New Targets Proposed for Shooting Down Neuropathic Pain

Neuropathic pain is a complex and puzzling state. Often described as a “burning” or “shooting” pain—and frequently associated with neurodegenerative diseases such as multiple sclerosis—it is still not clear what causes this mysterious sensation. A team of HMS researchers reports online in the Feb. 10 Nature Medicine that a pair of enzymes belonging to the family of matrix metalloproteases (MMPs) appears to have distinct roles in the induction and maintenance of neuropathic pain.

Courtesy Ru-Rong Ji

Enzyme ups and downs. Immunohistochemical staining shows that the matrix metalloprotease MMP-9 is rapidly upregulated within dorsal root ganglion (DRG) neurons of rats with spinal nerve damage (SNL, 1d), but not in naive or sham animals. MMP-2 is upregulated later in glial cells within the DRG of rats with spinal nerve damage (SNL, 10d).

Previous studies have indicated that inflammation induced by nerve injury plays a central role in pain pathogenesis, and MMPs are inflammatory mediators widely implicated in neuroinflammation and degeneration. “If MMPs are important in degenerative diseases, then they might also be involved in pain associated with nerve damage,” said Ru-Rong Ji, HMS associate professor of neurobiology at Brigham and Women’s Hospital.

Neuropathic pain is primarily managed with general analgesics such as morphine. But these only block pain temporarily since they fail to tackle the underlying pathology, which remains poorly understood.

To address this, Ji and colleagues conducted a comprehensive series of experiments looking at the involvement of two major MMPs—MMP-9 and MMP-2—in neuropathic pain. They began by harvesting tissue from the dorsal root ganglia of rats at various time points after inducing pain symptoms via spinal nerve damage and measuring MMP activity and localization.

Using a variety of analytical tools, the team was able to show that there was a time course difference between MMP-9 and MMP-2 as well as a localization difference. MMP-9 was expressed early and transiently within sensory neurons; MMP-2 was expressed later on in surrounding glial cells and had a far more persistent effect. “Based on this pattern, we hypothesized that MMP-9 would be critical for the induction phase,” said Ji, “and MMP2 would be critical for the maintenance phase of neuropathic pain.”

The team went on to explore the contributions of these MMPs to pain symptoms and to investigate possible underlying mechanisms. Using a multistrategy approach, Ji and his group demonstrated that MMP-9 acts as instigator, activating well-characterized downstream pain molecules such as IL-1 beta to cause rapid onset of pain symptoms. MMP-2 carries on what MMP-9 starts by taking over pain molecule activation and promoting the persistence of pain symptoms.

“Different phases of neuropathic pain were not clearly defined before. The traditional drugs blocked all neurotransmission so it didn’t matter if there were different phases,” said Ji. “We have not only identified MMPs as new players in neuropathic pain development and maintenance, but also that inhibition of MMP-9 and MMP-2 is highly effective in attenuating neuropathic pain. We could have new targets for treatment.”

—Yvonna Reekie

By Normalizing Feeder Vessels, Nitric Oxide Sets Up Tumors for Cancer Therapy

Cutting off the vascular supply to solid tumors to promote regression has not proven singularly effective in cancer patients because “in practice you can’t get rid of all of the blood vessels in the tumor,” explained Dai Fukumura, HMS associate professor of radiation oncology at Massachusetts General Hospital. “It is better to use an approach of normalizing the blood vessels to improve chemotherapy and radiation treatment.”

The tortuous, leaky vessels found in solid tumors have abnormal organization, structure, and function, posing barriers to delivery of antitumor agents. Inducing normal vessel morphology could enhance vascular function, thereby improving the efficacy of tumor drug therapy.

Fukumura and his colleague Rakesh Jain, HMS professor of radiation oncology (tumor biology) at MGH—and the person who first proposed the normalization concept using anti-angiogenic agents (see Focus, Feb. 2, 2007)—published a study online Feb. 17 in Nature Medicine detailing a novel mechanism for vascular normalization of solid tumors. They determined that creating an endothelial gradient of nitric oxide (NO), a gaseous mediator of neovascularization, was sufficient to normalize blood vessels in a mouse model of glioma, a brain tumor arising from glial tissue.

These tumors have diffuse NO expression resulting from two different forms of nitric oxide synthase, neuronal NOS (nNOS), generated by tumor cells, and endothelial NOS (eNOS), made by cells lining the vascular wall.

In tumors, NO made by nonvascular cells can hinder vessel maturation, contributing to abnormal structure and function. The authors hypothesize that “if NO could be localized selectively around blood vessels, the morphology and function of the tumor vasculature would be improved,” making a better vehicle for antitumor therapies.

The team used short hairpin RNA to block expression of nonvascular nNOS in glioma cells and evaluated the morphology and function of the blood vessels in the resulting tumors. The remaining vascular eNOS created an NO gradient that was restricted to blood vessels, causing them to be more numerous and less permeable than those found in tumors where nNOS was not silenced. In addition, tissue oxygenation was improved in the nNOS-silenced tumors. Together, these vascular alterations improved suppression of glioma growth and overall survival of tumor-bearing mice after they received radiation therapy.

“This is a proof of concept model that worked nicely with gliomas,” said Fukumura. “This may be useful in other diseases where abnormal blood vessels are involved.”

—Kafi Meadows

Cancer Drug Turns On Bone-forming Stem Cells

A drug for cancer that also encourages stem cells to differentiate into bone-forming cells could potentially be used for treating bone-degenerating disorders like osteoarthritis. An HMS research team led by David Scadden, the Gerald and Darlene Jordan professor of medicine at Massachusetts General Hospital and director of the hospital’s Center for Regenerative Medicine, published these results in the February Journal of Clinical Investigation.

A hematologist–oncologist, Scadden and his team were interested in seeing whether mesenchymal stem cells (MSCs) could be influenced to differentiate into a specific cell type. MSCs are found in the bone marrow and possess the capacity of developing into several tissue types, including cartilage, fat, and bone. Until recently, it has been unknown whether MSCs could be manipulated—pharmacologically or otherwise—to bias their differentiation toward a particular cell type.

Bortezomib (Velcade) is a drug currently used in treating multiple myeloma, an incurable cancer that targets plasma cells within bone marrow. A clinical study conducted last year indicated that the drug had the interesting side effect of boosting bone density in patients.

On the basis of this and a handful of in vitro studies, Siddhartha Mukherjee, first author on the current study and HMS instructor in medicine at the MGH Center for Regenerative Medicine, began to investigate whether bortezomib might also be useful for boosting bone density in bone-degenerating conditions.

He conducted experiments in mice—first implanting MSCs to see if bortezomib could bias their differentiation pattern, then investigating whether the drug could modify bone tissue turnover in a mouse model of osteoarthritis. He found that treatment with low doses of bortezomib had both the ability to bias the differentiation pattern of implanted MSCs toward bone-forming cells in vivo as well as to increase bone formation and reduce bone loss in the mouse model of osteoarthritis.

The researchers report that these results raise the possibility of a new approach for treating bone-degenerating conditions in humans. “The idea of finding a drug that can turn on a particular stem cell population to differentiate is conceptually very exciting,” said Scadden. “If it is possible, stem cell research could have an enormous impact on medical care.”

—Yvonna Reekie

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