Infection with mosquito-borne alphavirus induces selective loss of dopaminergic neurons, neuroinflammation and widespread protein aggregation

Collin M Bantle,Savannah M Rocha,Aaron T Phillips,Ken E Olson,Ronald B Tjalkens,Richard J Smeyne

doi:10.1038/s41531-019-0090-8

Abstract

Neuroinvasive infections with mosquito-borne alphaviruses such as Western equine encephalitis virus (WEEV) can cause post-encephalitic parkinsonism. To understand the mechanisms underlying these neurological effects, we examined the capacity of WEEV to induce progressive neurodegeneration in outbred CD-1 mice following non-lethal encephalitic infection. Animals were experientally infected with recombinant WEEV expressing firefly luciferase or dsRed (RFP) reporters and the extent of viral replication was controlled using passive immunotherapy. WEEV spread along the neuronal axis from the olfactory bulb to the entorhinal cortex, hippocampus and basal midbrain by 4 days post infection (DPI). Infection caused activation of microglia and astrocytes, selective loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) and neurobehavioral abnormalities. After 8 weeks, surviving mice displayed continued loss of dopamine neurons in the SNpc, lingering glial cell activation and gene expression profiles consistent with a neurodegenerative phenotype. Strikingly, prominent proteinase K-resistant protein aggregates were present in the the entorhinal cortex, hippocampus and basal midbrain that stained positively for phospho-serine129 α-synuclein (SNCA). These results indicate that WEEV may cause lasting neurological deficits through a severe neuroinflammatory response promoting both neuronal injury and protein aggregation in surviving individuals.

Highlights

Parkinson’s disease (PD) is characterized by loss of voluntary motor control due to the degeneration of dopaminergic neurons of the substantia nigra pars compacta (SNpc), associated with oxidative stress, glial cell activation and α-synuclein protein aggregation
To further characterize the neuronal tropism of Western equine encephalitis virus (WEEV), coronal and sagittal sections were stained with anti-tyrosine hydroxylase (TH) and imaged by fluorescence microscopy for colocalization with McRed, which strongly colocalized with TH + dopaminergic neurons in the SNpc (Fig. 2e–n). 3D confocal microscocy of the SNpc revealed widespread expression of dsRed in neurons throughout this nucleus (Fig. 2n)
We found that encephalitic infection with WEEV in outbred CD-1 mice by intranasal inoculation resulted in viral spread from the olfactory tracts to the basal midbrain

Summary

Introduction

Parkinson’s disease (PD) is characterized by loss of voluntary motor control due to the degeneration of dopaminergic neurons of the substantia nigra pars compacta (SNpc), associated with oxidative stress, glial cell activation and α-synuclein protein aggregation. The etiology of PD remains elusive but it is likely linked to interactions between genetic risk-factors, age and environmental stressors, including infectious agents.[1] Viruses, in particular, have been linked to PD and there have been several reports of parkinsonism observed among human survivors of encephalitic viral infection.[2] Exposure of the human central nervous system (CNS) to selected viruses have been shown to induce a phenotype that mimics the neuropathology and neurological dysfunction observed in cases of sporadic PD, and increasing evidence implicates viral infection of the CNS as a potentiating factor in multiple neurodegenerative diseases.[3,4,5] For example, following the 1918 “Spanish Flu” pandemic, nearly every patient who had an acute episode of encephalitis lethargica went on to develop post-encephalitic parkinsonism, a condition with neurological symptoms that closely resemble PD.[2,6,7,8] In addition, many neurotropic mosquito-borne viruses, including the West Nile virus (WNV), Japanese encephalitis virus (JEV), and St. Louis encephalitis virus have been shown to induce PD-like symptoms in humans and rodents.[9] Recent evidence has suggested that over 50% of patients who survive neuroinvasion with WNV exhibit a neurodegenerative phenotype later in life.[10]

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Conclusion