Temporal regulation of BMP signaling in patterning the tracheal cartilage and pharmacological approaches to prevent tracheomalacia

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ABSTRACT Tracheomalacia is the most common structural abnormality in the lower airway. The estimated incidence in children ranges from 1 in 1,500 to 1 in 2,500. Tracheomalacia is very often associated with congenital syndromic disorders and congenital tracheomalacia associated with impaired cartilage integrity often represents the ones with severe symptoms, less regression percentage and higher mortality rate. Currently, surgical intervention is the only option, which requires disproportionately high allocation of health care resources. The lack of knowledge on the tracheal cartilage development and the pathophysiological mechanisms lead to impaired tracheal cartilage integrity prevent searching for alternative interventions for congenital tracheomalacia. BMP signaling has been demonstrated as a critical growth factor necessary for chondrogenesis and cartilage development, however, how BMP signaling is regulated during tracheal cartilage development has not been investigated. EvC syndrome (OMIM 225500) is an autosomal recessive chondrodysplasia. Neonatal death subsequent to airway collapse has been documented in patients with EvC syndrome. Our preliminary studies using Evc2 mutant mice uncover the potential novel mechanisms on the regulated BMP signaling both in foregut mesenchyme and in tracheal mesenchyme. We have demonstrated that well-orchestrated BMP signaling in foregut and tracheal mesenchyme is necessary for tracheal cartilage developing into correct shape. Our preliminary studies lead us hypothesize that: Hedgehog-BMP axis at foregut mesenchyme determines the BMP signaling levels in tracheal mesenchyme and the subsequent chondrogenesis; after trachea and esophagus separation, extracellularly stored BMP ligands play a critical role in inducing BMP signaling in tracheal mesenchyme for subsequent chondrogenesis; and that administration of FK506 has the potential to correct the congenital defective tracheal cartilage in Evc2 mutant mice. This hypothesis will be investigated by the following three aims: 1) To demonstrate that the Hedgehog-BMP axis within the foregut mesenchyme is critical for tracheal cartilage development; 2) To evaluate roles of mesenchymally synthesized and extracellularly stored BMP ligands in inducing chondrogenic differentiation in tracheal mesenchyme; 3) To demonstrate that pharmacological elevation of BMP signaling will rescue the defects in tracheal cartilage development. The outcome of the proposed work will uncover the molecular networks govern the tracheal cartilage development, which will directly set up the knowledge base for the practical therapeutic solutions.