Transient receptor potential cation channel, subfamily V, member 4 and airway sensory afferent activation: Role of adenosine triphosphate

Abstract

Background: Sensory nerves innervating the airways play an important role in regulating various cardiopulmonary functions, maintaining homeostasis under healthy conditions and contributing to pathophysiology in disease states. Hypo-osmotic solutions elicit sensory reflexes, including cough, and are a potent stimulus for airway narrowing in asthmatic patients, but the mechanisms involved are not known. Transient receptor potential cation channel, subfamily V, member 4 (TRPV4) is widely expressed in the respiratory tract, but its role as a peripheral nociceptor has not been explored.

Objective: We hypothesized that TRPV4 is expressed on airway afferents and is a key osmosensor initiating reflex events in the lung.

Methods: We used guinea pig primary cells, tissue bioassay, in vivo electrophysiology, and a guinea pig conscious cough model to investigate a role for TRPV4 in mediating sensory nerve activation in vagal afferents and the possible downstream signaling mechanisms. Human vagus nerve was used to confirm key observations in animal tissues.

Results: Here we show TRPV4-induced activation of guinea pig airway-specific primary nodose ganglion cells. TRPV4 ligands and hypo-osmotic solutions caused depolarization of murine, guinea pig, and human vagus and firing of Aδ-fibers (not C-fibers), which was inhibited by TRPV4 and P2X3 receptor antagonists. Both antagonists blocked TRPV4-induced cough.

Conclusion: This study identifies the TRPV4-ATP-P2X3 interaction as a key osmosensing pathway involved in airway sensory nerve reflexes. The absence of TRPV4-ATP-mediated effects on C-fibers indicates a distinct neurobiology for this ion channel and implicates TRPV4 as a novel therapeutic target for neuronal hyperresponsiveness in the airways and symptoms, such as cough.

Keywords: ATP; Transient receptor potential; cough; hypotonicity; ion channels; sensory nerves; vagus.

Fig E1

Concentration-response curve of GSK2193874- against GSK1016790A (300 nmol/L)–induced depolarization in the guinea pig (N = 4). *P > .05, t test comparing agonist and antagonist responses in the same piece of nerve. V, Vehicle.

Fig E2

A and B, Neither HC67047 (10 μmol/L; Fig E2, A) nor GSK2193874 (10 μmol/L; Fig E2, B) had any effect on TRPV1 (capsaicin)– or TRPA1 (acrolein)–induced depolarization in the guinea pig. C and D, A similar result was seen in mouse vagus. Data are shown as means ± SEMs of 3 to 6 observations.

Fig E3

A, Representative examples of electrophoresis gels for PCR products generated by TRPV4 primers in separate jugular- and nodose-derived neurons. TRPV4 was expressed in 0 of 30 jugular neurons and 1 of 32 nodose neurons. The positive control tissue for TRPV4 was the kidney. B, Representative examples of electrophoresis gels for PCR products generated by P2X3 primers in separate jugular- and nodose-derived neurons. P2X3 was expressed in 23 of 30 cells from the jugular and 17 of 32 cells from the nodose ganglia. The positive control tissue for P2X3 was the trigeminal ganglia. C, Representative examples of electrophoresis gels for PCR products generated by P2X3 primers in separate jugular- and nodose-derived neurons. P2X2 was expressed in 1 of 30 jugular neurons and 13 of 32 of nodose neurons. All cells analyzed expressed both PGP9.5 and β-actin. +, Positive control; J, jugular-derived neurons; L, ladder; N, nodose-derived neurons; W, water control.

Fig E4

Example trace from an Aδ-fiber where the top panel indicates tracheal pressure (cm H₂O) and the bottom panel indicates single-fiber firing. αβ-MeATP (300 μmol/L) caused firing in Aδ-fibers but had no effect on bronchoconstriction.

Fig E5

Neither AF-353 (10 μmol/L) nor TNPATP (10 μmol/L) had any effect on capsaicin (A and B)– or acrolein (C and D)–induced depolarization in guinea pig vagus. Data are shown as means ± SEMs of n = 3 to 6 observations.

Fig E6

A, Example vehicle trace. After 2 reproducible responses to αβ-MeATP, vehicle (saline in 10% PEG400) was administered, and the following nerve responses to αβ-MeATP or GSK1016790A remained unaffected. B, Conversely, after AF-353 administration, αβ-MeATP response was significantly reduced, as was the response to GSK1016790A compared with vehicle control. C, Example trace after GSK2193874 administration where nerve firing to GSK1016790A was reduced; however, there was no effect on αβ-MeATP–induced firing.

Fig E7

Aerosol of GSK1016790A caused a concentration-dependent increase in the number of coughs in the conscious guinea pig. Vehicle had no effect on coughing. Data are shown as means ± SEMs of 4 observations.

Fig 1

A and B, Bright-field and pseudocolor images of an airway-specific neuron stained with the retrograde tracer DiI (Fig 1, A) and the same neuron exposed to GSK1016790A (100 nmol/L) showing a [Ca²⁺]_i level increase, with the right-hand side indicating images taken during real-time recording (Fig 1, B). C and D, Effect of GSK1016790A on [Ca²⁺]_i levels in DiI-stained neurons for responsive-only nodose (Fig 1, C) and jugular neurons (Fig 1, D). Data are presented as means ± SEMs of 4 to 11 observations and 4 to 6 guinea pigs. *Statistical significance (P < .05) compared with relevant control.

Fig 2

A-F, Effect of vehicle (0.1% DMSO), the TRPV4 agonists GSK1016790A and 4-αPDD, and hypo-osmotic solution on the activation of guinea pig (Fig 2, A-C) and mouse (Fig 2, D-F) isolated vagus nerve compared with capsaicin as a reference (n = 4-6). G, Activation of human vagus nerve by GSK1016790A (300 nmol/L), 4α-PDD (1 μmol/L), and −80 mOsm (n = 3). Data are presented as means ± SEMs. *Statistical significance (P < .05) compared with relevant control.

Fig 3

A-F, H, and I, Effect of the TRPV4 antagonist (HC067047) on either the TRPV4 agonists GSK1016790A (300 nmol/L)– or 4α-PDD (1 μmol/L)–induced or hypo-osmotic solution (–80 mOsm)–induced depolarization of guinea pig (Fig 3, A-C; n = 4-6), mouse (Fig 3, D-F; n = 4-6), and human (Fig 3, H) isolated vagus nerve (n = 2-3; example trace is shown in Fig 3, I). G, Depolarization of the vagus from Trpv4^−/− and wild-type (WT) control mice to GSK1016790A (300 nmol/L), 4α-PDD (1 μmol/L), hypo-osmotic solution (−80 mOsm), capsaicin (1 μmol/L), and acrolein (300 μmol/L; n = 4-6). Data are presented as means ± SEMs. *Statistical significance (P < .05) compared with relevant control.

Fig 4

A, Trace indicates changes in tracheal pressure (top) and action potential firing (bottom) of Aδ- and C-fibers in response to GSK1016790A (10 μg/mL). B and C, Total impulses were significantly increased after application of citric acid (CA; 0.3 mol/L) and GSK1016790A (10 μg/mL) in Aδ-fibers (Fig 4, B), but only capsaicin (Caps; 100 μmol/L) and citric acid (0.3 mol/L) significantly increased firing in C-fibers (Fig 4, C). D and E, Hypo-osmotic solution (−80 mOsm) significantly increased total impulses in Aδ-fibers but had no effect on C-fibers (Fig 4, D and E, respectively). Data are presented as means ± SEMs of 3 or 4 observations. Veh, Vehicle. *Statistical significance (P < .05), paired t test comparing responses before and after aerosol administration of agonist.

Fig 5

A and B, αβ-MeATP (10 μmol/L) caused an increase in [Ca²⁺]_i levels predominantly from guinea pig airway–specific nodose ganglion neurons (Fig 5, A) and a concentration-dependent depolarization of the guinea pig vagus nerve (Fig 5, B). C and D, This response was inhibited by the P2X3 antagonists AF-353 (10 μmol/L) and TNP-ATP (10 μmol/L) in both guinea pig (Fig 5, C) and donor human (Fig 5, D) vagus. E, Responses to GSK1016790A (300 nmol/L) were virtually abolished in Px1^−/− mice. KO, Knockout; WT, wild-type. F and G, Both GSK1016790A- and −80 mOsm–induced depolarization in guinea pig (Fig 5, F) and donor human (Fig 5, G) vagus was inhibited by AF-353 (10 μmol/L) and TNP-ATP (10 μmol/L). H, [Ca²⁺]_i signal induced by GSK1016790A (30 nmol/L) in the guinea pig airway–specific nodose ganglia neurons was also inhibited by AF-353 (10 μmol/L; N = 3; n = 4). Data are presented as means ± SEMs of 4 to 6 observations for guinea pig and 2 to 3 observations for human experiments. *Statistical significance (P < .05), unpaired t test comparing responses in Px1^−/− vagus with wild-type control or GSK1016790A-induced [Ca²⁺]_i responses in nodose neurons.

Fig 6

A-D, Effect of vehicle (10% PEG in saline, 10 mL/kg administered intraperitoneally) or AF-353 (30 mg/kg administered intraperitoneally) on firing of Aδ-fibers induced by αβ-MeATP (aerosolized at 300 μmol/L for 1 minute; shown in Fig 6, A and B, respectively) or hypo-osmotic solution (−80 mOsm aerosolized for 1 minute; Fig 6, C and D). E-H, Effect of vehicle (6% Cavitron in saline, 10 mL/kg administered intraperitoneally) or GSK2193874 (300 mg/kg administered intraperitoneally) on firing of Aδ-fibers induced by GSK1016790A (aerosolized at 100 ng/mL for 1 minute) or αβ-MeATP or hypo-osmotic solution (all n = 3). I-K, Effect of vehicle, TRPV4 (HC067047, 100 mg/kg; GSK2193874, 300 mg/kg administered intraperitoneally), or P2X3 (AF-353, 30 mg/kg administered intraperitoneally) antagonists on cough induced by the TRPV4 agonist GSK1016790A (30 μg/mL, aerosolized for 5 minutes) is shown, and coughs were counted for 10 minutes. Data were expressed as means ± SEMs. *Statistical significance (P < .05) compared with relevant control (n = 8-12). #Statistical significance (P < .05) comparing GSK1016790A to vehicle control.