Cell death in the olfactory epithelium

Olfactory dysfunction in passive vs active smoking

BACKGROUND

The aim of this study is to assess the olfactory functions of passive smokers compared to active smokers and nonsmokers.

METHODS

This prospective case-control study included 30 nonsmokers, 30 passive smokers, and 30 active smoker participants. All groups were matched for gender and age. The Sino-Nasal Outcome Test 22 (SNOT-22) and Sniffin' Sticks test battery were administered to all subjects. Threshold (T), discrimination (D), and identification (I) scores were noted. Olfactory function was subjectively assessed as 0: severe dysfunction and 5: no problem.

RESULTS

Overall, TDI scores of active smokers (24.78 ± 3.02) and passive smokers (24.90 ± 2.45) were significantly lower than nonsmokers (34.23 ± 3.46). There was no statistically significant difference between passive smokers and smokers (F (2,87) = 13.47, P < .001)). All subscores are negatively affected by active or passive smoking. The greatest impact of smoking was on threshold scores (η 2 T  = 0.719), followed by identification (η 2 I  = 0.353) and discrimination (η 2 D  = 0.282) scores. SNOT-22 and TDI scores were weakly (r = -.352) correlated as subjective assessment, and TDI scores were moderately correlated (r: .539) (P values < .001). Age and pack-years cigarette dosage had a negative effect on the TDI score (TDI = 26.386 - (0.084 × age) - (0.072 × Pack.Year)) according to stepwise linear regression model (F = 10.187; P = .001).

CONCLUSIONS

Passive smoking has nearly the same adverse effect on olfactory function as active smoking. The threshold scores are the most negatively affected. The olfactory effect of cigarette smoke may not be directly related to nasal inflammation. Olfactory neuronal pathways should be investigated to elucidate the exact pathophysiology.

LEVEL OF EVIDENCE

3b.

Development of the human olfactory system

This chapter focuses on the development of the human olfactory system. In this system, function does not require full neuroanatomical maturity. Thus, discrimination of odorous molecules, including a number within the mother's diet, occurs in amniotic fluid after 28-30 weeks of gestation, at which time the olfactory bulbs are identifiable by MRI. Hypoplasia/aplasia of the bulbs is documented in the third trimester and postnatally. Interestingly, olfactory axons project from the nasal epithelium to the telencephalon before formation of the olfactory bulbs and lack a peripheral ganglion, but the synaptic glomeruli of the future olfactory bulb serves this function. Histologic lamination of the olfactory bulb is present by 14 weeks, but maturation remains incomplete at term for neuronal differentiation, synaptogenesis, myelination, and persistence of the normal transitory fetal ventricular recess. Myelination occurs postnatally. Although olfaction is the only sensory system without direct thalamic projections, the olfactory bulb and anterior olfactory nucleus are, in effect, thalamic surrogates. For example, many dendro-dendritic synapses occur within the bulb between GABAergic granular neurons and periglomerular neurons. Moreover, bulbar synaptic glomeruli are analogous to peripheral ganglia of other sensory cranial nerves. The olfactory tract contains much gray as well as white matter. The olfactory epithelium and bulb both incorporate progenitor cells at all ages. Diverse malformations of the olfactory bulb can be detected by clinical examination, imaging, and neuropathology; indeed, olfactory reflexes of the neonate can be reliably tested. We recommend that such testing be routine in the neonatal neurologic examination, especially in children with brain malformations, endocrinopathies, chromosomopathies, genetic/metabolic disorders, and perinatal hypoxic/ischemic encephalopathy.

Olfactory Development, Part 2: Neuroanatomic Maturation and Dysgeneses

Olfactory axons project from nasal epithelium to the primitive telencephalon before olfactory bulbs form. Olfactory bulb neurons do not differentiate in situ but arrive via the rostral migratory stream. Synaptic glomeruli and concentric laminar architecture are unlike other cortices. Fetal olfactory maturation of neuronal differentiation, synaptogenesis, and myelination remains incomplete at term and have a protracted course of postnatal development. The olfactory ventricular recess involutes postnatally but dilates in congenital hydrocephalus. Olfactory bulb, tract and epithelium are repositories of progenitor stem cells in fetal and adult life. Diverse malformations of the olfactory bulb can be diagnosed by clinical examination, imaging, and neuropathologically. Cellular markers of neuronal differentiation and synaptogenesis demonstrate immaturity of the olfactory system at birth, previously believed by histology alone to occur early in fetal life. Immaturity does not preclude function.

Maturation and Dysgenesis of the Human Olfactory Bulb

The olfactory bulb with its unique architecture was studied for neuronal maturation in human fetuses. Neuroblasts stream into the olfactory bulb from the rostral telencephalon and secondarily migrate radially. The transitory olfactory ventricular recess regresses postnatally. Olfactory is the only sensory system without thalamic projections but incorporates intrinsic thalamic equivalents. The bulb is a repository of progenitor cells. Maturation of the bulb and tract was studied in 18 normal human fetuses of 16-41 weeks gestation; mid-gestational twins with hydrocephalus; 7 arrhinencephaly/holoprosencephaly; 2 olfactory dysgeneses. Multiple immunoreactivities were performed. Synaptophysin around mitral neurons, in a few synaptic glomeruli and concentric lamination of the outer granular layer, was seen at 16 weeks. Outer granular neurons exhibited NeuN at 16 weeks, only 2/3 were reactive at term. Concentric alternating sheets of granular neurons and their dendrodendritic synapses are seen during maturation. Calretinin reactivity is seen in neurons and neurites, primary olfactory nerve axons, periglomerular cells and neuroepithelial cells surrounding the ventricular recess; reactivity occurs later in synaptic glomeruli than with synaptophysin; not all glomeruli are strongly reactive even at term. Nestin- and vimentin-reactive bipolar progenitor cells were demonstrated at all ages and extend into the olfactory tract. Myelin is demonstrated by Luxol fast blue (LFB) only postnatally. In hydrocephalus, the olfactory recess is dilated. Mitral cell dispersion, disrupted glomeruli, heterotopia and maturational delay are seen in some dysgeneses. Malformations exhibit unique findings. Fusion of hypoplastic bulbs can occur. Abnormal architecture is seen in hemimegalencephaly. More documentation of olfactory dysgenesis is needed in other major brain malformations.

Histological and lectin histochemical studies on the olfactory mucosae of the Korean roe deer, Capreolus pygargus

The morphological features of the olfactory mucosae of Korean roe deer, Capreolus pygargus, were histologically studied using the ethmoid turbinates containing the olfactory mucosae from six roe deer (male, 2-3 years old). The ethmoid turbinates were embedded in paraffin, and histochemically evaluated in terms of the mucosal characteristics. Lectin histochemistry was performed to investigate the carbohydrate-binding specificity on the olfactory mucosa. Lectins, including Triticum vulgaris wheat germ agglutinin (WGA), Ulex europaeus agglutinin I (UEA-I), and soybean agglutinin (SBA) were used for the N-acetylglucosamine, fucose and N-acetylgalactosamine carbohydrate groups, respectively. Histologically, the olfactory mucosa, positioned mainly in the caudal roof of the nasal cavity, consisted of the olfactory epithelium and the lamina propria. The olfactory epithelium consisted of protein gene product (PGP) 9.5-positive olfactory receptor cells, galectin-3-positive supporting cells and basal cells. Bowman's glands in the lamina propria were stained by both the periodic acid Schiff reagent and alcian blue (pH 2.5). Two types of lectin, WGA and SBA, were labeled in free border, receptor cells, supporting cells and Bowman's glands, with the exception of basal cells, while UEA-I was labeled in free border, supporting cells and Bowman's glands, but not in receptor cells and basal cells, suggesting that carbohydrate terminals on the olfactory mucosae of roe deer vary depending on cell type. This is the first morphological study of the olfactory mucosa of the Korean roe deer to evaluate carbohydrate terminals in the olfactory mucosae.

Cellular and molecular mechanisms regulating embryonic neurogenesis in the rodent olfactory epithelium

Mechanisms that regulate cellular differentiation in developing embryos are maintained across multiple physiological systems, including the nervous system where neurons and glia are generated. The olfactory epithelium, which arises from the olfactory pit, is a stratified tissue in which the stepwise generation of neurons and support cells can easily be assessed and followed during embryogenesis and throughout adulthood. During olfactory epithelium morphogenesis, progenitor cells respond to factors that control their proliferation, survival, and differentiation in order to generate olfactory receptor neurons that detect odorants in the environment and glia-like sustentacular cells. The tight temporal regulation of expression of proneural genes in dividing progenitor cells, including Mash-1, Neurogenin-1, and NeuroD1, plays a central role in the production of olfactory receptor neurons. Multiple factors that either positively or negatively affect the generation of olfactory receptor neurons have been identified and shown to impinge on the transcriptional regulatory network in dividing progenitor cells. Several growth factors, such as FGF-8, act to promote neurogenesis by ensuring survival of progenitor cells that will give rise to olfactory receptor neurons. In contrast, other molecules, including members of the large TGF-β family of proteins, have negative impacts on neurogenesis by restricting progenitor cell proliferation and stalling their differentiation. Since recent reviews have focused on neurogenesis in the regenerating adult olfactory epithelium, this review describes neurogenesis at embryonic stages of olfactory epithelium development and summarizes our current understanding of how both cell intrinsic and extrinsic factors control this process.

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.

Insulin but not leptin protects olfactory mucosa from apoptosis

The mammalian olfactory mucosa (OM) is continually renewed throughout life. Owing to their position in the nasal cavity, OM cells are exposed to multiple insults, including high levels of odourants that can induce their death. OM regeneration is therefore essential to maintain olfactory function, and requires the tight control of both cell death and proliferation. Apoptosis has been implicated in OM cell death. Olfaction is one of the senses involved in food intake and depends on individual nutritional status. We have previously reported the influence of hormones related to nutritional status on odour perception and have shown that the OM is a target of insulin and leptin, two hormones known for their anti-apoptotic properties. In the present study, we investigated the potential anti-apoptotic effect of these metabolic hormones on OM cells. Both Odora cells (an olfactive cell line) and OM cells treated with etoposide, a p53 activity inducer, exhibited mitochondrial-dependent apoptosis that was inhibited by the pan-caspase inhibitor zVAD-fmk. Insulin, but not leptin, impaired this apoptotic effect. Insulin addition to the culture medium reduced p53 phosphorylation, caspase-3 and caspase-9 cleavage, and caspase-3 enzymatic activity induced by etoposide. The apoptotic wave observed in the OM after interruption of the neuronal connections between the OM and the olfactory bulb by bulbectomy was impaired by intranasal insulin treatment. These findings suggest that insulin may be involved in OM cellular dynamics, through endocrine and/or paracrine-autocrine effects of circulating or local insulin, respectively.

Cell proliferation and apoptosis in the olfactory epithelium of the shark Scyliorhinus canicula

To date, no study has been published on cell renewal in the olfactory epithelium of Chondrichthyes. Our work aimed at detecting proliferating cells (by Proliferating Cell Nuclear Antigen - PCNA immunohistochemistry) and apoptotic cells (by terminal uridine deoxynucleotidyl transferase nick end labeling method) in the olfactory epithelium of the shark Scyliorhinus canicula. PCNA immunoreactivity and mitotic figures were localized almost exclusively at the basal and apical thirds of the epithelial thickness. Double immunofluorescence for PCNA and OMP (a marker of mature olfactory neurons) showed that PCNA immunoreactivity is lacking in mature olfactory neurons, with the exception of crypt neurons. Crypt neurons, a cell type peculiar to fish, often showed PCNA immunoreactivity in the nucleus and may be involved in repair processes. The role of PCNA in mature crypt neurons requires further investigation to be clarified. Apoptosis was observed in sensory neurons and in basal cells. Our data highlight the presence of cell proliferation at different levels within the epithelium and the occurrence of apoptosis in both mature and proliferating cells.

Adult neurogenesis and neuronal regeneration in the brain of teleost fish

Whereas adult neurogenesis appears to be a universal phenomenon in the vertebrate brain, enormous differences exist in neurogenic potential between "lower" and "higher" vertebrates. Studies in the gymnotiform fish Apteronotus leptorhynchus and in zebrafish have indicated that the relative number of new cells, as well as the number of neurogenic sites, are at least one, if not two, orders of magnitude larger in teleosts than in mammals. In teleosts, these neurogenic sites include brain regions homologous to the mammalian hippocampus and olfactory bulb, both of which have consistently exhibited neurogenesis in all species examined thus far. The source of the new cells in the teleostean brain are intrinsic stem cells that give rise to both glial cells and neurons. In several brain regions, the young cells migrate, guided by radial glial fibers, to specific target areas where they integrate into existing neural networks. Approximately half of the new cells survive for the rest of the fish's life, whereas the other half are eliminated through apoptotic cell death. A potential mechanism regulating development of the new cells is provided by somatic genomic alterations. The generation of new cells, together with elimination of damaged cells through apoptosis, also enables teleost fish rapid and efficient neuronal regeneration after brain injuries. Proteome analysis has identified a number of proteins potentially involved in the individual regenerative processes. Comparative analysis has suggested that differences between teleosts and mammals in the growth of muscles and sensory organs are key to explain the differences in adult neurogenesis that evolved during phylogenetic development of the two taxa.

A novel role for jun N-terminal kinase signaling in olfactory sensory neuronal death

Olfactory sensory neurons (OSNs) represent a unique population of neurons in which death and regeneration are ongoing throughout adulthood, a feature that makes them an attractive model cell type for the investigation of neuronal death. However, the mechanism by which OSNs die remains elusive. Therefore, we developed a culture system for studying pathways involved in OSN death. Here, we show that inhibition of transcription or translation, by actinomycin D or cycloheximide, respectively, suppresses pathways leading to death, prolonging the survival of OSNs in culture. We discovered that caspase activity and jun N-terminal kinase (JNK) signaling both play a role in OSN death, and inhibition of JNK activity suppresses effector caspase (caspase-3) activation. Results from studies in culture were confirmed in vivo, in a mouse bulbectomy-induced OSN death model. These findings provide new insights into the nature of OSN death and a means of studying OSNs in vitro.

Derivation of an inhalation reference concentration based upon olfactory neuronal loss in male rats following subchronic acetaldehyde inhalation

Acetaldehyde inhalation induces neoplastic and nonneoplastic responses in the rodent nasal cavity. This experiment further characterizes the dose-response relationship for nasal pathology, nasal epithelial cell proliferation, and DNA-protein cross-link formation in F-344 rats exposed subchronically to acetaldehyde. Animals underwent whole-body exposure to 0, 50, 150, 500, or 1500 ppm acetaldehyde for 6 h/day, 5 days/wk for up to 65 exposure days. Respiratory tract histopathology was evaluated after 4, 9, 14, 30, and 65 exposure days. Acetaldehyde exposure was not associated with reduced body weight gain or other evidence of systemic toxicity. Histologic evaluation of the nasal cavity showed an increased incidence of olfactory neuronal loss (ONL) following acute to subchronic exposure to > or = 150 ppm acetaldehyde and increased olfactory epithelial cell proliferation following exposure to 1500 ppm acetaldehyde. The severity of the ONL demonstrated dose- and temporal-dependent behaviors, with minimal effects noted at 150-500 ppm acetaldehyde and moderately severe lesions seen in the highest exposure group, with increased lesion severity and extent as the exposure duration increased. Acetaldehyde exposure was also associated with inflammation, hyperplasia, and squamous metaplasia of the respiratory epithelium. These responses were seen in animals exposed to > or = 500 ppm acetaldehyde. Acetaldehyde exposure was not associated with increased DNA-protein cross-link formation in the respiratory or olfactory epithelium. A model of acetaldehyde pharmacokinetics in the nose was used to derive an inhalation reference concentration (RfC) of 0.4 ppm, based on the no-observed-adverse-effect level (NOAEL) of 50 ppm for the nasal pathology seen in this study.

Apoptosis and the insulin-like growth factor family in the developing olfactory epithelium

Vertebrate olfactory receptor neurons (ORN) are unique in that they are continually replaced throughout life. They die by apoptosis under physiological conditions at all stages during the life cycle, and apoptotic ORN are replaced by their progenitor cells. Apoptosis is linked with neurogenesis, of which pathway is regulated by a number of growth factors and neurotrophic factors. Members of the insulin-like growth factor (IGF) family have an anti-apoptotic effect on ORN, in addition to their ability to promote the proliferation, differentiation, and survival of these neurons. Expression of IGF and related molecules at both mRNA and protein levels in the olfactory epithelium have been reported. In this review article, we focus on apoptosis, IGF, and their related molecules in the developing olfactory epithelium.

Taurine deficiency and apoptosis: findings from the taurine transporter knockout mouse

Apoptosis is characterized by cell shrinkage, nuclear condensation, DNA-fragmentation and apoptotic body formation. Compatible organic osmolytes, e.g. taurine, modulate the cellular response to anisotonicity and may protect from apoptosis. Taurine transporter knockout mice (taut-/- mice) show strongly decreased taurine levels in a variety of tissues. They develop clinically important age-dependent diseases and some of them are characterized by apoptosis. Increased photoreceptor apoptosis leads to blindness of taut-/- mice at an early age. The taurine transporter may not be essential for the differentiation of photoreceptor cells, but many mature cells do not survive without an intact taurine transporter. The olfactory epithelium of taut-/- mice also exhibits structural and functional abnormalities. When compared with wild-types, taut-/- mice have a significantly higher proliferative activity of immature olfactory receptor neurons and an increased number of apoptotic cells. This is accompanied by electrophysiological findings indicating a reduced olfactory sensitivity. Furthermore, taut-/- and taut+/- mice develop moderate unspecific hepatitis and liver fibrosis beyond 1 year of age where hepatocyte apoptosis and activation of the CD95 system are pronounced.

Phenotype of the Taurine Transporter Knockout Mouse

This chapter reports present knowledge on the properties of mice with disrupted gene coding for the taurine transporter (taut-/- mice). Study of those mice unraveled some of the roles of taurine and its membrane transport for the development and maintenance of normal organ functions and morphology. When compared with wild-type controls, taut-/- mice have decreased taurine levels in skeletal and heart muscle by about 98%, in brain, kidney, plasma, and retina by 80 to 90%, and in liver by about 70%. taut-/- mice exhibit a lower body mass as well as a strongly reduced exercise capacity compared with taut+/- and wild-type mice. Furthermore, taut-/- mice show a variety of pathological features, for example, subtle derangement of renal osmoregulation, changes in neuroreceptor expression, and loss of long-term potentiation in the striatum, and they develop clinically relevant age-dependent disorders, for example, visual, auditory, and olfactory dysfunctions, unspecific hepatitis, and liver fibrosis. Taurine-deficient animal models such as acutely dietary-manipulated foxes and cats, pharmacologically induced taurine-deficient rats, and taurine transporter knockout mouse are powerful tools allowing identification of the mechanisms and complexities of diseases mediated by impaired taurine transport and taurine depletion (Chapman et al., 1993; Heller-Stilb et al., 2002; Huxtable, 1992; Lake, 1993; Moise et al., 1991; Novotny et al., 1991; Pion et al., 1987; Timbrell et al., 1995; Warskulat et al., 2004, 2006b). Taurine, which is the most abundant amino acid in many tissues, is normally found in intracellular concentrations of 10 to 70 mmol/kg in mammalian heart, brain, skeletal muscle, liver, and retina (Chapman et al., 1993; Green et al., 1991; Huxable, 1992; Timbrell et al., 1995). These high taurine levels are maintained by an ubiquitous expression of Na(+)-dependent taurine transporter (TAUT) in the plasma membrane (Burg, 1995; Kwon and Handler, 1995; Lang et al., 1998; Liu et al., 1992; Ramamoorthy et al., 1994; Schloss et al., 1994; Smith et al., 1992; Uchida et al., 1992; Vinnakota et al., 1997; Yancey et al., 1975). Taurine is not incorporated into proteins. It is involved in cell volume regulation, neuromodulation, antioxidant defense, protein stabilization, stress responses, and via formation of taurine-chloramine in immunomodulation (Chapman et al., 1993; Green et al., 1991; Huxtable, 1992; Timbrell et al., 1995). On the basis of its functions, taurine may protect cells against various types of injury (Chapman et al., 1993; Green et al., 1991; Huxtable, 1992; Kurz et al., 1998; Park et al., 1995; Stapleton et al., 1998; Timbrell et al., 1995; Welch and Brown, 1996; Wettstein and Häussinger, 1997). In order to examine the multiple taurine functions, murine models have several intrinsic advantages for in vivo research compared to other animal models, including lower cost, maintenance, and rapid reproduction rate. Further, experimental reagents for cellular and molecular studies are widely available for the mouse. In particular, mice can be easily genetically manipulated by making transgene and knockout mice. This chapter focuses on the phenotype of the TAUT-deficient murine model (taut-/-; Heller-Stilb et al., 2002), which may help researchers elucidate the diverse roles of taurine in development and maintenance of normal organ functions and morphology.

Recovery of the olfactory receptor neurons in the African Tilapia mariae following exposure to low copper level

Low levels of Cu(2+) are known to specifically cause olfactory neuron death in fish olfactory epithelium. This study investigated the morphological changes in the olfactory mucosa of the cichlid Tilapia mariae, after a 4-day exposure to different concentrations of Cu(2+) (20, 40 and 100 microg/l), and the regeneration time-frame, when fish exposed to 20 microg/l were returned to dechlorinated tap water. Light microscopy, combined with Fluoro Jade-B staining, permitted the observation of a dose-dependent damage which became less severe and more circumscribed to receptor cells when Cu(2+) concentration decreased. The regeneration process in the olfactory tissue was examined in fish after 0, 3, and 10 days of recovery in well water. Immunostaining with PCNA showed a massive mitotic activity in the basal region of the mucosa immediately after exposure was terminated. The mitotically produced elements were immature neurons since they expressed the neural growth-associated phosphoprotein GAP-43. After 3 days of recovery the nuclei had already completed their migration to the upper portion of the epithelium and mitotic activity was much less intensive. After 10 days the olfactory tissue did not present differences when compared to the control tissue. These results suggest that after 10 days the regeneration is completed and the integrity of the tissue restored.

Fine structural aspects of apoptosis in the olfactory epithelium

Vertebrate olfactory receptor neurons are unique because they are continually replaced throughout life. They die by apoptosis under physiological conditions at all stages in their life cycle, and the dead olfactory neurons are replaced by the progeny of dividing basal cells. Thus, in the olfactory epithelium apoptosis is involved in tissue homeostasis and may be a direct or indirect trigger of neurogenesis. In this study, we focused on morphological changes occurring in the olfactory epithelium, i.e., degradation of DNA, condensation of nuclear chromatin, condensation of cytoplasm, blebbing of cytoplasmic fragments, and disposal of the dying and dead cells as the final phase of apoptosis. Moreover, we addressed other stages of apoptosis examining the nature of the stimulus that provokes the apoptotic response, the signal or metabolic state, and transduction of the signal that sends the message to the effector apparatus, and the effector or execution phase, which includes the activation of proteases.

Chronic sinusitis and olfactory dysfunction

Chronic rhinosinusitis encompasses a group of disorders characterized by inflammation of the mucosa of the nose and paranasal sinuses of at least 12 weeks' duration. In addition to nasal obstruction and discharge, chronic sinusitis is a common cause of olfactory dysfunction. Smell loss can result in problems including safety concerns, hygiene matters, appetite disorders, and changes in emotional and sexual behavior. Although smell loss related to sinonasal disease is probably the most treatable form of olfactory dysfunction and treatment can improve olfactory sensation in the setting of sinusitis, most studies show that the effects are usually transient and incomplete.

Pathology of the Olfactory Epithelium: Smoking and Ethanol Exposure

OBJECTIVE

To investigate the effects of tobacco smoke on the olfactory epithelium. Cigarette smoking has been associated with hyposmia; however, the pathophysiology is poorly understood. The sense of smell is mediated by olfactory sensory neurons (OSNs) exposed to the nasal airway, rendering them vulnerable to environmental injury and death. As a consequence, a baseline level of apoptotic OSN death has been demonstrated even in the absence of obvious disease. Dead OSNs are replaced by the mitosis and maturation of progenitors to maintain sufficient numbers of neurons into adult life. Disruption of this balance has been suggested as a common cause for clinical smell loss. This current study will evaluate the effects of tobacco smoke on the olfactory mucosa, with emphasis on changes in the degree of OSN apoptosis.

STUDY DESIGN

A rat model was used to assess the olfactory epithelium after exposure to tobacco smoke.

METHODS

Rats were exposed to tobacco smoke alone (for 12 weeks), smoke plus dietary ethanol (for the final 5 weeks), or to neither (control). Immunohistochemical analysis of the olfactory epithelium was performed using an antibody to the active form of caspase-3. Positive staining for this form of the caspase-3 enzyme indicates a cell undergoing apoptotic proteolysis.

RESULTS

Control rats demonstrated a low baseline level of caspase-3 activity in the olfactory epithelium. In contrast, tobacco smoke exposure triggered a dramatic increase in the degree of OSN apoptosis that affected all stages of the neuronal lineage.

CONCLUSIONS

These results support the following hypothesis: smell loss in smokers is triggered by increased OSN death, which eventually overwhelms the regenerative capacity of the epithelium.

Olfactory neurons in bax knockout mice are protected from bulbectomy-induced apoptosis

Surgical ablation of the olfactory bulb (bulbectomy) triggers a massive wave of apoptosis in mature olfactory sensory neurons within the olfactory epithelium. The aim of the current study was to determine if this process is dependent on expression of the pro-apoptotic protein Bax. Immunohistochemical detection of caspase-3 activation and olfactory epithelial thickness was used to demonstrate and quantify neuronal apoptosis in bax knockout and wild type mice, following bulbectomy. Caspase-3 activation and epithelial thinning were both reduced in the bax knockout mouse compared to the wild type mouse, at least up to 9 days post-bulbectomy, indicating that apoptosis was inhibited not just delayed. This study demonstrates that Bax plays a major role in olfactory neuron apoptosis following surgical deafferentation.

Obesity-inducing amygdala lesions: examination of anterograde degeneration and retrograde transport

Small lesions centered in the posterodorsal region of the medial amygdala resulted in excessive weight gains in female rats. Unilateral lesions were nearly as effective as bilateral lesions in the first 48 h after surgery (+21 to +32 g). Assessment of lesion damage was done by both qualitative evaluation and by a quantitative grid-point counting method. The critical sites for weight gain were the intra-amygdaloid bed nucleus of the stria terminalis and the posterodorsal medial amygdaloid nucleus. Incidental damage to the overlying globus pallidus was negatively related to weight gain. The cupric silver method for demonstrating axonal degeneration was applied to brains with obesity-inducing lesions. A dense pattern of degenerating terminals was found in the lateral septum, amygdala, ventral striatum, and ventromedial hypothalamus. Degeneration in the paraventricular nucleus of the hypothalamus was scarce or absent. Small retrograde tracer injections made in either the intra-amygdaloid bed nucleus of the stria terminalis or in the posterodorsal medial amygdaloid nucleus labeled cells in the amygdala, lateral septum, and hypothalamus, reciprocating the anterograde projections from the amygdala to these areas. The data suggest that subdivisions of the posterodorsal amygdala participate in the regulation of feeding in a manner that is similar to the better-known role of this part of the brain in mediating reproductive behavior. Although topographical differences may exist within the amygdaloid and hypothalamic subdivisions regulating these two sexually dimorphic behaviors, the relays engaged by feeding-related connections and those related to reproduction are remarkably parallel.

Vitamin A deficiency leads to increased cell proliferation in olfactory epithelium of mature rats

We have shown previously that vitamin A deficiency (VAD) leads to the decreased expression of gene products that are specifically synthesized by mature neurons in the olfactory epithelium (OE) of adult rats. These results support the hypothesis that retinoic acid, a derivative of vitamin A, is required for neurogenesis and neuron replacement in vivo. VAD does not cause gross degeneration of the OE, raising the question: what types of cells continue to populate VAD OE? In this study, we compared the cell densities of VAD and VA-sufficient (VAS) OE and investigated whether cell proliferation is upregulated in VAD OE. The results show that (1) total cell number in VAD and VAS OE are comparable; (2) localized areas of hyperplasia are present in the basal regions of VAD, but not VAS, OE; (3) there is a substantial increase in the number of PCNA (proliferating cell nuclear antigen) positive cells in the basal region of VAD OE relative to VAS OE; and (4) there is a relative increase in the levels of mRNA encoding the transcription factor, MASH I, in VAD OE. We conclude that reduced availability of vitamin A derivatives, such as retinoic acid, leads to a loss of control over proliferation, hyperplasia, and increased numbers of pro-neural cells in vivo.

Neural progenitor cells of the neonatal rat anterior subventricular zone express functional GABA(A) receptors

The interneurons of the olfactory bulb arise from precursor cells in the anterior part of the neonatal subventricular zone, the SVZa, and are distinctive in that they possess a neuronal phenotype and yet undergo cell division. To characterize the differentiation of neonatal SVZa progenitor cells, we analyzed the complement of ionotropic neurotransmitter receptors that they express in vitro. For this analysis, we tested the sensitivity of SVZa progenitor cells to gamma-amino-n-butyric acid (GABA), adenosine triphosphate (ATP), kainate, N-methyl-D-aspartate (NMDA), and acetylcholine (ACh) after 1 day in vitro. SVZa progenitor cells had chloride currents activated by GABA and muscimol, the GABA(A) receptor-specific agonist, but were insensitive to ATP, kainate, NMDA, and ACh. In addition, GABA- or muscimol-activated chloride currents were blocked nearly completely by 30 microM bicuculline, the GABA(A) receptor-specific antagonist, suggesting that GABA(B) and GABA(C) receptors are absent. Measurements of the chloride reversal potential by gramicidin-perforated patch clamp revealed that currents generated by activation of GABA(A) receptors were inward, and thus, depolarizing. A set of complementary experiments was undertaken to determine by reverse transcription and polymerase chain reaction (RT-PCR) whether SVZa progenitor cells express the messenger RNA (mRNA) coding for glutamic acid decarboxylase 67 (GAD67), used in the synthesis of GABA and for GABA(A) receptor subunits. Both postnatal day (P0) SVZa and olfactory bulb possessed detectable mRNA coding for GAD67. In P0 SVZa, the GABA(A) receptor subunits detected with RT-PCR included alpha 2-4, beta 1-3, and gamma 2S (short form). By comparison, the P0 olfactory bulb expressed all of the subunits detectable in the SVZa and additional subunit mRNAs: alpha 1, alpha 5, gamma 1, gamma 2L (long form), gamma 3, and delta subunit mRNAs. Antibodies recognizing GABA, GAD, and various GABA(A) receptor subunits were used to label SVZa cells harvested from P0-1 rats and cultured for 1 day. The cells were immunoreactive for GABA, GAD, and the GABA(A) receptor subunits alpha 2-5, beta 1-3, and gamma 2. To relate the characteristics of GABA(A) receptors in cultured SVZa precursor cells to particular combinations of subunits, the open reading frames of the dominant subunits detected by RT-PCR (alpha 2-4, beta 3, and gamma 2S) were cloned into a mammalian cell expression vector and different combinations were transfected into Chinese hamster ovary-K1 (CHO-K1) cells. A comparison of the sensitivity to inhibition by zinc of GABA(A) receptors in SVZa precursor cells and in CHO-K1 cells expressing various combinations of recombinant GABA(A) receptor subunits suggested that the gamma 2S subunit was present and functional in the GABA(A) receptor chloride channel complex. Thus, SVZa precursor cells are GABAergic and a subset of the GABA(A) receptor subunits detected in the olfactory bulb was found in the SVZa, as might be expected because SVZa progenitor cells migrate to the bulb as they differentiate.

Tumor necrosis factor-alpha-induced apoptosis in olfactory epithelium in vitro: possible roles of caspase 1 (ICE), caspase 2 (ICH-1), and caspase 3 (CPP32)

We investigated the potential roles of three members of the interleukin-1beta-converting enzyme (ICE) protease family (caspases) in apoptosis in olfactory epithelium. By RT-PCR analysis, the mRNAs of caspase 1 (ICE), caspase 2 (ICH-1), and caspase 3 (CPP32) were detected in olfactory mucosa obtained from normal adults, E19 fetuses, and unilaterally bulbectomized rats. The transcript of caspase 2 disappeared in bulbectomized animals 3 and 5 days postoperatively, but reappeared 21 days postoperatively. This suggests that most of the caspase 2 transcript was in olfactory sensory neurons. We used TNF-alpha to induce cell death in organotypic cultures of E19 olfactory epithelium and assayed the ability of three caspase inhibitors to reverse the TNF-alpha effect. After 6 h of treatment with medium containing TNF-alpha, a 2.5-fold increase in apoptotic body number was observed throughout the olfactory epithelium. Pretreatment of the cultures with either of two irreversible caspase inhibitors (Z-VAD-fmk, Ac-YVAD-cmk) for 4 h, followed by a 6-h treatment with TNF-alpha plus an inhibitor, blocked TNF-alpha-induced cell death completely. Pretreatment with a third caspase inhibitor (Z-DEVD-fmk) in the same treatment schedule reduced the numbers of apoptotic cells significantly but not to the same extent as Z-VAD-fmk or Ac-YVAD-cmk. Increasing the dose of any of the inhibitors reduced the numbers of apoptotic figures below those of control cultures, indicating that the inhibitory response is dose dependent. Taken together, the results suggest that caspases 1, 2, and 3, and perhaps others that are blocked by the inhibitors we used, participate in TNF-alpha-induced cell death in vitro.

Immunolocalization of caspase proteolysis in situ: evidence for widespread caspase-mediated apoptosis of neurons and glia in the postnatal rat brain

Activation of a member of the caspase family of cysteine proteases is thought to be required for the execution of apoptosis in neurons and other cell types. We describe here an antibody (Ab127) reactive with a neoantigenic site on caspase substrate proteins degraded during apoptosis, and its characterization as a biochemical and histochemical probe for apoptosis-associated proteolysis in growth factor-deprived neural cells in vitro and the developing postnatal rat brain. Neuronally differentiated PC12 cells became strongly Ab127 immunoreactive only during apoptosis following nerve growth factor withdrawal. Apoptosis-associated caspase proteolysis was detectable on western blots as markedly increased immunoreactivity of a approximately 46,000 mol. wt polypeptide, a product also generated by caspase-3 treatment of cell-free extracts. In the postnatal rat brain, intense immunoreactivity indicative of caspase activation was exhibited by small proportions of neurons and glia in distinct regional and temporal patterns. The degenerating nature of these cells was confirmed by their argyrophilia, cytoplasmic immunoreactivity for c-jun and fragmented processes. Combined immunofluorescence and Hoechst 33342 staining demonstrated that cells immunopositive for caspase activation have apoptotic nuclear morphologies. Caspase proteolysis was observed throughout the neuraxis in a minority of progenitor cells in germinal zones, postmitotic neurons in the parenchyma, and glia in the corpus callosum and other white matter tracts, but was observed rarely in the adult brain. These data characterize a new approach for evaluating apoptosis in physiological and pathological neurodegeneration, and demonstrate that caspase-associated apoptosis is a widespread mechanism for the programmed death of neurons and glia in the postnatal rat brain.

Apoptosis in the development of the mouse olfactory epithelium

Apoptotic cells were detected in the mouse olfactory epithelium (OE) at different embryonic and postnatal stages by in situ nick translation (ISNT) and Tdt-mediated dUTP nick end-labeling (TUNEL) techniques. During development, the apoptotic process presented two peaks. One at E12 during the invagination of the olfactory placode and the second at E16 corresponding to olfactory axon synaptogenesis. Then, from E18, a sharp decrease in the number of apoptotic cells was observed and at E19 the apoptotic index reached low values that were maintained in postnatal stages, P1 and P8, and in the adult. Apoptotic nuclei belonged to mature as well as immature olfactory receptor neurons (ORNs). Indeed, double-labeling experiments evidenced apoptotic neurons immunopositive for olfactory marker protein (OMP), carnosine and GAP-43. According to our data, two apoptotic phases occur during early development. One is involved in the morphogenesis of the OE when this last is not yet, or poorly, connected to its target, the olfactory bulb (OB). The second peak of apoptosis is more closely dependent on the interplay between OE and OB.

Potassium currents in precursor cells isolated from the anterior subventricular zone of the neonatal rat forebrain

The progenitor cells from the anterior part of the neonatal subventricular zone, the SVZa, are unusual in that, although they undergo division, they have a neuronal phenotype. To characterize the electrophysiological properties of the SVZa precursor cells, recordings were made of potassium and sodium currents from SVZa cells that were removed from postnatal day 0-1 rats and cultured for 1 day. The properties of the delayed rectifier and A-type potassium currents were described by classical Hodgkin and Huxley analyses of activation and inactivation. In addition, cells were assessed under current clamp for their ability to generate action potentials. The A-type potassium current (IK(A)) was completely inactivated at a holding potential of -50 mV. The remaining potassium current resembled the delayed rectifier current (IK(DR)) in that it was blocked by tetraethylammonium (TEA; IC50 4.1 mM) and activated and inactivated slowly compared with IK(A). The conductance-voltage (G-V) curve revealed that G increased continuously from 0.2 nS at -40 mV to a peak of 2.6 nS at +10 or +20 mV, and then decreased for voltages above +30 mV. Activation time constants were largest at -40 mV ( approximately 11 ms) and smallest at 100 mV ( approximately 1.5 ms). The properties of IK(A) were studied in the presence of 20 mM TEA, to block IK(DR), and from a holding potential of -15 mV, to inactivate both IK(DR) and IK(A). IK(A) was then allowed to recover from inactivation to negative potentials during 200- to 800-ms pulses. Recovery from inactivation was fastest at -130 mV ( approximately 21 ms) and slowest at -90 mV ( approximately 135 ms). Inactivation was voltage independent from -60 to +60 mV with a time constant of approximately 15 ms. At steady state, IK(A) was half inactivated at -90 mV. GK(A) increased from 0.2 nS at -60 mV to a peak of 2.4 nS at +40 mV. Finally, the activation time constants ranged from approximately 1.9 ms at -50 mV to 0.7 ms at +60 mV. The properties of IK(A) resembled those of IK(A) found in differentiating cerebellar granule neurons. Most SVZa cells had sodium currents (28/32 cells). However, in current clamp 11 of 12 cells were incapable of generating action potentials from voltages of -30 to -100 mV, suggesting that the available current densities were too low to support excitability.

Olfactory mucosa of the South American armadillo Chaetophractus villosus: an ultrastructural study

The sense of olfaction in armadillos plays an important role, suggested by the great development of the nasal structures, olfactory bulbs, and related brain regions. The mammalian olfactory mucosa is a privileged site of neuronal death and regeneration during the whole life span. A detailed knowledge of its ultrastructure is convenient for gaining insight into the factors controlling those phenomena. We performed this work in species not previously studied in order to provide a firm basis for further research on those factors. No information is available on the histology and ultrastructure of the olfactory mucosa in the order Xenarthra to which armadillos belong. Samples from the endoturbinals of the armadillo Chaetophractus villosus were prepared for light and electron microscopic examination by the usual conventional means. The olfactory epithelium of Chaetophractus villosus shows the classical three types of cells: supporting cells, olfactory receptor neurons, and basal cells. The olfactory neurons and the basal cells were similar to that described in other species. Two different types of supporting cells are described. An outstanding characteristic of the supporting cells is the normal presence of abundant phagosomes, apical secretory granules, apocrine-like protrusions, and highly developed smooth endoplasmic reticulum. Apoptotic bodies are frequently found in the infranuclear cytoplasm of supporting cells. The ductular epithelium of Bowman's glands reveals secretory activity. The lamina propria shows mixed Bowman's glands. Great development of smooth endoplasmic reticulum is observed in the mucous acinar cells. Evidence for merocrine and apocrine mechanisms in the Bowman's glands is presented. The presence of apoptotic bodies and phagosomes in supporting cells suggests a participation in the cellular events induced by cell death and proliferation of the olfactory epithelium. The variety of characteristics exhibited by the supporting cells of the olfactory mucosa may contribute to a deeper understanding of their scarcely known functions.

Apoptosis in the cerebellum of adult teleost fish, Apteronotus leptorhynchus

While involvement of programmed cell death (apoptosis) in embryogenesis is well established, only very little is known about this phenomenon in later stages of development. Based primarily on indirect evidence, it has been proposed that during postembryonic development of fish cell death does not occur. We have re-addressed this issue by examining the gymnotiform fish Apteronotus leptorhynchus. This teleost exhibits a high degree of proliferative activity in the brain during adulthood. Most of these cells are born in the cerebellum, where they differentiate, migrate into specific target regions, and are added to the population of already existing cerebellar cells. By applying morphological criteria and an in situ technique for the detection of DNA fragmentation (a feature characteristic of apoptotic cells), we show here that a large number of cerebellar cells undergo apoptosis. The density of apoptotic cells is significantly higher in the granule cell layers of the subdivisions of the cerebellum than in the corresponding molecular layers. This finding is consistent with previous observations indicating a drastic reduction in areal density of newborn cells within these granule cell layers in a period 4-7 weeks after their generation. In the granule cell layers of two cerebellar subdivisions, the corpus cerebelli and the valvula cerebelli pars medialis, the areal density of apoptotic cells displays a significant negative correlation with body weight, thus pointing to a decrease in the number of apoptotic events with age. The results of our investigation provide clear evidence for the existence of apoptosis during adulthood in fish and underline the significance of this process in the postembryonic development of the brain.