How MUSE cells may generate multiple tissue types without behaving like embryonic stem cells
Part 4 of the PUR-FORM MUSE Cell Series
In Part 2 of this series, we looked at how MUSE cells may find injured tissue. In Part 3, we explored how they may survive once they get there.
That leads to the next question:
What can a MUSE cell actually become?
This is where MUSE cell biology becomes especially interesting.
MUSE cells are naturally occurring adult cells that express several markers normally associated with pluripotency and have demonstrated the ability to generate cell types representing all three major embryologic germ layers.
At the same time, their growth behavior differs markedly from that of embryonic stem cells and induced pluripotent stem cells.
That combination is why I prefer describing MUSE cells as pluripotent-like. They appear to possess some of the developmental flexibility associated with pluripotency without behaving like classical pluripotent stem cells.
What Does “Pluripotent” Actually Mean?
The word sounds complicated, but the concept is straightforward.
Very early in development, cells organize into three basic layers. Those layers eventually give rise to almost every tissue in the body.
Ectoderm gives rise to tissues including the brain, nervous system, and skin.
Mesoderm contributes to muscle, bone, connective tissue, blood, and blood vessels.
Endoderm contributes to organs and tissues including the liver, pancreas, lungs, and gastrointestinal tract.
A pluripotent cell can develop into cell types from all three germ layers.
Most adult stem and stromal cells are much more restricted. MUSE cells appear to be an exception.
Research has shown that individual MUSE cells can self-renew and differentiate into cell types representing ectoderm, mesoderm, and endoderm.
That is the basis for describing them as having pluripotent-like potential.
One Cell, Three Developmental Pathways
Under experimental conditions, MUSE cells have differentiated into cell types associated with all three germ layers.
Ectodermal lineages include neuron- and skin-related cells.
Mesodermal lineages include muscle, bone, connective tissue, and vascular cells.
Endodermal lineages include liver- and other organ-associated cell types.
This does not mean that a MUSE cell placed into the body can automatically become anything we want it to become. Developmental biology does not work that simply.
But it does suggest that MUSE cells possess a broader degree of cellular flexibility than we normally associate with conventional adult mesenchymal cells.
SSEA-3: An Important Marker
One of the markers most closely associated with MUSE cells is SSEA-3, short for stage-specific embryonic antigen-3.
Although the name originated in developmental biology, a small population of SSEA-3-positive cells also naturally occurs in adult tissues such as bone marrow, adipose tissue, and connective tissue.
Researchers commonly use SSEA-3 to help identify and enrich MUSE cells.
But a marker should never be confused with the entire identity of a cell.
Finding SSEA-3 tells us something about a cell. It does not tell us everything.
A true biological profile also includes characteristics such as stress resistance, self-renewal, expression of pluripotency-related genes, and multilineage differentiation.
That distinction is important when evaluating any product described as containing MUSE cells.
The Pluripotency Genes
MUSE cells also express several genes associated with developmental potential.
Three of the best known are Oct3/4, Sox2, and Nanog.
These genes are part of the regulatory network that helps maintain pluripotency.
Embryonic stem cells and induced pluripotent stem cells express these genes very strongly. MUSE cells express them as well, but generally at much lower levels.
That may be a clue to understanding their unusual biology.
MUSE cells seem to occupy a middle ground. They possess enough pluripotency-associated biology to demonstrate broad differentiation potential, but they do not display the highly proliferative behavior associated with classical pluripotent cells.
They Are Not Genetically Reprogrammed
An induced pluripotent stem cell, or iPSC, begins as a mature adult cell.
Researchers then genetically or molecularly reprogram that cell, pushing it back into a highly pluripotent state.
MUSE cells do not require that process.
They exist naturally within adult tissues and already possess their characteristic biology.
In other words, a MUSE cell is not simply an ordinary adult cell artificially converted into a pluripotent cell. It appears to represent a naturally occurring adult cell population with an unusual degree of developmental flexibility built in.
Can MUSE Cells Become Tissue-Specific Cells?
Laboratory studies suggest that they can.
MUSE cells have been differentiated toward neuronal, muscle, liver, bone, fat, and other cellular phenotypes.
Even more interesting is what has been observed after MUSE cells reach damaged tissue in animal models.
In experimental liver injury, intravenously administered MUSE cells have preferentially accumulated in damaged liver tissue and have subsequently expressed markers associated with liver cells.
Similar tissue-compatible differentiation has been reported in other preclinical models.
This suggests a potentially important sequence:
Find the injury. Survive the injury. Respond to the local environment. Potentially adopt characteristics appropriate to that tissue.
The Tissue May Help Tell the Cell What to Do
Cells do not make decisions in isolation.
They constantly receive information from their surroundings.
Growth factors, inflammatory molecules, and neighboring cells provide signals. The extracellular matrix provides both chemical and mechanical information.
A MUSE cell entering a damaged liver encounters a very different environment from one entering an injured brain, heart, or muscle.
Those local signals may influence how the cell behaves.
This fits into a larger concept we emphasize frequently at PUR-FORM:
The cell and its environment cannot really be separated.
Regeneration is an ecosystem.
The extracellular matrix, immune system, blood vessels, signaling molecules, and neighboring cells all influence what happens next.
Then Why Don’t MUSE Cells Form Teratomas?
This is where the story becomes particularly interesting.
Classical pluripotent cells have enormous developmental potential, but that potential poses a serious problem.
Embryonic stem cells and induced pluripotent stem cells can form teratomas, tumors containing disorganized mixtures of tissue types, when undifferentiated cells are transplanted under certain experimental conditions.
For years, pluripotency and teratoma formation were closely linked concepts.
MUSE cells appear to challenge that relationship.
Despite expressing pluripotency-associated markers and differentiating across all three germ layers, MUSE cells have not shown the same teratoma-forming behavior in commonly cited preclinical studies.
That apparent paradox is one reason MUSE cells have attracted so much scientific interest.
Telomerase May Be Part of the Explanation
One possible clue involves an enzyme called telomerase.
Every time most cells divide, the protective ends of their chromosomes, called telomeres, gradually shorten.
Telomerase helps maintain those telomeres and lets certain cells keep dividing for much longer.
Embryonic stem cells, iPS cells, and many cancer cells have very high telomerase activity.
MUSE cells appear different.
Studies have reported relatively low telomerase activity, much closer to that of ordinary adult cells than to that of highly proliferative pluripotent stem cells.
Their rate of cell division is also much more restrained.
Developmental flexibility and uncontrolled proliferation are not necessarily the same.
Chromosomal Stability Matters Too
Researchers have also reported normal karyotypes in studied MUSE-cell populations.
A karyotype looks at the number and general structure of a cell’s chromosomes.
That matters because genetic instability can become a concern when cells are extensively manipulated, reprogrammed, or expanded.
The combination of relatively restrained proliferation, low telomerase activity, and reported chromosomal stability provides additional biological context for why MUSE cells have not shown the same teratoma behavior as classical pluripotent cells in preclinical research.
That does not mean any cell therapy should be considered risk-free.
Long-term safety ultimately has to be established with long-term human data.
Why I Use the Term “Pluripotent-Like”
Some scientific publications refer to MUSE cells as pluripotent stem cells, while others use the term pluripotent-like.
For patients, I prefer pluripotent-like because it captures what is remarkable about these cells without implying that they are simply another version of an embryonic stem cell or iPSC.
MUSE cells appear to combine several features associated with pluripotency:
- expression of pluripotency-related markers
- self-renewal
- differentiation into lineages representing all three germ layers
Yet they also differ in important ways:
- they occur naturally in adult tissue
- they do not require genetic reprogramming
- they express pluripotency-associated genes at lower levels than classical pluripotent cells
- they exhibit relatively low telomerase activity
- they have not shown the same teratoma-forming behavior in commonly cited preclinical studies
Potential Is Not the Same as Proof
There is an important caution.
Showing that a cell can generate multiple cell types in a laboratory, or even demonstrate tissue-compatible differentiation in an animal model, is not the same as proving it can regenerate an entire human organ.
Biological potential differs greatly from proven clinical regeneration.
The science gives us good reasons to continue investigating MUSE cells.
It does not justify the claim that these cells can rebuild any tissue or cure every condition.
The human clinical evidence is still developing.
At PUR-FORM, I think that distinction is essential.
We can be excited about unusual biology while remaining clear about what has and has not yet been demonstrated.
The MUSE Story Is Starting to Connect
We can now begin to put the first four articles in this series together.
A regenerative cell needs more than one useful characteristic.
It needs a sequence of capabilities.
It must recognize injury. It must reach the injury. It must survive the environment it finds there. And once there, it must have the biological flexibility to contribute in some meaningful way.
That is why cell number alone cannot tell us everything about a regenerative product.
Cell identity matters.
Cell behavior matters.
And the biological capabilities of those cells after they enter the body may matter even more.
The unusual feature of MUSE cells is not simply that they are MSCs with one extra marker.
They appear to combine properties we do not usually expect to find together in an adult cell population:
stress endurance, injury-directed homing, broad differentiation potential, and comparatively restrained growth behavior.
That combination is what makes MUSE biology worth studying.
Dr. P
