Microglia, astrocytes, and even satellite glial cells have all been implicated in pain pathology. But the other major class of glia, oligodendrocytes, has not—until now. New research shows that, in mice, ablation of oligodendrocytes results in the rapid appearance of pain hypersensitivity that mimicked central pain, a neuropathic condition that can accompany stroke, spinal cord injury, or multiple sclerosis in people.
Oligodendrocytes are best known for the production of myelin, the layer of fatty insulation around axons that speeds neuron conductance. But evidence is mounting that oligodendrocytes provide neurons with much more than insulation, including metabolic support. In the new study, neither loss of myelin nor a ramped up immune response coincided with the development of pain. Rather, loss of support functions in the spinal cord appeared to be the critical factor. Without oligodendrocytes, neurons quickly developed axonal dysfunction that led to pain.
The work, led by Rohini Kuner at the University of Heidelberg, Germany, was published December 1 in Nature Communications. Maiken Nedergaard of the University of Rochester, US, who was not involved in the work but studies the role of glia in pain, called it “an extraordinary, interesting report.” The finding that pain onset in the mice preceded the inflammatory response and loss of myelin, Nedergaard told PRF in an email, was the report’s most exciting observation, because it suggests that oligodendrocyte dysfunction contributes directly to pain.
Pain comes after ablation
Co-author Ari Waisman at the Johannes Gutenberg University of Mainz, Germany, began by making transgenic mice that expressed the diphtheria toxin (DTX) receptor specifically in oligodendrocytes. When first authors Simon Gritsch and Jianning Lu treated the mature mice systemically with DTX, oligodendrocytes were selectively killed off.
The consequences were swift and striking. Within a few days of DTX treatment, the mice developed hypersensitivity to noxious mechanical and cold stimuli, as well as cold-evoked allodynia. Heat sensitivity, interestingly, was unchanged in the mice. Thirty days after oligodendrocyte ablation, the mice suffered loss of motor coordination, evident from their poor performance on the rotarod test.
As expected, DTX-induced ablation led to a sharp reduction in classical markers of oligodendrocytes within three days, including mRNA for oligodendrocyte-associated glycoprotein and proteolipid protein. By six days, staining for ASPA and adenomatous polyposis coli (APC), which label oligodendrocyte cell bodies, also fell, indicating oligodendrocyte death.
DTX treatment also led to demyelination—but not right away. Using electron microscopy (EM), co-author Klaus-Armin Nave, Max Planck Institute for Experimental Medicine in Göttingen, Germany, and his team saw the beginning signs of white matter demyelination in the spinal cord 24 days after DTX treatment. At the light microscopy level, though, myelin basic protein (MBP) and FluoroMyelin staining indicated that myelination was still quantitatively comparable to control-treated mice at 24 days. Demyelination progressed after that, and myelin was dramatically reduced in oligodendrocyte-ablated mice at 42 days post-treatment. (Earlier studies using oligodendrocyte ablation in the brain observed a similar time course of demyelination; see Pohl et al., 2011.)
Kuner started out with the hypothesis that demyelination might give rise to pathological pain; the link between central pain and demyelination is an outstanding question in diseases such as multiple sclerosis (MS). But Kuner found the pain phenotype in treated mice when myelin still seemed to be intact. “We saw a complete mismatch in the timing,” Kuner told PRF.
It is important to remember, Kuner said, that oligodendrocytes and myelin are not the same thing. Myelin is a fatty protein complex secreted by oligodendrocytes and can remain in place after oligodendrocytes die, which seemed to be the case here.
Nave told PRF, “The entire myelin compartment can appear to pinch off from the dying oligodendrocyte and survive for some time as a cushion around the axon, even supporting axonal energy metabolism for a significant amount of time.”
Dwight Bergles, who studies oligodendrocytes at Johns Hopkins University, Baltimore, US, says if myelin integrity is truly maintained at the time when pain occurs, then it suggests that some active process mediated by oligodendrocytes is necessary to prevent axonal dysfunction, rather than simply the presence of the myelin wrap.
Immune reactivity also could have disrupted neuronal function to cause the pain sensitivity, but that was not the case. Mice with the ablation did not show increased lymphocytes in the CNS—neither B nor T cells—at six or 24 days after treatment. Microglia reactivity, which has been implicated as a cause of neuropathic pain, also did not occur within the rapid window when pain developed. Although microglia did increase in number by six days post-treatment, signs of harmful reactivity did not appear until 24 days.
The researchers also looked for signs of spinal neuron death but did not see any at six, 16, or 24 days post-DTX treatment. However, structural defects in the axons appeared within days, and a marker for axon pathology called APP was markedly increased just days after treatment, suggesting that axon transport and other functions may be rapidly compromised when oligodendrocytes are missing.
Oligodendrocytes: not just for making myelination
Previous work by Nave and others has begun to uncover the roles of oligodendrocytes other than myelination, including providing neurons with fuel (see related news story at MS Discovery Forum; also Nave, 2010). But while energy metabolism is an important aspect of axon-oligodendrocyte interaction in white matter tracts, it is not the only one, Nave told PRF in an email. “Others include waste removal, detoxification, and neurotrophic supply, which will undoubtedly be impaired once oligodendrocytes are killed and not quickly replaced.”
Oligodendrocytes might also help neurons regulate the balance of ions directly outside their membrane, which would impact neuronal excitability. “We know remarkably little about the role of oligodendrocytes in processes like ion regulation,” Bergles told PRF, “but if myelin segments are isolated from their cell bodies, it seems likely that ion homeostasis and metabolic transporter activity in these segments could be impaired, leading to alterations in axonal repolarization,” which could dramatically disturb neuronal physiology.
How exactly oligodendrocytes support axons in the spinothalamic tract and elsewhere in the nervous system remains to be seen. Bergles pointed out that because the ablation affected the entire nervous system, the pain may have resulted from changes in the spinal cord or elsewhere in the brain.
“We do not know the cause of chronic pain in these mutants,” Nave told PRF, but some possible explanations could involve upset ion gradients or spatial potassium buffering, which could lead to local potassium accumulation, depolarization, and hyperactivity in pain-transmitting tracts. An imbalance between activity in excitatory and inhibitory spinal cord circuitry could also contribute to the pain phenotype. (See PRF related news story about new findings regarding spinal circuitry.)
Future work will address how the glial cells support neurons, but in the meantime, the new findings suggest that oligodendrocyte dysfunction could play a key role in central pain disorders even in the absence of demyelination or inflammation. “It appears that, in the central nervous system, you do not need demyelination to cause dysfunction and pain,” Kuner said. “As soon as the normal function of oligodendrocytes was gone, then pain came on.”
Stephani Sutherland, PhD, is a neuroscientist, yogi, and freelance writer in Southern California, US.
Image: Immunostaining of myelin basic protein in the spinal cord of mice six days after oligodendrocyte ablation. Courtesy of the Kuner lab.
