Developmental expression patterns of the signaling adapters FRS-2 and FRS-3 during early embryogenesis

Two fibroblast growth factor (FGF) receptor substrates (FRS2 and FRS3) are involved in downstream signaling from activated FGF receptors and neurotrophin-activated Trk receptors. Despite the importance of signaling from these factors in embryogenesis, FRS2 and FRS3 expression patterns during development are unknown. In this study we characterize the expression of FRS2 and FRS3 from E7 to parturition and in adult murine tissues. Both are first detected in whole E8.5 CD1 mouse embryos. FRS2 is detected as early as E7 in the developing syncytiotrophoblast, later in the neural tube (NT) and in many adult and fetal tissues. FRS3 is more restricted in location than FRS2 (fetal NT, heart, stomach, liver and some adult tissues), and is expressed predominantly in the ventricular layer of the developing NT and brains of murine embryos.

Neural crest stem cells undergo cell-intrinsic developmental changes in sensitivity to instructive differentiation signals

Rat neural crest stem cells (NCSCs) prospectively isolated from uncultured E14.5 sciatic nerve and transplanted into chick embryos generate fewer neurons than do NCSCs isolated from E10.5 neural tube explants. In addition, they differentiate primarily to cholinergic parasympathetic neurons, although in culture they can also generate noradrenergic sympathetic neurons. This in vivo behavior can be explained, at least in part, by a reduced sensitivity of sciatic nerve-derived NCSCs to the neurogenic signal BMP2 and by the observation that cholinergic neurons differentiate at a lower BMP2 concentration than do noradrenergic neurons in vitro. These results demonstrate that neural stem cells can undergo cell-intrinsic changes in their sensitivity to instructive signals, while maintaining multipotency and self-renewal capacity. They also suggest that the choice between sympathetic and parasympathetic fates may be determined by the local concentration of BMP2.

NRAGE, a novel MAGE protein, interacts with the p75 neurotrophin receptor and facilitates nerve growth factor-dependent apoptosis

The mechanisms employed by the p75 neurotrophin receptor (p75NTR) to mediate neurotrophin-dependent apoptosis are poorly defined. Two-hybrid analyses were used to identify proteins involved in p75NTR apoptotic signaling, and a p75NTR binding partner termed NRAGE (for neurotrophin receptor-interacting MAGE homolog) was identified. NRAGE binds p75NTR in vitro and in vivo, and NRAGE associates with the plasma membrane when NGF is bound to p75NTR. NRAGE blocks the physical association of p75NTR with TrkA, and, conversely, TrkA overexpression eliminates NRAGE-mediated NGF-dependent death, indicating that interactions of NRAGE or TrkA with p75NTR are functionally and physically exclusive. NRAGE overexpression facilitates cell cycle arrest and permits NGF-dependent apoptosis within sympathetic neuron precursors cells. Our results show that NRAGE contributes to p75NTR-dependent cell death and suggest novel functions for MAGE family proteins.

Direct binding of the signaling adapter protein Grb2 to the activation loop tyrosines on the nerve growth factor receptor tyrosine kinase, TrkA

We demonstrate that the signaling adapter, Grb2, binds directly to the neurotrophin receptor tyrosine kinase, TrkA. Grb2 binding to TrkA is independent of Shc, FRS-2, phospholipase Cgamma-1, rAPS, and SH2B and is observed in in vitro binding assays, yeast two-hybrid assays, and in co-immunoprecipitation assays. Grb2 binding to TrkA is mediated by the central SH2 domain, requires a kinase-active TrkA, and is phosphotyrosine-dependent. By analyzing a series of rat TrkA mutants, we demonstrate that Grb2 binds to the carboxyl-terminal residue, Tyr(794), as well as to the activation loop tyrosines, Tyr(683) and Tyr(684). By using acidic amino acid substitutions of the activation loop tyrosines on TrkA, we can stimulate constitutive kinase activity and TrkA-Shc interactions but, importantly, abolish TrkA/Grb2 binding. Thus, in addition to providing the first evidence of direct Grb2 binding to the neurotrophin receptor, TrkA, these data provide the first direct evidence that the activation loop tyrosines of a receptor tyrosine kinase, in addition to their essential role in kinase activation, also serve a direct role in the recruitment of intracellular signaling molecules.

Transient Notch activation initiates an irreversible switch from neurogenesis to gliogenesis by neural crest stem cells

The genesis of vertebrate peripheral ganglia poses the problem of how multipotent neural crest stem cells (NCSCs) can sequentially generate neurons and then glia in a local environment containing strong instructive neurogenic factors, such as BMP2. Here we show that Notch ligands, which are normally expressed on differentiating neuroblasts, can inhibit neurogenesis in NCSCs in a manner that is completely dominant to BMP2. Contrary to expectation, Notch activation did not maintain these stem cells in an uncommitted state or promote their self-renewal. Rather, even a transient activation of Notch was sufficient to cause a rapid and irreversible loss of neurogenic capacity accompanied by accelerated glial differentiation. These data suggest that Notch ligands expressed by neuroblasts may act positively to instruct a cell-heritable switch to gliogenesis in neighboring stem cells.

Subpopulations of rat B2(+) neuroblasts exhibit differential neurotrophin responsiveness during sympathetic development

Sympathetic neurons comprise a population of postmitotic, tyrosine hydroxylase expressing cells whose survival is dependent upon nerve growth factor (NGF) both in vivo and in vitro. However, during development precursors to rat sympathetic neurons in the thoracolumbar region are not responsive to NGF because they lack the signal transducing NGF receptor, trkA. We have previously shown that acquisition of trkA expression is sufficient to confer a functional response to NGF. Here we describe four subpopulations of thoracolumbar sympathetic neuroblasts which are mitotically active and unresponsive to NGF at E13.5 of rat gestation, but differ based upon their neurotrophic responsiveness in vitro. The survival in culture of the largest sympathetic subpopulation is mediated by neurotrophin-3 (NT-3) or glial-derived neurotrophic factor (GDNF), whereas the cell survival of two smaller subpopulations of neuroblasts are mediated by either solely GDNF or solely NT-3. Finally, we identify a subpopulation of sympathetic neuroblasts in the thoracolumbar region whose survival, exit from the cell cycle, induction of trkA expression, and consequent acquisition of NGF responsiveness in culture appear to be neurotrophin independent and cell autonomous. These subpopulations reflect the diversity of neurotrophic actions that occur in the proper development of sympathetic neurons.

Distinct human NUMB isoforms regulate differentiation vs. proliferation in the neuronal lineage

Neuronal cell fate decisions are directed in Drosophila by NUMB, a signaling adapter protein with two protein-protein interaction domains: a phosphotyrosine-binding domain and a proline-rich region (PRR) that functions as an SH3-binding domain. Here we show that there are at least four human NUMB isoforms and that these serve two distinct developmental functions in the neuronal lineage: differentiation (but not proliferation) is promoted by human NUMB protein isoforms with a type I (short) PRR. In contrast, proliferation (but not differentiation) is directed by isoforms that have a type II (long) PRR. The two types of PRR may promote distinct intracellular signaling pathways downstream of the NOTCH receptor during mammalian neurogenesis.

Activity-dependent interaction of the intracellular domain of rat trkA with intermediate filament proteins, the beta-6 proteasomal subunit, Ras-GRF1, and the p162 subunit of eIF3

Many responses to nerve growth factor (NGF) are regulated through the receptor tyrosine kinase trkA. To understand more fully the functions of trkA in NGF responsive cells, we have expressed the intracellular domain of rat trkA as a fusion protein with the yeast gal4 transcription factor, and used the fusion protein to probe rat and mouse cDNA libraries by the yeast two-hybrid system. We have identified a direct interaction between the intracellular domain of trkA and two members of the intermediate filament (IF) family of proteins, the guanine-nucleotide exchange protein Ras-GRF1, the p162 subunit of eIF3, and the beta-6 proteasome subunit. The interactions are dependent on an active trkA kinase, and RasGRF1, the beta-6 proteasomal subunit, and peripherin are directly phosphorylated by trkA. The interaction with trkA is not affected by mutations at either Tyr499 or Tyr794, the two major phosphotyrosine residues essential to the activation and receptor binding of Shc, FRS-2/SNT, and phospholipase Cgamma-1, and it is highly specific in vitro for trkA, with little or no binding observed with trkB and/or trkC. The results show that trkA may play a regulatory role in a variety of cellular functions in addition to neuritogenesis, including regulated protein degradation and transcriptional activation.

The signaling adapter FRS-2 competes with Shc for binding to the nerve growth factor receptor TrkA. A model for discriminating proliferation and differentiation

We have isolated a human cDNA for the signaling adapter molecule FRS-2/suc1-associated neurotrophic factor target and shown that it is tyrosine-phosphorylated in response to nerve growth factor (NGF) stimulation. Importantly, we demonstrate that the phosphotyrosine binding domain of FRS-2 directly binds the Trk receptors at the same phosphotyrosine residue that binds the signaling adapter Shc, suggesting a model in which competitive binding between FRS-2 and Shc regulates differentiation versus proliferation. Consistent with this model, FRS-2 binds Grb-2, Crk, the SH2 domain containing tyrosine phosphatase SH-PTP-2, the cyclin-dependent kinase substrate p13(suc1), and the Src homology 3 (SH3) domain of Src, providing a functional link between TrkA, cell cycle, and multiple NGF signaling effectors. Importantly, overexpression of FRS-2 in cells expressing an NGF nonresponsive TrkA receptor mutant reconstitutes the ability of NGF to stop cell cycle progression and to stimulate neuronal differentiation.

Mammalian NUMB is an evolutionarily conserved signaling adapter protein that specifies cell fate

BACKGROUND

Drosophila numb was originally described as a mutation affecting binary divisions in the sensory organ precursor (SOP) lineage. The numb gene was subsequently shown to encode an asymmetrically localized protein which is required for binary cell-fate decisions during peripheral nervous system development. Part of the Drosophila NUMB protein exhibits homology to the SHC phosphotyrosine-binding (PTB) domain, suggesting a potential link to tyrosine-kinase signal transduction.

RESULTS

A widely expressed mammalian homologue of Drosophila numb (dnumb) has been cloned from rat and is referred to here as mammalian Numb (mNumb). The mNUMB protein has a similar overall structure to dNUMB and 67 sequence similarity. Misexpression of mNumb in Drosophila during sensory nervous system precursor cell division causes identical cell fate transformations to those produced by ectopic dNUMB expression. In vitro, the mNUMB PTB domain binds phosphotyrosine-containing proteins, and SH3 domains of SRC-family tyrosine kinases bind to mNUMB presumably through interactions with proline-rich regions in the carboxyl terminus. Overexpression of full-length mNUMB in the multipotential neural crest stem cell line MONC-1 dramatically biases its differentiation towards neurons, whereas overexpression of the mNUMB PTB domain biases its differentiation away from neuronal fates.

CONCLUSIONS

Our results demonstrate that mNUMB is an evolutionarily conserved functional homologue of dNUMB, and establish a link to tyrosine-kinase-mediated signal transduction pathways. Furthermore, our results suggest that mNUMB and dNUMB are new members of a family of signaling adapter molecules that mediate conserved cell-fate decisions during development.

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.

Neurotrophins regulate sequential changes in neurotrophin receptor expression by sympathetic neuroblasts

We have examined the mechanisms controlling the induction of the two NGF receptors, trkA and p75, in proliferating neuroblasts immuno-isolated from thoracolumbar embryonic sympathetic ganglia. Contrary to prior studies, we find that induction of p75 follows rather than precedes that of trkA; this late induction is consistent with the fact that p75 functions at relatively late stages of sympathetic development. trkA induction is apparently not controlled by a cell-intrinsic mechanism. Rather, this receptor is induced by environmental signals including NT-3, which also acts as an interim survival factor for these neuroblasts. trkA induction by NT-3 is consequent to its promotion of mitotic arrest, as anti-mitotic drugs also efficiently induce trkA. p75 expression is induced in trkA-expressing cells by NGF. Thus, the development of sympathetic neurons involves sequential actions of different neurotrophins, which also regulate the expression of their own and each other's receptors.

CNTF, FGF, and NGF collaborate to drive the terminal differentiation of MAH cells into postmitotic neurons

The differentiation of neuronal cell progenitors depends on complex interactions between intrinsic cellular programs and environmental cues. Such interactions have recently been explored using an immortalized sympathoadrenal progenitor cell line, MAH. These studies have revealed that depolarizing conditions, in combination with exposure to FGF, can induce responsiveness to NGF. Here we report that CNTF, which utilizes an intracellular signaling pathway distinct from that of both FGF and NGF, can collaborate with FGF to promote efficiently the differentiation of MAH progenitor cells to a stage remarkably reminiscent of NGF-dependent, postmitotic sympathetic neurons. We also find that similar collaborative interactions can occur during transdifferentiation of normal cultured chromaffin cells into sympathetic neurons.

Expression of trk in MAH cells lacking the p75 low-affinity nerve growth factor receptor is sufficient to permit nerve growth factor-induced differentiation to postmitotic neurons

We have transfected MAH cells, an immortalized sympathoadrenal progenitor cell line, with a plasmid encoding the 140-kDa Trk protein, a nerve growth factor (NGF) receptor with protein-tyrosine kinase activity. NGF promotes neurite outgrowth and proliferation from such cells, indicating that Trk is sufficient to mediate such responses in the absence of significant levels of the endogenous 75-kDa low-affinity NGF receptor (p75). These initial NGF responses are indistinguishable from those evoked by basic fibroblast growth factor (bFGF). However, NGF is sufficient to promote terminal differentiation of a approximately 8% of trk-transfected MAH cells to postmitotic, NGF-dependent neurons, whereas all cells eventually die in medium with bFGF. Other environmental signals (such as depolarization or ciliary neurotrophic factor) can cooperate with NGF to enhance production of postmitotic NGF-dependent neurons in trk-transfected MAH cells. The terminal differentiation of sympathetic neurons thus involves sequential and cooperative actions of different growth and neurotrophic factors, as well as cell-intrinsic changes in the response to these factors.

p75LNGFR regulates Trk signal transduction and NGF-induced neuronal differentiation in MAH cells

We have examined NGF-induced signal transduction events and neuronal differentiation in MAH cells, a neuronal progenitor cell line, in which the expression of the two NGF receptors, p140trk (Trk) and p75LNGFR (p75), has been independently manipulated. Coexpression of a large molar excess of p75 substantially enhances the NGF-induced tyrosine autophosphorylation of Trk, compared with cells expressing Trk alone. MAH cells expressing both Trk and p75 stop dividing and acquire a mature neuronal morphology more rapidly and with greater efficiency than MAH cells expressing Trk alone. These biochemical and biological influences of p75 are not observed using a mutant form of NGF that binds Trk but not p75. These data provide evidence that p75 can modulate signal transduction through Trk in a neuronal progenitor cell context and that such modulation has functional consequences for the neuronal differentiation pathway induced by NGF.

Membrane depolarization induces p140trk and NGF responsiveness, but not p75LNGFR, in MAH cells

Nerve growth factor (NGF) is required for the maturation and survival of sympathetic neurons, but the mechanisms controlling expression of the NGF receptor in developing neuroblasts have not been defined. MAH cells, an immortalized sympathoadrenal progenitor cell line, did not respond to NGF and expressed neither low-affinity NGF receptor (p75) nor p140trk messenger RNAs. Depolarizing concentrations of potassium chloride, but none of a variety of growth factors, induced expression of p140trk but not p75 messenger RNA. A functional response to NGF was acquired by MAH cells under these conditions, suggesting that expression of p75 is not essential for this response. Depolarization also permitted a relatively high proportion of MAH cells to develop and survive as neurons in fibroblast growth factor and NGF. These data establish a relation between electrical activity and neurotrophic factor responsiveness in developing neurons, which may operate in the functioning of the mature nervous system as well.

Posttranscriptional regulation of myelin protein gene expression

Regulation of myelin protein gene expression occurs at many different levels including transcription, mRNA translocation, translation, and posttranslational modification of myelin proteins prior to their assembly into the membrane. Translocation of myelin basic protein (MBP) mRNAs into oligodendrocyte processes was observed in vivo and in primary cultures, but no such translocation was observed for the mRNAs encoding the proteolipid protein (PLP) or myelin-associated glycoprotein. More than 99% of the mRNAs encoding 2'3'-cyclic nucleotide phosphodiesterase (CNP) remained associated with cell bodies. In the jimpy mutant, MBP mRNA translocation appeared to be impaired, but translocation occurred normally in quaking brains in vivo. We have found that steroids, such as glucocorticoids, stimulate the translation of MBP and PLP mRNAs in cell-free systems and inhibit the translation of CNP mRNA. This pattern of regulation is consistent with compositional changes noted in myelin during development. We have localized a nine nucleotide segment within the 5'-untranslated region of the MBP mRNA that is involved in the action of steroids on translation of this mRNA. We have also determined that the protein synthetic step modulated by the steroids is chain initiation, enhancing the rate at which new ribosomal subunits bind to the MBP mRNAs.

Translational regulation by steroids. Identification of a steroid modulatory element in the 5'-untranslated region of the myelin basic protein messenger RNA

Although steroids have been implicated in post-transcriptional regulation, their effects on mRNA translation rates have been uncertain. We have used a cell-free translation system programmed with synthetic messages to show that steroids can alter the translation rates of a number of myelin protein mRNAs and the mRNA encoding a non-myelin protein, the estrogen receptor. Through the use of deletion analysis, site-directed mutagenesis, and chimeric mRNAs we have identified a 9-nucleotide segment in the 5'-untranslated region of one myelin protein mRNA that is necessary for steroid action. Steroid-mediated translational regulation is discussed in terms of myelination where subtle developmental changes in protein composition of the membrane have significant consequences on its morphology and function. We propose that the modulation of mRNA translation rates by steroids is a more general phenomenon that may serve as another mechanism by which steroids can regulate gene expression.

Posttranscriptional events in the expression of myelin protein genes

A number of posttranscriptional events may be involved in regulating the expression of the myelin protein genes. One such event in the expression of the myelin basic protein (MBP) gene is the translocation of MBP mRNAs from oligodendrocyte cell bodies to their processes. This translocation can be observed in vivo and in primary mixed glial cell cultures. In jimpy brains the translocation of MBP mRNA appears to be disrupted, so that most of the mRNA remains associated with cell bodies. This apparent failure of translocation may account for the lack of incorporation of newly synthesized MBP into jimpy myelin. In quaking myelin, where MBP assembly is also defective, translocation appears to be normal, suggesting that incorporation of MBP into the membrane also is regulated posttranslationally. We have identified a number of the structural features of MBP mRNAs that influence the efficiencies with which they are translated and may be involved in regulating the levels of individual MBP produced. We also found that glucocorticoids stimulate the translation of MBP and PLP mRNAs and inhibit the translation of CNP mRNA in cell-free systems. Our results suggest that this pattern of translational regulation may be physiologically meaningful, especially during maturation of myelin. The mechanism by which the steroids modulate translation of these messages appears to be novel. Analysis of the effect of steroids on cRNAs produced from engineered MBP cDNA constructs has permitted the identification of a nine nucleotide element involved in this steroid modulation within the 5' untranslated region of the MBP mRNA.

Expression of a novel transcript of the myelin basic protein gene

A cDNA (M41) corresponding to a mouse myelin basic protein (MBP) mRNA with a longer 5'-untranslated region than predicted from earlier studies of MBP gene structure has been isolated and characterized. The additional 5'-untranslated region is encoded by two previously unidentified exons upstream of the major transcription start site of the gene. Using a DNA probe specific for M41-MBP mRNAs, Northern blot analysis indicated that expression of this transcript follows a developmental course in mouse brain similar to that of the majority of MBP mRNAs, but that the level of expression varies between brain and spinal cored. Expression of MBP mRNAs similar to the mouse M41-MBP also was identified in rat brain. The results suggest that the structure of the MBP gene is more complex than originally thought, containing at least two more exons. There appears to be at least one more MBP gene promoter that directs the synthesis of a subset of MBP mRNAs with a unique 5'-untranslated region.

Translational regulation of myelin protein synthesis by steroids

Various steroids, including glucocorticoids, were observed to exert a direct effect on the rates of translation of several myelin-protein specific transcripts in a cell-free, reticulocyte lysate system. Hydrocortisone caused a twofold stimulation in the translation of mRNAs of myelin basic protein and proteolipid protein. It inhibited the translation of 2',3'-cyclic nucleotide 3'-phosphodiesterase mRNA by 50%, and had no effect on the translation of a number of other mRNAs. The data suggest that steroid-mediated translational regulation may serve as a novel mechanism to modulate the expression of myelin protein genes at the translational level.