A mystery unraveled: nontumorigenic pluripotent stem cells in human adult tissues

Abstract

Introduction: Embryonic stem cells and induced pluripotent stem cells have emerged as the gold standard of pluripotent stem cells and the class of stem cell with the highest potential for contribution to regenerative and therapeutic application; however, their translational use is often impeded by teratoma formation, commonly associated with pluripotency. We discuss a population of nontumorigenic pluripotent stem cells, termed Multilineage Differentiating Stress Enduring (Muse) cells, which offer an innovative and exciting avenue of exploration for the potential treatment of various human diseases.

Areas covered: This review discusses the origin of Muse cells, describes in detail their various unique characteristics, and considers future avenues of their application and investigation with respect to what is currently known of adult pluripotent stem cells in scientific literature. We begin by defining cell potency, then discuss both mesenchymal and various reported populations of pluripotent stem cells, and finally delve into Muse cells and the characteristics that set them apart from their contemporaries.

Expert opinion: Muse cells derived from adipose tissue (Muse-AT) are efficiently, routinely and painlessly isolated from human lipoaspirate material, exhibit tripoblastic differentiation both spontaneously and under media-specific induction, and do not form teratomas. We describe qualities specific to Muse-AT cells and their potential impact on the field of regenerative medicine and cell therapy.

Keywords: Muse cells; human pluripotent stem cells; nontumorigenic; regenerative medicine.

Figure 1. Multilineage Differentiating Stress Enduring (Muse) cell characterization by pluripotency markers and morphology

Immunostaining indicates that Muse cells express the pluripotency markers: (A) stage-specific embryonic antigen-3, (B) OCT3/4, (D) SOX2 and (E) Nanog. (C) Muse cells derived from adipose tissue (Muse-AT) cells grow in suspension, forming cell clusters as well as individual cells (red arrows). (F) Muse-AT cells were grown as adherent cells in the presence of myocyte differentiation medium. Formation of myocytes was detected using an antihuman MSA antibody. Nuclei were stained with DAPI (blue). (G) Muse-AT cells were grown as adherent cells in the presence of hepatocyte differentiation medium. Formation of hepatocytes was detected using an antihuman a-fetoprotein antibody. Nuclei were stained with DAPI (blue). (H) Isolated Muse-AT cells were grown for 7 days as nonadherent cells and then cultured for an additional 7 days as adherent cells. Neural-like cells were detected by immunofluorescence using an antihuman MAP2 antibody. Nuclei were stained with DAPI (blue). (Original magnification was 600× for figures F – H). Reproduced from [18].

Figure 2. Nontumorigenicity of Multilineage Differentiating Stress Enduring (Muse) cells

(A) Embryonic stem cells infused into immunodeficient mice (SCID mice) testes, formed teratomas within 8 – 12 weeks. Histological analysis showed that the teratoma contained (B) muscle tissue, (C) intestine-like structure and (D) keratinized skin. (E) Muse cell-transplanted testes did not generate teratomas similar to untreated testes. (F,G) Testis injected with Muse cells maintained normal structure even 6 months postinjection. (H) Muse cells derived from adipose tissue cells showed normal karyotype. Scale bars = 100 μm. Reproduced from [17].

Figure 3. Multilineage Differentiating Stress Enduring (Muse) cells integrated into damaged tissue

GFP-positive human Muse cells integrated into a (A) skin injury model, (B) spinal cord injury model, (C) fulminant hepatitis model, and (D) a degenerated muscle model. GFP-Muse cells were positive for (A) cytokeratin 14, (B) neurofilament albumin, (C) trypsin and (D) dystrophin. Muse cells were also positive for the human cell marker, antihuman Golgi complex, confirming that the positive cells were of human origin. Scale bars = 50 μm (A, B) and 100 μm (C, D). GFP: Green fluorescent protein. Reproduced from [17].

Figure 4. Identification of the top 10 functional groups of all differentially expressed genes (twofold or higher) in Muse cells derived from adipose tissue (Muse-AT) cells versus adipose stem cells (ASCs)

Fischer’s exact test was used to calculate a p-value determining the probability of the association between the genes in the data set with functional groups and canonical pathways. Functional groups are displayed along the x-axis, while the y-axis displays logarithm of p values calculated by Fisher exact between the ratio of the number of genes differentially expressed genes (twofold or higher) in Muse-AT versus ASCs in a given functional group or pathway divided by total number of genes that make up that functional group or pathway with a threshold for statistical significance set at 0.05. The analysis was performed by Ingenuity Pathways analysis software. Reproduced from [18].