The diverse functions of the DEG/ENaC family: linking genetic and physiological insights

Abstract

The DEG/ENaC family of ion channels was defined based on the sequence similarity between degenerins (DEG) from the nematode Caenorhabditis elegans and subunits of the mammalian epithelial sodium channel (ENaC), and also includes a diverse array of non-voltage-gated cation channels from across animal phyla, including the mammalian acid-sensing ion channels (ASICs) and Drosophila pickpockets. ENaCs and ASICs have wide ranging medical importance; for example, ENaCs play an important role in respiratory and renal function, and ASICs in ischaemia and inflammatory pain, as well as being implicated in memory and learning. Electrophysiological approaches, both in vitro and in vivo, have played an essential role in establishing the physiological properties of this diverse family, identifying an array of modulators and implicating them in an extensive range of cellular functions, including mechanosensation, acid sensation and synaptic modulation. Likewise, genetic studies in both invertebrates and vertebrates have played an important role in linking our understanding of channel properties to function at the cellular and whole animal/behavioural level. Drawing together genetic and physiological evidence is essential to furthering our understanding of the precise cellular roles of DEG/ENaC channels, with the diversity among family members allowing comparative physiological studies to dissect the molecular basis of these diverse functions.

Keywords: DEG/ENaCs; acid-sensing ion channels; degenerins; epithelial Na+ channels; pickpockets.

Figure 1. DEG/ENaC/ASIC channel structure

A, schematic representation of subdomain architecture of an acid‐sensing ion channel (adapted from Grunder & Chen, 2010). B, schematic representation of trimeric channel structure of chicken ASIC1, where the individual subdomains are coloured as in A. For easier visualization, a subdomain architectural schematic representation is shown for only one subunit. Each subunit has a large globular extracellular domain and a transmembrane domain (TMD) consisting of two transmembrane helices (TM1 and TM2). The N‐ and the C‐terminal regions are relatively short in many family members. Key residues in the acidic pocket and at the entrance to the pore, which are crucial for the binding of divalent cations, are shown in the inset. C, structure of the homotrimer determined at low pH, thus capturing the channel in the desensitized conformation (Protein Date Bank (PDB): 6VTK; Yoder & Gouaux, 2020). Protein backbone is shown in cartoon representation while glycans are represented as sticks. Individual protomers are coloured orange, green and purple. D, top view of the channel, viewed from the extracellular region. E and F, slice through the TMD displaying the upper gate (E) contributed by Asp433 from all three subunits, whereas the lower gate (F) is constituted by His29 from each monomer. G, cartoon representation of the single monomer from the trimeric channel shown in C, where the individual subdomains are coloured as in the schematic representations shown in A and B. Images representing protein structures were generated with UCSF ChimeraX (Pettersen et al. 2021).

Figure 2. C. elegans gentle touch

A, a behavioural assay for the escape response. When an animal is touched on the anterior body, for example by a hair, it responds by reversing, followed by the execution of a deep turn, enabling it to change the direction of locomotion. Touch on the posterior body produces an opposite response, i.e. accelerated forward movement. B, the touch receptor neurons (TRNs). The TRNs, comprising ALM (anterior lateral microtubule) left and right, AVM (anterior ventral microtuble) and their posterior counterparts, PLM left and right and PVM, are required for the response to gentle body touch. Inset shows a typical calcium transient in ALM, in response to a 1 s gentle touch stimulus (grey bar), recorded in an intact animal using a genetically encoded calcium indicator (in this case, the ratiometric indicator Cameleon; Walker & Schafer, 2020). C, presumed components of the mechanotransduction machinery, identified from mapping Mec (gentle touch defective) mutants. mec‐4 and mec‐10 encode DEG/ENaC subunits; mec‐1, mec‐5 and mec‐9 encode components of the extracellular matrix required for proper MEC‐4 localization; mec‐7 and mec‐12 encode microtubule subunits and mec‐2 and mec‐6 encode stomatin and paroxonase family members, respectively, which increase channel currents and appear to function in membrane translocation.

Figure 3. Ppk modulation of Drosophila courtship behaviour

Contact detection of pheromones by gustatory hairs on the male foreleg. The hairs contain pheromone‐sensing gustatory neurons, expressing Ppks, with complementary response profiles: F cells (orange) detect female pheromones, stimulating male–female courtship; M cells (green) detect male pheromones, inhibiting male–male courtship. F cells express Ppk25/Ppk29/Ppk23 heteromeric channel; in M cells Ppk25 is thought to be replaced by an unknown subunit. Two alternative models for the role of Ppk channels are presented. In model 1, the Ppk25/Ppk29/Ppk23 heterotrimer directly senses female pheromone. In model 2, unknown olfactory receptors detect pheromone and their activation leads to Ca²⁺ influx, which in turn activates the Ppk channel complex, leading to signal amplification (based on Pikielny, 2012).