Modeling of Ca 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 Ca signaling induced by brief pulses of GPCR agonists in mesenchymal stromal cells from the human adipose tissue (AD-MSCs). The experimental findings argued for IP/Ca-regulated Ca release via IP receptors (IPRs) as a key mechanism mediating agonist-dependent Ca transients. The consistent signaling circuit was proposed to formalize coupling of agonist binding to Ca mobilization for mathematical modeling. The model properly simulated the basic phenomenology of agonist transduction in AD-MSCs, which mostly produced single Ca 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 IP uncaging elicited similar Ca transients but IP pulses released Ca without pronounced delay. This suggested that IP production was rate-limiting in agonist transduction. In a subpopulation of AD-MSCs, brief agonist pulses elicited Ca bursts crowned by damped oscillations. With properly adjusted parameters of IPR inhibition by cytosolic Ca, the model reproduced such oscillatory Ca responses as well. GEM-GECO1 and R-CEPIA1er, the genetically encoded sensors of cytosolic and reticular Ca, respectively, were co-expressed in HEK-293 cells that also responded to agonists in an "all-or-nothing" manner. The experimentally observed Ca 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 Ca signaling in AD-MSCs.