Zooming-in on protein teamwork

November 03, 2017

FRANKFURT. The surface of every cell contains receptors that react to external signals similar to a "gate". In this way, the cells of the innate immune system can differentiate between friend and foe partly through their "toll-like receptors" (TLRs). Two parts of this gate often work together here, as researchers at Goethe University Frankfurt and their British colleagues have now found out with the help of a new super-resolution optical microscopy technique.

When the German Nobel Prize winner Christiane Nüsslein-Volhard discovered receptors in the fruit fly (Drosophila melanogaster) in the 1990s that transduced signals from the cell surface into a cellular response, she was amazed. She nicknamed the receptors "toll" (amazing) and this term has meanwhile become firmly established in scientific literature. Since then, similar receptors (toll-like receptors) have also been discovered in animals and humans. They recognize bacteria, viruses and fungi and thus ensure that our body reacts to infections in a suitable way. By contrast, de-regulated TLRs can lead to chronic inflammatory conditions and cancer.

Experiments conducted so far indicated that TLRs are activated by a chemical signal that causes two proteins to cluster together as dimers. This process, which is known as "dimerization", appears to play a pivotal role in a cell's fate: It can decide whether the cell survives, dies or moves within the body. Because dimerization takes place on a molecular scale that cannot be captured using conventional microscopy techniques, researchers have to date been dependent on indirect measuring methods. These were, however, prone to error and yielded diverging results. This has now changed thanks to the new super-resolution optical microscopy technique.

In the forthcoming issue of Science Signaling, the working groups led by Professor Mike Heilemann of Goethe University Frankfurt and by Dr. Darius Widera and Dr. Graeme Cottrell of the University of Reading in England describe how they have studied the organization of the TLR4 receptor on the cell surface in molecular resolution. In a first step, they used a super-resolution microscope with a resolution about 100 times better than a standard fluorescence microscope. Since this was still not sufficient to make single receptor molecules in a tiny protein dimer visible, the researchers developed a more sophisticated analysis of the optical signal. In this way they were able to zoom in closer on the super-resolution images and examine under which conditions TLR4 forms a monomer or a dimer. The researchers could also detect which chemical signals from different pathogens modulate the receptors' patterns.

The researchers hope that their work will lead in future to a better understanding of how TLR dimerization affects the decision between the life or death of a cell. It might also be possible to determine how pharmaceutical ingredients targeted at TLRs influence the behavior of cancer cells. "It is also conceivable that this approach will help us in future to understand better the fundamental biological processes that regulate the immune system in health and disease. At the same time, this microscopy method is also applicable to other membrane proteins and many similar questions," explains Professor Mike Heilemann from the Institute of Physical and Theoretical Chemistry at Goethe University Frankfurt.
-end-
Publication:

Carmen L. Krüger, Marie-Theres Zeuner, Graeme S. Cottrell, Darius Widera, Mike Heilemann: Quantitative single-molecule imaging of TLR4 reveals ligand-specific receptor dimerization, Science Signaling, doi: 10.1126/scisignal.aan1308

A picture can be downloaded under http://www.muk.uni-frankfurt.de/68944753

Caption: Left: Conventional light microscopy is an useful tool in visualising biological structures and processes. However, its resolution is not sufficient to study events occurring at molecular scale. The image on the left shows the nuclei of brain tumour cells (yellow: nuclei containing DNA) with Toll-like receptors 4 localised at the cell surface (cyan spots). Although many TLR4 can be clearly seen, the spatial resolution does not allow determination of single receptor units. Middle: Super-resolution microscopy greatly improves the spatial resolution and allows detection of single TLR4 clusters (cyan) at the surface of the cells. However, even at this superior resolution, it is not possible to distinguish between monomers and dimers of the receptor. Right: Crystal structure of a TLR4 dimer. The novel analysis method developed by the consortium is able to provide information allowing differentiating between receptor monomers and dimers.

Image rights: Widera/Heilemann

Further information: Professor Mike Heilemann, Institute of Physical and Theoretical Chemistry, Faculty of Biochemistry, Chemistry and Pharmacy, Riedberg Campus, Tel.: +49(0)69-798- 29736, Heilemann@chemie.uni-frankfurt.de.

Goethe University Frankfurt

Related Immune System Articles from Brightsurf:

How the immune system remembers viruses
For a person to acquire immunity to a disease, T cells must develop into memory cells after contact with the pathogen.

How does the immune system develop in the first days of life?
Researchers highlight the anti-inflammatory response taking place after birth and designed to shield the newborn from infection.

Memory training for the immune system
The immune system will memorize the pathogen after an infection and can therefore react promptly after reinfection with the same pathogen.

Immune system may have another job -- combatting depression
An inflammatory autoimmune response within the central nervous system similar to one linked to neurodegenerative diseases such as multiple sclerosis (MS) has also been found in the spinal fluid of healthy people, according to a new Yale-led study comparing immune system cells in the spinal fluid of MS patients and healthy subjects.

COVID-19: Immune system derails
Contrary to what has been generally assumed so far, a severe course of COVID-19 does not solely result in a strong immune reaction - rather, the immune response is caught in a continuous loop of activation and inhibition.

Immune cell steroids help tumours suppress the immune system, offering new drug targets
Tumours found to evade the immune system by telling immune cells to produce immunosuppressive steroids.

Immune system -- Knocked off balance
Instead of protecting us, the immune system can sometimes go awry, as in the case of autoimmune diseases and allergies.

Too much salt weakens the immune system
A high-salt diet is not only bad for one's blood pressure, but also for the immune system.

Parkinson's and the immune system
Mutations in the Parkin gene are a common cause of hereditary forms of Parkinson's disease.

How an immune system regulator shifts the balance of immune cells
Researchers have provided new insight on the role of cyclic AMP (cAMP) in regulating the immune response.

Read More: Immune System News and Immune System Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.