The Breakthroughs and Caveats of Using Human Pluripotent Stem Cells in Modeling Alzheimer's Disease

Abstract

Modeling Alzheimer's disease (AD) using human-induced pluripotent stem cells (iPSCs) is a field now spanning 15 years. Developments in the field have shown a shift in using simple 2D cortical neuron models to more advanced tri-cultures and 3D cerebral organoids that recapitulate more features of the disease. This is largely due to development and optimization of new cell protocols. In this review, we highlight recent major breakthroughs in the AD field and the implications this has in modeling AD using iPSCs (AD-iPSCs). To date, AD-iPSCs have been largely used to recapitulate and study impaired amyloid precursor protein (APP) processing and tau phosphorylation in both familial and sporadic AD. AD-iPSCs have also been studied for varying neuronal and glial dysfunctions. Moreover, they have been useful for discovering new molecular mechanisms, such as identifying proteins that bridge APP processing with tau phosphorylation and for identifying molecular pathways that bridge APP processing dysfunction with impaired cholesterol biosynthesis. Perhaps the greatest use of AD-iPSCs has been in discovering compounds via drug screening, that reduce amyloid beta (Aβ) in neurons, such as the anti-inflammatory compound, cromolyn, and antiparasitic drugs, avermectins. In addition, high content screening using AD-iPSCs has led to the identification of statins that can reduce levels of phosphorylated tau (p-Tau) in neurons. Some of these compounds have made it through to testing in human clinical trials. Improvements in omic technologies including single cell RNA sequencing and proteomics as well as advances in production of iPSC-cerebral organoids and tri-cultures is likely to result in the further discovery of new drugs and treatments for AD. Some caveats remain in the field, including, long experimental conditions to create mature neurons, high costs of media that limit research capabilities, and a lack of reproducibility using current iPSC-cerebral organoid protocols. Despite these current limitations, AD-iPSCs remain an excellent cellular model for studying AD mechanisms and for drug discovery.

Keywords: APP processing; Alzheimer’s disease; astrocytes; disease modelling; drug discovery; induced pluripotent stem cells; microglia; neurons; tau phosphorylation.

Figure 1

Highlighted major breakthroughs in the Alzheimer’s disease (AD) field over the last decade. New forms of AD have been identified including a sporadic form called LATE [2] and a familial form referred to as the Uppsala APP mutation [3] (A). New gene loci associated with development of AD have been identified, with a few of the recent loci from [4] and [5] shown here (B). Researchers have identified the structural insights of amyloid beta (Aβ) showing fibrils differ between familial and sporadic AD [6] (C). Progress has recently been made in finding treatments for AD including Lecanemab and Aducanumab, which have recently been approved and marketed in the US. Both target the removal of Aβ but have different preferences in their ability to remove either soluble protofibrils or insoluble fibers (D). Created with BioRender.com.

Figure 2

Modeling of Alzheimer’s disease using iPSCs. Human iPSCs are produced from fibroblasts and differentiated into neurons (A) that can be cultured in simple 2D conditions, triple cultures, or in 3D as organoids (B). Human AD-iPSCs have been used to study several different pathologies including amyloid precursor protein (APP) processing and increased amyloid beta 42 (Aβ42), tau phosphorylation and neuronal dysfunction (C). AD-iPSCs have also been used to screen and find candidate compounds that can reduce Aβ42 and tau (D). Created by BioRender.com.

Figure 3

Perturbed organelle and neuron function have been reported in both familial and sporadic AD-iPSC neurons. Mitochondrial dysfunction, reduced autophagy, perturbed numbers of synapses, decreased neurite lengths and branching as well as perturbed excitability have all been reported and reflect known phenotypes shown in the AD brain. However, not all iPSC neurons show these phenotypes and variabilities between cell lines and risk genes/mutations have been noted. Created with BioRender.com.