Coronavirus disease 2019 (COVID-19) continues to pose a significant global health threat, with the risk of severe health complications, including long COVID. As the world seeks effective strategies to mitigate, cure, or prevent COVID-19, lactoferrin (LF) and its derivative lactoferricin (LFC) have garnered increasing attention for their potential antiviral properties. These glycoproteins have demonstrated promising inhibitory effects against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus responsible for COVID-19.
The primary entry point for SARS-CoV-2 into human cells is the angiotensin-converting enzyme 2 (ACE2). The virus binds to ACE2 through its spike protein (S-protein), specifically the receptor-binding domain (RBD) within the N-terminal subunit (S1) of the S-protein. After attachment, the virus undergoes a priming process, predominantly facilitated by the host transmembrane protease serine 2 (TMPRSS2). This priming is crucial for the fusion of viral and cellular membranes, enabling the virus to enter the host cell. Once inside, viral RNA is replicated and packaged into new virions. LF and LFC have shown potential in interfering with SARS-CoV-2 at various stages of this pathway.
Human lactoferrin (hLF), also known as lactotransferrin, is a multifunctional glycoprotein found in human milk and other body fluids, as well as in the secondary granules of neutrophils. It is a member of the transferrin family and is known for its antibacterial, antifungal, antiviral, antiparasitic, antioxidant, antitumor, anti-inflammatory, and immunomodulatory activities. These activities primarily depend on LF’s ability to sequester iron ions and its potent binding capacity through its positively charged N-terminal region. Lactoferricin, derived from this N-terminal region through pepsin digestion, retains some of LF’s biological activities and exhibits additional functions due to its unique structural properties.
| Medical Concept | Simplified Explanation | Relevant Details | Examples/Additional Information |
|---|---|---|---|
| Coronavirus (COVID-19) | A contagious virus causing respiratory illness, similar to the flu, but often more severe. | Spread through respiratory droplets, causing symptoms like fever, cough, and shortness of breath. | Symptoms can range from mild to severe, and in some cases, it can be fatal. |
| Lactoferrin (LF) | A protein found in human milk and other body fluids that helps fight infections. | Has antibacterial, antiviral, and anti-inflammatory properties. | Found in high amounts in colostrum, the first form of breast milk. |
| Lactoferricin (LFC) | A smaller piece of lactoferrin with strong antimicrobial effects. | Produced when lactoferrin is broken down by stomach enzymes. | More potent than lactoferrin in fighting infections. |
| SARS-CoV-2 | The specific virus that causes COVID-19. | Uses a protein on its surface (spike protein) to enter human cells. | The full name is Severe Acute Respiratory Syndrome Coronavirus 2. |
| Spike Protein (S-protein) | A part of the SARS-CoV-2 virus that helps it attach to and enter human cells. | Binds to receptors on human cells to initiate infection. | Target for many vaccines and treatments. |
| Angiotensin-Converting Enzyme 2 (ACE2) | A protein on the surface of many human cells that SARS-CoV-2 uses to enter the cells. | Acts as the main entry point for the virus into human cells. | Found in the lungs, heart, blood vessels, and other organs. |
| Transmembrane Protease Serine 2 (TMPRSS2) | An enzyme that helps activate the SARS-CoV-2 spike protein, enabling the virus to enter cells. | Essential for the virus to fuse with the cell membrane and infect the cell. | Target for some COVID-19 treatments. |
| Heparan Sulfate Proteoglycans (HSPGs) | Molecules on cell surfaces that can also help SARS-CoV-2 attach to cells. | Provide an additional route for the virus to bind to and enter cells. | May influence the severity of the infection. |
| Viral RNA | The genetic material of SARS-CoV-2, which it uses to replicate inside human cells. | Encodes all the information needed to make new virus particles. | Target for PCR tests used to diagnose COVID-19. |
| Iron Sequestration | The process of binding and holding onto iron, which bacteria and viruses need to grow. | Lactoferrin can bind iron, making it unavailable to pathogens and limiting their growth. | Part of the body’s natural defense mechanism against infections. |
| Antiviral Properties | The ability of a substance to prevent or treat viral infections. | Lactoferrin and lactoferricin can block viruses from entering cells and replicating. | Includes a wide range of substances, from drugs to natural proteins. |
| Antimicrobial Activity | The ability to kill or inhibit the growth of microorganisms, including bacteria, viruses, and fungi. | Lactoferrin has broad antimicrobial effects due to its ability to bind iron and disrupt cell walls. | Used in medical treatments and food preservation. |
| Antioxidant Properties | The ability to neutralize harmful free radicals in the body, which can damage cells and tissues. | Lactoferrin helps protect cells from oxidative stress and inflammation. | Important for maintaining overall health and preventing chronic diseases. |
| Immunomodulatory Effects | The ability to modify or regulate one or more immune functions. | Lactoferrin can enhance the immune response to infections and reduce inflammation. | Potential therapeutic uses in autoimmune diseases and chronic inflammation. |
In the context of SARS-CoV-2, LF and LFC are believed to interfere with the virus through multiple mechanisms. These include directly blocking the interaction between the S-protein and heparan sulfate proteoglycans (HSPGs) on target cell membranes, inhibiting virus priming, and hampering RNA replication. Previous studies have shown that synthetic peptide pLF1, derived from the N-terminus of LF, can inhibit the proteolytic activity of serine proteases, such as plasmin, elastase, and TMPRSS2. This inhibition is not observed with the full-length LF, suggesting that the unique conformation of free LFC contributes to this activity. Despite this, both the N-terminal LFC and full-length LF have been shown to reduce SARS-CoV-2 infection by about 50%.
Further research has revealed that LF directly binds to the S-protein of SARS-CoV-2, with binding sites mapped to the N-terminal region of LF and the RBD of the S-protein. This binding may explain the observed protective effects of LF and LFC against SARS-CoV-2 infection and suggests that these glycoproteins could serve as cost-effective supplemental tools in managing COVID-19.
LF has been demonstrated to block the cell entry of many viruses, including herpes simplex virus, human immunodeficiency virus, dengue virus, and various coronaviruses. In SARS-CoV-2, LF has been shown to prevent the interaction between the viral S-protein and target cell membranes by binding to HSPG. Human LF, with approximately 700 amino acids and a molecular weight of around 80 kDa, consists of two homologous lobes and a highly positively charged N-terminal region, distinguishing it from other members of the transferrin family.
The synthetic peptide pLF1, derived from LF’s N-terminal region, effectively blocks the binding between the S-protein and target cells. The highly positively charged LFC, released from LF by pepsin cleavage in the gut, contributes to LF’s antimicrobial activity. Structural studies have revealed that the conformation of free N-terminal LFC differs significantly from its structure within intact LF. This unique structure, influenced by the high net positive charge and the position of cationic residues, plays a crucial role in the effectiveness of LF, LFC, and LF-derived peptides in blocking the S-protein of SARS-CoV-2.
Lactoferrins from different species exhibit high homology but may differ in their antiviral potencies due to slight differences in the tertiary structure and charge of their N-termini. For instance, bovine LFC (bLFC) is considered more potent than human LFC (hLFC). Most studies have focused on the antiviral properties of bLFC and hLFC, but limited research has also explored LFC from other species, such as pigs, mice, goats, and camels. Future molecular docking simulations could help identify LFC variants with the highest affinity for the S-protein, enhancing their effectiveness against SARS-CoV-2.
Interestingly, the C-terminal peptide pLF3 also partially blocks the interaction between LF and the S-protein, indicating that both ends of LF are involved in this interaction. The inhibitory effect of LF on SARS-CoV-2 infection appears to depend on its iron saturation state. However, since LFC, pLF1, and pLF3 lack iron-binding capacity, it is likely that iron saturation does not directly influence LF’s binding to the S-protein. Intact LF may block the infection through other mechanisms.
Direct binding between LF and the S-protein of SARS-CoV-2 has been suggested in previous studies. Using a pull-down approach, researchers identified LF binding to various S-protein variants. In silico molecular docking simulations proposed a model structure for this binding, implicating the RBD in the interaction. This model suggests that both the N- and C-lobes of LF are involved in contact with the S-protein. However, these studies used non-glycosylated forms of LF, raising questions about the suggested contact sites, as the corresponding parts of hLF and bLF contain glycosylation sites for relatively long sugar chains.
The antiviral properties of LF and LFC are further supported by their ability to bind to host cell receptors for various viruses. For example, LF binds to receptors for herpes simplex virus, human immunodeficiency virus, dengue virus, and coronaviruses. In the case of SARS-CoV-2, LF’s ability to block the interaction between the S-protein and HSPG on target cell membranes highlights its potential as a supplemental tool in preventing COVID-19 infection.
Overall, the multifunctional properties of LF and its derivative LFC offer promising avenues for mitigating SARS-CoV-2 infection. Their ability to interfere with multiple stages of the viral life cycle, coupled with their broad-spectrum antiviral activities, positions them as valuable candidates for supplemental pharmacological tools in managing COVID-19. Continued research into the molecular mechanisms underlying their antiviral effects, as well as the exploration of LFC variants from different species, could enhance our understanding and utilization of these glycoproteins in the ongoing fight against COVID-19.
reference : https://www.mdpi.com/1424-8247/17/8/1021
