January 12 2021
Neuroinvasion of SARS-CoV-2 in human and mouse brain

Although COVID-19 is considered to be primarily a respiratory disease, SARS-CoV-2 affects multiple organ systems including the central nervous system (CNS). Yet, there is no consensus on the consequences of CNS infections. Here, we used three independent approaches to probe the capacity of SARS-CoV-2 to infect the brain. First, using human brain organoids, we observed clear evidence of infection with accompanying metabolic changes in infected and neighboring neurons. However, no evidence for type I interferon responses was detected. We demonstrate that neuronal infection can be prevented by blocking ACE2 with antibodies or by administering cerebrospinal fluid from a COVID-19 patient. Second, using mice overexpressing human ACE2, we demonstrate SARS-CoV-2 neuroinvasion in vivo. Finally, in autopsies from patients who died of COVID-19, we detect SARS-CoV-2 in cortical neurons and note pathological features associated with infection with minimal immune cell infiltrates. These results provide evidence for the neuroinvasive capacity of SARS-CoV-2 and an unexpected consequence of direct infection of neurons by SARS-CoV-2.

Discussion
We examined the potential for SARS-CoV-2 to infect neural tissues of both mice and human origin and demonstrate potential consequences of its neuroinvasion. Our results suggest that neurological symptoms associated with COVID-19 may be related to consequences of direct viral invasion of the CNS. Specifically, our work experimentally demonstrates that the brain is a site for high replicative potential for SARS-CoV-2. We further show that SARS-CoV-2 causes significant neuronal death in human brain organoids. Using electron microscopy, we identified viral particles budding from the ER, indicating the virus’s ability to use the neuron cell machinery to replicate. Similar to neuronal loss observed in patient autopsies (Solomon et al., 2020), we noticed large numbers of cells dying in the organoid; however, this neuronal death did not colocalize directly with virus infection. Single-cell RNA-seq of the infected organoids showed metabolic changes in neurons without IFN or IFN-stimulated gene signatures, indicating that the neuroinvasive consequence of SARS-CoV-2 is unique compared with other neurotropic viruses such as ZIKV. Closer examination showed diverging metabolic changes in infected versus neighboring cells, suggesting that the infected cells can cause local changes to their microenvironment, affecting survival of nearby cells. It is possible that viral infection induces locally hypoxic regions, which aids in lowering the threshold for tissue damage in the context of an already oxygen-deprived state.

While ACE2 expression levels in the human brain are still being investigated, we showed that ACE2 is expressed at the protein level and is functionally required for SARS-CoV-2 infection in human brain organoids. Further, we detected robust antiviral antibody presence in the CSF of a COVID-19 patient who presented with acute neurological symptoms. This finding suggests that, at least in some patients with COVID-19 and neurological symptoms, there is robust antibody response against the virus within the CSF. In the in vivo setting of the CNS with vasculature and immune cells, neuronal death could have cascading downstream effects in causing and amplifying CNS inflammation.

Although our rodent model does not use endogenous ACE2 expression, it has been previously reported that even mouse-adapted SARS-CoV is still neurotropic in wild-type mice, and SARS-CoV-2 is neurotropic in mice with hACE2 expression from the endogenous locus (Roberts et al., 2007; Sun et al., 2020). Using mouse models, we demonstrate for the first time that SARS-CoV-2 neuroinvasion in mice can have significant remodeling of brain vasculature, providing a potential link between the hypoxia and what we see in both the human organoid and the patient brains.

Similar to previous reports of acute hypoxic ischemic damage without microthrombi in postmortem brain of COVID-19 patients (Solomon et al., 2020), we also found presence of ischemic damage and microinfarcts in postmortem brain samples of COVID-19 patients. In our study, we observed evidence of SARS-CoV-2 infection within the regions of micro–ischemic infarcts, suggesting the possibility of neuroinvasion-associated ischemia and vascular anomalies, consistent with what we observed in mice. However, a limitation of our study is that autopsy samples from only a small number of patients were examined, providing a snapshot of case reports from several patients rather than a generalizable phenomenon. Future studies are needed to examine whether there are other cases of neuroinvasion in the CNS, and the predisposition for such infection. Although we are unable to determine the exact relationship between neuroinvasion and ischemic infarcts, we pose a possible hypothesis from our findings in the patients, mice, and infections of human brain organoids: that SARS-CoV-2 neuroinvasion may cause locally hypoxic regions and disturbance of vasculature, and the disruption of brain vasculature can make vulnerable ischemic infarcts and regions more susceptible to viral invasion (Fig. 8 E). Our findings expand the utility of human brain organoids, beyond modeling fetal brains, and highlight the importance of using a variety of approaches to best model physiology of the human brain.

In future studies, identifying the route of SARS-CoV-2 invasion into the brain, in addition to determining the sequence of infection in different cell types in the CNS, will help validate the temporal relationship between SARS-CoV-2 and ischemic infarcts in patients. It may be through the nasal cavity–to-CNS connection through the cribriform plate, olfactory epithelium and nerve, or viremia, but regardless, the brain should be considered a SARS-CoV-2–susceptible organ system upon respiratory exposure (Baig and Sanders, 2020; Coolen et al., 2020).

Altogether, our study provides clear demonstration that neurons can become a target of SARS-CoV-2 infection, with devastating consequences of localized ischemia in the brain and cell death, highlighting SARS-CoV-2 neurotropism and guiding rational approaches to treatment of patients with neuronal disorders.


March 8, 2021
By Andrew E. Budson, MD, Contributor

As a cognitive behavioral neurologist, I’ve been hearing from many individuals who are complaining of “brain fog” after infection with COVID-19. So I thought it was worth discussing exactly what COVID-19 brain fog is, and some things to do that might help clear it.

Let’s start by trying to understand brain fog. Brain fog is not a medical or scientific term; it is used by individuals to describe how they feel when their thinking is sluggish, fuzzy, and not sharp. We all experience this feeling from time to time. Perhaps you couldn’t think clearly when you were sick with the flu or another illness. Maybe you were jet-lagged and your thinking was sluggish because it felt like it was 2 AM. Or perhaps you took an antihistamine or another medication that made your thinking fuzzy for a few hours. In each case you probably just waited to get back to normal, whether that meant recovering from your illness, adjusting to the new time zone, or waiting for the side effects of the medication to wear off. But what if your thinking didn’t return to normal?

While the COVID-19 pandemic continues to rage in parts of the world, it is slowly retreating in the U.S. There are now three FDA-authorized vaccines, including one for children as young as 12. The vaccines are proving to be nearly as effective in the real world as they were in clinical trials. The CDC has relaxed some prevention measures, particularly for people who are fully vaccinated, and especially outdoors. Meanwhile, scientists continue to explore treatments and to keep an eye on viral variants. Recently I received an email from a man who described how he is still struggling with “cognitive challenges” since recovering from the virus in the spring of 2020. His doctor ran him through a checkup and a battery of tests. Everything was normal, yet his cognitive challenges remain. Like this man, many people who have recovered from the acute, life-threatening effects of COVID-19, but still don’t feel that their thinking and memory are back to normal.

There are many ways that COVID-19 can damage the brain. As I described in a previous blog post, some can be devastating, such as encephalitis, strokes, and lack of oxygen to the brain. But other effects may be more subtle, such as the persistent impairment in sustained attention noted by Chinese researchers. In addition to direct effects on the brain, COVID-19 can also have long-term effects on other organ systems. So-called long haulers can have other lingering symptoms including fatigue, body aches, inability to exercise, headache, and difficulty sleeping. Some of these problems may be due to permanent damage to their lungs, heart, kidneys, or other organs. Damage to these organs — or even just the symptoms by themselves — can impair thinking and memory and cause brain fog. For example, how can you think clearly if you’re feeling fatigued and your body is aching? How can you concentrate if you were up half the night and awoke with a headache?

【読む・観る・理解を深める】
➡ mRNAワクチンに対する懸念を表明して、社会的に抹殺されたかに見えた Dr. Byram Bridle がメディアに復帰してきました。
➡ ワクチン研究で有名な米国ソーク研究所は、コロナワクチンの mRNA が産生するスパイクタンパクが血栓を育成し、健康に害を及ぼす可能性を指摘しました。
➡ メッセンジャーRNAの技術を開発したロバート・マローン博士がスパイクタンパクのリスクについて言及しています。ガセネタであることを祈ります。
➡ カナダのブライドル准教授が「We made a big mistake」とワクチンの危険性を告発し、ロイターは全否定しました。どうしてロイターは、ソーク研究所に確認しなかったんだろう?
➡ コロナ問題やワクチン問題を、科学的・体系的に理解したい方は、「科学的事実①:はじめに」から「新型コロナウイルス感染症に関する科学的事実(第三版:2021.5.24)」をお読みください。