Development of a neural rosette formation assay (RoFA) to identify neurodevelopmental toxicants and to characterize their transcriptome disturbances.

The first in vitro tests for developmental toxicity made use of rodent cells. Newer teratology tests, e.g. developed during the ESNATS project, use human cells and measure mechanistic endpoints (such as transcriptome changes). However, the toxicological implications of mechanistic parameters are hard to judge, without functional/morphological endpoints. To address this issue, we developed a new version of the human stem cell-based test STOP-tox(UKN). For this purpose, the capacity of the cells to self-organize to neural rosettes was assessed as functional endpoint: pluripotent stem cells were allowed to differentiate into neuroepithelial cells for 6 days in the presence or absence of toxicants. Then, both transcriptome changes were measured (standard STOP-tox(UKN)) and cells were allowed to form rosettes. After optimization of staining methods, an imaging algorithm for rosette quantification was implemented and used for an automated rosette formation assay (RoFA). Neural tube toxicants (like valproic acid), which are known to disturb human development at stages when rosette-forming cells are present, were used as positive controls. Established toxicants led to distinctly different tissue organization and differentiation stages. RoFA outcome and transcript changes largely correlated concerning (1) the concentration-dependence, (2) the time dependence, and (3) the set of positive hits identified amongst 24 potential toxicants. Using such comparative data, a prediction model for the RoFA was developed. The comparative analysis was also used to identify gene dysregulations that are particularly predictive for disturbed rosette formation. This 'RoFA predictor gene set' may be used for a simplified and less costly setup of the STOP-tox(UKN) assay.

Design and evaluation of bi-functional iron chelators for protection of dopaminergic neurons from toxicants.

While the etiology of non-familial Parkinson's disease (PD) remains unclear, there is evidence that increased levels of tissue iron may be a contributing factor. Moreover, exposure to some environmental toxicants is considered an additional risk factor. Therefore, brain-targeted iron chelators are of interest as antidotes for poisoning with dopaminergic toxicants, and as potential treatment of PD. We, therefore, designed a series of small molecules with high affinity for ferric iron and containing structural elements to allow their transport to the brain via the neutral amino acid transporter, LAT1 (SLC7A5). Five candidate molecules were synthesized and initially characterized for protection from ferroptosis in human neurons. The promising hydroxypyridinone SK4 was characterized further. Selective iron chelation within the physiological range of pH values and uptake by LAT1 were confirmed. Concentrations of 10-20 µM blocked neurite loss and cell demise triggered by the parkinsonian neurotoxicants, methyl-phenyl-pyridinium (MPP+) and 6-hydroxydopamine (6-OHDA) in human dopaminergic neuronal cultures (LUHMES cells). Rescue was also observed when chelators were given after the toxicant. SK4 derivatives that either lacked LAT1 affinity or had reduced iron chelation potency showed altered activity in our assay panel, as expected. Thus, an iron chelator was developed that revealed neuroprotective properties, as assessed in several models. The data strongly support the role of iron in dopaminergic neurotoxicity and suggests further exploration of the proposed design strategy for improving brain iron chelation.

Large-Scale Production of Human iPSC-Derived Macrophages for Drug Screening.

Tissue-resident macrophages are key players in inflammatory processes, and their activation and functionality are crucial in health and disease. Numerous diseases are associated with alterations in homeostasis or dysregulation of the innate immune system, including allergic reactions, autoimmune diseases, and cancer. Macrophages are a prime target for drug discovery due to their major regulatory role in health and disease. Currently, the main sources of macrophages used for therapeutic compound screening are primary cells isolated from blood or tissue or immortalized or neoplastic cell lines (e.g., THP-1). Here, we describe an improved method to employ induced pluripotent stem cells (iPSCs) for the high-yield, large-scale production of cells resembling tissue-resident macrophages. For this, iPSC-derived macrophage-like cells are thoroughly characterized to confirm their cell identity and thus their suitability for drug screening purposes. These iPSC-derived macrophages show strong cellular identity with primary macrophages and recapitulate key functional characteristics, including cytokine release, phagocytosis, and chemotaxis. Furthermore, we demonstrate that genetic modifications can be readily introduced at the macrophage-like progenitor stage in order to interrogate drug target-relevant pathways. In summary, this novel method overcomes previous shortcomings with primary and leukemic cells and facilitates large-scale production of genetically modified iPSC-derived macrophages for drug screening applications.

Stage-specific metabolic features of differentiating neurons: Implications for toxicant sensitivity.

Developmental neurotoxicity (DNT) may be induced when chemicals disturb a key neurodevelopmental process, and many tests focus on this type of toxicity. Alternatively, DNT may occur when chemicals are cytotoxic only during a specific neurodevelopmental stage. The toxicant sensitivity is affected by the expression of toxicant targets and by resilience factors. Although cellular metabolism plays an important role, little is known how it changes during human neurogenesis, and how potential alterations affect toxicant sensitivity of mature vs. immature neurons. We used immature (d0) and mature (d6) LUHMES cells (dopaminergic human neurons) to provide initial answers to these questions. Transcriptome profiling and characterization of energy metabolism suggested a switch from predominantly glycolytic energy generation to a more pronounced contribution of the tricarboxylic acid cycle (TCA) during neuronal maturation. Therefore, we used pulsed stable isotope-resolved metabolomics (pSIRM) to determine intracellular metabolite pool sizes (concentrations), and isotopically non-stationary 13C-metabolic flux analysis (INST 13C-MFA) to calculate metabolic fluxes. We found that d0 cells mainly use glutamine to fuel the TCA. Furthermore, they rely on extracellular pyruvate to allow continuous growth. This metabolic situation does not allow for mitochondrial or glycolytic spare capacity, i.e. the ability to adapt energy generation to altered needs. Accordingly, neuronal precursor cells displayed a higher sensitivity to several mitochondrial toxicants than mature neurons differentiated from them. In summary, this study shows that precursor cells lose their glutamine dependency during differentiation while they gain flexibility of energy generation and thereby increase their resistance to low concentrations of mitochondrial toxicants.

Major changes of cell function and toxicant sensitivity in cultured cells undergoing mild, quasi-natural genetic drift.

Genomic drift affects the functional properties of cell lines, and the reproducibility of data from in vitro studies. While chromosomal aberrations and mutations in single pivotal genes are well explored, little is known about effects of minor, possibly pleiotropic, genome changes. We addressed this question for the human dopaminergic neuronal precursor cell line LUHMES by comparing two subpopulations (SP) maintained either at the American-Type-Culture-Collection (ATCC) or by the original provider (UKN). Drastic differences in susceptibility towards the specific dopaminergic toxicant 1-methyl-4-phenylpyridinium (MPP+) were observed. Whole-genome sequencing was performed to identify underlying genetic differences. While both SP had normal chromosome structures, they displayed about 70 differences on the level of amino acid changing events. Some of these differences were confirmed biochemically, but none offered a direct explanation for the altered toxicant sensitivity pattern. As second approach, markers known to be relevant for the intended use of the cells were specifically tested. The "ATCC" cells rapidly down-regulated the dopamine-transporter and tyrosine-hydroxylase after differentiation, while "UKN" cells maintained functional levels. As the respective genes were not altered themselves, we conclude that polygenic complex upstream changes can have drastic effects on biochemical features and toxicological responses of relatively similar SP of cells.

Conversion of Nonproliferating Astrocytes into Neurogenic Neural Stem Cells: Control by FGF2 and Interferon-γ.

Conversion of astrocytes to neurons, via de-differentiation to neural stem cells (NSC), may be a new approach to treat neurodegenerative diseases and brain injuries. The signaling factors affecting such a cell conversion are poorly understood, and they are hard to identify in complex disease models or conventional cell cultures. To address this question, we developed a serum-free, strictly controlled culture system of pure and homogeneous "astrocytes generated from murine embryonic stem cells (ESC)." These stem cell derived astrocytes (mAGES), as well as standard primary astrocytes resumed proliferation upon addition of FGF. The signaling of FGF receptor tyrosine kinase converted GFAP-positive mAGES to nestin-positive NSC. ERK phosphorylation was necessary, but not sufficient, for cell cycle re-entry, as EGF triggered no de-differentiation. The NSC obtained by de-differentiation of mAGES were similar to those obtained directly by differentiation of ESC, as evidenced by standard phenotyping, and also by transcriptome mapping, metabolic profiling, and by differentiation to neurons or astrocytes. The de-differentiation was negatively affected by inflammatory mediators, and in particular, interferon-γ strongly impaired the formation of NSC from mAGES by a pathway involving phosphorylation of STAT1, but not the generation of nitric oxide. Thus, two antagonistic signaling pathways were identified here that affect fate conversion of astrocytes independent of genetic manipulation. The complex interplay of the respective signaling molecules that promote/inhibit astrocyte de-differentiation may explain why astrocytes do not readily form neural stem cells in most diseases. Increased knowledge of such factors may provide therapeutic opportunities to favor such conversions. Stem Cells 2016;34:2861-2874.

Switching from astrocytic neuroprotection to neurodegeneration by cytokine stimulation.

Astrocytes, the largest cell population in the human brain, are powerful inflammatory effectors. Several studies have examined the interaction of activated astrocytes with neurons, but little is known yet about human neurotoxicity under such situations and about strategies of neuronal rescue. To address this question, immortalized murine astrocytes (IMA) were combined with human LUHMES neurons and stimulated with an inflammatory (TNF, IL-1) cytokine mix (CM). Neurotoxicity was studied both in co-cultures and in monocultures after transfer of conditioned medium from activated IMA. Interventions with >20 drugs were used to profile the model system. Control IMA supported neurons and protected them from neurotoxicants. Inflammatory activation reduced this protection, and prolonged exposure of co-cultures to CM triggered neurotoxicity. Neither the added cytokines nor the release of NO from astrocytes were involved in this neurodegeneration. The neurotoxicity-mediating effect of IMA was faithfully reproduced by human astrocytes. Moreover, glia-dependent toxicity was also observed, when IMA cultures were stimulated with CM, and the culture medium was transferred to neurons. Such neurotoxicity was prevented when astrocytes were treated by p38 kinase inhibitors or dexamethasone, whereas such compounds had no effect when added to neurons. Conversely, treatment of neurons with five different drugs, including resveratrol and CEP1347, prevented toxicity of astrocyte supernatants. Thus, the sequential IMA-LUHMES neuroinflammation model is suitable for separate profiling of both glial-directed and directly neuroprotective strategies. Moreover, direct evaluation in co-cultures of the same cells allows for testing of therapeutic effectiveness in more complex settings, in which astrocytes affect pharmacological properties of neurons.

In vitro acute and developmental neurotoxicity screening: an overview of cellular platforms and high-throughput technical possibilities.

Neurotoxicity and developmental neurotoxicity are important issues of chemical hazard assessment. Since the interpretation of animal data and their extrapolation to man is challenging, and the amount of substances with information gaps exceeds present animal testing capacities, there is a big demand for in vitro tests to provide initial information and to prioritize for further evaluation. During the last decade, many in vitro tests emerged. These are based on animal cells, human tumour cell lines, primary cells, immortalized cell lines, embryonic stem cells, or induced pluripotent stem cells. They differ in their read-outs and range from simple viability assays to complex functional endpoints such as neural crest cell migration. Monitoring of toxicological effects on differentiation often requires multiomics approaches, while the acute disturbance of neuronal functions may be analysed by assessing electrophysiological features. Extrapolation from in vitro data to humans requires a deep understanding of the test system biology, of the endpoints used, and of the applicability domains of the tests. Moreover, it is important that these be combined in the right way to assess toxicity. Therefore, knowledge on the advantages and disadvantages of all cellular platforms, endpoints, and analytical methods is essential when establishing in vitro test systems for different aspects of neurotoxicity. The elements of a test, and their evaluation, are discussed here in the context of comprehensive prediction of potential hazardous effects of a compound. We summarize the main cellular characteristics underlying neurotoxicity, present an overview of cellular platforms and read-out combinations assessing distinct parts of acute and developmental neurotoxicology, and highlight especially the use of stem cell-based test systems to close gaps in the available battery of tests.

Functional and phenotypic differences of pure populations of stem cell-derived astrocytes and neuronal precursor cells.

Availability of homogeneous astrocyte populations would facilitate research concerning cell plasticity (metabolic and transcriptional adaptations; innate immune responses) and cell cycle reactivation. Current protocols to prepare astrocyte cultures differ in their final content of immature precursor cells, preactivated cells or entirely different cell types. A new method taking care of all these issues would improve research on astrocyte functions. We found here that the exposure of a defined population of pluripotent stem cell-derived neural stem cells (NSC) to BMP4 results in pure, nonproliferating astrocyte cultures within 24-48 h. These murine astrocytes generated from embryonic stem cells (mAGES) expressed the positive markers GFAP, aquaporin 4 and GLT-1, supported neuronal function, and acquired innate immune functions such as the response to tumor necrosis factor and interleukin 1. The protocol was applicable to several normal or disease-prone pluripotent cell lines, and the corresponding mAGES all exited the cell cycle and lost most of their nestin expression, in contrast to astrocytes generated by serum-addition or obtained as primary cultures. Comparative gene expression analysis of mAGES and NSC allowed quantification of differences between the two cell types and a definition of an improved marker set to define astrocytes. Inclusion of several published data sets in this transcriptome comparison revealed the similarity of mAGES with cortical astrocytes in vivo. Metabolic analysis of homogeneous NSC and astrocyte populations revealed distinct neurochemical features: both cell types synthesized glutamine and citrate, but only mature astrocytes released these metabolites. Thus, the homogeneous cultures allowed an improved definition of NSC and astrocyte features.

Definition of the Neurotoxicity-Associated Metabolic Signature Triggered by Berberine and Other Respiratory Chain Inhibitors.

To characterize the hits from a phenotypic neurotoxicity screen, we obtained transcriptomics data for valinomycin, diethylstilbestrol, colchicine, rotenone, 1-methyl-4-phenylpyridinium (MPP), carbaryl and berberine (Ber). For all compounds, the concentration triggering neurite degeneration correlated with the onset of gene expression changes. The mechanistically diverse toxicants caused similar patterns of gene regulation: the responses were dominated by cell de-differentiation and a triggering of canonical stress response pathways driven by ATF4 and NRF2. To obtain more detailed and specific information on the modes-of-action, the effects on energy metabolism (respiration and glycolysis) were measured. Ber, rotenone and MPP inhibited the mitochondrial respiratory chain and they shared complex I as the target. This group of toxicants was further evaluated by metabolomics under experimental conditions that did not deplete ATP. Ber (204 changed metabolites) showed similar effects as MPP and rotenone. The overall metabolic situation was characterized by oxidative stress, an over-abundance of NADH (>1000% increase) and a re-routing of metabolism in order to dispose of the nitrogen resulting from increased amino acid turnover. This unique overall pattern led to the accumulation of metabolites known as biomarkers of neurodegeneration (saccharopine, aminoadipate and branched-chain ketoacids). These findings suggest that neurotoxicity of mitochondrial inhibitors may result from an ensemble of metabolic changes rather than from a simple ATP depletion. The combi-omics approach used here provided richer and more specific MoA data than the more common transcriptomics analysis alone. As Ber, a human drug and food supplement, mimicked closely the mode-of-action of known neurotoxicants, its potential hazard requires further investigation.

Identification of the bacterial metabolite aerugine as potential trigger of human dopaminergic neurodegeneration.

The causes of nigrostriatal cell death in idiopathic Parkinson's disease are unknown, but exposure to toxic chemicals may play some role. We followed up here on suggestions that bacterial secondary metabolites might be selectively cytotoxic to dopaminergic neurons. Extracts from Streptomyces venezuelae were found to kill human dopaminergic neurons (LUHMES cells). Utilizing this model system as a bioassay, we identified a bacterial metabolite known as aerugine (C10H11NO2S; 2-[4-(hydroxymethyl)-4,5-dihydro-1,3-thiazol-2-yl]phenol) and confirmed this finding by chemical re-synthesis. This 2-hydroxyphenyl-thiazoline compound was previously shown to be a product of a wide-spread biosynthetic cluster also found in the human microbiome and in several pathogens. Aerugine triggered half-maximal dopaminergic neurotoxicity at 3-4 µM. It was less toxic for other neurons (10-20 µM), and non-toxic (at <100 µM) for common human cell lines. Neurotoxicity was completely prevented by several iron chelators, by distinct anti-oxidants and by a caspase inhibitor. In the Caenorhabditis elegans model organism, general survival was not affected by aerugine concentrations up to 100 µM. When transgenic worms, expressing green fluorescent protein only in their dopamine neurons, were exposed to aerugine, specific neurodegeneration was observed. The toxicant also exerted functional dopaminergic toxicity in nematodes as determined by the "basal slowing response" assay. Thus, our research has unveiled a bacterial metabolite with a remarkably selective toxicity toward human dopaminergic neurons in vitro and for the dopaminergic nervous system of Caenorhabditis elegans in vivo. These findings suggest that microbe-derived environmental chemicals should be further investigated for their role in the pathogenesis of Parkinson's disease.

Dynamic Metabolic and Transcriptional Responses of Proteasome-Inhibited Neurons.

Proteasome inhibition is associated with parkinsonian pathology in vivo and degeneration of dopaminergic neurons in vitro. We explored here the metabolome (386 metabolites) and transcriptome (3257 transcripts) regulations of human LUHMES neurons, following exposure to MG-132 [100 nM]. This proteasome inhibitor killed cells within 24 h but did not reduce viability for 12 h. Overall, 206 metabolites were changed in live neurons. The early (3 h) metabolome changes suggested a compromised energy metabolism. For instance, AMP, NADH and lactate were up-regulated, while glycolytic and citric acid cycle intermediates were down-regulated. At later time points, glutathione-related metabolites were up-regulated, most likely by an early oxidative stress response and activation of NRF2/ATF4 target genes. The transcriptome pattern confirmed proteostatic stress (fast up-regulation of proteasome subunits) and also suggested the progressive activation of additional stress response pathways. The early ones (e.g., HIF-1, NF-kB, HSF-1) can be considered a cytoprotective cellular counter-regulation, which maintained cell viability. For instance, a very strong up-regulation of AIFM2 (=FSP1) may have prevented fast ferroptotic death. For most of the initial period, a definite life-death decision was not taken, as neurons could be rescued for at least 10 h after the start of proteasome inhibition. Late responses involved p53 activation and catabolic processes such as a loss of pyrimidine synthesis intermediates. We interpret this as a phase of co-occurrence of protective and maladaptive cellular changes. Altogether, this combined metabolomics-transcriptomics analysis informs on responses triggered in neurons by proteasome dysfunction that may be targeted by novel therapeutic intervention in Parkinson's disease.

A Novel Mitochondria-Targeting Iron Chelator Neuroprotects Multimodally via HIF-1 Modulation Against a Mitochondrial Toxin in a Dopaminergic Cell Model of Parkinson's Disease.

Coumarins are plant-derived polyphenolic compounds belonging to the benzopyrones family, possessing wide-ranging pharmaceutical applications including cytoprotection, which may translate into therapeutic potential for multiple diseases, including Parkinson's disease (PD). Here we demonstrate the neuroprotective potential of a new polyhydroxyl coumarin, N-(1,3-dihydroxy-2-(hydroxymethyl)propan-2-yl)-2-(7-hydroxy-2-oxo-2H-chromen-4-yl)acetamide (CT51), against the mitochondrial toxin 1-methyl-4-phenylpyridinium (MPP+). MPP+'s mechanism of toxicity relates to its ability to inhibit complex I of the mitochondrial electron transport chain (METC), leading to adenosine triphosphate (ATP) depletion, increased reactive oxygen species (ROS) production, and apoptotic cell death, hence mimicking PD-related neuropathology. Dopaminergic differentiated human neuroblastoma cells were briefly pretreated with CT51, followed by toxin exposure. CT51 significantly restored somatic cell viability and neurite processes; hence, the drug targets cell bodies and axons thereby preserving neural function and circuitry against PD-related damage. Moreover, MPP+ emulates the iron dyshomeostasis affecting dopaminergic neurons in PD-affected brains, whilst CT51 was previously revealed as an effective iron chelator that preferentially partitions to mitochondria. We extend these findings by characterising the drug's interactive effects at the METC level. CT51 did not improve mitochondrial coupling efficiency. However, voltammetric measurements and high-resolution respirometry analysis revealed that CT51 acts as an antioxidant agent. Also, the neuronal protection afforded by CT51 associated with downregulating MPP+-induced upregulated expression of hypoxia-inducible factor 1 alpha (HIF-1α), a protein which regulates iron homeostasis and protects against certain forms of oxidative stress after translocating to mitochondria. Our findings support the further development of CT51 as a dual functioning iron chelator and antioxidant antiparkinsonian agent.

Discovery of Orally Available and Brain Penetrant AEP Inhibitors.

Alzheimer's Disease (AD) is the most widespread form of dementia, with one of the pathological hallmarks being the formation of neurofibrillary tangles (NFTs). These tangles consist of phosphorylated Tau fragments. Asparagine endopeptidase (AEP) is a key Tau cleaving enzyme that generates aggregation-prone Tau fragments. Inhibition of AEP to reduce the level of toxic Tau fragment formation could represent a promising therapeutic strategy. Here, we report the first orthosteric, selective, orally bioavailable, and brain penetrant inhibitors with an irreversible binding mode. We outline the development of the series starting from reversible molecules and demonstrate the link between inhibition of AEP and reduction of Tau N368 fragment both in vitro and in vivo.

Generation of an hiPSC-Derived Co-Culture System to Assess the Effects of Neuroinflammation on Blood-Brain Barrier Integrity.

The blood-brain barrier (BBB) regulates the interaction between the highly vulnerable central nervous system (CNS) and the peripheral parts of the body. Disruption of the BBB has been associated with multiple neurological disorders, in which immune pathways in microglia are suggested to play a key role. Currently, many in vitro BBB model systems lack a physiologically relevant microglia component in order to address questions related to the mechanism of BBB integrity or the transport of molecules between the periphery and the CNS. To bridge this gap, we redefined a serum-free medium in order to allow for the successful co-culturing of human inducible pluripotent stem cell (hiPSC)-derived microglia and hiPSC-derived brain microvascular endothelial-like cells (BMECs) without influencing barrier properties as assessed by electrical resistance. We demonstrate that hiPSC-derived microglia exposed to lipopolysaccharide (LPS) weaken the barrier integrity, which is associated with the secretion of several cytokines relevant in neuroinflammation. Consequently, here we provide a simplistic humanised BBB model of neuroinflammation that can be further extended (e.g., by addition of other cell types in a more complex 3D architecture) and applied for mechanistic studies and therapeutic compound profiling.

Examination of microcystin neurotoxicity using central and peripheral human neurons.

Microcystins (MC) are a group of cyanobacterial toxins that comprises MC-LF and other cyclic heptapeptides, best known as potent hepatotoxicants. Cell culture and epidemiological studies suggest that MC might also affect the nervous system when there is systemic exposure, e.g., via drinking water or food. We asked whether in vitro studies with human neurons could provide estimates on the neurotoxicity hazard of MC-LF. First, we used LUHMES neurons, a well-established test system for neurotoxicants and neuropathological processes. These central nervous system cells express OATP1A2, a presumed carrier of MC-LF, and we observed selective neurite toxicity in the µM range (EC20 = 3.3 µM ≈ 3.3 µg/mL). Transcriptome changes pointed towards attenuated cell maintenance and biosynthetic processes. Prolonged exposure for up to four days did not increase toxicity. As a second model, we used human dorsal root ganglia-like neurons. These peripheral nervous system cells represent parts of the nervous system not protected by the blood-brain barrier in humans. Toxicity was observed in a similar concentration range (EC20 = 7.4 µM). We conclude that MC-LF poses a potential neurotoxic hazard in humans. The adverse effect concentrations observed here were orders of magnitude higher than those presumed to be encountered after normal nutritional or environmental exposure. However, the low µM concentrations found to be toxic are close to levels that may be reached after very excessive algae supplement intake.

CD22 Blockage Restores Age-Related Impairments of Microglia Surveillance Capacity.

Microglia, the innate immune cells of the brain, are essential for maintaining homeostasis by their ramified, highly motile processes and for orchestrating the immune response to pathological stimuli. They are implicated in several neurodegenerative diseases like Alzheimer's and Parkinson's disease. One commonality of these diseases is their strong correlation with aging as the highest risk factor and studying age-related alterations in microglia physiology and associated signaling mechanism is indispensable for a better understanding of age-related pathomechanisms. CD22 has been identified as a modifier of microglia phagocytosis in a recent study, but not much is known about the function of CD22 in microglia. Here we show that CD22 surface levels are upregulated in aged versus adult microglia. Furthermore, in the amyloid mouse model PS2APP, Aß-containing microglia also exhibit increased CD22 signal. To assess the impact of CD22 blockage on microglia morphology and dynamics, we have established a protocol to image microglia process motility in acutely prepared brain slices from CX3CR1-GFP reporter mice. We observed a significant reduction of microglial ramification and surveillance capacity in brain slices from aged versus adult mice. The age-related decrease in surveillance can be restored by antibody-mediated CD22 blockage in aged mice, whereas surveillance in adult mice is not affected by CD22 inhibition. Moreover to complement the results obtained in mice, we show that human iPSC-derived macrophages exhibit an increased phagocytic capacity upon CD22 blockage. Downstream analysis of antibody-mediated CD22 inhibition revealed an influence on BMP and TGFß associated gene networks. Our results demonstrate CD22 as a broad age-associated modulator of microglia functionality with potential implications for neurodegenerative disorders.

Ligand-dependent kinase activity of MERTK drives efferocytosis in human iPSC-derived macrophages.

Removal of apoptotic cells by phagocytes (also called efferocytosis) is a crucial process for tissue homeostasis. Professional phagocytes express a plethora of surface receptors enabling them to sense and engulf apoptotic cells, thus avoiding persistence of dead cells and cellular debris and their consequent effects. Dysregulation of efferocytosis is thought to lead to secondary necrosis and associated inflammation and immune activation. Efferocytosis in primarily murine macrophages and dendritic cells has been shown to require TAM RTKs, with MERTK and AXL being critical for clearance of apoptotic cells. The functional role of human orthologs, especially the exact contribution of each individual receptor is less well studied. Here we show that human macrophages differentiated in vitro from iPSC-derived precursor cells express both AXL and MERTK and engulf apoptotic cells. TAM RTK agonism by the natural ligand growth-arrest specific 6 (GAS6) significantly enhanced such efferocytosis. Using a newly-developed mouse model of kinase-dead MERTK, we demonstrate that MERTK kinase activity is essential for efferocytosis in peritoneal macrophages in vivo. Moreover, human iPSC-derived macrophages treated in vitro with blocking antibodies or small molecule inhibitors recapitulated this observation. Hence, our results highlight a conserved MERTK function between mice and humans, and the critical role of its kinase activity in homeostatic efferocytosis.

Incorporation of stem cell-derived astrocytes into neuronal organoids to allow neuro-glial interactions in toxicological studies.

Human cell-based neural organoids are increasingly being used for investigations of neurotoxicity, and to study the pathophysiology of neurodegenerative diseases. Here, we present a fast and robust method to generate 3D cultured human dopaminergic neurons (LUHMES) for toxicity testing and long-term culture. Moreover, a plating step was introduced to allow generation of neurite networks with defined 2D orientation and several mm length, while all cell bodies (somata) remained in a 3D, dome-like structure. These cultures, named here 2.5D (for 2.5 dimensional), offer new approaches to quantify toxicant effects on organoids by standard technology and high throughput. For instance, the system reacted to the parkinsonian model toxicants MPP+, rotenone, MG-132 and the ferroptosis-inducer erastin. Moreover, stable incorporation of human stem cell-derived astrocytes or microglia was possible. Added astrocytes stabilized the post mitotic state of the LUHMES neurons and thereby allowed the formation of a stable micro-physiological system. We observed neuroprotection against the proteasome inhibitor MG-132 and the ferroptosis-inducer erastin by such glia. This exemplifies the crucial protective role of astrocytes in neurodegeneration. The modularity of the system was further employed to incorporate microglia together with astrocytes into the organoids. Such ratio-defined, three cell type-based organoids will allow new approaches to study human pathophysiology and toxicology of the nervous system.