Are MSCs angiogenic cells? New insights on human nestin-positive bone marrow-derived multipotent cells

Abstract

Recent investigations have made considerable progress in the understanding of tissue regeneration driven by mesenchymal stromal cells (MSCs). Data indicate the anatomical location of MSC as residing in the "perivascular" space of blood vessels dispersed across the whole body. This histological localization suggests that MSCs contribute to the formation of new blood vessels in vivo. Indeed, MSCs can release angiogenic factors and protease to facilitate blood vessel formation and in vitro are able to promote/support angiogenesis. However, the direct differentiation of MCSs into endothelial cells is still matter of debate. Most of the conflicting data might arise from the presence of multiple subtypes of cells with heterogeneous morpho functional features within the MSC cultures. According to this scenario, we hypothesize that the presence of the recently described Mesodermal Progenitor Cells (MPCs) within the MSCs cultures is responsible for their variable angiogenic potential. Indeed, MPCs are Nestin-positive CD31-positive cells exhibiting angiogenic potential that differentiate in MSC upon proper stimuli. The ISCT criteria do not account for the presence of MPC within MSC culture generating confusion in the interpretation of MSC angiogenic potential. In conclusion, the discovery of MPC gives new insight in defining MSC ancestors in human bone marrow, and indicates the tunica intima as a further, and previously overlooked, possible additional source of MSC.

Keywords: adult stem cells; angiogenesis; bone marrow; endothelial differentiation; in vivo MSC; mesenchymal stromal cells; neo-vascolarization; nestin.

Figure 1

The angiogenic potential of MSC cultures is controversial. (A) Mesodermal Progenitor Cells (MPCs) are CD90-, Nestin+ progenitors of MSCs. These cells remain in a quiescent state presenting a typical fried egg-shape, condensed chromatin and podosomal structures. When mesengenic differentiation is induced, MPCs differentiate into “early MSCs.” Early MSCs slowly proliferate, express Nestin and CD90 (a MSC marker) and modify their shape because of Actin re-organization in focal adhesion complexes. Under persistent stimulation toward mesengenic differentiation, “early MSC” become “late MSC” showing exponentially growth, fibroblastoid shape and the ability to differentiate into skeletal tissues (fat, bone and cartilage). (B) According to this hierarchical model, MPCs can be considered a putative progenitor of the mesenchymal lineage in vivo, being present in the bone marrow mononuclear fraction. When supplemented with media containing FBS, these cells rapidly differentiate toward the mesengenic lineage and form asynchronous and heterogeneous cultures fulfilling the ISCT MSC criteria of definition. The undetected and unpredictable presence of sub-populations of MPCs, in culture could explain the variable angiogenic potential described for MSC in the literature. Conversely, the isolation of MPC and their subsequent differentiation using optimized protocol can allow the generation of synchronized and homogeneous mesenchymal stromal cells with a reproducible angiogenic potential.

Figure 2

Nestin expression in human bone marrow biopsies. (A) Consistent expression of Nestin is detectable in the tunica intima of large vessels, arterioles and venules (dark brown color). Few cells of the sub-endothelium and adventitia show a weakly positive Nestin staining (light brownish color). Furthermore, Nestin is highly expressed in the sinusoids and two colors immunofluorescence reveals that Nestin is positive in lumen-facing cells surrounded by CD146+/Nestin− pericytes. (B) Schematic representation of possible tissue distribution of MPC (represented in green) and other bone marrow vessels-residing cells.