Distinct adhesion-independent functions of ß-catenin control stage-specific sensory neurogenesis and proliferation.

BACKGROUND: ß-catenin plays a central role in multiple developmental processes. However, it has been difficult to study its pleiotropic effects, because of the dual capacity of ß-catenin to coordinate cadherin-dependent cell adhesion and to act as a component of Wnt signal transduction. To distinguish between the divergent functions of ß-catenin during peripheral nervous system development, we made use of a mutant allele of ß-catenin that can mediate adhesion but not Wnt-induced TCF transcriptional activation. This allele was combined with various conditional inactivation approaches. RESULTS: We show that of all peripheral nervous system structures, only sensory dorsal root ganglia require ß-catenin for proper formation and growth. Surprisingly, however, dorsal root ganglia development is independent of cadherin-mediated cell adhesion. Rather, both progenitor cell proliferation and fate specification are controlled by ß-catenin signaling. These can be divided into temporally sequential processes, each of which depends on a different function of ß-catenin. CONCLUSIONS: While early stage proliferation and specific Neurog2- and Krox20-dependent waves of neuronal subtype specification involve activation of TCF transcription, late stage progenitor proliferation and Neurog1-marked sensory neurogenesis are regulated by a function of ß-catenin independent of TCF activation and adhesion. Thus, switching modes of ß-catenin function are associated with consecutive cell fate specification and stage-specific progenitor proliferation.

Temporal control of neural crest lineage generation by Wnt/ß-catenin signaling.

Wnt/ß-catenin signaling controls multiple steps of neural crest development, ranging from neural crest induction, lineage decisions, to differentiation. In mice, conditional ß-catenin inactivation in premigratory neural crest cells abolishes both sensory neuron and melanocyte formation. Intriguingly, the generation of melanocytes is also prevented by activation of ß-catenin in the premigratory neural crest, which promotes sensory neurogenesis at the expense of other neural crest derivatives. This raises the question of how Wnt/ß-catenin signaling regulates the formation of distinct lineages from the neural crest. Using various Cre lines to conditionally activate ß-catenin in neural crest cells at different developmental stages, we show that neural crest cell fate decisions in vivo are subject to temporal control by Wnt/ß-catenin. Unlike in premigratory neural crest, ß-catenin activation in migratory neural crest cells promotes the formation of ectopic melanoblasts, while the production of most other lineages is suppressed. Ectopic melanoblasts emerge at sites of neural crest target structures and in many tissues usually devoid of neural crest-derived cells. ß-catenin activation at later stages in glial progenitors or in melanoblasts does not lead to surplus melanoblasts, indicating a narrow time window of Wnt/ß-catenin responsiveness during neural crest cell migration. Thus, neural crest cells appear to be multipotent in vivo both before and after emigration from the neural tube but adapt their response to extracellular signals in a temporally controlled manner.

Lineage-specific requirements of beta-catenin in neural crest development.

Beta-catenin plays a pivotal role in cadherin-mediated cell adhesion. Moreover, it is a downstream signaling component of Wnt that controls multiple developmental processes such as cell proliferation, apoptosis, and fate decisions. To study the role of beta-catenin in neural crest development, we used the Cre/loxP system to ablate beta-catenin specifically in neural crest stem cells. Although several neural crest-derived structures develop normally, mutant animals lack melanocytes and dorsal root ganglia (DRG). In vivo and in vitro analyses revealed that mutant neural crest cells emigrate but fail to generate an early wave of sensory neurogenesis that is normally marked by the transcription factor neurogenin (ngn) 2. This indicates a role of beta-catenin in premigratory or early migratory neural crest and points to heterogeneity of neural crest cells at the earliest stages of crest development. In addition, migratory neural crest cells lateral to the neural tube do not aggregate to form DRG and are unable to produce a later wave of sensory neurogenesis usually marked by the transcription factor ngn1. We propose that the requirement of beta-catenin for the specification of melanocytes and sensory neuronal lineages reflects roles of beta-catenin both in Wnt signaling and in mediating cell-cell interactions.

Cell-intrinsic and cell-extrinsic cues regulating lineage decisions in multipotent neural crest-derived progenitor cells.

Multipotent stem cells must generate various differentiated cell types in correct number and sequence during neural development. In the peripheral nervous system (PNS), this involves the formation of postmigratory progenitor cell types which maintain multipotency and are able to give rise to neural and non-neural cells in response to instructive growth factors. We propose that fate restrictions in such progenitor cells are controlled by the combinatorial interaction of different extracellular signals, including community effects in response to both neurogenic and gliogenic factors. In addition, distinct progenitor cell types display intrinsic differences which modulate their response to the extracellular environment. Thus, a progenitor cell is apparently able to integrate multiple intrinsic and extrinsic cues and thereby to choose fates appropriate for its location. Fate analysis of genetically modified progenitor cells will help to identify the molecules involved. This approach appears promising given the identification of multipotent progenitor cells from the mouse PNS and the availability of genetics in the mouse system.

Sox10 promotes the formation and maintenance of giant congenital naevi and melanoma.

Giant congenital naevi are pigmented childhood lesions that frequently lead to melanoma, the most aggressive skin cancer. The mechanisms underlying this malignancy are largely unknown, and there are no effective therapies. Here we describe a mouse model for giant congenital naevi and show that naevi and melanoma prominently express Sox10, a transcription factor crucial for the formation of melanocytes from the neural crest. Strikingly, Sox10 haploinsufficiency counteracts Nras(Q61K)-driven congenital naevus and melanoma formation without affecting the physiological functions of neural crest derivatives in the skin. Moreover, Sox10 is also crucial for the maintenance of neoplastic cells in vivo. In human patients, virtually all congenital naevi and melanomas are SOX10 positive. Furthermore, SOX10 silencing in human melanoma cells suppresses neural crest stem cell properties, counteracts proliferation and cell survival, and completely abolishes in vivo tumour formation. Thus, SOX10 represents a promising target for the treatment of congenital naevi and melanoma in human patients.

Instructive role of Wnt/beta-catenin in sensory fate specification in neural crest stem cells.

Wnt signaling has recently emerged as a key factor in controlling stem cell expansion. In contrast, we show here that Wnt/beta-catenin signal activation in emigrating neural crest stem cells (NCSCs) has little effect on the population size and instead regulates fate decisions. Sustained beta-catenin activity in neural crest cells promotes the formation of sensory neural cells in vivo at the expense of virtually all other neural crest derivatives. Moreover, Wnt1 is able to instruct early NCSCs (eNCSCs) to adopt a sensory neuronal fate in a beta-catenin-dependent manner. Thus, the role of Wnt/beta-catenin in stem cells is cell-type dependent.