A Small Molecule Mimicking a Phosphatidylinositol (4,5)-Bisphosphate Binding Pleckstrin Homology Domain

Inositol phospholipids have emerged as important key players in a wide variety of cellular functions. Among the seven existing inositol phospholipids, phosphatidylinositol (4,5)-bisphosphate (PI(4,5)P₂) has attracted much attention in recent years due to its important role in numerous cellular signaling events and regulations, which in turn impact several human diseases. This particular lipid is recognized in the cell by specific lipid binding domains, such as the Pleckstrin-homology (PH) domain, which is also employed as a tool to monitor this important lipid. Here, we describe the synthesis and biological characterization of a small molecule that mimics the PH domain as judged by its ability to bind specifically to only PI(4,5)P₂ and effectively compete with the PH domain <i>in vitro</i> and in a cellular environment. The binding constant of this small molecule PH domain mimetic (PHDM) was determined to be 17.6 ± 10.1 μM, similar in potency to the PH domain. Using NIH 3T3 mouse fibroblast cells we demonstrated that this compound is cell-permeable and able to modulate PI(4,5)P₂-dependent effects in a cellular environment such as the endocytosis of the transferrin receptor, loss of mitochondria, as well as stress fiber formation. This highly PI(4,5)P₂-specific chemical mimetic of a PH domain not only is a powerful research tool but might also be a lead compound in future drug developments targeting PI(4,5)P₂-dependent diseases such as Lowe syndrome

Nuclear envelope remnants are enriched in polyphosphoinositides.

<p>Lipid analysis of nuclear envelope remnants. (A) Lipids extracted from <i>L. pictus</i> demembranated sperm cells were separated by HPLC on a normal phase column and characterized by ESI-MS/MS using the precursor ion scans of sphingomyelin (SM), phosphatidylglycerol (PtdGly), phosphatidylethanolamine (PtdEth), phosphatidic acid (PtdAc), phosphatidylserine (PtdSer), phosphatidylcholine (PtdCho), phosphatidylinositol (PtdIns) or using the multiple ion scans of phosphatidylinositolphosphate (PtdInsP), phosphatidylinositolbisphosphate (PtdInsP₂) and phosphatidylinositoltrisphosphate (PtdInsP₃). Phospholipids were quantified using 12∶0/12∶0 (SM, PtdGly, PtdEth, PtdAc, PtdSer and PtdCho) or 16∶0/16∶0 (PtdIns, PtdInsP, PtdInsP₂ and PtdInsP₃) internal standards. Data expressed as mean±SEM (n = 3). (B) Alkyl-acyl versus diacyl phosphoinositides species distribution in nuclear envelope remnants. Mole percentages of diacyl species (green) and alkyl-acyl species (blue) were quantified from the multiple ion scans for each phosphoinositide class: PtdInsP, PtdInsP₂ and PtdInsP₃. 38% of PtdInsP, 15% of PtdInsP₂ and 49% of PtdInsP₃ are diacyl species. The PtdInsP₂ is predominantly alkyl-acyl phosphoinositide. Data expressed as mean±SEM (n = 3).</p

Nuclear envelope remnants are relatively fluid membranes despite their enrichment in cholesterol.

<p>The top solid-state deuterium NMR spectrum corresponds to POPC-²H₃₁ small unilamellar vesicles (SUV) acquired at 10°C post equilibrium at 40°C for 22h. The second spectrum corresponds to POPC-²H₃₁ MLVs containing 30 mol% cholesterol acquired at 10°C after equilibration at 40°C. The bottom two spectra are of 0.1% nuclei incubated with SUVs of deuterated lipid (POPC-²H₃₁) for 30 min at 40°C. Labelled nuclear envelope remnants were equilibrated at 40°C for 22h and the ²H NMR spectrum was acquired post-equilibrium at 10°C for 4h (NER Tfin₁) and 20h (NER Tfin₂). The dashed lines show the plateau quadrupolar splitting enlargement of labelled nuclear envelope remnants post-equilibrium. Arrows show the plateau quadrupolar splittings used to calculate the order parameters. NMR spectra are representative of those obtained in a duplicate experiment.</p

Nuclear envelope remnants contain two membranes that line the acrosomal and centriolar fossae.

<p>(A) <i>P. lividus</i> sperm cells were fixed in the presence of 1% (w/v) tannic acid. The plasma membrane (PM) and the mitochondrial membranes (MM) are shown. AV: acrosomal vesicle, N: nucleus, F: flagellum. The nuclear envelope is tightly apposed to the chromatin but cup-like structures with nuclear envelope remnants can be seen at the poles (arrows). (B and C) <i>S. purpuratus</i> 0.1% nuclei were incubated in egg cytoplasm supplemented with ATP-GS and fixed in the presence of 1% (w/v) tannic acid. Electron dense structures (arrows) are shown in the centriolar (B) and acrosomal fossae (C). The two bilayers appear to have variable amounts of electron dense material between them. (D) Cryosections of <i>S. purpuratus</i> 0.1% nuclei prefixed in 4% (v/v) formaldehyde for 3h on ice show two membranes in the centriolar fossa (arrows). (E) <i>S. purpuratus</i> 0.1% nuclei were incubated in egg cytoplasm in the presence of an ATP-generating system, fixed in 2.5% (v/v) glutaraldehyde in the presence of 1% (w/v) tannic acid and viewed by TEM. The glancing cross section of the centriolar fossa shows the nuclear envelope remnants and an egg membrane vesicle (arrow) associated with the nuclear envelope remnants. Bars are 500nm (A), 400nm (B) and 200nm (C, D and E). The data are representative of nuclei observed in at least 3 experiments on independent nuclei preparations.</p

Nuclear envelope remnant phospholipid species are mainly polyunsaturated and arachidonyl.

<p>PtdCho, PtdEth and PtdIns species extracted from <i>L. pictus</i> 0.1% nuclei were characterized using the precursor ion scans of +184m/z, −196m/z and −241m/z respectively. Both alkyl-acyl (denoted by ‘a’) and diacyl species were mostly arachidonyl on their sn₂ positions. Lipid species are listed by descending order of abundance.</p

Engineering <i>de Novo</i> Membrane-Mediated Protein–Protein Communication Networks

Mechanical properties of biological membranes are known to regulate membrane protein function. Despite this, current models of protein communication typically feature only direct protein–protein or protein–small molecule interactions. Here we show for the first time that, by harnessing nanoscale mechanical energy within biological membranes, it is possible to promote controlled communication between proteins. By coupling lipid–protein modules and matching their response to the mechanical properties of the membrane, we have shown that the action of phospholipase A₂ on acyl-based phospholipids triggers the opening of the mechanosensitive channel, MscL, by generating membrane asymmetry. Our findings confirm that the global physical properties of biological membranes can act as information pathways between proteins, a novel mechanism of membrane-mediated protein–protein communication that has important implications for (i) the underlying structure of signaling pathways, (ii) our understanding of <i>in vivo</i> communication networks, and (iii) the generation of building blocks for artificial protein networks

Poly-phosphoinositides in 0.1% nuclei and whole sperm are enriched in the acrosomal and centriolar fossae.

<p><i>L. pictus</i> 0.1% nuclei (left) and whole live sperm (right) were incubated with the Texas Red labelled MARCKS peptide and visualised by fluorescence microscopy. The punctate staining of the acrosomal and centriolar fossae is typical of the majority of nuclei observed in experiments on two independent sperm and 0.1% nuclei preparations.</p

Cholesterol removal from nuclear envelope remnants inhibits membrane fusion events of nuclear envelope formation.

<p>(A) 0.1% <i>L. pictus</i> nuclei were treated with 10mM MβCD or untreated (control). Nuclei were filipin stained, normalized to an equal nuclei concentration and excited at 360nm. The fluorescence intensity of the emission peak at 479/480nm was measured, and data were normalized to the control value. Data shown are mean±SEM of four experiments conducted in duplicate. (B) 0.1% <i>L. pictus</i> nuclei were treated with 10mM MβCD (white) or untreated (black). Nuclear envelope precursor MVs were subsequently bound to nuclei (ATP), and in parallel reactions their fusion was triggered with GTP to complete envelope formation. At least 20 nuclei were scored for the presence of a fully-formed nuclear envelope in 3 independent experiments. Data shown are mean±SEM.</p

Order parameters (2S_CD) for deuterium labelled POPC incorporated into nuclear envelope remnants and model membranes (MLVs).

<p>Cholesterol/cholesteryl ester and phospholipid concentrations were determined by colorimetry. Cholesterol contents are expressed as molar percentages relative to total phospholipids. Nuclear envelope remnant order parameter was calculated from the ²H NMR spectrum of labelled nuclear envelope remnants acquired post-equilibrium (T_fin). POPC MLVs containing 0, 5, 15 or 30 mol% cholesterol values are taken from Garnier-Lhomme <i>et al.</i><a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0004255#pone.0004255-GarnierLhomme2" target="_blank">[34]</a>. Deuterium spectra of labelled nuclear envelope remnants and MLVs were acquired at 10°C after equilibration at 40°C.</p

Quantification of NE phenotype in single cell experiments of HeLa cells co-expressing LBR and DGKε.

<p>Treatment with rapalogue shows that 76% of cells co-expressing LBR and DGKε at cytokinesis had an incomplete NE, the mutant DGKε shows that 94.7% of the cells had a complete NE. The DAG-containing SUV rescue experiments show 89.7% of the cells had a complete NE at cytokinesis.</p