Abstract
Life is maintained in a sea water-like internal environment. The homeostasis of this environment is dependent on osmosensory system translation of hydromineral information into osmotic regulatory machinery at system, tissue and cell levels. In the osmosensation, hydromineral information can be converted into cellular reactions through osmoreceptors, which changes thirst and drinking, secretion of antidiuretic vasopressin (VP), reabsorption of water and salt in the kidneys at systemic level as well as cellular metabolic activity and survival status at tissue level. The key feature of osmosensation is the activation of mechanoreceptors or mechanosensors, particularly transient receptor potential vallinoid (TRPV) and canonical (TRPC) family channels, which increases cytosolic Ca2+ levels, activates osmosensory cells including VP neurons and triggers a series of secondary reactions. TRPV channels are sensitive to both hyperosmotic and hyposmotic stimuli while TRPC channels are more sensitive to hyposmotic challenge in neurons. The activation of TRP channels relies on changes in cell volume, membrane stretch and cytoskeletal reorganization as well as hydration status of extracellular matrix (ECM) and activity of integrins. Different families of TRP channels could be activated differently in response to hyperosmotic and hyposmotic stimuli in different spatiotemporal orders, leading to differential reactions of osmosensory cells. Together, they constitute the osmosensory machinery. The activation of this osmoreceptor complex is also associated with the activity of other osmolarity-regulating organelles, such as water channel protein aquaporins, Na-K-2Cl cotransporters, volume-sensitive anion channels, sodium pump and purinergic receptors in addition to intercellular interactions, typically astrocytic neuronal interactions. In this article, we review our current understandings of the composition of osmoreceptors and the processes of osmosensation.
Highlights
Life is maintained in a sea water-like internal environment
The key feature of osmosensation is the activation of mechanoreceptors or mechanosensors, transient receptor potential vallinoid (TRPV) and canonical (TRPC) family channels, which increases cytosolic Ca2+ levels, activates osmosensory cells including VP neurons and triggers a series of secondary reactions
We propose that during hyperosmotic stress, extracellular matrix (ECM) binds with excessive cation, buffers osmotic stress, reduces membrane stretch (Figure 1Ba) while accelerating water efflux to reduce intracellular volume (Figure 1Bb), and increases the interactions between integrin, actin filament and microtubules (Sims et al, 1992; Martinac, 2014)
Summary
Osmosensation requires functioning of osmoreceptors that detect hydromineral disturbance and initiate osmoregulation (Knepper et al, 2015). The central osmosensation involves local neuronal activity, astrocytic plasticity, blood-borne factors, direct osmotic stimuli and autoregulation (Scott and Brown, 2010; Wang et al, 2011; Pedrino et al, 2014); the essential requirements for osmosensation are still the ability of osmosensory cells to sense hydromineral changes. Noteworthy is that other neurons outside of this central osmosensory system can sense changes in osmotic pressure, such as oxytocin neurons in the SON and PVN (Kortus et al, 2016) and hippocampal neurons (Arranz et al, 2014). Many types of tissues and cells have the capacity of osmosensation (Pedrino et al, 2014), typically seen in the digestive tract (Zhu et al, 2001) that could change VP neuronal activity through medulla-mediated viscerosensory inputs (Rinaman, 2007). The activation of osmosensory system can change thirst and drinking, secretion of antidiuretic VP, and reabsorption of water and salt in the kidneys (Wang et al, 2011; Danziger and Zeidel, 2015) as well as VP gene transcription following increase in cAMP (Arima et al, 2001), cellular metabolic activity and survival status (Moeckel et al, 2006; Hollborn et al, 2015), thereby helping the body and its parts to restore hydromineral balance
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