IN VIVO TWO-PHOTON IMAGING OF THE EFFECTS OF TAUOPATHY AND AMYLOIDOPATHY ON SYNAPSE DYNAMICS IN THE RTG4510 AND J20 TRANSGENIC MODELS RESPECTIVELY

Abstract

A progressive loss of synapses occurs at the early clinical stages of Alzheimer’s Disease (AD) and has been correlated with cognitive deficits in AD patients. However, it is relatively unknown how synapse dynamics are affected by the two main pathological hallmarks of AD; the intracellular accumulation of tau and the extracellular accumulation of amyloid β protein, Here we used in vivo two-photon microscopy to assess the temporal dynamics of axonal boutons and dendritic spines in transgenic mouse models of human tauopathy (rTg4510) and amyloidopathy (J20). The Tg4510 model expresses the P301L tau mutation downstream of a tetracycline-operon-responsive element, whilst the J20 expresses both the Indiana (V717F) and Swedish (K670N, M671L) mutation on the amyloid precursor protein gene. Following a craniotomy, adeno-associated virus expressing green fluorescent protein (GFP) was injected into layer 2/3 of the somatosensory cortex to enable the visualisation of neurons and a cranial window was implanted for long-term imaging. GFP-labelled neurons were imaged in Tg4510s, J20s and wild-type littermate controls during a time period which spanned the onset of pathology. The gross morphology of axons and dendrites and the dynamics of their synaptic structures were assessed as the pathology progressed. In the rTg4510 experiments, a third group of animals received doxycycline to determine the effects of suppressing the pathogenic P301L transgene. In the J20 experiments, the density and size of amyloid plaques and the effects of plaque proximity on synapse density were also assessed. In rTg4510s, gross morphological changes such as the presence of dystrophic neurites were visible as the tauopathy progressed and these were found to have a distinctive morphological phenotype prior to neurite degeneration. Alongside this, synapse instability and loss were also observed and could be prevented by suppressing the P301L transgene. In J20 animals, amyloid plaques increased in size and density over time and spine density was affected by the dendrite’s proximity to plaques. Both tauopathy and amyloidopathy had effects on synapses as the respective pathology progressed. These results will inform subsequent drug discovery studies to identify novel therapies to stabilize synapse loss in AD.