Regulation of nestin expression by thrombin and cell density in cultures of bone mesenchymal stem cells and radial glial cells.

BACKGROUND: Bone marrow stromal cells and radial glia are two stem cell types with neural phenotypic plasticity. Bone marrow mesenchymal stem cells can differentiate into osteocytes, chondrocytes and adipocytes, but can also differentiate into non-mesenchymal cell, i.e. neural cells in appropriate in vivo and in vitro experimental conditions. Likewise, radial glial cells are the progenitors of many neurons in the developing cortex, but can also generate astrocytes. Both cell types express nestin, an intermediate filament protein which is the hallmark of neural precursors. RESULTS: In this study, we demonstrate that thrombin, a multifunctional serine protease, stimulates the growth of radial glial cells (RG) and mesenchymal stem cells (MSCs) in a dose-dependent manner. In RG, the mitogenic effect of thrombin is correlated with increased expression of nestin but in MSCs, this mitogenic effect is associated with nestin down-regulation. Both cell types express the PAR-1 type receptor for Thrombin and the effect of Thrombin on both cell types can be mimicked by its analogue TRAP-6 activating specifically this receptor subtype or by serum which contains various amount of thrombin. Moreover, we also demonstrate that serum deprivation-induced expression of nestin in MSCs is inhibited by high cell density (> 50,000 cells/cm2). CONCLUSION: This work shows that thrombin stimulates the growth of both RG and MSCs and that nestin expression by MSCs and RG is regulated in opposite manner by thrombin in vitro. Thrombin effect is thus associated in both cell types with a proliferating, undifferentiated state but in RG this involves the induction of nestin expression, a marker of immaturity for neural progenitors. In MSCs however, nestin expression, as it corresponds to a progression from the mesenchymal "undifferentiated", proliferating phenotype toward acquisition of a neural fate, is inhibited by the mitogenic signal.

Astrocytic and neuronal fate of mesenchymal stem cells expressing nestin.

Classically, bone marrow mesenchymal stem cells (MSC) differentiate in vivo or in vitro into osteocytes, chondrocytes, fibroblasts and adipocytes. Recently, it was reported by several groups that MSC can also adopt a neural fate in appropriate in vivo or in vitro experimental conditions. However, it is unclear if those cells are really able to differentiate into functional neural cells and in particular into functional neurons. Some observations suggest that a cell fusion process underlies the neural fate adoption by MSC in vivo and first attempts to reproduce in vitro this neural fate decision in MSC cultures were unsuccessful. More recently, however, in several laboratories including ours, differentiation of MSC cultivated from adult rat bone marrow into astrocytes and neuron-like cells was demonstrated. More precisely, we stressed the importance of the expression by MSC of nestin, an intermediate filament protein associated with immaturity in the nervous system, as a pre-requisite to adopting an astrocytic or a neuronal fate in a co-culture paradigm. Using this approach, we have also demonstrated that the MSC-derived neuron-like cells exhibit several electrophysiological key properties classically devoted to neurons, including firing of action potentials. In this review, we will discuss the neurogenic potential of MSC, the factor(s) required for such plasticity, the molecular mechanism(s) underlying this neural plasticity, the importance of the environment of MSC to adopt this neural fate and the therapeutic potential of these observations.