The contribution of activated astrocytes to Aβ production: Implications for Alzheimer's disease pathogenesis

Background: beta-Amyloid (A beta) plays a central role in Alzheimer's disease (AD) pathogenesis. Neurons are major sources of A beta in the brain. However, astrocytes outnumber neurons by at least five-fold. Thus, even a small level of astrocytic A beta production could make a significant contribution to A beta burden in AD. Moreover, activated astrocytes may increase A beta generation. beta-Site APP cleaving enzyme 1 (BACE1) cleavage of amyloid precursor protein (APP) initiates A beta production. Here, we explored whether pro-inflammatory cytokines or A beta 42 would increase astrocytic levels of BACE1, APP, and beta-secretase processing, implying a feed-forward mechanism of astrocytic A beta production. Methods: Mouse primary astrocytes were treated with combinations of LPS, TNF-alpha, IFN-gamma, and IL-1 beta and analyzed by immunoblot and ELISA for endogenous BACE1, APP, and secreted A beta 40 levels. Inhibition of JAK and iNOS signaling in TNF-alpha+IFN-gamma-stimulated astrocytes was also analyzed. In addition, C57BL/6J or Tg2576 mouse astrocytes were treated with oligomeric or fibrillar A beta 42 and analyzed by immunoblot for levels of BACE1, APP, and APPs beta sw. Astrocytic BACE1 and APP mRNA levels were measured by TaqMan RT-PCR. Results: TNF-alpha+IFN-gamma stimulation significantly increased levels of astrocytic BACE1, APP, and secreted A beta 40. BACE1 and APP elevations were post-transcriptional at early time-points, but became transcriptional with longer TNF-alpha+IFN-gamma treatment. Despite a similar to 4-fold increase in astrocytic BACE1 protein level following TNF-alpha+IFN-gamma stimulation, BACE1 mRNA level was significantly decreased suggesting a post-transcriptional mechanism. Inhibition of iNOS and JAK did not reduce TNF-alpha+IFN-gamma-stimulated elevation of astrocytic BACE1, APP, and A beta 40, except that JAK inhibition blocked the APP increase. Finally, oligomeric and fibrillar A beta 42 dramatically increased levels of astrocytic BACE1, APP, and APPs beta sw through transcriptional mechanisms, at least in part. Conclusions: Cytokines including TNF-alpha+IFN-gamma increase levels of endogenous BACE1, APP, and A beta and stimulate amyloidogenic APP processing in astrocytes. Oligomeric and fibrillar A beta 42 also increase levels of astrocytic BACE1, APP, and beta-secretase processing. Together, our results suggest a cytokine-and A beta 42-driven feed-forward mechanism that promotes astrocytic A beta production. Given that astrocytes greatly outnumber neurons, activated astrocytes may represent significant sources of A beta during neuroinflammation in AD.