Neuronal deficits, not involving motor neurons, in mice lacking BDNF and/or NT4

Role of neurotrophins and trk receptors in the development and maintenance of sensory neurons: an overview

The neurotrophins are a family of polypeptide neuronal growth factors related to the prototypical neurotrophic factor, nerve growth factor (NGF). In mammals this gene family encompasses NGF, brain-derived neurotrophic factor (BDNF) and neurotrophins-3 and -4/5, (NT-3, NT-4/5). The neurotrophins initiate signal transduction in responsive cells by ligand induced dimerization and activation of one of the Trk family of receptor tyrosine kinases; NGF being specific for TrkA, BDNF and NT-4/5 for TrkB, and TrkC the preferred receptor for NT-3. In accord with differential patterns of distribution of Trk receptors in peripheral ganglia, the neurotrophins show both distinct and overlapping specificity towards subpopulations of sensory neurons of both neural crest and neural placode origin. In vitro and in vivo studies, and transgenic mice baring targeted null mutations of the neutrophin genes have established that BDNF, NT-3 and NT-4/5, like NGF, play critical roles as classical target-derived survival factors for subclasses of developing sensory neurons. However, much broader effects of neurotrophins on sensory neurons are now evident, including paracrine and autocrine actions on neuroblast proliferation, phenotypic differentiation, and survival and regeneration in the adult. This article provides an overview of the discovery and properties of the neurotrophin family, their receptors and their actions and specificity for both distinct and overlapping subpopulations of spinal and cranial sensory neurons.

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.

Binding of neurotrophin-3 to p75LNGFR, TrkA, and TrkB mediated by a single functional epitope distinct from that recognized by trkC

Neurotrophins regulate differentiation and survival of vertebrate neurons through binding to members of the Trk family of receptor tyrosine kinases and to a common low affinity receptor, p75LNGFR. The specificity of neurotrophin action is determined by their selective interaction with the different members of the Trk family; TrkA, TrkB, and TrkC serve as cognate receptors for nerve growth factor, brain-derived neurotrophic factor, and neurotrophin-3 (NT-3), respectively. Unlike nerve growth factor and brain-derived neurotrophic factor, NT-3 can to some extent also bind and activate non-cognate TrkA and B receptors, although the physiological relevance of these interactions is unclear. Previous studies established that neurotrophins use an extended surface for binding to cognate Trk receptors, while binding to p75LNGFR is mediated by a localized cluster of positively charged residues. Here we show that the binding site of NT-3 to its non-preferred receptors TrkA and TrkB is dominated by two positively charged residues, Arg-31 and His-33, previously shown to constitute a main determinant of binding to p75LNGFR. Simultaneous mutation of these two residues into Ala completely abolished NT-3 binding and signaling through TrkA and greatly diminished binding and activation of TrkB. However, NT-3 binding and signaling through its cognate receptor TrkC was unaffected by the mutation. These results show that binding of NT-3 to p75LNGFR, TrkA, and TrkB is mediated by a common determinant, which is distinct from that recognized by TrkC and also different and more localized than the one recognized by TrkA and TrkB in their cognate ligands. Thus, although homologous regions in all neurotrophins are used for binding to Trk receptors, a given Trk may actually contact different residues in different neurotrophins. The mutant NT-3 described here may be of greater advantage than native NT-3 when a trophic activity needs to be specifically targeted to TrkC-expressing neurons and provides a monospecific neurotrophin for future therapeutic development.

Transgenic and knockout mice in the study of neurodegenerative diseases

Accurate animal models are essential for detailed analysis of the mechanisms underlying human neurodegenerative diseases. In addition, they can offer useful paradigms for the development and evaluation of new therapeutic strategies. We review the most popular techniques for modification of the mammalian genome in vivo, and provide a critical evaluation of the available transgenic mouse models for several neurological conditions of humans, including prion diseases, human retroviral diseases, Alzheimer's disease, and motor neuron diseases.

Grafts of fibroblasts genetically modified to secrete NGF, BDNF, NT-3, or basic FGF elicit differential responses in the adult spinal cord

Neuronal and axonal responses to neurotrophic factors in the developing spinal cord have been relatively well characterized, but little is known about adult spinal responses to neurotrophic factors. We genetically modified primary rat fibroblasts to produce either nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), or basic fibroblast growth factor (bFGF), then grafted these neurotrophic factor-secreting cells into the central gray matter of the spinal cord in adult rats. Spinal cord lesions were not made prior to grafting. From 2 wk to 6 mo later, sensory neurites of dorsal root origin extensively penetrated NGF-, NT-3-, and bFGF-producing grafts, whereas BDNF-secreting grafts elicited no growth responses. Putative noradrenergic neurites also penetrated NGF-secreting cell grafts. Local motor and corticospinal motor axons did not penetrate any of the neurotrophic factor-secreting grafts. These results indicate that unlesioned or minimally lesioned adult spinal cord sensory and putative noradrenergic populations retain significant neurotrophic factor responsiveness, whereas motor neurites are comparatively resistant even to those neurotrophic factors to which they exhibit survival dependence during development. Grafts of genetically modified cells can be a useful tool for characterizing neurotrophic factor responsiveness in the adult spinal cord and designing strategies to promote axonal regeneration after injury.

Expression and regulation of brain-derived neurotrophic factor and neurotrophin-3 mRNAs in distinct avian motoneuron subsets

We performed a detailed study of the expression of neurotrophin-3 and brain-derived neurotrophic factor transcripts in spinal motoneurons using in situ hybridization of serially sectioned chick embryos aged 3 to 8 days (E3 to E8). Neurotrophin-3 mRNA is detected in motoneuron subsets from E3.5 to E4 only in brachial segments of the neural tube and from E5 in both brachial and lumbar regions. Expression of brain-derived neurotrophic factor mRNA is first evident on E5 in a subset of brachial level motoneurons and from E6 also in motoneurons located in the rostral-most portion of the lateral motor column, as well as in the tail-innervating region of the spinal cord. Analysis along the rostrocaudal extent of the brachial lateral motor column reveals an overlap zone of expression of both neurotrophins of about two segments. In transverse sections of this region, it is observed that neurotrophin-3-positive motoneurons preferentially occupy the lateral part of the column, whereas brain-derived neurotrophic factor-producing motoneurons are localized in a more medial position. These results show that the two factors are synthesized at discrete axial levels of the spinal cord by distinct motoneuron subpopulations. Since brain-derived neurotrophic factor mRNA is expressed within the brachial but not the lumbar lateral motor column, we tested the possibility that brain-derived neurotrophic factor expression is regulated by the type of peripheral target, that is, the wing or the leg. Unilateral transplantation of a wing bud instead of a leg bud and vice versa, prior to the onset of peripheral innervation, failed to alter the original pattern of brain-derived neurotrophic factor mRNA observed in either level of the axis. Thus, the early synthesis of brain-derived neurotrophic factor by subsets of spinal motoneurons is independent of the type of peripheral target and may instead reflect intrinsic differences between motoneuron populations.

Decreased expression of TrkB and TrkC mRNAs in spinal motoneurons of aged rats

Several studies have indicated that a decrease in availability and/or responsiveness to neurotrophin(s) may be of importance in ageing and disease-related neurodegeneration. Using in situ hybridization we have studied the mRNA expression of the full-length neurotrophin receptors TrkB and TrkC in spinal cord motoneurons of aged rats (30 months old) with symptoms of hindlimb incapacity and in young adult rats (2-3 months old). The labelling intensity for TrkB of the individual cell profile was decreased by 25% (P < 0.001) in both the cervical and lumbar motor nuclei of aged rats. In thoracic motoneurons of aged and young adult rats the difference in expression of TrkB mRNA was smaller (down by 15%; P < 0.05). The labelling for TrkC mRNA was much weaker than that for TrkB in both aged and young adult rats, but TrkC mRNA expression also seemed to decrease. Comparison of the different motor nuclei along the spinal cord axis revealed that the motoneurons of the L6/S1 nuclei showed the strongest hybridization signal for the two Trk receptors in both aged and young adult rats. The possibility that a decrease in TrkB mRNA may contribute to age-related motor disturbances is discussed.

The expression of trkB and p75 and the role of BDNF in the developing neuromuscular system of the chick embryo

The neurotrophin, brain-derived neurotrophic factor, prevents motoneuron cell death during the normal development of the chick embryo. Brain-derived neurotrophic factor is a ligand for the low-affinity NGF receptor, p75, and for the high-affinity neurotrophin receptor, trkB. If motoneurons respond directly to brain-derived neurotrophic factor then they must possess at least one, and possibly both, of these receptors during the period of naturally occurring cell death. Histological sections from the lumbar region of chick embryos were probed for the presence of trkB and p75 mRNA using digoxigenin-labeled anti-sense RNA probes. p75 mRNA was present in spinal cord motoneurons at stages of development that correlate with motoneuron cell death. Immunohistochemical localization also revealed that p75 protein was present in motoneurons, primarily along the ventral roots and developing intramuscular nerves. In contrast trkB mRNA was not present in chick motoneurons until after the process of cell death was underway. The timing of trkB expression suggested that some motoneurons, i.e., those that die prior to the onset of trkB expression, may be insensitive to brain-derived neurotrophic factor. This was confirmed by comparing the number of surviving motoneurons following different in vivo treatment paradigms. The evidence indicates that motoneurons undergo a temporal shift in sensitivity to brain-derived neurotrophic factor.

Prenatal and postnatal requirements of NT-3 for sympathetic neuroblast survival and innervation of specific targets

Postnatal homozygous neurotrophin-3 mutant mice display a loss of about half the sympathetic superior cervical ganglion (SCG) neurons (Ernfors, P., Lee, K.-F., Kucera, J. and Jaenisch, R. (1994a) Cell 77, 503–512; Farinas, I., Jones, K. R., Backus, C., Wang, X. Y. and Reichardt, L. F. (1994) Nature 369, 658–661). We found that this loss is caused by excessive apoptosis of sympathetic neuroblasts leading to a failure to generate a normal number of neurons during neurogenesis. NT-3 was also found to be required postnatally. In Nt-3−/− mice, sympathetic fibers failed to invade pineal gland and external ear postnatally; whereas other targets of the external and internal carotid nerves, including the submandibular gland and the iris, displayed a normal complement of sympathetic innervation. Sympathetic fibers of mice carrying one functional copy of the Nt-3 gene (Nt-3+/− mice) invaded the pineal gland, but failed to branch and form a ground plexus. Cultured neonatal sympathetic neurons responded to NT-3 by neurite outgrowth and mRNA upregulation of the NT-3 receptor, trkC. Exogenously administered NT-3 promoted sympathetic growth and rescued the sympathetic target deficit of the mutant mice. We conclude that NT-3 is required for the survival of sympathetic neuroblasts during neurogenesis and for sympathetic innervation and branching in specific targets after birth.

Role of neurotrophic factors in neuronal development

The spectrum of potential biological roles for neurotrophic factors in development and maturation of the nervous system continues to widen. Careful analysis of the phenotypes of knock-out mice has been used to test directly the 'neurotrophic hypothesis', and the role of members of the transforming growth factor beta superfamily--in particular, glial cell line derived neurotrophic factor--in regulating neuronal survival has become apparent. The effects of neurotrophin-3 on early neuronal differentiation and maturation have proved to be both multiple and complex. Neurotrophic factors are also emerging as potential regulators of synapse stabilization and function.

Expression of neurotrophin trk and p75 receptors in quail embryos undergoing gastrulation and neurulation

We have previously demonstrated the presence of mRNA for the full-length neurotrophin receptors trkA, trkB and trkC in quail embryos from stages 1 through 6 using reverse transcription followed by the polymerase chain reaction (RT-PCR; Yao et al. [1994] Dev. Biol. 165: 727-730). Furthermore, we showed that mRNA for the neurotrophins brain-derived neurotrophic factor and neurotrophin-3 was present from stage 1 onward, while nerve growth factor mRNA began to be expressed at stage 5. In the present study, wholemount in situ hybridization was used to localize full-length trk mRNA in embryos from stages 3 through 10. Structures expressing trkC mRNA included the primitive streak and Hensen's node, the neural plate or notochord, somites and the rostral neural tube. trkA and trkB mRNA were expressed at much lower levels than trkC mRNA; however, staining was detected on the primitive streak and Hensen's node. In addition to trk mRNA, we have also demonstrated the presence of full-length Trk protein in embryos from stages 3 through 11, suggesting that the trk mRNA detected at these early stages is translated into functional cell surface receptors. To support this hypothesis, we have shown that neurotrophins can induce phosphorylation of Trk on tyrosine residues, at least at stage 11. We also detected mRNA and protein for the nontyrosine kinase neurotrophin receptor, p75, at similar stages. The presence of neurotrophin receptors, as well as neurotrophin mRNA, in embryos undergoing gastrulation and neurulation leads to speculation that neurotrophins may be playing a role in these processes.

Influence of growth factors on neuronal differentiation

With so many neurotrophins and receptors now known, how is our picture of neurotrophism changing? Recent studies on knockout mice have confirmed our expectations of neurotrophin action in neuronal development. A notable exception is the activation of TrkB, on motor neurons, by an unknown ligand. It is also clear that some neurotrophins have diverse activities and influence early developmental stages. There are interesting new data concerning the role of p75, the low affinity neurotrophin receptor, as a modulator of neurotrophin activity. Even more exciting are new studies on glia-derived neurotrophic factor (GDNF) which demonstrate that this growth factor acts as a potential protector of motor neurons and striatal dopaminergic neurons.

NT-4-mediated rescue of lateral geniculate neurons from effects of monocular deprivation

Altering the balance of activity between the two eyes during the critical period for visual-system development profoundly affects competitive interactions among neurons in the lateral geniculate nucleus and primary visual cortex. Neurons in the lateral geniculate nucleus that are deprived of activity by closing or silencing one eye atrophy as a result of competition with non-deprived neurons for some critical factor(s) presumed to be present in the cortex. Based on their actions in the developing visual system, neurotrophins are attractive candidates for such factors. We tested whether neurotrophins mediate intracortical competition of afferents from the lateral geniculate nucleus by using monocular deprivation and a new method for highly localized, in vivo delivery of neurotrophins. This method allowed unambiguous identification of neurons that were exposed to neurotrophin. Here we report that only one neurotrophin, the TrkB ligand NT-4, rescued neurons in the lateral geniculate nucleus from the dystrophic effects of monocular deprivation.

Mice lacking the CNTF receptor, unlike mice lacking CNTF, exhibit profound motor neuron deficits at birth

Ciliary neurotrophic factor (CNTF) supports motor neuron survival in vitro and in mouse models of motor neuron degeneration and was considered a candidate for the muscle-derived neurotrophic activity that regulates motor neuron survival during development. However, CNTF expression is very low in the embryo, and CNTF gene mutations in mice or human do not result in notable abnormalities of the developing nervous system. We have generated and directly compared mice containing null mutations in the genes encoding CNTF or its receptor (CNTFR alpha). Unlike mice lacking CNTF, mice lacking CNTFR alpha die perinatally and display severe motor neuron deficits. Thus, CNTFR alpha is critical for the developing nervous system, most likely by serving as a receptor for a second, developmentally important, CNTF-like ligand.

Peripheral neuropathies and neurotrophic factors: animal models and clinical perspectives

A large body of data exists showing that a wide variety of neurotrophic factors can promote the survival or growth of different neuronal populations in vitro. More recently, several studies have been published on the survival-promoting effects of particular factors in animal models of peripheral neuropathies. Thus, the effect of axotomy on neuropeptide expression in dorsal root ganglion cells is partially reversed by nerve growth factor treatment, and the effect on choline acetyltransferase expression in motoneurones is partially reversed by glial-derived neurotrophic factor, neurotrophin-4/5 and brain-derived neurotrophic factor. Nerve growth factor also ameliorates some of the changes seen in sensory neurones in animal models of diabetic neuropathy and small fibre cytostatic drug neuropathy, whereas neurotrophin-3 has been found to reverse some changes in large sensory neurones associated with cisplatin neurotoxicity. The results of these studies provide grounds for optimism in the clinical uses of such factors, and, indeed, several clinical studies are now under way.

Neurotrophins regulate dendritic growth in developing visual cortex

Although dendritic growth and differentiation are critical for the proper development and function of neocortex, the molecular signals that regulate these processes are largely unknown. The potential role of neurotrophins was tested by treating slices of developing visual cortex with NGF, BDNF, NT-3, or NT-4 and by subsequently visualizing the dendrites of pyramidal neurons using particle-mediated gene transfer. Specific neurotrophins increased the length and complexity of dendrites of defined cell populations. Basal dendrites of neurons in each cortical layer responded most strongly to a single neurotrophin: neurons in layer 4 to BDNF and neurons in layers 5 and 6 to NT-4. In contrast, apical dendrites responded to a range of neurotrophins. On both apical and basal dendrites, the effects of the TrkB receptor ligands, BDNF and NT-4, were distinct. The spectrum of neurotrophic actions and the laminar specificity of these actions implicate endogenous neurotrophins as regulatory signals in the development of specific dendritic patterns in mammalian neocortex.

Structural determinants of neurotrophin action

Five decades of research on NGF have led to the discovery of a small family of evolutionarily conserved proteins, which have vital functions in the survival and neuronal development of specific neuronal populations. The generation of mice lacking neurotrophin expression has recapitulated classic experiments using anti-NGF antibodies to dissect the physiological effects of trophic factor deprivation (73). Very similar outcomes resulted from both the NGF immunodepletion experiments and the transgenic mouse experiments. The genetic results also verify the structural predictions made from binding results in heterologous cells. The findings in cell culture and animal experiments clearly indicate the efficacy of neurotrophic factors for promoting the survival of prominent neuronal populations such as sensory and motor neurons. The high degree of conservation of neurotrophin structure is accompanied by a surprising variation in the amino acid contacts used by each neurotrophin with p75 and the trk receptor family members. It is this variation that may provide specificity for each ligand-receptor complex. The future challenge will be to make use of this knowledge to design effective therapeutic strategies to treat neurodegeneration and nerve injury.