Neural crest boundary cap cells constitute a source of neuronal and glial cells of the PNS

Intracerebroventricular administration of nerve growth factor induces gliogenesis in sensory ganglia, dorsal root, and within the dorsal root entry zone

Previous studies indicated that intracerebroventricular administration of nerve growth factor (NGF) leads to massive Schwann cell hyperplasia surrounding the medulla oblongata and spinal cord. This study was designed to characterize the proliferation of peripheral glial cells, that is, Schwann and satellite cells, in the trigeminal ganglia and dorsal root ganglia (DRG) of adult rats during two weeks of NGF infusion using bromodeoxyuridine (BrdU) to label dividing cells. The trigeminal ganglia as well as the cervical and lumbar DRG were analyzed. Along the entire neuraxis a small number of dividing cells were observed within these regions under physiological condition. NGF infusion has dramatically increased the generation of new cells in the neuronal soma and axonal compartments of sensory ganglia and along the dorsal root and the dorsal root entry zone. Quantification of BrdU positive cells within sensory ganglia revealed a 2.3- to 3-fold increase in glial cells compared to controls with a similar response to NGF for the different peripheral ganglia examined. Immunofluorescent labeling with S100β revealed that Schwann and satellite cells underwent mitosis after NGF administration. These data indicate that intracerebroventricular NGF infusion significantly induces gliogenesis in trigeminal ganglia and the spinal sensory ganglia and along the dorsal root entry zone as well as the dorsal root.

Neurotrophin signalling and transcription programmes interactions in the development of somatosensory neurons

Somatosensory neurons of the dorsal root ganglia are generated from multipotent neural crest cells by a process of progressive specification and differentiation. Intrinsic transcription programmes active in somatosensory neuron progenitors and early post-mitotic neurons drive the cell-type expression of neurotrophin receptors. In turn, signalling by members of the neurotrophin family controls expression of transcription factors that regulate neuronal sub-type specification. This chapter explores the mechanisms by which this crosstalk between neurotrophin signalling and transcription programmes generates the diverse functional sub-types of somatosensory neurons found in the mature animal.

A local source of FGF initiates development of the unmyelinated lineage of sensory neurons

The principle by which unmyelinated primary sensory neurons transducing thermal, itch and pain perception are specified in early development is unknown. These classes of sensory neurons diversify from a common population of late-born neurons, which initiate expression of Runt homology domain transcription factor RUNX1 and the nerve growth factor receptor TrkA. Here, we report that signals emanating from within the mouse dorsal root ganglion mediated partly by early-born neurons destined to a myelinated phenotype participate in fating late-born RUNX1(+)/TrkA(+) neurons. Inductive factors include FGFs via activation of FGF receptor 1 (FGFR1). Consistently, FGF2 is sufficient to induce expression of RUNX1, and Fgfr1 conditional mutant mice display deficits in fating of the common population of late-born RUNX1(+)/TrkA(+) neurons that develop into unmyelinated neurons. Thus, the distinct lineages of sensory neurons are acquired in response to increasing FGF levels provided by a rising number of born neurons.

Basic fibroblast growth factor is a key factor that induces bone marrow mesenchymal stem cells towards cells with Schwann cell phenotype

Bone marrow mesenchymal stem cells (MSCs) can be differentiate towards a Schwann cells (SCs) lineage when exposed to pre-inducing reagents β-mercaptoethanol (BME) and retinoic acid (RA), followed by inducing factors: forskolin (FSK), basic fibroblast growth factor (bFGF), platelet derived growth factor (PDGF), and heregulin (HRG). However, the underlying mechanisms remain unclear. Here, we investigated the individual effects of these inducing factors on the differentiation of MSCs towards SC phenotype in rats. We show that the omission of either HRG or PDGF from the induction medium is not sufficient to change the SC-like phenotype or the expression level of the SC marker, S100β. However, the omission of bFGF from the induction medium effectively blocked neural induction of the MSCs. Moreover, only bFGF was found to inhibit MSC proliferation during differentiation. To clarify the mechanism responsible for the effect of bFGF, we also investigated the activation of the extracellular signal-regulated kinase (ERK) pathway in the induced cells. Our results suggest that morphological changes in MSCs induced by bFGF depend on the activation of ERK, and bFGF may be an indispensable factor that induces MSCs to differentiate into cells with SCs phenotype.

Tissue interactions in neural crest cell development and disease

The neural crest is a transient population of migratory cells in the embryo that gives rise to a wide variety of different cell types, including those of the peripheral nervous system. Dysfunction of neural crest cells (NCCs) is associated with multiple diseases, such as neuroblastoma and Hirschsprung disease. Recent studies have identified NCC behaviors during their migration and differentiation, with implications for their contributions to development and disease. Here, we describe how interactions between cells of the neural crest and lineages such as the vascular system, as well as those involving environmental signals and microbial pathogens, are critically important in determining the roles played by these cells.

The structural role of radial glial endfeet in confining spinal motor neuron somata is controlled by the Reelin and Notch pathways

Neuronal migration is a fundamental biological process that enables the precise positioning of neurons to form functional circuits. Cortical neurons migrate along glial scaffolds formed by radial glia guided by Reelin ligand. However, it is unclear whether the Reelin-directed behavior of radial glia is also critical for positioning the spinal neurons. Here we demonstrate a novel role of radial glia that confines motor neurons within the neural tube and is promoted by Reelin and Notch signaling. Spinal radial glia express the Dab1 adaptor for Reelin signaling and are surrounded by Reelin. In reeler mice, in which Reelin is absent, ectopic motor neurons are found outside the neural tube, although they appear to maintain their identity. Boundary cap (BC) cells, Schwann cell precursors and the basal lamina at motor exit points are intact, whereas the glia limitans of radial glia are disorganized and detached from the basement membrane. The sparse and irregular radial scaffold is wide enough to allow motor somata to pass. Forced activation of Notch signaling rescued the structural defects in radial glia in reeler mice and the appearance of extraspinal neurons. In the absence of Reelin, Notch intracellular domain (NICD) protein level was reduced. In addition, disrupting the radial glia scaffold by destroying its polarity induced ectopic motor neurons in chick embryos. These findings suggest that activation of the Notch pathways by Reelin is required to establish the radial glial scaffold, a structure that actively constrains motor neuron somata and specifies the CNS-PNS boundary.

Smad-interacting protein 1 affects acute and tonic, but not chronic pain

BACKGROUND

Smad-interacting protein 1 (also named Zeb2 and Zfhx1b) is a transcription factor that plays an important role in neuronal development and, when mutated, causes Mowat-Wilson syndrome (MWS). A corresponding mouse model carrying a heterozygous Zeb2 deletion was comprehensively analysed in the German Mouse Clinic. The most prominent phenotype was the reduced pain sensitivity. In this study, we investigated the role of Zeb2 in inflammatory and neuropathic pain.

METHODS

For this, we tested mutant Zeb2 animals in different models of inflammatory pain like abdominal constriction, formalin and carrageenan test. Furthermore, we studied the pain reactivity of the mice after peripheral nerve ligation. To examine the nociceptive transmission of primary sensory dorsal root ganglia (DRG) neurons, we determined the neuronal activity in the spinal dorsal horn after the formalin test using staining of c-Fos. Next, we characterized the neuronal cell population in the DRGs and in the sciatic nerve to study the effect of the Zeb2 mutation on peripheral nerve morphology.

RESULTS

The present data show that Zeb2 is involved in the development of primary sensory DRG neurons, especially of C- and Aδ fibres. These alterations contribute to a hypoalgesic phenotype in inflammatory but not in neuropathic pain in these Zeb2(+/-) mice.

CONCLUSION

Our data suggest that the under-reaction to pain observed in MWS patients results from a reduced responsivity to nociceptive stimulation rather than an inability to communicate discomfort.

Plastic fantastic: Schwann cells and repair of the peripheral nervous system

Repair in the peripheral nervous system (PNS) depends upon the plasticity of the myelinating cells, Schwann cells, and their ability to dedifferentiate, direct axonal regrowth, remyelinate, and allow functional recovery. The ability of such an exquisitely specialized myelinating cell to revert to an immature dedifferentiated cell that can direct repair is remarkable, making Schwann cells one of the very few regenerative cell types in our bodies. However, the idea that the PNS always repairs after injury, in contrast to the central nervous system, is not true. Repair in patients after nerve trauma can be incredibly variable, depending on the site and type of injury, and only a relatively small number of axons may fully regrow and reinnervate their targets. Recent research has shown that it is an active process that drives Schwann cells back to an immature state after injury and that this requires activity of the p38 and extracellular-regulated kinase 1/2 mitogen-activated protein kinases, as well as the transcription factor cJun. Analysis of the events after peripheral nerve transection has shown how signaling from nerve fibroblasts forms Schwann cells into cords in the newly generated nerve bridge, via Sox2 induction, to allow the regenerating axons to cross the gap. Understanding these pathways and identifying additional mechanisms involved in these processes raises the possibility of both boosting repair after PNS trauma and even, possibly, blocking the inappropriate demyelination seen in some disorders of the peripheral nervous system.

Differential cellular localization of antioxidant enzymes in the trigeminal ganglion

Because of its high oxygen demands, neural tissue is predisposed to oxidative stress. Here, our aim was to clarify the cellular localization of antioxidant enzymes in the trigeminal ganglion. We found that the transcriptional factor Sox10 is localized exclusively in satellite glial cells (SGCs) in the adult trigeminal ganglion. The use of transgenic mice that express the fluorescent protein Venus under the Sox10 promoter enabled us to distinguish between neurons and SGCs. Although both superoxide dismutases 1 and 2 were present in the neurons, only superoxide dismutase 1 was identified in SGCs. The enzymes relevant to hydrogen peroxide degradation displayed differential cellular localization, such that neurons were endowed with glutathione peroxidase 1 and thioredoxin 2, and catalase and thioredoxin 2 were present in SGCs. Our immunohistochemical finding showed that only SGCs were labeled by the oxidative damage marker 8-hydroxy-2'-deoxyguanosine, which indicates that the antioxidant systems of SGCs were less potent. The transient receptor potential vanilloid subfamily member 1 (TRPV1), the capsaicin receptor, is implicated in inflammatory hyperalgesia, and we demonstrated that topical capsaicin application causes short-lasting mechanical hyperalgesia in the face. Our cell-based assay revealed that TRPV1 agonist stimulation in the presence of TRPV1 overexpression caused reactive oxygen species-mediated caspase-3 activation. Moreover, capsaicin induced the cellular demise of primary TRPV1-positive trigeminal ganglion neurons in a dose-dependent manner, and this effect was inhibited by a free radical scavenger and a pancaspase inhibitor. This study delineates the localization of antioxidative stress-related enzymes in the trigeminal ganglion and reveals the importance of the pivotal role of reactive oxygen species in the TRPV1-mediated caspase-dependent cell death of trigeminal ganglion neurons. Therapeutic measures for antioxidative stress should be taken to prevent damage to trigeminal primary sensory neurons in inflammatory pain disorders.

Targeted knock-down of cellular prion protein expression in myelinating Schwann cells does not alter mouse prion pathogenesis

In naturally acquired transmissible spongiform encephalopathies, the pathogenic agents or prions spread from the sites of initial peripheral uptake or replication to the brain where they cause progressive and fatal neurodegeneration. Routing via the peripheral nervous system is considered to be one of the main pathways to the central nervous system. Replication of prions in Schwann cells is viewed as a potentially important mechanism for efficient prion spread along nerves. Here we used a Cre-loxP mouse transgenetic approach to disrupt host-encoded prion protein (PrP(C)) specifically in myelinating Schwann cells. Despite the use of infection routes targeting highly myelinated nerves, there was no alteration in mouse prion pathogenesis, suggesting that conversion-dependent, centripetal spread of prions does not crucially rely on PrP(C) expressed by myelinating Schwann cells.

Signals and switches in Mammalian neural crest cell differentiation

Neural crest cells (NCCs) comprise a multipotent, migratory cell population that generates a diverse array of cell and tissue types during vertebrate development. These include cartilage and bone, tendons, and connective tissue, as well as neurons, glia, melanocytes, and endocrine and adipose cells; this remarkable lineage potential persists into adult life. Taken together with a limited capacity for self-renewal, neural crest cells bear the hallmarks of stem and progenitor cells and are considered to be synonymous with vertebrate evolution. The neural crest has provided a system for exploring the mechanisms that govern developmental processes such as morphogenetic induction, cell migration, and fate determination. Today, much of the focus on neural crest cells revolves around their stem cell-like characteristics and potential for use in regenerative medicine. A thorough understanding of the signals and switches that govern mammalian neural crest patterning is central to potential therapeutic application of these cells and better appreciation of the role that neural crest cells play in vertebrate evolution, development, and disease.

Gross anatomy and development of the peripheral nervous system

The nervous system is divided into the central nervous system (CNS) composed of the brain, the brainstem, the cerebellum, and the spinal cord and the peripheral nervous system (PNS) made up of the different nerves arising from the CNS. The PNS is divided into the cranial nerves III to XII supplying the head and the spinal nerves that supply the upper and lower limbs. The general anatomy of the PNS is organized according to the arrangement of the fibers along the rostro-caudal axis. The control of the development of the PNS has been unravelled during the last 30 years. Motor nerves arise from the ventral neural tube. This ventralization is induced by morphogenetic molecules such as sonic hedgehog. In contrast, the sensory elements of the PNS arise from a specific population of cells originating from the roof of the neural tube, namely the neural crest. These cells give rise to the neurons of the dorsal root ganglia, the autonomic ganglia and the paraganglia including the adrenergic neurons of the adrenals. Furthermore, the supportive glial Schwann cells of the PNS originate from the neural crest cells. Growth factors as well as myelinating proteins are involved in the development of the PNS.

Reconstitution of the central and peripheral nervous system during salamander tail regeneration

We show that after tail amputation in Ambystoma mexicanum (Axolotl) the correct number and spacing of dorsal root ganglia are regenerated. By transplantation of spinal cord tissue and nonclonal neurospheres, we show that the central spinal cord represents a source of peripheral nervous system cells. Interestingly, melanophores migrate from preexisting precursors in the skin. Finally, we demonstrate that implantation of a clonally derived spinal cord neurosphere can result in reconstitution of all examined cell types in the regenerating central spinal cord, suggesting derivation of a cell with spinal cord stem cell properties.

Engineering an integrated cellular interface in three-dimensional hydrogel cultures permits monitoring of reciprocal astrocyte and neuronal responses

This study reports a new type of three-dimensional (3D) tissue model for studying interactions between cell types in collagen hydrogels. The aim was to create a 3D cell culture model containing separate cell populations in close proximity without the presence of a mechanical barrier, and demonstrate its relevance to modeling the axon growth-inhibitory cellular interfaces that develop in the central nervous system (CNS) in response to damage. This provides a powerful new tool to determine which aspects of the astroglial scar response and subsequent neuronal regeneration inhibition are determined by the presence of the other cell types. Astrocytes (CNS glia) and dissociated dorsal root ganglia (DRG; containing neurons and peripheral nervous system [PNS] glia) were seeded within collagen solution at 4 °C in adjacent chambers of a stainless steel mould, using cells cultured from wild-type or green fluorescent protein expressing rats, to track specific populations. The divider between the chambers was removed using a protocol that allowed the gels to integrate without mixing of the cell populations. Following setting of the gels, they were maintained in culture for up to 15 days. Reciprocal astrocyte and neuronal responses were monitored using confocal microscopy and 3D image analysis. At DRG:astrocyte interfaces, by 5 days there was an increase in the number of astrocytes at the interface followed by hypertrophy and increased glial fibrillary acidic protein expression at 10 and 15 days, indicative of reactive gliosis. Neurons avoided crossing DRG:astrocyte interfaces, and neuronal growth was restricted to the DRG part of the gel. By contrast, neurons were able to grow freely across DRG:DRG interfaces, demonstrating the absence of a mechanical barrier. These results show that in a precisely controlled 3D environment, an interface between DRG and astrocyte cultures is sufficient to trigger reactive gliosis and inhibition of neuronal regeneration across the interface. Different aspects of the astrocyte response could be independently monitored, providing an insight into the formation of a glial scar. This technology has wide potential for researchers wishing to maintain and monitor interactions between adjacent cell populations in 3D culture.

Differentiating neural crest stem cells induce proliferation of cultured rodent islet beta cells

AIMS/HYPOTHESIS

Efficient stimulation of cycling activity in cultured beta cells would allow the design of new strategies for cell therapy in diabetes. Neural crest stem cells (NCSCs) play a role in beta cell development and maturation and increase the beta cell number in co-transplants. The mechanism behind NCSC-induced beta cell proliferation and the functional capacity of the new beta cells is not known.

METHODS

We developed a new in vitro co-culture system that enables the dissection of the elements that control the cellular interactions that lead to NCSC-dependent increase in islet beta cells.

RESULTS

Mouse NCSCs were cultured in vitro, first in medium that stimulated their proliferation, then under conditions that supported their differentiation. When mouse islet cells were cultured together with the NCSCs, more than 35% of the beta cells showed cycle activity. This labelling index is more than tenfold higher than control islets cultured without NCSCs. Beta cells that proliferated under these culture conditions were fully glucose responsive in terms of insulin secretion. NCSCs also induced beta cell proliferation in islets isolated from 1-year-old mice, but not in dissociated islet cells isolated from human donor pancreas tissue. To stimulate beta cell proliferation, NCSCs need to be in intimate contact with the beta cells.

CONCLUSIONS/INTERPRETATION

Culture of islet cells in contact with NCSCs induces highly efficient beta cell proliferation. The reported culture system is an excellent platform for further dissection of the minimal set of factors needed to drive this process and explore its potential for translation to diabetes therapy.

Postembryonic neuronal addition in zebrafish dorsal root ganglia is regulated by Notch signaling

Background

The sensory neurons and glia of the dorsal root ganglia (DRG) arise from neural crest cells in the developing vertebrate embryo. In mouse and chick, DRG formation is completed during embryogenesis. In contrast, zebrafish continue to add neurons and glia to the DRG into adulthood, long after neural crest migration is complete. The molecular and cellular regulation of late DRG growth in the zebrafish remains to be characterized.

Results

In the present study, we use transgenic zebrafish lines to examine neuronal addition during postembryonic DRG growth. Neuronal addition is continuous over the period of larval development. Fate-mapping experiments support the hypothesis that new neurons are added from a population of resident, neural crest-derived progenitor cells. Conditional inhibition of Notch signaling was used to assess the role of this signaling pathway in neuronal addition. An increase in the number of DRG neurons is seen when Notch signaling is inhibited during both early and late larval development.

Conclusions

Postembryonic growth of the zebrafish DRG comes about, in part, by addition of new neurons from a resident progenitor population, a process regulated by Notch signaling.

Specification of neural crest into sensory neuron and melanocyte lineages

Elucidating the mechanisms by which multipotent cells differentiate into distinct lineages is a common theme underlying developmental biology investigations. Progress has been made in understanding some of the essential factors and pathways involved in the specification of different lineages from the neural crest. These include gene regulatory networks involving transcription factor hierarchies and input from signaling pathways mediated from environmental cues. In this review, we examine the mechanisms for two lineages that are derived from the neural crest, peripheral sensory neurons and melanocytes. Insights into the specification of these cell types may reveal common themes in the specification processes that occur throughout development.

Molecular interactions underlying the specification of sensory neurons

Sensory neurons of the dorsal root ganglion (DRG) respond to many different kinds of stimulus. The ability to discriminate between the diverse types of sensation is reflected by the existence of functionally and morphologically specialized sensory neurons. This neuronal diversity is created in a step-wise process extending well into postnatal life. Here, we review the hierarchical organization and the molecular process involving interactions between environmental growth factors, used and reused in different developmental contexts in self-reinforcing and cross-inhibitory mechanisms, and intrinsic gene programs that underlie the progressive diversification of sensory progenitors into specialized neurons. The recent advance in knowledge of sensory neuron specification may provide mechanistic principles that could extend to other parts of the nervous system.

Motor axon exit from the mammalian spinal cord is controlled by the homeodomain protein Nkx2.9 via Robo-Slit signaling

Mammalian motor circuits control voluntary movements by transmitting signals from the central nervous system (CNS) to muscle targets. To form these circuits, motor neurons (MNs) must extend their axons out of the CNS. Although exit from the CNS is an indispensable phase of motor axon pathfinding, the underlying molecular mechanisms remain obscure. Here, we present the first identification of a genetic pathway that regulates motor axon exit from the vertebrate spinal cord, utilizing spinal accessory motor neurons (SACMNs) as a model system. SACMNs are a homogeneous population of spinal MNs with axons that leave the CNS through a discrete lateral exit point (LEP) and can be visualized by the expression of the cell surface protein BEN. We show that the homeodomain transcription factor Nkx2.9 is selectively required for SACMN axon exit and identify the Robo2 guidance receptor as a likely downstream effector of Nkx2.9; loss of Nkx2.9 leads to a reduction in Robo2 mRNA and protein within SACMNs and SACMN axons fail to exit the spinal cord in Robo2-deficient mice. Consistent with short-range interactions between Robo2 and Slit ligands regulating SACMN axon exit, Robo2-expressing SACMN axons normally navigate through LEP-associated Slits as they emerge from the spinal cord, and fail to exit in Slit-deficient mice. Our studies support the view that Nkx2.9 controls SACMN axon exit from the mammalian spinal cord by regulating Robo-Slit signaling.

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.

Sox2 and Mitf cross-regulatory interactions consolidate progenitor and melanocyte lineages in the cranial neural crest

The cellular origin and molecular mechanisms regulating pigmentation of head and neck are largely unknown. Melanocyte specification is controlled by the transcriptional activity of Mitf, but no general logic has emerged to explain how Mitf and progenitor transcriptional activities consolidate melanocyte and progenitor cell fates. We show that cranial melanocytes arise from at least two different cellular sources: initially from nerve-associated Schwann cell precursors (SCPs) and later from a cellular source that is independent of nerves. Unlike the midbrain-hindbrain cluster from which melanoblasts arise independently of nerves, a large center of melanocytes in and around cranial nerves IX-X is derived from SCPs, as shown by genetic cell-lineage tracing and analysis of ErbB3-null mutant mice. Conditional gain- and loss-of-function experiments show genetically that cell fates in the neural crest involve both the SRY transcription factor Sox2 and Mitf, which consolidate an SCP progenitor or melanocyte fate by cross-regulatory interactions. A gradual downregulation of Sox2 in progenitors during development permits the differentiation of both neural crest- and SCP-derived progenitors into melanocytes, and an initial small pool of nerve-associated melanoblasts expands in number and disperses under the control of endothelin receptor B (Ednrb) and Wnt5a signaling.

Analysis of neural crest migration and differentiation by cross-species transplantation

Avian embryos provide a unique platform for studying many vertebrate developmental processes, due to the easy access of the embryos within the egg. Chimeric avian embryos, in which quail donor tissue is transplanted into a chick embryo in ovo, combine the power of indelible genetic labeling of cell populations with the ease of manipulation presented by the avian embryo. Quail-chick chimeras are a classical tool for tracing migratory neural crest cells (NCCs). NCCs are a transient migratory population of cells in the embryo, which originate in the dorsal region of the developing neural tube. They undergo an epithelial to mesenchymal transition and subsequently migrate to other regions of the embryo, where they differentiate into various cell types including cartilage, melanocytes, neurons and glia. NCCs are multipotent, and their ultimate fate is influenced by 1) the region of the neural tube in which they originate along the rostro-caudal axis of the embryo, 2) signals from neighboring cells as they migrate, and 3) the microenvironment of their ultimate destination within the embryo. Tracing these cells from their point of origin at the neural tube, to their final position and fate within the embryo, provides important insight into the developmental processes that regulate patterning and organogenesis. Transplantation of complementary regions of donor neural tube (homotopic grafting) or different regions of donor neural tube (heterotopic grafting) can reveal differences in pre-specification of NCCs along the rostro-caudal axis. This technique can be further adapted to transplant a unilateral compartment of the neural tube, such that one side is derived from donor tissue, and the contralateral side remains unperturbed in the host embryo, yielding an internal control within the same sample. It can also be adapted for transplantation of brain segments in later embryos, after HH10, when the anterior neural tube has closed. Here we report techniques for generating quail-chick chimeras via neural tube transplantation, which allow for tracing of migratory NCCs derived from a discrete segment of the neural tube. Species-specific labeling of the donor-derived cells with the quail-specific QCPN antibody allows the researcher to distinguish donor and host cells at the experimental end point. This technique is straightforward, inexpensive, and has many applications, including fate-mapping, cell lineage tracing, and identifying pre-patterning events along the rostro-caudal axis. Because of the ease of access to the avian embryo, the quail-chick graft technique may be combined with other manipulations, including but not limited to lens ablation, injection of inhibitory molecules, or genetic manipulation via electroporation of expression plasmids, to identify the response of particular migratory streams of NCCs to perturbations in the embryo's developmental program. Furthermore, this grafting technique may also be used to generate other interspecific chimeric embryos such as quail-duck chimeras to study NCC contribution to craniofacial morphogenesis, or mouse-chick chimeras to combine the power of mouse genetics with the ease of manipulation of the avian embryo.

Neural crest stem cells: discovery, properties and potential for therapy

Neural crest (NC) cells are a migratory cell population synonymous with vertebrate evolution. They generate a wide variety of cell and tissue types during embryonic and adult development including cartilage and bone, connective tissue, pigment and endocrine cells as well as neurons and glia amongst many others. Such incredible lineage potential combined with a limited capacity for self-renewal, which persists even into adult life, demonstrates that NC cells bear the key hallmarks of stem and progenitor cells. In this review, we describe the identification, characterization and isolation of NC stem and progenitor cells from different tissues in both embryo and adult organisms. We discuss their specific properties and their potential application in cell-based tissue and disease-specific repair.

Boundary cap cells are peripheral nervous system stem cells that can be redirected into central nervous system lineages

Boundary cap cells (BC), which express the transcription factor Krox20, participate in the formation of the boundary between the central nervous system and the peripheral nervous system. To study BC stemness, we developed a method to purify and amplify BC in vitro from Krox20(Cre/+), R26R(YFP/+) mouse embryos. We show that BC progeny are EGF/FGF2-responsive, form spheres, and express neural crest markers. Upon growth factor withdrawal, BC progeny gave rise to multiple neural crest and CNS lineages. Transplanted into the developing murine forebrain, they successfully survived, migrated, and integrated within the host environment. Surprisingly, BC progeny generated exclusively CNS cells, including neurons, astrocytes, and myelin-forming oligodendrocytes. In vitro experiments indicated that a sequential combination of ventralizing morphogens and glial growth factors was necessary to reprogram BC into oligodendrocytes. Thus, BC progeny are endowed with differentiation plasticity beyond the peripheral nervous system. The demonstration that CNS developmental cues can reprogram neural crest-derived stem cells into CNS derivatives suggests that BC could serve as a source of cell type-specific lineages, including oligodendrocytes, for cell-based therapies to treat CNS disorders.

The role of the transcription factor Rbpj in the development of dorsal root ganglia

Background

The dorsal root ganglion (DRG) is composed of well-characterized populations of sensory neurons and glia derived from a common pool of neural crest stem cells (NCCs), and is a good system to study the mechanisms of neurogenesis and gliogenesis. Notch signaling is known to play important roles in DRG development, but the full scope of Notch functions in mammalian DRG development remains poorly understood.

Results

In the present study, we used Wnt1-Cre to conditionally inactivate the transcription factor Rbpj, a critical integrator of activation signals from all Notch receptors, in NCCs and their derived cells. Deletion of Rbpj caused the up-regulation of NeuroD1 and precocious neurogenesis in DRG early development but led to an eventual deficit of sensory neurons at later stages, due to reduced cell proliferation and abnormal cell death. In addition, gliogenesis was delayed initially, but a near-complete loss of glia was observed finally in Rbpj-deficient DRG. Furthermore, we found P75 and Sox10, which are normally expressed exclusively in neuronal and glial progenitors of the DRG after the NCCs have completed their migration, were co-expressed in many cells of the DRG of Rbpj conditional knock-out mice.

Conclusions

Our data indicate that Rbpj-mediated canonical Notch signaling inhibits DRG neuronal differentiation, possibly by regulating NeuroD1 expression, and is required for DRG gliogenesis in vivo.

Specific functions for ERK/MAPK signaling during PNS development

We have established functions of the stimulus-dependent MAPKs, ERK1/2 and ERK5, in DRG, motor neuron, and Schwann cell development. Surprisingly, many aspects of early DRG and motor neuron development were found to be ERK1/2 independent, and Erk5 deletion had no obvious effect on embryonic PNS. In contrast, Erk1/2 deletion in developing neural crest resulted in peripheral nerves that were devoid of Schwann cell progenitors, and deletion of Erk1/2 in Schwann cell precursors caused disrupted differentiation and marked hypomyelination of axons. The Schwann cell phenotypes are similar to those reported in neuregulin-1 and ErbB mutant mice, and neuregulin effects could not be elicited in glial precursors lacking Erk1/2. ERK/MAPK regulation of myelination was specific to Schwann cells, as deletion in oligodendrocyte precursors did not impair myelin formation, but reduced precursor proliferation. Our data suggest a tight linkage between developmental functions of ERK/MAPK signaling and biological actions of specific RTK-activating factors.

Peripheral somatosensation: a touch of genetics

The somatosensory system processes information that organisms 'feel': joint position, muscle stretch, pain, pressure, temperature, and touch. The system is composed of a diverse array of peripheral nerve endings specialized to detect these sensory modalities. Several recent discoveries have shed light on the genetic pathways that control specification and differentiation of these neurons, how they accurately innervate their central and peripheral targets, and the molecules that enable them to detect mechanical stimuli. Here, we review the cadre of genes that control these processes, focusing on mechanosensitive neurons and support cells of the skin that mediate different aspects of the sense of touch.

Somatosensory mechanisms in zebrafish lacking dorsal root ganglia

Dorsal root ganglion (DRG) sensory neurons transmit all somatosensory information from the trunk region of the body. erbb3 mutant zebrafish do not form DRG neurons because the neural crest cells that generate them migrate aberrantly. Here we report that homozygous erbb3 mutants appear to swim and feed normally, and that they survive through adulthood, despite never forming DRG neurons. The source of sensory compensation in adult erbb3 mutants remains unknown, although it may be from lateral line ganglion neuromasts which are reduced, but present, in erbb3 mutants. We also provide new information about the development of DRG neurons in wild-type juvenile zebrafish.

Hepatocyte growth factor-Met signaling is required for Runx1 extinction and peptidergic differentiation in primary nociceptive neurons

Nociceptors in peripheral ganglia display a remarkable functional heterogeneity. They can be divided into the following two major classes: peptidergic and nonpeptidergic neurons. Although RUNX1 has been shown to play a pivotal role in the specification of nonpeptidergic neurons, the mechanisms driving peptidergic differentiation remain elusive. Here, we show that hepatocyte growth factor (HGF)-Met signaling acts synergistically with nerve growth factor-tyrosine kinase receptor A to promote peptidergic identity in a subset of prospective nociceptors. We provide in vivo evidence that a population of peptidergic neurons, derived from the RUNX1 lineage, require Met activity for the proper extinction of Runx1 and optimal activation of CGRP (calcitonin gene-related peptide). Moreover, we show that RUNX1 in turn represses Met expression in nonpeptidergic neurons, revealing a bidirectional cross talk between Met and RUNX1. Together, our novel findings support a model in which peptidergic versus nonpeptidergic specification depends on a balance between HGF-Met signaling and Runx1 extinction/maintenance.

Regional differences in neural crest morphogenesis

Neural crest cells are pluripotent cells that emerge from the neural epithelium, migrate extensively, and differentiate into numerous derivatives, including neurons, glial cells, pigment cells and connective tissue. Major questions concerning their morphogenesis include: 1) what establishes the pathways of migration and 2) what controls the final destination and differentiation of various neural crest subpopulations. These questions will be addressed in this review. Neural crest cells from the trunk level have been explored most extensively. Studies show that melanoblasts are specified shortly after they depart from the neural tube, and this specification directs their migration into the dorsolateral pathway. We also consider other reports that present strong evidence for ventrally migrating neural crest cells being similarly fate restricted. Cranial neural crest cells have been less analyzed in this regard but the preponderance of evidence indicates that either the cranial neural crest cells are not fate-restricted, or are extremely plastic in their developmental capability and that specification does not control pathfinding. Thus, the guidance mechanisms that control cranial neural crest migration and their behavior vary significantly from the trunk. The vagal neural crest arises at the axial level between the cranial and trunk neural crest and represents a transitional cell population between the head and trunk neural crest. We summarize new data to support this claim. In particular, we show that: 1) the vagal-level neural crest cells exhibit modest developmental bias; 2) there are differences in the migratory behavior between the anterior and the posterior vagal neural crest cells reminiscent of the cranial and the trunk neural crest, respectively; 3) the vagal neural crest cells take the dorsolateral pathway to the pharyngeal arches and the heart, but the ventral pathway to the peripheral nervous system and the gut. However, these pathways are not rigidly specified because of prior fate restriction. Understanding the molecular, cellular and behavioral differences between these three populations of neural crest cells will be of enormous assistance when trying to understand the evolution of the neck.

Pathology and pathogenesis of sensory neuropathy in Friedreich's ataxia

Friedreich's ataxia (FRDA) causes a complex neuropathological phenotype with characteristic lesions of dorsal root ganglia (DRG); dorsal spinal roots; dorsal nuclei of Clarke; spinocerebellar and corticospinal tracts; dentate nuclei; and sensory nerves. This report presents a systematic morphological analysis of sural nerves obtained by autopsy of six patients with genetically confirmed FRDA. The outstanding lesion consisted of lack of myelinated fibers whereas axons were present in normal numbers. On cross-sections, only 11% of all class III-beta-tubulin-positive axons were myelinated in FRDA, contrasting with 36% in normal control nerves. Despite their paucity, thin myelinated fibers assembled compact sheaths containing the peripheral myelin proteins PMP-22, P(0), and myelin basic protein. The nerves displayed major modifications in Schwann cells that were apparent by laminin 2 and S100alpha immunocytochemistry. Few S100alpha-immunoreactive cells remained detectable whereas laminin 2 reaction product was abundant. The normal honeycomb-like distribution of laminin 2 around myelinated fibers was replaced by confluent regions of reaction product that enveloped clusters of closely apposed thin axons. Electron microscopy not only confirmed the lack of myelin but also showed abnormal Schwann cells and axons. Ferritin localized to normal Schwann cell cytoplasm. In the sensory nerves of patients with FRDA, the distribution of this protein strongly resembled laminin 2, but there was no net increase of the total ferritin-reactive area. Ferroportin reaction product occurred in all axons of sural nerves in FRDA, which was at variance with dorsal spinal roots. In the pathogenesis of sensory neuropathy in FRDA, two mechanisms are likely: hypomyelination due to faulty interaction between axons and Schwann cells; and slow axonal degeneration. Neurons of DRG, satellite cells, Schwann cells, and axons of sensory nerves and dorsal spinal roots derive from the neural crest, and hypomyelination in FRDA may be attributed to defects of regulation or migration of shared precursor cells. Sural nerves in FRDA showed no convincing change in ferritin and ferroportin, militating against local iron dysmetabolism. The result stands out in contrast to the previously reported changes in dorsal spinal roots of patients with FRDA.

Boundary cap cells are highly competitive for CNS remyelination: fast migration and efficient differentiation in PNS and CNS myelin-forming cells

During development, boundary cap cells (BC) and neural crest cell (NCC) derivatives generate Schwann cells (SC) of the spinal roots and a subpopulation of neurons and satellite cells in the dorsal root ganglia. Despite their stem-like properties, their therapeutic potential in the diseased central nervous system (CNS) was never explored. The aim of this work was to explore BC therapeutic potential for CNS remyelination. We derived BC from Krox20(Cre) x R26R(Yfp) embryos at E12.5, when Krox20 is exclusively expressed by BC. Combining microdissection and cell fate mapping, we show that acutely isolated BC are a unique population closely related but distinct from NCC and SC precursors. Moreover, when grafted in the demyelinated spinal cord, BC progeny expands in the lesion through a combination of time-regulated processes including proliferation and differentiation. Furthermore, when grafted away from the lesion, BC progeny, in contrast to committed SC, show a high migratory potential mediated through enhanced interactions with astrocytes and white matter, and possibly with polysialylated neural cell adhesion molecule expression. In response to demyelinated axons of the CNS, BC progeny generates essentially myelin-forming SC. However, in contact with axons and astrocytes, some of them generate also myelin-forming oligodendrocytes. There are two primary outcomes of this study. First, the high motility of BC and their progeny, in addition to their capacity to remyelinate CNS axons, supports the view that BC are a reservoir of interest to promote CNS remyelination. Second, from a developmental point of view, BC behavior in the demyelinated CNS raises the question of the boundary between central and peripheral myelinating cells.

Dicer in Schwann cells is required for myelination and axonal integrity

Dicer is responsible for the generation of mature micro-RNAs (miRNAs) and loading them into RNA-induced silencing complex (RISC). RISC functions as a probe that targets mRNAs leading to translational suppression and mRNA degradation. Schwann cells (SCs) in the peripheral nervous system undergo remarkable differentiation both in morphology and gene expression patterns throughout lineage progression to myelinating and nonmyelinating phenotypes. Gene expression in SCs is particularly tightly regulated and critical for the organism, as highlighted by the fact that a 50% decrease or an increase to 150% of normal gene expression of some myelin proteins, like PMP22, results in peripheral neuropathies. Here, we selectively deleted Dicer and consequently gene expression regulation by mature miRNAs from Mus musculus SCs. Our results show that in the absence of Dicer, most SCs arrest at the promyelinating stage and fail to start forming myelin. At the molecular level, the promyelinating transcription factor Krox20 and several myelin proteins [including myelin associated glycoprotein (MAG) and PMP22] were strongly reduced in mutant sciatic nerves. In contrast, the myelination inhibitors SOX2, Notch1, and Hes1 were increased, providing an additional potential basis for impaired myelination. A minor fraction of SCs, with some peculiar differences between sensory and motor fibers, overcame the myelination block and formed unusually thin myelin, in line with observed impaired neuregulin and AKT signaling. Surprisingly, we also found signs of axonal degeneration in Dicer mutant mice. Thus, our data indicate that miRNAs critically regulate Schwann cell gene expression that is required for myelination and to maintain axons via axon-glia interactions.

CNS/PNS boundary transgression by central glia in the absence of Schwann cells or Krox20/Egr2 function

CNS/PNS interfaces constitute cell boundaries, because they delimit territories with different neuronal and glial contents. Despite their potential interest in regenerative medicine, the mechanisms restricting oligodendrocytes and astrocytes to the CNS and Schwann cells to the PNS in mammals are not known. To investigate the involvement of peripheral glia and myelin in the maintenance of the CNS/PNS boundary, we have first made use of different mouse mutants. We show that depletion of Schwann cells and boundary cap cells or inactivation of Krox20/Egr2, a master regulatory gene for myelination in Schwann cells, results in transgression of the CNS/PNS boundary by astrocytes and oligodendrocytes and in myelination of nerve root axons by oligodendrocytes. In contrast, such migration does not occur with the Trembler(J) mutation, which prevents PNS myelination without affecting Krox20 expression. Altogether, these data suggest that maintenance of the CNS/PNS boundary requires a Krox20 function separable from myelination control. Finally, we have analyzed a human patient affected by a congenital amyelinating neuropathy, associated with the absence of the KROX20 protein in Schwann cells. In this case, the nerve roots were also invaded by oligodendrocytes and astrocytes. This indicates that transgression of the CNS/PNS boundary by central glia can occur in pathological situations in humans and suggests that the underlying mechanisms are common with the mouse.

Patterned assembly and neurogenesis in the chick dorsal root ganglion

The birth of small-diameter TrkA+ neurons that mediate pain and thermoreception begins approximately 24 hours after the cessation of neural crest cell migration from progenitors residing in the nascent dorsal root ganglion. Although multiple geographically distinct progenitor pools have been proposed, this study is the first to comprehensively characterize the derivation of small-diameter neurons. In the developing chick embryo we identify novel patterns in neural crest cell migration and colonization that sculpt the incipient ganglion into a postmitotic neuronal core encapsulated by a layer of proliferative progenitor cells. Furthermore, we show that this outer progenitor layer is composed of three spatially, temporally, and molecularly distinct progenitor zones, two of which give rise to distinct populations of TrkA+ neurons.

Neural crest-derived stem cells display a wide variety of characteristics

A recent burst of findings has shown that neural crest-derived stem cells (NCSCs) can be found in diverse mammalian tissues. In addition to their identification in tissues that are known to be derived from the neural crest, recent studies have revealed NCSCs in tissues that are not specifically derived from the neural crest, such as bone marrow. NCSCs can express a wide range of characteristics, and which properties are expressed mainly depends on their tissue sources and the ontogenic stage of the animal. The identification of NCSCs in various tissues opens an entirely new avenue of approach to developing autologous cell replacement therapies for use in regenerative medicine. In this review, we discuss the origin, migration, and lineage potential of NCSCs from various mammalian tissue sources.

A selective glial barrier at motor axon exit points prevents oligodendrocyte migration from the spinal cord

Nerve roots have specialized transition zones that permit axon extension but limit cell movement between the CNS and PNS. Boundary cap cells prevent motor neuron soma from following their axons into the periphery, thereby contributing to a selective barrier. Transition zones also restrict movement of glial cells. Consequently, axons that cross the CNS-PNS interface are insulated by central and peripheral myelin. The mechanisms that prevent the migratory progenitors of oligodendrocytes and Schwann cells, the myelinating cells of the CNS and PNS, respectively, from crossing transition zones are not known. Here, we show that interactions between myelinating glial cells prevent their movements across the interface. Using in vivo time-lapse imaging in zebrafish we found that, in the absence of Schwann cells, oligodendrocyte progenitors cross ventral root transition zones and myelinate motor axons. These studies reveal that distinct mechanisms regulate the movement of axons, neurons, and glial cells across the CNS-PNS interface.

Glial precursors clear sensory neuron corpses during development via Jedi-1, an engulfment receptor

During the development of peripheral ganglia, 50% of the neurons that are generated undergo apoptosis. How the massive numbers of corpses are removed is unknown. We found that satellite glial cell precursors are the primary phagocytic cells for apoptotic corpse removal in developing mouse dorsal root ganglia (DRG). Confocal and electron microscopic analysis revealed that glial precursors, rather than macrophages, were responsible for clearing most of the dead DRG neurons. Moreover, we identified Jedi-1, an engulfment receptor, and MEGF10, a purported engulfment receptor, as homologs of the invertebrate engulfment receptors Draper and CED-1 expressed in the glial precursor cells. Expression of Jedi-1 or MEGF10 in fibroblasts facilitated binding to dead neurons, and knocking down either protein in glial cells or overexpressing truncated forms lacking the intracellular domain inhibited engulfment of apoptotic neurons. Together, these results suggest a cellular and molecular mechanism by which neuronal corpses are culled during DRG development.

Regulation of boundary cap neural crest stem cell differentiation after transplantation

Success of cell replacement therapies for neurological disorders will depend largely on the optimization of strategies to enhance viability and control the developmental fate of stem cells after transplantation. Once transplanted, stem/progenitor cells display a tendency to maintain an undifferentiated phenotype or differentiate into inappropriate cell types. Gain and loss of function experiments have revealed key transcription factors which drive differentiation of immature stem/progenitor cells toward more mature stages and eventually to full differentiation. An attractive course of action to promote survival and direct the differentiation of transplanted stem cells to a specific cell type would therefore be to force expression of regulatory differentiation molecules in already transplanted stem cells, using inducible gene expression systems which can be controlled from the outside. Here, we explore this hypothesis by employing a tetracycline gene regulating system (Tet-On) to drive the differentiation of boundary cap neural crest stem cells (bNCSCs) toward a sensory neuron fate after transplantation. We induced the expression of the key transcription factor Runx1 in Sox10-expressing bNCSCs. Forced expression of Runx1 strongly increased transplant survival in the enriched neurotrophic environment of the dorsal root ganglion cavity, and was sufficient to guide differentiation of bNCSCs toward a nonpeptidergic nociceptive sensory neuron phenotype both in vitro and in vivo after transplantation. These findings suggest that exogenous activation of transcription factors expression after transplantation in stem/progenitor cell grafts can be a constructive approach to control their survival as well as their differentiation to the desired type of cell and that the Tet-system is a useful tool to achieve this.

HeyL regulates the number of TrkC neurons in dorsal root ganglia

The basic-helix-loop-helix transcription factor HeyL is expressed at high levels by neural crest progenitor cells (NCPs) that give rise to neurons and glia in dorsal root ganglia (DRG). Since HeyL expression was observed in these NCPs during the period of neurogenesis, we generated HeyL null mutants to help examine the factor's role in ganglion neuronal specification. Homozygous null mutation of HeyL reduced the number of TrkC(+) neurons in DRG at birth including the subpopulation that expresses the ETS transcription factor ER81. Conversely, null mutation of the Hey paralog, Hey1, increased the number of TrkC(+) neurons. Null mutation of HeyL increased expression of the Hey paralogs Hey1 and Hey2, suggesting that HeyL normally inhibits their expression. Double null mutation of both Hey1 and HeyL rescued TrkC(+) neuron numbers to control levels. Thus, the balance between HeyL and Hey1 expression regulates the differentiation of a subpopulation of TrkC(+) neurons in the DRG.

Proliferation and differentiation of glial and neuronal progenitors in the development of human spinal ganglia

Development and differentiation of the spinal ganglia were investigated in 10 human embryos and foetuses, ranging in age between 5th and 10th developmental weeks. The aim of the study was to estimate the spatial and temporal appearance, percentage and duration of proliferation process among neural crest cells and differentiating glial cells and neurons. The process of proliferation and differentiation of cell lineages from neural crest to neurons or glial cell was analysed using immunohistochemical and immunofluorescence methods in paraffin sections. Quantification of reacting cells was performed by counting the ratio of cells stained or double-stained to specific antibodies in the number of total cell population. Data were expressed as mean+/-SD, while the difference between dorsal and ventral parts of the spinal ganglia were analysed by the Mann-Whitney test. The Ki-67 proliferation marker had the strongest expression in the 5th and 6th developmental weeks (42% of positive cells), showing also significantly higher proliferation rate in the dorsal parts of the spinal ganglia than in the ventral parts (Mann-Whitney, p=0.003). During further development, the number of proliferating cells subsequently decreased to 32% in the foetal period. A majority of the proliferating cells expressed neural crest marker nestin (71.5%) or glial cell marker S100 protein (17%). Neurons (stained with PGP9.5 marker) showed no signs of proliferation. Some cells co-expressed both neural crest cells and glial cell markers. Our results indicate the highest proliferation activity of the progenitor neural crest cells, which slightly decreased with progression of spinal ganglia differentiation. On the contrary, glial cells displayed increasing proliferation activity at later developmental stages, thus conforming significance of gliogenesis during human spinal ganglia development. Although neurogenesis was not found during the investigated period, we could not exclude the possibility of neuronal differentiation from neural crest cells, or even immature glial cells.

Ion fluxes and neurotransmitters signaling in neural development

The brain develops and functions in a complex ionic milieu, which is a prerequisite for neurotransmitter function and neuronal signaling. Neurotransmitters and ion fluxes are, however, important not only in neuronal signaling, but also in the control of neural differentiation, and in this review, we highlight the recent advances in our understanding of how the gamma-amino butyric acid (GABA) neurotransmitter and ion fluxes are relevant for cell cycle control and neural differentiation. Conversely, proteins previously associated with ion transport across membranes have been endowed with novel ion-independent functions, and we discuss this in the context of gap junctions in cell adhesion and of the neuron-specific K(+)-Cl(-) cotransporter KCC2 in dendritic spine development. Collectively, these findings provide a richer and more complex picture of when ion fluxes are needed in neural development and when they are not.

How does the Schwann cell lineage form tumors in NF1?

Neurofibromas are benign tumors of peripheral nerve that occur sporadically or in patients with the autosomal dominant tumor predisposition syndrome neurofibromatosis type 1 (NF1). Multiple neurofibroma subtypes exist which differ in their site of occurrence, their association with NF1, and their tendency to undergo transformation to become malignant peripheral nerve sheath tumors (MPNSTs), the most common malignancy associated with NF1. Most NF1 patients carry a constitutional mutation of the NF1 tumor suppressor gene. Neurofibromas develop in these patients when an unknown cell type in the Schwann cell lineage loses its remaining functional NF1 gene and initiates a complex series of interactions with other cell types; these interactions may be influenced by aberrant expression of growth factors and growth factor receptors and the action of modifier genes. Cells within certain neurofibroma subtypes subsequently accumulate additional mutations affecting the p19(ARF)-MDM2-TP53 and p16INK4A-Rb signaling cascades, mutations of other as yet unidentified genes, and amplification of growth factor receptor genes, resulting in their transformation into MPNSTs. These observations have been validated using a variety of transgenic and knockout mouse models that recapitulate neurofibroma and MPNST pathogenesis. A new generation of mouse models is also providing important new insights into the identity of the cell type in the Schwann cell lineage that gives rise to neurofibromas. Our improving understanding of the mechanisms underlying the pathogenesis of neurofibromas and MPNSTs raises intriguing new questions about the origin and pathogenesis of these neoplasms and establishes models for the development of new therapies targeting these neoplasms.

The function of RhoGTPases in axon ensheathment and myelination

RhoGTPases are molecular switches that integrate extracellular signals to perform diverse cellular responses. This ability relies on the network of proteins regulating RhoGTPases activity and localization, and on the interaction of RhoGTPases with many different cellular effectors. Myelination is an ideal place for RhoGTPases regulation, as it is the result of fine orchestration of many stimuli from at least two cell types. Recent work has revealed that RhoGTPases are required for Schwann cells to sort, ensheath, and myelinate axons. Here, we will review these recent advances showing the critical roles for RhoGTPases in various aspects of Schwann development and myelination, including the recent discovery of their involvement in Charcot-Marie-Tooth disease. Comparison with potential roles of RhoGTPases in central nervous system myelination will be drawn.

Development of the Schwann cell lineage: from the neural crest to the myelinated nerve

The myelinating and nonmyelinating Schwann cells in peripheral nerves are derived from the neural crest, which is a transient and multipotent embryonic structure that also generates the other main glial subtypes of the peripheral nervous system (PNS). Schwann cell development occurs through a series of transitional embryonic and postnatal phases, which are tightly regulated by a number of signals. During the early embryonic phases, neural crest cells are specified to give rise to Schwann cell precursors, which represent the first transitional stage in the Schwann cell lineage, and these then generate the immature Schwann cells. At birth, the immature Schwann cells differentiate into either the myelinating or nonmyelinating Schwann cells that populate the mature nerve trunks. In this review, we will discuss the biology of the transitional stages in embryonic and early postnatal Schwann cell development, including the phenotypic differences between them and the recently identified signaling pathways, which control their differentiation and maintenance. In addition, the role and importance of the microenvironment in which glial differentiation takes place will be discussed.