On the surface, a stem cell-based cure for Parkinson’s disease seems pretty straight-forward. This age-related neurodegenerative disorder, which leads to progressively worsening tremors, slowness of movement and muscle rigidity, is caused by the death of a specific type of nerve cell, or neuron, that produces the chemical dopamine in a specific region of the brain. So it would seem that simply transplanting stem cell-derived dopamine-producing neurons (DA neurons) in the brains of Parkinson’s patients to replace the lost cells would restore dopamine levels and alleviate Parkinson’s symptoms.
Easier said than done
Well, it hasn’t turned out to be that easy. After initial promising results using fetal brain stem cell transplants in the 80’s and 90’s, larger clinical trials showed no significant benefit and even led to a worsening of symptoms in some patients. One potential issue with those early trials could have been variable cell composition of the fetal cell-based therapy. On top of that, the availability of fetal tissue is limited and the quantities of transplantable cells obtained from these samples are very low.
More recently, researchers have been busy at generating more pure populations of DA neurons from human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs). Great progress has been made so far, but the field is still hampered by not being able to make enough DA neurons from hESCs and iPSCs in a timely manner.
Cutting out the pluripotent “middle man”
This week a research team at the University of Buffalo reported in Nature Communications about a much more efficient method for producing DA neurons. It’s a finding that could provide a strong push towards stem cell-based therapy development for Parkinson’s disease.
The team bypassed the need to start with hESCs or iPSCs and instead converted skin cells directly into DA neurons. A thorough analysis of the cells confirmed that they were functional and matched that characteristics of the specific dopamine neurons that are lost in Parkinson’s.
This direct reprogramming of skin cells into DA neurons as well as other cells is a technique pioneered by several independent researchers including some of our own grantees. This method is thought to have a few advantages over the specialization of immature hESCs or iPSCs into tissue-specific cells. Not only is the direct reprogramming process faster it also doesn’t require cell division so there’s less concern about the introduction of DNA mutations and the potential of tumor formation. Another plus for direct reprogramming is the possibility of inducing the direct conversion of one cell type into another inside the body rather than relying on the manipulation of hESCs and iPSCs in the lab. Still, despite these advantages the efficiency of direct reprogramming is still very low. That’s where the University of Buffalo team comes into the picture.
Bringing down the gatekeeper
The researchers led by physiology and biophysics professor Jian Feng, made a few key modifications to increase the efficiency of the current skin cell to DA neuron direct reprogramming methods. They first reduced the level of a protein call p53. This protein has several nicknames like “guardian of our genes” and “tumor suppressor” because it plays critical roles in controlling cell division and DNA repair and, in turn, helps keep a clamp on cell growth.
Reducing the presence of p53 during the direct reprogramming process led to a much more efficient conversion of skin cells to DA neurons. And because the conversion from a skin cell to a neuron happens quickly – just a couple days – timing the introduction of cell nutrients specific to neurons had to be carefully watched. Together, these tweaks improved upon previous studies as Feng mentioned in a University of Buffalo press release:
“The best previous method could take two weeks to produce 5 percent dopamine neurons. With ours, we got 60 percent dopamine neurons in ten days.”
IMHO (In my humble opinion)
I imagine there’s a lot more work ahead to get this method of deriving DA neurons from skin ready for the clinic. This reprogramming technique relied on the introduction of neuron-specific genes into the skin cells using a deactivated virus as the means of delivery. Even though the virus is inactive, its viral DNA randomly inserts into the cells’ chromosomes which can turn on genes that cause cancer. Therefore, a non-viral version of this method would need to be developed for clinical use.
Also, as mentioned earlier, since p53 inhibits tumors by suppressing uncontrolled cell division, it would be important to make sure that a reduction of p53 didn’t lead to any long-term negative consequences, like the transplantation of potentially cancerous cells into the patient.
Still, this dramatic increase in efficiency for making functional DA neurons and the identification of p53 as a key control point for direct reprogramming are very exciting developments for a disease field that is committed to finding cures for its patients.