Neurodegenerative diseases: the virus-brain connection

Researchers Isidoro Martínez González and Salvador Resino García, both from the Carlos III Health Institute (ISCIII), have published an article where scientific evidence links the presence of certain viruses with diseases such as Alzheimer's, Parkinson's and ALS

The human brain is a complex organ and vulnerable to external threats, and pathogenic viruses are no exception. In fact, in recent years, scientific studies have shown that there is a strong connection between viral infections and neurodegenerative diseases, such as Alzheimer's, Parkinson's, and amyotrophic lateral sclerosis (ALS).

These diseases are characterized by the progressive loss of neurons, leading to cognitive decline, motor problems, and eventually death. Although aging, genetics, and environmental factors contribute to their development, viral infections may play a major role in their development. This suggests that viruses are not just temporary invaders, but can also cause long-term neurological damage.

Neurodegenerative mechanisms

Viruses can cause neurodegeneration through various mechanisms, both direct and indirect:

• Direct infection: Some viruses, such as rabies and polio, can invade and directly damage neurons, causing their death.
• Neuroinflammation: Viral infections often trigger an immune response, causing chronic inflammation. Immune cells in the brain, such as the microglia, release inflammatory substances that damage neurons. This prolonged inflammation can promote the accumulation of misfolded proteins, such as those found in Alzheimer's disease and Parkinson's disease.

• Mitochondrial dysfunction: Mitochondria – the “powerhouses” of cells – are essential for neuronal health. Viruses such as herpes simplex (HSV-1), the HIV and the Zika can alter their function, causing oxidative stress, DNA damage and problems with energy production. This contributes to neuronal death and cognitive decline.
• Protein misfolding: Viruses can interfere with the mechanisms that control the quality of proteins in cells. For example, HSV-1 is able to alter the autophagy, a process that removes toxic proteins, leading to their accumulation in neurons.
• Disruption of the blood-brain barrier: many viruses can damage this barrier protective of the brain, allowing the entry of harmful substances and immune cells, which worsens inflammation.
• Immune evasion and persistence: Some viruses, such as Epstein-Barr virus (VEB) and the cytomegalovirus (CMV), can remain dormant in the brain, periodically reactivating and causing damage. This persistent viral presence can lead to chronic inflammation and neurodegeneration.

The role of viruses in Alzheimer's

La Alzheimer disease, the most common neurodegenerative condition, is characterized by the accumulation of amyloid-beta plaques and tau tangles.

Some studies suggest that HSV-1, a common cause of cold sores, could accelerate the development of Alzheimer'sThis virus, which remains latent in neurons, has been found in amyloid plaques, indicating that it could be related to their formation. In addition, it can cause hyperphosphorylation of the tau protein, a key process in Alzheimer's, and trigger a chronic inflammatory response in the brain, with the potential to damage neurons and contribute to neurodegeneration.

Secondly, influenza viruses and SARS-CoV-2, which causes Covid-19, have also been linked to Alzheimer's-like symptoms. In patients with Covid-19, elevated levels of biomarkers associated with neurodegenerative disease have been observed, suggesting possible long-term neurological consequences.

And HIV, for its part, is associated with neurocognitive disorders that share characteristics with Alzheimer's.

Parkinson's Disease

El parkinson's It is characterized by the loss of dopamine-producing (dopaminergic) neurons and the presence of Lewy bodies, formed by aggregates of the protein alpha-synuclein.

Viruses such as hepatitis C (HCV), Epstein-Barr (EBV) and influenza A have been related with a higher risk of developing this disease. Neuroinflammation, neuronal toxicity and an autoimmune response that attacks dopaminergic neurons are among the mechanisms through which viruses could contribute to its appearance.

Amyotrophic lateral sclerosis (ALS)

La ELA It is a neurodegenerative disease that damages motor neurons, the nerve cells that control our muscles. Although the exact causes are not fully understood, viral infections have been shown to play a role in its development.

An interesting finding is the involvement of human endogenous retroviruses (HERVs), ancient viral remnants that integrated into our genome thousands of years ago. Normally, they remain dormant, but in people with ALS, a specific type of HERV, the HERV-K, is more active, suggesting that the resuscitation of these viruses could contribute to the death of motor neurons.

In addition, viruses such as Coxsackievirus B3 (CVB3) have been linked to the disease. Animal studies show that CVB3 can trigger immune responses that lead to motor neuron degeneration.

Finally, the COVID-2 It has also been associated with faster progression of ALS in some patients, reinforcing the link between viral infections and neurodegenerative conditions.

Some researchers believe that viruses could trigger an autoimmune response in which the immune system mistakenly attacks cells in the nervous system, accelerating neurodegeneration. For example, it is thought that multiple sclerosis It is triggered when immune cells attack the myelin sheath surrounding nerve fibers. This damage interferes with the normal transmission of electrical impulses along the nerves.

Implications for treatment and prevention

Understanding the viral role in neurodegeneration opens up new possibilities for prevention and treatment. Vaccination against viruses such as influenza, herpesviruses and SARS-CoV-2 could reduce the risk of neurodegenerative diseases associated with these pathogens. In addition, antiviral drugs, such as acyclovir for HSV-1, have shown promise in reducing the risk of Alzheimer's.

Anti-inflammatory and immunomodulatory treatments may also help slow the progression of these conditions. Future research should focus on developing biomarkers to detect early virus-induced neurodegeneration and exploring new antiviral strategies to protect the brain in the long term.

In conclusion, the evidence linking viral infections to neurodegenerative diseases suggests that viruses are not just a temporary threat, but can leave a lasting impact on the nervous system. By further investigating these connections, scientists could discover new ways to prevent and treat these ailments, improving the quality of life for millions of people around the world.

Source: The Conversation