Distinct patterns of APP processing in the CNS in autosomal-dominant and sporadic Alzheimer disease

Abstract

Autosomal-dominant Alzheimer disease (ADAD) is a genetic disorder caused by mutations in Amyloid Precursor Protein (APP) or Presenilin (PSEN) genes. Studies from families with ADAD have been critical to support the amyloid cascade hypothesis of Alzheimer disease (AD), the basis for the current development of amyloid-based disease-modifying therapies in sporadic AD (SAD). However, whether the pathological changes in APP processing in the CNS in ADAD are similar to those observed in SAD remains unclear. In this study, we measured β-site APP-cleaving enzyme (BACE) protein levels and activity, APP and APP C-terminal fragments in brain samples from subjects with ADAD carrying APP or PSEN1 mutations (n = 18), patients with SAD (n = 27) and age-matched controls (n = 22). We also measured sAPPβ and BACE protein levels, as well as BACE activity, in CSF from individuals carrying PSEN1 mutations (10 mutation carriers and 7 non-carrier controls), patients with SAD (n = 32) and age-matched controls (n = 11). We found that in the brain, the pattern in ADAD was characterized by an increase in APP β-C-terminal fragment (β-CTF) levels despite no changes in BACE protein levels or activity. In contrast, the pattern in SAD in the brain was mainly characterized by an increase in BACE levels and activity, with less APP β-CTF accumulation than ADAD. In the CSF, no differences were found between groups in BACE activity or expression or sAPPβ levels. Taken together, these data suggest that the physiopathological events underlying the chronic Aβ production/clearance imbalance in SAD and ADAD are different. These differences should be considered in the design of intervention trials in AD.

Fig. 1

Brain BACE1 protein levels and activity in ADAD, SAD patients and controls. BACE1 protein levels and activity were measured in brain homogenates from ADAD and SAD patients, and from young (YC) and elderly (EC) controls. There was an increase in BACE1 protein levels (a **p = 0.01) and activity (b *p = 0.04) in the frontal cortex of SAD cases compared to age-matched elderly controls. No differences were detected between ADAD cases and age-matched controls in either brain BACE1 protein levels or activity (p = 0.91 and p = 0.42, respectively). There was a significant increase in BACE1 protein levels (a *p = 0.03) but not in BACE1 activity (b, n.s., p = 0.12) in SAD relative to ADAD cases. Western blot analyses using the BACE-specific antibody BC05 confirmed the increased in BACE expression in SAD compared to ADAD (c) and the lack of differences between ADAD and controls (d). Representative blots are shown

Fig. 2

CSF BACE1 activity in PSEN1 mutation carriers, SAD patients and controls. a CSF BACE1 activity was measured in non-mutation carriers controls (YC), PSEN1 mutation carriers (MC), elderly controls (EC) and SAD patients. No differences were found between groups in CSF BACE1 activity. b CSF sAPPβ levels were determined in YC, MC, EC and SAD patients. No differences were found between SAD cases and age-matched controls or between MC and YC. c CSF BACE1 activity and sAPPβ levels showed a positive correlation in the entire subject sample (ρ = 0,501, p < 0.001). d CSF sAPPα levels were determined in YC, MC, EC and SAD patients. No differences were found between SAD cases and age-matched controls or between MC and YC. e CSF sAPPβ and sAPPα levels showed a strong positive correlation in the entire subject sample (ρ = 0,799, p < 0.0001). f Western blot analyses of BACE1 protein levels using the specific anti-BACE1 antibody D10E5 showed no differences between PSEN1 mutation carriers and non-carriers (YC). The specificity of the D10E5 was determined by using brain samples from 7-day-old (P7) wt and BACE 1−/− mice (Fig. S1). A representative blot is shown

Fig. 3

Brain APP β-CTF levels are elevated in ADAD. APP β-CTF levels were measured in membrane fractions in brain homogenates from ADAD, SAD and controls (a). ADAD cases showed higher APP β-CTF levels than age-matched controls and SAD (**p < 0.01). These differences were confirmed by Western blot in samples from patients with ADAD, SAD and controls. APP CTF accumulation was observed in ADAD cases compared to age-matched controls (b) and to SAD cases (c, d). APP CTF accumulation was also observed by Western blot in SAD cases compared to controls despite the fact that it did not reach statistical significance in the ELISA assay (e)

Fig. 4

APP accumulates in dystrophic neurites in ADAD. Immunohistochemistry for Aβ, APP and ubiquitin on representative brain sections from ADAD subjects carrying the APP I716F (a1–a3), the PSEN1 E120G (b1–b3), PSEN1 L286P (c1–c3) mutations and from one patient with SAD (d1–d3). Note frequent Aβ deposits, APP- and ubiquitin-positive bulbous dystrophic neurites in subjects with APP I716F and PSEN1 E120G mutations in contrast to the nearly lack of APP-positive neurites in subject with the PSEN1 L286P mutation where cotton-wool plaques predominate. In the latter case, ubiquitin immunostains delicate intermingled neurites (c3). In the SAD case, abundant mature Aβ deposits (d1) contrast with the few APP-(d2) and prominent ubiquitin-immunoreactive dystrophic neurites. Bar 50 µm