Part 2
The biological signaling system that may help MUSE cells find injured tissue
One of the most important questions in regenerative medicine is also one of the most obvious:
If we place regenerative cells into the bloodstream, how do they know where to go?
Simply administering cells intravenously does not ensure they will reach an injured heart, brain, joint, muscle, or other damaged tissue. To participate in repair, a cell must survive the circulation, recognize the injury, and somehow move toward it.
This is one of the features that makes MUSE cells particularly interesting.
MUSE cells appear to use a signaling pathway involving a molecule called sphingosine-1-phosphate, or S1P, and a receptor on the MUSE cell called S1PR2. The names sound complicated, but the idea is straightforward: injured tissue produces a chemical distress signal, and the MUSE cell has a receptor that can detect it.
I often think of it as a kind of biological GPS.
The Injured Tissue Sends Out a Distress Signal
When tissue is injured, its environment changes almost immediately. Oxygen may fall, inflammation may increase, cell membranes may become damaged, blood vessels may respond, and the extracellular matrix surrounding the cells may change. At the same time, the injured area begins releasing chemical signals.
One of those signaling molecules is S1P.
S1P is a naturally occurring bioactive lipid with many normal roles in the body, including blood-vessel function and immune cell movement. It is not unique to MUSE cells or to injury. But after tissue damage, changes in S1P signaling can help create what is essentially a chemical trail leading toward the injured area.
MUSE cells appear particularly capable of recognizing that trail because they express a receptor called S1PR2 on their surface.
The MUSE Cell Has the Receiver
Think of S1P as the signal and S1PR2 as the antenna that detects it. As a MUSE cell travels through the circulation, it can respond to an S1P gradient and move toward the area where the signal is stronger.
Scientists call this chemotaxis. A simpler way to say it is that the cell follows the chemical signal toward the injury.
This is very different from imagining an intravenously administered cell floating randomly through the bloodstream and simply hoping it ends up in the right place.
Tissue injury → S1P signal → S1PR2 detection → MUSE-cell migration → injured tissue
Actual biology is more complicated than a five-step diagram. Other receptors, inflammatory signals, adhesion molecules, blood vessels, and tissue conditions almost certainly play roles, too. But the S1P-S1PR2 pathway gives researchers something very important: a plausible mechanism explaining how MUSE cells may locate areas of injury.
How Do We Know the Pathway Matters?
One important experiment involved rabbits after an acute heart attack. Researchers gave MUSE cells intravenously and tracked where the cells traveled. The cells preferentially accumulated in the damaged heart; in that particular model, about 14.5% of the labeled MUSE cells were estimated to have engrafted in the heart three days after administration.
That number should not be assumed to apply to people or to other organs. The more important part of the experiment was what happened when researchers interfered with the S1P-S1PR2 pathway.
When S1PR2 signaling was blocked – either with a specific antagonist or by reducing S1PR2 expression in the MUSE cells – migration and homing to the injured heart fell substantially.
This matters because it does more than show that MUSE cells can be found in damaged tissue. It provides evidence that the S1P-S1PR2 pathway helps them get there.
The Body May Already Use This System Naturally
Perhaps the most fascinating part of the story is that MUSE-cell homing may not be something cell therapy created. MUSE cells already exist naturally within our bodies.
Human studies suggest that after a major injury, the body may increase the number of MUSE cells circulating in the bloodstream.
In one study of 79 patients after an acute heart attack, circulating MUSE-cell numbers rose rapidly and peaked at about one day. Researchers also measured plasma S1P levels, and circulating MUSE-cell numbers correlated with S1P. Patients who developed larger increases in circulating MUSE cells also tended to show more favorable changes in heart function and remodeling later.
That does not prove that MUSE cells caused the improvement. Observational studies can show relationships, but they cannot prove cause and effect. Still, the finding is biologically intriguing.
A separate study of 29 patients with ischemic stroke also found that circulating SSEA-3-positive MUSE cells increased during the acute phase after the stroke.
Together, these observations suggest that MUSE cells may participate in an endogenous injury-response system already built into human biology.
The concept can be simplified this way: an injury occurs, distress signaling increases, MUSE cells are mobilized into the circulation, and some of those cells may then be directed toward damaged tissue.
That is a very different way of thinking about regenerative cells. Rather than viewing MUSE cells only as something that can be prepared and administered therapeutically, we can also think of them as part of a repair system that the body may already attempt to deploy when significant injury occurs.
Why This Matters for IV Cell Therapy
Intravenous cell therapy has an obvious advantage: cells placed into the bloodstream have access to the entire circulation. But systemic delivery also creates a major challenge. The cells still have to find the target.
Many intravenously administered cell populations become distributed through organs such as the lungs, liver, and spleen. Simply placing cells into the bloodstream therefore does not guarantee meaningful delivery to damaged tissue.
A cell that can respond to an injury-associated chemical gradient represents a different biological proposition. MUSE cells appear to have an injury-recognition system that may help them respond to biochemical signals coming from damaged areas.
That does not mean every MUSE cell reaches the injury site. Homing is unlikely to be perfect and probably varies by the type, severity, and timing of tissue damage; vascular access; the condition of the cells; and other biological variables we do not yet fully understand.
But there is an important difference between a cell simply circulating through the body and a cell that appears capable of responding to a signal generated by tissue damage.
This May Not Be Limited to the Heart
The S1P-S1PR2 pathway has also been studied in other forms of injury. In a 2024 mouse study of radiation-induced intestinal damage, radiation increased S1P signaling within the injured intestine. After researchers gave human bone-marrow-derived MUSE cells intravenously, the cells preferentially homed to the damaged intestinal tissue.
When researchers blocked S1PR2, both MUSE-cell homing and much of the observed therapeutic effect were diminished.
This does not mean MUSE cells have been proven to treat radiation injury in people. It does, however, strengthen the idea that S1P-S1PR2 signaling may represent a broader injury-recognition mechanism rather than one confined to a single organ.
Could We Someday Mobilize Our Own MUSE Cells?
This raises an even more interesting question. Instead of only administering MUSE cells from outside the body, could we eventually learn how to encourage the body to mobilize more of its own MUSE cells?
Recent animal research has begun exploring this possibility. In a rabbit heart-attack model, researchers used an S1PR2 agonist – a compound designed to stimulate the receptor involved in MUSE-cell mobilization and homing. Stimulation of the pathway increased circulating endogenous MUSE cells and increased their recruitment to damaged heart tissue. It was also associated with smaller infarcts and improved measures of cardiac function.
This remains preclinical research and should not be interpreted as a treatment available to patients today. But the concept is fascinating.
Future regenerative medicine may involve not only giving reparative cells, but also learning how to activate, mobilize, and direct the repair systems already present within us.
Finding the Injury Is Only the Beginning
Reaching injured tissue does not automatically mean that tissue will heal. Homing is only the first step.
Once a MUSE cell arrives, it still faces a difficult environment. The tissue may have poor oxygen supply, intense inflammation, high oxidative stress, and a damaged or fibrotic extracellular matrix.
So the larger biological sequence may look something like this:
Injury → distress signal → recognition → homing → survival → signaling → potential repair
The S1P-S1PR2 pathway helps explain the first part of that story: how a MUSE cell may recognize that an injury exists and find its way there. The next question is equally important: what happens when the MUSE cell actually arrives?
Why This Biology Matters
In regenerative medicine, we spend a great deal of time discussing what is inside the syringe: How many cells are present? Are they viable? Where did they come from? How were they processed? Those are all important questions.
But there is another question that may ultimately be just as important: what can the cell actually do once it enters the body?
Can it recognize an injury? Can it reach it? Can it survive when it arrives? Can it communicate with the surrounding tissue? Can it participate in the repair process?
These questions tell us much more about the biological quality of a regenerative cell than cell count alone.
That is why the S1P-S1PR2 pathway is such an important part of the MUSE-cell story. It offers a potential explanation for something regenerative medicine has struggled with for years: how might an intravenously administered cell find the tissue that needs it?
We still do not know much. Most of the detailed mechanistic evidence comes from laboratory and animal studies, and larger human studies are needed to determine how consistently therapeutic MUSE cells home to different tissues, how long they persist, and whether greater homing translates into better clinical outcomes.
But a defined injury-recognition pathway gives MUSE-cell biology something extremely valuable: a biologically sensible mechanism.
Dr. P
