A Muse cell (Multi-lineage differentiating stress-enduring cell) is an endogenous non-cancerous pluripotent stem cell. They reside in the connective tissue of nearly every organ including the umbilical cord, bone marrow and peripheral blood, and are collectable from commercially obtainable mesenchymal cells such as human fibroblasts, bone marrow-mesenchymal stem cells and adipose-derived stem cells as 1~several percent of the total population. Muse cells are able to generate cells representative of all three germ layers from a single cell both spontaneously and under cytokine induction. Expression of pluripotency genes and triploblastic differentiation are self-renewable over generations. Muse cells do not undergo teratoma formation when transplanted into a host environment in vivo. This can be explained in part by their intrinsically low telomerase activity, eradicating the risk of tumorigenesis through unbridled cell proliferation. They were discovered in 2010 by Mari Dezawa and her research group. Clinical trials for acute myocardial infarction, stroke, epidermolysis bullosa, spinal cord injury, amyotrophic lateral sclerosis, acute respiratory distress syndrome (ARDS) related to novel coronavirus (SARS-CoV-2) infection, are conducted. A hysician-led clinical trial for neonatal hypoxic-ischemic encephalopathy was also started. The summary results of a randomized double-blind placebo-controlled clinical trial in patients with stroke was announced.
Characteristics Stress-tolerant. Do not show tumorigenicity. Resistant to genotoxic stresses due to efficient sensing of DNA damage and activation of DNA repair systems. Can be isolated as cells positive for SSEA-3, a well known human embryonic stem cell marker. Needless to inform that the positive SSEA-3 cells or Muse cells are fresh for maximum one day after sorting and if you sort the same cells after 5 days for instance, you only collect about the same percentage of Muse or positive signal you got before first sorting . Pluripotent stem cells, which can generate various kinds of the cells representative of all three germ layers have the ability to self-renew. Non-tumorigenic. Low telomerase activity. Nearly all organs produce sphingosine-1-phosphate (S1P) upon damage. Muse cells expressing S1P receptor 2 selectively migrate to the site of damage by intravenous or local injections. Replenish new functional cells through spontaneous differentiation into tissue-compatible cells after homing to damaged tissue. Repair tissue by systemic administration. Comprise ~0.03% of bone marrow transplantation and several % of mesenchymal stem cell transplantation. Have immunosuppressive and immunomodulatory effect. Pluripotent stem cells can be directly obtained from normal human tissues without using artificial manipulations such as gene introduction. Intravenous injection of donor-Muse cells can be directly used for treatment without HLA-matching test or immunosuppressant treatment due to the specific immune privilege.
Markers Muse cells are identified as cells positive for SSEA-3+, a well-known marker for undifferentiated human ES cells. Their size is 13~15 μm in diameter. Muse cells do not express CD34 (markers for hematopoietic stem cells, adipose stem cells, VSELs) and CD117 (hematopoietic stem cells markers), Snai1 and Slug (skin-derived precursors markers), CD271 and Sox10 (neural crest-derived stem cells markers), NG2 and CD146 (perivascular cells) or CD31 and von Willebrand factor (endothelial progenitor markers). This indicates that Muse cells do not belong to previously investigated stem cell types.
Differentiation capacity
In vitro Muse cells can differentiate into: Ectodermal- (cells positive for nestin, NeuroD, Musashi, neurofilament, MAP-2, melanocyte markers (tyrosinase, MITF, gf100, TRP-1, DCT)), Mesodermal- (brachyury, Nkx2-5, smooth muscle actin, osteocalcin, oil red-(+) lipid droplets, desmin) Endodermal- (GATA-6, α-fetoprotein, cytokeratin-7, albumin) lineages both spontaneously and under cytokine induction. After homing to damage tissue by S1P-S1PR2, Muse cells phagocytose damaged cells, similar to macrophages. They then recycle the differentiation signals (e.g., transcription factors) obtained from the damaged cells and rapidly differentiate into the same cell type as the damaged cell.
In vivo Muse cells are shown to home into the damage site by S1P-S1P receptor 2 axis and spontaneously differentiate into tissue-compatible cells according to the microenvironment to contribute to tissue regeneration when infused into the blood stream as shown in animal models with fulminant hepatitis, partial hepatectomy, muscle degeneration, epidermolysis bullosa, skin injury, stroke and spinal cord injury. The spontaneous differentiation into tissue-compatible cells was explained by the phagocytosis-dependent differentaition: after homing to damaged tissue, Muse cells phagocytose damaged cells, similar to macrophages, recycle the differentiation signals (e.g., transcription factors) obtained from the damaged cells and rapidly differentiate into the same cell type as the damaged cell.
Non-tumorigenicity
Low telomerase activity Muse cells are characterized by low telomerase activity, not a strong indicator of tumorigenicity. Hela cells and human fibroblast-derived iPS cells showed high telomerase activity while Muse was at nearly the same level as that in somatic cells such as fibroblasts (these data are shown without running control for the telomerase activity, the comparison is not scientific thought). This indicates the non-tumorigenic nature of Muse cells.
Expression of genes related to pluripotency and cell cycle The expression 'pattern' of genes related to pluripotency in Muse cells was almost the same as that in ES and iPS cells, while the expression 'level' was much higher in ES and iPS cells and that in Muse cells. In contrast, genes related to cell cycle progression and tumorigenicity in Muse cells were at the same level as those in somatic cells, while the same genes were very high in ES and iPS cells. These gene expression pattern and level may explain why Muse cells are pluripotent but without tumorigenic activity.
Transplantation into mouse testes Unlike ES and iPS cells, transplanted Muse cells in testes of immunodeficient mice -a commonly used experiment to test the tumorigenicity of stem cells- have not been reported to form teratomas, even after six months. Thus, Muse cells are pluripotent but are non-tumorigenic. Similarly, epiblast stem cells cultured under certain conditions also do not form teratomas in testes, even though they show pluripotency in vitro. Thus, pluripotent stem cells do not always show teratoma formation when transplanted in vivo.
Tissue repair Muse cells act as tissue repairing cells in vivo. When systemically administrated, naive Muse cells (without cytokine treatment or gene introduction) migrate to damaged site, home into the site and spontaneously differentiate into tissue-compatible cells to replenish new functional cells. This phenomenon was observed by the infusion of green fluorescent protein-labeled naive human Muse cells into animal models with fulminant hepatitis, partial hepatectomy, muscle degeneration, skin injury, stroke and spinal cord injury. Infused Muse cells integrated into each damaged tissue and differentiated into human albumin- and human anti-trypsin-expressing hepatocytes in the liver, human dystrophin-expressing cells in the muscle, neurofilament and MAP-2-expressing neuronal cells in the spinal cord and brain, desmoglein-3-, cytokerain14- and cytokeratin 15-expressing epidermal cells in the skin, glomerular cells in the kidney, corneal epithelial cells in the cornea and physiologically functional cardiac cells in the heart, respectively.
Muse cells have great advantages for regenerative medicine. Without need of cytokine induction or artificial gene manipulation, Muse cells are capable of repairing tissues when directly infused into the blood stream. Hence, the clinical applications of Muse cells appear promising.
Basic characteristics Pluripotency, namely pluripotent marker expression, triploblastic differentiation and self-renewability, are recognized in Muse cells directly collected from BM aspirates, indicating that their characteristics are not newly acquired by in vitro manipulation nor are they modified under culture conditions.
Location in vivo Muse cells are not generated by stress, cytokine induction or exogenous gene transfection. They are preexisting pluripotent stem cells that normally reside in the bone marrow, peripheral blood and connective tissue of every organ including the umbilical cord. ] In the bone marrow, they represent one out of 3000 mono-nucleated cells. Other than mesenchymal tissues, Muse cells locate in connective tissue of every organ and in the peripheral blood.
Dual features of pluripotent stem cells/macrophages Muse cells are pluripotent-like, express pluripotency genes at moderate levels, exhibit triploblastic-lineage differentiation, and self-renew at the single-cell level. After homing to damaged tissue, Muse cells phagocytose damaged cells, similar to macrophages. They then recycle the differentiation signals (e.g., transcription factors) obtained from the damaged cells and rapidly differentiate into the same cell type as the damaged cell. Because Muse cells are pluripotent-like, they can differentiate into multiple cell types that comprise various tissues.
Formation of clusters similar to embryoid body of ES cells in suspension In cell suspension, Muse cells begin to proliferate and to form clusters that are very similar to embryoid bodies formed from ES cells in suspension. Muse cell clusters are positive for pluripotency indicators such as alkaline phosphatase reactivities, Nanog, Oct3/4, Sox2 and PAR4. One of remarkable properties of Muse cells is that they are capable of forming clusters from a single cell in suspension. A single Muse cell-derived cluster is shown to spontaneously generate cells representative of all three germ layers on a gelatin-coated dish, proving the pluripotency of Muse cells.
Proliferation speed Muse cells proliferate at a speed of ~1.3 day/cell division in adherent culture. This is slightly slower than that of human fibroblasts (~1 day/cell division).
Self-renewal Muse cells are able to self-renew, maintaining their proliferative activity, pluripotency marker expression and a normal karyotype.
Sources Muse cells can be collected from bone marrow aspirate, whose collection is a well known procedure done daily in clinics. They can also be isolated from skin fibroblasts obtained via skin biopsy, adipose tissue obtained by liposuction and from the umbilical cord; a safe and non-invasive procedure often used for cosmetic surgery interventions Easy accessibility of Muse cells allows them to be auto- or allo-transplanted in regenerative clinical applications. Muse cells are also isolated from commercially available mesenchymal cell cultures, which ensure their availability and accessibility. General Sources. Muse cells can be obtained from: Bone marrow aspirate Adipose tissue and liposuction Dermis Umbilical cord Commercially available culture cells such as: Bone marrow-derived mesenchymal stem cells Fibroblasts Adipose-derived stem cells Bone marrow: Bone marrow mononucleated cells contain ~0.03% of SSEA-3 positive Muse cells. This ratio corresponds to one out of 3000 mono-nucleated cells. Dermis: Muse cells, detected as SSEA-3-positive cells, are located sparsely in the connective tissues of organs. In the human dermis, Muse cells are located in the connective tissues distributed in the dermis and hypodermis. Their location is not related to particular structures such as blood vessels or dermal papilla.