Stem Cell- and Cell-Based Therapies for Ischemic Stroke

Abstract

Stroke is the second cause of disability worldwide as it is expected to increase its incidence and prevalence. Despite efforts to increase the number of patients eligible for recanalization therapies, a significant proportion of stroke survivors remain permanently disabled. This outcome boosted the search for efficient neurorestorative methods. Stem cells act through multiple pathways: cell replacement, the secretion of growth factors, promoting endogenous reparative pathways, angiogenesis, and the modulation of neuroinflammation. Although neural stem cells are difficult to obtain, pose a series of ethical issues, and require intracerebral delivery, mesenchymal stem cells are less immunogenic, are easy to obtain, and can be transplanted via intravenous, intra-arterial, or intranasal routes. Extracellular vesicles and exosomes have similar actions and are easier to obtain, also allowing for engineering to deliver specific molecules or RNAs and to promote the desired effects. Appropriate timing, dosing, and delivery protocols must be established, and the possibility of tumorigenesis must be settled. Nonetheless, stem cell- and cell-based therapies for stroke have already entered clinical trials. Although safe, the evidence for efficacy is less impressive so far. Hopefully, the STEP guidelines and the SPAN program will improve the success rate. As such, stem cell- and cell-based therapy for ischemic stroke holds great promise.

Keywords: clinical trials; exosomes; extracellular vesicles; ischemic stroke; mesenchymal stem cells; miRNAs; neural stem cells; neuroregeneration.

Figure 1

Various mechanisms contribute to cell loss in the penumbral area. Reactive oxygen species (ROS) and increased cytosolic calcium damage mitochondria, and lead to opening of the mitochondrial permeability transition pore (MPTP) and release of cytochrome c and apoptosis inducing factor (AIF). Increased calcium also activates calpains, which cleave pro-apoptotic factors such as Bid and Bax, promoting their mitochondrial translocation and further permeabilization of the mitochondrial membrane. Cytochrome c activates the caspase cascade and leads to caspase-dependent apoptosis, while AIF and second mitochondrion-derived activator of caspase/direct inhibitor of apoptosis-binding protein with low pI (SMAC/DIABLO) activate caspase-independent apoptosis. ROS also damage DNA and activate poly(ADP-ribose) polymerases (PARP), leading to the production of poly(ADP-ribose) or PAR, which promotes the nuclear translocation of AIF and chromatin degradation, leading to parthanatos. Another consequence of ROS production is microglial activation, which together with excessive stimulation of N-methyl-D-aspartate receptors (excitotoxicity) activate the membrane death receptors, leading to caspase cleavage, as well as to receptor interacting kinase 1 (RIP1) phosphorylation, followed by the phosphorylation of RIP3 and mixed lineage kinase domain-like (MLKL), leading to oligomerization of phosphorylated MLKL at plasma membranes and cell rupture (necroptosis). Acidification, common in the penumbral area of cerebral infarction, promotes the association of acid-sensing ion channel 1a (ASIC1a) with RIP1 and the activation of the latter. In addition, an acidic environment, together with calpain activation, permeabilizes the lysosomal membrane and allows for the release of lysosomal proteases, cathepsins, and hydrolases into the cytosol, leading to autolysis.

Figure 2

Embryonic pluripotent stem cells (ESCs) are derived from the inner layer of the blastocyst, Neural multipotent stem cells are obtained from human fetal cortex, mesencephalon, or spinal cord. Mesenchymal multipotent stem cells can be harvested from bone marrow, umbilical cord and placenta, or from adipose tissue. There is also the possibility of obtaining induced pluripotent stem cells (iPSCs) via transduction of the four OSKM genes: octamer-binding transcription factor 4 (Oct4), sex-determining region Y-box 2, (Sox2), the Krupellike factor 4 (Klf4), and the avian myelocytomatosis viral oncogene homolog (c-myc). Once harvested, stem cells are cultured in special culture medium, where they release exosomes carrying proteins, DNA, messenger RNA (mRNA), microRNAs (miRNA), and mitochondrial DNA (mtDNA). Once delivered to the brain by various routes, they differentiate into neurons, astrocytes, and oligodendrocytes, and release various growth factors (such as brain derived neurotrophic factor-BDNF or vascular endothelial growth factor-VEGF) and anti-inflammatory cytokines (interleukins IL-6, IL-10, or tumor necrosis factor β), which modulate neuroinflammation and promote angiogenesis, neurogenesis, neural differentiation, and synaptogenesis.