Microfilaments and Microtubules Alternately Coordinate the Multistep Endosomal Trafficking of Classical Swine Fever Virus

The cytoskeleton, as a ubiquitous structure in the cells, plays an important role in the processes of virus entry, replication, and survival. However, the action mechanism of the cytoskeleton in the invasion of pestivirus into host cells remains unclear. In this study, we systematically dissected the key roles of the main cytoskeleton components, namely microfilaments and microtubules, in the endocytosis of the porcine pestivirus classical swine fever virus (CSFV). We observed the dynamic changes of actin filaments in CSFV entry. Confocal microscopy showed that CSFV invasion induced the dissolution and aggregation of stress fibers, resulting in the formation of lamellipodia and filopodia. Chemical inhibitors and RNA interference were used to find that the dynamic changes of actin were caused by an EGFR-PI3K/MAPK-RhoA/Rac1/Cdc42-cofilin signaling pathway, which regulates the microfilaments to help CSFV entry. Furthermore, colocalization of the microfilaments with clathrin and Rab5 (early endosome), as well as that of microtubules with Rab7 (late endosome) and Lamp1 (lysosome) revealed that microfilaments were activated and rearranged to help CSFV trafficking to the early endosome after endocytosis. Subsequently, recruitment of microtubules by CSFV also assisted membrane fusion of the virions from the late endosome to the lysosome with the help of a molecular motor, dynein. Unexpectedly, vimentin, which is an intermediate filament, had no effect on CSFV entry. Taken together, our findings comprehensively revealed the molecular mechanisms of cytoskeletal components that regulated CSFV endocytosis and facilitated further understanding of pestivirus entry, which would be conducive to exploration of antiviral molecules to control classical swine fever. IMPORTANCE Endocytosis, an essential biological process mediating cellular internalization events, is often exploited by pathogens for their entry into target cells. Previously, we reported different mechanisms of CSFV endocytosis into the porcine epithelial cells (PK-15) and macrophages (3D4/21); however, the details of microfilaments/microtubules mediated virus migration within the host cells remained to be elucidated. In this study, we found that CSFV infection induced rearrangement of actin filaments regulated by cofilin through an EGFR-PI3K/MAPK-RhoA/Rac1/Cdc42 pathway. Furthermore, we found that CSFV particles were trafficked along actin filaments in early and late endosomes, and through microtubules in lyso