Property-based design of a glucosylceramide synthase inhibitor that reduces glucosylceramide in the brain.

Synthesis inhibition is the basis for the treatment of type 1 Gaucher disease by the glucosylceramide synthase (GCS) inhibitor eliglustat tartrate. However, the extended use of eliglustat and related compounds for the treatment of glycosphingolipid storage diseases with CNS manifestations is limited by the lack of brain penetration of this drug. Property modeling around the D-threo-1-phenyl-2-decanoylamino-3-morpholino-propanol (PDMP) pharmacophore was employed in a search for compounds of comparable activity against the GCS but lacking P-glycoprotein (MDR1) recognition. Modifications of the carboxamide N-acyl group were made to lower total polar surface area and rotatable bond number. Compounds were screened for inhibition of GCS in crude enzyme and whole cell assays and for MDR1 substrate recognition. One analog, 2-(2,3-dihydro-1H-inden-2-yl)-N-((1R,2R)-1-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-1-hydroxy-3-(pyrrolidin-1-yl)propan-2-yl)acetamide (CCG-203586), was identified that inhibited GCS at low nanomolar concentrations with little to no apparent recognition by MDR1. Intraperitoneal administration of this compound to mice for 3 days resulted in a significant dose dependent decrease in brain glucosylceramide content, an effect not seen in mice dosed in parallel with eliglustat tartrate.

Lysosomal phospholipase A2 is selectively expressed in alveolar macrophages.

Lung surfactant is the surface-active agent comprised of phospholipids and proteins that lines pulmonary alveoli. Surfactant stabilizes the alveolar volume by reducing surface tension. Previously, we identified a lysosomal phospholipase A2, termed LPLA2, with specificity toward phosphatidylcholine and phosphatidylethanolamine. The phospholipase is localized to lysosomes, is calcium-independent, has an acidic pH optimum, and transacylates ceramide. Here, we demonstrate that LPLA2 is selectively expressed in alveolar macrophages but not in peritoneal macrophages, peripheral blood monocytes, or other tissues. Other macrophage-associated phospholipase A2s do not show a comparable distribution. LPLA2 is of high specific activity and recognizes disaturated phosphatidylcholine as a substrate. The lysosomal phospholipase A2 activity is six times lower in alveolar macrophages from mice with a targeted deletion of the granulocyte macrophage colony-stimulating factor (GM-CSF), a model of impaired surfactant catabolism, compared with those from wild-type mice. However, LPLA2 activity and protein levels are measured in GM-CSF null mice in which GM-CSF is expressed as a transgene under the control of the surfactant protein C promoter. Thus LPLA2 may be a major enzyme of pulmonary surfactant phospholipid degradation by alveolar macrophages and may be deficient in disorders of surfactant metabolism.

A turn in the road: How studies on the pharmacology of glucosylceramide synthase inhibitors led to the identification of a lysosomal phospholipase A2 with ceramide transacylase activity.

A series of inhibitors of glucosylceramide synthesis, the PDMP based family of compounds, has been developed as a tool for the study of sphingolipid biochemistry and biology. During the course of developing more active glucosylceramide synthase inhibitors, we identified a second site of inhibitory activity for PDMP and its structural homologues that accounted for the ability of the inhibitors to raise cell and tissue ceramide levels. This inhibitory activity was directed against a previously unknown pathway for ceramide metabolism, viz. the formation of 1- O -acylceramide. In this pathway the addition of a fatty acyl group to the primary hydroxyl of ceramide occurs through a transacylation with either phosphatidylethanolamine or phosphatidylcholine as a substrate. However, both in the absence and presence of ceramide, water serves as an acceptor for the fatty acid. Thus the enzyme may be considered to be a phospholipase A2. The enzyme is unique in that it has an acidic pH optimum and is localized to lysosomes by cell fractionation. More recently, the 1- O -acylceramide synthase has been purified, sequenced, and cloned. This phospholipase A2 was discovered to be structurally homologous to lecithin cholesterol acyltransferase (LCAT). However, this phospholipase A2 does not recognize cholesterol and lacks the defined lipoprotein-binding domain present in LCAT. We now refer to this enzyme as lysosomal phospholipase A2 (LPLA2). Although acidic phospholipase A2 activities have been previously identified, LPLA2 appears to be the first lysosomal PLA2 to have been sequenced. This new phospholipase A2 lacks an obvious and proven biological function.

Cloning and characterization of a lysosomal phospholipase A2, 1-O-acylceramide synthase.

Recently, a novel enzyme, 1-O-acylceramide synthase (ACS), was purified and characterized from bovine brain. This enzyme has both calcium-independent phospholipase A(2) and transacylase activities. The discovery of this enzyme led us to propose a new pathway for ceramide metabolism in which the sn-2-acyl group of either phosphatidylethanolamine or phosphatidylcholine is transferred to the 1-hydroxyl group of ceramide. In this study, the partial amino acid sequences from the purified enzyme revealed that the enzyme contains amino acid sequences identical to those of human lecithin:cholesterol acyltransferase-like lysophospholipase (LLPL). The coding sequences of the mouse, bovine, and human genes were obtained from the respective kidney cDNAs by PCR. The open reading frames of LLPL were cloned into pcDNA3 to generate carboxyl-terminally tagged proteins. The expression of mouse LLPL in COS-7 cells demonstrated that transfected cells had higher transacylase and phospholipase A(2) activities than did non-transfected cells. Immunoprecipitation confirmed that LLPL had ACS activity. There were no significant lecithin:cholesterol acyltransferase and lysophospholipase activities in the mouse LLPL-transfected cells under either acidic or neutral conditions. Amino acid sequences from cDNAs of mouse, human, and bovine LLPLs demonstrated a signal peptide cleavage site, one lipase motif (AXSXG), and several N-linked glycosylation sites in each LLPL molecule. The replacement of serine with alanine in the lipase motif of mouse LLPL resulted in elimination of enzyme activity, indicating that the serine residue is part of the catalytic site. Deglycosylation of mouse, human, and bovine LLPLs yielded core proteins with a molecular mass of 42 kDa without change in enzyme activities. LLPL was post-translationally modified by signal peptide cleavage and N-linked glycosylation, and each mature LLPL had the same size core protein. Subcellular fractionation demonstrated that ACS activity co-localized with N-acetylglucosaminidase. Therefore, LLPL encodes a novel lysosomal enzyme, ACS.