Jonathon Cummock

 Introduction: Alzheimer’s disease (AD) is a progressive, debilitating neuropsychiatric disorder characterized by multifaceted decline in cognitive and behavioral functions^{1, 11}. Key neuronal pathology includes amyloid-beta plaques and/or neurofibrillary tangles^{1, 11}. A well-known feature of AD is the marked reduction and death of cholinergic neurons in the basal forebrain with subsequent reduction in acetylcholine concentration³. Investigating stem cell therapy, scientists have generated cholinergic neurons from human induced pluripotent stem cells (hiPSC) for experimental transplantation in animal models^{4, 5, 6, 10}. Two highlighted studies using different derivation protocols resulting in functional and restorative neuronal grafts demonstrate a promising potential transplantation therapy to restore cognition^{4, 10}. Methods: Using a novel embryoid body non-adherent differentiation (NAdD) protocol, researchers generated basal forebrain cholinergic neurons (bfCN)¹⁰. Creating a physical 3D non-adherent differentiation system using chemically modified media to prevent cell adhesion and monolayer formation resulted in the production of secreted hedgehog ligand (SHH), promoting a ventral telencephalic phenotype expressing basal forebrain transcription factors. Validation of bfCN differentiation was performed with immunocytochemistry, PCR, gene expression, acetylcholine release, calcium imaging, and electrophysiology. Human embryonic stem cells were compared for control. hiPSC-derived neurospheres were experimentally transplanted into the medial septum of 6 adult rats. In another study, hiPSC-derived cholinergic neurons were cultured using an embryoid body adherent monolayer differentiation protocol⁴. They were subsequently transplanted into a PDAPP transgenic mouse, differentiated to cholinergic neurons, and ultimately harvested for analysis with PCR and immunofluorescence staining at various time points for characterization. Cognitive ability was tested using the Morris Water Maze. Results: With the NAdD protocol, researchers reproducibly generated high numbers of neurons, over 90% on average being ChAT-positive. The bfCNs also expressed high levels of p75^NTR protein, a key mediator in cholinergic signaling and hippocampal innervations in the basal forebrain^{2, 10}. Transplantation of the hiPSC-derived neurospheres into adult rats resulted in successful and viable grafts, with no evidence of teratoma formation. The hiPSC-derived bfCNs were proved to release acetylcholine and be electrically active with neuronal action potential firing. With the adherent differentiation protocol, transplantation of the hiPSC-derived cholinergic neurons restored spatial memory learning in the PDAPP transgenic mice^{4, 6}. Conclusion/Discussion: The successful transplantation and recapitulation of cholinergic neurons with the in-vivo models resulted in the improvement of cognitive function without teratoma formation, holding great promise and potential for future stem cell therapy^{4, 10}. Advances in understanding the transcriptional regulators of distinct neuronal populations along with disease specific environmental triggers are needed to ensure the safety of stem cell transplantation therapy^{7, 8, 9}.

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