Recent advances in nanotechnology have stimulated novel applications in biomedicine where nanoparticles (NPs) are used to achieve drug delivery and photodynamic therapy. In chemotherapeutic cancer treatment, tumor-specific drug delivery is a topic of considerable research interest for achieving enhanced therapeutic efficacy and for mitigating adverse side effects. Most anticancer agents are incapable of distinguishing between benign and malignant cells, and consequently cause systematic toxicity during cancer treatment. Owing to their small size, ligand-coated NPs can be efficiently directed toward, and subsequently internalized by tumor cells through ligand-receptor recognition and interaction (see Fig. 1), thereby offering an effective approach for specific targeting of tumor cells. For example, branching dendrimers have recently been identified as potential candidates for site-specific drug carriers.[2] NPs have also been exploited in other biomedical applications such as bioimaging [3,4] and biosensing. [5,6] It has been demonstrated that florescent quantum dots are efficient in tumor cell imaging, recognition, and tracking, [3,4] and that gold NPs are capable of detecting small proteins. [5,6] To enable rational design of such NP-based agents, it is essential to understand the underlying mechanisms that govern the transmembrane transport and invagination of NPs in biological cells. In this communication, we present a thermodynamic model for receptormediated endocytosis of ligand-coated NPs. We identify an optimal NP radius at which the cellular uptake reaches a maximum of several thousand at physiologically relevant parameters, and we show that the cellular uptake of NPs is regulated by membrane tension, and can be elaborately controlled by particle size. The optimal NP radius for endocytosis is on the order of 25−30 nm, which is in good agreement with prior estimates. [7] Theoretical models [7−11] have provided insights into the dynamics of receptor-mediated endocytosis based on energetic and kinetic considerations, primarily in the context of virus budding. Lerner et al.[8] argued that the discreteness of membrane wrapping via ligandreceptor binding results in a corrugated energy landscape for NP wrapping, which governs the kinetics of endocytosis. In contrast, Gao et al. [7] proposed that the endocytic rate is limited by

Parasitization by malaria-inducing Plasmodium falciparum leads to structural, biochemical, and mechanical modifications to the host red blood cells (RBCs). To study these modifications, we investigate two intrinsic indicators: the refractive index and membrane fluctuations in P. falciparum-invaded human RBCs (Pf-RBCs). We report experimental connections between these intrinsic indicators and pathological states. By employing two noninvasive optical techniques, tomographic phase microscopy and diffraction phase microscopy, we extract three-dimensional maps of refractive index and nanoscale cell membrane fluctuations in isolated RBCs. Our systematic experiments cover all intraerythrocytic stages of parasite development under physiological and febrile temperatures. These findings offer potential, and sufficiently general, avenues for identifying, through cell membrane dynamics, pathological states that cause or accompany human diseases.erythrocyte ͉ febrile temperature ͉ malaria ͉ mechanical properties ͉ optical techniques D uring the intraerythrocytic development, the malaria parasite Plasmodium falciparum causes structural, biochemical, and mechanical changes to host red blood cells (RBCs). Major structural changes include the formation of parasitophorus vacuoles that surround the growing parasite in their host RBCs, loss of cell volume, and the appearance of small, nanoscale protrusions or ''knobs,'' on the membrane surface (1). From the biochemical standpoint, a considerable amount of hemoglobin (Hb) is digested by parasites during intraerythrocytic development and converted into insoluble polymerized forms of heme, known as hemozoin (2, 3). Hemozoin appears as brown crystals in the vacuole of parasite in later maturation stages of P. falciparum-invaded human RBCs (Pf-RBCs).Two major mechanical modifications are loss of RBC deformability (4-6) and increased cytoadherence of the invaded RBC membrane to vascular endothelium and other RBCs (7). These changes lead to sequestration of RBCs in microvasculature in the later stages of parasite development, which is linked to vital organ dysfunction in severe malaria. In the earlier stage, where some loss of deformability occurs, Pf-RBCs continue to circulate in the bloodstream.Membrane dynamics of RBCs can be influenced by human disease states. Fluctuations in phospholipid bilayer and attached spectrin network are known to be altered by cytoskeletal defects, stress, and actin-spectrin dissociations arising from metabolic activity linked to adenosine 5Ј-triphosphate (ATP) concentration (8-12). Proteins transported from invading organisms, such as the virulent malaria-inducing parasite P. falciparum, to specific binding sites in the spectrin network are considered to introduce significant alterations to RBC membrane dynamics and mechanical response (13,14). These changes could provide insights into possible mechanistic pathways in the pathogenesis of malaria, because the parasite alters biophysical properties of RBCs during its intraerythrocyte stage that lasts up to 48 ...

The human erythrocyte (red blood cell, RBC) demonstrates extraordinary ability to undergo reversible large deformation and fluidity. Such mechanical response cannot be consistently rationalized on the basis of fixed connectivity of the cell cytoskeleton that comprises the spectrin molecular network tethered to phospholipid membrane. Active topological remodeling of spectrin network has been postulated, although detailed models of such dynamic reorganization are presently unavailable. Here we present a coarsegrained cytoskeletal dynamics simulation with breakable protein associations to elucidate the roles of shear stress, specific chemical agents, and thermal fluctuations in cytoskeleton remodeling. We demonstrate a clear solid-to-fluid transition depending on the metabolic energy influx. The solid network's plastic deformation also manifests creep and yield regimes depending on the strain rate. This cytoskeletal dynamics model offers a means to resolve long-standing questions regarding the reference state used in RBC elasticity theory for determining the equilibrium shape and deformation response. In addition, the simulations offer mechanistic insights into the onset of plasticity and void percolation in cytoskeleton. These phenomena may have implication for RBC membrane loss and shape change in the context of hereditary hemolytic disorders such as spherocytosis and elliptocytosis.cytoskeleton remodeling ͉ computer simulation ͉ fluidization ͉ plasticity ͉ spectrin-actin dissociation D uring its 120-day life span, a red blood cell (RBC) circulates a million times in human body, often squeezing through narrow capillaries. The erythrocyte's remarkable mechanical properties originate from the unique architecture of its cell wall, which is the main load bearing component as there are no stress fibers inside the cell. The cell wall comprises a spectrin tetramer network tethered to a phospholipid bilayer (1, 2). It is very flexible, and yet resilient enough to recover the biconcave shape whenever the cell is quiescent (see Fig. 1). It has been proposed that the spectrin network connectivity is not fixed and can experience extensive remodeling as the RBC undergoes large deformation. Experiments show that, under physiological conditions, the spectrin tetramers in an unstressed intact cell wall exist in rapid dynamic equilibrium with the dimers, and that shear-induced cell wall deformation displaces the balance in favor of the dimers (3). Here, the cell wall behaves as a weak elastic solid at low strain levels, whereas it can be fluidized beyond a certain level of shear deformation, similar to soft matter such as emulsions and colloidal pastes (4, 5). Upon unloading, the dynamic stability is shifted toward tetramers and the stiffness of the cell wall increases. Experiments also suggest that cytoskeleton remodeling can be regulated by biochemical factors such as Ca 2ϩ and ATP (adenosine 5Ј-triphosphate) concentrations (6, 7). It has been postulated that phosphorylation affects spectrin-actin binding and produces changes i...

Biological membranes are involved in numerous intriguing biophysical and biological cellular phenomena of different length scales, ranging from nanoscale raft formation, vesiculation, to microscale shape transformations. With extended length and time scales as compared to atomistic simulations, solvent-free coarse-grained membrane models have been exploited in mesoscopic membrane simulations. In this study, we present a one-particle-thick fluid membrane model, where each particle represents a cluster of lipid molecules. The model features an anisotropic interparticle pair potential with the interaction strength weighed by the relative particle orientations. With the anisotropic pair potential, particles can robustly self-assemble into fluid membranes with experimentally relevant bending rigidity. Despite its simple mathematical form, the model is highly tunable. Three potential parameters separately and effectively control diffusivity, bending rigidity, and spontaneous curvature of the model membrane. As demonstrated by selected examples, our model can naturally simulate dynamics of phase separation in multicomponent membranes and the topological change of fluid vesicles.

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The membrane of the red blood cell (RBC) consists of spectrin tetramers connected at actin junctional complexes, forming a two-dimensional (2D) sixfold triangular network anchored to the lipid bilayer. Better understanding of the erythrocyte mechanics in hereditary blood disorders such as spherocytosis, elliptocytosis, and especially, sickle cell disease requires the development of a detailed membrane model. In this study, we introduce a mesoscale implicit-solvent coarse-grained molecular dynamics (CGMD) model of the erythrocyte membrane that explicitly describes the phospholipid bilayer and the cytoskeleton, by extending a previously developed two-component RBC membrane model. We show that the proposed model represents RBC membrane with the appropriate bending stiffness and shear modulus. The timescale and self-consistency of the model are established by comparing our results with experimentally measured viscosity and thermal fluctuations of the RBC membrane. Furthermore, we measure the pressure exerted by the cytoskeleton on the lipid bilayer. We find that defects at the anchoring points of the cytoskeleton to the lipid bilayer (as in spherocytes) cause a reduction in the pressure compared with an intact membrane, whereas defects in the dimer-dimer association of a spectrin filament (as in elliptocytes) cause an even larger decrease in the pressure. We conjecture that this finding may explain why the experimentally measured diffusion coefficients of band-3 proteins are higher in elliptocytes than in spherocytes, and higher than in normal RBCs. Finally, we study the effects that possible attractive forces between the spectrin filaments and the lipid bilayer have on the pressure applied on the lipid bilayer by the filaments. We discover that the attractive forces cause an increase in the pressure as they diminish the effect of membrane protein defects. As this finding contradicts with experimental results, we conclude that the attractive forces are moderate and do not impose a complete attachment of the filaments to the lipid bilayer.

Super-resolution microscopy recently revealed that, unlike the soma and dendrites, the axon membrane skeleton is structured as a series of actin rings connected by spectrin filaments that are held under tension. Currently, the structure-function relationship of the axonal structure is unclear. Here, we used atomic force microscopy (AFM) to show that the stiffness of the axon plasma membrane is significantly higher than the stiffnesses of dendrites and somata. To examine whether the structure of the axon plasma membrane determines its overall stiffness, we introduced a coarse-grain molecular dynamics model of the axon membrane skeleton that reproduces the structure identified by super-resolution microscopy. Our proposed computational model accurately simulates the median value of the Young’s modulus of the axon plasma membrane determined by atomic force microscopy. It also predicts that because the spectrin filaments are under entropic tension, the thermal random motion of the voltage-gated sodium channels (Nav), which are bound to ankyrin particles, a critical axonal protein, is reduced compared to the thermal motion when spectrin filaments are held at equilibrium. Lastly, our model predicts that because spectrin filaments are under tension, any axonal injuries that lacerate spectrin filaments will likely lead to a permanent disruption of the membrane skeleton due to the inability of spectrin filaments to spontaneously form their initial under-tension configuration.

We present a two-component coarse-grained molecular-dynamics model for simulating the erythrocyte membrane. The proposed model possesses the key feature of combing the lipid bilayer and the erythrocyte cytoskeleton, thus showing both the fluidic behavior of the lipid bilayer and the elastic properties of the erythrocyte cytoskeleton. In this model, three types of coarse-grained particles are introduced to represent clusters of lipid molecules, actin junctions, and band-3 complexes, respectively. The proposed model facilitates simulations that span large length scales (approximately micrometers) and timescales (approximately milliseconds). By tuning the interaction potential parameters, we were able to control the diffusivity and bending rigidity of the membrane model. We studied the membrane under shearing and found that at a low shear strain rate, the developed shear stress was due mainly to the spectrin network, whereas the viscosity of the lipid bilayer contributed to the resulting shear stress at higher strain rates. In addition, we investigated the effects of a reduced spectrin network connectivity on the shear modulus of the membrane.