Showing papers by "Steven Clarke published in 2005"

Activation of the PI3K/Akt signal transduction pathway and increased levels of insulin receptor in protein repair‐deficient mice

Abstract: Summary Protein L -isoaspartate ( D -aspartate) O -methyltransferase is an enzyme that catalyses the repair of isoaspartyl damage in proteins. Mice lacking this enzyme ( Pcmt1 –/– mice) have a progressive increase in brain size compared with wild-type mice ( Pcmt1 +/+ mice), a phenotype that can be associated with alterations in the PI3K/Akt signal transduction pathway. Here we show that components of this pathway, including Akt, GSK3β and PDK-1, are more highly phosphorylated in the brains of Pcmt1 –/– mice, particularly in cells of the hippocampus, in comparison with Pcmt1 +/+ mice. Examination of upstream elements of this pathway in the hippocampus revealed that Pcmt1 –/– mice have increased activation of insulin-like growth factor-I (IGF-I) receptor and/or insulin receptor. Western blot analysis revealed an approximate 200% increase in insulin receptor protein levels and an approximate 50% increase in IGF-I receptor protein levels in the hippocampus of Pcmt1 –/– mice. Higher levels of the insulin receptor protein were also found in other regions of the adult brain and in whole tissue extracts of brain, liver, heart and testes of both juvenile and adult Pcmt1 –/– mice. There were no significant differences in plasma insulin levels for adult Pcmt1 –/– mice during glucose tolerance tests. However, they did show higher peak levels of blood glucose, suggesting a mild impairment in glucose tolerance. We propose that Pcmt1 –/– mice have altered regulation of the insulin pathway, possibly as a compensatory response to altered glucose uptake or metabolism or as an adaptive

Increased cell proliferation and granule cell number in the dentate gyrus of protein repair-deficient mice.

Abstract: Recent studies have demonstrated that mice lacking protein L-isoaspartate (Daspartate) O-methyltransferase (Pcmt1‐/‐ mice) have alterations in the insulin-like growth factor-I (IGF-I) and insulin receptor pathways within the hippocampal formation as well as other brain regions. However, the cellular localization of these changes and whether the alterations might be associated with an increase in cell number within proliferative regions, such as the dentate gyrus, were unknown. In this study, stereological methods were used to demonstrate that these mice have an increased number of granule cells in the granule cell layer and hilus of the dentate gyrus. The higher number of granule cells was accompanied by a greater number of cells undergoing mitosis in the dentate gyrus, suggesting that an increase in neuronal cell proliferation occurs in this neurogenic zone of adult Pcmt1‐/‐ mice. In support of this, increased doublecortin labeling of immature neurons was detected in the subgranular zone of the dentate gyrus. In addition, double immunofluorescence studies demonstrated that phosphorylated IGF-I/insulin receptors in the subgranular zone were localized on immature neurons, suggesting that the increased activation of one or both of these receptors in Pcmt1‐/‐ mice could contribute to the growth and survival of these cells. We propose that deficits in the repair of isoaspartyl protein damage leads to alterations in metabolic and growth-receptor pathways, and that this model may be particularly relevant for studies of neurogenesis that is stimulated by cellular damage. J. Comp. Neurol. 493:

Diet-dependent survival of protein repair-deficient mice ☆

Abstract: Protein l-isoaspartyl (d-aspartyl) O-methyltransferase (PCMT1) is a protein-repair enzyme, and mice lacking this enzyme accumulate damaged proteins in multiple tissues, die at an early age from progressive epilepsy and have an increased S-adenosylmethionine (AdoMet) to S-adenosylhomocysteine (AdoHcy) ratio in brain tissue. It has been proposed that the alteration of AdoMet and AdoHcy levels might contribute to the seizure phenotype, particularly as AdoHcy has anticonvulsant properties. To investigate whether altered AdoMet and AdoHcy levels might contribute to the seizures and thus the survivability of the repair-deficient mice, a folate-deficient amino acid-based diet was administered to the mice in place of a standard chow diet. We found that the low-folate diet significantly decreases the AdoMet/AdoHcy ratio in brain tissue and results in an almost threefold extension of mean life span in the protein repair-deficient mice. These results indicate that the increased AdoMet/AdoHcy ratio may contribute to the lowered seizure threshold in young PCMT1-deficient mice. However, mean survival was also extended almost twofold for mice on a control folate-replete amino acid-based diet compared to mice on the standard chow diet. Survival after 40 days was similar in the mice on the low- and high-folate amino acid-based diets, suggesting that the survival of older PCMT1-deficient mice is not affected by the higher brain AdoMet/AdoHcy ratio. Additionally, the surviving older repair-deficient mice have a significant increase in body weight when compared to age-matched normal mice, independent of the type of diet. This weight increase was not accompanied by an increase in consumption levels, indicating that the repair-deficient mice may also have an altered metabolic state.