Mechanisms of Multistability in Neuronal Models

Multistability is a fundamental attribute of the dynamics of neuronal systems under normal and pathological conditions. The mechanism of bistability of bursting and silence is not well understood and to our knowledge has not been experimentally recorded in single neurons. We considered four models. Two of them described the dynamics of a leech heart interneuron: the canonical model and a low-dimensional model. The other two models described mammalian pacemakers from the respiratory center. We investigated the low-dimensional model and identified six different types of multistability of dynamical regimes. We described six generic mechanisms underlying the co-existence of oscillatory and silent regimes. The mechanisms are based either on a saddle equilibrium or a saddle periodic orbit. The stable manifold of the saddle equilibrium or the saddle orbit sets the threshold between the regimes. In the two models of the leech interneuron the range of the controlling parameters supporting the co-existence of bursting and silence is limited by the Andronov-Hopf and homoclinic bifurcations (Malashchenko, Master Thesis 2007). The bistability was found in a narrow range of the leak currents\u27 parameters. Here, we introduced a propensity index to bistability as the width of the range on a bifurcation diagram; we investigated how the propensity index was affected by modifications of the ionic currents, and found that conductances of only two currents substantially affected the index. The increase of the conductance of the hyperpolarization-activated current, Ih, and the reduction of the fast Ca2+ current, ICaF, notably increased the propensity index. These findings define modulatory conditions under which we suggest the bistability of bursting and silence could be experimentally revealed in leech heart interneurons. We hypothesize that this mechanism could be commonly found in a large variety of neuronal models. We applied our techniques to models of vertebrate neurons controlling respiratory rhythm, which represent two types of inspiratory pacemakers of the Pre-Bӧtzinger Complex. We showed that both types of neurons could exhibit bistability of bursting and silence in accordance with the mechanism which we described

A Mechanism of Co-Existence of Bursting and Silent Regimes of Activities of a Neuron

The co-existence of bursting activity and silence is a common property of various neuronal models. We describe a novel mechanism explaining the co-existence of and the transition between these two regimes. It is based on the specific homoclinic and Andronov-Hopf bifurcations of the hyper- and depolarized steady states that determine the co-existence domain in the parameter space of the leech heart interneuron models: canonical and simplified. We found that a sub-critical Andronov-Hopf bifurcation of the hyperpolarized steady state gives rise to small amplitude sub-threshold oscillations terminating through the secondary homoclinic bifurcation. Near the corresponding boundary the system can exhibit long transition from bursting oscillations into silence, as well as the bi-stability where the observed regime is determined by the initial state of the neuron. The mechanism found is shown to be generic for the simplified 4D and the original 14D leech heart interneuron models

Six Types of Multistability in a Neuronal Model Based on Slow Calcium Current

Background: Multistability of oscillatory and silent regimes is a ubiquitous phenomenon exhibited by excitable systems such as neurons and cardiac cells. Multistability can play functional roles in short-term memory and maintaining posture. It seems to pose an evolutionary advantage for neurons which are part of multifunctional Central Pattern Generators to possess multistability. The mechanisms supporting multistability of bursting regimes are not well understood or classified. Methodology/Principal Findings: Our study is focused on determining the bio-physical mechanisms underlying different types of co-existence of the oscillatory and silent regimes observed in a neuronal model. We develop a low-dimensional model typifying the dynamics of a single leech heart interneuron. We carry out a bifurcation analysis of the model and show that it possesses six different types of multistability of dynamical regimes. These types are the co-existence of 1) bursting and silence, 2) tonic spiking and silence, 3) tonic spiking and subthreshold oscillations, 4) bursting and subthreshold oscillations, 5) bursting, subthreshold oscillations and silence, and 6) bursting and tonic spiking. These first five types of multistability occur due to the presence of a separating regime that is either a saddle periodic orbit or a saddle equilibrium. We found that the parameter range wherein multistability is observed is limited by the parameter values at which the separating regimes emerge and terminate. Conclusions: We developed a neuronal model which exhibits a rich variety of different types of multistability. We described a novel mechanism supporting the bistability of bursting and silence. This neuronal model provides a unique opportunity to study the dynamics of networks with neurons possessing different types of multistability

STIMULATING EFFECT OF HIGH DOSE HEPARIN ON MIGRATION ACTIVITY AND MSC STEMNESS PRESERVATION IN THE PRESENCE OF BONE-SUBSTITUTING MATERIALS

Synthetic materials used in regenerative medicine, upon implantation, induce the development of an inflammatory reaction necessary for the effective regeneration of damaged bone tissue. Implant contact with tissues is accompanied by the deposition of blood proteins and interstitial fluid on its surface, contributing to the activation of the complement system, components of innate immunity, initiating coagulation hemostasis, leading to the formation of a fibrin clot. An extracellular matrix based on fibrin, collagen and elastin forms on the implant’s surface, which provides the basis for the formation of tissue structure through the adhesion of stem cells to the forming bone callus before the formation of bone regenerate. To prevent the development of postoperative pathological conditions caused by hypercoagulable syndrome, therapeutic strategies are used to use anticoagulants (heparin, warfarin). However, their use limits the normal formation of a fibrin clot in vivo. This can slow down the migration of mesenchymal stem cells (MSC) and disrupt the formation of callus, inhibiting the processes of osseointegration of the implant and bone healing. The study’s goal was to study the effect of heparin in a gradient of low and high concentrations on the migration activity and stem capacity of human MSCs under in vitro cultivation conditions. According to the results of flow cytometry, it was revealed that high concentrations of heparin (130, 260 IU/ml) in a 2D cultivation model contribute to an increase in the number of cells expressing surface markers CD73 and CD90, which indicates that MSCs retain high clonogenic potential. A 3D model of in vitro cultivation with the addition of heparin and osteosubstituting implants bearing a CF coating with a roughness index of Ra = 2.6-4.9 μm contributed to preserving the “stemness” character of MSCs through the expression of surface markers CD73 and CD90. According to the results obtained using the xCELLigence system, heparin at a later time (from 20-40 hours) increases the invasion of MSCs through micropores that simulate the state of the blood vessel walls. However, in the presence of HAP nanoparticles that mimic the remodeling processes of the mineral bone matrix and/or resorption of bone cement, the effect of heparin was less pronounced. The results can be used in the field of regenerative medicine associated with the introduction of MSCs. The data can serve as a prerequisite for developing new therapeutic strategies for surgical patients with a high risk of postoperative thrombosis after osteosynthesis

Mesenchymal stem cells: a brief review of classis concepts and new factors of osteogenic differentiation

Molecular genetic mechanisms, signaling pathways, cultural conditions, factors, and markers of osteogenic differentiation of mesenchymal stem cells (MSC) are actively studied despite numerous works in this area of cellular technologies. This is largely due to the accumulating contradictions in seemingly classical knowledge, as well as permanent updating of the results in the field. In this regard, we focused on the main classical concepts and some new factors and mechanisms that have a noticeable regulatory effect on the differentiation potential of postnatal MSCs. The present review considers the significance of MSC sources for their differentiation capacity, as well as the role of the cellular microenvironment. The issues of classification, terminology, and functional activity of MSCs from various sources are discussed. The paracrine potential of MSCs in tissue regeneration has been considered; sufficient importance of inflammation in osteogenesis is noted, in particular, the presence of inflammatory cytokines and chemokines in the lesion focus, produced not only by microenvironmental cells but also by blood cells, including mononuclear leukocytes, migrating to the affected site. An important role in this review is given to biomechanical signals and to influence of conformational changes in cell cytoskeleton (cell shape) upon MSC differentiation, since the morphological features of cells and the structure of cytoskeleton are modulated by interactions of the cell surface with environmental factors, including hydrostatic pressure, fluid flow, compression/stretching loads. The data are presented concerning elasticity of extracellular matrix being a determining factor of cell differentiation. We conclude that one should switch from point studies of individual gene effects to multiple measurements of the gene-regulatory profile and biomolecules responsible for multiple, still poorly studied osteogenic factors of endogenous and exogenous origin. Among cornerstones in future (epi)genetic studies will be to decide if osteomodulatory effects are realized through specific signaling pathways and/or via cross-signaling with known genes controlling osteogenic differentiation of MSCs

Significance of nutrient media choice for the long-term cultures of leukemic T-lymphoblasts

Correct choice of nutrient media for culturing different types of cells in various applications is one of the most important aspects of modern biotechnology, since chemical composition of the culture media largely contains the necessary metabolites to support certain cells’ growth lines outside the body. Jurkat line of human leukemic T-lymphoblast-like cells (hereinafter Jurkat T-cells) is actively used for in vitro modeling of intracellular signaling and activation of normal blood T-lymphocytes mediated by the T-cell receptor/CD3/ CD4 complex in toxicological studies of immune and secretory responses, to test medicinal substances and ions. Also, Jurkat T-cells are widely used for ex vivo testing in immunology, oncology, toxicology, orthopedics, and traumatology. The existing standards and numerous studies are mainly based on short-term in vitro cultivation of Jurkat T-cells in RPMI 1640 nutrient medium. Meanwhile, the issues of long-term maintenance of the growth of Jurkat T-cells culture are poorly presented in the research literature. This study aimed for studying the activity of Jurkat T-cells over 7 to 14 days of in vitro culture and comparing the relative value of RPMI 1640 and αMEM media for the behavior of immunocompetent tumor cells. Using flow cytometry, multiplex analysis, and phase contrast Cell-IQ microscopy, the proportions of living cells and those dying by apoptosis and necrosis, secretion of cytokines and chemokines, and the dynamics of cell biomass propagation were studied. It was found that the αMEM medium in the complete nutrient medium, as compared with RPMI 1640, is more appropriate to in vitro promotion of cell viability (increased proportion of viable cells by 13.5% at the day 14), their secretory ability for 23 из 27 tested biomolecules, shortened adaptation time (на 32%) in culture before growth initiation, 5-fold increase of the Jurkat Т-cell cellularity by the day 7. Potential significance of the chemical components of nutrient media and secreted biomolecules for these results is discussed. As based on the results obtained, we concluded on superior properties of αMEM medium for long-term in vitro cultures of Jurkat T-cells. Consequently, the in vitro testing of medical devices intended for long-term contact with the body, including those for cancer patients, using Jurkat T-cell leukemia line in RPMI 1640 medium, may lead to wrong predictions on their biocompatibility and potential antitumor activity

Human Mesenchymal Stem Cells as a Carrier for a Cell-Mediated Drug Delivery

A number of preclinical and clinical studies have demonstrated the efficiency of mesenchymal stromal cells to serve as an excellent base for a cell-mediated drug delivery system. Cell-based targeted drug delivery has received much attention as a system to facilitate the uptake a nd transfer of active substances to specific organs and tissues with high efficiency. Human mesenchymal stem cells (MSCs) are attracting increased interest as a promising tool for cell-based therapy due to their high proliferative capacity, multi-potency, and anti-inflammatory and immunomodulatory properties. In particular, these cells are potentially suitable for use as encapsulated drug transporters to sites of inflammation. Here, we studied the in vitro effects of incorporating synthetic polymer microcapsules at various microcapsule-to-cell ratios on the morphology, ultrastructure, cytokine profile, and migration ability of human adipose-derived MSCs at various time points post-phagocytosis. The data show that under appropriate conditions, human MSCs can be efficiently loaded with synthesized microcapsules without damaging the cell’s structural integrity with unexpressed cytokine secretion, retained motility, and ability to migrate through 8 ?m pores. Thus, the strategy of using human MSCs as a delivery vehicle for transferring microcapsules, containing bioactive material, across the tissue–blood or tumor–blood barriers to facilitate the treatment of stroke, cancer, or inflammatory diseases may open a new therapeutic perspective