAO model, RHE, emodin, and kaempferol, the principal blood-entering constituents of the TCM formula DDD, exerted dual regulatory effects on metabolism and programmed cell death by modulating multiple pathways and processes, including PI3K/Akt signaling, autophagy, and glycerophospholipid metabolism.¹³³

Innovative delivery systems enhance additively effects: self-assembled carrier-free nanoassemblies of berberin and RHE with 3D porous structures exerted superior cholinesterase inhibition, Aβ aggregation regulation, and ROS elimination compared to single components or physical mixtures. Moreover, the resultant BER-RHE complex exhibited Fickian diffusion-controlled sustained-release properties, synergistic biological activities, and mild neurotoxicity.²³⁶ Similarly, nanoplatforms like K8@Fe-Rh/Pda NPs (RHE + polydopamine) simultaneously targeted Aβ aggregation (via polydopamine) and mitochondrial OS (via RHE-mediated SIRT1/PGC-1α activation), reducing neuronal apoptosis and improving cognition in APP/PS1 mice through BBB-penetrating and Aβ-targeted delivery.¹⁰⁸

Additionally, polyherbal formulations such as BHD (which contains emodin) exhibited multi-target effects via estrogen signaling (ESR1) and cholinergic (AChE inhibition) mechanisms, potentially complementing anthraquinones as adjuvants to standard therapies.¹⁸⁷ Anthraquinone-based multi-target strategies also extended to PD: emodin inhibited ferroptosis via dual mechanisms (iron chelation and mitochondrial UQCRC1 activation), stabilizing electron transport and restoring redox homeostasis in PD models, highlighting coordinated modulation of iron-dependent cell death and mitochondrial function.⁴⁰

These examples collectively demonstrated that anthraquinone-based combination therapies addressed the complexity of NDDs by simultaneously modulating overlapping pathological cascades, thereby offering superior efficacy compared to single-target approaches.

Challenges

Despite encouraging preclinical results, current studies on anthraquinone compounds for NDDs face several limitations and research gaps, such as efficacy and safety in clinical trials. For instance, emodin lacks human clinical data regarding its potential to ameliorate AD phenotypes. Clinical trials are therefore needed to assess its benefits in AD and mild cognitive impairment, as well as to thoroughly evaluate its toxic effects at therapeutic doses in humans. This is crucial because higher doses (500–1500 mg/kg body weight) in animal studies have been associated with toxicity, including inflammation and hepatotoxicity.^237,238 Notably, the clinical applicability of emodin remains constrained by its toxicity, and the absence of randomized controlled trials in humans assessing pure emodin prevents accurate determination of its tolerable dosage. Machine-learning-based modeling of serum alanine aminotransferase levels indicated that emodin may transition from hepatoprotective to hepatotoxic effects at doses above 45.74 mg/kg/day or with treatment durations exceeding 30.41 days.²³⁹ The dose-dependent neuroprotective effects of RHE observed in experimental models need to be translated into human studies, with clinical trials required to confirm both efficacy and safety in IS.²⁹

Addressing bioavailability issues remains a major challenge for anthraquinones due to their poor intestinal absorption, extensive first-pass metabolism, and low cell membrane permeability.^240,241 A comprehensive pharmacokinetic analysis by Zhou et al demonstrated that emodin exhibits low absolute bioavailability (~3.2%) in rats, with approximately 56% of the administered dose excreted unchanged in feces.²⁴² Glucuronidation metabolism seems to be a primary factor contributing to the exceedingly low oral bioavailability of emodin.²⁴³ Piperine markedly increased the in vivo bioavailability of emodin by inhibiting glucuronidation; nevertheless, the underlying mechanisms of this enhancement remain to be fully elucidated.²⁴⁴ Nano-formulations represent a promising research direction because they can improve pharmacokinetics, enhance tissue distribution, and increase bioavailability by reducing particle size and improving absorption and solubility relative to traditional formulations.²⁴⁵ Fang et al conceived and fabricated AE@ZIF-8 NPs, a novel class of biomineralized nanocarrier, which improved AE’s penetration through the BBB and reduced the toxicity.²⁴⁶

Gender-related limitations and potential endocrine disruption risks represent another challenge in the clinical application of anthraquinones. Firstly, sex-dependent differences in pharmacokinetics may arise from variations in physiological and biochemical factors, including body weight, plasma volume, gastric emptying time, plasma protein concentrations, and drug transporter activity.²⁴⁷ In the subchronic toxicity study of mice, female mice exhibited faster emodin metabolism and higher levels of glucuronidated metabolites than male mice.²⁴⁸ The US National Toxicology Program demonstrated that emodin exposure led to heightened occurrences of renal tubule pigmentation in both male and female mice, as well as increased instances of nephropathy in female mice.²⁴⁹ Moreover, higher hepatic accumulation of emodin had been observed in female rats, which is associated with increased hepatotoxicity and may be attributed to sex-related differences in hepatic distribution.²⁵⁰ Secondly, certain anthraquinones demonstrated weak phytoestrogenic activity, raising concerns regarding potential endocrine disruption. In vitro studies have demonstrated that emodin and AE suppress breast cancer cell proliferation by inhibiting ERα activation, downregulating ERα protein expression, and exerting anti-estrogenic effects.²⁵¹ These findings suggest that low-dose exposure may exert estrogen-like effects and disrupt endocrine homeostasis, whereas prolonged or high-dose exposure could raise safety concerns for hormone-sensitive individuals, particularly women of reproductive age. In summary, the clinical utility of anthraquinones necessitates careful balancing of therapeutic benefits against potential risks, underscoring the need for additional epidemiological and clinical investigations in humans.

Conclusions

The multi-target and multi-pathway nature of anthraquinones’ intervention in NDDs stems from their intricate mechanisms. Pharmacological evidence from diverse in vitro and in vivo disease models demonstrates improvements in cognitive and motor functions, reductions in pathological hallmarks, and modulation of multiple molecular targets. Synergistic strategies combining anthraquinones with other bioactive compounds or advanced drug-delivery systems are considered promising approaches for enhancing therapeutic efficacy and targeting multiple pathological pathways. Nevertheless, challenges remain in clinical validation, understanding detailed mechanisms, optimizing bioavailability, and addressing safety concerns. In the future, researchers may focus on the specific efficacy and safety of anthraquinones in NDDs, whether as standalone supplements or components of complex plant extracts.