Cells actively help to capture and incorporate influenza viruses. Here, a cell is shown, with a virus in the centre of the image.
Illustration: Emma Hyde / ETH Zürich
Researchers from Switzerland and Japan, led by Professor Yohei Yamauchi of Eidgenössische Technische Hochschule Zürich (ETH Zürich), have developed a microscopy technique that enables real-time, high-resolution observation of how a virus gains entry to a cell. Their findings are described in the Proceedings of the National Academy of Sciences of the USA (PNAS).
The process, in which a virus exploits the pathways cells normally use to take in larger molecules such as hormones, cholesterol, or iron, involves the active cooperation of the cell as it reaches out to engulf the viral particle. This mechanism is triggered by receptors on the cell surface, to which viruses bind while ‘surfing’ along the membrane, seeking regions rich in receptors to form a stable attachment.
In other words, creationists often portray this as an “irreducibly complex” system, supposedly dependent on all components being present from the outset, requiring what they call “complex specified information” in both virus and cell to produce the receptors and binding proteins. Discovery Institute fellows Michael J. Behe and William A. Dembski present this as evidence of intelligent design.
Their argument depends on a statistical sleight of hand: they treat the entire process as though it originated in a single event involving one cell and one virus, then calculate improbabilities for each step and multiply them together, producing a vanishingly small likelihood of the whole mechanism arising spontaneously. This ignores the fact that evolution operates in populations — often large ones — across long periods, where components accumulate gradually over generations, dramatically increasing the probability of multiple features emerging together in the same lineage.
It also overlooks the billions of years during which viruses and cells have co-evolved. As multicellular organisms evolved ever more sophisticated ways of receiving and responding to external signals and substances, viruses simultaneously improved their ability to exploit those mechanisms.
But to the scientifically illiterate target audience of the ID-creationism industry, evolution is imagined as a single event rather than a continuous process, leaving them oblivious to the misuse of probability and the underlying mathematical errors.
Creationists trying to use this argument for intelligent design usually respond to biologists pointing out the obvious fact that they just presented their putative god as some sort of celestial malevolence, by retreating into Bible literalism and religious fundamentalism and invoking mythical 'Fall', so betraying the claims of the Discovery Institute and its fellows that ID is real science, not bible-literalist creationism dressed in a lab coat, as a lie.
The ETH Zürich-led team’s research is summarised in an ETH Zürich news item by Fabio Bergamin.
How influenza viruses enter our cells
For the first time, researchers have observed live and in high resolution how influenza viruses infect living cells. This was possible thanks to a new microscopy technique, which could now help to develop antiviral therapies in a more targeted manner.
In brief
- For the first time, a new high-resolution microscopy technique has allowed researchers to watch live as influenza viruses infect cells.
- The international team led by ETH Zurich found that the cells actively promote virus uptake.
- This technique could now help to develop antiviral therapies in a more targeted manner.
Fever, aching limbs and a runny nose – as winter returns, so too does the flu. The disease is triggered by influenza viruses, which enter our body through droplets and then infect cells.
Researchers from Switzerland and Japan have now investigated this virus in minute detail. Using a microscopy technique that they developed themselves, the scientists can zoom in on the surface of human cells in a Petri dish. For the first time, this has allowed them to observe live and in high resolution how influenza viruses enter a living cell.
Led by Yohei Yamauchi, Professor of Molecular Medicine at ETH Zurich, the researchers were surprised by one thing in particular: the cells are not passive, simply allowing themselves to be invaded by the influenza virus. Rather, they actively attempt to capture it.
“The infection of our body cells is like a dance between virus and cell.
Professor Yohei Yamauchi, corresponding author.
Molecular Medicine Laboratory
Institute of Pharmaceutical Sciences
Department of Chemistry and Applied Biosciences
Eidgenössische Technische Hochschule Zürich
Zürich, Switzerland.
Viruses surf on the cell surface
Of course, our cells gain no advantage from a viral infection or from actively participating in the process. The dynamic interplay takes place because the viruses commandeer an everyday cellular uptake mechanism that is essential for the cells. Specifically, this mechanism serves to channel vital substances, such as hormones, cholesterol or iron, into the cells.
Like these substances, influenza viruses must also attach to molecules on the cell surface. The dynamics are like surfing on the surface of the cell: the virus scans the surface, attaching to a molecule here or there, until it has found an ideal entry point – one where there are many such receptor molecules located close to one another, enabling efficient uptake into the cell.
Once the cell’s receptors detect that a virus has attached itself to the membrane, a depression or pocket forms at the location in question. This depression is shaped and stabilised by a special structural protein known as clathrin. As the pocket grows, it encloses the virus, leading to the formation of a vesicle. The cell transports this vesicle into its interior, where the vesicle coating dissolves and releases the virus.
Previous studies investigating this key process used other microscopy techniques, including electron microscopy. As these techniques entailed the destruction of the cells, they could only ever provide a snapshot. Another technique that is used – known as fluorescence microscopy – only allows low spatial resolution.
Combined techniques, including for other viruses
The new technique, which combines atomic force microscopy (AFM) and fluorescence microscopy, is known as virus-view dual confocal and AFM (ViViD-AFM). Thanks to this method, it is now possible to follow the detailed dynamics of the virus’s entry into the cell.
Video: Nicole Davidson / ETH Zurich.
Accordingly, the researchers have been able to show that the cell actively promotes virus uptake on various levels. In this way, the cell actively recruits the functionally important clathrin proteins to the point where the virus is located. The cell surface also actively captures the virus by bulging up at the point in question. These wavelike membrane movements become stronger if the virus moves away from the cell surface again.
The new technique therefore provides key insights when it comes to the development of antiviral drugs. For example, it is suitable for testing the efficacy of potential drugs in a cell culture in real time. The study authors emphasise that the technique could also be used to investigate the behaviour of other viruses or even vaccines.
Publication:
What ID advocates never seem to notice is that, in arguing that such mechanisms must have been deliberately engineered, they are attributing to their designer a system in which viruses are given exquisitely tailored tools for invading the very cells it supposedly created. If one insists that this is intentional design, then one must also accept that the designer crafted the molecular equivalent of lockpicks and battering rams, optimised for breaching living tissue. It is difficult to reconcile this with any notion of benevolence.Significance
Influenza A viruses (IAVs) continue to cause epidemics worldwide due to their high mutability. Nevertheless, the initial step of infection, viral uptake into cells, has been challenging to observe directly with conventional microscopy techniques. Here, we developed a hybrid imaging system combining atomic force microscopy and confocal microscopy with enhanced mechanical functionality and minimal invasiveness to directly visualize nanoscale dynamics of IAV and cell membranes during viral uptake into living cells. This system enables the analysis of IAV lateral diffusion resulting from IAV–membrane interactions and characteristic membrane morphological changes induced by IAV during endocytosis. Our approach offers a method to rapidly assess the impact of viral mutations on host cell entry, which is critical for understanding emerging IAV variants.
Abstract
Influenza A virus (IAV) entry into host cells begins with interactions between the viral envelope proteins hemagglutinin (HA)/neuraminidase (NA) and sialic acid moieties on the cell plasma membrane. These interactions drive IAV’s lateral diffusion along the cell membrane and trigger membrane morphological changes required for endocytosis. However, directly visualizing these dynamic processes, which are crucial for IAV entry, has been challenging using conventional microscopy techniques. In this study, we enabled live-cell observation of nanoscale morphological dynamics of IAV and the cell membrane by reducing the mechanical invasiveness of atomic force microscopy (AFM). A customised cantilever with less than half the spring constant of conventional cantilevers enabled virus-view AFM imaging that preserved IAV–membrane interactions. By combining virus-view AFM with confocal microscopy, we performed correlative morphological and fluorescence observations of IAV lateral diffusion and endocytosis in living cells. Variations in diffusion coefficients of single virions suggested heterogeneity in sialic acid density on the cell membrane. NA inhibition decreased diffusion coefficients, while reduced sialic acid density increased them. The timing of clathrin accumulation at virion binding sites coincided with a decrease in diffusion coefficients, a relationship that was maintained independent of NA activity or sialic acid density. As clathrin assembly progressed, ~100-nm-high membrane bulges emerged adjacent to the virus, culminating in the complete membrane envelopment of the virus at peak clathrin accumulation. Our virus-view AFM will deepen our understanding of various virus–cell interactions, facilitate the evaluation of drug effects and promote future translational research.
Influenza A virus (IAV) is an enveloped RNA virus with two key surface glycoproteins: hemagglutinin (HA) and neuraminidase (NA). The virus surface contains 300 to 400 HA and 40 to 50 NA molecules (1). IAV envelope proteins comprise at least 18 HA and 11 NA subtypes (2), which enable IAV to infect various host species including humans, birds, pigs, bats, and other animals (3). These envelope proteins play crucial roles in IAV infection of host cells. They interact with sialic acids on cell surface glycolipids and glycoproteins (4) or with major histocompatibility complex class II (MHC class II) molecules (5–7). HA binds to sialic acids at the terminal ends of glycan chains on the cell surface. The HA–sialic acid interactions are inherently weak, with dissociation constants typically in the millimolar range (0.9 to 68.4 × 10−3 M) (8–10). However, multivalent binding of multiple HAs to sialic acids enables IAV to stably adhere to the cell membrane (11, 12). Meanwhile, NA catalyzes the cleavage of sialic acids (13), inhibiting stable adhesion of IAV to the cell membrane. Through these mechanisms, HA and NA effectively regulate the attachment and detachment of IAV to the cell membrane.
The competitive action between HA and NA allows IAV to diffuse laterally along the cell membrane surface topology (). This lateral diffusion represents a critical dynamic macroscopic phenomenon reflecting virus–membrane interactions. However, conventional microscopy techniques have struggled to detect IAV movement on the 10-nm-thick cell membrane, resulting in limited visualization success (15–18).
HA-NA-sialic acid interactions also trigger endocytosis involving morphological changes of the cell membrane. When diffusing IAV binds to functional receptors such as EGFR (19) and Cav1.2 (20) through sialic acids, it initiates the recruitment and assembly of the endocytic machinery including clathrin, actin, and dynamin. IAV utilizes multiple entry pathways including clathrin-mediated endocytosis (CME), macropinocytosis, and both clathrin-independent and dynamin-independent mechanisms (16, 21–23). IAV primarily utilizes CME for cellular entry (16, 21). Previous imaging of membrane dynamics using atomic force microscopy (AFM) has revealed that in IAV-free CME, clathrin-coated membrane invaginations (pits) larger than 100 nm in diameter form (24, 25). This is accompanied by the emergence of actin-dependent membrane bulges that develop on one side of the pit and eventually lead to its closure. Although electron microscopy has provided morphological snapshots of pits during IAV internalization (26), the membrane dynamics during IAV internalization via CME have yet to be successfully visualized.
AFM enables mechanical imaging of sample morphology with nanometer-scale resolution (27, 28). Since the development of high-speed AFM in 2001 (29), this technique has contributed significantly to molecular dynamics analysis (30–36). Additionally, the advent of cell-imaging AFM in 2013 has enabled advances in membrane dynamics analysis (37, 38). The integration of cell-imaging AFM combined with confocal microscopy has provided unique capabilities for observing nanoscale membrane morphological changes in living cells (24, 25). Despite these advances, a major challenge persists: the mechanical interference of the cantilever with biological samples. Visualizing the dynamic processes of IAV lateral diffusion and internalization requires an innovative technology capable of simultaneously observing the nanoscale morphology of the 10-nm-thick cell membrane and the 100-nm spherical IAV interacting with cell surface sialic acid-bearing glycolipids and proteins. Given that multivalent IAV–membrane interaction forces are relatively weak, ranging from 10 to 25 pN (39), achieving low-invasive imaging capabilities is critical.
In this study, we address and overcome the challenge of mechanical interference by enhancing the low invasiveness of AFM through the use of a customised soft cantilever. In combination with confocal microscopy, low-invasive AFM enables simultaneous live-cell imaging of both morphology and fluorescence. The redesigned cantilever minimizes disruption of IAV–membrane interactions, allowing accurate observation of viral dynamics. Using this system, we investigated the lateral diffusion of single IAV particles under various conditions, including NA inhibition, reduced cell surface sialic acid density, and different viral subtypes. We also analyzed membrane morphological changes before and during IAV endocytosis. While fluorescently labeled IAV was primarily used, we also demonstrate our AFM’s capability to track unlabeled viruses. This virus-view dual confocal and AFM, called ViViD-AFM, enables correlative morphological and fluorescence imaging of IAV–membrane dynamics, providing nanoscopic insights into HA-NA-sialic acid interactions.
A. Yoshida, Y. Uekusa, T. Suzuki, M. Bauer, N. Sakai, & Y. Yamauchi
Enhanced visualization of influenza A virus entry into living cells using virus-view atomic force microscopy
Proc. Natl. Acad. Sci. U.S.A. 122 (38) e2500660122, https://doi.org/10.1073/pnas.2500660122 (2025).
Copyright: © [year] The authors.
Published by [publisher]. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)
Indeed, by rejecting evolution as the explanation for viral entry, ID proponents corner themselves into an uncomfortable theological stance: their designer not only equipped viruses with the machinery to exploit cellular signalling, but also ensured that cells remained vulnerable to such exploitation. The result is an ecosystem in which suffering, disease, and death are not unfortunate consequences of natural processes but deliberate design choices.
This is, of course, why mainstream biology requires no such designer. Co-evolution naturally explains why cells have receptors essential for communication and nutrient uptake, while viruses have, over immense timescales, adapted to hijack those same pathways. No malevolent architect is required—only the simple, iterative logic of variation, selection, and replication.
Yet the ID movement persistently overlooks this simpler, evidence-based account, preferring instead an argument that—if taken seriously—presents their putative creator as either unable to prevent viral parasitism or fully complicit in engineering it. Neither option supports the benevolent, omnipotent designer they hope to defend.
