Modeling pulmonary alveolar microlithiasis by epithelial deletion of the Npt2b sodium phosphate cotransporter reveals putative biomarkers and strategies for treatment

Pulmonary alveolar microlithiasis (PAM) is a rare, autosomal recessive lung disorder associated with progressive accumulation of calcium phosphate microliths. Inactivating mutations in SLC34A2, which encodes the NPT2b sodiumdependent phosphate cotransporter, has been proposed as a cause of PAM.Weshow that epithelial deletion ofNpt2b in mice results in a progressive pulmonary process characterized by diffuse alveolar microlith accumulation, radiographic opacification, restrictive physiology, inflammation, fibrosis, and an unexpected alveolar phospholipidosis. Cytokine and surfactant protein elevations in the alveolar lavage and serum of PAM mice and confirmed in serum from PAM patients identify serum MCP-1 (monocyte chemotactic protein 1) and SP-D (surfactant protein D) as potential biomarkers.Microliths introduced by adoptive transfer into the lungs of wild-typemice produce markedmacrophagerich inflammation and elevation of serum MCP-1 that peaks at 1 week and resolves at 1 month, concomitant with clearance of stones. Microliths isolated by bronchoalveolar lavage readily dissolve in EDTA, and therapeutic wholelung EDTA lavage reduces the burden of stones in the lungs. A low-phosphate diet prevents microlith formation in young animals and reduces lung injury on the basis of reduction in serum SP-D. The burden of pulmonary calcium deposits in established PAM is also diminished within 4 weeks by a low-phosphate diet challenge. These data support a causative role for Npt2b in the pathogenesis of PAM and the use of the PAMmouse model as a preclinical platform for the development of biomarkers and therapeutic strategies

Selected contribution: long-term effects of beta(2)-adrenergic receptor stimulation on alveolar fluid clearance in mice.

Stimulation of active fluid transport with beta-adrenergic receptor (betaAR) agonists can accelerate the resolution of alveolar edema. However, chronic betaAR-agonist administration may cause betaAR desensitization and downregulation that may impair physiological responsiveness to betaAR-agonist stimulation. Therefore, we measured baseline and terbutaline- (10(-3) M) stimulated alveolar fluid clearance in mice that received subcutaneously (miniosmotic pumps) either saline or albuterol (2 mg. kg(-1). day(-1)) for 1, 3, or 6 days. Continuous albuterol administration increased plasma albuterol levels (10(-5) M), an effect that was associated with 1) a significant decrease in betaAR density and 2) attenuation, but not ablation, of maximal terbutaline-induced cAMP production. Forskolin-mediated cAMP-release was unaffected. Continuous albuterol infusion stimulated alveolar fluid clearance on day 1 but did not increase alveolar fluid clearance on days 3 and 6. However, terbutaline-stimulated alveolar fluid clearance in albuterol-treated mice was not reduced compared with saline-treated mice. Despite significant reductions in betaAR density and agonist-mediated cAMP production by long-term betaAR-agonist exposure, maximal betaAR-agonist-mediated increase in alveolar fluid clearance is not diminished in mice