Role of the carboxy-terminal domain of human apolipoprotein AI in high-density-lipoprotein metabolism--a study based on deletion and substitution variants in transgenic mice

Transfer of C-terminal residues of human apolipoprotein A-I to insect apolipophorin III creates a two-domain chimeric protein with enhanced lipid binding activity

Apolipophorin III (apoLp-III) is an insect apolipoprotein (18kDa) that comprises a single five-helix bundle domain. In contrast, human apolipoprotein A-I (apoA-I) is a 28kDa two-domain protein: an α-helical N-terminal domain (residues 1-189) and a less structured C-terminal domain (residues 190-243). To better understand the apolipoprotein domain organization, a novel chimeric protein was engineered by attaching residues 179 to 243 of apoA-I to the C-terminal end of apoLp-III. The apoLp-III/apoA-I chimera was successfully expressed and purified in E. coli. Western blot analysis and mass spectrometry confirmed the presence of the C-terminal domain of apoA-I within the chimera. While parent apoLp-III did not self-associate, the chimera formed oligomers similar to apoA-I. The chimera displayed a lower α-helical content, but the stability remained similar compared to apoLp-III, consistent with the addition of a less structured domain. The chimera was able to solubilize phospholipid vesicles at a significantly higher rate compared to apoLp-III, approaching that of apoA-I. The chimera was more effective in protecting phospholipase C-treated low density lipoprotein from aggregation compared to apoLp-III. In addition, binding interaction of the chimera with phosphatidylglycerol vesicles and lipopolysaccharides was considerably improved compared to apoLp-III. Thus, addition of the C-terminal domain of apoA-I to apoLp-III created a two-domain protein, with self-association, lipid and lipopolysaccharide binding properties similar to apoA-I. The apoA-I like behavior of the chimera indicate that these properties are independent from residues residing in the N-terminal domain of apoA-I, and that they can be transferred from apoA-I to apoLp-III.

Overexpression of CD39 in mouse airways promotes bacteria-induced inflammation

In airways, the ecto-nucleoside triphosphate diphosphohydrolase CD39 plays a central role in the regulation of physiological mucosal nucleotide concentrations and likely contributes to the control of inflammation because accelerated ATP metabolism occurs in chronic inflammatory lung diseases. We sought to determine whether constant elevated CD39 activity in lung epithelia is sufficient to cause inflammation and whether this affects the response to acute LPS or Pseudomonas aeruginosa exposure. We generated transgenic mice overexpressing human CD39 under the control of the airway-specific Clara cell 10-kDa protein gene promoter. Transgenic mice did not develop any spontaneous lung inflammation. However, intratracheal instillation of LPS resulted in accelerated recruitment of neutrophils to the airways of transgenic mice. Macrophage clearance was delayed, and the amounts of CD8(+) T and B cells were augmented. Increased levels of keratinocyte chemoattractant, IL-6, and RANTES were produced in transgenic lungs. Similarly, higher numbers of neutrophils and macrophages were found in the lungs of transgenic mice infected with P. aeruginosa, which correlated with improved bacteria clearance. The transgenic phenotype was partially and differentially restored by coinstillation of P2X(1) or P2X(7) receptor antagonists or of caffeine with LPS. Thus, a chronic increase of epithelial CD39 expression and activity promotes airway inflammation in response to bacterial challenge by enhancing P1 and P2 receptor activation.

The pituitary adenylate cyclase-activating polypeptide is a physiological inhibitor of platelet activation

The pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide of the vasoactive intestinal peptide/secretin/glucagon superfamily. Studies in two related patients with a partial trisomy 18p revealed three copies of the PACAP gene and elevated PACAP concentrations in plasma. The patients suffer from severe mental retardation and have a bleeding tendency with mild thrombocytopenia, and their fibroblasts show increased PACAP mRNA levels. The PACAP receptor (vasoactive intestinal peptide/pituitary adenylate cyclase-activating peptide receptor 1 [VPAC1]) in platelets and fibroblasts is coupled to adenylyl cyclase activation. Accordingly, we found increased basal cAMP levels in patients' platelets and fibroblasts, providing a basis for the reduced platelet aggregation in these patients. Megakaryocyte-specific transgenic overexpression of PACAP in mice correspondingly increased PACAP release from platelets, reduced platelet activation, and prolonged the tail bleeding time. In contrast, the PACAP antagonist PACAP(6-38) or a monoclonal PACAP antibody enhanced the collagen-induced aggregation of normal human platelets, and in PACAP knockout mice, an increased platelet sensitivity toward collagen was found. Thus, we found that PACAP modulates platelet function and demonstrated what we believe to be the first hemostatic defect associated with PACAP overexpression; our study suggests the therapeutic potential to manage arterial thrombosis or bleeding by administration of PACAP mimetics or inhibitors, respectively.

Overexpression of the platelet P2X1 ion channel in transgenic mice generates a novel prothrombotic phenotype

We have generated transgenic mice overexpressing the human P2X(1) ion channel in the megakaryocytic cell lineage. Platelets from transgenic mice exhibited a gain of P2X(1) ionotropic activity as determined by more prominent P2X(1)-mediated Ca(2+) influx and platelet shape change. P2X(1) overexpression enhanced platelet secretion and aggregation evoked by low doses of collagen, convulxin, or the thromboxane A(2) mimetic U46619. In contrast, transgenic platelet responses to adenosine diphosphate (ADP) or thrombin were normal. Perfusing whole blood from transgenic mice over collagen fibers at a shear rate of 1000 seconds(-1) resulted in increased P2X(1)-dependent aggregate formation and phosphatidylserine exposure. Platelet hyperreactivity to collagen was correlated with up-regulated extracellular signal-regulated kinase 2 (ERK2) phosphorylation. Accordingly, the MEK1/2 inhibitor U0126 potently inhibited the collagen-induced aggregation of transgenic platelets when stirred or when perfused over a collagen surface. In a viscometer, shear stress caused potent aggregation of transgenic platelets under conditions in which wild-type platelets did not aggregate. In an in vivo model of thromboembolism consisting of intravenous injection of a low dose of collagen plus epinephrine, transgenic mice died more readily than wild-type mice. Preinjection of U0126 not only fully protected transgenic mice against thrombosis, it also enhanced the survival of wild-type mice injected with a higher collagen dose. Hence, the platelet P2X(1) ion channel plays a role in hemostasis and thrombosis through its participation in collagen-, thromboxane A(2)-, and shear stress-triggered platelet responses. Activation of the ERK2 pathway is instrumental in these processes.

Arg123-Tyr166 domain of human ApoA-I is critical for HDL-mediated inhibition of macrophage homing and early atherosclerosis in mice

Atherosclerosis was studied in apolipoprotein E (apoE) knockout mice expressing human apolipoprotein A-I (apoA-I) or an apoA-I/apolipoprotein A-II (apoA-II) chimera in which the Arg123-Tyr166 central domain of apoA-I was substituted with the Ser12-Ala75 segment of apoA-II. High density lipoprotein (HDL) cholesterol levels were identical in apoA-I and apoA-I/apoA-II mice, but at 4 months, plaques were 2.7-fold larger in the aortic root of the apoA-I/apoA-II mice (P<0.01). The macrophage-to-smooth muscle cell ratio of lesions was 2.1-fold higher in apo-I/apoA-II mice than in apoA-I mice (P<0.01). This was due to a 2.7-fold higher (P<0.001) in vivo macrophage homing in the aortic root of apoA-I/apoA-II mice. Plasma platelet-activating factor acetyl hydrolase activity was lower (P<0.01) in apoA-I/apoA-II mice, resulting in increased oxidative stress, as evidenced by the higher titer of antibodies against oxidized low density lipoprotein (P<0.01). Increased oxidative stress resulted in increased stimulation of ex vivo macrophage adhesion by apoA-I/apoA-II beta-very low density lipoprotein and decreased inhibition of beta-very low density lipoprotein-induced adhesion by HDL from apoA-I/apoA-II mice. The cellular cholesterol efflux capacity of HDL from apoA-I/apoA-II mice was very similar to that of apoA-I mice. Thus, the Arg123-Tyr166 central domain of apoA-I is critical for reducing oxidative stress, macrophage homing, and early atherosclerosis in apoE knockout mice independent of its role in HDL production and cholesterol efflux.

The Arg123-Tyr166 central domain of human ApoAI is critical for lecithin:cholesterol acyltransferase-induced hyperalphalipoproteinemia and HDL remodeling in transgenic mice

High density lipoprotein (HDL) metabolism and lecithin:cholesterol acyltransferase (LCAT)-induced HDL remodeling were investigated in transgenic mice expressing human apolipoprotein (apo) AI or an apoAI/apoAII chimera in which the Arg123-Tyr166 domain of apoAI was substituted with the Ser12-Ala75 domain of apoAII. Expression of apoAI and of the apoAI/apoAII chimera resulted in a respective 3. 5-fold and 2.9-fold increase of HDL cholesterol. Human LCAT gene transfer into apoAI-transgenic mice resulted in a 5.1-fold increase of endogenous LCAT activity. This increase was associated with a 2. 4-fold increase of the cholesterol ester-to-free cholesterol ratio of HDL, a shift from HDL(3) to HDL(2), and a 2.4-fold increase of HDL cholesterol levels. Agarose gel electrophoresis revealed that human LCAT gene transfer into human apoAI-transgenic mice resulted in an increase of pre-beta-HDL and of pre-alpha-HDL. In contrast, human LCAT gene transfer did not affect cholesterol levels and HDL distribution profile in mice expressing the apoAI/apoAII chimera. Mouse LCAT did not "see" a difference between wild-type and mutant human apoAI, whereas human LCAT did, thus localizing the species-specific interaction in the central domain of apoAI. In conclusion, the Arg123-Tyr166 central domain of apoAI is not critical for in vivo lipoprotein association. It is, however, critical for LCAT-induced hyperalphalipoproteinemia and HDL remodeling independent of the lipid-binding properties of apoAI.

Substitution of the carboxyl-terminal domain of apo AI with apo AII sequences restores the potential of HDL to reduce the progression of atherosclerosis in apo E knockout mice

HDL metabolism and atherosclerosis were studied in apo E knockout (KO) mice overexpressing human apo AI, a des- (190-243)-apo AI carboxyl-terminal deletion mutant of human apo AI or an apo AI-(1-189)-apo AII-(12-77) chimera in which the carboxyl-terminal domain of apo AI was substituted with the pair of helices of apo AII. HDL cholesterol levels ranked: apo AI/apo E KO approximately apo AI-(1-189)-apo AII- (12-77)/apo E KO > > des-(190-243)-apo AI/apo E KO > apo E KO mice. Progression of atherosclerosis ranked: apo E KO > des-(190-243)-apo AI/apo E KO > > apo AI-(1-189)- apo AII-(12-77)/apo E KO approximately apo AI/apo E KO mice. Whereas the total capacity to induce cholesterol efflux from lipid-loaded THP-1 macrophages was higher for HDL of mice overexpressing human apo AI or the apo AI/apo AII chimera, the fractional cholesterol efflux rate, expressed in percent cholesterol efflux/microg apolipoprotein/h, for HDL of these mice was similar to that for HDL of mice overexpressing the deletion mutant and for HDL of apo E KO mice. This study demonstrates that the tertiary structure of apo AI, e.g., the number and organization of its helices, and not its amino sequence is essential for protection against atherosclerosis because it determines HDL cholesterol levels and not cholesterol efflux. Amino acid sequences of apo AII, which is considered to be less antiatherogenic, can be used to restore the structure of apo AI and thereby its antiatherogenicity.