Alterations in the neurotrophic factors BDNF, GDNF and CNTF in the regenerating olfactory system

Glial cells in the mammalian olfactory bulb

The mammalian olfactory bulb (OB), an essential part of the olfactory system, plays a critical role in odor detection and neural processing. Historically, research has predominantly focused on the neuronal components of the OB, often overlooking the vital contributions of glial cells. Recent advancements, however, underscore the significant roles that glial cells play within this intricate neural structure. This review discus the diverse functions and dynamics of glial cells in the mammalian OB, mainly focused on astrocytes, microglia, oligodendrocytes, olfactory ensheathing cells, and radial glia cells. Each type of glial contributes uniquely to the OB's functionality, influencing everything from synaptic modulation and neuronal survival to immune defense and axonal guidance. The review features their roles in maintaining neural health, their involvement in neurodegenerative diseases, and their potential in therapeutic applications for neuroregeneration. By providing a comprehensive overview of glial cell types, their mechanisms, and interactions within the OB, this article aims to enhance our understanding of the olfactory system's complexity and the pivotal roles glial cells play in both health and disease.

HB-EGF promotes progenitor cell proliferation and sensory neuron regeneration in the zebrafish olfactory epithelium

Maintenance and regeneration of the zebrafish olfactory epithelium (OE) are supported by two distinct progenitor cell populations that occupy spatially discrete stem cell niches and respond to different tissue conditions. Globose basal cells (GBCs) reside at the inner and peripheral margins of the sensory OE and are constitutively active to replace sporadically dying olfactory sensory neurons (OSNs). In contrast, horizontal basal cells (HBCs) are uniformly distributed across the sensory tissue and are selectively activated by acute injury conditions. Here we show that expression of the heparin-binding epidermal growth factor-like growth factor (HB-EGF) is strongly and transiently upregulated in response to OE injury and signals through the EGF receptor (EGFR), which is expressed by HBCs. Exogenous stimulation of the OE with recombinant HB-EGF promotes HBC expansion and OSN neurogenesis in a pattern that resembles the tissue response to injury. In contrast, pharmacological inhibition of HB-EGF membrane shedding, HB-EGF availability, and EGFR signaling strongly attenuate or delay injury-induced HBC activity and OSN restoration without affecting maintenance neurogenesis by GBCs. Thus, HB-EGF/EGFR signaling appears to be a critical component of the signaling network that controls HBC activity and, consequently, repair neurogenesis in the zebrafish OE.

Human olfactory epithelium-derived stem cells ameliorate histopathological deficits and improve behavioral functions in a rat model of cerebellar ataxia

Cell replacement therapy (CRT) is one of the most effective approaches used to alleviate symptoms of neurodegenerative syndromes such as cerebellar ataxia (CA). Human olfactory epithelium mesenchymal stem cells (OE-MSCs) have been recognized as a promising candidate for CRT, due to their distinctive features including immunomodulatory properties and ease of accessible compared to other types of MSCs. Hence, the main goal of our study was to explore the impacts of OE-MSCs transplantation on behavioral, structural, and histological deficiencies in a rat model of CA. After obtained an informed consent from volunteers, OE-MSCs were obtained from their nasal cavity. Then, OE-MSCs were characterized by the positive expression of CD73, CD90, and CD105 as MSCs as well as nestin and vimentin as primitive neuroectodermal stem cells markers. Then, the animals were randomized into three control, 3-acetylpyridine (3-AP) treated, and 3-AP + cell groups. In both experimental groups, the rats received intraperitoneal injection of 3-AP (75 mg/kg), followed by the implantation of OE-MSCs into the cerebellum of 3-AP + cell group. The impact of engrafted OE-MSCs on motor coordination and performance along with biochemical, immunohistochemical, and stereological changes in the cerebellum of the rat models of CA were investigated. According to our findings, the administration of 3-AP decreased the cerebellar GSH concentration. The injection of 3-AP also altered the morphological characteristics of the cerebellar Golgi cells. On the other hand, OE-MSCs transplantation improved motor coordination in CA. Besides, the implantation of OE-MSCs reduced caspase-3 expression and microglia proliferation in the cerebellum upon 3-AP administration. Finally, the transplant of OE-MSCs protected Purkinje cells against 3-AP toxicity. In sum, the present study revealed considerable advantages of OE-MSCs in managing CA animal model.

Diving into the streams and waves of constitutive and regenerative olfactory neurogenesis: insights from zebrafish

The olfactory system is renowned for its functional and structural plasticity, with both peripheral and central structures displaying persistent neurogenesis throughout life and exhibiting remarkable capacity for regenerative neurogenesis after damage. In general, fish are known for their extensive neurogenic ability, and the zebrafish in particular presents an attractive model to study plasticity and adult neurogenesis in the olfactory system because of its conserved structure, relative simplicity, rapid cell turnover, and preponderance of neurogenic niches. In this review, we present an overview of the anatomy of zebrafish olfactory structures, with a focus on the neurogenic niches in the olfactory epithelium, olfactory bulb, and ventral telencephalon. Constitutive and regenerative neurogenesis in both the peripheral olfactory organ and central olfactory bulb of zebrafish is reviewed in detail, and a summary of current knowledge about the cellular origin and molecular signals involved in regulating these processes is presented. While some features of physiologic and injury-induced neurogenic responses are similar, there are differences that indicate that regeneration is not simply a reiteration of the constitutive proliferation process. We provide comparisons to mammalian neurogenesis that reveal similarities and differences between species. Finally, we present a number of open questions that remain to be answered.

Inhibition of focal adhesion kinase increases adult olfactory stem cell self-renewal and neuroregeneration through ciliary neurotrophic factor

Constant neuroregeneration in adult olfactory epithelium maintains olfactory function by basal stem cell proliferation and differentiation to replace lost olfactory sensory neurons (OSNs). Understanding the mechanisms regulating this process could reveal potential therapeutic targets for stimulating adult olfactory neurogenesis under pathological conditions and aging. Ciliary neurotrophic factor (CNTF) in astrocytes promotes forebrain neurogenesis but its function in the olfactory system is unknown. Here, we show in mouse olfactory epithelium that CNTF is expressed in horizontal basal cells, olfactory ensheathing cells (OECs) and a small subpopulation of OSNs. CNTF receptor alpha was expressed in Mash1-positive globose basal cells (GBCs) and OECs. Thus, CNTF may affect GBCs in a paracrine manner. CNTF−/− mice did not display altered GBC proliferation or olfactory function, suggesting that CNTF is not involved in basal olfactory renewal or that they developed compensatory mechanisms. Therefore, we tested the effect of increased CNTF in wild type mice. Intranasal instillation of a focal adhesion kinase (FAK) inhibitor, FAK14, upregulated CNTF expression. FAK14 also promoted GBC proliferation, neuronal differentiation and basal stem cell self-renewal but had no effective in CNTF−/− mice, suggesting that FAK inhibition promotes olfactory neuroregeneration through CNTF, making them potential targets to treat sensorineural anosmia due to OSN loss.

Effects of isotretinoin on the olfactory function in patients with acne

Background:

Isotretinoin is a synthetic analog of vitamin A. Recent studies support a role for retinoic acid in the recovery of olfactory function following injury in mice.

Objective:

This study aimed at determining the effect of isotretinoin on olfactory function in patients who have acne and are otherwise healthy.

Methods:

Forty-five patients (aged 25-40 years) with acne were included in the study. All patients underwent a rhinological examination. Olfactory function was assessed by the Sniffin' Sticks Test. The test was assessed at baseline and in the third month of isotretinoin treatment.

Results:

Isotretinoin improved the performance of patients in the olfactory test. The SST score increased from 8.7±1.09 to 9.5±1.19 (p<0.001), prevalence of hyposmia decreased from 40% to 24% and normosmia increased from 60% to 75% (p=0.059). The percentage of patients whose olfactory function was categorized as "good" increased from 6% to 21.3%. This increase was statistically significant (p<0.05).

Study limitations:

Absence of a control group is one of the limitations of this study. Also, we did not evaluate patients with smell test after stopping isotretinoin treatment.

Conclusion:

We examined the effect of systemic isotretinoin on olfactory function. It can be concluded from the present investigation that isotretinoin therapy improves the sense of smell.

Effects of Human Alpha-Synuclein A53T-A30P Mutations on SVZ and Local Olfactory Bulb Cell Proliferation in a Transgenic Rat Model of Parkinson Disease

A transgenic Sprague Dawley rat bearing the A30P and A53T α-synuclein (α-syn) human mutations under the control of the tyrosine hydroxylase promoter was generated in order to get a better understanding of the role of the human α-syn mutations on the neuropathological events involved in the progression of the Parkinson's disease (PD). This rat displayed olfactory deficits in the absence of motor impairments as observed in most early PD cases. In order to investigate the role of the mutated α-syn on cell proliferation, we focused on the subventricular zone (SVZ) and the olfactory bulbs (OB) as a change of the proliferation could affect OB function. The effect on OB dopaminergic innervation was investigated. The human α-syn co-localized in TH-positive OB neurons. No human α-syn was visualized in the SVZ. A significant increase in resident cell proliferation in the glomerular but not in the granular layers of the OB and in the SVZ was observed. TH innervation was significantly increased within the glomerular layer without an increase in the size of the glomeruli. Our rat could be a good model to investigate the role of human mutated α-syn on the development of olfactory deficits.

Purinergic signaling regulates cell proliferation of olfactory epithelium progenitors

In the olfactory epithelium (OE) continuous neurogenesis is maintained throughout life. The OE is in direct contact with the external environment, and its cells are constantly exposed to pathogens and noxious substances. To maintain a functional sense of smell the OE has evolved the ability to permanently replenish olfactory receptor neurons and sustentacular cells lost during natural turnover. A cell population residing in the most basal part of the OE, the so-called basal cells (BCs), keep up this highly regulated genesis of new cells. The population of BCs is thought to include both the stem cells of the OE and various progenitor cells. In recent years a number of regulatory factors that positively and/or negatively regulate the proliferation within the OE have been identified, but a thorough comprehension of the complex interplay of these regulatory factors and the role of the different epithelial cell types is still illusive. Combining labeling techniques, immunohistochemistry, electron microscopy, functional calcium imaging, and a bromo-2'-deoxyuridine incorporation assay, we show for the first time that purinergic receptors are expressed in BCs of the OE of larval Xenopus laevis and that nucleotide-induced Ca(2+) signaling in these cells is involved in the regulation of the cell turnover in the OE. Our data contribute to a better understanding of the regulation of the cell turnover in the OE in particular and also of how the proliferation of neuronal progenitor cells is regulated in general.

Effects of systemic transplantation of adipose tissue-derived stem cells on olfactory epithelium regeneration

OBJECTIVE/HYPOTHESIS

The purpose of the study was to investigate the effect of intravenous adipose tissue-derived stem cell (ADSC) transplantation on olfactory epithelium regeneration following transection of the olfactory nerve in rats.

STUDY DESIGN

This was a experimental study using primary cultures of mesenchymal stem cells derived from animal adipose tissue with histological analysis of animal olfactory tissue.

METHODS

All rats underwent unilateral transection of the olfactory nerve to induce degeneration of olfactory epithelium, and then were observed for regeneration according to time sequences. ADSCs were cultivated from neck adipose tissue of rats, and systemically injected into the experimental group. The control group was injected with phosphate buffered solution, instead of ADSCs. After 30 days, regeneration of olfactory epithelium was observed with olfactory marker protein (OMP) and proliferating cell nuclear antigen. To observe the characteristics of the transplanted ADSCs, olfactory epithelium was stained with von Willebrant factor and OMP.

RESULTS

After olfactory nerve transection, mature olfactory cells disappeared in 5 days, but gradually regained their thickness with increased cell numbers at approximately 10 to 15 days. By 30 days post-transection, the thickness and cellular composition of epithelium was almost restored to baseline levels pretransection. However, OMP expressions remained decreased compared with day 0 or 3. Systemically injected ADSCs were transplanted into the olfactory epithelium and survived beyond 4 weeks. The ADSCs promoted regeneration of olfactory epithelium in the animal model and differentiated into olfactory receptor neurons and endothelial cells.

CONCLUSIONS

Our findings suggest the feasibility of ADSC transplantation as a treatment for head trauma-related olfactory dysfunction.

Leukemia inhibitory factor promotes olfactory sensory neuronal survival via phosphoinositide 3-kinase pathway activation and Bcl-2

Leukemia inhibitory factor (LIF), a neuropoietic cytokine, has been implicated in the control of neuronal development. We previously reported that LIF plays a critical role in regulating the terminal differentiation of olfactory sensory neurons (OSNs). Here, we demonstrate that LIF plays a complementary role in supporting the survival of immature OSNs. Mature OSNs express LIF, which may be elaborated in a paracrine manner to influence adjacent neurons. LIF null mice display more apoptotic immature neurons than do their wild-type littermates. LIF treatment of dissociated OSNs in vitro significantly reduces the apoptosis of immature OSNs. Double immunocytochemical analysis indicates that the survival of immature OSNs is dependent on the presence of LIF. LIF activates the phosphoinositide 3-kinase (PI3K) pathways and induces the expression of the antiapoptotic molecule Bcl-2 in OSNs, whereas inhibition of the PI3K pathway blocks LIF-dependent OSN survival and Bcl-2 induction. Thus, LIF plays a central role in maintaining the size and integrity of the population of immature neurons within the olfactory epithelium; this population is critical to the rapid recovery of olfactory function after injury. LIF may play a similar role elsewhere in the CNS and thus be important for manipulation of stem cell populations for therapeutic interventions.

Amphibian larvae and zinc sulphate: a suitable model to study the role of brain-derived neurotrophic factor (BDNF) in the neuronal turnover of the olfactory epithelium

The vertebrate olfactory system has fascinated neurobiologists over the last six decades because of its ability to replace its neurons and synaptic connections continuously throughout adult life, under both physiological and pathological conditions. Among the factors that are proposed to be involved in this regenerative potential, brain-derived neurotrophic factor (BDNF) is a candidate for having an important role in the neuronal turnover in the olfactory epithelium (OE) because of its well-documented neurogenic and trophic effects throughout the nervous system. The aim of the present study was to generate a suitable model to study the participation of BDNF in the recovery of the OE after injury in vivo. We developed an experimental design in which the OE of Rhinella arenarum tadpoles could be easily and selectively damaged by immersing the animals in ZnSO(4) solutions of various concentrations for differing time periods. Image analysis of histological sections showed that different combinations of each of these conditions produced statistically different degrees of injury to the olfactory tissue. We also observed that the morphology of the OE was restored within a few days of recovery after ZnSO(4) treatment. Immunohistochemical analysis of BDNF was performed with an antiserum whose specificity was confirmed by Western blotting, and which showed drastic changes in the abundance and distribution pattern of this neurotrophin in the damaged olfactory system. Our results thus suggest that BDNF is involved in the regeneration of the OE of amphibian larvae, and that our approach is suitable for further investigations of this topic.

Effect of Ninjin-yoei-to (Rensheng-Yangrong-Tang) on olfactory behavior after olfactory nerve transection

Ninjin-yoei-to (NYT), a Japanese traditional medicine, is used to treat athrepsia due to surgery, anorexia, cold constitution, and anemia. There are reports of the effects of NYT on the nervous system; however, there have been no behavioral studies of the effect of NYT on olfactory function. The olfactory system undergoes continuous replacement of sensory neurons. Morphologic and behavioral studies have shown that the olfactory system recovers after bilateral olfactory nerve transection (BNX). However, in the humans, olfactory function does not always recover. In this study, we examined the effect of oral NYT on behavioral recovery after BNX. Fourteen mice were subjected to BNX. The regular diet was mixed with 2% NYT (NYT diet). Mice were separated into two groups; seven mice were fed the regular diet (control group), and seven mice were fed the NYT diet (NYT group). NYT was administered beginning 7 days prior to BNX and continuing for 35 days after BNX. Mice in both groups had free access to food and water. Olfactory function was evaluated by testing each mouse's ability to avoid cotton balls treated with acetic acid. After BNX, mice lost their ability to avoid cotton balls treated with acetic acid. In the control group, the time for behavioral recovery after BNX was 28 days. In the NYT group, the time for behavioral recovery after BNX was 21 days. NYT hastened behavioral recovery after BNX. NYT may have therapeutic benefits for patients with olfactory disorders.

Exuberant neuronal convergence onto reduced taste bud targets with preservation of neural specificity in mice overexpressing neurotrophin in the tongue epithelium

A mouse fungiform taste bud is innervated by only four to five geniculate ganglion neurons; their peripheral fibers do not branch to other buds. We examined whether the degree or specificity of this exclusive innervation pattern is influenced by brain-derived neurotrophic factor (BDNF), a prominent lingual neurotrophin implicated in taste receptoneural development. Labeled ganglion cells were counted after injecting single buds with different color markers in BDNF-lingual-overexpressing (OE) mice. To evaluate the end-organs, taste buds and a class of putative taste receptor cells were counted from progeny of BDNF-OE mice crossbred with green fluorescent protein (GFP) (gustducin) transgenic mice. Fungiform bud numbers in BDNF-OE mice are 35%, yet geniculate neuron numbers are 195%, of wild-type mice. Neurons labeled by single-bud injections in BDNF-OE animals were increased fourfold versus controls. Injecting three buds, each with different color markers, resulted in predominantly single-labeled ganglion cells, a discrete innervation pattern similar to controls. Thus, hyper-innervation of BDNF-OE buds involves many neurons innervating single buds, not increased fiber branching. Therefore, both wild-type and BDNF-OE mice exhibit, in fungiform buds, the same, "discrete" receptoneural pattern, this despite dramatic neurotrophin overexpression-related decreases in bud numbers and increases in innervation density. Hyperinnervation did not affect GFP positive cell numbers; proportions of GFP cells in BDNF-OE buds were the same as in wild-type mice. Total numbers of ganglion cells innervating buds in transgenic mice are similar to controls; the density of taste input to the brain appears maintained despite dramatically reduced receptor organs and increased ganglion cells.

Target regulation of V2R expression and functional maturation in vomeronasal sensory neurons in vitro

Vomeronasal receptors from the V1R and V2R gene families mediate the detection of chemical stimuli such as pheromones via the vomeronasal organ (VNO). The differential expression of vomeronasal receptors might contribute in part to a variety of pheromonal effects, which are different sexually, developmentally and even individually. However, little is known about the mechanisms controlling vomeronasal receptor expression. Cultured vomeronasal sensory neurons (VSNs) bear phenotypic resemblance to the intact VNO but they remain immature. Because indices of VSN maturation are increased by coculture with the target cells for VSNs, accessory olfactory bulb (AOB) neurons, AOB neurons may regulate vomeronasal receptor expression and functional maturation in VSNs. To test this hypothesis, we examined the expression of V2R-type vomeronasal receptors (VR1 and VR4) and chemosensory responsiveness in VNOs cocultured with AOB neurons. Immunoblot and immunocytochemical analysis revealed that the coculture of VNOs with AOB neurons resulted in a greater expression of VR1 and VR4 after 10 days than VNOs cultured alone. Moreover, calcium imaging analysis showed that cocultured VNOs responded to urine components applied iontophoretically into their cavities with a time course similar to the V2R expression, in contrast to singly cultured VNOs that displayed no response. These results demonstrate that AOB neurons induce the expression of vomeronasal receptors in VSNs, allowing them to function.

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.

Glial cell line-derived neurotrophic factor promotes olfactory ensheathing cells migration

Olfactory ensheathing cells (OECs) are a unique type of macroglia with axonal growth-promoting properties. The migrating ability of OECs in CNS is essential for neural regeneration. However, little is known about the extracellular and intracellular factors that regulate OEC migration. In the present study, we examined the effects of glial cell line-derived neurotrophic factor (GDNF) on OECs migration. Initially, the "scratch" migration assay, Boyden chamber assay, and explant migration assay showed that GDNF could promote OECs migration in vitro. Treatment of OECs with GDNF also induced cytoskeleton reorganization and up-regulated expression of cytoskeleton proteins. GDNF-induced OECs migration was demonstrated depending on GFRalpha-1 and Ret receptor, and activation of JNK and Src signaling cascades. Furthermore, GDNF was found to promote implanted OECs migration in a spinal cord hemisection injury model. Together, we report, to our knowledge for the first time, that GDNF stimulate OECs migration in vitro and in vivo.

Transient expression of tyrosine hydroxylase promoter/reporter gene constructs in the olfactory epithelium of transgenic mice

Maturation and survival of olfactory receptor neurons (ORNs) are hypothesized to depend on trophic support from the olfactory bulb during both development and regeneration of the olfactory epithelium (OE). The current study characterized transgene expression in two independently derived transgenic mouse lines in which 9 kb of tyrosine hydroxylase (TH) promoter was utilized to drive either enhanced green fluorescent protein (TH/eGFP) or LacZ (TH/beta-gal) reporters. Transgene expression, found primarily on dorsal aspects of the OE, the dorsal septum and endoturbinate II, resembled the Zone one distribution of olfactory receptor genes. Labeled cells were ovoid to fusiform with dendrites that projected to the epithelial surface but only rarely exhibited discernable cilia. Axons were short and did not extend beyond the basal lamina. As only a subpopulation of the cells contained olfactory marker protein, indicative of ORN maturation, the transgene expressing cells were likely immature neuronal precursors. Demonstration of transgene expression without either TH mRNA or protein was consistent with low basal level transcriptional activity of endogenous TH that may reflect differences between TH and reporter protein stability. Molecules identifying specific olfactory-derived cell populations, PDE2 and LHRH, also did not co-localize with either reporter. A higher than predicted proportion of apoptotic neonatal transgene-expressing cells accounted for their apparent paucity in adult mice. These studies support the concept that transgene expressing cells exhibiting morphological and biochemical characteristics of presumptive ORNs are unable to mature and undergo apoptotic cell death possibly because they lack trophic support.

Effects of adenoviral vector-mediated BDNF expression on the bulbectomy-induced apoptosis of olfactory receptor neurons

The expression of adenoviral vector (Ad)-mediated lacZ and brain-derived neurotrophic factor (BDNF) in mouse olfactory epithelium (OE) was examined, and the effect of BDNF on the survival of the bulbectomized OE was evaluated. A recombinant adenovirus, Ax1CAlacZ, was administrated into the mouse OE after bulbectomy, and the expression of a transferred E. coli beta-galactosidase (beta-gal) gene was confirmed by X-gal staining. The expression and effects of exogenous BDNF in the OE after bulbectomy were examined using immunohistochemistry and the TUNEL method. The adenoviral vector-mediated expression of beta-gal in the mouse OE was detectable for up to 14 days after bulbectomy in vivo. The Ad-mediated expression of BDNF was also observed in the OE after bulbectomy. Exogenously induced BDNF suppressed the degenerative changes of bulbectomized OE. TUNEL staining indicated that the exogenous BDNF enhanced the survival of the bulbectomized OE by inhibiting apoptosis. Ad-mediated expression of BDNF in the mouse nasal mucosa alleviated degenerative changes in bulbectomized OE. Ad-mediated transfer of neurotrophic factors might be applicable in the treatment of olfactory disorders.

Immunolocalization of retinoic acid receptors in the mammalian olfactory system and the effects of olfactory denervation on receptor distribution

All-trans retinoic acid (ATRA), a metabolite of vitamin A, binds to retinoic acid receptors (RARs) to mediate gene transcription in target cells. We previously found that an ATRA supplement enhanced olfactory recovery rate in adult mice after olfactory bulb deafferentation. In this study, we examined the cellular localization of RARalpha, RARbeta, and RARgamma and the effects of surgery and ATRA treatment using immunocytochemistry. Mice received a left olfactory nerve transection with the right side serving as internal control. One day after surgery, the mice were given either ATRA mixed with sesame oil or just sesame oil. In the unoperated olfactory bulb, only RARalpha immunoreactivity (ir) was observed. In the unoperated right olfactory epithelium, RARalpha-ir was found in flask-shaped cells located in the supporting cell layer, in cell clusters above the basal cell layer, in cells in the lamina propria, in some respiratory cells and in the olfactory bulb. The flask-shaped cells did not immunostain for either neurons or sustentacular cells. RARbeta-ir was localized only in the respiratory cells while no RARgamma-ir was observed in the olfactory epithelium. The density of RARalpha-ir cells was higher in the operated left olfactory epithelium and highest after ATRA treatment. This study demonstrates the presence of RARs in the olfactory system, provides additional support that the ATRA-signaling pathway may be involved in the recovery of the olfactory epithelium after injury, and suggests a role for an unstudied cell type in that process.

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.

Atrial natriuretic peptide type C induces a cell-cycle switch from proliferation to differentiation in brain-derived neurotrophic factor- or nerve growth factor-primed olfactory receptor neurons

With the discovery of postnatal stem cells within the brain, it has become important to understand how extracellular factors might affect the maturation of neuronal precursors in the postnatal brain. Neurotrophic factors are known to play a role in neuronal development but display pleiotrophic effects, in part because of their physiological interactions with other factors. One factor positioned to interact with neurotrophins in the brains of postnatal animals is atrial C-type natriuretic peptide (CNP). In this study, we used olfactory receptor neurons (ORNs) as a model, because their precursors demonstrate the most robust and functional postnatal neurogenesis of those systems thus far described. We examined the effects of brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) and the interactions of these neurotrophins and CNP in postnatal olfactory neuronal precursors. Results obtained using mice with targeted deletion of the gene for BDNF indicated that BDNF is a neuroproliferation-inducing and survival factor for ORN precursors. These roles were confirmed in vitro using primary cultures of ORNs. NGF was found to be a proliferation-inducing factor but not a survival factor. The addition of CNP to either BDNF- or NGF-treated neuronal precursors resulted in an inhibition of proliferation and the promotion of maturation. These effects were accompanied by changes in cell-cycle proteins that suggest possible mechanisms for these effects. Thus, CNP may function in the postnatal brain to regulate the exit from the cell cycle in neuronal precursor cells.

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.

Reconstructing smell

Odorant signal transduction and neurogenesis are fundamental properties of the olfactory epithelium. Many preparations have been used to elucidate some of the mechanisms underlying these properties. In this article, we briefly review these research areas and describe some of the techniques used to obtain the data. We focus specifically on the cell-culture paradigm and the data obtained from various immortal cell lines in their attempts to reconstruct the olfactory epithelium in vitro.

Isolation and in vitro differentiation of conditionally immortalized murine olfactory receptor neurons

Two major challenges exist in our understanding of the olfactory system. One concerns the enormous combinatorial code underlying odorant discrimination by odorant receptors. The other relates to neurogenesis and neuronal development in the olfactory epithelium. To address these issues, continuous cell cultures containing olfactory receptor neurons (ORNs) were obtained from olfactory epithelia of H-2K(b)-tsA58 transgenic mice. ORNs were detected and characterized by immunocytochemistry, RT-PCR, and Western blot for the markers Galpha(olf), adenylyl cyclase III, the olfactory cyclic nucleotide-gated channel subunits, and olfactory marker protein. In culture, epidermal growth factor and nerve growth factor stimulated proliferation, and brain-derived neurotrophic factor and neurotrophin-3 induced cellular maturation. Clonal cell lines were isolated by fluorescence-activated cell sorting with anti-neural cell adhesion molecule antibodies, and of 144 single cells plated, 39 clones were expanded, propagated, and stored in liquid nitrogen. All attempts at recovery of clonal lines from frozen stocks have been successful. The most thoroughly characterized clone, 3NA12, expressed ORN markers and responded to stimulation by single odorants. Each odorant activated approximately 1% of cells in a clonal line, and this suggests that many different odorant receptors may be expressed by these clonal cells. Therefore, these cell lines and the method by which they have been obtained represent a significant advance in the generation of olfactory cell cultures and provide a system to investigate odorant coding and olfactory neurogenesis.

Olfactory receptor neurons exist as distinct subclasses of immature and mature cells in primary culture

The processes of neuronal differentiation and survival are key questions in neurobiology. The olfactory system possesses unique regenerative capacity, as its neurons are continually replaced throughout adulthood from a maintained population of precursor cells. Primary cultures of olfactory epithelium enriched in olfactory neurons would provide a useful model to study the processes of neurogenesis, differentiation and senescence. To determine whether immature olfactory neurons could be isolated in primary culture and to investigate the mechanisms underlying these processes, culture conditions which selectively favored the presence of immature olfactory neurons were optimized. Using low plating densities, a population of cells was identified which, by reverse transcription-polymerase chain reaction, demonstrated messages for olfactory neuronal markers, including Golf, olfactory cyclic nucleotide-gated channel and olfactory marker protein, as well as the p75 low-affinity nerve growth factor receptor. Immunocytochemical analysis showed that these putative immature olfactory neurons possessed immunoreactivity to G(olf), neuron-specific tubulin, neural cell adhesion molecule, synaptophysin and neurofilament. These neurons were defined as olfactory receptor neuron-1 cells. Under these conditions, a separate class of rarely occurring cells with different morphology demonstrated immunoreactivity to mature markers, such as adenylyl cyclase III and olfactory marker protein. Electrophysiologically, these cells displayed properties consistent with those of acutely dissociated olfactory receptor neurons. Another class of rarer cells which represented less than 2% of cells in culture demonstrated immunoreactivity to glial fibrillary acidic protein. These cultures can serve as a model for in vitro analysis of olfactory receptor neuronal development and maintenance, and provide a potential substrate for the development of cell lines.