key: cord-0734357-bo1k8alu
authors: Francesca, Pistollato; Mauro, Petrillo; Clerbaux, Laure-Alix; Leoni, Gabriele; Ponti, Jessica; Bogni, Alessia; Brogna, Carlo; Cristoni, Simone; Sanges, Remo; Mendoza-de Gyves, Emilio; Fabbri, Marco; Querci, Maddalena; Soares, Helena; Munoz Pineiro, Amalia; Whelan, Maurice; Van de Eede, Guy
title: Effects of spike protein and toxin-like peptides found in COVID-19 patients on human 3D neuronal/glial model undergoing differentiation: possible implications for SARS-CoV-2 impact on brain development
date: 2022-05-05
journal: Reprod Toxicol
DOI: 10.1016/j.reprotox.2022.04.011
sha: c76657549316cacfacd39062c23f3fd5b8e57394
doc_id: 734357
cord_uid: bo1k8alu

The possible neurodevelopmental consequences of SARS-CoV-2 infection are presently unknown. In utero exposure to SARS-CoV-2 has been hypothesized to affect the developing brain, possibly disrupting neurodevelopment of children. Spike protein interactors, such as ACE2, have been found expressed in the fetal brain, and could play a role in potential SARS-CoV-2 fetal brain pathogenesis. Apart from the possible direct involvement of SARS-CoV-2 or its specific viral components in the occurrence of neurological and neurodevelopmental manifestations, we recently reported the presence of toxin-like peptides in plasma, urine and fecal samples specifically from COVID-19 patients. In this study, we investigated the possible neurotoxic effects elicited upon 72-hour exposure to human relevant levels of recombinant spike protein, toxin-like peptides found in COVID-19 patients, as well as a combination of both in 3D human iPSC-derived neural stem cells differentiated for either 2 weeks (short-term) or 8 weeks (long-term, 2 weeks in suspension + 6 weeks on MEA) towards neurons/glia. Whole transcriptome and qPCR analysis revealed that spike protein and toxin-like peptides at non-cytotoxic concentration differentially perturb the expression of SPHK1, ELN, GASK1B, HEY1, UTS2, ACE2 and some neuronal-, glia- and NSC-related genes critical during brain development. Additionally, exposure to spike protein caused a decrease of spontaneous electrical activity after two days in long-term differentiated cultures. The perturbations of these neurodevelopmental endpoints are discussed in the context of recent knowledge about the key events described in Adverse Outcome Pathways relevant to COVID-19, gathered in the context of the CIAO project (https://www.ciao-covid.net/).

Coronavirus disease (COVID-19) resulting from severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection is still a public health issue. The effect of SARS-CoV-2 infection in pregnant women is of particular concern as studies suggest that they are at increased risk for severe COVID-19 associated with adverse fetal outcomes [1] .

It is well documented that infections during pregnancy can increase the risk for the offspring to develop neurodevelopmental disorders [2] , and in utero exposure to SARS-CoV-2 has been hypothesized to affect the developing brain [3, 4] , possibly disrupting neurodevelopment [5, 6] .

However, the mechanism underlying these poor outcomes are still unknown, and may be due to the exacerbated pro-inflammatory environment of the pregnant mother, or to vertical transmission of SARS-CoV-2. SARS-CoV-2 has been suggested to cross the placenta and to infect syncytiotrophoblast of the placental barrier, as shown by molecular and immunohistochemical analyses and electron microscopy, with an antibody-dependent transcytosis mediated by FcRn (Neonatal Fc receptor) hypothesized as a potential mechanism underlying placental invasion [7, 8] .

Moreover, anti-SARS-CoV-2 IgM antibodies have been detected in the blood of new-borns delivered by caesarean from mothers found positive for SARS-CoV-2, which suggests SARS-CoV-2 in utero exposure, or impaired placenta barrier with placental immune responses due to maternal respiratory SARS-CoV-2 infection [9, 10] . Since IgM antibodies do not cross the placenta except if disruption occurs and IgM represent the first immune response component against SARS-CoV-2, IgM antibodies in newborns may also indicate the possibility of recent exposure to SARS-CoV-2 of the fetus in the womb [11] .

J o u r n a l P r e -p r o o f 4 Additionally, transcytosis of opsonized or free viruses or viral particle transfer by infected blood cells could also cause placental infection and vertical transmission of the virus [12, 13] . Case reports documented placental SARS-CoV-2 infection [14, 15] . Although detection of RNA is considered not to be enough to conclude vertical transmission, a systematic review of the current literature aimed to evaluate the possibility of vertical transmission based on early RNA detection of SARS-CoV-2 after birth, and concluded that vertical transmission of the virus is very rare as observed in a minority of pregnant women tested positive for SARS-CoV-2 during the third trimester, with rates of infection similar to those of other pathogens causing congenital infections [16, 17] . Other recent systematic review studies suggest either that that vertical transmission of the virus is not strongly supported by clinical evidence, e.g., [18] , or that vertical transmission may be possible, although the likelihood of its occurrence is generally low, e.g., [19, 20] .

In addition to the presence of viral particles, spike proteins (S1 and S2) have also been found immunolocalized in cytotrophoblast and syncytiotrophoblast cells of the placental villi, in one asymptomatic pregnant woman tested positive for SARS-CoV-2 [21] . In this study, viral RNA was detected in the amniotic fluid and S proteins were detected in the fetal membrane at 8-13 gestational weeks [21] . This study provides evidence of persistent placental infection by SARS-CoV-2, which could be causative of hydrops fetalis and intrauterine fetal demise during the first trimester of pregnancy [21] . In another study, placental vasculopathy (which could lead to fetal growth reduction and other complications) and the presence of SARS-CoV-2 across the placenta have been reported in a pauci-symptomatic pregnant woman [22] , suggesting the possibility of viral vertical transmission during early pregnancy. Transplacental transmission of SARS-CoV-2 has been described also at later gestational weeks, e.g., in a 34-week pregnant woman, where the virus was found in the placenta as well as in several tissues of the fetus, who died as a consequence of severe placental thromboembolism [23] . Along the same line, transplacental viral transmission has been also detected in a woman at 35 weeks of gestation found positive for SARS-CoV-2, who showed neurological J o u r n a l P r e -p r o o f 5 issues and delivered a baby presenting irritability, axial hypertonia, poor feeding and opisthotonos [24] . On the contrary, Garcia-Flores et al. showed that SARS-CoV-2 infection during pregnancy was associated with humoral and cellular immune responses in the maternal blood, as well as with altered cytokine profile in umbilical cord blood, although in the absence of placental infection [25] .

Besides, the potential mechanisms of SARS-CoV-2 entry remain unclear in both placenta and fetal brain with variable findings reported [11, 26, 27] . In the placenta, the canonical ACE2 receptor was shown highly expressed during early gestation, then at negligible mRNA levels at full term, although term placentas from COVID-19 affected women showed increased ACE2 expression compared to healthy term placentas [17] . The study also showed that ACE2 protein is present in placenta despite low transcript levels. Noteworhty, by using an in vitro placental model, it has been shown that SARS-CoV-2 can infect the human placenta, and that ACE2 expression levels are directly associated with the release of SARS-CoV-2 [28] .

In the human fetal brain, spike protein interactors, i.e., ACE2, TMPRSS2, FURIN and the recently discovered ZDHHC5, GOLGA7, and ATP1A1, have been found expressed. In particular, ACE2 and classical TMPRSS2 co-factor have been found expressed at a low level (being undetectable in some brain regions). Besides, the alternative receptors and co-factors FURIN, ZDHHC5, GOLGA7 and ATP1A1 are expressed at a high level in the fetal brain, and could play a direct or indirect role in potential SARS-CoV-2 fetal brain pathogenesis, especially during the 2 nd and 3 rd trimesters of pregnancy [29] .

It is still unclear whether human placenta is susceptible to SARS-CoV-2 infection under normal In a previous study [30] , we reported the presence of toxin-like peptides in plasma, urine and fecal samples specifically and exclusively from symptomatic COVID-19 patients. In particular, the sequences of these (oligo)peptides (70 to 115, depending on the analysed samples) map with the sequences of known neurotoxic substances, i.e., conotoxins, metalloproteinases, prothrombin activators, phospholipases A2, and coagulation factors, which can be found in animal venoms and that are characterized by a high specificity and affinity towards human receptors, ion channels, and transporters of the CNS, like the nicotinic acetylcholine receptor [30] . We speculated that these toxin-like peptides could be involved in the reported COVID-19 neurological clinical manifestations.

In addition, it is presently unclear whether these toxin-like peptides can cross the placenta. While the origin of these peptides is presently unknown, several hypotheses can be formulated, e.g.: (i) these peptides may be coded by specific SARS-CoV-2 RNA regions [31] ; (ii) SARS-CoV-2 may be able to replicate in bacteria in a 'bacteriophage-like' manner [32] ; (iii) bacteria, may produce and secrete these peptides in reaction to the presence of the virus through not fully defined mechanisms, including the involvement of rRNA [33] , or small bacterial non-coding RNAs [34] ; or (iv) a combination of the aforementioned mechanisms [30] . Noteworthy, while animal toxins have been discussed as potential drug candidates for the treatment of human diseases, including neurodegenerative diseases, cardiovascular diseases, cancer, neuropathic pain, and autoimmune diseases [35, 36] , their possible detrimental effects on the developing brain have not been fully explored.

Several in vivo and in vitro models have been used to study SARS-CoV-2 mediated neurological and neuropathological modes of action. For instance, human cerebral organoids have been proven suitable to investigate SARS-CoV-2 infective mechanisms [37] [38] [39] [40] [41] . Human induced pluripotent stem cell (iPSC)-derived 3D models (e.g., neurospheres, brain spheres, organoids) can mimic key features of human foetal brain development [42] [43] [44] [45] [46] [47] , and therefore may be used to assess the neurodevelopmental effects of SARS-CoV-2 and its components. For instance, by using human brain J o u r n a l P r e -p r o o f 7 organoids, it has been shown that SARS-CoV-2 can cause impairment of excitatory synaptogenesis, affecting astrocytes' synaptogenic functions [48] .

While the possible neurodevelopmental consequences of SARS-CoV-2 infection are still not fully understood, the Adverse Outcome Pathway (AOP) framework could help improve interpretation and application of scientific understanding of COVID-19 pathological mechanisms [49] . In particular, the investigation of the molecular and cellular mechanisms underlying SARS-CoV-2 effects could be linked to the key events (KEs) described in COVID-19-relevant AOPs, which have been recently developed or are still under development in the context of the so called 'CIAO' project [49] [50] [51] . The project aims at modelling the pathogenesis of COVID-19 by exploiting the AOP framework approach. A similar strategy has been adopted also in the context of developmental neurotoxicity (DNT) testing, where mechanistic understanding of the molecular/cellular effects triggered by developmental neurotoxicants (e.g., deregulation of neural progenitor cell proliferation, neuronal and glial differentiation, migration, neurite outgrowth, synaptogenesis and neuronal network formation and function) could be anchored to KEs identified in the existing DNT relevant AOPs, as recently described [52] [53] [54] . Human 3D neuronal/glial models combined with emerging knowledge described in AOPs relevant for COVID-19 could enable mechanistic understanding of SARS-CoV-2 and viral components' effects.

In this study, we investigated the possible neurotoxic effects elicited by 72 hour (h) exposure to spike protein (recombinant S1+S2), toxin-like peptides found in symptomatic COVID-19 patients, and a combination of both on short-and long-term differentiated cultures of human iPSC-derived neural stem cells (NSCs) differentiated towards a mixed culture of neurons/glia as 3D neurospheres, as previously described [55] . Whole transcriptome (by RNA-seq) was assessed after 72h exposure in short-term differentiated cultures (2-week old neurospheres), impact on viability and selected gene expression by qPCR analysis were evaluated in both short-and long-term cultures, while effects on the generation of spontaneous electrical activity by microelectrode array (MEA) were analysed in J o u r n a l P r e -p r o o f 8 long-term differentiated cultures (2-weeks + 6-weeks on MEA). The perturbations of neurodevelopmental endpoints described in this in vitro study are discussed in the context of recent knowledge about the molecular and cellular KEs described in AOPs relevant to COVID-19associated brain disorders, gathered in the context of the CIAO project and under development in the online platform AOP-Wiki (https://aopwiki.org/).

The IMR90 cell line, a female human fetal lung fibroblast line obtained from a clinically normal 16

week old fetus, was originally developed at Coriell [56] with karyotype of a normal diploid female (46, XX) . IMR90 fibroblasts were directly reprogrammed at I-Stem (Evry, France https://www.istem.eu/en/) by retroviral transduction of OCT4 and SOX2 using pMIG vectors (Addgene). Frozen colony fragments of IMR90-hiPSCs were kindly provided by Prof. Marc Peschanski (I-Stem). HiPSC colonies were phenotypically characterized by analysis of colony/cell morphology, analysis of PSC-specific markers (by immoncytochemistry and flow cytometry), as well as gene expression analysis of pluripotency-related genes and alkaline phosphate activity, as detailed in [57] . Neural stem cells (NSCs) were derived from IMR90-hiPSCs [57, 58] , and differentiated as 3D neurospheres towards a mixed culture of neurons and astrocytes, in the presence of neuronal differentiation (ND) medium (i.e., Neurobasal Medium, N2 Supplements, B-27 Supplements, Penicillin/Streptomycin (50 U/mL), L-Glutamine (2 mM), laminin (mouse protein, 1 µg/mL), BDNF (2.5 ng/mL) and GDNF (1 ng/mL), all from ThermoFisher Scientific), as previously described [55] (submitted). Briefly, to generate neurospheres, NSCs were passaged using trypsin/EDTA and plated onto ultra-low adherent 6-well plates, at a density of 1 x 10 6 /mL (2 J o u r n a l P r e -p r o o f 9 mL/well) in the presence of ND medium. Already after 24h, neurospheres were visible, and reached an average diameter of 200-250 µm after 1 week. If present, large, necrotic spheres were manually discarded, and medium was refreshed twice/week. Neurospheres were differentiated for minimum 2 weeks (up to 6 weeks), and plates were kept at 37°C, 5% CO 2 under constant gyratory shaking (86 rpm) on a plate shaker. Two-week old neurospheres were characterized by the presence of 31% β-IIItubulin + neurons, 31% GFAP + astrocytes, 18% CNPase + oligodendrocytes, while 41% β-III-tubulin + , 39% GFAP + and 11% CNPase + cells were found in 6-week old cultures, as shown by flow cytometry analysis described in [55] . The percentage of remaining nestin + NSCs was 48% and 8% in 2-week and 6-week old neurospheres, respectively [55] .

Additionally, neurosphere cultures were characterized by the presence of a mixed culture of VGlut1 + glutamatergic, GABRE + GABAergegic and TH + dopaminergic neurons (see representative 40x

confocal images of 4-week old neurospheres in Supplementary Figure 1 ). Electrical activity analysis by multi-well microelectrode array (MEA) showed that this culture is mainly responsive to the modulatory effects of CNQX, an AMPA receptor antagonist, indicating the presence of functional excitatory glutamatergic neurons, whilst Gabazine (a GABAA receptor antagonist) and Muscimol (a selective GABAA receptor agonist) did non cause significant changes of spontaneous electrical activity [55] . Further details about the protocol and the characterization of the model are provided in [55] .

Two-week old neurospheres were manually transferred onto ultra-low adherence 24-well plates using a stereomicroscope (30 neurospheres/1 mL/well/) and exposed for 72h to recombinant 2019-nCoV S1+S2 ECD protein with His tag (hereafter called 'Spike' or 'S') (Sigma-Merck, cat. Figure 2) , as described below. The stock concentration of the Peptide solution was 140 µg/mL (tested using Bradford approach) in ~ 50 µL volume, which corresponded to 10X, 50X and 100X the average level of toxin-like peptides found in faecal, blood and urine samples, respectively, of COVID-19 patients described in [30] . The stock solution was thus diluted 10 times with sterile PBS 1X (without calcium and magnesium) and filtered with a 0.22 µm pore filter to prevent bacterial contamination of cell cultures. After 72h exposure to Spike and Peptides, cell viability was analysed as described below (compounds were not refreshed during the 72h). In parallel experiments, 3D neurospheres were exposed to Spike (10 µg/mL), Peptides (0.548 µg/mL) and a combination of both ('P+S'), and after 72h neurospheres were collected for whole transcriptome analysis by RNA-seq (Bioclavis) as described below. To measure electrical activity, 2-

week old 3D neurospheres were manually transferred using a stereomicroscope onto 24-well MEA plates and further differentiated for at least 6 additional weeks (total of 8-9 weeks in differentiation); then, cultures were exposed for 72h to non-cytotoxic concentrations of Spike (5 and 10 µg/mL), Peptides (0.548 µg/mL) or a combination of both ('P+S'). Electrical activity was recorded every day up to 72h as detailed below.

The mixture of Peptides was obtained from bacterial samples, grown as detailed in [30] . Briefly, bacterial cells were grown in NutriSelect™ Plus nutrient broth (Merck). Following the protocol recommended by the supplier, the medium was prepared as follows: 25 g were dissolved in 1 L of double distilled water and dispensed into tubes, which were sterilized by 15 min autoclave at 121°C. J o u r n a l P r e -p r o o f 11 All steps were conducted at temperature lower than 8°C, protected from direct light. Final composition of the medium was Peptone (15 g/L), Yeast extract (3.0 g/L), Sodium chloride (6.0 g/L). D(+)-Glucose (1.0 g/L), pH 7.5 at 25 °C. Tubes/flasks with growth broth and bacteria were placed in an orbital shaker at 37°C, and the liquid culture was left to grow for 30 days, monitoring bacterial growth by optical densities (OD) analysis using a (spectro)photometer absorbance microplate reader.

After 30 days, the Peptides mixture was extracted and purified as follows: after a centrifugation at 13,000 g for 10 min, 700 µL were collected from each ml of supernatant, and filtered with a 0.2 µm filter. The obtained solution was checked by means of mass spectrometry (peptide proton rearrangement were considered during data acquisition and elaboration) to confirm the presence of toxin-like peptides. The total peptide and protein concentration were analysed using the Bradford approach. The identification of toxin-like peptides was obtained by using Liquid Chromatography-Surface Activated Chemical Ionization -Cloud Ion Mobility Mass Spectrometry (LC-SACI-CIMS), as described in [30] . The full set of manually reviewed venom proteins and toxins from UniprotKB database, mixed with a subset of non-venom proteins and toxins, was used as reference protein dataset in order to give statistical significance to the results. The identified toxin-like peptides include those reported in the list of representative toxin-like peptides mapped on 36 candidate protein sequences belonging to Chordata, Echinodermata and Mollusca, is detailed in Table 1 in [30] , with information retrieved from UniprotKB and NCBI Taxonomy databases. This list is not expected to be exhaustive: in fact, several (oligo-) peptides (between 70 and 115, depending on the analysed sample [30] ) matched with different animal venom proteins and toxins like conotoxins, phospholipases A2, and metalloproteinases (86% of assignments have a -log(e) > 25). Apart from these peptides mapped on toxins of animal origin, no signal with statistical significance and attributed to any known bacterial toxin was observed. 

After 72h exposure to different concentrations of Spike or Peptides, cell viability was measured by incubating 3D neurospheres with CellTiter-Blue® Reagent (final 1:6 dilution in cell culture medium) at 37°C and 5% CO 2 for 4 hours. After incubation, 100 µL medium/reagent were transferred into new 96-well plates accounting also for wells containing blanc solution (ND medium with CellTiter Blue reagent), and fluorescence was measured at 530-560 nm-/590 nm (excitation/emission) in a multiwell fluorimetric reader (Tecan). After blanc subtraction, data were normalised to the mean of control cells (i.e., cells in ND medium).

Three-week old neurospheres were fixed with 4% formaldehyde for 25 min, washed twice with PBS 1X (w/o calcium and magnesium) for 7 min, and stored in PBS 1X at 4°C prior to staining.

Neurospheres were incubated in PBS 1X containing 0.1% Triton-X-100 and 3. replicates were considered, and at least 5 neurospheres were imaged for each experimental replicate.

To assess the effects of Spike (5 and 10 µg/mL), Peptides (0.548 µg/mL) and a combination of both interval to end burst of 50 ms, minimum interval between bursts of 100 ms) were done using the "Multiwell-Analyzer" software, analysing the full recording and considering only active wells (i.e., wells characterized by at least 3 active channels, each active channel with minimum 10 spikes/min, and a minimum amplitude of 10 µV) (manual instructions are available at https://www.multichannelsystems.com/sites/multichannelsystems.com/files/documents/manuals/Mul tiwell-MEA-System_Manual.pdf) [59] . The average of spikes number and bursts number of selected active electrodes within each well were normalized to their respective T0 (i.e., cells not yet exposed to compounds). Four independent biological replicates were done, with 3-4 internal replicates per condition.

Analysis of ACE2, MAP2 and GFAP gene expression by qPCR was performed in NSCs undergoing differentiation as 3D neurospheres (control culture) collecting samples for RNA isolation after 1, 2, 3, 4, 5 and 6 weeks of differentiation. Gene expression analysis of all other genes indicated in Table   1 was carried out in short-term and long-term differentiated cultures after 72 h exposure to Peptides 0.548 µg/mL, Spike 10 µg/mL, and P+S.

J o u r n a l P r e -p r o o f 15 RNA was isolated using the RNAqueous®-Micro Kit (ThermoFisher) according to manufacturer's instructions, and 500 ng of total RNA were reverse transcribed by using the High Capacity cDNA Reverse Transcription Kit (as directed, ThermoFisher). qPCR reactions were run in duplicate using TaqMan® Gene Expression Master Mix (ThermoFisher) and the TaqMan gene expression assays indicated in Table 1 . Amplification efficiencies of primers/probes were directly verified by the manufacturer (Thermo-Fisher) and were in the range of 100% (+/-10%) when measured over a 6-log dilution range (additional information are available at https://assets.thermofisher.com/TFS-Assets/LSG/Application-Notes/cms_040377.pdf) [60] . Figure 1B -D) or Ctr (for all genes shown in Figures 4 and 5) were used to normalize the data (ΔΔCt Method). Three independent biological replicates were performed.

Analysis of gene expression by RNA-seq was performed in 2-week old neurospheres exposed for 72h to Spike (10 µg/mL), Peptides (0.548 µg/mL) and a combination of both ('P+S') (vs Control) as briefly described. Neurospheres were decanted in 1. Prism 9. For all graphs, an asterisk over a data point indicates a significant difference with the control group or as indicated (* p < 0.05, ** p < 0.01, *** p < 0.001).

As expression of angiotensin-converting enzyme 2 (ACE2) receptor is critical to initiate viral entry into the cells through spike protein [64, 65] , we verified the expression of ACE2 in hiPSC-derived NSCs undergoing differentiation towards a mixed culture of neuronal and glial cells as 3D neurospheres over 6 weeks (Figure 1 ). Cultures underwent a progressive increase of MAP2 and GFAP gene expression over time ( Figure 1A and B, adapted from [55] ); ACE2 appeared twice more expressed already after 1 week of differentiation (albeit not significant), and its expression progressively increased during differentiation ( Figure 1C ).

J o u r n a l P r e -p r o o f 18 ACE2 was found more expressed in astrocytes (positive for glial fibrillary acidic protein, GFAP, green), and at a lower level in neurons (stained with β-III-tubulin, red), as revealed by immunocytochemistry and confocal imaging analysis of 2-, 3-and 6-week old neurospheres ( Figure   1D ). After 72h, no impact on cell viability was observed in 3D neurospheres exposed to either different Peptides ( Figure 2B ) or Spike ( Figure 2C ), and no changes of neurosphere size could be observed (not shown).

We further assessed the effects of 72h exposure to non-cytotoxic and physiologically relevant Figure 2G ). By imposing a cut-off at 0.05, three genes, i.e., SPHK1, ELN and GASK1B showed to be differentially expressed, and with this cut-off no differences were found with respect to data reported in Figure 2D -I.

Notably, while SPHK1, GASK1B and HEY1 upregulation was similar comparing samples exposed to Spike and P+S, the increase in ELN gene expression was significantly greater in cells exposed to P+S compared to Spike only (p < 0.05). On the contrary, Urotensin-II (UTS2) gene was down-regulated by both Peptides and P+S in a similar manner ( Figure 2H ). Samples were characterized by homogeneous reads counts per sample and low variability, as shown by principal component analysis (PCA) ( Figure 2I ). 

We further explored the effects of Spike and Peptides on the generation of spontaneous electrical activity in long-term differentiated cultures by using the MEA technology. To this aim, neurospheres were pre-differentiated for 2 weeks in suspension, before being transferred onto MEA plates for recording of spontaneous electrical activity. Cultures were differentiated for at least 6 additional weeks on MEA before exposing them to Spike alone (5 and 10 µg/mL), Peptides alone (0.548 µg/mL) or a combination of both (P+S) vs Control cultures. Electrical activity was recorded 1 min after adding compounds (acute effects) and after 1-, 2-and 3-day exposure ( Figure 3A ). Under these exposure conditions, no cytotoxic effects were observed, and about 34% increase of cell viability was recorded upon exposure to Spike (10 µg/mL) alone and in combination with Peptides (P+S) ( Figure 3B ).

MEA data showed that Peptides induced a decrease of spontaneous electrical activity (~30% decrease of spike rate) after 2d, whilst Spike at the concentration of 5 µg/mL had no significant impact on electrical activity ( Figure 3C , D, E). Spike protein at the higher tested concentration (10 µg/mL) caused a decrease of both spike rate (by ~57%) and overall number of bursts (by ~66%) after 2d ( Figure 3D , E). Combined exposure to both P + S-5µg/mL caused a significant decrease of both spike rate (by ~50%) and bursts (by ~63%) after 1d, and a similar decrease was observed after 2d exposure to P+S-10µg/mL (spike rate by ~43% and bursts by ~61%) (Figure 3D , E). The effects elicited by Spike-10 alone were similar to those elicited by the combined exposure to P+S-10µg/mL; on the other hand, the combined exposure to P+S-5µg/mL had a greater impact on spontaneous electrical activity formation than either Spike-5 alone or Peptides alone, which may suggest a potentiated effect.

J o u r n a l P r e -p r o o f 22 Figure 3 . Effects of spike protein and toxin-like peptides on electrical activity in long-term differentiated 3D neuronal/glial models. (A) NSCs were expanded, passaged and differentiated for 2 weeks as 3D neurospheres in suspension before being transferred to 24-well MEA plates for at least 6 additional weeks (total of 8-9 weeks in differentiation). Cultures were exposed for 72h to Spike alone (5 and 10 µg/mL), Peptides alone (0.548 µg/mL) or a combination of both, and electrical activity was recorded for 5 min before (T0), 1 min after adding compounds (acute), and after 1-, 2and 3-day exposure. (B) Cell viability analyses of cultures after 72h exposure as indicated in A (values were normalised to Ctr). (C) Representative phase contrast images of cell cultures on 24-well MEA plates exposed to compounds as described in A. (D, E) Bar graphs showing spike rate (i.e., number of spikes/sec) (D), and number of bursts (E) of cultures exposed to compounds as described in A (data were normalized to T0). For all analyses, mean ± S.E.M. of 4 biological replicates. Figure 4E) .

Notably, the expression of ACE2 did not significantly change in short-term differentiated cultures (not shown). However, in long-term cultures, ACE2 expression was found downregulated upon exposure to Peptides, while this downregulation was milder upon exposure to P+S; on the contrary, Spike protein caused a very modest but significant upregulation of ACE2 ( Figure 4F ). 

Finally, we analysed gene expression of a set of genes expressed by different neuronal subtypes and glial cells in both short-(2-weeks) and long-term differentiated cultures (2-weeks + 6-weeks on MEA) comparing the effects of Peptides alone (0.548 µg/mL), Spike alone (10 µg/mL), and P+S vs Ctr cultures. These data showed that Spike caused a slight decrease of GAP43 and GRIA2 in 2-week old neurospheres, while a very modest decrease of GRIA1 was seen after exposure to P+S in longterm differentiated cultures ( Figure 5A, B) .

The GABAergic gene GABRA3 was upregulated in both culture types after Peptide exposure (although not significantly in short-term differentiated culture), while GABRB3 resulted slightly downregulated after Spike exposure in short-term differentiated culture ( Figure 5A, B) .

The dopaminergic genes NR4A2 and TH were found differentially regulated in short-vs long-term differentiated cultures, with NR4A2 undergoing downregulation in short-term differentiated cultures and found upregulated in long-term differentiated cultures exposed to Peptides, and TH showing a tendency towards an increase in short-term differentiated culture, and a significant decrease in long-

term differentiated cultures at all tested conditions ( Figure 5A, B) .

Morever, the cholinergic gene SLC18A3 resulted downregulated upon exposure to Spike and P+S (in short-term differentiated), and Peptides and P+S (in long-term differentiated cultures), whilst CHAT and SLC5A7 were found upregulated in short-term differentiated cultures upon exposure to P+S ( Figure 5A, B) .

We also looked at the expression of some glia-related genes. While expression of BMPR2 did not change at either conditions, GFAP expression showed a tendency toward an increase in short-term differentiated culture exposed to P+S (not significant), whilst long-term differentiated cultures . Noteworthy, MBP expression increased by about 3-fold in short-term differentiated cultures exposed to Peptides and P+S, while its expression was modestly upregulated by Peptides in longterm differentiated cultures. The oligodendrocyte marker OLIG1 was found significantly downregulated in short-term differentiated cells exposed to Spike and P+S, and resulted downregulated by Peptides and P+S in long-term differentiated cultures ( Figure 5C, D) .

The increase in cell viability relative to control cultures observed in neurospheres exposed to Spike proteins ( Figure 3B ) may reflect possible differences in cell proliferation. In line with this hypothesis, we assessed in both short-and long-term differentiated neurosphere cultures the expression of NES and PAX6, which are expressed in dividing NSCs [67, 68] . While the expression of both genes did not change in 2- week old neurospheres at any tested condition, PAX6 was found significantly downregulated by both Spike and P+S in long-term cultures, whilst NES expression did not significantly change ( Figure 5E , F). Both culture types were exposed at the end of differentiation period to toxin-like Peptides alone (0.548 µg/mL), Spike protein alone (10 µg/mL) and both compounds (P+S) vs Control for 72h. Data were normalized to reference genes ACTB and GAPDH, and further normalized on Ctr (unexposed cells). For all analyses, mean ± S.E.M. of 3 biological replicates.

In a previous study [30] , we reported the presence of toxin-like peptides in plasma, urine and fecal samples exclusively from COVID-19 patients. At present, the origin of these Peptides, whether they can cross the placenta, and what possible detrimental effects they may have on human perinatal development and the developing brain are unknown. In this study, we investigated the neurotoxic effects elicited upon 72h exposure to Spike, Peptides mixture found in COVID-19 patients, and a combination of both (P+S) on human iPSC-derived NSCs differentiated towards a mixed culture of neurons/glia as 3D neurospheres at different differentiation stages. Expression of both glia and neuronal-related genes increased during differentiation in this 3D model, whose characterization is described in [55] . Notably, glia-related genes and proteins were observed as early as 2 weeks of differentiation [55] , whilst other studies on hiPSC-derived neuronal and glial cell derivatives cultured as 3D organoids or neurospheres have shown that several weeks in differentiation were needed to obtain mature glia (e.g., [46, 69, 70] ). Differences in temporal occurrence of gene expression and protein level changes may be linked to genetic, epigenetic and phenotypic differences in test systems, as well as differences in media formulation and experimental design. Notably, expression of ACE2 was found upregulated during differentiation in a time dependent manner, with ACE2 protein observed mainly in astrocytes, which suggests that 3D neurospheres may plausibly be infected by SARS-CoV-2 virus via Spike-ACE2 interaction. Our data show that Spike protein at 10 µg/mL caused a decrease of spontaneous electrical activity after 2d exposure in long-term differentiated cultures, and when combined with Peptides a similar decrease was observed. Notably, while 5 µg/mL Spike did not cause significant perturbations of electrical activity, when combined with Peptides, potentiated effects could be hypothesized, with a decrease of spike rate and bursts observed after 1d. Noteworthy, network connectivity perturbation observed in long-term differentiated cultures may be associated with the observed dysregulation of some critical neuronal-, glia-and NSC-related genes. 

GABRA3 gene expression (which resulted upregulated in both short-and long-term cultures upon exposure to Peptides) constitutes the dominant subunit in the forebrain tissue at birth [71] . GABRA3

upregulation has been associated with testosterone-mediated impulsive behaviour in rats [72] , and was observed upon exposure to the psychostimulant methamphetamine [73] . On the other hand, glutamatergic gene expression (GAP43, GRIA1, GRIA2, GRIA3) was minimally modulated in both culture types at either conditions.

TH is known to catalyse the conversion of L-tyrosine to L-3,4-dihydroxyphenylalanine (L-DOPA), which is the initial and rate-limiting step in the biosynthesis of dopamine, noradrenaline, and adrenaline. TH resulted strongly downregulated particulalry in long-term cultures exposed to Peptides and Spike together. Noteworthy, TH downregulation has been observed in Parkinson's disease (PD) [74] and in early parkinsonism [75] .

Also downregulation of NR4A2 (Nurr1, a nuclear receptor essential for the differentiation, survival and maintenance of midbrain dopaminergic neurons), which was observed in short-term differentiated cultures upon exposure to all tested compounds, is associated with PD as observed in both PD patients and animal models [76] , as well as neuroinflammation and neuronal cell death [77] .

SLC18A3 (which was found downregulated mainly by Spike in short-term cultures, and Peptides in long-term cultures) encodes the Vesicular acetylcholine transporter (VAChT), which transports acetylcholine into secretory vesicles for release into the extracellular space [78] . A deficit of VAChT, induced by SLC18A3 variants, are associated with the presynaptic congenital myasthenic syndrome, which is functionally characterized by electrodecrement on low-frequency repetitive stimulation and a prolonged period of postactivation exhaustion, as shown by electrophysiology studies conducted on congenital myasthenic syndrome affected patients [79] .

J o u r n a l P r e -p r o o f 28 The observed OLIG1 downregulation induced by Spike in short-term differentiated cultures and by Peptides in long-term differentiated cultures, as well as the upregulation of MBP mainly elicited by Peptides in both short-and long-term differentiated cultures, suggest a dysregulation of oligodendrocyte development and maturation. Indeed, OLIG1 controls oligodendrocyte precursor differentiation into myelin-forming oligodendrocytes during development, and together with SOX10, activates MBP transcription [80] .

Additionally, the tendency towards an upregulation of GFAP expression induced by P+S (not significant) in short-term cultures and the upregulation mainly induced by Spike in long-term cultures, could be associated with astrogliosis and microglial activation. In line with this, transgenic mice overexpressing wild type GFAP that develop encephalopathy showed upregulation of genes involved in glutathione metabolism, peroxide detoxification, and iron homeostasis at 3 months of age [81] , as well as increased activation of cytokine, cytokine receptor genes, and complement components. With ageing, these transcripts resulted further elevated, with additional induction of macrophage-specific markers, indicating activation of microglia [81] [82] [83] .

Additionally, PAX6 (which was found downregulated by Spike in long-term cultures), is an important transcription factor that controls NSC proliferation, multipotency, neurogenesis and cortical development [68] , and its mutation or deletion has been shown to cause major brain defects and several neurodevelopmental disorders in the developing embryo [84] . Exposure to Spike protein may therefore have an impact on NSC self-renewal/proliferation.

Altogether these data suggest that Peptides and Spike protein in short-and long-term differentiated cultures, differentially affect genes involved in NSC self-renewal/proliferation, neuronal and glial differentiation, which may ultimately be associated with the observed perturbation of electrical activity.

J o u r n a l P r e -p r o o f 29

Whole transcriptome analysis in short-term differentiated cultures revealed the upregulation of SPHK1, ELN, GASK1B, and HEY1 upon exposure to Spike or P+S, as well as the downregulation of UTS2 upon exposure to Peptides and P+S. qPCR analysis of these five genes showed that these genes were deregulated by tested compounds in a similar manner also in long-term differentiated cultures.

Upregulation of SPHK1 expression may be linked to induction of a pro-survival, pro-inflammatory mechanism in short-term and long-term cultures upon exposure to Spike and P+S. SPHK1 is known to control cell survival, migration and inflammation [85, 86] , and its deregulation has been observed in various inflammatory and immune related-diseases, such as hypertension, atherosclerosis,

Alzheimer's disease, inflammatory bowel disease, rheumatoid arthritis, and asthma [87] [88] [89] . It is also known to regulate microglial phagocytosis [88] , and to modulate inflammation during cerebral ischemia/reperfusion injury [90, 91] . Moreover, SPHK1/S1PR2 signaling axis is closely associated with the course of Temporal Lobe Epilepsy [92] . The sphingolipid rheostat has been shown to play a role in viral replication, immune response modulation, and the maintenance of blood vessel integrity [93, 94] .

The gene ELN (found upregulated especially by P+S in short-and long-term cultures) codes for elastin, which, together with elastases, play an important role in the aging of the arterial wall, skin and other connective tissues [95, 96] , and elastin-derived peptides have been shown to potentiate atherosclerotic plaque formation [97] . ELN is also responsible for the vascular and connective tissue features of Williams syndrome, a relatively rare microdeletion disorder, characterized by cardiovascular disease, distinctive craniofacial appearance, intellectual disability and hypersociability [98] . Notably, ELN was found to play a neuroprotective role in response to preterm ischemia-hypoxia brain damage in a rat model [99] . To our knowledge, no studies have to date J o u r n a l P r e -p r o o f 30 investigated the possible role of ELN in relation to SARS-CoV-2 associated brain (neurodevelopmental) sequelae.

Another gene found upregulated upon exposure to Spike and P+S in short-and long-term differentiated cultures is GASK1B (also known as FAM198A or C3orf41), which is expressed in nerve and epithelium during development [100] , and is an integral active component of the Golgi apparatus, known to play a fundamental role in SARS-CoV-2 virion assembly [101] . Additionally, GASK1B is one of the most important caveolae-associated proteins, whose secretion plays an important role in the caveolae biogenesis pathway [102] . Notably, caveolae are involved in numerous membrane functions, including membrane trafficking and lipid metabolism, cell motility, and viral infection [103, 104] . Contrary to other coronaviridae, which use caveolae for internalization, SARS-CoV-2 seems to preferentially undergo clathrin-mediated endocytosis;

however, the role of caveolae in SARS-CoV-2 remains disputed [105] , with different plausible modes of entry depending also on the cell type [106] .

HEY1 (found upregulated mainly upon exposure to P+S in short-term cultures) is a major Notch3 effector controlling NSC stemness in the vertebrate adult brain [107] . Overexpression of HEY1 has been shown to promote astrocyte differentiation and to inhibit neuronal differentiation in murine neural progenitor cells [108] . HEY1 resulted upregulated in long-term cultures exposed to Spike and more prominently P+S; together with PAX6 down-regulation observed under the same conditions, this suggests dysregulation of neuronal/glial differentiation processes in our test system mainly occurring as a consequence of Spike exposure.

Finally, Urotensin-II (UTS2) was found downregulated by Peptides and P+S in short-term cultures;

in long-term cultures a tendency towards UTS2 downregulation was observed upon Peptides exposure, resulting very slightly upregulated upon P+S exposure. UTS2 is an 11-aminoacid neuropeptide that interacts with the urotensin receptor (UT), a specific G-protein coupled receptor J o u r n a l P r e -p r o o f 31 [109] . UTS2 has both vasoconstrictor and vasodilatory actions, modulates cell proliferation, profibrosis, neuroendocrine activity, controls insulin resistance, and has carcinogenic and inflammatory effects, playing a role in the onset and development of inflammatory diseases [110] . The 'UTS2-UT' system is widely distributed in cardiovascular tissue, the nervous system (especially cardiovascular control centres), the kidney, and the respiratory tract, and its downregulation has been linked to numerous pathophysiological conditions [109] . Interestingly, UTS2 has been found as an ACE2 and TMPRSS2 correlated gene by KEGG pathways analysis in several brain regions, including hypothalamus, insula, amygdala, myelencephalon, and parabrachial nuclei of pons, as shown by human brain gene-expression analyses and immunohistochemistry [111] . Additionally, UTS2

receptors have been found expressed in presynaptic cholinergic terminals in a subset of motor neuronal and non-motor neuronal perikarya, as well as in non-cholinergic nerve terminals, as observed in the ventral horn of the adult mouse cervical spinal cord [112] . In our study, downregulation of UTS2 was mainly triggered by Peptides, which could be linked to Peptides' affinity towards nicotinic acetylcholine receptor [30] .

In both short-and long-term cultures, apart from the decrease of UTS2 (triggered by Peptides), upregulation of SPHK1, ELN, GASK1B and HEY1 were mainly triggered by Spike, and these effects were slightly greater in cell cultures exposed to both Spike and Peptides (P+S), suggesting a possible exacerbating role of Peptides.

Noteworthy, expression of ACE2 resulted downregulated in long-term differentiated cultures exposed to Peptides. ACE2 expression starts to increase already after 1-2 weeks of differentiation in our neuronal/glial model; along the same line, ACE2 expression has been shown in neurons and glial cells in the brain (in particular in the brain stem and cardiovascular regulatory areas) [113] , and may play a key role in the neural invasion of SARS-CoV-2 [114] . Downregulation of ACE2 has been shown to occur in response to SARS-CoV-2 cellular invasion [115] . Notably, ACE2 downregulation plays an important role in COVID-19 severity, with an imbalance of the renin-angiotensin system and consequential increase in the levels of substrates (e.g., angiotensin II, apelin-13, dynorphin-13), and a decrease of products (e.g., angiotensin (1-7), angiotensin (1-9), apelin-12, and dynorphin- 12) in the human body. Substrates accumulation can cause inflammation, angiogenesis, thrombosis, neuronal and tissue damage; on the other hand, ACE2 products' depletion can reduce the antiinflammatory, anti-thrombotic and anti-angiogenic responses [116] . In particular, angiotensin (1-7) has a neuroprotective role, and its decrease may be associated with oxidative stress and neuronal cell death [117] . Moreover, ACE2 downregulation has been associated with glutamate-induced excitotoxicity in primary mouse cortical neurons [118] .

The observed transcriptional and functional developmental neurotoxic effects described in this study are in line with mechanistic knowledge described in COVID-19 relevant AOPs under development in the context of the CIAO project [119] [120] [121] and in other neuro-related AOPs. Potential penetration of SARS-CoV-2 and of contaminating toxin-like peptides present in maternal blood through placenta and blood-brain barriers may trigger neuroinflammation in the developing fetus, as suggested by increase in GFAP expression and SPHK1, along with the decrease of NR4A2 and UTS2. AOs relevant to the developing brain/fetus as this mechanistic knowledge may help clarify the mechanisms underlying possible SARS-CoV-2 impact on the developing brain.

The crucial question is still the possibility for SARS-CoV-2 to penetrate the developing fetus. Two studies have described placental viral invasion shown by immunohistochemistry and electron microscopy analyses [7, 8] . On the contrary, other studies have reported negative results for the presence of the virus in the neonates and placenta of pregnant women affected by SARS-CoV-2 [17] .

Additionally, other studies reported about the presence of SARS-CoV-2 in one infant and one fetus affected from COVID-19 [10, 21] .

In general, recent systematic reviews suggest that the likelihood of vertical transmission of SARS-CoV-2, which is only known for late third trimester infections, may be low [16, [18] [19] [20] 122 ].

However, the cytokine storm and hyperinflammation observed in pregnant women severely affected by COVID-19 [17, 25, 123] may cause perturbation of placenta integrity; prolonged fever, hypoxia, hypertension and medication side effects may exacerbate this phenomenon ( Figure 6 ). It is presently unclear whether vertical transmission might also vary with different SARS-CoV-2 variants, which remains an open question for the future.

In the context of the CIAO project, several modulating factors, such as age, diet, gut microbiota, preexisting comorbidities, environmental pollutants, etc., were investigated as factors modulating COVID-19 symptomatology [50] . Toxin-like peptides may represent additional J o u r n a l P r e -p r o o f 34 exacerbating/detrimental modulatory factors possibly influencing SARS-CoV-2 impact on brain development. Figure 6 . Summary of the effects possibly triggered by SARS-CoV-2 Spike protein and toxin-like Peptides, along with Key Events (KEs) describing these mechanisms, and additional modulating factors. Under each KE, the endpoints (genes and electrical activity) that have been found deregulated in this study are reported. Dashed lines and question marks indicate still unknown (not yet verified) processes (image created with BioRender.com).

In this study, we could not test the direct effects of SARS-CoV-2 viral particles for biosafety related reasons, thus limited our investigation to the effects of Spike protein, a critical component of viral structure.

J o u r n a l P r e -p r o o f 35 Spike protein alone has been shown in one study to induce several damages associated with COVID-19, including damage to the lungs and arteries, inflammation of endothelial cells lining the pulmonary artery walls, together with impairment of mitochondrial function, decrease of ACE2 expression and eNOS activity, and increase of glycolysis as observed in vitro (on endothelial cells treated for 24h with 4 μg/mL Spike) and in vivo (upon intratracheal administration of a pseudovirus expressing Spike protein to Syrian hamsters) [124] . Spike proteins (S1 and active trimer, 15 and 30 nM) have been found to induce mitochondrial damage in brain endothelial cells [125] , and spike protein epitopes have been shown to interact with human toll-like receptor 8 (TLR 8), brain targeted Vascular Cell adhesion Molecules (VCAM1) proteins, Zonula Occludens (ZO), and some glia specific proteins (i.e., NDRG2 and Apo-S100B), which can lead to neuroinflammation [126] .

Moreover, 10 nM SARS-CoV-2 viral spike proteins have been shown to alter BBB functions and induce pro-inflammatory response after 24h in primary human brain microvascular endothelial cells (hBMVECs) cultured in 2D and 3D [127] . Radioiodinated spike S1 (12.5 ng per 300,000 c.p.m.

BSA) has been found to cross the BBB and enter the parenchymal brain space in intravenously injected mice [128] .

The concentrations of Spike protein tested in this study (5 and 10 µg/mL), are similar to what has been observed in the plasma of some COVID-19 patients (2.5-17.5 µg/mL) [66] . To the best of our knowledge, there are no published studies reporting the levels of spike protein in pregnant women affected by COVID-19. With regards to levels of spike protein found upon vaccination with mRNAvaccines, very few studies are available. Ogata et al., by using a Quanterix assay, found that spike protein levels in the blood of people vaccinated with mRNA-1273 was < 50 pg/mL after vaccination [129] , i.e., several orders of magnitude lower than the levels of spike tested in our study. On the other hand, Cognetti and Miller, by using a Disposable Photonics platform, found that 1-3 days after injection of mRNA vaccine BNT162b2, spike levels underwent an average shift of 15 pm, which corresponds to a maximum concentration of 14.6 μg/mL, returning to a baseline level in less than a J o u r n a l P r e -p r o o f 36 month [130] . The discrepancy between the two studies is high and not yet validated. Besides and again, the important missing data is the concentration of Spike proteins in cord blood or placenta.

Results reported in the present study are preliminary and would require further investigations, such as to assess the molecular and cellular effects induced by repeated dose exposure to Spike protein and Peptides, as well as the kinetics of spike protein entry into the cells. However, emerging evidence prompts the importance to further investigate and monitor the presently unknown long-term effects elicited by SARS-CoV-2 viral components on brain development. Use of S proteins, nonreplicating SARS-CoV2 pseudoviruses and SARS-CoV-2 viruses might be informative as well to discriminate the impact of S protein binding on the receptor compared to viral entry and the infectious process.

Additionally, in this study we only assessed the expression of ACE2. Future analyses should aim to investigate the expression of other spike protein interactors (i.e., FURIN, ZDHHC5, GOLGA7 and ATP1A1) that are expressed at a high level in the fetal brain [29] , in order to understand their role as alternative brain entry factors for SARS-CoV-2.

The functional and transcriptional perturbations described in the present study could contribute understanding some of the mechanisms underlying the neurodevelopmental manifestations that could be possibly associated with severe COVID-19. Our approach shows that the use of human in vitro models is crucial to gather insights about spike protein and SARS-CoV-2 effects and the role played 

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☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work

The authors would like to thank Dr. Marc Peschanski (I-Stem, Évry, France) for providing IMR90-hiPSCs, and Dr. Anna Navarro Cuenca for providing the license for the use of BioRender.com.

The authors have no conflict of interests to declare.