| In humans, aging often is associated with a decline in cognitive function. The neurological changes associated with normal aging are subtle compared to those of age-related neurodegenerative diseases, such as Alzheimer's disease. Progress toward an understanding of the molecular mechanisms underlying the initiation and progression of age-related neuronal decline could be hastened by the development of experimental systems that quickly test early and true symptoms (rather than the correlative downstream effects, such as pathology) of neuronal decline and disease. In contrast to muscle degradation, the nervous system of C. elegans is structurally remarkably well-preserved, leaving open the question of how to define age-related changes in neuronal function. To solve this problem, I have established a novel system to study associative learning, short-term associative memory, and long-term associative memory in C. elegans: chemotaxis assays measure worms' abilities to learn a positive association of a neutral chemoattractant with food.;I found that long-term, but not short-term, associative memory is dependent on transcription and translation, and the C. elegans homolog of the transcription factor CREB, crh-1. When applying our assays for cognitive function to aging worms, I found that worm learning and long-term associative memory decrease with age and are improved differentially by reduced insulin signaling and caloric restriction. Analysis of crh-1 gene expression and protein levels revealed that CREB levels strongly correlate with long-term associative memory performance.;Using CREB-reporter GFP expressing animals, I found that neuronal CREB transcriptional activity increases in the SIA and the AIM interneurons with long-term associative memory training, a pattern that differs from CREB activation during starvation. AIM morphological mutants, and AIY/AIA interneuron mutants display defective memory, suggesting that these cells are required for long-term associative memory. It is known that the relatively uncharacterized AIM interneurons are a site of serotonin biosynthesis; I demonstrated using genetic and pharmacological studies that serotonin signaling is required after training for C. elegans long-term associative memory. Transcriptional analysis identified CREB targets that are changed during long-term memory training, which include genes involved in vesicle trafficking, ion channels, synaptogenesis, RNA binding, and uncharacterized genes, many of which, we have shown to be required for long-term associative memory. Additionally, downstream targets of CREB are activated in other neurons, including glia, likely through CREB's activation of the JNK pathway.;I also applied our associative learning and memory assays to a neurodegenerative model of Alzheimer's Disease, transgenic C. elegans expressing mutant human tau pan-neuronally. Tau mutants are defective in associative learning and memory, and these disabilities are ameliorated by reduced insulin signaling, as well as resveratrol, a polyphenolic molecule that increases lifespan in many organisms. These results have encouraged us to develop a screen for mutations that increase learning and memory with age using Mos1-mediated mutagenesis in C. elegans. Identification of these mutants will open doors to the study of new treatments for learning and memory defects due to age-related neurodegenerative diseases, which can be tested with our learning and memory assays in worms before moving to higher organisms. |
