Respiratory virus mRNA as competitive viral RNA increases host factors expression and facilitates viral infection and replication

Viruses that infect the lungs, such as the flu and the virus behind COVID-19, are masters of hijacking the human body. To make copies of themselves, these invaders must commandeer the cell's internal machinery, forcing it to build new viral particles instead of maintaining the cell's own health. For decades, scientists have understood that viruses do this by producing massive amounts of their own genetic material, known as RNA, which acts as a blueprint for building viral proteins. However, a new study from researchers at the Chinese Academy of Sciences reveals that these viral blueprints do more than just instruct the cell to build the virus; they actively interfere with the cell's own internal communication system. This interference involves a specific type of small molecule called microRNA, which normally acts as a brake, slowing down the production of certain human proteins. The researchers discovered that the viruses produce so much of their own RNA that it acts like a sponge, soaking up these braking molecules and leaving the human proteins free to run at full speed, a state that the virus then exploits to replicate more efficiently.

The study focuses on two major respiratory viruses: the H1N1 strain of influenza and SARS-CoV-2. In the case of the flu virus, the researchers found that a specific segment of the virus's genetic code, called the PA segment, contains a sequence that perfectly matches a human microRNA known as hsa-miR-10a-5p. Under normal circumstances, this microRNA binds to a human protein called CAMK2B and suppresses its activity. However, when the flu virus infects a cell, it floods the environment with PA RNA. This viral RNA competes with the human protein's instructions for the microRNA, effectively stealing the microRNA's attention. With the microRNA occupied by the viral RNA, the human CAMK2B protein is released from its suppression and becomes highly active. The researchers demonstrated that this active CAMK2B protein then attaches a chemical tag, a phosphate group, to a viral protein called M1. This tagging process acts as a key, unlocking the cell's nuclear exit door and allowing the virus's genetic material to leave the nucleus and assemble into new viruses. Without this competition, the virus replicates much more slowly.

The researchers confirmed this mechanism by creating a version of the flu virus where the PA segment could no longer bind to the microRNA. When this modified virus infected cells, it failed to activate the human CAMK2B protein effectively, and its ability to replicate dropped significantly compared to the wild-type virus. The team also observed that the virus produces a vast excess of its own RNA compared to the amount of microRNA available; in their experiments, the viral RNA was present in quantities that allowed it to effectively outcompete the human genetic instructions. This suggests that the virus does not need to mutate its own proteins to become more efficient; it simply overwhelms the cell's regulatory system with sheer volume.

A similar strategy was observed with SARS-CoV-2, the virus responsible for the global pandemic. In this case, the virus uses a different section of its genetic code, specifically the 3' untranslated region (3' UTR), to compete for a different microRNA called hsa-miR-1307-3p. Normally, this microRNA keeps a human protein named CCZ1B in check. When SARS-CoV-2 infects a cell, its abundant 3' UTR RNA binds to the microRNA, preventing it from suppressing CCZ1B. The resulting increase in CCZ1B activity helps the cell transport a human receptor called ACE2 from the interior of the cell to its surface. Since ACE2 is the door through which SARS-CoV-2 enters the cell, having more of it on the surface makes the cell much more susceptible to infection. The researchers found that when they blocked the microRNA or increased the viral RNA, the amount of ACE2 on the cell surface rose, facilitating viral entry. Conversely, when they reduced the activity of CCZ1B, the virus struggled to infect the cells.

The study also looked at how these mechanisms might explain differences in how various strains of these viruses behave. The researchers analyzed the genetic sequences of different flu subtypes and SARS-CoV-2 variants to see if these competitive binding sites were preserved. They found that the binding sites for the flu virus were highly conserved in strains known for high replication rates, such as H1N1 and H7N9. For SARS-CoV-2, the binding site was present in earlier variants like Alpha, Beta, and Gamma, as well as the initial Omicron subvariant, but it showed mutations or deletions in later Omicron subvariants. This suggests that the virus may have evolved to rely less on this specific mechanism in newer strains, or that other factors have taken over. The researchers also noted that while the SARS-CoV-2 spike and nucleocapsid proteins were found to increase inflammatory signals in the cell, the 3' UTR specifically drove the increase in the CCZ1B protein, highlighting that different parts of the virus's genetic code play distinct roles in manipulating the host.

By mapping out these interactions, the researchers provided a quantitative view of how viral RNA levels influence human protein levels. They showed that the relationship follows a predictable pattern where the amount of human protein increases as the amount of viral RNA increases, but not in a simple straight line. This mathematical relationship helps explain why some viruses are more successful at replicating than others; it is not just about how well the virus binds to a cell, but how effectively its genetic material can disrupt the cell's internal regulatory network. The findings suggest that the sheer volume of viral RNA produced during an infection is a critical factor in the virus's ability to manipulate the host. This competition for microRNAs appears to be a widespread strategy used by respiratory viruses to turn the host cell's own defenses into tools for their own replication.

The implications of this discovery extend beyond understanding how these specific viruses work. The researchers identified that this competitive mechanism is not limited to just one or two genes but likely involves a network of interactions between viral RNA, microRNAs, and dozens of human proteins. They visualized these connections and found that many human factors that help viruses replicate are likely regulated by this same "sponge" effect. This means that the virus is not just a passive passenger in the cell but an active manipulator that reshapes the cellular environment by flooding it with its own genetic instructions. The study also points out that mutations in the non-coding regions of the virus, which do not change the viral proteins themselves, could still drastically alter the virus's ability to infect and spread by changing how well it competes for these microRNAs. This offers a new perspective on how viral strains evolve and why some become more infectious than others.

Ultimately, the research provides a clearer picture of the molecular tug-of-war that occurs when a respiratory virus infects a human cell. It shows that the virus wins not only by building its own parts but by disabling the cell's ability to regulate its own parts. The viral RNA acts as a competitive agent, sequestering the small regulatory molecules that would otherwise keep the cell's machinery in check. This leaves the cell's own proteins, which the virus needs for its own replication, free to function at high levels. The study confirms that this mechanism is a fundamental part of how these viruses operate, offering a new angle for understanding viral pathogenesis and potentially identifying new ways to disrupt the infection cycle by targeting these specific interactions between viral and human RNA.