Heat shock genes and the response to environmental stress during zebrafish development.

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Heat shock proteins are expressed in cells following exposure to environmental stress such as elevated temperature or toxic metals, and help cells survive these periods of stress. Several hsps also play important roles as molecular chaperones that normally help other proteins fold and reach their correct destination in the cell. It is important for a cell to be able to regulate where, when, and how much a heat shock protein is expressed, and it does this largely be regulating the expression of the gene that codes for the hsp. Toxic metals such as mercury and cadmium are a significant concern, and have negative effects on developing embryos. Mercury causes a range of problems, including visual defects in both humans and fish. Cadmium can damage the olfactory organ of both humans and fish, causing a loss of the sense of smell. A major question regarding toxic metals is whether they accumulate more in some cells and tissues than others, and whether the chemical form of the metal affects this process. Our work uses the zebrafish embryo as a model to examine the effects of environmental stress on development. Zebrafish are a popular model system since much of their development occurs in a similar way to human development, in addition to being an excellent model to look at fish development itself. We have used synchrotron x-rays to examine where in zebrafish embryos the toxic metal mercury accumulates. We found that one of the major targets is cells of sensory systems, in particular parts of the eye, olfactory organ, and a specialized sensory organ in fish known as the lateral line. We have also examined a specific heat shock protein known as hsp70. Interestingly, hsp70 is expressed normally during eye development, and is required for normal eye formation. This gene is also expressed in both the olfactory organ and lateral line following exposure to another toxic metal, cadmium. Experiments in this proposal will examine the impact of exposing zebrafish embryos to different types of mercury, and whether the cells that take up large amounts of mercury are also the most sensitive. This will include looking at whether cells in the sensory organs are more prone to death, and whether the proper function of the organs is disrupted. We will also examine how the hsp70 gene is turned on and off in mercury and cadmium treated embryos, and during normal development of the eye lens. Some of the work will use a transgenic fish model we have developed that fluoresces bright green in cells that express hsp70, allowing for very rapid detection of stressed cells. In summary, the work will allow us to develop an integrated model of how regulation of the hsp70 is gene is fine-tuned during normal development and following exposure of embryos to environmental stress, and how this relates to other events occurring in target cells ranging from toxic metal uptake and accumulation to functional defects in target organs. This work will be important to our understanding of how cells in an embryo respond to and survive exposure to environmental stress at the molecular and cellular level. We will be able to correlate impacts on sensory cells and their function with i) the actual amount of a toxic metal in these cells, ii) how the hsp70 gene is activated in response to this stress, and iii) how this activation may assist embryonic cells in surviving the stress event. It will also allow us to develop a better understanding of how the hsp70 gene is expressed during normal development of the eye, and further elucidate the importance of this event. This work will further determine if our transgenic fish strain can be used as a rapid fluorescent predictor of negative outcomes in some sensory organs.