Tensegrity structures are special architectures made by floating compressed struts kept together by a continuous system of tensioned cables. Their existence in a mechanically stable form is decided by the possibility of finding geometrical configurations such that pre-stressed tendons and bars can ensure self-equilibrium of the forces transmitted through the elastic network, the overall stiffness of which finally depends on both the rigidity of the compressed elements and the cables' pre-stress. The multiplicity of shapes that tensegrity structures can assume and their intrinsic capability to be deployable and assembled, so storing (and releasing) elastic energy, have motivated their success as paradigm -pioneeringly proposed three decades ago by the intuition of Donald E. Ingber-to explain some underlying mechanisms regulating dynamics of living cells. The interlaced structure of the cell cytoskeleton, constituted by actin microfilaments, intermediate filaments and microtubules which continuously change their spatial organization and pre-stresses through polymerization/depolymerization processes, seems in fact to steer migration, adhesion and arXiv:1904.04610v1 [physics.bio-ph] 9 Apr 2019 cell division by obeying the tensegrity construct. Even though rough calculations lead to estimate discrepancies of less than one order of magnitude when comparing axial stiffness of actin filaments (cables) and microtubules (struts) and recent works have shown bent microtubules among stretched filaments, no one has yet tried to remove the standard hypothesis of rigid struts in tensegrity structures when used to idealize the cell cytoskeleton mechanical response. With reference to the 30-element tensegrity cell paradigm, we thus introduce both compressibility and bendability of the struts and accordingly rewrite the theory to simultaneously take into account geometrical non-linearity (i.e. large deformations) and hyper-elasticity of both tendons and bars, so abandoning the classical linear stress-strain constitutive assumptions. By relaxing the hypothesis of rigidity of the struts, we demonstrate that some quantitative confirmations and many related extreme and somehow counterintuitive mechanical behaviors actually exploited by cells for storing/releasing energy, resisting to applied loads and deforming by modulating their overall elasticity and shape through pre-stress changes and instability-guided configurational switching, can be all theoretically found. It is felt that the proposed new soft-strut tensegrity model could pave the way for a wider use of engineering models in cell mechanobiology and in designing bio-inspired materials and soft robots.

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Acylpeptide hydrolase (APEH) is a ubiquitous cytosolic protease that plays an important role in the detoxification of oxidised proteins. In this work, to further explore the physiological role of this enzyme, two apeh cDNAs were isolated from the Chionodraco hamatus icefish, which lives in the highly oxygenated Antarctic marine environment. The encoded proteins (APEH-1Ch and APEH-2Ch) were characterised in comparison with the uniquely expressed isoform from the temperate fish Dicentrarchus labrax (APEH-1Dl) and the two APEHs from the red-blooded Antarctic fish Trematomus bernacchii (APEH-1Tb and APEH-2Tb). Homology modelling and kinetic characterisation of the APEH isoforms provided new insights into their structure/function properties. APEH-2 isoforms were the only ones capable of hydrolysing oxidised proteins, with APEH-2Ch being more efficient than APEH-2Tb at this specific function. Therefore, this ability of APEH-2 isoforms is the result of an evolutionary adaptation due to the pressure of a richly oxygenated environment. The lack of expression of APEH-2 in the tissues of the temperate fish used as the controls further supported this hypothesis. In addition, analysis of gene expression showed a significant discrepancy between the levels of transcripts and those of proteins, especially for apeh-2 genes, which suggests the presence of post-transcriptional regulation mechanisms, triggered by cold-induced oxidative stress, that produce high basal levels of APEH-2 mRNA as a reserve that is ready to use in case of the accumulation of oxidised proteins.

In the present work, the overall nonlinear elastic behavior of a 1D multi-modular structure incorporating possible imperfections at the discrete (micro-scale) level, is derived with respect to both tensile and compressive applied loads. The model is built up through the repetition of n units, each one comprising two rigid rods having equal lengths, linked by means of pointwise constraints capable to elastically limit motions in terms of relative translations (sliders) and rotations (hinges). The mechanical response of the structure is analyzed by varying the number n of the elemental moduli, as well as in the limit case of infinite number of infinitesimal constituents, in light of the theory of (first order) Structured Deformations (SDs), that interprets the deformation of any continuum body as the projection, at the macroscopic scale, of geometrical changes occurring at the level of its sub-macroscopic elements. In this way, a wide family of nonlinear elastic behaviors is generated by tuning internal microstructural parameters, the tensile buckling and the classical Euler's Elastica under compressive loads resulting as special cases in the so-called continuum limit -say when n → ∞. Finally, by plotting the results in terms of first Piola-Kirchhoff stress versus macroscopic stretch, it is for the first time demonstrated that such SDs-based 1D models can be helpfully used to generalize some standard hyperelastic behaviors by additionally taking into account instability phenomena and concealed defects.