Muse Cells: Nontumorigenic Pluripotent Stem Cells Present in Adult Tissues-A Paradigm Shift in Tissue Regeneration and Evolution

Abstract

Muse cells are a novel population of nontumorigenic pluripotent stem cells, highly resistant to cellular stress. These cells are present in every connective tissue and intrinsically express pluripotent stem markers such as Nanog, Oct3/4, Sox2, and TRA1-60. Muse cells are able to differentiate into cells from all three embryonic germ layers both spontaneously and under media-specific induction. Unlike ESCs and iPSCs, Muse cells exhibit low telomerase activity and asymmetric division and do not undergo tumorigenesis or teratoma formation when transplanted into a host organism. Muse cells have a high capacity for homing into damaged tissue and spontaneous differentiation into cells of compatible tissue, leading to tissue repair and functional restoration. The ability of Muse cells to restore tissue function may demonstrate the role of Muse cells in a highly conserved cellular mechanism related to cell survival and regeneration, in response to cellular stress and acute injury. From an evolutionary standpoint, genes pertaining to the regenerative capacity of an organism have been lost in higher mammals from more primitive species. Therefore, Muse cells may offer insight into the molecular and evolutionary bases of autonomous tissue regeneration and elucidate the molecular and cellular mechanisms that prevent mammals from regenerating limbs and organs, as planarians, newts, zebrafish, and salamanders do.

Figure 1

Muse-AT cells can differentiate into mesodermal, endodermal, and ectodermal cell lineages. Muse-AT cells can grow in suspension, forming spheres or cell clusters as well as individual cells (see white arrows) both expressing characteristic pluripotent stem cell markers, such as Oct4 (Texas Red) and TRA-1-60 (Green Fluorescence GFP), while nuclei were stained with DAPI (blue) (a); myogenic differentiation medium; the formation of myocytes (mesodermal origin) was detected using an anti-human MSA antibody (b); hepatogenic differentiation medium; formation of hepatocytes (endodermal origin) was detected using an anti-cytokeratin 7 antibody (c); neural differentiation medium; neural-like cells (ectodermal origin) were detected by immunofluorescence using an anti-human MAP2 antibody (d). Nuclei were stained with DAPI (blue) (original magnification 600x). (b)–(d) From Figures 3(C), 4(C), and 5(D) [5].

Figure 2

Properties of Muse-AT cells. Nontumorigenicity of Muse-AT cells. Embryonic stem (ES) cells injected into immunodeficient mice (SCID mice) testes, formed teratomas within 8 to 12 weeks (a). Histological analysis showed that the teratoma contained muscle tissue, intestine-like structure, and keratinized skin (b). Muse-AT cells transplanted into testes did not form teratomas even 6 months after injection, similar to untreated testes (d). Testis injected with Muse-AT cells maintained normal structure (e). Yin-Yang balance between Let7 and Lin28: ES and IPS expressed much higher levels of Lin28 versus Let7 (c). Muse-AT cells and neoblasts expressed much higher levels of Let7 versus Lin28 (f). Differences in expression of pluripotent stem cell genes between Muse cells, ES, iPS, and non-Muse cells determined by qRT-PCR (g). MicroRNA Let-7 is an upstream regulator of CDA3, CDC16, DZIP1, SSR1, RFC3, RFC5, MCM6, NUF2, BRCA1, BUB1B, and CDK6 (h). Normal karyotype of human Muse-AT cells is indicated by 23 intact pair of chromosomes with a pair XX chromosome indicating female origin of the cells (i). (a)-(b) From Figure 3: [6]. (g) From Table 1 [7]. ∗ indicates pluripotent stem cell markers.

Figure 3

Planarians, zebrafish, newts, and salamanders have the capacity to self-regenerate damaged appendages and critical organs.