Abstract
Sphingolipids are essential constituents of the plasma membrane (PM) and play an important role in signal transduction by modulating clustering and dynamics of membrane receptors. Changes in lipid composition are therefore likely to influence receptor organisation and function, but how this precisely occurs is difficult to address given the intricacy of the PM lipid-network. Here, we combined biochemical assays and single molecule dynamic approaches to demonstrate that the local lipid environment regulates adhesion of integrin receptors by impacting on their lateral mobility. Induction of sphingomyelinase (SMase) activity reduced sphingomyelin (SM) levels by conversion to ceramide (Cer), resulting in impaired integrin adhesion and reduced integrin mobility. Dual-colour imaging of cortical actin in combination with single molecule tracking of integrins showed that this reduced mobility results from increased coupling to the actin cytoskeleton brought about by Cer formation. As such, our data emphasizes a critical role for the PM local lipid composition in regulating the lateral mobility of integrins and their ability to dynamically increase receptor density for efficient ligand binding in the process of cell adhesion.
Highlights
The cellular levels of-sphingolipids and cholesterol as well as the expression of lipid metabolizing enzymes are altered in a variety of diseases including cancer[19,20], in response to external stimuli such as pathogens[21] or induced by drug treatment[22]
While several studies have reported a dependency of TM receptor mobility on the plasma membrane (PM) cholesterol, Cer and SM content[40,41,42], it is still unknown whether these lipids contribute to the regulation of LFA-1 mobility and whether LFA-1 function is sensitive to changes in PM lipid composition
Using super-resolution microscopy we demonstrated that LFA-1 binding to its ligand ICAM-1 on monocytes depended on cholesterol and its spatial proximity to GM1 and glycosylphosphatidyl-inositol anchored proteins (GPI-APs) nanodomains[3,38]
Summary
The cellular levels of (glyco)-sphingolipids and cholesterol as well as the expression of lipid metabolizing enzymes are altered in a variety of diseases including cancer[19,20], in response to external stimuli such as pathogens[21] or induced by drug treatment[22]. Formation and dynamics of Cer-enriched membrane platforms has been documented in living cells[28], where they can influence function and avidity state of several receptors[21]. An α and a β subunit form a functional heterodimer, and their regulation occurs via conformational changes that alter affinity for their ligands[32] or via dynamic redistribution within the membrane that locally increases the receptor density (i.e. valency) leading to increased avidity[33]. We demonstrated an essential role of cholesterol in mediating LFA-1-GPI-AP interactions at the nanoscale and in the formation of larger raft-based adhesion sites upon ligand binding[3,38]. By combining SPT and biochemical assays, our results reveal that PM lipid alteration by induction of SMase activity can negatively affect integrin function by compromising lateral mobility, interfering with leukocyte adhesion
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