Fibroblast growth factors and insulin growth factors combine to promote survival of rat Schwann cell precursors without induction of DNA synthesis

Phagocytosis by Peripheral Glia: Importance for Nervous System Functions and Implications in Injury and Disease

The central nervous system (CNS) has very limited capacity to regenerate after traumatic injury or disease. In contrast, the peripheral nervous system (PNS) has far greater capacity for regeneration. This difference can be partly attributed to variances in glial-mediated functions, such as axon guidance, structural support, secretion of growth factors and phagocytic activity. Due to their growth-promoting characteristic, transplantation of PNS glia has been trialed for neural repair. After peripheral nerve injuries, Schwann cells (SCs, the main PNS glia) phagocytose myelin debris and attract macrophages to the injury site to aid in debris clearance. One peripheral nerve, the olfactory nerve, is unique in that it continuously regenerates throughout life. The olfactory nerve glia, olfactory ensheathing cells (OECs), are the primary phagocytes within this nerve, continuously clearing axonal debris arising from the normal regeneration of the nerve and after injury. In contrast to SCs, OECs do not appear to attract macrophages. SCs and OECs also respond to and phagocytose bacteria, a function likely critical for tackling microbial invasion of the CNS via peripheral nerves. However, phagocytosis is not always effective; inflammation, aging and/or genetic factors may contribute to compromised phagocytic activity. Here, we highlight the diverse roles of SCs and OECs with the focus on their phagocytic activity under physiological and pathological conditions. We also explore why understanding the contribution of peripheral glia phagocytosis may provide us with translational strategies for achieving axonal regeneration of the injured nervous system and potentially for the treatment of certain neurological diseases.

Axo-glial interdependence in peripheral nerve development

During the development of the peripheral nervous system, axons and myelinating Schwann cells form a unique symbiotic unit, which is realized by a finely tuned network of molecular signals and reciprocal interactions. The importance of this complex interplay becomes evident after injury or in diseases in which aspects of axo-glial interaction are perturbed. This Review focuses on the specific interdependence of axons and Schwann cells in peripheral nerve development that enables axonal outgrowth, Schwann cell lineage progression, radial sorting and, finally, formation and maintenance of the myelin sheath.

Schwann Cell Precursors; Multipotent Glial Cells in Embryonic Nerves

The cells of the neural crest, often referred to as neural crest stem cells, give rise to a number of sub-lineages, one of which is Schwann cells, the glial cells of peripheral nerves. Crest cells transform to adult Schwann cells through the generation of two well defined intermediate stages, the Schwann cell precursors (SCP) in early embryonic nerves, and immature Schwann cells (iSch) in late embryonic and perinatal nerves. SCP are formed when neural crest cells enter nascent nerves and form intimate relationships with axons, a diagnostic feature of glial cells. This involves large-scale changes in gene expression, including the activation of established glial cell markers. Like early glia in the CNS, radial glia, SCP retain developmental multipotency and contribute to other crest-derived lineages during embryonic development. SCP, as well as closely related cells termed boundary cap cells, and later stages of the Schwann cell lineage have all been implicated as the tumor initiating cell in NF1 associated neurofibromas. iSch are formed from SCP in a process that involves the appearance of additional differentiation markers, autocrine survival circuits, cellular elongation, a formation of endoneurial connective tissue and basal lamina. Finally, in peri- and post-natal nerves, iSch are reversibly induced by axon-associated signals to form the myelin and non-myelin Schwann cells of adult nerves. This review article discusses early Schwann cell development in detail and describes a large number of molecular signaling systems that control glial development in embryonic nerves.

Fibroblast growth factor 21 facilitates peripheral nerve regeneration through suppressing oxidative damage and autophagic cell death

Seeking for effective drugs which are beneficial to facilitating axonal regrowth and elongation after peripheral nerve injury (PNI) has gained extensive attention. Fibroblast growth factor 21 (FGF21) is a metabolic factor that regulates blood glucose and lipid homeostasis. However, there is little concern for the potential protective effect of FGF21 on nerve regeneration after PNI and revealing related molecular mechanisms. Here, we firstly found that exogenous FGF21 administration remarkably promoted functional and morphologic recovery in a rat model of sciatic crush injury, manifesting as persistently improved motor and sensory function, enhanced axonal remyelination and regrowth and accelerated Schwann cells (SCs) proliferation. Furthermore, local FGF21 application attenuated the excessive activation of oxidative stress, which was accompanied with the activation of nuclear factor erythroid‐2‐related factor 2 (Nrf‐2) transcription and extracellular regulated protein kinases (ERK) phosphorylation. We detected FGF21 also suppressed autophagic cell death in SCs. Additionally, treatment with the ERK inhibitor U0126 or autophagy inhibitor 3‐MA partially abolishes anti‐oxidant effect and reduces SCs death. Taken together, these results indicated that the role of FGF21 in remyelination and nerve regeneration after PNI was probably related to inhibit the excessive activation of ERK/Nrf‐2 signalling‐regulated oxidative stress and autophagy‐induced cell death. Overall, our work suggests that FGF21 administration may provide a new therapy for PNI.

Isolation of Schwann Cell Precursors from Rodents

Schwann cell precursors are the first defined stage in the generation of Schwann cells from the neural crest and represent the glial cell of embryonic nerves. Highly pure cultures of these cells can be obtained by enzymatic dissociation of nerves dissected from the limbs of 14- or 12-day-old rat and mouse embryos, respectively. Since Schwann cell precursors, unlike Schwann cells, are acutely dependent on axonal signals for survival, they require addition of trophic factors, typically β-neuregulin-1, for maintenance in cell culture. Under these conditions they convert to Schwann cells on schedule, within about 4 days.

Neurofibromatosis Type 1 and tumorigenesis: molecular mechanisms and therapeutic implications

Neurofibromatosis Type 1 (NF1) is a common autosomal dominant disease characterized by complex and multicellular neurofibroma tumors, and less frequently by malignant peripheral nerve sheath tumors (MPNSTs) and optic nerve gliomas. Significant advances have been made in elucidating the cellular, genetic, and molecular biology involved in tumor formation in NF1. Neurofibromatosis Type 1 is caused by germline mutations of the NF1 tumor suppressor gene, which generally result in decreased intracellular neurofibromin protein levels, leading to increased cascade Ras signaling to its downstream effectors. Multiple key pathways are involved with the development of tumors in NF1, including Ras/mitogen-activated protein kinase (MAPK) and Akt/mammalian target of rapamycin (mTOR). Interestingly, recent studies demonstrate that multiple other developmental syndromes (in addition to NF1) share phenotypic features resulting from germline mutations in genes responsible for components of the Ras/MAPK pathway. In general, a somatic loss of the second NF1 allele, also referred to as loss of heterozygosity, in the progenitor cell, either the Schwann cell or its precursor, combined with haploinsufficiency in multiple supporting cells is required for tumor formation. Importantly, a complex series of interactions with these other cell types in neurofibroma tumorigenesis is mediated by abnormal expression of growth factors and their receptors and modification of gene expression, a key example of which is the process of recruitment and involvement of the NF1+/– heterozygous mast cell. In general, for malignant transformation to occur, there must be accumulation of additional mutations of multiple genes including INK4A/ARF and P53, with resulting abnormalities of their respective signal cascades. Further, abnormalities of the NF1 gene and molecular cascade described above have been implicated in the tumorigenesis of NF1 and some sporadically occurring gliomas, and thus, these treatment options may have wider applicability. Finally, increased knowledge of molecular and cellular mechanisms involved with NF1 tumorigenesis has led to multiple preclinical and clinical studies of targeted therapy, including the mTOR inhibitor rapamycin, which is demonstrating promising preclinical results for treatment of MPNSTs and gliomas.

Cellular therapies in motor neuron diseases

Amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA) are prototypical motor neuron diseases that result in progressive weakness as a result of motor neuron dysfunction and death. Though much work has been done in both diseases to identify the cellular mechanisms of motor neuron dysfunction, once motor neurons have died, one of potential therapies to restore function would be through the use of cellular transplantation. In this review, we discuss potential strategies whereby cellular therapies, including the use of stem cells, neural progenitors and cells engineered to secrete trophic factors, may be used in motor neuron diseases. We review pre-clinical data in rodents with each of these approaches and discuss advances and regulatory issues regarding the use of cellular therapies in human motor neuron diseases.

Anin vivoanalysis of Schwann cell programmed cell death in embryonic mice: the role of axons, glial growth factor, and the pro-apoptotic geneBax

Building upon previous in vitro studies, the present investigation involves an in vivo examination of Schwann cell programmed cell death (PCD) and development in the brachial spinal ventral roots of embryonic mice. The period of Schwann cell PCD was found to occur between embryonic days (E) 11.5 and 18.5, which is in close coincidence with the PCD period of associated brachial motoneurons (E13.5-E18.5). Additionally, Schwann cells exhibited a peak in proliferation at E11.5, and differentiation from the precursor to the immature Schwann cell stage between E12.5 and E14.5. Axon-mediated Schwann cell survival was demonstrated in vivo by excitotoxic elimination of motoneurons and their axons, via NMDA treatment in utero. This treatment increased apoptotic Schwann cell death within degenerating ventral roots. Conversely, in utero co-treatment of glial growth factor (GGF) with NMDA resulted in decreased Schwann cell death, a finding which supports previous reports of the promotion of Schwann cell survival by GGF. Analysis of mice lacking Bax, a pro-apoptotic Bcl-2 protein, revealed that Schwann cell PCD occurred independently of Bax. However, owing to the lack of motoneuron PCD in Bax-knockout mice, and the corresponding increase in the number of ventral root axons, a decrease in Schwann cell PCD was observed during the normal period of motoneuron PCD. In conclusion, our findings regarding the regulation of Schwann cell development in vivo are consistent with the conclusions from in vitro studies, including a dependency on axons for survival and proliferation signals, timing of differentiation, and a dependency on GGF.

Molecular, genetic, and cellular pathogenesis of neurofibromas and surgical implications

Neurofibromatosis 1 (NF1) is a common autosomal dominant disease characterized by complex and multicellular neurofibroma tumors. Significant advances have been made in the research of the cellular, genetic, and molecular biology of NF1. The NF1 gene was identified by positional cloning. The functions of its protein product, neurofibromin, in RAS signaling and in other signal transduction pathways are being elucidated, and the important roles of loss of heterozygosity and haploinsufficiency in tumorigenesis are better understood. The Schwann cell was discovered to be the cell of origin for neurofibromas, but understanding of a more complicated interplay of multiple cell types in tumorigenesis, specifically recruited heterogeneous cell types such as mast cells and fibroblasts, has important implications for surgical therapy of these tumors. This review summarizes the most recent NF1 and neurofibroma literature describing the pathogenesis and treatment of nerve sheath tumors. Understanding the biological underpinnings of tumorigenesis in NF1 has implications for future surgical and medical management of neurofibromas.

Transplantation of Schwann cells to subarachnoid space induces repair in contused rat spinal cord

Schwann cell transplantation is well known to induce repair in the injured spinal cord which disables millions of injured patients throughout the world. An ideal route of delivering the grafted Schwann cells to the spinal cord should neither cause more injury nor reinitiate inflammatory events and also provide a favorable milieu to the grafted cells. In this study, we have utilized subarachnoid route to transplant Schwann cells and evaluated their effects in a contusive model of spinal cord injury. Adult rats weighing 100-140 g were experimentally injured by crushing the spinal cord with a titanium clip and then divided into four groups (Tracing, Control, Medium-treated and Schwann cell-treated). Cultured Schwann cells (5x10(4) cells in 5 microl) or medium were injected to the animals of corresponding groups via subarachnoid space at the injured site 7 days after injury. In tracing group, Schwann cells (labeled with Hoechst) demonstrated their presence within spinal cord 7 days after transplantation. Evaluation of locomotor performance of animals for 60 days after injury showed that animals treated with Schwann cells had significant improvement (P<0.01). Similarly, the axon density at the site of injury was significantly higher. The results indicate the efficacy of subarachnoid route for the transplantation of Schwann cells in inducing repair of the contused spinal cord. We conclude that this route can be useful for the transplantation of Schwann cells and offers a hope for the patients suffering from spinal cord injury.

Cerebrospinal fluid insulin-like growth factor (IGF-1) and insulin-like growth factor binding protein (IGFBP-2) in children with acute lymphoblastic leukemia

BACKGROUND

Insulin-like growth factor-1 (IGF-1) has specific effects on axonal growth and myelination, low CSF IGF-1 levels being found in some severe neurologic diseases. We studied the levels of CSF IGF-1 and IGF binding protein-2 (IGFBP-2) in children with ALL to find out whether these levels correlated with any of the neurological deficits observed.

METHODS

IGF-1 and IGFBP-2 levels were prospectively measured by radioimmunoassay in the CSF of 14 children with ALL throughout the ALL chemotherapy. These were compared with the levels of 16 control subjects and of patient groups with severe neurological diseases.

RESULTS

During induction, the children with ALL had subnormal CSF IGF-1 levels which improved after 2 months. In seven individuals, two with severe vincristine polyneuropathy, the subnormal levels persisted throughout the chemotherapy.

CONCLUSIONS

Our findings suggest impairment of the IGF-1 trophic system during induction by a mechanism so far unknown. Correlation with disturbed neuronal function could not be statistically proven.

Identification and Characterization of ZFP-57, a Novel Zinc Finger Transcription Factor in the Mammalian Peripheral Nervous System

To isolate new zinc finger genes expressed at early stages of peripheral nerve development, we have used PCR to amplify conserved zinc finger sequences. RNA from rat embryonic day 12 and 13 sciatic nerves, a stage when nerves contain Schwann cell precursors, was used to identify several genes not previously described in Schwann cells. One of them, zinc finger protein (ZFP)-57, proved to be the homologue of a mouse gene found in F9 teratocarcinoma cells. Its mRNA expression profile within embryonic and adult normal and transected peripheral nerves, and its distribution in the rest of the nervous system is described. High levels of expression are seen in embryonic nerves and spinal cord. These drop rapidly during the first few weeks after birth, a pattern mirrored in other parts of the nervous system. ZFP-57 localizes to the nucleus of Schwann and other cells. The sequence contains an N-terminal Krüppel-associated box (KRAB) domain and ZFP-57 constructs containing green fluorescent protein reveal that the protein colocalizes with heterochromatin protein 1alpha to centromeric heterochromatin in a characteristic speckled pattern in NIH3T3 cells. The KRAB domain is required for this localization, because constructs lacking it target the protein to the nucleus but not to the centromeric heterochromatin. When fused to a heterologous DNA binding domain, the KRAB domain of ZFP-57 represses transcription, and full-length ZFP-57 represses Schwann cell transcription from myelin basic protein and P(0) promoters in co-transfection assays. Zfp-57 mRNA is up-regulated in Schwann cells in response to leukemia inhibitory factor and fibroblast growth factor 2.

Gene therapy for ALS delivers

Amyotrophic lateral sclerosis (ALS) is a fatal, progressive neurodegenerative disease that kills motor neurons. Despite a long disappointing history of human trials with neurotrophins, including insulin-like growth factor 1 (IGF-1), Kaspar and colleagues have successfully slowed disease in transgenic ALS mice by forcing motor neurons to produce IGF-1 following retrograde delivery of recombinant adeno-associated virus (AAV) injected into muscle. With the clinical safety of both IGF-1 and AAV already established, this provides real hope for an effective treatment of ALS.

The trunk neural crest and its early glial derivatives: a study of survival responses, developmental schedules and autocrine mechanisms

Regulation of survival during gliogenesis from the trunk neural crest is poorly understood. Using adapted survival assays, we directly compared crest cells and the crest-derived precursor populations that generate satellite cells and Schwann cells. A range of factors that supports Schwann cells and glial precursors does not rescue crest, with the major exception of neuregulin-1 that rescues crest cells provided they contact the extracellular matrix. Autocrine survival appears earlier in developing satellite cells than Schwann cells. Satellite cells also show early expression of S100beta, BFABP and fibronectin and early survival responses to IGF-1, NT-3 and PDGF-BB that in developing Schwann cells are not seen until the precursor/Schwann cell transition. These experiments define novel differences between crest cells and early glia and show that entry to the glial lineage is an important point for regulation of survival responses. They show that survival mechanisms among PNS glia differ early in development and that satellite cell development runs ahead of schedule compared to Schwann cells in several significant features.

Insulin-like growth factor-1 rescues the mutated FGF receptor 3 (G380R) expressing ATDC5 cells from apoptosis through phosphatidylinositol 3-kinase and MAPK

An activated mutation in the FGFR3 gene causes ACH. To examine the effects of IGF-1, which is an important mediator of GH, on apoptosis, we analyzed a chondrogenic cell line expressing the FGFR3 mutants. Our findings that IGF-1 prevented the apoptosis through PI3K and MAPK pathways may explain how GH treatment improves the disturbed bone growth in ACH.

INTRODUCTION

Achondroplasia (ACH), which is caused by a point mutation of the fibroblast growth factor receptor 3 (FGFR3) gene in the transmembrane domain (G380R), is one of the most common genetic forms of dwarfism. Recently, using a chondrogenic cell line, ATDC5, we have showed that the constitutively active FGFR3 mutants induced an apoptosis of chondrocytes. We have also reported that growth hormone (GH) treatment increased the growth rate in achondroplasia in parallel with the increment of serum levels of insulin-like growth factor (IGF)-1, suggesting an important role of IGF-1 in skeletal development. In this study, to clarify the mechanism by which GH treatment improved the phenotype of ACH patients, we examined the possible effects of IGF-1 on an apoptosis induced by FGFR3 mutant in ATDC5.

MATERIALS AND METHODS

Using adenovirus vector, wildtype or mutant FGFR3 (G380R) was introduced into ATDC5. Analysis of apoptosis was estimated by TUNEL assay. Expression levels of apoptosis-related genes and activation of signaling molecules were analyzed by immunoblot.

RESULTS

MTT assay showed that the cell number was reduced in ATDC5 cells expressing the mutant FGFR3 (G380R; ATDC5-mtR3 cells), suggesting that ATDC5-mtR3 cells might fall into apoptosis. IGF-1, which is an important mediator of GH, restored cell proliferation and reduced apoptosis in ATDC5-mtR3 cells. IGF-1 also decreased the ratio of Bax/Bcl-2 in the cells. To investigate which signaling cascade is responsible for antiapoptotic effects of IGF-1, we examined the role of phosphatidylinositol 3-kinase (PI3K) and MAPK in ATDC5-mtR3 cells. Specific inhibitors of PI3K or MAPK blocked the antiapoptotic effects of IGF-1 in ATDC5-mtR3 cells.

CONCLUSIONS

Our findings, showing IGF-1 prevents the apoptosis induced by FGFR3 mutation through the PI3K pathway and MAPK pathway, explain the mechanisms by which GH treatment improves the disturbed bone growth in ACH.

Diabetic neuropathy and nerve regeneration

Diabetic neuropathy is the most common peripheral neuropathy in western countries. Although every effort has been made to clarify the pathogenic mechanism of diabetic neuropathy, thereby devising its ideal therapeutic drugs, neither convinced hypotheses nor unequivocally effective drugs have been established. In view of the pathologic basis for the treatment of diabetic neuropathy, it is important to enhance nerve regeneration as well as prevent nerve degeneration. Nerve regeneration or sprouting in diabetes may occur not only in the nerve trunk but also in the dermis and around dorsal root ganglion neurons, thereby being implicated in the generation of pain sensation. Thus, inadequate nerve regeneration unequivocally contributes to the pathophysiologic mechanism of diabetic neuropathy. In this context, the research on nerve regeneration in diabetes should be more accelerated. Indeed, nerve regenerative capacity has been shown to be decreased in diabetic patients as well as in diabetic animals. Disturbed nerve regeneration in diabetes has been ascribed at least in part to all or some of decreased levels of neurotrophic factors, decreased expression of their receptors, altered cellular signal pathways and/or abnormal expression of cell adhesion molecules, although the mechanisms of their changes remain almost unclear. In addition to their steady-state changes in diabetes, nerve injury induces injury-specific changes in individual neurotrophic factors, their receptors and their intracellular signal pathways, which are closely linked with altered neuronal function, varying from neuronal survival and neurite extension/nerve regeneration to apoptosis. Although it is essential to clarify those changes for understanding the mechanism of disturbed nerve regeneration in diabetes, very few data are now available. Rationally accepted replacement therapy with neurotrophic factors has not provided any success in treating diabetic neuropathy. Aside from adverse effects of those factors, more rigorous consideration for their delivery system may be needed for any possible success. Although conventional therapeutic drugs like aldose reductase (AR) inhibitors and vasodilators have been shown to enhance nerve regeneration, their efficacy should be strictly evaluated with respect to nerve regenerative capacity. For this purpose, especially clinically, skin biopsy, by which cutaneous nerve pathology including nerve regeneration can be morphometrically evaluated, might be a safe and useful examination.

In vivo analysis of Schwann cell programmed cell death in the embryonic chick: regulation by axons and glial growth factor

The present study uses the embryonic chick to examine in vivo the mechanisms and regulation of Schwann cell programmed cell death (PCD) in spinal and cranial peripheral nerves. Schwann cells are highly dependent on the presence of axons for survival because the in ovo administration of NMDA, which excitotoxically eliminates motoneurons and their axons by necrosis, results in a significant increase in apoptotic Schwann cell death. Additionally, pharmacological and surgical manipulation of axon numbers also affects the relative amounts of Schwann cell PCD. Schwann cells undergoing both normal and induced PCD display an apoptotic-like cell death, using a caspase-dependent pathway. Furthermore, axon elimination results in upregulation of the p75 and platelet-derived growth factor receptors in mature Schwann cells within the degenerating ventral root. During early development, Schwann cells are also dependent on axon-derived mitogens; the loss of axons results in a decrease in Schwann cell proliferation. Axon removal during late embryonic stages, however, elicits an increase in proliferation, as is expected from these more differentiated Schwann cells. In rodents, Schwann cell survival is regulated by glial growth factor (GGF), a member of the neuregulin family of growth factors. GGF administration to chick embryos selectively rescued Schwann cells during both normal PCD and after the loss of axons, whereas other trophic factors tested had no effect on Schwann cell survival. In conclusion, avian Schwann cells exhibit many similarities to mammalian Schwann cells in terms of their dependence on axon-derived signals during early and later stages of development.

Schwann cells as regulators of nerve development

Myelinating and non-myelinating Schwann cells of peripheral nerves are derived from the neural crest via an intermediate cell type, the Schwann cell precursor [K.R. Jessen, A. Brennan, L. Morgan, R. Mirsky, A. Kent, Y. Hashimoto, J. Gavrilovic. The Schwann cell precursor and its fate: a study of cell death and differentiation during gliogenesis in rat embryonic nerves, Neuron 12 (1994) 509-527]. The survival and maturation of Schwann cell precursors is controlled by a neuronally derived signal, beta neuregulin. Other factors, in particular endothelins, regulate the timing of precursor maturation and Schwann cell generation. In turn, signals derived from Schwann cell precursors or Schwann cells regulate neuronal numbers during development, and axonal calibre, distribution of ion channels and neurofilament phosphorylation in myelinated axons. Unlike Schwann cell precursors, Schwann cells in older nerves survive in the absence of axons, indicating that a significant change in survival regulation occurs. This is due primarily to the presence of autocrine growth factor loops in Schwann cells, present from embryo day 18 onwards, that are not functional in Schwann cell precursors. The most important components of the autocrine loop are insulin-like growth factors, platelet derived growth factor-BB and neurotrophin 3, which together with laminin support long-term Schwann cell survival. The paracrine dependence of precursors on axons for survival provides a mechanism for matching precursor cell number to axons in embryonic nerves, while the ability of Schwann cells to survive in the absence of axons is an absolute prerequisite for nerve repair following injury. In addition to providing survival factors to neurones and themselves, and signals that determine axonal architecture, Schwann cells also control the formation of peripheral nerve sheaths. This involves Schwann cell-derived Desert Hedgehog, which directs the transition of mesenchymal cells to form the epithelium-like structure of the perineurium. Schwann cells thus signal not only to themselves but also to the other cellular components within the nerve to act as major regulators of nerve development.

Transforming growth factor beta (TGFbeta) mediates Schwann cell death in vitro and in vivo: examination of c-Jun activation, interactions with survival signals, and the relationship of TGFbeta-mediated death to Schwann cell differentiation

In some situations, cell death in the nervous system is controlled by an interplay between survival factors and negative survival signals that actively induce apoptosis. The present work indicates that the survival of Schwann cells is regulated by such a dual mechanism involving the negative survival signal transforming growth factor beta (TGFbeta), a family of growth factors that is present in the Schwann cells themselves. We analyze the interactions between this putative autocrine death signal and previously defined paracrine and autocrine survival signals and show that expression of a dominant negative c-Jun inhibits TGFbeta-induced apoptosis. This and other findings pinpoint activation of c-Jun as a key downstream event in TGFbeta-induced Schwann cell death. The ability of TGFbeta to kill Schwann cells, like normal Schwann cell death in vivo, is under a strong developmental regulation, and we show that the decreasing ability of TGFbeta to kill older cells is attributable to a decreasing ability of TGFbeta to phosphorylate c-Jun in more differentiated cells.

SpL201: a conditionally immortalized Schwann cell precursor line that generates myelin

Dramatic progress has been made over recent years toward the elucidation of the mechanisms regulating lineage determination and cell survival in the developing peripheral nervous system. However, our understanding of Schwann cell development is limited. This is partly due to the difficulties in culturing primary Schwann cell precursor cells, the earliest developmental stage of the Schwann cell lineage defined to date. Both the inability to maintain cultured Schwann cell precursor cells in an undifferentiated state and the technical difficulties involved in their isolation have hampered progress. We have conditionally immortalized rat Schwann cell precursor cells using a retrovirally encoded EGFR/neu fusion protein to circumvent these problems and to generate a source of homogeneous cells. The resulting SpL201 cell line expresses p75 and nestin, two proteins expressed by neural crest-derived cells, as well as peripheral myelin protein 22, protein zero, and Oct-6 as markers of the Schwann cell lineage. When cultured in EGF-containing medium, the SpL201 cells proliferate and maintain an undifferentiated, Schwann cell precursor cell-like state. The cell line is dependent on EGF for survival but can differentiate into early Schwann cell-like cells in response to exogenous factors. Like primary rat Schwann cells, SpL201 cells upregulate Oct-6 and myelin gene expression in response to forskolin treatment. Furthermore, the SpL201 cell line can form myelin in the presence of axons in vitro and is capable of extensively remyelinating a CNS white matter lesion in vivo. Thus, this cell line provides a valuable and unique tool to study the Schwann cell lineage, including differentiation from the Schwann cell precursor cell stage through to myelination.

Neuregulin signaling through a PI3K/Akt/Bad pathway in Schwann cell survival

beta-Neuregulin (betaNRG) is a potent Schwann cell survival factor that binds to and activates a heterodimeric ErbB2/ErbB3 receptor complex. We found that NRG receptor signaling rapidly activated phosphoinositide 3-kinase (PI3K) in serum-starved Schwann cells, while PI3K inhibitors markedly exacerbated apoptosis and completely blocked NRG-mediated rescue. NRG also rapidly signaled the phosphorylation of mitogen-activated protein kinase (MAPK) and the serine/threonine kinase Akt. The activation of Akt and MAPK in parallel pathways downstream from PI3K resulted in the phosphorylation of Bad at different serine residues. PI3K inhibitors that blocked NRG-mediated rescue also blocked the phosphorylation of Akt, MAPK, and Bad. However, selective inhibition of MEK-dependent Bad phosphorylation downstream from PI3K had no effect on NRG-mediated survival. Conversely, ectopic expression of wild-type Akt not only enhanced Bad phosphorylation but also enhanced autocrine- and NRG-mediated Schwann cell survival. Taken together, these results demonstrate that NRG receptor signaling through a PI3K/Akt/Bad pathway functions in Schwann cell survival.

The angiogenic factor midkine is aberrantly expressed in NF1-deficient Schwann cells and is a mitogen for neurofibroma-derived cells

Loss of the tumor suppressor gene NF1 in neurofibromatosis type 1 (NF1) contributes to the development of a variety of tumors, including malignant peripheral nerve sheath tumors (MPNST) and benign neurofibromas. Of the different cell types found in neurofibromas, Schwann cells usually provide between 40 and 80%, and are thought to be critical for tumor growth. Here we describe the identification of growth factors that are upregulated in NF1-/- mouse Schwann cells and are potential regulators of angiogenesis and cell growth. Basic fibroblast growth factor (FGF-2), platelet-derived growth factor (PDGF) and midkine (MK) were found to be induced by loss of neurofibromin and MK was further characterized. MK was induced in human neurofibromas, schwannomas, and various nervous system tumors associated with NF1 or NF2; midkine showed an expression pattern overlapping but distinct from its homolog pleiotrophin (PTN). Immunohistochemistry revealed expression of MK in S-100 positive Schwann cells of dermal and plexiform neurofibromas, and in endothelial cells of tumor blood vessels, but not in normal blood vessels. Furthermore, MK demonstrated potent mitogenic activity for human systemic and brain endothelial cells in vitro and stimulated proliferation and soft agar colony formation of human MPNST derived S100 positive cells and fibroblastoid cells derived from an NF1 neurofibroma. The data support a possible central role for MK as a mediator of angiogenesis and neurofibroma growth in NF1. Oncogene (2001) 20, 97 - 105.

Endothelins control the timing of Schwann cell generation in vitro and in vivo

Schwann cell precursors, derivatives of the neural crest, generate Schwann cells in a process that is tightly timed, well characterized, and directly controlled by axonal signals, in particular beta-neuregulins. Here we provide evidence that endothelins (ETs) are also important for survival and lineage progression in this system. We show that ETs promote rat Schwann cell precursor survival in vitro without stimulation of DNA synthesis. Using ET receptor agonists and antagonists, we demonstrate that this action of ET is mediated by the ET(B) receptor. RT-PCR reveals the presence of ET and ET receptor mRNA in the developing rat PNS. We showed previously that in vitro beta-neuregulins promote the generation of Schwann cells from precursors on schedule and that this process can be accelerated by fibroblast growth factor 2. Here we show that although ETs promote long-term precursor survival the transition of precursors to Schwann cells is delayed. Moreover, ETs block the maturation effects of beta-neuregulins. In spotting lethal rats, in which functional ET(B) receptors are absent, we find accelerated expression of the Schwann cell marker S100 in developing nerves. These observations indicate that complex growth factor interactions control the timing of Schwann cell development in embryonic nerves and that ETs act as negative regulators of Schwann cell generation.

Platelet-derived growth factor-BB supports the survival of cultured rat Schwann cell precursors in synergy with neurotrophin-3

To understand the intimate axon-Schwann cell relationship required for the accurate development and regeneration of the peripheral nervous system (PNS), it is important to elucidate the repertoire of growth factors involved in this tightly regulated bi-directional dialogue. We focused on the identification and functional characterization of receptor tyrosine kinases (RTKs) in Schwann cells to gain insights into the corresponding growth factor ligands, which may be regulating the highly controlled differentiation of the Schwann cell lineage. Using an RT-PCR based differential display approach, we have identified 17 tyrosine kinases in embryonic rat sciatic nerves during the crucial transition from Schwann cell precursors to early Schwann cells. In this study, we have examined the expression and function of TrkC and the platelet-derived growth factor (PDGF) receptors alpha and beta on Schwann cell precursor cells. These receptors are expressed on freshly isolated Schwann cell precursors, and we show that PDGF-BB is able to rescue a subpopulation of these cells from apoptotic cell death in vitro. Furthermore, the TrkC-ligand neurotrophin-3 (NT-3) can act synergistically to potentiate this effect. However, PDGF-BB and NT-3 do not induce Schwann cell precursor proliferation or differentiation. Our data are consistent with a model suggesting that a combination of growth factors that include PDGF-BB and NT-3 are acting in concert and in synergy to regulate early Schwann cell development.

Insulin-like growth factor-I prevents caspase-mediated apoptosis in Schwann cells

Both neurons and glia succumb to programmed cell death (PCD) when deprived of growth factors at critical periods in development or following injury. Insulin-like growth factor-I (IGF-I) prevents apoptosis in neurons in vitro. To investigate whether IGF-I can protect Schwann cells (SC) from apoptosis, SC were harvested from postnatal day 3 rats and maintained in serum-containing media until confluency. When cells were switched to serum-free defined media (DM) for 12-72 h, they underwent PCD. Addition of insulin or IGF-I prevented apoptosis. Bisbenzamide staining revealed nuclear condensation and formation of apoptotic bodies in SC grown in DM alone, but SC grown in DM plus IGF-I had normal nuclear morphology. The phosphatidylinositol 3-kinase (PI 3-K) inhibitor LY294002 blocked IGF-I-mediated protection. Caspase-3 activity was rapidly activated upon serum withdrawal in SC, and the caspase inhibitor BAF blocked apoptosis. These results suggest that IGF-I rescues SC from apoptosis via PI 3-K signaling which is upstream from caspase activation.

The role of POU domain proteins in the regulation of mammalian pituitary and nervous system development

POU domain proteins represent a subfamily of homeodomain-containing transcription factors that are expressed in many animal orders in a number of distinct regions in the developing and adult organism. In mammals, the expression profiles of these factors have suggested roles for class I, class III, and class IV POU domain proteins in the development, maintenance, and function of the endocrine and nervous systems. The genetic characterizations of the functions of these proteins during the generation, differentiation, and maturation of cells comprising these tissues have revealed a requirement for the individual actions of these transcription factors in the development of various elements of the anterior pituitary, the brain, and the somatosensory, vestibular/cochlear, and visual systems.

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.

Schwann cell development in embryonic mouse nerves

Previously we proposed that Schwann cell development from the neural crest is a two-step process that involves the generation of one main intermediate cell type, the Schwann cell precursor. Until now Schwann cell precursors have only been identified in the rat, and much remains to be learned about these cells and how they generate Schwann cells. Here we identify this cell in the mouse and analyze its transition to form Schwann cells in terms of timing, molecular expression, and extracellular signals and intracellular pathways involved in survival, proliferation, and differentiation. In the mouse, the transition from precursors to Schwann cells takes place 2 days earlier than in the rat, i.e., between embryo days 12/13 and 15/16, and is accompanied by the appearance of the 04 antigen and the establishment of an autocrine survival circuit. Beta neuregulins block precursor apoptosis and support Schwann cell generation in vitro, a process that is accelerated by basic fibroblast growth factor 2. The development of Schwann cells from precursors also involves a change in the intracellular survival signals utilized by neuregulins: To block precursor death neuregulins need to signal through both the mitogen-activated protein kinase and the phosphoinositide-3-kinase pathways although neuregulins support Schwann cell survival by signaling through the phosphoinositide-3-kinase pathway alone. Last, we describe the generation of precursor cultures from single 12-day-old embryos, a prerequisite for culture studies of genetically altered precursors when embryos are non-identical with respect to the transgene in question.

A role for insulin-like growth factor-I in the regulation of Schwann cell survival

During postnatal development in the peripheral nerve, differentiating Schwann cells are susceptible to apoptotic death. Schwann cell apoptosis is regulated by axons and serves as one mechanism through which axon and Schwann cell numbers are correctly matched. This regulation is mediated in part by the provision of limiting axon-derived trophic molecules, although neuregulin-1 (NRG-1) is the only trophic factor shown to date to support Schwann cell survival. In this report, we identify insulin-like growth factor-I (IGF-I) as an additional trophin that can promote Schwann cell survival in vitro. We find that IGF-I, like NRG-1, can prevent the apoptotic death of postnatal rat Schwann cells cultured under conditions of serum withdrawal. Moreover, we show that differentiating Schwann cells in the rat sciatic nerve express both the IGF-I receptor (IGF-I R) and IGF-I throughout postnatal development. These results indicate that IGF-I is likely to control Schwann cell viability in the developing peripheral nerve and, together with other findings, raise the interesting possibility that such survival regulation may switch during postnatal development from an axon-dependent mechanism to an autocrine and/or paracrine one.

Control of Schwann cell survival and proliferation: autocrine factors and neuregulins

Postnatal rat Schwann cells secrete factors that prevent the programmed cell death (PCD) of low-density Schwann cells in serum-free culture. These autocrine survival signal(s) do not promote Schwann cell proliferation. Moreover, while NRG and bFGF, which promote proliferation, both rescue a subpopulation of neonatal Schwann cells from PCD, they do not rescue freshly isolated Schwann cells from older animals; other known protein factors tested also do not mimic the autocrine signal. These results suggest that Schwann cells switch their survival dependency around the time of birth from axonal signals such as NRG to autocrine signals. Such an arrangement would be advantageous for the regeneration of peripheral axons following injury. We also compared NRG-induced Schwann cell proliferation using autocrine signals or serum to promote survival. The autocrine signals increase the rate of NRG-stimulated proliferation of low-density Schwann cells in serum-free medium, whereas serum inhibits proliferation by inhibiting both the production of survival signals and the expression of erbB2 and erbB3 receptors; these inhibitions are all reversed by forskolin. In contrast, forskolin has no effect on proliferation when the cells are exposed to high levels of autocrine factors.

Fibroblast growth factors as multifunctional signaling factors

The fibroblast growth factor (FGF) family consists of at least 15 structurally related polypeptide growth factors. Their expression is controlled at the levels of transcription, mRNA stability, and translation. The bioavailability of FGFs is further modulated by posttranslational processing and regulated protein trafficking. FGFs bind to receptor tyrosine kinases (FGFRs), heparan sulfate proteoglycans (HSPG), and a cysteine-rich FGF receptor (CFR). FGFRs are required for most biological activities of FGFs. HSPGs alter FGF-FGFR interactions and CFR participates in FGF intracellular transport. FGF signaling pathways are intricate and are intertwined with insulin-like growth factor, transforming growth factor-beta, bone morphogenetic protein, and vertebrate homologs of Drosophila wingless activated pathways. FGFs are major regulators of embryonic development: They influence the formation of the primary body axis, neural axis, limbs, and other structures. The activities of FGFs depend on their coordination of fundamental cellular functions, such as survival, replication, differentiation, adhesion, and motility, through effects on gene expression and the cytoskeleton.

Origin and early development of Schwann cells

Cellular events leading to the generation of Schwann cells from the neural crest have recently been clarified and it is now possible to outline a relatively simple model of the Schwann cell lineage in the rat and mouse. Neural crest cells have to undergo three main developmental transitions to become mature Schwann cells. These are the formation of Schwann cell precursors from crest cells, the formation of immature Schwann cells from precursors and, lastly, the postnatal and reversible generation of non-myelin- and myelin-forming Schwann cells. Axonal signals involving neuregulins are important regulators of these events, in particular of the survival, proliferation, and differentiation of Schwann cell precursors. Transcription factors likely to be involved in the developmental transitions are beginning to be identified. These include Oct-6, Krox-20, and Pax-3 but also members of the basic helix-loop-helix family, Sox 10, and the cAMP response element binding protein CREB.

The neuron-glia signal beta neuregulin induces sustained CREB phosphorylation on Ser-133 in cultured rat Schwann cells

beta neuregulins (also called NDF, GGF, ARIA, and heregulins) are neuron-derived molecules that are likely to be responsible for Schwann cell precursor survival, proliferation, and maturation in vivo and in vitro. Although the receptors to which beta neuregulins bind have been defined, little is known about the transcription factors these important ligands activate. Using antibodies, quantitative imaging methods and Western blotting, we show that beta neuregulin induces a high level of phosphorylation of the transcription factor cyclic AMP response element binding protein (CREB) on Ser-133 in cultured rat Schwann cells and that the phosphorylation is prolonged over several hours. In contrast, neurotrophins, CNTF, FGF-2, EGF, and TGF beta induce little or no phosphorylation of CREB despite the fact that receptors for these factors are present on Schwann cells. As expected CREB phosphorylation was detected following cAMP elevation, and it was also induced by elevation of cytoplasmic Ca2+, endothelin 1, and PDGF-BB. The signal was lower than that seen in response to beta neuregulin, and transient, unlike the sustained CREB activation induced by beta neuregulin. Our results suggest that the sustained phosphorylation of CREB on Ser-133 may contribute to the broad spectrum of effects that beta neuregulins have on cells of the Schwann cell lineage and that the CREB pathway may be important for transduction of neuregulin signals in Schwann cells.

Anti-apoptotic signaling by a colony-stimulating factor-1 receptor/insulin receptor chimera with a juxtamembrane deletion

The intracellular mechanisms used by insulin and insulin-like growth factors to block programmed cell death are unknown. To identify receptor structures and signaling pathways essential for anti-apoptotic effects on cells, we have created a chimeric receptor (colony-stimulating factor-1 receptor/insulin receptor chimera (CSF1R/IR)) connecting the extracellular, ligand-binding domain of the colony-stimulating factor-1 (CSF-1) receptor to the transmembrane and cytoplasmic domains of the insulin receptor. Upon activation with CSF-1, the CSF1R/IR phosphorylates itself and intracellular substrates in a manner characteristic of normal insulin receptors. CSF-1 treatment protected cells expressing the CSF1R/IR from staurosporine-induced apoptosis. A chimeric receptor (CSF1R/IRDelta960) with a deletion of 12 amino acids from its juxtamembrane domain was constructed and expressed. CSF-1-treated cells expressing the CSF1R/IRDelta960 are unable to phosphorylate IRS-1 and Shc (Chaika, O. V., Chaika, N., Volle, D. J., Wilden, P. A. , Pirrucello, S. J., and Lewis, R. E. (1997) J. Biol. Chem. 272, 11968-11974). CSF-1 stimulated glucose uptake, mitogen-activated protein kinases, and IRS-1-associated phosphatidylinositol 3' kinase in cells expressing the CSF1R/IR but not in cells expressing the CSF1R/IRDelta960. Surprisingly, the CSF1R/IRDelta960 was as effective as the CSF1R/IR in mediating CSF-1 protection of cells from staurosporine-induced apoptosis. These observations indicate that the anti-apoptotic effects of the insulin receptor cytoplasmic domain can be mediated by signaling pathways distinct from those requiring IRS-1 and Shc.

Widespread elimination of naturally occurring neuronal death in Bax-deficient mice

The proapoptotic molecule BAX is required for death of sympathetic and motor neurons in the setting of trophic factor deprivation. Furthermore, adult Bax-/- mice have more motor neurons than do their wild-type counterparts. These findings raise the possibility that BAX regulates naturally occurring cell death during development in many neuronal populations. To test this idea, we assessed apoptosis using TUNEL labeling in several well-studied neural systems during embryonic and early postnatal development in Bax-/- mice. Remarkably, naturally occurring cell death is virtually eliminated between embryonic day 11.5 (E11.5) and postnatal day 1 (PN1) in most peripheral ganglia, in motor pools in the spinal cord, and in the trigeminal brainstem nuclear complex. Additionally, reduction, although not elimination, of cell death was noted throughout the developing cerebellum, in some layers of the retina, and in the hippocampus. Saving of cells was verified by axon counts of dorsal and ventral roots, as well as facial and optic nerves that revealed 24-35% increases in axon number. Interestingly, many of the supernumerary axons had very small cross-sectional areas, suggesting that the associated neurons are not normal. We conclude that BAX is a critical mediator of naturally occurring death of peripheral and CNS neurons during embryonic life. However, rescue from naturally occurring cell death does not imply that the neurons will develop normal functional capabilities.

In vitro and in vivo behaviour of NDF-expanded monkey Schwann cells

Schwann cells, the myelin-forming cells of the peripheral nervous system may play a major role in the regeneration and remyelination not only of the peripheral but also of the central nervous system. The discovery of the mitogenicity of human recombinant forms of neuregulins (glial growth factors) on primate Schwann cells allows us to envisage a considerable expansion of these cells in culture with a view to autologous transplantation in the central nervous system. To assay this possibility, we used human recombinant neu-differentiation factor beta (NDFbeta) to expand monkey Schwann cells derived from perinatal and adult nerve biopsies. We report that NDFbeta containing the epidermal growth factor (EGF)-like domain (residues 177-228) is a potent mitogen for monkey Schwann cells but is more effective on perinatal than adult Schwann cells. Moreover, continuous treatment with NDFbeta, does not seem to prevent Schwann cells differentiation into myelin-forming cells after their transplantation into the demyelinated mouse spinal cord. These observations, in addition to the close similarities of in vitro behaviour which exist between human and monkey Schwann cells, indicate that monkey Schwann cells could be an ideal tool to study the potential and limits of autologous transplantation in a non-human primate model of central nervous system demyelination.

Ureteric bud cells secrete multiple factors, including bFGF, which rescue renal progenitors from apoptosis

Kidney development requires reciprocal interactions between the ureteric bud and the metanephrogenic mesenchyme. Whereas survival of mesenchyme and development of nephrons from mesenchymal cells depends on signals from the invading ureteric bud, growth of the ureteric bud depends on signals from the mesenchyme. This codependency makes it difficult to identify molecules expressed by the ureteric bud that regulate mesenchymal growth. To determine how the ureteric bud signals the mesenchyme, we previously isolated ureteric bud cell lines (UB cells). These cells secrete soluble factors which rescue the mesenchyme from apoptosis. We now report that four heparin binding factors mediate this growth activity. One of these is basic fibroblast growth factor (bFGF), which is synthesized by the ureteric bud when penetrating the mesenchyme. bFGF rescues three types of progenitors found in the mesenchyme: precursors of tubular epithelia, precursors of capillaries, and cells that regulate growth of the ureteric bud. These data suggest that the ureteric bud regulates the number of epithelia and vascular precursors that generate nephrons by secreting bFGF and other soluble factors.

Embryonic Schwann cell development: the biology of Schwann cell precursors and early Schwann cells

The cellular events leading to the generation of Schwann cells from the neural crest have recently been clarified and it is now possible to outline a relatively simple model of the Schwann cell lineage in the rat and mouse. Neural crest cells have to undergo 3 main developmental transitions to become mature Schwann cells. These are the formation of Schwann cell precursors from crest cells, the formation of immature Schwann cells from precursors and, lastly, the postnatal and reversible generation of non-myelin- and myelin-forming Schwann cells. Axonal signals involving neuregulins are important regulators of these events, in particular of the survival, proliferation and differentiation of Schwann cell precursors.

Overexpression of FGF-2 modulates fiber cell differentiation and survival in the mouse lens

During mammalian embryogenesis, the ocular lens forms through a temporally and spatially regulated pattern of differentiation which is thought to be coordinated at least in part by the FGF-1 and FGF-2 members of the fibroblast growth factor (FGF) family. Previous transgenic experiments in which FGF-1 or dominant negative FGF receptors were overexpressed in the lens indicated that FGF-1 could induce differentiation while differentiated lens cells rely upon FGF signaling for their survival. In this study, we asked if the 17.5 kDa FGF-2 protein was capable of inducing differentiation of lens cells in transgenic mice. Unexpectedly, differentiation was inhibited by lens-specific expression of a transgene encoding a secreted form of the 17.5 kDa bovine FGF-2 protein under the transcriptional control of the murine alphaA-crystallin promoter (alphaAIgFGF-2 transgenic mice). To address the possibility that FGF-2 functions as a modulator of fiber cell survival, alphaAIgFGF-2 transgenic mice were crossed to transgenic mice exhibiting extensive apoptosis in the lens due to the functional inactivation of the retinoblastoma protein (alphaAE7 transgenic mice). The level of apoptosis in the lenses of double transgenic mice was substantially reduced as compared to the level in lenses from alphaAE7 only mice. These studies indicate that FGF-2 can act as a modulator of the later stages of differentiation including fiber cell survival. Additionally, they imply that control of lens development by FGFs is a complex process in which FGF-1 and FGF-2 play distinct roles.

Decreased skin sensory innervation in transgenic mice overexpressing insulin-like growth factor-II

Cutaneous sensory innervation was studied in transgenic mice overexpressing insulin-like growth factor II using a keratin promoter. The skin area of these animals is enlarged providing increased target for sensory neurons. L4 dorsal root ganglion cell counts revealed that the total number of sensory neurons was the same in transgenics as control animals. Levels of nerve growth factor per unit weight of skin were also unchanged. The cutaneous nerves of the hindlimb were immunostained with the pan-neuronal marker PGP 9.5 in transgenic and control mice at postnatal day 0 and 21. The innervation in transgenic mice was markedly reduced, particularly in superficial dermis and epidermis and in some areas innervation was completely absent. The effect was greatest in distal skin regions and increased with age. Since insulin-like growth factor II has been reported to be a sensory neurotrophic factor, its effect on neurite outgrowth was tested on embryonic day 14 and 18 mouse lumbar dorsal root ganglion explants in culture. Under these conditions insulin-like growth factor II (5-100 ng/ml) did not have strong growth promoting activity and at embryonic day 18, in the presence of 5-10 ng/ml nerve growth factor, neurite outgrowth was suppressed by insulin-like growth factor II. The results show that increased skin target and availability of nerve growth factor per se do not alter the number of innervating sensory neurons. However, reduced sensory terminal arborization and skin hypoinnervation does occur in the presence of excess insulin-like growth factor-II. It is possible that insulin-like growth factor-II inhibits terminal axon growth directly via receptors on sensory neurons or peripheral glia.

ARIA: a neuromuscular junction neuregulin

Motor neurons influence the expression and the distribution of acetylcholine receptors in skeletal muscle. Molecules that mediate this carefully choreographed interaction have recently been identified. One of them, ARIA, is a polypeptide purified from chicken brain on the basis of its ability to stimulate the synthesis of muscle acetylcholine receptors. The predicted amino acid sequence suggests that ARIA is synthesized as a transmembrane precursor protein and that it is a member of a family of ligands that activate receptor tyrosine kinases related to the epidermal growth factor receptor. Certain features of the ligand family (the neuregulins) and their receptors (erbBs) are reviewed. Evidence that ARIA plays an important role at developing and mature neuromuscularjunctions is discussed.

Axonal interactions regulate Schwann cell apoptosis in developing peripheral nerve: neuregulin receptors and the role of neuregulins

Programmed cell death during development resulting from the lack of appropriate survival factors has been demonstrated in both neurons and oligodendrocytes and occurs mostly in the form of apoptosis. We now demonstrate that Schwann cells in the rat sciatic nerve undergo apoptosis during early postnatal development and that the amount of apoptosis is markedly increased by axotomy. The apoptotic Schwann cells express the low-affinity nerve growth factor receptor but not myelin-related proteins, indicating that they are in the premyelinating state. Apoptosis resulting from normal development or from axotomy can be inhibited markedly by exogenous neuregulin. Consistent with this, the neuregulin receptor components erbB2 and erbB3 are expressed and phosphorylated in developing sciatic nerve. These data suggest that Schwann cell number in developing peripheral nerve is regulated by apoptosis through competition for axonally derived neuregulin.