Structure and expression of the nerve growth factor gene in Xenopus oocytes and embryos

NGF in Early Embryogenesis, Differentiation, and Pathology in the Nervous and Immune Systems

The physiology of NGF is extremely complex, and although the study of this neurotrophin began more than 60 years ago, it is far from being concluded. NGF, its precursor molecule pro-NGF, and their different receptor systems (i.e., TrkA, p75NTR, and sortilin) have key roles in the development and adult physiology of both the nervous and immune systems. Although the NGF receptor system and the pathways activated are similar for all types of cells sensitive to NGF, the effects exerted during embryonic differentiation and in committed mature cells are strikingly different and sometimes opposite. Bearing in mind the pleiotropic effects of NGF, alterations in its expression and synthesis, as well as variations in the types of receptor available and in their respective levels of expression, may have profound effects and play multiple roles in the development and progression of several diseases. In recent years, the use of NGF or of inhibitors of its receptors has been prospected as a therapeutic tool in a variety of neurological diseases and injuries. In this review, we outline the different roles played by the NGF system in various moments of nervous and immune system differentiation and physiology, from embryonic development to aging. The data collected over the past decades indicate that NGF activities are highly integrated among systems and are necessary for the maintenance of homeostasis. Further, more integrated and multidisciplinary studies should take into consideration these multiple and interactive aspects of NGF physiology in order to design new therapeutic strategies based on the manipulation of NGF and its intracellular pathways.

NGF and IL-1beta are co-localized in the developing nervous system of the frog, Xenopus laevis

NGF, a neurotrophic factor best known for its role in promoting cell survival, regulates many neurodevelopmental processes, including synaptic plasticity, neurite outgrowth and programmed cell death. Although there is a large amount of data regarding NGF in the developing nervous system of many species, there is little known about its regulation and role in the frog, Xenopus laevis. In this report, immunocytochemistry was used to characterize NGF protein expression in developing tadpoles. Protein expression was analyzed in tadpoles from stage 44/45 through stage 50, a period of development characterized by extensive neurite outgrowth, neuronal differentiation and an initial period of programmed cell death. Similar to other species, NGF was expressed in sensory cells and tissues, including the inner ear, eye, olfactory system, lateral line organs, papillae in the oral cavity, and gills tufts. In addition, NGF was expressed in specific cells in the central nervous system, cranial and dorsal root ganglia, spinal sensory and motoneurons, and muscle tissues in the tail and body cavity. In the mammalian nervous system, the cytokine, interleukin-1beta (IL-1beta) induces expression of NGF. In this report, double-label immunocytochemistry was used to determine the relationship between NGF and IL-1beta. Results showed most cell types and/or tissues that expressed NGF also expressed IL-1beta. However, NGF was typically associated with cellular and nuclear membranes, whereas IL-1beta appeared in the cytoplasm and nucleolus. The nuclear localization of IL-1beta supports the idea that it regulates gene transcription in the frog. The appearance of NGF and IL-1beta in the same cells suggests they may interact to influence neural development.

Expression of Nerve Growth Factor (NGF), and Its Receptors trkA and p75 in Ovaries of the Cyclic Golden Hamster (Mesocricetus auratus) and the Regulation of Their Production by Luteinizing Hormone

In the present study, changes in localization of nerve growth factor (NGF) and its receptors, trkA and p75 in the ovary were investigated during the estrous cycle of the golden hamster. The effect of LH surge on changes in localization of NGF, trkA and p75 in the ovary was also investigated. NGF and its receptors trkA and p75 were localized in oocytes, granulosa cells and theca cells of various stages of follicles throughout the estrous cycle. NGF and its two receptors were also present in numerous interstitial cells and luteal cells. The number of interstitial cells staining positively for NGF and its two receptors was greater in ovaries of day 1 (day 1=day of ovulation) than the other days during the estrous cycle. Treatment with the antiserum against luteinizing hormone releasing hormone (LHRH-AS) at 1100 h on day 4 completely blocked ovulation. There were few positive reactions for NGF and its two receptors in interstitial cells 24 hr after LHRH-AS injection. The effect of LHRH-AS treatment was blocked by a single injection of 10 IU human chorionic gonadotropin. The distinct widespread distribution of NGF and its two receptors in the ovary of golden hamsters suggest that NGF may be an important growth factor for regulation of ovarian function. Furthermore, the LH surge may be an important factor for inducing production of NGF and its two receptors in interstitial cells of the cyclic golden hamster.

Nerve growth factor signals via preexisting TrkA receptor oligomers

Nerve growth factor (NGF) promotes neuronal survival and differentiation by activating TrkA receptors. Similar to other receptor tyrosine kinases, ligand-induced dimerization is thought to be required for TrkA receptor activation. To study this process, we expressed TrkA receptors in Xenopus laevis oocytes and analyzed their response to NGF by using a combination of functional, biochemical, and structural approaches. TrkA receptor protein was detected in the membrane fraction of oocytes injected with TrkA receptor cRNA, but not in uninjected or mock-injected oocytes. Application of NGF to TrkA receptor-expressing oocytes promoted tyrosine phosphorylation and activated an oscillating transmembrane inward current, indicating that the TrkA receptors were functional. Freeze-fracture electron microscopic analysis demonstrated novel transmembrane particles in the P-face (protoplasmic face) of oocytes injected with TrkA cRNA, but not in uninjected or mock injected oocytes. Incubating TrkA cRNA-injected oocytes with the transcriptional inhibitor actinomycin D did not prevent the appearance of these P-face particles or electrophysiological responses to NGF, demonstrating that they did not arise from de novo transcription of an endogenous Xenopus oocyte gene. The appearance of these particles in the plasma membrane correlated with responsiveness to NGF as detected by electrophysiological analysis and receptor phosphorylation, indicating that these novel P-face particles were TrkA receptors. The dimensions of these particles (8.6 x 10 nm) were too large to be accounted for by TrkA monomers, suggesting the formation of TrkA receptor oligomers. Application of NGF did not lead to a discernible change in the size or shape of these TrkA receptor particles during an active response. These results indicate that in Xenopus oocytes, NGF activates signaling via pre-formed TrkA receptor oligomers.

Expression and function of Xenopus laevis p75(NTR) suggest evolution of developmental regulatory mechanisms

Neurotrophins signal through two different classes of receptors, members of the trk family of receptor tyrosine kinases, and p75 neurotrophin receptor (p75(NTR)), a member of the tumor necrosis factor receptor family. While neurotrophin binding to trks results in, among other things, increased cell survival, p75(NTR) has enigmatically been implicated in promoting both survival and cell death. Which of these two signals p75(NTR) imparts depends on the specific cellular context. Xenopus laevis is an excellent system in which to study p75(NTR) function in vivo because of its amenability to experimental manipulation. We therefore cloned partial cDNAs of two p75(NTR) genes from Xenopus, which we have termed p75(NTR)a and p75(NTR)b. We then cloned two different cDNAs, both of which encompass the full coding region of p75(NTR)a. Early in development both p75(NTR)a and p75(NTR)b are expressed in developing cranial ganglia and presumptive spinal sensory neurons, similar to what is observed in other species. Later, p75(NTR)a expression largely continues to parallel p75(NTR) expression in other species. However, Xenopus p75(NTR)a is additionally expressed in the neuroepithelium of the anterior telencephalon, all layers of the retina including the photoreceptor layer, and functioning axial skeletal muscle. Finally, misexpression of full length p75(NTR) and each of two truncated mutants in developing retina reveal that p75(NTR) probably signals for cell survival in this system. This result contrasts with the reported role of p75(NTR) in developing retinae of other species, and the possible implications of this difference are discussed.

Brain-derived neurotrophic factor and TrkB tyrosine kinase receptor gene expression in zebrafish embryo and larva

The genes that encode the neurotrophin family of secreted polypeptides and the Trk family of high affinity neurotrophin transmembrane protein tyrosine kinase receptors are induced at the time of neurogenesis in mammals and are known to play critical roles in nervous system development. We show here that in contrast to mammals, the genes encoding the neurotrophin brain-derived neurotrophic factor (BDNF) and the neurotrophin receptor TrkB are expressed throughout embryonic development in the zebrafish. At the embryonic stages preceding transcription of endogenous genes all cells contain BDNF transcripts and immunoreactive BDNF and the trkB transcripts lack the region that encodes a kinase domain. As development proceeds, progressively fewer cells contain BDNF transcripts and by the time of neurogenesis the trkB transcripts encode a kinase-domain. In the 4-day-old larva, a small subset of specialized sensory cells on the surface and cells in deeper structures including the gill arches, fin, and cloaca express the BDNF gene at high levels in a promoter-specific fashion. This progressive restriction of BDNF gene expression must involve an extinction of BDNF gene transcription in some and induction of high levels of transcription in a promoter-specific fashion in other cells.

Regulation of N- and L-type Ca2+ channels in adult frog sympathetic ganglion B cells by nerve growth factor in vitro and in vivo

To examine mechanisms responsible for the long-term regulation of Ca2+-channels in an adult neuron, changes in whole cell Ba2+ current (IBa) were examined in adult bullfrog sympathetic ganglion B cells in vitro. Cells were cultured at low density in defined, serum free medium. After 15 days, total IBa was similar to the initial value, whereas IBa density was reduced by approximately 36%, presumably due to an increase in neuronal surface area. By contrast, IBa density remained constant after 6-15 days in the presence of murine beta-NGF (200 ng/ml), and total IBa was almost doubled. Inclusion of cytosine arabinoside (Ara-C; 10 microM) to inhibit proliferation of nonneuronal cells, did not affect the survival of neurons in the absence of nerve growth factor (NGF) nor did it attenuate IBa. Ara-C did not prevent the effect of NGF on IBa. There were three independent components to the action of NGF; during 6-9 days, it increased omega-conotoxin-GVIA-sensitive N-type IBa (IBa,N); increased nifedipine-sensitive L-type IBa (IBa,L) and decreased inactivation of the total Ba2+ conductance (gBa). The latter effect involved a selective decrease in the amplitude of one of the four kinetic components that describe the inactivation process. Total IBa was also 55.8% larger than control in the somata of B cells acutely dissociated from leopard frogs that had received prior subcutaneous injections of NGF. By contrast, injection of NGF antiserum decreased total IBa by 29.4%. There was less inactivation of gBa in B cells from NGF-injected animals than in cells from animals injected with NGF antiserum (P < 0.001). These data suggest that NGF-like molecule(s) play(s) a role in the maintenance of IBa in an adult amphibian sympathetic neuron; the presence of NGF may allow the neuron to maintain a constant relationship between cell size and current density. They also show that IBa inactivation in an adult neuron can be modulated in a physiologically relevant way by an extracellular ligand.

FGF signaling and target recognition in the developing Xenopus visual system

We report that the growth cones of Xenopus retinal ganglion cells express fibroblast growth factor receptors (FGFRs) and that bFGF stimulates neurite extension from cultured retinal neurons. Furthermore, bFGF is abundant in the developing optic tract but is reduced in the optic tectum. To test whether FGF signaling plays a role in axonal guidance in vivo, bFGF was exogenously applied to the developing optic pathway in "exposed brain" preparations. FGF-treated retinal axons navigate normally through the optic tract, but the majority veer aberrantly at the tectal border and bypass the target. Our results implicate FGF signaling in target recognition and suggest that diminished levels of bFGF in the tectum cause arriving axons to slow their growth.

Neurotrophin-3 acquires NGF-like activity after exchange to five NGF amino acid residues: molecular analysis of the sites in NGF mediating the specific interaction with the NGF high affinity receptor

Despite the large sequence similarity around 55-60% among the known NGF-related neurotrophins, the members display different activities on different subset of neurons. Recent studies have shown that the various neurotrophins are ligands with high affinity to different receptors of the Trk family of tyrosine kinase receptors. We wanted to elucidate what specific parts of NGF replaced in neurotrophin-3 (NT-3) would result in NGF-like receptor binding and biological activity. By studying evolutionarily conserved amino acid sequences not shared by NT-3 and NGF and excluding parts which have been examined in earlier work with NGF and BDNF chimeras as well as taking advantage of the crystallographic data available for NGF, we decided to exchange three specific blocks of two or three amino acids in the human NT-3 backbone for the corresponding residues in NGF. The NGF residues Asn-Ile-Asn (43-45), Val-Phe (48,49) and Gln-Ala-Ala (96-98) were combined in pairs and are all shown to contribute NGF-like activity in the context of NT-3. The most efficient NGF-like transformation was obtained by the exchange of Pro-Val and Leu-Val-Gly in NT-3 to the NGF residues Val-Phe and Gln-Ala-Ala. This mutant reached 90% NGF activity, based on survival of sympathetic neurons, stimulation of fibre outgrowth from sympathetic ganglia, the ability to block high affinity NGF binding to PC12 cells and phosphorylation of gp140trk. Thus, the three mutants with paired combinations of the NGF residues as well as the NT-3 housing all three blocks of NGF residues were able to mimic NGF activity. This activity is gained, although the mutated neurotrophin proteins do not lose the original NT-3 activity as ascertained by the stimulation of neurite outgrowth from the Remak ganglion. The three mutated sites are situated in two beta-loops at one end of the NGF molecule, forming a cleft that could specifically interact with high affinity to the signalling NGF receptor gp140trk.

The conservation of neurotrophic factors during vertebrate evolution

Neurotrophic factors are a family of extracellular ligands that affect the differentiation, survival (by inhibition of apoptosis) and maintenance of function of neuronal cells in vertebrates. The family includes nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and neurotrophin-4/5 (NT-4/5). The survival specificities of NGF and BDNF for different classes of chick neurons are maintained from the fish to the mammalian proteins, implying a conserved interaction with neuronal cell surface receptors (of the Trk family). However, the quantitative effect of a fish neurotrophin can differ significantly from that of the mammalian orthologue.

Molecular cloning of a cDNA encoding a nerve growth factor precursor from the krait, Bungarus multicinctus

NGFs have been isolated from the venom of many snakes. Here we report the isolation and the sequencing of a nearly full-length NGF cDNA from the Bungarus multicinctus venom gland cDNA library. The structure of the predicted krait precursor resembles that of cobra and of other animals, with a highly conserved mature NGF protein at the carboxy-terminus. Prepro part of the precursors are less conserved. The krait one possesses the presumptive 18 amino residues terminal signal sequence and the two long stretches corresponding to functional domains which are highly conserved alternating with large poorly conserved regions. We discuss particularities in the sequence of the precursor in the krait which may in part explain some earlier results obtained concerning this factor in the krait venom.

Function and evolution in the NGF family and its receptors

The gene family of neurotrophins includes nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4). Recently, neurotrophin-5 (NT-5), a possible mammalian homologue to NT-4 described in the frog Xenopus, has been cloned in man and rat. The neurotrophins stimulate survival and differentiation of a range of target neurons by binding to cell surface receptors. The structure of NGF has recently been clarified from crystallographic data. The similarities between the different neurotrophins are substantial with the variable regions, giving specificity to each of the family members, being localized to some exposed loop regions. Low-affinity binding (Kd of 10(-9) M) of all tested neurotrophins is mediated via a 75 K glycoprotein (LNGFR) that has been cloned and characterized. A 140 K tyrosine protein kinase encoded by the proto-oncogene trk has been found to bind NGF with high affinity (Kd of 10(-11) M) and to evoke the cellular neurotrophic responses. In addition, a protein encoded by the trk-related gene trkB has been shown to bind BDNF. Recently, a third member of the trk family, trkC, has been cloned and demonstrated to function as a high-affinity receptor for NT-3. The expression of trk and LNGFR mRNA are co-localized in the rat brain to the medial septal nucleus and the nucleus of Broca's diagonal band containing the NGF-responsive magnocellular cholinergic neurons projecting to hippocampus and cerebral cortex. In sharp contrast, the pattern of expression of trkB is widely spread in many areas of the cortex as well as lateral septum. The trkB protein might serve general functions in large areas of the cortex. Site-directed mutagenesis and expression of recombinant chimaeric neurotrophin proteins have made it possible to localize a likely region for the interaction between NGF and the LNGFR. This region could be altered, resulting in the total loss of LNGFR binding by the mutant NGF protein without affecting the binding to the trk receptor which was sufficient for the full biological activity. Cladistic analysis of likely phylogenies within the neurotrophins shows BDNF and NT-4 to be most closely related whereas NGF may be the sister group to NT-3, BDNF, and NT-4. Neurotrophins offer obvious clinical possibilities for treatment of neurodegenerative diseases.

Disruption of the low affinity receptor-binding site in NGF allows neuronal survival and differentiation by binding to the trk gene product

Nerve growth factor (NGF), like many other growth factors and hormones, binds to two different receptor molecules on responsive cells. The product of the proto-oncogene trk, p140trk, is a tyrosine kinase receptor that has been identified as a signal-transducing receptor for NGF, while the role of the low affinity NGF receptor, p75NGFR, in signal transduction is less clear. The crystal structure of NGF has recently been determined, although structures involved in receptor binding and biological activity are unknown. Here we show that Lys-32, Lys-34, and Lys-95 form a positively charged interface involved in binding to p75NGFR. Simultaneous modification of Lys-32 with either of the two other lysines resulted in loss of binding to p75NGFR. Despite the lack of binding to p75NGFR, these mutants retained binding to p140trk and biological activity, demonstrating a functional dissociation between the two NGF receptors.