Translation of a unique transcript for protein isoaspartyl methyltransferase in haploid spermatids: implications for protein storage and repair

l-Isoaspartyl Methyltransferase Deficiency in Zebrafish Leads to Impaired Calcium Signaling in the Brain

Isomerization of l-aspartyl and l-asparaginyl residues to l-isoaspartyl residues is one type of protein damage that can occur under physiological conditions and leads to conformational changes, loss of function, and enhanced protein degradation. Protein l-isoaspartyl methyltransferase (PCMT) is a repair enzyme whose action initiates the reconversion of abnormal l-isoaspartyl residues to normal l-aspartyl residues in proteins. Many lines of evidence support a crucial role for PCMT in the brain, but the mechanisms involved remain poorly understood. Here, we investigated PCMT activity and function in zebrafish, a vertebrate model that is particularly well-suited to analyze brain function using a variety of techniques. We characterized the expression products of the zebrafish PCMT homologous genes pcmt and pcmtl. Both zebrafish proteins showed a robust l-isoaspartyl methyltransferase activity and highest mRNA transcript levels were found in brain and testes. Zebrafish morphant larvae with a knockdown in both the pcmt and pcmtl genes showed pronounced morphological abnormalities, decreased survival, and increased isoaspartyl levels. Interestingly, we identified a profound perturbation of brain calcium homeostasis in these morphants. An abnormal calcium response upon ATP stimulation was also observed in mouse hippocampal HT22 cells knocked out for Pcmt1. This work shows that zebrafish is a promising model to unravel further facets of PCMT function and demonstrates, for the first time in vivo, that PCMT plays a pivotal role in the regulation of calcium fluxes.

Is the oxidative stress theory of aging dead?

Currently, the oxidative stress (or free radical) theory of aging is the most popular explanation of how aging occurs at the molecular level. While data from studies in invertebrates (e.g., C. elegans and Drosophila) and rodents show a correlation between increased lifespan and resistance to oxidative stress (and in some cases reduced oxidative damage to macromolecules), direct evidence showing that alterations in oxidative damage/stress play a role in aging are limited to a few studies with transgenic Drosophila that overexpress antioxidant enzymes. Over the past eight years, our laboratory has conducted an exhaustive study on the effect of under- or overexpressing a large number and wide variety of genes coding for antioxidant enzymes. In this review, we present the survival data from these studies together. Because only one (the deletion of the Sod1 gene) of the 18 genetic manipulations we studied had an effect on lifespan, our data calls into serious question the hypothesis that alterations in oxidative damage/stress play a role in the longevity of mice.

13 Protein L-isoaspartyl, D-aspartyl O-methyltransferases: Catalysts for protein repair

Protein L-isoaspartyl, D-aspartyl O-methyltransferases (PIMTs) are ancient enzymes distributed through all phylogenetic domains. PIMTs catalyze the methylation of L-isoaspartyl, and to a lesser extent D-aspartyl, residues arising from the spontaneous deamidation and isomerization of protein asparaginyl and aspartyl residues. PIMTs catalyze the methylation of isoaspartyl residues in a large number of primary sequence configurations, which accounts for the broad specificity of the enzyme for protein substrates both in vitro and in vivo. PIMT-catalyzed methylation of isoaspartyl substrates initiates the repair of the polypeptide backbone in its damaged substrates by a spontaneous mechanism that involves a succinimidyl intermediate. The repair process catalyzed by PEVITs is not completely efficient, however, leaving open the possibility that unidentified enzymatic activities cooperate with PIMT in the repair process. Structurally, PIMTs are members of the class I family of AdoMet-dependent methyltransferases. PIMTs have a unique topological arrangement of strands in the central β sheet that provides a signature for this class of enzymes. The regulation and physiological significance of PIMT has been studied in several model organisms. PIMTs are constitutively synthesized by cells, but they can be upregulated in response to conditions that are potentially damaging to protein structures, or when proteins are stored for prolonged periods of time. Disruption of PIMT genes in bacteria and simple eukaryotes produces subtle phenotypes that are apparent only under stress. Loss of PIMT function in transgenic mice leads to fatalepilepsy, suggesting that PIMT function is particularly important to neurons in mammals.

Improved rotorod performance and hyperactivity in mice deficient in a protein repair methyltransferase

The protein L-isoaspartate (D-aspartate)-O-methyltransferase participates in the repair of age-induced protein damage by initiating the conversion of abnormal aspartyl residues within proteins to normal L-aspartyl residues. Previous studies have shown that mice deficient in the gene encoding this enzyme (Pcmt1-/-) accumulate damaged proteins, have altered levels of brain S-adenosylmethionine (AdoMet) and S-adenosylhomocysteine (AdoHcy), and suffer from epileptic seizures that result in death at an average age of about 42 days. In this study, we found that the behavior of Pcmt1-/- mice is abnormal in comparison to their wild-type (Pcmt1+/+) and heterozygous (Pcmt1+/-) littermates in two standard quantitative behavioral assays - the accelerating rotorod and the open-field test. On the accelerating rotorod, we found Pcmt1-/- mice actually perform significantly better than their heterozygous and wild-type littermates, a situation that has only been infrequently described in the literature and has not been described to date for epilepsy-prone mice. The Pcmt1-/- mice show, however, hyperactivity in the open-field test that becomes more pronounced with age, with a partial habituation with time in the chamber. Additionally, these mice demonstrate a strong thigmotaxic movement pattern. We present evidence that these phenotypes are not related to the alterations of the AdoMet/AdoHcy ratio in the brain and thus may be a function of the accumulation of damaged proteins. These results implicate a role for this enzyme in motor coordination and cerebellum development and suggest the importance of the function of the repair methyltransferase in hippocampal-dependent spatial learning.