Inhibition of pathogenic tau signaling via blocking of the phosphatase-activating domain by novel small molecules

Tau pathology is a major feature of Alzheimer's disease (AD) and multiple other adult-onset neurodegenerative diseases. Aberrant exposure of an N-terminal phosphatase-activating domain (PAD) is characteristic of pathological tau, representing a toxic gain of function. Exposure of the PAD in pathological tau leads to dysregulation of protein phosphatase 1/glycogen synthase kinase 3 (PP1/ GSK3β) signaling, inhibition of fast axonal transport, synaptic dysfunction, and altered transcription, along with other pathological consequences. Previous studies showed that TNT1, an antibody against the PAD, blocked toxicity of pathogenic forms of tau.

In this article, we describe a high-throughput screen for small molecules that block TNT1 binding to the PAD in an AlphaLISA screen and bind specifically to the PAD in surface plasmon resonance assays. Candidate PAD ligands (PADis) were identified, and initial biochemical and biophysical optimization produced PADis with increased affinity and selectivity. Three candidate PADis were evaluated in neuronal (rat E18 embryonic cortical neurons) and non-neuronal cells (HEK293T human embryonic kidney cells) using a nano-bioluminescence resonance energy transfer (nanoBRET) assay to assess PP1 binding and cell toxicity.

All three compounds prevented PP1 binding to PAD and neurite degeneration due to pathological tau in primary cultured cortical neurons. The final candidates had an IC₅₀ value between 10 and 20 nM in neurons with low cytotoxicity, CC₅₀ > 75 μM in primary cultured neurons, and 40-100 μM in non-neuronal cells. PADi treatment of primary cultured neurons transfected with pathogenic tau restored axonal growth and prevented neurodegeneration. These studies establish a novel approach to therapeutics for Alzheimer's disease and tauopathies.