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.
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Mentions: GFAP was among the most robustly up-regulated protein based on the relative intensity of a number of peptides independently identified. Furthermore, the intensity of the MS signal in NBH samples also indicated that the protein is among the more abundant proteins profiled even in control samples. Immunocytochemical staining for GFAP in hippocampal sections from NBH and ME7 animals shows the abundance of astrocytes in both cohorts and a markedly increased staining in the ME7 samples (Fig. 3, A–D, compared with E–H). Three of the other candidate proteins that we observed to be up-regulated in ME7 animals included Clusterin, EAAT-2, and Prdx6, which are all known to be predominantly expressed in astrocytes (Fig. 3, I–N) (34–36). Immunoreactivity for these molecules is also observed in astrocytes outside the hippocampus (data not shown). We used double immunocytochemistry of Prdx6 and GFAP to confirm whether this protein is expressed and induced in astrocytes or other cells in ME7 animals. Our data revealed co-localization of Prdx6 and GFAP albeit with weaker immunoreactivity of Prdx6 than GFAP; Prdx6 was expressed in NBH tissue (Fig. 4, A–D) but was markedly increased in the ME7 animals (Fig. 4, E–J). No cells other than astrocytes appear to express Prdx6. ImageJ co-localization is shown in white at the far right, illustrating the pixels having both significant red and green signals (Fig. 4, I–J).