Colchicine-induced cell death and proliferation in the olfactory epithelium and vomeronasal organ of the mouse

Immune responses in the injured olfactory and gustatory systems: a role in olfactory receptor neuron and taste bud regeneration?

Sensory cells that specialize in transducing olfactory and gustatory stimuli are renewed throughout life and can regenerate after injury unlike their counterparts in the mammalian retina and auditory epithelium. This uncommon capacity for regeneration offers an opportunity to understand mechanisms that promote the recovery of sensory function after taste and smell loss. Immune responses appear to influence degeneration and later regeneration of olfactory sensory neurons and taste receptor cells. Here we review surgical, chemical, and inflammatory injury models and evidence that immune responses promote or deter chemosensory cell regeneration. Macrophage and neutrophil responses to chemosensory receptor injury have been the most widely studied without consensus on their net effects on regeneration. We discuss possible technical and biological reasons for the discrepancy, such as the difference between peripheral and central structures, and suggest directions for progress in understanding immune regulation of chemosensory regeneration. Our mechanistic understanding of immune-chemosensory cell interactions must be expanded before therapies can be developed for recovering the sensation of taste and smell after head injury from traumatic nerve damage and infection. Chemosensory loss leads to decreased quality of life, depression, nutritional challenges, and exposure to environmental dangers highlighting the need for further studies in this area.

Single or Repeated Ablation of Mouse Olfactory Epithelium by Methimazole

Odor-detecting olfactory sensory neurons residing in the nasal olfactory epithelium (OE) are the only neurons in direct contact with the external environment. As a result, these neurons are subjected to chemical, physical, and infectious insults, which may be the underlying reason why neurogenesis occurs in the OE of adult mammals. This feature makes the OE a useful model for studying neurogenesis and neuronal differentiation, with the possibility for systemic as well as local administration of various compounds and infectious agents that may interfere with these cellular processes. Several different chemical compounds have been shown to cause toxic injury to the OE, which can be used for OE ablation. We, and others, have found that the systemic administration of the hyperthyroid drug, methimazole, reliably causes olfactotoxicity as a side effect. Here, we outline an OE lesioning protocol for single or repeated ablation by methimazole. A single methimazole administration can be used to study neuroepithelial regeneration and stem cell activation, while repeated ablation and regeneration of OE enable the study of tissue stem cell exhaustion and generation of tissue metaplasia.

Structural and ultrastructural studies on the developing vomeronasal sensory epithelium in the grass snake Natrix natrix (Squamata: Colubroidea)

The sensory olfactory epithelium and the vomeronasal sensory epithelium (VSE) are characterized by continuous turnover of the receptor cells during postnatal life and are capable of regeneration after injury. The VSE, like the entire vomeronasal organ, is generally well developed in squamates and is crucial for detection of pheromones and prey odors. Despite the numerous studies on embryonic development of the VSE in squamates, especially in snakes, an ultrastructural analysis, as far as we know, has never been performed. Therefore, we investigated the embryology of the VSE of the grass snake (Natrix natrix) using electron microscopy (SEM and TEM) and light microscopy. As was shown for adult snakes, the hypertrophied ophidian VSE may provide great resolution of changes in neuron morphology located at various epithelial levels. The results of this study suggest that different populations of stem/progenitor cells occur at the base of the ophidian VSE during embryonic development. One of them may be radial glia-like cells, described previously in mouse. The various structure and ultrastructure of neurons located at different parts of the VSE provide evidence for neuronal maturation and aging. Based on these results, a few nonmutually exclusive hypotheses explaining the formation of the peculiar columnar organization of the VSE in snakes were proposed.

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

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

Histopathological analysis of the olfactory epithelium of zebrafish (Danio rerio) exposed to sublethal doses of urea

Chronic renal disease is known to alter olfactory function, but the specific changes induced in olfactory organs during this process remain unclear. Of the uraemic toxins generated during renal disease, high levels of urea are known to induce hyposmic conditions. In this study, the effects of environmental exposure to elevated concentrations of urea (7, 13.5 and 20 g L(-1)) on the sensory mucosa of zebrafish in acute toxicity and chronic toxicity tests were described. It was observed that lamellae maintained structural integrity and epithelial thickness was slightly reduced, but only following exposure to the highest concentrations of urea. Pan-neuronal labelling with anti-Hu revealed a negative correlation with levels of urea, leading to investigation of whether distinct neuronal subtypes were equally sensitive. Using densitometric analysis of immunolabelled tissues, numbers of Gα olf-, TRPC2- and TrkA-expressing cells were compared, representing ciliated, microvillous and crypt neurons, respectively. The three neuronal subpopulations responded differently to increasing levels of urea. In particular, crypt cells were more severely affected than the other cell types, and Gα olf-immunoreactivity was found to increase when fish were exposed to low doses of urea. It can be concluded that exposure to moderate levels of urea leads to sensory toxicity directly affecting olfactory organs, in accordance with the functional olfactometric measurements previously reported in the literature.

A unique method for the isolation of nasal olfactory stem cells in living rats

Stem cells are attractive tools to develop new therapeutic strategies for a variety of disorders. While ethical and technical issues, associated with embryonic, fetal and neural stem cells, limit the translation to clinical applications, the nasal stem cells identified in the human olfactory mucosa stand as a promising candidate for stem cell-based therapies. Located in the back of the nose, this multipotent stem cell type is readily accessible in humans, a feature that makes these cells highly suitable for the development of autologous cell-based therapies. However, preclinical studies based on autologous transplantation of rodent olfactory stem cells are impeded because of the narrow opening of the nasal cavity. In this study, we report the development of a unique method permitting to quickly and safely biopsy olfactory mucosa in rats. Using this newly developed technique, rat stem cells expressing the stem cell marker Nestin were successfully isolated without requiring the sacrifice of the donor animal. As an evidence of the self-renewal capacity of the isolated cells, several millions of rat cells were amplified from a single biopsy within four weeks. Using an olfactory discrimination test, we additionally showed that this novel biopsy method does not affect the sense of smell and the learning and memory abilities of the operated animals. This study describes for the first time a methodology allowing the derivation of rat nasal cells in a way that is suitable for studying the effects of autologous transplantation of any cell type present in the olfactory mucosa in a wide variety of rat models.

Peripheral olfactory sensitivity in rodents after treatment with docetaxel

OBJECTIVES/HYPOTHESIS

Clinical studies have documented that cytotoxic chemotherapy is often associated with body weight loss and decreased enjoyment of food. Besides taste, olfaction plays a role in food intake. We assessed whether systemic chemotherapeutic cancer treatment compromises olfactory function in rats and mice treated with docetaxel (Taxotere; Sanofi-Aventis, Paris, France).

STUDY DESIGN

Randomized, controlled trials on mice and rats.

METHODS

Male mice received a single and male rats either a single, two, or three docetaxel administrations. Olfactory function was tested by means of electroolfactograms (EOGs) from the chemosensory epithelium of the nasal septum and the endoturbinates. We evaluated and compared the magnitude of EOG responses evoked by different odorants recorded at different time points after treatment.

RESULTS

In both animal species, docetaxel administration reduced body weight gain, thus evidencing the general toxic effect of the drug. In both animal species, the olfactory mucosa remained responsive to stimulation of odorants during the whole course of experiment, but treatment revealed regional differences of docetaxel susceptibility and induced marked transitory electrophysiological changes. In mice and rats a significant transitory decrease in EOG response magnitude occurred after a single administration. Unexpectedly, in rats we also observed an increase of the olfactory response following the second administration of the drug.

CONCLUSIONS

Docetaxel exerts a neurotoxic effect on olfactory epithelia of rodents at doses similar to human doses, thus inducing transitory functional alterations. Although moderate, they are consistent with the hypothesis of a dysfunction of olfactory function. Further experiments are needed to elucidate the origin of the electrophysiological effects and their impact on the olfactory perception.

Fate of marginal neuroblasts in the vomeronasal epithelium of adult mice

Chemical stimuli are sensed through the olfactory and vomeronasal epithelia, and the sensory cells of both systems undergo neuronal turnover during adulthood. In the vomeronasal epithelium, stem cells adjacent to the basal lamina divide and migrate to replace two classes of sensory neurons: apical neurons that express G(i2alpha)-linked V1R vomeronasal receptors and project to the anterior accessory olfactory bulb, and basal neurons that express G(oalpha)-linked V2R receptors and project to the posterior accessory olfactory bulb. Most of the dividing cells are present in the margins of the epithelium and only migrate locally. Previous studies have suggested that these marginal cells may participate in growth, sensory cell replacement or become apoptotic before maturation; however, the exact fate of these cells have remained unclear. In this work we investigated the fate of these marginal cells by analyzing markers of neurogenesis (bromodeoxyuridine incorporation), apoptosis (caspase-3), and neuronal maturation (olfactory marker protein and Neurotrace Nissl stain). Our data reveal a pool of dividing cells in the epithelial margins that predominantly give rise to mature neurons and only rarely undergo apoptosis. Newly generated cells are several times more numerous than apoptotic cells. These marginal neuroblasts could therefore constitute a net neural addition zone during adulthood.

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.

Neurogenesis, migration, and apoptosis in the vomeronasal epithelium of adult mice

The location of neurogenesis and the direction of migration of neurons in the adult mouse vomeronasal organ is controversial. Cell division occurs at the center, and particularly, at the edges of the epithelium. Newly generated cells at the center of the epithelium participate in neurogenesis, however, it is unknown to what extent dividing cells at the edges participate in growth, become apoptotic or mature into neurons. Premitotic cells were labeled with bromodeoxyuridine (BrdU) in adult mice and animals allowed to survive for different postinjection periods. The terminal deoxynucleotidyl transferase-mediated biotinylated dUTP nick end-labeling (TUNEL) method was used to show the distribution of apoptotic cells. The vertical and horizontal position of BrdU-labeled cells was analyzed as a function of postinjection survival time. Vertical and horizontal migration of BrdU-labeled cells were detected. Cells in the central portions of the epithelium migrated vertically to become neurons as demonstrated by co-expression of olfactory marker protein. Cells at the edges migrated horizontally very slowly (less than 10% of the distance from the edge to the center of the epithelium per month), thus indicating that these cells participate in cell renewal exclusively in marginal regions. Neural turnover in the mouse vomeronasal epithelium, therefore appears to occur through a process of vertical migration. Data on the distribution of apoptotic cells indicate that a number of dividing cells throughout the epithelium, but particularly at the edges, die before becoming functional neurons. Accordingly, most dividing cells at the edges probably constitute a reservoir of stem cells dying before differentiation.

Retinol-induced apoptosis in larval pancreas of Bufo bufo

Tadpoles of Bufo bufo at the end of premetamorphosis were exposed to the action of retinol diluted in the rearing water at two concentrations: 20 and 40 microM, for 48 h. Assuming that retinol causes growth suppression by cell death induction, the TUNEL reaction for the detection of apoptotic cells was performed on paraffin sections of pancreas. The results showed that retinol induced a significant dose-dependent increase of number of apoptotic cells with respect to controls, in which apoptosis was scarce. Electron microscopic observations of treated specimens showed that the effects of retinol mostly occurred on exocrine cells: condensed crescent-shaped nuclear chromatin, dark cytoplasm with long projections; conversely, mitochondria and endoplasmic reticulum appeared unaffected.

Lectin-induced apoptosis of mature olfactory receptor cells

Previous studies showed that uptake of the lectin conjugate, wheat germ agglutinin-horseradish peroxidase (WGA-HRP) by olfactory receptor cells results in a thinning of the olfactory epithelium (OE) and increased turnover of globose basal cells. To ascertain the cell-type lost as well as the time course and mechanism of the loss, the current study measured changes in the number of dendritic knobs, olfactory marker protein (OMP) expression and assessed TUNEL labeling as an indicator of apoptosis. Electron microscopic analysis of the number of dendritic knobs showed that the largest reduction occurred at 1 week after intranasal irrigation with WGA-HRP. This data in conjunction with decreased OMP staining provided evidence for a loss of mature receptor neurons. TUNEL labeling, especially in more superficial aspects of the OE, peaked at 18 hr after WGA-HRP application suggesting that the lectin-conjugate produced a rapid induction of apoptotic cell death that was complete by 3 days. Measurement of tyrosine hydroxylase (TH) activity in the olfactory bulb, a sensitive measure of deafferentation, showed that innervation reached a nadir at about 1 week and that reinnervation was complete by 4 weeks. These findings demonstrate that internalization of WGA-HRP by some receptor cells results in their death by apoptosis and a subsequent deafferentation of the olfactory bulb.

Morphological and histochemical changes in the regenerating vomeronasal epithelium

Receptor cell degeneration and regeneration within the vomeronasal organ (VNO) of the rat was studied using both electron microscopy and histochemical methods. Electron microscopy was employed to examine the morphological changes along the surface of the sensory epithelium, and histochemical markers were used to monitor the changes in the epithelial cell layers. Transection of the vomeronasal nerves induced selective degeneration of the receptor cells, and within six days, a significant decrease in the number of receptor cells was observed. During the subsequent stage of receptor cell regeneration, cilia and bud-like structures characteristic of a developing sensory epithelium were seen. By day 15, thin microvilli covering the surface of the receptor cells reappeared in the sensory epithelium. The neural cell adhesion molecule (NCAM) and two vomeronasal system-specific lectins; 1) Bandeiraea simplicifolia lectin (BSL-I) and 2) Vicia villosa agglutinin (VVA) were used as the histochemical markers. NCAM immunoreactivity on the surface of the epithelium was observed to be decreased significantly six days after nerve transection, and was restored during receptor cell regeneration (day 15). The reactivity of the two lectins, BSL-I and VVA, was decreased slightly during degeneration, but was still detectable at the time of maximum receptor cell degeneration (day 6). Lectin reactivity was restored to control levels by day 15. These findings suggest that (1) NCAM is a useful marker for vomeronasal receptor cells and that the vomeronasal system-specific lectins may bind to both receptor and supporting cells and (2) degeneration of vomeronasal receptor cells occurs during the first week (day 6) following nerve transection and the receptor cell population begins to recover within 15 days. The morphological changes observed during receptor cell regeneration suggest that the stages of VNO receptor cell regeneration are similar to those observed during development.

Retinoic acid enhances the rate of olfactory recovery after olfactory nerve transection

In the olfactory system, retinoic acid (RA) plays an important role in development and may affect growth in the adult animal. To explore the potential effects of RA on recovery after injuries, adult mice were trained in a buried food paradigm and were given a single oral supplement of RA after olfactory nerve transection. Results demonstrate that RA accelerates the recovery of olfactory functions after injury.

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.

Supplementary report on experimental autumn crocus (Colchicum autumnale L.) poisoning in cattle: morphological evidence of apoptosis

Previously we reported that tissue destruction characterized by the presence of karyopyknotic, karyorrhectic and mitotically arrested cells was seen in alimentary epithelial cells and lymphocytes in the lymphoid and hemopoietic systems of cattle experimentally administered with autumn crocus (Colchicum autumnale L.). This report deals with the mechanism of acute cellular injury following experimental autumn crocus poisoning in cattle as demonstrated by the in situ DNA strand break analysis and electron microscopy. The analyses revealed that cellular injury caused by autumn crocus was closely associated with apoptosis.