Gretchen J. Darlington, Ph.D.
Baylor College of Medicine

Identification of Candidate Genes for Longevity in Long Lived Mouse Models.

Few mammalian models of aging have the phenotype of increased life span. Virtually all work has focused on food restricted (FR) rodents. Although many changes in physiologic function and gene expression have been identified, it is difficult to determine which changes contribute significantly to life span extension. Additional models are much needed. Mutant mice with pituitary insufficiency provide a novel system in which to identify genes that contribute to longevity. The Ames dwarf (df) mouse model has a dramatically extended life span--the greatest extension of any mammalian model (Brown-Borg, Nature, 1996). The normal life span for this strain is 718 to 723 days whereas, the homozygous mutant animals carrying the Prop-1 transcription factor mutation live an average of 1,076 to 1,206 days (males and females, respectively). Life span extension in dietary restricted animals is usually in the 40-50% range. Differences in the expression of pituitary specific genes have been examined in the Ames dwarf and defects in the development of the differentiated cell types in the pituitary have been well characterized. Three of the five pituitary cell types, lactotrophs, somatotrophs, and thyrotrophs, fail to differentiate fully and are greatly reduced in number in the df animals, leading to a triple deficiency of growth hormone, thyrotropin, and prolactin.

The hypothesis for the life span increase in the two long-lived mouse models is that genes governing metabolic processes and/or stress responses are responsible for the increased longevity. Further, the reduced level of hormone production by the pituitary of the Ames dwarf animals results in alterations in liver, adipose, and muscle specific genes that govern metabolic homeostasis leading to increased life span. We will compare the metabolic and physiologic status of dwarf and control mice in order to better understand energy utilization and expenditure in these animals. We will use microarray analysis of cDNAs to identify genes whose expression is similar between the dwarf and FR models, but that differ from ad libitum fed animals in these three metabolically active tissues. We further hypothesize that the extent of altered expression of these genes will correlate with the extension of life span in dwarf and food restricted models (i.e., a greater degree of alteration in the longer-lived dwarf model). This evaluation will identify many genes that may influence longevity. Criteria for the identification of likely candidates from the pool of differentially expressed loci will include similar alterations in expression in all long-lived models examined, the degree of difference in expression, and the known or projected function of the gene product.

We also propose to identify additional long-lived mouse strains. Two additional mouse models already exist that have single gene mutations that result in a partial pituitary deficiency state; these are the growth hormone releasing hormone receptor (the Little mouse) and the thyroid stimulating hormone receptor, Tshr (the Petite mouse). These mutant lines potentially provide additional models in which to examine longevity and gene expression levels to correlate age and candidate gene function. We would determine whether the Little or Tshr dwarf also has an extended life span. The mutations are in different genes, but lead to similar, although not identical, pituitary deficiency states. If the Little or Tshr mice have increased life spans, they should share altered expression of the relevant longevity genes with the df and FR mice. Through the identification of groups of genes whose altered expression correlates with increased life span in multiple animal models, the number of candidate genes will be narrowed, permitting us to focus on the most likely candidates for increased life span.

Changes in gene expression in a particular metabolic pathway would signify an important role for that pathway. The rate limiting gene in the pathway whose expression is altered in the models of increased life span could then be manipulated to create the expression levels observed in the longevity models by cell ablation, transgenic, knockout and knock-in strategies in order to demonstrate a causal role.

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