iPSC-derived neurons were found to display reduced neuronal connectivity.

Scientists have managed to culture viable neurons from induced pluripotent stem cells generated directly from the skin fibroblasts of schizophrenia patients. The researchers, led by a team at the Salk Institute for Biological Science and Pennsylvania State University, claim studies with the cells have confirmed the neurons are both genetically and morphologically different to neurons derived from nonschizophrenic people.  

They suggest the ability to culture patient-derived neurons will provide new insights into the development and mechanisms of schizophrenia and provide a new in vitro model for screening potential drug candidates. The research is published in Nature in a paper titled “Modelling schizophrenia using human induced pluripotent stem cells.”

Schizophrenia has a worldwide prevalence of 1%, and there is a strong genetic component, with an estimated heritability of 80–85%, report lead author Fred Gage, Ph.D., and colleagues. Post mortem studies have revealed reduced brain volume, cell size, spine density, and abnormal neural distribution in the prefrontal cortex and hippocampus of schizophrenic brain tissue. Neuropharmacological studies have in addition implicated dopaminergic, glutamatergic, and GABAergic activity in schizophrenia.

The authors admit, though, that despite this accumulated knowledge the cell types affected in schizophrenia and the molecular mechanisms underlying the disease state remain unclear. In order to further investigate the cellular and molecular basis of schizophrenia at the level of the neuron, the team reprogrammed fibroblasts taken from four schizophrenia patients into iPSCs, and then triggered these to differentiate into neurons.

A rabies viral tracer allowed them to compare these schizophrenia patient-derived neurons from those derived from healthy individuals. The results showed that the schizophrenia-derived neurons connected less frequently with each other and put out fewer cell body projections.

The researchers treated the neurons with five antipsychotic drugs during the last three weeks of neuronal differentiation, to see which improved neuronal connectivity. Of the five tested—clozapine, loxapine, olanzapine, risperidone, and thioridazine—only loxapine significantly increased neuronal connectivity. However, they point out, “optimization of the concentration and timing of drug administration may improve the effects of the other antipsychotic medications.”

“These drugs are doing a lot more than we thought they were doing,” notes co-author Kristen Brennand, also at the Salk Institute. “But now, for the very first time, we have a model system that allows us to study how antipsychotic drugs work in live, genetically identical neurons from patients with known clinical outcomes, and we can start correlating pharmacological effects with symptoms.”

The team in addition carried out comparative gene-expression studies on the schizophrenia-derived and normal neurons. These highlighted 596 genes that were differentially expressed by the schizophrenia-derived neurons. 25% of these had previously been linked with the disease, but a number were involved in pathways not previously linked with schizophrenia, including NOTCH signaling, SLIT/ROBO axon guidance, EFNA-mediated axon growth, cell adhesion, and transcriptional silencing.

The authors suggest their data support a watershed model of schizophrenia, whereby a number of different combinations of misfunctioning genes could be capable of disrupting the key pathways involved. They expect that studying more cases of schizophrenia using neurons generated from patients will whittle down the huge pot of potentially critical genes to a smaller subset that are consistently affected and identify perhaps just a handful of essential pathways that cause schizophrenia when disrupted.

“This is the first time that a complex mental disease has been modeled in live human cells,” claims Dr. Gage, who holds the Vi and John Adler chair for research on age-related neurodegenerative diseases at the Salk’s Laboratory of Genetics. “This model not only affords us the opportunity to look at live neurons from schizophrenia patients and healthy individuals to understand more about the disease mechanism but also to screen for drugs that may be effective in reversing it.”  

Dr. Brennand adds, “Nobody knows how much the environment contributes to the disease. By growing neurons in a dish, we can take the environment out of the equation, and start focusing on the underlying biological problems.”

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