Loss of MECP2 Leads to Activation of P53 and Neuronal Senescence

To determine the role for mutations of MECP2 in Rett syndrome, we generated isogenic lines of human induced pluripotent stem cells, neural progenitor cells, and neurons from patient fibroblasts with and without MECP2 expression in an attempt to recapitulate disease phenotypes in vitro. Molecular profiling uncovered neuronal-specific gene expression changes, including induction of a senescence-associated secretory phenotype (SASP) program. Patient-derived neurons made without MECP2 showed signs of stress, including induction of P53, and senescence. The induction of P53 appeared to affect dendritic branching in Rett neurons, as P53 inhibition restored dendritic complexity. The induction of P53 targets was also detectable in analyses of human Rett patient brain, suggesting that this disease-in-a-dish model can provide relevant insights into the human disorder.

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Development of a patient-specific model of human Rett syndrome

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Loss of function of MECP2 leads to induction of p53

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MECP2 null neurons show evidence of cellular senescence

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Inhibition of p53 can restore dendritic branching in MECP2 null neurons

Identifying gene expression modules that define human cell fates

Using a compendium of cell-state-specific gene expression data, we identified genes that uniquely define cell states, including those thought to represent various developmental stages. Our analysis sheds light on human cell fate through the identification of core genes that are altered over several developmental milestones, and across regional specification. Here we present cell-type specific gene expression data for 17 distinct cell states and demonstrate that these modules of genes can in fact define cell fate. Lastly, we introduce a web-based database to disseminate the results.

ADAR1 is involved in the regulation of reprogramming human fibroblasts to induced pluripotent stem cells

Adenosine-to-inosine (A-to-I) RNA editing is a post-transcriptional, site-specific modification process that is catalyzed by Adenosine Deaminase Acting on RNA (ADAR) gene family members. Since ADARs act on double-stranded RNA, most A-to-I editing occurs within repetitive elements, particularly Alu elements, as the result of the inherent property of these sequences to fold and form double strands. ADAR1-mediated A-to-I RNA editing was recently implicated in the regulation of human embryonic stem cells (hESCs). Spontaneous and neuronal differentiation of hESC was shown to result in a decrease in A-to-I editing levels. Knockdown of ADAR1 in hESCs results in an elevation of the expression of differentiation-related genes. In addition, we found that hESCs over-expressing ADAR1 could not be generated. The current study shows that the editing levels of induced pluripotent stem cells (iPSCs) change throughout reprogramming, from a source cell level to a level similar to that of hESCs. Up- or down-regulation of the ADAR1 level in human foreskin fibroblast (HFF) cells before induction of reprogramming results in varied reprogramming efficiencies. Furthermore, HFF-iPSC early clones derived from source cells in which the ADAR1 level was down-regulated lose their iPSC properties shortly after iPSC colony formation and instead exhibit characteristics of cancer cells. Taken together, our results imply a role for ADAR1 in the regulation of pluripotency induction as well as in the maintenance of early iPSC properties.

Altered A-to-I RNA editing in human embryogenesis

Post-transcriptional events play an important role in human development. The question arises as to whether Adenosine to Inosine RNA editing, catalyzed by the ADAR (Adenosine Deaminase acting on RNA) enzymes, differs in human embryogenesis and in adulthood. We tested the editing of various target genes in coding (FLNA, BLCAP, CYFIP2) and non-coding sequences at their Alu elements (BRCA1, CARD11, RBBP9, MDM4, FNACC), as well as the transcriptional levels of the ADAR1 enzymes. This analysis was performed on five fetal and adult human tissues: brain, heart, liver, kidney, and spleen, as well as on human embryonic stem cells (hESCs), which represent the blastocyst stage in early human development. Our results show substantially greater editing activity for most adult tissue samples relative to fetal ones, in six of the eight genes tested. To test the effect of reduced A-to-I RNA editing activity in early human development we used human embryonic stem cells (hESCs) as a model and tried to generate hESC clones that overexpress the ADAR1–p110 isoform. We were unable to achieve overexpression of ADAR1–p110 by either transfection or lentiviral infection, though we easily generated hESC clones that expressed the GFP transgene and overexpressed ADAR1-p110 in 293T cells and in primary human foreskin fibroblast (HFF) cells. Moreover, in contrast to the expected overexpression of ADAR1-p110 protein following its introduction into hESCs, the expression levels of this protein decreased dramatically 24–48 hr post infection. Similar results were obtained when we tried to overexpress ADAR1-p110 in pluripotent embryonal carcinoma cells. This suggests that ADAR1 protein is substantially regulated in undifferentiated pluripotent hESCs. Overall, our data suggest that A-to-I RNA editing plays a critical role during early human development.

Multipotent vasculogenic pericytes from human pluripotent stem cells promote recovery of murine ischemic limb

BACKGROUND

Pericytes represent a unique subtype of microvessel-residing perivascular cells with diverse angiogenic functions and multilineage developmental features of mesenchymal stem cells. Although various protocols for derivation of endothelial and/or smooth muscle cells from human pluripotent stem cells (hPSC, either embryonic or induced) have been described, the emergence of pericytes in the course of hPSC maturation has not yet been elucidated.

METHODS AND RESULTS

We found that during hPSC development, spontaneously differentiating embryoid bodies give rise to CD105(+)CD90(+)CD73(+)CD31(-) multipotent clonogenic mesodermal precursors, which can be isolated and efficiently expanded. Isolated and propagated cells expressed characteristic pericytic markers, including CD146, NG2, and platelet-derived growth factor receptor β, but not the smooth muscle cell marker α-smooth muscle actin. Coimplantation of hPSC-derived endothelial cells with pericytes resulted in functional and rapid anastomosis to the murine vasculature. Administration of pericytes into immunodeficient mice with limb ischemia promoted significant vascular and muscle regeneration. At day 21 after transplantation, recruited hPSC pericytes were found incorporated into recovered muscle and vasculature.

CONCLUSIONS

Derivation of vasculogenic and multipotent pericytes from hPSC can be used for the development of vasculogenic models using multiple vasculogenic cell types for basic research and drug screening and can contribute to angiogenic regenerative medicine.

Molecular characterization and functional properties of cardiomyocytes derived from human inducible pluripotent stem cells

In view of the therapeutic potential of cardiomyocytes derived from induced pluripotent stem (iPS) cells (iPS-derived cardiomyocytes), in the present study we investigated in iPS-derived cardiomyocytes, the functional properties related to [Ca(2+) ](i) handling and contraction, the contribution of the sarcoplasmic reticulum (SR) Ca(2+) release to contraction and the b-adrenergic inotropic responsiveness. The two iPS clones investigated here were generated through infection of human foreskin fibroblasts (HFF) with retroviruses containing the four human genes: OCT4, Sox2, Klf4 and C-Myc. Our major findings showed that iPS-derived cardiomyocytes: (i) express cardiac specific RNA and proteins; (ii) exhibit negative force-frequency relations and mild (compared to adult) post-rest potentiation; (iii) respond to ryanodine and caffeine, albeit less than adult cardiomyocytes, and express the SR-Ca(2+) handling proteins ryanodine receptor and calsequestrin. Hence, this study demonstrates that in our cardiomyocytes clones differentiated from HFF-derived iPS, the functional properties related to excitation-contraction coupling, resemble in part those of adult cardiomyocytes.

Enhanced reprogramming and cardiac differentiation of human keratinocytes derived from plucked hair follicles, using a single excisable lentivirus

Induced pluripotent stem cells (iPSCs) represent an ideal cell source for future cell therapy and regenerative medicine. However, most iPSC lines described to date have been isolated from skin fibroblasts or other cell types that require harvesting by surgical intervention. Because it is desirable to avoid such intervention, an alternative cell source that can be readily and noninvasively isolated from patients and efficiently reprogrammed, is required. Here we describe a detailed and reproducible method to derive iPSCs from plucked human hair follicle keratinocytes (HFKTs). HFKTs were isolated from single plucked hair, then expanded and reprogrammed by a single polycistronic excisable lentiviral vector. The reprogrammed HFKTs were found to be very sensitive to human embryonic stem cell (hESC) growth conditions, generating a built-in selection with easily obtainable and very stable iPSCs. All emerging colonies were true iPSCs, with characteristics typical of human embryonic stem cells, differentiated into derivatives of all three germ layers in vitro and in vivo. Spontenaeouly differentiating functional cardiomyocytes (CMs) were successfully derived and characterized from these HFKT-iPSCs. The contracting CMs exhibited well-coordinated intracellular Ca²+ transients and contractions that were readily responsive to β-adrenergic stimulation with isoproterenol. The introduction of Cre-recombinase to HFKT-iPSC clones was able to successfully excise the integrated vector and generate transgene-free HFKT-iPSC clone that could be better differentiated into contracting CMs, thereby revealing the desired cells for modeling human diseases. Thus, HFKTs are easily obtainable, and highly reprogrammed human cell source for all iPSC applications.

Fourtwistgenes in zebrafish, four expression patterns

Twist genes code for regulatory bHLH proteins essential for embryonic development and conserved across the metazoa. There are four genes that constitute the zebrafish twist family: twist1a, twist1b, twist2--orthologs of the mammalian twist1 and twist2 genes; and twist3--a gene from a new clade that does not exist in mammals. Presented here are their embryonic mRNA expression profiles. The study extends the known conservation of twist developmental patterns in tetrapods to the fish, e.g., expression in cephalic neural crest, sclerotome and lateral plate mesoderm. Some other expression domains are unique, like hypochord and dorsal aorta; some, like the notochord, may be ancestral patterns retained from protochordates; and the expression in invaginating/migrating cells may have been retained from the jellyfish. Perhaps this is one of the more ancient functions of twist--conserved from diploblasts to humans--to facilitate cell movement.