Preconditioning stem cells for in vivo delivery

Abstract

Stem cells have emerged as promising tools for the treatment of incurable neural and heart diseases and tissue damage. However, the survival of transplanted stem cells is reported to be low, reducing their therapeutic effects. The major causes of poor survival of stem cells in vivo are linked to anoikis, potential immune rejection, and oxidative damage mediating apoptosis. This review investigates novel methods and potential molecular mechanisms for stem cell preconditioning in vitro to increase their retention after transplantation in damaged tissues. Microenvironmental preconditioning (e.g., hypoxia, heat shock, and exposure to oxidative stress), aggregate formation, and hydrogel encapsulation have been revealed as promising strategies to reduce cell apoptosis in vivo while maintaining biological functions of the cells. Moreover, this review seeks to identify methods of optimizing cell dose preparation to enhance stem cell survival and therapeutic function after transplantation.

Keywords: aggregate formation; encapsulation; hydrogel; preconditioning; stem cells.

FIG. 1.

Molecular mechanisms of environmental preconditioning of stem cells. Chronic exposure to hypoxia prevents hypoxia-inducible factor (HIF)-1α degradation, by inhibition of its ubiquitination by prolyl hydrolxylase. HIF-1α stabilization reduces oxidative phosphorylation, leading to the opening of mitoK_ATP channels and consequently the activation of protein kinase C (PKC). PKC activates nuclear factor kappa beta (NFκβ) signaling, leading to the enhanced expression of antioxidant and anti-apoptotic proteins (MnSOD, Bcl-2, etc.). NFκβ also enhances trophic functions of the cells (i.e., secretion of VEGF, FGF, BDNF, etc.). The chronic exposure to oxidative stress (e.g., H₂O₂) induces a transient release of reactive oxygen species (ROS) from mitochondria, leading to the activation of extracellular signal-regulated kinases (ERK). ERK activation promotes the expression of anti-apoptotic proteins. Heat shock treatment promotes the expression of heat shock proteins (HSPs), which activate the phosphoinositide 3-kinase (PI3K)/AKT signaling. The PI3K/AKT signaling induces the expression of anti-oxidants, anti-apoptotic factors, and trophic factors. Nutrient deprivation activates mammalian target of rapamycin (mTOR) signaling which also leads to the activation of AKT. VEGF, vascular endothelial growth factor; FGF, fibroblast growth factor; BDNF, brain-derived neurotrophic factor; IGF, insulin-like growth factor; HGF, hepatocyte growth factor; TCA cycle, tricarboxylic acid cycle.

FIG. 2.

Stem cell aggregate formation as a preconditioning treatment. (A) Mechanism of stem cell aggregate formation. Stem cells organize and sort out the structure based on the degree of cadherin expression, according to the differential adhesion hypothesis. (B) Formation of stem cell aggregates promotes the secretion of extracellular matrix (ECM) and trophic factors, as well as creating a mildly hypoxic environment. Stem cell aggregation also could avoid anoikis, promote the expression of antioxidant and anti-apoptotic proteins, and enhance the trophic functions.

FIG. 3.

Stem cell encapsulation as a preconditioning treatment. (A) Liquid core/solid shell encapsulation promotes aggregate formation and provides mass transport of biomolecules and immune isolation. (B) Stem cell encapsulation in nonadhesive hydrogels promotes aggregate formation and provides mass transport of biomolecules and immune isolation. (C) Stem cell encapsulation in adhesive hydrogels (i.e., containing integrin- and MMP-binding sites) promotes stem cell adhesion and provides mass transport of biomolecules and immune isolation.