Genetic Disruption of □pdl and □pdlr Phospholipases in Mice Leads to Impaired Platelet Adhesion and Aggregation

Platelet adhesion and aggregation are critical events in thrombosis. Bioactive phospholipid LPA (lysophosphatidic acid) has been identified as an important agonist for platelet aggregation. Plasma LPA can be generated from phospholipid substrates by phospholipase A¹ (PLA¹) and phospholipase D (PLD). We previously identified two novel PLA¹ enzymes, designated lpdl (lpd lipase) and lpdlr (lpdl related lipase). Together with phosphatidylserine phospholipase A¹ (PS-PLA¹), these three phospholipases form a unique PLA¹ lipase subfamily. Phospholipids are important structural components of cellular membranes. Recent studies demonstrate that lipid raft microdomains on the platelet membrane contribute to platelet activation. The gathering of membrane lipid rafts is necessary for ADP-mediated platelet activation. Disruption of lipid raft results in a reduction of ADP-induced platelet aggregation. In addition, it is reported that alteration of membrane lipid composition also affects the function of platelet β3 integrin (GPIIbIIIa). Thus, phospholipases may play an important role in platelet function. However, the roles of lpdl and lpdlr in platelet activation and thrombosis are unknown. To study the function of lpdl and lpdlr, we have recently knocked out both lpdl and lpdlr genes in mice by deleting their exons I and II, including their translation start codon ATG. RT-PCR confirmed that neither lpdl nor lpdlr gene is expressed in homozygous lpdl^−/− or lpdlr^−/− mice but expressed in wild-type (WT) tissues, indicating successful knockout of these phospholipases. Next, optical platelet aggregometry was used to assess platelet aggregation in platelet rich plasma (PRP). Platelet aggregation was induced with 5, 10 and 20 μM ADP in pooled PRP (3×10⁸ platelets/mL) from age- and sex-matched WT, heterozygous and homozygous mice. Our results demonstrated that both the lpdl^−/− and lpdlr^−/− mice have decreased platelet aggregation after ADP stimulation. We further studied thrombus formation in lpdl^−/− mice using an ex vivo perfusion chamber model on collagen type I-coated rectangular glass microcapillary tubes. Our preliminary data showed that both platelet adhesion and aggregation were impaired in the lpdl^−/− mice. We are repeating these experiments and are examining lpdlr^−/− mice using the same perfusion chamber model. In vivo thrombosis models with intravital microscopy and the mechanisms of how lpdl and lpdlr enzymes regulate platelet activation will also be investigated. These studies could potentially identify a novel pathway for regulation of platelet function, which may lead to development of new diagnostic and/or therapeutic methods for atherothrombosis. (Drs. D. Wang and A. Reheman contribute equally to this work)