The Surface Markers and Identity of Human

Running head: Markers and Identity of MSCs

The Surface Markers and Identity of Human Mesenchymal Stem Cells Feng-Juan Lv 1, 3, 4 , Rocky S. Tuan 2 , Kenneth M.C. Cheung 1, 3, 4* , Victor Y.L. Leung 1, 3, 4* 1.

Depart ment of Orthopaedics and Traumatology, Li Ka Shing Faculty of Medicine, The Un iversity of Hong Kong, 21 Sassoon Road, Hong Kong, Ch ina

2.

Center for Cellular and Mo lecular Engineering, Depart ment of Orthopaedic Surgery, Un iversity of Pittsburgh School of Medicine, PA 15219, U.S.A.

3.

Stem Cell & Regenerative Medicine Consortium, The University of Hong Kong, Hong Kong SAR, China

4.

Center for Reproduction, Development and Growth, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong SAR, China

*

Joint corresponding authors:

Dr. Victor Y.L. Leung 9/F, Lab Block, Depart ment of Orthopaedics and Traumatology, Li Ka Sh ing Faculty of Medicine, The University of Hong Kong, 21 Sassoon Road, Hong Kong SAR, Ch ina Email: [email protected]

Tel: +852 2819 9589

Fax: +852 2818 5210

Prof. Kenneth M.C. Cheung 9/F, Lab Block, Depart ment of Orthopaedics and Traumatology, Li Ka Sh ing Faculty of Medicine, The University of Hong Kong, 21 Sassoon Road, Hong Kong SAR, Ch ina Email: [email protected] k

Tel: +852 2255 4254

Fax: +852 2817 4392

Other author’s email address: Dr. Feng-Juan Lv Dr. Rocky S. Tuan

fengjuan.lv@g mail.co m, [email protected] rst13@p itt.edu

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Author Contribution: Feng-Juan Lv: Conception and design, Collection and/or assembly of data, interpretation and analysis of data, Manuscript writ ing, Final approval o f manuscript Rock S. Tuan: Interpretation and analysis of data, Manuscript writ ing, Final approval of manuscript Kenneth M.C. Cheung: Financial support, Administrative support, Manuscript writ ing, Final approval of manuscript Victor Y.L. Leung: Financial support, Administrative support, Manuscript writ ing, Final approval of manuscript

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Abstract: The concept of mesenchymal stem cells (MSCs) is becoming increasingly obscure due to the recent findings of heterogeneous populations with different levels of stemness within MSCs isolated by traditional p lastic-adherence. MSCs were orig inally identified in bone marro w and later detected in many other t issues. Currently, no cloning based on single surface marker is capable o f isolating cells that satisfy the minimal criteria of MSCs fro m various tissue environments . Markers that associate with the stemness of MSCs await to be elucidated. A nu mber of candidate MSC surface markers or markers possibly related to their stemness have been brought forward so far, including Stro-1, SSEA -4, CD271 and CD146, yet there is a large difference in their exp ression in various sources of MSCs . The exact identity of MSCs in vivo is not yet clear, although reports have suggested they may have a fibroblastic or pericytic orig in. In this review we rev isit the reported expression of surface mo lecules in M SCs fro m various sources, aiming to assess their potential as MSC markers and define the crit ical panel for future investigation. We also discuss the relationship of MSCs to fibroblasts and pericytes in an attempt to shed light on their identity in vivo.

Key word: Mesenchymal stem cells; surface epitopes, CD271, CD146; markers; pericytes; niche; regenerative med icine

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INTRODUCTION Mesenchymal stem cells (MSCs) were first identified fro m bone marrow mononuclear cells (BM -MNCs) as fibroblastic colony-forming units (CFU-Fs). Due to their mu lt ipotency and paracrine effect [1, 2], M SCs are ideal candidates for regenerative med icine [3, 4]. Currently, there is no consensus on a single surface molecule to identify MSCs fro m various sources. The min imu m criteria of MSCs [5] include: (1) remain plastic-adherent under standard culture conditions; (2) express CD105, CD73 and CD90, and lack exp ression of CD45, CD34, CD14 or CD11b, CD79a or CD19 and HLA-DR; (3) d ifferentiate into osteoblasts, adipocytes and choncytes in vitro. Other surface antigens generally expressed by MSC include CD13, CD29, CD44, and CD10 [6][7]. Although bone marrow (BM ) is the most widely recognized source of MSCs , recent research has identified alternative sources of MSC-like cells, including ad ipose tissue (AT) [8], placenta [9], dental pu lp [10], synovial membrane [11], peripheral b lood [12], periodontal ligament [13], endometriu m [14], umbilical cord (UC) [15] and umbilical cord b lood (UCB)[16, 17]. In fact, evidence has suggested that MSCs may be p resent virtually in any vascularized tissues throughout the whole body [18]. Genuine MSCs are expected to possess both clonogenicity and tri-potency. However, only a fraction of CFU-Fs fro m p lastic adherence isolated MSCs (PA-MSCs) exh ibited mu ltipotency [19], indicating that PA-MSCs co mprised a heterogeneous population of cells with different lineage co mmit ment [19], which may relate to their in vivo environment. Th is is reflected in the differences in the protein expression profile, cytokine profile o r d ifferentiation potency of various sources of MSCs (rev iewed in [20]). For examp le, The percentage of BM CFU-Fs with osteogenic potency was higher than that with adipogenic potency [21]. Similarly, ectopic t ransplantation of BM-MSCs resulted in heterotopic bone tissue formation, while dental pulp -derived MSCs generated reparative dentin-like t issue [22]. It is now 4

widely accepted that in the MSCs population, only a proportion of cells satisfy the ‘MSC’ criteria at single cell level, wh ile the other cells are more co mmitted. There is also a more immature population in MSCs wh ich are emb ryonic stem cell (ESC)-like and express Oct-4 and So x2 [23]. So far, the markers proposed for MSCs fall into two categories: sole markers and stemness markers. A sole marker is an alternative MSC selection tool to plastic adherence, which alone is sufficient to identify or purify MSC-like cells fro m their in vivo environment [5]. A ‘stemness’ marker is able to identify a subset of MSCs with high CFU-Fs and tri-lineage potential, or even identify ESC-like population. Ideally, such ‘stemness’ marker may facilitate selection and therefore enrich ment of subpopulation that exhib it superior CFU-Fs and multipotency. Based on the nature of the two different types of markers, the sole markers are normally high ly expressed, while the stemness markers may be moderately detected. The majority of MSC markers are identified for BM-M SCs. To date, however, whether these markers can be applied to other sources of MSCs is not very clear. Moreover, the exact in situ identity of MSCs is not entirely clear, although reports have suggested they may have a fibroblastic or pericytic origin. This review attempt to address these issues through a comprehensive analysis of the current findings on MSC isolated fro m various tissues via the use of single surface markers. Moreover, the capacity of stemness markers representing the subset of more primitive MSCs is revisited and the origin of MSCs is discussed.

Sole and Stemness Markers of MS Cs A number of molecu les have been suggested as MSC markers , as shown in Table 1. A mong them, Stro-1, CD271, SSEA-4 and CD146 are the ones that have received the most attention and adopted in 5

studies as markers to sort MSCs. The expression of these four molecu les in various sources of MSCs is listed in Table 2. Stro-1 Stro-1 is one of the most well known markers for M SCs. Stro-1 is a cell membrane single pass type I

protein that translocates from the endoplasmic reticulum to the cell membrane in response to the depletion of intracellular calcium [24]. By co mbining negative selection against glycophorin-A, Stro-1 enriched CFU-Fs fro m bone marrow with mu ltipotency [25]. However, it did not enrich CFU-Fs fro m hu man endometrial stro ma [14]. The degree of ho mogeneity of the Stro-1 selected MSCs was further enhanced 1000 fold by positive selection for CD106 co mpared to PA-M SCs [26]. Injection of human Stro -1(+) but not Stro-1(-) BM-MNCs into rat myocard iu m led to arteriogenesis and functional cardiac recovery [27]. Further in vivo research demonstrated that Stro-1(-) MSCs supported higher HSC engraft ment in NOD/SCID mice wh ile no support was detected by Stro-1(+) MSCs. However, Stro-1(+) MSCs exh ibited greater capability in ho ming to spleen, bone marro w and kidney [28]. Conditioned med iu m fro m Stro-1(+) MSCs could induce a greater degree of cardiac vascular repair than PA-MSCs [26]. These suggest that Stro-1 may be involved in clonogenicity, and play a role in homing and angiogenesis of MSCs. However, Stro-1 is not universally expressed in all reported types of MSCs. Stro -1 is expressed in dental- [10], synovial memb rane- [11], decidua parietalis- derived MSCs [29] and mu ltipotent dermal fibroblasts [30]. AT- [31], UCB- [32], UC- [16], peripheral blood- derived MSCs [33] are negative/low for Stro-1 expression. It is reported that placenta-derived MSCs gradually lose Stro-1 expression in culture [34]. In contrast, however, the expression of Stro-1 in BM-M SCs increases with culture t ime [35]. 6

The potential of Stro-1 as an MSC marker is limited in several ways. It is unclear whether Stro-1 expression correlates with mu ltipotency. Stro-1 is also unsuitable as a sole marker to separate MSCs fro m its harboring t issue, at least not fro m BM , as greater than 95% of Stro-1(+) cells in the human BM were g lycophorin A expressing nucleated erythroid cells [25]. Moreover, Stro-1 exp ression appears not universal for various MSC types .

CD271 CD271 (also named as lo w-affin ity nerve growth factor receptor, LNGFR) is a receptor for neurotrophins, which stimulate neuronal cells to survive and differentiate. CD271 has been used to select CFU-Fs fro m BM-M NCs. The percentage of CD90(+) CD105(+) CD45(-)CD34(-)CD79(-)

cells in BM-MNCs coincided with the amount of CD271(+) cell subset (0.54%) [36]. CFU-Fs could only be generated fro m CD271(+) subsets of CD45(-)glycophorin-A(-) human BM-MNCs while the CD271(-) fraction showed no residual CFU-F activ ity [7, 37]. CD271(+) BM MSCs were shown to have enhanced capability in pro moting HSC engraft ment compared to PA-MSCs [38], and also induced superior chondral repair than the CD271(-) BM MSCs [39]. These together suggest a role of CD271 in maintaining clonogenicity and function of MSCs. However, the majority of the CD271(+) cells were found not to co-express CD90 and CD73, the two general markers of MSCs. The percentage of CD90 and CD73 positive cells was found to be very low (90%) but not in CD45(-) BM cells. It is also expressed in AT-MSCs and UC-M SCs [72], but not on foreskin fibroblasts. However, a portion of CD34(+) or CD19(+) BM cells also express GD2 [71], suggesting GD2 expression is not limited with in BM -MSCs.

3G5 3G5 is a pericyte marker. Khan reported that p lastic-adherence isolated BM-MSCs were negative for

CD271, CD56 and Stro-1 but positive for 3G5 [73]. To date, 3G5 expression is detected on BM-MSCs, dental pulp- [74], and decidua parietalis- derived MSCs [29]. Shi and Gronthos showed that a minor population of BM-MSCs positive for Stro-1 expression is also positive for 3G5 [74]. 3G5 positivity accounts for 14% of BM CFU-Fs and 63% of dental pulp CFU-Fs [74]. However, only 54% of BM cells express 3G5, eliminating its potential as a sole marker [74].

SSEA-3 Stage-specific emb ryonic antigen-3 (SSEA -3) is a pluripotent stem cell marker. Recently, evidence showed that a minor subset of SSEA-3(+)CD105(+) cells in MSCs, namely mu ltilineage-differentiat ing stress enduring (MUSE) cells, are able to differentiate into ectodermal, endodermal, and mesodermal 11

lineage cells in vivo [23]. Induced pluripotent stem cells (iPSCs) were only found to be derived fro m the MUSE cell subset in fibroblasts but not the none-MUSE subset [75], suggesting that SSEA-3 and CD105 exp ressing MSCs ( ≈ 1%) as progenitor cells reminiscent of, but not identical to,

pluripotent-like ESCs. MUSE cells co-exp ressed some other p luripotency markers including Nanog, Oct3/4, PA R-4, So x2 [23]. Since MSCs are strongly positive for CD105, M USE cells can be represented as the SSEA-3(+) subset of MSCs. M USE cells are not tumo rigenic and can differentiate in vivo without prior genetic manipulat ion or gro wth receptor induction [23], hence they may have practical advantages for regenerative med icine.

SUSD2 (W5C5) Type 1 integral membrane protein Sushi domain containing 2 (SUSD2) has been recently reported to enrich for CFU-Fs and tri-potency fro m endometriu m- [76], and BM-derived MSCs [77]. SUSD2 can be detected by the W5C5 antibody [77]. SUSD2 is not exp ressed in hematopoietic cells. In the endometriu m, it is predo minantly expressed in perivascular regions. W5C5(+) cells are also capable of producing endometrial stro mal-like t issue in vivo [76]. Whether SUSD2 being a co mmon M SC marker remains to be consolidated.

Others There are also some other MSC sole or stemness markers proposed by some researchers which are well investigated. Stro-4, MSCA-1, CD56, CD200, and PODXL have been p roposed as MSC markers by their CFU-Fs enrich ment capacity. The antibody Stro-4 identified the beta isoform of heat shock protein-90. It was found exp ressed in BM-, dental pulp-, periodontal ligament-, and AT- derived MSCs, 12

and enriched CFU-Fs fro m both human and ovine BM by 16- and 8-folds compared to BM -MNCs [78]. MSCA-1 (mesenchymal stem cell antigen-1) is identical to t issue non-specific alkaline phosphatase [57]. Co mpared to unsorted BM-MNCs, MSCA-1 selection resulted in a 90-fold increase in enrich ment of CFU-Fs and a 180-fo ld increase when co-selected for CD56 [79]. Another surface molecu le CD200 was reported [80] to enrich CFU-Fs fro m BM -MNCs to 333-fold. PODXL, a sialo mucin in the CD34 family, was also reported to decrease in high-density cultures which have lower clonogenicity and differentiat ion potency compared to less confluent cultures [35]. Unlike the above molecules, neuron-glial antigen 2 (NG2), So x11 and TM4SF1 were proposed largely based on their expression. NG2 is first observed on the surface of neural progenitors and is a pericyte marker which expression is also shared by BM-MSCs [18] [81] [82]. So x11, a transcription factor previously identified in neural progenitor cells, was found to significantly decrease during MSC passages and knockdown of Sox11 with siRNA decreased the proliferat ion and osteogenic differentiat ion potential of MSCs [83]. TM4SF1 is another surface p rotein highly exp ressed in BM -, UCB-, and AT- M SCs which is not detected in mononuclear cells and fib roblasts, suggesting it may be a potential marker for M SC selection [84].

Comparison of MS Cs Isolated by Different Markers With the array of potential markers identified in MSCs, it is still unclear whether the se markers define different or overlapping subpopulations of MSCs. One of the reasons is that the phenotypic or functional differences among the MSC subpopulations selected by different markers are still poorly understood. Here we aim to review studies that were designed to compare various MSC populations sorted in parallel and d irect ly fro m single sources with respect to their co -exp ression of MSC markers, 13

CFU enrich ment capacity or differentiat ion potential. CFU-Fs enrichment capacity Delorme et al [80] reported that, among 9 molecules, CD73, CD130, CD146, CD2 00, and integrinαV/5 were able to enrich CFU-Fs fro m CD235a(-)/CD45(-)/ CD11b (-) BM -MNCs, while CD49b, CD90, and CD105 showed less enrich ment. A mong various methods of MSC isolation fro m BM-M NCs, including plastic adherence, RosetteSep-isolation, and CD105(+) and CD271(+) selection [50], CD271(+) fract ion showed the highest number of CFU -Fs colonies. Double selection for CD56 or MSCA-1 enriched CFU-Fs to 3- or -fold, respectively in CD271(bright) BM-M NCs [79]. Sch wab et al [14] found that CD146 but not Stro-1 or CD133 selection enriched CFU-Fs fro m human endo metrial stromal cells. Sort ing of CD34(-)CD45(-) [85] or CD45(-) [58] hu man BM -MNCs with CD271 and CD146 revealed that CFU-Fs units remained exclusively in CD271(+) population regard less of CD146 expression, with a tendency towards more CFU-Fs in CD271(+)CD146(+) cells relative to CD271(+)CD146(-) cells. CD146(+) subsets accounted for 96% of CFU-Fs in unfractionated human dental pulp cells, while Stro-1(+) and 3G5(+) subsets accounted for around 80% and 60%, respectively [74]. In addition, the cloning efficiency of W5C5(+)CD146(+) cells was found significantly higher

than CD140b(+)CD146(+) cells. W5C5hi cells had a high clonal capacity equivalent to W5C5+CD146+ cells [76]. Taken together, these findings imply that CD146 and CD271 positivity indicates superior CFU-F capacity in M SCs. These findings are summarized in Tab le 3. Differentiation potential Battula et al [79] reported that chondrocytes and pancreatic-like islets are predominantly induced from MSCA-1(+)CD56(+) BM-MNCs whereas adipocytes emerge exclusively fro m M SCA -1(+)CD56(-) subsets, indicating that CD56 is involved in differentiat ion tendency. Jarocha et al [50] reported that 14

CD271(+) or CD105(+) M SCs have lower lineage marker exp ression than PA-MSCs after osteogenic, chondrogenic and adipogenic induction. Arufe et al [86] reported that when co mparing CD73, CD106, or CD271 positive human synovial memb ranes cells , CD271(+) cells are h ighly chondrogenic, whereas the CD73(+) cells are less chondrogenic and the CD106(+) cells mostly undifferentiated after induction. Vaculik et al [64] reported that CD271(+) but not SSEA-4(+) dermal cells exh ibit osteogenic and chondrogenic differentiation potential. Dermal SSEA -4(+) cells, in contrast, are only responsive to adipogenic induction. In adult hu man BM-MNCs, CD271(+)CD146(-/low) and CD271(+)CD146(+) subsets [58] show a similar capacity to differentiate and to support hematopoiesis, but the two subsets have been found at different s ites; CD271(+)CD146(-/lo w) cells are bone-lining, while CD271(+)CD146(+) cells have a perivascular localization , suggesting that the two subsets play different ro les in HSC niche. The function of surface markers in mu lt ipotency enrichment is summarized in Tab le 4. Surface marker co-expression Relevant studies on the degree of co-expression of surface markers on MSCs is summarized in Tab le 5. The expression of CD106 and CD146 was found to be restricted to the MSCA -1(+)CD56(-) MSCs and CD166 to MSCA-1(+)CD56(+/-) M SCs [79]. Vaculik [64] reported that in human dermis, the expression pattern of SSEA-4 is almost analogous to CD271. Both were found only weakly expressed and co-expressed with CD45. Van Landuyt and Qu irici also reported the detection o f CD34 exp ression on CD271(+) subpopulation of human synovial and BM-MSCs [40] [37]. In human dermis, CD73 and CD105 are co-expressed [64]. A minor population of the hu man dermis CD73(+) cells is CD90(-). Dermis CD271(+) cells were CD73(+) and CD105(+), whereas the majority of CD271(+) cells are CD90(-) [64]. Similarly, in t wo other reports, only a minor subset of the CD271(+) cells express CD90, 15

CD73 (