SARS-CoV-2 spike protein induces TLR4-mediated long-term cognitive dysfunction recapitulating post-COVID syndrome in mice, 2023

Brazilian team

Introduction

There does seem to be evidence that virus particles do get into the brain, and for some time after the acute infection.

The paper did not say in the abstract or Introduction, but I'm assuming TLRs is Toll-like receptors. So, they are suggesting there is evidence for the CoV-2 spike protein binding to TLR4, which then produces an inflammatory response.


Results - injection of spike protein into mice
Temporary reduction in cognitive performance

Mice infused with the CoV-2 protein performed ok on what is described as a cognitive task 6 or 7 days after infusion (See Figure 1B), but didn't later (Figures 1C, D), whereas the control mice performed ok throughout. They say that by 60 days (1E), the spike protein treated mice had regained their cognitive ability.



The authors suggest that this is a pure cognition test, rather than related to a general sickness behaviour, as measures such as time spent exploring the objects weren't different. The results do seem to reflect what they report; the asterisks indicate significant differences between the time spent exploring novel objects versus familiar objects ( in charts B and E for the spike protein treatment, in all the charts for the control treatment).

They also administered the spike protein subcutaneously (rather than direct into the brain) and report that the results of the memory test were similar. They also tested the mice with a water maze; they report similar results.


Reduction in synaptic density
They looked to see if there was a loss of synaptic density in the hippocampus. They didn't find any differences compared to the controls in the early stages (Fig 1J-N), but did later (Fig 1O-S). They found that synaptic loss was not caused by neuron cell death. I don't know what co-localised puncta is, but there does seem to be a significant difference between the result for the treated group at the early time (N) and the later time (S).



I'm going to take a break. There is a lot in this paper. Of course a big question is 'why do the effects we see in Long Covid often persist beyond the 60 days when the mice in these experiments seem to come right during that time?'.

I should really read the whole paper before I start to post about it, to produce a coherent summary but I suspect that if I did that, I wouldn't end up posting much. So, you get a sort of stream of consciousness, a record of what I find interesting as I read through.

 

I suppose potentially it might. The researchers tried a lower dose of the protein (10-fold smaller), and it didn't affect memory at all, so dose seems important. I haven't looked at and thought about what dose they used, but it's possible that the dosage used in this study in mice is not a reasonable model for what happens in humans during a Covid-19 infection except in the most extreme of circumstances.

If a noticeable impact on cognition was a major risk from vaccination, I think we would have heard, even if, as here in mice, it was a fairly short-lived effect.

 

The spike protein triggers neuroinflammation
The researchers tried applying the protein directly to the neutrons - that didn't seem to injure the neurons or affect synaptic density.

In the early stage after injection of the spike protein into the brain didn't change the number or shape of microglia, or increased expression of a range of inflammatory genes (TNF, IL-1b, IL-6, INF-b, IFNAR1) but levels of IFNAR2 mRNA did decrease. At the late stage, mRNA levels of TNF, IL-1b, IFNa and IFNb and IFNAR2 increased in the hippocampus. Protein levels of TNF and IL-1b were higher in the hippocampus too. Hippocampal expression of IL-6 and IFN-y cytokines and the receptor IFNAR1 were not affected. They also found increased serum levels of TNF at the late stage which returned to normal by day 60.

They looked at sections of the hippocampus to see if there was evidence of gliosis (glial cell types include microglia and astrocytes; gliosis is when glial cells proliferate in injured areas of the brain). There was no evidence of microgliosis at the early stage, but, at the late stage, they found an increased number of Iba-1-positive cells (Fig. 2K-M) and a predominance of cells with ameboid morphology in the hippocampus (Fig. 2K,L, N), as well as significantly higher TMEM119 immunoreactivity in the DG hippocampal subregion of Spike-infused mice (Supplementary Fig. 5N-P).

They concluded that the cognitive impairment that they believe happens after infusion of the spike protein is accompanied by microglial activation and neuroinflammation.

Synaptic phagocytosis by microglia
They found evidence that activated microglia are pruning off and internalising synapse, as is known to occur in dementia and viral encephalitis.

This is Figure 3A, 3B. The images are from the late stage. In 3A, from mice hippocampus tissue treated with the control infusion, the synapse is intact, and the microglia is branched and looking normal. In 3b, which had the spike infusion, it is said that synapse bits are inside a microglia. Yeah, if you squint it's plausible.


Here are measures of co-location of the synapse and microglia markers in two different parts of the hippocampus (C and D). They also looked at concentrations of complement 1q (C1q), which they say is involved in the tagging of synapses for engulfment by the microglia. They found no increase of C1q at the early stage, but a big increase at the late stage.



So, so far, so good. But it's not terribly surprising that brains react when you inject a pathogen protein into them, or even that cognitive function is associated with that. The big news so far, I think, is that sub-dermal infusion can possibly cause the same issues, that the pathogen protein crosses the BBB. And still we don't know if the problem could possibly become chronic (although this paper might not address that).

It might worth looking to see if there are any ME/CFS studies relevant to these findings - I'm not sure how much microglial activation has been found.

 

Just wanted to park this diagram here, as it shows how a pathogen or pathogenic material (the LPS at the top left) can impact TLR4 (mentioned in this paper), and how that could then go on to activate the itaconate shunt, down regulating glycolysis.

Source: The role of itaconate in host defense and inflammation