Ruotong Ren

Aging stem cells. A Werner syndrome stem cell model unveils heterochromatin alterations as a driver of human aging.

Heterochromatin in aging stem cells Analysis of human aging syndromes, such as Werner syndrome (WS), may lead to greater understanding of both premature and normal aging. Zhang et al. generated isogenic WS-specific human embryonic stem cell lines (see the Perspective by Brunauer and Kennedy). WS-mesenchymal stem cells displayed features characteristic of premature aging, including heterochromatin disorganization. WRN protein thus functions in the maintenance of heterochromatin, and heterochromatin alterations may represent a driving force of human aging. Science, this issue p. 1160; see also p. 1093 Stabilization of heterochromatin by WRN protein safeguards human mesenchymal stem cells from aging. [Also see Perspective by Brunauer and Kennedy] Werner syndrome (WS) is a premature aging disorder caused by WRN protein deficiency. Here, we report on the generation of a human WS model in human embryonic stem cells (ESCs). Differentiation of WRN-null ESCs to mesenchymal stem cells (MSCs) recapitulates features of premature cellular aging, a global loss of H3K9me3, and changes in heterochromatin architecture. We show that WRN associates with heterochromatin proteins SUV39H1 and HP1α and nuclear lamina–heterochromatin anchoring protein LAP2β. Targeted knock-in of catalytically inactive SUV39H1 in wild-type MSCs recapitulates accelerated cellular senescence, resembling WRN-deficient MSCs. Moreover, decrease in WRN and heterochromatin marks are detected in MSCs from older individuals. Our observations uncover a role for WRN in maintaining heterochromatin stability and highlight heterochromatin disorganization as a potential determinant of human aging.

Targeted Gene Correction Minimally Impacts Whole-Genome Mutational Load in Human-Disease-Specific Induced Pluripotent Stem Cell Clones

The utility of genome editing technologies for disease modeling and developing cellular therapies has been extensively documented, but the impact of these technologies on mutational load at the whole-genome level remains unclear. We performed whole-genome sequencing to evaluate the mutational load at single-base resolution in individual gene-corrected human induced pluripotent stem cell (hiPSC) clones in three different disease models. In single-cell clones, gene correction by helper-dependent adenoviral vector (HDAdV) or Transcription Activator-Like Effector Nuclease (TALEN) exhibited few off-target effects and a low level of sequence variation, comparable to that accumulated in routine hiPSC culture. The sequence variants were randomly distributed and unique to individual clones. We also combined both technologies and developed a TALEN-HDAdV hybrid vector, which significantly increased gene-correction efficiency in hiPSCs. Therefore, with careful monitoring via whole-genome sequencing it is possible to apply genome editing to human pluripotent cells with minimal impact on genomic mutational load.

SIRT6 safeguards human mesenchymal stem cells from oxidative stress by coactivating NRF2

SIRT6 belongs to the mammalian homologs of Sir2 histone NAD+-dependent deacylase family. In rodents, SIRT6 deficiency leads to aging-associated degeneration of mesodermal tissues. It remains unknown whether human SIRT6 has a direct role in maintaining the homeostasis of mesodermal tissues. To this end, we generated SIRT6 knockout human mesenchymal stem cells (hMSCs) by targeted gene editing. SIRT6-deficient hMSCs exhibited accelerated functional decay, a feature distinct from typical premature cellular senescence. Rather than compromised chromosomal stability, SIRT6-null hMSCs were predominately characterized by dysregulated redox metabolism and increased sensitivity to the oxidative stress. In addition, we found SIRT6 in a protein complex with both nuclear factor erythroid 2-related factor 2 (NRF2) and RNA polymerase II, which was required for the transactivation of NRF2-regulated antioxidant genes, including heme oxygenase 1 (HO-1). Overexpression of HO-1 in SIRT6-null hMSCs rescued premature cellular attrition. Our study uncovers a novel function of SIRT6 in maintaining hMSC homeostasis by serving as a NRF2 coactivator, which represents a new layer of regulation of oxidative stress-associated stem cell decay.

Modelling Fanconi anemia pathogenesis and therapeutics using integration-free patient-derived iPSCs

Fanconi anaemia (FA) is a recessive disorder characterized by genomic instability, congenital abnormalities, cancer predisposition and bone marrow (BM) failure. However, the pathogenesis of FA is not fully understood partly due to the limitations of current disease models. Here, we derive integration free-induced pluripotent stem cells (iPSCs) from an FA patient without genetic complementation and report in situ gene correction in FA-iPSCs as well as the generation of isogenic FANCA-deficient human embryonic stem cell (ESC) lines. FA cellular phenotypes are recapitulated in iPSCs/ESCs and their adult stem/progenitor cell derivatives. By using isogenic pathogenic mutation-free controls as well as cellular and genomic tools, our model serves to facilitate the discovery of novel disease features. We validate our model as a drug-screening platform by identifying several compounds that improve hematopoietic differentiation of FA-iPSCs. These compounds are also able to rescue the hematopoietic phenotype of FA patient BM cells.

Visualization of aging-associated chromatin alterations with an engineered TALE system

Visualization of specific genomic loci in live cells is a prerequisite for the investigation of dynamic changes in chromatin architecture during diverse biological processes, such as cellular aging. However, current precision genomic imaging methods are hampered by the lack of fluorescent probes with high specificity and signal-to-noise contrast. We find that conventional transcription activator-like effectors (TALEs) tend to form protein aggregates, thereby compromising their performance in imaging applications. Through screening, we found that fusing thioredoxin with TALEs prevented aggregate formation, unlocking the full power of TALE-based genomic imaging. Using thioredoxin-fused TALEs (TTALEs), we achieved high-quality imaging at various genomic loci and observed aging-associated (epi) genomic alterations at telomeres and centromeres in human and mouse premature aging models. Importantly, we identified attrition of ribosomal DNA repeats as a molecular marker for human aging. Our study establishes a simple and robust imaging method for precisely monitoring chromatin dynamics in vitro and in vivo.

Modeling xeroderma pigmentosum associated neurological pathologies with patients-derived iPSCs

ABSTRACTXeroderma pigmentosum (XP) is a group of genetic disorders caused by mutations of XP-associated genes, resulting in impairment of DNA repair. XP patients frequently exhibit neurological degeneration, but the underlying mechanism is unknown, in part due to lack of proper disease models. Here, we generated patient-specific induced pluripotent stem cells (iPSCs) harboring mutations in five different XP genes including XPA, XPB, XPC, XPG, and XPV. These iPSCs were further differentiated to neural cells, and their susceptibility to DNA damage stress was investigated. Mutation of XPA in either neural stem cells (NSCs) or neurons resulted in severe DNA damage repair defects, and these neural cells with mutant XPA were hyper-sensitive to DNA damage-induced apoptosis. Thus, XP-mutant neural cells represent valuable tools to clarify the molecular mechanisms of neurological abnormalities in the XP patients.

A Novel Suppressive Effect of Alcohol Dehydrogenase 5 in Neuronal Differentiation

Background: The role of ADH5 in neuronal development and differentiation remains unknown. Results: ADH5 denitrosated HDAC2 and thus negatively regulates neurite growth of hippocampal neurons and neuronal differentiation of hNSCs. Conclusion: ADH5 is a novel suppressor of neuronal differentiation. Significance: These results advance our understanding of the role of ADH5 in neuronal differentiation. Alcohol dehydrogenase 5 (ADH5) is a conserved enzyme for alcohol and aldehyde metabolism in mammals. Despite dynamic expression throughout neurogenesis, its role in neuronal development remains unknown. Here we present the first evidence that ADH5 is a negative regulator of neuronal differentiation. Gene expression analyses identify a constant reduction of ADH5 levels throughout neuronal development. Overexpression of ADH5 reduces both development and adult neuronal differentiation of mouse neurons. This effect depends on the catalytic activity of ADH5 and involves ADH5-mediated denitrosation of histone deacetylase 2 (HDAC2). Our results indicate that ADH5 counteracts neuronal differentiation of human neural stem cells and that this effect can be reversed by pharmacological inhibition of ADH5. Based on these observations, we propose that ADH5 is a novel suppressor of neuronal differentiation and maturation. Inhibition of ADH5 may improve adult neurogenesis in a physiological or pathological setting.

Genetic enhancement in cultured human adult stem cells conferred by a single nucleotide recoding

Genetic enhancement in cultured human adult stem cells conferred by a single nucleotide recoding

Regulation of Stem Cell Aging by Metabolism and Epigenetics

Stem cell aging and exhaustion are considered important drivers of organismal aging. Age-associated declines in stem cell function are characterized by metabolic and epigenetic changes. Understanding the mechanisms underlying these changes will likely reveal novel therapeutic targets for ameliorating age-associated phenotypes and for prolonging human healthspan. Recent studies have shown that metabolism plays an important role in regulating epigenetic modifications and that this regulation dramatically affects the aging process. This review focuses on current knowledge regarding the mechanisms of stem cell aging, and the links between cellular metabolism and epigenetic regulation. In addition, we discuss how these interactions sense and respond to environmental stress in order to maintain stem cell homeostasis, and how environmental stimuli regulate stem cell function. Additionally, we highlight recent advances in the development of therapeutic strategies to rejuvenate dysfunctional aged stem cells.

ATF6 safeguards organelle homeostasis and cellular aging in human mesenchymal stem cells

Loss of organelle homeostasis is a hallmark of aging. However, it remains elusive how this occurs at gene expression level. Here, we report that human mesenchymal stem cell (hMSC) aging is associated with dysfunction of double-membrane organelles and downregulation of transcription factor ATF6. CRISPR/Cas9-mediated inactivation of ATF6 in hMSCs, not in human embryonic stem cells and human adipocytes, results in premature cellular aging, characteristic of loss of endomembrane homeostasis. Transcriptomic analyses uncover cell type-specific constitutive and stress-induced ATF6-regulated genes implicated in various layers of organelles’ homeostasis regulation. FOS was characterized as a constitutive ATF6 responsive gene, downregulation of which contributes to hMSC aging. Our study unravels the first ATF6-regulated gene expression network related to homeostatic regulation of membrane organelles, and provides novel mechanistic insights into aging-associated attrition of human stem cells.