Solute-excluded volumes near the Novikoff cell surface

Sequence-Specific Mucins for Glycocalyx Engineering

Few approaches exist for the stable and controllable synthesis of customized mucin glycoproteins for glycocalyx editing in eukaryotic cells. Taking advantage of custom gene synthesis and a biology-by-parts approach to cDNA construction, we build a library of swappable DNA bricks for mucin leader tags, membrane anchors, cytoplasmic motifs, and optical reporters, as well as codon-optimized native mucin repeats and newly designed domains for synthetic mucins. We construct a library of over 50 mucins, each with unique chemical, structural, and optical properties and describe how additional permutations could readily be constructed. We apply the library to explore sequence-specific effects on glycosylation for engineering of mucins. We find that the extension of the immature α-GalNAc Tn-antigen to Core 1 and Core 2 glycan structures depends on the underlying peptide backbone sequence. Glycosylation could also be influenced through recycling motifs on the mucin cytoplasmic tail. We expect that the mucin parts inventory presented here can be broadly applied for glycocalyx research and mucin-based biotechnologies.

Physical Principles of Membrane Shape Regulation by the Glycocalyx

Cells bend their plasma membranes into highly curved forms to interact with the local environment, but how shape generation is regulated is not fully resolved. Here, we report a synergy between shape-generating processes in the cell interior and the external organization and composition of the cell-surface glycocalyx. Mucin biopolymers and long-chain polysaccharides within the glycocalyx can generate entropic forces that favor or disfavor the projection of spherical and finger-like extensions from the cell surface. A polymer brush model of the glycocalyx successfully predicts the effects of polymer size and cell-surface density on membrane morphologies. Specific glycocalyx compositions can also induce plasma membrane instabilities to generate more exotic undulating and pearled membrane structures and drive secretion of extracellular vesicles. Together, our results suggest a fundamental role for the glycocalyx in regulating curved membrane features that serve in communication between cells and with the extracellular matrix.

ATP depletion during mitotic arrest induces mitotic slippage and APC/CCdh1-dependent cyclin B1 degradation

ATP depletion inhibits cell cycle progression, especially during the G1 phase and the G2 to M transition. However, the effect of ATP depletion on mitotic progression remains unclear. We observed that the reduction of ATP after prometaphase by simultaneous treatment with 2-deoxyglucose and NaN₃ did not arrest mitotic progression. Interestingly, ATP depletion during nocodazole-induced prometaphase arrest resulted in mitotic slippage, as indicated by a reduction in mitotic cells, APC/C-dependent degradation of cyclin B1, increased cell attachment, and increased nuclear membrane reassembly. Additionally, cells successfully progressed through the cell cycle after mitotic slippage, as indicated by EdU incorporation and time-lapse imaging. Although degradation of cyclin B during normal mitotic progression is primarily regulated by APC/C^Cdc20, we observed an unexpected decrease in Cdc20 prior to degradation of cyclin B during mitotic slippage. This decrease in Cdc20 was followed by a change in the binding partner preference of APC/C from Cdc20 to Cdh1; consequently, APC/C^Cdh1, but not APC/C^Cdc20, facilitated cyclin B degradation following ATP depletion. Pulse-chase analysis revealed that ATP depletion significantly abrogated global translation, including the translation of Cdc20 and Cdh1. Additionally, the half-life of Cdh1 was much longer than that of Cdc20. These data suggest that ATP depletion during mitotic arrest induces mitotic slippage facilitated by APC/C^Cdh1-dependent cyclin B degradation, which follows a decrease in Cdc20 resulting from reduced global translation and the differences in the half-lives of the Cdc20 and Cdh1 proteins.

An investigation into the effects of cellular energy depletion reveals a potential mechanism by which tumors evade chemotherapy. Adenosine triphosphate (ATP) is the primary energetic currency for many biological processes, and ATP depletion generally stalls the cell cycle that regulates proliferation. However, researchers led by Jae-Ho Lee of South Korea’s Ajou University School of Medicine discovered that ATP-depleted cells can sometimes bypass roadblocks in the cell division process. Before dividing, cells synthesize duplicates of every chromosome, and Lee’s team treated cells with chemotherapy agents that stall cell division by preventing separation of these duplicates. Surprisingly, subsequent ATP depletion allowed these cells to bypass this arrested state and re-enter the cell cycle, albeit with twice as much DNA as normal. Since many cancerous cells experience ATP depletion, this ‘escape hatch’ could help tumors survive treatment.

Physical biology of the cancer cell glycocalyx

The glycocalyx coating the outside of most cells is a polymer meshwork comprising proteins and complex sugar chains called glycans. From a physical perspective, the glycocalyx has long been considered a simple 'slime' that protects cells from mechanical disruption or against pathogen interactions, but the great complexity of the structure argues for the evolution of more advanced functionality: the glycocalyx serves as the complex physical environment within which cell-surface receptors reside and operate. Recent studies have demonstrated that the glycocalyx can exert thermodynamic and kinetic control over cell signalling by serving as the local medium within which receptors diffuse, assemble and function. The composition and structure of the glycocalyx change markedly with changes in cell state, including transformation. Notably, cancer-specific changes fuel the synthesis of monomeric building blocks and machinery for production of long-chain polymers that alter the physical and chemical structure of the glycocalyx. In this Review, we discuss these changes and their physical consequences on receptor function and emergent cell behaviours.

An interpretation of 14C-urea and 14C-primidone extraction in isolated rabbit lungs

We measured the venous concentration versus time curves of 14C-urea and 14C-primidone after rapid bolus injections of a vascular reference indicator, fluorescein isothiocyanate dextran, and one of the two 14C-labeled indicators in isolated rabbit lungs perfused with Krebs-Ringer bicarbonate solution containing 4.5% bovine serum albumin at flow rates (F) of 6.67, 3.33, 1.67, and 0.83 ml/sec and with nearly constant microvascular pressure and total lung vascular volume. When we calculated the permeability-surface area product, PS, from the 14C-urea and 14C-primidone outflow curves using the Crone model, the estimates of the PS product were directly proportional to F. However, the fractional change in the PS with flow was different for the two indicators. We also estimated the PS from the same 14C-urea and 14C-primidone data using an alternative model that includes perfusion heterogeneity, estimated in a previous study, and flow-limited and barrier-limited extravascular volumes accessible to both urea and primidone. This model was able to fit the outflow curves of either 14C-urea or 14C-primidone at all four flows studied with one flow-independent PS for each indicator. The ability of the new model to explain the 14C-urea and 14C-primidone data with no flow-dependent change in PS suggests that a change in PS with F estimated using other models such as the Crone model is not sufficient for capillary surface area recruitment.

Analysis of steric partition behavior of molecules in membranes using statistical physics. Application to gel chromatography and electrophoresis

The principles of statistical physics are used to formulate general expressions for the steric partition behavior of molecules in both random and ordered membrane structures that may be applied to any shape of the solute and/or the volume-excluding element of the membrane. These expressions fully define partitioning in terms of the volume excluded to point molecules and to finite-sized molecules. The mean effective exclusion volume for a molecule is calculated as a function of a global interaction energy, which varies with position, conformation, and orientation of the molecule. It allows consideration of electrostatic and other nonsteric factors. To test the model, specific partition functions are derived for several simple geometries describing the membrane and solute. Frequently, the derived expressions agree with past analyses; however, a new expression describing partitioning within an random network of fibers is derived. It agrees with past results only in the limit of low exclusion volumes. With greater volume exclusions, past results greatly overestimate the partition function. It is applied to gel electrophoresis and chromatography and survives testing with available experimental data. Unlike past analyses, it predicts nonlinear Ferguson plots for agarose gel electrophoresis. In addition, an analytical expression predicting the minimum radius of a sphere excluded from a random fiber matrix is derived, tested, and found to agree with experimental data.

Endothelial cell permeability to water

We have calculated diffusional permeability coefficients for tracer water and for [14C]antipyrine in endothelial cells. With these values and those from studies in whole lungs we set a range for diffusional water permeability coefficients of the intact endothelium.

Passage of ions and dextran molecules across the rete mirabile of the eel. The effects of charge

The countercurrent-perfused rete mirabile of the eel is a preparation in which capillary permeability values can be determined accurately in broadscale fashion. To provide insight into charge effects on transendothelial passage, the permeability values of small inorganic cations (labeled sodium and rubidium, and stable potassium) and anions (labeled chloride, iodide, sulfate, and ferrocyanide) were compared to those expected for neutral solutes with approximately matched diffusion coefficients, and that of a neutral dextran fraction was compared to that of a negatively charged dextran sulfate with a similar diffusion coefficient. In the small ion experiments, the labeled iodide values were unexpectedly high, apparently due to the contamination of the labeled iodide solution with I-3. The permeabilities of the rest of the ions clustered at a level about 0.5 of the values which would have been expected for neutral solutes with similar diffusion coefficients. The decrement was interpreted to reflect the presence of both positive and negative charges along the transendothelial pathway, which effectively decrease the dimensions of the limiting part of the pathway for the charged microions relative to that accessible to comparable nonelectrolytes. The larger negatively charged dextran sulfate was also reduced in its passage, in comparison with its matching neutral dextran; this was taken to indicate the presence of a larger scale average net negative charge along its pathway. The data indicate the presence of a staggering of positive and negative charges along the transendothelial pathway accessible to the microions, and a net negative charge in the more restricted part of the pathway available to the dextrans.