Keywords: SARS-Cov-2 spike protein; mRNA vaccines; prion and prion-like diseases; p53; Wip1; autophagy; aging; senescence; COVID-19.
1. Introduction and Background
A significant percentage of patients suffering from SARS-CoV-2 develop
neurological and cognitive impairments, sometimes lasting long after the
infection has cleared. This condition has been named “long haul COVID
disease,” or simply “long COVID,” also known as “PASC” (Post-Acute
Sequelae of SARS CoV-2 infection). An international study quantified
persistent long-COVID symptoms among 3,762 individuals following a
SARS-CoV-2 infection. These authors wrote: “Memory and cognitive
dysfunction, experienced by over 88% of respondents, were the most
pervasive and persisting neurologic symptoms in this cohort, equally
common across all ages, and with substantial impact on work and daily
life. Memory and cognitive dysfunction, together with other commonly
reported neuropsychiatric symptoms, may point to larger neurological
issues involving both the central and peripheral nervous system”
[1]. A post-mortem study of the brains of three patients who died
from severe COVID-19 showed a large number of activated microglia that
were associated with overexpression of inflammatory markers, including
Interleukin-1β (IL-1β) and IL-6. The authors suggested that oxidative
stress induced a glial-mediated neuroinflammatory response leading to
neuronal injury [2].
A growing consensus attributes these symptoms to neurotoxic effects of
the spike glycoprotein, particularly the S1 subunit [3]. The
receptor-binding domain of SARS-CoV-2 spike S1 protein binds to heparin
and to heparin-binding proteins [4]. Idrees and Kumar wrote in their
conclusion: ”Our results indicate stable binding of the S1 protein to
these aggregation-prone proteins which might initiate aggregation of
brain protein and accelerate neurodegeneration” [4]. A study
evaluating the amyloidogenic potential of the spike protein verified
that the spike protein can cause amyloid-like fibrils to appear after
the protein has been subjected to proteolysis. A specific segment that
appeared following proteolysis, spike 194-213 (FKNIDGYFKI), was
demonstrated both theoretically and experimentally to be amyloidogenic
[5]. A study by Kruger et al. found that proteolysis
resistant fibrin amyloid microclots accumulate in the blood in
association with PASC, and this also suggests that the spike protein has
amyloidogenic properties [6].
Direct experimental evidence of S1’s toxic effects in the brain comes
from studies conducted by a team of Korean researchers, published in
2022 [7]. In the experiment, S1 subunits were introduced directly
into the dorsal hippocampus of mice, and it was shown that the mice
subsequently suffered from anxiety-like behavior and cognitive deficits.
Further experiments both in vivo and in vitro found that
the effects were mediated by microglia, which became activated following
exposure. The microglia released excitatory cytokines, in particular
IL-1β. IL-1β expression was upregulated more than seven-fold in the
hippocampi of the exposed mice. Morphologically, the microglia of the
exposed mice acquired the features of reactive microglia.
In this paper, we attempt to trace the likely biological pathways by
which neuronal damage occurs in response to the spike protein,
particularly S1. We will argue based on the emerging literature that
toll-like receptor 4 signaling is central to the destructive reaction
process. An important intermediary is the MAPK cascade. MAPK comprises
four distinct pathways, a) the extracellular signal regulated kinase 1
and 2 (ERK1/2), b) the ERK-big MAP kinase 1(BMK1), c) the c-Jun
NH2-terminal kinases (JNK) or stress activated protein kinases (SAPKs),
and d) the p38 MAPKs. The ERK pathways are stimulated by growth factors,
hormones and pro-inflammatory stimuli whereas the JNK and p38 MAPK are
activated by cellular and environmental stress signals in addition to
pro-inflammatory stimuli [8,9]. It is these latter two pathways that
we will argue play a primary role in spike protein neurotoxicity.
Recent neurotoxicity studies indicate that the SARS-CoV-2 S1 subunit
induces neuro-inflammation in microglial cells, a special type of
macrophage in the central nervous system (CNS) [10,11]. The
neuroinflammatory response is mediated by p38 MAPK and nuclear factor
κ-light chain enhancer of activated B cells (NF-κB) activation, mainly
through the pattern recognition receptor TLR4. In addition, the SARS
CoV-2 S1 subunit elicits a pro-inflammatory response in murine and human
macrophages by activating TLR4 receptor signaling. In this signaling
process, both JNK and p38 are activated by phosphorylation [12]. It
is important to note that infectious prions also activate the p38 MAPK
pathway to induce their neurotoxicity effects [13]. The spike
protein has prion-like characteristics that may contribute to its
neurotoxicity. We will return to this topic in great detail later.