Unit step and impulse function equations to simplify the solution of engineering problems

A unit step equation is proposed that when differentiated by elementary calculus yields the impulse function and when the resulting impulse function equation is integrated by elementary calculus yields the proposed unit step equation. Using these two equations, a two stage methodology is presented for the simplification of the solution of problems involving the impulse function

Scanning and non-scanning surface plasmon microscopy to observe cell adhesion sites

We observe adhesion sites of a cell on a substrate with high resolution. Since this observation requires interfacial measurements between the cell and the substrate, we employ scanning localized surface plasmon microscopy. We experimentally show that focal adhesion sites of a mouse muscle cell can be observed without fluorescent labeling. We also show that a non-scanning surface plasmon microscope combined with the scanning localized surface plasmon microscope contributes to observing an entire cell adhesion site and identify regions of interest

Dynamics of Sleep-Wake Transitions During Sleep

We study the dynamics of the awakening during the night for healthy subjects and find that the wake and the sleep periods exhibit completely different behavior: the durations of wake periods are characterized by a scale-free power-law distribution, while the durations of sleep periods have an exponential distribution with a characteristic time scale. We find that the characteristic time scale of sleep periods changes throughout the night. In contrast, there is no measurable variation in the power-law behavior for the durations of wake periods. We develop a stochastic model which agrees with the data and suggests that the difference in the dynamics of sleep and wake states arises from the constraints on the number of microstates in the sleep-wake system.Comment: Final form with some small corrections. To be published in Europhysics Letters, vol. 57, issue no. 5, 1 March 2002, pp. 625-63

Mapping the energy landscape of biomolecules using single molecule force correlation spectroscopy (FCS): Theory and applications

In the current AFM experiments the distribution of unfolding times, P(t), is measured by applying a constant stretching force f_s from which the apparent unfolding rate is obtained. To describe the complexity of the underlying energy landscape requires additional probes that can incorporate the dynamics of tension propagation and relaxation of the polypeptide chain upon force quench. We introduce a theory of force correlation spectroscopy (FCS) to map the parameters of the energy landscape of proteins. In the FCS the joint distribution, P(T,t) of folding and unfolding times is constructed by repeated application of cycles of stretching at constant fs, separated by release periods T during which the force is quenched to f_q<f_s. During the release period, the protein can collapse to a manifold of compact states or refold. We show that P(T,t) can be used to resolve the kinetics of unfolding as well as formation of native contacts and to extract the parameters of the energy landscape using chain extension as the reaction coordinate and P(T,t). We illustrate the utility of the proposed formalism by analyzing simulations of unfolding-refolding trajectories of a coarse-grained protein S1 with beta-sheet architecture for several values of f_s, T and f_q=0. The simulations of stretch-relax trajectories are used to map many of the parameters that characterize the energy landscape of S1.Comment: 23 pages, 9 figures; accepted to Biophysical Journa

Three classes of inhibitory amino acid terminals in the cochlear nucleus of the guinea pig

Electron microscopic postembedding immunocytochemistry was used to analyze and assess the synaptic distribution of glycine (GLY) and γ-amino butyric acid (GABA) immunoreactivities in the guinea pig cochlear nucleus (CN). Three classes of endings were identified containing immunolabeling for glycine, GABA, or both glycine and GABA (GLY/GABA). All classes were similar in that the terminals contained pleomorphic vesicles and formed symmetric synapses with their postsynaptic targets. A fourth class, which labeled with neither antibody, contained round vesicles and formed asymmetric synapses. Glycine endings predominated in the ventral CN, while GLY/GABA endings were prevalent in the dorsal CN. GABA endings were the least common and smallest in size. Glycine, GLY/GABA, and GABA endings differed in their proportions and patterns of distribution on the different classes of projection neurons in the CN, including spherical bushy, type I stellate/multipolar, and octopus cells in the ventral CN and fusiform cells in the dorsal CN. The vast majority of anatomically-defined, putative inhibitory endings contain GLY, GABA, or both, suggesting that most of the inhibition in the cochlear nucleus is mediated by these three cytochemically and, probably, functionally distinct classes of endings. The results of this study also suggest that a large proportion of the GABA available for inhibition in the CN coexists in terminals with glycine. © 1996 Wiley-Liss, Inc.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/50069/1/2_ftp.pd

Elastic interactions of active cells with soft materials

Anchorage-dependent cells collect information on the mechanical properties of the environment through their contractile machineries and use this information to position and orient themselves. Since the probing process is anisotropic, cellular force patterns during active mechanosensing can be modelled as anisotropic force contraction dipoles. Their build-up depends on the mechanical properties of the environment, including elastic rigidity and prestrain. In a finite sized sample, it also depends on sample geometry and boundary conditions through image strain fields. We discuss the interactions of active cells with an elastic environment and compare it to the case of physical force dipoles. Despite marked differences, both cases can be described in the same theoretical framework. We exactly solve the elastic equations for anisotropic force contraction dipoles in different geometries (full space, halfspace and sphere) and with different boundary conditions. These results are then used to predict optimal position and orientation of mechanosensing cells in soft material.Comment: Revtex, 38 pages, 8 Postscript files included; revised version, accepted for publication in Phys. Rev.

Compartmentalisation and localisation of the translation initiation factor (eIF) 4F complex in normally growing fibroblasts

Previous observations of association of mRNAs and ribosomes with subcellular structures highlight the importance of localised translation. However, little is known regarding associations between eukaryotic translation initiation factors and cellular structures within the cytoplasm of normally growing cells. We have used detergent-based cellular fractionation coupled with immunofluorescence microscopy to investigate the subcellular localisation in NIH3T3 fibroblasts of the initiation factors involved in recruitment of mRNA for translation, focussing on eIF4E, the mRNA cap-binding protein, the scaffold protein eIF4GI and poly(A) binding protein (PABP). We find that these proteins exist mainly in a soluble cytosolic pool, with only a subfraction tightly associated with cellular structures. However, this "associated" fraction was enriched in active "eIF4F" complexes (eIF4E.eIF4G.eIF4A.PABP). Immunofluorescence analysis reveals both a diffuse and a perinuclear distribution of eIF4G, with the perinuclear staining pattern similar to that of the endoplasmic reticulum. eIF4E also shows both a diffuse staining pattern and a tighter perinuclear stain, partly coincident with vimentin intermediate filaments. All three proteins localise to the lamellipodia of migrating cells in close proximity to ribosomes, microtubules, microfilaments and focal adhesions, with eIF4G and eIF4E at the periphery showing a similar staining pattern to the focal adhesion protein vinculin

Strain driven fast osseointegration of implants

BACKGROUND: Although the bone's capability of dental implant osseointegration has clinically been utilised as early as in the Gallo-Roman population, the specific mechanisms for the emergence and maintenance of peri-implant bone under functional load have not been identified. Here we show that under immediate loading of specially designed dental implants with masticatory loads, osseointegration is rapidly achieved. METHODS: We examined the bone reaction around non- and immediately loaded dental implants inserted in the mandible of mature minipigs during the presently assumed time for osseointegration. We used threaded conical titanium implants containing a titanium2+ oxide surface, allowing direct bone contact after insertion. The external geometry was designed according to finite element analysis: the calculation showed that physiological amplitudes of strain (500–3,000 ustrain) generated through mastication were homogenously distributed in peri-implant bone. The strain-energy density (SED) rate under assessment of a 1 Hz loading cycle was 150 Jm-3 s-1, peak dislocations were lower then nm. RESULTS: Bone was in direct contact to the implant surface (bone/implant contact rate 90%) from day one of implant insertion, as quantified by undecalcified histological sections. This effect was substantiated by ultrastructural analysis of intimate osteoblast attachment and mature collagen mineralisation at the titanium surface. We detected no loss in the intimate bone/implant bond during the experimental period of either control or experimental animals, indicating that immediate load had no adverse effect on bone structure in peri-implant bone. CONCLUSION: In terms of clinical relevance, the load related bone reaction at the implant interface may in combination with substrate effects be responsible for an immediate osseointegration state

Distinct pathways in the over-expression of matrix metalloproteinases in human fibroblasts by relaxation of mechanical tension.

peer reviewedaudience: researcherThe aim of the work was to analyze, on a comparative basis, the signaling pathways operating in the regulation of a panel of matrix metalloproteinases (MMP) expressed by human dermal fibroblasts submitted to mechanical stress relaxation by cytochalasin D (CD) and in a retracting collagen gel (RCG). The mRNA steady-state level of MMPs was measured by a quantitative RT-PCR procedure using a synthetic RNA as internal standard. In monolayer, most MMPs were barely detected, except MMP-2. Disruption of the actin stress fibers by CD induced a moderate increase of MMP-2 mRNA and a much larger stimulation of MMP-3, -9, -13 and -14 mRNAs. In RCG, a significant up-regulation of these MMPs was also observed although to a lower extent than in CD-treated monolayers. Among the investigated MMPs, the MMP-8 and -11 were not reproducibly detected. MMP-2 was processed to its active form both by CD and in RCG. The CD-induced up-regulation of gene expression was largely repressed by blocking protein synthesis by cycloheximide for all the MMPs, by inhibiting the tyrosine-kinases of the src family by herbimycin A for all MMPs, except MMP-2, and by inhibiting the TPA-inducible PKC isoforms by bisindoyl maleimide for all MMPs, except MMP-14. The up-regulation induced by stress relaxation in RCG was protein synthesis-dependent for MMP-2 and MMP-13, tyrosine kinases-dependent for MMP-3 and MMP-13, as previously described for MMP-1. Inhibiting TPA-inducible PKC did not affect any MMP in RCG except MMP-13, which was strongly induced. The processing of MMP-2 was tyrosine kinases-dependent but PKC-independent. Inhibitors of the ERK1,2 and p38 MAP kinases pathways diversely affected the MMPs expression. Inhibiting the Rho-kinase activity by Y-27632 was inactive. These results point to the potent regulation operated by the status of the cytoskeleton on the cell phenotype, and to distinct regulatory pathways involved in the control of different MMPs expression