During aging, organisms lose the ability to respond adaptively to stressful extremes in the internal and external environment. One of the best examples of adaptation to stress at the molecular level is the transsynaptic induction of tyrosine hydroxylase (TH) gene expression in adrenal gland and sympathetic ganglia after exposure to stressful stimuli. Tyrosine hydroxylase is the rate-limiting enzymatic step in catecholamine synthesis. To date, most of the information accumulated on the regulation of TH gene expression pertains to transcriptional mechanisms. Our studies reveal that increased levels of TH mRNA during aging result from increased TH mRNA and that with increasing age there is a profound decrement in the transsynaptic induction of TH protein and activity. It is important to note that this decrement in induction of TH protein and activity does not generalize to other adrenal neurotransmitter synthesizing enzymes that we have examined. Significantly, the effects of aging on TH gene expression were attenuated by dietary restriction, the only treatment known to increase both median and maximum life-span of mammals. Most studies of changes in gene expression during aging have focused on the transcriptional level. However, there are a growing list of genes that are reported to be selectively affected at the posttranscriptional level by aging. For a number of genes, including the TH gene, interactions between cis- acting elements and trans-acting factors in the 3' untranslated region (3'UTR) are implicated in gene-specific regulation of both mRNA stability and translation. In the research described in this proposal, we will test the hypothesis that specific sequences within the TH mRNA 3'UTR interact with mRNA binding proteins to regulate TH mRNA stability and translation. We hypothesize that aging animals are impaired in the ability to induce TH in response to stress because of alterations in posttranscriptional mechanisms related to the TH 3'UTR. We will test this hypothesis by first investigating these mechanisms in vitro in cultured adrenal chromaffin cells and then by studying these mechanisms in vivo in relation to the effects of stress and dietary restriction on adrenal medullary TH gene expression in animals 4-30 months of age.
The specific aims of the proposal are as follows:
Aim #1. To identify cis- and trans-acting factors in the 3' untranslated region (UTR) that regulate TH mRNA stability and translation.
Aim #2. To assess the role of the TH 3'UTR in age-related changes in TH mRNA stability and translation.
Aim #3. To assess the effects of dietary restriction on mechanisms related to the TH 3'UTR that regulate TH mRNA stability and translation in the adrenal gland of rats of different ages.
