Olive Leaves and Hibiscus Flowers Extracts-Based Preparation Protect Brain from Oxidative Stress-Induced Injury

Oxidative stress (OS) arising from tissue redox imbalance, critically contributes to the development of neurodegenerative disorders. Thus, natural compounds, owing to their antioxidant properties, have promising therapeutic potential. Pres phytum (PRES) is a nutraceutical product composed of leaves- and flowers-extracts of Olea europaea L. and Hibiscus sabdariffa L., respectively, the composition of which has been characterized by HPLC coupled to a UV-Vis and QqQ-Ms detector. As PRES possess antioxidant, antiapoptotic and anti-inflammatory properties, the aim of this study was to assess its neuroprotective effects in human neuroblastoma SH-SY5Y cells and in rat brain slices subjected to OS. PRES (1–50 µg/mL) reverted the decrease in viability as well as the increase in sub-diploid-, DAPI-and annexin V-positive-cells, reduced ROS formation, recovered the mitochondrial potential and caspase-3 and 9 activity changes caused by OS. PRES (50–100 µg/mL) neuroprotective effects occurred also in rat brain slices subjected to H2O2 challenge. Finally, as the neuroprotective potential of PRES is strictly related to its penetration into the brain and a relatively good pharmacokinetic profile, an in-silico prediction of its components drug-like properties was carried out. The present results suggest the possibility of PRES as a nutraceutical, which could help in preventing neurodegenerative diseases.


Introduction
In the last decades, a wealth of literature has suggested that despite their clinical heterogeneity neurodegenerative disorders share many common features in the etiology [1]. The accumulation of misfolded proteins is the common hallmark, which occurs alongside neuronal loss in specific brain areas, caused by mitochondrial function impairment, enhancement in endoplasmic reticulum stress as well as neuroinflammation [2]. High levels of oxidative stress (OS) are usually described in the brain of patients suffering from neurodegenerative conditions, and increased formation of reactive oxygen species (ROS) has been proposed as one of the potential crucial steps in their development [3].
Indeed, ROS are not considered as mere "triggering factors", but rather they are likely to worsen the progression of the disease being produced also during the brain inflammatory response [4,5].
The lack of disease-modifying therapies along with the growing number of cases, makes it critical to search for new approaches for treating different neurodegenerative diseases. The possibility of lowering ROS levels may be a promising strategy to delay or prevent neurodegeneration. In this view, many natural compounds endowed with antioxidant (but not only) properties are currently under clinical investigations (for a review see for example [6,7]).
The pharmaceutical formulation Pres Phytum ® (PRES) is a 13:2 (w/w) mix of the extracts prepared from Olea europea L. leaves and Hybiscus sabdariffa L. calyces, respectively, marketed as a food supplement (www.nutraceutica.it). This formulation, the composition of which has been fully characterized [8] (see also below) possesses antioxidant and cytoprotective effects [8,9], mainly due to the phenolic compounds found in both plants which includes hydroxytyrosol, oleuropein, flavonoids, chalcones, tannins, and hibiscus acid. Most of them possess neuroprotective properties along with many other positive effects [10][11][12][13]. Therefore, the aim of the present work was to assess the neuroprotective properties of PRES toward OS-mediated injury in human neuroblastoma SH-SY5Y cells. Cell viability, apoptosis markers, ROS formation, as well as changes in the mitochondrial potential were assessed after H 2 O 2 challenge. Activities of caspase-9 (intrinsic pathway), caspase-8 (extrinsic pathway) and caspase-3 (executioner pathway) were also evaluated. To explore the effects of PRES in a tissue context where the structural and synaptic organization of the original tissue is preserved, thus allowing a better extrapolation in terms of neuroprotection, rat brain slices subjected to OS were also used. Finally, as natural compounds have always faced problems, including limited absorption, rapid metabolism or systemic elimination, as well as inability to penetrate the brain, which might limit their protective activity, in silico physicochemical and pharmacokinetic properties of main PRES components were predicted.

Apoptosis Assays
Cell cycle and sub-G0/G1 population analysis was performed by using flow cytometry. SH-SY5Y (2.5 × 10 5 cells/mL, final volume of 2 mL) treated as detailed before, were trypsinized, collected by centrifugation (1500 rpm, 10 min), and carefully rinsed before fixing them with 70% ethanol. Before flow cytometry analysis, cells were treated with RNase (0.1 mg/mL, 30 min) and then stained with propidium iodide (PI) (20 µg/mL, 30 min. Red fluorescence (DNA) was detected by FACScan flow cytometer (BD Biosciences, San Jose, CA, USA) in the FL2 channel, acquiring 10 4 cells/sample. Cell Quest software v. 3.0 (BD Biosciences, San Jose, CA, USA) was used to calculate the percentage of apoptotic cell in the sub-G0/G1 peak and in the other cell cycle phases [16,17] DAPI (4 ,6-diamidino-2-phenylindole) staining kit (Life Tecnologies Italia, Monza, Italy) was used for assessing nuclear morphology. Kit's suggested protocol, with minor changes (i.e., staining with 0.5 µM DAPI for 10 min, followed by two washing in PBS and one in water) was followed [16,17]. For quantification, nuclei with fragmented and condensed DNA were scored as apoptotic cells by a blind, expert operator, who randomly chose 5-6 fields from at least three independent experiments. [18].
The Alexa fluor 488™-Annexin V/PI double staining kit (Life Tecnologies Italia, Monza, Italy) was used according to the manufacturer's protocol and data acquisition was performed as previously reported [17]. Cells were scored according the following criteria: PI negative, AV negative: viable cells. AV positive, PI negative, early apoptotic cells. AV positive, PI positive, late apoptotic cells. AV negative, PI positive, necrotic cells.

Effects of PRES on SH-SY5Y Cell Viability
The effects of PRES per se (0-100 µg/mL for 24 h) on SH-SY5Y cells viability were evaluated to find suitable concentrations for assessing neuroprotective properties.
As reported in Figure 1, the extract was devoid of cytotoxic effects up to 50 µg/mL, while a significant reduction in cell viability (~28%) was detected at 100 µg/mL. This was confirmed also by phase-contrast light microscope examination of the cells, which highlighted grade 1 signs of cytotoxicity (i.e., ~20% of cells were round, tended to detach and did not show intracytoplasmic granules). Therefore, the neuroprotective effects of the extract were explored in the range 0-50 µg/mL.

PRES Reverted OS-Induced Decrease in SH-SY5Y Cells Viability
To reproduce OS, SH-SY5Y cells were treated with H2O2 (25 µM for 1 h followed by 24 h with medium). A decrease in cell viability of about 50% occurred upon the H2O2 challenge ( Figure 2), while the 1 h pretreatment with PRES caused a concentration-dependent protection. Although not significantly, cells viability was partially recovered already at 1 µg/mL, while maximal neuroprotection was attained in the 5-50 µg/mL concentration range (25.8-34.1%, p < 0.01 vs. H2O2).

PRES Reverted OS-Induced Decrease in SH-SY5Y Cells Viability
To reproduce OS, SH-SY5Y cells were treated with H 2 O 2 (25 µM for 1 h followed by 24 h with medium). A decrease in cell viability of about 50% occurred upon the H 2 O 2 challenge (Figure 2), while the 1 h pretreatment with PRES caused a concentration-dependent protection. Although not significantly, cells viability was partially recovered already at 1 µg/mL, while maximal neuroprotection was attained in the 5-50 µg/mL concentration range (25.8-34.1%, p < 0.01 vs. H 2 O 2 ). Antioxidants 2020, 9, x FOR PEER REVIEW 5 of 16

Effects of PRES on SH-SY5Y Cell Viability
The effects of PRES per se (0-100 µg/mL for 24 h) on SH-SY5Y cells viability were evaluated to find suitable concentrations for assessing neuroprotective properties.
As reported in Figure 1, the extract was devoid of cytotoxic effects up to 50 µg/mL, while a significant reduction in cell viability (~28%) was detected at 100 µg/mL. This was confirmed also by phase-contrast light microscope examination of the cells, which highlighted grade 1 signs of cytotoxicity (i.e., ~20% of cells were round, tended to detach and did not show intracytoplasmic granules). Therefore, the neuroprotective effects of the extract were explored in the range 0-50 µg/mL.

PRES Reverted OS-Induced Decrease in SH-SY5Y Cells Viability
To reproduce OS, SH-SY5Y cells were treated with H2O2 (25 µM for 1 h followed by 24 h with medium). A decrease in cell viability of about 50% occurred upon the H2O2 challenge ( Figure 2), while the 1 h pretreatment with PRES caused a concentration-dependent protection. Although not significantly, cells viability was partially recovered already at 1 µg/mL, while maximal neuroprotection was attained in the 5-50 µg/mL concentration range (25.8-34.1%, p < 0.01 vs. H2O2). PRES neuroprotective effects were pointed out also by phase-contrast images. Compared to the control SH-SY5Y cells, those treated with H 2 O 2 tended to round up and to detach from the well, showed shrinkage and membrane blebbing that are normally associated with apoptotic cell death ( Figure 2 and Figure S1). PRES-treated cells instead appeared progressively healthy in both shape and number upon increasing PRES concentration.

PRES Reduced Apoptotic-Mediated SH-SY5Y Cell Death Caused by OS
Cell cycle analysis showed that after OS the percentage of sub G 0 /G 1 hypodiploid, apoptotic cells was significantly higher (+12.0%, p < 0.001 vs. untreated cells), while that of G 0 /G 1 phase decreased (−9.1%, p < 0.001 vs untreated cells) (Figure 3a). Interestingly, flow cytometry data also indicated that PRES (5-50 µg/mL) exerted neuroprotection, as the percentage of cell in sub G0/G1-and G 0 /G 1 -phase were brought back to basal values. Finally, the amounts of cells in S and G 2 /M phase were mostly unchanged in both OS and OS + PRES conditions. Nuclear apoptosis, assessed by using the florescent dye DAPI, confirmed previous results ( Figure 3b). Antioxidants 2020, 9, x FOR PEER REVIEW 6 of 16 are reported as means ± SEMs. § § p < 0.01, § § § p < 0.001 vs. untreated cells; ** p < 0.01, *** p < 0.001 vs. H2O2 (one-way ANOVA and Bonferroni post-test).
PRES neuroprotective effects were pointed out also by phase-contrast images. Compared to the control SH-SY5Y cells, those treated with H2O2 tended to round up and to detach from the well, showed shrinkage and membrane blebbing that are normally associated with apoptotic cell death (Figures 2 and S1). PRES-treated cells instead appeared progressively healthy in both shape and number upon increasing PRES concentration.

PRES Reduced Apoptotic-Mediated SH-SY5Y Cell Death Caused by OS
Cell cycle analysis showed that after OS the percentage of sub G0/G1 hypodiploid, apoptotic cells was significantly higher (+12.0%, p < 0.001 vs. untreated cells), while that of G0/G1 phase decreased (−9.1%, p < 0.001 vs untreated cells) (Figure 3a). Interestingly, flow cytometry data also indicated that PRES (5-50 µg/mL) exerted neuroprotection, as the percentage of cell in sub G0/G1-and G0/G1-phase were brought back to basal values. Finally, the amounts of cells in S and G2/M phase were mostly unchanged in both OS and OS + PRES conditions. Nuclear apoptosis, assessed by using the florescent dye DAPI, confirmed previous results (Figure 3b). H2O2 caused, in fact, a huge increase in the number of DAPI positive cells with fragmented and condensed nuclei, while in the presence of PRES a concentration-dependent reduction of these cells was observed, achieving the maximum effect at 5-50 µg/mL PRES.
The externalization of phosphatidyl serine (Annexin V/PI assay) was used to further assess PRES neuroprotection. As shown in Figure 4, the H2O2 challenge resulted in a marked increase in early and late apoptotic cells, as well as in those necrotic, while the population of viable cells dropped. The externalization of phosphatidyl serine (Annexin V/PI assay) was used to further assess PRES neuroprotection. As shown in Figure 4, the H 2 O 2 challenge resulted in a marked increase in early and late apoptotic cells, as well as in those necrotic, while the population of viable cells dropped.
In contrast, PRES-pretreated SH-SY5Y cells showed significant resistance to OS-induced cytotoxicity, especially when using the natural extract in the 5-50 µg/mL concentration range. In this case, viable, early apoptotic, and necrotic cells gradually returned to control values upon increasing PRES concentration, while the late apoptotic population seemed not to be affected. In contrast, PRES-pretreated SH-SY5Y cells showed significant resistance to OS-induced cytotoxicity, especially when using the natural extract in the 5-50 µg/mL concentration range. In this case, viable, early apoptotic, and necrotic cells gradually returned to control values upon increasing PRES concentration, while the late apoptotic population seemed not to be affected.

PRES Hampered the Formation of ROS Along with Loss in Mitochondria Membrane Potential Caused by OS
PRES lowered in a concentration-and time-dependent pattern intracellular ROS formation caused by H2O2 (Figure 5a), as highlighted by DCF-DA assay. Significant protection already occurred at 1 µg/mL after 24 h, while maximum effect was achieved at 10-50 µg/mL at both tested times. Finally, PRES per se had negligible effects on ROS production. The mitochondrial membrane potential (Ψm) is closely linked to functional activity and loss of Ψm (depolarization) is an initial sign of apoptosis, being a result of mitochondrial uncoupling. The change in JC-1 fluorescence from red to green was used to assess Ψm changes. ΔΨm of hydrogenperoxide-treated cells was reduced by 36% after 24 h (Figure 5b). PRES restored Ψm values and the maximal effect was achieved at 10 µg/mL. These results suggest that PRES preserves mitochondrial The mitochondrial membrane potential (Ψm) is closely linked to functional activity and loss of Ψm (depolarization) is an initial sign of apoptosis, being a result of mitochondrial uncoupling. The change in JC-1 fluorescence from red to green was used to assess Ψm changes. ∆Ψm of hydrogen-peroxide-treated cells was reduced by 36% after 24 h (Figure 5b). PRES restored Ψm values and the maximal effect was achieved at 10 µg/mL. These results suggest that PRES preserves mitochondrial function, thus probably hampering the initiation of mitochondrial-dependent apoptosis.

The Increase in Caspase Activity Caused by OS Was Reduced by PRES
Caspase-9 and -8 are the key enzymes of the apoptotic mitochondria-and the extrinsic receptor-mediated pathways, respectively. These, in turn, activate caspase-3 [21]. To investigate whether PRES could revert the activation of these apoptotic pathways caused by OS, specific fluorescent caspase-3, -8 and -9 substrates were used. As 10 and 50 µg/mL were equally active in the previous assays, PRES was tested in the range 1-10 µg/mL. The effects of PRES per se (10 µg/mL) were also assessed.
The mitochondrial membrane potential (Ψm) is closely linked to functional activity and loss of Ψm (depolarization) is an initial sign of apoptosis, being a result of mitochondrial uncoupling. The change in JC-1 fluorescence from red to green was used to assess Ψm changes. ΔΨm of hydrogenperoxide-treated cells was reduced by 36% after 24 h (Figure 5b). PRES restored Ψm values and the maximal effect was achieved at 10 µg/mL. These results suggest that PRES preserves mitochondrial function, thus probably hampering the initiation of mitochondrial-dependent apoptosis.

The Increase in Caspase Activity Caused by OS Was Reduced by PRES
Caspase-9 and -8 are the key enzymes of the apoptotic mitochondria-and the extrinsic receptormediated pathways, respectively. These, in turn, activate caspase-3 [21]. To investigate whether PRES could revert the activation of these apoptotic pathways caused by OS, specific fluorescent caspase-3, -8 and -9 substrates were used. As 10 and 50 µg/mL were equally active in the previous assays, PRES was tested in the range 1-10 µg/mL. The effects of PRES per se (10 µg/mL) were also assessed.
Results showed that the activity of both cleaved caspase-3 and -9, but not that of caspase-8, were significantly increased upon OS ( Figure 6). The activation of the mitochondrial pathway was significantly reverted by PRES at 1 or 10 µg/mL as both caspase-3 and caspase-9 activity regained basal values. Finally, PRES per se did not affect caspase-3, -8 and -9 activity.

PRES Neuroprotection Occurred Also in Rat Brain Slices Subjected to OS
PRES neuroprotection was also assessed in rat brain slices, in which the main structural and synaptic organization of the original tissue is conserved [22]. As a first step, the effects of PRES per se (0-200 µg/mL) on brain slices viability (MTT assay) and tissue edema were evaluated. The extract was devoid of cytotoxic properties in the range of the tested concentrations (Figure 7a,b): however, as a slight, not significant reduction in slices viability was observed upon 200 µg/mL, PRES neuroprotective activity was tested in the range of 0-100 µg/mL. For these experiments, PRES pretreatment (1-100 µg/mL, 1 h) was performed, followed by an OS challenge (H 2 O 2 1 mM, 1 h): afterward, slice viability and tissue edema were assessed. OS caused a significant reduction in slice viability (54.1 ± 2.6%, p < 0.001), which was accompanied by an increase in tissue edema (54.12 ± 15.6%, p < 0.001) (Figure 7c,d).
Interestingly PRES reverted the effects of OS at 50 and 100 µg/mL, while 10 µg/mL was ineffective.

In Silico Prediction of PRES Component's Drug-Like Properties
As the neuroprotective potential of PRES is strictly related to its penetration into the brain and a relatively good pharmacokinetic profile, an in-silico prediction of PRES component's drug-like properties was carried out (Table 1). Moreover, oleuropein has been assessed either as glycoside or

In Silico Prediction of PRES Component's Drug-Like Properties
As the neuroprotective potential of PRES is strictly related to its penetration into the brain and a relatively good pharmacokinetic profile, an in-silico prediction of PRES component's drug-like properties was carried out (Table 1). Moreover, oleuropein has been assessed either as glycoside or as aglycone, being the latter the main form assimilated in the shape of oleuropein, and endowed of biological and pharmacological properties, including a remarkable antioxidant activity.
In particular, quantitative structure-activity relationship studies, considering CNS-active drugs, have suggested a set of physicochemical parameters that CNS compounds should possess. They are molecular weight <450 Dalton, a number of hydrogen bond acceptors <7, a polar surface area <90 Å, a ClogP <5, and a number of torsions ≤10. Considering these features, oleuropein has a good probability of entering the CNS (Table 1), considering that the majority of the predicted values fall within the above-described ranges. Moreover, the QPPMDCK value, which gives an estimation of the diffusion into MDCK cells (models of the BBB), shows that oleuropein would have adequate brain diffusion features. This is further supported by the predicted brain/blood partition coefficient for orally administered drugs (QPlogBB). Interestingly the same component is also predicted to overcome the gut/blood barrier (see QPPCaco in Table 1) thereby supporting its oral bioavailability. Apart from oleuropein, the other components are not predicted to easily cross the BBB except for elenolic acid glucoside and hibiscus acid, for which the QPlogBB (Table 1) indicates the potential to be brain-penetrant. Overall, from these calculations, oleuropein, elenolic acid glucoside, and hibiscus acid are the three PRES components that would show the highest possibility to exert their biological functions at CNS.

Discussion
The present work has assessed neuroprotective properties of PRES toward OS-mediated injury in both human neuroblastoma SH-SY5Y cells and in rat brain slices. OS-induced injury was selected as it is strictly involved in the mechanisms linked to neurodegeneration, either as a cause or as a crucial factor of the downstream cascade which leads to neuronal death [23]. The present findings demonstrated that OS caused by H 2 O 2 challenge induced ROS production accompanied by apoptotic death of SH-SY5Y cells, shown as an increase in hypodiploid cells (cell cycle analysis), early and late apoptotic cells (AV/PI staining), as well as in chromatin condensation and DNA fragmentation (DAPI staining). The fluorescent probe JC-1 was also used to investigate the effects of OS on mitochondria membrane potential.
A membrane depolarization occurred, strongly suggesting the triggering of the apoptotic intrinsic, mitochondrial-mediated pathway, that involves the activation of the downstream caspase-9, which leads to active caspase-3 and -7 [21]. The results showed an increase in caspase-9 and caspase-3, but not caspase-8, activity following H 2 O 2 treatment, thus confirming this hypothesis. The present findings agree with other reports on the same cell line [24,25], but not with [26], which reported caspase-8-mediated extrinsic pathway activation, and [27], which described both intrinsic and extrinsic involvement. This discrepancy can be explained by considering the different concentrations and times of exposure of H 2 O 2 used to induce OS, the types of assays and different parameters used in each study to assess the injury, as already examined by [27].
Results here reported demonstrated that PRES may have potential in preventing OS-mediated neuronal injury since it can revert loss in cell viability (MTT assay). When neurons are exposed to oxygen and nitrogen species, a huge amount of energy is required to protect tissues from the stress. This causes mitochondrial malfunction, with the release of cytochrome C and other mitochondrial proteins, which in turn drive apoptosis [28]. PRES reverted the formation of ROS and apoptotic cells, the loss in MPP as well as caspase-3 and caspase-9 activation. Interestingly, PRES cytoprotective and antioxidant effects occurred at the same concentrations (5-50 µg/mL) in both SH-SY5Y and HUVEC cells exposed to the same challenge [8].
To better understand the mechanism underlying PRES neuroprotection, it is crucial to refer to its main components. PRES composition, characterized by HPLC coupled to a UV-Vis and QqQ-Ms detector, has already been reported [8,9] and the main components, fully detailed in Materials and Methods, consist in elenolic acid glucoside (23.65 ± 0.79 mg/g), oleuropein (215.1 ± 1.64 mg/g), oleuropein isomer (51.09 ± 0.35 mg/g) for Olea europaea L. leaves, and hibiscus acid (139.2 ± 0.47 mg/g) for Hibiscus sabdariffa L. calyces. These compounds possess antioxidant properties [29][30][31] and this could be at the basis of the ability of PRES to revert ROS formation and thus to elicit neuroprotection. Numerous in vitro and in vivo reports have proven oleuropein-derived numerous positive effects in consideration of its antioxidant, antimicrobial, antiviral, hepatoprotective, antitumoral, antiaging, cardioprotective, and anti-inflammatory properties [29,32]. In terms of neuroprotection, the contributions of oleuropein are its dose-dependent, strong, antioxidant property linked to metal ions chelating activity, the ability to scavenge nitric oxide, to decrease ROS and RNS, and to reduce lipid peroxidation [33]. Also its antiapoptotic properties as well as the ability to activate the intracellular ubiquitin/proteasome pathway, which drive the degradation of damaged or misfolded proteins, have been proven to contribute [33,34]. Inhibition of inflammatory enzymes, such as lipoxygenases, and reduction of inflammatory mediators (TNF-α, NF-kB, IL-1β and IL-6) have been also described [32]. Being oleuropein and its isomer the most representative from a quantitative point of view, it is reasonable to hypothesize that PRES-mediated neuroprotection is mainly driven by oleuropein itself. Interestingly, however, the most recent experimental studies in SH-SY5Y cells subjected to different injuries have shown that oleuropein exert neuroprotection at concentrations 20-50 µM (see, for example, [35,36]). The present results showed that this compound is active at 2-4 µM, thus suggesting a contribution of other components of PRES. Among these, elenolic acid glucoside and hibiscus acid, the most representative from a quantitative point of view after oleuropein, might play a role. While the neuroprotective activity of elenolic acid glucoside is elusive, that of hibiscus acid is instead known. This compound was proven to ameliorate cognitive dysfunction in vivo via antioxidant effect, to boost the cholinergic system, to inhibit tau hyperphosphorylation pathways by regulating the expression of p-JNK, p-tau, and c-PARP [37]. Moreover, in a mice model of multiple sclerosis, hibiscus acid is neuroprotective by ameliorating inflammation and OS [38]. Taken together we can conclude that PRES neuroprotective activity toward OS-induced SH-SY5Y apoptotic death, is due to the antioxidant, as well as antiapoptotic, properties of its components, which might act synergistically. This is further supported by the observation that in HUVEC cells cytoprotective and antioxidant properties of the single extracts from Olea europaea L. leaves and Hibiscus sabdariffa L. calyces were less active in reverting OS-mediated injury than the 13:2 (w/w) mixture (i.e., PRES used in the present study) [8].
SH-SY5Y cells results were further supported by those obtained in rat brain slices, an in vitro experimental model that maintains functional synapses along with three-dimensional tissue architecture, and widely used to assess neuroprotection [39]. PRES still exerted protection, although at higher concentrations than in SH-SY5Y cells (50-100 µg/mL). This can be explained by considering that in brain slices, target(s) site(s) is(are) reached after PRES permeation across the membranes and this could require higher concentration in comparison to cultured cells, where the targets are certainly more accessible.
In the case of neuroprotective compounds, the ability to access the brain is a crucial point. For these reasons, we have performed an in-silico prediction of PRES component's drug-like properties. These studies confirmed that among all the PRES components, oleuropein has the highest chance to be orally bioavailable and cross the blood-brain barrier. Additionally, to some extent, elenolic acid glucoside and hibiscus acid would have a comparable chance of distributing in the brain. Nevertheless, it should be also outlined that the ADME properties of this nutraceutical product, and consequently its biological effect in vivo, might be ascribed to the concurrent presence of all the PRES components. Finally, both oleuropein and hibiscus acid have shown interesting and promising neuroprotective effects also in in vivo models of neurodegenerative diseases [10,[40][41][42][43]. Moreover, oleuropein or its active metabolite, hydroxytyrosol, is found in a significant amount in the brain following oleuropein peripheral administration [44,45]. These observations, along with in silico data of the present study, support the possibility of the PRES main component to cross the blood-brain barrier, supporting its use as a neuroprotective agent.

Conclusions
Effective treatments for neurodegenerative disorders represent a significant challenge for medicine. Preventive strategies can decrease not only the risk of developing the disease, but also the progression of their symptoms [46]. A possible approach is to use a multi-target therapy, based on the use of compounds able to act simultaneously on more than one target, with the final goal of achieving a synergistic action and, thus, a better therapeutic efficacy. Natural derivatives constitute an important source of multi-target compounds, and these might drive toward a novel, interesting direction the discovery of novel compounds for delaying the onset or the progression of neurodegenerative disorders [47]. Their importance is further supported by the synergistic effect often caused by the combination of the different components [48,49]. Intake of phytochemicals on a regular basis might in fact boost the antioxidant system, thus increasing neuronal cell survival and improving physical and mental activity [47,50]. The present findings highlight interesting neuroprotective properties of PRES, probably due to the multi-target, synergic effects of its main components. The possible good penetration in the brain constitutes an added value to its potential use as a nutraceutical, which could help in preventing neurodegenerative diseases.