2004 Global Researcher Conference Proceeding
April 09 - 11, 2004
| Conference: | 2004 Global Researcher Conference |
|---|---|
| Title: | Bypassing the vasopressin receptor in aquaporin 2 trafficking: from cell biology to potential therapy of nephrogenic diabetes insipidus |
| Authors: | Brown, Dennis; Lu, MD, PhD, Hua A.J.; Bouley, Ph.D., Richard |
| Institutions: | Massachusetts General Hospital, Massachusetts General Hospital and Harvard Medical School |
AQP2 is a fascinating protein whose sorting signals can be interpreted by different cell types to achieve apical or basolateral membrane insertion, in both regulated and constitutive trafficking pathways. In addition to the well-known cAMP-mediated, stimulatory effect of vasopressin on AQP2 membrane insertion, other signaling and trafficking events can also lead to AQP2 membrane accumulation via cAMP-independent mechanisms. These include, 1) elevation of cGMP, mediated by sodium nitroprusside (SNP) (a nitric oxide donor), atrial natriuretic factor, and L-arginine (via nitric oxide synthase); 2) disruption of the actin cytoskeleton; 3) inhibition of the clathrin-mediated endocytotic arm of the AQP2 recycling pathway by dominant negative dynamin expression and by membrane cholesterol depletion with methy-beta-cyclodextrin. Recent data also indicate that AQP2 recycles constitutively in epithelial cells, that it can be inserted into different membrane domains in different cell types both in vitro and in vivo, and that these pathways can be modulated by factors including hypertonicity. The roles of accessory proteins including small GTPases and SNARE proteins in AQP2 membrane insertion is also being uncovered. We have also used cGMP phosphodiesterase inhibition as a means of inducing cell surface accumulation of AQP2. These data indicate that activating the vasopressin receptor (V2R) is not essential to achieve AQP2 membrane insertion. Understanding cAMP-independent mechanisms for membrane insertion of AQP2 is especially relevant to the therapeutic bypassing of the mutated, dysfunctional vasopressin receptor in patients with X-linked nephrogenic diabetes insipidus.
The most well known path by which AQP2 finds its way to the cell membrane begins when the hormone, arginine vasopressin (AVP) binds with the vasopressin 2 receptor (V2R). When this happens, the cellular level of cyclic andenosine monophosphate (cAMP) increases. This activates protein kinase A (PKA), which in turn stimulates other parts of the molecular cascade that results in AQP2 moving from the cell interior to insert itself in the cell membrane. People with X-linked NDI have faulty V2Rs and thus the molecular cascade that gets AQP2 to the cell membranes does not occur.
Fortunately, scientific research into AQP2 is revealing that there are several other ways AQP2 can travel from the cell interior to the cell membrane: 1) elevating levels of cyclic guanoside monophosphate (cGMP), 2) disrupting the actin cytoskeleton that reinforces the cytoplasm in a cell’s interior, 3) inhibiting the natural return of the AQP2 from the cell membrane to the cell interior are all ways by which AQP2 can be accumulated in the cell membrane. In addition, recent research reveals that AQP2 can also cycle to and from the membrane without any need for hormonal signals in epithelial cells. Finally, Brown, et al., are discovering the role of accessory proteins in the movement of AQP2. In short, they are finding that there are other avenues by which AQP2 can be accumulated in the cell membrane than being stimulated to do so by V2R.
Pursuing this research may result in a way in which AQP2 can be induced to accumulate in the principal cells of the kidney in people with X-linked NDI.