TRPC3 regulates release of brain-derived neurotrophic factor from human airway smooth muscle

Abstract

Exogenous brain-derived neurotrophic factor (BDNF) enhances Ca2+ signaling and cell proliferation in human airway smooth muscle (ASM), especially with inflammation. Human ASM also expresses BDNF, raising the potential for autocrine/paracrine effects. The mechanisms by which ASM BDNF secretion occurs are not known. Transient receptor potential channels (TRPCs) regulate a variety of intracellular processes including store-operated Ca2+ entry (SOCE; including in ASM) and secretion of factors such as cytokines. In human ASM, we tested the hypothesis that TRPC3 regulates BDNF secretion. At baseline, intracellular BDNF was present, and BDNF secretion was detectable by ELISA of cell supernatants or by real-time fluorescence imaging of cells transfected with GFP-BDNF vector. Exposure to the pro-inflammatory cytokine TNFα (20 ng/ml, 48h) or a mixture of allergens (ovalbumin, house dust mite, Alternaria, and Aspergillus extracts) significantly enhanced BDNF secretion and increased TRPC3 expression. TRPC3 knockdown (siRNA or inhibitor Pyr3; 10µM) blunted BDNF secretion, and prevented inflammation effects. Chelation of extracellular Ca2+ (EGTA; 1mM) or intracellular Ca2+ (BAPTA; 5µM) significantly reduced secreted BDNF, as did knockdown of SOCE proteins STIM1 and Orai1 or plasma membrane caveolin-1. Functionally, secreted BDNF had autocrine effects suggested by phosphorylation of high-affinity tropomyosin related kinase TrkB receptor, prevented by chelating extracellular BDNF with chimeric TrkB-Fc. These data emphasize the role of TRPC3 and Ca2+ influx in regulation of BDNF secretion by human ASM and the enhancing effects of inflammation. Given BDNF effects on Ca2+ and cell proliferation, BDNF secretion may contribute to altered airway structure and function in diseases such as asthma.

Airway smooth muscle in airway reactivity and remodeling: what have we learned?

“…However, there are now considerable data that ASM also expresses the TrkB and TrkC receptors for BDNF/neurotrophin 4 and neurotrophin 3, respectively (200,213,236,239,268,291,345). Here, it appears that ASM itself can release BDNF (200,324) both at baseline and in response to inflammatory stimuli. Importantly, BDNF can nongenomically enhance ASM [Ca 2ϩ ] i (236) and potentiate the effects of receptors for neurally derived factors such as the neurokinins (200).…”

“…Although it is not clear whether ASM-derived chemokines serve any other function beyond the expected recruitment of inflammatory cells, it is nonetheless clear that ASM per se plays an important role in the inflamed airway. In addition to these typical inflammatory factors, there is also emerging evidence that growth factors such as VEGF and BDNF can be secreted by ASM cells (5,160,200,324), which may have autocrine effects in terms of cell proliferation as well as contractility. The overall relevance of ASM-derived factors from a clinical perspective lies in the ability of therapies such as glucocorticoids in blunting the secretion of these factors and thus inflammation and its effects as a whole.…”

The Roles of Rasd1 small G proteins and leptin in the activation of TRPC4 transient receptor potential channels

“…It seems that each growth factor activates each TRPC channel. PDGF activates TRPC6, 24 BDNF TRPC3, 25,26 and VEGF TRPC5. 27 The physiological role of these channels was recently established, demonstrating that TRPC4 and TRPC6 are the molecular candidates for non-selective cation channels activated by the muscarinic receptor stimulation (mIcat) in visceral smooth muscle cells.…”

Secreted Brain-Derived Neurotrophic Factor and Asthma Severity

“…Altered BDNF levels have been reported in asthma, allergy, and even lung cancer (3,4). A possible mechanistic role for BDNF has been suggested in airway inflammation, remodeling, and hyperreactivity (3,5,6). All these reports obviously raise questions regarding cell sources versus targets of BDNF.…”

“…Accordingly, ASM-derived BDNF could explain the dichotomy between total BDNF in sputum versus lack of BDNF production by epithelial cells. In terms of release, we demonstrated that BDNF secretion is Ca 21 -dependent and involves TRPC3 channels (6), which are upregulated in asthma (6). Accordingly, enhanced ASM-derived BDNF secretion may be relevant in asthma.…”