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The transformation of one cell type into another, such as turning a skin cell into a neuron, traditionally involves a complex process where the original cell is first converted into a pluripotent stem cell before being differentiated into the desired cell type. However, researchers at MIT have innovated a more streamlined approach that directly converts skin cells into neurons, thus bypassing the stem cell stage entirely.
In experiments utilizing mouse skin cells, the team has created a highly efficient conversion method that can yield over 10 neurons from a single skin cell. If this technique can be adapted for human cells, it could lead to a significant advancement in producing motor neurons on a large scale, which may offer new treatments for spinal cord injuries or conditions that hinder mobility.
“Our results indicate that we can produce a quantity of neurons that warrants investigation into their potential as candidates for cell replacement therapies,” asserts Katie Galloway, the W. M. Keck Career Development Professor in Biomedical Engineering and Chemical Engineering.
As a critical step in assessing the therapeutic potential of these neurons, the research team successfully generated motor neurons and integrated them into the brains of mice, where they established connections with existing tissues.
Galloway is the senior author of two groundbreaking studies detailing this method, both published in the journal Cell Systems, with MIT graduate researcher Nathan Wang as the lead author.
From Skin to Neurons
Nearly two decades ago, Japanese scientists discovered that by introducing four specific transcription factors into skin cells, they could reprogram them into induced pluripotent stem cells (iPSCs). iPSCs possess the ability to differentiate into various other cell types similar to embryonic stem cells. Although effective, this conventional method typically spans several weeks and often results in incomplete transitions to fully developed cell types.
“One of the ongoing challenges in cellular reprogramming is that cells can often remain stuck in intermediate states,” Galloway explains. “Our approach utilizes direct conversion, enabling us to shift from a somatic cell directly to a motor neuron, circumventing the iPSC phase.”
While Galloway’s research and that of her colleagues have previously achieved direct conversion, the rates were disappointingly low, often under 1 percent. In earlier studies, Galloway employed a combination of eight genes, delivered via multiple viral vectors, complicating the process of ensuring consistent gene expression levels across the cells.
In one of the recent studies published in Cell Systems, Galloway’s team outlined a more efficient method that facilitated the conversion of skin cells to motor neurons using only three transcription factors, in addition to two genes promoting a robust cell proliferation state.
Beginning with the original six transcription factors, the researchers systematically removed each one to identify a successful trio: NGN2, ISL1, and LHX3, which effectively enabled the conversion to neurons.
Following the optimization to three genes, the researchers utilized a single modified virus for their delivery, ensuring proper expression levels in each skin cell. Additionally, they introduced genes encoding p53DD and a mutated version of HRAS to stimulate extensive cell division before conversion, achieving substantial neuron yields—approximately 1,100 percent.
“When transcription factors are expressed at elevated levels in nonproliferative cells, reprogramming rates tend to be low. In contrast, hyperproliferative cells show heightened receptiveness to the transcription factors, akin to a primed state for conversion,” Galloway remarks.
The team also developed an alternative set of transcription factors suitable for direct conversion in human cells, albeit with reduced efficiency of 10 to 30 percent. This procedure takes about five weeks, which is more rapid than the conventional iPSC-based approach.
Implanting Cells
Upon establishing the optimal gene combination for conversion, the researchers focused on the delivery method, which is discussed in the second Cell Systems paper.
Through trials with three different viral delivery systems, they found that a retrovirus yielded the highest conversion efficiency. Additionally, by reducing cell density during culturing, they successfully enhanced the overall yield of motor neurons to over 1,000 percent within a two-week timeframe for mouse cells.
Collaborating with Boston University, the researchers then investigated their ability to successfully engraft these motor neurons into a targeted area of the mouse brain known as the striatum—an area integral to motor function.
After two weeks, many of the implanted neurons survived and appeared to form synaptic connections with native brain cells. When evaluated in vitro, these cells demonstrated measurable electrical activity and calcium signaling, indicating functional engagement with other neurons. The next steps involve exploring the feasibility of implanting these neurons into the spinal cord.
The MIT team aims to enhance the efficiency of this conversion process for human cells, potentially facilitating the mass production of neurons for therapeutic applications in spinal cord injury treatment and conditions like amyotrophic lateral sclerosis (ALS). As clinical trials are currently underway using neurons derived from iPSCs for ALS treatments, an increase in the availability of cells for such therapies would foster broader testing and development opportunities, according to Galloway.
This research was supported by the National Institute of General Medical Sciences and the National Science Foundation Graduate Research Fellowship Program.
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