Modulation of Acid-sensing Ion Channel 1a by Intracellular pH and Its Role in Ischemic Stroke.

An important contributor to brain ischemia is known to be extracellular acidosis, which activates acid-sensing ion channels (ASICs), a family of proton-gated sodium channels. Lines of evidence suggest that targeting ASICs may lead to novel therapeutic strategies for stroke. Investigations of the role of ASICs in ischemic brain injury have naturally focused on the role of extracellular pH in ASIC activation. By contrast, intracellular pH (pHi) has received little attention. This is a significant gap in our understanding because the ASIC response to extracellular pH is modulated by pHi, and activation of ASICs by extracellular protons is paradoxically enhanced by intracellular alkalosis. Our previous studies show that acidosis-induced cell injury in in vitro models is attenuated by intracellular acidification. However, whether pHi affects ischemic brain injury in vivo is completely unknown. Furthermore, whereas ASICs in native neurons are composed of different subunits characterized by distinct electrophysiological/pharmacological properties, the subunit-dependent modulation of ASIC activity by pHi has not been investigated. Using a combination of in vitro and in vivo ischemic brain injury models, electrophysiological, biochemical, and molecular biological approaches, we show that the intracellular alkalizing agent quinine potentiates, whereas the intracellular acidifying agent propionate inhibits, oxygen-glucose deprivation-induced cell injury in vitro and brain ischemia-induced infarct volume in vivo Moreover, we find that the potentiation of ASICs by quinine depends on the presence of the ASIC1a, ASIC2a subunits, but not ASIC1b, ASIC3 subunits. Furthermore, we have determined the amino acids in ASIC1a that are involved in the modulation of ASICs by pHi.

Regarding the charmed-strange member of the 2³S1 meson state.

By employing the mass relations derived from the mass matrix and Regge trajectory, we investigate the masses of charmed and charmed-strange members of the 2³S1 meson. The masses are compared with the values predicted by other theoretical approaches and experimental data. The results may be useful for the discovery of the unobserved meson and the determination of the quantum number of the newly discovered states.

NPA motifs play a key role in plasma membrane targeting of aquaporin-4.

The two highly conserved NPA motifs (asparagine-proline-alanine, NPA) are the most important structural domains that play a crucial role in water-selective permeation in aquaporin water channels. However, the functions of NPA motifs in aquaporin (AQP) biogenesis remain largely unknown. Few AQP members with variations in NPA motifs such as AQP11 and AQP12 do not express in the plasma membrane, suggesting an important role of NPA motifs in AQP plasma membrane targeting. In this study, we examined the role of the two NPA motifs in AQP4 plasma membrane targeting by mutagenesis. We constructed a series of AQP4 mutants with NPA deletions or single amino acid substitutions in AQP4-M1 and AQP4-M23 isoforms and analyzed their expression patterns in transiently transfected FRT and COS-7 cells. Western blot analysis showed similar protein bands of all the AQP4 mutants and the wild-type AQP4. AQP4 immunofluorescence indicated that deletion of one or both NPA motifs resulted in defective plasma membrane targeting, with apparent retention in endoplasmic reticulum (ER). The A99T mutant mimicking AQP12 results in ER retention, whereas the A99C mutant mimicking AQP11 expresses normally in plasma membrane. Furthermore, the AQP4-M1 but not the M23 isoform with P98A substitution in the first NPA motif can target to the plasma membrane, indicating an interaction of N-terminal sequence of AQP4-M1 with the first NPA motif. These results suggest that NPA motifs play a key role in plasma membrane expression of AQP4 but are not involved in AQP4 protein synthesis and degradation. The NPA motifs may interact with other structural domains in the regulation of membrane trafficking during aquaporin biogenesis.

Water channel activity of plasma membrane affects chondrocyte migration and adhesion.

1. Recent studies indicate that the aquaporin-1 (AQP1) water channel is expressed in human and equine articular chondrocytes. The role of AQP1 in chondrocyte function has not been characterized. In the present study, we investigated the expression of the AQP1 water channel in cultured articular chondrocytes from wild-type (AQP1(+/+)) and AQP1-knockout (AQP1(-/-)) mice and characterized its function in chondrocyte proliferation, migration and adhesion. 2. Expression of AQP1 mRNA and protein was identified in freshly isolated neonatal AQP(+/+) chondrocytes. Immunofluorescence localized the AQP1 protein to the plasma membrane of AQP(+/+) chondrocytes in primary cultures. Relative plasma membrane water permeability of AQP1(+/+) chondrocytes was approximately 1.6-fold higher than that of AQP1(-/-) chondrocytes. 3. The chondrocyte proliferation rate was not affected by AQP1 deletion. However, the serum-induced transwell migration rate of AQP1(-/-) chondrocytes was markedly reduced compared with AQP1(+/+) chondrocytes (16.2 +/- 0.2 vs 27.1 +/- 0.3%, respectively; P < 0.01). Cell adhesion to type II collagen-coated plates was also significantly reduced in AQP1(-/-) chondrocytes compared with AQP1(+/+) chondrocytes (38.1 +/- 0.3 vs 51 +/- 1%, respectively; P < 0.01). 4. The results provided direct evidence that AQP1-mediated plasma membrane water permeability plays an important role in chondrocyte migration and adhesion.

Aquaporins as potential drug targets.

The aquaporins (AQP) are a family of integral membrane proteins that selectively transport water and, in some cases, small neutral solutes such as glycerol and urea. Thirteen mammalian AQP have been molecularly identified and localized to various epithelial, endothelial and other tissues. Phenotype studies of transgenic mouse models of AQP knockout, mutation, and in some cases humans with AQP mutations have demonstrated essential roles for AQP in mammalian physiology and pathophysiology, including urinary concentrating function, exocrine glandular fluid secretion, brain edema formation, regulation of intracranial and intraocular pressure, skin hydration, fat metabolism, tumor angiogenesis and cell migration. These studies suggest that AQP may be potential drug targets for not only new diuretic reagents for various forms of pathological water retention, but also targets for novel therapy of brain edema, inflammatory disease, glaucoma, obesity, and cancer. However, potent AQP modulators for in vivo application remain to be discovered.