Efficient recycling of waste is not only one of the most important challenges that we face to protect our environment, also each and every cell in our body needs to recycle waste to avoid its accumulation.
However: human cells do much better than human beings! Almost nothing is wasted in cells and all non-functional or superfluous components including organelles, misfolded proteins, but also toxic protein aggregates such as prions are decomposed into their building blocks. Such building blocks, for example amino and fatty acids as well as nucleotides, are re-used to synthesise new cellular components – a perfect cycle.
What happens if this cycle slows down or even stops has been the focus of intense current and past research of many laboratories worldwide. This combined effort revealed that many human diseases including neurodegeneration, cancer, metabolic diseases such as diabetes but also autoimmune diseases and infectious diseases are related to or triggered by inefficient cellular recycling.
How do cells recycle cytoplasmic material?
The diversity of cellular waste in terms of size and shape demands a versatile and adaptive system that is able to select and to sequester this material and to transport it to lysosomes for degradation. The related process has been termed “autophagy” which means self digestion. The beauty of the pathway is that a membrane cisterna is formed which captures cargo and encloses it entirely. This membrane is flexible enough to capture cargo of various shapes and can be expanded to enclose even large cargo. Sealing of this membrane structure gives rise to an autophagosome that delivers its content to lysosomes.
Our research aims to reveal the fundamental molecular mechanisms of autophagy and how dysfunctions of these mechanisms trigger the development of human diseases. We are particularly interested in the question why dysfunctional autophagy contributes to the development of neurodegenerative diseases. To address these questions, we are using a combination of cell biology, biochemistry, high resolution microscopy, electron tomography and genome editing tools.
During normal conditions, autophagy is mainly selective to avoid degradation of functional cytoplasmic components. Upon cytotoxic stresses or starvation, however, this selectivity is lost and autophagy starts to degrade bulk cytoplasm. What appears to be detrimental for the cell under normal conditions protects cells if they are challenged. The degradation of bulk cytoplasm restores building blocks and provides energy to maintain vital cellular functions during starvation and stress. But unleashing autophagy needs to be tightly controlled to avoid excessive degradation which could eventually lead to cellular death. We could recently show how cells control this important switch ().
The switch from selective to non-selective autophagy requires also an ajustement of the physical properties of the autophagic membrane. In selective autophagy, this membrane needs to be flexible to adapt to the shape of its cargo. However, to capture bulk cytoplasm, the membrane needs to expand without templating cargo. We previously demonstrated that a protein shell at the surface of autophagic membranes functions as scaffold to support autophagosome maturation (). Our hypothesis is that this protein shell provides physical support to stabilise the intrinsically flexible autophagic membrane.
Autophagy in the test tube
Our major strength is to combine in vitro reconstitution techniques with advanced and cutting edge in vivo methods including fluorescent microscopy (TIRF, FLIM, live cell imaging) and correlative electron microscopy. To recapitulate the biogenesis of autophagosomes in the test tube, we established a wide variety of different model membrane systems including giant, large, and small unilamellar vesicles and supported lipid bilayers. We used this technique to reconstitute the autophagic scaffold on membranes and to investigate the properties of this scaffold by e.g. fluorescent recovery after photobleaching.
The movie shows such an experiment with a giant unilamellar vesicles (membrane, red) at which the protein shell (green) was reconstituted. FRAP (fluorescence recovery after photobleaching) revealed that the proteins which make up the shell are immobile, whereas the lipids of the membrane diffuse freely in and out of the bleached area. This example demonstrates the power of our combinatorial approach in discovering fundamental molecular mechanisms in autophagy.
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World Soccer Championships 2018
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