Targeting a key gateway can help treat chronic pain, shows study

Something like a quarter of the world's population suffers from chronic pain at some point. As opposed to acute pain – for example, that feeling after hitting your finger with a hammer – chronic pain may not even have a clear cause, and it can linger for years or lifetimes.

The burden of chronic pain includes damage to mental and physical health, lower productivity, and drug addiction.

Now, a study led by researchers at the Weizmann Institute of Science suggests an original approach to treating this affliction by targeting a key gateway – one that leads to the activation of genes in the peripheral nerve cells that are involved in many forms of chronic pain. The findings of this study were published in Science.

Pain starts in the sensory neurons, which pass information from the skin to the central nervous system. Damage to these neurons, chronic injury, or disease can cause the neurons to "short circuit" and continually send pain messages.

Prof. Mike Fainzilber of the Department of Biomolecular Sciences investigates molecules that regulate the messaging activities taking place within these nerve cells. These molecules – importins – are found in every cell, acting as conduits between the cell nucleus and its cytoplasm, shuttling molecules in and out of the nucleus and thus controlling access to the genes.

This role takes on special significance in the peripheral nerve cells, which have long, thin bodies where molecular messages can take hours to get from nerve endings to cell nuclei. Some of the importins Prof. Fainzilber and his team have identified, for example, relay messages about injury to the body of the nerve cell, initiating repair mechanisms.

To determine whether importins are involved in chronic neuropathic pain, the researchers, led by Dr. Letizia Marvaldi in Prof. Fainzilber's group, first set out to screen a number of mouse lines with mutant importins. The mice were generated by the lab of Prof. Dr. Michael Bader at the Max-Delbruck Center in Berlin, who collaborated on this European Research Council-supported research.

Behavioral screens on these different mouse lines revealed that importin alpha-3 was the only importin implicated in controlling pain pathways. The scientists then sought to identify the gene expression pattern associated with long-lasting pain in peripheral nerve cells, and determine how it tied into importin alpha-3 activity.

Analyses of differences in the expression patterns between normal neurons and neurons lacking importin alpha-3 directed Prof. Dr. Marvaldi's attention to c-Fos, a protein that importin alpha-3 brings into the nucleus. c-Fos is a transcription factor – a molecule that raises or lowers the expression of numerous genes. Further experiments in mice showed that c-Fos accumulates in the nuclei of peripheral nerve cells in mice suffering from chronic pain.

The scientists next used specialized viruses as tools to reduce or disable importin alpha-3 or c-Fos in mouse peripheral nerve cells. These mice had much reduced responses to chronic pain situations than regular mice.

Further research showed that importin alpha-3 is critical in late and chronic pain. c-Fos is also involved in earlier pain responses, but seems to enter the nucleus by other means at those earlier stages. This suggests that blocking importin alpha-3 activity might be especially well-suited to preventing lasting, chronic pain.

The research team then took their findings to the next level, asking how easily the results can be translated to clinical application.

The group utilized a specialized database, the Connectivity Map (CMap) from the Broad Institute of MIT and Harvard, which reveals connections between drugs and gene expression patterns. CMap enabled the researchers to identify around 30 existing drugs that might target the importin alpha-3 – c-Fos pathway.

Almost two-thirds of the compounds they identified were not previously known to be associated with pain relief. The team chose two – a cardiotonic drug (cardiotonics improve contraction of the heart muscle) and an antibiotic – and tested them in mice. The result: injection with these compounds provides relief of neuropathic pain symptoms in mice.

The compounds we identified in this database search are a kind of fast track – proof that drugs already approved for other uses in patients can probably be repurposed to treat chronic pain. Clinical trials could be conducted in the near future, as these compounds have already been shown to be safe in humans.

Dr Letizia Marvaldi. Professor, Weizmann Institute of Science

"We are now in a position to conduct screens for new and better drug molecules that can precisely target this chain of events in the sensory neurons," says Prof. Fainzilber. "Such targeted molecules might have fewer side effects and be less addictive than current treatments, and they could provide new options for reducing the burden of chronic pain."

Also participating in this research were Dr. Nicolas Panayotis, Dr. Stefanie Alber, Dr. Shachar Y. Dagan, Dr. Nataliya Okladnikov, Dr. Indrek Koppel, Agostina Di Pizio, Didi-Andreas Song, Yarden Tzur, Dr. Marco Terenzio, Dr. Ida Rishal, and Dr. Dalia Gordon, all of the Department of Biomolecular Sciences at the Weizmann Institute; Dr. Franziska Rother of the Max-Delbruck Center, Berlin and the University of Lübeck, Germany; and Prof. Dr. Enno Hartmann of the University of Lübeck.