Plasma glial fibrillary acidic protein is an early marker of Aβ pathology in Alzheimer’s disease

Although recent clinical trials targeting amyloid-β (Aβ) in Alzheimer’s disease (AD) have shown promising results, there is increasing evidence suggesting that understanding alternative disease pathways that interact with Aβ metabolism and amyloid pathology might be important to halt the clinical deterioration. In particular, there is evidence supporting a critical role of astroglial activation and astrocytosis in AD. However, to this date, no studies have assessed whether astrocytosis is independently related to Aβ or tau pathology, respectively, in vivo. To address this question, we determined the levels of the astrocytic marker glial fibrillary acidic protein (GFAP) in plasma and cerebrospinal fluid (CSF) of 217 Aβ-negative cognitively unimpaired individuals, 71 Aβ-positive cognitively unimpaired individuals, 78 Aβ-positive cognitively impaired individuals, 63 Aβ-negative cognitively impaired individuals and 75 patients with a non-AD neurodegenerative disorder from the Swedish BioFINDER-2 study. Subjects underwent longitudinal Aβ (18F-flutemetamol) and tau (18F-RO948) positron emission tomography (PET) as well as cognitive testing. We found that plasma GFAP concentration was significantly increased in all Aβ-positive groups compared with subjects without Aβ pathology (p<0.01). In addition, there were significant associations between plasma GFAP with higher Aβ-PET signal in all Aβ-positive groups, but also in cognitively normal individuals with normal Aβ values (p<0.001), which remained significant after controlling for tau-PET signal. Furthermore, plasma GFAP could predict Aβ-PET positivity with an area under the curve of 0.76, which was greater than the performance achieved by CSF GFAP (0.69) and other glial markers (CSF YKL-40: 0.64, sTREM2: 0.71). Although correlations were also observed between tau-PET and plasma GFAP, these were no longer significant after controlling for Aβ-PET. In contrast to plasma GFAP, CSF GFAP concentration was significantly increased in non-AD patients compared to other groups (p<0.05) and correlated with Aβ-PET only in Aβ-positive cognitively impaired individuals (p=0.005). Finally, plasma GFAP was associated with both longitudinal Aβ-PET and cognitive decline, and mediated the effect of Aβ-PET on tau-PET burden, suggesting that astrocytosis secondary to Aβ aggregation might promote tau accumulation. Altogether, these findings indicate that plasma GFAP is an early marker associated with brain Aβ pathology but not tau aggregation, even in cognitively normal individuals with a normal Aβ status. This suggests that plasma GFAP should be incorporated in current hypothetical models of AD pathogenesis and be used as a non-invasive and accessible tool to detect early astrocytosis secondary to Aβ pathology.