Repair of tissues by adult stem/progenitor cells (MSCs): controversies, myths, and changing paradigms

Abstract

Research on stem cells has progressed at a rapid pace and, as might be anticipated, the results have generated several controversies, a few myths and a change in a major paradigm. Some of these issues will be reviewed in this study with special emphasis on how they can be applied to the adult stem/progenitor cells from bone marrow, referred to as MSCs.

Figure 1

Surface epitopes reversibly present in low density cultures enriched for rapidly self-renewing MSCs (RS-MSCs), but not as the cells were expanded toward confluency. Human MSCs (hMSCs) passage 1 that were plated at 100 cells/cm² and incubated for 5 days or 9 days to generate passage 2 MSCs. To prepare passage 3 MSCs, 9-day cultures were lifted with trypsin/EDTA and replated at 100 cells/cm² for incubation for 5 or 9 days. (a) RT-PCR assays. (b) Western blot assays. (c) Assays by immunocytochemistry. Bar = 200 µm. (d) Double-immunostaining for podocalyxin-like protein (PODXL) (red) and the five other surface proteins (green). Nuclei were labeled with DAPI (blue). Upper panels: bar = 200 µm. Lower panels: bar = 50 µm. Arrows: regions of the cells in which PODXL and other proteins are colocalized. With permission from ref. 31.

Figure 2

Demonstration of an in vitro niche found in single cell–generated colony of human MSCs (hMSCs). hMSCs were plated on slides for laser microdissection at 2 cells/cm² and incubated for 12 days without medium change. The colonies in the figure were stained with crystal violet for illustrative purposes only. (a) Intact colony. (b) Colony after IN region was captured by laser microdissection. (c) Heat map and gene ontologies from microarray data from IN and OUT samples from four colonies (numbers 1, 2, 4, and 5) using 199 differentially expressed genes. Bar in a, and b = 500 µm.

Figure 3

Schematic summarizing changing properties of human MSCs (hMSCs) expanded in culture. MSCs in suspension are spherical with multiple podia⁹⁴ but become spindle-shaped after adherence to culture surfaces. The cells then undergo a lag period followed by a rapid growth with doubling time (Dt) of <12 hour that is driven by expression of the Wnt inhibitor Dkk-1. During the rapid growth period, the small rapidly self-renewing MSCs (RS-MSCs) express a characteristic set of surface epitopes, including podocalyxin-like protein (PODXL) (Figure 1). They are weakly positive to STRO-1 and are highly clonogenic.³¹ As single cell–derived colonies expand, they develop distinct inner and outer regions that define in vitro niches. The colonies reach a near stationary phase with Dt of over 20 hour. The cells begin to lose expression of PODXL, express higher levels of STRO-1, and decrease in clonogenicity. If lifted before confluency and replated at low density, the sequence is repeated through four to seven passages.

Figure 4

Schematic summarizing the incompletely explained effects of MSCs on tissue repair after intravenous infusion into mice. As indicated, there are several unanswered questions as to how intravenously infused MSCs can modulate systemic immune responses or repair distal tissues.