Mechano-sensitivity of ENaC: may the (shear) force be with you
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The epithelial Na+ channel (ENaC) is the rate-limiting step for Na+ absorption in various vertebrate epithelia and deeply enmeshed in the control of salt and water homeostasis. The phylogenetic relationship of ENaC molecules to mechano-sensitive Degenerins from Caenorhabditis elegans indicates that ENaC might be mechano-sensitive as well. Primarily, it was suggested that ENaC might be activated by membrane stretch. However, this issue still remains to be clarified because controversial results were published. Recent publications indicate that shear stress represents an adequate stimulus, activating ENaC via increasing the single-channel open probability. Basing on the experimental evidence published within the past years and integrating this knowledge into a model related to the mechano-sensitive receptor complex known from C. elegans, we introduce a putative mechanism concerning the mechano-sensitivity of ENaC. We suggest that mechano-sensitive ENaC activation represents a nonhormonal regulatory mechanism. This feature could be of considerable physiological significance because many Na+-absorbing epithelia are exposed to shear forces. Furthermore, it may explain the wide distribution of ENaC proteins in nonepithelial tissues. Nevertheless, it remains a challenge for future studies to explore the mechanism how ENaC is controlled by mechanical forces and shear stress in particular.