Direct cellular reprogramming in Caenorhabditis elegans: facts, models, and promises for regenerative medicine

Developmental Plasticity and Cellular Reprogramming in Caenorhabditis elegans

While Caenorhabditis elegans was originally regarded as a model for investigating determinate developmental programs, landmark studies have subsequently shown that the largely invariant pattern of development in the animal does not reflect irreversibility in rigidly fixed cell fates. Rather, cells at all stages of development, in both the soma and germline, have been shown to be capable of changing their fates through mutation or forced expression of fate-determining factors, as well as during the normal course of development. In this chapter, we review the basis for natural and induced cellular plasticity in C. elegans We describe the events that progressively restrict cellular differentiation during embryogenesis, starting with the multipotency-to-commitment transition (MCT) and subsequently through postembryonic development of the animal, and consider the range of molecular processes, including transcriptional and translational control systems, that contribute to cellular plasticity. These findings in the worm are discussed in the context of both classical and recent studies of cellular plasticity in vertebrate systems.

The multipotency-to-commitment transition in Caenorhabditis elegans-implications for reprogramming from cells to organs

In animal embryos, cells transition from a multipotential state, with the capacity to adopt multiple fates, into an irreversible, committed state of differentiation. This multipotency-to-commitment transition (MCT) is evident from experiments in which cell fate is reprogrammed by transcription factors for cell type-specific differentiation, as has been observed extensively in Caenorhabditis elegans. Although factors that direct differentiation into each of the three germ layer types cannot generally reprogram cells after the MCT in this animal, transcription factors for endoderm development are able to do so in multiple differentiated cell types. In one case, these factors can redirect the development of an entire organ in the process of "transorganogenesis". Natural transdifferentiation also occurs in a small number of differentiated cells during normal C. elegans development. We review these reprogramming and transdifferentiation events, highlighting the cellular and developmental contexts in which they occur, and discuss common themes underlying direct cell lineage reprogramming. Although certain aspects may be unique to the model system, growing evidence suggests that some mechanisms are evolutionarily conserved and may shed light on cellular plasticity and disease in humans.

Wound healing, cellular regeneration and plasticity: the elegans way

Regeneration and wound healing are complex processes that allow organs and tissues to regain their integrity and functionality after injury. Wound healing, a key property of epithelia, involves tissue closure that in some cases leads to scar formation. Regeneration, a process rather limited in mammals, is the capacity to regrow (parts of) an organ or a tissue, after damage or amputation. What are the properties of organs and the features of tissue permitting functional regrowth and repair? What are the cellular and molecular mechanisms underlying these processes? These questions are crucial both in fundamental and applied contexts, with important medical implications. The mechanisms and cells underlying tissue repair have thus been the focus of intense investigation. The last decades have seen rapid progress in the domain and new models emerging. Here, we review the fundamental advances and the perspectives that the use of C. elegans as a model have brought to the mechanisms of wound healing and cellular plasticity, axon regeneration and transdifferentiation in vivo.

Programming and reprogramming the brain: a meeting of minds in neural fate

In early April 2017, over 130 delegates met in Munich, Germany, to discuss the latest research in the development and reprogramming of cells of the nervous system. The conference, which was organised by Abcam and entitled 'Programming and Reprogramming the Brain', was a great success, and provided an excellent snapshot of the current state of the field, and what the challenges are for the future. This Meeting Review provides a summary of the talks presented and the major themes that emerged from the conference.

The Caenorhabditis elegans Excretory System: A Model for Tubulogenesis, Cell Fate Specification, and Plasticity

The excretory system of the nematode Caenorhabditis elegans is a superb model of tubular organogenesis involving a minimum of cells. The system consists of just three unicellular tubes (canal, duct, and pore), a secretory gland, and two associated neurons. Just as in more complex organs, cells of the excretory system must first adopt specific identities and then coordinate diverse processes to form tubes of appropriate topology, shape, connectivity, and physiological function. The unicellular topology of excretory tubes, their varied and sometimes complex shapes, and the dynamic reprogramming of cell identity and remodeling of tube connectivity that occur during larval development are particularly fascinating features of this organ. The physiological roles of the excretory system in osmoregulation and other aspects of the animal's life cycle are only beginning to be explored. The cellular mechanisms and molecular pathways used to build and shape excretory tubes appear similar to those used in both unicellular and multicellular tubes in more complex organs, such as the vertebrate vascular system and kidney, making this simple organ system a useful model for understanding disease processes.

Fine-tuning of chromatin composition and Polycomb recruitment by two Mi2 homologues during C. elegans early embryonic development

BACKGROUND

The nucleosome remodeling and deacetylase complex promotes cell fate decisions throughout embryonic development. Its core enzymatic subunit, the SNF2-like ATPase and Helicase Mi2, is well conserved throughout the eukaryotic kingdom and can be found in multiple and highly homologous copies in all vertebrates and some invertebrates. However, the reasons for such duplications and their implications for embryonic development are unknown.

RESULTS

Here we studied the two C. elegans Mi2 homologues, LET-418 and CHD-3, which displayed redundant activities during early embryonic development. At the transcriptional level, these two Mi2 homologues redundantly repressed the expression of a large gene population. We found that LET-418 physically accumulated at TSS-proximal regions on transcriptionally active genomic targets involved in growth and development. Moreover, LET-418 acted redundantly with CHD-3 to block H3K4me3 deposition at these genes. Our study also revealed that LET-418 was partially responsible for recruiting Polycomb to chromatin and for promoting H3K27me3 deposition. Surprisingly, CHD-3 displayed opposite activities on Polycomb, as it was capable of moderating its LET-418-dependent recruitment and restricted the amount of H3K27me3 on the studied target genes.

CONCLUSION

Although closely homologous, LET-418 and CHD-3 showed both redundant and opposite functions in modulating the chromatin environment at developmental target genes. We identified the interplay between LET-418 and CHD-3 to finely tune the levels of histone marks at developmental target genes. More than just repressors, Mi2-containing complexes appear as subtle modulators of gene expression throughout development. The study of such molecular variations in vertebrate Mi2 counterparts might provide crucial insights to our understanding of the epigenetic control of early development.

Simultaneous expression of multiple proteins under a single promoter in Caenorhabditis elegans via a versatile 2A-based toolkit

Caenorhabditis elegans is a powerful in vivo model in which transgenesis is highly developed. However, while the analysis of biological phenomena often require the expression of more than one protein of interest, no reliable tool exists to ensure efficient concomitant and equivalent expression of more than two polypeptides from a single promoter. We report the use of viral 2A peptides, which trigger a "ribosomal-skip" or "STOP&GO" mechanism during translation, to express multiple proteins from a single vector in C. elegans. Although none of the viruses known to infect C. elegans contain 2A-like sequences, our results show that 2A peptides allow the production of separate functional proteins in all cell types and at all developmental stages tested in the worm. In addition, we constructed a toolkit including a 2A-based polycistronic plasmid and reagents to generate 2A-tagged fosmids. 2A peptides constitute an important tool to ensure the delivery of multiple polypeptides in specific cells, enabling several novel applications such as the reconstitution of multi-subunit complexes.