Of salamanders, mice and men - digit regeneration mechanisms revealed

Regenerated mouse digit tip/Yuval Rinkevich

 Guest blogger Uta Grieshammer is a science officer at CIRM

A form of regeneration that has captured the imagination of scientists and the general public for many decades occurs in certain salamanders, as they have the remarkable ability to regrow a severed leg. Leg regeneration is unusual not only because it is so rare among vertebrates, but also because the underlying mechanism is thought to be quite different from that operating during the regeneration of organs in mammals.

Although mammalian legs do not regenerate, the very tips of our fingers and toes, and those of mice, do sometimes regrow. A new study published August 24 in Nature from Irv Weissman’s lab at Stanford University, and partially funded by CIRM, now comes to the surprising conclusion that the mechanism at work during mouse digit tip regeneration more resembles that of our other organs rather than the way that salamanders’ legs have been thought to regrow.

Although it may not be obvious, many of the organs in a healthy person regenerate themselves throughout life, some more than others. Our whole blood forming system and our gut, for instance, turn over relatively rapidly, whereas only about half of our heart cells are replaced in our entire life. Some organs, such as our skin, muscles, and bones, also have a reasonable capacity for repair after injury, if the damage was relatively small, while much of our liver will be faithfully replaced after a large portion has been removed. Other organs, though, replace cells lost to insult or disease only poorly or not at all, such as the heart following a heart attack. The hope is, if we figure out how the regeneration superstars of our body, or those of salamanders, accomplish their remarkable feats, we can use that knowledge to coax their less talented brethren into action.

Scientists have three basic models for how regeneration occurs: 1) tissue-specific stem cells within the organ divide and mature into the additional tissue, 2) mature cells divide to produce more of themselves without contributing to other cell types, or 3) mature cells lose their specificity, become more like embryonic cells and form a blastema with the ability to divide and form the original cell type and also other cell types in the regenerating tissue.

Weissman’s work with postdoctoral scholar Yuval Rinkevich, who was first author on the paper, shows that during digit regeneration in mice, the third model is not the right one. Remaining skin cells only make new skin cells, bone cells only make new bone. This comes a bit as a surprise, as this appears to be very different from the blastema mechanism thought to be used by salamanders to regrow their limbs. However, recent experiments from Elly Tanaka’s group in Dresden have challenged that long held model, showing that blastema cells in regrowing salamander legs do not typically adopt fates different from those of the cells they’re derived from. These studies then suggest that regeneration of limbs and digits in salamanders and mice, respectively, does occur through related mechanisms, just not the one originally thought. In both cases, repopulating cells do not switch cell fate, but whether regenerated digit or limb cells are derived from tissue-specific stem cells or from mature cells remains an open question for both species, although the Weissman paper makes a cogent argument that their data are consistent with the stem cell model, at least for some of the cell types involved in mouse digit regeneration.

Concerted efforts by scientists studying animals such as salamanders and mice will likely lead to an ever more accurate picture of limb and digit regeneration, thereby laying the groundwork for translating these findings to human cells, and eventually to human treatments.

CIRM funding: Irv Weissman (RC1-00354)
Nature, August 24, 2011

Weeding out the tumor-forming cells from potential stem cell therapies

CIRM grantees at Stanford University have removed some of the risk of therapies based on human embryonic stem cells or reprogrammed adult cells, known as iPS cells.

Both of these cells types are known as pluripotent, which means that the cells can go on to form all the mature cells of the human body. The problem is that those cells also form tumors called teratomas. In the process of developing new therapies, scientists first prod the stem cells into a more mature cell type, such as a neural progenitor for spinal cord injury, an insulin-producing pancreatic cell for diabetes or retinal cell for forms of blindness. Then, they go through a laborious process to show that no tumor-forming cells still remain in that batch of cells that they hope to use in therapies.

The new technique, published August 14 in Nature Biotechnology, provides a novel way of identifying cells that are potentially tumorigenic and removing them from a batch of cells. Krista Conger at Stanford wrote about this paper:

"The ability to do regenerative medicine requires the complete removal of tumor-forming cells from any culture that began with pluripotent cells," said Irving Weissman, MD, director of the Stanford Institute for Stem Cell Biology and Regenerative Medicine. "We've used a combination of antibodies to weed out the few undifferentiated cells that could be left in the 10 or 100 million differentiated cells that make up a therapeutic dose."

Weissman pointed out that the production of therapeutic cells from pluripotent stem cells for regenerative medicine was a major goal of Proposition 71, the ballot measure that established the California Institute for Regenerative Medicine to allocate $3 billion to advance stem cell science. CIRM funded this research.

…

"Commonly used differentiation protocols for embryonic stem and iPS cells often give rise to mixed cultures of cells," said research associate Micha Drukker, PhD. "Because even a single undifferentiated cell harbors the ability to become a teratoma, we sought to develop a way to remove these cells before transplantation."

If other research groups repeat these findings, the technique could reduce some of the risk of therapies based on pluripotent cells.

Nature Biotechnology, August 14, 2011

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Aggressive breast cancer treated with bone marrow stem cells

Last week brought a paper by Stanford researchers that has been a long, long time coming. It shows that 12-14 years after the experimental treatment, women with metastatic breast cancer benefited from high dose chemotherapy followed by transplantation of their own blood-forming stem cells. The paper was published online July 15 in Biology of Blood and Marrow Transplantation.

Back at the time when the group, which included CIRM grantee Irv Weissman, carried out this trial, doctors were rejecting high-dose chemotherapy for people with metastatic breast cancer. That therapy destroys the cancer, but also destroys the patient's bone marrow, which produces all blood and immune cells. That side effect would be deadly, but doctors can reinject bone marrow cells taken from patients before chemotherapy. This is the process that is used today for many types of cancers. However, doctors were finding that the whole bone marrow also contained some breast cancer cells. If those cells survived the transplantation they could spread and form a new, deadly cancer. So much for the chemotherapy.

Back when the Stanford scientists carried out their trial (between 1996 to 1998) Weissman had recently figured out how to purify the blood-forming stem cells in the bone marrow that are responsible for rebuilding the blood system. He and the team thought they could pull out just those cells from the patient's blood and use those cells to save the blood system after high-dose chemotherapy. If it worked, the chemotherapy would destroy the cancer, and the purified stem cells would save the blood system without reintroducing cancer cells lurking in the blood.

It all sounded good, but they were not sure whether their idea had worked until now. What they learned is that 23 percent of the women in their trial are still alive, compared to 9 percent of women who received unpurified stem cells.

A Stanford press release about the work quotes Weissman:

“Even with this small sample size, this paper demonstrates much-better overall and progression-free survival in those patients who received cancer-free stem cells.”

Senior author on the paper Judith Shizuru said in the release:

“Most people in the oncology community feel that this issue is a done deal, that high-dose chemotherapy does not work for patients with breast cancer. But our study suggests that the high-dose therapy strategy can be modified to include the use of cancer-free purified blood stem cells to yield better overall outcomes in women with advanced breast cancer.”

The authors are encouraging scientists to revisit high-dose chemotherapy for other cancers where it isn't traditionally offered. If it shows benefit for those patients it could open up a new form of therapy for a wide range of cancers.

This paper also highlights something that will continue to be true of all forms of stem cell research: It takes a long time to learn whether a therapy was truly effective. A decade from now we'll know whether the stem cell trials of today really worked. It's slow and frustrating, but papers like this one make the wait worth while.

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More on the Berlin patient, stem cells, and a cure for HIV

The response to the story about the Berlin patient who was reported cured of  HIV has been incredible, but in this case it’s also a little troubling. What I see in comments on news stories or in tweets is that we have an uphill battle in terms of educating people about stem cells.

The stem cells used to eliminate this man’s HIV infection were from the bone marrow. Essentially, what he got was just a really great bone marrow transplant, because the transplanted cells were resistant to HIV. The bone marrow houses the blood-forming stem cells that create the entire blood system. Those blood-forming stem cells were the first stem cells to be identified, back in 1998 by Stanford’s Irv Weissman.

CIRM’s two disease teams are working on even more sophisticated -- and safer -- bone marrow transplants that could bring this same cure to thousands of people who need it. (Summaries of those teams are available here and here.)

What concerns me is that many of the comments on news stories or Tweets are tying this HIV therapy to embryonic stem cells or even abortion. Just so we’re clear here, an aborted fetus has no embryonic stem cells. None. Human embryonic stem cells come from IVF embryos left over after a couple completes their family. (Paul Knoepfler of UC Davis has a good description of how those human embryonic stem cell lines are created.) These embryonic stem cells are amazing — they can form any tissue in the body, which can become cures or unlock the mysteries of how diseases form.

In the case on the Berlin patient, it was adult, blood-forming stem cells that were the basis of the cure. For other diseases, the cure may come from embryonic or reprogrammed iPS cells. At this point, we don’t know which cell type will cure which disease. All we know is that we need cures, and like the Chilean miners who dug three rescue holes to have one succeed, we’re working all angles in order to be successful in our quest for cures.

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Knocking out leukemia stem cells

A good report about bone marrow transplantation in progress comes from the National Hockey League, of all places. Mandi Schwartz, a Yale women’s hockey player, was diagnosed with acute myeloid leukemia and is being treated at the Seattle Cancer Care Alliance.

In the name of full disclosure, that’s the center that cured my mother’s lymphoma, so let’s just say I’m a fan.

NHL.com wrote a nice description of how bone marrow transplantation knocks out leukemia stem cells. Acute myeloid leukemia is a cancer that begins in the bone marrow stem cells — the cells that continuously produce new blood and immune cells throughout a person’s life. A bone marrow transplant essentially replaces the cancerous leukemia stem cells with new ones from a donor, like an organ transplant but with bone marrow. The NHL writes:

Engraftment, which was the next phase in her recovery, is needed in order for the transplanted stem cells to begin to grow in her bone marrow and manufacture new blood cells and immune cells... Complete recovery of a new immune system can take a year or longer depending on any complications as a result of the transplant.

Bone marrow transplants like this one are effective, but dangerous. The process of eliminating a person’s diseased bone marrow leaves the person extremely weak and prone to infections. This danger is why several CIRM grantees are working on a less toxic way of killing off the diseased bone marrow stem cells (here's a list of our awards targeting blood cancers). Irving Weissman at Stanford University has found molecules on the surface of the stem cells underlying acute myelogenous leukemia. He has a CIRM disease team award to develop a chemotherapy that could destroy those cells in a way that’s far less toxic than bone marrow transplant.

If that research is successful, future people like Schwartz may recover from acute myelogenous leukemia with fewer side effects. As someone who has seen a family member battle cancer, fewer side effects for an effective therapy is a winning combination.

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Basic research and the search for cures

Two interesting reports today discuss the relationship between basic research and the kind of translational research that is the most visible sign of progress toward cures.

In his blog, the director of the bay area biotech collaborative QB3 Regis Kelly writes about public speakers at the Translational Medicine Alliance speaking out against basic research. He says:

Repeatedly mentioned with disdain was the amount of money that went into R01 grants, the single investigator grants that are the backbone of fundamental research in the US. Over 80% of the grants go to R01s, it was said, while only 2 to 5% go to translational research, the mantra of the meeting participants.

This is the same type of research funded by CIRM’s Basic Biology Awards and in some New Faculty Awards. The basic discoveries that come out of this kind of research are considered to be the fuel in the pipeline leading to new cures: No new ideas? No new therapies.

Kelly advocates rather than ignoring the complaints, “put our house in order and mount a major public education campaign to validate our position.” That is, do what can be done to make basic research as efficient and effective as possible then explain to members of the public just what they are getting for their money.

Or we can discount the criticisms as uninformed foolishness, and do nothing. That could be suicidal!

Kelly’s point is a good one. Most people have at least one friend or family member with a critical disease, and those people want to see new cures coming from publically funded research. Until the relationship between basic research and new cures is made clearer people will likely continue pushing for less basic research.

While Kelly defends basic research, the head of the National Cancer Institute Harold Varmus held a brainstorming session to figure out the basic questions in cancer research. According to Science magazine:

Once the list is finished, Varmus might hold a special competition to invite proposals for several questions and fund, say, 15 of the best ideas. Research funding may be tight, but "we've got over a $5 billion budget," Varmus says. Nothing has been decided, though. Right now, he says, "we're just trying to have a conversation that evolves into something useful."

Stanford’s Irv Weissman, who was at Varmus’ session, gave an excellent talk to the CIRM governing board about the value of basic research. In his case, it was a basic discovery about stem cell biology that led to cancer research that’s now moving toward the clinic.

Stem cell videos make the grade

One amazing aspect of living in the era of social media is the incredible way information spreads. A butterfly batting its little orange wings in a monarch grove in Santa Cruz could influence a tweet of a blogger heard ‘round the world.

Or, in CIRM’s case, a few videos playing on YouTube could be used by a teacher heard 'round the world. In the past week a video about the difficulties of differentiating stem cells into therapeutically useful cell types has popped up in the curriculum of Harrison College, which offers a number of online and classroom courses. The video, which has been watched hundreds of times in the past week by those students, features Mark Mercola of Sanford-Burnham Medical Research Institute who is working to differentiate cardiac cells from human embryonic stem cells. Here’s that video:


In the past, a video about iPS cells featuring Jerome Zack from UCLA has made its way into college curricula, as has a video discussing the different types of stem cells with Stanford University’s Irv Weissman. These videos are all part of a stem cell basics CIRM put together to help educate people about stem cell research both in written form and in short videos.

Given the misperceptions of stem cell research in the public and in the media its nice to see these videos getting discovered and used for educational purposes.

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Hope for CIRM leukemia disease team

The clock is ticking on the 14 CIRM Disease Team projects issued last October, which are working under a four-year deadline to hit the clinic. The $20 million acute myeloid leukemia project headed up by Irv Weissman of Stanford University just reported some promising progress.

Weissman and his team are developing a chemotherapy drug that binds to a protein found on leukemia stem cells, killing the cells. The protein, called CD47, is also found on other cancer stem cells.

The work, which was published in the Sept. 3 issue of Cell, was focused on non-Hodgkins lymphoma. The group gave mice with non-hodgkins lymphoma the molecule that blocks CD47 in addition to another antibody, and cured the disease in 60 percent of cases.

According to a Stanford press release:

The researchers point out that, although the CIRM grant focuses on investigating anti-CD47 therapies for acute myeloid leukemia, the drug development process will result in an antibody that could also be used for other cancers. They focused their preliminary investigations on non-Hodgkin’s lymphoma because they were curious as to how the anti-CD47 antibody would work with rituximab, which also binds to human lymphoma cells.

The release goes on to say that the researchers hope to try the therapy in other cancers:

The researchers are moving forward to conduct tests on other CD47-expressing cancer cells, which include acute leukemia, bladder and several other cancer stem cells. They speculate that they might see a similar synergistic effect between anti-CD47 and other cancer-specific monoclonal antibodies currently in clinical use. They are also moving ahead as quickly as possible to bring the anti-CD47 antibody treatment to trials in human patients.

Recently, CIRM disease teams targeting brain tumors (blogged about here) and HIV/AIDS (blogged about here) have also had some preliminary success.

You can learn more about the CIRM disease team program in our October press release.

If you aren’t familiar with cancer stem cells, here’s a video with Catriona Jameison, a cancer stem cell scientist at University of California, San Diego, talking about their role in cancer.



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The competition that isn't: Adult vs. embryonic stem cells

The past few days have sent the blogosphere -- especially the anti-embryonic stem cell blogosphere -- abuzz over a story by the Associated Press with the headline "Adult Stem Cell Research Far Ahead of Embryonic."

It's true. At this time there are many adult stem cell trials and only one embryonic stem cell trial underway. But what the story makes clear, if you read past the headline, is that adult stem cells were first out of the gate and are therefore first to trial. The story does highlight several of the exciting applications of adult stem cells. What it doesn't do is suggest that embryonic stem cells aren't of value.

Consider this: Irv Weissman of Stanford University (and multiple CIRM grantee) discovered the first adult stem cells in the bone marrow of mice in 1988. Ten years later, James Thomson of the University of Wisconsin created the first human embryonic stem cells.

With that timeline in mind, the AP story quotes Hank Greely, Stanford law professor and long-time follower of stem cell research, as saying:

"Give it another five years and I'll be surprised if we don't have some substantial progress" beyond the initial safety studies of embryonic stem cell research.

CIRM funds both adult and embryonic stem cell research. That's because at this stage of the research it's too early to know which cells will be the best therapeutic option for different diseases and conditions. In fact, many of the very people leading adult stem cell trials (including Weissman) advocate also pursuing embryonic stem cell research and have signed an open letter endorsing all forms of stem cell research. You can look up names of researchers who have signed here. Many of those names will be familiar as adult stem cell researchers in the AP story.

Those who oppose embryonic stem cell research are quick to appoint the first stem cell discovered as the leader. But in this case first doesn't mean best, and we won't know which cell type is best for many years. In fact, the best cellular therapy may depend on the disease -- adult cells for one disease, embryonic for another, and small molecules discovered through stem cell research for still other diseases.

At CIRM, we're excited about all potential therapies to end chronic disease and injury. Some of those therapies may come from adult stem cells. Others may come from embryonic stem cells or reprogrammed iPS cells. Whatever the origin of the cellular therapy, CIRM hopes that by funding all avenues of stem cell research we will push the field ever closer to the best therapies for disease and injury.

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Cancer stem cells at the heart of melanoma

A team led by Irving Weissman at Stanford University School of Medicine has found the cancer-initiating stem cells in melanoma. Weissman has CIRM Comprehensive and Disease Team Awards relating to his cancer stem cell work.

According to a Stanford press release:

The finding is significant because the existence of such a cell in the aggressive skin cancer has been a source of debate. It may also explain why current immunotherapies are largely unsuccessful in preventing disease recurrence in human patients.

That's because any therapy that kills the bulk of the tumor without eliminating the cancer stem cells won't be effective:

Any therapy that doesn’t wipe out these elite cancer stem, or initiating, cells has no chance of completely eradicating the disease even if it destroys nearly all other tumor cells. That’s why, say proponents, it can be relatively easy to get a patient into remission, but extremely difficult to prevent the cancer stem cells from roaring back and causing a relapse months or years later.

The work was published in the July 1, 2010 issue of Nature.

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