Inositol polyphosphates: a new frontier for regulating gene expression.

Highly phosphorylated, soluble inositides are an emerging family of potential eukaryotic second messengers. The mechanisms for generating an outstanding diversity of mono- and pyrophosphorylated inositides have been recently elucidated and require a series of conserved lipases, kinases, and phosphatases. With several of the inositol kinases and the phospholipase C having access to the nucleus, roles for inositides in nuclear functions have been suggested. In support of this hypothesis, multiple studies have revealed the protein machines that are modulated by these inositides and found specific roles in nuclear physiology. In this paper, we review a novel paradigm for regulating gene expression at distinct steps by different inositide isomers. We discuss discoveries showing inositol polyphosphate regulation of gene expression at the level of transcription, chromatin remodeling, messenger ribonucleic acid (mRNA) editing, and mRNA export. Recent structural studies of inositol polyphosphate-binding proteins suggest the inositides modulate protein function as essential structural cofactors, triggers for allosteric or induced fit structural changes, and direct antagonistic competitors for other inositide ligands. We propose that the cell orchestrates the localized production of soluble inositol polyphosphates and inositol pyrophosphates to direct decisive and rapid signaling events. These insights also illustrate how extracellular stimuli might faithfully trigger the correct synchrony between gene expression steps and coordinate nuclear responses to changes in cellular environments.