Modelling neurodegenerative diseases in human iPSC-derived neurons
Neurodegeneration is a pathology shared by a varied class of diseases, and many of the mutations that are known to cause such diseases have been linked to protein aggregation and autophagy dysfunction. Improvements to gene editing and neuronal differentiation strategies have enabled the derivation of in vitro disease models using human iPSC-derived neurons to provide a more accurate understanding of how disease mutations affect neuronal health. I first sought to model the polyglutamine disease spinal and bulbar muscular atrophy (SBMA), detailed in Chapter 4. Using iPSCs derived from a healthy donor and an SBMA patient, the CAG repeat of the endogenous androgen receptor (AR) gene was CRISPR-edited to encode a series of lengths or an early stop codon. However, AR expression was silenced upon transcription factor-mediated differentiation to a lower motor neuron-like state, and chemical differentiation prevented ligand-induced AR nuclear translocation. Deriving the cell lines in this work highlighted that purification of transgenic cells is a key bottleneck to gene editing. I therefore adapted a synthetic marker gene that presents a streptavidin binding peptide (SBP) tag on the extracellular membrane, detailed in Chapter 3. Expression of this tag in iPSCs enabled transient fluorophore staining and effective sorting of mixed populations with magnetic streptavidin beads. Finally, Chapter 5 establishes L1CAM as a novel autophagy modulator; iPSC-derived neurons showed that reduction of the L1CAM transcript with shRNAs, but not genetic knockout of the L1CAM protein, is sufficient to reduce transcription of the ATG8 gene family, which are core components of macroautophagy. This work exemplified both the strengths and weaknesses of iPSC-derived neurons; namely, they are tractable and able to recapitulate neural phenotypes, but deriving new model lines requires a high initial investment, so adequate proof-of-concept is crucial.