Basal expression of bone morphogenetic protein receptor is reduced in mild asthma

Rationale: Despite increasing recognition of bone morphogenetic protein (BMP) signaling in tissue remodeling, the expression pattern of ligands and signaling pathways remain undefined in the asthmatic airway. Objectives: To determine expression of BMP ligands (BMP-2, BMP-4, and BMP-7) and type I and type II receptors (ALK-2, ALK-3, ALK-6, and BMPRII) as well as evidence for activation of BMP signaling via detection of phosphorylated Smad1/5 (pSmad1/5) expression in asthmatic airways at baseline (compared with nonasthmatic controls), and after allergen challenge. Methods: Bronchial biopsies were obtained from 6 nonasthmatic control volunteers, and 15 atopic patients with asthma (median age, 25 yr; median FEV1% predicted, 97%) at baseline, then at 24 hours and 7 days after allergen challenge. Expression of BMP ligands, receptors, and signaling was analyzed using immunohistochemistry. Measurements and Main Results: BMP ligand expression did not differ between asthmatic and control airways at baseline. Compared with the normal airway, there was significant down-regulation of ALK-2 (P = 0.001), ALK-6 (P = 0.0009), and BMPRII (P = 0.009) expression in asthma. Allergen challenge was associated with marked and sustained up-regulation of BMP-7 in airway epithelium (P = 0.017) and infiltrating inflammatory cells (P = 0.071) (predominantly in eosinophils, but also CD4(+) T cells, mast cells, and macrophages). Upregulation of pSmad1/5 expression (P = 0.031), ALK-2 (P = 0.002), and ALK-6 (P < 0.001) was observed indicating active signaling. Conclusions: BMP receptor expression is down-regulated in the asthmatic airway, which may impede repair responses. Allergen provocation increases expression of the regulatory ligand BMP-7, activates BMP signaling, and increases receptor expression, all of which may contribute to repair and control of inflammation.