Circadian rhythms and the light-dark cycle interact to regulate amyloid plaque accumulation and tau phosphorylation...

Circadian disruption has long been appreciated as a downstream consequence of Alzheimer's Disease in humans. However, an upstream role for behavioral circadian disruption in regulating AD pathology remains an open question.

To determine the role of the central circadian clock in the suprachiasmatic nucleus (SCN) in regulating amyloid pathology, we generated mice harboring deletion of the critical clock gene Bmal1 in GABAergic neurons using VGAT-iCre, which is expressed in >95% of SCN cells, and crossed this line to the 5xFAD amyloid mouse model. To examine the role of the light-dark cycle in this process, we aged these mice in either regular 12:12 light-dark (LD) or constant darkness (DD) conditions. Transcriptional, behavioral, and physiological rhythms were examined in VGAT-iCre; Bmal1 ^fl/fl; 5xFAD (VGAT-BMAL1KO;5xFAD) mice under varying light conditions. Amyloid plaque deposition, peri-plaque tau phosphorylation, glial activation, and transcriptomic changes were examined.

VGAT-BMAL1KO;5xFAD mice showed loss of SCN BMAL1 expression and severe disruption of behavioral rhythms in both LD and DD, with loss of day-night rhythms in consolidated sleep and blunting of rhythmic clock gene expression in the brain. Surprisingly, VGAT-BMAL1KO;5xFAD mice kept under LD showed reduced total plaque accumulation and peri-plaque tau phosphorylation, compared to Cre-negative controls. These changes were gated by the light-dark cycle, as they were absent in VGAT-BMAL1KO;5xFAD mice kept in DD conditions. Total plaque accumulation was also reduced in control 5xFAD mice kept in DD as compared to LD, suggesting a general effect of light-dark cycle on amyloid aggregation. Expression of murine presenilin 1 (Psen1), as well as amyloidogenic cleavage of amyloid precursor protein, were suppressed in VGAT-BMAL1KO;5xFAD specifically under LD conditions.

In 5xFAD mice, the central circadian clock and the light-dark cycle interact to regulate amyloid pathology. Disruption of the central clock in the presence of a light-dark cycle may reduce APP cleavage and plaque formation. These results call into question the proposed simple positive feedback loop between circadian rhythm disruption and Alzheimer's Disease pathology.