The life of a sleeping muscle stem cell is very busy

For biological processes, knowing when to slow down is as important as knowing when to step on the accelerator. Take for example muscle stem cells. In a healthy state, these cells mostly lay quiet and rarely divide but upon injury, they bolt into action by dividing and specializing into new muscle cells to help repair damaged muscle tissue. Once that mission is accomplished, the small pool of muscle stem cells is replenished through self-renewal before going back into a dormant, or quiescent, state.

muscle stem cell

Muscle stem cell (pink with green outline) sits along a muscle fiber. Image: Michael Rudnicki/OIRM

“Dormant” may not be the best way to describe it because a lot of activity is going on within the cells to maintain its sleepy state. And a better understanding of the processes at play in a dormant state could reveal insights about treating aging or diseased muscles which often suffer from a depletion of muscle stem cells. One way to analyze cellular activity is by examining RNA transcripts which are created when a gene is turned “on”. These transcripts are the messenger molecules that provide a gene’s instructions for making a particular protein.

By observing something, you change it
In order to carry out the RNA transcript analyses in animal studies, researchers must isolate and purify the stem cells from muscle tissue. The worry here is that all of the necessary poking of prodding of the cells during the isolation method will alter the RNA transcripts leading to a misinterpretation of what is actually happening in the native muscle tissue. To overcome this challenge, Dr. Thomas Rando and his team at Stanford University applied a recently developed technique that allowed them to tag and track the RNA transcripts within living mice.

The CIRM-funded study reported today in Cell Reports found that there are indeed significant differences in results when comparing the standard in vitro lab method to the newer in vivo method. As science writer Krista Conger summarized in a Stanford Medical School press release, those differences led to some unexpected results that hadn’t been observed previously:

“The researchers were particularly surprised to learn that many of the RNAs made by the muscle stem cells in vivo are either degraded before they are made into proteins, or they are made into proteins that are then rapidly destroyed — a seemingly shocking waste of energy for cells that spend most of their lives just cooling their heels along the muscle fiber.”

It takes a lot of energy to stay ready
Dr. Rando thinks that these curious observations do not point to an inefficient use of a cell’s resources but instead, “it’s possible that this is one way the cells stay ready to undergo a rapid transformation, either by blocking degradation of RNA or proteins or by swiftly initiating translation of already existing RNA transcripts.”

The new method provides Rando’s team a whole new perceptive on understanding what’s happening behind the scenes during a muscle stem cell’s “dormant” state. And Rando thinks the technique has applications well beyond this study:

Rando

Thomas Rando

“It’s so important to know what we are and are not modeling about the state of these cells in vivo. This study will have a big impact on how researchers in the field think about understanding the characteristics of stem cells as they exist in their native state in the tissue.”

 

 

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