An immunoglobulin-like domain determines the specificity of neurotrophin receptors.

The neurotrophins influence survival and maintenance of vertebrate neurons in the embryonic, early post-natal and post-developmental stages of the nervous system. Binding of neurotrophins to receptors encoded by the gene family trk initiates signal transduction into the cell. trkA interacts preferably with nerve growth factor (NGF), trkB with brain-derived neurotrophic factor (BDNF) and neurotrophin-4/5 (NT-4/5) and trkC with neurotrophin-3 (NT-3). By constructing 17 different chimeras and domain deletions of the human trk receptors and analyzing their binding affinities to the neurotrophins we have shown that an immunoglobulin-like domain located adjacent to the transmembrane domain is the structural element that determines the interaction of neurotrophins with their receptors. Chimeras of trkC where this domain was exchanged for the homologous sequences from trkB or trkA gained high affinity binding to BDNF or NGF respectively, while deletion of this domain in trkC or trkA abolished binding to NT-3 or NGF respectively. This domain alone retained affinities to neurotrophins similar to the full-length receptors and when expressed on NIH 3T3 cells in fusion with the kinase domain showed neurotrophin-dependent activation.     

Differential expression of mRNAs for neurotrophins and their receptors after axotomy of the sciatic nerve

The neurotrophin family includes NGF, brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4). Previous studies have demonstrated that expression of NGF and its low-affinity receptor is induced in nonneuronal cells of the distal segment of the transected sciatic nerve suggesting a role for NGF during axonal regeneration (Johnson, E. M., M. Taniuchi, and P. S. DeStefano. 1988. Trends Neurosci. 11:299-304). To assess the role of the other neurotrophins and the members of the family of Trk signaling neurotrophin receptors, we have here quantified the levels of mRNAs for BDNF, NT-3, and NT-4 as well as mRNAs for trkA, trkB, and trkC at different times after transection of the sciatic nerve in adult rats. A marked increase of BDNF and NT-4 mRNAs in the distal segment of the sciatic nerve was seen 2 wk after the lesion. The increase in BDNF mRNA was mediated by a selective activation of the BDNF exon IV promoter and adrenalectomy attenuated this increase by 50%. NT-3 mRNA, on the other hand, decreased shortly after the transection but returned to control levels 2 wk later. In Schwann cells ensheathing the sciatic nerve, only trkB mRNA encoding truncated TrkB receptors was detected with reduced levels in the distal part of the lesioned nerve. Similar results were seen using a probe that detects all forms of trkC mRNA. In the denervated gastrocnemius muscle, the level of BDNF mRNA increased, NT-3 mRNA did not change, while NT-4 mRNA decreased. In the spinal cord, only small changes were seen in the levels of neutrophin and trk mRNAs. These results show that expression of mRNAs for neurotrophins and their Trk receptors is differentially regulated after a peripheral nerve injury. Based on these results a model is presented for how the different neurotrophins could cooperate to promote regeneration of injured peripheral nerves.     

The binding epitopes of neurotrophin-3 to its receptors trkC and gp75 and the design of a multifunctional human neurotrophin.

Survival and maintenance of vertebrate neurons are influenced by neurotrophic factors which mediate their signal by binding to specific cell surface receptors. We determined the binding sites of human neurotrophin-3 (NT-3) to its receptors trkC and gp75 by mutational analysis and compared them to the analogous interactions of nerve growth factor (NGF) with trkA and gp75. The trkC binding site extends around the central beta-strand bundle and in contrast to NGF does not make use of non-conserved loops and the six N-terminal residues. The gp75 epitope is dominated by loop residues and the C-terminus of NT-3. A novel rapid biological screening procedure allowed the identification of NT-3 mutants that are able to signal efficiently through the non-preferred receptors trkA and trkB, which are specific for NGF and BDNF respectively. Mutation of only seven residues in NT-3 resulted in a human neurotrophin variant which bound to all receptors of the trk family with high affinity and efficiently supported the survival of NGF-, BDNF- and NT-3-dependent neurons. Our results suggest that the specificity among neurotrophic factors is not solely encoded in sequence diversity, but rather in the way each neurotrophin interacts with its preferred receptor.     

Cutting edge: clonally restricted production of the neurotrophins brain-derived neurotrophic factor and neurotrophin-3 mRNA by human immune cells and Th1/Th2-polarized expression of their receptors.

Neurotrophins, such as neurotrophin-3 (NT-3) and brain-derived neurotrophic factor (BDNF), are potent regulators of neuronal functions. Here we show that human immune cells also produce NT-3 mRNA, secrete BDNF, and express their specific receptors trkB and trkC. The truncated trkB receptor, usually expressed in sensory neurons of the central nervous system, was also constitutively expressed in unstimulated Th cells. Full-length trkB was detectable in stimulated PBMC, B cell lines, and Th1, but not in Th2 and Th0 cell clones. Clonally restricted expression was also observed for trkC, until now not detected on blood cells. The Th1 cytokine IL-2 stimulated production of trkB mRNA but not of trkC, whereas the Th2 cytokine IL-4 enhanced NT-3 but not BDNF mRNA expression. Microbial Ags, which influence the Th1/Th2 balance, could therefore modulate the neurotrophic system and thereby affect neuronal synaptic activity of the central nervous system.     

神经生长因子家族及其受体研究进展

过去几年在神经营养因子、受体和神经元细胞程序性死亡的研究领域中取得了几项引人注目的进展：（１）神经生长因子（ＮＧＦ）基因家族的其他一些成员包括脑源性神经营养因子（ＢＤＮＦ）、神经营养素－３（ＮＴ－３）、神经营养素－４（ＮＴ－４）、神经营养素－５（ＮＴ－５）的发现;（２）神经生长因子三维结构及功能和进化之关系的阐明;（３）定性了两种神经生长因子受体Ｐ７５＾ＮＧＦＲ和原癌基因ｐ１４０＾ｔｒｋＡ以及相关     

Neurotrophic factor regulation of developing avian oculomotor neurons: differential effects of BDNF and GDNF.

Neurotrophic factors support the development of motoneurons by several possible mechanisms. Neurotrophins may act as target-derived factors or as afferent factors derived from the central nervous system (CNS) or sensory ganglia. We tested whether brain-derived neurotrophic factor (BDNF), neurotrophin 3 (NT-3), neurotrophin 4 (NT-4), and glial cell line-derived neurotrophic factor (GDNF) may be target-derived factors for neurons in the oculomotor (MIII) or trochlear (MIV) nucleus in chick embryos. Radio-iodinated BDNF, NT-3, NT-4, and GDNF accumulated in oculomotor neurons via retrograde axonal transport when the trophic factors were applied to the target. Systemic GDNF rescued oculomotor neurons from developmental cell death, while BDNF and NT-3 had no effect. BDNF enhanced neurite outgrowth from explants of MIII and MIV nuclei (identified by retrograde labeling in ovo with the fluorescent tracer DiI), while GDNF, NT-3, and NT-4 had no effect. The oculomotor neurons were immunoreactive for BDNF and the BDNF receptors p75(NTR) and trkB. To determine whether BDNF may be derived from its target or may act as an autocrine or paracrine factor, in situ hybridization and deprivation studies were performed. BDNF mRNA expression was detected in eye muscles, but not in CNS sources of afferent innervation to MIII, or the oculomotor complex itself. Injection of trkB fusion proteins in the eye muscle reduced BDNF immunoreactivity in the innervating motoneurons. These data indicate that BDNF trophic support for the oculomotor neurons was derived from their target.     

K-252b Potentiation of Neurotrophin-3 Is trkA Specific in Cells Lacking p75NTR

Abstract: K-252b potentiates the neurotrophic effects of neurotrophin-3 (NT-3) in primary cultures of rat central cholinergic and peripheral sensory neurons and in a rat pheochromocytoma PC12 cell line. The ligand and receptor specificity, and role of the low-affinity neurotrophin receptor (p75^NTR) in the potentiation response induced by K-252b, are unknown. To address the issues of ligand and receptor specificity of K-252b potentiation, we have examined neurotrophin-induced DNA synthesis ([³H]thymidine incorporation) in NIH3T3 cells expressing trkA, trkB, or trkC . Neither NT-3 nor K-252b alone could stimulate mitogenic activity in the trkA -overexpressing clone. However, coaddition of K-252b (EC₅₀ of ∼2 n M ) with 10–100 ng/ml NT-3 led to incorporation of [³H]thymidine in trkA expressing cells to a level induced by optimal concentrations of nerve growth factor (NGF). The K-252b- and NT-3-induced [³H]thymidine incorporation correlated with an increase in the tyrosine autophosphorylation of the trkA receptor as well as tyrosine phosphorylation of trk -associated phospholipase C-γ1 and SH2-containing proteins. K-252b did not potentiate submaximal doses of NGF, or maximal doses of brain-derived neurotrophic factor (BDNF) or neurotrophin-4/5 (NT-4/5) in trkA -expressing cells. Furthermore, K-252b did not potentiate DNA synthesis by submaximal doses of BDNF, NT-4/5, or NT-3 in trkB - or trkC -expressing NIH3T3 cells, suggesting that the potentiation profile for K-252b was specific for NT-3 in trkA -expressing cells. We found no expression of p75^NTR in the trk -expressing NIH3T3 cells. This is the first demonstration that K-252b potentiates a trkA -mediated biological nonneuronal response by NT-3 that occurs independent of p75^NTR and appears to be both ligand and receptor specific.     

Expression of neurotrophins and their receptors in peripheral lung cells of mice

Neurotrophins play an essential role in nerve systems. Recent reports indicated that neurotrophins [nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4/5 (NT-4/5)] have numerous effects on non-neural cells, especially on immune cells. However, whether lung cells express neurotrophins and/or their receptors (TrkA for NGF, TrkB for BDNF and NT-4/5, and TrkC for NT-3) has never been systematically investigated. We investigated constitutive expression of neurotrophin family and their Trk receptor family in alveolar macrophages and other peripheral lung cells of mice. New findings were: (1) RT-PCR for neurotrophins and their receptors detected NT-3 and NT-4/5 in alveolar macrophages, BDNF, NT-4/5, trkA, the truncated form of trkB, and trkC in lung homogenate, but no trks in alveolar macrophages, (2) immunohistochemistry for neurotrophin receptors detected TrkA in capillary cells, the truncated form of TrkB, and TrkC in interstitial macrophages, (3) immunoelectron microscopy for TrkC revealed expression of TrkC on the surface of interstitial macrophages, and (4) in situ hybridization for neurotrophins detected BDNF in interstitial macrophages and alveolar type I cells, NT-3 in alveolar macrophages, and NT-4/5 in alveolar and interstitial macrophages. These findings indicate that a previously unknown signal trafficking occurs through neurotrophins in peripheral lung.     

Differential effects of the trophic factors BDNF, NT-4, GDNF, and IGF-I on the isthmo-optic nucleus in chick embryos

The isthmo-optic nucleus (ION) of chick embryos is a model system for the study of retrograde trophic signaling in developing CNS neurons. The role of brain-derived neurotrophic factor (BDNF) is well established in this system. Recent work has implicated neurotrophin-4 (NT-4), glial cell line-derived neurotrophic factor (GDNF), and insulin-like growth factor I (IGF-I) as additional trophic factors for ION neurons. Here it was examined in vitro and in vivo whether these factors are target-derived trophic factors for the ION in 13- to 16-day-old chick embryos. Unlike BDNF, neither GDNF, NT-4, nor IGF-I increased the survival of ION neurons in dissociated cultures identified by retrograde labeling with the fluorescent tracer DiI. BDNF and IGF-I promoted neurite outgrowth from ION explants, whereas GDNF and NT-4 had no effect. Injections of NT-4, but not GDNF, in the retina decreased the survival of ION neurons and accelerated cell death in the ION. NT-4-like immunoreactivity was present in the retina and the ION. Exogenous, radiolabeled NT-4, but not GDNF or IGF-I, was retrogradely transported from the retina to the ION. NT-4 transport was significantly reduced by coinjection of excess cold nerve growth factor (NGF), indicating that the majority of NT-4 bound to p75 neurotrophin receptors during axonal transport. Binding of NT-4 to chick p75 receptors was confirmed in L-cells, which express chick p75 receptors. These data indicate that GDNF has no direct trophic effects on ION neurons. IGF-I may be an afferent trophic factor for the ION, and NT-4 may act as an antagonist to BDNF, either by competing with BDNF for p75 and/or trkB binding or by signaling cell death via p75.     

Neurotrophins and their receptors in rat peripheral trigeminal system during maxillary nerve growth

We examined the expression of the neurotrophins (NTFs) and their receptor mRNAs in the rat trigeminal ganglion and the first branchial arch before and at the time of maxillary nerve growth. The maxillary nerve appears first at embryonic day (E)10 and reaches the epithelium of the first branchial arch at E12, as revealed by anti-L1 immunohistochemistry. In situ hybridization demonstrates, that at E10- E11, neurotrophin-3 (NT-3) mRNA is expressed mainly in the mesenchyme, but neurotrophin-4 (NT-4) mRNA in the epithelium of the first branchial arch. NGF and brain-derived neurotrophic factor (BDNF) mRNAs start to be expressed in the distal part of the first brachial arch shortly before its innervation by the maxillary nerve. Trigeminal ganglia strongly express the mRNA of trkA at E10 and thereafter. The expression of mRNAs for low-affinity neurotrophin receptor (LANR), trkB, and trkC in trigeminal ganglia is weak at E10, but increases by E11-E12. NT-3, NT-4, and more prominently BDNF, induce neurite outgrowth from explant cultures of the E10 trigeminal ganglia but no neurites are induced by NGF, despite the expression of trkA. By E12, the neuritogenic potency of NGF also appears. The expression of NT-3 and NT-4 and their receptors in the trigeminal system prior to target field innervation suggests that these NTFs have also other functions than being the target-derived trophic factors.     

Making and breaking the innervation of the ear: neurotrophic support during ear development and its clinical implications

Analyses of single and double mutants of members of the neurotrophin family and their receptors are reviewed. These data demonstrate that the two neurotrophins, brain-derived neurotrophic factor (BDNF) and neurotrophin 3 (NT-3), and their high-affinity receptors trkB and trkC, are the sole support for the developing afferent innervation of the ear. Neurotrophins are first expressed in the otocyst around the time afferent sensory neurons become postmitotic. They are crucial for the survival of certain topologically distinct populations of sensory neurons. BDNF supports all sensory neurons to the semicircular canals, most sensory neurons to the saccule and utricle, and many sensory neurons to the apex and middle turn of the cochlea. In contrast, NT-3 supports few sensory neurons to the utricle and saccule, all sensory neurons to the basal turn of the cochlea and most sensory neurons to the middle and apical turn. Some topologically restricted effects reflect the pattern of neurotrophin distribution as revealed by in situ hybridization (e.g., loss of all innervation to the semicircular canal sensory epithelia in BDNF or trkB mutants). However, other topologically restricted effects cannot be explained on the basis of current knowledge of neurotrophin or neurotrophin receptor distribution. Data on mutants also support the notion that BDNF may play a role in neonatal plastic reorganization of the pattern of innervation in the ear and possibly the brainstem. In contrast, data obtained thus far on the ability of neurotrophins to rescue adult sensory neuron after insults to cochlear hair cells are less compelling. The ear is a model system to test the interactions of the two neurotrophins, BDNF and NT-3, with their two high-affinity receptors, trkB and trkC.     

Expression of neurotrophin receptors trkB and trkC and their ligands in rat adrenal gland and the intermediolateral column of the spinal cord

Neurotrophins and their trk receptors constitute major classes of signaling molecules with important actions in the developing and adult nervous system. With regard to the sympathoadrenal cell lineage, which gives rise to sympathetic neurons and chromaffin cells, neurotrophin-3 (NT-3) and nerve growth factor (NGF) are thought to influence developing sympathetic neurons. Neurotrophin requirements of chromaffin cells of the adrenal medulla are less well understood than those for NGF. In order to provide the bases for understanding of putative functions of neurotrophins for the development and maintenance of chromaffin cells and their preganglionic innervation, in situ hybridization has been used to study the expression of brain-derived neurotrophic factor (BDNF) and NT-3, together with their cognate receptors trkB and trkC, in the adrenal gland and in the intermediolateral column (IML) of the spinal cord. BDNF is highly expressed in the embryonic adrenal cortex and later in cells of the cortical reticularis zone. Adrenal medullary chromaffin cells fail to express detectable levels of mRNAs for BDNF, NT-3, and their cognate receptors trkB and trkC. Neurons in the IML express BDNF and trkB, and low levels of NT-3 and trkC. Our data make it unlikely that BDNF and NT-3 serve as retrograde trophic factors for IML neurons but suggest roles of BDNF and NT-3 locally within the spinal cord and possibly for sensory nerves of the adrenal cortex.     

Neurotrophic factor regulation of developing avian oculomotor neurons: Differential effects of BDNF and GDNF

Neurotrophic factors support the development of motoneurons by several possible mechanisms. Neurotrophins may act as target‐derived factors or as afferent factors derived from the central nervous system (CNS) or sensory ganglia. We tested whether brain‐derived neurotrophic factor (BDNF), neurotrophin 3 (NT‐3), neurotrophin 4 (NT‐4), and glial cell line–derived neurotrophic factor (GDNF) may be target‐derived factors for neurons in the oculomotor (MIII) or trochlear (MIV) nucleus in chick embryos. Radio‐iodinated BDNF, NT‐3, NT‐4, and GDNF accumulated in oculomotor neurons via retrograde axonal transport when the trophic factors were applied to the target. Systemic GDNF rescued oculomotor neurons from developmental cell death, while BDNF and NT‐3 had no effect. BDNF enhanced neurite outgrowth from explants of MIII and MIV nuclei (identified by retrograde labeling in ovo with the fluorescent tracer DiI), while GDNF, NT‐3, and NT‐4 had no effect. The oculomotor neurons were immunoreactive for BDNF and the BDNF receptors p75^NTR and trkB. To determine whether BDNF may be derived from its target or may act as an autocrine or paracrine factor, in situ hybridization and deprivation studies were performed. BDNF mRNA expression was detected in eye muscles, but not in CNS sources of afferent innervation to MIII, or the oculomotor complex itself. Injection of trkB fusion proteins in the eye muscle reduced BDNF immunoreactivity in the innervating motoneurons. These data indicate that BDNF trophic support for the oculomotor neurons was derived from their target. © 1999 John Wiley & Sons, Inc. J Neurobiol 41: 295–315, 1999     

Ruffini endings are absent from the periodontal ligament of trkB knockout mice

To clarify the role of neurotrophin receptors in the development of Ruffini endings, periodontal ligaments and trigeminal ganglia of trkA, trkB, and trkC knockout mice were immunostained for protein gene product 9.5 (PGP 9.5), calcitonin gene-related peptide (CGRP), parvalbumin (PV), and calretinin (CR). Innervation patterns of PGP 9.5- and CGRP-immunoreactive fibers were examined in the periodontal ligament of the knockout mice. PGP 9.5-positive fibers in the incisal periodontal ligaments of trkA and trkC knockout mice form Ruffini endings distinguished by dendritic ramifications and branches. However, Ruffini endings were not present in the periodontal ligament of trkB knockout mice. Only free nerve endings were observed in tissue of trkB knockout mice. Compared with trkA and trkC knockouts, the proportion of CR-positive neurons in mandibular and maxillary regions of the trigeminal ganglion of trkB knockout mice is decreased. These findings indicate that the development of periodontal Ruffini endings is regulated by trkB-dependent and CR-coexpressing neurons.     

Neurotrophins promote the survival and development of neurons in the cerebellum of hypothyroid rats in vivo

The development of cerebellar cortex is strongly impaired by thyroid hormone (T3) deficiency, leading to altered migration, differentiation, synaptogenesis, and survival of neurons. To determine whether alteration in the expression of neurotrophins and/or their receptors may contribute to these impairments, we first analyzed their expression using a sensitive RNAse protection assay and in situ hybridization; second, we administered the deficient neurotrophins to hypothyroid animals. We found that early hypothyroidism disrupted the developmental pattern of expression of the four neurotrophins, leading to relatively higher levels of NGF and neurotrophin 4/5 mRNAs and to a severe deficit in NT-3 and brain-derived neurotrophic factor (BDNF) mRNA expression, without alteration in the levels of the full-length tyrosine kinase (trk) B and trkC receptor mRNAs. Grafting of P3 hypothyroid rats with cell lines expressing high levels of neurotrophin 3 (NT-3) or BDNF prevented hypothyroidism-induced cell death in neurons of the internal granule cell layer at P15. In addition, we found that NT-3, but not BDNF, induced the differentiation and/or migration of neurons in the external granule cell layer, stimulated the elaboration of the dendritic tree by Purkinje cells, and promoted the formation of the mature pattern of synaptic afferents to Purkinje cell somas. Thus, our results indicate that both granule and Purkinje neurons require appropriate levels of NT-3 for normal development in vivo and suggest that T3 may regulate the levels of neurotrophins to promote the development of cerebellum.     

Biochemical and functional interactions between the neurotrophin receptors trk and p75NTR.

Neurotrophins bind to two structurally unrelated receptors, the trk tyrosine kinases and the neurotrophin receptor p75(NTR). Ligand activation of these two types of receptor can lead to opposite actions, in particular the prevention or activation of programmed cell death. Many cells co-express trk receptors and p75(NTR), and we found that p75(NTR) was co-precipitated with trkA, trkB and trkC in cells transfected with both receptor types. Co-precipitation of p75(NTR) was not observed with the epidermal growth factor receptor. Experiments with deletion constructs of trkB (the most abundant trk receptor in the brain) and p75(NTR) revealed that both the extracellular and intracellular domains of trkB and p75(NTR) contribute to the interaction. Blocking autophosphorylation of trkB substantially reduced the interactions between p75(NTR) and trkB constructs containing the intracellular, but not the extracellular, domains. We also found that co-expression of p75(NTR) with trkB resulted in a clear increase in the specificity of trkB activation by brain-derived neurotrophic factor, compared with neurotrophin-3 and neurotrophin-4/5. These results indicate a close proximity of the two neurotrophin receptors within cell membranes, and suggest that the signalling pathways they initiate may interact soon after their activation.     

Dietary restriction enhances neurotrophin expression and neurogenesis in the hippocampus of adult mice

The adult brain contains small populations of neural precursor cells (NPC) that can give rise to new neurons and glia, and may play important roles in learning and memory, and recovery from injury. Growth factors can influence the proliferation, differentiation and survival of NPC, and may mediate responses of NPC to injury and environmental stimuli such as enriched environments and physical activity. We now report that neurotrophin expression and neurogenesis can be modified by a change in diet. When adult mice are maintained on a dietary restriction (DR) feeding regimen, numbers of newly generated cells in the dentate gyrus of the hippocampus are increased, apparently as the result of increased cell survival. The new cells exhibit phenotypes of neurons and astrocytes. Levels of expression of brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) are increased by DR, while levels of expression of high-affinity receptors for these neurotrophins (trkB and trkC) are unchanged. In addition, DR increases the ratio of full-length trkB to truncated trkB in the hippocampus. The ability of a change in diet to stimulate neurotrophin expression and enhance neurogenesis has important implications for dietary modification of neuroplasticity and responses of the brain to injury and disease.     

Expression patterns of neurotrophic factor mRNAs in developing human teeth

Neurotrophic factors regulate survival, differentiation, growth and plasticity in the nervous system. In addition, based on their specific and shifting temporospatial expression patterns, neurotrophic factors have been implicated in morphogenetic events during tooth development in rodents. To determine whether these findings in rodents could be related to humans, we have now studied nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), neurotrophin-4 (NT-4), glial cell-line derived neurotrophic factor (GDNF), and neurturin (NTN) mRNA expression patterns in developing human teeth during gestational weeks 6.5-11. Using in situ hybridization histochemistry, we found distinct and specific patterns of neurotrophin and GDNF mRNA expression in the developing human teeth. NGF mRNA labeling was weak and confined predominantly to the dental papilla. BDNF mRNA labeling was stronger than NGF mRNA and was seen in the mesenchyme located lateral to the dental organ, as well as in epithelial structures (inner dental epithelium and enamel knot). NT-3 mRNA was observed in the dental papilla and in the area of the cervical loop. NT-4 mRNA was expressed in both oral and dental epithelia in all stages studied. GDNF mRNA was found in the dental follicle and at different sites in the inner dental epithelium. Weak NTN mRNA labeling was also found in the developing teeth. Based on these findings, we suggest that neurotrophins, GDNF and NTN might be involved in morphogenetic events during early stages of tooth development in humans. Protein gene product (PGP) 9.5-immunoreactive nerve fibers were observed in the dental follicle by 11 weeks coinciding with the labeling for neurotrophic factor mRNAs in this structure. This suggests that these neurotrophic factors might be involved in the innervation of dental structures. The rich expression of neurotrophic factors in developing dental tissues suggests that developing, or possibly adult, dental tissue might be used as an allograft source of trophic support for diseases of the nervous system.