Abstract
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.