Age is the strongest environmental risk factor for Alzheimer’s, Parkinson’s, and other neurodegenerative disorders, and scientists often wish they could control the process when studying various disease models. Now they may get their wish. In the December 5 Cell Stem Cell, researchers led by Lorenz Studer at the Sloan-Kettering Institute, New York, report a method for speeding up aging in neurons. Using a protein that causes the premature aging disorder progeria, the researchers artificially aged cells derived from healthy people and from patients with inherited forms of PD. The combination of progerin and PD genes revealed parkinsonian features in neurons that previous cell models lacked.
“This is a really exciting idea that addresses a major problem in the field. There is a tremendous need for cell models that recapitulate the phenotypes of aging,” said Asa Abeliovich of Columbia University, New York, who was not involved in the study.
Deriving cellular models of disease from patient samples is a promising application of new technologies that reprogram cells. For example, induced pluripotent stem cells (iPSCs) reprogrammed from skin cells can be turned into neurons, or in some cases even directly converted into certain neuronal types. Recently, these approaches have been used to make human neuron cultures that recapitulate some key pathological features from patients with Alzheimer’s disease (see Aug 2011 news story; Jan 2012 news story), Parkinson’s disease (Nguyen et al., 2011; Chung et al., 2013), and amyotrophic lateral sclerosis (Sareen et al., 2013).
However, scientists wonder to what extent these cells model disease in older adults because the reprogramming technique resets the cells to a developmentally immature state. “If you take the gene expression profiles [of reprogrammed neurons] … they match those [found in neurons] in early fetal development,” Studer told Alzforum. "At that stage, many of the diseases we are interested in have no obvious phenotype.”
First author Justine Miller and colleagues looked for biological changes that occur during aging and then tried to reproduce those in patient-derived cells. They first defined a set of age-associated markers that clearly differentiated fibroblasts taken from young (age 11) and old (ages 71 or more) donors. These included abnormally shaped nuclei, loss of a nuclear lamina-associated protein and heterochromatin markers, more DNA damage, and mitochondrial stress. When the scientists created iPSCs, they noticed that the cells all had a “young” marker profile regardless of the donor’s age. With differentiation back to fibroblasts, the cells from old donors reacquired none of the morphological and biochemical characteristics of old age. The authors noted that “iPSC-derived cells do not maintain a memory of their age.”
To re-establish these aging characteristics, Studer looked to accidents of nature. He focused on Hutchinson-Gilford progeria syndrome (HGPS). In this rare genetic disorder, a mutation leads to the production of progerin, a shortened pathogenic version of the lamin A protein found in the nuclear envelope. Children with the mutation age dramatically faster than normal and most die in their teens. Induced pluripotent stem cells from progeria patients also rejuvenate with reprogramming, but iPSC-derived fibroblasts do reacquire the markers of advanced age upon differentiation, as reported in previous studies and confirmed by Studer's group. This suggests that progerin expression is sufficient to induce an aged phenotype in iPSC-derived cells.
To test this, Miller and colleagues overexpressed progerin for three to five days in fibroblasts and midbrain dopaminergic (mDA) neurons derived from iPSCs made from young and old individuals. This treatment recapitulated those features of aging seen in the original fibroblasts from old individuals and progeria patients, regardless of the age of the iPSC donors. The iPSC neurons developed a subset of the aging markers, including damaged DNA and dysfunctional mitochondria. In addition, neurites had begun to degenerate.
Next, the researchers tried the same treatment in an iPSC-based model of a late-onset neurodegenerative disease. They overexpressed progerin in iPSC-mDA neurons from patients with familial PD mutations in either PINK1 or Parkin. With the progerin, several characteristics of PD-related pathology emerged that were not there in PD-only iPSC-mDA neurons, including degenerating dendrites. They also saw increased apoptotic cell death and decreased phosphorylation of the kinase AKT and its downstream signaling targets, which are hypothesized to reduce neuronal survival in PD. "We’re quite convinced that we can copy many aspects of an aged cell. Obviously we cannot claim it is perfect phenocopying … but it’s pretty good for such a short manipulation,” said Studer.
The findings suggest that progerin could help scientists model age-related PD phenotypes over the short term in neuron cultures. What about longer exposure to progerin in a more physiological setting? The researchers injected control or PD iPSC-mDA neurons, with or without progerin overexpression, into the striatum of a mouse model of PD (6-hydroxydopamine lesion). Three months later, most of the mice improved in a rotarod test of motor control. Animals that did not improve had received PD iPSC-neurons overexpressing progerin, and the grafted brain tissue of these animals showed that many of the mDA neurons had died. Six months after transplantation, those neurons that remained had neurite degeneration, enlarged mitochondria, and signs of Lewy body precursors.
Although this technique suggests a way to artificially age iPSC-based disease models, some researchers wondered if these cells’ youth could also be informative. Perhaps it could reveal vulnerabilities that contribute to pathology. “We don't know the significance of early changes, which may be mechanistically very important in the development of the disease,” said Larry Goldstein, University of California, San Diego, "The later pathology might be a secondary effect of early changes.” Thus, comparing progerin-treated and -untreated iPSC neurons may yield insight into the disease.
Progerin does not drive neurodegeneration in progeria or other diseases, hence more validation in additional cell types and disease models is needed to assess what exactly progerin can contribute to the study of age-related neurodegenerative diseases. “How well this will work in modeling aging-related diseases still needs to be vetted, but this is definitely something that should be tried,” said Abeliovich.—Linda Lee
Linda Lee is a freelance writer in Somerville, Massachusetts.
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