Analysis of the hippocampal proteome in ME7 prion disease reveals a predominant astrocytic signature and highlights the brain-restricted production of clusterin in chronic neurodegeneration.
Bottom Line: The observed changes associated with reactive glia highlighted some specific proteins that dominate the proteome in late-stage disease.Four of the up-regulated proteins (GFAP, high affinity glutamate transporter (EAAT-2), apo-J (Clusterin), and peroxiredoxin-6) are selectively expressed in astrocytes, but astrocyte proliferation does not contribute to their up-regulation.This does not preclude an important role of Clusterin in late-stage disease, but it cautions against the assumption that brain levels provide a surrogate peripheral measure for the progression of brain degeneration.
Affiliation: From the Centre for Biological Sciences and.
Prion diseases are characterized by accumulation of misfolded protein, gliosis, synaptic dysfunction, and ultimately neuronal loss. This sequence, mirroring key features of Alzheimer disease, is modeled well in ME7 prion disease. We used iTRAQ(TM)/mass spectrometry to compare the hippocampal proteome in control and late-stage ME7 animals. The observed changes associated with reactive glia highlighted some specific proteins that dominate the proteome in late-stage disease. Four of the up-regulated proteins (GFAP, high affinity glutamate transporter (EAAT-2), apo-J (Clusterin), and peroxiredoxin-6) are selectively expressed in astrocytes, but astrocyte proliferation does not contribute to their up-regulation. The known functional role of these proteins suggests this response acts against protein misfolding, excitotoxicity, and neurotoxic reactive oxygen species. A recent convergence of genome-wide association studies and the peripheral measurement of circulating levels of acute phase proteins have focused attention on Clusterin as a modifier of late-stage Alzheimer disease and a biomarker for advanced neurodegeneration. Since ME7 animals allow independent measurement of acute phase proteins in the brain and circulation, we extended our investigation to address whether changes in the brain proteome are detectable in blood. We found no difference in the circulating levels of Clusterin in late-stage prion disease when animals will show behavioral decline, accumulation of misfolded protein, and dramatic synaptic and neuronal loss. This does not preclude an important role of Clusterin in late-stage disease, but it cautions against the assumption that brain levels provide a surrogate peripheral measure for the progression of brain degeneration.
Related in: MedlinePlus
Mentions: The increased expression of the individual proteins could simply reflect an increase in astrocyte number as a consequence of disease. There is evidence showing that GFAP content is increased in astrocytes during reactive gliosis (37, 38) and also evidence astrocytes may proliferate in some disease states (39). To address this issue, we used Ki67 as a marker of cell proliferation and GFAP double labeling to investigate if there was proliferation of astrocytes during ME7 disease. Using this approach, we observed that in NBH animals Ki67 immunoreactivity is low (data not shown) relative to ME7 animals (Fig. 5). Although there was increased Ki67 in ME7 animals, we did not observe significant co-localization of Ki67 and GFAP (Fig. 5, arrowheads). In instances of Ki67 and GFAP apposition, this staining appeared in associated cells (Fig. 5, arrows). This is consistent with our recent report in which we demonstrate that the microglial population was the main cell type proliferating during prion disease (21). This suggests that the increased level of the astrocytic protein in the hippocampus of ME7 animals is not determined by their proliferation (Fig. 5).