How does botulinum toxin - one of the most potent toxins known to man - survive the gut to wreak neurological havoc elsewhere?

Researchers have discovered how one of the most potent toxins known to man can survive in the human stomach and digestive tract without being destroyed by a highly acidic environment or chewed to pieces by enzymes. The finding could open the way to developing potential antidotes to the toxin - botulinum - by attacking its survival mechanism.

Botulinum neurotoxin (BONT) is produced by some Clostridium bacteria and it can cause paralysis by destroying a key protein receptor involved in the release of the neurotransmitter acetylcholine. This powerful physiological effect can be harnessed for therapeutic applications - as in the iconic anti-wrinkle drug Botox (onabotulinumtoxin A). But it remains a problem as BONT is often ingested accidentally through contaminated food.

Scientists have known for many years that botulinum is secreted by the bacteria in the form of a non-toxic complex of several proteins, and that at least one of these is believed to protect the key toxic protein within the complex. However, the mechanism of this protection has remained elusive.


Source: © Rongsheng Jin

Structure of the combined BoNT/Ai-NTNHA-A complex

Now, a team from the Sanford-Burnham Medical Research Institute in California and the Medical School of Hannover in Germany, led by Rongsheng Jin, has cracked the problem and identified how a ’bodyguard’ protein protects its BONT master before releasing it into bloodstream. 

The researchers focused on one of the proteins secreted with BONT - a protein called NTNHA. The team then genetically engineered a bacterium to produce only BONT and NTNHA, crystallised the complex and imaged the crystal with X-rays to give a detailed picture of how the two proteins interact.

’We were surprised to see that NTNHA, which is not toxic, turned out to be remarkably similar to botulinum neurotoxin. It’s composed of three parts, just like a copy of the toxin itself. The two proteins hug each other and interlock with what looks like a handshake,’ says Jin. This tight grip serves to hide the sensitive portions of the toxin and protect them from attack by acid and by digestive enzymes.

The team also identified residues on the NTNHA protein that are sensitive to pH - effectively acting as pH sensors. ’In the low pH of the digestive tract, the bodyguard protein remains tightly interlocked with the toxin protein,’ says Jin. ’But when it enters the bloodstream and the pH rises, the structure changes and the toxin is released.’

Knowledge of how the bodyguard exerts its protective influence might allow small molecules to be developed that can interfere with its action and act as an antidote to the poison, Jin suggests. ’This is something we are looking into.’

Commenting on the work, Bal Ram Singh, director of the Botulinum Research Center at the University of Massachusetts Dartmouth in the US, says that the high similarity of the two proteins suggests, intriguingly, the possibility of a common origin, perhaps through gene duplication. Singh adds, ’Physiological and therapeutic implications will need more study involving the whole complex that contains at least four other proteins - and demonstrating that this M-complex [BONT and NTNHA] is stable in physiological pH conditions, which is 7 rather than 6 used in this study’.

Simon Hadlington