Functions of Tau protein in human neural cells

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Project Summary/Abstract Tau protein, encoded by the MAPT gene, is a neuronally enriched protein with an established role as a microtubule-binding protein. Abnormal accumulation of tau protein is a neuropathological hallmark of several neurodegenerative diseases, including Alzheimer’s disease (AD) and frontotemporal dementia (FTD). Dominant mutations in the MAPT gene are present in inherited FTD, indicating that tau is causal in neurodegenerative disease. However, whether these mutations lead to loss-of-function, gain-of-function or the acquisition of a novel function is unknown. Furthermore, although there are no mutations in MAPT that cause inherited AD, how alterations in tau function contribute to dysfunction in neural cells and pathological aggregation of tau protein are still incompletely understood. These issues have not been resolved, largely because the functions of the tau protein have not been conclusively determined. While highly expressed in neurons, tau is expressed in other CNS cell types and has been linked to cellular localizations ranging from the nucleolus to the plasma membrane. Furthermore tau-interacting proteins display a range of cellular functions including gene regulation, membrane transport, RNA binding and metabolism and cytoskeletal elements. Together this suggests a role for tau protein in central nervous system cells beyond microtubule stabilization. Many studies have examined the effects of pathological tau, but only a limited number of studies have investigated the cellular functions of wild type, endogenous tau. Examining cellular phenotypes in the absence of tau is one approach to understand its normal function. To date, most studies have studied tau deficiency using rodent models or transformed human cell lines and these studies show various and sometimes conflicting results. In this study, we will use our expertise in human induced pluripotent stem cell technology to study loss of tau function in human neural cultures and cortical organoids. We will use CRISPR/Cas9 to disrupt the human tau reading frame, generating tau-KO cell lines. We will then pursue both unbiased transcriptomic profiling and hypothesis-based experiments examining molecular and physiological consequences in neurons and astrocytes deficient in tau expression. We have generated pilot data indicating a strong up-regulation of pathways involved in neuroimmune function in tau-KO cortical cultures compared to isogenic WT controls and we have validated our findings with shRNAs targeting MAPT. Our proposed experiments will use hiPSC-derived neurons and astrocytes to test cell autonomous and non-cell autonomous responses to tau deficiency. These tau deficiency phenotypes include altered granulostasis and the accumulation of double-stranded RNA, as suggested by our preliminary studies. We will also assess electrophysiological function of tau deficient neurons cultured with wild-type astrocytes and wild-type neurons cultured with tau deficient astrocytes. To our knowledge, there are no reports of tau-KO in hiPSC-derived neural cell types, therefore the data generated by this study has the potential to significantly contribute to understanding the role of tau in the central nervous system.