Efficient Direct Conversion of Skin Cells to Motor Neurons for Advanced Therapies

Transforming one type of cell into another, such as converting a skin cell into a neuron, typically involves inducing the skin cell into a pluripotent stem cell and then differentiating it into a neuron. However, researchers at MIT have developed a more streamlined process that bypasses the stem cell stage altogether, allowing for direct conversion of skin cells into neurons.

Innovative Direct Conversion Method

By working with mouse cells, the researchers have created a highly efficient method that can yield more than 10 neurons from a single skin cell. If this process can be replicated in human cells, it could pave the way for generating significant quantities of motor neurons, which may be utilized in therapies for spinal cord injuries or mobility impairing diseases.

Katie Galloway, the W. M. Keck Career Development Professor in Biomedical Engineering and Chemical Engineering, emphasizes the potential of these cells for cell replacement therapies. “We were able to achieve yields that allow us to investigate whether these cells can serve as viable candidates for therapeutic applications. This is where advanced reprogramming technologies can lead us,” she states.

Therapeutic Implantation and Integration

As a preliminary step towards using these cells in therapeutic settings, the research team demonstrated the ability to generate motor neurons and successfully implant them into mouse brains, where they integrated with the surrounding tissue.

This innovative approach builds on nearly two decades of research, starting with studies from Japan that showed skin cells could be transformed into induced pluripotent stem cells (iPSCs) using four transcription factors. While effective, this method is time-consuming and often results in incomplete cell maturation.

Galloway notes, “One of the significant challenges in reprogramming is that cells can become stuck in intermediate states. Our method employs direct conversion, enabling a seamless transition from somatic cells directly to motor neurons without passing through the iPSC stage.”

Refining Gene Combinations for Efficiency

Previous attempts at direct conversion yielded low success rates of under 1 percent. In prior research, Galloway used a combination of six transcription factors alongside two proteins to encourage cell proliferation, which complicated the expression levels of each gene across individual cells.

In their recent findings published in Cell Systems, Galloway and her team introduced a simplified strategy that allows for the conversion of skin cells to motor neurons utilizing just three transcription factors along with two genes that promote cell proliferation.

Through iterative experimentation, they identified the optimal trio of transcription factors—NGN2, ISL1, and LHX3—that efficiently facilitate the conversion to neurons. With this reduction in gene number, the researchers were able to use a single modified virus to deliver all three factors effectively.

“When transcription factors are expressed at high levels in non-proliferative cells, reprogramming rates tend to be low. However, hyperproliferative cells are more amenable to reprogramming, making them more responsive to transcription factor levels.” – Katie Galloway

Additionally, by employing another virus to introduce genes encoding p53DD and a mutated version of HRAS, they enabled the skin cells to proliferate significantly before commencing conversion to neurons. This process led to an impressive yield increase of approximately 1,100 percent.

Advancements in Human Cell Conversion

Furthermore, the team has successfully adapted this direct conversion approach for human cells using a different combination of transcription factors, achieving efficiency rates between 10 and 30 percent over about five weeks—faster than the traditional iPSC pathway.

After determining the optimal gene combination for conversion, the researchers focused on refining delivery methods, which was detailed in their second Cell Systems paper. Their experiments revealed that retroviruses provided the most efficient conversion rates. Additionally, reducing cell density during culture enhanced overall neuron yield.

Functional Integration and Future Prospects

Following these advancements, the team collaborated with colleagues at Boston University to test the engraftment of these motor neurons into mice. The cells were delivered to the striatum region of the brain responsible for motor control and other functions.

Two weeks post-implantation, many of the neurons had survived and appeared to form connections with existing brain cells. When cultured in vitro, these neurons exhibited measurable electrical activity and calcium signaling, indicating functional communication capabilities. The researchers are now optimistic about exploring similar implants into the spinal cord.

The MIT team also aims to enhance the efficiency of converting human cells, which could lead to abundant neuron generation for treating spinal cord injuries or motor control disorders like ALS. Current clinical trials are testing iPSC-derived neurons for ALS treatment; expanding the availability of such cells could facilitate broader testing and development for clinical applications.

This groundbreaking research was supported by funding from the National Institute of General Medical Sciences and the National Science Foundation Graduate Research Fellowship Program.