Showing papers by "Angelika Amon published in 2018"

MitoCPR-A surveillance pathway that protects mitochondria in response to protein import stress.

Abstract: INTRODUCTION Mitochondria provide cells with energy and numerous essential metabolites such as lipids, amino acids, iron sulfur clusters, and heme. All mitochondrial functions rely on import of proteins into the organelle because the mitochondrial proteome is almost exclusively encoded by nuclear genes. Given the central importance of mitochondria for cell viability, it is not surprising that cells mount a nuclear response when mitochondrial functions are compromised. These mitochondria-to-nucleus signaling pathways include the mtUPR (mitochondrial unfolded protein response), which triggers expression of mitochondrial chaperones when mitochondrial protein folding is defective, and the UPRam (unfolded protein response activated by mistargeting of proteins) and mPOS (mitochondrial precursor over-accumulation stress) pathways, which reduce translation and induce degradation of unimported proteins in the cytosol when mitochondrial import is impaired. Even though mitochondrial import is central to all mitochondrial functions, no response to protein import defects had been described that protects mitochondria during this stress. RATIONALE To determine how cells respond to defects in mitochondrial protein import, we first developed a system in budding yeast with which to specifically inhibit this process. We found that overexpression of proteins that rely on a bipartite signal sequence for their mitochondrial localization inhibited mitochondrial import and led to the accumulation of mitochondrial precursors. Protease protection and carbonate extraction assays that were performed on isolated mitochondria revealed that these unimported proteins accumulated on the mitochondrial surface and in the import channel known as the translocase. RESULTS Having developed a system that allowed us to specifically inhibit mitochondrial protein import, we examined the cellular response to this defect. Transcriptome analysis of cells overexpressing bipartite signal–containing proteins identified a gene expression pattern related to the multi-drug resistance response. We termed this response mitochondrial compromised protein import response (mitoCPR). mitoCPR was triggered by protein import defects but not other mitochondrial deficiencies, such as respiratory failure, and was mediated by the transcription factor Pdr3. Our analyses further showed that mitoCPR was critical for the protection of mitochondria during import stress. Cells lacking PDR3 did not mount a mitoCPR during import stress and accumulated higher levels of unimported proteins on the organelle surface as compared with those of wild-type cells. Consequently, pdr3 Δ cells exhibited decreased respiratory function and loss of mitochondrial DNA when mitochondrial import was restored. Our results also shed light on the mechanism by which mitoCPR protected mitochondria. Upon mitochondrial import stress, Pdr3 induced expression of Cis1. Coimmunoprecipitation analyses showed that Cis1 recruited the AAA + adenosine triphosphatase Msp1 to the translocase by binding to the translocase receptor Tom70. There, the two proteins mediated the clearance and proteasomal degradation of proteins that failed to be imported into mitochondria. CONCLUSION We discovered a mitochondrial import surveillance mechanism in budding yeast. This surveillance mechanism, mitoCPR, is activated when mitochondrial import is stalled in order to induce the removal of mitochondrial proteins accumulating on the mitochondrial surface. Clearance of precursors is critical for maintaining mitochondrial functions during import stress. We propose that mitoCPR could be especially important when the import machinery is overwhelmed, as may occur in situations that require the rapid expansion of the mitochondrial compartment.

Deregulation of the G1/S-phase transition is the proximal cause of mortality in old yeast mother cells.

Abstract: Budding yeast cells produce a finite number of daughter cells before they die. Why old yeast cells stop dividing and die is unclear. We found that age-induced accumulation of the G1/S-phase inhibitor Whi5 and defects in G1/S cyclin transcription cause cell cycle delays and genomic instability that result in cell death. We further identified extrachromosomal rDNA (ribosomal DNA) circles (ERCs) to cause the G1/S cyclin expression defect in old cells. Spontaneous segregation of Whi5 and ERCs into daughter cells rejuvenates old mothers, but daughters that inherit these aging factors die rapidly. Our results identify deregulation of the G1/S-phase transition as the proximal cause of age-induced proliferation decline and cell death in budding yeast.

A somatic evolutionary model of the dynamics of aneuploid cells during hematopoietic reconstitution

Abstract: Aneuploidy is associated with many cancers. Recent studies demonstrate that in the hematopoietic stem and progenitor cell (HSPC) compartment aneuploid cells have reduced fitness and are efficiently purged from the bone marrow. However, early phases of hematopoietic reconstitution following bone marrow transplantation provide a window of opportunity whereby aneuploid cells rise in frequency, only to decline to basal levels thereafter. Here we demonstrate by Monte Carlo modeling that two mechanisms could underlie this aneuploidy peak: rapid expansion of the engrafted HSPC population and bone marrow microenvironment degradation caused by pre-transplantation radiation treatment. Both mechanisms reduce the strength of purifying selection acting in early post-transplantation bone marrow. We explore the contribution of other factors such as alterations in cell division rates that affect the strength of purifying selection, the balance of drift and selection imposed by the HSPC population size, and the mutation-selection balance dependent on the rate of aneuploidy generation per cell division. We propose a somatic evolutionary model for the dynamics of cells with aneuploidy or other fitness-reducing mutations during hematopoietic reconstitution following bone marrow transplantation.