Induced Pluripotent Stem Cells Lead Neuroscientists to the Cause of Neuron Loss in Parkinson’s Disease

Salk Institute scientists have made induced pluripotent stem cells (iPSCs) from patients with early-onset Parkinson’s disease (PD) in order to study precisely what goes wrong in the brains of PD patients. Their findings may lead to new ways to diagnose and even treat PD.

At the Salk Institute for Biological Studies in La Jolla, CA, Juan Carlos Izpisua Belmonte and his colleagues have examined the effects of mutations in a gene that encodes the leucine-rich repeat kinase 2 (LRRK2) protein on cultured neurons. LRRK2 mutations are responsible for approximately 2% of all inherited and sporadic cases of PD in North American Caucasian populations and up to 20% of all PD cases in Ashkenazi Jewish patients and approximately 40% of all PD cases in patients of North African Berber Arab ancestry. Therefore, the LRRK2 gene product plays a central role in PD pathology.

When iPSCs derived from PD patients who carry LRRK2 mutations, they were differentiated into neurons that were cultured in the laboratory. Cultured neurons from PD patients show profound disruption of the nuclear membrane and this undoes all nuclear architecture, which leads to cell death.

According to Dr. Izpisua Belmonte, “This discovery helps explain how PD, which had traditionally been associated with loss of neurons that produce dopamine and subsequent motor impairment, could lead to locomotor dysfunction and other common non-motor manifestations, such as depression and anxiety. Similarly, current clinical trials explore the possibility of neural stem cell transplantations to compensate for dopamine deficits. Our work provides the platform for similar trials by using patient-specific corrected cells. It identifies degeneration of the nucleus as a previously unknown player in PD.”

Izpisua Belmonte and his colleagues were also able to confirm that these disruptions of the nuclear membrane also occur in brain tissue from deceased PD patients. While it is still unclear if these disruptions to the nuclear membrane are the result of PD or are a cause of PD, Izpisua Belmonte’s lab used gene replacement techniques that were initially developed and perfected in work with mouse ESCs to fix the mutation in the PD patient-derived iPSCs. When they fixed the mutation, the disruptions to the nuclear membrane failed to form. Belmonte thinks that this could open the door for drug treatments of PD patients, although he did speculate as to how a pharmacological agent might mitigate abnormal nuclear architecture.

These results underscore the power of using iPSCs to model genetic diseases. As Belmonte noted, “We can model disease using these cells in ways that are not possible using traditional research methods, such as established cell lines, primary cultures and animal models.”

Another finding that nicely comports with data from clinical observations of PD patients is the tendency for patients to become progressively worse as they age. Likewise, in their cultured neurons differentiated from that were iPSCs derived from PD patients, Belmonte and his group observed progressively greater deformities in the nuclear membranes of the cells as they aged.

“This means that, over time, the LRRK2 mutation affects the nucleus of neural stem cells, hampering [sic] both their survival and their ability to produce neurons. It is the first time to our knowledge that human neural stem cells have been shown to be affected during Parkinson’s pathology due to aberrant LRRK2. Before development of these reprogramming technologies, studies on human neural stem cells were elusive because they needed to be isolated directly from the brain,” said Belmonte.

Belmonte further opined that dysfunctional neural stem cell populations that are afflicted with LRRK2 mutations might also contribute to other health issues associated with this particular form of PD, which includes depression, anxiety, and the inability to smell.

Modeling diseases with iPSCs also has an added bonus, since this model system can effectively recapitulate the effects of aging. Since unique dysfunctions result from aging, there are very few ways to model such events. However, using cultured cells made from iPSCs can bypass this problem, since the age-related pathologies will typically show up in culture.