The most dire consequences of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)
infection may not manifest during the initial infection but appear later and are disproportionately
associated with compromised neural function. Central nervous system (CNS) complications
often appear after the acute phase but persist and become prominent later and last
for weeks to months (i.e., long COVID). Indeed, while severe neurological diseases
accompanying COVID-19 are rare during the acute phase (1, 2), 7.3 to 33.62% of COVID-19
survivors experience long COVID (3, 4), with diverse neurological and neuropsychiatric
complications such as headaches, fatigue, anosmia, cognitive impairment, and depression.
In addition, surprising brain pathological changes, such as loss of gray matter in
several regions of cortex, have been identified in COVID-19 survivors (5). With hundreds
of millions of people being affected by SARS-CoV-2 worldwide, concerns have been raised
in regard to public health and socioeconomic burdens. One of the key questions remains
whether SARS-CoV-2 can infect brain cells directly. To address this, Andrews et al.
(6) investigated the primary root of infection in the CNS and presented evidence that
SARS-CoV-2 preferentially infects astrocytes in the brain through noncanonical mechanisms.
This indicates that the neurological symptoms associated with long COVID arise indirectly
from increased neuroinflammation and nonautonomous neuronal death.
Foremost to understanding SARS-CoV-2’s impact on the CNS is distinguishing which cells
are impacted by this virus and whether these effects are direct or indirect. Given
the complexity and diversity of the CNS symptoms, discerning whether health impacts
arise from direct infection of the nervous system or indirect hypoxia, coagulopathy,
and systemic inflammation that are able to trigger downstream neuroinflammatory responses
is essential. Central to addressing these issues is determining which CNS populations
are directly infected by the virus. To date, inferences of SARS-CoV-2 infection have
largely come from examination of postmortem tissue. Neurotropism, referring to the
ability of a virus to infect and replicate in neural tissue, of SARS-CoV-2 remains
unclear. Recently, groups have utilized human pluripotent stem cell (hPSC)-based models
as a convenient tool to investigate neurotropism of SARS-CoV-2 on various brain cell
types and perform large-scale screening of therapeutic targets (7). Interestingly,
a study using neural-perivascular organoids identified pericytes in the vascular unit
potentially mediate the entry of SARS-CoV-2 into the brain and the spread to astrocytes
(8), whose endfeet are in contact with pericytes. In PNAS, Andrews et al. (6) further
explored the neurotropism of SARS-CoV-2 using hPSC-derived cortical organoids, as
well as primary human cortical tissues during development and in adulthood (Fig. 1).
Bolstering the earlier report, they found that SARS-CoV-2 preferentially infects and
replicates and propagates in astrocytes, particularly those adjacent to infected vasculature.
Notably, this vulnerability of astrocytes to SARS-CoV-2 not only exists during development
but also extends to adulthood. In contrast, neurons and microglia are less likely
to be directly infected. Importantly, while microglia and astrocytes are both reactivated,
a direct dosage-sensitive effect of SARS-CoV-2 is only observed in reactive astrocytes.
Fig. 1.
Astrocytes are the primary targets of SARS-CoV-2 in the brain. SARS-CoV-2 is shown
to preferentially infect astrocytes over neurons in primary and organoid cortical
cultures, resulting in astrocyte reactivation and non-cell-autonomous neuronal death.
Furthermore, BSG/CD147 and DPP4 are found to be key molecular mediators of SARS-CoV-2
infection in cortical astrocytes. Abbreviations: BBB, blood–brain barrier; CSF, cerebrospinal
fluid; CVO, circumventricular organs.
Many studies have suggested that SARS-CoV-2 relies on its obligate receptor to enter
cells (9). The canonical SARS-CoV-2 receptor, angiotensin-converting enzyme 2 (ACE2),
is the major entry receptor for SARS-CoV-2 in many cell types, such as nasal epithelial
cells, endothelial cells, and pericytes (10). However, ACE2 expression is undetectable
in cortical astrocytes pre- or postinfection. If not ACE2, what are the molecular
mechanisms mediating SARS-CoV-2 infection on cortical astrocytes? Andrews et al. (6)
first investigated the expression of neuropilin 1 (NRP1), a host factor that can enhance
transmembrane serine protease 2 (TMPRSS2)-mediated entry of wild-type SARS-CoV-2 (11).
However, NRP1, like ACE2, was not detected in cortical cells that are infected. The
receptor basigin (BSG/CD147), which is abundantly coexpressed with SARS-CoV-2 proteases
furin (FURIN) and cathepsin B (CTSB) in pericytes and astrocytes, represents an alternative
route for SARS-CoV-2 entry (12). In addition, dipeptidyl peptidase 4 (DPP4), the main
receptor of Middle East respiratory syndrome–related coronavirus, has been suggested
as a binding target for SARS-CoV-2 (13). In PNAS, Andrews et al. (6) demonstrate that
DPP4 and BSG/CD147 are able to mediate SARS-CoV-2 infection in astrocytes. Specifically,
while knockdown of BSG/CD147 or treatment with DPP4 inhibitor (vildagliptin) significantly
reduces SARS-CoV-2 infection, conversely overexpressing BSG/CD147 or DPP4 increases
infection in vitro. Both double-stranded RNA (dsRNA)+ cells and N+ cells are increased
with DPP4 overexpression, while only dsRNA+ cells are increased with BSG/CD147 overexpression.
Taken together, these results suggest that the molecular mechanisms underlying the
neurotropism of SARS-CoV-2 are likely mediated by DPP4 and BSG/CD147, which enhances
SARS-CoV-2 entry and is important for replication, respectively.
What are the functional consequences of SARS-CoV-2 infection of cortical astrocytes?
First of all, Andrews et al. (6) found that infected astrocytes show increased reactivity
and cellular stress. Moreover, non-cell-autonomous inflammatory effects are found
in SARS-CoV-2–infected cultures, such as an increase in reactive microglia and an
overall loss of neurons by apoptosis. Studies have suggested that astrocytes are critical
support cells in the regulation of brain energy, metabolism, and microenvironment
(14). Interestingly, BSG/CD147 is also a key part of the astrocyte metabolic pathways,
providing energy support to neurons (15). Therefore, SARS-CoV-2 infection in astrocytes
may cause neuronal death indirectly through inflammation and dysfunction of brain
energy metabolism.
To summarize, Andrews et al. (6) found that SARS-CoV-2 can infect brain astrocytes
via DPP4 and BSG/CD147, resulting in elevated inflammation and neuronal death. Although
very few published autopsy studies have reported detection of SARS-CoV-2 infection
in the brain, authors of a preprint from Brazil who analyzed 26 postmortem brains
from individuals who died with COVID-19 found that 5 of them had genetic viral components,
as well as SARS-CoV-2 spike protein in the brain (16). Moreover, the majority of these
SARS-CoV-2 spike+ cells (65.93%) were astrocytes (16), suggesting astrocytes are the
main target cell type in the brain. In circumventing the brain’s immune-privileged
status, possible neuroinvasive routes for SARS-CoV-2 include the olfactory system,
cranial nerves, dysfunctional blood–brain/cerebrospinal fluid barrier, and circumventricular
organs (10, 17). In infecting astrocytes, it is likely that the virus circulates in
the brain vasculature and infects pericytes, which in turn is spread into astrocytes
through their endfeet. Astrocytic infection results in dysfunction of their metabolic
homeostasis, enhancing neuroinflammation and impacting energy support for neurons
indirectly. These could contribute to COVID-19–associated CNS complications. More
severe neurological and neuropsychiatric symptoms may result from neuronal death or
synaptic loss in brain regions that are more vulnerable to pathogenesis, inflammation,
or energy deficiency. Since the CNS complications are diverse within different individuals
and viral infection is a dynamic process, it will be important to further investigate
the correlation between viral load within different brain regions, the neuroimmune
response levels, and the CNS symptoms with a larger clinical sample size. Furthermore,
it will also be crucial to compare different SARS-CoV-2 variants with regard to their
ability to infect brain cell types and understand their correlations to CNS symptoms.
If SARS-CoV-2 infection of brain astrocytes in COVID-19 patients is the proximal cause
of the observed neurological dysfunctions, the question arises as to how to prevent
viral entry preinfection, as well as the means to relieve the ensuing CNS symptoms
postinfection. Possible strategies may include, but are not limited to, the blockage
of viral infection or replication in pericytes or astrocytes and interventions to
reduce neuroinflammation. To relieve the symptoms, approaches to augment metabolic
energy to the brain as a means to restore brain homeostasis may also have promise.
Although brain organoids do not yet fully replicate the complete cellular and regional
diversity in the brain, they are a powerful tool to understand the mechanism of viral
tropism and screen for treatment targets. Together, the combination of clinical observations,
the in vitro use of human tissues, and organoid cultures will help us better understand
COVID-19 pathology in humans and facilitate the development of treatments against
SARS-CoV-2 infection.