Numerous studies have investigated the central nervous system (CNS) involvement in COVID-19. The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was detected in some samples of brain tissue. In contrast, other studies did not detect the virus, but they did detect evidence of widespread immune activation in the brain parenchyma. In this study, a consortium of authors, led by scientists from Germany, used mouse model and postmortem tissue samples from COVID-19 convalescents who died from other causes long after their COVID-19 infection to investigate the presence of the SARS-CoV-2 in the skull-meninges-brain axis.
The SARS-CoV-2 is an enveloped, positive-sense, single-stranded RNA virus. Its genome encodes four structural proteins, namely the spike (S), envelope (E), nucleocapsid (N), and membrane (M) proteins. The S protein is a glycosylated homotrimer that comprises two subunits: S1, responsible for receptor binding, and S2 which mediates membrane fusion. The S1 and S2 subunits are produced by proteolytic cleavage of the full-length S protein. The S1 subunit may interact with epithelial and endothelial cells independently of the virion. It seems that SARS-CoV-2 uses various neuroinvasive strategies and pathways to invade the CNS, such as infection of the nasal olfactory epithelium and axonal transport along the olfactory nerve, retrograde axonal transport, invasion by compromising the blood-brain barrier (BBB), and the use of infected hematopoietic cells as “Trojan horses” (hematogenous route).
The skull bone marrow is characterized as a reservoir for myeloid cells through the channels between the skull marrow and meninges. Also, the CNS border meninges were found to contain a lymphopoietic niche. This suggests that cell reservoirs localized in the skull and meninges are involved in the skull-meninges-brain axis and consequently, in various neurological diseases. A recent study revealed that the bone marrow, as the primary site of hematopoiesis, has an unrecognized role in multiple sclerosis, promoting intimate interactions between autoreactive T-cells that migrate into the bone marrow via the CXCL12- CXCR4 axis, and hematopoietic stem cells and progenitor cells. https://discovermednews.com/autoreactive-t-cells-in-the-bone-marrow-of-ms-patients/
About the Study and Results
After the intravenous injection of fluorescently labeled SARSCoV-2 S1 protein into mice, a distribution of the S1 protein was identified in all tissues targeted by the virus. The S1 protein was accumulated in the skull marrow niches, skull-meninges connection, meninges, and the brain parenchyma, reaching the meninges and the brain parenchyma after 30 minutes. Co-staining for vessel visualization showed that the S1 protein was mostly localized in the brain blood vessels.
Additionally, the S1 binding was found in most organs. High concentrations were found close to the liver, kidney, and lung blood vessels. The S1 protein was also detected in the testis, ovary, spleen, and bone marrow of other bones, such as the tibia and femur.
The intravenous injection of the S1 protein into mice also caused broad proteome changes in the skull marrow, meninges, and brain, showing its immunogenicity in the absence of other viral components. Dysregulated were proteins involved in complement and coagulation cascade, neutrophil degranulation, neutrophil extracellular traps (NETs) formation, and PI3K-AKT signaling pathway with key proteins involved, PI3K (phosphatidylinositol 3-kinase) and Akt (protein kinase B). A dysregulation of the complement and coagulation pathways was observed in both, the skull marrow and the brain. The S1 also increased the expression of amyloid precursor protein in the brain 3 and 28 days after the intravenous injection.
Postmortem examination of tissue samples from COVID-19 convalescents who died from other causes long after their COVID-19 infection revealed that SARS-CoV-2 RNA and the N protein were detected in 8/16 skull samples and 6/12 meninge samples by reverse transcription polymerase chain reaction (rt-PCR) test. All samples of the prefrontal cortex were rt-PCR negative for SARS-CoV-2 N protein. In contrast, the SARSCoV-2 S protein was present not only in the skull marrow niches, skull-meninges connections, and meninges but also in blood vessels and the vicinity of cortical neurons positive for NeuN protein, which is localized in nuclei and perinuclear cytoplasm of most of the neurons in the central nervous system of mammals.
The authors noted that the connection between the skull marrow and meninges very likely contributes to the presence of the S protein in the brain and meninges in the absence of viral load, suggesting that the S protein is a residue from a previous brain infection, or, that highly immunogenic S protein from COVID-19 vaccine is a possible cause that triggers infection-independent effects.
Conclusion
This study showed that SARSCoV-2 S protein was found in the skull-meninges-brain axis in mice after the intravenous injection of fluorescently labeled SARSCoV-2 S1 and human postmortem skull samples from COVID-19 convalescents. Of note, human brain samples were PCR negative for SARS-CoV-2 N protein. The presence of S1 protein in the mouse skull marrow triggered a broad proteomic change in the skull marrow, meninges, and brain.
The findings of this study suggest an alternative route for the SARSCoV-2 S protein entry into the CNS, wherein it might reach first the skull marrow and meninges before entering the brain.
This study has been published on a preprint server and is currently under review.
Journal Reference
Rong Z, Mai H, Kapoor S, et al. SARS-CoV-2 Spike Protein Accumulation in the Skull-Meninges-Brain Axis: Potential Implications for Long-Term Neurological Complications in post-COVID-19. bioRxiv preprint. https://doi.org/10.1101/2023.04.04.535604
