Molecular basis for activation of cardiovascular sensory afferents

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For decades, studies ofbaroreceptor activity have depended on measurements of action potentials in single fibers or whole nerve. We had no insight into the molecular components of the mechanoelectrical transducers that initiate depolarization and trigger action potentials. In fact, transduction of mechanical stimuli is one of the least understood of the vertebrate senses. Our goal has been to define the molecular basis for mechanical activation of arterial and cardiac sensory afferents. In earlier studies we defined the characteristics of aortic baroreceptor neurons (BRNs) in culture. These channels are cation-selective, non voltage-gated, and blocked by amiloride or gadolinium. However, their molecular identity remains unknown. A candidate family of evolutionary-conserved ion channels, the degenerin/epithelial Na+-channels (DEG/ENaC), was discovered in a genetic screen for mechanosensitive genes in C. elegans. During the past 4 years we made important discoveries to advance our hypothesis that DEG/ENaC channels function as the mechanoelectrical transducer in mammalian meehanoreceptors: 1) DEG/ENaC subunits are expressed in mechanoreceptive neurons and in their sensory terminals. 2) The functions of BRNs, both in vivo and in vitro are reduced by inhibitors of DEG/ENaC channels. 3) Most important, targeted disruption of a DEG/ENaC subunit in mice reduced mechanosensation in aortic BRNs and in cutaneous mechanoreceptors but did not abolish it. We believe the mammalian mechanosensitive channels may be a heteromultimeric complex of multiple DEG/ENaC proteins, along with associated intra and extracellular "tethering" proteins. Thus, our first hypothesis is aimed at defining the subunits of the DEG/ENaC family and associated proteins that form the mechanosensitive complex in BRNs. Additionally, we have evidence that DEG/ENaC channels also play an important role in cardiac sensory neurons, not only as mechanosensors, but also as H+-sensors in the setting of myocardial ischemia. Thus, these channels could be the mediators of activation of cardiac sympathetic afferents, causing the pronounced reflex increase in sympathetic outflow in heart failure states. Therefore, our second hypothesis is aimed at defining the proton- and mechano-sensitive DEG/ENaC channels of cardiac sensory afferents in dorsal root ganglia (sympathetic afferents) and nodose ganglia (vagal afferents) and determining their function under normal physiological and in myocardial ischemia and heart failure.