TO THE EDITOR
We have carefully thoroughly reviewed the study reported by Wang et al[1] featured in the World Journal of Gastroenterology. Parkinson’s disease (PD), a multifocal neurodegenerative disorder, involved the aggregation and accumulation of alpha-synuclein (α-syn) in the substantia nigra of the central nervous system (CNS), as well as in other neural architectures[2]. PD was once the fastest-growing neurological disease globally. As more conditions like diabetic neuropathy are included in research, its age-standardized disability-adjusted life years growth rate has been surpassed. However, PD’s absolute disease burden continues to rise significantly. It remains a key contributor to the global burden of neurological diseases, especially affecting older adults[3]. Gastrointestinal (GI) symptoms are counted among the earliest clinical presentations of PD, often arising ahead of the onset of motor impairments. Closely linked to PD-associated GI dysfunction is the gut-brain axis, a pathway that reflects bidirectional regulatory crosstalk between the CNS and the GI tract under both physiological and pathological conditions[4]. However, existing studies regarding the link between PD and GI dysfunction remain subject to certain limitations, and the exact molecular mechanisms governing this connection have not been fully elucidated.
Findings derived from the study carried out by Wang et al[1] have furnished vital perspectives and novel therapeutic approaches for the management of PD. Notably, the ability of rhapontin to mitigate gut-brain axis dysregulation in PD via activating nuclear factor erythroid 2-related factor 2 (NRF2) is systematically validated for the first time in this work, thus laying the foundation for identifying a completely new therapeutic target for the disease. Through the combination of network pharmacology-based predictions and experimental validation, the study’s scientific rigor and credibility have been markedly improved. Furthermore, the key advancement of this investigation resides in clarifying that rhapontin’s anti-inflammatory activity is restricted to the intestinal tract and bears no direct association with the brain-derived NRF2 pathway, a finding that offers robust experimental evidence for PD’s “enteric pathology hypothesis”[5]. Overall, these results provide a fundamental theoretical basis for rhapontin to be regarded as a potential candidate drug for managing PD-associated GI dysfunction.
Nevertheless, we hold several exploratory viewpoints concerning specific aspects of this research. First, at the gut microbiota level, the study only explored the relationship between rhapontin and PD-associated GI dysfunction. It failed to systematically integrate and investigate the gut-brain axis with PD, nor did it assess rhapontin’s impact on microbiota composition or function, making it difficult to establish causal relationships in the current microbiota research. Numerous studies have confirmed the pivotal role of the gut-brain axis in regulating PD-related GI dysfunction via interconnected pathophysiological processes. For instance, imbalances in gut microbiota composition can impair the integrity of the intestinal barrier, resulting in the occurrence of a “leaky gut”. Through this mechanism, lipopolysaccharides can stimulate immune cell and secrete pro-inflammatory, which in turn elicits systemic inflammation, as well as augments the permeability of the blood-brain barrier (BBB). Consequently, peripheral inflammatory cues are able to penetrate the CNS, subsequently leading to the accumulation of pathological α-syn within substantia nigra and the vagus nerve [dorsal motor nucleus of the vagus (DMV)]’s dorsal motor nucleus. Furthermore, the existence of this pathological α-syn enhances the DMV’s mitochondrial fragmentation, ultimately driving the pathological progression of PD[6]. Moreover, gut-to-brain transmission of pathogenic α-syn can induce nigrostriatal dopaminergic degeneration and thereby elevate the susceptibility to PD[7-9]. Animal research substantiates this hypothesis, showing that fecal microbiota transfer from PD patients initiates dopaminergic neuron aggregation alongside α-syn aggregation[10]. Notably, progressive depletion of substantia nigra pars compacta dopamine (DA) neuron is recognized as a cardinal feature of PD[11]. Moreover, gut microbiota regulate DA and serotonin synthesis to sustain neurotransmitter homeostasis, while short-chain fatty acids confer DA and neuroprotection through mitigating intestinal inflammation, diminishing oxidative stress, and strengthening the BBB integrity[6,10-14]. Ultimately, substantia nigra dopaminergic neurons degeneration may originate from gut microbial dysbiosis (an imbalanced gut microbiome), characterized by aggravated oxidative stress, activation pathway of Toll-like receptor 4/nuclear factor kappa-B (NF-κB) and compromised energy metabolism in mitochondria[6]. Taken together, these mechanisms form a central regulatory network via which the gut-brain axis is involved in the pathogenic process of PD. Yet, in this current research, the investigation focused solely on rhapontin’s impact on PD-associated GI dysfunction, with no systematic and in-depth exploration of the integration between the gut-brain axis and PD itself. Furthermore, while gut microbiota plays a critical role in the gut-brain axis associated with PD[15-17], establishing causal relationships in existing microbiota-related studies remains a challenge. Whether rhapontin indirectly activates NRF2 through modulating specific microbial taxa or directly exerts anti-inflammatory activities remains ambiguous. In future investigations, quantification of microbe-derived metabolites and assessment of fecal microbial community alterations represent key objectives achievable through metagenomic or 16SrRNA-sequencing techniques. Mechanistic validation might be conducted via two key models, including fecal microbiota transplantation and antibiotic-induced microbial depletion, so, as to thoroughly explore gut-brain interaction and defined rhapontin’s pharmacodynamics. Secondly, the study lacked detailed elaboration on rhapontin’s molecular mechanism of action. As a stilbenoid glucoside compound, rhapontin is derived from the medicinal plant rhubarb (Rheum rhabarbarum), specifically its rhizomes.
Various studies focusing on the anti-inflammatory mechanisms of rhapontin have confirmed that this component can exert synergistic regulatory effects across multiple signaling pathways. For example, in lipopolysaccharide-induced endothelial cell inflammation models, rhapontin can effectively inhibit the synthesis of nitric oxide and tumor necrosis factor-α (TNF-α), while also significantly downregulating cyclooxygenase-2, inducible nitric oxide synthase, and nicotinamide adenine dinucleotide phosphate oxidase-related genes. This effect primarily depends on the inhibitory function of rhapontin in mitogen-activated protein kinase (MAPK) and NF-κB pathways[18]. Regarding antioxidant modulation, rhapontin can activate signaling pathway of NRF2. Diabetic retinopathy studies demonstrate NRF2 activation not only diminishes matrix metalloproteinase-2 and TNF-α level, but also elevate tissue inhibitor of metalloproteinase-1 and interleukin (IL)-10 expression, concurrently blocking NF-κB nuclear translocation[19]. Regarding its anti-fibrotic activity, rhapontin exerts such effects via a two-pronged mechanism: On the one hand, it activates adenosine monophosphate-activated protein kinase (AMPK); On the other hand, suppression of transforming growth factor-β (TGF-β)/Smad cascade activity is observed. The synergistic interaction of these two mechanisms can effectively reverse the aberrant deposition of extracellular matrix, thus halting the progression of pulmonary fibrosis[20,21]. Furthermore, existing evidence has confirmed that rhapontin can improve the function of the colonic epithelial barrier by activating sirtuin 1 (SIRT1) signaling[22]. Taken together, these results collectively shed light on rhapontin’s comprehensive therapeutic actions, which are attained via the coordinated modulation of signaling cascades encompassing NF-κB/MAPK, AMPK/TGF-β/Smad, and SIRT1, thus exerting simultaneous targeting of inflammation, oxidative stress, and fibrosis. Additionally, based on relevant literature, it is hypothesized that NF-κB inhibition may create conditions for NRF2-mediated antioxidant responses[23], and that SIRT1/AMPK signaling may directly activate NRF2 through phosphorylation[24,25].
Findings from the current investigation have validated that rhapontin exerts robust anti-inflammatory activity via reducing pro-inflammatory cytokines levels, including TNF-α, IL-6, and IL-1β. Simultaneously, rhapontin activates NRF2 signaling pathway, which in turn contributes to facilitate GI function. However, this study lacked exploration of rhapontin’s potential to inhibit pulmonary fibrosis progression through AMPK activation and suppression of TGF-β/Smad cascade. Furthermore, the regulatory mechanism of SIRT1 signaling pathway in enhancing the function of colonic epithelial barrier remains uninvestigated. Moreover, despite employing network pharmacology to predict shared PD-rhapontin targets and identifying top 10 hub genes through via the protein-protein interaction (PPI) network, the study did not elaborate on the associations between these hub targets and NRF2. It is advisable to supplement these target-NRF2 correlations so, as to assist readers in grasping the target screening process. Additionally, the screening procedure for NRF2 as a hub target was not clearly clarified in the research. Thus, it is proposed that the article elaborate on NRF2’s degree centrality and betweenness centrality within the common target set of PD and rhapontin, as well as its correlation scores with PD’s core pathological pathways (inflammation, oxidative stress, and gut-brain axis regulation) based on metascape-based functional enrichment analysis. Furthermore, the study lacks in-depth mechanistic elaboration on the specific signaling pathways (NF-κB, SIRT1/AMPK) associated with NRF2, including the regulatory mechanisms of its upstream and downstream molecules this aspect necessitates additional in-depth exploration. Furthermore, the article lacks a clear clarification of the process for identifying hub target NRF2. To elucidate the mechanism of rhapontin’s anti-inflammatory activity in PD, future investigations should prioritize validation of NRF2 interactions with rhapontin’s key targets, such as KEAP1, p62, or other PPI network partners, using Western blotting (WB) and co-immunoprecipitation. Additionally, methods like quantitative real-time polymerase chain reaction or RNA sequencing will provide to examine NRF2-regulated downstream genes. These integrated approaches will establish a comprehensive framework for assessing rhapontin’s therapeutic potential in PD.
In addition, when assessing the experimental segment of Wang et al’s study[1], several aspects merit additional consideration. About the open field test, it lacks clear labeling of the specific experimental group corresponding to each result, making it difficult to interpret. Furthermore, the WB images are insufficiently clear and fail to show a distinct trend in the outcomes a factor that may impede the drawing of definitive conclusions. Second, with respect to the experimental methodology, the assessment of motor function in mice relied solely on two behavioral paradigms: The pole-climbing test and the open field test. While these assessment methods have some referential value, they may not fully capture the entire spectrum of behavioral deficits in PD mice. Incorporating supplementary behavioral assays such as the Morris water maze and eight-arm radial maze (designed to evaluate spatial learning and memory capacities), as well as the elevated plus maze (used to assess anxiety-like behaviors) would enable a more comprehensive evaluation of motor and cognitive functions in PD mouse models[26]. Additionally, The 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced mouse model used in the original study, while capable of simulating central dopaminergic neuron damage in PD, fails to align with the “gut-origin” hypothesis: Its pathology originates in the CNS rather than the gut, inadequately simulates core gut-brain axis components such as intestinal barrier and microbiota, and poorly reflects gut-related non-motor symptoms like early PD GI dysfunction. This may hinder the extrapolation of conclusions to the clinical “gut-first” pathological process. Subsequent studies could use models such as intestinal α-syn preconditioning to enhance simulation authenticity[6,27].
To summarize, the research group spearheaded by Wang et al[1] has undertaken innovative, pioneering research efforts, thereby offering valuable perspectives on rhapontin’s therapeutic potential for managing GI dysfunction associated with PD. This work is thoroughly deserving of acknowledgment and commendation. Simultaneously, we anticipate that the authors will, in future studies, integrate multi-omics strategies: Analyze changes in gut microbiota via metagenomic sequencing, combine metabolomics (e.g., liquid chromatograph mass spectrometer/mass spectrometer) to detect characteristic metabolites, then correlate transcriptomic data of colonic tissues, screen core microbiota and metabolites associated with NRF2 activation, clarify the “rhapontin-microbiota-metabolite-NRF2” regulatory axis, and provide a basis for targeted therapy.
