Viral infections have been implicated in precipitating neurological sequelae, including symptoms of parkinsonism. Mechanisms of viral encephalopathies proposed include viral affinity to the midbrain catecholaminergic neurons in the substantia nigra and locus coeruleus, which are lost in Parkinson’s disease; and indirect action of the viral infection through inflammatory cytokines or glia activation.

Study: COVID-19 infection enhances susceptibility to oxidative-stress induced parkinsonism. Image Credit: sruilk/Shutterstock

In the context of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the precise etiology (direct or indirect) for post-encephalic parkinsonism has not been ascertained yet. However, previous preclinical studies have suggested that SARS-CoV-2 infection is a predisposing factor for the development of Parkinson’s disease.

The study

A recent study published in bioRxiv* preprint server explored whether individuals who have recovered from mild to moderate or severe coronavirus disease 2019 (COVID-19) may have an increased risk for developing Parkinson’s disease in the future.

For this study, SARS-CoV-2 (isolate USA-WA1/2020), obtained from bei RESOURCES (Manassas, VA), was propagated in Vero cells using a 10-30% sucrose gradient in an ultracentrifuge. The yield was titrated using the classic Reed and Muench formula.

Here, SARS-CoV2 susceptible mice (B6.Cg-Tg(K18-ACE2)2Prlmn/J, Strain #034860, hACE2 mice) were randomly assigned to a dose-finding study and then to four study arms. First, in a dose-finding study, mice were tested at three doses of virus, and the optimal dose was selected. Overall, 18 mice were intranasally infected with SARS-CoV-2 at the optimal study dose (strain USA-1), while eleven were subjected to a sham procedure.

After 31 days, six recovered SARS-CoV2 and five sham-treated mice were challenged with a mitochondrial toxin – 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), known to induce some of the characteristic pathologies of Parkinson’s disease.

Findings

Tests for antibody titers 45 days after infection revealed significant antibody response in both groups (naïve and MPTP-treated) infected with SARS-CoV-2. In comparison, there was no detectable antibody titer in animals who were intranasally administered saline alone.

A subtoxic dosage of MPTP (10mg/kg x 4, ip injections every 2 hours) empirically induced a small inflammatory response but no dopamine neuron death. The effects of these subtoxic levels of MPTP on inflammation and dopaminergic neuron death were examined in the animals infected with the moderate dose. No SNpc DA neuron loss could be detected after saline, SARS-CoV-2 alone, or MPTP alone. The subtoxic MPTP dose induced a significant – 23% or 19%, greater loss of substantia nigra pars compacta dopaminergic neurons in mice infected with 4 x 103 TCID50 SARS-CoV-2.

A quantitative, stereological analysis was conducted to analyze the neuroinflammatory response induced by SARS-CoV-2 of the total, resting, and activated microglia in the SNpc 45 days after SARS-CoV-2 infection. The results revealed no change in the number of microglia in any of the experimental groups after SARS-CoV-2 infection. MPTP treatment after SARS-CoV-2 infection rendered a three-fold increase in activated microglia.

It was also found that SARS-CoV-2 infection alone did not induce central nervous system (CNS) inflammation nor SNpc neuron death. These results suggested this virus could not directly precipitate Parkinson’s disease. Nevertheless, systemic infection sensitizes the SNpc dopaminergic neurons to mitochondrial stress, which cannot induce neuron loss. This post-infection sensitization of the SNpc dopaminergic neurons persists transiently after the resolution of the viral infection without any physical manifestation of a direct viral inflammatory effect in the SNpc.

The authors stated that it is important to evaluate the sensitivity of this mechanism to viral load and its heterogeneity across variants. Furthermore, it is essential to investigate whether this viral sensitization can be mitigated by any of the current treatments for COVID-19 infection.

From the examples of H1N1 vaccination and immediate Oseltamivir phosphate treatment, vaccination and antiviral therapies could modify this mechanism. Nonetheless, for the large global population who survived COVID-19 without vaccination, the future consequences of the infection have to be determined. It was suggested that healthcare personnel and governments must prepare for long-term COVID-19 complications.

*Important notice

bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.