Improving drug development for Alzheimer’s disease and other neurodegenerative diseases by using human induced stem cells
Drug development for neurodegenerative diseases has largely depended on the availability of humanized animal models, as well as cell lines and culture systems derived from animals. Such models attempt to simulate complex human conditions, but their success in therapeutics has been under extensive debate. Unfortunately, the availability of optimal human material for such work is negatively impacted by the difficulty in sourcing neuronal tissue from patients. Undeniably, traditional models, such as rodents, have led the progress in uncovering the underlying mechanisms behind neurodegenerative disorders for decades.
However, it has become increasingly evident that there are key differences between rodent and human neurobiology which impact our ability to translate drug efficacy from rodent systems to humans. For example, many rodent models only exhibit Alzheimer’s Disease (AD) pathophysiology after a large accumulation of mutations associated with the human disease, while they still lack important phenotypes, such as the extensive loss of neurons, that is characteristic in humans (Drummond & Wisniewski, 2017) (LaFerla & Green, 2012). What is more, a species-specific genetic background can limit their pharmacological application in many cases. Such issues are thought to be the underlying factors behind the limited success rates of drugs developed through animal model testing, reported at less than 12% at clinical trial stages (Paul et al., 2010).
Progress in stem cell research and regenerative biomedicine provide a new path for carrying out drug testing on human cells. Initially, the potential of this work was limited by the difficulties associated with sourcing and working with human embryonic stem cells (ESC), such as the consideration of ethical aspects and the difficulty of accessing patient specific material. However, the discovery of the “Yamanaka factors” revolutionised the field. Takahashi, Yamanaka and colleagues showed that only four transcription factors can be used to reprogram fibroblasts and blood cells to pluripotent cells that are highly similar to ESC (Takahashi et al., 2007)(Takahashi & Yamanaka, 2006). Human induced pluripotent stem cells (iPSC) have since provided an unprecedented opportunity for drug discovery, bringing down multiple barriers in acquiring efficient human cell culture systems (Shi et al., 2016). Most importantly, it is easy to source the material needed for iPSC reprogramming and it has become possible to acquire patient specific cells, paving the road to personalised medicine, while avoiding the ethical issues of ESC (Kondo et al., 2017) (Shi, Yanhong Inoue, Haruhisa Wu, Joseph C. Yamanaka, 2017).
In recent years, the use of iPSC has powered cutting-edge developments on AD research and drug discovery. Currently there is a focus on the development of complex combinatorial systems based on iPSC-derived cell populations that include 3D cultures and mini-brain organoids that provide simplified proxies of the human brain (Tan et al., 2021) (Choi et al., 2016). Nevertheless, iPSC systems not only provide a research and development platform, but are also potential therapies that could be administered through the transplantation of stem cells or derived cell populations in patients (Hayashi et al., 2020).
There is now available multiple protocols for the differentiation of iPSC towards various specific neuronal fates, including CA3 hippocampal neurons (Sarkar et al., 2018), cerebral cortical neurons (Shi et al., 2012) , medium spiny neurons (Naujock et al., 2016) and dopaminergic neurons (Zhang et al., 2014) (Mahajani et al., 2019). In addition, iPSC-derived neurons, astrocytes, and microglia derived from familial AD patients have been shown to successfully replicate AD-related pathophysiology, such as tau phosphorylation, endosome enlargement or impaired amyloid beta clearance, which are associated with APOE polymorphisms (Lin et al., 2018) (Wang et al, 2018). Genetic engineering experiments have also demonstrated the value of iPSC-derived neurons for both understanding and treating familial AD. For example, correction of PSEN1 and FAD1 mutations in induced neurons leads to a reversal of amyloid beta phenotypes. The above also highlight the potential of iPSC-derived systems for personalized drug screening, targeted to the genetic background of different patients (Kondo et al., 2017). Finally, while much progress has been made in the field, there is still vibrant ongoing work on optimizing iPSC-derived systems, such as obtaining neuronal populations with high purity and specificity (Kondo et al., 2017), as well as defining small molecules that could control cell reprogramming, differentiation or survival of iPSC, increasing their potential as druggable agents.
In the scope of EuroNeurotrophin, iPSC technology presents an opportunity for testing the action and efficacy of promising candidates on a human cell system. Hence, efforts have focused on testing the neurogenic or neuroprotective action of selected microneurotrophin compounds on human iPSC-derived cortical neurons. This innovative approach can help us assess a new way to test compounds for their ability to induce neurogenesis on human tissue by the selective activation of neurotrophin receptor pathways, inducing specific differentiation of precursor cells and decreasing cell death of human neuronal populations. This work provides a testing platform that is much more translational for potential human applications and promotes a key goal of the consortium, adhering to the 3R principle: Replacement, Reduction and Refinement in animal research.
About the Author
ESR12: Despoina Charou
Despoina is hosted at the Foundation for Research and Technology Hellas and her work focuses on testing synthetic and natural neurotrophin mimetics on specific neurotrophin-dependent cellular populations. The molecular mechanisms and functions leading to an increase in adult neurogenesis through the induction of endogenous neural stem cell proliferation and survival, which is affected in AD, will be assessed.