Diseases that are hard to study, such as Alzheimer’s, schizophrenia, and autism can be examined more safely and effectively thanks to an innovative new method for making mature brain cells from reprogrammed skin cells. Gong Chen, the Verne M. William Chair in Life Sciences and professor of biology at Penn State University and the leader of the research team that designed this method said this: “The most exciting part of this research is that it offers the promise of direct disease modeling, allowing for the creation, in a Petri dish, of mature human neurons that behave a lot like neurons that grow naturally in the human brain.”
Chen’s method could lead to customized treatment for individual patients that are based on their own genetic and cellular profile. Chen explained it this way: “Obviously we do not want to remove someone’s brain to experiment on, so recreating the patient’s brain cells in a Petri dish is the next best thing for research purposes and drug screening.”
In previous work, scientists at the University of Wisconsin in James Thomson’s laboratory and in Shinya Yamanaka’s laboratory at Kyoto University in Kyoto, Japan discovered a way to reprogram adult cells into pluripotent stem cells. Such stem cells are called induced pluripotent stem cells or iPSCs. To make iPSCs, scientists infect adult cells with genetically engineered viruses that introduce four specific genes (OCT4, SOX2, KLF4 and cMYC for those who are interested). These genes encode transcription factors, which are proteins that bind to DNA or to the machinery that directly regulates gene expression. These transcription factors turn on those genes (e.g., OCT4, NANOG, REX1, DNMT3β and SALL4, and OCT4) that induce pluripotency, which means the ability to form any adult cell type. Once in the pluripotent state, iPSCs can be cultured and grown life embryonic stem cells and can differentiate into adult cell types and tissues.
As Chen explained, “A pluripotent stem cell is a kind of blank slate.” Chen continued, “During development, such stem cells differentiate into many diverse specialized cell types, such as a muscle cell, a brain cell, or a blood cell. So, after generating iPSCs from skin cells, researchers then can culture them to become brain cells, or neurons, which can be studies safely in a Petri dish.”
Chen’s team invented a protocol to differentiate iPSCs into mature human neurons much more effectively than previous protocols. This generates cells that behave neurons in our own brains and can be used to model the individualized disease of a single patient.
In the brain, neurons rarely work alone, but instead are usually in close proximity to star-shaped cells called astrocytes. Astrocytes are very abundant cells and they assist neuron function and mediate neuronal survival. “Because neurons are adjacent to astrocytes in the brain, we predicted that this direct physical contact might be an integral part of neuronal growth and health,” said Chen. To test this hypothesis, Chen and his colleagues began by culturing iPSCs-derived neural stem cells, which are stem cells that have the potential to become neurons. These cells were cultured on top of a one-cell-thick layer of astrocytes sop that the two cell types were physically touching each other.
“We found that these neural stem cells cultured on astrocytes differentiated into mature neurons much more effectively,” Chen said. This contrasts Chen’s method with other neural stem cells that were cultured alone in a Petri dish. As Chen put it, the astrocytes seems to be “cheering the stem cells on, telling them what to do, and helping them to fulfill their destiny to become neurons.”
While this sounds a little cheesy, it is undeniable that the astrocyte layer increases the efficiency of neuronal differentiation of iPSCs. Personalized medicine is moving beyond the gene level, to the level of cellular organization and tissue physiology, and iPSCs are showing the way.