A Messenger of Pain
This past weekend I sliced my finger while attempting to slice a carrot. The Wusthof cut through the nail on my left pinkie, aggravating a piece of flesh that, until that point, had successfully avoided combat. And while it took a healthy dose of Advil PM to fall asleep that night, I now get the fascination and comfort of watching my finger reconstitute, pain receding hourly.
Many don’t have it so easy. According to WebMD, chronic pain affects 100 million Americans. An especially unfortunate number of those people have a condition known as neuropathic pain, in which even normal contact — such as a tap on the shoulder — results in severe pain.
Neuropathic pain is the result of damage to the somatosensory nervous system, which is headquartered in the brain and spinal cord but sends tentacle-like sensory neurons to almost every part of the body.
Nerve damage to any part of this web, by a stroke, a piercing knife cut, or as a side effect of chemotherapy, can result in neuropathic pain.
One thing that never changes with neuropathic pain is that there is no good way to treat it.
“You can block the calcium channels, you can block the neurotransmitter reuptake… none of them actually work well,” says Dr. Zhonghui Guan, an Assistant Professor at UCSF who treats chronic pain patients and studies the mechanisms of pain in the lab of Dr. Allan Basbaum.
The treatments fail largely because so little is known about the cellular and molecular mechanisms behind neuropathic pain. The antidepressants often used to treat neuropathic pain, the ones that block neurotransmitter reuptake, don’t even do a good job of treating depression.
Research led by Guan and published on December 7 in Nature Neuroscience has identified a protein essential in activating the neuropathic pain condition. Optimistic about taking the discovery into the clinic, Guan has filed a patent based on the findings.
Like treating neuropathic pain, studying it in the lab is not easy. It involves a complex nervous system that cannot be recapitulated in a petri dish with human cells. As is the case with metastasis, often the most effective way to study a complex biological phenomenon is in a mouse because all the parts similar to humans are there and coordinated.
Neuropathic pain can be induced in a mouse by performing surgery to sever a peripheral sensory nerve. To test if the injury generated a chronic pain condition, you poke the mouse hindpaw with filaments of increasing thickness. After nerve injury, a much thinner filament is required to provoke a response. This behavior, known as mechanical hypersensitivity, is exactly what is seen in human neuropathic pain patients.
Note that the spot where the mice are poked is very far from the spot where the injury occurred.
“If I put [the filament] in my body, the injury is induced somewhere around the thigh and where we’re testing stimulation is the bottom of the foot,” explains Guan.
This reflects a consensus in the field that neuropathic pain involves changes to the spinal cord, the processing center.
It’s analogous to something like this: a jewel thief breaks in the window of a museum. The window is alarmed, so a signal is sent to a central computer that in turn sends signals out to guards all over the museum, who become hyper-alert. These are like the sensory neurons in the mouse hindpaw that become sensitive to very thin filaments.
Researchers suspect that for neuropathic pain, a problem with the central computing system leads to a state of constant alert, causing the guards to overreact to innocuous phenomena: a bird flying into the museum window, perhaps.
For this model to be correct, two things have to happen. First, the peripheral nerve injury activates the central nervous system. Second, this activation leads to mechanical hypersensitivity all over the body. The Nature Neuroscience paper has shed light on the first problem.
A note on anatomy, or how the alarm system is wired: the sensory neuron of interest to Guan is the dorsal root ganglion (DRG), which extends one arm all the way to the periphery of the mouse, where the injury is being made, and a second arm into the spinal cord, where it comes into close proximity with a type of cell called a microglial cell.
Microglial cells are the only immune cells in the central nervous system, and thus have an essential role in its balance.
“Microglia are probably involved in almost every neurological disease,” says Guan.
Neuropathic pain is no exception, as nerve injury can activate the microglia and in turn cause hypersensitivity. But how does injury activate the microglia in the spinal cord?
As Guan phrased his question: “The neuron has to send something to the spinal cord, so what is that something?”
In this day and age, if you want to scan for a biological element in an unbiased way, you generally use RNA sequencing, a technique that has come up again and again in this column, proving its utility in all kinds of fields.
Since proteins are responsible for almost all cellular functions, and since all proteins have to be translated from RNA, looking at which RNAs are being made and in what quantities can tell you so much about how a cell responds to a stimulus.
RNA sequencing allowed these researchers to look at the DRG cells and ask: how do the cells respond to injury?
The researchers saw that nerve injury led to increased production of a protein called CSF1. CSF1 is a protein that has already been studied and is known to help microglia divide. It is a signaling protein that can communicate information from one cell to another. The receiving cell recognizes CSF1 through a cell surface receptor, conveniently named CSF1 receptor, or CSF1R.
Corresponding to CSF1 production in the injured nerve, Guan and company saw increased production of CSF1R in the spinal cord, specifically on the microglia.
As powerful as it is, RNA sequencing usually just generates hypotheses. Simply observing increased amounts of two RNAs does not actually validate how the corresponding proteins function. To do this, Guan and colleagues removed the CSF1 gene from the DRGs. When they did this, nerve injury no longer resulted in microglial activation, a two-part process that increases the number of microglia and turns on certain genes in each cell.
Guan describes the microglia as an army, which overlays nicely with the two-part activation: “each soldier becomes agitated, ready to fight, but at the same time you want reinforcements.”
Removing CSF1 also eliminated the hypersensitivity that followed nerve injury in these mice. In a sense, it got rid of the chronic pain.
On the flip side, injecting CSF1 into the mice without injuring any nerves at all resulted in both microglial activation and hypersensitivity.
Since CSF1 plays such an essential role in the cellular communications that underlie neuropathic pain, the human CSF1 stands out as a promising molecular target for therapies for human neuropathic pain.
Indeed, this is why Guan has filed his patent and may realize his dream of using his lab research to help the patients he sees regularly, a synergy that until now has eluded physicians and scientists in the field of chronic pain.
“The basic research and the clinical practice are like two pieces of paper: they don’t connect and they have difficulty understanding each other.”
With this assessment of chronic pain science, Guan may be underselling how much progress he and his colleagues have already made.