Pathological substrate of memory impairment in multiple system atrophy

AIMS

Synaptic dysfunction in Parkinson's disease is caused by propagation of pathogenic α-synuclein between neurons. Previously, in multiple system atrophy (MSA), pathologically characterised by ectopic deposition of abnormal α-synuclein predominantly in oligodendrocytes, we demonstrated that the occurrence of memory impairment was associated with the number of α-synuclein-positive neuronal cytoplasmic inclusions (NCIs) in the hippocampus. In the present study, we aimed to investigate how abnormal α-synuclein in the hippocampus can lead to memory impairment.

METHODS

We performed pathological and biochemical analyses using a mouse model of adult-onset MSA and human cases (MSA, N = 25; Parkinson's disease, N = 3; Alzheimer's disease, N = 2; normal controls, N = 11). In addition, the MSA model mice were examined behaviourally and physiologically.

RESULTS

In the MSA model, inducible human α-synuclein was first expressed in oligodendrocytes and subsequently accumulated in the cytoplasm of excitatory hippocampal neurons (NCI-like structures) and their presynaptic nerve terminals with the development of memory impairment. α-Synuclein oligomers increased simultaneously in the hippocampus of the MSA model. Hippocampal dendritic spines also decreased in number, followed by suppression of long-term potentiation. Consistent with these findings obtained in the MSA model, post-mortem analysis of human MSA brain tissues showed that cases of MSA with memory impairment developed more NCIs in excitatory hippocampal neurons along with α-synuclein oligomers than those without.

CONCLUSIONS

Our results provide new insights into the role of α-synuclein oligomers as a possible pathological cause of memory impairment in MSA.

Phosphorylation of Tau at Threonine 231 in Patients With Multiple System Atrophy and in a Mouse Model

Multiple system atrophy (MSA) is a sporadic neurodegenerative disorder pathologically characterized by the presence of glial cytoplasmic inclusions (GCIs). Some MSA patients exhibit motor deficits with accompanying cognitive impairment. Of note, some patients suffering from MSA with longer disease duration have AT8-positive signals, which correspond to phosphorylated tau (P-tau) at 202/205 (P-tau202/205). However, P-tau sites other than the AT8 antibody epitope antibody are less well studied. Here, we focused on the effect of α-synuclein (Syn) expression on the phosphorylation of tau in MSA model mice. Among the 6 kinds of antibodies against P-tau, we confirmed that antibodies against P-tau at 231 (P-tau231) were phospho-specific and found that P-tau231 level was increased in parallel with disease progression in MSA model mice. Additional studies of human brains revealed that P-tau231 was mainly expressed in the temporal cortex in MSA brains and that its expression level was significantly higher in MSA patients than in controls. Immunohistochemical analysis showed that anti-P-tau231-, but not AT8, antibodies mainly immunolabeled hippocampal CA2/3 pyramidal neurons, and some GCIs in MSA. These data suggest that P-tau231 occurs in MSA differently from P-tau202/205.