Neurotrophins in the Lung

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Airway hyperreactivity (AHR) and remodeling in asthma involve increased airway smooth muscle (ASM) contractility, mass, and extracellular matrix (ECM) driven by inflammation. ASM actively secretes growth factors that modulate airway structure/function via autocrine/paracrine influences. In previous cycles, we identified brain-derived neurotrophic factor (BDNF) as an ASM-derived factor with autocrine enhancement of ASM contractility, proliferation and fibrosis. Within this purview, we discovered glial-derived neurotrophic factor (GDNF) and a related member neurturin (NRTN) as novel growth factors in the airway that promote inflammation effects. GDNF and NRTN have protective roles in the nervous system but there is minimal to no information on GDNF or NRTN in airway biology or asthma, particularly for ASM. Preliminary studies show that A) Human ASM expresses and secretes GDNF and NRTN in response to agonist, with increased release by TNFα or TGFβ and in asthmatic ASM; B) GDNF and NRTN receptors Ret, GFRα1 and GFRα2 are present in ASM with increased expression in inflammation/asthma; C) Exogenous GDNF and NRTN have pleiotropic effects on ASM, enhancing [Ca2+]cyt and contractility, promoting ECM formation, and intriguingly ER stress, mitochondrial fission, mitochondrial Ca2+ and respiration; D) GDNF and NRTN can interact via GFRα1. In vivo studies in mixed allergen (MA) mouse models of asthma show 1) GDNF enhances airway reactivity; 2) Ret inhibition or chelation of GDNF blunt MA effects on AHR and remodeling. Based on these data, we propose an overall hypothesis that ASM expression and autocrine signaling by GDNF ligand family contributes to AHR and remodeling in asthma. We will test this concept via four Aims, focusing particularly on the novel role of ASM-derived GDNF and NRTN. Our Aims are: Aim 1: To examine mechanisms of upstream regulation of GDNF vs. NRTN in human ASM; Aim 2: To examine mechanisms by which GDNF vs. NRTN enhance Ca2+/contractility in human ASM in the context of inflammation and asthma; Aim 3: To examine mechanisms by which GDNF vs. NRTN enhance remodeling in human ASM in the context of inflammation and asthma; Aim 4: To examine in vivo importance of GDNF vs. NRTN in the context of AHR and remodeling using a mixed allergen mouse model of asthma. Aims 1-3 utilize human epithelium-denuded ASM tissues and isolated ASM cells from mild or moderate asthmatics vs. non-asthmatics to examine signaling mechanisms by which inflammatory mediators enhance GDNF/NRTN production (Aim 1), the receptor and intracellular pathways by which these ligands influence contractility (Aim 2) vs. ER stress, mitochondrial structure/function and proliferation/ECM (Aim 3). Aim 4 applies the MA model to mice where GDNF vs. NRTN is enhanced or inhibited, particularly in smooth muscle and explores changes in airway structure, ECM composition, and mechanics. Clinical significance lies in establishing the role of ASM-derived growth factors such as GDNF or NRTN that influence multiple aspects of asthma pathophysiology and are appealing therapeutic targets.