Study of calcium signaling in non-excitable cells

Voltage-gated ion channels as novel regulators of epithelial ion transport in the osmoregulatory organs of insects

Voltage-gated ion channels (VGICs) respond to changes in membrane potential (Vm) and typically exhibit fast kinetic properties. They play an important role in signal detection and propagation in excitable tissues. In contrast, the role of VGICs in non-excitable tissues like epithelia is less studied and less clear. Studies in epithelia of vertebrates and invertebrates demonstrate wide expression of VGICs in epithelia of animals. Recently, VGICs have emerged as regulators of ion transport in the Malpighian tubules (MTs) and other osmoregulatory organs of insects. This mini-review aims to concisely summarize which VGICs have been implicated in the regulation of ion transport in the osmoregulatory epithelia of insects to date, and highlight select groups for further study. We have also speculated on the roles VGICs may potentially play in regulating processes connected directly to ion transport in insects (e.g., acid-base balance, desiccation, thermal tolerance). This review is not meant to be exhaustive but should rather serve as a thought-provoking collection of select existing highlights on VGICs, and to emphasize how understudied this mechanism of ion transport regulation is in insect epithelia.

Rotavirus-related systemic diseases: clinical manifestation, evidence and pathogenesis

Rotaviruses, double-stranded, non-enveloped RNA viruses, are a global health concern, associated with acute gastroenteritis and secretory-driven watery diarrhoea, especially in infants and young children. Conventionally, rotavirus is primarily viewed as a pathogen for intestinal enterocytes. This notion is challenged, however, by data from patients and animal models documenting extra-intestinal clinical manifestations and viral replication following rotavirus infection. In addition to acute gastroenteritis, rotavirus infection has been linked to various neurological disorders, hepatitis and cholestasis, type 1 diabetes, respiratory illness, myocarditis, renal failure and thrombocytopenia. Concomitantly, molecular studies have provided insight into potential mechanisms by which rotavirus can enter and replicate in non-enterocyte cell types and evade host immune responses. Nevertheless, it is fair to say that the extra-intestinal aspect of the rotavirus infectious process is largely being overlooked by biomedical professionals, and there are gaps in the understanding of mechanisms of pathogenesis. Thus with the aim of increasing public and professional awareness we here provide a description of our current understanding of rotavirus-related extra-intestinal clinical manifestations and associated molecular pathogenesis. Further understanding of the processes involved should prove exceedingly useful for future diagnosis, treatment and prevention of rotavirus-associated disease.

Lipid droplets and their interactions with other organelles in liver diseases

Lipid droplets are cellular organelles used for lipid storage with a hydrophobic core of neutral lipids enclosed by a phospholipid monolayer. Besides presenting as giant single organelles in fat tissue, lipid droplets are also widely present as a multitude of small structures in hepatocytes, where they play key roles in health and disease of the liver. In addition to lipid storage, lipid droplets are also directly involved in lipid metabolism, membrane biosynthesis, cell signaling, inflammation, pathogen-host interaction and cancer development. In addition, they interact with other cellular organelles to regulate cellular biology. It is fair to say that the exact functions of lipid droplets in cellular physiology remain largely obscure. Thus prompted, here we aim to analyze the corpus of contemporary biomedical literature to create a framework as to how the role of lipid droplets in hepatocyte physiology and pathophysiology should be understood. The resulting framework should help understanding the interaction of lipid droplets with other organelles in important liver diseases, including fatty liver disease, viral hepatitis and liver cancer and direct further research directions.

On the electrical conductivity of alginate hydrogels

Hydrogels have been extensively used in the field of biomedical applications, offering customizable natural, synthetic or hybrid materials, particularly relevant in the field of tissue engineering. In the bioelectronics discipline, hydrogels are promising mainly as sensing platforms with or without encapsulated cells, showing great potential in healthcare and medicine. However, to date there is little data in the literature which characterizes the electrical properties of tissue engineering materials which are relevant to bioelectronics. In this work, we present electrical characterization of alginate hydrogels, a natural polysaccharide, using a four-probe method similar to electrical impedance spectroscopy. The acquired conductance data show distinct frequency-dependent features that change as a function of alginate and crosslinker concentration reflecting ion kinetics inside the measured sample. Furthermore, the presence of NIH 3T3 fibroblasts encapsulated in the hydrogels matrix was found to alter the artificial tissue's electrical properties. The method used provides valuable insight to the frequency-dependent electrical response of the resulting systems. It is hoped that the outcome of this research will be of use in the development of cell/electronic interfaces, possibly toward diagnostic biosensors and therapeutic bioelectronics.

A bio-hybrid tactile sensor incorporating living artificial skin and an impedance sensing array

The development of a bio-hybrid tactile sensor array that incorporates a skin analogue comprised of alginate encapsulated fibroblasts is described. The electrical properties are modulated by mechanical stress induced during contact, and changes are detected by a ten-channel dual-electrode impedance sensing array. By continuously monitoring the impedance of the sensor array at a fixed frequency, whilst normal and tangential loads are applied to the skin surface, transient mechanotransduction has been observed. The results demonstrate the effectiveness and feasibility of the preliminary prototype bio-hybrid tactile sensor.

Calcium influx in mammalian eggs

Calcium (Ca2+) signals are involved in the regulation of oocyte maturation and play a critical role during fertilization. In the egg, Ca2+is stored in the lumen of the endoplasmic reticulum and a signal is generated when the stored Ca2+is released through specialized channels in the membrane of the endoplasmic reticulum to elevate the free Ca2+concentration in the cytoplasm. Extracellular Ca2+is also important, indicated by the fact that the mobilization of luminal Ca2+is typically followed by Ca2+entry across the plasma membrane. The transmembrane Ca2+flux replenishes the endoplasmic reticulum, and thus, it is essential to sustain prolonged Ca2+signals. It also seems to be responsible for the stimulation of important signaling cascades required for complete egg activation. Characterization of the pathway that mediates Ca2+entry implies that its major components include STIM1, a protein that senses the filling status of the stores, and ORAI1, a channel protein located in the plasma membrane. Defining the mechanism and functions of Ca2+entry will not only lead to a better understanding of egg physiology but may also help improving the efficiency of a number of assisted reproductive technologies.

Stochastic hybrid modeling of intracellular calcium dynamics

Deterministic models of biochemical processes at the subcellular level might become inadequate when a cascade of chemical reactions is induced by a few molecules. Inherent randomness of such phenomena calls for the use of stochastic simulations. However, being computationally intensive, such simulations become infeasible for large and complex reaction networks. To improve their computational efficiency in handling these networks, we present a hybrid approach, in which slow reactions and fluxes are handled through exact stochastic simulation and their fast counterparts are treated partially deterministically through chemical Langevin equation. The classification of reactions as fast or slow is accompanied by the assumption that in the time-scale of fast reactions, slow reactions do not occur and hence do not affect the probability of the state. Our new approach also handles reactions with complex rate expressions such as Michaelis-Menten kinetics. Fluxes which cannot be modeled explicitly through reactions, such as flux of Ca(2+) from endoplasmic reticulum to the cytosol through inositol 1,4,5-trisphosphate receptor channels, are handled deterministically. The proposed hybrid algorithm is used to model the regulation of the dynamics of cytosolic calcium ions in mouse macrophage RAW 264.7 cells. At relatively large number of molecules, the response characteristics obtained with the stochastic and deterministic simulations coincide, which validates our approach in the limit of large numbers. At low doses, the response characteristics of some key chemical species, such as levels of cytosolic calcium, predicted with stochastic simulations, differ quantitatively from their deterministic counterparts. These observations are ubiquitous throughout dose response, sensitivity, and gene-knockdown response analyses. While the relative differences between the peak-heights of the cytosolic [Ca(2+)] time-courses obtained from stochastic (mean of 16 realizations) and deterministic simulations are merely 1%-4% for most perturbations, it is specially sensitive to levels of G(βγ) (relative difference as large as 90% at very low G(βγ)).

A kinetic model for calcium dynamics in RAW 264.7 cells: 1. Mechanisms, parameters, and subpopulational variability

Calcium (Ca(2+)) is an important second messenger and has been the subject of numerous experimental measurements and mechanistic studies in intracellular signaling. Calcium profile can also serve as a useful cellular phenotype. Kinetic models of calcium dynamics provide quantitative insights into the calcium signaling networks. We report here the development of a complex kinetic model for calcium dynamics in RAW 264.7 cells stimulated by the C5a ligand. The model is developed using the vast number of measurements of in vivo calcium dynamics carried out in the Alliance for Cellular Signaling (AfCS) Laboratories. Ligand binding, phospholipase C-beta (PLC-beta) activation, inositol 1,4,5-trisphosphate (IP(3)) receptor (IP(3)R) dynamics, and calcium exchange with mitochondria and extracellular matrix have all been incorporated into the model. The experimental data include data from both native and knockdown cell lines. Subpopulational variability in measurements is addressed by allowing nonkinetic parameters to vary across datasets. The model predicts temporal response of Ca(2+) concentration for various doses of C5a under different initial conditions. The optimized parameters for IP(3)R dynamics are in agreement with the legacy data. Further, the half-maximal effect concentration of C5a and the predicted dose response are comparable to those seen in AfCS measurements. Sensitivity analysis shows that the model is robust to parametric perturbations.