Genetic variants of phospholipase C-γ2 alter the phenotype and function of microglia and confer differential risk for Alzheimer's disease.

Genetic association studies have demonstrated the critical involvement of the microglial immune response in Alzheimer's disease (AD) pathogenesis. Phospholipase C-gamma-2 (PLCG2) is selectively expressed by microglia and functions in many immune receptor signaling pathways. In AD, PLCG2 is induced uniquely in plaque-associated microglia. A genetic variant of PLCG2, PLCG2P522R, is a mild hypermorph that attenuates AD risk. Here, we identified a loss-of-function PLCG2 variant, PLCG2M28L, that confers an increased AD risk. PLCG2P522R attenuated disease in an amyloidogenic murine AD model, whereas PLCG2M28L exacerbated the plaque burden associated with altered phagocytosis and Aß clearance. The variants bidirectionally modulated disease pathology by inducing distinct transcriptional programs that identified microglial subpopulations associated with protective or detrimental phenotypes. These findings identify PLCG2M28L as a potential AD risk variant and demonstrate that PLCG2 variants can differentially orchestrate microglial responses in AD pathogenesis that can be therapeutically targeted.

Harnessing Clickable Acylated Glycerol Probes as Chemical Tools for Tracking Glycerolipid Metabolism.

We report the use of clickable monoacylglycerol (MAG) analogs as probes for the labeling of glycerolipids during lipid metabolism. Incorporation of azide tags onto the glycerol region was pursued to develop probes that would label glycerolipids, in which the click tag would not be removed through processes including acyl chain and headgroup remodeling. Analysis of clickable MAG probes containing acyl chains of different length resulted in widely variable cell imaging and cytotoxicity profiles. Based on these results, we focused on a probe bearing a short acyl chain (C4 -MAG-N3 ) that was found to infiltrate natural lipid biosynthetic pathways to produce click-tagged versions of both neutral and phospholipid products. Alternatively, strategic blocking of the glycerol sn-3 position in probe C4 -MEG-N3 served to deactivate phospholipid tagging and focus labeling on neutral lipids. This work shows that lipid metabolic labeling profiles can be tuned based on probe structures and provides valuable tools for evaluating alterations to lipid metabolism in cells.

Cellular Labeling of Phosphatidylserine Using Clickable Serine Probes.

Phosphatidylserine (PS) is a key lipid that plays important roles in disease-related biological processes, and therefore, the means to track PS in live cells are invaluable. Herein, we describe the metabolic labeling of PS in Saccharomyces cerevisiae cells using analogues of serine, a PS precursor, derivatized with azide moieties at either the amino (N-l-SerN3) or carbonyl (C-l-SerN3) groups. The conservative click tag modification enabled these compounds to infiltrate normal lipid biosynthetic pathways, thereby producing tagged PS molecules as supported by mass spectrometry studies, thin-layer chromatography (TLC) analysis, and further derivatization with fluorescent reporters via click chemistry to enable imaging in yeast cells. This approach shows strong prospects for elucidating the complex biosynthetic and trafficking pathways involving PS.

"Fix and Click" for Assay of Sphingolipid Signaling in Single Primary Human Intestinal Epithelial Cells.

Capillary electrophoresis with fluorescence detection (CE-F) is a powerful method to measure enzyme activation in single cells. However, cellular enzymatic assays used in CE-F routinely utilize reporter substrates that possess a bulky fluorophore that may impact enzyme kinetics. To address these challenges, we describe a "fix and click" method utilizing an alkyne-terminated enzyme activation reporter, aldehyde-based fixation, and a click chemistry reaction to attach a fluorophore prior to analysis by single-cell CE-F. The "fix and click" strategy was utilized to investigate sphingolipid signaling in both immortalized cell lines and primary human colonic epithelial cells. When the sphingosine alkyne reporter was loaded into cells, this reporter was metabolized to ceramide (31.6 ± 3.3% peak area) without the production of sphingosine-1-phosphate. In contrast, when the reporter sphingosine fluorescein was introduced into cells, sphingosine fluorescein was converted to sphingosine-1-phosphate and downstream products (32.8 ± 5.7% peak area) without the formation of ceramide. Sphingolipid metabolism was measured in single cells from both differentiated and stem/proliferative human colonic epithelium using "fix and click" paired with CE-F to highlight the diversity of sphingosine metabolism in single cells from primary human colonic epithelium. This novel method will find widespread utility for the performance of single-cell enzyme assays by virtue of its ability to temporally and spatially separate cellular reactions with alkyne-terminated reporters, followed by the assay of enzyme activation at a later time and place.

Fluorogenic XY-69 in Lipid Vesicles for Measuring Activity of Phospholipase C Isozymes.

Mammalian phospholipase C (PLC) isozymes are major signaling nodes that regulate a wide range of cellular processes. Dysregulation of PLC activity has been associated with a growing list of human diseases such as cancer and Alzheimer's disease. However, methods to directly and continuously monitor PLC activity at membranes with high sensitivity and throughput are still lacking. We have developed XY-69, a fluorogenic PIP2 analog, which can be efficiently hydrolyzed by PLC isozymes either in solution or at membranes. Here, we describe the optimized assay conditions and protocol to measure the activity of PLC-γ1 (D1165H) with XY-69 in lipid vesicles. The described protocol also applies to other PLC isozymes.

A High-Throughput Assay to Identify Allosteric Inhibitors of the PLC-γ Isozymes Operating at Membranes.

The two phospholipase C-γ (PLC-γ) isozymes are major signaling hubs and emerging therapeutic targets for various diseases, yet there are no selective inhibitors for these enzymes. We have developed a high-throughput, liposome-based assay that features XY-69, a fluorogenic, membrane-associated reporter for mammalian PLC isozymes. The assay was validated using a pilot screen of the Library of Pharmacologically Active Compounds 1280 (LOPAC1280) in 384-well format; it is highly reproducible and has the potential to capture both orthosteric and allosteric inhibitors. Selected hit compounds were confirmed with secondary assays, and further profiling led to the interesting discovery that adenosine triphosphate potently inhibits the PLC-γ isozymes through noncompetitive inhibition, raising the intriguing possibility of endogenous, nucleotide-dependent regulation of these phospholipases. These results highlight the merit of the assay platform for large scale screening of chemical libraries to identify allosteric modulators of the PLC-γ isozymes as chemical probes and for drug discovery.

Labeling of Phosphatidylinositol Lipid Products in Cells through Metabolic Engineering by Using a Clickable myo-Inositol Probe.

Phosphatidylinositol (PI) lipids control critical biological processes, so aberrant biosynthesis often leads to disease. As a result, the capability to track the production and localization of these compounds in cells is vital for elucidating their complex roles. Herein, we report the design, synthesis, and application of clickable myo-inositol probe 1 a for bioorthogonal labeling of PI products. To validate this platform, we initially conducted PI synthase assays to show that 1 a inhibits PI production in vitro. Fluorescence microscopy experiments next showed probe-dependent imaging in T-24 human bladder cancer and Candida albicans cells. Growth studies in the latter showed that replacement of myo-inositol with probe 1 a led to an enhancement in cell growth. Finally, fluorescence-based TLC analysis and mass spectrometry experiments support the labeling of PI lipids. This approach provides a promising means for tracking the complex biosynthesis and trafficking of these lipids in cells.

Calcium-Responsive Liposomes via a Synthetic Lipid Switch.

Liposomal drug delivery would benefit from enhanced control over content release. Here, we report a novel avenue for triggering release driven by chemical composition using liposomes sensitized to calcium-a target chosen due to its key roles in biology and disease. To demonstrate this principle, we synthesized calcium-responsive lipid switch 1, designed to undergo conformational changes upon calcium binding. The conformational change perturbs membrane integrity, thereby promoting cargo release. This was shown through fluorescence-based release assays via dose-dependent response depending on the percentage of 1 in liposomes, with minimal background leakage in controls. DLS experiments indicated dramatic changes in particle size upon treatment of liposomes containing 1 with calcium. In a comparison of ten naturally occurring metal cations, calcium provided the greatest release. Finally, STEM images showed significant changes in liposome morphology upon treatment of liposomes containing 1 with calcium. These results showcase lipid switches driven by molecular recognition principles as an exciting avenue for controlling membrane properties.

Artificial Membrane Fusion Triggered by Strain-Promoted Alkyne-Azide Cycloaddition.

Artificial systems for controlled membrane fusion applicable for drug delivery would ideally use triggers that are orthogonal to biology. To apply the strain-promoted alkyne-azide cycloaddition (SPAAC) to drive membrane fusion, oxo-dibenzocyclooctyne (ODIBO)-lipid 1 was designed, synthesized, and studied alongside azadibenzocyclooctyne (ADIBO)-lipids 2-4 to assess fusion with liposomes containing azido-lipid 5. Lipids 1-2 were first shown to be effective for liposome derivatization. Next, fusion was evaluated using liposomes containing 1 and varying ratios of PC and PE via a FRET dilution fusion assay, and a 1:1 PC-to-PE ratio yielded the greatest signal change attributed to fusion. Finally, lipids 1-4 were compared, and 1 yielded the greatest triggering of fusion, while 2-4 yielded varying efficacies depending on the structural features of each lipid. Fusion was further validated through STEM studies showing larger multilamellar assemblies after liposome mixing, and FRET assay results supporting the mixing of liposome aqueous contents. This work provides a platform for triggered fusion toward drug delivery applications and an understanding of the effects of lipid structure and membrane composition on fusion.