Single particle tracking reveals SARS-CoV-2 regulating and utilizing dynamic filopodia for viral invasion.

Although severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) entry mechanism has been explored, little is known about how SARS-CoV-2 regulates the subcellular structural remodeling to invade multiple organs and cell types. Here, we unveil how SARS-CoV-2 boosts and utilizes filopodia to enter the target cells by real-time imaging. Using SARS-CoV-2 single virus-like particle (VLP) tracking in live cells and sparse deconvolution algorithm, we uncover that VLPs utilize filopodia to reach the entry site in two patterns, "surfing" and "grabbing", which avoid the virus from randomly searching on the plasma membrane. Moreover, combining mechanical simulation, we elucidate that the formation of virus-induced filopodia and the retraction speed of filopodia depend on cytoskeleton dynamics and friction resistance at the substrate surface caused by loading-virus gravity, respectively. Further, we discover that the entry process of SARS-CoV-2 via filopodia depends on Cdc42 activity and actin-associated proteins fascin, formin, and Arp2/3. Together, our results highlight that the spatial-temporal regulation of actin cytoskeleton by SARS-CoV-2 infection makes filopodia as a highway for virus entry and potentiates it as an antiviral target.

Actin nucleator formins regulate the tension-buffering function of caveolin-1.

Both the mechanosensitive actin cytoskeleton and caveolae contribute to active processes such as cell migration, morphogenesis, and vesicular trafficking. Although distinct actin components are well studied, how they contribute to cytoplasmic caveolae, especially in the context of mechano-stress, has remained elusive. Here, we identify two actin-associated mobility stereotypes of caveolin-1 (CAV-1)-marked intracellular vesicles, which are characterized as 'dwelling' and 'go and dwelling'. In order to exploit the reason for their distinct dynamics, elongated actin-associated formin functions are perturbed. We find drastically decreased density, increased clustering, and compromised motility of cytoplasmic CAV-1 vesicles resulting from lacking actin nucleator formins by both chemical treatment and RNA silencing of formin genes. Furthermore, hypo-osmosis-stimulated diminishing of CAV-1 is dramatically intensified upon blocking formins. The clustering of CAV-1 vesicles when cells are cultured on soft substrate is also aggravated under formin inhibition condition. Together, we reveal that actin-associated formins are essential for maintaining the dynamic organization of cytoplasmic CAV-1 and importantly its sensitivity upon mechanical challenge. We conclude that tension-controlled actin formins act as a safety valve dampening excessive tension on CAV-1 and safeguarding CAV-1 against mechanical damage.

How Physical Factors Coordinate Virus Infection: A Perspective From Mechanobiology.

Pandemics caused by viruses have threatened lives of thousands of people. Understanding the complicated process of viral infection provides significantly directive implication to epidemic prevention and control. Viral infection is a complex and diverse process, and substantial studies have been complemented in exploring the biochemical and molecular interactions between viruses and hosts. However, the physical microenvironment where infections implement is often less considered, and the role of mechanobiology in viral infection remains elusive. Mechanobiology focuses on sensation, transduction, and response to intracellular and extracellular physical factors by tissues, cells, and extracellular matrix. The intracellular cytoskeleton and mechanosensors have been proven to be extensively involved in the virus life cycle. Furthermore, innovative methods based on micro- and nanofabrication techniques are being utilized to control and modulate the physical and chemical cell microenvironment, and to explore how extracellular factors including stiffness, forces, and topography regulate viral infection. Our current review covers how physical factors in the microenvironment coordinate viral infection. Moreover, we will discuss how this knowledge can be harnessed in future research on cross-fields of mechanobiology and virology.