MGH-AbbVie AD Progression Study

Alzheimer’s disease neuropathological changes (ADNC) are accompanied by prominent morphological and functional changes in astrocytes, collectively termed reactive astrogliosis. While transcriptomic studies have begun to unveil the molecular underpinnings of reactive astrocytes, whether astrocyte gene expression parallels the severity of ADNC or differs between Aβ plaques and pTau neurofibrillary tangles remains unknown.

Citation

Alberto Serrano-Pozo, Zhaozhi Li, Maya E. Woodbury, Clara Muñoz-Castro, Astrid Wachter, Rojashree Jayakumar, Annie G. Bryant, Ayush Noori, Lindsay A. Welikovitch, Miwei Hu, Karen Zhao, Fan Liao, Gen Lin, Timothy Pastika, Joseph Tamm, Aicha Abdourahman, Taekyung Kwon, Rachel E. Bennett, Robert V. Talanian, Knut Biber, Eric H. Karran, Bradley T. Hyman, Sudeshna Das. Astrocyte transcriptomic changes along the spatiotemporal progression of Alzheimer’s disease. bioRxiv 2022.12.03.518999; doi: 10.1101/2022.12.03.518999

Study Design

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Using an enrichment strategy consisting of depleting NEUN+ neurons and OLIG2+ oligodendrocytes via fluorescently assisted cell sorting (FACS) we obtained a total of 629,755 astrocyte nuclei from five brain regions of 32 individuals with autopsy findings along the normal aging-AD neuropathological continuum. The five brain regions were selected to represent the hierarchical spreading of pTau NFTs along the brain networks as categorized with the Braak NFT staging, and included entorhinal cortex [EC], inferior temporal gyrus [ITG, BA20], dorsolateral prefrontal cortex [PFC, BA46], visual association cortex [V2, BA18/19], and primary visual cortex [V1, BA17].

Our strategy to enrich astrocyte nuclei was effective as indicated by the low numbers of neuronal and oligodendroglial nuclei identified. Astrocyte nuclei were identified by the expression levels of the marker genes ADGRV1, ALDH1L1, AQP4, and GFAP, resulting in over 100,000 astrocyte nuclei in each brain region with approx. 6,000 average total number of reads and approx. 2,500 average total number of genes detected, which is orders of magnitude higher than previous studies.

Since astrocytes react to nearby Aβ plaques and pTau NFTs, we quantified the local burden of ADNC in adjacent tissue samples by measuring the 3D6+ Aβ -immunoreactive % area fraction via immunohistochemistry and the pTau/total-tau ratio via ELISA. To reflect the progression of AD neuropathological changes, we then grouped the 32 donors into four pathology stages based on their global semiquantitative measures of neuritic plaques (CERAD NP score) and NFTs (Braak NFT stages) complemented with these immunohistochemical and biochemical quantitative measures of local Aβ and pTau burdens. The four pathology stages were:

1. Low pathology controls with no NPs, Braak NFT stage 0/I/II).
2. Intermediate pathology with sparse or moderate NPs, Braak NFT stage II/III.
3. ADNC with moderate or frequent NPs, Braak NFT stage V.
4. ADNC with moderate or frequent NPs, Braak NFT stage VI.

We separated the latter two groups because, by definition, the primary visual cortex (region V1) bears NFTs only in Braak NFT stage VI. The quantitative measures of ADNC across brain regions of the 32 donors grouped in four stages are shown below. Within each stage, the Aβ plaque load was relatively constant across brain regions except for higher levels in PFC in late stages. By contrast, the pTau/Tau ratio was highest in EC and demonstrated the expected pattern EC>ITG>PFC>V2>V1 in all stages, consistent with the stereotypical hierarchical accumulation of NFTs along neural networks.