Targeting intracellular tau with a gene-encoded single-chain antibody promotes neuronal homeostasis and ameliorates tau pathology

The intraneuronal aggregation of tau is a key driver of pathogenesis in tauopathies such as Alzheimers disease. Passive immunotherapy is a promising strategy for targeting tau, with several tau-specific antibodies having demonstrated the ability to reduce tau pathology and improve behavioural deficits in tau transgenic mouse models. Despite preclinical promise, however, conventional antibodies have limited access to the cell cytoplasm where tau pathology itself originates and accumulates to cause downstream neuronal dysfunction. As such, conventional antibodies are typically limited to targeting extracellular tau, failing to address the primary site of tau pathogenesis. This challenge can be overcome by intracellular antibodies or intrabodies, small antibody fragments that can be expressed within cells to target intracellular antigens like tau. Here, we have generated a single-chain variable fragment derived from the N-terminal tau-specific antibody, RNJ1, and investigated its potential as an intrabody to reduce tau pathology and restore neuronal function in a tau transgenic mouse model. The RNJ1 intrabody successfully engaged intracellular tau and reduced total tau and phosphorylated tau inclusions in brains of tau transgenic mice. Furthermore, treatment with the RNJ1 intrabody in female tau transgenic mice induced restoration of various protein pathways important for cellular homeostasis, thus promoting the restoration of neuronal function. Our findings underscore the therapeutic utility of targeting intracellular tau in disease, providing novel insights into the potential mechanisms by which intrabodies ameliorate tau pathology.