Modeling Calcium Signaling in S. cerevisiae Highlights the Role and Regulation of the Calmodulin-Calcineurin Pathway in Response to Hypotonic Shock

Abstract

Calcium homeostasis and signaling processes in Saccharomyces cerevisiae, as well as in any eukaryotic organism, depend on various transporters and channels located on both the plasma and intracellular membranes. The activity of these proteins is regulated by a number of feedback mechanisms that act through the calmodulin-calcineurin pathway. When exposed to hypotonic shock (HTS), yeast cells respond with an increased cytosolic calcium transient, which seems to be conditioned by the opening of stretch-activated channels. To better understand the role of each channel and transporter involved in the generation and recovery of the calcium transient—and of their feedback regulations—we defined and analyzed a mathematical model of the calcium signaling response to HTS in yeast cells. The model was validated by comparing the simulation outcomes with calcium concentration variations before and during the HTS response, which were observed experimentally in both wild-type and mutant strains. Our results show that calcium normally enters the cell through the High Affinity Calcium influx System and mechanosensitive channels. The increase of the plasma membrane tension, caused by HTS, boosts the opening probability of mechanosensitive channels. This event causes a sudden calcium pulse that is rapidly dissipated by the activity of the vacuolar transporter Pmc1. According to model simulations, the role of another vacuolar transporter, Vcx1, is instead marginal, unless calcineurin is inhibited or removed. Our results also suggest that the mechanosensitive channels are subject to a calcium-dependent feedback inhibition, possibly involving calmodulin. Noteworthy, the model predictions are in accordance with literature results concerning some aspects of calcium homeostasis and signaling that were not specifically addressed within the model itself, suggesting that it actually depicts all the main cellular components and interactions that constitute the HTS calcium pathway, and thus can correctly reproduce the shaping of the calcium signature by calmodulin- and calcineurin-dependent complex regulations. The model predictions also allowed to provide an interpretation of different regulatory schemes involved in calcium handling in both wild-type and mutants yeast strains. The model could be easily extended to represent different calcium signals in other eukaryotic cells.

Luminometric Studies of Yeast Response to Complex Environmental Calcium Variations Demonstrate Sensing of External Calcium Ion Changes

“…In this expression, the flux is permanent and increases with external concentration increments. Our observations indicate that the use of the model does not account for important features of the system and, [4]. In the model the probability of transition between states is determined by the difference in osmotic pressure between the environment and the cytosol.…”

“…In Ref. [4] the pressure difference is the key factor, but many other mechanisms are plausible. We will explore these models in future work.…”

“…The response of yeast to changes in external calcium concentration has been extensively investigated using diverse experimental techniques as well as modeling approaches [1] [2] [3] [4]. Key components of the network that regulates cytosolic calcium have been identified [5] [6] though important details remain to be clarified.…”

The dynamic hypoosmotic response ofVibrio choleraerelies on the mechanosensitive channel MscS

“…The model is presented as a system of interdependent Ordinary Differential Equations. The time course of the osmotic response is generated by numeric integration of the model using the COPASI software (Hoops et al, 2006; Mendes et al, 2009), an advanced simulator for biochemical networks that has been used previously to model osmoregulation in yeast (Spolaor et al, 2022).…”

Signaling Pathway Governed by Lipid Derived Molecules as Secondary Messenger

“…Phosphoinositol-4,5-bisphosphate is broken down by PLC into diglycerides and Inositol Triphosphate. The following spreads throughout the cytoplasm attach to its receptor that is mostly found on the ER, which is the greatest and most in controlled intercell Ca 2+ source (Spolaor et al, 2022). The protoplasmic Ca 2+ rises that follows a variety of configurations that are highly organised.…”