Mesenchymal stem cells as trophic mediators of neural differentiation

Abstract

Intense excitement and optimism surrounds the rapidly-expanding field of stem cell research, owing to their high capacity for self-renewal and intrinsic ability to differentiate into mature cell lineages. Although it may be envisioned that embryonic stem cells will be of significantly greater therapeutic value than their adult stem cell counterparts, the use of embryonic stem cells is fraught with both technical and ethical challenges and, as such, significant impetus has been placed on adult stem cell-based research. In particular, mesenchymal stem cells (MSCs) present as exciting candidates for potential use in cellular therapies and tissue engineering strategies.

MSCs are defined at the functional level in terms of their ability to differentiate into mesodermal derivatives such as bone and fat. However, this functional definition is evolving, and there is considerable evidence to suggest that MSCs have a key role within their niche involving the release and/or uptake of soluble factors and cytokines, significantly influencing the behaviour of other cell types within the niche. Both facets of MSC behaviour are valuable from a clinical perspective, and have been examined in the present thesis.

The most obvious and realistically-achievable clinical application of MSCs at present is in the treatment of osseous and adipose tissue defects. However, before the use of MSCs in the clinic becomes more commonplace, it is crucial to gain a more comprehensive understanding of the complex molecular and cellular mechanism(s) by which MSCs commit to a given fate and undergo differentiation to produce mature, fully-functional derivatives. Much of our present knowledge is derived from studies performed on the highly unnatural, 2D environment of tissue culture plastic. The present study assessed the behaviour of MSCs cultured on AlvetexTM, a novel, 3D scaffold manufactured by ReInnervate, with particular emphasis on the ability of MSCs to undergo osteogenic and adipogenic differentiation. Results obtained suggest that AlvetexTM may provide a more realistic and physiologically-relevant system in which to study osteogenesis and adipogenesis, in a manner more pertinent to that which occurs in vivo.

Furthermore, the ability of MSCs to influence the behaviour of other cell types via the release of trophic factors and cytokines was examined, with particular emphasis on the nervous system. An in vitro conditioned media model was developed in order to investigate the influence(s) of MSC-derived soluble factors/cytokines on neural development and plasticity, using the adult rat hippocampal progenitor cell (AHPC) line as a model system. Results obtained suggest that, under defined conditions, MSCs secreted a complement of soluble factors/cytokines that induce AHPCs to commit to and undergo astrogenesis. This effect was characterised at both the cellular and molecular level. The specific complement of bioactive factors secreted by MSCs has been investigated using a combination of targeted transcriptional profiling and shotgun proteomics, and several putative candidate factors have been identified for further investigation.