Endothelin evokes distinct calcium transients in neuronal and non-neuronal cells of rat olfactory mucosa primary cultures

Endothelin-1 axes in the framework of predictive, preventive and personalised (3P) medicine

Endothelin-1 (ET-1) is involved in the regulation of a myriad of processes highly relevant for physical and mental well-being; female and male health; in the modulation of senses, pain, stress reactions and drug sensitivity as well as healing processes, amongst others. Shifted ET-1 homeostasis may influence and predict the development and progression of suboptimal health conditions, metabolic impairments with cascading complications, ageing and related pathologies, cardiovascular diseases, neurodegenerative pathologies, aggressive malignancies, modulating, therefore, individual outcomes of both non-communicable and infectious diseases such as COVID-19. This article provides an in-depth analysis of the involvement of ET-1 and related regulatory pathways in physiological and pathophysiological processes and estimates its capacity as a predictor of ageing and related pathologies,a sensor of lifestyle quality and progression of suboptimal health conditions to diseases for their targeted preventionand as a potent target for cost-effective treatments tailored to the person.

Endothelin increases the proliferation of rat olfactory mucosa cells

The olfactory mucosa holds olfactory sensory neurons directly in contact with an aggressive environment. In order to maintain its integrity, it is one of the few neural zones which are continuously renewed during the whole animal life. Among several factors regulating this renewal, endothelin acts as an anti-apoptotic factor in the rat olfactory epithelium. In the present study, we explored whether endothelin could also act as a proliferative factor. Using primary culture of the olfactory mucosa, we found that an early treatment with endothelin increased its growth. Consistently, a treatment with a mixture of BQ₁₂₃ and BQ₇₈₈ (endothelin receptor antagonists) decreased the primary culture growth without affecting the cellular death level. We then used combined approaches of calcium imaging, reverse transcriptase-quantitative polymerase chain reaction and protein level measurements to show that endothelin was locally synthetized by the primary culture until it reached confluency. Furthermore, in vivo intranasal instillation of endothelin receptor antagonists led to a decrease of olfactory mucosa cell expressing proliferating cell nuclear antigen (PCNA), a marker of proliferation. Only short-term treatment reduced the PCNA level in the olfactory mucosa cells. When the treatment was prolonged, the PCNA level was not statistically affected but the expression level of endothelin was increased. Overall, our results show that endothelin plays a proliferative role in the olfactory mucosa and that its level is dynamically regulated. This study was approved by the Comité d’éthique en expérimentation animale COMETHEA (COMETHEA C2EA -45; protocol approval #12-058) on November 28, 2012.

The olfactory mucosa, first actor of olfactory detection, is sensitive to glucocorticoid hormone

The olfactory mucosa (OM) is the primary site of odorant detection, and its axonal projections relay information to brain structures for signal processing. We have previously observed that olfactory function can be affected during a prolonged stress challenge in Wistar rats. The stress response is a neuroendocrine retro-controlled loop allowing pleiotropic adaptive tissue alterations, which are partly mediated through the release of glucocorticoid hormones. We hypothesised that, as part of their wide-ranging pleiotropic effects, glucocorticoids might affect the first step of olfactory detection. To study this, we used a number of approaches ranging from the molecular detection and functional characterisation of glucocorticoid receptors (GRs) in OM cells, to the study of GR acute activation in vivo at the molecular, electrophysiological and behavioural levels. In contrast to previous reports, where GR was reported to be exclusive in olfactory sensory neurones, we located functional GR expression mostly in olfactory ensheathing cells. Dexamethasone (2 mg/kg) was injected intraperitoneally to activate GR in vivo, and this led to functional odorant electrophysiological response (electro-olfactogram) and OM gene expression changes. In a habituation/cross-habituation test of olfactory sensitivity, we observed that DEX-treated rats exhibited higher responsiveness to a complex odorant mixture. These findings support the idea that olfactory perception is altered in stressed animals, as glucocorticoids might enhance odour detection, starting at the first step of detection.

IL-17c is involved in olfactory mucosa responses to Poly(I:C) mimicking virus presence

At the interface of the environment and the nervous system, the olfactory mucosa (OM) is a privileged pathway for environmental toxicants and pathogens towards the central nervous system. The OM is known to produce antimicrobial and immunological components but the mechanisms of action of the immune system on the OM remain poorly explored. IL-17c is a potent mediator of respiratory epithelial innate immune responses, whose receptors are highly expressed in the OM of mice. We first characterized the presence of the IL-17c and its receptors in the OM. While IL-17c was weakly expressed in the control condition, it was strongly expressed in vivo after intranasal administration of polyinosinic-polycytidylic (Poly I:C), a Toll Like Receptor 3 agonist, mimicking a viral infection. Using calcium imaging and electrophysiological recordings, we found that IL-17c can effectively activate OM cells through the release of ATP. In the longer term, intranasal chronic instillations of IL-17c increased the cellular dynamics of the epithelium and promoted immune cells infiltrations. Finally, IL-17c decreased cell death induced by Poly(I:C) in an OM primary culture. The OM is thus a tissue highly responsive to immune mediators, proving its central role as a barrier against airway pathogens.

The Endothelin-A Receptor Antagonist Zibotentan Induces Damage to the Nasal Olfactory Epithelium Possibly Mediated in Part through Type 2 Innate Lymphoid Cells

Zibotentan, an endothelin-A receptor antagonist, has been used in the treatment of various cardiovascular disorders and neoplasia. Castrated athymic nude mice receiving zibotentan for a preclinical xenograft efficacy study experienced weight loss, gastrointestinal bloat, and the presence of an audible respiratory click. Human side effects have been reported in the nasal cavity, so we hypothesized that the nasal cavity is a target for toxicity in mice receiving zibotentan. Lesions in the nasal cavity predominantly targeted olfactory epithelium in treated mice and were more pronounced in castrated animals. Minimal lesions were present in vehicle control animals, which suggested possible gavage-related reflux injury. The incidence, distribution, and morphology of lesions suggested direct exposure to the nasal mucosa and a possible systemic effect targeting the olfactory epithelium, driven by a type 2 immune response, with group 2 innate lymphoid cell involvement. Severe nasal lesions may have resulted in recurrent upper airway obstruction, leading to aerophagia and associated clinical morbidity. These data show the nasal cavity is a target of zibotentan when given by gavage in athymic nude mice, and such unanticipated and off-target effects could impact interpretation of research results and animal health in preclinical studies.

Endothelin impacts on olfactory processing in rats

In the olfactory epithelium, olfactory sensitive neurons and their axons are surrounded by glia-like cells called sustentacular cells, which maintain both the structural and ionic integrity of the olfactory mucosa. We have previously found that endothelin-1 (ET-1) can uncouple sustentacular cell gap junctions in vitro similarly as carbenoxolone, a known gap junction uncoupling agent. The role of gap junctions in odorant transduction remains controversial and we explored here if ET-1 naturally produced by the olfactory mucosa could impact odorant detection. Using calcium imaging on olfactory mucosa explant, we first confirmed that ET-1 uncouples gap junctions in an olfactory mucosa preparation preserving the tissue integrity. We next measured the olfactory epithelium responses to odorant stimulation using electro-olfactogram recordings. While the amplitude of the response was not modified by application of ET-1 and carbenoxolone, its repolarizing phase was slower after both treatments. We finally examined the behavioral performances of rat pups in an orientation test based on maternal odor recognition after intranasal instillations of ET-1 or carbenoxolone. While rat pups performances were decreased after ET-1 treatment, it was unchanged after carbenoxolone treatment. Overall, our results indicate that ET-1 modulates olfactory responses at least partly through gap junction uncoupling.

Endothelin uncouples gap junctions in sustentacular cells and olfactory ensheathing cells of the olfactory mucosa

Several factors modulate the first step of odour detection in the rat olfactory mucosa (OM). Among others, vasoactive peptides such as endothelin might play multifaceted roles in the different OM cells. Like their counterparts in the central nervous system, the olfactory sensory neurons are encompassed by different glial-like non-neuronal OM cells; sustentacular cells (SCs) surround their cell bodies, whereas olfactory ensheathing cells (OECs) wrap their axons. Whereas SCs maintain both the structural and ionic integrity of the OM, OECs assure protection, local blood flow control and guiding of olfactory sensory neuron axons toward the olfactory bulb. We previously showed that these non-neuronal OM cells are particularly responsive to endothelin in vitro. Here, we confirmed that the endothelin system is strongly expressed in the OM using in situ hybridization. We then further explored the effects of endothelin on SCs and OECs using electrophysiological recordings and calcium imaging approaches on both in vitro and ex vivo OM preparations. Endothelin induced both robust calcium signals and gap junction uncoupling in both types of cells. This latter effect was mimicked by carbenoxolone, a known gap junction uncoupling agent. However, although endothelin is known for its antiapoptotic effect in the OM, the uncoupling of gap junctions by carbenoxolone was not sufficient to limit the cellular death induced by serum deprivation in OM primary culture. The functional consequence of the endothelin 1-induced reduction of the gap junctional communication between OM non-neuronal cells thus remains to be elucidated.

Neural peptidase endothelin-converting enzyme 1 regulates endothelin 1-induced pruritus

In humans, pruritus (itch) is a common but poorly understood symptom in numerous skin and systemic diseases. Endothelin 1 (ET-1) evokes histamine-independent pruritus in mammals through activation of its cognate G protein-coupled receptor endothelin A receptor (ETAR). Here, we have identified neural endothelin-converting enzyme 1 (ECE-1) as a key regulator of ET-1-induced pruritus and neural signaling of itch. We show here that ETAR, ET-1, and ECE-1 are expressed and colocalize in murine dorsal root ganglia (DRG) neurons and human skin nerves. In murine DRG neurons, ET-1 induced internalization of ETAR within ECE-1-containing endosomes. ECE-1 inhibition slowed ETAR recycling yet prolonged ET-1-induced activation of ERK1/2, but not p38. In a murine itch model, ET-1-induced scratching behavior was substantially augmented by pharmacological ECE-1 inhibition and abrogated by treatment with an ERK1/2 inhibitor. Using iontophoresis, we demonstrated that ET-1 is a potent, partially histamine-independent pruritogen in humans. Immunohistochemical evaluation of skin from prurigo nodularis patients confirmed an upregulation of the ET-1/ETAR/ECE-1/ERK1/2 axis in patients with chronic itch. Together, our data identify the neural peptidase ECE-1 as a negative regulator of itch on sensory nerves by directly regulating ET-1-induced pruritus in humans and mice. Furthermore, these results implicate the ET-1/ECE-1/ERK1/2 pathway as a therapeutic target to treat pruritus in humans.

Activation of SRE and AP1 by olfactory receptors via the MAPK and Rho dependent pathways

Whereas the activation of MAPKs (mitogen activated kinases) and Rho dependant pathways by GPCR (G protein coupled receptors) has been the subject of many studies, its implication in the signalling of olfactory receptors, which constitute the largest GPCR family, has been far less analysed. Using an in vitro heterologous system, we showed that odorant activated ORs activate SRE containing promoters via the ERK pathway. We also demonstrated that RhoA and Rock kinases but not Rac were involved in ORs-induced SRE/SRF activation and that AP1 was activated, via JNK and p38 MAPKinase. Using real time PCR we found that mOR23, RnI7 and CfOR12A07 induced elevated levels of transcription factors ELK-4, srf, c-fos and c-jun mRNAs whereas mOREG induced an elevated transcription levels of c-fos and c-jun mRNA only. We showed also that odorant activated ORs stimulate the downstream MAPKs and Rho pathways in primary cultures of rat olfactory sensory neurons (OSNs). Similar results were also obtained with OE (olfactory epithelium) extracts prepared from rats exposed to odorants in vivo. Finally, we showed the important role of the AKT and MAPK signalling pathways in OSNs survival. Taken together, these data provide direct evidence that the binding of odorants onto their ORs activates the MAPK and Rho signalling pathways that are involved in OSNs survival events. This suggests that these pathways could be implicated in the regulation of OSNs homeostasis.

Peripheral modulation of smell: fact or fiction?

Despite studies dating back 30 or more years showing modulation of odorant responses at the level of the olfactory epithelium, most descriptions of the olfactory system infer that odorant signals make their way from detection by cilia on olfactory sensory neurons to the olfactory bulb unaltered. Recent identification of multiple subtypes of microvillar cells and identification of neuropeptide and neurotransmitter expression in the olfactory mucosa add to the growing body of literature for peripheral modulation in the sense of smell. Complex mechanisms including perireceptor events, modulation of sniff rates, and changes in the properties of sensory neurons match the sensitivity of olfactory sensory neurons to the external odorant environment, internal nutritional status, reproductive status, and levels of arousal or stress. By furthering our understanding of the players mediating peripheral olfaction, we may open the door to novel approaches for modulating the sense of smell in both health and disease.

Endothelin as a neuroprotective factor in the olfactory epithelium

In mammals, the olfactory sensory neurons are the only ones directly in contact with an aggressive environment. Thus, the olfactory mucosa is one of the few neuronal zones which are continuously renewed during adulthood. We have previously shown that endothelin is locally matured in the olfactory mucosa and that olfactory sensory neurons preferentially express ETB receptors, while ETA receptors are rather present in non neuronal olfactory mucosa cells. In addition to its vasoactive effect, the endothelin system is known for its pleiotropic effects including the modulation of cell population dynamics. We thus examined its potential neuroprotective effect in the olfactory mucosa using a primary culture of olfactory sensory neurons lying on non neuronal cells. While a serum deprivation led to a massive decrease of the density of olfactory sensory neurons in the primary cultures, endothelin 1 (ET-1) rescued part of the neuronal population through both ETA and ETB receptors. This effect was mainly anti-apoptotic as it reduced cleaved caspase-3 signal and nuclear condensation. Furthermore, the olfactory epithelium of ETB-deficient rats displayed increased apoptosis. These results strongly suggest that ET-1 acts as an anti-apoptotic factor on olfactory sensory neurons, directly through ETB and indirectly by limiting non neuronal cells death through ETA.

The interdependence of endothelin-1 and calcium: a review

The 21-amino-acid peptide ET-1 (endothelin-1) regulates a diverse array of physiological processes, including vasoconstriction, angiogenesis, nociception and cell proliferation. Most of the effects of ET-1 are associated with an increase in intracellular calcium concentration. The calcium influx and mobilization pathways activated by ET-1, however, vary immensely. The present review begins with the basics of calcium signalling and investigates the different ways intracellular calcium concentration can increase in response to a stimulus. The focus then shifts to ET-1, and discusses how ET receptors mobilize calcium. We also examine how disease alters calcium-dependent responses to ET-1 by discussing changes to ET-1-mediated calcium signalling in hypertension, as there is significant interest in the role of ET-1 in this important disease. A list of unanswered questions regarding ET-mediated calcium signals are also presented, as well as perspectives for future research of calcium mobilization by ET-1.