Authors: Gerard J. NuovoCynthia MagroToni ShafferHamdy AwadDavid SusterSheridan MikhailBing HeJean-Jacques MichailleBenjamin LiechtyEsmerina Tili Annals of Diagnostic Pathology
ABS T R A C T
Neurologic complications of symptomatic COVID-19 are common. Brain tissues from 13 autopsies of people who died of COVID-19 were examined. Cultured endothelial and neuronal cells were incubated with and wild type mice were injected IV with different spike subunits. In situ analyses were used to detect SARS-CoV-2 proteins and he host response. In 13/13 brains from fatal COVID-19, A (spike, envelope, and membrane proteins
without viral RNA) were present in the endothelia of microvessels ranging from 0 to 14 ositive cells/200× field (mean 4.3). The pseudo virions strongly co-localized with caspase-3, ACE2, IL6, TNFα, and C5b-9. The sur- rounding neurons demonstrated increased NMDAR2 and neuronal NOS plus decreased MFSD2a and SHIP1 proteins. Tail vein injection of the full length S1 spike subunit in mice led to neurologic signs (increased thirst, stressed behavior) not evident in those injected with the S2 subunit. The S1 subunit localized to the endothelia of micro vessels in the mice brain and showed co-localization with caspase-3, ACE2, IL6, TNFα, and C5b-9. The surrounding neurons showed increased neuronal NOS and decreased MFSD2a. It is concluded that ACE2+ endothelial damage is a central part of SARS-CoV2 pathology and may be induced by the spike protein alone. Thus, the diagnostic pathologist can use either hematoxylin and eosin stain or immunohistochemistry for caspase 3 and ACE2 to document the endothelial cell damage of COVID-19.
- Introduction
Neurologic complications are now well recognized in symptomatic COVID-19 [1-5]. Estimates of neurologic findings in COVID-19 range from 7% in a study of hospitalized patients in Wuhan China to 69% among ICU patients in France [5]. The neurologic symptoms and signs nclude lethargy, delirium, confusion, irritability, and difficulty
concentrating which may persist for months [1-5]. Possible etiologies for the,neurologic manifestations include micro- and macro ischemic strokes associated with the pro-thrombotic state of severe COVID-19[1,6,7], the viral-associated cytokine storm [2], direct neuronal invasion [3,4], and so-called bystander injury to neurons [3,4]. Although direct infection of SARS-CoV2 in the brain and neurons has been re-
ported, most studies have documented that viral RNA is rarely detected in the brain or CSF of people with neurologic manifestations of COVID-9 [3,4].Our group has reported that the viral pneumonia that is the essential event behind moderate/severe COVID-19 is associated with a large amount of infectious virus in the lung that infects primarily alve olarmacrophages and endothelial cells and, to a lesser degree, alveolar
pneumocytes [6-8]. This infection induces the alveolar capillaries toorm thromboses due to complement activation, which is called micro-angiopathy [6-8]. It has been postulated that the subsequent death of the intrapulmonary virus releases pseudovirions (spike, envelope, and☆ No external funding. The authors have no conflicts of interest to report.
- Corresponding author at: GNOME Diagnotics, 1476 Manning Parkway, Powell, OH, USA.
E-mail address: nuovo.1@osu.edu (G.J. Nuovo).
Contents lists available at ScienceDirect
Annals of Diagnostic Pathology
journal homepage: www.elsevier.com/locate/anndiagpath
https://doi.org/10.1016/j.anndiagpath.2020.151682
Gerard J. NuovoCynthia MagroToni ShafferHamdy AwadDavid SusterSheridan MikhailBing HeJean-Jacques MichailleBenjamin LiechtyEsmerina Tili1-551-516723434346-86-8
Annals of Diagnostic Pathology 51 (2021) 151682
2membrane proteins) disassociated from the viral RNA. The circulating
pseudovirions can then attach to endothelia with the ACE2 receptor via
the spike protein. It has been documented that the microvascular beds
with the greatest number of ACE2 receptors in endothelia include, be-
sides the lung, the skin’s deeper dermal vessels/subcutaneous fat, brain,
and liver [6-8]. Various investigators have shown that the endocytosed
spike glycoprotein, even in the absence of viral RNA, can induce a
caspase-3 mediated cell death, complement activation which could lead
to a hypercoagulable state, and the increase of many cytokines/proteins
associated with severe COVID-19, including TNFα, IL6, IL8, IL1β, and
p38 [6-9]. It is possible that the neurologic manifestations of moderate/
severe COVID-19 may reflect a microencephalitis induced by comple-
ment and cytokine mediated endothelial damage triggered by circu-
lating pseudovirions. This study examined the effects of various subunits
of the spike protein alone (without infectious virus) on human endo-
thelial cells in culture and after intravenous injection into mice as well as
performing a comprehensive analysis of thirteen brains obtained from
people with neurologic manifestations who died of COVID-19. The key
finding of this study for the diagnostic anatomic pathologist is that
endothelial cell damage, which can be visible on hematoxylin and eosin
stains or with immunohistochemistry for caspase 3, is highly correlated
with the pathophysiology of COVID-19.
- Methods
2.1. Formalin-fixed, paraffin embedded human brain samples
Autopsy brain tissues were available from 13 patients who died of
COVID-19. They ranged in age from 28 to 92 (mean 69; 7 men and 6
women) and had neurologic symptoms/signs that included lethargy,
confusion, and irritability. The formalin fixed, paraffin embedded tis-
sues were from the frontal or temporal lobe, the hippocampus, midbrain,
or the pons. Ten aged matched cases from patients who had died of non-
neurologic diseases prior to 2019 served as negative controls; CNS
findings were described as unremarkable.
2.2. Immunohistochemistry
Immunohistochemistry was done as per a previously published
protocol [8,10]. In brief, optimal conditions for each antibody were
determined by testing various dilutions and pretreatment conditions.
The antibodies used follow; ABCAM, Cambridge MA (IL6, TNFα,
NMDAR2, SHIP1, caspase 3, and C5b-9), Enzo Life Sciences (neuronal
NOS, MFSD2a), ACE2 (PROSCI, Poway CA) (cat # 3215) and optimal
pretreatment in each case was 30 min with the Leica EDTA antigen
retrieval solution.
Detection of the SARS-CoV-2 spike glycoprotein, membrane and/or
envelope proteins was as previously described [8,10]. In brief, the Leica
Bond Max automated platform was used with the primary antibodies
(PROSCI) (membrane, cat # 3527), (spike; cat # 3525) and (envelope;
cat # 3521) after antigen retrieval for 30 min. The specific PROSCI
antibodies used to detect the S1 subunit, the truncated S1 subunit, and
the S2 subunit were: 9083, 9087, and 9123, respectively. The HRP
conjugate from Enzo Life Sciences (Farmingdale, New York, USA) was
used in place of the equivalent reagent from Leica as this has been shown
to substantially reduce background [10].
2.3. In situ hybridization
Detection of SARS-CoV-2 RNA was done using the ACD RNAscope
(Newark, California, USA) probe (Cat No. 848561-C3) through a pre-
viously published protocol [8,10].
2.4. Co-expression and statistical analyses
Co-expression analyses were done using the Nuance/InForm system
whereby each chromogenic signal is separated, converted to a
fluorescence-based signal, then mixed to determine what percentage of
cells were expressing the two proteins of interest as previously described
[8,10].
The number of positive cells/200× field was counted with the
InForm software or manually in 10 fields/tissue. Manual and InForm
computer-based counts were equivalent. Statistical analysis was done
using the InStat Statistical Analysis Software (version 3.36) and a paired
t-test (also referred to as a “repeated measure t-test”). The null hy-
pothesis was rejected if the significance level was below 5%.
2.5. Recombinant spike proteins
The recombinant spike proteins, all from PROSCI, were: Val16-
Arg685 (cat #10-300) = full length S1 subunit; Arg319-Phe541 (10-
303) = truncated S1 subunit (contains only the receptor binding
domain) and full length S2 subunit = (Ser686–Pro1273 (10-426).
2.6. Cell culture
HUVEC (human umbilical vein/vascular endothelium) (CRL-1730)
and RAW 264.7 (ATCC® TIB-71™) murine macrophage cells, was pur-
chased from the ATCC. Motor Neuron-Like Cell line MN1 (also known as
NSC-34) is a hybrid cell line produced by the fusion of motor neurons
from the spinal cords of mouse embryos with mouse neuroblastoma cells
N18TG2 [11]. The cells were grown in RPMI-1640 Medium, 5% heat
inactivated FBS, and Penicillin/Streptomycin used at 1×.
2.7. Mouse IV injection studies
Nine months old female C57BL/6 mice (purchased from Jackson
laboratory, Maine) were tail vein injected with different spike peptides
and euthanized using CO2 after five days. Ten μg/mouse of Spike S1 and
ten μg/mouse of S1peptide that contains only the Receptor binding
domain (RBD) were injected in 3 mice per group, except for the controls
(n = 4). Spike S2 was used at 3 μg /mouse. The peptides were diluted in
PBS; the total volume of injection was 150 μl. Mice were monitored
every 12 h. Blood smears were prepared right after euthanasia and fixed
in formalin overnight; the organs were fixed in 10% buffered formalin
for 3 days. All procedures were approved by and performed in accor-
dance with The Ohio State University’s Institutional Lab Animal Care
and Use Committee (Columbus, Ohio). - Results
3.1. Human COVID-19 brain findings
The molecular and histologic changes were analyzed in the 26
COVID-19 brain tissues from 13 patients (2 tissues/person) and 10 age-
matched controls in a blinded fashion. The hematoxylin and eosin
findings revealed two differences: 1) Microvessels of the brain of COVID-
19 patients showed perivascular edema as defined by a zone of edema at
least 50 μm in size around a capillary. 2) Less commonly, microvessels in
the COVID-19 cases had endothelial cells that were degenerated,
mummified, and detached, showed focal basement membrane duplica-
tion, and/or microthrombi. The perivascular edema was evident in be-
tween 5.3% and 25.9% of the microvessels in the COVID-19 brains
(Fig. 1).
Next, the distribution of the infectious virus versus the pseudovirion
(defined as spike protein with other capsid proteins without viral RNA)
was analyzed. In eight cases, autopsy lung tissue was also available and,
in each case, both viral RNA and viral capsid proteins were detected.
Viral RNA was detected in 2/26 brain tissues in the COVID-19 patients
and, in each case, was seen in less than 3 cells/cm; the cells had the
cytology of microglia (data not shown). Viral spike protein was detected
in 26/26 brain tissues and none of the controls. Over 95% of the cells
G.J. Nuovo et al.
6-86-98108101081081011Fig. 1
Annals of Diagnostic Pathology 51 (2021) 151682
3with spike protein were the endothelial cells of microvessels (Fig. 1)
(Table 1). Scattered cells directly outside the affected microvessels had
detectable spike protein (Fig. 1). Serial section analyses (tissues four
microns apart) of spike protein and ACE2 showed the same distribution
(Fig. 1). Co-expression data confirmed that spike and ACE2 strongly co-
localized. Viral spike, envelope, and membrane proteins showed the
same distribution and co-localized (data not shown).
The histologic findings suggested that endocytosis of the spike pro-
tein could induce endothelial cell damage. The cases and controls were
each tested for activated caspase-3 and read in a blinded fashion
(Table 1). Note the highly significant correlation between caspase-3
expression and SARS-CoV2 capsid protein within the brain micro-
vessels. Co-expression confirmed that caspase-3 and spike glycoprotein
co-localized in endothelial cells (Fig. 1).
Cytokine expression was examined in the 26 tissues/13 cases and 10
controls and focused on IL6 and TNFα, since these two cytokines are
increased in severe COVID-19 [7,8]. The data presented in Table 1
highlights the significant association between each cytokine and SARS-
CoV-2 protein detection in the microvessels (each p < 0.001). Scattered
extravascular cells did show signal for each cytokine, but over 95% of
the positive cells were endothelia and the cytokines co-localized with
the spike glycoprotein (Fig. 1).
Next examined was if there was an effect on neuronal function pre-
sumably as a sequela to the SARS-CoV-2 associated microangiopathy.
MFSD2A plays an essential role in the blood brain barrier (BBB), and
supply of omega-3 fatty acids to neurons [12-14]. The normal brain
cases showed strong expression of MFSD2A in over 90% of neurons
(Table 1). In comparison, the percentage of neurons with detectable
MFSD2A decreased significantly (p < 0.001) in the COVID-19 brains
(Table 1) (Fig. 2). SHIP1 (Src-homology-2 containing inositol phos-
phatase) is another key brain protein that has a strong anti-
inflammatory effect [15]. It was strongly expressed in the normal
brains in over 85% of neurons. As seen in Table 1, there was a significant
decreased expression of SHIP1 in the COVID-19 brain tissues (p value
<0.001) (Fig. 2).
In addition, Glutamate NMDA receptor 2 (NMDAR2) and neuronal
nitric oxide synthetase (nNOS) have each been implicated as markers of
neuronal dysfunction [15-20]. NMDAR2 was not detected in the normal
brain tissues. It was expressed in the COVID-19 brains where it was
evident in neurons adjacent to SARS-CoV-2 positive microvessels
(Table 1 and Fig. 2). Similarly, the percentage of neurons with nNOS was
significantly greater (p < 0.001) in the COVID-19 brain tissues compared
to the controls (Table 1 and Fig. 2).
Fig. 1. Histologic and molecular correlates of COVID-19 in human brains.
Panel A shows the microvessels in normal brain. In comparison, many of the capillaries in COVID-19 brain tissues show marked perivascular edema (panel B). Serial
section analyses of the COVID-19 brain shows that the endothelial cells of the microvessels contained the spike glycoprotein (panel C), the ACE2 receptor (panel D)
and IL 6 (panel F), but not viral RNA (panel E). The fluorescent yellow signal marks co-localization of the spike protein with IL6 (panel G) and caspase 3 (panel H),
respectively, in these endothelial cells. Each magnification is 800× with DAB (brown) signal (panels C, E, F) or Fast Red (red) (panel D). (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 1
Quantification of biomarkers associated with endothelialopathy of the brains in people who died of COVID-19 and in the mice with IV injection of different spikeprotein subunits.
Protein Human normal brains
mean with SD
COVID-19 brains
mean with SD
Mouse normal brains
mean with SD
S1 subunit brains
mean with SD
Truncated S1 subunit brains
mean with SD
S2 subunit brains
mean with SD
Spike protein
SARS-CoV-2
0a 4.3 (0.9) 0 1.3 (0.4) 0 0
Caspase 3 0.4 (0.1) 4.8 (1.5) 0 1.7 (0.5) 0.3 (0.1) 0
IL6 0 5.1 (1.3) 0 1.2 (0.2) 0 0
TNFα 0 5.9 (1.5) 0 1.8 (0.6) 0 0
MFSD2a 94.7 (5.1)b 19.1 (7.3) 76.9 (6.1) 30.2 (7.9) 56.8 (9.0) 73.9 (8.2)
SHIP1 95.2 (6.0) 17.2 (2.9) 57.4 (9.6) 39.9 (10.3) 40.2 (13.3) 33.9 (11.4)
NMDAR2 0 9.9 (3.9) 0 2.3 (0.8) 1.9 (1.0) 2.0 (0.7)
Neuronal NOS 5.5 (1.8) 69.2 (10.1) 3.8 (1.3) 55.2 (7.1) 9.1 (3.3) 10.3 (2.9)
a Counts for spike, caspase 3, IL6 and TNFα are the number of + endothelial cells/200× field with a minimum of 10 fields counted/case. The controls and cases were
read in a blinded fashion.
b Counts for MFSD2a, NMDAR2, neuronal NOS, and SHIP1 are the % of positive neurons/200× field with a minimum of 10 fields counted/case. The controls and
cases were read in a blinded fashion.
G.J. Nuovo et al.
Fig. 1Fig. 1Fig. 1Fig. 178Fig. 112-14Fig. 215Fig. 215-20Fig. 2Fig. 2
Annals of Diagnostic Pathology 51 (2021) 151682
43.2. Spike subunit incubation in cell lines
Three different cell lines were incubated with the different subunits
of the SARS-CoV-2 spike protein for two days: RAW264.7 (murine,
macrophage), MN1 (murine, motorneuron), and HUVEC (human,
endothelial). Macroscopic changes included increased adhesion and
degeneration of the HUVEC cells and degeneration in the MN1 cell lines.
The immunohistochemistry data is presented in Table 2. Note that the
RAW264.7 cells were negative for ACE2 expression and showed no ev-
idence of the spike protein internalization nor caspase-3 induction. The
HUVEC and MN1 cell lines each had strong ACE2 expression. The S1
subunit was detected in 11.1% (HUVEC) and 5.1% (MN1) of the cells.
Co-expression analyses showed that the MN and HUVEC cells positive
for S1 subunit also expressed caspase-3 (Fig. 3).
Increasing the concentration of S1 spike protein from 70 to 350 ng/
ml resulted in increased HUVEC cell degeneration and adhesion, as well
as a significant increase in the % of cells with the spike subunit and
caspase-3 (Table 2 and Fig. 3).
The HUVEC cells were then treated with 70 ng/ml of the truncated
S1 subunit and the full length S2 subunit. Scattered degenerative
changes were seen with the latter. As seen in Table 2, the % of cells with
identifiable spike subunit or caspase-3 was significantly less than for the
full length S1 subunit (p < 0.001).
3.3. Spike subunit injection into mouse
Mice were tail vein injected with the different spike peptides (S1 full
length subunit, truncated S1 subunit, and full length S2 subunit) and
euthanized using CO2 after five days. The mice injected with the S2
subunit showed no behavioral changes; the mice injected with the S1
subunit had significantly increased thirst (consumed 11 g of hydrogel/
mouse at day 1 versus 3 g/mouse for sham injected) and showed stress-
related behavior that included eating the plastic support in their cages.
The mice injected with the truncated S1 subunit showed less thirst than
the mice who received the full S1 subunit (4.5 g hydrogel/mouse at day
1) and no stressed behavior.
At autopsy, no gross changes were evident. The major organs were
analyzed for the spike subunit as well as IL6, TNFα, C5b-9, and caspase 3
blinded to the experimental conditions. The mice treated with the S2
subunit showed no evidence of the spike protein internalization and no
increased expression of IL6, TNFα, C5b-9, or caspase 3 (Table 1 and
Fig. 4). The mice treated with the full length S1 subunit showed the viral
protein primarily in the brain, subcutaneous fat of the skin, and liver.
Over 90% of S1 spike positive cells were endothelia in microvessels
(Fig. 4). ACE2 analyses by immunohistochemistry showed that the three
organs with the strongest expression in the microvessels were the skin/
subcutaneous fat, brain, and liver. Co-expression analyses confirmed
that S1 spike subunit co-localized with ACE2 (data not shown). Endo-
thelial cell damage, with increased caspase-3, as well as TNFα, IL6, and
C5b-9 expression, was seen in the microvessels of the skin and brain in
the mouse model with co-localization with the S1 spike protein (Fig. 4).
The mice treated with the truncated S1 subunit showed scattered
cells positive for this spike protein but not in the endothelia. Rather, the
viral protein was seen in cells in the brain and liver that had the
morphology of macrophages and microglia, respectively. In these mice
no increased caspase-3, IL6, or TNFα expression was noted compared to
the controls (Table 1); rare C5b-9 positive endothelial cells were evident
but far fewer compared to the mice treated with the full S1 spike subunit
Fig. 2. Molecular changes in neurons in COVID-19 human brains.
Panels A and C shows the strong neuronal expression of MFSD2A and SHIP1 in normal brain, respectively. The signal is much decreased for each in the COVID-19
brain (panels B/D). Neither nNOS nor NMDAR2 (panels E/H) were strongly expressed in normal brains, respectively. Each protein was markedly increased in COVID-
19 brains (panels F, G, respectively; note the nuclear localization of nNOS). Each magnification is 600× with DAB (brown) signal (panels A,B,G,H) or Fast Red (red)
(panels C,D,E,F). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 2
Quantification of cell line data after incubation with different subunits of the SARS-CoV2 spike protein.
Protein
tested
RAW RAW 70 ng/
ml spike S1
HUVEC HUVEC 70 ng/
ml spike S1
HUVEC 350 ng/
ml spike S1
HUVEC 70 ng/ml
spike S1 truncated
HUVEC 70 ng/
ml spike S2
Motor
neurons
Motor neurons 70
ng/ml spike S1
ACE2a 2.2
(0.5)
2.7 (0.7) 97.3
(0.4)
96.2 (0.8) 75,6 (2.0) 97.2 (0.4) 95.1 (0.8) 95.0 (1.0) 94.5 (0.4)
Caspase 3 0 0 0.2 (0.1) 4.1 (0.9) 14.2 (3.8) 0.6 (0.1) 0.9 (0.3) 0 3.5 (1.2)
Spike
protein
0 0 0 11.1 (1.9) 26.7 (4.2) 1.2 (0.6) 0.2 (0/1) 0 5.1 (1.4)
a Counts for ACE2, caspase 3, and the spike protein are the % of positive cells/200× field with a minimum of 10 fields; several thousands of cells were scored per each
data point with n = 3.
G.J. Nuovo et al.
Fig. 3Fig. 3Fig. 4Fig. 4Fig. 4
Annals of Diagnostic Pathology 51 (2021) 151682
5(data not shown).
The mouse brain data is presented in Table 1 in parallel with the
human brain data. Only the mice treated with the full length S1 subunit
showed a significant increase in caspase-3, IL6, C5b-9, and TNFα in the
brain. Given the marked changes in MFSD2a, SHIP1, NMDAR2, and
nNOS in the human COVID-19 brains, the same proteins were analyzed
in the mice brains. Note the significant decrease in MFSD2a and increase
in nNOS in the mice brains exposed to the full length S1 subunit, but not
in the other mice (Table 1 and Fig. 4) (p < 0.001). However, the sig-
nificant change in the human COVID-19 brain tissues with regard to
NMDAR2 or SHIP1 expression were not evident in the mice brains
treated with the full length S1 subunit. - Discussion
The two main findings of this study were: 1) human COVID-19 cases
show diffuse pauci-inflammatory microvessel endothelial damage in the
brain and other organs including the skin [6-8] from the endocytosis of
circulating viral spike protein that induces C5b-9, caspase-3 and cyto-
kine production that is associated with a microencephalopathy. This
Fig. 3. Cytologic and molecular correlates of the S1 subunit of the spike protein in cell lines.
Panel A shows the cytology of untreated HUVEC cells; these endothelial cells strongly express ACE2 (panel B). Treatment with the S1 subunit of spike protein induced
cell aggregation and degeneration (panel C). The S1 subunit was not endocytosed by the RAW cells (panel D) and was evident in the HUVEC cells where it co-
localized with caspase 3 as seen in panels F-H. Panel F is the isolated spike data (fluorescent green), panel G the isolated caspase 3 data (fluorescent red) and
panel H the merged image with fluorescent yellow marking the co-localization of the two protein. The MN cells likewise showed co-localization of the spike protein
and caspase 3 (panel E). The magnifications are 600× (panels A-D) and 1000× (panels E-F) with DAB (brown) signal (panel D). (For interpretation of the references
to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Histologic and molecular results in mice after tail vein injection of different spike subunits.
Panel A shows the capillaries of normal mouse brain in which no perivascular space is evident (box). The microvessels of the mice brains after spike S1 subunit
injection do show edema (panel B, box). The spike S1 subunit was evident in the capillaries of the brain (panel C) and deep fat of the skin (panel F) as was activated
caspase 3 (panel D). However, mice injected with the spike S2 subunit showed no endocytosis in the endothelia in the brain nor caspase-3 activation (panel E). The
fluorescent yellow in panel F indicates that the S1 spike subunit did co-localize with caspase-3. As in the human COVID-19 brains, the mouse brains showed increase
nNOS after spike S1 subunit injection (panel G) but not S2 subunit injection (panel H). Complement cascade activated, documented by C5b-9 expression, was seen in
the capillaries of the brain (panel I) and deep fat of the skin (panel J) after S1 injection. Magnifications are at 600× (panels A, B, G, H) or at 1000× (other panels)
with DAB (brown) signal. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
G.J. Nuovo et al.
Fig. 46-8
Annals of Diagnostic Pathology 51 (2021) 151682
6encephalopathy is marked, in part, by neuronal dysfunction, evidenced
by increased nNOS and NMDAR2 plus the reduction of key neuronal
proteins that include MFSD2a and SHIP1. 2) Injection of the S1 full
length spike subunit into the tail vein of mice, but not the S2 subunit or
truncated S1 subunit, induces an equivalent microvascular encepha-
lopathy that shares with the human COVID-19 brain disease the endo-
cytosis of the S1 subunit in ACE2+ endothelia, caspase 3, C5b-9, TNFα,
and IL6 activation, and the over-expression of nNOS and much reduced
expression of MFSD2a. The much milder clinical effects of the truncated
S1 subunit suggests that the N-terminal region plays a role in spike S1-
induced CNS injury.
The human and murine CNS data are consistent with the term
“neurovascular unit” that has been used to reflect the intimate associa-
tion between neuronal circuits and their associated microvessels [21-
23]. MFSD2A plays an essential role in this unit in the blood brain
barrier (BBB) and in the transport of the fatty acid docosahexaenoic acid
to neurons [12-14]. NMDAR2 signaling is a marker of neuronal
dysfunction in this unit, indicative of an increased ratio of excitatory
versus inhibitory impulses [16,17]. nNOS plays a more complex role in
the unit where increased nuclear expression, as evident in the COVID-19
brain tissues, can facilitate mitochondrial biogenesis to compensate for
reduced energy production [18-20]. In mice, increased nNOS can signal
increased stress-related behavior [20], as seen in this study. We had
previously reported that the SUR1 and TRPM4, which are markers of
blood brain barrier dysfunction, were increased in the endothelial cells
in the COVID-19 brains and that they co-localized with the spike protein
[8]. These findings were similar to that described in acute spinal cord
injury associated with either open repair or endovascular stent repair of
an aortic aneurysm model, where microvascular injury is also postulated
to play a role in pathogenesis [24].
It is well documented that the spike protein of SARS-CoV-2 is
composed of two subunits, S1 and S2. The S1 subunit contains a
receptor-binding domain that recognizes and binds to ACE 2, while the
S2 subunit, after cleavage from the S1 subunit, mediates viral cell
membrane fusion and RNA entry by forming a six-helical bundle via the
two-heptad repeat domain [25,26]. Wang et al. [9] did demonstrate that
the truncated form of the S1 subunit, containing the ACE2 receptor
domain (RBD), could be internalized in embyronic kidney cells in vitro
without the N-glycosylation that occurs in the intact virus. Their study
showed that the HEK293 cells would internalize the RBD spike protein
with ACE2. Although we showed similar findings, our data suggested
that the full length S1 subunit was more effective in this regard both in
vitro and in vivo. This may reflect the different cell lines used: embry-
onic kidney cells versus endothelial cells. It has also been shown that
binding of the spike