Cytokines play a vital role in coordinating immune and inflammatory responses. Unlike growth factor receptors with a tyrosine kinase, cytokine receptors have no intrinsic tyrosine kinase activity. Based on their structure, cytokine receptors are classified into several groups. High affinity receptors for IL-2, IL-3, IL-5, IL-6, and GM-CSF are composed of at least two distinct subunits, alpha and beta. The alpha subunits are primary cytokine binding proteins, and the beta subunits are required for formation of high affinity binding sites as well as for signal transduction. The GM-CSF, IL-3, and IL-5 receptors appear to share the same beta subunit in human, and therefore cross-talk among these cytokines may occur at the receptor level. High affinity receptors presumably are linked to various signal transduction pathways that lead to different cytokine functions. Differential expression of the cytokine receptors as well as reorganization of intracellular signalling pathways are critical for development of hemopoietic cells.

BSTRACTAlthough extracellular matrix (ECM) stiffness is an important aspect of the extracellular microenvironment and is known to direct the lineage specification of stem cells and affect cancer progression, the molecular mechanisms that sense ECM stiffness have not yet been elucidated. In this study, we show that the proline-rich linker (PRL) region of vinculin and the PRL-region-binding protein vinexin are involved in sensing the stiffness of ECM substrates. A rigid substrate increases the level of cytoskeleton-associated vinculin, and the fraction of vinculin stably localizing at focal adhesions (FAs) is larger on rigid ECM than on soft ECM. Mutations in the PRL region or the depletion of vinexin expression impair these responses to ECM stiffness. Furthermore, vinexin depletion impairs the stiffness-dependent regulation of cell migration. These results suggest that the interaction of the PRL region of vinculin with vinexin a plays a crucial role in sensing ECM stiffness and in mechanotransduction.

No abstract

Cells sense the rigidity of their substrate; however, little is known about the physical variables that determine their response to this rigidity. Here, we report traction stress measurements carried out using fibroblasts on polyacrylamide gels with Young's moduli ranging from 6 to 110 kPa. We prepared the substrates by employing a modified method that involves N-acryloyl-6-aminocaproic acid (ACA). ACA allows for covalent binding between proteins and elastomers and thus introduces a more stable immobilization of collagen onto the substrate when compared to the conventional method of using sulfo-succinimidyl-6-(4-azido-2-nitrophenyl-amino) hexanoate (sulfo-SANPAH). Cells remove extracellular matrix proteins off the surface of gels coated using sulfo-SANPAH, which corresponds to lower values of traction stress and substrate deformation compared to gels coated using ACA. On soft ACA gels (Young's modulus <20 kPa), cell-exerted substrate deformation remains constant, independent of the substrate Young's modulus. In contrast, on stiff substrates (Young's modulus >20 kPa), traction stress plateaus at a limiting value and the substrate deformation decreases with increasing substrate rigidity. Sustained substrate strain on soft substrates and sustained traction stress on stiff substrates suggest these may be factors governing cellular responses to substrate rigidity.

Kinetics of volume phase transition in poly(N-isopropylacrylamide) (NIPA) gels jumped from a low-temperature swollen phase to a high-temperature shrunken phase was studied as functions of NIPA monomer and crosslinker concentrations. We found for the first time a clear kinematical boundary at which the shrinking relaxation time of gels changes discontinuously by 102–104 times, and that the profile of the boundary correlates with the sol-gel transition line and the contour line of turbidity of gels. A “morphological” boundary which characterizes the emergence of the bubble formation on gel surface was also determined. The theoretical calculation of the phase diagram on the basis of the mean field theory shows qualitatively that the shrinking speed of gels could be connected with the depth of the thermodynamic region of the spinodal instability (K+4μ/3=0) into which they are transferred where K and μ are the bulk and the shear moduli, respectively. A mechanism of discontinuous change of the shrinking speed is discussed in connection with the thermodynamic properties as well as the inhomogeneity of network structure.

No abstract

1Physical properties of the extracellular matrix (ECM) can control cellular phenotypes 2 via mechanotransduction, which is the process of translation of mechanical stresses into 3 biochemical signals. While current research is clarifying the relationship between 4 mechanotransduction and cytoskeleton or adhesion complexes, the contribution of transcription 5 factors to mechanotransduction is not well understood. The results of this study revealed that the 6 transcription factor NF-B, a major regulator for immunoreaction and cancer progression, is 7 responsive to substrate stiffness. NF-B activation was temporarily induced in H1299 lung 8 adenocarcinoma cells grown on a stiff substrate but not in cells grown on a soft substrate. 9 Although the activation of NF-B was independent of the activity of integrin β1, an 10 ECM-binding protein, the activation was dependent on actomyosin contractions induced by 11 phosphorylation of myosin regulatory light chain (MRLC). Additionally, the inhibition of 12 MRLC phosphorylation by Rho kinase inhibitor Y27632 reduced the activity of NF-B. We also 13 observed substrate-specific morphology of the cells, with cells grown on the soft substrate 14 appearing more rounded and cells grown on the stiff substrate appearing more spread out. 15Inhibiting NF-B activation caused a reversal of these morphologies on both substrates. These 16 results suggest that substrate stiffness regulates NF-B activity via actomyosin contractions, 17 resulting in morphological changes.

Recent studies reveal that the mechanical environment influences the behavior and function of various types of cells, including stem cells. However, signaling pathways involved in the mechanical regulation of stem cell properties remain largely unknown. Using polyacrylamide gels with varying Young's moduli as substrates, we demonstrate that mouse embryonic stem cells (mESCs) are induced to differentiate on substrates with defined elasticity, involving the Src-ShcA-MAP kinase pathway. While the dual inhibition of mitogen-activated protein (MAP) kinase and glycogen synthase kinase 3 (GSK3), termed ''2i,'' was reported to sustain the pluripotency of mESCs, we find it to be substrate elasticity dependent. In contrast, Src inhibition in addition to 2i allows mESCs to retain their pluripotency independent of substrate elasticity. The alternative dual inhibition of Src and GSK3 (''alternative 2i'') retains the pluripotency and self-renewal of mESCs in vitro and is instrumental in efficiently deriving mESCs from preimplantation mouse embryos. In addition, the transplantation of mESCs, maintained under the alternative 2i condition, to immunodeficient mice leads to the formation of teratomas that include differentiation into three germ layers. Furthermore, mESCs established with alternative 2i contributed to chimeric mice production and transmitted to the germline. These results reveal a role for Src-ShcA-MAP kinase signaling in the mechanical regulation of mESC properties and indicate that alternative 2i is a versatile tool for the maintenance of mESCs in serumfree conditions as well as for the derivation of mESCs.