It’s not an infection, it’s an invasion!: An army of exosomes loaded with non-coding RNAs reaches endothelial shores

On 6 June, 1944 (also termed “the D-day”), the Allied army irrupted into the shores of western France in what was called the Invasion of Normandy, during the last stages of World War II. The operation began with hundreds of amphibian landing vehicles emerging from the sea and unloading their cargo – the Infantry divisions that bravely attempted the assault – on beaches where German troops had settled batteries of weapons that caused the death of almost two thirds of the landing troops. However, the operation resulted in success, and this bloody episode is today recalled as the beginning of the end of the occupation of France by the Nazis, becoming a turning point in History. We have seen these scenes depicted in many films, and it will serve us to exemplify the elaborate strategy used by a little worm who fancies living inside the intestine of mice.

It is well known that some living organisms have developed very curious behaviors in order to live using other being’s resources: making profit of their leftovers, sharing their bodies in an exchange of mutual advantages, or even living inside others without asking permission. The latter is typical of the parasites, who have evolved to counteract the defenses of their hosts in peculiar and diverse ways. And as much as molecular biology deepens into the properties and capabilities of organic molecules, these strategies are shown to be much more complex as one can initially think. The little worm Heligmosomoides polygyrus is fond of living inside the intestinal tract of mice, for instance. Obviously, the mouse is not asked about his opinion about hosting worms, eggs and larvae, and its defensive system tries to avoid this unwanted visitors. Vertebrate animals have evolved a complex system of immunity that recognizes foreigners, fires the alarms and triggers a chain of molecular and cellular responses that – luckily – will eliminate the threat. Then, how manages this little nematode to live a happy life (or at least as happy as living inside a rodent’s intestine can be), to mate and breed descendants, that will finally infect the mouse faeces in order to continue with this cycle of filthy existence? Buck and collaborators ¹ asked themselves this question, and have found amazing details in the strategy used by the worm to mock the animal’s defenses. Which takes us back to the D-day.

In our story, the worm becomes the ally army, trying to enter the French territory (the mouse’s intestine) through the shore (the intestinal epithelium). And like in the beaches of Normandy, the attack is preceded by an assault of carrier transports loaded with the infantry troops. These transports are exosomes, a specific kind of vesicles (like the ones used to secrete insulin from inside a pancreatic cell, or neurotransmitters from inside a neuron) that contain concrete molecules that the worm will use to evade the animal’s defensive troops (the German army). Exosomes can be observed in their way from the nematode, until they are engulfed by the rodent’s endothelial cells, ignorant of the dangerous content they are about to internalize. Because a closer look and a couple of biochemical analysis and genetic sequencing techniques reveal the content of the exosomes: the carrier transport is filled with a battery of RNA molecules.

Particularly, the most abundant soldiers are short, non-coding RNAs known as microRNAs. We already talked about the properties of RNAs as signaling molecules in the series of posts Life and deeds of RNA. Nowadays, the functional capacity of these molecules to silence the specific activation of certain genes is well known. Curiously, several microRNAs from the worm’s army are evolutionarily related to others present in the mouse. And hence, they have a genetic target inside the mouse’s genome. These microRNAs are the enemy in disguise, allied soldiers disguised as German officers, granting them the possibility of breaking through all defenses and sabotaging one of the key weapons of the enemy army. And that is literally what they do: the worm’s exosomes are internalized by the mouse’s cells, and their RNA content is able to target specific genes which are usually regulated by extremely similar mouse RNAs.

But not any genes; the targets, DUSP1 and IL3, are genes involved in inflammatory response, genes that participate in triggering the defensive response against multicellular parasites. Now we can go back to our analogy and recall the initial scenes of Saving private Ryan, where the few lucky soldiers who make it through the curtain of bullets coming from the shore, go straight towards the firing turrets handled by German troops: they break into those bunkers and destroy these machine-guns as a first step to make the path clear for the rest of soldiers. Once these genes are attacked by the worm’s non-coding RNAs, they are downregulated, the immune response turns slow and weak, and the worm can advance until taking calmly and without further trouble the enemy’s shore. The invasion succeeds.

The work directed by Amy H. Buck, elegantly described in Nature Communications, precisely recapitulates the steps of this peculiar, silent invasion, as did the brave reporters that accompanied the Second World War armies, taking the pictures and recording the filming that decades later granted Spielberg’s faithful and breathtaking re-interpretation of this bloody episode of human History. In the case of the invader worm, the researchers have cultured the animals, put them in contact with rodent cells, and analyzed the secretory products; they have identified all the types of RNA inside the vesicles; compared the RNA content with that of the mouse; tracked the way of the secreted exosomes, using fluorescence microscopy techniques and careful labeling; and not only the identity of the little soldiers inside the carrier exosomes has been unveiled, but also their targets into the enemy cells has been discovered. How these small RNAs perform their silencing task still generates many questions, since other RNA’s and RNA-processing proteins travel together with them, and the role of these side-kicks is still unknown. But the increasing number of examples in which different sorts of RNA molecules grant a strategy for organisms that live inside others promises new and dazzling surprises. This multidisciplinary job, an investigation coordinated from different fronts, has torn apart an invasive strategy that shreds light for understanding the intricate ways of those myriad armies that menace our species and others. The molecular resources for a parasitic invasion seem limitless; but the human wit to dissect them, to understand them, and to finally counter-act them, is equally amazing.