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SUMMARYSenescence-accelerated mice (SAMP8) serve as a model for Alzheimerʼs disease (AD) as they exhibit early loss of memory and increased amyloid precursor protein (APP) expression. APP is a ubiquitous membrane protein that is physiologically processed by site-specific proteolysis firstly by α-or β-secretases, releasing a large fragment called APP S that contains most of the extracellular sequences of APP, a small extracellular stub, the transmembrane region and the cytoplasmic tail of APP (ʻAICDʼ-APP intracellular domain). These are subsequently cleaved by γ-secretase at multiple sites in the transmembrane region, releasing small peptides, Aβ 1-40 and Aβ 1-42 , the major components of AD-associated amyloid fibrils. γ-secretase is a high-molecular-mass complex composed of presenilin-1 (PS1), nicastrin, APH-1 and Pen-2. As PS1 has been shown to play a critical role in facilitating γ-secretase activity, and mutations in this protein are associated with familial AD (FAD), we have cloned it from SAMP8 mouse hippocampus and compared its sequence with those of other species. Furthermore, changes in the expression of PS1 with age in the hippocampal tissue of SAMP8 were studied. The results showed that the SAMP8 PS1 cDNA sequence is identical to that of normal mice. However, its expression in the hippocampus of SAMP8 exhibited an increase, while CD-1 mice, a strain that does not exhibit premature memory loss, showed no change with age. An increased amount or mutation(s) in PS1, which alters the stoichiometric balance of the γ-secretase complex, may be the cause of aberrant or increased processing of APP, resulting in Aβ accumulation leading to loss of memory.Key words: aging, amyloid, memory, presenilin, scenecence. THE JOURNAL OF EXPERIMENTAL BIOLOGY 495Increase in PS1 may cause memory loss of other species, including the house mouse, in order to locate any mutations that may be presented in the SAMP8 mouse sequence. Many mutations that are linked to FAD occur in human PS1 but only a few occur in APP. Therefore, even a single change in the PS1 sequence in SAMP8 mouse may explain the cause of its loss of memory at an earlier age. While no change in sequence was noticed in the SAMP8 PS1, protein expression increased with age in these mice, providing a possible mechanism for the increase in hippocampal Aβ. MATERIALS AND METHODS MaterialsAll fine chemicals were obtained from Sigma Chemical Company, St Louis, MO, USA. Sequencing grade [ 35 S]ATP was obtained from ICN Biomedicals, Costa Mesa, CA, USA. PVDF membranes for western blotting were supplied by BioRad Laboratories, Hercules, CA, USA. Lipofectamine was supplied by BRL-GIBCO, Gaithersburg, MD, USA. Antibody that detects the PS1 holoprotein was obtained from Santa Cruz Biotechnology, Inc., Santa Cruz, CA, USA. GAPDH antibodies are from Calbiochem, San Diego, CA, USA. 4-12% Bis-tris gels with MES buffer were from Invitrogen, Carlsbad, CA, USA. Senescence-accelerated P8 line (SAMP8) and CD-1 mice were from our in-house colonies. Hippocampal tissues were dissected from ...