The peptidyl transferase antibiotic sparsomycin can induce efficient ribosomal translocation in the absence of EF-G and GTP. However, how the antibiotic facilitates the translocation is unclear. Here, two models of antibiotic-induced translocation are considered, in which the interaction of the antibiotic with the peptidyl-tRNA in the A/P state, besides behaving as a pawl in the “Brownian ratchet”, also has an effect of compromising the interaction of the 30S subunit with the mRNA-tRNA complex (Model I) or facilitating the forward 30S head rotation which results in the ribosomal unlocking (Model II). It is shown that although the results obtained with Model I can explain the available experimental data on the translocation through the single-stranded mRNA, they are inconsistent with the experimental data on the translocation through the mRNA duplex; by contrast, the results obtained with Model II can explain quantitatively all of the available experimental data. With model II it is further shown that the efficient translocation induced by the binding of sparsomycin results mainly from the facilitation of the forward 30S head rotation, whereas the effect as the pawl plays only a little role. In addition, it is noted that the mRNA movement is brought about by the reverse intersubunit rotation rather than driven directly by the forward 30S head rotation, and the mechanism of antibiotic-induced translocation is similar to that of EF-G-catalyzed translocation.

Tissue engineering emerges as a viable method to replace cartilage damaged by osteoarthritis or trauma. A key element for cartilage tissue engineering is to elucidate growth factor exposure regimes that enhance cartilage production from multipotent progenitor cells. To address this, we identify growth factor conditions that promote chondrogenesis of human mesenchymal stem cells cultured under static conditions. We identified transient exposure to bone morphogenic protein-2 in the presence of transforming growth factor beta-1 as a viable condition that promotes expression of chondrogenic phenotype for human mesenchymal stem cells seeded onto polycaprolactone scaffolds. We then use this growth factor regime as a proof of concept to test a new shear and perfusion bioreactor. This novel bioreactor design applies shear stress stimulation on the surface of constructs while allowing tissue perfusion. Constructs harvested from the bioreactor after 8 days showed cartilage development primarily near the construct surface exposed to shear stress. By day 14, cells, collagen deposition and proteoglycan deposition is observed throughout the scaffold, indicative of cartilage development.

Collective cell migration (CCM) can be described as a large scale coordinated movement of cells that are in close proximity with each other. It is a phenomenon that is observed not only in physiological processes such as that found in embryogenesis and wound healing but also in pathophysiological processes such as cancer metastasis. Some of the factors influencing this concerted process include cell–cell adhesion, cell–substrate interaction and mechanical cues such as geometrical constraints among others. Here, we review recent research work done to investigate CCM of adherent cells. We highlight the classical example of an in vitro cell monolayer to illustrate our current understanding of the different mechanobiological mechanisms involved as these cells respond to the mechanical cues present in their environment. It is hoped that such understanding may potentially lead to better therapeutic strategies for diseases such as cancer and for tissue engineering and repair.

A key step of adaptive immune responses is the T lymphocyte capacity to detect the presence of foreign antigens on specialized cells with high speed and specificity during contacts lasting a few minutes. Much evidence suggests that there is a deep link between the lifetime of molecular interactions between T cell receptors and ligands and T cell activation, but the precise mechanisms of bond formation and dissociation remain incompletely understood. Previous experiments done with interference reflection microscopy/reflection interference contrast microscopy disclosed transverse motions with several nanometer average amplitude of micrometer size membrane zones. More recently, total internal reflection fluorescence microscopy was used to show that the initial interaction between primary T lymphocytes and model surfaces involved the tip of microvilli (typically 0.2 µm2 area) generating apparent contacts of a few seconds that allowed cells to detect ligands of their membrane receptors. Here we show that these microvilli displayed minimal lateral displacements but quantitative fluorescence measurement suggested the occurrence of spontaneous transverse fluctuations of order of 67 nm amplitude during 1-s observation periods. This may play a major role in membrane receptor engagement and ensuing signal generation.

Human pluripotent stem cells (hPSCs) - including embryonic stem cells (hESCs) and induced pluripotent stem cells (hiPSCs) - are very promising candidates for cell therapies, tissue engineering, high throughput pharmacology screens, and toxicity testing. These applications require large numbers of high quality cells; however, scalable production of human pluripotent stem cells and their derivatives at a high density and under well-defined conditions has been a challenge. We recently reported a simple, efficient, fully defined, scalable, and good manufacturing practice (GMP) compatible 3D culture system based on a thermoreversible hydrogel for hPSC expansion and differentiation. Here, we describe additional design rationale and characterization of this system. For instance, we have determined that culturing hPSCs as a suspension in a liquid medium can exhibit lower volumetric yields due to cell agglomeration and possible shear force-induced cell loss. By contrast, using hydrogels as 3D scaffolds for culturing hPSCs reduces aggregation and may insulate from shear forces. Additionally, hydrogel-based 3D culture systems can support efficient hPSC expansion and differentiation at a high density if compatible with hPSC biology. Finally, there are considerable opportunities for future development to further enhance hydrogel-based 3D culture systems for producing hPSCs and their progeny.

Cardiovascular diseases, including hypertension and the evolution of an atherosclerotic plaque alter the mechanics of the sub-endothelium. The extent to which either arterial softening or stiffening exacerbates or reduces mononuclear leukocyte infiltration from the bloodstream into the wall remains unclear. Mononuclear cell (MNC) transmigration was observed using an in vitro model of the inflamed human vascular endothelium on variable substrate stiffness. Briefly, human aortic endothelial cells were allowed to grow to confluency on gels of 1, 3, 5, 280 kPa, and on glass (~70 GPa), and then activated with TNF-α. Isolated human MNCs were allowed to transmigrate across the inflamed endothelium for 1 h. Fewer MNC transmigrated on soft compared to stiff substrates, yet the relative expression of ICAM-1 and VCAM-1, and the fraction of MNCs that become activated (or changed shape) did not change. Following MNC transendothelial migration the distribution of VCAM-1 is translocated from the apical to basal plasma membrane as revealed through immunofluorescence; since less MNCs transmigrate on soft subendothelial substrates, translocation of VCAM-1 was not observed. These results herein highlight that stiffer subendothelial substrates actually increase MNC transmigration, yet transmigration on stiff substrates can be abrogated by blocking ICAM-1.

Since the first in vitro observations of movement driven by molecular motors by Sheetz and Spudich in 1983, 30 years of work have brought us toward a better foundation in understanding how molecular motors play roles in different organisms and how they can be used in biotechnology applications. Recent advances in this area were highlighted in a session ‘‘Biomolecular Motors and Force Generation’’ at the first US Association for Computational Mechanics Thematic Conference on Multiscale Methods and Validation in Medicine and Biology held in February 2012. This event featuring 15 oral presentations, including a keynote speech by George Oster, generated fruitful discussions among researchers in the field and led us to conceive of this special issue. A majority of biomolecular motors are ATPdependent, coupling ATP hydrolysis energy to mechanical work. These motors include cytoskeletal myosin, kinesin, and dynein, as well as nucleic acidbased translocases, ATP-dependent chaperones, and ATP synthase. Each molecular motor generates forward movement along its specific substrate (myosin on actin, kinesin and dynein on microtubule, ATPdependent chaperones on peptides, nucleic acid-based translocases on DNA or RNA, and ATP synthase on the central shaft). The motor–substrate interactions and their corresponding functions, and the interactions between individual motors are one focus of this special issue. For the interaction between myosin and actin filaments, Yasuhiro Inoue and Taiji Adachi illustrate how actin–myosin catch bonds affect actomyosin aggregate formation. Using computer simulations they found that the catch-bond behavior facilitates the assembly of myosin in a concentration dependent manner. The actin–myosin interaction thus can influence the properties of actin–myosin networks at a larger scale which in turn affect cellular morphogenesis. In muscle, the interaction between a myosin motor and an actin filament can be affected by the binding of neighboring myosins to the actin filament. This cooperative effect is not captured by classical models based on linear partial differential equations. Sam Walcott’s contribution is a new model which includes these cooperative effects and is an exciting step towards a complete multi-scale description of muscle contraction. Travis Stewart and his colleagues from the research group of Josh Baker describe an experimental study which shows that the details of actin–myosin interactions affect muscle shortening speeds not only by defining the detachment rates but also by influencing the attachment kinetics. In contrast to actomyosin’s cooperativity, kinesins often hinder each other except under high loads, and the detailed mechanism behind this load-dependent cooperativity is unclear. Uppulury et al. used a stochastic model to identify biochemical and structural parameters affecting this negative cooperativity. This contribution continues the advances made by Anatoly Kolomeisky’s and Michael Diehl’s groups in the understanding of kinesin-driven intracellular transport. The behavior of two elastically coupled motors is studied theoretically by Florian Berger et al., who derived explicit results to describe the general dynamics of pairs of biomolecular motors. These results enable the prediction of distinct behaviors of different motor species and build upon prior work by Reinhard Lipowsky and Stefan Klumpp. A second focus of this special issue is kinesin’s counterpart in microtubule-associated transport, the dynein motor. Recently solved crystal structures of dynein facilitate further analysis of dynein activities. Dynein belongs to the AAA+ protein superfamily, forming a ring-shaped structure with multiple catalytic sites. Wonmuk Hwang and Matthew Lang used a coarse-grained model to study dynein as well as ClpX and HslU, two ATP-dependent peptide translocases. They found that nucleotide-free and nucleotide occupied subunits work together to generate force while enabling a variety of motor designs. Nandini Shekar and Jun Wu review the accumulated knowledge and its Address correspondence to Henry Hess, Department of Biomedical Engineering, Columbia University, New York, NY, USA. Electronic mail: hh2374@columbia.edu, jcliao@columbia.edu Cellular and Molecular Bioengineering, Vol. 6, No. 1, March 2013 ( 2012) pp. 1–2 DOI: 10.1007/s12195-012-0268-1

Intracellular transport is a fundamental biological process during which cellular materials are driven by enzymatic molecules called motor proteins. Recent optical trapping experiments and theoretical analysis have uncovered many features of cargo transport by multiple kinesin motor protein molecules under applied loads. These studies suggest that kinesins cooperate negatively under typical transport conditions, although some productive cooperation could be achieved under higher applied loads. However, the microscopic origins of this complex behavior are still not well understood. Using a discrete-state stochastic approach we analyze factors that affect the cooperativity among kinesin motors during cargo transport. Kinesin cooperation is shown to be largely unaffected by the structural and mechanical parameters of a multiple motor complex connected to a cargo, but much more sensitive to biochemical parameters affecting motor-filament affinities. While such behavior suggests the net negative cooperative responses of kinesins will persist across a relatively wide range of cargo types, it is also shown that the rates with which cargo velocities relax in time upon force perturbations are influenced by structural factors that affect the free energies of and load distributions within a multiple kinesin complex. The implications of these later results on transport phenomena where loads change temporally, as in the case of bidirectional transport, are discussed.

Durable osseointegration of metallic bone implants requires that progenitor cells attach, proliferate and differentiate on the implant surface. Previously, we demonstrated superior biocompatibility of human mesenchymal stromal cells (MSCs) cultivated on ultrasmooth tantalum (Ta) as compared to titanium (Ti) surface. The aim of this study was to extend the previous investigation of biocompatibility by monitoring temporal gene expression of MSCs on topographically comparable smooth Ta and Ti surfaces using whole-genome gene expression analysis. Total RNA samples from telomerase-immortalized human MSCs cultivated on plain sputter-coated surfaces of Ta or Ti for 1, 2, 4, and 8 days were hybridized to n = 16 U133 Plus 2.0 arrays (Affymetrix®). Functional annotation, cluster and pathway analyses were performed. The vast majority of genes were differentially regulated after 4 days of cultivation and genes upregulated by MSCs exposed to Ta and Ti were predominantly related to the processes of differentiation and transcription, respectively. Functional annotation analysis of the 1,000 temporally most significantly regulated genes suggests earlier cellular differentiation on Ta compared to Ti surface. Key genes related to osteogenesis and cell adhesion were upregulated by MSCs exposed to Ta. We further identified differentially regulated candidate transcription factors, e.g., NRF2, EGR1, IRF-1, IRF-8, NF-Y, and p53 as well as relevant signaling pathways, e.g., p53 and mTOR, indicating e.g., differences in the Ta- and Ti-induced oxidative stress reactions at the cell/biomaterial interface. These findings suggest that Ta is a promising material for bone implants.

In January of 2011, the Biomedical Engineering Society (BMES) and the Society for Physical Regulation in Biology and Medicine (SPRBM) held its inaugural Cellular and Molecular Bioengineering (CMBE) conference. The CMBE conference assembled worldwide leaders in the field of CMBE and held a very successful Round Table discussion among leaders. One of the action items was to collectively construct a white paper regarding the future of CMBE. Thus, the goal of this report is to emphasize the impact of CMBE as an emerging field, identify critical gaps in research that may be answered by the expertise of CMBE, and provide perspectives on enabling CMBE to address challenges in improving human health. Our goal is to provide constructive guidelines in shaping the future of CMBE.