3 Diverse roles of protein arginine methyltransferases

Inhibition of coactivator-associated arginine methyltransferase 1 modulates dendritic arborization and spine maturation of cultured hippocampal neurons

An improved understanding of the molecular mechanisms in synapse formation provides insight into both learning and memory and the etiology of neurodegenerative disorders. Coactivator-associated arginine methyltransferase 1 (CARM1) is a protein methyltransferase that negatively regulates synaptic gene expression and inhibits neuronal differentiation. Despite its regulatory function in neurons, little is known about the CARM1 cellular location and its role in dendritic maturation and synapse formation. Here, we examined the effects of CARM1 inhibition on dendritic spine and synapse morphology in the rat hippocampus. CARM1 was localized in hippocampal post-synapses, with immunocytochemistry and electron microscopy revealing co-localization of CARM1 with post-synaptic density (PSD)-95 protein, a post-synaptic marker. Specific siRNA-mediated suppression of CARM1 expression resulted in precocious dendritic maturation, with increased spine width and density at sites along dendrites and induction of mushroom-type spines. These changes were accompanied by a striking increase in the cluster size and number of key synaptic proteins, including N-methyl-d-aspartate receptor subunit 2B (NR2B) and PSD-95. Similarly, pharmacological inhibition of CARM1 activity with the CARM1-specific inhibitor AMI-1 significantly increased spine width and mushroom-type spines and also increased the cluster size and number of NR2B and cluster size of PSD-95. These results suggest that CARM1 is a post-synaptic protein that plays roles in dendritic maturation and synaptic formation and that spatiotemporal regulation of CARM1 activity modulates neuronal connectivity and improves synaptic dysfunction.

16 Inhibition of mammalian protein methyltransferases by 5'-methylthioadenosine (MTA): A mechanism of action of dietary same?

5'-deoxy-5'-methylthioadenosine (5'-methylthioadenosine, MTA) is a naturally occurring metabolite. As an experimental reagent, it has proved useful in providing investigators a window onto the role of protein methylation reactions in intact cells, although its mode of action is poorly understood in most cases. This chapter reevaluates its utility as a reagent. It appears now that MTA is at best a poor direct inhibitor of methyltransferases and that its effectiveness in intact cells may depend on its ability to inhibit S-adenosyl-l-homocysteine hydrolase. This chapter reviews recent evidence that points to an important role for MTA as an intermediary in the beneficial pharmaceutical action of orally ingested S-adenosyl-l-methionine (AdoMet, SAMe). These new results suggest that oral AdoMet may function not by enhancing the activity of cellular methyltransferases, as has been previously surmised, but by inhibiting their action. Such inhibition, particularly of protein methyltransferases involved in intracellular communication, may attenuate signal transduction pathways otherwise leading to inflammatory damage to tissues.

Structural dynamics of protein lysine methylation and demethylation

Lysine methylation plays a central role in the "histone code" that regulates chromatin structure, impacts transcription, and responds to DNA damage. A single lysine can be mono-, di-, tri-methylated, or unmethylated, with different functional consequences for each of the four forms. This review (written in the early 2006) described structural aspects of methylation of histone lysine residues by two enzyme families with entirely different structural scaffolding (the SET proteins and Dot1p), and the protein motifs that recognize (decode) these methyl-lysine signals. We also discuss the recently discovered protein lysine demethylating enzymes (LSD1 and JmjC domains).