Preprint: a human neuron cell model traces tau from seeding to synaptic failure
Preprint: a human neuron cell model traces tau from seeding to synaptic failure
On September 12, a preprint described two neuron models derived from human iPSCs, reprogrammed cells that can be grown into neural tissue in the laboratory. Laboratory-grown tau filaments were introduced into the cultures, where they served as seeds, triggering the accumulation of pathological tau inside the neurons.
Tau protein stabilizes microtubules, the internal filaments along which neurons transport cargo to their processes and synapses. In tau pathology, phosphate groups attach to the protein, causing it to change shape and assemble into insoluble filaments. The authors built two cultures to track the early steps of this process using normal human tau. In the first, neurons retained their endogenous tau and accumulated aggregates slowly. In the second, neurons additionally produced a normal tau variant; aggregation developed over 7–28 days, making this system suitable for biochemical assays, structural imaging, and compound screening.
Phosphoproteomics, the simultaneous measurement of phosphate marks across many proteins, pointed to MARK2, a kinase that places such marks on tau. When the researchers substituted individual amino acids in tau, substitutions at sites acted on by MARK2 and GSK3 altered aggregation, while substitutions at the site recognized by the AT8 antibody did not. The authors therefore classify AT8 as a late marker of the process.
Cryo-electron tomography, an imaging method that reveals structure inside a frozen cell, detected tau fibrils in synaptic regions near vesicles that carry chemical signals between neurons. In seeded cultures, presynaptic proteins assembled into unusual clusters, and multielectrode arrays recorded less network activity. In this cell system, tau fibrils were accompanied by synaptic remodeling and reduced network activity.
On 384-well plates the authors screened approximately 800 compounds. Around 80 altered aggregation; 80% of the selected active compounds passed a secondary screen using the MC1 antibody, which recognizes tau that has already changed shape and assembled into aggregates rather than merely carrying a phosphate mark. Compounds inhibiting PI3K, mTOR, and GSK3, proteins through which cells relay signals, reduced aggregates and abnormal clustering of a synaptic protein in this cell model. A molecular FRET biosensor that reports the onset of tau assembly showed that PI3K and mTOR inhibitors weakened the early signal after seeding. The authors attribute this either to reduced uptake of seeds by the cells or to weaker initiation of aggregation. GSK3 blockade reduced tau phosphorylation at sites linked to aggregation. This model allows interventions to be tested separately at each step, from tau seeding through synaptic activity.