Exocytosis is an essential cellular activity, particularly evident in neuronal network. The secretion of proteins and other molecules, packed into lipid membranes, or granules, starts in the endoplasmic reticulum (ER), involves the Golgi apparatus, and ends on the surface of the plasma membrane. The transport of molecules to be released outside the cells requires a lot of specialized transport proteins that organize a “cargo” system across the cytoplasm. But cargo proteins are not the only actors in the transport system: many lipids actively participate in the recruitment of cargo proteins, in the structural modifications of lipid granules, and in the interactions with plasma membrane.
After having been produced in the ER, proteins destined to secretion are recruited by the trans compartment of Golgi apparatus (trans Golgi network, TGN). Lipid granules bud from TGN through a complex interaction between cytosolic regulatory proteins and lipids on TGN membranes. The composition of the TGN lipid bilayer shows differences between the internal layer, called lumenal, and the cytosolic one. For example, lumenal layer is rich with sphingolipids, while cytosolic layer is rich with phosphatidylserine. The difference in lipid composition is present in the secretory granules as well, and it has important structural functions, as it will be explicated later.
The formation of secretory granules is mediated by key lipid metabolites, such as diacylglycerol (DAG), phosphatidic acid (PA), and phosphoinositides. For example, DAG accumulates on TGN membrane, and its conical shape is essential for generating a curvature on the membrane that leads to the budding of the granules.
Moreover, not all the secretory granules have the same lipid composition. This is because lipids with different physical properties, such as glycerolipids and sphingolipids, get separated between them in TGN. This lipid segregation forms the so called “lipid microdomains”, in which there is a high concentration of just some types of lipids, such as sphingolipids and cholesterol. Also, the microdomains recruit specific proteins that on one hand help to form the secretory granules, and on the other hand enter the composition of the same granules.
Once the secretory granules have been formed, they start a journey towards the cell plasma membrane. The granules are transported through the cytoplasm by interactions with microtubules. During the transport, granule undergo two main maturation processes: acidification and size reduction.
Acidification is the result of an increasing proton pump density that drop pH off from 6.5 to 5.0, approximately. This process might be controlled by phosphoinositides, that are lipids with a key role in granule movement and signal transduction. At the same time, granules reduce their size by undergoing a loss of ions (NA+, K+, Cl–) and water. Specifically, the water loss is mediated by a protein called aquaporin, that is recruited by TGN microdomains during the early formation of secretory granules.
Shrunk granules are now mature: they have budded from TGN, have travelled through cytoplasm, and they are now ready to release their content outside the cell through exocytosis, that is a well regulated process that starts after an appropriate signal.
Exocytosis first step is the granule docking to the plasma membrane. Granules are recruited to exocytotic sites and, once again, the formation of lipid microdomains on plasma membrane trigger the final steps of the secretion process. In a manner similar to that described for granule budding, cholesterol, phosphatidylinositol and sphingolipids concentrate in small clusters that recruit specific proteins called SNARE. SNARE proteins form complexes that let granule membrane proteins to join the docking site on the plasma membrane.
Once docked, further interactions are needed between SNARE proteins and granule membrane lipids to make them become ready for secretion. Molecular mechanisms are not clearly understood yet, however it is known that phosphoinositides and unsaturated lipids such as arachidonic acid facilitate exocytosis through the interaction with the SNARE complex.
Finally, all is prepared for secretion. Now, exocytosis just depends on the structural interaction between the lipids of the outer granule layer and the lipids of the inner plasma membrane layer. The most accepted model is something similar to the ligand-receptor model of proteins: granule cone-shaped lipids (such as cholesterol and DAG) contact with the inverted conical lipids on plasma membrane (gangliosides, phosphatidylserine), thus promoting the fusion of membranes and the release of granule content.
Lipids have a key role in every step of the secretory pathway. They not only have structural properties, but also a functional role, as they direct interact with the proteins involved in this process. An alteration of lipid metabolism or a bad diet can have harmful effects on the secretory pathway.
Tanguy E. et al. Lipids implicated in the journey of a secretory granule: from biogenesis to fusion. J. Neurochem. (2016) 137, 904-912
The elaboration of this post has been financed by the project PI15/01082, as a part of the National Plan of I+D+I and co-financed by the ISCIII – General Deputy Direction for Evaluation and Development of Health Research – and the European Regional Development Fund (ERDF).