Quantitative profiling of the endonuclear glycerophospholipidome of murine embryonic fibroblasts

A reliable method for purifying envelope-stripped nuclei from immortalized murine embryonic fibroblasts (iMEFs) was established. Quantitative profiling of the glycerophospholipids (GPLs) in envelope-free iMEF nuclei yields several conclusions. First, we find the endonuclear glycerophospholipidome differs from that of bulk membranes, and phosphatidylcholine (PtdCho) and phosphatidylethanolamine species are the most abundant endonuclear GPLs by mass. By contrast, phosphatidylinositol (PtdIns) represents a minor species. We also find only a slight enrichment of saturated versus unsaturated GPL species in iMEF endonuclear fractions. Moreover, much lower values for GPL mass were measured in the iMEF nuclear matrix than those reported for envelope-stripped IMF-32 nuclei. The collective results indicate that the nuclear matrix in these cells is a GPL-poor environment where GPL occupies only approximately 0.1% of the total nuclear matrix volume. This value suggests GPL accommodation in this compartment can be satisfied by binding to resident proteins. Finally, we find no significant role for the PtdIns/PtdCho-transfer protein, PITPα, in shuttling PtdIns into the iMEF nuclear matrix.

Lamellipodia are crucial for haptotactic sensing and response

Haptotaxis is the process by which cells respond to gradients of substrate-bound cues, such as extracellular matrix proteins (ECM); however, the cellular mechanism of this response remains poorly understood and has mainly been studied by comparing cell behavior on uniform ECMs with different concentrations of components. To study haptotaxis in response to gradients, we utilized microfluidic chambers to generate gradients of the ECM protein fibronectin, and imaged the cell migration response. Lamellipodia are fan-shaped protrusions that are common in migrating cells. Here, we define a new function for lamellipodia and the cellular mechanism required for haptotaxis - differential actin and lamellipodial protrusion dynamics lead to biased cell migration. Modest differences in lamellipodial dynamics occurring over time periods of seconds to minutes are summed over hours to produce differential whole cell movement towards higher concentrations of fibronectin. We identify a specific subset of lamellipodia regulators as being crucial for haptotaxis. Numerous studies have linked components of this pathway to cancer metastasis and, consistent with this, we find that expression of the oncogenic Rac1 P29S mutation abrogates haptotaxis. Finally, we show that haptotaxis also operates through this pathway in 3D environments.

p16INK4a reporter mice reveal age-promoting effects of environmental toxicants

While murine-based systems to identify cancer-promoting agents (carcinogens) are established, models to identify compounds that promote aging (gerontogens) have not been described. For this purpose, we exploited the transcription of p16INK4a, which rises dynamically with aging and correlates with age-associated disease. Activation of p16INK4a was visualized in vivo using a murine strain that harbors a knockin of the luciferase gene into the Cdkn2a locus (p16LUC mice). We exposed p16LUC mice to candidate gerontogens, including arsenic, high-fat diet, UV light, and cigarette smoke and serially imaged animals to monitor senescence induction. We show that exposure to a high-fat diet did not accelerate p16INK4a expression, whereas arsenic modestly augmented, and cigarette smoke and UV light potently augmented, activation of p16INK4a-mediated senescence. This work provides a toxicological platform to study mammalian aging and suggests agents that directly damage DNA promote molecular aging.

Zebrafish class 1 phosphatidylinositol transfer proteins: PITPbeta and double cone cell outer segment integrity in retina

Phosphatidylinositol transfer proteins (PITPs) in yeast co-ordinate lipid metabolism with the activities of specific membrane trafficking pathways. The structurally unrelated metazoan PITPs (mPITPs), on the other hand, are an under-investigated class of proteins. It remains unclear what biological activities mPITPs discharge, and the mechanisms by which these proteins function are also not understood. The soluble class 1 mPITPs include the PITPalpha and PITPbeta isoforms. Of these, the beta-isoforms are particularly poorly characterized. Herein, we report the use of zebrafish as a model vertebrate for the study of class 1 mPITP biological function. Zebrafish express PITPalpha and PITPbeta-isoforms (Pitpna and Pitpnb, respectively) and a novel PITPbeta-like isoform (Pitpng). Pitpnb expression is particularly robust in double cone cells of the zebrafish retina. Morpholino-mediated protein knockdown experiments demonstrate Pitpnb activity is primarily required for biogenesis/maintenance of the double cone photoreceptor cell outer segments in the developing retina. By contrast, Pitpna activity is essential for successful navigation of early developmental programs. This study reports the initial description of the zebrafish class 1 mPITP family, and the first analysis of PITPbeta function in a vertebrate.

The pathologies associated with functional titration of phosphatidylinositol transfer protein α activity in mice

Phosphatidylinositol transfer proteins (PITPs) bind phosphatidylinositol (PtdIns) and phosphatidylcholine and play diverse roles in coordinating lipid metabolism/signaling with intracellular functions. The underlying mechanisms remain unclear. Genetic ablation of PITPalpha in mice results in neonatal lethality characterized by intestinal and hepatic steatosis, spinocerebellar neurodegeneration, and glucose homeostatic defects. We report that mice expressing a PITPalpha selectively ablated for PtdIns binding activity (Pitpalpha(T59D)), as the sole source of PITPalpha, exhibit phenotypes that recapitulate those of authentic PITPalpha nullizygotes. Analyses of mice with graded reductions in PITPalpha activity reveal proportionately graded reductions in lifespan, demonstrate that intestinal steatosis and hypoglycemia are apparent only when PITPalpha protein levels are strongly reduced (>or=90%), and correlate steatotic and glucose homeostatic defects with cerebellar inflammatory disease. Finally, reconstitution of PITPalpha expression in the small intestine substantially corrects the chylomicron retention disease and cerebellar inflammation of Pitpalpha(0/0) neonates, but does not rescue neonatal lethality in these animals. These data demonstrate that PtdIns binding is an essential functional property of PITPalpha in vivo, and suggest a causal linkage between defects in lipid transport and glucose homeostasis and cerebellar inflammatory disease. Finally, the data also demonstrate intrinsic neuronal deficits in PITPalpha-deficient mice that are independent of intestinal lipid transport defects and hypoglycemia.

Defects in yolk sac vasculogenesis, chorioallantoic fusion, and embryonic axis elongation in mice with targeted disruption of Yap65

YAP is a multifunctional adapter protein and transcriptional coactivator with several binding partners well described in vitro and in cell culture. To explore in vivo requirements for YAP, we generated mice carrying a targeted disruption of the Yap gene. Homozygosity for the Yap(tm1Smil) allele (Yap-/-) caused developmental arrest around E8.5. Phenotypic characterization revealed a requirement for YAP in yolk sac vasculogenesis. Yolk sac endothelial and erythrocyte precursors were specified as shown by histology, PECAM1 immunostaining, and alpha globin expression. Nonetheless, development of an organized yolk sac vascular plexus failed in Yap-/- embryos. In striking contrast, vasculogenesis proceeded in both the allantois and the embryo proper. Mutant embryos showed patterned gene expression domains along the anteroposterior neuraxis, midline, and streak/tailbud. Despite this evidence of proper patterning and tissue specification, Yap-/- embryos showed developmental perturbations that included a notably shortened body axis, convoluted anterior neuroepithelium, caudal dysgenesis, and failure of chorioallantoic fusion. These results reveal a vital requirement for YAP in the developmental processes of yolk sac vasculogenesis, chorioallantoic attachment, and embryonic axis elongation.

Use of mass spectrometry-based lipidomics to probe PITPalpha (phosphatidylinositol transfer protein alpha) function inside the nuclei of PITPalpha+/+ and PITPalpha-/- cells

The mammalian phospholipid exchange protein PITPalpha (phosphatidylinositol transfer protein alpha), found in both extranuclear and endonuclear compartments, is thought in part to facilitate nuclear import of the PtdIns (phosphatidylinositol) consumed in the generation of proliferation-associated endonuclear diacylglycerol accumulations. Unlike phosphatidylcholine, endonuclear PtdIns is not synthesized in situ. However, despite progressive postnatal lethality of PITPalpha ablation in mice, PITPalpha(-/-) MEF (mouse embryonic fibroblasts) lack an obviously impaired proliferative capacity. We used ESI-MS (tandem electrospray ionization-MS) to monitor incorporation of the deuterated phospholipid precursors, choline-d(9) and inositol-d(6), into molecular species of whole cell and endonuclear phosphatidylcholine and PtdIns over 24 h to assess the contribution of PITPalpha to the nuclear import of PtdIns into MEF cells. In cells labelled for 1, 3, 6, 12 and 24 h fractional inositol-d(6) incorporation into whole-cell PtdIns species was consistently higher in PITPalpha(-/-) MEF implying greater flux through its biosynthetic pathway. Moreover, endonuclear accumulation of PtdIns-d(6) was apparent in the PITPalpha(-/-) cells and mirrored that in PITPalpha(+/+) cells. Together, these results suggest that the essential endonuclear PtdIns import via PITPalpha can be accommodated by other mechanisms.

Mice lacking phosphatidylinositol transfer protein-alpha exhibit spinocerebellar degeneration, intestinal and hepatic steatosis, and hypoglycemia

Phosphatidylinositol transfer proteins (PITPs) regulate the interface between lipid metabolism and cellular functions. We now report that ablation of PITP alpha function leads to aponecrotic spinocerebellar disease, hypoglycemia, and intestinal and hepatic steatosis in mice. The data indicate that hypoglycemia is in part associated with reduced proglucagon gene expression and glycogenolysis that result from pancreatic islet cell defects. The intestinal and hepatic steatosis results from the intracellular accumulation of neutral lipid and free fatty acid mass in these organs and suggests defective trafficking of triglycerides and diacylglycerols from the endoplasmic reticulum. We propose that deranged intestinal and hepatic lipid metabolism and defective proglucagon gene expression contribute to hypoglycemia in PITP alpha-/- mice, and that hypoglycemia is a significant contributing factor in the onset of spinocerebellar disease. Taken together, the data suggest an unanticipated role for PITP alpha in with glucose homeostasis and in mammalian endoplasmic reticulum functions that interface with transport of specific luminal lipid cargoes.

Genetic ablation of phosphatidylinositol transfer protein function in murine embryonic stem cells

Phosphatidylinositol transfer proteins (PITPs) regulate the interface between signal transduction, membrane-trafficking, and lipid metabolic pathways in eukaryotic cells. The best characterized mammalian PITPs are PITP alpha and PITP beta, two highly homologous proteins that are encoded by distinct genes. Insights into PITP alpha and PITP beta function in mammalian systems have been gleaned exclusively from cell-free or permeabilized cell reconstitution and resolution studies. Herein, we report for the first time the use of genetic approaches to directly address the physiological functions of PITP alpha and PITP beta in murine cells. Contrary to expectations, we find that ablation of PITP alpha function in murine cells fails to compromise growth and has no significant consequence for bulk phospholipid metabolism. Moreover, the data show that PITP alpha does not play an obvious role in any of the cellular activities where it has been reconstituted as an essential stimulatory factor. These activities include protein trafficking through the constitutive secretory pathway, endocytic pathway function, biogenesis of mast cell dense core secretory granules, and the agonist-induced fusion of dense core secretory granules to the mast cell plasma membrane. Finally, the data demonstrate that PITP alpha-deficient cells not only retain their responsiveness to bulk growth factor stimulation but also retain their pluripotency. In contrast, we were unable to evict both PITP beta alleles from murine cells and show that PITP beta deficiency results in catastrophic failure early in murine embryonic development. We suggest that PITP beta is an essential housekeeping PITP in murine cells, whereas PITP alpha plays a far more specialized function in mammals than that indicated by in vitro systems that show PITP dependence.

Functional characterization of a mammalian Sac1 and mutants exhibiting substrate-specific defects in phosphoinositide phosphatase activity

The Saccharomyces cerevisiae SAC1 gene was identified via independent analyses of mutations that modulate yeast actin function and alleviate the essential requirement for phosphatidylinositol transfer protein (Sec14p) activity in Golgi secretory function. The SAC1 gene product (Sac1p) is an integral membrane protein of the endoplasmic reticulum and the Golgi complex. Sac1p shares primary sequence homology with a subfamily of cytosolic/peripheral membrane phosphoinositide phosphatases, the synaptojanins, and these Sac1 domains define novel phosphoinositide phosphatase modules. We now report the characterization of a rat counterpart of Sac1p. Rat Sac1 is a ubiquitously expressed 65-kDa integral membrane protein of the endoplasmic reticulum that is found at particularly high levels in cerebellar Purkinje cells. Like Sac1p, rat Sac1 exhibits intrinsic phosphoinositide phosphatase activity directed toward phosphatidylinositol 3-phosphate, phosphatidylinositol 4-phosphate, and phosphatidylinositol 3,5-bisphosphate substrates, and we identify mutant rat sac1 alleles that evoke substrate-specific defects in this enzymatic activity. Finally, rat Sac1 expression in Deltasac1 yeast strains complements a wide phenotypes associated with Sac1p insufficiency. Biochemical and in vivo data indicate that rat Sac1 phosphatidylinositol-4-phosphate phosphatase activity, but not its phosphatidylinositol-3-phosphate or phosphatidylinositol-3, 5-bisphosphate phosphatase activities, is essential for the heterologous complementation of Sac1p defects in vivo. Thus, yeast Sac1p and rat Sac1 are integral membrane lipid phosphatases that play evolutionary conserved roles in eukaryotic cell physiology.

Yeast Sec14p deficient in phosphatidylinositol transfer activity is functional in vivo

Yeast phosphatidylinositol transfer protein (Sec14p) is essential for Golgi secretory function. It is widely accepted, though unproven, that phosphatidylinositol transfer between membranes represents the physiological activity of phosphatidylinositol transfer proteins (PITPs). We report that Sec14pK66,239A is inactivated for phosphatidylinositol, but not phosphatidylcholine (PC), transfer activity. As expected, Sec14pK66,239A fails to meet established criteria for a PITP in vitro and fails to stimulate phosphoinositide production in vivo. However, its expression efficiently rescues the lethality and Golgi secretory defects associated with sec14-1ts and sec14 null mutations. This complementation requires neither phospholipase D activation nor the involvement of a novel class of minor yeast PITPs. These findings indicate that PI binding/transfer is remarkably dispensable for Sec14p function in vivo.

Phosphatidylinositol transfer proteins: the long and winding road to physiological function

Phosphatidylinositol transfer proteins (PITPs) have historically been thought to help execute lipid-sorting events by transporting phospholipid monomers between membrane bilayers. Recent data, however, indicate unanticipated roles for PITPs in the coordination and/or coupling of phospholipid metabolism with vesicle trafficking and the downregulation of signal-transduction reactions. We are only now beginning to appreciate both the identities of PITP-dependent cellular reactions and the intriguing mechanisms by which PITPs execute their functions in eukaryotic cells.

A phosphatidylinositol 3-kinase and phosphatidylinositol transfer protein act synergistically in formation of constitutive transport vesicles from the trans-Golgi network

Current evidence suggests that phosphatidylinositol (PI) kinases and phosphatidylinositol transfer protein (PITP) are involved in driving vesicular traffic from yeast and mammalian trans-Golgi network (TGN). We have tested the interaction between these cytosolic proteins in an assay that measures the formation of constitutive transport vesicles from the TGN in a hepatocyte cell-free system. This reaction is dependent on a novel PI 3-kinase, and we now report that, under conditions of limiting cytosol, purified PI 3-kinase and PITP functionally cooperate to drive exocytic vesicle formation. This synergy was observed with both yeast and mammalian PITPs, and it also extended to the formation of PI 3-phosphate. These collective findings indicate that the PI 3-kinase and PITP synergize to form a pool of PI 3-phosphate that is essential for formation of exocytic vesicles from the hepatocyte TGN.

The phosphatidylinositol transfer protein domain of Drosophila retinal degeneration B protein is essential for photoreceptor cell survival and recovery from light stimulation

The Drosophila retinal degeneration B (rdgB) gene encodes an integral membrane protein involved in phototransduction and prevention of retinal degeneration. RdgB represents a nonclassical phosphatidylinositol transfer protein (PITP) as all other known PITPs are soluble polypeptides. Our data demonstrate roles for RdgB in proper termination of the phototransduction light response and dark recovery of the photoreceptor cells. Expression of RdgB's PITP domain as a soluble protein (RdgB-PITP) in rdgB2 mutant flies is sufficient to completely restore the wild-type electrophysiological light response and prevent the degeneration. However, introduction of the T59E mutation, which does not affect RdgB-PITP's phosphatidylinositol (PI) and phosphatidycholine (PC) transfer in vitro, into the soluble (RdgB-PITP-T59E) or full-length (RdgB-T59E) proteins eliminated rescue of retinal degeneration in rdgB2 flies, while the light response was partially maintained. Substitution of the rat brain PITPalpha, a classical PI transfer protein, for RdgB's PITP domain (PITPalpha or PITPalpha-RdgB chimeric protein) neither restored the light response nor maintained retinal integrity when expressed in rdgB2 flies. Therefore, the complete repertoire of essential RdgB functions resides in RdgB's PITP domain, but other PITPs possessing PI and/or PC transfer activity in vitro cannot supplant RdgB function in vivo. Expression of either RdgB-T59E or PITPalpha-RdgB in rdgB+ flies produced a dominant retinal degeneration phenotype. Whereas RdgB-T59E functioned in a dominant manner to significantly reduce steady-state levels of rhodopsin, PITPalpha-RdgB was defective in the ability to recover from prolonged light stimulation and caused photoreceptor degeneration through an unknown mechanism. This in vivo analysis of PITP function in a metazoan system provides further insights into the links between PITP dysfunction and an inherited disease in a higher eukaryote.

Phospholipid metabolism and membrane dynamics

Genetic and biochemical approaches are shedding new light on the distinct physiological functions of specific phospholipid metabolic pathways and the mechanisms by which phospholipids are mobilized between intracellular compartments. In particular, phosphatidylinositol-transfer proteins have recently been revealed to play fascinating and unanticipated roles in the coordination of phospholipid metabolism with vesicle-trafficking and signal-transducing reactions.

Mutant rat phosphatidylinositol/phosphatidylcholine transfer proteins specifically defective in phosphatidylinositol transfer: implications for the regulation of phospholipid transfer activity

The mammalian phosphatidylinositol/phosphatidylcholine transfer proteins (PI-TPs) catalyze exchange of phosphatidylinositol (PI) or phosphatidylcholine (PC) between membrane bilayers in vitro. We find that Ser-25, Thr-59, Pro-78, and Glu-248 make up a set of rat (r) PI-TP residues, substitution of which effected a dramatic reduction in the relative specific activity for PI transfer activity without significant effect on PC transfer activity. Thr-59 was of particular interest as it is a conserved residue in a highly conserved consensus protein kinase C phosphorylation motif in metazoan PI-TPs. Replacement of Thr-59 with Ser, Gln, Val, Ile, Asn, Asp, or Glu effectively abolished PI transfer capability but was essentially silent with respect to PC transfer activity. These findings identify rPI-TP residues that likely cooperate to form a PI head-group binding/recognition site or that lie adjacent to such a site. Finally, the selective sensitivity of the PI transfer activity of rPI-TP to alteration of Thr-59 suggests a mechanism for in vivo regulation of rPI-TP activity.

Phospholipid transfer activity is relevant to but not sufficient for the essential function of the yeast SEC14 gene product

To investigate several key aspects of phosphatidylinositol transfer protein (PI-TP) function in eukaryotic cells, rat PI-TP was expressed in yeast strains carrying lesions in SEC14, the structural gene for yeast PI-TP (SEC14p), whose activity is essential for Golgi secretory function in vivo. Rat PI-TP expression effected a specific complementation of sec14ts growth and secretory defects. Complementation of sec14 mutations was not absolute as rat PI-TP expression failed to rescue sec14 null mutations. This partial complementation of sec14 lesions by rat PI-TP correlated with inability of the mammalian protein to stably associate with yeast Golgi membranes and was not a result of rat PI-TP stabilizing the endogenous sec14ts gene product. These collective data demonstrate that while the in vitro PI-TP activity of SEC14p clearly reflects some functional in vivo property of SEC14p, the PI-TP activity is not the sole essential activity of SEC14p. Those data further identify an efficient Golgi targeting capability as a likely essential feature of SEC14p function in vivo. Finally, the data suggest that stable association of SEC14p with yeast Golgi membranes is not a simple function of its lipid-binding properties, indicate that the amino-terminal 129 SEC14p residues are sufficient to direct a catalytically inactive form of rat PI-TP to the Golgi and provide the first evidence to indicate that a mammalian PI-TP can stimulate Golgi secretory function in vivo.