Degenerating synaptic boutons in prion disease: microglia activation without synaptic stripping

A growing body of evidence suggests that the loss of synapses is an early and major component of a number of neurodegenerative diseases. Murine prion disease offers a tractable preparation in which to study synaptic loss in a chronic neurodegenerative disease and to explore the underlying mechanisms. We have previously shown that synaptic loss in the hippocampus underpins the first behavioral changes and that there is a selective loss of presynaptic elements. The microglia have an activated morphology at this stage but they have an anti-inflammatory phenotype. We reasoned that the microglia might be involved in synaptic stripping, removing synapses undergoing a degenerative process, and that this gives rise to the anti-inflammatory phenotype. Analysis of synaptic density revealed a progressive loss from 12 weeks post disease initiation. The loss of synapses was not associated with microglia processes; instead, we found that the postsynaptic density of the dendritic spine was progressively wrapped around the degenerating presynaptic element with loss of subcellular components. Three-dimensional reconstructions of these structures from Dual Beam electron microscopy support the conclusion that the synaptic loss in prion disease is a neuron autonomous event facilitated without direct involvement of glial cells. Previous studies described synapse engulfment by developing and injured neurons, and we suggest that this mechanism may contribute to developmental and pathological changes in synapse numbers.