HUMAN CORTICAL BRAIN ORGANOIDS EXPRESSING P301L TAU REVEAL EARLY TAU-DRIVEN NETWORK HYPEREXCITABILITY IN FTDP-17

Microtubule-associated protein tau is essential for neuronal integrity, yet pathological tau drives cognitive decline in primary tauopathies such as Frontotemporal Dementia with Parkinsonism linked to chromosome 17 (FTDP-17). Among MAPT mutations, the P301L variant, accounting for ~25–40% of familial FTDP-17 cases, provides a genetic model for investigating early disease mechanisms. It preferentially affects excitatory neurons and is associated with synaptic dysfunction and network disorganisation. Hyperphosphorylated tau has been linked to epileptiform activity, suggesting that tau-driven alterations in neuronal activity (NA) precede overt neurodegeneration. Evidence indicates a bidirectional relationship between tau and NA: endogenous tau modulates neuronal excitability, while neuronal activity promotes tau phosphorylation and extracellular release; however, how pathogenic tau mutations shape NA at presymptomatic stages remains unclear. To address this, we developed an in vitro platform using human cortical brain organoids (hCBOs) derived from embryonic stem cells. Organoids were transduced with adeno-associated viruses expressing either P301L-tau or full-length non-mutated human tau (2N4R), and tau pathology and network activity were assessed. P301L-tau expression increased the 4R/3R tau ratio, elevated hyperphosphorylated tau levels (AT8, pSer422), and induced tau aggregation detected by MC1 and Gallyas staining. Functionally, P301L-expressing hCBOs showed significantly increased mean firing rates and reduced interspike intervals compared to controls, while overall network organisation remained preserved. Notably, these organoids exhibited reverberating super bursts indicative of network hyperexcitability. These findings demonstrate that pathogenic tau mutations drive functional alterations in NA, generating hyperexcitable networks susceptible to seizure-like dynamics, and establish hCBOs as a platform for dissecting tauopathy mechanisms and therapeutic targeting.