NGF Is an Essential Survival Factor for Bronchial Epithelial Cells during Respiratory Syncytial Virus Infection

Abstract

BackgroundOverall expression of neurotrophins in the respiratory tract is upregulated in infants infected by the respiratory syncytial virus (RSV), but it is unclear where (structural vs. inflammatory cells, upper vs. lower airways) and why, these changes occur. We analyzed systematically the expression of neurotrophic factors and receptors following RSV infection of human nasal, tracheal, and bronchial epithelial cells, and tested the hypothesis that neurotrophins work as innate survival factors for infected respiratory epithelia.MethodologyExpression of neurotrophic factors (nerve growth factor, NGF; brain-derived neurotrophic factor, BDNF) and receptors (trkA, trkB, p75) was analyzed at the protein level by immunofluorescence and flow cytometry and at the mRNA level by real-time PCR. Targeted siRNA was utilized to blunt NGF expression, and its effect on virus-induced apoptosis/necrosis was evaluated by flow cytometry following annexin V/7-AAD staining.Principal FindingsRSV infection was more efficient in cells from more distal (bronchial) vs. more proximal origin. In bronchial cells, RSV infection induced transcript and protein overexpression of NGF and its high-affinity receptor trkA, with concomitant downregulation of the low-affinity p75NTR. In contrast, tracheal cells exhibited an increase in BDNF, trkA and trkB, and nasal cells increased only trkA. RSV-infected bronchial cells transfected with NGF-specific siRNA exhibited decreased trkA and increased p75NTR expression. Furthermore, the survival of bronchial epithelial cells was dramatically decreased when their endogenous NGF supply was depleted prior to RSV infection.Conclusions/SignificanceRSV infection of the distal airway epithelium, but not of the more proximal sections, results in overexpression of NGF and its trkA receptor, while the other p75NTR receptor is markedly downregulated. This pattern of neurotrophin expression confers protection against virus-induced apoptosis, and its inhibition amplifies programmed cell death in the infected bronchial epithelium. Thus, pharmacologic modulation of NGF expression may offer a promising new approach for management of common respiratory infections.

Nanoparticles increase human bronchial epithelial cell susceptibility to respiratory syncytial virus infection via nerve growth factor-induced autophagy

“…Since the lower airways are the natural target of RSV infection, we carried our experiments on human bronchial epithelial cells (Othumpangat et al. 2009). …”

“…Finally, RSV‐infected bronchial cells pre‐exposed to nanoparticles showed a greater increase in the percentage of necrotic cells, with approximately one fifth of cells staining positive for propidium iodide by 24 h, but also maintained a lower rate of apoptosis by upregulating NGF‐TrkA signaling (Othumpangat et al. 2009). Similarly, RSV infection of HEp‐2 cells induces the antiapoptotic factor IEX‐1L (Domachowske et al.…”

“…2009). In particular, the prototypical nerve growth factor (NGF) and its tropomyosin receptor kinase A (TrkA) high‐affinity receptor are overexpressed in RSV‐infected airways leading to persistent changes in the distribution and reactivity of sensory and motor nerves, which results in nonspecific airway hyperreactivity (Piedimonte 2003).…”

“…Our previous studies have also shown that RSV interferes with apoptotic pathways of host airway epithelial cells via NGF‐mediated mechanisms (Othumpangat et al. 2009). …”

Nerve Growth Factor and Sepsis

“…It was determined that NGF prevents apoptosis in a dose-dependent manner, and when anti-NGF antibody is applied, NGF's antiapoptotic effect is completely blocked [77]. NGF has been reported to have a protective effect on respiratory syncytial virus (RSV)-induced apoptosis of airway epithelial cells, and therefore, it has been proposed as a new approach to the maintenance of respiratory tract infections [78].…”