(B–C) HRCT images: diffuse symmetric pattern of microlithiasis, consisting of both discrete nodules and areas of ground glass attenuation of the lung parenchyma. (A) Chest radiograph: symmetric pattern of diffuse fine micronodules in both lungs. Homozygous mutations in the gene SLC34A2 coding for NaPi-IIb are responsible for pulmonary alveolar microlithiasis, a rare disease characterized by the deposition of calcium phosphate microliths throughout the lungs. NaPi-IIa and NaPi-IIc are expressed mostly on the brush border membrane of the proximal tubule. Consequently, phosphate transport by NaPi-IIa and NaPi-IIb is electrogenic whereas NaPi-IIc is electroneutral. These transporters mediate the translocation of one divalent inorganic phosphate (HPO4 (2-)) together with two (NaPi-IIc) or three sodium ions (NaPi-IIa and NaPi-IIb), respectively. The importance of these channels is highlighted by their tissue-specific activity, regulatory pathways, and by the pathology associated with genetic variants. Both proteins are ubiquitous and considered as “housekeeping” transport protein. The SLC20 family has two members, SLC20A1 (also known as PiT-1, OMIM *137570) and SLC20A2 (also known as PiT-2 OMIM: *158378, #213600 AD). Two protein families, SLC20 and SLC34, act as Na +-dependent, secondary-active co-transporters to transport Pi across cell membranes.
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