Expression of a novel secreted factor, Seraf indicates an early segregation of Schwann cell precursors from neural crest during avian development

The neural crest gives rise to glial cells in the peripheral nervous system. Among the peripheral glia, Schwann cells form the myelin often wrapping the peripheral axons. Compared to other crest-derived cell lineages such as neurons, the analysis of fate determination and subsequent differentiation of Schwann cells is not well advanced, partly due to the lack of early markers of this phenotype. In this study, we have identified a gene, uniquely expressed in avian embryo Schwann cell precursors, which encodes a novel secreted factor, designated Seraf (Schwann cell-specific EGF-like repeat autocrine factor). Expression of Seraf and P0 delineates the earliest phase of Schwann cell differentiation. Seraf binds to neural crest cells and Schwann cells, and affects the distribution of Schwann cells, when introduced to chicken embryos during neural crest migration. Our results suggest an autocrine function of Seraf and provide a significant step to understand the developmental processes of Schwann cell lineage.     

Membrane-bound neuregulin1 type III actively promotes Schwann cell differentiation of multipotent Progenitor cells

Many steps of peripheral glia development appear to be regulated by neuregulin1 (NRG1) signaling but the exact roles of the different NRG1 isoforms in these processes remain to be determined. While glial growth factor 2 (GGF2), a NRG1 type II isoform, is able to induce a satellite glial fate in neural crest stem cells, targeted mutations in mice have revealed a prominent role of NRG1 type III isoforms in supporting survival of Schwann cells at early developmental stages. Here, we investigated the role of NRG1 isoforms in the differentiation of Schwann cells from neural crest-derived progenitor cells. In multipotent cells isolated from dorsal root ganglia, soluble NRG1 isoforms do not promote Schwann cell features, whereas signaling by membrane-associated NRG1 type III induces the expression of the Schwann cell markers Oct-6/SCIP and S100 in neighboring cells, independent of survival. Thus, axon-bound NRG1 might actively promote both Schwann cell survival and differentiation.     

P0 and PMP22 mark a multipotent neural crest-derived cell type that displays community effects in response to TGF-beta family factors.

Protein zero (P0) and peripheral myelin protein 22 (PMP22) are most prominently expressed by myelinating Schwann cells as components of compact myelin of the peripheral nervous system (PNS), and mutants affecting P0 and PMP22 show severe defects in myelination. Recent expression studies suggest a role of P0 and PMP22 not only in myelination but also during embryonic development. Here we show that, in dorsal root ganglia (DRG) and differentiated neural crest cultures, P0 is expressed in the glial lineage whereas PMP22 is also detectable in neurons. In addition, however, P0 and PMP22 are both expressed in a multipotent cell type isolated from early DRG. Like neural crest stem cells (NCSCs), this P0/PMP22-positive cell gives rise to glia, neurons and smooth-muscle-like cells in response to instructive extracellular cues. In cultures of differentiating neural crest, a similar multipotent cell type can be identified in which expression of P0 and PMP22 precedes the appearance of neural differentiation markers. Intriguingly, this P0/PMP22-positive progenitor exhibits fate restrictions dependent on the cellular context in which it is exposed to environmental signals. While single P0/PMP22-positive progenitor cells can generate smooth muscle in response to factors of the TGF-(beta) family, communities of P0/PMP22-positive cells interpret TGF-(beta) factors differently and produce neurons or undergo increased cell death instead of generating smooth-muscle-like cells. Our data are consistent with a model in which cellular association of postmigratory multipotent progenitors might be involved in the suppression of a non-neural fate in forming peripheral ganglia.     

Neural crest progenitors and stem cells: from early development to adulthood

In the vertebrate embryo, the neural crest forms transiently in the dorsal neural primordium to yield migratory cells that will invade nearly all tissues and later, will differentiate into bones and cartilages, neurons and glia, endocrine cells, vascular smooth muscle cells and melanocytes. Due to the amazingly diversified array of cell types it produces, the neural crest is an attractive model system in the stem cell field. We present here in vivo and in vitro studies of single cell fate, which led to the discovery and the characterization of stem cells in the neural crest of avian and mammalian embryos. Some of the key issues in neural crest cell diversification are discussed, such as the time of segregation of mesenchymal vs. neural/melanocytic lineages, and the origin and close relationships between the glial and melanocytic lineages. An overview is also provided of the diverse types of neural crest-like stem cells and progenitors, recently identified in a growing number of adult tissues in animals and humans. Current and future work, in which in vivo lineage studies and the use of injury models will complement the in vitro culture analysis, should help in unraveling the properties and function of neural crest-derived progenitors in development and disease.     

Sequential specification of neurons and glia by developmentally regulated extracellular factors

Cortical progenitor cells give rise to neurons during embryonic development and to glia after birth. While lineage studies indicate that multipotent progenitor cells are capable of generating both neurons and glia, the role of extracellular signals in regulating the sequential differentiation of these cells is poorly understood. To investigate how factors in the developing cortex might influence cell fate, we developed a cortical slice overlay assay in which cortical progenitor cells are cultured over cortical slices from different developmental stages. We find that embryonic cortical progenitors cultured over embryonic cortical slices differentiate into neurons and those cultured over postnatal cortical slices differentiate into glia, suggesting that the fate of embryonic progenitors can be influenced by developmentally regulated signals. In contrast, postnatal progenitor cells differentiate into glial cells when cultured over either embryonic or postnatal cortical slices. Clonal analysis indicates that the postnatal cortex produces a diffusible factor that induces progenitor cells to adopt glial fates at the expense of neuronal fates. The effects of the postnatal cortical signals on glial cell differentiation are mimicked by FGF2 and CNTF, which induce glial fate specification and terminal glial differentiation respectively. These observations indicate that cell fate specification and terminal differentiation can be independently regulated and suggest that the sequential generation of neurons and glia in the cortex is regulated by a developmental increase in gliogenic signals.     

Glial cells more than support cells?

Glial cells are the most abundant cells in the human brain and have long been considered as passive supporting cells for neurons. In contrast to the extensive studies on various neuronal functions in the nervous system, we still have limited knowledge about glial cells. Recently a number of pioneering studies have provided convincing evidence that glia play active roles in development and function of the central nervous system. This review discusses recent advances in our understanding of the molecular mechanisms underlying glial cell differentiation. We then highlight some of the novel findings about glial function, i.e. the role of glia in synaptogenesis and the intricate relationship between astrocytes and adult neural stem cells. Finally, we summarize the emerging studies that implicate abnormalities in the formation or maintenance of glia leading to severe brain diseases, such as Alexander disease, glioblastoma and multiple sclerosis, and potential therapeutic strategies to tackle these diseases.     

Fate determination of neural crest cells by NOTCH-mediated lateral inhibition and asymmetrical cell division during gangliogenesis

Avian trunk neural crest cells give rise to a variety of cell types including neurons and satellite glial cells in peripheral ganglia. It is widely assumed that crest cell fate is regulated by environmental cues from surrounding embryonic tissues. However, it is not clear how such environmental cues could cause both neurons and glial cells to differentiate from crest-derived precursors in the same ganglionic locations. To elucidate this issue, we have examined expression and function of components of the NOTCH signaling pathway in early crest cells and in avian dorsal root ganglia. We have found that Delta1, which encodes a NOTCH ligand, is expressed in early crest-derived neuronal cells, and that NOTCH1 activation in crest cells prevents neuronal differentiation and permits glial differentiation in vitro. We also found that NUMB, a NOTCH antagonist, is asymmetrically segregated when some undifferentiated crest-derived cells in nascent dorsal root ganglia undergo mitosis. We conclude that neuron-glia fate determination of crest cells is regulated, at least in part, by NOTCH-mediated lateral inhibition among crest-derived cells, and by asymmetric cell division.     

Neuregulins in Glial Cells

The role of growth factors in controlling the development of glial cells in both the peripheral and central nervous systems has been investigated for a number of years. The recent discovery of a new family of growth factors termed the neuregulins (NRGs) has led to an explosion of information concerning the putative role of these growth factors in the development of Schwann cells (SC), oligodendrocytes (OLG), and astrocytes. Many of these previous studies have focused on the effects of exogenous NRGs on glial cell development and differentiation. We now review the evidence that these glial cells themselves produce NRGs and discuss the major implications of these findings with respect to glial cell development and diseases which affect glial cell function. We also discuss the potential role of endogenous NRGs following neural injury.     

Isolation of a stem cell for neurons and glia from the mammalian neural crest.

We have isolated mammalian neural crest cells using a monoclonal antibody to the low affinity NGF receptor, and established conditions for the serial propagation of these cells in clonal culture to assess their developmental potential. This analysis indicates that, first, single mammalian neural crest cells are multipotent, able to generate at least neurons and Schwann cells like their avian counterparts. Second, multipotent neural crest cells generate multipotent progeny, indicating that they are capable of self-renewal and therefore are stem cells. Third, multipotent neural crest cells also generate some clonal progeny that form only neurons or glia, suggesting the production of committed neuroblasts and glioblasts. Manipulation of the substrate alters the fate of the multipotent cells. These findings have implications for models of neural crest development in vivo, and establish a system for studying the generation of cellular diversity by a multipotent stem cell in vitro.     

In the beginning: Generating neural crest cell diversity

Neural crest cells (NCCs) are migratory cells that delaminate from the neural tube early in development and then disseminate throughout the embryo to give rise to a wide variety of cell types that are key to the vertebrate body plan. During their journey from the neural tube to their peripheral targets, NCCs progressively differentiate, raising the question of when the fate of an individual NCC is sealed. One hypothesis suggests that the fate of a NCC is specified by target-derived signals emanating from the environment they migrate through, while another hypothesis proposes that NCCs are already specified to differentiate along select lineages at the time they are born in the neural tube, with environmental signals helping them to realize their prespecified fate potential. Alternatively, both mechanisms may cooperate to drive NCC diversity. This review highlights recent advances in our understanding of prespecification during trunk NCC development.Key words: neural crest cell, multipotent, prespecification, neuropilin, semaphorin, migration, cell fate