Differential effects of combined trk receptor mutations on dorsal root ganglion and inner ear sensory neurons

Sortilin Modulates Schwann Cell Signaling and Remak Bundle Regeneration Following Nerve Injury

Peripheral nerve regeneration relies on the ability of Schwann cells to support the regrowth of damaged axons. Schwann cells re-differentiate when reestablishing contact with the sprouting axons, with large fibers becoming remyelinated and small nociceptive fibers ensheathed and collected into Remak bundles. We have previously described how the receptor sortilin facilitates neurotrophin signaling in peripheral neurons via regulated trafficking of Trk receptors. This study aims to characterize the effects of sortilin deletion on nerve regeneration following sciatic crush injury. We found that Sort1^–/– mice displayed functional motor recovery like that of WT mice, with no detectable differences in relation to nerve conduction velocities and morphological aspects of myelinated fibers. In contrast, we found abnormal ensheathment of regenerated C-fibers in injured Sort1^–/– mice, demonstrating a role of sortilin for Remak bundle formation following injury. Further studies on Schwann cell signaling pathways showed a significant reduction of MAPK/ERK, RSK, and CREB phosphorylation in Sort1^–/– Schwann cells after stimulation with neurotrophin-3 (NT-3), while Schwann cell migration and myelination remained unaffected. In conclusion, our results demonstrate that loss of sortilin blunts NT-3 signaling in Schwann cells which might contribute to the impaired Remak bundle regeneration after sciatic nerve injury.

The dynamics of trkA expression in the bovine ovary are associated with a luteotrophic effect of ovulation-inducing factor/nerve growth factor (OIF/NGF)

BACKGROUND

Ovulation-inducing factor in semen (OIF/NGF) influences ovulation and CL form and function in camelids and, remarkably, in cows. To test the hypothesis that the luteotrophic effect of OIF/NGF is mediated by an increase in trkA receptors in the ovulatory follicle and early CL, a study was designed to characterize the spatial and temporal distribution of trkA in ovarian follicles and CL at known stages of the bovine estrous cycle.

METHODS

Sexually mature cattle (n = 14) were examined daily by transrectal ultrasonography to determine the day of ovulation (Day 0), and assigned randomly to be unilaterally ovariectomized on Day 2, 4, 6 or in the pre-ovulatory period just before or after exogenous LH treatment. After a complete interovulatory interval, the cows were re-assigned to a different day-group on which the remaining ovary was removed. Sections of ovarian tissue representing the dominant follicle, largest subordinate follicle, and the CL were processed for immunofluorescent detection and quantification of trkA receptor.

RESULTS

TrkA immuno-fluorescence in ovarian tissues was restricted to follicles and the CL (no reaction in stroma or vessels), and was restricted to the cytoplasm (no nuclear staining). The trkA staining intensity, area of staining, and proportion of cells stained was greater in both theca and granulosa layers of dominant follicles than in that of subordinate follicles (P ≤ 0.05) in all day-groups except the Pre-LH group. Among dominant follicles, a progressive reduction in the immuno-positive reaction was detected from Day 2 to Day 6. Among subordinate follicles, immuno-reactivity remained low and unchanged except a rise in the Pre-LH group. The number of immuno-positive cells was greater in early developing CL (Days 2 and 4 combined) than in mature or regressing stage CL (Day 6, Pre- and Post-LH combined; P = 0.01). The intracellular distribution of trkA was more diffuse and widespread in dominant than subordinate follicles, particularly on Day 2 and Post-LH (P < 0.05).

CONCLUSIONS

Distinct differences in trkA expression between dominant and subordinate follicles, particularly when circulating progesterone is minimal (early luteal development and after luteolysis) is consistent with a local role of OIF/NGF in follicle selection and early luteogenesis.

Sortilins in neurotrophic factor signaling

The sortilin family of Vps10p-domain receptors includes sortilin, SorLA, and SorCS1-3. These type-I transmembrane receptors predominate in distinct neuronal tissues, but expression is also present in certain specialized non-neuronal cell populations including hepatocytes and cells of the immune system. The biology of sortilins is complex as they participate in both cell signaling and in intracellular protein sorting. Sortilins function physiologically in signaling by pro- and mature neurotrophins in neuronal viability and functionality. Recent genome-wide association studies have linked members to neurological disorders such as Alzheimer's disease and bipolar disorder and outside the nervous system to development of coronary artery disease and type-2 diabetes. Particularly well described are the receptor functions in neuronal signaling by pro- (proNT) and mature (NT) neurotrophins and in the processing/metabolism of amyloid precursor protein (APP).

Neurotrophin signalling and transcription programmes interactions in the development of somatosensory neurons

Somatosensory neurons of the dorsal root ganglia are generated from multipotent neural crest cells by a process of progressive specification and differentiation. Intrinsic transcription programmes active in somatosensory neuron progenitors and early post-mitotic neurons drive the cell-type expression of neurotrophin receptors. In turn, signalling by members of the neurotrophin family controls expression of transcription factors that regulate neuronal sub-type specification. This chapter explores the mechanisms by which this crosstalk between neurotrophin signalling and transcription programmes generates the diverse functional sub-types of somatosensory neurons found in the mature animal.

Expression of TrkC receptors in the developing brain of the Monodelphis opossum and its effect on the development of cortical cells

In this study, we investigated the distribution, localization and several various functions of TrkC receptors during development of the Monodelphisopossum brain. Western blotting analysis showed that two different forms of the TrkC receptor, the full-length receptor and one of its truncated forms, are abundantly expressed in the opossum brain. The expression of TrkC receptors was barely detected in the brain of newborn opossums. At postnatal day (P) 3, the expression of full-length TrkC remained at low levels, while moderate expression of the TrkC truncated form was detected. The expression levels of both forms of this protein gradually increased throughout development, peaking at P35. We found that in different neocortical areas located both at the rostral and caudal regions of the cortex, up to 98% of BrdU-labeled cells forming cortical layers (II-VI) had prominently expressed TrkC. To assess which developmental processes of cortical cells are regulated by TrkC receptors, three different shRNAs were constructed. The shRNAs were individually tested in transfected cortical progenitor cells grown on culture plates for 2 days. The effects of the shRNA-TrkC constructs were similar: blockade of TrkC receptors decreased the number of Ki67-positive and apoptotic cells, and it did not change the number of TUJ-positive neurons in vitro. Thus, the lack of TrkC receptors in cultured progenitor cells provided insight on the potential role of these receptors in the regulation of proliferation and cell survival but not in the differentiation of cortical cells.

Challenges for stem cells to functionally repair the damaged auditory nerve

INTRODUCTION

In the auditory system, a specialized subset of sensory neurons are responsible for correctly relaying precise pitch and temporal cues to the brain. In individuals with severe-to-profound sensorineural hearing impairment these sensory auditory neurons can be directly stimulated by a cochlear implant, which restores sound input to the brainstem after the loss of hair cells. This neural prosthesis therefore depends on a residual population of functional neurons in order to function effectively.

AREAS COVERED

In severe cases of sensorineural hearing loss where the numbers of auditory neurons are significantly depleted, the benefits derived from a cochlear implant may be minimal. One way in which to restore function to the auditory nerve is to replace these lost neurons using differentiated stem cells, thus re-establishing the neural circuit required for cochlear implant function. Such a therapy relies on producing an appropriate population of electrophysiologically functional neurons from stem cells, and on these cells integrating and reconnecting in an appropriate manner in the deaf cochlea.

EXPERT OPINION

Here we review progress in the field to date, including some of the key functional features that stem cell-derived neurons would need to possess and how these might be enhanced using electrical stimulation from a cochlear implant.

The convergence of cochlear implantation with induced pluripotent stem cell therapy

According to 2010 estimates from The National Institute on Deafness and other Communication Disorders, approximately 17% (36 million) American adults have reported some degree of hearing loss. Currently, the only clinical treatment available for those with severe-to-profound hearing loss is a cochlear implant, which is designed to electrically stimulate the auditory nerve in the absence of hair cells. Whilst the cochlear implant has been revolutionary in terms of providing hearing to the severe-to-profoundly deaf, there are variations in cochlear implant performance which may be related to the degree of degeneration of auditory neurons following hearing loss. Hence, numerous experimental studies have focused on enhancing the efficacy of cochlear implants by using neurotrophins to preserve the auditory neurons, and more recently, attempting to replace these dying cells with new neurons derived from stem cells. As a result, several groups are now investigating the potential for both embryonic and adult stem cells to replace the degenerating sensory elements in the deaf cochlea. Recent advances in our knowledge of stem cells and the development of induced pluripotency by Takahashi and Yamanaka in 2006, have opened a new realm of science focused on the use of induced pluripotent stem (iPS) cells for therapeutic purposes. This review will provide a broad overview of the potential benefits and challenges of using iPS cells in combination with a cochlear implant for the treatment of hearing loss, including differentiation of iPS cells into an auditory neural lineage and clinically relevant transplantation approaches.

Sortilin and SorLA regulate neuronal sorting of trophic and dementia-linked proteins

Sortilin and SorLA are members of the Vps10p domain receptor family, the Sortilins, which comprise five type I transmembrane receptors differentially expressed in neuronal tissues of the central and peripheral nervous system. Since the identification of sortilin in 1997, members of this receptor family are recognized as sorting receptors primarily in the trans-Golgi network, interacting with a wide range of ligands comprising other transmembrane receptors as well as soluble proteins from neurotrophic factors to enzymes targeted for lysosomes. Specifically, the involvement of sortilin in neutrophin signaling in healthy and injured neurons is increasingly recognized, as well as the impact of SorLA on the cellular processing of amyloid precursor protein, an important component in Alzheimer's disease. The current understanding of these issues as well as the recent recognition of a molecular link between sortilin and frontotemporal dementia is addressed in this present review.

Specification of neural crest into sensory neuron and melanocyte lineages

Elucidating the mechanisms by which multipotent cells differentiate into distinct lineages is a common theme underlying developmental biology investigations. Progress has been made in understanding some of the essential factors and pathways involved in the specification of different lineages from the neural crest. These include gene regulatory networks involving transcription factor hierarchies and input from signaling pathways mediated from environmental cues. In this review, we examine the mechanisms for two lineages that are derived from the neural crest, peripheral sensory neurons and melanocytes. Insights into the specification of these cell types may reveal common themes in the specification processes that occur throughout development.

A comparison of in vitro treatments for directing stem cells toward a sensory neural fate

PURPOSE

Low numbers of primary auditory neurons (ANs) may compromise the clinical performance of a cochlear implant. The focus of this research is to determine whether stem cells can be used to replace the ANs lost following deafness. To successfully replace these neurons, stem cells must be capable of directed differentiation into a sensory neural lineage in vitro and, subsequently, of survival and integration into the deafened cochlea.

MATERIALS AND METHODS

In this study, we compared three in vitro treatments for directing the differentiation of mouse embryonic stem cells toward a sensory neural fate using neurotrophins, conditioned media from early post-natal cochlear epithelium, or media containing BMP4.

RESULTS

In all treatments, stem cells were first exposed to retinoic acid, which was sufficient to induce Brn3a-positive patterning in 8-day differentiated embryoid bodies. After a further 8 days of differentiation in adherent culture conditions, BMP4 media-treated cultures produced higher proportions of cells expressing sensory neural markers in comparison to both the conditioned media and neurotrophin treatments, including significantly greater numbers of cells expressing peripherin (P ≤ .001), tyrosine receptor kinase B (P ≤ .001), and β-III tubulin (P ≤ .001).

CONCLUSIONS

This study illustrated that combined treatment with retinoic acid and BMP4 was most effective at directing differentiation of mouse stem cells into sensory-like neurons in vitro. This finding further supports the role of bone morphogenetic proteins in the differentiation of sensory neurons from neural progenitors, and provides a basis for allotransplantation studies for auditory neuron replacement in the deaf mouse cochlea.

The spiral ganglion: connecting the peripheral and central auditory systems

In mammals, the initial bridge between the physical world of sound and perception of that sound is established by neurons of the spiral ganglion. The cell bodies of these neurons give rise to peripheral processes that contact acoustic receptors in the organ of Corti, and the central processes collect together to form the auditory nerve that projects into the brain. In order to better understand hearing at this initial stage, we need to know the following about spiral ganglion neurons: (1) their cell biology including cytoplasmic, cytoskeletal, and membrane properties, (2) their peripheral and central connections including synaptic structure; (3) the nature of their neural signaling; and (4) their capacity for plasticity and rehabilitation. In this report, we will update the progress on these topics and indicate important issues still awaiting resolution.

Structure and development of cochlear afferent innervation in mammals

In mammals, sensorineural deafness results from damage to the auditory receptors of the inner ear, the nerve pathways to the brain or the cortical area that receives sound information. In this review, we first focused on the cellular and molecular events taking part to spiral ganglion axon growth, extension to the organ of Corti, and refinement. In the second half, we considered the functional maturation of synaptic contacts between sensory hair cells and their afferent projections. A better understanding of all these processes could open insights into novel therapeutic strategies aimed to re-establish primary connections from sound transducers to the ascending auditory nerve pathways.

Peripheral somatosensation: a touch of genetics

The somatosensory system processes information that organisms 'feel': joint position, muscle stretch, pain, pressure, temperature, and touch. The system is composed of a diverse array of peripheral nerve endings specialized to detect these sensory modalities. Several recent discoveries have shed light on the genetic pathways that control specification and differentiation of these neurons, how they accurately innervate their central and peripheral targets, and the molecules that enable them to detect mechanical stimuli. Here, we review the cadre of genes that control these processes, focusing on mechanosensitive neurons and support cells of the skin that mediate different aspects of the sense of touch.

Sortilin associates with Trk receptors to enhance anterograde transport and neurotrophin signaling

Binding of target-derived neurotrophins to Trk receptors at nerve terminals is required to stimulate neuronal survival, differentiation, innervation and synaptic plasticity. The distance between the soma and nerve terminal is great, making efficient anterograde Trk transport critical for Trk synaptic translocation and signaling. The mechanism responsible for this trafficking remains poorly understood. Here we show that the sorting receptor sortilin interacts with TrkA, TrkB and TrkC and enables their anterograde axonal transport, thereby enhancing neurotrophin signaling. Cultured DRG neurons lacking sortilin showed blunted MAP kinase signaling and reduced neurite outgrowth upon stimulation with NGF. Moreover, deficiency for sortilin markedly aggravated TrkA, TrkB and TrkC phenotypes present in p75(NTR) knockouts, and resulted in increased embryonic lethality and sympathetic neuropathy in mice heterozygous for TrkA. Our findings demonstrate a role for sortilin as an anterograde trafficking receptor for Trk and a positive modulator of neurotrophin-induced neuronal survival.

PLCγ-activated signalling is essential for TrkB mediated sensory neuron structural plasticity

Background

The vestibular system provides the primary input of our sense of balance and spatial orientation. Dysfunction of the vestibular system can severely affect a person's quality of life. Therefore, understanding the molecular basis of vestibular neuron survival, maintenance, and innervation of the target sensory epithelia is fundamental.

Results

Here we report that a point mutation at the phospholipase Cγ (PLCγ) docking site in the mouse neurotrophin tyrosine kinase receptor TrkB (Ntrk2) specifically impairs fiber guidance inside the vestibular sensory epithelia, but has limited effects on the survival of vestibular sensory neurons and growth of afferent processes toward the sensory epithelia. We also show that expression of the TRPC3 cation calcium channel, whose activity is known to be required for nerve-growth cone guidance induced by brain-derived neurotrophic factor (BDNF), is altered in these animals. In addition, we find that absence of the PLCγ mediated TrkB signalling interferes with the transformation of bouton type afferent terminals of vestibular dendrites into calyces (the largest synaptic contact of dendrites known in the mammalian nervous system) on type I vestibular hair cells; the latter are normally distributed in these mutants as revealed by an unaltered expression pattern of the potassium channel KCNQ4 in these cells.

Conclusions

These results demonstrate a crucial involvement of the TrkB/PLCγ-mediated intracellular signalling in structural aspects of sensory neuron plasticity.

Development and neuronal dependence of cutaneous sensory nerve formations: Lessons from neurotrophins

Null mutations of genes from the NGF family of NTs and their receptors (NTRs) lead to loss/reduction of specific neurons in sensory ganglia; conversely, cutaneous overexpression of NTs results in skin hyperinnervation and increase or no changes in the number of sensory neurons innervating the skin. These neuronal changes are paralleled with loss of specific types of sensory nerve formations in the skin. Therefore, mice carrying mutations in NT or NTR genes represent an ideal model to identify the neuronal dependence of each type of cutaneous sensory nerve ending from a concrete subtype of sensory neuron, since the development, maintenance, and structural integrity of sensory nerve formations depend upon sensory neurons. Results obtained from these mouse strains suggest that TrkA positive neurons are connected to intraepithelial nerve fibers and other sensory nerve formations depending from C and Adelta nerve fibers; the neurons expressing TrkB and responding to BDNF and NT-4 innervate Meissner corpuscles, a subpopulation of Merkell cells, some mechanoreceptors of the piloneural complex, and the Ruffini's corpuscles; finally, a subpopulation of neurons, which are responsive to NT-3, support postnatal survival of some intraepithelial nerve fibers and Merkel cells in addition to the muscle mechanoreceptors. On the other hand, changes in NTs and NTRs affect the structure of non-nervous structures of the skin and are at the basis of several cutaneous pathologies. This review is an update about the role of NTs and NTRs in the maintenance of normal cutaneous innervation and maintenance of skin integrity.

Expression of brain-derived neurotrophic factor and TrkB in the lateral line system of zebrafish during development

The neuromasts of the lateral line system are regarded as a model to study the mechanisms of hearing, deafness, and ototoxicity. The neurotrophins (NTs), especially brain-derived neurotrophic factor (BDNF), and its signaling receptor TrkB are involved in the development and maintenance of neuromasts. To know the period in which the BDNF/TrkB complex has more effects in the neuromast biology, the age-related changes were studied. Normal zebrafish from 10 to 180 days post-fertilization (dpf), as well as transgenic ET4 zebrafish 10 and 20 dpf, was analyzed using qRT-PCR, western blot, and immunohistochemistry. BDNF and TrkB mRNAs followed a parallel course, peaking at 20 dpf, and thereafter progressively decreased. Specific immunoreactivity for BDNF and TrkB was found co-localized in all hairy cells of neuromasts in 20 and 30 dpf; then, the number of immunoreactive cells decreased, and by 180 dpf BDNF remains restricted to a subpopulation of hairy cells, and TrkB to a few number of sensory and non-sensory cells. At all ages examined, TrkB immunoreactivity was detected in sensory ganglia innervating the neuromasts. The present results demonstrate that there is a parallel time-related decline in the expression of BDNF and TrkB in zebrafish. Also, the patterns of cell expression suggest that autocrine/paracrine mechanisms for this NT system might occur within the neuromasts. Because TrkB in lateral line ganglia did not vary with age, their neurons are potentially capable to respond to BDNF during the entire lifespan of zebrafish.

BDNF is essentially required for the early postnatal survival of nociceptors

Neurotrophins promote the survival of specific types of neurons during development and ensure proper maintenance and function of mature responsive neurons. Significant effects of BDNF (Brain-Derived Neurotrophic Factor) on pain physiology have been reported but the contribution of this neurotrophin to the development of nociceptors has not been investigated. We present evidence that BDNF is required for the survival of a significant fraction of peptidergic and non-peptidergic nociceptors in dorsal root ganglia (DRG) postnatally. Bdnf homozygous mutant mice lose approximately half of all nociceptive neurons during the first 2 weeks of life and adult heterozygotes exhibit hypoalgesia and a loss of 25% of all nociceptive neurons. Our in vitro analyses indicate that BDNF-dependent nociceptive neurons also respond to NGF and GDNF. Expression analyses at perinatal times indicate that BDNF is predominantly produced within sensory ganglia and is more abundant than skin-derived NGF or GDNF. Function-blocking studies with BDNF specific antibodies in vitro or cultures of BDNF-deficient sensory neurons suggest that BDNF acts in an autocrine/paracrine way to promote the early postnatal survival of nociceptors that are also responsive to NGF and GDNF. Altogether, the data demonstrate an essential requirement for BDNF in the early postnatal survival of nociceptive neurons.

Nerve growth factor mediates mechanical allodynia in a mouse model of type 2 diabetes

C57BLKS db/db (db/db) mice develop a neuropathy with features of human type 2 diabetic neuropathy. Here, we demonstrate that these mice develop transient mechanical allodynia at the early stage of diabetes. We hypothesized that nerve growth factor (NGF), which enhances the expression of key mediators of nociception (i.e. substance P [SP] and calcitonin gene-related peptide), contributes to the development of mechanical allodynia in these mice. We found that NGF, SP, and calcitonin gene-related peptide gene expression is upregulated in the dorsal root ganglion (DRG) of db/db mice before or during the period that they develop mechanical allodynia. There were more small- to medium-sized NGF-immunopositive DRG neurons in db/db mice than in control db+ mice; these neurons also expressed SP, consistent with its role in nociception. Nerve growth factor expression in the hind paw skin was also increased in a variety of dermal cell types and nerve fibers, suggesting the contribution of a peripheral source of NGF to mechanical allodynia. The upregulation of NGF coincided with enhanced tropomyosin-related kinase A receptor phosphorylation in the DRG. Finally, an antibody against NGF inhibited mechanical allodynia and decreased the numbers of SP-positive DRG neurons in db/db mice. These results suggest that inhibition of NGF action is a potential strategy for treating painful diabetic neuropathy.

Role of neurotrophin signalling in the differentiation of neurons from dorsal root ganglia and sympathetic ganglia

Manipulation of neurotrophin (NT) signalling by administration or depletion of NTs, by transgenic overexpression or by deletion of genes coding for NTs and their receptors has demonstrated the importance of NT signalling for the survival and differentiation of neurons in sympathetic and dorsal root ganglia (DRG). Combination with mutation of the proapoptotic Bax gene allows the separation of survival and differentiation effects. These studies together with cell culture analysis suggest that NT signalling directly regulates the differentiation of neuron subpopulations and their integration into neural networks. The high-affinity NT receptors trkA, trkB and trkC are restricted to subpopulations of mature neurons, whereas their expression at early developmental stages largely overlaps. trkC is expressed throughout sympathetic ganglia and DRG early after ganglion formation but becomes restricted to small neuron subpopulations during embryogenesis when trkA is turned on. The temporal relationship between trkA and trkC expression is conserved between sympathetic ganglia and DRG. In DRG, NGF signalling is required not only for survival, but also for the differentiation of nociceptors. Expression of neuropeptides calcitonin gene-related peptide and substance P, which specify peptidergic nociceptors, depends on nerve growth factor (NGF) signalling. ret expression indicative of non-peptidergic nociceptors is also promoted by the NGF-signalling pathway. Regulation of TRP channels by NGF signalling might specify the temperature sensitivity of afferent neurons embryonically. The manipulation of NGF levels "tunes" heat sensitivity in nociceptors at postnatal and adult stages. Brain-derived neurotrophic factor signalling is required for subpopulations of DRG neurons that are not fully characterized; it affects mechanical sensitivity in slowly adapting, low-threshold mechanoreceptors and might involve the regulation of DEG/ENaC ion channels. NT3 signalling is required for the generation and survival of various DRG neuron classes, in particular proprioceptors. Its importance for peripheral projections and central connectivity of proprioceptors demonstrates the significance of NT signalling for integrating responsive neurons in neural networks. The molecular targets of NT3 signalling in proprioceptor differentiation remain to be characterized. In sympathetic ganglia, NGF signalling regulates dendritic development and axonal projections. Its role in the specification of other neuronal properties is less well analysed. In vitro analysis suggests the involvement of NT signalling in the choice between the noradrenergic and cholinergic transmitter phenotype, in the expression of various classes of ion channels and for target connectivity. In vivo analysis is required to show the degree to which NT signalling regulates these sympathetic neuron properties in developing embryos and postnatally.

Cross talk between vestibular neurons and Schwann cells mediates BDNF release and neuronal regeneration

It is now well-established that an active cross-talk occurs between neurons and glial cells, in the adult as well as in the developing and regenerating nervous systems. These functional interactions not only actively modulate synaptic transmission, but also support neuronal growth and differentiation. We have investigated the possible existence of a reciprocal interaction between inner ear vestibular neurons and Schwann cells maintained in primary cultures. We show that ATP released by the extending vestibular axons elevates intracellular calcium levels within Schwann cells. Purinergic activation of the Schwann P2X(7) receptor induces the release of neurotrophin BDNF, which occurs via a regulated, tetanus-toxin sensitive, vesicular pathway. BDNF, in turn, is required by the vestibular neuron to support its own survival and growth. Given the massive release of ATP during tissue damage, cross-talk between vestibular neurons and Schwann cells could play a primary role during regeneration.

Specification of sensory neuron cell fate from the neural crest

How distinct cell fates are generated from initially homogeneous cell populations is a driving question in developmental biology. The neural crest is one such cell population that is capable of producing an incredible array of derivatives. Cells as different in function and form as the pigment cells in the skin or the neurons and glia of the peripheral nervous system are all derived from neural crest. How do these cells choose to migrate along distinct routes, populate defined regions of the embryo and differentiate into specific cell types? This chapter focuses on the development of one particular neural crest derivative, sensory neurons, as a model for studying these questions of cell fate specification. In the head, sensory neurons reside in the trigeminal and epibranchial ganglia, while in the trunk they form the spinal or dorsal root ganglia (DRG). The development of the DRG will be the main focus of this review. The neurons and glia of the DRG derive from trunk neural crest cells that coalesce at the lateral edge of the spinal cord (Fig. 1). These neural crest cells migrate along the same routes as neural crest cells that populate the autonomic sympathetic ganglia located along the dorsal aorta. Somehow DRG precursors must make the decision to stop and adopt a sensory fate adjacent to the spinal cord rather than continuing on to become part of the autonomic ganglia. Moreover, once the DRG precursors aggregate in their final positions there are still a number of fate choices to be made. The mature DRG is composed of many neurons with different morphologies and distinct biochemical properties as well as glial cells that support these neurons.

The TrkC receptor induces apoptosis when the dependence receptor notion meets the neurotrophin paradigm

The TrkC/NT-3 receptor/ligand pair is believed to be part of the classic neurotrophic theory claiming that neuronal death occurs by default when neurotrophic factors become limited, through loss of survival signals. Here, we show that TrkC is a dependence receptor and, as such, induces caspase-dependent apoptotic death in the absence of NT-3 in immortalized cells, a proapoptotic activity inhibited by the presence of NT-3. This proapoptotic activity of TrkC relies on the caspase-mediated cleavage of the intracellular domain of TrkC, which permits the release of a proapoptotic fragment. This fragment induces apoptosis through a caspase-9-dependent mechanism. Finally, we show that the death of dorsal root ganglion (DRG) neurons provoked by NT-3 withdrawal is inhibited when TrkC-proapoptotic activity is antagonized. Thus, the death of neurons upon disappearance of NT-3 is not only due to a loss of survival signals but also to the active proapoptotic activity of the unbound TrkC dependence receptor.

Implanted embryonic sensory neurons project axons toward adult auditory brainstem neurons in roller drum and Stoppini co-cultures

Previously we have shown in vivo the survival, migration and integration of embryonic dorsal root ganglion (DRG) neurons that were grafted into the inner ear and peripheral auditory nervous system. In order to evaluate relevant factors determining integration of sensory neurons further into the central auditory nervous system, complementary in vitro techniques are necessary. The advantages of in vitro systems are that a large number of factors including various grafts and different conditions can be efficiently examined for. Hence, we co-cultured 300 microm thick postnatal rat brainstem slices containing the cochlear nucleus including the central part of the 8th cranial nerve with mouse embryonic DRG neurons. The organotypic co-cultures were either grown on coverslips using the roller drum method described by Gähwiler or on membranes according to the interface method described by Stoppini. Neurons in the cochlear nucleus were labeled with DiI. The results demonstrate that (1) brainstem slices survive for up to 5 weeks in culture, and that (2) co-cultures of embryonic sensory neurons and brainstem show a high degree of neuronal survival, and that (3) survival and axonal outgrowth from the implanted embryonic neurons are dependent on the presence of the brainstem slice rather than on exogenous NGF and that (4) implanted embryonic neurons send axons toward neurons in the cochlear nucleus.

Rebuilding lost hearing using cell transplantation

OBJECTIVE

The peripheral auditory nervous system (cochlea and auditory nerve) has a complex anatomy, and it has traditionally been thought that once the sensorineural structures are damaged, restoration of hearing is impossible. In the past decade, however, the potential to restore lost hearing has been intensively investigated using molecular and cell biological techniques, and we can now part with such a pessimistic view. In this review, we examine an important field in hearing restoration research: cell transplantation.

METHODS

Most efforts in this field have been directed to the replacement of hair cells by transplantation to the cochlea. Here, we focus on transplantation to the auditory nerve, from the side of the cerebellopontine angle rather than the cochlea.

RESULTS

Delivery of cells to the cochlea is potentially damaging, and nerve cells transplanted distally to the Schwann-glial transitional zone (cochlear side) may become inhibited when they reach the transitional zone. The auditory nerve is probably the most suitable route for cell transplantation.

CONCLUSION

The auditory nerve occupies an important position not only in neurosurgery but also in various diseases in other disciplines, and several lines of recent evidence indicate that it is a key target for hearing restoration. It is familiar to most neurosurgeons, and the recent advances in the molecular and cell biology of inner-ear development are of direct importance to neurorestorative medicine. In this article, we review the anatomy, development, and molecular biology of the auditory nerve and cochlea, with emphasis on the advances in cell transplantation.

Angiopoietin-1 promotes neurite outgrowth from dorsal root ganglion cells positive for Tie-2 receptor

The effects of vascular factors on the nervous system are still poorly investigated. Angiopoietin-1 (Ang-1), an endothelial cell growth factor with influences on blood vessel stabilization, has been recently reported to prevent apoptosis in a neuroblastoma cell line via a pathway dependent on Tie-2 receptor. The present study focuses on the effect of Ang-1 on cultured dorsal root ganglion (DRG) cells isolated from 1-day-old rats. Three-day-old DRG cultures were exposed to Ang-1 treatment under serum-free condition for another 5 days and stained with antibodies against neurofilament (NF) 200 protein. Neurite length and density increased compared with those of controls. Double-immunofluorescence staining demonstrated the co-localization of the Tie-2 receptor in some NF-200-positive perikarya. The reverse transcription/polymerase chain reaction technique identified Tie-2 receptor mRNA in intact DRG and in Ang-1-stimulated DRG cell cultures, but not in a Schwann cell line or in primary astrocyte cultures. Western blotting confirmed that the expression of NF 68 protein in cultures treated with Ang-1 or nerve growth factor was higher than that in cultures treated with medium alone. When the Tie-2 receptor was blocked with anti-Tie-2 receptor antibody, neurite outgrowth was severely impeded. Induction of trkA-receptor protein expression was observed to be dependent on the presence of Tie-2 receptors. We conclude that Ang-1 promotes neurite outgrowth from DRG cells positive for Tie-2 receptor. The signalling pathway appears to involve transactivation of the trkA receptor.

Substance P expression in TRPV1 and trkA-positive dorsal root ganglion neurons innervating the mouse lung

In the present study, the co-localisation of substance P (SP) with the vanilloid receptor TRPV1 and the neurotrophin receptor tyrosine kinase trkA was analysed in airway-specific murine dorsal root ganglion (DRG) neurons. DRG neurons labelled with Fast Blue were predominantly found at the segmental levels T2-T5. Immunoreactivity for the receptor TRPV1 was localized to 12% of Fast Blue labelled DRG neurons. Double-labelling immunohistochemistry revealed that a substantial number of them also co-express SP (7.6 +/- 1.1% (mean +/- S.E.M.)), whereas neurons with immunoreactivity for TRPV1 only were found in 4.4 +/- 1.3% of the retrogradely labelled neuronal population. Further analysis of retrogradely labelled neurons showed that their majority expressed trkA (62.8 +/- 1.4%), neurofilament protein 68-kDa (64.8 +/- 1.5%) or glutamate alone (19.5 +/- 1.9%). SP was always expressed in trkA-positive neurons. Based on the extent of co-localization of SP with the receptors TRPV1 and trkA in DRG airway neurons, the present study indicates that the DRG pathway may have effects on the magnitude of neurogenic inflammation in airway diseases such as asthma.

NGF stimulates extensive neurite outgrowth from implanted dorsal root ganglion neurons following transplantation into the adult rat inner ear

Neuronal tissue transplantation is a potential way to replace degenerated spiral ganglion neurons (SGNs) since these cells cannot regenerate in adult mammals. To investigate whether nerve growth factor (NGF) can stimulate neurite outgrowth from implanted neurons, mouse embryonic dorsal root ganglion (DRG) cells expressing enhanced green fluorescent protein (EGFP) were transplanted into the scala tympani of adult rats with a supplement of NGF or artificial perilymph. DRG neurons were observed in the cochlea for up to 6 weeks postoperatively. A significant difference was identified in the number of DRG neurons between the NGF and non-NGF groups. In the NGF group, extensive neurite projections from DRGs were found penetrating the osseous modiolus towards the spiral ganglion. These results suggest the possibility that embryonic neuronal implants may become integrated within the adult auditory nervous system. In combination with a cochlear prosthesis, a neuronal implantation strategy may provide a possibility for further treatment of profoundly deaf patients.

PDGF signaling in cells and mice

Since its discovery over three decades ago, platelet-derived growth factor (PDGF) has been a model system for learning how growth factors regulate biological processes. For the first several decades investigators used cells grown in tissue culture. More recently, PDGF signaling has also been investigated in mice. This review outlines the advances in these two systems, and highlights some of the directions for future investigation.

Regulation of neuronal death and calcitonin gene-related peptide by fibroblast growth factor-2 and FGFR3 after peripheral nerve injury: Evidence from mouse mutants

The presence and regulation of basic fibroblast growth factor and its high-affinity tyrosine kinase receptor FGFR3 in sensory neurons during development and after peripheral nerve injury suggest a physiological role of the fibroblast growth factor-2 system for survival and maintenance of sensory neurons. Here we investigated L5 spinal ganglia of intact and lesioned fibroblast growth factor-2 knock-out and FGFR3 knock-out mice. Quantification of sensory neurons in intact L5 spinal ganglia revealed no differences between wild-types and mutant mice. After sciatic nerve axotomy, the normally occurring neuron loss in wild-type mice was significantly reduced in both knock-out strains suggesting that fibroblast growth factor-2 is involved in neuronal death mediated via FGFR3. In addition, the number of chromatolytic and eccentric cells was significantly increased in fibroblast growth factor-2 knock-out mice indicating a transient protection of injured spinal ganglia neurons in the absence of fibroblast growth factor-2. The expression of the neuropeptide calcitonin gene-related peptide in sensory neurons of intact fibroblast growth factor-2 knock-out and FGFR3 knock-out mice was not changed in comparison to adequate wild-types. Fibroblast growth factor-2 wild-type and FGFR3 wild-type mice showed a lesion-induced decrease of calcitonin gene-related peptide-positive neurons in ipsilateral L5 spinal ganglia whereas the loss of calcitonin gene-related peptide-immunoreactive sensory neurons is reduced in the absence of fibroblast growth factor-2 or FGFR3, respectively. In addition, FGFR3 wild-type and knock-out mice displayed a contralateral reduction of the neuropeptide after axotomy. These results suggest that endogenous fibroblast growth factor-2 and FGFR3 are crucially involved in the regulation of survival and calcitonin gene-related peptide expression of lumbar sensory neurons after lesion, but not during development.

Absence of Meissner corpuscles in the digital pads of mice lacking functional TrkB

The TrkB-expressing sensory neurons seem to be involved in touch and other discriminative sensibilities. Thus, several slowly and rapidly adapting cutaneous mechanoreceptors, as well as muscle spindles, are reduced or absent in the territory of the trigeminal nerve in functionally TrkB-deficient mice. Whether this also occurs in the cutaneous or muscular territories of dorsal root ganglia has not been analyzed. Here we used immunohistochemistry and transmission-electron microscopy to analyze the impact of a mutation in the gene coding for TrkB on Meissner and Pacinian corpuscles, and muscle spindles. The animals were studied at the post-natal days 15 and 25, because at this time all the mechanoreceptors examined are fully developed. Typical Meissner's corpuscles, displaying S-100 protein immunoreactivity, were found in the digital pads of wild-type and TrkB+/- mice whereas they were absent in the TrkB-/- animals. Regarding Pacinian corpuscles, the mutation in the trkB gene does not alter either the immunohistochemical or the ultrastructural characteristics. Finally, in muscle spindles the arrangement of the intrafusal muscle fibers and nerve fibers was unchanged in the mutated animals. Nevertheless, about 10% of muscle spindles showed increased number of the intrafusal cells (between 6 and 12) and were supplied by more than one large myelinic nerve fiber. The present results strongly suggest that TrkB-expressing sensory neurons in dorsal root ganglia, like those of the trigeminal ganglion, are responsible for the development and maintenance of several rapidly adapting cutaneous mechanoreceptors, i.e. Meissner's corpuscles.

Survival of neuronal tissue following xenograft implantation into the adult rat inner ear

The poor regenerative capacity of the spiral ganglion neurons of the mammalian inner ear has initiated research on how to assist the functional recovery of the injured auditory system. A possible treatment is to use a biological implant with a potential to establish central or peripheral synaptic contacts to develop into a functional auditory unit. The feasibility of this approach was tested by xenograft implantation of dorsal root ganglion (DRG) neurons from embryonic days 13 to 14, mouse expressing either LacZ or enhanced green fluorescent protein (EGFP) into the scala tympani of the adult rat inner ear. Transplanted DRG neurons survived in the scala tympani for a postoperative period ranging from 3 to 10 weeks, as verified by histochemical detection of LacZ, EGFP fluorescence and immunohistochemical labeling of the neuronal markers neurofilament and Thy 1.2. DRG neurons were found close to structures near the sensory epithelium (the organ of Corti) as well as adjacent to the spiral ganglion neurons with their peripheral dendrites. These results illustrate not only the survival of xenografted DRG neurons in the adult inner ear but also the feasibility of a neuronal transplantation strategy in the degenerated auditory system, thereby creating possibilities to replace spiral ganglion neurons.

Effect of Brn-3a deficiency on parvalbumin-, calbindin D-28k-, calretinin- and calcitonin gene-related peptide-immunoreactive primary sensory neurons in the trigeminal ganglion

Immunohistochemistry for parvalbumin, calbindin D-28k, calretinin and calcitonin gene-related peptide (CGRP) was performed on the trigeminal ganglion and oro-facial tissues in Brn-3a wildtype and knockout mice at embryonic day 18.5 and postnatal day 0. In wildtype mice, the trigeminal ganglion contained abundant parvalbumin-, calbindin D-28k- and CGRP-immunoreactive neurons while the ganglion was almost devoid of calretinin-immunoreactive neurons. In Brn-3a knockout mice, a 63% decrease of parvalbumin-immunoreactive neurons was detected. In contrast, the absence of Brn-3a dramatically increased the number of calbindin D-28k-immunoreactive (3.5-fold increase) and calretinin-immunoreactive neurons (91-fold increase). The number of CGRP-immunoreactive neurons, however, was not altered by the Brn-3a deficiency. Cell size analysis indicated that loss of Brn-3a increased the proportions of small (<100 microm (2)) parvalbumin-, calbindin D-28k- and CGRP-immunoreactive neurons while it decreased those of large (>200 microm(2)) immunoreactive cells. Calretinin-immunoreactive neurons were either small or medium (100-200 microm (2)) in mutant mice. The oro-facial tissues contained parvalbumin-, calbindin D-28k- and CGRP-immunoreactive fibers, but not calretinin-immunoreactive ones in wildtype mice. In Brn-3a knockout mice, the number of parvalbumin-immunoreactive fibers markedly decreased in the infraorbital nerve and parvalbumin-immunoreactive endings disappeared in the vibrissa. In contrast, the number of calbindin D-28k-immunoreactive fibers increased significantly in the infraorbital and mental nerves. In addition, calbindin D-28k-immunoreactive endings appeared in the vibrissa. As well, some fibers showed calretinin-immunoreactivity in the infraorbital nerve of the mutant. However, no obvious change of CGRP-immunoreactive fibers was observed in the oro-facial region of knockout mice. Taken together, our data suggest that Brn-3a deficiency has effects on the expression of neurochemical substances in the trigeminal ganglion.

Influence of neurotrophins on the synaptogenesis of inner hair cells in the deaf Bronx waltzer (bv) mouse organ of Corti in culture

The Bronx waltzer (bv) deaf mouse is characterized by massive degeneration of the primary auditory receptors, the inner hair cells, which occurs during the time of expected afferent synaptogenesis. The process is associated with degeneration and protracted division of the normally postmitotic afferent spiral ganglion neurons. To investigate the potential role of neurotrophins in the afferent synaptogenesis of inner hair cells, we exposed bv newborn cochleas in organotypic culture to brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and nerve growth factor (NGF), and also to gamma aminobutyric acid (GABA), for up to 8 days. The study was done using light and electron microscopy. Only about 20% of the inner hair cells survived in culture, regardless of the treatment, similar to the number in the intact mutant in our colony. Depending on the exogenous treatment, this population consisted of either innervated ultrastructurally normal cells or denervated dedifferentiated cells wrapped-in lieu of nerve endings-by the supporting inner phalangeal and border cells. In the control and GABA cultures, inner hair cells were mostly denervated. BDNF and NT-3 alone or combined increased synaptogenesis and hair cell survival only during the first 3 days (by about 10%); however, the cells became denervated by 8 postnatal (PN). Only NGF induced stable innervation and differentiation of neurosensory relationships, including supernumerary innervation characteristic of the intact bv. Denervation among the remaining 20% of inner hair cells induced a reactive wrapping by inner phalangeal and border cells which evidently extended inner hair cell survival. Immunocytochemical studies of these reactive supporting cells were done in the intact (8 PN) mutant cochlea. The supporting cells that provide sustenance to the denervated inner hair cells displayed strong BDNF (and possibly NT-3) immunoreactivity. Subsequently, we revealed the presence of all three neurotrophins in the inner hair cell region of the developing (1-8 PN) cochlea of the normal ICR mouse. The inner hair cells expressed all three neurotrophins; BDNF prevailed in the inner phalangeal cells, NT-3 in the pillar cells and inner phalangeal cells, and NGF in the pillar cells.

IN CONCLUSION

initially, the 80% loss of inner hair cells is apparently caused by their failed afferent synaptogenesis. Exogenous neurotrophins influence synaptogenesis in the bv in culture, but NGF alone is successful in promoting stable neurosensory relationships. The presence of neurotrophins in supporting cells in the normal and degenerating cochlea indicates their role in the sustenance of inner hair cells.

Auditory system development: primary auditory neurons and their targets

The neurons of the cochlear ganglion transmit acoustic information between the inner ear and the brain. These placodally derived neurons must produce a topographically precise pattern of connections in both the inner ear and the brain. In this review, we consider the current state of knowledge concerning the development of these neurons, their peripheral and central connections, and their influences on peripheral and central target cells. Relatively little is known about the cellular and molecular regulation of migration or the establishment of precise topographic connection to the hair cells or cochlear nucleus (CN) neurons. Studies of mice with neurotrophin deletions are beginning to yield increasing understanding of variations in ganglion cell survival and resulting innervation patterns, however. Finally, existing evidence suggests that while ganglion cells have little influence on the differentiation of their hair cell targets, quite the opposite is true in the brain. Ganglion cell innervation and synaptic activity are essential for normal development of neurons in the cochlear nucleus.

A compendium of mouse knockouts with inner ear defects

Genetically engineered strains of mice, modified by gene targeting (knockouts), are increasingly being employed as alternative effective research tools in elucidating the genetic basis of human deafness. An impressive array of auditory and vestibular mouse knockouts is already available as a valuable resource for studying the ontogenesis, morphogenesis and function of the mammalian inner ear. This article provides a current catalog of mouse knockouts with inner ear morphogenetic malformations and hearing or balance deficits resulting from ablation of genes that are regionally expressed in the inner ear and/or within surrounding tissues, such as the hindbrain, neural crest and mesenchyme.

Regulation of neurogenesis by neurotrophins in developing spinal sensory ganglia

Neurons and glia in spinal sensory ganglia derive from multipotent neural crest-derived stem cells. In contrast to neural progenitor cells in the central nervous system, neural crest progenitors coexist with differentiated sensory neurons all throughout the neurogenic period. Thus, developing sensory ganglia are advantageous for determining the possible influence of cell-cell interactions in the regulation of precursor proliferation and neurogenesis. Neurotrophins are important regulators of neuronal survival in the developing vertebrate nervous system and, in addition, they appear to influence precursor behavior in vitro. Studies in mice carrying mutations in neurotrophin genes provide a good system in which to analyze essential actions of these factors on the different developing neural populations.

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

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

Neurotrophins: roles in neuronal development and function

Neurotrophins regulate development, maintenance, and function of vertebrate nervous systems. Neurotrophins activate two different classes of receptors, the Trk family of receptor tyrosine kinases and p75NTR, a member of the TNF receptor superfamily. Through these, neurotrophins activate many signaling pathways, including those mediated by ras and members of the cdc-42/ras/rho G protein families, and the MAP kinase, PI-3 kinase, and Jun kinase cascades. During development, limiting amounts of neurotrophins function as survival factors to ensure a match between the number of surviving neurons and the requirement for appropriate target innervation. They also regulate cell fate decisions, axon growth, dendrite pruning, the patterning of innervation and the expression of proteins crucial for normal neuronal function, such as neurotransmitters and ion channels. These proteins also regulate many aspects of neural function. In the mature nervous system, they control synaptic function and synaptic plasticity, while continuing to modulate neuronal survival.

Essential role of BETA2/NeuroD1 in development of the vestibular and auditory systems

BETA2/NeuroD1 is a bHLH transcription factor that is expressed during development in the mammalian pancreas and in many locations in the central and peripheral nervous systems. During inner ear ontogenesis, it is present in both sensory ganglion neurons and sensory epithelia. Although studies have shown that BETA2/NeuroD1 is important in the development of the hippocampal dentate gyrus and the cerebellum, its functions in the peripheral nervous system and in particular in the inner ear are unclear. Mice carrying a BETA2/NeuroD1 null mutation exhibit behavioral abnormalities suggestive of an inner ear defect, including lack of responsiveness to sound, hyperactivity, head tilting, and circling. Here we show that these defects can be explained by a severe reduction of sensory neurons in the cochlear-vestibular ganglion (CVG). A developmental study of CVG formation in the null demonstrates that BETA2/NeuroD1 does not play a primary role in the proliferation of neuroblast precursors or in their decision to become neuroblasts. Instead, the reduction in CVG neuron number is caused by a combination both of delayed or defective delamination of CVG neuroblast precursors from the otic vesicle epithelium and of enhanced apoptosis both in the otic epithelium and among those neurons that do delaminate to form the CVG. There are also defects in differentiation and patterning of the cochlear duct and sensory epithelium and loss of the dorsal cochlear nucleus. BETA2/NeuroD1 is, thus, the first gene to be shown to regulate neuronal and sensory cell development in both the cochlear and vestibular systems.

Cellular studies of auditory hair cell regeneration in birds

A decade ago it was discovered that mature birds are able to regenerate hair cells, the receptors for auditory perception. This surprising finding generated hope in the field of auditory neuroscience that new hair cells someday may be coaxed to form in another class of warm-blooded vertebrates, mammals. We have made considerable progress toward understanding some cellular and molecular events that lead to hair cell regeneration in birds. This review discusses our current understanding of avian hair cell regeneration, with some comparisons to other vertebrate classes and other regenerative systems.

Positive and negative interactions of GDNF, NTN and ART in developing sensory neuron subpopulations, and their collaboration with neurotrophins

Glial cell line-derived neurotrophic factor (GDNF), neurturin (NTN) and neublastin/artemin (ART) are distant members of the transforming growth factor beta family, and have been shown to elicit neurotrophic effects upon several classes of peripheral and central neurons. Limited information from in vitro and expression studies has also substantiated a role for GDNF family ligands in mammalian somatosensory neuron development. Here, we show that although dorsal root ganglion (DRG) sensory neurons express GDNF family receptors embryonically, they do not survive in response to their ligands. The regulation of survival emerges postnatally for all GDNF family ligands. GDNF and NTN support distinct subpopulations that can be separated with respect to their expression of GDNF family receptors, whereas ART supports neurons in populations that are also responsive to GDNF or NTN. Sensory neurons that coexpress GDNF family receptors are medium sized, whereas small-caliber nociceptive cells preferentially express a single receptor. In contrast to brain-derived neurotrophic factor (BDNF)-dependent neurons, embryonic nerve growth factor (NGF)-dependent nociceptive neurons switch dependency to GDNF, NTN and ART postnatally. Neurons that survive in the presence of neurotrophin 3 (NT3) or neurotrophin 4 (NT4), including proprioceptive afferents, Merkel end organs and D-hair afferents, are also supported by GDNF family ligands neonatally, although at postnatal stages they lose their dependency on GDNF and NTN. At late postnatal stages, ART prevents survival elicited by GDNF and NTN. These data provide new insights on the roles of GDNF family ligands in sensory neuron development.

Severe peripheral sensory neuron loss and modest motor neuron reduction in mice with combined deficiency of brain-derived neurotrophic factor, neurotrophin 3 and neurotrophin 4/5

Neurotrophins are a family of structurally and functionally related proteins that regulate neuronal survival during development. In the peripheral nervous system (PNS), both in vitro and in vivo studies have shown that neurotrophins are potent factors for the survival of various sensory neurons and sympathetic neurons. However, it is not clear whether all PNS neurons are neurotrophin-dependent. In the central nervous system (CNS), studies using injury models show that neurotrophins promote the survival of CNS neurons. But mice lacking individual neurotrophins or a combination of BDNF and NT4 did not show significant CNS neuronal loss. Here we derived mice lacking three neurotrophins, brain-derived growth factor (BDNF), neurotrophin-3 (NT3), and neurotrophin-4 (NT4) to study the effect of triple neurotrophin deficiency on peripheral and central neurons. These triple-deficient mice did not nurse and died within 12 hours after birth. Neuronal cell counts showed that triple mutant pups lacked most of their peripheral sensory neurons and had a statistically significant reduction of motor neurons in several motor nuclei. Our results suggest that neurotrophins are essential for the survival of most peripheral sensory neurons and affect the survival of a small portion of motor neurons during embryogenesis.

The neurotrophins act synergistically with LIF and members of the TGF-beta superfamily to promote the survival of spiral ganglia neurons in vitro

A number of growth factor families have been implicated in normal inner ear development, auditory neuron survival and protection. Several growth factors, including transforming growth factor-beta5 (TGF-beta5) and TGF-beta3, neurotrophin-3 (NT-3), brain-derived neurotrophic factor (BDNF) and leukemia inhibitory factor (LIF) were tested for their ability, individually or in combination, to promote auditory neuron survival in dissociated cell cultures of early rat post-natal spiral ganglion cells (SGCs). The results indicate that at discrete concentrations all growth factors act in an additive fashion and some in synergy when promoting neuronal survival. These findings support the hypothesis that growth factors from different families may be interdependent when sustaining neuronal integrity.

Trks and p75 genes are differentially expressed in the inner ear of human embryos. What may Trks and p75 null mutant mice suggest on human development?

Recent work has shown the expression of Neurotrophins low (p75) and high affinity (Trk's A, B, and C) receptors in the developing inner ear sensory neurons of chick and mouse. Likewise the biological significance of such receptor expression was demonstrated by using both Trks and Neurotrophins null mutant mice. The present study was conducted to determine the expression of Trks and p75 proteins in the human inner ear throughout development. Hence to assess the potential role of Neurotrophins in the development of auditory and vestibular specific innervation in man. In other words, we intend to address the issue whether or not what null mutant mice for Trks and p75 have revealed on inner ear development may be relevant for human embryos. Fifty-two inner ears and their cochleovestibular ganglions (CVG) from human embryos and fetuses, ranging from 5 to 24 weeks of pregnancy were analyzed. Both Western blot and immunocytochemistry on frozen sections were used as complementary procedures. Quantitative Western blot studies revealed that Trk-B and C immunoreactivity (IR) appeared by embryonic week 5 in CVG neurons, increased at high levels between embryonic weeks 7 and 12, and later on, in 15 week-old specimens and older began to decrease to minimal levels. Trk-A IR was detected at just moderate levels during 5 and 7 weeks reflecting the presence of NGF high affinity receptors only at these earlier developmental ages. The p75 IR was detected at high degrees in the early stage of the 5th week and at abundant levels in all studied inner ears from the 7th to the 24th pregnancy week. These Western blot observations were corroborated by immunocytochemistry on frozen sections, which also revealed a major distribution of both p75 and Trks on neuronal bodies while p75 appears localized on supporting cells. Our findings reveal a tight correlation between p75 and Trks expression throughout human development and specific inner ear developmental events, such as target-dependent neuronal cell death and afferent hair cells innervation. That kind of association of p75 and Trks temporal pattern with distinctive steps in inner ear developmental schedule, is a feature shared between human embryos and other mammals, such as mouse. Based on the present results and considering them together with the reported phenotype of p75 and Trks null mutant mice, we hypothesize that p75 and Trk receptors, as well as, their binding Neurotrophins may be essential in human inner ear development. Accordingly, they may be required molecules for sensory epitheliums innervation and target-dependent neuronal cell death, during embryogenesis and even early postnatal life, in man.

Development of the vertebrate ear: insights from knockouts and mutants

The three divisions of the ear (outer, middle and inner) each have an important role in hearing, while the inner ear is also crucial for the sense of balance. How these three major components arise and coalesce to form the peripheral elements of the senses of hearing and balance is now being studied using molecular-genetic approaches. This article summarizes data from studies of knockout and mutant animals in which one or more divisions of the ear are abnormal. The data confirm that development of all three divisions of the ear depends on the genes involved in hindbrain segmentation and segment identity. Genes that are regionally expressed in the inner ear can, when absent or mutated, yield selective ablation of specific inner-ear structures or cell types.

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.