Brain-Derived Neurotrophic Factor Enhances Calcium Regulatory Mechanisms in Human Airway Smooth Muscle

Abstract

Neurotrophins (NTs), which play an integral role in neuronal development and function, have been found in non-neuronal tissue (including lung), but their role is still under investigation. Recent reports show that NTs such as brain-derived neurotrophic factor (BDNF) as well as NT receptors are expressed in human airway smooth muscle (ASM). However, their function is still under investigation. We hypothesized that NTs regulate ASM intracellular Ca2+ ([Ca2+]i) by altered expression of Ca2+ regulatory proteins. Human ASM cells isolated from lung samples incidental to patient surgery were incubated for 24 h (overnight) in medium (control) or 1 nM BDNF in the presence vs. absence of inhibitors of signaling cascades (MAP kinases; PI3/Akt; NFκB). Measurement of [Ca2+]i responses to acetylcholine (ACh) and histamine using the Ca2+ indicator fluo-4 showed significantly greater responses following BDNF exposure: effects that were blunted by pathway inhibitors. Western analysis of whole cell lysates showed significantly higher expression of CD38, Orai1, STIM1, IP3 and RyR receptors, and SERCA following BDNF exposure, effects inhibited by inhibitors of the above cascades. The functional significance of BDNF effects were verified by siRNA or pharmacological inhibition of proteins that were altered by this NT. Overall, these data demonstrate that NTs activate signaling pathways in human ASM that lead to enhanced [Ca2+]i responses via increased regulatory protein expression, thus enhancing airway contractility.

Brain-derived neurotrophic factor enhances cholinergic contraction of longitudinal muscle of rabbit intestine via activation of phospholipase C

“…Finally, we measured the peak tonic contraction in response to CCh and SP, whereas in the mouse intestine and colon the contractile agonists elicited an increase in ongoing phasic contractions. It is noteworthy that similar to our findings in gut smooth muscle, preexposure of rat and human airway smooth muscle to BDNF causes an augmentation of muscarinic contractions (1,40,48,49,62).…”

“…Thirdly, we used two TrkB-selective agonists, LM22A4 and 7,8-dihydroxyflavone (1,5,28), and showed that these resulted in augmentation of peak CCh contraction similar to that seen with BDNF (Fig. 4).…”

“…In line with this finding, several reports indicate that BDNF mediates its biological effect in muscle through TrkB receptor activation. For example, BDNF and 7,8-dihydroxyflavone enhanced cholinergic neurotransmission via activation of TrkB rather than the p75NTR receptor in mice diaphragm muscle and rat and human airway smooth muscle (1,37,40) and the augmentation of cholinergic contraction by BDNF in airway smooth muscle is strongly inhibited by K252a and a TrkB antibody (49,57). Finally, we directly measured the ability of BDNF to activate (i.e., phosphorylate) TrkB receptors in cultured rabbit jejunal smooth muscle cells devoid of other cell types.…”

“…The participation of TrkB was further tested with two selective TrkB agonists LM 22A4 and 7,8-dihydroxyflavone, which do not interact with other Trk receptors and have been shown to be effective TrkB agonists in other systems (1,5,28,37). The same approach was used with these agonists as described for BDNF.…”

“…In a similar manner, BDNF is upregulated in airway smooth muscle during airway inflammation and causes a hypercontractility of airway smooth muscle (1,40,48,49,57). This raises the possibility that the increased BDNF expression in gut smooth muscle may account in part for the hypercontractility of the gut muscularis during inflammation.…”