Leveling Waddington: the emergence of direct programming and the loss of cell fate hierarchies.

For decades, Waddington's concept of the 'epigenetic landscape' has served as an educative hierarchical model to illustrate the progressive restriction of cell differentiation potential during normal development. While still being highly valuable in the context of normal development, the Waddington model falls short of accommodating recent breakthroughs in cell programming. The advent of induced pluripotent stem (iPS) cells and advances in direct cell fate conversion (also known as transdifferentiation) suggest that somatic and pluripotent cell fates can be interconverted without transiting through distinct hierarchies. We propose a non-hierarchical 'epigenetic disc' model to explain such cell fate transitions, which provides an alternative landscape for modelling cell programming and reprogramming.

Leveling Waddington: the emergence of direct programming and the loss of cell fate hierarchies.

For decades, Waddington's concept of the 'epigenetic landscape' has served as an educative hierarchical model to illustrate the progressive restriction of cell differentiation potential during normal development. While still being highly valuable in the context of normal development, the Waddington model falls short of accommodating recent breakthroughs in cell programming. The advent of induced pluripotent stem (iPS) cells and advances in direct cell fate conversion (also known as transdifferentiation) suggest that somatic and pluripotent cell fates can be interconverted without transiting through distinct hierarchies. We propose a non-hierarchical 'epigenetic disc' model to explain such cell fate transitions, which provides an alternative landscape for modelling cell programming and reprogramming.

Retigabine, a potassium channel opener, restores thalamocortical neuron functionality in a murine model of autoimmune encephalomyelitis.

BACKGROUND: Multiple Sclerosis (MS) is an autoimmune neurodegenerative disease, whose primary hallmark is the occurrence of inflammatory lesions in white and grey matter structures. Increasing evidence in MS patients and respective murine models reported an impaired ionic homeostasis driven by inflammatory-demyelination, thereby profoundly affecting signal propagation. However, the impact of a focal inflammatory lesion on single-cell and network functionality has hitherto not been fully elucidated. OBJECTIVES: In this study, we sought to determine the consequences of a localized cortical inflammatory lesion on the excitability and firing pattern of thalamic neurons in the auditory system. Moreover, we tested the neuroprotective effect of Retigabine (RTG), a specific Kv7 channel opener, on disease outcome. METHODS: To resemble the human disease, we focally administered pro-inflammatory cytokines, TNF-α and IFN-γ, in the primary auditory cortex (A1) of MOG35-55 immunized mice. Thereafter, we investigated the impact of the induced inflammatory milieu on afferent thalamocortical (TC) neurons, by performing ex vivo recordings. Moreover, we explored the effect of Kv7 channel modulation with RTG on auditory information processing, using in vivo electrophysiological approaches. RESULTS: Our results revealed that a cortical inflammatory lesion profoundly affected the excitability and firing pattern of neighboring TC neurons. Noteworthy, RTG restored control-like values and TC tonotopic mapping. CONCLUSION: Our results suggest that RTG treatment might robustly mitigate inflammation-induced altered excitability and preserve ascending information processing.

Human-specific ARHGAP11B ensures human-like basal progenitor levels in hominid cerebral organoids.

The human-specific gene ARHGAP11B has been implicated in human neocortex expansion. However, the extent of ARHGAP11B's contribution to this expansion during hominid evolution is unknown. Here we address this issue by genetic manipulation of ARHGAP11B levels and function in chimpanzee and human cerebral organoids. ARHGAP11B expression in chimpanzee cerebral organoids doubles basal progenitor levels, the class of cortical progenitors with a key role in neocortex expansion. Conversely, interference with ARHGAP11B's function in human cerebral organoids decreases basal progenitors down to the chimpanzee level. Moreover, ARHGAP11A or ARHGAP11B rescue experiments in ARHGAP11A plus ARHGAP11B double-knockout human forebrain organoids indicate that lack of ARHGAP11B, but not of ARHGAP11A, decreases the abundance of basal radial glia-the basal progenitor type thought to be of particular relevance for neocortex expansion. Taken together, our findings demonstrate that ARHGAP11B is necessary and sufficient to ensure the elevated basal progenitor levels that characterize the fetal human neocortex, suggesting that this human-specific gene was a major contributor to neocortex expansion during human evolution.

Human cerebral organoids reveal progenitor pathology in EML1-linked cortical malformation.

Malformations of human cortical development (MCD) can cause severe disabilities. The lack of human-specific models hampers our understanding of the molecular underpinnings of the intricate processes leading to MCD. Here, we use cerebral organoids derived from patients and genome edited-induced pluripotent stem cells to address pathophysiological changes associated with a complex MCD caused by mutations in the echinoderm microtubule-associated protein-like 1 (EML1) gene. EML1-deficient organoids display ectopic neural rosettes at the basal side of the ventricular zone areas and clusters of heterotopic neurons. Single-cell RNA sequencing shows an upregulation of basal radial glial (RG) markers and human-specific extracellular matrix components in the ectopic cell population. Gene ontology and molecular analyses suggest that ectopic progenitor cells originate from perturbed apical RG cell behavior and yes-associated protein 1 (YAP1)-triggered expansion. Our data highlight a progenitor origin of EML1 mutation-induced MCD and provide new mechanistic insight into the human disease pathology.

Cerebral organoids to unravel the mechanisms underlying malformations of human cortical development.

Genetic studies identified multiple mutations associated with malformations of cortical development (MCD) in humans. When analyzing the underlying mechanisms in non-human experimental models it became increasingly evident, that these mutations accumulate in genes, which functions evolutionary progressed from rodents to humans resulting in an incomplete reflection of the molecular and cellular alterations in these models. Human brain organoids derived from human pluripotent stem cells resemble early aspects of human brain development to a remarkable extent making them an attractive model to investigate MCD. Here we review how human brain organoids enable the generation of fundamental new insight about the underlying pathomechanisms of MCD. We show how phenotypic features of these diseases are reflected in human brain organoids and discuss challenges and future considerations but also limitations for the use of human brain organoids to model human brain development and associated disorders.

Voltammetric Approach for Characterizing the Biophysical and Chemical Functionality of Human Induced Pluripotent Stem Cell-Derived Serotonin Neurons.

Depression is quickly becoming one of the world's most pressing public health crises, and there is an urgent need for better diagnostics and therapeutics. Behavioral models in animals and humans have not adequately addressed the diagnosis and treatment of depression, and biomarkers of mental illnesses remain ill-defined. It has been very difficult to identify biomarkers of depression because of in vivo measurement challenges. While our group has made important strides in developing in vivo tools to measure such biomarkers (e.g., serotonin) in mice using voltammetry, these tools cannot be easily applied for depression diagnosis and drug screening in humans due to the inaccessibility of the human brain. In this work, we take a chemical approach, ex vivo, to introduce a human-derived system to investigate brain serotonin. We utilize human induced pluripotent stem cells differentiated into serotonin neurons and establish a new ex vivo model of real-time serotonin neurotransmission measurements. We show that evoked serotonin release responds to stimulation intensity and tryptophan preloading, and that serotonin release and reuptake kinetics resemble those found in vivo in rodents. Finally, after selective serotonin reuptake inhibitor (SSRI) exposure, we find dose-dependent internalization of the serotonin reuptake transporters (a signature of the in vivo response to SSRI). Our new human-derived chemical model has great potential to provide an ex vivo chemical platform as a translational tool for in vivo neuropsychopharmacology.

In Vitro Recapitulation of Developmental Transitions in Human Neural Stem Cells.

During nervous system development, early neuroepithelial stem (NES) cells with a highly polarized morphology and responsiveness to regionalizing morphogens give rise to radial glia (RG) cells, which generate region-specific neurons. Recently, stable neural cell populations reminiscent of NES cells have been obtained from pluripotent stem cells and the fetal human hindbrain. Here, we explore whether these cell populations, similar to their in vivo counterparts, can give rise to neural stem (NS) cells with RG-like properties and whether region-specific NS cells can be generated from NES cells with different regional identities. In vivo RG cells are thought to form from NES cells with the onset of neurogenesis. Therefore, we cultured NES cells temporarily in differentiating conditions. Upon reinitiation of growth factor treatment, cells were found to enter a developmental stage reflecting major characteristics of RG-like NS cells. These NES cell-derived NS cells exhibited a very similar morphology and marker expression as primary NS cells generated from human fetal tissue, indicating that conversion of NES cells into NS cells recapitulates the developmental progression of early NES cells into RG cells observed in vivo. Importantly, NS cells generated from NES cells with different regional identities exhibited stable region-specific transcription factor expression and generated neurons appropriate for their positional identity. Stem Cells 2019;37:1429-1440.

Excitation-induced ataxin-3 aggregation in neurons from patients with Machado-Joseph disease.

Machado-Joseph disease (MJD; also called spinocerebellar ataxia type 3) is a dominantly inherited late-onset neurodegenerative disorder caused by expansion of polyglutamine (polyQ)-encoding CAG repeats in the MJD1 gene (also known as ATXN3). Proteolytic liberation of highly aggregation-prone polyQ fragments from the protective sequence of the MJD1 gene product ataxin 3 (ATXN3) has been proposed to trigger the formation of ATXN3-containing aggregates, the neuropathological hallmark of MJD. ATXN3 fragments are detected in brain tissue of MJD patients and transgenic mice expressing mutant human ATXN3(Q71), and their amount increases with disease severity, supporting a relationship between ATXN3 processing and disease progression. The formation of early aggregation intermediates is thought to have a critical role in disease initiation, but the precise pathogenic mechanism operating in MJD has remained elusive. Here we show that L-glutamate-induced excitation of patient-specific induced pluripotent stem cell (iPSC)-derived neurons initiates Ca(2+)-dependent proteolysis of ATXN3 followed by the formation of SDS-insoluble aggregates. This phenotype could be abolished by calpain inhibition, confirming a key role of this protease in ATXN3 aggregation. Aggregate formation was further dependent on functional Na(+) and K(+) channels as well as ionotropic and voltage-gated Ca(2+) channels, and was not observed in iPSCs, fibroblasts or glia, thereby providing an explanation for the neuron-specific phenotype of this disease. Our data illustrate that iPSCs enable the study of aberrant protein processing associated with late-onset neurodegenerative disorders in patient-specific neurons.

Arylsulfatase A Overexpressing Human iPSC-derived Neural Cells Reduce CNS Sulfatide Storage in a Mouse Model of Metachromatic Leukodystrophy.

Metachromatic leukodystrophy (MLD) is an inherited lysosomal storage disorder resulting from a functional deficiency of arylsulfatase A (ARSA), an enzyme that catalyzes desulfation of 3-O-sulfogalactosylceramide (sulfatide). Lack of active ARSA leads to the accumulation of sulfatide in oligodendrocytes, Schwann cells and some neurons and triggers progressive demyelination, the neuropathological hallmark of MLD. Several therapeutic approaches have been explored, including enzyme replacement, autologous hematopoietic stem cell-based gene therapy, intracerebral gene therapy or cell-based gene delivery into the central nervous system (CNS). However, long-term treatment of the blood-brain-barrier protected CNS remains challenging. Here we used MLD patient-derived induced pluripotent stem cells (iPSCs) to generate long-term self-renewing neuroepithelial stem cells and astroglial progenitors for cell-based ARSA replacement. Following transplantation of ARSA-overexpressing precursors into ARSA-deficient mice we observed a significant reduction of sulfatide storage up to a distance of 300 µm from grafted cells. Our data indicate that neural precursors generated via reprogramming from MLD patients can be engineered to ameliorate sulfatide accumulation and may thus serve as autologous cell-based vehicle for continuous ARSA supply in MLD-affected brain tissue.

Small molecules enable highly efficient neuronal conversion of human fibroblasts.

Forced expression of proneural transcription factors has been shown to direct neuronal conversion of fibroblasts. Because neurons are postmitotic, conversion efficiencies are an important parameter for this process. We present a minimalist approach combining two-factor neuronal programming with small molecule-based inhibition of glycogen synthase kinase-3ß and SMAD signaling, which converts postnatal human fibroblasts into functional neuron-like cells with yields up to >200% and neuronal purities up to >80%.

Embryonic stem cell-based modeling of tau pathology in human neurons.

Alterations in the microtubule (MT)-associated protein, tau, have emerged as a pivotal phenomenon in several neurodegenerative disorders, including frontotemporal dementia and Alzheimer's disease. Although compelling lines of evidence from various experimental models suggest that hyperphosphorylation and conformational changes of tau can cause its aggregation into filaments, the actual tau species and effective mechanisms that conspire to trigger the degeneration of human neurons remain obscure. Herein, we explored whether human embryonic stem cell-derived neural stem cells can be exploited to study consequences of an overexpression of 2N4R tau (two normal N-terminal and four MT-binding domains; n-tau) versus pseudohyperphosphorylated tau (p-tau) directly in human neurons. Given the involvement of tau in MT integrity and cellular homeostasis, we focused on the effects of both tau variants on subcellular transport and neuronal survival. By using inducible lentiviral overexpression, we show that p-tau, but not n-tau, readily leads to an MC-1-positive protein conformation and impaired mitochondrial transport. Although these alterations do not induce cell death under standard culture conditions, p-tau-expressing neurons cultured under non-redox-protected conditions undergo degeneration with formation of axonal varicosities sequestering transported proteins and progressive neuronal cell death. Our data support a causative link between the phosphorylation and conformational state of tau, microtubuli-based transport, and the vulnerability of human neurons to oxidative stress. They further depict human embryonic stem cell-derived neurons as a useful experimental model for studying tau-associated cellular alterations in an authentic human system.

Forebrain Eml1 depletion reveals early centrosomal dysfunction causing subcortical heterotopia.

Subcortical heterotopia is a cortical malformation associated with epilepsy, intellectual disability, and an excessive number of cortical neurons in the white matter. Echinoderm microtubule-associated protein like 1 (EML1) mutations lead to subcortical heterotopia, associated with abnormal radial glia positioning in the cortical wall, prior to malformation onset. This perturbed distribution of proliferative cells is likely to be a critical event for heterotopia formation; however, the underlying mechanisms remain unexplained. This study aimed to decipher the early cellular alterations leading to abnormal radial glia. In a forebrain conditional Eml1 mutant model and human patient cells, primary cilia and centrosomes are altered. Microtubule dynamics and cell cycle kinetics are also abnormal in mouse mutant radial glia. By rescuing microtubule formation in Eml1 mutant embryonic brains, abnormal radial glia delamination and heterotopia volume were significantly reduced. Thus, our new model of subcortical heterotopia reveals the causal link between Eml1's function in microtubule regulation and cell position, both critical for correct cortical development.

Presenilin-1 L166P mutant human pluripotent stem cell-derived neurons exhibit partial loss of γ-secretase activity in endogenous amyloid-ß generation.

Alzheimer's disease (AD) is the most frequent cause of dementia. There is compelling evidence that the proteolytic processing of the amyloid precursor protein (APP) and accumulation of amyloid-ß (Aß) peptides play critical roles in AD pathogenesis. Due to limited access to human neural tissue, pathogenetic studies have, so far, mostly focused on the heterologous overexpression of mutant human APP in non-human cells. In this study, we show that key steps in proteolytic APP processing are recapitulated in neurons generated from human embryonic and induced pluripotent stem cell-derived neural stem cells (NSC). These human NSC-derived neurons express the neuron-specific APP(695) splice variant, BACE1, and all members of the γ-secretase complex. The human NSC-derived neurons also exhibit a differentiation-dependent increase in Aß secretion and respond to the pharmacotherapeutic modulation by anti-amyloidogenic compounds, such as γ-secretase inhibitors and nonsteroidal anti-inflammatory drugs. Being highly amenable to genetic modification, human NSCs enable the study of mechanisms caused by disease-associated mutations in human neurons. Interestingly, the AD-associated PS1(L166P) variant revealed a partial loss of γ-secretase function, resulting in the decreased production of endogenous Aß40 and an increased Aß42/40 ratio. The PS1(L166P) mutant is also resistant to γ-secretase modulation by nonsteroidal anti-inflammatory drugs. Pluripotent stem cell-derived neurons thus provide experimental access to key steps in AD pathogenesis and can be used to screen pharmaceutical compounds directly in a human neuronal system.

Human-induced pluripotent stem cells form functional neurons and improve recovery after grafting in stroke-damaged brain.

Reprogramming of adult human somatic cells to induced pluripotent stem cells (iPSCs) is a novel approach to produce patient-specific cells for autologous transplantation. Whether such cells survive long-term, differentiate to functional neurons, and induce recovery in the stroke-injured brain are unclear. We have transplanted long-term self-renewing neuroepithelial-like stem cells, generated from adult human fibroblast-derived iPSCs, into the stroke-damaged mouse and rat striatum or cortex. Recovery of forepaw movements was observed already at 1 week after transplantation. Improvement was most likely not due to neuronal replacement but was associated with increased vascular endothelial growth factor levels, probably enhancing endogenous plasticity. Transplanted cells stopped proliferating, could survive without forming tumors for at least 4 months, and differentiated to morphologically mature neurons of different subtypes. Neurons in intrastriatal grafts sent axonal projections to the globus pallidus. Grafted cells exhibited electrophysiological properties of mature neurons and received synaptic input from host neurons. Our study provides the first evidence that transplantation of human iPSC-derived cells is a safe and efficient approach to promote recovery after stroke and can be used to supply the injured brain with new neurons for replacement.

A rosette-type, self-renewing human ES cell-derived neural stem cell with potential for in vitro instruction and synaptic integration.

An intriguing question in human embryonic stem cell (hESC) biology is whether these pluripotent cells can give rise to stably expandable somatic stem cells, which are still amenable to extrinsic fate instruction. Here, we present a pure population of long-term self-renewing rosette-type hESC-derived neural stem cells (lt-hESNSCs), which exhibit extensive self-renewal, clonogenicity, and stable neurogenesis. Although lt-hESNSCs show a restricted expression of regional transcription factors, they retain responsiveness to instructive cues promoting the induction of distinct subpopulations, such as ventral midbrain and spinal cord fates. Using lt-hESNSCs as a donor source for neural transplantation, we provide direct evidence that hESC-derived neurons can establish synaptic connectivity with the mammalian nervous system. Combining long-term stability, maintenance of rosette-properties and phenotypic plasticity, lt-hESNSCs may serve as useful tool to study mechanisms of human NSC self-renewal, lineage segregation, and functional in vivo integration.

Asymmetric Notch activity by differential inheritance of lysosomes in human neural stem cells.

Symmetric and asymmetric cell divisions are conserved strategies for stem cell expansion and the generation of more committed progeny, respectively. Here, we demonstrate that in human neural stem cells (NSCs), lysosomes are asymmetrically inherited during mitosis. We show that lysosomes contain Notch receptors and that Notch activation occurs the acidic lysosome environment. The lysosome asymmetry correlates with the expression of the Notch target gene HES1 and the activity of Notch signaling in the daughter cells. Furthermore, an asymmetry of lysosomes and Notch receptors was also observed in a human organoid model of brain development with mitotic figures showing preferential inheritance of lysosomes and Notch receptor in that daughter cell remaining attached to the apical membrane. Thus, this study suggests a previously unknown function of lysosomes as a signaling hub to establish a bias in Notch signaling activity between daughter cells after an asymmetric cell division of human NSCs.

Diverse maturity-dependent and complementary anti-apoptotic brakes safeguard human iPSC-derived neurons from cell death.

In humans, most neurons are born during embryonic development and have to persist throughout the entire lifespan of an individual. Thus, human neurons have to develop elaborate survival strategies to protect against accidental cell death. We set out to decipher the developmental adaptations resulting in neuronal resilience. We demonstrate that, during the time course of maturation, human neurons install a complex and complementary anti-apoptotic signaling network. This includes i.) a downregulation of central proteins of the intrinsic apoptosis pathway including several caspases, ii.) a shift in the ratio of pro- and anti-apoptotic BCL-2 family proteins, and iii.) an elaborate regulatory network resulting in upregulation of the inhibitor of apoptosis protein (IAP) XIAP. Together, these adaptations strongly increase the threshold for apoptosis initiation when confronted with a wide range of cellular stressors. Our results highlight how human neurons are endowed with complex and redundant preemptive strategies to protect against stress and cell death.

Lineage selection of functional and cryopreservable human embryonic stem cell-derived neurons.

A major prerequisite for the biomedical application of human embryonic stem cells (hESC) is the derivation of defined and homogeneous somatic cell types. Here we present a human doublecortin (DCX) promoter-based lineage-selection strategy for the generation of purified hESC-derived immature neurons. After transfection of hESC-derived neural precursors with a DCX-enhanced green fluorescent protein construct, fluorescence-activated cell sorting enables the enrichment of immature human neurons at purities of up to 95%. Selected neurons undergo functional maturation and are able to establish synaptic connections. Considering that the applicability of purified hESC-derived neurons would largely benefit from an efficient cryopreservation technique, we set out to devise defined freezing conditions involving caspase inhibition, which yield post-thaw recovery rates of up to 83%. Combined with our lineage-selection procedure this cryopreservation technique enables the generation of human neurons in a ready-to-use format for a large variety of biomedical applications.

Drug discovery in psychopharmacology: from 2D models to cerebral organoids
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Psychiatric disorders are a heterogeneous group of mental illnesses associated with a high social and economic burden on patients and society. The complex symptomatology of these disorders, coupled with our limited understanding of the structural and functional abnormalities affecting the brains of neuropsychiatric patients, has made it difficult to develop effective medical treatment strategies. With the advent of reprogramming technologies and recent developments in induced pluripotent stem (iPS) cell-based protocols for differentiation into defined neuronal cultures and 3-dimensional cerebral organoids, a new era of preclinical disease modeling has begun which could revolutionize drug discovery in psychiatry. This review provides an overview of iPS cell-based disease models in psychiatry and how these models contribute to our understanding of pharmacological drug action. We also propose a refined iPSC-based drug discovery pipeline, ranging from cell-based stratification of patients through improved screening and validation steps to more precise psychopharmacology.
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