Berschneider 2016 Thesis
|Berschneider K (2016) Connecting the functions of the proteasome and mitochondria in the lung. Dissertation p161.|
Abstract: In living cells the cellular proteome is under constant remodeling as cells adapt to altered physiological states or as cellular proteins become misfolded and need to be degraded. The cellular processes which grant maintenance of a functional proteome are collectively called proteostasis. The proteasome constitutes one of the major intracellular protein degradation systems and is responsible for the turnover of damaged or unwanted proteins with almost 90% of the intracellular proteins being potential proteasome substrates. Especially for misfolded proteins, a timely degradation is necessary to avoid proteotoxic stress, e.g. by formation of protein aggregates. Degradation of proteins by the proteasome system is a very energy demanding process. Hence it was speculated that proteasomal function is interconnected with the function of mitochondria. Additionally, a correlation between mitochondrial and proteasomal dysfunction exists since a decline in the function of both systems is well recognized as hallmark of aging. However, it is unclear how proteasomal and mitochondrial function are directly linked together, especially in settings of chronic mitochondrial respiratory chain dysfunction. Moreover, limited knowledge is available to which extent proteasomal and mitochondrial functions contribute to aging processes specifically in lung tissue during healthy aging or in the development of age-related lung diseases such as chronic obstructive pulmonary disease.
• Keywords: MLE12 Mouse lung epithelial cells
Labels: MiParea: Respiration, mtDNA;mt-genetics, Comparative MiP;environmental MiP, Exercise physiology;nutrition;life style, Pharmacology;toxicology Pathology: Aging;senescence, COPD
Organism: Mouse Tissue;cell: Lung;gill Preparation: Permeabilized cells
Coupling state: LEAK, ROUTINE, ET Pathway: ROX HRR: Oxygraph-2k
In the first part of this study, the connection between mitochondrial dysfunction and the proteasome was assessed by analyzing proteasome function in the so-called “mtDNA mutator” mouse model, a model of chronic mitochondrial respiratory chain dysfunction. A strong interconnection between mitochondrial and proteasomal function was confirmed in mouse embryonic fibroblasts of mtDNA mutator mice as in these cells decreased proteasomal activity and reduced levels of assembled 26S and 30S proteasomes was found based on a pronounced mitochondrial respiratory chain dysfunction. Importantly, human dermal fibroblasts from patients with single mutations in mitochondrial genes accordingly showed reduced proteasome activity. Furthermore, it was shown that in addition to acute regulation by reactive oxygen species or ATP levels, further signaling mechanisms exist, which connect mitochondrial and proteasomal function in chronic settings.
Second, to assess mitochondrial alterations in response to environmental challenges in the lung, the effect of cigarette smoke exposure on mitochondrial function and quality control in alveolar epithelial cells was analyzed. Murine alveolar epithelial cells exhibited pronounced mitochondrial hyperfusion after treatment with cigarette smoke extract, which was accompanied by increased metabolic activity. Cigarette smoke extract-induced mitochondrial hyperfusion was not associated with a protein stress response at mitochondria. Furthermore, no alteration in mitochondrial protein quality control by the proteasome was observed. Therefore, mitochondrial hyperfusion seems to be an adaptive pro-survival response of alveolar epithelial cell mitochondria to nontoxic concentrations of cigarette smoke extract.
Finally, the specific involvement of both systems, proteasome and mitochondria, in healthy aging of the lung was assessed. Therefore typical features of healthy lung aging and proteasome function were analyzed in young and aged wildtype, proteasome reporter and immunoproteasome knockout mice. Hereby, it was observed that immunoproteasome subunits were upregulated in the lungs of aged mice and the caspase-like proteasome activity was concomitantly decreased. However, aged knockout mice for the immunoproteasome subunits LMP2 or LMP7 showed no alteration in proteasome activities while exhibiting typical lung aging phenotypes. This suggests that immunoproteasome function is dispensable for physiological lung aging in mice. These results indicate that healthy aging of the lung does not involve impairment of proteasome function. Furthermore, no connection between mitochondrial dysfunction and lung aging was found in prematurely aging mtDNA mutator mice.
Altogether, this study confirms the hypothesis that the functions of the proteasome and mitochondria are closely connected. Furthermore, it shows that mitochondria as well as the proteasome system provide some spare capacity, which enables them to remain functional upon mild insults during healthy aging or exposure to mild environmental hazards. However, due to the close interconnection of both systems, the simultaneous burden of an age-related functional decline together with noxious environmental exposures such as cigarette smoke might push proteasomal and mitochondrial dysfunction beyond a damage threshold thereby contributing to the pathogenesis of disease states or to a combined functional decline of both systems as described in aging.