Motor neuron derivation from human embryonic and induced pluripotent stem cells: experimental approaches and clinical perspectives

Abstract

Motor neurons are cells located in specific areas of the central nervous system, such as brain cortex (upper motor neurons), brain stem, and spinal cord (lower motor neurons), which maintain control over voluntary actions. Motor neurons are affected primarily by a wide spectrum of neurological disorders, generally indicated as motor neuron diseases (MNDs): these disorders share symptoms related to muscular atrophy and paralysis leading to death. No effective treatments are currently available. Stem cell-derived motor neurons represent a promising research tool in disease modeling, drug screening, and development of therapeutic approaches for MNDs and spinal cord injuries. Directed differentiation of human pluripotent stem cells - human embryonic stem cells (hESCs) and human induced pluripotent stem cells (hiPSCs) - toward specific lineages is the first crucial step in order to extensively employ these cells in early human development investigation and potential clinical applications. Induced pluripotent stem cells (iPSCs) can be generated from patients' own somatic cells (for example, fibroblasts) by reprogramming them with specific factors. They can be considered embryonic stem cell-like cells, which express stem cell markers and have the ability to give rise to all three germ layers, bypassing the ethical concerns. Thus, hiPSCs constitute an appealing alternative source of motor neurons. These motor neurons might be a great research tool, creating a model for investigating the cellular and molecular interactions underlying early human brain development and pathologies during neurodegeneration. Patient-specific iPSCs may also provide the premises for autologous cell replacement therapies without related risks of immune rejection. Here, we review the most recent reported methods by which hESCs or iPSCs can be differentiated toward functional motor neurons with an overview on the potential clinical applications.

Figure 1

Schematic representation of the role of morphogens during neural tube formation in vivo. Color gradients are indicative of the expression levels of each morphogen. Bone morphogen protein (BMP) can be found in high concentrations in the dorsal part of the neural tube (light green): its levels decrease along the ventral part. In contrast, Sonic hedgehog (Shh) is more concentrated in the ventral part (orange), but it is not expressed in the dorsal one. Fibroblast growth factor (FGF) is highly expressed in the anterior (purple) and posterior (dark green) parts of the neural tube. Retinoic acid (RA) levels of expression decrease in the posterior part (light blue), where high concentrations of both FGF and Wnt can be found. MN, motor neuron.

Figure 2

Generation of human motor neurons from human embryonic and induced pluripotent stem cells. A schematic representation for motor neuron (MN) generation in vitro is shown. The first step in pluripotent stem cell - human embryonic stem cell (hESC) and human induced pluripotent stem cell (hiPSC) - differentiation is the attainment of embryoid bodies (EBs) in suspension or neural rosettes in adhesion conditions. These neural precursors can be successfully differentiated in MNs (characterized by specific features) with different multistage experimental protocols. hESC- or iPSC-derived MNs are a promising research tool to model and study in vitro pathological mechanisms underlying MN diseases in humans. These MNs could also represent an appealing source for autologous cell replacement. RA, retinoic acid; Shh, Sonic hedgehog.