Tooth pulp tissue promotes neurite outgrowth from rat trigeminal ganglia in vitro

Tgfbr2 in Dental Pulp Cells Guides Neurite Outgrowth in Developing Teeth

Transforming growth factor β (TGFβ) plays an important role in tooth morphogenesis and mineralization. During postnatal development, the dental pulp (DP) mesenchyme secretes neurotrophic factors that guide trigeminal nerve fibers into and throughout the DP. This process is tightly linked with dentin formation and mineralization. Our laboratory established a mouse model in which Tgfbr2 was conditionally deleted in DP mesenchyme using an Osterix promoter-driven Cre recombinase (Tgfbr2 ^cko ). These mice survived postnatally with significant defects in bones and teeth, including reduced mineralization and short roots. Hematoxylin and eosin staining revealed reduced axon-like structures in the mutant mice. Reporter imaging demonstrated that Osterix-Cre activity within the tooth was active in the DP and derivatives, but not in neuronal afferents. Immunofluorescence staining for β3 tubulin (neuronal marker) was performed on serial cryosections from control and mutant molars on postnatal days 7 and 24 (P7, P24). Confocal imaging and pixel quantification demonstrated reduced innervation in Tgfbr2 ^cko first molars at both stages compared to controls, indicating that signals necessary to promote neurite outgrowth were disrupted by Tgfbr2 deletion. We performed mRNA-Sequence (RNA-Seq) and gene onotology analyses using RNA from the DP of P7 control and mutant mice to investigate the pathways involved in Tgfbr2-mediated tooth development. These analyses identified downregulation of several mineralization-related and neuronal genes in the Tgfbr2 ^cko DP compared to controls. Select gene expression patterns were confirmed by quantitative real-time PCR and immunofluorescence imaging. Lastly, trigeminal neurons were co-cultured atop Transwell filters overlying primary Tgfbr2 ^f/f DP cells. Tgfbr2 in the DP was deleted via Adenovirus-expressed Cre recombinase. Confocal imaging of axons through the filter pores showed increased axonal sprouting from neurons cultured with Tgfbr2-positive DP cells compared to neurons cultured alone. Axon sprouting was reduced when Tgfbr2 was knocked down in the DP cells. Immunofluorescence of dentin sialophosphoprotein in co-cultured DP cells confirmed reduced mineralization potential in cells with Tgfbr2 deletion. Both our proteomics and RNA-Seq analyses indicate that axonal guidance cues, particularly semaphorin signaling, were disrupted by Tgfbr2 deletion. Thus, Tgfbr2 in the DP mesenchyme appears to regulate differentiation and the cells' ability to guide neurite outgrowth during tooth mineralization and innervation.

Analysis of Tooth Innervation in Microfluidic Coculture Devices

Innervation plays a key role in the development, homeostasis, and regeneration of organs and tissues. However, the mechanisms underlying these phenomena are not well understood yet. In particular, the role of innervation in tooth development and regeneration is neglected. Cocultures constitute a valuable method to investigate and manipulate the interactions between nerve fibers and teeth in a controlled and isolated environment. Microfluidic systems for allow cocultures of neurons and different cell types in their appropriate culture media, while permitting the passage of axons from one compartment to the other. Here we describe how to isolate and coculture developing trigeminal ganglia and tooth germs in a microfluidic coculture system. This protocol describes a simple and flexible way to coculture ganglia/nerves and their target tissues and to study the roles of specific molecules on such interactions in a controlled and isolated environment.

A Co-Culture Method to Study Neurite Outgrowth in Response to Dental Pulp Paracrine Signals

Tooth innervation allows teeth to sense pressure, temperature and inflammation, all of which are crucial to the use and maintenance of the tooth organ. Without sensory innervation, daily oral activities would cause irreparable damage. Despite its importance, the roles of innervation in tooth development and maintenance have been largely overlooked. Several studies have demonstrated that DP cells secrete extracellular matrix proteins and paracrine signals to attract and guide TG axons into and throughout the tooth. However, few studies have provided detailed insight into the crosstalk between the DP mesenchyme and neuronal afferents. To address this gap in knowledge, researchers have begun to utilize co-cultures and a variety of techniques to investigate these interactions. Here, we demonstrate the multiple steps involved in co-culturing primary DP cells with TG neurons dispersed on an overlying transwell filter with large diameter pores to allow axonal growth through the pores. Primary DP cells with the gene of interest flanked by loxP sites were utilized to facilitate gene deletion using an Adenovirus-Cre-GFP recombinase system. Using TG neurons from the Thy1-YFP mouse allowed for precise afferent imaging, with expression well above background levels by confocal microscopy. The DP responses can be investigated via protein or RNA collection and analysis, or alternatively, through immunofluorescent staining of DP cells plated on removable glass coverslips. Media can be analyzed using techniques such as proteomic analyses, although this will require albumin depletion due to the presence of fetal bovine serum in the media. This protocol provides a simple method that can be manipulated to study the morphological, genetic, and cytoskeletal responses of TG neurons and DP cells in response to the controlled environment of a co-culture assay.

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.

Sources of dentin-pulp regeneration signals and their modulation by the local microenvironment

Many aspects of dentin pulp tissue regeneration have been investigated, and it has been shown that dentin pulp has a high regeneration capacity. This seems to be because of the presence of progenitor cells and inductive regeneration signals from different origins. These signals can be liberated after the acidic dissolution of carious dentin as well as from pulp fibroblasts and endothelial cells in cases of traumatic injury. Thus, both carious lesions and pulp cells provide the required mediators for complete dentin-pulp regeneration including reparative dentin secretion, angiogenesis, and innervation. Additionally, all dentin pulp insults including carious "infection," traumatic injuries, application of restorative materials on the injured dentin pulp, and subsequent apoptosis are known activators of the complement system. This activation leads to the production of several biologically active fragments responsible for the vascular modifications and the attraction of immune cells to the inflammatory/injury site. Among these, C5a is involved in the recruitment of pulp progenitor cells, which express the C5a receptor. Thus, in addition to dentin and pulp cells, plasma should be considered as an additional source of regeneration signals.

Pulp Fibroblasts Control Nerve Regeneration through Complement Activation

Dentin-pulp regeneration is closely linked to the presence of nerve fibers in the pulp and to the healing mechanism by sprouting of the nerve fiber’s terminal branches beneath the carious injury site. However, little is known about the initial mechanisms regulating this process in carious teeth. It has been recently demonstrated that the complement system activation, which is one of the first immune responses, contributes to tissue regeneration through the local production of anaphylatoxins such as C5a. While few pulp fibroblasts in intact teeth and in untreated fibroblast cultures express the C5a receptor (C5aR), here we show that all dental pulp fibroblasts, localized beneath the carious injury site, do express this receptor. This observation is consistent with our in vitro results, which showed expression of C5aR in lipoteichoic acid–stimulated pulp fibroblasts. The interaction of C5a, produced after complement synthesis and activation from pulp fibroblasts, with the C5aR of these cells mediated the local brain-derived neurotropic factor (BDNF) secretion. Overall, this activation guided the neuronal growth toward the lipoteichoic acid–stimulated fibroblasts. Thus, our findings highlight a new mechanism in one of the initial steps of the dentin-pulp regeneration process, linking pulp fibroblasts to the nerve sprouting through the complement system activation. This may provide a useful future therapeutic tool in targeting the fibroblasts in the dentin-pulp regeneration process.

Analysis of Developing Tooth Germ Innervation Using Microfluidic Co-culture Devices

Innervation plays a key role in the development, homeostasis and regeneration of organs and tissues. However, the mechanisms underlying these phenomena are not well understood yet. In particular, the role of innervation in tooth development and regeneration is neglected. Several in vivo studies have provided important information about the patterns of innervation of dental tissues during development and repair processes of various animal models. However, most of these approaches are not optimal to highlight the molecular basis of the interactions between nerve fibres and target organs and tissues. Co-cultures constitute a valuable method to investigate and manipulate the interactions between nerve fibres and teeth in a controlled and isolated environment. In the last decades, conventional co-cultures using the same culture medium have been performed for very short periods (e.g., two days) to investigate the attractive or repulsive effects of developing oral and dental tissues on sensory nerve fibres. However, extension of the culture period is required to investigate the effects of innervation on tooth morphogenesis and cytodifferentiation. Microfluidics systems allow co-cultures of neurons and different cell types in their appropriate culture media. We have recently demonstrated that trigeminal ganglia (TG) and teeth are able to survive for a long period of time when co-cultured in microfluidic devices, and that they maintain in these conditions the same innervation pattern that they show in vivo. On this basis, we describe how to isolate and co-culture developing trigeminal ganglia and tooth germs in a microfluidic co-culture system.This protocol describes a simple and flexible way to co-culture ganglia/nerves and target tissues and to study the roles of specific molecules on such interactions in a controlled and isolated environment.

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.

Coordination of tooth morphogenesis and neuronal development through tissue interactions: lessons from mouse models

In addition to being an advantageous model to investigate general molecular mechanisms of organ formation, the tooth is a distinct target organ for peripheral nerve innervation. These nerves are required for the function and protection of the teeth and, as shown in fish, also for their regeneration. This review focuses on recent findings of the local tissue interactions and molecular signaling mechanisms that regulate the early nerve arrival and patterning of mouse mandibular molar tooth sensory innervation. Dental sensory nerve growth and patterning is a stepwise process that is intimately linked to advancing tooth morphogenesis. In particular, nerve growth factor and semaphorin 3A serve as essential functions during and are iteratively used at different stages of tooth innervation. The tooth germ controls development of its own nerve supply, and similar to the development of the tooth organ proper, tissue interactions between dental epithelial and mesenchymal tissues control the establishment of tooth innervation. Tgf-β, Wnt, and Fgf signaling, which regulate tooth formation, are implicated to mediate these interactions. Therefore, tissue interactions mediated by conserved signal families may constitute key mechanism for the integration of tooth organogenesis and development of its peripheral nerve supply.

Nanofibers implant functionalized by neural growth factor as a strategy to innervate a bioengineered tooth

Current strategies for jaw reconstruction require multiple procedures, to repair the bone defect, to offer sufficient support, and to place the tooth implant. The entire procedure can be painful and time-consuming, and the desired functional repair can be achieved only when both steps are successful. The ability to engineer combined tooth and bone constructs, which would grow in a coordinated fashion with the surrounding tissues, could potentially improve the clinical outcomes and also reduce patient suffering. A unique nanofibrous and active implant for bone-tooth unit regeneration and also the innervation of this bioengineered tooth are demonstrated. A nanofibrous polycaprolactone membrane is functionalized with neural growth factor, along with dental germ, and tooth innervation follows. Such innervation allows complete functionality and tissue homeostasis of the tooth, such as dentinal sensitivity, odontoblast function, masticatory forces, and blood flow.

Dentin regeneration in vitro: the pivotal role of supportive cells

The elaboration of dentin-pulp engineering strategies requires the investigation of not only progenitor cell potentials but also their interactions with other non-progenitor "supportive" cells. Under severe caries lesions, progenitor cells may be activated by growth factors released after the acidic dissolution of carious dentin. However, dentin regeneration has also been observed after traumatic injuries without any significant dentin dissolution. This raises questions about the origin of signals involved in progenitor cell activation, migration, and differentiation. Study models such as the entire tooth culture and co-cultures of pulp and endothelial cells highlighted the role of interactions between the different pulp cell types and the pivotal role they play in dentin regeneration. Injured pulp fibroblasts secrete growth factors involved in progenitor cell activation and differentiation as well as neoangiogenesis which may pave the pathways for progenitor cell migration. This appears to be the first paper to focus on this very important field in dental pulp biology.

Implanted adult human dental pulp stem cells induce endogenous axon guidance

The human central nervous system has limited capacity for regeneration. Stem cell-based therapies may overcome this through cellular mechanisms of neural replacement and/or through molecular mechanisms, whereby secreted factors induce change in the host tissue. To investigate these mechanisms, we used a readily accessible human cell population, dental pulp progenitor/stem cells (DPSCs) that can differentiate into functionally active neurons given the appropriate environmental cues. We hypothesized that implanted DPSCs secrete factors that coordinate axon guidance within a receptive host nervous system. An avian embryonic model system was adapted to investigate axon guidance in vivo after transplantation of adult human DPSCs. Chemoattraction of avian trigeminal ganglion axons toward implanted DPSCs was mediated via the chemokine, CXCL12, also known as stromal cell-derived factor-1, and its receptor, CXCR4. These findings provide the first direct evidence that DPSCs may induce neuroplasticity within a receptive host nervous system.

Target finding of pain nerve fibers: Neural growth mechanisms in the tooth pulp

The tooth pulp has a dense sensory innervation which, upon stimulation, conveys sensory signals perceived as pain. This innervation, which originates from the trigeminal ganglion, is established through a series of regulated steps during development, and represents an interesting example of tissue targeting by pain-specific nerves. We have investigated various potentially neurotrophic and neurorepulsive influences during this process. The dental papilla/pulp appears to secrete neurite growth inhibitory molecular factors at early stages, which prevent nerve fibers from entering the tissue at what appears to be inappropriate timepoints. Later, a shift from repulsive to attractive factors apparently takes place, and nerve fibers then enter the tooth. When nerve fibers have invaded the dental mesenchyme, a complicated interplay of secreted and membrane-bound factors probably directs the nerve terminals to appropriate sites. Laminin-8 (alpha4beta1gamma1, Lm-411), which is produced by pulpal cells, emerges as an important candidate molecule in this context. Insights into the interactions between the dental pulp nerve fibers and their environment may become important in the search for novel ways to ameliorate pain in the tooth, as well as at other sites.

BDNF promotes target innervation of Xenopus mandibular trigeminal axons in vivo

BACKGROUND

Trigeminal nerves consist of ophthalmic, maxillary, and mandibular branches that project to distinct regions of the facial epidermis. In Xenopus embryos, the mandibular branch of the trigeminal nerve extends toward and innervates the cement gland in the anterior facial epithelium. The cement gland has previously been proposed to provide a short-range chemoattractive signal to promote target innervation by mandibular trigeminal axons. Brain derived neurotrophic factor, BDNF is known to stimulate axon outgrowth and branching. The goal of this study is to determine whether BDNF functions as the proposed target recognition signal in the Xenopus cement gland.

RESULTS

We found that the cement gland is enriched in BDNF mRNA transcripts compared to the other neurotrophins NT3 and NT4 during mandibular trigeminal nerve innervation. BDNF knockdown in Xenopus embryos or specifically in cement glands resulted in the failure of mandibular trigeminal axons to arborise or grow into the cement gland. BDNF expressed ectodermal grafts, when positioned in place of the cement gland, promoted local trigeminal axon arborisation in vivo.

CONCLUSION

BDNF is necessary locally to promote end stage target innervation of trigeminal axons in vivo, suggesting that BDNF functions as a short-range signal that stimulates mandibular trigeminal axon arborisation and growth into the cement gland.

Tissue interactions in the regulation of axon pathfinding during tooth morphogenesis

Like many other organs, the tooth develops as a result of the epithelial-mesenchymal interactions. In addition, the tooth is a well-defined peripheral target organ for sensory trigeminal nerves, which are required for the function and protection of the teeth. Dental trigeminal axon growth and patterning are tightly linked with advancing tooth morphogenesis and cell differentiation. This review summarizes recent findings on the regulation of dental axon pathfinding, which have provided evidence that the development of tooth trigeminal innervation is controlled by epithelial-mesenchymal interactions. The early dental epithelium possesses the information to instruct tooth nerve supply, and signals mediating these interactions are part of the signaling networks regulating tooth morphogenesis. Tissue interactions, thus, appear to provide a central mechanism of spatiotemporally orchestrating tooth formation and dental axon navigation and patterning.

Laminins 2 (α2β1γ1, Lm-211) and 8 (α4β1γ1, Lm-411) are synthesized and secreted by tooth pulp fibroblasts and differentially promote neurite outgrowth from trigeminal ganglion sensory neurons

The tooth pulp innervation originates from the trigeminal ganglion (TG) and represents an illustrative example of tissue targeting by sensory nerves. Pulpal fibroblasts strongly promote neurite outgrowth from TG neurons in vitro. In the present study, we have investigated the possible participation of laminins (LNs), potent neuritogenic extracellular matrix components. Immunohistochemistry of human tooth pulp demonstrated expression of LN alpha1, alpha2, alpha4, alpha5, beta1 and gamma1, and laminin-binding integrin alpha3, alpha6, beta1 and beta4 chains in nerves. Though faintly stained for laminins in situ, pulpal fibroblasts reacted, once cultured and permeabilized, with antibodies to LN alpha2, alpha4, beta1 and gamma1 chains by flow cytometry. The cells also expressed the corresponding mRNAs and were able to assemble and secrete LN-2 (alpha2beta1gamma1, Lm-211) and LN-8 (alpha4beta1gamma1, Lm-411). LN-8 displayed a chondroitin sulphate (CS) modification in its alpha4 chain. In functional assays, mouse LN-1 (alpha1beta1gamma1, Lm-111) and recombinant human (rh) LN-8, but not native or rhLN-2, strongly promoted neurite outgrowth from TG neurons, mimicking the effect of cultured pulp fibroblast. Altogether, the results indicate that LN-2 and LN-8 are synthesized by tooth pulp fibroblasts and differentially promote neurite outgrowth from TG neurons. LN-8 may contribute to sensory innervation of teeth and other tissues during development and/or regeneration.

Expression and localization of reelin in human odontoblasts

Reelin is a large extracellular matrix (ECM) glycoprotein strongly expressed during embryonic development in the central nervous system and involved in architectonic brain development. It could participate in axon plasticity processes or adhesion-recognition between nerve fibers in adulthood. Previously identified from a subtractive cDNA library of fully differentiated human odontoblasts, reelin might be involved in the relationship between dental nerves and odontoblasts in as so far the latter are in close association with pulpal nerve fibers. Here, we show by in situ hybridization and immunohistochemistry that reelin is specifically expressed by human odontoblasts in vivo and in vitro and that an intense expression of the reelin gene is detected in odontoblasts in comparison with pulpal cells (PC). Co-cultures of rat trigeminal ganglion (TG) and odontoblasts allow to mimic odontoblast innervation and demonstrate that neurites contact these cells with reelin molecules as observed in vivo in human dental pulp. Moreover, by RT-PCR, we show that both reelin receptors (namely apolipoprotein E receptor [ApoER-2], very low density lipoprotein receptor [VLDLR] and cadherin-related neuronal receptor [CNR]) and the cytoplasmic adapter Disabled-1 implicated in the reelin signal transduction, were expressed by trigeminal ganglion. On the basis of these data, we suggest that reelin might be an extracellular matrix molecule involved in the terminal innervation of the dentin-pulp complex, promoting adhesion between dental nerve endings and odontoblasts.

Glial cell line-derived neurotrophic factor (GDNF) from adult rat tooth serves a distinct population of large-sized trigeminal neurons

Glial cell line-derived neurotrophic factor (GDNF) mediates trophic effects for specific classes of sensory neurons. The adult tooth pulp is a well-defined target of sensory trigeminal innervation. Here we investigated potential roles of GDNF in the regulation of adult trigeminal neurons and the dental pulp nerve supply of the rat maxillary first molar. Western blot analysis and radioactive 35S-UTP in situ hybridization revealed that GDNF in the dental pulp and its mRNAs were localized with Ngf in the coronal pulp periphery, in particular in the highly innervated subodontoblast layer. Retrograde neuronal transport of iodinated GDNF and Fluorogold (FG) from the dental pulp indicated that GDNF was transported in about one third of all the trigeminal dental neurons. Of the GDNF-labelled neurons, nearly all (97%) were large-sized (> or =35 microm in diameter). Analysis of FG-labelled neurons revealed that, of the trigeminal neurons supporting the adult dental pulp, approximately 20% were small-sized, lacked isolectin B4 binding and did not transport GDNF. Of the large-sized dental trigeminal neurons approximately 40% transported GDNF. About 90% of the GDNF-accumulating neurons were negative for the high-temperature nociceptive marker VRL-1. Our results show that a subclass of large adult trigeminal neurons are potentially dependent on dental pulp-derived GDNF while small dental trigeminal neurons seems not to require GDNF. This suggests that GDNF may function as a neurotrophic factor for subsets of nerves in the tooth, which apparently mediate mechanosensitive stimuli. As in dorsal root ganglia both small- and large-sized neurons are known to be GDNF-dependent; these data provide molecular evidence that the sensory supply in the adult tooth differs, in some aspects, from the cutaneous sensory system.

Neurites from trigeminal ganglion explants grown in vitro are repelled or attracted by tooth-related tissues depending on developmental stage

Although neurite attracting factors are present in the developing dental pulp and trigeminal ganglion (TG) axons can respond to such factors, nerve fibres do not enter the tooth pulp until a late developmental stage compared with surrounding tissues supplied by the TG. This suggests that the dental pulp secretes neurite growth inhibitory molecules. Semaphorins represent one group of substances, which can inhibit/repel growing neurites. The aims of the present study were to investigate if dental tissue explants inhibit/repel neurite growth from TGs at some developmental stages in vitro, and if so, to seek evidence for or against a participation of semaphorins in that interaction. By co-culturing mandibular or dental epithelial and mesenchymal tissue explants and TGs in collagen gels, we found that embryonic day 11 (E11) mandibular and E13 dental mesenchymal explants repel neurites from corresponding TGs. Repulsion was replaced by attraction if tissues from late embryonic or early postnatal mice (E17-postnatal day 5) were used. Using semi-quantitative reverse transcription/polymerase chain reaction we showed that a number of semaphorins were expressed by tooth-related mesenchyme collected from embryonic and postnatal mice. The expression of some semaphorins (3A, 3C, 3F, 4F, 5B, 6A, 6B and 6C) was high early in development and then decreased in a temporal pattern that correlated with neurite inhibitory/repulsive effects of dental mesenchyme observed in co-cultures. The expression of other semaphorins increased with development (3B, 4A and 7A), whilst others varied irregularly or remained at a fairly constant level (3E, 4B, 4C, 4D, 4G and 5A). Immunohistochemistry was used to determine if tooth-related nerve fibres possess neuropilins. This revealed that axons surrounding embryonic tooth buds express neuropilin-1, but not neuropilin-2. In postnatal teeth, nerve fibres located within the tooth pulp were immunonegative for neuropilin-1 and neuropilin-2. We conclude that developing mandibular/dental mesenchyme can inhibit/repel neurite growth in vitro. Our results support the hypothesis that semaphorins may be involved in this interaction.

Responses of rat trigeminal neurones to dental pulp cells or fibroblasts overexpressing neurotrophic factors in vitro

The adult dental pulp is innervated by sensory trigeminal axons and efferent sympathetic axons. Rat trigeminal ganglia extend neurites when co-cultivated in vitro with pulpal tissue explants, suggesting that pulpal cells secrete soluble molecules that stimulate the growth of trigeminal ganglion axons. In addition, cultured pulpal cells produce mRNAs for neurotrophins and glial cell line-derived neurotrophic factor-family members. These data suggest that neurotrophic factors are involved in the formation of a pulpal innervation. Here, we examine how pulpal cells and 3T3 fibroblasts overexpressing certain neurotrophic factors (nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3, neurotrophin-4, glial cell line-derived neurotrophic factor or neurturin) influence survival and growth of single trigeminal ganglion neurones in vitro in quantitative terms. The results show that most of the neurotrophic factor-overexpressing fibroblasts induce similar neuronal soma diameters, but higher survival rates and neurite lengths compared with pulpal cells. With respect to neurite growth pattern, trigeminal ganglion neurones co-cultured with fibroblasts overexpressing nerve growth factor develop a geometry that is most similar to that seen in co-cultures with pulpal cells. We conclude that none of the fibroblasts overexpressing neurotrophic factors can fully mimic the effects of pulpal cells on trigeminal ganglion neurones, and that nerve growth factor promotes a neurite growth pattern most similar to the picture seen in co-cultures with pulpal cells.

Impaired dental cytodifferentiation in glial cell-line derived growth factor (GDNF) deficient mice

Glial cell line-derived neurotrophic factor promotes the survival of multiple neuron types in the central and peripheral nervous system. Moreover, it plays a key role in the development of the enteric nervous system and in the kidney organogenesis. Glial cell line-derived neurotrophic factor and their receptors are expressed in the developing tooth as well as in the trigeminal ganglion. However, the precise role of this growth factor in tooth morphogenesis and cell differentiation, or in the development of trigeminal ganglion cells, is still elusive. Using structural and ultrastructural techniques we analyzed in detail the first molar tooth germ of glial cell line-derived neurotrophic factor deficient mice as well as the neuronal density in trigeminal ganglion. The length and width of first molar tooth germ in knockout deficient animals showed no differences in the knockout animals in comparison with age-matched heterozygous or wild-type littermates. Nevertheless, in mice lacking glial cell line-derived neurotrophic factor, both ameloblasts and odontoblasts failed to fully develop and differentiate, and the enamel matrix and predentin layers were absent. On the other hand, the number of trigeminal sensory neurons and the structure of the nerves supplying first molar tooth germ were largely normal. Present results suggest a new non-neuronal role for glial cell line-derived neurotrophic factor in tooth development. Glial cell line-derived neurotrophic factor seems not to be involved in tooth initiation and morphogenesis, whereas it seems essential for cytodifferentiation. Conversely, neither development of trigeminal neuron nor nerve fibers supplying teeth are directly dependent on glial cell line-derived neutrophic factor.

Dental pulp cells produce neurotrophic factors, interact with trigeminal neurons in vitro, and rescue motoneurons after spinal cord injury

Interactions between ingrowing nerve fibers and their target tissues form the basis for functional connectivity with the central nervous system. Studies of the developing dental pulp innervation by nerve fibers from the trigeminal ganglion is an excellent example of nerve-target tissue interactions and will allow specific questions regarding development of the dental pulp nerve system to be addressed. Dental pulp cells (DPC) produce an array of neurotrophic factors during development, suggesting that these proteins might be involved in supporting trigeminal nerve fibers that innervate the dental pulp. We have established an in vitro culture system to study the interactions between the dental pulp cells and trigeminal neurons. We show that dental pulp cells produce several neurotrophic factors in culture. When DPC are cocultured with trigeminal neurons, they promote survival and a specific and elaborate neurite outgrowth pattern from trigeminal neurons, whereas skin fibroblasts do not provide a similar support. In addition, we show that dental pulp tissue becomes innervated when transplanted ectopically into the anterior chamber of the eye in rats, and upregulates the catecholaminergic nerve fiber density of the irises. Interestingly, grafting the dental pulp tissue into hemisected spinal cord increases the number of surviving motoneurons, indicating a functional bioactivity of the dental pulp-derived neurotrophic factors in vivo by rescuing motoneurons. Based on these findings, we propose that dental pulp-derived neurotrophic factors play an important role in orchestrating the dental pulp innervation.

Rat tooth pulp cells elicit neurite growth from trigeminal neurones and express mRNAs for neurotrophic factors in vitro

Molecular factors control the developmental ingrowth of axons to the tooth pulp. Here we examine the ability of pulpal cells to induce neurite outgrowth from neonatal rat trigeminal neurones (TGNs) in vitro. We found that TGNs emitted neurites and formed networks of branches in relation to pulpal cells. Neurones co-cultured with a mixture of pulpal cells and 3T3 fibroblasts formed networks exclusively in relation to the pulpal cells. Cultivated pulpal cells and pulpal tissue produced mRNAs for all neurotrophins and members of the glial cell line-derived neurotrophic factor family. Hence, rat pulpal cells have neuritogenic effects on single TGNs in vitro, that may be associated with secretion of neurotrophic factors.