Cell plasticity in Caenorhabditis elegans: from induced to natural cell reprogramming

Achieving controlled reprogramming of differentiated cells into a desired cell type would open new opportunities in stem-cell biology and regenerative medicine. Experimentation on cell reprogramming requires a model in which cell conversion can be induced and tracked individually. The tiny nematode, Caenorhabditis elegans, owing to its known cellular lineage, allows the study of direct cell type conversion with a single-cell resolution. Indeed, recent advances have shown that despite its invariant cell lineage, cellular identities can be reprogrammed, leading to cell conversion in vivo. In addition, natural transdifferentiation events occur in the worm, providing a powerful model for the study of cellular plasticity in a physiological cellular microenvironment. Here, we review pioneer studies on induced and naturally occurring reprogramming events in C. elegans and the new notions that have emerged.     

Reprogramming cellular identity for regenerative medicine

Although development leads unidirectionally toward more restricted cell fates, recent work in cellular reprogramming has proven that one cellular identity can strikingly convert into another, promising countless applications in biomedical research and paving the way for modeling diseases with patient-derived stem cells. To date, there has been little discussion of which disease models are likely to be most informative. Here, we review evidence demonstrating that, because environmental influences and epigenetic signatures are largely erased during reprogramming, patient-specific models of diseases with strong genetic bases and high penetrance are likely to prove most informative in the near term. We also discuss the implications of the new reprogramming paradigm in biomedicine and outline how reprogramming of cell identities is enhancing our understanding of cell differentiation and prospects for cellular therapies and in vivo regeneration.     

Partial reprogramming as a therapeutic approach for heart disease: A state-of-the-art review

Heart disease such as myocardial infarction is the first cause of mortality in all countries. Today, cardiac cell-based therapy using de novo produced cardiac cells is considered as a novel approach for cardiac regenerative medicine. Recently, an alchemy-like approach, known as direct reprogramming or direct conversion, has been developed to directly convert somatic cells to cardiac cells in vitro and in vivo. This cellular alchemy is a short-cut and safe strategy for generating autologous cardiac cells, and it can be accomplished through activating cardiogenesis- or pluripotency-related factors in noncardiac cells. Importantly, pluripotency factors-based direct cardiac conversion, known as partial reprogramming, is shorter and more efficient for cardiomyocyte generation in vitro. Today, this strategy is achievable for direct conversion of mouse and human somatic cells to cardiac lineage cells (cardiomyocytes and cardiac progenitor cells), using transgene free, chemical-based approaches. Although, heart-specific partial reprogramming seems to be challenging for in vivo conversion of cardiac fibroblasts to cardiac cells, but whole organism-based in vivo partial reprogramming ameliorates cellular and physiological hallmarks of aging and prolongs lifespan in mouse. Notably, cardiac cells produced using partial reprogramming strategy can be a useful platform for disease modeling, drug screening and cardiac cell-based therapy, once the safety issues are overcome. Herein, we discuss about all progresses in de novo production of cardiac cells using partial reprogramming-based direct conversion, as well as give an overview about the potential applications of this strategy in vivo and in vitro.     

Autophagy regulates cytoplasmic remodeling during cell reprogramming in a zebrafish model of muscle regeneration

Cell identity involves both selective gene activity and specialization of cytoplasmic architecture and protein machinery. Similarly, reprogramming differentiated cells requires both genetic program alterations and remodeling of the cellular architecture. While changes in genetic and epigenetic programs have been well documented in dedifferentiating cells, the pathways responsible for remodeling the cellular architecture and eliminating specialized protein complexes are not as well understood. Here, we utilize a zebrafish model of adult muscle regeneration to study cytoplasmic remodeling during cell dedifferentiation. We describe activation of autophagy early in the regenerative response to muscle injury, while blocking autophagy using chloroquine or Atg5 and Becn1 knockdown reduced the rate of regeneration with accumulation of sarcomeric and nuclear debris. We further identify Casp3/caspase 3 as a candidate mediator of cellular reprogramming and Fgf signaling as an important activator of autophagy in dedifferentiating myocytes. We conclude that autophagy plays a critical role in cell reprogramming by regulating cytoplasmic remodeling, facilitating the transition to a less differentiated cell identity.     

Restoring Stem Cell Function in Aged Tissues by Direct Reprogramming?

Adult stem cells are responsible for the cellular turnover of many organs, and an impairment in their function leads to aging and disease. In efforts to reverse the process of tissue stem cell aging, we speculate on the promise and challenges of in vivo direct reprogramming strategies.     

Teaching cells new tricks

The direct conversion of one differentiated cell type into another--a process referred to as transdifferentiation--would be beneficial for producing isogenic (patient's own) cells to replace sick or damaged cells or tissue. Adult stem cells display a broader differentiation potential than anticipated and might contribute to tissues other than those in which they reside. As such, they could be worthy therapeutic agents. Recent advances in transdifferentiation involve nuclear transplantation, manipulation of cell culture conditions, induction of ectopic gene expression and uptake of molecules from cellular extracts. These approaches open the doors to new avenues for engineering isogenic replacement cells. To avoid unpredictable tissue transformation, nuclear reprogramming requires controlled and heritable epigenetic modifications. Considerable efforts remain to unravel the molecular processes underlying nuclear reprogramming and evaluate stable of the changes in reprogrammed cells.     

Lineage Specifiers： New Players in the Induction of Pluripotency

Pluripotency-associated factors and their rivals,lineage specifers,have long been considered the determining factors for the identity of pluripotent and differentiated cells,respectively.Therefore,factors that are employed for cellular reprogramming in order to induce pluripotency have been identifed mainly from embryonic stem cell(ESC)-enriched and pluripotency-associated factors.Recently,lineage specifers have been identifed to play important roles in orchestrating the process of restoring pluripotency.In this review,we summarize the latest discoveries regarding cell fate conversion using pluripotency-associated factors and lineage specifers.We highlight the value of the‘‘seesaw’’model in defning cellular identity,opening up a novel scenario to consider pluripotency and lineage specifcation.     

A poor imitation of a natural process: A call to reconsider the iPSC engineering technique

Reprogramming somatic cells into a pluripotent state is expected to initiate a new era in medicine. Because the precise underlying mechanism of reprogramming remains unclear, many efforts have been made to optimize induced pluripotent stem cell (iPSC) engineering. However, satisfactory results have not yet been attained. In this review, we focus on recent roadblocks in iPSC reprogramming engineering, such as the inefficiency of the process, tumorigenicity and heterogeneity of the generation. We conclude that cell reprogramming is a naturally occurring phenomenon rather than a biological technique. We will only be able to mimic the natural process of reprogramming when we fully understand its underlying mechanism. Finally, we highlight the alternative method of direct conversion, which avoids the use of iPSCs to generate cell materials for patient-specific cell therapy.     

Human adult pluripotency: Facts and questions

Cellular reprogramming and induced pluripotent stem cell(IPSC) technology demonstrated the plasticity of adult cell fate, opening a new era of cellular modelling and introducing a versatile therapeutic tool for regenerative medicine.While IPSCs are already involved in clinical trials for various regenerative purposes, critical questions concerning their medium-and long-term genetic and epigenetic stability still need to be answered. Pluripotent stem cells have been described in the last decades in various mammalian and human tissues(such as bone marrow, blood and adipose tissue). We briefly describe the characteristics of human-derived adult stem cells displaying in vitro and/or in vivo pluripotency while highlighting that the common denominators of their isolation or occurrence within tissue are represented by extreme cellular stress. Spontaneous cellular reprogramming as a survival mechanism favoured by senescence and cellular scarcity could represent an adaptative mechanism. Reprogrammed cells could initiate tissue regeneration or tumour formation dependent on the microenvironment characteristics. Systems biology approaches and lineage tracing within living tissues can be used to clarify the origin of adult pluripotent stem cells and their significance for regeneration and disease.     

Cellular reprogramming for clinical cartilage repair

The repair of articular cartilage needs a sufficient number of chondrocytes to replace the defect tissue, and therefore, expansion of cells is generally required. Chondrocytes derived by cellular reprogramming may provide a solution to the limitations of current (stem) cell-based therapies. In this article, two distinct approaches—induced pluripotent stem cell (iPSC)-mediated reprogramming and direct lineage conversion—are analysed and compared according to criteria that encompass the qualification of the method and the derived chondrocytes for the purpose of clinical application. Progress in iPSC generation has provided insights into the replacement of reprogramming factors by small molecules and chemical compounds. As follows, multistage chondrogenic differentiation methods have shown to improve the chondrocyte yield and quality. Nevertheless, the iPSC ‘detour’ remains a time- and cost-consuming approach. Direct conversion of fibroblasts into chondrocytes provides a slight advantage over these aspects compared to the iPSC detour. However, the requirement of constitutive transgene expression to inhibit hypertrophic differentiation limits this approach of being translated to the clinic. It can be concluded that the quality of the derived chondrocytes highly depends on the characteristics of the reprogramming method and that this is important to keep in mind during the experimental set-up. Further research into both reprogramming approaches for clinical cartilage repair has to include proper control groups and epigenetic profiling to optimize the techniques and eventually derive functionally stable articular chondrocytes.     

A poor imitation of a natural process

Reprogramming somatic cells into a pluripotent state is expected to initiate a new era in medicine. Because the precise underlying mechanism of reprogramming remains unclear, many efforts have been made to optimize induced pluripotent stem cell (iPSC) engineering. However, satisfactory results have not yet been attained. In this review, we focus on recent roadblocks in iPSC reprogramming engineering, such as the inefficiency of the process, tumorigenicity and heterogeneity of the generation. We conclude that cell reprogramming is a naturally occurring phenomenon rather than a biological technique. We will only be able to mimic the natural process of reprogramming when we fully understand its underlying mechanism. Finally, we highlight the alternative method of direct conversion, which avoids the use of iPSCs to generate cell materials for patient-specific cell therapy.