UV-induced DNA damage is a major cause of age-related skin diseases and photoaging. Although the body can develop defenses over time to UV stress (i.e., melanin concentration and thickening of the stratum corneum), there is a fundamental gap in understanding whether cells can mount an effective immediate protective response to UV. We recently discovered an immediate and robust UV-induced DNA protection mechanism that involves a global chromatin compaction triggered by calcium influx. The chromatin compaction and DNA protection responses were demonstrated in human HeLa cells, and the compaction was also demonstrated in NIH2/4 mouse embryonic fibroblasts and in the roundworm C. elegans. Our long-term goal is to understand the molecular basis of this mechanism, and to investigate if it can be manipulated to increase our natural protection from UV damage. We will explore whether this mechanism declines with age like other stress resistance mechanisms. The objectives of this application are to develop the C. elegans as a model system to probe the age-dependence of this stress resistance mechanism and to use human epidermal melanocytes in combination with C. elegans to gain the first glimpse into the molecular pathway of this UV- induced DNA protective chromatin compaction. The central hypothesis is that chromatin compaction is triggered by a conserved molecular machinery of a photoreceptor acting through the G?q/11-coupled phototransduction pathway to activate a calcium influx. We hypothesize that this is an evolutionarily conserved response that is less efficient in aged organisms. The rationale for choosing C. elegans is that it is a well- established model system for studying aging with superb genetic and developmental tools. Human primary epidermal melanocytes were chosen because they are human skin cells in which a specific pathway involving photoreceptors and G?q/11 was shown to control a UV-induced calcium influx. The specific aims for this research are: 1) Test whether UV-induced chromatin compaction protects DNA from further damage in human epidermal melanocytes (HEMs) and in young and old C. elegans. We will UV irradiate HEMs and C. elegans and detect the rate of removal of photoproducts by Southwestern blotting (for HEMs and C. elegans) and in vivo (for C. elegans). 2) Determine whether UVR-induced chromatin compaction involves the G?q/11-coupled phototransduction pathway and calcium influx in human epidermal melanocytes and C. elegans. This aim will be achieved by RNAi/siRNA knock downs in C. elegans and human primary epidermal melanocytes. The proposed research is innovative because it explores a previously undescribed DNA defense mechanism from UV radiation and it sets as a goal establishing C. elegans as a model system for relating this phenomenon to aging. Adding to our innovative capacity is the ongoing cross-pollination between a C. elegans lab and a mammalian chromatin lab. The results will be significant because new potential targets that control the DNA- protective pathway will be discovered which can lead to better pharmaceutical strategies to protect from UV.