Dissection of the Influenza A Virus Endocytic Routes Reveals Macropinocytosis as an Alternative Entry Pathway

de Vries, Erik; Tscherne, Donna M.; Wienholts, Marleen J.; Cobos-Jiménez, Viviana; Scholte, Florine; García-Sastre, Adolfo; Rottier, Peter J. M.; de Haan, Cornelis A. M.

Despite this extensive list of arguments, viral entry by macropinocytosis needs to be considered with caution. The characteristics of the DYNA-IND route of cell entry by IAV are similar, but not identical to the macropinocytic entry routes taken by other viruses, like two different strains of vaccinia virus and by coxsackie virus B [34, 35] . As is shown in Table 1 and discussed in more detail below, the macropinocytic pathways used by each of these viruses have a few unique characteristics. This may very well reflect the growing notion that macropinocytosis represents a number of differentially induced and regulated processes, rather than being a single endocytic pathway [13, 14] . Macropinocytosis has collectively been described as an inducible form of endocytosis by which fluid-phase cargo travels via non-coated, relatively large and heterogeneous organelles that have emanated from extensive protrusions (e.g lamellar ruffles, circular ruffles or retracting blebs) of the plasma membrane [13] . In the case of DYNA-IND IAV entry more extensive studies using electron microscopy will be required to study the morphology of membrane protrusions with which IAV may associate. In addition, live cell imaging microscopy will be required to characterize the exact itinerary that is taken by IAV virions traveling via a macropinocytic process. This is especially important as different routes of IAV entry are likely to converge at some point in the endocytic pathway. Although unlikely, co-localization of IAV particles with fluid-phase dextran as shown in Fig. 8B may thus represent a situation occurring after convergence of several different routes. The use of microscopy to study macropinocytosis is however complicated by the lack of specific membrane-associated markers for any early step of this endocytic process.