Alpha- and beta-secretase activity as a function of age and beta-amyloid in Down syndrome and normal brain

Abstract

Aged individuals with Down syndrome (DS) develop Alzheimer's disease (AD) neuropathology by the age of 40 years. The purpose of the current study was to measure age-associated changes in APP processing in 36 individuals with DS (5 months-69 years) and in 26 controls (5 months-100 years). Alpha-secretase significantly decreased with age in DS, particularly in cases over the age of 40 years and was stable in controls. The levels of C-terminal fragments of APP reflecting alpha-secretase processing (CTF-alpha) decreased with age in both groups. In both groups, there was significant increase in beta-secretase activity with age. CTF-beta remained constant with age in controls suggesting compensatory increases in turnover/clearance mechanisms. In DS, young individuals had the lowest CTF-beta levels that may reflect rapid conversion of beta-amyloid (Abeta) to soluble pools or efficient CTF-beta clearance mechanisms. Treatments to slow or prevent AD in the general population targeting secretase activity may be more efficacious in adults with DS if combined with approaches that enhance Abeta degradation and clearance.

Fig. 1

Insoluble Aβ is higher in DS brain than in control brain after the age of 40 years of age. Both Aβ1–40 (A) and Aβ1–42 (B) are consistent in young controls, aged controls and in young individuals with DS. However, DS cases over the age of 40 years (old DS) show significantly higher formic acid extracted Aβ relative to the other three groups. Individual data points are shown, horizontal lines represent means and the top of the box illustrates the 75th percentile and the bottom the 25th percentile. Young controls <40 years, old controls >40 years, young DS <40 years, old DS >40 years.

Fig. 2

Progressive rise in insoluble Aβ42 with age and a rapid rise in Aβ40 after 40 years of age in DS. Aβ40 remains constant with age in controls (A) but shows a rapid increase after the age of 40 years in DS (B). Aβ1–42 remains constant with age in controls (C) and rises progressively with age in DS (D).

Fig. 3

A decrease in the ratio of Aβ1–42:Aβ1–40 occurs in individuals with DS after the age of 40 years. (A) The ratio of Aβ1–42:Aβ1–40 remains constant at greater than 80% in control subjects. (B) In contrast, the ratio of Aβ1–42 to Aβ1–40 is stable until approximately 40 years of age in DS and then shows a steep drop with increasing age.

Fig. 4

Average alpha-secretase activity remains relatively stable in controls and in DS. (A) Alpha-secretase activity was similar in control and DS cases under the age of 40 years and over the age of 40 years. Further, control cases over the age of 90 years were not different from younger controls. Individual data points are shown, horizontal lines represent means and the top of the box illustrates the 75th percentile and the bottom the 25th percentile. Young controls <40 years, old controls >40 years, young DS <40 years, old DS >40 years. (B) Alpha-secretase is plotted as a function of age and shows relatively consistent activity levels in both controls and DS. Note that a subset of DS cases over 40 years of age has lower alpha-secretase activity.

Fig. 5

Beta-secretase activity increases with age in both controls and in DS cases. (A) Beta secretase activity increases with age in both controls and in DS, however, no further increases are observed in control cases when the age range is extended to include subjects over 90 years of age. Note that a subset of old DS cases had higher levels of beta-secretase than other cases. Individual data points are shown, horizontal lines represent means and the top of the box illustrates the 75th percentile and the bottom the 25th percentile. Young controls <40 years, old controls <40 years, young DS <40 years, old DS >40 years. (B) Beta-secretase activity is plotted as a function of age and illustrates the relatively parallel rise in activity with age in both controls and DS but maintenance of activity levels in individuals over 90 years of age.

Fig. 6

APP CTF-alpha and CTF-beta with age in controls and in DS. (A) A western blot illustrates CTF-alpha and CTF-beta decreasing and increasing, respectively in both controls and DS. β-actin illustrates relatively equal loading across all cases. (B) Quantification of Western blots after correcting for protein loading and expressed as a proportion of total full length APP from the larger set of samples reveals an age dependent decrease in CTF-alpha to total APP in old controls and a similar decline in old DS cases. (C) In contrast, CTF-beta to total APP did not appear to increase appreciably in control cases and appeared lower in young DS cases when quantified. (D) The ratio of CTF-beta to alpha rose with age in controls and in DS but was also lower overall in young DS cases. (E) In controls, CTFs of APP as a result of alpha- or beta-secretase shows stable CTF beta levels relative to total APP and a progressive decline in CTF-alpha. (F) In DS, a similar effect was observed except that the differences were more pronounced particularly in cases under 40 years of age. (G) The percentage of CTF-beta to CTF-alpha rose with age in controls but was variable in cases over 60 years of age. (H) In DS, the percentage of CTF-beta to alpha increased progressively with age. To detect CTF, the antibody CT20 was used and the specificity has been described previously [48].

Fig. 7

Steady state levels of APP show no direct relationship with age but are higher overall in the DS cases. (A) Full length APP was detected using 22C11 in this illustration showing that overall, individuals with DS had higher levels of APP but there were no apparent trends towards an increase or decrease in APP with age in either group. Total APP corrected for actin protein loading is plotted as a function of age for controls (B) and for individuals with DS (C) showing a lack of change with increasing age.