Although understanding cells' responses to mechanical stimuli is seen as increasingly important for understanding cell biology, how to best measure, interpret, and model cells' mechanical properties remains unclear. We determine the frequency-dependent shear modulus of cultured mammalian cells by using four different methods, both unique and well established. This approach clarifies the effects of cytoskeletal heterogeneity, ATP-dependent processes, and cell regional variations on the interpretation of such measurements. Our results clearly indicate two qualitatively similar, but distinct, mechanical responses, corresponding to the cortical and intracellular networks, each having an unusual, weak power-law form at low frequency. The two frequency-dependent responses we observe are remarkably similar to those reported for a variety of cultured mammalian cells measured with different techniques, suggesting it is a useful consensus description. Finally, we discuss possible physical explanations for the observed mechanical response.cell mechanics ͉ cytoskeleton ͉ mechanotransduction ͉ microrheology T he important role of mechanical and physical cues in determining cell behavior is increasingly recognized. Cell shape can modulate cell differentiation (1), whereas substrate stiffness can affect tissue morphogenesis (2) and myoblast differentiation (3). The mechanisms, however, by which mechanical cues lead to molecular and biochemical responses remain largely undetermined. One approach to studying such mechano-sensing processes is to understand the mechanical properties of cells' constitutive molecules individually. Although protein conformation can respond to locally applied, small-scale mechanical signals such as molecular tension (4), how these signals may be combined to sense larger-scale mechanical properties remains unclear (5). Ultimately, an integrated physical description of cytoskeletal mechanics will be required to connect these molecular and cellular levels of description.In soft-matter and polymer physics, the mechanical response and dynamics of supramolecular assemblies are determined by using rheology, the study of the frequency-dependent elastic and viscous behavior of deformable materials. Reliably interpreting rheology measurements on living cells, however, has proven notoriously difficult. Only after decades of experimental effort have different cell measurements begun to report comparable responses, although many differences remain (6). The rheology of cells is typically inferred from either deformation in response to an applied force (termed active microrheology) or the Brownian motion of embedded or attached tracer particles (termed passive microrheology). The results of both approaches depend on theoretical models for the deformation geometry or coupling between the tracer and the cell. Passive methods will also be confounded by any non-Brownian tracer motion such as intracellular trafficking or cell crawling. The comparison of cell mechanics measurements based on different methods is currently ...

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The culminating step of the intraerythrocytic development of Plasmodium falciparum, the causative agent of malaria, is the spectacular release of multiple invasive merozoites on rupture of the infected erythrocyte membrane. This work reports for the first time that the whole process, taking place in time scales as short as 400 milliseconds, is the result of an elastic instability of the infected erythrocyte membrane. Using high-speed differential interference contrast (DIC) video microscopy and epifluorescence, we demonstrate that the release occurs in 3 main steps after osmotic swelling of the infected erythrocyte: a pore opens in ϳ 100 milliseconds, ejecting 1-2 merozoites, an outward curling of the erythrocyte membrane is then observed, ending with a fast eversion of the infected erythrocyte membrane, pushing the parasites forward. It is noteworthy that this last step shows slight differences when infected erythrocytes are adhering. We rationalize our observations by considering that during the parasite development, the infected erythrocyte membrane acquires a spontaneous curvature and we present a subsequent model describing the dynamics of the curling rim. Our results show that sequential erythrocyte membrane curling and eversion is necessary for the parasite efficient angular dispersion and might be biologically essential for fast and numerous invasions of new erythrocytes. (Blood. 2011;117(15):4118-4124) IntroductionThe phylum Apicomplexa includes a large number of notorious human and animal pathogens such as Plasmodium falciparum, the causative agent of severe cases of malaria, killing millions of people every year and reported to be clinically responsible for death since Pharaon times. 1 Host cell invasion by these obligate intracellular parasites occurs by active penetration of the host cell with the formation of a parasitophorous vacuole (PV). All along the erythrocytic cycle, and to achieve the needs of their growth, multiplication, and final release, Plasmodium parasites highly modify both the host cell membrane and their PV. Newly formed invasive parasites then escape from the host cell after the sequential opening of the PV and the host RBC membranes. 2 Although some parasite proteins, and particularly a cascade of proteolytic activities, 3 have been shown to be implicated, the dynamics of merozoite release, the crucial and very last step of the parasite intraerythrocytic development, and its mechanisms still remain to be deciphered. 4 It is still commonly referred to as an "explosive" event, 2,5,6 35 years after the seminal video microscopy work of Dvorak et al. 7 However, what happens to the membrane during this "explosion" and, in particular, how parasite displacements can reach several times the parasite body size in a split second without any swimming appendices or inertia are questions that have not yet received convincing answers because of the lack of direct observations. Hypothetical mechanisms have been proposed but not proven, such as the shredding of the membrane because of the osmotic...