Accumulating evidence has shown that cell dedifferentiation or reprogramming is a pivotal procedure for animals to deal with injury and promote endogenous tissue repair. Tissue damage is a critical factor that triggers cell dedifferentiation or reprogramming in vivo. By contrast, microenvironmental changes, including the loss of stem cells, hypoxia, cell senescence, inflammation and immunity, caused by tissue damage can return cells to an unstable state. If the wound persists in the long‑term due to chronic damage, then dedifferentiation or reprogramming of the surrounding cells may lead to carcinogenesis. In recent years, extensive research has been performed investigating cell dedifferentiation or reprogramming in vivo, which can have significant implications for wound repair, treatment and prevention of cancer in the future. The current review summarizes the molecular events that are known to drive cell dedifferentiation directly following tissue injury and the effects of epigenetic modification on dedifferentiation or reprogramming in vivo. In addition, the present review explores the intracellular mechanism of endogenous tissue repair and its relationship with cancer, which is essential for balancing the risk between tissue repair and malignant transformation after injury.

SignificanceOutside the neurogenic niches, the adult brain lacks multipotent progenitor cells. In this study, we performed a series of in vivo screens and reveal that a single factor can induce resident brain astrocytes to become induced neural progenitor cells (iNPCs), which then generate neurons, astrocytes, and oligodendrocytes. Such a conclusion is supported by single-cell RNA sequencing and multiple lineage-tracing experiments. Our discovery of iNPCs is fundamentally important for regenerative medicine since neural injuries or degeneration often lead to loss/dysfunction of all three neural lineages. Our findings also provide insights into cell plasticity in the adult mammalian brain, which has largely lost the regenerative capacity.

In vivo cell fate conversions have emerged as potential regeneration-based therapeutics for injury and disease. Recent studies reported that ectopic expression or knockdown of certain factors can convert resident astrocytes into functional neurons with high efficiency, region specificity, and precise connectivity. However, using stringent lineage tracing in the mouse brain, we show that the presumed astrocyte-converted neurons are actually endogenous neurons. AAV-mediated co-expression of NEUROD1 and a reporter specifically and efficiently induces reporter-labeled neurons. However, these neurons cannot be traced retrospectively to quiescent or reactive astrocytes using lineage-mapping strategies. Instead, through a retrograde labeling approach, our results reveal that endogenous neurons are the source for these viral-reporter-labeled neurons. Similarly, despite efficient knockdown of PTBP1 in vivo, genetically traced resident astrocytes were not converted into neurons. Together, our results highlight the requirement of lineage-tracing strategies, which should be broadly applied to studies of cell fate conversions in vivo.

Adult neurogenesis plays critical roles in maintaining brain homeostasis and responding to neurogenic insults. However, the adult mammalian spinal cord lacks an intrinsic capacity for neurogenesis. Here we show that spinal cord injury (SCI) unveils a latent neurogenic potential of NG2+ glial cells, which can be exploited to produce new neurons and promote functional recovery after SCI. Although endogenous SOX2 is required for SCI-induced transient reprogramming, ectopic SOX2 expression is necessary and sufficient to unleash the full neurogenic potential of NG2 glia. Ectopic SOX2-induced neurogenesis proceeds through an expandable ASCL1+ progenitor stage and generates excitatory and inhibitory propriospinal neurons, which make synaptic connections with ascending and descending spinal pathways. Importantly, SOX2-mediated reprogramming of NG2 glia reduces glial scarring and promotes functional recovery after SCI. These results reveal a latent neurogenic potential of somatic glial cells, which can be leveraged for regenerative medicine.

Overexpression of Oct3/4, Klf4, Sox2, and c-Myc (OKSM) transcription factors can de-differentiate adult cells in vivo. While sustained OKSM expression triggers tumorigenesis through uncontrolled proliferation of toti- and pluripotent cells, transient reprogramming induces pluripotency-like features and proliferation only temporarily, without teratomas. We sought to transiently reprogram cells within mouse skeletal muscle with a localized injection of plasmid DNA encoding OKSM (pOKSM), and we hypothesized that the generation of proliferative intermediates would enhance tissue regeneration after injury. Intramuscular pOKSM administration rapidly upregulated pluripotency (Nanog, Ecat1, and Rex1) and early myogenesis genes (Pax3) in the healthy gastrocnemius of various strains. Mononucleated cells expressing such markers appeared in clusters among myofibers, proliferated only transiently, and did not lead to dysplasia or tumorigenesis for at least 120 days. Nanog was also upregulated in the gastrocnemius when pOKSM was administered 7 days after surgically sectioning its medial head. Enhanced tissue regeneration after reprogramming was manifested by the accelerated appearance of centronucleated myofibers and reduced fibrosis. These results suggest that transient in vivo reprogramming could develop into a novel strategy toward the acceleration of tissue regeneration after injury, based on the induction of transiently proliferative, pluripotent-like cells in situ. Further research to achieve clinically meaningful functional regeneration is warranted.

Neuronal subtype is largely fixed in the adult mammalian brain. Here, however, we unexpectedly reveal that adult mouse striatal neurons can be reprogrammed into dopaminergic neuron-like cells (iDALs). This in vivo phenotypic reprogramming can be promoted by a stem cell factor (SOX2), three dopaminergic neuron-enriched transcription regulators (NURR1, LMX1A, and FOXA2), and a chemical compound (valproic acid). Although the site of action of the reprogramming factors remains to be determined, immunohistochemistry and genetic lineage mappings confirm striatal neurons as the cell origin for iDALs. iDALs exhibit electrophysiological properties stereotypical to endogenous dopaminergic rather than striatal neurons. Together, these results indicate that neuronal phenotype can be reengineered even in the adult brain, implicating a therapeutic strategy for neurological diseases.

Regenerative capacities vary enormously across the animal kingdom. In contrast to most cold-blooded vertebrates, mammals, including humans, have very limited regenerative capacity when it comes to repairing damaged or degenerating tissues. Here, we review the main mechanisms of tissue regeneration, underlying the importance of cell dedifferentiation and reprogramming. We discuss the significance of cell fate and identity changes in the context of regenerative medicine, with a particular focus on strategies aiming at the promotion of the body\'s self-repairing mechanisms. We also introduce some of the most recent advances that have resulted in complete reprogramming of cell identity in vivo. Lastly, we discuss the main challenges that need to be addressed in the near future to develop in vivo reprogramming approaches with therapeutic potential.

Reprogramming technology has enabled the fate conversion of terminally differentiated somatic cells into pluripotent stem cells or into another differentiated state. A dynamic reorganization of epigenetic regulation takes place during cellular reprogramming. Given that reprogramming does not require changes in the underlying genome, the technology can be used to actively modify epigenetic regulation. Although reprogramming has been investigated mostly at the cellular level in vitro, studies have reported that somatic cells are reprogrammable in multicellular organisms in vivo. In vivo reprogramming provides a potential strategy for regenerative medicine. Notably, recent studies using in vivo reprogramming technology to alter epigenetic regulation at organismal levels have revealed unappreciated epigenetic mechanisms in various biological phenomena, including cancer development, tissue regeneration, aging, and rejuvenation in mammals. Moreover, in vivo reprogramming technology can be applied to abrogate epigenetic aberrations associated with aging and cancer, which raises the possibility that the technology could provide a potential strategy to control the fate of detrimental cells such as senescent cells and cancer cells in vivo. Here, we review recent progress and future perspectives of in vivo reprogramming.

Converting resident glia into functional and subtype-specific neurons in vivo by delivering reprogramming genes directly to the brain provides a step forward toward the possibility of treating brain injuries or diseases. To date, it has been possible to obtain GABAergic and glutamatergic neurons via in vivo conversion, but the precise phenotype of these cells has not yet been analyzed in detail. Here, we show that neurons reprogrammed using Ascl1, Lmx1a, and Nurr1 functionally mature and integrate into existing brain circuitry and that the majority of the reprogrammed neurons have properties of fast-spiking, parvalbumin-containing interneurons. When testing different combinations of genes for neural conversion with a focus on pro-neural genes and dopamine fate determinants, we found that functional neurons can be generated using different gene combinations and in different brain regions and that most of the reprogrammed neurons become interneurons, independently of the combination of reprogramming factors used.

Reprogramming of differentiated cells into induced pluripotent stem cells has been recently achieved in vivo in mice. Telomeres are essential for chromosomal stability and determine organismal life span as well as cancer growth. Here, we study whether tissue dedifferentiation induced by in vivo reprogramming involves changes at telomeres. We find telomerase-dependent telomere elongation in the reprogrammed areas. Notably, we found highly upregulated expression of the TRF1 telomere protein in the reprogrammed areas, which was independent of telomere length. Moreover, TRF1 inhibition reduced in vivo reprogramming efficiency. Importantly, we extend the finding of TRF1 upregulation to pathological tissue dedifferentiation associated with neoplasias, in particular during pancreatic acinar-to-ductal metaplasia, a process that involves transdifferentiation of adult acinar cells into ductal-like cells due to K-Ras oncogene expression. These findings place telomeres as important players in cellular plasticity both during in vivo reprogramming and in pathological conditions associated with increased plasticity, such as cancer.