Mitochondrial DNA and senescence in Podospora anserina

Control of mitochondrial integrity in ageing and disease

Various molecular and cellular pathways are active in eukaryotes to control the quality and integrity of mitochondria. These pathways are involved in keeping a 'healthy' population of this essential organelle during the lifetime of the organism. Quality control (QC) systems counteract processes that lead to organellar dysfunction manifesting as degenerative diseases and ageing. We discuss disease- and ageing-related pathways involved in mitochondrial QC: mtDNA repair and reorganization, regeneration of oxidized amino acids, refolding and degradation of severely damaged proteins, degradation of whole mitochondria by mitophagy and finally programmed cell death. The control of the integrity of mtDNA and regulation of its expression is essential to remodel single proteins as well as mitochondrial complexes that determine mitochondrial functions. The redundancy of components, such as proteases, and the hierarchies of the QC raise questions about crosstalk between systems and their precise regulation. The understanding of the underlying mechanisms on the genomic, proteomic, organellar and cellular levels holds the key for the development of interventions for mitochondrial dysfunctions, degenerative processes, ageing and age-related diseases resulting from impairments of mitochondria.

A genome-wide longitudinal transcriptome analysis of the aging model Podospora anserina

Aging of biological systems is controlled by various processes which have a potential impact on gene expression. Here we report a genome-wide transcriptome analysis of the fungal aging model Podospora anserina. Total RNA of three individuals of defined age were pooled and analyzed by SuperSAGE (serial analysis of gene expression). A bioinformatics analysis identified different molecular pathways to be affected during aging. While the abundance of transcripts linked to ribosomes and to the proteasome quality control system were found to decrease during aging, those associated with autophagy increase, suggesting that autophagy may act as a compensatory quality control pathway. Transcript profiles associated with the energy metabolism including mitochondrial functions were identified to fluctuate during aging. Comparison of wild-type transcripts, which are continuously down-regulated during aging, with those down-regulated in the long-lived, copper-uptake mutant grisea, validated the relevance of age-related changes in cellular copper metabolism. Overall, we (i) present a unique age-related data set of a longitudinal study of the experimental aging model P. anserina which represents a reference resource for future investigations in a variety of organisms, (ii) suggest autophagy to be a key quality control pathway that becomes active once other pathways fail, and (iii) present testable predictions for subsequent experimental investigations.

Deceleration of fusion-fission cycles improves mitochondrial quality control during aging

Mitochondrial dynamics and mitophagy play a key role in ensuring mitochondrial quality control. Impairment thereof was proposed to be causative to neurodegenerative diseases, diabetes, and cancer. Accumulation of mitochondrial dysfunction was further linked to aging. Here we applied a probabilistic modeling approach integrating our current knowledge on mitochondrial biology allowing us to simulate mitochondrial function and quality control during aging in silico. We demonstrate that cycles of fusion and fission and mitophagy indeed are essential for ensuring a high average quality of mitochondria, even under conditions in which random molecular damage is present. Prompted by earlier observations that mitochondrial fission itself can cause a partial drop in mitochondrial membrane potential, we tested the consequences of mitochondrial dynamics being harmful on its own. Next to directly impairing mitochondrial function, pre-existing molecular damage may be propagated and enhanced across the mitochondrial population by content mixing. In this situation, such an infection-like phenomenon impairs mitochondrial quality control progressively. However, when imposing an age-dependent deceleration of cycles of fusion and fission, we observe a delay in the loss of average quality of mitochondria. This provides a rational why fusion and fission rates are reduced during aging and why loss of a mitochondrial fission factor can extend life span in fungi. We propose the ‘mitochondrial infectious damage adaptation’ (MIDA) model according to which a deceleration of fusion–fission cycles reflects a systemic adaptation increasing life span.

Mitochondria are organelles that play a central role as ‘cellular power plants’. The cellular organization of these organelles involves a dynamic spatial network where mitochondria constantly undergo fusion and fission associated with the mixing of their molecular content. Together with the processes of mitophagy and biogenesis of mitochondrial mass, this results into a cellular surveillance system for maintaining their bioenergetic quality. The accumulation of molecular damage in mitochondria is associated with various human disorders and with aging. However, how these processes affect aging and how they can be reconciled with existing aging theories is just at the beginning to be considered. Mathematical modeling allows simulating the dynamics of mitochondrial quality control during aging in silico and leads to the ‘mitochondrial infectious damage adaptation’ (MIDA) model of aging. It reconciles a number of counterintuitive observations obtained during the last decade including infection-like processes of molecular damage spread, the reduction of fusion and fission rates during cellular aging, and observed life span extension for reduced mitochondrial fission. Interestingly, the MIDA model suggests that a reduction in mitochondrial dynamics rather than being merely a sign or even cause of aging, may actually reflect a systemic adaptation to prolong organismic life span.

Differential proteomic profiling of mitochondria from Podospora anserina, rat and human reveals distinct patterns of age-related oxidative changes

According to the 'free radical theory of ageing', the generation and accumulation of reactive oxygen species are key events during ageing of biological systems. Mitochondria are a major source of ROS and prominent targets for ROS-induced damage. Whereas mitochondrial DNA and membranes were shown to be oxidatively modified with ageing, mitochondrial protein oxidation is not well understood. The purpose of this study was an unbiased investigation of age-related changes in mitochondrial proteins and the molecular pathways by which ROS-induced protein oxidation may disturb cellular homeostasis. In a differential comparison of mitochondrial proteins from young and senescent strains of the fungal ageing model Podospora anserina, from brains of young (5 months) vs. older rats (17 and 31 months), and human cells, with normal and chemically accelerated in vitro ageing, we found certain redundant posttranslationally modified isoforms of subunits of ATP synthase affected across all three species. These appear to represent general susceptible hot spot targets for oxidative chemical changes of proteins accumulating during ageing, and potentially initiating various age-related pathologies and processes. This type of modification is discussed using the example of SAM-dependent O-methyltransferase from P. anserina (PaMTH1), which surprisingly was found to be enriched in mitochondrial preparations of senescent cultures.