Plasticité phénotypique des mélanocytes et des cellules de Schwann

Phenotypic plasticity of neural crest-derived melanocytes and Schwann cells

Inserm U894 Équipe Plasticité Gliale, Centre Psychiatrie et Neurosciences, 2 ter rue d’Alésia, 75014 Paris, France

Auteur correspondant : Elisabeth Dupin, elisabeth.dupin@inserm.fr

Reçu : 1 Février 2011

Résumé

La notion selon laquelle la différenciation cellulaire serait unidirectionnelle et irréversible a été contestée dans plusieurs systèmes biologiques. Des travaux récents ont révélé qu’il était même possible, à l’aide de seulement quelques gènes, de manipuler des cellules différenciées afin de changer leur phénotype ou de leur conférer des propriétés de cellules souches. Nous avons abordé cette question de la stabilité de l’état différencié d’une cellule, dans le modèle des lignages issus de la crête neurale (CN). Cette structure ectodermique transitoire de l’embryon de vertébrés donne naissance, entre autres dérivés, aux mélanocytes de la peau et aux cellules de Schwann des nerfs périphériques. Ces deux types cellulaires ont pour origine un progéniteur commun, lui-même issu de cellules souches multipotentes présentes aux stades précoces du développement de la CN. Nous avons analysé le devenir de mélanocytes et de cellules de Schwann isolés chez l’embryon de caille lorsque ces cellules sont maintenues en culture clonale in vitro en présence d’un facteur mitogène, l’endothéline-3 (EDN3). Ces conditions induisent la prolifération et la dé-différenciation rapide des mélanocytes et des cellules de Schwann, qui engendrent alors à la fois des cellules gliales, des mélanocytes et des myofibroblastes. Par des expériences de sous-clonages in vitro, nous avons pu démontrer que cette plasticité phénotypique s’effectuait par un retour vers un état plus immature, équivalent à celui d’un progéniteur multipotent et doué d’auto-renouvellement présent dans la CN. Ainsi, après avoir été isolées de leur tissu d’origine, les cellules pigmentaires et les cellules de Schwann dérivées de la CN font preuve d’une grande instabilité phénotypique et sont capables, sous l’influence d’un nouvel environnement in vitro et en absence de modification génétique induite, d’acquérir ou de ré-activer des propriétés de cellules souches.

Abstract

Melanocytes, the pigmented cells of the skin, and the glial Schwann cells lining peripheral nerves are developmentally derived from an early and transient ectodermal structure of the vertebrate embryo, the neural crest, which is also at the origin of multiple neural and non-neural cell types. Besides melanocytes and neural cells of the peripheral nervous system, the neural crest cells give rise to mesenchymal cell types in the head, which form most of the craniofacial skeleton, dermis, fat tissue and vascular musculo-connective components. How such a wide diversity of differentiation fates is established during embryogenesis and is later maintained in adult tissues are among key questions in developmental and stem cell biology. The analysis of the developmental potentials of single neural crest cells cultured in vitro led to characterizing multipotent stem/progenitor cells as well as more restricted precursors in the early neural crest of avian and mammalian embryos. Data support a hierarchical model of the diversification of neural crest lineages through progressive restrictions of multipotent stem cell potentials driven by local environmental factors. In particular, melanocytes and glial Schwann cells were shown to arise from a common bipotent progenitor, which depends upon the peptide endothelin-3 for proliferation and self-renewal ability. In vivo, signaling by endothelin-3 and its receptor is also required for the early development of melanocytes and proper pigmentation of the vertebrate body. It is generally assumed that, after lineage specification and terminal differentiation, specialized cell types, like the melanocytes and Schwann cells, do not change their identity. However, this classic notion that somatic cell differentiation is a stable and irreversible process has been challenged by emerging evidence that dedifferentiation can occur in different biological systems through nuclear transfer, cell fusion, epigenetic modifications and ectopic gene expression. This review considers the issue of whether neural crest-derived lineages are endowed with some phenotypic plasticity. Emphasis is put on the ability of pigment cells and Schwann cells to dedifferentiate and reprogram their fate in vitro. To address this question, we have studied the clonal progeny of differentiated Schwann cells and melanocytes after their isolation from the sciatic nerve and the back skin of quail embryos, respectively. When stimulated to proliferate in vitro in the presence of endothelin-3, both cell types were able to dedifferentiate and produce alternative neural crest-derived cell lineages. Individual Schwann cells isolated by FACS, using a glial-specific surface marker, gave rise in culture to pigment cells and myofibroblasts/smooth muscle cells. Treatment of the cultures with endothelin-3 was required for Schwann cell conversion into melanocytes, which involved acquisition of multipotency. Moreover, Schwann cell plasticity could also be induced in vivo: following transplantation into the branchial arch of a young chick host embryo, dedifferentiating Schwann cells were able to integrate the forming head structures of the host and, specifically, to contribute smooth muscle cells to the wall of cranial blood vessels. We also analyzed the in vitro behavior of individual pigment cells obtained by microdissection and enzymatic treatment of quail epidermis at embryonic and hatching stages. In single cell cultures treated with endothelin-3, pigment cells strongly proliferated while rapidly dedifferentiating into unpigmented cells, leading to the formation of large colonies that comprised glial cells and myofibroblasts in addition to melanocytes. By serially subcloning these primary colonies, we could efficiently propagate a bipotent glial-melanocytic precursor that is generated in the progeny of the melanocytic founder. These data therefore suggest that pigment cells have the ability to revert back to the state of self-renewing neural crest-like progenitors. Altogether, these studies have shown that Schwann cells and pigment cells display an unstable status of differentiation, which can be disclosed if these differentiated cells are displaced out of their native tissue. When challenged with new environmental conditions in vitro, differentiated Schwann cells and pigment cells can reacquire stem cell properties of their neural crest ancestors. Notably, suchreprogramming was achieved through the effect of a single exogenous factor and without the need of any induced genetic modification. Deciphering the cellular and molecular mechanisms that regulate the plasticity and maintenance of neural crest-derived differentiated cells is likely to be an important step towards the understanding of the neurocristopathies and cancers that target neural crest derivatives in humans.