Stem Cell Reports

BackgroundGains of chromosome 20q11.21 are among the most common culture-acquired abnormalities in human pluripotent stem cells (hPSC), conferring a well-defined survival advantage while altering differentiation capacity. However, it remains unclear whether this advantage persists during differentiation, how the aneuploidy alters ectodermal and retinal pigment epithelium (RPE) lineage specification, and which genes within the minimal amplicon drive these effects. MethodsWe used three isogenic human embryonic stem cell line pairs (wild-type and 20q11.21 gain) and assessed their behaviour in two neuroectoderm differentiation systems: directed neuroectoderm induction (dual SMAD inhibition) and long-term spontaneous RPE differentiation. Competitive dynamics were measured in mixed cultures, and lineage outcomes were analysed using immunostaining, gene expression profiling and single-cell RNA sequencing. To identify driver genes, we generated BCL2L1 and ID1 overexpression lines and tested their effects under both directed and spontaneous differentiation conditions. ResultsAcross all lines and conditions, 20q cells expanded from a minor fraction to dominate mixed cultures, indicating that their competitive advantage persists beyond the undifferentiated state. Despite this dominance, pure 20q cells failed to specify to neuroectoderm or RPE. Single-cell transcriptomics revealed consistent diversion toward non-neural ectodermal and extraembryonic fates. Mechanistically, overexpression of BCL2L1 and ID1 alone or in combination impaired neuroectoderm specification, while synergistic effect of both genes promoted non-neural ectodermal outcomes under directed differentiation conditions. In spontaneous differentiation, both genes could disrupt differentiation. ConclusionsThe 20q11.21 gain couples a persistent survival advantage with a disruption of neural and RPE lineage competence, redirecting cells toward alternative ectodermal and extraembryonic fates. These effects arise from the combined action of two dosage-sensitive genes BCL2L1 and ID1 within the amplicon, illustrating how regional gene dosage can reshape developmental signalling responses in hPSC.

Advances in transcriptome profiling have revealed transcriptomic differences across different cellular states. However, functional interpretation requires precise perturbation tools and experimental frameworks. This study benchmarked two widely used modalities: CRISPR interference (CRISPRi) and Cas13d/CasRx. A standardized workflow was established to generate human pluripotent stem cells (PSCs) with inducible ZIM3-dCas9 or CasRx expression. The cell lines were subjected to flow cytometry, copy number, and immunocytochemical analyses. The knockdown performance was validated via robust OCT4 suppression and the expected downstream effects on pluripotency genes. Time-course measurements indicated that CRISPRi produced faster and stronger repression but slower recovery after inducer withdrawal. In contrast, CasRx yielded slower and typically weaker knockdown with rapid reversibility. Furthermore, a key limitation of CRISPRi was demonstrated using the ATF5-NUP62 locus, wherein CRISPRi could co-repress genes with overlapping promoter regions. In contrast, CasRx avoids these limitations and supports isoform-resolved targeting of circular and alternatively spliced transcripts, albeit with variable efficiency. These results provide practical guidance for selecting complementary knockdown tools to improve the interpretability of transcriptomic function studies. MOTIVATIONAdvances in transcriptome profiling have enabled the detection of subtle cell type-specific differences. However, mechanistic interpretation still depends on perturbation tools that can modulate transcripts with high precision and efficiency. Recent CRISPR-based modalities, CRISPRi and Cas13/CasRx, function as robust and orthogonal methods to achieve the knockdown of specific gene targets. However, a standardized approach for cell line preparation and comparative studies on their relative performances and limitations remains unclear. Consequently, this study presents a standardized workflow for generating cell lines that support high-efficiency knockdown using CRISPRi and CasRx. Moreover, it compares the trade-offs in potency, reversibility, and isoform resolution, along with a practical overview of method-specific pitfalls to guide tool selection and data interpretation in future studies. HIGHLIGHTSO_LIDoxycycline-inducible AAVS1 knock-in human PSC platforms for CRISPRi (ZIM3-dCas9) and CasRx (RfxCas13d) were generated to enable standardized RNA perturbation experiments. C_LIO_LIThe prepared cell lines demonstrated strong OCT4 knockdown, with expected downstream effects on the expression of another pluripotency gene, NANOG. C_LIO_LIA comparison of knockdown characteristics and their reversibility revealed rapid and sustained repression with CRISPRi, whereas slow but rapid recovery was observed with CasRx. C_LIO_LIA CRISPRi-specific off-target effect arising from TSS proximity/overlap (ATF5-NUP62) was identified, whereas CasRx achieved ATF5 knockdown without collateral repression of the neighboring NUP62 gene. C_LIO_LICasRx enables isoform-resolved knockdown of structural isoforms (circHIPK3 vs. linear HIPK3 mRNA) and splice isoforms (RAB6A-iso1 vs. RAB6A-iso2). C_LI

The neural crest (NC) is a transient stem cell population which migrates throughout the developing embryo to contribute to diverse tissues dependent on axial origin. For example, cranial NC can give rise to bone and cartilage, while more posterior NC populations give rise to peripheral nervous system and neuroendocrine tissues. Perturbations in neural crest development can lead to severe congenital anomalies and cancers, with over 700 neurocristopathies reported. In humans, early NC development remains poorly understood due to the inaccessibility of tissue samples, thus necessitating the development of in vitro models. Currently, a limited number of NC organoid protocols are available, but these mainly focus on cranial NC and lack relevant tissue architecture. Here, we describe a novel bioengineered pipeline to derive human pluripotent stem cell (hPSC)-derived neuroepithelial organoids, "neurocrestoids" featuring physiologically-relevant tissue architecture. We show that neurocrestoids recapitulate the dynamics of induction, delamination, and migration of human neural crest cells (NCCs), and can be directly compared to murine NC explants for cross-species validation. Organoids express an array of HOX genes indicating the successful generation of cranial, vagal and trunk NCCs. Moreover, we have integrated our neurocrestoids with a customised micropatterned substrate suitable for live visualisation and guided separation of SOX10-positive migratory human NCCs. Our "NCC migration on-chip" are reproducible across multiple hPSC lines and should be scalable for future diagnostic and therapeutic applications, significantly improving our ability to study human NC pathologies.

The ability to revascularize target tissues and organs through cell-based therapy would provide a novel approach for the treatment of a range of ischemic disorders including cardiovascular diseases, stroke and peripheral artery disease. Towards this goal, we have identified a human pluripotent stem cell (hPSC)-derived vascular progenitor (VP) population generated via an epicardial intermediate with functional engraftment properties. VP cells efficiently engraft the mammary fat pad and hind limb skeletal muscle of NSG recipient mice and form vessel-like structures that integrate with the host vasculature. In an ischemic hind limb mouse model, VPs generate extensive vascular grafts that improve perfusion, restore some function and preserve muscle integrity over a three-month period post-transplant. Single-cell transcriptomic and flow cytometric analyses show that the VP population, initially identified by the co-expression of CD140b, CD13 and KDR, displays an epicardial lineage signature and expresses a spectrum of genes and proteins indicative of vascular progenitor stage cells. Together, these findings demonstrate that it is possible to revascularize both normal and ischemic tissue through the transplantation of an appropriate hPSC-derived progenitor and in doing so, lay the foundation for developing cell-based therapy approaches to treat ischemic diseases. Graphical Abstract LegendHuman pluripotent stem cells are differentiated through an epicardial intermediate to generate vascular progenitor (VP) cells characterized by expression of CD140b, CD13 and KDR. These VP cells demonstrate the capacity to engraft both mammary fat pad and skeletal muscle tissue where they form stable perfused vascular networks. In a hindlimb ischemia model, VP cell transplantation restores blood flow and improves functional outcomes. eTOC BlurbFernandes et al. develop a protocol to generate engraftable vascular progenitors from human pluripotent stem cells through an epicardial intermediate. These cells form functional vessels in vivo, restore perfusion in ischemic tissue, and demonstrate tissue-specific adaptation while maintaining endothelial identity, providing a foundation for therapeutic revascularization. HighlightsO_LIA staged differentiation protocol generates vascular progenitors (VPs) from hPSCs via an epicardial intermediate. C_LIO_LIVP cells form stable, perfused vascular networks following transplantation into multiple tissue sites. C_LIO_LIVP cell therapy with or without VEGF nanoparticles restores perfusion and improves functional outcomes in hindlimb ischemia. C_LIO_LISingle-cell analysis reveals tissue-specific adaptation while maintaining endothelial identity. C_LI

The human olfactory epithelium (OE) represents a lifelong source of neural progenitor cells and has been proposed as an accessible model to investigate molecular alterations associated with neurodevelopmental disorders in postnatal individuals. Globose basal cells are considered the immediate neuronal progenitors within the OE, and several studies have attempted to culture these cells from nasal exfoliates. However, the actual contribution of neurogenic lineages in these cultures remains largely unquantified. Here, we cultured human nasal explants using an established protocol and characterized the resulting cell populations by immunohistochemistry and single-cell RNA sequencing. Integration with primary in vivo OE datasets revealed that these cultures are predominantly composed of mesenchymal-like cells, with limited representation of globose basal cells and neurons, and low expression of canonical neuronal markers. Using curated gene sets associated with neurodevelopmental disorders and malformations of cortical development, we assessed the extent to which disease-relevant transcriptional programs are captured in OE-derived cultures. While disease-associated genes are enriched in neurogenic lineages in vivo, their representation in mesenchymal cells is reduced. Together, our results challenge the assumption that standard OE culture systems faithfully model neurogenic compartments and suggest that current approaches may need refinement to recover neurogenic lineages.

Ischaemic heart disease remains the leading cause of mortality worldwide, yet no therapies prevent cardiomyocyte death during acute ischaemia-reperfusion injury (IRI). Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) provide a platform for modelling cardiac injury, but their immature phenotype limits the physiological fidelity of in vitro models. Here, we systematically evaluated how experimental variables used during preparation of hiPSC-CM endpoint assays influence cardiomyocyte maturation and susceptibility to IRI. Integrating literature mining, molecular profiling, statistical genetics, and functional assays, we examined the effects of replating conditions, backbone media, metabolic substrates, and signalling modulators. We define the relationship between culture conditions and metabolic supplements in determining contractile maturation and sensitivity to IRI. Notably, we show that metabolic composition of the backbone medium establishes the baseline maturation state and determines responsiveness to additional maturation cues. These findings identify metabolic environment as a dominant regulator of injury susceptibility and provide a framework for improving the physiological fidelity of hiPSC-CM models of cardiac ischaemia.

Development of motoneurons from stem cells is characterized by a change from glycolytic to oxidative metabolism. Since this transition remains poorly understood, we examined it at five distinct differentiation stages from hiPSC to motoneuron. While a direct comparison of hiPSCs and mature motoneurons confirmed the expected glycolytic-to-oxidative shift, the intermediate stages showed that the conversion was not monotonic. After an initial drop of glycolysis at the hiPSC-to-neuroepithelial transition, late neuroepithelial cells showed intermittent peaks of the glycolytic marker lactate dehydrogenase A and the metabolic regulator TIGAR. Furthermore, the lactate-produced-to-glucose-consumed ratio remained elevated. A fully oxidative phenotype was only assumed upon progress from neural progenitors to motoneurons, portrayed by a definitive drop of the lactate-produced-to-glucose-consumed ratio, an increase of mitochondrial membrane charging, and shifts from lactate dehydrogenase A to B, from pyruvate dehydrogenase to anaplerotic pyruvate carboxylase, and from Mitofusin 1 to 2. Together, our data show that metabolic maturation in human motoneurons does not occur as a simple switch. Instead, it unfolds through distinct stages in a directional yet nonlinear manner.

Despite lacking a robust interferon response, pluripotent stem cells remain highly resistant to viral infection, in part through the constitutive expression of immune genes traditionally classified as interferon-stimulated genes. While interferon signaling has been shown to be incompatible with the maintenance of pluripotency, the molecular mechanisms underlying this relationship remain poorly understood. Here, we investigate the transcriptional response of human embryonic stem cells (hESCs) to infection with a potent activator of the interferon response, an influenza A virus mutant lacking the viral NS1 protein. Single-cell RNA sequencing revealed that while most hESCs remain unresponsive to infection, a distinct subpopulation expresses type I and III interferons. Notably, only interferon-expressing cells mounted a robust antiviral response, characterized by strong induction of interferon-stimulated genes. In contrast to the bulk hESC population, interferon responding cells exhibited reduced expression of core pluripotency factors as well as negative regulators of interferon signaling, such as SOCS1 and SPRY4. Depletion of SOCS1 enabled hESCs to respond robustly to interferon stimulation, showing that this negative regulator is a key suppressor of interferon signaling in pluripotent stem cells. We further show that SOCS1 and additional negative regulators of IFN signaling are intrinsically expressed in hESCs and are transcriptionally controlled by pluripotency factors, such as NANOG, SOX2 and OCT4. Together, our findings support a model in which pluripotency factors regulate intrinsic immune gene expression, including negative regulators of interferon signaling, thereby suppressing canonical interferon signaling to preserve pluripotency while maintaining antiviral resistance. IMPORTANCEBy combining single-cell transcriptomics with functional studies, we demonstrate that the pluripotency transcriptional program active in pluripotent stem cells coordinately regulates pluripotency factors, antiviral genes, and negative regulators of interferon signaling. This integrated control enables pluripotent stem cells to achieve effective protection against viral infection while preserving their differentiation potential, providing new insights into how innate immunity is selectively constrained in pluripotent stem cells. These findings have important implications for stem cell-based therapies, where transient modulation of antiviral responses without disrupting pluripotency could improve therapeutic efficacy. More broadly, this work advances our understanding of interferon regulation that could inform the development of antiviral strategies that enhance protective immune responses while limiting harmful or unwanted inflammatory signaling.

Loss-of-function mutations in the ABCA4 gene cause Stargardt disease (STGD1), the most common inherited macular dystrophy leading to progressive central vision loss. Here, we generated hiPSC-derived retinal organoids harboring a premature stop codon in exon-24 of ABCA4 to evaluate the impact of this mutation on mRNA and protein levels in a human model. Immunofluorescence analysis revealed the absence of ABCA4 protein in the mutant photoreceptor outer segment discs, while single-cell RNA sequencing detected no major transcriptional alterations in rods and cones. Unexpectedly, differential gene expression and pathway enrichment analyses of Muller glial cells (MGCs) and astrocytes highlighted disruption of neuronal development, microenvironment of glial cells, intercellular communication, and programmed cell death pathways. These findings suggest that ABCA4 might play a role in maintaining the retinal microenvironment homeostasis, and that the early transcriptomic response of MGCs and astrocytes preceding photoreceptor degeneration could contribute to Stargardt disease development. Significance StatementHuman induced pluripotent stem cell (hiPSC)-derived retinal organoids provide a powerful platform to investigate inherited retinal diseases. In this study, we generated ABCA4-mutant hiPSC lines and differentiated them into retinal organoids to model Stargardt disease. Despite complete loss of ABCA4 protein from the photoreceptor outer segment discs, rods and cones exhibited minimal transcriptional alterations. In contrast, the ABCA4 variant triggered changes in the glial cell homeostasis, suggesting that Muller glial cells and astrocytes might exhibit an early response to photoreceptor dysfunction in the absence of ABCA4. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=139 SRC="FIGDIR/small/718110v1_ufig1.gif" ALT="Figure 1"> View larger version (15K): org.highwire.dtl.DTLVardef@1d8dbf6org.highwire.dtl.DTLVardef@51239corg.highwire.dtl.DTLVardef@f8fbdborg.highwire.dtl.DTLVardef@5f0b44_HPS_FORMAT_FIGEXP M_FIG Human induced pluripotent stem cells (hiPSCs) were engineered to generate ABCA4-mutant cell lines, later differentiated into retinal organoids as a model of Stargardt disease. The organoids showed loss of ABCA4 from the outer segment discs of rod and cone photoreceptors, while the mutant Muller glial cells and astrocytes exhibited transcriptional changes in pathways involved in neuronal development, microenvironment, and programmed cell death. C_FIG

Human iPSC-derived neuronal networks are increasingly being employed in basic and applied research to enhance translation. Astrocytes are essential for neuronal network function, but are often not included, or replaced with mouse astrocytes, which compromises translation. Current protocols produce hiPSC-derived astrocytes by stepwise differentiation using small molecules and cytokines, or by forward programming by inducing transcription factors introduced by lentiviral transduction. Here we created a stable, inducible hiPSC line capable of producing iAstrocytes by introducing the transcription factors NFIB and SOX9 into the AAVS1 locus of the BIHi005-A hiPSC line. iAstrocytes induced from this line upregulated astrocytic genes over four weeks in culture, expressed GFAP and S100B and exhibited spontaneous calcium waves and responses to ATP and CPA. In co-cultures, iAstrocytes supported the growth and function of mature iNeuron networks. Pre- and post-synaptic markers and synchronous neuronal activity measured by high-density multi-electrode array recordings and neuronal calcium imaging, appeared by four weeks. The use of iAstrocytes will help to standardize the use of human astrocytes to support human neural networks and enhance translation.

Sickle cell disease (SCD) is caused by a point mutation in the {beta}-globin gene that promotes hemoglobin polymerization, leading to chronic hemolytic anemia, vaso-occlusive episodes, and progressive organ damage. The most efficacious therapies focus on reactivating fetal hemoglobin (HbF) expression to mitigate the pathological effects of sickle hemoglobin (HbS) polymerization. However, the predominantly used HbF inducer, hydroxyurea (HU), exhibits substantial interpatient variability in efficacy, and curative approaches such as gene therapy remain inaccessible to the vast majority of patients. Although all SCD patients share the same causative HBB glu7val mutation, differences in genetic background significantly influence disease severity and therapeutic response. We describe a SCD-specific induced pluripotent stem cell (iPSC) platform as a renewable and scalable preclinical model to interrogate treatment responses across the genetically diverse SCD patient population. By generating patient-specific iPSC-derived erythroblasts (iEry) representing distinct SCD genetic backgrounds, we demonstrate that this system faithfully recapitulates the heterogeneous HbF induction observed clinically in response to HU. Moreover, this platform enables the identification and evaluation of alternative therapeutic agents for HU non-responders and provides sufficient resolution to dissect drug-specific effects on erythroid differentiation and cellular phenotypes. Together, these findings support the use of iPSC-derived erythroid models as a versatile tool to advance precision therapeutic strategies for SCD. KEY POINTS- SCD iPSC-derived erythroid cells (iEry) reflect the diversity in HU-mediated HbF induction seen in SCD patients - SCD iEry recapitulate patient-specific treatment responses and can be used to identify therapeutic alternatives for HU non-responders - iEry provide a versatile platform to study the impact of novel HbF inducers on erythroid cell characteristics and differentiation parameters

At the Wisconsin National Primate Research Center, we have identified a family of rhesus carrying the microtubule-associated protein tau (MAPT) R406W mutation linked to frontotemporal dementia (FTD). Rhesus induced pluripotent stem cells (RhiPSCs) derived from these monkeys present a unique opportunity for in vitro modeling and comparison with cells derived from MAPT R406W human carriers. Here, we report the development of a reproducible method to generate RhiPSCs compliant with the standards of the International Society for Stem Cell Research (ISSCR) to support in vitro modeling of FTD-MAPT R406W. Our stepwise approach identified efficient methods for fibroblast derivation, fibroblast reprogramming to RhiPSC, and RhiPSC maintenance over continued culture. To derive fibroblasts from MAPT wild type (WT) and R406W monkeys, a combination of manual processing and overnight enzymatic digestion was required to maximize the number of low passage fibroblasts available for reprogramming. Fibroblast reprogramming to RhiPSC using Sendai viral vectors versus oriP/EBNA1 episomal plasmids revealed the latter as most efficient. Electroporation conditions for oriP/EBNA1 reprogramming were optimized to maximize plasmid uptake and cell survival. Ultimately, eight RhiPSC lines were derived from 4 donor rhesus monkeys (n=2 WT, n=2 R406W; two clonal lines per donor) and fully characterized according to ISSCR standards. RhiPSC stemness and genetic stability was best maintained on mouse embryonic fibroblast feeders in Universal Primate Pluripotency Stem Cell medium, as opposed to Essential 12 medium supplemented with IWR1, which produced cytogenetic abnormalities. Rhesus neural progenitor cells were generated using a monolayer protocol and expressed PAX6 and NESTIN after 21 days of differentiation. Our reliable method will be useful to labs seeking to derive RhiPSCs for preclinical studies. Overall, the RhiPSCs generated from MAPT R406W carriers will be a critical resource for evaluating the molecular underpinnings of tau-related neurodegeneration across primate species.

Neuromodulators influence critical functions of the developing human brain and regulate behavioral states. Dysfunction of neuromodulatory systems is often involved in neuropsychiatric disease and many drugs for these conditions act on these signaling pathways. Recent advances in stem cell biology have made it possible to derive a wide range of cells across the developing human nervous system in regionalized organoids and to functionally integrate them into assembloids, however they currently do not systematically incorporate neuromodulation. Here, we generated human midbrain-hindbrain organoids (hMHO) from human induced pluripotent stem (hiPS) cells and fused them with human cortical organoids (hCO) to form neuromodulatory assembloids (hNMA). We focus on serotonin (5-hydroxytryptamine, 5-HT) as one key neuromodulator and found characteristic gene expression patterns and electrophysiological properties of serotonergic neurons (5-HT neurons) in the hMHO. In hNMA, 5-HT neurons projected into hCO, released 5-HT and modulated cortical network activity. To explore the applicability of this system in human disease, we studied 22q11.2 deletion syndrome (22q11.2DS), a common microdeletion associated with high risk for neuropsychiatric disease and defects in 5-HT signaling. We found aberrant 5-HT dynamics in hNMA from patient hiPS cell lines that were rescued by administration of a selective serotonin reuptake inhibitor (SSRI). Taken together, hNMA can be used to study human 5-HT dynamics and uncover disease phenotypes which could facilitate therapeutic development.

Ex vivo expansion of mobilized peripheral blood (mPB) hematopoietic stem cells (HSCs) represents a promising approach to advance cell and gene therapy strategies yet is hampered by loss of stem cell function when applying commonly used culture protocols. We performed in-depth characterization of mPB expansion cultures by single cell RNA sequencing, which highlighted differentiation trajectories with preservation of lineage fidelity in committed progenitors. Defining a putative HSC cluster allowed an estimation of transduction efficiency in ex vivo cultures, which correlated with long-term gene marking in xenografts and patients enrolled in a gene therapy study. We then developed a clinically translatable, GMP-compliant process to expand lentivirus (LV)-transduced HSCs from mPB of pediatric patients and adult donors, by biologically informed protocol improvements of cytokine supplementation, media choice, timing of LV transduction and combinations of small molecules preventing the activation of differentiation programs. Our optimized process outperforms validated state-of-the-art cord blood expansion protocols when applied to mPB. LV integration site analysis and genomic barcode-based clonal tracking provided definitive proof for symmetric HSC self-renewal divisions occurring during ex vivo culture. These results warrant clinical testing of this HSC transduction/expansion process in an upcoming clinical gene therapy trial for autosomal recessive osteopetrosis (EU CT 2024-518972-30). One Sentence SummaryA mobilized peripheral blood HSC expansion protocol optimized for gene therapy allows robust polyclonal long-term engraftment of LV-transduced cells.

BackgroundCalcium release deficiency syndrome (CRDS) is a recently described inherited channelopathy caused by loss-of-function variants in RYR2. Clinically, CRDS patients present with lethal ventricular arrhythmias which are not reproduced on exercise stress testing, unlike catecholaminergic polymorphic ventricular tachycardia. A hallmark trigger identified for CRDS mimics a long-burst, long-pause, short-coupled extra-stimulus (LBLPS) programmed electrical stimulation protocol, which was experimentally validated in humans and mouse models. Moreover, application of a long-burst, long-pause (LBLP) protocol alone can induce an abnormal repolarization on the first sinus beat that is unique to CRDS. However, the electrophysiological basis of CRDS in human cardiac tissue, including other triggers, are not fully understood, and whether clinically relevant arrhythmias can be observed in human stem cell models remains unknown. MethodsWe performed electrophysiological and arrhythmia inducibility studies using clinically relevant programmed electrical stimulation protocols in two-dimensional cardiac tissue generated from metabolically matured human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) carrying the CRDS variant RyR2-E4146D. High spatiotemporal optical mapping and multielectrode arrays were used for electrophysiological phenotyping. ResultsAt baseline, E4146D+/- monolayers showed no arrhythmias, similar to controls. During rapid pacing, E4146D+/- promoted electrical vulnerability by reducing the threshold for action potential duration (APD) alternans and Ca2+ alternans and increasing the propensity for spatial discordance of alternans. In response to LBLP pacing, E4146D+/- monolayers often demonstrated an abnormal repolarization response characterized by spatially dispersed APD prolongation and large Ca2+ release. Notably, LBLPS pacing produced early-after depolarization (EAD)-driven triggered activity resulting in re-entrant tissue conduction patterns, explaining the short-coupled ectopy driven arrhythmias seen in CRDS patients. Similar arrhythmias were observed when EADs developed during spatially discordant alternans. Lastly, flecainide showed efficacy in suppressing arrhythmia inducibility for the here studied variant. ConclusionsWe developed the first hiPSC model for CRDS which recapitulates clinically observed and inducible arrhythmias. Our model provides novel insights into tissue-level, re-entrant arrhythmias, which are initiated by EADs during electrically vulnerable states in CRDS human cardiac tissue and can be suppressed by flecainide. This model provides the framework for studying other CRDS variants and complex arrhythmias in hiPSC-CMs and establishes a human-based new approach method (NAM) for drug and gene therapy development for CRDS. CLINICAL PERSPECTIVEO_ST_ABSWhat is new?C_ST_ABS{blacksquare} We developed the first human stem cell-derived cardiomyocyte (hiPSC-CM) tissue model for calcium release deficiency syndrome (CRDS) which recapitulates its hallmark clinical features, including inducible ventricular arrhythmias with programmed electrical stimulation and post-pacing repolarization abnormalities. {blacksquare}Using genome edited and metabolically matured hiPSC-CMs combined with high spatiotemporal optical mapping, we show that tissue-level arrhythmias are initiated by early-after depolarizations (EADs) which develop during electrically vulnerable states, leading to re-entrant conduction patterns. We comprehensively characterize the features of EAD-induced triggered activity, showing that these ectopic beats promote re-entry through slower conduction velocities and shorter action potential durations. This uncovers how EAD-induced short-coupled ectopy leads to malignant ventricular arrhythmias in CRDS patients, and establishes the phenotype for future hiPSC-CM investigations. {blacksquare}We identified flecainide as an effective agent in suppressing arrhythmias on single cell and tissue levels in hiPSC-CMs for this CRDS variant, reproducing clinical results. What are the clinical implications?{blacksquare} CRDS has only recently been described as a unique channelopathy caused by loss-of-function RYR2 variants, and much of its triggers and mechanisms in human cardiomyocytes remain unclear. The arrhythmias observed are often not related to exercise, and exercise stress testing does not reproduce these abnormalities. No human models exist to date which closely recapitulate the triggers shown to induce tissue-level arrhythmias in patients and mouse models. Our model demonstrates that programmed electrical stimulation, without pharmacological {beta}-adrenergic stimulation, can reliably induce the same arrhythmias seen clinically, enabling accurate disease modeling and drug development. {blacksquare}Combining programmed electrical stimulation in cardiac tissue derived from genome-edited hiPSC-CMs with high spatiotemporal optical mapping is a robust and novel approach to identify the mechanisms of complex, tissue-level arrhythmias which remain underexplored, such as short-coupled ventricular fibrillation, in a patient-specific and translational manner.

The mammalian retina lacks meaningful regenerative capacity, and degeneration usually leads to irreversible vision loss. Although lower vertebrates regenerate retinal neurons through Muller glia, this capacity has generally been considered absent in humans. Using long-term organotypic retinal cultures from 39 adult donors, we show that defined humoral cues alone are sufficient to unlock a latent neurogenic program in human Muller cells. FGF-2 treatment and GSK-3 inhibition induced robust proliferation across both peripheral and central retina, with 79.09 {+/-} 6.32% of dividing cells identified as Muller glia, some completing multiple cell cycles. Single-cell transcriptomics revealed activation of progenitor-like and neuronal differentiation pathways, whereas immunohistochemistry demonstrated expression of early and late neuronal markers spanning all major retinal lineages. Newly generated cells expressed markers of cone, rod, bipolar, horizontal, amacrine, and ganglion cell identities, together with evidence of early synaptogenesis. These findings reveal an intrinsic regenerative potential in adult human Muller glia, with implications for future vision-restoration strategies in degenerative retinal disease. SummaryAdult human Muller glia retain an intrinsic capacity for proliferation and neural lineage commitment independent of donor age or gender. In long-term organotypic cultures of human donor retina, defined humoral cues, without genetic manipulation, induce Muller glia proliferation and the onset of neuronal differentiation. These findings reveal intrinsic neurogenic potential in human Muller glia and provide a human-relevant platform for retinal regeneration studies.

Parvalbumin-expressing cortical interneurons play a critical role in maintaining the balance between excitatory and inhibitory signalling and are essential for cognition, with dysfunction implicated in numerous brain disorders. Although human pluripotent stem cells have enabled the generation of diverse human neuronal types in vitro, including cortical interneurons, parvalbumin-expressing interneurons - unlike somatostatin-expressing interneurons - remain difficult to generate reliably and consistently. Here, we demonstrate the efficient and reproducible generation of parvalbumin-expressing cortical interneurons in vitro within 50 days of differentiation. Parvalbumin mRNA and protein were detected without forced gene expression, cell sorting, rodent co-culture or intracerebral transplantation, approaches commonly required by previous protocols. Single-cell transcriptomic analyses validated neuronal identity and authenticity, revealing enrichment for gene expression signatures of parvalbumin-expressing cortical interneurons in vivo. Together, these findings establish a robust method that facilitates interneuron research by enabling the reliable generation of authentic human parvalbumin-expressing cortical interneurons within a short time frame. eTOC blurbAzzouni et al. present a rapid and reproducible protocol for generating authentic human parvalbumin-expressing cortical interneurons from pluripotent stem cells in just 50 days, without forced gene expression or co-culture. Single-cell transcriptomics confirm robust acquisition of in vivo-like PVALB interneuron identity, enabling new opportunities for human interneuron research. HighlightO_LIOptimising SHH and WNT modulators enables consistent PVALB interneuron generation. C_LIO_LI10% of cells express PVALB mRNA within 50 days of 2D differentiation from hPSCs. C_LIO_LIPVALB expression occurs without gene forcing, sorting, co-culture or grafting. C_LIO_LIComparison of gene expression to in vivo interneurons confirms PVALB authenticity. C_LI Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=192 SRC="FIGDIR/small/710579v1_ufig1.gif" ALT="Figure 1"> View larger version (32K): org.highwire.dtl.DTLVardef@1a53d13org.highwire.dtl.DTLVardef@14cd131org.highwire.dtl.DTLVardef@3a0f9corg.highwire.dtl.DTLVardef@1d7034d_HPS_FORMAT_FIGEXP M_FIG C_FIG

Rejuvenating aging human skin is a major therapeutic goal, but objective, quantitative measures of intrinsic aging are limited. We performed a cross-sectional histological study of UV-protected buttock and abdominal skin in adults spanning multiple decades of life to identify features that reliably track age. Epidermal thickness measured between rete ridges was unchanged, but rete ridge size declined linearly with age: ridges became shorter and thinner in both sites, though rete ridge number decreased only in the abdomen. Consistent with these structural changes, proliferative cells (Ki67+) per ridge and expression of integrin {beta}4 (ITGB4), a putative stem-cell marker, were reduced in aged skin. We combined these biomarkers into a predictive model that estimated skin age more accurately than any single marker. To test whether the model detects longitudinal change, we analyzed aged abdominal skin before and after xenografting onto young or aged mice, a procedure previously reported to rejuvenate human skin in young but not aged recipient mice. Both individual biomarkers and the imaging model indicated rejuvenation regardless of host age; however, notably, engraftment efficiency was lower in aged hosts, with surviving grafts showing younger histological phenotypes. These results provide quantitative criteria for assessing intrinsic skin aging and suggest that the process of engraftment itself is sufficient to induce rejuvenation-like changes.

Artery endothelial cells (ECs) arise through different pathways, including differentiation from mesodermal cells (vasculogenesis) or from already established vein or capillary plexus ECs (angiogenesis), the latter being most common during embryonic development and regeneration. Understanding the vein-to-artery (v2a) transition could improve revascularization therapies, but progress is limited by a lack of human models. Here, we develop a human pluripotent stem cell (hPSC) differentiation protocol that models the v2a EC conversion. Comparing v2a and mesoderm-to-artery (m2a) transcriptomes with publicly available single cell RNA sequencing (scRNA-seq) data from human embryos showed they reflected angiogenesis- and vasculogenesis-derived artery ECs, respectively. This reductionist system revealed that VEGF activation alongside PI3K inhibition was sufficient for vein ECs to acquire arterial identity within 48 hours. We model a critical step in vascular development and define the minimal signals required for artery differentiation from veins, providing a framework to promote this conversion in revascularization or therapeutic contexts.

The establishment of genetic circuits in pluripotent stem cells (PSCs) allows to model and manipulate developmental events. However, prototyping complex circuitry remains challenging, due to limitations in screening circuit components and transgene silencing. Here, we introduce KELPE: PSCs with two silencing-resistant insulated genomic landing pads targeted to genomic safe harbour sites. KELPE cells enable the stable integration of multiple transgenes into the same genomic region, facilitating fair comparisons of genetic circuit components. We demonstrate this by fine-tuning "synthetic neighbour-labelling" technologies. We first generate optimised PUFFFIN PSCs, which report on cell-cell interactions by fluorescently labelling wild-type neighbours. We then generate new synNotch "receiver" PSCs, which can trigger expression of any transgene following interaction with a synthetic ligand presented by "sender" cells of interest. We describe an optimised circuit syntax that abolishes ligand-independent transgene induction in receiver PSCs, and showcase this by synthetically programming cell death in receiver cells engineered to express a toxin following interaction with sender cells. In summary, we describe a new cell line that facilitates silencing-resistant transgene expression and prototyping of synthetic biology tools in a developmentally-relevant model.