Part 3
Why MUSE cells may survive where other regenerative cells struggle
In Part 2, we looked at how MUSE cells may use injury signals such as S1P to recognize damaged tissue and migrate toward it.
But finding the injury is only half the journey.
Once a regenerative cell reaches damaged tissue, it enters an environment that can be extremely difficult to survive. Oxygen may be low. Inflammation may be intense. Reactive oxygen species can accumulate. Nutrients may be limited. Proteins, mitochondria, cell membranes, and DNA may all be under stress.
In other words, the very place where we want a regenerative cell to work may also be one of the hardest places for that cell to stay alive.
That is where the name MUSE becomes particularly meaningful. MUSE stands for Multilineage-differentiating Stress-Enduring cell. Stress endurance is not simply an interesting laboratory characteristic. It may be one of the most important clues to understanding what makes these cells biologically different.
An Injury Is a Hostile Environment
Think of sending a repair crew into a building immediately after a major fire. Finding the building matters, but the workers still have to function amid heat, smoke, debris, and structural damage.
Something similar happens at the cellular level. After a heart attack, stroke, significant inflammation, trauma, or other serious tissue injury, blood flow may be disrupted and oxygen levels can fall. Inflammatory signals increase. Oxidative stress rises. Damaged cells release debris, and the extracellular matrix, the structural framework surrounding our cells, can also become disrupted.
Many transplanted cells find these conditions difficult to tolerate. Poor survival after cell administration has long been recognized as a major challenge for regenerative cell therapies.
In laboratory and preclinical studies, MUSE cells appear unusually resistant to several of these stresses. They have been described as a stress-tolerant population within mesenchymal tissues that can survive conditions that eliminate many surrounding cells.
A regenerative cell cannot participate in repair if it dies shortly after it arrives.
Stress Helped Reveal MUSE Cells
The way researchers learned about MUSE cells is itself fascinating.
When researchers mixed mesenchymal cell populations and exposed them to unusually severe laboratory stress, many cells died. A small population survived.
Those surviving cells were enriched for the population that became known as MUSE cells. Researchers have used conditions such as prolonged enzymatic exposure, nutrient deprivation, low temperature, and hypoxia to select for these unusually stress-resistant cells.
The important point is that severe stress is useful for selecting and enriching cells that already possess unusual stress tolerance. It is not simply turning an ordinary MSC into a MUSE cell.
That is why I think of MUSE cells as a naturally occurring survivor population within adult mesenchymal tissues.
What Does “Stress-Enduring” Really Mean?
Stress-enduring does not mean indestructible.
MUSE cells can still be injured or killed. The distinction is that, under experimental conditions, they appear better able than many non-MUSE mesenchymal cells to withstand certain forms of damaging stress and recover.
One particularly interesting example involves DNA damage.
DNA is constantly being challenged by normal metabolism, inflammation, oxidative stress, radiation, and environmental exposures.
When damage occurs, a cell can repair it, temporarily stop dividing, enter a dysfunctional state called senescence, or, if the injury is too severe, activate apoptosis, the cell’s programmed self-destruct pathway.
Researchers directly comparing MUSE cells with non-MUSE mesenchymal stromal cells after chemical and physical DNA damage found that MUSE cells showed less apoptosis and less senescence and activated their DNA-damage response more effectively.
That begins to explain how these cells may remain functional under conditions that are more damaging to other cells.
A Better Cellular Repair System
One DNA-repair system studied in MUSE cells is called non-homologous end joining, or NHEJ.
The name is complicated, but the idea is straightforward: if both strands of DNA break, the cell needs a way to reconnect them.
In experimental studies, MUSE cells increased NHEJ repair activity after DNA damage and appeared to recognize and respond to DNA injury efficiently.
MUSE cells do not appear merely to tolerate cellular damage. They also have active systems to detect and repair some of that damage.
Handling Oxidative Stress
DNA is only one part of the cell that can be damaged.
Another major challenge inside injured tissue is oxidative stress.
Our cells naturally produce reactive oxygen species, or ROS. At normal levels, ROS can serve useful signaling functions. But when inflammation, ischemia, or other severe stress drives ROS too high, they can damage cell membranes, proteins, mitochondria, and DNA.
Proteomic studies, which examine thousands of proteins within cells, have found that MUSE cells are enriched in pathways involved in reactive-oxygen handling, cellular stress responses, protein trafficking, and the ubiquitin-proteasome system.
Cellular Housekeeping Matters
Cells manufacture thousands of different proteins, and some eventually become damaged, misfolded, or unnecessary.
The cell therefore needs a quality-control system to identify and remove those proteins.
One major system is the ubiquitin-proteasome pathway. Ubiquitin acts like a molecular tag, marking proteins for the proteasome, where they can be broken down and recycled.
Why does this matter in injured tissue?
Because oxidative stress can damage many parts of a cell at once, including proteins, membranes, mitochondria, and DNA. A cell that can identify damage, repair what it can, and remove what it cannot has a better chance of remaining functional.
Proteomic studies suggest that MUSE cells have active pathways involved in this type of cellular quality control.
Survival Comes Before Regeneration
This leads to one of the most important concepts in MUSE-cell biology.
We often ask what a regenerative cell can become or what signals it can release. But those abilities matter only if the cell remains biologically functional long enough to use them.
The sequence matters.
First, the cell has to find the injury.
Then it has to survive the injury.
Only then can signaling, differentiation, and tissue support become relevant.
The process can be viewed as a biological chain:
Homing → hostile microenvironment → stress endurance → survival → potential repair support
Break the chain at any point, and the regenerative opportunity may be reduced.
Stress Resistance Is Not the Same as Uncontrolled Growth
There is an obvious question: if a cell can resist stress, repair DNA, and avoid apoptosis, could that also make it more likely to grow uncontrollably?
This is one of the unusual aspects of MUSE biology.
MUSE cells display some pluripotent-like characteristics, yet their growth behavior differs markedly from classical embryonic stem cells and induced pluripotent stem cells.
Published studies have reported low telomerase activity, normal karyotypes, and no teratoma formation in commonly cited preclinical models.
Stress resistance, therefore, should not automatically be confused with unlimited proliferation.
That distinction, how MUSE cells can display broad differentiation potential while behaving differently from classical pluripotent stem cells, is important enough that we will devote Part 4 of this series to it.
What Does This Mean Clinically?
Biologically, stress endurance makes sense.
If we want a regenerative cell to function within ischemic, inflamed, oxidatively stressed tissue, the ability to tolerate those conditions could be valuable.
But this is also where we need to separate mechanistic evidence from clinical proof.
Much of what we know about DNA repair, oxidative-stress handling, and cellular quality-control mechanisms in MUSE cells comes from laboratory and preclinical research.
We cannot yet say that a particular DNA-repair pathway or antioxidant mechanism is the reason a patient will improve after MUSE-cell administration.
These mechanistic findings are important, but they should not be read as proof of clinical benefit.
What we can say is that stress resistance is a repeatedly observed characteristic of MUSE cells, and research has begun to identify biological systems that may help explain it.
A Different Way to Think About Cell Quality
Regenerative medicine has traditionally spent a great deal of time discussing cell counts.
- How many cells are in the vial?
- How many are alive?
- Those numbers matter, but they do not tell us everything.
- Increasingly, I think we also need to ask:
- Can the cell recognize damaged tissue?
- Can it reach that tissue?
- Can it tolerate low oxygen and oxidative stress?
- Can it repair damage to its own cellular machinery?
- Can it remove damaged proteins and remain functional?
- And can it do all of this without uncontrolled growth?
Those questions shift the discussion from simply how many cells we have to what those cells are biologically capable of doing.
MUSE cells are intriguing because several of these characteristics appear to exist together in the same naturally occurring adult cell population.
Part 2 explained how a MUSE cell may find an injury. Part 3 explains why finding it is not enough.
A regenerative cell has to survive the environment it enters long enough to do something useful.
That ability to survive may be one of the most important and underappreciated features of MUSE-cell biology.
Dr. P
