Differential aberrant structural synaptic plasticity in axons and dendrites ahead of their degeneration in tauopathy

Neurodegeneration driven by aberrant tau is a key feature of many dementias. Pathological stages of tauopathy are characterised by reduced synapse density and altered synapse function. Furthermore, changes in synaptic plasticity have been documented in the early stages of tauopathy suggesting that they may be a driver of later pathology. However, it remains unclear if synapse plasticity is specifically linked to the degeneration of neurons. This is partly because, in progressive dementias, pathology can vary widely from cell-to-cell along the prolonged disease time-course. To overcome this variability, we have taken a longitudinal experimental approach to track individual neurons through the progression of neurodegenerative tauopathy. Using repeated in vivo 2-photon imaging in rTg4510 transgenic mice, we have measured structural plasticity of presynaptic terminaux boutons and postsynaptic spines on individual axons and dendrites over long periods of time. By following individual neurons, we have measured synapse density across the neuronal population and tracked changes in synapse turnover in each neuron. We found that tauopathy drives a reduction in density of both presynaptic and postsynaptic structures and that this is partially driven by degeneration of individual axons and dendrites that are spread widely across the disease time-course. Both synaptic loss and neuronal degeneration was ameliorated by reduction in expression of the aberrant P301L transgene, but only if that reduction was initiated early in disease progression. Notably, neurite degeneration was preceded by alterations in synapse turnover that contrasted in axons and dendrites. In dendrites destined to die, there was a dramatic loss of spines in the week immediately before degeneration. In contrast, axonal degeneration was preceded by a progressive attenuation of presynaptic turnover that started many weeks before axon disappearance. Therefore, changes in synapse plasticity are harbingers of degeneration of individual neurites that occur at differing stages of tau-driven neurodegenerative disease, suggesting a cell or neurite autonomous process. Furthermore, the links between synapse plasticity and degeneration are distinct in axonal and dendritic compartments. Key findings Tauopathy driven by tau P301L in rTg4510 mice causes a progressive decrease in density of presynaptic terminaux boutons and postsynaptic dendritic spines in cortical excitatory neurons. Longitudinal imaging of individual axons and dendrites shows that there is a huge diversity of effects at varying times in different cells. Decreases in overall synapse density are driven partly, but not exclusively, by degeneration of dendrites and axons that are distributed widely across the time-course of disease. Suppression of pathological P301L tau expression can ameliorate accumulation of tau pathology, synapse loss and neurodegeneration, but only if administered early in disease progression. Neurite degeneration is preceded by aberrant structural synaptic plasticity in a cell-specific way that is markedly different in dendrites and axons. Degeneration of dendrites is immediately preceded by dramatic loss of dendritic spines. Axonal loss is characterised by a progressive attenuation of presynaptic bouton plasticity that starts months before degeneration.