Jiwon Jang

Nrf2, a regulator of the proteasome, controls self-renewal and pluripotency in human embryonic stem cells.

Nuclear factor, erythroid 2‐like 2 (Nrf2) is a master transcription factor for cellular defense against endogenous and exogenous stresses by regulating expression of many antioxidant and detoxification genes. Here, we show that Nrf2 acts as a key pluripotency gene and a regulator of proteasome activity in human embryonic stem cells (hESCs). Nrf2 expression is highly enriched in hESCs and dramatically decreases upon differentiation. Nrf2 inhibition impairs both the self‐renewal ability of hESCs and re‐establishment of pluripotency during cellular reprogramming. Nrf2 activation can delay differentiation. During early hESC differentiation, Nrf2 closely colocalizes with OCT4 and NANOG. As an underlying mechanism, our data show that Nrf2 regulates proteasome activity in hESCs partially through proteasome maturation protein (POMP), a proteasome chaperone, which in turn controls the proliferation of self‐renewing hESCs, three germ layer differentiation and cellular reprogramming. Even modest proteasome inhibition skews the balance of early differentiation toward mesendoderm at the expense of an ectodermal fate by decreasing the protein level of cyclin D1 and delaying the degradation of OCT4 and NANOG proteins. Taken together, our findings suggest a new potential link between environmental stress and stemness with Nrf2 and the proteasome coordinately positioned as key mediators. Stem Cells 2014;32:2616–2625

Primary Cilium-Autophagy-Nrf2 (PAN) Axis Activation Commits Human Embryonic Stem Cells to a Neuroectoderm Fate

Under defined differentiation conditions, human embryonic stem cells (hESCs) can be directed toward a mesendoderm (ME) or neuroectoderm (NE) fate, the first decision during hESC differentiation. Coupled with lineage-specific G1 lengthening, a divergent ciliation pattern emerged within the first 24 hr of induced lineage specification, and these changes heralded a neuroectoderm decision before any neural precursor markers were expressed. By day 2, increased ciliation in NE precursors induced autophagy that resulted in the inactivation of Nrf2 and thereby relieved transcriptional activation of OCT4 and NANOG. Nrf2 binds directly to upstream regions of these pluripotency genes to promote their expression and repress NE derivation. Nrf2 suppression was sufficient to rescue poorly neurogenic iPSC lines. Only after these events had been initiated did neural precursor markers get expressed at day 4. Thus, we have identified a primary cilium-autophagy-Nrf2 (PAN) control axis coupled to cell-cycle progression that directs hESCs toward NE.

Haploinsufficiency of BAZ1B contributes to Williams syndrome through transcriptional dysregulation of neurodevelopmental pathways

Williams syndrome (WS) is a neurodevelopmental disorder caused by a genomic deletion of ∼28 genes that results in a cognitive and behavioral profile marked by overall intellectual impairment with relative strength in expressive language and hypersocial behavior. Advancements in protocols for neuron differentiation from induced pluripotent stem cells allowed us to elucidate the molecular circuitry underpinning the ontogeny of WS. In patient-derived stem cells and neurons, we determined the expression profile of the Williams-Beuren syndrome critical region-deleted genes and the genome-wide transcriptional consequences of the hemizygous genomic microdeletion at chromosome 7q11.23. Derived neurons displayed disease-relevant hallmarks and indicated novel aberrant pathways in WS neurons including over-activated Wnt signaling accompanying an incomplete neurogenic commitment. We show that haploinsufficiency of the ATP-dependent chromatin remodeler, BAZ1B, which is deleted in WS, significantly contributes to this differentiation defect. Chromatin-immunoprecipitation (ChIP-seq) revealed BAZ1B target gene functions are enriched for neurogenesis, neuron differentiation and disease-relevant phenotypes. BAZ1B haploinsufficiency caused widespread gene expression changes in neural progenitor cells, and together with BAZ1B ChIP-seq target genes, explained 42% of the transcriptional dysregulation in WS neurons. BAZ1B contributes to regulating the balance between neural precursor self-renewal and differentiation and the differentiation defect caused by BAZ1B haploinsufficiency can be rescued by mitigating over-active Wnt signaling in neural stem cells. Altogether, these results reveal a pivotal role for BAZ1B in neurodevelopment and implicate its haploinsufficiency as a likely contributor to the neurological phenotypes in WS.

Polycation-mediated enhancement of retroviral transduction efficiency depends on target cell types and pseudotyped Env proteins: implication for gene transfer into neural stem cells.

Polycations such as polybrene (PB) are routinely used for most retroviral vector-mediated gene transfer studies because they can increase the infectivity of retroviruses. However, it was not systematically determined if addition of the polycation is an essential prerequisite for all retroviral transductions. To test this, we measured the effects of the polycation on transduction efficiency using various combinations of target cells and pseudotyped viral envelope (Env) proteins. Here, we show polycations do not always increase retroviral transduction efficiency and that their enhancing effect depends on both the type of target cells and Env proteins. The findings presented here also suggest that high transduction rates can be achieved in primary neural stem cells in vitro and in vivo by choosing an appropriate Env protein for pseudotyping without using polycations which are potentially toxic to primary cells and may change the intrinsic characteristics of cells.

A retroviral vector suitable for ultrasound image-guided gene delivery to mouse brain

Gene transfer to the early-stage embryonic brain using the ultrasound image-guided gene delivery (UIGD) technique has proven to be valuable for investigating brain development. Thus far, this technology has been restricted to the study of embryonic neurogenesis. When this technique is designed to be employed for the study in adult animals, a long-term stable gene expression will be required. We attempted to develop a retroviral vector suitable for expressing exogenous genes in the brains of postnatal and adult mice in the context of the UIGD technique. Retroviral vectors containing four different long terminal repeats (LTRs) (each from Moloney murine leukemia virus (MoMLV), murine stem cell virus (MSCV), myeloproliferative sarcoma virus (MPSV) and spleen focus-forming virus (SFFV)) were compared using the well-known CE vector having the EF1α internal promoter as a control. The MS vector containing MSCV LTR produced a higher viral titer and a higher level of gene expression than other vectors including CE. The MS vector drove the gene expression in cultured neural stem cells for 3 weeks. Furthermore, the MS vector could efficiently deliver the gene to the mouse central nervous system, as transgene expression was found in various regions of the brains and spinal cords as well as in all major neural cell types. The data from an in vivo luciferase imaging analysis showed that the gene expression from the MS vector was sustainable for almost 3 months. Our data suggested that the MS vector would be suitable to construct mice containing the transgene expressed in the brain or spinal cord in a quick and cost-effective manner.

Recording action potential propagation in single axons using multi-electrode arrays

The small caliber of central nervous system (CNS) axons makes routine study of axonal physiology relatively difficult. However, while recording extracellular action potentials from neurons cultured on planer multi-electrode arrays (MEAs) we found activity among groups of electrodes consistent with action potential propagation in single neurons. Action potential propagation was evident as widespread, repetitive cooccurrence of extracellular action potentials (eAPs) among groups of electrodes. These eAPs occurred with invariant sequences and inter-electrode latencies that were consistent with reported measures of action potential propagation in unmyelinated axons. Within co-active electrode groups, the inter-electrode eAP latencies were temperature sensitive, as expected for action potential propagation. Our data are consistent with these signals primarily reflecting axonal action potential propagation, from axons with a high density of voltage-gated sodium channels. Repeated codetection of eAPs by multiple electrodes confirmed these eAPs are from individual neurons and averaging these eAPs revealed sub-threshold events at other electrodes. The sequence of electrodes at which eAPs co-occur uniquely identifies these neurons, allowing us to monitor spiking of single identified neurons within neuronal ensembles. We recorded dynamic changes in single axon physiology such as simultaneous increases and decreases in excitability in different portions of single axonal arbors over several hours. Over several weeks, we measured changes in inter-electrode propagation latencies and ongoing changes in excitability in different regions of single axonal arbors. We recorded action potential propagation signals in human induced pluripotent stem cell-derived neurons which could thus be used to study axonal physiology in human disease models. Significance Statement Studying the physiology of central nervous system axons is limited by the technical challenges of recording from axons with pairs of patch or extracellular electrodes at two places along single axons. We studied action potential propagation in single axonal arbors with extracellular recording with multi-electrode arrays. These recordings were non-invasive and were done from several sites of small caliber axons and branches. Unlike conventional extracellular recording, we unambiguously identified and labelled the neuronal source of propagating action potentials. We manipulated and quantified action potential propagation and found a surprisingly high density of axonal voltage-gated sodium channels. Our experiments also demonstrate that the excitability of different portions of axonal arbors can be independently regulated on time scales from hours to weeks.

The Role of Chromatin Density in Cell Population Heterogeneity during Stem Cell Differentiation

We incorporate three-dimensional (3D) conformation of chromosome (Hi-C) and single-cell RNA sequencing data together with discrete stochastic simulation, to explore the role of chromatin reorganization in determining gene expression heterogeneity during development. While previous research has emphasized the importance of chromatin architecture on activation and suppression of certain regulatory genes and gene networks, our study demonstrates how chromatin remodeling can dictate gene expression distribution by folding into distinct topological domains. We hypothesize that the local DNA density during differentiation accentuate transcriptional bursting due to the crowding effect of chromatin. This phenomenon yields a heterogeneous cell population, thereby increasing the potential of differentiation of the stem cells.

Development of murine leukemia virus-based retroviral vectors with a minimum possibility of cis-activation

The possibility of insertional mutagenesis in retroviral gene therapy can be reduced by using a vector lacking the enhancer sequence present in the U3 of the long-terminal repeats. However, such vectors suffer from many pitfalls. We attempted to improve a murine leukemia virus-based retroviral vector containing the enhancer-free U3, first by making it easier to construct a producer line and then by introducing the cellular RPL10 promoter as an internal promoter. The reverse orientation of the expression cassette of the transgene was found to give higher transducing titer and higher-level gene expression. The deletion analysis revealed that the 54-bp-long sequence of U3 (34 and 20 bp present at 5′ and 3′ extreme ends, respectively) was sufficient for the functions of retroviral vectors. The data from the in vitro cell culture assay indicated that the final construct, ROK, containing all these features, had little cis-activation activity, even if it was placed right upstream from the RNA start site of the neighboring gene. Our data suggested that the newly developed vector might provide increased safety, while still producing high viral titer from a stable producer line and high-level gene expression in various target cells including human CD34+ stem cells.

Development of an in vitro cell culture assay system for measuring the activation of a neighbouring gene by the retroviral vector.

The use of retroviral vectors has shown an actual clinical benefit in a few inherited diseases. However, the occurrence of cases of leukemia after the X‐SCID gene therapy trial raised concerns about the safety of insertional mutagenesis inherent to the biology of the retrovirus. Although the retrovirus has long been known to integrate into the host chromosome, and thus have the potential to activate the nearby gene, there has been no convenient method of studying or assaying such a cis‐activation phenomenon.

Ultrasound backscatter microscopy image-guided intraventricular gene delivery at murine embryonic age 9.5 and 10.5 produces distinct transgene expression patterns at the adult stage.

In utero injection of a retroviral vector into the embryonic telencephalon aided by ultrasound backscatter microscopy permits introduction of a gene of interest at an early stage of development. In this study, we compared the tissue distribution of gene expression in adult mice injected with retroviral vectors at different embryonic ages in utero. Following ultrasound image-guided gene delivery (UIGD) into the embryonic telencephalon, adult mice were subjected to whole-body luciferase imaging and immunohistochemical analysis at 6 weeks and 1 year postinjection. Luciferase activity was observed in a wide range of tissues in animals injected at embryonic age 9.5 (E9.5), whereas animals injected at E10.5 showed brain-localized reporter gene expression. These results suggest that mouse embryonic brain creates a closed and impermeable structure around E10. Therefore, by injecting a transgene before or after E10, transgene expression can be manipulated to be local or systemic. Our results also provide information that widens the applicability of UIGD beyond neuroscience studies.