Neurodegenerative diseases represent a significant unmet medical need in our aging society. There are no effective treatments for most of these diseases, and we know comparatively little regarding pathogenic mechanisms. Among the challenges faced by those involved in developing therapeutic drugs for neurodegenerative diseases, the syndromes are often complex, and small animal models do not fully recapitulate the unique features of the human nervous system. Human induced pluripotent stem cells (iPSCs) are a novel technology that ideally would permit us to generate neuronal cells from individual patients, thereby eliminating the problem of species-specificity inherent when using animal models. Specific phenotypes of iPSC-derived cells may permit researchers to identify sub-types and to distinguish among unique clusters and groups. Recently, iPSCs were used for drug screening and testing for neurologic disorders including Alzheimer’s disease (AD), amyotrophic lateral sclerosis (ALS), spinocerebellar atrophy (SCA), and Zika virus infection. However, there remain many challenges still ahead, including how one might effectively recapitulate sporadic disease phenotypes and the selection of ideal phenotypes and for large-scale drug screening. Fortunately, quite a few novel strategies have been developed that might be combined with an iPSC-based model to solve these challenges, including organoid technology, single-cell RNA sequencing, genome editing, and deep learning artificial intelligence. Here, we will review current applications and potential future directions for iPSC-based neurodegenerative disease models for critical drug screening.

Amyotrophic lateral sclerosis (ALS) is a progressive nervous system disease that causes motor neuron (MN) degeneration and results in patient death within a few years. To recapitulate the cytopathies of ALS patients’ MNs, SOD1^G85R mutant and corrected SOD1^G85G isogenic-induced pluripotent stem cell (iPSC) lines were established. Two SOD1 mutant ALS (SOD1^G85R and SOD1^D90A), two SOD1 mutant corrected (SOD1^G85G and SOD1^D90D), and one sporadic ALS iPSC lines were directed toward MNs. After receiving ~90% purity for MNs, we first demonstrated that SOD1^G85R mutant ALS MNs recapitulated ALS-specific nerve fiber aggregates, similar to SOD1^D90A ALS MNs in a previous study. Moreover, we found that both SOD1 mutant MNs showed ALS-specific neurite degenerations and neurotransmitter-induced calcium hyperresponsiveness. In a small compound test using these MNs, we demonstrated that gastrodin, a major ingredient of Gastrodia elata, showed therapeutic effects that decreased nerve fiber cytopathies and reverse neurotransmitter-induced hyperresponsiveness. The therapeutic effects of gastrodin applied not only to SOD1 ALS MNs but also to sporadic ALS MNs and SOD1^G93A ALS mice. Moreover, we found that coactivation of the GSK3β and IGF-1 pathways was a mechanism involved in the therapeutic effects of gastrodin. Thus, the coordination of compounds that activate these two mechanisms could reduce nerve fiber cytopathies in SOD1 ALS MNs. Interestingly, the therapeutic role of GSK3β activation on SOD1 ALS MNs in the present study was in contrast to the role previously reported in research using cell line- or transgenic animal-based models. In conclusion, we identified in vitro ALS-specific nerve fiber and neurofunctional markers in MNs, which will be useful for drug screening, and we used an iPSC-based model to reveal novel therapeutic mechanisms (including GSK3β and IGF-1 activation) that may serve as potential targets for ALS therapy.

Retinal pigmented epithelial (RPE) cells possess high mitochondria content for energy production, which is required for phagocytosis and vision cycle metabolism. The mitochondrial integrity in RPE cells helps the homeostasis of photoreceptor turnover and prevents retina aging and degeneration. Mitochondrial transplantation benefits the recovery of several acute inflammatory diseases, leading us to investigate the effects of mitochondrial transplantation on retina degeneration. Allogeneic mitochondria were isolated and delivered into the vitreous chamber in the Royal College of Surgeons (RCS) rats, which exhibit inherited and early-onset retina degeneration. The progress of retina degeneration was examined with optical coherence tomography (OCT) and visual evoked potential (VEP) to determine the retina thickness and integrity of afferent electrical signals from affected eyes, respectively. We found that mitochondria engraftment moderately attenuated the degeneration of retinal layers in RCS rats by histological examination. This result was consistent with the OCT measurement of retina thickness around the optic disc. The VEP analysis revealed that the peak one (N1) latency, representing the arriving time of electrical impulse from the retina to cortex, was substantially maintained as the normal value after the mitochondrial transplantation. This result suggests that the intra-vitreous transplanted mitochondria ameliorate the degeneration of photoreceptors in RCS rats and might be potential for clinical application.

Human pluripotent stem cell (hPSC)-derived motor neurons (MNs) act as models for motor neuron diseases (MNDs), such as amyotrophic lateral sclerosis (ALS) or spinal muscular atrophy. However, the MN differentiation efficiency and viability following cryopreservation require further development for application in large-scale studies and drug screening. Here, we developed a robust protocol to convert hPSCs into MN cryopreservation stocks (hPSCs were converted into >92% motor neural progenitors and >91% MNs). Near-mature MNs were cryopreserved at a high thawing survival rate and 89% MN marker expression on day 32. Moreover, these MNs exhibited classical electrophysiological properties and neuromuscular junction (NMJ) formation ability within only 4–6 days after thawing. To apply this platform as an MND model, MN stocks were generated from SOD1^G85R, SOD1^G85G isogenic control, and sporadic ALS hPSC lines. The thawed ALS MNs expressed ALS-specific cytopathies, including SOD1 protein aggregation and TDP-43 redistribution. Thus, a stable and robust protocol was developed to generate ready-to-use cryopreserved MNs without further neuronal maturation processes for application in MND mechanistic studies, NMJ model establishment, and large-scale drug screening.

The neuromuscular junction (NMJ) is essential for transmitting signals from motor neurons (MNs) to skeletal muscles (SKMs), and its dysfunction can lead to severe motor disorders. However, our understanding of the NMJ is limited by the absence of accurate human models. Although human induced pluripotent stem cell (iPSC)-derived models have advanced NMJ research, their application is constrained by challenges such as limited differentiation efficiency, lengthy generation times, and cryopreservation difficulties. To overcome these limitations, we developed a rapid human NMJ model using cryopreserved MNs and SKMs derived from iPSCs. Within 12 days of coculture, we successfully recreated NMJ-specific connectivity that closely mirrors in vivo synapse formation. Using this model, we investigated amyotrophic lateral sclerosis (ALS) and replicated ALS-specific NMJ cytopathies with SOD1 mutant and corrected isogenic iPSC lines. Quantitative analysis of 3D confocal microscopy images revealed a critical role of MNs in initiating ALS-related NMJ cytopathies, characterized by alterations in the volume, number, intensity, and distribution of acetylcholine receptors, ultimately leading to impaired muscle contractions. Our rapid and precise in vitro NMJ model offers significant potential for advancing research on NMJ physiology and pathology, as well as for developing treatments for NMJ-related diseases.

Traditional Chinese medicine has been practiced for centuries in East Asia. Herbs are used to maintain health and cure disease. Certain Chinese herbs are known to protect and improve the brain, memory, and nervous system. To apply ancient knowledge to modern science, some major natural therapeutic compounds in herbs were extracted and evaluated in recent decades. Emerging studies have shown that herbal compounds have neuroprotective effects or can ameliorate neurodegenerative diseases. To understand the mechanisms of herbal compounds that protect against neurodegenerative diseases, we summarize studies that discovered neuroprotection by herbal compounds and compound-related mechanisms in neurodegenerative disease models. Those compounds discussed herein show neuroprotection through different mechanisms, such as cytokine regulation, autophagy, endoplasmic reticulum (ER) stress, glucose metabolism, and synaptic function. The interleukin (IL)-1β and tumor necrosis factor (TNF)-α signaling pathways are inhibited by some compounds, thus attenuating the inflammatory response and protecting neurons from cell death. As to autophagy regulation, herbal compounds show opposite regulatory effects in different neurodegenerative models. Herbal compounds that inhibit ER stress prevent neuronal death in neurodegenerative diseases. Moreover, there are compounds that protect against neuronal death by affecting glucose metabolism and synaptic function. Since the progression of neurodegenerative diseases is complicated, and compound-related mechanisms for neuroprotection differ, therapeutic strategies may need to involve multiple compounds and consider the type and stage of neurodegenerative diseases.