The morphogenesis of mouse vallate gustatory epithelium and taste buds requires BDNF-dependent taste neurons

Role of bone marrow-derived stem cells versus insulin on filiform and fungiform papillae of diabetic albino rats (light, fluorescent and scanning electron microscopic study)

BACKGROUND

Diabetes mellitus (DM) is a chronic metabolic disease characterized by high blood glucose levels. DM affects many body's organs and caused by insulin production deficiency or by the ineffectiveness of the produced insulin. Administration of exogenous insulin is required for management of type I DM; however, it does not cure the disease. Bone marrow-mesenchymal stem cells (BM-MSCs) have been highlighted to offer a novel cell based approach for treatment of diabetes because of their anti-diabetic effect, direct differentiation into a variety of cell types, or release of paracrine factors.

AIM

To examine the effect of BM-MSCs versus insulin on true filiform and fungiform papillae of diabetic rats.

MATERIALS AND METHODS

Fifty six male Wistar albino rats weighing 200-250 g were equally divided into: Control group (Gp I): Rats did not receive any drug. Diabetic group (Gp II): Rats received a single intra-peritoneal injection of streptozotocin (40 mg/kg). BM-MSCs treated diabetic group (Gp III): After DM confirmation; rats received a single intravenous injection of BM-MSCs (million units) through tail vein. Insulin treated diabetic group (Gp IV): After DM confirmation; rats received a daily subcutaneous injection of insulin (5IU/kg). After four weeks, half of the tongue specimens were processed and stained by Hematoxyline & Eosin and Anti-proliferating cell nuclear antigen (Anti-PCNA) then examined by light microscope. Fluorescent microscope was used to detect homing of injected labeled BM-MSCs in rats' filiform and fungiform papillae. While the other half were examined by scanning electron microscope.

RESULTS

True filiform and fungiform papillae of Gp II showed significant histological and morphological alterations. In treated groups, Gp III and Gp IV, both papillae showed marked improvements, being more noticeable in Gp IV. There was a significant increase in the number of Anti-PCNA positive cells and a significant decrease in fasting blood glucose level in Gp III and Gp IV in comparison to Gp II.

CONCLUSIONS

DM had degenerative effects on true filiform and fungiform papillae. Administration of BM-MSCs reduced the deleterious effects of DM on both papillae. Insulin injection caused more obvious improvements in both papillae of diabetic rats than BM-MSCs.

Innovative Dental Stem Cell-Based Research Approaches: The Future of Dentistry

Over the past decade, the dental field has benefited from recent findings in stem cell biology and tissue engineering that led to the elaboration of novel ideas and concepts for the regeneration of dental tissues or entire new teeth. In particular, stem cell-based regenerative approaches are extremely promising since they aim at the full restoration of lost or damaged tissues, ensuring thus their functionality. These therapeutic approaches are already applied with success in clinics for the regeneration of other organs and consist of manipulation of stem cells and their administration to patients. Stem cells have the potential to self-renew and to give rise to a variety of cell types that ensure tissue repair and regeneration throughout life. During the last decades, several adult stem cell populations have been isolated from dental and periodontal tissues, characterized, and tested for their potential applications in regenerative dentistry. Here we briefly present the various stem cell-based treatment approaches and strategies that could be translated in dental practice and revolutionize dentistry.

Ephrin-B/EphB Signaling Is Required for Normal Innervation of Lingual Gustatory Papillae

The innervation of taste buds is an excellent model system for studying the guidance of axons during targeting because of their discrete nature and the high fidelity of innervation. The pregustatory epithelium of fungiform papillae is known to secrete diffusible axon guidance cues such as BDNF and Sema3A that attract and repel, respectively, geniculate ganglion axons during targeting, but diffusible factors alone are unlikely to explain how taste axon terminals are restricted to their territories within the taste bud. Nondiffusible cell surface proteins such as Ephs and ephrins can act as receptors and/or ligands for one another and are known to control axon terminal positioning in several parts of the nervous system, but they have not been studied in the gustatory system. We report that ephrin-B2 linked β-galactosidase staining and immunostaining was present along the dorsal epithelium of the mouse tongue as early as embryonic day 15.5 (E15.5), but was not detected at E14.5, when axons first enter the epithelium. Ephrin-B1 immunolabeling was barely detected in the epithelium and found at a somewhat higher concentration in the mesenchyme subjacent to the epithelium. EphB1 and EphB2 were detected in lingual sensory afferents in vivo and geniculate neurites in vitro. Ephrin-B1 and ephrin-B2 were similarly effective in repelling or suppressing outgrowth by geniculate neurites in vitro. These in vitro effects were independent of the neurotrophin used to promote outgrowth, but were reduced by elevated levels of laminin. In vivo, mice null for EphB1 and EphB2 exhibited decreased gustatory innervation of fungiform papillae. These data provide evidence that ephrin-B forward signaling is necessary for normal gustatory innervation of the mammalian tongue.

Single Lgr5- or Lgr6-expressing taste stem/progenitor cells generate taste bud cells ex vivo

Leucine-rich repeat-containing G protein-coupled receptor 5 (Lgr5) and its homologs (e.g., Lgr6) mark adult stem cells in multiple tissues. Recently, we and others have shown that Lgr5 marks adult taste stem/progenitor cells in posterior tongue. However, the regenerative potential of Lgr5-expressing (Lgr5(+)) cells and the identity of adult taste stem/progenitor cells that regenerate taste tissue in anterior tongue remain elusive. In the present work, we describe a culture system in which single isolated Lgr5(+) or Lgr6(+) cells from taste tissue can generate continuously expanding 3D structures ("organoids"). Many cells within these taste organoids were cycling and positive for proliferative cell markers, cytokeratin K5 and Sox2, and incorporated 5-bromo-2'-deoxyuridine. Importantly, mature taste receptor cells that express gustducin, carbonic anhydrase 4, taste receptor type 1 member 3, nucleoside triphosphate diphosphohydrolase-2, or cytokeratin K8 were present in the taste organoids. Using calcium imaging assays, we found that cells grown out from taste organoids derived from isolated Lgr5(+) cells were functional and responded to tastants in a dose-dependent manner. Genetic lineage tracing showed that Lgr6(+) cells gave rise to taste bud cells in taste papillae in both anterior and posterior tongue. RT-PCR data demonstrated that Lgr5 and Lgr6 may mark the same subset of taste stem/progenitor cells both anteriorly and posteriorly. Together, our data demonstrate that functional taste cells can be generated ex vivo from single Lgr5(+) or Lgr6(+) cells, validating the use of this model for the study of taste cell generation.

Microfluidics co-culture systems for studying tooth innervation

Innervation plays a key role in the development and homeostasis of organs and tissues of the orofacial complex. Among these structures, teeth are peculiar organs as they are not innervated until later stages of development. Furthermore, the implication of neurons in tooth initiation, morphogenesis and differentiation is still controversial. Co-cultures constitute a valuable method to investigate and manipulate the interactions of nerve fibers with their target organs in a controlled and isolated environment. Conventional co-cultures between neurons and their target tissues have already been performed, but these cultures do not offer optimal conditions that are closely mimicking the in vivo situation. Indeed, specific cell populations require different culture media in order to preserve their physiological properties. In this study we evaluate the usefulness of a microfluidics system for co-culturing mouse trigeminal ganglia and developing teeth. This device allows the application of specific media for the appropriate development of both neuronal and dental tissues. The results show that mouse trigeminal ganglia and teeth survive for long culture periods in this microfluidics system, and that teeth maintain the attractive or repulsive effect on trigeminal neurites that has been observed in vivo. Neurites are repealed when co-cultured with embryonic tooth germs, while postnatal teeth exert an attractive effect to trigeminal ganglia-derived neurons. In conclusion, microfluidics system devices provide a valuable tool for studying the behavior of neurons during the development of orofacial tissues and organs, faithfully imitating the in vivo situation.

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.

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.

Expression of tryptophan hydroxylase in developing mouse taste papillae

Gustatory papillae and associated taste buds receive and process chemical information from the environment. In mammals, their development takes place during the late phase of embryogenesis. However, the cellular factors that regulate the differentiation of taste papillae remain largely unknown. Here, we show by quantitative real time RT-PCR that both isoforms of tryptophan hydroxylase (TPH1 and TPH2), the first and rate limiting enzyme of serotonin (5-HT) synthesis, are expressed in developing circumvallate papillae. Immuno-staining experiments further indicated that TPH is localized both in gustatory fibers and in differentiated taste receptor cells. These results point to the synthesis of 5-HT in gustatory papillae, and allow one to hypothesize that the development of taste buds might be modulated by serotonin.

Epithelial overexpression of BDNF and NT4 produces distinct gustatory axon morphologies that disrupt initial targeting

Most fungiform taste buds fail to become innervated when BDNF or NT4 is overexpressed in the basal layer of tongue epithelium. Here, we examined when and how overexpression of BDNF and NT4 disrupt innervation to fungiform papillae. Overexpression of either factor disrupted chorda tympani innervation patterns either before or during the initial innervation of fungiform papillae. NT4 and BDNF overexpression each disrupted initial innervation by producing different gustatory axon morphologies that emerge at distinct times (E12.5 and E14.5, respectively). Chorda tympani nerve branching was reduced in NT4 overexpressing mice, and neuronal fibers in these mice were fasciculated and remained below the epithelial surface, as if repelled by NT4 overexpression. In contrast, many chorda tympani nerve branches were observed near the epithelial surface in mice overexpressing BDNF, and most were attracted to and invaded non-taste filiform papillae instead of gustatory papillae. These results suggest that BDNF, but not NT4, normally functions as a chemoattractant that allows chorda tympani fibers to distinguish their fungiform papillae targets from non-gustatory epithelium. Since BDNF and NT4 both signal through the p75 and TrkB receptors, trophin-specific activation of different internal signaling pathways must regulate the development of the distinct gustatory axon morphologies in neurotrophin-overexpressing mice.

Building sensory receptors on the tongue

Neurotrophins, neurotrophin receptors and sensory neurons are required for the development of lingual sense organs. For example, neurotrophin 3 sustains lingual somatosensory neurons. In the traditional view, sensory axons will terminate where neurotrophin expression is most pronounced. Yet, lingual somatosensory axons characteristically terminate in each filiform papilla and in each somatosensory prominence within a cluster of cells expressing the p75 neurotrophin receptor (p75NTR), rather than terminating among the adjacent cells that secrete neurotrophin 3. The p75NTR on special specialized clusters of epithelial cells may promote axonal arborization in vivo since its over-expression by fibroblasts enhances neurite outgrowth from overlying somatosensory neurons in vitro. Two classical observations have implicated gustatory neurons in the development and maintenance of mammalian taste buds--the early arrival times of embryonic innervation and the loss of taste buds after their denervation in adults. In the modern era more than a dozen experimental studies have used early denervation or neurotrophin gene mutations to evaluate mammalian gustatory organ development. Necessary for taste organ development, brain-derived neurotrophic factor sustains developing gustatory neurons. The cardinal conclusion is readily summarized: taste buds in the palate and tongue are induced by innervation. Taste buds are unstable: the death and birth of taste receptor cells relentlessly remodels synaptic connections. As receptor cells turn over, the sensory code for taste quality is probably stabilized by selective synapse formation between each type of gustatory axon and its matching taste receptor cell. We anticipate important new discoveries of molecular interactions among the epithelium, the underlying mesenchyme and gustatory innervation that build the gustatory papillae, their specialized epithelial cells, and the resulting taste buds.

Distinct expression pattern of insulin-like growth factor family in rodent taste buds

The insulin-like growth factor (IGF) system is an important regulator of growth and differentiation in a variety of tissues. In the present study, the expression of IGF family members in the taste buds of mice and rats was examined. By reverse transcriptase polymerase chain reaction (RT-PCR) analysis, mRNA of IGF-I and -II, IGF-I receptor (IGF-IR), insulin receptor (insulin R), and IGF-binding protein (IGFBP)-2, -3, -4, -5, and -6 was detected in the taste bud-containing epithelium of the circumvallate papillae of mice. As suggested by the study using degenerate PCR (McLaughlin [2000] J. Neurosci. 20:5679-5688), IGF-IR was expressed in most of the taste bud cells of adult mice, as found by immunohistochemistry, and in those of postnatal day (P) 6 mice by in situ hybridization. Insulin R, which has strong homology to IGF-IR, was also detected in most of the taste bud cells of mice by immunohistochemistry and in situ hybridization. IGF-I immunoreactivity was detected in a few taste bud cells and in the epithelium surrounding taste buds. Northern blot analysis revealed that the amount of IGF-I mRNA in taste bud-containing epithelium was very low compared with that in liver. IGF-II immunoreactivity was weakly detected in mouse taste buds and the surrounding epithelium. In the rat tissue, a subset of the taste bud cells was positive for IGF-II. Among the six IGFBPs, IGFBP-2, -5, and -6 were detected in the mouse taste buds: IGFBP-2 and -5 immunoreactivity was seen in the majority of the taste bud cells, whereas IGFBP-6 immunoreactivity was found in the nerve fibers innervating the taste buds. In situ hybridization study also revealed that IGFBP-2 and -5 mRNA was synthesized in the taste buds of P6 mice and that the expression of these mRNAs overlapped in von Ebner's glands. These data reveal that IGF-I and -II might be produced in taste bud cells and (or) surrounding lingual epithelium and act through IGF-IR and insulin R locally in a paracrine and autocrine manner. The activity of these IGFs may be modulated through their interaction with IGFBP-2, -5, and 6.

Role of IGFBPs in the morphogenesis of lingual papillae

The expression of insulin-like growth factor binding proteins (IGFBPs) during the morphogenesis of lingual papillae of mice was examined by in situ hybridization. Among seven mouse IGFBPs, IGFBP-1, -6, and -7 mRNAs were not expressed in the tongue tissue. At E12, though no papillae have formed yet, IGFBP-2, -4, and -5 were expressed in the entire tongue epithelium. At E14, fungiform papillae appeared in the anterior region and circumvallate papillae were distinguished in the posterior region. Strong expression of IGFBP-5 was observed in the apical region of both fungiform and circumvallate papillae. At this stage, the epithelial elevation of filiform papillae was not clear; but IGFBP-5 was expressed in the apex. At E15, foliate papillae were distinguished and IGFBP-5 was expressed in the dorsal epithelium of ridges. In filiform papillae, IGFBP-3 was expressed in the core of the connective tissue. At E17, the expression of IGFBP-5 disappeared from the apical region of fungiform, filiform, foliate, and circumvallate papillae, whereas that of IGFBP-2 remained. This finding suggests that IGFBP-5 and -2 function to cause evagination of the epithelium into a raised structure. In the epithelium of trenches of foliate and circumvallate papillae, strong expression of IGFBP-4 was observed at E15 and E17. As previously suggested from a study on postnatal mice (Suzuki et al. J Comp Neurol 2005;482:74-84), IGFBP-4 acts in the epithelial invagination to form the trenches, grooves, or furrows of lingual papillae during development.

The effect of brain-derived neurotrophic factor on sensory and autonomic function after lingual nerve repair

Brain-derived neurotrophic factor (BDNF) is important in the response to peripheral nerve injury and may enhance regeneration. We have assessed its role in the functional recovery of sensory afferents and autonomic efferents after repair of the chorda tympani and lingual nerves in the cat. Six months after entubulation repair, with or without the incorporation of BDNF at the repair site, the recovery of secretomotor and vasomotor efferents was determined by recording salivary flow from the submandibular gland and temperature changes on the tongue surface, each evoked by stimulation of the repaired nerve. Electrophysiological recordings from the lingual and chorda tympani nerves proximal to the repair were undertaken to characterise mechanosensitive, thermosensitive, and gustatory afferents. When compared with data from uninjured control animals, both repair groups showed changes in receptor sensitivity and spontaneous discharge, and persistent reductions in conduction velocity, proportion of gustatory and thermosensitive units, rate of salivary secretion, and vasomotor responses. Comparisons between the outcome of repair with or without BDNF revealed few differences. In the BDNF group, fewer units in the chorda tympani responded to gustatory or thermal stimuli and the sensitivity of the gustatory units was lower. The conduction velocity of afferents in the lingual nerve was also lower, but the mechanoreceptive field size was higher. Thus, despite its known trophic role in the gustatory system, BDNF had not enhanced recovery of these or other fibre populations. We conclude that the application of BDNF to a site of lingual nerve repair has a negative effect on the long-term outcome.

Each sensory nerve arising from the geniculate ganglion expresses a unique fingerprint of neurotrophin and neurotrophin receptor genes

Neurons in the geniculate ganglion, like those in other sensory ganglia, are dependent on neurotrophins for survival. Most geniculate ganglion neurons innervate taste buds in two regions of the tongue and two regions of the palate; the rest are cutaneous nerves to the skin of the ear. We investigated the expression of four neurotrophins, nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin 3 (NT-3), and NT-4, and five neurotrophin receptors, trkA, trkB, trkC, p75, and truncated trkB (Trn-B) in single sensory neurons of the adult rat geniculate ganglion associated with the five innervation fields. For fungiform papillae, a glass pipette containing biotinylated dextran was placed over the target papilla and the tracer was iontophoresed into the target papilla. For the other target fields, Fluoro-Gold was microinjected. After 3 days, geniculate ganglia were harvested, sectioned, and treated histochemically (for biotinylated dextran) or immunohistochemically (for Fluoro-Gold) to reveal the neurons containing the tracer. Single labeled neurons were harvested from the slides and subjected to RNA amplification and RT-PCR to reveal the neurotrophin or neurotrophin receptor genes that were expressed. Neurons projecting from the geniculate ganglion to each of the five target fields had a unique expression profile of neurotrophin and neurotrophic receptor genes. Several individual neurons expressed more than one neurotrophin receptor or more than one neurotrophin gene. Although BDNF is significantly expressed in taste buds, its primary high affinity receptor, trkB, was not prominently expressed in the neurons. The results are consistent with the interpretation that at least some, perhaps most, of the trophic influence on the sensory neurons is derived from the neuronal somata, and the trophic effect is paracrine or autocrine, rather than target derived. The BDNF in the taste bud may also act in a paracrine or autocrine manner on the trkB expressed in taste buds, as shown by others.

Support of trigeminal sensory neurons by nonneuronal p75 neurotrophin receptors

The p75 neurotrophin receptor (p75NTR) binds all four mammalian neurotrophins, including neurotrophin-3 (NT-3) required for the development of select sensory neurons. This study demonstrated that many gustatory and somatosensory neurons of the tongue depend upon p75NTR. Each of thousands of filiform papillae at the front of the tongue as well as each somatosensory prominence at the back of the tongue has a small cluster of p75NTR-positive epithelial cells that is targeted by somatosensory innervation. This expression of p75NTR by epithelial target cells required NT-3 but not adult innervation. NT-3-secreting cells were adjacent to the p75NTR-positive target cells of each somatosensory organ, as demonstrated in NT-3(lacZneo) transgenic mice. In NT-3 null mutant mice, there were few lingual somatosensory neurons. In p75NTR null mutant mice, the lingual somatosensory axons were likewise absent or had deficient terminal arborizations. Cell culture indicated that substrate p75NTR can influence neuronal outgrowth. Specifically, dissociated trigeminal sensory neurons more than doubled their neurite lengths when grown on a lawn of p75NTR-overexpressing fibroblasts. This enhancement of neurite outgrowth by fibroblast p75NTR raises the possibility that epithelial target cell p75NTR may help to promote axonal arborization in vivo. The co-occurrence in p75NTR null mice of a 35% reduction in geniculate ganglion taste neurons and a shortfall of taste buds is consistent with the established role of gustatory innervation in prompting mammalian taste receptor cell differentiation.

Brain-derived neurotrophic factor is present in adult mouse taste cells with synapses

Brain-derived neurotrophic factor (BDNF), one of the members of the nerve growth factor family of neurotrophins, is expressed in developing gustatory papillae and is thought to be the neurotrophin that supports gustatory innervation during development. BDNF expression does not cease after development but continues in some taste cells of adult mice. To determine which types of taste cells produce BDNF, we undertook an immunohistochemical study of taste cells in BDNF(LacZ) gene targeted "knock-in" adult mice. In these mice, beta-galactosidase (beta-gal) immunoreactivity is an indicator of cells that produce BDNF transcripts. In the tongues of adult BDNF(LacZ) mice, beta-gal (BDNF) is present in long slender taste cells, as well as pyriform taste cells. Bromodeoxyuridine labeling experiments in BDNF(LacZ) mice indicate that BDNF is not present in taste cells that are younger than 3 days postmitotic. BDNF mainly colocalizes with markers of type II and type III taste cells: ubiquitin carboxyl terminal hydrolase (PGP 9.5), serotonin (5-HT), neural cell adhesion molecule (N-CAM), synaptic associated protein of 25 kDa (SNAP-25), and to a lesser extent with alpha-gustducin. beta-Gal immunoreactivity is not associated with blood group H antigen, a marker of type I taste cells. We conclude that BDNF is absent from basal cells and type I (blood group H antigen immunoreactive) taste cells but is present in differentiated type II and type III taste cells. The presence of SNAP-25 in BDNF-expressing cells suggests a role for BDNF in synaptic formation and transmission.

Brain-derived neurotrophic factor-, neurotrophin-3-, and tyrosine kinase receptor-like immunoreactivity in lingual taste bud fields of mature hamster

The neurotrophins brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), as well as their respective tyrosine kinase (Trk) receptors, TrkB and TrkC, influence peripheral target cell innervation, survival, and proliferation. In the mature taste system the role of neurotrophins and their receptors is not known. The mature hamster is an intriguing model because anterior lingual fungiform, unlike posterior lingual foliate and circumvallate, taste buds survive denervation. In light of this difference, we examined whether the degree of neurotrophin- or neurotrophin receptor-like immunoreactivity (IR) normally differs among lingual gemmal fields. In single- and double-labeled immunofluorescent experiments, 3,209 taste bud sections (profiles) from 13 hamsters were examined for immunopositive gemmal cells or nerve fibers using antibodies to BDNF and NT-3, their respective receptors TrkB and TrkC, and the neural marker ubiquitin c-terminal hydrolase L-1 [protein gene product (PGP) 9.5]. In each gemmal field, more than 75% of taste bud profiles showed immunopositivity to BDNF, NT-3, and TrkB. Across bud fields, BDNF-, TrkB-, and BDNF/TrkB-like IR, as well as PGP 9.5 and PGP 9.5/BDNF-like IR in centrally located, fungiform bud cells was greater (P < 0.0001 to P < 0.002) than in circumvallate or foliate buds. Within bud fields, the number of BDNF-like, labeled bud cells/bud profile was greater than that for NT-3-like IR in fungiform (P < 0.0002) and foliate (P < 0.0001) buds. TrkC was immunonegative in gemmal cells. The average density of TrkB- and TrkC-like fiber IR was more pronounced in fungiform than posterior gemmal-bearing papillae. Thus, fungiform papillae, whose taste buds are least affected by denervation, exhibit specific neurotrophin and receptor enrichment.

Brain-derived neurotrophic factor-, neurotrophin-3-, and tyrosine kinase receptor-like immunoreactivity in lingual taste bud fields of mature hamster after sensory denervation

Unlike lingual taste buds in most mammals, fungiform buds on the anterior tongue of mature hamster survive sensory denervation. The role of the neurotrophin ligands, brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), and their respective tyrosine kinase (Trk) receptors, TrkB and TrkC, in denervated taste buds is not known. The present report investigates changes in the degree of gemmal cell immunoreactivity (IR) (i.e., number of immunoreactive cells/bud profile) and density of nerve fiber-IR of these markers in unilaterally denervated mature hamsters. The fungiform bud field after chorda tympani/lingual nerve resection is compared with the nerve-dependent, posterior tongue foliate and circumvallate bud fields after glossopharyngeal nerve resection. Four weeks post lesion, the number of denervated fungiform buds matched that on the unoperated side, whereas denervated foliate and circumvallate bud counts decreased by 72% and 38%, respectively. In taste buds that survived on the posterior tongue, the degree of foliate bud cell BDNF-, NT-3-, and TrkB-like IR, and circumvallate bud cell BDNF- and NT-3-like IR, significantly decreased compared with the unoperated side. In contrast, for anterior tongue fungiform bud cells, the degree of neurotrophin- and receptor-like IR was relatively less affected: NT-3- and TrkB-like IR were unchanged; BDNF-like IR, although significantly decreased, was also maintained. Moreover, TrkB-like fiber IR was essentially eliminated within and surrounding fungiform buds. Hence, NT-3-, BDNF-, and TrkB-like IR in fungiform gemmal cells may reflect an autocrine capacity promoting survival. Because TrkC-like IR in bud cells is absent (i.e., immunonegative), and sparse in fibers intragemmally and perigemmally, NT-3 may also bind to bud cell TrkB so as to sustain fungiform gemmal cell viability post denervation.

Development of anterior gustatory epithelia in the palate and tongue requires epidermal growth factor receptor

We characterized the gustatory phenotypes of neonatal mice having null mutations for epidermal growth factor receptor (egfr(-/-)), brain-derived neurotrophic factor (bdnf(-/-)), or both. We counted the number and diameter of fungiform taste buds, the prevalence of poorly differentiated or missing taste cells, and the incidence of ectopic filiform-like spines, each as a function of postnatal age and anterior/posterior location. Egfr(-/-) mice and bdnf(-/-) mice had similar reductions in the total number of taste buds on the anterior portions of the tongue and palate. Nonetheless, there were significant differences in their gustatory phenotypes. EGFR deficiency selectively impaired the development of anterior gustatory epithelia in the mouth. Only bdnf(-/-) mice had numerous taste buds missing from the foliate, vallate, and posterior fungiform papillae. Only egfr(-/-) fungiform taste papillae had robust gustatory innervation, markedly reduced cytokeratin 8 expression in taste cells, and a high incidence of a filiform-like spine. Egfr/bdnf double-null mutant mice had a higher frequency of failed fungiform taste bud differentiation. In bdnf(-/-) mice taste cell development failed because of sparse gustatory innervation. In contrast, in young egfr(-/-) mice the abundance of axons innervating fungiform papillae and the normal numbers of geniculate ganglion neurons implicate gustatory epithelial defects rather than neural defects.

Brain-derived neurotrophic factor and TrkB tyrosine kinase receptor gene expression in zebrafish embryo and larva

The genes that encode the neurotrophin family of secreted polypeptides and the Trk family of high affinity neurotrophin transmembrane protein tyrosine kinase receptors are induced at the time of neurogenesis in mammals and are known to play critical roles in nervous system development. We show here that in contrast to mammals, the genes encoding the neurotrophin brain-derived neurotrophic factor (BDNF) and the neurotrophin receptor TrkB are expressed throughout embryonic development in the zebrafish. At the embryonic stages preceding transcription of endogenous genes all cells contain BDNF transcripts and immunoreactive BDNF and the trkB transcripts lack the region that encodes a kinase domain. As development proceeds, progressively fewer cells contain BDNF transcripts and by the time of neurogenesis the trkB transcripts encode a kinase-domain. In the 4-day-old larva, a small subset of specialized sensory cells on the surface and cells in deeper structures including the gill arches, fin, and cloaca express the BDNF gene at high levels in a promoter-specific fashion. This progressive restriction of BDNF gene expression must involve an extinction of BDNF gene transcription in some and induction of high levels of transcription in a promoter-specific fashion in other cells.

Epithelial overexpression of BDNF or NT4 disrupts targeting of taste neurons that innervate the anterior tongue

Brain-derived neurotrophic factor (BDNF) and neurotrophin-4 (NT4) are essential for the survival of geniculate ganglion neurons, which provide the sensory afferents for taste buds of the anterior tongue and palate. To determine how these target-derived growth factors regulate gustatory development, the taste system was examined in transgenic mice that overexpress BDNF (BDNF-OE) or NT4 (NT4-OE) in basal epithelial cells of the tongue. Overexpression of BDNF or NT4 caused a 93 and 140% increase, respectively, in the number of geniculate ganglion neurons. Surprisingly, both transgenic lines had severe reduction in fungiform papillae and taste bud number, primarily in the dorsal midregion and ventral tip of the tongue. No alterations were observed in taste buds of circumvallate or incisal papillae. Fungiform papillae were initially present on tongues of newborn BDNF-OE animals, but many were small, poorly innervated, and lost postnatally. To explain the loss of nerve innervation to fungiform papillae, the facial nerve of developing animals was labeled with the lipophilic tracer DiI. In contrast to control mice, in which taste neurons innervated only fungiform papillae, taste neurons in BDNF-OE and NT4-OE mice innervated few fungiform papillae. Instead, some fibers approached but did not penetrate the epithelium and aberrant innervation to filiform papillae was observed. In addition, some papillae that formed in transgenic mice had two taste buds (instead of one) and were frequently arranged in clusters of two or three papillae. These results indicate that target-derived BDNF and NT4 are not only survival factors for geniculate ganglion neurons, but also have important roles in regulating the development and spatial patterning of fungiform papilla and targeting of taste neurons to these sensory structures.

Gustatory innervation and bax-dependent caspase-2: participants in the life and death pathways of mouse taste receptor cells

In the adult mouse tongue, an average of 11% of the gustatory receptor cells are replaced each day. In investigating homeostatic cell death mechanisms in gustatory renewing epithelium, we observed that taste receptor cells were selectively immunopositive for the bcl-2 family death factor, Bax, and for the protease Caspase-2 (Nedd2/Ich1). We determined that 8-10% of the taste receptor cells of the vallate papilla were Bax positive and that 11% were Caspase-2 positive. Some of these immunopositive taste cells had apoptotic morphological defects. Within the subset of vallate taste cells immunopositive for either Caspase-2 or Bax, up to 79% coexpressed both death factors. Bax and Caspase-2 first appeared in occasional vallate taste receptor cells on the same postnatal day-the day after birth. bax null mutation markedly reduced gustatory Caspase-2 immunoexpression. These observations suggest that taste cell death pathways utilize p53, Bax, and Caspase-2 to dispose of aged receptor cells. Apart from reducing Caspase-2 expression, Bax deficiency also altered taste organ development. bax(-/-) mice had a more profusely innervated vallate papilla, which grew to be 25% longer and taller, with the mean taste bud containing more than twice the normal number of taste cells. This augmentation of taste organ development with increased innervation is complementary to the well-documented reduction in taste organ development with sparse innervation. We propose that additional taste neurons survived programmed cell death in Bax-deficient mice, thereby providing an inductive boost to vallate gustatory development.

Taste cell function. Structural and biochemical implications

Maintenance of constant relations between receptor cell types and branching from a single gustatory nerve fiber during normal cell turnover and regeneration requires cell-cell recognition likely mediated by timed expression of molecules at surfaces of taste bud cells, nerve endings, and in extracellular matrix. These processes assure stability of gustatory quality representation during intragemmal remodeling. Coincidentally, features of gemmal cell lifespan, including elongation, differentiation, and migration prior to apoptosis, must also be orchestrated by molecular signals. This article reviews the potential roles played by a variety of molecular markers for some relevant classes of proteins, peptides, and enzymes, which were presumed to be important for carrying out these gustatory cellular functions.

An in vitro model for the study of taste papillae morphogenesis using branchial arch explants

It is generally accepted that innervation is required for the maintenance of taste papillae and taste buds, but it is not entirely clear what role, if any, innervation plays in papillae and taste bud formation. Events in taste papillae formation and differentiation take place almost entirely in utero and, therefore, the study of the role of innervation in these events requires a suitable in vitro model. In the past, investigators have made use of various culture techniques to study mammalian taste papillae development in vitro and the role of innervation in this process with varying success. All of these models examined papillae development in isolated tongue or tongue fragments and have lacked the ability to manipulate the innervation of developing taste papillae in these explants. We have established a protocol for an in vitro model of taste papillae morphogenesis using branchial arch explants and roller tube culture methodology. Our results demonstrate that this model supports the morphogenesis of the circumvallate papilla with an integrated nerve. In addition, the use of branchial arch explants allows the inclusion or exclusion of geniculate and petrosal ganglia to examine directly the effects of the presence or absence of innervation on papillae formation and maintenance.

An in vitro model for the study of the role of innervation in circumvallate papillae morphogenesis

The following study was done to demonstrate the reliability of an in vitro model for use in the study of early events and the role of innervation in mouse circumvallate papillae development. Gestational day (gd)-11 fetuses were partially dissected to produce explants that included the mandibular, hyoid, third and fourth branchial arches and their ganglia. In ganglionectomized explants, the nodose ganglia and either the geniculate, petrosal or both ganglia were removed. Explants were cultivated in roller tube culture for 24, 48, 72, and 96 h of culture and examined for the presence of papillary structures. Innervation was verified by immunostaining for neural cell adhesion molecule (NCAM). In all control explants, circumvallate papillae had formed by 72 h in culture. These papillae were innervated by fibers originating in petrosal or nodose ganglia, although, in a small number, fibers from the geniculate also contributed. Circumvallate papillae also formed in some explants in which either the geniculate or petrosal ganglia had been removed. However, placodal structures failed to mature into papillary structures even by 96 h in explants in which both ganglia had been removed. Our results demonstrate that an in vitro model using branchial arch explants supports the morphogenesis of an epithelial placode through the formation of a definite papillary structure, the circumvallate papilla, with an integrated nerve. Our results also indicate that, whereas the initial stages in gustatory papillae formation, the formation of a placode, are nerve-independent, the maturation of the placodal structure to form a papilla requires the presence of an intact nerve.

Neurotrophin receptors in the geniculate ganglion

We examined the distribution of the high affinity neurotrophin receptors (trkA, trkB, and trkC) in the rat geniculate ganglion. Previous work had shown that during early (prenatal) development, trkB and its two ligands, BDNF and NT-4/5, were most important for survival of almost all neurons. Using nested polymerase chain reaction (PCR), we showed that trkA, trkB, and trkC transcripts were expressed, and the mRNAs for trkB and trkC were more abundant than that for trkA. We modified and improved the method for direct reverse transcription in situ PCR and localized trkB mRNA in approximately one third of the neurons in the ganglion. Immunohistochemical data confirmed that approximately the same fraction of neurons was immunoreactive with antibody vs. trkB, and an approximately equal fraction was immunoreactive with trkC antibody. These results are consistent with the notion that both BDNF/trkB and NT-3/trkC play important roles in maintenance of the geniculate ganglion neurons and possibly the peripheral taste system in the young postnatal rat.

Expression of sonic hedgehog, patched, and Gli1 in developing taste papillae of the mouse

Lingual taste buds form within taste papillae, which are specialized structures that develop in a characteristic spatial and temporal pattern. To investigate the signaling events responsible for patterning and morphogenesis of taste papillae, the authors examined the time course and distribution of expression of several related developmental signaling genes as well as the time course of innervation of taste papillae in mouse embryos from embryonic day 12 (E12) to E18. Lingual expression of the signaling molecule Sonic hedgehog (Shh), its receptor Patched (Ptc), and the Shh-activated transcription factor Gli1 were assayed by using in situ hybridization. Shh is expressed broadly in the lingual epithelium at E12 but becomes progressively restricted to developing circumvallate and fungiform papillary epithelia. Shh is expressed specifically within the central cells of the papillary epithelium starting at E13.5 and persisting through E18. Ptc and Gli1 expression follow a pattern similar to that of Shh. Compared with Shh, Ptc is expressed in larger regions surrounding the central papillary cells and also in the mesenchyme underlying Shh-expressing epithelium. Innervation of taste papillae was examined by using the panneuronal antibody to ubiquitin carboxyl terminal hydrolase (protein gene product 9.5). Nerves reach the basal lamina of developing taste papillae at E14 to densely innervate the papillary epithelium by E16. Thus, the pattern of Shh expression within developing taste papillae is established prior to innervation, ruling out neuronal induction of papillae. The results suggest that the Shh signaling pathway may be involved in: 1) establishing papillary boundaries in taste papilla morphogenesis, 2) papillary epithelial-mesenchymal interactions, and/or 3) specifying the location or development of taste buds within taste papillae.

Taste neurons have multiple inductive roles in mammalian gustatory development

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