The ASCs as well as other mesenchymal stem cells derived from several tissue types were shown to express in vivo and vitro a gamut of surface markers, the specific combos of which may ultimately define distinct repertoires [ 26 , 31 , 33 , 46 – 50 ]. Previous studies suggested that some of the markers may be involved in the ASC functionality, such as angiogenesis, differentiation, migration, plus secretome [ 1 , 27 , 50 – 52 ]. These approaches were based on the choice of single surface markers and characterization directly after selecting, giving thus a limited emphasis to the co-expression patterns [ 29 , 31 , 45 , 48 , 53 – 55 ]. In continuation of these efforts we sought to acquire insight into how more complex surface marker variants are preserved during culturing and whether they have a potential to provide meant for progeny populations with enhanced specific functionality. Such research would further our understanding of the significance of different combinations associated with more and less frequent markers that specify the under the radar subpopulations. It is plausible that overrepresentation of unique repertoires in certain donors would provide at least partial explanation for great inter-individual differences in particular regenerative applications. Consequently, specific donors or even subpopulations could be rationally selected to provide for optimal medical response.
Based on previous inspections [ 14 , 15 , 20 – 23 ], we identified CD73, CD90, CD105, CD34, CD146, and CD271 as six surface markers along with major relevance for determination of ASCs. In this research, not all combinations appear equally permissible, but 95% associated with early ASCs (P0) are found in the compartment defined simply by CD73 + 90 + 105 + phenotype. Of these tissues, 54% does not express any of the other studied markers, 16% co-express CD34, 15% co-express CD146, and 7% co-express both of them. To study the effect of in vitro tradition on the properties of discrete subpopulations, we opted to incorporate a minor SP4 CD146+271+ in favor of the only CD34 + variant since our own previous investigation indicated frequent occurrence of this phenotype, which range from 5 to 20% [ 46 ]. Low frequency from the minor combination CD146 + CD271 + in the current study underscores significance associated with factors underlying interindividual variability, in this case most likely those associated with age [ 47 ].
Oddly enough, our findings show that in the progeny population coming from the single sorted profile, a distribution of phenotypic variants reappears so as to resemble to original pre-sorted human population (P0). Depending on the originally selected markers, all downstream outlines thus as a result of culturing gained, lost, or gained plus lost one or more markers, ultimately resembling each other. This indicates that the restricted heterogeneity rendered by distinct physiologically important repertoires is critical for stability and viability of ASC ethnicities, and further raises the question about possible mechanisms involved in the shared intervariant stimulatory interactions enabling survival in vitro. Significantly, despite obvious resemblance, the downstream lines may screen differential functionality under appropriate conditions. Selection for the CD73 + CD90 + CD105 + CD34 − 146 + 271 − phenotype (SP2) clearly resulted in a line with increased endothelial stimulation. Given the scope of epitopes selected in this particular study, it seems that of the minor markers, the CD146 is essential and sufficient to provide for augmented paracrine support associated with endothelial cells. An explanation for this functional significance of the CD146 probably stems from the fact that it has been identified as a cell adhesion molecule and a hallmark epitope of endothelium regulators, the particular pericytes. Pericytes are perivascular cells that have been located perivascularly in almost all tissues [ 29 ], and in addition to a big part in angiogenesis through an interplay with endothelial cells [ 21 , 28 , 56 – 58 ], they have been found to influence homeostasis through shrinkage of blood vessels [ 21 ], or regulate coagulation plus lymphocyte activation and migration [ 59 ]. Nevertheless, to verify the existence of true pericyte phenotype, the presence of additional markers, like the α -SMA, NG2, and PDGF-B receptor and lack of CD31, CD34, CD45, and CD56 would have to be verified [ 21 , 27 – 30 , 57 ].
There is no apparent explanation why selection of SP2 repertoire would produce such useful outcome when all other progeny lines also contained this particular variant in a substantial proportion. The level of CD146 expression might be of importance, as was suggested previously by some researchers [ 31 , 33 ], but that such simple quantitative indication plays a role does not seem to be supported by our information. The answer thus may reside in differences between relative dimensions of individual repertoires within the progeny lines or in the manner how CD146 expressed in the context of other epitopes defines the selected parental population. The issue of more described subpopulations appears intricate, since some previous data confront our findings by highlighting the CD34 + CD146 + phenotype because highly pro-angiogenic [ 45 ]. Clearly, further studies are essential to shed more light on the biological significance associated with complex repertoires, especially in terms of molecular events mixed up in secretion of paracrine factors. By the same token, it could be of interest to determine how significant is the association of CD146 with the regenerative potential in the clinical setting. Since, as stated above, this phenotype plays a role in revascularization processes, it can be envisaged that in conjunction with appropriate preconditioning regimen, such as hypoxia [ 8 , 15 , 16 , 60 – 63 ], substantially improved therapeutic outcome would be achieved.