The tau isoform 1N4R confers vulnerability of MAPT knockout human iPSC-derived neurons to amyloid beta and phosphorylated tau-induced neuronal dysfunction.

Human tau protein, composed of six brain-specific isoforms, is a major driver of Alzheimer's disease (AD). The role of its isoforms however remains unclear and human AD models are scarce. We generated human MAPT- (tau-) knockout (KO) induced pluripotent stem cells (iPSC) using CRISPR/Cas9, differentiated these into glutamatergic neurons, and assessed isoform-specific functions of tau in these neurons. We used omic- approaches, live-cell imaging, subcompartmental analysis, and lentivirus-based reintroduction of specific tau isoforms to investigate isoform-mediated neuronal dysfunction in an AD model. Tau KO human iPSC-derived neurons showed decreased neurite outgrowth and axon initial segment length and, notably, resisted amyloid beta oligomer (AβO)-induced neuronal activity reduction. Introducing the 1N4R-tau isoform, but not other isoforms, confers AβO vulnerability and increases KxGS phosphorylation of tau, without altering neuronal activity or microtubule modifications. While tau KO impacts neuronal development and activity, tau-KO also confers resistance against AβO insult. 1N4R-tau likely mediates AβO-induced and phosphorylated tau toxicity, representing a novel prime therapeutic target for AD. Tau knockout alters neurite growth and axon initial segment formation in human neurons. Tau isoforms show differential axonal localization in human neurons. Tau depletion protects against amyloid beta oligomer (AβO)-mediated neurotoxicity. 1N4R tau mediates AβO-induced toxicity in human neurons.