Wrinkling instabilities for biologically relevant fiber-reinforced composite materials with a case study of Neo-Hookean/Ogden–Gasser–Holzapfel bilayer

Biomechanics and Modeling in Mechanobiology - Wrinkling is a ubiquitous surface phenomenon in many biological tissues and is believed to play an important role in arterial health. As arteries are...     

Apolipoprotein E receptor 2 interactions with the N-methyl-D-aspartate receptor

In our previous studies we showed that apoE treatment of neurons activated ERK 1/2 signaling, and activation was blocked by treatment with inhibitors of the low density lipoprotein receptor family, the N-methyl-d-aspartate (NMDA) receptor antagonist MK 801, and calcium channel blockers. We hypothesized an interaction between the low density lipoprotein receptor family members and the NMDA receptor. In the present study, we confirmed through co-immunoprecipitation experiments an interaction between the apoE receptor, ApoEr2, and NMDAR1 through their extracellular domains. We also found that the PDZ1 domain of PSD95, a postsynaptic scaffolding protein, interacted with the C terminus of ApoEr2 via an alternatively spliced, intracellular exon. This interaction between ApoEr2 and PSD95 in neurons was modulated by NMDA receptor activation and an ApoEr2 ligand. We also found that the PDZ2 domain of PSD95 interacted with the NR2A and NR2B subunits of NMDA receptors. Full-length PSD95 increased cell surface levels of ApoEr2 and its cleavage, resulting in increases in secreted ApoEr2 and C-terminal fragments of ApoEr2. These studies suggest that ApoEr2 can form a multiprotein complex with NMDA receptor subunits and PSD95.     

Directed evolution of a new catalytic site in 2-keto-3-deoxy-6-phosphogluconate aldolase from Escherichia coli

BACKGROUND: Aldolases are carbon bond-forming enzymes that have long been identified as useful tools for the organic chemist. However, their utility is limited in part by their narrow substrate utilization. Site-directed mutagenesis of various enzymes to alter their specificity has been performed for many years, typically without the desired effect. More recently directed evolution has been employed to engineer new activities onto existing scaffoldings. This approach allows random mutation of the gene and then selects for fitness to purpose those proteins with the desired activity. To date such approaches have furnished novel activities through multiple mutations of residues involved in recognition; in no instance has a key catalytic residue been altered while activity is retained. RESULTS: We report a double mutant of E. coli 2-keto-3-deoxy-6-phosphogluconate aldolase with reduced but measurable enzyme activity and a synthetically useful substrate profile. The mutant was identified from directed-evolution experiments. Modification of substrate specificity is achieved by altering the position of the active site lysine from one beta strand to a neighboring strand rather than by modification of the substrate recognition site. The new enzyme is different to all other existing aldolases with respect to the location of its active site to secondary structure. The new enzyme still displays enantiofacial discrimination during aldol addition. We have determined the crystal structure of the wild-type enzyme (by multiple wavelength methods) to 2.17 A and the double mutant enzyme to 2.7 A resolution. CONCLUSIONS: These results suggest that the scope of directed evolution is substantially larger than previously envisioned in that it is possible to perturb the active site residues themselves as well as surrounding loops to alter specificity. The structure of the double mutant shows how catalytic competency is maintained despite spatial reorganization of the active site with respect to substrate.     

A bacterial selection for the directed evolution of pyruvate aldolases

A novel bacterial in vivo selection for pyruvate aldolase activity is described. Pyruvate kinase deficient cells, which lack the ability to biosynthetically generate pyruvate, require supplementation of exogenous pyruvate when grown on ribose. Supplementation with pyruvate concentrations as low as 50 microM rescues cell growth. A known substrate of the KDPG aldolases, 2-keto-4-hydroxy-4-(2'-pyridyl)butyrate (KHPB), also rescues cell growth, consistent with retroaldol cleavage by KDPG aldolase and rescue through pyruvate release. An initial round of selection against 2-keto-4-hydroxyoctonate (KHO), a nonsubstrate for wild-type aldolase, produced three mutants with intriguing alterations in protein sequence. This selection system allows rapid screening of mutant enzyme libraries and facilitates the discovery of enzymes with novel substrate specificities.     

Glycerol-induced membrane stiffening: the role of viscous fluid adlayers

Lipid interfaces, ranging from cell membranes to thin surfactant layers that stabilize lung alveoli, are integral to living systems. Such interfaces are often subjected to mechanical forces, and because of their membrane-like geometry, they can easily deform by bending into localized folds. In this work, we explore the role of small molecules (i.e., glycerol) on the mechanical stability of model lung surfactant monolayers. We demonstrate that the presence of glycerol increases local monolayer bending stiffness by orders of magnitude. Our x-ray and neutron reflectivity measurements indicate that water is preferentially depleted, or glycerol is preferentially enriched, at the lipid headgroup/solvent interface, and that this glycerol-enriched layer extends beneath the monolayer with an adsorption free energy of −2.5 to −4.6 kJ/mol. The dramatic change in membrane bending stiffness in the presence of the sugar adlayer is understood in terms of two models: 1), lipid antiplasticization by glycerol; and 2), a continuum mechanical model of the viscous adlayer.     

Dehydroepiandrosterone formation is independent of cytochrome P450 17α-hydroxylase/17, 20 lyase activity in the mouse brain

Cytochrome P450 17α-hydroxylase/17, 20 lyase (CYP17) is a microsomal enzyme reported to have two distinct catalytic activities, 17α-hydroxylase and 17, 20 lyase, that are essential for the biosynthesis of peripheral androgens such as dehydroepiandrosterone (DHEA). Paradoxically, DHEA is present and plays a role in learning and memory in the adult rodent brain, while CYP17 activity and protein are undetectable. To determine if CYP17 is required for DHEA formation and function in the adult rodent brain, we generated CYP17 chimeric mice that had reduced circulating testosterone levels. There were no detectable differences in cognitive spatial learning between CYP17 chimeric and wild-type mice. In addition, while CYP17 mRNA levels were reduced in CYP17 chimeric compared to wild-type mouse brain, the levels of brain DHEA levels were comparable. To determine if adult brain DHEA is formed by an alternative Fe²⁺-dependent pathway, brain microsomes were isolated from wild-type and CYP17 chimeric mice and treated with FeSO₄. Fe²⁺ caused comparable levels of DHEA production by both wild-type and CYP17 chimeric mouse brain microsomes; DHEA production was not reduced by a CYP17 inhibitor. Taken together these in vivo studies suggest that in the adult mouse brain DHEA is formed via a Fe²⁺-sensitive CYP17-independent pathway.     

Elasticity and geometry: a computational model of the Heineke–Mikulicz strictureplasty

Crohn’s disease is a challenging inflammatory process with a propensity for focal gastro-intestinal tract inflammation and stricture. Surgically, Crohn’s is often treated with resection. However, a subtype of diffuse disease with multiple strictures is treated by strictureplasty procedures in hope of avoiding short-gut syndrome. Prior work by Pocivavsek et al. defined the geometry of a Heineke–Mikulicz strictureplasty. Here, we bring this analysis one step closer to clinical and biological relevance by calculating the mechanical stresses and strains that the strictureplasty deformation generates on a model intestinal wall. The small bowel is simulated as a linearly elastic isotropic deformable cylindrical shell using finite element modeling. Data show a divergence in elastic response between the anti-mesenteric and mesenteric halves. The anti-mesenteric surface shows a bending dominated elastic response that correlates with the prior purely geometric analysis. However, the mesenteric half is not a neutral bystander during strictureplasty formation, as geometric arguments predict. Strong in-plane stretching strains develop in a rim around the image of the transverse closure, which may impact local perfusion and serve as sites of disease recurrence. Lastly, nearly all the deformation energy is stored in the central vertex stitch, placing this part at highest risk of dehiscence. This study enhances our understanding of mechanical response in complex nonlinear cylindrical geometries like the surgically manipulated intestinal tract. The developed framework serves as a platform for future addition of more complex clinically relevant parameters to our model, including real tissue properties, anisotropy, blood supply modeling, and patient deriver anatomic factors.     

Inhibition of c-Jun N-terminal kinase increases apoE expression in vitro and in vivo

Apolipoprotein E (apoE), a ligand for the low-density lipoprotein receptor family, has been implicated in modulating glial inflammatory responses and the risk of neurodegeneration associated with Alzheimer’s disease. Glial cells activated by lipopolysaccharide (LPS) have decreased apoE levels and we recently showed that apoE itself can modulate the inflammatory response by reducing c-Jun N-terminal kinase (JNK) activation. Reduced JNK phosphorylation is vital to overcome the LPS-induced decrease in apoE expression, suggesting that JNK inhibition may be an effective way to increase apoE protein and protract its anti-inflammatory properties. This study investigates the impact of JNK inhibition on apoE production using two JNK inhibitors. Our work in primary glia and in vivo in mice injected with JNK inhibitor demonstrates that inhibition of JNK may be an effective way to increase apoE expression.