Caspase-2 is required for dendritic spine and behavioural alterations in J20 APP transgenic mice.

Caspases have critical roles in Alzheimer's disease pathogenesis. Here we show that caspase-2 is required for the cognitive decline seen in human amyloid precursor protein transgenic mice (J20). The age-related changes in behaviour and dendritic spine density observed in these mice are absent when they lack caspase-2, in spite of similar levels of amyloid beta (Aß) deposition and inflammation. A similar degree of protection is observed in cultured hippocampal neurons lacking caspase-2, which are immune to the synaptotoxic effects of Aß. Our studies suggest that caspase-2 is a critical mediator in the activation of the RhoA/ROCK-II signalling pathway, leading to the collapse of dendritic spines. We propose that this is controlled by an inactive caspase-2/RhoA/ROCK-II complex localized in dendrites, which dissociates in the presence of Aß, allowing for their activation and entry in the spine. These findings directly implicate caspase-2 as key driver of synaptic dysfunction in Alzheimer's disease and offer novel therapeutic targets.

Cross-linking of cell surface amyloid precursor protein leads to increased ß-amyloid peptide production in hippocampal neurons: implications for Alzheimer's disease.

The accumulation of the ß-amyloid peptide (Aß) in Alzheimer's disease (AD) is thought to play a causative role in triggering synaptic dysfunction in neurons, leading to their eventual demise through apoptosis. Aß is produced and secreted upon sequential cleavage of the amyloid precursor protein (APP) by ß-secretases and γ-secretases. However, while Aß levels have been shown to be increased in the brains of AD patients, little is known about how the cleavage of APP and the subsequent generation of Aß is influenced, or whether the cleavage process changes over time. It has been proposed that Aß can bind APP and promote amyloidogenic processing of APP, further enhancing Aß production. Proof of this idea has remained elusive because a clear mechanism has not been identified, and the promiscuous nature of Aß binding complicates the task of demonstrating the idea. To work around these problems, we used an antibody-mediated approach to bind and cross-link cell-surface APP in cultured rat primary hippocampal neurons. Here we show that cross-linking of APP is sufficient to raise the levels of Aß in viable neurons with a concomitant increase in the levels of the ß-secretase BACE1. This appears to occur as a result of a sorting defect that stems from the caspase-3-mediated inactivation of a key sorting adaptor protein, namely GGA3, which prevents the lysosomal degradation of BACE1. Together, our data suggest the occurrence of a positive pathogenic feedback loop involving Aß and APP in affected neurons possibly allowing Aß to spread to nearby healthy neurons.

Reversal of long-term dendritic spine alterations in Alzheimer disease models.

Synapse loss is strongly correlated with cognitive impairment in Alzheimer's disease (AD). We have previously reported the loss of dendritic spines and the presence of dystrophic neurites in both the hippocampi of transgenic mice overexpressing amyloid precursor protein (APP) and in the human brain affected with AD. In the studies reported here we have asked whether the acute alterations in dendritic spines induced by Abeta, as well as the chronic loss of spine density seen in hAPP transgenic mice, are reversible by treatments that restore the cAMP/PKA/CREB signaling pathway or proteasome function to control levels. The results show that both rolipram and TAT-HA-Uch-L1 restore spine density to near control conditions, even in elderly mice. The results suggest that changes in dendritic structure and function that occur after Abeta elevation are reversible even after long periods of time, and that one could envision therapeutic approaches to AD based on this restoration that could work independently of therapies aimed at lowering Abeta levels in the brain.

Improving the generation of genomic-type transgenic mice by ICSI.

Transgenes included in genomic-type constructs, such as yeast artificial chromosomes (YAC), P1-derived artificial chromosomes, or bacterial artificial chromosomes (BAC), are normally correctly expressed, according to the endogenous expression pattern of the homologous locus, because their large size usually ensures the inclusion of all regulatory elements required for proper gene expression. The use of these large genomic-type transgenes is therefore the method of choice to overcome most position effects, commonly associated with standard-type transgenes, and to guarantee the faithful transgene expression. However, in spite of the different methods available, including pronuclear microinjection and the use of embryonic stem cells as vehicles for genomic transgenes, the generation of transgenic animals with BACs and, particularly, with YACs can be demanding, because of the low efficiencies requiring extensive microinjection sessions and/or higher number of oocytes. Recently, we have explored the use of intracytoplasmic sperm injection (ICSI) into metaphase II oocytes as an alternative method for the generation of YAC transgenic mice. Our results suggest that the use of transgenic strategies based on ICSI significantly enhances the efficiency of YAC transgenesis by at least one order of magnitude.

Effect of transgene concentration, flanking matrix attachment regions, and RecA-coating on the efficiency of mouse transgenesis mediated by intracytoplasmic sperm injection.

Intracytoplasmic sperm injection (ICSI) of DNA-loaded sperm cells has been shown to be a valuable tool for the production of transgenic animals, especially when DNA constructs with submegabase magnitude are used. In order to optimize and to understand the mechanism of the ICSI-mediated transgenesis, we have evaluated the impact of transgene DNA concentration, transgene flanking with nuclear matrix attachment regions (MARs), and the use of recombinase A (RecA)-coated DNA on the efficiency of mouse transgenesis production by ICSI. Presented data include assays with three DNA constructs; an enhanced green fluorescent protein (EGFP) plasmid of 5.4 kb, this plasmid flanked with two MAR elements (2.3 Kb of the human beta-interferon domain boundaries), and a yeast artificial chromosome (YAC) construct of approximately 510 kb (the largest transgenic construct introduced by ICSI that we have seen reported). ICSI-mediated transgenesis was done in the B6D2 mouse strain using different concentrations for each construct. Analysis of generated data indicated that ICSI allows the use of higher DNA concentrations than the ones used for pronuclear microinjection, however, when a certain threshold is exceeded, embryo/fetal viability decrease dramatically. In addition, independently of the transgene concentration tested, transgene flanking with MAR sequences did not have a significant impact on the efficiency of this transgenesis method. Finally, we observed that although the overall efficiency of ICSI-mediated transgenesis with fresh spermatozoa and RecA-complexed DNA was similar to the one obtained with the common ICSI-mediated transgenesis approach with frozen-thawed spermatozoa and RecA free DNA, this method was not as efficient in maintaining a low frequency of founder animal mosaicism, suggesting that different mechanisms of transgene integration might result from each procedure.

Generation of yeast artificial chromosome transgenic mice by intracytoplasmic sperm injection.

Genomic-type transgenes are usually expressed in appropriate spatial- and temporal-specific manners. The largest genomic transgenes can be prepared using yeast artificial chromosomes (YACs). Normally, YAC transgenic mice are produced by standard pro-nuclear microinjection, although other methods, involving the use of embryonic stem (ES) cells, have been also devised. To overcome the difficulty and time extension of ES cell-type approaches and to improve the rather usual low efficiency of YAC DNA transgenesis by pronuclear microinjection, that is mostly dependent on the YAC DNA quality of samples, we have devised an updated intracytoplasmic sperm injection (ICSI) method for the stable incorporation of YACs into the germ line of mice. DNA transgenesis efficiencies achieved are often 10 times greater than those usually obtained by standard microinjection, thus enabling the identification of either more transgenic founder animals and the use of reduced numbers of individuals in animal experimentation.

Neuronal specific regulatory elements in apolipoprotein E gene proximal promoter.

Systemic and local changes in apolipoprotein E (ApoE) quantity have been related with Alzheimer's and other neurodegenerative diseases, showing the relevance of maintaining physiological ApoE levels. However, APOE transcription has not been extensively studied in neural cells. In this report, we study the transcriptional activity of different APOE proximal promoter regions and their binding to nuclear proteins from human neural (astrocytoma and neuroblastoma) and peripheral (hepatoma and lymphoma) cell lines. We define several regions with a negative regulatory effect in all the cells and a region with a strong positive activity in neuroblastoma cells. Additionally, we show that the -219T/G polymorphism produces variations in APOE transcriptional activity, with the G allele showing higher activity.

Efficient generation of transgenic mice with intact yeast artificial chromosomes by intracytoplasmic sperm injection.

The production of animals with large transgenes is an increasingly valuable tool in biotechnology and for genetic studies, including the characterization and manipulation of large genes and polygenic traits. In the present study, we describe an intracytoplasmic sperm injection (ICSI) method for the stable incorporation and phenotypic expression of large yeast artificial chromosomes (YAC) constructs of submegabase and megabase magnitude. By coinjecting spermatozoa and YACs into metaphase II oocytes, we were able to produce founders exhibiting germline transmission of an intact and functional transgene of 250 kilobases, carrying the mouse tyrosinase locus, used here as a reporter gene to rescue the albinism of recipient mice. More than 35% transgenesis was obtained for this YAC transgene. When compared with the pronuclear microinjection standard method, the efficiency of the ICSI-mediated YAC transfer system was significantly greater. In summary, we describe, for the first time, stable incorporation in the host genome and correct phenotypic expression of large DNA constructs mediated by ICSI.