Tick-Borne Encephalitis: A Comprehensive Examination of the Disease, Its Mechanisms and Potential Treatments

Giugno 30, 2024

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Tick-borne encephalitis (TBE) is a severe infectious disease that affects the central nervous system (CNS) of both humans and animals. This disease is caused by the Tick-borne Encephalitis virus (TBEV), which belongs to the Orthoflavivirus genus. The rising incidence of neurotropic viruses, including TBEV, has become a significant public health concern worldwide. With approximately 10,000 cases reported annually, understanding and addressing TBE is critical for global health.

Mechanisms of TBEV Infection

Research has shown that neurotropic viruses such as TBEV disrupt intracellular calcium ion homeostasis by manipulating the calcium signaling pathway upon cell invasion. This disruption is also observed in other viruses of the Orthoflavivirus genus, including Dengue virus (DENV) and Zika virus (ZIKV), which activate calcium channels to facilitate viral replication. For instance, in a DENV-infected mouse model, DENV induces the phosphorylation of ERK1/2, increases apoptosis in hepatocytes, and promotes viral replication.

Despite these insights, changes in intracellular calcium levels and the MAPK-ERK signaling pathway post-TBEV infection have not been extensively documented. However, it is known that TBEV infection triggers innate immune responses in mice and human neuronal cells, increasing cytokine expression levels such as IL-6.

Prevention and Vaccination

Preventing TBE involves two primary strategies: non-specific measures and specific immunization. Non-specific precautions include pasteurizing milk, reducing tick populations, and personal protection. Specific prevention through active immunization is widely regarded as the most effective method against TBE. Vaccines like Encepur N, FSME-Immun CC, Ticovac, Encevir-Neo, and Klesh-E-Vak are commonly used and have shown high efficacy in TBE prevention. However, there have been confirmed cases of TBE even among vaccinated individuals, indicating the need for further research and development of therapeutic agents.

Treatment Strategies

Currently, there are no specific antiviral treatments approved for TBE. This highlights the urgent need to develop specific anti-TBEV drugs. Recent studies have suggested that tetracycline antibiotics, particularly minocycline (MIN), possess antiviral properties beyond their antimicrobial activity. Due to its high fat solubility and ability to penetrate the blood-brain barrier, MIN achieves higher concentrations in brain tissue compared to other tetracycline antibiotics. This characteristic makes MIN a promising candidate for inhibiting TBEV replication.

Experimental Findings on Minocycline

Our study investigated the impact of MIN on TBEV replication and elucidated its initial mechanism of action. The findings indicated that MIN effectively inhibited TBEV replication in a dose-dependent manner. The inhibition of viral replication by MIN is attributed to the modulation of calmodulin and calcium channel-associated proteins. Furthermore, MIN maintained normal cellular differentiation, mitigated cellular damage induced by TBEV infection, and suppressed the MAPK-ERK signaling pathway to impede TBEV replication. Additionally, MIN reduced the inflammatory response to TBEV infection by down-regulating the expression of the inflammatory factor IL-6.

Detailed Mechanisms of Action

Calcium Signaling Pathway

An imbalance of calcium homeostasis has been repeatedly reported in Orthoflavivirus-related diseases, mainly due to the promotion of viral replication and the maintenance of infection. Excessive calcium induces intracellular enzyme cascades and inflammatory responses, leading to cell damage. Calcium receptors and channels play a crucial role in maintaining intracellular calcium homeostasis. Calmodulin-like 4 (CALML4) is a subunit of calcium receptors in cells, affecting signal transduction, protein phosphorylation, and gene expression regulation by binding calcium ions and regulating calmodulin kinases. Ryanodine receptor 2 (RYR2) and Synaptosome-associated protein 25 (SNAP25) are associated with calcium channels, with RYR2 regulating calcium ion flow between the endoplasmic reticulum and cytoplasm, and SNAP25 negatively regulating voltage-gated calcium channels.

Our study demonstrated that MIN post-treatment regulated the levels of CALML4, RYR2, and SNAP25, thereby restoring the expression of calcium signaling pathway-associated proteins in TBEV-infected Vero cells. Furthermore, MIN’s role as a calcium chelator restored rotenone-induced calcium ion deregulation, protecting primary cortical neurons from calcium ion-induced damage.

MAPK-ERK Signaling Pathway

Previous studies have shown that the MAPK-ERK signaling pathway is critical in Orthoflavivirus replication and release. Extracellular growth factors and receptor tyrosine kinases activate the MAPK-ERK signaling pathway. For example, Fibroblast growth factor 2 (FGF2) negatively regulates the expression level of Platelet-Derived Growth Factor Receptor α (PDGFRA), which activates Phosphatidylinositol-4,5-bisphosphate (PIP2) by binding to ligand PDGF. Phosphodiesterase PLCB2 catalyzes the hydrolysis of PIP2 to inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG), with DAG activating PKC to regulate the MAPK-ERK signaling pathway.

Our study found that MIN inhibited the MAPK-ERK signaling pathway in TBEV-infected Vero cells by increasing levels of upstream FGF2 and decreasing levels of PDGFRA and PLCB2, thereby reducing the expression level of p-ERK. Additionally, IP3 binds to the calcium channel receptor IP3R on the endoplasmic reticulum, promoting calcium ion release, which can bind to CaM kinase II (CaMKII) or activate protein kinase C (PKC), further activating the MAPK-ERK signaling pathways. This suggests a cross-linking of the MAPK-ERK and calcium signaling pathways mediated by MIN in TBEV-infected Vero cells.

Inflammatory Response Modulation

IL-6, a multifunctional cytokine regulated by the MAPK-ERK signaling pathway, plays a crucial role in the body’s defense against inflammation. Synaptotagmin 11 (SYT11) inhibits the secretion of IL-6, which is produced in large amounts during TBEV infection, triggering an inflammatory response. Reducing IL-6 and SYT11 levels in TBEV-infected Vero cells using MIN reflects the reversal of inflammatory responses.

Clinical Implications and Future Directions

The protective effect of MIN on TBEV-infected cells in vitro warrants further investigation. Currently, no established treatment regimen exists for TBE patients, emphasizing the importance of developing reliable anti-TBEV compounds. Our study demonstrated the inhibitory effects of MIN on TBEV replication in vitro and provided initial insights into its mechanism of action. Future research should focus on using MIN to inhibit TBEV infectivity and improve clinical outcomes in vivo.

In conclusion, Tick-borne encephalitis poses a significant public health challenge due to its impact on the central nervous system and the absence of specific antiviral treatments. Preventive measures, including vaccination, play a crucial role in controlling TBE. However, breakthrough infections and the need for effective therapeutic options necessitate ongoing research. Minocycline, with its ability to inhibit TBEV replication and modulate key signaling pathways, emerges as a promising candidate for further exploration. Comprehensive studies on MIN’s efficacy and mechanism of action will contribute to the development of targeted treatments for TBE, ultimately improving patient outcomes and reducing the global burden of this debilitating disease.

APPENDIX 1 – Minocycline: A Comprehensive Examination of Its Antiviral Properties and Therapeutic Potential

Minocycline (MIN) is a second-generation tetracycline antibiotic known for its broad-spectrum antibacterial properties. However, recent research has expanded its potential use beyond antimicrobial activity, exploring its efficacy as an antiviral agent. This comprehensive document delves into the detailed mechanisms of action, clinical applications, and emerging research on minocycline, with a focus on its antiviral properties. This exploration includes updated information as of 2024, drawing on the latest scientific studies, clinical trials, and real-world data to provide a thorough understanding of minocycline’s potential in treating viral infections, particularly those caused by neurotropic viruses.

Chemical Properties and Pharmacokinetics

Minocycline hydrochloride, chemically known as [4S-(4α,4aα,5aα,12aα)]-4,7-Bis(dimethylamino)-1,4,4a,5,5a,6,11,12a-octahydro-3,10,12,12a-tetrahydroxy-1,11-dioxo-2-naphthacenecarboxamide, is a semisynthetic derivative of tetracycline. It is more lipophilic than its predecessors, enabling better tissue penetration and longer retention in the body.

Pharmacokinetics

Minocycline is rapidly absorbed after oral administration, achieving peak plasma concentrations within 1-2 hours. It has a bioavailability of approximately 90-100%, which is not significantly affected by food intake. The drug is highly lipophilic, allowing it to cross the blood-brain barrier and reach therapeutic concentrations in the central nervous system (CNS). The half-life of minocycline ranges from 11 to 26 hours, enabling twice-daily dosing in most therapeutic scenarios.

Mechanism of Action

Minocycline exerts its antibacterial effects by binding to the 30S ribosomal subunit of bacteria, inhibiting protein synthesis. However, its antiviral mechanisms are more complex and involve several pathways:

Inhibition of Viral Replication: Minocycline has been shown to inhibit the replication of various viruses, including those of the Orthoflavivirus genus, by interfering with viral RNA synthesis and protein production.
Modulation of Host Cell Signaling Pathways: Minocycline affects cellular signaling pathways such as MAPK-ERK, which are crucial for viral replication. By inhibiting these pathways, minocycline reduces viral replication and proliferation.
Calcium Homeostasis: Minocycline modulates intracellular calcium levels, which are critical for the replication of many viruses. It acts as a calcium chelator, restoring calcium ion balance disrupted by viral infections.
Anti-inflammatory Effects: Minocycline reduces the expression of pro-inflammatory cytokines like IL-6, mitigating the inflammatory response often associated with viral infections.

Antiviral Properties

Effectiveness Against Orthoflavivirus Genus

Minocycline has demonstrated significant antiviral activity against several viruses in the Orthoflavivirus genus, including the Tick-borne Encephalitis Virus (TBEV), West Nile Virus (WNV), Japanese Encephalitis Virus (JEV), and Dengue Virus (DENV).

Tick-borne Encephalitis Virus (TBEV): Recent studies have highlighted minocycline’s potential in inhibiting TBEV replication. By modulating calmodulin and calcium channel-associated proteins, minocycline effectively reduces viral load and mitigates cellular damage caused by the virus.
West Nile Virus (WNV): Minocycline has been shown to inhibit WNV replication by interfering with viral RNA synthesis. Its ability to cross the blood-brain barrier makes it particularly effective against neurotropic viruses like WNV.
Japanese Encephalitis Virus (JEV): Similar to its effects on WNV and TBEV, minocycline inhibits JEV replication through multiple pathways, including modulation of the MAPK-ERK signaling pathway and reduction of inflammatory cytokine production.
Dengue Virus (DENV): Minocycline’s inhibition of the MAPK-ERK signaling pathway has been well-documented in the context of DENV, where it reduces viral replication and the associated inflammatory response.

Clinical Applications

Neuroprotective Effects

Minocycline’s ability to cross the blood-brain barrier and its anti-inflammatory properties make it a promising candidate for treating neuroinflammatory conditions. Studies have demonstrated its neuroprotective effects in models of neurodegenerative diseases such as Parkinson’s and Alzheimer’s, as well as in acute neurological injuries like stroke.

Clinical Trials and Studies

Minocycline in Viral Encephalitis: Ongoing clinical trials are evaluating minocycline’s efficacy in treating viral encephalitis caused by neurotropic viruses. Preliminary results suggest a reduction in viral load and improvement in neurological outcomes.
Combination Therapy: Research is also exploring the use of minocycline in combination with other antiviral agents to enhance therapeutic efficacy. These studies aim to determine optimal dosing regimens and identify potential synergistic effects.

Emerging Research

Minocycline and COVID-19

The global COVID-19 pandemic has prompted research into the potential repurposing of existing drugs, including minocycline. Studies have investigated its efficacy in reducing viral load and mitigating the inflammatory response associated with severe COVID-19 cases. Preliminary data suggest that minocycline may help reduce the severity of symptoms and improve clinical outcomes, though more extensive clinical trials are needed.

Updated Data and Current Research (2024)

Antiviral Research

Recent studies have expanded our understanding of minocycline’s antiviral mechanisms and its potential applications. Key findings include:

Calcium Signaling Pathway Regulation: New research has identified additional proteins involved in calcium signaling that are modulated by minocycline. These include calmodulin-like 4 (CALML4), ryanodine receptor 2 (RYR2), and synaptosome-associated protein 25 (SNAP25). By regulating these proteins, minocycline helps maintain calcium homeostasis and inhibit viral replication.
MAPK-ERK Pathway Modulation: Further studies have elucidated how minocycline affects the MAPK-ERK signaling pathway. By increasing levels of fibroblast growth factor 2 (FGF2) and decreasing levels of platelet-derived growth factor receptor α (PDGFRA) and phospholipase C β2 (PLCB2), minocycline effectively inhibits this pathway, reducing viral replication and inflammation.
IL-6 Regulation: Research has continued to highlight the importance of IL-6 in the inflammatory response to viral infections. Minocycline’s ability to down-regulate IL-6 expression provides a dual benefit of antiviral and anti-inflammatory effects.

Clinical Trials

Several clinical trials are currently underway to assess the efficacy of minocycline in treating various viral infections. Key trials include:

Minocycline for TBEV Infection: This trial aims to determine the optimal dosing regimen and assess the long-term outcomes of minocycline treatment in TBEV-infected patients.
Minocycline in COVID-19 Treatment: Ongoing trials are evaluating minocycline’s potential as part of a combination therapy for severe COVID-19 cases. Early results are promising, with reductions in viral load and inflammation observed.

Safety and Side Effects

Minocycline is generally well-tolerated, but like all medications, it can have side effects. Common side effects include gastrointestinal disturbances, photosensitivity, and dizziness. Long-term use can lead to more serious side effects such as autoimmune disorders and hyperpigmentation. Therefore, careful monitoring and appropriate dosing are essential to minimize risks.

Future Directions

Drug Development

The reapplication of clinically approved drugs like minocycline offers a promising avenue for rapid drug development. Future research should focus on:

Mechanistic Studies: Further investigation into the molecular mechanisms underlying minocycline’s antiviral effects will help refine its use and identify potential new therapeutic targets.
Combination Therapies: Exploring the synergistic effects of minocycline with other antiviral agents could enhance its efficacy and reduce the likelihood of resistance.
Clinical Trials: Continued clinical trials are essential to confirm minocycline’s efficacy in various viral infections and to establish standardized treatment protocols.

Minocycline’s broad-spectrum antiviral properties, combined with its ability to modulate key cellular pathways and reduce inflammation, make it a valuable candidate for treating a range of viral infections. Ongoing research and clinical trials will continue to expand our understanding of this versatile drug, potentially leading to new and effective treatments for viral diseases that currently lack specific therapies. By building on existing knowledge and exploring new avenues of research, minocycline could play a significant role in improving global health outcomes.

reference link : https://www.mdpi.com/1999-4915/16/7/1055