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Human induced Pluripotent Stem Cell (hiPSC) derived neural cells offer great potential for modelling neurological diseases and toxicities and have found application in drug discovery and toxicology. As part of the European Innovative Medicines Initiative (IMI2) NeuroDeRisk (Neurotoxicity De-Risking in Preclinical Drug Discovery), we here explore the Ca 2+ oscillation responses of 2D and 3D hiPSC derived neuronal networks of mixed Glutamatergic/GABAergic activity with a compound set encompassing both clinically as well as experimentally determined seizurogenic compounds. Both types of networks are scored against Ca 2+ responses of a primary mouse cortical neuronal 2D network model serving as an established comparator assay. Parameters of frequency and amplitude of spontaneous global network Ca 2+ oscillations and the drug-dependent directional changes to these were assessed, and predictivity of seizurogenicity scored using contingency table analysis. In addition, responses between models were compared between both 2D models as well as between 2D and 3D models. Concordance of parameter responses was best between the hiPSC neurospheroid and the mouse primary cortical neuron model (77% for frequency and 65% for amplitude). Decreases in spontaneous Ca 2+ oscillation frequency and amplitude were found to be the most basic shared determinants of risk of seizurogenicity between the mouse and the neurospheroid model based on testing of clinical compounds with documented seizurogenic activity. Increases in spontaneous Ca 2+ oscillation frequency were primarily observed with the 2D hIPSC model, though the specificity of this effect to seizurogenic clinical compounds was low (33%), while decreases to spike amplitude in this model were more predictive of seizurogenicity. Overall predictivities of the models were similar, with sensitivity of the assays typically exceeding specificity due to high false positive rates. Higher concordance of the hiPSC 3D model over the 2D model when compared to mouse cortical 2D responses may be the result of both a longer maturation time of the neurospheroid (84-87 days for 3D vs. 22-24 days for 2D maturation) as well as the 3-dimensional nature of network connections established. The simplicity and reproducibility of spontaneous Ca 2+ oscillation readouts support further investigation of hiPSC derived neuronal sources and their 2- and 3-dimensional networks for neuropharmacological safety screening., Competing Interests: Declaration of Competing Interest The authors are employees of their respective companies and confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome. Mention of trade names or commercial products does not constitute endorsement or recommendation for use., (Copyright © 2023. Published by Elsevier Inc.)

Introduction: In the framework of the IMI2-NeuroDeRisk consortium, three in vitro electrophysiology assays were compared to improve preclinical prediction of seizure-inducing liabilities., Methods: Two cell models, primary rat cortical neurons and human induced pluripotent stem cell (hiPSC)-derived glutamatergic neurons co-cultured with hiPSC-derived astrocytes were tested on two different microelectrode array (MEA) platforms, Maestro Pro (Axion Biosystems) and Multiwell-MEA-System (Multi Channel Systems), in three separate laboratories. Pentylenetetrazole (PTZ) and/or picrotoxin (PTX) were included in each plate as positive (n = 3-6 wells) and ≤0.2% DMSO was used as negative controls (n = 3-12 wells). In general, concentrations in a range of 0.1-30 μM were tested, anchored, when possible, on clinically relevant exposures (unbound C max ) were tested. Activity thresholds for drug-induced changes were set at 20%. To evaluate sensitivity, specificity and predictivity of the cell models, seizurogenic responses were defined as changes in 4 or more endpoints. Concentration dependence trends were also considered., Results: Neuronal activity of 33 compounds categorized as positive tool drugs, seizure-positive or seizure-negative compounds was evaluated. Acute drug effects (<60 min) were compared to baseline recordings. Time points < 15 min exhibited stronger, less variable responses to many of the test agents. For many compounds a reduction and cessation of neuronal activity was detected at higher test concentrations. There was not a single pattern of seizurogenic activity detected, even among tool compounds, likely due to different mechanisms of actions and/or off-target profiles. A post-hoc analysis focusing on changes indicative of neuronal excitation is presented., Conclusion: All cell models showed good sensitivity, ranging from 70 to 86%. Specificity ranged from 40 to 70%. Compared to more conventional measurements of evoked activity in hippocampal slices, these plate-based models provide higher throughput and the potential to study subacute responses. Yet, they may be limited by the random, spontaneous nature of their network activity., Competing Interests: Declaration of Competing Interest The authors declare no conflicts of interest. All co-authors are employed by pharmaceuticals or biotechnology companies interested in developing novel in vitro assays to improve human health and in assessing methodological improvements to this end, without bias. This research was conducted in the context of the IMI2: NeuroDeRisk non-competitive industry-academic consortium funded by the European Medicines Agency. Mention of trade names or commercial products does not constitute endorsement or recommendation for use., (Copyright © 2023 Merck Sharp & Dohme LLC., a subsidiary of Merck & Co., Inc., Rahway, NJ, USA, The Author(s). Published by Elsevier Inc. All rights reserved.)

Pax5 is a critical regulator of B-cell commitment. Here, we identified direct Pax5 target genes by streptavidin-mediated ChIP-chip analysis of pro-B cells expressing in vivo biotinylated Pax5. By binding to promoters and enhancers, Pax5 directly regulates the expression of multiple transcription factor, cell surface receptor and signal transducer genes. One of the newly identified enhancers was shown by transgenic analysis to confer Pax5-dependent B-cell-specific activity to the Nedd9 gene controlling B-cell trafficking. Profiling of histone modifications in Pax5-deficient and wild-type pro-B cells demonstrated that Pax5 induces active chromatin at activated target genes, while eliminating active chromatin at repressed genes in committed pro-B cells. Pax5 rapidly induces these chromatin and transcription changes by recruiting chromatin-remodelling, histone-modifying and basal transcription factor complexes to its target genes. These data provide novel insight into the regulatory network and epigenetic regulation, by which Pax5 controls B-cell commitment.

Human ESC and iPSC are an attractive source of cells of high quantity and purity to be used to elucidate early human development processes, for drug discovery, and in clinical cell therapy applications. To efficiently differentiate pluripotent cells into a pure population of hematopoietic progenitors we have developed a new 2-dimensional, defined and highly efficient protocol that avoids the use of feeder cells, serum or embryoid body formation. Here we showed that a single matrix protein in combination with growth factors and a hypoxic environment is sufficient to generate from pluripotent cells hematopoietic progenitors capable of differentiating further in mature cell types of different lineages of the blood system. We tested the differentiation method using hESCs and 9 iPSC lines generated from different tissues. These data indicate the robustness of the protocol providing a valuable tool for the generation of clinical-grade hematopoietic cells from pluripotent cells.

V(H)-DJ(H) recombination of the immunoglobulin heavy chain (Igh) locus is temporally and spatially controlled during early B cell development, and yet no regulatory elements other than the V(H) gene promoters have been identified throughout the entire V(H) gene cluster. Here, we discovered regulatory sequences that are interspersed in the distal V(H) gene region. These conserved repeat elements were characterized by the presence of Pax5 transcription factor-dependent active chromatin by binding of the regulators Pax5, E2A, CTCF, and Rad21, as well as by Pax5-dependent antisense transcription in pro-B cells. The Pax5-activated intergenic repeat (PAIR) elements were no longer bound by Pax5 in pre-B and B cells consistent with the loss of antisense transcription, whereas E2A and CTCF interacted with PAIR elements throughout early B cell development. The pro-B cell-specific and Pax5-dependent activity of the PAIR elements suggests that they are involved in the regulation of distal V(H)-DJ(H) recombination at the Igh locus., (Copyright © 2011 Elsevier Inc. All rights reserved.)

Induced pluripotent stem cells (iPSCs) provide an unprecedented opportunity for modeling of human diseases in vitro, as well as for developing novel approaches for regenerative therapy based on immunologically compatible cells. In this study, we employed an OP9 differentiation system to characterize the hematopoietic and endothelial differentiation potential of seven human iPSC lines obtained from human fetal, neonatal, and adult fibroblasts through reprogramming with POU5F1, SOX2, NANOG, and LIN28 and compared it with the differentiation potential of five human embryonic stem cell lines (hESC, H1, H7, H9, H13, and H14). Similar to hESCs, all iPSCs generated CD34(+)CD43(+) hematopoietic progenitors and CD31(+)CD43(-) endothelial cells in coculture with OP9. When cultured in semisolid media in the presence of hematopoietic growth factors, iPSC-derived primitive blood cells formed all types of hematopoietic colonies, including GEMM colony-forming cells. Human induced pluripotent cells (hiPSCs)-derived CD43(+) cells could be separated into the following phenotypically defined subsets of primitive hematopoietic cells: CD43(+)CD235a(+)CD41a(+/-) (erythro-megakaryopoietic), lin(-)CD34(+)CD43(+)CD45(-) (multipotent), and lin(-)CD34(+)CD43(+)CD45(+) (myeloid-skewed) cells. Although we observed some variations in the efficiency of hematopoietic differentiation between different hiPSCs, the pattern of differentiation was very similar in all seven tested lines obtained through reprogramming of human fetal, neonatal, or adult fibroblasts with three or four genes. Although several issues remain to be resolved before iPSC-derived blood cells can be administered to humans for therapeutic purposes, patient-specific iPSCs can already be used for characterization of mechanisms of blood diseases and for identification of molecules that can correct affected genetic networks.

Objective: Cellular and molecular changes that occur during the genesis of the hematopoietic system and hematopoietic stem cells in the human embryo are mostly inaccessible to study and remain poorly understood. To address this gap we have exploited the human embryonic stem cell (hESC) system to molecularly characterize the global transcriptomes of the two functionally discreet and phenotypically separable populations of multipotent hematopoietic cells that first appear when hESCs are induced to differentiate on OP9 cells., Materials and Methods: We prepared long serial analysis of gene expression libraries from lin-CD34+CD43+CD45- and lin-CD34+CD43+CD45+ subsets of primitive hematopoietic cells derived in vitro from hESCs, sequenced them to a depth of 200,000 tags and compared their content with similar libraries prepared from highly purified populations of very primitive human fetal liver and cord blood hematopoietic cells., Results: Comparison of libraries obtained from hESC-derived lin-CD34+CD43+CD45- and lin-CD34+CD43+CD45+ revealed differences in their expression of genes associated with myeloid development, cellular biosynthetic processes, and cell-cycle regulation. Further comparisons with analogous data for primitive hematopoietic cells isolated from first-trimester human fetal liver and newborn cord blood showed an apparent similarity between the transcriptomes of the most primitive hESC- and in vivo-derived populations, with the main differences involving genes that regulate HSC self-renewal and homing, chromatin remodeling, AP1 transcription complex genes, and noncoding RNAs., Conclusion: These data suggest that primitive hematopoietic cells are generated from hESCs in vitro by processes similar to those operative during human embryogenesis in vivo, although some differences were also detected.

The transcription factor Pax5 represses B lineage-inappropriate genes and activates B cell-specific genes in B lymphocytes. Here we have identified 170 Pax5-activated genes. Conditional mutagenesis demonstrated that the Pax5-regulated genes require continuous Pax5 activity for normal expression in pro-B and mature B cells. Expression of half of the Pax5-activated genes is either absent or substantially reduced upon Pax5 loss in plasma cells. Direct Pax5 target genes were identified based on their protein synthesis-independent activation by a Pax5-estrogen receptor fusion protein. Chromatin immunoprecipitation (ChIP) of Pax5 together with chromatin profiling by ChIP-on-chip analysis demonstrated that Pax5 directly activates the chromatin at promoters or putative enhancers of Pax5 target genes. The Pax5-activated genes code for key regulatory and structural proteins involved in B cell signaling, adhesion, migration, antigen presentation, and germinal-center B cell formation, thus revealing a complex regulatory network that is activated by Pax5 to control B cell development and function.

The transcription factor Pax5 (BSAP) is required for the expression of a B-cell-specific genetic program and for B-cell differentiation, and also to suppress genes of alternative lineages. The molecular mechanism by which repression of myeloid genes occurs during early B-lineage restriction is unknown and in this study we addressed this question. One of the genes repressed by Pax5 in B cells is the colony-stimulating factor receptor 1 gene (csf1r or c-fms). We examined the changes in chromatin caused by Pax5 activity, and we show that Pax5 is directly recruited to c-fms resulting in the rapid loss of RNA polymerase II binding, followed by loss of transcription factor binding and DNaseI hypersensitivity at all cis-regulatory elements. We also show that Pax5 targets the basal transcription machinery of c-fms by interacting with a binding site within the major transcription start sites. Our results support a model by which Pax5 does not lead to major alterations in chromatin modification, but inhibits transcription by interfering with the action of myeloid transcription factors.

The subnuclear location and chromatin state of the immunoglobulin heavy-chain (IgH) locus have been implicated in the control of VDJ recombination. VH-to-DJH rearrangement of distal, but not proximal V(H) genes, furthermore, depends on the B-lineage commitment factor Pax5 (BSAP). He e we demonstrate that ectopic Pax5 expression from the Ikaros promote induces proximal rather than distal VH-DJH rearrangements in Ik(Pax5/+) thymocytes, thus recapitulating the loss-of-function phenotype of Pax5-/- pro-B cells. The phenotypic similarities of both cell types include (1) chromatin accessibility of distal VH genes in the absence of VH-DJH rearrangements, (2) expression of the B-cell-specific regulator EBF, (3) central location of IgH alleles within the nucleus, and (4) physical separation of distal VH genes from proximal segments in an extended IgH locus. Reconstitution of Pax5 expression in Pax5-/- pro-B cells induced large-scale contraction and distal VH-DJH rearrangements of the IgH locus. Hence, VH-DJH recombination is regulated in two steps during early B-lymphopoiesis. The IgH locus is first repositioned from its default location at the nuclear periphery toward the center of the nucleus, which facilitates proximal VH-DJH recombination. Pax5 subsequently activates locus contraction and distal VH-DJH rearrangements in collaboration with an unknown factor that is present in pro-B cells, but absent in thymocytes.