Keeping elderly cells old to understand the aging process

Aging is a key risk factor for many diseases, particularly disorders of the brain like Alzheimer’s or Parkinson’s, which primarily occur in the elderly. So a better understanding of the aging process should provide a better understanding of these neurodegenerative diseases.

The induced pluripotent stem cell (iPSC) technique makes it possible to grow human brain cells, or neurons, in the lab from elderly patient skin samples. Unfortunately, this method has a major pitfall when it comes to aging research: reprogramming skin cells back into the embryonic stem cell-like state of iPSCs strips away many of their old age-related characteristics.

Based on data published last week in Cell Stem Cell, Salk Institute researchers used a different technique called direct reprogramming as a means to keep old cells old. This alternative method sidesteps the need to make iPSCs (which brings cells all the way back to the pluripotent state) and instead converts a skin cell directly into the desired cell type.

First author Jerome Mertens and senior author Rusty Gage (Courtesy of the Salk Institute for Biological Studies).

First author Jerome Mertens and senior author Rusty Gage (Courtesy of the Salk Institute for Biological Studies).

iPSC and direct reprogramming go head-to-head

The study, funded in part by CIRM, relied on skin samples from people ranging in age from newly born to 89 years. The team generated iPSC and iPSC-derived neurons from these samples. They also made so-called induced neurons (iNs) from the skin cells using the direct reprogramming method. Other CIRM grantees have pioneered direct reprogramming of skin into nerve cells (see link below).

Skin cell samples from elderly human donors are directly converted into induced neurons (iNs), shown. (image: Courtesy of the Salk Institute for Biological Studies)

Skin cells from elderly human donors are directly converted into induced neurons (iNs), shown. (Image courtesy of the Salk Institute for Biological Studies).

When comparing skin cells from donors younger than 40 years old versus cells from the over 40 group, the team found several genes had age-dependent activity patterns. Those differences virtually disappeared in the iPSCs and iPSC-derived neurons from the same individuals. However, unlike iPSCs, direct reprogramming of the skin cells to neurons (iNs) hung on to age-dependent differences in gene activity.

Loss of RanBP17 protein a fountain of youth in reverse

A deeper analysis identified one gene called RanBP17 whose activity levels declined with increased age of the donor in both the original skin cells and those directly converted into iNs. But when those same donor skin cells were turned into iPSCs or even iPSC-derived neurons, RanBP17 levels in the older cells were no longer reduced and were on par with RanBP17 levels in the younger cells. In follow up experiments, a reduction in RanBP17 protein led to glitches in the transport of proteins into the cell’s nucleus, which other studies have linked to neurodegenerative diseases as well as the aging process.

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Gene expression patterns of age-related factors like RanBP17 are maintained in induced neurons but not iPSCs. (Mertens et al., 2015)

Altogether, these results encourage researchers to select iNs over iPSC-derived neurons when it comes to faithfully representing the aging process of brain cells. Based on a Salk Institute press release, you can tell that professor Martin Hezter, a contributing author, is excited about future studies with iNs:

By using this powerful approach, we can begin to answer many questions about the physiology and molecular machinery of human nerve cells–not just around healthy aging but pathological aging as well.

 


Related links:

CIRM-Funded Scripps Team Replicates Pain in a Lab Dish; Seeks New Treatments for Chronic Sufferers

Pain hurts but it also protects. Thanks to nerve cells called sensory neurons, which weave their nerve fibers throughout our skin and other tissues, we are alerted to dangerous events like touching a hot plate or even to the sense of having a full bladder.

However, trauma such as a spinal cord injury or diseases like HIV and diabetes can damage sensory neurons and cause chronic pain that debilitates rather than protects those affected. Sadly, conventional pain treatments are usually not effective for the stinging, burning, tingling and numbness associated with this type of pain. Clearly, new innovations are needed.

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These induced sensory neurons could be useful in the testing of potential new therapies for pain, itch and related conditions. Credit: Baldwin Lab, The Scripps Research Institute

Last week, a CIRM-funded research team from The Scripps Research Institute, reported in Nature Neuroscience that they developed a technique, which induces human skin cells to transform into sensory neurons in a petri dish. Up until now, the field mostly relied on mouse studies due to the difficulty of collecting and growing human sensory neurons in the lab. This may explain the lack of success in clinical trials for treating chronic pain. As co-lead author Joel Blanchard, a PhD candidate in Kristin Baldwin’s laboratory, stated in the institute’s press release:

“Mouse models don’t represent the full diversity of the human response. [With these human sensory neurons] we can start to understand how individuals respond uniquely to pain, cold, itch and so on.”

Kevin Eade, research associate, and Joel Blanchard, graduate student, co-lead authors of the report  Credit: Cindy Brauer, The Scripps Research Institute

Kevin Eade, research associate, and Joel Blanchard, graduate student, co-lead authors of the report. Credit: Cindy Brauer, The Scripps Research Institute

To generate the nerve cells, the Baldwin research team inserted, into human skin cells, a combination of genes known to produce proteins that are key controllers of sensory neuron function. The resulting cells had the appearance of sensory neurons and responded appropriately when exposed to heat in the form of the active ingredient in chili peppers as well as activating a cold response when exposed to menthol. Adding more confidence to these results, an independent research team from the Harvard Stem Cell Institute reported in the same Nature Neuroscience   issue and in a press release that they too had successfully generated human sensory neurons from skin cells.

This direct reprogramming of one cell type directly into another is a variant of the induced pluripotent stem cell (iPS) technique in which a cell, often skin, is first reprogrammed into an embryonic stem cell-like state and then coaxed to form into virtually any cell type of the body.

Now that the Baldwin lab has nailed down the recipe for making human sensory neurons, they now can seek out treatments to bring relief to chronic pain sufferers. Dr. Baldwin looks forward to this future work:

Kristin Baldwin, Associate Professor Department of Molecular and Cellular Neuroscience. Credit: The Scripps Research Institute

Kristin Baldwin
Associate Professor
Credit: The Scripps Research Institute

“This method is rapid, robust and scalable. Therefore we hope that these induced sensory neurons will allow our group and others to identify new compounds that block pain and itch and to better understand and treat neurodegenerative disease and spinal cord injury.”

Watch the short video below to hear from a pioneer of direct reprogramming of nerve cells, CIRM grantee Marius Wernig of Stanford University: