The availability of both the mouse and human genome sequences allows for the systematic discovery of human gene function through the use of the mouse as a model system. To accelerate the genetic determination of gene function, we have developed a sequence-tagged gene-trap library of >270,000 mouse embryonic stem cell clones representing mutations in ≈60% of mammalian genes. Through the generation and phenotypic analysis of knockout mice from this resource, we are undertaking a functional screen to identify genes regulating physiological parameters such as blood pressure. As part of this screen, mice deficient for the Wnk1 kinase gene were generated and analyzed. Genetic studies in humans have shown that large intronic deletions in WNK1 lead to its overexpression and are responsible for pseudohypoaldosteronism type II, an autosomal dominant disorder characterized by hypertension, increased renal salt reabsorption, and impaired K+ and H+ excretion. Consistent with the human genetic studies, Wnk1 heterozygous mice displayed a significant decrease in blood pressure. Mice homozygous for the Wnk1 mutation died during embryonic development before day 13 of gestation. These results demonstrate that Wnk1 is a regulator of blood pressure critical for development and illustrate the utility of a functional screen driven by a sequence-based mutagenesis approach

apeutic intervention for diseases such as atherosclerosis, pancreatitis, or dyslipidemia associated with metabolic syndrome or type II diabetes ( 1-3 ). Central to triglyceride metabolism is lipoprotein lipase (LPL), an extracellular enzyme primarily located in the vascular beds of many tissues ( 3,4 ). LPL catalyzes the hydrolysis of the triglyceride component of chylomicrons (CM) and VLDL, which constitute the major forms of triglycerides in plasma ( 3, 5 ). Although LPL is expressed in many different tissues, the enzyme is expressed at high levels in metabolically active tissues, such as adipose, cardiac muscle, and skeletal muscle, where fatty acids released by the action of LPL are stored or used ( 4 ).LPL appears to be regulated by a variety of mechanisms. Several apolipoproteins associated with CM and VLDL, including apolipoprotein CII (APOC2) and apolipoprotein AV (APOA5), stimulate LPL activity ( 6-9 ) apparently by increasing its V max ( 10,11 ). In contrast, apolipoproteins CI (APOC1) and CIII (APOC3) can inhibit LPL activity ( 7,12 ). LPL is inherently unstable and proteins or other factors that either stabilize or destabilize LPL are likely to play a role in regulating its in vivo activity ( 13 ). The active form of LPL exists as a head-totail homodimer, which dissociates into metastable monomers. These monomers can reassociate to form catalytically active LPL or they can undergo conformational changes, forming inactive, stable monomers. The spontaneous in- Our understanding of how triglyceride (TG) metabolism is regulated is essential for designing avenues of ther-

Angiopoietin-like 3 (ANGPTL3) and angiopoietin-like 4 (ANGPTL4) are secreted proteins that regulate triglyceride (TG) metabolism in part by inhibiting lipoprotein lipase (LPL).Recently, we showed that treatment of wild-type mice with monoclonal antibody (mAb) 14D12, specific for ANGPTL4, recapitulated the Angptl4 knock-out (؊/؊) mouse phenotype of reduced serum TG levels. In the present study, we mapped the region of mouse ANGPTL4 recognized by mAb 14D12 to amino acids Gln 29 -His 53 , which we designate as specific epitope 1 (SE1). The 14D12 mAb prevented binding of ANGPTL4 with LPL, consistent with its ability to neutralize the LPL-inhibitory activity of ANGPTL4. Alignment of all angiopoietin family members revealed that a sequence similar to ANGPTL4 SE1 was present only in ANGPTL3, corresponding to amino acids Glu 32 -His 55 . We produced a mouse mAb against this SE1-like region in ANGPTL3. This mAb, designated 5.50.3, inhibited the binding of ANGPTL3 to LPL and neutralized ANGPTL3-mediated inhibition of LPL activity in vitro. Treatment of wild-type as well as hyperlipidemic mice with mAb 5.50.3 resulted in reduced serum TG levels, recapitulating the lipid phenotype found in Angptl3 ؊/؊ mice. These results show that the SE1 region of ANGPTL3 and ANGPTL4 functions as a domain important for binding LPL and inhibiting its activity in vitro and in vivo. Moreover, these results demonstrate that therapeutic antibodies that neutralize ANGPTL4 and ANGPTL3 may be useful for treatment of some forms of hyperlipidemia.Lipoprotein lipase (LPL) 5 plays a pivotal role in lipid metabolism by catalyzing the hydrolysis of plasma triglycerides (TGs).LPL is likely to be regulated by mechanisms that depend on nutritional status and on the tissue in which it is expressed (1-3). Two secreted proteins, angiopoietin-like 3 (ANGPTL3) and angiopoietin-like 4 (ANGPTL4), play important roles in the regulation of LPL activity (4, 5). ANGPTL3 and ANGPTL4 consist of a signal peptide, an N-terminal segment containing coiled-coil domains, and a C-terminal fibrinogen-like domain. The N-terminal segment as well as full-length ANGPTL3 and ANGPTL4 have been shown to inhibit LPL activity, and deletion of the N-terminal segment of ANGPTL3 and ANGPTL4 resulted in total loss of LPL-inhibiting activity (6, 7). These observations clearly indicate that the N-terminal region of ANGPTL4 contains the functional domain that inhibits LPL and affects plasma lipid levels. The coiled-coil domains have been proposed to be responsible for oligomerization (8); however, it is not known whether the coiled-coil domains directly mediate the inhibition of LPL activity.To define the physiological role of ANGPTL4 more clearly, we characterized the pharmacological consequences of ANGPTL4 inhibition in mice treated with the ANGPTL4-neutralizing monoclonal antibody (mAb) 14D12 (9). Injection of mAb 14D12 significantly lowered fasting TG levels in C57BL/6J mice relative to levels in C57BL/6J mice treated with an isotypematched anti-KLH control (KLH) mAb (9). These reduced TG...

Two members of the angiopoietin-like family of proteins, ANGPTL3 and ANGPTL4, have been shown to play important roles in modulating lipoprotein metabolism in the body. Both proteins were found to suppress lipoprotein lipase (LPL) activity in vitro as well as in vivo. However, their mechanisms of inhibition remained poorly understood. Using enzyme kinetic analysis with purified recombinant proteins, we have found key mechanistic differences between ANGPTL3 and ANGPTL4. ANGPTL3 reduced LPL catalytic activity but did not significantly alter its self-inactivation rate. In contrast, ANGPTL4 suppressed LPL by accelerating the irreversible inactivation of LPL. Furthermore, heparin was able to overcome the inhibitory effect of ANGPTL3 on LPL but not that of ANGPTL4. Site-directed mutagenesis demonstrated the critical function of Glu 40 in ANGPTL4. In contrast, when cysteine residues involved in disulfide bond formation were mutated to serines, ANGPTL4 retained its activity. Taken together, our data provide a more detailed view of the structure and mechanisms of these proteins. The finding that ANGPTL3 and ANGPTL4 inhibit LPL activity through distinct mechanisms indicates that the two proteins play unique roles in modulation of lipid metabolism in vivo. Lipoprotein lipase (LPL)2 is an essential enzyme that catalyzes the hydrolysis of triglycerides to generate free fatty acids and monoacylglycerol (for review, see Refs. 1 and 2). It is synthesized and secreted by adipocytes, macrophages, and muscle cells and then bound to the vascular endothelium by heparin sulfate proteoglycans and GPIHBP1 (glycosylphosphatidylinositol-anchored high density lipoprotein-binding protein 1), a recently discovered protein (3, 4). LPL anchored as such releases free fatty acids and monoacylglycerol from triglycerides carried by chylomicron and very low density lipoprotein particles (5-9) and thus plays a major role in lipid metabolism. LPL-deficient subjects have severe hypertriglyceridemia and increased risk of arteriosclerosis (10). In contrast, subjects with slightly increased LPL activity were found to have lower triglyceride levels and decreased risk of cardiovascular diseases (11).It is well known that LPL is rapidly inactivated in vivo, but the underlying mechanism is unknown (12, 13). Recently, two secreted proteins were found to inhibit LPL activity both in vitro and in vivo. These two proteins, known as ANGPTL3 and ANGPTL4, are members of the angiopoietin-like protein family (14 -17). They share 31% overall sequence homology, with an N-terminal domain containing a coiled-coil region and a C-terminal fibrinogen-like domain that is cleaved off in vivo (16,18). Both proteins are found to inhibit LPL activity in vitro (16,18). Overexpression of ANGPTL3 and ANGTPL4 in mice led to extremely high blood levels of triglycerides and cholesterol (15, 19 -21). Knock-out of either gene in mice results in much lower blood levels of these lipids (14,(17)(18)(19)(22)(23)(24). Furthermore, post-heparin plasma LPL activity is significantly elevated in...

The amyloid precursor protein (APP) is a broadly expressed transmembrane protein that has a significant role in the pathogenesis of Alzheimer's disease (AD). APP can be cleaved at multiple sites to generate a series of fragments including the amyloid β (Aβ) peptides and APP intracellular domain (AICD). Although Aβ peptides have been proposed to be the main cause of AD pathogenesis, the role of AICD has been underappreciated. Here we report that APP induces AICD-dependent cell death in Drosophila neuronal and non-neuronal tissues. Our genetic screen identified the transcription factor forkhead box O (FoxO) as a crucial downstream mediator of APP-induced cell death and locomotion defect. In mammalian cells, AICD physically interacts with FoxO in the cytoplasm, translocates with FoxO into the nucleus upon oxidative stress, and promotes FoxO-induced transcription of pro-apoptotic gene Bim. These data demonstrate that APP modulates FoxO-mediated cell death through AICD, which acts as a transcriptional co-activator of FoxO.

The p75 neurotrophin receptor (p75NTR) is a known mediator of β-amyloid (Aβ)-induced neurotoxicity implicated in Alzheimer's disease (AD). Here, we demonstrate that death receptor 6 (DR6) binds to p75NTR and is a component of the p75NTR signaling complex responsible for Aβ-induced cortical neuron death. Cortical neurons isolated from either DR6 or p75NTR null mice are resistant to Aβ-induced neurotoxicity. Blocking DR6 function in cortical neurons by anti-DR6 antibodies that block the binding of DR6 to p75NTR receptor complex or by a dominant negative DR6 construct lacking the cytoplasmic signaling death domain attenuates Aβ-induced caspase 3 activation and cell death. DR6 expression is upregulated in AD cortex and correlates with elevated neuronal death. Targeting the disruption of the DR6/p75NTR complex to prevent Aβ cytotoxicity represents a new approach for the treatment of neurodegenerative disorders such as AD.

U-937 human leukemia cells were selected for resistance to doxorubicin in the presence or absence of a specific drug modulator that inhibits the activity of P-glycoprotein (Pgp), encoded by the multidrug-resistance gene (MDR1). Parental cells expressed low basal levels of the multidrug-resistanceassociated gene (MRP1) and major vault protein (MVP) mRNAs and no MDR1 mRNA. Two doxorubicin-resistant cell lines were selected. Both drug-resistant cell lines upregulated the MVP mRNA level 1.5-fold within 1 cell passage. The MVP mRNA level continued to increase over time as the doxorubicin selection pressure was increased. MVP protein levels generally paralleled the mRNA levels. The 2 high molecular weight vault protein mRNAs were always expressed at constitutive levels. Fully formed vault particles consisting of the MVP, the 2 high molecular weight proteins and the vault RNA assembled and accumulated to increased levels in drug-selected cells. MVP induction is therefore the rate-limiting step for vault particle formation in U-937 cells. Key words: vault; MVP; LRP; U-937; multidrug resistanceVaults are ribonucleoprotein particles that are conserved throughout evolution in diverse phylogeny including mammals, avians, amphibians and the slime mold. 1 They were first observed in preparations of clatharin-coated vesicles and were named based on their structural similarity to arched cathedral ceilings. 2 Vault particles have a mass of approximately 13 MDa and are composed of multiple copies of 3 proteins and a unique, untranslated RNA. The major vault protein (MVP) constitutes 70% of the total mass of the particle. The remaining mass comprises vault RNA and 2 high molecular weight proteins, vault poly(ADP-ribose) polymerase 3 and telomerase-associated protein 1. 4 The MVP has also been referred to as the lung resistance-associated protein (LRP). 5 Several groups have documented that the cellular level of MVP is an excellent predictor of multidrug resistance (MDR) in cancer cell lines and in clinical tumors. 6 However, it is not known whether vault particles act alone or in combination with other drug-resistance factors to confer a multidrug-resistance phenotype, or whether vaults are purely a marker for the phenotype. Previous studies have noted that MVP is upregulated early during drug selection. 7-10 Many cancer cell lines upregulate not only MVP but also MRP1, MDR1 and other drug transporters. In revertant MDR cancer cell lines, the number of vault particles decreases. 11 Previously, human U-937 myeloid leukemia cells were selected for drug resistance in the presence of increasing concentrations of doxorubicin (Dox). 12 By Northern blot analysis, parental U-937 cells expressed a low level of MRP1 and expressed no MDR1. In the presence of Dox, the level of MRP1 first increased followed by a later increase in MDR1 expression. None of the sublines showed gene amplification. Verapamil was able to significantly modulate IC 50 values thus implicating these ATP binding cassette (ABC) drug transporters in the drug-resistance pa...

No abstract