Knowledge continues to evolve from the medical and research community exploring common symptoms of COVID-19 infection, including fever, dry cough, and shortness of breath that seem to quickly escalate in severity in more susceptible populations, for respiratory and nervous system ailments. Neurological symptoms including neuroinflammation, encephalopathy, acute necrotizing (hemorrhagic) encephalopathy (ANE), and ischemic stroke have been observed.
A symptom even common to asymptomatic individuals carrying the infection, referred to as “silent spreaders,” is a loss or attenuation of smell and taste, indicative of a peripheral nervous system (PNS) effect that is now being used to identify and prevent unwanted viral spreading from these carriers.
SARS-CoV-2, similar to SARS and MERS, is one of several taxonomically-related coronaviruses able to infect humans and shown to affect the brain in some people. Researchers have also isolated SARS-CoV-2 from brain tissues using techniques like immunohistochemistry at autopsies, confirming viral infection of the neurons in a few cases. Infection to the central nervous system (CNS) is thought to be a rare event; however, easily surpassing the impact of earlier coronaviruses on the human population, COVID-19 continues to infect millions globally and has been responsible for hundreds of thousands of deaths with no FDA- approved vaccines currently available.
Active areas of investigation
The infectious nature of SARS-CoV-2 and onslaught of global infection have made it difficult to gather cumulative and cohesive data that can speak to COVID-19 and its mode of action that elicits neurological damage to its patients. Pathologists are looking to establish clinical signs and clues that establish SARS-CoV-2 as being a direct contributor of CNS infections or if the symptoms observed are an indirect effect of resulting substantial systemic inflammation by overactive cytokines and immune cells.
Others have sought to explore presence of ACE2 receptors and assess if the virus can be found in the endothelial linings that may implicate the blood-brain barrier (BBB) as the mode of entry (Figure 1).
Figure 1. Graphical abstract, exploring potential routes of HCoV entry into the CNS. https://doi.org/10.3390/v12010014
Recently, a group published a case study of what they determined were viral particles (assuming of SARS-CoV-2) in small vesicles of endothelial cells, indicating a potential pathogen entry-transit across the brain microvascular endothelial cells.
Furthermore, they noted viral particles within vesicles in the cytoplasm of neural cell bodies within the frontal lobe sections as examined by transmission electron microscopy post-mortem of an elderly Hispanic male who presented with a positive SARS-CoV-2 nasopharyngeal swab test. Researchers are reviewing past SARS and MERS studies to help answer questions regarding the neurotropic properties of this new virus using both animal and in vitro models.
Animal models and in vitro tools that may accelerate your COVID-19 research
In vivo animal models are commonly used to provide valuable insight that is currently not easily collected in a controlled manner in human patients, especially amidst pandemics. Among its potential findings are
- Mechanisms of viral entry;
- Discovery of molecular biomarkers, key cellular pathways or qualitative/quantitative phenotypic observations upon CNS infection;
- Elucidation into direct neurological damage by a virus or indirect injury due to secondary inflammation, and;
- Establishment of both short-term and long-term effects of pathogenicity that could supplement knowledge to the frontline and potentially help support recommendations of different therapies based on different presenting symptoms.
Mice are popular animal models for studying human disorders and viral infections. ACE2 has been described as being the receptor to which SARS-CoV-2 attaches to the cell membrane for subsequent infection. It has been described to be abundant in epithelia of the lung, as well as arterial and venous endothelial cells in organs, including the brain.
Researchers are similarly relying on past studies exploring related coronaviruses of the same genus to understand possible mechanisms of neurological infection. In one study, researchers used non-invasive imaging in infected mice using a recombinant HCoV-OC43 containing a luciferase reporter gene injected intranasally (I.N.) into mice and assessed viral propagation using bioluminescence (Figure 2). In a different study, a different viral construct rOC43-ns2DelRluc was inoculated intranasally, and mouse brain and spinal cord were imaged using immunofluorescence staining.
Both studies imply the importance of the potential temporal and spatial insight into viral spread that can be obtained from animal models that may help assess efficacy of treatments, especially those that may in part target the neuroinvasive properties of HCoVs.
Figure 2. Non-invasive in vivo imaging of recombinant HCoV-OC43 with luciferase reporter gene injected intra-nasally into mice. Bioluminescence imaging (BLI) via intraperitoneal injection of D-luciferin was done on the Xenogen VIVO Vision IVIS 100 system (PerkinElmer) to assess the spread of the virus into the brain and spinal cord. Taken directly from Figure 3, https://doi.org/10.3390/v12010014.
Within the research community, many agree that neuroinflammation likely plays a critical role in observed neurological symptoms. The activation of a systemic inflammatory storm with its accompanying cytokines, chemokines, and other biomolecular signals may have a compound role in weakening the BBB, which could further promote neuroinflammation to negatively impact brain homeostasis.
The exact role of other cells, including astroglial cells, microglia cells, astrocytes, et al. in this inflammatory response to viral infection is also an area of active research. Researchers have relied on no wash technologies like amplified luminescent proximity homogeneous assay (ALPHA) and Homogenous Time Resolved Fluorescence (HTRF) assays to detect inflammatory factors in the past when looking at neuroinflammation and treatments that may lessen pro-inflammatory expression levels (Figure 3).
Figure 3. Minocycline has been shown to block microglial activation, and LPS treatment is used in many cell models to elicit a proinflammatory response. Here, authors explored the anti-inflammatory effects of minocycline (MNO) with and without retinol (ROL) on TNF-a and IL-6 synthesis in human microglial-like cells using HTRF assays to measure TNA-a and IL-6 cytokine levels (PerkinElmer). Interestingly, by itself, LPS did not induce a response (a, c) and only after addition of metabolically competent synaptosomal preparations from rat brain as a natural source of RA-degrading enzymes to the cell culture media did they see decreased expression when cells were subjected to LPS-stimulation (b, d). VEH = vehicle. Taken directly from Figure 4, https://doi.org/10.3390/v12010014.
Understanding the role of subtle or clear neurological symptoms may help with development of a potential screen for early neurological signs as a possible predictive factor of a different management plan for those patients not presenting with any CNS-related symptoms. Additionally, the link between SARS-CoV-2 and the CNS is important to further understand for the establishment of effective treatments, especially for drugs which may not easily penetrate the BBB.
As we continue to research the mechanisms behind SARS-CoV-2 and its global impact, understanding the extent of CNS involvement in COVID-19 infections will be critical in supporting both immediate as well as long-term health consequences.
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