(2) and London mutation(s) (3) alter APP processing, causing increased production of the A␤ peptide of 42 amino acids (4), hypothesized to be pivotal in AD pathology (1, 5). Early onset familial AD caused by mutations in the presenilin genes supports this hypothesis, because they increase production of A␤ (42) peptide (6, 7) due to the gain of an unknown function (8). The extensive cell biological definition of the metabolic effects of the different mutations in APP in vitro requires matching analysis of their physiological impact in vivo. Transgenic mice with wild type and different mutant forms of APP have been generated and the original, most wanted end point, i.e. AD-like amyloid plaques in mouse brain, was obtained (9, 10), accompanied by cognitive deficits (11) and by hyperphosphorylation of protein tau (12). In other transgenic mouse strains overexpression of APP caused behavioral, synaptotrophic, and neurodegenerative effects, accelerated senescence, and premature death, in the absence of amyloid deposits (13-16). Intracellular expression of the A␤ peptide yielded mice with extensive neuronal loss but no amyloidosis (17). Overexpression of the C-terminal domain of APP caused neuronal degeneration (18), whereas in another model, pre-amyloid deposits, hippocampal cell loss, and cognitive deficits were documented (19).We have generated additional transgenic mouse strains, expressing human APP, either wild type or the London or Swedish clinical mutations, from the neuron-specific mouse thy-1 gene promoter. Their phenotype was analyzed by biochemical, histochemical, behavioral, electrophysiological, and pharmacological methods. Measurements of different APP metabolites in brain demonstrated that increased A␤(42) levels correlated with the formation of amyloid plaques in the brain of old APP/London transgenic mice. The plaques were extensively characterized immunohistochemically and displayed many aspects typically observed in the brain of AD patients. As opposed to plaques that developed only after at least 12 months of age, other deficits were observed from 3 months onwards and included cognitive impairment, decreased long term potentiation, differential glutamatergic responses, aggression, and neophobia, among others. These signs were largely independent of the actual isoform or mutant of APP that was expressed, were not correlated with a single APP metabolite, and are dissociated in time from plaque formation. These mice will be good models to study both early and late, neuropathological, and clinical aspects related to Alzheimer's disease. EXPERIMENTAL PROCEDURESGeneration of Transgenic Mice-cDNA coding for human wild type APP (695 isoform), the Swedish (K670N,M671L) mutant (770 isoform), and the London (V642I) mutant (695 isoform) were cloned in the pTSC vector in the mouse thy-1 gene (16). The purified, linearized minigenes were microinjected into prenuclear embryos from superovulated FVB/N females.Antibodies-Rabbit antisera B11/4 and B12/4, generated against a *

In the brain of Alzheimer's disease (AD) patients, neurotoxic amyloid peptides accumulate and are deposited as senile plaques. A major therapeutic strategy aims to decrease production of amyloid peptides by inhibition of gamma-secretase. Presenilins are polytopic transmembrane proteins that are essential for gamma-secretase activity during development and in amyloid production. By loxP/Cre-recombinase-mediated deletion, we generated mice with postnatal, neuron-specific presenilin-1 (PS1) deficiency, denoted PS1(n-/-), that were viable and fertile, with normal brain morphology. In adult PS1(n-/-) mice, levels of endogenous brain amyloid peptides were strongly decreased, concomitant with accumulation of amyloid precursor protein (APP) C-terminal fragments. In the cross of APP[V717I]xPS1 (n-/-) double transgenic mice, the neuronal absence of PS1 effectively prevented amyloid pathology, even in mice that were 18 months old. This contrasted sharply with APP[V717I] single transgenic mice that all develop amyloid pathology at the age of 10-12 months. In APP[V717I]xPS1 (n-/-) mice, long-term potentiation (LTP) was practically rescued at the end of the 2 hr observation period, again contrasting sharply with the strongly impaired LTP in APP[V717I] mice. The findings demonstrate the critical involvement of amyloid peptides in defective LTP in APP transgenic mice. Although these data open perspectives for therapy of AD by gamma-secretase inhibition, the neuronal absence of PS1 failed to rescue the cognitive defect, assessed by the object recognition test, of the parent APP[V717I] transgenic mice. This points to potentially detrimental effects of accumulating APP C99 fragments and demands further study of the consequences of inhibition of gamma-secretase activity. In addition, our data highlight the complex functional relation of APP and PS1 to cognition and neuronal plasticity in adult and aging brain.

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Testis regression was induced in male Syrian hamsters by chronic exposure to diethylstilbestrol (DES), and estradiol-17 beta agonist. Experimental groups (n = 4-5) were killed at increasing time intervals over a period of 6 mo after initiation of treatment. Apoptosis in testes was demonstrated by in situ analysis of DNA fragmentation. Cell proliferation was monitored by immunostaining nuclei of S-phase cells after pulse labeling with 5-bromo-2'-deoxyuridine. Levels of FSH and testosterone, measured by RIA fell rapidly in DES-treated hamsters. In parallel, testis weight and seminiferous tubule area underwent an 80% decrease during the first 2 wk of DES administration. The composition of seminiferous epithelium was also drastically affected by DES, since it became progressively confined to Sertoli cells, spermatogonia, and spermatocytes. Testis regression was associated with an important increase of apoptosis, which started 3 days after the beginning of DES administration. Apoptosis was still 10- to 50-fold higher than in control testes by the end of treatment; it affected primarily spermatocytes and, to a much lesser extent, spermatogonia. Cell proliferation was not inhibited by chronic DES administration. In conclusion, these data indicate that apoptosis can by itself account for estrogen-induced testis regression.

Presenilins, whose mutant forms are the most common cause of early onset familial Alzheimer's disease, are involved in two very distinct processes: (i) proteolytic activity as ␥-secretase acting on amyloid precursor protein to produce amyloid peptides and (ii) storage of Ca 2؉ in the endoplasmic reticulum (ER). In particular, absence of presenilin-1 (PS1) was claimed to potentiate capacitative calcium entry (CCE), i.e. the mechanism of replenishment of ER Ca 2؉ stores. However, until now, evidence in favor of the latter role has been obtained only in isolated or cultured cells and not on neurons in situ. Here, we studied the strength of the synapses between Schaffer's collaterals and CA1 neurons in hippocampal slices when they were submitted first to Ca 2؉ -free medium containing thapsigargin and subsequently to normal artificial cerebrospinal fluid, a procedure known to trigger CCE. We demonstrate that Ca 2؉ influx via the CCE mechanism is sufficient to trigger robust long term potentiation of the synapses in hippocampal slices from transgenic mice with a postnatal, neuronspecific ablation of PS1, but remarkably not from wildtype mice. Our data establish for the first time in neurons confined in normal neuronal networks that PS1 acts on the refilling mechanism of ER Ca 2؉ stores.