Induced pluripotent stem cell lines that are created from the somatic cells of people with neurological diseases are beginning to pave a pathway for researchers to understand pathophysiology and screen for new drug candidates.
Recent work has characterized disease phenotypes and, in some cases, tested therapies in neurons that were differentiated from induced pluripotent stem cells (iPSCs) derived from patients with sporadic and familial Alzheimer’s disease, Huntington’s disease, and familial Parkinson’s disease.
These are heady times for this field in general and for neurological disease in particular. "It is really exciting when you start to be able to see relevant pathophysiologies in the dish," said Matthew Huentelman, Ph.D., head of the neurobehavioral research unit in the neurogenomics division at the Translational Genomics Research Institute, Phoenix.
Dr. Yadong Huang
Other investigators are trying to avoid one of the problems that has been plaguing this line of research: cell teratoma formation in vivo iPSC and embryonic stem cells. Their current approach is to reprogram fibroblasts into multipotent induced neural stem cells (iNSCs) that can then be differentiated into whatever neural progenitor cells the investigators wish to culture (neurons, oligodendroglia, and astroglia). This method also may lead to more efficient differentiation into the desired cell type, according to Dr. Yadong Huang, an associate investigator at the Gladstone Institute of Neurological Disease at the University of California, San Francisco.
Dr. Huang was the senior author on a study that used a single factor, rather than three or more, to directly reprogram human fibroblasts into multipotent iNSCs that can self-renew for 40-50 generations. This self-renewal ability, which is not present in the direct reprogramming of fibroblasts into neurons, makes them better to use for pathophysiology and drug testing studies (Cell Stem Cell 2012;11:100-9).
Only in the past year or so have people begun working in this middle ground between iPSCs and directly induced neuronal cells, so "it is important for studies in all three techniques to go forward now in parallel because each has advantages and disadvantages, at least at this point," Dr. Huang said in an interview.
"These initial studies ... are really encouraging because [they set us up] to dream about how we might leverage these cellular technologies to perform high-throughput drug screening," noted Dr. Huentelman.
In one study of Alzheimer’s disease, investigators developed iPSC cell lines from one patient with sporadic disease and two patients with familial disease caused by a mutation in the gene for amyloid precursor protein (APP). In each case, the investigators observed three major biochemical markers of Alzheimer’s (Nature 2012 Jan. 25 [doi:10.1038/nature10821]).
A separate study of familial Alzheimer’s disease with patients with presenilin-1 mutations, which was recently presented at the Alzheimer’s Association International Conference 2012, found substantial phenotypic differences between control neurons derived from iPSCs from unaffected family members, such as an increased ratio of amyloid-beta-42 to amyloid-beta-40 and an enhanced cell death response to apoptotic stimuli.
Even greater success has been reported with human Huntington’s disease neurons derived from iPSCs. In one study, researchers correlated the severity of phenotype in neurons with the number of CAG expansions observed in the huntingtin gene, as is observed in humans. However, some of the phenotypes were observed only in cell lines with the longest CAG expansion (Cell Stem Cell 2012 June 28 [doi:10.1016/j.stem.2012.04.027]). A separate group of researchers reported success in replacing the CAG expansion with a normal repeat length, which corrected the Huntington’s disease phenotype of mitochondrial deficits, lower levels of brain-derived neurotrophic factor, and altered cell signaling proteins (Cell Stem Cell 2012 June 28 [doi:10.1016/j.stem.2012.04.026]).
Another group of investigators reported the pharmacologic rescue of mitochondrial deficits in iPSC-derived neural cells from patients with familial Parkinson’s disease caused by mutations in either PINK1 or LRRK2. Neural cells with PINK1 mutations that received low doses of mitochondrial stressors were rescued by both the antioxidant coenzyme Q10 and an LRRK2 inhibitor, whereas those with an LRRK2 mutation were partially rescued by rapamycin or an LRRK2 inhibitor. The researchers said that they hope to apply their observations on cellular responses to stress in mutation-carrying, iPSC-derived neural cells to sporadic forms of Parkinson’s disease (Sci. Transl. Med. 2012 July 4 [doi:10.1126/scitranslmed.3003985]).
"I think where the real excitement might come is how relevant these models systems are in helping us dissect the more sporadic forms of these diseases, because, frankly, that’s where we need some additional ability to control our investigations," Dr. Huentelman said.
Dr. Matthew Huentelman
However, he noted that iPSC-derived cell lines take a lot of time and money to produce. In addition, all of the studies reported so far suffer from a small sample size – mostly one or two patients – as well as the potential for diagnostic inaccuracy. "Accuracy is good for these disorders, but it is not 100%," he said.