Stem cell stories that caught our eye: developing the nervous system, aging stem cells and identical twins not so identical

Here are the stem cell stories that caught our eye this week. Enjoy!

New theory for how the nervous system develops.

There’s a new theory on the block for how the nervous system is formed thanks to a study published yesterday by UCLA stem cell scientists in the journal Neuron.

The theory centers around axons, thin extensions projecting from nerve cells that transmit electrical signals to other cells in the body. In the developing nervous system, nerve cells extend axons into the brain and spinal cord and into our muscles (a process called innervation). Axons are guided to their final destinations by different chemicals that tell axons when to grow, when to not grow, and where to go.

Previously, scientists believed that one of these important chemical signals, a protein called netrin 1, exerted its influence over long distances in a gradient-like fashion from a structure in the developing nervous system called the floor plate. You can think of it like a like a cell phone tower where the signal is strongest the closer you are to the tower but you can still get some signal even when you’re miles away.

The UCLA team, led by senior author and UCLA professor Dr. Samantha Butler, questioned this theory because they knew that neural progenitor cells, which are the precursors to nerve cells, produce netrin1 in the developing spinal cord. They believed that the netrin1 secreted from these progenitor cells also played a role in guiding axon growth in a localized manner.

To test their hypothesis, they studied neural progenitor cells in the developing spines of mouse embryos. When they eliminated netrin1 from the neural progenitor cells, the axons went haywire and there was no rhyme or reason to their growth patterns.

Left: axons (green, pink, blue) form organized patterns in the normal developing mouse spinal cord. Right: removing netrin1 results in highly disorganized axon growth. (UCLA Broad Stem Cell Research Center/Neuron)

A UCLA press release explained what the scientists discovered next,

“They found that neural progenitors organize axon growth by producing a pathway of netrin1 that directs axons only in their local environment and not over long distances. This pathway of netrin1 acts as a sticky surface that encourages axon growth in the directions that form a normal, functioning nervous system.”

Like how ants leave chemical trails for other ants in their colony to follow, neural progenitor cells leave trails of netrin1 in the spinal cord to direct where axons go. The UCLA team believes they can leverage this newfound knowledge about netrin1 to make more effective treatments for patients with nerve damage or severed nerves.

In future studies, the team will tease apart the finer details of how netrin1 impacts axon growth and how it can be potentially translated into the clinic as a new therapeutic for patients. And from the sounds of it, they already have an idea in mind:

“One promising approach is to implant artificial nerve channels into a person with a nerve injury to give regenerating axons a conduit to grow through. Coating such nerve channels with netrin1 could further encourage axon regrowth.”

Age could be written in our stem cells.

The Harvard Gazette is running an interesting series on how Harvard scientists are tackling issues of aging with research. This week, their story focused on stem cells and how they’re partly to blame for aging in humans.

Stem cells are well known for their regenerative properties. Adult stem cells can rejuvenate tissues and organs as we age and in response to damage or injury. However, like most house hold appliances, adult stem cells lose their regenerative abilities or effectiveness over time.

Dr. David Scadden, co-director of the Harvard Stem Cell Institute, explained,

“We do think that stem cells are a key player in at least some of the manifestations of age. The hypothesis is that stem cell function deteriorates with age, driving events we know occur with aging, like our limited ability to fully repair or regenerate healthy tissue following injury.”

Harvard scientists have evidence suggesting that certain tissues, such as nerve cells in the brain, age sooner than others, and they trigger other tissues to start the aging process in a domino-like effect. Instead of treating each tissue individually, the scientists believe that targeting these early-onset tissues and the stem cells within them is a better anti-aging strategy.

David Sadden, co-director of the Harvard Stem Cell Institute.
(Jon Chase/Harvard Staff Photographer)

Dr. Scadden is particularly interested in studying adult stem cell populations in aging tissues and has found that “instead of armies of similarly plastic stem cells, it appears there is diversity within populations, with different stem cells having different capabilities.”

If you lose the stem cell that’s the best at regenerating, that tissue might age more rapidly.  Dr. Scadden compares it to a game of chess, “If we’re graced and happen to have a queen and couple of bishops, we’re doing OK. But if we are left with pawns, we may lose resilience as we age.”

The Harvard Gazette piece also touches on a changing mindset around the potential of stem cells. When stem cell research took off two decades ago, scientists believed stem cells would grow replacement organs. But those days are still far off. In the immediate future, the potential of stem cells seems to be in disease modeling and drug screening.

“Much of stem cell medicine is ultimately going to be ‘medicine,’” Scadden said. “Even here, we thought stem cells would provide mostly replacement parts.  I think that’s clearly changed very dramatically. Now we think of them as contributing to our ability to make disease models for drug discovery.”

I encourage you to read the full feature as I only mentioned a few of the highlights. It’s a nice overview of the current state of aging research and how stem cells play an important role in understanding the biology of aging and in developing treatments for diseases of aging.

Identical twins not so identical (Todd Dubnicoff)

Ever since Takahashi and Yamanaka showed that adult cells could be reprogrammed into an embryonic stem cell-like state, researchers have been wrestling with a key question: exactly how alike are these induced pluripotent stem cells (iPSCs) to embryonic stem cells (ESCs)?

It’s an important question to settle because iPSCs have several advantages over ESCs. Unlike ESCs, iPSCs don’t require the destruction of an embryo so they’re mostly free from ethical concerns. And because they can be derived from a patient’s cells, if iPSC-derived cell therapies were given back to the same patient, they should be less likely to cause immune rejection. Despite these advantages, the fact that iPSCs are artificially generated by the forced activation of specific genes create lingering concerns that for treatments in humans, delivering iPSC-derived therapies may not be as safe as their ESC counterparts.

Careful comparisons of DNA between iPSCs and ESCs have shown that they are indeed differences in chemical tags found on specific spots on the cell’s DNA. These tags, called epigenetic (“epi”, meaning “in addition”) modifications can affect the activity of genes independent of the underlying genetic sequence. These variations in epigenetic tags also show up when you compare two different preparations, or cell lines, of iPSCs. So, it’s been difficult for researchers to tease out the source of these differences. Are these differences due to the small variations in DNA sequence that are naturally seen from one cell line to the other? Or is there some non-genetic reason for the differences in the iPSCs’ epigenetic modifications?

Marian and Vivian Brown, were San Francisco’s most famous identical twins. Photo: Christopher Michel

A recent CIRM-funded study by a Salk Institute team took a clever approach to tackle this question. They compared epigenetic modifications between iPSCs derived from three sets of identical twins. They still found several epigenetic variations between each set of twins. And since the twins have identical DNA sequences, the researchers could conclude that not all differences seen between iPSC cell lines are due to genetics. Athanasia Panopoulos, a co-first author on the Cell Stem Cell article, summed up the results in a press release:

“In the past, researchers had found lots of sites with variations in methylation status [specific term for the epigenetic tag], but it was hard to figure out which of those sites had variation due to genetics. Here, we could focus more specifically on the sites we know have nothing to do with genetics. The twins enabled us to ask questions we couldn’t ask before. You’re able to see what happens when you reprogram cells with identical genomes but divergent epigenomes, and figure out what is happening because of genetics, and what is happening due to other mechanisms.”

With these new insights in hand, the researchers will have a better handle on interpreting differences between individual iPSC cell lines as well as their differences with ESC cell lines. This knowledge will be important for understanding how these variations may affect the development of future iPSC-based cell therapies.

Wiping out a cell’s identity shifts cellular reprogramming into high gear

Blog CAF-1 chromatin

The packaging of DNA into chromatin (image credit: Felsenfeld and Groudine, Nature 2013

If stretched out end to end, the DNA in just one cell of your body would reach a whopping six feet in length. A complex cellular structure called chromatin – made up of coils upon coils of DNA and protein – makes it possible to fit all that DNA into a single cell nucleus that’s only 0.0002 inches in diameter.

Chromatin: more than meets the eye
Once thought to merely play a structural role, mounds of data have shown that chromatin is also a critical regulator of gene activity. In fact, it’s a key component to maintaining a cell’s identity. So, for example, in the nucleus of a skin cell, genes related to skin function tend to lie within stretches of DNA having a loosely coiled chromatin structure. This placement makes the skin-related genes physically more accessible to become activated. But genes related to, say, heart, liver or brain cell function in that same skin cell tend to remain silent within tightly packaged, inaccessible chromatin.


Depiction of (a) loosely packaged, accessible chromatin (red is DNA; blue is protein) vs (b) tightly packaged inaccessible chromatin. (Image credit: Interface Focus (2012) 2, 546–554)

As that skin cell divides and its DNA is replicated, there are various proteins that assemble and maintain the same chromatin positioning in their daughter cells, which helps them know they are skin cells. This cellular memory isn’t easy to erase, and it’s one of the reasons for the low efficiency when reprogramming a skin cell back into an embryonic stem cell-like state, also known as the induced pluripotent stem cell (iPSC) technique.

Blocking a DNA roadblock increases iPSC efficiency
So researchers at Harvard and in Vienna asked what if you blocked proteins responsible for arranging the chromatin – would it make it easier to generate iPSCs? The answer is a resounding “yes” based on data reported last Thursday in Nature. While previous studies asking the very same question have shown decent increases in iPSC reprogramming efficiency, this current research achieved orders of magnitude higher efficiency.

Using two independent screening methods, the research team systematically blocked the activity of hundreds of genes that play a role in the packaging of chromatin structure and maintaining cellular memory. These inhibition experiments were carried out in skin cells that were in the process of being reprogrammed into iPSCs. In both screening approaches, the inhibition of two proteins, collectively called chromatin assembly factor 1 (CAF-1), led to large increases in reprogramming efficiency.

Blog CAF -1 105305_web

Induced pluripotent stem cell (iPS cell) colonies were generated after researchers at Harvard Stem Cell Institute suppressed the CAF1 gene. (Image credit: Sihem Chaloufi)

Inhibiting CAF-1 potently erases cell memory
While the inhibition of genes previously identified to block reprogramming led to a three to four-fold increase in iPSC generation, inhibition of CAF-1 dramatically increased efficiency 50 to 200 fold. Also, compared to a typical reprogramming time of nine days, in skin cells with CAF-1 inhibition, the first iPSCs were observed in just four days.

The increased ease of manipulating cells also applies to direct reprogramming. This alternative reprogramming method skips the iPSC process altogether and instead directly converts one adult cell type into another. In this case, the researchers were able to convert skin cells into neurons and one immune system cell type (B cells) into another (macrophages).

In a Harvard press release posted on Monday, co-first author Sihem Chaloufi, a postdoc in Konrad Hochedlinger’s lab at Harvard, succinctly described the overall finding:

“The cells forget who they are, making it easier to trick them into becoming another type of cell.”

This potent erasing of cell memory via CAF-1 inhibition could make it easier to derive many different cells types from iPSCs or direct reprogramming for use in drug testing, modeling human disease in a lab dish as well as scaling up production of future cell therapies.

Scientists Reach Yet Another Milestone towards Treating Type 1 Diabetes

There was a time when having type 1 diabetes was equivalent to a death sentence. Now, thanks to advances in science and medicine, the disease has shifted from deadly to chronic.

But this shift, doctors argue, is not good enough. The disease still poses significant health risks, such as blindness and loss of limbs, as the patients get older. There has been a renewed effort, therefore, to develop superior therapies—and those based on stem cell technology have shown significant promise.

Human stem cell-derived beta cells that have formed islet like clusters in a mouse. Cells were transplanted to the kidney capsule and photo was taken two weeks later by which time the beta cells are making insulin and have cured the mouse's diabetes. [Credit: Douglas Melton]

Human stem cell-derived beta cells that have formed islet like clusters in a mouse. Cells were transplanted to the kidney capsule and photo was taken two weeks later by which time the beta cells are making insulin and have cured the mouse’s diabetes. [Credit: Douglas Melton]

Indeed, CIRM-funded scientists at San Diego-based Viacyte, Inc. recently received FDA clearance to begin clinical trials of their VC-01 product candidate that delivers insulin via healthy beta cells contained in a permeable, credit card-sized pouch.

And now, scientists at Harvard University have announced a technique for producing mass quantities of mature beta cells from embryonic stem cells in the lab. The findings, published today in the journal Cell, offer additional hope for the millions of patients and their families looking for a better way to treat their condition.

The team’s ability to generate billions of healthy beta cells—cells within the pancreas that produce insulin in order to maintain normal glucose levels—has a particular significance to the study’s senior author and co-scientific director of the Harvard Stem Cell Institute, Dr. Doug Melton. 23 years ago, his infant son Sam was diagnosed with type 1 diabetes and since that time Melton has dedicated his career to finding better therapies for his son and the millions like him. Melton’s daughter, Emma, has also been diagnosed with the disease.

Type 1 diabetes is an autoimmune disorder in which the body’s immune system systematically targets and destroys the pancreas’ insulin-producing beta cells.

In this study, the team took human embryonic stem cells and transformed them into healthy beta cells. They then transplanted them into mice that had been modified to mimic the signs of diabetes. After closely monitoring the mice for several weeks, they found that their diabetes was essentially ‘cured.’ Said Melton:

“You never know for sure that something like this is going to work until you’ve tested it numerous ways. We’ve given these cells three separate challenges with glucose in mice and they’ve responded appropriately; that was really exciting.”

The researchers are undergoing additional pre-clinical studies in animal models, including non-human primates, with the hopes that the 150 million cells required for transplantation are also protected from the body’s immune system, and not destroyed.

Melton’s team is collaborating with Medical Engineer Dr. Daniel G. Anderson at MIT to develop a protective implantation device for transplantation. Said Anderson of Melton’s work:

“There is no question that the ability to generate glucose-responsive, human beta cells through controlled differentiation of stem cells will accelerate the development of new therapeutics. In particular, this advance opens the doors to an essentially limitless supply of tissue for diabetic patients awaiting cell therapy.”

Perfecting the use of stem cells as drug delivery mules shows promise in brain tumors

Stem cells loaded with cancer-killing herpes virus (red) attacking a brain tumor cell (green). Courtesy HSCI

The innate tendency of stem cells to seek out inflammation—combined with the fact that our bodies see tumors as inflammation—has led many teams to try to harness stem cells as delivery vehicles for cancer therapies. CIRM funds a team at City of Hope in Duarte, California that aims to treat brain tumors with stem cells loaded with an agent that can be turned into a form of chemotherapy.

Now, a team at the Harvard Stem Cell Institute and Massachusetts General Hospital have used stem cells to revive a therapy, once considered highly promising, that failed in early clinical trials.

A number of viruses have the ability to kill tumors. In particular, some viruses naturally kill rapidly dividing cells like those found in tumors. But as so often happens, early success in mice did not carry over to the first trials in humans. Researchers reasoned that our body’s immune system cleared out the virus before it could do its deadly deed with the cancer cells.

In this study the Harvard team decided to shield the virus inside stem cells. They then encased the stem cells in a gel that they had previously shown could enhance the ability of the stem cells to remain alive after transplantation.

In a mouse model of glioblastoma, the most common form of brain cancer, the combination extended the life of the mice after the cells were placed at the site where the tumors were surgically debulked, a procedure performed on most human patients with the cancer.

The herpes virus they used had been tagged with an imaging protein that allowed the team to verify that the stem cells lived long enough for the virus to replicate and kill the residual tumor left behind after surgery. It is this residual cancer than makes this brain tumor almost uniformly fatal.

Because a few cancer cells are resistant to the herpes virus, the team gave their killer a second weapon, using genetic engineering techniques to help the virus kill cancer cells. The lead researcher, Khalid Shah, discussed the combination therapy in an article in Genetic Engineering & Biotechnology News:

“Our approach can overcome problems associated with current clinical procedures. The work will have direct implications for designing clinical trials using oncolytic viruses, not only for brain tumors, but for other solid tumors.”

Shah predicted the process would enter clinical trials in two to three years. They published the current research in the Journal of the National Cancer Institute.

Don Gibbons