Emerging experimental evidence suggests that the spread of tau pathology in the brain in Tauopathies reflects
the propagation of abnormal tau species along neuroanatomically connected brain areas. This propagation could occur
through a “prion-like” mechanism involving transfer of abnormal tau seeds from a “donor cell” to a “recipient cell” and
recruitment of normal tau in the latter to generate new tau seeds. This review critically appraises the evidence that the
spread of tau pathology occurs via such a “prion-like” mechanism and proposes a number of recommendations for
directing future research. Recommendations for definitions of frequently used terms in the tau field are presented in an
attempt to clarify and standardize interpretation of research findings. Molecular and cellular factors affecting tau
aggregation are briefly reviewed, as are potential contributions of physiological and pathological post-translational
modifications of tau. Additionally, the experimental evidence for tau seeding and “prion-like” propagation of tau
aggregation that has emerged from cellular assays and in vivo models is discussed. Propagation of tau
pathology using “prion-like” mechanisms is expected to incorporate several steps including cellular uptake,
templated seeding, secretion and intercellular transfer through synaptic and non-synaptic pathways. The
experimental findings supporting each of these steps are reviewed. The clinical validity of these
experimental findings is then debated by considering the supportive or contradictory findings from patient
samples. Further, the role of physiological tau release in this scenario is examined because emerging data shows that tau is
secreted but the physiological function (if any) of this secretion in the context of propagation of pathological tau seeds is
unclear. Bona fide prions exhibit specific properties, including transmission from cell to cell, tissue to tissue and organism to
organism. The propagation of tau pathology has so far not been shown to exhibit all of these steps and how this
influences the debate of whether or not abnormal tau species can propagate in a “prion-like” manner is discussed.
The exact nature of tau seeds responsible for propagation of tau pathology in human tauopathies remains
controversial; it might be tightly linked to the existence of tau strains stably propagating peculiar patterns of
neuropathological lesions, corresponding to the different patterns seen in human tauopathies. That this is a property
shared by all seed-competent tau conformers is not yet firmly established. Further investigation is also required to
clarify the relationship between propagation of tau aggregates and tau-induced toxicity. Genetic variants identified as
risks factors for tauopathies might play a role in propagation of tau pathology, but many more studies are needed to
document this. The contribution of selective vulnerability of neuronal populations, as an alternative to prion-like
mechanisms to explain spreading of tau pathology needs to be clarified. Learning from the prion field will be helpful
to enhance our understanding of propagation of tau pathology. Finally, development of better models is expected to
answer some of these key questions and allow for the testing of propagation-centred therapies