More than two years after the global COVID-19 pandemic, it is clear that infection with severe acute respiratory syndrome coronavirus type-2 (SARS-CoV-2) can lead to a new disease called post-acute COVID syndrome or long COVID. The most frequent, persistent and disabling symptoms of long COVID are neurological. These symptoms include headaches, visual and olfactory dysfunction, gait disturbances, paresthesia, coordination problems, and cognitive impairments, such as concentration and memory problems and confusion. Their presence results in a long-term impairment of functional ability. https://discovermednews.com/why-does-long-covid-look-like-a-neurological-disease/
There is no doubt that SARS-CoV-2 affects the central nervous system (CNS), but the molecular and cellular mechanisms are still elusive. In vivo studies have shown that SARS-CoV-2 spike protein can enter the brain and cause death of hippocampal neurons through the glial cell activation.
Early studies suspected direct SARS-CoV-2 infection in the CNS, and, based on the first theory, SARS-CoV-2 enters the CNS through migration into the nasal cavity and the olfactory tract.
However, some researchers have speculated that the spike (S) protein enters the CNS by itself, crossing the blood–brain barrier (BBB). This pathogenetic process would go on in the absence of neutralizing antibodies as they do not cross the BBB. The researchers noted that SARS-CoV-2 S proteins are often cleaved from the virus by host cell proteases. Once cleaved, subunits S1 and S2 are not covalently held by disulfide bonds and so S1 could be shed from virions. Shed S1 could cross the BBB without necessarily involving the crossing of intact viral particles.
The findings suggest that the S proteins interact not only with the angiotensin-converting enzyme 2 (ACE2), but also with several other host receptors including neuropilin-1 and CD147. The ACE2 and CD147 receptors are expressed on glial cells, while the ACE2 and neuropilin-1 are relatively highly expressed in the hippocampus compared to other brain regions.
About the studies
The study of Buzhdygan T, et al used postmortem brain tissue to show that the S1 subunit of the S protein changes BBB function in the 2D static and 3D microfluidic models of the human BBB, a platform that more closely resembles the physiological conditions at this CNS interface.
The S1 subunit of the S protein contains a receptor-binding domain that initially binds to cell-surface receptors, setting the stage for viral internalization. The results showed that the S1 subunit of the S protein promoted a loss of BBB integrity. The ACE2 receptors are found in various vessel calibers in the cortex, and, therefore, the authors suggested that the S protein could trigger an inflammatory response in the brain endothelial cells contributing to an altered BBB state.
(Buzhdygan T et al. Neurobiology of Disease, 146 (2020) 105131). https://doi.org/10.1016/j.nbd.2020.105131)
In vivo study of Rhea EM, et al explored whether radioiodinated S1 subunit of the SARS-CoV-2 S protein (I-S1) can cross the BBB after intravenous and intranasal administration in male mice.
The results showed uptake of I-S1 in all tissues. The much higher uptake in liver compared to kidney suggests that I-S1 is cleared from blood predominantly by the liver. The authors noted that a number of receptors probably participate in the uptake of S1.
After intravenous administration, radiolabeled S1 readily crossed the BBB and entered brain regions and the parenchymal space. A dissection of whole brain into 10 regions showed that I-S1 entered all brain regions, with no statistically significant differences between them. The inflammation induced by lipopolysaccharide injection increased the amount of I-S1 that entered the brain. The authors speculated that this increase was due to the disruption of the BBB.
After intranasal administration, I-S1 was found in all brain regions, but I-S1 levels in the olfactory bulb and hypothalamus were higher than in other brain regions. However, whole-brain I-S1 level, expressed as a percentage of the administered dose, was approximately 10 times higher after intravenous injection than after intranasal administration.
(Rhea EM. Nature Neuroscience, 2021, 24, 368–78). https://doi.org/10.1038/s41593-020-00771-8
The in vivo study by Oh J, et al investigated whether the injection of S1 protein in the mouse hippocampus affects brain functions.
The results showed cognitive deficits and anxiety-like behavior due to the death of hippocampal cells. However, the S1 subunits did not directly cause the death of hippocampal cells. The S1 subunits activated hippocampal glial cells to express interleukin-1 beta (IL-1β), which contributed to the death of hippocampal neurons. The same process of inflammatory glia activation, IL-1β expression and subsequent death of neuronal cells have been observed in other neurotropic virus infections like the human immunodeficiency virus type 1 and Japanese encephalitis.
The ACE2 and CD147 receptors that interact with the S1 are expressed on glial cells. In addition, the ACE2 and neuropilin-1 are relatively highly expressed in the hippocampus, which may increase the susceptibility of the hippocampus to the effects of the S1 subunit. The hippocampus is important for cognition, learning and affective functions of the brain. The death of hippocampal neurons can lead to cognitive deficit, memory impairment, anxiety and depression. This could explain why hippocampus-dependent neurological and psychological symptoms were observed in patients with long COVID syndrome, or as a side effects following COVID-19 immunization.
Oh J et al. Scientific Reports 2022; 12: 5496. https://doi.org/10.1038/s41598-022-09410-7