8.2 Sensing Osmotic Stress/Shock
KEY CONCEPTS
By the end of this section, you will be able to do the following:
- Apply your understanding of cell signalling pathways (Chapter 5.6) to explain two examples of signal reception and transduction involved in detecting osmotic stress.
- Evaluate why high extracellular Ca2+ concentration can be a good early indicator of hyperosmotic stress.
- Explain why plasma membrane stretching can be a good indicator of hypoosmotic stress.
There are various ways to detect osmotic stress, including detecting changes in ion concentrations. Common signs of osmotic stress are membrane stretching, macromolecule crowding, a change in cell volume, and a change in ion/solute concentrations. An early effect of osmotic stress could be membrane stretching while a later effect of osmotic stress could be macromolecule crowding (e.g. protein aggregation). In this chapter section, we will describe one mechanism each for detecting hyperosmotic and hypoosmotic stress, although there are others. If you need to review the basics of cell signalling, refer back to Chapter 5.6.
Calcium-Sensing Receptors Detecting Hyperosmotic Conditions
In hyperosmotic environments, like the ocean, some organisms use calcium-sensing receptors (CaSR) to detect extracellular calcium (Ca2+) concentrations, one indicator of environmental osmolarity. This mechanism is started by the binding of Ca2+ to CaSR in the cell membrane (Figure 8.3) of cells that are in contact with the external environment (e.g., gill cells of a tilapia fish). Ca2+ binding to CaSR is an indication of high external osmolarity, and stimulates phospholipase-C (PLC) inside the cell to catalyze the formation of a second messenger called inositol triphosphate (IP3). IP3 then binds to its receptor channel on the surface of the endoplasmic reticulum (ER), which triggers the release of Ca2+ from the ER to the cytosol. Ca2+ in the cytosol then activates various signaling pathways by binding to calmodulin (CaM) or other calcium-binding proteins, which then stimulate cellular responses that help with the response to the hyperosmotic environment. To terminate the signalling pathway, excess Ca2+ is pumped back into the ER by SarcoEndoplasmic Reticulum Calcium ATPase (SERCA), where it is stored until needed again.
Cell Stretch Receptors Detect Hypoosmotic Conditions
In hypoosmotic environments, the plasma membrane tends to stretch due to the influx of water causing cellular swelling. One of the receptor proteins in the plasma membrane that can detect and respond to stretching is integrin (Figure 8.4). Integrin plays an important role in connection between the extracellular matrix and the cytoskeleton of many animal cells. When the plasma membrane stretches, it causes a conformational change in integrin and other integral membrane proteins, resulting in an intracellular activation of guanine exchange factors (GEFs). These GEFs lead to the activation of the Rho (Ras homologous) protein by stimulating Rho to bind to GTP (guanosine triphosphate) instead of GDP. Activated Rho then activates Rho kinase (ROCK) to phosphorylate several substrates leading to various cellular responses.