Creating pain in the lab
Up to one in five people are afflicted by chronic pain, but our ability to understand and develop better treatments for the condition has been hampered for a long time by the fact that it's been impossible to study human pain nerves in the lab. Now, a team at Harvard have found a way to genetically reprogram skin cells called fibroblasts, and turn them into pain-sensing nerve cells that can be studied in a dish. Chris Smith spoke to Harvard scientist Clifford Woolf, who led the study...
Clifford - I have studied pain neurobiology for most of my career and one of the challenges has been how to study pain in humans when it's not possible to take tissue samples from a living patient. Therefore, we know very little about the way in which the nervous systems of humans operate to produce pain. And so, the goal of our study was to see if we could transform cells that are easy to obtain and turn them into cells that are extremely difficult to obtain, specifically, those that are involved in the generation of pain.
Chris - What cell type specifically were you trying to turn from what into what?
Clifford - Our studying material, fibroblasts. These are the common cells that are the glue of most tissues in the body and are particularly prominent in the skin. Our aim was to convert these directly into a set of cells we call nociceptors and these are the sensory neurons in the periphery, in our skin, in our muscle, in our joints, that respond to tissue damaging stimuli and initiate the sensation that we call pain.
Chris - How do you do that reprogramming effect where you get the cells to turn from these very common skin fibroblasts into much more specialised pain-detecting nerve cells?
Clifford - This is an example if you like of cellular alchemy. The way to do this is to switch off all those genes that make a fibroblast a fibroblast and then switch on those genes that make a pain neuron a pain neuron. The way we do this is by expressing in the fibroblasts a set of proteins called transcription factors which bind to DNA and in this way, they control the expression of whole networks of genes. They switch on many genes and switch off others and we aimed our studies to use transcription factors that would uniquely cause those genes that make a pain neuron a pain neuron to be expressed in cells that had previously been fibroblasts.
Chris - How like real nerve cells are these reprogrammed skin cells that through your study, you have arrived at?
Clifford - We defined the property of these cells in terms of the genes they express, the morphology or shape of the cells, their function, their capacity to for example, generate action potentials which is the electrical activity that enables processing in the nervous system, and by the unique characteristics that are present in pain neurons. For example, pain neurons express an ion channel called TRPV1 which is the ion channel that is activated by noxious heat stimuli about 42 degrees and above which causes us to say, "Ouch!" in response to an intense heat experience. The same channel is activated by capsaicin which is the pungent ingredient in chili peppers. The reason a chili pepper is hot is because it activates this channel. So, we for example could find that our pain neurons responded to capsaicin with a vigorous burst of action potentials representing if you like, them saying, "Ouch!" in the dish.
Chris - So, this really do look like they're recapitulating what happens in the real person in the real situation. I know you've said that it's difficult to model that situation of what goes on in a human in a dish because it's so hard to get real samples. So now, you can do that. What sorts of questions will you be looking to your dish neurons to try and answer?
Clifford - These cells obviously will not recapitulate the full pain experience a human being has, but they certainly, we think, are very useful model to study the initiation of pain. With that in mind, we think they're going to be extraordinarily useful to study drugs that can reduce pain - analgesics. We also think they could be extremely useful to help identify toxic chemicals that can damage these neurons. One particular class of this are cancer chemotherapeutic agents which are known to produce both pain neuropathy which is killing of these sensory neurons in patients. These side effects are so severe that many patients terminate potentially life-saving therapy because of these severe adverse effects. We were able to show that these neurons that we made responded to at least one class of cancer agents in a way that indicated to us that we may be able to detect which individuals have a higher risk of adverse effects. We also hope that we may be able to identify who's at risk of developing chronic pain. Not everyone has the same risk for example after surgery. Some patients have no further adverse effects, whereas others go on to develop severe incapacitating pain that persists for many years. If we can model at least some aspects of that, again, we may be able to identify those patients who need to be very aggressively managed to reduce the risk of developing chronic pain.