Rac1 modification by an electrophilic 15-deoxy Δ(12,14)-prostaglandin J2 analog.

Vascular endothelial cells (ECs) are important for maintaining vascular homeostasis. Dysfunction of ECs contributes to cardiovascular diseases, including atherosclerosis, and can impair the healing process during vascular injury. An important mediator of EC response to stress is the GTPase Rac1. Rac1 responds to extracellular signals and is involved in cytoskeletal rearrangement, reactive oxygen species generation and cell cycle progression. Rac1 interacts with effector proteins to elicit EC spreading and formation of cell-to-cell junctions. Rac1 activity has recently been shown to be modulated by glutathiolation or S-nitrosation via an active site cysteine residue. However, it is not known whether other redox signaling compounds can modulate Rac1 activity. An important redox signaling mediator is the electrophilic lipid, 15-deoxy-Δ(12,14)-prostaglandin J2 (15d-PGJ2). This compound is a downstream product of cyclooxygenase and forms covalent adducts with specific cysteine residues, and induces cellular signaling in a pleiotropic manner. In this study, we demonstrate that a biotin-tagged analog of 15d-PGJ2 (bt-15d-PGJ2) forms an adduct with Rac1 in vitro at the C157 residue, and an additional adduct was detected on the tryptic peptide associated with C178. Rac1 modification in addition to modulation of Rac1 activity by bt-15d-PGJ2 was observed in cultured ECs. In addition, decreased EC migration and cell spreading were observed in response to the electrophile. These results demonstrate for the first time that Rac1 is a target for 15d-PGJ2 in ECs, and suggest that Rac1 modification by electrophiles such as 15d-PGJ2 may alter redox signaling and EC function.

A model of caching depth: implications for scatter hoarders and plant dispersal.

The ability of foragers to detect stored seeds and nuts is known to decrease with increasing depth at which scatter hoarders bury these food items. Depth of burial of seeds and nuts is known to influence the ability of seedlings to emerge and establish if seeds are not discovered by foragers. These two patterns are combined in a model of caching depth, and implications of the model for food hoarders and plant propagules are explored. Optimal caching depth for hoarders is the depth where the probable recoverable energy in a cache discounted by the energy to make and retrieve a cache is greatest. Optimal burial depth for a propagule is where the probability of it escaping detection by a forager times the probability that it will germinate, emerge, and survive in the absence of predation is maximized. Certain aspects of the cache depth model were tested for yellow pine chipmunks caching the seed of antelope bitterbrush in field and laboratory experiments. The depths yielding the greatest probability of establishment of bitterbrush seedlings was 10-30 mm, with seeds buried 20 mm deep having the greatest success. Yellow pine chipmunks, the primary dispersal agent of bitterbrush seeds in this Sierra Nevada study area, cached most seeds between 5 and 20 mm deep, in the upper portion of the range yielding the greatest bitterbrush establishment. The partial agreement between chipmunk caching behavior and bitterbrush requirements may be largely fortuitous, but certain evolutionary adjustments by the plants may result in a closer match between seedling requirements and the behavior of seed-caching animals.

Scatter-hoarding rodents and marsupials: convergent evolution on diverging continents.

Neotropical rainforests host a rich community of fruit-eating animals, among them neotropical scatter-hoarding rodents that bury seeds in soil. These animals perform an important community-building process because the seeds germinate and establish seedlings away from the parent plant. In three recent studies, researchers have demonstrated a comparable role for paleotropical rodents in southeast Asia. With the discovery that a frugivorous kangaroo behaves similarly to its neotropical rodent counterpart, a new level has been reached in our understanding of the evolution of tropical forests.