Lipid metabolism in phosphatidylinositol transfer protein α-deficient vibrator mice

Mammalian diseases of phosphatidylinositol transfer proteins and their homologs

Inositol and phosphoinositide signaling pathways represent major regulatory systems in eukaryotes. The physiological importance of these pathways is amply demonstrated by the variety of diseases that involve derangements in individual steps in inositide and phosphoinositide production and degradation. These diseases include numerous cancers, lipodystrophies and neurological syndromes. Phosphatidylinositol transfer proteins (PITPs) are emerging as fascinating regulators of phosphoinositide metabolism. Recent advances identify PITPs (and PITP-like proteins) to be coincidence detectors, which spatially and temporally coordinate the activities of diverse aspects of the cellular lipid metabolome with phosphoinositide signaling. These insights are providing new ideas regarding mechanisms of inherited mammalian diseases associated with derangements in the activities of PITPs and PITP-like proteins.

Biochemical and biological functions of class I phosphatidylinositol transfer proteins

Phosphoinositides function in a diverse array of cellular activities. They include a role as substrate for lipid kinases and phospholipases to generate second messengers, regulators of the cytoskeleton, of enzymes and of ion channels, and docking sites for reversible recruitment of proteins to membranes. Mammalian phosphatidylinositol transfer proteins, PITPalpha and PITPbeta are paralogs that share 77% sequence identity and contain a hydrophobic cavity that can sequester either phosphatidylinositol or phosphatidylcholine. A string of 11 amino acid residues at the C-terminal acts as a "lid" which shields the lipid from the aqueous environment. PITPs in vitro can facilitate inter-membrane lipid transfer and this requires the movement of the "lid" to allow the lipid cargo to be released. Thus PITPs are structurally designed for delivering lipid cargo and could thus participate in cellular events that are dependent on phosphatidylinositol or derivatives of phosphatidylinositol. Phosphatidylinositol, the precursor for all phosphoinositides is synthesised at the endoplasmic reticulum and its distribution to other organelles could be facilitated by PITPs. Here we highlight recent studies that report on the three-dimensional structures of the different PITP forms and suggest how PITPs are likely to dock at the membrane surface for lipid delivery and extraction. Additionally we discuss whether PITPs are important regulators of sphingomyelin metabolism, and finally describe recent studies that link the association of PITPs with diverse functions including membrane traffic at the Golgi, neurite outgrowth, cytokinesis and stem cell growth.

Subcellular localization of phosphatidylinositol synthesis

It is well-established that the endoplasmic reticulum is the major site of phosphatidylinositol (PtdIns) synthesis. The PtdIns synthetic ability of other organelles, such as plasma membrane and nucleus, remains controversial. In the present study, we re-examine this question by comparing PtdIns synthesis in isolated cytoplasts (enucleated cells) with that in corresponding karyoplasts (nuclei surrounded by plasma membrane but lacking most cytoplasmic components). We report that cytoplasts are competent to carry out both basal and stimulated PtdIns synthesis as well as polyphosphoinositide hydrolysis, while karyoplasts can neither synthesize PtdIns nor hydrolyze phosphoinositides in response to agonists. The karyoplasts are, however, capable of synthesizing phosphatidylcholine (PtdCho), as previously reported. From these data, we conclude that PtdIns synthesis is limited to cytoplasmic components, and cannot be sustained by either plasma membrane or nucleus under conditions that permit robust PtdCho synthesis.

Phosphorylation of a distinct structural form of phosphatidylinositol transfer protein alpha at Ser166 by protein kinase C disrupts receptor-mediated phospholipase C signaling by inhibiting delivery of phosphatidylinositol to membranes

Phosphatidylinositol transfer protein alpha (PITPalpha) participates in the supply of phosphatidylinositol (PI) required for many cellular events including phospholipase C (PLC) beta and gamma signaling by G-protein-coupled receptors and receptor-tyrosine kinases, respectively. Protein kinase C has been known to modulate PLC signaling by G-protein-coupled receptors and receptor-tyrosine kinases, although the molecular target has not been identified in most instances. In each case phorbol myristate acetate pretreatment of HL60, HeLa, and COS-7 cells abrogated PLC stimulation by the agonists formyl-Met-Leu-Phe, ATP, and epidermal growth factor, respectively. Here we show that phosphorylation of PITPalpha at Ser166 resulted in inhibition of receptor-stimulated PLC activity. Ser166 is localized in a small pocket between the 165-172 loop and the rest of the protein and was not solvent-accessible in either the PI- or phosphatidylcholine-loaded structures of PITPalpha. To allow phosphorylation at Ser166, a distinct structural form is postulated, and mutation of Thr59 to alanine shifted the equilibrium to this form, which could be resolved on native PAGE. The elution profile observed by size exclusion chromatography of phosphorylated PITPalpha from rat brain or in vitro phosphorylated PITPalpha demonstrated that phosphorylated PITPalpha is structurally distinct from the non-phosphorylated form. Phosphorylated PITPalpha was unable to deliver its PI cargo, although it could deliver phosphatidylcholine. We conclude that the PITPalpha structure has to relax to allow access to the Ser166 site, and this may occur at the membrane surface where PI delivery is required for receptor-mediated PLC signaling.