Activation of the metabolic sensor AMP-activated protein kinase inhibits aquaporin-2 function in kidney principal cells.

Aquaporin-2 (AQP2) is essential to maintain body water homeostasis. AQP2 traffics from intracellular vesicles to the apical membrane of kidney collecting duct principal cells in response to vasopressin [arginine vasopressin (AVP)], a hormone released with low intravascular volume, which causes decreased kidney perfusion. Decreased kidney perfusion activates AMP-activated kinase (AMPK), a metabolic sensor that inhibits the activity of several transport proteins. We hypothesized that AMPK activation also inhibits AQP2 function. These putative AMPK effects could protect interstitial ionic gradients required for urinary concentration during metabolic stress when low intravascular volume induces AVP release. Here we found that short-term AMPK activation by treatment with 5-aminoimidazole-4-carboxamide-1-ß-d-ribofuranoside (AICAR; 75 min) in kidney tissue prevented baseline AQP2 apical accumulation in principal cells, but did not prevent AQP2 apical accumulation in response to the AVP analog desmopressin (dDAVP). Prolonged AMPK activation prevented AQP2 cell membrane accumulation in response to forskolin in mouse collecting duct mpkCCDc14 cells. Moreover, AMPK inhibition accelerated hypotonic lysis of Xenopus oocytes expressing AQP2. We performed phosphorylation assays to elucidate the mechanism by which AMPK regulates AQP2. Although AMPK weakly phosphorylated immunoprecipitated AQP2 in vitro, no direct AMPK phosphorylation of the AQP2 COOH-terminus was detected by mass spectrometry. AMPK promoted Ser-261 phosphorylation and antagonized dDAVP-dependent phosphorylation of other AQP2 COOH-terminal sites in cells. Our findings suggest an increasing, time-dependent antagonism of AMPK on AQP2 regulation with AICAR-dependent inhibition of cAMP-dependent apical accumulation and AVP-dependent phosphorylation of AQP2. This inhibition likely occurs via a mechanism that does not involve direct AQP2 phosphorylation by AMPK.

Vacuolar H+-ATPase apical accumulation in kidney intercalated cells is regulated by PKA and AMP-activated protein kinase.

The vacuolar H(+)-ATPase (V-ATPase) in type A kidney intercalated cells is a major contributor to acid excretion in the collecting duct. The mechanisms of V-ATPase-trafficking regulation in kidney intercalated cells have not been well-characterized. In developmentally related epididymal clear cells, we showed previously that PKA, acting downstream of soluble adenylyl cyclase (sAC), induces V-ATPase apical membrane accumulation. These PKA-mediated effects were inhibited by activators of the metabolic sensor AMP-activated kinase (AMPK) in clear cells. Here, we examined the regulation of V-ATPase subcellular localization in intercalated cells by PKA and AMPK in rat kidney tissue slices ex vivo. Immunofluorescence labeling of kidney slices revealed that the PKA activator N(6)-monobutyryl cAMP (6-MB-cAMP) induced V-ATPase apical membrane accumulation in collecting duct intercalated cells, whereas the V-ATPase had a more cytosolic distribution when incubated in Ringer buffer alone for 30 min. V-ATPase accumulated at the apical membrane in intercalated cells in kidney slices incubated in Ringer buffer for 75 min, an effect that was prevented by treatment with PKA inhibitor (mPKI). The V-ATPase distribution was cytosolic in intercalated cells treated with the carbonic anhydrase inhibitor acetazolamide or the sAC inhibitor KH7, effects that were overridden by 6-MB-cAMP. Preincubation of kidney slices with an AMPK activator blocked V-ATPase apical membrane accumulation induced by 6-MB-cAMP, suggesting that AMPK antagonizes cAMP/PKA effects on V-ATPase distribution. Taken together, our results suggest that in intercalated cells V-ATPase subcellular localization and therefore its activity may be coupled to acid-base status via PKA, and metabolic status via AMPK.

Guillain-Barré syndrome as a paraneoplastic manifestation of small-cell carcinoma of lung.

Guillain-Barré syndrome (GBS) encompasses the variants of acute immune-mediated polyneuropathies usually preceded by an infection. A few case reports have associated GBS to neoplastic diseases. It remains unclear whether these are merely coincidental or represent paraneoplastic phenomena. The clinical features of GBS associated with oncological cases do not appear to differ from post-infectious GBS. We report a 74-year-old man in whom small cell carcinoma of lung (SCLC) was diagnosed during a presentation with GBS. Treatment with chemotherapy for SCLC and intravenous immunoglobulins led to complete neurological recovery and tumor regression.