Inhibition of Endocytosis Causes Phosphorylation (S256)-Independent Plasma Membrane Accumulation of AQP-2
|Title:||Inhibition of Endocytosis Causes Phosphorylation (S256)-Independent Plasma Membrane Accumulation of AQP-2|
|Authors:||Lu, MD, PhD, Hua A.J.; Sun, Tian-Xiao; Bouley, Ph.D., Richard; Blackburn, Karen; McLaughlin, M.D., Margaret Elise; Brown, Dennis|
|Publisher:||American Journal of Physiology: Renal Physiology|
|Date Published:||February 01, 2004|
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This translation by the NDI Foundation is to assist the lay reader. To provide a clear, accessible interpretation of the original article, we eliminated or simplified some technical detail and complicated scientific language. We concentrated our translation on those aspects of the article dealing directly with NDI. The NDI Foundation thanks the researchers for their work toward understanding and more effectively treating this disorder.
© Copyright NDI Foundation 2007 (JC)
The movement of AQP2 from within the cell interior to the cell membrane is called exocytosis. Its movement back to the cell interior after serving its function is called endocytosis. The precise mechanics of both AQP2 exocytosis and endocytosis are not fully understood, but until recently the movement of AQP2 to the cell membrane was thought to be dependent on the molecular process initiated when AVP binds with V2R.
However, recent research using laboratory cell cultures indicates that AQP2 can travel to the cell membrane independently of the AVP/V2R initiated process. For example, AQP2 can reach cell culture membrane when cyclic guanosine monophosphate (cGMP) is stimulated, and it has also been shown that AQP2 moves continuously between the cell membrane and cell interior independent of any sort of hormonal stimulation.
Lu, et al., have investigated ways in which AQP2 can gather in the cell membrane independent of AVP. They have shown AQP2 could be prevented from traveling back to the cell interior by inhibiting the process that normally takes it back to the cell interior. In their present study, Lu, et al., used a method of inhibiting AQP2 endocytosis – membrane cholesterol depletion with methyl-b-cyclodextrin (mbCD) – that took only a matter of minutes to produce its effects. This resulted in an accumulation of AQP2 in the cell membrane that was as similar in number and almost as fast as the accumulation of AQP2 caused by AVP.
The researchers were also able to determine whether phosphorylation was necessary for cell membrane accumulation of AQP2. Though phosphorylation is definitely for AQP2 membrane accumulation when it uses the pathway stimulated by AVP/V2R, the research indicating that AQP2 also traveled to and from the cell membrane independently of any hormone suggested that phosphorylation might not always be necessary.
Using two different types of laboratory cell cultures, Lu, et al., determined that they could inhibit AQP2 from returning to the cell interior in a manner that results in rapid accumulation of AQP2 in the cell membrane. Further, this accumulation was independent of AQP2 phosphorylation at Serine 256. (In the classic model that begins with AVP/V2R binding, AQP2 must be phosphorylated at a specific place on it: the serine amino acid residue that is the 256th residue in the chain of amino acid residues that comprise the AQP2 protein.)
The picture the researchers’ study indicates is that AQP2 travels to the cell membrane by at least two pathways: one that is regulated by the hormone AVP, and another that needs no hormone to signal the AQP2 to move to the membrane. In this latter, constitutive, pathway, AQP2s are constantly traveling to and from the cell membrane. The fact that the AQP2s that take this pathway can rapidly accumulate in the membrane when their return trip is inhibited may have important implications for people with X-linked NDI. This is because people with X-linked NDI do not respond to AVP, and thus their AQP2 cannot accumulate in the cell membrane as it should. Should a treatment protocol arise from this research that allows rapid accumulation of AQP2 in the cell membrane by working with another AQP2 pathway, it could prove very beneficial for people with X-linked NDI.