A power outage not only leaves us sitting in the dark, it also affects many other things: we are no longer able to communicate with the outside world or cook food. The scenario resulting from severe damage to the mitochondria, which are the body’s energy producers, is fairly similar. Especially neurons are very sensitive to disturbances and many different functions are no longer able to work efficiently when the power supply is impaired. Prof. Dr. Chris Meisinger and his team at the Institute of Biochemistry and Molecular Biology at the University of Freiburg have found that amyloid-beta peptides block important enzymes in the mitochondria, resulting in mitochondrial dysfunction.
Amyloid-beta (Aβ), a peptide consisting of around 40 amino acids, has once again attracted the attention of the global scientific community. Aβ forms upon the sequential cleavage of the amyloid precursor protein (APP), an integral membrane protein whose function is not yet known in detail. In the healthy brain, the fragments are broken down and eliminated. It is assumed that APP plays a key role in the formation of synapses and that healthy Aβ levels control the excitability of neurons, thus protecting them from harmful hyperactivity.
However, the accumulation of larger Aβ clusters in the brain has a neurotoxic effect and can lead to the death of the neurons – so the hypothesis goes. It is assumed that the increased production of Aβ is, amongst other things, due to genetic factors and elevated cholesterol levels. However, the exact mechanism of the neurotoxicity of Aβ is not yet known. A widely held scientific opinion is that this is because Aβ is not optimally degraded and removed. Aβ is still regarded as the major culprit of the appearance of the senile plaques found in the brains of Alzheimer’s patients and people with Down syndrome. However, evidence is still lacking. Researchers have found that the formation of plaques does not correlate 100% with the progression of Alzheimer’s disease. In addition, the exact organelle where Aβ is produced and its exact physiological role are still contentious. With Meisinger’s findings, there is reason to believe that Aβ damages the nerve cells in a different way than previously thought.
This hypothesis is based on the observation that Aβ peptides are not exclusively released into the extracellular space. Prof. Dr. Chris Meisinger from the Institute of Biochemistry and Molecular Biology at the University of Freiburg is not specialised in the pathogenesis of Alzheimer’s disease, but for many years he has been studying the mechanisms of mitochondrial protein biogenesis. “There is evidence that Aβ also accumulates inside cells, resulting in cellular dysfunction. And the most remarkable thing is that Aβ is also found in the mitochondria,” says Meisinger going on to explain that this was the reason why he and his team developed a greater interest in Aβ. Meisinger believes that it is necessary to completely revise some of the hypotheses, work models and existing Alzheimer’s therapies. Meisinger and his colleagues carried out experiments with transgenic yeast cells and found that the majority of Aβ, which is expressed in the nucleus and translated into protein in the cytosol, emigrates into the mitochondria. Meisinger and his team are mainly focused on the translocation of proteins into the mitochondria. More than 90 percent of all mitochondrial proteins are encoded in the cell nucleus and translated into large preproteins by cytosolic ribosomes. The mitochondrial import machinery – the TOM (translocase of outer mitochondrial membrane) and TIM (translocase of inner mitochondrial membrane) enzymes – transports the proteins to their final destination. In order to be recognized by the import receptors on the outer mitochondrial membrane, the preproteins need to be equipped with an N-terminal signal sequence. This ‘passport’ sequence is 20 to 80 amino acids long and has a positive charge. The signal sequence is proteolytically removed by the mitochondrial processing peptidase (MPP) shortly after the import of the preprotein through the pores of the membrane, releasing mature proteins with a functional conformation. The leftover presequences are cut in pieces and eliminated by specific peptidases (PreP in mammals and Cym1 in yeast).
“Presequences that are not properly degraded will accumulate and in turn block the enzyme that normally cleaves off these signal peptides,” says Meisinger. It seems that the coupling of signal peptide degradation and processing is necessary for the protein maturation machinery to work properly. Dysfunctional preprotein maturation leads to the accumulation of immature proteins in the mitochondria, which has toxic consequences for the affected cell. The preproteins of other essential proteins cannot fold properly, are instable and cannot exert their specific function. “This impairs cellular respiration; the cells consume less oxygen than normal and also produce less ATP. In short, the mitochondria are no longer able to fulfil their role as the cell’s power stations,” says Meisinger.
The crux is that in addition to the signal peptides, Aβ is also a known substrate for the proteolytic proteases. The researchers found that Aβ inhibits the degradation of presequence peptides by PreP (Cym1), resulting in the accumulation of immature mitochondrial preproteins and processing intermediates. “This shows that the processing of the signal peptides is crucial for the stability and proper function of proteins,” says Meisinger. In the mitochondria, Aβ impairs the action of MPP so that it is less effective in cutting off the signal sequences. Preproteins are unable to mature and are broken down prematurely. The entire mitochondrial proteome is thrown off balance; crucial proteins in all functional areas such as the respiratory chain, TCA cycle and β oxidation are lacking.
The researchers then came up with the idea of using genetically modified yeast that produced larger quantities of Aβ in order to find out whether this led to the presumed feedback inhibition of MPP. They found a large number of preproteins and hardly any mature proteins, and were thus able to confirm that the impaired turnover of presequence peptides results in the feedback inhibition of presequence processing enzyme. “The result is that we have a model that helps us explain why the presence of excessive Aβ affects so many different functions,” says the molecular biologist. “It also shows how Aβ is able to impair mitochondrial performance, namely that it interferes with the step all mitochondrial proteins have to go through on their way from the cytosol into the mitochondria.”
It makes no difference whether the respiratory chain complex or the enzyme superoxide dismutase is affected first. Mitochondrial dysfunction always leads to the loss of the body’s most important energy producers. This has drastic consequences, especially for the nervous system, which is an organ that consumes a lot of energy. An energy loss of 15 percent or more is fatal and drives the cell into apoptosis. Meisinger, who works with yeasts, mouse models and human post-mortem material, has observed that immature preproteins accumulate in the brains of Alzheimer patients. His goal is to develop a diagnostic test that enables a more effective and earlier diagnosis of Alzheimer’s. The researchers have found immature mitochondrial proteins in blood cells from Alzheimer’s patients and have already developed an antibody that specifically recognizes the signal peptide of preproteins. “Blood tests that are able to identify preproteins would become useful diagnostics to differentiate between healthy people and people suffering from Alzheimer’s.
Further information:Prof. Dr. Chris MeisingerInstitute of Biochemistry and Molecular BiologyUniversity of FreiburgStefan-Meier-Str. 1779104 FreiburgTel.: +49 (0)761 / 203 - 5287Fax: +49 80)761/ 203 - 5261E-mail: chris.meisinger(at)biochemie.uni-freiburg.de