Bone Marrow Stem Cells Treat Chronic Pain


Nerve damage as a result of type 2 diabetes, surgical amputation, chemotherapy and other conditions can lead to chronic pain. Such chronic pain can resist painkiller medications and other treatments and is debilitating.

New studies from scientists at Duke University with mice have shown that injections of bone marrow-derived stem cells might be able to relieve this type of chronic, neuropathic pain. This study was recently published in the Journal of Clinical Investigation and might be the springboard for advanced cell-based therapies to treat chronic pain conditions, lower back pain and spinal cord injuries.

Ru-Rong Ji, professor of anesthesiology and neurobiology at the Duke School of Medicine and his team used bone marrow stromal cells (BMSCs) that were isolated from bone marrow aspirations. BMSCs have been shown in a variety of clinical trials and basic research experiments to produce an array of healing factors and can differentiate into many cell types of cells in the body. BMSCs are being tested in small-scale clinical studies with people who suffer from inflammatory bowel disease, heart damage and stroke. BMSCs might also be useful for treating pain, but it’s not clear how they work.

“Based on these new results, we have the know-how and we can further engineer and improve the cells to maximize their beneficial effects,” said Professor Ji. In his team’s study, stromal cells were used to treat mice with pain caused by nerve damage. The cells were delivered by means of lumbar puncture, which infused the BMSCs into the cerebrospinal fluid (CSF) that bathes the spinal cord.

cerebrospinal-fluid-brain-flow

Mice treated with the bone marrow stromal cells were much less sensitive to painful stimuli after their nerve injury in comparison with untreated mice.

“This analgesic effect was amazing,” Ji said. “Normally, if you give an analgesic, you see pain relief for a few hours, at most a few days. But with bone marrow stem cells, after a single injection we saw pain relief over four to five weeks.”

When the spinal cords of the treated animals were examined in detail, Ji and others observed that the injected stem cells had clustered together along the nerve cells in the spinal cord.

To understand how the stem cells alleviated pain, Ji and his coworkers measured levels of anti-inflammatory molecules that have been linked to pain suppression. One of these molecules in particular, TGF-β1, was present in higher amounts in the CSF of the stem cell-treated animals compared with the untreated animals.

Immune cells typically secrete TGF-β1, which is a small protein, and it is found at low concentrations throughout the body. According to Professor Ji, people with chronic pain have been shown to possess too little TGF-β1.

In the new study, when Ji and others chemically neutralized TGF-β1 in the stem cell-treated animals, the pain-killing benefit of the infused BMSCs was reversed. This suggests that the secretion of this protein by BMSCs was a major reason these are able to abate neuropathic pain. When Ji and his crew directly injected TGF-β1 into the CSF, it provides significant pain relief, but only for a few hours, according to Ji.

However, infused BMSCs, remain at the site of infusion for as long as three months after their administration. This is just the right length of time for the cells to persist, according to Ji, because if the stem cells permanently persisted in the CSF, they have an increased risk of becoming cancerous.

Even more significantly, infused BMSCs also migrate to the site of injury. The ability of these cells to migrate to the site of injury depends on a molecule secreted by the injured nerve cells called CXCL12 (which, incidentally, has also previously been linked to neuropathic pain). CXCL12 (also known as stromal cell-derived factor-1) acts as a homing signal, since BMSCs have on their cell surfaces, a receptor for CXCL12 called CXCR4, CXCL12 acts as a kind of stem cell attractant.

In the next set of experiments, Ji and his colleagues would like to find a way to make the stromal cells more efficient. “If we know TGF-β1 is important, we can find a way to produce more of it,” Ji said. Additionally, the cells may produce other pain-relieving molecules, and Ji’s group is working to identify those.

The Nooks and Crannies in Bone Marrow that Nurture Stem Cells


Stems cells in our bodies often require a specific environment to maximize their survival and efficiency. These specialized locations that nurture stem cells is called a stem cell niche. Finding the right niche for a stem cell population can go a long way toward growing more stem cells in culture and increasing their potency.

To that end, a recent discovery has identified the distinct niches that exist in bone marrow for hematopoietic stem cells (HSCs), which form the blood cells in our bodies.

A research team from Washington university School of Medicine in St. Louis has shown that stem niches in bone marrow can be targeted, which may potentially improve bone marrow transplants and cancer chemotherapy. Drugs that support particular niches could encourage stem cells to establish themselves in the bone marrow, which would greatly increase the success rate of bone marrow transplants. Alternatively, tumor cells are known to hide in stem cell niches, and if drugs could disrupt such niches, then the tumor cells would be driven from the niches and become more susceptible to chemotherapeutic agents.

Daniel Link, the Alan A. and Edith L. Wolff Professor of Medicine at Washington University, said, “Our results offer hope for targeting these niches to treat specific cancers or to impress the success of stem cell transplants. Already, we and others are leading clinical trials to evaluate whether it is possible to disrupt these niches in patients with leukemia or multiple myeloma.”

Working in mice, Link and his colleagues deleted a gene called CXCL12, only in “candidate niche stromal cell populations.” CXCL12 which encodes a receptor protein known to be crucial for maintaining HSC function, including retaining HSCs in the bone marrow, controlling  HSCs activity, and repopulating the bone marrow with HSCs after injury.

CXCL12 crystal structure
CXCL12 crystal structure

CXCL12 signaling pathways

In bone marrow, HSCs are surrounded by a whole host of cells, and it is difficult to precisely identify which type of cells serve as the niche cells. These bone marrow cells are known collectively as “stroma,” but there are several different types of cells in stroma. Cells that have been implicated in the HSC niche include endosteal osteoblasts (osteoblasts are bone-making cells and the endosteum in the layer of connective tissue that lines the inner cavity of the bone), perivascular stromal cells (cells that hang out around blood vessels), CXCL12-abundant reticular cells, leptin-receptor-positive stromal cells, and nestin–positive mesenchymal progenitors. Basically, there are a lot of cells in the stroma and figuring out which one is the HSC niche is a big deal.

bone marrow stromal cells

When HSCs divide, they form two cells, one of which replaces the HSC that just divided and a new cells called a hematopoietic progenitor cell (HPC), which can divide and differentiate into either a lymphoid progenitor or a myeloid progenitor. The lymphoid progenitor differentiates into either a B or T lymphocyte and the myeloid progenitor differentiates into a red blood cell, or other types of white blood cells (neutrophil, basophil, macrophage, platelet or eosinophil). As the cells become more differentiated, they lose their capacity to divide.

HSC differentiation

Deleting CXCL12 from mineralizing osteoblasts (bone making cells) did nothing to the HSCs or those cells that form lymphocytes (lymphoid progenitors). Deletion of Cxcl12 from osterix-expressing stromal cells, which include CXCL12-abundant reticular cells and osteoblasts, causes mobilization of hematopoietic progenitor cells (HPCs) from the bone marrow into the bloodstream, and loss of B-lymphoid progenitors, but HSC function is normal. Cxcl12 deletion from blood vessel cells causes a modest loss of long-term repopulating activity. Deletion of Cxcl12 from nestin-negative mesenchymal progenitors causes a marked loss of HSCs, long-term repopulating activity, and lymphoid progenitors. All of these data suggest that osterix-expressing stromal cells comprise a distinct niche that supports B-lymphoid progenitors and retains HPCs in the bone marrow. Also, the expression of CXCL12 from stromal cells in the perivascular region, including endothelial cells and mesenchymal progenitors, supports HSCs.

Link summarized his results this way: “What we found was rather surprising. There’s not just one niche for developing blood cells in the bone marrow. There’s a distinct niche for stem cells, which have the ability to become any blood cell in the body, and a separate niche for infection-fighting cells that are destined to become T cells and B cells.”

These data provide the foundation for future investigations whether disrupting these niches can improve the effectiveness of cancer chemotherapy.

In a phase 2 study at Washington University, led by oncologist Geoffrey Uy, assistant professor of medicine, Link and his team are evaluating whether the drug G-CSF (granulocyte colony stimulating growth factor) can alter the stem cell niche in patients with acute lymphoblastic leukemia and whose disease is resistant to chemotherapy or has recurred. The FDA approved this drug more than 20 years ago to stimulate the production of white blood cells in patients undergoing chemotherapy, who have often weakened immune systems and are prone to infections.

Uy and his colleagues want to evaluate G-CSF if it is given prior to chemotherapy. Patients enrolled at the Siteman Cancer Center will receive G-CSF for five days before starting chemotherapy, and the investigators will determine whether it can disrupt the protective environment of the bone marrow and make cancer cells more sensitive to chemotherapy.

This trial is ongoing, and the results are not yet in, but Link’s work has received a welcome corroboration of his work. A companion paper was published in the same issue of Nature by Sean Morrison, the director of the Children’s Medical Center Research Institute at the University of Texas Southwestern Medical Center in Dallas. Morrison and his team used similar methods as Link and his colleagues and came to very similar conclusions.

Link said, “There’s a lot of interest right now in trying to understand these niches. Both of these studies add new information that will be important as we move forward. Next, we hope to understand how stem cells niches can be manipulated to help patients undergoing stem cell transplants.”