Purinergic nucleotides, including ATP and adenosine, are important neuromodulators of peripheral auditory and visual sensory systems (Thorne and Housley, 1996). ATP released by the olfactory epithelium (OE) after noxious stimuli provides a physiological source for a neuromodulatory substance independent of efferent innervation. Here we show that multiple subtypes of purinergic receptors are differentially expressed in olfactory receptor neurons and sustentacular support cells. Activation of purinergic receptors evoked inward currents and increases in intracellular calcium in cultured mouse olfactory receptor neurons. A mouse olfactory epithelial slice preparation and confocal imaging were used to measure changes in intracellular calcium in response to odors, purinergic receptor (P2R) agonists, or combined odor + P2R agonists. Pharmacological studies show that both P2Y and P2X receptor activation by exogenous and endogenous ATP significantly reduces odor responsiveness. Moreover, purinergic receptor antagonists increase the odor-evoked calcium transient, providing direct evidence that endogenous ATP modulates odor sensitivity via activation of multiple purinergic receptor subtypes in olfactory receptor neurons. Odor activation of G-protein-coupled receptors results in increased cAMP production, opening of cyclic nucleotide-gated channels, influx of Ca2+ and Na+, depolarization of the membrane, and activation of voltage- and Ca2+-gated ion channels. On-cell current-clamp recordings of olfactory receptor neurons from neonatal mouse slices revealed that ATP reduced cyclic nucleotide-induced electrical responses. These data also support the idea that ATP modulates odor sensitivity in mammalian olfactory neurons. Peripheral ATP-mediated odor suppression is a novel mechanism for reduced olfactory sensitivity during exposure to olfactotoxins and may be a novel neuroprotective mechanism.

Mutations in CHD7, a chromodomain gene, are present in a majority of individuals with CHARGE syndrome, a multiple anomaly disorder characterized by ocular Coloboma, Heart defects, Atresia of the choanae, Retarded growth and development, Genital hypoplasia and Ear anomalies. The clinical features of CHARGE syndrome are highly variable and incompletely penetrant. Olfactory dysfunction is a common feature in CHARGE syndrome and has been potentially linked to primary olfactory bulb defects, but no data confirming this mechanistic link have been reported. On the basis of these observations, we hypothesized that loss of Chd7 disrupts mammalian olfactory tissue development and function. We found severe defects in olfaction in individuals with CHD7 mutations and CHARGE, and loss of odor evoked electro-olfactogram responses in Chd7 deficient mice, suggesting reduced olfaction is due to a dysfunctional olfactory epithelium. Chd7 expression was high in basal olfactory epithelial neural stem cells and down-regulated in mature olfactory sensory neurons. We observed smaller olfactory bulbs, reduced olfactory sensory neurons, and disorganized epithelial ultrastructure in Chd7 mutant mice, despite apparently normal functional cilia and sustentacular cells. Significant reductions in the proliferation of neural stem cells and regeneration of olfactory sensory neurons in the mature Chd7(Gt/+) olfactory epithelium indicate critical roles for Chd7 in regulating neurogenesis. These studies provide evidence that mammalian olfactory dysfunction due to Chd7 haploinsufficiency is linked to primary defects in olfactory neural stem cell proliferation and may influence olfactory bulb development.

In the CNS, ATP induces the synthesis and release of neurotrophic factors, cell proliferation, and differentiation. The olfactory system is one site where multipotent progenitor cells continue to proliferate and differentiate into neurons throughout life. We tested the hypothesis that ATP initiates proliferation in olfactory epithelium by measuring 5-bromo-2-deoxyuridine incorporation. Adult mice were pre-treated intraperitoneally or intranasally with saline or purinergic receptor antagonists (pyridoxal-phosphate-6-azophenyl-2′,4′-disulfonate + suramin) 30 min prior to nasal instillation of ATP, UTP, ATPγS or saline (0 hr). Mice received three injections of 5-bromo-2-deoxyuridine between 42–46 hr, and were sacrificed at 2, 9 or 16 days post ATP instillation. ATP, UTP or ATPγS significantly increased 5-bromo-2-deoxyuridine incorporation compared to intranasal saline controls in groups pre-treated with saline. Saline, ATP, UTP or ATPγS instillation did not significantly increase 5-bromo-2-deoxyuridine incorporation in groups pre-treated with purinergic receptor antagonists. Similar results were observed in neonates and in a cultured slice preparation. Intranasal instillation of ATP also increased the protein levels of proliferating cell nuclear antigen in adults. Pre-treatment with purinergic receptor antagonists inhibited the ATP-induced increase in proliferating cell nuclear antigen. In adults, a subset of the cells that incorporated 5-bromo-2-deoxyuridine were immunoreactive to neuronal markers MASH 1, GAP43, and OMP at 2, 9, and 16 days, respectively. Collectively, these data indicate that purinergic receptor activation induces proliferation and neuronal differentiation in the mouse olfactory epithelium. We propose that extracellular ATP released upon injury could induce proliferation and promote the neuroregeneration of the olfactory epithelium.

Sustentacular cells have structural features that allude to functions of secretion, absorption, phagocytosis, maintenance of extracellular ionic gradients, metabolism of noxious chemicals, and regulation of cell turnover. We present data detailing their dynamic activity. We show, using a mouse olfactory epithelium slice model, that sustentacular cells are capable of generating two types of calcium signals: intercellular calcium waves where elevations in intracellular calcium propagate between neighboring cells, and intracellular calcium oscillations consisting of repetitive elevations in intracellular calcium confined to single cells. Sustentacular cells exhibited rapid, robust increases in intracellular calcium in response to G-protein coupled muscarinic and purinergic receptor stimulation. In a subpopulation of sustentacular cells, oscillatory calcium transients were evoked. We pharmacologically characterized the properties of purinergic-evoked increases in intracellular calcium. Calcium transients were elicited by release from intracellular stores and were not dependent on extracellular calcium. BAPTA-AM, a cytosolic calcium chelator, and cyclopiazonic acid, an endoplasmic reticulum Ca 2+ -ATPase inhibitor irreversibly blocked the purinergic-induced calcium transient. Phospholipase C (PLC) antagonist U73122 inhibited the purinergic-evoked calcium transient. 2-aminoethoxydiphenyl borate (2-APB), an inositol-1,4,5-trisphosphate (IP 3 ) receptor antagonist, and the ryanodine receptor (RyR) antagonists tetracaine and ryanodine, inhibited the UTP-induced calcium transients. Collectively, these data suggest that activation of the PLC pathway, IP 3 -mediated calcium release, and subsequent calcium-induced-calcium release is involved in ATP-elicited increases in intracellular calcium. Our findings indicate that sustentacular cells are not static support cells, and, like glia in the central nervous system, have complex calcium signaling.

No abstract

Calcium-dependent release of neurotrophic factors plays an important role in the maintenance of neurons, yet the release mechanisms are understudied. The inositol triphosphate (IP3) receptor is a calcium release channel that has a physiological role in cell growth, development, sensory perception, neuronal signaling and secretion. In the olfactory system, the IP3 receptor subtype 3 (IP3R3) is expressed exclusively in a microvillous cell subtype that is the predominant cell expressing neurotrophic factor neuropeptide Y (NPY). We hypothesized that IP3R3-expressing microvillous cells secrete sufficient NPY needed for both the continual maintenance of the neuronal population and for neuroregeneration following injury. We addressed this question by assessing the release of NPY and the regenerative capabilities of wild type, IP3R3+/−, and IP3R3−/− mice. Injury, simulated using extracellular ATP, induced IP3 receptor-mediated NPY release in wild-type mice. ATP-evoked NPY release was impaired in IP3R3−/− mice, suggesting that IP3R3 contributes to NPY release following injury. Under normal physiological conditions, both IP3R3−/− mice and explants from these mice had fewer progenitor cells that proliferate and differentiate into immature neurons. Although the number of mature neurons and the in vivo rate of proliferation were not altered, the proliferative response to the olfactotoxicant satratoxin G and olfactory bulb ablation injury was compromised in the olfactory epithelium of IP3R3−/− mice. The reductions in both NPY release and number of progenitor cells in IP3R3−/− mice point to a role of the IP3R3 in tissue homeostasis and neuroregeneration. Collectively, these data suggest that IP3R3 expressing microvillous cells are actively responsive to injury and promote recovery.

In the CNS, ATP is released upon injury and promotes neuroproliferation via purinergic receptors. In the olfactory epithelium, ATP promotes the synthesis and release of neurotrophic factor NPY in neonates and induces neuroproliferation in neonatal and adult mice. We tested the hypothesis that NPY is involved in ATP-induced neuroproliferation in adult mice olfactory epithelium. Intranasal instillation of ATP significantly increased protein levels and number of NPY + cells. Pre-intranasal instillation of purinergic receptor antagonist PPADS significantly reduced ATP-induced upregulation of NPY. Intranasal instillation of NPY-Y1 receptor antagonist BIBP3226 following ATP instillation significantly inhibited the ATP-induced increase in BrdU incorporation, suggesting that NPY is released after ATP instillation and activates Y1 receptors to promote neuroproliferation. These data indicate that ATP initiates neuroproliferation via NPY upregulation, NPY release, and Y1 receptor activation, and suggests that the olfactory epithelium is good model to study neuroregenerative mechanisms in the CNS.

The electrical properties of sustentacular cells (SCs) in the olfactory epithelium (OE) were investigated in tissue slices taken from neonatal mice (P0-P4). Conventional whole-cell recordings were obtained from SCs and also from olfactory receptor neurones (ORNs) in situ. SCs had a larger apparent cell capacitance (C cell ) (18.6 ± 0.5 pF) than ORNs (4.4 ± 0.4 pF) and a lower apparent membrane resistance (R m ) (160 ± 11 MΩ versus 664 ± 195 MΩ, respectively). When corrected for a seal resistance of 1 GΩ, these mean R m values were increased to 190 MΩ and 2 GΩ in SCs and ORNs, respectively. SCs generated a TTX (1 µM)-resistant voltage-activated Na + current (I Na ) that had a peak density at −38 mV of −44 pA pF −1 and supported action potential firing. Peak current density of I Na in neurones was 510 ± 96 pA pF