An international group of researchers led by Pompeu Fabra University has discovered the nanomachine that controls constitutive exocytosis: the uninterrupted delivery of spherical molecular packages to the cell surface. This is an essential activity present in virtually all organisms to preserve cell fitness and other vital functions such as communication with the cell's exterior, cell growth and division. According to Oriol Gallego, who has led the research, "despite being one of the largest nanomachines in the cell, its short lifespan and dynamism made it very challenging to capture". The discovery of this flexible and transient 'nanocourier' has required the combined power of multiple microscopes and artificial intelligence, yielding unprecedented information of a key process that occurs billions of times per day in our bodies. Moreover, the understanding of exocytosis may have profound implications for the treatment of some infections and rare diseases.
Every day, every cell of our body transports between 10,000 –100,000 of these spherical packages to the cell surface to fulfil cellular processes that require the release or display of any molecule on the outside of the cell, such as the secretion of enzymes and hormones, repairing wounds on the cell surface or simply because the cell needs to grow, move or change its shape. Therefore, the delivery of packages to the surface is essential because it is linked to many vital processes that the cell undergoes daily.
"The function of this nanocourier is so important that it is very rare to find it mutated in patients as its alteration would normally impair the viability of the embryo", says Oriol Gallego, leader of the Biophysics in Cell Biology Group at the UPF Department of Medicine and Life Sciences ( MELIS ).
Though vital for the cell, this process could not be studied in detail until today, when Oriol Gallego's lab, in collaboration with Carlo Manzo (Universitat de Vic), Daniel Castaño (Instituto Biofisika) and Jonas Ries (Max Perutz Labs), reported the discovery of the 'nanocourier' in the journal Cell. By combining some of the most advanced light and electron microscopes with image analysis using artificial intelligence, they have resolved the 3D organization of this nanomachine, and they have filmed how it quickly changes its structure during the delivery of spherical packages.
Imaging the nanocourier at work
At the core of this nanomachine, the concerted motion of seven protein assemblies builds a flexible ring that holds the spherical packages in place upon their arrival at their destination: the cell surface. "We have named this nanocourier ExHOS, standing for exocyst higher-order structure", explains Marta Puig-Tintó, one of the main authors of the study. "The ExHOS features three checkpoints and a mechanism of disassembly that ensures that the delivery of molecular packages continues at the required speed".
"It is as if every time the cell needs to deliver a heavy package, a team of seven strong couriers work together to do so", explains Sasha Meek, also a main author. "Because the package is so heavy, they can't just drop it all at once and have to lower it in three steps. And when they're finished, they need confirmation of receipt so that the team of couriers can break up and go on to make other deliveries", the young researcher adds.
The ExHOS in context
Deepening the understanding of exocytosis goes far beyond the mere desire to know, and could one day affect many fields of applied science. For instance, plants need the ExHOS to defend cells against microbial invasion. Hence, many phytopathogens have developed mechanisms to attenuate plant immunity by attacking the ExHOS. A good example is Magnaporthe oryzae, also known as rice blast fungus, which causes the loss of up to a third of the world's rice production.
In humans, several viruses such as SARS-CoV-2, HIV, or pathogenic bacteria, like Salmonella, behave similarly and hijack exocytosis during infection. Even mild alterations of ExHOS components are linked to human diseases. Though infrequent, mutations in components of the nanocourier cause rare diseases related to neurodevelopmental disorders. In other cases, the ExHOS participates in cell invasion in metastatic cancers.
A revolution in bioimaging
"Despite being small in size, the cell interior is a vast space full of enigmatic nanomachines that have never been observed because of the limitations of current microscopy tools", Marta Puig-Tintó comments. "But I think that the future lies in integrating various imaging technologies with the power of new computational tools like AI to "make the invisible visible"", she continues.
"With these new opportunities, we have unveiled a fundamental and vital cellular process", Gallego remarks. "It's like explaining how oxygen is exchanged during breathing or how the periodicity of the heartbeat is maintained. It might not have an immediate application, but the discovery of this nanomachine will facilitate future research to find solutions to severe biomedical and biotechnological problems", the scientist concludes.
