Tumor Necrosis Factor-Alpha Induces Airway Hypersensitivity: Role of Ceramide

Airway hypersensitivity, characterized by exaggerated sensory and reflex responses to several stimuli, is a common feature of airway inflammatory diseases such as asthma. Tumor necrosis factor-alpha (TNFα) is critical proinflammatory cytokine and plays an important role in the pathogenesis of allergic asthma. Originally, studies for the role of TNFα in asthma have been focused on its regulation of immune responses. However, our recent study demonstrated that a brief treatment (9 min) of TNFα sensitized isolated lung vagal sensory C-neurons with two characteristics: 1) TRPV1 selective: sensitization only occurred in responses to TRPV1 activator; 2) biphasic (two distinct phases): sensitization occurred immediately (immediate phase) after TNFα pretreatment, declined to baseline, and reached another peak after 60-90 min (delayed phase). However, the mechanism underlying the TNFα-induced hypersensitivity in lung vagal sensory C-neurons remains unclear. Ceramide is a key signaling molecular for TNFα-associated immune response during airway inflammation. Thus, this proposal aims to investigate whether ceramide plays a role in the TNFα-induced hypersensitivity of lung vagal sensory C-neurons and airway hypersensitivity. The proposed study of the 1st-year aims to study 1) whether ceramide is a critical messenger in TNFα-induced sensitization of lung vagal sensory C-neurons; if yes, 2) whether ceramide plays an important role in the TNFα-induced airway hypersensitivity; 3) TNFα receptor subtypes responsible for this ceramide contribution; and 4) whether ceramide mimics TNFα also sensitize these neurons selective to TRPV1 stimuli. The 2nd-year will focus on 1) the cellular signal transduction pathways of ceramide involved in this sensitization: ceramide-1-phosphate, sphingosine-1-phosphate (S1P) and NADPH oxidase-derived reactive oxygen species; and 2) the interaction between these signal transduction pathways with cyclooxygenase-2, a key contributor of lung vagal C-neuron hypersensitivity. Neuronal responses will be determined using Ca2+ imaging and whole-cell perforated patch clamp techniques in rat cultured primary lung vagal sensory C-neurons. Intracellular levels of various synthetic enzymes and products for the related signal transduction will be measured. Pharmacological inhibitors and scavenger will be used as experimental interventions. Airway reflex will be determined, in vivo preparation, by breathing pattern changes to TNFα inhalation in anesthetized, spontaneously breathing rats. Our preliminary results reveal that 1) TNFα enhanced Ca2+ influx evoked by TRPV1 stimuli in lung vagal sensory C-neurons; 2) the TNFα-induced neuronal sensitization was significantly decrease after block the ceramide generation from sphingomyelin; and 3) TNFα-induced sensitization of inward current responses was suppressed after inhibitors of S1P and NADPH oxidase. These results suggest the feasibility of this proposed study. The findings of this proposed study may provide a new insight into potential therapies of airway hypersensitivity.