A novel contribution of autophagic proteins to the process of cell death

Background. Autophagocytosis, also known as a "self eating process" is a mechanism that is primarily used by eukaryotic cells to recycle nutrients rapidly at the onset of environmental stresses such as amino acid or nitrogen starvation. Despite the fact that this process has been known for a number of decades, its mechanism of action remains poorly described. One of the biggest challenges facing this field is the relationship between this "pro-life" phenomenon and the increasingly indisputable evidence linking autophagy and cell death. One of the reasons autophagic cell death (ACD) is questioned is that many of the examples supporting it were obtained using mutants with defects in normal cell death pathways, while many of the examples showing that ACD could be prevented used inhibitors that also affected other processes. One way to resolve this controversy is to develop a new model for studying ACD that can be manipulated genetically. One candidate organism for such studies is Saccharomyces cerevisiae. Since autophagy plays many roles in eukaryotic cell and human pathologies, the results from this analysis could identify novel pathways and gene targets leading to the development of new drugs and treatments.;Methods. In order to examine autophagy functions in response to different stresses I have performed phenotypic analyses on 112 specifically chosen yeast deletion mutants. To distinguish between apoptosis and necrosis I used Annexin V/PI co-staining. Genetic screening for suppressors of cell death identified the yeast vacuolar protein sorting 70 (VPS70) and other members of the transferrin receptor-like protein family (TfRL) including the plant altered meristem program 1 (AMP1) and human prostate specific membrane antigen (PSMA). By means of biochemical examination employing western blot analysis together with changes in the in vivo localization of fluorescent reporters, I monitored how different autophagy pathways are influenced by death-inducing treatments and how the presence of TfRLs are able to alter this response.;Results. As a result of these analyses I first determined that yeast treated with zinc underwent a form of necrosis as they died that was facilitated by a group of autophagy proteins. Secondly, this same treatment also induced other autophagy proteins to carry out a process in opposition to the "pro-death" set so as to prolong yeast survival. Thirdly, I have shown that these two processes also operated when cells were exposed to other stresses such as nitrogen or leucine starvation. Finally, apoptosis and necrosis could be suppressed by any one of three TfRLs including the mammalian Psma.;Conclusions. Based on data I have gathered, I have concluded that Psma and other tested members of the TfRL family possess in common a novel biological activity which can prolong cell survival during exposure to several unrelated stresses. Significantly, none of the experiments found evidence of ACD operating independently of apoptosis and necrosis.