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. 2023 Apr 27;97(4):e0014423.
doi: 10.1128/jvi.00144-23. Epub 2023 Apr 11.

SARS-CoV-2 Infection of Human Neurons Is TMPRSS2 Independent, Requires Endosomal Cell Entry, and Can Be Blocked by Inhibitors of Host Phosphoinositol-5 Kinase

Affiliations

SARS-CoV-2 Infection of Human Neurons Is TMPRSS2 Independent, Requires Endosomal Cell Entry, and Can Be Blocked by Inhibitors of Host Phosphoinositol-5 Kinase

Pinja Kettunen et al. J Virol. .

Abstract

2019 coronavirus disease (COVID-19) is a disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). In addition to respiratory illness, COVID-19 patients exhibit neurological symptoms lasting from weeks to months (long COVID). It is unclear whether these neurological manifestations are due to an infection of brain cells. We found that a small fraction of human induced pluripotent stem cell (iPSC)-derived neurons, but not astrocytes, were naturally susceptible to SARS-CoV-2. Based on the inhibitory effect of blocking antibodies, the infection seemed to depend on the receptor angiotensin-converting enzyme 2 (ACE2), despite very low levels of its expression in neurons. The presence of double-stranded RNA in the cytoplasm (the hallmark of viral replication), abundant synthesis of viral late genes localized throughout infected cells, and an increase in the level of viral RNA in the culture medium (viral release) within the first 48 h of infection suggested that the infection was productive. Productive entry of SARS-CoV-2 requires the fusion of the viral and cellular membranes, which results in the delivery of the viral genome into the cytoplasm of the target cell. The fusion is triggered by proteolytic cleavage of the viral surface spike protein, which can occur at the plasma membrane or from endosomes or lysosomes. We found that SARS-CoV-2 infection of human neurons was insensitive to nafamostat and camostat, which inhibit cellular serine proteases, including transmembrane serine protease 2 (TMPRSS2). Inhibition of cathepsin L also did not significantly block infection. In contrast, the neuronal infection was blocked by apilimod, an inhibitor of phosphatidyl-inositol 5 kinase (PIK5K), which regulates early to late endosome maturation. IMPORTANCE COVID-19 is a disease caused by the coronavirus SARS-CoV-2. Millions of patients display neurological symptoms, including headache, impairment of memory, seizures, and encephalopathy, as well as anatomical abnormalities, such as changes in brain morphology. SARS-CoV-2 infection of the human brain has been documented, but it is unclear whether the observed neurological symptoms are linked to direct brain infection. The mechanism of virus entry into neurons has also not been characterized. Here, we investigated SARS-CoV-2 infection by using a human iPSC-derived neural cell model and found that a small fraction of cortical-like neurons was naturally susceptible to infection. The productive infection was ACE2 dependent and TMPRSS2 independent. We also found that the virus used the late endosomal and lysosomal pathway for cell entry and that the infection could be blocked by apilimod, an inhibitor of cellular PIK5K.

Keywords: COVID-19; PIK5; SARS-CoV-2; apilimod; astrocyte; brain; central nervous system infections; iPSC; long COVID; neuron.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

FIG 1
FIG 1
Characterization of iPSC-derived neurons and astrocytes. (A) Representative images of immunocytochemical staining of iPSC-derived NGN2-neurons with MAP2, TUBB3, GABA, CUX1, and VGLUT1 antibodies. Scale bar, 50 μm. (B) Representative images of immunocytochemical staining of iPSC-derived astrocytes with GFAP, S100β, and AQP4 antibodies. Scale bar, 100 μm. (C) Mean normalized mRNA expression of astrocyte markers GFAP and S100β in iPSCs and iPSC-derived astrocytes. Values are normalized to human GAPDH expression. N = 4 cell lines. (D) Quantification of the mean firing rate (in hertz), percentage of electrodes partaking in bursts, percentage of electrodes partaking in network bursts, and network burst duration (in seconds) in iPCS-derived neuron-astrocyte cocultures grown for the indicated times. N = 3 cell lines. (E) Representative images of MEA recordings from neuron-astrocyte cocultures at days 21, 30, 31, and 35. (F) Representative fluorescence images of indicated cell lines after immunocytochemical staining using antibodies against ACE2. Scale bar, 1,000 μm. (G) Mean normalized mRNA expression of ACE2, TMPRSS2, and NRP1 in Caco2, Caco2-ACE2, A549, neuron, and astrocyte cell lines. Values were normalized to human GAPDH expression. N = 2 to 4 cell lines.
FIG 2
FIG 2
Infection of iPSC-derived neural cultures by SARS-CoV-2 is mainly dependent on the ACE2 receptor and does not spread efficiently. (A to C) Representative images of hiPCS-derived neuron-astrocyte cocultures infected with SARS-CoV-2 Wuhan virus for 48 h. Infected cells were identified by immunofluorescence detection (green) of the viral protein N (A and B) or N and double-stranded RNA colocalizing in the same infected cell (C). The image in panel B represents an enlarged area from the white boxed area in panel A. Nuclei were visualized with Hoechst DNA staining and neurons by immunodetection on MAP2 protein (magenta). (D) Percentage of SARS-COV-2-infected neurons in neuron-astrocyte cocultures at 24, 48, and 120 hpi. Neurons were identified by automated classification based on the fluorescence of MAP2. Astrocytes, identified as MAP2-negative cells, were never found infected. N = 6 wells per treatment. For each group, more than 10,000 cells were imaged and analyzed. (E) Representative fluorescence images of infected neuron-astrocyte cocultures from the experiment shown in panel D, at the indicated time points. SARS-CoV-2-infected cells (green, indicated by white arrows) identified by immunodetection of viral N protein, neurons by MAP2 (magenta), and nuclei (blue) by Hoechst DNA staining. (F) qRT-PCR of the extracellular medium withdrawn at the indicated time points after infection of neuron-astrocyte cocultures, using primers against SARS-COV-2 genome. Values are expressed as the number of genome copies per milliliter by comparison with a standard curve using known amounts of viral RNA. N = 6 wells per group. Significance was tested between 0 hpi and 24 hpi or 48 hpi (not significant [n.s.]). (G) Infection of Vero E6 cells with medium collected from infected neuron-astrocyte cocultures at 0, 24, or 48 hpi, or with the indicated amounts of a virus stock previously produced in Vero E6-TMPRSS2 cells (250 to 750 PFU or 25 to 75 PFU were used to infect each well of 96-well plates). N = 6 wells per group. Significance is shown in comparison to 0 hpi. (H) Representative immunofluorescent images of VERO E6 cells infected as described for panel G. Nuclei (blue) were stained with Hoechst, and infected cells (magenta) were identified by SARS-CoV-2 N protein immunofluorescence detection. (I) Percentage of SARS-COV-2-infected neurons in monocultures at 24 and 48 hpi. (J) Quantification of SARS-CoV-2 infection at 24 hpi in neurons that were treated with PBS (vehicle) or 2 μg/mL (low), 5 μg/mL (medium), or 20 μg/mL (high) anti-ACE2 or anti-actinin antibodies 1 h prior to infection. N = 6 wells per group. Significance is shown in comparison to the vehicle group. All scale bars are 100 μm. Columns and bars represent means ± SEM. Data were analyzed with an ordinary one-way ANOVA followed by Dunnett’s multiple-comparison test (C), an ordinary one-way ANOVA (D and E), or an ordinary one-way ANOVA followed by Tukey’s multiple-comparison test (J). *, P < 0.05; **, P < 0.005; ***, P < 0.0005.
FIG 3
FIG 3
SARS-CoV-2 infection of iPSC-derived neural cultures is blocked by inhibition of PIK5K but not serine proteases. (A and B) Treatment of neuron-astrocyte cocultures with 0.2 to 1 μM apilimod, 25 μM nafamostat, or a combination at 30 min prior to infection with SARS-CoV-2 at an MOI of 1.5. Analysis was at 24 hpi (A) and 48 hpi (B). N = 6 samples per group. (C) Treatment of Caco-2-ACE2 cells with 25 μM nafamostat, 1 μM apilimod, a combination, or anti-ACE2 antibody 30 min prior to infection with SARS-CoV-2 at an MOI of 2.5. Analysis was at 24 hpi. N = 3 samples per group. (D) Representative immunofluorescence images of SARS-CoV-2 infection (SARS-CoV-2 N protein) in Caco-2-ACE2 cells at 24 hpi at an MOI of 2.5, treated with the indicated drugs. Blue, Hoechst 33342; red, SARS-CoV-2 N. (E) Representative immunofluorescence images of SARS-CoV-2 infection (SARS-CoV-2 N protein) in neuron-astrocyte cocultures at 48 hpi at an MOI of 15, treated with the indicated drugs. Blue, Hoechst 33342; magenta, MAP2; green, SARS-CoV-2 N. (F) Treatment of neuron-astrocyte cocultures with 2 μM apilimod, 50 μM camostat, or a combination of the two drugs, 30 min prior to infection with SARS-CoV-2 at an equivalent MOI of 1.5. Cells were analyzed at 48 hpi after immunofluorescence imaging and image analysis. N = 3 wells per group. (G) Cytotoxicity assay in neuron-astrocyte cocultures treated for the indicated times with apilimod at concentrations ranging from 0.2 to 2 μM, or with 9 μM UCN-01. All scale bars are 100 μm. Columns and bars represent means ± SEM, respectively. Data were analyzed by an ordinary one-way ANOVA followed by Dunnett’s multiple-comparison test. *, P ≤ 0.05; **, P ≤ 0.01; ***, P ≤ 0.001; ****, P ≤ 0.0001. Significance is shown compared to the vehicle group.
FIG 4
FIG 4
Cathepsin L inhibitor SB412515 does not significantly inhibit SARS-CoV-2 neuron infection. (A) Quantification of SARS-CoV-2 infection in neuron-astrocyte cocultures (48 hpi), Vero E6, A549-ACE2-TMRPSS2, and Caco2-ACE2 cells (24 hpi) treated with 10 μM cathepsin L inhibitor SB412515 starting 30 min before infection. (B) Representative immunofluorescence images of cells infected and treated as described for panel A. Blue, nuclei stained with Hoechst; magenta, infected cells; yellow, neurons (identified by immunofluorescence detection of SARS-VOV-2 N and MAP2 proteins, respectively). Columns and bars represent means ± SEM, respectively. Data were analyzed by an ordinary one-way ANOVA followed by Dunnett’s multiple-comparison test. *, P ≤ 0.05; **, P ≤ 0.01; ***, P ≤ 0.001; ****, P ≤ 0.0001. Significance is shown compared to the vehicle group.

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