SARS-CoV-2 spike protein induces long-term TLR4-mediated synapse and cognitive loss 

A new study by researchers from the Federal University of Rio de Janeiro- Brazil has found that various cognitive issues and impairment that is manifestated in Long COVID is actually caused by the SARS-CoV-2 spike protein that induces long-term TLR4-mediated synapse issues.

The study findings were published on a preprint server and is currently being peer reviewed.https://www.biorxiv.org/content/10.1101/2022.06.07.495149v2

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is considered a respiratory pathogen, but the impact of the infection on extrapulmonary tissues is of high concern. Coronavirus disease 2019 (COVID-19) is associated with unpredictable and variable outcomes, and while most patients show a positive recovery after the acute stages, others experience a myriad of acute and long term neurological dysfunctions(1).

Cognitive impairment is a well-characterized feature of the post-COVID syndrome, referred to as “long COVID” or brain fog(2). Mounting evidence suggests that COVID-induced neurological symptoms are mediated by multiple mechanisms, including brain hypoxia and systemic inflammation even in patients with mild symptoms(3, 4).

Despite some findings indicating that SARS-CoV-2 can reach and directly impact the brain, others indicate that the virus can rarely cross the blood-brain-barrier (BBB)(5, 6). Nevertheless, whether brain presence of SARS-CoV-2 viral particles and/or its products is a crucial event for the development of cognitive impairment in post-COVID patients remains unknown.

SARS-CoV-2 spike (S) protein plays a pivotal role in COVID-19 pathogenesis and is the main target for vaccine development. This viral surface protein is a homotrimer composed of two functional domains, also known as subunits (S1 and S2), as they are generated by proteolytic cleavage of S protein after virus binding to enzyme 2 angiotensin-converting (ACE2), which mediates cell entry(7).

During SARS-CoV-2 infection, cells produce and release variable amounts of viral particles and proteins, including the S protein(7, 8). The S1 was shown to cross the BBB, reaching different memory-related regions of the brain in a mouse model of SARS-CoV-2 infection(9). Inflammation and increased BBB permeability were also shown in in vitro models of S1 exposure(10).

Likewise, the protein was detected in the central nervous system (CNS) of COVID-19 patients, irrespective of viral RNA detection(11, 12). In addition, increased levels of proinflammatory cytokines and brain gliosis have been reported in severe COVID-19 patients(13,

14). Nonetheless, proof concerning the acute and chronic impact of S protein on COVID-19 brain dysfunction and its underlying mechanisms are still lacking.

Most experimental studies investigating the effects of SARS-CoV-2 have focused on acute infection, especially on peripheral tissues. Few studies have used experimental models to evaluate the possible mechanism of post-COVID syndrome. Here, we developed a mouse model of intracerebroventricular (icv) of S exposure to understand the role of this protein in late cognitive impairment after viral infection. Here, we infused S protein in the brains of mice and performed a long-term (45 days) follow-up of the behavioral, neuropathological, and molecular consequences.

We report late cognitive impairment, synapse loss, and microglial engulfment of presynaptic terminals after icv infusion of S protein. Early TLR4 blockage prevented S-associated detrimental effects on synapse and memory. We also demonstrated that the single nucleotide polymorphism (SNP) rs10759931, linked with increased TLR4 expression is associated with long-term cognitive impairment in mild COVID-19-recovered patients. Collectively, these findings show that S protein impacts the mouse CNS, independent of virus infection, and identify TLR4 as a key mediator and interesting target to investigate the long-term cognitive dysfunction both in humans and rodents.