Just a short while ago it was impossible to study the fundamental processes of living human brain cells (neurons)…but that is not true any more. Three years ago, a pair of Japanese investigators, Kazutoshi Takahashi and Shinya Tamanaka, developed a method to turn skin cells into stem cells. In so doing, they opened a new chapter in biological psychiatric research.
Stem cells are the basic, non-specialized cells that have the potential to turn into any of the hundreds of different types of cells that we have in our bodies. Every stem cell has a full set of genes. However, only a subset of genes is activated in any individual stem cell as it matures. The particular subset that becomes activated sets the cell on a course of specific purpose, development, and function; a process called differentiation. Stem cells programmed to activate muscle genes differentiate into muscle cells, those programmed to activate neuronal genes differentiate into neurons, and so on. We call the capacity of the stem cells to differentiate into any type of cell “pluripotency”.
All differentiated cells are derived from pluripotent stem cells. It is important to remember that, despite the effects of differentiation, and while only a subset of genes is active in each cell, a full set of genes continues to be present in every cell in our bodies.
You can imagine, therefore, that if you could roll back the “differentiation” clock that turned a particular stem cell into a skin cell, and return it to its pluripotent state, you might be able to switch its destiny and turn it into a neuron or any other cell type. Rolling back the differentiating clock is exactly what Takahashi and Yamanaka did. They created a special gene recipe that brings a fully differentiated cell back to its pluripotent state. They call this re-created stem cell an “induced pluripotent stem cell” or iPSC. Since their landmark discovery, some investigators have reported that iPSCs can also be made from hair follicles and white blood cells.
The main use envisioned for iPSCs is as a substitute for embryonic stem cells. Embryonic stem cells show promise for regenerating tissue damaged by disease or trauma, such as spinal cord injury. However, the controversy over harvesting embryonic stem cells from embryos poses a major obstacle for this research. The use of iPSC’s bypasses the controversy and throws open the doors of exploration. In addition to tissue regeneration, researchers envision another use for iPSCs; to develop models for human diseases that are otherwise impossible to study in the laboratory, such as being able to work on living neurons derived from patients with various psychiatric disorders. JBRF sponsored researchers want to do just that: capitalize on the new iPSC technology to grow neurons from the skin or hair follicles of children with pediatric bipolar disorder (PBD), thereby providing a unique opportunity to understand the fundamental deficit at the heart of our children’s problems.
There is essentially no limit to the types of experiments that can be carried out using neurons derived from iPSCs. However, before we explain to you what our investigators propose to do, we must first give a brief explanation about how neurons speak to each other.
Imagine a relay race and picture the point at which one runner hands the baton to the other. Transferring the baton is somewhat similar to how one neuron sends its information to the next neuron. The hand off occurs in a space between the two cells called the “synapse” and instead of a baton, the cells use “neurotransmitters”.
There are two basic classes of neurotransmitters. One class is responsible for telling the next cell to get excited; the other to settle down. The most common neurotransmitter that belongs to the first class is glutamate, while GABA is the major inhibitory transmitter. Since “go” and “stop” (excitation and inhibition) underlie almost every message passed between neurons, these two neurotransmitters are the most abundant in the brain. There are other neurotransmitters that modulate neuronal function, which many of you are probably familiar with: dopamine, serotonin and norepinephrine, for example.
Many studies indicate that glutamatergic and GABAergic neurons (those that release glutamate and GABA) are impaired in bipolar disorder. Therefore, the JBRF consortium of investigators proposes to create both glutamatergic and GABAergic neurons from iPSCs. The research they propose to do with these neurons focuses on avenues that they feel will provide the best opportunity for new drug development in the future. They are:
- Gene Expression Profiling
- Analysis of synaptic function in glutamatergic and GABAergic neurons.
Gene expression profiling:
Although every cell contains every gene, only a subset of genes is activated in cells. Thus neuronal genes are active in neurons and muscle genes in muscles. This gene activity status is referred to as an expression profile. Disease usually alters a cell’s expression profile, thereby adversely affecting cellular function. Using state of the art techniques called microarray analysis and massively parallel sequencing, investigators can evaluate the expression pattern of every gene in a quantifiable manner. They can then compare the expression profiles of neurons derived from subjects with bipolar disorder to those of neurons derived from control subjects to detect underlying differences and aberrant molecular pathways.
Such studies are very informative. For example, a similar type of analysis identified a molecular signature in a very malignant form of breast cancer. That marker became the target for new drug development. The highly specific, novel medication that resulted from that process is only effective in breast cancers that express the marker. What was previously one of the worst types of breast cancer is now one of the most treatable.
We hope to be able to repeat this sort of enlightening and productive search in PBD.
Gene expression profiling in psychiatric disorders is not new. However, iPSC technology provides a distinct advantage. Previously, investigators were only able to gather and study post-mortem brain tissue. In that situation, it was not always possible to know if influences other than the disease may have affected the cell’s expression profile, such as illegal drugs, medications, alcohol and cigarettes. Using iPSC technology, investigators will be able to obtain pure samples of cells that have not been affected by these substances, since any effect these might have on gene expression are erased when iPSCs are cultivated.
Synaptic Function in Glutamatergic and GABAergic Neurons:
Synapses are very complicated structures. Individual neurons in the brain can form synapses with thousands of other neurons. It has been estimated that in the adult brain, there may be as many as one quadrillion synapses. The elaborate system of communication formed by these synaptic connections defines a person’s consciousness, memories, skills, and behavior.
Many investigators in biological psychiatry are discovering that some of the genes implicated in neuropsychiatric disorders are directly involved in building the brain’s synaptic architecture. The fundamental functions of the synapse; to transfer electrical and biochemical information from neuron to neuron and to modify itself in response to stimulation, cannot be analyzed in autopsy samples. The opportunity to study these basic processes in living cells using iPSC technology will be instrumental in the identification of specific glutamate and GABA-related abnormalities in PBD.
Our Unique Positioning to Carry Out A Successful Study
Our clinical and basic research team is uniquely positioned to carry out a successful study in this novel and exciting area of research.
JBRF basic research investigators are currently conducting an iPSC study in patients with schizophrenia and a condition called Velo-Cardio-Facial Syndrome. The study is one of a small handful of iPSC-related studies selected by the National Institute of Mental Health (NIMH) for funding. The reviewers were particularly impressed with the experts in neuroscience, cell biology and biostatistics who were assembled for the study. The same research team is prepared to work together to initiate a similar study in patients with PBD.
On the clinical side, the discovery of the Fear-of-Harm (FOH) phenotype (link to FOH phenotype) adds a unique dimension to the iPSC project in PDB. Through the identification of children who are clinically homogeneous for the FOH trait and those who are not, insight into underlying neuronal pathways responsible for these clinical states will be possible when gene expression profiling is carried out and glutamatergic and GABAergic neurons are analyzed, and differences between these clinical subtypes are identified. The JBRF is eager to find the means to fund this very exciting project.