Assassin's tricks revealed in nature
Assassin's tricks revealed in nature
A team of researchers from Birkbeck, University of London, and Melbourne have shown how a protein called perforin punches holes in, and kills, rogue cells in our bodies. Their discovery of the mechanism of this assassin is published today in the science journal Nature.
'The first major step in understanding perforin came from the discovery by the Australian group, as well as researchers from the Netherlands and the US, that it is related to a family of bacterial toxins, including pneumolysin, which we had previously studied,' says Professor Helen Saibil of Birkbeck. 'We went on to obtain an electron microscopy map of perforin pores, and we could see some similarities to pneumolysin pores but we couldn’t interpret the structure in detail.'
'Perforin is our body’s weapon of cleansing and death,' says Professor James Whisstock from Monash University. 'It breaks into cells that have been hijacked by viruses or turned into cancer cells and allows toxic enzymes in, to destroy the cell from within. Without it our immune system can't destroy these cells. Now we know how it works, we can start to fine tune it to fight cancer, malaria and diabetes,' he says.
The first observations that the human immune system could punch holes in target cells was made by the Nobel laureate Jules Bordet over 110 years ago. But how?
Researchers from Birkbeck College in London, Monash University and the Peter MacCallum Cancer Institute in Melbourne collaborated on the 10-year study to unravel the molecular structure and function of perforin - the protein responsible. The structure was revealed with the help of the Australian Synchrotron, and with powerful electron microscopes at Birkbeck. Combining the detailed structure of a single perforin molecule with the electron microscopy reconstruction of a ring of perforins forming a hole in a model membrane reveals how this protein assembles to punch holes in cell membranes.
The new research has confirmed that the important parts of the perforin molecule are quite similar to those in toxins deployed by bacteria such as anthrax, listeria and streptococcus. 'The molecular structure has survived for close to two billion years, we think,' says Professor Joe Trapani, head of the Cancer Immunology Program at the Peter MacCallum Cancer Institute.
When they tried to match up the crystal structure to the less detailed images of the pore obtained by electron microscopy, the researchers were in for a surprise. 'We were initially puzzled to find that the molecule seemed to fit into our pore structure in an inside-out orientation compared to pneumolysin pores,' Saibil says. However, labelling experiments confirmed that mammalian cells use the pore 'the wrong way round' compared to the bacterial system.
'Remarkably, the same basic pore forming machine, used both by the immune system for defense and by bacteria for attack, works in opposite orientations in these two systems,' Saibil concludes.
The weapon of death is a powerful molecule. If perforin isn’t working properly the body can’t fight infected cells. And there is evidence from mouse studies that defective perforin leads to an upsurge in malignancy, particularly leukaemia.
Perforin is also the culprit when the wrong cells are marked for elimination, either in autoimmune disease conditions, such as early onset diabetes, or in tissue rejection following bone marrow transplantation. So the Australian researchers are now investigating ways to boost perforin for more effective cancer protection and therapy for acute diseases such as cerebral malaria. And with the help of a grant from the Wellcome Trust they are working on potential inhibitors to suppress perforin and counter tissue rejection.
The lead authors are Natalya Lukoyanova from Birkbeck and Ruby Law from Monash University, Ilia Voskoboinik from the Peter MacCallum Cancer Centre and the University of Melbourne. The project leaders are:
- Helen Saibil (Birkbeck)
- James Whisstock (Monash)
- Joe Trapani (Peter Mac)
The research was supported by the above institutions, the NHMRC, the ARC, the UK Biotechnology and Biological Sciences Research Council and the Wellcome Trust.