Liver specification of human iPSC-derived endothelial cells transplanted into mouse liver

Background & Aims

Liver sinusoidal endothelial cells (LSECs) are important in liver development, regeneration, and pathophysiology, but the differentiation process underlying their tissue-specific phenotype is poorly understood and difficult to study because primary human cells are scarce. The aim of this study was to use human induced pluripotent stem cell (hiPSC)-derived LSEC-like cells to investigate the differentiation process of LSECs.

Methods

hiPSC-derived endothelial cells (iECs) were transplanted into the livers of Fah-/-/Rag2-/-/Il2rg-/- mice and assessed over a 12-week period. Lineage tracing, immunofluorescence, flow cytometry, plasma human factor VIII measurement, and bulk and single cell transcriptomic analysis were used to assess the molecular and functional changes that occurred following transplantation.

Results

Progressive and long-term repopulation of the liver vasculature occurred as iECs expanded along the sinusoids between hepatocytes and increasingly produced human factor VIII, indicating differentiation into LSEC-like cells. To chart the developmental profile associated with LSEC specification, the bulk transcriptomes of transplanted cells between 1 and 12 weeks after transplantation were compared against primary human adult LSECs. This demonstrated a chronological increase in LSEC markers, LSEC differentiation pathways, and zonation. Bulk transcriptome analysis suggested that the transcription factors NOTCH1, GATA4, and FOS have a central role in LSEC specification, interacting with a network of 27 transcription factors. Novel markers associated with this process included EMCN and CLEC14A. Additionally, single cell transcriptomic analysis demonstrated that transplanted iECs at 4 weeks contained zonal subpopulations with a region-specific phenotype.

Conclusions

Collectively, this study confirms that hiPSCs can adopt LSEC-like features and provides insight into LSEC specification. This humanised xenograft system can be applied to further interrogate LSEC developmental biology and pathophysiology, bypassing current logistical obstacles associated with primary human LSECs.

Impact and implications

Liver sinusoidal endothelial cells (LSECs) are important cells for liver biology, but better model systems are required to study them. We present a pluripotent stem cell xenografting model that produces human LSEC-like cells. A detailed and longitudinal transcriptomic analysis of the development of LSEC-like cells is included, which will guide future studies to interrogate LSEC biology and produce LSEC-like cells that could be used for regenerative medicine.

Engineering transplantable human lymphatic and blood capillary networks in a porous scaffold

Due to a relative paucity of studies on human lymphatic assembly in vitro and subsequent in vivo transplantation, capillary formation and survival of primary human lymphatic (hLEC) and blood endothelial cells (hBEC) ± primary human vascular smooth muscle cells (hvSMC) were evaluated and compared in vitro and in vivo. hLEC ± hvSMC or hBEC ± hvSMC were seeded in a 3D porous scaffold in vitro, and capillary percent vascular volume (PVV) and vascular density (VD)/mm² assessed. Scaffolds were also transplanted into a sub-cutaneous rat wound with morphology/morphometry assessment. Initially hBEC formed a larger vessel network in vitro than hLEC, with interconnected capillaries evident at 2 days. Interconnected lymphatic capillaries were slower (3 days) to assemble. hLEC capillaries demonstrated a significant overall increase in PVV (p = 0.0083) and VD (p = 0.0039) in vitro when co-cultured with hvSMC. A similar increase did not occur for hBEC + hvSMC in vitro, but hBEC + hvSMC in vivo significantly increased PVV (p = 0.0035) and VD (p = 0.0087). Morphology/morphometry established that hLEC vessels maintained distinct cell markers, and demonstrated significantly increased individual vessel and network size, and longer survival than hBEC capillaries in vivo, and established inosculation with rat lymphatics, with evidence of lymphatic function. The porous polyurethane scaffold provided advantages to capillary network formation due to its large (300-600 μm diameter) interconnected pores, and sufficient stability to ensure successful surgical transplantation in vivo. Given their successful survival and function in vivo within the porous scaffold, in vitro assembled hLEC networks using this method are potentially applicable to clinical scenarios requiring replacement of dysfunctional or absent lymphatic networks.

A soft and ultrasensitive force sensing diaphragm for probing cardiac organoids instantaneously and wirelessly

Time-lapse mechanical properties of stem cell derived cardiac organoids are important biological cues for understanding contraction dynamics of human heart tissues, cardiovascular functions and diseases. However, it remains difficult to directly, instantaneously and accurately characterize such mechanical properties in real-time and in situ because cardiac organoids are topologically complex, three-dimensional soft tissues suspended in biological media, which creates a mismatch in mechanics and topology with state-of-the-art force sensors that are typically rigid, planar and bulky. Here, we present a soft resistive force-sensing diaphragm based on ultrasensitive resistive nanocracked platinum film, which can be integrated into an all-soft culture well via an oxygen plasma-enabled bonding process. We show that a reliable organoid-diaphragm contact can be established by an 'Atomic Force Microscope-like' engaging process. This allows for instantaneous detection of the organoids' minute contractile forces and beating patterns during electrical stimulation, resuscitation, drug dosing, tissue culture, and disease modelling.

Sustained subcutaneous delivery of secretome of human cardiac stem cells promotes cardiac repair following myocardial infarction

AIMS

To establish pre-clinical proof of concept that sustained subcutaneous delivery of the secretome of human cardiac stem cells (CSCs) can be achieved in vivo to produce significant cardioreparative outcomes in the setting of myocardial infarction.

METHODS AND RESULTS

Rats were subjected to permanent ligation of left anterior descending coronary artery and randomized to receive subcutaneous implantation of TheraCyte devices containing either culture media as control or 1 × 106 human W8B2+ CSCs, immediately following myocardial ischaemia. At 4 weeks following myocardial infarction, rats treated with W8B2+ CSCs encapsulated within the TheraCyte device showed preserved left ventricular ejection fraction. The preservation of cardiac function was accompanied by reduced fibrotic scar tissue, interstitial fibrosis, cardiomyocyte hypertrophy, as well as increased myocardial vascular density. Histological analysis of the TheraCyte devices harvested at 4 weeks post-implantation demonstrated survival of human W8B2+ CSCs within the devices, and the outer membrane was highly vascularized by host blood vessels. Using CSCs expressing plasma membrane reporters, extracellular vesicles of W8B2+ CSCs were found to be transferred to the heart and other organs at 4 weeks post-implantation. Furthermore, mass spectrometry-based proteomic profiling of extracellular vesicles of W8B2+ CSCs identified proteins implicated in inflammation, immunoregulation, cell survival, angiogenesis, as well as tissue remodelling and fibrosis that could mediate the cardioreparative effects of secretome of human W8B2+ CSCs.

CONCLUSIONS

Subcutaneous implantation of TheraCyte devices encapsulating human W8B2+ CSCs attenuated adverse cardiac remodelling and preserved cardiac function following myocardial infarction. The TheraCyte device can be employed to deliver stem cells in a minimally invasive manner for effective secretome-based cardiac therapy.

A Tissue Engineering Chamber for Continuous Pulsatile Electrical Stimulation of Vascularized Cardiac Tissues In Vivo

Background: Cardiomyocytes derived from pluripotent stem cells are immature. Maturation of cardiomyocytes is a multifactorial dynamic process that involves various factors in vivo that cannot be fully recapitulated in vitro. Here, we report a novel tissue engineering chamber with an integrated electrical stimulator and electrodes that will allow wireless electrical stimulation of cardiac tissue in vivo. Materials and Methods: Immunocompromised rats were implanted with tissue engineering chambers containing the stimulator and electrodes, and control chambers (chambers with electrical stimulator but without the electrodes) in the contralateral limb. Each chamber contained cardiomyocytes derived from human induced pluripotent stem cells (iPSCs). After 7 days of chamber implantation, the electrical stimulators were activated for 4 h per day, for 21 consecutive days. Results: At 4 weeks postimplantation, cardiomyocytes derived from human iPSCs survived, were assembled into compact cardiac tissue, and were perfused and vascularized by the host neovessels. Conclusion: This proof-of-principle study demonstrates the biocompatibility of the tissue engineering chamber with integrated electrical stimulator and electrodes. This could be utilized to study the influence of continuous electrical stimulation on vascularized cardiac or other tissues in vivo.

Bio-engineering a tissue flap utilizing a porous scaffold incorporating a human induced pluripotent stem cell-derived endothelial cell capillary network connected to a vascular pedicle

Tissue flaps are used to cover large/poorly healing wounds, but involve complex surgery and donor site morbidity. In this study a tissue flap is assembled using the mammalian body as a bioreactor to functionally connect an artery and vein to a human capillary network assembled from induced pluripotent stem cell-derived endothelial cells (hiPSC ECs). In vitro: Porous NovoSorb™ scaffolds (3 mm × 1.35 mm) were seeded with 200,000 hiPSC ECs ± 100,000 human vascular smooth muscle cells (hvSMC), and cultured for 1-3 days, with capillaries formed by 24 h which were CD31⁺, VE-Cadherin⁺, EphB4⁺, VEGFR2⁺ and Ki67⁺, whilst hvSMCs (calponin⁺) attached abluminally. In vivo: In SCID mice, bi-lateral epigastric vascular pedicles were isolated in a silicone chamber for a 3 week 'delay period' for pedicle capillary sprouting, then reopened, and two hiPSC EC ± hvSMCs seeded scaffolds transplanted over the pedicle. The chamber was either resealed (Group 1), or removed and surrounding tissue secured around the pedicle + scaffolds (Group 2), for 1 or 2 weeks. Human capillaries survived in vivo and were CD31⁺, VE-Cadherin⁺ and VEGFR2⁺. Human vSMCs remained attached, and host mesenchymal cells also attached abluminally. Systemically injected FITC-dextran present in human capillary lumens indicated inosculation to host capillaries. Human iPSC EC capillary morphometric parameters at one week in vivo were equal to or higher than the same parameters measured in human abdominal skin. This 'proof of concept' study has demonstrated that bio-engineering an autologous human tissue flap based on hiPSC EC could minimize the use of donor flaps and has potential applications for complex wound coverage. STATEMENT OF SIGNIFICANCE: Tissue flaps, used for surgical reconstruction of wounds, require complex surgery, often associated with morbidity. Bio-engineering a simpler alternative, we assembled a human induced pluripotent stem cell derived endothelial cell (hiPSC ECs) capillary network in a porous scaffold in vitro, which when transplanted over a mouse vascular pedicle in vivo formed a functional tissue flap with mouse blood flow in the human capillaries. Therefore it is feasible to form an autologous tissue flap derived from a hiPSC EC capillary network assembled in vitro, and functionally connect to a vascular pedicle in vivo that could be utilized in complex wound repair for chronic or acute wounds.

Characterization of isolated liver sinusoidal endothelial cells for liver bioengineering

BACKGROUND

The liver sinusoidal capillaries play a pivotal role in liver regeneration, suggesting they may be beneficial in liver bioengineering. This study isolated mouse liver sinusoidal endothelial cells (LSECs) and determined their ability to form capillary networks in vitro and in vivo for liver tissue engineering purposes.

METHODS AND RESULTS

In vitro LSECs were isolated from adult C57BL/6 mouse livers. Immunofluorescence labelling indicated they were LYVE-1⁺/CD32b⁺/FactorVIII⁺/CD31^-. Scanning electron microscopy of LSECs revealed the presence of characteristic sieve plates at 2 days. LSECs formed tubes and sprouts in the tubulogenesis assay, similar to human microvascular endothelial cells (HMEC); and formed capillaries with lumens when implanted in a porous collagen scaffold in vitro. LSECs were able to form spheroids, and in the spheroid gel sandwich assay produced significantly increased numbers (p = 0.0011) of capillary-like sprouts at 24 h compared to HMEC spheroids. Supernatant from LSEC spheroids demonstrated significantly greater levels of vascular endothelial growth factor-A and C (VEGF-A, VEGF-C) and hepatocyte growth factor (HGF) compared to LSEC monolayers (p = 0.0167; p = 0.0017; and p < 0.0001, respectively), at 2 days, which was maintained to 4 days for HGF (p = 0.0017) and VEGF-A (p = 0.0051). In vivo isolated mouse LSECs were prepared as single cell suspensions of 500,000 cells, or as spheroids of 5000 cells (100 spheroids) and implanted in SCID mouse bilateral vascularized tissue engineering chambers for 2 weeks. Immunohistochemistry identified implanted LSECs forming LYVE-1⁺/CD31^- vessels. In LSEC implanted constructs, overall lymphatic vessel growth was increased (not significantly), whilst host-derived CD31⁺ blood vessel growth increased significantly (p = 0.0127) compared to non-implanted controls. LSEC labelled with the fluorescent tag DiI prior to implantation formed capillaries in vivo and maintained LYVE-1 and CD32b markers to 2 weeks.

CONCLUSION

Isolated mouse LSECs express a panel of vascular-related cell markers and demonstrate substantial vascular capillary-forming ability in vitro and in vivo. Their production of liver growth factors VEGF-A, VEGF-C and HGF enable these cells to exert a growth stimulus post-transplantation on the in vivo host-derived capillary bed, reinforcing their pro-regenerative capabilities for liver tissue engineering studies.

Mitochondrial fission protein Drp1 inhibition promotes cardiac mesodermal differentiation of human pluripotent stem cells

Human induced pluripotent stem cells (iPSCs) are a valuable tool for studying the cardiac developmental process in vitro, and cardiomyocytes derived from iPSCs are a putative cell source for personalized medicine. Changes in mitochondrial morphology have been shown to occur during cellular reprogramming and pluripotent stem cell differentiation. However, the relationships between mitochondrial dynamics and cardiac mesoderm commitment of iPSCs remain unclear. Here we demonstrate that changes in mitochondrial morphology from a small granular fragmented phenotype in pluripotent stem cells to a filamentous reticular elongated network in differentiated cardiomyocytes are required for cardiac mesodermal differentiation. Genetic and pharmacological inhibition of the mitochondrial fission protein, Drp1, by either small interfering RNA or Mdivi-1, respectively, increased cardiac mesoderm gene expression in iPSCs. Treatment of iPSCs with Mdivi-1 during embryoid body formation significantly increased the percentage of beating embryoid bodies and expression of cardiac-specific genes. Furthermore, Drp1 gene silencing was accompanied by increased mitochondrial respiration and decreased aerobic glycolysis. Our findings demonstrate that shifting the balance of mitochondrial morphology toward fusion by inhibition of Drp1 promoted cardiac differentiation of human iPSCs with a metabolic shift from glycolysis towards oxidative phosphorylation. These findings suggest that Drp1 may represent a new molecular target for future development of strategies to promote the differentiation of human iPSCs into cardiac lineages for patient-specific cardiac regenerative medicine.

Three Dimensional Collagen Scaffold Promotes Intrinsic Vascularisation for Tissue Engineering Applications

Here, we describe a porous 3-dimensional collagen scaffold material that supports capillary formation in vitro, and promotes vascularization when implanted in vivo. Collagen scaffolds were synthesized from type I bovine collagen and have a uniform pore size of 80 μm. In vitro, scaffolds seeded with primary human microvascular endothelial cells suspended in human fibrin gel formed CD31 positive capillary-like structures with clear lumens. In vivo, after subcutaneous implantation in mice, cell-free collagen scaffolds were vascularized by host neovessels, whilst a gradual degradation of the scaffold material occurred over 8 weeks. Collagen scaffolds, impregnated with human fibrinogen gel, were implanted subcutaneously inside a chamber enclosing the femoral vessels in rats. Angiogenic sprouts from the femoral vessels invaded throughout the scaffolds and these degraded completely after 4 weeks. Vascular volume of the resulting constructs was greater than the vascular volume of constructs from chambers implanted with fibrinogen gel alone (42.7±5.0 μL in collagen scaffold vs 22.5±2.3 μL in fibrinogen gel alone; p<0.05, n = 7). In the same model, collagen scaffolds seeded with human adipose-derived stem cells (ASCs) produced greater increases in vascular volume than did cell-free collagen scaffolds (42.9±4.0 μL in collagen scaffold with human ASCs vs 25.7±1.9 μL in collagen scaffold alone; p<0.05, n = 4). In summary, these collagen scaffolds are biocompatible and could be used to grow more robust vascularized tissue engineering grafts with improved the survival of implanted cells. Such scaffolds could also be used as an assay model for studies on angiogenesis, 3-dimensional cell culture, and delivery of growth factors and cells in vivo.

Identification of P-Rex1 as a novel Rac1-guanine nucleotide exchange factor (GEF) that promotes actin remodeling and GLUT4 protein trafficking in adipocytes

Phosphoinositide 3-kinase (PI3K) signaling promotes the translocation of the glucose transporter, GLUT4, to the plasma membrane in insulin-sensitive tissues to facilitate glucose uptake. In adipocytes, insulin-stimulated reorganization of the actin cytoskeleton has been proposed to play a role in promoting GLUT4 translocation and glucose uptake, in a PI3K-dependent manner. However, the PI3K effectors that promote GLUT4 translocation via regulation of the actin cytoskeleton in adipocytes remain to be fully elucidated. Here we demonstrate that the PI3K-dependent Rac exchange factor, P-Rex1, enhances membrane ruffling in 3T3-L1 adipocytes and promotes GLUT4 trafficking to the plasma membrane at submaximal insulin concentrations. P-Rex1-facilitated GLUT4 trafficking requires a functional actin network and membrane ruffle formation and occurs in a PI3K- and Rac1-dependent manner. In contrast, expression of other Rho GTPases, such as Cdc42 or Rho, did not affect insulin-stimulated P-Rex1-mediated GLUT4 trafficking. P-Rex1 siRNA knockdown or expression of a P-Rex1 dominant negative mutant reduced but did not completely inhibit glucose uptake in response to insulin. Collectively, these studies identify a novel RacGEF in adipocytes as P-Rex1 that, at physiological insulin concentrations, functions as an insulin-dependent regulator of the actin cytoskeleton that contributes to GLUT4 trafficking to the plasma membrane.

Analysis of phosphatidylinositol 3,4,5 trisphosphate 5-phosphatase activity by in vitro and in vivo assays

Phosphatidylinositol 3,4,5 trisphosphate [PtdIns(3,4,5)P3] is a potent membrane-bound signaling molecule transiently synthesized by phosphoinositide 3-kinase (PI3-kinase) in response to extracellular agonists. PtdIns(3,4,5)P3 signals need to be strictly controlled. PtdIns(3,4,5)P3 recruits and binds effectors that function in oncogenic signaling pathways. PtdIns(3,4,5)P3 activates cell proliferation, growth, and migration as well as regulating insulin signaling. The inositol polyphosphate 5-phosphatase family of enzymes dephosphorylate and thereby modulate PtdIns(3,4,5)P3 levels, attenuating PI3-kinase-dependent signaling. PtdIns(3,4,5)P3 5-phosphatase enzyme activity can be assessed in vitro by analysis of the hydrolysis of radiolabeled or fluorescently labeled PtdIns(3,4,5)P3 and in vivo by visualization of the recruitment and turnover of the PtdIns(3,4,5)P3-specific biosensor GFP-PH/ ARNO or other PtdIns(3,4,5)P3 binding proteins at the plasma membrane.

Regulation of FcγR-stimulated phagocytosis by the 72-kDa inositol polyphosphate 5-phosphatase: SHIP1, but not the 72-kDa 5-phosphatase, regulates complement receptor 3–mediated phagocytosis by differential recruitment of these 5-phosphatases to the phagocytic cup

Macrophages phagocytose particles to resolve infections and remove apoptotic cells. Phosphoinositide 3-kinase generates phosphatidylinositol-3,4,5-trisphosphate [PtdIns(3,4,5)P3] is restricted to the phagocytic cup, promoting phagocytosis. The PtdIns(3,4,5)P3 5-phosphatase (5-ptase) Src homology 2 (SH2) domain-containing inositol-5-phosphatase 1 (SHIP1) inhibits phagocytosis. We report here that another PtdIns(3,4,5)P3-5-ptase, the 72-kDa-5-phosphatase (72-5ptase), inhibits Fcγ receptor (FcγR)– but not complement receptor 3 (CR3)–mediated phagocytosis, affecting pseudopod extension and phagosome closure. In contrast, SHIP1 inhibited FcγR and CR3 phagocytosis with greater effects on CR3-stimulated phagocytosis. The 72-5ptase and SHIP1 were both dynamically recruited to FcγR-stimulated phagocytic cups, but only SHIP1 was recruited to CR3-stimulated phagocytic cups. To determine whether 5-ptases focally degrade PtdIns(3,4,5)P3 at the phagocytic cup after specific stimuli, time-lapse imaging of specific biosensors was performed. Transfection of dominant-negative 72-5ptase or 72-5ptase small interfering RNA (siRNA) resulted in amplified and prolonged PtdIns(3,4,5)P3 at the phagocytic cup in response to FcγR- but not CR3-stimulation. In contrast, macrophages from Ship1−/−/AktPH-GFP transgenic mice exhibited increased and sustained PtdIns(3,4,5)P3 at the cup in response to CR3 activation, with minimal changes to FcγR activation. Therefore, 72-5ptase and SHIP1 exhibit specificity in regulating FcγR- versus CR3-stimulated phagocytosis by controlling the amplitude and duration of PtdIns(3,4,5)P3 at the phagocytic cup.

Phosphatidylinositol 3-phosphate [PtdIns3P] is generated at the plasma membrane by an inositol polyphosphate 5-phosphatase: endogenous PtdIns3P can promote GLUT4 translocation to the plasma membrane

Exogenous delivery of carrier-linked phosphatidylinositol 3-phosphate [PtdIns(3)P] to adipocytes promotes the trafficking, but not the insertion, of the glucose transporter GLUT4 into the plasma membrane. However, it is yet to be demonstrated if endogenous PtdIns(3)P regulates GLUT4 trafficking and, in addition, the metabolic pathways mediating plasma membrane PtdIns(3)P synthesis are uncharacterized. In unstimulated 3T3-L1 adipocytes, conditions under which PtdIns(3,4,5)P3 was not synthesized, ectopic expression of wild-type, but not catalytically inactive 72-kDa inositol polyphosphate 5-phosphatase (72-5ptase), generated PtdIns(3)P at the plasma membrane. Immunoprecipitated 72-5ptase from adipocytes hydrolyzed PtdIns(3,5)P2, forming PtdIns(3)P. Overexpression of the 72-5ptase was used to functionally dissect the role of endogenous PtdIns(3)P in GLUT4 translocation and/or plasma membrane insertion. In unstimulated adipocytes wild type, but not catalytically inactive, 72-5ptase, promoted GLUT4 translocation and insertion into the plasma membrane but not glucose uptake. Overexpression of FLAG-2xFYVE/Hrs, which binds and sequesters PtdIns(3)P, blocked 72-5ptase-induced GLUT4 translocation. Actin monomer binding, using latrunculin A treatment, also blocked 72-5ptase-stimulated GLUT4 translocation. 72-5ptase expression promoted GLUT4 trafficking via a Rab11-dependent pathway but not by Rab5-mediated endocytosis. Therefore, endogenous PtdIns(3)P at the plasma membrane promotes GLUT4 translocation.

The SH2 domain containing inositol polyphosphate 5-phosphatase-2: SHIP2

Phosphoinositides are membrane-bound signaling molecules that recruit, activate and localize target effectors to intracellular membranes regulating apoptosis, cell proliferation, insulin signaling and membrane trafficking. The SH2 domain containing inositol polyphosphate 5-phosphatase-2 (SHIP2) hydrolyzes phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) generating phosphatidylinositol 3,4-bisphosphate (PtdIns(3,4)P2). Overexpression of SHIP2 inhibits insulin-stimulated phosphoinositide 3-kinase (PI3K) dependent signaling events. Analysis of diabetic human subjects has revealed an association between SHIP2 gene polymorphisms and type 2 diabetes mellitus. Genetic ablation of SHIP2 in mice has generated conflicting results. SHIP2 knockout mice were originally reported to show lethal neonatal hypoglycemia resulting from insulin hypersensitivity, but in addition to inactivating the SHIP2 gene, the Phox2a gene was also inadvertently deleted. Another SHIP2 knockout mouse has now been generated which inactivates the SHIP2 gene but leaves Phox2a intact. These animals show normal insulin and glucose tolerance but are highly resistant to weight gain on high fat diets, exhibiting an obesity-resistant phenotype. Therefore, SHIP2 remains a significant therapeutic target for the treatment of both obesity and type 2 diabetes.

SHIP-2 forms a tetrameric complex with filamin, actin, and GPIb-IX-V: localization of SHIP-2 to the activated platelet actin cytoskeleton

The platelet receptor for the von Willebrand factor (VWF) glycoprotein Ib-IX-V (GPIb-IX-V) complex mediates platelet adhesion at sites of vascular injury. The cytoplasmic tail of the GPIbalpha subunit interacts with the actin-binding protein, filamin, anchoring the receptor in the cytoskeleton. In motile cells, the second messenger phosphatidylinositol 3,4,5 trisphosphate (PtdIns(3,4,5)P3) induces submembraneous actin remodeling. The inositol polyphosphate 5-phosphatase, Src homology 2 domain-containing inositol polyphosphate 5-phosphatase-2 (SHIP-2), hydrolyzes PtdIns(3,4,5)P3 forming phosphatidylinositol 3,4 bisphosphate (PtdIns(3,4)P2) and regulates membrane ruffling via complex formation with filamin. In this study we investigate the intracellular location and association of SHIP-2 with filamin, actin, and the GPIb-IX-V complex in platelets. Immunoprecipitation of SHIP-2 from the Triton-soluble fraction of unstimulated platelets demonstrated association between SHIP-2, filamin, actin, and GPIb-IX-V. SHIP-2 associated with filamin or GPIb-IX-V was active and demonstrated PtdIns(3,4,5)P3 5-phosphatase activity. Following thrombin or VWF-induced platelet activation, detection of the SHIP-2, filamin, and receptor complex decreased in the Triton-soluble fraction, although in control studies the level of SHIP-2, filamin, or GPIb-IX-V immunoprecipitated by their respective antibodies did not change following platelet activation. In activated platelets spreading on a VWF matrix, SHIP-2 localized intensely with actin at the central actin ring and colocalized with actin and filamin at filopodia and lamellipodia. In spread platelets, GPIb-IX-V localized to the center of the platelet and showed little colocalization with filamin at the plasma membrane. These studies demonstrate a functionally active complex between SHIP-2, filamin, actin, and GPIb-IX-V that may orchestrate the localized hydrolysis of PtdIns(3,4,5)P3 and thereby regulate cortical and submembraneous actin.

Inositol polyphosphate 5-phosphatases: lipid phosphatases with flair

Recent studies have identified the inositol polyphosphate 5-phosphatases as a large family of signal modifying enzymes comprising 10 mammalian and 4 yeast family members. A number of investigations including gene-targeted deletion of 5-phosphatases in mice have demonstrated that these enzymes regulate many important cellular events including hematopoietic cell proliferation and activation, insulin signaling, endocytosis, and actin polymerization.

Mammalian inositol polyphosphate 5-phosphatase II can compensate for the absence of all three yeast Sac1-like-domain-containing 5-phosphatases

Phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)] plays a complex role in generating intracellular signalling molecules, and also in regulating actin-binding proteins, vesicular trafficking and vacuolar fusion. Four inositol polyphosphate 5-phosphatases (hereafter called 5-phosphatases) have been identified in Saccharomyces cerevisiae: Inp51p, Inp52p, Inp53p and Inp54p. Each enzyme contains a 5-phosphatase domain which hydrolyses PtdIns(4,5)P(2), forming PtdIns4P, while Inp52p and Inp53p also express a polyphosphoinositide phosphatase domain within the Sac1-like domain. Disruption of any two yeast 5-phosphatases containing a Sac1-like domain results in abnormalities in actin polymerization, plasma membrane, vacuolar morphology and bud-site selection. Triple null mutant 5-phosphatase strains are non-viable. To investigate the role of PtdIns(4,5)P(2) in mediating the phenotype of double and triple 5-phosphatase null mutant yeast, we determined whether a mammalian PtdIns(4,5)P(2) 5-phosphatase, 5-phosphatase II, which lacks polyphosphoinositide phosphatase activity, could correct the phenotype of triple 5-phosphatase null mutant yeast and restore cellular PtdIns(4,5)P(2) levels to near basal values. Mammalian 5-phosphatase II expressed under an inducible promoter corrected the growth, cell wall, vacuolar and actin polymerization defects of the triple 5-phosphatase null mutant yeast strains. Cellular PtdIns(4,5)P(2) levels in various 5-phosphatase double null mutant strains demonstrated significant accumulation (4.5-, 3- and 2-fold for Deltainp51Deltainp53, Deltainp51Deltainp52 and Deltainp52Deltainp53 double null mutants respectively), which was corrected significantly following 5-phosphatase II expression. Collectively, these studies demonstrate the functional and cellular consequences of PtdIns(4,5)P(2) accumulation and the evolutionary conservation of function between mammalian and yeast PtdIns(4,5)P(2) 5-phosphatases.

Cloning and characterization of a 72-kDa inositol-polyphosphate 5-phosphatase localized to the Golgi network

The inositol-polyphosphate 5-phosphatase enzyme family removes the 5-position phosphate from both inositol phosphate and phosphoinositide signaling molecules. We have cloned and characterized a novel 5-phosphatase, which demonstrates a restricted substrate specificity and tissue expression. The 3.9-kb cDNA predicts for a 72-kDa protein with an N-terminal proline rich domain, a central 5-phosphatase domain, and a C-terminal CAAX motif. The 3. 9-kilobase mRNA showed a restricted expression but was abundant in testis and brain. Antibodies against the sequence detected a 72-kDa protein in the testis in the detergent-insoluble fraction. Indirect immunofluorescence of the Tera-1 cell line using anti-peptide antibodies to the 72-kDa 5-phosphatase demonstrated that the enzyme is predominantly located to the Golgi. Expression of green fluorescent protein-tagged 72-kDa 5-phosphatase in COS-7 cells revealed that the enzyme localized predominantly to the Golgi, mediated by the N-terminal proline-rich domain, but not the C-terminal CAAX motif. In vitro, the protein inserted into microsomal membranes on the cytoplasmic face of the membrane. Immunoprecipitated recombinant 72-kDa 5-phosphatase hydrolyzed phosphatidylinositol 3,4,5-trisphosphate and phosphatidylinositol 3, 5-bisphosphate, forming phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3-phosphate, respectively. We propose that the novel 5-phosphatase hydrolyzes phosphatidylinositol 3,4, 5-trisphosphate and phosphatidylinositol 3,5-bisphosphate on the cytoplasmic Golgi membrane and thereby may regulate Golgi-vesicular trafficking.