Modeling of Ca2+ transients initiated by GPCR agonists in mesenchymal stromal cells

The integrative study that included experimentation and mathematical modeling was carried out to analyze dynamic aspects of transient Ca²⁺ signaling induced by brief pulses of GPCR agonists in mesenchymal stromal cells from the human adipose tissue (AD-MSCs). The experimental findings argued for IP₃/Ca²⁺-regulated Ca²⁺ release via IP₃ receptors (IP₃Rs) as a key mechanism mediating agonist-dependent Ca²⁺ transients. The consistent signaling circuit was proposed to formalize coupling of agonist binding to Ca²⁺ mobilization for mathematical modeling. The model properly simulated the basic phenomenology of agonist transduction in AD-MSCs, which mostly produced single Ca²⁺ spikes upon brief stimulation. The spike-like responses were almost invariantly shaped at different agonist doses above a threshold, while response lag markedly decreased with stimulus strength. In AD-MSCs, agonists and IP₃ uncaging elicited similar Ca²⁺ transients but IP₃ pulses released Ca²⁺ without pronounced delay. This suggested that IP₃ production was rate-limiting in agonist transduction. In a subpopulation of AD-MSCs, brief agonist pulses elicited Ca²⁺ bursts crowned by damped oscillations. With properly adjusted parameters of IP₃R inhibition by cytosolic Ca²⁺, the model reproduced such oscillatory Ca²⁺ responses as well. GEM-GECO1 and R-CEPIA1er, the genetically encoded sensors of cytosolic and reticular Ca²⁺, respectively, were co-expressed in HEK-293 cells that also responded to agonists in an "all-or-nothing" manner. The experimentally observed Ca²⁺ signals triggered by ACh in both compartments were properly simulated with the suggested signaling circuit. Thus, the performed modeling of the transduction process provides sufficient theoretical basis for deeper interpretation of experimental findings on agonist-induced Ca²⁺ signaling in AD-MSCs.

Attenuation of calmodulin regulation evokes Ca2+ oscillations: evidence for the involvement of intracellular arachidonate-activated channels and connexons

Intracellular Са²⁺ controls its own level by regulation of Ca²⁺ transport across the plasma and organellar membranes, often acting via calmodulin (CaM). Drugs antagonizing CaM action induce an increase in cytosolic Ca²⁺ concentration in different cells. We have found persistent Са²⁺ oscillations in cultured white adipocytes in response to calmidazolium (CMZ). They appeared at [CMZ] > 1 μM as repetitive sharp spikes mainly superimposed on a transient or elevated baseline. Similar oscillations were observed when we used trifluoperazine. Oscillations evoked by 5 μM CMZ resulted from the release of stored Ca²⁺ and were supported by Са²⁺ entry. Inhibition of store-operated channels by YM-58483 or 2-APB did not change the responses. Phospholipase A₂ inhibited by AACOCF₃ was responsible for initial Ca²⁺ mobilization, but not for subsequent oscillations, whereas inhibition of iPLA₂ by BEL had no effect. Phospholipase C was partially involved in both stages as revealed with U73122. Intracellular Са²⁺ stores engaged by CMZ were entirely dependent on thapsigargin. The oscillations existed in the presence of inhibitors of ryanodine or inositol 1,4,5-trisphosphate receptors, or antagonists of Ca²⁺ transport by lysosome-like acidic stores. Carbenoxolone or octanol, blockers of hemichannels (connexons), when applied for two hours, prevented oscillations but did not affect the initial Са²⁺ release. Incubation with La³⁺ for 2 or 24 h inhibited all responses to CMZ, retaining the thapsigargin-induced Ca²⁺ rise. These results suggest that Ca²⁺-CaM regulation suppresses La³⁺-sensitive channels in non-acidic organelles, of which arachidonate-activated channels initiate Ca²⁺ oscillations, and connexons are intimately implicated in their generation mechanism.

Toggle switches, pulses and oscillations are intrinsic properties of the Src activation/deactivation cycle

Src-family kinases (SFKs) play a pivotal role in growth factor signaling, mitosis, cell motility and invasiveness. In their basal state, SFKs maintain a closed autoinhibited conformation, where the Src homology 2 domain interacts with an inhibitory phosphotyrosine in the C-terminus. Activation involves dephosphorylation of this inhibitory phosphotyrosine, followed by intermolecular autophosphorylation of a specific tyrosine residue in the activation loop. The spatiotemporal dynamics of SFK activation controls cell behavior, yet these dynamics remain largely uninvestigated. In the present study, we show that the basic properties of the Src activation/deactivation cycle can bring about complex signaling dynamics, including oscillations, toggle switches and excitable behavior. These intricate dynamics do not require imposed external feedback loops and occur at constant activities of Src inhibitors and activators, such as C-terminal Src kinase and receptor-type protein tyrosine phosphatases. We demonstrate that C-terminal Src kinase and receptor-type protein tyrosine phosphatase underexpression or their simultaneous overexpression can transform Src response patterns into oscillatory or bistable responses, respectively. Similarly, Src overexpression leads to dysregulation of Src activity, promoting sustained self-perpetuating oscillations. Distinct types of responses can allow SFKs to trigger different cell-fate decisions, where cellular outcomes are determined by the stimulation threshold and history. Our mathematical model helps to understand the puzzling experimental observations and suggests conditions where these different kinetic behaviors of SFKs can be tested experimentally.

Ca2+ dependence of Ca2+ release from intracellular stores of intact and permeabilized Ehrlich ascites tumour cells

Effects of Ca2+ ions on the mobilization of Ca2+ from intracellular stores of intact and permeabilized (15 microM digitonin) Ehrlich ascites tumour cells (EATC) have been compared. For permeabilized cells, the dependences of the initial rate and amplitude of Ca2+ mobilization evoked by the addition of 100 nM inositol 1,4,5-trisphosphate (IP3) on preexisting [Ca2+] were bell-shaped within a [Ca2+] range 10(-7)-10(-6) M with the maxima at [Ca2+] = 166 nM. In intact cells, different concentrations of free cytosolic Ca2+ ([Ca2+]i) were produced using low (up to 0.005%) concentrations of digitonin which selectively increased the permeability of the plasma membrane. Stimulation of cells by exogenous ATP at [Ca2+]i = 10(-8)-10(-6) M resulted in Ca2+ mobilization the rate and amplitude of which were maximal at 102-115 nM Ca2+. The experimental Ca2+ dependences were fit by a model which includes channel opening upon Ca2+ binding and transition to the inactive states upon Ca2+ binding to the closed and open channel forms. Three inactivation types (including two particular cases) demonstrate a slight priority of inhibitory binding of Ca2+ only to the open channel, but predict markedly different parameter values. We conclude that an increase in [Ca2+] can stimulate IP3-induced mobilization, but in intact EATC, deviations of [Ca2+]i from the resting level (about 100 nM) attenuate responses to the agonist stimulation.

Quantal Ca2+ release and inactivation in a model of the inositol 1,4,5-trisphosphate receptor involving transformation of the ligand by the receptor

A model explaining quantal Ca2+ release as an intrinsic property of the inositol 1,4,5-triphosphate (IP3) receptor has been put forward. The model is based on the hypothesis that the IP3 receptor can catalyze a transformation of the IP3 molecule differing from its conventional metabolism. A simple kinetic mechanism is considered, in which IP3-induced Ca2+ channel opening is followed by the step of IP3 conversion and channel closure. Examination of the resulting mathematical model shows that it can reproduce well both partial release of stored Ca2+ and the same responsiveness to subsequent IP3 additions. On incorporation of an additional closed state of the channel, the model describes also a time-dependent channel inactivation at a high IP3 dose. Temperature sensitivity of the catalytic step accounts for the reported elimination of quantal responses and inactivation at low temperature. The transformation product is surmised to be a positional or stereo isomer of IP3.

[Substrate inhibition as a cause of oscillations in an open irreversible enzymic reaction S1 + S2 in the presence of E(R,T) leads to S1' + S2'. A mathematical model]

A mathematical model of an open irreversible reaction S1 + S2 (formula: see text) catalysed by an olygomeric enzyme E(R, T) has been analysed. It is assumed that the enzyme undergoes the concerted conformational transitions R in equilibrium T in conformity with the theory of Monod, Wyman and Changeux, and one of the substrates (S2) produces inhibition of the enzyme, thus shifting the equilibrium between the two enzyme forms in the direction of T formation. A simple graphical explanation is given to the hysteresis of the input characteristic approximately v ([S1]) (approximately v is the reaction rate at d[S2]/dt=O) which gives rise to self--oscillations. The hysteresis occurs both in the case of allosteric and isosteric substrate inhibition.