Phosphate metabolism

Phosphates are absorbed from foodstuff. Specialized channel proteins called sodium-phosphate transporters 2b (or NaPi2b) located at the surface of the epithelial cells of the small intestine are performing the task. About 1.5 g of phosphate are captured daily by this process by a normal adult.

 

 

 

Once in the bloodstream, phosphates can be absorbed by organs and tissues. A part of it is stored in bones.

Then, phosphates reach the kidneys where most of it is filtered out of the blood. But before being eliminated in the urine, another channel protein (NaPi2a) similar to the one that capture phosphate from foodstuff bring it back to bloodstream in a process called 'reabsorption'. This steps of filtration and reabsorption taking place in the kidney are crucial for the maintenance of phosphate levels.

 

 

 

Regulation of phosphate metabolism - where it occurs

 

Basically, regulation of body's phosphate level can occur at different levels:

 

-at the site of entry (absorption): it is accomplished by modulating the number of NaPi2b channels. this mechanism is under regulation by calcitriol (1,25)-OH vitamin D3 (or 1,25-D3).

 

-at different sites of use inside the body. Organs and tissue will 'fish' phosphates in the bloodstream to fulfill their needs. Bone is a heavy consumer of phosphates both for maintenance of the structure and for the activity of their cells. Parathyroid is a special organ sensitive to the calcium (and phosphorus) levels. When calcium rises in the blood, parathyroid will release the hormone PTH.

 

-at the site of elimination. In the kidneys, phosphate are first filtered out of the system (filtration). Then, a specialized mechanism called reabsorption occurs: phosphates are again filtered out of the urine and reinjected into the bloodstream. this is a major source of regulation of the blood phopshate. NaPi2a channels of the proximal tubules are doing this job. Napi2a is under the regulation of calcitriol (1,25-D3) and of FGF23. 

 

-the main regulators of phosphate are 1,25-D3, PTH and FGF23. FGF23 is synthesized by bone cells called osteocytes and released in the bloodstream. It serves as a signal that is understood only by kidney cells (those posessing the appropriate receptor called FGFR1-Klotho) . By an unknown mechanism, FGF 23 is inhibited by PHEX. We will see more of the regulatory mechanism in the next section below.

 

 

Regulation of phosphate metabolism - how it occurs.

We will look at little bit a the interplay between the different actors of the phosphate regulation. In the figure above, red arrows means a positive effect (upregulation, stimulation) and obviously blue arrows means a downregulation (inhibition).

 

Starting from bone cells: the protein Phex (mutated in XLH) exerts a dowregulation on the synthesis of FGF23. So, the level of FGF23 in the blood is moderate. FGF23 is sensed by kidney cells posessing the co-receptor for FGF23. This triggers a signal resulting in downregualtion of 1,25-D3 and NaPi2a. 1,25-D3 is stimulating intestinal absorption of phosphate (via NaPi2b) and NaPi2a is responsible for the re-aborption of phosphate in the kidney. But since the FGF23 level is moderate, so is the phosphate wasting signal and hence, phosphate levels stay balanced.

 

In XLH, Phex is mutated and cannot exert its influence on FGF23. As a consequence, there is an overproduction of FGF23. So, the phosphate wasting signal is enhanced and this results in the main symptoms of hypophosphatemia.