Neurotrophin-3 is expressed in a discrete subset of olfactory receptor neurons in the mouse

Cannabinoid receptor signaling induces proliferation but not neurogenesis in the mouse olfactory epithelium

The olfactory epithelium actively generates neurons through adulthood, and this neurogenesis is tightly regulated by multiple factors that are not fully defined. Here, we examined the role of cannabinoids in the regulation of neurogenesis in the mouse olfactory epithelium. In vivo proliferation and cell lineage studies were performed in mice (C57BL/6 and cannabinoid type 1 and 2 receptor deficient strains) treated with cannabinoids directly (WIN 55,212–2 or 2-arachidonylglycerol ether) or indirectly via inhibition of cannabinoid hydrolytic enzymes. Cannabinoids increased proliferation in neonatal and adult mice, and had no effect on proliferation in cannabinoid type 1 and 2 receptor deficient adult mice. Pretreatment with the cannabinoid type1 receptor antagonist AM251 decreased cannabinoid-induced proliferation in adult mice. Despite a cannabinoid-induced increase in proliferation, there was no change in newly generated neurons or non-neuronal cells 16 d post-treatment. However, cannabinoid administration increased apoptotic cell death at 72 hours post-treatment and by 16 d the level of apoptosis dropped to control levels. Thus, cannabinoids induce proliferation, but do not induce neurogenesis nor non-neuronal cell generation. Cannabinoid receptor signaling may regulate the balance of progenitor cell survival and proliferation in adult mouse olfactory epithelium.

Differential localization of NT-3 and TrpM5 in glomeruli of the olfactory bulb of mice

Olfactory sensory neurons that express transient receptor potential channel M5 (TrpM5) or neurotrophin-3 (NT-3) project to defined clusters of glomeruli situated ventrally in the main olfactory bulb. Using genetically labeled mice, we investigated whether expression of NT-3-driven βgal and TrpM5-driven GFP marked overlapping sets of glomeruli and whether expression of these markers was coordinated. Our results indicate that these markers largely characterize independent sets of olfactory sensory neuron axons and glomeruli. Further, in glomeruli in which both TrpM5-GFP and NT-3-βgal labeled axons occur, they are expressed independently. The nature of staining for these two markers also differs within glomeruli. Within each labeled TrpM5-positive glomerulus, the level of TrpM5-GFP expression was similar throughout the glomerular neuropil. In contrast, NT-3-driven βgal expression levels are heterogeneous even within heavily labeled glomeruli. In addition, a population of very small TrpM5-GFP positive glomeruli is apparent while no similar populations of NT-3-βgal glomeruli are evident. Taken together, these data suggest that TrpM5 and NT-3 characterize two largely independent receptor populations both conveying odorant information to the ventral olfactory bulb.

P75 nerve growth factor receptors modulate development of GnRH neurons and olfactory ensheating cells

Temporal and spatial localization of nerve growth factor receptor (p75NGFR) in the developing olfactory system and gonadotropin-releasing hormone-1 (GnRH) system was characterized and its role analyzed using p75NGFR null mice and nasal explants. Prenatally, p75NGFR was expressed by GnRH neurons and olfactory ensheathing cells (OECs). In p75NGFR null mice, no change in the number of GnRH cells was detected as compared to wild-type. However, in null mice, a shift in the distribution of GnRH neurons was found, with a small population of GnRH cells migrating further caudally toward the median eminence. Additionally, a reduction of both GAD67 positive olfactory axons and GFAP positive OEC fibers occurred. Acute administration of a p75NGFR blocker to GnRH cells maintained in vitro increased migration rate, consistent with the change in distribution detected in p75NGFR null mice. Chronic inhibition of p75NGFR caused an attenuation of olfactory axon fasciculation and a decrease in OEC density, again mimicking the changes detected in null mice. However, a reduction in GnRH cell number was found after chronic treatment that not observed in KO animals suggesting indirect changes occur during chronic treatment in vitro and/or a compensatory mechanism occurs in vivo that prevents loss of GnRH neurons in the absence of p75NGFR.

Anterograde trafficking of neurotrophin-3 in the adult olfactory system in vivo

The olfactory system continuously incorporates new neurons into functional circuits throughout life. Axons from olfactory sensory neurons (OSNs) in the nasal cavity synapse on mitral, tufted and periglomerular (PG) cells in the main olfactory bulb, and low levels of turnover within the OSN population results in ingrowth of new axons under normal physiological conditions. Subpopulations of bulb interneurons are continually eliminated by apoptosis, and are replaced by new neurons derived from progenitors in the adult forebrain subventricular zone. Integration of new neurons, including PG cells that are contacted by sensory axons, leads to ongoing reorganization of adult olfactory bulb circuits. The mechanisms regulating this adaptive structural plasticity are not all known, but the process is reminiscent of early nervous system development. Neurotrophic factors have well-established roles in controlling neuronal survival and connectivity during development, leading to speculation that trophic interactions between OSNs and their target bulb neurons may mediate some of these same processes in adults. A number of different trophic factors and their cognate receptors are expressed in the adult olfactory pathway. Neurotrophin-3 (NT3) is among these, as reflected by beta-galactosidase expression in transgenic reporter mice expressing lacZ under the NT3 promoter. Using a combination of approaches, including immunocytochemistry, real-time PCR of laser-captured RNA, and adenovirus-mediated gene transfer of NT3 fusion peptides in vivo, we demonstrate that OSNs express and anterogradely transport NT3 to the olfactory bulb. We additionally observe that in mice treated with adenovirus encoding NT3 tagged with hemagglutinin (HA), a subset of bulb neurons expressing the TrkC neurotrophin receptor are immunoreactive for HA, suggesting their acquisition of the fusion peptide from infected sensory neurons. Our results therefore provide evidence that OSNs may serve as an afferent source of trophic signals for the adult mouse olfactory bulb.

Differential expression of RET receptor isoforms in the olfactory system

The glial cell line-derived neurotrophic factor (GDNF) family supports neurons by activating the tyrosine kinase receptor RET. The two main isoforms of RET, RET9 and RET51, differ in their carboxyl termini and have been implicated with distinct functions in the enteric and central nervous systems. Previously we reported the cellular localization of GDNF, neurturin and RET9 in the olfactory system [Maroldt H, Kaplinovsky T, Cunningham AM (2005) J Neurocytol 34:241-255]. In the current study, we examined immunohistochemical expression of RET9 and RET51 in neonatal and adult rat olfactory neuroepithelium (ON) and bulb to explore their potential functional roles. In the ON, RET9 was expressed by olfactory receptor neurons (ORNs) throughout the olfactory neuroepithelial sheet, whereas RET51 was restricted to ORNs situated in ventromedial and ventrolateral regions. Within these regions, RET51 was expressed by a subset of RET9-expressing ORNs. In olfactory bulb, RET9 expression was primarily on cell bodies, including olfactory ensheathing and periglomerular cells, and again, RET51 was expressed by a subset of RET9-expressing cells. RET51 was identified on axons in the olfactory nerve layer and glomerular neuropil, but only in the ventromedial and ventrolateral regions of the bulb. This regionalization correlated with the predicted axonal projection from expressing regions of the ON. RET51 was also expressed on dendrites in the external plexiform layer and glomerular neuropil. These results suggest RET9 may be the predominant functional isoform in the ON while RET51 plays a more selective role in a restricted region of the olfactory neuroepithelial sheet. In the bulb, RET9 is likely the main functional isoform while RET51 may be important in axonal and dendritic function/targeting.

Smooth-muscle-specific expression of neurotrophin-3 in mouse embryonic and neonatal gastrointestinal tract

Vagal gastrointestinal (GI) afferents are essential for the regulation of eating, body weight, and digestion. However, their functional organization and the way that this develops are poorly understood. Neurotrophin-3 (NT-3) is crucial for the survival of vagal sensory neurons and is expressed in the developing GI tract, possibly contributing to their survival and to other aspects of vagal afferent development. The identification of the functions of this peripheral NT-3 thus requires a detailed understanding of the localization and timing of its expression in the developing GI tract. We have studied embryos and neonates expressing the lacZ reporter gene from the NT-3 locus and found that NT-3 is expressed predominantly in the smooth muscle of the outer GI wall of the stomach, intestines, and associated blood vessels and in the stomach lamina propria and esophageal epithelium. NT-3 expression has been detected in the mesenchyme of the GI wall by embryonic day 12.5 (E12.5) and becomes restricted to smooth muscle and lamina propria by E15.5, whereas its expression in blood vessels and esophageal epithelium is first observed at E15.5. Expression in most tissues is maintained at least until postnatal day 4. The lack of colocalization of beta-galactosidase and markers for myenteric ganglion cell types suggests that NT-3 is not expressed in these ganglia. Therefore, NT-3 expression in the GI tract is largely restricted to smooth muscle at ages when vagal axons grow into the GI tract, and when vagal mechanoreceptors form in smooth muscle, consistent with its role in these processes and in vagal sensory neuron survival.

Long-term survival of olfactory sensory neurons after target depletion

Life-long addition and elimination of neurons within the adult olfactory epithelium and olfactory bulb allows for adaptive structural responses to sensory experience, learning, and recovery after injury. The interdependence of the two structures is highlighted by the shortened life span of sensory neurons deprived of bulb contact, and has prompted the hypothesis that trophic cues from the bulb contribute to their survival. The specific identity and source of these signals remain unknown. To investigate the potential role of target neurons in this support, we employed a neurotoxic lesion to selectively remove them while preserving the remaining nerve projection pathway, and examined the dynamics of sensory neuron proliferation and survival. Pulse-labeling of progenitors with bromodeoxyuridine showed that, as with surgical bulb removal, increased apoptosis in the epithelium triggered accelerated production of new neurons after chemical depletion of target cells. Rather than undergoing premature death, a large subpopulation of these neurons survived long term. The combination of increased proliferation and extended survival resulted in essentially normal numbers of new sensory neurons surviving for as long as 5 weeks, with an accompanying restoration of olfactory marker protein expression. Changes in neurotrophic factor expression levels as measured by quantitative polymerase chain reaction (Q-PCR), and in bulb cell populations, including the addition of new neurons generated in the subventricular zone, were observed in the injured bulb. These data indicate that olfactory sensory neurons can adapt to reductions in their normal target field by obtaining sufficient support from remaining or alternative cell sources to survive and maintain their projections.

Neurotrophin expression in the adult olfactory epithelium

Published reports of neurotrophin expression in the olfactory system are incomplete because of missing data and conflicting results. Previous studies used a variety of fixation procedures and antibodies on different species and different ages. The aim of the present study was to examine expression of neurotrophins and their receptors using optimized methodologies: five methods of fixation, multiple antibodies, a variety of immunochemical protocols, and RT-PCR. We show here that (i) transcripts for all neurotrophins and their receptors are found in the adult olfactory epithelium; (ii) all neurotrophins are expressed in the supporting cells and the neuronal layers of the undisturbed adult olfactory epithelium while NT4 is found additionally in the horizontal basal cells; (iii) neurotrophin immunoreactivity required a fixative that included parabenzoquinone (not used in previous studies of olfactory tissue); (iv) TrkB and TrkC are restricted to the globose basal cell and neuron layers while TrkA is found in the horizontal basal cells and in the supporting cells where it co-localizes with the low affinity receptor for NGF (p75NTR). These findings confirm that neurotrophins are produced within the olfactory epithelium, suggesting autocrine and paracrine regulation of olfactory neurogenesis.

Detection of neurotrophic factors in taste buds by laser capture microdissection, immunohistochemistry, and in situ hybridization

Neurotrophic factors are thought to function in the survival and maintenance of the taste buds and nerve fibers innervating them. Laser capture microdissection (LCM) coupled with the reverse transcription polymerase chain reaction (RT-PCR) was performed to detect the mRNA of neurotrophic factors and their receptors in the taste buds of adult mouse circumvallate papillae. Results showed mRNAs of the ciliary neurotrophic factor (CNTF), its receptor (CNTFR), glial cell line-derived neurotrophic factor (GDNF), GDNF family receptors alpha-1 (GFRalpha-1), GFRalpha-2, and RET tyrosine kinase receptor (RET), neurotrophin (NT)3, NT4/5, tyrosine kinase (Trk) C, nerve growth factor (NGF), and TrkA were expressed in the isolated taste buds. Among these neurotrophic factors, GDNF, GFRalpha-1, GFRalpha-2, NT3, NT4/5, NGF, and TrkA were previously found in the taste buds immunohistochemically and were detected at the mRNA level in the present study. The present immunohistochemical study revealed that CNTF, CNTFR, and the RET tyrosine kinase receptor, which binds GDNF family/ receptor complexes, were also expressed in the taste buds. However, by in situ hybridization, mRNAs of CNTF and RET were not detected in the taste buds from adult mice although they were found in those from early postnatal mice. CNTFR mRNA did not show any specific pattern in the taste buds. Moreover, mRNA expressions of NT4/5 and TrkC was re-examined by in situ hybridization; however no specific pattern was found for them in the taste buds. In summary, LCM is a useful tool for the detection of a relatively small amount of mRNA, such as that of neurotrophic factors and receptors in the taste buds.

Macrophage depletion in the murine olfactory epithelium leads to increased neuronal death and decreased neurogenesis

Apoptosis of olfactory sensory neurons (OSNs) induced by olfactory bulbectomy (OBX) leads to the activation of resident macrophages within the olfactory epithelium (OE). These macrophages phagocytose degenerating OSNs and secrete chemokines, which recruit additional macrophages into the OE, and cytokines/growth factors, which regulate basal cell proliferation and differentiation and maturation of OSNs. In this study we apply for the first time the use of liposome-encapsulated clodronate to selectively deplete macrophages during the OSN degeneration/regeneration cycle in order to elucidate the role(s) of macrophages in regulating cellular mechanisms that lead to apoptosis and neurogenesis. Mice were injected intranasally and intravenously with either liposome-encapsulated clodronate or empty liposomes prior to and after OBX or sham OBX. At 48 hours after surgery the numbers of macrophages in the OE of both sham and OBX clodronate-treated mice were significantly reduced compared to liposome-treated controls (38% and 35%, respectively, P < 0.05). The reduction in macrophage numbers was accompanied by significant decreases in OE thickness (22% and 21%, P < 0.05), the number of mOSNs (1.2- and 1.9-fold, P < 0.05), and basal cell proliferation (7.6- and 3.8-fold, P < 0.005) in sham and OBX mice, respectively, compared to liposome-treated controls. In OBX mice there was also increased immunoreactivity for active caspase-3 in the OE and olfactory nerves of clodronate-treated OBX mice compared to liposome-treated controls. These results indicate that macrophages modulate the OSN population in the normal and target-ablated murine OE by influencing neuronal survival and basal cell proliferation, resulting in neurogenesis and replacement of mature OSNs.

Immunohistochemical expression of two members of the GDNF family of growth factors and their receptors in the olfactory system

The glial cell line-derived (GDNF) family of trophic factors, GDNF, neurturin, persephin and artemin, are known to support the survival and regulate differentiation of many neuronal populations, including peripheral autonomic, enteric and sensory neurons. Members of this family of related ligands bind to specific GDNF family receptor (GFR) proteins, which complex and signal through the Ret receptor tyrosine kinase. We showed previously that GDNF protein was detectable in olfactory sensory neurons (OSNs) in the olfactory neuroepithelium (ON). In this immunohistochemical study, we localized GDNF, neurturin, GFRalpha1, GFRalpha2 and Ret in the adult rat ON and olfactory bulb. We found that GDNF and Ret were widely expressed by immature and mature OSNs, while neurturin was selectively expressed in a subpopulation of OSNs zonally restricted in the ON. The GFRs had differential expression, with mature OSNs and their axons preferentially expressing GFRalpha1, whereas progenitors and immature neurons more avidly expressed GFRalpha2. In the bulb, GDNF was highly expressed by the mitral and tufted cells, and by periglomerular cells, and its distribution generally resembled that of Ret, with the exception that Ret was far more predominant on fibers than cell bodies. Neurturin, in contrast, was present at lower levels and was more restricted in its expression to the axonal compartment. GFRalpha2 appeared to be the dominant accessory protein in the bulb. These data are supportive of two members of this neurotrophic family, GDNF and neurturin, playing different physiological roles in the olfactory neuronal system.

A genetic approach for investigating vagal sensory roles in regulation of gastrointestinal function and food intake

Sensory innervation of the gastrointestinal (GI) tract by the vagus nerve plays important roles in regulation of GI function and feeding behavior. This innervation is composed of a large number of sensory pathways, each arising from a different population of sensory receptors. Progress in understanding the functions of these pathways has been impeded by their close association with vagal efferent, sympathetic, and enteric systems, which makes it difficult to selectively label or manipulate them. We suggest that a genetic approach may overcome these barriers. To illustrate the potential value of this strategy, as well as to gain insights into its application, investigations of CNS pathways and peripheral tissues involved in energy balance that benefited from the use of gene manipulations are reviewed. Next, our studies examining the feasibility of using mutations of developmental genes for manipulating individual vagal afferent pathways are reviewed. These experiments characterized mechanoreceptor morphology, density and distribution, and feeding patterns in four viable mutant mouse strains. In each strain a single population of vagal mechanoreceptors innervating the muscle wall of the GI tract was altered, and was associated with selective effects on feeding patterns, thus supporting the feasibility of this strategy. However, two limitations of this approach must be addressed for it to achieve its full potential. First, mutation effects in tissues outside the GI tract can contribute to changes in GI function or feeding. Additionally, knockouts of developmental genes are often lethal, preventing analysis of mature innervation and ingestive behavior. To address these issues, we propose to develop conditional gene knockouts restricted to specific GI tract tissues. Two genes of interest are brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), which are essential for vagal afferent development. Creating conditional knockouts of these genes requires knowledge of their GI tract expression during development, which little is known about. Preliminary investigation revealed that during development BDNF and NT-3 are each expressed in several GI tract regions, and that their expression patterns overlap in some tissues, but are distinct in others. Importantly, GI tissues that express BDNF or NT-3 are innervated by vagal afferents, and expression of these neurotrophins occurs during the periods of axon invasion and receptor formation, consistent with roles for BDNF or NT-3 in these processes and in receptor survival. These results provide a basis for targeting BDNF or NT-3 knockouts to specific GI tract tissues, and potentially altering vagal afferent innervation only in that tissue (e.g., smooth muscle vs. mucosa). Conditional BDNF or NT-3 knockouts that are successful in selectively altering a vagal GI afferent pathway will be valuable for developing an understanding of that pathway's roles in GI function and food intake.

Ciliary neurotrophic factor-immunoreactivity in olfactory sensory neurons

Ciliary neurotrophic factor (CNTF) has been implicated in processes of neuroprotection, axonal regeneration and synaptogenesis in the lesioned CNS. In the olfactory system, which is characterized by particularly robust neuroplasticity throughout life, the concentration of CNTF is high even under physiological conditions. In the present study, the cellular localization of CNTF-immunoreactivity was studied in the rat and mouse olfactory epithelium. In both species, individual olfactory sensory neurons (ONs) displayed intense CNTF-immunoreactivity. The number of CNTF-ir ONs varied interindividually in rats and was lower in mice than in rats. In olfactory epithelia of mice expressing beta-galactosidase under control of the CNTF promoter, cells of the ON layer were immunoreactive for the reporter protein. CNTF-ir ONs were olfactory marker protein-positive and growth associated protein 43-negative. CNTF-ir ONs lacked apoptotic markers, and the number of specifically labeled ONs was apparently unchanged after light chemical lesioning of the epithelium, indicating that CNTF-immunoreactivity was not associated with ON death. Electron microscopy of CNTF-ir ON axons in innervated olfactory bulb glomeruli documented that they formed typical ON axonal synapses with target neurons. Three dimensional reconstructions of bulb pairs showed a striking similarity of the positions of glomeruli innervated by CNTF-ir ON axons in left and right bulbs of individual animals and interindividually. The number of innervated glomeruli differed interindividually in rats and was lower in mice than in rats. The results show that in rodents CNTF-immunoreactivity occurs in a subset of mature, functionally competent ONs. The localization of target glomeruli suggests that CNTF-immunoreactivity may be associated with the expression and/or activation of specific olfactory receptor proteins.

Effects of glossopharyngeal nerve section on the expression of neurotrophins and their receptors in lingual taste buds of adult mice

The expression of neurotrophins and neurotrophin receptors is essential for the proper establishment and function of many sensory systems. To determine which neurotrophins and neurotrophin receptors are expressed in taste buds, and in taste buds of mice following denervation, antibodies directed against the neurotrophins and their receptors were applied to adult mouse gustatory tissue. Immunohistochemistry reveals that nerve growth factor (NGF)-like immunoreactive (LIR), tyrosine kinase (trk) A-LIR, trkB-LIR, and p75-LIR elongated, differentiated taste cells are present within all lingual taste buds, whereas neither neurotrophin (NT)-3- nor trkC-LIR was detected in taste cells. Double-label immunohistochemistry using markers of different taste cell types in brain-derived neurotrophic factor (BDNF)LacZ mice reveals that BDNF (beta-gal) and trkB colocalize, mainly in type III taste cells. NGF, pro-NGF, and trkA coexist in type II taste cells, i.e., those expressing phospholipase Cbeta2 (PLCbeta2). p75-LIR also is present in both BDNF and NGF taste cell populations. To determine the neural dependence of neurotrophin expression in adult taste buds, glossopharyngeal nerves were cut unilaterally. During the period of denervation (10 days to 3 weeks), taste buds largely disappear, and few neurotrophin-expressing cells are present. Three weeks after nerve transection, nerve fascicles on the operated side of the tongue exhibit BDNF-LIR, NGF-LIR, and ubiquitin carboxyl terminal hydrolase (PGP 9.5)-LIR. However, BDNF-LIR staining intensity but not NGF-LIR or PGP 9.5-LIR is increased in nerve fascicles on the operated compared with the unoperated side. Five weeks following nerve transection, NT and NT receptor expression resumes and appears normal in taste buds and nerves. These results indicate that neurotrophin expression in taste buds is dependent on gustatory innervation, but expression in nerves is not dependent on contact with taste buds.

Neurotrophin-3 signaling maintains maturational homeostasis between neuronal populations in the olfactory epithelium

Neurons within the olfactory system undergo functional turnover throughout life. This process of cell death and compensatory neurogenesis requires feedback between neuronal populations of different developmental ages. We examined the role of NT-3 in this process. NT-3 was localized within both the olfactory bulb and olfactory epithelium. Mice null for NT-3 showed increased numbers of immature neurons, without change in the number of mature neurons. This was due to compensatory alterations in apoptosis of mature and immature neuronal populations. Using a primary olfactory neuronal culture, NT-3 was found to directly activate the PI3K/Akt pathway and indirectly activate the MAPK and PLC pathways. Activated PI3K/Akt promoted mature neuronal survival and induced the release of secondary factors, which activated the MAPK and PLC pathways to reduce neuronal precursor proliferation and inhibit neuronal maturation. These effects of NT-3 serve to maintain homeostasis between neuronal populations within the olfactory epithelium.