Extensive research on marine algae, especially on their health-promoting properties, has been conducted. Various ingredients with potential biomedical applications have been discovered and extracted from marine algae. Alginate oligosaccharides are low molecular weight alginate polysaccharides present in cell walls of brown algae.
Carbohydrate Polymers 271 (2021) 118408 Contents lists available at ScienceDirect Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol Potential applications of alginate oligosaccharides for biomedicine – A mini review Mingpeng Wang a, Lei Chen a, *, Zhaojie Zhang b a b College of Life Science, Qufu Normal University, Qufu 273100, China Department of Zoology and Physiology, University of Wyoming, Laramie, Wyoming, USA A R T I C L E I N F O A B S T R A C T Keywords: Alginate oligosaccharides Brown algae Health-promoting Biomedicine Extensive research on marine algae, especially on their health-promoting properties, has been conducted Various ingredients with potential biomedical applications have been discovered and extracted from marine algae Alginate oligosaccharides are low molecular weight alginate polysaccharides present in cell walls of brown algae They exhibit various health benefits such as anti-inflammatory, anti-microbial, anti-oxidant, anti-tumor and immunomodulation Their low-toxicity, non-immunogenicity, and biodegradability make them an excellent material in biomedicine Alginate oligosaccharides can be chemically or biochemically modified to enhance their biological activity and potential in pharmaceutical applications This paper provides a brief overview on alginate oligosaccharides characteristics, modification patterns and highlights their vital health promoting properties Introduction Brown algae are a large group of multicellular algae and one of the essential and integral components of the marine ecosystem (Lavaud & Goss, 2014) They are biologically diverse, with thousands of different species, forming the dominant vegetation in the intertidal and subtidal zone of rocky shores (Bartsch et al., 2008) They live from coastalestuarine to deep-sea regions and have many unique features, such as fast growth, distinctive structure, strong adaptiveness, and wide distri bution Their ecological significance is based in part on their contribu tions to marine biomass and marine carbon cycling (Lavaud & Goss, 2014) Brown algae provide food and habitats for many other organisms and promote the prosperity of the entire marine biosphere (de Mesquita et al., 2018; Santelices, 2007) Brown algae are also economically important for human beings as a kind of food, especially in Asian countries In addition, they are used as natural feed or fertilizer due to their high mineral and trace elements, and as a source of biological products such as alginates, mannitol and iodine (Afonso et al., 2019; Arioli et al., 2015; Overland et al., 2019; Salehi et al., 2019) Brown algae have tremendous potential as a source of novel functional com ponents that are not present in terrestrial plants More recently, the valuable ingredients extracted from brown algae, e.g alginates, fucoi dan and laminaran, have been explored for nutrient and drug develop ment (Ford et al., 2020; Garcia-Vaquero et al., 2018; Generali´c Mekini´c et al., 2019; Gunathilaka et al., 2020; Saraswati et al., 2019; Thanh et al., 2013; Zou et al., 2019) Alginate is a linear acidic polysaccharide distributed widely in cell walls of brown algae (Sari-Chmayssem et al., 2015; Synytsya et al., 2015) Alginate consists of hexuronic acid residues β-D-mannuronic acid (M) and α-L-guluronic acid (G) with exclusively → glycosidic link ages Its chelation, gelation, and hydrophilic properties have led to its wide application in food, cosmetic and biomedical industries (Donati & Paoletti, 2009; Lee & Mooney, 2012) Increasing evidence indicates that, when used as a therapeutic adjuvant, drug carrier, wound healing ma terial and biological scaffold, alginate could improve antitumor immune efficacy in ovarian cancer, melanoma, liver cancer, and breast cancer (Fan et al., 2019) However, the direct therapeutic effects of alginate in biomedical applications have been greatly limited due to its macromo lecular structure, poor solubility and low bioavailability (Zhu et al., 2020) Alginate can be digested chemically or enzymatically, producing alginate oligosaccharides (AOS), which have lower molecular weights and lower viscosity AOS have better solubility and bioavailability (Trincone, 2015) As a consequence, particular interest has been focused on AOS due to their better pharmacological activities and beneficial effects in biomedicine In this paper, we present an overview of the structure, biological activities and modification patterns of AOS We also discuss the recent developments of using AOS in treating chronic and degenerative * Corresponding author at: Qufu Normal University, 57 Jingxuan West Road, Qufu, China E-mail address: leichen_2018@qfnu.edu.cn (L Chen) https://doi.org/10.1016/j.carbpol.2021.118408 Received April 2021; Received in revised form 23 June 2021; Accepted July 2021 Available online July 2021 0144-8617/© 2021 The Author(s) Published by Elsevier Ltd This (http://creativecommons.org/licenses/by-nc-nd/4.0/) is an open access article under the CC BY-NC-ND license M Wang et al Carbohydrate Polymers 271 (2021) 118408 Fig The structure of alginate and AOS: monomers; chain conformation and fragments of AOS product ≤ n ≤ 25 diseases, modulating human gut microbiota and promoting curative effect of traditional drugs as therapeutic adjuvant or drug carrier formation of poly G (Mackie et al., 1983; Plazinski, 2011) Generally, the poly G involving axial linkage is more rigid than the equatorially linked poly M Structure and modification patterns of alginate oligosaccharides 2.2 Production of AOS from alignate 2.1 Structure of AOS At present, the three main methods for AOS production are acid hydrolysis (AH), oxidative degradation (OD) and enzymatic digestion (ED) Each method has its own advantages and limitations The main feature of acid hydrolysis of alginate is that it results in random cleavage along the polysaccharide chains and produces AOS fragments with unmodified hexuronic acid residues at both termini (Fig 2) Therefore, the AOS produced by acid hydrolysis can maintain the inherent structure of alginate Acid hydrolysis has been widely used for producing a series of AOS with different degrees of polymerization because of its low cost, ease of control, simplicity and availability However, only under high temperature and pressure conditions can AOS with molecular weight below 4000 be obtained through acid hydrolysis In addition, a large number of inorganic salts are generated during the final neutralization stage of acid hydrolysis The application of acid hydrolysis has been limited due to equipment corrosion, high energy consumption and residue waste pollution Another chemical degradation method is oxidative degradation, which has higher reaction efficiency and high yield compared to acid hydrolysis As an easily degradable reagent that creates only water as an oxidation by-product, hydrogen peroxide (H2O2) has been widely used to produce functional AOS with high purity and quality As shown in Fig 2, AOS residues are easily ring-opened at the reducing end to form carboxyl group during the oxidative degradation This additional carboxyl radical could induce novel bioactivities of oxidative AOS Zhou et al reported that guluronate oligosaccharide (GOS) prepared by oxidative degradation (GOS-OD), but not GOS produced by acid hy drolysis or enzymatic digestion, significantly reduced the lipopolysac charide (LPS)-stimulated overproduction of nitric oxide (NO) in RAW As a degradation product of alginate, AOS are a mixture of linear oligomers, which consist of β-D-mannuronic acid (M) and α-L-guluronic acid (G) (Fig 1) at different ratios and degrees of polymerization (DP) (Rioux & Turgeon, 2015) The overall composition of the two uronic acids and their distribution along the oligomer chain vary widely depending on the species of algae and influence the properties of AOS As shown in Fig 1, three types of oligomer blocks (2 ≤ DP ≤ 25) are typically obtained: M, G, and mixed MG blocks, according to different source and degradation methods Structurally, the monomeric units M and G are a pair of C-5 epimers due to their different orientation of carboxyl group at the C-5 position (Grasdalen, 1983) The molecular formula of M or G units is C6H10O7 and the conversion relationships of AOS relative molecular weight (Mr) and the degree of polymerization can be described as shown in Eq (1): Mr = DP⋅Mr (C6 H10 O7 ) − (DP − 1)⋅Mr (H2 O) (1) When the atomic weights of carbon, hydrogen and oxygen, and the molecular weights of H2O are plugged into the formula, the DP can be calculated using Eq (2): DP = (Mr − 18)/176 (2) Finally, the epimerization at C-5 results in significant differences in spatial structure and physicochemical properties of their oligomers, respectively The equatorial configuration of the β-1,4-glycosidic bond predestines a stretched chain conformation of poly M while the axial linkage of the α-1,4-glycosidic bond implicates the tendency to helix M Wang et al Carbohydrate Polymers 271 (2021) 118408 Fig The different structures of AOS fragments produced by three degradation methods and hypothetical structure of modified AOS 264.7 cells (Zhou et al., 2015) The existence of the additional carboxyl group may play an important role in the NO-inhibitory and subsequent anti-inflammation effect of GOS-OD The enzymatic digestion may be the most promising method for producing AOS due to its key advantages such as site-specific cleavage reaction, mild reaction conditions, efficient reaction rate, and high re action yields Alginate lyases are key enzymes that specially catalyze the degradation of alginate They break the O–C4 bond to uronic acid residues through a β-elimination reaction that leads to formation of the 4,5-unsaturated hexuronic acid residue at the non-reducing terminus (Fig 2) This unsaturated terminal structure with a C4–C5 double bond can be detected at 230 nm The unsaturated AOS have various novel bioactivities such as antioxidant, anti-tumor, neuroprotective and immuno-stimulation (Iwamoto et al., 2003; Iwamoto et al., 2005; Tusi et al., 2011) Compared with saturated AOS, the unsaturated AOS showed efficient anti-obesity effects in high-fat diet (HFD)-fed mice through activating the AMPK signaling pathway (Li et al., 2019) Although the enzymatic digestion of alginate has various advantages and bright application prospects, most studies are still at laboratory level The discovery and development of novel enzymes with high yield, activity and stability is still in high demand for the achievement of in dustrial production of AOS 2.3 Modification of AOS Recently, various approaches, i.e vanadylation, sulfation, selenyla tion or oxidation, have been used to modify the backbone of AOS and improve their physicochemical and biological properties For example, vanadyl AOS (VAOS) presents a higher antioxidant activity than un modified AOS in hydroxyl and DPPH radical scavenging systems (Liu et al., 2015) In addition, VAOS exhibited strong anti-proliferation ac tivities against human hepatoma cell line BEL-7402 VAOS could markedly inhibit tumor progression in non-small cell lung cancer (NSCLC) (Zhou et al., 2018) It was synthesized through slowly adding vanadium (IV) oxide sulfate hydrate into the AOS solution under con dition of constant stirring and pH 12 during whole process The vana dium content of VAOS could reach to about 3.0% According to FT-IR spectral analysis, introduction of vanadyl groups changed the absorp tion peak of different functional groups of the oligosaccharide chain – O and C–O A similar infrared shift has also been such as C–O–C, C– M Wang et al Carbohydrate Polymers 271 (2021) 118408 found in FT-IR spectra of vanadyl (IV)/chondroitin sulfate A (CSA) complex (Etcheverry et al., 1994) Although the exact structure of VAOS remains to be determined, it is believed that VAOS is a new type of coordination compound rather than a covalent compound based on the available data As previously reported, the FT-IR spectra in this study indicate the hypothetical structure of VAOS in the form of coordination through the hydroxyl oxygen of carboxylate group and the glycosidic oxygen of G or M moieties (Etcheverry et al., 1994; Etcheverry et al., – C–O occupy 1997) As shown in Fig 2, both the C–O–C and the O– two coordination positions This speculated coordination structure may play a key role in improving the bioavailability of vanadium and bioactivity of AOS The definite structure-function relationship of VAOS requires further investigation A recent study of selenium-containing alginate oligosaccharides (Bi et al., 2020) showed that the modification of AOS was driven by two reaction steps: Poly M and SO3-Py were each suspended in dimethyl methanamide, and then the two solutions were mixed and reacted to obtain the sulfonated Poly M (S-PM) in the sulfonation step; while in the selenylation step, the sulfur in S-PM was partially replaced by Se through reacting with Na2SeO3 in the presence of excess barium chloride (BaCl2) in 5% HNO3 at 60 ◦ C for h According to the FT-IR spectra of – O and C–O–S PM, S-PM, and Se-PM, the specific absorption of S– detected only in S-PM and Se-PM, confirming that the sulfation substi tution was successfully prepared Furthermore, the spectrum of Se-PM is similar to that of S-PM, which suggests that their carbon-skeleton structure was the same In addition, the measured S content in Se-PM decreased to about 60% of that in S-PM, implying that some S bound is replaced by Se Therefore, they speculated that Se might be in the form of -SeO3 covalently bound to sites that were originally occupied by S (Fig 2) As a new covalent compound, Se-PM combines the advantages of Se and AOS and exhibits various bioactivities including antioxidation, anti-inflammation and neuroprotection that are superior to those of Se itself or Se-free oligosaccharides (Bi et al., 2019; Bi, Lai, Cai, et al., 2018; Bi, Lai, Han, et al., 2018) The resultant low molecular weight Se-PM exhibited enhanced neuroimmunoregulatory activity in LPS-induced BV2 microglia probably due to the covalent structure formed during the replacement process that could attenuate the nitric oxide (NO) and prostaglandin E2 (PGE2) secretion, as well as the inducible NO synthase-20 (iNOS) and cyclooxygenase-2 (COX-2) expression, depending on the treatment dose A different kind of sulfated mannuronate oligosaccharides (S-MOS) were developed by reacting mannuronate oligomers with ClSO3H in formamide (Liu et al., 2005) 13C-nuclear magnetic resonance (NMR) analysis suggested that sulfate modification of mannuronate occurs at the hydroxyl groups of C-2 and partial C-3 with different degrees of substitution by the same degradation patterns, still exhibited distinct biological ac tivities Interestingly, AOS with anti-obesity effects tend to have a lower average DP, usually no more than in many reports (Guo et al., 2016, 2017; Li et al., 2019; Nakazono et al., 2016; Wang et al., 2020) 2.4.2 G/M ratio Xu et al reported that the unsaturated guluronate oligosaccharides (GOS) prepared by enzymatic digestion exhibited macrophagesactivating effect in mouse immune response while GOS prepared by other methods or mannuronate oligosaccharides (MOS) showed very low or no such effects (Xu, Wu, et al., 2014) A series of studies have shown the potential of the guluronate rich alginate OligoG CF-5/20 (containing >85% G residues) as an effective treatment in chronic res piratory disease (Nordgård & Draget, 2011; Pritchard et al., 2016; Pritchard et al., 2019; Sletmoen et al., 2012) The OligoG could directly interact with mucin and reduce its linearization and flexibility, leading to effective detachment of cystic fibrosis (CF) mucus (Ermund et al., 2017; Pritchard et al., 2016) MOS on the other hand, plays important roles in the treatment of human melanoma and Alzheimer's disease (AD) MOS produced by Mspecific alginate lyase strongly inhibits anchorage-independent colony formation of human melanoma cells compared to polymannuronate and GOS (Belik et al., 2020) It is suggested that MOS might be a potential drug candidate for synergistic tumor therapy More recently, two groups demonstrated that MOS (DP2-DP11) could significantly inhibit the ag gregation of amyloid-β (Aβ) oligomer and Aβ fibril formation, although their respective MOS samples were prepared by different methods (Bi et al., 2021; Wang et al., 2019) Their results suggested the mannuronate component and proper DP may play pivotal roles in alleviating AD In addition, G/M ratio may determine spatial conformation of AOS and affect their gelation, mechanical properties and biological activity As mentioned in 2.1, G block has a tendency to form a helical confor mation while M block has a relatively straight chain-like conformation due to their different linkage at C-1 and C-4 (Mackie et al., 1983; Pla zinski, 2011) Alginate with higher M residues was characterized by higher tensile strength and percent of elongation than alginate with dominated G bocks (Costa et al., 2018) It is well-known that alginates containing G-blocks can form strong hydrogels in the presence of diva lent cations such as calcium in the so-called egg-box model (Grant et al., 1973) However, several reports also indicate that such cooperative ionotropic gelation only occurs when the length of the G blocks involved in the dimerization exceeds a certain length (Skjåk-Bræk et al., 1986; Smidsrød & Haug, 1972) For example, and ± contiguous G resi dues are required to form stable junction zones for Sr2+- and Ca2+induced gelation respectively (Stokke et al., 1991; Stokke et al., 1993) Thus, AOS with low G/M ratio and DP might not form stable crosslinking conformation even in the presence of divalent cations and maintain high water solubility which is conducive to their delivery in vivo while AOS with higher DP and GM ratio may maintain gel-forming property similar to alginate, which makes them have potential application as drug car riers G blocks normally form stiffer, brittle, and mechanically more stable gels On the contrary, M blocks form a softer and more elastic gel (Ashikin et al., 2010) As a potential drug carrier candidate, MOS were added in an alginate-based drug delivery system, which improved me chanical properties and antifungal activity of the whole delivery system (Szekalska et al., 2019) The flexible conformation of MOS may facilitate penetration into bacterial cells and makes them show better antimi crobial effect than GOS, which usually forms stiff chains (Hu et al., 2005) 2.4 Structure-function relatioship of AOS The size, composition and structure of AOS vary due to algal species (Guo et al., 2016; Li, Jiang, Guan, & Wang, 2011), degradation patterns (Li et al., 2019; Wang et al., 2019) and modification methods (Bi et al., 2020; Liu et al., 2015) The structural features of AOS, including degrees of polymerization, the G/M ratio, residue structure and spatial confor mation, are responsible for biological functions of AOS The following are some examples that reflect structure-function relationship of AOS 2.4.1 DP AOS DP5 (pentamer composed of randomly arranged G and M) ob tained by enzymatic digestion rather than AOS DP2, DP3 or DP4, exhibited significant inhibitory functions on the growth of osteosarcoma cells in vitro (Chen et al., 2017) Unsaturated guluronate oligomers (DP3–DP6) significantly enhanced the bacterial phagocytosis of mac rophages, especially GOS DP5 (the guluronate pentamer) showed the maximum enhancement among all oligomers measured (Xu, Bi, et al., 2014) These studies suggested that AOS with different DP, even derived 2.4.3 Special terminal structure AOS derived by enzymatic digestion usually have unsaturated structure at non-reducing end while AOS obtained by oxidative degra dation are always ring-opened at reducing end to form carboxyl groups (Fig 2) The unsaturated and oxidative terminal structure are key fac tors determining biological function of AOS M Wang et al Carbohydrate Polymers 271 (2021) 118408 (Wang et al., 2019) These studies suggest that the molecular mecha nisms of AOS are greatly affected by different terminal structures It also suggests that there is a direct relationship between structural characteristics of AOS and their biological activities The molecular size, G/M ratio and terminal structure play important roles in determining the biological functions and action modes of AOS In order to obtain AOS products with stable structure and unique function, several important aspects need to be considered: 1) The source of alginate; The G/M ratio, content and order of alginate are always different due to algal species and their respective environments Algi nate derived from Laminaria japonica with an M/G ratio of 1.86 (Guo et al., 2016) was degraded by alginate lyase from Pseudomonas sp HZJ 216 for h at 30 ◦ C, producing AOS with DP to However, when the alginate source changed to another brown seaweed, Laminaria sp with M/G ratio of 2.28, AOS products were mainly oligomers of DP and under exactly the same degradation conditions (Li, Jiang, Guan, & Wang, 2011) Therefore, the uniform alginate source is a prerequisite for obtaining stable AOS 2) Production technology; Production techniques greatly affect the structural properties and biological functions of AOS products For example, the type of alginate lyase is the most essential factor affecting the final product in enzymatic technique An alginate lyase that could specially produce AOS with DP to was reported (Huang et al., 2013) Recently, two mannuronate-specific alginate lyase were found in a marine bacterium Formosa algae and the human gut microbe Bacteroides cellulosilyticus (Belik et al., 2020; Stender et al., 2019) These alginate lyases can be applied to produce special AOS products The development and standardization of efficient and stable production technology is the key to ensure the quality, yield and variety of AOS products 3) Purification and characterization methods; In most cases, AOS are mixtures of oligomers with varied DP, different G/M ratios and sequences Thus, it is very important to develop purification and analytical methods for qualification and quantification of AOS Nowadays, FT-IR and NMR spectroscopy are widely used to obtain Table Anti-tumor activities of AOS Mechanism of anti-tumor Cancer style and Cell line Reference Inhibition of cell growth and colony formation Human bone cancer cell MG-63 Chen et al (2017) Belik et al (2020) Han et al (2019) Yang et al (2017) Arlov et al (2015) Zhou et al (2018) Arlov et al (2015) Human melanoma cells SK-MEL-5, SK-MEL-28, and RPMI-7951 Human prostate cancer cells DU145 and PC-3 Human aneurysm Human myeloma RPMI-8226 cells Induction of cell apoptosis Reduction of angiogenesis induced by HGF Human non-small cell lung cancer cells A549 and LTEP-a-2 Human myeloma RPMI-8226 cells The unsaturated mannuronate oligosaccharides (UMOS) predomi nantly contains mannuronic acids (M/G ratio = 2.12) and exhibits su perior anti-obesity effect via suppressing the accumulation of triglycerides and improving the intestinal microflora (Kim, 2018) The unsaturated alginate oligosaccharides (UAOS) also exhibited significant anti-obesity effect (Li et al., 2019) Moreover, this anti-obesity effect was only related to the unsaturated structure but independent of the G or M composition MOS derived from enzymatic digestion and oxidative degradation exhibited a similar inhibition to Aβ oligomer aggregation However, the underlying molecular mechanisms of these two kind MOS in the treat ment of AD were different, although their DP and G/M ratio were very similar The unsaturated MOS enhanced autophagy to promote clear ance of amyloid precursor protein (APP) and Aβ in AD cell models (Bi et al., 2021) while the oxidative MOS reconstituted gut microbiota and improved anti-neuroinflammation responses to inhibit AD progression Fig The proposed mechanism of AOS for suppressing the tumorigenicity of prostate cancer cells Adapted according to Han et al (2019) M Wang et al Carbohydrate Polymers 271 (2021) 118408 precise and key information for AOS structure (Bi et al., 2020; Liu et al., 2005; Liu et al., 2015; Lundqvist et al., 2012) High performance liquid chromatography (HPLC), mass spectrometry (MS) and related tech niques are used to purify and characterize AOS (Fu et al., 2013; Jona than et al., 2013; Zhang et al., 2006) and chest pain, pulse-less legs, persistent cough and wound infection were also reduced The clinical trial demonstrated that AOS with DP to effectively reduced aneurysm recurrence after EVAR As a bioactive small molecule, AOS could cause changes of related signal pathway such as the expression of miR-29b (a small noncoding RNA) and toll-like re ceptor signaling According to their results, the expression levels of miR29b in aortic aneurysm patients were significantly reduced after AOS treatment Consequently, it affected the toll-like receptor (TLR) signaling pathway involving variety of downstream related factors such as mitogen-activated protein kinase (MAPK), nuclear factor kappa B (NF-κB), interleukin (IL-1) β, and interleukin (IL-6) They concluded that AOS can prevent the regeneration of aneurysms by reducing the level of TLR4, NF-κB, IL-1β, and IL-6 via the inhibition of miR-29b Myeloma, also known as plasma cell tumor, is a malignant tumor originating from the plasma cells in the bone marrow The major char acteristic of myeloma cells is the high expression of the cell surface heparin sulfate proteoglycan syndecan-1 (Sdc-1) (Gambella et al., 2015) Human growth factor (HGF) interacts with Sdc-1 and increases downstream HGF signaling, promoting angiogenesis, cell migration and tumor growth (Aref et al., 2003) Blocking the interaction between HGF and Sdc-1 can effectively inhibit the proliferation of tumor cells Arlov et al found that sulfated AOS could directly bind to HGF, preventing HGF from interacting with Sdc-1 (Arlov et al., 2015) The sulfated AOSbound HGF was released from the surface of myeloma cells when appropriate sizes of sulfated AOS were used In contrast, no HGF release was observed upon treatment with non-sulfated AOS A recent study showed that VAOS, a novel modified coordination compound, have an effective inhibitory effect on NSCLC in both cultured cells and mouse models with transplanted tumor cells (Zhou et al., 2018) It was further confirmed that VAOS (12.5, 25, 50 μM) could induce apoptosis of NSCLC cells by activating protein kinase B (AKT) to increase intracellular reactive oxygen species (ROS) levels Phosphorus colorimetric analysis showed that VAOS significantly inhibited the dephosphorylation activity of phosphatase and tensin homolog on chromosome 10 (PTEN), another member of the protein tyrosine phos phatases (PTPases)-upstream factor of AKT In addition, ectopic PTEN overexpression decreased VAOS-induced apoptosis In vivo, VAOS treatment (intraperitoneal injection, 30 mg/kg for weeks) triggered the hyperactivation of AKT through significant reduction of phosphatase activity of PTEN, leading to ROS accumulation and apoptosis of NSCLC cells Potential applications of AOS for biomedicine 3.1 Cancer therapies Cancer is a major public health problem worldwide In the last de cades, numerous active substances with anti-cancer effects have been reported As a natural product from marine algae, AOS and their de rivatives have various anti-cancer effect (Table 1) Han and colleagues reported on AOS's capacity to inhibit prostate cancer cell growth via the Hippo (Ste20-like protein kinase, mutations in this gene lead to tissue overgrowth, or a “hippopotamus”-like pheno type) /YAP (Yes-associated protein) pathway (Han et al., 2019) AOS trigger the activation or suppression of regulatory processes in prostate cancer cells involving various transcription factors and effectors (Fig 3) First, AOS could activate the Hippo signaling pathway, which negatively regulates YAP activity through a cascade of phosphorylation Reduced expression of the oncogene YAP and an increase in phosphorylated YAP protein were observed in prostate cancer cells in response to AOS treatment The recruitment of both the coactivator YAP and c-Jun (a component of the transcription factor activator protein-1, belongs to the Jun family containing c-Jun, JunB, and JunD) into the upstream response region of the sialyltransferase ST6Gal-1 promoter was largely blocked due to the absence of YAP As a result, a decrease of ST6Gal-1, which plays a fundamental role in growth, migration, and invasion of prostate cancer cells was detected at different levels of transcription, translation and sialylation both in cultured cells and in a xenograft mouse model Finally, proliferation of human prostate cancer cells was attenuated at a non-cytotoxic concentration of AOS (0.5 mg/ml for cell lines; 2.5 mg/kg for mouse models) via repression of the Hippo/YAP/cJun pathway and sialylation caused by downregulation of ST6Gal-1 gene expression (Han et al., 2019) In a clinical trial, AOS could significantly suppress aneurysm recur rence caused by endovascular aortic repair (EVAR) (Yang et al., 2017) Compared with the control, the size of residual aneurysms was signifi cantly reduced after 2-year treatment with AOS (oral administration, 10 mg/day) The incidence of EVAR-related adverse effects, such as back Fig The proposed mechanism of Se-PM for anti-inflammation effect Adapted according to Bi, Lai, Cai, et al., 2018 and Bi, Lai, Han, et al., 2018) M Wang et al Carbohydrate Polymers 271 (2021) 118408 Fig The proposed mechanism of AOS for anti-oxidation effect Adapted according to Jiang et al (2021) and Zhao, Han, et al (2020) 3.2 Anti-inflammation Se-PM, a modified AOS derivate, significantly attenuated the in flammatory response in LPS-activated murine macrophage RAW264.7 cells, primary microglia and astrocytes by suppressing the activation of NF-κB and MAPK signaling pathways (Bi, Lai, Cai, et al., 2018; Bi, Lai, Han, et al., 2018) The cytotoxicity of Se-PM was measured by CCK-8 assay It showed that Se-PM did not exert any cytotoxic effects on pri mary glial cells when the concentration of Se-PM reached 0.8 mg/ml As shown in Fig 4, Se-PM treatment significantly decreased phosphoryla tion of NF-κB inhibitor (IκB-α), Akt p38 kinases (p38), extracellular signal-regulated kinases (ERK) and c-Jun N-terminal kinases (JNK) It lead to reduced phosphorylation and nuclear translocation of tran scription factors, especially p65 Therefore, the expressions of down stream target genes iNOS and COX-2 were dramatically reduced As a result, the production of related pro-inflammatory mediators, including ROS, NO, PGE2 and the secretion of pro-inflammatory cytokines, including TNF-α, IL-1β and IL-6 also decreased in cells incubated with Se-PM Finally, Se-PM inhibited the inflammatory response in different types of LPS-triggered cells, respectively Moreover, Se-PM remarkably suppressed carrageenan-induced pro-inflammatory cytokine production and LPS-triggered microglial and astrocytic activation in different mouse models Se-PM might block the interaction between LPS and re ceptors on the surface of LPS-induced in RAW264.7 cells These studies might contribute to the comprehensive understanding of the potential health benefits of Se-PM for alleviating the prolonged and excessive inflammation induced by human degenerative diseases, including can cer, cardiovascular diseases, type diabetes, and arthritis Frequent uses of anticancer drugs cause a series of side effects, especially gastrointestinal inflammation A variety of agents, such as probiotics, selenium, volatile oils and prebiotics have been used to reduce pharmacotherapy-induced intestinal disruption (Araújo et al., 2015; Justino et al., 2014; Lee et al., 2017; Ting et al., 2017; Yao et al., 2013 However, little success has been reported in these efforts Novel bioactive substances are urgently needed to assist the recovery of inflammation after cancer therapy It has been shown that AOS could effectively alleviate the inflam mation caused by busulfan, a drug used for patients with chronic myeloid leukemia (Zhao, Feng et al., 2020) AOS also enhances integrity and migration ability of IPEC-J2 cells (porcine small intestinal cell line) (Xiong et al., 2020) The underlying molecular mechanisms are multi faceted and wide-ranging, including the regulation of AOS on cellular transcriptome, and the subsequent responses of different types of in testinal cells SiRNA trials in IPEC-J2 cells confirmed that AOS per formed their function by interacting with mannose receptors on the cell surface Consequently, the extensive remodeling of transcriptomes in different type small intestine cells was observed The researchers iden tified 184 active and differentially expressed genes, including multiple transcription factors and associated cofactors Finally, AOS (oral gavage, 10 mg/kg for weeks) rescued the developmental time line of different types of small intestinal cells (enterocytes, goblet, Paneth, tuft cells) disrupted by busulfan, and their respective functions involving micro villi organization, cell junction, antibacterial humoral response, and others In addition, the improved plasma metabolome further verified that AOS could recover small intestinal function in patients undergoing anticancer chemotherapy Moreover, fecal microbiota transplantation (FMT) from AOS-treated mice was an effective measure for alleviating small intestine mucositis through reconditioning gut microbiota and improving blood metabolome on a multi-omics scale (Zhang et al., 2020) In another study, it showed that AOS alleviated TNF-α-induced inflammatory injury by decreasing pro-inflammatory cytokine (IL-6 and TNF-α) concentrations and TNF receptor (TNFR1)-mediated apoptosis rate in TNF-α-treated IPEC-J2 cells (Wan et al., 2020) 3.3 Anti-microbial activities Microbial infections represent one of the major causes of human diseases The biofilm-forming abilities and the resulting increased drug resistance of pathogens make them a formidable challenge and remain difficult to overcome (Ceri et al., 1999; Moskowitz et al., 2004) AOS have exhibited great potential in the treatment of microbial infections caused by the most common opportunistic pathogens including Pseu domonas aeruginosa, Acinetobacter baumannii, and Candida species (Pritchard, Jack, Powell, et al., 2017; Pritchard, Powell, Jack, et al., M Wang et al Carbohydrate Polymers 271 (2021) 118408 Fig The proposed mechanism of AOS for anti-obesity effect Adapted according to Li et al (2019) and Wang et al (2020) 2017; Stokniene et al., 2020; Tøndervik et al., 2014) Powell et al reported that the OligoG CF-5/20 could significantly reduce the biomass, thickness and density of biofilm of Pseudomonas aeruginosa during early biofilm formation or even after biofilm estab lishment (Powell et al., 2018) They revealed that the OligoG CF-5/20 not only inhibited bacterial biofilm growth but also destroyed the well-defined structure of established biofilm They further demonstrated that OligoG CF-5/20 could rapidly diffuse into the whole biofilm and make the biofilm collapse from the inside, due to disrupting the struc ture of extracellular polysaccharides matrix and DNA-Ca2+-DNA bridges, both of which are essential for biofilm formation and matura tion (Bales et al., 2013; Flemming & Wingender, 2010) In addition, the synergistic use of OligoG CF-5/20 with antibiotics enhanced the efficacy of antibiotics Most recently, Stokniene et al has reported the artificial bi-functional compounds of OligoG CF-5/20–polymyxins exhibited prolonged antimicrobial and antibiofilm activities against multidrugresistant Gram-negative bacterial pathogens compared to parent anti biotic whilst reducing antibiotic toxicity to human (Stokniene et al., 2020) OligoG CF-5/20 also showed an effective inhibitory effect on in fections caused by Candida albicans through reducing the candidal growth, mycelium formation and invasion (Pritchard, Powell, Khan, et al., 2017) Moreover, the gene expression and protein production of key phospholipase as the major virulence factor was significantly decreased in OligoG pre-treated C albicans ATCC 90028 cells of the development of various degenerative diseases, especially atherosclerosis Studies have shown that AOS treatment can significantly enhance the activity of antioxidant enzymes and the accumulation of free radical scavengers, including superoxide dismutase (SOD), catalase (CAT) and glutathione (GSH) in human umbilical vein endothelial cells (HUVECs) (Jiang et al., 2021; Zhao, Han, et al., 2020) AOS treatment also reduced H2O2-induced ROS accumulation, the production of malondialdehyde (MDA), one of the final products of lipid peroxidation, and the secretion of endothelin-1 (ET-1), a signaling molecule to stimulate ROS and su peroxide generation In addition, AOS protected HUVECs against oxidative stress-induced apoptosis by regulating expression of genes involving in the caspase-mediated apoptosis pathway and integrinα/FAK/PI3K pathway (Fig 5) These studies suggested that AOS can protect endothelial cells, due to their effective antioxidant and anti apoptotic activities, providing a promising therapeutic strategy for preventing and treating atherosclerosis 3.5 Anti-obesity Obesity is a metabolic disorder characterized by excessive accumu lation of body fat, leading to a variety of diseases, such as hypertension, hyperlipidemia, diabetes and even cancer It has become a major problem threatening human health In addition to general recommen dations with regard to proper diet and exercise, several different ap proaches to treat obesity have been proposed, such as the use of functional food supplements that have anti-obesity effects As a naturally-derived food additive with various beneficial effects, AOS might be a candidate for the treatment of obesity (Guo et al., 2016, 2017; Li et al., 2019; Nakazono et al., 2016) Li et al investigated the anti-obesity effects of the unsaturated AOS (UAOS) from the enzymatic degradation of Laminaria japonicais, in a high-fat diet (HFD) mouse model (Li et al., 2019) They found that UAOS showed stronger anti-obesity effects than acid hydrolyzed saturated AOS (SAOS), judged by the greater reduction in body and liver weights, 3.4 Anti-oxidation Oxidative stress reflects an imbalance between the systemic mani festation of ROS and the innate antioxidant defense systems Distur bances in the normal redox state of cells can cause toxic effects through the production of peroxides and free radicals that damage proteins, lipids, and DNA Oxidative stress leads to cell dysfunction and apoptosis via driving disruptions in normal mechanisms of cellular signaling In humans, oxidative stress is thought to be associated with the occurrence M Wang et al Carbohydrate Polymers 271 (2021) 118408 Fig The proposed mechanism of AOS for AD therapies effect Adapted according to Wang et al., 2019) adipose tissue mass, serum and liver lipid contents Similar studies have also shown that the UAOS provides greater biological activity (Guo et al., 2016, 2017; Nakazono et al., 2016) UAOS causes significant in crease on both AMPKα and acetyl-CoA carboxylase (ACC) phosphory lation in adipocytes, suggesting that UAOS acts as an anti-obesity agent through AMPK signaling In another study, it is observed that AOS treatment induces reduction in size of adipocytes and lipid metabolism improvement, such as decrease of TG and low-density lipoprotein cholesterol (LDL-C) levels and inhibition of lipogenesis genes expression (Wang et al., 2020) AOS alleviated HFD-induced metabolic disorders and inflammation via modulating gut microbial communities, and the consequent release of microbiota-dependent short-chain fatty acids (SCFAs) and reduction of endotoxin levels AOS treatment recovered the HFD-disturbed gut microbiota by increasing the abundance of specific and beneficial gut microbiota, including Akkermansia muciniphila, Lactobacillus reuteri and Lactobacillus gasseri (Fig 6) These results showed that the inhibitory effect of AOS on obesity was closely related to their regulation of in testinal microorganism Recently, several studies have shown that AOS plays an important role with different action modes in the pathogenesis of AD The effects of two different forms of AOS, MOS (derived from enzymatic digestion) and Se-PM (modified with Se) on the treatment of AD were evaluated by Bi et al (2020) and Bi et al (2021) Both MOS and Se-PM significantly inhibited the aggregation of Aβ1–42 oligomer, which is suggested to be the most neurotoxic form, and suppressed Aβ1–42, APP and BACE1 expression in N2a-sw cells In addition, ROS production and oxidative stress levels were decreased by MOS and Se-PM These results suggested that the Se and the unsaturated double bonds in the AOS might be a vital factor in exerting their biological function Subse quently, they found that MOS could activate autophagy via suppressing mTOR signaling pathway, promoting clearance of intracellular accu mulation of APP and Aβ in AD cell models; while Se-PM could attenuate cell apoptosis and improve cell survival of N2a-sw cells via reducing the expression of cytochrome c and enhancing the mitochondrial membrane potential Studies revealed that GV-971, a kind of MOS derived from oxidative degradation, exerts its role in treating AD by targeting the gut-brain axis (Wang et al., 2019) GV-971 could improve cognitive functions by remodeling gut microbiota, reducing the production of abnormal me tabolites, especially phenylalanine and isoleucine, preventing the inva sion of peripheral immune cells to the brain, inhibiting neuroinflammation, and reducing brain Aβ deposition and tau hyper phosphorylation (Fig 7) A phase clinical trial showed that GV-971 significantly reversed cognitive impairment in patients with mild to moderate AD In addition, GV-971 had been demonstrated to cross the blood-brain barrier, and directly bind to multiple subregions of Aβ to inhibit the formation of Aβ fibril and destabilize the Aβ aggregates into 3.6 Alzheimer's disease therapies Alzheimer's disease (AD) is a chronic neurodegenerative disease AD is characterized by the deposition of amyloid-β (Aβ) in the brain forming plaques (plaques); and the hyperphosphorylation of tau protein causing neurofibrillary tangles (NFTs) and loss of neurons, accompanied by glial cell proliferation (Jana & Pahan, 2010) The Aβ peptide is generated from amyloid precursor protein (APP) by β-secretase (BACE) and γ-secretases cutting orderly (Lazarov & Demars, 2012; Mattson, 2004) M Wang et al Carbohydrate Polymers 271 (2021) 118408 non-toxic monomers The elaboration of the action mechanism of GV971 undoubtedly provides an important scientific basis for the indepth understanding of targeting gut microbiota as a novel treatment strategy for AD, and a detailed experimental basis for the research and development of similar drugs as a novel treatment path for AD At present, the GV-971 has already been marketed in China and approved by the FDA in proceeding international Phase clinical trials et al., 2017; Mou & Miao, 2019) AOS were also applied in aquaculture industry due to their beneficial effects on growth performance, immu nity, and disease resistance of tilapia (Oreochromis niloticus) (Van Doan et al., 2016) 3.9 Safety and commercialization of AOS Biosafety assessment of AOS is important for their potential appli cations in biomedical field None of AOS (-ED, -AH, and -OD) exhibited cytotoxic effects on RAW264.7 cells at a concentration of mg/ml in WST-8 assay (Xu, Wu, et al., 2014) AOS with concentration of mg/ml did not cause toxicity to endothelial cells, isolated from human aneu rysm (Yang et al., 2017) Zhao et al found that AOS treatment at different doses (0.05, 0.1, 0.2, 0.4, and 0.8 mg/ml) showed no cyto toxicity on HUVECs (Zhao, Han, et al., 2020) No obvious cell toxicity and mutagenicity of AOS were observed in mice after 31 days of oral administration at dosage of 600 mg/mouse/day (Ogawa et al., 2001) A double-blind, randomized, placebo controlled, days, dose-escalation phase I study (NCT00970346) to test the in vivo safety and tolera bility of inhaled OligoG CF-5/20 in healthy volunteers have been completed, as well as a multicenter, randomized, placebo controlled, crossover phase II study (NCT01465529) with 28 days treatment periods to evaluate the safety, tolerability and preliminary efficacy of OligoG CF5/20 in subjects with CF chronically colonized with Pseudomonas aeru ginosa At present, OligoG CF-5/20 is in phase IIb/III clinical trials in CF patients In the recently completed phase III trial (NCT02293915), GV971 (900 mg twice a day for 24 weeks) was demonstrated to meet the primary endpoint, with statistical significance (p < 0.001) No serious adverse events were observed, with similar incidence rate between GV971 and placebo group (Wang et al., 2019) Nowadays, GV-971, a lowmolecular-weight mannuronic acid oligomer, has been manufactured into sodium oligomannate capsules and successfully marketed in China In general, all these studies demonstrated that the doses of AOS had no obvious toxicity and side-effects for different cell lines, mouse models and human patients, which indicated safety of AOS for utilization as food supplements, drug carriers or pharmaceutical ingredients How ever, the future applications of AOS in biomedical field still require a large number of perspective efforts on the metabolism and safety of all AOS products, due to their diverse composition, variable structure and multiple modification styles Commercialization is a good way to realize large-scale application of AOS products At present, there are only a few commercially available AOS-based biomedical products The GV-971 related products have been marketed in China after more than 20 years of research and development GV-971 is the first new Alzheimer drug to receive regu latory approval globally since 2003 This typical and successful case will provide great inspiration and a wealth of practical experience for the development of AOS-based drugs in future In addition, the drug candidate Oligo G CF-5/20 has proven to be safe Its serial products are being developed by a pharmaceutical company named AlgiPharma, which has successfully completed five clinical trials (NCT00970346; NCT01465529; NCT01991028; NCT02157922; NCT02453789) including a drug deposition study in CF patients Multiple formulations for inhalation, oral, and topical administration for the treatment of respiratory diseases and microbial infections are expected to be ready in the near future 3.7 Drug carrier Considering that alginate hydrogels are difficult to degrade in vivo, Park et al proposed AOS as a drug matrix in oral sustained release formulations (Park et al., 2021) They simulated the passage of an oral drug through the stomach into intestine via controlling pH of the reac tion solution in vitro AOS obtained by enzyme digestion could form spherical gel containing lysosome through liquid dropping test and protect the lysosome from degradation or hydrolysis under acidic con ditions (pH 1.2) Then, the gel was dissolved and the embedded lyso some was released at near-neutral pH (pH 6.8) The morphological integrity and antimicrobial activity of lysosomes were largely preserved Therefore, they suggested that AOS have potential as oral delivery sys tem of drugs, proteins, and lysosomes for the treatment of metabolic diseases In addition, AOS could alleviate enterotoxigenic E coli-induced intestinal mucosal damage in weaned pigs, and improve intestinal morphology and growth (Wan et al., 2016, 2018) The above studies indicate that AOS may not only serve as carriers to deliver special drugs, but also act as stimulators on intestinal microecology In another study, AOS with high M content were used as matrices for drug delivery and enhancing the antifungal activity of posaconazole (POS) (Szekalska et al., 2019) They used the freeze-thaw technique to prepare AOS gels This method involves solvent crystallization during freezing, which led to the compression of the space of AOS chains, increased forces between AOS chains, allowing their connection and formation of the hydrogels after thawing In this way, the conventional antifungal drugs were embedded in AOS with appropriate concentra tions and proportions in order to obtain optimal mucoadhesive and prolonged drug release Finally, it was shown that POS-containing mucoadhesive films, based on AOS for buccal delivery, possessed larger zones of inhibition and reduced the growth of all tested Candida spp In general, AOS have improved water solubility and bioavailability due to their short chain length Although AOS are different from alginate in molecular size, they still maintain the properties of forming effective gel similar to alginate by adjusting the AOS size, proportion and con centration and using proper preparation technique These studies indi cated that AOS related gels also had good mucoadhesive and swelling properties and prolonged drug release effect, all of which are important and necessary for a drug carrier (Park et al., 2021; Szekalska et al., 2019) AOS have additional properties such as antimicrobial effect, enhancing drug efficacy and inducing physiologic and pathophysiologic stress response (Liu et al., 2019; Stokniene et al., 2020; Tusi et al., 2011) Therefore, AOS, as a kind of natural and emerging drug carrier with dual curative effect, have a broad application prospect and are worth further research and development 3.8 Other medical effects In addition to the above multiple pharmacological benefits, AOS have other medical effects AOS can alleviate pulmonary hypertension in mice via restoration of the TGFβ1/p-Smad2 signaling pathway and restraining the activation of P-selectin/p38MAPK/NF-κB (Feng, Hu, et al., 2020; Hu et al., 2019) AOS could prevent D-galactose-mediated cataracts in C57BL/6 J mice through inhibiting oxidative stress and upregulating genes related to antioxidant system (Feng, Yang, et al., 2020) Two studies showed that AOS exhibited protection effect on senescent cardiomyocytes and could alleviate myocardial reperfusion injury (Guo Conclusion and perspective In summary, the physicochemical and biochemical properties of AOS, such as small molecular size, low viscosity, high water solubility, and intestinal absorption, make them suitable for preparation in a va riety of dosage forms for different administration modes, including inhalation, intraperitoneal injection (i.p.) and oral gavage (Pritchard et al., 2016; Wang et al., 2019; Zhou et al., 2018) AOS could perform their unique biological functions in three modes, as extracellular 10 M Wang et al Carbohydrate Polymers 271 (2021) 118408 Table Summary of different AOS and their action modes Source Method Extracellular signaling molecules Alginate sodium Enzymatic digestion Size Ingredient and doses Function Mechanism Reference DP 2–10 G, M 0–2.5 mg/ kg G mg/ml for cell mg/ mouse Se-PM 0.8 mg/ml mg/ mouse Inhibit prostate cancer cell growth Activate the Hippo/YAP pathway block the recruitment of YAP and c-Jun attenuates α2,6-sialylation modification Stimulates TLR4/Akt/NF-κB, TLR4/ Akt/mTOR and MAPK signaling pathways Han et al (2019) Neuroimmunoregulatory activity Anti-inflammation Neuroprotection Attenuate NF-κB and MAPK Signaling pathways activation and proinflammatory mediator production; inhibit N2a-sw cell apoptosis Bi, Lai, Cai, et al (2018), Bi, Lai, Han, et al (2018), Bi et al (2019), and Bi et al (2020) Jiang et al (2021) Alginate (sigma) Alginate lyase from Pseudoalteromonas sp DP 2–8 Alginate (sigma) Acid hydrolysis sulfation, selenylation Mr 3000 Alginate Sinopharm Chemical Reagent Co., Ltd AOS Qingdao BZ Oligo Biotech Co Ltd AOS Qingdao BZ Oligo Biotech Co Ltd Alginate Qingdao Bright Moon Seaweed Group Co., Ltd Laminaria digitata Immobilized AlgL17 DP 1–4 G, M 0–1 mg/ml Antioxidant Upregulation of ROS scavenging activities and attenuation of the caspasemediated apoptosis pathway NMa DP 2–10 G, M 0–0.8 mg/ ml Protects endothelial cells against oxidative stress injury Zhao, Han, et al (2020) Alginate lyase from Pseudomonas sp HZJ 216 Mr 1200 DP 2–6 DP 2–6 M/G, 1/2.6 200 mg/kg Prevents acute doxorubicin cardiotoxicity Regulated integrin-a/FAK/PI3K pathway against oxidative stress-induced apoptosis Suppressing oxidative stress and endoplasmic reticulum-mediated apoptosis G, M 0.6 mg/ml Alleviating inflammatory injury in intestinal epithelial cells Reducing TNFR1-dependent apoptosis Wan et al (2020) Enzymatic digestion acidic hydrolysis alginate lyase Aly08 from Vibrio sp Mr 3500 DP 2,3 G, M Improve epidermal homeostasis in aged skin effective anti-obesity effects Increases stem cell proliferation and self-renewal in aged keratinocytes Activate AMPK signaling via increasing both AMPK and ACC phosphorylation Charruyer et al (2016) Li et al., 2019 NM Mr 2600–3200 Antimicrobial Inhibition of microbial growth, disturb biofilm formation and persistence and reduce resistance to antibiotic treatment alter the structure of mucus, modulate mucin assembly, and reduce the elastic and viscous properties of sputum from CF patients Khan et al (2012), Pritchard, Powell, Jack, et al (2017), Pritchard, Powell, Khan, et al (2017) Ermund et al., 2017, Pritchard et al (2016), Pritchard et al (2019) Laminaria japonicais Laminaria hyperborea Alginate lyase from Streptomyces violaceoruber Potent prebiotic agents AOS Nanjing NM Junlan Biotechnology Company Laminaria alginate lyase Aly08 japonicais from Vibrio sp Alginate sodium Oxidative degradation G, M 0–400 mg/ kg OligoG (CF-5/20) G>85% 0.2, 2, and 10% Improve innate immunity Treatment of chronic respiratory disease Fang et al (2017) Guo et al (2016) DP 1–4 G, M 500 mg/kg Improves lipid metabolism and inflammation in HFD-fed mice Modulate gut microbiota release of microbiota-dependent SCFAs decrease of endotoxin Wang et al (2020) Mr 420 DP 2–10 G, M 400 mg/kg M (GV-971) 50, 100 mg/ kg Attenuated obesity-related metabolic abnormalities Inhibit AD progression Modulate gut microbiota Li et al (2020) Remodels gut microbiota and suppresses gut bacterial amino acids-shaped neuroinflammation Geng et al (2019) and Wang et al (2019) Drug carrier or wound dressings AOS Xi'an Obtained from Haoxuan Laminaria japonica Biotech Co., Ltd Mr 2900 Increase the activity of antifungal drugs Alginate lyase NM (sigma) Brown seaweed (sigma) Enzymolysis Optimal mucoadhesive and swelling properties and prolonged drug release affected the growth reduction of Candida spp Exhibit pH-sensitive characteristics keep lysozyme antimicrobial activity Szekalska et al (2019) Alginate sodium M 48% G 16% MG 36% 1, 2% (w/w) NM 2% (w/v) NM NM Promote wound healing debridement effect Provide weak acid wet for wound healing environment prevent tissue adhesion and infection reduce the adverse reactions of antiinflammatory drugs Miao et al (2019) a In vivo oral drug delivery NM: Not Mentioned 11 Park et al., 2016 M Wang et al Carbohydrate Polymers 271 (2021) 118408 signaling molecules to induce host biological reactions; as prebiotics to stimulate and improve the interactions between symbiotic microbiota and host metabolism; as drug adjuvant or carrier to enhance the efficacy of drug itself As shown in Table 2, different sources of raw materials, preparation methods and modification styles determinate the structure and properties of final AOS products, which affect their beneficial functions and special effects The doses of AOS vary according to the applications, such as 0–1 mg/ml for cell line treatments, 2.5–500 mg/ kg/day or mg/mouse for xenograft mouse models treatments, and 0.2–10% as drug carrier or antimicrobials The challenges on current AOS research include: 1) Most studies are limited to the laboratory level Only a few studies involve in clinical trials in humans; 2) Production of AOS is far from being industrialized As for enzymatic digested AOS, the low yield and hydrolysis efficiency of current alginate lyases are the key restricting factors for industrial production; 3) AOS production cost is too high Consequently, drugs based on AOS, especially those that are not yet included in the medical insurance, are expensive and increase the economic burden on patients In spite of these challenges, AOS have great research significance, development value and a broad application prospect due to their strong structural plasticity, high bioavailability and diverse beneficial effects It is worth investing more efforts for their potential applications In future studies, researchers should focus on the following aspects: 1) More clinical trials in humans need to be operated to provide detailed experimental evidences 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