Distinct isoforms of neuregulin are expressed in mesenchymal and neuronal cells during mouse development

Effect of neu differentiation factor isoforms on neonatal oligodendrocyte function

Previous studies have suggested that neu differentiation factor (NDF), a member of the neuregulin (NRG) family of growth factors, may regulate the development of PNS and CNS glial cells. There is limited information concerning the potential role of NDF on the development of neonatal (immature) oligodendrocytes (OLG) into adult OLG. We now report the effect of the two major isoform families of NDF (NDF alpha and NDF beta) on the development of cultured rat neonatal OLG. Immunocytochemical and western blot analyses of neonatal OLG using anti-erb-B antibodies revealed that these immature OLG express all four members of NRG (erb-B) receptors. Treatment of neonatal OLG with varying concentrations of either NDF alpha or NDF beta did not have a mitogenic effect on cultured neonatal OLG. Pretreatment of immature OLG with either of the NDF isoforms also did not influence the subsequent mitogenicity of other known OLG mitogens. However, treatment of neonatal OLG with either isoform of NDF influenced the survival of these cells by protecting the cells from apoptosis. Additionally, treatment of neonatal OLG with either NDF alpha or NDF beta resulted in more extensive process formation compared to control, non-treated OLG.

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.

Isoform-specific expression and function of neuregulin

Neuregulin (also known as NDF, heregulin, ARIA, GGF or SMDF), induces cell growth and differentiation. Biological effects of neuregulin are mediated by members of the erbB family of tyrosine kinase receptors. Three major neuregulin isoforms are produced from the gene, which differ substantially in sequence and in overall structure. Here we use in situ hybridization with isoform-specific probes to illustrate the spatially distinct patterns of expression of the isoforms during mouse development. Ablation of the neuregulin gene in the mouse has demonstrated multiple and independent functions of this factor in development of both the nervous system and the heart. We show here that targeted mutations that affect different isoforms result in distinct phenotypes, demonstrating that isoforms can take over specific functions in vivo. Type I neuregulin is required for generation of neural crest-derived neurons in cranial ganglia and for trabeculation of the heart ventricle, whereas type III neuregulin plays an important role in the early development of Schwann cells. The complexity of neuregulin functions in development is therefore due to independent roles played by distinct isoforms.

Role of GGF/neuregulin signaling in interactions between migrating neurons and radial glia in the developing cerebral cortex

During neuronal migration to the developing cerebral cortex, neurons regulate radial glial cell function and radial glial cells, in turn, support neuronal cell migration and differentiation. To study how migrating neurons and radial glial cells influence each others' function in the developing cerebral cortex, we examined the role of glial growth factor (a soluble form of neuregulin), in neuron-radial glial interactions. Here, we show that GGF is expressed by migrating cortical neurons and promotes their migration along radial glial fibers. Concurrently, GGF also promotes the maintenance and elongation of radial glial cells, which are essential for guiding neuronal migration to the cortex. In the absence of GGF signaling via erbB2 receptors, radial glial development is abnormal. Furthermore, GGF's regulation of radial glial development is mediated in part by brain lipid-binding protein (BLBP), a neuronally induced, radial glial molecule, previously shown to be essential for the establishment and maintenance of radial glial fiber system. The ability of GGF to influence both neuronal migration and radial glial development in a mutually dependent manner suggests that it functions as a mediator of interactions between migrating neurons and radial glial cells in the developing cerebral cortex.

Effects of delayed re-innervation on the expression of c-erbB receptors by chronically denervated rat Schwann cells in vivo

We propose that chronically denervated Schwann cells may be less able to respond to axonal signals than their acutely denervated counterparts, and that this lack of sensitivity may be one reason why axons fail to regenerate into chronically denervated nerve stumps. To test this proposal we have used in situ hybridization, and quantitative and qualitative immunohistochemistry to compare the expression of c-erbB2 and c-erbB4 receptors in Schwann cells denervated for up to 6 months in vivo, with that seen in Schwann cells denervated for similar periods of time but then exposed to regenerating axons. The results were correlated with the extent of axonal regeneration in each experimental group as assessed from transverse sections which had been double-immunolabelled using anti S-100 and anti-beta tubulin III antibodies. Since c-erbBs are receptors for neuronally derived neuregulins we probed the appropriate axotomised DRG neurons for expression of GGF2 mRNA. When the denervated distal stumps were anastomosed to acutely transected proximal stumps, GGF expression in DRGs increased transiently during the first week: we assume that secreted GGF2 derived from regrowing axon sprouts would have been available to Schwann cells in all distal stumps. Endoneurial cell proliferation (predominantly Schwann cell proliferation); levels of expression of c-erbB receptors by Schwann cells, and the degree to which axons regenerated into the distal stumps all decreased as the period of prior denervation increased: the longer the time of denervation, the lower the expression of c-erbBs in Schwann cells, and the smaller the percentage of bands of Bungner which were re-innervated.

The mesenchymal winged helix transcription factor Fkh6 is required for the control of gastrointestinal proliferation and differentiation

The winged helix transcription factor Fkh6 is expressed in the mesoderm of the gastrointestinal tract directly adjacent to the endoderm-derived epithelium. Homozygous null mice for Fkh6 showed postnatal growth retardation secondary to severe structural abnormalities of the stomach, duodenum, and jejunum. Dysregulation of epithelial cell proliferation in these organs resulted in an approximately fourfold increase in the number of dividing intestinal epithelial cells and marked expansion of the proliferative zone. As a consequence, the tissue architecture of the stomach and small intestine was distorted, with abnormal crypt structure, formation of mucin filled cysts, and lengthening of villi. Changes in the cellular phenotype and composition of the gastric and intestinal epithelia also suggests that epithelial cell-lineage allocation or differentiation may be affected by loss of Fkh6. From the analysis of a number of potential signaling molecules, we found Bmp2 and Bmp4 expression reduced in the gastrointestinal tract of Fkh6 mutant mice, suggesting that Fkh6 directs a signaling cascade that mediates communication between the mesenchyme and endoderm of the gut to regulate cell proliferation.

Neuregulins stimulate the functional expression of Ca2+-activated K+ channels in developing chicken parasympathetic neurons

The developmental expression of macroscopic Ca2+-activated K+ currents (IK[Ca]) in chicken ciliary ganglion (CG) neurons is dependent in part on trophic factors released from preganglionic nerve terminals. Neuregulins are expressed in the preganglionic neurons that innervate the chicken CG and are therefore plausible candidates for this activity. Application of 1 nM beta1-neuregulin peptide for 12 hr evokes a large (7- to 10-fold) increase in IK[Ca] in embryonic day 9 CG neurons, even in the presence of a translational inhibitor. A similar posttranslational effect is produced by high concentrations (10 nM) of epidermal growth factor and type alpha transforming growth factor but not by 10 nM alpha2-neuregulin peptide or by neurotrophins at 40 ng.ml-1. beta1-neuregulin treatment for 12 hr also confers Ca2+ sensitivity onto large-conductance (285 pS) K+ channels observed in inside-out patches. beta-Neuregulins have no effect on voltage-activated Ca2+ currents of CG neurons. These data support the hypothesis that beta-neuregulins mediate the trophic effects of preganglionic nerve terminals on the electrophysiological differentiation of developing CG neurons.

Maintenance of acetylcholine receptor number by neuregulins at the neuromuscular junction in vivo

ARIA (for acetylcholine receptor-inducing activity), a protein purified on the basis of its ability to stimulate acetylcholine receptor (AChR) synthesis in cultured myotubes, is a member of the neuregulin family and is present at motor endplates. This suggests an important role for neuregulins in mediating the nerve-dependent accumulation of AChRs in the postsynaptic membrane. Nerve-muscle synapses have now been analyzed in neuregulin-deficient animals. Mice that are heterozygous for the deletion of neuregulin isoforms containing an immunoglobulin-like domain are myasthenic. Postsynaptic AChR density is significantly reduced, as judged by the decrease in the mean amplitude of spontaneous miniature endplate potentials and bungarotoxin binding. On the other hand, the mean amplitude of evoked endplate potentials was not decreased, due to an increase in the number of quanta released per impulse, a compensation that has been observed in other myasthenic states. Thus, the density of AChRs in the postsynaptic membrane depends on immunoglobulin-containing neuregulin isoforms throughout the life of the animal.

Receptor tyrosine kinase-dependent neural crest migration in response to differentially localized growth factors

How different neural crest derivatives differentiate in distinct embryonic locations in the vertebrate embryo is an intriguing issue. Many attempts have been made to understand the underlying mechanism of specific pathway choices made by migrating neural crest cells. In this speculative review we suggest a new mechanism for the regulation of neural crest cell migration patterns in avian and mammalian embryos, based on recent progress in understanding the expression and activity of receptor tyrosine kinases during embryogenesis. Distinct subpopulations of crestderived cells express specific receptor tyrosine kinases while residing in a migration staging area. We postulate that the differential expression of receptor tyrosine kinases by specific subpopulations of neural crest cells allows them to respond to localized growth factor ligand activity in the embryo. Thus, the migration pathway taken by neural crest subpopulations is determined by their receptor tyrosine kinase response to the differential localization of their cognate ligand.

Expression of neuregulins and their putative receptors, ErbB2 and ErbB3, is induced during Wallerian degeneration

Schwann cell dedifferentiation and proliferation is a prerequisite to axonal regeneration in the injured peripheral nervous system. The neuregulin (NRG) family of growth and differentiation factors may play a particularly important role in this process, because these axon-associated molecules are potent Schwann cell mitogens and differentiation factors in vitro. We have examined Schwann cell DNA synthesis and the expression of NRGs and their receptors, the erbB membrane tyrosine kinases, in rat sciatic nerve, sensory ganglia, and spinal cord 0-30 d postaxotomy. Analysis of NRG cDNAs from these tissues revealed several novel splice variants and showed that cells endogenous to injured nerve express NRG mRNAs. A selective induction of mRNAs encoding the glial growth factor (GGF) subfamily of NRGs occurs in nerve beginning 3 d postaxotomy and thus coincides with the onset of Schwann cell DNA synthesis. In later stages of Wallerian degeneration, however, Schwann cell mitogenesis markedly decreases, whereas elevated GGF expression persists. Of the four known erbB kinases, Schwann cells express both erbB2 and erbB3 receptors over the entire interval studied. Expression of erbB2 and erbB3 is coordinately induced in response to axotomy, indicating that Schwann cell responses to NRGs may be modulated by changes in receptor density. Neuregulin (including transmembrane precursors) and erbB protein are associated with Schwann cells postaxotomy. Thus, in contrast to the concept of NRGs as axon-associated mitogens, our findings suggest that NRGs produced by Schwann cells themselves may be partially responsible for Schwann cell proliferation during Wallerian degeneration, probably acting via autocrine or paracrine mechanisms.

Expression of neuregulins and their putative receptors, ErbB2 and ErbB3, is induced during Wallerian degeneration

Schwann cell dedifferentiation and proliferation is a prerequisite to axonal regeneration in the injured peripheral nervous system. The neuregulin (NRG) family of growth and differentiation factors may play a particularly important role in this process, because these axon-associated molecules are potent Schwann cell mitogens and differentiation factors in vitro. We have examined Schwann cell DNA synthesis and the expression of NRGs and their receptors, the erbB membrane tyrosine kinases, in rat sciatic nerve, sensory ganglia, and spinal cord 0-30 d postaxotomy. Analysis of NRG cDNAs from these tissues revealed several novel splice variants and showed that cells endogenous to injured nerve express NRG mRNAs. A selective induction of mRNAs encoding the glial growth factor (GGF) subfamily of NRGs occurs in nerve beginning 3 d postaxotomy and thus coincides with the onset of Schwann cell DNA synthesis. In later stages of Wallerian degeneration, however, Schwann cell mitogenesis markedly decreases, whereas elevated GGF expression persists. Of the four known erbB kinases, Schwann cells express both erbB2 and erbB3 receptors over the entire interval studied. Expression of erbB2 and erbB3 is coordinately induced in response to axotomy, indicating that Schwann cell responses to NRGs may be modulated by changes in receptor density. Neuregulin (including transmembrane precursors) and erbB protein are associated with Schwann cells postaxotomy. Thus, in contrast to the concept of NRGs as axon-associated mitogens, our findings suggest that NRGs produced by Schwann cells themselves may be partially responsible for Schwann cell proliferation during Wallerian degeneration, probably acting via autocrine or paracrine mechanisms.

Chicken acidic leucine-rich EGF-like domain containing brain protein (CALEB), a neural member of the EGF family of differentiation factors, is implicated in neurite formation

Chicken acidic leucine-rich EGF-like domain containing brain protein (CALEB) was identified by combining binding assays with immunological screens in the chicken nervous system as a novel member of the EGF family of differentiation factors. cDNA cloning indicates that CALEB is a multidomain protein that consists of an NH2-terminal glycosylation region, a leucine-proline-rich segment, an acidic box, a single EGF-like domain, a transmembrane, and a short cytoplasmic stretch. In the developing nervous system, CALEB is associated with glial and neuronal surfaces. CALEB is composed of a 140/130-kD doublet, an 80-kD band, and a chondroitinsulfate-containing 200-kD component. The latter two components are expressed in the embryonic nervous system and are downregulated in the adult nervous system. CALEB binds to the extracellular matrix glycoproteins tenascin-C and -R. In vitro antibody perturbation experiments reveal a participation of CALEB in neurite formation in a permissive environment.

Neuregulins and neuregulin receptors in neural development

The neuregulins are a family of closely related proteins that play important roles in neural and cardiac development, as well as in mammary carcinogenesis. The pleiotropic activities of these molecules are transduced by a set of receptor protein tyrosine kinases that exhibit structural similarity to the receptor for epidermal growth factor. Recent results have demonstrated essential roles for the neuregulins and their receptors in regulating cell number, determining cell fate, and establishing pattern in the developing central and peripheral nervous systems.

Retinal ganglion cell axons drive the proliferation of astrocytes in the developing rodent optic nerve

We show that the proliferation of astrocytes in the developing rodent optic nerve absolutely depends on axons and that this axonal influence depends on axonal transport but not on axonal electrical activity. We also show that purified retinal ganglion cells stimulate DNA synthesis in optic nerve astrocytes in culture and that the effect can be mimicked by fibroblast growth factor but not by neuregulins or several other growth factors. Taken together with previous findings, our present results indicate that axons promote glial cell proliferation and survival in the developing optic nerve by at least three distinct mechanisms.

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.

Glial growth factor 2, a soluble neuregulin, directly increases Schwann cell motility and indirectly promotes neurite outgrowth

Schwann cells proliferate, migrate, and act as sources of neurotrophic support during development and regeneration of peripheral nerves. Recent studies have demonstrated that neuregulins, a family of growth factors secreted by developing motor and peripheral neurons, influence Schwann cell development. In this study, we use three distinct assays to show that glial growth factor 2 (GGF2), a secreted neuregulin, exerts multiple effects on mature Schwann cells in vitro. At doses submaximal for proliferation, GGF2 increases the motility of Schwann cells cultured on peripheral nerve cryosections. Furthermore, in a novel bioassay, focal application of GGF2 causes directed migration in conventional monolayer cultures of directed migration of Schwann cells. At higher doses, GGF2 causes proliferation, as described previously. In a new explant culture system designed to emulate entubulation repair of transected peripheral nerves, GGF2 concentrations greater than necessary to saturate the mitotic response induce the secretion by Schwann cells of activities that promote sympathetic neuron survival and outgrowth. These findings support a model in which neuregulins secreted by peripheral neurons are key components of reciprocal neuron-glia interactions that are important for peripheral nerve development and regeneration.

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.

The regulation of Krox-20 expression reveals important steps in the control of peripheral glial cell development

The zinc finger transcription factor gene Krox-20 is expressed in Schwann cells and is required for the myelination of peripheral nerves. We show that the regulation of Krox-20 expression in peripheral glial cells reveals three important steps in the development and differentiation of these cells. (i) Expression of Krox-20 in Schwann cells requires continuous neuronal signalling via direct axonal contact. Therefore Krox-20 appears to be a key component of the transduction cascade linking axonal signalling to myelination. (ii) Krox-20 inducibility is acquired by Schwann cells at the time that they are formed from their precursors. Diffusible factor(s) synthesised by the neural tube can mediate this transition and can be mimicked by NDFbeta or a combination of CNTF and bFGF. Furthermore, the neural tube activity is blocked by a hybrid protein containing the NDF-binding domain of the ErbB4 receptor, strongly implicating NDF in the physiological transition. (iii) In sensory ganglia, the microenvironment is capable of negatively regulating Krox-20, presumably by preventing the conversion of satellite glial cells toward a Schwann cell-like phenotype.

Neu differentiation factor/neuregulin isoforms activate distinct receptor combinations

The multiple isoforms of Neu differentiation factor (NDF/neuregulin) induce a pleiotropic cellular response that is isoform-specific and cell type-dependent. The molecular basis of this heterogeneity was addressed by comparing the two major groups of isoforms, alpha and beta. Both groups bind to the catalytically impaired receptor tyrosine kinase ErbB-3, whose mitogenic stimulation by NDF requires transactivation by other ErbB proteins, either ErbB-1 or ErbB-2. By expressing each pair of receptors in interleukin 3-dependent myeloid cells, we found that both isoforms induced mitogenic signals in cells co-expressing the combination of ErbB-3 with ErbB-2. However, only the beta isoform stimulated cells that expressed both ErbB-3 and ErbB-1, and neither isoform was active on cells expressing ErbB-3 alone. Both isoforms bind to all ErbB-3-expressing cells, albeit with different affinities, but the co-stimulatory mitogenic effect is correlated with the ability of each auxiliary receptor to transphosphorylate ErbB-3. These results imply that NDF isoforms differ in their ability to induce receptor heterodimers; whereas both types of isoforms signal through ErbB-3/ErbB-2 heterodimers, only beta isoforms are able to stabilize ErbB-3/ErbB-1 heterodimers.

Glial growth factor 2, a soluble neuregulin, directly increases Schwann cell motility and indirectly promotes neurite outgrowth

Schwann cells proliferate, migrate, and act as sources of neurotrophic support during development and regeneration of peripheral nerves. Recent studies have demonstrated that neuregulins, a family of growth factors secreted by developing motor and peripheral neurons, influence Schwann cell development. In this study, we use three distinct assays to show that glial growth factor 2 (GGF2), a secreted neuregulin, exerts multiple effects on mature Schwann cells in vitro. At doses submaximal for proliferation, GGF2 increases the motility of Schwann cells cultured on peripheral nerve cryosections. Furthermore, in a novel bioassay, focal application of GGF2 causes directed migration in conventional monolayer cultures of directed migration of Schwann cells. At higher doses, GGF2 causes proliferation, as described previously. In a new explant culture system designed to emulate entubulation repair of transected peripheral nerves, GGF2 concentrations greater than necessary to saturate the mitotic response induce the secretion by Schwann cells of activities that promote sympathetic neuron survival and outgrowth. These findings support a model in which neuregulins secreted by peripheral neurons are key components of reciprocal neuron-glia interactions that are important for peripheral nerve development and regeneration.

GGF/neuregulin is a neuronal signal that promotes the proliferation and survival and inhibits the differentiation of oligodendrocyte progenitors

We show that GGF/neuregulin is a mitogen for prooligodendrocytes (O4+/O1- cells), oligodendrocytes (O4+/O1+ cells), and type-2 astrocytes. Heregulin beta 1, another neuregulin isoform, is also mitogenic. The proliferative effect of glial growth factor (GGF) does not require, but is greatly potentiated by, serum factors. GGF also promotes the survival of pro-oligodendrocytes under serum-free conditions. High levels of GGF reversibly inhibit the differentiation and lineage commitment of oligodendrocyte progenitors and, in differentiated cultures, result in loss of O1 and myelin basic protein expression. All three erbB receptors are expressed by progenitors and are activated by GGF; the relative abundance of these receptors changes during differentiation. Finally, cortical neurons release a soluble mitogen for pro-oligodendrocytes that is specifically blocked by antibodies to GGF. These results implicate the neuregulins in the neuronal regulation of oligodendrocyte progenitor proliferation, survival, and differentiation.

Motogenic and morphogenic activity of epithelial receptor tyrosine kinases

Receptor tyrosine kinases play essential roles in morphogenesis and differentiation of epithelia. Here we examined various tyrosine kinase receptors, which are preferentially expressed in epithelia (c-met, c-ros, c-neu, and the keratin growth factor [KGF] receptor), for their capacity to induce cell motility and branching morphogenesis of epithelial cells. We exchanged the ligand-binding domain of these receptors by the ectodomain of trkA and could thus control signaling by the new ligand, NGF. We demonstrate here that the tyrosine kinases of c-met, c-ros, c-neu, the KGF receptor, and trkA, but not the insulin receptor, induced scattering and increased motility of kidney epithelial cells in tissue culture. Mutational analysis suggests that SHC binding is essential for scattering and increased cell motility induced by trkA. The induction of motility in epithelial cells is thus an important feature of various receptor tyrosine kinases, which in vivo play a role in embryogenesis and metastasis. In contrast, only the c-met receptor promoted branching morphogenesis of kidney epithelial cells in three-dimensional matrices, which resemble the formation of tubular epithelia in development. Interestingly, the ability of c-met to induce morphogenesis could be transferred to trkA, when in a novel receptor hybrid COOH-terminal sequences of c-met (including Y14 to Y16) were fused to the trkA kinase domain. These data demonstrate that tubulogenesis of epithelia is a restricted activity of tyrosine kinases, as yet only demonstrated for the c-met receptor. We predict the existence of specific substrates that mediate this morphogenesis signal.

Neuregulins with an Ig-like domain are essential for mouse myocardial and neuronal development

Neuregulins are ligands for the erbB family of receptor tyrosine kinases and mediate growth and differentiation of neural crest, muscle, breast cancer, and Schwann cells. Neuregulins contain an epidermal growth factor-like domain located C-terminally to either an Ig-like domain or a cysteine-rich domain specific to the sensory and motor neuron-derived isoform. Here it is shown that elimination of the Ig-like domain-containing neuregulins by homologous recombination results in embryonic lethality associated with a deficiency of ventricular myocardial trabeculation and impairment of cranial ganglion development. The erbB receptors are expressed in myocardial cells and presumably mediate the neuregulin signal originating from endocardial cells. The trigeminal ganglion is reduced in size and lacks projections toward the brain stem and mandible. We conclude that IgL-domain-containing neuregulins play a major role in cardiac and neuronal development.

Alternative neural crest cell fates are instructively promoted by TGFbeta superfamily members

How growth factors influence the fate of multipotent progenitor cells is not well understood. Most hematopoietic growth factors act selectively as survival factors, rather than instructively as lineage determination signals. In the neural crest, neuregulin instructively promotes gliogenesis, but how alternative fates are determined is unclear. We demonstrate that bone morphogenic protein 2 (BMP2) induces the basic-helix-loop-helix protein MASH1 and neurogenesis in neural crest stem cells. In vivo, MASH1+ cells are located near sites of BMP2 mRNA expression. Some smooth muscle differentiation is also observed in BMP2. A related factor, transforming growth factor beta1 (TGFbeta1), exclusively promotes smooth muscle differentiation. Like neuregulin, BMP2 and TGFbeta1 act instructively rather than selectively. The neural crest and hematopoietic systems may therefore utilize growth factors in different ways to generate cellular diversity.

Effects of GGF/neuregulins on neuronal survival and neurite outgrowth correlate with erbB2/neu expression in developing rat retina

We have examined the expression of neuregulin and its putative receptors, erbB2/neu, erbB3 and erbB4/tyro2 during retinal development, and tested several potential functions of this class of molecules in dissociated rat retinal cell cultures. At least one form of neuregulin is expressed in the retina, from the earliest stages of retinal development examined; in addition, all three of the known receptors are expressed by retinal neurons in a developmentally regulated manner. When added to cultures of embryonic or neonatal rat retinal cells, neuregulin (rhGGF2) promotes survival and neurite extension from retinal neurons in a dose-dependent manner. These results indicate that in addition to their well described effects on glia, the neuregulins also have direct effects on central nervous system neurons.

A reciprocal cell-cell interaction mediated by NT-3 and neuregulins controls the early survival and development of sympathetic neuroblasts

Neurotrophin 3 (NT-3) can support the survival of some embryonic sympathetic neuroblasts before they become nerve growth factor dependent. We show that NT-3 is produced in vivo by nonneuronal cells neighboring embryonic sympathetic ganglia. NT-3 mRNA is produced by these nonneuronal cells in vitro and is up-regulated by platelet-derived growth factor, ciliary neurotrophic factor, and glial growth factor 2 (a neuregulin). Nonneuronal cell-conditioned medium promotes survival and induces TrkA expression in isolated sympathetic neuroblasts, and this activity is blocked by anti-NT-3 antibody. Neuroblasts also enhance NT-3 production by nonneural cells. Neuroblasts synthesize several forms of neuregulin, and antibodies to neuregulin attenuate the effect of the neuroblasts on the nonneuronal cells. These data suggest a reciprocal cell-cell interaction, in which neuroblast-derived neuregulins promote NT-3 production by neighboring nonneuronal cells, which in turn promotes neuroblast survival and further differentiation.

The importance of the microenvironment in breast cancer progression: recapitulation of mammary tumorigenesis using a unique human mammary epithelial cell model and a three-dimensional culture assay

The extracellular matrix (ECM) is a dominant regulator of tissue development and homeostasis. "Designer microenvironments" in culture and in vivo model systems have shown that the ECM regulates growth, differentiation, and apoptosis in murine and human mammary epithelial cells (MEC) through a hierarchy of transcriptional events involving the intricate interplay between soluble and physical signaling pathways. Furthermore, these studies have shown that these pathways direct and in turn are influenced by the tissue structure. Tissue structure is directed by the cooperative interactions of the cell-cell and cell-ECM pathways and can be modified by stromal factors. Not surprisingly then, loss of tissue structure and alterations in ECM components are associated with the appearance and dissemination of breast tumors, and malignancy is associated with perturbations in cell adhesion, changes in adhesion molecules, and a stromal reaction. Several lines of evidence now support the contention that the pathogenesis of breast cancer is determined (at least in part) by the dynamic interplay between the ductal epithelial cells, the microenvironment, and the tissue structure (acini). Thus, to understand the mechanisms involved in carcinogenesis, the role of the microenvironment (ECM as well as the stromal cells) with respect to tissue structure should be considered and studied. Towards this goal, we have established a unique human MEC model of tumorigenesis, which in concert with a three-dimensional assay, recapitulates many of the genetic and morphological changes observed in breast in cancer in vivo. We are currently using this system to understand the role of the microenvironment and tissue structure in breast cancer progression.

Multiple essential functions of neuregulin in development

Neuregulin (also called NDF, heregulin, GGF and ARIA) is a member of the EGF family which induces growth and differentiation of epithelial, glial and muscle cells in culture. The biological effects of the factor are mediated by tyrosine kinase receptors. Neuregulin can bind directly to erbB3 and erbB4 and receptor heterodimerization allows neuregulin-dependent activation of erbB2 (refs 1, 2, 5). A targeted mutation in mice reveals multiple essential roles of neuregulin in development. Here we show that neuregulin -/- embryos die during embryogenesis and display heart malformations. In addition, Schwann cell precursors and cranial ganglia fail to develop normally. The phenotype demonstrates that in vivo neuregulin acts locally and frequently in a paracrine manner. All cell types affected by the mutation express either erbB3 or erbB4, indicating that either of these tyrosine kinase receptors can be a component in recognition and transmission of essential neuregulin signals.

Requirement for neuregulin receptor erbB2 in neural and cardiac development

The receptor erbB2/neu is a member of the epidermal growth factor receptor (EGFR or erbB) family that also includes erbB3 and erbB4. Amplification of the erbB2/neu gene is found in many cancer types and its overexpression is correlated with a poor prognosis for breast and ovarian cancer patients. Investigation of the biology of erbB2 led to the identification of a family of ligands termed neuregulins which included the neu-differentiation factors, the heregulins, a ligand with acetylcholine-receptor-inducing activity and glial growth factor. Several lines of evidence suggest that heterodimerization of erbB2 with other erbB receptors is required for neuregulin signalling. Here we investigate the developmental role of erbB2 in mammalian development in mice carrying an erbB2 null allele. We find that mutant embryos die before E11, probably as a result of dysfunctions associated with a lack of cardiac trabeculae. Development of cranial neural-crest-derived sensory ganglia was markedly affected. DiI retrograde tracing revealed that the development of motor nerves was also compromised. Our results demonstrate the importance of erbB2 in neural and cardiac development.

The cellular response to neuregulins is governed by complex interactions of the erbB receptor family

Deregulated signaling by the four members of the epidermal growth factor receptor tyrosine kinase family (erbB family) is implicated in the genesis or progression of human cancers. However, efforts to analyze signaling by these receptors have been hampered by the diversity of ligands and extensive interreceptor cross talk. We have expressed the four human erbB family receptors, singly and in pairwise combinations, in a pro-B-lymphocyte cell line (Ba/F3) and investigated the range of interactions activated by the epidermal growth factor homology domain of the agonist neuregulin beta. The results provide the first comprehensive analysis of the response of this receptor family to a single peptide agonist. This peptide induced complex patterns of receptor tyrosine phosphorylation and regulation of Ba/F3 cell survival and proliferation. These data demonstrate the existence of several previously undocumented receptor interactions driven by neuregulin.

Neuregulins and their receptors

The recent identification of an activator for the ErbB2/Neu receptor has uncovered a new family of polypeptide growth factors that undoubtedly play a major role in the regulation of neuronal growth and differentiation. These factors, called the neuregulins, are expressed in neural and mesenchymal tissues, and activate members of the epidermal growth factor family of receptor tyrosine kinases. The identification and characterization of the neuregulins and their receptors will facilitate the dissection of the biochemical pathways regulating nervous system development.

Sequential requirement of hepatocyte growth factor and neuregulin in the morphogenesis and differentiation of the mammary gland

We have examined the role of two mesenchymal ligands of epithelial tyrosine kinase receptors in mouse mammary gland morphogenesis. In organ cultures of mammary glands, hepatocyte growth factor (HGF, scatter factor) promoted branching of the ductal trees but inhibited the production of secretory proteins. Neuregulin (NRG, neu differentiation factor) stimulated lobulo-alveolar budding and the production of milk proteins. These functional effects are paralleled by the expression of the two factors in vivo: HGF is produced in mesenchymal cells during ductal branching in the virgin animal; NRG is expressed in the mesenchyme during lobulo-alveolar development at pregnancy. The receptors of HGF and NRG (c-met, c-erbB3, and c-erbB4), which are expressed in the epithelial cells, are not regulated. In organ culture, branching morphogenesis and lobulo-alveolar differentiation of the mammary gland could be abolished by blocking expression of endogenous HGF and NRG by the respective antisense oligonucleotides; in antisense oligonucleotide-treated glands, morphogenesis could again be induced by the addition of recombinant HGF and NRG. We thus show that two major postnatal morphogenic periods of mammary gland development are dependent on sequential mesenchymal-epithelial interactions mediated by HGF and NRG.

Changes in retinal cell fate induced by overexpression of EGF receptor

The differentiation of multipotential progenitor cells in the vertebrate retina into photoreceptors, neurons and glial cells is regulated in part by cell-cell signalling. Transforming growth factor (TGF)-alpha is one of the extracellular signals implicated in the control of several aspects of retinal development, including proliferation and cell fate. The way cells interpret pleiotropic signals such as TGF-alpha is influenced by the level of expression of epidermal growth factor receptor (EGF-R) in some cell lines. To address the influence of receptor level on responses of retinal progenitor cells to TGF-alpha, additional copies of EGF-Rs were introduced in vitro and in vivo with a retrovirus. Normally in vitro, low concentrations of TGF-alpha stimulated proliferation whereas high concentrations biased choice of cell fate, inhibiting differentiation into rod photoreceptors while promoting differentiation into Müller glial cells. We report here that introduction of extra EGF-Rs into progenitor cells in vitro reduced the concentration of TGF-alpha required for changes in rod and Müller cell differentiation but did not enhance proliferation. Introduction of extra EGF-Rs in vivo increased the proportion of clones that contained Müller glial cells, suggesting that receptor level is normally limiting. These findings demonstrate that responsiveness to extracellular signals during development can be modulated by the introduction of additional receptors, and suggest that the level of expression of receptors for these signals contributes to the regulation of cell fate.

Sensory and motor neuron-derived factor. A novel heregulin variant highly expressed in sensory and motor neurons

The heregulin family of polypeptides arise as splice variants from a single gene and share a conserved epidermal growth factor (EGF)-like domain thought to be the major determinant of their biological activities. We report here the cloning of a novel member of this family, termed sensory and motor neuron-derived factor or SMDF, which is highly expressed in sensory and motor neurons in human and rodent species. It contains a C-terminal beta-type EGF-like domain and an unique N-terminal sequence which lacks an Ig-like domain and is distinct from all known heregulin variants. Mammalian cell-expressed SMDF activates tyrosine phosphorylation of a 185-kDa protein in cell lines expressing p185erbB2, indicating that it is biologically active. Analyses of expression patterns suggest that, unlike other heregulin variants, SMDF is expressed mainly in the nervous system. In situ hybridization signals with the unique SMDF sequence probe and with a probe to the conserved EGF-like domain are comparable, suggesting that SMDF is the predominant isoform expressed in sensory and motor neurons. Expression of SMDF is maintained in both adult motor neurons and dorsal root ganglion neurons. These findings suggest that SMDF may mediate biological responses such as Schwann cell proliferation and acetylcholine receptor induction in the peripheral nervous system.

Axon-induced mitogenesis of human Schwann cells involves heregulin and p185erbB2

The ability of sensory axons to stimulate Schwann cell proliferation by contact was established in the 1970s. Although the mitogen responsible for this proliferation has been localized to the axon surface and biochemically characterized, it has yet to be identified. Recently a family of proteins known as heregulins (HRGs) has been isolated, characterized, and shown to interact with a number of class 1 receptor tyrosine kinases, including the erbB2, erbB3, and erbB4 gene products. These factors include glial growth factor, a Schwann cell mitogen. We have tested the effects of antibodies against components of this system (HRG beta 1 and p185erbB2) in cocultures of rat sensory neurons and human (or rat) Schwann cells to elucidate the role of these proteins in axon-induced Schwann cell proliferation. 2C4, a monoclonal antibody specific for the human p185erbB2 receptor tyrosine kinase, bound to the surface of human Schwann cells and reduced human Schwann cell incorporation of [3H]thymidine by > 90% compared with untreated controls in this coculture system. This antibody had no effect on rat Schwann cell incorporation of [3H]thymidine under similar conditions. A polyclonal antibody raised against HRG beta 1 reduced human and rat Schwann cell incorporation of [3H]thymidine by nearly 80% and up to 49%, respectively, relative to controls. These results imply that a HRG, or a HRG-like molecule, is a component of the axonal mitogen. This mitogen is presented to Schwann cells by axons and induces proliferation through an interaction that involves p185erbB2 on Schwann cells.

Characterization of the scatter factor/hepatocyte growth factor gene promoter. Positive and negative regulatory elements direct gene expression to mesenchymal cells

Scatter factor/hepatocyte growth factor (SF/HGF) and its receptor c-Met represent a paracrine signaling system involved in mesenchymal-epithelial interactions during development and during tumor progression. We have examined the promoters of the mouse and human SF/HGF genes by deletion mapping followed by CAT assays as well as by gel retardation and footprinting analysis. The promoter sequences are highly conserved (89.5% identity) up to position -453 from the major transcription start site but diverged considerably further upstream. Both promoters are active in mesenchymal but not epithelial cells thus reflecting the expression pattern of the SF/HGF gene in cells in vitro and in vivo. We have here identified two regulatory sequences in the SF/HGF promoter: a negative element at positions -239 to -258 and a positive element near the major transcription start site; specific deletions destroyed the activities of these elements. We were not able to localize elements on the SF/HGF promoter region that mediate the previously described effects of transforming growth factor beta, 12-O-tetradecanoylphorbol-13-acetate, and coculture of epithelial cells on SF/HGF gene expression. This study represents a first step toward understanding the intricately regulated and cell type-specific expression of the paracrine acting SF/HGF.

Factors controlling growth, motility, and morphogenesis of normal and malignant epithelial cells

Factors that control epithelial growth, motility, and morphogenesis play important roles in malignancy and in normal development. Here we discuss the molecular nature and the function of two types of molecules that control the development and maintenance of epithelia: Components that regulate epithelial cell adhesion; and soluble factors and their receptors that regulate growth, motility, differentiation, and morphogenesis. In development, the establishment of epithelial cell characteristics and organization is crucially dependent on cell adhesion and the formation of functional adherens junctions. The integrity of adherens junctions is frequently disturbed late in tumor progression, and the resulting loss of epithelial characteristics correlates with the metastatic potential of carcinoma cells. Various soluble factors that induce epithelial growth, motility, or differentiation in cell culture, function via tyrosine kinase receptors. We concentrate here on receptors that are expressed exclusively or predominantly on epithelia, and on ligands that are derived from the mesenchyme. In development, these receptors and their ligands function in mesenchymal-epithelial interactions, which are known to govern growth, morphogenesis, and differentiation of epithelia. During tumor development, mutations or overexpression of the receptors are frequently observed; these alterations contribute to the development and progression of carcinomas.

Neural development. Fate diverted

Signals that alter cell fate are probably crucial in metazoan development. Glial growth factor may play such a role in the mammalian neural crest, by regulating the generation of neurons and Schwann cells.