A case of cellular alchemy: lineage reprogramming and its potential in regenerative medicine

Abstract

The field of regenerative medicine is rapidly gaining momentum as an increasing number of reports emerge concerning the induced conversions observed in cellular fate reprogramming. While in recent years, much attention has been focused on the conversion of fate-committed somatic cells to an embryonic-like or pluripotent state, there are still many limitations associated with the applications of induced pluripotent stem cell reprogramming, including relatively low reprogramming efficiency, the times required for the reprogramming event to take place, the epigenetic instability, and the tumorigenicity associated with the pluripotent state. On the other hand, lineage reprogramming involves the conversion from one mature cell type to another without undergoing conversion to an unstable intermediate. It provides an alternative approach in regenerative medicine that has a relatively lower risk of tumorigenesis and increased efficiency within specific cellular contexts. While lineage reprogramming provides exciting potential, there is still much to be assessed before this technology is ready to be applied in a clinical setting.

Introduction

The exploration of cellular plasticity has taken an exciting turn in recent years, as amassing evidence has emerged, challenging ancient assumptions that lineage fate decisions once determined are permanent. As of late, there has been an influx of information demonstrating the incredible versatility of an adult somatic cell. Examples can be taken from a number of studies, including the most recent works seen in the field of regenerative medicine.

Lineage reprogramming is a term used to describe the conversion of a mature, terminally differentiated cell type into another mature cell type within or outside of its lineage of origin without undergoing dedifferentiation. The notion of cell plasticity holds the potential to provide researchers and physicians the opportunity to offer patient-specific treatments and transform the way we treat a number of diseases requiring cell replacement resulting from liver failure, heart disease, diabetes, Parkinson's disease, Alzheimer's disease, and blood diseases. While the field has taken rather large strides in a relatively short amount of time, there is still much to be assessed before lineage reprogramming will be seen in the clinic.

Regenerative medicine has seen a recent upward surge of investigation with regard to lineage reprogramming in the past few years, examples of which can be found in Table 1. A large part of this increase could arguably be attributed to studies demonstrating that mouse embryonic and adult fibroblasts can be reprogrammed into an embryonic-like state using a cocktail of transcription factors—often referred to as the ‘Yamanaka factors’—Oct3/4, Sox2, c-Myc, and Klf4, under embryonic stem cell culture conditions (Takahashi and Yamanaka, 2006). In the following years, several groups found it possible to convert adult human fibroblasts into induced pluripotent stem (iPS) cells using the same Yamanaka factors (Maherali et al., 2007; Okita et al., 2007; Takahashi et al., 2007; Wernig et al., 2007; Park et al., 2008) and through a different set of factors, consisting of Oct4, Sox2, Nanog, and Lin28 (Yu et al., 2007, 2009; Li et al., 2009a). It was discovered that the oncogene, c-Myc could be omitted (Nakagawa et al., 2008; Wernig et al., 2008) and that concordant results could be achieved using only Oct4 and Sox2, enabled by the histone deacetylase inhibitor, valporic acid (Huangfu et al., 2008). It was also found that the G9a histone methyltransferase inhibitor, BIX01294, and the L-channel agonists, Bayk8644, could enable the induction of iPS cells using Oct4 and Klf4 (Shi et al., 2008). Interestingly, similar reprogramming outcomes could be achieved using Oct4 and KLF4 with the small molecules CHIR99021, a glycogen synthase kinase-3 inhibitor and tranylcypromine, an H3K4 inhibitor (Li et al., 2009b). Other studies found that it is possible to achieve iPS cell induction using Oct4 and either Klf4 or c-Myc (Kim et al., 2008), omit both factors in neural stem cell populations using Oct4 alone (Kim et al., 2009), or Oct 4 and ID3 (Moon et al., 2011). The induction of iPS cells could also be achieved using Oct4 in combination with small molecules (Zhu et al., 2010; Chen et al., 2011; Li et al., 2011; Wang et al., 2011; Yuan et al., 2011). The variety of factor combinations and cell types used to generate iPS cells has been many (Cheng et al., 2011; Jiang et al., 2011) and has served to aid in the reassessment of cell plasticity and the potential cell plasticity harnesses for therapeutic use. iPS cells have played an active role in reviving an interest in lineage reprogramming studies. On the other hand, lineage reprogramming offers another means of directing the cellular program in an efficient, potentially less-tumorigenic, and genetically stable manner.

Table 1.

Examples of experimental lineage reprogramming.

Initial cell type	Target cell type	Factors	Reference
Mouse extra embryonic mesoderm	Cardiomyocytes	Gata4, Tbx5, and Baf60c	Takeuchi and Bruneau (2009)
Mouse cardiac- and tail tip fibroblasts	Cardiomyocytes	Gata4, Mef2c, and Tbx5	Ieda et al. (2010)
Mouse embryonic fibroblasts	Cardiomyocytes	Oct 4, Sox2, Klf4, and c-Myc	Efe et al. (2011)
Cochlea	Cochlear inner hair cells	Atoh 1	Izumikawa et al. (2005)
Human embryonic fibroblasts and human fetal lung fibroblasts	Dopaminergic neurons	Ascl1, Brn2 (Pou3f2), and Myt1l, Lmx1a, FoxA2	Pfisterer et al. (2011)
Adult mouse tail-tip fibroblasts and human embryonic fibroblasts	Dopaminergic neurons	Ascl1, Lmx1a, and Nurr1	Caiazzo et al. (2011)
Human adult skin fibroblasts	Glutamatergic neurons	Ascl1, Brn2 (Pou3f2), Myt1l, Zic1, and Olig2	Qiang et al. (2011)
Mouse embryonic fibroblasts and dermal fibroblasts	Hepatocyte-like cells	Hnf4α, Foxa1, Foxa2, and Foxa3	Sekiya and Suzuki (2011)
β-cells	Macrophage-like cells	C/EBPα and PU.1	Xie et al. (2004)
Mouse embryonic fibroblasts and mouse adult fibroblasts	Macrophage-like cells	C/EBPα/β and PU.1	Feng et al. (2008)
Mouse embryonic (and human embryonic) fibroblasts	Motor neurons	Ascl1, Brn2, Myt1l, Lhx3, Ngn2, Isl1, and Hb9 [NeuroD1]	Son et al. (2011)
Human dermal fibroblasts	Myeloid, erythroid, megakaryocytic progenitors	Oct4	Szabo et al. (2010)
Mouse embryonic fibroblasts	Myoblasts	MyoD1	Davis et al. (1987)
Mouse fibroblasts	Neural progenitors	Oct4, Sox2, Klf4, and c-Myc	Kim et al. (2011a,b)
Mouse embryonic fibroblasts	Neurons	Ascl1, Brn2 (Pou3f2), and Myt1l	Vierbuchen et al. (2010)
Human fetal, postnatal fibroblasts and human dermal (11-year-old donor) fibroblasts	Neurons	Ascl1, Brn2 (Pou3f2), and Myt1l, NeuroD1	Pang et al. (2011)
Human fetal fibroblasts and adult dermal (30-year-old donor) fibroblasts	Neurons	[Ascl1, Brn2], NeuroD2, miR9, and miR125	Yoo et al. (2011)
Mouse postnatal liver hepatocytes	Neurons	Ascl1, Brn2 (Pou3f2), and Myt1l	Marro et al. (2011)
Adult human primary dermal fibroblasts	Neurons	Brn2, Myt1l, and miR124	Ambasudhan et al. (2011)
Mouse sertoli cells	Neural stem cells	Ascl1, Ngn2, Hes1, Id1, Pax6, Brn2, Sox2, c-Myc, and Klf4	Sheng et al. (2012)
Exocrine cells	β-like cells	Ngn3, Pdx1, and Mafa	Zhou et al. (2008)

This table provides a brief outline summarizing some of the current factors and somatic cell types used in lineage reprogramming studies.

Natural conversion

Newt lens epithelial cells

Possibly one of the most well-established examples of lineage reprogramming in vertebrates occurs in the eyes of the newt. For over a century, it has been known that after complete removal of the eye lens, from the adult or tadpole-stage newt, the dorsal portion of the iris-pigmented epithelium is gradually converted into a new lens. The newly regenerated lens is not only capable of restoring the vision of the affected eye but is indistinguishable from the original lens (Yamada, 1966). Interestingly, it has been noted that shortly following lens removal or limb amputation, the iPS cell reprogramming factors, Sox2, Klf4, and c-Myc, are temporarily upregulated thus initiating the regeneration process. The absence of Oct4 and Nanog throughout this event could attest to the possibility that pigmented lens epithelial cells circumvent conversion to a pluripotent state (Maki et al., 2009). Understanding these natural phenomena will aid us on a quest to discover the mechanisms associated with lineage reprogramming and help us to untangle the hierarchal sequence of events which occurs during reprogramming process.

Epithelial-to-mesenchymal transition

An epithelial–mesenchymal transition (EMT) allows for polarized epithelial cells to undergo changes in which they assume a mesenchyme phenotype. Examples can be found in cases of tissue repair, inflammation, and cancer (Kalluri and Weinberg, 2009). EMT has been classified into three types. One type refers to the non-invasive and non-fibrotic EMTs characteristic of implantation, embryogenesis, and organ development. An example can be taken from gastrulation. During gastrulation, epithelial cells convert into mesenchymal cells, making EMTs crucial for germ layer formation. A second type is associated with tissue regeneration and wound healing. Such an example can be seen in fibrogenesis of the adult kidney when tubular epithelial cells lose their apical-basal polarity and migrate into interstitial organ tissue. While the third type is seen in advanced staged cancers that become metastatic (Kalluri and Weinberg, 2009). Reprogramming brings about concerns regarding the potential for tumorigenesis. It has been suggested that induction of an EMT is likely to be an important mechanism for the progression of carcinomas (Kalluri and Weinberg, 2009). Further unfolding mechanisms associated with lineage reprogramming in general and EMT specifically may be critical to prevent cancer onset and progression.

Rectal to motor neuron transition in Caenorhabditis elegans

Using a model organism such as Caenorhabditis elegans allows researchers to unambiguously follow the mechanisms associated with lineage reprogramming in real time. In C. elegans, the epithelial rectal cell, ‘Y’, migrates from the rectum anterodorsally, after withdrawing from the epithelium to become the motor neuron named PDA. Further evaluation demonstrated that rather than the ‘Y’ cell passing through a state with expression profiles indicative of a mixture of both the initial ‘Y’ cell and target PDA cell type, it momentarily assumed a phenotype of its own. Moreover, BrdU staining suggests the reprogramming event occurs without cell division (Richard et al., 2011). Though it may be a feature unique to C. elegans, these results allude to the possibility of lineage reprogramming occurring without risk of tumor formation as it does not appear that the process creates an intermediate cell of mixed identity with the ability to divide, a feature often seen in cancer cells (Richard et al., 2011).

Experimental conversion

With many examples of lineage reprogramming events found to occur naturally, scientists have explored appropriating this concept to therapeutic treatment and disease modeling.

Myogenic conversion by MyoD

The myoblast determination gene 1, MyoD1, belongs to the basic-helix-loop-helix (bHLH) family of transcription factors and is often referred to as a ‘master regulator’ of muscle-derived cells. This began with the observation of the ability of 5-azacytidine—an inhibitor of DNA methyltransferases—to produce a conversion to stably determined but undifferentiated states of myogenic, adipogenic, and chondrogenic origin (Konieczny and Emerson, 1984). Following was the discovery that under forced MyoD1 expression, 10T1/2 fibroblasts were converted to myoblasts (Davis et al., 1987). The possible existence of a ‘master regulator’ in which fate conversion could be achieved became highly popular because MyoD1 was shown to initiate myogenic programs in a variety of cell types, including fibroblasts, smooth muscles, adipocytes, and kidney cells. While these examples of MyoD-invoked conversions are mesodermal in origin; hepatocytes, which are endodermal in origin, have been shown to cross germ layer boundaries and follow the myogenic program when exposed to MyoD (Li et al., 2005).

Though successful conversion has been demonstrated in vitro, in-depth in vivo support has been lacking. Clinical trials involving myoblast transplantation has proved unsuccessful due to low migratory ability and poor survival rates of cells (Darabi et al., 2009). In a separate study involving transplantation of myoblasts to patients with Duchenne's muscular dystrophy, transplanted cells failed to improve muscular strength of patients (Mendell et al., 1995). On the other hand, others were able to generate positive results through the use of satellite-enriched population (Collins et al., 2005; Montarras et al., 2005; Cerletti et al., 2008). While the transplantation of satellite-enriched populations is promising, many tissues and diseases requiring transplantation of cells may not have this form of support. Therefore, lineage reprogramming to progenitor cells may be more applicable as they are expandable and capable of conversion not only to the target cell types, but also to supporting cell types that are crucial to the survival and function of the target cell types.

Auditory cell conversion by Atoh

Sensorineural hearing loss is commonly attributed to the degeneration of cochlear sensory (hair) cells as a result of sound exposures, aging, genetic disorders, ototoxic drugs, infections, and auditory hyperstimulation amongst other causes. It is an irreversible process requiring the regeneration of new functional sensory hair cells (Izumikawa et al., 2005). It was demonstrated that the generation of new hair cells is possible when Atoh1, a bHLH transcription factor, was adenovirally transduced into the epithelial layer of the cochlea in deaf animals. Specifically, differentiated non-sensory cells of the auditory epithelium were able to be induced into functional inner hair cells that improved hearing in deaf animals by the expression of Atoh1 (Izumikawa et al., 2005). This study, along with other lineage reprogramming examples, suggests that the expression of key developmental regulatory genes in mature tissues may provide a potential strategy for cell replacement therapy in situ.

Reprogramming to pancreatic cell types

Lineage reprogramming to pancreatic cells is particularly interesting because of its potential applications towards the management and treatment of diabetes. It has been shown that differentiated exocrine cells return to a partially dedifferentiated state with the ability to be converted into ductal cells (Rooman et al., 2000, 2001), hepatocyte-like cells (Lardon et al., 2004a), and insulin-producing cells (Lardon et al., 2004b). Based on similarities seen in embryonic neurogenesis and pancreatic endocrinogenesis, leukemia inhibitory factor (LIF) and epidermal growth factor were used to direct the exocrine pancreatic cells to insulin-producing β-cells (Baeyens et al., 2004). It was later shown that adenoviral transduction of transcription factors, Ngn3, Pdx1, and Mafa, into the pancreas of adult mice could achieve similar results in vivo (Zhou et al., 2008). This study provides another example of converting fully differentiated cells into another differentiated cell type using developmental transcription factors in situ. The induced β-cells closely resemble primary islet β-cells. However, the induced cells did not organize into islet structures, thus lacked insulin-secreting function (Zhou et al., 2008). Further studies are required to produce functional β-cells with full glucose responsiveness.

Reprogramming to hepatocyte-like cells

Recently demonstrated was the ability to generate hepatocyte-like cells from mouse embryonic and dermal fibroblasts using transcription factors Hnf4α plus Foxa1, Foxa2, or Foxa3 (Sekiya and Suzuki, 2011). Induced hepatocytes shared similar traits to wild-type hepatocytes and were able to reconstitute hepatic tissues upon transplantation and support hepatic function in vivo (Sekiya and Suzuki, 2011). Moreover, genetically modified induced hepatocytes could repair hepatic defects after transplantation, providing a potential therapeutic tactic for liver diseases. This study serves as a useful reference for studying molecular mechanisms of cellular plasticity and provides a powerful system for developing therapeutic strategies directed at liver diseases.

Direct conversion into cellular tissue of the cardiac muscle

In an effort to direct cell replacement strategies towards heart muscle regeneration to treat afflictions such as heart disease and stroke (Takeuchi and Bruneau, 2009) endeavored to find a remedy by demonstrating that Gata4, Tbx5, and Baf6c, a cardiac specific subunit of the BAF chromatin remodeling complexes, could direct differentiation from the mouse amnion into beating cardiomyocytes. Henceforth, mouse cardiac fibroblasts and tail-tip fibroblasts were shown to be reprogrammed into functional, beating cardiomyocytes using Gata4, Mef2c, and Tbx5 resulting in similar global gene profiles between induced and heart-derived cardiomyocytes (Ieda et al., 2010).

Lineage reprogramming studies typically aim to circumvent progression through an unstable intermediate stage. However, several groups have demonstrated the importance of strategically coercing ‘unstable’ intermediates to obtain lineage-specific progenitors. In a recent study, it was shown that iPS cell reprogramming factors—Oct4, Sox2, and Klf4—could be used to initiate the cardiac program in mouse fibroblasts and be further directed to cardiomyocytes both rapidly and efficiently (Efe et al., 2011). Using small molecules and cytokines vital to the regulation of TGFβ, BMP, hedgehog, Wnt, and Notch pathways, they were able to direct Oct4, Sox2, and Klf4-transduced unstable intermediates into cardiomyocyte lineage. The omission of LIF paired with the inhibition of the Jak-Stat pathway serves to bypass pluripotency and regulate unstable intermediates (Bao et al., 2009; Niwa et al., 2009; Buecker et al., 2010; Bichsel et al., 2011). Interestingly, this process is comparable to what occurs during blastema regeneration in frogs and zebrafish as the onset of lineage reprogramming and limb regeneration share a common pattern of brief low-level expression of reprogramming factors, chiefly Oct4 and Sox2 (Stadtfeld, 2008; Christen et al., 2010; Efe et al., 2011). Moreover, it is suggested that Oct4 is not directly involved in the activation of lineage specific genes, but rather erases cellular identity. The enhancement of cardiogenesis through the regulation of key cellular pathways via small molecules and cytokines during strategic points of the lineage reprogramming event results in a more efficient and rapid conversion (Efe et al., 2011), attesting to the need to further explore the role of signal transduction and pathway regulation taking place as the cell takes on a new fate. While this study provides a strategy to direct conventional reprogramming towards lineage-specific cardiogenesis in an efficient manner, transplantation of the induced cells still needs to be performed to ensure that the induced cells are functional in vivo and incapable of teratoma formation.

Progenitor conversion using Oct4

Early work done within the hematopoietic system illustrates an interest for cells to take on a hematopoietic program because of its usefulness in disease modeling and cell therapy. Xie et al. (2004) demonstrated that forced expression of C/EBPα and PU.1 in committed B-cells could upregulate macrophage-specific genes. Later, it was evidenced that fate committed cells could be reprogrammed from outside of the hematopoietic system to cells of myeloid and lymphoid lineages via the conversion of mouse fibroblasts to macrophage-like cells using PU.1 and C/EBPα/β (Feng et al., 2008). More recently Szabo et al. (2010) showed that fibroblasts can convert into multipotent hematopoietic progenitors of myeloid, erythroid, and megakaryocytic lineages using Oct4-induced reprogramming. Unlike human iPS cell-derived hematopoietic cells demonstrated elsewhere (Chang et al., 2006), these CD45⁺ fibroblasts could acquire the adult globin protein, conveying that the POU domain of Oct proteins plays a redundant role in hematopoietic conversion, thus enabling the activation of the hematopoietic program. Although lymphoid fate was not detected, future studies aim to combine Oct4, Oct1, and Oct2 along with lymphoid-specific culture conditions to obtain a lymphoid conversion.

Neuronal subtypes and glial cells

Lineage reprogramming into the different subsets of neural cells has proved to be quite popular in recent years. Vierbuchen et al. (2010) used Ascl1, Brn2 (Pou3f2), and Myt1l to reprogram mouse embryonic fibroblasts into neurons. Later, it was demonstrated that mouse liver hepatocytes could also undergo a reprogramming event to become functional neurons using the same transcription factors (Marro et al., 2011). The addition of Lmx1a and Foxa2 to the previously mentioned cocktail (Vierbuchen et al., 2010) could specifically direct fibroblasts to dopaminergic neurons, the neuronal cell subtype involved in motor function and lost in Parkinson's disease (Pfisterer et al., 2011). Following initial experiments in mice, the addition of NeuroD1 greatly enhanced the conversion of human fibroblasts to neurons produced by Ascl1, Brn2, and Myt1 (Pang et al., 2011). Ascl1, Lmx1a, and Nurr1 were subsequently shown to convert human fibroblasts isolated from a Parkinson's disease patient to dopaminergic neurons with the ability to persist and integrate into the host tissue (Caiazzo et al., 2011). Another study used a cocktail of seven transcription factors (Ascl1, Brn2, Myt1l, Lhx3, Ngn2, Isl1, and Hb9) to convert human fibroblasts to cholinergic motor neurons of the central nervous system (Son et al., 2011).

Alzheimer's disease patients could benefit tremendously from the conversion of skin-derived fibroblasts to neurons. In support of this idea, direct reprogramming from fibroblasts to glutamatergic neurons using Ascl1, Brn2, Myt1, zinc-finger protein 1 (Zic1), and oligodendrocyte marker 2 (Olig2) was achieved (Qiang et al., 2011). After applying the conversion protocol to a panel of fibroblasts derived from patients with PSEN-1 and PSEN-2 mutation-caused Alzheimer's disease, an acting role for the beta amyloid peptide (Aβ), a peptide involved in the etiology of Alzheimer's disease, was evidenced (Lahiri and Maloney, 2010; Qiang et al., 2011). In this way, lineage reprogramming could serve in disease modeling, providing a means to explore the pathological mechanisms of human diseases.

Interestingly, microRNAs can be used in lineage reprogramming as well. Human fetal fibroblasts were converted to MAP2 expressing neuron-like cells using miR-9/9* and miR-124 (Yoo et al., 2011). When combined with NeuroD2, followed by Ascl1, and Myt1l, the conversion was significantly enhanced (Yoo et al., 2011). Induced neurons were also reprogrammed from fibroblasts using miR124, Brn2, and Myt1l (Ambasudhan et al., 2011). These studies suggest the significance of including microRNA tag teams in the reprogramming cocktail to aid in resetting the epigenetic clock and demonstrate the ability of microRNAs to work synergistically with transcription factors to induce reprogramming.

In an effort to create a more standardized lineage reprogramming procedure, Kim et al. (2011a) used an iPS cell factor-based direct reprogramming strategy, consisting of Oct4, Sox2, Klf4, and c-Myc. After illustrating their ability to efficiently and rapidly direct reprogramming into cardiomyocytes (Efe et al., 2011), the same group was able to show that fibroblasts could also be converted to neural progenitors using a similar methodology. These studies demonstrate that lineage reprogramming can be accomplished in a rapid and consistent manner within the specific cellular contexts and suggest that a common shared reprogramming pathway may be used to convert a wide variety of target cells. However, the neural progenitor cells lost their ability to form colonies within three to five passages in this study and in vivo function of the resultant neural progenitors remains to be determined (Kim et al., 2011a). Recently, multipotent neural stem cells were derived from sertoli cells using nine transcription factors, Ascl1, Ngn2, Hes1, Id1, Pax6, Brn2, Sox2, c-Myc, and Klf4 (Sheng et al., 2012). These cells were able to differentiate into functional neurons both in vitro and in vivo. Moreover, 4–6 weeks after transplantation, no tumorigenesis was observed (Sheng et al., 2012). These results suggest that induced neural stem cells may provide a potential cell source for replacement therapy. However, further efforts are needed to produce self-renewable neural stem cells that are able to provide more abundant cell sources and thus are more applicable for cell therapy.

As sustained engraftment and functional recovery are crucial to the potential of lineage reprogramming in regenerative medicine, more studies now include in vivo evidence to support this potentiality. For instance, human induced neurons were demonstrated to integrate into neuronal circuitry in embryonic mouse brains (Qiang et al., 2011). Induced motor neurons displayed the ability to survive in vivo, integrate into the developing chick spinal cord, and correctly respond to axon guidance cues (Son et al., 2011). The functional potential of induced dopaminergic neurons was studied in a Parkinson's disease animal model. The transplanted induced neurons exhibited integrated grafts and led to significant functional recovery in the 6-hydroxydopamine lesioned mice (Kim et al., 2011b). Moreover, induced neural stem cells were able to differentiate into neurons that matured and formed functional connections with host neurons after transplantation into adult mouse brains (Sheng et al., 2012). These engraftment results are consistent with an earlier study showing the ability of induced neurons to survive in the brain and exhibit physiological activities (Watanabe et al., 2004). These studies together suggest that lineage reprogramming has the potential to provide therapeutic cell sources for cell replacement therapies, in addition to their applications in disease modeling and drug discovery.

Perspective

Regenerative medicine holds great potential to transform the way we approach disease treatment, and in turn the way health is restored back to the patient (Figure 1). Lineage reprogramming provides promising therapeutic potential for the replacement, repair, and restoration of damaged tissues and organs. While it has made significant progress since its beginnings, there is still a need to identify key mechanisms and pathways involved in lineage reprogramming and determine safe delivery methods of cells or reprogramming factors for therapeutic applications. Efforts are also needed to generate lineage-specific progenitors through lineage reprogramming, in addition to terminally differentiated cells that most current research is focused on. In addition, evidence of transplantability, prolonged engraftment, and functional recovery is crucial for the characterization of reprogrammed cells. It will be exciting to see how the field progresses as lineage reprogramming makes its way towards becoming therapeutically applicable.

Figure 1.

A possible strategy for the use of lineage reprogramming in therapeutics. Following the isolation of primary cells from the patient, cells will be reprogrammed using reprogramming factors. Once the target cell type is generated, the cells will undergo further characterization and correction to ensure that it is safe to transplant back into the patient.

Funding

This work was supported by NIH NINDS (R01 NS059546 and RC1 NS068370) and California Institute for Regenerative Medicine (TR2-01832).

Conflict of interest: none declared.