Anisotropic stiffness and tensional homeostasis induce a natural anisotropy of volumetric growth and remodeling in soft biological tissues

Biomechanics and Modeling in Mechanobiology - Growth in soft biological tissues in general results in anisotropic changes of the tissue geometry. It remains a key challenge in biomechanics to...     

Cellular mechanosensitivity to substrate stiffness decreases with increasing dissimilarity to cell stiffness

Computational modelling has received increasing attention to investigate multi-scale coupled problems in micro-heterogeneous biological structures such as cells. In the current study, we investigated for a single cell the effects of (1) different cell-substrate attachment (2) and different substrate modulus \(\textit{E}_\mathrm{s}\) on intracellular deformations. A fibroblast was geometrically reconstructed from confocal micrographs. Finite element models of the cell on a planar substrate were developed. Intracellular deformations due to substrate stretch of \(\lambda =1.1\), were assessed for: (1) cell-substrate attachment implemented as full basal contact (FC) and 124 focal adhesions (FA), respectively, and \(\textit{E}_\mathrm{s}\,=\,\)140 KPa and (2) \(\textit{E}_\mathrm{s}\,=\,10\), 140, 1000, and 10,000 KPa, respectively, and FA attachment. The largest strains in cytosol, nucleus and cell membrane were higher for FC (1.35\(\text {e}^{-2}\), 0.235\(\text {e}^{-2}\) and 0.6\(\text {e}^{-2}\)) than for FA attachment (0.0952\(\text {e}^{-2}\), 0.0472\(\text {e}^{-2}\) and 0.05\(\text {e}^{-2}\)). For increasing \(\textit{E}_\mathrm{s}\), the largest maximum principal strain was 4.4\(\text {e}^{-4}\), 5\(\text {e}^{-4}\), 5.3\(\text {e}^{-4}\) and 5.3\(\text {e}^{-4}\) in the membrane, 9.5\(\text {e}^{-4}\), 1.1\(\text {e}^{-4}\), 1.2\(\text {e}^{-3}\) and 1.2\(\text {e}^{-3}\) in the cytosol, and 4.5\(\text {e}^{-4}\), 5.3\(\text {e}^{-4}\), 5.7\(\text {e}^{-4}\) and 5.7\(\text {e}^{-4}\) in the nucleus. The results show (1) the importance of representing FA in cell models and (2) higher cellular mechanical sensitivity for substrate stiffness changes in the range of cell stiffness. The latter indicates that matching substrate stiffness to cell stiffness, and moderate variation of the former is very effective for controlled variation of cell deformation. The developed methodology is useful for parametric studies on cellular mechanics to obtain quantitative data of subcellular strains and stresses that cannot easily be measured experimentally.     

Cellular stress response cross talk maintains protein and energy homeostasis

Maintenance of cellular homeostasis depends upon several pathways that allow a cell to respond and adapt to both environmental stress and changes in metabolic status. New work in this issue of The EMBO Journal reveals a mechanism of cross talk between heat shock factor 1 (HSF1), the primary regulator of the proteotoxic stress response, and AMP‐activated protein kinase (AMPK), the primary sensor in the metabolic stress response.     

Glomerular filtration and podocyte tensional homeostasis: importance of the minor type IV collagen network

Biomechanics and Modeling in Mechanobiology - The minor type IV collagen chain, which is a significant component of the glomerular basement membrane in healthy individuals, is known to assemble...     

miR-181a and inflammation: miRNA homeostasis response to inflammatory stimuli in vivo

Inflammatory stimuli are usually associated with homeostatic responses, which have an important function in protecting the body from excessive inflammatory damage. Previous studies reported the anti-inflammatory effect of miR-181a. The current study utilized two animal models of inflammation, induced by either lipopolysaccharides (LPS) or streptozotocin. We demonstrated that inflammatory stimuli significantly increase miR-181a expression, concurrently with inflammatory factors. In addition, the knock down of toll-like receptor 4 (TLR-4) by small interfering RNA in LPS-induced Raw264.7 cells significantly reduces the expression of both miR-181a and inflammatory factors. Furthermore, patients with inflammatory response show increased expression of miR-181a, which is strongly correlated with the expression of interleukin (IL)-1β, IL-6, and tumor necrosis factor alpha. These data indicate that the up-regulation of miR-181a may be associated with homeostatic response to inflammatory stimuli by TLR-4 pathway activation. Therefore, miR-181a may serve as a novel marker for inflammatory response.     

Cellular and mitochondrial iron homeostasis in vertebrates

Iron plays an essential role in cellular metabolism and biological processes. However, due to its intrinsic redox activity, free iron is a potentially toxic molecule in cellular biochemistry. Thus, organisms have developed sophisticated ways to import, sequester, and utilize iron. The transferrin cycle is a well-studied iron uptake pathway that is important for most vertebrate cells. Circulating iron can also be imported into cells by mechanisms that are independent of transferrin. Once imported into erythroid cells, iron is predominantly consumed by the mitochondria for the biosynthesis of heme and iron sulfur clusters. This review focuses on canonical transferrin-mediated and the newly discovered, non-transferrin mediated iron uptake pathways, as well as, mitochondrial iron homeostasis in higher eukaryotes. This article is part of a Special Issue entitled: Cell Biology of Metals.     

Smooth muscle stiffness variation due to external longitudinal oscillations

This research focuses on an in vitro investigation of the stiffness changes of contracted airway smooth muscles (ASM) subjected to external longitudinal oscillations. ASM tissues were dissected from excised pig tracheas and stimulated by a chemical stimulus (acetylcholine, 10(-3) M) to produce maximum contractions. The tissues were then systematically excited with external oscillations. Various frequencies, amplitudes and durations were used in the experiments to determine stiffness changes in response to these variations. Force changes were recorded to reflect the muscle stiffness changes. Two stiffness definitions were used to quantify the results, dynamic stiffness to reflect variations during oscillation and static stiffness to reflect the net effect of oscillation. Under isometric contractions, these two stiffnesses were determined before, during and after oscillations. Incorporating an empirical stiffness equation, a two-dimensional finite element model (FEM) was developed to generalize the tissue responses to oscillation. The main outcomes from this work are: the dynamic stiffness has the tendency to decrease as the frequency and/or amplitude of external oscillation increases; the static stiffness has the tendency of decreasing with an increase in the frequency and/or amplitude of excitation until it reaches almost a constant value for frequencies at and above 25 Hz. The difference in the behavior of the dynamic and static stiffness changes may be attributed to the effect of elasticity and mass inertia that are involved in the dynamic motion. The findings of this research are in agreement with the hypothesis that oscillations exert a direct action on the contractile processes by causing an increased rate of actin-myosin detachments.     

Cellular stress response: From homeostatic to allostatic perspective

For the vertebrates the more flexible allostasis frameworks tend to overlap the homeostatic model in the study of stress-induced physiological changes and whole-body adaptation. I hypothesise the possibility to extend this paradigm to stress-induced rearrangements of cellular networks.     

Cellular response to endoplasmic reticulum stress: a matter of life or death

The proper functioning of the endoplasmic reticulum (ER) is critical for numerous aspects of cell physiology. Accordingly, all eukaryotes react rapidly to ER dysfunction through a set of adaptive pathways known collectively as the ER stress response (ESR). Normally, this suite of responses succeeds in restoring ER homeostasis. However, in metazoans, persistent or intense ER stress can also trigger programmed cell death, or apoptosis. ER stress and the apoptotic program coupled to it have been implicated in many important pathologies but the regulation and execution of ER stress-induced apoptosis in mammals remain incompletely understood. Here, we review what is known about the ESR in both yeast and mammals, and highlight recent findings on the mechanism and pathophysiological importance of ER stress-induced apoptosis.     

Cellular response to low adhesion nanotopographies

This review focuses on how cells respond to low-adhesion nanotopographies. In order to do this, fabrication techniques, how cells may locate nanofeatures through the use of filopodia and possible mechanotransductive mechanisms are discussed. From this, examples of low-adhesion topographies and sizes and arrangements that may lead to low-adhesion are discussed. Finally, it is hypothesized as to how specifically low-adhesion materials may fit into the outlined mechanotransductive mechanisms.     

IgA response to symbiotic bacteria as a mediator of gut homeostasis

Colonization of germ-free mice with a normal gut microbiota elicits bacteria-specific IgA antibody responses. The effects of these responses on microbial and host biology remain poorly defined. Therefore, we developed a gnotobiotic mouse model where the microbiota is reduced to one bacterial species, and the antibody repertoire to a single, monoclonal IgA against the bacterium's capsular polysaccharide. Bacteroides thetaiotaomicron was introduced into germ-free wild-type, immunodeficient Rag1(-/-), or Rag1(-/-) mice harboring IgA-producing hybridoma cells. Without IgA, B. thetaiotaomicron elicits a more robust innate immune response and reacts to this response by inducing genes that metabolize host oxidative products. IgA reduces intestinal proinflammatory signaling and bacterial epitope expression, thereby balancing suppression of the oxidative burst with the antibody's negative impact on bacterial fitness. These results underscore the adaptive immune system's critical role in establishing a sustainable host-microbial relationship. Immunoselection of bacterial epitope expression may contribute to the remarkable strain-level diversity in this ecosystem.     

Regulation of single sodium channels in renal tissue: a role in sodium homeostasis

The kidney is responsible for the maintenance of an organism's body solute and water balance (i.e., Na+ homeostasis). The distal nephron and the cortical collecting duct (CCD) (an example of a tight epithelium) are important sites of regulatory control over the rate of Na+ reabsorption. The Na+ channel, a specialized protein located in the apical membrane of CCD cells, is the specific site of transepithelial Na+ movement. Na+ entry into the cell across the apical membrane occurs by passive diffusion of Na+ down an electrochemical gradient. We have used the patch-voltage clamp method to examine single-channel conductance events of the amiloride-sensitive apical Na+ channel in A6 cells, a model of CCD. Two types of Na+ channel were identified. One type was characterized by low selectivity (Na+ to K+) and high conductance, the other by high selectivity and low conductance. The type and frequency of channel observed depended on the transporting state of the epithelium. In a tissue with poor transport rates, the low-selectivity type of channel was prevalent (the other type of channel was present, but in a very low density). Therefore, the poorly transporting tissue had an overall low apical Na+ conductance. In a tissue with high transport rates, the highly selective channel appeared to be predominant. In this case the net result was a highly Na+ conductive apical membrane.     

Cellular calcium mobilization in response to phosphoinositide delivery

Intracellular calcium [Ca(2+)](i) is mobilized in many cell types in response to activation of phosphoinositide (PIP(n)) signaling pathways involving PtdIns(4,5)P(2) or PtdIns(3,4,5)P(3). To further explore the relationship between increases in intracellular PIP(n) concentrations and mobilization of [Ca(2+)](i), each of the seven phosphorylated phosphoinositides (PIP(n)s) were delivered into cells and the metabolism and physiological effects of the exogenously administered PIP(n)s were determined. The efficient cellular delivery of fluorophore-tagged and native PIP(n)s was accomplished using histone protein, neomycin, and dendrimeric polyamines. PtdIns(4,5)P(2) fluorophore-tagged analogs with short- and long-acyl chains were substrates for cellular enzymes in vitro and for phospholipases in stimulated fibroblasts. PtdIns(4)P, PtdIns(3,4)P(2) and PtdIns(4,5)P(2), each induced calcium mobilization rapidly after exogenous addition to fibroblasts. PtdIns(3,4,5)P(3) induced a significant, but smaller increase in intracellular calcium. These observations suggest that PIP(n)s other than PtdIns(4,5)P(2) or PtdIns(3,4,5)P(3) may have direct roles in signaling involving [Ca(2+)](i).     

Response of a single cell to an external electric field.

The response of a cell to an external electric field is investigated using dimensional analysis and singular perturbation. The results demonstrate that the response of a cell is a two-stage process consisting of the initial polarization that proceeds with the cellular time constant (< 1 microseconds), and of the actual change of physiological state that proceeds with the membrane time constant (several milliseconds). The second stage is governed by an ordinary differential equation similar to that of a space-clamped membrane patch but formulated in terms of intracellular rather than transmembrane potential. Therefore, it is meaningful to analyze the physiological state and the dynamics of a cell as a whole instead of the physiological states and the dynamics of the underlying membrane patches. This theoretical result is illustrated with an example of an excitation of a cylindrical cell by a transverse electric field.     

Contractile equilibration of single cells to step changes in extracellular stiffness

Extracellular stiffness has been shown to alter long timescale cell behaviors such as growth and differentiation, but the cellular response to changes in stiffness on short timescales is poorly understood. By studying the contractile response of cells to dynamic stiffness conditions using an atomic force microscope, we observe a seconds-timescale response to a step change in extracellular stiffness. Specifically, we observe acceleration in contraction velocity (μm/min) and force rate (nN/min) upon a step decrease in stiffness and deceleration upon a step increase in stiffness. Interestingly, this seconds-timescale response to a change in extracellular stiffness is not altered by inhibiting focal adhesion signaling or stretch-activated ion channels and is independent of cell height and contraction force. Rather, the response timescale is altered only by disrupting cytoskeletal mechanics and is well described by a simple mechanical model of a constant velocity actuator pulling against an internal cellular viscoelastic network. Consistent with the predictions of this model, we find that an osmotically expanding hydrogel responds to step changes in extracellular stiffness in a similar manner to cells. We therefore propose that an initial event in stiffness sensing is establishment of a mechanical equilibrium that balances contraction of the viscoelastic cytoskeleton with deformation of the extracellular matrix.