Generation of induced neurons via direct conversion in vivo. Proc Natl Acad Sci U S A. 2013;110:7038-7043. [PMID: 23530235 DOI: 10.1073/pnas.1303829110]

Advances in individualized and regenerative medicine

Molecular and cell biology have resulted in major advances in our understanding of disease pathogenesis as well as in novel strategies for the diagnosis, therapy and prevention of human diseases. Based on modern molecular, genetic and biochemical methodologies it is on the one hand possible to identify for example disease-related point mutations and single nucleotide polymorphisms. On the other hand, using high throughput array and other technologies, it is for example possible to simultaneously analyze thousands of genes or gene products (RNA and proteins), resulting in an individual gene or gene expression profile ('signature'). Such data increasingly allow to define the individual disposition for a given disease and to predict disease prognosis as well as the efficacy of therapeutic strategies in the individual patient ('individualized medicine'). At the same time, the basic discoveries in cell biology, including embryonic and adult stem cells, induced pluripotent stem cells, genetically modified cells and others, have moved regenerative medicine into the center of biomedical research worldwide with a major translational impact on tissue engineering as well as transplantation medicine. All these aspects have greatly contributed to the recent advances in regenerative medicine and the development novel concepts for the treatment of many human diseases, including liver diseases.

In vivo conversion of astrocytes to neurons in the injured adult spinal cord

Spinal cord injury (SCI) leads to irreversible neuronal loss and glial scar formation, which ultimately result in persistent neurological dysfunction. Cellular regeneration could be an ideal approach to replenish the lost cells and repair the damage. However, the adult spinal cord has limited ability to produce new neurons. Here we show that resident astrocytes can be converted to doublecortin (DCX)-positive neuroblasts by a single transcription factor, SOX2, in the injured adult spinal cord. Importantly, these induced neuroblasts can mature into synapse-forming neurons in vivo. Neuronal maturation is further promoted by treatment with a histone deacetylase inhibitor, valproic acid (VPA). The results of this study indicate that in situ reprogramming of endogenous astrocytes to neurons might be a potential strategy for cellular regeneration after SCI.

Development-inspired reprogramming of the mammalian central nervous system

In 2012, John Gurdon and Shinya Yamanaka shared the Nobel Prize for the demonstration that the identity of differentiated cells is not irreversibly determined but can be changed back to a pluripotent state under appropriate instructive signals. The principle that differentiated cells can revert to an embryonic state and even be converted directly from one cell type into another not only turns fundamental principles of development on their heads but also has profound implications for regenerative medicine. Replacement of diseased tissue with newly reprogrammed cells and modeling of human disease are concrete opportunities. Here, we focus on the central nervous system to consider whether and how reprogramming of cell identity may affect regeneration and modeling of a system historically considered immutable and hardwired.

Pluripotent stem cells in regenerative medicine: challenges and recent progress

After years of incremental progress, several recent studies have succeeded in deriving disease-relevant cell types from human pluripotent stem cell (hPSC) sources. The prospect of an unlimited cell source, combined with promising preclinical data, indicates that hPSC technology may be on the verge of clinical translation. In this Review, we discuss recent progress in directed differentiation, some of the new technologies that have facilitated the success of hPSC therapies and the remaining hurdles on the road towards developing hPSC-based cell therapies.

In vivo direct reprogramming of reactive glial cells into functional neurons after brain injury and in an Alzheimer's disease model

Loss of neurons after brain injury and in neurodegenerative disease is often accompanied by reactive gliosis and scarring, which are difficult to reverse with existing treatment approaches. Here, we show that reactive glial cells in the cortex of stab-injured or Alzheimer's disease (AD) model mice can be directly reprogrammed into functional neurons in vivo using retroviral expression of a single neural transcription factor, NeuroD1. Following expression of NeuroD1, astrocytes were reprogrammed into glutamatergic neurons, while NG2 cells were reprogrammed into glutamatergic and GABAergic neurons. Cortical slice recordings revealed both spontaneous and evoked synaptic responses in NeuroD1-converted neurons, suggesting that they integrated into local neural circuits. NeuroD1 expression was also able to reprogram cultured human cortical astrocytes into functional neurons. Our studies therefore suggest that direct reprogramming of reactive glial cells into functional neurons in vivo could provide an alternative approach for repair of injured or diseased brain.

Regenerative medicine: transdifferentiation in vivo

A major challenge in regenerative medicine is the generation of functionally effective target cells to replace or repair damaged tissues. Transdifferentiation in vivo is a novel strategy to achieve cell fate conversion within the native physiological niche; this technology may provide a time- and cost-effective alternative for applications in regenerative medicine and may also minimize the concerns associated with in vitro culture and cell transplantation.

Transdifferentiation of adipogenically differentiated cells into osteogenically or chondrogenically differentiated cells: phenotype switching via dedifferentiation

Reprogramming is a new wave in cellular therapies to achieve the vital goals of regenerative medicine. Transdifferentiation, whereas the differentiated state of cells could be reprogrammed into other cell types, meaning cells are no more locked in their differentiated circle. Hence, cells of choice from abundant and easily available sources such as fibroblast and adipose tissue could be converted into cells of demand, to restore the diseased tissues. Before diverting this new approach into effective clinical use, transdifferentiation could not be simply overlooked, as it challenges the normal paradigms of biological laws, where mature cells transdifferentiate not only within same germ layers, but even across the lineage boundaries. How unipotent differentiated cells reprogram into another, and whether transdifferentiation proceeds via a direct cell-to-cell conversion or needs dedifferentiation. To address such questions, MSC were adipogenically differentiated followed by direct transdifferentiation, and subsequently examined by histology, immunohistochemistry, qPCR and single cell analysis. Direct cellular conversion of adipogenic lineage cells into osteogenic or chondrogenic resulted in mixed culture of both lineage cells (adipogenic and new acquiring osteogenic/chondrogenic phenotypes). On molecular level, such conversion was confirmed by significantly upregulated expression of PPARG, FABP4, SPP1 and RUNX2. Chondrogenic transdifferentiation was verified by significantly upregulated expression of PPARG, FABP4, SOX9 and COL2A1. Single cell analysis did not support the direct cell-to-cell conversion, rather described the involvement of dedifferentiation. Moreover, some differentiated single cells did not change their phenotype and were resistant to transdifferentiation, suggesting that differentiated cells behave differently during cellular conversion. An obvious characterization of differentiated cells could be helpful to understand the process of transdifferentiation.

Stem cells: potential and challenges for kidney repair

Renal damage resulting from acute and chronic kidney injury poses an important problem to public health. Currently, patients with end-stage renal disease rely solely on kidney transplantation or dialysis for survival. Emerging therapies aiming to prevent and reverse kidney damage are thus in urgent need. Although the kidney was initially thought to lack the capacity for self-repair, several studies have indicated that this might not be the case; progenitor and stem cells appear to play important roles in kidney repair under various pathological conditions. In this review, we summarize recent findings on the role of progenitor/stem cells on kidney repair as well as discuss their potential as a therapeutic approach for kidney diseases.

In vivo reprogramming of astrocytes to neuroblasts in the adult brain

Adult differentiated cells can be reprogrammed into pluripotent stem cells or lineage-restricted proliferating precursors in culture; however, this has not been demonstrated in vivo. Here, we show that the single transcription factor SOX2 is sufficient to reprogram resident astrocytes into proliferative neuroblasts in the adult mouse brain. These induced adult neuroblasts (iANBs) persist for months and can be generated even in aged brains. When supplied with BDNF and noggin or when the mice are treated with a histone deacetylase inhibitor, iANBs develop into electrophysiologically mature neurons, which functionally integrate into the local neural network. Our results demonstrate that adult astrocytes exhibit remarkable plasticity in vivo, a feature that might have important implications in regeneration of the central nervous system using endogenous patient-specific glial cells.

Reprogramming in vivo produces teratomas and iPS cells with totipotency features

Reprogramming of adult cells to generate induced pluripotent stem cells (iPS cells) has opened new therapeutic opportunities; however, little is known about the possibility of in vivo reprogramming within tissues. Here we show that transitory induction of the four factors Oct4, Sox2, Klf4 and c-Myc in mice results in teratomas emerging from multiple organs, implying that full reprogramming can occur in vivo. Analyses of the stomach, intestine, pancreas and kidney reveal groups of dedifferentiated cells that express the pluripotency marker NANOG, indicative of in situ reprogramming. By bone marrow transplantation, we demonstrate that haematopoietic cells can also be reprogrammed in vivo. Notably, reprogrammable mice present circulating iPS cells in the blood and, at the transcriptome level, these in vivo generated iPS cells are closer to embryonic stem cells (ES cells) than standard in vitro generated iPS cells. Moreover, in vivo iPS cells efficiently contribute to the trophectoderm lineage, suggesting that they achieve a more plastic or primitive state than ES cells. Finally, intraperitoneal injection of in vivo iPS cells generates embryo-like structures that express embryonic and extraembryonic markers. We conclude that reprogramming in vivo is feasible and confers totipotency features absent in standard iPS or ES cells. These discoveries could be relevant for future applications of reprogramming in regenerative medicine.

Reshaping the brain: direct lineage conversion in the nervous system

During embryonic development, cells in an uncommitted pluripotent state undergo progressive epigenetic changes that lock them into a final restrictive differentiated state. However, recent advances have shown that not only is it possible for a fully differentiated cell to revert back to a pluripotent state, a process called nuclear reprogramming, but also that differentiated cells can be directly converted from one class into another without generating progenitor intermediates, a process known as direct lineage conversion. In this review, we discuss recent progress made in direct lineage reprogramming of differentiated cells into neurons and discuss some of the therapeutic implications of the findings.