RESUMEN
OBJECTIVE: The most common pathogenesis for familial Alzheimer's disease (FAD) involves misprocessing (or alternative processing) of the amyloid precursor protein (APP) by γ-secretase due to mutations of the presenilin 1 (PS1) gene. This misprocessing/alternative processing leads to an increase in the ratio of the level of a minor γ-secretase reaction product (Aß42) to that of the major reaction product (Aß40). Although no PS1 mutations are present, altered Aß42/40 ratios are also observed in sporadic Alzheimer's disease (SAD), and these altered ratios apparently reflect deposition of Aß42 as amyloid. METHODS: Using immunoprecipitation-mass spectrometry with quantitative accuracy, we analyzed in the cerebrospinal fluid (CSF) of various clinical populations the peptide products generated by processing of not only APP but also an unrelated protein, alcadein (Alc). Alc undergoes metabolism by the identical APP α-secretases and γ-secretases, yielding a fragment that we have named p3-Alc(α) because of the parallel genesis of p3-Alc(α) peptides and the p3 fragment of APP. As with Aß, both major and minor p3-Alc(α) s are generated. We studied the alternative processing of p3-Alc(α) in various clinical populations. RESULTS: We previously reported that changes in the Aß42/40 ratio showed covariance in a linear relationship with the levels of p3-Alc(α) [minor/major] ratio in media conditioned by cells expressing FAD-linked PS1 mutants. Here we studied the speciation of p3-Alc(α) in the CSF from 3 groups of human subjects (n = 158): elderly nondemented control subjects; mild cognitive impairment (MCI) subjects with a clinical dementia rating (CDR) of 0.5; SAD subjects with CDR of 1.0; and other neurological disease (OND) control subjects. The CSF minor p3-Alc(α) variant, p3-Alc(α) 38, was elevated (p < 0.05) in MCI subjects or SAD subjects, depending upon whether the data were pooled and analyzed as a single cohort or analyzed individually as 3 separate cohorts. INTERPRETATION: These results suggest that some SAD may involve alternative processing of multiple γ-secretase substrates, raising the possibility that the molecular pathogenesis of SAD might involve γ-secretase dysfunction.
Asunto(s)
Enfermedad de Alzheimer/complicaciones , Secretasas de la Proteína Precursora del Amiloide/líquido cefalorraquídeo , Trastornos del Conocimiento/líquido cefalorraquídeo , Trastornos del Conocimiento/etiología , Anciano , Anciano de 80 o más Años , Secretasas de la Proteína Precursora del Amiloide/genética , Péptidos beta-Amiloides/líquido cefalorraquídeo , Precursor de Proteína beta-Amiloide/líquido cefalorraquídeo , Femenino , Humanos , Inmunoprecipitación , Masculino , Fragmentos de Péptidos/líquido cefalorraquídeo , Espectrometría de Masa por Láser de Matriz Asistida de Ionización Desorción , Espectrometría de Masas en TándemRESUMEN
Alcadeins (Alcs) constitute a family of neuronal type I membrane proteins, designated Alc(alpha), Alc(beta), and Alc(gamma). The Alcs express in neurons dominantly and largely colocalize with the Alzheimer amyloid precursor protein (APP) in the brain. Alcs and APP show an identical function as a cargo receptor of kinesin-1. Moreover, proteolytic processing of Alc proteins appears highly similar to that of APP. We found that APP alpha-secretases ADAM 10 and ADAM 17 primarily cleave Alc proteins and trigger the subsequent secondary intramembranous cleavage of Alc C-terminal fragments by a presenilin-dependent gamma-secretase complex, thereby generating "APP p3-like" and non-aggregative Alc peptides (p3-Alcs). We determined the complete amino acid sequence of p3-Alc(alpha), p3-Alc(beta), and p3-Alc(gamma), whose major species comprise 35, 37, and 31 amino acids, respectively, in human cerebrospinal fluid. We demonstrate here that variant p3-Alc C termini are modulated by FAD-linked presenilin 1 mutations increasing minor beta-amyloid species Abeta42, and these mutations alter the level of minor p3-Alc species. However, the magnitudes of C-terminal alteration of p3-Alc(alpha), p3-Alc(beta), and p3-Alc(gamma) were not equivalent, suggesting that one type of gamma-secretase dysfunction does not appear in the phenotype equivalently in the cleavage of type I membrane proteins. Because these C-terminal alterations are detectable in human cerebrospinal fluid, the use of a substrate panel, including Alcs and APP, may be effective to detect gamma-secretase dysfunction in the prepathogenic state of Alzheimer disease subjects.