cells Article MUSE Stem Cells Can Be Isolated from Stromal Compartment of Mouse Bone Marrow, Adipose Tissue, and Ear Connective Tissue: A Comparative Study of Their In Vitro Properties Domenico Aprile 1 , Nicola Alessio 1 , Ibrahim H. Demirsoy 1 , Tiziana Squillaro 1, Gianfranco Peluso 2, Giovanni Di Bernardo 1,3 and Umberto Galderisi 1,3,4,* 1 Department of Experimental Medicine, Luigi Vanvitelli Campania University, 80138 Naples, Italy; [email protected] (D.A.); [email protected] (N.A.); [email protected] (I.H.D.); [email protected] (T.S.); [email protected] (G.D.B.) 2 Research Institute on Ecosystems (IRET), CNR, 80131 Naples, Italy; [email protected] 3 Center for Biotechnology, Sbarro Institute for Cancer Research and Molecular Medicine, Temple University, Philadelphia, PA 19122, USA 4 Genome and Stem Cell Center (GENKOK), Erciyes University, 38280 Kayseri, Turkey * Correspondence: [email protected]; Tel.: +39-081-566-7858 Abstract: The cells present in the stromal compartment of many tissues are a heterogeneous popu- lation containing stem cells, progenitor cells, fibroblasts, and other stromal cells. A SSEA3(+) cell subpopulation isolated from human stromal compartments showed stem cell properties. These cells, known as multilineage-differentiating stress-enduring (MUSE) cells, are capable of resisting Citation: Aprile, D.; Alessio, N.; stress and possess an excellent ability to repair DNA damage. We isolated MUSE cells from different Demirsoy, I.H.; Squillaro, T.; Peluso, mouse stromal compartments, such as those present in bone marrow, subcutaneous white adipose G.; Di Bernardo, G.; Galderisi, U. MUSE Stem Cells Can Be Isolated tissue, and ear connective tissue. These cells showed overlapping in vitro biological properties. The from Stromal Compartment of Mouse mouse MUSE cells were positive for stemness markers such as SOX2, OCT3/4, and NANOG. They Bone Marrow, Adipose Tissue, and also expressed TERT, the catalytic telomerase subunit. The mouse MUSE cells showed spontaneous Ear Connective Tissue: A commitment to differentiation in meso/ecto/endodermal derivatives. The demonstration that mul- Comparative Study of Their In Vitro tilineage stem cells can be isolated from an animal model, such as the mouse, could offer a valid Properties. Cells 2021, 10, 761. alternative to the use of other stem cells for disease studies and envisage of cellular therapies. https://doi.org/10.3390/cells10040761 Keywords: mesenchymal stem cells; differentiation; adipose tissue; fibroblasts; bone marrow Academic Editor: Debabrata Banerjee Received: 15 February 2021 Accepted: 29 March 2021 1. Introduction Published: 30 March 2021 Mesenchymal stromal cells (MSCs), a heterogeneous population found in stromal Publisher’s Note: MDPI stays neutral tissues throughout the body, contain stem cells, progenitor cells, fibroblasts, and other with regard to jurisdictional claims in stromal cells. MSCs are involved in the homeostasis of the body and perform important published maps and institutional affil- functions through the secretion of bioactive molecules [1]. iations. In 2010, Dezawa’s research group [2] identified a subpopulation of SSEA3(+) multilin- eage stem cells within MSCs isolated from human bone marrow. These cells, known as multilineage-differentiating stress-enduring (MUSE) cells, are endogenous cells capable of resisting stress and possess an excellent ability to repair DNA damage [3]. The identification of MUSE cells has been demonstrated both in vitro and in vivo, Copyright: © 2021 by the authors. confirming that these cells are an endogenous tissue population. In humans, MUSE Licensee MDPI, Basel, Switzerland. This article is an open access article cells have been isolated from MSCs of bone marrow and subcutaneous fat, where they distributed under the terms and represent 2–3% of the population, and from skin fibroblasts where they constitute 1% of conditions of the Creative Commons the population [4,5]. Attribution (CC BY) license (https:// MUSE cells are self-renewable, not tumorigenic, and can home within damaged tissues. creativecommons.org/licenses/by/ In this context, it has been hypothesized that they may contribute to the daily repair of 4.0/). tissues and organs through spontaneous differentiation in tissue constituent cells [6,7]. Cells 2021, 10, 761. https://doi.org/10.3390/cells10040761 https://www.mdpi.com/journal/cells Cells 2021, 10, 761 2 of 15 Due to the abovementioned properties, MUSE cells are considered to be suitable for regenerative medicine; in fact, it has been proven that they are capable of differentiating into neural cells and facilitate reconstruction after lacunar infarction in immunodeficient mice [8]. MUSE cells also provided restorative effects and robust functional recovery in a rabbit acute myocardial infarction model [9]. There are several findings showing the existence of multilineage stem cell subtypes within MSCs, but studies confirming their existence are controversial [10,11]. In this scenario, the isolation of such stem cells in several mammals may confirm that stromal tissues host multilineage stem cells. Characterization of MUSE cells isolated from animal tissues is still missing. Recently, only mouse adipose tissue-derived MUSE cells have been extracted and identified for their neuronal differentiation potential in vitro [12], but a thorough analysis of their biological properties has never been performed, and the presence of these cells in different stromal districts has not been verified. In this study, we isolated SSEA3(+) cells from three different mouse stromal districts —bone marrow, subcutaneous fat, and fibroblasts—and analyzed their biological properties to ascertain if they fulfill the MUSE stem cell phenotype. 2. Materials and Methods 2.1. Animals Five C57BL/6 inbred male mice of 3 weeks of age were purchased from Charles River (Wilmington, MA, USA). The animals were handled in compliance with the protocols ap- proved by the Animal Care and Use Committee of University of Campania Luigi Vanvitelli and approved by the Italian Ministry of Health (number 1036/2020-PR). The mice were euthanized by cervical dislocation, and tissue samples were harvested for the experiments indicated below. 2.2. Mouse MSCs and Fibroblasts’ Isolation We harvested MSCs from the bone marrow of the femurs and tibias of mice by inserting a 21-gauge needle into the shaft of the bone and flushing it with D-MEM (Microgem, Napoli, Italy). The cells from one animal were plated onto two 100-mm dishes with D- MEM low glucose (Microgem) containing 15% ES-FBS (Euroclone, Pero, Italy). After 48 h, we discarded the nonadherent cells and washed the adherent ones with PBS 1X (Microgem). We then incubated the cells for 7 to 10 days in a proliferating medium in order to reach confluence (P0). We collected MSCs from 300 mg of subcutaneous white adipose tissue surrounding the hips of animals and fibroblasts from the ear connective tissue of the animals. Tissues were digested in a D-MEM solution containing collagenase type II (1 mg/mL) (Sigma, St. Louis, MI, USA) for 30 min at 37 ◦C. Samples were filtered on cell strainers (70 µm mesh), centrifuged, and washed three times with PBS 1X (Microgem). Cells were plated into 100-mm dishes with D-MEM low glucose (Microgem) containing 15% ES-FBS (Euroclone). We then incubated the cells for 7 to 10 days in a proliferating medium in order to reach confluence (P0). The cells were later trypsinized and expanded until the 3rd in vitro passage (P3). 2.3. Harvest of Mouse Tissues and Organs We harvested leg muscle tissue, liver, and brain. The tissue and organs were col- lected in cryostat embedding medium (Bioptica, Milano, Italy) or in RNAlater (Qiagen, Hilden, Germany) and stored at −80 ◦C until immunocytochemistry analysis or mRNA expression analysis. 2.4. Culture of MUSE Cells Confluent MSCs and fibroblasts were collected by 0.25% trypsin-EDTA (Sigma) and were subjected to cell sorting to isolate MUSE cells as described previously [13,14]. In brief, Cells 2021, 10, 761 3 of 15 cells were suspended in FACS buffer, which contained 0.5% bovine serum albumin (BSA) (Sigma) and 2 mM EDTA-2H2O (Sigma) in FluoroBrite DMEM (ThermoFisher, Waltham, MA, USA), and were incubated with the anti-mouse SSEA-3 antibody (IBL International, Hamburg, Germany) for 1 h on ice. Cells were then washed three times with FACS buffer and centrifuged at 400× g for 5 min. Afterwards, cells were incubated with a secondary antibody, anti-rabbit IgM-FITC (ImmunoReagents, Raleigh, NC, USA) for 1 h on ice, and subsequently washed three times again. Magnetic-activated cell sorting (MACS) was used to collect SSEA-3(+) cells and SSEA-3(−) non-MUSE cells according to the manufacturer’s instructions (Miltenyi, Bergisch Gladbach, Germany). Subsequently, cells were incubated with anti-FITC microbeads (Miltenyi) for 15 min in ice and later washed with FACS buffer. Cells were then loaded on LS columns for magnetic separation. The collected SSEA-3 positive cells represented MUSE cells and were cultivated in suspension on poly-2-hydroxyethyl methacrylate- (pHEMA) (Sigma) coated petri dishes in a DMEM low glucose medium (Microgem) containing 10% ES-FBS (Euroclone), 4 mM L-glutamine (Sigma, St. Louis, MO, USA), 100 U/mL penicillin-streptomycin (HiMedia, Düsseldorf, Germany), and 2.6% MethoCult (STEMCELL Technologies, Vancouver, BC, ◦ Canada) at 37 C and 5% CO2 for 10 days. At the end, the cells were plated for successive experiments or subjected to analysis. The collected