Neuron death occurs with certain botulinum toxin types

Boston - Not all botulinum toxins are equally safe for neurons, according to a new study by Harvard researchers.

According to the study’s senior author, Harvard Medical School researcher Min Dong, Ph.D., two of seven of the botulinum toxins, types C and E, cause neuron death. And while generally safe for healthy neurons, botulinum toxin type A, used in Botox (onabotulintumtoxinA, Allergan) and Dysport (abobotulinumtoxinA, Medicis), causes neuron death in the lab when scientists introduce mutations to a key neuron protein.

Dr. Dong and colleagues published the study Feb. 12 in Nature Communications.

“We investigated the seven types of toxins,” Dr. Dong says. “All seven relax muscles by blocking synaptic vesicle exocytosis. This is very well established. We looked at whether any of them, in addition to blocking synaptic vesicle exocytosis, might cause damage to survival of neurons.”

Dr. Dong says he was intrigued by earlier research suggesting botulinum toxin type C caused neuron death. His study, which looked at both rats’ neurons cultured from embryonic rat brains and human motor neurons differentiated from human embryonic stem cells, not only confirms that research but also exposes potential dangers of using type E toxin and helps to establish a cellular mechanism.

It is known that all botulinum toxins shut down signals from neurons to muscles by attacking a complex composed of three proteins inside neurons. The researchers have discovered that two of these three proteins, known as syntaxin 1 and SNAP-25, not only release signals from neurons, but also perform the essential housekeeping process of recycling the neuron’s plasma membranes. Type C toxin attacks syntaxin 1, while type E toxin attacks SNAP-25. The resulting blockage of plasma membrane recycling leads to death of neurons.

“You have types A, C, and E that cleave … SNAP-25. Then, you have type C that cleaves two proteins: SNAP-25 and syntaxin 1,” Dr. Dong says.  

In essence, cleavage of SNAP-25 and syntaxin 1 not only block neurotransmitter release but also induce neuron death. Why? Dr. Dong explains SNAP-25 and syntaxin 1 have a secondary role: to mediate the recycling of the plasma membrane of neurons. Recycling of the plasma membrane is essential for neuron survival.

In contrast to syntaxin 1 and SNAP-25, the third protein in the complex, known as synaptobrevin, is not involved in the recycling process. This explains why type B, D, F and G toxins, all of which attack synaptobrevin, do not cause neuron death, according to this study.

What about type A?

The researchers suggest that unlike types C and E, type A does not generally cause neuron death, despite cleaving SNAP-25. Type E cleaves a larger fragment from SNAP-25 than type A, causing more extensive damage to the function of SNAP-25. In contrast, SNAP-25 cleaved by type A can still support the neuron’s essential recycling of plasma membranes.

But the safety of type A toxin is not absolute, the scientists found. Once SNAP-25 has been cleaved by type A, its ability to tolerate additional mutations and defects in neurons is reduced. In fact, when Dr. Dong and colleagues introduced a mild mutation into SNAP-25 - which under normal conditions is tolerated - and then introduced botulinum toxin type A, the neuron died.

“Whether this can occur in rare cases in patients needs to be studied further,” he says.

Using information in practice

The use of botulinum toxin type A in clinical applications is quite safe, although, questions remain about whether a patient with a mild mutation in the SNAP-25 sequence might be susceptible to losing neuron function after being treated with botulinum toxin A. If such a patient exists, it would be a rare occurrence, Dr. Dong says.

Another comfort regarding the use of type A for cosmetic purposes, according to Dr. Dong, is that the cytotoxicity effect is dose dependent.

“In order to observe this effect, even for types C and E, we have to use a relatively higher level of toxin to treat the neurons and get rid of basically every molecule of SNAP-25 and syntaxin 1 in neurons. The level of toxin we used under experimental conditions is much higher than what is used in the clinic, especially by dermatologists,” Dr. Dong says.

Dermatologists, however, should be cautious of using types C and E to treat patients, according to Dr. Dong. While the toxins are not yet used commercially by dermatologists, there are small-scale studies looking at the use of type C for muscle relaxing, the researcher says.