Epibranchial placode-derived neurons produce BDNF required for early sensory neuron development

Exosome-shuttled miR-126 mediates ethanol-induced disruption of neural crest cell-placode cell interaction by targeting SDF1

During embryonic development, 2 populations of multipotent stem cells, cranial neural crest cells (NCCs) and epibranchial placode cells (PCs), are anatomically adjacent to each other. The coordinated migration of NCCs and PCs plays a major role in the morphogenesis of craniofacial skeletons and cranial nerves. It is known that ethanol-induced dysfunction of NCCs and PCs is a key contributor to the defects of craniofacial skeletons and cranial nerves implicated in fetal alcohol spectrum disorder (FASD). However, how ethanol disrupts the coordinated interaction between NCCs and PCs was not elucidated. To fill in this gap, we established a well-designed cell coculture system to investigate the reciprocal interaction between human NCCs (hNCCs) and human PCs (hPCs), and also monitored the migration behavior of NCCs and PCs in zebrafish embryos. We found that ethanol exposure resulted in a disruption of coordinated hNCCs-hPCs interaction, as well as in zebrafish embryos. Treating hNCCs-hPCs with exosomes derived from ethanol-exposed hNCCs (ExoEtOH) mimicked ethanol-induced impairment of hNCCs-hPCs interaction. We also observed that SDF1, a chemoattractant, was downregulated in ethanol-treated hPCs and zebrafish embryos. Meanwhile, miR-126 level in ExoEtOH was significantly higher than that in control exosomes (ExoCon). We further validated that ExoEtOH-encapsulated miR-126 from hNCCs can be transferred to hPCs to suppress SDF1 expression in hPCs. Knockdown of SDF1 replicated ethanol-induced abnormalities either in vitro or in zebrafish embryos. On the contrary, overexpression of SDF1 or inhibiting miR-126 strongly rescued ethanol-induced impairment of hNCCs-hPCs interaction and developmental defects.

Intra-amniotic transplantation of brain-derived neurotrophic factor-modified mesenchymal stem cells treatment for rat fetuses with spina bifida aperta

Background

Spina bifida aperta (SBA) is a relatively common clinical type of neural tube defect. Although prenatal fetal surgery has been proven to be an effective treatment for SBA, the recovery of neurological function remains unsatisfactory due to neuron deficiencies. Our previous results demonstrated that intra-amniotic transplanted bone marrow mesenchymal stem cells (BMSCs) could preserve neural function through lesion-specific engraftment and regeneration. To further optimize the role of BMSCs and improve the environment of defective spinal cords so as to make it more conducive to nerve repair, the intra-amniotic transplanted BMSCs were modified with brain-derived neurotrophic factor (BDNF-BMSCs), and the therapeutic potential of BDNF-BMSCs was verified in this study.

Methods

BMSCs were modified by adenovirus encoding a green fluorescent protein and brain-derived neurotrophic factor (Ad-GFP-BDNF) in vitro and then transplanted into the amniotic cavity of rat fetuses with spina bifida aperta which were induced by all-trans-retinoic acid on embryonic day 15. Immunofluorescence, western blot and real-time quantitative PCR were used to detect the expression of different neuron markers and apoptosis-related genes in the defective spinal cords. Lesion areas of the rat fetuses with spina bifida aperta were measured on embryonic day 20. The microenvironment changes after intra-amniotic BDNF-BMSCs transplantation were investigated by a protein array with 90 cytokines.

Results

We found that BDNF-BMSCs sustained the characteristic of directional migration, engrafted at the SBA lesion area, increased the expression of BDNF in the defective spinal cords, alleviated the apoptosis of spinal cord cells, differentiated into neurons and skin-like cells, reduced the area of skin lesions, and improved the amniotic fluid microenvironment. Moreover, the BDNF-modified BMSCs showed a better effect than pure BMSCs on the inhibition of apoptosis and promotion of neural differentiation.

Conclusion

These findings collectively indicate that intra-amniotic transplanted BDNF-BMSCs have an advantage of promoting the recovery of defective neural tissue of SBA fetuses.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-022-03105-6.

Elp1 is required for development of visceral sensory peripheral and central circuitry

Cardiovascular instability and a blunted respiratory drive in hypoxic conditions are hallmark features of the genetic sensory and autonomic neuropathy, familial dysautonomia (FD). FD results from a mutation in the gene ELP1, the encoded protein of which is a scaffolding subunit of the six-subunit Elongator complex. In mice, we and others have shown that Elp1 is essential for the normal development of neural crest-derived dorsal root ganglia sensory neurons. Whether Elp1 is also required for development of ectodermal placode-derived visceral sensory receptors, which are required for normal baroreception and chemosensory responses, has not been investigated. Using mouse models for FD, we here show that the entire circuitry underlying baroreception and chemoreception is impaired due to a requirement for Elp1 in the visceral sensory neuron ganglia, as well as for normal peripheral target innervation, and in their central nervous system synaptic partners in the medulla. Thus, Elp1 is required in both placode- and neural crest-derived sensory neurons, and its reduction aborts the normal development of neuronal circuitry essential for autonomic homeostasis and interoception.

This article has an associated First Person interview with the first author of the paper.

Summary: Our data indicate that Elp1 is required in both placode- and neural crest-derived sensory neurons, and that it exerts comparable effects, including survival, axonal morphology and target innervation in both lineages.

Effect of Sonic hedgehog on the proliferation in mouse tongue epithelial cells

OBJECTIVE

To investigate Sonic hedgehog (Shh) effects on proliferation and apoptosis of tongue epithelial cells in embryonic and ageing mice.

MATERIALS AND METHODS

Embryonic day 13.5 (E13.5), E14.5, E16.5 and postnatal day 0.5 (PN0.5) K14-Cre;Shh^fl/fl mice, and E14.5, E16.5, PN0.5, PN90.5 and postnatal 1.5 years (PN1.5Y) wild-type (Wt) mice were employed. Scanning electron microscopy, haematoxylin-eosin and immunohistochemistry were performed. Gel beads containing exogenous Shh protein were embedded in the tongue of PN90.5 and PN1.5Y Wt mice. Three days later, 5-bromodeoxyuridine (BrdU), proliferating cell nuclear antigen (PCNA) immunohistochemical and TUNEL staining were performed.

RESULTS

The number of fungiform papillae was decreased with age. The numbers of BrdU- and PCNA-positive cells were the highest at PN0.5 and the lowest at PN1.5Y. Compared with Wt mice, K14-Cre;Shh^fl/fl mice had decreased PCNA-positive cells in the epithelium, a smaller tongue volume, and fewer papillae at PN0.5. At E14.5, the number of BrdU-positive cells was decreased in K14-Cre;Shh ^fl/fl mice. At PN1.5Y, the number of apoptotic cells in tongue tissue exposed to Shh protein was less than that in the BSA group and the numbers of BrdU- and PCNA-positive proliferating cells were increased.

CONCLUSION

Shh maintains cell proliferation and reduces apoptosis during tongue development and ageing.

BDNF and NGF signals originating from sensory ganglia promote cranial motor axon growth

After exiting the hindbrain, branchial motor axons reach their targets in association with sensory ganglia. The trigeminal ganglion has been shown to promote motor axon growth from rhombomeres 2/3 and 4/5, but it is unknown whether this effect is ganglion specific and through which signals it is mediated. Here, we addressed these questions by co-cultures of ventral rhombomere 8 explants with cranial and spinal sensory ganglia in a collagen gel matrix. Our results show that all cranial sensory ganglia and even a trunk dorsal root ganglion can promote motor axon growth and that ganglia isolated from older embryos had a stronger effect on the axonal growth than younger ones. We found that brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) are necessary and sufficient for this effect. Altogether, our results demonstrate that the promoting effect of sensory ganglia on cranial motor axon growth is stage dependent, but not ganglion specific and is mediated by BDNF and NGF signals.

The transcription factor Phox2b distinguishes between oral and non-oral sensory neurons in the geniculate ganglion

Many basic characteristics of gustatory neurons remain unknown, partly due to the absence of specific markers. Some neurons in the geniculate ganglion project to taste regions in the oral cavity, whereas others innervate the outer ear. We hypothesized that the transcription factor Phox2b would identify oral cavity-projecting neurons in the geniculate ganglion. To test this possibility, we characterized mice in which Phox2b-Cre mediated gene recombination labeled neurons with tdTomato. Nerve labeling revealed that all taste neurons projecting through the chorda tympani (27%) and greater superficial petrosal nerves (15%) expressed Phox2b during development, whereas non-oral somatosensory neurons (58%) in the geniculate ganglion did not. We found tdTomato-positive innervation within all taste buds. Most (57%) of the fungiform papillae had labeled innervation only in taste buds, whereas 43% of the fungiform papillae also had additional labeled innervation to the papilla epithelium. Chorda tympani nerve transection eliminated all labeled innervation to taste buds, but most of the additional innervation in the fungiform papillae remained. Some of these additional fibers also expressed tyrosine hydroxylase, suggesting a sympathetic origin. Consistent with this, both sympathetic and parasympathetic fibers innervating blood vessels and salivary glands contained tdTomato labeling. Phox2b-tdTomato labels nerve fascicles in the tongue of the developing embryo and demonstrates a similar stereotyped branching pattern DiI-labeling.

BDNF is required for taste axon regeneration following unilateral chorda tympani nerve section

Taste nerves readily regenerate to reinnervate denervated taste buds; however, factors required for regeneration have not yet been identified. When the chorda tympani nerve is sectioned, expression of brain-derived neurotrophic factor (BDNF) remains high in the geniculate ganglion and lingual epithelium, despite the loss of taste buds. These observations suggest that BDNF is present in the taste system after nerve section and may support taste nerve regeneration. To test this hypothesis, we inducibly deleted Bdnf during adulthood in mice. Shortly after Bdnf gene recombination, the chorda tympani nerve was unilaterally sectioned causing a loss of both taste buds and neurons, irrespective of BDNF levels. Eight weeks after nerve section, however, regeneration was differentially affected by Bdnf deletion. In control mice, there was regeneration of the chorda tympani nerve and taste buds reappeared with innervation. In contrast, few taste buds were reinnervated in mice lacking normal Bdnf expression such that taste bud number remained low. In all genotypes, taste buds that were reinnervated were normal-sized, but non-innervated taste buds remained small and atrophic. On the side of the tongue contralateral to the nerve section, taste buds for some genotypes became larger and all taste buds remained innervated. Our findings suggest that BDNF is required for nerve regeneration following gustatory nerve section.

Insulin-Like Growth Factors Are Expressed in the Taste System, but Do Not Maintain Adult Taste Buds

Growth factors regulate cell growth and differentiation in many tissues. In the taste system, as yet unknown growth factors are produced by neurons to maintain taste buds. A number of growth factor receptors are expressed at greater levels in taste buds than in the surrounding epithelium and may be receptors for candidate factors involved in taste bud maintenance. We determined that the ligands of eight of these receptors were expressed in the E14.5 geniculate ganglion and that four of these ligands were expressed in the adult geniculate ganglion. Of these, the insulin-like growth factors (IGF1, IGF2) were expressed in the ganglion and their receptor, insulin-like growth factor receptor 1 (IGF1R), were expressed at the highest levels in taste buds. To determine whether IGF1R regulates taste bud number or structure, we conditionally eliminated IGF1R from the lingual epithelium of mice using the keratin 14 (K14) promoter (K14-Cre::Igf1r^lox/lox). While K14-Cre::Igf1r^lox/lox mice had significantly fewer taste buds at P30 compared with control mice (Igf1r^lox/lox), this difference was not observed by P80. IGF1R removal did not affect taste bud size or cell number, and the number of phospholipase C β2- (PLCβ2) and carbonic anhydrase 4- (Car4) positive taste receptor cells did not differ between genotypes. Taste buds at the back of the tongue fungiform taste field were larger and contained more cells than those at the tongue tip, and these differences were diminished in K14-Cre::Igf1r^lox/lox mice. The epithelium was thicker at the back versus the tip of the tongue, and this difference was also attenuated in K14-Cre::Igf1r^lox/lox mice. We conclude that, although IGFs are expressed at high levels in the taste system, they likely play little or no role in maintaining adult taste bud structure. IGFs have a potential role in establishing the initial number of taste buds, and there may be limits on epithelial thickness in the absence of IGF1R signaling.

Pharyngeal arch deficiencies affect taste bud development in the circumvallate papilla with aberrant glossopharyngeal nerve formation

BACKGROUND

The pharyngeal arches (PAs) generate cranial organs including the tongue. The taste placodes, formed in particular locations on the embryonic tongue surface, differentiate into taste buds harbored in distinct gustatory papillae. The developing tongue also has a complex supply of cranial nerves through each PA. However, the relationship between the PAs and taste bud development is not fully understood.

RESULTS

Ripply3 homozygous mutant mice, which have impaired third/fourth PAs, display a hypoplastic circumvallate papilla and lack taste buds, although the taste placode is normally formed. Formation of the glossopharyngeal ganglia is defective and innervation toward the posterior tongue is completely missing in Ripply3 mutant embryos at E12.5. Moreover, the distribution of neuroblasts derived from the epibranchial placode is severely, but not completely, atenuated, and the neural crest cells are diminished in the third PA region of Ripply3 mutant embryos at E9.5-E10.5. In Tbx1 homozygous mutant embryos, which exhibit another type of deficiency in PA development, the hypoplastic circumvallate papilla is observed along with abnormal formation of the glossopharyngeal ganglia and severely impaired innervation.

CONCLUSIONS

PA deficiencies affect multiple aspects of taste bud development, including formation of the cranial ganglia and innervation to the posterior tongue.

Is there a relationship between brain-derived neurotrophic factor for driving neuronal auditory circuits with onset of auditory function and the changes following cochlear injury or during aging?

Brain-derived neurotrophic factor, BDNF, is one of the most important neurotrophic factors acting in the peripheral and central nervous system. In the auditory system its function was initially defined by using constitutive knockout mouse mutants and shown to be essential for survival of neurons and afferent innervation of hair cells in the peripheral auditory system. Further examination of BDNF null mutants also revealed a more complex requirement during re-innervation processes involving the efferent system of the cochlea. Using adult mouse mutants defective in BDNF signaling, it could be shown that a tonotopical gradient of BDNF expression within cochlear neurons is required for maintenance of a specific spatial innervation pattern of outer hair cells and inner hair cells. Additionally, BDNF is required for maintenance of voltage-gated potassium channels (KV) in cochlear neurons, which may form part of a maturation step within the ascending auditory pathway with onset of hearing and might be essential for cortical acuity of sound-processing and experience-dependent plasticity. A presumptive harmful role of BDNF during acoustic trauma and consequences of a loss of cochlear BDNF during aging are discussed in the context of a partial reversion of this maturation step. We compare the potentially beneficial and harmful roles of BDNF for the mature auditory system with those BDNF functions known in other sensory circuits, such as the vestibular, visual, olfactory, or somatosensory system.

Neural crest and placode interaction during the development of the cranial sensory system

In the vertebrate head, the peripheral components of the sensory nervous system are derived from two embryonic cell populations, the neural crest and cranial sensory placodes. Both arise in close proximity to each other at the border of the neural plate: neural crest precursors abut the future central nervous system, while placodes originate in a common preplacodal region slightly more lateral. During head morphogenesis, complex events organise these precursors into functional sensory structures, raising the question of how their development is coordinated. Here we review the evidence that neural crest and placode cells remain in close proximity throughout their development and interact repeatedly in a reciprocal manner. We also review recent controversies about the relative contribution of the neural crest and placodes to the otic and olfactory systems. We propose that a sequence of mutual interactions between the neural crest and placodes drives the coordinated morphogenesis that generates functional sensory systems within the head.

A fate-map for cranial sensory ganglia in the sea lamprey

Cranial neurogenic placodes and the neural crest make essential contributions to key adult characteristics of all vertebrates, including the paired peripheral sense organs and craniofacial skeleton. Neurogenic placode development has been extensively characterized in representative jawed vertebrates (gnathostomes) but not in jawless fishes (agnathans). Here, we use in vivo lineage tracing with DiI, together with neuronal differentiation markers, to establish the first detailed fate-map for placode-derived sensory neurons in a jawless fish, the sea lamprey Petromyzon marinus, and to confirm that neural crest cells in the lamprey contribute to the cranial sensory ganglia. We also show that a pan-Pax3/7 antibody labels ophthalmic trigeminal (opV, profundal) placode-derived but not maxillomandibular trigeminal (mmV) placode-derived neurons, mirroring the expression of gnathostome Pax3 and suggesting that Pax3 (and its single Pax3/7 lamprey ortholog) is a pan-vertebrate marker for opV placode-derived neurons. Unexpectedly, however, our data reveal that mmV neuron precursors are located in two separate domains at neurula stages, with opV neuron precursors sandwiched between them. The different branches of the mmV nerve are not comparable between lampreys and gnatho-stomes, and spatial segregation of mmV neuron precursor territories may be a derived feature of lampreys. Nevertheless, maxillary and mandibular neurons are spatially segregated within gnathostome mmV ganglia, suggesting that a more detailed investigation of gnathostome mmV placode development would be worthwhile. Overall, however, our results highlight the conservation of cranial peripheral sensory nervous system development across vertebrates, yielding insight into ancestral vertebrate traits.

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The first detailed fate-map for placode-derived sensory neurons in a jawless fish.

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Pax3 is a pan-vertebrate marker for ophthalmic trigeminal placode-derived neurons.

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Maxillomandibular trigeminal neuron precursors are located in two separate domains.

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Confirmation that lamprey neural crest cells contribute to cranial sensory ganglia.

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Results overall highlight conservation of cranial sensory nervous system development.

BDNF and NT4 play interchangeable roles in gustatory development

A limited number of growth factors are capable of regulating numerous developmental processes, but how they accomplish this is unclear. The gustatory system is ideal for examining this issue because the neurotrophins brain-derived neurotrophic factor (BDNF) and neurotrophin-4 (NT4) have different developmental roles although both of them activate the same receptors, TrkB and p75. Here we first investigated whether the different roles of BDNF and NT4 are due to their differences in temporal and spatial expression patterns. Then, we asked whether or not these two neurotrophins exert their unique roles on the gustatory system by regulating different sets of downstream genes. By using Bdnf(Nt4/Nt4) mice, in which the coding region for BDNF is replaced with NT4, we examined whether the different functions of BDNF and NT4 are interchangeable during taste development. Our results demonstrated that NT4 could mediate most of the unique roles of BDNF during taste development. Specifically, caspase-3-mediated cell death, which was increased in the geniculate ganglion in Bdnf(-/-) mice, was rescued in Bdnf(Nt4/Nt4) mice. In BDNF knockout mice, tongue innervation was disrupted, and gustatory axons failed to reach their targets. However, disrupted innervation was rescued and target innervation is normal when NT4 replaced BDNF. Genome wide expression analyses revealed that BDNF and NT4 mutant mice exhibited different gene expression profiles in the gustatory (geniculate) ganglion. Compared to wild type, the expression of differentiation-, apoptosis- and axon guidance-related genes was changed in BDNF mutant mice, which is consistent with their different roles during taste development. However, replacement of BDNF by NT4 rescued these gene expression changes. These findings indicate that the functions of BDNF and NT4 in taste development are interchangeable. Spatial and temporal differences in BDNF and NT4 expression can regulate differential gene expression in vivo and determine their specific roles during development.

Taste neurons consist of both a large TrkB-receptor-dependent and a small TrkB-receptor-independent subpopulation

Brain-derived neurotrophic factor (BDNF) and neurotrophin-4 (NT-4) are two neurotrophins that play distinct roles in geniculate (taste) neuron survival, target innervation, and taste bud formation. These two neurotrophins both activate the tropomyosin-related kinase B (TrkB) receptor and the pan-neurotrophin receptor p75. Although the roles of these neurotrophins have been well studied, the degree to which BDNF and NT-4 act via TrkB to regulate taste development in vivo remains unclear. In this study, we compared taste development in TrkB^−/− and Bdnf^−/−/Ntf4^−/− mice to determine if these deficits were similar. If so, this would indicate that the functions of both BDNF and NT-4 can be accounted for by TrkB-signaling. We found that TrkB^−/− and Bdnf^−/−/Ntf4^−/− mice lose a similar number of geniculate neurons by E13.5, which indicates that both BDNF and NT-4 act primarily via TrkB to regulate geniculate neuron survival. Surprisingly, the few geniculate neurons that remain in TrkB^−/− mice are more successful at innervating the tongue and taste buds compared with those neurons that remain in Bdnf^−/−/Ntf4^−/− mice. The remaining neurons in TrkB^−/− mice support a significant number of taste buds. In addition, these remaining neurons do not express the TrkB receptor, which indicates that either BDNF or NT-4 must act via additional receptors to influence tongue innervation and/or targeting.

Cranial neural crest cells form corridors prefiguring sensory neuroblast migration

The majority of cranial sensory neurons originate in placodes in the surface ectoderm, migrating to form ganglia that connect to the central nervous system (CNS). Interactions between inward-migrating sensory neuroblasts and emigrant cranial neural crest cells (NCCs) play a role in coordinating this process, but how the relationship between these two cell populations is established is not clear. Here, we demonstrate that NCCs generate corridors delineating the path of migratory neuroblasts between the placode and CNS in both chick and mouse. In vitro analysis shows that NCCs are not essential for neuroblast migration, yet act as a superior substrate to mesoderm, suggesting provision of a corridor through a less-permissive mesodermal territory. Early organisation of NCC corridors occurs prior to sensory neurogenesis and can be recapitulated in vitro; however, NCC extension to the placode requires placodal neurogenesis, demonstrating reciprocal interactions. Together, our data indicate that NCC corridors impose physical organisation for precise ganglion formation and connection to the CNS, providing a local environment to enclose migrating neuroblasts and axonal processes as they migrate through a non-neural territory.

Hmx1 is required for the normal development of somatosensory neurons in the geniculate ganglion

Hmx1 is a variant homeodomain transcription factor expressed in the developing sensory nervous system, retina, and craniofacial mesenchyme. Recently, mutations at the Hmx1 locus have been linked to craniofacial defects in humans, rats, and mice, but its role in nervous system development is largely unknown. Here we show that Hmx1 is expressed in a subset of sensory neurons in the cranial and dorsal root ganglia which does not correspond to any specific sensory modality. Sensory neurons in the dorsal root and trigeminal ganglia of Hmx1dm/dm mouse embryos have no detectable Hmx1 protein, yet they undergo neurogenesis and express sensory subtype markers normally, demonstrating that Hmx1 is not globally required for the specification of sensory neurons from neural crest precursors. Loss of Hmx1 expression has no obvious effect on the early development of the trigeminal (V), superior (IX/X), or dorsal root ganglia neurons in which it is expressed, but results in marked defects in the geniculate (VII) ganglion. Hmx1dm/dm mouse embryos possess only a vestigial posterior auricular nerve, and general somatosensory neurons in the geniculate ganglion are greatly reduced by mid-gestation. Although Hmx1 is expressed in geniculate neurons prior to cell cycle exit, it does not appear to be required for neurogenesis, and the loss of geniculate neurons is likely to be the result of increased cell death. Fate mapping of neural crest-derived tissues indicates that Hmx1-expressing somatosensory neurons at different axial levels may be derived from either the neural crest or the neurogenic placodes.

Neural crest-derived horizontal basal cells as tissue stem cells in the adult olfactory epithelium

Horizontal basal cells (HBCs) have garnered attention as tissue stem cells of the olfactory epithelium (OE); however, these cells' exact lineage and their contributions to OE regeneration remain unknown. Neural crest-derived cells (NCDCs) have been shown to possess stem cell properties and to participate in the normal development of the OE. However, the contributions of NCDCs to both normal and regenerating adult OE remain unclear. In this study, we investigated the contribution of NCDCs to the OE, focusing particularly on HBCs. Using immunohistochemistry, we observed the OE of P0-Cre/EGFP mice expressing EGFP-tagged NCDCs at several stages of normal development along with regenerated OE following methimazole treatment. We observed EGFP expression in the HBCs, sustentacular cells (SUSs), Bowman's glands, olfactory receptor neurons (ORNs), and olfactory ensheathing cells of 6-week-old mice. No ectopic Cre expression was identified. Although HBCs at late embryonic stages were placode-derived (i.e., EGFP-negative), we found that EGFP+ HBCs alternatively increased with the decrease of placode-derived HBCs during maturation. In regenerated OE, the percentages of neural crest-derived ORNs and SUSs significantly increased compared with normal OE. These results suggest that NCDCs contribute greatly to the adult HBC population and that they are important for the maintenance of the OE.

Neurotrophin-4 regulates the survival of gustatory neurons earlier in development using a different mechanism than brain-derived neurotrophic factor

The number of neurons in the geniculate ganglion that are available to innervate taste buds is regulated by neurotrophin-4 (NT-4) and brain-derived neurotrophic factor (BDNF). Our goal for the current study was to examine the timing and mechanism of NT-4-mediated regulation of geniculate neuron number during development. We discovered that NT-4 mutant mice lose 33% of their geniculate neuronal cells between E10.5 and E11.5. By E11.5, geniculate axons have just reached the tongue and do not yet innervate their gustatory targets; thus, NT-4 does not function as a target-derived growth factor. At E11.5, no difference was observed in proliferating cells or the rate at which cells exit the cell cycle between NT-4 mutant and wild type ganglia. Instead, there was an increase in TUNEL-labeling, indicating an increase in cell death in Ntf4(-/-) mice compared with wild types. However, activated caspase-3, which is up-regulated in the absence of BDNF, was not increased. This finding indicates that cell death initiated by NT-4-removal occurs through a different cell death pathway than BDNF-removal. We observed no additional postnatal loss of taste buds or neurons in Ntf4(-/-) mice. Thus, during early embryonic development, NT-4 produced in the ganglion and along the projection pathway inhibits cell death through an activated caspase-3 independent mechanism. Therefore, compared to BDNF, NT-4 plays distinct roles in gustatory development; differences include timing, source of neurotrophin, and mechanism of action.

Neural crest and ectodermal cells intermix in the nasal placode to give rise to GnRH-1 neurons, sensory neurons, and olfactory ensheathing cells

The origin of GnRH-1 cells and olfactory ensheathing cells has been controversial. Genetic Cre-lox lineage tracing of the neural crest (NC) versus ectodermal contribution to the developing nasal placode was performed using two complementary mouse models, the NC-specific Wnt1Cre mouse line and an ectodermal-specific Crect mouse line. Using these lines we prove that the NC give rise to the olfactory ensheathing cells and subpopulations of GnRH-1 neurons, olfactory and vomeronasal cells. These data demonstrate that Schwann cells and olfactory ensheathing cells share a common developmental origin. Furthermore, the results indicate that certain conditions that impact olfaction and sexual development, such as Kallmann syndrome, may be in part neurocristopathies.