Dependence of developing group Ia afferents on neurotrophin-3

Proprioceptive afferents survive in the masseter muscle of trkC knockout mice

Peripheral innervation patterns of proprioceptive afferents from dorsal root ganglia and the mesencephalic trigeminal nucleus were assessed in trkC-deficient mice using immunohistochemistry for protein gene product 9.5 and parvalbumin. In trkC knockout mice, spinal proprioceptive afferents were completely absent in the limb skeletal muscles, M. biceps femoris and M. gastrocnemius, as previously reported. In these same animals, however, proprioceptive afferents from mesencephalic trigeminal nucleus innervated masseter muscles and formed primary endings of muscle spindles. Three wild-type mice averaged 35.7 spindle profiles (range: 31-41), six heterozygotes averaged 32.3 spindles (range: 27-41), and four homozygotes averaged 32.8 spindles (range: 26-42). Parvalbumin and Nissl staining of the brain stem showed approximately 50% surviving mesencephalic trigeminal sensory neurons in trkC-deficient mice. TrkC-/- mice (n = 5) had 309.4 +/- 15.9 mesencephalic trigeminal sensory cells versus 616.5 +/- 26.3 the sensory cells in trkC+/+ mice (n = 4). These data indicate that while mesencephalic trigeminal sensory neurons are significantly reduced in number by trkC deletion, they are not completely absent. Furthermore, unlike their spinal counterparts, trigeminal proprioceptive afferents survive and give rise to stretch receptor complexes in masseter muscles of trkC knockout mice. This indicates that spinal and mesencephalic trigeminal proprioceptive afferents have different neurotrophin-supporting system during survival and differentiation. It is likely that one or more other neurotrophin receptors expressed in mesencephalic trigeminal proprioceptive neurons of trkC knockout mice compensate for the lack of normal neurotrophin-3 signaling through trkC.

Proportions of slow myosin heavy chain-positive fibers in muscle spindles and adjoining extrafusal fascicles, and the positioning of spindles relative to these fascicles

Chicken leg muscles were examined to calculate the percentages of slow myosin heavy chain (MHC)-positive fibers in spindles and in adjacent extrafusal fascicles, and to clarify how the encapsulated portions of muscle spindles are positioned relative to these fascicles. Unlike mammals, in chicken leg muscles slow-twitch MHC and slow-tonic MHC are expressed in intrafusal fibers and in extrafusal fibers, suggesting a close developmental connection between the two fiber populations. In 8-week-old muscles the proportions of slow MHC-positive extrafusal fibers that ringed muscle spindles ranged from 0-100%. In contrast, proportions of slow MHC-positive intrafusal fibers in spindles ranged from 0-57%. Similar proportions in fiber type composition between intrafusal fibers and surrounding extrafusal fibers were apparent at embryonic days 15 and 16, demonstrating early divergence of extrafusal and intrafusal fibers. Muscle spindles were rarely located within single fascicles. Instead, they were commonly placed where several fascicles converged. The frequent extrafascicular location of spindles suggests migration of intrafusal myoblasts from developing clusters of extrafusal fibers toward the interstitium, perhaps along a neurotrophic gradient established by sensory axons that are advancing in the connective tissue matrix that separates adjoining fascicles.

Development and specification of muscle sensory neurons

Although the response properties and synaptic projections of muscle sensory neurons have been studied extensively, relatively little is known about how these sensory neurons develop their unique phenotypes during embryonic life. The explosion of new information on neurotrophins, however, has revealed that neurotrophin 3 (NT3) is critically involved in several aspects of this development, including the initial differentiation, survival, and perhaps even the terminal arborizations of muscle sensory neurons within the spinal cord. The ETS family of transcription factors, recently shown to be expressed in these sensory neurons, may help specify their choice of synaptic targets in the central nervous system.

Neurotrophin actions during the development of the peripheral nervous system

Neurotrophins are important regulators of the development and maintenance of the vertebrate nervous system. Besides their well-established role in promoting neuronal survival during development, in vitro data suggest that they can regulate proliferation, survival, and differentiation of precursor cells. Analysis of the developing peripheral nervous system in mouse strains carrying mutations in genes encoding the neurotrophins and their receptors indicate, however, that lack of neurotrophin signalling results in specific neuronal deficits that are primarily due to neuronal death. Many of these deficits occur before final target encounter.

A role for p75 receptor in neurotrophin-3 functioning during the development of limb proprioception

Neurotrophin-3 is indispensable for the development of limb proprioceptive neurons and their end organs, muscle spindles. To determine whether the low-affinity p75 receptor potentiates the actions of neurotrophin-3, we examined the development of the proprioceptive system in p75 null mutant mice that had either normal or decreased tissue levels of neurotrophin-3. Postnatal mice lacking both copies of the p75 gene had fewer sensory neurons in dorsal root ganglia, but normal complements of muscle spindles in fast hindlimb muscles, although the slow soleus muscle showed a 50% loss of spindles. However, compound mutants lacking both copies of the p75 gene as well as one copy of the neurotrophin-3 gene displayed a dystonic/ataxic phenotype similar to that observed previously in neurotrophin-3 null mutants devoid of proprioception. The compound mutants also exhibited a commensurate loss of parvalbumin-expressing (proprioceptive) neurons in dorsal root ganglia. The degree of deficiency of spindles (and presumably proprioceptive neurons) in the compound mutants exceeded the sum of deficits in single mutants lacking either both copies of p75 genes or one copy of neurotrophin-3 gene, suggesting a synergistic interaction between the p75 receptor and neurotrophin-3. Neuronal deficits in the compound mutants were present prior to embryonic day 14, indicating an early role for the p75 receptor in sensory neuronogenesis. Collectively, these data indicate that the p75 receptor is not essential for the survival and differentiation of most limb proprioceptive neurons when neurotrophin-3 is expressed at normal levels. However, the p75 receptor may act in synergy with neurotrophin-3 to enhance the survival of proprioceptive neurons when tissue levels of neurotrophin-3 are a limiting factor.

Development of fusimotor innervation correlates with group Ia afferents but is independent of neurotrophin-3

Fusimotor neurons, group Ia afferents and muscle spindles are absent in mutant mice lacking the gene for neurotrophin-3 (NT3). To partition the effect of Ia afferent or spindle absence from that of NT3 deprivation on fusimotor neuron development, we examined the fusimotor system in a mutant mouse (NesPIXpNT3) that lacks Ia afferents and spindles, but has normal or elevated tissue levels of NT3 during embryogenesis. Fusimotor fibers were absent in lumbar ventral spinal roots, and limb muscles were devoid of Ia afferents and spindles in adult NesPIXpNT3 mice. In contrast, no deficiency in motoneuron numbers was observed in the trigeminal nucleus which contains cell bodies of motor axons innervating muscles of mastication. Spindles and Ia afferents were also present in the masticatory muscles. Thus, the development and/or survival of fusimotor neurons correlates with the presence of Ia afferents and/or spindles, and not with the amount of NT3 in the spinal cord or muscle.

Taste neurons have multiple inductive roles in mammalian gustatory development

The embryonic loss of brain-derived neurotropic factor (BDNF)-dependent taste axons in bdnf null mutant mice secondary impairs the development of gustatory epithelia and taste buds. In normal mice gustatory development continues for at least two weeks postnatally as axons promote taste bud formation. We conclude that taste axons in the fungiform, foliate, vallate and nasopalate papillae: i) promote papilla development, and ii) establish competent gustatory cells and iii) mature taste buds. Hence, gustatory innervation contributes critically to at least three of the multiple inductive interactions controlling the development of mammalian gustatory structures.

Neurotrophic factors in the tongue: expression patterns, biological activity, relation to innervation and studies of neurotrophin knockout mice

How taste buds develop and how they become innervated has been a matter of debate for a long time. Brain-derived neurotropic factor (BDNF) and neurotrophin-3 (NT3) mRNA expression patterns suggested a possible involvement in lingual gustatory and somatosensory innervation. Studies of null-mutated mice showed that BDNF-/- mice had few abnormal taste buds and were unable to discriminate between primary tastes. NT3-/- mice had a severe loss of lingual somatosensory innervation. These novel findings may have clinical implications in rare human conditions such as familial dysautonomia and/or in more common cases of problems with loss of taste and sensation in the mouth such as those seen after injury to the nerves, either by accident or following oral/facial surgery. Knowledge about which proteins that are required to stimulate nerve fibers to grow into mucous membranes of the oral cavity during development suggests that these same proteins might become helpful in stimulating regeneration of injured nerves in patients, perhaps helping them to regain lost taste and sensory functions. Here, the presence of glial cell-derived neurotrophic factor (GDNF) families of neurotrophic factors and receptors in the tongue is also discussed. Further, a model for the development and innervation of taste buds in mammals is proposed.

Endogenous neurotrophin-3 supports the survival of a subpopulation of sensory neurons in neonatal rat

Neurotrophin-3 promotes the differentiation and supports the survival of neuroblasts derived from the neural crest in early development. Neurotrophin-3 also plays an important role in the differentiation and survival of a subpopulation of large sensory neurons after their axons arrive at their targets. Proprioception and mechanoception are lost after gene deletion of neurotrophin-3 or its high-affinity receptor, TrkC. However, the function of neurotrophin-3 during late development and in mature animals is not clear. We have used an antiserum, specific for neurotrophin-3, to neutralize endogenous neurotrophin-3 in postnatal rats to determine its role in late sensory neuron development. Administration of the antiserum for a period of two weeks, but not one week, resulted in a 20% reduction in the number of primary sensory neurons in the dorsal root ganglia and a 19% reduction in the number of myelinated axons in the saphenous nerve. The size distribution histogram also indicated that a subpopulation of large neurons was lost by the neurotrophin-3 antiserum treatment. This neuronal loss was accompanied by reduced cell soma sizes and weights of the ganglia. Immunoreactivities for calbindin and calretinin were reduced in the trigeminal and dorsal root ganglia and nerve fibres surrounding whisker hair follicles. The number of Merkel cells in touch domes labelled with quinacrine and the number of parvalbumin-immunoreactive neurons in the dorsal root ganglia were significantly reduced by the antibody treatment. In contrast, the number of muscle spindles in the gastrocnemius muscle is not reduced by the neurotrophin-3 antiserum. Together, these results indicate that a subpopulation of primary sensory neurons in the neonatal rat requires neurotrophin-3 for their survival and expression of calcium binding proteins. In addition, Merkel cells in touch domes also require neurotrophin-3 for their survival. Thus, endogenous neurotrophin-3 in neonatal rats is critical for the survival and function of a subpopulation of primary sensory neurons and Merkel cells.

Sensory ataxia and muscle spindle agenesis in mice lacking the transcription factor Egr3

Muscle spindles are skeletal muscle sensory organs that provide axial and limb position information (proprioception) to the central nervous system. Spindles consist of encapsulated muscle fibers (intrafusal fibers) that are innervated by specialized motor and sensory axons. Although the molecular mechanisms involved in spindle ontogeny are poorly understood, the innervation of a subset of developing myotubes (type I) by peripheral sensory afferents (group Ia) is a critical event for inducing intrafusal fiber differentiation and subsequent spindle formation. The Egr family of zinc-finger transcription factors, whose members include Egr1 (NGFI-A), Egr2 (Krox-20), Egr3 and Egr4 (NGFI-C), are thought to regulate critical genetic programs involved in cellular growth and differentiation (refs 4-8, and W.G.T. et al., manuscript submitted). Mice deficient in Egr3 were generated by gene targeting and had gait ataxia, increased frequency of perinatal mortality, scoliosis, resting tremors and ptosis. Although extrafusal skeletal muscle fibers appeared normal, Egr3-deficient animals lacked muscle spindles, a finding that is consistent with their profound gait ataxia. Egr3 was highly expressed in developing muscle spindles, but not in Ia afferent neurons or their terminals during developmental periods that coincided with the induction of spindle morphogenesis by sensory afferent axons. These results indicate that type I myotubes are dependent upon Egr3-mediated transcription for proper spindle development.

Nerve growth factor: two receptors, multiple functions

Nerve growth factor (NGF) was characterized over 4 decades ago, and like the other neurotrophins subsequently discovered, it is best known for its trophic role, including the prevention of programmed cell death in specific populations of neurones in the peripheral nervous system. This property can be accounted for by the activation of a tyrosine kinase receptor. NGF also regulates neuronal function, as illustrated by its role in pain and inflammation, and in synaptic plasticity. Finally, NGF recently was shown to activate the neurotrophin receptor p75 (p75NTR), a receptor with no intrinsic catalytic activity and with similarities to members of the tumor necrosis factor receptor family. During normal development, the activation of p75NTR by NGF actually kills cells in the central nervous system. One remarkable property of NGF is then that it controls cell numbers in opposite ways in the developing nervous system, a result of its unique ability to activate two different receptor types.

Afferent-inherent regulation of myosin heavy chain isoforms in rat muscle spindles

Whether afferents exert their morphogenetic influence on spindles through release of trophic factors at intrafusal fiber junctions or via participation in proprioceptive pathways which modulate the motor activity to muscles was investigated by comparing myosin heavy chain (MHC) expression in intrafusal fibers after ablation of afferents (deafferentation, or DA) to the extensor digitorum longus (EDL) of adult rats or after ablation of the corresponding central processes of afferents to the spinal cord (central-process ablation, or CPA). DA and CPA elicited an exaggerated pedal plantarflexion, and hypertrophy of the EDL concomitant with atrophy of the soleus in the affected hindlimb. Frequencies and patterns of expression of seven MHCs expressed by intrafusal fibers in CPA muscles were indistinguishable from normal rats. However, frequencies and patterns of expression of several MHCs were abnormal following DA. Thus factors transported anterogradely from afferents to intrafusal fibers may regulate MHC expression in intrafusal fibers.

Neurotrophin-3-enhanced nerve regeneration selectively improves recovery of muscle fibers expressing myosin heavy chains 2b

The purpose of this study was to evaluate the effect of neurotrophin 3 (NT-3) enhanced nerve regeneration on the reinnervation of a target muscle. Muscle fibers can be classified according to their mechanical properties and myosin heavy chain (MHC) isoform composition. MHC1 containing slow-type and MHC2a or 2b fast-type fibers are normally distributed in a mosaic pattern, their phenotype dictated by motor innervation. After denervation, all fibers switch to fast-type MHC2b expression and also undergo atrophy resulting in loss of muscle mass. After regeneration, discrimination between fast and slow fibers returns, but the distribution and fiber size change according to the level of reinnervation. In this study, rat gastrocnemius muscles (ipsilateral and contralateral to the side of nerve injury) were collected up to 8 mo after nerve repair, with or without local delivery of NT-3. The phenotype changes of MHC1, 2a, and 2b were analyzed by immunohistochemistry, and fiber type proportion, diameter, and grouping were assessed by computerized image analysis. At 8 mo, the local delivery of NT-3 resulted in significant improvement in gastrocnemius muscle weight compared with controls (NT-3 group 47%, controls 39% weight of contralateral normal muscle; P < 0.05). NT-3 delivery resulted in a significant increase in the proportion (NT-3 43.3%, controls 35.7%; P < 0.05) and diameter (NT-3 87.8 micron, controls 70.8 micron; P < 0.05) of fast type 2b fibers after reinnervation. This effect was specific to type 2b fibers; no normalization was seen in other fiber types. This study indicates that NT-3-enhanced axonal regeneration has a beneficial effect on the motor target organ. Also, NT-3 may be specifically affecting a subset of motoneurons that determine type 2b muscle fiber phenotype. As NT-3 was topically applied to cut nerves, our data suggest a discriminating effect of the neurotrophin on neuro-muscular interaction. These results would imply that muscle fibers may be differentially responsive to other neurotrophic factors and indicate the potential clinical role of NT-3 in the prevention of muscle atrophy after nerve injury.

Introduction of a neurotrophin-3 transgene into muscle selectively rescues proprioceptive neurons in mice lacking endogenous neurotrophin-3

To clarify the role of muscle-derived neurotrophin-3 (NT-3) in the development of sensory neurons, we generated transgenic mice selectively overexpressing NT-3 in skeletal muscles under the control of a myogenin promoter (myo-NT-3 mice). The myo-NT-3 transgene was then bred into an NT-3 null mutant (-/-) line to generate myo-NT-3, NT-3(-/-) mice in which NT-3 was expressed in muscles, but not elsewhere. Transient overexpression of NT-3 in developing muscles increased the number of proprioceptive neurons as well as the density of both their central and peripheral projections, resulting in more Ia afferents in spinal cord and more spindles (end organs of Ia afferents) in muscles. NT-3 expression restricted to muscles was sufficient to secure the development of proprioceptive neurons and their central and peripheral projections in myo-NT-3, NT-3(-/-) mice. The loss of nonproprioceptive neurons observed in NT-3(-/-) mice was not reversed by the transgene, suggesting that these neurons are regulated by NT-3 from sources other than muscle. We conclude that target-derived rather than intraganglionic NT-3 is preeminent in supporting the development of proprioceptive neurons. The level of NT-3 in developing muscles may be the principal factor determining the number of proprioceptive neurons in dorsal root ganglions and spindles in skeletal muscles of adults.

Cellular delivery of neurotrophin-3 promotes corticospinal axonal growth and partial functional recovery after spinal cord injury

The injured adult mammalian spinal cord shows little spontaneous recovery after injury. In the present study, the contribution of projections in the dorsal half of the spinal cord to functional loss after adult spinal cord injury was examined, together with the effects of transgenic cellular delivery of neurotrophin-3 (NT-3) on morphological and functional disturbances. Adult rats underwent bilateral dorsal column spinal cord lesions that remove the dorsal corticospinal projections or underwent more extensive resections of the entire dorsal spinal cord bilaterally that remove corticospinal, rubrospinal, and cerulospinal projections. Long-lasting functional deficits were observed on a motor grid task requiring detailed integration of sensorimotor skills, but only in animals with dorsal hemisection lesions as opposed to dorsal column lesions. Syngenic primary rat fibroblasts genetically modified to produce NT-3 were then grafted to acute spinal cord dorsal hemisection lesion cavities. Up to 3 months later, significant partial functional recovery occurred in NT-3-grafted animals together with a significant increase in corticospinal axon growth at and distal to the injury site. These findings indicate that (1) several spinal pathways contribute to loss of motor function after spinal cord injury, (2) NT-3 is a neurotrophic factor for the injured corticospinal projection, and (3) functional deficits are partially ameliorated by local cellular delivery of NT-3. Lesions of the corticospinal projection may be necessary, but insufficient in isolation, to cause sensorimotor dysfunction after spinal cord injury in the rat.

Limb proprioceptive deficits without neuronal loss in transgenic mice overexpressing neurotrophin-3 in the developing nervous system

The role of neurotrophin-3 (NT3) during sensory neuron development was investigated in transgenic mice overexpressing NT3 under the control of the promoter and enhancer regions of the nestin gene, an intermediate filament gene widely expressed in the developing nervous system. Most of these mice died during the first postnatal day, and all showed severe limb ataxia suggestive of limb proprioceptive dysfunction. Tracing and histological analyses revealed a complete loss of spindles in limb muscles, absence of peripheral and central Ia projections, and lack of cells immunoreactive to parvalbumin in the dorsal root ganglion (DRG). Despite these deficits, there was no neuronal loss in the DRG of these mice. At birth, transgenic DRG showed increased neuron numbers, and displayed a normal proportion of neurons expressing substance P, calcitonin gene-related peptide and the NT3 receptor trkC. Transgenic dorsal roots exhibited an increased number of axons at birth, indicating that all sensory neurons in transgenic mice projected to the dorsal spinal cord. Despite the absence of central Ia afferents reaching motorneurons, several sensory fibers were seen projecting towards ectopic high levels of NT3 in the midline of transgenic spinal cords. These findings suggest novel roles for NT3 in differentiation of proprioceptive neurons, target invasion and formation of Ia projections which are independent from its effects on neuronal survival.

Neurotrophin-3 delivered locally via fibronectin mats enhances peripheral nerve regeneration

A better understanding of the mechanisms of nerve regeneration could improve the outcome of surgical nerve repair. We have previously shown that axonal regeneration is increased by nerve growth factor. Neurotrophin-3 (NT-3) belongs to the same family as nerve growth factor but acts on a distinct neuron subpopulation. As little is known about its role following nerve injury, we have investigated the effect of NT-3 delivered via fibronectin mats, previously shown to support nerve regeneration comparable to nerve grafts. NT-3 stimulation (0.1-1000 ng/ml) of neurite extension from embryonic chick dorsal root ganglia in vitro has shown that fibronectin can bind and release bioactive NT-3. Fibronectin mats impregnated with NT-3 (500 ng/ml) were grafted into 1 cm sciatic nerve defects in adult Lewis rats. Plain mats were used as controls. Computerized quantification of penetration distance, volume of axonal regeneration and myelinated fibre counts was undertaken using immunostaining for axonal markers (growth-associated protein 43, calcitonin gene-related peptide, substance P, vasoactive intestinal peptide and neuropeptide tyrosine), or S100 or thionine blue staining up to 8 months postoperatively. The maximal effect of NT-3 occurred at day 15, when for GAP43-immunostained axons both penetration distance (NT-3, 6.10 +/- 0.42 mm; control, 4.11 +/- 0.41 mm; P < 0.01) and staining area (NT-3, 0.137 +/- 0.012 mm2; control, 0.077 +/- 0.018 mm2; P < 0.05) were significantly increased. Similar results were found for each neuronal subpopulation investigated. By 8 months after repair, the NT-3 group supported a significantly greater number of myelinated axons (NT-3, 7003 +/- 402; control, 4932 +/- 725; P < 0.05) of similar diameter and g-ratio to controls. These results demonstrate the contribution of NT-3 to the increase of nerve regeneration promoted by growth factors.

Differential regulation of mRNAs for GDNF and its receptors Ret and GDNFR alpha after sciatic nerve lesion in the mouse

Glial cell line-derived neurotrophic factor (GDNF), first characterized for its effect on dopamine uptake in central dopaminergic neurons, appears to be a powerful neurotrophic factor for motor neurons. GDNF has recently been shown to signal through a multisubunit receptor. This receptor is composed of a ligand-binding subunit, called GDNF receptor alpha (GDNFR alpha), and a signalling tyrosine kinase subunit, Ret. To gain further insight into GDNF function, we investigated the expression of GDNF and its receptors after nerve lesion in adult mice. Analysis of expression in muscle, nerve and spinal cord by RNase protection assay and in situ hydridization revealed that, in adult non-lesioned mice, GDNF mRNA was expressed in the nerve and GDNFR alpha mRNA in the nerve and the spinal cord, while the expression of Ret was restricted to spinal cord motor neurons. After a sciatic nerve crush a rapid increase in GDNF mRNA was observed in the distal part of the nerve and a delayed elevation in the muscle, while GDNFR alpha mRNA was up-regulated in the distal part of the sciatic nerve but not in proximal nerve or spinal cord. The lesion also induced a rapid increase in Ret mRNA expression, but the increase was observed only in spinal cord motor neurons and in dorsal root ganglion neurons. A pattern of expression of GDNF and its receptors similar to that seen after lesion in the adult was detected during embryonic development. Administration of GDNF enhanced sciatic nerve regeneration measured by the nerve pinch test. Taken together, these results suggest that GDNF has an important role during regeneration after nerve damage in the adult.

Neurotrophin-3 promotes the differentiation of muscle spindle afferents in the absence of peripheral targets

The neurons of the dorsal root ganglia (DRG) that supply muscle spindles require target-derived factors for survival. One necessary factor for these neurons is neurotrophin-3 (NT3). To determine whether NT3 can promote the survival of these neurons in the absence of other target-derived factors, we analyzed the effects of exogenous NT3 after early limb bud deletion in the chick. In control embryos, limb bud deletion eliminated approximately 90% of the trkC-positive (trkC+) neurons in lumbar DRG on the deleted side. In addition, the deletion led to a dramatic loss of collateral sensory projections to motoneurons. Exogenous NT3 restored a normal population of trkC+ neurons in lumbar DRG on the deleted side and increased the number of trkC+ neurons in DRG with normal targets (contralateral lumbar and thoracic). The effect was highly selective; NT3 increased the number of trkC+ neurons without significantly changing the number of either trkA+ or trkB+ neurons. The effect of NT3 was attributable to the rescue of DRG neurons from cell death, because exogenous NT3 reduced the number of pyknotic nuclei without significantly altering proliferation. Analysis of spinal projections showed further that many of the trkC+ neurons rescued by NT3 projected to the ventral spinal cord. These neurons thus had central projections characteristic of muscle spindle afferents. Together, our results indicate that NT3 signaling is both necessary and sufficient for the development of the proprioceptive phenotype, even in the absence of other signals from limb muscle.

Neurotrophin regulation of the developing nervous system: analyses of knockout mice

The neurotrophins, NGF, BDNF, NT3 and NT4, are one family in a growing repertoire of neurotrophic factors. The neurotrophins have long been implicated in neuronal survival and recent studies from mice with targeted disruptions of the neurotrophin genes confirm this role, but also reveal that the action of the neurotrophins is more complex, and in some instances more interactive, than originally envisaged. Lack of functional NGF, BDNF and NT3 genes results in severe neuronal deficits and an early postnatal death. However, NT4 is unique among the neurotrophins and while the absence of NT4 does result in limited sensory neuron loss these mice do not die early, suggesting that NT4-dependent neurons are not critical for survival. Phenotypic analyses of mice lacking neurotrophin receptors, TrkA, B and C, confirm that TrkA is the functional receptor for NGF, TrkB acts as the primary receptor for BDNF and NT4, and NT3 signals primarily through TrkC. However, the finding that TrkC mutant mice have a less dramatic phenotype than their NT3 counterparts implicates NT3 in signaling via receptors other than TrkC. Further studies, using combinatorial Trk and neurotrophin deletions, reveal that while BDNF and NT4 subserve distinct neuron populations in most cases, other neuron sub-populations can be supported by either BDNF or NT4, providing evidence for compensatory actions between neurotrophins. As a mechanism to explain programmed cell death that occurs in the developing nervous system, recent studies examining neurotrophin gene-dosage effects suggest that the availability of neurotrophins, NGF, BDNF and NT3, may be limiting for some neuron populations. In addition, the proposed switch in neurotrophin dependency for some neuron populations is now being determined using neurotrophin mutant mice. We discuss these and other recent findings on neurotrophin requirements for the developing nervous system.

Lack of neurotrophin-3 results in death of spinal sensory neurons and premature differentiation of their precursors

To understand mechanisms resulting in the absence of two-thirds of spinal sensory neurons in mice lacking NT-3, we have compared dorsal root ganglia development in normal and mutant embryos. The reduction in neurons, achieved by E13, results from several deficits: first, elevated neuronal apoptosis significantly reduces neuronal numbers; second, elevated neurogenesis between E11 and E12, without changes in rates of precursor proliferation or apoptosis, depletes the precursor pool; consequently, the reduced precursor pool prevents increases in neuronal numbers between E12 and E13, when most neurons are born in normal animals. Although deficits occur before final target innervation, we show that NT-3 is expressed at all stages in regions accessible to these neurons or their axons and is only restricted to final targets after innervation.

Coculture of rat embryonic proprioceptive sensory neurons and myotubes

With the aim to study the cellular mechanism underlying the process of muscle spindle regeneration, dorsal root ganglia (DRG) neurons derived from 16-day rat embryos were cocultured with developing myotubes in a compartmentalized culture device. To accomplish the selective survival and neurite formation of the proprioceptive subpopulation, the neurotrophic factor, neurotrophin-3, was added to the culture medium. It appeared that the proprioceptive DRG neurons could develop specialized, Ia afferent terminal-like contacts with myotubes. However, these interactions were scarce and did not result in the induction of differentiation of the contacted myotubes into intrafusal fibers as normally occurs during in vivo development. The present coculture setup apparently lacks appropriate regulatory factors essential for the proper matching of sensory axons and intrafusal fiber precursors and the induction of a functional sensory myoneural connection.

Synchronous onset of NGF and TrkA survival dependence in developing dorsal root ganglia

Determinations of dorsal root ganglion (DRG) neuron loss in nerve growth factor (NGF) and neurotrophin-3 (NT-3) null mutant mice have supported the concept that neurons can switch neurotrophin dependence by revealing that many neurons must require both of these factors acting either sequentially or simultaneously during development. The situation is complex, however, in that NT-3(-/-) mutant mice show far greater neuron loss than mice deficient in the NT-3 receptor TrkC, suggesting that NT-3 may support many DRG neurons via actions on the NGF receptor TrkA. To assess the possibility of ligand-receptor cross-talk as a developmental mechanism, we have compared the onset of survival dependence of lumbar DRG neurons on NT-3, TrkC, NGF, and TrkA signaling in mice deficient in these molecules as a result of gene targeting. At embryonic day 11.5 (E11.5), virtually all lumbar DRG cells express TrkC mRNA and many require NT-3 and TrkC signaling for survival. In contrast, although many lumbar DRG cells also express TrkA at E11.5, there is little survival dependence on TrkA signaling. By E13.5, most lumbar DRG cells have downregulated TrkC mRNA. The onset of survival dependence on NGF and TrkA-signaling is concurrent and of equal magnitude at E13.5, demonstrating that NT-3 alone does not support DRG neurons via TrkA, nor can NT-3 compensate for the loss of NGF. We conclude that many murine DRG cells require NT-3 activation of TrkA is unimportant to these early NT-3 survival-promoting actions. We suggest that the discrepancy in cell loss between NT-3(-/-) and trkC(-/-) mutants is attributable to the ability of NT-3 to support DRG neurons via TrkA in the artificial situation where TrkC is absent.

Most classes of dorsal root ganglion neurons are severely depleted but not absent in mice lacking neurotrophin-3

During development, many neurons in the dorsal root ganglia require neurotrophin-3 for survival. However, it is not known precisely which subpopulations of sensory neurons, other than the proprioceptive afferents, are neurotrophin-3 dependent in vivo. In this study, using a battery of neurochemical markers that label different subpopulations of dorsal root ganglion neurons, we found a widespread, about 60-65% loss of cells in most subpopulations in neurotrophin-3 deficient mice. Intermediate losses were found in the heterozygous mutant mice consistent with a gene dosage effect. In agreement with this, the cell size distribution between the homozygous mutant and wild type mice was virtually identical. The loss of small neurons containing calcitonin gene-related peptide, substance P and thiamine monophosphatase activity suggests that many unmyelinated primary afferents are also lost in the mutant animals. The fact that many different sensory neuron subpopulations are lost to the same extent in neurotrophin-3 deficient mice is consistent with the proposed early role of neurotrophin-3 during neurogenesis. Interestingly, calretinin immunoreactive neurons, which contribute a minor subpopulation, were not affected suggesting that neurotrophin-3 independent regulation of neurogenesis occurs in addition to prominent neurotrophin-3 dependent mechanisms.

Cutaneous overexpression of NT-3 increases sensory and sympathetic neuron number and enhances touch dome and hair follicle innervation

Target-derived influences of nerve growth factor on neuronal survival and differentiation are well documented, though effects of other neurotrophins are less clear. To examine the influence of NT-3 neurotrophin overexpression in a target tissue of sensory and sympathetic neurons, transgenic mice were isolated that overexpress NT-3 in the epidermis. Overexpression of NT-3 led to a 42% increase in the number of dorsal root ganglia sensory neurons, a 70% increase in the number of trigeminal sensory neurons, and a 32% increase in sympathetic neurons. Elevated NT-3 also caused enlargement of touch dome mechanoreceptor units, sensory end organs innervated by slowly adapting type 1 (SA1) neurons. The enlarged touch dome units of the transgenics had an increased number of associated Merkel cells, cells at which SA1s terminate. An additional alteration of skin innervation in NT-3 transgenics was an increased density of myelinated circular endings associated with the piloneural complex. The enhancement of innervation to the skin was accompanied by a doubling in the number of sensory neurons expressing trkC. In addition, measures of nerve fibers in cross-sectional profiles of cutaneous saphenous nerves of transgenics showed a 60% increase in myelinated fibers. These results indicate that in vivo overexpression of NT-3 by the epidermis enhances the number of sensory and sympathetic neurons and the development of selected sensory endings of the skin.

Neurotrophins and the specification of neuronal phenotype

Nerve growth factor, brain derived neurotrophic factor and neurotrophin-3 all influence sensory neurons derived from the dorsal root ganglia. Traditionally these neurotrophins have been thought of as survival factors for sensory neurons during their development. Recent evidence from experiments where the in vivo levels of these proteins has been manipulated indicates that they may influence the development of specific sensory neuron phenotypes. In this review these experiments are discussed in relation to the mechanisms by which neurotrophins could influence the phenotypic fate of sensory neurons. The first mechanism requires that when a neuron becomes dependent for survival on a neurotrophin the availability of the factor simply influences the number of neurons surviving with a certain modality. This model requires that neurotrophin responsiveness is a determinant of the possible modalities that the neuron may acquire. The second mechanism requires that the availability of a given neurotrophin influences how many neurons can differentiate into different sensory neuron phenotype independent of survival. The available experimental data is discussed in relation to these two models.

Specific subtypes of cutaneous mechanoreceptors require neurotrophin-3 following peripheral target innervation

Neurotrophin-3 (NT-3) is required for the development of most sensory neurons of the dorsal root ganglia. Using electrophysiological techniques in mice with null mutations of the NT-3 gene, we show that two functionally specific subsets of cutaneous afferents differentially require this factor: D-hair receptors and slowly adapting mechanoreceptors; other cutaneous receptors were unaffected. Merkel cells, which are the end organs of slowly adapting mechanoreceptors, are virtually absent in 14-day-old homozygous mutants and are severely reduced in adult NT-3 heterozygous animals. This loss of Merkel cells, together with their innervation, happens in the first postnatal weeks of life, in contrast to muscle spindles and afferents, which are never formed in the absence of NT-3. Thus, NT-3 is essential for the maintenance of specific cutaneous afferents known to subserve fine tactile discrimination in humans.