2  Application prospects of polysaccharides against coronavirus
Polysaccharides are macromolecular compounds obtained mainly from plants, algae, and even animals [15]. The antiviral properties of polysaccharides are not only a simple function of their charge density and chain length, but also their detailed structural characteristics [19]. Coronaviruses, such as SARS-CoV, MERS-CoV, and novel SARS-CoV-2, cause high mortality and pose a severe threat to humans and animals health, creating a need for effective inhibitors [20]. Polysaccharides, which are commonly used active ingredients in traditional Chinese medicine, have a great application prospect in the prevention and treatment of coronavirus based on their broad-spectrum antiviral activities and unique antiviral mechanisms. The presence of carbohydrate-binding agents can strongly inhibit coronaviruses, including transmissible gastroenteritis virus, infectious bronchitis virus (IBV), feline coronaviruses serotypes I and II, mouse hepatitis virus (MHV), and PRRSV [21].

2.1  Source and structure of polysaccharides
The main sources of polysaccharides are endogenous glycosaminoglycans (GAGs), Marine polysaccharides and terrestrial plant polysaccharides, especially polysaccharides from Chinese herbal medicines. GAGs are naturally-derived linear polysaccharides that are expressed in the intracellular compartments, cell surface, and extracellular environments, and they interact with various molecules to regulate many cellular processes associated with health and disease [22]. GAGs are comprised distinct O-linked disaccharide units, which are typically composed of a combination of iduronic acid, glucuronic acid, glucosamine, galactose or galactosamine monosaccharides [23,24]. The widely studied GAGs mainly include chondroitin sulfate (CS), heparan sulfate (HS) and heparin (HP) in animal tissues (Fig. 3 ) [22,25]. GAG chains are in most cases sulfated, except hyaluronan (HA) (Fig. 3), which are biodegradable and non-immunogenic in the body [26,27]. The chemical structures of typical GAGs are shown in Fig. 3.
Fig. 3 The structures of several polysaccharides (GAGs, marine polysaccharides, and terrestrial plant polysaccharides).
Marine organisms are rich sources of polysaccharides. Chitosan is a linear, positive-charged, alkaline polysaccharide repeating by glucosamine and N-acetylglucosamine (Fig. 3) [28,29], derived from the shells of shrimps and crustacean or the cell walls of fungi [30,31]. Marine algae products have been applied in traditional Chinese herbal medicine for a long time [11], and contain a variety of polysaccharides, including carrageenan, fucoidan, and alginate. Carrageenans are sulphated linear polysaccharides composed of repeating disaccharide units with alternating 3-linked β-d-galactopyranose (G-units) and 4-linked α-galactopyranose (D-units) or 3,6-anhydro-α- galactopyranose (AnGal-units) [[32], [33], [34]], which are extracted from certain red algae containing 15–40% ester sulfate with an average molecular weight above 100 kDa [35,36]. The three commercial most important and widely distributed carrageenans are iota (ι-, G4S-DA2S), kappa (κ-, G4S-DA) and lambda (λ-, G2S-D2S, 6S)-carrageenan (Fig. 3) [37]. Fucoidan is a fucose-enriched and sulfated polysaccharide extracted from brown algae [11,38], which is composed of L-fucose, sulfate groups and small proportions of D-xylose, D-mannose, D-galactose, and D-glucuronic acid in different sources of brown algae (Fig. 3) [[38], [39], [40]]. Alginate, an acidic and linear polysaccharide extracted from brown algae, is consisted of alternating β-D-mannuronic acid (M) and α- L-guluronic acid (G) residues [41]. Polyguluronate sulfate (PGS) (Fig. 3) is a low molecular weight sulfated brown algae polysaccharide obtained by chemical sulfation of polyguluronate (PG) with about 1.5 sulfate per sugar residue [42,43].
Astragalus polysaccharide (APS) is the most important bioactive component isolated from a Chinese traditional herbal medicine of Astragalus membranaceus, which is composed of glucose, mannose, d-glucose, and D-galactose (Fig. 3) [[44], [45], [46]]. Radix Isatidis (RI) is also a kind of traditional Chinese herbal medicine with significant antiviral effect, and polysaccharide is its main active component [47,48]. The polysaccharide from RI is mainly composed of mannose, glucose, galactose and arabinose [49]. Mushrooms are used as food for long time in China, and also are drugs in the Orient centuries [50]. Lentinus edodes is one of the most widely edible mushrooms, and is popularly consumed as health foods in Asian countries [50,51]. Among the bioactive components of mushrooms, the Lentinus edodes polysaccharide (lentinan, LNT) is the most extensively investigated with many immune processes, which is generally described as biological response modifiers [52,53]. It consists of a β-(1 → 3)-glucan backbone with β-(1 → 6)-glucosyl side-branching units terminated by mannosyl or galactosyl residues (Fig. 3) [50,51]. Recently, LNT has been widely used as an alternative medicine and dietary supplement in the world [50].

2.2  Anti-coronavirus activity of GAGs
Cell surface GAGs serve as co-receptors by increasing the local concentration of pathogens, so that they can more efficiently interact with their entry receptors. Most coronavirus receptors of carbohydrate are mainly negatively charged, such as sulfated GAGs or glycans containing sialic acid [54,55]. S protein concentrated outside the virus contains the receptor binding domains (RBDs) at the N-terminal, such as MHV-CoV N-RBD and SARS C-RBD with their receptor (Fig. 2B–D) [7,56,57]. The coronavirus NL63 (CoV-NL63), and SARS-CoV use angiotensin-converting enzyme 2 (ACE2) as a primary receptor for infection of target cells (Fig. 2) [56,58,59]. Phylogenetically, SARS-CoV-2 is almost identical to SARS-CoV, sharing 79.6% genomic sequence identity [60], and use the same cell entry receptor, ACE2, as SARS-CoV [8,60]. During infection, CoV first binds host cell via interaction between its S1-RBD and the cell membrane receptor, triggering conformational changes in the S2 subunit that result in virus fusion and entry into the target cell. Viral RNA gradually forms mature virions through replication, transcription, and synthesis, and then is released from host cell (Fig. 4 ) [8,[60], [61], [62]]. However, the expression of ACE2 is not sufficient for infection, and HSPGs play important roles in the entry of some pathogens such as SARS-CoV [[63], [64], [65]]. A soluble HS was used to assess whether the attachment of HCoV-NL63 was mediated by HS proteoglycans. Flow cytometric analysis showed that the adhesion of virus to LLC-Mk2 cells was completely inhibited in the presence of soluble HS, indicating the role of this molecule in adhesion to susceptible cells and possible also in cell entry [54]. Both CoV-NL63 and SARS-CoV initially bind to the HS on the cell surface, and virus entry depends on the HS interaction, indicating that HS can inhibit virus attachment and entry [22,54].
Fig. 4 Life cycle of highly pathogenic human CoVs. These CoVs enter host cells by first binding to their respective cellular receptors via the surface S protein. Viral genomic RNA is released and translated into viral polymerase proteins. Viral RNA and nucleocapsid (N) structural protein are replicated, transcribed, or synthesized in the cytoplasm, whereas other viral structural proteins, including S, membrane (M), and envelope (E), are transcribed then translated in the endoplasmic reticulum (ER) and transported to the Golgi. The viral RNA–N complex and S, M, and E proteins are further assembled in the ER–Golgi intermediate compartment (ERGIC) to form a mature virion, then released from host cells [62].
Natural products of HS and the allied polysaccharide, heparin, are involved and prevent infection by a range of viruses including S-associated coronavirus strain HSR1 [66]. HS is known to bind CoV surface proteins and to be used by coronavirus for its attachment to target cells [54]. Currently, there are no commercially available medicinal products designed to treat and/or prevent infections associated with the novel SARS-CoV-2 coronavirus outbreak. The surface plasmon resonance and circular dichroism were used to measure the interaction between the SARS-CoV-2 Spike S1 protein RBD (SARS-CoV-2 S1 RBD) and heparin. Additionally, basic amino acids are known to dictate the binding between proteins and heparin. Primary sequence analysis of the expressed protein domain and analysis of the modeled SARS-CoV-2 S1 RBD structure show that there are several potential heparin binding sites, and more importantly, theses patches of basic amino acids are exposed on the protein surface. This study has implications for the rapid development of a first-line therapeutic by repurposing heparin and for next-generation, tailor-made, GAG-based antivirals [66].

2.3  Anti-coronavirus activity of marine polysaccharides
Marine polysaccharides, such as carrageenan, PGS, chitosan, and their derivatives, show good inhibitory activity against various viruses, which provides a reference for their research on coronavirus. Iota-carrageenan containing lozenges show highly active against human rhinovirus (HRV), influenza virus A H1N1, and HCoV OC43 throughout the entire dissolution process, and are a promising therapy against viral infections of throat [67]. The cationically modified chitosan, N-(2-hydroxypropyl)-3-trimethylammonium chitosan chloride (HTCC), shows significant inhibition against the human coronavirus HCoV-229E, HCoV-OC43, HCoV-NL63, and HCoV-HKU1, and its hydrophobically modified derivative (HM-HTCC) is a potent inhibitor of the coronavirus HCoV-NL63, indicating that HTCC polymers based on chitosan are effective inhibitors of all low-pathogenic human coronaviruses [68]. Acute viral upper respiratory tract infection, also known as common cold, is mainly caused by respiratory viruses such as rhinovirus, coronavirus, influenza virus [[69], [70], [71]]. Clinical trials applying iota-carrageenan nasal spray have shown to reduce the duration of a virus-confirmed common cold. Carrageenan nasal spray shows significant antiviral efficacy in three virus subgroups, HRV, human coronavirus, and influenza A virus (IAV), and the highest effectiveness was observed in human corona virus-infected patients. The reduced duration of disease was 3 days (p < 0.01), and the number of relapses was three times less (p < 0.01) in carrageenan treated corona-virus -infected patients compared to control patients [70].
After the outbreak of SARS in 2003, many survivors developed residual pulmonary fibrosis with increased severity in older patients. Pulmonary fibrosis is caused by a hyperactive host response to lung injury mediated by epidermal growth factor receptor (EGFR) signaling in animal models (Fig. 5 ). Inhibition of EGFR signaling can prevent an excessive fibrotic response to SARS-CoV and other respiratory viral infections [72]. Moreover, sulfated polysaccharides such as fucoidan and sulfated rhamnan, can interfere or inhibit the expression and activation of EGFR pathway, which may help to suppress coronavirus [73,74]. The understanding of how polysaccharides play a role in EGFR and other pro-fibrotic pathways after viral infection will provide new ideas for COVID-19 treatment.
Fig. 5 The illustration about potential role of EGFR in lung fibrosis. Physical injury or a pathogen ① initiates the wound healing response by damaging healthy tissue, releasing EGFR ligands ② and activating the EGFR pathway. This results in an exaggerated wound healing response leading to a fibrotic lung ③. The early use of inhibitors like tyrosine kinase ④ could prevent the normal progress of wound healing and fibrosis [72].

2.4  Anti-coronavirus activity of traditional Chinese medicine polysaccharides
Traditional Chinese herbal medicine is widely used in the prevention and treatment of viral infectious diseases in China [75]. Some Chinese herbs contain potential anti-SARS-CoV-2 active compounds, especially Hedysarum multijugum maxim, coptidis rhizoma, and forsythiae fructus, which have been catalogued for treating viral respiratory infections [76]. This provides a basis for the application of traditional Chinese medicine polysaccharides in coronavirus. The avian coronavirus causes infectious bronchitis (IB), which is one of the most serious diseases affecting the avian industry worldwide. APS can significantly reduce the replication of IBV in infected chicken embryo kidney (CEK) cells in a dose-dependent manner. The titer of IBV-specific antibodies, lymphocyte proliferation, and the expression levels of interleukin (IL)-1β, IL-2, IL-8, and TNF-a in APS treatment groups were higher than those in the control group. These data suggest that APS enhances the immune response to IBV vaccination in chickens, and is a potential therapeutic agent for inhibiting IBV [77,78]. During the outbreak of SARS coronaviruses in China, RI, as a Chinese medicinal herb, was prepared as an antiviral drug [79]. Polysaccharides isolated from RI have been shown to stimulate the expression of cytokines, such as IL-2 and interferon (INF)-γ, thereby regulating and enhancing non-specific immunological function, humoral immunity and cellular immunity in mice to play antiviral effects [48]. Active compounds derived from cultured Lentinula edodes mycelia (AHCC) is an α-glucan-based standardized mushroom extract that has been extensively investigated as an immunostimulant both in animals and in humans affected by influenza virus, herpes virus, avian influenza virus (AIV), human papillomavirus (HPV), hepatitis B virus (HBV), and human immunodeficiency virus (HIV) by promoting a regulated and protective immune response [80]. Due to its action in promoting a protective response to a wide range of viral infections, which can support its use in the prevention of diseases provoked by a human pathogenic coronavirus, including COVID-19 [80].