Mitochondrial K+/H+ exchanger

Potassium (K+) is the most abundant ion in cells. It tends to leak into mitochondria with their inside negative charge, thereby causing their swelling and eventually rupture of the organelle. To prevent this damage of structures, mitochondria are assumed to extrude potassium by a K+/H+ exchange system. Many physiological studies supported this model, but experimental evidence was scarce because the gene(s) encoding the K+/H+ exchange system could not be found. We have now identified a gene in the yeast and human genome, named MKH1, which encodes a component of the K+/H+ exchange system. This proposal deals with three aspects: First, using budding yeast as a model organism, we want to further functionally characterize the protein encoded by MKH1, describe the molecular composition of the K+/H+ exchange system and identify novel genes contributing to K+ homeostasis of mitochondria. Second, using human cell culture we will study the the role of mitochondrial K+ homeostasis in the initiation of programmed cell death (apoptosis). We have observed this connection of K+ homeostasis and apoptosis for the first time when we studied genetically induced loss of MKH1. Our genetic approach is novel and gives access to the question how mitochondrial defects might initiate apoptosis. Third, cell cultures of patients with Wolf-Hirschhorn syndrome, caused by a hemizygous deletion including MKH1, will be included in our studies. This work thus may help to understand phenotypic aspects of a genetic disease.

MITOCHONDRIA WITH FUNCTIONAL AND/OR MORPHOLOGICAL DEFECTS ARE SUBJECTED TO PROCESSES OF SELECTIVE AUTOPHAGIC DEGRADATION (`MITOPHAGY`) IN THE VACUOLE (FUNGI) OR THE LYSOSOME (MAMMALIA). IN CASE OF DRAMATIC DEFECTS OF MITOCHONDRIA MITOPHAGY MAY LEAD ON TO PROGRAMMED CELLS DEATH (APOPTOSIS). UNLIKE PREVIOUS STUDIES OF MITOPHAGY WE HAVE FOR THE FIRST TIME THIS PROCESS INDUCED BY DEPLETION OF A MITOCHONDRIAL PROTEIN AND THE USE OF A NOVEL INDICATOR SYSTEM: DOWNREGULATION OF THE SYNTHESIS OF THE YEAST PROTEIN YOL027 (MDM38, A COMPONENT OF THE MITOCHONDRIAL K+/H+ EXCHANGERS) LEADS TO THE ACCUMULATION OF K+ (AND WATER), RESULTING IN SWELLING OF MITOCHONDRIA. AS A CONSEQUENCE THE MITOCHONDRIAL NETWORK GETS DESTROYED AND ITS FRAGMENTS ARE INGESTED AND DEGRADED BY THE VACUOLE. SIMILAR STUDIES WITH THE MDM HOMOLOGOUS PROTEIN IN HUMAN CELLS (LETM1) EQUALLY REVEAL THE DEGRADATION OF MITOCHONDRIA (IN THE LYSOSOME), BUT ALSO INCREASED RATES OF PROGRAMMED CELL DEATH. OUR EXPERIMENTAL APPROACH HAS THE ADVANTAGE OF INDUCING MITOPHAGY WITHOUT PARTICIPATION OF THE GENERAL CELLULAR AUTOPHAGIC PROCESSES. IT CONSTITUTES AN EXCELLENT BASIS FOR THE IDENTIFICATION OF GENES AND PROTEINS WHICH GOVERN THIS PROCESS.