Dabbling with Piezo2 for mechanosensation.

Nature has devised various strategies for sensing food so species can coexist and exploit different types of prey. Sharks can detect amino acids in blood as low as 1 ppb, while hawks and buzzards scan the Earth from a height of ∼10,000 feet looking for rodents. However, not all birds rely on their keen sense of sight to acquire food. Albatross hover above the water to smell floating food and foraging ducks utilize sense of touch to capture insects beneath the water. In PNAS, Schneider et al. (1) examine the molecular alterations in vertebrates that lead to diverse types of feeding behavior. They study foraging birds that rely on distinct sensory input: visual (chickens) vs. tactile (ducks and geese). Dabbling ducks utilize their soft bills/beaks to sense food without visual or olfactory cues just like primates can feel touch through fingertips. Sense of touch is primitive and crucial for survival but still the least-understood sensory modality in the vertebrates. Significant efforts are underway to understand mechanosensation at a cellular and molecular level (2). The hairless skin of mammals detects touch and vibration via specialized corpuscles (Meissner and Pacinian) innervated by rapidly adapting mechanoreceptors. In ducks, the tactile information is carried out by Herbst and Grandry corpuscles located in the glabrous skin of bills, tongue, and oral cavity. In a previous study, the authors established that adult duck’s trigeminal neurons produce robust mechanically activated electrical responses in vitro. In addition, trigeminal ganglia from several species of adult tactile foraging birds contain significantly large-diameter cells, which suggested but did not prove the presence of large mechanoreceptor populations (3). The focus of the current study by Schneider et al. is to build on previous work by characterizing molecular and cellular mechanisms in the bill and trigeminal system of ducks. Another aspect is to decipher molecular alterations between tactile and visual foraging birds. The authors picked late-stage duck embryos as a model system since their development is largely complete in ovo and easily accessible to experimental manipulation. They performed ex vivo techniques to stimulate the bill and record electrical activity from the intact neurons within trigeminal ganglia. Immunostaining of duck’s embryo and electrophysiological analysis confirmed that rapidly adapting mechanoreceptors in duck bill are functional well before hatching. It has been shown that the neurotrophic factor receptor TrkA underlies the development of most thermoreceptors and nociceptors, while TrkB is responsible for mechanoreceptors (4). Schneider et al. analyzed the expression of TrkA and TrkB in the trigeminal of ducks and chicken and compared it to the reported quantifications in chickens (5) mice and rats (6, 7). Both embryonic and adult duck neurons express significantly higher TrkB (67%) compared with TrkA (7%) (Table 1). Therefore, unlike mice or chickens that do not require tactile methods to find food, duck neurons are programmed to develop more mechanoreceptors than nociceptors and thermoreceptors. To complement the results of TrkB abundance and mechanoreceptors, the authors focused on expression of mechanotransduction channel Piezo2 (8). Since its discovery in 2010, Piezo2 has been established to detect light touch (9–11), proprioception (12), and lung inflation (13) in mouse models partly based on its robust expression in sensory neurons. Indeed, mechanically gated Piezo2 channels are found in 69% of duck trigeminal neurons, while nociceptors and thermoreceptors TRPV1 and TRPM8 were expressed in 16% and 2% of the cells (Table 1). On the contrary, in chicken trigeminal neurons, Piezo2, TRPV1, and TRPM8 were present in 35%, 37%, and 10% of the cells, respectively, distributions similar to those found in mice (14).