Regulation of expression of mRNAs encoding the nerve growth factor receptors p75 and trkA in developing sensory neurons

Timing of neuronal death in trkA, trkB and trkC mutant embryos reveals developmental changes in sensory neuron dependence on Trk signalling

The sensory neurons of the embryonic mouse trigeminal ganglion are supported in culture by different neurotrophins at successive stages of development. Initially the neurons survive in response to BDNF and NT3 and later switch to becoming NGF-dependent (Buchman, V. I. and Davies, A. M. (1993), Development 118, 989–1001). To determine if this in vitro switch in neurotrophin responsiveness is physiologically relevant, we studied the timing of neuronal death in the trigeminal ganglia of embryos that are homozygous for null mutations in the trkA, trkB and trkC genes, which encode receptor tyrosine kinases for NGF, BDNF and NT3, respectively. In wild-type embryos, the number of pyknotic nuclei increased from E11 to peak between E13 and E14, and decreased gradually at later ages, becoming negligible by birth. Neuronal death in the trigeminal ganglia of trkA−/− embryos also peaked between E13 and E14, but was almost threefold greater than in wild-type embryos at this stage. Whereas there was no significant difference between the number of pyknotic nuclei in trkA−/− and wild-type embryos at E11 and E12, there was a substantial increase in the number of pyknotic nuclei in the trigeminal ganglia of trkB−/− at these earlier stages. Counts of the total number of neurons in E13 trigeminal ganglia revealed a marked decrease in trkB−/− but not trkA−/− or trkC−/− embryos. Consistent with the later onset of excessive neuronal death in trkA−/− embryos, there was a marked decrease in the neuronal complement of the trigeminal ganglia of trkA−/− embryos at E15. These results demonstrate that TrkB signalling is required for the in vivo survival of many trigeminal neurons during the early stages of target field innervation before they become NGF-dependent.

Analysis of low affinity neurotrophin receptor (p75) expression in glia of the CNS-PNS transition zone following dorsal root transection

Peripheral nerves exit from the brain through the transition zone where oligodendroyctes and astrocytes of the central nervous system (CNS) and Schwann cells of the peripheral nervous system (PNS) are in close proximity. In this zone, the same axons are ensheathed by oligo-dendrocytes and Schwann cells. We examined, in adult rats, the expression of the low affinity neurotrophin receptor (p75) in central glia and Schwann cells in response to lesion of lumbar dorsal roots. In normal rats, scattered p75-immunoreactive glial cells were present in the CNS-PNS transition zone. A marked increase of p75 immunoreactivity occurred in Schwann cells near the transition zone from 4 days to at least 3 weeks after dorsal root transection. In contrast, the p75 immunoreactivity remained unchanged in central glia. The differential expression of p75 in the two types of glial cells was sharply demarcated at the CNS-PNS border. Our results are consistent with earlier observations that axon damage is less potent in its ability to induce central glial expression of p75, and further, suggests a possible mechanism for the failure of regenerating dorsal root axons growing into the spinal cord.

Distinct modulatory actions of TGF-beta and LIF on neurotrophin-mediated survival of developing sensory neurons

The neurotrophins nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) are important for the regulation of survival and differentiation of distinct, largely non-overlapping populations of embryonic sensory neurons. We show here that the multifunctional cytokine transforming growth factor-beta (TGF-beta) fails to maintain sensory neurons cultured from embryonic day (E) 8 chick dorsal root ganglia (DRG), although DRG neurons are immunoreactive for the TGF-beta receptor type II, which is essential for TGF-beta signaling. However, in combination with various concentrations of NT-3 and NT-4, but not NGF, TGF-beta 3 causes a further significant increase in neuron survival. In DRG cell cultures treated with NGF, NT-3, and NT-4, a neutralizing antibody to TGF-beta decreases neuron survival suggesting that endogenous TGF-beta in these cultures affects the efficacies of neurotrophins. Consistent with this notion and a modulatory role of TGF-beta in neurotrophin functions is the observation that TGF-beta 2 and -beta 3 immunoreactivities and TGF-beta 3 mRNA are located in embryonic chick DRG in close association with neurons from E5 onwards. We also show that leukemia inhibitory factor (LIF) significantly decreases NGF-mediated DRG neuron survival. Together, these data indicate that actions and efficacies of neurotrophins are under distinct control by TGF-beta and LIF in vitro, and possibly also in vivo.

Can the neurotrophic hypothesis explain degeneration and loss of plasticity in mature and ageing autonomic nerves?

The causes of age-related degeneration in the peripheral nervous system remain unclear. The search for clues has focused on developmental mechanisms and particularly on the neurotrophic hypothesis and its principal player, nerve growth factor, reduced levels of which are thought to cause degeneration of some autonomic and central neurons in old age. Nerve growth factor may well be important in the mature and ageing nervous system, but recent experiments on sympathetic nerves in ageing rats suggest that lack of NGF is not the only limiting factor in neuronal growth and survival. Other candidates include laminin, which is bound in the extracellular matrix and may act in synergy with NGF to regulate neuronal maintenance and growth in maturity. Reduced, region-specific patterns of availability of one or both of these substances may underlie age-related degeneration in autonomic nerves. Different combinations of these factors may influence particular aspects of neuronal plasticity, such as collateral sprouting and regeneration. In addition to extrinsic factors, it appears increasingly likely that altered neuronal responsiveness to neurotrophic factors in old age contributes to structural and functional deficits in autonomic nerves.

The neurotrophic hypothesis: where does it stand?

In the developing peripheral nervous system many neurons die shortly after their axons reach their target fields. This loss is thought to match the number of neurons to the size and requirements of their target fields because altering target field size before innervation affects the number of neurons that survive. The neurotrophic hypothesis provides an explanation for how target fields influence the size of the neuronal populations that innervate them. This hypothesis arose from work on nerve growth factor (NGF), the founder member of the neurotrophin family of secreted proteins. Its principal tenet is that the survival of developing neurons depends on the supply of a neurotrophic factor that is synthesized in limiting amounts in their target fields. The neurotrophic hypothesis has, however, been broadened by the demonstration that multiple neurotrophic factors regulate the survival of certain populations of neurons. For example, some neurons depend on several different neurotrophic factors which may act concurrently or sequentially during target field innervation. In addition, there are aspects of neurotrophin action that do not conform with the classic neurotrophic hypothesis. For example, the dependence of some populations of sensory neurons on particular neurotrophins before significant neuronal death takes place raises the possibility that the supply of these neurotrophins is not limiting for survival at this stage of development. There is also evidence that at stages before and after sensory neurons depend on target-derived neurotrophins for survival, neurotrophins act on at least some sensory neurons by an autocrine route. Yet despite the growing wealth of information on the multiple roles and modes of action of neurotrophic factors, the neurotrophic hypothesis has remained the best explanation for how neuronal target fields in the developing peripheral nervous system regulate their innervation density.

Expression of the trk family of neurotrophin receptors in developing and adult dorsal root ganglion neurons

Expression of trk receptors is a major determinant of neurotrophin responsiveness of sensory neurons. Although it has been apparent for some time that subpopulations of dorsal root and trigeminal ganglion neurons respond in vitro to each of the members of the neurotrophin family, the extent to which functionally distinct subclasses of sensory neurons are dependent on the actions of different neurotrophins for their development and function remains an active area of investigation. One step towards elucidating the role of various neurotrophins in development and function of sensory neurons has been to examine the distribution of trk receptors on sensory neurons. These studies have clearly revealed that members of the trk family are differentially expressed in functionally distinct populations of both developing and mature sensory neurons and, further, have provided evidence consistent with a shift in neurotrophin responsiveness during the development of sensory 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.

Role of nerve growth factor in the expression of trkA mRNA in cultured embryonic rat basal forebrain cholinergic neurons

Using a quantitative reverse transcription-polymerase chain reaction (RT-PCR), we studied the regulation of trkA mRNA expression in serum-free, cultured basal forebrain neurons from 17-day fetal rats. Besides increasing choline acetyltransferase (ChAT) activities, nerve growth factor (NGF) strikingly induced trkA gene expression in a time- and NGF concentration-dependent manner. Therefore, NGF might play a critical role in trkA gene expression during the development of basal forebrain cholinergic neurons. Furthermore, to investigate whether this up-regulation is connected with the trophic effects on basal forebrain cholinergic neurons, we examined the effects of some other neurotrophic agents (BDNF, NT-3, bFGF, CNTF, and 40 mM KCI) upon ChAT activity and trkA gene expression. Some neurotrophic factors increased ChAT activities to the same degree as NGF, whereas they did not stimulate trkA mRNA expression so potently. NT-3 plus K252b promotes the tyrosine phosphorylation of TrkA in PC12 cells and increases ChAT activity in cultured basal forebrain cholinergic neurons like NGF (Knusel et al., J Neurochem 59: 715-722, 1992). We found that NT-3 plus K252b upregulated the level of trkA mRNA. These results suggested that the expression of trkA mRNA is regulated directly by its specific ligand NGF, rather than neurotrophic effects upon basal forebrain cholinergic neurons and that the up-regulation is connected to a molecular event initiated by the binding of NGF to the TrkA receptor.

Dependence of developing group Ia afferents on neurotrophin-3

At birth, group Ia proprioceptive afferents and muscle spindles, whose formation is Ia afferent-dependent, are absent in mice carrying a deletion in the gene for neurotrophin-3 (NT-3-/-). Whether Ia afferents contact myotubes, resulting in the formation of spindles which subsequently degenerate, or whether Ia afferents and spindles never form was examined in NT-3-/- mice at embryonic days (E) 10.5-18.5 by light and electron microscopy. Three sets of data indicate that Ia neurons do not develop and spindles do not form in NT-3-deficient mice. First, peripheral projections of Ia afferents did not innervate hindlimbs of NT-3-/- mice, as reflected by a deficiency of nerve fibers in limb peripheral nerves and an absence of afferent nerve-muscle contacts and spindles in the soleus muscle at E13.5-E18.5. Second, central projections of Ia afferents did not innervate the spinal cord in the absence of NT-3, as shown by an atrophy of the dorsal spinal roots and absence of afferent projections from limb musculature to spinal motor neurons at E13.5 or E15.5. Lastly, the lumbar dorsal root ganglia (DRGs) at E10.5-E14.5, the stages of development that precede or coincide with the innervation of the spinal cord and hindlimbs by Ia afferents, were 20-64% smaller in mutant than in wild-type mice, presumably because the cell bodies of Ia neurons were absent in embryos lacking NT-3. The failure of Ia neurons to differentiate and/or survive and Ia afferent projections to form in early fetal mice lacking NT-3 suggests that NT-3 may regulate neuronal numbers by mechanisms operating prior to neurite outgrowth to target innervation fields. Thus, developing Ia neurons may be dependent on NT-3 intrinsic to the DRGs before they reach a stage of potential dependence on NT-3 retrogradely derived from skeletal muscles or spinal motor neurons.

High specificity of neurotrophins in the embryonic chicken trigeminal system

Studies of cell lines and some cultured neurons have demonstrated potential cross-talk between neurotrophins and their receptors; high concentrations of neurotrophins can exhibit either agonist or antagonistic actions on heterologous neurotrophin receptors. We have studied neurotrophin discrimination among the sensory neurons of the embryonic chicken trigeminal system. We show that nerve growth factor (NGF) at a concentration that is six orders of magnitude greater than that required to promote the survival of NGF-dependent dorsomedial trigeminal ganglion (DMTG) neurons has no effect on the survival of brain-derived neurotrophic factor (BDNF)-dependent trigeminal mesencephalic nucleus (TMN) neurons and does not affect the dose-response relationship of these neurons to BDNF. A similar high level of neurotrophin-3 neither promotes the survival of BDNF-dependent ventrolateral trigeminal ganglion neurons nor affects the dose response of these neurons to BDNF. High levels of BDNF have a negligible effect on the survival of mid-embryonic DMTG neurons. These results show that some neurons are able to discriminate completely between neurotrophins at very high concentrations, indicating that neurotrophin responses can be far more highly specific than previously appreciated.

Role of Bcl-2 in the brain-derived neurotrophic factor survival response

Developing neurons die if they fail to obtain an adequate supply of neurotrophins from their targets but how neurotrophins suppress cell death is not known. Although over-expression of exogenous Bcl-2 can prevent the death of cultured neurons deprived of members of the nerve growth factor family of neurotrophins it is not known if this effect is physiologically relevant. To determine if Bcl-2 participates in the neurotrophin survival response we used antisense bcl-2 RNA to inhibit endogenous Bcl-2 expression. Here we show that brain-derived neurotrophic factor (BDNF)-dependent neurons are killed by antisense bcl-2 RNA in the presence of BDNF. However, when these neurons were supported with ciliary neurotrophic factor (CNTF) their survival was not affected by antisense bcl-2 RNA. Likewise, the survival of CNTF-dependent ciliary neurons was not affected by antisense bcl-2 RNA. Our findings suggest that Bcl-2 is required for the BDNF survival response and that alternative, Bcl-2-independent survival mechanisms operate in sensory and parasympathetic neurons exposed to CNTF.

A potential interaction of p75 and trkA NGF receptors revealed by affinity crosslinking and immunoprecipitation

Nerve growth factor binds independently to two transmembrane receptors, the p75 neurotrophin receptor and the p140trk (trkA) tyrosine kinase receptor, which are both co-expressed in the majority of neuronal cells that respond to NGF. Previous findings have suggested that appropriate co-expression of the two receptors gives rise to high affinity NGF binding sites and increased neurotrophin responsiveness; however, evidence demonstrating a direct interaction between the two receptors in cell lines has been lacking. Here we have utilized affinity crosslinking agents with 125I-NGF to detect an association of trkA and p75 receptors in embryonic spinal cord and brain tissues enriched in the two receptors. Although multimeric complexes of trkA and p75 were not detected by affinity crosslinking, immunoprecipitation of cross-linked NGF-receptor complexes with trk-specific antibodies resulted in selective immunoprecipitation of crosslinked p75. Our results indicate that the trkA and p75 receptors can potentially interact, and that such an association may be responsible for the generation of high affinity NGF binding sites.

Developmentally regulated expression of mRNA for neurotrophin high-affinity (trk) receptors within chick trigeminal sensory neurons

To investigate the distribution of neurons within the developing trigeminal sensory system which express mRNA for each of the three known high-affinity neurotrophin receptors (trk, trkB and trkC), we have performed in situ hybridization histochemistry on serial sections through the trigeminal ganglion and trigeminal mesencephalic nucleus at various ages of development using specific antisense oligonucleotide probes. We show that trkC mRNA is first expressed in the chicken embryo at stage 13, in presumptive neurons prior to the formation of the ganglion, that trkB mRNA labelling is initially observed within peripheral neurons slightly later, at stage 19, and that trk mRNA expression is not detectable until around embryonic day 3.5 (stage 21/22). The neurons which exhibit mRNA labelling for each of the high-affinity receptors occupy discrete regions within the ganglion, indicating that the ganglion comprises distinct neuronal subpopulations, each of which has a different capacity to respond to the different neurotrophins. Neurons which express trk mRNA are confined to the proximal region of the ganglion, whereas those which express trkB mRNA and trkC mRNA are located in two distinct regions within the distal aspect and also within the trigeminal mesencephalic nucleus. From the estimation of the number of neurons which exhibit labelling between embryonic days 9 and 18, we determined that the expression of mRNA for the high-affinity receptors changes during embryonic development of the ganglion. This is consistent with the observed differences in the response to neurotrophins in vitro.

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.

The p75 neurotrophin receptor

The low-affinity p75 molecule and trk tyrosine kinases serve as receptors for target-derived neurotrophins. While the mechanism by which receptor tyrosine kinases impart intracellular signaling has become well understood, the precise roles of the p75 receptor are not fully defined. The p75 neurotrophin receptor belongs to a family of transmembrane molecules which also serve as receptors for the tumor necrosis factor family of cytokines. Each receptor shares a common extracellular structure highlighted by conserved cysteine-rich repeats. Because NGF, BDNF, NT-3, and NT-4/5 bind to p75 with similar affinity, p75 may either act as a common subunit in a neurotrophin receptor complex with trk family members, or act by independent mechanisms to mediate biological actions of each neurotrophin.

The role of neurotrophins in the developing nervous system

Neurotrophins were originally identified by their ability to promote the survival of developing neurons. However, recent work on these proteins indicates that they may also influence the proliferation and differentiation of neuron progenitor cells and regulate several differentiated traits of neurons throughout life. Moreover, the effects of neurotrophins on survival have turned out to be more complex than originally thought. Some neurons switch their survival requirements from one set of neurotrophins to another during development, and several neurotrophins may be involved in regulating the survival of a population of neurons at any one time. Much of our understanding of the developmental physiology of neurotrophins has come from studying neurons of the peripheral nervous system. Because these neurons and their progenitors are segregated into anatomically discrete sites, it has been possible to obtain these cells for in vitro experimental studies from the earliest stage of their development. The recent generation of mice having null mutations in the neurotrophin and neurotrophin receptor genes has opened up an unparalleled opportunity to assess the physiological relevance of the wealth of data obtained from these in vitro studies. Here I provide a chronological account of the effects of members of the NGF family of neurotrophins on cells of the neural lineage with special reference to the peripheral nervous system.

The survival of NGF-dependent but not BDNF-dependent cranial sensory neurons is promoted by several different neurotrophins early in their development

Recent work has shown that the survival of the nerve growth factor (NGF)-dependent trigeminal ganglion neurons of the mouse embryo is promoted by brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) during the early stages of target field innervation (Buchman and Davies, (1993) Development, 118, 989–1001). The present study was undertaken to ascertain if responsiveness to multiple neurotrophins is a universal feature of the early stages of neuronal development or is restricted to only certain kinds of neurons. To address this issue, we took advantage of the accessibility, from an early developmental stage, of several populations of cranial sensory neurons in the chicken embryo that depend for survival on just one or two known neurotrophins during the phase of naturally occurring cell death. During the mid-embryonic period (E10 to E12) when the number of sensory neurons is declining due to naturally occurring neuronal death, the neurons of the jugular ganglion and the dorsomedial part of the trigeminal ganglion (DMTG) were supported by NGF, the neurons of the ventrolateral part of the trigeminal ganglion (VLTG) were supported by BDNF and the nodose ganglion contained a major subset of neurons supported by BDNF and a minor subset supported by NT-3. Earlier in development (E6), the survival of DMTG and jugular neurons was additionally promoted by BDNF and NT-3. In contrast, E6 VLTG neurons did not exhibit a survival response to either NGF or NT-3, and E6 nodose neurons did not exhibit a survival response to NGF.(ABSTRACT TRUNCATED AT 250 WORDS)

Intrinsic programmes of growth and survival in developing vertebrate neurons

Neurons become dependent on a supply of target-derived neurotrophins for survival when their axons reach their targets in development. Because the distance axons have to grow varies from one population of neurons to another, the timing of dependence on neurotrophins likewise varies. Although it would be expected that the simplest way of getting the timing right would be for the target to provide a suitable signal to the arriving axons, detailed studies on developing cranial sensory neurons suggest that these neurons are programmed, before differentiation, to acquire dependence at the correct time independently of external signals. Neurons not only partly compensate for different target distances by extending axons more rapidly the further they have to grow, but possess an intrinsic clock that switches on dependence at the right time in accordance with the time it normally takes their axons to reach their targets. Here the experimental evidence for these intrinsic programmes of growth and survival is reviewed, the rationale that might have favoured their evolution is discussed, and parallels are drawn with other developing neuronal systems.

p75-deficient embryonic dorsal root sensory and neonatal sympathetic neurons display a decreased sensitivity to NGF

To understand the role of low-affinity neurotrophin receptor p75 in neural development, we previously generated mice carrying a null mutation in the p75 locus (Lee, K. F., Li, E., Huber, L. J., Landis, S. C., Sharpe, A. H., Chao, M. V. and Jaenisch, R. (1992) Cell 69, 737–749). To elucidate the mechanisms leading to deficits in the peripheral nervous system in p75 mutant mice, we have employed dissociated cultures to examine the responses of p75-deficient dorsal root ganglion (DRG) and superior cervical ganglion (SCG) neurons to different neurotrophins. We found that p75-deficient DRG and SCG neurons displayed a 2- to 3-fold decreased sensitivity to NGF at embryonic day 15 (E15) and postnatal day 3 (P3), respectively, ages that coincide with the peak of naturally occurring cell death. Furthermore, while p75-deficient E15 DRG neurons did not change their response specificity to BDNF, NT-3, and NT-4/5, P3 SCG neurons became more responsive to NT-3 at higher concentrations (nanomolar ranges). These results may help explain the deficits in the peripheral nervous system in p75 mutant mice and provide evidence that p75 can modulate neurotrophin sensitivity in some neurons.

Neurotrophic factors. Switching neurotrophin dependence

Recent studies have revealed an unexpected switch in the survival requirements of neurons, from one set of neurotrophins to another, during the early stages of target-field innervation.

The role of neurotrophins during successive stages of sensory neuron development

Neurotrophins comprise a family of basic homodimeric proteins. The isolation of the first two neurotrophins, nerve growth factor and brain-derived neurotrophic factor, was based on the ability of these proteins to promote the survival of embryonic neurons. However, the identification of additional neurotrophins by homology screening together with recent work on these proteins has shown that neurotrophins do more than just regulate neuronal survival. Neurotrophins influence the proliferation and differentiation of neuron progenitor cells and regulate the expression of several differentiated traits of neurons throughout life. Moreover, the influence of neurotrophins on survival is more complex than originally thought; some neurons switch their survival requirements from one set of neurotrophins to another during development and several neurotrophins may be involved in regulating the survival of a population of neurons at any one time. Most of what is known of the developmental physiology of neurotrophins has come from studying neurons of the peripheral nervous system. Quite apart from the accessibility of these neurons and their progenitor cell populations, investigation of the actions of neurotrophins on several well-characterised populations of sensory neurons has permitted the age-related changes in the effects of neurotrophins to be interpreted in the appropriate developmental context. In this review I provide a chronological account of the action of neurotrophins in neuronal development with special reference to sensory neurons.