The uses of 3D human brain organoids for neurotoxicity evaluations: A review

Upregulation of Intracellular Zinc Ion Level after Differentiation of the Neural Stem/Progenitor Cells In Vitro with the Changes in Gene Expression of Zinc Transporters

We measured the intracellular zinc ion concentration of murine fetal neural stem/progenitor cells (NSPCs) and that in the differentiated cells. The NSPCs cultured with 1.5 μM Zn2+ proliferated slightly faster than that in the zinc-deficient medium and the intracellular zinc concentration of the NSPCs and that of their differentiated cells (DCs) cultured with 1.5 μM Zn2+ was 1.34-fold and 2.00-fold higher than those in the zinc-deficient medium, respectively. The zinc transporter genes upregulated over the 3.5-fold change were Zip1, Zip4, Zip12, Zip13, ZnT1, ZnT8, and ZnT10 whereas the only downregulated one was Zip8 during the differentiation of NSPCs to DCs. The cell morphologies of both NSPCs and DCs in the low oxygen culture condition consisting of 2%O2 and 5%CO2, the high carbon dioxide condition consisting of 21%O2 and 10%CO2, and the normal condition consisting of 21%O2 and 5%CO2 were essentially the same each other. The expression of Zip4, Zip8, Zip12, and Zip14 was not drastically changed depending on the O2 and CO2 concentrations.

Molecular mechanisms of silver nanoparticle-induced neurotoxic injury and new perspectives for its neurotoxicity studies: A critical review

Silver nanoparticles (AgNPs) garnered significant attention and applications in the field of nanotechnology due to their unique physicochemical properties. However, with the increasing exposure of AgNPs in the environment and biological systems, concerns about their potential neurotoxicity have also risen. Recent studies on the neurotoxic effects and mechanisms of AgNPs have often relied on traditional toxicological research methods and perspectives. This reliance has limited the extrapolation of these findings to the human brain environment and hindered a deep understanding of the neurotoxicity of AgNPs. This review first outlines the molecular mechanisms of AgNPs-induced neurotoxic injury from a traditional research perspective, identifying oxidative stress, inflammatory responses, and autophagy disorders as key areas of current research. Related molecular signaling pathways, including the nuclear transcription factor-κB (NF-κB) signaling pathway, the nuclear factor erythroid 2-related factor 2 (Nrf2) signaling pathway, and the calcium signaling pathway, have been implicated in the neurotoxic injury process induced by AgNPs. Subsequently, we elucidated the unique advantages of the 3D brain organoids applied to the neurotoxicity study of AgNPs by drawing on relevant studies in the same field. We also emphasize that establishing a standardized 3D brain organoids construction platform is a crucial prerequisite for its widespread application. Furthermore, we suggest that future studies should explore the neurotoxicity mechanisms of AgNPs through the lenses of "adaptive homeostasis" and "structure-activity relationship analysis". In conclusion, the neurotoxicity of AgNPs should be comprehensively evaluated by integrating new research techniques and perspectives, ultimately allowing these nanoparticles to better serve human society.

Microinstrumentation for Brain Organoids

Brain organoids are three-dimensional aggregates of self-organized differentiated stem cells that mimic the structure and function of human brain regions. Organoids bridge the gaps between conventional drug screening models such as planar mammalian cell culture, animal studies, and clinical trials. They can revolutionize the fields of developmental biology, neuroscience, toxicology, and computer engineering. Conventional microinstrumentation for conventional cellular engineering, such as planar microfluidic chips; microelectrode arrays (MEAs); and optical, magnetic, and acoustic techniques, has limitations when applied to three-dimensional (3D) organoids, primarily due to their limits with inherently two-dimensional geometry and interfacing. Hence, there is an urgent need to develop new instrumentation compatible with live cell culture techniques and with scalable 3D formats relevant to organoids. This review discusses conventional planar approaches and emerging 3D microinstrumentation necessary for advanced organoid-machine interfaces. Specifically, this article surveys recently developed microinstrumentation, including 3D printed and curved microfluidics, 3D and fast-scan optical techniques, buckling and self-folding MEAs, 3D interfaces for electrochemical measurements, and 3D spatially controllable magnetic and acoustic technologies relevant to two-way information transfer with brain organoids. This article highlights key challenges that must be addressed for robust organoid culture and reliable 3D spatiotemporal information transfer.

Enhanced electrophysiological activity and neurotoxicity screening of environmental chemicals using 3D neurons from human neural precursor cells purified with PSA-NCAM

The assessment of neurotoxicity for environmental chemicals is of utmost importance in ensuring public health and environmental safety. Multielectrode array (MEA) technology has emerged as a powerful tool for assessing disturbances in the electrophysiological activity. Although human embryonic stem cell (hESC)-derived neurons have been used in MEA for neurotoxicity screening, obtaining a substantial and sufficiently active population of neurons from hESCs remains challenging. In this study, we successfully differentiated neurons from a large population of human neuronal precursor cells (hNPC) purified using a polysialylated neural cell adhesion molecule (PSA-NCAM), referred to as hNPCPSA-NCAM+. The functional characterization demonstrated that hNPCPSA-NCAM+-derived neurons improve functionality by enhancing electrophysiological activity compared to total hNPC-derived neurons. Furthermore, three-dimensional (3D) neurons derived from hNPCPSA-NCAM+ exhibited reduced maturation time and enhanced electrophysiological activity on MEA. We employed subdivided population analysis of active mean firing rate (MFR) based on electrophysiological intensity to characterize the electrophysiological properties of hNPCPSA-NCAM+-3D neurons. Based on electrophysiological activity including MFR and burst parameters, we evaluated the sensitivity of hNPCPSA-NCAM+-3D neurons on MEA to screen both inhibitory and excitatory neuroactive environmental chemicals. Intriguingly, electrophysiologically active hNPCPSA-NCAM+-3D neurons demonstrated good sensitivity to evaluate neuroactive chemicals, particularly in discriminating excitatory chemicals. Our findings highlight the effectiveness of MEA approaches using hNPCPSA-NCAM+-3D neurons in the assessment of neurotoxicity associated with environmental chemicals. Furthermore, we emphasize the importance of selecting appropriate signal intensity thresholds to enhance neurotoxicity prediction and screening of environmental chemicals.

Comparison of developmental toxicity of graphene oxide and graphdiyne to zebrafish larvae

Graphdiyne (GDY) is a new member of family of carbon-based 2D nanomaterials (NMs), but the environmental toxicity is less investigated compared with other 2D NMs, such as graphene oxide (GO). In this study, we compared with developmental toxicity of GO and GDY to zebrafish larvae. It was shown that exposure of zebrafish embryos from 5 h post fertilization to GO and GDY for up to 5 days decreased hatching rate and induced morphological deformity. Behavioral tests indicated that GO and GDY treatment led to hyperactivity of larvae. However, blood flow velocity was not significantly affected by GO or GDY. RNA-sequencing data revealed that both types of NMs altered gene expression profiles as well as gene ontology terms and KEGG pathways related with metabolism. We further confirmed that the NMs altered the expression of genes related with lipid droplets and autophagy, which may be account for the delayed development of zebrafish larvae. At the same mass concentrations, GO induced comparable or even larger toxic effects compared with GDY, indicating that GDY might be more biocompatible compared with GO. These results may provide novel understanding about the environmental toxicity of GO and GDY in vivo.

The uses of zebrafish (Danio rerio) as an in vivo model for toxicological studies: A review based on bibliometrics

An in vivo model is necessary for toxicology. This review analyzed the uses of zebrafish (Danio rerio) in toxicology based on bibliometrics. Totally 56,816 publications about zebrafish from 2002 to 2023 were found in Web of Science Core Collection, with Toxicology as the top 6 among all disciplines. Accordingly, the bibliometric map reveals that "toxicity" has become a hot keyword. It further reveals that the most common exposure types include acute, chronic, and combined exposure. The toxicological effects include behavioral, intestinal, cardiovascular, hepatic, endocrine toxicity, neurotoxicity, immunotoxicity, genotoxicity, and reproductive and transgenerational toxicity. The mechanisms include oxidative stress, inflammation, autophagy, and dysbiosis of gut microbiota. The toxicants commonly evaluated by using zebrafish model include nanomaterials, arsenic, metals, bisphenol, and dioxin. Overall, zebrafish provide a unique and well-accepted model to investigate the toxicological effects and mechanisms. We also discussed the possible ways to address some of the limitations of zebrafish model, such as the combination of human organoids to avoid species differences.

Long-term exposure to cadmium disrupts neurodevelopment in mature cerebral organoids

Cadmium (Cd) is a pervasive environmental pollutant. Increasing evidence suggests that Cd exposure during pregnancy can induce adverse neurodevelopmental outcomes. However, due to the limitations of neural cell and animal models, it is challenging to study the developmental neurotoxicity and underlying toxicity mechanism of long-term exposure to environmental pollutants during human brain development. In this study, chronic Cd exposure was performed in human mature cerebral organoids for 49 or 77 days. Our study found that prolonged exposure to Cd resulted in the inhibition of cerebral organoid growth and the disruption of neural differentiation and cortical layer organization. These potential consequences of chronic Cd exposure may include impaired GFAP expression, a reduction in SOX2+ neuronal progenitor cells, an increase in TUJ1+ immature neurons, as well as an initial increase and a subsequent decrease in both TBR2+ intermediate progenitors and CTIP2+ deep layer cortical neurons. Transcriptomic analyses revealed that long-term exposure to Cd disrupted zinc and copper ion homeostasis through excessive synthesis of metallothionein and disturbed synaptogenesis, as evidenced by inhibited postsynaptic protein. Our study employed mature cerebral organoids to evaluate the developmental neurotoxicity induced by long-term Cd exposure.

Exposure of zebrafish (Danio rerio) to graphene oxide for 6 months suppressed NOD-like receptor-regulated anti-virus signaling pathways

Environmental exposure to graphene oxide (GO) is likely to happen due to the use and disposal of these materials. Although GO-induced ecological toxicity has been evaluated before by using aquatic models such as zebrafish, previous studies typically focused on the short-term toxicity, whereas this study aimed to investigate the long-term toxicity. To this end, we exposed zebrafish to GO for 6 months, and used RNA-sequencing to reveal the changes of signaling pathways. While GO exposure showed no significant effects on locomotor activities, it induced histological changes in livers. RNA-sequencing data showed that GO altered gene expression profiles, resulting in 82 up-regulated and 275 down-regulated genes, respectively. Through the analysis of gene ontology terms and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways, we found that GO suppressed the signaling pathways related with immune systems. We further verified that GO exposure suppressed the expression of genes involved in anti-virus responses possibly through the inhibition of genes involved in NOD-like receptor signaling pathway. Furthermore, NOD-like receptor-regulated lipid genes were also inhibited, which may consequently lead to decreased lipid staining in fish muscles. We concluded that 6 month-exposure to GO suppressed NOD-like receptor-regulated anti-virus signaling pathways in zebrafish.

Chronic exposure to (2 R,6 R)-hydroxynorketamine induces developmental neurotoxicity in hESC-derived cerebral organoids

(R,S)-ketamine (ketamine) has been increasingly used recreationally and medicinally worldwide; however, it cannot be removed by conventional wastewater treatment plants. Both ketamine and its metabolite norketamine have been frequently detected to a significant degree in effluents, aquatic, and even atmospheric environments, which may pose risks to organisms and humans via drinking water and aerosols. Ketamine has been shown to affect the brain development of unborn babies, while it is still elusive whether (2 R,6 R)-hydroxynorketamine (HNK) induces similar neurotoxicity. Here, we investigated the neurotoxic effect of (2 R,6 R)-HNK exposure at the early stages of gestation by applying human cerebral organoids derived from human embryonic stem cells (hESCs). Short-term (2 R,6 R)-HNK exposure did not significantly affect the development of cerebral organoids, but chronic high-concentration (2 R,6 R)-HNK exposure at day 16 inhibited the expansion of organoids by suppressing the proliferation and augmentation of neural precursor cells (NPCs). Notably, the division mode of apical radial glia was unexpectedly switched from vertical to horizontal division planes following chronic (2 R,6 R)-HNK exposure in cerebral organoids. Chronic (2 R,6 R)-HNK exposure at day 44 mainly inhibited the differentiation but not the proliferation of NPCs. Overall, our findings indicate that (2 R,6 R)-HNK administration leads to the abnormal development of cortical organoids, which may be mediated by inhibiting HDAC2. Future clinical studies are needed to explore the neurotoxic effects of (2 R,6 R)-HNK on the early development of the human brain.

An update on the biological effects of quantum dots: From environmental fate to risk assessment based on multiple biological models

Quantum dots (QDs) are zero-dimension nanomaterials with excellent physical and chemical properties, which have been widely used in environmental science and biomedicine. Therefore, QDs are potential to cause toxicity to the environment and enter organisms through migration and bioenrichment effects. This review aims to provide a comprehensive and systematic analysis on the adverse effects of QDs in different organisms based on recently available data. Following PRISMA guidelines, this study searched PubMed database according to the pre-set keywords, and included 206 studies according to the inclusion and elimination criteria. CiteSpace software was firstly used to analyze the keywords of included literatures, search for breaking points of former studies, and summarize the classification, characterization and dosage of QDs. The environment fate of QDs in the ecosystems were then analyzed, followed with comprehensively summarized toxicity outcomes at individual, system, cell, subcellular and molecular levels. After migration and degradation in the environment, aquatic plants, bacteria, fungi as well as invertebrates and vertebrates have been found to be suffered from toxic effects caused by QDs. Aside from systemic effects, toxicity of intrinsic QDs targeting to specific organs, including respiratory system, cardiovascular system, hepatorenal system, nervous system and immune system were confirmed in multiple animal models. Moreover, QDs could be taken up by cells and disturb the organelles, which resulted in cellular inflammation and cell death, including autophagy, apoptosis, necrosis, pyroptosis and ferroptosis. Recently, several innovative technologies, like organoids have been applied in the risk assessment of QDs to promote the surgical interventions of preventing QDs' toxicity. This review not only aimed at updating the research progress on the biological effects of QDs from environmental fate to risk assessment, but also overcame the limitations of available reviews on basic toxicity of nanomaterials by interdisciplinarity and provided new insights for better applications of QDs.

Exposure to MoS2 nanosheets or bulk activated Kruppel-like factor 4 in 3D Caco-2 spheroids in vitro and mouse intestines in vivo

MoS₂ nanosheets (NSs) are novel 2D nanomaterials (NMs) being used in many important fields. Recently, we proposed the need to evaluate the influences of NMs on Kruppel-like factors (KLFs) even if these materials are relatively biocompatible. In this study, we investigated the influences of MoS₂ NSs or bulk on KLF4 signaling pathway in 3D Caco-2 spheroids in vitro and mouse intestines in vivo. Through the analysis of our previous RNA-sequencing data, we found that exposure to MoS₂ NSs or bulk activated KLF4 expression in 3D Caco-2 spheroids. Consistently, these materials also activated KLF4-related gene ontology (GO) terms and down-regulated a panel of KLF4-downstream genes. To verify these findings, we repeatedly exposed mice to MoS₂ NSs or bulk materials via intragastrical administration (1 mg/kg bodyweight, once a day, for 4 days). It was shown that oral exposure to these materials decreased bodyweight, leading to relatively higher organ coefficients. As expected, exposure to both types of materials increased Mo elements as well as other trace elements, such as Zn, Fe, and Mn in mouse intestines. The exposure also induced morphological changes of intestines, such as shortening of intestinal villi and decreased crypt depth, which may result in decreased intestinal lipid staining. Consistent with RNA-sequencing data, we found that material exposure increased KLF4 protein staining in mouse intestines and decreased two KLF4 downstream proteins, namely extracellular signal-regulated kinase (ERK) and serine/threonine kinase (AKT). We concluded that MoS₂ materials were capable to activate KLF4-signaling pathway in intestines both in vivo and in vitro.

Prolonged Differentiation of Neuron-Astrocyte Co-Cultures Results in Emergence of Dopaminergic Neurons

Dopamine is present in a subgroup of neurons that are vital for normal brain functioning. Disruption of the dopaminergic system, e.g., by chemical compounds, contributes to the development of Parkinson's disease and potentially some neurodevelopmental disorders. Current test guidelines for chemical safety assessment do not include specific endpoints for dopamine disruption. Therefore, there is a need for the human-relevant assessment of (developmental) neurotoxicity related to dopamine disruption. The aim of this study was to determine the biological domain related to dopaminergic neurons of a human stem cell-based in vitro test, the human neural progenitor test (hNPT). Neural progenitor cells were differentiated in a neuron-astrocyte co-culture for 70 days, and dopamine-related gene and protein expression was investigated. Expression of genes specific for dopaminergic differentiation and functioning, such as LMX1B, NURR1, TH, SLC6A3, and KCNJ6, were increasing by day 14. From day 42, a network of neurons expressing the catecholamine marker TH and the dopaminergic markers VMAT2 and DAT was present. These results confirm stable gene and protein expression of dopaminergic markers in hNPT. Further characterization and chemical testing are needed to investigate if the model might be relevant in a testing strategy to test the neurotoxicity of the dopaminergic system.

The cytological and electrophysiological effects of silver nanoparticles on neuron-like PC12 cells

The aim of this study was to investigate the toxic effects and mechanism of silver nanoparticles (SNPs) on the cytological and electrophysiological properties of rat adrenal pheochromocytoma (PC12) cells. Different concentrations of SNPs (20 nm) were prepared, and the effects of different application durations on the cell viability and electrical excitability of PC12 quasi-neuronal networks were investigated. The effects of 200 μM SNPs on the neurite length, cell membrane potential (CMP) difference, intracellular Ca2+ content, mitochondrial membrane potential (MMP) difference, adenosine triphosphate (ATP) content, and reactive oxygen species (ROS) content of networks were then investigated. The results showed that 200 μM SNPs produced grade 1 cytotoxicity at 48 h of interaction, and the other concentrations of SNPs were noncytotoxic. Noncytotoxic 5 μM SNPs significantly increased electrical excitability, and noncytotoxic 100 μM SNPs led to an initial increase followed by a significant decrease in electrical excitability. Cytotoxic SNPs (200 μM) significantly decreased electrical excitability. SNPs (200 μM) led to decreases in neurite length, MMP difference and ATP content and increases in CMP difference and intracellular Ca2+ and ROS levels. The results revealed that not only cell viability but also electrophysiological properties should be considered when evaluating nanoparticle-induced neurotoxicity. The SNP-induced cytotoxicity mainly originated from its effects on ATP content, cytoskeletal structure and ROS content. The decrease in electrical excitability was mainly due to the decrease in ATP content. ATP content may thus be an important indicator of both cell viability and electrical excitability in PC12 quasi-neuronal networks.

The uses of transcriptomics and lipidomics indicated that direct contact with graphene oxide altered lipid homeostasis through ER stress in 3D human brain organoids

The potential uses of graphene-based nanomaterials (NMs) in various fields lead to the concern about their neurotoxicity, considering that graphene-based NMs are capable to cross blood brain barrier (BBB) and enter central nervous system (CNS). Although previous studies reported the possibility of graphene-based NM exposure to alter lipid homeostasis in animals or cultured neurons, recent studies suggested the need to use 3D human brain organoids for mechanism-based toxicological studies as this model might better recapitulate the complex human brains. Herein, we used multi-omics techniques to investigate the mechanisms of graphene oxide (GO) on lipid homeostasis in a novel 3D brain organoid model. We found that 50 μg/mL GO induced cytotoxicity but not superoxide. RNA-sequencing data showed that 50 μg/mL GO significantly up-regulated and down-regulated 80 and 121 genes, respectively. Furthermore, we found that GO exposure altered biological molecule metabolism pathways including lipid metabolism. Consistently, lipidomics data supported dose-dependent alteration of lipid profiles by GO in 3D brain organoids. Interestingly, co-exposure to GO and endoplasmic reticulum (ER) stress inhibitor 4-phenylbutyric acid (4-PBA) decreased most of the lipid classes compared with the exposure of GO only. We further verified that exposure to GO promoted ER stress marker GRP78 proteins, which in turn activated IRE1α/XBP-1 axis, and these changes were partially or completely inhibited by 4-PBA. These results proved that direct contact with GO disrupted lipid homeostasis through the activation of ER stress. As 3D brain organoids resemble human brains, these data might be better extrapolated to humans.

Silver nanoparticles exposure induces developmental neurotoxicity in hiPSC-derived cerebral organoids

Silver nanoparticles (AgNPs) are used in various research fields. Although the neurotoxicity of AgNPs has been explored in animal models and 2D cell-culture models, including human stem cells, these models cannot accurately mimic the development of the human brain. Therefore, the potential mechanisms of AgNPs-induced developmental neurotoxicity in humans are still largely unclear. In this study, cerebral organoids derived from induced pluripotent stem cells were treated with 0.1 μg/mL or 0.5 μg/mL AgNPs for 7 days. At the low concentration (0.1 μg/mL), AgNPs increased the cell proliferation and inhibited the neural apoptosis in the organoids, but impaired the cilium assembly and elongation, which may perturb the cell cycle and induce abnormal cerebral-organoid growth. Conversely, at the high concentration (0.5 μg/mL), AgNPs significantly inhibited cell proliferation and induced apoptosis in cerebral organoids. High-concentration AgNPs reduced the expression and co-localization of the cytoskeleton proteins F-actin, myosin, and tubulin, thereby perturbing neurite growth. In conclusion, AgNPs exposure induces developmental neurotoxic effects in cerebral organoids and is thus a potential congenital risk factor.

The SH-SY5Y human neuroblastoma cell line, a relevant in vitro cell model for investigating neurotoxicology in human: focus on organic pollutants

Investigation of the toxicity triggered by chemicals on the human brain has traditionally relied on approaches using rodent in vivo models and in vitro cell models including primary neuronal cultures and cell lines from rodents. The issues of species differences between humans and rodents, the animal ethical concerns and the time and cost required for neurotoxicity studies on in vivo animal models, do limit the use of animal-based models in neurotoxicology. In this context, human cell models appear relevant in elucidating cellular and molecular impacts of neurotoxicants and facilitating prioritization of in vivo testing. The SH-SY5Y human neuroblastoma cell line (ATCC® CRL-2266^TM) is one of the most used cell lines in neurosciences, either undifferentiated or differentiated into neuron-like cells. This review presents the characteristics of the SH-SY5Y cell line and proposes the results of a systematic review of literature on the use of this in vitro cell model for neurotoxicity research by focusing on organic environmental pollutants including pesticides, 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD), flame retardants, PFASs, parabens, bisphenols, phthalates, and PAHs. Organic environmental pollutants are widely present in the environment and increasingly known to cause clinical neurotoxic effects during fetal & child development and adulthood. Their effects on cultured SH-SY5Y cells include autophagy, cell death (apoptosis, pyroptosis, necroptosis, or necrosis), increased oxidative stress, mitochondrial dysfunction, disruption of neurotransmitter homeostasis, and alteration of neuritic length. Finally, the inherent advantages and limitations of the SH-SY5Y cell model are discussed in the context of chemical testing.

Intratracheal instillation of graphene oxide decreases anti-virus responses and lipid contents via suppressing Toll-like receptor 3 in mouse livers

Recent studies revealed a causal relationship between Toll-like receptors (TLRs) and lipid droplet biogenesis. Interestingly, it has been reported before that nanomaterials (NMs) were capable to modulate TLRs, but it remains unclear if NMs could affect lipid levels via TLR signaling pathways. In this study, we investigated the influences of airway exposure to graphene oxide (GO) on TLR3 signaling pathways and lipid levels in mouse livers. Intratracheal instillation of GO (0.1, 1, and 5 mg/kg, once a day, totally 5 days) induced inflammatory cell infiltrations as indicated by hematoxylin-eosin (H&E) staining and fibrosis as indicated by Masson staining in lungs, accompanying with decreased TLR3 proteins. Consistently, a TLR3-regulated anti-virus protein, namely interferon induced protein with tetratricopeptide repeats 1 (IFIT1), as well as two TLR3-regulated lipid proteins, namely radical S-adenosyl methionine domain containing 2 (RSAD2) and perilipin 2 (PLIN2), were decreased in lungs. The protein levels of interferon-β in serum were also decreased. In livers, GO exposure induced disorganization of liver cells but not fibrosis. In agreement with the trends observed in lungs, TLR3, IFIT1, RSAD2, and PLIN2 proteins were decreased in livers. As a possible consequence, GO exposure dose-dependently decreased lipid levels in livers as indicated by oil red O and BODIPY 493/503 staining. We concluded that airway exposure to GO decreased anti-virus responses and lipid levels in mouse livers via the suppression of TLR3.