The fish gill is a multipurpose organ that, in addition to providing for aquatic gas exchange, plays dominant roles in osmotic and ionic regulation, acid-base regulation, and excretion of nitrogenous wastes. Thus, despite the fact that all fish groups have functional kidneys, the gill epithelium is the site of many processes that are mediated by renal epithelia in terrestrial vertebrates. Indeed, many of the pathways that mediate these processes in mammalian renal epithelial are expressed in the gill, and many of the extrinsic and intrinsic modulators of these processes are also found in fish endocrine tissues and the gill itself. The basic patterns of gill physiology were outlined over a half century ago, but modern immunological and molecular techniques are bringing new insights into this complicated system. Nevertheless, substantial questions about the evolution of these mechanisms and control remain.

The transcription factor SKN-1 protects Caenorhabditis elegans from stress and promotes longevity. SKN-1 is regulated by diverse signals that control metabolism, development, and stress responses, but the mechanisms of regulation and signal integration are unknown. We screened the C. elegans genome for regulators of cytoprotective gene expression and identified a new SKN-1 regulatory pathway. SKN-1 protein levels, nuclear accumulation, and activity are repressed by the WD40 repeat protein WDR-23, which interacts with the CUL-4/DDB-1 ubiquitin ligase to presumably target the transcription factor for proteasomal degradation. WDR-23 regulates SKN-1 target genes downstream from p38 mitogen-activated protein kinase, glycogen synthase kinase 3, and insulin-like receptor pathways, suggesting that phosphorylation of SKN-1 may function to modify its interaction with WDR-23 and/or CUL-4/DDB-1. These findings define the mechanism of SKN-1 accumulation in the cell nucleus and provide a new mechanistic framework for understanding how phosphorylation signals are integrated to regulate stress resistance and longevity.In response to xenobiotic and oxidative stress, eukaryotic cells activate conserved pathways that increase the expression of phase II detoxification enzymes that scavenge free radicals, synthesize glutathione, and catalyze conjugation reactions that increase xenobiotic solubility and excretion (20). Phase II detoxification plays a central role in preventing age-related diseases, such as cancer and neurodegeneration (34, 39), and in mediating the multidrug resistance of pathogenic fungi, helminthes, and tumor cells (30,44,57).Phase II detoxification in Caenorhabditis elegans is controlled by the transcription factor SKN-1 (1), which promotes stress resistance and longevity (1,2,31,55). In nonstressed animals, SKN-1 is constitutively localized in the nuclei of hypothalamus-like (ASI) neurons, where it is required for life span extension by dietary restriction (5). SKN-1 is absent from the nuclei of other cell types except during exposure to oxidative stress and xenobiotics, which induces its accumulation in intestinal-cell nuclei, where it activates the expression of phase II detoxification genes (1,2,15,27,55). Despite the central role of SKN-1 in stress resistance and longevity, the mechanisms that control nuclear accumulation of the transcription factor are unknown.Phosphorylation of SKN-1 by glycogen synthase kinase 3 (GSK-3) inhibits nuclear accumulation (2). Nuclear accumulation is also inhibited by phosphorylation via SGK-1, AKT-1, and AKT-2 kinases downstream from the insulin-like receptor DAF-2 (55). Conversely, accumulation of SKN-1 in the nucleus is promoted by phosphorylation by a p38 mitogen-activated protein kinase (MAPK) cascade (23) and the activities of at least four other protein kinases (31). Phosphorylation of SKN-1 by these diverse kinases allows C. elegans to integrate phase II gene expression with metabolism, development, stress, and aging (55). However, the mechanisms by which phosphorylation...

Although extracellular matrices function as protective barriers to many types of environmental insult, their role in sensing stress and regulating adaptive gene induction responses has not been studied carefully...

Choe KP, Strange K. Genome-wide RNAi screen and in vivo protein aggregation reporters identify degradation of damaged proteins as an essential hypertonic stress response. Am J Physiol Cell Physiol 295: C1488 -C1498, 2008. First published October 1, 2008 doi:10.1152/ajpcell.00450.2008.-The damaging effects of hypertonic stress on cellular proteins are poorly defined, and almost nothing is known about the pathways that detect and repair hypertonicityinduced protein damage. To begin addressing these problems, we screened ϳ19,000 Caenorhabditis elegans genes by RNA interference (RNAi) feeding and identified 40 that are essential for survival during acute hypertonic stress. Half (20 of 40) of these genes encode proteins that function to detect, transport, and degrade damaged proteins, including components of the ubiquitin-proteasome system, endosomal sorting complexes, and lysosomes. High-molecular-weight ubiquitin conjugates increase during hypertonic stress, suggesting a global change in the ubiquitinylation state of endogenous proteins. Using a polyglutamine-containing fluorescent reporter, we demonstrate that cell shrinkage induces rapid protein aggregation in vivo and that many of the genes that are essential for survival during hypertonic stress function to prevent accumulation of aggregated proteins. High levels of urea, a strong protein denaturant, do not cause aggregation, suggesting that factors such as macromolecular crowding also contribute to protein aggregate formation during cell shrinkage. Acclimation of C. elegans to mild hypertonicity dramatically increases the osmotic threshold for protein aggregation, demonstrating that protein aggregation-inhibiting pathways are activated by osmotic stress. Our studies demonstrate that hypertonic stress induces protein damage in vivo and that detection and degradation of damaged proteins are essential mechanisms for survival under hypertonic conditions.

Cells adapt to stressors by activating mechanisms that repair damage and protect them from further injury. Stress-induced damage accumulates with age and contributes to age associated diseases. Increased age attenuates the ability to mount a stress response, but little is known about the mechanisms by which this occurs. To begin addressing this problem, we studied hormesis in the nematode Caenorhabditis elegans. When exposed to a low concentration of the xenobiotic juglone, young worms mount a robust hormetic stress response and survive a subsequent exposure to a higher concentration of juglone that is normally lethal to naïve animals. Old worms are unable to mount this adaptive response. Microarray and RNAi analyses demonstrate that an altered transcriptional response to juglone is responsible in part for the reduced adaptation of old worms. Many genes differentially regulated in young versus old animals are known or postulated to be regulated by the FOXO homologue DAF-16 and the Nrf2 homologue SKN-1. Activation of these pathways is greatly reduced in juglone stressed old worms. DAF-16- and SKN-1-like transcription factors play highly conserved roles in regulating stress resistance and longevity genes. Our studies provide a foundation for developing a molecular understanding of how age affects cytoprotective transcriptional pathways.