Bio314: Advanced Cell Biology
Bio314: Advanced Cell Biology |[pic] | |
|Project Description: | |
Characterization of a Mutation in C. elegans, affecting the Unfolded Protein Response.
Introduction
The unfolded protein response (UPR) is a mechanism by which eukaryotic cells respond to stresses which result in the accumulation of unfolded proteins in the endoplasmic reticulum (ER). Cells which fail to respond to such stresses ultimately die. This occurs because the unfolded proteins aggregate with one another and eventually fill the entire ER with insoluble aggregated protein.
A general description of the response can be found in text by Alberts et al. on page106. Recently the UPR in C. elegans has been studied by Shen et al (2001). These authors showed that an intact UPR is required for C. elegans to survive stresses that inhibit proper protein folding in the ER (treatment with dithiothereitol or Tunicamysin). They have identified the genes that participate in the signal pathway which activates the transcription of ER stress protein genes. The gene ire-1, codes for a trans-membrane endonuclease which when activated by unfolded proteins in the ER lumen can signal through the ER membrane to the cytoplasm. When the endonuclease activity of the Ire-1 protein is activated by the presence of unfolded protein it cuts and splices the cytoplasmic mRNA transcribed from the xbp-1gene, activating it for translation into active Xbp-1 protein. Xbp-1 is a transcription factor. This protein
specifically activates transcription of the ER protein folding protein genes (notably hsp-4) by binding to a control element called an X box.
Calfon et al (2002), screened for mutations which make C. elegans sensitive to Tunicamysin and dithiothereitol (DTT). These mutations were presumed to affect the worm’s ability to activate the UPR. They isolated a line of worms that appeared to have no UPR. They verified this by introducing a GFP fusion gene hsp-4::GFP into the germ line of these worms. Hsp-4 is the worm homologue of the human ER chaperone grp78 (Bip). When the transgenic worms were treated with DTT or Tunicamysin they were unable to induce the GFP:: Hsp-4 fusion protein (they did not fluoresce more than controls). The presence of the fusion gene in the mutants, however, is verified by the fact that they do express the gene at a low level under non stressed conditions.
Experimental Question
Your project is to determine whether the mutation which inactivates the UPR in these worms is in the ire-1 gene or in the xbp-1 gene. This will be accomplished by:
1. Analysis of the phenotype of the worms with respect to survival under ER stress and inducibility of Hsp-4.
2. Testing the worms for Ire-1 activity by looking for xbp-1 mRNA splicing in the mutants as compared to normal worms in response to ER stress.
We will conduct the following experiments:
• We will examine the survival curves for normal and mutant worms on plates with Tunicamysin or DTT.
• We will assay the stressed and non stressed worms for the expression of hsp-4 as an indicator for the induction of the UPR.
• We will extract total RNA from normal and mutant worms which are either stressed or unstressed.
• We will use reverse transcriptase PCR to specifically amplify xbp-1 mRNA.
• We will use agarose gel electrophoresis to measure the size of the xbp-1 reverse transcripts. The gels will be examined to determine if the mRNA has been spliced (short mRNA) or un-spliced (long mRNA).
• If the mutants do not splice xbp-1 mRNA). This would suggest that the mutation is in the ire-1 endonuclease gene.
• If the mutants do splice the xbp-1 mRNA then it would suggest that the mutation is in the xbp-1 gene (i.e. the Xbp-1 protein does not properly activate the transcription of the molecular chaperone genes (such as hsp-4) involved in the UPR.
References Cited
Alberts et al. Molecular Biology of the Cell, Fourth edition, Garland Science, New York (2002) p 706
Calfon et al (2002) Nature 415:92-96
Shen et al. (2001) Cell 107:893-903
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