Part 2 Marine Natural Products: Promising Candidates in The Modulation Of Gut-Brain Axis Towards Neuroprotection

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The neuroprotective property of astaxanthin has been recently noticed passing through its anti-inflammatory, anti-apoptotic, and antioxidant effects besides maintenance of neural plasticity [156–158]. This collection of features, candidate astaxanthin as a therapeutic agent in neurodegenerative diseases. Although, because of its anti-inflammatory and antioxidative behavior, it has been investigated in cardiovascular health, metabolic syndrome, management of gastric ulcers, and malignancy, all of which share a common characteristic of inflammation and oxidative stress [159]. In building to cell membrane against oxidative stress, which is being exacerbated by aging, astaxanthin plays an important part by applying associated antioxidant agents like SOD, heme oxygenase-1 (HO-1), and GSH. Its unique structure of ketone-bearing ionone rings stabilizes radicals synergistically with the polyene backbone and improves antioxidative ability [160]. It was demonstrated that ROS could be depleted in vitro by using astaxanthin [161]. Suppression of microglial activation and as a result, lowering the production of cytotoxic compounds, is another function of astaxanthin [118]. Released NO from microglia during inflammation, reacts with superoxide leading to the formation of peroxynitrite that is a ROS, and destructing proteins/lipids/DNA is the final consequence [162]. It was demonstrated that the expression and release of IL-6, COX-2 was down-regulated by astaxanthin in an LPS-induced activated microglia, in vitro [163]. The host immune system can be boosted by the administration of astaxanthin. The result was the stimulation of T cells which results in the production o IFN-γ, B cells to end up to secretion of IgA, and natural killer cells, as well as generation of IFN-c and IL-6 [164].

In a recent study by Wu et al., astaxanthin made a shift in the GM towards the suppression of intestinal inflammation/oxidative stress pathways by inhibiting colon NLRP3 inflammasome activation [165]. Overall, on one hand, astaxanthin potentially attenuates neuronal dysregulated pathways. Besides, it modulated GM towards the modulation of intestinal inflammation/oxidative stress/apoptosis. Meanwhile, the expression of in- inflammatory factors including TNF-α and INF-γ decreased, while the expression of IL-10 increased. In another study, by regulating cecal microbiota, astaxanthin meaningfully decreased the expression of MyD88, TLR4, and p-p65, while increasing the p65 expression [166]. So, considering the simultaneous modulatory role of astaxanthin on the gut and brain, it introduces astaxanthin as a great candidate in regulating the gut-brain axis towards neuroprotection.

Lycopene is another carotenoid through which the gut and brain potentiate their beneficial relationship. It exists in vegetables and red fruits, such as red pepper, papaya, tomato, watermelon, and also in algae. Following 5 days of administration of diets on lycopene, the related plasma amount of lycopene is significantly increased towards distribution in blood and brain. Lycopene seems to be responsible for gut-brain regulation then is particularly discharged undigested from the human body [167]. Studies indicated that the GM balance has a vital contribution to the occurrence/development of colitis, which affects brain function by the gut physiology regulation [3,168]. As a carotenoid, it has shown a critical role in colitis and the associated behavioral disorders. It is shown that 40 days of lycopene therapy (50 mg/kg body weight/day) in male mice prevented gut damage and inflammatory responses induced by dextran sulfate sodium (DSS). Lycopene attenuated DSS-induced dysfunctions of anxiety-like behavior and depression by suppressing synaptic damages, inhibiting neuroinflammation, and increasing the expressions of neurotrophic factor and postsynaptic-density protein. Additionally, lycopene resulted in GM remodeling of colitis mice by reducing the relative abundance of Proteobacteria and elevating the relative abundance of Lactobacillus and Bifidobacterium. It has also stimulated the SCFAs production and suppressed the LPS permeability in colitis mice [169]. Recent results demonstrated that GM germinates SCFAs in Bacteroides and some others. Among those, Prevotella, Ruminococcus, Bacteroides, Clostridium, and Streptococcus produce acetic acid, Bacteroides, Coprococcus, and Ruminococcus, germinate propionic acid, and Lachnospiraceae Bifidobacterium, and Coprococcus harvest butyric acid [170]. The cell wall of Gram-negative bacteria (e.g., Proteobacteria and Bacteroides) is primarily composed of LPS, which is an activator antigen of TLR4 in triggering inflammatory responses [169]. The dietary supply of Bifidobacteria and Lactobacilli has also shown promising effects on behavioral disorders like depression [168]. Growing reports have indicated that the destruction of the gut barrier would also result in a gut permeability increment. Subsequently, the generation of some pathogenic compounds (e.g., LPS) is followed by passing through intestinal epithelium and the blood entrance, to trigger neuroinflammation [171]. Reports have revealed that microglia activation and pro-inflammatory cytokines (e.g., IL-1 and TNF-α) could dysregulate synaptic plasticity towards emotional disorders and depression-like behaviors [127]. Thus, lycopene attenuates DSS-induced behavioral disorders and colitis through making a balance in the microbe's gut-brain axis, thereby reducing brain microglia activation and gut/brain inflammatory cytokines [172]. In agreement with recent studies, a low level of BDNF may increase stress susceptibility to affect the structure of forebrain neurons. Therefore, it has been an auspicious marker of a positive antidepressant response of related drugs [3]. Moreover, lycopene has also protected the brain against behavioral disorders by modulating the brain expression of BDNF, as well as associated synaptic plasticity. Bidirectionally, it is controlling the DSS-induced change of GM metabolites including SCFAs and LPS. Thus, lycopene is reshaping the GM of DSS-induced colitis mice, indicating that gut-brain axis balance is an underlying mechanism [167]. Overall, the studies illustrated that lycopene could ameliorate DSS-induced colitis, as well as associated anxiety-like symptoms and depression. It could be clarified by the helpful effects of lycopene on gut barrier integrity as well as its modulatory roles on GM and associated production of metabolites [172].

As previously mentioned, lycopene has shown beneficial effects in CNS. Its activity in reducing oxidative stress and tertbutyl hydroperoxide-induced cell apoptosis promised using lycopene in AD. Lycopene has a variety of activities including raising GSH/GSSG enzyme levels, maintenance of mitochondrial membrane potential, depletion of ROS [173], reduction of inflammatory cytokine levels, and downregulation of TLR4 and NF-κB p65 mRNA and protein expressions [174]. Besides, the effects of lycopene on neurological recovery and anti-inflammation activities were also investigated on rat models for the management of spinal cord ischemia/reperfusion injury [175]. During a clinical trial, lycopene increased the relative abundance of GM profiles, such as Bifidobacterium adolescentis and Bifidobacterium longum [176]. So, according to the critical role of lycopene in modulating GM and associated mediators of the gut-brain axis, it would be a promising agent towards neuroprotection.

5.2. Polysaccharides: Fucoidan, Chitosan, Alginate, and Laminarin

Other anti-inflammatory molecules that could be obtained from algal are polysaccharides, including chitosan, laminarin, fucoidan, and alginate [177]. Attenuation of NF-κB and ERK/MAPK/Akt pathways in BV2 microglia induced by LPS resulted in depressed anti-inflammatory responses by a sulfated polysaccharide named fucoidan [178]. In an in vivo study by Shang et al., fucoidan attenuated the GM in mice by increasing the abundance of Ruminococcaceae and Lactobacillus [179]. In another study, it alleviated colonic inflammation and GM dysbiosis by inhibiting IL-1β, IL-6, and TNF-α, while increasing IL-10 [180]. In a more recent study, feeding fucoidan from Okinawa mozuk altered the intestinal microbiota composition of adult zebrafish, which was characterized by the emergence and predominance of multiple bacterial affiliated with Comamonadaceae and Rhizobiaceae. They also found decreased expression of intestinal IL-1β [181]. Shi et al., also showed that fucoidan from Acaudina molpadioides increased the abundance of short SCFAs producer Coprococcus, Butyricicoccus, and Rikenella, through which mitigated intestinal mucosal injury [182]. In a recent double-blinded, placebo-controlled study, combination therapy of fucoidan increased fecal SCFAs, Pseudocatenulatum, Bifidobacterium, Bacteroides intestinalis, Eubacterium siraeum, while decreasing Prevotella copri [183]. Additional studies have also focused on the potential of fucoidan in modulating GM by targeting antioxidative mediators (e.g., Nrf2, and GSH/GSSG) [184]. Sulfated oligosaccharides of gran algae Ulva Lactuca and Enteromorpha prolifera decreased pro-inflammatory agents, reduced p53 and fork-head box protein O1 (FoxO1) genes, and caused overexpression of Sirt1 gene in SAMP8 mice [185]. Additionally, it upregulated the expression of BDNF through the ERK/CREB/tropomyosinrelated kinase receptor B (TrkB) pathway and antioxidant enzymes such as HO-1, NAD(P)H quinine oxidoreductase-1 (NQO-1), and glutamate-cysteine ligase catalytic subunit (GCLC) via Akt [186].

The second most abundant polysaccharide is chitin after cellulose [187]. The deacetylated derivative, chitosan, has shown several therapeutic applications, including antimicrobial/antifungal [188,189], antioxidant [190], and antitumor [191] effects. Many studies showed the neuroprotective, antioxidative, and anti-inflammatory properties of chitosan. Deacetylation degree and chain size are two determinants of these properties [192]. In a study by He et al., chitooligosaccharides were used as a protectant against oxidative stress caused by H2O2, and the lowest concentration with the highest efficacy was reported as 0.02 mg/mL. It was also demonstrated that carboxymethylated chitosan had a satisfactory effect in protecting Schwann cells against hydrogen peroxide-induced damage through a mitochondrial-dependent pathway [193]. In most recent reports, chitosan potentially modulated GM [194] by suppressing Helicobacter, promoting Akkermansia [195], and decreasing serum levels of IL-6 and IL-1β [196]. Yu et al., also showed that chitosan supplementation improved the Prevotella in the cecal of pigs [197]. Another polysaccharide that has been extracted from algae is selenopolymannuronate was prepared from alginate-derived polymannuronate. Alginate is the major structural polysaccharide of brown macroalgae, which is widely used as food additives and functional food ingredients owing to the desired physicochemical properties and well-recognized beneficial effects on gut ecology [198]. Bacteroides ovatus is responsible for the fermentation of alginate in the gut. The fermentation outcome of alginate by gut microorganisms is SCFAs, which acts as an energy provider for intestinal and immune cells [199]. Treatment with alginate reduced IL-1β and CD11c inflammatory markers, and improved the growth of GM Lactobacillus gasseri, Lactobacillus reuteri, and Akkermansia muciniphila in mice [200]. Such beneficial roles of alginate on GM were also reported in other studies [201]. Additionally, alginate-derived oligosaccharides downregulated pro-inflammatory agent’s expression and enzymes in LPS/Aβ-induced BV2 microglia. This oligosaccharide also decreased the expression of TLR4 and NF-κB [202]. From the mechanistic point, alginate has targeted oxidative pathways (e.g., ROS) and neuroinflammation (e.g. TLR4) to protect neuronal damages. So, regulation of GM and neuronal dysregulated pathways by alginate pave the road in attaining a promising gut-brain axis [203]. Other polysaccharides which could be fermented by GM include Laminarin [204]. Brown seaweeds, such as Laminariaceae as the source and Laminaria, Saccharina, or Eisenia species as secondary [205] contain a linear polysaccharide named laminarin. Studies showed its neuroprotective properties in cerebral ischemic events through gliosis reduction and controlling pro-inflammatory microglia. In an in vivo study, laminarin meaningfully decreased GM Firmicutes while increased Bacteroidetes [206]. On the other hand, laminarin shows potential neuroprotective effects by attenuation of inflammation and oxidative stress. In this regard, laminarin reduced IL-1β, TNF-α, while increasing SOD, IL-4, and IL-13 in aged gerbils [207]. So, by targeting GM and triggering anti-inflammatory and antioxidant mediators, laminarin could pave the road in the modulation of the gut-brain axis towards neuroprotection. Besides, carrageenan derived from red algae is a high molecular weight biopolymer molecule, involves linear sulfated galactans [205]. κ-carrageenan extracted from Hypnea musciformis red algae showed the neuroprotective property in neurotoxicity caused via 6-hydroxydopamine on SH-SY5Y cells by regulating mitochondria transmembrane potential and decreasing activity of caspase-3 [208]. Porphyrin from Porphyra yezoensis suppressed NO generation in LPS-stimulated RAW264.7 cells by downregulation of iNOS expression [209]. Besides, sulfated polysaccharide fractions from Porphyra hairiness had antioxidative properties and avoided lipid peroxidation in rat liver microsome [131].

In general, marine-derived polysaccharides either modulate GM or suppress neuronal dysregulations towards the attenuation of the gut-brain axis and developing neuroprotection.

5.3. Macrolactins/Anthraquinones: Macrolactin A

Another example of marine natural products is macro actions. Those are polyene cyclic backbone consisting of 24-membered ring lactones with alterations such as the binding of glucose β-pyranoside. Around 32 macro actions are discovered so far including, 7-O-succinyl prolactin A, macro actions A–Z, 7-O-succinyl prolactin F, 7-O-malonyl prolactin A, and three ether-containing prolactin A. Their sources are marine sediment and soil isolates. Macrolactin A has exhibited antibacterial effects against S. aureus and B. subtilis at a concentration of 5 and 20 µg/disc, respectively, with the ability to inhibit B16-F10 murine melanoma cancer cells in vitro, mammalian Herpes simplex viruses, and protection of lymphoblast cells in opposition to HIV by inhibiting virus replication [210]. A Gram-positive bacterium which is an habitant of the deep-sea sediments is another source of prolactin A. This compound has antibacterial, antiviral, and cytotoxic activities. It shows potent inhibition of mammalian herpes simplex virus (type I and II), protection of T cells against HIV replication, and suppression of B16-F10 murine melanoma cells in in vitro assays [211]. Marine microorganism-derived macro actions inhibit inflammatory mediators through modulating HO-1/Nrf2 and suppressing TLR4 [212]. Yan et al. showed that treatment with macro actions inhibited the mRNA expressions of iNOS, IL-1β, and IL-6, in vitro [213]. Those all could introduce macro actions as promising modulators of the gut-brain axis.

In addition to prolactin A, anthraquinones, like carotenoids, have antioxidative properties [88,214] and this property is owed to the quinoid structure that makes them capable to take part in reduction-oxidation reactions [215]. The antioxidant/pro-oxidant [216], antimicrobial/antiviral/antiparasitic [217], immunomodulatory [218], diuretic/laxative [109], vasorelaxant [219], lipid/glucose-lowering [220], and estrogenic activities [221] of anthraquinones extracted from marine sources are also reported [222]. These classes of marine drugs have shown potential neuroprotective responses by suppressing inflammatory/apoptotic/oxidative pathways [223]. Referring to the gut-brain axis, anthraquinones meaningfully promoted the dominant growth of Akkermansia muciniphila and suppressed the growth of Clostridium tyrobutyricum and Clostridium butyricum as well as butyrate-producing bacteria, thereby decreasing butyrate levels [224].

So, considering the neuroprotective role of macrolactins/anthraquinones as well as their modulatory effects on GM could make their acceptable future on the gut-brain axis towards neuroprotection.

5.4. Diterpenes/Sesquiterpenes: Lobocrasol, Excavatolide B, Crassumol E, and Zonarol

Soft corals are reservoirs of other marine-derived structurally named diterpenes as secondary metabolites with antioxidative properties. They participate in immune responses via regulation of the NF-κB signaling pathway at different levels [225,226]. A diterpene from cultured Taiwan gorgonian Briareum excavatum, called Excavatolide B, could suppress mRNA expression of COX-2 and iNOS in LPS-induced macrophage-activated mouse models through anti-inflammatory effects [227]. NF-κB activation is induced by TNF-α at sites of inflflammation in various diseases, and thereby strong inhibition of NF-κB activation was observed by Lobocrasols A and B and other cembranoid diterpenes (crassumol E and (1R,4R,2E,7E,11E)-cembra-2,7,11-trien4-ol) that were obtained from methanolic extract of the Vietnamese soft coral Lobophytum crassum [228]. Novel diterpenoid compounds from L. crissum also showed anti-inflammatory effects by inhibiting COX-2 and iNOS [229]. Subsequently, as a sesquiterpene, control is derived from Dictyopteris undulata marine source with potential antioxidant effects (increasing NQO-1, HO-1, and PRDX4) to modulate the gut-brain axis [139,230]. These effects could introduce diterpenes/sesquiterpenes as promising therapeutic agents in modulating gut-brain axis and neuronal dysfunction.

5.5. Phytosterols: Fucosterol and Solomonsterol A

Steroids with strong anti-inflammatory activity and immune response suppression could be found in marine sponges. Receptors of farnesoid X and pregnane X were modulated by steroidal molecules [231]. Agonists of pregnane X receptors demonstrated promising effects in lowering intestinal inflammation and NF-κB activity [232]. Fucosterol, as a phytosterol, with a diverse range of activities including, antioxidant, anti-inflammatory, anticholinesterase, neuroprotection is extracted from brown algae. Besides, it can inhibit acetyl- and butyryl-cholinesterase (AChE and BChE) and β-secretase (an enzyme involved in Aβ production, which is a key player in AD) and diminish Aβ-induced neuronal death [233]. Recent studies have proved the potential of fucosterol to be used in neurodegenerative diseases. Although its precise mechanism is unknown. Due to its cholesterol-like structure, targets like intracellular proteins are reachable for this compound, and they can cross the cell membrane. Literature evidence showed that the target proteins of fucosterol are involved in inflammatory pathways like TNF, hypoxia-inducible factor 1-alpha (HIF-1), NF-κB, and vascular endothelial growth factor (VEGF) signaling. NF-κB as a transcription factor takes part in inflammatory and immune responses and so has a major role in neurodegeneration diseases. In research, it was shown that fucosterol has a weakening effect on inflammation induced by LPS in RAW 264.7 macrophages [234]. Cholesterol homeostasis in the brain is regulated by LXRs which are nuclear receptors with cholesterol-sensing ability. After activation of these receptors, they inhibit the production of pro-inflammatory cytokines such as TNF-α and IL-1β. Besides LXRs affecting the expression of genes related to lipid metabolism, they also block inflammatory gene expression initiated by TLR activation [234].

PI3K/Akt pathway in parallel to MAPK signaling pathway modulates growth and survival of the cell. These pathways downstream compounds like CREB, Bcl-2, caspase-9, IKK, and NF-κB also participate in cell survival processes. It has been demonstrated that CREB which is a transcription regulator sensing the upstream signal from PI3K/Akt pathway was targeted by fucosterol.

CREB has a pro-survival activity through upregulation of Bcl-2. Cholinergic, dopaminergic, and serotonergic synapses are highly abundant with CREB, ensuing that fucosterol might have a major effect on neuronal growth, survival, and activity. Maturation of evolving neurons, growth, and survival of mature neurons are dependent on the neurotrophin signaling pathway. Therefore, any abnormality in this pathway will cause neurodegeneration. Evidence showed that neurotrophin mimetic agents could be used in the therapy of AD. TrkB as a receptor of BDNF is one of the targets of the fucosterol which confirms its neurotrophin mimetic activity [234].

Aβ1-42-induced cytotoxicity through activating TrkB-mediated ERK1/2 signaling in primary hippocampal neurons could be inhibited by fucosterol, based on a work done by Oh and colleagues in an in vitro assay. Translation of in vitro results was done by showing the effects of fucosterol on the attenuation of Aβ1-42-stimulated mental impairment in aging rats [15]. Molecular docking and binding energy calculation in an in silico analysis confirmed strong binding affifinity of fucosterol to TrkB which could be assumed as extra evidence of BDNF-mimetic activity of fucosterol.

Antioxidant activity of the fucosterol was proved by its effect of raising the levels of antioxidant enzymes such as glutathione peroxidase (GPX1), SOD, CAT, and HO-1 via Nrf2 activation and in silico data on TrkB binding [235]. Synaptic plasticity can be regulated by Ca2+ signaling at glutamatergic synapses. Synaptic proteins, such as GluN2A and AChE which have an affifinity to fucosterol are associated with Ca2+ signaling, thus influencing synaptic plasticity that affects memory and cognition. Based on the above information, fucosterol as a GluN2A NMDA receptor agonist could be used to improve AD patients [234].

The cholinergic deficit has a role in the pathology of AD. AChE inhibitor, by increasing availability of the acetylcholine, can compensate this deficiency, so fucosterol with AChE inhibitory property, (confirmed by molecular docking findings) can be used efficiently in the treatment of AD patients [236]. TLRs are key role-players of innate immunity by detecting pathogen-associated molecular patterns. Pathogenesis of chronic diseases including neurodegeneration is associated with the malfunction of TLR signaling. An in silico analysis with TLRs as protein target showed interaction of the fucosterol with both TLR2 and TLR4 which leads to the idea of using it in inflflammation-induced neurodegeneration.

Levels of antioxidant activity of SOD, GPx, and CAT were raised in rat models induced by fucosterol [237]. Jung and colleagues showed that fucosterol avoided ROS generation in tert-butyl hydroperoxide (t-BHP)-induced RAW264.7 macrophages [238]. Also, in another research, it was concluded that HepG2 cells were protected against oxidation via fucosterol [239], and the mechanism by which the lung epithelial cells were protected was the elevation of SOD, CAT, and HO-1, and nuclear translocation of Nrf2 [240]. The glycoprotein of U. pinnatifid elevated the activity of SOD (53.45%) and the activity of xanthine oxidase was reduced by 82.05%. Diphlorethohydroxycarmalol and 6,60 -bieckol from Ishige okamurae showed antioxidant activity and decreased ROS level in RAW264.7 cells [131].

During the consequent studies, it was observed that inflammation induced by LPS was attenuated via using fucosterol in RAW 264.7 macrophage [112] and alveolar macrophage [241]. LPS- or Aβ-mediated neuroinflammation in stimulated microglial cells also was attenuated by fucosterol. Ecklonia spp. produces phlorotannins such as director, phlorofucofuroeckol A [242], phlorofucofuroeckol B 6,60 -bieckol, and 8,80 -bieckol with anti-inflammatory properties via suppression of NF-κB and MAPK pathways [243].

As previously mentioned, some other classes of marine natural products also play critical roles in modulating the gut-brain axis. Of those, neoechinulin B, as an alkaloid extracted from Eurotium sp. SF-5989 marine fungi. is in this way through suppressing NF-κB and p38MAPK towards inhibiting neuroinflammation [244].

In general, carotenoids, polysaccharides, phytosterols, terpenoids, phenolic compounds, and alkaloids extracted from marine sources, possess several biological activities and health benefits. Figure 1 displays the chemical structure of the aforementioned candidate marine natural products capable of modulating the gut-brain axis towards neuroprotection.

They have the potential of modulating GM towards neuroprotection, thereby attenuating the gut-brain axis. Table 2 indicates marine-derived compounds, as well as associated sources towards neuroprotective responses.

The bidirectional relationship of GM and brain, as well as the critical role of marine natural products, are provided in Figure 2. As described, marine natural products are modifying the GM towards modulation of critical dysregulated inflammatory/apoptotic/oxidative stress pathways in neurodegenerative diseases, thereby revealing neuroprotection.

6. Conclusions

In addition to the role of GM in modulating dysbiosis, its critical role in preventing/treating neurodegenerative diseases(e.g., AD, ASD, ALS, MS, and PD), as well as associated complications is undeniable. Growing evidence implicates the gut-brain axis as a possible key target in the attenuation of neurological disorders.

Marine natural products are multi-target agents in a simultaneous modulation of intestinal and supra-intestinal disorders. Accordingly, carotenoids, polysaccharides, phytosterols, terpenoids, phenolic compounds, and alkaloids either by indirect modulation of GM or by direct suppression of neuroinflammation/apoptosis/oxidative stress, could be considered a potential/efficient strategy in combating neurodegenerative diseases. To do such, marine natural products potentially reduce the relative abundance of harmful GM, while increasing beneficial GM towards modulating the inflammatory mediators (e.g., NF-kB, TNF-α, ILs, COX-2, and TLRs), apoptosis(e.g., caspase, Bax/Bcl-2), and oxidative stress (e.g., ROS, Nrf2, HO-1, and AREs) in the gut. These compounds also regulate associated critical pathways in the gut, including PI3K/Akt/mTOR, MAMPs, BDNE, and ERK/CREB/MAPK. Considering the bidirectional relationship between gut and brain, modulation of these mediators/signaling mediators in the gut would result in their regulation in the brain towards neuroprotection.

A further area of research should focus on pre-clinical studies to reveal the precise molecular communication of the gut-brain axis, followed by well-controlled clinical trials. Such studies will help to investigate the more potential marine-derived natural products in the prevention, management, and treatment of the gut-brain axis towards neuroprotection.