On the Origin and Impact of Mesenchymal Stem Cell Heterogeneity: New Insights and Emerging Tools for Single Cell Analysis
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EuropeanCM McLeod Cells and and RL Materials Mauck Vol. 34 2017 (pages 217-231) DOI: 10.22203/eCM.v034a14 MSC heterogeneity and single ISSN cell 1473-2262 analysis ON THE ORIGIN AND IMPACT OF MESENCHYMAL STEM CELL HETEROGENEITY: NEW INSIGHTS AND EMERGING TOOLS FOR SINGLE CELL ANALYSIS C.M. McLeod1,2,3 and R.L. Mauck1,2,3* 1 Department of Bioengineering, University of Pennsylvania, Philadelphia, PA 19104, USA 2 McKay Orthopaedic Research Laboratory, Department of Orthopaedic Surgery, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA 19104, USA 3 Translational Musculoskeletal Research Center, Philadelphia VA Medical Center, Philadelphia, PA 19104, USA. Abstract Mesenchymal stem cells (MSCs) display substantial cell-to-cell variation. This heterogeneity manifests among donors, among tissue sources, and within cell populations. Such pervasive variability complicates the use of MSCs in regenerative applications and may limit their therapeutic efficacy. Most conventional assays measure MSC properties in bulk and, as a consequence, mask this cell-to-cell variation. Recent studies have identified extensive variability amongst and within clonal MSC populations, in dimensions including functional differentiation capacity, molecular state (e.g. epigenetic, transcriptomic, and proteomic status), and biophysical properties. While the origins of these variations remain to be elucidated, potential mechanisms include in vivo micro-anatomical heterogeneity, epigenetic bistability, and transcriptional fluctuations. Emerging tools for single cell analysis of MSC gene and protein expression may yield further insight into the mechanisms and implications of single cell variation amongst these cells, and ultimately improve the clinical utility of MSCs in tissue engineering and regenerative medicine applications. This review outlines the dimensions across which MSC heterogeneity is present, defines some of the known mechanisms that govern this heterogeneity, and highlights emerging technologies that may further refine our understanding and improve our clinical application of this unique cell type. Key words: Stem cells - differentiation, tissue engineering/regenerative medicine, cells/tissue – analytical methods, genetics – gene expression, proteomics. *Address for correspondence: Robert L. Mauck, PhD, McKay Orthopaedic Research Laboratory, University of Pennsylvania, 424 Stemmler Hall, 36th Street and Hamilton Walk, Philadelphia, PA 19104, USA Telephone: 1-215-898-3294 FAX: 1-215-573-2133 E-mail: [email protected] Introduction stem cells (MSCs) are readily obtained from adult tissue, expand well in culture, and can undergo In tissue engineering applications, the structure chondrogenic differentiation. However, even with and function of the extracellular matrix (ECM) the most effective differentiation protocols, MSCs are crucial determinants of the success or failure generally fail to fully match the performance of of an engineered construct. The ECM is created chondrocytes (Huang et al., 2010; Johnstone et al., and maintained by resident cells, and hence, the 2013). Such gaps in performance between MSCs choice of cell source strongly influences construct and differentiated cell types likely exist, in part, as performance. For example, in engineered cartilage, a consequence of marked variation in the ability of the quality and organisation of the matrix produced individual MSCs to undergo lineage commitment. differs substantially based on the cell types employed. Some MSCs robustly undergo differentiation while Chondrocytes, the cells resident in native cartilage, others fail to do so (Huang et al., 2010). While these excel at producing robust extracellular matrices underperforming, alternatively performing or non- in vitro, even under nutrient limiting conditions responsive subpopulations hinder the maturation (Johnstone et al., 2013; Kock et al., 2012). Unfortunately, of engineered tissues, their poor performance is chondrocytes are difficult to obtain in sufficient often masked by bulk assays that pool signal across number, and chondrogenically induced stem cells entire cell populations (Fig. 1). Recently, given are often used as an alternative. Mesenchymal the advent of single cell methods and a growing 217 www.ecmjournal.org CM McLeod and RL Mauck MSC heterogeneity and single cell analysis appreciation that ensemble measurements can mask important variation, new findings have begun to delineate MSC heterogeneity. Here, we review the current understanding of heterogeneity among and within MSC populations, and discuss how single cell techniques may be used to further parse this Fig. 1. Bulk observations can mask heterogeneity, variability. including a) ‘tail’ observations, b) small subpopulations, and c) bimodal behaviour. Inspired by Altschuler and Wu, 2010. Mesenchymal stem cell heterogeneity The defining properties of mesenchymal stem cells As a cell type, MSCs are defined by three criteria. umbilical cord and dental pulp (Erices et al., 2000; MSCs must: 1) be plastic adherent; 2) express the Gronthos et al., 2000; Zuk et al., 2001) to name a few. surface markers CD105, CD73 and CD90, and lack In standard isolation techniques, adipose or bone expression of CD45, CD14 or CD11b, CD79 or CD19 marrow aspirates are progressively centrifuged, and HLA-DR; and 3) be capable of differentiating and filtered before being plated into culture. A into osteoblasts, adipocytes, and chondroblasts small fraction of the cells (the presumed MSCs) will (Bourin et al., 2013; Dominici et al., 2006). These adhere to the tissue culture plastic, and proliferate. criteria are periodically updated, and the reader is Both bone marrow- and adipose-derived MSCs are referred to the website of the International Society readily available (Estes et al., 2010), yet they originate for Cellular Therapy MSC Subcommittee for the most from stem cell niches that provide distinct biological, up-to-date information (Web ref.1). Even so, this chemical and mechanical cues. Tissue-dependent operational definition does not necessarily define a variation in differentiation capacity, surface markers homogenous population of multipotent progenitors. and transcriptional and proteomic profiles is widely Instead, it describes a heterogeneous group of studied, and the reader is referred to recent reviews cells that demonstrate variability among tissues of for comparisons of MSCs across tissue sources (Kern origin, among individual donors, amongst clonal et al., 2006; Mattar and Bieback, 2015; Strioga et al., subpopulations, and at the single cell level (Fig. 2). 2012). Even when derived from the same tissue of origin, MSCs exhibit heterogeneity on multiple levels MSCs demonstrate tremendous donor-to-donor While MSCs were first isolated from bone marrow variability. Intuitively, donor health may influence (Friedenstein, 1976; Johnstone et al., 1998; Pittenger the availability and functional potential of MSCs et al., 1999), they have since been identified in many (Kuznetsov et al., 2009; Wang et al., 2013). Similarly, connective tissues, including adipose tissue, the as donors age, MSC availability, self-renewal capacity Phinney et al., 1999 Johnstone et al., 1998 Muraglia et al., 2000 Lee et al., 2014 D’Ippolito et al., 1999 Pittenger et al., 1999 Larsen et al., 2010 Marble et al., 2014 Kuznetsov et al., 2009 Erices et al., 2000 Russell et al., 2011 Freeman et al., 2015 Mindaye et al., 2013 Gronthos et al., 2000 González-Cruz et al., 2012 Cote et al., 2016 Zuk et al., 2001 Selich et al., 2016 Li et al., 2016 Fig. 2. MSC heterogeneity exists at multiple levels, including among donors, tissues, clonal subpopulations, and single cells. Selected works are highlighted at each of these levels. www.ecmjournal.org 218 CM McLeod and RL Mauck MSC heterogeneity and single cell analysis and differentiation potential have been reported to decline (D’Ippolito et al., 1999; Katsara et al., 2011; Stenderup et al., 2003). Surprisingly, however, even MSCs isolated from young, healthy donors exhibit stark differences in their proliferation rate, differentiation capacity, and ultimate clinical utility (Phinney et al., 1999). This functional variation extends to the molecular status of these cells (Mindaye et al., 2013; Mindaye et al., 2015). For example, mass spectroscopy of MSCs isolated from six donors revealed that only 62 % of all identified proteins were found in at least half of the donors, and only 13 % of identified proteins were found in cells from each donor (Mindaye et al., 2013). Such donor- to-donor variation complicates use, and motivates a more detailed investigation of MSC variability. Further study revealed that donor- and tissue- dependent differences are superimposed upon cell-to-cell variation amongst MSCs within a single population. For example, multiple bone marrow aspirates isolated from the same donor over a period of six months, or bilaterally from a donor at a single time point, yield MSC cultures that proliferate at different rates (Phinney et al., 1999). Even within a single isolate, cell-to-cell variation in MSC phenotype becomes evident during culture expansion and downstream use (Fig. 3, Fig. 4). This variation is commonly examined by comparing clonal subpopulations (groups of cells that are not only genetically identical, but also recently derived from single parent cells) (Huang, 2009). Fig. 3. Example of MSC-to-MSC variation in MSCs readily form clones, and their clonogenicity extracellular matrix production. Confocal cross- can be observed