A collaboration between American and Japanese scientists has discovered and characterized a new stem cell population from human fat that do not cause tumors and can differentiate into derivatives from ectoderm, mesoderm, and endoderm.
Multilineage Differentiating Stress-Enduring or Muse cells are found in bone marrow and the lower layers of the skin (dermis). Muse cells are a subpopulation of mesenchymal stem cells, and even express a few mesenchymal stem cell-specific genes (e.g., CD105, a cell-surface protein specific to mesenchymal stem cells). However, Muse cells also express cell surface proteins normally found in embryonic stem cells (e.g., stage-specific embryonic antigen-3, SSEA-3). Additionally, Muse cells have the ability to self-renew, and differentiate into cell types from all three embryonic germ layers, ectoderm (which forms skin and brain), mesoderm, (which forms muscle, bone, kidneys, gonads, heart, blood vessels, adrenal glands, and connective tissue), and endoderm (which forms the gastrointestinal tract and its associated tissues). Finally, Muse cells can home to damaged sites and spontaneously differentiate into tissue-specific cells as dictated by the microenvironment in which the cells find themselves.
A new publication by Fumitaka Ogura and others from Tohoku University Graduate School of Medicine in Sendai, Japan and Saleh Heneidi from the Medical College of Georgia (Augusta, Georgia), and Gregorio Chazenbalk from the David Geffen School of Medicine at UCLA has shown that Muse cells also exist in human fat.
The source of cells came from two places: commercially available fat tissue and freshly collected fat from human subjects, collected by means of liposuction. After growing these cells in culture, the mesenchymal stem cells and Muse cells grew steadily over the 3 weeks. Then the Dezawa research group used fluorescence-activated cell sorting (FACS) to isolate from all these cells those cells that express SSEA-3 on their cell surfaces.
FACS uses antibodies conjugated to dyes that can bind to specific cell proteins. Once the antibodies bind to cells, the cells are sluiced through a small orifice while they are illuminated by the laser. The laser activates the dyes if the cell fluoresces, one door opens and the other closes. The cell goes to one test tube. If the cell does not fluoresce, then the door stay shut and another door opens and the cell goes into a different test tube. In this way, cells with a particular cell-surface protein are isolated from other cells that do not have that cell-surface protein.
In addition to expression SSEA-3, the fat-based Muse cells expressed other mesenchymal stem cell-specific cell-surface proteins (CD29, CD90), but they did not express proteins usually thought to be diagnostic for fat-based mesenchymal stem cells (MSCs) such as CD34 and CD146. Muse cells also expressed pluripotency genes (Nanog, Oct3/4, PAR4, Sox2, and Tra-1-81). The Muse cells grew in small clusters and some cell expressed ectodermal-specific genes (neurofilament, MAP2), others expressed mesodermal-specific genes (smooth muscle actin, NKX2) and endodermal-specific genes (alpha-fetoprotein, GATA6). These data suggested that the cultured Muse cells were poised to form either ectoderm, mesodermal, or endodermal derivatives.
When transplanted into mice with non-functional immune systems, the Muse cells never formed any tumors or disrupted the normal structure of the nearly tissues. When placed in differentiating media, fat-derived Muse cells differentiated into cells with neuron-like morphology that expressed neuron-specific genes (Tuj-1), liver cells, and fat. When compared with Muse cells from bone marrow or skin, the fat-derived Muse cells were better at making bone, fat, and muscle, but not as good as bone marrow Muse cells at making neuronal cell types, but not as good at making glial cells. Many of these assays were based on gene expression experiments and not more rigorous tests. Therefore, the results of these experiments might be doubtful until they are corroborated by more rigorous experiments.
These cells are expandable and apparently rather safe to use. More work needs to be done in order to fully understand the full regenerative capacity of these cells and protocols for handling them must also be developed. However, hopefully pre-clinical experiments in rodents will give way to larger animal experiments. If these are successful, then maybe human trials come next. Here’s to hoping.