Bioengineered stem cells in neural development and neurodegeneration research

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ARTICLE IN PRESS Model

RR 452 1–10

Ageing Research Reviews xxx (2013) xxx– xxx

Contents lists available at SciVerse ScienceDirect

Ageing Research Reviews

j ourna l h om epage: www.elsev ier .com/ locate /ar r

eview

ioengineered stem cells in neural development andeurodegeneration research

hauna H. Yuan ∗, Mason Shanerniversity of California, San Diego, Department of Neurosciences, 9500 Gilman Dr. MC 0624, MTF Room 151, La Jolla, CA 92093-0624, United States

a r t i c l e i n f o

rticle history:eceived 21 November 2012eceived in revised form 5 April 2013ccepted 9 April 2013vailable online xxx

eywords:tem cells

a b s t r a c t

The recent discovery of a simple method for making induced pluripotent stem cells (iPSC) from humansomatic cells was a major scientific advancement that opened the way for many promising new devel-opments in the study of developmental and degenerative diseases. iPSC have already been used tomodel many different types of neurological diseases, including autism, schizophrenia, Alzheimer’s dis-ease and Parkinson’s disease. Because of their pluripotent property, iPSC offer the possibility of modelinghuman development in vitro. Their differentiation seems to follow the developmental timeline and obeysenvironmental cues. Clinically relevant phenotypes of neurodegenerative pathologies have also been

eurodegenerationeural developmentuman disease modelransplantation

observed using iPSC derived human neuronal cultures. Options for treatment are still some way off.Although some early research in mouse models has been encouraging, major obstacles remain for neuralstem cell (NSC) transplantation therapy. However, iPSC now offer the prospect of an unlimited amount ofhuman neurons or astrocytes for drug testing. The aim of this review is to summarize the recent progressin modeling neural development and neurological diseases using iPSC and to describe their applicationsfor aging research and personalized medicine.

© 2013 Elsevier B.V. All rights reserved.

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 002. There is more than one way to derive human neurons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

2.1. Generation of induced pluripotent stem cells (iPSC) from human somatic cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 002.2. Generation of induced neurons (iN) from human dermal fibroblasts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 002.3. Generation of induced neural stem cells (iNSC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

3. Application of pluripotent stem cells (PSC) in studying human neural development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 003.1. Developmental milestones and patterns are recapitulated in the PSC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

3.1.1. PSC form three-dimensional structures mimicking the laminar structure of the brain and follow the developmental time-line 003.1.2. Transplantation of NSC shows guidance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

3.2. Modeling developmental neurological diseases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 003.2.1. Using iPSC to model a monogenetic neural developmental disorder: Rett syndrome . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 003.2.2. Reprogramming of FMR1 fibroblasts reveals that some disease epigenetic modifications are resistant to reprogramming . . . . 00

4. Application of iPSC and transdifferentiated neurons in studying human neurodegenerative diseases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 004.1. Modeling neurodegenerative diseases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

4.1.1. Modeling Alzheimer’s disease with iPSC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 004.1.2. Modeling Huntington’s disease with iPSC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 004.1.3. Modeling ALS with iPSC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 004.1.4. Modeling Machado-Joseph disease with iPSC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

4.2. Modeling aging with iPSC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

Please cite this article in press as: Yuan, S.H., Shaner, M., Bioengineered

Ageing Res. Rev. (2013), http://dx.doi.org/10.1016/j.arr.2013.04.002

4.3. Synapses, neuronal connections, axonal transport, can these be d4.4. Limitations of PSC for modeling neurodegenerative diseases . . . . .

4.4.1. Erasure of epigenetic marks during reprogramming . . . . .

∗ Corresponding author. Tel.: +1 858 822 0626; fax: +1 858 822 2050.E-mail addresses: [email protected] (S.H. Yuan), [email protected] (M. Shaner).

568-1637/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.arr.2013.04.002

stem cells in neural development and neurodegeneration research.

emonstrated? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

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ARTICLE IN PRESSG Model

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4.4.2. Aging as a factor for disease modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 005. Application of PSC in therapeutic development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

5.1. Transplantation as a therapy for neurodegenerative diseases. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 005.1.1. Transplantation therapy in Alzheimer’s disease . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 005.1.2. Transplantation therapy in Parkinson’s disease. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 005.1.3. Transplantation therapy in amyotrophic lateral sclerosis (ALS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 005.1.4. Source of cell types for transplantation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 005.1.5. Transplantation of stem cells in clinical trials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

5.2. Drug screening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 006. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

6.1. Implications of iPSC in development and aging research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 006.2. Implications of iPSC in personalized disease modeling and medicine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

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. Introduction

The recent discovery of a simple method for making inducedluripotent stem cells (iPSC) from somatic cells was a major scien-ific advancement that opened the way for many promising newevelopments in the study of human developmental and degen-rative diseases. Sir John Gordon and Shinya Yamanaka have justeen awarded the Nobel Prize in 2012 for Medicine for theirontribution to the reprogramming technology, highlighting theignificance of this scientific breakthrough. Since the publicationf Dr. Yamanaka’s work on generation of induced pluripotent stemells (iPSC), significant progress has already been made as humanPSCs have been used to model many different types of neurolog-cal diseases, including autism, schizophrenia, Alzheimer’s diseasend Parkinson’s disease. The aim of this review is to summarize theecent progress in neurological disease modeling using iPSC and toescribe their applications and implications in aging research. Weill first describe the current methodology for generating neurons,

hen we will discuss the different in vitro systems attempting toodel neurodevelopmental and neurodegenerative diseases. Weill also cover how these models could be applied in replacement

herapy and medicinal drug development.

. There is more than one way to derive human neurons

Currently, there are three different methods to derive neu-ons from somatic cells. Somatic cells such as skin fibroblasts cane reprogrammed to become iPSC and then differentiated intoeurons and glia. They can also be transdifferentiated to become

nduced neural stem cells (iNSC), or directly transdifferentiatedo become induced neurons (iN). It is useful to understand theotentials and limitations for each of these three methods, for their

nherent properties and downstream applications are quite differ-nt. In this section, we will discuss these three different methods.

.1. Generation of induced pluripotent stem cells (iPSC) fromuman somatic cells

The generation of iPSC was first developed by Yamanaka’s group.hey performed a screen of 24 candidate genes, based on theypothesis that these genes controlled embryonic stem cell iden-ity. They narrowed the candidates down to four factors, Oct3/4,ox2, Klf4 and c-Myc. When these factors were introduced to mousembryonic fibroblasts (MEF) with virus, the MEFs were repro-rammed to iPSCs (Takahashi and Yamanaka, 2006). The cells were



orphologically similar to the mouse embryonic stem cells andxhibited pluripotent stem cell markers, SSEA-3, SSEA-4, Tra-1-60nd Tra-1-81. The pluripotent potential of the iPSC was tested byeratoma assays, which showed that the iPSC can differentiate to

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

the three different germ layers, namely ectoderm, mesoderm andendoderm. Following their initial discovery in the mouse, the groupsubsequently showed that the same four factors could turn humanadult dermal fibroblasts to iPSC (Takahashi et al., 2007). Similarly,the human iPSCs have a morphology resembling the human embry-onic stem cells, such that they grow in condensed clusters, withscanty cytoplasms, and they express pluripotent stem cell markers.Furthermore, they can be differentiated into cell types with threegerm layers, for example, beta-III-tubulin positive neuron-like cells.

The reprogramming of human fibroblasts was reproduced by Yuet al. (Yu et al., 2007), using slightly different factors, Oct4, Sox2,Nanog and Lin28. It has also been shown that iPSC could be derivedfrom other types of somatic cells, for example, pancreatic beta-cells(Stadtfeld et al., 2008), liver (Aasen et al., 2008), and keratinocytes(Aoi et al., 2008). However in some terminally differentiated cells,such as B-cell lymphocytes (Hanna et al., 2008) and neurons (Kimet al., 2011b), suppression of p53 is necessary to improve the repro-gramming efficiency, suggesting that although cell type may not berestrictive for reprogramming, the efficiencies may be significantlydifferent and some cell types may require additional factors.

2.2. Generation of induced neurons (iN) from human dermalfibroblasts

In addition to reprogramming somatic cells into iPSC, methodsof transdifferentiation have been developed to directly reprogramone somatic cell type into another. Transdifferentiation is a pro-cess by which one cell type is transformed to take on the identityof another cell type. In contrast to reprogramming, where one celltype is first reprogrammed to the pluripotent state and then dif-ferentiated to the cell type of interest, transdifferentiation directlytransforms one cell type into another (Slack, 2007). Some exam-ples include retinal epithelial cells converted into muscle-like cells(Choi J Fau - Costa et al., 2013), and mature B lymphocytes con-verted into macrophages (Xie et al., 2004). Douglas Melton’s grouphas used a similar approach to that applied in the discovery of thefour factors in iPSC to identify the combination of transcriptionfactors important to the development of islet cells, which whenadded to pancreatic exocrine cells, can transdifferentiate them intopancreatic islet cells in vivo (Zhou et al., 2008).

Several groups have shown that human fibroblasts can be trans-differentiated into neurons (Ambasudhan et al., 2011; Pang et al.,2011; Qiang et al., 2011). Vierbuchen et al., using a similar approachto that used by Yamanaka and Melton’s groups, screened 19 geneswhich were expressed in neurons specifically and involved in neu-


ral development or epigenetic modifications. They found that theBrn2 (also known as Pou3f2), Ascl1 and Myt1l transcription factors,which are key regulators of neural lineage, could convert mouseembryonic stem cells and postnatal mouse fibroblasts into neurons

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Vierbuchen et al., 2010). With the addition of NeuroD1, a basicelix–loop–helix transcription factor, human dermal fibroblastsould also be transdifferentiated into neurons. The process tookbout 3 weeks. The neurons exhibited electrophysiologic proper-ies, demonstrating that they could fire an action potential whentimulated with an electric current. When the iNs were co-culturedith mouse cortical cultures, they developed spontaneous activi-

ies and were sensitive to the GABAA receptor inhibitor Picrotoxinnd to AMPA (a-amino-3-hydroxy-5-methyl-4-isoxazole propioniccid), and the receptor blocker CNQX (Pang et al., 2011), demon-trating that these methods are sufficient to generate differentubtypes of neurons.

.3. Generation of induced neural stem cells (iNSC)

In addition to turning fibroblasts into neurons directly by trans-ifferentiation, other researchers have demonstrated that it isossible to turn fibroblasts directly into neural stem cells (NSC).everal groups have reported transdifferentiation of mouse fibrob-asts into NSCs with various combinations of transcription factorsHan et al., 2012; Kim et al., 2011a; Lujan et al., 2012; Ringt al., 2012; Thier et al., 2012). It seems that Sox2 is essential inhese cocktails. Ring et al. showed that by introducing Sox2 alone,broblasts could turn into NSCs. The NSCs showed a self-renewalroperty, and could differentiate into neurons, astrocytes and oligo-endrocytes. The neurons had proper electrical activities and couldorm synapses (Ring et al., 2012). Kim et al. used transient transfec-ion of the four reprogramming factors, Oct4, Sox2, Klf4 and c-MycKim et al., 2011a). They demonstrated that the iNSC derived fromxogenous expression of Sox2, Klf4 and c-Myc, (which are nor-ally expressed in NSC), were enough for transdifferentiation, and

ustained expression was not necessary. Their iNSC could be main-ained in culture up to 50 passages (Thier et al., 2012). Ring et al.howed that iNSC could also be made from human fetal fibroblasts.

hen the iNSCs were transplanted into the mouse brain, they didot form tumors. It is not known whether these methods couldork with human adult fibroblasts. It is also not clear that the iNSC

re patternable and have the potential to generate all the differentinds of neurons in the brain.

. Application of pluripotent stem cells (PSC) in studyinguman neural development

Our understanding of development has been significantly aidedy studies performed in lower organisms, for human development

s difficult to study in vivo due to the inaccessibility of the humanissues. Because of their pluripotent property, the human PSC offerhe possibility of modeling human development, assuming thathey retain their developmental program in vitro. Recently, sev-ral lines of research suggest that PSC do partially recapitulatehe developmental program and can self-organize to form three-imensional structures mimicking the shape of an organ. Theirifferentiation seems to follow the developmental timeline andbey environmental cues. This suggests that they may be usefulor disease modeling, organogenesis, and replacement therapy inransplantation applications.

.1. Developmental milestones and patterns are recapitulated inhe PSC

We have a very limited ability to study neural developmentirectly in humans. The study of human subjects is limited by



bvious physical and ethical constraints. With the use of geneticngineering in other species, from c-elegans to rodents, we haveained a general understanding of how the nervous system devel-ps. Now, with the application of the PSC method we may finally

PRESSh Reviews xxx (2013) xxx– xxx 3

be able to study the development of the human nervous systemin a human PSC model. The key question is, do the cells derivedfrom PSC mimick the basic principles of human neural develop-ment? There is some encouraging evidence showing that the PSCdo recapitulate the time-line of neural development and that theyalso follow the endogenous guidance cue to arrive at their properdestination (Eiraku et al., 2008; Gaspard et al., 2008).

3.1.1. PSC form three-dimensional structures mimicking thelaminar structure of the brain and follow the developmentaltime-line

Sasai’s group demonstrated that neurospheres formed by PSCwhen left growing in culture will over time lateralize into a threedimensional structure, similar to the six layers in the cortex.They noticed that N-cadherin+ neural progenitors first accumu-late in the Sox1+ outer regions, then these progenitors graduallyexpanding inward. When the neurospheres were sectioned, thegroup observed by immunofluorescent staining that Reelin+ andTbr1+/Reelin−/Bf1+ neurons appeared on days 7 and 8, whileCtip2+/Emx1+ neurons were seen only on and after day 10.Brn2+/Bf1+ cells were substantially increased during days 10–13.These Brn2+ neurons also expressed other upper CP markers suchas Cux1 and Satb2 (Eiraku et al., 2008). The group extendedtheir study, by conducting a birth-date analysis, pulsing BrdUduring days 8–14 and analyzing the BrdU positive cells on day16. They found the birth of Reelin+ (layer I), Tbr1+/Bf1+ (layerVI), Ctip2+/Emx1+ (layer V), and Brn2+/Tuj1+ (layer II/III) neuronspeaked on days 8–10, 9–10, 10–11, and 12–13, respectively. Theseresults demonstrated that the layer-specific neurons from mouseembryonic stem cell (mESC)-derived cortical progenitors were gen-erated in the same temporal order previously observed in theembryonic mouse cortex.

More recent work from Sasai’s group suggests that human ESChave the ability to self-organize and form three-dimensional struc-tures resembling the optic cup (Nakano et al., 2012). They observedthat the human embryonic stem cell (hESC)-derived optic cup islarger than the mouse ESC-derived optic cup, likely due to speciesdifferences. The hESC-derived neural retina can form multilayeredtissue, including both rods and cones. This work further confirmsthat the PSC can self-organize to form structures resembling thenervous system. This is a promising finding suggesting that it maybe possible to use PSC for organogenesis in transplantation appli-cations and disease modeling.

3.1.2. Transplantation of NSC shows guidanceAnother line of work from Gaspard et al. using mouse embry-

onic stem cells (mESC), has demonstrated that the NSC derivedfrom the mESC pass through the normal stages of developmentalprogression and can generate neurons that follow the developmen-tal timeline(Gaspard et al., 2008). They differentiated the mESCwith a sonic hedgehog inhibitor, cyclopramine, to drive the pro-genitors to the dorsal cortical fate. They then evaluated thesepyramidal neurons to see if they express layer specific mark-ers. Although they were able to observe lower layer specificmarkers over time, they were not able to detect markers asso-ciated with upper layers, suggesting that development of upperlayer neurons requires in vivo conditions or a three dimensionalcue.

Using a TAU (MAPT)-GFP knock-in ESC line, Gaspard et al. stud-ied the axonal projections of grafted ESC derived neurons. Afterdifferentiating the TAU-GFP mESC for 12–17 days, the differenti-ated cells were grafted into the frontal cortex of neonatal mice.


They noticed that the neurons had projections which followed cor-tical efferents, suggesting that the neurons generated had corticalidentity. Furthermore, they observed that the projections werearea specific. For example, transplanted cells in the cortex only

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rojected to the visual and limbic area, but not to the motor oromatosensory cortices. These are striking findings which hinthat the neural progenitors derived from the ESCs can differenti-te into a variety of neurons and still contain the cues for properrojection to the target area. This could have ramifications foreveloping potential replacement therapy under optimal circum-tances.

.2. Modeling developmental neurological diseases

PSC lines modeling pediatric neurological disorders have beeneveloped and demonstrated to be a useful addition to the cur-ent developmental disease model system. This topic has beeneviewed elsewhere (Kunkanjanawan et al., 2011, Chailangkarn,012 #1600). We will cover a few of the studies as examples ofuccesses, dilemmas and new opportunities.

.2.1. Using iPSC to model a monogenetic neural developmentalisorder: Rett syndrome

Rett syndrome is a pediatric neural developmental disease,ffecting almost exclusively females. It is mainly due to mutationsn the methyl-CpG-binding protein-2 (MECP2) gene, located on the

chromosome. MECP2 binds to methylated DNA and is thoughto regulate gene expression. Normal MECP2 protein is vital for life,s male infants with MECP2 mutation do not survive. The mouseodel has contributed to our current understanding of MECP2 in

ett syndrome; however, the mouse model does not fully recapit-late the pathological features.

Muotri’s group reprogrammed dermal fibroblasts from patientsith Rett Syndrome due to MECP2 mutation and found that theutant neurons recapitulated several phenotypes of the disease.

hey had fewer synapses and less spines compared to the control.he neurons also seemed to have less connectivity compared tohe control, as they had lower activity dependent calcium signal-ng and decreased frequency of spontaneous postsynaptic currentsMarchetto et al., 2010). This work demonstrates the possibility oftudying synaptic and network dysfunction in iPSC autism models.

.2.2. Reprogramming of FMR1 fibroblasts reveals that someisease epigenetic modifications are resistant to reprogramming

Fragile X is the leading cause of inheritable intellectual disabil-ty. It is due to trinucleotide CCG repeat in the 5′ untranslated regionf the Fragile X Mental Retardation (FMR1) gene, resulting in theoss of FMR1 expression. FMR1 is a RNA binding protein, whichrafficks RNAs from the nucleus to the synapses, and is required forormal neural development. The PSC model is an attractive wayo study Fragile X, since the regulation of the early events of FMR1ilencing could not be reproduced using other animal models. Eigest al. generated hESC with the trinucleotide expansion detectedy preimplantation genetic diagnosis. They found that FMR1 wasxpressed by the ESC, and the promoter was unmethylated. How-ver, the FMR1 promoter becomes methylated upon differentiationnd is associated with the decrement of FMR1 protein (Eiges et al.,007). Interestingly, two other groups reprogrammed somatic cellsith the trinucleotide expansion to iPSC. They found that the FMR1romoter was methylated and there was an absence of FMR1xpression (Urbach et al., 2010, Sheridan, 2011 #6). This suggestshat the methylation at the promoter of FMR1 is due to the trin-cleotide repeat and is resistant to the reprogramming process.heridan et al. went on to show that the FMR1 promoter is methy-ated and FMR1 expression was absent in the neurons and astroytes



erived from the mutant iPSC. The inconsistency between the hESCnd hiPSC lines derived from Fragile X carriers demonstrates somef the limitations and potentials of using hPSC in disease model-ng.

PRESSh Reviews xxx (2013) xxx– xxx

4. Application of iPSC and transdifferentiated neurons instudying human neurodegenerative diseases

Even though neurodegenerative pathologies may take years toaccumulate in the brain, clinically relevant phenotypes have beenobserved in a relatively short time within the in vitro iPSC derivedneuronal cultures. When iPSCs were differentiated the resultingcells expressed premature aging phenotypes, including acceler-ated senescence, shortening of telomeres and dysmorphic nuclei.Although prolonged culturing in vitro is problematic it is also possi-ble to age the human PSC derivatives by injecting them into animalbrains. The use of iPSC would not be recommended for the studyof diseases which are highly affected by environmental factors andmay not have a strong genetic contribution, since most of the epige-netic marks are erased during re-programming. However, humaniPSC have been used successfully to study many different types ofneurodegenerative diseases.

4.1. Modeling neurodegenerative diseases

4.1.1. Modeling Alzheimer’s disease with iPSCAlzheimer’s disease (AD) is an age-related neurodegenerative

disease. Patients develop gradual and insidious memory loss, whichprogresses to inability to perform daily activities. The pathologytypically starts in the entorhinal cortex and hippocampus, thenspreads to the cerebral cortex (Braak and Braak, 1991). However,recently a minor subtype was described which originates in thehippocampus (Murray et al., 2011). The pathological hallmarks arean accumulation of extracellular amyloid plaques and intracellularneurofibrillary tangles. The severity of the inclusions seems to cor-relate with disease severity, although the tangles correlate betterthan the plaques.

AD can be caused by mutations in Presenilin-1 (PS1), Presenilin-2 (PS2), or Amyloid precursor protein (APP). These mutations areinherited in an autosomal dominant fashion and thus are calledfamilial AD (FAD). PS1 forms the catalytic component of the �-secretase, which has multiple substrates, including APP. Aftercleavage by �-secretase (BACE), the C-terminus fragment of APPis cleaved by �-secretase to form the A� peptide and AICD (APPintracellular domain). In the PS1 FAD mutations, the A�42 to A�40ratio is elevated, in general either due to increased A�42 produc-tion or a decrease of A�40 production. The A�42/40 ratio has beenobserved in human fibroblasts and in mouse transgenic models.Both A�42 and A�40 are found in the amyloid plaques. However,it is thought that A�42 is more amyloidogenic, and its reductionmay serve as a cerebral spinal fluid (CSF) biomarker for confirmingan AD diagnosis, possibly due to the decreased removal of A�42from the brain parenchyma (Hulstaert et al., 1999). In addition tothe FAD mutations, patients with Down’s Syndrome also developmemory disorders and dementia in their 40s and 50s, presumablydue to duplication of APP, located on chromosome 21. In fact, peo-ple with duplication of the APP gene also develop AD, likely due tothe overproduction of APP (Rovelet-Lecrux et al., 2007).

iN cells have been generated from fibroblasts carrying thePresenilin-1 mutation (Qiang et al., 2011). These neurons exhibit anelevated A�42/40 ratio compared to the control. When treated with�-secretase inhibitor, DAPT, these neurons have reduced secretedA� and reduced A� containing endosomes.

Israel et al. showed that neurons derived from iPSC originat-ing from patients with APP duplication have elevated A�40 (Israelet al., 2012). They also showed that a patient with sporadic AD can


have elevated A�40 and enlarged endosomes. Interestingly, theyfound an elevated level of phospho Tau, which is thought to bethe precursor to neurofibrillary tangle formation, in lines with ele-vated A�. This phenomenon is associated with elevated activated

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SK� and could be alleviated by BACE inhibitor treatment, but not-secretase inhibitor treatment.

Shi et al. showed that neuronal cultures derived from Down’syndrome iPSC and ESC form extracellular plaques after threeonths in culture (Shi et al., 2012). They were thioflavin analog

TA1 positive. This evidence shows that the clinically relevant phe-otypes could be observed in the in vitro iPSC derived neuronalultures. Even though these pathologies take years to accumulaten the brain, they could be seen in a relatively short amount of timen culture using these systems.

.1.2. Modeling Huntington’s disease with iPSCHuntington’s disease (HD) is caused by neurodegeneration of

he basal ganglia, caused by an expanded CAG repeat in the Hunt-ngtin gene.

Because of its monogenetic nature, several groups haveodeled HD with iPSC (An et al., 2012; Park et al., 2008;

he†Hd†iPsc†Consortium, 2012). An et al. found that the neuraltem cells derived form the HD iPSC were more vulnerable to oxida-ive stress than control iPSC. These defects could be reversed whenhe HD iPSC CAG repeat was corrected by genetic engineering (Ant al., 2012).

The HD consortium found that lines with higher CAG repeats hadore severe pathological phenotypes than the lines with less CAG

epeats (The†Hd†iPsc†Consortium, 2012). They found that the NSCsith higher CAG repeat did not form functional neurons and grad-ally died, as compared to NSCs with lower CAG repeats. They alsoound morphological changes in the neural progenitor cells (NPC)ith HD lines, in that they had less binding in phalloidin, suggest-

ng that they had changes in the actin cytoskeleton. And they notedhat the NPCs from the HD lines had a decreased ATP/ADP ratio,uggesting compromise in the energy metabolism.

Because of the autosomal dominant nature of HD, there is exten-ive pre-implantation screening for it during in vitro fertilization, sohat the embryos in the blastocyst stage which carry the long CAGepeat might potentially become available for research purposes.hese embryos could be used for the generation of embryonic stemell lines for HD research. Although iPSC and hESC are similar, theESC could be used as a comparison to the iPSC lines.

.1.3. Modeling ALS with iPSCALS (Amyotrophic Lateral Sclerosis), also known as Lou Gehrig’s

isease, is a neurodegenerative disease with symptoms primarilyue to the demise of motor neurons. This disease strikes people inheir 40s, starting with weakness such as difficulty buttoning shirts,nd slurring of speech. As the disease progresses, the weaknesspreads to involve proximal muscles, resulting in difficulty ambu-ating, lifting weight, chewing and swallowing. Ultimately, patientsie of respiratory distress from the failure of the diaphragms. Theisease originates with the motor neurons in the spinal cord, andbout 10% of ALS is familial. About 1% of all ALS cases are caused by auperoxide dismutase 1 (SOD1) mutation. It is not clear how SOD1auses ALS. There is evidence suggesting that the sick astrocytesn the spinal cord may poison the motor neurons (Clement et al.,003). Patients’ symptoms can progress quickly; from the time ofiagnosis to death can be as short as two years.

When astrocytes derived from ALS iPSC, were co-cultured withhe neurons, they showed a toxic effect on motor neurons, but nothe interneurons (Di Giorgio et al., 2008, 2007). Furthermore, thisrocess may be due to activation of NOX2 to produce superoxide

n the astrocytes, and could be mitigated by treatment with NOX2



nhibitor, apocynin (Marchetto et al., 2008). These results suggest non-cell autonomous effect that is specific to the motor neuronsnd that finding treatments to protect the motor neuron loss in ALSould potentially ameliorate the disease (Di Giorgio et al., 2008).


Multiple other causes of familial type of ALS have been found,such as FUS/TLS, TDP43, and most recently c9ORF. Many of thesegenes involve RNA binding, and potentially the regulation of geneexpression. It will be very interesting to see if the neurons derivedfrom fibroblasts carrying these other genetic causes show similarphenotypes as the SOD1.

4.1.4. Modeling Machado-Joseph disease with iPSCMachado-Joseph disease or Spinocerebellar ataxia type 3 (SCA3)

is a rare, autosomal dominant neurodegenerative disease, whichcauses progressive cerebellar ataxia. It is caused by abnormalCAG expansion in the ATXN3 gene. Koch et al. generated iPSCfrom the skin cells of patients with Machado-Joseph disease(Koch et al., 2011). They found that neurons differentiated fromthe iPSC formed insoluble protein aggregates upon stimulation.The authors hypothesized that calcium dependent activation ofproteases causes the cleavage of ATXN3 leading to protein aggre-gation as a result. When they stimulated the neuronal culture,they observed elevation of ATXN3 cleavage and ATXN3 insolu-ble aggregates. This process was dependent on the strength ofthe stimulation, as the aggregation correlated with the concentra-tion of l-glutamine, and the depolarization induced calcium influxthrough voltage-gated calcium channels. Interestingly, the authorsdid not observe inclusion bodies or increased cell death. This combi-nation of observations suggests that the excitation induced proteinaggregation is an early event, which could lead to the developmentof inclusion bodies and cell death when the neurons could no longertolerate the burden of aggregated proteins.

4.2. Modeling aging with iPSC

We provided examples above for using iPSC to model age-related neurodegeneration, indicating that it is possible to observewhat was thought to be age-dependent phenotypes in only a fewweeks in vitro. Research has also shown that diseases caused byaccelerated aging can be modeled with iPSC. Hutchinson–Gilfordprogeria syndrome (HGPS) is a rare and fatal premature agingdisease in humans, which is characterized by premature arte-riosclerosis and degeneration of vascular smooth muscle cells(SMCs). HGPS is caused by a single point mutation in the laminA (LMNA) gene, resulting in the generation of progerin, which is atruncated splicing mutant of lamin A. The accumulation of progerinleads to various aging-associated nuclear defects including disor-ganization of nuclear lamina and loss of heterochromatin.

Liu et al. reprogrammed dermal fibroblasts from patients withHGPS. They found that progerin was expressed in the patient fibrob-lasts; however, it was not expressed in the embryonic stem cells,nor in the iPSC, consistent with the theory that progerin expres-sion is cell type specific and reprogramming resets its expressionlevel (Liu et al., 2011). When HGPS iPSCs were differentiated toSMC, progerin expression was upregulated and increased over timewith passaging. In addition, the HGPS SMC exhibited prematureaging phenotypes, including accelerated senescence, shorteningof telomeres and dysmorphic nuclei. The group performed a pro-teomic analysis using MudPit and found that progerin interactedwith DNA-dependent protein kinase catalytic subunit (DNAPKcs).Interestingly, they also observed that there was down regulation ofDNAPKcs in HGPS cells, which is dependent on the accumulationof progerin in differentiated cells. DNAPKcs may be important inthe disease pathogenesis, for knockdown of DNAPKcs also recapit-ulated premature aging. Liu et al. demonstrated that aging could


potentially be accelerated by causing mutations in genes whichcontrol aging, suggesting that such manipulations of the aging pro-cess could potentially accelerate the manifestation of what wasonce thought to be age-related phenotypes. If accelerated aging

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s feasible, such process would make the iPSC based models moreowerful for modeling age-dependent diseases.

.3. Synapses, neuronal connections, axonal transport, can thesee demonstrated?

As mentioned before, neurons derived from iPSC or directransdifferentiation from fibroblasts (iN cells) harbor many of theeuronal properties. These differentiated neurons contain sodiumnd potassium channels, can fire action potentials when stim-lated, and exhibit properties of spontaneous activities (Israelt al., 2012; Marchetto et al., 2010; Qiang et al., 2011). Fromhe transplantation work by Gaspard et al., it is even possibleo study differentiation into different subtypes of neurons andxon guidance (Gaspard et al., 2008). The differentiation of neu-ons does seem to follow developmental timelines. Therefore, its not surprising that the neurons derived from three-week cul-ure closely resemble the expression profile of human embryonicrains (Israel et al., 2012). Human neurons are distinct from otherypical model systems such as rodents, in that they can be over

meter long. Therefore, using hPSC to generate human neuronso study axonal transport could be particularly fruitful, since the

echanisms of regulation in humans could potentially be differ-nt.

.4. Limitations of PSC for modeling neurodegenerative diseases

.4.1. Erasure of epigenetic marks during reprogrammingFor neurodegenerative diseases, epigenetic modifications aris-

ng from environmental exposure to toxins, lifestyle factors, ando-morbidities could be significant in disease modification andanifestation. However, several studies suggest that during re-

rogramming most of the epigenetic marks are erased (Elden et al.,010; Stadtfeld et al., 2010). This prevents modeling of any epige-etic changes related to environmental factors that occur duringn individual’s lifetime. Therefore, the use of iPSC would not beecommended for the study of diseases which are highly affectedy environmental factors and may not have a strong genetic con-ribution (Grskovic et al., 2011). The status of the preservation ofpigenetic marks is still unknown in the iN and iNSC system. Ifhe epigenetic marks are preserved during the transdifferentiationuring the making of iN and iNSC, then these systems would beuperior than the iPSC for studying the epigenetic contribution toiseases. This issue has yet to be resolved.

.4.2. Aging as a factor for disease modelingThe majority of neurodegenerative diseases are age-dependent.

ge makes a contribution to disease development and manifesta-ion for a variety of reasons. Some of these involve environmentalactors, possibly causing the epigenetic changes just discussed, thatccumulate over a lifetime. Because so many of these factors mightontribute to disease development, it is very difficult, if not impos-ible, to model aging itself using PSC. Prolonged culturing of PSCn vitro is prone to contamination. Current culture conditions areoorly controlled for long-term culture and could be compromisedy excessive cell death. It may be possible, however, to age theuman PSC derivatives by injecting them into animal brains. Pro-ocols for injecting NSC into rodent brains have been reported by

ultiple groups (Kriks et al., 2011; Muotri et al., 2005; Tabar et al.,005); therefore, this seems to be highly feasible. It is difficult,hough, to track these cells in vivo, and new methods need to be



eveloped to observe them. It is also difficult to interpret the results,or there may be different read outs due to the varied genetic back-rounds of the recipient animals. This is another area that needsurther development.


5. Application of PSC in therapeutic development

As PSCs hold the promise to become any cell type of thebody, this potentially could be applied in development of ther-apy. It has been proposed that PSC derived transplantable cellswould be considered autologous (self to self); therefore, long-termimmuno-supression may not be necessary. Furthermore, thesepatient-specific cells could be used for drug-development to screenfor drugs that are most suitable to the patient in terms of their effi-cacy and toxicity profile. In this section we will discuss the evidencefor using PSC derivatives in transplantation and drug screening.

5.1. Transplantation as a therapy for neurodegenerative diseases

Although some early research in mouse models has beenencouraging, major obstacles remain for NSC transplantation ther-apy. Neurodegenerative diseases by their nature tend to spreadover time, making it difficult to target specific regions for therapyand suggesting the importance of early intervention.

5.1.1. Transplantation therapy in Alzheimer’s diseaseAlzheimer’s disease pathology generally originates in the

entorhinal cortex and the hippocampus, but then spreads to otherregions of the brain as the disease progresses (Braak and Braak,1991). Therefore, transplantation as a therapy for Alzheimer’s dis-ease does not seem likely to be an effective solution once the diseasehas begun to spread. However, in recent work by Blurton-Joneset al., they showed that NSCs slowed the disease progression in anAD mouse model.

Blurton-Jones et al. transplanted GFP labeled NSCs into the hip-pocampal region bilaterally in triple transgenic mice (3xTg-AD)that express pathogenic forms of the amyloid precursor proteinand presenilin (Blurton-Jones et al., 2009). The group observedthat deficits in cognitive function in the aged 3xTg-AD mice, mea-sured by Morris Water Maze and context-dependent novel objectrecognition, were improved with transplantation without alteringAß or tau pathology. The authors found that there was a signifi-cant increase in hippocampal synaptic density, mediated by brainderived neurotrophic factor (BDNF). The authors found that BDNFwas secreted by the NSCs, because when BDNF was knocked downby shRNA in the NSCs, and the behavioral improvement in the 3xTg-AD mice could no longer be observed. These results are intriguing,because they suggest that even in a widespread pathology, a neu-rodegenerative disease might possibly be modulated by stem celltransplantation therapy. This might offer reasons to hope that stemcell therapy may provide a regenerative approach to combat AD.Further studies are warranted to fully evaluate the efficacy, toxicityand tumor formation potential of such a therapy.

5.1.2. Transplantation therapy in Parkinson’s diseaseThe hallmark of Parkinson’s disease pathology is loss of

dopaminergic neurons in the substansia nigra, causing extrapyra-midal movement disorders, such as tremors, bradykinesia andrigidity. Because the cell loss seems to be more restricted toone area, there has been a great interest in transplanting neu-rons derived from iPSC using various Parkinson’s disease mousemodels. Multiple clinical trials transplanting fetal ventral mesen-cephalic (fVM) tissue into the striatum demonstrated that suchtransplantation is feasible. Patients in general benefited fromthe transplantation and required less medication; however somedeveloped significant dyskinesia as a side effect (Lindvall andBjörklund, 2004). Recently, in a follow up evaluation using fMRI,


three patients were shown to have a normal level of dopamine inthe striatum, but a low serotonergic level in the non-cholinergicneurons, highlighting the fact that the neuronal loss in PD is not lim-ited to dopaminergic neurons alone (Politis et al., 2012). Although

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Fig. 1. Disease-in-a-dish model. Human dermal fibroblasts can be generated fromskin biopsies. These fibroblasts subsequently can be converted to specific cells ofinterest, such as neurons and glia, that may be particularly vulnerable in neurode-velopmental or neurodegenerative diseases through either reprogramming to iPSCor direct conversion to NSC or neurons. These methods allow researchers to studynot only disease mechanisms, but also use for development of therapies. The infor-

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he initial symptoms of Parkinson’s disease seem to be limited tootor impairments, neurodegeneration also involves other areas as

atients age. Studies from Braak and Braak show that Parkinson’sisease may start outside of the midbrain (Braak and Del Tredici,008; Del Tredici et al., 2002). However, stem cell therapy couldtill be warranted if significant improvement of motor symptomsould be demonstrated.

.1.3. Transplantation therapy in amyotrophic lateral sclerosisALS)

Multiple studies demonstrated that in ALS, astrocytes couldctively damage the motor neurons, causing neuronal demiseClement et al., 2003; Di Giorgio et al., 2008, 2007; Marchettot al., 2008). Leopore et al. showed that transplantation of healthylial restricted progenitors (GRP) derived from human fetal tissuento the cervical cord of SOD1G93A rats led to significant improve-

ent in their survival, with reduced motor neuron loss and declinen fore-limb motor and respiratory physiological function (Leporet al., 2011, 2008). The transplanted GRP initially expressed mainlyestin, and later in vivo it became glial fibrillary acidic protein

GFAP) positive, which is a glial marker. These results suggest thathe transplantation of NSC or astrocytes derived from NSC coulde a viable therapy for treating devastating neurodegenerative dis-ases such as ALS.

.1.4. Source of cell types for transplantationThe most ideal cell type for transplantation is probably highly

ependent on the disease etiology and the therapeutic goals of eachf the study designs. However, it is important to ensure that theells are highly pure and of GMP (good manufacturing practice)uality. We have developed a method, which enriches neurons,eural stem cells and glial progenitors, based on cell surface signa-ures (Yuan et al., 2011). It is also important to know the likelihoodf survival and integration of the different cell types. We investi-ated this problem, by comparing the efficiency of grafting sortedSC, differentiated neuronal culture and sorted neurons into rat

pinal cord. We found that the sorted neurons had the lowest andhe NSC had the highest rate of survival (unpublished data).

.1.5. Transplantation of stem cells in clinical trialsNSC derived from human spinal cord has already been

n a clinical safety study in ALS patients. Neuralstem com-leted its Phase I NSC transplantation in ALS patients,nd is now proceeding with recruitment of Phase II trialhttp://www.clinicaltrials.gov/ct2/show/NCT01348451). Aighly publicized first FDA approved clinical trial using oligo-endrocyte progenitors derived from hESC for spinal cord

njury sponsored by Geron was halted after one year of thetudy. The study was stopped not due to adverse effects,ut strictly as a business decision. The four patients whoeceived transplantation are still being followed. There wereo reports of any changes in their symptoms, nor of any adverseffects (http://clinicaltrials.gov/ct2/show/study/NCT01217008).dvanced cell technology is currently evaluating hESC derivedetinal pigmented epithelium (RPE) in an FDA approved Phase I/IIrial for the treatment of age-related macular degeneration (AMD)http://clinicaltrials.gov/ct2/show/study/NCT01217008).

.2. Drug screening

Until now, researchers did not have the resources to acquire annlimited amount of human neurons or astrocytes for drug testing.



owever, iPSC models have demonstrated that such drug testing isow possible.

Currently the cell types used for drug screening, whether ofhemicals or small organic molecules, have included animal cells

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mation gathered from these models can be further confirmed or applied toward thediagnosis or design of therapies for patients.

or immortalized cell lines overexpressed with mutant proteins. Forexample, in Alzheimer’s disease, the typical cell lines used are CHO(Chinese Hamster Ovary) or neuroblastoma SH5YSY lines overex-pressed with Presenilin-1 or APP mutations. These cells are easy togrow and they are scalable for screening a large number of com-pounds. However, these cell lines may not share the same behaviorsas neurons or astrocytes, which are the primary cell types at riskin neurodegenerative diseases. Furthermore, the overexpression ofthese mutations may not truly reflect the cellular responses to com-pounds the body would have at a physiological level. The use of iPSCoffers an alternative model that can more accurately recapitulatethe in vivo conditions of the disease, improving the efficacy of thedrug screen, making it more likely that candidate drugs which haveperformed well in vitro will also function in vivo.

Several reviews have been written regarding the potential appli-


cations of bioengineered stem cells for development of therapy(Ebert et al., 2012, Almad, 2012 #1608, Grskovic et al., 2011 #1173).Recent work by Lee et al. utilized neural crest precursors derived

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ARTICLE IN PRESSG Model

ARR 452 1–10

8 S.H. Yuan, M. Shaner / Ageing Research Reviews xxx (2013) xxx– xxx

Fig. 2. Overview of the current methods of bioengineered stem cells. The ESCs are derived from blastocysts, whereas the iPSC, iN and iNSC are generated from somatic cells.Both the ESC and iPSC have the potential to generate different cell types, whereas iNSC and induced neurons derived through direct conversion are fate restricted. Thesed ese cf ve them cells

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ifferences in the methodology and the cells of origin have implications on how throm cells derived through the reprogramming or transdifferentiation methods ha

odel systems that limit the research in specific cells, whereas the pluripotent stem

rom iPSC generated from patients with familial dysautonomia (FD)o demonstrate the feasibility of large scale-screening of a chemicalibrary, including 6912 compounds (Lee et al., 2012). The authorsdentified eight compounds that rescued the expression of inhibitorf kappa light polypeptide gene enhancer in B-cells (IKBKAP), whichs deficient in FD, and the remediation of the cellular phenotype.he authors found one of the eight compounds induced IKBKAPranscription through modulation of intracellular cAMP levels.

Peng et al. evaluated 44 known compounds, examining theesponse to rotenone-induced toxicity to dopaminergic neuronserived from iPSC (Peng et al., 2013). Of the 44 compounds, 16ere found to be protective. This offers the potential that such a

creen could be expanded to a large, high-throughput screen, inost-mitotic neurons.

. Discussion

.1. Implications of iPSC in development and aging research



It is surprising that many of the phenotypes associated witheurodegenerative diseases could be observed in relatively youngeurons, after a few months in culture, though it often takes manyecades for them to appear in humans. Our understanding of

ells could be used. For example, the disease mechanisms and therapies developed potential to be specific to the individual. The direct conversion methods result in

allow the study of multiple cell types.

neurodegenerative diseases has been shaped by the fact that, bythe time symptoms have developed, there are only a small numberof healthy neurons left alive. Neurodegeneration is a slow process,which begins ten to twenty years before the onset of disease(Bateman et al., 2012). Yet some of the abnormalities have beenobserved as early as in utero in the familial form of Alzheimer’sdisease (Cataldo et al., 2001). These disease related phenotypescould represent the beginning or early developmental stage of thedisease. Pluripotent stem cells offer a new opportunity to studythese early events and the mechanism of disease progression.

6.2. Implications of iPSC in personalized disease modeling andmedicine

Up until now, disease modeling has been involved with knowngenetic causes. Modeling sporadic diseases, where the geneticcauses are usually multiple and unclear, has not been feasible. It isnow possible to model sporadic diseases using iPSC. These patientderived cells embody the combined genetic risk factors which con-


tribute to patients’ development of disease. For example, diseasessuch as autism, where multiple defective genes may be involvedin the development of disease, could potentially be modeled usingiPSC by reprogramming patient fibroblasts. The iPSC method will

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lso save time required to develop the models. For example, araditional animal model would take years to make in polygeneticiseases, for it would require several rounds of targeting to get allf the mutations into the germline of the animal (Fig. 1).

It is also possible to create iPSC models for individuals andtudy how that person could develop disease. This could become

tool for assessing risk factors for the development of disease.uch information could be useful in modification of risk factors, sohat individual’s future development of disease could be avoided.he personalized disease model-in-a-dish, could also potentiallye used for testing drug responses and toxicity, to avoid treatmentailure and adverse events.

Now that there are multiple methods to derive the cell type ofnterest, the method of choice is highly dependent on the researchuestion and the efficiency of the method (Fig. 2). The main advan-age of iPSC is that these cells are by definition pluripotent stemells; therefore, they have the potential to differentiate into allhe different types of cells of the human body. This allows theesearcher to perform studies on experimental paradigms involv-ng multiple types of cells. For example, one could study drugoxicity in different end organs, such as liver cells and cardiac

yocytes. However, getting from the iPSC to useful terminallyifferentiated cells could take months, involving both reprogram-ing and differentiation processes. Direct conversion of one type

f somatic cell, such as the fibroblast, into another type such aseurons, would speed up the generation of the desired terminallyifferentiated population. However, neurons do not live alone in therain; therefore, this method has very specific and limited utility.onversion of the fibroblast into a neural stem cell (NSC), whichould give rise to both the neuron and the glia, is preferable ifhe research requires both neurons and the glia. In the past fewears, multiple groups have demonstrated that each of these cellypes can be derived, providing a set of diverse and useful tools foresearchers.

These engineered cells are derived from individuals, allowingesearchers to study phenomena associated with that specific indi-idual. This could be very useful, for example, when attempting toetermine why some individuals develop disease while others doot.

. Conclusions

The past few years have been an exciting time for stem cell biol-gy. The discovery of induced pluripotent stem cells (iPSC) hasiven new impetus to the stem cell field. The hope is that thisill ultimately lead to the creation of individualized patient mod-

ls which can predict disease and assist in its treatment. We arelready seeing the effect of whole genome sequencing on person-lized treatment in cancer patients. Perhaps stem cells may alsoave such an effect on the way we prevent, diagnose and treat ouratients in the future.

cknowledgements

Due to limitation of space, the authors apologize the omissionf other relevant manuscripts in the topics covered in this review.his work was supported by U01AG10483.

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Bioengineered stem cells in neural development and neurodegeneration research

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