Variations in plume activity reveal the dynamics of water-filled faults on Enceladus.

After discovering a jet activity near the south pole of Saturn's moon Enceladus, the Cassini mission demonstrated the existence of a subsurface water ocean with a unique sampling opportunity through flybys. Diurnal variations in the observed brightness of the plume suggest a tidal control, although the existence and timing of two activity maxima seem to contradict stress analysis predictions. Here, we re-interpret the observed plume variability by combining a 3D global model of tidal deformation of the fractured ice shell with a 1D local model of transport processes within south-polar faults. Our model successfully predicts the observed plume's temporal variability by combining two independent vapour transport mechanisms: slip-controlled jet flow and normal-stress-controlled ambient flow. Moreover, it provides a possible explanation for the differences between the vapour and solid emission rates during the diurnal cycle and the observed fractionation of the various icy particle families. Our model prediction could be tested by future JWST observations targeted when Enceladus is at different positions on its orbit and could be used to determine the optimal strategy for plume material sampling by future space missions.

Mathematical Models in the Description of Pregnane X Receptor (PXR)-Regulated Cytochrome P450 Enzyme Induction.

The pregnane X receptor (PXR) is a drug/xenobiotic-activated transcription factor of crucial importance for major cytochrome P450 xenobiotic-metabolizing enzymes (CYP) expression and regulation in the liver and the intestine. One of the major target genes regulated by PXR is the cytochrome P450 enzyme (CYP3A4), which is the most important human drug-metabolizing enzyme. In addition, PXR is supposed to be involved both in basal and/or inducible expression of many other CYPs, such as CYP2B6, CYP2C8, 2C9 and 2C19, CYP3A5, CYP3A7, and CYP2A6. Interestingly, the dynamics of PXR-mediated target genes regulation has not been systematically studied and we have only a few mechanistic mathematical and biologically based models describing gene expression dynamics after PXR activation in cellular models. Furthermore, few indirect mathematical PKPD models for prediction of CYP3A metabolic activity in vivo have been built based on compartmental models with respect to drug⁻drug interactions or hormonal crosstalk. Importantly, several negative feedback loops have been described in PXR regulation. Although current mathematical models propose these adaptive mechanisms, a comprehensive mathematical model based on sufficient experimental data is still missing. In the current review, we summarize and compare these models and address some issues that should be considered for the improvement of PXR-mediated gene regulation modelling as well as for our better understanding of the quantitative and spatial dynamics of CYPs expression.

Allosteric antibody inhibition of human hepsin protease.

Hepsin is a type II transmembrane serine protease that is expressed in several human tissues. Overexpression of hepsin has been found to correlate with tumour progression and metastasis, which is so far best studied for prostate cancer, where more than 90% of such tumours show this characteristic. To enable improved future patient treatment, we have developed a monoclonal humanized antibody that selectively inhibits human hepsin and does not inhibit other related proteases. We found that our antibody, hH35, potently inhibits hepsin enzymatic activity at nanomolar concentrations. Kinetic characterization revealed non-linear, slow, tight-binding inhibition. This correlates with the crystal structure we obtained for the human hepsin-hH35 antibody Fab fragment complex, which showed that the antibody binds hepsin around α3-helix, located far from the active centre. The unique allosteric mode of inhibition of hH35 is distinct from the recently described HGFA (hepatocyte growth factor activator) allosteric antibody inhibition. We further explain how a small change in the antibody design induces dramatic structural rearrangements in the hepsin antigen upon binding, leading to complete enzyme inactivation.

Comparative erythropoietin receptor binding kinetics of C.E.R.A. and epoetin-beta determined by surface plasmon resonance and competition binding assay.

C.E.R.A., a continuous erythropoietin (EPO) receptor activator, has been developed to provide stable maintenance of hemoglobin levels at once-monthly dosing intervals and smooth and steady anemia correction. The comparative EPO receptor binding properties of C.E.R.A. and epoetin-beta were assessed by surface plasmon resonance using soluble recombinant EPO receptors and by competition binding on cultured UT-7 cells. Calculated equilibrium dissociation constants (surface plasmon resonance assay) for C.E.R.A. and epoetin-beta were 140 and 2.9 nmol/l, respectively. Respective IC(50) values (competition binding assay) were 200 and 1.5 nmol/l. Compared with epoetin-beta, C.E.R.A. has approximately 50- to 100-fold lower affinity for EPO receptor binding sites. Analysis of the equilibrium binding curves indicates that the difference in affinity is mainly due to slower association. The different receptor binding properties of C.E.R.A. may enable continuous stimulation of erythropoiesis and, combined with a long half-life and slow systemic clearance, permit administration at extended intervals.

Structural basis of the adaptive molecular recognition by MMP9.

Matrix metalloproteinase (MMPs) are critical for the degradation of extracellular matrix components and, therefore, need to be regulated tightly. Almost all MMPs share a homologous C-terminal haemopexin-like domain (PEX). Besides its role in macromolecular substrate processing, the PEX domains appear to play a major role in regulating MMP activation, localisation and inhibition. One intriguing property of MMP9 is its competence to bind different proteins, involved in these regulatory processes, with high affinity at an overlapping recognition site on its PEX domain. With the crystal structure of the PEX9 dimer, we present the first example of how PEX domains accomplish these diverse roles. Blade IV of PEX9 mediates the non-covalent and predominantly hydrophobic dimerisation contact. Large shifts of blade III and, in particular, blade IV, accompany the dimerisation, resulting in a remarkably asymmetric homodimeric structure. The asymmetry provides a novel mechanism of adaptive protein recognition, where different proteins (PEX9, PEX1, and TIMP1) can bind with high affinity to PEX9 at an overlapping site. Finally, the structure illustrates how the dimerisation generates new properties on both a physico-chemical and functional level.