Polyphenols and Their Impact on the Prevention of Neurodegenerative Diseases and Development

Studies on mice have shown that curcumin and piperine consumption decreases MAO-A and MAO-B-dependent monoaminergic neurotransmitters decomposition, which results in elevated serotonin and dopamine level in animal brain tissue, exhibiting natural antidepressant activity, as well as enhancing the effect of subthreshold doses of antidepressant drugs [31]. In another study curcumin in rat diet improved their reaction to stress restoring the hippocampal BDNF (brain-derived neurotrophic factor) and CREB (cAMP response element-binding protein) signaling [32]. What is more curcumin ingestion exhibited neuroprotective activity in rats after fluid percussion injury, where curcumin-rich diet caused normalization in BDNF level, and related CREB and synapsin1-dependent synaptic plasticity. In the brain polyphenols improve hippocampal neurogenesis, learning skills and memory. It was observed that polyphenol-rich diet boosts BDNF amount, thereby regulating the maintenance and formation of new synapses and improving memory formation [27,28]. As enhancers of BDNF level dietary polyphenols also contribute to the increased neurons survival during inflammation. Considering the CREB/BDNF-dependent neuronal activity of dietary polyphenols, promising results were obtained from experiments conducted on animals consuming berry juice [33]. In this study, blueberry polyphenols’ consumption was associated with improvement in spatial working memory and cognitive functions. Observed effects were related with enhanced hippocampal plasticity and CREB-dependent BDNF production [33]. Blueberry polyphenols consumption also improved object recognition in rats, which was linked with lower oxidative stress due to the lower NFĸB (nuclear factor kappa B) level in different brain areas [29]. Apart from berry juice polyphenols, also tea flavanols were able to improve brain functions in mice with cognitive deficits [34,35]. Animal studies have suggested the utility of plant-derived polyphenolic compounds in amyloid aggregation-related disorders like AD or PD. Experiments conducted on rats have shown dopaminergic neuroprotection provided by quercetin in rats. Its biological effect was to maintain the tyrosine hydroxylase amount in cytoplasm which, in turn, was supporting dopamine synthesis. Quercetin-induced reduction in ROS formation as well as decrease in proinflammatory cytokines’ synthesis and apoptosis ratio in brain tissue were also noticed [30]. This raises the possibility of using some polyphenols for slowing Parkinson’s progression. As reported by Hamaguchi et al. plant polyphenols may interfere with Aβ (amyloid beta) aggregation, mitigating AD pathology [36]. Resveratrol, a polyphenolic antioxidant enriched in grape, can cross the blood-brain barrier and exert protective effects against cerebral ischemic injury [37]. In developing mice received sevoflurane exposure (the most commonly used inhaled anesthetic in pediatric anesthesia), resveratrol pretreatment ameliorated cognitive impairment by modulating SIRT1-NF-κB pathway in microglia [38]. Animal studies investigating the action of plant-derived polyphenols on brain functions were summarized in Table 1.

The randomized control trials focus on the impact of plant polyphenols on the human brain and check their role in improving the functions of a healthy brain, preserving memory and cognitive functions lost during aging, or mitigating the symptoms of neurodegenerative diseases (Table 2). Dietary polyphenols were reported to improve brain activity in depression. Curcumin from Curcuma longa, as well as eugenol, piperine, quercetin, and resveratrol, were proven to act as reversible inhibitors of MAO—an enzyme involved in serotonin degradation [44]. Thus, their use may improve the activity of serotonin-, dopamine-, or norepinephrine-utilizing synapses, contributing to mental well-being. Polyphenol compounds in the diet may modulate the activity of the human brain, contributing to reduced outcomes in neuropsychiatric disorders. In a randomized clinical trial study, a diet supplemented with bark-extracted polyphenols from French maritime pine significantly decreased ADHD (attention deficit hyperactivity disorder) symptoms in children, improving concentration and attention after a 4-week-long supplementation with Pycnogenol^® (Geneva, Switzerland) [45]. A plant polyphenol-rich diet not only mitigates the decline of cognitive functions during aging, but also these compounds are also considered helpful in the prevention and treatment of neuroinflammatory and neurodegenerative disorders. Numerous dietary polyphenols exhibit anti-inflammatory and antioxidant activity, which makes them potentially active in mitigating AD development [40,46,47,48,49,50,51]. However, the results of AD-related experiments are not clear. Numerous reports describe the impact of various polyphenols on APP (amyloid precursor protein) processing, aggregation, and toxicity of Aβ aggregates. Curcumin, myricetin, or gallic acid were reported to interfere with Aβ aggregation by inhibiting the nucleation and/or the elongation phase of Aβ aggregates formation [52,53,54,55]. Furthermore, numerous in vitro studies report numerous polyphenol activities with potent beneficial actions in AD. Curcumin and resveratrol were reported to reduce ROS generation, hyperphosphorylation of tau protein and improve SH-SY5Y cell viability in the presence of Aβ oligomers [21]. Thus, these polyphenols may be considered interesting agents supporting AD treatment, however, recent findings describe their pro-oxidative cytotoxic properties [24,55]. Nevertheless, dietary polyphenol intake seems to be an important factor in decreasing the risk of AD and dementia development during aging. An observational study conducted on the French population in the Bordeaux region indicates the importance of specific plant polyphenol-rich products’ consumption in significantly lowering the chance for dementia and AD development [56].

Intensive studies, in which plant-derived polyphenols were used as therapeutic agents useful in the treatment and prevention of PD, were carried out. In fact, it has been reported that dietary polyphenols present in Mediterranean and Asian diets decrease the risk of PD development [57,58]. Polyphenolic acids, especially gallic acid and its derivatives, were reported to significantly improve cells’ viability and reduce apoptosis, which resulted from 6-hydroxydopamine-induced oxidative stress and may be beneficial for PD [59]. It was reported that men consuming huge amounts of flavonoids and anthocyanins exhibit a significantly lower risk of PD development [57,60]. Beneficial activity in PD was also reported for quercetin [61]. Studies on the effectiveness of green tea polyphenols administration in mild PD gave confusing results. After 6 months of green tea polyphenol administration, the PD symptoms measured with a unified PD rating scale improved compared to the control, but surprisingly, no effect was observed after 12 months of green tea administration [23].

Many studies have revealed that a nutritional model based on increased consumption of antioxidants has the potential to ameliorate the cognitive impairment associated with neurodegenerative disorders (Table 3). It has been shown that diets rich in anti-inflammatory and antioxidant compounds, in combination with limiting the consumption of calories, reduce the decline in cognitive functions, the risk of cardiovascular disorders, and neurodegenerative diseases [17,62,63,64,65,66]. Moreover, in recent years, there has been a growing interest in the influence of a particular diet as a potentially modifiable factor in CNS disorders. It is now well known that the effect of certain nutritional patterns is a significant influencer in the protection against CNS impairment and nutrition in terms of brain protection, and this is receiving more and more attention [64,65]. After analyzing the prohealth effects of various eating patterns, it was postulated that their beneficial effects result from synergistic interactions between individual dietary components [66].

Randomized clinical trials have shown that the improvement of cognitive functions resulting from the consumption of foods rich in flavonoids is independent of age and health conditions (Table 3). Barfoot et al. determined the acute effect of a blueberry smoothie on particular aspects of executive functions in healthy schoolchildren (n = 54, aged 7–10), which is a critical period in the development of the CNS. It was noted that 2 h after the intervention, the reaction time of the modified attention network task (MANT) test in the study group was significantly shorter (compared to placebo) without affecting the accuracy of the response, and the verbal memory assessed by the auditory verbal learning test (AVLT) after consuming the blueberry cocktail was augmented. However, there was no difference between the groups in reading performance [60]. In turn, Whyte et al. assessed the acute effect of a smoothie of berries (blueberries, raspberries, and strawberries) on the improvement of executive functions and moods in young adults. The study enrolled 40 healthy subjects (aged 20–30) who consumed either a cocktail or a placebo. Cognitive functions were evaluated within 6 h of consumption. It was shown that cognitive performance during the working day (after 6 h) in the placebo group decreased due to fatigue, while in the study group, it was still maintained. Moreover, in the group receiving the cocktail, attention was significantly improved, both 2 and 4 h after consumption, compared to placebo. On the other hand, the blueberry intervention had no effect on the mood [63]. Whereas beneficial effects on cognition in the elderly were noted by Bensalem et al. and Godos et al. [64,67]. Bensalem et al. assessed the influence of polyphenol-rich grape and blueberry extract (PEGB) on memory. A total of 215 healthy elderly people (aged 60–70 years) who received 600 mg/day of PEGB or placebo were enrolled in the study; the duration of the intervention was 6 months. It was found that after 6 months of supplementation, cognitive functions decreased compared to the initial result, without any significant difference in the study and the control group. However, it was observed that after 6 months of PEGB consumption in the study group, recall memory was improved (p = 0.006) in comparison with placebo (without any differences in the baseline results in both groups), whereas, it was found that PEGB did not affect recognition or working memory. Importantly, the cohort was stratified into quartiles based on the initial primary outcome of the CANTAB (the Cambridge neuropsychological test automated battery), paired associate learning (PAL), and visuospatial learning and episodic memory tests; thus, a subgroup with cognitive impairment was distinguished. Within this subgroup, PEGB significantly improved cognitive performance compared to placebo. Thus, it was postulated that PEGB supplementation containing 58 mg of flavonoids; flavonols, flavan-3-ols, and anthocyanins (in particular quercetin, malvidin, chlorogenic acid, ferulic acid, gallic acid, and resveratrol) significantly enhance cognitive performance in elderly people with significant cognitive impairment [17]. The MEAL study, comprised of 2044 adult men and women from southern Italy, has proven that a higher dietary intake of flavonoids is positively linked with cognitive functioning. It has been demonstrated that a diet rich in flavonoids negatively correlates with cognitive dysfunction among the adult population (OR 0.39; 95% CI 0.15–1.00). Moreover, there is a positive correlation between the intake of flavonoid subclasses: anthocyanins, flavan-3-ols, flavonols, catechins, and cognitive functions. In the case of individual polyphenol compounds, an inverse association between the level of dietary intake and impaired cognitive status was found only for quercetin [64].

Recent years’ findings suggest the selectivity of endothelial transport for polyphenol derivatives depends on their structure and chemical modifications. Figueira and co-workers described the differences in the transport of berry juice polyphenols’ derivatives across the BBB depending on their structure and chemical modifications. They observed enhanced transport of methylated or sulfated gallic acid and catechol derivatives across the BBB, whereas the transport of similarly modified pyrogallol derivatives was less effective. The authors, in their elegant work, have also described the activity of BCRP (breast cancer resistance protein)-transporter proteins belonging to ABC (ATP-binding cassette)-type efflux transporters in human immortalized brain endothelium cells, actively limiting the accumulation of polyphenol derivatives in the brain tissue [8]. Similar efflux activity of the BCRP transporter was also described to limit quercetin accumulation [101]. Finally, brain endothelial cells were also noticed to metabolize polyphenols during their transport from plasma, which may contribute to their lower availability for neurons [8]. More lipophilic polyphenols or their metabolites are supposed to be more motile through the endothelial cells depending on their passive permeation; however, the presence of specific efflux activity should always be considered.

The major obstacle to the application of polyphenols in the prevention of neurodegenerative disorders is their bioavailability. The presence of the BBB limits the transport of many compounds into the brain; thus, it is the key regulator of the entry and retention of polyphenols in the brain. The BBB is a highly selective barrier protecting the CNS from toxic compounds and pathogens, isolating the periphery from the brain [102]. Transport across the BBB occurs via six main routes: paracellular diffusion, lipid-mediated diffusion, carrier-mediated transport, receptor-mediated transcytosis, absorptive-mediated transcytosis, and active efflux transport [103]. The permeability of BBB endothelial cells to polyphenols is mainly influenced by the expression of efflux transporters and solute carriers [104].

The permeation of polyphenols across the BBB has been studied by Youdim and co-workers [101,105]. They suggested that polyphenols’ ability to cross the transmembrane in vitro depends on the degree of their lipophilicity, where less polar polyphenols or their metabolites (e.g., methylated molecules) have greater brain uptake than the more polar ones. However, it is not clear whether the main route of polyphenol transport into the CNS is simple diffusion or carrier-mediated transport [106]. It was also observed that polyphenols are able to modulate the activity of some ABC drug transporters, which are efflux pumps. Flavonoids have been shown to inhibit the ABC transporters [107,108,109]. Moreover, some flavonoids are themselves substrates for various ABC transporters, such as P-gp (P-glycoprotein), MRP2 (multidrug-resistant protein type 2), and BCRP [110]. Such interactions dramatically limit their bioavailability. Youdim and coworkers have shown that quercetin can cross the BBB when coadministered with a P-gp or BCRP inhibitor. It was suggested that this compound was able to enter the CNS, possibly by passive diffusion due to its hydrophobicity, but was then exported by efflux transporters [101]. Although most polyphenols are ligands for efflux transporters, they may still penetrate the brain in substantial amounts to exert protective function, as was shown for resveratrol [111].

Numerous in vitro and in vivo studies have pointed out that polyphenols’ ability to cross the BBB is affected by their chemical modifications, e.g., methylation, sulfation, or glucuronidation [9,112]. Methylation or glucuronidation of selected polyphenols or their metabolites increases their permeation across the BBB models in vitro [9]. Studies utilizing animal models have revealed that polyphenols accumulate in the brain in a nonregion-specific manner. Moreover, it was shown that these compounds are able to cross the BBB and accumulate in brain tissue independently of their route of administration [113,114,115,116]. Among others, curcumin has been shown to be transported to the CNS, where it induces neuroprotection by antiapoptotic mechanisms and mitochondrial protection [51,117,118]. Additionally, resveratrol was shown to cross the BBB and incorporate itself into the brain tissue [37]. Moreover, phenolic acids, such as caffeic acid, accumulate in the CNS and exert neuroprotective effects during cerebral ischemic injury [43,119]. The ability of caffeic acid to cross the BBB has also been observed in the CSF of patients with neurological disorders [68].

The use of dietary patterns with foods rich in flavonoids may prevent cognitive impairment due to the well-known multimolecular mechanisms of their action. What is more, based on recent discoveries, an impact on neuronal cells due to passing the BBB by flavonoid metabolites is considered a possible mechanism through which certain flavonoids could exert a direct neuroprotective effect [101]. However, epidemiological evidence is still limited, and there are discrepancies between in vitro and in vivo evidence on the neuroprotective role of flavonoid compounds. In vitro models are an important tool for assessing the penetration ability of xenobiotics across the BBB, but they do not fully reflect the physiological conditions. The main limitation is the inability to determine the contribution of efflux transporters involved in restricting the passage of compounds into the brain from the blood. Thus, the use of only in vitro models is insufficient to accurately determine both qualitative and quantitative penetration of substances into the human brain [25]. Due to highly invasive methods of obtaining samples from the CNS, there is a large deficit in research concerning the concentration of polyphenols in the cerebrospinal fluid (CSF) and brain of humans. Thus, most of the studies on the CSF bioavailability of phenolic compounds are based on in vitro and animal model studies, but only human data can fully provide knowledge on the usefulness of using these compounds in CNS diseases [25].

The current state of knowledge suggests that the passage of polyphenols and their metabolites through the BBB depends on their lipophilicity [101]. However, to establish the necessary conditions for human BBB penetration, it is still crucial to define their active forms, the activity locations, and the mechanism of neuroprotective actions (Table 4) [105].

High therapeutic hopes are associated with the use of resveratrol in the prevention of neurodegenerative diseases. Particularly important neuroprotective properties of resveratrol are related to anti-inflammatory, -oxidative, -apoptotic, and -aging activities [124], which have been investigated in several clinical trials [125,126,127,128,129]. It has been demonstrated that the administration of resveratrol was associated with a dose-dependent increase in cerebral blood flow and deoxyhemoglobin levels in healthy adults [125], improvement of memory performance through augmentation of glucose metabolism, and increased functional connectivity of the hippocampus in the overweight elderly [126], as well as enhancement of cognition function [127]. Moreover, resveratrol supplementation reduced the level of glycosylated hemoglobin A1c, retained the volume of the hippocampus, and improved resting-state functional connectivity in patients with mild cognitive impairment [129]. In preclinical studies, it has been observed that resveratrol penetrates the BBB [37]. This was confirmed in phase II multicenter randomized controlled trial that investigated the effect of 52-week supplementation with resveratrol (500 mg/day, with a dose increase of 500 mg every 13 weeks, final dose 2000 mg/day) in patients with mild-to-moderate AD (n = 119). Resveratrol, as well as its major metabolites: 3-O-glucuronidated-resveratrol (3GRES), 4-O-glucuronidated-resveratrol (4GRES), and 3-sulfated-resveratrol (SRES), were detected in the CSF and thus penetrated the BBB. After a 52-week-long intervention, the mean CSF concentrations of resveratrol, 3GRES, 4GRES, and SRES for the entire population were 0.45 ± 0.14 ng/mL, 8.3 ± 1.3 ng/mL, 11.2 ± 1.7 ng/mL, and 12.6 ± 1.6 ng/mL, respectively. Moreover, it has been found that the concentration of Aβ40 (the 40-amino acid isoform of β-amyloid being a diagnostic biomarker of AD), both in plasma and CSF, showed a smaller decrease in the study group as compared to the control. Whereas there were no differences between the groups in the level of Aβ42 (the 42-amino acid isoform of β-amyloid being a diagnostic biomarker of AD) in CSF, tau in CSF, phospho-tau 181 in CSF, as well as in functional tests [121]. Similarly, the recent study by Gu et al. confirmed the neuroprotective effect of resveratrol at a dose of 500 mg/day in patients with AD (mild-to-moderate), after 52 treatments. They demonstrated that this polyphenol regulated a progressive decline in Aβ40 levels in both CSF and plasma compared to placebo as well as based on an MRI scan; they showed that the brain volume was reduced in the test group compared to the placebo (p = 0.011). Moreover, it was observed that in the group receiving trans-resveratrol, the worsening in motor performance (based on the Alzheimer’s Disease Cooperative Study Activities of Daily Living Scale) was significantly reduced [130]. Although high-dose resveratrol is safe, well-tolerated, penetrates the BBB, and has direct effects on the CNS [121,130], the interpretation of changes in biomarker levels should be approached with caution. Due to small, pilot studies, it is necessary to conduct multicenter randomized trials that will allow for establishing the benefits of resveratrol in neurodegenerative disease.

The beneficial effects of curcumin on CNS diseases have been demonstrated in preclinical and clinical studies [47,122,131]. Its neuroprotective effect is mainly based on strong antioxidant and anti-inflammatory properties, as well as inhibiting the aggregation of misfolded proteins. In clinical studies, it has been observed that supplementation with curcumin, both singly and in combination therapy, mainly with coenzyme Q7 and omega-3 acids, reduces the severity, duration, and frequency of migraine attacks [132,133], the occurrence of motor disorders in Parkinson’s disease [129], and improves the clinical status of patients with amyotrophic lateral sclerosis [131]. The only available study assessing the levels of curcumin and its metabolites in CSF has not demonstrated a beneficial effect in AD patients. Ringman et al., in the randomized controlled trial—the Curcumin C3 Complex (^®)—assessed the effect of oral curcumin supplementation in patients with mild-to-moderate AD (n = 36). The studies lasted 24 weeks, with an open-label extension of 48 weeks. Patients received curcumin at a dose of 2 g/day, 4 g/day, or placebo for 24 weeks, then for the next 24 weeks, patients in the curcumin group continued the dose, while patients in the placebo group received curcumin at a dose of 2 g/day or 4 g/day. The purpose of this study was to investigate both curcumin’s effectiveness and bioavailability. They found that curcumin was well tolerated, but no clinical or biochemical improvement was observed. In contrast, in a pharmacokinetic study using liquid chromatography/tandem mass spectrometry, they showed that both native curcumin, tetrahydrocurcumin, and their glucuronidated metabolites were absent in CSF, suggesting that it does not penetrate the BBB [114]. The discrepancy in data on the beneficial effects of curcumin on the CNS implies a further examination of its neurorestorative activity, with particular emphasis on the pharmacological profile in humans. On the other hand, in vitro and preclinical studies have shown that quercetin, the most abundant naturally occurring flavonoid, has a neuroprotective effect through its antioxidant, anti-inflammatory, and cell toxicity-inhibitory activity [115,132,133,134]. However, the beneficial effects of quercetin in CNS diseases have not been confirmed in clinical trials because the limitation may be the low cerebral bioavailability of quercetin [135,136]. Nevertheless, intensive research on increasing the bioavailability of quercetin prompted Ishisaka et al. to investigate the cerebral localization of the main dietary metabolite of quercetin—quercetin-3-O-glucuronide (Q3GA). In a post-mortem study using human brain tissue with or without infarction using the immunohistochemical method, significant immunoreactivity was found in the cells of the choroid plexus, which form the structural basis of the blood-CSF barrier, and in foamy macrophages in recent stroke brains. The results of the immunohistochemical study implied an in vitro examination of the consequences of Q3GA cellular accumulation using brain capillary endothelial RBEC1, macrophage-like RAW264, and microglial MG6 cells. It has been demonstrated that immune cells may be a potential target of Q3GA because greater accumulation of Q3GA was observed in MG6 and RAW264 cells than in RBEC1. Importantly, this study noted that in MG6 and RBEC1 cells, Q3GA was deconjugated to its native and methylated forms. In addition, it has been found that the deconjugated forms in LPS-treated macrophages, as opposed to Q3GA, had anti-inflammatory effects by inhibiting the JNK pathway (c-Jun N-terminal kinase). Thus, it has been observed that quercetin glucuronide penetrates the BBB in humans and may accumulate in some types of immune cells [137]. Grabska-Kobylecka et al. showed that some polyphenols can pass through the BBB in humans. They examined the presence of 12 phenolic compounds in CSF, and collected data for diagnostic purposes from 28 patients (age 46 ± 16 years; 18 women and 10 men) with neurological diseases (multiple sclerosis, polyneuropathy, mononeuropathy, meningitis, brain tumor, epilepsy, amyotrophic lateral sclerosis, and Friedreich’s ataxia). Additionally, 30 min prior to the lumbar puncture, plasma was collected from each patient, and both plasma and CSF were assembled after an overnight fast. The identification of polyphenols in CSF and plasma was performed by combining solid-phase extraction with high-performance liquid chromatography. It has been shown that caffeic acid, homovanilic acid, and 3-hydroxyphenylacetic acid were present in CSF and plasma, while hippuric acid, 3,4-dihydrobenzoic acid, and dihydrocaffeic acid were detected in plasma only [68]. The sources of caffeic acid, homovanilic acid, and 3-hydroxyphenylacetic acid can be some vegetables and fruits, and they can also be metabolites of more complex polyphenols ingested. Nevertheless, homovanilic acid and 3-hydroxyphenol acetic acid can also be produced endogenously in the brain as products of catecholamine metabolism [134,135,136,137]. However, according to the current state of knowledge, caffeic acid is not produced endogenously, and its presence is indicative of an exogenous origin. Importantly, in the above study, no correlation between the concentration of phenolic acids in plasma and CSF was found, which may also suggest that the penetration of polyphenols through the BBB is not a result of facilitated or passive transport [68]. In turn, Zini et al. did not find detectable amounts of flavan-3-ols and their metabolites in CSF 2 h after green tea intake, while in the plasma collected 1 h after consumption, flavan-3-ol methyl, glucuronide, and sulfate metabolites: (2)-epigallocatechin glucuronide, (2)-epicatechin glucuronide, (2)-epicatechin sulfate, and methyl-(2)-epicatechin sulfate were identified by HPLC-MS2 analysis. The study included six patients, aged 34–61 (three women and three men), who had a lumbar puncture performed for diagnostic purposes due to the suspicion of multiple sclerosis. This study did not confirm green tea flavan-3-ol’s direct effect on the CNS. However, as the authors conclude, a serious limitation of this study is the small size of the group and a single dose of green tea [120].

An important issue in the direct action of polyphenols on the CNS is not only their penetration through the BBB, but also their brain metabolism and localization. However, as already mentioned, for methodological reasons, conducting mechanistic research in humans is extremely difficult, and knowledge on this subject is very limited. Nevertheless, human studies are crucial to unequivocally determining the effectiveness of antioxidant use in the prevention and treatment of CNS diseases.

To improve the bioavailability of phenolic compounds, nanoencapsulation technology can be used. The encapsulation process is the packing of small active particles (solid, liquid, or gas) into a matrix or shell, which is a secondary material [138]. That way, particles with diameters ranging from 1 to 1000 nm are formed. In such small capsules, the surface-volume ratio is increased. Nanoencapsulation provides significant protection against drastic conditions and allows for control of the release rate at the absorption site. The solubility of the encapsulated end product is improved, as well as toxicity to peripheral organs is reduced, and the degradation process in the gastrointestinal tract can be minimized. Other features of nanoparticles that should be emphasized are: nonthrombogenic, nonimmunogenic, and noninflammatory. It is essential that the encapsulation process does not affect or alter the medication’s properties. The following nanoscale delivery systems can be used to encapsulate polyphenols: nanocapsules, nanospheres, micelles, cyclodextrins, solid lipid nanoparticles, and liposomes [139].

Phenolics can be nanoencapsulated by natural nanocarriers, such as cyclodextrins, chitosan, casein, or nanocrystals. The dietary bioflavonoid quercetin can reduce inflammatory processes and exert neuroprotective effects when administered in vivo. The main anti-Alzheimer’s disease properties of quercetin include the inhibition of Aβ (amyloid-β) aggregation [140] and tau phosphorylation [141]. Quercetin, characterized by low solubility and poor absorption, is extensively metabolized in the gut after oral administration in humans. It has low BBB penetrability and limited bioavailability [142].

In order to improve its permeation across the BBB and its bioavailability to enhance neuroprotective effects, Testa and coworkers formulated b-cyclodextrin-dodecylcarbonate-based nanoparticles containing quercetin [49]. Cyclodextrin is a cyclic oligosaccharide in which 6, 7, or 8 glucopyranose units are joined by a glycosidic bond. The bioavailability of such a product is improved, as well as stability and other functional properties [143]. Such nanoparticles were used in an in vitro study on human neuroblastoma SH-SY5Y cells. Quercetin-loaded nanoparticles were noncytotoxic, and their anti-inflammatory effect in SH-SY5Y cells was stronger than that of free quercetin. The new therapeutic approach applied by the authors may allow for more effective use of the neuroprotective properties of quercetin for preventing/reducing AD progression. The study conducted by the Aluani group determined the neuroprotective activity of quercetin encapsulated in chitosan-alginate nanoparticles in two in vitro models: H₂O₂ induced oxidative stress in human neuroblastoma cells SH-SY5Y and 6-hydroxydopamine (6-OHDA) induced neurotoxicity in rat brain synaptosomes [144]. Naturally occurring polysaccharide, chitosan, is known for its high biodegradability, biocompatibility, and low toxicity. Quercetin-loaded nanoparticles, especially chitosan-based ones, were protective against oxidative stress in both models of neuronal toxicity. Thus, limitations associated with the clinical use of quercetin could be overcome, and its neuroprotective effect could be enhanced due to its incorporation into chitosan-based nanoparticles.

The first study investigating the protective and therapeutic effects of quercetin nanoparticles (QNPs) in an animal AD model was performed by Rifaai et al. [145]. AD was induced in rats by the oral administration of AlCl₃. AlCl₃ application triggered degenerative effects in the rat’s hippocampus, which were minimized by QNPs used as prophylaxis along with AlCl₃. In addition, the AlCl₃-induced neurofibrillary tangles and amyloid plaque formation were almost abolished in the same group of animals, confirming the prophylactic effect of QNPs in the AD model. Additionally, proliferation in the hippocampal dentate gyrus was preserved when rats were treated with QNPs. The authors demonstrated that the nanoparticle form of quercetin increased its bioavailability to the affected neurons and reduced the damaging effect of AlCl₃ on hippocampal neurons at the molecular, cellular, and subcellular levels.

To induce the bioavailability and solubility of quercetin, Jajin et al. used the magnetic targeting of superparamagnetic nanoparticles [146]. The authors developed quercetin-conjugated superparamagnetic iron oxide nanoparticles (QT-SPIONs) and tested their effects on AD-like symptoms induced by AlCl₃ in a rat model. What they discovered was that QT-SPION conjugates in all experiments were significantly more effective than quercetins alone and led to better bioavailability for quercetins. The entrance of quercetin into the brain tissue in the conjugate form was increased. QT-SPION augmented the expression levels of antioxidant enzymes (SOD1, CAT, and GPX1) reduced iNOS, along with an increase in the expression level of the antiapoptotic gene (BCL2) and a decrease in the proapoptotic gene (BAX). Experiments on rats with the application of aluminum to induce memory impairment have shown that treatment with QT-SPION recovered the memory impairment developed by AlCl₃ and the appearance of AD-like symptoms. A significant reduction in the expression level of the amyloid precursor protein gene in the group treated with QT-SPION was also demonstrated, which proves the potential of QT-SPION to inhibit AD development and progression caused by aluminum exposure. The obtained results make quercetin-conjugated nanoparticles, a nanosized delivery system, able to penetrate the BBB with beneficial effects on neurodegeneration, especially effective in preventing the progression of AD symptoms in long-term usage.

The most promising tool to aid in therapy for the treatment of AD is solid lipid nanoparticles (SLNs), a colloidal drug carrier system composed of a hydrophobic lipid core composed of an emulsion (oil in water) with lipids. The solid core, coated with aqueous surfactants, contains the drug, which is dissolved or dispersed [137]. For brain applications, the size of SLNs should not exceed 200 nm to cross the tight endothelial cells of the BBB, thus ensuring a high degree of tissue penetration.

The most important mechanisms making SLNs an attractive delivery system include low intrinsic cytotoxicity, the ability to bypass liver and spleen filtration, increased penetration to the BBB, the controlled release of the incorporated drug, and high stability [147,148]. Loureiro et al., in experiments on human brain-like endothelial cells, worked on developing an efficient system to increase the bioavailability of resveratrol, a natural polyphenolic flavonoid with known neuroprotective effects [149]. They took advantage of well-characterized morphologic lipid nanoparticles functionalized with an OX26 monoclonal antibody with the ability to bind cells expressing the transferrin receptors, TfR, present in the BBB. Such nanostructures were used as carriers for resveratrol, grape skin, and grape seed extracts. The authors demonstrated that encapsulated extracts were effectively transported into the brain, where the formation of Aβ(1–42) aggregates was inhibited, potentially influencing the progression of Alzheimer’s disease. The authors proved that the functionalization of SLN by coupling with the OX-26 antibody allows for targeting polyphenols or other drugs in the brain, where such nanoparticles are stable for at least one month and are taken by BBB cells.

Solid lipid nanoparticles were also used by Sandhir and colleagues to encapsulate curcumin (C-SLNs), and their neuroprotective efficacy was evaluated as an intervention in Huntington’s disease [150]. A rat model of 3-nitropropionic acid-induced HD in C-SLN-treated animals showed a notable increase in the activity of mitochondrial complexes, cytochrome, and reduced GSH levels. Additionally, reductions in mitochondrial swelling, lipid peroxidation, protein carbonyls, and ROS were observed when compared to nontreated rats. A significant improvement in neuromotor coordination has also been noted. These results proved that C-SLNs developed by this group have the potential to ameliorate the mitochondrial impairments in HD and could provide a potential therapeutic intervention in other neurodegenerative disorders.

Kundu et al. demonstrated in a study conducted with the use of curcumin and piperine-coloaded glyceryl monooleate (GMO) lipid nanoparticles coated with various surfactants demonstrated that such dual drug-loaded NPs were able to cross the BBB in a Parkinson’s disease mouse model [151]. The authors observed in in vitro experiments that alpha S protein aggregates inhibition, reduces cytotoxicity, oxidative stress, and apoptosis, and activates autophagy-mediated protein degradation induced by dual drug-loaded GMO nanoparticles compared to their native counterparts. Moreover, rotenone-induced motor coordination impairment was restored, and dopaminergic neuronal degeneration was restrained in a PD mouse model. In this case, a nanodelivery system is a promising tool to deliver drugs to the brain and enhance the bioavailability of multiple therapeutic agents for effective PD management.

Polymeric nanoparticles are composed of a solid core of a dense polymer, natural (proteins and polysaccharides) or synthetic (poly(lactide), poly(lactic acid) PLA, poly(glycolic acid) (PGA), poly(d,l-lactide-co-glycolic acid) (PLGA), poly(alkyl cyanoacrylate) (PACA) [152]. The selection of an appropriate polymer for the production of nanoparticles depends on the needs of a specific application and the characteristics of the drug being introduced [153]. The encapsulated lipophilic drug can be solubilized in an oil or aqueous core surrounded by the polymeric shell (nanocapsules), dispersed throughout the polymeric network, or located on the surface of the polymeric matrix (nanospheres) [154]. A promising strategy to reduce degradability and increase the effective delivery of phenolics could be nanoencapsulation with biopolymeric nanoparticles, e.g., poly (lactic-co-glycolic acid) (PLGA). PLGA, commonly used in nanomedicine, is a synthetic biocompatible copolymer composed of lactic and glycolic acid polyesters, approved by the Food and Drug Administration (FDA, Silver Spring, MD, USA) for therapeutic use in pharmaceutical products [155]. The possibility of forming nanoparticles, micelles, and microspheres, together with their complete biodegradability and well-tolerable properties, makes them a tool for controlled, targeted delivery of small drugs into the brain through different transport mechanisms [156].

There are several studies reporting the use of polyphenols encapsulated with PLGA nanopolymer as drugs transported to the brain for the treatment of brain diseases, especially Alzheimer’s and Parkinson’s disease. Among them, nanoparticles loaded with curcumin drew the attention of researchers. Mathew and coworkers synthesized water-soluble curcumin nanoparticles encapsulated in PLGA. The authors found by using an in vitro AD model that nanocurcumin particles bound to Aβ aggregates and mediated their dissociation, along with antioxidative properties and noncytotoxicity [157]. Another study by Mathew et al. showed that functionalized curcumin-loaded PLGA nanoparticles conjugated with Tet-1 peptide, known for its high affinity for neurons, were also able to disaggregate amyloid proteins. The targeting of the nanoparticles with the Tet-1 peptide enhanced their uptake by neuronal cells. Curcumin-PLGA-Tet-1 nanoparticles retained their antioxidant properties and were not cytotoxic [158]. The research of this group proved that nanoformulations with encapsulated curcumin may have a beneficial impact in order to address problems related to the poor bioavailability and pharmacokinetics of this antioxidant. Another group took advantage of the curcumin-encapsulated PLGA nanoparticles. In a study conducted on neural stem cells (NSC) and adult rat brains, Tiwari and colleagues demonstrated that Cur-PLGA-NPs are able to induce NSC proliferation as well as neuronal differentiation as compared to bulk curcumin, which proves that PLGA nanoparticles internalize into the NSC and nanocurcumin reaches the brain [159]. The authors proved that curcumin-loaded nanoparticles induced neurogenesis, as confirmed by the experiments carried out with the use of an amyloid β-induced rat model of AD. The results also demonstrated that neurogenesis was mediated by activating the canonical Wnt/beta-catenin pathway. PLGA nanoencapsulation could be a promising approach to delivering curcumin to the brain.

Additionally, Ray and coworkers studied the properties of a polymeric nanoparticle formulation of curcumin, NanoCurc, in in vitro and in vivo experiments and demonstrated its neuroprotective effect [160]. In vitro, NanoCurc-treated human SK-N-SH cells were protected from H₂O₂-mediated ROS insults. In vivo, NanoCurc was injected intraperitoneally in athemic mice, and following such treatment, significant curcumin levels were observed in the brain of this cohort versus controls, together with decreased levels of H₂O₂, caspase activity, and increased glutathione concentrations. This is another study whose results confirm the increase in curcumin’s bioavailability when used in a water-soluble encapsulated form. The potential of NanoCurc to protect, preserve, and rescue human neuronal cells against oxidative damage may have implications in the treatment of neurodegenerative diseases. In turn, Gosh et al. investigated PLGA-nanoencapsulated quercetin as a drug carrier for oral treatment of ischemia-reperfusion-induced neuronal damage in young and old Swiss Albino rats [161]. Such therapy resulted in an improvement in neuronal count in the hippocampal subfields, where massive damage to neuronal cells was observed after three days of continuous reperfusion following ischemia. Moreover, the downregulation of iNOS and caspase 3 activities was demonstrated as a result of oral nanoencapsulated quercetin. The delivery of this potent herb-origin antioxidant to the brain may protect against oxidative stress that causes ischemic neuronal damage. Sun and coworkers tested PLGA-functionalized quercetin nanoparticles (PLGA@QT) and investigated their effect on the bioavailability of this potent flavonoid to be used in AD treatment [162]. First, the authors confirmed in an in vitro and in vivo cytotoxicity assay on the neuronal cell line SH-SY5Y and female BALB/c nude mice that such quercetin nanoformulations are nontoxic. Second, PLGA@QT inhibited and disassembled Aβ42 fibrils. They also determined that PLGA@QT nanoparticles reversed cognition and memory impairments when injected into APP/PS1 mice insulted by AD. Furthermore, the improvement of spatial memory was also observed in a dose-dependent manner. The observations made make quercetin encapsulated in PLGA a potent inhibitor against Aβ42 aggregation and cytotoxicity with improved therapeutic efficacy for AD treatment.

Another example of widely used nanosized carriers for targeted drug delivery are liposomes, phospholipid spherical vesicles consisting of at least one concentric lipid bilayer that mimics the natural biomembrane that encloses aqueous spaces. Liposomes, together with solid lipid nanoparticles, are representative of lipid-based nanoparticles. Their sizes range from 30 nm to several micrometers, but the optimum size to pass through the BBB and provide efficient drug delivery is 20–100 nm. All lipophilic, hydrophilic, and amphiphilic molecules can be encapsulated, transferred, released, and protected from early inactivation and degradation [163]. Priprem et al. developed intranasal quercetin liposomes and tested them in male Wistar rats [164]. The elevated plus maze test showed anxiolytic-enhancing effects of quercetin liposomes, while the Morris water maze test revealed cognitive function improvement in rats compared with free quercetin treatment animals. It is worth emphasizing that administration through the intranasal route led to the direct delivery of quercetin to the brain, bypassing the BBB. The intranasal path provides more efficient delivery to the brain of lower quercetin doses and a lack of cytotoxicity compared to oral dosage.

α-Mangostin (α-M) is another polyphenolic compound that, apart from numerous pharmacological effects, slows down the progression of Alzheimer’s disease by affecting the amyloidogenic process [165]. Its neuroprotective effect is limited because of its poor penetration through the blood-brain barrier. In the study by Chen et al., liposome modification by transferrin was applied, thus the penetration of the drug across the BBB was facilitated via receptor-mediated endocytosis [166]. Such an application was used to examine α-mangostin transport in the Tf(α-M) liposome through the BBB and its distribution in the rats’ brains. The transport of the Tf(α-M) liposome through the BBB was confirmed by researchers in an in vitro BBB model constructed of cocultured astrocytes and the bEnd3 cells. α-M accumulation in the brain was confirmed in in vivo experiments on rats’ brains after intravenous administration. The authors showed the improvement of the brain delivery of α-mangostin by transferrin-modified liposome constructs and proved that Tf liposomes can serve as a potential carrier of the drug against AD.