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Tyrosine phosphorylation evokes functional changes in a variety of ion channels. Modulation of the actin cytoskeleton also affects the function of some channels. Little is known about how these avenues of ion channel regulation may interact. We report that the potassium channel Kv1.2 associates with the actin-binding protein cortactin and that the binding is modulated by tyrosine phosphorylation. Immunocytochemical and biochemical analyses show that Kv1.2 and cortactin co-localize to the cortical actin cytoskeleton at the leading edges of the cell. Binding assays using purified recombinant proteins reveal a 19-amino acid span within the carboxyl terminus of Kv1.2 that is necessary for direct cortactin binding. Phosphorylation of specific tyrosines within the C terminus of Kv1.2 attenuates that binding. In HEK293 cells, activation of the M1 muscarinic acetylcholine receptor evokes tyrosine phosphorylationdependent suppression of Kv1.2 ionic current. We show that M1 receptor activation also reduces the interaction of cortactin with Kv1.2 and that mutant Kv1.2 channels deficient for cortactin binding exhibit strongly attenuated ionic current. These results demonstrate a dynamic, phosphorylation-dependent interaction between Kv1.2 and the actin cytoskeleton-binding protein cortactin and suggest a role for that interaction in the regulation of Kv1.2 ionic current.Ion channels regulate a wide range of cellular processes, including development (1, 2), and neuronal plasticity (3, 4). Accordingly, the activity of nearly all ion channels is under some form of post-translational control, most commonly direct phosphorylation of the ion channel protein (5-7). The earliest and most widely reported examples involve serine/threonine phosphorylation; however, it is also now recognized that tyrosine phosphorylation is an important means of ion channel regulation (8). The nicotinic acetylcholine receptor was the first ligand-gated ion channel found to be regulated by tyrosine phosphorylation (9). The shaker family potassium channel Kv1.2, which is expressed abundantly in cardiac muscle (10) and neurons (11-13), was the first voltage-gated channel found to be so regulated.Kv1.2 ␣-subunit protein becomes tyrosine-phosphorylated upon the activation of M1 muscarinic acetylcholine receptors (mAChRs), 1 leading to a profound suppression of Kv1.2 ionic current (14). Part of the mechanism for such suppression involves phosphorylation of the N-terminal tyrosine Tyr 132 . However, phosphorylation of additional tyrosines within Kv1.2 is also likely to be required, since Y132F mutant channels are only partially resistant to tyrosine phosphorylation and ionic current suppression. Little is known about the signaling proteins involved in tyrosine phosphorylation-dependent suppression of Kv1.2 ionic current; however, binding of the activated form of the guanine nucleotide-binding protein RhoA to Kv1.2 is required (15).RhoA is an important regulator of actin dynamics (16 -18), and the physical association between RhoA and Kv1.2 suggests a link bet...

The intentional adulteration of pet food with melamine and derivatives, including cyanuric acid, has been implicated in the kidney failure and death of a large number of cats and dogs in the United States. Although individually these compounds present low toxicity, coexposure can lead to the formation of melamine cyanurate crystals in the nephrons and eventual kidney failure. To determine the dose-response for nephrotoxicity upon coadministration of melamine and cyanuric acid, groups of male and female F344 rats (six animals per sex per group) were fed 0 (control), 7, 23, 69, 229, or 694 ppm of both melamine and cyanuric acid; 1388 ppm melamine; or 1388 ppm cyanuric acid in the diet for 7 days. No toxicity was observed in the rats exposed to the individual compounds, whereas anorexia and a statistically significant increase in blood urea nitrogen and serum creatinine levels was observed in the animals treated with 229 and 694 ppm melamine and cyanuric acid. The kidneys of these animals were grossly enlarged and pale yellow. Large numbers of crystalline structures deposited in the tubules were seen on sections in kidneys from all rats in these treatment groups. No significant changes were detected in the remaining treatment groups exposed to both melamine and cyanuric acid. In the melamine-only treatment group, 5 of 12 rats had scattered crystals present in renal tubules when examined by wet mount. These were not observed by histopathology. The observed adverse effect level (8.6 mg/kg bw [body weight]/day) and benchmark dose modeling data (8.4-10.9 mg/kg bw/day) determined in this study suggest that the tolerable daily intake values derived from studies conducted with melamine alone may underestimate the risk from coexposures to melamine and cyanuric acid.

ABSTRACT. A study was undertaken to determine the effects of incremental levels of dietary restriction (DR) in rats. Survival, growth, reproductive, and dietary intake (DI)

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