Einfluss des Transkriptionsfaktors Miz1 auf die Regulation des Autophagieprozesses in Fibroblasten

Autophagie ist ein physiologischer und evolutionär hoch konservierter Prozess, bei dem zelluläre Bestandteile sequestriert und schließlich recycelt werden. Er ermöglicht es der Zelle, ihr Überleben unter Stressbedingungen zu sichern und sich veränderten Umweltbedingungen anzupassen. Hierfür werde...

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Bibliographic Details
Main Author: Berliner, Sophie
Contributors: Elsässer, Hans-Peter (Prof. Dr.) (Thesis advisor)
Format: Doctoral Thesis
Published: Philipps-Universität Marburg 2012
Online Access:PDF Full Text
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Autophagy is a physiological and evolutionary highly conserved process in which cellular compo- nents are sequestered and then recycled. It enables the cell to secure its survival under starvation conditions and adapt to changing environmental situations. Cytoplasmic components are engulfed by a bilipidal membrane layer through formation of a phagophore which elongates gradually and finally closes to form an autophagosome. Fusion of the autophagosome with a lysosome enables the degradation of the sequestered cytoplasmic material by lysosomal enzymes. Numerous proteins of autophagy-related genes are involved in the induction of autophagosome formation as well as in the steps of maturation. Previous work provided evidence that the transcription factor Miz1 is involved in the regulation of the autophagic process. Results of this work show that withdrawal of amino acids (incubation with EBSS) induces gene expression of Miz1 which hints to a role of Miz1 in the adaptation to starvation conditions. The importance of Miz1 for the autophagic process was confirmed on a morphological level. Electron microscopy showed a failure of maturation of autophagic vacuoles in cells lacking Miz1. Moreover, it could be shown that this defect could be abrogated by substitution of human Miz1. At the same time incubation with EBSS restored the ability for autophagosomal maturation in ∆POZ cells. These results indicate a different regulation of the autophagic process during basal or stimulated autophagy. After induction of autophagy the inhibition of autophagosomal maturation seems to be overcome by mechanisms independent of Miz1. To further identify which steps of the autophagic process are influenced by Miz1 a series of gene expression analysis of autophagy-associated genes was carried out using cells with and without fully functional Miz1. Experiments were conducted in mouse embryonic fibroblasts that were ei- ther immortalized through 3T3 protocol (3T3 MEFs) or through knockdown of p19ARF by shRNA (MEFsARF ). Results of this work show that the expression of autophagy-associated genes Atg9b and Atg16L2 is regulated through Miz1 during basal autophagy. Moreover, evidence could be ob- tained that further genes of the ubiquitin-like conjugation system might be regulated through Miz1. It could therefore be shown that Miz1 has an effect on certain steps of autophagy on a molecular basis and that this influence is exerted mainly on steps of maturation of autophagosoms. These results obtained through analysis of gene expression complement morphological results showing severe maturation defects of autophagosomes in cells containing solely the truncated Miz1. Initia- tion of autophagy seems to take place largely independently of Miz1. This can be concluded from unchanged levels of gene expression of ULK1 and ULK2 in cells with or without Miz1. The pre- vious findings were confirmed by the fact that autophagic vacuoles formed both in cells containing and in cells not containing fully functional Miz1. It is well known that Miz1 exerts its transcriptional function in cooperation with several partners. It could be shown that Miz1 regulates gene expression of its most important partner Myc. Moreover the results obtained indicate that ARF, another partner of Miz1 and Myc, has substantial influence on Miz1: ARF seems to affect both gene expression of Miz1 and regulation of gene expression of other genes through Miz1. To further define the influence of Miz1 on autophagy it might be useful to conduct more detailled morphological analysis. Results of this work suggest that analysis of gene expression of single Atg genes is not suitable for detecting mechanisms of acute induced autophagy. The results of this work suggest that autophagy is not regulated through altered expression patterns of Atg genes. Possibly it is the expression of Miz1 that is first adapted in response to incubation with EBSS whereas expression of autophagy-associated genes is modified only later. This would imply that modifications of gene expression of Atg genes are not detected when conducting experiments six hours after inducing autophagy because they only take place at a later date.