From Entry to Dysfunction: SARS-CoV-2 Neuropathogenesis in Peripheral Sensory and Autonomic Neurons and Its Implications

Abstract

During the COVID-19 pandemic, reports of neurological symptoms affecting the central nervous system (CNS) and peripheral nervous system (PNS) became common, suggesting the virus may be neuroinvasive. These observations prompted studies to determine how SARS-CoV-2, the causative virus, invades the nervous system. Early studies focused exclusively on CNS invasion and found that SARS-CoV-2 infects sustentacular cells and olfactory sensory neurons in the nasal neuroepithelium, enters the olfactory bulb, and spreads throughout the brain. Autopsy studies later suggested that viral entry is also mediated by failure of the blood brain barrier due to inflammation. Missing from these studies was an assessment of the susceptibility of the PNS to infection and to its role in CNS invasion. In this dissertation I show that both sensory and autonomic neurons in the PNS are susceptible to SARS-CoV-2 infection, with replication occurring in neurons, satellite glial cells that support these neurons, and functionally connected CNS tissues. The studies herein show that SARS-CoV-2 invades the PNS directly, before the onset of viremia, and establishes a productive transient infection within peripheral neurons. Our studies in K18-hACE2 mice, wild-type mice, and golden Syrian hamsters, as well as primary neuronal cultures, show the presence of viral RNA, proteins, and infectious virus in PNS neurons and supporting cells as early as 18 hours post infection. Given that infection occurred in neurons not expressing human angiotensin converting enzyme 2 (hACE2), the host protein used as a receptor for viral entry, we identify neuropilin-1 (NRP-1) as a factor facilitating viral entry into these neurons. As the COVID-19 pandemic progressed, numerous variants of SARS-CoV-2 emerged, calling into question the impact of viral variants on the neuroinvasive potential of these variants relative to ancestral SARS-CoV-2. This dissertation further shows, using SARS-CoV-2 XBB1.5, a contemporary Omicron variant, that the virus establishes a productive infection with distinct replication kinetics that is more attenuated when compared to ancestral WA1/2020. These findings show that SARS-CoV-2 replication dynamics in the PNS may differ across variants, potentially influencing severity and persistence of neurological symptoms. Additionally, as the COVID-19 pandemic progressed, numerous animal models were tested for their suitability to study SARS-CoV-2. This dissertation reports results on the susceptibility of guinea pigs to intranasal infection with SARS-CoV-2 as a model for studying PNS involvement in COVID-19. Despite their utility in studying SARS-CoV-1, we found guinea pigs are resistant to intranasal infection with ancestral SARS-CoV-2 WA1/2020, highlighting limitations in their use for investigating neurological sequelae associated with SARS-CoV-2 infection and calling into question their continued use for evaluation of SARS-CoV-2 therapeutics. This dissertation provides compelling evidence that SARS-CoV-2 directly infects the PNS and establishes a productive infection in peripheral neurons, leading to sensory impairments. This work lays the foundation for future study to determine how neuronal infection contributes to the broader spectrum of neurological symptoms observed in COVID-19, including trigeminal neuralgia, Horner syndrome, and radicular pain. These insights should inform future therapeutic strategies to mitigate the neurological sequelae of infection with SARS-CoV-2 as it transitions from a pandemic virus to endemicity.