Skipping a Step: Turning Brain Cells Directly into Neurons

It was once commonly believed that “what you see is what you get” with the human brain. As in, the brains cells that you are born with are the only ones you’ll have for the rest of your life because they can’t regenerate.

The discovery of brain stem cells in the late 90s disproved this notion and established that the brain can replace old cells and repair damage after injury. The brain’s regenerative capacity is limited, however. Consequently, patients suffering from neurodegenerative diseases like Alzheimer’s and Parkinson’s can’t rely on their brain stem cells to repopulate all of the sick and dying neurons in their brains.

This is where cellular reprogramming technology could come to the rescue. Induced pluripotent stem cells (iPSCs) generated from patient skin cells by cellular reprogramming can be turned into many types of brain cells to study diseases in a petri dish, as well as to test drugs and develop stem cell therapies. Eventually, the hope is to transplant iPSC-derived brain cells back into patient brains to treat or cure degenerative diseases.

Making neurons directly from other brain cells

Another form of cellular reprogramming, offers a more direct approach to generating populations of healthy brain cells. Using a similar technique to iPSCs, scientists can use specific factors to directly reprogram skin cells or other brain cells into neurons without making them go through the pluripotent stem cell state. By skipping a step in the reprogramming process, researchers save time, money, and energy – and it could result in safer cells.

The group used small molecules to directly reprogram human astrocytes into neurons that could be transplanted into mice. (Zhang et al., 2015)

The group used small molecules to directly reprogram human astrocytes into neurons that could be transplanted into mice. (Zhang et al., 2015)

While direct reprogramming of skin and non-neuronal brain cells into neurons has been published before, a study in Cell Stem Cell by a group at Penn State University last week described a new-and-improved method to make properly functioning neurons from brain astrocytes. Astrocytes are a type of glial cell that are abundant in the brain. They provide neurons with support, nutrients, and aid following injury.

Led by senior author Gong Chen, the group bathed human astrocytes in a cocktail of small molecules that turned the astrocytes into neurons in less than 10 days. These neurons survived in a dish for more than 5 months and were able to send electrical signals to each other (a sign that they were functional). Even more exciting was that the directly reprogrammed neurons survived and functioned properly when they were transplanted into the brains of mice.

When they studied the biological mechanism behind their direct reprogramming method, they found that the small molecule cocktail turned off the activity of astrocyte-specific genes in the astrocytes and turned on neuron-specific genes to convert them into neurons.

Human astrocytes (left) were directly reprogrammed into neurons (right). (Zhang et al., 2015)

Human astrocytes (left) were directly reprogrammed into neurons (right). (Zhang et al., 2015)

This discovery is great news for the reprogramming field as using small molecule reprogramming instead of the commonly used transcription-factor based reprogramming (which involves using viruses that can damage or alter the genome) is a more attractive method with broader applications­ for making neurons that can be transplanted into humans.

Direct reprogramming makes new neurons in the brain

But wait, there’s more! An article from TheScientist reported that multiple groups at the Society for Neuroscience (SFN) conference  in Chicago presented results on directly converting glial cells into neurons in mouse brains rather than in a dish.

One group from the Johannes Gutenberg University used two transcription factors, proteins that control which genes are turned on or off in the human genome, to directly reprogram mouse astrocytes into neurons. By producing more of Sox2 and Ascl1 in the cortex of the mouse brain than would normally be found there, they were able to turn 15% of the glial cells in that area into neurons.

The function of these directly reprogrammed neurons remains to be determined, but the lead scientist, Sophie Peron, told TheScientist:

“That’s the next step. Now that we have a system to get these cells converted we are currently studying their connectivity, functionality, and precise characteristics.”

Two other groups also reported similar findings when they worked with a type of glial cell called reactive astrocytes. These cells are specifically activated during injury to jumpstart the healing process. The first group from the University of Texas Southwestern used the factor Sox2 to directly reprogram reactive astrocytes into neurons in mice, while the group from Penn State University – mentioned earlier in this blog – did the same thing, but using a different factor, NeuroD1.

The Penn State group went further to test their direct reprogramming method in a mouse model of stroke and found that NeuroD1-reprogrammed neurons reduced cell death and tissue scarring after stroke.

Lead scientist Yuchen Chen said:

“These findings suggest that direct reprogramming of glial cells into functional neurons may provide a completely new approach for brain repair after stroke. Our next step is to analyze whether the glia-neuron conversion technology can facilitate functional recovery in stroke animals.”

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Stay on Target: Scientists Create Chemical ‘Homing Devices’ that Guide Stem Cells to Final Destination

When injecting stem cells into a patient, how do the cells know where to go? How do they know to travel to a specific damage site, without getting distracted along the way?

Scientists are now discovering that, in some cases they do but in many cases, they don’t. So engineers have found a way to give stem cells a little help.

As reported in today’s Cell Reports, engineers at Brigham and Women’s Hospital (BWH) in Boston, along with scientists at the pharmaceutical company Sanofi, have identified a suite of chemical compounds that can help the stem cells find their way.

Researchers identified a small molecule that can be used to program stem cells (blue and green) to home in on sites of damage. [Credit: Oren Levy, Brigham and Women's Hospital]

Researchers identified a small molecule that can be used to program stem cells (blue and green) to home in on sites of damage. [Credit: Oren Levy, Brigham and Women’s Hospital]

“There are all kinds of techniques and tools that can be used to manipulate cells outside the body and get them into almost anything we want, but once we transplant cells we lose complete control over them,” said Jeff Karp, the paper’s co-senior author, in a news release, highlighting just how difficult it is to make sure the stem cells reach their destination.

So, Karp and his team—in collaboration with Sanofi—began to screen thousands of chemical compounds, known as small molecules, that they could physically attach to the stem cells prior to injection and that could guide the cells to the appropriate site of damage. Not unlike a molecular ‘GPS.’

Starting with more than 9,000 compounds, the Sanofi team narrowed down the candidates to just six. They then used a microfluidic device—a microscope slide with tiny glass channels designed to mimic human blood vessels. Stem cells pretreated with the compound Ro-31-8425 (one of the most promising of the six) stuck to the sides. An indication, says the team, Ro-31-8425 might help stem cells home in on their target.

But how would these pre-treated cells fare in animal models? To find out, Karp enlisted the help of Charles Lin, an expert in optical imaging at Massachusetts General Hospital. First, the team injected the pre-treated cells into mouse models each containing an inflamed ear. Then, using Lin’s optical imaging techniques, they tracked the cells’ journey. Much to their excitement, the cells went immediately to the site of inflammation—and then they began to repair the damage.

According to Oren Levy, the study’s co-first author, these results are especially encouraging because they point to how doctors may someday soon deliver much-needed stem cell therapies to patients:

“There’s a great need to develop strategies that improve the clinical impact of cell-based therapies. If you can create an engineering strategy that is safe, cost effective and simple to apply, that’s exactly what we need to achieve the promise of cell-based therapy.”