Abstract

1. Amyloid Suppresses Induction of Genes Critical for Memory Consolidation in APP+PS1 Transgenic Mice Chad A. Dickey1, Marcia N. Gordon1, Jerimiah E. Mason1, Nedda J. Wilson1, David M. Diamond2, John F. Guzowski3 & Dave Morgan1 1 Alzheimer’s Disease Research Laboratory, Department of Pharmacology, University of South Florida, College of Medicine 12901 Bruce B. Downs Blvd, MDC 9, Tampa, Fl 33612 2 Departments of Psychology and Pharmacology, College of Arts and Sciences, University of South Florida, and Medical Research, Veterans Hospital, 4202 E. Fowler Ave. Tampa, FL 33620 3 Department of Neurosciences, Basic Medical Sciences Building, Room 145, University of New Mexico, Health Sciences Center, Albuquerque, NM 87131 2.Different Sensitivity to Amyloid Accumulation by Age in APP+PS1 Mice Early significant reductions of immediate-early gene expression were seen at 6 months followed by significant reductions in other plasticity related genes by 18-months, when memory deficits were detectable by radial arm water maze. 5.Induction of Immediate Early Gene Expression is Impaired in APP+PS1 Mice After a five minute exposure to a novel enriched environment, IEG induction was significantly impaired in APP+PS1 mice compared to non-transgenic littermates. Abstract Mice transgenic for mutated forms of the amyloid precursor protein (APP) plus presenilin-1 (PS-1) genes gradually develop amyloid pathology as they age resulting in reliable memory deficits by 15 months. Several genes critical for learning and memory are down-regulated in memory deficient transgenic mice compared to non-transgenic littermates. The down-regulation of these genes is strongly linked to the presence of amyloid, as gene expression is normal in APP+PS1 mice at an age prior to amyloid deposition, and the extent of gene repression worsens as amyloid deposits accumulate. We have also found that while 18 month old singly transgenic PS1 mice do not exhibit reductions in expression of any of the mRNAs we investigated, singly transgenic APP mice have less repression of gene expression than doubly transgenic APP+PS1 mice, consistent with their differences in amyloid deposition. We also find that some of the repressed genes, typically in the immediate early gene (IEG) category, are more sensitive to the presence of amyloid than the constitutively expressed synaptic genes that are critical for neural plasticity. Lastly, using a novel environment to stimulate IEG expression, we demonstrate that the abundance of several IEG mRNAs in the APP+PS1 mice are reduced during induction and not at the basal expression level. Therefore, an inability to induce expression as robustly as non-transgenic littermates may preclude the APP+PS1 mice from having adequate memory function. These data imply that amyloid deposition can lead to repression of multiple genes linked to synaptic plasticity and that the repression of these genes and the loss of their dynamic nature may account for the memory loss characteristic of early stage Alzheimer's disease. Immediate-early Genes Plasticity Related Genes Summary Non-Tg Basal Non-Tg Induced APP+PS1 Basal APP+PS1 Induced Relative expression is derived from dividing target mRNA by 18S rRNA following qRT-PCR. % relative expression was determined by dividing each transgenic value by an average of the non-transgenic values. * indicates p-value is <0.05. Relative expression is derived from dividing target mRNA by 18S rRNA following qRT-PCR. % relative expression was determined by dividing each transgenic value by an average of the non-transgenic non-induced values. * indicates p-value is <0.05 between non-tg induced and APP+PS1 induced. 3.Contributions of Individual Transgenes to mRNA Reduction in APP+PS1 Mice Significant reductions of immediate-early gene expression were seen in both APP only and APP+PS1 mice, however significant reductions in other plasticity related genes were only observed in the doubly transgenic mice. PS1 only mice maintain expression similar to non-transgenic mice. 1. Question: What Mechanisms Account for Memory Loss in APP+PS1 Mice? • 6. Conclusions • We have presented strong evidence that A itself causes specific reductions in gene expression critically linked to proper memory function in APP+PS1 mice • There is a variable sensitivity of these genes to A • IEGs are reduced in response to lower levels of A, prior to detectable memory dysfunction • Other plasticity related genes are reduced only at the time when memory loss is detectable • These reductions are not ascribed to transgene over-expression • It is the induction of the IEGs that is reduced, not the low-level basal expression • No Significant Neuron Loss • No Significant Synapse Loss • Changes in Gene Expression? • Gene Expression Profiling Reveals Reductions in Several Plasticity Related Genes in Memory Deficient APP+PS1 Mice • Reductions in Plasticity Related Genes are Confirmed and Extended by qRT-PCR • Reductions are Restricted to Amyloid Containing Regions • Reductions are Age-Dependent • Expression is Further Reduced with Increasing Amyloid Load • Induction by Environmental Novelty is Repressed in APP+PS1 Mice • Protein Function Loss follows mRNA Reductions PS1 only APP only APP+PS1 4.Differential Expression in APP+PS1 Mice in Regions with Different Amyloid Load and Composition Significant reductions of immediate-early gene expression were seen in hippocampus, cortex and caudate nucleus with reductions in other plasticity related genes seeming predominantly exclusive to the hippocampus of 18-month old APP+PS1 mice. Caudate Nucleus Posterior Cortex Hippocampus Relative expression is derived from dividing target mRNA by 18S rRNA following qRT-PCR. % relative expression was determined by dividing each transgenic value by an average of the non-transgenic values. * indicates p-value is <0.05 between transgenic and non-transgenic. † indicates significance p-value is <0.05 between APP only and APP+PS1 mice (3) or cortex and hippocampus (4). This work was supported by the National Institute of Aging (NIA) & National Institutes of Health (NIH) AG18478