WNKs are potassium-sensitive kinases.

With no lysine (K) (WNK) kinases regulate epithelial ion transport in the kidney to maintain homeostasis of electrolyte concentrations and blood pressure. Chloride ion directly binds WNK kinases to inhibit autophosphorylation and activation. Changes in extracellular potassium are thought to regulate WNKs through changes in intracellular chloride. Prior studies demonstrate that in some distal nephron epithelial cells, intracellular potassium changes with chronic low- or high-potassium diet. We, therefore, investigated whether potassium regulates WNK activity independent of chloride. We found decreased activity of Drosophila WNK and mammalian WNK3 and WNK4 in fly Malpighian (renal) tubules bathed in high extracellular potassium, even when intracellular chloride was kept constant at either ∼13 mM or 26 mM. High extracellular potassium also inhibited chloride-insensitive mutants of WNK3 and WNK4. High extracellular rubidium was also inhibitory and increased tubule rubidium. The Na+/K+-ATPase inhibitor, ouabain, which is expected to lower intracellular potassium, increased tubule Drosophila WNK activity. In vitro, potassium increased the melting temperature of Drosophila WNK, WNK1, and WNK3 kinase domains, indicating ion binding to the kinase. Potassium inhibited in vitro autophosphorylation of Drosophila WNK and WNK3, and also inhibited WNK3 and WNK4 phosphorylation of their substrate, Ste20-related proline/alanine-rich kinase (SPAK). The greatest sensitivity of WNK4 to potassium occurred in the range of 80-180 mM, encompassing physiological intracellular potassium concentrations. Together, these data indicate chloride-independent potassium inhibition of Drosophila and mammalian WNK kinases through direct effects of potassium ion on the kinase.

TRPV1 temperature activation is specifically sensitive to strong decreases in amino acid hydrophobicity.

Several transient receptor potential (TRP) ion channels can be directly activated by hot or cold temperature with high sensitivity. However, the structures and molecular mechanism giving rise to their high temperature sensitivity are not fully understood. One hypothesized mechanism assumes that temperature activation is driven by the exposure of hydrophobic residues to solvent. This mechanism further predicts that residues are exposed to solvent in a coordinated fashion, but without necessarily being located in close proximity to each other. However, there is little experimental evidence supporting this mechanism in TRP channels. Here, we combined high-throughput mutagenesis, functional screening, and deep sequencing to identify mutations from a total of ~7,300 TRPV1 random mutant clones. We found that strong decreases in hydrophobicity of amino acids are better tolerated for activation by capsaicin than for activation by hot temperature, suggesting that strong hydrophobicity might be specifically required for temperature activation. Altogether, our work provides initial correlative support for a previously hypothesized temperature mechanism in TRP ion channels.

Directionality of temperature activation in mouse TRPA1 ion channel can be inverted by single-point mutations in ankyrin repeat six.

Several transient receptor potential (TRP) ion channels are activated with high sensitivity by either cold or hot temperatures. However, structures and mechanism that determine temperature directionality (cold versus heat) are not established. Here we screened 12,000 random mutant clones of the cold-activated mouse TRPA1 ion channel with a heat stimulus. We identified three single-point mutations that are individually sufficient to make mouse TRPA1 warm activated, while leaving sensitivity to chemicals unaffected. Mutant channels have high temperature sensitivity of voltage activation, specifically of channel opening, but not channel closing, which is reminiscent of other heat-activated TRP channels. All mutations are located in ankyrin repeat six, which identifies this domain as a sensitive modulator of thermal activation. We propose that a change in the coupling of temperature sensing to channel gating generates this sensitivity to warm temperatures. Our results demonstrate that minimal changes in protein sequence are sufficient to generate a wide diversity of thermal sensitivities in TRPA1.

Allelic specificity of Ube3a expression in the mouse brain during postnatal development.

Genetic alterations of the maternal UBE3A allele result in Angelman syndrome (AS), a neurodevelopmental disorder characterized by severe developmental delay, lack of speech, and difficulty with movement and balance. The combined effects of maternal UBE3A mutation and cell type-specific epigenetic silencing of paternal UBE3A are hypothesized to result in a complete loss of functional UBE3A protein in neurons. However, the allelic specificity of UBE3A expression in neurons and other cell types in the brain has yet to be characterized throughout development, including the early postnatal period when AS phenotypes emerge. Here we define maternal and paternal allele-specific Ube3a protein expression throughout postnatal brain development in the mouse, a species that exhibits orthologous epigenetic silencing of paternal Ube3a in neurons and AS-like behavioral phenotypes subsequent to maternal Ube3a deletion. We find that neurons downregulate paternal Ube3a protein expression as they mature and, with the exception of neurons born from postnatal stem cell niches, do not express detectable paternal Ube3a beyond the first postnatal week. By contrast, neurons express maternal Ube3a throughout postnatal development, during which time localization of the protein becomes increasingly nuclear. Unlike neurons, astrocytes and oligodendrotyes biallelically express Ube3a. Notably, mature oligodendrocytes emerge as the predominant Ube3a-expressing glial cell type in the cortex and white matter tracts during postnatal development. These findings demonstrate the spatiotemporal characteristics of allele-specific Ube3a expression in key brain cell types, thereby improving our understanding of the developmental parameters of paternal Ube3a silencing and the cellular basis of AS.

Surface-mediated production of hydroxyl radicals as a mechanism of iron oxide nanoparticle biotoxicity.

Emerging applications of nanosized iron oxides in nanotechnology introduce vast quantities of nanomaterials into the human environment, thus raising some concerns. Here we report that the surface of γ-Fe(2)O(3) nanoparticles 20-40 nm in diameter mediates production of highly reactive hydroxyl radicals (OH(•)) under conditions of the biologically relevant superoxide-driven Fenton reaction. By conducting comparative spin-trapping EPR experiments, we show that the free radical production is attributed primarily to the catalytic reactions at the nanoparticles' surface rather than being caused by the dissolved metal ions released by the nanoparticles as previously thought. Moreover, the catalytic centers on the nanoparticle surface were found to be at least 50-fold more effective in OH(•) radical production than the dissolved Fe(3+) ions. Conventional surface modification methods such as passivating the nanoparticles' surface with up to 935 molecules of oleate or up to 18 molecules of bovine serum albumin per iron oxide core were found to be rather ineffective in suppressing production of the hydroxyl radicals. The experimental protocols developed in this study could be used as one of the approaches for developing analytical assays for assessing the free radical generating activity of a variety of nanomaterials that is potentially related to their biotoxicity.