Exploring Motor Neuron Diseases Using iPSC Platforms

Abstract

The degeneration of motor neurons is a pathological hallmark of motor neuron diseases (MNDs), but emerging evidence suggests that neuronal vulnerability extends well beyond this cell subtype. The ability to assess motor function in the clinic is limited to physical examination, electrophysiological measures, and tissue-based or neuroimaging techniques which lack the resolution to accurately assess neuronal dysfunction as the disease progresses. Spinal muscular atrophy (SMA), spinal and bulbar muscular atrophy (SBMA), hereditary spastic paraplegia (HSP), and amyotrophic lateral sclerosis (ALS) are all MNDs with devastating clinical outcomes that contribute significantly to disease burden as patients are no longer able to carry out normal activities of daily living. The critical need to accurately assess the cause and progression of motor neuron dysfunction, especially in the early stages of those diseases, has motivated the use of human iPSC-derived motor neurons (hiPSC-MN) to study the neurobiological mechanisms underlying disease pathogenesis and to generate platforms for therapeutic discovery and testing. As our understanding of MNDs has grown, so too has our need to develop more complex in vitro models which include hiPSC-MN co-cultured with relevant non-neuronal cells in 2D as well as in 3D organoid and spheroid systems. These more complex hiPSC-derived culture systems have led to the implementation of new technologies, including microfluidics, multielectrode array, and machine learning which offer novel insights into the functional correlates of these emerging model systems.

Keywords: ALS; hereditary spastic paraplegia; motor neuron disease; spinal and bulbar muscular atrophy; spinal muscular atrophy; stem cell.

Graphical Abstract

Figure 1.

Motor neuronopathies and their corresponding levels of MN damage. Spinal muscular Atrophy (SMA) primarily affects spinal MNs and their subsequent innervation of muscle. Human iPSCs have been created to model SMA type 1. Spinal and bulbar muscular atrophy (SBMA) also affects spinal MNs with sparing of CSMNs. CAG trinucleotide expansion repeats in the androgen receptor produce the phenotype. Hereditary spastic paraplegia (HSP) primarily affects CSMN with iPSC developed from a few of the more than 80 genetic subtypes. Amyotrophic lateral sclerosis (ALS) variably affects both CSMN and spinal MNs producing wide ranges in phenotypic presentation. While hiPSC-derived spinal MNs have been made for numerous FALS and SALS subtypes, far fewer models of CSMN biology exist. Green (normal MN), Red (affected MN subtype). Abbreviations: CSMNs, corticospinal motor neurons; FALS, familial amyotrophic lateral sclerosis; hiPSC, human induced pluripotent stem cells; iPSC, induced pluripotent stem cells; MNs, motor neurons; SALS, sporadic amyotrophic lateral sclerosis.

Figure 2.

iPSC-based platforms for motor neuron disease modeling. (A) Healthy control hiPSC-MN (green) can be cultured either alone or with non-neuronal cell types (astrocytes in this case) carrying disease mutations (orange) to establish the significance of non-cell-autonomous effects on cell survival. Neurite outgrown from MNs can also be measured anatomically and temporally using microfluidic chambers and longitudinal imaging. (B) Organoids and spheroids are being used to study either individual anatomical regions (cerebral) of MND pathology or combined to create entire pathways involved in disorders like ALS (corticospinal-muscle). (C) Increasingly sophisticated methods for manipulating (optogenetics) and analyzing electrophysiological of hiPSC-MN (MEA) or force-generating properties of hiPSC-muscle (micropillars) are being used. (D) Automated cell counting and machine learning strategies allow for longitudinal and unbiased analyses of cell survival, morphology, and molecular pathway investigation. Abbreviations: ALS, amyotrophic lateral sclerosis; hiPSC-MN, human iPSC-derived motor neurons; iPSC, induced pluripotent stem cells; MND, motor neuron disease; MNs, motor neurons.